Amphoteric Arsenic in Gan

Reprint from APPLIED PHYSICS LETTERS 90 (2007) 181934 Amphoteric arsenic in GaN U. Wahla Instituto Tecnológico e Nuclear, Estrada Nacional 10, 2686-953 Sacavém, Portugal, and Centro de Física Nuclear da Universidade de Lisboa, Avenida Professor Gama Pinto 2, 1649-003 Lisboa, Portugal J. G. Correia Instituto Tecnológico e Nuclear, Estrada Nacional 10, 2686-953 Sacavém, Portugal, and Centro de Física Nuclear da Universidade de Lisboa, Avenida Professor Gama Pinto 2, 1649-003 Lisboa, Portugal, and CERN-PH, 1211 Geneva 23, Switzerland J.P. Araújo Departamento de Física, Universidade do Porto, Rua do Campo Alegre 687, 4169-007 Porto, Portugal E. Rita and J.C. Soares Centro de Física Nuclear da Universidade de Lisboa, Avenida Professor Gama Pinto 2, 1649-003 Lisboa, Portugal The ISOLDE collaboration CERN-PH, 1211 Geneva 23, Switzerland (Received 15 March 2007; accepted 11 April 2007; published online 4 May 2007) We have determined the lattice location of implanted arsenic in GaN by means of conversion electron emission channeling from radioactive 73As. We give direct evidence that As is an amphoteric impurity, thus settling the long-standing ques- tion as to whether it prefers cation or anion sites in GaN. The amphoteric character of As and the fact that AsGa “anti- sites” are not minority defects provide additional aspects to be taken into account for an explanantion of the so-called “miscibility gap” in ternary GaAs1−x Nx compounds, which cannot be grown with a single phase for values of x in the range 0.1<x<0.99. © 2007 American Institute of Physics. [DOI: 10.1063/1.2736299] The growth and properties of ternary semiconductors of scale with the amount of radioactive 71As and 72As resulting the nitride family have been under intense investigation ever from radioactive decay. since nitrides emerged as blue-light emitting devices in the The amphoteric nature of As in GaN was first proposed by 1990s. The particular interest in the ternary nitrides results Guido et al [18], who suggested that As could fill up both Ga from the fact that they allow adjusting the semiconductor band and N vacancies and thus reduce yellow band emission and gap to values not available in pure compounds. One of the carrier scattering in GaN. Subsequently, the 2.6 eV blue lumi- ternary systems under study is GaAs1−x Nx, which shows some nescence [12, 14-16, 19] and a deep level ~0.8 eV below the intriguing properties such as large band gap bowing parameter conduction band have been attributed to AsGa [20, 21]. While [1-3]. Unfortunately the growth of GaAs1−x Nx compounds en- there are indications that the AsGa:AsN ratio changes with the counters significant difficulties, one of the reasons being that overall Ga:N:As stoichiometry [13, 15], so far no direct evi- GaAs crystallizes in the cubic zinc blende structure while the dence for so-called AsGa “anti-sites” exists, in particular it is most stable polytype of GaN is hexagonal wurtzite. However, unknown whether AsGa might be a minority or majority defect while, on the As-rich side of the phase diagram, it is possible to in lightly As-doped GaN. incorporate up to ~10-15% of N into cubic GaAs [3, 4], on the A large number of theoretical studies have investigated the N-rich side not more than ~1% of As in GaN have been properties of GaAs1−x Nx alloys [1, 2, 22-29] and also the nature achieved [5-8]. In the intermediate region, usually the coexis- of As dopants in hexagonal [2, 22-24, 27, 30, 31] and cubic tence of hexagonal N-rich GaAsN and cubic As-rich GaNAs GaN [28, 31-34]. However, most of these studies considered phases is observed. only isoelectronic As atoms substituting for N, thus preserving GaN which is lightly As-doped is also highly interesting the cation-anion chemical bonds, and neglecting the possibility due to the fact that it shows intense blue luminescence centered of AsGa anti-sites. Only Van De Walle and Neugebauer [31] around 2.6 eV, as observed already many years ago in As- and Ramos et al [34] pointed out that As probably acts as an implanted GaN by Pankove and Hutchby [9] and Metcalfe et al amphoteric impurity, which may substitute for both N and Ga. [10], and subsequently also found in GaN doped with As dur- In this work we have determined the lattice location of ra- 