Norwegian Journal of development of the International Science No 44/2020
21
PHYSICAL SCIENCES MODEL OF A STREAMER DISCHARGE CHANNEL IN MONOCRYSTALLINE CDS
Kulikov V. Doctor of Physical and Mathematical Sciences. Professor Tomsk Agricultural Institute – a Branch of the Novosibirsk State Agrarian University,Tomsk, Russia Yakovlev V. Doctor of Physical and Mathematical Sciences. Professor National Research Tomsk Polytechnic University, Tomsk, Russia Bobkova L. Candidate of Chemical Sciences. Associate Professor National Research Tomsk State University, Tomsk, Russia Abstract Partial breakdown of a crystalline cadmium sulfide sample in a pulsed inhomogeneous electric field was considered. Streamer discharge channels are oriented in the equivalent planes of sulfur ions (11 2 0), (1 2 10), ( 2 110) and along the c axis of the crystal. It has been shown that the crystallographic orientation, radial stability and high velocity of streamer discharge channels are satisfactorily described in terms of the mechanism of cascade Auger transitions, taking into account the crystallochemical symmetry of the cadmium sulfide lattice. Keywords: streamer discharge in semiconductors, electrical breakdown of crystals Introduction Partial streamer breakdown is observed in single crystals of semiconductor compounds of the type A2B6, when they are excited by high-voltage (~50-150 kV) nanosecond pulses [1-8]. Brightly glowing tracks propagate in certain crystallographic directions without destroying the substance. The velocity increases super linearly with the growth of voltage applied and ranges from ~108 to 109 cm/s. The length of channels is ~5-10 mm at a diameter of ~2-3 m. The radiative transition mechanism is related to the recombination of electron-hole pairs in the discharge channel. High concentration of nonequilibrium charge carriers inside the channel creates the conditions for population inversion and stimulated emission. A reversible electrical breakdown can be applied in the development of streamer pulsed lasers [1, 5-8]. Advances in the physics and technology of semiconductor lasers connected with the understanding of streamer discharge formation, including streamer propagation anisotropy, high concentrations of carriers and high channel velocity. In Refs. [5, 6], streamer discharge is considered as a conducting channel with a region of strong electric field in the front section. The field front movement is determined by the tunnel-impact mechanism, where tunneling generates primary charge carriers with their subsequent impact multiplication [5, 9]. In the adopted model of substance atom ionization by accelerated electrons, the avalanche multiplication of charge carriers makes it possible to provide a significant concentration (~ 1018 cm-3) of nonequilibrium carriers [5], though it does not reflect the crystallographic orientation of breakdown channels. A high velocity of streamers (up to 3∙109 cm/s [5, 6]) does not go well with the drift velocity of avalanches. The latter does not exceed ~107 cm/s in CdS, using the data from Ref. [5].
In general, the properties of streamer discharges in wide-bandgap semiconductors and breakdown channels in ion compound crystals [10] are similar. The electrical breakdown of alkali-halide crystals, quartz has the following consistent patterns: crystallographic orientation and anodic nature, high channel front velocity (~107-108 cm/s) and breakdown current density (~104 A/cm2). They are satisfactorily described in terms of the mechanism based on cascade Auger transitions in valence band of the dielectric [1116]. This paper presents a description of crystallographic orientations of partial electrical breakdown channel propagation in a single crystal of cadmium sulfide. It also discusses the formation of a streamer discharge in terms of the mechanism of free electron generation by means of interatomic Auger transitions. Experimental results and discussion In the study, we used single crystals of hexagonal CdS, grown by vapor deposition, using the DavydovMarkov method (Platan, Fryazino town) with electrical resistivity of ~109 Om∙cm. To investigate the crystallographic orientation of discharges, a wafer 6 mm in thickness, positioned in the (0001) plane, was cut out from the single-crystal ingot and ground. After cleaving the crystal sides, the dimensions of the sample were ~6×13×23 mm3. The method of generating a strong electric field (~106 V/cm) using an electron beam of the accelerator is described in Ref. [11]. The accelerator had the following parameters: average electron energy ~250 keV, duration of beam current pulse (at half-height) ~18 ns, current density ~100 A/cm2. The sample was positioned between two electrodes, the upper being a point electrode, the lower – a round aluminum plate ~1 mm in thickness and ~9 mm in diameter (Fig. 1). An electron beam was applied to the lower electrode. In this case, under irradiation, a
