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Abstract: We report the key galvanomagnetic properties of Hall and magnetoresistor cross-shaped sensors with lateral dimensions 2mm×3mm. The measured parameters of these devices gave an interesting insight into their behavior at temperatures ranging from LHe to room temperature. The large changes of the galvanomagnetic parameters versus magnetic field and temperature allow these devices to be used as field or temperature sensors. [Copyright &y& Elsevier]

Abstract: Lead iodide PbI2 is a promising material for X-ray and γ detectors. We report on the development and optimization of the method of direct synthesis for the preparation of PbI2 single crystals, purification by zone melting, and growth technique by using the Bridgman–Stockbarger method. We further report on addition of silver as admixture during synthesis in order to study its influence on properties of the prepared material, and on the possible formation of polytypes. The prepared lead iodide has been characterized by measurements of electrical resistivity and low-temperature photo-luminescence. X-ray diffraction has been used to monitor the crystallographic structure. It has been shown that Ag admixtures have changed resistivity of the PbI2 and has not influenced the basic polytype. [Copyright &y& Elsevier]

Abstract: The structural transformations in InSb crystals irradiated with fast neutrons () and the full spectrum of reactor neutrons with the ratio of the slow to fast neutron fluxes ϕ sn/ϕ fn≈1 are studied. Two ranges on the dose dependence can be selected according to the influence of fast neutron irradiation on the lattice parameter. Increase of the lattice parameter is not observed at the low flux (). As it follows from the X-ray diffuse scattering analysis, vacancy-type clusters dissociate in this range, and the number of small interstitial-type clusters simultaneously increases in this range. At the lattice parameter linearly increases with flux. A great number of small interstitial- and vacancy-type clusters, which can trap Sn impurity atoms, are formed. The heat treatment of irradiated samples up to 400°C completely recovers the values of the lattice parameter. [Copyright &y& Elsevier]

Abstract: We investigated the electrical and structural qualities of Mg-doped p-type In0.01Ga0.99N and GaN layers grown under different growth conditions by metalorganic chemical vapor deposition. It was found that we obtained a lower hole concentration and rough surface by reducing the growth temperature down to 930°C in the case of p-GaN. A slight improvement of the hole concentration was obtained for Mg-doped In0.01Ga0.99N grown at 840°C, but this was accompanied by pit formation on the surface. In0.19Ga0.81N/GaN and In0.25Ga0.75N/GaN multiple-quantum-well (MQW) light-emitting diodes (LED) with such different p-layers were also grown. It was found from photoluminescence studies that the optical and structural properties of the MQW in the LED structure were improved by reducing the growth temperature of the p-layer due to reduced thermal annealing effect of the active region during p-layer growth. However, it was also found the electroluminescence intensity was lower for the LEDs with p-In0.01Ga0.99N layers grown at 840°C. [Copyright &y& Elsevier]

Abstract: Research activity on ZnO has increased over the past few years, with particular interest in potential electronic and optical device applications such as transparent field-effect transistors (FETs) and light-emitting diodes (LEDs). High-quality bulk and epitaxial samples have been prepared using a variety of growth techniques; however, progress on ZnO-based devices has been limited by the lack of reliable and reproducible p-type doping. Unintentionally doped ZnO films usually exhibit n-type conduction, generally attributed to interstitial H or Zn, oxygen vacancies, or substitutional impurities such as AlZn serving as shallow donors. Recent efforts have demonstrated p-type conduction using N, P, As and Sb as acceptor dopants, with hole concentrations as high as 1019 cm−3. In this work, the electrical properties of N- and P-doped p-type ZnO are characterized by temperature-dependent Hall-effects and photo-Hall-effects. An MBE-grown ZnO:N homoepitaxial layer exhibits weak p-type conduction with an acceptor energy E A≈90meV in the dark and n-type photoconduction with a peak electron mobility at low temperature μ n>850cm2/Vs under blue/UV light. This n-type photoconductivity persists for days when the sample is maintained in the dark, under vacuum, at room temperature. A sputtered ZnO:P film shows degenerate p-type conduction with p≈4×1018 cm−3 and a hole mobility μ p≈3cm2/Vs in the dark at room temperature. Under blue/UV light exposure, this P-doped sample undergoes a classic-mixed conduction transition from p-type to n-type where the carrier concentration exhibits a singularity, the Hall mobility (μ H) goes to zero and both change sign as the temperature is increased. However, the n-type photoconductivity persists and no transition from n- back to p-type is observed upon subsequent cooling. Sequential, 400K anneals with the sample in the dark and under vacuum cause the mixed conduction transition to reappear and shift to progressively higher temperatures, ultimately returning the sample to its original p-type state. A surface-layer model provides qualitative agreement with the observed behavior. [Copyright &y& Elsevier]

