Table of contents

The electrical and physical characteristics of (CeO₂)_0.67(Al₂O₃)_0.33 (CAO), for use in metal-oxide-semiconductor gate dielectric applications were investigated. The CAO thin films have been deposited at 650 °C in different oxygen pressures by pulsed laser deposition. The CAO thin film was found to exhibit excellent characteristics such as atomic-scale smooth surface, thin interfacial layer, high accumulation capacitance and low leakage current density. This demonstrates that CAO thin film is a promising gate dielectric replacing SiO₂ in future for its good physical and electrical properties.

Blue and white organic light-emitting diodes (OLEDs) based on 4,4'-bis(2,2' diphenyl vinyl)-1,1'-biphenyl (DPVBi) were reported. Devices with a configuration of indium tin oxide (ITO)/N,N'-bis-(1-naphthyl)-N,N'-diphenyl-1,1'-biphenyl-4,4'-diamine (NPB)/DPVBi/tris-(8-hydroxyquinoline)aluminium (Alq₃)/LiF/Al have been fabricated. The thickness of the DPVBi used in the OLED has a significant influence on the colour and efficiency. When the thickness of DPVBi is more than 10 nm, the pure blue emission with a peak at 456 nm is observed. And white emission was obtained in the device with a 10 nm DPVBi without doping. The power efficiencies of the blue and white OLEDs are 1.10 and 2.94 lm W⁻¹, respectively.

In the conventional process of liquid phase deposition, silicon dioxide cannot be deposited on silicon nitride and tungsten silicide substrates. In the present study, this limitation is removed by adding aqueous ammonia to the hydrofluorosilicic acid source. A selective or blanket deposition can be controlled. The deposition rate is also enhanced.

Gold diffusion at 300 °C and 400 °C in 2 µm thick undoped wurtzite GaN grown by the metal–organic chemical vapour deposition technique has been studied by using secondary ion mass spectroscopy (SIMS) depth profiles. The diffusion coefficients D(300 °C) = 1.03 × 10⁻¹⁵ cm² s⁻¹, D(400 °C) = 1.69 × 10⁻¹⁴ cm² s⁻¹ and the activation energy Q = 0.93 eV are obtained. The dominant mechanism of Au diffusion in GaN is probably the diffusion of an impurity by the interstitial mode. Comparison with Au diffusion in other semiconductors is given and discussed.

Te-doped cadmium telluride (CdTe) films were deposited on ITO/glass substrates using the close spaced sublimation (CSS) method. The films were characterized by x-ray diffraction (XRD), the x-ray fixed-quantity (XRF) method, scanning electron microscopy (SEM) and the Hall effect. The XRD and SEM results show that appropriate Te doping would be favourable to the growth of CdTe crystallite. The Hall effect measurements indicate that the conductivity of CdTe films could be dramatically improved by Te doping. The work presented here suggests that p-type doping CdTe films can be produced using this deposition method.

An interesting hydrogen sensing Pd/InGaP metal-semiconductor (MS) Schottky diode has been fabricated and studied. Both the steady state and the transient condition of the hydrogen adsorption process are investigated. Even at room temperature, an extremely low hydrogen concentration of 15 ppm H₂/air can be detected. In addition, the wide operating temperature range of 250 K of the studied Pd/InGaP hydrogen sensor is found. From experimental results, it is shown that the variation of Schottky barrier height increases with the increase of the operating temperature and hydrogen concentration. As the operation temperature is elevated, the water formation effect is also studied in the quasi-equilibrium region under the transient condition.

In this work, Pd–GaAs Schottky diodes have been fabricated by a novel electroless plating technique. A scanning electron microscope (SEM) and Raman spectra are used to characterize the surface morphology of Pd film and Pd–GaAs interface, respectively. Effects of plating variables including concentrations of PdCl₂, N₂H₄ and bulk plating bath, as well as the plating time, on the Pd surface morphology and current–voltage (I–V) characteristics are investigated. From experimental results, it is revealed that I–V characteristics of Pd–GaAs diodes are strongly influenced by the Pd grain size. The Schottky barrier height is increased with decreasing Pd grain size and particle size distribution by lowering the plating temperature and concentrations of PdCl₂, N₂H₄ and bulk plating bath. Moreover, in the presence of sodium, I–V characteristics of the studied diodes are obviously inferior. Based on these results, the high performance Pd–GaAs Schottky diodes can be obtained by appropriately manipulating the plating conditions.

