Gert Denninger

Shallow donors in GaN: A magnetic double resonance investigation

Abstract The residual shallow donor in GaN is investigated by electron spin resonance and the Overhauser shift double resonance technique. From the resolved quadrupolar splitting, we determine the electric field gradients at the Ga and the N site. The hyperfine interaction, as determined simultaneously from the Overhauser shift and the Knight shift, supports a delocalized donor wavefunction with an effective Bohr radius of 20–30 A.

Optically detected nuclear magnetic resonance and Knight shift in AlxGa1−xAs/GaAs heterostructures

Abstract In Al x Ga 1− x As/GaAs heterostructures the NMR and the Knight shift of 69 Ga, 71 Ga and 75 As has been detected optically due to the enhanced sensitivity compared to conventional NMR. From the Knight shifts results a ratio 1.4 of the conduction electron |4;ψ( r k )| 2 on the V - and III -site. The nuclear spin lattice relaxation times for each nuclear species were determined separately. These times are discussed in comparison with results from other measurements.

Nuclear spin relaxation in AlGaAsGaAs heterostructures observed via optically detected magnetic resonance (ODMR) experiments

Abstract The resonant field at which an electron spin resonance (ESR) occurs may be shifted by an effective magnetic field, which is due to spin polarized nuclei. This shift, known as Overhausershift, is caused by the field of all polarized nuclei without respect to their isotopic number. Irradiation of the nuclear magnetic resonance (NMR) frequency of some of the isotopes during the ESR experiment allows to measure the nuclear field of only one kind. The time dependent evolution of the Overhausershift is according to the nuclear spin relaxation time. In undoped semiconductors e.g. in AlGaAs GaAs heterostructures, typical time constants are determined to be between 10 minutes and one hour; irradiation with visible light which changes the number of free carriers affects the relaxation process. We further show that the ratio of the relaxation times of the two Ga-isotopes 69Ga and 71Ga behaves as a quadrupolar relaxation rather than relaxation via Fermi contact interaction with free carriers. This result is supported by optically detected NMR measurements, which exhibit a quadrupolar splitting of the NMR resonance line.

A magnetic double-resonance investigation of the quadrupole interaction in ZnO single crystals

Abstract Shallow donors in ZnO are used to investigate the quadrupole interaction in single crystals. From the resolved quadrupole splitting, we determine the electric field gradients at the 67 Zn and the 17 O site. A magnetic double-resonance technique both increases the sensitivity and the accuracy of the field gradient determination. The data for ZnO are compared to previous results on GaN.

Optically detected conduction electron spin resonance and Overhauser shift in p-GaAlAs/GaAs-heterostructures

Abstract The spin resonance of the conduction electrons in a single GaAs/GaAlAs heterostructure has been detected by measuring the degree of circular polarization of the luminescence in a microwave magnetic resonance field at 8.4 GHz. The effective g -factor is g ∗ = -047 ± 0.01, slightly different from the bulk value ⋍ -0.44. Additionally, the shift of the conduction electron spin resonance frequency due to the polarization of nuclei (Overhauser shift) has been observed for the first time in a single heterostructure.

Double resonance calibration of g factor standards: Carbon fibers as a high precision standard

The g factor of paramagnetic defects in commercial high performance carbon fibers was determined by a double resonance experiment based on the Overhauser shift due to hyperfine coupled protons. Our carbon fibers exhibit a single, narrow and perfectly Lorentzian shaped ESR line and a g factor slightly higher than gfree with g=2.002644=gfree·(1+162ppm) with a relative uncertainty of 15ppm. This precisely known g factor and their inertness qualify them as a high precision g factor standard for general purposes. The double resonance experiment for calibration is applicable to other potential standards with a hyperfine interaction averaged by a process with very short correlation time.