Table of contents

A surface acoustic wave (SAW) based ultraviolet (UV) light detector configured in the form of a SAW oscillator is investigated. The SAW oscillator consists of a ZnO/LiNbO₃ bilayer structure, where ZnO is used as UV light sensing material and LiNbO₃ is used to excite the surface acoustic waves. The SAW oscillator is characterized for its functional performance at different V_cc, the voltage to the oscillator circuit, under different levels of UV light illumination. The changes in the amplitude and the frequency shift of the SAW oscillator output exhibit a linear variation with UV light intensity. The SAW UV detector is shown to be highly sensitive at lower V_cc. A low level UV intensity of 450 nW cm⁻² is easily detectable and the voltage responsivity is estimated to be ∼24 kV W⁻¹.

A fully depleted SiGe-on-insulator (SGOI) substrate has been fabricated by a modified Ge condensation technique. The characteristics of the SiGe/buried-oxide (BOX) interface and the influence on the electrical performance of the SGOI substrate are investigated. The long oxidation time during the fabrication of the fully depleted (FD) SGOI substrate leads to Ge atom pileup at the SiGe/BOX interface, and consequently a large number of interfacial trapped charges are produced. Furthermore, our results disclose that Ge diffusion into the BOX increases the interfacial trapped charges and the fixed oxide charges during oxidation. As a result, the electrical properties of the FD-SGOI substrate degrade due to the poor SiGe/BOX interface.

Anatase TiO₂ hydrosol was prepared by hydrolyzation of tetrabutyl titanate followed by acidic peptization of the precipitate under 70 °C. The method was highly simplified and the growth of TiO₂ nanocrystallites that bothered traditional preparations was avoided. The photocatalytic activity of the hydrosol was confirmed through the photocatalytic degradation of methyl blue dye under visible light irradiation.

The adsorbed molecular species, such as H₂O and I₂, that produce electron acceptor states on the TiO₂ surface are found to generate 1/f noise in the electric current through nanocrystalline films of TiO₂. It is suggested that the trapping and detrapping of electrons at the surface states is the cause of this noise. When the TiO₂ film surface is coated with dyes, the passivation of the active surface sites suppresses 1/f noise. Implications of 1/f noise on the functioning of the dye-sensitized solar cell, notably the effect of adsorbed iodine in inducing recombinations, are discussed.

Self-heating induced by vias and metal/Si contacts in VLSI circuits is undoubtedly one of the most important reliability issues for microelectronic integrated circuits. In this study the thermal characteristics of Ni/poly-Si contacts subject to an electrical current stressing were investigated. The self-heating induced contact temperature rises were measured for the Ni/poly-Si contacts having different contact resistivity, poly-Si sheet resistance and contact geometry. A current crowding phenomenon inside the contact window was analysed numerically based on a two-dimensional computation model. The temperature rise at the Ni/poly-Si contacts was found to increase proportionally with the heating power density of the contact. The extent of current crowding inside the contact window shows a strong dependence on R_s/ρ_c ratio and contact size. The effect of contact misalignment on the thermal properties of the contact is also discussed.

The effective mass (m*) of two-dimensional electrons in silicon metal–oxide–semiconductor field-effect transistors (MOSFETs), obtained from measurements of the thermal damping of Shubnikov–de Haas oscillations, has been studied as a function of electron density (n_s) for samples with physical gate oxide thicknesses (d_ox) of 4.7 nm and 3.1 nm. For the latter at a low electron density, the ratio (a_Br_s)/d_ox (where r_s is the interaction parameter and a_B is the Bohr radius in the semiconductor) exceeded 2 and the modification of the electron–electron interaction potential by the presence of the metallic gate was expected to be manifested as a change in the interaction-driven enhancement of the effective mass with increasing r_s. The deduced mass enhancement in both thin-oxide samples is well described by m*/m_b = 0.96 + γr_s, where m_b is the bare band mass within the plane of confinement, and γ is a constant. Although the results from both samples are in good quantitative agreement with previous experiments on thicker-oxide MOSFETs, a small but significant difference in the extracted value of γ between the thin-oxide samples was observed. This difference cannot, however, be unambiguously interpreted as a true renormalization of m* caused by the screening effect of the gate.

This work examines stress-induced leakage current (SILC) in both ultrathin silicon oxynitride and hafnium silicate dielectric layers for future MOS technology nodes. SILC is confirmed to be sense voltage dependent and is observed to have a dependence on bulk oxide traps for both dielectric layers. A possible explanation for the sense voltage dependence is provided. SILC is found to be a greater problem in the HfSiON layers, because of its magnitude relative to the initial current. This results in the leakage current density quickly becoming greater than that for SiON. The SILC is found to have a transient component in the HfSiON layers, indicating the presence of slow electron traps. Finally, the correlation between SILC and drive current reduction is demonstrated. It is concluded that in the high-k layers investigated in this study, SILC seems more of a problem than dielectric breakdown.