Abstract: The defect structure of CdTe doped with Yb and co-doped with Ge was investigated by a set of optical (photoluminescence, absorption, photoconductivity), galvanomagnetic and thermoelectric methods. The results can be explained by a model, in which Yb acts as a deep donor with the energy level at E V+0.3eV corresponding to the Yb2+/Yb3+ electronic transition. Introduction of Yb with concentration 1019 cm−3 in the melt results in a decrease of electrically and optically active acceptor defects in the as-grown crystals and causes a decrease of electrical resistivity of CdTe:Ge. A line at 1.585eV related to an exciton bound to a complex of Yb and Cd vacancy was observed both in photoluminescence and photoconductivity spectra. [Copyright &y& Elsevier]

Abstract: Pyrite (FeS2) crystals doped with As, Ni and Co were synthesized with chemical vapor transport over an 18cm horizontal gradient of 700–600°C in evacuated quartz tubes, from a mixture of FeS and S, with FeBr3 as a transport agent. Sulfur fugacity and thus S:Fe stoichiometry was constrained by the Fe1− x S/FeS2− y buffer. As, Ni and Co concentrations were ∼3–800ppm, ∼200–1500ppm and ∼ 450–3700ppm, respectively. Semiconducting properties were measured at room temperature using a van der Pauw and Hall measurement system. Ni and Co-doped pyrite are n-type while As-doped pyrite tends to be p-type. Resistivity for Co-doped pyrite ranged from 0.009 to 0.02Ωcm while for Ni- and As-doped pyrite, resistivity ranged from 2 to 17Ωcm. Undoped pyrite resistivity ranged from 15 to 85Ωcm. Carrier concentration was similar for undoped and Ni-doped pyrite, ranging from 1015 to 1016.6 cm−3, while for Co-doped pyrite it ranged from 1018.7 to 1019.3 cm−3 and for As-doped pyrite it ranged from 1014 to 1018 cm−3. Hall mobility was similar for Co and Ni-doped pyrite ranging from 60 to 270cm2 v−1 s−1 while for undoped pyrite it ranged from 8 to 70cm2 v−1 s−1. Hall mobility for As-doped pyrite ranged from 55.0 to 0.2cm2 v−1 s−1 for electrons and from 0.1 to 11.3cm2 v−1 s−1 for holes with the exception of one sample (of 22). These values should be viewed more as trends than as definitive. The results obtained for Ni, Co, and undoped pyrite are similar to those reported in the literature while results for As-doped synthetic pyrite have not previously been reported. [Copyright &y& Elsevier]

Abstract: The magneto-transport properties of ferromagnetic Ga1−x Mn x As epilayers with Mn mole fractions in the range of x≈2.2–4.4% were investigated through Hall effect measurements. The magnetic field-dependent Hall mobility for a metallic sample with x≈2.2% in the temperature range of T=0–300K was analyzed by magnetic field-dependent mobility model including an activation energy of Mn acceptor level. This model provides outstanding fits to the measured data up to T=300K. It was found that the acceptor levels with activation energies of 112meV at B=0Oe decreased to 99meV at B=5kOe in the ferromagnetic region. The decrease in acceptor activation energy was due to the spin splitting of the Mn acceptor level in the ferromagnetic region, and was responsible for increase in carrier concentration. [Copyright &y& Elsevier]

Abstract: Synchrotron radiation photoelectron spectroscopy (SRPES) has been used to study the electronic structure of the Au/GaN(0001) system at the initial growth stage. The peak fitting of Au4f7/2 core-level and the energy shift of valence band indicate that Au–Ga alloy were formed in the interface reaction. According to the Ga3d signal intensity attenuation vs. the gold film thickness, the early growth mode is considered to be 3D mode above the reaction layer. By using the Linear Augmented Plane Wave method the density of states (DOS) for GaN and Au bulk are calculated within the framework of local functional theory. The theoretical results agree with the valence band structure quite well. The mechanism of interface reaction is discussed based on the experimental and theoretical results. [Copyright &y& Elsevier]

Abstract: We report on the improvement of the electrical characteristics of GaN-based light-emitting diodes (LEDs) fabricated with Ag (1nm)/indium tin oxide (ITO) (250nm) p-type contacts by using Ni nanodots. As-deposited Ag/ITO contacts with and without the Ni nanodots give non-linear electrical behaviour. However, annealing the samples at 530–630°C for 1min in air results in ohmic behaviour. The reverse leakage current characteristics of near UV LEDs are much improved when the Ni nanodots are used. The output power (at 20mA) of LEDs fabricated with the Ni nanodots is enhanced by a factor of 1.34 as compared with that of LEDs made without the Ni nanodots. [Copyright &y& Elsevier]

Abstract: Amorphous gallium arsenide (a-GaAs) films were prepared from their constituents using the co-evaporation technique. The temperature dependence of DC conductivity for the as-deposited (fresh) As-rich and that of thermally annealed films over the temperature range 373–553K is measured. Results of the characteristic electrical parameters indicate non-monotonic behavior during the thermal annealing range 373–423K and a monotonic one in the range 423–553K. Results of the density of structural defect states, using the space-charge-limited current model, are given and discussed in relation to the nature and distribution of defect states regarding their dependence on the thermal treatment. [Copyright &y& Elsevier]

