The results of study detector structures based on epitaxial layers of GaAs, compensated by Cr, are presented. It is shown that distribution of electric field straight in active region of the structures affected on it amplitude spectrums of γ-quantum.

The method of scanning Kelvin probe force microscopy has been used to study the electric field distribution in GaAs-based p +-π-n-n + detector structures. In the active layer volume, two maxima in the field strength profiles have been found, which are localized in the regions of p +-π and π-n junctions. A volt-age drop on the π-n junction expands the region of collection of nonequilibrium holes, thus increasing the charge collection efficiency for the absorption of γ photons with an energy of 59.5 keV.

The electric-field strength profiles are calculated for the structures used in X-ray detectors produced by diffusion of the deep acceptor Cr impurity into an n-GaAs epitaxial layer. The effects of diffusion temperature, external bias, and the ratio of electron and hole emission coefficients on the distribution of electric-field strength in the structure are examined.

The growth of GaAs epitaxial structures for X- and gamma-ray detectors and the device characteristics have been investigated. Conditions of reproducible LPE growth of GaAs layers more than 100 mum thick on substrates of 40 mum diameter have been established. Complex doping with tin and deep Cr-acceptor have been studied for the liquid-phase epitaxial growth in a wide temperature range. A method has been developed for GaAs:Sn,Cr layer growth with the resistivity in the range (10(7)-10(9))Omega cm and with thickness up to 550 mum.Detector p-i-n structures have been fabricated on the base of high-resistivity GaAs layers. The electric field distribution and current flow mechanisms in the diodes have been studied. Diodes have been fabricated with a dark current density of 1 x 10(-7) A/cm(2) at a reverse bias voltage of 100 V.Alpha particle and gamma-ray spectra of the detectors have been measured. An X-ray image has been obtained with a strip detector based on LPE layers. (C) 2004 Elsevier B.V. All rights reserved.

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 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)μm, their resistivities lie in the interval ρ=(5×106–2.5×108)ohmcm. 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.

The effect of an inverted p -region along the free surface of n -Al x Ga 1− x Sb on the reverse current of p−n structures from the given solid solution is analyzed. Expressions which describe “collection” of the inverted layer current on the cylindrical surface of an n -region are discussed. The contribution of the near-surface and bulk components to the reverse current of p−n structures with a semi-infinite n -region is estimated. For structures with a two-layer n -region of finite thickness we have calculated the dependence of the near-surface current on the voltage across the p−n structure, the thickness of the n -region, and its composition and doping level. We have compared the calculated current-voltage characteristics with experiment using a Al 0.15 Ga 0.85 Sb p−n structure as an example.

The effect of a p inversion layer along the free surface of a solid solution and the reverse current for metal/ n -Al x Ga 1− x Sb surface barrier structures are analyzed. An algorithm is proposed for numerical solution of the system of equations which describes the “collection” of the current along the unbounded planar surface of a structure with a metallic contact of radius r c . During the calculations of the properties for surface barrier structures it was assumed that the positions of the Fermi level at the boundary with the metal and at the free n - Al x Ga 1− x Sb surface are about the same. Calculations of the near-surface component of the reverse current I s for the structures with this assumption exceed the bulk component I b by more than an order of magnitude. The dependences of I s and I b and the effective radius of the collection region of the current on the composition of the solid solution are found. We compared the calculated current-voltage characteristics with the experimental ones for Pd/Al 0.1 Ga 0.9 Sb structures.

The surface properties of frequency oscillations of n-AlxGa1−xSb solid solution are discussed. The dependence of the barrier height for electrons at the metal/n-AlxGa1−xSb boundary on the composition of the solid solution is considered. In studying the characteristics of the surface-barrier structure, it is shown that the position of the Fermi level at the boundary with the metal and at the n-AlxGa1−xSb free surface is similar. Pinning of the Fermi level close to the top of the valence band in the solid solution with 0≤x≤0.3 leads to the appearance of a p-type inverse layer at the surface. Experimental proof of the existence of this layer is provided. The hole concentration at the n-AlxGa1−xSb surface is estimated, along with the thickness and surface conductivity of the inverse layer.

The manganese doping of In1−xGaxAsyP1-y(0 ≦ y ≦ 1) films grown by liquid-phase epitaxy has been investigated. A study has been made of the influence of the solid-solution composition and of the method of introducing the manganese into the solution melt on the electrical properties of the films, and also on the surface morphology, the optical absorption edge, and the mismatch of the film and substrate lattice constants. It is shown that the hole density in the films decreases with decreasing y. The reasons for this are the decrease in the distribution coefficient of the acceptor manganese (from 0.3 to 0.001), the increase in the degree of compensation of the acceptors with donors (from 0.2 to 0.9), and the increase in the acceptor ionization energy. The composition of the InGaAsP:Mn film and the morphology of its surface depend on the method of preparation of the melt, this being explained by the strong interaction of the manganese atoms with the other components of the liquid phase.

Thermodynamic computations of the critical supercooling of a melt are performed for the case of heteroepitaxy of a solid AlxGa1−xSb solution on a GaSb substrate for which there should be no substrate etching. Three kinds of supercoolings are examined, where Δ cr (1) is the supercooling for which the change in the system Gibbs energy should equal zero because of dissolution, Δ cr (1) is the supercooling for which the diminution in the system Gibbs energy due to substrate dissolution equals the energy being liberated during crystallization of the AlxGa1−xSb solid solutions layer. Finally, the influence of the specific free interphasal energy of the substrate-melt interface on the result of computing the critical supercooling (the supercooling Δ cr (1) is considered.