| Zinc sulfide (ZnS), as an important wide band-gap semiconductor compound of the II-VI groups, has attracted considerable attention due to its photoluminescence and electroluminescence, which enable wide applications in the fields of flat-panel displays, electroluminescent devices, sensors, lasers and light emitting diodes. The electron affinity of ZnS is about3.9eV and the optical band gap of ZnS is about3.74eV at room temperature, which makes it possible to be transparent to almost all wavelengths of the solar spectrum. Therefore, it may be possible to improve the cell performance by replacing CdS with ZnS-based buffer material to realize high-efficiency CdTe or CIGS solar cell. In addition, the lattice structure of ZnS thin film also plays an important role on the physical properties of the optoelectronic devices, it is due to the formation of the semiconductor heterojunction needs two kinds of materials with a low rate of lattice mismatch, and only two kinds of materials with similar lattice constants can be used to make a semiconductor heterojunction with excellent properties. Aa far as ZnS is concerned, the more stable structure is cubic phase at room temperature, and the temperature of phase transformation is1020℃. Preparing hexagonal ZnS thin films at such a high temperature necessarily leads to increase production costs. Therefore, in order to reduce production costs, it is of grave importance to develop a method in preparation of hexagonal ZnS thin films at low temperature.In this paper, we explore the preparation conditions of ZnS thin films, and then successfully prepared the hexagonal ZnS thin films and wurtzite-type ZnS/(CdS/ZnS)n (n=2,4,8) multilayer heterostructure films with a low rate of lattice mismatch by PLD method. The discussions are presented as follows.Firstly, the ZnS ceramics were successfully synthesized by the solid-state reaction method under different conditions, and the spectra were investigated from the measurements of photoluminescence. The transformation of structure from cubic (zinc-blende) to hexagonal (wurtzite) was observed with X-ray diffraction. The structure transformation of ZnS ceramics from zinc-blende below900℃to hexagonal wurtzite over1100℃was observed. Both the sintering temperature and time greatly influence the quality of ZnS ceramics; the hexagonal wurtzite ZnS ceramics with much better quality were obtained at the sintering temperature of1100℃and for100min sintering time, when the relative density reaches94%, and the stoichiometry of Zn and S is close to1:1. And from the room temperature photoluminescence (PL) measurements of these ZnS ceramics four emission bands located at～342,～407,～488and～525nm were observed. Thereinto, the emission band at405nm is related to the sulfur vacancies; the emission band at480nm is attributed to the zinc vacancies states; and the green emission at525nm, which may be related to line or planar defects.Secondly ZnS thin films have successfully prepared on uncoated and Au-coated sapphire substrates by pulsed laser deposition. The substrate temperature and Au catalyst had an important effect on the crystal growth of ZnS thin films. We have achieved controllable growth for the crystalline phase of ZnS film through appropriate adjustment of substrate temperature during the growth process and coating an Au-layer on sapphire in advance. SEM characterization confirms that ZnS film with Au coating had a smoother surface and few defects. PL spectra revealed two emission bands centered at～405nm and～520nm, respectively.Thirdly, high-quality ZnS thin films were successfully synthesized on uncoated and Au-coated Si (100) substrates by pulsed laser deposition (PLD). Results indicated that the films deposited on uncoated Si substrate had mixture of zinc-blende (cubic) and wurtzite (hexagonal) phases with the RMS of the surface roughness of～17.1nm, when deposited on Au-coated Si substrate, the films only had the wurtzite phase with RMS～13.3nm. After annealing treatment, ZnS thin films showed more uniform, more continuous and denser microstructures with fewer line or surface defects, and the RMS are9.95nm and4.361nm for the ZnS film deposited on uncoated and Au-coated substrates, respectively. Room temperature photoluminescence measurements with325nm excitation showed four emission bands located at405nm,480nm,525nm and588nm, respectively. The origin of the first three PL peaks are in accordance with the ZnS ceramics and yellow emission at588nm may be related to interfacial defects.In conclusion, wurtzite-type ZnS/(CdS/ZnS)n (n=2,4,8) multilayer heterostructure films have successfully prepared on sapphire substrate by pulsed laser deposition (PLD) at a low growth temperature of100℃. For these ZnS/CdS multilayer heterostructures in our experiments, the mismatch between their lattice parameters is only～3.0%. The cross-sectional FESEM characterization showed the mutually contacted CdS and ZnS thin layers and little interdiffusion between them in our samples. The obtained samples have a high optical transparency of above85%in entire visible range in these multilayer structures. In addition, comprehensive studies on Raman scattering were presented, and the characteristic first and second order longitudinal optical phonon modes for CdS, and multiphonon processes for ZnS were resolved, indicating high crystalline ZnS/(CdS/ZnS)n (n=2,4,8) multilayers. Furthermore, room temperature PL measurements showed two emission peaks in the blue (～405nm) and green (-504nm) wavelength range, respectively, and the PL peak emission intensity enhanced with increase in film layers for ZnS/CdS structures. In addition, the temperature-dependence of PL spectra for the samples proved that the origin of the green emission band, which is related to the near band edge (NBE) emission of CdS. |
PDF Full Text Request