About the 1996 Electronic Materials Conference: Friday Morning Sessions (June 28)

Session BB: Quantum Effect Materials: Quantum Dots I

8:20AM, BB1

"Auger Processes in Nanosize Semiconductor Quantum Dots:" Al. L. EFROS, V.A. Kharchenko, M. Rosen, Nanostructure Optics Section, Naval Research Laboratory, Washington, DC 20375; Institute for Theoretical Atomic and Molecular Physics, Harvard-Smithsonian Center for Astrophysics, 60 Garden St., MS 14, Cambridge, MA 02138

The role of Auger processes in nanosize semiconductor crystals is discussed. Three types of processes are considered: Auger autoionization of nanocrystals containing two electron-hole pairs, nonradiative Auger recombination in nanocrystals containing one external charge, and Auger-like thermalization processes in semiconductor quantum dots with only one electron-hole pair.

Auger autoionization of nanocrystals plays an important role in the process of optical degradation of the photoluminescence in semiconductor doped glasses (photo-darkening effect), colloidal nanocrystals, and porous Si. After such ionization (which is a result of the optical excitation of two electron-hole pairs in a single nanocrystal), the nonradiative Auger recombination is completely dominant over radiative recombination in the charged nanocrystal. The rates of these processes (Auger autoionization and Auger recombination) have been studied as a function of energy band parameters and nanocrystal size. The abrupt potential barrier at the temperature energy threshold known for bulk semiconductors. Our results are compared with the observed degradation both of the PL in semiconductor doped glasses and of the optically induced polarization of the PL in porous Si.

In semiconductor quantum dots with an electron level spacing much larger than typical phonon energies, Auger-like processes break the phonon bottleneck and lead to the fast thermalization of carriers. This process is described as the rapid transfer of the electron energy to scattering of a hot electron with a heavy hole in semiconductor quantum wells (the second most efficient relaxation process there). The relaxation time and its dependence on the nanocrystal energy band parameters and size is obtained. A value of ~2ps has been calculated for the electron thermalization for spherical CdSe nanocrystals, which is in good agreement with available experimental data.

8:40AM, BB2

"Self-Assembled InP Islands Grown on GaP:" Y. NABETANI, K. Sawada, Y. Furukawa, A. Wakahara, A. Sasaki, Department of Electronic Science and Engineering, Kyoto University, Kyoto 606, Japan

Quantum dots(QDs) by self-assembled islands grown at the initial stage of lattice-mismatched heteroepitaxy has been attracting with great interest. We have proposed that InAs islands grown on GaAs can be utilized as a mesoscopic structure and reported that they exhibit characteristics of QD. Among several combinations of lattice-mismatched heteroepitaxy, InP QD is one of the promising materials for optical applications in a visible region. Thus far, InP islands have been grown on InGaP which is lattice-matched to GaAs substrate. In this case, however, inherent phenomena at InGaP surface such as strain inhomogeneity, alloy ordering, and In segregation affect the InP island formation.

In this paper, we have grown InP on GaP to investigate the InP island formation and its application for QD. The GaP substrate was chosen rather than InGaP lattice-matched to GaAs which absorbs the light from the InP QD, and further the island formation is free from intricate interactions as occurred at the interface of InP/InGaP. Samples were grown by atmospheric pressure organometallic vapor phase epitaxy(OMVPE) using tertiarybuthylphosphine(TBP) as a P source. The decomposition temperature of TBP is much lower than that of PH . Trimethylindium(TMI) and trimethylgallium(TMG) were used for group III sources. Undoped GaP was grown on (001) nominally-oriented GaP substrate at 720deg.C at first. Then, several monolayers of InP was grown at 420~550deg.C. Growth rate of InP was 0.25ML/s and V/III flow ratio was 10~1000. The structure of InP was observed by atomic force microscope(AFM) and transmission electron microscope(TEM).

InP was grown two-dimensionally at least to 1.0ML with the V/III flow ratio of 52 at the growth temperature of 550deg.C. On the contrary, well-formed islands were observed when the InP layer thickness exceeds 1.25ML. Island lateral size was 5000 è and the height was 1000 è at 1.8ML. Island density was 1 x 10 cm . Island surface was terminated by four {11x} facets dominantly and some large islands had four {110} facets in addition. Only two-dimensional nuclei and steps were observed between islands. The island density becomes 4.5 x 10 cm when InP layer thickness was 8ML, though the island size remained same. With increasing V/III flow ratio from 10 to 1000, the island becomes smaller. Islands of 2500 è in lateral size and 300 è in height were obtained at V/III flow ratio of 1000. Plan-view TEM observation revealed the existence of misfit dislocations in InP island. Dislocation in island is undesirable for the QD application. The most influential growth parameter for island size was the growth temperature. The island grown at 420deg.C was less than 500 è in diameter. By reducing the island size, we could succeed to form InP islands where no misfit dislocation was found. InP growth at such low temperature was achieved by use of TBP. The island size is almost comparable to that of InP island grown on InGaP in which quantum effect has been confirmed.

