The following papers will be presented at the 8th Biennial Workshop on OMVPE, on Thursday morning, April 17th, 1997. The calendar of events describes the entire technical program.
R.D. Dupuis, University of Texas at Austin, Austin TX
The Shadow Masked MOVPE Growth and Optical Properties of GaAs Microlenses: G. Peake and S.D. Hersee, Center for High Technology Materials, University of New Mexico, Albuquerque, NM 87131
The large emitting aperture of surface emitting lasers (SELs) reduces the divergence of the output beam considerably compared to that of edge emitting lasers. However, there are many applications (e.g. parallel optical interconnects) where a more tightly collimated or even focused output beam is required for optimum coupling of the SEL output into a small aperture. This paper reports preliminary results of our use of shadow masked MOVPE growth (SMMG) to fabricate GaAs microlenses and microlens arrays, which can be integrated onto SEL devices. The paper will briefly describe our novel SMMG process and the mask geometry used and will then concentrate on the physical and optical properties of the microlenses that have been fabricated. Extending from our previous description of SMMG growth in 2 dimensions  an important result demonstrated by this work is that SMMG can be used to produce smooth surfaced 3-dimensional objects (in this case microlenses) that are completely free from facetting. The paper will compare the measured focal length and intensity distribution at the focal point with theoretically predicted values, for a range of lenses with focal lengths as small 30 microns. These preliminary measurements indicate that while spherical abberations are apparent these lenses already offer the potential for significant improvements in the optical-coupling of SELs.
Step-Free InAs Quantum Well Selectively Grown on GaAs (111)B Substrate: Toshio Nishida and Naoki Kobayashi, NTT Basic Research Laboratories, 3-1, Morinosato Wakamiya, Atsugi-shi, Kanagawa, Japan 243-01
We investigated possibility to form step-free quantum well structure. A step-free In As monolayer was grown in selectively grown mesa by controlling surface phase applying in-situ monitoring of surface photo-absorption (SPA). We selectively grew 200-nm-thick GaAs at openings in SiO2 mask film on GaAs (ll1)B substrate at 800 degree, and cooled sample to 630 degree keeping (2x2)-like As stabilized surface. Atomic force microscope (AFM) observation clarified fully stepfree surfaces are formed on S-micron-wide mesa. This flatness is due to the finite growth area, because monolayer steps exist on planar area. We formed one-monolayer-thick InAs on this step-free surface and capped by 7 ML of GaAs grown under (2x2)-like condition which effectively suppress indium segregation. Further, GaAs barrier layer was grown using flow-rate modulation (FME) method to enhance migration under slightly (root19 x rootl 9)-like condition. We evaluated the quantum level of step-free InAs layer by using spatially resolved photoluminescenc (PL) measurement. Uniform PL intensity and no double layer peak confirmed the formation of step-free InAs quantum well.
Effect of Growth Interrupts on Size-Homogeneity of Coherent Stranski-Krastanow Islands: Niclas Carlsson, Jonas Johansson, Lars Samuelson and Werner Seifert, Solid State Physics, Lund University, Box 118, S-221 00 Lund, Sweden
The use of spontaneous self-organization effects is an efficient way to produce nano-structures, as for instance quantum dots. Our results for InP/GaInP/GaAs show that surface densities and sizes of InP dots are mainly determined by the deposition conditions. The nucleation conditions determine the density of the dots and the available excess material per dot determines their size. The high homogeneity in size is caused by a growth regulating effect due to the local strain which builds up around the 3D islands. We demonstrate that this effect leads to a high homogeneity in size of the dots only for the case that the materials transport is dominated by surface diffusion, consuming excess material from the wetting layer. When material is provided directly from the vapour towards already formed coherent islands, one amplifies small differences in size, loosing in this way the size homogeneity. An explanation is given by considering the development of the local chemical potential with the formation of 3D islands.
