Short period InN/nGaN superlattices. Experiment versus theory T. Suski a , I. Gorczyca *a , G. Staszczak a , X.Q. Wang b , N. E. Christensen c , A. Svane c , E. Dimakis d , and T.D. Moustakas e a Institute of High Pressure Physics, UNIPRESS, 01-142 Warsaw, Poland b State Key Laboratory of Artificial Microstructure and Mesoscopic Physics, Beijing, China c Department of Physics and Astronomy, Aarhus University, DK-8000 Aarhus C, Denmark d Physics Department, University of Crete, 71003, Heraklion-Crete, Greece. e Boston University, Boston, Massachusetts 02215, USA ABSTRACT Measurements of photoluminescence and its dependence on hydrostatic pressure are performed on a set of InN/nGaN superlattices with one InN monolayer, and with different numbers of GaN monolayers (n from 1 to 40). The emission energies, E PL , measured at ambient pressure, are close to the value of the band gap, E g , in bulk GaN, in agreement with other experimental findings. The pressure dependence of the emission energies, dE PL /dp, however, resembles that of the InN energy gap. Further, the magnitudes of both E PL and dE PL /dp are significantly higher than those obtained from ab- initio calculations for 1InN/nGaN superlattices. Some causes of these discrepancies are suggested...Detailed analysis of the electronic band structure of 1InN/5GaN superlattice is performed showing that the built-in electric field plays an important role in the mInN/nGaN structures. It strongly influences the valence- and conduction-band profiles and thus determines the effective band gap. Keywords: InN/GaN, quantum well, photoluminescence, band structure 1. INTRODUCTION Fabrication of short-period mInN/nGaN superlattices (SLs) consisting of small numbers of monolayers (MLs) m and n, is one of the possible ways to perform band gap engineering in the blue-green range of the spectrum. A potential advantage of employing these SLs is that difficulties in obtaining high-quality InGaN quantum wells (QWs) with large In-content may be avoided. In x Ga 1-x N structures with x larger than ∼0.25 tend to exhibit compositional fluctuations. This effect can to a great extent be avoided in InN/GaN short-period SLs. With the recent advances in growth technology very thin InN QWs in a GaN matrix with 1 to 5 MLs of InN have been synthesized. 1-3 Yoshikawa et al. 1,2 produced structures where 1- 2 MLs of InN were pseudomorphically embedded in a thick GaN matrix. Dimakis et al. 3 fabricated 1 InN ML thick InN/GaN multiple QWs (MQWs). An increase in the number of InN layers causes strain relaxation and introduces related structural defects. The emission energies, E PL , derived from photoluminescence (PL) experiments 1,2 performed on the MQWs fabricated by Yoshikawa et al. 1,2 are close to the GaN band gap (3.4 eV), respectively 3.41 eV, 3.11 eV and 2.8 eV for MQWs with 1, 3, 5 InN MLs. PL measurements performed by Dimakis et al. 3 on 1 ML thick InN/GaN MQWs found E PL = 3.26 eV. Assuming that E PL in the MQW structures is close to the band-to-band radiative transition, and that this emission energy corresponds to the band gap, a discrepancy occurs when comparing it to calculated band gaps. The E PL obtained for the MQW with one InN ML (3.41 eV - 3.26 eV) is significantly higher than the calculated 4,5 band gap (2.2 eV) for the 1InN/nGaN SL in the limit of large n. This problem has motivated us to further investigations. We performed photoluminescence measurements on a set of InN/GaN samples with one InN ML, and with 1 to 40 GaN MLs. Trying to identify the character and origin of energy transitions we extend the research by including the effects of applying external pressure. Knowledge of the band gap pressure coefficient, dE g /dp, usually helps in determination of the nature *iza@unipress.waw.pl ; phone +48 22 876 0354 Please verify that (1) all pages are present, (2) all figures are correct, (3) all fonts and special characters are correct, and (4) all text and figures fit within the red margin lines shown on this review document. Complete formatting information is available at http://SPIE.org/manuscripts Return to the Manage Active Submissions page at http://spie.org/app/submissions/tasks.aspx and approve or disapprove this submission. Your manuscript will not be published without this approval. Please contact author_help@spie.org with any questions or concerns. 8625 - 16 V. 2 (p.1 of 7) / Color: No / Format: A4 / Date: 1/11/2013 4:55:51 AM SPIE USE: ____ DB Check, ____ Prod Check, Notes: