Table of contents

Preface to the Proceedings of the 33rd International Conference on the Physics of Semiconductors, Beijing, 2016

Shaoyun Huang¹, Yingjie Xing¹, Yang Ji², Dapeng Yu³, and Hongqi Xu¹

¹Beijing Key Laboratory of Quantum Devices, Key Laboratory for the Physics and Chemistry of Nanodevices and Department of Electronics, Peking University, Beijing 100871, China

²SKLSM, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China

³State Key Laboratory for Mesoscopic Physics, Department of Physics, Peking University, Beijing 100871, China

From July 31^st to August 5^th, 2016, the 33rd International Conference on the Physics of Semiconductors (ICPS 2016) was held in Beijing, China, with a great success. The International Conference on the Physics of Semiconductors began in the 1950's and is a premier biennial meeting for reporting all aspects of semiconductor physics including electronic, structural, optical, magnetic and transport properties. Reflecting the state of the art developments in semiconductor physics, ICPS 2016 served as an international forum for scholars, researchers, and specialists across the globe to discuss future research directions and technological advancements. The main topics of ICPS 2016 included:

• Material growth, structural properties and characterization, phonons

• Wide-bandgap semiconductors

• Narrow-bandgap semiconductors

• Carbon: nanotubes and graphene

• 2D Materials beyond graphene

• Organic semiconductors

• Topological states of matter, topological Insulators and Weyl semimetals

• Transport in heterostructures

• Quantum Hall effects

• Spintronics and spin phenomena

• Electron devices and applications

• Optical properties, optoelectronics, solar cells

• Quantum optics, nanophotonics

• Quantum information

• Other topics in semiconductor physics and devices

• Special topic: Majorana fermions in solid state

All papers published in this volume of IOP Conference Series: Materials Science and Engineering have been peer reviewed through processes administered by the proceedings Editors. Reviews were conducted by expert referees to the professional and scientific standards expected of a proceedings journal published by IOP Publishing.

We report the growth of InAs/GaSb core-shell heterostructure nanowires with smooth sidewalls on Si substrates using metal-organic chemical vapor deposition (MOCVD) with no assistance from foreign catalysts. Sb adatoms were observed to strongly influence the morphology of the GaSb shell. In particular, Ga droplets form on the nanowire tips when a relatively low TMSb flow rate is used, whereas the droplets are missing and the radial growth of the GaSb is enhanced due to a reduction in the diffusion length of the Ga adatoms when the TMSb flow rate is increased. Moreover, transmission electron microscopy measurements revealed that the GaSb shell coherently grew on the InAs core without any misfit dislocations.

We study the charge transport mechanism in ferroelectric Hf_0.5Zr_0.5O₂ thin films. Transport properties of Hf_0.5Zr_0.5O₂ are described by phonon-assisted tunnelling between traps. Comparison with transport properties of amorphous Hf_0.5Zr_0.5O₂ demonstrates that the transport mechanism does not depend on the structure. The thermal and optical trap energies 1.25 eV and 2.5 eV, respectively, in Hf_0.5Zr_0.5O₂ were determined based on comparison of experimentally measured data on transport with simulations within phonon-assisted tunnelling between traps. We found that the trap density in ferroelectric Hf_0.5Zr_0.5O₂ is slightly less than one in amorphous Hf_0.5Zr_0.5O₂. A hypothesis that oxygen vacancies are responsible for the charge transport in Hf_0.5Zr_0.5O₂ is confirmed by ab initio simulation of electronic structure.

We study the charge transport mechanism of electron via traps in thermal SiO₂ on silicon. Electron transport is limited by phonon-assisted tunnelling between traps. Charge flowing leads to oxygen vacancies generation, and the leakage current increases. Long-time annealing at high temperatures decreased the leakage current to initial values due to oxygen vacancies recombination with interstitial oxygen. Taking into account results of ab initio simulations, we found that the oxygen vacancies act as electron traps in SiO₂.

In this work GaAs nanowires were grown by self-assisted growth method with completely identical growth parameters, such as growth temperature, growth time, Ga and As flux, on GaAs (111)B and Si (111) substrates using Molecular Beam Epitaxy (MBE). All samples were then characterized by Scanning Electron Microscope (SEM), Energy-dispersive X-ray spectroscopy (EDX), and X-ray Diffraction (XRD). The results from both substrates were compared in order to understand the effect of substrate type on nanowires.

The ability to explore electronic structure and their role in determining material's macroscopic behaviour is essential to explain and engineer functions of material and device. Graphene/hexagonal boron nitride heterostructure (G/h-BN) has become a model system to study the emergent behaviour in 2D van der Waals heterostructure. Here by employing angle-resolved photoemission spectroscopy with spatial resolution ∼ 100 nm (Nano-ARPES), we give a full description on the electronic structure of G/h-BN, demonstrating the power of Nano-ARPES to detect the microscopic inhomogeneity of electronic structure for different materials.

Two different layer orders of p-Si/Al (as structure #1) and Al/p-Si (as structure #2) thin films were deposited on n-type mono-crystalline Silicon wafer and quartz glass by magnetron sputtering at 200 °C. The fabricated films were annealed at different temperatures for various durations. Then, they were analyzed by XRD, Raman and SEM methods. According to XRD results the largest grain size of both structures was smaller than 20 nm. The Raman spectra of the samples annealed at 1000 °C determined a crystallization ratio of 98% and 75% of structures #1 and #2 respectively. SEM images confirmed that the crystallization happened for structure #1 at lower temperatures than for structure #2. The impact of annealing on electrical and photovoltaic performance of the samples were studied after the fabrication of metal contact by sputtering of a few hundred nanometers of aluminium. The highest measured V_oc were 360 and 402 mV, and the best J_sc were 2.2 and 0.12 mA/cm², for structure #1 and #2 respectively.

We report original method of formation Ga(In)N/AlN quantum dots with low density by ammonia MBE on the (0001)AlN surface by using a decomposition process of Ga(In)N thin layer. Low density of quantum dots have been obtained in the range 10⁷-10⁹ cm^-2. Single quantum dots photoluminescence lines corresponding to exciton and biexciton transitions were observed in micro-photoluminescence spectra.

Organometal trihalide perovskites have been demonstrated as excellent light absorbers for high efficiency solar cell [1]. In this work, Glass/ITO/PEDOT:PSS/Perovskite/PCBM/Ag planar heterojunction structure was designed and investigated. For the reference cell, the efficiency of 12.5 % was achieved. An relative low J_sc was exhibited at the result, mainly owing to the low conductivity of the electric transport layer – PEDOT:PSS. By controlling the amount of DMSO doped in PEDOT:PSS, a superior device was obtained with the efficiency of 13.6% although the roughness of the PEDOT:PSS layer was also increasing with doping DMSO. Furthermore, another key factor for a perovskite solar cell is the high crystallization of the perovskite layer[2]. Solvent annealing was adopted to improve the crystallization in this work. With dropping IPA around the solar cell and annealing for 20 minute, a surprising characteristics of the device with the efficiency of 15%, V_OC of 0.99 V, Fill Factor of 67.9%, J_SC of 22.3mA/cm² was obtained.

