PLD Growth And Properties Of Non-polar ZnOS Thin Films

It has been a hot topic of the research about ZnO,since it has many excellent performances in photovoltaic,piezoelectric,pyroelectric,ferroelectric and other areas.Since the 1990 s,Room-temperature ultraviolet(UV)laser emission of ZnO microcrystallite thin films is reported,ZnO as a new type of photoelectric information functional materials caused the research upsurge.So far most reports have focused on c-plane oriented ZnO-based alloy semiconductors.However,the conventional c-plane oriented ZnO has a strong polarization field along the c-direction from the spontaneous and piezoelectric polarization,which may lead to separation of electrons and holes at space in quntum well and it will reduce the internal quantum efficiency of the photovoltaic device,and this phenomenon is defined as quantum confined Stark effect.Fortunately,in hexagonal structure,the growth direction in a plane or m-plane avoids quantum confined Stark effect due to the electric field perpendicular to the direction of polarization.On the other hand,anionic moiety substituted ZnOX(X = S,Se,etc.)research ternary alloy is also increasingly rise,by changing the content of anions in the film.It's important to study non-polar ZnOX both can avoids quantum confined Stark effect and regulate bandgap to realise high efficiency ZnO based photovoltaic device.In this paper,we use pulsed laser deposition to grow ZnOS films on a plane and m-plane sapphire substrate,respectively,and the ZnS ceramics used as ablation target,O2 as reaction gas.Substrate temperature and oxygen pressure have been optimized as growth conditions,and the structure and properties has been studied systematically on the non-polar ZnOS film.The main contents and results are as follows:1.High quality non-polar a-plane ZnOS films were deposited epitaxially on r-sapphire substrates by pulsed laser ablating a ZnS ceramic target in O2 atmosphere with a fixed substrates of 600?.The in-plane orientation relationship between ZnOS films and sapphire substrates was revealed by ZnOS[1100]||Al2O3[1120]and ZnOS[0001]|| Al2O3[1101].The lattice quality is anisotropy,the quality is best along the m-axis and worst along the c-axis.Both lattice constants a and c expand with increasing S content in the ZnOS alloys.Large lattice misfit along either ZnOS[0001]or ZnOS[1100]indicated a domain matching rather than lattice matching epitaxial mode for a-plane ZnOS on r-plane sapphire.Optical measurements show high transparency of the films in visible range.The band gap of ZnOS becomes narrower with S fraction increasing up to 43.18%,which behaves similar to the band gap of c-plane ZnOS films grown on c-plane sapphire,and Eg(ZnOS(x))= 3.275-3.575x +4.72x2(0%?x?43.18%)?2.The ZnOS thin films were fabricated epitaxially on m-plane sapphire by pulsed laser deposition.The growth experiments were carried out at a fixed O2 partial pressure of 2.5 Pa and substrate temperatures between 300 and 700 ?.X-ray diffraction(XRD),X-ray photoelectron spectroscopy,atomic force microscopy and UV-Vis-IR spectrophotometry were used to characterize the films.The experimental results indicate that the substrate temperatures during growing influence obviously the sulfur content in the ZnOS films.The structural,surface morphology,sulfur content and optical properties of the films at different substrate temperatures were investigated.The XRD analysis reveals that the ternary films change from multi-orientation to single-orientation,when the substrate temperature increases from 300 to 400 0C.The epitaxial ZnOS(1010)thin films are obtained at substrate temperature of above 400?.i.e.ZnOS[1010]||Al2O3[1010]with S content below 28.2 at.%.The XRD ?-scans demonstrate that the films possess an in-plane relationship with the substrate of ZnOS[0001]|| Al2O3[2110]and ZnOS[2110]|| Al2O3[0001].The transmittance of the films is about 90%in the visible range.The optical absorption edge increases from 2.94 eV to 3.05 eV with increase of the S content in the films from 0.115 to 0.282 at.%,and Eg(ZnOS(x))= 3.22-0.01973x + 0.000357x2(11.5%?x?28.2%).The lattice quality is also anisotropy,and the quality is best along the a-axis and worst along the c-axis.3.Non-polar a-plane and m-plane ZnOS films were grown epitaxially on r-sapphire and m-sapphire substrates respectively by pulsed laser deposition with a ZnO buffer layer.The ZnO buffer layers were deposited with a pure ZnO ceramic target at different temperatures,the ZnOS films were deposited with a ZnS ceramic target in O2 atmosphere at different temperatures,and we got single phase non-polar ZnOS films when both of the substrate temperature are higher.The in-plane orientation relationship between the ZnOS film,ZnO buffer layer,and the sapphire substrate was revealed by X-ray diffraction phi-scan measurements.Both a-plane and m-plane ZnOS films show a W-shaped variation of rocking-curve half-width as a function of phi,indicating an in-plane anisotropic crystallographic mosaicity.The transmittance of the films is about 90%in the visible range,and bandgaps of ZnOS estimated from the optical absorption edge are almost same.