Optical Properties Of Metastable Shallow Acceptors In Mg‐Doped GaN Layers Grown
By Metal‐Organic Vapor Phase Epitaxy
G. Pozina, C. Hemmingsson, J. P. Bergman, T. Kawashima, H. Amano, I. Akasaki, A. Usui, and B. Monemar
Citation: AIP Conference Proceedings 1199, 110 (2010); doi: 10.1063/1.3295320 View online: http://dx.doi.org/10.1063/1.3295320
View Table of Contents: http://scitation.aip.org/content/aip/proceeding/aipcp/1199?ver=pdfcov Published by the AIP Publishing
Articles you may be interested in
Optical properties of C-doped bulk GaN wafers grown by halide vapor phase epitaxy J. Appl. Phys. 116, 223503 (2014); 10.1063/1.4903819
Electrical Properties Of GaN Layers Grown By Metal Organic Vapor Phase Epitaxy (MOVPE) AIP Conf. Proc. 1399, 35 (2011); 10.1063/1.3666245
Structural and optical inhomogeneities of Fe doped GaN grown by hydride vapor phase epitaxy J. Appl. Phys. 104, 123712 (2008); 10.1063/1.3040702
Effect of annealing on metastable shallow acceptors in Mg-doped GaN layers grown on GaN substrates Appl. Phys. Lett. 92, 151904 (2008); 10.1063/1.2909541
Optical absorption properties of Mg-doped GaN nanocolumns J. Appl. Phys. 98, 104303 (2005); 10.1063/1.2133900
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 130.236.173.112 On: Tue, 12 May 2015 07:24:36
Optical Properties Of Metastable Shallow Acceptors In
Mg-Doped GaN Layers Grown By Metal-Organic Vapor Phase
Epitaxy
G. Pozina
1, C. Hemmingsson
1, J.P. Bergman
1, T. Kawashima
2, H. Amano
2, I.
Akasaki
2, A. Usui
3, and B. Monemar
11
Department of Physics, Chemistry and Biology (IFM), Linköping University, S-581 83 Linköping, Sweden
2
Department of Materials Science and Engineering, Meijo University, 1-501 Shiogamaguchi, Tempaku-ku, Nagoya 468-8502, Japan
3
R&D Division, Furukawa Co. Ltd., Tsukuba, Ibaraki 305-0856, Japan
Abstract. GaN layers doped by Mg show a metastable behavior of the near-band-gap luminescence caused by electron
irradiation or UV excitation. At low temperatures < 30 K the changes in luminescence are permanent. Heating to room temperature recovers the initial low temperature spectrum shape completely. Two acceptors are involved in the recombination process as confirmed by transient PL. In as-grown samples a possible candidate for the metastable acceptor is CN, while after annealing a second more stable acceptor related to Mg became active.
Keywords: GaN, Mg-doped, metastability, shallow acceptors. PACS: 78.55.Cr, 78.47.Cd, 78.60.Hk
INTRODUCTION
Despite many years study, the properties of p-type GaN are not completely understood. For example, there are at least two shallow acceptors in Mg-doped GaN [1, 2] which demonstrate a metastable behavior in PL [3]. Previous studies [4, 5] have also shown the presence of the metastable defect-related UV luminescence in cathodoluminescence (CL) which can not be observing in photoluminescence (PL). In this work we report metastable optical properties of two shallow acceptors studied by PL and by time-resolved PL (TRPL) in high quality 1 µ m-thick GaN layers grown by metal-organic vapor phase epitaxy and doped by Mg with concentrations 1x1019 - 1x1020 cm-3.
EXPERIMENTAL DETAILS
We have studied as-grown samples and after annealing during 10 min at 800 °C in N2 atmosphere.
For TRPL, the optical excitation has been done with the third harmonics (λe = 266 nm) from a Ti:sapphire
femtosecond pulsed laser with a frequency of 75 MHz or 250 KHz.
The samples show metastability, which means that the shape of the PL or CL spectra depends on the excitation time before detection. That is why we introduce the delay time tD, i.e. the interval between
the beginning of excitation and the start of detection.
