AVS2004 Session SC+EM-ThM: Wide Bandgap Semiconductors

Thursday, November 18, 2004 8:20 AM in Room 304B

Thursday Morning

Start	Invited?	Item
8:20 AM		SC+EM-ThM-1 HVP-CVD: A Novel Chemical Vapor Deposition Approach for Zinc Oxide Synthesis T.M. Barnes, J. Leaf, C. Fry, C.A. Wolden (Colorado School of Mines) Zinc oxide (ZnO) is a versatile II-VI semiconductor that has generated tremendous interest due to its unique combination of optical, electronic and mechanical properties. High vacuum plasma-assisted chemical vapor deposition (HVP-CVD) is introduced as a novel technique for the deposition of zinc oxide and the study of the associated surface chemistry. An inductively coupled plasma (ICP) source was used for the generation of atomic oxygen and other radicals. Radicals from the ICP source and organometallic precursors diffuse into a high vacuum environment where they combine to form metal oxide thin films on a heated substrate. The process is differentiated from conventional CVD approaches in that the collisionless environment precludes gas-phase reactions with the metal precursor. Advantages of the HVP-CVD approach were demonstrated for zinc oxide growth using dimethyl zinc. Notable achievements include high growth rates of highly oriented material, room temperature formation of (002)oriented ZnO, and nitrogen doping. The underlying process chemistry was investigated using a combination of in-situ diagnostics and ex-situ materials characterization. Optical emission spectroscopy (OES) and quadrupole mass spectrometry (QMS) were used to monitor the ICP source and deposition environment, respectively. In the case of intrinsic ZnO, the growth kinetics were found to be first order in dimethyl zinc and zero order in atomic oxygen. QMS analysis suggests that growth proceeds through an elegant pathway in which the methyl ligands simply desorb without oxidation. Nitrogen doping was achieved by replacing oxygen with N₂O in the ICP source. The mechanism of nitrogen incorporation is discussed in light of film properties and characterization of the deposition environment.
8:40 AM		SC+EM-ThM-2 Realization of Mg(x=0.15)Zn(1-x=0.85)O based Metal-Semiconductor-Metal UV Detector on Quartz and Sapphire S. Hullavarad (University of Maryland); R. Vispute (Bluewave Semiconductors, Inc.); T. Venkatesan, S. Dhar, I. Takeuchi (University of Maryland) MgZnO is a novel oxide based UV sensitive material. The band gap of MgxZn1-xO can be tuned by varying the composition of Mg to achieve band gaps corresponding to UV-A, UV-B, UV-C regions of UV spectrum. This material is of significant importance for various applications in flame sensors, UV index monitors and missile plume detection. The interesting property that makes this material unique is its existence in multiple phases for different Mg compositions. This allows picking up desired Mg composition corresponding to suitable UV sensitive window and growing on lattice matched substrate. In this paper we present the growth of MgZnO on non-conventional substrates like quartz and on sapphire for comparison of the device reliability. MgZnO films are characterized by X-Ray Diffraction, UV-Visible spectroscopy and Rutherford Back Scattering - channeling techniques. We are reporting for the first time the highly oriented growth of MgZnO on quartz by Pulsed Laser Deposition technique with a RBS channeling yield of 50% showing highly ordering. The morphology of the films is studied by Atomic Force Microscopy. The metal-semiconductor-metal device was fabricated on the MgZnO film to study the device response under proper UV irradiation.
9:00 AM	Invited	SC+EM-ThM-3 A Non-Traditional Approach to Low-Temperature Nitride Thin Film Deposition and Nanoscale Device Fabrication M.A. Hoffbauer, A.H. Mueller, E.A. Akhadov, M.A. Petruska, V.I. Klimov (Los Alamos National Laboratory) Using energetic neutral atoms to control interfacial chemistry opens new opportunities for low temperature materials processing and device fabrication at the nanoscale. We are developing a unique low-temperature thin film growth and etching technology, exclusive to LANL, called Energetic Neutral Atom Beam Lithography/Epitaxy (ENABLE) that utilizes neutral reactive atomic species (e.g. N and O) with kinetic energies comparable to chemical bonds strengths (a few eV) for growing nitride and oxide thin films at low temperatures and for etching very high-aspect-ratio features into polymers. Co-depositing metals onto substrates simultaneously exposed to energetic N-atoms permits device quality GaN-based semiconducting films to be grown at temperatures ranging from ambient to greater than 500 C on a variety of substrates (including plastics). Characterization details regarding film crystallinity, epitaxy, stoichiometry, and optical properties will be discussed. Low-temperature GaN film deposition permits semiconducting nanocrystals (NCs) synthesized by colloidal chemistry and having size-controlled emission wavelengths to be encapsulated in a GaN-based matrix. Devices based on p-i-n structures show direct charge injection and electroluminescence from the NCs. Using energetic oxygen atoms to selectively etch various nanoscale features in polymeric films yields sub-100 nm features with aspect ratios exceeding 35:1. Examples will be shown where ENABLE is used to directly grow patterned nitride thin film structures by combining the capability to etch polymer templates with subsequent low-temperature thin film growth. Future prospects and challenges for low-temperature ENABLE-based nanoscale fabrication along with progress towards an efficient multicolor (white) light source based on a NC LED device will be presented.
