Semi-insulating GaAs Detectors Research Articles

Experimental analysis of the electric field distribution in semi-insulating GaAs detectors via alpha particles

Semi-insulating gallium arsenide (SI GaAs) detectors offer a promising alternative to commercially available silicon detectors. They demonstrate superior radiation hardness and provide improved efficiency for gamma and X-ray detection, primarily attributed to their higher density. In this study, we examined 350 μm thick SI GaAs detectors featuring front-side Ti/Pt/Au Schottky contacts with varying contact areas, complemented by back-side Ni/AuGe/Au ohmic contacts spanning the entire area. First, the reverse current-voltage characteristics of the prepared detectors were measured. The dependence of the reverse current and the breakdown voltage on the Schottky contact area was revealed. As the contact area decreases, the reverse current decreases and the breakdown voltage increases. The detection performance of the detectors was evaluated by alpha spectrometry using an 241Am source. After irradiation of the detectors from the Schottky electrode, the measured alpha spectra show an increasing CCE with decreasing Schottky contact area. Finally, the correlation between the applied bias voltage and the extent of the active detector area from the edge of the detector contact was investigated.

Spreading of an active region of semi-insulating GaAs detectors after radiation degradation

The radiation hardness of GaAs detectors against high-energy electrons goes up to a few MGy. Their degradation is mainly connected to the decrease of the charge collection efficiency and the increase of the reverse current. On the other hand, an improvement in detection efficiency was observed at low degradation doses. The explanation that this could be caused by an enlargement of the detector active area due to spreading of the collecting electric field caused by radiation-induced defects is studied in this paper. We have used the alpha particles of 241Am, which interact in the GaAs surface layer, to study the enlargement of the detector active region after its degradation by 5 MeV electrons with doses ranging from 24 to 2000 kGy. The results show that the electric field spreads behind the Schottky contact metallization edges not only with increasing applied reverse bias but also with rising cumulative dose of radiation degradation in the dose range from 24 up to 100 kGy, followed by a slight reduction in area size. The electric collecting field keeps larger dimensions than before detector degradation for doses up to 600 kGy. For higher doses than 1000 kGy, the active area of the detector was reduced below its initial size before degradation. Moreover, the improvement of detection efficiency with increasing bias applied becomes weaker with increasing degradation dose. Radiation degradation affects the electric field distribution in semi-insulating GaAs detectors and the results obtained may provide useful knowledge to preparation of multi-pixel GaAs structures for imaging and particle tracking.

Semi-insulating GaAs detectors degraded by 8 MeV electrons up to 1500 kGy

Radiation degradation of semi-insulating GaAs detectors by 8 MeV electrons up to doses of 1500 kGy is studied in this paper. The influence of irradiation on GaAs material parameters and on spectrometric and electrical properties of fabricated detectors is evaluated. The detector material was degraded before contact preparation, which ensured separation of radiation degradation solely to the bulk material, excluding the contact degradation. The positron annihilation lifetime spectroscopy (PALS) was involved to characterize the substrate material together with galvanometric measurements. Radiation-induced mono-vacancies were clearly identified by PALS in the irradiated materials with increasing concentration up to 2.8 × 1016 cm−3 at maximal applied dose. In correlation with defect concentration the electron Hall mobility decreased with dose down to 3270 cm2 V−1 s−1 and resistivity increased up to 5.22 × 108 Ω cm at 1500 kGy. The bulk material properties influenced the parameters of fabricated detectors. The detectors lost their current blocking behaviour at 1000 kGy according to current-voltage measurements. The charge collection efficiency during alpha-particle and gamma ray measurements almost exponentially decreased with applied dose from initial 40% down to 5% at 1500 kGy in the case of alpha spectrometry and from 48% to 12% at 500 kGy for gamma spectrometry.