Abstract: We grew single crystals of the new dilute magnetic semiconductor p-Sb2− x Cr x Te3 (x=0,0.0115,0.0215) and investigated their transport, magneto-transport and magnetic properties in the temperature interval 1.7 – 300K. In high fields we clearly observed Shubnikov– de Haas oscillations, that show a decrease in hole concentration for Cr-doped samples. A negative magnetoresistance and anomalous Hall effect are observed in Cr-doped samples. Ferromagnetic order is found below the Curie temperature T c≈5.8K (x=0.0215) and T c≈2.0K (x=0.0115). In the ferromagnetic phase we observe large magnetic anisotropy with the easy-axis parallel to the C 3 crystallographic axis. [Copyright &y& Elsevier]

Abstract: We investigated the photoluminescence (PL) characteristics of ZnO whiskers grown without a catalyst by the hot wall expitaxy method. In the low-temperature PL measurement, free exciton (n=1, 2) emissions appeared as a shoulder. Exciton emissions bound to the neutral donor (D0X) and acceptor (A0X) dominated the edge emission region while phonon replicas of D0X and donor–acceptor pair (DAP) emission dominated the lower energy region. Furthermore, no defect-related band was observed, indicating high crystal quality. With increased temperature, the A0X (I9) peak decreased abruptly and was then suppressed above 130K. Finally, the room temperature PL spectrum was dominated by DAP emission and free exciton emission appeared faintly around 3.308eV. Based on these results, the room temperature bandgap energy was estimated to be 3.368eV, which takes the exciton binding energy 60meV. [Copyright &y& Elsevier]

Abstract: Fabrication of thin film transistor (TFT) using rf sputtered ZnO as channel layer is described in this paper. Deposition condition of the ZnO channel layer is investigated. It is found that metal mesh shielding of the substrate and higher oxygen partial pressure help improve the on–off ratio of the device. Levinson’s expression of drain current is successfully applied to the transfer character of the device. TFT described here would likely find its use in large area FPD to cooperate with display elements that need large current driven. [Copyright &y& Elsevier]

Abstract: The metal–insulator (MI) transition induced by a magnetic field was evidenced for the first time in compensated n-type GaSb layers grown by molecular beam epitaxy. The free electron densities were in the low 1016 cm−3 range or even slightly lower, so that the zero-field 3D electron gas was degenerate and, at the B MI magnetic field of the MI transition, it populates only the spin-split 0(+) Landau level (extreme quantum limit). On the metallic side of the MI transition a T 1/3 dependence of the conductivity was assumed to fit the low-T data and to estimate the B MI value, which resulted of 9.1T in the purest sample. The MI transition manifests in a strong increase of the diagonal resistivity with the magnetic field, but not of the Hall coefficient, suggesting that the apparent electron density is practically constant, whereas the mobility varies strongly. The evidence of a maximum in the temperature dependence of the Hall coefficient has been explained through a two channels transport mechanism involving localized and extended states. [Copyright &y& Elsevier]

Abstract: The 32-strips linear detector based on semi-insulating GaAs is fabricated. The dimensions of each strip are 170×2500×200μm3. Analog and digital read-out electronic card with 24 channels is connected to the linear detector. The surface mounted device (SMD) components with automatic assembling of electronic devices for read-out front-end electronics are used. Testing of counting ability of each strip with micro-collimated (φ∼250μm) X-ray source up to 60kV is carried out. The maximum counting rate is about 1.0×105 cps. The results for gradual decrease of linearity on approaching the maximum counting rate and the dispersion of the intensity registered in individual channels are presented. The crosstalk between neighbouring strips is found negligible. [Copyright &y& Elsevier]

Abstract: Indium nitride films, grown at 375 °C by low-pressure metal–organic chemical vapor deposition, have been annealed at temperatures ranging from 400 °C to 475 °C under an ambient ammonia atmosphere. According to our X-ray diffraction spectra of the annealed InN films, the integrated intensity of the InN (0002) diffraction peak reaches its minimum value at the temperature of 425 °C, but the full-width at half maximum of the InN (0002) peak shows an opposite change with the annealing temperature. These results indicate that the InN film annealed at 425 °C has the worst structural quality. Scanning electron microscope and X-ray photoelectron spectroscope measurements demonstrate that as the annealing temperature is increased to 425 °C, the density of indium grains on the surface increases greatly due to the volatilization of nitrogen atoms. Above 425 °C, the density of the indium grains begins to decrease, resulting from the desorption of indium. The indium grains formed during this annealing process are then supposed to be responsible for the degraded structural quality in the annealed InN. [Copyright &y& Elsevier]

Abstract: Hall effect, electric conductivity, and charge carriers mobility in n-ZnSe single crystals doped with gold during the process of a long-term high-temperature annealing in Zn+Au melt with various Au contents were investigated in the temperature range from 77 to 300K. It has been established that, at low gold concentration, Au atoms form mainly donor-type interstitial Aui defects. The increase of Au concentration in Zn+Au melt leads to the formation of both simple AuZn defects and associative acceptors (AuZn–Aui), (AuZn–DZn), and (AuZn–VSe). These defects determine electrical properties of the crystals and they are responsible for the complex structure of excitonic and impurity radiation spectra. The influence of dopant concentration on both electrical and luminescent properties of n-ZnSe:Zn:Au crystals is investigated. The observed variations of electrical and luminescent properties are due to amphoteric properties of gold impurity in zinc selenide. [Copyright &y& Elsevier]

