Bulk Semi-insulating GaAs Research Articles

Magneto-optical and ODEPR investigations of native defects in substrate-free LT-MBE grown GaAs

As-grown substrate-free LT-GaAs (200°C) and annealed samples (up to 660°C) have been investigated with magnetic circular dichroism of absorption (MCDA) and optically detected electron paramagnetic resonance (MCDA-EPR) in K- and W-bands. MCDA-EPR spectra of all samples reveal several As Ga-related defects with different excitation spectra, that indicate their different structures in as-grown and annealed material. A strong change occurs in samples annealed above 400°C. The ODEPR spectra show another yet unknown defect which we attribute to a Ga vacancy-related defect. Samples annealed at 500°C exhibit EL2-like MCDA spectra. From the above band gap MCDA spectra of the EL2-like defects evidence is found for their low symmetry. The bleaching properties and spin-lattice relaxation times of the different As Ga-related defects were investigated and compared with the properties of the EL2 defect in bulk semi-insulating GaAs.

Higher Picosecond Photoresponsivity Realized by Introducing Hole-Capturing Levels of Iron in GaAs

Control of transient carrier populations has been achieved by the use of carrier-capturing properties of relevant individual deep levels. Slow contribution from holes to transient photocurrents has been successfully suppressed so that the fast component of electron current becomes dominant. This has been realized by introducing deep acceptor levels of iron into undoped semi-insulating GaAs bulk material. Photoresponses to picosecond light pulses with and without iron doping were compared. A sharp peak at the leading edge, whose width is less than 100 ps, is increased for the iron-doped one. An additional cause of this could be reduced electron capture due to a concomitant decrease of deep donors such as electron levels No. 2 (EL2) and No. 6 (EL6). Iron-diffused epitaxial material, which has much lower concentrations of grown-in deep levels, showed the same electron-dominated characteristics, confirming the effectiveness of the hole-capturing nature of the iron level.

SI-GaAs detectors with epitaxial junction

High leakage currents and incomplete charge collection limit the performance of detectors obtained from bulk semi-insulating (SI) GaAs. In this contest, a crucial role is played by the electron limiting contact. We have investigated the effect of an epitaxial p-type layer deposited on the semi-insulating substrate in place of the usual Schottky contact. Electrical characterization evidences an effect of the p-layer only at low voltages, but radiation detection experiments show an improvement of the charge collection efficiency, which is function of the doping level of the p-layer. In particular both /spl alpha/ and X irradiations give evidence of gain mechanisms in the collected charge. From these results, the strong relation between the structure of the electron-blocking interface and the charge collection properties of these detectors comes out.

Generation of EL2 defects by a 6-MeV proton irradiation of semi-insulating GaAs

The defects introduced in a bulk semi-insulating (SI) GaAs by a 6-MeV proton irradiation at 300 K were investigated with the electron paramagnetic resonance (EPR) detected via the magnetic circular dichroism of the optical absorption. EL2 defects (EL2 0 and EL2 +) are introduced by proton irradiation in contrast to electron irradiation, the total introduction rate being 100±50 cm −1. The radiation-induced EL2 defects could not be completely bleached. They have properties similar to the EL2 defects in a low temperature grown molecular beam epitaxy material.

Fast interstitial diffusion in As-pressure annealed GaAs

Abstract Electronic properties and microstructure of bulk GaAs crystals can be varied over a large range by thermal annealing at different As-gas pressures. Deep-level transient Fourier spectroscopy, Hall effect and DSL etching have been applied to analyse originally semi-insulating GaAs bulk samples after annealing between 800 and 1100°C at As-pressures between 0 and 3 bar. The electrical resistivity, electronic mobility and the EL2 concentrations increased with increasing As-pressure at all temperatures although at levels differing by orders of magnitude. Initially n-type samples are converted to p-type at pressures below 0.5 bar and temperatures above 1000°C. Published results of the Oda-group could be confirmed. As-precipitates disappear at high temperature and reappear at low-temperature annealing. We propose long-range and fast As-interstitial transport between surface and bulk combined with short-range Ga-vacancy migration with dislocations as sources and sinks as elementary processes to explain the observations. The discrepancy with the very small literature values of tracer diffusion coefficients can be resolved by assuming that rapidly diffusing marked interstitials which enter the lattice at the crystal surface tend to exchange sites with unmarked lattice species after which they are immobilized.

