R. Goldhahn

Universality of electron accumulation at wurtzite c- and a-plane and zinc-blende InN surfaces

Electron accumulation is found to occur at the surface of wurtzite (112¯0), (0001), and (0001¯) and zinc-blende (001) InN using x-ray photoemission spectroscopy. The accumulation is shown to be a universal feature of InN surfaces. This is due to the low Г-point conduction band minimum lying nsignificantly below the charge neutrality level.

Molecular beam epitaxy of phase pure cubic InN

Cubic InN layers were grown by plasma assisted molecular beam epitaxy on 3C-SiC (001) substrates at growth temperatures from 419to490°C. X-ray diffraction investigations show that the layers have zinc blende structure with only a small fraction of wurtzite phase inclusions on the (111) facets of the cubic layer. The full width at half maximum of the c-InN (002) x-ray rocking curve is less than 50arcmin. The lattice constant is 5.01±0.01A. Low temperature photoluminescence measurements yield a c-InN band gap of 0.61eV. At room temperature the band gap is about 0.56eV and the free electron concentration is about n∼1.7×1019cm−3.

Determination of the anisotropic dielectric function for wurtzite AlN and GaN by spectroscopic ellipsometry

We demonstrate that variable angle spectroscopic ellipsometry (SE) is capable of determining both ordinary and extraordinary dielectric functions (DFs) in the transparent region of group III nitride wurtzite films even if the optical axis is oriented normal to the surface plane. In contrast to the so far used prism coupling technique, the SE method is neither restricted to layers deposited on lower refractive index substrates nor to available laser wavelengths. Application to AlN and GaN films grown on 6H-SiC substrates yields experimental data which can be represented in simple analytical form. The difference in energy dispersion for ordinary and extraordinary DFs is related to the effective optical band gap depending on the light polarization. Extrapolation of data to lower photon energies allows estimation of the ordinary and extraordinary high-frequency dielectric constants.

Model for the thickness dependence of electron concentration in InN films

A model for the influence of different contributions to the high electron concentration in dependence on the film thickness of state-of-the-art InN layers grown by molecular-beam epitaxy is proposed. Surface accumulation has a crucial influence for InN layers 10μm.

Refractive index and gap energy of cubic InxGa1−xN

Spectroscopic ellipsometry studies have been carried out in the energy range from 1.5 to 4.0 eV in order to determine the complex refractive indices for cubic InGaN layers with various In contents. The films were grown by molecular-beam epitaxy on GaAs(001) substrates. By studying GaN films, we prove that for the analysis of optical data, a parametric dielectric function model can be used. Its application to the InGaN layers yields, in addition, the composition dependence of the average fundamental absorption edge at room temperature. From the latter, a bowing parameter of 1.4 eV is deduced.

Dielectric function of zinc-blende AlN from 1 to 20 eV: Band gap and van Hove singularities

The dielectric function (DF) of phase-pure cubic AlN films is determined by ellipsometry. The sharp onset of the imaginary part of the DF defines the direct absorption edge corresponding to a conduction-to-valence band spacing at the center of the Brillouin zone (BZ) of 5.93 eV. Phonon-assisted transitions lead to the pronounced absorption tail below this edge from which the indirect gap of zinc-blende AlN is estimated with 5.3 eV. Transitions due to four additional critical points of the BZ are resolved at higher photon energies. The high-frequency and static dielectric constants are determined with 4.25 and 8.07, respectively.

Optical constants of cubic GaN in the energy range of 1.5-3.7 eV

The refractive index and extinction coefficient of cubic GaN in the energy range of 1.5–3.7 eV were determined with high accuracy using combined reflectivity and spectroscopic ellipsometry studies of layers grown by molecular beam epitaxy on GaAs(001). A comparison of the experimental reflectivity data with theoretical calculations demonstrates that the data analysis has to be performed by taking into account both surface roughness and a nonabrupt substrate–film interface. In the transparent region the refractive index of cubic GaN was found to be slightly higher than that of the hexagonal modification.

Reflectivity study of hexagonal GaN films grown on GaAs: Surface roughness, interface layer, and refractive index

Detailed reflectivity studies of hexagonal GaN films grown by molecular beam epitaxy on GaAs substrates have been carried out in the energy range from 1.4 to 3.8 eV at room temperature. Measurements using ambient media with different refractive indexes verify that the reflectivity is strongly influenced by a surface roughness. Furthermore, the optical data give a clear evidence for the formation of an interface layer (mixture of GaAs with voids) between the film and the substrate which agrees well with transmission electron microscopy observations. A quantitative analysis has been performed by correcting the reflectivity spectra for the surface roughness and making use of a two-layer model to take into account the interface layer. This procedure yields for all samples the same energy dependent refractive index of GaN despite the differing surface and interface layer properties. The obtained values of the root mean squared roughness are close to the atomic force microscopy data. Growth on (001) substrates ...

Influence of the surface potential on electrical properties of AlxGa1−xN/GaN heterostructures with different Al-content: Effect of growth method

The influence of the growth method on the surface potential and thus on the sheet carrier concentration of GaN capped AlxGa1−xN/GaN heterostructures was evaluated. Nominally undoped low pressure metal-organic vapor-phase (MOVPE) and plasma-assisted molecular beam epitaxial (PA-MBE) grown structures with an Al-content between 12% and 30% yield carrier concentrations from 3.6×1012 to 1.2×1013u2002cm−2. A difference of the concentrations for a fixed Al-content was found between the different epitaxial techniques. This result indicates unambiguously different surface potentials determined quantitatively from the carrier concentration, and is verified in addition by the results of photoreflectance spectroscopy. The GaN surface potentials of MOVPE and PA-MBE grown samples amounts to (0.26±0.04) and (0.61±0.10)u2002eV irrespective of the Al-content of the barrier layer. After device fabrication, we find that due to the identical surface potential defined by the Ni Schottky gate, the threshold voltage for a given Al-cont...

Impact of GaN cap thickness on optical, electrical, and device properties in AlGaN/GaN high electron mobility transistor structures

We systematically investigate Al0.22Ga0.78N/GaN high electron mobility transistors with GaN cap layer thicknesses of 0, 1, and 3 nm. All samples have electron mobilities around 1700u2002cm2/Vs and sheet carrier concentrations around 8×1012u2002cm−2 as determined by Hall effect measurements. From photoreflectance measurements we conclude that the electric field strength within the AlGaN barrier increases with GaN cap layer thickness leading to a broadening of the transition peaks as determined by spectroscopic ellipsometry. The surface potential as determined by photoreflectance varies in the range between 0.585 and 0.249 eV dependent on the thickness of the GaN cap. Device results show a significant decrease in Ohmic contact resistance, an increase in ideality factor, a decrease in gate and drain leakage currents, an increase in gain, and an increase in power added efficiency with increasing cap layer thickness. Finally, devices with GaN cap show an improved direct current reliability compared to their counterpar...