Matthias Wieneke

InGaN/GaN light-emitting diodes on Si(1?1?0) substrates grown by metal?organic vapour phase epitaxy

We report on the implementation of InGaN/GaN-based light-emitting diodes (LEDs) structures on Si(1?1?0) oriented substrates grown by metal?organic vapour phase epitaxy. The total thickness of the completely crack-free device structures on Si(1?1?0) substrates amounts to 2.5??m exhibiting a comparable or even better layer quality than identical LED structures grown on standard Si(1?1?1) substrates. This result can be explained by a more suited epitaxial relation between the c-plane of the high-temperature AlN seed layer and the Si(1?1?0) surface. The crystallographic structure of the LEDs was analysed by x-ray diffraction and the optical properties were investigated by photo- and electroluminescence. The improved crystallographic quality on Si(1?1?0) goes in line with a higher peak intensity in the photoluminescence measurements. Furthermore, a bright bluish light emission at 490?nm is obtained by an electrical excitation at room temperature.

Anisotropy of effective electron masses in highly doped nonpolar GaN

The anisotropic effective electron masses in wurtzite GaN are determined by generalized infrared spectroscopic ellipsometry. Nonpolar (112¯0) oriented thin films allow accessing both effective masses, m⊥* and m∥*, by determining the screened plasma frequencies. A n-type doping range up to 1.7 × 1020 cm−3 is investigated. The effective mass ratio m⊥*/m∥* is obtained with highest accuracy and is found to be 1.11 independent on electron concentration up to 1.2 × 1020 cm−3. For higher electron concentrations, the conduction band non-parabolicity is mirrored in changes. Absolute values for effective electron masses depend on additional input of carrier concentrations determined by Hall effect measurements. We obtain m⊥*=(0.239±0.004)m0 and m∥*=(0.216±0.003)m0 for the parabolic range of the GaN conduction band. Our data are indication of a parabolic GaN conduction band up to an energy of approximately 400 meV above the conduction band minimum.

Eliminating stacking faults in semi-polar GaN by AlN interlayers

We report on the elimination of stacking faults by the insertion of low-temperature AlN interlayers in nearly (1016) and (11¯04) oriented semi-polar GaN grown by metalorganic vapor phase epitaxy on Si(112) and Si(113), respectively. The elimination of these defects is visualized by cathodoluminescence (CL) as well as scanning transmission electron microscopy (STEM) and STEM-CL. A possible annihilation mechanism is discussed which leads to the conclusion that the elimination mechanism is most likely valid for all layers with (11¯01) surfaces, enabling heteroepitaxial semi- and non-polar GaN free from stacking faults.

Observation of individual stacking faults in GaN microcrystals by x-ray nanodiffraction

X-ray nanodiffraction was used for the investigation of basal stacking faults in a-GaN microcrystallites. The method made it possible to find the positions of individual stacking faults in a chosen crystallite, and the resulting positions were compared with the observation of individual faults by electron channeling contrast in scanning electron microscopy. The x-ray diffraction data revealed that the faults occur in closely positioned pairs; the stacking faults in a pair have opposite displacement vectors.

Direct microscopic correlation of crystal orientation and luminescence in spontaneously formed nonpolar and semipolar GaN growth domains

We present a direct microscopic correlation between local optical properties, characterized by spectrally resolved cathodoluminescence microscopy and the microscopic crystallographic orientation determined by electron backscatter diffraction at identical sample positions of nonpolar and semipolar GaN growth domains simultaneously formed during metal-organic vapor phase epitaxy on the same r-plane sapphire substrate. The luminescence from all nonpolar, (1120) grown crystallites is dominated by the basal plane stacking fault luminescence, while all crystallites having semipolar (1126) orientation show a luminescence characterized by pure excitonic emission, i.e., without any contribution of stacking faults, and with an order of magnitude enhanced quantum efficiency.

a-plane GaN Shear Wave Thin Film Resonator

This paper focuses on metal-organic-vapor-phase-epitaxial (MOVPE) grown on a-plane gallium-nitride (GaN), representing a novel approach for piezoelectric materials with good prospects for quartz-crystal-microbalance (QMB), like sensors with respect to its biocompatibility and frequency filter applications. Material characteristics of gallium-nitride as well as the processing of shear wave resonators and their acoustical characteristics are as well discussed

