AVS2004 Session SC-TuP: Poster Session
Tuesday, November 16, 2004 4:00 PM in Room Exhibit Hall B
Tuesday Afternoon
Time Period TuP Sessions | Topic SC Sessions | Time Periods | Topics | AVS2004 Schedule
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SC-TuP-1 Epitaxial Growth of GaN on Sapphire by RF-MOMBE
S.Y. Kuo, C.C. Kei (National Science Council, Taiwan); C.K. Chao (National Central University, Taiwan); J.S. Chen (National Science Council, Taiwan); S.Y. Huang (National Taiwan University, Taiwan); C.N. Hsiao (National Science Council, Taiwan) A self-designed radio-frequency plasma metal organic molecular beam epitaxy (RF-MOMBE) system is developed to study III-nitride semiconductors. The surface morphologies and crystal structures were monitored by in-situ reflection high energy electron diffraction (RHEED). Up to 4 inches substrates of sapphire, silicon and other materials were available. Followed by substrate nitridation, a low temperature buffer layer was deposited to facilitate growth of III-nitride films on large lattice mismatched substrates. The III-nitride films were characterized by photoluminescence spectrometry, x-ray diffraction, scanning electron microscopy and transmission electron microscopy. The effect of growth temperature and III-V ratio on crystallinity and surface morphologies of III-nitride films was prominent. High quality III-nitride films were achieved by well optimized condition. |
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SC-TuP-2 Comparison of Ga- and N-polar GaN Surfaces
R.P. Bhatta, B.D. Thoms (Georgia State University); C.R. Eddy, Jr., R.T. Holm, R.L. Henry (Naval Research Laboratory) Growth of wurtzite GaN(0001) thin films by metalorganic chemical vapor deposition (MOCVD) may produce either Ga- or N-polar surfaces resulting in differences in surface termination, electronic structure, and chemical reactivity. GaN surfaces of both polarities have been characterized, in particular by their reaction with atomic hydrogen and deuterium. Auger Electron Spectroscopy (AES) and Low Energy Electron Diffraction (LEED) showed that the surfaces were clean and ordered. Electron Energy Loss Spectroscopy (EELS) and High Resolution Electron Energy loss Spectroscopy (HREELS) were used to characterize the electronic and vibrational properties of the surfaces. After cleaning the N-polar surface by sputtering with 1 keV nitrogen ions and annealing to 900 °C, HREELS showed Fuchs-Kliewer phonons, N-H stretching vibrations, and combination losses, which indicate that hydrogen is present on the surface after annealing. HREELS following exposure to atomic hydrogen (deuterium) confirmed the assignment of surface N-H (N-D) and showed no sign of surface Ga-H (Ga-D) species. In addition, heating to temperatures from 400 to 900 °C was not sufficient to remove the surface N-H species. In contrast, HREELS of the Ga-polar surface after sputtering and annealing showed Fuchs-Kliewer phonons but not any adsorbate loss peaks, which indicates a hydrogen-free surface. HREELS following exposure to atomic hydrogen (deuterium) reveals Ga-H (Ga-D) stretching vibrations along with combination modes, but no N-H (N-D) vibrational modes are observed. Hydrogen is desorbed completely from this surface below 400 °C. In addition to differences in surface termination and chemical reactivity, EELS was used to characterize differences in the electronic structure of the hydrogenated Ga- and N- polar surfaces. |
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SC-TuP-3 Ohmic Contacts to AlGaN
S.E. Mohney, H.J. Wang, M.A. Horsey (The Pennsylvania State University); B.A. Hull (Cree, Inc.) Research on ohmic contacts to AlxGa1-xN of high Al fraction (x) is presented. The work was motivated by the requirements for low resistance ohmic contacts for short-wavelength devices. In the work on ohmic contacts to n-AlxGa1-xN with x = 0.6, we have achieved reproducible contact resistivities of 4 x 10-6 Ohm-cm2 with V/Al/V/Au contacts, even without reactive ion etching prior to metallization. These contacts exhibit relatively smooth surface morphologies. We have also investigated the interfacial reactions in these contacts using transmission electron microscopy and Auger electron spectroscopy to provide more information on the mechanism for forming a low resistance ohmic contact. For contacts to p-AlxGa1-xN (x = 0.45), we have identified annealing conditions required to achieve ohmic contacts, and we discuss the need for passivation of the p-AlxGa1-xN surface in order to avoid degradation from exposure to light, due we believe to traps at the semiconductor surface. We also describe the relationship between the resistance of the contacts and the interfacial reactions between the contacts and p-AlxGa1-xN. |
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SC-TuP-4 Fabrication and Properties of Thin Film Photon Sieve Diffractive Lens
