A: Chromium compensated GaAs sensors have been characterized using the chargeintegrating readout chip JUNGFRAU. Due to its low noise performance and 75 × 75 µm 2 pixel size, JUNGFRAU enables a precise measurement of the charge (of either polarity) with a high spatial resolution.Several sensor parameters like dark current, noise and spectral performance as well as the charge transport properties of the electrons have been determined. The short lifetime of holes in GaAs:Cr gives rise to an effect where pixels adjacent to a pixel with a photon hit show a strong negative signal when being absorbed close to the readout electrode. This so-called 'crater effect' has been simulated and allows an estimation of the hole lifetime in GaAs:Cr.

Chromium compensated GaAs or GaAs:Cr sensors provided by the Tomsk State University (Russia) were characterized using the low noise, charge integrating readout chip JUNGFRAU with a pixel pitch of 75 × 75 µm2 regarding its application as an X-ray detector at synchrotrons sources or FELs. Sensor properties such as dark current, resistivity, noise performance, spectral resolution capability and charge transport properties were measured and compared with results from a previous batch of GaAs:Cr sensors which were produced from wafers obtained from a different supplier. The properties of the sample from the later batch of sensors from 2017 show a resistivity of 1.69 × 109 Ω/cm, which is 47% higher compared to the previous batch from 2016. Moreover, its noise performance is 14% lower with a value of (101.65 ± 0.04) e- ENC and the resolution of a monochromatic 60 keV photo peak is significantly improved by 38% to a FWHM of 4.3%. Likely, this is due to improvements in charge collection, lower noise, and more homogeneous effective pixel size. In a previous work, a hole lifetime of 1.4 ns for GaAs:Cr sensors was determined for the sensors of the 2016 sensor batch, explaining the so-called “crater effect” which describes the occurrence of negative signals in the pixels around a pixel with a photon hit due to the missing hole contribution to the overall signal causing an incomplete signal induction. In this publication, the “crater effect” is further elaborated by measuring GaAs:Cr sensors using the sensors from 2017. The hole lifetime of these sensors was 2.5 ns. A focused photon beam was used to illuminate well defined positions along the pixels in order to corroborate the findings from the previous work and to further characterize the consequences of the “crater effect” on the detector operation.

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.

The investigation results of GaAs:Cr X-ray sensor noise characteristics are presented. Measured samples were 3*3 mm 2 and thickness in the range of 300-500 µm. It is shown that the proposed method can be used to reveal the nature of dominant noise and calculate the energy resolution of structures. This technique allows estimation of the optimal operating voltage of GaAs:Cr sensors and characterization of detector material.

A: Currently, semiconductors with high atomic number Z arouse strong interest in construction of X-ray sensors. One of the most prospective materials are presented by elements from the group A III B V . Gallium arsenide compensated with chromium (HR-GaAs:Cr) is one of these materials and exhibits unique characteristics. The sensors based on HR-GaAs:Cr demonstrate high absorption efficiency. The response of HR-GaAs:Cr sensors to subnanosecond X-and β-ray pulses of 28 ÷ 52 keV from an accelerator of runaway electrons are described in this research. The samples have symmetric metal-semiconductor-metal structure. The active area of the samples was 0.09 cm 2 and the thickness of sensitive layer was 145 ÷ 500 µm. Experimental characteristics of pulses were compared with theoretical estimations. An optimal thickness of sensitive layer of HR-GaAs:Cr sensors was determined, which allowed us to obtain the minimal possible value of output pulse duration ≈ 1 ns.