The recent results for the self-diffusivities, D(Ga) and D(Sb), of Ga and Sb in GaSb obtained by Bracht et al (Bracht H, Nicols S P, Walukjewicz W, Silveira J P, Briones F and Haller E E 2000 Nature408 69 and Bracht H, Nicols S P, Haller E E, Silveira J P and Briones F 2001 J. Appl. Phys.89 5393) are compared and related to the earlier measurements by Weiler and Mehrer (Weiler D and Mehrer H 1984 Phil. Mag. A 49 309). It is proposed that the differences between the two sets of data are due to higher concentrations of hydrogen impurity in the samples of Weiler and Mehrer. The experimental evidence indicates that the diffusion mechanisms associated with D(Ga) and D(Sb) both have two parallel mechanisms. For D(Ga) the native defects involved are the Frenkel pair, Ga_iV_Ga, and the Ga vacancy, V_Ga. For D(Sb) one mechanism is due to the defect pair Sb_iV_Ga and the second to either the vacancy pair V_GaV_Sb or the triple defect V_GaGa_SbV_Ga. It is proposed that the mobilities of all these defects, excepting Ga_iV_Ga, are enhanced in the presence of hydrogen as an impurity in the GaSb lattice. On this basis the differences in the data obtained by Bracht et al and Weiler and Mehrer can be reconciled. It is also shown that measured free hole concentrations identify Ga²⁻_Sb as the residual acceptor in GaSb and that undoped GaSb is intrinsic at diffusion anneal temperatures.

Amorphous hydrogenated silicon nitride films have been deposited by the electron cyclotron resonance plasma technique, using N₂ and SiH₄ as precursor gases. The gas flow ratio, deposition temperature and microwave power have been varied in order to study their effect on the properties of the films, which were characterized by Rutherford back-scattering spectrometry, elastic recoil detection analysis (ERDA), Fourier transform infrared spectroscopy and ellipsometry. All samples show N/Si ratios near or above the stoichiometric value (N/Si = 1.33). The hydrogen content determined from ERDA measurements is significantly higher than the amount detected by infrared spectroscopy, evidencing the presence of non-bonded H.

As the N₂/SiH₄ gas flow ratio is increased (by decreasing the SiH₄ partial pressure), the Si content decreases and the N–H concentration increases, while the N content remains constant, resulting in an increase of the N/Si ratio. The decrease of the Si content causes a decrease of the refractive index and the density of the film, while the growth ratio also decreases due to the limiting factor of the SiH₄ partial pressure. The infrared Si–N stretching band shifts to higher wavenumbers as the N–H concentration increases.

The increase of deposition temperature promotes the release of H, resulting in a higher incorporation of N and Si into the film and a decrease of the N/Si ratio. The effect of increasing the microwave power is analogous to increasing the N₂/SiH₄ ratio, due to the increase in the proportion of nitrogen activated species.

We have studied the experimental linear relationship between barrier heights (BHs) and ideality factors for Pb/p-type Si(100) Schottky contacts with a doping density of about 10¹⁵ cm⁻³. The BH for the Pb/p-type Si(100) diodes from the current–voltage (I–V) characteristics varied from 0.686 to 0.735 eV, the ideality factor n varied from 1.054 to 1.191, and from capacitance–voltage (C⁻²–V) characteristics the BH varied from 0.751 to 0.928 eV. The experimental BH distributions obtained from the I–V and C⁻²–V characteristics were fitted by a Gaussian function, and their mean BH values were found to be 0.709 and 0.799 eV, respectively. The laterally homogeneous BH value of approximately 0.741 eV for the H-terminated Pb/p-type Si(100) Schottky diodes was obtained from the linear relationship between experimental effective BHs and ideality factors.

CdS films were prepared by chemical bath deposition (CBD) at 60 °C without stirring. The crystallographic structure of the films and the size of the crystallites in the films were studied by x-ray diffraction. The energy gap of the films was found to decrease by annealing. The best annealing temperature for CBD grown CdS films was found to be 350 °C from optical properties. The crystallite sizes were found to increase and the x-ray diffraction patterns were seen to sharpen by annealing. The optical properties of the films were seen to be dependent on the film thicknesses. The band edge sharpness of optical absorption was seen to oscillate by thermal annealing. Self-oxidation and sulfur evaporation were found to be responsible for this oscillating behaviour. Annealing over 400 °C was seen to degrade the optical properties of the film.

We have calculated the resonant and nonresonant third-order nonlinearity χ⁽³⁾ for third harmonic generation (THG) and four wave mixing (FWM) in two triple quantum well structures. The first structure is designed to access both THG (χ⁽³⁾(3ω)) and FWM (χ⁽³⁾(ω4)) at mid-infrared wavelengths in a single device. It is found that χ⁽³⁾(3ω) = 1.8 × 10⁻¹⁴ (m V⁻¹)² at a wavelength near 9.6 µm and χ⁽³⁾(ω4) = 1.5 × 10⁻¹⁴ (m V⁻¹)² at 3.8 µm. In the second structure near infrared FWM and nonresonant THG can be obtained using strain-compensated InGaAs-InAlAs material. In the latter structure χ⁽³⁾(ω4) exhibits a peak as a function of biasing corresponding to the energy levels being made equally spaced due to the triply resonant four wave mixing phenomenon. Also, it has been shown that high conversion efficiency can be obtained at the above condition. A nonresonant χ⁽³⁾(3ω) of 1.6 × 10⁻¹⁵ (m V⁻¹)² is calculated at 5 µm and shown to have appreciable values at near infrared wavelengths for increased bias.

