Wen-Chung Tsai

PERSISTENT PHOTOCONDUCTIVITY IN SIGE/SI QUANTUM WELLS

Persistent photoconductivity (PPC) has been observed in boron-doped Si1−xGex/Si quantum wells. The decay kinetics of the PPC effect can be well described by a stretched-exponential function, Ippc(t)=Ippc(0)exp[−(t/τ)β](0<β<1), which is usually observed in many disorder materials. Through the studies of the PPC effect under various conditions, such as different temperature, different photon energy of photoexcitation, and different Ge content, we identify that the alloy potential fluctuations induced by compositional disorder are the origin of the PPC effect in Si1−xGex/Si quantum wells.

Pulsed KrF laser annealing of Ni/Si0.76Ge0.24 films

Pulsed KrF laser annealing can suppress the island structure formation and Ge segregation associated with the interfacial reactions of Ni/Si0.76Ge0.24. For the Ni/Si0.76Ge0.24 films annealed at an energy density of 0.1–0.3 J/cm2 nickel germanosilicide associated with the amorphous overlayer was formed, while at energy densities above 0.4 J/cm2 cellular structures of Ge-deficient Si1−xGex islands surrounded by Ni(Si1−xGex)2 due to the constitutional supercooling occurred. For the continuous Ni(Si1−xGex) films grown at 200 °C, subsequent laser annealing at a higher energy density of 0.6–1.0 J/cm2 caused transformation into homogeneous Ni(Si0.76Ge0.24)2 films without island structure and Ge deficiency which readily appeared on furnace annealing at temperatures above 400 °C. At energy densities above 1.6 J/cm2 the same cellular structures as described above were also noted.

Low-Temperature Epitaxial Growth of Silicon and Silicon-Germanium Alloy by Ultrahigh-Vacuum Chemical Vapor Deposition

Low-temperature epitaxy of silicon and silicon-germanium alloy via an ultrahigh-vacuum chemical vapor deposition system was investigated. Bistable conditions were observed for silicon epitaxial growth performed within the temperature range of 550° C to 800° C. The activation energy of the SiGe growth rate was found to decrease as the germanium composition increased. The germanium atomic molar fraction in these epitaxial layers was tightly controlled by a computer-controlled gas source switching system. Si/SiGe superlattice structures of 20-period 5 nm Si/12 nm SiGe layers were grown to demonstrate the excellent controllability of this growth technique.

Interfacial reactions of the Co/Si1−xGex system

Abstract Thermal reactions of Co(200)/Si0.76Ge0.24(1500)/Si and Co(200)/Si0.54Ge0.46(1000)/Si systems in a vacuum of 1−2×10−6 Torr were studied. At temperatures above 200 °C Ge segregation appeared even though no silicides and/or germanosilicides were formed. At a temperature of 225–550 °C Co(Si1 − yGey) was formed, in which the Ge concentration was deficient. The formation temperatures of CoSi2 in the Co/Si1 − xGex systems, where x = 0.24 and 0.46, were above 575 °C, being relatively higher than that in the Co/Si system. At temperatures above 500 °C the island structure, Ge segregation to the surface of the exposed Si1 − xGex films, and the penetration of reacted layer into the Si substrate occurred. At temperatures above 700 °C a SiC layer was grown on the film surface. For the Si0.54Ge0.46 films the penetration of the reacted layer into the Si substrate occurred even at 350 °C owing to the wave structure of the as-grown Si0.54Ge0.46 films. A Si layer interposed between Co and Si0.76Ge0.24 films is an effective scheme to grow a continuous CoSi2 contact at 550–600 °C without inducing Ge segregation and hence the strain relaxation in the Si0.76Ge0.24 films.

Interfacial reactions of Ni on Si0.76Ge0.24 and Si by pulsed laser annealing

Pulsed KrF laser annealing and vacuum annealing on the interfacial reactions of Ni/Si 0.76 Ge 0.24 and Ni/Si were studied. For the Ni/ Si 0 76 Ge 0 24 films annealed at temperatures above 300°C, some Ge-rich Si 1 x Ge x grains were formed between the Ge-deficient Ni germanosilicide grains. resulting in the island structure. For Ni/Si films homogeneous epitaxial NiSi 2 films could be grown even at 600°C. Ni silicide ( germanosilicide) associated with the amorphous overlayer was generally formed at lower energy densities for Ni/Si, NiSi/Si. Ni/Si 0.76 Ge 0.24 4 and Ni(Si 1-x Ge x )/Si 0.76 Ge 0.24 systems, respectively. At higher energy densities constitutional supercooling occurred. The energy densities at which constitutional supercooling appeared were higher for NiSi and Ni(Si 1 x Ge x ) than for Ni. For the continuous Ni(Si 1 -x Ge x ) films grown at 200°C in a vacuum furnace, subsequent laser annealing at an energy density of 0.6-1.0 J cm 2 have shown to render homogeneous Ni ( Si 0 76 Ge 0 24 ) 2 and Si 0 76 Ge 0 24 films without the island structure and Ge segregation.