N. Bradman, H. Chung, M. Davidson, P.H. Holloway, K. Woo, D. Tanner, S. Selcuk, A. Hebard (University of Florida); O. Shenderova, A. Shenderova, G.E. McGuire (International Technology Center) Photon sieves are diffractive lens that offer better focusing than Fresnel zone plates and are lighter weight and lower volume than refractive lens. The photon sieve lens in this study consists of a transparent substrate coated with an opaque metal thin film with a pattern of holes in a circular array. The size of the holes range from above to below the wavelength of light being focused. Fabrication of the photon sieve lens will be described, which consists of a thin electron beam metallization, pattern writing by electron beam lithography, and pattern development by reactive ion etching. In addition to significant reductions in weight and volume, photon sieves offer a larger field of view than refractive optics, but they suffer from poor transmission. Surface plasmon resonant coupling will be reported to improve the transmission by factors of two to five at the resonant wavelength. The coupling of the surface plasmon resonant structure with the photon sieve pattern will be discussed. Transmission data for traditional and modified photon sieves will be presented and compared. |
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SC-TuP-5 Etch Characteristics of Sapphire using Inductively Coupled Plasmas
Y.J. Sung, T. Jang, K.K. Choi, S.H. Chae, Y.H. Kim, J.S. Kwak, O.H. Nam, Y. Park (Samsung Advanced Institute of Technology, South Korea) Sapphire has been widely used as a substrate for epitaxial growth of GaN-based optoelectronic devices due to its high chemical and thermal stability. Therefore it is essential to understand the dry etching technique in order to fabricate advanced optoelectronic devices. Recently, researches on the various etching of sapphire such as inductively coupled plasma etching, ion beam etching, chemical wet etching after ion implantation, reactive ion etching, etc. have been carried out and have focused on achievement of high etch rates and high selectivity over etch mask. In case of GaN-based laser diodes and light emitting diodes, various sapphire etching methods have been applied to techniques such as epitaxial lateral overgrowth(ELO) and lateral epitaxial patterned sapphire substrates(LEPS) to reduce dislocation density in GaN layer. However, until now there have been only a few works on the etching of sapphire and its properties. In this study, inductively coupled plasmas were used to etch sapphire. The effects of etch parameters such as gas combination of BCl3/ Cl2 and BCl3/Cl2/Ar, inductive power (400- 800Watts), bias voltage (-100--300Volts), and operational pressure (3-30mTorr) on the etch characteristics such as etch selectivity and etch properties of sapphire were investigated. To investigate the etch mechanism of sapphire, optical emissions from the plasmas during the etch process were monitored in situ and the sapphire surface composition after the etching was observed by X-ray photoelectron spectroscopy. |
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SC-TuP-6 Exact-Exchange-Based Quasiparticle Calculations of II-VI Compounds and Group III Nitrides
P. Rinke (Fritz-Haber-Institut der Max-Planck-Gesellschaft, Germany); A. Qteish (Yarmouk University, Jordan); J. Neugebauer (University of Paderborn, Germany); M. Scheffler (Fritz-Haber-Institut der Max-Planck-Gesellschaft, Germany) We present a systematic ab initio study of the electronic structure for a wide range of II-VI compounds and group III nitrides in the zinc-blende structure. The challenge from a computational point of view is to capture the exchange-correlation effects arising from the shallow semicore d-electrons of the cation, which we explicitly treat as valence states in our pseudopotential approach. In order to correctly describe the dominant exchange interaction we apply density-functional theory (DFT) in the exact-exchange (EXX) approximation. Although the EXX Kohn-Sham bandstructure compares well with experiment for standard semiconductors1 the bandstructure as measured by photoemission is a property of the excited system. We therefore apply many-body perturbation theory in the GW approximation to the EXX groundstate. The hirarchy of our approach allows us to systematically investigate the role of exchange and correlation in these materials from first principles. Our results show that it is crucial to treat exchange and correlation on the same footing at every stage. To achieve this in our EXX calculations we employ the newly developed EXX-pseudopotentials2. Furthermore we obtain GW bandstructures in very good agreement with existing all-electron GW calculations. Our results indicate that, in contrast to common believe, a pseudopotential approach including only the d-electrons of the semicore shell in question, is sufficient to accurately describe the electronic structure if treated in the EXX+GW formalism. |
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SC-TuP-7 Transparent Conducting Oxides Based on ZnO by Reactive Sputtering