Abstract: The electrical behaviors of two large volume CZT coplanar detectors in the voltage and temperature application range are presented in this work. Two different regions are treated: bulk and anode surface. curves were acquired at different temperatures in these two regions. Experimental results were analyzed by treating the device as a metal–semiconductor Schottky barrier. Two different formulations were used for modeling this structure: the interfacial-layer thermionic-diffusion model, considered as a complete and general theory, and the diffusion model as a more specific approach. It will be shown that the diffusion model is sufficient to reproduce the results obtained for biases and temperatures normally used in operation. A detailed description of this model is provided. In the anode face, the diffusion model is compared with the space-charge-limited current theory. Differences found in the two regions studied are outlined in this paper. Finally, the influence of the electrical parameters in the design of future detectors is discussed. [Copyright &y& Elsevier]

Abstract: The present work describes a portable digital X-ray scanner based on bulk undoped semi-insulating (SI) GaAs monolithic strip line detectors. The scanner operates in “quantum” imaging mode (“single photon counting”), with potential improvement of the dynamic range in contrast of the observed X-ray images. The “heart” of the scanner (detection unit) is based on SI GaAs strip line detectors. The measured detection efficiency of the SI GaAs detector reached a value of over 60% (compared to the theoretical one of ∼75%) for the detection of 60keV photons at a reverse bias of 200V. The read-out electronics consists of 20 modules fabricated using a progressive SMD technology with automatic assembly of electronic devices. Signals from counters included in the digital parts of the modules are collected in a PC via a USB port and evaluated by custom developed software allowing X-ray image reconstruction. The collected data were used for the creation of the first X-ray “quantum” images of various test objects using the imaging software developed. [Copyright &y& Elsevier]

Abstract: In this work, we investigate the operating conditions for RF plasma sources that employ nitrogen–argon gas mixtures for the molecular beam epitaxy growth of dilute nitride, narrow-bandgap semiconductors. The effects of RF power and gas flow rate variations are discussed, wherein a ‘critical’ RF power level is identified at 300W. Growths performed at RF powers below this critical setting resulted in noticeably better quality material, which was determined from a comparison of pre- and post-anneal room temperature photoluminescence measurements. In addition to the RF power, the gas flow rate directly influences the growth chamber pressure, which in turn also affects the material quality. A modified pumping configuration was used to decouple the chamber pressure from the gas flow rate. In all growths, lower chamber pressures resulted in better material quality, irregardless of the power setting used. These results suggest that operating the nitrogen RF plasma source at its lowest possible power setting and lowest gas flow rate, while still maintaining a stable plasma, will result in the best quality of dilute nitride materials. [Copyright &y& Elsevier]

Abstract: Reactive plasma deposition (RPD) is a technique for depositing a thin film on a substrate using a pressure–slope type plasma ion gun. This method offers the advantage of low-ion damage, low deposition temperature, large area deposition and high growth rates. Ga-doped zinc oxide (ZnO) thin film was grown on a moving glass substrate by RPD. Evaporation of very small quantity of tungsten from anode electrode by plasma collision lets the resistivity of grown ZnO transparent conductive oxide (TCO) film to increase. However, no reduction of carrier concentration was observed but only reduction of carrier mobility. It indicates that reduction of evaporation of tungsten from anode electrode induces increase of carrier mobility without any increase of carrier concentration. After installation of an anode cooling system in order to avoid the tungsten evaporation, increase of the mobility (37 cm2/Vsec) was observed and the lowest resistivity (2.0×10−4 Ω cm) film was obtained from large size grown ZnO TCO of 200×200 mm at low growth temperature of 200 °C with high growth rate of 24 μm/h. [Copyright &y& Elsevier]

Abstract: The molecular beam epitaxy (MBE) technique was used for ZnO epitaxial growth and growth properties were evaluated. Intrinsic ZnO epilayers with mobility of 120cm2/(Vs) and carrier concentrations of 7×1016cm-3 were obtained. ZnO on Si, bandgap engineering using Se and nitrogen doping were carried out for research of device application using this growth technique. We found a large bowing parameter of 12.7eV in ZnOSe, new ZnOSSe semiconductor that is lattice matched to Si and able to change the bandgap from UV to IR, and co-doping phenomenon of N and Ga doped ZnO. [Copyright &y& Elsevier]

Abstract: We study the transport properties of molecular beam epitaxial films with and , 0.5 and 7%. We find that the hole concentration decreases with increasing Cr content, which can be attributed to increasing hole localisation in the impurity band. Our data indicate that carrier hopping or tunnelling are the main conduction mechanisms in the samples. We find no evidence of ferromagnetic ordering in the films co-doped with Mn and Cr. [Copyright &y& Elsevier]

