Papers

AVS2001 Session SC-MoM: Band-Engineered Electronic Materials

Monday, October 29, 2001 9:40 AM in Room 124

Monday Morning

Start	Invited?	Item
9:40 AM		SC-MoM-1 III-N-V: A Novel Class of Compound Semiconductors for Electronic and Photonic Applications C.W. Tu (University of California, San Diego) Recently there is much interest in III-N-V compound semiconductors, because only a small amount of nitrogen incorporation (less than 5%) in conventional GaAs- and InP-based III-V compounds results in very large bandgap bowing, which is mainly from the downward movement of the conduction band edge. We demonstrated that InNAsP/GaInAsP single-quantum-well microdisk lasers exhibit a large characteristic temperature, as a result of a large conduction band discontinuity and electron confinement in the quantum well. An application of more interest is using GaInNAs/GaAs heterostructures on GaAs substrates for 1.3 micron edge-emitting and vertical-cavity surface-emitting lasers (VCSELs), utilizing the well developed GaAs/AlAs distributed Bragg reflectors (DBRs). We demonstrated even longer wavelength (1.5 micron) room-temperature photoluminescence (PL) from self-assembled GaInNAs QDs. We have also utilized the low-bandgap GaInNAs as the base of a heterojunction bipolar transistor (HBT), which exhibits a 0.4 V reduction in the turn-on voltage for low-power applications. Besides bandgap bowing, incorporating nitrogen in GaP also results in a change in the band structure. We have demonstrated that with only 0.5% nitrogen, the Ga(N)P bandgap changes from being indirect to direct, with strong PL emission in the red (650 nm). We have fabricated light-emitting diodes (LEDs) from GaNP/GaP heterostructures grown with one-step epitaxy, which is simpler than the commercial process of GaAs substrate removal and wafer bonding to a transparent GaP substrate for high-brightness AlInGaP LEDs. We are also exploring GaInNP on GaAs for HBT applications because of near-zero conduction band offset.
10:20 AM		SC-MoM-3 Microstructure and Optical Properties of (InGa)(AsN) Alloys and Nanostructures X. Weng, S. Clarke, A. Daniel, J. Holt, S. Krishna, S. Kumar (University of Michigan, Ann Arbor); J. Sipowska (University of Michigan, Flint); V. Rotberg, R. Clarke, A. Francis, P.K. Bhattacharya, R.S. Goldman (University of Michigan, Ann Arbor) Mixed anion nitride-arsenide compound semiconductor heterostructures are promising for devices with emission or detection wavelengths throughout the near infrared range. However, a limited miscibility of InGaAsN on the anion sublattice leads to the formation of phase separation-induced alloy nanostructures.^1,2. We have synthesized InGaAsN alloys and nanostructures by N ion implantation into GaAs and InAs, with a variety of implantation and rapid thermal annealing conditions. We have analyzed the composition, structure, and properties of the resulting alloys and nanostructures, using nuclear reaction analysis, transmission electron microscopy (TEM), x-ray energy dispersive spectrometry, x-ray diffraction, photoluminescence, and cathodoluminescence spectroscopy. For 50 keV N ion implanted GaAs and InAs substrates, high resolution cross-sectional TEM reveals ~5nm diameter amorphous nanostructures surrounded by crystalline matrices. For 100 keV N ion implanted GaAs epilayers, crystalline nanostructures surrounded by disordered matrices are apparent. Electron and x-ray diffraction indicate that these nanostructures are cubic phases with lattice parameters similar to that of pure GaN. The crystalline nanostructures exhibit significant photoluminescence in the near infrared range. The apparent lowering of the fundamental band gap of the GaN nanostructures is consistent with strain-induced band gap narrowing of a GaN-rich cluster.¹. We will discuss the mechanisms of formation and coarsening of these nanos truc tures, correlations between their optical and structural properties, and comparisons with similar alloys and nanostructures synthesized by molecular beam epitaxy. ¹R. S. Goldman et al., Appl. Phys. Lett. 69, 3698 (1996), J. Electr. Mater. 26, 1342 (1997). ²H. P. Xin et al., Appl. Phys. Lett. 74, 2337 (1999).
