Papers

AVS2004 Session EM-TuA: Defects and Interfaces in Electronic Materials and Devices

Tuesday, November 16, 2004 1:20 PM in Room 304B

Tuesday Afternoon

Start	Invited?	Item
1:20 PM		EM-TuA-1 Spectroscopic & Microscopic Characterization of Multi-layered Superconducting Thin Films S.M. Mukhopadhyay, S. Krishnaswami, S. Vemulakonda (Wright State University) Correlation of chemical composition profile, microstructure & electrical properties of thin film superconductors will be presented in this talk. Studies have been performed to compare the chemical and compositional profiles, grain texture and critical current densities of Y-Ba-Cu-Oxide layers grown by PLD and MOCVD on single crystals and on polycrystalline metal substrates and different buffer (diffusion barrier) layers. X-ray Photoelectron Spectroscopy (XPS) has been used in conjunction with ion beam sputtering to obtain composition and chemistry information at different depths into the film. It is seen that variations in overall composition such as cationic ratios and diffusion of buffer layer species can occur in some types of film. MOCVD samples grown on IBAD textured buffer layers have higher copper concentration that may be linked to higher grain boundary content. Some films grown on polycrystalline metal-substrates (buffered with YSZ and CeO2) show Zr diffusion into the YBCO region whereas those grown on single crystal YSZ (buffered with CeO2) do not. The durability and stability of these films, and their dependence on starting microstructures has been compared and will be presented.
1:40 PM		EM-TuA-2 Characterization of Hydrogen Complex Formation in InP:Zn (100) M.D. Williams, S.A. Yasharahla, S.C. Williams, N. Jallow (Clark Atlanta University) Ultraviolet photoemission spectroscopy is used to study the kinetics of the H-Zn complex formation in Zn doped InP (100). Hydrogen injected into the material electronically passivates the local carrier concentration.¹ Reverse-biased anneals of the InP show a dramatic change in the work function of the material with increasing temperature. Spectral features are also shown to be sensitive to sample temperature. To our knowledge, we show the first view of hydrogen retrapping at the surface using photoemission. A simpe work function analysis shows the state of passivation of the material. ¹ W. C. Dautremont-Smith, Mat. Res. Soc. Symp. Proc. 104, 313 (1988).
2:00 PM		EM-TuA-3 In situ Studies of Stacking Fault Formation in Silicon Carbide A. Galeckas, J. Linnros (Royal Institute of Technology, Sweden); P. Pirouz (Case Western Reserve University) Silicon carbide (SiC) has long been recognized as one of the leading semiconductors for high-power applications. Recently, commercial implementation of 4H-SiC material in high-voltage bipolar electronics has seriously been obstructed by a phenomenon causing degradation of electrical characteristics. In this paper we will address the internal causes of degradation by presenting an overview of in situ studies of structural instabilities within active area of SiC device. Several applications of a novel time-resolved pump-probe imaging spectroscopy technique, recently developed by our group for studies of stacking fault (SF) nucleation and expansion dynamics, will be presented in conjunction with conventional TEM measurement results. More specifically, in our method an intense optical excitation is employed to provide excess carriers and to trigger recombination-enhanced motion of dislocations (REDM), whereas dynamics of defects is monitored by time-lapse imaging of the background luminescence, set by a low-intensity probe cw-excitation. We report optically induced nucleation, multiplication and rapid expansion of SF defects and provide experimental evidence of their detrimental impact on carrier transport properties. Our results show that degradation phenomenon is not restricted to the device’s active area – stacking faults likewise nucleate and expand on the outside as well as in the standalone epilayers. This fact essentially discards common assumptions that heavy doping of P+ layer and residual mechanical stress at P+/N junction are almost certain SF nucleation sources. In contrast, statistical analysis of PL imaging results strongly suggests that the pre-existing growth defects, for the most part threading screw dislocations, are the major nucleation sources of SFs. Temperature and excitation dependencies of optically stimulated dislocation glide have been quantified, providing an activation enthalpy of 0.25±0.05 eV, which is consistent with energy estimated from the electrical REDM experiments. We discuss the observed differences of glide velocities in terms of possibly different origin of dislocation partials (silicon-core versus carbon-core) and propose the anti-phase defects (APD) on the dislocation partials as likely sites for the kink-pair nucleation. Finally, in view of the important experimental observation that similar formations of SFs could be optically-induced also in p-type material and in 6H polytype, we conclude that degradation phenomenon seems to be rather a general problem of hexagonal SiC material than that of a bipolar device.
