Session Chairman: K. Wang, 66-147 Engineering IV, Electrical Engineering Department, UCLA, Los Angeles, CA 90095-1594
Co-Chairman: A. Sasaki, Department of Electronic Science and Engineering, Kyoto University, Kyoto 606, Japan
"Enhanced Photoluminescence of Symmetrically Strained Ge/Si Disordered Superlattices:" AKIHIRO WAKAHARA, Yoshihiro Nomura, Motonori Ishii, Akio Sasaki, Department of Electronic Science and Engineering, Kyoto University, Kyoto 606-01, Japan
We have investigated the enhanced luminescence by carrier localization in a SiGe/Si disordered superlattice (d-SL) in which disordering is artificially introduced in the growth direction and induces the carrier localization. When the SiGe/Si d-SLs are grown on Si substrate, we could not observe localization effects on the relaxation of the momentum conservation, because the minimum energy states in these superlattices appear in the [[Delta]]-valleys perpendicular to the growth direction "z". We grew symmetrically-strained Ge/Si d-SLs on a strain-free SiGe layers to achieve the localization in [[Delta]]z valley and observed enhanced photoluminescence compared with an ordered superlattice (o-SL).
Two types of superlattices, i.e., Ge(4ML)/Si(6ML) o-SL and Ge(mML)/Si(nML) d-SL with m=2,4,6 and n=3,6,9 ML (mono-atomic layer) were grown on strain-relaxed Si0.6Ge0.4/n--Si(001) substrates at 500deg.C by using solid source molecular beam epitaxy. The values of m and n appear disorderly along the growth direction, and with equal appearance probability. Thus, the macroscopic compositions of the o-SL and d-SL are the same, but their microscopic structures are different. The sample consisted of a 50 nm of Si0.6Ge0.4 cap-layer, the superlattice layer, a 300 nm of Si0.6Ge0.4 cladding layer, Si/Si0.6Ge0.4/Si carrier blocking layer, Ge/Si short period strained superlattice (SPSS), and 400 nm of Si0.6Ge0.4 buffer layer on Si substrate. The SPSS is inserted for the reduction of the threading dislocations.
The photoluminescence (PL) spectrum was measured at low temperature by using He gas flow cryostat and Ar-ion laser as an excitation source. The PL properties of the d-SL were compared with those of the o-SL. PL spectra of both d- and o-SLs indicate a no-phonon emission but not clear phonon replicas. The PL peak of d-SL shifts to lower energy than that of the corresponding o-SL, and the red-shift energy is about 150 meV. The PL intensity of d-SL is about 10 times stronger than that of o-SL. These results are quite different from that observed from Ge/Si SLs grown on Si substrate, i.e., strong phonon-replica, small red-shift ([[Delta]]E-10 meV), and weak enhancement of PL intensity (only a few times). It is the first successful result that the luminescence enhancement by disordering in Ge/Si superlattice is similar to that reported by the AlAs/GaAs and AlP/GaP d-SLs. The PL intensity of d-SLs indicates smaller temperature dependence than that of o-SLs. These improvement of the PL properties by the d-SL structure may be due to the strong localization which could achieve by taking the direction of localization parallel to the growth direction. Further studies for increasing the light emission of SiGe will be continued.
"Direct and Indirect Bandgap Behavior in Si-Ge Strained Alloys and Superlattices:" T.P. PEARSALL, A. DiVergilio, S. Vaidyanathan, Department of Electrical Engineering, University of Washington, Seattle, WA 98195; L. Colace, Dipartimento Ingeneria Elettronica, Terza Università di Roma, Rome, Italy; H. Presting, Daimler-Benz Forschung, Wilhelm-Runge-Strasse 11, D-89081 Ulm, Germany; Erich Kasper, Institut für Halbleitertechnik, University of Stuttgart, D-70174 Stuttgart, Germany
We have used photocurrent spectroscopy at 300K, 77K and 4.2K to study optical absorption processes of 12 (001) Si-Ge strained layer superlattices and alloys with an average composition of 50% Si - 50% Ge. The energy dependence of the optical absorption is proportional to the density of states. The energy dependence of the density of states gives direct information about direct versus indirect bandgap behavior.
Using photocurrent spectroscopy, we observed an evolution toward higher energies of the threshold in the photocurrent spectra as the period of the superlattices decreases, with the spectrum of the shortest period superlattices (2:2) approaching that of the alloy. In agreement with earlier work by Olajos, et al,(1) our photocurrent spectra do not distinguish between the energy dependence of photoresponse of a superlattice and the energy dependence of the photoresponse for an alloy of the same average composition.
Measurements on the same samples using optical absorption indicates that a significant difference exists between the superlattices and the alloy. Hence photocurrent spectroscopy and optical absorption are not equivalent methods for assessing the energy dependence of the density of states. Our results show that optical absorption samples the density of states at the band edge. These results show that all Ge-Si superlattices show a mixture of direct-gap and indirect-gap behavior, whereas the alloy shows the signature of an indirect gap semiconductor. Within the limits of present technology, it is unlikely that pure direct-gap behavior in the superlattices can be observed.
1. J. Olajos, J. Engvall, H.G. Grimmeiss, U. Menczigar, G. Abstreiter, H. Kibbel, E. Kasper, and H.Presting, Phys. Rev. B46, 12857(1992).
