Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session L18: Focus Session: Wide Band Gap Semiconductors II |
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Sponsoring Units: DMP Chair: Niti Goel, Univ. of Notre Dame Room: LACC 406A |
Tuesday, March 22, 2005 2:30PM - 2:42PM |
L18.00001: Doping and defects in AlN and InN Chris Van de Walle First-principles calculations have significantly contributed to our understanding of defects and impurities in GaN. Here I will present new results for AlN and InN. UV light emitters require AlGaN alloys with high Al content, in which doping becomes increasingly problematic. In addition, bulk AlN is being considered as a substrate material, prompting an investigation of the origin of various absorption lines. For n-type doping, DX-center formation turns shallow impurities such as O, Si, or Ge into deep centers. I will present detailed configuration-coordinate diagrams that summarize the prospects of using these impurities for doping. Al vacancies, as well as their complexes with oxygen, can also occur, and I will discuss specific proposals for the optical absorption lines typically observed at 2.8 eV and 4.5 eV. Turning to InN, a major challenge is to control the n-type conductivity observed in nominally undoped material. Calculations show that point defects such as nitrogen vacancies are unlikely to be responsible for this unintentional conductivity. It is more plausibly caused by impurities such as oxygen. In the case of InN we have found that an even more common impurity can act as a shallow donor, namely hydrogen. Detailed results for the atomic and electronic structure of hydrogen in various configurations will be discussed. [Preview Abstract] |
Tuesday, March 22, 2005 2:42PM - 2:54PM |
L18.00002: Effect of surface facets on the efficiency of InGaN/GaN quantum wells grown by molecular-beam epitaxy Haipeng Tang, Soufien Haffouz, Jennifer Bardwell, Robin Williams, Sylvain Raymond, Jean Lapointe The pronounced enhancement of indium incorporation efficiency and quantum efficiency for InGaN/GaN quantum wells due to rough, faceted surface of the GaN templates is reported. We studied the growth of InGaN/GaN quantum wells by RF plasma MBE on two types of GaN templates, i.e. MOCVD GaN templates and ammonia- MBE GaN templates. The latter was grown in situ with a growth system equipped for both ammonia- MBE and RF plasma MBE. Unlike the smooth (0002) surface of GaN templates grown by MOCVD, the surface of the templates grown by ammonia-MBE is defined by {\{}10-1$m${\}} pyramidal facets causing significant surface roughness. Possible mechanisms for the enhanced indium incorporation efficiency due to these surface facets, such as the possible indium migration to the extremities of the facets forming quantum dots, are discussed. We also investigated InGaN/GaN quantum wells grown on selectively grown GaN micro-pyramids with well-defined {\{}10-12{\}} or {\{}10-11{\}} facets. Micro-PL measurements aimed at resolving the emissions from the quantum wells on the facets and quantum dots at the tips of the micro-pyramids will be discussed. [Preview Abstract] |
Tuesday, March 22, 2005 2:54PM - 3:06PM |
L18.00003: Electron scattering in bulk InN Leon Hsu, Wladek Walukiewicz Recent improvements in the quality of single-crystal InN have made controlled doping studies possible. Irradiation by a variety of beams (electron, proton, He) can be used to produce n-type bulk crystals of InN with carrier concentrations ranging from mid-10$^{17}$ cm$^{-3}$ to more than 10$^{20}$ cm$^{-3}$. We have calculated electron mobilities in InN, incorporating the standard mechanisms of acoustic and optical phonons, Coulomb scattering, and scattering from resonant defect states, and compared them with room temperature experimental values. The mobility in samples with carrier concentrations below the 10$^{18}$ cm$^{-3}$ level is limited by scattering from optical phonons and Coulomb centers. At concentrations between 10$^{18}$ cm$^{-3}$ and 10$^{20}$ cm$^{-3}$, the mobility is dominated by Coulomb scattering alone. At the highest concentrations, above 10$^{20}$ cm$^{-3}$, scattering from defects with energy levels resonant with the conduction band begins to play an important role. Our results also suggest that the defects from which the electrons originate may be multiply-charged. This work was partially supported by the US DOE under Contract No. DE-AC03-76SF00098. [Preview Abstract] |
Tuesday, March 22, 2005 3:06PM - 3:42PM |
