Papers

AVS1997 Session EM-ThM: SiGe Heterostructures

Thursday, October 23, 1997 8:20 AM in Room C1/2

Thursday Morning

Start	Invited?	Item
8:20 AM		EM-ThM-1 Atomic-Layer Growth of Si on Ge(100) Using SiH₄ J. Murota, M. Sakuraba, T. Watanabe, T. Matsuura (Tohoku University, Japan) Reaction mechanism of SiH₄ on Ge(100) was investigated by ultraclean cold-wall low pressure CVD. A self-limited atomic layer of Si was deposited on the Ge surface at 260°C at the SiH₄ pressure of 500 Pa. The time and pressure dependence of the adsorption and reaction of SiH₄ can be explained by Langmuir-type kinetics. The saturated surface concentration of the deposited Si atoms depends on the preheating conditions which affect the surface H-termination and reconstruction. A single atomic layer was found for preheating at 350°C in Ar, where the H-free reconstructed surface was observed by FTIR/RAS and RHEED, and at 260°C in H₂, where the H terminated unreconstructed surface was observed before SiH₄ exposure. However, in the case of preheating in H₂ at 350°C, where the H-terminated dimer structure was formed on the Ge surface, the Si atom concentration hardly reached that of the single atomic layer. Thus, it is concluded that the density of the SiH₄ reaction sites on the H terminated surface with the dimer structure is lower than that of the H-terminated unreconstructed surface and the H-free surface. The SiH₄ reaction on the Ge surface is believed to induce desorption of the hydrogen atoms bonded to the Ge atoms on the top surface, and is suppressed on the remaining dimer structure and the deposited Si atoms with the Si-H₂ structure.
8:40 AM		EM-ThM-2 Effects of H Coverage on Ge Segregation during Si_1-xGe_x Gas-Source Molecular Beam Epitaxy H. Kim, N. Taylor, J.R. Abelson, J.E. Greene (University of Illinois, Urbana-Champaign) The effects of H coverage on Ge segregation during Si_1-x Ge_x gas-source molecular beam epitaxy (GS-MBE) were investigated using D₂ temperature programmed desorption (TPD). Si_1-x Ge_x films with x = 0.01-0.30 were grown from Si₂H₆ /Ge₂H₆ mixtures at T_s = 450-800°C. As- deposited layers were annealed at the growth temperature for 30 secs and exposed to atomic deuterium at 200°C until saturation coverage. D₂ TPD spectra were fit using four peaks corresponding, in order of decreasing activation energy, to desorption from Si monodeuterides, Ge-Si mixed-dimer monodeuteride, Si dideuteride and Ge monodeuteride. Steady-state Ge surface coverages were determined from the TPD data as a function of T_s and x. In contrast to solid-source MBE films grown in this temperature regime, the tendency for Ge segregation during GS-MBE decreases with decreasing T_s due to the increasing H coverage. The results were well described by a model accounting for Ge/Si site exchange and θ_H. The Ge segregation enthalphy varied from -0.28 eV at high growth temperatures, T_s ≥ 800°C, where the steady-state hydrogen coverage θ_H approaches zero to -0.1 eV at T_s ≤ 450°C where θ_H is nearly one.
