Optoelectronic Materials and Devices

This paper discusses the design of electrically-pumped AlGaN-based in-plane lasers emitting at ∼290 nm. Our laser design utilizes strained Al_0.5Ga_0.5N quantum wells, and a novel polarization engineered AlGaN/InGaN/AlGaN-based tunnel junction. The low ­resistive tunnel junction is used as an intracavity contact in the device in place of the resistive p-type contact; which leads to improved hole injection and a reduced threshold voltage. Hence, room-temperature continuous-wave laser operation could be enabled. Strategies to improve the performance of the tunnel junction contact through the incorporation of low concentrations of boron in the highly-doped AlGaN tunnel junction layers as a means to increase the polarization sheet charge are also discussed.

Slow light in the Technologically important devices InGaAs/GaAs quantum dot (QD) Vertical Cavity Semiconductor Optical Amplifiers (QD-VCSOAs) is studied. The presented work benefits from the possibility offered by QD-VCSOA structures for operation exclusively at the amplifying regime without reaching threshold unlike Quantum Well (QW) VCSOAs. The mathematical formulation of the main slow down metric based on gain dispersion is presented. The effects of various functional (current, input signal) and structural (inhomogeneity, cavity length, density of dots etc) properties on the optical delay properties of the QD-VCSOA are identified and relevant design strategies are outlined. The numerical investigation concludes that considerably higher Group Delays can be achieved in QD-VCSOAs compared to QW-VCSOAs by means of static (i.e. structural design) and dynamic (i.e. injection current) properties. The results highlight the flexibility (tunability) offered by the QDs in tailoring SL properties of the VCSOA.

Based on an InGaN/GaN light emitting diode (LED) structure, monolithically integrated vertical driving metal-oxide-semiconductor field-effect transistors (MOSFETs) were designed and experimentally implemented using a selective area growth (SAG) method. A simple p-GaN/n-GaN stack was selectively regrown on top of the LED wafer to realize an n/p/n structure for the vertical MOSFET fabrication. The integrated vertical MOSFET, which can effectively modulate the injection current through the serially connected LED, exhibited high performance such as an enhancement-mode (E-mode) operation with a relatively high output current density. However, on-resistance (R_ON) degradation was observed in the fabricated vertical MOSFET at a low drain bias level (V_DS < 2 V). Through a 2D TCAD simulation, the origin of the high R_ON was revealed to be an electron barrier induced by the LED's p-AlGaN electron blocking layer (EBL). The simulation results also demonstrated that it can be improved by band engineering of the EBL.

High quality AlAs_1−xBi_x layers with Bi composition of 3%–10.5% have been successfully grown by molecular beam epitaxy. The Bi incorporation is confirmed by Rutherford backscattering spectroscopy. For a 400 nm thick AlAsBi layer, the strain relaxation occurs when the Bi composition is larger than 6.5%. Flux ratio is calculated from Knudsen-cell model and Maxwell equation, according to the geometrical relationship of our equipment. The Bi incorporation increases with increasing the As–Al flux ratio as well as the Bi flux. The extrapolation lattice constant of hypothetic zincblende AlBi alloy is about 6.23 Å.

In this work, the electronic bandstructure of GaAs_1−xBi_x/GaAs single quantum well (QW) samples grown by molecular beam epitaxy is investigated by photomodulated reflectance (PR) measurements as a function of Bi content (0.0065 ≤ x ≤ 0.0215) and substrate orientation. The Bi composition is determined via simulation of high-resolution x-ray diffraction measurement and is found to be maximized in the 2.15%Bi and 2.1%Bi samples grown on (100) and (311)B GaAs substrates. However, the simulations indicate that the Bi composition is not only limited in the GaAsBi QW layer but extends out of the GaAsBi QW towards the GaAs barrier and forms a GaAsBi epilayer. PR spectra are fitted with the third derivative function form (TDFF) to identify the optical transition energies. We analyze the TDFF results by considering strain-induced modification on the conduction band (CB) and splitting of the valence band (VB) due to its interaction with the localized Bi level and VB interaction. The PR measurements confirm the existence of a GaAsBi epilayer via observed optical transitions that belong to GaAsBi layers with various Bi compositions. It is found that both Bi composition and substrate orientation have strong effects on the PR signal. Comparison between TDFF and calculated optical transition energies provides a bandgap reduction of 92 meV/%Bi and 36 meV/%Bi and an interaction strength of the isolated Bi atoms with host GaAs valence band (C_BiM ) of 1.7 eV and 0.9 eV for (100) and (311)B GaAs substrates, respectively.

