Subpicosecond carrier llifetime in GaAs grown by molecular beam epitaxy at low temperatures (1991)

(Received 3 1 May 1991; accepted for publication 23 September 1991) Epitaxial GaAs grown by molecular beam epitaxy (MBE) at low substrate temperatures is observed to have a significantly shorter carrier lifetime than GaAs grown at normal substrate temperatures. Using femtosecond time-resolved-reflectance techniques, a sub- picosecond ( ~0.4 ps) carrier lifetime has been measured for GaAs grown by MBE at -200°C and annealed at 600 “C. With the same material as a photoconductive switch we have measured electrical pulses with a full-width at half-maximum of 0.6 ps using the technique of electro-optic sampling. Good responsivity for a photoconductive switch is observed, corresponding to a mobility of the photoexcited carriers of - 120-150 cm”/V s. GaAs grown by MBE at 200 “C! and annealed at 600 “C is also semi-insulating, which results in a low dark current in the switch application. The combination of fast recombination lifetime, high carrier mobility, and high resistivity makes this material ideal for a number of . subpicosecond photoconductive applications. The development of ultrashort-pulse mode-locked la- ser systems has resulted in new techniques for the genera- tion and detection of picosecond and subpicosecond elec- trical transients. l-3 Among these, the use of semiconductor photoconductive switches are the most popular, because these devices can be used to efficiently generate signals and to generate and detect electrical transients in guided media or free space. Also, the semiconductor growth and process- ing techniques available for tailoring the properties of these materials enhance their versatility. The minimum attain- able electrical pulsewidth from a photoconductive element is limited by a number of factors such as the laser pulse- width, circuit parameters of the generation and detection site, and the carrier lifetime in the semiconductor. With the use of femtosecond lasers and photolithographically de- fined millimeter-wave co-planar structures, the limits to speed imposed by the first two factors can be reduced. To shorten the carrier lifetime of a semiconductor layer, im- purity doping of the semiconductor,4 growth of polycrys- talline or amorphous material,5 and damage by ion implantation6 can be used. Earlier we reported that photo- conductive switches based upon GaAs grown by molecular beam epitaxy (MBE) at low temperatures showed fast re- sponse (1.6 ps) and good responsivity in unoptimized structures.’ In this letter we extend our earlier study’ of the photoresponse of low-temperature (LT) GaAs using both a femtosecond transient reflectance technique and photo- conductive switching measurements. From both experi- ments we have observed a subpicosecond carrier lifetime for LT-GaAs grown at -200 “C. The 2+m-thick, (lOO)-oriented epitaxial tilrns dis- cussed here were grown by MBE at substrate temperatures of 400, 350, 300, 260,200, and 190 “C. For all the growths an As4 source was used, and the samples were mounted on the same MO block using In solder. The growth rate was 1.0 pm/h, and the As/Ga beam-equivalent-pressure ratio was 10. Pieces of the LT-GaAs samples were annealed inside the growth chamber under an As overpressure, just after the cmompletion of the growth, by raising the substrate temperature to 600 “C for 10 min. A number of papers have reported novel material properties of as-grown and annealed LT-GaAs layers, es- pecially those grown at -200 “C.“-” For photoconductive- switch applications, the most relevant properties of both as-grown and annealed 200 “C LT-GaAs are that the ma- terials are crystalline and yet contain a high density ( :> 10” cm 3 ) of point defects as As antisites, As inter- stitials, and Ga-related vacancies.“,” In addition to a high density of point defects, annealed 200 “C LT-GaAs grown in the Lincoln Laboratory MBE system also contains small ( < 5 nm) As precipitatesI at densities of -3~ lOI cm -3. The aforementioned point defects can act as recom- bination and trapping centers. Assuming simple Shockley- Read-Hall theory for the recombination mechanism of the photoexcited carriers, and using a density N- lOi cm ’ for the deep levels, a capture cross section u- lo- I3 cm2 (a typical value for deep levels in GaAs), and thermal velocity u,h at T = 300 K, we estimate that the carrier lifetime r = l/(Nmu,) in as-grown and annealed LT- GaAs to be less than 1 ps. Although as-grown LT-GaAs is relatively conducting (p- 10 s1 cm) at room temperature, annealed LT-GaAs is semi-insulating (p- 10’ fl cm).” Despite the high density of point defects and As precipitates, the Hall mobility at room temperature in annealed LT-GaAs is relatively high ( - 1000 cm”/V s).” Therefore, LT-GaAs grown at .- 200 “C and subsequently in situ annealed has the desired properties of a fast photoconductor; namely, a short carrier