Modification of GaAs/AIGaAs growth-interrupted interfaces through changes in ambient conditions during growth

The effect of the ambient conditions in the growth chamber of the molecular beam epitaxy machine during the growth of GaAs/Ales,Gae(i,As structures was investigated. Both growth-interrupted (120 s at each heterointerface) and uninterrupted surfaces and interfaces were evaluated using a growth temperature of 580 “C. Two ambient conditions were studied: (a) 1 x 10-r’ Torr 02; and (b) ultrahigh vacuum (UHV, 5 x lo‘* Torr, with no intentional introduction of contaminants). A striking difference was observed in both the 1.7 K photoluminescence (PL) spectra of single quantum well (SQW) structures and UHV scanning tunneling microscopy (STM) of surfaces, which were grown under ambient condition (a) as opposed to (b). When consecutive growth-interrupted SQW samples were grown with different well widths (25 and 28 A) under condition (a), the emission energy splitting into several peaks was observed, indicating discrete thickneLes of the well. However, the peak energies shifted as the laser spot was scanned across each sample. Additionally, the peak energy shifted from 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 sample, or from sample to sample for a given well width. Furthermore, the PL observations are supported by UHV-STM results. UHV-STM images indicated a very rough surface with large islands containing small terraces on top (a bimodal distribution) for condition (a). Conversely, when samples were grown under condition (b), only large islands were observed. For growth interrupted GaAs surfaces, 400 AX 600 A islands were observed, and for Ale35Gae65A~, they were 150 AX400 A, with a one-monolayer step in between islands. These data are consistent with abrupt interfaces with only a single-mode distribution for growth-interrupted surfaces. On the other hand, UHV-STM images of uninterrupted GaAs surfaces grown under condition (b) showed islands that were 40-60 A across. Photoluminesce spectra of a similarly grown SQW sample showed only a single broad emission line, consistent with an interface configuration of many steps which are smaller than the exciton diameter. The results show that interface roughness is sensitive to background OZ.