GaAs detectors for hard X-ray astronomy

Particle detectors fabricated from bulk semi-insulating (SI) GaAs of 4 different suppliers are investigated. I-V, C-V characteristics, material transport parameters as well as inhomogeneities are studied and correlated with detector performances. It was observed that the charge collection efficiency and the energy resolution of detectors tested by 5.48 MeV α-particles and 122 keV X-rays is strongly affected by physical properties of the base material. It is concluded that detectors fabricated from the graded bulk SI GaAs material are suitable for application in hard X-ray astronomy.

High-resolution resistivity mapping of bulk semi-insulating GaAs by point-contact technique

Electrical non-uniformities connected wih the cellular structure of dislocations have been investigated by high-resolution point-contact measurements both in liquid encapsulated Czochralski (LEC)-grown and in vertical gradient freeze (VGF)-grown GaAs. Comparing maps of point-contact current with those of the photoluminescence intensity, in most cases good agreement was observed. The reason for this correlation with regard to relevant defect levels is discussed. Improvements in the homogeneity of semi-insulating GaAs wafers by different ingot-annealing processes are clearly proved by the point-contact technique.

Material analysis and characterization on zone refined and zone leveled vertical zone melt GaAs for radiation spectrometers

GaAs is a wide band gap (1.42 eV) semiconductor that has shown promise as a room temperature operated γ-ray detector. A practical γ-ray detector would be large in volume, hence the resistivity of the material must be high to ensure large depletion volumes and low leakage currents. Commercially available semi-insulating (SI) bulk GaAs is compensated by a balance between native defect deep donors (EL2) and residual dopant impurities. The high concentrations of electrically active deep and shallow levels are believed to contribute to electric field distortions observed in γ-ray detectors fabricated from SI bulk GaAs. Hence, the controlled reduction of native defects and contaminant impurities may yield improved bulk GaAs for γ-ray detectors. Custom grown vertical zone melt (VZM) bulk GaAs is presently under investigation as an alternative material for room temperature operated γ-ray detectors. The VZM technique allows for zone refinement (ZR) and zone leveling (ZL) of the GaAs ingots. Custom growth of the material allows for controlled changes in the bulk crystal, including deliberate reductions in EL2 and impurity concentrations. Comparisons are made to commercial vertical gradient freeze (VGF) and liquid encapsulated Czochralski (LEC) bulk GaAs.

Semi-insulating bulk GaAs as a semiconductor thermal-neutron imaging device

Thermal-neutron Schottky barrier detector arrays fabricated from semi-insulating bulk GaAs are presently being tested. The devices use a film of 10B to convert the incident thermal-neutron field into α particles and lithium ions, either of which interact in the detector. Bulk GaAs Schottky barrier detectors are relatively radiation hard to thermal neutrons and γ rays and have shown reasonably good energy resolution for α particles. Additionally, reverse biased radiation detectors fabricated from semi-insulating bulk GaAs have been shown to have truncated electric field distributions, resulting in the formation of a high field active region and a considerably lower field dead region. The device is sensitive only to electron-hole pairs excited in the high field region, thus the truncated field effect is advantageous as an inherent γ-ray discrimination feature for neutron detectors. Preliminary results show no indication of device degradation after over 2400 hr in a thermal-neutron beam from a reactor. Images have been formed of 1, 1.5, and 2 mm holes and crosses from 2 mm thick Cd templates.

Semi-insulating bulk GaAs thermal neutron imaging arrays

Prototype thermal neutron imaging arrays have been fabricated from semi-insulating (SI) bulk GaAs. The arrays are 1 mm square Schottky diodes arranged in a 5/spl times/5 matrix. GaAs Schottky barrier radiation detectors are relatively radiation hard and can withstand higher neutron and gamma ray exposure fields than MOS-based Si diode imaging arrays. The devices use /sup 10/B to convert incident thermal neutrons to energetic Li ions and alpha particles. The truncated field effect observed with SI bulk GaAs detectors produces high and low field regions in the device. Electron-hole pairs produced in the active (or high field) region of the device contribute to the observed induced charge, whereas electron-hole pairs produced in the low field region contribute very little to the induced charge. The effect is manipulated to reduce the background gamma ray interaction rate in the devices. Preliminary results show no indication of device degradation after exposure to a total thermal neutron fluence of 1.73/spl times/10/sup 13/ n/cm/sup 2/. Images have been formed of 1, 1.5, and 2 mm holes and crosses from 2 mm thick Cd templates.

