Theodore I. Kamins

Strong quantum-confined Stark effect in germanium quantum-well structures on silicon

Silicon is the dominant semiconductor for electronics, but there is now a growing need to integrate such components with optoelectronics for telecommunications and computer interconnections. Silicon-based optical modulators have recently been successfully demonstrated; but because the light modulation mechanisms in silicon are relatively weak, long (for example, several millimetres) devices or sophisticated high-quality-factor resonators have been necessary. Thin quantum-well structures made from III-V semiconductors such as GaAs, InP and their alloys exhibit the much stronger quantum-confined Stark effect (QCSE) mechanism, which allows modulator structures with only micrometres of optical path length. Such III-V materials are unfortunately difficult to integrate with silicon electronic devices. Germanium is routinely integrated with silicon in electronics, but previous silicon–germanium structures have also not shown strong modulation effects. Here we report the discovery of the QCSE, at room temperature, in thin germanium quantum-well structures grown on silicon. The QCSE here has strengths comparable to that in III-V materials. Its clarity and strength are particularly surprising because germanium is an indirect gap semiconductor; such semiconductors often display much weaker optical effects than direct gap materials (such as the III-V materials typically used for optoelectronics). This discovery is very promising for small, high-speed, low-power optical output devices fully compatible with silicon electronics manufacture.

Polycrystalline silicon for integrated circuit applications

1 Deposition.- 1.1 Introduction..- 1.2 Thermodynamics and kinetics.- 1.3 The deposition process.- 1.4 Gas-phase and surface processes.- 1.4.1 Convection.- 1.4.2 The boundary layer.- 1.4.3 Diffusion through the boundary layer.- 1.4.4 Reaction.- 1.4.5 Steady state.- 1.5 Reactor geometries.- 1.5.1 Low-pressure, hot-wall reactors.- 1.5.2 Atmospheric-pressure, cold-wall reactor.- 1.6 Reaction.- 1.6.1 Decomposition of silane.- 1.6.2 Surface adsorption.- 1.6.3 Deposition rate.- 1.6.4 Rate-limiting step.- 1.7 Deposition of doped films.- 1.7.1 n-type deposited films.- 1.7.2 p-type deposited films.- 1.7.3 Electrostatic model.- 1.8 Step coverage.- 1.9 Enhanced deposition techniques.- 1.10 Summary.- 2 Structure.- 2.1 Nucleation.- 2.1.1 Amorphous surfaces.- 2.1.2 Single-crystal surfaces.- 2.2 Surface diffusion and structure.- 2.2.1 Subsurface rearrangement.- 2.3 Evaluation techniques.- 2.4 Grain structure.- 2.5 Grain orientation.- 2.6 Optical properties.- 2.6.1 Index of refraction.- 2.6.2 Absorption coefficient.- 2.6.3 Ultraviolet surface reflectance.- 2.6.4 Use of optical properties for film evaluation.- 2.7 Etch rate.- 2.8 Stress.- 2.9 Thermal conductivity.- 2.10 Structural stability.- 2.10.1 Recrystallization mechanisms.- 2.10.2 Undoped or lightly doped films.- 2.10.3 Heavily doped films.- 2.10.4 Implant channeling.- 2.10.5 Amorphous films.- 2.11 Epitaxial realignment.- 2.12 Summary.- 3 Dopant Diffusion and Segregation.- 3.1 Introduction.- 3.2 Diffusion mechanism.- 3.2.1 Diffusion along a grain boundary.- 3.2.2 Diffusion in polycrystalline material.- 3.3 Diffusion in polysilicon.- 3.3.1 Arsenic diffusion.- 3.3.2 Phosphorus diffusion.- 3.3.3 Antimony diffusion.- 3.3.4 Boron diffusion.- 3.3.5 Limits of applicability.- 3.4 Diffusion from polysilicon.- 3.5 Interaction with metals.- 3.5.1 Aluminum.- 3.5.2 Other metals and silicides.- 3.6 Dopant segregation at grain boundaries.- 3.6.1 Theory of segregation.- 3.6.2 Experimental data.- 3.7 Summary.- 4 Oxidation.- 4.1 Introduction.- 4.2 Oxide growth on polysilicon.- 4.2.1 Oxidation of undoped films.- 4.2.2 Oxidation of doped films.- 4.2.3 Effect of grain boundaries.- 4.2.4 Effects of device geometry.- 4.2.5 Oxide-thickness evaluation.- 4.3 Conduction through oxide on polysilicon.- 4.3.1 Interface features.- 4.3.2 Deposition conditions.- 4.3.3 Oxidation conditions.- 4.3.4 Dopant concentration and annealing.- 4.3.5 Carrier trapping.- 4.4 Summary.- 5 Electrical Properties.- 5.1 Introduction.- 5.2 Undoped polysilicon.- 5.3 Moderately doped polysilicon.- 5.3.1 Carrier trapping at grain boundaries.- 5.3.2 Carrier transport.- 5.3.3 Trap concentration and energy distribution.- 5.3.4 Thermionic field emission.- 5.3.5 Grain-boundary barriers.- 5.3.6 Limitations of models.- 5.3.7 Segregation and trapping.- 5.3.8 Summary.- 5.4 Grain-boundary modification.- 5.5 Heavily doped polysilicon films.- 5.5.1 Solid solubility.- 5.5.2 Method of doping.- 5.5.3 Stability.- 5.5.4 Mobility.- 5.5.5 Future trends.- 5.6 Minority-carrier properties.- 5.6.1 Lifetime.- 5.6.2 Switching characteristics.- 5.7 Summary.- 6 Applications.- 6.1 Introduction.- 6.2 Silicon-gate technology.- 6.2.1 Threshold voltage.- 6.2.2 Polysilicon interconnections.- 6.2.3 Process compatibility.- 6.2.4 New structures.- 6.2.5 Gettering.- 6.2.6 Gate-oxide reliability.- 6.3 Nonvolatile memories.- 6.4 High-value resistors.- 6.5 Fusible links.- 6.6 Polysilicon contacts.- 6.6.1 Reduction of junction spiking.- 6.6.2 Diffusion from polysilicon.- 6.7 Bipolar integrated circuits.- 6.7.1 Vertical npn bipolar transistors.- 6.7.2 Lateral pnp bipolar transistors.- 6.8 Device isolation.- 6.8.1 Dielectric isolation.- 6.8.2 Epi-poly isolation.- 6.8.3 Trench isolation.- 6.8.4 Summary.- 6.9 Trench capacitors.- 6.10 Polysilicon diodes.- 6.11 Polysilicon transistors.- 6.12 Polysilicon sensors.- 6.13 Summary.

