Effect of ZnO Layers on Transport and Relaxation of Charge in Porous Silicon–Silicon Structures

Zinc oxide nanostructures have been grown by electrochemical deposition on porous silicon–silicon substrate. The effect of electrolyte temperature on the morphology of grown ZnO arrays was observed. Temperature dependencies of the electrical conductivity for the structures based on porous silicon were investigated in 80–325 K range. The results are analyzed within the model of disordered semiconductors and the activation energy of charge transport is determined. It is shown that ZnO layers cause the decrease of the electrical conductivity activation energy in 140–250 К temperature range. Electric conductivity was also shown to be dependent on the morphology of ZnO arrays. Based on the spectra of thermally stimulated depolarization current, the localized electron states in the experimental samples are found. The trap levels are distributed quasi-continuously on the activation energy and exist in the ranges of 0.2–0.3, 0.4–0.45, 0.5–0.55 and 0.6–0.65 eV. ZnO nanocrystals grown on porous silicon substrates modify density of states in different energy ranges.


Introduction
With the rapid growth of nanotechnology, zinc oxide (ZnO) gained significant importance as a functional semiconducting material. ZnO with its wide direct bandgap of 3.37 eV and large exciton binding energy of 60 meV is a prospective material for use in various electronic and optoelectronic applications [1][2][3]. As far as optoelectronics is considered, ZnO main benefits are green and blue luminescence, transparency of nanostructured ZnO layers in the visible, efficient photoelectric conversion in the UV, antireflective properties and compatibility with traditional electrode materials [4][5][6][7][8]. ZnO also shows interesting catalytic [9], sensor [10,11] and piezoelectric properties [3,12].
Porous silicon (PS) is a perfect candidate to serve as a substrate for the deposition of ZnO nanocrystals, since sponge-like geometrical structure of PS allows for ZnO to infiltrate into the substrate with large specific surface area. It is generally thought that PS provides an advantage for growing high-quality ZnO nanostructured arrays, decreasing mechanical stress caused by large mismatches in the lattice constant and thermal expansion coefficients [26].
PS is prepared by etching a single Si crystal with the formation of small cavities, resulting in thick walls between the pores that may have several nanometers in size [27,28]. PS has a number of useful properties. Owing to wider bandgap, intense visible photoluminescence, and a large surface area per unit volume, PS nanostructures are promising for photoelectronics, touch-sensor electronics and biomedical technologies [29][30][31][32]. Therefore, significant efforts have been devoted to the growth of ZnO nanostructures on the PS substrates [16,21,26,33]. ZnO-PS nanocomposites have been used for white light emission, photodetectors and to tune ZnO grain size for possible sensing applications. Less attention so far has been paid to the studies of charge transport in ZnO-PS nanosystems. Understanding the conductivity mechanisms, however, is important from a perspective of LED or photoelectric conversion applications.
In the present research, ZnO nanostructures were grown by means of electrodeposition method on the PS-silicon 41 substrate. Specific attention was focused on exploring the influence of ZnO layers on the electrical properties of PS nanostructures. Studies of the relaxation processes in the structures based on PS were performed using thermally stimulated depolarization (TSD) spectroscopy.

