A. V. Tyazhev

GaAs radiation imaging detectors with an active layer thickness up to 1 mm

Unlike conventional GaAs detector structures that use a space charge region (SCR) of a barrier structure, we propose to form a detector structure of resistor type made of GaAs compensated with Cr. In this case, the electric field distribution, ξ(x), is not screened by the ion concentration in the SCR but it is defined only by the uniformity of the distribution of the resistance value in the structure. The experimental results on measurements of the electrophysical characteristics and the electric field distribution are presented. It is shown that in these structures the electric field distribution is uniform through the whole high-resistive layer with a thickness up to 1 mm. The possibility of achieving high values of charge collection efficiency of gamma-radiation is demonstrated.

Performance of a Medipix3RX Spectroscopic Pixel Detector With a High Resistivity Gallium Arsenide Sensor

High resistivity gallium arsenide is considered a suitable sensor material for spectroscopic X-ray imaging detectors. These sensors typically have thicknesses between a few hundred μm and 1 mm to ensure a high photon detection efficiency. However, for small pixel sizes down to several tens of μm, an effect called charge sharing reduces a detectors spectroscopic performance. The recently developed Medipix3RX readout chip overcomes this limitation by implementing a charge summing circuit, which allows the reconstruction of the full energy information of a photon interaction in a single pixel. In this work, we present the characterization of the first Medipix3RX detector assembly with a 500 μm thick high resistivity, chromium compensated gallium arsenide sensor. We analyze its properties and demonstrate the functionality of the charge summing mode by means of energy response functions recorded at a synchrotron. Furthermore, the imaging properties of the detector, in terms of its modulation transfer functions and signal-to-noise ratios, are investigated. After more than one decade of attempts to establish gallium arsenide as a sensor material for photon counting detectors, our results represent a breakthrough in obtaining detector-grade material. The sensor we introduce is therefore suitable for high resolution X-ray imaging applications.

GaAs resistor structures for X-ray imaging detectors

Abstract Unlike conventional GaAs detector structures, which operation is based on the use of a space charge region of a barrier structure, we propose to form a detector structure of resistor type. In this case, the electric field distribution, ξ ( x ), is not screened by the ion concentration in the SCR but it is defined only by the uniformity of the resistance value distribution in the structure. The experimental results on charge collection efficiency for the detector irradiation with α, β, γ-radiation are presented. It is shown that the amplitude spectrum shape in the case of interaction with γ-radiation is defined mainly by the electron component of the charge. The simulation of the detector response function confirms it. It is established that, despite of hole trapping, it is possible to achieve high values of charge collection efficiency of γ-radiation. Explanation of the charge collection efficiency dependence on a type of ionizing radiation is made. Problems of design of the detector with high charge collection efficiency and low dark current are discussed.

GaAs structures for X-ray imaging detectors

Abstract A comparative analysis of characteristics of detector structures fabricated by means of technology of epitaxial growth of an undoped high-resistive GaAs layer as well as structures based on SI-GaAs compensated with Cr during a diffusion process is presented in this work. Advantages and disadvantages of the proposed methods of formation of high-resistive layers, their electrophysical characteristics and properties are examined. Limit parameters of the detector structures which can be achieved by using a combination of technological methods are analyzed.

The LAMBDA photon-counting pixel detector and high-Z sensor development

Many X-ray experiments at third-generation synchrotrons benefit from using single-photon-counting detectors, due to their high signal-to-noise ratio and potential for high-speed measurements. LAMBDA (Large Area Medipix3-Based Detector Array) is a pixel detector system based on the Medipix3 readout chip. It combines the features of Medipix3, such as a small pixel size of 55 μm and flexible functionality, with a large tileable module design consisting of 12 chips (1536 × 512 pixels) and a high-speed readout system capable of running at 2000 frames per second. To enable high-speed experiments with hard X-rays, the LAMBDA system has been combined with different high-Z sensor materials. Room-temperature systems using GaAs and CdTe systems have been produced and tested with X-ray tubes and at synchrotron beamlines. Both detector materials show nonuniformities in their raw image response, but the pixel yield is high and the uniformity can be improved by flat-field correction, particularly in the case of GaAs. High-frame-rate experiments show that useful information can be gained on millisecond timescales in synchrotron experiments with these sensors.

