Amorphous Silicon-Based Superlattices and their Potentials to Device Applications
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AMORPHOUS SILICON-BASED SUPERLATTICES AND THEIR POTENTIALS TO DEVICE APPLICATIONS MASATAKA HIROSE, NAOKI MURAYAMA, SEIICHI MIYAZAKI, AND YOHJI IHARA, Department of Electrical Engineering, Hiroshima University, Higashihiroshima 724, Japan ABSTRACT Two types of amorphous silicon (a-Si:H)/silicon nitride (a-Si 3 N4 :H) multilayers have been prepared either by turning-off the plasma at each step of individual layer growth (step by step deposition) or by quick gas switching without interrupting the plasma (continuous deposition). It is found that the continuous deposition causes a significant mixing of nitrogen into the a-Si:H well layers and deteriorates the compositional abruptness in the a-Si:H/a-Si 3 N4 :H interface. Also, undesirable incorporation of nitrogen into the a-Si:H well layers tends to induce nonradiative recombination centers near the interface. 1. INTRODUCTION Quantum mechanical effects in crystalline semiconductor superlattices have been extensively studied so far for understanding the fundamental physics as well as for utilizing the structures as novel electron devices. In recent years, ultra-thin layered structures fabricated from the amorphous silicon (a-Si:H) family have also shown many interesting properties which could be interpreted in terms of the quantum size effects as in the case of crystalline heterojunction superlattices [1-5]. The heterojunction interfaces in amorphous silicon-based multilayers appear to be atomically smooth [4.6] although a significant amount of interface states and space charge doping effect in the interface region were often, observed. In this paper, two types of a-Si:H/a-Sil-xNx:H multilayers were prepared and their physical properties were compared: In type I the individual layer was continuously deposited by quick switching of the gas compositions without turning-off the discharge (continuous deposition), while in type II the plasma was turned-off after individual layer growth and the reactor was completely purged after a-Si 3 N4 :H deposition (step by step deposition). The structural, optical, and electrical properties of such superlattices are systematically investigated to reveal how these properties are influenced by the compositional mixing in the heterojunction interface. 2. SAMPLE PREPARATION Amorphous semiconductor superlattices consisting of alternating layers of a-Si:H and stoichiometric a-Si 3 N4 :H have been fabricated by using an rf capacitively-coupled reactor. Quartz, corning 7059 glass, and c-Si substrates were placed on the grounded electrode with a diameter of 12 cm separated by a 4 cm gap from the powered electrode. Deposition conditions for the potential well and barrier layers are summarized in Table I. Stoichiometric a-Si 3 N4 :H barrier layers have been grown from a SiH 4 +NH 3 gas mixture with a molar ratio of [NH 3 ]/[SiH4 ] = 10. The growth rate for a-Si:H well layer is 0.17 A/sec and for a-Si 3 N4 :H barrier layer 0.33 A/sec. The residence time TR of the source gases in the reactor is estimated to be 1.6 sec which is short enough compared
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