Halo ion implantation was adopted to reduce the short channel effect (SCE) of a buried channel p-MOSFET device on pseudomorphic Si_0.70Ge_0.30 layers. The strained pseudomorphic Si_0.70Ge_0.30 layer of 10 nm thickness, with a Si cap layer on top, was grown using molecular beam epitaxy. The results show an overall reduction in threshold voltage (V_th) roll-off in both Si and pseudomorphic SiGe devices. Halo implantation of As⁺, 120 keV and dose 2 × 10¹³ cm⁻², was successfully used to reduce roll-off for the 2 nm Si cap SiGe device by 0.3 V. However, it was found that halo implantation causes the reverse short channel effect (RSCE) on the devices, which can result in V_th roll-up with reducing channel length. The effect of the RSCE becomes greater with increasing Si cap thicknesses of the SiGe devices. Also, non-halo implanted devices were found to demonstrate the RSCE. This is due to excess interstitials from the source/drain implant and their subsequent diffusion, which causes virtual halo dopant to form in non-halo implanted devices.

In this paper, we propose a channel structure for a promising switch pHEMT with excellent isolation characteristics based on the distribution of electric field intensity beneath the Schottky contact in the transistor. Using the proposed device channel structure, SPST and SPDT switches were designed and fabricated, applicable to 2.4 GHz and 5.8 GHz WLAN systems. We discuss the relationship between dc characteristics and switch parameters in this paper in detail. The developed SPST switch exhibits a low insertion loss of 0.26 dB and a high isolation of 34.3 dB with a control voltage of 0 V/−3 V at 5.8 GHz. The SPDT also shows a good performance of 0.85 dB insertion loss and 31.5 dB isolation under the same conditions. The measured power-handling capability at 2.4 GHz reveals that the SPDT has an output power of 27 dBm at the 1 dB compression point and a third-order intercept point of more than 46 dBm.

Post-growth treatments, such as annealing, sulfurization, etching as well as ageing, were performed on CuInS₂ films prepared by RF reactive sputtering. Their effects on the structural, optical and electrical properties of the films were studied by means of x-ray diffraction (XRD) and scanning electron microscopy (SEM), optical transmission, and Hall effect measurement, respectively. Heating under vacuum at 500 °C for a certain duration causes recrystallization of the as-sputtered films. The secondary Cu–In phases coexisting in the films sputtered with an insufficient H₂S flow during sputtering can be eliminated by annealing in H₂S atmosphere at 500 °C for suitable duration. Meanwhile, the film structural as well as optical properties are enhanced. The electrical properties of the as-grown films changed dramatically with ageing in air, and annealing in vacuum or air. KCN etching removed Cu_xS segregations on the film surfaces and returned the film electrical property to its initial state.

We have shown that process effects induced by extending the post-exposure bake temperature in the process flow of chemically amplified photoresists can lead to significant improvements in depth-of-focus (DOF) and exposure latitude (EL) and small geometry printing capability. Due to improved acid dose contrasts and a balanced optimization of acid diffusion in the presence of quencher, PEB temperature increase has enabled the printing of iso and semi-dense space of 0.2 µm and below with a large DOF, using binary masks and 248 nm lithography without expensing the iso-dense bias. The results and findings of a full patterning process in a device flow, with different PEB temperatures as a process enhancement, are presented. The main objective of this study is to demonstrate how, using KrF lithography with binary masks and no optical proximity correction (OPC) nor other reticle enhancement technique (RET), the process latitude can be improved. Lithographic process latitudes, intra-field critical dimension (CD) uniformity and resist profiles of different PEB processes are evaluated. Then, the after-etch profiles are also investigated to ensure the feasibility of this technique.

We describe a way to obtain the degradation, induced by proton and electron irradiations, of solar cell parameters (short-circuit current, open-circuit voltage and maximum power) versus fluence, directly from the calculation of the characteristics of the cell and of the irradiation-induced recombination centres. The calculation can be performed for any energy of the irradiating particle and for any specific thicknesses and doping levels of the base and emitters. The validity of this approach is illustrated in the case of GaAs cells of different origins and extended to GaInP cells. It will allow us to deduce the degradation of multijunction cells.

