PREPARATION OF N-Si-P-GaSe HETEROJUNCTIONS BASED ON AN AMORPHOUS GaSe LAYER WITHOUT IMPURITIES AND STUDY OF THEIR ELECTRICAL PROPERTIES †

,


INTRODUCTION
Even though semiconductor materials have been studied for a long time, these studies are continued in this period with modern methods.The main reason for this is that they have different functional properties.Due to the change in functions under the influence of the size effect, extensive research is being carried out on these materials at the atomic level [1][2][3][4][5][6][7].The study of heterojunctions in these materials is of particular importance [8][9][10].Because the obtained results further increase their application areas.The study of physical processes in semiconductor structures is related to increasing the efficiency of photoconverters and reducing the cost of solar cells.The research conducted to increase the efficiency of solar cells is one of the important directions of solar energy.One of the promising solutions to this problem is the creation of thin-layer structures.GaSe crystal, which is of practical importance in small-sized electronic systems, occupies a special place among A3B6 type compounds.The high drift mobility (~7000 cm 2 /V‧s) of electrons in ultra-thin GaSe nanosheets expands its application possibilities [11][12][13][14].On the other hand, the width of the forbidden band gap of GaSe crystal nanolayers strongly depends on the thickness of the layer, and the variation of optical and photoelectric properties with control of thickness increases their application in optoelectronics [15][16][17][18][19].To create effective photoconverter structures based on GaSe crystal, Si-based amorphous and single crystals as well as complex semiconductor compounds are used [16].Spectral distribution of photosensitivity in a-Si:H − n-InSe and Si -GaN(O) structures obtained by a new technological method was investigated and the prospects of obtained heterojunctions were shown [17,18].Since the creation of a potential barrier in disordered semiconductors in the aforementioned structures is based on the classical potential barrier model, the role of deep energy levels localized near the Fermi level in the formation of p-n junctions is not taken into account, so there is a need to conduct new research in this direction [19].There exist several experimental and theoretical studies for the physical properties of GeS crystal but so far, its thin film properties are still not sufficiently studied.
The aim of the study is investigation of the important characteristics of the heterocontact (n-Si − p-GaSe) created based on Si single crystal and p-GaSe thin layer (amorphous and polycrystalline).

A METHOD FOR PRODUCING A HETEROCONTACT AND MEASUREMENT METHODOLOGY
During the creation of the photosensitive structure, a chemically cleaned (100) oriented n-Si (KEF-7,5) single crystal was taken as a substrate.As the second component, scraps of GaSe single crystal grown by the Bridgman method were used.GaSe scraps were deposited on a cold Si (thickness ~50 μm) substrate by thermal evaporation method in a vacuum (~10 -6 mmHg).The thickness of the obtained thin layers was ~ 0.8-2 µm.To determine the crystallization temperature of the thin layer, it was thermally annealed in a vacuum at a temperature of t = 200 °C for a period of τ = 2-3 hours.The optical absorption coefficient, dark conductivity and stationary photoconductivity were measured in the thin film Preparation of N-Si-P-GaSe Heterojunctions Based on an Amorphous GaSe Layer...