NANOMETER THICK SI/SIGE STRAINED-LAYER SUPERLATTICES GROWN BY AN ULTRAHIGH-VACUUM CHEMICAL-VAPOR-DEPOSITION TECHNIQUE

High quality Si/Si1−xGex superlattices having layers as thin as 1.5 nm have been grown by an ultrahigh vacuum/chemical vapor deposition system. High‐resolution double‐crystal x‐ray diffraction, and conventional and high‐resolution cross‐sectional transmission electron microscopy were used to evaluate the crystalline quality of these superlattices. A dynamical x‐ray simulation program was employed to analyze the experimental rocking curves. Excellent matches between experimental rocking curves and simulated ones were obtained for all superlattices with various periodicity. A cross‐sectional transmission electron micrograph of an 80 period Si(4.2 nm)/Si0.878Ge0.122 (1.5 nm) superlattice, in which each individual layers was clearly resolved, demonstrated the capability of this growth technique for nanometer thick layer deposition.

Characterization of the Si/SiGe heterojunction diode grown by ultrahigh vacuum chemical vapor deposition

The unipolar Si/SiGe heterojunction diode grown by ultrahigh vacuum chemical vapor deposition at 550 °C is demonstrated. The dark current density measured at 77 K is (2.5±0.1)×10−7 A/cm2 for the barrier height of 176±8 meV, at a reverse bias of 1 V. The barrier heights are measured from the activation analysis of the saturation current and compared to the theoretical values. The barrier height decreases as the thickness of the SiGe strained layer exceeds the critical thickness.

Light emission from the porous boron δ-doped Si superlattice

We report the first study on the porous boron δ-doped Si superlattice. Visible photoluminescence (PL) was observed with multiple peaks from the porous boron δ-doped Si superlattice at room temperature. In the electroluminescence (EL) experiment, a bright yellow light emission was observed from the porous boron δ-doped Si superlattices. However, a weak red light emission was also observed from the conventional porous Si which is anodized at the same etching condition. As a result, the structure of the porous boron δ-doped Si superlattice has the ability of controlling the quantum size in porous Si and enhancing the light intensity from porous Si.

Quantum confinement effects of Si/SiGe strained-layer superlattices grown by an ultrahigh vacuum/chemical vapor deposition technique

Well-resolved band-edge luminescence is observed for Si0.86Ge0.14/Si strained-layer superlattices grown by an ultrahigh vacuum/chemical vapour deposition technique at 550 ‡C. High-resolution double-crystal X-ray diffraction (HRXRD) and cross-sectional transmission electron microscopy (XTEM) were used to determine the strain and other parameters for these strained-layer superlattices. Quantum confinement is observed for a SiGe well as thin as 1.3 nm. The blue shift of the emission peaks with decreasing well width is found to be in good agreement with theoretical calculation.

Local electric field effects in a SiGe quantum well investigated by photoluminescence

Abstract We describe photoluminescence and admittance spectroscopy of p-type Si/Si 0.75 Ge 0.25 /Si quantum-well structures with the SiGe quantum well surrounded by undoped Si spacer layers of various thickness. Holes confined in the SiGe quantum well create a local electric field, which induces potential barriers for holes in the surrounding Si, and a potential well for electrons in the vicinity of the SiGe quantum-well region. Decreasing the thickness of one of the Si spacers from 30 nm to 5 nm increases the local electric field and shifts the SiGe-related near-band-edge photoluminescence spectrum to higher photon energies. This can be explained by a reduced exciton binding energy due to exciton polarization. The polarization is caused by the increasingly asymmetrical potential well for electrons and holes for the thinner Si spacer layers. In addition, admittance spectroscopy was carried out in order to measure the potential barriers for the confined holes for various thicknesses of the Si spacer layers. For thicker Si spacer layers, the results are in agreement with the photoluminescence data. For thinner Si spacer layers, thermally activated tunnelling of holes via the potential barrier was observed. Our interpretations are supported by theoretical calculations.