Previously, much attention was paid mainly to cadmium telluride CdTe, chalcopyrite compounds CuInSe , CuGaInSe , which were step by step replacing traditional crystal silicon plates . By replacing In/Ga by Zn/Sn in chalcopyrite, kesterite structure was obtained. This structure represents an alternative perspective material for light-absorbing layer in thin-filmed solar elements. Kesterite thin-filmed compounds Cu ZnSnSe attract attention of researchers due to their cost, environmental safety, and high efficiency of photovoltaic devices . Present research attempts are pointed on alloying ZnSe film with Cu or making triple compounds Cu-Zn-Se (CZSe) including solid solutions Zn Cu Se (х 0,10), which are able to replace multicomponent chalcopyrite and kesterite structures.
One of the typical representatives of A B compounds is ZnSe, which can be used for creation of semiconductor electronic devices and information display systems, as active laser medium, color TV displays, optical light modulators, and other optoelectronics devices. From the ecological point of view, it is very important that ZnSe is a nontoxic material due to absence of heavy metals in its compound.
The thin filmed copper (I) selenide is under increased attention of specialists in solar energy, micro- and optoelectronics, because it can express wide spectrum of semiconductor and specific electrophysical properties due to its elemental composition and microstructure. Band gap of copper (I) selenide thin films with nonstoichiometric compound Cu Se is 1.1–2.3 eV. These values are optimal for using Cu Se films as an absorbing layer and as a precursor for multicomponent kesterite Cu ZnSnSe structure. The present work is about chemical bath deposition of ZnSe and Cu Se thin films, which are the precursors for Cu ZnSe structure, and exploration of their elemental composition, morphology, and conductivity type.
Thin films of copper (I) selenide were obtained by chemical deposition from two reaction mixtures of different compositions. In the first case, the film was deposed from aqueous solution of copper chloride CuCl ·2H O and sodium selenosulfate Na SeSO with hydrochloric hydroxylamine NH OH HCl. The film obtained from this solution will be further referred to as C Se-1. In the second variant of synthesis, copper selenide film was obtained in the same conditions, but with addition of potassium thiocyanate KSCN to the solution as ligand for copper ions (Cu Se-2). To create reducing medium for conversion bivalent copper Cu into monovalent one Cu , hydrochloric hydroxylamine NH OH·HCl was added to the reaction mixture, because it has sufficient value of reducing potential .
Zinc selenide thin films obtained by chemical bath deposition method from the reaction mixture containing zinc chloride ZnCl , sodium citrate Na C H O (Na Cit), hydrochloric hydroxylamine NH OH HCl, and sodium selenosulfate Na SeSO were used as a chalcogenizer. To create alkaline medium sodium hydroxide, NaOH was used.
Sodium selenosulfate Na SeSO was obtained by dissolution of amorphous selenium in solution of sodium sulfite Na SO at the temperature of 363 K. As a substrate material for copper selenide thin films deposition, sitall plates ST-50-1 with size of 30 24 mm were used. The plates were degreased in advance. The synthesis of films was carried out at the temperature of 373 K in hermetic molybdenum glass reactors, which contained substrates fixed in specially prepared fluoroplastic holders. Reactors were placed in thermostat TS-TB-10, which ensured accuracy of maintaining the temperature with an error 0.1 . Duration of the synthesis was 120 min.
Films thickness was measured with interference microscope (Linnik's microinterferometer) MII–4M with measure error of 22%.
Study of structure-morphological characteristics and elemental compound was carried out by scanning electron microscopy method using MIRA3LMV microscope at accelerating voltage of electron bundle of 10 kV and scanning electron microscope JOEL JSM-5900 LV with attachment for energy-dispersive (EDX) analysis (EDS IncaEnergy 250). Accuracy in determination of the film's elemental composition was approximately 10%.
Conductivity type of deposed layers was determined using hot probe having the sign of thermoelectric power compared with semiconductor silicon of KDB brand.
Thermodynamics calculations allowed revealing concentration areas and optimal pH values for the formation of mono- and bivalent copper (Figures 1 & 2) and zinc (Figure 3) selenide, as well as hydroxide phases. It made significantly easier to form the reaction mixtures concentration composition.
As a result of chemical deposition at the temperature of 363 K from reaction mixtures containing 0.2 mol/L of CuCl , 0.1 mol/L NH OH HCl, 0.04 mol/L Na SeSO (рН = 3.3), and with addition of 0.001 mol/L of KSCN (pH = 4.0) to bath with the same composition, we synthesized homogenous mirror films Cu Se-1 and Cu Se-2, grey colored, with good adhesion to sitall substrate (Figure 4).
As a result of chemical deposition at 353 K from reaction mixture containing 0.03 mol/L ZnCl , 0.35 mol/L Na C H O , 0.36 mol/L NH OH HCl, 0.99 mol/L NaOH, and 0.04 mol/L Na SeSO (рН = 10.5), mirror films of ZnSe were synthesized having lemon-yellow color with good adhesion to sitall substrate (Figure 5).
Thickness of synthesized layers varied from 100 to 800 nm depending on the reaction mixture composition and temperature of the synthesis.
The results of EDX-analysis made us to conclude that in deposed layers there was almost exact ratio equal to two between main elements Cu/Se. Therefore, by chemical bath deposition in used reaction mixture, stoichiometric layers of monovalent copper selenide Cu Se were obtained. Stoichiometry ratio of basic elements Zn and Se (51.61 and 48.36 at. %, correspondingly) is not respected. Nonstoichiometry can be related with presence of oxygen-containing phases in form of zinc hydroxide in the film. This was predicted by thermodynamic calculation: with pH 10 almost 50% of metal exists in the form of neutral hydroxo-complex Zn(OH) .
According to carried out research, freshly-deposited Cu Se and ZnSe layers have a hole-type conductivity.
Analysis of ion equilibrium at the temperature 298 K in the reaction systems `CuCl – NH OH HCl – Na SeSO , `CuCl – NH OH HCl – Na SeSO – KSCN', and `ZnCl – Na C H O – NH OH HCl – H O' established prevailing copper and zinc complex forms. The border conditions and concentration areas of mono- and bivalent copper, as well as zinc selenides formation were determined. The possibility of arising admixture phases in the form of copper and zinc hydroxides was evaluated. Alkaline area of pH was shown to be the most favorable for chemical copper (I) selenide solid phase deposition.
Energy dispersive analysis established that using sodium selenosulfate as chalcogenizer and presence of hydrochloric hydroxylamine in the solution provided creation of reducing medium and transformation of bivalent copper into monovalent state, leading to the formation of Cu Se solid phase. It was established that using sodium selenosulfate as chalcogenizer provided formation of ZnSe solid phase characterized by nonstoichiometry of selenium, which could be related to formation of zinc oxygen-containing phase of Zn(OH) apart from metal selenide.
Copper (I) selenide films with stoichiometric composition were obtained by chemical bath deposition from the explored reaction mixtures on sitall substrates. Thickness of the films depended on initial conditions, being in the range of 100–500 nm; the films had a good adhesion to the substrate. According to electron microscopy, the films were formed from crystallites with size of 80–450 nm.
From citrate-hydroxylamine reaction mixture by chemical bath deposition, semiconductor films of zinc selenide were obtained. The films had a good adhesion to sitall substrate; their thickness was 100–800 nm depending on the given conditions. According to electron microscopy, ZnSe film consisted of globular formations with average size in substrate plane 200–500 nm. These formations, in their turn, were made of nanoglobules with 30–50 nm size.
Thermoelectric power research proved that obtained Cu Se and ZnSe semiconductor layers had a hole type conductivity.