Transient electron transport in the III–V compound semiconductors gallium arsenide and gallium nitride

A three-valley Monte Carlo simulation approach is used for a detailed comparative analysis of the transient electron transport that occurs within bulk zinc blende gallium arsenide and bulk wurtzite gallium nitride. We find that in both cases that the electron drift velocity and the average electron energy field-dependent “settling times” are strongly correlated, and that the electric field resulting in the shortest electron transit-time is a function of the channel length. The calculated dependence of the peak transient electron drift velocity on the applied electric field can be used for the design optimization of short-channel high-frequency devices.     

Investigation of MBE grown GaAs/AlGaAs/InGaAs heterostructures

This paper reports on the influence of the In mole fraction variation (0.1≤x≤0.25) of MBE grown pseudomorphic GaAs/Al_yGa_1−yAs/In_xGa_1−xAs heterostructures on the material quality and the performance of the fabricated devices. For x=0.1–0.15, the carrier mobility in the samples was 4500 cm² V⁻¹ s (at 300 K) and 37 000 cm²V⁻¹ s (at 77 K) and decreased significantly at low temperatures as x was increasing up to 0.25. Transistors with gate length of 0.8 m and In_0.1Ga_0.9As channels exhibited transconductances of 200–220 mSm mm⁻¹ and output conductances of 0.15–0.20 mSm mm⁻¹, while gate-source breakdown voltages were 27–28 V. Delay times of the designed and fabricated ICs frequency dividers by 2 were 130–140 ps.     

Characterization of GaN epitaxial layers on SiC substrates with AlxGa1−xN buffer layers

High quality GaN epitaxial layers were obtained with Al_xGa_1−xN buffer layers on 6H–SiC substrates. The low-pressure metalorganic chemical vapor deposition (LP-MOCVD) method was used. The 500 Å thick buffer layers of Al_xGa_1−xN (0≤x≤1) were deposited on SiC substrates at 1025°C. The FWHM of GaN (0004) X-ray curves are 2–3 arcmin, which vary with the Al content in Al_xGa_1−xN buffer layers. An optimum Al content is found to be 0.18. The best GaN epitaxial film has the mobility and carrier concentration about 564 cm² V⁻¹ s⁻¹ and 1.6×10¹⁷ cm⁻³ at 300 K. The splitting diffraction angle between GaN and Al_xGa_1−xN were also analyzed from X-ray diffraction curves.     

Heavy ion radiation damage in double-sided silicon strip detectors

A ²⁵²Cf fission fragment source was used to produce heavy-ion radiation damage in a double-sided silicon strip detector. It was found that a good quality fission fragment spectrum (as determined by the peak to valley ration N_L/N_V) could not be achieved for radiation incident on the p⁺ face of the detector. However, for radiation incident on the n⁺ face, the ratio N_L/N_V remained adequate up to an accumulated dose of 4×10⁶ fragments mm⁻². For the measurement of alphas, typical resolution deteriorated from an initial 30 keV FWHM to 50 keV FWHM at a dose of 8×10⁶ fragments mm⁻² for incident on the n⁺ face, and 6×10⁶ for radiation incident on the p⁺ face. The interstrip resistance in one region of the n⁺ face broke down completely after a relatively small radiation doses incident on that face. Further investigation of this is still required.     

Fabrication and characterisation of p-type GaN metal-semiconductor-metal ultraviolet photoconductors grown by MBE

We have fabricated interdigital metal-semiconductor-metal ultraviolet photoconductors using p-type GaN grown by MBE. The material had a hole concentration of 10¹⁸ cm⁻³ and a mobility of 5 cm² V⁻¹ s⁻¹. The spectral response of the detectors has been measured and it shows a peak at 364.2 nm (3.402 eV) possibly caused by excitonic effects. The transient response of the photodetector cannot be described by a single time constant. The rise and fall times of the photoresponse are different indicating that the theory usually applied to GaN photoconductors is not valid.     

Structure-composition-property dependence in reactive magnetron sputtered ZnO thin films

Thin films of zinc oxide (ZnO) were prepared by dc reactive magnetron sputtering on glass substrates at various oxygen partial pressures in the range 1×10⁻⁴–6×10⁻³ mbar and substrate temperatures in the range 548–723 K. The variation of cathode potential of zinc target on the oxygen partial pressure was explained in terms of target poisoning effects. The stoichiometry of the films has improved with the increase in the oxygen partial pressure. The films were polycrystalline with wurtzite structure. The films formed at higher substrate temperatures were (0 0 2) oriented. The temperature dependence of Hall mobility of the films formed at various substrate temperatures indicated that the grain boundary scattering of charge carriers was predominant electrical conduction mechanism in these films. The optical band gap of the films increased with the increase of substrate temperature. The ZnO films formed under optimized oxygen partial pressure of 1×10⁻³ mbar and substrate temperature of 663 K exhibited low electrical resistivity of 6.9×10⁻² Ω cm, high visible optical transmittance of 83%, optical band gap of 3.28 eV and a figure of merit of 78 Ω⁻¹ cm⁻¹.     

