Optical Absorption of Cr⁴⁺ in GaAs:Cr

Abstract

Optical absorption experiments are reported on p-type GaAs:Cr: Zn from which the spectral shape of the absorption due to stable Cr4+ is derived in the energy region from the onset a t about 0.4 eV up to the GaAs absorption edge. This absorption is interpreted as due to the photoionization transition Cr4+ + hv -Cr3+ + holevb superimposed by the simultaneously occurring transition Cr3+ + hv -Cr2+ + holevb. Its spectral shape a t T = 78 K can well be fitted by a calculated one taking into account the uppermost three valence bands, the *T1(F) ground state of Cr*+ and the ST, ground state of Cr2+ located within the gap, and the electron-phonon coupling of these localized states. The fit yields the optical threshold energy E3 = 0.68 eV and the broaden-

Photoinduced Optical Absorption of Cr4+ in Semi‐Insulating GaAs:Cr

“…During illumination the absorption spectra of Cr4+, Cr3+, and Cr2+ are superimposed in the energy region 0.8 to 1.5 eV. However, the spectral shape of the absorption due to the presence of Cr4+ in the energy region 0.4 to 1.5 eV is known from experimentals on p-conducting GaAs : Cr : Zn where all chromium is in the Cr4+ charge state [8]. Therefore, the Crg absorption can be extrapolated into the energy region 0.8 to 1.5 eV as the dashed curve 2' in Fig.…”

“…1) represents the absorption due to Cr3+ and Cr2+ present in the sample during illumination and their concentrations [Cr2+Ii and [Cr3+Ii can be evaluated. The concentration of Crg is obtained from the absorption coefficient of curve 2 a t 0.75 eV and the absorption cross section of Cr4+ given in [8]. Carrying out these measurements on more than 20 si GaAs : Cr samples in each case the following; relations were found :…”

“…From recent experiments on p-conducting GaAs : Cr : Zn, in which all chromium is in the charge state Cr4+, the spectral shape of the optical absorption due to the presence of Cr4+ in the whole energy region from its onset a t 0.4 eV up to the absorption edge of GaAs a t 1.52 eV has been derived and analyzed in terms of photoionization transitions [8]. This knowledge allows now to make a more sophisticated analysis of the photoinduced optical absorption of Cr4+.…”

Electrical and Optical Properties of Chromium Doped GaP

“…The DLOS spectrum measured at T ¼ 173 K (see Fig. 4) exhibits in the energy region 0:62 eV E 1:7 eV the same characteristic spectral shape as the optical absorption spectrum of GaAs : Cr 4þ [26]. As the Cr 3þ/4þ donor level is empty, electrons can be excited from the valence band into this level, the electronic configuration of which is the ground state 4 T 1 (F) of Cr 3þ .…”

“…It has to be noted that the thresholds seen in the optical absorption or DLOS spectra are not identical with the optical excitation energies in the photoionization processes (see the analysis of the photoionisation spectra involving Cr 4þ and Cr 3þ in GaAs [26]). Therefore, the Franck-Condon shifts estimated here from the DLOS spectra are only rough approximations.…”