9:00 AM		EM-ThM-3 SiGe Materials and Devices J.O. Chu, K. Ismail, S.J. Koester, K.Y. Lee (IBM T.J. Watson Research Center); M. Arafa (University of Illinois) The recent advances in the growth and bandgap engineering of Si/SiGe heterostructures have allowed the silicon technology to extend into device regimes previously associated with only III-V compound semiconductor. This presentation will discuss the growth of Si and SiGe materials using the ultra-high vacuum-chemical vapor deposition (UHV-CVD) process. The basic overall growth kinetics for Si and SiGe films will be described. For device applications, both p- and n-type doping concentrations ranging from 10¹⁶ - 10²⁰ can be precisely controlled using in-situ doping from B₂H₆ or PH₃ as the dopant source for p- and n-type, respectively. Moreover, very abrupt doping profiles for phosphorous doping in the levels of >1¹⁹ will be presented. The strain created by the lattice mismatch between Si and SiGe alloys has been harnessed to grow strained quantum wells with enhanced transport properties. In compressively-strained SiGe layers with high Ge content (>70%), enhanced hole mobility (about 5 times higher than the conventional silicon p-mos) has been demonstrated. Similiarly, strained Si channels which can be achieved by growing the Si layer on a relaxed SiGe buffer have also yielded enhanced electron and hole mobility about 3 times higher than in bulk Si. Furthermore, microwave device performance of our SiGe FETs will be presented. The very high operating speed of such devices reflects the enhancement in transport caused by the strain. Power-delay products compared to standard Si MOS are 3-6 times lower in our devices. Finally in order to address the remaining device problems such as parasitic junction capacitance and low power operation, several novel strained SOI structures and schemes will be presented based upon selective local oxidation and bond and etch-back techniques.
9:40 AM		EM-ThM-5 Surface Morphological Evolution in Si_0.7Ge_0.3/Si(001) Structures Grown by Gas Source-Molecular Beam Epitaxy T. Spila, P. Desjardins, N. Taylor, D.G. Cahill, J.E. Greene (University of Illinois, Urbana-Champaign); S. Guillon, R.A. Masut (Ecole Polytechnique de Montréal, Canada) Si_0.7Ge_0.3 layers were grown on Si(001) with miscuts ≤ 0.3° by GS-MBE from Si₂H₆ and Ge₂H₆ mixtures. Surface morphological evolution was determined from AFM and x-ray reflectivity measurements, while the overall strain state was obtained from high-resolution x-ray reciprocal lattice maps, as a function of layer thickness (h = 10-1000 nm) and growth temperature (T_s = 450-550 °C). AFM observations show that the surface morphology is described by well separated growth mounds. The square-root of the height difference correlation function G^1/2, directly related to the surface width, and the characteristic lateral mound separation d both exhibit power law dependencies on layer thickness, G^1/2 α h^beta and d α h^gamma. No change in slope was observed due to film relaxation at h = h_c which we interpret as an indication that roughening during low-temperature GS-MBE is controlled more strongly by the steady-state H coverage than by strain. The surface morphology of the films grown at 500 °C evolves from growth mounds to an island structure with a superimposed cross-hatched 110 misfit dislocation structure for h > h_c. However, ß and γ remain constant at 1 and 0.8±0.1, respectively. For the films grown at 550 °C, d decreases, but the thickness evolution, dominated by a large number of small [105] facets, is still well described by γ = 0.8. Surface roughening, however, rapidly becomes controlled by the presence of small number of deep pits yielding ß = 2.4±0.4
10:00 AM		EM-ThM-6 Formation of a SiGe/Si Heterojunction Diode by Ultrahigh Vacuum Electron Cyclotron Resonance Chemical Vapor Deposition S.H. Hwang, K.W. Whang, S.J. Joo, J.W. Park, E. Yoon (Seoul National University, Korea) Low-temperature (< 510 C) SiGe epilayers were successfully grown by ultrahigh vacuum electron cyclotron resonance chemical vapor deposition (UHV-ECRCVD). In this process, source and dopant gases were dissociated by downstream ECR hydrogen plasma. Process parameters such as microwave power, magnet current, total pressure, substrate dc bias, etc., were properly optimized to obtain dislocation-free SiGe epilayers on Si, as confirmed by transmission electron microscopy and Shimmel etching. To evaluate their electrical properties, mesa-type SiGe/Si heterojunction diodes were fabricated and their I-V characteristics were measured. The Ge content of the SiGe epilayers was 15%, and they were in situ doped with B at 2x10¹⁹. I-V characteristics of the SiGe/Si heterojunction diodes with different epilayer thicknesses were measured, and they were compared with the strain relaxation behavior of the SiGe epilayers. When the thickness of the SiGe epilayer exceeded the critical thickness, the ideality factor of the diode was greater than two and the reverse leakage current was very high, implying the existence of many recombination-generation centers in the epilayer. However, when the thickness of the SiGe epilayer was below the critical thickness, the forward I-V characteristic of a dislocation-free SiGe/Si diode was nearly ideal and its reverse leakage current ranged from 6 to 43uA/cm², depending on the size of the diodes. It is suggested that the SiGe epilayers grown by UHV-ECRCVD may be suitable for novel electronic device fabrication at low temperatures.
