CuIn(Sb)Se₂ chalcopyrites are widely used in various photovoltaic devices, which is associated with a high absorption coefficient of radiation in a wide range of wavelengths (ultraviolet, visible and near infrared spectral ranges) [1,2]. But for the use of these materials in real devices it is important to obtain thin films of a corresponding stoichiometric composition of a large area with minimal technology costs [3]. Such small area films are obtained by sputtering a massive chalcopyrite target with pulsed lasers [4], but this is a very expensive technology due to the high cost of laser and vacuum technology. Therefore, other cheaper physical methods for the synthesis of thin chalcopyrite films can be promising, for example, when sputtering massive electrodes in a strong electric field of a high-current nanosecond discharge in air or other gases due to the formation of ectons [5] or sputtering electrodes with plasma ions. The results of the use of such an approach to the sputtering of transition metals in the form of a study of the characteristics of the discharge and the properties of the synthesized nanostructures of copper, zinc and iron oxides are given in [6-9] and summarized in the monograph [10]. The results of the study of the spatial, electrical, and optical characteristics of an overstressed nanosecond discharge in atmospheric air between chalcopyrite electrodes, as well as estimates of the density and temperature of electrons, showed that chalcopyrite films with an identical stoichiometric composition of the electrode material (CuInSe₂) could be obtained in this way [11-15].

Nanosecond and sub nanosecond discharges in which it is possible to obtain beams of "runaway electrons" and the accompanying X-ray radiation can be fairly uniform and suitable for deposition of thin films on different substrates. But their characteristics and plasma parameters are most studied when using only metal electrodes with a distance between them of 1-15cm [16-18]. Less studied are nanosecond discharges between electrodes from chalcopyrite-type semiconductor clusters under conditions of strong overstress of the discharge gap when the distance between the electrodes is d = 1-3 mm in electrode systems with moderate non-uniformity of the electric field strength when using an oxygen-free gaseous medium (He, Ne, Ar, N₂) in the pressure range 5-300 kPa. This paper presents the results of a spectroscopic study of the decomposition of a CuInSe₂ molecule in a strong electric field of a nanosecond discharge with injection of the chalcopyrite-based electrode vapor into nitrogen due to sputtering of the electrodes.

Chalcopyrite electrodes (CuInSe₂) were installed in a sealed Plexiglas discharge chamber (Figure 1). The distance between the electrodes was 1 mm. The discharge chamber was pumped out to a residual air pressure of 5-10 Pa and then filled with nitrogen to pressures in the range of 5.3-202 kPa. The diameter of the cylindrical semiconductor electrodes was 5 mm. The radius of rounding of the working end part of the electrodes was 3 mm. To ignite the discharge, bipolar high-voltage nanosecond pulses with amplitudes of positive and negative polarity in the range of 10-40 kV were applied to the electrodes. The pulse repetition rate of the modulator was in the range of 40-1000 Hz. The main part of the experiments was carried out at a frequency f = 100 Hz. The voltage pulses at the discharge gap and the discharge current were measured using a wide-band capacitive divider, a Rogowski coil, and a 6-LOR-04 wide-band oscilloscope. 

Figure 1: Scheme of a discharge device for ignition of discharge and synthesis of thin films of chalcopyrite on the glass surface: 1 - gas-discharge chamber of plexiglass, 2 - upper flange, 3 - hermetic metal inputs, 4 - connection for connection to a vacuum gas mixing system, 5 - electrodes of chalcopyrite, 6 - glass substrate with a film on the basis of dispersed chalcopyrite electrodes, 7 - system of fastening of a plate.

The time resolution of this registration system was 2-3 ns. To record the plasma emission spectra, an MDR-2 monochromator and an FEU-106 photomultiplier were used. The radiation of the discharge plasma was analyzed in the spectral range 200-650 nm. In more detail, the system for obtaining and studying the characteristics of the discharge is given in [10,15]. At the time of the existence of an overstressed nanosecond discharge in a pulsed-periodic regime (f = 40-200 Hz), for 2-3 hours on an insulating substrate mounted at a distance of 3 cm from the center of the discharge gap, the deposition of a gray-color film from the sputtering products of the electrodes material was recorded on the basis of the CuInSe₂ compound. The obtained samples of chalcopyrite based films were investigated for transmission in a wide range of radiation wavelengths using deuterium and heat lamps (200-850 nm). The study of the transmission spectra, deposited on quartz substrates, of chalcopyrite films was carried out using a spectral complex based on the MDR-23 monochromator at room temperature in accordance with the method described in [19].