We report the preparation of a rhombohedral phase (space group $R\bar{3}c$) AlFeO₃ nanoparticles (NPs) and its magnetic and ferroelectric properties. The high purity rhombohedral phase AFO NPs of an average size 50 nm were prepared by sol – gel route followed by annealing at 700 °C. The NPs show antiferromagnetic nature at room temperature and near room temperature, weak ferroelectric properties. The coexistence of magnetic and ferroelectric features indicate that AFO NPs can be used as a prospective lead free multiferroic material for the spintronics applications.

Ag colloidal nanoparticles are used as a catalyst for growth of GaAs nanowires by the molecular beam epitaxy on the Si(111) and GaAs(111)B substrate surfaces. The scanning electron microscopy measurements revealed that the nanowire formation occurs in different ways on different substrates, but the parameters of the synthesized nanowires open great prospects for their further use.

AlN thin film was epitaxial grown on c-plane sapphire substrate by pulsed laser deposition. To reduce structural defects from largely lattice mismatched substrate, MgO or ZnO buffer layer was inserted between AlN and sapphire. Crystal structure and surface morphology of as prepared AlN were characterized by XRD, AFM, and SEM. It was found that buffer layers significantly improve crystalline quality of AlN, especially using ZnO. Furthermore, a general and steady wet chemical process was developed to selectively etch away ZnO layer, so that high quality free-standing AlN thin film was obtained. This film could be transferred onto any other host substrates such as Si, quartz, etc. Moreover, with no clamping effect from the substrate, the as-prepared free-standing AlN thin films may find potential applications in high sensitivity piezoelectric devices, flexible wearable detectors and so on.

Doping in Si nano-crystals (Si NCs) is currently a great challenge to develop the high-performance nano-devices. Here, we fabricated P-doped Si NCs by PECVD in Si/SiO₂ multilayered structures after annealing at high temperature. It is demonstrated experimentally that P dopants can be incorporated into Si NCs substitutionally. Furthermore, we found that the photoluminescence properties of Si NCs can be drastically modified by P doping in Si NCs/SiO₂ multilayers with the ultra-small size. A subband light emission centred at 1200nm is observed, which can be ascribed to the radiative recombination via the P-induced deep level in the gap of the Si NCs. Interestingly, it is also found that the subband emission can be enhanced obviously by B and P co-doping. Our results suggest that doping plays an important role in the electronic structures and optoelectronic characteristics of Si NCs, which provide a new route to realize the Si NCs-based optical and electronical devices.

We report on the growth of single crystal InAs NWs on Si/SiO_x substrates by chemical vapor deposition (CVD). By adjusting growth parameters, the diameters, morphology, length and the proportion of superlattice ZB InAs NWs (NWs) can be controlled on a Si/SiO_x substrate. Our work provides a low-cost route to grow and phase-engineer single crystal InAs NWs for a wide range of potential applications.

Pure and N₂ doped ZnO thin films of thickness ranging ∼300-500nm with 5,10,15,20,25, and 50 sccm inflow ratios of N₂ are deposited on soda-lime glass by means of RF magnetron sputtering system, and observed the dependence of optical properties of ZnO by the function of doping with the help of spectroscopic ellipsometer. And found that the N₂-inflow highly affects the optical properties of ZnO thin films. Even the high transmittance of about 97% is achieved and absorbance graph also shows that slight variation in N₂ inflow affects the absorbance, which is maximum with in UV region. Optical conductivity of ZnO is also observed high with the increase of N₂ inflow. With 25sccm N₂ inflow conductivity rose to the maximum value of about 1.4 × 10⁷Ω^-1cm^-1, with 15sccm inflow of N₂ conductivity value is 2.0 × 10⁶Ω^-1cm^-1 in the visible region. This is a strong contribution towards next generation photovoltaic devices.

Physical vapor transport (PVT) is the most successful and widely used approach for bulk aluminum nitride (AlN) single crystals. During the process of PVT growing AlN crystals, crucible materials, the growth setup, and the growth parameters (e.g., temperature distribution, growth pressure) are crucial. This work proposes a detailed study on the PVT growth of single AlN crystals with sizes ranging from nanometers to centimeters. AlN crystals with different sizes are grown by spontaneous nucleation. Furthermore, it discusses and contrasts the growth conditions and mechanisms of AlN crystals with different sizes. The structural and optical properties of the AlN crystals are also involved.

We report the variation of the yellow luminescence (YL) in GaN bulk single crystals by gamma-ray irradiation. The crystals are irradiated at room temperature with gamma-rays of 1.17 and 1.33 MeV from a cobalt-60 source. Gamma-ray dose is 160 kGy. The resistivity varies from 30 Ωcm for an un-irradiated sample to 10⁴ Ωcm for gamma-ray irradiated one. The nitrogen displacement in gamma-ray irradiated samples is observed by Rutherford backscattering channeling experiments using proton beam, suggesting the existence of the deep energy level relating to interstitial nitrogen atoms. The YL from the un-irradiated GaN with a peak at 557 nm (2.22 eV) is observed at around 440 nm to 800 nm, whereas that of the gamma-ray irradiated GaN shows a peak at 532 nm (2.33 eV) although the YL spectrum is almost overlapped with un-irradiated ones. Compton electrons emitted by the gamma-ray irradiation induce the shallow donor located at about 50 meV bellow the conduction band. This energy level is close to that of nitrogen vacancy. The modification of YL is attributed to a transition from the shallow donor induced by the gamma-ray irradiation to the native gallium vacancy.

Persistent Photoconductivity (PPC) in hydorogen-ion implanted (001) oriented KNbO₃ bulk single crystals (perovskite structure at room temperature; ferroelectric with a band gap of 3.16 eV) is studied in air at room temperature to prevent the crystallinity degradation caused by the phase transition. Hydrogen is implanted into KNbO₃ bulk single crystals using the energy (the peak ion fluence) of 500 keV (5.0 × 10¹⁵ cm^-2). The resistivity varies from ∼10⁸ Ω/□ for an un-implanted KNbO₃ sample to 2.3 × 10⁵ Ω/□ for as-implanted one. suggesting the presence of donors consisting of hydrogen interstitial and oxygen vacancy. The PPC is clearly observed with ultraviolet and blue LEDs illumination rather than green and infrared, suggesting the release of electrons from the metastable conductive state below the conduction band relating to the charge states of the oxygen vacancy as observed in electron irradiated ZnO.

In this report, are shown the results of high temperature resistivity and Hall Effect studies of Mg-doped GaN epilayers. The samples studied were grown on (0001) (c-plane) sapphire by molecular beam epitaxy and 0.5 μm GaN:Mg layers have been achieved on low temperature buffers of GaN (30 nm) and AlN ( 150 nm). The experiments were carried out in the temperature range from 300 K up to 900 K. Up to about 870 K a typical thermally activated conduction process has been observed with the activation energy value E_A = 215 meV. However, for higher temperatures, an annealing effect is observed in all the investigated samples. The increase of the free carrier concentration as a function of time leads to an irreversible decrease of sample resistivity of more than 60%.