Time-resolved optical orientation experiments have been performed in dilute bismide structures. Bulk layers with bismuth fractions in the range 1%–3.8% and quantum wells with bismuth fractions in the range 2.4%–7% were investigated. A clear decrease of the electron spin relaxation time is evidenced in both cases when the bismuth content increases. These results can be well interpreted by the increased efficiency of the spin relaxation mechanisms due to the bismuth induced larger spin-orbit interaction in these alloys.

We present a characterisation of a GaInNAs/GaNAs quantum well-based photodetector with a bottom distributed Bragg reflector (quasi-cavity). The detector is designed to be used at the 1.3 μm optical fibre communication window. The quantum efficiency of the photodetector is measured as 24% at 1286 nm under −2 V applied reverse bias. As the reverse bias voltage is increased, a carrier multiplication-related increase and oscillations are observed in the voltage-dependent responsivity curve. The observed carrier multiplication is explained by the high electrical field-induced impact ionisation mechanism in the pin junction region, while the observed voltage-dependent oscillations are explained by the Franz–Keldysh effect (FKE). At the wavelength of 1286 nm, which is close to the absorption wavelength of the active region of the photodetector, FKE-related oscillations (FKOs), start at very low reverse bias values and the responsivity of the photodetector is dominated by FKOs. On the other hand, FKOs quench at higher wavelengths and an impact ionisation-related increase at the voltage-dependent responsivity curve dominates. At λ = 1310 nm, only impact ionisation mechanisms have an effect over the R (V) curve. The multiplication factor for 1310 nm is calculated as M = 12 at room temperature. The applied electric field and excitation wavelength dependence of the absorption coefficient is calculated and a good match with the experimental results at the applied voltages is achieved.

In this paper we discuss the development of new semiconductor materials and approaches to overcome the fundamental limitations of well established (Al, In)GaAs/InP and InGaAsP/InP infrared-emitting lasers. We consider three approaches; dilute nitride InGaAsN-based structures; InAs-based quantum dot/dash structures and the most recently emerging dilute bismide (GaAsBi, InGaAsBi), all of which may be grown on either GaAs or InP substrates. These material systems provide a range of possibilities for band engineering and strain control, thereby giving new routes to improve device efficiency, overcoming existing limitations of device performance and to develop range of new cost-efficient devices with improved characteristics. However, all of these approaches have common difficulties related to establishing optimised growth conditions to produce high quality material for device fabrication. Particularly, in this paper we compare and contrast the effects of inhomogeneous carrier distribution in these systems and discuss the influence of this on the physical properties of lasers developed using these approaches.

While recent work developing GaAsBi for opto-electronic applications has shown promise, it has also indicated that the large valence band offset of GaAsBi/GaAs may cause undesirable hole-trapping in GaAsBi quantum wells. In this work, hole-trapping is demonstrated to be the cause of the reduced depletion width of GaAsBi/GaAs multi-quantum well solar cell devices under illumination. Modelling of the quantum confinement energies in these devices shows how the carrier escape times vary as functions of temperature, providing a tool for the design for future GaAsBi based photovoltaic devices.

We present a theoretical analysis and optimisation of the properties and performance of mid-infrared semiconductor lasers based on the dilute bismide alloy In_xGa_1−xAs_1−yBi_y, grown on conventional (001) InP substrates. The ability to independently vary the epitaxial strain and emission wavelength in this quaternary alloy provides significant scope for band structure engineering. Our calculations demonstrate that structures based on compressively strained In_xGa_1−xAs_1−yBi_y quantum wells (QWs) can readily achieve emission wavelengths in the 3–5 μm range, and that these QWs have large type-I band offsets. As such, these structures have the potential to overcome a number of limitations commonly associated with this application-rich but technologically challenging wavelength range. By considering laser structures having (i) fixed QW thickness and variable strain, and (ii) fixed strain and variable QW thickness, we quantify key trends in the properties and performance as functions of the alloy composition, structural properties, and emission wavelength, and on this basis identify routes towards the realisation of optimised devices for practical applications. Our analysis suggests that simple laser structures—incorporating compressively strained In_xGa_1−xAs_1−yBi_y QWs and unstrained ternary In_0.53Ga_0.47As barriers—which are compatible with established epitaxial growth, provide a promising route to realising InP-based mid-infrared diode lasers.