Haematite thin films were prepared by spraying ethanolic solution of iron tri-chloride onto tin-doped indium oxide (ITO) coated glass substrates. The spray time, spray rate, pressure of carrier gas, etc were optimized to obtain nanocrystalline thin films. The films were adherent, smooth, compact and pinhole free. The films were characterized for their structural, morphological, optical and compositional properties. X-ray diffraction studies show that films are nanocrystalline α-Fe₂O₃ having haematite phase. SEM shows that the film consists of elliptical grains randomly distributed with average grain size of about 40–45 nm. The optical band gap was found to be 2.0 eV. Compositional uniformity was confirmed by Auger emission spectroscopy. Annealing the films at 380 °C for half an hour increases the particle size.

Ultraviolet Schottky photodetectors based on n-4H–SiC (N_d − N_a = 4 × 10¹⁵ cm⁻³) epitaxial layers of high purity have been fabricated. Their spectral sensitivity range is 3.2–5.3 eV peaking at 4.9 eV (quantum efficiency is about ∼0.3 electron/photon), which is close to the bactericidal ultraviolet radiation spectrum. The temperature dependence of the quantum efficiency of 4H–SiC Schottky structure has been investigated to determine the temperature stability and the mechanism of the photoelectric conversion process. At low temperatures (78–175 K) the quantum efficiency increases with increasing temperature for all photon energy values and then tends to saturate. We suppose that some imperfections in the space-charge region act as traps that capture both photoelectrons and photoholes. After some time the trapped electron–hole pairs recombine due to the tunnelling effect. At high temperatures (more than 300 K), the second enhancement region of the quantum efficiency is observed in the photon energy range of 3.2–4.5 eV. It is connected with a phonon contribution to indirect optical transitions between the valence band and the M-point of the conduction band. When the photon energy is close to a direct optical transition threshold this enhancement region disappears. This threshold is estimated to be 4.9 eV. At photon energies more than 5 eV a drastic fall of the quantum efficiency has been observed throughout the temperature interval. We propose that in this case the photoelectrons and photoholes are bound to form hot excitons in the space-charge region due to the Brillouin zone singularity, and do not contribute to the following photoelectroconversion process.

This paper investigates the electrical characteristics of NiSi Schottky barrier diodes (SBD). A single-step rapid thermal process (RTP) and a two-step RTP were employed to form the SBDs. The diode structures were designed so as to minimize edge leakage. The two-step silicidation process resulted in a significant reduction of the reverse leakage current density, suggesting an improvement of the interface characteristics. Temperature-dependent current–voltage measurements were used to analyse the interface characteristics. The two-step RTP process reduces the density of non-ideal micro-contacts with low barrier height, which are responsible for the reverse leakage current.

Recent progress in the development of transparent thin-film transistors for integration with flexible displays is discussed. Specifically, the fabrication and properties of ZnO-based thin-film transistors on glass are described. Top-gate-type thin-film transistors with transparent n-type ZnO as the active channel layer have been fabricated via wet photolithography processing. The ZnO layers were deposited using pulsed laser deposition. A low leakage current of 10⁻⁷ A cm⁻² was realized with amorphous HfO₂ or (Ce, Tb)MgA₁₁O₁₉ as the gate dielectric. N-channel depletion-mode operation was shown for the undoped ZnO thin-film transistors. Phosphorus-doped ZnO and (Zn, Mg)O were also utilized as channel materials in order to realize a reduction in carrier density. The current–voltage measurements demonstrate an enhancement-mode device operation for the thin-film transistors with P-doped (Zn, Mg)O as the active channel layer and HfO₂ serving as the gate dielectric.

The behaviour of copper dopant in p-Hg_1−xCd_xTe single crystals is analysed for in-diffusion in the samples with heterogeneous distribution of Hg vacancies and for ion beam milling. For the case of low Hg vacancy, p-Hg_1−xCd_xTe (x ∼ 0.2) material, copper is found to diffuse by a relay-race interstitial mechanism at low temperature. When the Cu-doped p-Hg_1−xCd_xTe samples are exposed to ion milling, the p–n conversion of conductivity type occurs in a substantially thick subsurface layer (∼10 µm). Unstable donor centres such as copper in interstitials rapidly relax so that residual donors control the conductivity in a converted layer after several hours. The possible causes of the incomplete p-type reconversion are discussed.

The scattering of hole spin during hole intra- and intervalence band transitions due to optical deformation potential interaction in tetrahedral semiconductors is considered. Matrix elements for transitions mediated by longitudinal and transverse optical phonons are presented. It is shown that for this type of scattering, the spin relaxation time can be found from hole momentum scattering time.