Lower Ti/Al/Ni/Au Ohmic contact resistance on AlGaN/GaN with wider rapid thermal annealing (RTA) temperature window was achieved using recessed Ohmic contact structure based on self-terminating thermal oxidation assisted wet etching technique (STOAWET), in comparison with conventional Ohmic contacts. Even at lower temperature such as 650°C, recessed structure by STOAWET could still obtain Ohmic contact with contact resistance of 1.97Ω·mm, while conventional Ohmic structure mainly featured as Schottky contact. Actually, both Ohmic contact recess and mesa isolation processes could be accomplished by STOAWET in one process step and the process window of STOAWET is wide, simplifying AlGaN/GaN HEMT device process. Our experiment shows that the isolation leakage current by STOAWET is about one order of magnitude lower than that by inductivity coupled plasma (ICP) performed on the same wafer.

Cavity-exciton polaritons have attracted much interest because these light-matter quasiparticles are very promising for various optoelectronic applications. Bragg-polaritons have been discussed as new tools for tailoring light-matter interactions. These structures can be created by incorporating quantum wells (QWs) periodically into a distributed Bragg reflector (DBR). The advantage of this sample type is the high number of QWs which can be embedded into the sample in order to increase the Rabi-splitting energy. Calculations show, that the coverage of the sample with a thin metal layer results in an increase of the temperature stability of the strong coupling regime. In addition, this concept enables a specific spectral variation of the cavity resonance allowing for the manipulation of the light-matter interaction.

Al_xGa_1-xN epilayers with x ranging from 0.20 to 0.50 have been grown on c-plane sapphire substrate by metal-organic chemical vapor deposition. The thickness, composition, strain and stress values of the AlGaN were determined by high resolution X-ray diffraction. The optical properties of the epilayers were studied using photoluminescence (PL) and reflectivity measurements. The effect of the stress on the bandgap can be explained by room temperature PL. The temperature dependent PL result shows the well-known "S-shape" behavior.

In this paper, we proposed to use step heterojunctions emitter spacer (SHES) and InGaN sub-quantum well in AlGaN/GaN/AlGaN double barrier resonant tunnelling diodes (RTDs). Theoretical analysis of RTD with SHES and InGaN sub-quantum well was presented, which indicated that the negative differential resistance (NDR) characteristic was improved. And the simulation results, peak current density J_P=82.67 mA/μm², the peak-to-valley current ratio PVCR=3.38, and intrinsic negative differential resistance R_N=-0.147Ω at room temperature, verified the improvement of NDR characteristic brought about by SHES and InGaN sub-quantum well. Both the theoretical analysis and simulation results showed that the device performance, especially the average oscillator output power presented great improvement and reached 2.77mW/μm² magnitude. And the resistive cut-off frequency would benefit a lot from the relatively small R_N as well. Our works provide an important alternative to the current approaches in designing new structure GaN based RTD for practical high frequency and high power applications.

AlGaN/GaN high-electron mobility transistor's (HEMT's) off-state breakdown is investigated using conventional three-terminal off-state breakdown I–V measurement. Competition between gate leakage and source-injection buffer leakage (SIBL) is discussed in detail. It is found that the breakdown is dominated by source-injection which is sensitive to gate voltage and gate length at large gate-to-drain spacing (Lgd > 7μm), where a threshold drain voltage of the occurrence of the SIBL current in GaN buffer exists, and after this threshold voltage the SIBL current continually increased till the buffer breakdown. Our analysis showed that due to the punch-through effect in the buffer, a potential barrier between 2DEG and GaN buffer at the source side mainly controlled by the drain voltage determines the buffer leakage current and the occurrence of the following buffer breakdown, which could explain the experimentally observed breakdown phenomenon.

We explain the 1/4-cycle phase shift of minima in the microwave-induced magnetoresistance oscillations in high mobility two dimensional electron systems. We calculate the minima positions obtaining an exact coincidence with the experimental ones. We find that the physical origin of the 1/4-cycle phase shift is a delay of $\frac{\pi }{2}$ of the radiation-driven Landau guiding center with respect to radiation, demonstrating the oscillating nature of the Landau states under irradiation.

Lithographically fabricated mesoscopic wires and stadia, coupled to a decohering classical environment by two side-wires, were studied using low-temperature quantum transport to obtain quantum phase coherence lengths. Longer phase coherence lengths were observed in longer wires, an effect due to environmental coupling decoherence being mitigated by averaging over longer wire lengths. In stadia the dominant decoherence mechanism appears as device-device, here stadium-wire, coupling decoherence rather than environmental coupling decoherence. Consistency between the experimental results and a decoherence mechanism due to stadium-wire coupling is found in the relation between lower decoherence rate and relative geometrical similarity between the stadia and their connecting wires.

In this paper, magneto-transport properties of the LPE-grown and anodic oxidated p-type Hg_1-xCd_xTe(x=0.237) films have been studied by using maximum entropy mobility spectrum analysis (ME-MSA) technique. It can be found that the high-mobility electron (μ_e∼2 × 10⁴cm²/Vs) has considerable contributions to the conduction of anodic oxidated Hg_1-xCd_xTe(x=0.237) film, but not in LPE-grown Hg_1-xCd_xTe(x=0.237) film. The high-mobility electron maintains dominant contributions from 11k to 150k, which can be attributed to two-dimensional electron gas in the inversion layer of anodic oxidated p-type Hg_1-xCd_xTe(x=0.237) film. In addition, we also observe the nonphysical contributions of low mobility electrons (μ_e∼0.08 × 10⁴cm²/Vs) in mobility spectrum of both LPE-grown and anodic oxidated p-type HgCdTe films. The low-mobility electrons, so-called mirror peaks, can be interpreted as a consequence of magnetic freeze-out of holes in vacancy-doped HgCdTe, which disappeared at T=150k.

The existence of a range of an anomalous growth in charge carrier mobility under the transition to heavy doping is established for Bi₂Te₃ -Sb₂Te₃ solid solutions. This confirms our suggestion about the universal character of critical phenomena accompanying the transition from impurity discontinuum to impurity continuum. The experimental results are analyzed in terms of percolation theory taking into account alloy scattering and spatial correlations of impurity centers.

It was recently shown that a finite imbalance between electron densities in the K and K' valleys of bilayer graphene induces a magnetoelectric coupling. Here we explore ramifications of this electronically tunable magnetoelectric effect for the optical conductivity and dielectric permittivity of this material. Our results augment current understanding of longitudinal magnetoresistance and magnetocapacitance in unconventional materials.

