Band-edge photoluminescence of CuGaSe2 films grown by molecular beam epitaxy

Abstract

Slightly Cu-rich CuGaSe 2 films were grown on ͓001͔ oriented GaAs substrates by molecular beam epitaxy. Photoluminescence of the films showed a remarkable emission peaked at 1.71 eV at low temperature, which is attributed to recombination of free excitons and bound excitons. The dissociation energy of free excitons and their localization energy to a center are found to be 16.2 and 3.3 meV, respectively. The band-gap energy E g is estimated to be 1.7310 eV at low temperature. It is suggested that the temperature variation of E g is dominated by interaction with phonons of 26 meV which corresponds to the mean energy of the optical phonons in CuGaSe 2 .

Optical properties of CuGaSe2 and CuAlSe2 layers epitaxially grown on Cu(In0.04Ga0.96)Se2 substrates

“…This value is very large for the free exciton emission exhibiting linear dependence on excitation intensity. 28 Such superliner dependence of the free and bound exciton lines has been reported for the MBE-grown CuGaSe 2 /GaAs, 29 and iodine transported CuGaS 2 at 77 K. 30 The reason for this is not clear at the present time. The other PL peak at 1.68 and 1.63 eV has n value of 1.3-1.4 and ϳ1.3, respectively.…”

“…PL peaks due to transitions from the conduction band to acceptor ͑interstitial Se atom: E A ϭ40 meV) at 1.68 eV 31,32 and donor ͑Se vacancy: E D ϭ80 meV) to valence band at 1.63 eV 33,34 have been reported. The peak at 1.62 eV ͑corresponding to the peak at 1.63 eV͒ found in Cu-rich CuGaSe 2 /GaAs prepared by MBE 29,35 has been assigned to both the D -A pair transition and the donor to valence band transition associated with the selenium vacancy (E D ϭ108 meV). PL peaks at 1.63 and 1.68 eV have been found in CuGaSe 2 /GaAs grown by metalorganic molecular beam epitaxy, and these peaks were assigned to D -A pair emission ͑1.68 eV: E D ϩE A ϭ81 meV and 1.63 eV: E D ϩE A ϭ123 meV) at low temperature ͑8 K͒ and the free to bound emission at higher temperature ͑77 K͒.…”

Electronic defects in Cu(In,Ga)Se2 : Towards a comprehensive model

“…The early work of Niki et al [33][34][35] was based on high-quality thin films and crystals and allowed pioneering investigation of the temperature and intensity dependencies of the observed PL spectra. Since then more agreement has been reached for the attribution of the various PL peaks to specific electronic transitions: for the excitons in CuInSe 2 see, e.g., [20,[35][36][37][38][39][40]; for those in CuGaSe 2 see, e.g., [19,38,[41][42][43][44][45][46]; for the DA or the related free-to-bound (FB) transitions in CuInSe 2 see, e.g., [18,20,33,37,47]; and in CuGaSe 2 see, e.g., [19,42,[47][48][49]. When investigating the temperature dependence of the luminescence spectra we observe the change from DA luminescence to FB luminescence.…”

Excitation power and temperature dependence of excitons in CuInSe2

“…directly proportional to the total number of excitons, which in turn depends on the product of the number of holes and electrons, each of which is proportional to the excitation power P. 32 Since bound excitons are in thermal equilibrium with free excitons, the concentration of bound excitons is proportional to that of free excitons. 33 Therefore, values of k between 1 and 2 are also expected for bound exciton transitions, assuming the concentration of defects is high enough and no saturation occurs. For the A and B free excitons, the AB line as well as for the components of the M1 bound exciton, power coefficients 1 < k 1 < 2 have been determined, as summarised in Table I.…”