Growth And Properites Of Novel Semiconductor Materials

As the representatives of novel semiconductor materials developed in recentyears, Indium Nitride (InN), hydrogened nanocrystalline silicon (nc-Si:H) andgaphene have attracted great interests. In this thesis, we discusse the epitaxial growthof InN, nc-Si:H and graphene thin films by using radio-frequency magnetronsputtering, plasma enhanced chemical vapor deposition (PECVD) and face-to-face(FTF) expitaxial growth method, respectively. With the aid of various analysismethods, such as X-ray diffraction (XRD), atom force microscopy (AFM), lowenergy electron diffraction (LEED), Raman spectra and angle-resolved photoemissionspectroscopy (ARPES), the film growth mechanisums and the the effects ofprocessing parameters on material structure are studied. Moreover, we also investigatecarrier transport properties of InN and nc-Si:H by employing temperature-andmagnetic field-dependent Hall measurements as well as weak localization andquantum interference theories.As a member of III-V compound semiconductors, InN shows great applicationprospect in optoelectronic devices, ultra-high speed microelectronics devices,ultra-high frequency microwave devices and photoelectric integration. Because InNhas low thermal decomposition temperature (600℃), it is critical to process thegrowth in the circumstance with low temperature. Here, RF magnetron sputteringtechnique is applied. By comparing crystallization status and surface morphology of samples prepared under different conditions, we discuss the effects of sputtering pressure and substrate temperature on InN quality. It is found that10mTorr is the best sputtering pressure, in which InN with the highest crystallinity forms. The optimized substrate temperature varies with different sputtering pressure. In general, InN films grown in low temperature shows better quality.During the process of growth, it is inevitable that various impurities and defects will be introduced into films, which can cause structural disorder. By the aid of temperature-and field-dependent Hall measurements, the effects of disorder on carrier transport properties have been studied. The weak localization theory predicts that under low temperature, lattice vibration is suppressed while elastic scatterings dominate over inelastic scatterings, and the phase memory of electron wave causes strong quantum interference that enhance the probability with which an electron returns to its original position after a series of scattering events over a closed path. In magnetic fields, electron wave gains an additional phase which breaks the time reversal symmetry, resulting in positive magneto-conductance. If spin-orbit interaction exists in the system, electron spin relaxation will cause negative magneto-conductance. With this theory model, we extract electron inelastic dephasing time from the experimental data. Considering that electron-phonon (e-ph) scattering is the dominant inelastic process in InN films, we investigate the dependence of e-ph scattering rate (τe-ph-1) on disorder. In InN semiconducting disorder system, the e-ph scattering rate is characterized by the dependence τe-ph-1∝T310in the pure case (qT1>1) and inclines to obey the law of τe-ph-1∝T21-1as the films because disordered (qT1<1). This result complies well with the theoretical prediction proposed by Sergeev et al., and provides experimental evidence for their theory.Silicon quantum dot film represents the low dimension direction of semiconductor materials. Hydrogenated nanocrystalline silicon (nc-Si:H) thin film, a new low-dimentional artificial semiconductor material, where nano-scaled silicon grains are embedded in Si:H amorphous tissues, is a natural quantum dots (QDs) system.As a widely used industrial technology, PECVD is one of the common methods to grow nc-Si:H. The thermal equilibrium equation demonstrates that the formation of nc-Si:H film is the result of constrain and competition between the forword and reverse reactions. The forword reaction is the process in which silane (SiH4) is decomposed and silicon thin film deposites on Si substrate; while the reverse reaction is the one in which hydrogen plasma etches away those weak bonding Si-Si in the film. Hydrogen plays an important role during the formation of Si nanocrystal grains, by providing the energy for crystallization and participating in the control of nucleation and growth of the grains.The formation of Si nano-crystal not only improves the structural disorder, but also causes lots of unique electricial and electric transport properties of nc-Si:H. In this thesis, researches on electron spin relaxation mechanism are carried out by measureing low temperature Hall effects. The experiement shows that in low temperature, quasi-2D hopping conducting behavior is the dominant transport mechanism, and weak anti-localization phenomena has been observed. We successfully fit the magneto-conductivity data with HLN theory and extract two important transport parameters-inelastic scattering rate τi-1and spin-orbit scattering rate τso-1. The variation of τi-1with different temperature indicates that the dephasing process in Si coupled QDs is dominated by the small energy transfer mechanisum, in which triple channel interaction plays an important role. We also observe that the spin-orbit scattering rate decreases exponentially with1/T and tends to a steady value with T below3K. This behavior demonstrates that spin relaxation in Si couple QDs structure happens not only in the hopping process from one dot to another but also in the inner of Si QDs.At the end of this thesis, we study the synthesis of graphene. Graphene is a flat monolayer of carbon atoms tightly packed into a two-dimensional (2D) honeycomb lattice, and is a basic building block for graphitic materials of all other dimensionalities. It can be wrapped up into OD fullerenes, rolled into1D nanotubes or stacked into3D graphite. As a unique zero gap semiconductor, since its independentform was discovered in nature in2004, graphene has become the research focus in thefield of chemistry, material science and physics, due to its mechanical, thermal,electrical, optical and other aspects of excellent properties as well as great applicationpotentials.Large-scale template growth of graphene is the main obstacle hindering itsapplication. Therefore, how to prepare high-quality large-scale graphene is the focusof current research. Among all the synthesis methods, epitaxial growth on siliconcarbide (SiC) has been considered as one of the most effective way to realizeindustrical preparation and production. In this method, single crystal SiC is heated tohigh temperature to decompose, in which silicon atoms evaporate from the surface,while carbon atoms are left. After a series of reconstructions, graphene forms. Usually,graphene prepared under vacuum circumstance will not be larger than100nm. In thisthesis, we propose a new method, called FTF (face-to-face) epitaxial method, andsuccessfully grow graphene flakes in mirometer scales. Two pieces SiC are orientedso that the Si-terminated surfaces face each other and simultaneously heated. Undercertain temperature, both pieces act as sources and sinks of SiC on the opposingsurface, and homoepitaxial growth of SiC occurs, which improves the initial surfacemorphology before graphene forms. On the other hand, the close proximity of the twosurfaces partially traps Si atoms which sublimate from each heated surface, restrictingthe net rate of Si sublimation from the substrates, which allows large pieces ofgraphene to be formed.This work is suppoted by the National Science Foundation of China underconstract Nos.10734020and11074169, National Major Basic Research Project of2010CB933702, and Shanghai Key Project of06JC14039.