Electronic structure describes the distribution of electronic states in reciprocal space, being one of the most fundamental concepts in condensed matter physics, since it determines the electrical, optical and magnetic behaviours of materials. Due to its two-dimensional honeycomb lattice with covalent bonding, pristine graphene exhibits unsurpassed in-plane stiffness and stable structural properties. Here by employing angle-resolved photoemission spectroscopy with spatial resolution ∼ 100 nm (Nano-ARPES), we discuss in detail the structural and electronic properties of graphene grown on cooper by chemical vapour deposition (CVD). Our results reveal the spatial inhomogeneity of graphene film, demonstrating the power of Nano-ARPES to detect the microscopic inhomogeneity of electronic structure for different materials.

Charge carrier mobility is a central transport property in nanoscale electronics. Carbon nanotubes (CNTs) are supposed to have high carrier mobility. The preparation methods of CNTs have been greatly improved, but the defects always exist. This work presented first-principle investigations on the charge carrier mobility of carbon nanotubes containing several intrinsic defects. The charge carrier mobilities of zigzag (10, 0) tubes with Stone–Wales, mono vacant and 5/8/5 defects were studied as an example to explore the role of defects. Most carrier mobilities were decreased, but several values of mobility are unexpectedly increased upon the appearance of the defects. This interesting result is discussed based on the changes of the stretching modulus, the effective mass of the carrier and deformation potential constant induced by the defects.

Ultrathin sheets of MoS₂ are a newly discovered 2D semiconductor that holds great promise for nanoelectronics. Understanding the pattern of current flow will be crucial for developing devices. In this talk, we present images of current flow in MoS₂ obtained with a Scanned Probe Microscope (SPM) cooled to 4 K. We previously used this technique to image electron trajectories in GaAs/AlGaAs heterostructures and graphene. The charged SPM tip is held just above the sample surface, creating an image charge inside the device that scatters electrons. By measuring the change in resistance ΔR while the tip is raster scanned above the sample, an image of electron flow is obtained. We present images of electron flow in an MoS₂ device patterned into a hall bar geometry. A three-layer MoS₂ sheet is encased by two hBN layers, top and bottom, and patterned into a hall-bar with multilayer graphene contacts. An SPM image shows the current flow pattern from the wide contact at the end of the device for a Hall density n = 1.3×10¹² cm^-2. The SPM tip tends to block flow, increasing the resistance R. The pattern of flow was also imaged for a narrow side contact on the sample. At density n = 5.4×10¹¹ cm^-2; the pattern seen in the SPM image is similar to the wide contact. The ability to image electron flow promises to be very useful for the development of ultrathin devices from new 2D materials.

Two-dimensional transition-metal dichalcogenides such as MoS₂ are promising materials for next-generation nano-electronic devices. The physical properties of MoS₂ are determined by layer number according to the variation of band-gap. Here, we synthesize large-size bilayer-MoS₂ with triangle and hexagonal nanosheets in one step by chemical vapor deposition, Monolayer and bilayer-MoS₂ back-gate field effect transistors are also fabricated and the performance including mobility and on/off ratios are compared. The bilayer-MoS₂ back-gate field effect transistor shows superior performance with field effect mobility of ∼21.27cm²V^-1s^-1, and I_on/I_off ratio of ∼3.9×10⁷.

The physics of excitons, electron-hole pairs that are bound together by their mutual Coulomb attraction, can to great extent be understood in the framework of the quantum-mechanical hydrogen model. This model has recently been challenged by spectroscopic measurements on two-dimensional transition-metal dichalchogenides that unveil strong deviations from a hydrogenic spectrum. Here, we show that this deviation is due to the particular relativistic character of electrons in this class of materials. Indeed, their electrons are no longer described in terms of a Schrödinger but a massive Dirac equation that intimately links electrons to holes. Dirac excitons therefore inherit a relativistic quantum spin-1/2 that contributes to the angular momentum and thus the exciton spectrum. Most saliently, the level spacing is strongly reduced as compared to the hydrogen model, in agreement with spectroscopic measurements and ab-initio calculations.

Nonequilibrium current fluctuation is theoretically studied through a multilevel quantum dot in the Coulomb blockade regime, where the higher-order tunneling processes, "cotunneling," are dominant in the transport. The current fluctuation shows a Poisson (super-Poisson) noise in the elastic (inelastic) cotunneling regime when the bias voltage eV between the leads is smaller (larger) than the excitation energy Δ from the ground level in the quantum dot. The super-Poisson noise is remarkably enhanced by the degeneracy of the excited states around the beginning of inelastic cotunneling at eV ≈ Δ, qualitatively in good agreement with a recent experimental result. An analytical expression is given for the current fluctuation as a function of degeneracy.

We investigate the excitonic dephasing of transition metal dichalcogenides, namely MoS₂, MoSe₂ and WSe₂ atomic monolayer thick and bulk crystals, in order to understand the factors that determine the optical coherence in these materials. Coherent nonlinear optical spectroscopy, temperature dependent absorption combined with theoretical calculations of the phonon spectra, reveal the important role electron-phonon interactions plat in dephasing process. The temperature dependence of the electronic band gap and the excitonic linewidth combined with 'ab initio' calculations of the phonon energies and the phonon density of state reveal strong interaction with the E' and E" phonon modes.

Despite the great progress made recently in spectroscopic imagery and even the remarkable success achieved, the challenge still remain concerning the precise determination of the chemical and electronic imagery of advances materials, which usually are available as heterogeneous large crystals or tiny homogeneous monocrystals. Here we report, a recently developed novel X-ray microscope, labelled, k-microscope or Nano-ARPES (Nano Angle Resolved Photoelectron Spectroscopy) particularly well suited to provide both high resolved chemical and electronic information in the real and reciprocal space of complex materials with nano-scale resolution.

Two-dimensional (2D) MoS₂ materials possess indirect-to-direct bandgap tunability, and have enjoyed wide applications in electronics and optoelectronics. Most of investigators have ubiquitously synthesized these materials by using the chemical vaporization deposition (CVD) method. Here we have adopted MoO₃ source materials to synthesize MoS₂ on 280-nm SiO₂/Si substrates via molecular beam epitaxy (MBE). We have obtained triangular nucleation, tens-of-micron domain, and monolayer MoS₂. This MBE technique can be applied to synthesizing other members of semiconducting layered transition metal dichalcogenides (TMDCs), such as WS₂, MoSe₂, and WSe₂ materials.

Recently, the study of topological crystalline insulators has become of great interest in condensed matter physics. In this work, we present infrared magneto-optical investigation of such novel quantum states in narrow gap rocksalt IV-VI semiconductor Pb_1-xSn_xTe which exhibits a topological phase transition at a critical value x_c≈0.40. High-mobility Bi-doped trivial and non-trivial (111)-Pb_1-xSn_xTe (0≤x≤0.56) films grown on BaF₂ substrates by MBE were examined at 4.5K and magnetic fields B=0-15T. Massive and massless Dirac fermion models are used to analyze transmission spectra. We are able to determine the band parameters of the bulk and the topological surface states of such material.

