Showing papers by "W. Shan published in 2005"

Nature of room-temperature photoluminescence in ZnO

Abstract: The temperature dependence of the photoluminescence (PL) transitions associated with various excitons and their phonon replicas in high-purity bulk ZnO has been studied at temperatures from 12 K to above room temperature (320 K). Several strong PL emission lines associated with LO phonon replicas of free and bound excitons are clearly observed. The room temperature PL spectrum is dominated by the phonon replicas of the free exciton transition with the maximum at the first LO phonon replica. The results explain the discrepancy between the transition energy of free exciton determined by reflection measurement and the peak position obtained by the PL measurement.

Fermi-level stabilization energy in group III nitrides

Abstract: Energetic particle irradiation is used to systematically introduce point defects into In{sub 1-x}Ga{sub x}N alloys over the entire composition range. Three types of energetic particles (electrons, protons, and {sup 4}He{sup +}) are used to produce a displacement damage dose spanning five decades. In InN and In-rich InGaN the free electron concentration increases with increasing irradiation dose but saturates at a sufficiently high dose. The saturation is due to Fermi level pinning at the Fermi Stabilization Energy (E{sub FS}), which is located at 4.9 eV below the vacuum level. Electrochemical capacitance-voltage (ECV) measurements show that the pinning of the surface Fermi energy at E{sub FS} is also responsible for the surface electron accumulation in as-grown InN and In-rich InGaN alloys. The results are in agreement with the amphoteric defect model that predicts that the same type of native defects are responsible for the Fermi level pinning in both cases.

On the crystalline structure, stoichiometry and band gap of InN thin films

Abstract: Detailed transmission electron microscopy, x-ray diffraction (XRD), and optical characterization of a variety of InN thin films grown by molecular-beam epitaxy under both optimized and nonoptimized conditions is reported. Optical characterization by absorption and photoluminescence confirms that the bandgap of single-crystalline and polycrystalline wurtzite InN is 0.70±0.05eV. Films grown under optimized conditions with an AlN nucleation layer and a GaN buffer layer are stoichiometric, single-crystalline wurtzite structure with dislocation densities not exceeding mid-1010cm−2. Nonoptimal films can be polycrystalline and display an XRD diffraction feature at 2θ≈33°; this feature has been attributed by others to the presence of metallic In clusters. Careful indexing of wide-angle XRD scans and selected area diffraction patterns shows that this peak is in fact due to the presence of polycrystalline InN grains; no evidence of metallic In clusters was found in any of the studied samples.

Pressure-dependent photoluminescence study of ZnO nanowires

Abstract: The pressure dependence of the photoluminescence (PL) transition associated with the fundamental band gap of ZnO nanowires has been studied at pressures up to 15 GPa. ZnO nanowires are found to have a higher structural phase transition pressure around 12 GPa as compared to 9.0 GPa for bulk ZnO. The pressure-induced energy shift of the near band-edge luminescence emission yields a linear pressure coefficient of 29.6 meV/GPa with a small sublinear term of -0.43 meV/GPa{sup 2}. An effective hydrostatic deformation potential -3.97 eV for the direct band gap of the ZnO nanowires is derived from the result.

Multiphonon Resonance Raman Scattering in InGaN

Abstract: June 28, 2005 Multiphonon Resonance Raman Scattering in InGaN J. W. Ager III a) , W. Walukiewicz a) , W. Shan a) , K. M. Yu a) , S. X. Li, a),b) E. E. Haller a),b) , H. Lu c) , and W. J. Schaff c) a) Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, Department of Materials Science and Engineering, University of California, Berkeley, Department of Electrical and Computer Engineering, Cornell University, Ithaca, New California 94720 b) California 94720 c) York 14853 PACS: 78.30.Fs, 78.66.Fd, 63.20.-e ABSTRACT In In x Ga 1-x N epitaxial films with 0.37 0.37. Page 1 of 25

Effect of Native Defects on Optical Properties of InxGa1-xN Alloys

Abstract: The energy position of the optical absorption edge and the free carrier populations in In{sub x}Ga{sub 1-x}N ternary alloys can be controlled using high energy {sup 4}He{sup +} irradiation. The blue shift of the absorption edge after irradiation in In-rich material (x > 0.34) is attributed to the band-filling effect (Burstein-Moss shift) due to the native donors introduced by the irradiation. In Ga-rich material, optical absorption measurements show that the irradiation-introduced native defects are inside the bandgap, where they are incorporated as acceptors. The observed irradiation-produced changes in the optical absorption edge and the carrier populations in In{sub x}Ga{sub 1-x}N are in excellent agreement with the predictions of the amphoteric defect model.