10:40 AM		SC-MoM-4 Measurement of Charge Separation Potentials In GaAs(1-x)N(x) S.W. Johnston, R.K. Ahrenkiel (National Renewable Energy Laboratory); C.W. Tu, Y.G. Hong (University of California, San Diego) The ternary alloy GaAs(1-x)N(x) is interesting as a semiconductor that can be grown epitaxially on GaAs. As is well known, the bandgap can be reduced by as much as 0.4 eV by changing the nitrogen concentration from 0% to 3%. We measured the spectral response and photoconductive lifetime of the alloys, as a function of temperature. In this work, the films were grown by gas-source molecular beam epitaxy on semi-insulating GaAs substrates. All measurements were made using the contactless, resonant-coupled photoconductive decay (RCPCD) method. Our data shows that the spectral or excitation spectra of GaAs1-xNx alloys consists of photoconductive band tails that extend well into the infrared (beyond the nominal bandgap). For example, the photoconductive bandtails extend to about 1.8 mm for GaAs(0.97)N(0.03). The primary photoconductive decay times are in the range of 200 to 300 ns. At temperatures below about 200 K, the decay rate begins to decrease with lowered temperature. By plotting the inverse lifetime versus 1/T, one generates the standard Arrhenius plot of a thermally activated process. These data fits produce activation energies that increase with the N-content. The activation energies, DE, for compositions x = 0.011, 0.023, and 0.033 are 67, 72, and 83 meV, respectively. These energies represent the potential barriers which inhibit recombination. The increase of DE with x is indicative of charge separation being related to N-atom clustering. Our model suggests that these barriers originate from the inhomogenous band structure produced by the random distribution of the nitrogen impurity.
11:00 AM		SC-MoM-5 Characterization of GaPN Layers Grown by Molecular Beam Epitaxy on Si Substrates M.A. Santana-Aranda, M. Melendez-Lira, M. Lopez-Lopez (Centro de Investigacion y de Estudios Avanzados del IPN, Mexico); K. Momose, H. Yonezu (Toyohashi University of Technology, Japan); S. Jiménez-Sandoval (Cinvestav-IPN, Unidad Querétaro, Mexico) The heteroepitaxial growth of III-V-N alloys with high crystal quality on Si substrates could make possible the monolithic integration of III-V-N based light-emitting devices with the Si based microelectronics. However the III-V-N/Si heteroepitaxy present serious problems like the lattice mismatch and the difference in thermal expansion coefficients. In order to solve these problems, we have been studying the growth of GaPN with a N concentration of 2% that is lattice-matched to the Si substrate. The epilayers were grown by molecular beam epitaxy employing an RF plasma source to produce active nitrogen species. First, a 3.2µm Si homoepitaxial layer was grown on the substrate, followed by a thin (20nm) GaP layer to avoid the strong interaction of nitrogen with Si. Then, the GaP_0.98N_0.02layer was grown with a thickness of 400nm. In order to avoid the generation of crystal defects induced by the different thermal expansion coefficients the epilayer was capped first with a 16nm thick GaP layer and finally with a 300nm thick Si layer. The structural and optical properties of this sample were compared with those of a GaPN layer on GaP/Si but without the capping layers, and with those of a GaPN layer on a GaP substrate. Transmission electron microscopy showed that the capped heterostructure was free of crystal defects. While the other samples presented dislocations and cracks. The photoluminescence at 10K associated to GaP was blue shifted in the capped structure confirming that the GaP layers were coherently strained to the Si lattice. Raman spectroscopy showed narrow peaks in the capped structure reflecting the high structural quality of this sample, for the other samples the Raman peaks were wide suggesting the presence of disorder.
