Technology of production and photoelectric characteristics of AlB 10 heterojunctions based on silicon

. Currently, the usage of electronic devices with diverse applications is prevalent in numerous commercial, industrial, electrical, and military sectors. The development of new semiconductors is essential for the advancement of highly sensitive, fast-responding, multifunctional, and high-precision devices and installations. Thin films based on semiconductor monocrystalline substrates are being obtained by scientists in leading research centers worldwide. Production technologies are being improved, optimal conditions are being determined, and the structural and unique physical properties of the obtained thin films are being studied. Additionally, research is being conducted to broaden the light absorption spectra of gas, temperature, and pressure-sensitive heterostructures. In this work, the results of a low-temperature technology development for obtaining films of refractory materials based on the presence of the eutectic state in boron-metal oxide systems are presented. The electro-physical properties of the obtained AlB 10 films are investigated over a wide temperature range. The discussion revolves around the development of the technology for manufacturing metal-dielectric-metal (MDM) structures based on AlB 10 . The results of investigating the volt-ampere characteristics (VAC) of Ni-AlB 10 -Ni and Al-AlB 10 -n-Si structures are presented. The method employed for obtaining AlB 10 films involves thermal evaporation of a B+Al 2 O 3 mixture in a vacuum at a temperature of 1350-1400°C.


Introduction
Highly promising materials for the creation of semiconductor devices capable of operating in extreme conditions, such as high temperatures, aggressive environments, and radiation fields, are β-rhombohedral boron (β-boron) and borides.Additionally, these materials have garnered interest from industries such as metallurgy, nuclear energy, and rocket technology [1][2][3].The distinctive semiconductor properties of β-boron and borides stem from the specific nature of their complex crystalline structure.By combining electrical, optical, and thermal properties, they occupy an intermediate position between crystalline and amorphous semiconductors, giving rise to a new class of materials known as quasi-amorphous semiconductors [4][5][6].As a result, there has been a growing interest in boron and borides.Firstly, due to the discovery and investigation of quasicrystals and amorphous metallic alloys exhibiting orientation and hexagonal ordering.Secondly, the comparison of icosahedral borides with new high-temperature superconductors is possible because of the established bipolar hopping conductivity mechanisms in certain borides (e.g., boron carbide).Depending on the complexity of their crystalline structure, these materials undergo a transition from properties typical of crystals to properties typical of amorphous semiconductors [7].The number of atoms in the elementary cells of these materials varies by two orders of magnitude (from 12 to 1600).Furthermore, they all possess the properties of refractory materials: high melting temperatures exceeding 200°C, high hardness, and resistance to chemical interactions [6][7][8][9].

The methodology and experimental results
The following block diagram (fig. 1) was used for the initial I-V characteristics (currentvoltage characteristics) of thin AlB10 films and heterostructures based on AlB10: The main components of the setup are the power supply unit (PSU), control panel (CP), thermostat, and measurement block (MB).
When conducting the I-V characteristics measurements of the heterostructure, a shielded measurement cell was utilized and placed in the thermostat, enabling the maintenance of a stable temperature ranging from room temperature to 300 degrees Celsius.The contact between the sample and the upper aluminum electrode was established using a microprobe positioned on a micromanipulator within the thermostat.The control panel (CP) regulated both the temperature of the thermostat and the voltage applied to the sample from the power supply unit (PSU).The measurement block (MB) was employed to determine the temperature of the thermostat and the magnitude of the voltage supplied to the structures.
The deposition of AlB10 films onto Ni or n-Si substrates was performed through the thermal evaporation method, employing a B+Al2O3 mixture under vacuum conditions at temperatures ranging from 1350 to 1400 degrees Celsius.Additionally, AlB10 films were deposited onto n-Si substrates, with the substrate temperatures varying within the range of 80 to 450 degrees Celsius.The film deposition rate at the evaporation temperatures was maintained at 10-15 Å/s.

