Tak Kin Chu

Growth and interfacial chemistry of insulating (100) barium fluoride on gallium arsenide

The epitaxial growth of insulating BaF2 films on (100) and (111)‐oriented GaAs substrates has been investigated. (100)‐oriented BaF2 was successfully deposited on (100) GaAs at temperatures as low as room temperature, in contrast to a previous report. This was accomplished by first establishing a chemically reacted template layer a few monolayers thick at the BaF2/GaAs interface. These films consistently exhibited epitaxial reflection high‐energy electron diffraction patterns with three‐dimensional growth modes for a wide range of incident BaF2 flux rates. The epitaxial quality of the (100) films was, however, temperature dependent. A film deposited on a (111) wafer at 600 °C was (111) oriented and showed two‐dimensional growth. X‐ray photoelectron spectroscopy studies of the interface chemistry indicate the existence of a Ba state other than that of the BaF2, authenticating the relevance of the template layer. The (100) BaF2 films are insulating, with a breakdown field of ∼1×106 V/cm.

Heteroepitaxial deposition of Group IIa fluorides on gallium arsenide

Abstract The epitaxial deposition of fluoride films has been a subject of research interest because of their potential for integrated electronic and electro-optic device applications. We report here our recent results on the investigation of (100)BaF 2 -GaAs growth. Our approach differs from the conventional method by addressing the problem at the atomic layer level, especially at the interface between the deposited material and the substrate. These investigations have revealed that an interfacial chemical reaction is important in the heteroepitaxy process. As a result of this chemical reaction, an atomic Ba layer is formed on the GaAs surface. It is this Ba-template layer that enables two dimensional, molecular layer-by-layer growth of the BaF 2 film. Films thus grown are of a high epitaxial quality which appears to be limited only by the quality of the GaAs surface. For BaF 2 on GaAs, (100) growth is favored over (111) growth. This is contrary to earlier results by other investigators. It is concluded that the conventional approach to heteroepitaxial growth, relying on lattice match and surface energies is not applicable, at least for the fluoride films. This can be understood from the atomic and molecular structure of Ba and BaF 2 . Implications of these results to the understanding of the heteroepitaxial process, especially involving large lattice mismatches, and to the development of new materials and technologies will be discussed.

Determination of growth mechanisms of MBE grown BaF2 on Si(100) by target angle dependence of RBS yields

Abstract The angular dependence of 2.0 MeV 4He1+ Rutherford backscattering spectroscopy (RBS) yields have been used to determine the growth mechanism of epitaxial BaF2 films grown on Si(100) substrates by molecular beam epitaxy (MBE). RBS yields from uniformly thick layers are characterized by a (cos φ)−1 dependence, where φ is the angle between the incident beam and the target normal. Deviations from this relationship have been attributed to layers which are composed of islands, rather than films of uniform thickness. A series of BaF2 films of increasing deposition time were examined in this way. The results of this analysis show that the BaF2 at first grows in small islands which eventually coalesce into a uniform epitaxial layer.

Heavy ion backscattering spectroscopy (HIBS) analysis of gallium arsenide diffusion into barium fluoride layers grown by molecular beam epitaxy

Abstract Heavy Ion Backscattering Spectroscopy (HIBS) using 12 MeV 12C ions was used to examine GaAs (1 0 0) and GaAs (1 1 1) layers grown by Molecular Beam Epitaxy (MBE) on BaF2 (1 0 0) and BaF2 (1 1 1) layers, respectively. HIBS was the only available technique able to verify the Ga As ratio in these samples because X-ray Photoelectron Spectroscopy (XPS) is too surface sensitive for this task and the lighter ions used in Rutherford Backscattering Spectroscopy (RBS) cannot resolve the Ga and As signals. A modified RBS analysis program was used to analyze the HIBS spectra. Using the stopping power data of Ziegler et al., the HIBS analysis overestimated the thickness of a BaF2 layer by about 14% when compared to RBS analysis of the same layer. HIBS was able to resolve the Ga and As peaks, as well as the two isotopes of Ga. This enhanced mass resolution made the determination of the GaAs layer stoichiometry possible, and introduced spectral features that aided in fitting HIBS simulation spectra to the data points. HIBS analysis of GaAs/BaF2/Si heteroepitaxies also showed diffusion of Ga and As into BaF2. This diffusion is temperature dependent and therefore subject to control. The information gathered from the HIBS analysis was used to confirm information obtained by in situ MBE diagnostic techniques, and in the case of the diffusion studies, provided information that was not available by those surface sensitive techniques.

Heavy ion beam backscattering spectroscopy of GaAs/BaF2/GaAs heterostructures

Heavy Ion Backscattering Spectroscopy (HIBS) using 12.4 MeV Carbon ion beams was used to examine GaAs/BaF2/GaAs heterostructures deposited on GaAs substrates by Molecular Beam Epitaxy (MBE). The ability of the heavier ions to separate the backscattered signals of Arsenic and Gallium, as well as the individual isotopes of Gallium, was used to verify the 50/50 stoichiometry of the MBE grown GaAs layers. This information is not readily available using traditional insitu MBE diagnostic techniques. HIBS spectra were analyzed using a modification of an existing Rutherford Backscattering Spectroscopy (RBS) analysis program. The HIBS spectra were also instrumental in identifying interdiffusion of BaF2 and GaAs layers taking place during MBE growth. This information was used to modify the MBE growth process to achieve sharper interfaces between the BaF2 and GaAs layers.

An inexpensive, low-energy, ionized gas source for molecular beam epitaxy applications

Many molecular beam epitaxy (MBE) applications require a source of ionized gas atoms or molecules. However, the high gas pressures and high kinetic energies associated with many standard gas sources can be detrimental to MBE deposition. These disadvantages are addressed here by an ionized gas source fabricated from a common laboratory ionization gauge. The source described here produces 100 eV gas ions while maintaining vacuums of better than 10−8 mbar. This source has the additional advantage of being inexpensive and simple to construct. Design, construction, and operation of the source will be presented.