Chris M. Gronet

Limited reaction processing: Silicon epitaxy

We introduce a new technique, limited reaction processing, in which radiant heating is used to provide rapid, precise changes in the temperature of a substrate to control surface reactions. This process was used to fabricate thin layers of high quality epitaxial silicon. Abrupt transitions in doping concentration at the epitaxial layer/substrate interface were achieved for undoped films deposited on heavily doped substrates.

Si/Si/sub 1-x/Ge/sub x/ heterojunction bipolar transistors produced by limited reaction processing

Si/Si/sub 1-x/Ge/sub x/ heterojunction transistors (HBTs) fabricated by a chemical vapor deposition (CVD) technique are reported. A rapid thermal CVD limited-reaction processing (LRP) technique was used for the in situ growth of all three device layers, including a 20-mm Si/sub 1-x/Ge/sub x/ layer in the base. The highest current gains observed ( beta =400) were for a Si/Si/sub 1-x/Ge/sub x/ HBT with a base doping of 7*10/sup 18/ cm/sup -3/ near the junction and a shallow arsenic implant to form ohmic contacts and increase current gain. Ideal base currents were observed for over six decades of current and the collector current remained ideal for nearly nine current decades starting at 1 pA. The bandgap difference between a p-type Si layer doped to 5*10/sup 17/ cm/sup -3/ and the Si/sub 1-x/Ge/sub x/(x=0.31) base measured 0.27 eV. This value was deduced from the measurements of the temperature dependence of the base current and is in good agreement with published calculations for strained Si/sub 1-x/Ge/sub x/ layers on Si.<<ETX>>

A 14% efficient nonaqueous semiconductor/liquid junction solar cell

We describe the most efficient semiconductor/liquid junction solar cell reported to date. Under W‐halogen (ELH) illumination, the device is a 14% efficient two‐electrode solar cell fabricated from an n‐type silicon photoanode in contact with a nonaqueous electrolyte solution. The cell′s central feature is an ultrathin electrolyte layer which simultaneously reduces losses which result from electrode polarization, electrolyte light absorption, and electrolyte resistance. The thin electrolyte layer also eliminates the need for forced convection of the redox couple and allows for precise control over the amount of water (and other electrolyte impurities) exposed to the semiconductor. After one month of continuous operation under ELH light at 100 mW/cm^2, which corresponds to the passage of over 70 000 C/cm^2, thin‐layer cells retained over 90% of their efficiency. In addition, when made with Wacker Silso cast polycrystalline Si, cells yield an efficiency of 9.8% under simulated AMl illumination. The thin‐layer cells employ no external compensation yet surpass their corresponding experimental (three‐electrode) predecessors in efficiency.

Growth of GeSi/Si strained‐layer superlattices using limited reaction processing

SiGe/Si superlattices were grown using limited reaction processing. Each multilayer structure was fabricated in situ by changing the gas composition between high‐temperature cycles. Commensurate SiGe alloy layers as thin as 15 nm were reproducibly deposited and were examined using transmission electron microscopy, sputtering Auger electron spectroscopy, and Rutherford backscattering. Si/SiGe interfaces are abrupt to within a few monolayers, establishing for the first time the use of a chemical vapor deposition technique to fabricate abrupt GeSi/Si‐based heterostructures.

Small-geometry, high-performance, Si-Si/sub 1-x/Ge/sub x/ heterojunction bipolar transistors

Si-Si/sub 1-x/Ge/sub x/ heterojunction bipolar transistors (HBTs) with very heavily doped bases, fabricated using electron-beam lithography to obtain very small feature sizes, are discussed. Emitter, base, and collector epitaxial layers were grown in situ in a lamp-heated, chemical-vapor-deposition reactor. Transistors with common-emitter current gain of approximately 50 and f/sub t/ of about 28 GHz have been obtained. Analysis indicates that the frequency response is limited by parasitic resistances and capacitances in the simple demonstration structure used, rather than by the intrinsic device characteristics. Simple ring oscillators have been fabricated using HBTs in the inverse-active mode of operation.<<ETX>>

Limited reaction processing: Growth of Si1−xGex/Si for heterojunction bipolar transistor applications