2:40 PM		EM-TuA-5 Spectroscopic Investigation of Epitaxial Dielectrics on SiC J. Choi, C.M. Tanner, R. Puthenkovilakam, J.P. Chang (University of California, Los Angeles) SiC has attracted much attention as a promising wide-bandgap semiconductor material due to its excellent physical and electrical properties. Among other SiC- based devices, group-III nitrides on SiC have demonstrated great device performance as blue light-emitting diodes and high electron mobility transistors. In these devises, inclusion of an AlN buffer layer improves the performance due to its good lattice match with SiC substrate and relatively low misfit with the III-nitride layers. In this work, we demonstrate the deposition of HfO₂, a well known high-k gate dielectric material for silicon based devices, by an atomic layer deposition process on AlN/SiC. During the thin-film growth, the crystallinity of the epitaxial layers is monitored by in-situ reflection high-energy electron diffraction (RHEED) and the thin film composition is characterized by in-situ x-ray photoelectron spectroscopy (XPS). To examine the characteristic electrical properties, capacitance-voltage and current-voltage measurements are performed on HfO₂/AlN/SiC and HfO₂/SiC capacitors fabricated by photolithographic patterning of metal electrodes. The correlation between growth conditions, stoichiometry and crystallinity of the eptaxial layer, and device performance will be discussed. Finally, we present first-principle calculations of the valence band structures of AlN and SiC as well as band alignment at the interface, in comparison with the experimentally determined band alignment by XPS. The experimental band offset is determined by measuring the core-level to valence-band maximum binding energy difference for AlN and SiC at an interface, subsequently scaling them with respect to core-level separation between bulk Si and Al, and agrees well with the theoretical prediction.
3:00 PM		EM-TuA-6 Variation of WNx Gate Work Function after High Temperature Annealing P.-C. Jiang, J.S. Chen (National Cheng Kung University, Taiwan) The selection of the gate electrode material has become a critical issue as the dimension of CMOS devices continues to scale down. According to the 2001 ITRS roadmap, WNx is a promising material as the gate electrode for the PMOS transistor, because the WNx has a Fermi level near the silicon valence band. In this work, we investigate the variation of metal work function (Î¦m) of reactive sputtered WNx films after annealing at different temperatures (400-600â"f). The annealing atmosphere is forming gas (N2:H2=135:15) and the annealing time is 30min. By measuring the C-V curves of the MOS structure with oxides of different thickness, we can derive the Î¦m of the WNx gate. After annealing, the phases of the WNx films had changed and their nitrogen contents also varied due to the out-diffusion of nitrogen to the ambient. The variation of nitrogen content in WNx upon annealing will lead to the change of Î¦m. By measuring the I-V curves of the MOS structure with reverse bias, the leakage current increases with decreasing the Î¦m. As a result, the Î¦m not only affects the threshold voltage of the device but also changes the leakage current of the oxide layer, and the nitrogen content in WNx is the key point to determine the Î¦m.
3:20 PM		EM-TuA-7 New Mechanisms for Controlling Transistor Junction Formation through Surface Chemistry E.G. Seebauer, K. Dev, C.T.Z. Kwok, R.D. Bratz (University of Illinois at Urbana-Champaign) Forming extremely shallow pn junctions with very low electrical resistance is becoming an insurmountable stumbling block to the continued scaling of microelectronic device performance according to Moore's Law. We have developed a technology based on surface chemistry that holds great promise for simultaneously reducing junction depth and increasing activation for dopants implanted into silicon. The approach uses the surface as a large controllable "sink" that removes Si interstitials selectively over dopant interstitials. We have discovered two distinct ways to employ adsorption at the surface for this task: adjusting the intrinsic loss rate of interstitials to the surface, and reducing the degree of electrical charge built up at the surface. We control the interstitial loss rate to the surface by saturating dangling bonds using adsorbed nitrogen, introduced as ammonia, hydrazine, or a similar simple gas before implantation or the subsequent annealing step. To demonstrate such effects, we have measured SIMS profiles of isotopically labeled Si (mass 30) implanted into a Si host lattice depleted in this isotope, The annealed profiles with an atomically clean surface change relatively little from the as-implanted profile. The annealed profiles with adsorbed N change more, and the profiles with a rather thick layer of adsorbed O has change the most. Electrical charge that builds up at dangling bonds on a surface couples to charged defects in the underlying bulk through an electric field near the surface that strongly repels the bulk defects. The strong repulsion in the near-surface region in effect makes the surface more reflecting of interstitials, thereby changing the concentration profile of the bulk defects and correspondingly in the dopant profile. We have shown by the optical method of photoreflectance that nitrogen adsorption greatly reduces the charge buildup at the surface, and therefore reduces the surface repulsion effect.
3:40 PM		EM-TuA-8 Annealing Effect on Dielectric Properties of Ti Doped K(Ta,Nb)O₃ Thin Film using PLD H.-J. Bae, J. Sigman (University of Florida); L.A. Boatner (Oak Ridge National Laboratory); D.P. Norton (University of Florida) K(Ta,Nb)O₃ (KTN) has been widely studied for optical wave guides and tunable microwave applications. In this study, the reduction of dielectric loss by acceptor ion doping is investigated for epitaxial thin films. Acceptor doping traps electrons due to oxygen vacancies that form during oxide film growth. Epitaxial KTa_0.524Nb_0.446Ti_0.03O₃ films with 3% Ti were investigated. The KTN:Ti films were grown at 750°C using pulsed laser deposition. The dielectric properties of Ti doped KTN films were measured for unannealed and annealed films. Annealing temperatures ranged from 800°C and 1000°C in an oxygen ambient. The dielectric properties of KTN:Ti films obtained after annealing are compared with that for as-deposited KTN:Ti film and undoped KTN films. The crystallinity, surface morphology, and electrical properties were investigated using x-ray diffraction, atomic force microscopy, and dielectric response. This latter involved capacitance measurements of as-deposited and annealed films, measured at 1MHz using interdigitated electrodes. Tunability and dielectric loss of as-deposited KTN:Ti film were determined to be 10% and 0.0134, respectively. For films annealed at 800°C and 900°C, the dielectric loss decreased but with a decrease in tunability as well.