C. Kirchner

ZnMgO epilayers and ZnO-ZnMgO quantum wells for optoelectronic applications in the blue and UV spectral region

We have investigated the properties of ZnMgO epilayers and ZnO–ZnMgO quantum well structures grown by metalorganic vapor-phase epitaxy. A well-controlled incorporation of magnesium, x⩽0.10, could be confirmed resulting in a blueshift of the photoluminescence emission wavelength of the Zn1−xMgxO layers up to 200meV. Using ZnMgO as barrier material, ZnO–ZnMgO quantum well structures with different well widths have then been fabricated. The confinement effect in the ZnO quantum wells leads to the expected increase of the corresponding quantum well emission energy with decreasing well width. A comparison to calculations also suggests a further enhancement of the exciton binding energy in the quantum wells of up to 90meV.

Optical and structural analysis of ZnCdO layers grown by metalorganic vapor-phase epitaxy

The development of ZnO-based semiconductor devices requires band gap engineering. Ternary Zn1−xCdxO allows reduction of the band gap relative to ZnO, which would be necessary for devices emitting visible light. We have analyzed the structural and optical properties of Zn1−xCdxO layers grown by metalorganic vapor-phase epitaxy. A narrowing of the fundamental band gap of up to 300 meV has been observed, while introducing a lattice mismatch of only 0.5% with respect to binary ZnO. Photoluminescence, high-resolution x-ray diffraction, and spatially resolved cathodoluminescence measurements revealed a lateral distribution of two different cadmium concentrations within the Zn1−xCdxO layers.

Evaluation of the temperature stability of AlGaN/GaN heterostructure FETs

Temperature stress experiments up to 800/spl deg/C have been applied to AlGaN/GaN FETs grown by MOVPE on sapphire and their individual technological building blocks. It was found that the temperature limit is given by the irreversible degradation of the intrinsic active heterostructure material itself during operation above 600/spl deg/C. The irreversible degradation was observed for both unconnected and electrically operated devices during temperature stress.

Optical investigations on the annealing behavior of gallium- and nitrogen-implanted ZnO

Gallium and nitrogen ions have been implanted into ZnO crystals and metal organic vapor phase epitaxy grown ZnO layers. Postimplantation annealing behavior in the temperature range between 200 and 900 °C has been studied by means of Raman scattering and low-temperature photoluminescence. The temperature for healing of the implantation-induced defects was found to be 800 °C. Implanted gallium acts as donor with a donor binding energy ED of 53 meV, thus allowing the control of n-type doping in ZnO. From photoluminescence measurements of the donor-acceptor pair transition of a series of nitrogen-implanted ZnO samples we estimate the binding energy EA of the nitrogen acceptor between 163 and 196 meV. Electrical characterization of nitrogen-implanted samples shows a behavior ranging from low n-type to highly compensated. But no unambiguous and reproducible type conversion could be achieved.

Temperature dependence of the E2 and A1(LO) phonons in GaN and AlN

The frequencies and dampings of the zone-center optical phonons E2 and A1(LO) in wurtzite-type GaN and AlN layers have been measured by Raman spectroscopy in the temperature range from 85 to 760 K. The GaN layer was grown by metalorganic vapor phase epitaxy and the AlN layer by molecular beam epitaxy both on sapphire substrate. The experimentally obtained frequencies and dampings are modeled by a theory taking into account the thermal expansion of the lattice, a symmetric decay of the optical phonons into two and three phonons of lower energy, and the strain in the layers induced by the different thermal expansion coefficients of layer and substrate. The results were used to determine the local temperature of a GaN pn diode in dependence on the applied voltage.

Photoinduced oxide film formation on n-type GaN surfaces using alkaline solutions

We report on photoassisted wet chemical formation of thin oxide films on n-GaN layers in potassium hydroxide based electrolytes at room temperature. The kinetics of the oxide formation and dissolution were examined via photocurrent transients. The tendency of the photocurrent to level out during photoelectrochemical etching experiments is associated with a quasiequilibrium state at the semiconductor/electrolyte interface. Homogeneous oxide films were grown in weak alkaline solutions (11<pH<13) under potentionstatic control with oxidation rates of up to 250 nm/h and characterized by Auger electron spectroscopy. Consequences on wet photochemical etch strategies are discussed.

Origin and consequences of a high stacking fault density in epitaxial ZnO layers

Transmission electron microscopy was applied to study ZnO grown by metalorganic vapor phase epitaxy on Al2O3(0001) substrates. The defect structure of the material is dominated by an extraordinary high density of small stacking faults with extensions between 5 and 25 nm which induce a bright small-scale speckle contrast under weak-beam imaging conditions. The stacking faults are terminated by Frank partial dislocations with Burgers vectors of the type 1/6 〈2203〉. The precipitation of interstitial atoms is the most likely process for the generation of the stacking faults, which are characterized by an additional (0002) plane. The high stacking fault density can be considered as an indicator for high point defect concentrations, which are expected to aggravate the control of the electrical conductivity.

Influences of biaxial strains on the vibrational and exciton energies in Zno

We have investigated the structural, optical, and vibrational properties of strained heteroepitaxial ZnO layers by high resolution x-ray diffraction, reflectivity, and Raman measurements. The ZnO layers were grown by metalorganic vapor phase epitaxy on sapphire substrates under varying growth conditions. A Poisson number of μ=0.303 and phonon deformation-potential parameters of a=−690 cm−1, b=−940 cm−1 for the high-energy E2 optical phonon mode have been determined. The shift of the excitonic resonances due to the strain in the layers agrees well with the experimentally determined Poisson ratio using the deformation-potentials D1–D4 determined by Wrzesinski and Frohlich [Phys. Rev. B 56, 13087 (1997)].

Analysis of ZnO and ZnMgO nanopillars grown by self-organization

In this contribution we analyse the structural and optical properties of ZnO as well as ZnMgO nanopillars grown catalyst-free by metalorganic vapour-phase epitaxy. The nanostructures were grown directly onto different substrate materials with various orientations. The nanopillars deposited on a-plane sapphire show the best vertical c-axis alignment and have a typical diameter of about 50 nm and a height of several micrometres, depending on growth time. We achieved well ordered, almost completely c-axis oriented pillars, as confirmed by scanning electron microscopy and high resolution x-ray diffraction. Photoluminescence measurements revealed very narrow donor-bound exciton emission lines with half widths as small as 0.5 meV. In order to investigate the possibility of a combination of band gap engineering and nanopillar growth, ZnMgO nanopillars were also grown. The Mg incorporation was confirmed by photoluminescence measurements and a blue shift of the band gap of up to 170 meV could be achieved for the nanopillars with the highest Mg concentration.

Homoepitaxial growth of GaN by metalorganic vapor phase epitaxy: A benchmark for GaN technology

Carefully optimized low-pressure metalorganic vapor phase epitaxy is used for homoepitaxial growth on distinctively pretreated GaN bulk single crystal substrates. Thereby, outstanding structural and optical qualities of the material have been achieved, exhibiting photoluminescence linewidths for bound excitons as narrow as 95 μeV. These extremely sharp lines reveal fine structures, not reported for GaN. Additionally, all three free excitons as well as their excited states are visible in low-temperature photoluminescence at 2 K. These transitions are clearly identified by reflectance measurements. X-ray diffraction analysis of these layers reveal about 20 arcsec linewidth for the (0004) reflex using CuKα radiation.