Papers

AVS2001 Session SC-TuM: Semiconductor Interfaces and Thin Films

Tuesday, October 30, 2001 8:20 AM in Room 124

Tuesday Morning

Start	Invited?	Item
8:20 AM		SC-TuM-1 Nanometer-scale Studies of Possible Dislocation Charging at GaN Interfaces J.P. Pelz, H.-J. Im, Y. Ding, E.R. Heller (The Ohio State University); B. Heying, J.S. Speck (University of California, Santa Barbara); W.J. Choyke (University of Pittsburgh) Threading dislocations (TDs) in III-Nitride films are thought to be electrically active and of major concern for device applications. Several recent studies of GaN and AlGaN/GaN films suggested that TDs might develop significant fixed negative charge (up to 1 e- every c-axis lattice spacing) at or near the TD core. We have quantified possible dislocation charging near metal/GaN interfaces using ultra high vacuum Ballistic Electron Emission Microscopy (BEEM) measurements of identifiable TDs, which were compared with electrostatic modeling of conduction band (CB) bending due to fixed local negative charge. Surprisingly, measurements of TDs in GaN films (grown by molecular beam epitaxy (MBE) under Ga droplet conditions) do not indicate any negative charge at TDs close to the metal-GaN interface, with an estimated upper limit of ~0.25 (e-)/c along the TDs. In contrast, we generally observe a mild decrease in the local CB at TDs as well as at step edges, which may be due to piezoelectric surface charge induced by local stress variations. We will discuss on-going measurements of near-interface dislocation charging of III-Nitride films grown under different conditions (MBE Ga-poor, MOCVD, etc.) to investigate how growth conditions affect local dislocation charging and local transport behavior. Time permitting, we will also discuss on-going measurements of how the Schottky Barrier height on metal/SiC contacts depends on SiC polytype, interface orientation, and deposited metal. This work was supported by the Office of Naval Research.
9:00 AM		SC-TuM-3 Nanometer-scale Studies of Phase Separation in Compound Semiconductor Alloys B. Shin, A. Lin, K. Lappo, R.S. Goldman (University of Michigan); M.C. Hanna, S. Francoeur, A.G. Norman, A. Mascarenhas (National Renewable Energy Laboratory) Thin films of compound semiconductor alloys can be grown with a wide range of band gap energies and lattice constants, useful for the development of novel electronic and optoelectronic devices. In most of these systems, growth conditions have been reported for which phase separation occurs. Yet, the thermodynamic versus kinetic origin of phase separation, as well as the experimental conditions for determining the presence of phase separation has been the subject of debate for nearly 20 years.^1,2 In thin films of compound semiconductor alloys, both the difference in binary bond lengths and the film/substrate misfit are expected to play a significant role in the initiation of alloy phase separation. In this work, we have examined phase separation in the misfit-free InAlAs/InP system using ultra-high vacuum cross-sectional scanning tunneling microscopy (XSTM) and x-ray reciprocal space mapping. For p-doped thin InAlAs layers, XSTM reveals the presence of isotropic non-uniformities which consist of nanometer-sized clusters. For thicker, undoped InAlAs layers, longer wavelength quasi-periodic modulations perpendicular to the growth direction are apparent. These lateral modulations are observed in both topographic and conductance XSTM images, suggesting that they are due to a combination of compositional and strain variations. A signature of these modulations is also apparent in x-ray reciprocal space maps. Interestingly, the modulation wavelengths increase with film thickness and are notably lower than those reported for similar films grown at higher temperatures.³ Together, these results suggest that phase separation is a thermally activated kinetic process which may be significantly affected by the presence of impurities such as dopants. ¹G. B. Stringfellow, J. Cryst. Growth 65, 454 (1983). ²A. Zunger and S. Mahajan, in Handbook on Semiconductors (North-Holland, Amsterdam, 1994), Vol. 3, p. 1399. ³H. K. Cho et al, Mat. Sci. Eng. B 64, 174 (1999).
9:20 AM		SC-TuM-4 Scanning Tunneling Spectroscopy on Adsorbate Induced Two Dimensional Electron Systems on InAs(110) J. Klijn, M. Morgenstern, Chr. Meyer, D. Haude, R. Wiesendanger (Hamburg University, Germany) Two dimensional electron systems (2DES) are usually prepared in heterostructures and thus buried below the surface. Scanning probe methods are consequently restricted to resolutions above 100 nm.¹ To increase the spatial resolution, we have prepared an adsorbate induced 2DES close to the surface on n-type InAs(110).² We measured the local density of states of this 2DES in zero and in finite magnetic field (T=6K, B<=6T). The data at zero field reflect the scattered wave functions at the residual dopants. The corresponding Fourier transformation identifies the k-vector of the undisturbed 2DES as the strongest contribution. However, mixing with other k-vectors due to the potential scattering leads to broadening of the k-space features. The magnetic field data show Landau quantization of the 2DES and exhibit distinct changes in the local density of states. ¹ S.H. Tessmer et al., Nature 392(1998)51; N.B. Zhitenev et al., Nature 404(2000)473; P. Weitz et al., appl.Surf.Sci. 157(2000)349 ² M. Morgenstern et al., Phys. Rev.B 61(2000)13805.