Growth and Characterization of Self-Assembled InAs/InP (001) Nanometer-Sized Coherent Islands by Metalorganic Vapor Phase Epitaxy: H. Marchand, P. Desjardins, S. Guillon, J-E. Paultre, Z. Bougrioua, R. Yip, R.A. Masut, Groupe de recherche en physique et technologie des couches minces (GCM) and Departement de genie physique, Ecole Polytechnique de Montreal, C.P. 6079, Succursale Centre-Ville, Montreal, Quebec, Canada H3C 3A7
The growth of self-assembled coherent islands has been the subject of intense investigation due to the expected zero-dimensional density of states and low defect density for these quantum confined semiconductor systems. Most studies have focused on the deposition of In(Ga)As on GaAs(001) either by molecular beam epitaxy or by metalorganic vapor phase epitaxy (MOVPE), while the formation of InAs islands on InP(001) has only been recently demonstrated using molecular beam epitaxy and chemical beam epitaxy. We show that the controlled deposition of InAs/lnP(001) self-assembled coherent islands is also possible using MOVPE and present structural and optical characterization results using atomic force microscopy (AFM), transmission electron microscopy (TEM), high-resolution x-ray diffraction (HRXPD), and photoluminescence spectroscopy (PL). The growth was performed in a low-pressure MOVPE reactor using TMIn, TBAs and phosphine as precursors. The growth temperature was varied between 450 and 600°C, and the InAs deposition time ranged between 3 and 12 s with a constant growth rate of approximately 0.7 monolayer/s. To investigate the kinetics of island formation we used a growth interruption sequence (times varying between 0-250 s) after the deposition of the InAs layer. Depending on the growth conditions, the average island diameter and areal density determined by AFM and TEM are 20-60 nm and 0.44 x 1010 cm-2, respectively, with a relatively narrow diameter distribution. The TEM micrographs indicate a significant amount of elastic relaxation of the strain in the coherent islands as well as in the surrounding InP matrix. The 77 K PL displays a broad emission between 0.7 and 1.0 eV attributed to the islands, and a sharper peak at 1.24 eV attributed to a wetting layer 2 monolayer in thickness.
Nanoscale InP and InAsP Islands Grown by MOVPE: Oleg V. Kovalenkov, Dmitry A. Vinokurov, Daniil A. Livshits, II'ya S. Tarasov, Nikolai A. Bert, and Zhores I. Alferov, A.F. Ioffe Physico-Technical Institute RAS, 26 Polytechnicheskaya str., St. Petersburg, 194021, Russia
We present the growth of InP and, for the first time, InAsP self-assembled nanoscale islands embedded in In0.5Ga0.5P by low pressure MOVPE. Growth of our samples started with the deposition of a 500nm-thick Ga0.5In0.5P lower barrier followed by nominally from 0.7 to 15 monolayers (MLs) for InP islands deposition and from 0.7 to 7.5 MLs in case of InAsP. The As composition was varied from 0 to 0.5. After 5s growth interruption the islands were finally overgrown by a 50nm-thick upper Ga0.5In0.5P. A nominal growth rates of 1.4 ML per second (ML/s) and 0.7 ML/s were for the barriers and QDs layer, respectively. The structures have been investigated by photoluminescence (PL) and transmission electron microscopy (TEM) methods. The dots density and lateral size can approximately be estimated to be (1-5)*109cm-2 and 80 nm, respectively. The PL peak position for InP QDs in the range from 2 to 15 MLs remains nearly constant in contrast to the quantum well peak which continuously shifts to lower energy with growing layer thickness. This indicates that formed by the three-dimensional Stranski-Krastanov growth mode the coherent 3D islands have developed up to their critical dimension, which is determined by the strain equilibrium between matrix material, wetting layer and islands. The integrated efficiency of the radiative recombination from dots reaches 30% at liquid nitrogen temperature (even for quantity of deposited QDs material as small as 3 monolayers).
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