Fumitaka Goto

Structural and optical characterization of CdS films grown by photochemical deposition

CdS thin films are grown by photochemical deposition from an aqueous solution and characterized by x-ray diffraction (XRD), Raman spectroscopy, photoluminescence measurement, and optical transmission spectroscopy. The films are deposited at room temperature and annealed at temperatures up to 500u200a°C. The as-deposited film is dominantly zinc blende cubic. The cubic phase remains dominant until the annealing temperature becomes higher than 400u200a°C. By the annealing at 450u200a°C, the XRD pattern turns to that of hexagonal phase. Moreover, its peak width decreases and the near-band-edge luminescence begins to be observed. The band gap is decreased by annealing below 400u200a°C and then abruptly increased by the annealing at 450u200a°C. This annealing behavior of the band gap is interpreted considering the quantum size effects, the band tail due to disorder, and the cubic-hexagonal transition.

A New Technique of Compound Semiconductor Deposition from an Aqueous Solution by Photochemical Reactions.

CdS films were successfully formed on a glass substrate in an aqueous solution containing S2O32- and Cd2+ ions by photochemical reaction. S2O32- ions in the growth solution absorb ultra-violet light of wavelengths shorter than about 300 nm, and the excited S2O32- ions supply sulfur atoms and electrons to the metal ions such as Cd2+. Thus, the formation reaction of the sulfide semiconductor occurs in the only illuminated region. Photochemically deposited CdS thin films were polycrystalline of hexagonal structure. The composition of the films became stoichiometric by the annealing at temperatures higher than 300° C.

Defect reduction in electrochemically deposited CdS thin films by annealing in 02

Abstract Using the electrochemical deposition method, CdS thin films were deposited from acid solutions (pH = 2.5) containing CdS0 4 and Na 2 S 2 0 3 on indium-oxide coated glass substrates. These films were annealed in N 2 , air, or O 2 atmosphere at 200–500°C for 30 min. Photoluminescence spectra were measured at 77 K. For the films annealed in N 2 , the band edge emission became weaker and the luminescence due to defects shifted to longer wavelengths as the annealing temperature was raised above 300°C. However, for the films annealed in air or O 2 , the band edge emission was observed strongly irrespective of the annealing temperature and the luminescence due to defects was weak. Thus the O 2 annealing is useful for the defects reduction.

Photochemical Deposition of CdS from Aqueous Solutions

CdS was deposited on a glass substrate by photochemical deposition (PCD). The substrate was held in an aqueous solution containing and and was irradiated with a mercury lamp. Then a CdS film was deposited in the irradiated region of the substrate. The deposition rate depended on various experimental parameters, such as pH of the solution, light intensity, and stirring speed, which were investigated. Here we discuss the deposition mechanism. It was shown that the deposition rate is determined by (i) density of nucleation sites on the substrate, (ii) light intensity, and (iii) transport of the ions in the solution. In the stirred solution, the former two factors are dominant. It was also demonstrated that a pattern can be made on the film using a mask. This proves that PCD has good controllability and thus is well suited for fabrication of devices such as solar cells.

Raman and photoluminescence characterizations of electrochemically deposited ZnxCd1-xS layers

Abstract CdS, ZnS, and Zn x Cd 1 − x S alloy were deposited from acidic solutions by the electrochemical technique, and characterized mainly by optical methods. Raman scattering was observed even for as-deposited films, and the alloy composition x was determined from the longitudinal-optical phonon frequency. Photoluminescence (PL) was detected at 77 K after annealing at 300 °C. The peak energy for Zn x Cd 1 − x S is larger than that of CdS, as expected from the larger bandgap of the alloy. The PL peak was red-shifted by annealing at higher temperature, which was attributed to defect formation during the annealing.

Electrical and Optical Properties of CdS Films Grown by Photochemical Deposition From Aqueous Solutions

Recently, we have established a novel deposition technique for semiconductor thin films, namely photochemical deposition (PCD), in which the compound formation is activated by ultraviolet illumination. In the present work, the electrical properties of the PCD-CdS films grown under different conditions are analyzed and presented. Their resistivity is high at room temperature and decreases with increasing temperature with an activation energy of about 0.9 eV.

Photoluminescence and Raman Scattering of Electrochemically Deposited CdS Films

CdS films were electrochemically deposited from acidic solutions containing CdSO4 and Na2S2O3. Photoluminescence spectra of these films were measured using the 325 nm line of a He–Cd laser as the light source at 77 K. For the films annealed at 300° C, we observed overlapping peaks at 488 and 502 nm. The peak at 488 nm was assigned to the band-edge emission of CdS. The peak at 502 nm was thought to be due to point defects generated by annealing. As the annealing temperature was raised, the band-edge emission became weaker and the luminescence due to the point defects shifted to longer wavelengths. These results were compared with the results of Raman scattering.