A.P. Vorobiev

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.

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.

GaAs X-ray coordinate detectors

Abstract GaAs coordinate detectors for X-ray systems of scanning type have been developed. Two modifications of the detectors have been constructed for ionizing radiation detection in parallel and perpendicular to the electric field direction in the detector bulk. The detectors have 64 active regions with 100xa0μm pitch. In order to reach the minimum value of coupling capacitance and resistance coupling between active regions, the latter is separated by narrow gaps on unit cells. The gaps are made by means of reactive ion etching. The detectors are fabricated on the base of vapor-phase epitaxial material 40–45xa0μm in thickness with free carriers concentration of p 0 =10 12 xa0cm −3 . Input capacitance of a detector cell is 0.25xa0pF, dark current is 30–50xa0pA per cell. The detectors have almost 100% charge collection efficiency for all types of ionizing radiation. High resolution of the detector allows to detect structures of a spectrum from the 241 Am source in the energy range of 14–17xa0keV.

Epitaxial structures based on compensated GaAs for γ- and X-ray detectors

Abstract The growth and characteristics of the detector p–i–n-structures fabricated by means of the epitaxial methods are discussed. High-resistivity i-layers containing chromium as a compensated impurity have been grown on the n-GaAs substrates by the liquid-phase epitaxy method. The layer thicknesses are (150–250)xa0μm, their resistivities lie in the interval ρ =(5×10 6 –2.5×10 8 )xa0ohmxa0cm. The thin Zn-doped p-layer has been grown upon the i-layer by the vapour-phase epitaxy method. The electric field profiles in the p–i–n-diodes have been measured. The reverse current-voltage characteristics of the diodes have been analysed. The sensitivity of the structures to β- and γ- radiations has been investigated.

Modeling of processes of charge division and collection in GaAs detectors taking into account trapping effects

Abstract A mathematical modeling of processes of charge division and collection in GaAs detectors has been carried out. The charge carriers behavior has been described by the system of continuity equations and Poissons equation. We studied a case when semi-insulating (SI) GaAs contains deep centers of only one type with the abrupt cross-section dependence on field. A concentration of empty traps was taken at a level of 10 15 –10 16 xa0cm −3 . All the parameters of deep centers correspond to the EL2 center which is dominant in SI GaAs. Interaction with γ-rays and electrons were simulated by electron–hole packets of corresponding dimensions. It was shown that dynamics of charge division in the case of trapping and without trapping is determined by three factors: (1) the electric field strength value in the region of electron–hole pairs creation; (2) a concentration of non-equilibrium charge carriers in that region (i.e. the ionizing radiation type); (3) the region dimensions.

GaAs as a material for particle detectors

Among the possible semiconductor materials for ionizing radiation detectors, GaAs looks very promising due to its high carrier mobility, wide band gap and high density. A comparative analysis of physical and electrical characteristics of GaAs semi-insulating layers (SI-GaAs) fabricated by means of different technological methods is given in this work. The main parameters of detector structures and multi-element detectors on the base of SI-GaAs are presented.

Charge collection in X-ray pixel detectors based on semi-insulating GaAs doped with Cr

Abstract An analysis of the performance of X-ray pixel detectors based on semi-insulating GaAs doped with Cr has been carried out. An analytic form for electron and hole currents has been obtained. The detector based on our material has high charge collection efficiency only when positive potential is applied to pixels, in contrast to semi-insulating materials (LEC SI-GaAs). A mean charge of a pixel contact is calculated for photon energy of 30 and 60xa0keV. The maximum mean charge is 3000 e for the detector thickness of 300xa0μm and photon energy of 30xa0keV and 2300 e for the detector thickness of 600–800xa0μm and photon energy of 60xa0keV.

Charge collection in X-ray pixel detectors based on SI-GaAs doped with Cr

Abstract An analysis of the performance of X-ray pixel detectors based on SI-GaAs doped with Cr has been carried out. An analytic form for electron and hole currents of a pixel has been obtained. The detector based on our material has high charge-collection efficiency only when the pixel contacts are anodes in contrast to LEC SI-GaAs. The mean charge collected on pixel contact is calculated for radiation energies of 30 and 60xa0keV (charge is averaged over all monoenergetic photons irradiating the detector cell).

Modeling of processes of charge separation in a GaAs detector

Abstract The problem of charge separation in a GaAs detector has been solved by means of mathematical modeling. Dynamics of the process for a three-dimensional detector is described in this work in detail. It is shown that in case of interaction with alpha particles, the plasma time can reach several nanoseconds in a strong electric field. In a weak electric field the plasma time exceeds the charge carrier lifetime. The time of separation of charge carriers created by a gamma photon in a strong electric field is 100xa0ps. The results of calculation are confirmed by experimental data.