Abstract: InP high electron mobility transistor (HEMT) structures with In0.53Ga0.47As channels and In0.52Al0.48As barriers were grown by molecular beam epitaxy. A GaTe source was used as an n-type dopant. Conventional structures with 50–100Å InAlAs(Te) layers and Te-delta-doped structures were investigated. Both types of structures exhibited good transport characteristics, with mobilities of 8000–10,000cm2/V-s and sheet densities of 1–4×1012/cm2. Fluorination studies showed similar behavior for Si- and Te-doped HEMT structures, with donor deactivation resulting in substantial reductions in mobility and carrier density after exposure to fluorine. [Copyright &y& Elsevier]

Abstract: Using plasma-assisted molecular beam epitaxy (MBE), GaN x Sb1− x films have been grown onto GaSb and GaAs substrates over a range of growth temperatures (310–460°C). The films showed excellent crystalline quality and a nitrogen incorporation of 0–1.75% measured by X-ray diffraction. These materials should enable access to wavelengths in the 2–4μm range. [Copyright &y& Elsevier]

Abstract: This study investigated the atomic layer deposition (ALD) behavior of zinc oxide (ZnO) films on SiO2/Si substrates using Zn(C2H5)2 and O3 as the precursor and oxidant, respectively, at substrate temperatures ranging from 230 to 300 °C, and the electrical properties of the thin films. The self-limiting ALD mechanism of ZnO thin film growth was confirmed in the whole temperature region, and the growth rate decreased from 0.19 nm/cycle at 230 °C to 0.16 nm/cycle at 300 °C. ZnO films with a c-axis preferential orientation were obtained at these relatively low temperatures. The films contained a lower oxygen concentration compared to the films grown with the same precursor and H2O oxidant. However, the ZnO film grown with the O3 oxidant at 300 °C showed approximately a resistivity three orders of magnitude higher than that of the ZnO film using the H2O oxidant, due to higher Si impurity concentration in the ZnO films. Si diffusion into the ZnO films must be due to the stronger oxidation power of O3 compared to H2O. [Copyright &y& Elsevier]

Abstract: The InP/GaAsSb/InP double heterostructure bipolar transistors with 20-nm-thick base layer are passivated by low-temperature deposited SiN x . The SiN x passivation results in an increase of the current gain, seen from common-emitter I–V characteristics. An improvement of the Early voltage is also observed after the passivation. The study of the forward Gummel plots shows that the collector current keeps unchanged, while the base current decreases after SiN x passivation. A decrease of the base current ideality factor is found by the SiN x passivation. The passivation also results in an improvement of the base–collector junction. In contrast, the leakage current of the base–collector junction increases tow orders after passivation. [Copyright &y& Elsevier]

Well c-axis oriented 320 nm and 160 nm thick ZnO thin films were deposited with pulsed laser deposition on Pt/Ti/SiO 2 /Si(100) substrates. Diode characteristics of Au/ZnO/Pt in electrical conductivity were observed. The conductivity in forward bias was sensitive to the hydrogen annealing but almost insensitive in reverse bias. The conductivity of 320 nm thick film in forward bias increased from 6.1 × 10 -5 Ω -1 cm -1 in as-deposited film to 3.3 × 10 -3 Ω -1 cm -1 in hydrogen annealed one at 200 &circle; C. The observed dielectric constant 70 was much higher than that of ceramic ZnO. Dielectric anomaly at about 1 kHz was observed and attributed to the space charge. [ABSTRACT FROM AUTHOR]

Abstract: We investigated the photoconductivity of GaAs:Si epilayer on SI-GaAs specimens. We measured the current from the time we switched on the illumination and after switching it off as a function of time at various temperatures from 250 to 330K. A small persistent photoconductivity starts to appear from 250K. At 330K a clear positive persistent photoconductivity (PPPC) appears. The persistent photoconductivity increases with temperature. Then the specimens were irradiated with α-particles from an Am241 source at 330K. A change in persistent photoconductivity is observed from the dose of 6×1012particles/cm2. At higher doses not only the current increases, but also the PPC duration. We try to explain these phenomena on the base of the existing theoretical models. [Copyright &y& Elsevier]

Abstract: The growth of B12As2 by chemical vapor deposition on 6H–SiC substrates using hydrides B2H6 and AsH3 as the reactants is described. The growth rate increased from 1.5μm/h at 1100°C to a maximum of 5μm/h at 1400°C, and decreased at higher temperatures. X-ray diffraction indicates that the deposits were amorphous when the deposition temperature was below 1150°C. Strongly c-axis oriented crystalline B12As2 films were obtained at temperatures higher than 1150°C. The orientation relationship of the B12As2 on 6H–SiC was . The surface morphology of the B12As2 film grown at 1150°C consisted of isolated triangular crystallites. A continuous film forms as the growth temperature is progressively increased up to 1450°C. [Copyright &y& Elsevier]

