Effect of Silicon Surface Treatment on the Electrical and Photoelectric Properties of Nanostructured MoOx/n-Si Heterojunctions

The paper presents the results of studies of the effect of silicon surface treatment on the electrical and photoelectric properties of nanostructured MoOx/n-Si heterojunctions. The nanostructured heterojunctions MoOx/n-Si, were prepared by deposition of thin films of molybdenum oxide (n-type conductivity) by reactive magnetron sputtering in the universal vacuum system Leybold Heraeus L560 on the nanostructured silicon substrates (n-type conductivity), which were made by chemical etching with the assistance of silver nanoparticles. Dark and light volt-ampere (I – V) characteristics of the heterojunctions under study were measured, the value of the potential barrier height, the values of the serial Rs and the shunt Rsh resistance at room temperature were determined. It was established that the silicon surface treatment does not affect the potential barrier height, but significantly affects the values of serial Rs and shunt Rsh resistance. The electrical and photoelectric properties of the obtained structures were investigated, the dominant mechanisms of current transfer through the heterostructures under forward bias are well described in the framework of emission-recombination and tunneling models with the presence of interface states. The main mechanism for the charge carrier transport through heterojunctions with the reverse bias is the Frenkel–Pool emission. Investigation of photoelectric properties of heterojunctions MoOx/n-Si was carried out at illumination by white light with intensity Popt = 80 mW/сm2. It was established that the heterostructure No.5 MoOx/n-Si with grown nanowires and etched silver nanoparticles has a maximum open-circuit voltage Voc = 0.17 V, short-circuit current density Isc = 10 mA/cm2. The possibilities of using the obtained heterostructures as photodiodes were analyzed.

In recent years the interest to the research of semiconductor heterojunctions created on the basis of thin films of transition metal oxides has considerably grown up. Molybdenum oxide (MoO x ) has found practical application in electronics and in photovoltaic devices long ago due to high work function of electron and good physical properties: it has a high transmission factor in the visible part of the spectrum [1], a low electrical resistivity [2], the band gap width E g > 3 eV [3,4]. In our previous work [5] we showed the possibility to form planar photosensitive heterostructures based on silicon and molybdenum oxide. It is known that the expansion of the scope of such heterostructures application is possible by creating nanostructured surfaces of the base material.
The silicon-based nanostructures are attribute components of the up-to-date instrument engineering in the field of electronics, optoelectronics, chemical sensors, as well as for the conversion and accumulation of solar energy. The silicon surface, which is modified by arrays of nanowires, has a low reflection coefficient and a large active area, what allows its successful practical application [6]. Judging by the aforesaid, the creation and study of nanostructured MoOx/Si heterojunctions is of considerable scientific and practical interest.
The objective of the work is to study the effect of silicon surface treatment on the electrical and photoelectric properties of nanostructured MoO x /n-Si heterojunctions.

EXPERIMENTAL PART
To grow nanowires the monocrystalline silicon of n-type conductance with surface orientation (100) and thickness of 330 μm was used. The resistivity and concentration of the crystals charge carriers at the temperature (295 K) made: ρ = 6 Ohm•cm and n = 7.4•10 14 cm -3 , respectively. The depth of the burial level -Fermi for the base material (E с -E F = 0.27 eV) was determined from the expression for the concentration of equilibrium electrons: n = 2(2πm n kT/h 2 ) 3/2 exp(-(E C -E F )/kT).
To remove the native oxide and to purify the surface from the contaminants the silicon plates were chemically etched in 5% solution of hydrofluoric acid (HF) in bi-distilled water for 5 minutes. At the start of the nanowires growing the silicon substrates were washed in an ultrasonic bath in bi-distilled water and in acetone, after washing the substrates were etched in a solution of sulfuric acid and 30% of hydrogen peroxide (H 2 O 2 ) in the appropriate ratio (3:1) to remove the organic contaminants. After purification our samples were immersed in pre-prepared aqueous solutions of 0.02M AgNO 3 and of 5M HF in the ratio (1:1) for 5-10 seconds in order to deposit silver nanoparticles on the substrates [5][6].
After deposition of silver nanoparticles some part of the substrates were taken to create heterostructures, and the others for the next steps to create nanowires. The next step to create nanowires was etching of silicon substrates with silver nanoparticles in the solution of 5M HF and 30% H 2 O 2 in the ratio (10:1). Also, after creating nanowires on the 34 Effect of Silicon Surface Treatment on the Electrical and Photoelectric Properties...

