Jingxi Sun

p-GaN surface treatments for metal contacts

The chemical bonding and electronic properties of wet, chemically treated p-GaN surfaces were studied using synchrotron radiation photoemission spectroscopy. Chlorine-based chemical bonding was identified on the conventional HCl-treated p-GaN surface, which is associated with a shift of the surface Fermi level toward the conduction band edge by ∼0.9 eV with respect to the thermally cleaned surface. Compared to the HCl-treated surface, the surface Fermi level on the KOH-treated surface lies about ∼1.0 eV closer to the valence band edge, resulting in a much smaller surface barrier height to p-type materials than the HCl-treated surface. The smaller surface barrier height to p-GaN after KOH treatment can lead to a lower contact resistivity and can play an important role in lowering the metal contact resistivity to p-GaN.

n-GaN surface treatments for metal contacts studied via x-ray photoemission spectroscopy

The surface chemistry and electronic properties of n-GaN surfaces were studied via x-ray photoemission spectroscopy before and after wet chemical treatments. Shifts of the surface Fermi level were measured with the change in position of the Ga 3d core level peak. HCl treatment of n-GaN led to a 0.9 eV shift of the surface Fermi level toward the conduction band minimum, while KOH treatment led to a 0.3 eV shift of the surface Fermi level toward the valance band maximum. These shifts lead to a reduction in the surface barrier for HCl-treated n-GaN and for KOH-treated p-GaN, potentially improving contact resistance. The changes in surface chemistry indicate that a N (or Ga) deficiency with HCl(KOH) treatment alters the surface state density through the formation of donor (acceptor)-like states.

Role of interfacial compound formation associated with the use of ZnO buffers layers in the hydride vapor phase epitaxy of GaN

ZnO buffer layers have been used in the hydride vapor phase epitaxy of GaN in order to improve the initial nucleation and growth of the GaN and hence the subsequent materials properties. The specific role of the ZnO buffer layer was investigated by x-ray photoelectron spectroscopy and x-ray diffraction. The improvements in the GaN growth behavior are attributed to the formation of a thin surface layer of ZnAl2O4 that results from a reaction-diffusion process between the ZnO and Al2O3. This layer can provide an improved lattice match to GaN as well as change in surface energy affecting the initial growth of the GaN.

Chemical bonding and electronic properties of SeS2-treated GaAs(100)

SeS2-passivated n-type GaAs (100) surfaces, formed by treatment of GaAs in SeS2:CS2 solution at room temperature, were studied with high-resolution core-level photoemission spectroscopy excited with synchrotron radiation source. The SeS2-treated surface consists of a chemically stratified structure of several atomic layers thickness. Arsenic-based sulfides and selenides reside in the outermost surface layer while gallium-based selenides are adjacent to the bulk GaAs substrate. The shift of the surface Fermi level within the band gap was monitored during controlled thermal annealing, allowing for the identification of the specific chemical entities responsible for the reduction in surface band bending. Arsenic-based species are removed at low annealing temperature with little shift of the Fermi level. Gallium-based selenides are shown to be associated with the unpinning of the surface Fermi level.

Demonstration of near-field scanning photoreflectance spectroscopy

A near-field scanning optical microscope (NSOM) was developed to perform photoreflectance (PR) spectroscopy experiments at high spatial resolution (∼1 μm). Representative PR spectra are shown, along with an image illustrating the capability of observing contrast in images due to the strength of a PR feature. It was found that sufficiently high intensity light from the NSOM tip can produce photovoltages large enough to limit the spatial resolution of the electric field determination by PR. The photovoltage effect is measured as a function of light intensity, and the results are discussed in terms of a simple photovoltage expression.

Comparative study of GaN growth process by MOVPE

A comparative study of two different MOVPE reactors used for GaN growth is presented. Computational fluid dynamics (CFD) was used to determine common gas phase and fluid flow behaviors within these reactors. This paper focuses on the common thermal fluid features of these two MOVPE reactors with different geometries and operating pressures that can grow device-quality GaN-based materials. The study clearly shows that several growth conditions must be achieved in order to grow high quality GaN materials. The high-temperature gas flow zone must be limited to a very thin flow sheet above the susceptor, while the bulk gas phase temperature must be very low to prevent extensive pre-deposition reactions. These conditions lead to higher growth rates and improved material quality. A certain range of gas flow velocity inside the high-temperature gas flow zone is also required in order to minimize the residence time and improve the growth uniformity. These conditions can be achieved by the use of either a novel reactor structure such as a two-flow approach or by specific flow conditions. The quantitative ranges of flow velocities, gas phase temperature, and residence time required in these reactors to achieve high quality material and uniform growth are given.