Time-resolved photoluminescence spectroscopy has been applied to determine the nature of the energy gap of InAlAs/AlAsSb multiple quantum well structures. While the InAlAs buffer layer exhibits a decay time of the order of 1 ns, which is typical for direct gap semiconductors, the decay time of the InAlAs/AlAsSb multiple quantum well structures is prolonged by more than two orders of magnitude. This observation is direct evidence for the presence of an indirect energy gap. The decay time

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... s with increasing InAlAs layer thickness indicating the decreasing overlap of electron and hole wave functions. Type II staggered quantum well structures exhibit unique optical properties due to confinement of electrons and holes in the different layers. In these structures, tunnelingassisted electron-hole recombination occurs across the type II heterointerface, resulting in light emission below the energy gaps of the constituent materials. The respective optical transition is indirect in real space, but direct in reciprocal space. InAlAs/InP quantum well structures lattice matched to InP substrates represent the most widely studied type II material system. 1-5 Electric-field induced excitons and a new type of optical bistability were observed for InAlAs/InP type II multiple quantum well ͑MQW͒ diodes. 6, 7 Recently, the enhancement of the recombination lifetime has been reported for several type II single quantum well systems, e.g., for GaP/AlP/GaP 8 and tensile strained InGaAs. 9 In 0.52 Al 0.48 As/AlAs 0.56 Sb 0.44 quantum well structures lattice matched to InP substrates are also expected to exhibit a type II staggered band configuration. In this material system, electrons are confined to the InAlAs layers, while the holes are localized in the AlAsSb layers as indicated in Fig. 1 . Recently, we reported the successful growth of high-quality In 0.52 Al 0.48 As/AlAs 0.56 Sb 0.44 type II MQW layers by molecular beam epitaxy ͑MBE͒. 10 Light emission at about 1.28 eV, which is below the energy gaps of InAlAs and AlAsSb, was observed and identified to occur near the InAlAs/AlAsSb heterointerfaces. However, the type II character of the emission was only inferred from the spectral position of the emission lines. The optical properties of type II emission are expected to differ considerably from that of bulk material and/or type I quantum well structures. In particular, the type II emission should exhibit a prolonged electron/hole recombination lifetime in comparison with type I structures, because the overlap of the electron and hole wave functions is strongly reduced. In this letter, we present direct evidence of the type II character of the InAlAs/AlAsSb MQW obtained from timeresolved photoluminescence ͑PL͒ spectroscopy. The radiative lifetime was strongly increased in the type II MQW compared to the InAlAs bulk-like buffer layer. Furthermore, the lifetime increases with increasing InAlAs layer thickness. The InAlAs/AlAsSb MQW structures consist of ten periods grown by MBE on Fe-doped ͑100͒ InP substrates. A 0.3 m thick, undoped InAlAs buffer layer was incorporated between the substrate and the MQW structure. In and Al metals were used to generate the group III beams, while the tetramers As 4 and Sb 4 were emitted by the group V beam sources. The group III beam and the Sb beam were supplied using conventional effusion cells ͑K cells͒, while the As beam was supplied using a valved As cell. The substrate temperature during the growth was 505°C, which was monitored by a calibrated infrared pyrometer. Prior to the growth, the InP substrate surface was thermally cleaned at 515°C for 1 min under an arsenic vapor pressure of 1ϫ10 Ϫ3 Pa. The a͒ Permanent address: RIAST, Osaka Prefecture University, 1-2 Gakuen-cho, Sakai, Osaka 599, Japan. b͒ Electronic mail: htg@pdi-berlin.de FIG. 1. Schematic diagram of the type II band alignment in InAlAs/AlAsSb multiple quantum well structures. The letters C and V indicate the edge of the conduction and valence band, respectively. The vertical arrows indicate the spatially indirect recombination at the InAlAs/AlAsSb heterointerfaces and the direct recombination in the InAlAs buffer layer.