Rf. Kopf et al., MODIFICATION OF GAAS ALGAAS GROWTH-INTERRUPTED INTERFACES THROUGH CHANGES IN AMBIENT CONDITIONS DURING GROWTH/, Journal of applied physics, 74(10), 1993, pp. 6139-6145
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.