J. Nissilä

THE INFLUENCE OF MG DOPING ON THE FORMATION OF GA VACANCIES AND NEGATIVE IONS IN GAN BULK CRYSTALS

Gallium vacancies and negative ions are observed in GaN bulk crystals by applying positron lifetime spectroscopy. The concentration of Ga vacancies decreases with increasing Mg doping, as expected from the behavior of the VGa formation energy as a function of the Fermi level. The concentration of negative ions correlates with that of Mg impurities determined by secondary ion mass spectrometry. We thus attribute the negative ions to MgGa−. The negative charge of Mg suggests that Mg doping converts n-type GaN to semi-insulating mainly due to the electrical compensation of ON+ donors by MgGa− acceptors.

Digital measurement of positron lifetime

A prototype of a digital positron lifetime apparatus is presented. We demonstrate that direct digitizing of detector pulses and subsequent simple analysis with software, can be used to replace the conventional analog electronics chain (constant-fraction discriminators, time-to-amplitude converter and multichannel analyzer). In this work, we use a fast digital oscilloscope. The quality of the lifetime data is shown to be as good as with a usual apparatus. For a pulsed positron lifetime beam the digital system is particularly suitable as no coincidence detection is needed and only one analog pulse has to be analyzed.

High pressure fabrication and processing of GaN:Mg

Results on introduction of magnesium to GaN by three methods are presented. They consists of (i) high pressure growth of bulk, single crystals of GaN from GaMg melt, (ii) diffusion of Mg to bulk GaN and to layers of GaN:Al2O3 at high temperatures and high pressures and (iii) implantation of Mg to bulk, single crystals and to layers of GaN:Al2O3 and subsequent high-pressure annealing. Applied pressure is in the range of 10‐15 kbar and temperatures between 1200‐1500°C. The growth of bulk, highly Mg doped crystals leads to the semi-insulating material with the characteristic blue photoluminescence band at about 3 eV. High pressure annealing of bulk crystals as well as GaN:Al2O3 layers in (N2 Mg) atmosphere leads to the increase in the Mg incorporation with the highest diffusivity observed for GaN:Al2O3 layers. The performed experiments give an evidence of the importance of the defect (dislocations) in diffusion of Mg in the GaN semiconductor. Moreover, incorporation of Mg impurity appears to be higher on (00.1) Ga-face of the wurtzite GaN crystals than on the (00. 1) N-face. We demonstrate also a strong enhancement of the blue-photoluminescence intensity in high pressure annealed GaN:Al2O3 layers (N2 Mg atmosphere) and Mg-implanted and high pressure annealed GaN crystals and layers.

Observation of Ga vacancies and negative ions in undoped and Mg-doped GaN bulk crystals

Abstract Gallium vacancies and negative ions are observed in GaN bulk crystals by applying positron lifetime spectroscopy. The concentration of Ga vacancies decreases with increasing Mg doping, as expected from the behavior of the V Ga formation energy as a function of the Fermi level. The concentration of negative ions correlates with that of Mg impurities determined by secondary ion mass spectrometry. We thus attribute the negative ions to Mg − Ga . The negative charge of Mg suggests that Mg doping converts n-type GaN to semi-insulating mainly due to the electrical compensation of O N + donors by Mg Ga − acceptors.

Successful implementation of fast preamplifiers in a positron lifetime spectrometer

A method to improve the long-term stability of fast coincidence apparatuses, like a positron lifetime spectrometer, is reported. Ageing of the photomultiplier tubes (PMT), i.e., nonreversible degradation of the gain, can be slowed down by lowering the supply voltages over the PMTs and by compensating the lower gain with fast preamplifiers set at the anodes. The timing characteristics of the PMTs can be preserved by using voltage dividers with which the voltage in the input optics of the PMTs remains high enough for good photoelectron collection efficiency (for XP2020 above 300 V). With this setup, the anode current and the rate of gain degradation can be reduced at least by a factor of 20 with no loss in the time resolution of the spectrometer.

Evidence on a bond-breaking relaxation in the bistable centers In and Ga in CdF2

Abstract Positron annihilation experiments reveal an open-volume defect in the deep state atomic configurations of bistable donors In and Ga in CdF 2 . The size of the open volume is at least half of a monovacancy. The results are similar to those obtained previously for the DX centers in the covalent system Al x Ga 1− x As. It is therefore likely that the bond-breaking mechanism (substitutional to interstitial atomic motion) responsible for metastability of point defects in covalent semiconductors is more universal and its validity extends to even some highly ionic compounds, like CdF 2 .

Sample illumination facility for slow positron beam studies and its application to the photoionization cross-section of the DX center in AlxGa1-xAs

Abstract We have built a sample illumination facility for slow positron beam studies. It enables the illumination of the sample with monochromatic light in visible and near-IR wavelength range with the maximum photon flux of 10 13 –10 15 s −1 cm −2 . Light is guided from the monochromator into the vacuum chamber by an optical fibre bundle and focused onto the sample with a mirror and a lens. We have applied the new illumination facility in studying the photoionization cross-section of the DX center in AlGaAs. The measurements were done using photon energies in the range of 1.0–1.6 eV and the cross-sections were calculated from the behaviour of the positron annihilation parameters as a function of the photon fluence. The resulting photoionization cross-sections were found to coincide well with the results from the photo-capacitance measurements.