10:20 AM		EM-ThM-7 High-Performance SiGe/Si Heterostructures Produced by Double-Energy Si⁺ and Ge⁺, and Ge⁺ and Ge²⁺ Ion Implantations Z. Xia (Nokia, Salo, Finland); E. Ristolainen (Tampere University of Technology, Finland); K. Jones, P.H. Holloway (University of Florida, Gainesville) Si_1-xGe_x/Si heterostructures were formed on Si (100) wafers by using double-energy Si⁺ and Ge⁺ implantation, and double-energy Ge⁺ and Ge²⁺ implantation. In addition, the incorporation of Ge into Si, following material models have been developed: the density of states in the valence and conduction bands, the bandgap, the effective intrinsic carrier concentration, charge carrier mobilities, depletion widths and capacitance of p-n junctions. As high dose implantation inevitably cause end-of-range (EOR) damage beyond original amorphous/crystalline (a/c) interfaces after annealing, two double-energy implantation processes were adopted in this work: using low energy and high dose Ge⁺ implantation to form a compositionally graded SiGe alloy layer, and using high energy and low dose Si⁺ or Ge²⁺ implantation to form a deep amorphous layer. The spatial separation between Ge concentration maximum and a/c is significantly increased. Subsequent solid phase epitaxy (SPE) localizes EOR beyond a/c far away from peak Ge positions. Transmission electron microscope (TEM) measurements show that there are mainly two kinds of residual defects remained after SPE: (i) near-surface region defects, and (ii) EOR defects. The near-surface defects are mostly hairpin dislocations, and the EOR defects are dislocation loops. Furthermore, this technology offers the opportunity to use Si_1-xGe_x in a selective area. In this work, we will present RBS channeling results on solid phase epitaxy (SPE) of Si_1-xGe_x layers that were amorphized by either the double-energy Si⁺ and Ge⁺ implantation or the double-energy Ge⁺ and Ge²⁺ implantation as well as the usual single-energy Ge⁺ implantation. In addition, results observed by using a transmission electron microscope will be also presented to reveal defect details in a cross-sectional view.
10:40 AM		EM-ThM-8 Atomic-Layer Etching Control of Si and Ge Using an Ultraclean ECR Plasma T. Matsuura, T. Sugiyama, J. Murota (Tohoku University, Japan) Atomic-layer etching control of a SiGe system was investigated by alternate chlorine adsorption and low-energy Ar⁺ ion irradiation using an ultraclean ECR plasma system ¹. The surface after interrupting atomic-layer etching was analyzed by XPS and FTIR. From the initial surface after HF-treatment, hydrogen termination on Si and Ge was removed by Ar⁺ ion irradiation, and that on Ge was removed, while not on Si, by chlorine molecular supply. In a condition with chlorine molecular supply (≥ about 0.02Pa^.s) within a small amount of Ar⁺ ion irradiation (about 4x10¹⁵ cm^-2), the saturated etch rate per cycle of the (100) surface of Si, Ge, and Si_0.5Ge_0.5 each was 1/4 atomic-layer thickness. Langmuir-type adsorption of chlorine describes very well the chlorine pressure and supplying time dependence of the Si atomic-layer etch rate. Crystal orientation dependence of the saturated etch rate was observed for Si and explained by the surface bond structure. When Ar⁺ ion irradiation was increased further under a condition of saturated chlorine molecular adsorption, the etch rate per cycle tended to increase with Ar⁺ ion irradiation up to a saturation value of the single atomic-layer thickness at an irradiation of about 2x10¹⁶cm^-2 Ar⁺ ions with an energy higher than about 13eV. These results suggest a unified atomic-layer etching mechanism of a SiGe system with chlorine adsorption and energy-dependent Ar⁺ ion induced reaction. ¹T.Matsuura, J.Murota, Y.Sawada, and T.Ohmi, Appl. Phys. Lett. 63, 2803 (1993).