73 ing growth [5, 11-16]. The chemical nature of the 2.6 eV blue dioactive As (t1/2 = 80.3 d) in single-crystalline thin films of luminescence and the fact that it results from optical centers GaN by means of the emission channeling technique [35] and involving one As atom only was unambiguously proven by we present direct experimental evidence that As acts in fact as means of the radiotracer photoluminescence (PL) work of an amphoteric impurity. Emission channeling is based on the Stötzler et al [17], who studied the luminescence along the fact that charged particles from nuclear decay (α, β−, β+, con- radioactive decay chains 71As→71Ge→71Ga and version electrons) experience channeling or blocking effects 72Se→72As→72Ge. Following the ion implantation of 71As and along major crystallographic axes and planes. The resulting 72Se they observed the intensity of the 2.6 eV luminescence to 1 Reprint from APPLIED PHYSICS LETTERS 90 (2007) 181934 experiment simulation SGa+SN sites simulation SGa sites simulation SN sites -2 -1 0 1 2 -2 -1 0 1 2 -2 -1 0 1 2 -2-1012 (a) (e) [0001] (a) [0001] (d) [0001] 2 1.20 - 1.27 1.39 - 1.47 1.18 - 1.23 1.30 - 1.39 1.12 - 1.18 1 1.14 - 1.20 1.08 - 1.14 1.22 - 1.30 1.07 - 1.12 1.13 - 1.22 1.01 - 1.07 (1120) 1.01 - 1.08 0 1.05 - 1.13 0.96 - 1.01 0.95 - 1.01 0.97 - 1.05 0.91 - 0.96 -1 0.88 - 0.95 0.88 - 0.97 0.85 - 0.91 0.82 - 0.88 0.80 - 0.88 0.80 - 0.85 -2 0.75 - 0.82 0) 11 (b) [1102] (e) [1102] (0 (f) [1102] 2 1.39 - 1.47 1.14 - 1.18 (b) 1.24 - 1.30 1.30 - 1.39 1.10 - 1.14 1 1.18 - 1.24 1.22 - 1.30 1.06 - 1.10 1.12 - 1.18 1.13 - 1.22 1.02 - 1.06 (1120) 0 1.06 - 1.12 1.05 - 1.13 0.98 - 1.02 1.00 - 1.06 0.97 - 1.05 0.94 - 0.98 -1 0.94 - 1.00 0.88 - 0.97 0.90 - 0.94 0.88 - 0.94 0.80 - 0.88 0.86 - 0.90 -2 0.82 - 0.88 (c) [1101] (f) [1101] 1) 02 (g) [1101] 1.43 - 1.52 1.49 - 1.58 (2 2 (c) 1.23 - 1.28 1.34 - 1.43 1.40 - 1.49 1 1.17 - 1.23 1.25 - 1.34 1.31 - 1.40 1.12 - 1.17 1.15 - 1.25 1.22 - 1.31 (1120) 0 1.07 - 1.12 1.06 - 1.15 1.12 - 1.22 1.02 - 1.07 0.97 - 1.06 1.03 - 1.12 -1 0.97 - 1.02 0.88 - 0.97 0.94 - 1.03 0.79 - 0.88 0.85 - 0.94 -2 0.91 - 0.97 0.86 - 0.91 -2 -1 0 1 2 -2-1012[deg] 1) (h) [2113] 01 2 (1 FIG. 2. (Color online) Theoretical emission channeling patterns for 1.35 - 1.43 (d) 1 1.27 - 1.35 substitutional SGa and SN sites. (a)-(c): patterns for 100% of emitter atoms on S sites. (d)-(f): patterns for 100% on S sites. (0110) 0 1.20 - 1.27 Ga N 1.12 - 1.20 -1 1.04 - 1.12 0.97 - 1.04 [37, 38]), and a diversity of interstitial sites resulting from dis- -2 0.89 - 0.97 0.81 - 0.89 placements along or off the c-axis. The theoretical emission -2 -1 0 1 2 -2 -1 0 1 2 [deg] channeling patterns were calculated by means of the “many- beam” theory of electron diffraction in single-crystals [35]. FIG. 1. (Color online) Angular distribution of conversion electron emis- Details with respect to the structural properties of GaN used in sion yields from 73As→73Ge in GaN following 300°C annealing around the simulations have been given previously [37]. the (a) [0001], (b) [1102], (c) [1101] and (d) [2113] axis. (e)-(h): best The experimental emission patterns along the [0001], fits of the channeling patterns corresponding to 57(7)% of probe atoms [1102], [1101] and [2113] directions, following 300°C anneal- at substitutional SGa and 48(6)% at SN sites. ing, are shown in Figs. 1 (a)-(d). While the channeling effect along [0001] shows that the emitter atoms are located along the anisotropic emission yield from the crystal surface character- c-axis, this does not yet allow distinguishing between SGa and izes the lattice site occupied by the probe atoms during decay. SN sites since both are aligned with the [0001] axis. However, The production and ion implantation of 73As were per- it is obvious from a simple visual comparison of the experi- formed at CERN’s on-line isotope separator facility ISOLDE. mental patterns for off-surface directions [Figs. 1 (b)-(d)] to the The samples were ~1-2 μm-thick epitaxial wurtzite GaN layers simulated angular-dependent emission yields for emitter atoms grown on sapphire into which 73As was implanted at room on substitutional SGa and substitutional SN sites, which are temperature with 60 keV energy at fluences of 0.8- shown in Fig.

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