In this paper, we investigated the magnetic topological insulator (MTI) Cr-doped Bi₂Se₃ film using first principles calculations based on the density functional theory (DFT). The band structure of Cr doped 3QL-Bi₂Se₃ film was calculated comparing with pure Bi₂Se₃ film. Our results demonstrate that the doping of Cr atom changes the degenerate surface state of pure Bi₂Se₃, inducing the ferromagnetism. Under the external electric field, the band gap of pure Bi₂Se₃ films is determined by the charge transfer and the effect of spin-orbital coupling (SOC). For the MTI, the electric field will redistribute the electrons and enhance the magnetism. Our results will further promote the development of the electronic and spintronic applications of topological insulator.

This report studied the magneto-transport properties in the hybrid structure which combines thin flakes of 3D topological insulator (TI) Bi₂Te₃ with topside indium superconducting electrodes. The observed anomalous magnetoresistance suggests two critical transitions. The first transition, obtained approximately at T = 3.4 K, is attributed to superconductivity in the indium electrodes. While the second transition, observed at lower temperature, is attributed to the proximity effect generated from the interface between the TI and the superconductor.

Using high-resolution Nano-Angle Resolved Photoemission Spectroscopy (Nano-ARPES), we have determined the electronic structure of the surface and bulk states of topological insulator Sb₂Te₃ nanowires, which have been also characterized by magnetoresistance measurements. The observed Aharonov-Bohm-type oscillations could be unambiguously related to the transport by topological protected surface states directly recorded by photoemission. We have measured Nano-ARPES on individual nanowires of a few nanometers wide to provide direct evidence of the existence of the nontrivial topological surface states, as well as their doping. Our findings are consistent with theoretical predictions and confirm that the surface states of intrinsically doped unidimensional topological insulator nanowires are responsible for the quantum transport.

The goal of the present work is to reveal effects accompanying the band inversion in Pb_1-xSn_xTe solid solutions by measuring heat capacity and microhardness. The objects of the study are Pb_1-xSn_xTe alloys with Sn concentrations in the range of x = (0.59 – 0.68), near the composition corresponding to the transition to a gapless state close to room temperature. It was established that in the Pb_1-xSn_xTe solid solutions, the transition to the bulk gapless state with the band inversion is manifested through the appearance of peaks in the dependences of specific heat and microhardness on composition at a fixed temperature.

A physical model describing the piezoelectric-effect-mediated influence of bending of a thin suspended cantilever with a two-dimensional electron gas on the conductivity is proposed. The model shows that the conductivity change is almost entirely caused by the rapid change in mechanical stress near the boundary of suspended and non-suspended areas, rather than by the stress itself. An experiment confirming that the electromechanical coupling is associated with the piezoelectric effect is performed. The experimentally measured conductance sensitivity to the cantilever's vibrations agree with the developed physical model.

To improve the quality of hybrid p-polymer/n-zinc oxide (ZnO) nanowire heterojunctions a thin polystyrene (PS)-based passivating interlayer is deposited on plain aqueous-chemically grown ZnO nanowires by spin coating. The structural investigation of the PS-coated nanowires via scanning electron microscopy shows that the concentration of PS in the solution affects the thickness of the PS layer. For vertically aligned nanowires a concentration of 0.125 g in 10 ml toluene is needed to get thin homogeneous passivating layers. Photoluminescence measurements of the PS-coated nanowires show a decrease of the deep-level emission depending on the PS concentration which indicates a passivation on the nanowire surface without affecting the general optical properties of the ZnO. Current-voltage characteristics of these samples show a decreased hysteresis and an increased current with decreasing PS concentration/PS layer thickness. However, a reasonably thin layer reduces the tunnelling through trap states of the ZnO nanowires.

The longitudinal and transverse piezoresistance coefficients of Ge at room temperature are represented graphically as a function of the crystal directions for orientation (001), (110) and (211) planes. Many valley model of conduction band and stress decoupling decoupling of the degenerate valence band into two bands of prolate and oblate ellipsoidal energy surface are shown to explain origin of the piezoresistance. One this basis, comparison between piezoresistance coefficient and theoretical model is discussed.

The impact of how to model phonon scattering on hole transport in Si nanowires was studied based on Boltzmann's transport equation. Boundary conditions for atomistic description of phonons in nanowires and approximation by bulk acoustic and optical phonons were analyzed in terms of their impacts on high-field hole transport. The boundary conditions for phonons influence the drift velocity and momentum relaxation time, especially at low electric field, but the energy relaxation time hardly depends on the boundary conditions. The impacts by the change of boundary conditions can be approximated by the change of the strength of acoustic phonon scattering in bulk phonon picture, though the behavior of energy relaxation and distribution function of holes can not be reproduced by bulk phonon approximation.

The magneto-transport characteristics of the negative conductivity/resistivity state in the microwave photo-excited two-dimensional electron system (2DES) is examined through a numerical solution of the associated boundary value problem. The results suggest, surprisingly, that a bare negative diagonal conductivity/resistivity state in the 2DES under photo-excitation should yield a positive diagonal resistance along with a sign reversal in the Hall voltage.

A systematic comparative study of radiation-induced magneto-resistance oscillations using circularly polarized- and linearly polarized microwaves was carried out on the high mobility GaAs/AlGaAs heterostructure two dimensional electron system (2DES). The results showed that, the sinusoidal sensitivity in the amplitude of the radiation-induced magnetoresistance oscillations observed under launcher rotation for linearly polarized microwaves, is remarkably absent in the similar experiment carried out with circularly polarized microwaves.

The electron and phonon transport in hybrid graphene-BN zigzag nanoribbons are investigated by the nonequilibrium Green's function method combined with density functional theory calculations. A 100% spin-polarized electron transport in a large energy window around the Fermi level is found and this behavior is independent of the ribbon width as long as there contain 3 zigzag carbon chains. The phonon transport calculations show that the ratio of C-chain number to BN-chain number will modify the thermal conductance of the hybrid nanoribbon in a complicated manner.

We present an experimental study aimed at extracting the microwave radiation-induced magnetoresistance oscillations from the bell-shape giant magnetoresistance in high mobility GaAs/AlGaAs devices using a multi-conduction model. The results show that the multi-conduction model describes the observed giant magnetoresistance effect and the model helps to extract radiation-induced magnetoresistance oscillations, over a wider parameter space.

The splitting of the delta-layers in the DA-pHEMT heterostructures has resulted in the increase of the spacer's effective thickness and growth of the low-field 2DEG mobility from 4000÷5000 cm² V^-1 s^-1 up to 6500 cm² V^-1 s^-1 at the temperature of 300 K and 2DEG density of 4.0×10¹² cm^-2. The 2DEG mobility in the δ-splitted DA-pHEMT heterostructures almost coincides with the mobility in standard pHEMT heterostructures, but the 2DEG density in the DA-pHEMT heterostructures is approximately twice higher. The additional potential barrier in the DA-pHEMT heterostructures formed by the acceptors causes the reduction of the real-space transfer effect. Therefore, the drift saturation velocity in these heterostructures is higher than the drift saturation velocity in standard pHEMT heterostructures.