9:00AM, BB3+

"Deposition of Self-Assembled InP Islands on Different Surfaces: Effects of Composition, Orientation, and Roughness:" C.M. REAVES, R.I. Pelzel, W.H. Weinberg, S.P. DenBaars, Center for Quantized Electronic Structures, University of California, Santa Barbara, CA 93106

Quantum dots can be used to study a number of physical processes and to improve electronic devices. One quantum dot fabrication technique that has emerged is the utilization of coherent islands that form during lattice-mismatched semiconductor epitaxy. A number of semiconductor material systems have been observed to exhibit the coherent Stranski-Krastanov growth mode. When appropriate combinations of materials are used, the resulting structures are self-assembled quantum dots. One of the materials that can be used for the quantum dot is InP. When deposited on a material with the GaAs lattice constant, three sizes of InP islands are observed. The sizes of these islands depend on a variety of growth parameters (e.g., substrate temperature and deposition rate). One growth parameter that has been explored in this study is the nature of the deposition surface. The composition of the surface, the roughness of the surface, and the orientation of the surface have all been observed to affect the evolution and size of the islands.

For the case of InP deposited on a Ga InP/GaAs(100) surface at an estimated growth rate of 1 monolayer/second, the growth is initially two dimensional with the formation of three-dimensional islands after ~ 2s. For deposition on a GaAs(100) surface, the InP growth continues in a two-dimensional fashion for approximately 5 s before the formation of three-dimensional islands. For deposition on a moderately smooth GaInP(100) surface (interface width ~ 5 è), the base area of one type of island is measured to be 4 x 10 um , whereas for deposition on a rough GaInP(100) surface (interface width ~ 25è), the base area is measured to be 10 x 10 um . In comparing islands formed on different orientations, we observe differences in island size and island density. InP islands on GaInP(311)A surfaces are smaller and more dense than the islands formed on GaInP(100). The impact of surface composition, surface roughness, and orientation on the evolution and resulting size of the InP islands will be detailed. These results offer insights into heteroepitaxy as well as demonstrate flexibility in island sizes and available material systems for quantum dots.

9:20AM, BB4+

The Role of Quantum-Confined Excitons vs. Defects in Luminescence of Ion Beam Synthesized Si Nanocrystals in SiO2 Matrices: KYU S. MIN, Kirill V. Shcheglov, C.M. Yang, Harry A. Atwater, Thomas J. Watson Laboratory of Applied Physics, California Institute of Technology, Pasadena, CA 91125

Synthesis of light emitting Si nanocrystals in thermal Si0 films is carried out using a ULSI process-compatible approach of precipitation from a supersaturated solid solution of Si in Si0 made by Si+ ion implantation. Upon annealing at temperatures greater than 800deg.C, the films exhibit intense visible photoluminescence at room temperature. The presence of Si nanocrystals in the samples that display luminescence has been confirmed by high resolution transmission electron microscopy. As it has been reported previously, defects in the matrix also display visible photoluminescence. For this reason, it has been unclear whether the visible photoluminescence observed in many reports can be attributed to matrix defects or excitonic recombination in Si nanocrystals. In order to isolate the luminescence originating from defects in the Si0 matrix, we have performed Xe implantation into Si0 films with similar atomic displacement profiles. Photoluminescence spectra from annealed Xe -implanted Si0 clearly indicated that -defects in the matrix produce a strong luminescence band peaked at energies greater than 2 eV, which is also observed for annealed Si -implanted samples.

We have found that hydrogen passivation can quench the matrix defect luminescence, thus enabling other sources of luminescence to be distinguished from the matrix defect contribution. We have thus performed deuterium passivation of our films by performing low energy (<1KeV) D implantation on annealed Si -implanted samples. Deuterium was chosen instead of hydrogen since the implanted D concentration can be precisely determined via elastic recoil ion scattering spectrometry. Room temperature D implantation beyond a dose of 3.0x10 /cm completely quenches the matrix defect luminescence. The remaining luminescence band, which can be attributed to Si nanocrystals, initially remains unchanged. Then, upon further annealing a low temperatures (<600deg.C), the intensity of the remaining band is shown to increase by as much as a factor of two. After suppression of the luminescence band that originates from matrix defects we were able to observe "red shifts" in PL peak energy as a function of different processing parameters. After deuteration, the photoluminescence peak energy has been shown to red shift by more than 0.3 eV by varying the annealing temperature from 800deg.C to 1200deg.C, which can be expected to yield an increase in the nanocrystal size. Similarly, red shifts of more than 0.5 eV has been observed by varying the implantation dose from 1x10 /cm to 5x10 /cm. In order to investigate whether there is any effort of nanocrystal size on the energy band gap, we also have performed high resolution X-ray photoelectron spectroscopy. The measured Si 2p binding energy attributable to nanocrystalline Si is greater than that of bulk Si by more than 1 eV, suggesting a valence band shift relative to bulk Si. In summary, the observed red shifts and valence band shifts could be explained by a quantum confinement model.

