The material presented in this thesis concerns the growth and characterization of Si-based electronic materials through the use of molecular beam epitaxy (MBE). In particular, the Si1-xGex/Si and V1-xSix/Si material systems are studied because of their potential application in novel, heteroepitaxial device structures grown on crystalline Si substrates. Our work with the Si1-xGex/Si material system involves a study of the kinetics of strain relaxation for coherently strained Si1-xGex alloys grown on (100) Si at extremely low temperatures. In addition, we measure the strain dependence of the (100) Si/Ge valence band offset through the use of x-ray photoelectron spectroscopy (XPS). Further study of Si1-xGex alloys reveals a previously unreported (2 x 8) surface reconstruction, and for highly metastable alloys, the occurrence of chemical segregation during growth. We also report the successful nucleation and growth of superconducting V3Si on (111) Si, the first step toward the realization of epitaxial superconductor/semiconductor heterostructures. Finally, we make use of the knowledge gained in our preliminary studies and present the first electrical characterization of p-Si1-xGex/n-Si heterojunction interband-tunnel (HIT) diodes, demonstrating an enhancement of negative differential resistance (NDR) as a function of Ge concentration in the p-type Si1-xGex layer.
In Chapter 2, we present a study of strain relaxation in metastable Si1-xGex/Si superlattices and Si1-xGex alloys. The samples prepared for this study are grown at unusually low growth temperatures (~ 365°C) to thicknesses significantly in excess of previously established critical thicknesses for growth at higher temperatures. 0-20 x-ray diffraction (0-20 XRD) is used to study the relaxation process as the samples are annealed in a sequence of isochronal anneal steps. Significant relaxation is observed at anneal temperatures as low as 370°C. In addition, we compare the relaxation behavior of Si1-xGex/Si superlattices and Si1-xGex alloys designed to possess the same average properties and report that the superlattices relax to a lesser extent than the corresponding alloys. Finally, alloy relaxation is described as a thermally activated, first-order kinetic process described by a single activation energy of approximately 2.0 eV.
In Chapter 3, the strain dependence of the (100) Si/Ge valence band offset is measured through the use of XPS. Coherently strained Si, strained Ge, and symmetrically strained Si/Ge superlattices are grown on relaxed Si1-xGex buffer layers and transferred in ultrahigh vacuum (UHV) to the XPS chamber. Si 2p and Ge 3d core level to valence-band edge, and core level to core-level energy separations are measured as a function of in-plane lattice constant. High resolution x-ray diffraction (HRXRD) is used to measure the strain in these samples. For the valence band offset, we measure 0.83 ± 0.11 eV for Ge strained to (100) Si and 0.22 ± 0.13 eV for Si strained to (100) Ge.
High-quality, coherently strained Si1-xGex alloy layers are studied in Chapter 4 using HRXRD and ex situ transmission electron diffraction (TEM). Several samples are grown at extremely low temperatures (310-330°C) by MBE. Sample thicknesses and alloy concentrations are chosen to span a range beginning just below to significantly above critical thicknesses previously reported for this system. HRXRD observations demonstrate a high degree of coherency in the as-grown structures, since measurements of the lattice constant parallel to the sample surface (a∥) consistently yield the value for the (100) Si substrate. HRXRD from (004) planes used to measure a⊥ typically yield a spectrum with several peaks for growths in excess of the critical thickness and single peaks for those below the critical thickness. The high degree of coherency observed in these samples suggests that chemical segregation is responsible for the observed x-ray peaks.
In Chapter 5, the surfaces of Si1-xGex alloys are studied using reflection high-energy electron diffraction (RHEED) and low-energy electron diffraction (LEED) techniques. Si1-xGex films are grown on Si (100) substrates by MBE at temperatures between approximately 230 and 550°C, with alloy compositions ranging from x = 0.11-0.30. RHEED and LEED patterns from samples within this compositional range and at temperatures between 350°C and 550°C exhibit the usual Si-like (2 x 1) surface reconstruction patterns modified by the appearance of new, n/8-order diffracted beams. The n/8-order beams are observed for both coherently strained and unstrained films. Upon annealing and recooling, they appear to degrade reversibly within the temperature range 600-700°C. The additional fractional orders are interpreted as an 8-fold-periodic modulation in electron scattering factor, which is due to spatial correlation (ordering) of Ge atoms along the dimer chains of a (2 x 1) surface reconstruction. Possible physical origins of the Ge odering are discussed.
A study of the growth parameters governing the nucleation of metastable superconducting A15 V3Si on Si and Al2O3 is presented in Chapter 6. Nominally, 500Å films of V1-xSix are produced through codeposition of V and Si onto heated (111) Si and 1102 Al2O3 substrates. Samples are prepared in a custom-built UHV chamber containing dual a-beam evaporation sources and a high-temperature substrate heater. V and Si fluxes are adjusted to result in the desired average film composition. V0.75Si0.25 films prepared at temperatures in excess of 550°C on Si show significant reaction with the substrate and are nonsuperconducting, while similar films grown on l2O3 exhibit superconducting transition temperatures (Tc) approaching bulk values for V3Si (16.6 - 17.1 K). Codeposition at temperatures between 350 and 550°C results in superconducting films on Si substrates, while growth at lower temperatures results in nonsuper-conducting films. Lowering the growth temperature to 400°C is shown through ex situ TEM and Auger compositional profiling to minimize the reaction with the Si substrate while still activating the surface-migration processes needed to nucleate A15 V3Si. Variation of film composition about x = 0.25 is shown to result in nonsuperconducting films for high x and superconducting films with Tc approaching the bulk V value (5.4K) for low x. Finally, lowering the V0.75Si0.25 deposition rate is shown to raise Tc.
In Chapter 7, we present an electrical characterization of the first p-Si1-xGex/n-Si HIT diodes. Equilibrium band-bending calculations are used to predict qualitatively an enhancement of NDR as a function of Ge concentration in the Si1-xGex alloy layers. Experimentally, measurements of I-V curves from HIT diodes compared with a Si interband-tunnel diode confirm the enhancement effect. Finally, differentiation of the current-voltage relationship through the use of inelastic electron tunnelling spectroscopy (LETS) reveals previously unobserved phonon peaks in the HIT diodes. The possible origin of these features is discussed.
