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.
| Date of Award | 2 Oct 2008 |
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| Original language | English |
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| Awarding Institution | - City University of Hong Kong
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| Supervisor | Wenjun ZHANG (Supervisor) |
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