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T. S. Iwayama, S.N. Nakao, K. Saitoh, Appl. Phys. Lett. 65, 1814 (1994)

9:20AM, BB4+

"The Role of Quantum-Confined Excitons vs. Defects in Luminescence of Ion Beam Synthesized Si Nanocrystals in SiO2 Matrices:" KYU S. MIN, Kirill V. Shcheglov, C.M. Yang, Harry A. Atwater, Thomas J. Watson Laboratory of Applied Physics, California Institute of Technology, Pasadena, CA 91125

The two dimensional electron gas (2DEG) properties of high quality AlGaN/GaN heterostructures grown on SiC and sapphire substrates have been characterized and compared. The III-V nitride layers were grown by metal-organic vapor phase epitaxy (MOVPE) on 6H-SiC and c-plane sapphire, utilizing insulating buffer schemes optimized for each type of substrate. A GaN layer with a nominal thickness of 3 um was grown on the buffer and followed by the growth of a 500 è Al GaN donor layer. The structural qualities of the III-V nitride layers were measured by double crystal x-ray diffraction ([[omega]]-scan). The full-width-at-half-maximum of the GaN (0004) reflection for the GaN layer grown on 6H-SiC was 1.8 arcmins, compared to 5 arcmins for GaN grown on sapphire. C-V measurements confirmed that the majority of carriers were located near the AlGaN/GaN interface.

Temperature dependent Hall effect measurements were made from 10 to 300K. A temperature independent mobility indicative of the presence of a 2DEG was observed in all samples below 80K. The sample grown on SiC had the highest low temperature mobility, 7500 cm /Vs at 10K; the carrier concentration was 6x10 cm . The mobilities of the samples grown on sapphire were lower, the largest being 5700 cm /Vs at 10K. Shubnikov-de Haas experiments on AlGaN/GaN heterostructure grown on SiC showed strong magnetoresistance oscillations starting at 3T. Oscillations were observed up to 10K. These are believed to be the lowest field and highest temperature at which Shubnikov-de Haas oscillations have been observed in AlGaN/GaN heterostructures. The results of analysis of the magnetic field and temperature dependences of the oscillations will be reported.

9:40AM, BB5

"Two Dimensional Electron Gas Properties of AlGaN/GaN Heterostructures Grown by MOVPE on SiC and Sapphire Substrates:" J. M. REDWING, J.S. Flynn, M.A. Tischler, S. Elhamri, M. Ahoujja, R.S. Newrock, W.C. Mitchel, A. Saxler, Advanced Technology Materials, Inc., 7 Commerce Dr., Danbury, CT 06810; Department of Physics, University of Cincinnati, Cincinnati, OH 45221, Wright Laboratory, WL/MLPO, W-PAFB, OH 45433

The two dimensional electron gas (2DEG) properties of high quality AlGaN/GaN heterostructures grown on SiC and sapphire substrates have been characterized and compared. The III-V nitride layers were grown by metal-organic vapor phase epitaxy (MOVPE) on 6H-SiC and c-plane sapphire, utilizing insulating buffer schemes optimized for each type of substrate. A GaN layer with a nominal thickness of 3 um was grown on the buffer and followed by the growth of a 500 è Al GaN donor layer. The structural qualities of the III-V nitride layers were measured by double crystal x-ray diffraction ([[omega]]-scan). The full-width-at-half-maximum of the GaN (0004) reflection for the GaN layer grown on 6H-SiC was 1.8 arcmins, compared to 5 arcmins for GaN grown on sapphire. C-V measurements confirmed that the majority of carriers were located near the AlGaN/GaN interface.

Temperature dependent Hall effect measurements were made from 10 to 300K. A temperature independent mobility indicative of the presence of a 2DEG was observed in all samples below 80K. The sample grown on SiC had the highest low temperature mobility, 7500 cm /Vs at 10K; the carrier concentration was 6x10 cm . The mobilities of the samples grown on sapphire were lower, the largest being 5700 cm /Vs at 10K. Shubnikov-de Haas experiments on AlGaN/GaN heterostructure grown on SiC showed strong magnetoresistance oscillations starting at 3T. Oscillations were observed up to 10K. These are believed to be the lowest field and highest temperature at which Shubnikov-de Haas oscillations have been observed in AlGaN/GaN heterostructures. The results of analysis of the magnetic field and temperature dependences of the oscillations will be reported.