Characterization of photon counting pixel detectors based on semi-insulating GaAs sensor material

Hybrid semiconductor pixel detectors are considered of high interest for synchrotron applications like diffraction and imaging experiments. However, at photon energies above 30 keV, high-Z sensor materials have to be used due to the weak absorption of the most commonly used sensor material, for instance silicon wafers with a thickness of a few hundred μm. Besides materials like CdTe and Ge, semi-insulating, chromium compensated SI-GaAs(Cr) proves to be a promising sensor material for applications with X-rays in the mid-energy range up to ~60 keV. In this work, material characterisation of SI-GaAs(Cr) wafers by electrical measurements and synchrotron white beam topography as well as the characterization and application of pixel detector assemblies based on Medipix readout chips bump-bonded to 500 μm thick SI-GaAs(Cr) sensors are presented. The results show a very homogeneous material with high resistivity and good electrical properties of the electrons as well as a very promising imaging performance of the detector assemblies.

Chromium-compensated GaAs detector material and sensors

Results obtained from numerical calculations of and experimental studies on the pulse height distribution inherent in ionizing radiation gallium arsenide sensors as a function of the design features of the devices and electrophysical characteristics of the detector material are presented. It is shown that the pulse height distribution is defined by the distribution pattern of the nonequilibrium charge carrier lifetime and by the electric field profile in the bulk of the sensor. Investigations on the detector sensitivity to X-ray energies in the range between 40 and 150 keV were performed. The sensor polarization was found to produce only a marginal effect compensated by an increase in the bias voltage. Prototype pixel sensors measuring 256 × 256 and 512 × 768 pixels with a 55 μm pitch and a 500 μm thick sensitive layer were produced. The dependence of the photocurrent and count rate on the X-ray radiation intensity and bias voltage applied to the sensor was examined. In the 40–80 keV energy range, the maximum count rate amounted to 800 kHz/pixel for a negative sensor bias voltage of 800 V. The sensors are demonstrated to provide spatial resolution varying with the pixel pitch and to enable high-quality X-ray images to be obtained.

Investigation of the radiation hardness of GaAs sensors in an electron beam

A compact and finely grained sandwich calorimeter is designed to instrument the very forward region of a detector at a future e+e- collider. The calorimeter will be exposed to low energy e+e - pairs originating from beamstrahlung, resulting in absorbed doses of about one MGy per year. GaAs pad sensors interleaved with tungsten absorber plates are considered as an option for this calorimeter. Several Cr-doped GaAs sensor prototypes were produced and irradiated with 8.5-10 MeV electrons up to a dose of 1.5 MGy. The sensor performance was measured as a function of the absorbed dose.

Ga 2 O 3 Films Formed by Electrochemical Oxidation

The effect of oxygen plasma on gallium oxide films formed by electrochemical oxidation of n-GaAs wafers with a donor concentration Nd = (1–2) × 1016 cm−3 has been investigated. It is shown that the treatment in an oxygen plasma at a temperature of 50–90°C increases the concentration of β-phase crystallites, which causes an increase in the permittivity, a decrease in the dielectric dissipation factor, and a change in the conductivity of GaAs-〈gallium oxide〉-metal structures.

Investigating the suitability of GaAs:Cr material for high flux X-ray imaging

Semi-insulating wafers of GaAs material with a thickness of 500μm have been compensated with chromium by Tomsk State University. Initial measurements have shown the material to have high resistivity (3 × 109Ωcm) and tests with pixel detectors on a 250 μm pitch produced uniform spectroscopic performance across an 80 × 80 pixel array. At present, there is a lack of detectors that are capable of operating at high X-ray fluxes (> 108 photons s-1 mm-2) in the energy range 5–50 keV. Under these conditions, the poor stopping power of silicon, as well as issues with radiation hardness, severely degrade the performance of traditional detectors. While high-Z materials such as CdTe and CdZnTe may have much greater stopping power, the formation of space charge within these detectors degrades detector performance. Initial measurements made with GaAs:Cr detectors suggest that many of its material properties make it suitable for these challenging conditions. In this paper the radiation hardness of the GaAs:Cr material has been measured on the B16 beam line at the Diamond Light Source synchrotron. Small pixel detectors were bonded to the STFC Hexitec ASIC and were irradiated with 3 × 108 photons s-1 mm-2 monochromatic 12 keV X-rays up to a maximum dose of 0.6 MGy. Measurements of the spectroscopic performance before and after irradiation have been used to assess the extent of the radiation damage.