Experiment
In the experimental studies we used PS samples that were fabricated by means of electrochemical etching of single-crystalline (100) silicon substrates with a thickness of 400 μm in ethanol solution of hydrofluoric acid. The silicon substrates have the n-type conductivity with the specific resistance of 4.5 Ohm·cm. In order to obtain homogeneous layers, gold films were preliminary deposited on back surfaces of the substrates with the aid of a thermo-vacuum technique. These films also served as contacts in further measurements. Anodic current density was set to 30 mA/cm 2 throughout the etching period. The duration of etching process was 10 min. To ensure the availability of holes in the surface layer of n-Si, which is necessary for anodic reactions to occur and allows the formation of PS [27], the working surface of a silicon plate was irradiated with white light during the entire process of electrochemical etching.
In order to obtain ZnO layers on the PS surface, electrochemical deposition method was used. The method has proven itself effective in the growth of ZnO nanostructures on Si and other electrically conductive substrates [5,8]. Two-electrode electrochemical cell with water solution of 0.05 M Zn(NO 3 ) 2 ·6H 2 O and 0.1 M NaNO 3 was used in the deposition process. The deposition potential of -1.4 V was applied to the porous silicon-silicon substrate, which served as a working electrode. The platinum net played a role of the additional electrode. Duration of growth for ZnO nanostructures was 10-20 min. Electrolyte was heated up to 65-80 о С in the deposition process, since the temperature is one of the defining factors influencing ZnO morphology. During the electrochemical deposition, the reagents penetrate into the pores of silicon and zinc oxide is synthesized right on the surface of the electrode and inside the pores. After the process of electrodeposition, the experimental samples were washed in distilled water and dried in air. For the purpose of experimental studies, ZnO-PS-Si structures obtained by deposition of zinc oxide onto PS at τ = 10 min, t = 65 o С (sample A), τ = 20 min, t = 75 o С (sample B) and τ = 20 min, t = 80 o С (sample C) were used.
ZnO-PS-Si hybrid structures were characterized by scanning electron microscopy (SEM). After SEM investigation, the contact with a diameter of ~3 mm was deposited on the surface of PS and ZnO layer using conductive paste. Hence, the investigations of electrical properties of experimental samples were carried out in the geometry in which the current flows across the structure and perpendicular to the surface.
Digital RLC-meter working at 1 MHz frequency was employed to study temperature dependencies of electric conductivity of PS-based structures. Samples under study were placed into the cryostat kept under 10 -3 mmhg vacuum. In the TSD experiment, experimental structures were polarized (polarization voltage was U = 5 V) at room temperature and then cooled down to liquid nitrogen temperature. Investigation of the temperature dependence of the depolarization current of preliminary polarized samples was carried out in the absence of the external electric field at linear heating from 80 to 325 K. The heating rate was 0.1 K/sec.

