Jessica Chai

Band structure of ZnO from resonant x-ray emission spectroscopy

Soft x-ray emission and absorption spectroscopy of the O K-edge are employed to investigate the electronic structure of wurtzite ZnO(0001). A quasiparticle band structure calculated within the GW approximation agrees well with the data, most notably with the energetic location of the Zn 3d-O 2p hybridized state and the anisotropy of the absorption spectra. Dispersion in the band structure is mapped using the coherent k-selective part of the resonant x-ray emission spectra. We show that a more extensive mapping of the bands is possible in the case of crystalline anisotropy such as that found in ZnO.

MBE growth and characterization of Mn-doped InN

The majority of InN doping studies have primarily focused on Mg, as it has previously been used to successfully realize p-type GaN. Here, we consider an alternative dopant—Mn—as a possible acceptor candidate in InN. Magnetotransport, x ray photoelectron spectroscopy, and photoluminescence were used to investigate electrical and optical properties of a series of Mn-doped InN thin films grown using molecular beam epitaxy. Evidence of acceptor behavior was observed only for moderate (1017 cm−3) doping levels. At a doping level around 1017 cm−3, light hole features appear in the quantitative mobility spectrum analysis, the surface Fermi level shifts downwards towards the valence band, and low energy features appear in the low temperature photoluminescence spectra.

X-Ray Photoelectron Spectroscopy Study of Oxide Removal Using Atomic Hydrogen for Large-Area II–VI Material Growth

This study investigates using atomic hydrogen to clean GaSb (211)B and (111)B substrates as an alternative to thermal desorption under an Sb overpressure. X-ray photoelectron spectroscopy measurement verified the oxide removal on the atomic hydrogen-cleaned GaSb. Atomic force microscopy was used to characterize the surface morphologies of GaSb after atomic hydrogen cleaning with various conditions. All substrates investigated contained a high density of pits that became larger as higher deoxidation temperatures were used, with or without atomic hydrogen. Growth of homoepitaxial GaSb (100) and (211)B was used to compare stoichiometry changes with various oxide removal conditions, and growth effects on sequential epilayers.

Kinetic surface roughening and wafer bow control in heteroepitaxial growth of 3C-SiC on Si(111) substrates

A thin, chemically inert 3C-SiC layer between GaN and Si helps not only to avoid the “melt-back” effect, but also to inhibit the crack generation in the grown GaN layers. The quality of GaN layer is heavily dependent on the unique properties of the available 3C-SiC/Si templates. In this paper, the parameters influencing the roughness, crystalline quality, and wafer bow are investigated and engineered to obtain high quality, low roughness 3C-SiC/Si templates suitable for subsequent GaN growth and device processing. Kinetic surface roughening and SiC growth mechanisms, which depend on both deposition temperature and off-cut angle, are reported for heteroepitaxial growth of 3C-SiC on Si substrates. The narrower terrace width on 4° off-axis Si enhances the step-flow growth at 1200 °C, with the roughness of 3C-SiC remaining constant with increasing thickness, corresponding to a scaling exponent of zero. Crack-free 3C-SiC grown on 150-mm Si substrate with a wafer bow of less than 20 μm was achieved. Both concave and convex wafer bow can be obtained by in situ tuning of the deposited SiC layer thicknesses. The 3C-SiC grown on off-axis Si, compared to that grown on on-axis Si, has lower surface roughness, better crystallinity, and smaller bow magnitude.

Vertically Conductive Single-Crystal SiC-Based Bragg Reflector Grown on Si Wafer

Single-crystal silicon carbide (SiC) thin-films on silicon (Si) were used for the fabrication and characterization of electrically conductive distributed Bragg reflectors (DBRs) on 100 mm Si wafers. The DBRs, each composed of 3 alternating layers of SiC and Al(Ga)N grown on Si substrates, show high wafer uniformity with a typical maximum reflectance of 54% in the blue spectrum and a stopband (at 80% maximum reflectance) as large as 100 nm. Furthermore, high vertical electrical conduction is also demonstrated resulting to a density of current exceeding 70 A/cm2 above 1.5 V. Such SiC/III-N DBRs with high thermal and electrical conductivities could be used as pseudo-substrate to enhance the efficiency of SiC-based and GaN-based optoelectronic devices on large Si wafers.

