Materials Science and Engineering B 174 (2010) 263–265 Contents lists available at ScienceDirect Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb Magnesium doped GaN grown by MOCVD C. Guarneros ∗ , V. Sánchez Ingeniería Eléctrica, Sección Electrónica del Estado Sólido, Centro de Investigación y de Estudios Avanzados del Instituto Politécnico Nacional, Av. I.P.N. 2508, San Pedro Zacatenco, 07360 México, D.F., Mexico article info Article history: Received 31 August 2009 Received in revised form 5 February 2010 Accepted 9 March 2010 Keywords: Gallium nitride Magnesium Annealing Photoluminescence Hall Effect abstract We have studied the optical and electrical characteristics of undoped and doped GaN layers. The n- and p-type layers have been prepared by low pressure MOCVD technique. Photoluminescence (PL) studies were carried at low temperature. In the PL spectra of undoped GaN layer, a low intensity band edge emission and a broad yellow emission band were observed. The donor–acceptor pair (DAP) emission and its phonon replicas were observed in Mg lightly doped GaN layer. The dominance of the blue and the yellow emissions increased in the PL spectra as the Mg concentration was increased. The X-ray diffraction was employed to study the structure of the layers. Both the undoped and the doped layers exhibited hexagonal structure. The samples were annealed and significant changes were not observed in Hall Effect and in the PL measurements, so we suggest that there is no need of a thermal annealing for magnesium acceptor activation. © 2010 Elsevier B.V. All rights reserved. 1. Introduction The III-nitride semiconductors with their wide band gap have become very important materials for optoelectronic applications. There is special interest in gallium nitride (GaN) growth for the application of the blue light emitting diodes (LEDs), blue laser diodes (LDs) and also as the basis for heterojunction bipolar tran- sistors (HBTs) and high temperature electronics using devices such as high electron mobility transistors (HEMTs). In order to produce these devices, both n- and p-type doping are required [1]. Magnesium was the first p-type dopant of GaN, and is now the most widely used and studied Ga substitutional dopant. However, growth techniques such as metalorganic chemical vapor deposi- tion (MOCVD) introduce large concentrations of hydrogen into the growing material. In p-type material, hydrogen behaves as a donor and forms neutral (Mg–H) ◦ complexes with acceptors suppress- ing the formation of self-compensating native point defects (e.g. V N ) [2]. Therefore, post-growth annealing is required in order to activate those acceptors. These processes are either low energy electron beam irradiation (LEEBI) or a N 2 -ambient thermal anneal- ing above 600 ◦ C. The activation process involves the dissociation of acceptor–hydrogen complexes rather than the removal of separate compensating donors [3]. Depending on the growth conditions and the magnesium con- centration various types of emission were observed in GaN:Mg. For light Mg doping (Mg concentration 10 17 to 10 18 atom/cm 3 ) and at ∗ Corresponding author. Tel.: +52 01 55 5747 3780; fax: +52 01 55 5747 3978. E-mail address: cesyga@yahoo.com.mx (C. Guarneros). low temperatures, the set of bands at about 3.27 eV with 92 meV longitudinal optical (LO)-phonon replicas are observed. These are related to the bound exciton associated with the decay of exci- ton bound at neutral acceptor sites and donor–acceptor pair (DAP) emission due to optical transitions from shallow-donor to a shallow Mg acceptor [4,5]. For heavier magnesium doping, which is required to obtain p- type material (Mg concentration 10 19 atom/cm 3 ), samples often exhibit an ultraviolet band at 3.1–3.2 eV, which can overlap with 3.27 eV DAP emission. Moreover, quite often a strong blue lumi- nescence band at 2.7–2.9 eV is observed in conjunction with 3.1 eV band or completely dominates the spectrum. The former is due to the optical transitions from the conduction band to the Mg accep- tor. And the latter is attributed to transitions from the conduction band to a deep acceptor level or/and Mg complex [1,6,7]. 2. Experimental procedure In order to investigate Mg doping characteristics in GaN, two kinds of GaN films with different hole concentrations obtained by varying the amount of Cp 2 Mg flow rate were grown. P-type GaN:Mg layers were deposited on (0 0 0 1) oriented (Al 2 O 3 ) sapphire sub- strates using low pressure MOCVD horizontal quartz reactor [8]. A low temperature GaN buffer layer (520 ◦ C) was grown using timethylgallium (TMGa) and ammonia (NH 3 ) as Ga and N pre- cursors, respectively. Bis-(cyclopentadienyl)magnesium (Cp 2 Mg) was used as p-type doping source. After deposition of the GaN buffer layer, the substrate temperature was raised to 900 ◦ C to grow GaN epitaxial layer. The layers of p-GaN were grown at 900 ◦ C for 90 min with a Cp 2 Mg flow rate to 2.03 × 10 -7 mole/min and 0921-5107/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2010.03.022