Orientation Dependence Of Surface Passivation For Semi-Insulating Gaas

AbstractWe report on the orientation dependence ((100), (110) and (111) ) of photoluminescence (PL), photoreflectance (PR) and Surface Photo-Voltage (SPV) for sulfur passivated bulk semiinsulating (SI) GaAs. Near band gap PL peak intensities (bound-exciton and acceptor-related) were enhanced following (NH4)2S or S2Cl2 treatment of GaAs for all orientations. The reduction of surface recombination velocity (from PL data) was orientation dependent and especially pronounced for the case of (111)A and (111)B orientations. The effect of thin dielectric layers deposited on S-treated surfaces was also investigated, particularly for (100) and (111)A orientations. SPV data shows a strong increase in the above band gap signal after both Streatment and dielectric film deposition, which was higher than that measured for only S-treated surfaces. PR data showed an increase in the interfacial electric field following deposition of dielectric film. The results of absolute S-surface coverage measurements using particle-induced X-ray emission measurements were correlated with the optical characteristics.

Comparison of indium-doped and undoped GaAs minimum ionising particle detectors

Schottky diode particle detectors were fabricated on bulk semi-insulating GaAs to test the material for use as a minimum ionising particle (m.i.p.) detector. Using indium doped LEC GaAs and undoped Vertical Bridgman (VB) GaAs, detectors were fabricated which had large breakdown voltages. A high charge collection efficiency (c.c.e.) of up to 85% was obtained. Commercially available undoped LEC GaAs from different suppliers had lower breakdown voltages and a correspondingly lower c.c.e. was obtained. All the detectors fabricated could be operated at biases up to breakdown and still show spectra with good separation of signal and noise. The indium doped LEC GaAs detectors had a leakage current of 30 nA mm −2 at −1350 V bias and the undoped VB GaAs detectors had a leakage current of 75 nA mm −2 at −1250 V. With the VB material a threshold bias was found which optimised the resolution. At higher bias the c.c.e. kept increasing linearly with bias but the noise level increased more rapidly until it dominated the signal. This increase in noise observed before breakdown was less in the indium doped LEC detectors where the m.i.p. signal was clearly seperated from noise.

Semi-insulating GaAs layers grown by molecular-beam epitaxy

Temperature dependent conductivity and Hall-effect measurements were carried out on molecular-beam epitaxial GaAs layers grown at 420 °C and separated from the substrate. The layers exhibit semi-insulating properties with a room temperature resistivity of (6–7)×106 Ω cm. The electron Hall mobility is proportional to temperature as ∼T−1.1 and the room temperature value is 5900 cm2 V−1 s−1. From the (nHT−3/2) vs T−1 plot, a donor activation energy of 0.68 eV, different than in bulk semi-insulating GaAs, has been evaluated. A similar activation energy is observed in as-grown and annealed GaAs layers grown at 200–350 °C. This indicates that the high resistivity of GaAs grown at low temperatures might be explained by deep donor defects rather than As precipitates.

Magnetic Circular Dichroism of Low-Temperature-Grown AlxGa1-xAs

AbstractWe study magnetic circular dichroism of absorption (MCDA) of LT AlxGa1−xAs as a function of aluminum content. The MCDA spectrum of LT AlxGa1−xAs is distinctly different from the MCDA spectrum of LT GaAs, which has one paramagnetic and one diamagnetic peak at 0.95 eV and 1.18 eV, respectively. As the aluminum content increases, the spectrum of LT AlxGa1−xAs is dominated by a diamagnetic peak similar to the 1.18eV peak of the EL20-like defects in LT GaAs. However, the peak shifts to higher energies as x increases. The photoquenching and temperature dependence of this peak indicates an association with the EL2 defect. The paramagnetic peak observed in LT GaAs also shifts to higher energies but faster and eventually merges with the diamagnetic peak as the Al content increases. The study of the MCDA spectrum of LT AlxGa1−xAs as a function of aluminum content allows a better understanding of the MCDA phenomena of LT AlxGa1−xAs and LT GaAs, as well as the EL2-related transitions in bulk semi-insulating GaAs.

