GaAs Radiation Research Articles

Space charge formation in chromium compensated GaAs radiation detectors

Space charge formation in chromium-compensated GaAs sensors is investigated by the laser-induced transient current technique applying pulsed and DC bias. Formation of non-standard space charge manifested by an appearance of both negatively and positively charged regions in DC biased sensors was revealed during 5 ms after switching bias. Using Monte Carlo simulations of current transients we determined enhanced electron lifetime τ = 150 ns and electron drift mobility μd = 3650 cm2 Vs−1. We developed and successfully applied a theoretical model based on fast hole trapping in the system with spatially variable hole conductivity.

Particles-on-surface sensor with potential barriers embedded in a semiconductor target

Recently, we calculated an effect of surface charges on potential barriers embedded inside a Si target. As a result, electrons and holes transport in a semiconductor target could be controlled by ionized atoms or molecules situated inside the surface adsorbate layer. Since GaAs possesses higher electron mobility than Si, in this paper we calculate the potential distribution and charge carriers motion dependences on the GaAs target surface charge value and dopant concentration. Computer simulation was implemented using the software COMSOL Multiphysics. The obtained results can help to improve a design of GaAs radiation detector, particle detector, surface charge detector, biochemical sensor, etc.

Processing and characterization of epitaxial GaAs radiation detectors

GaAs devices have relatively high atomic numbers (Z=31, 33) and thus extend the X-ray absorption edge beyond that of Si (Z=14) devices. In this study, radiation detectors were processed on GaAs substrates with 110–130μm thick epitaxial absorption volume. Thick undoped and heavily doped p+ epitaxial layers were grown using a custom-made horizontal Chloride Vapor Phase Epitaxy (CVPE) reactor, the growth rate of which was about 10μm/h. The GaAs p+/i/n+ detectors were characterized by Capacitance Voltage (CV), Current Voltage (IV), Transient Current Technique (TCT) and Deep Level Transient Spectroscopy (DLTS) measurements. The full depletion voltage (Vfd) of the detectors with 110μm epi-layer thickness is in the range of 8–15V and the leakage current density is about 10nA/cm2. The signal transit time determined by TCT is about 5ns when the bias voltage is well above the value that produces the peak saturation drift velocity of electrons in GaAs at a given thickness. Numerical simulations with an appropriate defect model agree with the experimental results.

Supercontinuum generation from 2 to 20 μm in GaAs pumped by picosecond CO₂ laser pulses.

We report on the generation of supercontinuum radiation from 2 to 20 μm in a 67 mm long GaAs crystal pumped by a train of 3 ps CO2 laser pulses. Temporal measurements indicate that sub-picosecond pulse splitting is involved in the production of such wide-bandwidth radiation in GaAs. The results show that the observed spectral broadening is heavily influenced by four-wave mixing and stimulated Raman scattering.

Carrier density dependence of the nonlinear absorption of intense THz radiation in GaAs

We study the carrier density dependence of nonlinear terahertz (THz) absorption due to field-induced intervalley scattering in photoexcited GaAs using the optical-pump/THz-probe technique. The intervalley scattering in GaAs is strongly dependent on the photo-carrier density. As the carrier density is increased from 1 × 10(17) to 4.7 × 10(17) cm(-3), the nonlinear absorption bleaching increases. However, if the carrier density is increased further above 4.7 × 10(17) cm(-3), the trend reverses and the bleaching is reduced. The initial increase in absorption bleaching is because, unlike low THz field, high THz field experiences intervalley scattering and nonparabolicity of the conduction band. On the other hand, a simple electron transport model shows that the reduction in intervalley scattering is mainly due to the increase in the electron-hole scattering rate with the increase in the carrier density. This increase in the electron-hole scattering rate limits the maximum kinetic energy attainable by the electrons and thus reduces the observed nonlinear absorption.

High sensitivity detection of radio-frequency modulated magnetic moment in semiconductors

An experimental setup has been realized to measure weak magnetic moments which can be modulated at radio frequencies (~1-5 MHz). Using an optimized radio-frequency (RF) pickup coil and lock-in amplifier, an experimental sensitivity of 10(-15) Am(2) corresponding to 10(-18) emu has been demonstrated with a 1 s time constant. The detection limit at room temperature is 9.3 × 10(-16) Am(2)/√Hz limited by Johnson noise of the coil. The setup has been used to directly measure the magnetic moment due to a small number (~7 × 10(8)) of spin polarized electrons generated by polarization modulated optical radiation in GaAs and Ge.

Efficiency of generation of terahertz radiation in GaAs, InAs, and InSb crystals

Pulsed generation of terahertz radiation in GaAs, InAs, and InSb semiconductors under the action of femtosecond laser radiation at the wavelength of 775 nm is studied. The generation efficiency is shown to increase by a factor of 2.7 if InAs is placed into a magnetic field of 0.8 T. An increase in the generation efficiency by the mechanism of optical rectification in InAs samples with the 〈111〉 orientation is demonstrated.