Abstract: Wide band-gap group-III nitrides are important for the design of optical devices in the blue and blue–green region. Owing to their wurtzite structure, these materials have a strong inherent polarization field that affects carrier distribution, exciton stability and hence influences the optical properties of the devices. So far, carriers have been assumed to have a sheet-like character. In this paper a non sheet-like distribution function for these quasi two-dimensional carriers is proposed that incorporates the effect of the polarization field. Here GaN/InGaN/GaN and AlGaN/GaN/AlGaN quantum wells have been studied. The polarization field causes the electron and hole wave functions to separate out, thus causing decrease of emission strength and strong reduction of exciton binding energy. This treatment explains well the qualitative nature of carrier distribution in the well. The polarization field changes with GaN mole fraction present in the tertiary nitride layer. The effect of mole fraction on carrier distribution has also been studied. It is found that, inside the well, the hole distribution changes a little more with change in mole fraction than the electron distribution, but for all practical purposes the net change in the distribution pattern is negligible. [Copyright &y& Elsevier]

Abstract: Nitride-based GaN/In0.12Ga0.88N heterostructure field effect transistors (HFETs) with semi-insulating Mg-doped GaN current blocking layers were successfully fabricated. The as-grown layers exhibited a very smooth surface. The DC characteristics of these HFETs were substantially improved over those previously reported. Devices with a 0.75μm gate length showed a maximum drain current (IDS) of 421mA/mm, and a maximum gm of about 85.6mS/mm at VGS between -0.5 and -3V. The device which had a very good pinch-off with source–drain (S–D) leakage current was negligibly small, in the range of 10-1–10-2mA/mm. The RF characteristic of devices showed the current gain cutoff frequency, fT, was 7.45GHz and the maximum oscillation frequency, fMAX, was 12.36GHz. [Copyright &y& Elsevier]

Magnetic properties of III1-xMnxV ferromagnetic alloys depend critically on the distribution of Mn++ ions over the different sites which this ion can occupy in the host III–V lattice. The reason for this is that only Mn++ ions at substitutional group-III sites, MnIII, provide both the localized spins and (since they are acceptors) also the free carriers needed to mediate the ferromagnetic interaction between these spins. Mn++ ions occupying interstitial sites, on the other hand, are double donors, which compensate the substitutional Mn acceptors, thus reducing the hole concentration; and, in addition, the Mn interstitials form antiferromagnetic pairs with the substitutional Mn++ ions, thus canceling their magnetic moments. Both these effects result in lowering the Curie temperature of the III1-xMnxV alloys. In this paper we show that the manner in which Mn enters the III–V lattice is determined by the Fermi level (i.e., by the electronic processes within the material) during the growth process itself. To demonstrate this, we describe a series of growth experiments that involve annealing, co-doping of III1-xMnxV alloys with Be, as well as remote Be-doping (modulation doping) of Al1-yGayAs/Ga1-xMnxAs/Al1-yGayAs heterostructures. [Copyright &y& Elsevier]

The effect of all standard scattering mechanisms, including scattering by acoustic and optical phonons, background impurities and dislocation, on two-dimensional electron gas (2DEG) transport in GaN/AlGaN heterostructures is presented. In order to compare the experimental results with the theory, the simple analytical formulas have been used for a 2DEG confined in a triangular well. At high temperatures electron mobility is ultimately limited by optical phonon scattering. At intermediate temperatures acoustic deformation potential and piezoelectric scattering are the dominant scattering mechanisms. The dislocation scattering prevails at low temperatures and the 2DEG mobility is affected strongly by high density of dislocations. We also investigate the transport to quantum lifetime ratios due to charge dislocations. It is found that ratio is larger for dislocation scattering than for impurity scattering. [Copyright &y& Elsevier]

Wide band gap nitride semiconductors, with potential use in optical devices in the blue region of the optical spectra and as high-power electrical devices, are gaining importance widely (Phys. Stat. Sol. (a) 187(1) (2001) 33; Phys. Stat. Sol. (a) 200 (2003) 118; Phys. Stat. Sol. (b) 228 (2001) 99; InT. J. Numer. Model.: Electron. Networks Dev. Fields 14 (2001) 303; J. Appl. Phys. 94 (2003) 3110; J. Appl. Phys. 94 (2003) 2779; Appl. Phys. Lett. 80 (2002) 3967; J. Appl. Phys. 87 (2000) 965). The carriers confined in the nitride heterojunctions owe their origin to the polarization field present in the structure as much as the dopants in the barrier (Rev. Mod. Phys. 54 (1982) 466; 56 (1997) R10024; Jpn. J. Appl. Phys. 41 (2002) 2531). Polarization being an important property of the nitride materials, affects the carrier confinement significantly. In this paper a quasi-two-dimensional model has been developed on the basis of a non sheet-like carrier distribution function and evaluating the quasi-Fermi level at the heterointerface. The model explains the carrier confinement studied experimentally in an AlGaN/GaN heterojunction (190 (2002) 65). [Copyright &y& Elsevier]

By measuring the ac impedance, we measure for the first time the crossover from diffusive (ωτ<1) to ballistic (ωτ>1) transport as a function of frequency (dc to 5 GHz) in a dc contacted 2d electron gas in the low electric field limit, where τ is the momentum scattering time. The geometry is an “ungated HEMT”, meaning current flows through an ohmic contact into a 2d electron gas, laterally through the 2d electron gas, and out into a second ohmic contact; the 2d electron gas itself is not gated. At the measurement temperature (4.2 K), the low field mobility is 3.2 × 106 cm2/V s. We also measure for the first time the frequency dependent contact impedance in this ballistic limit. [Copyright &y& Elsevier]

