Recent Progress in Microcrystalline Semiconductor Thin Films
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them, has been most extensively studied because of its advantages such as the capability of lowtemperature processes in a range of 150 - 3000 C and also the easiness to deposit a large area film, which is compatible with the a-Si:H technology. There have been reported a lot of variations in PECVD so far. As for a reactor srtucture, in addition to conventional capacitively-coupled (diode)[5] and inductively-coupled reactors [3], a triode reactor has been used for controlling the amount of ionic species impinging on the surface [11] and/or selecting long-lifetime precursors from the SiH4 plasma [121. Very high frequency glow discharge (VHF-GD), in a frequency range of 50 - 120 MHz, has pruduced Rc-Si:H films of larger-size grains with higher deposition rates than the conventional 13.56-MHz GD [13]. Higher deposition rates have also been achieved by electron cyclotron wave resonance (ECWR) for plasma excitation [14] and plasma-gun CVD [151. SiFnHm or SiF4,I2 has been used as source gases with emphasis on chemical annealing in the film growth process [16]. Remote plasma [17] and hot filament [18] techniques emphasize soft reaction on the growing surface free from ion bombardment. Those techniques and approaches have led to a significant improvement in film properties of RcSi:H and also improved our understanding of the growth mechanisms. However, in all of the above preparation methods, excitation reactions of all the gases are coupled, and depositon reactions compete with other surface processes such as diffusion and desorption of adsorbates and etching. In this sense, the use of time-modulated gas flows enables us to separate the gas-phase reaction from the surface modification process. Actually, gas-flow modulation methods have been widely introduced for preparing itc-Si:H in a layer-by-layer (LBL) manner, and also for deeper understanding of the microscopic process of the film growth [19-22]. STRUCTURAL PROPERTIES Macroscopic observations as well as microscopic characterizations have been extensively made on the structure of itc-Si:H films using a variety of techniques : X-ray diffraction [23], highresolution TEM [13,24], atomic force microscopy (AFM) [25], scanning tunneling microscopy (STM) [12], spectroscopic ellipsometry [26,27], Raman scattering [28-32], infrared absorption [5, 33], proton NMR [34, 35] and ESR [36, 37]. All of those structural investigations show that vcSi:H films are highly inhomogeneous, being composed of small crystallites with an average grain size 8 of about 40 - 400 A, embedded in an amorphous matrix with some fraction of voids. The relative amount of crystalline phase contained in the film is described by the crystalline volume fraction Xc, which can vary from a few percent up to higher than 90% in fully-crystallized specimens. Hydrogen atoms, most of which are bonded with silicon atoms, are included in the film with the content CH of about 3 - 15 at.% in a spatially-inhomogeneous manner, as is described below. Morphology Macroscopic inhomogeneity along the growth direction of Rc-Si:H f
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