The result of studies of resonant tunnelling of charge carriers in InGaN/GaN unipolar structure is presented. Authors show that at temperatures below 150 K the multiple negative-differential resistance regions are observed on a reverse current-voltage characteristic which is typical for inhomogeneous distribution field for sample with 6 nm barrier thickness. At GaN barrier thickness 3 and 12 nm resonant tunneling were not observed.

In this work, the transport of tunnel field-effect transistor (TFET) based on vertically stacked hereto-structures from 2D transition metal dichalcogenide (TMD) materials is investigated by atomistic quantum transport simulations. WTe2-MoS2 combination was chosen due to the formation of a broken gap hetero-junction which is desirable for TFETs. There are two assumptions behind the MoS2-WTe2 hetero-junction tight binding (TB) model: 1) lattice registry. 2) The S − Te parameters being the average of the S − S and Te − Te parameters of bilayer MoS2 and WTe2. The computed TB bandstructure of the hetero-junction agrees well with the bandstructure obtained from density functional theory (DFT) in the energy range of interest for transport. NEGF (Non-Equilibrium Green's Function) equations within the tight binding description is then utilized for device transfer characteristic calculation. Results show 1) energy filtering is the switching mechanism; 2) the length of the extension region is critical for device to turn off; 3) MoS2-WTe2 interlayer TFET can achieve a large on-current of 1000µA/µm with V_DD = 0.3V, which suggests interlayer TFET can solve the low ON current problem of TFETs and can be a promising candidate for low power applications.

Zitterbewegung (ZB; trembling motion) is one of the peculiar phenomena predicted in the Dirac equation in that the velocity and the spatial coordinate cannot be well defined kinetic constant. While the ZB in vacuum is far out of reach, feasibility of observation in solids has been discussed though very few experiments have been reported yet. We here report the ZB in two-dimensional electron gas with strong Rashba-type spin-orbit coupling causes large fluctuation in conductance. Such an experiment was impossible until recent development of point contact injectors with spin-polarised electrons. Numerical calculations on a simple model successfully simulate the experimental observations.

Microwave radiation-induced magneto-resistance oscillations are examined under bichromatic excitation for various frequency combinations in order to obtain a better understanding of the lineshape observed in the dual excitation experiment of the high mobility GaAs/AlGaAs 2D electron system. Here, we examine superposition- or lack thereof- in the lineshape observed in the bichromatic experiment, and report a trend observed between the monochromatic and bichromatic responses of the oscillatory diagonal resistance.

We report the evolution of the phase shift, θ₀, extracted from traces of the diagonal resistance, R_xx, vs. the linear polarization angle, θ, at oscillatory extrema of the microwave radiation induced magnetoresistance oscillations over the 36 ≤ f ≤ 40 GHz band in GaAs/AlGaAs system. A reference phase shift for the linear polarization angle in the vicinity of the specimen is obtained with the help of a sensitive carbon resistor. We fit an empirical cosine square law to the sinusoidal responses of R_xx vs. θ to extract the phase shift θ₀. The quasi-continuous variation θ₀ vs. f trace suggests a preferable polarization orientation for the specimen, and the f− and B− independence of overall average of θ₀.

Remotely sensed microwave reflection was measured concurrently with standard magnetotransport in photo-excited high mobility GaAs/AlGaAs heterostructure. Experiments indicate strong reflection resonance on both sides of the magnetic field axis for linearly polarized microwave/terahertz photo-excitation over the examined frequency 30 < f < 330 GHz band. In addition, there is evidence for electronic heating in the vicinity of cyclotron resonance (CR), which is indicated by reduced amplitude of the Shubnikov-de Haas oscillations. Effective mass extracted from the measurements was found to equal the CR mass within experimental error.

We have investigated the magnetic dipole moments of interstitial oxygen molecules (O₂) and their interactions in CdSe by using spin-dependent first-principle calculations based on the local density functional (DFT) theory. We constructed supercells of Cd₆₄Se₆₄ by repeating the primitive cell Cd₂Se₂ 4×4×2 times. Then an O₂ molecule aligned along c-axis was added to the center of the CdSe cage. The charge densities of the complexes were then computed for both spin-up and spin-down electrons. Their difference indicates that O₂ molecule in CdSe is paramagnetic, although its magnetic dipole moment is lower than that in free space. Two such O₂ molecules were then placed either (a) parallel to each other in side-by-side supercells or (b) in neighboring supercells on top of each other. The computed energies of the resultant magnetic structures suggest that the two O₂ magnetic moments interact anti-ferromagnetically in case (a) but do not interact in (b).

The Aharonov-Casher (AC) effect is theoretically examined in a mesoscopic ring with an embedded quantum dot in a multi-terminal geometry. We examine a realistic model for such a system with spin-orbit interaction in the ring. A spin-polarized current can be generated in a three-terminal device even in the absence of magnetic field, whereas it cannot in a two-terminal device. The spin polarization is enhanced by the resonant tunneling around the Coulomb peak and Kondo effect in the Coulomb blockade regime. The efficiency for the spin polarization is discussed in realistic devices.

Experimental studies of spin transport in a two-dimensional electron gas hosted by a triple GaAs/AlGaAs quantum well are reported. Using time-resolved Kerr rotation, we observed the precession of the spin polarization about a current-controlled spin-orbit magnetic field. Spatially-resolved imaging showed a large variation of the electron g-factor and the drift transport of coherent electron spins over distances exceeding half-millimetre in a direction transverse to the electric field.

In this paper, we report the effects of Mn²⁺ and the defect on ferromagnetism in Mn-doped ZnO thin films prepared by radio frequency magnetron sputtering at different annealing temperature below 600 °C. The result of superconducting quantum interference device reveals that the sample which annealed at 500 °C realized ferromagnetism till room temperature. The X-ray diffraction results shows that the samples are wurtzite structure and the sample annealed at optimum temperature gets relatively better crystallinity. X-ray photoelectron spectroscopy results show clear evidence of oxygen vacancy increment for the samples which annealed at 500 °C. Our results indicate that Mn²⁺ ions replace the Zn sites and no evidence for either metallic Mn or Mn-related oxide in the samples. It was concluded that the existence of ferromagnetism was closely related to the concentration of Mn²⁺ and the defect of the ZnO matrix.

In this paper, a novel 200V lateral IGBT on thin SOI layer with a band-to-band tunneling junction near the anode is proposed. The structure and the operating mechanism of the proposed IGBT are described and discussed. Its main feature is that the novel IGBT structure has a unique abrupt doped p++/n++ tunneling junction in the side of the anode. By utilizing the reverse bias characteristics of the tunneling junction, the proposed IGBT can achieve excellent reverse conducting performance. Numerical simulations suggest that a low reverse conduction voltage drop V_R=−1.6V at a current density of 100A/cm² and a soft factor S=0.63 of the build-in diode are achieved.