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silicon substrate with intercalated silver nanoparticles, some part of the substrates was taken to create heterostructures. The final procedure was etching silver nanoparticles from the substrate itself using nitric acid.
Thin MoO x films were deposited on the surface of nanostructured monocrystalline Si (size 5×5×0.36 mm) with orientation (100) in the universal vacuum device Leybold Heraeus L560 by reactive magnetron sputtering of a pure molybdenum target in the atmosphere of argon and oxygen mixture under the forward-current voltage. Before starting the spraying process the vacuum chamber was pumped out to the residual pressure of 5·10 -3 Pa. The partial pressures of argon and oxygen were 0.24 Pa and 0.034 Pa, respectively, at the constant magnetron power of 30 W. In the course of spraying process the substrate temperature is maintained at 573 K. The spraying process lasted for 3 minutes.
After finishing the process of deposition of the MoO x thin films the vacuum chamber was gradually cooled to room temperature and then was opened to replace the molybdenum target with ITO target (In 2 O 3 -SnO 2 90:10 by mass). The deposition of ITO thin 0.15 μm-thick films, which were used as an antireflection coating, was carried out by the method of magnetron sputtering of the ITO target in the Argon atmosphere under the direct-current voltage.
During the process of deposition the argon pressure in the vacuum chamber was ~ 0.4 Pa. The installed power of the magnetron was ~ 30 W. The deposition process lasted for ~ 5 min. at the substrate temperature of ~ 420 K. To provide the frontal electric contact with the thin MoO x film the conducting paste was used. To avoid recombination on the rear side of silicon and to ensure a good set of photogenerated charge carriers, we used substrates that previously had rear contact with the built-in internal field. The contact was created by sputtering a layer of the intrinsic hydrogenated amorphous silicon (a-Si:H) with the thickness of ~ 10 nm to passivate the substrate surface, the next layer of hydrogenated amorphous silicon n + (a-Si:H) with the thickness of ~ 20 nm, which was heavily doped with phosphorus, to create isotype junction with the barrier height of ~ 0.1 eV on the rear side of Si, and the last layer of Al was deposited by thermal evaporation (the block diagram is shown in the inset of Fig. 1).

RESULTS AND DISCUSSION ELECTRICAL PROPERTIES OF NANOSTRUCTURED HETEROJUNCTIONS MoOx/n-Si
The MoO x /n-Si heterostructures were prepared using the method of reactive magnetron sputtering of molybdenum oxide thin films on the single-crystal n-Si substrates with different surface treatment (Table 1). The MoOx/n-Si heterostructure: nanowires with intercalated silver nanoparticles were grown on Si substrates. Sample No. 5 The MoOx/n-Si heterostructure: nanowires were grown on Si substrates and silver nanoparticles were etched. Fig. 1 shows the volt-ampere characteristics and dependences of the differential resistance [7] on the applied bias for the MoO x /n-Si heterojunctions with different treatment of Si surface, listed in Table 1.  Table 2 presents the major parameters of heterojunctions subject to the volt-ampere characteristics: heterojunction rectification factor k, the value of the potential barrier height φ 0 , and the value of the series and shunt resistance of heterojunctions. As Fig. 1 and Table 2 show, that the Si surface treatment does not affect the height of the potential barrier since its value depends on the difference in the work function of the heterojunction components, and its value can be affected by the electric charge localized on the surface energy states. However, the surface treatment significantly affects the value of the series resistance R s . Table 2 shows that the substrate surface treatment in HF (sample 2) causes a decrease in the series resistance due to the removal of the native oxide (SiO 2 ) from the substrate surface, but to determine more precisely the reasons for the series resistance growth an additional research is required. The lowest value of the series resistance for sample No. 5 is stipulated by an increase in the active area of the developed heterojunction surface and by a decrease in the effective length of the base material [8], while the highest values of the rectification factor and shunt resistance indicate that heterostructure No. 5 is the best among all the above-listed ones.

