Papers

AVS2001 Session SC-MoA: GaN Surfaces, Interfaces, and Devices

Monday, October 29, 2001 2:00 PM in Room 124

Monday Afternoon

Start	Invited?	Item
2:00 PM		SC-MoA-1 The Influence of Active Nitrogen Species and Substrate Polarity on the Growth and Doping of GaN Grown by RF Plasma-assisted Molecular Beam Epitaxy T.H. Myers, A.J. Ptak (West Virginia University); L.T. Romano (Xerox Palo Alto Research Center) Growth of GaN by molecular beam epitaxy (MBE) is typically limited by increased desorption of Ga from the growing surface. We will report on the relative reactivity of the various active nitrogen species produced by rf plasma sources: low and high energy ions, atoms and metastables. Studies of growth rate as a function of temperature suggest the GaN surface is prone to "attack" by neutral and ionic atomic nitrogen above 700oC, promoting decomposition. This leads directly to the observed lower than expected temperature for a significant decrease in growth rate, while this decrease is not observed when the active nitrogen flux consists primarily of nitrogen metastables. Dramatically improved electrical properties are observed in epilayers grown using nitrogen metastables. Similar to most compound semiconductors, GaN lacks inversion symmetry that leads to different chemical terminations on different crystal planes. The two most common surfaces, the (0001) (or Ga-polar) and the (000-1) (or N-polar) surfaces exhibit quite different properties. Results illustrating the differences in basic growth kinetics related to surface polarity will be presented. Surface polarity also has a pronounced influence on doping. A pronounced dependence of Mg incorporation on surface polarity was observed in a series of Mg step-doped epitaxial GaN layers. Measurements indicate surface accumulation of Mg occurs during growth, with stable accumulations of close to a monolayer of Mg on the Ga-polarity surface. This surface layer can cause surface inversion to occur. Beryllium incorporation was also studied for both Ga-polarity and N-polarity GaN. Unlike magnesium, surface polarity-related incorporation differences were less pronounced for Be. Preliminary results, however, indicate a strong dependence of compensation, e.g. point defect formation, on surface polarity. Polarity related issues for O-incorporation will also be discussed.
2:40 PM		SC-MoA-3 "Functionalizing" the GaN(0001) Surface: The Chemisorption of Organic Amines V.M. Bermudez (Naval Research Laboratory) The emergence of "molecular electronics" has led to interest in the synthesis of hybrid organic/semiconductor structures. Surfaces "functionalized" by attachment of unsaturated hydrocarbons provide the possibility of subsequently building complex electro- or photoactive molecular films using, e.g., photochemical or cycloaddition reactions. Previous work¹ with NH₃ shows that amines are highly reactive with the Ga-polar GaN(0001) surface. Here we use mainly XPS, UPS and ELS to study chemisorption of amines involving π-bonded hydrocarbons, focusing on a primary amine (aniline, C₆H₅-NH₂) and a secondary amine (3-pyrroline, C₄H₆NH) having N in a 5-member ring with one C=C bond. Dosing near 300 K with either amine causes rapid elimination of the GaN 3.4 eV surface-state loss in ELS and growth of a C=C π-π^* loss at 6.5 eV (aniline) or 7.2 eV (3-pyrroline). The background-corrected xray-excited C KLL and N KLL band areas indicate a saturation coverage of about 0.40 molecules per surface Ga site for either species. In contrast, benzene (C₆H₆) does not adsorb under these conditions. HeII UPS shows rich orbital structure, differing from that of either free molecule, which has been analyzed with the aid of ab-initio DFT calculations. The results suggest adsorption via the amine N-atom with the hydrocarbon ring remaining intact. UPS has also been used to measure changes in band bending and in electron affinity (δχ) with adsorption. This permits construction of energy-level diagrams showing the alignment of the molecular HOMO and LUMO with the GaN band edges. For aniline (3-pyrroline), the surface dipole layer leads to δχ of about -0.55 (-0.92) eV, vs. the clean-surface χ of 3.3 eV. This large reduction in χ may be useful in electron emission devices. ¹ V.M. Bermudez, Chem. Phys. Lett. 317 (2000) 290.
3:20 PM		SC-MoA-5 STM Study of Dislocation-Mediated Surface Morphology of GaN Grown by ECR-Plasma Assisted MBE Y. Cui, L. Li (University of Wisconsin) The surface morphology of GaN films grown on the 6H-SiC substrates by ECR-plasma assisted molecular beam epitaxy was studied by reflection high-energy electron diffraction (RHEED) and in situ scanning tunneling microscopy (STM). Clean SiC substrates were prepared by a two-step method of etching in hydrogen atmosphere at 1600 °C and annealing under Si beam in ultrahigh vacuum at 950 °C. These processes remove the polishing damages of the SiC substrates. The resulting surfaces are composed of atomically flat terraces that are separated by triple-layer steps. At temperature between 550 and 600 °C and plasma power of 30 W, two-dimensional growth was observed. The surface morphology of the films can be characterized by two dislocation-mediated structures: pinned steps and spiral hillocks. Straight-pinned steps along the {1120} directions were found for film thickness of 500 Å, created due to the emergence of screw and mixed dislocations at the crystal surface from the bulk of the film. By counting the number of the steps, the dislocation density is estimated to be in the order of 10¹⁰ cm^-2. At film thickness greater than 1000 Å, these pinned steps grow outward and around the dislocation, forming spiral hillocks with a density in the order of 10⁸ cm^-2. The reduction of the density is explained by annihilation of the dislocations during the formation of the spirals. These results and their implications for GaN epitaxy will be presented at the meeting. This research is supported by NSF DMR-0094105.
