(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

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

The stoichiometry, crystallinity, defect concentration, and the excess As lattice location in GaAs layers grown by molecular beam epitaxy at low growth temperatures (≤300 °C) were studied using ion beam techniques. The excess As concentration in the layers was measured by particle induced x‐ray emission and was found to increase as the growth temperature was lowered. Excess As concentrations up to 1.5 at. % were measured in layers grown at 190 °C. After annealing at temperatures higher than 400 °C under As overpressure, the excess As atoms coalesce to form As precipitates as revealed by transmission electron microscopy. Ion channeling on the unannealed layers grown at 200 °C revealed that they have good crystalline quality with a large fraction of the excess As atoms sitting at interstitial sites close to the normal As sites in the lattice. The rest of the excess As atoms are believed to be in an AsGa antisite position.

Characterization of GaAs layers grown by low temperature molecular beam epitaxy using ion beam techniques

This paper describes TEM characterization of bulk GaN crystals grown at 1500–1800Kin the form of plates from a solution of atomic nitrogen in liquid gallium under high nitrogen pressure (up to 20 kbars). The x-ray rocking curves for these crystals were in the range of 20–30 arc-sec. The plate thickness along thec axis was about 100 times smaller than the nonpolar growth directions. A substantial difference in material quality was observed on the opposite sides of the plates normal to thec direction. On one side the surface was atomically flat, while on the other side the surface was rough, with pyramidal features up to 100 nm high. The polarity of the crystals was determined using convergent-beam electron diffraction. The results showed that, regarding the long bond between Ga and N along the c-axis, Ga atoms were found to be closer to the flat side of the crystal, while N atoms were found to be closer to the rough side. Near the rough side, within 1/10 to 1/4 of the plate thickness, there was a high density of planar defects (stacking faults and dislocation loops decorated by Ga/void precipitates). A model explaining the defect formation is proposed.

Structural characterization of bulk GaN crystals grown under high hydrostatic pressure

We show that it is possible to regrow an amorphous GaAs layer created by high dose As implantation at room temperature. If implantation parameters are carefully selected, As precipitates may be formed in the regrown layer with structural characteristics the same as those observed in semi‐insulating GaAs grown by molecular beam epitaxy at low temperature. Transmission electron microscopy has been used to study the structure of these precipitates in connection with the structural defects which are seen in the layer. This process appears promising for the formation of low cost semi‐insulating GaAs layers.

Formation of As precipitates in GaAs by ion implantation and thermal annealing

The relationship between the structural quality of low-temperature GaAs layers and the photoexcited carrier lifetime has been studied. Transmission electron microscopy, x-ray rocking curves, time-resolved reflectance methods, and photoconductive-switch-response measurements were used for this study. For a variety of samples grown at temperatures in the vicinity of 200°C, subpicosecond carrier lifetimes were observed both in as-grown layers, as well as in the same layers after post-annealing and formation of As precipitates. These results suggest that the carrier lifetime, which was found to be shorter in the as-grown layers than in the annealed ones, might be related to the density of AsGa antisite defects present in the layers. The annealed layers which contained structural defects before annealing appeared to exhibit the longest carrier lifetime due to gettering of As on these defects (and formation of relatively large As precipitates) and depletion of extra As (AsGa) defects from the layer. It was found as well that the responsivity of detectors fabricated on these layers depended strongly on the structural quality of the layers, with the greatest response obtained not for the layers with the fewest defects, but for the layers with 107–108/cm2 of pyramidal defects.

Structure and carrier lifetime in LT-GaAs

The crystal structure and orientation of As precipitates in annealed low-temperature GaAs (LT-GaAs) layers have been investigated by transmission electron microscopy. Three types of As precipitates were identified in layers grown by molecular beam epitaxy at substrate temperatures from 180° to 210° C. In the monocrystalline LT-GaAs layers small “pseudocubic” As precipitates (2–3 nm diameter) coherent with the GaAs lattice were observed. These precipitates lose their coherency when a certain critical size is exceeded. Precipitates of similar sizes are occasionally found for which a TEM lattice image cannot be obtained. These precipitates are believed to be amorphous. Larger As precipitates with a hexagonal structure (>4 nm diameter) were also found in the layers. These hexagonal As precipitates were observed to be largest near structural defects. The effect of these precipitates on the structure and on the electronic properties of the host GaAs is discussed.

Microstructure of annealed low-temperature-grown GaAs layers

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Microstructure of annealed low-temperature-grown GaAs layers