S. Shokhovets

Determination of Group III Nitride Film Properties by Reflectance and Spectroscopic Ellipsometry Studies

Reflectance measurements under normal incidence of light and variable angle spectroscopic ellipsometry (SE) studies are applied for characterizing hexagonal GaN films grown by molecular beam epitaxy on both GaAs (111)B and 6H-SiC substrates. A comparison of the reflectance above the band gap with model calculations for a smooth film yields definitely the root mean square surface roughness. By analyzing the envelopes of the reflectance spectra, the influence of a buffer layer and/or a non-abrupt substrate/film interface is verified and qualitative conclusions about the interface properties are deduced which are emphasized by transmission electron microscopy studies. On this basis a suitable model for data fitting is established which contains only a small number of parameters to be adjusted. From the fit, the complex refractive index versus photon energy as well as the thicknesses of the active layer and the interlayers are obtained. A meticulous analysis of the SE data below the band gap shows that from those studies the extraordinary refractive index of hexagonal GaN can be determined for the first time.

Study of the linear electro-optic effect in α-GaN by electroreflectance

Abstract The linear electro-optic (Pockels) effect for hexagonal GaN was studied by electrolytic electroreflectance spectroscopy. The electric field strength and space charge region parameters were extracted from Franz–Keldysh oscillations and photocurrent spectra. Data analysis yielded for the linear electro-optic coefficient r13 a value of (1.55±0.08) pm V−1 indicating an appreciable ionic contribution which can be related to the converse piezoelectric effect.

Momentum matrix element and conduction band nonparabolicity in wurtzite GaN

Comparing the imaginary part of the dielectric function of wurtzite GaN measured by spectroscopic ellipsometry at room temperature with calculations, we found the values of Ep=(19.8±0.5)eV, m0*=(0.201±0.005)m0, and F=−0.89±0.05 for the momentum matrix element, electron effective mass at the conduction band edge, and parameter F describing the influence of remote bands, respectively. We also observed a remarkable nonparabolicity of the conduction band. The data obtained are consistent with A sets of valence band parameters reported earlier providing a parameterization of band structure of wurtzite GaN in the vicinity of the Γ point.

Exciton quenching in Pt/GaN Schottky diodes with Ga- and N-face polarity

We observed a peculiar dependence of low-temperature electroreflectance spectra of Pt/GaN Schottky diodes with Ga- and N-face polarity on the bias voltage (so-called “rotation” spectra), indicating the quenching of discrete exciton states and the formation of an exciton dead layer (EDL) beneath the gate. Data analysis is carried out using the field-dependent dielectric function of GaN. It yields parameters of excitons for the zero-field limit and the depth of the EDL, as well as the surface band bending and the ionized impurity concentration.

Optical study of the -GaN/GaAs interface properties as a function of MBE growth conditions

Undoped hexagonal GaN films have been grown by molecular beam epitaxy on (111)B GaAs substrates. Both Ga flux and nitrogen plasma conditions were fixed, while the growth temperature was varied in the range from 650 up to 720C. Reflectivity measurements below and above the GaN band gap were employed in order to determine optical parameters characterizing the substrate/film interface properties. The refractive index of the interface layer was found to decrease with increasing growth temperature indicating the formation of a mixed medium in this region. This effect is related to the columnar growth of GaN and outdiffusion of substrate atoms via channels between the columns which promotes the formation of voids in the substrate/film interface. Low temperature initiated growth or nitridation of the substrate were observed to reduce the influence of elevated growth temperatures.

Optical characterisation of interface properties for hexagonal GaN grown by MBE on GaAs

A new optical method for the characterisation of substrate/film interface properties of GaN grown on GaAs is presented. It is based on reflectivity measurements over an extended spectral range. The influence of surface roughness is separated by a comparison of the experimental data with calculations for a smooth film above the band gap of GaN, while at lower energies, the deviations from the calculated one-layer behaviour (without interface) are strongly correlated to the effective refractive index of the interface nint and its thickness. It is shown that for GaAs substrates under certain MBE growth conditions, nint is mainly determined by the void fraction.

Study of Exciton Dead Layers in GaN Schottky Diodes with N and Ga-Face Polarity

For the first time, we observed exciton dead layers in Pt/GaN Schottky diodes with N and Ga-face polarity. We show that these layers determine the shape of electroreflectance and photocurrent spectra close to the band gap in dependence on the bias voltage. Analysis of experimental data using a spatially dependent dielectric function of GaN due to an inhomogeneous electric field in the depletion region yields the ionised impurity concentration and the surface band bending. In addition, exciton and broadening energies for the field-free limit are obtained.