"Electronic Structure of GaAs/Ge2 Superlattices:" JEFFREY RUFINUS, Department of Physics, 1150 University Avenue, University of Wisconsin-Madison, Madison, WI 53706; G.E. Crook, Department of Electrical Engineering, 1415 Engineering Dr., University of Wisconsin-Madison, Madison, WI 53706
The (GaAs)1-x(Ge2)x alloy has been predicted to have a direct bandgap for x 0.75,with a value as low as 0.7 eV at x 0.3. These predictions, combined with the close lattice match between Ge and GaAs, make (GaAs)(Ge2) alloys promising for "bandgap engineering" of optoelectronic devices in the 0.9 to 1.7um wavelength range. In order to compare the predicted band structure of the (GaAs)(Ge2) random alloys to that of GaAs/Ge superlattices, we have used a second-nearest-neighbor tight-binding model to calculate the electronic structure of (001)(GaAs)m/(Ge2)n superlattices with (m,n) ranging from 1 to 20.
We have found no correspondence between calculated band structures of the (GaAs)/(Ge2) superlattices and (GaAs)1-x(Ge2)x random alloys. The predicted superlattice bandgaps are small, as low as 0.11 eV for m=n=1. For small values of m, the (GaAs)m/(Ge2)n superlattice bandgaps are indirect, regardless of the valence band offset used in the calculation. For larger values of m, however, the superlattice bandgaps become direct for large values of valence band offset. This is significant in light of the wide range of band offset values (valence band offsets [[Delta]]Ev from 0.23 to 0.70 eV) which have been reported for the GaAs/Ge systems, and the reported dependence of the offset on growth conditions.
"Hole Relaxation Times in GaAs/Al0.35Ga0.35Ga0.65As Quantum Wells Measured by Femtosecond Time-Resolved Differential Transmission:" KIMBERLY L. SCHUMACHER1, D. Collings, R.T. Phillips, J.N. Schulman2, D. Ritchie, Klaus Ploog, University of Cambridge, Cavendish Laboratory, Madingley Road, Cambridge, England, CB3 0HE; 1presently at the University of Nottingham, Electrical and Electronic Engineering, Nottingham, England NG7 2RD; Hughes Research Laboratories, 3011 Malibu Canyon Road, Malibu, CA 90265; Paul-Drude-Institut für Festkörperelecktronik, Berlin, Germany
A direct measurement of the hole relaxation times as a function of the energy spacing between the first two confined subbands of a set of GaAs/Al0.35Ga0.65As quantum well samples was made using time-resolved differential transmission. The laser pump and probe energies were selected from a supercontinuum with a time resolution of less than 100fs, and the excitation density was less than 7.11010 cm-2. The energy difference between the first two confined conduction subbands was either , 0.7 , or 1.3 , where is equal to one LO phonon energy, or -36 meV. The results were analyzed using rate equations, considering the relative bleaching strengths of the effects on the relaxation of the carriers in the quantum wells, such as Coulomb screening, phase-space filling, and exchange and correlation-hole effects. The total intersubband scattering time for the sample with an inrtersubband spacing of 0.7 was 1.5 ps500fs, 2.5-3 ps 500fs for the sample, and 2 ps500fs for the 1.3 sample. A second experiment was performed on the sample with an intersubband spacing of that measured the temporal evolution of the differential transmission as a function of laser excitation density. The rate equation analysis revealed that the hole relaxation time increased with excitation density, ranging from -10 ps500fs at high excitation densities to -2ps+/-500fs at 10% of the high density.
L-Band Recombination in InxGa1-xP/InAlP Multiple Quantum Wells: D. PATEL, K. Interholzinger, P. Thiagarajan, G.Y. Robinson, C.S. Menoni, Department of Electrical Engineering, Colorado State University, Fort Collins, CO 80523
InGaP is one of the most attractive materials for the development of semiconductor lasers emitting in the yellow-red region of the optical spectrum. Current visible laser diode technology uses multiple quantum wells (MQWs), with InGaP wells and InAlP or InGaAlP barriers. This heterostructure combination coupled with the possibility of varying well width and composition, offers large flexibility in the selection of the operating wavelength as well as for tailoring of the output characteristics. Normally knowledge of the direct energy bandgap is sufficient for the selection of the operating wavelength. However, for specific requirements such as the reduction of the laser threshold current, it is necessary to know the separation among the conduction band minima, as to avoid any detrimental influences in the laser operation from the higher effective mass, indirect L1c and X1c valleys.
In this paper we report on the first direct observation of recombination from the L1c band in InxGa1-xP/In0.5Al0.5P MQWs and show that L1c becomes the conduction minima in narrow (35 Å) unstrained MQWs and also in highly tensile strained MQWs of 50 Å well width. The L-like behavior of the conduction band minima was identified from pressure dependent photoluminescence measurements at 50K. Carrier recombination from L1c was characterized by a pressure coefficient of (60+/-5) meV/GPa, considerably smaller than that measured for the lowest confined [[Gamma]]c state. The separation of L1c and states associated with [[Gamma]]1c and X1c, also observed in the experiments, was determined by extrapolating the high pressure data to ambient conditions. By correcting these values for confinement and strain effects, we determined the separation of the conduction band minima in bulk unstrained InxGa1-xP for In compositions x 0.48. L1c was determined to be (0.1+/-0.02)eV above [[Gamma]]1c and (0.18+/-0.04) eV below X1c for x=0.48. Decreasing x, decreased the [[Gamma]]c-L1c separation to (0.08+/-0.02) eV and (0.03+/-0.02)eV for x=0.41 and x=0.37, respectively. The L1c-X1c separation remained constant, within the error of the measurements, for x=0.41 and x=0.37 respectively.
Work supported by the National Science Foundation, Grants Nos. DMR 9321422 and
ECS-9502888 and AFOSR contract F49620-93-0021.
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