MECHANISMS OF CURRENT TRANSFER IN THE NANOSTRUCTURED МоOх/n-Si HETEROJUNCTIONS Forward biases
The analysis of the forward branches of I-V characteristics of МоO х /n-Si structures built in a semi-log scale (Fig. 2

), with the influence of series and shunt resistances taken into account, showed that the dependence ln[I-(V-IR s )/R sh ] = f(V-IR s ) comprises two straight portions, what indicates to the exponential dependence of the current on the voltage and to the presence of two dominant mechanisms of charge transfer in the voltage range under study. The values of the nonideality factor according to Δ ln[I-(V-IR s )/R sh ]/Δ(V-IR s )=e/nkT,
where n is the nonideality factor, determined for the both voltage portions are given in Table 3.
A small value of the potential barrier height φ 0 = 0.49 eV, as a rule, leads to the flow of the over-barrier current. In the voltage range (3kT/e < V < 0.2 V) the dependence I(V) is well described by the expression for the emissionrecombination mechanism of current transfer with the influence of the series and shunt resistances taken into account (direct recombination of charge carriers through the energy states on the interface, which is determined by the potential barrier height) [9,5]: where В 0 is the coefficient that depends weakly on the temperature, the coefficient n, as a rule, varies from 1 to 2, what correlates well with experimentally obtained values (Table 3). Table 3. MoO x /n-Si heterojunction parameters When the emission-recombination mechanism is dominant, it is considered that the recombination centers are uniformly distributed by energy and are concentrated in a narrow region near the interface.
Taking the logarithm of expression (1) we obtain:

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nkT From the last expression it is evident that the dependences ln(I-[V-IR s ]/R sh )= f(V-IR s ) should be approximated by straight lines with a slope, what is really observed (Fig. 2). In the voltage range 0.2 < V < 0.4 V a small constant slope (a large value of the nonideality factor n > 2) of the experimental dependences ln(I) = f (V) at different temperatures can be considered as an evidence of the tunneling nature of the current transfer mechanism [10]. Straight portions of I-V characteristics with identical slopes occur at sufficiently large biases, where the space charge region is sufficiently thin for direct tunnel effect, which is described by Newman formula for the tunneling mechanism of current transfer with the effect of series resistance taken into account [9,10]: 0 t I , α, β are constants. The experimental value of α was determined from the dependence ln (I) = f (V-IR s ) and is shown in Table 3.

Reverse biases
The dependence I rev (V) is well described in the framework of the model based on the Frenkel-Poole emission. The essence of the corresponding processes is the thermal excitation of charge carriers, captured by surface traps, facilitated by electric field [11].
The dependence of the electric field strength on the reverse voltage E(V) in the space charge region of the asymmetric heterojunction was estimated by the formula taken from [5].
The dependence of the reverse current on the voltage (0.12 < | V | < 2 V), plotted in coordinates ln(I rev ) = f | V | 1/2 is shown in the inset of Fig. 2 and is well approximated by straight lines, what confirms the validity of the proposed mechanism of the current transfer. Curve 4 (for the sample with the intercalated silver nanoparticles) deviates from the others, what is due to formation of silver oxide or silver-containing compounds with dielectric properties in this heterostructure, and as a result some additional traps or recombination centers are formed that affect the current flow. Figure 3 presents the dark and light I-V characteristics of MoO x /n-Si heterostructures. As Fig. 3 shows, at illumination with white light with the intensity of 80 mW/cm 2 the forward and reverse current I light increases as compared to their values in the dark zone I dark . The heterostructure parameters determined from the dependence I=f (V) had the following values (Table 4).

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Mykhailo Solovan, Taras Kovaliuk, et al.  A large ratio between the photocurrent and the dark reverse current I ph /I rev shows that the heterostructures under study can be used as photodiodes (Fig. 4). At the same time, Fig. 4 shows that for sample 4 (with intercalated silver nanoparticles) the photoelectric parameters are the lowest. Due to the negative effect of the intercalated silver nanoparticles on the current transfer mechanisms, as well as the barrier and photoelectric parameters of the obtained structures, these silver nanoparticles are to be etched in nitric acid when growing nanowires on silicon by the above described method. Table 4 shows that heterostructure No. 5 has the highest photoelectric parameters among the heterostructures under study, and after their refinement by introducing intermediate layers and high-quality passivation of dangling bonds in silicon they can be used as solar cells.

CONCLUSION
The effect of surface treatment of Si substrates on the electrical and photoelectric properties of MoOx/n-Si heterostructures was studied.
Photosensitive MoO x /n-Si heterojunctions were obtained by the method of reactive magnetron sputtering. The I-V characteristics were measured. The electrical properties and barrier parameters of the obtained heterostructures were investigated.
The studies have shown that the mechanisms of current transfer through the heterostructures under forward bias are well described in the framework of emission-recombination and tunnel models with the presence of interface states. The basic mechanism for the charge carrier transfer through the heterojunctions at reverse bias is Frenkel-Poole emission.