MODIFICATION OF GAAS ALGAAS GROWTH-INTERRUPTED INTERFACES THROUGH CHANGES IN AMBIENT CONDITIONS DURING GROWTH/

The effect of the ambient conditions in the growth chamber of the mole cular beam epitaxy machine during the growth of GaAs/Al0.35Ga0.65As st ructures was investigated. Both growth-interrupted (120 s at each hete rointerface) and uninterrupted surfaces and interfaces were evaluated using a growth temperature of 580-degrees-C. Two ambient conditions we re studied: (a) approximately 1 X 10(-10) Torr O2; and (b) ultrahigh v acuum (UHV, approximately 5 X 10(-11) Torr, with no intentional introd uction of contaminants). A striking difference was observed in both th e 1.7 K photoluminescence (PL) spectra of single quantum well (SQW) st ructures and UHV scanning tunneling microscopy (STM) of surfaces, whic h were grown under ambient condition (a) as opposed to (b). When conse cutive growth-interrupted SQW samples were grown with different well w idths (25 and 28 angstrom under condition (a), the emission energy spl itting into several peaks was observed, indicating discrete thicknesse s of the well. However, the peak energies shifted as the laser spot wa s scanned across each sample. Additionally, the peak energy shifted fr om sample to sample for the same nominal well width. On the other hand , when SQW samples were grown under condition (b), no variation in the emission energy was observed as the laser was scanned across the samp le, or from sample to sample for a given well width. Furthermore, the PL observations are supported by UHV-STM results. UHV-STM images indic ated a very rough surface with large islands containing small terraces on top (a bimodal distribution) for condition (a). Conversely, when s amples were grown under condition (b), only large islands were observe d. For growth interrupted GaAs surfaces, 400 angstrom X 600 angstrom i slands were observed, and for Al0.35Ga0.65As, they were 150 angstrom X 400 angstrom, with a one-monolayer step in between islands. These dat a are consistent with abrupt interfaces with only a single-mode distri bution for growth-interrupted surfaces. On the other hand, UHV-STM ima ges of uninterrupted GaAs surfaces grown under condition (b) showed is lands that were 40-60 angstrom across. Photoluminesce spectra of a sim ilarly grown SQW sample showed only a single broad emission line, cons istent with an interface configuration of many steps which are smaller than the exciton diameter. The results show that interface roughness is sensitive to background O2.