L18.00004: Surface and Bulk Properties of InN Invited Speaker: InN presents challenges and opportunities uncovered during the last 3 years. The bandgap of 0.65eV is much smaller than UV bandgaps for GaN or AlN. InN shares similar chemical and radiation resistant properties of these other wurtzite III-nitrides. In contrast, control of InN electrical conductivity is significantly more challenging. The greatest difference is seen in band bending at the free surface. In place of surface depletion, InN exhibits surface accumulation of electrons at sheet densities of 3-5x1013 cm-2 accompanied by a large surface electric field. The valence band to Fermi level energy difference is found to be 1.2eV (XPS) to 1.5eV (EELS,CV). Undoped InN is n-type. Electron densities are below donor impurity densities and fall with increased thickness. A large dislocation density, which falls with thickness, may play a role in creating the bulk electrons. Low mobility electron transport occurs in defective regions near the lattice mismatched interface with GaN or AlN buffer layers, while low defect density regions have mobility exceeding 2000 cm2/Vsec with non-degenerate carrier densities near 1x1017 cm-3. Electron effective mass is 0.045m0 in the minimum of a non-parabolic gamma conduction band. Velocity-field relations from single particle spectroscopy show velocity overshoot and negative differential mobility. THz wavelength radiation from short pulse laser excitation is generated from the surface of InN and GaN/InN interfaces due to large electric fields. InN electro-chemical sensing occurs by surface chemical interaction with InN surface electron accumulation. P-type doping of InN and GaInN with Mg is inferred by temperature variable conductivity, but surface electron conductivity dominates net Hall polarity. [Preview Abstract] |
Tuesday, March 22, 2005 3:42PM - 3:54PM |
L18.00005: A1(LO)phonon in degenerate InN semiconductor films J.S. Thakur, D. Haddad, R. Naik, G.W. Auner, V.M. Naik, H. Lu, W.J. Schaff We have studied the A$_{1}$(LO) structure of InN thin films from a low (n$_{e}$=6.7x10$^{17}$/cm$^{3}$ ) to a very high (n$_{e}$=9.6x10$^{20}$/cm$^{3 })$ carrier concentration using Raman scattering experiments. Theoretically we investigated this structure using a wavevector dependent dielectric function$\varepsilon (q,\omega )$ which takes into account the coupling of longitudinal-optical (LO) phonon and electrons with non-parabolic energy dispersion. Phonon-plasmon interaction cannot explain the origin of this structure. However, phonon interaction with electron-hole pair excitations forms a well-defined structure in $Im\varepsilon (q,\omega )^{-1}$ which emerges from the electron hole pairs spectrum when higher-energy coupled-mode becomes Landau damped. With increasing values of q, this structure moves towards the experimental value. This peak structure is formed by a weaker (relative to the plasmon) interaction between the LO-phonon and electron hole pair excitations. Experimentally it is observed that the energy of this structure increases with increasing value of electron density. [Preview Abstract] |
Tuesday, March 22, 2005 3:54PM - 4:06PM |
L18.00006: Near resonance enhanced Raman scattering and room temperature photoluminescence in highly degenerate InN films V.M. Naik, R. Naik, D.B. Haddad, J.S. Thakur, G.W. Auner, H. Lu, W.J. Schaff We report the results of near resonance enhanced Raman scattering and room temperature photoluminescence (PL) studies on highly degenerate (carrier concentration, $n_{e} \quad >$ 3 $\times $ 10$^{19}$ cm$^{-3})$, wurtzite InN films grown on $c$-plane sapphire substrates by plasma source molecular beam epitaxy. At room temperature, carrier concentration dependent strong PL emission is observed in the 1.4-1.8 eV range. These films show strong resonance enhanced first and second order Raman scattering under 785 nm (1.58 eV) excitation energy and not with 514.5 nm (2.41 eV) excitation, suggesting large shifts in the optical absorption edges due band filling effects in these highly degenerate InN samples. The PL emission peak energies and their dependence on the carrier concentration are consistent with observed optical absorption edges. The present results are compared and contrasted to the data on single crystalline, low degenerate InN films which show a bandgap energy of $\sim $0.7 eV. [Preview Abstract] |
Tuesday, March 22, 2005 4:06PM - 4:18PM |
L18.00007: Demonstration of strong photoluminescence in the deep-green/yellow region from InGaN/GaN multiple quantum wells grown on native AlN substrates Fatemeh Shahedipour-Sandvik, James Grandusky, Muhammad Jamil, Sandra Schujman, Keith Evans, Sridhar Srinivasan , Fernando Ponce Relatively intense deep-green/yellow photoluminescence emission at $\sim $570 nm is demonstrated for InGaN/GaN multi quantum well structures grown on native AlN substrates, showing potential to extend commercial III-N LED technology to longer wavelengths. Temperature- and excitation-power dependent photoluminescence and cathodoluminescence results show the presence of alloy compositional fluctuation in the active region despite the lower strain expected in the structure. This is contrary to what has been observed for lower In content InGaN/GaN MQWs on bulk GaN substrates with sharp interfaces and little alloy compositional fluctuation in the InGaN layers. Detailed optical characterization results will be presented . [Preview Abstract] |