GaN and AlGaN metal–semiconductor–metal photodetectors

GaN based interdigital metal–semiconductor–metal (MSM) photodetectors have been successfully fabricated. The MSM structures were patterned on highly resistive GaN and the ternary compound, AlGaN. For the highly resistive GaN detector, the lowest dark current is 0.1 nA and the UV responsivity of the device was about 460 A W⁻¹ at a DC bias of 5 V. The AlGaN with 24% Al exhibited larger gains of up to 10⁶ A W⁻¹ at 20 V, but at a very high dark current, 1 mA, and very long detector responses, greater than 60 s. The high gain in this device is not well understood. The preliminary measurements indicate that tunneling occurs at high electric fields since a negative temperature coefficient for the breakdown voltage was observed.     

Beam tests of a Fast-RICH prototype with VLSI readout electronics

In this report we discuss the Fast Ring Imaging Cherenkov technique that we have developed for application to proximity-focused LiF (or CaF₂) solid radiator and multiwire chamber photon detector with cathode-pad readout using TEA in CH₄ as photosensor. We describe the full-scale Prototype of 12 000 pads (5.334 × 6.604 mm²) we have built, and briefly the dedicated VLSI readout electronics we have developed. We report in detail the investigations we have performed in a hadron test beam at the CERN PS, and compare the results obtained to the expected performances. The maximum momentum for π/K separation at 3σ achieved in these tests is 2.86 GeV/c for LiF (2.39 GeV/c for CaF₂). The experimentally achieved Cherenkov merit factors, after correction for azimuthal angle acceptance, are N₀ = 65.5 cm⁻¹ (57.7 cm⁻¹), to be compared with 53.8 cm⁻¹ (50.2 cm⁻¹) from Monte Carlo calculations. Operation of the detector over several months has proven the technique reliable and robust, and suitable for application in high-luminosity hadron colliders like LHC, as well as e⁺e⁻ B-Factories like KEK (Japan), SLAC (USA), and Cornell (USA).     

Surface physical and chemical changes of pure iron after molybdenum ion implantation and their effects on the tribological behaviour I: Physical and chemical analysis

Molybdenum ions generated by a metal vapour vacuum arc (MEVVA) ion source were implanted into pure iron at doses of 1 × 10¹⁷ and 3 × 10¹⁷ ions cm⁻² with an extraction voltage of 45 kV. Auger electron spectroscopy (AES) sputtering depth profiles, X-ray photoelectron spectroscopy (XPS) analysis, X-ray diffraction (XRD) analysis, microhardness and the residual stress of the implanted specimen were studied. The results show that molybdenum atoms exist in the implanted layer at a maximum concentration 20 at.%. A new phase (Fe₃C) is formed in the specimens implanted higher doses due to carbon incorporation during sputtering of the natural oxide film from the implanted surface. The Fe₂Mo phase is formed in both dose regimes. Residual compressive stresses of 310 and 560 MPa were measured on the surfaces of the specimens after molybdenum ion implantation at 1 × 10¹⁷ and 3 × 10¹⁷ ions/cm² respectively due to a local expansion of the lattice in the near-surface region. Due to the existence of residual compressive stress and the formation of the new phases, the microhardness of pure iron specimens was increased from 264 to 325 and 333 kgf mm⁻² by molybdenum ion implantation at 1 × 10¹⁷ and 3 × 10¹⁷ ions cm⁻² respectively.     

Synthesis of hexagonal and cubic GaN thin film on Si (111) using a low-cost electrochemical deposition technique

Gallium nitride GaN thin films were deposited on Si (111) substrates using electrochemical deposition technique at 20 °C. SEM images and EDX results indicated that the growth of GaN films varies with the current density. XRD and Raman analyses showed the presence of hexagonal wurtzite and cubic zinc blende GaN phases with the crystallite size around 18-19 nm. Photoluminescence spectrum showed that the energy gaps of h-GaN/Si (111) and c-GaN/Si (111) were near 3.39 eV and 3.2 eV respectively at 300 K. Raman spectrum indicated the presence of mixed phonon modes of hexagonal and cubic GaN.     