Generation of difference-frequency radiation in GaAs in the field of a few-cycle laser pulse in the regime of weakly expressed chromatic dispersion

We derive a system of coupled modified Korteweg-de Vries equations in the approximation of unidirectional waves which describe the evolution of electric fields of mutually-orthogonal linearly polarized laser pulses with duration of several optical oscillations in a nonlinear crystal with quadratic nonlinearity in the regime of weakly expressed chromatic dispersion. An isotropic GaAs crystal is considered as a nonlinear crystal, and the direction of propagation of interacting pulses is assumed to coincide with the normal to the 〈110〉 plane of the crystal. The derived system of equations was solved numerically by the finite difference method. We obtained the dependence of the relative shift of the central wavelength of pumping pulse spectrum on the distance travelled. It is shown that the decrease in the crystal thickness down to values commensurate with the coherence length of difference-frequency radiation leads to an increase in conversion efficiency. In particular, it is shown that the efficiency of difference-frequency generation at 4.1 μm and 10 μm for the 30 fs duration of pumping at 1.98 μm and the 100 MV/m amplitude of the electric field is −51 dB and −14 dB, respectively.

Development and evaluation of semi-insulating GaAs detectors in hot plasmas diagnostics

This work deals with the evaluation of semi-insulating (SI) GaAs radiation detector applicable in the spectrometry of “soft” X-rays (1–20 keV) emitted by hot plasmas and evaluation of its performance. SI GaAs detector with thin base and sandwich electrodes configuration was fabricated. The detector chip was mounted onto common support together with the input external JFET and feedback elements and cooled down to 255 K by Peltier cooler. The first JFET stage was connected to the PR307SF hybrid charge-sensitive preamplifier. The shaping amplifier was assembled from the set of CREMAT hybrid circuits allowing variation of the shaping time (0.25–8.0 μs). The detector performance was evaluated by the measurement of the 241Am pulse-height spectra. Observed results are demonstrated and discussed.

On the development of portable X-ray CT mini-system using semi-insulating GaAs radiation imaging detectors

The paper describes the present status of the development of a portable, X-ray computer tomography (CT) mini-system, whose detection part is based on a double array of 2×1024 semi-insulating GaAs pixel detectors. The system can also work as a quantum X-ray scanner while the readout electronics allows for operation in single photon counting and in photon energy separation modes (one energy window). The maximum size of an object that can be evaluated using this system is 180 mm in diameter and 250 mm in length.

Quantum Imaging X—ray CT Systems Based on GaAs Radiation Detectors Using Perspective Imaging Reconstruction Techniques

LAN connection X-ray source Control and power electronic Quantum Imaging X–ray CT Systems Based on GaAs Radiation Detectors Using Perspective Imaging Reconstruction Techniques J. Přibil , B. Zaťko , I. Frollo , F. Dubecký , P. Scepko 3 and J. Mudroň 4 1 Institute of Measurement Science, SAS, Dubravska cesta 9, SK-841 04 Bratislava, Slovakia, umerprib@savba.sk 2 Institute of Electrical Engineering, SAS, Dubravska cesta 9, SK-841 04 Bratislava, Slovakia 3 T&N System, Ltd., Severna 5, SK-974 01 Banska Bystrica, Slovakia 4 MTC, a. s., Kuzmanyho 11, SK-031 01, Liptovský Mikulas, Slovakia

Role of new ohmic electrode metallization in detection performance of bulk semi-insulating GaAs radiation detectors

In this work, we concentrate on the study of the role of ohmic electrode metallization of radiation imaging detectors based on bulk semi-insulating (SI) GaAs. We fabricated SI GaAs radiation detectors with Ti/Pt/Au Schottky contact on one side and In/Au, Mg/Au and Gd/Au contacts on the other side of bulk material with the aim to form hole barrier Schottky contact with non-injecting effect. Current–voltage and noise characteristics and also detection performance of detectors are measured and evaluated. Observed results are discussed and compared with detector using standard AuGeNi non-alloyed ohmic metallization.

Participation of electron-phonon interaction in the ultrafast self-modulation of absorption of light in GaAs. Relation of modulation of absorption with the spectrum of stimulated radiation in GaAs

Ultrafast (varying for ∼ 1 ps) self-modulation of the absorption spectrum of light takes place during the picosecond-scale photogeneration of charge carriers and intense intrinsic stimulated radiation in GaAs. With the modulation, formation of local amplifications of absorption in the spectrum (juts), which are attributed to local depletion of electron populations in the conduction band, is implied. It is found experimentally that the location of the juts in the spectrum is repeated over the interval determined by the energy of the longitudinal optical (LO) phonon and masses of the electron and heavy hole. This circumstance confirms the previous assumption about the substantial role of the electron-(LO phonon) interaction in ultrafast self-modulation of the absorption spectrum. The previously established notion of the relation of the shape of modulation of the absorption spectrum with the shape of the time-integrated spectrum of intrinsic picosecond radiation is also expanded to the case when ultrafast self-modulation of the absorption spectrum manifests itself.

