真空中加热衬底对ZnSe MIS二极管电致发光的影响

本文研究了在制备ZnSe MIS二极管的绝缘层时,为了改善IS间的界面接触,在真空中加热ZnSe衬底,其结果虽然使电致发光的均匀性有所改善,却使原来的蓝色电致发光变为红色。文中着重研究了红色电致发光的起源,在液氮温度下出现的二个峰值为5350Å和6320Å的谱带应分别归结为ZnSe晶体中的铜绿(Cu-G)和铜红(Cu-R)发光中心。文中指出,真空中加热的条件,使ZnSe晶体中残留的Cu杂质从非发光中心状态转变为发光中心状态。因此,要改善ZnSe晶体蓝色电致发光的性能,进一步提高ZnSe晶体的纯度是十分重要的。

THE EFFECT OF HEAT TREATMENT FOR SUBSTRATE ZnSe IN VACUUM ON ELECTROLUMI-NESCENSE OF ZnSe MIS DIODES

There is a considerable interest in blue electroluminescence (EL) emitted by MIS diodes of ZnSe operated in forward-bias1-4].Recently it was reported4] that the homogeneity of blue EL has been improved. In this paper it is reported that the effect of heat treatment for substrate ZnSe during fabricating an insulating layer in vacuum on EL of ZnSe MIS diodes Nominally undoped ZnSe crystal; grown in this laboratory 6], were used in this study The boule crystals were grown using the method described by Cutter and Woods5] Dice with dimensions of 3×3×1.2mm3 were cut from the boules, and heated in molten zinc to reduce their resistivities. They were then polished and etched in bromine in methanol,it was followed that an oh-mic contact was made to one of the large area faces with a pellet of indium. A thick (500-1000Å) insulating layer of ZnSe was deposited on the opposite face by the evaporation heated by electron beam, during heating or not heating the substrate ZnSe Finally a circular gold electrode was evaporated on top of the ZnSe film EL emission spectra in forward-biased ZnSe MIS diodes were measured using a Model 44W spectrometer. Fig.1 and 2 showed EL emission spectra in ZnSe MIS diode at RT and LNT, respectively, when the substrate ZnSe was not (curve 1) and was heated (curve 2) during fabricating an insulating layer of ZnSe polycrystal. It was found that two deep center luminescence bands peaked at 6320A and 5350A appeared in EL emission spectra, when a substrate ZnSe was heated during the deposition of the insulating layer Fig.3 and 4 showed EL emission spectra in Cu-doped ZnSe MIS diodes. In comparing the EL emission spectra from the ZnSe MIS diodes with that from the Cu-doped ZnSe MIS diodes, it was obvious that two deep center luminescence bands peaked at 6320Å and 5350Å could be attributed to copper-red (Cu-R) and copper-green (Cu-G) centers of luminescence in ZnSe7,8]. It was reported that7,10] as for the atomic nature of centers, the Cu-R center was due to the donor-acceptor pair which was responsible for a substi-tutional copper ion(CuZn+)and a donorlike level of coactivator and the Cu-G center was responsible for an association of a substitutional coppor ion(CuZn+) with an interstitial copper ion (CuI+). Therefore both Cu-R and Cu-G centers included CuZn+ and Cu-G center Contained more amount of copper than Cu-R center Since purification of Ⅱ-Ⅵ compound semiconductors by firing treatment in molten zinc has been reported by Aven and Woodbury11] . This method has also been applied to ZnSe crystal in order to eliminate Zn vacancies and Cu impurities and reduce resistivities of ZnSe crystal However this firing treatment in molten zinc can not effectively remove residual copper impurity from ZnSe12]. The ZnSe MIS diodes, in which EL emission spectra contained the blue band only, did not include the copper centers of luminescence If the ZnSe dice were heated in vacuum,the Cu-G and Cu-R bands appeared in EL emission spectra. The results mean that although there were no copper center of luminescence in the ZnSe dice before heating the ZnSe dice, but copper impurity still existed in the ZnSe dice. When the ZnSe dice were heated in vacuum,zinc vacancies were formed on the surface of ZnSe dice due to Zn out diffusion, at same time residual copper impurity diffused into zinc vacancies and formed CuZn+. Thus it is possible to form Cu-R and Cu-G centers of luminescence. Fig.5 showed an insulating layer could be formed on the surface of ZnSe by heating the ZnSe dice in vacuum. It means that Zn vacancies could be produced by heating ZnSe dice in vacuum As the copper impurities diffused into Zn vacancies of the ZnSe dice and formed the copper center of luminescence during the deposition, the EL emitted from copper centers should be depended on both heating temperature and time. The observations shown in Fig 6 and 7 support this point of view. The ratio of luminescent intensity of blue band to that of copper centers of luminescence reduced with increasing heating temperature and time.