| As the most important part of the third generation wide band semiconductor materials, III-V compound semiconductor Gallium Nitride(Ga N) owns a direct wide bandgap(~ 3.4 e V), high electron mobility, good thermostability and chemical stability. Ga N has been widely used in the fields of light emitting diodes(LEDs), laser diodes(LDs), high electron mobility transistors(HEMTs), solar cells, et al. To our knowledge, Ga N is the most important semiconductor material that can realize high efficient blue LEDs and LDs in mass-production scale. Si is the most important and widely used semiconductor in modern electronic industry for its technical maturity, easy to integrate, elements abundance in earth’s crust and low prices. Advanced MOS technology spurs the silicon-based integrated circuit(IC) developing as Moore’s Law. However, Si has an indirect bandgap semiconductor which led to low emission efficiency, usually applying to optical and photoelectrical devices by compositing other direct-bandgap semiconductors. Therefore, combination of Ga N and Si can largely improve the capacity of optoelectronic devices, which may be of great help to information transfer in future. Due to the large lattice and thermal mismatch between Ga N and Si, conventional wafer-bonding or hetero-epitaxy to realize integration of Ga N and Si cannot obtain good quality Ga N films. An intermediated layer like metal alloy or Al N buffer or nano-patterned silicon substrate that relieve strain in three-dimensions are usually needing to obtain strong bonding or good crystal quality films. Realize low defect and crack density caused by lattice and thermal mismatch. We used silicon nanoporous pillar array(Si-NPA) as a functional substrate to grown Ga N nanocrystalline, realized direct contact of Ga N and Si, and construct a novel prototype device of Ga N/Si-NPA nanoheterojunction. The device showed rectification effect and yellow electroluminescence under forward bias. Based on these previous results, the Ga N growth mechanism, the construction of Ga N/Si-NPA devices, electrical properties and photodetecting properties are studied in detail in this dissertation. The mean researches conducted are list as follows.(1) Research on synthesis and Photoluminescence properties of Si-NPA.The Si-NPA was synthesized by hydrothermal method and the photoluminescence properties were measured under room temperature. The PL of Si-NPA shows a blue peak at about 430 nm and a red peak at about 640 nm. The blue PL peak is related to oxidation of Si-NPA. The luminescence intensity of blue peak increases with oxidation degree until reach saturation, and the oxidation degree increase with storage time until reach saturation to maximum. The energy of blue PL peak changes with exciting light. The blue PL peak is assigned to radiative recombination of oxygen-related defect state in the incompleted oxidation layer, and the energy of blue PL peak differs with exciting light due to electron transition of different defect energy band. The red PL peak is relatively stronger and the peak position is shift to red with the wavelength increase of exciting light. The red PL peak is related to band-to-band emission of nano-crystalline and quantum confinement effect. The feature size of nc-Si is about 4.5 nm and the energy band broadens even cracks due to quantum confinement effect. The indirect bandgap semiconductor silicon of 1.1 e V turns to direct bandgap of 2.1 e V and obtain the strong red PL peak at room temperature.(2) Study on the controllable growth and growth mechanisms of Ga N/Si-NPA.Using Si-NPA as deposition substrate, high purity metal Ga and ammonia gas as precursors, Ga N were grown by Chemical Vapor Deposition(CVD) technique and the role of Pt catalyst is studied. Ga N cannot be deposited on substrate without catalyst. Pt-Ga alloy is obtained by gathering Ga vapor in high temperature process without ammonia inlet. The self-organization effect of Pt-Ga alloy and ammonia pressure is essential to the morphology of as-grown Ga N. The saturation vapor pressure of Ga is low, which is hard for Ga atoms to deposits and nucleate. The enrichment of Ga obtained in Pt droplet by excellent solubility. Ga N is easy to synthesis by chemical reaction of enriched Ga and N. The effect of temperatures, ammonia pressure on the morphology and structure of obtained Ga N is also studied. Different nanostructures such as Ga N nanoparticals, nanorods, nanocone-strings and nanowires are obtained by changing growth conditions.The Ga N grown by vapor-liquid-solid mechanism under low ammonia flux:(i) enrichment of Ga source. Ga atoms can be gathering in Pt droplet due to excellent solubility.(ii) nucleary. The ammonia is decomposed under high temperature and Ga N is synthesized by enriched Ga and N atoms bonding together. However, Ga N depositing and nucleating is related to temperature. Low temperature led to matastable state nucleus and critical nucleus which cannot grow.(iii) nuclei growth. The nuclei are growing up by more Ga N deposited. Ga N is preferred to grow along c-axis to form nanowire and hexagonal prism under relative low temperature, which is called top growth model. Ga N is preferred grow perpendicular to c-axis to form nanocone and cone-strings.The vapor-solid mechanism is followed in Ga N growth under high ammonia flux, Ga vapor and N vapor impacting and bonding together to form Ga N molecules, and Ga N molecules depositing on the substrate and nucleating. Though the Ga N preferring to grow along c-axis, the growth speed of perpendicular to c-axis is also fast, which results big feature sized hexagonal prism and micro-wafer.Therefore, the morphology and structure of Ga N/Si-NPA can be regulated by control the growth conditions like temperature and ammonia flux rate, and the thickness of Ga N films can be controlled by growth time.(3) The luminescence properties and electrical transport mechanisms of Ga N/Si-NPA.We choose aluminum(Al) as anode electrode material by vacuum evaporation method, found Al electrode and polished Si-NPA is a good ohmic contact after annealed under argon atmosphere. ITO film used as cathode electrode material by vacuum magnetron sputtering and also obtain ohmic contact. The I-V curve of Ga N/Si-NPA has a obvious rectifying effect, and the thickness of depletion layer and build-in field depends on doping density and carries density. Under relative low forward bias, only the electrons that energy higher than peak barrier can transport to p-type Si-NPA from Ga N. An exponential relationship was formed between current and voltage under low forward bias, which can be described by heterojunction thermionic emission theory. Under relative higher forward bias, carriers injected by electrical field are beyond the equilibrium formed space charges, and space charges affect the space electrical field which could affect the drift current. A square relationship formed between current and voltage eventually, which can be satisfied with space-charge-limited-current model. While Ga N/Si-NPA is under reverse bias, the energy band turns tilt, which can make the valence band of Si-NPA higher than the conduction band of Ga N. The tunneling length ?x become short and the Zener tunneling taken place when the energy of electron in Si-NPA higher than energy gap.The electroluminescence spectrum of Ga N/Si-NPA covers visible light from 400-750 nm. The EL intensity changes with the applied voltage and the luminescence peak and central position fixed. The EL intensity is related with thickness of Ga N film that thick film results low luminescence intensity. The EL is assigned to radiative recombination of electron and hole from depletion layer of Ga N, therefore, thick film blocks the light emission from depletion layer and led to a low luminescence efficiency. The intensity increase with as applied voltage until reach saturation, and then decrease with the voltage increase continuously. When the applied voltage reach maximum, the device temperature rise due to high power applied, and the non-radiative recombination dominate results low luminescence efficiency. |
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