PHYSICAL REVIEW B VOLUME 42, NUMBER 9 15 SEPTEMBER 1990-II Experimental study of the I -X electron transfer in type-II (Al, Ga)As/A1As superlattices and multiple-quantum-well structures J. Feldmann Fachbereich Physik, Philipps Un-iversitat Marburg, Renthof 5, D 355-0Marburg, Federal Republic of Germany J. Nunnenkamp Max Plan-ck Insti-tut fu rFes'tkorperforschung, Heisenbergstrasse l, Postfach 800665, D 700-0 Stuttgart 80, Federal Republic of Germany G. Peter and E. Gobel Fachbereich Physik, Philipps Univ-ersitat Marburg, Renthof 5, D 3550M-arburg, Federal Republic of Germany J. Kuhl and K. Ploog Max Plane-k Instit-ut fiir Festkorperforschung, Heisenbergstrasse l, Postfach 800665, D 7000 S-tuttgart 80, Federal Republic of Germany P. Dawson and C. T. Foxon Philipps Research Laboratories, Redhill, Surrey RH1 5', United Kingdom (Received 2 March 1990) A detailed experimental study of the real-space I -X transfer in type-II GaAs/A1As short-period superlattices and in type-II Al, Ga&, As/A1As multiple-quantum-well structures is presented. Transfer times on a subpicosecond and picosecond time scale are observed. The time constants crit- ically depend on the thickness of the (Al, Ga)As layers, but not on the AlAs-layer thickness in the samples studied. The I -X transfer rate is determined by the spatial overlap of the I and X wave functions confined in the different layers. Intensity- and temperature-dependent measurements pro- vide insight into the scattering mechanism. We conclude that electron — LO-phonon scattering is the 0 dominant scattering process for samples with thick (Al, Ga)As layers ( & 100 A). In contrast, inter- face scattering due to the interface mixing potential (1 -X, mixing) and/or due to potential Auctua- tions caused by interface roughness (I -X, ~ mixing) probably dominates for samples with thin (Al, Ga) As layers ( ( 35 A ). I. INTRODUCTION The electrical and optical properties of semiconductor materials are strongly influenced by carrier-scattering processes. The relative importance of different scattering mechanisms, e. g. , carrier-carrier, carrier-phonon, carrier-defect, or carrier-impurity interaction, as well as their respective scattering rates, depends on band struc- ture, excess energy of the excited carriers, carrier density, and the interaction potential of the relevant mechanism. The recent progress in high-speed electronic semicon- ductor devices' and very large-scale integration has stimulated intensive research on intervalley-scattering phenomena. Under the inhuence of high electric fields applied in ultrafast electronic devices, electrons in the lowest conduction-band minimum (e. g. , at the I point for CxaAs) can be efFectively scattered to higher val- leys, e. g. , to the X and L points of the Brillouin zone for GaAs. These scattering processes are of fundamental im- portance for high-speed electronic devices, since the transfer of carriers from I to, e.g. , the X valleys results in a drastic reduction of the carrier mobility, owing to the large effective mass of carriers in the satellite valleys. Several groups have investigated I -X intervalley scattering of electrons in bulk semiconductors GaAs and Al„Ga, „As (Refs. 4 and 6 — 8) and measured scattering times as short as 30 — 80 fs. ' ' It is evident that the I -X scattering rate will vary as a function of the excess energy of the electron in the I valley with respect to the energy of the X minimum, and thus critically depends upon the band structure. The recent progress in growing quantum-well and superlattice (SL) structures provides great potential for tailoring these scattering rates. Variation of the GaAs well width in a (GaAs) /(A1As)„SL (m, n give the number of mono- layers with thickness ao =2. 83 A) results in an increased confinement energy of the I conduction-band states in the GaAs layer with decreasing well width, if coupling between adjacent wells is not significant. This allows us to tune the lowest I conduction-band state in the GaAs to lie above the X minima of the A1As layers and generate a type-II SL configuration for m +12. In these type-II SL's, the highest valence-band state is contained in the GaAs layers, whereas the lowest conduction-band state occurs in the A1As layers. In ternary- on binary- compound (AI„Ga&, As) /(A1As)„SL's, type-II behav- 42 5809 1990 The American Physical Society