Free Excitons in GaN
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Internet Journal of Nitride Semiconductor Research:
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Free Excitons in GaN B. Monemar, J.P. Bergman, I.A. Buyanova, W. Li, H. Amano and I. Akasaki MRS Internet Journal of Nitride Semiconductor Research / Volume 1 / January 1996 DOI: 10.1557/S1092578300001745, Published online: 13 June 2014
Link to this article: http://journals.cambridge.org/abstract_S1092578300001745 How to cite this article: B. Monemar, J.P. Bergman, I.A. Buyanova, W. Li, H. Amano and I. Akasaki (1996). Free Excitons in GaN . MRS Internet Journal of Nitride Semiconductor Research, 1, pp e2 doi:10.1557/S1092578300001745 Request Permissions : Click here
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M R S
Internet Journal o f
Nitride S emiconductor Research
Volume 1, Article 2
Free Excitons in GaN B. Monemar, J. P. Bergman, I. A. Buyanova, W. Li Department of Physics and Measurement Technology, Linköping University H. Amano, I. Akasaki Department of Electrical and Electronic Engineering, Meijo University This article was received on June 3, 1996 and accepted on July 8, 1996.
Abstract Optical spectra on free exciton properties for GaN are presented and discussed, in particular the influence of epitaxial strain and temperature. The exciton-phonon coupling is also manifested via the temperature dependence of the LO phonon replicas of the free exciton.
1. Introduction Like with most wide bandgap semiconductors excitons play an important role in the optical properties of GaN [1]. The fundamental near bandgap A, B and C-excitons in GaN are connected with the splitting in the valence band top of GaN into the three states Γ9, Γ7, and Γ7 [2] [3]. The energies of A, B and C excitons are quite sensitive to the heteroepitaxial strain in thin layers [4] [5] [6] [7] [8]. This strain is largely understood as due to the difference in thermal expansion coefficients between layer and substrate. The strain will also affect the temperature dependence of the exciton energies [4]. The polariton splitting of the excitonic states has not been spectrally resolved, but the lower polariton dispersion is manifested in the relative intensities of the excitonic photoluminescence (PL) lines and their phonon replicas.
2. Exciton spectra and epitaxial strain. Low temperature PL spectra for undoped GaN typically show (donor- or acceptor-related) bound exciton (BE) lines as well as the free exciton (FE) lines (A, B and C), provided a sufficiently high dynamics is used in the data collection [6]. Such PL spectra, together with reflectance data for the FE lines, provide accurate values for the exciton energies. Samples of a thickness of a few µm grown on sapphire substrates demonstrate a clear upshift of the lowest A and B excitons by 10-15 meV (depending on the sample), compared to a thick bulk-like GaN sample with a much reduced strain [4] [5] [7] [8] [9] [10] [11] (Figure 1). The upshift for the C exciton is found to be
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