4:00 PM		SC-MoA-7 Search for the Missing Group-III Flux during AlGaN OMVPE J.R. Creighton, M.E. Coltrin, R.P. Pawlowski (Sandia National Laboratories) At normal operating conditions, most AlGaN OMVPE reactors exhibit non-ideal behavior with respect to the group-III precursor concentration. The deposition rate can be considerably less than the predicted transport-limited rate, and the solid AlGaN alloy composition is typically a nonlinear function of the gas-phase composition. It is generally thought that gas-phase "parasitic" reactions between trimethylgallium (TMGa), trimethylaluminum (TMAl), and ammonia are responsible for removing group-III material from the deposition process. We have explored many possible mechanisms for the parasitic pathways using both experimental techniques and complex reactive flow simulations. As expected, TMGa and TMAl react with ammonia to form adducts, which we have unambiguously identified with mass spectroscopy and FTIR. We have measured the vapor pressure of the adducts and their mixtures near room temperature and found that physical condensation can be an important process, especially at higher reactor pressures and higher TMAl concentrations. However, over the 0-100°C range we have found no evidence of significant irreversible decomposition reactions, such as methane elimination, which have often been postulated to be the source of the decrease in group-III flux. As the temperature is raised in this range, the adducts simply dissociate back into the original reactants at rates consistent with equilibrium calculations. The lack of evidence for a low temperature parasitic reaction pathway is consistent with our reactive flow simulations, which indicate that the parasitic reaction pathway occurs at high temperatures near the growing surface. The simulations utilized deposition rate measurements from a rotating disk reactor over a wide range of operating conditions chosen to accentuate the differences between possible high-temperature and low-temperature pathways. Recent results examining possible high-temperature pathways will be presented.
4:20 PM		SC-MoA-8 Models, Concepts and Realizations of Pyroelectronic Heterostructure Devices P. Vogl, G. Zandler, S. Hackenbuchner, J.A. Majewski, O. Ambacher (Technische Universitaet Muenchen, Germany); K. Chu, V. Tilak, R. Dimitrov, L.F. Eastman (Cornell University) High field transport in semiconductors that possess high internal spontaneous electric fields opens up a new field of "pyroelectronics". The pyroelectric character of group-III-nitrides with wurtzite crystal structure yields a novel degree of freedom in designing and tayloring devices for modern microelectronic applications. We present both theoretical and experimental studies of III-nitride based high electron mobility field effects transistors (HEMT´s) that demonstrate these devices to be optimally suited for high power and high frequency transistors as well as microwave amplifiers. On the theory side, we have employed first principles calculations to determine spontaneous and piezoelectric polarization charges at interfaces and surfaces of GaN based heterostructures and superlattices. Based on this calculated set of electronic structure data, we have performed extensive high field transport simulations for submicron HEMT devices. In addition, we show how the built-in electric fields can be utilized to produce high hole densities as a function of gate voltage in these devices. Experimentally, undoped and pyroelectric AlGaN/GaN HEMT´s have been processed on c-Al₂O₃, Si(111) and 6H-SiC substrates that show excellent device characteristics. With gate length down to 150 nm, we have reached an electron transit velocity and intrinsic transit time frequency of 1.3x10⁷ cm/s and 106 GHz, respectively, at room temperature. Experimental results for the first microwave amplifier build by a combination of 4x4 AlGaN/GaN HEMTs will be presented.
5:00 PM		SC-MoA-10 Ni/Au Ohmic Contacts to p-GaN Epilayers B. Liu, E. Lambers, P.H. Holloway (University of Florida); T. Johnson, D. Guiterrez, K. Kidney, W.B. Alexander (Uniroyal Optoelectronics) The deposition and properties of evaporated thin Ni (5 nm)/Au (5 nm) contacts on MOCVD p-GaN has been studied. After annealing the bi-layer structure in an oxidizing environment, the contact is transformed from a Schottky to an ohmic behavior. The Ni underlying the Au top layer was found to have largely diffused to the surface and formed NiO during the anneal. The Au formed a porous film between the NiO and p-GaN which allowed between 50 and 85% transmission of 450 nm light, depending upon the deposition and annealing conditions. The porous Au network also allowed good sheet resistance with typical values of 300 Ω. X-ray photoelectron spectroscopy indicated that the NiO reached to the GaN interface, however a Ni-Ga-Au ternary metallic phase also formed at the interface. This phase modified the capillary forces and resulted in formation of the porous Au network. In the absence of this metallic phase, the Au film broke into islands with a very high sheet resistance. Formation of the porous network will be discussed in terms of a total energy model of thin film morphology. The consequences of forming NiO and forming the ternary metallic layer upon achieving an ohmic contact will also be discussed.