Zigzag zinc blende ZnS nanowires: Large scale synthesis and their structure evolution induced by electron irradiation

Large scale zigzag zinc blende single crystal ZnS nanowires have been successfully synthesized during a vapor phase growth process together with a small yield of straight wurtzite single crystal ZnS nanowires. AuPd alloy nanoparticles were utilized to catalyze a vapor-solid-solid growth process of both types of ZnS nanowires, instead of the more common vapor-liquid-solid growth process. Surprisingly, the vapor-phase grown zigzag zinc blende ZnS nanowires are metastable under high-energy electron irradiation in a transmission electron microscope, with straight wurtzite nanowires being much more stable. Upon exposure to electron irradiation, a wurtzite ZnO nanoparticle layer formed on the zigzag zinc blende ZnS nanowire surface with concomitant displacement damage. Both electron inelastic scattering and surface oxidation as a result of electron-beam heating occur during this structure evolution process. When prolonged higher-voltage electron irradiation was applied, local zinc blende ZnS nanowire bodies evolved into ZnS-ZnO nanocables, and dispersed ZnS-ZnO nanoparticle networks. Random AuPd nanoparticles were observed distributed on zigzag ZnS nanowire surfaces, which might be responsible for a catalytic oxidation effect and speed up the surface oxidation-induced structure evolution.      

Chemical substitution of Cd ions by Hg in CdSe nanorods and nanodots: Spectroscopic and structural examination

The chemical substitution of cadmium by mercury in colloidal CdSe quantum dots (QDs) and nanorods has been examined by absorption, photoluminescence and Raman spectroscopy. The crystalline structure of original CdSe QDs used for Cd/Hg substitution (zinc blende versus wurtzite) shows a strong impact on the optical and structural properties of resultant Cd_xHg_1−xSe nanocrystals. Substitution of Cd by Hg in isostructural zinc blende CdSe QDs converts them to ternary Cd_xHg_1−xSe zinc blende nanocrystals with significant NIR emission. Whereas, the wurtzite CdSe QDs transformed first to ternary nanocrystals with almost no emission followed by slow structural reorganization to a NIR-emitting zinc blende Cd_xHg_1−xSe QDs. CdSe nanorods with intrinsic wurtzite structure show unexpectedly intense NIR emission even at early Cd/Hg substitution stage with PL active zinc blende Cd_xHg_1−xSe regions.     

Microstructure and optical properties of GaN films on sapphire substrates

Transmission electron microscopy (TEM), double crystal X-ray diffraction (XRD), photoluminescence (PL) and Raman scattering measurement were applied to study the correlation between the microstructure and material properties of the GaN films grown by light radiation heating metalorganic chemical vapor deposition (LRH-MOCVD), using GaN buffer layer on sapphire substrates. Corresponding to the density of the threading dislocation (TD) increasing approximately one order, the yellow luminescence (YL) intensity was strengthened from negligible to two orders higher than the band-edge emission intensity. The full width of half maximum (FWHM) of GaN (0002) peak of the XRD rocking curve was widened from 11 to 15 min, and in Raman spectra, the width of E₂ mode is broadened from 5 cm⁻¹ to 7 cm⁻¹. A ‘zippers’ structure of GaN buffer layer was discovered by high-resolution electron microscope (HREM).     

Atomistic simulation of hydrocarbon diffusion in silicalite

Molecular dynamics simulation using a burchart-DREIDING force field has been employed to determine the self-diffusion coefficients of methane, ethane, propane, n-butane, isobutane, benzene, and pyridine in silicalite at 300 K. In the case of the alkanes, Einstein diffusion was observed early in a 500-ps simulation. Values of the diffusion coefficient for the alkanes ranged from about 9 × 10⁻⁴ cm² s⁻¹ for methane to 2 × 10⁻⁶ cm² s⁻¹ for isobutane at a loading of 0.5 molecule per unit cell (mpuc) or 2 molecules per simulation box. For the alkanes, there is good agreement between the results of simulation and experimental values of the self-diffusion coefficient obtained by microscopic techniques such as pulsed-field gradient NMR measurements. In the case of benzene and pyridine, anomalous diffusion was observed for time scales up to 9 ns and apparent diffusion coefficients (range of 10⁻¹¹ to 10⁻⁸ cm² s⁻¹) calculated form the Einstein relation increased with increased loading (up to 3.75 mpuc) and, as shown for pyridine, with increasing temperature. In the case of benzene where experimental data was available, the apparent diffusion coefficients calculated at high loadings were within an order of magnitude of experimental values.

