Effects of Deposition Temperature and Film Thickness on the Structural, Electrical, and Optical Properties of Germanium
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Effects of Deposition Temperature and Film Thickness on the Structural, Electrical, and Optical Properties of Germanium Thin Films William B. Jordan and Sigurd Wagner Department of Electrical Engineering, Princeton University, Princeton, NJ 08544 ABSTRACT Germanium films deposited on glass by plasma-enhanced chemical vapor deposition from germane and hydrogen grow in the structure succession of amorphous-nanocrystallineamorphous-nanocrystalline as the substrate temperature is raised from 30°C to 310°C. We ascribe the phase formation, from low to high temperature, to a sequence of low to high mobility of Ge growth species on a surface that is hydrogenated at low temperature but not hydrogenated at high temperature. We report some structural, optical, and electrical transport properties of Ge films as a function of deposition temperature and film thickness. INTRODUCTION Subsidiary to microcrystalline silicon (µc-Si:H), microcrystalline germanium (µc-Ge:H) has recently attracted interest for possible use in low gap solar cells and long-wavelength photodetectors due to its high optical absorption coefficient and conductivity, and its low optical gap. A working prototype µc-Ge:H p-i-n photodetector already has been demonstrated [1]. Similar to µc-Si:H, µc-Ge:H likely will not exhibit the light-induced degradation of amorphous material. We report the result of a study of the phase diagram of hydrogenated amorphous germanium (a-Ge:H) and nanocrystalline germanium (nc-Ge:H). When films are grown by plasmaenhanced chemical vapor deposition (PECVD) from germane (GeH4) and hydrogen, their structure exhibits a succession of a-nc-a-nc as the growth temperature is raised from 30°C to 310°C. a-Ge:H [2,3] and nc-Ge:H [4-13] remain poorly understood materials, but the a-Si:H – nc-Si:H phase diagram has been studied carefully [14,15] and at most three phases, a-nc-a, have been reported [16,17] to exist in temperature space. When deposited by PECVD from mixtures of silane with much hydrogen, silicon films typically begin growing with an amorphous (a) structure and then gradually evolve nanocrystalline (nc) as the film grows several 100-nm thick [15,18,19]. Nanocrystallization has been observed to begin directly on the substrate [20], and even to proceed backwards into the initially-deposited bottom amorphous layer [14]. These observations have led to the concept of an evolutionary phase diagram [15] for a-Si:H – nc-Si:H, which is applied as a non-equilibrium concept to describe the evolution from a-Si:H toward ncSi:H with film thickness and growth time, and as a function of the substrate material. One study has been performed for Ge and found four phases, a-nc-a-nc, over the temperature range of 150°C through 350°C [12,13]. We conducted a similar study over much of the same temperature range (30°C to 310°C), but at lower power, higher pressure, and higher hydrogen dilution, and found the same four phase transitions, but shifted to lower temperature. The succession of four phases accessible by variation of substrate temperature Tdep wi
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