Tuesday, March 22, 2005 4:18PM - 4:30PM |
L18.00008: True LDA Band Gaps of Wurtzite and Cubic Indium Nitride D. Bagayoko, G.L. Zhao, L. Franklin We report the calculated band gap of wurtzite and cubic indium nitride (InN). Our ab-initio computations employed a local density approximation (LDA) potential and the linear combination of Gaussian orbital (LCGO) formalism. The implementation of the Bagayoko, Zhao, and Williams (BZW) method led to \textit{true LDA band gaps of 0.88 eV and 0.65 eV for wurtzite and cubic indium nitrides}, respectively. When available, recent experimental electronic structures, density of states, band gaps, and effective masses agree with our findings. We discuss the need for the BZW approach in LCAO calculations purporting to implement the initial, density functional theory that is concerned with the description of \textit{only the ground state.} [Preview Abstract] |
Tuesday, March 22, 2005 4:30PM - 4:42PM |
L18.00009: Room Temperature Photoluminescence and Absorption in InAlN/AlN/Sapphire quantum dot structures Yuri Danylyuk, Dmitri Romanov, Gregori Auner We have grown InAlN self-assembled quantum dots (QD) on a AlN epitaxial layer with the average diameter of the QDs was as small as 20 nm and detected strong QD photoluminescence. The samples were grown by Plasma Source Molecular Beam Epitaxy on sapphire (0001) substrates. A 150 nm AlN buffer layer was grown at 400$^{\circ}$C and a 200-700 nm InAlN at 500$^{\circ}$C. In-situ RHEED scan mode measurements were used to define RHEED intensity oscillations, strain profiles, and coherence length profiles; they confirmed Stranski-Krastanov 3-dimentional regime of the film growth. The In composition of QDs is estimated 0.8 and 0.4 from the photoluminescence (PL) spectrum. Microscopic PL spectra were obtained using the 514.5 nm line of an Ar+ laser as excitation source. The laser spot was about 2-3 $\mu$m in diameter. We observe a reproducible double peak of very sharp and strong photoluminescence with FWHM of 1 meV at room temperature. We attribute these photoluminescence peaks to electron confinement in nano-hillocks of the InAlN film by the strong electric field of piezoelectric and spontaneous polarization; our model calculations of the localization energies agree with the experimental data. [Preview Abstract] |
Tuesday, March 22, 2005 4:42PM - 4:54PM |
L18.00010: Effects of Stoichiometry on Electrical and Optical Properties of InN Johnny C. Ho, Petra Specht, Xiaoyu Xu, Quing Yang, William Hong, Eicke R. Weber Indiumnitride is the least developed semiconductor among the group III -- nitride compounds. Although the commonly produced material has high defect concentrations it already exhibits high electron mobility which indicates a great potential for future high speed electronic applications. Recently, a series of research efforts has been focused on the clarification of the fundamental bandgap of InN. The variations in the bandgap measurements were mainly attributed to the Burstein-Moss energy shift, the presence of oxide precipitates, possible indium clusters and other stoichiometry related effects. The influence of the indium to nitrogen flux ratio on the electrical and optical properties of InN, grown by molecular beam epitaxy (MBE) is systematically investigated and presented in this paper. A sudden increase in the electron concentration was observed for the highest indium flux. Simultaneously, a red shift in the photoluminescence peak energy was recorded. A correlation of these findings with changes in the materials chemistry and/or the presence of defects such as indium clusters will be presented. [Preview Abstract] |
Tuesday, March 22, 2005 4:54PM - 5:06PM |
L18.00011: Coherent longitudinal optical phonon and plasmon coupling in the near-surface region of InN Y.-M. Chang, C. T. Chuang, C. T. Chia, K. T. Tsen, H. Lu, W. J. Schaff Coherent phonon spectroscopy of a high-quality InN epitaxial layer is carried out using time-resolved second-harmonic generation. Only coherent longitudinal optical phonon and plasmon coupling mode at 447 cm$^{-1}$ can be resolved in the spectrum. Its frequency shows no dependence on the photoinjected carrier density up to 1.5$\times $10$^{19}$ cm$^{- 3}$. This phenomenon is attributed to the hybridization of coherent A$_{1} $(LO) phonon with the intrinsic cold plasma accumulated in the near-surface region of InN, where the plasma density could reach the order of 10$^{20} $ cm$^{-3}$, much higher than the bulk carrier concentration, 1$\times $10$^ {18}$ cm$^{-3}$, determined by Hall effect measurement.. [Preview Abstract] |