Growth mechanism for alternating supply epitaxy: the unique pathway to achieve uniform silicon carbide films on multiple large-diameter silicon substrates

Low-cost large-diameter cubic silicon carbide (3C-SiC) film grown on silicon (Si) has been demonstrated to have a wide range of applications in photonics, electronics, photoelectrochemistry and micro-electro-mechanical system technologies. In this paper, the epitaxial growth of SiC on Si by low-pressure chemical vapour deposition is investigated. Two modes were employed to supply the precursors: the alternating supply and the simultaneous supply. Compared with SiC films grown at the same temperature by simultaneous supply epitaxy method, the SiC grown by alternating supply epitaxy (ASE) method has better crystallinity, smoother surface, and better thickness uniformity as confirmed by X-ray diffraction and atomic force microscopy characterisation. We propose the growth mechanism for ASE growth of 3C-SiC and validate it in detail experimentally. It is found that, Si deposition on SiC follows either Stranski–Krastanov mode or island growth mode, while SiC formation proceeds in two possible reaction paths: redistributing of the formed Si islands or smoothing of the formed SiC islands by decomposition migration process. Both reaction paths are driven by minimizing the surface free energy and reducing dangling bonds density. In summary, the key features of ASE are: (1) Si has a longer diffusion length and thus higher probability to adhere to a crystallographically favourable position; (2) undesirable gas phase reactions can be avoided. The obtained results indicate that ASE is a unique and economically viable method to prepare uniform 3C-SiC on multiple large-diameter Si wafers.

Silicon etching using only Oxygen at high temperature: An alternative approach to Si micro-machining on 150 mm Si wafers

Using a combination of low-pressure oxygen and high temperatures, isotropic and anisotropic silicon (Si) etch rates can be controlled up to ten micron per minute. By varying the process conditions, we show that the vertical-to-lateral etch rate ratio can be controlled from 1:1 isotropic etch to 1.8:1 anisotropic. This simple Si etching technique combines the main respective advantages of both wet and dry Si etching techniques such as fast Si etch rate, stiction-free, and high etch rate uniformity across a wafer. In addition, this alternative O2-based Si etching technique has additional advantages not commonly associated with dry etchants such as avoiding the use of halogens and has no toxic by-products, which improves safety and simplifies waste disposal. Furthermore, this process also exhibits very high selectivity (>1000:1) with conventional hard masks such as silicon carbide, silicon dioxide and silicon nitride, enabling deep Si etching. In these initial studies, etch rates as high as 9.2 μm/min could be achieved at 1150 °C. Empirical estimation for the calculation of the etch rate as a function of the feature size and oxygen flow rate are presented and used as proof of concepts.

Si Surface Preparation for Heteroepitaxial Growth of SiC Using In Situ Oxidation

To achieve high quality SiC growth on Si substrate, it is essential to get a smooth Si surface without forming SiC and graphitic islands during the surface cleaning and before the carbonisation process. In this paper, a novel in-situ surface cleaning method designed for the hetero-epitaxial growth of SiC on Si substrate is developed using a custom-made low-pressure chemical vapour deposition reactor. The results indicate that the combination of ramping in oxygen and subsequent flowing of SiH4 avoids the contamination of Si, enables the oxide layer to be removed smoothly, and subsequently creates a smooth Si surface with regular atomic steps. SiC grown on off-axis Si has better crystallinity and significantly smaller roughness than that grown on on-axis Si.

Sulfur passivation of surface electrons in highly Mg-doped InN

Electron accumulation with a sheet density greater than 1013 cm−2 usually occurs at InN surfaces. Here, the effects of treatment with ammonium sulfide ((NH4)2Sx) on the surface electronic properties of highly Mg-doped InN (>4×1018 cm−3) have been investigated with high resolution x-ray photoemission spectroscopy. The valence band photoemission spectra show that the surface Fermi level decreases by approximately 0.08 eV with (NH4)2Sx treatment, resulting in a decrease of the downward band bending and up to a 70% reduction in the surface electron sheet density.