Abstract Limited reaction processing (LRP) of silicon-based materials is reviewed as an alternative growth method to molecular beam epitaxy (MBE). LRP is a chemical vapor deposition technique which uses wafer temperature, rather than gas flow switching, to initiate and terminate growth. Processing takes place within a cold-wall, quartz reaction chamber, and gases are changed between successive lamp-heated growth cycles. In addition to minimizing thermal exposure, the technique allows individual layers in a multi-layer structure to be deposited at their optimum growth temperature. LRP is particularly well suited to the growth and processing of metastable layers such as strained Si 1− x Ge x on silicon. Several properties of LRP-grown Si 1− x Ge x are shown to be similar to those reported for MBE material, including qualitative islanding behavior and quantitative measurement of the onset of misfit dislocation formation. However, a direct comparison of thermal stability reveals larger numbers of misfit dislocations in MBE-grown films upon annealing. The electrical behavior of misfit dislocations in heterojunction diodes, and the growth and analysis of high-quality Si/Si 1− x Ge x /Si heterojunction bipolar transistors are also discussed.

Thin, highly doped layers of epitaxial silicon deposited by limited reaction processing

Limited reaction processing was used to deposit ultrathin, highly doped layers of epitaxial silicon. Multilayer structures consisting of alternating undoped and heavily boron‐doped regions were fabricated in situ. The interlayer doping profiles of these structures, as determined by secondary ion mass spectroscopy, are abrupt. Van der Pauw measurements indicate that the electrical characteristics of the p+ epitaxial films are comparable to bulk material.

Electrical and material quality of Si/sub 1-x/Ge/sub x//Si p-N heterojunctions produced by limited reaction processing

Si/sub 1-x/Ge/sub x//Si p-N heterojunctions prepared by a chemical vapor deposition technique, limited reaction processing (LRP) were characterized using DC electrical measurements, transmission electron microscopy (TEM), and X-ray topography. Heterojunctions with Si/sub 1-x/Ge/sub x/ layer thickness ranging from 52 to 295 nm and a constant Ge fraction of 23% were fabricated to study the effect of increasing the number of misfit dislocations on the device characteristics. Devices with the thinnest layers (<or=120 nm) display forward characteristics with ideality factors of 1.01 and reverse leakage current densities of less than 4 nA/cm for a 5-V reverse bias. These thin-layer devices have dislocation spacings greater than 10 mu m. Devices utilizing Si/sub 1-x/Ge/sub x/ layers thicker than 200 nm have forward characteristics which clearly display the presence of recombination currents, and reverse leakage current densities greater than 290 nA/cm/sup 2/ at -5 V. The dislocation spacing in these devices is less than 1 mu m. Ideal characteristics were found at room temperature in devices known to contain dislocations.<<ETX>>

Minority‐carrier properties of thin epitaxial silicon films fabricated by limited reaction processing

Generation lifetimes and diode properties have been measured in epitaxial silicon films grown by limited reaction processing. Generation lifetimes from 1.4 to 94  μs were measured by observing the recovery of MOS capacitors from deep depletion. Planar diodes fabricated in both n‐ and p‐type epitaxial films show excellent behavior in both forward and reverse bias. p‐n junctions formed by growing p‐type epitaxial silicon directly on an n‐type substrate show no evidence of excessive interface defects or traps.

n-type GaAs photoanodes in acetonitrile: Design of a 10. 0% efficient photoelectrode

n-type GaAs semiconductor/liquid junctions have been studied in acetonitrile (ACN) solvent with the ferrocene/ferricenium redox couple. Previously reported inefficiencies in this system are demonstrated to be due to bulk electron-hole recombination and not to recombination at the junction. Increases in minority-carrier collection length lead to increases in short circuit current of the n-GaAs/ferrocene/ferricenium cell in ACN, with photocurrent densities in excess of 21 mA/cm^2 at 88 mW/cm^2 of ELH-type tungsten-halogen irradiation. Properly prepared n-GaAs samples yield photoelectrode efficiencies of 10.0%±0.5% for conversion of natural sunlight (65 mW/cm^2) to electricity, with open circuit voltages Voc of 0.70–0.72 V, short circuit currents of 16–17 mA/cm^2, and fill factors of 0.52–0.56, when measured relative to the potential of a reversible reference electrode in the same solvent/redox couple/electrolyte solution.