Carbon and nitrogen co-implantation characteristics in Be-doped GaN with different dopant concentration ratios have been systematically investigated. It was found that the conductive characteristics of Be-doped GaN convert to p-type by co-implantation of C or N atoms and subsequent annealing, which are essentially related to the dopant concentration ratio and annealing conditions. The conversion could be attributed to the reduction of self-compensation and the shift of the surface Fermi level towards the valance band edge. The outcome was reasonably in agreement with the surface stoichiometric switching as determined by x-ray photoelectron spectroscopy measurements. From photoluminescence data, the activation energy of the Be acceptor level was evaluated to be about 145–155 meV, which is shallower than that of Mg acceptors. These experimental results indicated that co-implanting C or N with Be atoms into the selective area of GaN is an effective method to enhance the electrical activation efficiency of Be acceptors and to reduce the surface barrier height, which can help to decrease the metal contact resistivity to p-type GaN.

Porosity-induced birefringence in a GaP membrane is studied using a method based on the analysis of beats in unpolarized and polarized transmittance spectra as well as in angular dependence of optical spectra. Birefringence as high as n_e − n_o = 0.25 was measured in a porous GaP membrane with the degree of porosity close to 40%. The measured values of n_e and n_o are compared with those calculated in the frame of effective medium theory for a GaP membrane with appropriate morphology.

In this work the synthesis of zinc mercury selenide thin films (Zn_1−xHg_xSe) by electrodeposition is carried out. The films were deposited onto conducting glass (SnO₂) substrates from an aqueous solution bath containing ZnSO₄, HgCl₂ and SeO₂ at bath temperatures between 30 °C and 70 °C. The influence of deposition parameters such as electrolyte composition, deposition potential and temperature on the crystallinity and composition of the films is studied. It is found that the amount of mercury content in the solution bath and deposition potential control the composition and structure of the alloy films. The films were characterized by x-ray diffraction (XRD), energy dispersive x-ray analysis (EDAX), optical absorption and scanning electron microscope (SEM) studies. Photoelectrochemical solar cells studies using Zn_1−xHg_xSe thin films showed improved performance for annealed and etched electrodes and the results are discussed.

The thermal stability of various gate metallizations on AlGaAsSb/InAs, with and without an InAs cap, was investigated. A W/Au gate metallization was found to be a good candidate for stable gate metallizations directly on AlGaAsSb. Ti/Pt/Au (30/40/80 nm) and Co/Si/Co/Si/Co gates are thermally stable on InAs. Cross-sectional transmission electron microscopy showed that degradation in the current–voltage characteristics of aged samples is associated with metal/InAs reactions.

In an AlQ-based bilayer organic light-emitting diode, n-type silicon has been used as an anode, and semitransparent metals Sm (15 nm)/Au (15 nm) as a cathode. This device has much smaller currents at high voltages (>8 V) and a higher turn-on voltage than the device with an identical structure but an indium–tin oxide anode. By successively depositing ultra thin films of Au and AlQ on the n-Si surface, the device performances are improved significantly, reaching a power efficiency of 0.1 lm W⁻¹ and a current efficiency of 0.7 cd A⁻¹ at 15.9 V and 0.3 mA mm⁻². The mechanisms for the hole injection and performance improvement are discussed.

Polycrystalline cadmium-doped indium selenide thin films were obtained by the thermal co-evaporation of α-In₂Se₃ crystals and Cd onto glass substrates kept at a temperature of 200 °C. The temperature dependence of the optical band gap in the temperature region of 300–450 K and the room temperature refractive index, n(λ), of these films have been investigated. The absorption edge shifts to lower energy as temperature increases. The fundamental absorption edge corresponds to a direct energy gap that exhibits a temperature coefficient of −6.14 × 10⁻⁴ eV K⁻¹. The room temperature n(λ) which was calculated from the transmittance data allowed the identification of the oscillator strength and energy, static and lattice dielectric constants and static refractive index as 20.06 and 3.07 eV, 7.43 and 10.52 and 2.74, respectively.

The application of separation-by-implantation-of-oxygen (SIMOX) for silicon–germanium-on-insulator (SGOI) fabrication is always limited by the Ge loss caused by the high temperature annealing. A unique SIMOX method was introduced to fabricate SGOI and resolve the problem of Ge loss. During the process, a SiO₂ layer was formed pre-annealing to block the Ge out-diffusion. As the x-ray rocking curve and Raman spectra studies show, the Ge fraction of the SGOI was improved to 17 at.%. The final sample exhibits a planar and continuous buried oxide layer, sharp interfaces and a defect free top SiGe layer as the cross-sectional transmission-electron- microscopy (XTEM) and secondary-ion-mass-spectrometry studies show. The Rutherford backscattering spectroscopy demonstrated that the superficial SiGe layer was superior in quality (derived channelling yield of 10%). The results indicate that the additional step of thermal oxidation pre-annealing is vital to resolve the problem of Ge loss and the modified SIMOX process is applicable for SGOI fabrication.