We analyze the microstructure of a series of Ga(Sb, Bi)/GaSb quantum wells (QW) with varying Bi content and QW thicknesses using transmission electron microscopy. The structures are grown on GaSb(001) substrates by low-temperature molecular beam epitaxy. Although Ga(Sb, Bi) is regarded as a highly-mismatched alloy, the material is of remarkable quality and no extended defects or nanoclusters are detected in the pseudomorphic layers. Regardless their different Bi content (ranging from 4% to 11%) all QW samples seem to be homogeneous in composition and no composition fluctuations are detected. The QW/barriers are nevertheless affected by thickness fluctuations, where the thickness slightly varies from QW to QW but also laterally. Despite the samples exhibit well-defined QW/barrier interfaces, we note that the Ga(Sb, Bi)-on-GaSb interface is rougher than the GaSb-on-Ga(Sb, Bi) one. The morphological smoothing effect at the GaSb-on-Ga(Sb, Bi) interface is attributed to the well-known surfactant behavior of Bi in connection with Bi surface segregation, as evidenced from experimental Bi distribution profiles.

Using continuous-wave optical pumping of a spin-VCSEL at room temperature, we find high spin amplification of the pump close to threshold within the communications wavelength window, here at 1300 nm. This facilitates a strong switch from left to right circularly polarised light emission, which has potential applications in polarisation encoding for data communications. We use a simple spin flip model to fit the experimental results and discuss the VCSEL parameters that affect this amplification.

A model of the Elliot process for spin relaxation is developed that explicitly incorporates the Dresselhaus spin-splitting of the conduction band in semiconductors lacking an inversion symmetry. It is found that this model reduces to existing models in bulk if the scattering matrices are constructed from a superposition of eigenstates. It is shown that the amplitude for intra-sub-band spin relaxation disappears in quantum wells on the basis of existing models. However, an amplitude due to the Dresselhaus spin-splitting remains, becoming increasingly important as the well becomes narrower. It is also shown that this component does not disappear for scattering between spin states at the same wavevector. It is concluded that for quantum wells and lower dimensional semiconductors that this modified model should be used in analysis of the spin dynamics.

We report on the optical properties of InAs/AlSb/GaSb based Type-II superlattice N-structure with p on n configuration. The detector structure is designed to operate in the mid wavelength infrared range with 50% cut-off wavelength of 4.88 μm at 79 K. Electronic properties of N-structure such as heavy hole–light hole splitting energies are optimized by a first principles approach taking into account InAlAs interface bonding between InAs/AlSb layers. A responsivity of 1.13 A/W at 3.2 μm was measured under zero bias, with a corresponding quantum efficiency of 44%. An analytical model was developed in order to identify the contribution of quantum efficiency (QE) constituents. The overall QE contains a 56% contribution from the quasi-neutral p-region, 36% from the quasi-neutral n-region and 8% from the depletion region.

In this work, we have investigated the structural and optical properties of GaAs_(1−x)Bi_x/GaAs single quantum wells (QWs) grown by molecular beam epitaxy on GaAs (311)B substrates using x-ray diffraction, atomic force microscopy, Fourier-transform Raman (FT-Raman) and photoluminescence spectroscopy techniques. The FT-Raman results revealed a decrease of the relative intensity ratio of transverse and longitudinal optical modes with the increase of Bi concentration, which indicates a reduction of the structural disorder with increasing Bi incorporation. In addition, the PL results show an enhancement of the optical efficiency of the structures as the Bi concentration is increased due to important effects of exciton localization related to Bi defects, nonradiative centers and alloy disorder. These results provide evidence that Bi is incorporated effectively into the QW region. Finally, the temperature dependence of the PL spectra has evidenced two distinct types of defects related to the Bi incorporation, namely Bi clusters and pairs, and alloy disorder and potential fluctuation.

Knowledge about the temperature dependence of the fundamental band-gap energy of semiconductors is very important and constitutes the basis for developing semiconductor devices that work in a wide range of temperatures. Since the 90s, it has been suggested that III−V dilute bismides have temperature insensitive band gaps, and this fact might be important for semiconductor lasers, whose wavelength stays nearly constant through ambient temperature variations. Here, we have reviewed the available information about temperature evolution of the band gap for various III−V dilute bismide semiconductors. It is well known from many experimental results that the band-gap energy decreases monotonically with increasing temperature, and such behavior can be described by the Varshni formula. It is highly desirable for the Varshni temperature coefficient (α parameter) to be very small, indicating temperature insensitivity of the band gap. In this article, information about the band-gap temperature sensitivity or insensitivity is collected for chosen III−V bismides and discussed.

