Semi-insulating GaAs Material Research Articles

First tests of Timepix detectors based on semi-insulating GaAs matrix of different pixel size

In this work, we have focused on Timepix detectors coupled with the semi-insulating GaAs material sensor. We used undoped bulk GaAs material with the thickness of 350 μm. We prepared and tested four pixelated detectors with 165 μm and 220 μm pixel size with two versions of technology preparation, without and with wet chemically etched trenches around each pixel. We have carried out adjustment of GaAs Timepix detectors to optimize their performance. The energy calibration of one GaAs Timepix detector in Time-over-threshold mode was performed with the use of 241Am and 133Ba radioisotopes. We were able to detect γ-photons with the energy up to 160 keV. The X-ray imaging quality of GaAs Timepix detector was tested with X-ray source using various samples. After flat field we obtained very promising imaging performance of tested GaAs Timepix detectors.

Chromium compensated gallium arsenide detectors for X-ray and γ-ray spectroscopic imaging

Semi-insulating GaAs material of 500μm thickness grown using the Liquid Encapsulated Czochralski (LEC) method has been compensated with chromium to produce high resistivity single crystals suitable for spectroscopic imaging applications. Results are presented for the performance of three small pixel detectors each with 80×80 pixels on a 250μm pitch, fabricated with metal contacts and bonded to a spectroscopic imaging ASIC. Current–voltage measurements demonstrated a material resistivity of 2.5×109Ωcm at room temperature. At an optimised bias voltage, the average energy resolution at 60keV (FWHM) was in the range 2.8–3.3keV per pixel. An analysis of the voltage dependent X-ray spectroscopy suggests that the electron mobility lifetime (μτe) for each detector is in the range 2.1–4.5×10−5cm2V−1. The spectroscopic imaging capability of the detectors is also demonstrated in X-ray absorption spectroscopy measurements.

Deep traps study of radiation-damaged semi-insulating GaAs detectors introduced by neutrons

Abstract Neutron detectors based on bulk semi-insulating (SI) GaAs represent an interesting and perspective solution due to its relatively high resistance against neutrons. Therefore SI GaAs material seems to be an excellent candidate for fabrication of a neutron imaging detector. Set of detectors based on SI GaAs material was bombarded with various integral neutron fluencies (10 11 –10 15 n cm −2 ). The performance of detectors was evaluated via measured spectra of 241 Am radionuclide sources. From the results a significant worsening in detection ability was observed at fluencies exceeding (over) ∼10 13 n cm −2 . These observations are explained by lattice defects in the base SI GaAs material or in the overall detector structure (electrode interfaces, surface) introduced by neutrons. Photo-induced current transient spectroscopy was used to determine changes in deep level states of bulk SI GaAs base material.

Performance of semi-insulating GaAs-based radiation detectors: Role of key physical parameters of base materials

In this work, the requirements of detector-grade semiconductor materials for radiation detectors, applicable in X-ray digital radiology, are identified. The study includes 12 various bulk semi-insulating (SI) GaAs single crystals grown by LEC and VGF methods, undoped and Cr-doped, obtained from 8 different suppliers. Conductivity, Hall, glow discharge mass spectrometry (GDMS), etch pit density (EPD), scanning electron beam induced current (S-EBIC), X-ray and laser scattering tomography (LST) techniques are used for the bulk SI GaAs material evaluation. The radiation detectors fabricated on these SI GaAs single crystals have been characterized by capacitance methods and their performances have been evaluated from detected pulse height spectra of 57Co. The correlation between the physical characteristics of the base materials and the performance of the detectors is demonstrated and discussed. Key detector-grade SI GaAs parameters, useful for material evaluation, are identified.

