Electrochemical Formation and Characterization of Cuprous Oxide Films on n-Type InP Porous Structures
Authors: Omi Sayaka , Kumazaki Yusuke , Sato Taketomo 
Published: February 2020

The high-density formation of semiconductor nano- and micro- structures has been intensely researched targeting applications for high conversion-efficiency solar cells. A semiconductor porous structure formed by the electrochemical process is a good candidate as a building block of energy conversion devices having large surface area and low photo-reflectance. Recently, we have reported the photoelectric-conversion device utilizing a Pt Schottky barrier formed on indium phosphide (InP) porous structure [1]. The photocurrents drastically increased in the Pt/InP porous device, whereas it remains an issue on the low photo-voltage. One possible approach to improve the conversion efficiency is to replace Schottky contacts with pn junctions which have a large built-in potential. In this study, we aimed to form the p-type cuprous oxide (Cu2O) films on n-type InP porous structures using the electrochemical deposition process.
The porous structures were electrochemically formed using a (001) n-type InP (n=1x1017 cm-3). The electrochemical cell has three electrodes: an n-type InP electrode as a working electrode (WE), a platinum (Pt) counter electrode (CE), and Ag/AgCl reference electrode (RE). These were electrically connected to the potentiostat with a pulse generator. The n-InP layer was first anodized at 5 V in a HCl-based electrolyte to form the porous structures. A disordered irregular layer formed and remained on the top of the ordered porous layer [2]. To remove this irregular layer, the porous surface was then photo-electrochemically etched at an anode bias of 1 V in the same electrolyte under illumination. After that, in order to form a thin Cu2O film on the wall inside the pores, the cathode bias, V
c, was applied to the WE in the electrolyte consisting of 0.5 M cupric sulfate and 3.0 M lactic acid. The conductivity and morphology of Cu2O films were strongly affected by the pH value of the electrolyte [3,4]. In this study, the pH of the Cu2O electrolyte was varied in range from 9 to 12 by adding a NaOH solution. The deposited films were characterized using a scanning electron microscope (SEM), X-ray diffraction (XRD), Auger electron spectroscopy (AES), UV-visible spectroscopy, and photo-electrochemical measurements.
In order to optimize the electrochemical process, Cu2O films were first deposited on indium tin oxide (ITO) plates by changing the V
c and pH values. From the SEM and photo-electrochemical measurements, the smooth and chemically-stable surface were obtained at V
c = -0.3 V and pH of 12.0. Under such an optimal condition, the top surface of Cu2O films was dominantly oriented in the (001) direction. After the deposition of Cu2O films for 30 min, the photocurrent measurements were conducted in a 1M NaCl electrolyte under the irradiation of monochromatic light. Figure 1 shows the photocurrents measured as function of light wavelength. As shown in the inset of Fig. 1, the cathode current was observed under illumination, but it wasn’t in the dark. This result suggests that the Cu2O film formed in this study showed p-type conductivity. The amount of photocurrents steeply increased around the wavelength of 580-620 nm whose photon energy was very consistent with the bandgap energy of Cu2O. These results suggest that the Cu2O films formed by the electrtochemical process are usefull as a light absorber to be formed on n-type semiconductors.
Figure 2(a) shows the cross-sectional image of the n-type InP porous structure formed by the electrochemical process. The average depth and diameter of the ordered pores is 7.1 µm and 100 nm, respectively. After the cathodic deposition, it was proved that Cu2O films were formed on n-type InP walls inside the pores, as shown in Fig. 2(b). Figure 2(c) shows the AES spectra obtained at the deep part of the InP porous sample after the Cu2O deposition. The AES signals from Cu and O were clearly observed at 920 eV and 510 eV, respectively. These results indicate that the electrochemical deposition of Cu2O films is very useful technique for the fromation of a pn junction on the high-density nanostructures with a good surface coverage.

[1] R. Jinbo et al., Thin Solid Films, 520 (2012) 5710.

[2] T. Sato et al., Electrochem. Solid-State Lett., 11 (2008) H111.

[3] R. Liu et al., Chem. Mater., 15 (2003) 4882.

[4] L. Wang et al., Electrochem. Solid-State Lett., 10 (2007) H248

Figure 1