This paper presents some studies performed with a state of the art large volume, 1.5 cm diameter × 1.0 cm thick, cylindrical CdZnTe coplanar detector with a new anode pattern design in the line of units previously reported. The absolute efficiency of the detector for different interaction depths is quantified. The detector''s ideal response is simulated with a Monte Carlo code and compared with experimental results. Some conclusions regarding interaction depth profiles, detector efficiency, the new anode pattern design and device quality are reported. These are further compared with the behavior of previous coplanar units. [Copyright &y& Elsevier]

Abstract: High temperature electrical conductivity (HTEC) as a function of temperature and component vapour pressure has been investigated in ZnS:Al and in CdSe:Al single crystals using a two zone resistance furnace and a vacuum sealed quartz ampoule with four tungsten or graphite electrodes. The activation energies of HTEC isobars and the slope values of HTEC isotherms in the large temperature region (500–1200 °C) and in a large component vapour pressure range were determined. The region of compensation of electrons with onefold ionized substitutional Al exists up to change with the region of self-compensation of native defects at high Cd vapour pressures in HTEC isotherms of CdSe:Al. These two electroneutrality conditions were never achieved in ZnS:Al crystals under investigation. The common feature for ZnS:Al isotherms at zinc vapour pressure and for CdSe:Al isotherms at selenium vapour pressure is the existence of self-compensation of Al defects. The equilibrium constant of the association of aluminium with cadmium vacancies in CdSe:Al was determined. The increasing role of holes appears similarly at low Zn vapour pressure values in ZnS:Al and at high selenium vapour pressure values in CdSe:Al. [Copyright &y& Elsevier]

In this work, basic tasks related to the spectrometric performance of X- and γ-ray radiation-imaging detectors based on semi-insulating (SI) GaAs are studied. The state of the art in this field is reported. The importance of a low-noise front-end read-out electronic chain is demonstrated. Four different charge-sensitive preamplifiers are tested and compared. The investigation of SI GaAs detectors considered for imaging applications is focused on the study of different electrode technologies. A blocking electrode is obtained by a surface Schottky barrier or an MBE-grown P+ layer. An ohmic N+ contact is fabricated using alloyed AuGeNi metallization or an advanced, quasi-ohmic non-injecting system created by a non-stoichiometric buffer grown by low-temperature MBE at the metal-semiconductor interface. This latter approach is considered as an improvement of the “non-alloyed” ohmic contact. The pulse height spectra obtained with 241Am and 57Co sources and corresponding detector energy resolution are evaluated. The role of detector geometry and temperature on the spectrometric performance is studied. [Copyright &y& Elsevier]

Results are presented of an experimental study of detectors based on LEC SI GaAs with various top Schottky contact arrangements. The study is concentrated on an improvement of relative detection efficiency, as an important detection property of detectors considered for application in digital radiography. The current–voltage and pulse height spectra measurements at 295 K using 241Am and 57Co gamma sources are demonstrated. Following the application of the detectors in digital radiology systems, the detector properties, such as leakage current, breakdown voltage, charge collection efficiency, relative energy resolution, peak to valley ratio, and relative detection efficiency are evaluated. Improvement of overall detector properties with decreasing top contact area (“small pixel effect”) is presented, and consequently it is shown that the connecting of more pixels into the top contact led to an increase of the relative detection efficiency. [Copyright &y& Elsevier]

Abstract: We report the results of our experimental and theoretical studies concerning the temperature dependence of electron mobility in a two dimensional electron gas (2DEG) confined at the GaN/AlGaN interface. Experimental mobility of about at 3.8K remains almost constant up to lattice temperature , it then decreases rapidly down to about at . The results are discussed using a theoretical model that takes into account the most important scattering mechanisms contributing to determine the mobility of electrons in 2DEG. We show that the polar optical phonon scattering is the dominant mechanism at high temperatures and the acoustic deformation potential and piezoelectric scatterings are dominant at the intermediate temperatures. At low temperatures, the Hall mobility is confined by both the interface roughness (IFR) and ionised impurity scattering. The correlation length and lateral size of roughness at the GaN/AlGaN heterointerface have been determined by fitting best to our low-temperature experimental data. [Copyright &y& Elsevier]

Room-temperature deposited SiNx passivation of InGaAs/InP heterostructure bipolar transistors (HBTs) with graded base was investigated. The passivation indicated that the surface recombination significantly affected the HBT performances. Both the base current and the base current ideality factor were found to decrease drastically after the SiNx passivation. The passivation also resulted in the increase of the current gain. The ideality factor of the surface recombination current of 2 was identified by the analyses of emitter–base diodes with different sizes. The emitter-size-dependent current gain showed that the improvement was due to the reduction of surface recombination. The SiNx passivation also suppressed the dependence on the perimeter-to-area ratio of the emitter junction. [Copyright &y& Elsevier]

