Relaxation of donor and acceptor bound excitons, (D°, X) and (A°, X), to a final state in which the impurity atoms are left in an internally excited state has been used in several laboratories as a tool in assigning the chemical identity of the impurity species. In many specific samples, however, this excited final state emission is obscured by the free to bound (e−, A°) or an interfering donor to acceptor (D°, A°) transition. Enhanced emission from excited final state relaxation processes in GaAs has been observed using selective excitation to increase the population of the appropriate initial states. Increases in emission intensity of approximately 8 for (D°, X)* and 30 for (A°, X)* were observed. The excitation wavelength dependence of the enhancement is explained in terms of instrumental parameters and free exciton absorption. The converse of the selective excitation experiment was performed in order to learn more about the mechanisms which populate the levels giving rise to the analytically interesting emission. Anti-Stokes emission was observed in the near-gap region in high purity GaAs for the first time. The emission intensity as a function of excitation wavelength data is discussed in terms of a simple parallel pathway model for upconversion. The model postulates two parallel mechanisms which may populate the emitting states, and the access to one or the other of the two is determined by the energy of the incident photon. The model explains both the absolute intensity behavior of the (D°, X) and (D+, X) emission and the intensity ratio trend as a function of excitation wavelength.
P. W. Bohn, T. D. Harris, R. Bhat, and H. M. Cox, "Selectively Excited Luminescence in GaAs," Appl. Spectrosc. 38, 417-422 (1984)