Stable ultrathin gold nanowires (Au NWs, 2nm diameter) were produced using wet chemical synthesis, which implied oleylamine molecules, and subsequently assembled onto gold electrodes. Transport properties of fabricated structures were studied using the noise spectroscopy technique. Lorentzian-shaped components were revealed in the noise spectra. The characteristic frequencies of these components were independent of the bias applied, which thus excludes self-heating of the nanowires. Noise peculiarities were shown to be the result of the presence of oleylamine molecules on the NWs. The evidence of oleylamine on the surface of the fabricated structures was confirmed by high-resolution transmission electron microscopy. Organic monolayers can strongly influence charge properties in nanosystems. These results should be taken into account in designing nanowire-based devices for molecular electronics.

Memristor is an emerging technology aimed at implementing neuromorphic computing in hardware system. Resistive random access memory (RRAM) is a kind of memristor with excellent performance, but abrupt switching in the set process influences the efficiency of neuromorphic system. In this study, we present an interface switching memristor device based on TiN/Si/TaOx/TiN stack and CMOS compatible fabrication process to achieve gradually resistive switching both in set and reset processes. The devices show a more than 10 switching window. The related switching mechanism is discussed.

Recently, Cu₂ZnSnS₄ (CZTS) with band gap about 1.50 eV is predicted to become an ideal light absorption material due to the abundant component elements in the crust being nontoxic and environmentally friendly. However, CZTS solar cells made by high temperature and vacuum-processed are at a perceived cost disadvantage in compared with solution-processed systems such as organic and hybrid solar cells. In this study, we propose a hybrid solar configurations with solution-processed CZTS nanocrystals and P3HT:PCBM bulk heterojunction. The forming double heterojunction, as charge can be separated at both the P3HT:PCBM and CZTS:PCBM interface is attributed to enhance the light harvesting efficiency. As a result, organic solar cells with CZTS nanocrystals show the higher efficiency 3.32 % compare to 2.65 % of reference organic solar cells. A 25 % improvement of power conversion efficiency is obtained by the increasing in short-circuit current and fill factor.

Three surface modifications of indium tin oxide (ITO) are experimentally investigated to improve the performance of small-molecule organic solar cells (OSCs) with an ITO/anode buffer layer (ABL)/copper phthalocyanine (CuPc)/fullerene/bathocuproine/Ag structure. An ultrathin Ag ABL and ultraviolet (UV)-ozone treatment of ITO independently improve the durability of OSCs against illumination stress. The thin pentacene ABL provides good ohmic contact between the ITO and the CuPc layer, thereby producing a large short-circuit current. The combined use of the abovementioned three modifications collectively achieves both better initial performance and durability against illumination stress.

We report the improved characteristic emission of Er³⁺ ions by co-doping wide-band-gap SnO₂ nanocrystals (NCs), which as sensitizers of Er³⁺ ions. It is found that the Er³⁺-related near infrared emission is enhanced by three orders of magnitude, which can be attributed to the effective energy transfer process occurs between SnO₂ nanocrystals and Er³⁺ ions. Meanwhile, we also prepare the Er³⁺-Yb³⁺ co-doped NaYF₄ NCs with Gd³⁺ dopant ions to obtain high-efficiency up-conversion emission by energy transfer from Yb³⁺ to Er³⁺. We demonstrate experimentally that Gd³⁺ dopant ions play key roles in the formation of β-NaYF₄ NCs which can improve the energy transfer efficiency from Yb³⁺ to Er³⁺ obviously. Consequently, the up-conversion emission is enhanced by more than five times.

Nd doped, Nd-Al co-doped anatase phase TiO₂ thin films were fabricated on silicon substrates by laser ablation and post annealing. The result of measuring PL spectra showed intense emissions from a luminescent centre of Nd³⁺ ions in these thin films. The luminescence of Nd-Al co-doped samples were stronger and broader than only Nd doped samples. Doping concentration of Nd determined suitable co-doping concentration of Al and post annealing temperature. Also there is the difference of shape of the PL spectra which indicates that the difference of local fine structure around Nd. Based on these facts, XAFS spectra was measured and suggested that the coordination around Nd was changed by this Al-co-doping.

The mid-infrared photo-induced absorption relaxation kinetics of the self-assembled array of GeSi quantum dots in Si matrix was studied in the conditions of short pulsed interband optical excitation. The measured absorption decay curves directly show the temporal evolution of the population of the QD ground states. The analysis of the experimental data allowed us to estimate characteristic recombination and capture times, and the electron localization energy in the vicinity of QD.

The influence of transverse and lateral electric field on the mid-infrared intersubband light absorption is experimentally investigated in tunnel-coupled GaAs/AlGaAs quantum wells. Observed absorption modulation in electric field is related to the electron redistribution between the quantum well states resulting in variation of space charge in the structure. This phenomenon in transverse electric field is connected with a change of potential profile, whereas in lateral field effect is caused by electron heating. Electron heating also leads to a modulation of near-infrared interband photoluminescence under lateral electric field.

We report photoluminescence investigations of heavily doped Al_xGa_1-xN:Si films grown by molecular beam epitaxy on sapphire substrates. The wide intensive defect-related band dominates in the photoluminescence spectra of Al_xGa_1-xN:Si films with the Al content higher than 0.38 covering the whole visible spectral range. This band is attributed to donor-acceptor and free electron-acceptor transitions involving the same acceptor. The acceptor ionization energy of about 1.87 eV for heavily doped AlN:Si was obtained, decrease of Al content leads to decrease of the acceptor ionization energy. The donor was assigned to the Si atom on the Ga/Al site; the acceptor might be (0/-) transition level of the C_N or (2-/3-) transition level of the V_Al.

We report on the observation of the circular photon drag effect in a bulk semiconductor. The photocurrent caused by a transfer of both linear and angular momenta of photons to charge carriers is detected in tellurium. Dependencies of the photocurrent on the light polarization and on the incidence angle agree with the symmetry analysis of the circular photon drag effect. Experimental spectral data on the photocurrent in mid-infrared range qualitatively agree with a microscopic model of the circular photon drag effect considering intersubband optical transitions of holes in tellurium.

We report on the studies of the surface plasmon polaritons in n-GaN epitaxial layers. The grating etched on the surface of the epitaxial layer is used to excite surface plasmon polaritons by means of terahertz photons. The experimental reflectivity spectrum for p-polarized radiation demonstrates a set of resonances associated with excitation of different surface plasmon polariton modes. Spectral peculiarities due to the diffraction effect have been also revealed. Emission of terahertz radiation is investigated under epilayer temperature modulation by electric current. The emissivity spectrum of the epitaxial layer with surface-relief grating shows peaks in the frequency ranges corresponding to the decay of the surface plasmon polariton modes. The characteristic features of the reflectivity and emissivity spectra are well described theoretically by a differential method with explicit integration scheme.

