"Growth Induced Low-Dimensional Quantized Micorstructure and Optical Property Enhancement in (111) A AlGaAs:" ALBERT CHIN, Y.S. Kao, K.Y. Hsieh, W.J. Chen, J.H. Kao, Department of Electronics Engineering, National Chiao Tung University, Hsinchu, Taiwan; Institute of Materials Science & Engineering, National Sun Yet-Sen University, Kaohsiung, Taiwan; Department of Mechanical Materials Engineering, National Yun-Lin Polytechnic Institute, Huwei, Taiwan
We have studied the growth induced long-range Al-rich and Ga-rich AlxGa1-xAs/AlyGa1-yAs superlattices in (111)A AlGaAs. The spontaneously formed quantized microstructures are observed by cross-sectional TEM in AlxGa1-xAs (X=0.3 to 0.4) grown on (111)A GaAs substrates at 600 to 720deg.C. In contrast, there are no such superstructures observed by TEM on simultaneously grown (100) oriented substrates. For 640deg.C grown (111)A Al0.30Ga0.70As, the separation of Ga-rich and Al-rich superlattices increase from less than 50Å to ~400Å as the growth thickness is increased. The effect of compositional modulation is reduced as growth temperature is increased. Although the long-range Al-rich and Ga-rich superlattices can be still observed from 700deg.C grown (111)A Al0.40Ga0.60As, the contrast between the bright Al-rich and dark Ga-rich AlGaAs is lower than that of 640deg.C grown (111)A AlGaAs. The separation of Ga-rich and Al-rich superlattices is also quite uniform for 700deg.C grown (111)A Al0.40Ga0.60As, which shows the formation of such superlattices that is due to the different surface migration velocities of Ga and Al adatoms on (111)A. The compositionally modulated superlattice observed by TEM is strongly related to the large peak energy red-shift and peak intensity enhancement of 15deg.K photoluminescence (PL) on (111)A orientations to that on (100). Similar reduced compositional modulation at high growth temperatures is also observed by PL. A narrow PL linewidth of 14.0 meV is measured from 720deg.C grown (111)A Al0.30Ga0.65As, which is the best reported value for AlGaAs grown on (111)A orientation. The red-shifts of PL peak energy for (111)A AlGaAs to that of (100) are 137, 31, and 2 meV for 600, 640, and 700deg.C grown samples, respectively. The PL integrated intensity enhancements to (100) are 200, 5, and 1.5 times for 600, 640, and 700deg.C grown (111)A AlGaAs, respectively. The large red shift and intensity enhancement also indicate the formation of quantized microstructures by Al-rich and Ga-rich superlattices. The 600deg.C grown (111)A AlAs/Al0.40Ga0.60As/AlAs quantum well shows two orders of PL intensity enhancement to that of (100), which can be used in super luminescent devices in red wavelength.
"Raman Studies of Spontaneously-Ordered GalnP2:" H.M.CHEONG, A. Mascarenhas, D. Friedman, J.M. Olson, National Renewable Energy Laboratory, 1617 Cole Blvd., Golden, CO 80401; P. Ernst, C. Geng, 4 Physikalisches Institut, Universität Stuttgart. D-70550 Stutgart, Germany
GaInP2 alloys grown by MOCVD on (001) GaAs substrates exhibit a spontaneous CuPt-type ordering of various degrees along the ordirections, depending on the growth conditions and the substrate misorientation. These structures resemble monolayer superlattices of Ga(0.5+h)In(0.5-h)P/Ga(0.5-h)In(0.5+h)P (0<=[[eta]]<=1) along the ordering direction. Due to this ordering, the symmetry of the crystal changes from the Td symmetry of zinc-blende to trigonal C3n symmetry. This change of symmetry should manifest itself in the changes in the Raman spectra of these alloys.
We have studied the Raman spectra of a series of GaInP2 samples with various degrees of ordering. We measured the Raman spectra in the (001) and (110) backscattering geometry, and in the right-angle scattering geometry between the (001) and surfaces. For disordered alloys and alloys with [[eta]]<=0.3, three major features are observed in the Raman spectra: a GaP-like LO phonon at 381cm- 1, an InP-like LO-phonon at 362 cm- 1, and TO phonon at 329 cm- 1. For ordered alloys with [[eta]]>0.3, three major changes are observed: 1) an extra peak appears at 353 cm- 1 when the LO phonon are allowed, and this peak is enhanced when the electric field of the excitation has a component along the ordering direction ; 2) in the acoustic phonon region, two-zone-boundary acoustic phonon modes, folded to the zone center due to the ordering, are observed at 60 and 204 cm- 1; 3) the LO phonon peaks move to higher energies.
The Td and C3n Raman selection rules are compared with the experimental results. It has been found that the enhancement of the extra peak at 353 cm- 1 when the excitation polarization has a component in the direction, does not follow either Td and C3v symmetries. The implications of this will be discussed.
"Magnetoluminescence Study on Effective Mass Anisotropy in Ordered GaInP Alloys:" YONG ZHANG, P. Ernst, F.A.J.M. Driessen, A. Mascarenhas, E.D. Jones, C.Geng, F. Scholz, H. Schweizer, National Renewable Energy Laboratory, 1617 Cole Blvd., Golden, CO 80401; Physikalisches Institut, Universtät Stuttgart, Germany; Sandia National Laboratories, PO Box 5800, Albuquerque, NM 87185
It has been well established that epitaxially grown GaxIn1-xP alloys (x ~ 0.5) exhibit CuPt ordering along the B directions under proper growth conditions. The ordering changes the semiconductor band structure. Band-gap reduction and valence-band splitting have been observed as predicted. Ordering induced changes in effective masses, especially effective mass anisotropy, have also been predicted. Previous magnetoluminescence studies, with magnetic field along the  growth direction, have shown a reduction in averaged exciton reduced mass or averaged conduction band mass. Because of the rhombohedral distortion along the B direction, an ordered alloy has ellipsoidal energy dispersions E(k) with a symmetry axis in the ordering direction. The aim of this work is to verify the effective mass anisotropy by studying magnetoluminescence with the magnetic field B aligned both in the ordering and growth directions.