Alpha-spectrometry by radiation-degraded semi-insulating GaAs detectors

A radiation hardness study of semi-insulating GaAs detectors against 5 MeV electrons is presented in this paper. The influence of cumulative dose in the range from 136 up to 2000 kGy on alpha-spectrometry of 241Am is evaluated for investigated detectors. The characterising parameter, the charge collection efficiency, was negatively affected by cumulative dose applied. At 200 V detector reverse bias it rapidly decreased from initial 45% it to 15% at 600 kGy proceeding with mild deterioration down to 6% at2000 kGy. The operability of detector was well preserved through whole range of doses. The alpha- and gamma-spectrometry of the same radioactive source are compared.

Radiation hardness limits in gamma spectrometry of semi-insulating GaAs detectors irradiated by 5 MeV electrons

A radiation hardness study of developed Schottky barrier semi-insulating (SI) GaAs detectors against 5 MeV electrons is described in this paper. The influence of cumulative dose (up to 1000 kGy) and the applied dose rate (20, 40 or 80 kGy/h) on spectrometric and electrical properties of detectors is presented. The spectrometric properties were evaluated from measured 241Am gamma spectra and the electrical properties were obtained measuring the current-voltage characteristics. The cumulative dose has negatively affected all spectrometric properties. The detector CCE (Charge Collection Efficiency) exponentially decreased with dose from initial 74% down to 24% at maximum applied dose of 1000 kGy. The relative energy resolution was increasing from 17% before irradiation up to almost 50% at maximum dose applied. The peak-to-valley ratio firstly increased from 10 to 17 at 24 kGy and then rapidly decreased below 2 at 200 kGy. The electrical properties were obtained from measured current-voltage characteristics of detectors. The breakdown voltage of detectors firstly decreased to 60% of its original value and at few hundred kGy it started to increase exceeding the initial value twice. The reverse current exhibited similar behaviour decreasing up to 200 kGy and then increasing. Concerning obtained results we can determine the limit of sufficient functionality of developed SI GaAs detectors against 5 MeV electrons to be 1000 kGy for 59.5 keV particles. The second investigated parameter of irradiation, the dose rate in chosen ranges, did not greatly alter the spectrometric properties of studied detectors.

From single GaAs detector to sensor for radiation imaging camera

Our research in the field of bulk SI (semi-insulating) GaAs detectors for digital imaging has taken almost 20 years. We have optimized the process of detector fabrication and improved the detector quality in terms of its detection properties. We have obtained the energy resolution of 7.2% for 122 keV photons decreased to 5.3% by cooling the detector to 255 K. The typical charge collection efficiency of our SI GaAs detectors is between 85 and 88%. Besides, we were developing various X-ray imaging systems. We started with moving one single detector, where we obtained a spatial resolution of 500 µm but the exposition period was of several hours. Later we prepared a digital X-ray scanner composed of 480 strip detectors in one line scanning an area of 12 × 15 cm2 and obtaining the spatial resolution of 250 µm. This fully homemade scanner shortened the exposition time to several minutes. Finally we have fabricated a two-dimensional system of 256 × 256 pixel SI GaAs detectors covering an area of 1.4 × 1.4 cm2 suitable for Timepix read-out chip developed by the MEDIPIX2 collaboration reaching a basic spatial resolution of 55 µm and exposition in the range of not more than a few seconds.

Comparison of semi-insulating GaAs and 4H-SiC-based semiconductor detectors covered by LiF film for thermal neutron detection

In this contribution we have focused on comparison of spectroscopic properties of semi-insulating (SI) GaAs and 4H-SiC detectors of thermal neutrons fabricated at the Institute of Electrical Engineering SAS in Piešťany. 6LiF reactive film has been applied on Schottky contact as a convertor of thermal neutrons to detectable charged particles (tritons and α particles). Optimal thickness of the 6LiF film has been determined for front side irradiation using MCNPX code to be ca 25 μm. From the energies deposited by secondary charged particles in the active volume of a detector, corresponding responses have been calculated for different thicknesses of the 6LiF conversion layer. The calculated responses have been compared with those collected by measurements using thermal neutrons generated by 239Pu-Be neutron source. Thermal neutron spectra have been recorded by SI GaAs and 4H-SiC detectors using different 6LiF film thicknesses. Generally, in spite of limited spectroscopic performances of the studied detectors, two hills related to tritons (2.73 MeV) and α particles (2.05 MeV) are discernible in all spectra beside a noise peak positioned in the low energy region. The specific differences between SI GaAs and 4H-SiC detector response to thermal neutrons will be closely discussed in the paper.