We have studied the influence of an AlGaN insertion layer in a GaN quantum well on the light emission from a strained GaN/AlGaN multiple-quantum well system. The structural properties of GaN/AlGaN multiquantum wells of the same composition were studied by using the reciprocal space mapping and simulation techniques of high-resolution x-ray diffraction. The lattice constants along the in-plane direction determined from the (1 0 5) reciprocal space maps were a = 3.161 Å for normal multiquantum wells and a = 3.152 Å for interlayer multiquantum wells. The spatial localization of the quantum well emission was unambiguously determined by monochromatic cathodoluminescence measurements. This observation indicates that the blue emission near 2.96 eV in the interlayer multiquantum wells originates from the MQW region, rather than from a deep level in the GaN buffer layer. These transitions could be a quantum confined Stark effect due to the piezoelectric field caused by the strain between the well and barrier.

We have demonstrated a 1.60 µm ridge-structure laser diode and electroabsorption modulator monolithically integrated with buried-ridge-structure dual-waveguide spot-size converters at the input and output ports for low-loss coupling to a cleaved single-mode optical fibre by means of selective area growth and asymmetric twin waveguide technologies. The devices emit in single transverse and quasi-single longitudinal modes with a side mode suppression ratio of 25.6 dB. These devices exhibit 3 dB modulation bandwidth of 15.0 GHz and modulator extinction ratios of 14.0 dB dc. The output beam divergence angles of the spot-size converter in the horizontal and vertical directions are as small as 7.3° × 10.6°, respectively, resulting in 3.0 dB coupling loss with a cleaved single-mode optical fibre.

Measurements of low-frequency transconductance dispersion at different temperatures and conductance deep level transient spectroscopic (CDLTS) studies of an AlGaAs/InGaAs pseudomorphic high electron mobility transistor (p-HEMT) were carried out. The experimental results show the presence of defect states at the AlGaAs/InGaAs hetero-interface. A mobility degradation model was developed to explain the low-frequency negative transconductance dispersion as well as the apparent 'hole' like peaks observed in the CDLTS spectra. This model incorporates a time-dependent change in the two-dimensional electron gas mobility due to ionized impurity scattering by the remaining charge states at the adjoining AlGaAs/InGaAs hetero-interface.

Electrical characteristics of sol–gel derived titanium dioxide (TiO₂) as insulating layers were investigated by making capacitance and leakage current measurements in metal–insulator–semiconductor configurations. The structure was fabricated by depositing 37 nm thick anatase TiO₂ films on p-type silicon (p-Si) substrates. The frequency dispersion of capacitance was attributed to the leaky behaviour of the TiO₂ dielectrics. Using an equivalent circuit, values of the frequency-independent dielectric constant, interfacial surface density and threshold voltage were estimated to be 13, 3 × 10¹⁴ m⁻³ and −0.085 V, respectively. The carrier diffusion was found to be primarily responsible for the diode leakage current at room temperature but the increase in the ideality factor with lowering temperature was believed to be due to fluctuations of barrier height at the TiO₂/p-Si interface.

The switching of high-voltage (1.5 kV) 4H-SiC thyristors by the dV/dt effect has been investigated for the first time in the temperature range from 300 to 504 K. At a rise time of the forward bias V(t) equal to 30 ns, the characteristic bias at which the structure under investigation can be switched on by the dV/dt effect decreases steadily from 289 V at room temperature (dV/dt ∼ 9.7 kV µs⁻¹) to 137 V at T = 504 K. The characteristic value of the critical charge per unit area, Q_cr, is ∼1.9 × 10⁻⁷ C cm⁻² at room temperature and also decreases steadily as the temperature increases.

Thin films of p-type ZnO:N have been obtained by thermally oxidizing zinc oxynitride films prepared by plasma enhanced chemical vapour deposition (PECVD). The p-type ZnO:N thin film with a hole concentration of 2.7 × 10¹⁶ cm⁻³ was obtained after an annealing process was conducted at 600 °C. A conductivity transition from n-type to p-type was observed, which was systematically researched via structural and compositional analyses. In terms of these analyses, it helped to better understand the properties and behaviour of nitrogen in ZnO. First, nitrogen was incorporated into ZnO films during the growth process to occupy oxygen positions, and also partly compensated some donors induced from non-stoichiometric (ZnO_1–x) composition. Second, the amount of activated nitrogen gradually increased in an oxidizing atmosphere and exceeded those donor states to realize an effective compensation, yielding p-type conductivity during the course of thermal oxidation.

