Optical Nonlinearities and Carrier Transport in GaAs: EL2 at High Excitation Levels

Abstract

Laser induced picosecond transient gratings are used to study carrier transport via free carrier and photorefractive nonlinearities in semi-insulating undoped GaAs bulk crystals. Carrier lifetime ( T~ = 1.5 ns), hole and ambipolar mobilities (ph = 410 cm2 V-' s-' , pa = 760cm2V-'s-') are measured directly and an electron mobility equal to 5200 cm2 V -' s -' is calculated. The optical properties of highly excited GaAs reveal deep donor EL2 transformation and intracenter absorption under picosecond excitation. Rotation of the diffracted beam polarization is observed for the first time in orientation of the grating vector along the crystallographic axis [OOl]. This signal is attributed to a strain field at dislocations in LEC-grown GaAs wafers.La diffraction de la lumiere sur les reseaux de porteurs libres et photortfractif engendrts par des impulsions picoseconde permet d'etudier les non-lintaritbs optiques ainsi que le transport des porteurs libres dans le GaAs semi-isolant. Le temps de vie (7R = 1,5 ns), la mobilitk des trow et la mobilite ambipolaire (ph = 410 cmz V-' s-l , pa = 760 cmz V-' s-') sont mesuries directement; la valeur de la mobilitd des electrons en est deduite (p, = 5200 cm2 V-' s-' ). Les proprietbs optique du GaAs sous fortes excitations rkvelent les transformations du niveau profond EL2 et I'absorption intracentre sous excitation picoseconde. Une rotation de la polarisation du faisceau diffracte est observee pour la premiere fois avec un vecteur reseau oriente suivant I'axe cristallographique [Ool]. Ce signal est attribut a un champ de contrainte a longue distance autour des dislocations dans le GaAs fabrique par la technique LEC.

Optical Nonlinearities in Bulk GaAs Determined by EL2 Defect

“…Calculation of absorption coefficient modulation shows that an amplitude grating may be created in EL2 traps due to their spatial photoionization [1]. Thus, both free carrier phase grating and amplitude grating may lead to slow, recombination governed grating decay, which is seen experimentally.…”

“…In case when one of recording beams was closed we realizedprobe-beam technique. The details of the experimental setups can be found elsewhere [1]. The samples used in the experiments were LEC-grown semi-insulating (SI) In-alloyed GaAs wafers with different density of EL2.…”

Dislocation density-dependent photorefractive effect in (001)-cut GaAs

“…5 This indicates that space-charge fields and carrier concentrations are coupled and would give a constant value of Esc/An. Thus v 1 § varies linearly with r*e. tion density is shown in Fig.…”

“…7 We compare the diffraction characteristics of semiinsulating GaAs (sample #1) in two different orientations of the grating vector (Kg /I [110] and Kg// [001] axes) to analyze the origin of the observed effect. 5 Dependencies of the diffracted beam energy I, versus excitation energy density Io (so-called exposure characteristics) and decay characteristics of the gratings (I1 versus probe beam delay time At) are measured.…”

“…The ratio E 1 /E 2 varies with excitation and time. An indication of a grating erasure by diffusion is given by a fast component whose time constant decreases with excitation and reaches its ambipolar limit , = K 2e T /up ± P//n = 43 ps TKg2kBT nl+ P at high excitations (n = p) 5 (n and p are the free-electron and hole densities). For example, for Kg// [110] we find the decay time of the PR grating TPR = 65 ps at 1o = 4 mJ cm" 2 ( Fig.…”