Results
Previous reports [1] indicated that the use of the electron diffraction method allowed for the determination of the amorphous structure of freshly cleaved films.Subsequent annealing at temperatures of 350-400 degrees Celsius for a minimum of 3 hours resulted in the formation of crystalline phases exhibiting hexagonal lattice parameters of a=7.835Å and c=15.91 Å.The band gap widths of the amorphous and crystalline films, calculated from the temperature dependence of electrical conductivity, were found to be 1.2 eV and 1.8 eV, respectively [2].The specific resistances for AlB10 films, which were calculated from the linear segments of the I-V curves, ranged from 10 9 to 10 10 ohm•cm, indicating their potential as dielectric coatings.A symmetrical shape in both current directions and three segments (ohmic, quadratic, and cubic) were observed in the I-V curve of the Ni-AlB10-Ni structure (Figure 2).The deposition of AlB10 films was conducted in a vacuum of no less than 10 -5 mmHg, followed by the application of aluminum or nickel films as ohmic contacts.
For the investigation of the I-V characteristics, the prepared structures of Ni-AlB10-Ni or Al-AlB10-nSi-Ni were placed in a shielded measurement cell, and the upper aluminum films were contacted using a microprobe positioned on a micromanipulator.The cell was then positioned in a thermostat, providing a stable temperature range from room temperature to 470 K.The analysis of transverse conductivity in film sandwich structures allows for the convenient study of the electrical conductivity mechanism in thin polycrystalline films.In this regard, when a transverse current flows through a film, the influence of intergranular regions diminishes if the film thickness is comparable to the crystal size, thereby approximating the experimental conditions to the ideal case of current flow in a single crystal (Figure 3).
The temperature-dependent I-V characteristics of thin AlB10 films were measured over a wide temperature range (296-470 K) to investigate the current transport mechanism.The measured sequence of I-V curves corresponds to currents that are limited by space charges (SCL3) [3] (Figure 4).According to the SCL3 theory, the concentration of free current carriers injected increases as the applied voltage increases.When this concentration becomes comparable to the equilibrium concentration n, Ohm's law is violated.The increase in voltage with temperature is attributed to the rise in carrier concentration.Based on this assumption, the concentration n can be calculated at different temperatures using the formula: At room temperature, a concentration of n=1,4•10 17 sm -3 was determined.For the estimation, ε=8.3 and d=1 μm (film thickness) were utilized.
A plot of ~( 1  )) was created (Figure 5a), from which the carrier activation energy ΔE = 0.05 eV was obtained.The band gap widths of the amorphous and crystalline films, calculated from the temperature dependence of electrical conductivity, were found to be 1.2 eV and 1.8 eV, respectively.The specific resistances of the AlB10 films, calculated from the linear segment of the I-V curve, ranged from 10 9 to 10 10 Ohm•cm, indicating their high potential as dielectric coatings.The I-V curve of the Al-AlB10-nSi heterostructure displayed a strongly asymmetric diode-like shape when the AlB10 films possessed both amorphous and crystalline structures (Figure 3), with a rectification coefficient of k=10 5 at U=2B.
In the forward direction, current flows through the structures when a positive polarity is applied to the aluminum electrode.The overall shape of the forward branches of the I-V curves, plotted in logarithmic coordinates (Figure 6), leads to the conclusion that in this case, monopoles current carrier injection occurs, and the current is limited by space charges (SCL3) due to the presence of traps in the dielectric and at the dielectric-semiconductor interface.The depth of traps, which is estimated using the formula: considering the computed values of θ and Nt, is determined to be Et = 0.33 eV.
Hence, monoenergetic traps with a concentration of Nr =1,7•10 16 cm -3 are involved in limiting the current, located at a depth of 0.33 eV from the bottom of the conduction band.

Discussion
When the negative terminal of the voltage is connected to the aluminum electrode, electron injection into the high-resistivity AlB10 takes place from the side of the aluminum electrode.The I-V curve plotted in  ~ √ coordinates displays a linear nature and exhibits a significant increase with temperature.This behavior of the I-V curve indicates a Schottky over-barrier conduction mechanism [4].To determine the height of the potential barrier between AlB10 and Al, temperature dependencies of the I-V curves were examined.By extrapolating the lines to their intersection with the current axis in the [  2  ⁄ ]] vs. U 1/2 dependence for two temperatures (Figure 5), the effective height of the potential barrier between AlB10 and Al was calculated using the formula: The calculated value is  В = 0,85 ± 0,07eV.
It should be noted that the I-V curve exhibits a distinct Schottky character up to a voltage of 10 V.However, with further increase in voltage, the shape of the I-V curve for this structure undergoes a change.In the region of breakdown fields, a slowing down of current growth is observed.This can be attributed to tunneling-limited impurity conductivity.the Фb0 values for the asdeposited and MIS diode were found as 0.77 and 0.73 eV at 300 K.That is, the Фb0 value for the D1 MIS diode at 300 K is lower than that for the as deposited.The case may be ascribed to the passivation effect of the Al2O3 thin film on interface defects to improve the interface inhomogeneity [9,10,12,33,[15][16][17][18][19][20].They [22] experimentally found that the Фb0 value decreased with increase in the interfacial Al2O3 thinfilm thickness from as-deposited diode to 2 nm over whole test temperature (five different temperatures).Thus, they [22] attributed this image to the fact that the insertion of Al2O3 improves the inhomogeneity of the Schottky interface.Their TEM results [22]  showed that a more flatness interface formed with the insertion of Al2O3 thin layer which reduces Ti diffuse into SiC to start the solid state reaction at the interface.Our findings for the D1 MIS and as-deposited diodes are similar to theirs [22].As mentioned above, we also prepared the MIS diodes with 5nm and 10nm Al2O3 layer thicknesses other than the diode with the 3nm interfacial layer.The barrier height values of D2 and D3 MIS diodes extracted higher than those of the asdeposited and D1 MIS diode.We evaluate the results related to these diodes.

Conclusion
In conclusion, it can be inferred that in the Al-AlB10-nSi-Ni heterojunction, under positive aluminum polarity, injection currents flow, and their characteristics are determined by monoenergetic traps of the donor type with an activation energy of 0.33 eV and a concentration of Nt=1,7•10 16 cm -3 .Under the negative polarity of the aluminum electrode, the main role is played by the transition between Al and AlB10, and the I-V curve is explained by the Schottky over-barrier mechanism.

Fig. 4 .
Fig. 4. I-V characteristics of the Al-AlB10-nSi-Ni structure for the crystalline (1) and amorphous (2) films in the forward and reverse directions of the current.