The device characteristics of organic light-emitting devices based on tris-(8-hydroxyqunoline) aluminium with a thin layer of MgF₂ inserted at the indium-tin-oxide (ITO) and organic interface or the organic and Mg:Ag cathode interface are investigated. A 1.0 nm MgF₂ thin layer can enhance electron injection when it is inserted only between organic electron transporting layer and Mg:Ag alloy cathode, but can block hole injection when it is inserted only between the ITO anode and organic hole transporting layer. By inserting MgF₂ at both sides on the ITO anode and under the Mg:Ag cathode, the current efficiency of the device is improved by 74%, and power efficiency is also improved by 18% at a current injection of 20 mA cm⁻², compared to the standard device without MgF₂ buffer layer. This is due to the increased electron and decreased hole injection, which results in more balanced electron and hole injection, and more efficient exciton formation. The increased electron injection can be well understood by the tunnelling effect model.

We demonstrate high efficiency polymer electrophosphorescent light-emitting diodes based on a new iridium complex tris{diphenyl-(4-pyridin-2-yl-phenyl)-amine}iridium (III) (Ir(dpppa)₃) doped into a poly (N-vinyl carbazole) (PVK) host. Electron transporting 1,3,5-tris(2-N-phenylbenzimidazolyl) benzene (TPBI) and hole transporting N,N-bis-(1-naphthyl)-N,N'-diphenyl-1,1'-biphenyl-4, 4'-diamine (NPB) were doped simultaneously into the host to improve the transport balance of the charge carriers and avoid the accumulation of the space charges caused by unbalanced charge transport and direct carrier trapping on Ir(dpppa)₃. The effect of the TPBI and NPB concentrations was examined. By optimizing the concentrations of the TPBI and NPB in the blend, the device showed a current efficiency as high as 25.2 cd A⁻¹ at a current density of 0.85 mA cm⁻².

We report on the InGaN multi-quantum well laser diodes (LDs) made by RF plasma-assisted molecular beam epitaxy (PAMBE). The laser operation was demonstrated in a temperature range from 297 K up to 360 K with pulsed current injections using 50 ns pulses at 0.25% duty cycle. The threshold current density and voltage for these LDs were 9 kA cm⁻² (680 mA) and 8.2 V respectively at 297 K. The slope efficiency is 0.35–0.47 W A⁻¹. A high output power of 1.1 W was obtained during pulse operation for 3.6 A and 8.7 V. We compare parameters of laser diodes with two and five InGaN/InGaN quantum wells. The new, low temperature growth mechanism which enhances surface adatom kinetics, together with bulk GaN low dislocation density substrates allowed us to grow high quality laser diode structures. Our result indicates that there are no intrinsic limitations in the growth of nitride-based optoelectronic devices by PAMBE.

We present a technique for the preparation of positively defined multiply connected electron waveguides on modulation-doped GaAs/AlGaAs heterostructures. This technique is based on a mix-and-match combination of electron-beam lithography (EBL) with standard photo lithography. Low-energy EBL on high-resolution negative-tone resist calixarene allows a nearly proximity-free positive definition of nanostructures with a minimum line width of about 25 nm. Subsequent to the EBL process the device leads and contacts are defined in photoresist with standard lithographic techniques. A single-step wet-chemical etch transfer enables the low-damage formation of isolated and multiply connected electron waveguides as well as large-area reservoirs. A 150 nm wide and 0.3 µm (1.2 µm) long quantum wire prepared by this technique shows quantized conductance with a maximum energy separation of 9.8 meV (10.9 meV) between the lowest one-dimensional subbands.

In this work, the influence of the properties of CdS thin films grown by chemical bath deposition upon the characteristics of CdS/CdTe solar cells, when varying the thiourea concentrations in the CdS bath solution, is studied. The important solar cell parameters such as short circuit current (J_sc), open circuit voltage (V_oc), fill factor (FF) and efficiency (η) were measured and it was noted that they improve for thiourea/CdCl₂ ratios (in the CdS deposition solution) up to 5, and drop for higher ratios in the range investigated. In addition, the ideal diode factor (n), saturation current (J_o) and series resistence (R_s) were studied under illumination and dark conditions. The results could be related to several factors, among them, the photoconductivity properties of the layers and the change of the CdS_1−xTe_x layer at the CdS–CdTe interface.

High performance molybdenum-doped indium oxide (IMO) films were deposited on slide glass substrates from metallic targets by using dc reactive magnetron sputtering at room temperature. The structural, electrical and optical properties have been investigated as functions of target composition and oxygen partial pressure. The deposited films were smooth and amorphous, as determined by scanning electron microscopy and x-ray diffraction, respectively. The results revealed that the as-deposited molybdenum-doped In₂O₃ films show good electrical property and high optical transmittance, as well as high infrared transmittance. The films prepared at oxygen partial pressure of 3.8 × 10⁻² Pa and with 2 wt% Mo-doped target are characteristic of high Hall mobility of 20.2 cm² V⁻¹ s⁻¹, carrier concentration of 5.2 × 10²⁰ cm⁻³, and the average optical transmittance excess 90% in the visible region from 400 to 700 nm. Thus IMO films may be a potential material for novel optoelectrical devices such as an organic light-emitting diode.