9:40 AM		SC-TuM-5 InAs Surface Passivation for Electronics and Biosensors D.Y. Petrovykh (University of Maryland, College Park); M.J. Yang, L.J. Whitman (Naval Research Laboratory) Many semiconductor-based chemical and biological sensors operate by detecting changes in the device conductivity caused by adsorption of organic or inorganic molecules on the sensor surface. The conductivity is affected by the surface charge induced by adsorbates, so sensors based on very thin films or nanostructures should be inherently more sensitive. InAs is a natural material for these applications, because the charge accumulation layer, naturally formed on its surface, provides intrinsic conductivity down to nm-scales. To be used in chemical/biological sensors, InAs films and nanostructures must be properly passivated and functionalized. Ammonium sulfide treatment, commonly used in GaAs processing, is known to effectively remove the oxide and other surface contaminants. We show that it also offers sub-5 nm etching, and the resulting S-passivated surface resists oxidation during short-term exposure to ambient air, or immersion in water (with a range of pH) or organic solvents. We will discuss the possibility of using alkanethiol films for longer-term stability and surface functionalization, including the use of selective deposition by dip-pen nanolithography. For sensor applications, it is also important to control the surface Fermi level pinning. We use conductivity measurements and electron spectroscopy to examine band bending in InAs films and the effects on the conductivity of different capping layers, passivation treatments and other common device processing steps. This work was carried out at the Naval Research Laboratory and supported by the Office of Naval Research, National Nanotechnology Initiative Program on the Nanoscience Basis for Miniaturized, Intelligent Sensors.
10:00 AM		SC-TuM-6 Formation of Co Silicides on Si_0.7Ge_0.3 Layer in the Presence of Thin Interposing Au Layer and Capping Ti Layer W.W. Wu, T.F. Chiang, S.L. Cheng, H.H. Lin, L.J. Chen (National Tsing Hua University, Taiwan, R.O.C.); H.H. Cheng, Y.H. Peng (National Taiwan University, Taiwan, R.O.C.) Strained SiGe alloys offer the possibility of bandgap engineering for silicon-base devices. Due to their high mobility, SiGe/Si heterostructures have been investigated for use as SiGe channels in MOSFET^'s as well as high speed and high transconductance MODFET^'s, elevated source-drain contacts and gate material in CMOS technologies. Silicide/Si_1-xGe_x/Si(001) heterostructures are promising structures for use in devices such as the heterojunction bipolar transistor and infrared detectors with high cutoff wavelength. Due to its low resistivity, low Schottky barrier, good thermal stability, and possibility of self-aligned formation at relatively low temperatures, CoSi₂ is an attractive contact material for submicron Si devices. However, in the Co/Si_0.7Ge_0.3 system, the CoSi₂ tended to agglomerate at relatively low temperatures. The formation of Co silicides on Si_0.7Ge_0.3 alloys with a thin interposing Au layer and capping Ti layer has been investigated. CoSi₂ was observed to be the only silicide phase in Si_0.7Ge_0.3 samples annealed at 650-950 °C with a thin interposing Au layer and capping Ti layer. The sequence of phase formation is the same as the reaction of Co with single-crystal Si. The presence of Au was found to decrease the formation temperature of CoSi₂ by about 300 °C compared to that of Co(30nm)/Si_0.7Ge_0.3 samples. In addition, a thin capping Ti layer improves the uniformity and thermal stability of CoSi₂ layer. For Ti(5nm)/Co(30nm)/Au(1nm)/Si_0.7Ge_0.3 system, the process window of CoSi₂ was extended to 650-950 °C. SIMS analysis indicated that a large amount of Au diffused from the Co/Si_0.7Ge_0.3 interface to disperse in CoSi₂ layer during annealing.
