Relation between exciton splittings, magnetic circular dichroism, and magnetization in wurtzite Ga1−xFexN

Abstract

The question of the correlation between magnetization, band splittings, and magnetic circular dichroism (MCD) in the fundamental gap region of dilute magnetic semiconductors is examined experimentally and through model calculations, taking the case of wurtzite Ga1−xFexN as an example. Magnetization and polarization-resolved reflectivity measurements have been performed down to 2 K and up to 7 T for x = 0.2%. Optical transitions originating from all three free excitons A, B and C, specific to the wurtzite structure, have been observed and their evolution with the magnetic field determined. It is demonstrated that the magnitude of the exciton splittings evaluated from reflectivity-MCD data can be overestimated by more than a factor of 2, as compared to the values obtained by describing the polarization-resolved reflectivity spectra with appropriate dielectric functions. A series of model calculations shows that the quantitative inaccuracy of MCD originates from a substantial influence of the magnetization-dependent exchange interactions not only on the spin splittings of excitons but also upon their linewidth and oscillator strength. At the same time, a method is proposed that allows to evaluate the field and temperature dependencies of the magnetization from MCD spectra. The accurate values of the excitonic splittings and of the magnetization reported here substantiate the magnitudes of the apparent sp − d exchange integrals in (Ga,Fe)N previously determined.

Upper bound for thes−dexchange integral inn-(Ga,Mn)N:Si from magnetotransport studies

“…According to this model, when Mn impurities are negatively charged, they repel the conduction band electrons, with the effect of reducing the apparent magnitude of s-d coupling. Surprisingly, small magnitudes of N 0 α were also found in studies of magnetoexcitons in semiinsulating, not n-type, (Ga,Mn)N [27] and (Ga,Fe)N [28]. We explain these observations with the occupation of mid-gap states of transition metal ions by trapped photoelectrons under steady state illumination, which-similarly to the case of n-type (Ga,Mn)Ndiminishes the s-d splitting of the conduction band.…”

“…By applying a multiparameter fitting procedure to magnetoreflectivity spectra the values N 0 α = (0±100) and (+100± 200) meV were determined for Ga 1−x Fe x N (x ≤ 0.21%) and Ga 1−x Mn x N (x ≤ 0.9%), respectively [26,27]. The former was reassessed by examining excitonic magnetic circular dichroism for Ga 1−x Fe x N with x = 0.2%, which resulted in N 0 α = (+50 ± 100) meV [28]. In contrast, results of more recent time-resolved Kerr rotation measurements that probe directly photoelectrons, not excitons, point to a standard value N 0 α = (+230 ± 20) meV for a series of Ga 1−x Mn x N films obtained by MOVPE with x up to 0.27% [29].…”

“…In the case of magnetically doped wurtzite (wz) GaN, exciton magnetospectroscopy [25][26][27] and magnetic circular dichroism [28] were employed to evaluate the sp-d exchange energies in films deposited by metal-organic vapor phase epitaxy (MOVPE). These methods provide the magnitude of N 0 (β − α) rather accurately.…”

Spinodal nanodecomposition in semiconductors doped with transition metals

“…It was suggested (Dietl, 2008a) that due to the strong p-d coupling, the molecular and virtual crystal approximations fail in oxides and nitrides, making the apparent exchange splitting the valence band, quantified by N 0 β (app) , reduced in absolute value and of opposite sign than expected, and observed in other II-VI DMSs. In this context, measurements of magnetization and magnetoreflectivity in the free exciton region for (Ga,Fe)N epilayers were reported (Pacuski et al, 2008;Rousset et al, 2013), and the obtained values for N 0 β (app) supported the theoretical expectations (Dietl, 2008a).…”

“…Brillouin paramagnetism of Ga-substitutional Fe 3+ ions: Because of a nonzero but relatively low solubility limit, a paramagnetic contribution from diluted Fe 3+ ions is always present (provided that the density of compensating donors is sufficiently small), as confirmed by a quantitative interpretation of EPR spectra in terms of weakly interacting localized spins S = 5/2 occupying Gasubstitutional sites 2006a). These spins give rise to the Brillouin-like dependence of magnetization on the magnetic field and temperature visible in magnetooptical (Pacuski et al, 2008;Rousset et al, 2013) and magnetization measurements Navarro-Quezada et al, 2010;Pacuski et al, 2008). Van Vleck paramagnetism due to Fe 2+ ions: In addition to the Brillouin-like term described previously, a term linear in the magnetic field was found to contribute to (Ga,Fe)N magnetization, and assigned to the Van Vleck paramagnetism of Fe 2+ ions that can be present in (Ga,Fe)N due to residual or purposely introduced donors Gosk et al, 2003; efficiency of Fe incorporation.…”