11:20 AM		SC-MoM-6 Nanoscale Phase Formation at Cu(In,Ga)Se₂ Surfaces Y.M. Strzhemechny, G.H. Jessen, J.I. Choi, L.J. Brillson (The Ohio State University); D.-X. Liao, A. Rockett (University of Illinois at Urbana-Champaign) Copper indium gallium diselenide (CIGS) exhibits unique optical properties that make it well suited for thin film devices as a polycrystalline material. The engineered electronic properties of CIGS heterojunctions depend sensitively on the CIGS near-surface region. The nanoscale electronic structure and chemistry of this layer and, specifically, point defect segregation and surface phases are thought to be critical to such device structures, yet they are relatively unexplored. We employed low-energy depth-resolved cathodoluminescence (CL) and Auger Electron Spectroscopy (AES) to measure the local band and defect properties as well as composition variations of CIGS films grown epitaxially with (001), (110), and (112) orientations on GaAs wafer substrates. CL spectra reveal near band edge (NBE) emissions of 1.10 to 1.15 eV, depending on growth and orientation as well as the presence of a deep-level transition with energy 0.89-0.93 eV confined to within a few hundred nm of the free CIGS surface. The NBE energies vs. AES compositions agree with the reported variation in band gap vs. Cu/(In+Ga) ratio. The deep defect-associated feature relative to the higher lying peak has maximum intensity at the surface and decreases exponentially into the bulk with a decay length of ca. 50 nm. Auger depth profiles for (110) and (001) orientations show 16% and 20%, respectively, depletion in Cu content within tens of nanometers of the surface. Deep level and NBE energies exhibit a strong dependence on surface Cu and Ga content. Both increase by tens of meV away from the free surface, in line with the increased band gap and changes in composition. The Cu-deficient surface layers are consistent with an ordered vacancy compound proposed for the CIGS surface. The observed gap and composition changes confirm the existence of a nanoscale surface phase whose properties can impact charge generation and recombination for solar energy-generating structures.
11:40 AM		SC-MoM-7 Metal-organic Vapor Phase Epitaxial Growth and Photoluminescent Properties of ZnMgO and ZnCdO Thin Films W.I. Park, G.-C. Yi (Pohang Univ. of Science and Technology, Korea (ROK)) ZnO, a wide-gap semiconductor oxide, has attracted considerable attention due to its large exciton binding energy (~60 meV) and bond strength, which might make reliable high efficiency photonic devices based on ZnO. Recently it has also been reported that ZnMgO was grown with maximum Mg incorporation up to 36 at.% without phase separation and that the room temperature luminescence energy in this film blue-shifted from 3.3 to 4.0 eV. Since a ZnMgO containing MgO over 4 at.% is in a thermodynamically metastable state, this result indicates that the solubility limit of Mg in ZnO depends on growth mechanisms as well as growth conditions. Meanwhile, current research on the growth of ZnO-related alloys is restricted to pulsed laser deposition and molecular beam epitaxy. Despite the epitaxial growth of high quality ZnO and related alloys using the methods, they might have disadvantages in mass production, due to high cost and low throughput. In this talk, we demonstrate that metal-organic vapor phase epitaxy (MOVPE), which has great advantages in terms of large area deposition and atomic composition control feasibility, is an excellent technique for the epitaxial growth of high quality ZnO and related alloy films. In addition, the structural and optical characterizations of ZnMgO and ZnCdO thin films will be reported. By increasing Mg content up to 47 at.%, the c-axis constant of ZnMgO films decreased from 0.521 nm to 0.515 nm and no significant phase separation in the ZnMgO films was observed as determined by x-ray diffraction measurements. Furthermore, the near-band-edge (NBE) emission peak position showed blue shifts of 100, 430, and 570 meV at Mg content levels of 9, 21, and 47 at.%, respectively. Photoluminescent properties of the ZnMgO and ZnCdO alloy films will also be discussed.