Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session D46: Focus Session: Wide Band Gap Semiconductors III |
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Sponsoring Units: DMP Chair: Robert Nemanich, North Carolina State University Room: Baltimore Convention Center 349 |
Monday, March 13, 2006 2:30PM - 3:06PM |
D46.00001: Step-Controlled Strain Relaxation in the Vicinal Surface Epitaxy of Nitrides Invited Speaker: |
Monday, March 13, 2006 3:06PM - 3:18PM |
D46.00002: Observation of Standing Waves at Steps on the GaN(0001) Pseudo (1x1) Surface by Scanning Tunneling Spectroscopy at Room Temperature Mike Harland, Lian Li Standing waves formed at steps of the GaN(0001) pseudo (1x1) surface are observed with scanning tunneling spectroscopy (STS) at room temperature. The oscillatory behavior of the \textit{dI}/\textit{dV} calculated for free electrons scattered by atomic steps agrees well with that measured by STS, and thus establishes that the surface state of the GaN(0001) pseudo (1x1) has a nearly free-electron like energy dispersion. [Preview Abstract] |
Monday, March 13, 2006 3:18PM - 3:30PM |
D46.00003: Novel low temperature surface reconstructions of GaN(000$\overline 1$): A LT-STM investigations Kendal Clark, Danda P. Acharya, Muhammad B. Haider, Arthur R. Smith, Saw-Wai Hla The nitrogen polar gallium-rich GaN (000$\overline 1 )$ samples are grown on sapphire (0001) substrates at sample temperature of 650 $^{0}$ C using r.f. N-plasma molecular beam epitaxy. During the growth, the surface is monitored by using reflection high energy electron diffraction and 3x3 and 6x6 reconstructed surface patterns have been observed. The freshly grown samples are then transferred to a low-temperature scanning tunneling microscope (LT-STM) system in an ultra-high-vacuum environment. Then the sample temperature is lowered to 75 K and 4.6 K for the STM investigations. The STM images reveal four novel low temperature reconstructions on this surface. We will also present voltage dependent STM images and tunneling spectroscopy data acquired on these structures. This work is financially supported by a NSF-NIRT grant no. DMR-0304314. (NIRT collaboration). [Preview Abstract] |
Monday, March 13, 2006 3:30PM - 3:42PM |
D46.00004: Dopant Adsorption and Incorporation at Irradiated GaN Surfaces Qiang Sun, Annabella Selloni, Thomas Myers, W. Alan Doolittle Mg and O are two of the common dopants in GaN, but, in spite of extensive investigation, the atomic scale understanding of their adsorption and incorporation is still incomplete. In particular, high-energy electron irradiation, such as occurring during RHEED, has been reported to have an important effect on the incorporation of these impurities, but no study has addressed the detailed mechanisms of this effect yet. Here we use DFT calculations to study the adsorption and incorporation of Mg and O at the Ga- and N-polar GaN surfaces under various Ga, Mg and O coverage conditions as well as in presence of light or electron beam-induced electronic excitation. We find that the adsorption and incorporation of the two impurities have opposite surface polarity dependence: substitutional Mg prefers to incorporate at the GaN(0001) surface, while O prefers to adsorb and incorporate at the N-polar surface. In addition, our results indicate that in presence of light irradiation the tendency of Mg to surface-segregate is reduced. The O adsorption energy on the N-polar surface is also significantly reduced, consistent with the experimental observation of a much smaller concentration of oxygen in the irradiated samples. [Preview Abstract] |
Monday, March 13, 2006 3:42PM - 3:54PM |
D46.00005: Imaging surface states of GaN(000$\overline 1$) at low temperatures Danda P. Acharya, Kendal Clark, Muhammad B. Haider, Arthur R. Smith, Nancy Sandler, Saw-Wai Hla It is known that the surface layer of nitrogen polar gallium-rich GaN (000$\overline 1 )$ is semi-metallic in contrast to the bulk GaN. Such semi-metallic behavior is predicted to be contributed by surface state bands [1]. Here, we are able to image two of the surface states associated with the GaN (000$\overline 1 )$ surface for the first time at liquid helium temperatures using a low-temperature scanning tunneling microscope (LT-STM). The samples, GaN(000$\overline 1 )$, are grown on sapphire (0001) substrates at sample temperature of 650 $^{0}$ C using r.f. N-plasma molecular beam epitaxy. The freshly grown samples are then transferred to LT-STM system in an ultrahigh vacuum environment for direct investigations. The voltage dependent STM images clearly reveal novel surface features associated with these structures, which are further confirmed by tunneling spectroscopy measurements. This work is financially supported by a NSF-NIRT grant, DMR-0304314. (NIRT collaboration). [1] J. Vac. Sci. Technol. B 16(4), 2241 (1998) [Preview Abstract] |