Role of electrode metallization in performance of semi-insulating GaAs radiation detectors

In the present work, a comparative study of semi-insulating (SI) GaAs radiation detectors with different blocking (Schottky) and ohmic contact metallization is presented. The detectors fabricated from “detector-grade” bulk SI GaAs are characterized by current–voltage measurements and their detection performance is evaluated from pulse-height spectra of 241Am and 57Co γ-sources. Observed results are evaluated and discussed. Importance of the optimized electrodes technology of SI GaAs detector with good performance is demonstrated.

Terahertz radiation from coherent confined optical phonons in GaAs/AlAs multiple quantum wells

AbstractWe have investigated the characteristics of the intense terahertz (THz) electromagnetic radiation in (GaAs)35/(AlAs)35 multiple quantum wells with an optical gating method, where the radiation source is coherent longitudinal optical (LO) phonons confined in the GaAs layers. It is found that the intensity of the THz wave from the coherent GaAs‐like LO phonon is enhanced under the condition that the pump‐pulse energy is resonant with the energies of the confined heavy‐hole and light‐hole excitons, which suggests the considerable contribution of the Fröhlich interaction to the generation mechanism. The period‐number dependence of the THz‐wave intensity demonstrates that the THz wave generated in each GaAs layer is constructively superimposed. Moreover, the temperature dependence of the THz‐wave intensity is discussed from an aspect of disturbance by thermal phonons. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Generation of even harmonics of sub-THz radiation in bulk GaAs in the presence of a static electric field

The static electric field effects on nonlinear carrier dynamics in low-doped GaAs bulk under the influence of an intense sub-terahertz field are studied by a three-dimensional multivalleys Monte Carlo simulation. The conversion efficiency is calculated by using the appropriate Maxwell equation for the propagation of an electromagnetic wave along a given direction in the medium. Production of odd and even harmonics due to the nonlinearity of the velocity-field relation is investigated.

Performance study of radiation detectors based on semi-insulating GaAs with P+ homo- and heterojunction blocking electrode

Abstract A preliminary study of technology and performance of radiation detectors based on bulk semi-insulating (SI) GaAs with P+ blocking electrodes is presented. Detectors with blocking electrode system formed either by P+(GaAs)/N–(SI GaAs) homojunction or P+(Al0.3Ga0.7As)/N–(SI GaAs) heterojunction are prepared. Room-temperature I–V characteristics, measurement and evaluation of pulse–height spectra using 241Am and 57Co radionuclide sources were performed for the characterization of the fabricated detectors. The aim of the study is to verify the possibility to improve the performances of SI GaAs radiation detectors based on the Schottky contact by using the more sophisticated P+–N junction technology. The obtained results and the possibility of future improvements of SI GaAs radiation detectors to be used in digital X-ray imaging are discussed.

Experimental analysis of the electric field distribution in GaAs radiation detectors

Abstract We describe the results of experimental studies of Liquid Encapsulated Czochralski (LEC) semi-insulating GaAs detectors with different contact areas. The detector active area spreading at different applied bias is proven by three various experiments. In the first, the scan of the detector electrode and adjacent area was performed using the 660 nm pulsed laser beam, with a spot diameter of 50 μm. The other experiments use the spectra measured by tested detectors, when irradiated by 241 Am α source (5.48 MeV). The experiments show relation between bias voltage applied and the range of active detector area from the edge of the detector contact.

Displacement damage correlation of proton and silicon ion radiation in GaAs

We present results of displacement damage correlation between 2 MeV protons and 22 MeV silicon ion irradiation damage in p/sup +/n GaAs solar cells. The radiation induced degradation of the photovoltaic response correlates well in terms of displacement damage dose.

Radiation Resistance Study of Semi-Insulating GaAs-Based Radiation Detectors to Extremely High Gamma Doses

In our previous paper [V. Necas et al.: Nucl. Inst. and Meth. A 458 (2001) 348-351] we reported on the study on radiation stability of semi-insulating (SI) LEG GaAs detectors to doses of photons from 60 Co up to 19.2 kGy. Later we presented a study, which covered radiation hardness to the same doses on the base of detector material itself, where strong dependence has been proved [T. Ly Anh et al., Proceedings of the XII th International Conference on Semiconducting and Insulating Materials (SIMC-XII-2002). Smolenice Castle, Slovakia (2002) 292-295 (0-7803-7418-5)]. In this paper we present both the key electrical and detection characteristics of SI GaAs radiation detectors prepared using substrates from four various supplies and two different types of contacts, which were exposed to several gamma doses from 60 Co up to the integral dose of about 1 MGy. The obtained results show that SI LEG GaAs detectors provide good spectroscopic performances and even their slight improvement after low to middle gamma irradiation doses (3 −10 kGy) was observed. Further dose exposure caused the degradation of detection properties with an extreme and following improvement depending on detector material properties. SI GaAs detector still retains its working capabilities even after very high doses applied, up to 1 MGy.