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Title: Growth and properties of high-quality textured and heteroepitaxial AlN thin films
Other Titles: Gao pin zhi zhi gou ji yi zhi wai yan dan hua lü bo mo de sheng zhang yu xing neng yan jiu
Authors: Yao, Zhiqiang (姚志強)
Department: Department of Physics and Materials Science
Degree: Doctor of Philosophy
Issue Date: 2008
Publisher: City University of Hong Kong
Subjects: Thin films -- Mechanical properties.
Notes: CityU Call Number: TA418.9.T45 Y37 2008
xxii, 131 leaves : ill. 30 cm.
Thesis (Ph.D.)--City University of Hong Kong, 2008.
Includes bibliographical references.
Type: thesis
Abstract: AlN (aluminium nitride) has a set of outstanding physical properties including the widest direct band gap (6.2 eV) among III-nitride semiconductor materials, high thermal conductivity (140-280 W/mK), great piezoelectric response, negative electron affinity, and dopability for both p- and n-type conductivities. The combination of these excellent properties makes AlN a promising material for extensive applications in, e.g., deep-ultraviolet (UV) light-emitting diodes (LEDs), laser diodes, bulk acoustic wave devices, silicon-on-insulator electronic devices, and field emission displays. An AlN (p-type/intrinsic/n-type) p-i-n homojunction LED with an emission wavelength of 210 nm, being the shortest one reported for LEDs so far, has been achieved. AlN is also widely employed as buffer layers for the heteroepitaxial growth of other III-nitride materials such as GaN on AlN due to their small lattice mismatch. Moreover, AlN is superior to ceramic films such as ZnO and Pb(Zr,Ti)O3 for applications in piezoelectric devices due to its high elastic modulus and small temperature coefficient of resonance frequency. Deposition of wurtzite AlN films with high crystal quality (free of amorphous interfacial layer, with the minimum film/substrate interface reaction, etc.) on Si (100) surfaces is still a changllenge for growth methods attempted including molecular beam epitaxy (MBE), metal-organic vapor phase epitaxy (MOVPE) and magnetron sputtering (MS). The ultrahigh growth temperature (up to 1000 oC or above) adopted by MBE and MOVPE induces a large interface reaction due to the eutectic temperature for the Al-Si system at 577 oC. In addition, the nitridation reaction of Si, which is known to be responsible for the formation of amorphous SiNx interlayer, is also serious at elevated substrate temperature. A thin disordered amorphous layer is always observed at the AlN/Si interface for the AlN films grown by sputtering on Si (100) substrates even at the substrate temperatures below 350 °C. The eixistance of interfacial amorphous nitride/oxide layer is believed to result in poor crystal quality of the AlN films deposited, however, the mechasnism how the crystal quality of AlN films is correlated to the interfacial structure and growth parameters are still not clear. In this work, the aforementioned problems have been solved by synergetic optimization of the substrate temperature and ion bombardment energy during sputtering. This work demonstrates that the AlN films can grow directly on Si (100) substrates without an amorphous interfacial layer. The interface reactions between AlN and Si are limited to only 1~3 atomic layers. The as-grown AlN films consist of well-defined columns, and each column is revealed to be a single crystal. A subsurface growth/relaxation process is proposed based on the observation of randomly oriented AlN crystallites embedded in the surface amorphous matrix layer. A comprehensive understanding of the microstructure of deposited films can contribute to further optimizing the film growth parameters. Thus far, there are only few reports on the microstructural analysis of AlN films synthesized by different methods, while the effects of parasitic defects on the performance of AlN-based devices have been overlooked. For example, in studying the luminescence properties of AlN, all non-band-edge emissions, i.e., defect-related luminescence, of the AlN polytypes were ascribed to point defects such as N vacancies or O impurities, irrespective of the crystal structures. The contribution of linear/planar defects induced by the mosaic texture has not been considered. In this work, AlN thin films with different orientation degrees, i.e., (0002) textured, (0002) textured with traces of non-(0002) components, and randomly oriented, were deposited on silicon substrates by the precise control of the nitrogen partial pressure during the deposition. Microstructural analyse by high-resolution transmission electron microscopy (HRTEM) show directly the linear and planar defects are abundantly anchored at large-angle grain boundaries in the poorly-oriented films. Based on photoluminescence (PL) measurements, it is suggested that these defects might possibly be responsible for the enhanced non-band-edge PL emission. Due to its high thermal conductivity (140-280 vs 1.4 W/mK for SiO2), AlN is expected to be an excellent substitutional material of conventional amorphous SiO2 insulation layer in GaN-based devices to improve the device performance and reliability, and to reduce self-heating effect. Moreover, it is also known that AlN single crystals have a significantly higher thermal conductivity than that of AlN polycrystalline bulks. In this work, single crystal AlN thin films were grown epitaxially on GaN substrates on a macroscopic scale by MS. The microscopic structure and orientation degree of the AlN epilayers were studied by HRTEM, high-resolution x-ray diffraction, and reciprocal spacing mapping. The analyse reveales that the AlN epilayers have high in-plane and out-of-plane orientation degrees and low defect densities. The electrical and optical properties of the AlN epilayers are also studied, and the results suggest that the AlN epilayers grown by sputtering may be employed in the fabrication of GaN-based light-emitting diode devices with increased efficiency. SrTiO3 (STO) is a ferroelectric in cubic perovskite structure and has been widely used as a substrate for growing oxides. As the lattice mismatch between cubic STO (111) and AlN (0001) is fairly small (~2.52% at room temperature) and the melting point (2080 °C) of STO is high, STO (111) surfaces are expected to be a potential substrate for heteroepitaxial growth of AlN films. Comparing with the conventional substrates for growing epitaxial AlN films, e.g., sapphire and SiC, STO substrates are transparent, conductive, and easily diced. In particular, (111) STO epitaxial films have readily been grown on Si substrates. The heteroepitaxial AlN/STO/Si sandwich structure is expected to be more efficient in reducing the self-heating effect in silicon-on-insulator devices due to the significantly improved thermal conductivity of single-crystal AlN films. Thus far there are few reports on the growth of AlN films on STO by MBE and pulsed laser ablation. However, the deposition of epitaxial AlN films on a macroscopic scale has not been identified. This is the first work reporting deposition of epitaxial AlN films on single-crystal STO (111) substrates by MS. It was found that substrate temperature is the predominant parameter for controlling the in-plane orientation of AlN films. Single-domain epitaxial AlN films were grown at moderate temperatures of 270-370 °C with a sharp interface and orientation relationship of AlN STO [2110] //[011] − − − and (0002)AlN//(111)STO. At temperatures above 470 °C, an additional 30° in-plane-rotated AlN domain appeared, and increased in percentage with increasing temperature. A model based on the reconstruction of STO (111) surfaces from (1×1) to ( 3 × 3)R30° was proposed to account for the formation of this new domain. In addition, the effects of HF pre-etching of the STO (111) substrates on the orientation degree of AlN films were also investigated.
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