Monday, March 13, 2006 3:54PM - 4:06PM |
D46.00006: Surface and interface studies of GaN growth on ZrB$_{2}$(0001)/Si(111) Yukiko Yamada-Takamura, Z. T. Wang, Y. Fujikawa, Q. K. Xue, T. Sakurai, J. Tolle, P.-L. Liu, A. V. G. Chizmeshya, J. Kouvetakis, I. S. T. Tsong ZrB$_{2}$ is a conductive, reflective, and lattice-matched buffer layer for GaN growth on Si. This study demonstrates the inherent suitability of ZrB$_{2}$ as an ideal buffer layer for growing inversion domain-free GaN films on Si using an UHV MBE-SPM system. Single crystal ZrB$_{2}$ films had been grown on Si(111) by the thermal decomposition of single molecular precursor Zr(BH$_{4})_{4}$. Annealing the film at 800$^{o}$C in UHV was enough to remove the surface oxides and recover the atomically smooth surface. The GaN grown on the oxide-free ZrB$_{2}$ by plasma-assisted MBE was found to be consistently N-polar independent of the growth conditions. The films were insulating and N-polarity was demonstrated by combined \textit{in situ }RHEED and non-contact AFM study. Various interface structures were proposed and as a result of first principles calculations, a single interface structure leading to N-polar GaN growth was found most stable for the wide range of growth conditions. Ref. Y. Yamada-Takamura \textit{et al.,} Phys. Rev. Lett. \textit{in press}. [Preview Abstract] |
Monday, March 13, 2006 4:06PM - 4:18PM |
D46.00007: Electronic characterization of in-grown 8H inclusions in 4H-SiC using Ballistic Electron Emission Microscopy (BEEM) K. -B. Park, W. Cai, J. P. Pelz, M. S. Miao, W. R. L. Lambrecht, X. Zhang, M. Skowronski, M. A. Capano Planar ``8H'' Stacking-fault inclusions (SFIs) formed during epilayer growth on an 8$^{o}$ miscut n-type 4H-SiC substrate were characterized using nm-resolution BEEM and first-principles electronic structure calculations. Enhanced BEEM current and a $\sim $0.39 eV lower local Schottky Barrier were observed along lines where these inclined SFIs intersect a thin Pt film, indicating that 8H SFIs are electron quantum wells (QWs), as observed for two types of cubic SFIs in 4H-SiC [1,2]. The measured $\sim $0.39 eV QW depth is close to our calculated value of $\sim $0.43 eV. We also observed the BEEM current amplitude and threshold voltage to be quite asymmetric with respect to the intersection lines of the SFIs with the Pt film. We will discuss on-going modeling to see if these asymmetries can be explained respectively by electron reflection from the inclined subsurface SFIs, and by the difference in spontaneous polarization between 8H- and 4H-SiC. Work supported by ONR. [1] Ding \textit{et al}., Phys. Rev. B\textbf{ 69}, 041305(R) (2004). [2] Park \textit{et al}., Appl. Phys. Lett. \textbf{87}, 232103 (2005). [Preview Abstract] |
Monday, March 13, 2006 4:18PM - 4:30PM |
D46.00008: Polarity effects on the electronic states of N- and Al-polar aluminum nitride surfaces Ji-Soo Park, Yingjie Tang, Robert Davis, Robert Nemanich N- and Al-polar (0001) wurtzite AlN films were grown on C- and Si-polar 6H-SiC substrates, respectively. The polarity was confirmed by KOH chemical etching and atomic force microscopy (AFM) imaging. The surfaces were cleaned in situ by annealing to $\sim $1100$^{o}$C in a flux of ammonia, and X--ray photoemission spectroscopy (XPS) and UV photoemission spectroscopy (UPS) were employed to measure the electronic states at the surfaces. The results indicated a similar electron affinity for both surfaces which appears to approach a value of zero. Furthermore, it was determined that the Fermi energy (Ec-Ef) was 2.4 eV below the conduction band for N-face AlN and 3.0 eV for Al-face AlN. The 0.6eV difference in the Fermi energy indicates a difference in band bending near the surface which is discussed in terms of the built-in field induced by the polarization bound surface charges. [Preview Abstract] |
