Luminescent Hydrogenated Nanocrystalline Silicon Films
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ABSTRACT Hydrogenated nanocrystalline silicon (nc-Si:H) films with room temperature luminescence have been prepared in a PECVD system. Heavily H2 diluted silane and large negative bias accompanying low deposition temperature are used to decrease the crystalline size. The films comprise nanocrystallites surrounded by an interfacial phase with wurtzite structure characterized by 495cm-1 feature in the Raman spectrum. The X-ray diffraction spectrum shows the average grain size is about 4-5nn. The room temperature photoluminescence (PL) spectrum consists of two peaks, one at 2.21eV which may be induced by the transitions of the quantum subbands in the nanocrystalline phase, the other at 2.84eV which may be attributed to some kinds of localized centers in the interfacial phase. The photoluminescence excitation (PLE) spectrum also shows two peaks, a low one at 3.4eV approaching the bulk like transitions between F 2 5 ,,-rI 5 c while the high energy envelope around 5.0eV has a complicated configuration, and might be related to both the bulk like transition between A3v-A3cand band transitions of (Si-H-12). chains. Introduction Early in 1980's, D. J. Wolford et all reported that heavily hydrogenated amorphous silicon films prepared by HOMOCVD could emit strong visible PL at room temperature and the integrated huminescence intensity was five times stronger than that of GaAs 0 .6 P0 .4 , a typical merchant luminescent material. Later, visible PL at room temperature from nc-Si:H films prepared by sputtering in hydrogen atmosphere at low temperature (-100K) could also be observed2, and the PL intensity and efficiency were comparable to that of porous silicon (PS) reported by L. T. Canham3 in 1990. All these observations show that a-Si:H and nc-Si:H films have promising future in the applications of optoelectronics as well as thin film transistors, imaging sensors and photovoltaic devices. Furthermore, in comparison with anodization PS, aSi:H and nc-Si:H films have advantages in large area preparation, technological controllability and compatibility with the traditional IC technique. But usual high-quality a-Si:H films prepared by plasma enhanced chemical vapor deposition (PECVD) have no such visible PL characteristic. To find out the reason for this and try to prepare luminescent nc-Si:H films by means of using PECVD method, we have made some experiments. Here, we present the progress in preparation and characterization of luminescent nc-Si:H films in ur laboratory, and especially put the stress on the optical properties of nc-Si:H films, such as the optical 769 Mat. Res. Soc. Symp. Proc. Vol. 358 01995 Materials Research Society
constants, optical bandgap, IR absorption, Raman scattering, X-ray diffraction, PL and PLE spectra. Experiment The samples were prepared by capacitively coupled rfPECVD method in a three-chambers system using a gas mixture of silane and hydrogen. By alternating the deposition parameters we have realized the transition of amorphous to nanocrystalline silicon, and obtained nc-Si:H films with a wide opt
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