Photoreflectance study of exciton energies and linewidths for homoepitaxial and heteroepitaxial GaN layers

Abstract

Photoreflectance has been applied to study the exciton energies and linewidths for hetero-and homoepitaxial GaN layers ͑i.e., GaN layers grown on sapphire and truly bulk GaN crystal obtained by ammonothermal method͒. In order to modulate the built-in electric field inside the samples and eliminate photoluminescence signal from photoreflectance spectra, the surface band bending was modulated by the laser light with the photon energy smaller than the energy gap of GaN, i.e., a 532 nm laser line. The Varshni-type and Bose-Einstein-type parameters that describe the temperature dependence of the exciton transition energies have been evaluated and compared for the two GaN epilayers. It has been concluded that the residual strain in the heteroepitaxial layer influences the exciton transition energy but does not influence the Varshni and Bose-Einstein parameters. It has been found that this strain significantly influences the exciton linewidth. The broadening parameter, which is associated with the temperature-independent mechanisms ͑⌫ 0 parameter corresponding to the exciton linewidth at 0 K͒, has been found to be ϳ1 meV for the homoepitaxial layer. For the heteroepitaxial layer this parameter is twice higher ͑this work͒ or few times higher ͑previous papers͒.

Optical and structural properties of m ‐plane GaN substrates grown by ammonothermal method and GaN epilayers grown on these substrates

“…Figure 8 shows reflectance spectra for various light polarizations. For unpolarized light three well resolved free exciton transitions (FXA, FXB, and FXC) are observed in reflectance spectrum at energies of E FXA =3.4776 eV, E FXB =3.4827 eV, and E FXC =3.5015 eV, which are typical for homoepitaxial epilayers [15,17]. The same exciton transitions are observed when the electric vector is perpendicular to c axis.…”

“…Since GaN epilayers grown on A-GaN substrate are strain free [15], it should be expected that the FXA exciton for m-plane GaN epilayer will be completely polarized in the epilayer plane, see for example the reflectance study for a-plane GaN films in Ref. [16].…”

Properties of truly bulk GaN monocrystals grown by ammonothermal method

“…High quality of homoepitaxial layers deposited on c-oriented A-GaN substrates was also confirmed by modulation spectroscopy studies: photoreflectance [16] (PR) and CER measurements [15]. The PR spectra for c-plane A-GaN crystals (Fig.…”

“…The PR spectra for c-plane A-GaN crystals (Fig. 4) exhibit the following features: 1) typical three resonances corresponding to intrinsic free exciton transitions (FXA, FXB and FXC) with energies evidently lower (3.4760 eV, 3.4817 eV, 3.4991 eV) than those reported for GaN layers grown on sapphire substrates (3.483 eV, 3.491 eV, 3.512 eV, respectively) [16], 2) exciton linewidth for the homoepitaxial layer are significantly smaller than those for heteroepitaxial layer; the observed transitions are also about 20-50% narrower than the exciton transitions reported in Ref. [17] measured by reflectivity for homoepitaxial layers grown on HNP crystals.…”

Contactless electroreflectance spectroscopy of optical transitions in low dimensional semiconductor structures

“…Photon energy of the pump beam is usually larger than the band gap of the semiconductor being under study. However, there is a possibility to use a below band gap modulation through the excitation of impurity or sur− face states [63,64]. In both cases photo−generated carriers can be trapped by surface states and in this way the surface band bending is modulated [1][2][3][4].…”