Hall Mobility in Chemically Deposited Polycrystalline Silicon

Hall‐mobility measurements have been performed on polycrystalline silicon films deposited on a silicon oxide surface by the thermal decomposition of silane. Samples with doping impurities added during deposition or by diffusion from a doped vapor‐deposited oxide showed similar behavior. For both n‐type and p‐type samples approximately 5 μ thick, the mobility reached a maximum value of about 40 cm2/V sec at a free carrier concentration of about 1018 cm−3 and decreased for both higher and lower carrier concentrations. The observed Hall mobility was generally higher in p‐type samples than in n‐type samples. The decrease in observed mobility with decreasing carrier concentration is attributed to the effects of high resistivity space‐charge regions surrounding grain boundaries in the polycrystalline material. The mobility was seen to increase as the film thickness increased for samples with similar doping, indicating a more ordered structure in thicker films.

Dopant segregation in polycrystalline silicon

Dopant segregation at grain boundaries in polycrystalline silicon has been investigated. Arsenic, phosphorus, and boron were ion implanted into low‐pressure, chemically‐vapor‐deposited polycrystalline‐silicon films. All films were then annealed at 1000 °C for 1 h, and some were subsequently further annealed at 800, 850, or 900 °C for 64, 24, or 12 h, respectively. For phosphorus and arsenic the room‐temperature resistivity of the films was found to be higher after annealing at lower temperatures. By successively annealing the same sample at lower and higher temperatures, the resistivity would repeatedly increase and decrease, indicating reversible dopant segregation at the grain boundaries. Hall measurements were used to estimate the number of active dopant atoms within the grains and the number of atoms segregated at the grain boundaries as a function of annealing temperature. A theory of segregation in systems of small particles has been developed. Using this theory, the heat of segregation of arsenic and phosphorus in polycrystalline silicon was calculated. For boron no appreciable segregation was observed.