GaAs and AlGaAs photodiodes for ionizing radiation detectors

In this work we present large area AlGaAs photodiodes for detecting the luminescent light of crystal scintillators in detectors of ionizing radiation, fabricated by low-temperature variant of the liquid-phase epitaxy (LPE) method. They exhibit a low leakage current (lower than 10 −12 A/cm 2), a high quantum efficiency for wide spectral range (400–900 nm) and a high operating temperature (up to 120 °C). P-i-n GaAs detectors on the base of high purity LPE GaAs material grown by LPE are suggested as an alternative to those on the base of semi-insulating (SI) GaAs material. The use of high purity GaAs instead SI GaAs material much reduces charge carrier trap concentrations and make it possible for p-i-n photodiode to obtain a high charge collection efficiency at low bias voltages. The calculated electrical characteristics show that low capacitance and low dark current can be realised in these photodiodes.

Photoconductive detection of millimeter waves using proton implanted GaAs

We show that ion-damaged GaAs may be used to detect millimeter electromagnetic waves in photoconductive sampling gates. The semi-insulating GaAs material which was implanted with a dose of 1014 cm−2 protons at an energy of 200 keV gave a signal to noise improvement of about 11.4 dB when compared with as-grown semi-insulating GaAs. The improvement is in spite of a reduced carrier mobility in the ion-implanted material and is due to the shorter carrier lifetime and formation of ohmic contacts with the ion-damaged semi-insulating GaAs.

The energy level of the EL2 defect in GaAs

The free electron concentration in semi-insulating GaAs material is larger than the intrinsic concentration. This is due to the contribution of electrons originating from EL2 defects. The magnitude of this contribution implies that the energy level associated with this defect lies near 0.6 eV and not at 0.75 eV, as commonly assumed. We examine the various values which have been determined for this energy level and underline the difference found between photoluminescence and electrical techniques. We finally use a technique derived from deep level transient spectroscopy to get a direct measurement of this energy level. We obtain a value of 0.58 eV, which is consistent with all observations.

Improvement in Semi-Insulating GaAs Material Quality: A Comparative Study of Defects with Deep Levels

Thermally stimulated currents (TSC) spectra and photocurrent (IPC) measurements were used for detection and evaluation of defects with deep levels in undoped semi-insulating (SI) GaAs crystals. Large number of liquid encapsulated Czochralski (LEC) grown materials, produced from late 80's till nowadays, provided from ten various sources were analysed. Deep levels were characterised by a new analytical method-simultaneous multiple peak analysis (SIMPA) of TSC spectra. For each deep trap its unique and reliable signature was determined, comprising activation energy, Ea, electron capture cross section, σn, as well as peak maxima, Tm, and trap's relative and absolute concentrations. It has been found that all measured TSC spectra, even having dramatically different shapes, can be excellently described with a limited set of 11 deep traps. Considerable improvement in quality of the SI GaAs materials produced during last decade was found. It reflects in concentration reduction of most defects and much better defect's distributional uniformity along the same as well as among different wafers, and in similarity of IPC transients.

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.

Evaluation of active layer properties and charge collection efficiency of GaAs particle detectors

According to Ramo's theorem the charge collection efficiency of a particle detector is mainly influenced by the field distribution between the contacts of a Schottky diode. In semi-insulating GaAs material a space charge layer is formed due to deep levels needed for the compensation of acceptors. In this paper the deep levels and their influence on the distribution of the electric field is studied experimentally by different methods of electrical characterization. It is found that the electrical active concentration of the midgap donor of ∼10 15 cm −3 at an energy of 0.67 eV below the conduction band is only about one tenth of its total concentration of ∼16 16 cm −3 as measured by infrared absorption. The Schottky barrier leakage current is found to be responsible for the variation of the electrically active deep centers and it therefore influences the charge collection efficiency (c.c.e.). The c.c.e. turns out to be inversely proportional to the active concentration of deep centers. These results are supported by our modelling of the c.c.e.: Using the transport and the Poisson equation the electrical field distribution can be calculated through the coupling of the quasi-Fermi levels and the compensation mechanism. The model calculations of charge collection efficiencies for both alpha particles and protons are confirmed by the experimental results. The work is performed within the framework of the RD8 project.

