Introduction: A transparent conducting oxide (TCO) thin film exhibiting high electrical conductivity and high visible light transparency has found considerable practical applications in solar cells and in transparent electronics [1]. Most of wide bandgap oxides and sulfides are inherently n-type. Therefore, realization of p-type TCOs can be a milestone to expand the utilization of TCOs in transparent electronics, because a wide variety of active functions of semiconductor elements come from pn junctions [2]. However, fabrication of p-type transparent materials has proved to be substantially challenging [3].

Methods and Results: In this work, Cu-doped Mg(OH)2 thin films are deposited electrochemically from an aqueous solution and annealed in air at 400 °C. The deposition solution contains Mg(NO3)2 and Cu(NO3)2. UV-visible experiments show high transmission larger than 90% for all the samples. According to X-ray photoelectron spectroscopy results, copper is in the Cu1+ charge state for as- deposited films whereas after annealing, its state is a mixture of Cu2+ and Cu1+. Those films are found to be amorphous by X-ray diffraction. n-type conductivity is identified for the as-deposited films by photoelectrochemical characterizations, and resistivity is of the order of 103 or 104 Ωcm. The annealed films are p-type or intrinsic with high resistivity of the order of 105 Ωcm.

Conclusions: Thus, both n-type and p-type Mg(OH)2 semiconductor films are fabricated with Cu doping and will be possibly applied for transparent electronics. According to the first-principles calculation, origin of n-type conduction could be Cu atoms at the interlayer sites whereas p-type conduction could be due to substitutional Cu.

Figure 1 shows the optical transmission spectra for the as-deposited and annealed samples. The transmission was around 90 % in the visible range. Figure 2 shows the conduction type of the films is n-type for as-deposited samples and after the annealing, for the samples with 1 and 5 mM Cu(NO₃)₂, there is no photoresponse, but for the sample with 10 mM Cu(NO₃)₂, clear negative photoresponse was observed, which indicates that the electron is the minority carrier, i.e., the conduction type is p-type.

Keywords: electrochemical deposition, conductivity, magnesium hydroxide.

References:

[1] Mallick A and Basak D 2018 Revisiting the electrical and optical transmission properties of co-doped ZnO thin films as n-type TCOs Prog. Mater. Sci. 96 86-110.

[2] Yanagi H, Inoue S, Ueda K, Kawazoe H, Hosono H and Hamada N 2000 Electronic structure and optoelectronic properties of transparent p-type conducting CuAlO2 J. Appl. Phys. 88 4159-4164.

[3] Quackenbush N F et al 2013 Origin of the Bipolar Doping Behavior of SnO from X-ray Spectroscopy and Density Functional Theory Chem. Mater. 25 3114-3123.