RESULTS AND DISCUSSIONS
As we reported previously [3], samples with low (< 5 x 1018 cm-3) or high Mg doping level (~1 x 1020 cm-3) show only a very weak metastable behavior in PL or CL. For GaN layers with Mg concentrations between 1x1019-5x1019 cm-3 the following changes in PL spectra are typical after a longer exposure to UV irradiation: a strong reduction of the acceptor bound exciton (ABE) line intensity and changes of the relative intensities of PL peaks in the region 3.26 eV-3.0 eV. Figure 1 shows PL spectra detected directly after beginning of excitation (tD=0 min) and after ~1 h
exposure to UV laser. The main effect of annealing is activation of a lower energy acceptor, thus all samples with the ABE1 line at 3.462 eV after annealing demonstrate instead ABE2 at the lower energy 3.453 eV. The effect of metastability is reduced in annealed samples, but does not disappear completely.
110
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 130.236.173.112 On: Tue, 12 May 2015 07:24:36
CREDIT LINE (BELOW) TO BE INSERTED ON THE FIRST PAGE OF EACH PAPER
CP1199, 29th International Conference on the Physics of Semiconductors, edited by M. J. Caldas and N. Studart © 2009 American Institute of Physics 978-0-7354-0736-7/09/$25.00
FIGURE 1. PL spectra measured at 2K for the as-grown
(solid lines) and annealed (dashed lines) samples are shown for two delay times after the beginning of the illumination by the laser light.
The following optical signatures are typical for the shallower acceptor A1: ABE1 at ~3.462 eV and a donor – acceptor pair (DAP) recombination at ~3.27 eV with its two LO phonon replicas. The deeper acceptor related to Mg is associated in PL with ABE2 at ~3.453 eV and with a broad band at ~ 3.1 eV.
FIGURE 2. TRPL spectra measured at 2K for the as-grown
(a) and annealed (b) samples detected directly after the beginning of the illumination (tD = 0 min). Spectra are
shown with time delay intervals ∆t = 60 ps.
To confirm the presence of two acceptors we have done TRPL measurements. We have observed a fast initial transfer (~ a few ps) between the free exciton A (FE) and ABE1 for the as-grown sample. Figure 2 shows TRPL spectra detected within the first 300 ps after the laser pulse income. Spectra are normalized and vertically shifted for clarity. The shallow ABE1 dominates PL for as-grown samples; however, there is a transformation of the spectrum from ABE1 to ABE2
seen after few min of exposure to the laser light. In annealed samples we could clearly see both ABE1 and ABE2 within the first 300 ps after PL excitation by the laser pulse with a complete domination of ABE2 after 2 ns. We have also observed that a broad DAP spectrum at ~3.1 eV appears and dominates PL after 0.5 µ s. Thus, TRPL measurements directly confirm the presence of two different shallow acceptors in Mg-doped GaN.
FIGURE 3. TRPL spectra for the as-grown (solid lines) and
annealed (dashed lines) samples detected after a 30 minutes exposure to the laser light. For convenience, only two spectra resolved by 2 ns are shown for each sample.
The temporal evolution of PL for the as-grown and annealed samples after exposure to the UV light during 30 min is summarized in Figure 3. Transient PL shows similar qualitative behavior in both cases. The intensity of ABE1 is weak and comparable with intensity of the ABE2 line even at ∆t= 0 in as-grown samples. The transfer between ABE1 and ABE2 takes only 150 ps, thus, the deeper ABE2 dominates PL at longer times.
The shallower acceptor A1 is not yet identified; however, according to theoretical prediction it might be related to CN [6]. The metastability of the shallow
acceptors with excitation time at low T can tentatively be explained by mobile H related effects.
This work was supported by the Swedish Energy Agency and the Swedish Research Council.
REFERENCES
1. B. Monemar, et al., Mat. Sci. Semicon. Process. 9, 168 (2006).
2. B. Monemar, et al., Physica B 376-377, 440 (2006). 3. G. Pozina, C. Hemmingsson, P.P. Paskov, J.P. Bergman,
B. Monemar, T. Kawashima, H. Amano, I. Akasaki, and A. Usui, Appl. Phys. Lett. 92, 151904 (2008).
4. G. Pozina, et al., B 401-402, 302 (2007).
5. G. Pozina, P.P. Paskov, J.P. Bergman, C. Hemmingsson, L. Hultman, B. Monemar, H. Amano, I. Akasaki, A. Usui, Appl. Phys. Lett. 91, 221901 (2007).
6. T.A.G. Eberlein, R. Jones, S. Öberg, P.R. Briddon, Appl. Phys. Lett. 91, 132105 (2007).)
111