Result and Discussion
Analysis of the surface and the cross section of ZnO-PS-Si structures were carried out using SEM methods in elastically reflected electrons mode and X-ray microanalysis ( Figure 1).
As one can see from Figure 1,а, the formation of narrow pores was observed. Pores were oriented perpendicular to the silicon surface. According to SEM studies of the porous layer, its thickness was about 20 μm. Average pore diameter is within the range of 100-1000 nm. Dense arrays of ZnO nanostructures formed a layer on the surface of PS and partially penetrated into the pores. Surface morphology of ZnO arrays depended on the temperature of the electrolyte and electrodeposition modes.
At 65 о С, the formation of ZnO nanowires network was observed. At elevated temperatures and prolonged electrodeposition durations, disordered arrays of zinc oxide nanocrystals were formed. At 80 о С, disordered nanorods of ZnO were formed; the smaller rods were grown on larger hexagonal ones, with sizes up to several microns (see Figure  1,d). The broad spectrum of ZnO morphological peculiarities is due to different kinetic parameters of different growth planes [3]. The important factor that allows to effectively control the shape and the size of ZnO arrays is the temperature of the electrolyte, since it determines the intensity of electrochemical processes, reagent diffusion rate and desorption rate of reaction products from the surface of grown ZnO [34]. The thickness of zinc oxide layer was increased with deposition duration. The X-ray surface microanalysis of the hybrid structures revealed the traces of silicon, zinc and oxygen. This fact proves that the surface of PS was covered by ZnO layer.  In order to clarify the mechanism of charge transport in PS-Si and ZnO-PS-Si structures, temperature dependencies of electric conductivity G in the range of 80-325 K were studied. Exponential increase in the conductivity of experimental samples at elevating temperatures was detected, as shown in Figure 2. This indicates the activation mechanism of charge transport in investigated structures [35]. Based on the temperature dependence of the resistivity, presented in (ln 1/G, 1/T) coordinates, one can estimate the activation energy for the conductivity (see inset in Figure 2). The activation energy for PS-Si structures in the low temperature range, as calculated by measuring the slope of ln G -1 (T -1 ) line was about 0.05 eV.
G(T) dependence for ZnO-PS-Si structures exhibits two characteristic regions: around 80-140 K and around 140-250 K, where activation energy for conductivity is different. In the low-temperature region the activation energy for conductivity in case of ZnO-PS-Si structures is 0.05 еV which is close to that of PS-Si. In 140-250 K region, the activation energy of charge transport in ZnO-PS-Si structures is dependent on the morphology of ZnO arrays and is found to be 0.03, 0.02 and 0.013 еV for samples A, B and C, respectively. This might be related to disordered character of ZnO-PS nanocomposite structure. Generally, disordered semiconductors not only have different conductivity activation energy values, but also allow for several simultaneous charge transport mechanism to be realized. The role of such mechanisms depends on the temperature range [36]. Besides, closer to ambient temperature we observed lower values of ZnO-PS-Si conductivity in comparison with PS-Si, what may due to larger electrical resistance of wide band gap zinc oxide nanocrystals. Moreover, electrical conductivity of the samples A, B and C decreased with the increase of the size of ZnO structures (see Figure 2).
Studies of the relaxation processes in the structures based on PS were performed using TSD spectroscopy. In the case of disordered systems, including PS, the trap levels are distributed quasi-continuously on the activation energy. To determine the activation energy of electrically active defects in experimental structures, the temperature dependencies of the depolarization current have been measured.
TSD spectra of PS-Si and ZnO-PS-Si show similar character. There occurs thermally stimulated (from LNT temperatures) emission of charge carriers, then one observes an increase in current with temperature and the band with the maximum current in 250-270 K range (Figure 3).  The analysis of the TSD spectra was performed based on the phenomenological theory of TSD currents for the disordered dielectrics [37]. The results of the calculation of the density of states energy distribution g(E), which was carried out by the numerical method based on the Tikhonov regularization algorithm, are presented in Figure 4. Approximation of the energy distribution with Gaussian functions suggests that the groups of trap levels, with differences in nature and activation energies in the ranges of 0.2-0.3, 0.4-0.45 and 0.6-0.65 eV are present.
Drawing the correlations with SiO 2, one can suggest that the 0.2-0.3 eV band corresponds to the activation energy of hydrogen ions H + [38]. It is likely that the bands at 0.4-0.45 and 0.6-0.65 eV are associated with the electrically active defects in the PS layer. In the density of states energy spectra for ZnO-PS-Si structures one can emphasize the additional band at 0.5-0.55 eV, which is probably related to the defects at Si-ZnO interface, because this band is observed for samples with different ZnO morphology. The layer of ZnO nanocrystals at PS surface is responsible also for the transformation of 0.2-0.3 eV band.

Conclusions
It has been shown that electrochemical deposition is an efficient method for controlled growth of the arrays of zinc oxide nanostructures on PS surface. The shape and size of such arrays can be largely tuned by changing temperature and temporal parameters of electric deposition. In turn, morphological features of the zinc oxide layers determine the electric conductivity of ZnO-PS-Si structures. Decrease in the conductivity of such structures as compared to the initial PS-Si sample at room temperature may be related to larger electrical resistance of wide band gap ZnO nanocrystals.
By performing complex studies of electrical conductivity and depolarization current in the wide temperature range, charge transport and relaxation processes in PS-based structures were analyzed. Activation mechanism of electrical conductivity was established and the respective activation energy for different temperature ranges was estimated. For PS-Si structure this energy is 0.05 eV in the 80-240 К temperature range. ZnO layers lead to the decrease of the electric conductivity activation energy in 140-250 К temperature range. Activation energy of charge transport in ZnO-PS-Si structures is within 0.013-0.03 еV and depends on the morphology of ZnO layers.
As follows from the analysis of TSD spectra of PS-Si structure, the energy distribution of the density of states for nonequilibrium carriers g(E) shows peaks in the ranges of 0.2-0.3, 0.4-0.45 and 0.6-0.65 eV. ZnO nanocrystals grown on porous silicon substrates modify the density of states in different energy ranges and lead to the appearance of the additional band at 0.5-0.55 eV. Such localized electron states influence the charge transport in the ZnO-PS nanostructures.