9:40 AM		SC+EM-ThM-5 Confined Epitaxial Growth of GaN for Defect Reduction and Device Development C.R. Eddy, Jr., R.T. Holm, R.L. Henry, M.E. Twigg, N.D. Bassim, L.M. Shirey, F.K. Perkins (Naval Research Laboratory); M.C. Peckerar (University of Maryland); E.J. Cukauskas (Sachs Freeman Associates) The family of III-V nitride materials has been the subject of many device technology development efforts, including visible and ultraviolet light emitters and detectors and high power rf transistors. Another area of interest is the development of vertically conducting device technologies for power electronics. In such applications it is critical that the threading dislocations inherent in this heteroeptixial materials system (no native substrate) be eliminated, particularly in the active region of the device. These threading dislocations have been identified as sources of leakage currents and premature failure of voltage blocking devices. In this work, an approach to reduce/eliminate vertical threading dislocations is described and initial results presented. The approach involves confined homo- or hetero-epitaxy of GaN materials using sputtered oxide masks to delineate growth regions. Growth is carried out using conventional MOCVD and conditions that inhibit lateral growth over the mask. The resulting confined epitaxial material is terminated with equilibrium crystal facets that form hexagonal mesas. The material contains a reduced dislocation density (approximately one order of magnitude as determined by TEM) compared to the underlying template layer for homoepitaxial growth. This reduction in dislocation density is believed to be the result of reduced strain in the epitaxial volume and the presence of the free surfaces represented by the sidewalls of the mesa. Characterization of pn junction diodes grown in this manner reveals significantly reduced leakage currents in as-grown structures (1 μA/cm²), which can be further reduced with application of passivation coatings. The approach is well suited to the development of distributed diode device technologies appropriate for power device applications. Issues such as doping variations in the confined epitaxial regions and the impact of additional device filtering techniques will also be presented.
10:00 AM		SC+EM-ThM-6 Conductive Atomic Force Microscopy Studies of Forward and Reverse Current Conduction in GaN Films A.A. Baski, J. Spradlin, S. Dogan, H. Morkoc (Virginia Commonwealth University) We have investigated the current conduction of homo- and heteroepitaxial GaN-based films using conductive atomic force microscopy (C-AFM). For the case of a homoepitaxial film grown by MBE on HVPE template, C-AFM shows premature current breakdown at the centers of hillocks associated with screw dislocations, consistent with the results of other groups. Local C-AFM current-voltage curves of such dislocations indicate a Frenkel-Poole mechanism for forward conduction on defective regions, as opposed to field emission on non-defective regions. In the case of heteroepitaxial GaN films grown on sapphire, C-AFM data do not show a straightforward correlation between topography and current conduction. We observe, however, that films with more rectifying Schottky behavior via standard I-V measurements produce forward and reverse bias C-AFM images with strong asymmetry. In addition, standard I-V data indicate a field emission (forward bias) or hopping (reverse bias) mechanism for a high quality rectifying film, and a Frenkel-Poole mechanism for a lower quality film. This behavior is consistent with the C-AFM I-V data of defective regions on the homoepitaxial film, which also show Frenkel-Poole conduction in forward bias.
10:20 AM	Invited	SC+EM-ThM-7 Thin-Film Diamond Electronics: Progress and Expectations J.E. Gerbi (Argonne National Laboratory) Diamond has long been thought to have the potential to revolutionize electronics due to its exceptional single-crystal properties, including extremely high thermal conductivities and carrier mobilities. Unfortunately, major roadblocks exist for both the growth of affordable single-crystal diamond, and especially for the doping of such material. Non-single crystal diamond, while suffering the degradation of some important single crystal diamond properties, does nevertheless show promise as an extremely useful electronic material. A review of such materials will be given, with emphasis on how the structure of various types of polycrystalline diamond affects key electronic properties. The ability to dope polycrystalline diamond in non-traditional ways will be discussed, as will the integration of diamond thin films in a device environment. Finally, prototype room-temperature device fabrication and performance will be discussed. The emphasis will be made that the field of d iamond electronics continues to make significant progress, especially for niche applications such as MEMS and chemical sensors. This work was supported by the DOE-Office of Science-Materials Science under Contract No. W-31-109-ENG-38.