Tuesday, March 22, 2005 5:06PM - 5:18PM |
L18.00012: Plasma-Assisted Molecular Beam Epitaxy Grown InN Epifilm Li-Wei Tu, C.L. Hsiao, K.R. Wang, M. Chen, Z.W. Jiang, Y.J. Tu High quality InN epitaxial layer is grown by plasma-assisted molecular beam epitaxy. Substrates used are c-plane sapphire and (111) Si wafer. Various characterizations are performed to investigate the crystal structure, compositions, electrical and optical properties. Hall measurements yield unintentional doping concentration in a range of 10$^{18}$ -- 10$^{20}$ cm$^{-3}$. Hall mobility reaches 1000 cm$^{2}$/Vs. High resolution x-ray rocking curve gives a full-width-at-half-maximum of $\sim $1000 arcsec for InN (0002). No oxygen signal can be detected with electron probe micro-analysis. Field emission scanning electron microscopy and atomic force microscopy show the flatness of the film surface. Raman scattering spectroscopy reveals Raman modes only from the hexagonal phase of InN. Extensive photoluminescence measurements are carried out to explore the bandgap of InN. Discussion on the results will be reported in detail. [Preview Abstract] |
Tuesday, March 22, 2005 5:18PM - 5:30PM |
L18.00013: In-plane optical anisotropy in In$_{x}$Ga$_{1-x}$N/GaN multiple quantum wells induced by Pockels effect Hsiu-Ju Chang, C. H. Chen, L. Y. Huang, Y. F. Chen, T. Y. Lin We have investigated the crystal orientation dependence of optical properties in In$_{X}$Ga$_{1-X}$N/GaN multiple quantum wells. The spectral peaks and intensity of the micro- photoluminescence signal for different crystal orientations were found to have sixfold symmetry. Quite interestingly, the refractive index, obtained from the interference pattern, also varies with the crystal orientation. The 60 degree periodic anisotropy of electronic transitions as well as optical parameters was interpreted in terms of the Pockels effect induced by the strong built-in field in nitride heterojunctions. The linear dependence of the change of the refractive index on electric field is consistent with the prediction of the Pockels effect. Our result provides an alternative solution to improve the designs of photonic and electronic devices based on nitride semiconductors. [Preview Abstract] |
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L18.00014: Optical properties of InGaN quantum dots Ahmad Alsaad, Heinz Schweizer Standard nitride field effect transistors rely on the polarization based 2DEG generation on GaN/AlGaN interfaces. The disadvantage of this structural approach (sensitivity against surface charges) shall be overcome by this approach of the quaternary InAlGaN material composition. With this approach we intend to realize 2DEG at zero polarization field thus controlling charge and mobility in the channel completely by doping the (In)AlGaN barrier and adjusting the spacer layer thickness. This approach should reduce the surface charge sensitivity dramatically open up a new degree of freedom in choosing optimized parameters for nitride FETs. [Preview Abstract] |
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L18.00015: X-ray Spectroscopy of InN heavily irradiated with He J.D. Denlinger, S.X. Li, R.E. Jones, K.M. Yu, J.W. Ager III, W. Walukiewicz, E.E. Haller, Hai Lu, W.J. Schaff We show that irradiation of InN with 2 MeV He ions produces a highly conducting n-type material. Electron concentration saturates at about 4$\times$10$^{20}$ cm$^{-3}$ for the ion dose of 800 $\mu$C. Nitrogen K-edge soft x-ray absorption (XAS) and emission (XES) spectroscopy is used to investigate modifications to the conduction band (CB) and valence band (VB) electronic structure of InN containing these very high concentrations of free electrons and defects. XAS, a probe of unoccupied CB states, shows a depletion of states near threshold absorption corresponding to free-carrier filling of the CB, and the creation of two new peaks for irradiated InN that correspond to $(i)$ the N-vacancy defect level, and $(ii)$ the formation of N-pairs. XES, a probe of occupied states, shows additional emission above the VB maximum, not present in non-irradiated InN, resulting from filled CB states and from elastic scattering. The elastic scattering intensity shows an enhancement for photon excitation at the localized defect level and the non-elastic CB emission is consistent with a band gap narrowing of $\approx$0.4 eV arising from free-carrier electron-electron and electron-ionized defect interactions. The results provide additional support for previously reported low energy gap and large Burstein-Moss shift in heavily doped InN \footnote{J. Wu {\it et~al.}, Phys. Rev. B {\bf 66}, 201403 (2002).}. [Preview Abstract] |
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