RESULTS AND DİSCUSSİON
Figure 1 shows the volt-farad characteristics of n-Si -p-GaSe structures on a cold substrate (1) and thermally annealed (2).It can be seen from Figure 1 that the dependence of C 2 ~ f(U) has a linear character in the thermally annealed sample (curve 2), while deviations from the linear dependence are observed in the sample with a cold base at high voltages.This shows that the distribution of impurity atoms in the region of bulk charges has a non-linear character (measurements were carried out at frequencies f = 50-100 kHz).The contact potential difference, p-n junction width, and electrostatic field intensity for both structures were calculated according to [19,21] and for the thermally annealed sample were ~ 0.5 V, 2•10 -4 cm, 3.2•10 4 V/cm, and for the sample with cold substrate were ~ 0.38 V, 3•10 -5 cm, 7.2•10 2 V/cm.
From the slope of the curves from VFC, the concentration of charge carriers was calculated according to [20] and was 2.3•10 15 cm -3 and 1.8•10 13 cm -3 , respectively.Based on the comparison of the parameters obtained during the calculation, it can be concluded that due to the weak interatomic bond in the GaSe layer obtained on a cold substrate, the nature of the distribution of ionized atoms in the range of bulk charges depends on the localized oxygen layer (dielectric layer) at the Si -αGaSe boundary during thermal evaporation.This fact is evident from the VFC extracted at different temperatures C 2 ~ f(U).Figure 1 shows the VFC of the n-Si−p-GaSe structure at different temperatures.It can be seen from the figure that with increasing temperature, the dependence of C 2 ~ f(U) in samples with a cold substrate (1) changes more sharply than in thermally annealed samples.The reason for this may be the high concentration of unstable defects.
Figure 2 shows the thermally annealed volt-ampere characteristics of the n-Si − p-GaSe structure at different temperatures.The slope of the dependence I ~ f(U) consists of two parts.In the region of U = 0-8 V, the diode coefficient is m ~ 1.1, and in the high voltage region it is m ~ 1.5.In the amorphous layered structure (2), small deviations are observed at values of reverse voltage U > 10 V, and the diode coefficient is close to m ~ 1.This may be due to the high layer and small barrier capacity of the layer, as shown in [19,22].
The VAC analysis of the n-Si-p-GaSe structure shows that the current flow mechanism in anisotropic n-Si-p-GaSe structures depends on the value of the contact potential difference that forms the area of bulk charges.As shown in [23], due to the high density of state in the vicinity of Fermi level (~10 18 cm -3 ), the height of the potential barrier decreases as a result of the change of the potential and electric field distribution in the area of bulk charges.This leads to a deterioration in the quality factor of the p-n junction relative to the structure undergoing crystallization.
Figure 4 shows the spectral characteristics of n-Si − p-GaSe structures (1,2-heated substrate; 3,4 cold substrate, GaSe surface illuminated) T = 300 K. From curve 1 in Figure 4, it can be seen that the "window effect" observed in the spectral range of 1.0-3.0eV during illumination from the wide-band surface of the heterojunction obtained on the heated substrate is related to the forbidden gap of the compounds.The width of the photosensitive region of the spectrum observed at the maximum photosensitivity hν = ~1.5 eV was ∆S ~ 70-800 meV. the sharp decrease in photocurrent at values of hν > 2.6 eV is due to the decrease in interband electron transitions.As can be seen from curve 2 in Figure 4 the photosensitivity of the heterojunction formed on the cold substrate decreases compared to curve 1 and reaches its maximum value at hν = 1.5 eV.The half-width of the spectrum decreases and becomes ∆S ~ 45-500 meV.The shortening of photosensitivity and spectral range is due to high electroactive local defect density at the contact boundary [23].
The photosensitivity and spectral width decrease when the heterojunction formed on the heated substrate during illumination from the Si side (3rd curve in Figure 4).In this case, the energy corresponding to the maximum value of the photocurrent shifts to the lower energy region and becomes hν = 1.2 eV.When the heterojunction formed on the cold substrate and is illuminated from the Si side, the obtained spectrum shifts to the long-wave region (curve 4) and the the Preparation of N-Si-P-GaSe Heterojunctions Based on an Amorphous GaSe Layer...