The optical and structure properties of the PbS nanoclusters in the Langmuir-Blodgett matrix as well as their properties after the matrix removal in the ammonium atmospheres have been studied. According to the electron microscopy data, it has been established that there are PbS nanoclusters formed on the surface of the samples. It has been found that the matrix removal results in a significant enlargement of the PbS nanocrystals accompanied by the shift of the luminescence into the infra-red range. It has been shown that the PL intensity of PbS quantum dots (QDs) in the matrix decreases with the temperature decrease due to the carrier transfer into defect QDs whereas the the luminescence intensity of the PbS nanocrystals without a matrix grows at the temperature increase.

A study of photoluminescence (PL) of InAlAs grown on InP by molecular beam epitaxy was performed in a wide range of temperatures and excitation intensities. A novel defect-related transition has been observed for the first time by 120–180 meV below the near band edge PL line of InAlAs. The novel band appears in the spectra only in a limited range of temperatures around 50–160 K. In that temperature range the band may dominate in the PL spectrum. The characteristics of the band show that it is related to recombination via deep centres located in potential wells created by the alloy clustering. We show that by establishing quasi-stoichiometric conditions on the growth surface we were able to grow layers which show virtually zero intensity of the defect-related transitions and luminescence efficiencies by 1–2 orders of magnitude greater than that in the samples grown under standard conditions.

Template assisted assembly of molecular nano arrays is one of the key steps towards molecular electronics and fullerene is one of the potential structural building blocks in fabrication of identical molecular nano arrays for miniature devices such as photovoltaic devices and single-molecule transistors. In this report, the reconstructed Au (111) with defect areas (steps) has been used as a template to assemble the highly ordered C₆₀ nano array at low coverage studied with scanning tunnelling microscopy (STM) in conjunction with density functional theory (DFT). The interaction between the substrate and C₆₀ nano arrays is strong enough to change the geometrical shape of C₆₀. As a result of strong interaction, the C₆₀ molecule appears to be deformed into ellipsoidal shape which causes the reduction of C₆₀ nano arrays on step sites of Au (111).

Properties of deep-level defects in CuGaSe₂ thin-film solar cells were investigated using transient photo-capacitance (TPC) spectroscopy. Two Gaussian-shaped deep-level defects centered at around 0.8 eV and 1.54 eV above the valence band were identified. The electronic structure of the two defects was illustrated by a configuration coordinate model to explain the thermal quenching effect in the two defects, which considered a large lattice distortion for the 0.8 eV defect while no distortion for the 1.54 eV defect.

This paper presents modeling and simulation of a multiple quantum well structure formed with Ge_0.95Sn_0.05 quantum wells separated by Ge_0.51Si_0.35Sn_0.14 barriers for the applications. These alloy compositions are chosen to satisfy two conditions simultaneously: type-I band alignment between Ge_0.95Sn_0.05/Ge_0.51Si_0.35Sn_0.14 and a lattice match between wells and barriers. This lattice match ensures that the strain-free structure can be grown upon a relaxed Ge_0.51Si_0.35Sn_0.14 buffer on a silicon substrate – a CMOS compatible process.

A electro-absorption modulator with the Ge_0.95Sn_0.05/Ge_0.51Si_0.35Sn_0.14 multiple quantum well structure based on quantum-confined Stark effect(QCSE) is demonstrated in theory. The energy band diagrams of the GeSiSn/GeSn multi-quantum-well structure at 0 and 0.5V bias are calculated, respectively. And the corresponding absorption coefficients as a function of cut-off energy for this multiple quantum well structure at 0 and 0.5Vbias are also obtained, respectively. The reduction of cut-off energy is observed with the applying of the external electric field, indicating a strong QCSE in the structure.

We study the transient Fano resonance of a semiconductor Si observed in the early time region of coherent phonon generation induced by an ultrafast pump laser. We particularly examine effects of the detuning on the transient Fano resonance, where the detuning is de ned by the difference between the central frequency of the pump laser and the band gap. It is clari ed that asymmetric pro les of transient induced photoemission spectra, implying the Fano resonance, strongly depend on the detuning. This is attributed to energetically adjacent bosonic states, whose energy levels are strongly in uenced by the detuning.

Dynamics of nuclear spin decoherence and nuclear spin flip-flops in self-assembled InGaAs/GaAs quantum dots are studied experimentally using optically detected nuclear magnetic resonance (NMR). Nuclear spin-echo decay times are found to be in the range 1-4 ms. This is a factor of ~3 longer than in strain-free GaAs/AlGaAs structures and is shown to result from strain-induced quadrupolar effects that suppress nuclear spin flip-flops. The correlation times of the flip-flops are examined using a novel frequency-comb NMR technique and are found to exceed 1 s, a factor of ~1000 longer than in strain-free structures. These findings complement recent studies of electron spin coherence and reveal the paradoxical dual role of the quadrupolar effects in self-assembled quantum dots: large increase of the nuclear spin bath coherence and at the same time significant reduction of the electron spin-qubit coherence. Approaches to increasing electron spin coherence are discussed. In particular the nanohole filled GaAs/AlGaAs quantum dots are an attractive option: while their optical quality matches the self-assembled dots the quadrupolar effects measured in NMR spectra are a factor of 1000 smaller.

In this work, the emission properties of double quantum dots driven by an intense monochromatic electromagnetic field, while undergoing resonant tunnelling, are investigated. We find the optically active energy transitions and their corresponding emission intensity, and compute resonance fluorescence spectra for different detunings between the direct and indirect exciton energies. The simulated emission exhibit either three, five, or seven peaks, tunable on demand. On the basis of the obtained results, our proposal offers efficient control of the resonance fluorescence of an artificial molecule, suitable for optoelectronic applications.

Faraday rotation (FR) can also be observed in a resonantly excited system without an external magnetic field. In a microcavity, polaritons spin population imbalance generates an effective magnetic field across the cavity mode, which results in an amplified FR on the emitted photon. Also, when the cavity is tuned to light hole (LH) exciton energy, an additional field momentum will be added to the effective magnetic field parallel to cavity mode, resulting in a aditional FR.

GaN-based vertical-cavity surface-emitting lasers (VCSELs) with high optical gain and short cavity lifetime are favorable for the generation of ultra-short pulses in the blue and green regions. In our previous works, 6 and 2 picosecond short-pulses have been generated from gain-switched InGaN VCSELs with 3- and 10-period InGaN/GaN quantum wells (QWs) in the active layers by using an up-conversion measurement system. To further increase the gain of the VCSEL for the generation of even shorter pulses, 20-period InGaN/GaN QWs samples were fabricated. The emission characteristics of these high-gain VCSELs were investigated and analyzed under the optical pumping at room temperature.

We investigate oxygen-deficient anatase using quantum-chemical simulation within the density functional theory. It was found that the most energetically favorable spatial configuration of an oxygen polyvacancy is a three-dimensional chain in crystallographic direction [100] or [010]. The ability of oxygen polyvacancy in the form of a chain to act as a conductive filament and to participate in the resistive switching is discussed.