E. S. Yang

Electronic processes at grain boundaries in polycrystalline semiconductors under optical illumination

The dependence of minority carrier lifetime (τ) on the doping concentration N<inf>d</inf>, grain size<tex>d</tex>and interface state density N<inf>is</inf>at the grain boundaries in (n-type) polycrystalline semiconductors has been calculated analytically. The recombination velocity at grain boundaries is enhanced by the diffusion potential V<inf>d</inf>adjacent to the boundaries, and ranges from<tex>\simeq 10^{2}</tex>to 10⁶cm . s^-1depending on N<inf>is</inf>and N<inf>d</inf>. Under illumination, the population of the interface states is altered considerably from its dark level and as a result, V<inf>d</inf>decreases to that value which maximizes recombination (equal concentrations of electrons and holes at the boundary). This causes τ to decrease with increasing N<inf>d</inf>. Sample calculations for polycrystalline silicon show that for low angle boundaries with interface state densities of<tex>\simeq 10^{11}</tex>cm^-2eV^-1, τ decreases from 10^-6to 10^-10s as the grain size is reduced from 1000 to 0.1 µm (for<tex>N_{d} = 10^{16}</tex>cm^-3). For a constant grain size, τ decreases with increasing N<inf>d</inf>. The open-circuit voltage of p-n junction solar cells decreases for<tex>\tau \leq 10^{-7}</tex>s, whereas that for Schottky barrier cells remains at its maximum value until<tex>\tau \lsim 10^{-8}</tex>s.

Effects of Ge concentration on SiGe oxidation behavior

The oxidation of SiGe layers grown by molecular beam epitaxy was studied. Auger depth profile showed that after oxidation, Ge was completely rejected from the oxide and Ge‐rich layers were formed. However, the Ge concentration in the SiGe layer was found to play an important role in the formation of these Ge‐Rich layers. For SiGe with Ge concentration below 50%, Si was preferentially oxidized and only one Ge‐rich layer was formed at the oxide/substrate interface. On the other hand, for SiGe with Ge concentration above 50%, two Ge‐rich layers were formed after oxidation with one at the oxide/substrate interface and the other at the oxide surface. X‐ray photoelectron spectroscopy studies showed that Ge at the oxide/substrate interface is in elemental form, while Ge at the oxide surface is in an intermediate oxidized state. A classical binary alloy oxidation theory is employed to explain the overall oxidation behavior qualitatively.

Negative capacitance at metal-semiconductor interfaces

A negative capacitance effect has been observed in metal‐semiconductor contacts. This phenomenon is explained by considering the loss of interface charge at occupied states below Fermi level due to impact ionization. A modified Shockley–Read treatment is proposed to interpret the experimental observations. In particular, a two‐energy‐level simplified model is presented to simulate the capacitance spectrum. The results are in good agreement with the experimental data.

MIS-Schottky theory under conditions of optical carrier generation in solar cells

The theory of MIS‐Schottky barriers and their electrical characteristics is examined for its application to solar cells. It is found that the interface behavior of contacts forward‐biased by illumination is qualitatively different from that of the same contacts biased in the dark by an applied forward voltage. Observed increases due to the interfacial layer in the open‐circuit voltage of the solar cell cannot therefore be associated with increases in the ’’n value’’ measured for the dark current, but rather are due to the different effects of this layer on the transport properties of majority and minority carriers. The theory predicts an optimum thickness for the interfacial layer above which the short‐circuit (minority‐carrier) current decreases, and the efficiency (fill factor) is degraded.

Interfacial reactions and Schottky barriers of Pt and Pd on epitaxial Si1−xGex alloys

The evolution of interfacial reactions during the deposition of Pt and Pd on epitaxial Si1−xGex alloys was studied using x‐ray photoelectron spectroscopy (XPS) for metal coverage up to 10 A. Auger electron depth profiling was performed on a thicker metal overlayer before and after in vacuo annealing to study the redistribution of composition in the reactions. We have found that Pt and Pd react mainly with Si to form silicides at 350 °C, leaving some Ge to segregate at the surface. These results were correlated with Schottky barrier height measurements. We found that the Schottky barrier heights of Pt/n‐Si0.8Ge0.2 and Pd/n‐Si0.8Ge0.2 are about the same, pinned at 0.68 eV, which is much smaller than those of n‐Si. These barrier heights are quite stable up to 550 °C.