11:00 AM		EM-ThM-9 Low Temperature Selective Heteroepitaxy of Heavily Doped Si_1-xGe_x on Si for Application to Ultrasmall Devices J. Murota, M. Sakuraba, T. Matsuura (Tohoku University, Japan) Mechanism and application of selective Si_1-xGe_x(0.2≤x≤0.8)/Si heteroepitaxy heavily-doped with P and B were investigated using an ultraclean low-pressure CVD at 550°C with a SiH₄(6.0Pa)-GeH₄(0.2-4.7Pa)-H₂ gas mixture with the PH₃ and B₂H₆ addition(1.25x10^-5-4.24x10^-2Pa). In a low dopant gas partial pressure region such as below about 1mPa, the dopant incorporation rate shows a linear dependence on the dopant gas partial pressure and increases with the Ge fraction. In case of P doping, in a higher PH₃ partial pressure region, the decrease of the deposition rate and the P concentration(C_P) and the increase of the Ge fraction were observed with increasing PH₃ partial pressure in the range about C_P=10²⁰cm^-3. The PH₃ partial pressure at which the change occurred shifted higher with increasing GeH₄ partial pressure. These heavy P doping characteristics can be explained by assuming that the high PH₃ partial pressure induces high coverage phosphorus adsorption which suppresses both the SiH₄ and GeH₄ adsorption/reactions on the surface, and such suppression is different at the Si-Si, Si-Ge and Ge-Ge pair sites on the surface. By applying in-situ doped selective epitaxy of Si_1-xGe_x to source/drain fabrication, Super Self-aligned ultraShallow junction Electrode MOSFET(S³EMOSFET)'s with an about 0.075µm gate length were fabricated and their normal transistor characteristics were observed. S³EMOSFET has very high potentials, because the effective channel length is almost the same as the fabricated gate length, and the source/drain junctions are extremely shallow.
11:20 AM		EM-ThM-10 Si/Si_1-x-yGe_xC_y Heterostructures - Fundamental Properties and Devices K. Eberl, O.G. Schmidt (Max-Planck-Institut für Festkörperforschung, Germany) Si_1-yC_y and Si_1-x-yGe_xC_y alloy layers with a carbon concentration of a few percent are prepared by solid source molecular beam epitaxy. Near band-edge photoluminescence (PL) is observed from Si/Si_1-yC_y multiple quantum well (MQW) structures. The band gap in the pseudomorphic films is reduced by about 65 meV per percent C. The PL indicate a type I heterostructure with the band offset being mainly in the conduction band. In Si_1-x-yGe_xC_y MQW's compressive strain caused by Ge is partially compensated by C alloying and the band gap increases with y. PL measurements from closely spaced Si_1-yC_y/Si_1-xGe_x layers show a lower transition energy than separate Si_1-yC_y and Si_1-xGe_x reference samples. This is attributed to spatially indirect PL transitions between the electrons confined in the Si_1-yC_y layers and the heavy holes located in the Si_1-xGe_x layers. Self assembled Ge quantum dots with lateral dimensions of 10 nm are achieved by C induced island formation during Ge deposition. The Ge dots are embedded in Si and show intensive no-phonon PL with the energy depending on the size of the Ge dots. In p-type modulation doped 8 nm thick Si_0.49Ge_0.49C_0.02 quantum wells we observe an improved hole mobility at room temperature and 77 K compared to corresponding samples without C, which is a consequence of the reduced strain in the layer due to substitutional C. The realisation of a Si_0.54Ge_0.45C_0.012 p-channel MODFET on Si is presented. Room temperature transconductances of g_me=57 mS/mm and a saturation currents of I_DSS=40mA/mm are demonstrated for a 0.75 µm gate-length non-recessed device.