Arsenic precipitates have been observed in GaAs low-temperature buffer layers (LTBLs) used as "substrates" for normal molecular beam epitaxy growth. Transmission electron microscopy has shown the arsenic precipitates to be hexagonal phase single crystals. The precipitates are about 6±4 nm in diameter with a density on the order of 10 17 precipitates per cm). The semi-insulating properties of the LTBL can be explained in terms of these arsenic precipitates acting as "buried" Schottky barriers with overlapping spherical depletion regions. The implications of these results on LTBL resistivity stability with respect to doping and anneal temperature will be discussed as will the possible role of arsenic precipitates in semi-insulating liquid-encapsulated Czochralski-grown bulk GaAs. Recently, a new type of semi-insulating GaAs epilayer, known as a low-temperature buffer layer (LTBL) was found to reduce "sidegating" or "backgating", an important parasitic problem associated with GaAs field-effect transistor circuit technology.l Even though there is currently much interest concerning possible applications of this material, there is a certain "mystery" about its chemistry, atomic structure, and electronic properties. Most of this mystery is well documented by Kaminska et a1. 2 It can
We have grown film structures by molecular beam epitaxy which include GaAs buffer layers grown at low substrate temperatures (250°C). The film structures have been examined using transmission electron microscopy. The layers grown at normal temperatures (600 °C) were free of defects or clusters. In contrast, the layer which 'Nas grown at low substrate temperatures contained precipitates which have been identified as hexagonal arsenic. The density of the arsenic precipitates is found to be very sensitive to the substrate temperature during growth.The growth of GaAs by molecular beam epitaxy (MEE) at low substrate temperatures has recently attracted much attention. 1·5 These low-temperature buffer layers (LTBLs) are highly resistive and have been shown to virtually eliminate side gating in GaAs integrated circuits. I ,2 L TELs have been found to contain an excess of arsenic. s We have used transmission electron microscopy (TEM) to examine film structures grown by MBE which include an L TEL. The layers grown at normal substrate temperatures (600 "C) were found to be free of defects. In contrast the layer grown at low substrate temperatures (250°C) was found to contain precipitates which have been identified as hexagonal arsenic. The formation of the arsenic precipitates is very sensitive to the growth conditions and post-growth "thermo-history" of the sample" This point is clearly evident from previous lack of observation of arsenic precipitates in as-grown LTBLs and L T-fiLs which were annealed at 600°C. 5 However, several groups have recently reported the observation of arsenic precipitates in L TBLs following growth of a layer at normal substrate temperatures on top of the LTBL or after an anneal following the growth of the L TBL. 68 In this letter we present the details of the MBE of our L TBLs and TEM analysis of our films.The film used in this work was grown in a Varian GEN II MBE system on a 2-in.-diam liquid-encapsulated Czochralski GaAs substrate. Some of the details of the film growth have been reported previously.3 However, the thermo-history of the sample plays a key role in the formation of the arsenic precipitates. Therefore, a more detailed description of the film growth will be presented so that the TEM results at various levels of the film structure can be correlated with the growth conditions"The substrate was degreased, etched in a 60 DC solution of 5:1:1 of HZS04:Hz02:H20 for 1 min and placed in a nonbonded substrate mount. The substrate was outgassed for 2 h at 200 DC in the entry chamber of the MBE, moved to the buffer chamber where it was outgassed for 1 h at 300 DC, and then loaded into the growth chamber. In the growth chamber, the sample was heated to 615 DC for 2 min (the surface oxides desorbed at 580°C) and then lowered to the initial growth temperature of 600 0c.The growth rates for all layers were 1 {Lm/h with a group V to group III beam equivalent pressure of 16. (The arsenic source was the tetramer As 4 .) Initially, 0.8 f-lm of undoped GaAs was grown. Then the substrate temperature ...
We have unpinned the Fermi level at the surface of both n- and p-type (100) GaAs in air. Light-induced photochemistry between GaAs and water unpins the surface Fermi level by reducing the surface state density. Excitation photoluminescence spectroscopy shows a substantial decrease in both surface band bending and surface recombination velocity in treated samples, consistent with a greatly reduced surface state density (≂1011 cm−2). Capacitance-voltage measurements on metal-insulator-semiconductor structures corroborate this reduction in surface state density and show that the band bending may be controlled externally, indicating an unpinned Fermi level at the insulator/GaAs interface. We discuss a possible unpinning mechanism.
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