We study terahertz (THz) generation from anisotropic streaming distribution of a two-dimensional electron gas (2DEG) in a GaN/AlGaN quantum well using the ensemble Monte Carlo method. Under proper electric fields, electrons do not have enough energy for intersubband and intervalley transitions. They make nearly cyclic motion in momentum space within the first subband, which gives rise to a spindle-shaped electron distribution. This effect is improved due to large polar optical phonon energy, strong electron–phonon interaction and much sharper increase of the polar optical phonon emission rate in GaN 2DEG. We demonstrate that electrons show dynamic negative differential mobility at the transit-time frequency and its harmonics at low temperatures due to this cyclic motion. The resonant frequency can be tuned by simply varying the applied electric field. We may thus use this effect to develop voltage-tunable THz masers.

We report our results on InGaNAs/GaAs vertical-cavity surface-emitting lasers (VCSELs) in the 1.3 µm range. The epitaxial structures were grown on (1 0 0) GaAs substrates by metalorganic chemical vapour deposition (MOCVD) or molecular beam epitaxy (MBE). The nitrogen composition of the InGa(N)As/GaAs quantum-well (QW) active region is 0–0.02. The long-wavelength (up to 1.3 µm) room-temperature continuous-wave (RT CW) lasing operation was achieved for MBE- and MOCVD-grown VCSELs. For MOCVD-grown devices with n- and p-doped distributed Bragg reflectors (DBRs), a maximum optical output power of 0.74 mW was measured for In_0.36Ga_0.64N_0.006As_0.994/GaAs VCSELs. A very low J_th of 2.55 kA cm⁻² was obtained for the InGaNAs/GaAs VCSELs. The MBE-grown devices were made with an intracavity structure. Top-emitting multi-mode 1.3 µm In_0.35Ga_0.65N_0.02As_0.98/GaAs VCSELs with 1 mW output power have been achieved under RT CW operation. A J_th of 1.52 kA cm⁻² has been obtained for the MBE-grown In_0.35Ga_0.65N_0.02As_0.98/GaAs VCSELs, which is the lowest threshold current density reported. The emission characteristics of the InGaNAs/GaAs VCSELs were measured and analysed.

Current–voltage measurements were used to study the electrical and hydrogen sensing characteristics of Pd Schottky contacts fabricated on ZnO. The ZnO films were grown on doped GaAs by MOCVD. By utilizing the GaAs substrate as a low resistance contact to the ZnO film, a remarkably low series resistance of the diodes was achieved. After exposure of the Schottky diodes to a hydrogen ambient, the current at reverse bias increased, indicating the sensitivity to hydrogen gas. In contrast to previous reports, the contacts are able to recover fully at room temperature. Annealing studies in hydrogen ambient show stability of these contacts up to temperatures of 150 °C.

We report on the material growth and device performance characterization of a strain-compensated In_0.54Ga_0.46As/In_0.51Al_0.49As quantum cascade laser at λ ∼ 8 µm. For 2 µs pulse at a 5 kHz repetition rate, laser action is achieved up to room temperature (30 °C). The tuning coefficient dλ/dT is 1.37 nm K⁻¹ between 83 K and 163 K and 0.60 nm K⁻¹ in the range from 183 K to 303 K. The peak output power is reported to be ∼11.3 mW per facet at 293 K and the corresponding threshold current density is 5.69 kA cm⁻².

The deposition behaviours of a silicon adatom on H-terminated Si (1 0 0) surfaces have been simulated by the empirical tight-binding (ETB) method. The adsorption energies of a single Si adatom on H-terminated Si (1 0 0) surfaces are specially mapped out in this paper, from which the favourite binding sites and possible diffusion pathways have been found. The energy barriers of the adatom diffusion are found to be higher than that on a clean surface, because the H atoms saturate the dangling bonds of the surface Si atoms. The variation of the diffusion anisotropy with the coverage transformation is also obtained.

