X-ray phase analysis of the samples was performed using the diffractometer Philips X'Pert PRO – MRD. CuKαl radiation (λ = 0.15406 nm) was used in these experiments. The voltage of 45 kV was applied to the tube anode, the current was 40 mA. Diffractograms were taken both in the symmetrical and grazing geometries with various slopes of the sample surface relatively to the direction of X-ray beam. It provided lowering the strong peak from the germanium substrate in orientation (111). The qualitative phase analysis of diffractograms was carried out using the database ICDD.
Studying the electrical resistance of surface layers was performed using the method of elastic contacts [10], which has the following important advantages as compared to the four-probe method:
- large area of the contact zone, which provides a low current density and, respectively, absence of Joule heating;
- low pressure onto the semiconductor surface, which enables to minimize distraction of this surface;
- applying the contacts with the maximum possible area covering the whole sample enables to avoid the influence of geometrical errors.
The theory of electrical contacts is represented in detail in [11]. The elastic contacts are of special interest. Investigations of them started from the work [12]. Composite materials, including those from nanomaterials, are widely researched for applying in electronic facilities as elastic contacting elements [13 – 16].
Application of the elastic contacts based on rubber having the anisotropic electrical conductivity is now widely used in liquid-crystal indicators (Liquid Cristal Displays – LCD). We used the contacts manufactured by the firm Fujitsu (Japan).
One of the promising fields of using this method is pulsed voltammetry [17] with its distinctive feature – duration of measuring pulses reaches here 100 ms or higher, while the interval between pulse trains exceeds 1 s. The pulsed methods for measuring the resistance of thin films as well as semiconductor surface layers find their wide application in recent years [18, 19].
Shortening the time for measurements by using short measuring pulses (single or serial with a high off-duty ratio) enables to minimize heating the thin films. The pulse methods allow changing both the amplitude and temporal parameters of pulses: their duration, repetition frequency and off-duty ratio. This enables to optimize the measuring methods for every type of surfaces – various kinds of metals and semiconductors, as well as composites and nanomaterials.
The structural scheme of stand for pulsed measurements of the surface resistance is shown in Fig. 1.
The measurements were performed in the following regime: pulses of the amplitude 4 to 10 V, frequency 160 kHz and off-duty ratio 3 to 8 from the generator enter the voltage divider that consists of the resistor 1…140 kOhm and the studied sample with applied elastic contacts. The signals from generator and the sample (midpoint of the voltage divider) enter the digital oscillograph (channels 1 and 2). The obtained values are used to calculate the surface resistance of the studied sample.
Before measurements, it was experimentally found that the zone of reliable contact begins from the pressure 20 kPa. After 3-fold increasing it, the resistance value decreases only by one third. Therefore, when measuring the resistance of fragile materials, it is sufficient to use the pressure 20…30 kPa that is two orders less than that used in the four-probe method.
The measurements of electrical resistance we carried out using the pulsed method on the 2101th reading at the scale of digital oscillograph. The instant voltage value at the generator output was kept the same. The pulses were taken from the generator based on the timer NE555, the amplitude of pulses was 4.84±0.04 V. The pressure applied to elastic contacts was 20 kPa.
Topology of the sample surface was studied using the atomic-force microscope.