Selectively Excited Blue Luminescence in heavily Mg doped p - type GaN

The emission at -2.8 eV from heavily doped p-GaN, known as the blue luminescence (BL), has been studied by selective excitation using a dye laser tunable between 2.7-3.0 eV. The peak position and intensity of the BL are found to exhibit an unusual dependence on the excitation photon energy. We have explained our results with a shallow-donor and deep-acceptors pair recombination model which includes potential fluctuations induced by heavy doping. We found a "critical energy" of -2.8 eV for the BL. Electron-hole pairs with energies above this energy are able to achieve quasithermal equilibrium while those w4th energies below 2.8 e\' are strongly "localiied". It is now well known that heavily doped p-type GaN often exhibits a strong broadband luminescence with a peak centered around 2.8-2.9 eV in the blue[l]. This blue luminescence (BL) peak behaves differcntly than the higher energy emission around 3.1-3.2 cV which has been attributed to recombination at shallow impurity centers. There is no consensus in the identification of the BL. Most authors agree that the BL originate from donor-acceptor pair recombination, however, they disagree on the nature of the donors and acceptors. For example, optically detector magnetic resonance (ODMR) study in a Mg-doped bulk sample has led to the suggestion that complexes involving deep donors are involved[2]. On the other hand, similar ODMR studies in thin film samples have led to the conclusion that deep acceptors and shallow donors are involved[3]. Recently, Suski [4] has measured the pressure dependence of the BL in bulk GaN samples and concluded that deep donors are involved. In this paper we report an investigation of the BL in a heavily doped GaN:Mg thin film sample grown by metal-organic chemical vapor deposition (MOCVD) on sapphire by selective excitation with a tunable dye laser. This technique has also been applied to study the yellow luminescence (YL) in n-type GaN[S]. Unlike the results in YL, the peak energy of the resonantly excited BL varies in an unusual way with the excitation photon energy. Our results have been explained by assuming that the BL contains at least two components. Those with photon energy (Am) less than -2.8 eV originate from strongly localized centers while those with hw >>2.8 eV come from interacting centers so that their carriers can achieve quasithermal equilibrium. Our GaN:Mg sample is a 2 microns thick film grown on sapphire by MOCVD. It was doped with around 1019 cm'3 of Mg atoms during growth, followed by annealing at 750°C for 4 minutes. The measured camer concentration is -2 x lOI7 ~ m . ~ . These values are typical of GaN samples which ha\re been reported to display the BL in the literature[6]. The photoluminescence (PL) of our sample at temperature T = 12K when excited by a HeCd laser at 325 nm exhibits a -0.3 eV wide BL peak centered around 2.8 eV. Since it resembles the BL reported by other authors[l], it will not be reproduced here. A Spectra Physics W Ar+-ion laser pumped dye laser (dye: Stilbene 420) with a tuning range of 2.67 0-7803-5814-7/00/$10.00