Benjamin Neuschl

Growth and Characterization of AlN and AlGaN Epitaxial Films on AlN Single Crystal Substrates

AlN and AlGaN epitaxial films were deposited by metal organic chemical vapor deposition on single crystal AlN substrates processed from AlN boules grown by physical vapor transport. Structural, chemical, and optical characterization demonstrated the high crystalline quality of the films and interfaces.

Direct determination of the silicon donor ionization energy in homoepitaxial AlN from photoluminescence two-electron transitions

We report on the identification of a two-electron transition for the shallow donor silicon in homoepitaxial aluminum nitride (AlN). One c-oriented sample was analyzed by low temperature photoluminescence spectroscopy on multiple excitation spots. We find a unique correlation of one single emission band, 76.6 meV below the free excitonic emission, with the luminescence of excitons bound to neutral silicon proving the identity as a two-electron transition. The assignment is confirmed by temperature dependent photoluminescence investigations. We find a donor ionization energy of (63.5 ± 1.5) meV for silicon in AlN.

Enforced c-axis growth of ZnO epitaxial chemical vapor deposition films on a-plane sapphire

To enforce perfect c-axis orientation of ZnO epitaxial films grown on a-plane sapphire, we first grew perfectly perpendicularly aligned (i.e., c axis oriented) ZnO nanopillars on such sapphire substrates, and then over-grew these by a closed epitaxial film using a modified chemical vapor deposition process at atmospheric pressure. X-ray diffraction and low temperature photoluminescence measurements confirm the desired epitaxial relationship and very high crystalline quality. This growth scheme is an efficient method to suppress dislocations and polycrystalline growth of ZnO and could work equally well for other heteroepitaxial epilayer/substrate systems.

Composition dependent valence band order in c-oriented wurtzite AlGaN layers

The valence band order of polar wurtzite aluminum gallium nitride (AlGaN) layers is analyzed for a dense series of samples, grown heteroepitaxially on sapphire substrates, covering the complete composition range. The excitonic transition energies, found by temperature dependent photoluminescence (PL) spectroscopy, were corrected to the unstrained state using input from X-ray diffraction. k⋅p theory yields a critical relative aluminum concentration xc=(0.09±0.05) for the crossing of the uppermost two valence bands for strain free material, shifting to higher values for compressively strained samples, as supported by polarization dependent PL. The analysis of the strain dependent valence band crossing reconciles the findings of other research groups, where sample strain was neglected. We found a bowing for the energy band gap to the valence band with Γ9 symmetry of bΓ9=0.85eV, and propose a possible bowing for the crystal field energy of bcf=−0.12eV. A comparison of the light extraction efficiency perpendic...

Cathodoluminescence of GaInN quantum wells grown on nonpolar a plane GaN: Intense emission from pit facets

GaxIn1−xN quantum wells grown by metal organic vapor phase epitaxy on a plane GaN grown on r plane sapphire substrate typically show relatively large surface pits. We show by correlation of low temperature photoluminescence, cathodoluminescence, scanning and transmission electron microscopy that the different semipolar side facets of these pits dominate the overall luminescence signal of such layers.

Polarity-controlled ultraviolet/visible light ZnO nanorods/p-Si photodetector

Vertically aligned ZnO nanorods of high quality were grown on p-type silicon substrate by a modified chemical vapor phase process. Low temperature photoluminescence measurements show a near band gap donor bound exciton full width at half maximum of less than 500 μeV, and only weak green defect luminescence proofing the superior optical quality of the ZnO structures. n-ZnO/p-Si photodetector devices were processed from these upright standing nanorods. The devices show bias dependent selective photon response: under forward bias, an ultraviolet to visible sensitivity ratio of three orders of magnitude was achieved, while under reverse bias ultraviolet and visible light contributed similarly to the electrical readout current. Thus, the characteristics of this detector element can be switched between “solar blind” and “broadband.”

Cathodoluminescence and photoluminescence study on AlGaN layers grown with SiNx interlayers

In this study the optical properties of high quality c-plane AlGaN layers grown on c-plane sapphire by metal organic vapor phase epitaxy have been investigated. Submonolayers of SiNx have been deposited in situ to reduce the dislocation density. After subsequent AlGaN growth atomic force microscopy shows hexagonal hillocks. They consist of differently oriented facets, which contain different amounts of Al as we find in low temperature scanning electron microscope cathodoluminescence measurements. In macroscopic photoluminescence measurements, this leads to doublet emission bands. Further AlGaN overgrowth planarizes the surface, both emission bands coalesce, and the defect density is reduced.

Very slow decay of a defect related emission band at 2.4 eV in AlN: Signatures of the Si related shallow DX state

We report on a defect related luminescence band at 2.4 eV in aluminum nitride bulk crystals, for which we find strong indications to be related to silicon DX centers. Time resolved photoluminescence spectroscopy using a sub-bandgap excitation reveals two different recombination processes with very long decay times of 13 ms and 153 ms at low temperature. Based on the results of temperature and excitation dependent photoluminescence experiments, the process with the shorter lifetime is assigned to a donor-acceptor pair transition involving the shallow silicon donor state, which can be emptied with a thermal dissociation energy of 65 meV. The slower process with a thermal quenching energy of 15 meV is assigned to the slightly deeper Si DX state known from electron paramagnetic resonance experiments, which is transferred back to the shallow donor state.

Oxygen vacancies induced DX center and persistent photoconductivity properties of high quality ZnO nanorods

Ultraviolet sensors based on homoepitaxially grown ZnO nanorods were fabricated using clean room technology. We study the spectral dependence and frequency dependence of the photoresponse of these rods at different temperatures and ambient conditions. Whereas the response for above-bandgap light is fast, we find a slow response to light below band gap and clear signatures of persistent photoconductivity. These findings are explained by switching oxygen vacancies by light from nonconductive to conductive state, whereas the oxygen vacancies undergo a large lattice relaxation. The threshold photon energy for this process is found to be 2.6 eV at room temperature.

Synchrotron-based photoluminescence excitation spectroscopy applied to investigate the valence band splittings in AlN and Al0.94Ga0.06N

We demonstrate that synchrotron-based photoluminescence excitation (PLE) spectroscopy is a versatile tool for determining valence band splittings of AlN and high aluminum content AlGaN. PLE results are independently confirmed by synchrotron-based spectroscopic ellipsometry. The splittings between the ordinary and the extraordinary absorption edges are found to be −240 meV and −170 meV for AlN and Al0.94Ga0.06N, respectively. These values differ from the crystal field energy due to residual strain.