Optical Deep-Level Transient Conductance Characterisation of Semi-Insulating Gallium Arsenide Treated with Hydrogen Plasma

Optical deep-level transient conductance spectroscopy (ODLTCS) has been employed to characterise Schottky devices fabricated on liquid encapsulation Czochralski (LEC) semi-insulating (SI) GaAs material which has been exposed to low-pressure microwave (2.42 GHz) H2 plasma. A range of substrate temperatures and exposure times to the plasma were tested. An increase in device conductance spectra amplitudes were observed after 60 min of H2 plasma treatment at 100° C, 300° C, and 400° C. In devices exposed to H2 plasma at 200° C, the conductance spectra decreased in magnitude with time and after 60 min and 120 min treatments, the spectra were identical to that of the control device. The change in device conductance under the above conditions suggests that there is (i) an optimal plasma condition for the passivation of defects by atomic H with the formation of neutral defect-H (DH) complexes, (ii) the reaction between defects and atomic H is time dependent, and (iii) the LEC SI GaAs materials are contaminated with H during the growth process. In contrast, the increase in magnitude of ODLTC spectrum after 200° C treatment with molecular H for 60 min confirms the role of passivation by the atomic H.

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.

Donor-Acceptor Pair as Opposed to Anion Antisite Metastability in Bulk Semi-Insulating GaAs: Electron Paramagnetic Resonance and Photoconductivity Data Analysis

We show that the time evolution of the paramagnetic signals and photoconductivity in the same semi-insulating GaAs material during illumination below the band gap at low temperature can both be analyzed within a charge transfer model. This model involves the formation of metastable donor-acceptor pairs due to free carrier trapping by initially ionized defects. This result again questions whether the common link between EL2 and the AsGa-related defects needs a configuration-related metastability.

Semi-insulating epitaxial GaAs

Semi-insulating GaAs material results from the compensation of residual acceptor impurities and intrinsic defects by the so-called EL2 donor defects which possess a level at midgap. A semi-insulating layer can therefore be produced if it is possible to introduce a large enough concentration of EL2 defects. We describe a vapor-phase epitaxy technique which allows to introduce an EL2 concentration in the range 1015–1016 cm−3. This technique is made to reach very large growth rates, based on the fact that the EL2 concentration increases with the growth rate. The variation of the EL2 concentration versus the various growth parameters (substrate temperature, partial pressure, growth rate) have been monitored. The mechanism by which the incorporation of EL2 defects occurs has been determined, thus allowing one to get a given EL2 concentration reproducibly. This technique of growth, being inexpensive and fast, requiring no toxic gases, appears to be a reasonable alternative to the production of semi-insulating substrates.

Surface-contamination-related thermal instability in GaAs

The instability of the electrical properties of semi-insulating GaAs materials has been investigated. The presence of chemical contaminants on the surface of the finished wafers is responsible. Simulated thermal conversion using intentional carbon-ion implantation indicates larger concentrations for the possible contaminants on the surface than that in the bulk. The contribution to instability due to preferential gettering of imperfections from bulk by the implant and annealing process was eliminated using intentional Ar-ion implantation. For the samples which exhibited a change in electrical conduction type, a linear relationship between the thickness of the type-converted surface layer and the square root of heat treatment time was observed. It is proposed that the causes for the instability are governed by conventional diffusion mechanisms with multiple electrically active species involved. A positive correlation was also observed between thermally induced electrical instability in ion-implanted material and a reduction in activation, peak carrier density, and Hall mobility.

Microtomography observation of precipitates in semi-insulating GaAs materials

Defects introduced in GaAs materials during growth and post-growth thermal processes are known to largely influence the specifications of transistors. Various techniques have been improved to detect these defects and obtain images of their organization. Laser scanning tomography provides us with macroscale images of fault structures, the details of which up to now have not been resolved. In this paper we show that extending this technique to a microscopic scale reveals small individual scatterers which are at the origin of the tomography images: these scatterers are microprecipitates located on dislocations or condensed on point defects. Several examples of undoped, In doped, and annealed materials are reported and discussed: apart from the decoration precipitates which look rather large, smaller and abundant microprecipitates are observed, even in dislocation-free materials; their number is found to be consistent with microetched pit density responsible for the ‘‘microroughness’’ of etched surfaces. Ingot annealed materials developing a conventional cell structure are also shown to contain clouds of such microprecipitates concentrated in the central region of the cells. It is likely that the type and the arrangement of these particles are strongly related to the dislocation network and to the thermal history. There is no doubt that the chemical species involved play an important role at a microscale level in the local electronic properties of the material.

