Photoluminescence and pressure effects in short period InN/nGaN superlattices

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. The emission energies, EPL, measured at ambient pressure, are close to the value of the band gap, Eg, in bulk GaN, in agreement with other experimental findings. The pressure dependence of the emission energies, dEPL/dp, however, resembles that of the InN energy gap. Further, the magnitudes of both EPL and dEPL/dp are significantly higher than those obtained from ab-initio calculations for 1InN/nGaN superlattices. Some causes of these discrepancies are suggested.

Bandgap behavior of InGaN/GaN short period superlattices grown by metal‐organic vapor phase epitaxy

“…Dimakis et al fabricated InN/GaN superlattices embedded in p‐n junction demonstrating electroluminescence peak at similar energies. Results of PL measurements obtained in different laboratories showed similar values of E PL , suggesting the origin of the radiative recombination in SPSLs as strongly influenced by the GaN barrier ( E PL = 2.8–3.3 eV). On the other hand, application of hydrostatic pressure showed a pressure shift of the photoluminescence energy, dE PL /d p of the investigated samples approaching values very similar to dE PL /d p ≈ dE g /d p of InN, that is, 28 meV GPa −1 , but much lower than dE PL /d p ≈ dE g /d p of GaN equal to 40 meV GPa −1 .…”

Band gaps in InN/GaN superlattices: Nonpolar and polar growth directions

“…6). The latter fall in the range of 3.2 eV-3.3 eV, 16 i.e., only slightly below the GaN band gap (3.4 eV). The experimental data exhibit a weak dependence on the effective InN content and disagree with the calculated band gaps for polar 1InN/nGaN SLs, which are substantially lower and depend strongly on the SL barrier width.…”

“…Lines are spline fits to guide the eye. The dashed curve corresponds to the calculations performed for In x Ga 1Àx N quasi-random alloys 17,29 with the asterisks marking the pure binary compounds, 30 16 (for polar structures) in Fig. 7.…”

“…The possible intermixing of InN and GaN was modeled by considering SLs of composition InGaN/GaN; but while it leads to larger band gaps than in the pure InN/GaN, they still remain smaller than the experimental PL energies. 16,20 In order to further improve our understanding of PL experiments on InN/GaN SLs, several additional effects should be considered in the modeling. These include excitonic effects and the occurrence of surface states, also their influence on the sample quality during the growth.…”

“…The measured pressure coefficients dE PL /dp range from 33.4 meV/GPa to 28.7 meV/GPa, showing the same trend as observed in In x Ga 1Àx N alloys, 17 i.e., for higher In content, the pressure coefficients approach the InN band gap pressure coefficient (28 meV/GPa). 16 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, E g , a discrepancy occurs when comparing it to the calculated E g value (2.2 eV) 18-20 for polar 1InN/nGaN SLs in the limit of large n. Several explanations were proposed to explain this discrepancy, including the suggestion that the inserted layer is not pure InN but actually consists of an alloy 16,20 of the two compounds (see also Ref. 19), that optical transitions are attributed to GaN excitons partially localized in the InN region, 13 and that the observed light emission originates from the recombination of carriers located in different spatial regions, i.e., from the GaN barrier to the InN quantum well.…”