Formation of arsenic precipitates in GaAs buffer layers grown by molecular beam epitaxy at low substrate temperatures

Abstract

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 ...

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“…LTG-GaAs is prepared through molecular beam epitaxial growth at 200 °C instead of the usual 600 °C. After growth, however, proper in situ or ex situ annealing at 600 °C is necessary in order increase the density of arsenic precipitates and hence resistivity of this layer [60]. A high density (>10 18 cm -3 ) of arsenic precipitates in LTG-GaAs results in excess arsenic-related point defects and a short carrier trapping time [61].…”

An Investigation on the Lattice Site Location of the Excess Arsenic Atoms in GaAs layers Grown by Low Temperature Molecular Beam Epitaxy

“…Subsequent annealing of these layers above 400°C in As ovetpressure results in a uniform semi-insulating property and the lowering of their lattice parameter to the value of stoichiometric GaAs grown at normal temperatures (-600°C). Transmission electron microscopy (TEM) investigation revealed that As precipitates with diameter ranging from 2 to 5 nm are formed in the layers after annealing [2][3][4][5].…”

“…GaAs thin films grown by Molecular Beam Epitaxy (MBE) at low growth temperature,T g (<400°C) have been subject to many investigations recently [1][2][3][4][5][6][7][8][9][10]. Murotani et.al.…”

Comment on ‘‘Mechanism responsible for the semi-insulating properties of low-temperature-growth GaAs’’ [Appl. Phys. Lett. 65, 3002 (1994)]

“…5 With anneal this excess arsenic beings to precipitate. 6 The final composite structure of semimetallic arsenic precipitates in a GaAs matrix can be controlled with the substrate temperature during MBE and the subsequent anneal. The substrate temperature during MBE sets the amount of excess As in the epilayer; 7,8 the subsequent anneal controls the precipitation of the excess arsenic and the amount of coarsening of the arsenic precipitates.…”