11:00 AM		SC+EM-ThM-9 X-ray Absorption and Emission Studies of Diamond Nanoparticles T. van Buuren (Lawrence Livermore National Laboratory); C. Bostedt (HASYLAB at DESY, Hamburg, Germany); T.M. Willey, R.W. Meulenberg (Lawrence Livermore National Laboratory); J.Y. Raty (University of Liege, Belgium); G. Galli, L.J. Terminello (Lawrence Livermore National Laboratory) Carbon nanoparticles, produced in detonations, are found to have a core of diamond with a coating fullerene-like carbon. X-ray diffraction and TEM show that the nanodiamonds are crystalline and approximately 4 nm in diameter. These nano-sized diamonds do not display the charateristic property of other group IV nanoparticles: a strong widening of the energy gap between the conduction and valence bands owing to quantum-confinement effects. For nano-sized diamond with a size distribution of 4 nm, there is no shift of the band energies relative to bulk diamond¹. The C 1s core exciton feature clearly observed in the K-edge absorption of bulk diamond is attenuated and broadened in the nanodiamond case due to increased overlap of the excited electron with the core hole in the small particle. Also the depth of the second gap in the nanodiamond spectra is shallower than that of bulk diamond. A feature at lower energy in the X-ray absorption spectra that is not present in the bulk samples is consistent with a fullerene like surface reconstruction. By exposing the diamond nanoparticles to an Argon / Oxygen plasma then annealing in a UHV environment, we have obtained a hydrogen free surface. The nanodiamonds processed in this manner show an increase fullerene type contribution in the carbon x-ray absorption pre-edge. High spatial resolution EELS measurements of the empty states of a single nanodiamond particle acquired with a filed emission TEM also show the core of the particle is bulk diamond like whereas the surface has a fullerene like structure. Density-functional theory calculations on clusters show an increase in bandgap only for clusters smaller than 1 nm, and confirm the fullerene-like surface reconstruction. This work is supported by the U.S. DOE, BES Materials Sciences under contract W-7405-ENG-48, LLNL. ¹ J. Y. Raty, G. Galli, C. Bostedt, T. van Buuren, L. J. Terminello, Phys. Rev. Lett. 90, p.401 (2003) .
11:20 AM		SC+EM-ThM-10 Direct Electronic Sensing of Biological Binding Events on Diamond Thin Films W. Yang (University of Wisconsin-Madison); J.E. Butler, J.N. Russell, Jr. (Naval Research Laboratory); R.J. Hamers (University of Wisconsin-Madison) The chemical and physical inertness of diamond combined with its semiconducting electrical properties make it an attractive substrate for biological sensing. We have investigated the direct covalent modification of diamond with a variety of biomolecules including DNA and antibodies, and have investigated the relationships between the interfacial structure and the resulting electrical response observed upon DNA hybridization and antibody-antigen binding. The biological sensitivity and selectivity on diamond surfaces have been optimized using traditional fluorescence methods. By using electrical impedance spectroscopy (EIS), we have been able to detect DNA hybridization and antigen-antibody interactions in real time with high sensitivity. EIS measurements allow us to map out the impedance response as functions of both frequency and potential, helping to separate the electrical properties of the diamond space-charge region, the molecular layers, and the remaining solution. At low frequencies binding can be detected by the ion diffusion through the modified interfacial layers. At higher frequencies binding can be detected through a field effect induced in the diamond. The field effect mechanism allows us to directly detect biological molecules based on the different molecular charges. Complementary measurements on n-type and p-type silicon confirm the overall picture of the transduction process. Our results suggest the possibility of fabricating biological FET devices on diamond thin films.
11:40 AM		SC+EM-ThM-11 Enormous Photocurrent for Hydrogenated Single-crystalline B-doped Diamond Epilayers Grown by Microwave-excited Plasma Chemical Vapor Deposition. Y.K. Koide (National Institute for Materials Science (NIMS), Japan) Diamond with energy band-gap of 5.5 eV is an attractive semiconductor for applying to a visible-blind photodetector operated at ultraviolet wavelength smaller than 240 nm. Although there have been several reports for challenges to develop such the photodetector (PD), tremendous efforts were made by applying polycrystalline diamond films to the PDs. Enormous photocurrent (PC), persistent photoconductivity (PPC), and relatively-large thermally-stimulated current (TSC) were reported by several groups to be observed for the polycrystalline diamond DUV-PDs, and the mechanisms of the PPC and TSC were proposed to be due to bulk deep levels, surface states, and/or interface states at grain boundaries in the polycrystalline diamond films. In order to understand the defect-related mechanism of the PPC and TSC, it is required to investigate the photoresponse properties of PD fabricated by single-crystalline diamond epilayer, which will lead to development of high-efficiency diamond DUV-PD with high reliability. Also, it is essential to investigate influences of diamond surfaces terminated by hydrogen and oxygen (named by hydrogenated and oxidized surfaces, respectively) on photoresponse properties, which will provide an important information on effect of surface states on the mechanism of PPC. The purpose of this paper is, as a first step to develop reliable, high-efficiency diamond PD, to explore the PC and PPC for homoepitaxial B-doped diamond epilayer with hydrogenated and oxidized surfaces. The PC's with gain larger than 105 and the long-term PPC were observed in illuminating UV-light to metal/p-diamond/ metal photodiodes fabricated on hydrogenated surfaces of the single-crystalline B-doped p-diamond epilayer. Saturation of PC with increasing applied voltages and phototransistor action against incident optical power densities were observed. The PC with large gain and the PPC were believed to be due to surface states on hydrogenated surface.