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Metastability of Oxygen Donors in Algan

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Metastability of Oxygen Donors in Algan VOLUME 80, NUMBER 18 PHYSICAL REVIEW LETTERS 4MAY 1998 Metastability of Oxygen Donors in AlGaN M. D. McCluskey, N. M. Johnson, C. G. Van de Walle, D. P. Bour, and M. Kneissl Xerox Palo Alto Research Center, Palo Alto, California 94304 W. Walukiewicz Center for Advanced Materials, Lawrence Berkeley National Laboratory, Berkeley, California 94720 (Received 22 December 1997) Experimental and theoretical evidence is presented for the metastability of oxygen donors in AlxGa12x N. 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 Al0.39Ga0.61N:O at temperatures below 150 K after exposure to light, with an optical threshold energy of 1.3 eV. A configuration coordinate diagram is obtained from first-principles calculations and yields values for the capture barrier, emission barrier, and optical threshold which are in good agreement with the experimental results. [S0031-9007(98)05950-X] PACS numbers: 71.55.Eq, 72.80.Ey, 73.20.Hb In the race to develop blue laser diodes, GaN-based de- relaxed along a f0001g direction. While Park and Chadi vices have emerged as the current leaders, with projected [12] predict that silicon can form DX centers in AlGaN, lifetimes of up to 10 000 hours [1]. An understanding of Van de Walle [11] has concluded that silicon is a shallow the role of dopants in group-III-nitride semiconductors is donor for the entire alloy range. essential for the realization of high-performance optoelec- In this Letter, we present experimental evidence that tronic devices. In addition to laser diodes, doping issues oxygen is a DX center in AlxGa12xN for x . 0.27, based in AlGaN alloys have important implications for the fab- on the Hall effect, persistent photoconductivity, and optical rication of wide band-gap devices such as ultraviolet de- threshold measurements. The results of first-principles tectors and high-temperature, high-power transistors. calculations for oxygen in AlxGa12xN are also presented Oxygen is an omnipresent impurity in the AlGaInN and compared with the experimental data. materials system; it is at least partly responsible for the AlxGa12xN epilayers were grown to a thickness of background n-type conductivity in nominally undoped 1 mm by metalorganic chemical vapor phase epitaxy on as-grown GaN. First-principles theoretical calculations c-plane sapphire substrates. The Al concentrations were [2] predicted that oxygen can occupy a substitutional determined by x-ray diffraction (XRD), by assuming nitrogen site sONd and act as a shallow donor, with a relaxed layers and Vegard’s law. The concentrations low formation energy under typical growth conditions. of silicon and oxygen impurities were measured by Results from secondary ion mass spectrometry (SIMS) SIMS. Al0.4Ga0.6N and Al0.5Ga0.5N epilayers were [3,4] have shown that in unintentionally doped GaN the implanted with 18O and 29Si ions at respective doses of concentration of free electrons is approximately equal 5 3 1014 cm23 and used as calibration standards. Unin- to the concentration of oxygen present in the material, tentionally doped AlxGa12xN shows oxygen and silicon consistent with the hypothesis that oxygen is a prevalent concentrations of approximately 1019 and 1018 cm23, donor. Layers of heteroepitaxially grown AlxGa12xN also respectively. Intentionally doped Al0.44Ga0.56N:Si has a exhibit n-type conductivity for x , 0.4 [5]. For x . 0.4, silicon concentration of 8 3 1018 cm23 and an oxygen 18 23 however, undoped AlxGa12xN is semi-insulating at room concentration of 3 3 10 cm . temperature [5]. The freeze-out of carriers has also been To determine the electron activation energies, variable- observed in GaN under hydrostatic pressures greater than temperature Hall effect measurements were performed in 20 GPa [6,7]. In this Letter, we present evidence that the the van der Pauw geometry with a magnetic field of low concentration of free electrons in Al-rich AlGaN is 17 kG. Arrhenius plots of electron concentration as a due to the formation of oxygen DX centers. function of inverse temperature for several AlxGa12xN DX centers have been intensively studied for over samples are shown in Fig. 1. The free-electron con- two decades [8]. In AlxGa12xAs alloys with x . 