10:20 AM		SC-TuM-7 Intrinsic Defects of Cl-doped ZnSe Epitaxial Layers Examined by Photothermal Spectroscopy K. Yoshino (Miyazaki University, Japan); M. Yoneta, K. Ohimori, H. Saito, M. Ohishi (Okayama University of Science, Japan) Photothermal (PT) measurements have recently been carried out as one of the new methods to study the physical properties of semiconductors. One of the great advantages of the PT measurements is that the nonradiative carrier recombination processes are measured directly. Therefore, the PT may complement a photoluminescence (PL) and PL excitation (PLE). Furthermore, the PT also is much easier than deep-level transient-capacitance spectroscopy (DLTS) since no electrodes are needed in the PT system. In our previous paper,¹ the PT measurements were carried out for nondoped and N-doped ZnSe epitaxial layers grown by molecular beam epitaxy (MBE), and we obtained the nonradiative carrier recombination centers in those samples. In this paper, we carried out the PT and PL measurements on Cl-doped ZnSe epitaxial layers from 80 to 300 K. The net carrier concentration is from 5.8 ´ 10 ¹⁷ to 2 ´ 10¹⁸ cm³. Three distinct peaks correspond to bandgap energy of ZnSe and two kinds of Cl related centers are observed. The activation energies of the Cl defects are estimated to be about 25 and 250 meV. The energy of 25 meV is known to be an activation energy of Cl atom in the Se site and the energy of 250 meV is not unknown. The emission due to the deep defect is not observed in the PL spectrum. Therefore, it indicates the defect with an activation energy of 250 meV acts the nonradiative carrier recombination center. ¹ K. Yoshino et al., J. Crystal Growth 214&215 (2000) 572.
10:40 AM		SC-TuM-8 Metal-Induced States and Polytype Transformations at SiC Interfaces S.P. Tumakha, L.J. Brillson, G.H. Jessen (Ohio State University); R.S. Okojie, D. Lukco (NASA-Glenn Research Center) We have used low energy electron-excited nanoluminescence (LEEN) spectroscopy to probe electronic structure at chemically-treated and metallized 4H and 6H-SiC interfaces. SiC high temperature electronics requires metal contacts with controllable barriers and minimal deep level electronic states. LEEN spectra over incident electron beam energies E from 0.5 to 5 keV identify the presence of localized states and their spatial distribution on a nanometer scale. With increasing E, the electron cascade and resultant generation of free electron-hole pairs peak at increasing depth ranging from 10 nm at 0.5 keV to 200 nm at 5 keV. The resultant band-to-band and band-to-defect luminescence is detected selectively at the intimate metal-SiC interface, the near-interface region extending tens of nanometers into the SiC, or the bulk SiC up to 0.2 microns into the solid. Pt/Ti/SiC junctions were prepared by standard cleaning, oxidation, and etching methods. 6H-SiC exhibits optical emission that varies with depth from the intimate interface and with surface chemical preparation. The depth-dependent spectra exhibit 2.9 eV near band edge (NBE) features of 6H-SiC for bulk excitation vs. a disordered and/or defected region within a few nm of the metal contact. Spectra from the near interface region indicate the existence of a SiC polytype with a higher band gap of ca 3.4 eV - resembling 4H-SiC as well as a discrete deep level, i.e., emission energy = 1.9 eV, for a specific surface treatment. Metal-induced features on 4H-SiC are similar. In addition, oxidation before or after metallization produces 2.5 eV emission extending hundreds of nm into the 4H bulk, characteristic of polytype conversion to 3C-SiC and confirmed by TEM. A strong impurity doping dependence suggests that oxidation or metallization-induced strain drives this transformation. The structural as well as electronic changes at SiC interfaces have significant device and processing implications.
11:00 AM		SC-TuM-9 Growth and Electrical Characterization of Ultra-Dense Phosphorous Delta-Doping Layers in Silicon T.-C. Shen, J.-Y. Ji (Utah State University); M. Zudov, R.-R. Du (University of Utah); J.S. Kline, J.R. Tucker (University of Illinois) If dopant atoms substitute a substantial fraction of a monolayer of the Si atoms within a crystal, the resulting 2D dopant sheet may provide unique electrical properties for novel nano-scale devices. We have demonstrated that depositing phosphine molecules onto Si(100) surfaces in ultrahigh vacuum, followed by 35-50ML of Si epitaxy at T<500K, can yield a conducting layer that does not freeze out even at 0.3K. At 1/4ML saturation coverage at room temperature, the positive phosphorous ions create very large electric fields in the growth direction, producing a tightly confined 2D electron system. Within the plane, however, wavefunctions for these bound electrons are expected to couple across relatively large distances of a few Bohr radii (~2.5nm for P-atom donors), opening up new possibilities for lateral tunnel junctions. Initial magnetotransport measurements reveal an electron density of ~ 2.6x10¹⁴cm^-2 (~1/4ML) at 0.3K indicating complete electrical activation of the donor layer. From 60 to 0.3K the sheet resistance grows logarithmically with decreasing T, yielding a resistance of 1.16 kΩ/sq and a mobility of 21cm²/Vs at 0.3K. Studies of the correlation between electrical characteristics and phosphine deposition parameters will be presented. In addition, a new paradigm for devices based on selectively patterned 2D electron/hole systems will be discussed.