Nature of Room-temperature Photoluminescence in ZnO

Abstract: The temperature dependence of the photoluminescence (PL) transitions associated with various excitons and their phonon replicas in high-purity bulk ZnO has been studied at temperatures from 12 K to above room temperature (320 K). Several strong PL emission lines associated with LO phonon replicas of free and bound excitons are clearly observed. The room temperature PL spectrum is dominated by the phonon replicas of the free exciton transition with the maximum at the first LO phonon replica. The results explain the discrepancy between the transition energy of free exciton determined by reflection measurement and the peak position obtained by the PL measurement.

Highly Mismatched Alloys for Intermediate Band Solar Cells

Abstract: It has long been recognized that the introduction of a narrow band of states in a semiconductor band gap could be used to achieve improved power conversion efficiency in semiconductor-based solar cells. The intermediate band would serve as a “stepping stone” for photons of different energy to excite electrons from the valence to the conduction band. An important advantage of this design is that it requires formation of only a single p-n junction, which is a crucial simplification in comparison to multijunction solar cells. A detailed balance analysis predicts a limiting efficiency of more than 50% for an optimized, single intermediate band solar cell. This is higher than the efficiency of an optimized two junction solar cell. Using ion beam implantation and pulsed laser melting we have synthesized Zn1-yMnyOxTe1-x alloys with x<0.03. These highly mismatched alloys have a unique electronic structure with a narrow oxygen-derived intermediate band. The width and the location of the band is described by the Band Anticrossing model and can be varied by controlling the oxygen content. This provides a unique opportunity to optimize the absorption of solar photons for best solar cell performance. We have carried out systematic studies of the effects of the intermediate band on the optical and electrical properties of Zn1-yMnyOxTe1-x alloys. We observe an extension of the photovoltaic response towards lower photon energies, which is a clear indication of optical transitions from the valence to the intermediate band.

Band Anticrossing and Related Electronic Structure in III–N-V Alloys

Abstract: This chapter presents the effect of N on the electronic band structure in dilute III-N-V nitrides in terms of a band anti-crossing interaction between highly localized N states and the extended conduction band states of the semiconductor matrix. The interaction leads to a splitting of the conduction band into two non-parabolic sub-bands. The downward shift of the lower sub-band edge relative to the valence band is responsible for the reduction of the fundamental band gap. The profound effects on the optical and electrical properties of the dilute nitrides, such as the significant increase in the electron effective mass and the drastic decrease in the electron mobility can all be quantitatively account for using this model. The band anti-crossing (BAC) model not only explains the unusual optical and electronic properties of Highly Mismatched Alloys (HMAs) but also to predict new effects that have been later experimentally confirmed. The approach is however, limited to the review of properties of group III-N-V alloys, it is emphasized that these alloys are only a sub group of a much broader class of materials whose electronic structure is determined by the anti-crossing interaction.

Group III-Nitride Materials for High Efficiency Photoelectrochemical Cells

Abstract: Two ternary alloys based on III-nitride semiconductor alloys are explored as potential components of photoelectrochemical cells (PECs) for the direct generation of hydrogen using solar energy. For In1-xGaxN, it will be shown using prior measurements of band offsets that spontaneous water splitting can occur for x up to 0.2 and potentially higher. Flat band potential and photocurrent measurements from an n-type epilayer with x = 0.37 will be presented. This initial data appears to indicate that the flat band potential lies just below the H+/H2 from pH 0 – 14. In the case of GaAsxN1-x we will demonstrate that the replacement of a few percent of As in N sublattice drives the bandgap down from the GaN value (3.4 eV) into a range that is attractive for PEC cells [1]. This band gap reduction is explained by the valence band anticrossing that pushes the valence band maximum up initially by 0.5 eV. From the point of view of a PEC cell, this reduces the gap (desirable for efficiency) without compromising the desired H+/H2 overpotential.

Electronic and Optical Properties of Energetic Particle-Irradiated In-rich InGaN

Abstract: We have carried out a systematic study of the effects of irradiation on the electronic and optical properties of InGaN alloys over the entire composition range. High energy electrons, protons, and {sup 4}He{sup +} were used to produce displacement damage doses (D{sub d}) spanning over five orders of magnitude. The free electron concentrations in InN and In-rich InGaN increase with D{sub d} and finally saturate after a sufficiently high D{sub d}. The saturation of carrier density is attributed to the formation of native donors and the Fermi level pinning at the Fermi Stabilization Energy (E{sub FS}), as predicted by the amphoteric native defect model. Electrochemical capacitance-voltage (ECV) measurements reveal a surface electron accumulation whose concentration is determined by pinning at E{sub FS}.