Unintentional doping of a-plane GaN by insertion of in situ SiN masks

Undoped a-plane GaN layers grown by metal-organic vapour phase epitaxy on sapphire (1?0???1?2) substrates using low temperature (LT) GaN seed layers and in situ SiN masks were characterized by Hall-effect measurements, CV-characteristics and photovoltage spectroscopy. With increasing deposition time of the SiN masks the electron concentrations of the GaN layers are enhanced. The dominant activation energy between 14 and 22?meV determined by temperature-dependent Hall effect is very similar to the donor silicon on gallium site. Two other activation energies at 30?meV and between 50 and 70?meV were found corresponding well with OGa and VN defects, respectively. The depth profiles of the net donor densities show a strong increase towards the substrate/LT-GaN/high temperature(HT)-GaN interface indicating diffusion of silicon from the SiN mask towards the surface. Therefore, the Si doping is attributed to the dissolution of the SiN masks during the following HT GaN layer growth. The Si doping from the SiN masks also explains the deterioration of the band bending within the LT-GaN/HT-GaN junction found by photovoltage spectroscopy.

Leakage currents and Fermi-level shifts in C- and Fe-doped GaN (Conference Presentation)

Due to its large band gap and excellent electrical properties, nitride-based heterostructures are rapidly becoming a material of choice for RF and power switching applications. However, these devices require a carbon or iron doped semi-insulating buffer to deliver high breakdown voltages and suppress off-state leakage currents. We have grown semi-insulating GaN using precursor-based metal-organic chemical vapor phase epitaxy by intentionally introducing carbon and iron impurities with doping concentration ranging from 1x10^17cm-3 to 5x10^18cm-3 to compensate residual donors. Scanning probe microscopy techniques, scanning surface potential microscopy (SSPM) and bias dependent electric force microscopy (EFM) are mainly used to compare contact potential differences and local potential mapping at the vicinity of dislocation regions. For reference n-type GaN layers doped with Si and Ge, and p-type GaN layers doped with Mg are also investigated. Skew and edge type dislocation densities are estimated from tilt and twist x-ray diffraction measurements using omega-scans for the (0002) reflection and grazing incidence in-plane geometry for the (101 0) reflection. The obtained values are in the range of low 108 cm-2 for screw-type and low 109 cm-2 for edge-type dislocations, independent of doping type and concentration. Locally probing dislocations by SSPM reveals a negative charge contrast with respect to the surrounding areas in C-doped samples increasing with doping concentration whereas Fe-doped samples exhibit no contrast. By investigating the contact potential by EFM, the combined effects of Fermi-level position and surface band bending due to surface states are determined. With the references of n-type and p-type GaN samples, the acceptor states introduced by carbon cause Fermi-level pinning below midgap position whereas acceptor-states by Fe impurities have to be energetically above midgap position. In vertical transport measurements, C-doped GaN layers with a dopant concentration of 4.6x10^18 cm-3 exhibit an up to 5 orders of magnitude lower dark current at room temperature and significantly higher thermal activations than Fe-doped samples with a comparable dopant concentration. In conclusion C-doped samples show superior properties in comparison to Fe-doped samples.

Leakage currents and Fermi-level shifts in GaN layers upon iron and carbon-doping

Semi-insulating GaN is a prerequisite for lateral high frequency and high power electronic devices to isolate the device region from parasitic conductive channels. The commonly used dopants for achieving semi-insulating GaN, Fe, and C cause distinct properties of GaN layers since the Fermi-level is located either above (Fe) or below (C) the midgap position. In this study, precursor-based doping of GaN in metalorganic vapor phase epitaxy is used at otherwise identical growth conditions to control the dopant concentrations in the layer. Using electric force microscopy, we have investigated the contact potentials of Fe- and C-doped samples with respect to a cobalt metal probe tip in dependence of on the dopant concentration. While in Fe-doped samples the sign of the contact potential is constant, a change from positive to negative contact potential values is observed at high carbon concentrations, indicating the shift of the Fermi-level below the midgap position. In vertical transport measurements, C-doped G...

X-ray Study of Step Induced Lateral Correlation Lengths in Thin AlGaN Nucleation Layers

A 15 nm thick high temperature AlGaN nucleation layer was grown on a slightly off-oriented c-plane sapphire substrate by metal–organic vapor phase epitaxy. The evaluation of this layer by high resolution X-ray diffraction yields a nucleation with a threefold distribution of the rotational disorder within about 4° in the ω-scan at the in-plane AlGaN(1100) reflection, whereas the ω-scan at the symmetric AlGaN(0002) reflection exhibits a very narrow correlation peak. In addition, two weaker signals could be determined in the symmetric ω-scan. The positions of these minor maxima with respect to the correlation peak depend on the azimuth angle of the incident X-ray beam. Based on the determined off-orientation angle and lateral coherence lengths the phenomenon is explained in terms of a stepped sapphire surface morphology.