Radiation hardness study of semi-insulating GaAs detectors against 5 MeV electrons

A radiation hardness study of Semi-Insulating (SI) GaAs detectors against 5 MeV electrons is described in this paper. The influence of two parameters, the accumulative absorbed dose (from 1 to 200 kGy) and the applied dose rate (20, 40 or 80 kGy/h), on detector spectrometric properties were studied. The accumulative dose has influenced all evaluated spectrometric properties and also negatively affected the detector CCE (Charge Collection Efficiency). We have observed its systematic reduction from an initial 79% before irradiation down to about 51% at maximum dose of 200 kGy. Relative energy resolution was also influenced by electron irradiation. Its degradation was obvious in the range of doses from 24 up to a maximum dose of 200 kGy, where an increase from 19% up to 31% at 200 V reverse voltage was noticed. On the other hand, a global increase of detection efficiency with accumulative absorbed dose was observed for all samples. Concerning the actual detector degradation we can assume that the tested SI GaAs detectors will be able to operate up to a dose of 300 kGy at least, when irradiated by 5 MeV electrons. The second investigated parameter of irradiation, the dose rate of chosen ranges, did not greatly alter the spectrometric properties of studied detectors.

Spectrometric properties of semi-insulating GaAs detectors irradiated by 5 MeV electrons at different dose rates

The radiation hardness of Semi-Insulating (SI) GaAs detectors against 5 MeV electrons is investigated in this paper. The influence of two parameters, the accumulative absorbed dose (from 1 to 120 kGy) and the applied dose rate (20, 40 or 80 kGy/h), on detector spectrometric properties was studied. The electron irradiation has negatively affected the detector CCE (Charge Collection Efficiency). Un-irradiated detectors exhibited the CCE of 79% at maximum operating reverse voltage of 300 V and reached the maximum CCE of 51% at 200 V after irradiation by a dose of 120 kGy. Relative energy resolution was also affected by electron irradiation. Its global degradation was observed in the range of doses from 24 up to 120 kGy, where an increase from 19% up to 39% at 200 V reverse voltage was noticed. On the other hand, a global increase of detection efficiency with dose, by about 30% at 120 kGy, was observed with all samples. We did not observe any significant influence of chosen dose rates applied during irradiation on investigated spectrometric properties of detectors.

Semi-insulating GaAs based detector of fast neutrons produced by D–T nuclear reaction

We have examined semi-insulating (SI) GaAs detectors with high density polyethylene (HDPE) conversion layer by a mono-energetic neutrons with kinetic energy of 16.755 MeV generated by a deuterium—tritium nuclear reaction. First, the influence of HDPE layer thickness on the relative detection efficiency of fast neutrons was studied. The MCNPX (Monte Carlo N-particle eXtended) code has been used to support the analysis of the experiment. The theoretical optimum thickness of the conversion layer was determined to 1.9 mm using the MCNPX code. The HDPE conversion layers of various thicknesses, in the range from 50 μ m to 3200 μ m, were glued on the top Schottky contact of SI GaAs detector in the experiment. The neutron detection efficiency was evaluated from measured spectra and compared to results from simulations. The experimental data showed very good agreement with simulation results. Then the effect of active detector thickness modified by detector reverse bias on neutron detection efficiency was studied. Finally, the effect of the angle of irradiation on neutron detection efficiency was evaluated exhibiting decreasing tendency with increasing deviation from perpendicular direction of impinging neutrons.

