Spatially resolved optical emission of cubic GaN/AlN multi-quantum well structures
- PDF / 17,050,661 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 55 Downloads / 182 Views
Spatially resolved optical emission of cubic GaN/AlN multi-quantum well structures D.J. As1, R. Kemper1, C. Mietze1, T. Wecker1, J.K.N. Lindner1, P. Veit2, A. Dempewolf2, F. Bertram2, J. Christen2 1 Department Physik, Universität Paderborn, Warburger Strasse 100, 33095 Paderborn, Germany 2 Institut für Experimentelle Physik, Universität Magdeburg, P.O. Box 4120, D-39016 Magdeburg, Germany ABSTRACT In this contribution we report on the optical properties of cubic AlN/GaN asymmetric multi quantum wells (MQW) structures on 3C-SiC/Si (001) substrates grown by radio-frequency plasma-assisted molecular beam epitaxy (MBE). Scanning transmission electron microscopy (STEM) and spatially resolved cathodoluminescence (CL) at room temperature and at low temperature are used to characterize the optical properties of the cubic AlN/GaN MQW structures. An increasing CL emission intensity with increasing film thickness due to the improved crystal quality was observed. This correlation can be directly connected to the reduction of the linewidth of x-ray rocking curves with increasing film thickness of the c-GaN films. Defects like stacking faults (SFs) on the {111} planes, which also can be considered as hexagonal inclusions in the cubic crystal matrix, lead to a decrease of the CL emission intensity. With low temperature CL line scans also monolayer fluctuations of the QWs have been detected and the observed transition energies agree well with solutions calculated using a one-dimensional (1D) Schrödinger-Poisson simulator. INTRODUCTION For advanced optoelectronic and electronic devices, like light emitting diodes (LEDs), or laser diodes (LDs) Al containing cladding layers or barriers are necessary. The commonly used hexagonal AlyGa1-yN/GaN heterostructures show an inherently strong spontaneous polarization oriented along the hexagonal c-axis as well as strain induced piezoelectric polarization. Such polarization induced electric fields in strained quantum wells can cause the spatial separation of electrons and holes resulting in a severe reduction of optical recombination efficiency [1]. Using the metastable cubic modifications of AlN and GaN such piezoelectric effects can be avoided if the samples are grown in (001) direction [2]. In general, one key issue in device fabrication is the improvement of the structural quality, because extended defects like dislocations reduce the device performance. Due to a lack of cubic GaN bulk substrates [3] 3C-SiC (001) with a mismatch of -3.5% between cubic GaN (cGaN) and 3C-SiC is the substrate of choice. Transmission electron microscopy (TEM) measurements provide the evidence that stacking faults (SFs) on the {111} planes are the predominant crystallographic defects in c-GaN films [4,5]. These SFs are local deviations from the cubic (111) stacking sequence to the (0001) stacking sequence in the wurtzite (WZ) phase. Therefore, SFs can lead to hexagonal inclusions [6]. In the opposite case, basal-plane SFs in a WZ crystal also produce cubic inclusions in the hexagonal crystal structure. Detailed
Data Loading...