In the electrode system “sphere-sphere” based on CuInSe₂ compound, at nitrogen pressures in the range of 5-200 kPa, the distance between the electrodes commensurate with the radius of curvature of the chalcopyrite electrodes, a high-current nanosecond discharge was sparked in the form of a sphere, which had a diffuse character, although it was initiated without using a separate preionization system. This is possible if the runaway electron beam, X-ray or ultraviolet radiation play the role of the pre-ionization system [17,18]. With a low nitrogen pressure (5.3 kPa), the generation threshold of runaway electrons in an overstressed nanosecond discharge plasma [20] is reached, and at atmospheric nitrogen pressures, preionization can occur from ultraviolet and X-ray radiation. The volume of the discharge is highly dependent on the repetition rate of the voltage pulses. The mode of operation of the plasma emitter in the form of a "point" lamp was achieved only at low repetition rates of the exciting pulses (f = 40-100 Hz). With increasing frequency from 40 to 1000 Hz, the volume of the emitting plasma increased from about 1 to 100 mm³. A typical oscillogram of the voltage across the discharge gap and the electrical pulsed power of an overstressed nanosecond discharge at low and atmospheric pressure of nitrogen in the discharge chamber are shown in (Figures 2 and 3).

Figure 2: Oscillogram of voltage between CuInSe₂ electrodes with nitrogen pressure 5.3 kPa (f = 100 Hz).

Figure 3: The pulsed power of an overstressed nanosecond discharge at nitrogen pressures of 5.3 (1) and 202 (2) kPa (1).

 In the experiment, voltage oscillations were observed across the gap with a half-period of ~ 10 ns, which are caused by the mismatch of the output impedance of the high-voltage pulse 

generator with the load. With a nitrogen pressure of 5.3 kPa, the maximum amplitude of voltage spikes reached 40-55 kV, and with increasing nitrogen pressure up to 101 kPa, the amplitude of voltage pulses of positive and negative polarity decreased to 40 kV. The current pulses had the form of current oscillations damping with time with the current spike amplitude of 130 -150 A. The main part of the electric power was introduced in the discharge in the first 100-200 ns. The maximum value of the pulsed electrical power introduced in the plasma reached ~ 7-9.3 MW. The maximum energy input was achieved at a nitrogen pressure of 202 kPa. In the time interval t = 200-400 ns, the discharge was maintained by pulsed power oscillations at a level of ~ 0.5-1.0 MW. The energy in the pulse that was introduced into the discharge was in the range of 350-375 mJ.

Such a behavior of the energy input, which is realized in the mode of mismatch of the output impedance of the generator of high voltage nanosecond voltage pulses with the resistance of the discharge plasma, contributes to the effective sputtering of the electrode material and its deposition on the glass substrate [21,22], as with low nitrogen pressure and high values ??of E/N (where: E is the electric field strength in the discharge gap, N is the density of the vapor-gas medium), when the ecton mechanism of electrodes’ sputtering can be effectively implemented [5], and at atmospheric pressures of nitrogen, when it is possible to turn on the sputtering processes of the electrodes under the action of nitrogen ions [10,15].

Optical characteristics of plasma based on CuInSe2 compound

(Figures 4 and 5) present the plasma emission spectra of an overstressed nanosecond discharge between the electrodes from the CuInSe₂ compound at pressures of nitrogen of 5.3 and 101 kPa. (Tables 1) present the results of the interpretation of the most intense spectral lines of the bands in the plasma emission spectra shown in (Figures 4 and 5).

Figure 4: The plasma emission spectrum of an overstressed nanosecond discharge at a nitrogen pressure of 5.3 kPa.

Figure 5: The emission spectrum of an overstressed nanosecond discharge at p (N₂) = 101 kPa.

With a low nitrogen pressure in the discharge chamber (p = 5.3 kPa), the emission spectrum of the discharge plasma in the wavelength range of 200-230 nm consisted of a group of closely spaced spectral lines of an atom and a single-charged copper ion, as well as low-intensity spectral lines of a single-charged indium ion ( 1-13; Table). The spectral lines of copper were approximately the same as for a plasma of an overstressed nanosecond discharge between copper electrodes at atmospheric pressure at an interelectrode distance of d = 2 mm [6,7]. When sputtering electrodes from a CuInSe₂ compound in atmospheric pressure air (d = 2 mm) in the same type of discharge, this group of spectral lines has not been previously studied [11,12,15].