Visual cortical activations on fMRI upon stimulation of the vision-implicated acupoints.

We used fMRI to reveal the visual cortical activations during conventional or electro-acupuncture over four vision-implicated acupoints in 18 healthy volunteers and compared the results with those obtained during direct visual stimulation. Positive activations were seen over the visual cortex during visual stimulation in all subjects, and similar activations were observed in 10 subjects during conventional acupuncture as well as in eight and seven subjects during electro-acupuncture at 2 and 20 Hz, respectively. Negative activations were also seen over the occipital lobes, temporal gyri and frontal gyri bilaterally in 13 subjects during conventional acupuncture. Thus, acupuncture may modulate the activity of relevant brain sites. Our results also suggest that electro-acupuncture is useful in future studies.

Optically induced electromagnetic radiation from semiconductor surfaces

Ultrafast electromagnetic radiation induced by a femtosecond laser beam from a semiconductor provides determination of the impurity doping concentration, carrier mobility, sign, and strength of the depletion field near the semiconductor surface.

Carrier transport across heterojunction interfaces

Abstract A general theory is presented to describe the carrier transport across heterojunction interfaces. In matching the boundary conditions at the interface, the conservation of total energy and perpendicular momentum is assumed and the difference of effective masses on two sides of the junction is taken into account. The quantum mechanical transmission coefficient is calculated by a combined numerical and WKB method. Application of the present model to an AlxGa1−xAsGaAs N-n heterojunction is performed and it gives rise to rectifying characteristics together with non-saturated reverse current. Comparison with the classical thermionic emission model is made to show the significance of tunneling and effect of quantum mechanical reflection.

On the low-temperature static and dynamic properties of high-performance silicon bipolar transistors

In a study performed over the temperature range of 400 to 77 K, Si bipolar transistors were found to have near-ideal characteristics at low temperatures with beta as high as 80 at 77 K. Detailed calculations indicate that the conventional theory of the temperature dependence of beta does not match the data. The discrepancy can be removed if it is assumed that a phenomenological thermal barrier to hole injection is present. Emitter-coupled logic (ECL) ring oscillators are functional at 85 K with no degradation in speed until about 165 K when compared to 358 K (85 degrees C). Calculations using a delay figure of merit indicate that f/sub T/, R/sub b/, and C/sub c/ are the delay components most affected by low-temperature operation. The feasibility of reduced logic swing operation of bipolar circuits at low temperatures is examined. It is found that successful ECL circuit operation at reduced logic swings is possible provided emitter resistance is kept small and can be used to enhance low-temperature power-delay performance. These data suggest that conventionally designed high-performance bipolar devices are suitable for the low-temperature environment. >

An fMRI study comparing brain activation between word generation and electrical stimulation of language-implicated acupoints

We compared the brain activation on functional magnetic resonance imaging (MRI) during word generation with the activation during electrical stimulation of two language‐implicated acupoints in 17 healthy, Mandarin‐speaking, Chinese male volunteers (age 19–26 years). All subjects were strongly right handed according to a handedness inventory. Using a standard functional MRI procedure and a word‐generation paradigm, significant activation was seen in the left and right inferior frontal gyri (BA 44, 45) as well as the left superior temporal gyrus (BA 22, 42). Stronger activation with a larger volume was seen in the left hemisphere. Electrical stimulation of either one of the two language‐implicated acupoints, SJ 8 (11 subjects) and Du 15 (6 subjects), without the word‐generation paradigm in the same cohort, produced significant activation in the right inferior frontal gyrus (BA 44, 46) and in the left and right superior temporal gyri (BA 22, 42), respectively. Nevertheless, no activation was seen in the left inferior frontal gyrus. In addition, electrical stimulation of the adjacent non‐acupoints did not produce any significant brain activation. Although our results support the notion of acupoint–brain activation, applying acupuncture at SJ 8 or Du 15 does not activate the typical language areas in the left inferior frontal cortex. Hum. Brain Mapping 18:233–238, 2003.