Monday, March 13, 2006 4:30PM - 4:42PM |
D46.00009: The Negative Electron Affinity Effect on the Thermionic Emission from Nitrogen Doped Diamond Yingjie Tang, Franz Koeck, Robert Nemanich The thermionic emission properties of microwave CVD nitrogen doped diamond films with a negative electron affinity (NEA) were investigated with thermionic emission spectroscopy (TES) and UV photoelectron emission spectroscopy (UPS). At a temperature of 230$^{o}$C thermionic electron emission was observed with a current density $\sim $1nA/cm$^{2}$ and a minimum kinetic energy of 1.6eV relative to the Fermi level. Both the thermionic and photoemission spectra display the same sharp cutoff edge corresponding to the conduction band minimum (CBM) which is detected because of the NEA properties of films. With increasing temperature, electron emission is observed below the CBM which is attributed to tunneling phenomena from nitrogen related states and sp2 bonded defects. The emission extends to $\sim $1.3eV below the conduction band which would be consistent with prior measurements of the negative electron affinity of diamond surfaces. The effect of the NEA on space charge and defect related electron emission is also discussed. [Preview Abstract] |
Monday, March 13, 2006 4:42PM - 4:54PM |
D46.00010: Study of Thermally Induced Damage and its Recovery in ZnO Single Crystals by Elastic Resonant Ion Channeling S. Dhar, D. Pugel, S.S. Hullavarad, R.D. Vispute, S.B. Ogale, T. Venkatesan ZnO is widely considered as the next generation wide-band gap material for optoelectronic device applications due to its excellent material properties. Although in recent years various studies on ZnO crystal have been made, very little is known about its response after thermal treatments. In this work, we studied the influence of thermal annealing on ZnO (0001) bulk single crystal in the temperature range of 30-1000 $^{o}$C by elastic resonant ion channeling and Rutherford backscattering (RBS) techniques using a 3.05 MeV He beam. After an isochronal1 h anneal, RBS-Ion channeling analysis showed the evolution of disorder in the Zn sub-lattice of the crystal with increasing annealing temperature. The maximum disorder was observed at 200 $^{o}$C above which it decreased. The induced disorder in the Zn sub-lattice at lower temperature ($<$ 400 $^{o}$C) can almost be recovered by further annealing above 800 $^{o}$C. Elastic resonant ion channeling analysis, which monitors the oxygen in the near-surface region clearly revealed the evolution of thermally induced disorder in the oxygen sub-lattice with increasing temperature. In contrast to the disorder recovery of Zn sub-lattice, the disorder in oxygen sub-lattice did not recover completely. Possible mechanism on the disorder and its recovery will be discussed. [Preview Abstract] |
Monday, March 13, 2006 4:54PM - 5:06PM |
D46.00011: Band offset measurements on Metal/Hf$_{1-x}$Si$_{x}$O$_{2}$/Si stacks Eric Bersch, Sylvie Rangan, Robert A. Bartynski, Eric Garfunkel The study of high-k gate dielectric stacks with metal gate electrodes is motivated by the need to reduce the size of MOSFET devices. Controlling the band offsets between the metal/high-k/Si layers of devices is critical to reducing the leakage current through the dielectric. In order to understand the band alignment at the interfaces we have measured, for each layer of the stack, the conduction band edge (CBE) using inverse photoemission, and the core levels binding energies and valence band edge (VBE) using photoemission. For the high dielectric constant alloys Hf$_{1-x}$Si$_{x}$O$_{2}$ (x=0, $\sim $0.5, 1), the gap increases with x, and we will discuss the effect of x on the CBO and VBO between the silicon and the dielectric. Upon metallization with Ru or Al, Ru does not affect the position of the CBE and VBE of the dielectric, while Al induces a strong shift ($\sim $0.5 eV toward higher binding energy) of the band edges. These results will be compared to other techniques, particularly internal photoemission. [Preview Abstract] |
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