The most intense group of spectral lines and bands was recorded in the spectral range of 290-410 nm (lines and bands 14-35; Table). Here the spectral lines of the copper atom (lines 16, 19-24, 26, 33) and the intense bands of the second positive system of the nitrogen molecule (bands 14, 18, 25, 27-32, 34, 35) were most characteristic. The presence of intense bands of the nitrogen molecule of the C³Π_u⁺-B³Π_g⁺ system and the spectral line of 427.99 nm Cu II indicates that, in addition to runaway electrons, the main body of the electron energy distribution function has electrons with energies of 9-18 eV, which are responsible for the emission of the nitrogen molecule in the spectral range 290- 410 nm.

The emission of atoms and singly charged indium ions in the visible region of the spectrum was represented by low-intensity spectral lines 410.17; 451.13 nm In I and 462.07; 609 nm In II. The characteristic spectral line of 500.52 nm N II, which is usually observed in the emission spectra of a nanosecond discharge in air [23-25], also appeared in the emission spectrum of overstressed nanosecond discharge plasma. In the yellow-red region of the spectrum, the radiation spectrum (Figure 4) shows the manifestation of a weak continuous spectrum in the form of a substrate for individual low-intensity spectral lines, which may be due to the emission of selenium molecules. With increasing nitrogen pressure from 5.3 to 101 kPa and, accordingly, reducing the value of the E/N parameter by about 20 times (from about 1.5 10⁴ V/cm mm Hg), the emission spectrum of the plasma of an  overstressed nanosecond discharge changes dramatically (Figure 5). This may be due to a decrease in the efficiency of the ecton sputtering mechanism of chalcopyrite electrodes in a weaker electric field and the activation of the sputtering mechanism of the electrodes under the impact of positive nitrogen ions (N ⁺, N₂ ⁺), which in excited states appear in the emission spectrum (Figure 5). In this case, the threshold for electron runaway is not reached in the discharge [20], the EEDF changes, and energy transfer processes from nitrogen molecules and nitrogen atoms in metastable states to chalcopyrite molecules or plasma destruction products in plasma become probable.

The main features of the plasma emission spectrum based on the nitrogen-CuInSe₂ vapor-gas mixture at p (N₂) = 101 kPa are associated with a significant increase in the intensity of the group of spectral lines Cu I, Cu II, In II in the wavelength range 200-250 nm, and also a decrease in their number. The intensity of the emission bands of the nitrogen molecule, at the same time, has sharply decreased. Instead of the bands of the nitrogen molecule in the spectral range of 290 - 460 nm, some intense spectral lines Cu I, In I, In II were recorded in the emission spectrum (lines 15-39; Table 1).

Table 1: The results of the identification of the most intense spectral lines of the products of the destruction of chalcopyrite in an overstressed nanosecond discharge (p(N2) = 5,3; 101  kPa).

Thus, it was shown that at a pressure of nitrogen in the range of 5.3 - 202 kPa between the electrodes from the CuInSe₂ compound with a distance between them of 1 mm, a sufficiently uniform nanosecond discharge is ignited with a pulsed electrical power of 7-9.3 MW and the energy input to the plasma in one pulse 0.35-0.44 J. The study of the spectral characteristics of chalcopyrite plasma showed that the most intense are the spectral lines of the copper atom in the range 200-230 nm and the spectral lines of the indium atom and copper ions and indium ions in the longer wavelength region of the spectrum; at the same time, as the nitrogen pressure increased, the intensities of the copper atom lines in the spectral range 200-230 nm and the intensities of the ion lines in the spectral range 230-460 nm increased. The nature of the emission spectrum suggests the existence of selective mechanisms for the formation of excited atoms and ions of copper and indium in the plasma, associated with the transfer of energy from metastable atoms and argon molecules. Based on the measured relative intensities of the spectral lines of atoms and copper and indium ions, it is possible to make estimates of the temperature and density of electrons in the plasma under study. To diagnose the deposition of chalcopyrite films in real time, the following most intense and separately located in the spectral range 300-460 nm lines can be used: 307.38 nm CuI, 329.05 nm CuI, 410.17 nm In I, 451.13 nm In I. The study of the relative transmission spectra of probe radiation in the 200-800 nm wavelength range of chalcopyrite films synthesized by the pulsed gas-discharge method in nitrogen and air showed that the lowest transmission is characteristic for films synthesized at atmospheric pressure of nitrogen and air, which is important for their use in photovoltaic devices.

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