Samples used in this study are a set of ordered GaInP samples that are different in the degree of ordering (ordering parameter [[eta]] ranges from 0 to 0.54). Photoluminescence and photoluminescence excitation spectra are taken at liquid helium temperature with the magnetic field varying from 0 - 14deg. Tesla. We observe that when B is along the ordering direction, the excitonic peaks in ordered samples have larger energy shifts compared to that in a disordered sample; for a ordered sample, the energy shift is smaller when B is along the growth direction than along the ordering direction. These observations agree with theoretical predictions that for the band edge excitonic state, the reduced mass in the plane perpendicular to the ordering direction is lighter than that in the ordering direction and lighter than that of a disordered alloy.
"Spontaneously Generated Lateral Superlattices:" J. MIRECKI MILLUNCHICK, E.D. Jones, Sandia National Laboratory, PO Box 5800, MS 1311, Albuquerque, NM 87185; Y. Zhang, P. Ahrenkiel, A. Mascarenhas, National Renewable Energy Laboratory, 1617 Cole Ave., Golden, CO 80401
During the synthesis of ultra-thin vertical superlattices comprised of lattice mismatched binary semiconductor alloys ((AC)n/(BC)n, n = 1,2,3) it has been observed that there exists a tendency towards self organized behavior. During MBE growth, the instability generated by the lattice mismatch strain drives the system to lower its elastic energy, resulting in the spontaneous generation of a lateral composition modulation wave, whereby the vertically averaged alloy composition is no longer ABC2 but oscillates as A1-dB1+dC2 where d >> [[Delta]]Cos(2[[pi]]/[[lambda]])z with [[Delta]] >> 0.2, [[lambda]] >> 200 Å and z directed along the  axes. Our spectroscopic measurements of the band edge electronic and the dielectric function confirm that the structure behaves as a lateral superlattice. However, the lateral two dimensional quantum confinement is not caused by the modulation of the band gap, since this is diminished by negative feedback from coherency strain in the lateral interface planes (the A rich and B rich regions are size mismatched). The large band gap regions are dilated whilst the small band gap regions are compressed due to the hydrostatic component of the lateral coherency strain wave, and this tends to lock the band gaps of the compositionally modulated regions. The axial components of the lateral coherency strain wave alters the curvature (in k space) of the valence bands and so the effective mass of holes alternates periodically with [[lambda]], varying by a factor of 24 between the regions that are under tension and compression respectively. The interplay between the competing effects of the spontaneously generated lateral composition modulation wave and the lateral coherency strain wave (in real space), as revealed in the electronic band structure (in k space) result in the formation of an effective-mass lateral superlattice.
"Spontaneously Ordering in (110)-InGaP Alloy:" A.M. MINTAIROV, N.A. Bert, A.S. Vlasov, A.F. Ioffe Physical-Technical Institute RAS, Polytechnicheskaya 26, 194021, St.-Petersburg, Russia; I.G. Malkina, B.N. Zvonkov, E.R. Linkova, Yu.N. Safyanov, Physical-Technical Institute of State University, N.Novgorod,Russia
Spontaneous ordering in semiconductors can appear due to both atomic sublattice (microscopic)  or phase separation (macroscopic)  ordering in semiconductor alloys or heterostructures. This phenomena is of great current interest in semiconductor physics.
In this paper we report the results of observation of [-110] sheet-like ordered domains in InxGa1-xP (x=0.49-0.55) grown on (110) GaAs substrates by metalorganic vapor phase epitaxy (MOVPE). This observed structure exhibits both microscopic and macroscopic types of ordering. The layers were grown by atmospheric pressure MOVPE from trimethylgalium, trimethylindium and phosphine at 650deg.C. The growth rate was about 1nm/s, the layer thickness was 0.8-2.2um. We performed transmission electron microscopy, photoluminescence and Raman measurements.
In-plane selected area diffraction patterns and bright-field image show that the investigated layers contain domains which are thin along the [-110] direction. The size of domains in this direction is 10- 20 nm, and is 0.1-0.5 um in the  direction. The domains are lattice matched to an InGaP alloy. They occupy ~ 10% of the total volume.
Several bands of internal vibrations of the ordered domains have been identified in the Raman spectra. The main bands appear the frequencies 310, 335, 359, and 379cm- 1 and show polarization properties differing from those of zinc blend structures. This is direct evidence of atomic ordering inside domains. The identification of their structure is in progress. The strong and wide emission bands at 1.6-1.9eV had extraordinary strong polarization that correlated with sheet domain orientation. The low-energy shifts of polarized band maxima were up to 300meV from unpolarized bands of disordered InGaP. The bands emitted from the (110) surface were polarized in the  direction, ones emitted from the (001) surface - in the  direction. The PL from the (-110) surface was not polarized.
1. A. Zunger, and S. Mahajan, in: Handbook on Semiconductors, Vol. 3, (Elsevier, Amsterdam, 1994)
2. I.P. Ipatova, V.G. Malyshkin, and V.A. Shchukin Phys. Low-Dim. Struct., 7, p. 1-14(1994).
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