Vacancies and Dominant Electrically Active Defects in Bulk Semi-Insulating GaAs

Positron Annihilation techniques have been used to investigate two important defects that occur naturally in semi-insulating (SI) Gallium arsenide. The growth and assessment of SI GaAs and the application of PA to defect analysis of this important material are reported

Bulk Photorefractive Semiconductors

The photorefractive (PR) effect has been studied for more than 25 years and many applications for optical signal processing such as correlation, real-time holography, dynamic interconnections, and optical memories have been developed. The main focus of study for the PR effect has been oxides (ferroelectrics and sillenites) in which the useful spectral range lies in the visible. Applications for telecommunication systems and eye-safe devices have required extending the spectral range into the near infrared (1.0 to 1.5 μm), and so the exploration of different materials. It has been shown that the bulk semi-insulating III-V semiconductors GaAs and InP, and more recently the II-VI compound CdTe, were efficient materials for this spectral range. III-V materials offer the advantage of availability as bulk semiinsulating materials of high crystalline perfection and homogeneity regarding their electrical properties due to their importance as substrate materials in micro and optoelectronic technology. However, these materials have not been optimized for PR applications, so quantitative analyses of PR experiments related to the specific material defect properties are necessary for further developments. It has equally been shown that the PR effect can be used as an efficient tool for materials characterization.

Below band-gap electroabsorption in bulk semi-insulating GaAs

Electroabsorption measurements of bulk semi-insulating GaAs using a pulsed applied voltage are presented for the wavelength range 890–920 nm and compared with results of the same measurement using a dc applied voltage. In the latter case, field inhomogeneities in the GaAs gives a spatially dependent absorption which thwarts extraction of the absorption coefficient versus electric field. We circumvent this problem by transiently measuring the absorption during a pulsed bias voltage when the field in the sample is nearly uniform.

Iron doped bulk semi-insulating GaAs

As an acceptor dopant with a solid:liquid distribution coefficient ks<1, iron is an example of an impurity which can be used in modest amounts to ensure that an adequate fraction of EL2 midgap defects are ionized along the length of a melt-grown GaAs crystal, as desired for semi-insulating behavior. The results of such deliberate doping with iron (when NFe is in the mid-1015 cm−3 range) are reported for crystals grown by both the liquid encapsulated Czochralski and the vertical gradient freeze methods. Except in the very tail region of such crystals (when NFe≳NEL2 and high resistivity p-type behavior results), GaAs with this modest iron modification to the compensation balance behaves with quite ordinary semi-insulating properties. The iron acceptors are then all ionized, and are optically ‘‘invisible.’’

Toward Modelling Correlated DLTS Spectra from Al/Plasma Oxide/Semi-Insulating GaAs(Cr) Devices

Metal—oxide—semiconductor (MOS) structures prepared on semi-insulating (SI) GaAs substrates are studied by a multichannel modified charge deep-level transient spectroscopy (DLTS) in the depletion mode. In this way an improvement in the selectivity, as compared with the standard DLTS, is obtained. Such an improvement is a prerequisite of a more effective decomposition of the total spectra into individual components, assuming that both the dielectric relaxation and trap-limited charge transients interfere with each other. In addition to the charge released from the dominant interface trap, the contribution from the thermal activation of the conductivity of the bulk SIGaAs material was modelled using an equivalent circuit scheme. The experimentally observed behaviour resembles strongly the results of our model calculations.

Photoreflectance spectroscopy of semi-insulating GaAs

Clear observations of photoreflectance (PR) spectra due to excitonic transitions in semi-insulating GaAs bulk materials are reported. The modulation mechanism is attributed to the electromodulation induced by the Dember effect. This study indicates that the PR spectroscopy provides an important method for characterizing the crystal quality of high-resistivity GaAs.