M.A. Santana-Aranda (Instituto Mexicano del Petroleo, México); M. Meléndez-Lira, M. Becerril-Silva, S.J. Jiménez-Sandoval (Centro de Investigación y Estudios Avanzados, México) Transparent conducting oxides have a broad variety of applications in the electric and optoelectronic industries. Applications like conducting window material for solar cells as well as active layers of LEDs and laser diodes emitting in the UV. We have employed reactive sputtering to deposit ZnO and Al:ZnO thin films on glass substrates. The target was formed with zinc and aluminum circular plates; we controlled the aluminum content changing the ratio of Al/Zn areas. Surface morphology and aluminum content were monitored with a scanning electron microscope. X ray diffraction and Raman spectroscopy measurements were performed to monitor the crystalline structure of the films. The energy of the absorption edge was monitored by optical transmission measurements. The changes observed in the absorption edge are related with the changes in the aluminum content of the films. Resistivity of the films was determined by electrical measurements, these results are also well correlated with the aluminum content. |
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SC-TuP-8 XPS and UPS Study of ZnO:N Thin Films
C.L. Perkins (National Renewable Energy Laboratory); X. Li (University of Texas at Arlington); S. Asher, T.J. Coutts, S.-H. Lee (National Renewable Energy Laboratory) There are strong motivations for obtaining nitrogen-doped p-type ZnO, including the possibilities of air-stable, high quality UV lasers, detectors, and efficient photocatalytic water splitting. Problems remain with the growth of this material however. Reproducibility of conductivity type is difficult, and the distributions and identities of nitrogen species in N-containing ZnO films are not well known. Although theory predicts that N can be incorporated in at least two different states, one of which, N2 occupying a position on the oxygen sublattice (N2O) should be a double shallow donor, and the other, NO, being the desired acceptor, there have been few determinations of the chemical states of nitrogen in ZnO:N materials. In order to gain a better understanding of their chemical and electronic properties, we have examined via XPS and UPS thin films of ZnO:N produced by two different methods. MOCVD films were grown using diethylzinc and nitric oxide. Sputtered films were produced with a Zn target and a mixture of O2 and N2. Core level photoemission shows that the films contain 0.5-2.5 % nitrogen, and that the nitrogen occupies at least four different chemical environments. With UPS the relative positions of the films' valence bands are determined with respect to the Fermi level. Results from the thin polycrystalline films are compared to initial results obtained on ZnO(0001) single crystals reacted in UHV with nitric oxide, and with data obtained from N-implanted Zn foil. Definitive XPS peak assignments are made for N2O, NO, and two other nitrogen chemical states that have not previously been identified in ZnO:N. |
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SC-TuP-9 Electrical Characteristics of Al-doped ZnO (Al-ZnO) Films and their Application to Al-ZnO/p-Si Solar Cells
T. Ichinohe, S. Masaki (Tokyo National College of Technology, Japan); K. Kawasaki (TDY Co., Ltd., Japan) We fabricated aluminum doped zinc oxide (Al-ZnO) films by DC-magnetron sputter-deposition. The films showed n-type semiconductor characteristics: they were electron carriers with a negative coefficient of resistivity. We attempted to fabricate a p-n junction structure using Al-ZnO films formed on a p/p+-Si epitaxial substrate. The Al-ZnO/p-Si structure showed rectified I-V characteristics, indicating a p-n junction has been fabricated. The photocurrent for reverse bias increased to several hundred times higher than dark current when a halogen lamp irradiated its surface. The open circuit voltage (Voc) increased when the halogen lamp power was increased, and the value saturated at about 0.2 V. We infer that the value of Voc is appropriate for built-in potential (0.3 eV) in the interface of Al-ZnO/p-Si, as estimated by the Kelvin-probe work function measurement system. Energy efficiency of the Al-ZnO/p-Si structure was about 1.2%, as estimated by the standard quasi-solar light exposure system. Reducing the resistivity of Al-ZnO films and the contact resistance of the cell structure can further increase the energy efficiency. |
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SC-TuP-10 Supermagnetron Plasma CVD of Amorphous CNx:H Films Using Rf Power-Ratio Control
H. Kinoshita, R. Ikuta, K. Sakurai (Shizuoka University, Japan) Amorphous CNx:H films were deposited on lower electrode using i-C4H10/N2 supermagnetron plasma CVD. 1 By a control of rf power-ratio, hard and soft CNx:H films were deposited at upper electrode rf power of 800W. Above the lower electrode rf power (LORF) of 40W, CNx:H films became hard and opaque. Below LORF of 20W, however, CNx:H films became soft and transparent. The optical band gap of hard film was below 0.8eV and that of soft film was above 1.9eV. Hardness of hard film was above 19GPa and that of soft film was about 7GPa. Electrical resistivity of hard film was low and that of soft film was high. Soft films showed white photoluminescence. |