Evidence for Oxygen DX Centers in AlGaN
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M.D. MCCLUSKEY*, N.M. JOHNSON*, C.G. VAN DE WALLE*, D.P. BOUR*, M. KNEISSL*, W. WALUKIEWICZ** *Xerox Palo Alto Research Center, 3333 Coyote Hill Rd., Palo Alto, CA 94304 "**LawrenceBerkeley National Laboratory, MS 2-200, 1 Cyclotron Rd., Berkeley, CA 94720 ABSTRACT Experimental and theoretical evidence is presented for oxygen DX centers in AlxGai-xN. As the aluminum content increases, Hall effect measurements reveal an increase in the electron activation energy, consistent with the emergence of a deep DX level from the conduction band. Persistent photoconductivity is observed in A0 39Gao. 61N:O at temperatures below 150 K after exposure to light, with an optical threshold energy of 1.3 eV, in excellent agreement with first-principles calculations. Unlike oxygen, silicon does not exhibit DX-like behavior, in agreement with previous theoretical predictions. INTRODUCTION Doping issues in AIGaN alloys have important implications for the fabrication of wide band-gap devices such as ultraviolet detectors and high-temperature, high-power transistors [1]. Oxygen is an omnipresent impurity in AIGaN alloys and is at least partly responsible for the background n-type conductivity in nominally undoped as-grown GaN. First-principles theoretical calculations [2] predicted that oxygen can occupy a substitutional nitrogen site (ON) and act as a shallow donor, with a low formation energy under typical growth conditions. Experiments [3,4] have verified that oxygen is a prevalent donor in as-grown GaN. AlGa1 .xN epilayers also exhibit n-type conductivity for x < 0.4 [5]. For x > 0.4, however, undoped AlGa1_xN is semi-insulating at room temperature [5]. The freeze-out of carriers has also been observed in GaN under hydrostatic pressures greater than 20 GPa [6,7]. In this paper, we present evidence that the low concentration of free electrons in Al-rich AlGaN is due to the formation of oxygen DX centers. DX centers have been intensively studied for over two decades [8]. In AI•Gaj.xAs alloys with x > 0.22, the DX center is the lowest-energy state of silicon donors. Chadi and Chang [9,10] proposed a model for the negatively charged DX center in which the Si atom is displaced into an interstitial position. Recent first-principles calculations [11,12] have predicted that oxygen forms DX centers in wurtzite AIN, with the oxygen atom relaxed along a [0001] direction. While Park and Chadi [12] predict that silicon can form DX centers in A1GaN, Van de Walle [11] has concluded that silicon is a shallow donor for the entire alloy range. In this paper we present experimental evidence that oxygen is a DX
531 Mat. Res. Soc. Symp. Proc. Vol. 512 ©1998 Materials Research Society
center in AIxGa 1 _.N for x>0.27, based on Hall effect, persistent photoconductivity, and optical threshold measurements.
EXPERIMENTAL DETAILS AlxGa1 ./N epilayers were grown to a thickness of I gm by metalorganic chemical vapor phase epitaxy (MOCVD) on c-plane sapphire substrates. The Al concentrations were determined by x-ray diffraction (XRD), by assuming relaxed layer
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