Electron-LO-phonon intrasubband scattering rates near a cylindrical cavity are calculated as functions of the axial applied magnetic field and the electric potential of the heterostructure. Confinement of phonon modes by the potential of the heterostructure is neglected and the scattering of electrons is assumed to be via bulk phonon modes. A direct consequence of having a forbidden region for the motion of the electrons leads to the lowest-order electron's energy subbands taking turns at becoming the ground state, following the sequence {m = 0, −1, −2, ..., −N} of azimuthal quantum numbers. The intrasubband scattering integrals are found to be characterized by strong quasi-periodic oscillations in their variations with the phonon radial wave number. For moderate to large radius of the cavity, the intrasubband scattering rates also exhibit a strong oscillatory behaviour, nevertheless highly aharmonic, in their variations with the applied magnetic field.

It has been demonstrated experimentally that the electron drift mobility of a two-dimensional electron gas in the channel of AlGaN/GaN metal-oxide-semiconductor heterostructure field effect transistors (MOSHFETs) may be appreciably higher than that in the conventional AlGaN/GaN HFETs for structures with a thin (10 nm) AlGaN barrier layer. For the conventional AlGaN/GaN heterostructures with barrier thickness of 10 nm, the maximum electron drift mobility in the 2D channel, μ_d, is equal to ∼600 cm² Vs⁻¹, which is much lower than the Hall mobility measured with the same wafer (μ_H ≈ 1400 cm² Vs⁻¹). Introduction of a 7 nm thick SiO₂ layer between the gate and the AlGaN layer to fabricate MOSHFETs leads to an increase in the electron drift mobility to μ_d ≈ 1400 cm² Vs⁻¹. Introduction of a SiO₂ layer with the same thickness into the AlGaN/GaN heterostructures with relatively thick (25 nm) AlGaN barrier does not affect the maximum electron drift mobility in the channel of the heterostructures. The results obtained correlate well with published data on the influence of the barrier thickness on the high-power performance of AlGaN/GaN HFETs.

Silicon oxide–silicon nitride (ON) stack layers have been formed by mercury sensitized photochemical vapour deposition (photo-CVD) of silicon nitride layers over thermally grown and photo-CVD deposited silicon oxide layers on p-type Si (100) substrates. The properties of these two groups of samples were studied using Fourier transform infrared spectroscopy (FTIR), x-ray photoelectron spectroscopy (XPS) and capacitance–voltage measurements. Photo-CVD deposited nitrogen-rich stoichiometric silicon nitrides over thermally grown oxides give less electronic state density with an increased Si–N bond concentration, and on the other hand photo-CVD deposited oxide–nitride stacks give less fixed oxide charges. The absence of Si–H bonds in nitrides indicates that all nitrides are hydrogenated. Oxygen incorporation in the nitride layer was studied from FTIR and XPS measurements, which in turn affects the properties of these stack layers.

Crystalline zinc sulfide (ZnS) thin films were prepared by chemical bath deposition (CBD) using the mixed aqueous solutions of zinc acetate, thiourea and tri-sodium citrate, where tri-sodium citrate was used as the complexing agent. The thin films were characterized by x-ray diffraction (XRD), scanning electron microscopy (SEM) and optical absorption. The as-deposited thin films were surface homogeneous with pure wurtzite structure and the optical band gap of the film was estimated to be 3.53 eV.

In this paper, a CMOS correlated double sampling and hold (CDSH) structure based on novel dynamic source follower for large format infrared focal-plane array (IRFPA) is proposed. The primary noise (fixed-pattern noise (FPN) and KTC reset noise, etc) of the CMOS image sensor is reduced, through the improved CDSH structure based on the novel dynamic source follower, and all pixels in the selected row can start integrating simultaneously; the readout process happens in the next row selection time, which can increase the frame frequency, and the output waveform is boxcar, which is easy-to-realize analog/digital conversion. SPICE simulation results have shown that the proposed improved CDSH structure can achieve the advantages of simple driving signal, large dynamic range in large format IRFPA with low power consumption.

