Materials Science and Engineering B82 (2001) 91 – 94 Deep levels in MOCVD n -type hexagonal gallium nitride studied by high resolution deep level transient spectroscopy P. Muret a, *, A. Philippe a , E. Monroy b , E. Mun˜oz b , B. Beaumont c , F. Omne`s c , P. Gibart c a LEPES -CNRS, BP166, 38042 Grenoble Cedex 9, France b Uniersidad Politecnica de Madrid, E.S.T.I. Telecomunicacion, Ciudad Uniersitaria, 28040 Madrid, Spain c CRHEA-CNRS, Parc Sophia Antipolis, 06560 Valbonne, France Abstract Deep level transient spectroscopy (DLTS) is performed in MOCVD  -GaN either doped with two different concentrations of Si or unintentionally doped. Capacitance transients are measured in Schottky diodes made of an Au or Ni rectifying contact and an Al/Ti ohmic contact, both on the top of the samples. Only two peaks are detected in each sample in the energy range from the conduction band edge down to 1.1 eV below it, respectively close to 0.50 and 0.92–1.05 eV, by Fourier Transform DLTS (FTDLTS) with concentrations not exceeding 3 ×10 15 cm −3 . These two results testify the high crystalline quality of the samples. The deeper level characteristics depend on the shallow impurity, either Si or the unintentional shallow donor, in deep states which comprise in fact a fine structure not evidenced by FTDLTS. A high resolution DLTS (HRDLTS) method is implemented to resolve this fine structure into several sub-levels which cannot be related to distinct chemical environments. The study of emission and capture kinetics confirms that at least three charge states ( +,0,-) are involved. It is concluded that MOCVD  -GaN comprises deep centers which are stabilized in such a form with a concentration in the range of a few 10 14 –10 15 cm −3 . © 2001 Elsevier Science B.V. All rights reserved. Keywords: Gallium nitride; Deep levels; DX centers; DLTS; Capture and emission kinetics www.elsevier.com/locate/mseb 1. Introduction Some studies using deep level transient spectroscopy (DLTS) techniques have been done previously in GaN prepared by molecular beam epitaxy (MBE) [1], metal- lorganic chemical vapor decomposition (MOCVD) [2,3] and hydride vapor phase epitaxy (HVPE) [4]. However, the microscopic nature of the defects is not always well understood. A major challenge consists in determining the origin of electrically active defects, and to know if those appearing as electron traps in n-type materials may be DX centers provided by the deep level form of donor impurities like silicon or oxygen. In the present study, FTDLTS and HRDLTS are applied to several n -type samples prepared by MOCVD, either doped with silicon or unintentionally doped. 2. Diode preparation and measurements Several samples have been prepared by MOCVD on sapphire substrates with an AlN buffer layer [5]. Shal- low doping concentrations N D calculated from capaci- tance – voltage characteristics are either 2.10 17 or 2.10 18 cm −3 in Si doped samples while unintentionally doped samples showed an N D value of 2.10 16 cm −3 , due likely to the oxygen donor both because oxygen is always present as traces in gases used in MOCVD and more energetically favorable than nitrogen vacancies [6,7]. Typical Hall mobilities are 350 cm 2 /Vs at 300 K. Schot- tky contacts were achieved by Au or Ni/Au Joule metallization while ohmic contacts by Ti/Al layers sur- rounding the circular diodes defined by photolithogra- phy [8]. Schottky diodes used in DLTS measurements show ideality factors near 1.1 or less and capacitances below 200 pF. Their series resistance is some 100  at 300 K and does not exceed 1.2 k at 120 K so that the diode time constant always stays smaller than 300 ns, * Corresponding author. Tel.: +33-47-6887893; fax: +33-47- 6887988. E-mail address: muret@lepes.polycnrs-gre.fr (P. Muret). 0921-5107/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII:S0921-5107(00)00780-7