Time-resolved Spectroscopy of the violet luminescence of undoped AlN
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Internet Journal Nitride Semiconductor Research
Time-resolved Spectroscopy of the violet luminescence of undoped AlN R. Freitag12, K. Thonke12, R. Sauer12, D. G. Ebling12 and L. Steinke12 1Universität 2University
Ulm, Abteilung Halbleiterphysik, Germany, of Freiburg,
(Received Wednesday, May 4, 2005; accepted Thursday, September 22, 2005)
We report on the time-resolved luminescence of the defect-related violet band from undoped AlN epitaxial layers grown on sapphire and SiC. For both measurements in photoluminescence and in cathodoluminescence a decay of algebraic nature at long times is observed. This is typical for donoracceptor pair transitions. We compare the behavior of this band to that of the generically yellow luminescence of GaN.
1 Introduction With the realization of gallium nitride laser diodes [1], GaN has attracted much attention and it is obvious that GaN will be an important semiconductor in optoelectronic applications. Although all the major semiconductor devices have now been realized with GaN, the material is far from being mastered. State-of-the art aluminum nitride layers grown by MBE or MOCVD on SiC or sapphire substrates typically show a broad band centered around 3 eV in luminescence measurements as the dominant feature. This band has been assigned to oxygen accommodating defects with donor-acceptor pair transitions [2] and to transitions within the optical bandtail states [3]. M. D. Bremser et al. [4] observe: (a) an increase of the violet luminescence (VL) with the density of defects (b) a sublinear shift of the VL of AlN to the yellow luminescence of GaN as a function of the decrease of the bandgap of AlxGa1-xN. In this paper we discuss the temperature dependent time-resolved spectroscopy of the VL band and compare it to the generically similar yellow luminescence band of GaN. 2 Results and Discussion The AlN samples investigated here were grown on sapphire and on SiC Substrates by molecular beam epitaxy (MBE). The time-resolved measurement of this luminescence has been done with a correlation method which has the advantage of a greater signal to noise ratio. It is approximately one order of magnitude higher than with the conventional single shot mode method (Figure 1). The exciting beam (electron or photon beam) is modulated with a pseudorandom binary sequence
which has a two level autocorrelation function. The crosscorrelation function of the system response and the binary sequence is, with a multiplicative and an additive constant, the pulse response of the system. The violet photoluminescence of AlN on sapphire excited by a HeCd-Laser at 325 nm is very weak because it is a subbandgap excitation. Therefore only below band edge states could absorb the laser and only a few percent can be absorbed by AlN. Nevertheless it was possible to excite a measurable violet photoluminescence which is red-shifted by about 30 nm compared with the violet cathodoluminescence (Figure 2). The result is an algebraic decay of the luminescence with I∼t-1.9±0.3 at 5 Kelvin at medium and long times ( Figu
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