A gallium arsenide bulk crystal highly enriched with gallium isotope ⁷¹Ga to 99.40 at% has been grown using the seedless horizontal Bridgman method. The crystal has a total impurity concentration of about 2 × 10¹⁷ atoms cm⁻³ and a high electrical resistivity ρ(300 K) ∼ 10³–10⁸ Ω cm. Thermal conductivity measurements have been made from 90 K to 300 K of the enriched ⁷¹GaAs sample and three different ^natGaAs samples with a natural isotope composition of gallium. The ⁷¹GaAs has a higher value of thermal conductivity due to reduced isotope scattering of phonons: the observed isotope effect amounts to 5% at room temperature, increasing to 13% with a temperature decrease down to 100 K.

The effect of the ionization threshold softness on the temperature variation of impact ionization coefficients is examined theoretically. It is found that increasing the softness reduces the temperature dependence of ionization, because temperature induced heating or cooling of the carrier distribution results in a smaller change in the ionization scattering rate sampled. This may explain the wide variation in the temperature dependence of breakdown voltage reported in the literature and the difficulty in modelling accurately the temperature dependence of the ionization process.

The thermal diffusivity of CuGa_1–xFe_xO₂ solid solutions with x = 0, 0.1, 0.2 and 1 has been investigated using an open photoacoustic cell (OPC). The thermal diffusivity of the samples is evaluated from the phase data associated with the photoacoustic signal as a function of modulation frequency in transmission detection configuration (TDC). Analysis is based on the theory developed by Rosencwaig and Gersho. The effect of thermoelastic bending on thick samples is studied. The results are confirmed by performing measurements on samples of different thicknesses.

Thermal emission characteristics of a wide gap semiconductor structure with an h–l junction have been studied with the view of application to long wavelength (8–12 µm) IR sources. The device performs emission modulation in the spectral range below the edge of fundamental absorption via modulation of the charge carrier concentration in the structure base due to the contact exclusion effect. It is experimentally and theoretically shown that the structure base doping level determines both the magnitude of IR signal and the radiating region length. Experimental studies have been carried out with the structures based on p-Ge at temperatures from 300 to 430 K. It is shown that the maximum active region length may achieve 1 cm with emission intensity of ∼mW cm⁻² and operation speed of ∼100 µs.

SnO₂:Sb sol–gel derived thin films doped with Tb³⁺ were deposited on porous silicon (PS) layers. Transmission electron microscopy observations, electron diffraction patterns and energy dispersive x-ray analysis revealed the crystallization of small crystallites of cassiterite embedded in the porous layer and the presence of Tb³⁺ ions in the SnO₂ crystallites. The photoluminescence spectrum shows that Tb³⁺ ions emit highly after annealing at 500 °C. Preliminary characterizations of the electrical and transient electroluminescence of the resulting nanocomposite structures are presented. We discuss the I–V of SnO₂:Tb³⁺/PS device under both forward and reverse bias conditions.

The space resolved photocurrent-imaging technique was used to investigate the degradation of dye sensitized solid-state TiO₂|dye|CuI solar cells. In contrast to dye sensitized liquid cells only blurred and not sharp boundaries between illuminated and dark areas were observed. The deterioration of the cell was found to be much faster than in liquid dye sensitized cells and does not involve the sensitizer.

A smart pixel that can function as a building block of an optical transceiver with circular dimensions is proposed using InGaP/GaAs double heterojunction bipolar transistors (DHBTs) and a GaAs/AlGaAs vertical cavity surface emitting laser (VCSEL). Fabricated DHBT showed a breakdown voltage of 13 V and a cut-off frequency of 35 GHz at I_c = 30 mA. Also a high-performance 850 nm infra-red VCSEL exhibited a threshold current of 3.5 mA with a maximum optical output of 4.8 mW at I_c = 20 mA and forward voltage of 1.8 V. The detector, comprised of the base-to-collector junction of the DHBT as a pin-PD, produced a photocurrent of 180 µA for a given input power of 0.3 mW. The extracted small-signal equivalent circuit parameters from measurement of these devices were used in PSPICE simulations to design an interface drive circuit for high-speed modulation. The overall OEIC circuit performance operates up to data rates of 2 Gb s⁻¹ at 1 mW output power with maximum output voltage of 2 V across the VCSEL and modulation current of 5 mA.

The effects of hydrostatic stress on the binding energy and the density of shallow-donor and shallow-acceptor impurity states in a GaAs–(Ga, Al)As quantum well are calculated using a variational procedure within the effective-mass approximation. Results are for different well widths and hydrostatic stresses, as a function of the impurity position along the growth direction of the structure. We have found that in the low-pressure regime the binding energy changes linearly for both donor and acceptor impurities, independently of the sizes of the well. However, for high pressures (greater than 13.5 kbar) this is valid for acceptors but not for donors due to the Γ-X crossover. We have shown that there are two special structures in the density of impurity states, one associated with on-centre and the other with on-edge impurities. Also, we have observed that the density of impurity states depends strongly on the applied hydrostatic stress.