V. A. Novikov

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.

Effect of Pt, Pd, Au additives on the surface and in the bulk of tin dioxide thin films on the electrical and gas-sensitive properties

The microstructure and properties of thin (∼100 nm) SnO2 films with noble metals Pt, Pd, Au additives, grown by dc magnetron deposition are studied. It is shown that the introduction of additives into the bulk and the deposition of dispersed catalysts on the semiconductor surface make it possible to control the sensor parameters in pure air and upon exposure to reduction (CO, H2, CH4) and oxidation (NO2) gases. Possible mechanisms for the effect of Pt, Pd, Au on the bulk and surface properties of tin dioxide are discussed. The technological conditions for film growth, which provide the selective detection of low concentrations (10–100 ppm) of CO and H2, below-explosive concentrations (0.5–2.5 vol %) of methane, and trace concentrations (0.05–5 ppm) of NO2 are determined.

Effect of gold on the properties of nitrogen dioxide sensors based on thin WO3 films

The microstructure and properties of gold-doped WO3 (WO3:Au) thin films before and after deposition of dispersed Au layers have been studied. It is shown that the γ-WO2.72 phase arises in WO3:Au layers, which leads to a significant increase in the film conductivity. Deposition of a dispersed gold layer results in an increase in the sensor response to NO2 by several times. The concentration dependences and the dynamics of sensor responses to nitrogen dioxide are described by the analytical expressions derived under the assumption that WO3 films contain grains connected by conducting bridges. An analysis of the experimental data using these expressions made it possible to determine the activation energies of NO2 adsorption and desorption and the adsorption heat.

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.

Fractal geometry of the surface potential in electrochemically deposited platinum and palladium films

The surface potentials are distributed very nonuniformly in electrochemically deposited thin films of palladium and platinum, and it reflects the grain structure of their surface profile. The Hausdorf-Bezikovich space dimensions of the surface potentials in these films much exceed the topological dimension of their projections, which is indicative of their fractal geometry. It is found that the surfaces of platinum and palladium films differ both in the spread of magnitudes of the surface-potential nonuniformities and in the shape and character of their distribution. This substantially affects the way of formation (geometry) of their potential profile and the values of their fractal dimensions. In addition, the fractal geometry of the potential profile of these film surfaces leads to the fact that the total electric charge of their surfaces varies much slower-it is proportional to the change in their linear dimensions to power (4 − Df), where 2 < Df < 3, instead of being proportional to the square of the change in the linear dimensions of the areas under investigation as in the two-dimension case. As a result, it is shown that, for the accurate designing of devices with metal components of submicrometer and nanometer sizes on the basis of thin films of platinum and palladium, it is necessary to take into account their fractal geometry.

Study of the Properties of the Surface of Gallium Arsenide by Scanning Atomic Force Microscopy

Using the method of atomic force microscopy, complex studies of the profile, potential distribution φ(x,y), and distributions of the phase contrast of the surface of n-GaAs subjected to various types of chemical treatment are carried out. The distribution of the potential and phase contrast at a microlevel, in general, correlates with the profile character. The surface treated in the solution H2SO4: H2O = 1: 10 is characterized by a high degree of nonuniformity with an average roughness of the main profile Δh≈ 10 nm. A considerable part of the surface is covered by hills 20–60 nm in height and 100–500 nm in diameter forming a specific substructure, which correspond to potential jumps as large as 50–60 mV against a general background of 0.77–0.80 V. At a nanolevel, correlation between the profile and phase contrast is clearly pronounced, but no correlation is found between the profile and potential distribution. Treatment of the surface of n-GaAs in a concentrated aqueous NH4OH solution leads to a decrease in the value of φ(x,y) by ∼0.2 V, and in its roughness by more than an order of magnitude (∼0.75 nm). The distribution of the profile and phase contrast over the surface is close to the ideal Gaussian distribution for relatively small areas of the surface (200 × 200 nm2). As the area increases, deviation from the Gaussian distribution becomes substantial because of smooth variation in the potential over the contact area. Conservation of the Gaussian character of the surface profile and a simultaneous rise in the average level of the roughness with an increase in the analyzed area indicates the fractal mechanism of formation of the surface profile.

Determination of the fractal dimension for the epitaxial n-GaAs surface in the local limit

Atomic-force microscopy studies of epitaxial n-GaAs surfaces prepared to deposit barrier contacts showed that major relief for such surfaces is characterized by a roughness within 3–15 nm, although “surges” up to 30–70 nm are observed. Using three independent methods for determining the spatial dimension of the surface, based on the fractal analysis for the surface (triangulation method), its section contours in the horizontal plane, and the vertical section (surface profile), it was shown that the active surface for epitaxial n-GaAs obeys all main features of behavior for fractal Brownian surfaces and, in the local approximation, can be characterized by the fractal dimension Df slightly differing for various measuring scales. The most accurate triangulation method showed that the fractal dimensions for the studied surface of epitaxial n-GaAs for measurement scales from 0.692 to 0.0186 μm are in the range Df = 2.490−2.664. The real surface area Sreal for n-GaAs epitaxial layers was estimated using a graphical method in the approximation δ → 0 δ is the measurement scale parameter). It was shown that the real surface area for epitaxial n-GaAs can significantly (ten times and more) exceed the area of the visible contact window.