Studies of Photon Energy Dependence of Transient Photocurrent in Semi-Insulating GaAs Materials by means of Photo-Deep-Level Fourier Spectroseopy and Photosensitivity Transient Spectroscopy

The photon energy dependence of the transient photocurrent in Cr-doped semi-insulating GaAs materials was investigated by photo-deep-level Fourier spectroscopy (photo-DLFS) and photosensitivity transient spectroscopy (PSTS). From the experimental results, it was found that photo-DLFS signals change systematically according to the change in the photon energy of monochromatic light; fourteen deep levels were detected. It is shown that measurements taken while continuously varying the photon energy are very useful in studying specified deep levels in cases where the signals of two or more deep levels overlap and in detecting the many deep levels by increasing the change in the occupancy probability at each deep level. Also, a photoquenching effect is shown to occur within the new photon energy range, 0.85–0.92 eV, by PSTS.

Characterization of EL2 distribution on semi-insulating GaAs wafer by optically assisted imperfection profile

An improved nondestructive characterization technique is developed to measure the relative density distribution of the EL2 level in undoped semi-insulating (SI) GaAs wafers at room temperature. Experimentally, the resistance of a small area of the wafer is measured twice, first with greater than band-gap illumination outside a small masked area and then with a narrow-band optical filter centered at 1074 nm in place of the masked area. The difference of the two measured resistances is shown to be proportional to the density of the EL2 level. By moving the masked area across the wafer while taking resistance measurements, the relative density variation of EL2 can be determined. A theoretical discussion based on the commonly used compensation model for undoped SI GaAs materials is presented to interpret the experimental data. A technique for applying electrical contacts to SI GaAs materials by ultrasonic soldering has been developed to achieve reproducible measurements. Although only GaAs materials were investigated, this optically assisted imperfection profile can be applied to study other high-resistivity semiconductors.

AsGaantisites and their relation to EL2 defects in GaAs

Photoquenching experiments of the magnetic circular dichroism and optically detected ESR and ENDOR signals of the As-antisite in various semi-insulating GaAs materials show that the isolated AsGa defects cannot be identified with the EL2 defect in either charge state. Investigations of the spin-lattice relaxation time T1 in conventional ESR and ODESR experiments show that the AsGa defects seen in conventional ESR are not isolated AsGa centres but most probably complexes of AsGa defects. Therefore the correlations established previously between EL2 and AsGa defects from conventional ESR experiments must be reinterpreted to be correlations between EL2 defects and complexes of AsGa defects.

High-purity semi-insulating GaAs material for monolithic microwave integrated circuits

Liquid-Encapsulated Czochralski (LEC) growth of large-diameter bulk GaAs crystals from pyrolytic boron nitride (PBN) crucibles has been shown to yield high crystal purity, stable high resistivities, and predictable direct ion-implantation characteristics. Undoped (≲low 1014cm-3chromium) and lightly Cr-doped (low 1015cm-3range) -GaAs crystals, synthesized and pulled from PBN crucibles contain residual shallow donor impurities typically in the mid 1014cm-3, exhibit bulk resistivities above 107Ω . cm, and maintain the high sheet resistances required for IC fabrication (>106Ω/□) after implantation anneal. Direct29Si channel implants exhibit uniform (± 5 percent) and predictable LSS profiles, high donor activation (75 percent), and 4800- to 5000-cm2/V . s mobility at the (1 to 1.5) × 1017cm-3peak doping utilized for power FET's. It has also been established that LEC crystals can provide the large-area, round