EEJP. 1 (2024)
short-wavelength border of photosensitivity corresponds to the band gap of the Si substrate.As can be seen from Figure 4, the red border of the spectral characteristics of n-Si − p-GaSe structures is determined by the forbidden gap of the substrate.Kinetic analysis of photocurrents in anisotropic n-Si − p-GaSe structures shows that the mechanism of photoconductivity depends on the distribution density of defects at the amorphous-crystalline boundary (HJ).Photosensitivity is reduced because photocarriers in the HJ, formed in a cold substrate illuminated from a wide bandgap, are trapped by local high-density levels formed at the interface.After thermal annealing (Fig. 4. curve 1) due to the orderly distribution of electroactive defects in the area of bulk charges, the height of the potential barrier for the main charge carriers increases (Fig. 1) and the photosensitivity of HJ increases several times.With the applying of an external voltage to HJ-(direct or opposite direction), the Fermi level position in the region of bulk charges changes, as well as the height of the potential barrier, as a result of which the diffusion and drift velocities of charge carriers are change.This causes a redistribution of charged defects in the area of bulk charges.From the comparison of the volt-farat characteristics of n-Si-p-GaSe structures, it can be seen that the processes occurring at the boundary during the amorphous GaSe -Si contact depend on the distribution of the electroactive density of states localized in the vicinity of the Fermi level.
Considering that broken bonds in amorphous layers are sensitive to external influences (temperature, electric field, ionizing rays), their recovery process is accelerated during thermal annealing and due to the redistribution of electroactive local levels, occurs equilibrium in the area of bulk charges.This leads to an increase in the height of the potential barrier in the structure.
To clarify the current flow mechanism, the zone diagram of the studied heterojunction was built according to Anderson's model (Figure 5).
Since there is no information about the electron affinity parameter for the GaSe layer, the main parameters of the zone diagram -breakdown (∆Ec and ∆E V ) were calculated according to the experimental values of the diffusion potential (V d1 and V d2 ) taking into account the Fermi level of the components and the energy values of the forbidden gap.For the calculation, the electron affinity χ n = χ p = 3.6 eV for the GaSe crystal was used [22,23].Based on the results of the mentioned literature, the breakdowns of the energy zones for the nSi-aGaSe heterojunction were calculated for conduction and valence bands ∆Ec ~ 1.55 eV, ∆E v ~ 0.15 eV respectively.The high density of state at the contact boundary of the HJ, formed based on the used model, is due to the incompatibility of the structural parameters of the components.Since partial crystallization is observed in the a-GaSe layer during thermal annealing, the photosensitivity of the structure increases due to a decrease in the density of states of the surface.The obtained results correspond to the values given for InSe − GaSe and Si − GaSe in [15,17].

CONCLUSİONS
Thus, based on VFC, the influence of the charge density distribution on the electrostatic parameters in the region of bulk charges in a heterostructure formed based on amorphous GeSe and crystalline n-Si was determined.It was found that an increase in the contact potential difference in the dependence of C 2 ~ f(V) for thermally annealed n-Si − p-GaSe sample is due to reduce the density of states near the Fermi level (from ~10 19 eV -1 cm -1 to 10 16 eV -1 cm -1 ) in the amorphous GaSe layer.In an amorphous GaSe layer, the distribution of potential and electric field intensity in the region of bulk charges changes due to the presence of the density of states of charged local levels located in the vicinity of the Fermi level.As a result of a decrease in the density of states at the contact boundary during thermal annealing, the intensity in the bulk charge region decreases, and the width of the region of bulk charges and the height of the potential barrier increase from 0.4 to 0.8.eV.

1 Figure 2 . 1 Figure 3 .
Figure 2.Current-voltage characteristics of the n-Si -p-GaSe structure (at different temperatures: T = 100 K (1' and 2'), T = 300 K (1 and 2), 1 and 1' GaSe obtained on a cold layer substrate, and 2 and 2' -a GaSe layer at a temperature of T = 473 K It can be seen from Figure2that at temperature T = 300 K and at low voltages, the dependence of I ~ f(U) obeys the law I ~ exp(eU/βkT) (β = 2-3), and at high voltages obey the law (0.3-0.4 V) I ~ AU m (m = 1.5-2).At low temperature (100 K), due to the increase of the voltage drop in the crystal, the Volt-Ampere characteristics (VAC) shift towards the high voltage region, but the exponential regularities are preserved.The analysis of the obtained experimental results shows that the rectifier capacitance of the p-n junction obtained on a cold substance is lower than that of the annealed

Figure 5 .
Figure 5. Band energy diagram of the n-Si-p-GaSe heterojunction