Radiation hardness of GaAs sensors against gamma-rays, neutrons and electrons

Radiation hardness of semi-insulating GaAs detectors against 60Co gamma-rays, fast neutrons and 5MeV electrons was compared. Slight improvements in charge collection efficiency (CCE) and energy resolution in FWHM (Full Width at Half Maximum) were observed at low doses with all kinds of radiation followed by their degradation. The effect occurred at a dose of about 10Gy of neutrons (CCE improved by 1%, FWHM by 5% on average), at 1kGy of electrons (FWHM decreased by 3% on average) and at 10kGy of gamma-rays (CCE raised by 5% and FWHM dropped by 8% on average), which is in agreement with the relative displacement damage of the used types of radiation. Gamma-rays of MeV energies are 1000-times less damaging than similar neutrons and electrons about 10-times more damaging than photons. On irradiating the detectors with neutrons and electrons, we observed a global increase in their detection efficiency, which was caused probably by enlargement of the active detector area as a consequence of created radiation defects in the base material. Detectors were still functional after a dose of 1140kGy of ∼1MeV photons, 104kGy of 5MeV electrons but only up to 0.576kGy of fast (∼2 to 30MeV) neutrons.

Semi-insulating GaAs detectors with HDPE layer for detection of fast neutrons from D–T nuclear reaction

Bulk semi-insulating (SI) GaAs detectors optimized for fast-neutron detection were examined using mono-energetic neutrons. The detectors have an active area of 7.36 mm2 defined by a multi-pixel structure of a AuZn Schottky contact allowing a relatively high breakdown voltage (300 V) sufficient for full depletion of the detector structure. The Schottky contact is covered by a HDPE (high density polyethylene) conversion layer, where neutrons transfer their kinetic energy to hydrogen atoms through elastic nuclear collisions. The detectors were exposed to mono-energetic neutrons generated by a deuterium (D)–tritium (T) nuclear reaction at a Van de Graaff accelerator. Neutrons reached a kinetic energy of 16.8 MeV when deuterons were accelerated by 1 MV potential. The influence of the HDPE layer thickness on the detection efficiency of the fast neutrons was studied. The thickness of the conversion layer varied from 50 [Formula: see text]m to 1300 [Formula: see text]m. The increase of the HDPE layer thickness led to a higher detection efficiency due to higher conversion efficiency of the HDPE layer. The effect of the active detector thickness modified by the detector reverse bias voltage on the detection efficiency was also evaluated. By increasing the detector reverse voltage, the detector active volume expands to the depth and also to the sides, slightly increasing the neutron detection efficiency.

GaAs detectors irradiated by electrons at different dose rates

The radiation hardness of Semi-Insulating (SI) GaAs detectors against high-energy electrons was investigated. The detectors were irradiated by 5 MeV electrons. The influence of two irradiation parameters, the total absorbed dose (up to 24 kGy) and the applied dose rate (20, 40 and 80 kGy/h), on their spectrometric properties was studied. An 241Am gamma-ray source was used to evaluate the spectrometric properties. The applied dose has negatively affected the detector CCE (Charge Collection Efficiency) and has influenced also the energy resolution. Nevertheless, a global increase of detection efficiency with the dose was observed. Three different dose rates used during irradiation did not affect the CCE, but in the range of doses from 4 to 16 kGy an influence of the applied dose rate upon two other parameters was observed. With higher dose rates, a steeper increase in the detection efficiency and significant worsening of energy resolution were achieved.

Influence of electron irradiation on properties of semi-insulating GaAs detectors

The effects of electron beam irradiation on electrical and spectrometric properties of semi-insulating (SI) GaAs detectors were studied. The electric properties were monitored by reverse and forward current–voltage characteristics. In general, a breakdown voltage decrease with the dose was observed. However, some samples showed a local increase in the breakdown voltage at doses between 5 and 10 kGy. The detector spectrometric properties (the charge collection efficiency (CCE), the energy resolution and the detection efficiency) were evaluated from measured spectra of the 241Am radionuclide gamma source before and after electron irradiation. The CCE and energy resolution showed minor changes after irradiation. The detection efficiency noticed an initial increase (up to a dose of 5 kGy) followed by a permanent decrease. At 30 kGy, the overall degradation of detector functionality was observed with all samples.

