Electron mobility-lifetime and resistivity mapping of GaAs:Cr wafers

Abstract

Previous works onchromium compensated gallium arsenide (GaAs:Cr) have shown high efficiency, good spatial and energy resolution, which is obviously connected with the high quality of material itself. The purpose of this research was to aggravate the diffusion process by increasing the annealing temperature and to observe whether there will be any degradation of material characteristics. The investigation of three 3-inch GaAs:Cr wafers with different annealing temperature of chromium was carried out. Resistivity and mobility-lifetime measurements were made using pad sensors made of these wafers. The I-V curves were built to estimate the resistivity across the wafer. Furthermore charge collection efficiency (CCE) measurements were carried out in order to estimate the µ e τ e product of GaAs:Cr. The resistivity mapping has showed a variation of resistivity across the wafer in the range from 1.25 × 10 9 to 5.5 × 10 8 Ohm cm. Although the third wafer showed quite good uniformity, the resistance didn't reached values higher than 3.5 × 10 8 Ohm cm. In spite of harsh diffusion conditions all the materials showed quite good CCE (about 90%) and µ e τ e more than 5 × 10 −5 cm 2 /V. Also a strong dependency between the resistivity and mobility-lifetime product was found only for one wafer. So the uniformity of µ e τ e product across the wafer can be stated independently of resistivity. More detailed information and discussion of experimental results is presented in the article.

Characterization of Chromium Compensated GaAs Sensors with the Charge-Integrating JUNGFRAU Readout Chip by Means of a Highly Collimated Pencil Beam

“…In order to measure the µ·τ product of either holes or electrons, fluorescence photons of molybdenum with an energy of 17.4 keV have been used, which deposit a well-defined amount of charge within the first few tens of microns of the sensor. The Hecht relationship modified by Hamann to take into account charge trapping and the small pixel effect (i.e., a position dependence of the signal induction, where most of the signal is induced close to the collecting electrode) has been used for fitting the data [ 2 , 15 , 19 , 20 ] (Equation (1)).…”

Characterization of chromium compensated GaAs as an X-ray sensor material for charge-integrating pixel array detectors

“…Measurements were done using a contactless measurement technique [21,22] at room temperature.Since the sensors are fabricated as photoresistors, the dark current is comparatively high and depends strongly on bias and temperature [23]. While this is not much of a problem for most photon counting detectors like the Medipix3 [18], it is an important parameter for integrating detectors, like the one used in this study.The mobility of electrons and holes and their lifetimes was measured at TSU to be 2500 cm 2 /Vs and 40 ns for electrons and 165 cm 2 /Vs and 1.1 ns for holes [22,23].Given the sensor thickness of 500 µm and a bias voltage of 200 V, we expect more than 88 % of electrons to reach the readout electrode before being trapped2, however the probability of holes reaching the electrode before being trapped is practically zero. Even though the small pixel effect [24] works in our favor if we collect electrons, we still expect a noticeable effect from the trapping and detrapping of holes.Discussing the theory behind these expected effects is beyond the scope of this paper.…”

“…The nominal average resistivity of the wafer used in this study was 0.5 GΩcm. Measurements were done using a contactless measurement technique [21,22] at room temperature.…”

“…The mobility of electrons and holes and their lifetimes was measured at TSU to be 2500 cm 2 /Vs and 40 ns for electrons and 165 cm 2 /Vs and 1.1 ns for holes [22,23].…”

Space charge formation in chromium compensated GaAs radiation detectors

“…Semiinsulating (SI) GaAs compensated by in-diffused chromium (GaAs:Cr) appears nowadays a promising X-ray detector material overcoming known drawbacks of Liquid Encapsulated Czochralski (LEC) SI-GaAs significantly debased by EL2 defect [1][2][3]. GaAs:Cr exhibits extended electron lifetime and long-term stability at applied bias, conserving simultaneously profitable properties of GaAsbased room-temperature radiation detectors, especially the relatively high average atomic number (Z=32), moderate energy band-gap (Eg=1.42 eV) and high electron drift mobility (μde=2500 cm 2 /Vs) [4][5]. One of the key problems for application of SI GaAs in high-performed radiation detectors is still space charge formation resulting in the internal electric field distortion.…”