The electronic structure of Eu1-xDyxTiO3 is investigated by means of soft X-ray absorption spectroscopy (XAS) near the Ti L2,3-edges. The spectrum of EuTiO3 shows a satellite structure in the high-energy side of four main peaks. The spectra analyzed by using a full-multiplet calculation and the observed features are almost reproduced by the cluster model. The satellite structure above four main peaks is due to the charge transfer (CT) from the O 2p orbital to the Ti 3d orbital. The intensity decreases with the composition of Dy. This tendency corresponds to the change of conductivity against x. This result means that the intensity of the charge transfer peak is strongly related to the conductivity in Eu1-xDyxTiO3. The charge transfer from the O 2p orbital to the Ti 3d orbital plays an important role in the conductivity in EuTiO3. According to the values of the parameters (charge transfer Δ, Coulomb interactions Udd, Udc) obtained by the theoretical calculation, EuTiO3 is an intermediate between the Mott-Hubbard-type and Charge-Transfer-type insulator. [Copyright &y& Elsevier]

Variable magnetic field Hall effect measurements were performed on InN samples grown by molecular beam epitaxy. Since mixed conduction effects due to both the presence of multiple conducting layers and sample inhomogeneity were present, standard measurements yielded only averaged results. The variable field approach allowed direct measurement of interfacial/surface conduction while also allowing determination of the bulk electrical properties. Analysis always indicated electrons with more than one value of mobility, often with a significant spread in the mobility. This suggests multiple conduction layers in the sample as well as sample inhomogeneity. Variable magnetic field analysis of a 7.5 μm thick InN sample suggests maximum mobilities greater than 4000 cm2/Vs in “bulk” InN. The quantitative mobility spectrum analysis technique, reported here for the first time on InN, indicated a continuous and significant spread in mobility for the bulk electron, likely with sample thickness. [Copyright &y& Elsevier]

We measured the magneto-piezoresistance of micromechanical cantilevers fabricated from two-dimensional InAs/AlGaSb heterostructures. The magneto-piezoresistance showed the features of conductance fluctuation and Schbnikov–de Haas oscillation, depending on the sample electron mobility, indicating strong quantum effects on the piezoresistance. A possible mechanism for these quantum effects is discussed based on the comparison between the magneto-piezoresistance and differential magnetoresistance curves. [Copyright &y& Elsevier]

We studied the structural, electrical, and mechanical properties of an InAs thin film grown on GaAs (1 1 1)A substrates by molecular beam epitaxy. In contrast to conventionally used (0 0 1) surfaces, where Stranski–Krastanov growth dominates the highly mismatched heteroepitaxy, layer-by-layer growth of InAs can be established. One of the largest advantages of this unique heteroepitaxial system is that it provides a two-dimensional electron gas system in the near-surface region without the problem of electron depletion. We review the fundamental properties and applications of this unique heteroepitaxial system. [Copyright &y& Elsevier]

In this paper we present the first results of thin film transistors produced completely at room temperature using ZnO as the active channel and silicon oxynitride as the gate dielectric. The ZnO-based thin film transistors (ZnO-TFT) present an average optical transmission (including the glass substrate) of 84% in the visible part of the spectrum. The ZnO-TFT operates in the enhancement mode with a threshold voltage of 1.8 V. A field effect mobility of 70 cm2/V s, a gate voltage swing of 0.68 V/decade and an on-off ratio of 5 × 105 were obtained. The combination of transparency, high field-effect mobility and room temperature processing makes the ZnO-TFT very promising for the next generation of invisible and flexible electronics. [Copyright &y& Elsevier]

The effect of temperature and applied electric field on transport properties of intrinsic nanocrystalline silicon thin films as well as p type doped with boron prepared by VH-PECVD have been studied. The conductivity of all samples, as a function of the exponential of T-1/4 measured at intermediate fields, presented a linear behavior in all the temperature ranges studied (270–450 K). Following the method proposed by Godet [C. Godet, J. Non-Cryst. Solids 299–302 (2002) 333], a linear relationship between the conductivity prefactor (σ00) and the characteristic temperature (T0) of Mott''s law was obtained for a group of quite different materials. From this, and using classical equations of Percolation Theory, the density of states near the Fermi level, the range of hopping, the activation energy for hopping, and the localization parameter were calculated. At low applied fields, dark conductivity becomes field dependent. The non-ohmic behavior of the conductivity observed is analyzed in terms of the hopping transport equations. [Copyright &y& Elsevier]

Fe doped semi-insulating and p-type InP were implanted with P+ ions to produce an excess of phosphorous atoms in the order of 0.1 at.%. Subsequent annealing in Ar ambient in the temperature interval 400–600 °C was performed for 30 s in a rapid thermal annealing system. Variable temperature (12–300 K) Hall-effect measurements have been used for characteristic defect energy level extraction. A large amount of negative free carriers have been observed after the thermal treatments. These electrons are attributed to the creation of PIn antisite defects with an energy level above the minimum of the conduction band. [Copyright &y& Elsevier]