Time Resolved Photoluminescence of Cubic Mg Doped GaN
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Abstract Mg doped cubic GaN layers were studied by steady state and time resolved photoluminescence. The blue emission due to Mg doping can be decomposed in three bands. The decay curves and the spectral shift with time delays indicates donor-acceptor pair behaviour. This can be confirmed by excitation density dependent measurements. Furthermore temperature dependent analysis shows that the three emissions have one impurity in common. We propose that this is an acceptor level related to the Mg incorporation and the three deep donor levels are due to compensation effects.
Introduction GaN is found to exist in a thermodynamically stable hexagonal and in a metastable cubic phase [I]. Substrates for the cubic material are much easier available, cleavage is less complicated and due to the higher symmetry superior electronic properties are expected. In order to achieve optoelectronic devices based on p-type cubic GaN it is important to study its fundamental properties. As in hexagonal samples, the basic properties like recombination channels of Mg doped cubic GaN samples are still not completely understood. Lightly and moderately doped samples show besides the excitonic transitions at 3.27eV, the donor-acceptor pair DAP transition at 3.15eV and a Mg related DAP recombination at 3.04eV [2]. For heavily doped samples ([Mg]>10 18cm ) deep Mg related transitions are observed. In this work these deep blue emissions are studied by steady state (SS) and time resolved (TR) photoluminescence.
Experiment The cubic GaN layers were grown by plasma-assisted molecular-beam epitaxy on semi-insulating (100) GaAs substrates at a substrate temperature of 720(C. Details of the growth procedure were reported elsewhere [3]. For Mg doping, a commercially available effusion cell with an orifice of 3mm was used. The thickness of the GaN layers varies between 700 and 900nm. Mg concentration and depth distribution was determined by secondary ion mass spectroscopy. In this work samples with Mg concentration from 2.3* 10"' to 5*101 cm-. were analysed. Results are compared with an non-intentionally doped (nid) sample. For steady state (SS) photoluminescence PL measuremenis we used the 325nm line of a HeCd laser. Excitation intensity was varied by neutral density (ND) filters. Time resolved (TR) measurements were carried out with a pulsed Xe lamp as an excitation source and a boxcar system for detection. The emission was dispersed and detected in both cases by a monochromator (I m, 1200/mm) and a photomultiplier, respectively. The samples were cooled down to 10K by a closed-cycle He cryostat. The temperature could
225 Mat. Res. Soc. Symp. Proc. Vol. 572 ©1999 Materials Research Society
he varied from 10K to room temperature by a heating resistance. The temperature was measured by a thermocouple.
Results and discussion Figure I shows SSPL spectra of a heavily Mg doped and a nid sample. In the nid layer there is a strong emission at 3.27eV and 3.15eV ascribed to excitonic and pair emission, respectively. In the heavily Mg doped samples the PL spe
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