• PDF / 449,904 Bytes
• 6 Pages / 612 x 792 pts (letter) Page_size
• 5 Downloads / 246 Views

DOWNLOAD

REPORT

---

L5.8.1

Deep Donor-Acceptor Pair Luminescence in Codoped GaN Bing Han, Joel M. Gregie, Melville P. Ulmer,1 and Bruce W. Wessels Department of Materials Science and Engineering and Materials Research Center, Northwestern University, Evanston, IL 60208, USA 1 Department of Physics and Astronomy, Northwestern University, Evanston, IL 60208, USA ABSTRACT Deep level defects formed in p-type GaN:Mg codoped with shallow donors have been investigated by photoluminescence (PL) spectroscopy. A donor-acceptor pair (DAP) luminescence band peaked at 2.45 eV dominates the room temperature PL spectrum in heavily codoped epilayers. A superlinear dependence of PL intensity on excitation density is observed for this band, with an exponent of 1.4~1.7. The intensity of this band increases with increasing temperature with a maximum at 264K. To explain the luminescent behavior a DAP model was developed whereby the recombination involves a deep donor and shallow Mg acceptor. The deep donor is tentatively attributed to a DX center. INTRODUCTION Advances in the epitaxial deposition of group III-V nitrides have led to high efficiency solid state devices such as UV-blue lasers and solar-blind UV detectors [1,2]. Despite their importance, much is still unknown about deep recombination centers and their properties in these materials and devices. For example, metastable defects have been observed in p-type GaN:Mg whose origin remains controversial [3-7]. Persistent photoconductivity effect has been observed in p-type GaN and attributed to the bistable Mg center [3,4]. Evidence for deep level defect formation has also been obtained from photoluminescence measurements. Deep level green luminescence [5-7] is often observed in p-type GaN:Mg, which has been attributed to transitions involving deep Mg complexes [6]. In the present study, codoping of p-type GaN with Mg and Si or O was investigated to determine its effect on the luminescent properties and in particular deep-level formation. In the codoped material, a broad DAP emission band at 2.45 eV was observed at room temperature. The band is tentatively attributed to recombination involving a deep DX state and a shallow Mg acceptor. EXPERIMENTAL DETAILS Co-doped p-type GaN films were grown by atmospheric pressure metal-organic vapor phase epitaxy (MOVPE) on c-plane sapphire substrates [10,11]. Film growth was initiated with a low temperature GaN nucleation layer, followed by a 0.5-1.0 µm semi-insulating (>100Ωcm) GaN:Mg buffer layer. Films were then doped with Mg and the donor O or Si. The codoped layer, 0.5-1 µm thick, was grown at 1030°C. The as-grown films were annealed for 15 minutes in N2 at 850°C to activate the Mg acceptors through the removal of hydrogen. Photoluminescence measurements were made between 18K and 300K using a closed-cycle He cryostat. A He-Cd laser was used as the above bandgap excitation source and the excitation intensity was varied over the range 10-5-1.2 W/cm2 using calibrated neutral density filters. Van der Pauw measurements were conducted at room temperature to determi