Observation of the exciton and Urbach band tail in low-temperature-grown GaAs using four-wave mixing spectroscopy

Abstract

Four-wave mixing (FWM) spectroscopy reveals clear signatures associated with the exciton, free carrier inter-band transitions, and the Urbach band tail in low-temperature-grown GaAs, providing a direct measure of the effective band gap as well as insight into the influence of disorder on the electronic structure. The ability to detect (and resolve) these contributions, in contrast to linear spectroscopy, is due to an enhanced sensitivity of FWM to the optical joint density of states and to many-body effects. Our experiments demonstrate the power of FWM for studying the near-band-edge optical properties and coherent carrier dynamics in low-temperature-grown semiconductors.

Signatures of four-particle correlations associated with exciton-carrier interactions in coherent spectroscopy on bulk GaAs

“…The high sensitivity of this technique to many-body effects stems from polarization diffraction contributions to the measured signal tied to Coulomb-induced coupling of the polarization on a given optical transition to other transitions 4 . TFWM has been used extensively to study the nonlinear optical response of semiconductor systems over the past two decades [8][9][10][11][12][13]23,[25][26][27][28][29][31][32][33][34] . Two-pulse, degenerate spectrally-resolved four-wave mixing experiments were carried out on a bulk GaAs sample held at 10 K using 1.55 eV, 30 fs pulses with a fullwidth at half maximum bandwidth of 78 meV.…”

Simultaneous observation of free and defect-bound excitons in CH3NH3PbI3 using four-wave mixing spectroscopy

“…Excitons, being charge-neutral spatially-localized excitations, have much longer T 2 times than delocalized free carrier and band tail transitions2930. This sensitivity to excitonic effects has been exploited to observe the fundamental exciton in LT-GaAs31, in which the optical band edge is strongly broadened by As Ga antisite defects preventing the observation of any signature of the exciton in linear absorption, as well as the exciton resonance at the spin-orbit split-off band gap in InP32, which is masked in linear spectroscopy by strong degenerate interband transitions associated with the lower-energy band gaps.…”

Control of the Urbach band tail and interband dephasing time with post-growth annealing in low-temperature-grown GaAs

“…Our experiments also show an increase in the interband dephasing time (T 2 ) with annealing temperature up to 500 • C. This indicates that arsenic point defects contribute strongly to carrier scattering in weakly-annealed LT-GaAs films. These scattering processes are diminished with annealing due to the reduction in the As i and As Ga point defects as precipitates begin to form, which has been found to occur for T a as low as 300 • C. 11,30 By extending our earlier FWM studies of as-grown films of LT-GaAs 27,28 to lower excitation powers, our experiments reveal a dip in the FWM response in the vicinity of the exciton. This dip is found to be insensitive to laser tuning and disappears when the sample is annealed at 550 • C, indicating that it is tied to the coexistence of band tail and exciton signal contributions, reminiscent of a Fano resonance.…”

“…This is due primarily to the need to apply nonlinear optical spectroscopy, 15 which enables the band tail states to be isolated from the strong defect-induced absorption that dominates the linear optical response of LT-GaAs. 20,[27][28][29] Here we report the application of four-wave mixing (FWM) spectroscopy to study the influence of annealing on the optical properties of LT-GaAs. The Urbach energy (E U ) was extracted in our experiments from the low-energy tail of the FWM spectrum for a range of annealing temperatures (T a ).…”

“…For these results, the excitation laser was tuned to 1.520 eV. The FWM signal for the LT-GaAs samples for T a ≤ 500 • C contains two main features: (i) a peak associated with the exciton resonance (at approximately 1.5 eV, 27,28 ); and (ii) a signal originating from the optically-excited interband continuum transitions at higher detection energies. The exciton response is peaked at zero delay, while the peak position of the interband response moves to larger values of interpulse delay as T a is increased.…”