0.22, centration of the Al0.44Ga0.56N:Si epilayer is n ­ 1 3 the DX center is the lowest-energy state of silicon 1019 cm23, which is very close to the concentration of donors. Chadi and Chang [9,10] proposed a model for silicon atoms measured by SIMS. The fact that the free- the negatively charged DX center in which the Si atom electron concentration is independent of temperature indi- is displaced into an interstitial position. Recent first- cates that the silicon donors have a small binding energy principles calculations [11,12] have predicted that oxygen such that the donor level is degenerate with the conduc- forms DX centers in wurtzite AlN, with the oxygen atom tion band. In the unintentionally oxygen-doped material, 4008 0031-9007y98y80(18)y4008(4)$15.00 © 1998 The American Physical Society VOLUME 80, NUMBER 18 PHYSICAL REVIEW LETTERS 4MAY 1998 FIG. 2. Dependence of the DX energy level on AlN concen- tration in Alx Ga12xN:O. The DX level is extrapolated to inter- sect the conduction band (CB) minimum at an Al concentration of x ­ 0.27. The increase in the donor binding energy is consistent with a deep DX level which has a lower energy than the FIG. 1. Free-electron concentration as a function of inverse conduction band minimum for x . 0.27 (Fig. 2). Since temperature for silicon- and oxygen-doped Al Ga N. x 12x the DX wave function is localized in real space, it is extended in k space and is not pinned to the conduction however, the free electrons freeze out with decreas- band minimum. To estimate the alloy dependence of ing temperature. The electron activation energies in- the conduction band minimum ECBM, the theoretical crease with increasing AlN concentration, which results AlxGa12xN valence band offset of 0.8x eV [15,16] is in freeze-out curves with progressively steeper slopes. subtracted from the expression for the band gap of To quantitatively model these results, we derived an AlxGa12xN [17], which yields expression for the free-electron concentration as a func- 2 tion of temperature. The oxygen is assumed to have three ECBM ­ 1.45x 1 0.53x , (3) stable charge states: a negatively charged DX state, a neu- where ECBM is in units of eV and is arbitrarily set to zero tral donor state, and a positive donor state. With the addi- for GaN. By linear extrapolation, the DX level intersects tional assumption of charge neutrality [13], in the regime the conduction band minimum at x ­ 0.27. This value is where n ø ND 2 NA, the free-electron concentration is significantly lower than the value of x ­ 0.4 predicted by given by Wetzel et al. [7,18] from Raman scattering experiments N 2 N 1y2 of GaN:O under hydrostatic pressure. That prediction, n N D A S k 2E k T ø c exps y Bd exps DX y B d , however, assumed that the pressure dependence of the ∑ND 1 NA ∏ (1) valence band is negligible. A more realistic approach is to consider the pressure dependence of the band gap. where n, N , and N are the free-electron, donor, and D A The band gap of GaN at the critical pressure (20 GPa) acceptor concentrations, respectively; S is the difference in entropy between the donor and DX configurations; k is approximately 0.8 eV higher than at ambient pressure B [19]. This value corresponds to an Al concentration of is Boltzmann’s constant; E is the energy difference DX x between the conduction band minimum and the DX level; , 0.3, in good agreement with our results. Persistent photoconductivity, a more direct manifesta- and T is the temperature in Kelvin. N is the conduction c tion of metastability, is observed in Al Ga N epilayers band effective density of states, given by x 12x for x $ 0.39 at temperatures below 150 K. The persistent p 3y2 2pm kBT photoconductivity of DX centers is attributed to the pho- Nc ­ 2 , (2) h2 toinduced transfer of the DX state into a metastable state. p µ ∂ where m ­ 0.2me is the electron effective mass. The metastable state has a lower binding energy than the The activation energy EDX was determined by least- DX state and therefore is more likely to contribute an elec- square fits of Eq. (1) to the Hall effect data. As shown in tron to the conduction band for a given temperature. As Fig. 1, the decrease in the free-electron concentration with shown in Fig. 3, at a temperature of 100 K and an applied increasing AlN content can be explained by an increase bias of 100 V, the current through an Al0.39Ga0.61N epi- in EDX . These results are in qualitative agreement with layer increases by over 2 orders of magnitude after expo- those of Polyakov et al. [14]. For x . 0.5, the resistivity sure to monochromatic light with a wavelength of 1.1 mm. was such that reliable Hall voltages could not be obtained. After the light is turned off, the current decreases as the 4009 VOLUME 80, NUMBER 18 PHYSICAL REVIEW LETTERS 4MAY 1998 FIG. 4. Optical cross section of oxygen DX centers in Al0.39Ga0.61N as a function of wavelength at 100 K. The optical threshold Eopt is approximately 1.3 eV. FIG.
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