We present the results of longitudinal carrier transport under a high electrical field in n- and p-type modulation-doped Ga_0.68In₀.₃₂N_yAs_1−y/GaAs (y = 0.009, 0.017) quantum well (QW) structures. Nitrogen composition-dependent drift velocities of electrons are observed to be saturated at $1.7\times {10}^{7}\,{\rm{cm}}\,{{\rm{s}}}^{-1}$ and $1.2\times {10}^{7}\,{\rm{cm}}\,{{\rm{s}}}^{-1}$ at 77 K for the samples with y = 0.009 and y = 0.017, respectively, while the drift velocities of holes do not saturate but slightly increase at the applied electric field in the range of interest. The hole drift velocity is observed to be higher than the electron drift velocity. The electron mobility exhibits an almost temperature-independent characteristic. On the other hand, the hole mobility exhibits a conventional temperature dependence of modulation-doped QW structures. As the temperature increases, the drift velocity of the electrons exhibits an almost an temperature-insensitive characteristic, but, on the other hand, for holes, drift velocity decreases approximately from 10⁷–10⁶ cm s⁻¹. It is observed that the drift velocities of electrons and holes are N-dependent and suppressed at higher electric fields. Furthermore, experimental results show that there is no evidence of negative differential velocity (NDV) behaviour for both n- and p-type samples. To explore the observed electron and hole drift velocity characteristic at high electric fields, we use a simple theoretical model for carrier transport, which takes into account the effect of non-drifting hot phonons. The mobility mapping technique (comparison method) is used to extract hot hole temperature in order to employ it in the non-drifted phonon distribution and to obtain the drift velocity–electric field curves. Then hot electron temperatures are obtained from the drift velocity–electric field curves as a fit parameter using non-drifted hot phonon dynamics. The analytical model is well-matched to the experimental ${\upsilon }_{{\rm{d}}}$–E curves, indicating that carrier-hot phonon scattering is the main reason for suppressing the NDV mechanism in GaInNAs/GaAs QW structures with a carrier density higher than 10¹⁷ cm⁻³.

Closed-form expressions are derived for the relationship between the polarisation of the output and that of the pump for spin-polarised vertical-cavity surface-emitting lasers. These expressions are based on the spin-flip model (SFM) combined with the condition that the carrier recombination time is much greater than both the spin relaxation time and the photon lifetime. Allowance is also included for misalignment between the principal axes of birefringence and dichroism. These expressions yield results that are in excellent agreement both with previously published numerical calculations and with further tests for a wide range of parameters. Trends with key parameters of the SFM are easily deduced from these expressions.

We provide a brief survey of the most recent results obtained by performing spatially selective hydrogen irradiation of dilute nitride semiconductors. The striking effects of the formation of stable N–H complexes in these compounds—coupled to the ultrasharp diffusion profile of H therein—can be exploited to tailor the structural (lattice constant) and optoelectronic (energy gap, refractive index, electron effective mass) properties of the material in the growth plane, with a spatial resolution of a few nm. This can be applied to the fabrication of site-controlled quantum dots (QDs) and wires, but also to the realization of the optical elements required for the on-chip manipulation and routing of qubits in fully integrated photonic circuits. The fabricated QDs—which have shown the ability to emit single photons—can also be deterministically coupled with photonic crystal microcavities, proving their inherent suitability to act as integrated light sources in complex nanophotonic devices.

Model expressions for the spin-orbit interaction in a quantum dot are obtained. The resulting form does not neglect cubic terms and allows for a generalized structural inversion asymmetry. We also obtain analytical expressions for the coupling between states for the electron–phonon interaction and use these to derive spin-relaxation rates, which are found to be qualitatively similar to those derived elsewhere in the literature. We find that, due to the inclusion of cubic terms, the Dresselhaus contribution to the ground state spin relaxation disappears for spherical dots. A comparison with previous theory and existing experimental results shows good agreement thereby presenting a clear analytical formalism for future developments. Comparative calculations for potential materials are presented.