Origin of the red luminescence in Mg-doped GaN

Abstract

Optically-detected magnetic resonance (ODMR) and positron annihilation spectroscopy (PAS) experiments have been employed to study magnesium-doped GaN layers grown by metal-organic vapor phase epitaxy. As the Mg doping level is changed, the combined experiments reveal a strong correlation between the vacancy concentrations and the intensity of the red photoluminescence band at 1.8 eV. The analysis provides strong evidence that the emission is due to recombination in which electrons both from effective mass donors and from deeper donors recombine with deep centers, the deep centers being vacancy-related defects.Deep defects play a key role in the performance limits and aging effects of GaN-based light-emitting devices. They also lead to photoluminescence (PL) at energies well below the band-gap. For example, PL and opticallydetected magnetic resonance (ODMR) studies [1,2,3,4] have suggested that deep defects are responsible for the red (1.8 eV) luminescence band which is often observed in Mg-doped GaN and that the band is due to recombination emission in which vacancy-dopant complexes are involved [1,2]. However, this proposal was mainly based on indirect evidence and on previous experience of II -VI compounds, and further experimental confirmation is therefore needed. The present study involved the use of both ODMR and positron annihilation spectroscopy (PAS) on the same set of samples covering a range of Mg doping levels and we have established a correlation between the ODMR spectra (obtained by monitoring the red PL) and the PAS results.ODMR is well established as a means of investigating centers involved in recombination processes in semiconductors [5,6]. For a detailed description of the technique and our ODMR system, see Ref. [7]. The ODMR was carried out at 14 GHz with the specimen at 2K. The PL was excited with a UV argon-ion laser (363.8/351.1 nm). The microwaves were chopped at 605 Hz and changes in the PL intensity caused by magnetic resonance were monitored at this frequency as the magnetic field was slowly swept. PAS with a slow positron beam is an effective tool for the investigation of open volume defects such as neutral or negatively charged vacancies in semiconductor films. When positrons annihilate electrons in semiconductors the resulting gamma ray energy spectrum, peaked at 511 keV, is Doppler-broadened (since the electrons have a range of momenta). The annihilation linewidth is characterized by quantities S (W ), defined as the central (wing) fraction of the line. The value of S (W ) is characteristic of the material under study, but * Electronic address: d.wolverson@bath.ac.uk is generally higher (lower) when vacancies are present [8]. Measurements of S (W ) can thus be used to monitor vacancy concentrations. In the present work, singledetector Doppler-broadening PAS was performed using a magnetic transport positron beam system [9]. Positrons were implanted into the layers at energies in the range 0.1 -30 keV, corresponding to mean depths up to 1.5 nm.Details of the growth of the GaN:Mg sampl...

Point defects in group-III nitride semiconductors studied by positron annihilation

“…Positron annihilation is a non-destructive characterization tool, and there is no limitation on the sample conductivities or the measurement temperatures. Vacancy-type defects in group-III nitride semiconductors have been investigated using this method [2][3][4][5][6][7][8][9][10][11][12], and the results show that positrons are a powerful probe for studying cation vacancies, vacancy clusters, and vacancy-impurity complexes.…”

Properties of the main Mg-related acceptors in GaN from optical and structural studies

“…noted and sometimes dominates the PL spectra. [10][11][12][13][14] We show such spectra in Fig. 5 for MOCVD grown samples, measured under different excitation conditions.…”

“…10 In the deep red spectral region there is a broad band observed at about 1.8 eV, suggested as related to a deep Mg-related complex defect. [12][13][14] In this paper, we will present additional PL and CL data obtained from MOCVD-grown GaN:Mg samples on low defect density bulk GaN substrates. These data allow a more definite identification of the above-mentioned characteristic ABE and DAP spectra for GaN:Mg.…”

Optically-detected magnetic resonance of spin-paired complexes emitting in the2.3eVspectral region in Mg-doped GaN

“…The ODMR spectra change drastically when the samples are annealed but here, we report only on spectra seen in unannealed material when monitoring emission in the green-yellow region ͑the same samples have previously been the subject of a positron annihilation investigation combined with an ODMR study of PL in the red region͒. 7 An ODMR study of these samples following annealing has also been reported previously 8 and has led to a detailed model for the Mg acceptor states. A description of the detailed behavior of the ODMR and PL signals under a range of annealing processes will be reported separately.…”

“…31 The present experiments provide the first evidence that interstitial gallium can be present in as-grown magnesium doped GaN, the data suggesting that it is associated with ͑and thus compensating͒ the magnesium acceptors. In a separate study of these specimens in which we have combined ODMR with positron annihilation spectroscopy, 7 we have found evidence that in as-grown Mg-doped GaN there exist a large number of vacancies. Annealing is found to reduce this number; at the same time, the spectra observed in the present study disappear.…”