Heats of adsorption were calculated by a canonical Monte Carlo method. For the alkanes (methane, ethane, propane, and n-butane), the magnitude of the heats of adsorption linearly increased with the number of alkane carbons in qualitative and quantitative agreement with experimental data. Heats of adsorption calculated for isobutane and benzene also agreed well with experimental values. No experimental values were available for pyridine for comparison.     

Sb Incorporation in Wurtzite and Zinc Blende InAs1−xSbx Branches on InAs Template Nanowires

The physical properties of material largely depend on their crystal structure. Nanowire growth is an important method for attaining metastable crystal structures in III–V semiconductors, giving access to advantageous electronic and surface properties. Antimonides are an exception, as growing metastable wurtzite structure has proven to be challenging. As a result, the properties of these materials remain unknown. One promising means of accessing wurtzite antimonides is to use a wurtzite template to facilitate their growth. Here, a template technique using branched nanowire growth for realizing wurtzite antimonide material is demonstrated. On wurtzite InAs trunks, InAs_1?xSb_x branch nanowires at different Sb vapor phase compositions are grown. For comparison, branches on zinc blende nanowire trunks are also grown under identical conditions. Studying the crystal structure and the material composition of the grown branches at different x_v shows that the Sb incorporation is higher in zinc blende than in wurtzite. Branches grown on wurtzite trunks are usually correlated with stacking defects in the trunk, leading to the emergence of a zinc blende segment of higher Sb content growing parallel to the wurtzite structure within a branch. However, the average amount of Sb incorporated within the branch is determined by the vapor phase composition.     

Difference between mechanochemical and thermal processes of polymorphic transformation of ZnS and PbO

Mechanochemical and thermal processes of the polymorphic transformations, litharge ? massicot and wurtzite ? zinc blende were compared. Massicot transformed readily to litharge with a slight mechanical stimulation at room temperature whereas the mechanochemical transformation in the opposite direction was almost impossible. The wurtzite ? zinc blende transformation was similar. On the other hand, simple heating of litharge at temperatures higher than 673K enabled the transformation into massicot without any difficulty. Wurtzite was also formed on heating zinc blende without any mechanical aids. The reason for this kind of apparent irreversibility of the transformation, being topotactic and including layer structures, is discussed.     

Unravelling the Elusive Antiferromagnetic Order in Wurtzite and Zinc Blende CoO Polymorph Nanoparticles

Although cubic rock salt‐CoO has been extensively studied, the magnetic properties of the main nanoscale CoO polymorphs (hexagonal wurtzite and cubic zinc blende structures) are rather poorly understood. Here, a detailed magnetic and neutron diffraction study on zinc blende and wurtzite CoO nanoparticles is presented. The zinc blende‐CoO phase is antiferromagnetic with a 3rd type structure in a face‐centered cubic lattice and a Néel temperature of T_N (zinc‐blende) ≈225 K. Wurtzite‐CoO also presents an antiferromagnetic order, T_N (wurtzite) ≈109 K, although much more complex, with a 2nd type order along the c‐axis but an incommensurate order along the y‐axis. Importantly, the overall magnetic properties are overwhelmed by the uncompensated spins, which confer the system a ferromagnetic‐like behavior even at room temperature.     

MBE grown InGaN quantum dots and quantum wells: effects of in-plane localization

InGaN/GaN heterostructure samples were grown by molecular beam epitaxy using ammonia as a nitrogen precursor. The growth of InGaN/GaN self-assembled quantum dots was monitored in situ by reflection high energy electron diffraction intensity oscillations. Atomic force microscopy scans showed a very high density of InGaN islands, 1×10¹¹ cm⁻², well above the dislocation density. This could explain the increased radiative efficiency of these samples compared to homogeneous quantum wells. Light emitting diodes (LEDs) with InGaN active layers buried in GaN were realized. Electroluminescence and photocurrent spectra of these LEDs evidence a strong Stokes shift that can be attributed to high localization of carriers in InGaN layers.     

Ion-burst method for positive and negative ion species measurements

A simple method for positive and negative ion species measurements utilizing ion bursts is presented. In this ion-burst method, the ion bursts are excited by a potential applied to a mesh grid immersed in the plasma. Since the velocity of the ion burst is dependent on the excitation voltage and the ion mass, the ion species can be determined from measurement of the ion-burst velocity. In the DC discharge of an Ar/SF₆ gas mixture, the positive ion species are found to be Ar⁺, SF₃⁺ and SF₅⁺, and the negative ion species are found to be F⁻ and SF₆⁻. These results are compared with the spectrum analysis using a quadrupole mass spectrometry system and show qualitative agreement with them.