Ti-catalyzed Si nanowires by chemical vapor deposition: Microscopy and growth mechanisms

Si nanowires grow rapidly by chemical vapor deposition on Ti-containing islands on Si surfaces when an abundant supply of Si-containing gaseous precursor is available. The density of wires is approximately the same as the density of the nucleating islands on the Si surface, although at least two different types of islands appear to correlate with very different wire growth rates. For the deposition conditions used, a minority of long, defect-free wires form, along with more numerous wires containing defects. Energy-dispersive x-ray spectroscopy shows that the Ti-containing nanoparticles remain at the tip of the growing wires. The estimated diffusion coefficient of Si in TiSi2 is consistent with the catalyzing nanoparticle remaining in the solid phase during nanowire growth.

DEPOSITION OF THREE-DIMENSIONAL GE ISLANDS ON SI(001) BY CHEMICAL VAPOR DEPOSITION AT ATMOSPHERIC AND REDUCED PRESSURES

This report summarizes observations of Ge island formation during growth on Si(001) by chemical vapor deposition from germane in the pressure range from 10 Torr to atmospheric pressure in a conventional epitaxial reactor. A four-step growth process is observed: (1) uniform pseudomorphic overlayer (“wetting’’ layer) formation; (2) three-dimensional island growth with a constant aspect ratio; (3) continued island growth with a constant diameter and increasing height; (4) rapid growth of larger, faceted islands. Ostwald ripening of the islands during continued heat treatment after terminating the deposition is slow compared to island formation and growth during deposition for the experimental conditions used.

Lithographic positioning of self-assembled Ge islands on Si(001)

Ge islands were deposited on Si(001) partially covered with patterned oxide. Selective Si was deposited on some wafers before Ge deposition to form raised Si(001) plateaus with well-defined sidewall facets. On narrow lines, the Ge islands locate preferentially at the edges of the raised Si(001) regions, and the preference is strongest on the narrowest patterns aligned along a 〈100〉 direction. For a 450 nm wide plateau aligned in this direction, all the islands are positioned along the edges of the pattern, with a 300 nm space near the center of the pattern free of Ge islands. The islands appear to be uniformly spaced along the pattern edges. On wider lines, several rows of islands are aligned near the edges of the pattern, with the order decreasing farther from the edge.

EVOLUTION OF GE ISLANDS ON SI(001) DURING ANNEALING

The evolution of the shape and size distributions of Ge islands on Si(001) during annealing after deposition has been studied at different temperatures and effective coverages. The initial distributions of square-based pyramids, elongated “hut” structures, faceted “dome-shaped” islands, and much larger “superdomes” depends on the deposition conditions. During annealing after deposition, the islands coarsen over a limited range of times and temperatures. Those pyramidal-shaped islands that grow transform to faceted, dome-shaped islands as they become larger. Initially dome-shaped islands that dissolve transform to a pyramidal shape as they become smaller during the process of dissolving. Outside of this coarsening regime, the islands can achieve a relatively stable, steady-state configuration, especially at lower temperatures. At higher temperatures, intermixing of Si into the Ge islands dominates, decreasing the strain energy and allowing larger islands to form. At lower and intermediate temperatures, the...

Photovoltaic retinal prosthesis with high pixel density

Retinal degenerative diseases lead to blindness due to loss of the “image capturing” photoreceptors, while neurons in the “image processing” inner retinal layers are relatively well preserved. Electronic retinal prostheses seek to restore sight by electrically stimulating surviving neurons. Most implants are powered through inductive coils, requiring complex surgical methods to implant the coil-decoder-cable-array systems, which deliver energy to stimulating electrodes via intraocular cables. We present a photovoltaic subretinal prosthesis, in which silicon photodiodes in each pixel receive power and data directly through pulsed near-infrared illumination and electrically stimulate neurons. Stimulation was produced in normal and degenerate rat retinas, with pulse durations from 0.5 to 4 ms, and threshold peak irradiances from 0.2 to 10 mW/mm2, two orders of magnitude below the ocular safety limit. Neural responses were elicited by illuminating a single 70 μm bipolar pixel, demonstrating the possibility of a fully-integrated photovoltaic retinal prosthesis with high pixel density.

Hydrogenation of transistors fabricated in polycrystalline-silicon films

Transistors have been fabricated with their active channels in thin films of polycrystalline silicon. A subsequent hydrogen plasma treatment has been used to improve the transistor properties significantly by reducing the number of electrically active grain-boundary defects. Plasma conditions to maximize the hydrogenation effect have been briefly investigated.