GaAs detectors irradiated by low doses of electrons

Semi-insulating (SI) GaAs detectors were irradiated by 5 MeV electrons up to a dose of 69 kGy, in order to test their radiationhardness. The electric and spectrometric stability of detectors was examinedas a function of the absorbed dose. Investigated detectors showed a verygood detector radiation resistance within a dose up to 40 kGy followed bydeterioration of some spectrometric and electric properties. However, thereverse current and the detector charge collection efficiency showed minimumchanges with the overall applied doses. The obtained results will be used asa preliminary study for further radiation-hardness investigations of GaAsdetectors against high energy electrons. This will complete our previousstudies of GaAs detector radiation hardness against fast neutrons andγ-rays.

Semi-insulating GaAs detectors optimized for fast neutron detection

Semi-insulating (SI) GaAs detectors with a HDPE (High Density PolyEthylene) conversion layer were optimized for detection of fast neutrons (from 0.5 MeV to 12 MeV). Based on previous simulations of neutron transport in HDPE and SI GaAs carried out by the Monte Carlo radiation transport computer code MCNPX (Monte Carlo N-Particle eXtended, version 2.5.0) we used a SI GaAs wafer with a larger thickness of 270 μm. The area of a single AuZn Schottky contact was enlarged from 6.25 mm2 to 7.36 mm2. Thanks to the pixel structure of the new metallization the breakdown voltage increased from 60 V to 280 V as deduced from the measured I−V characteristics. Various thicknesses of HDPE layers in the range from 100 μm to 2000 μm were used with SI GaAs detectors for neutron conversion. The measured relative detection efficiency for fast neutrons using SI GaAs detectors with various HDPE thicknesses varied from 0.07 to 0.12% at lower applied voltages, with a maximum for a 500 μm thick conversion layer.

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.

Transport properties in semiconductor-gas discharge electronic devices

Nonlinear electrical transport of semi-insulating (SI) GaAs detector in semiconductor-gas discharge IR image converter (SGDIC) are studied experimentally for a wide range of the gas pressures ( p = 28–55 Torr), interelectrode distances ( d = 445–525 μm) and inner electrode diameters ( D = 12–22 mm) of photocathode. The destabilization of homogeneous state observed in a planar dc-driven structure is due to nonlinear transport properties of GaAs photocathode. Experimental investigation of electrical instability in SGDIC structure was analyzed using hysteresis, N-shaped negative differential conductivity (NDC) current voltage characteristics (CVC) and dynamic behavior of current in a wide range of feeding voltage ( U = 590–1000 V) under different IR light intensities incident on cathode material. It is established that hysteresis are related to electron capture and emission from EL2 deep center on the detector substrate. We have experimentally investigated domain velocity and electron mobility based on well-understood transferred electron effect (TEE) for abovementioned nonlinear electrical characteristics of SI GaAs. The experimental findings are in good agreement with estimated results reported by other independent authors.

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.

A study of possibilities of development of GaAs detectors for high-power nanosecond X-ray pulses

It is proposed to use chromium-compensated semi-insulating GaAs detectors for detecting high-power nanosecond X-ray pulses. An X-ray facility based on a small direct-acting electron accelerator “Sinus-150” developed at the Institute of High-Current Electronics of the Siberian Branch of the Russian Academy of Sciences, Tomsk was used as a source of pulsed X-ray radiation. The detectors are shown to detect 5 ns pulses under exposure to 30 mR radiation during one pulse without distortions. In so doing, one-ampere currents flow through the active detector area 0.2 cm2. It is found that the physical processes limiting the working capacity of GaAs detectors are due to capture of nonequilibrium electrons and holes by deep centers. At the nonequilibrium charge-carrier concentrations higher than 5·1013 cm3, nonuniform distributions of the electric field are formed in the active region and these can result in distortion of the output detector signal shape.