Annealing dynamics of molecular-beam epitaxial GaAs grown at 200 °C

Abstract

By separating a 2-pm-thick molecular-beam-epitaxial GaAs layer grown at 200 "C from its 650~pm-thick substrate, we have been able to obtain accurate Hall-effect and conductivity data as functions of annealing temperature from 300 to 600 "C. At a measurement temperature of 300 K, analysis confirms that hopping conduction is much stronger than band conduction for all annealing temperatures. However, at higher measurement temperatures (up to 500 K), the band conduction becomes comparable, and a detailed analysis yields the donor and acceptor concentrations and the donor activation energy. Also, an independent absorption study yields the total and charged AsGa concentrations. Comparisons of all of these quantities as a function of annealing temperature T, show a new feature of the annealing dynamics, namely, that the dominant acceptor (probably Vo, related) strongly decreases and then increases as TA is increased from 350 to 450 "C. Above 450 "C, ND, NA , and [Asod all decrease, as is known from previous studies. 306

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.…”

“…As i and V Ga have been found to assist the diffusion of As i defects at temperatures as low as 300 • C. 11,30 The increase in T 2 observed here for T a ≤ 500 • C likely reflects an increase in the rate of diffusion of these point defects with temperature in the regime in which precipitates are beginning to form. 30 Our experiments, which highlight the high sensitivity of the interband dephasing time in LT-GaAs to changes in the density of point defects, indicate that scattering with these defects is a key factor in carrier relaxation and transport in LT-GaAs subject to annealing at temperatures below 500 • C. Our observation of a higher threshold annealing temperature for the reduction in the Urbach energy indicates that a substantial fraction of the point defects must be replaced by As clusters in order to significantly alter the potential landscape experienced by carriers in the crystal.…”

“…17 The point defects lead to rapid trapping of band electrons 18 and pin the Fermi level in the gap resulting in semi-insulating behaviour. 11,19 The band tail states contribute to a large optical nonlinearity in the vicinity of the band gap due to the spatial localization of the electron and hole wavefunctions. 15 In addition to these beneficial properties, the high density of defects also leads to undesirable effects.…”

“…11,16,[19][20][21][22][23][24][25] A variety of techniques have been used to study the point defect density as a function of the growth and annealing temperature, including Hall effect, 11 electron paramagnetic resonance, 22 Auger spectroscopy, 16 positron annihilation, 23 and scanning tunneling microscopy. 24,25 These studies have shown that annealing reactions involving defect complexes of As Ga , As i , and V Ga promote the diffusion of As i and the resulting reduction in point defect density through the formation of As precipitates.…”

“…10 The unique characteristics of this material that make it attractive for such applications (semi-insulating conductivity, 11 short carrier lifetime, 1,[12][13][14] and large band edge optical nonlinearity, 15 ) stem from the incorporation of up to 0.6% excess arsenic during growth. 16 This excess arsenic leads to a high density of As i and As Ga point defects and associated midgap states 11,16 as well as local potential fluctuations that lead to spatially localized states below the band gap (the so-called Urbach band tail). 17 The point defects lead to rapid trapping of band electrons 18 and pin the Fermi level in the gap resulting in semi-insulating behaviour.…”