The effects of impurity-free vacancy diffusion using a SiO₂ capping layer on the optical and optoelectronic properties of the In_0.53Ga_0.47As/In_0.52Al_0.48As multiple quantum wells (MQW) electroabsorption (EA) modulator structure are investigated. A significant improvement (about 20 times compared to an as-grown sample) of photoluminescence (PL) intensity was observed after thermal treatments at temperatures of above 700 °C. In this structure, however, a red shift was observed after rapid thermal annealing (RTA) using the SiO₂ capping layer, in contrast to the blue shift in conventional interdiffused MQW. A red shift of about 27 meV was obtained at an annealing temperature of 800 °C for 45 s without noticeable PL linewidth broadening. We believe that the red shift is attributed to the exchange between Ga of an InGaAs well and In of an InAlAs barrier. InGaAs/InAlAs MQW EA modulators were fabricated on as-grown and annealed substrates. The basic characteristics of the devices fabricated before and after RTA were evaluated by current–voltage and photocurrent measurements.

A numerical investigation on the transition of surface acoustic waves (SAWs) in Al_xGa_1−xN/GaN/SiC heterostructures to Lamb-like modes localized in the GaN layers is presented. The significantly large sound velocity in AlN in comparison to that in GaN leads to an expulsion of the SAWs from the surface Al_xGa_1−xN layer into the buried GaN layer when the SAW wavelength is roughly less than the thicknesses of the Al_xGa_1−xN and/or GaN layers. The confinement effect is examined as a function of Al composition in the top layer, and is found to be present even when the composition is as low as x ∼ 0.3.

We have fabricated metals (Au/Pt/Ti) Ohmic contacts on mesa structures of boron-doped homoepitaxial diamond films by metal deposition followed by various thermal annealing processes. Specific contact resistance was determined by characterizing the current–voltage relations from transmission line model (TLM) measurement. The lowest specific contact resistance of 1.3 × 10⁻⁵ Ω cm² at room temperature was obtained after 873 K annealing. The temperature dependence of the specific contact resistance of the (Au/Pt/Ti)/diamond contacts has confirmed the thermal stability of the contacts.

Two-dimensional simulations of surface charge effects in AlGaN/GaN HEMT and GaN MESFET devices are performed. The influence of charges of different magnitude, sign and spatial distribution at the ungated surface on drain current characteristics is studied. We have found that positive and negative surface charges of the same magnitude produce very different modifications of drain current. The relative influence of polarization fields, surface charges and free hole accumulation at the top surface is considered and gives rise to the opposite behaviour in HEMT and MESFET devices. The implications of our study in current collapse and related dispersion effects in GaN FETs are discussed.

Electrical and optical properties of low-temperature, plasma enhanced chemical vapour deposited films of TiO₂ have been studied; the source gases were TiCl₄ and O₂. The amorphous, as-deposited films had a dielectric constant ∼33 consistent with their measured density of 3.2 ± 0.2 g cm⁻³. Films deposited using a −41 V substrate bias contained the anatase phase and some rutile as evidenced from infrared spectroscopy and x-ray scattering. Annealing of these films at 600 °C resulted in a significant increase in the rutile content of the film.

We report on optical enhancement of the 655 nm photoluminescence (PL) band intensity in the core-shell CdSe/ZnS nano-particles (quantum dots) under a laser illumination. Kinetic curves and 80 K-to-room temperature dependences of the PL intensity reveal two parallel processes: a reversible enhancement when the PL intensity is recovered after the laser illumination is turned off, and a non-reversible permanent increase of the PL output. Experimental data evidence that the PL enhancement is attributed to the light-activated increase of the energy barrier for photo-generated carriers to escape quantum dot levels. A trap recharging and/or photo-chemical bond restructure play a possible role in the luminescence photo-enhancement.

A novel device of tandem MQW EAMs monolithically integrated with a DFB laser is fabricated by an ultra-low-pressure (22 mbar) selective area growth MOCVD technique. Experimental results exhibit superior device characteristics with low threshold of 19 mA, output light power of 4.5 mW, and over 20 dB extinction ratio when coupled to a single mode fibre. Moreover, over 10 GHz modulation bandwidth is developed with a driving voltage of 2 V. Using this sinusoidal voltage driven integrated device, 10 GHz repetition rate pulse with an actual width of 13.7 ps without any compression elements is obtained.

Enhancement in below bandgap room temperature infrared transmission has been observed in tellurium (Te)-doped GaSb bulk crystals. The effect of Te concentration on the transmission characteristics of GaSb has been experimentally and theoretically analysed. Undoped GaSb is known to exhibit p-type conductivity with residual hole concentration of the order of (1–2) × 10¹⁷ cm⁻³ at room temperature due to the formation of native defects. For such samples, inter-valence band absorption has been found to be the dominant absorption mechanism. The residual holes could be compensated by n-type dopants such as Te. With increasing Te concentration, free carrier absorption due to electrons and inter-valley transitions in the conduction subband become significant. The dependences of various absorption mechanisms as a function of wavelength have been discussed in this paper.