"Room Temperature Lasing From Excited Stated of InGaAs Quantum Dots:" RICHARD MIRIN, Arthur Gossard, John Bowers, ECE Department, MS100, University of California, , Santa Barbara, CA 93106
Quantum dots formed by highly strained epilayers undergoing a Stranski-Krastanow transition(2D-3D) have recently been demonstrated in several III-V semiconductor systems such as InGaAs/GaAs, GaSb/GaAs, and InP/GaInP. We have previously reported optical and structural characteristics of In0.3Ga0.7As quantum dots grown using alternating molecular beam epitaxy(MBE). This deposition method allows indium segregation to the growth front and the reflection high energy electron diffraction(RHEED) pattern shows that the 2D-3D transition occurs between 7.1 and 8.0 monolayers of deposition. We continue to deposit In0.3Ga0.7As until 22.1 monolayers have been deposited. The quantum dots demonstrate room temperature photoluminescence at 1.32 um with a full width at half maximum of only 28 meV. The dot density is 2-31010 cm-2 and the area coverage is about 40%. In this work, we demonstrate electrically-injected lasers with quantum dots in the active region.
The laser structure consists of the following: n+ GaAs substrate, 1.1 um n-Al0.7Ga0.3As, 0.1 um NID GaAs, 17.7 monolayers of In0.3Ga0.7As(grown with alternating MBE), 0.1 um NID GaAs, 1.1 um p-Al0.7Ga0.3As, and 500 Å p+-GaAs. Ti/Pt/Au is patterned into 50 um wide stripes for the p-contact. After wet etching a ridge using the p-metal as an etch mask, the samples are lapped and NiAuGe contacts are deposited on the backside. As a reference sample, we have grown the identical structure except we replace the InGaAs quantum dot layer with a 7.1 monolayer In0.3Ga0.7As quantum well grown with alternating MBE.
The lasers have been tested at temperatures ranging from 85-295 K. At 295 K, the quantum dot laser lases with a threshold current density of 1250 A/cm2 for a 1 mm long device. The ground state emission from the quantum dots is at about 1200 nm. The intensity at this wavelength increases until the current density reaches about 120A /cm2 at which point the ground state emission saturates and excited state luminescence at higher energy begins to dominate the spectrum. As the current density continues to increase, the excited state intensity also increases, and lasing is finally achieved at 1025 nm. At 85 K, the quantum dot laser has a threshold current density of 510 A/cm2. The ground state emission at 1150 nm saturates at about 32 A/cm2 and lasing occurs from an excited state at 993 nm.
By comparison, the quantum well reference laser has a lower threshold current density(230 A/cm2 at 295 K, 50 A/cm2 at 85 K) and shorter emission wavelength(944 nm at 295 K, 898 nm at 85 K). The higher threshold current density in the quantum dot laser means that the threshold carrier density is much larger in the quantum dot laser than in the reference quantum well laser. Higher carrier densities lead to shorter wavelength lasing because of band filling. Despite this effect, the reference quantum well laser lases at a much shorter wavelength than the quantum dot laser. This is a strong indication that the lasing in the quantum dot laser is coming from states associated with the quantum dots, as opposed to wetting layer states.
This research is supported by the NSF Center for Quantized Electronic Structures(QUEST), Grant No. DMR91-20007.
"Effects of Substrate Growth Temperature on Photoluminescence Linewidth and Intensity in Self-Organized InAs Quantum Dots on GaAs:" G.S. SOLOMON, S. Komarov, J.S. Harris, Jr., Solid State Laboratories, Stanford University, Stanford, CA 94305-4055
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.
"Self-assembled InAs Quantum Dots Grown on Patterned GaAs Substrates:" KANJI YOH, Toshiya Saitoh, Arata Tanimura, Research Center for Interface Quantum Electronics, Hokkaido University, North 13, West 8, Sapporo 060, Japan
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.
"Energy Shifts in Vertical-Stacked InAs Island Pairs: Hydrostatic Strain and Electronic Coupling:" MARK MILLER, Mats-Erik Pistol, Soren Jeppesen, Lars Samuelson, Department of Solid State Physics, Lund University, 221 00 Lund, Sweden
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.
"Phonon Enhanced Carrier Relaxation Between Ground and Excited States in InAs Self Assembled Quantum Dots:" K.H. SCHMIDT, G. Medeiros-Ribeiro, P.M. Petroff, Center for Quantized Electronic Structures (QUEST) and Materials Department, University of California, Santa Barbara, CA 93106
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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