K. Nauka

Phase-change recording medium that enables ultrahigh-density electron-beam data storage

An ultrahigh-density electron-beam-based data storage medium is described that consists of a diode formed by growing an InSe/GaSe phase-change bilayer film epitaxially on silicon. Bits are recorded as amorphous regions in the InSe layer and are detected via the current induced in the diode by a scanned electron beam. This signal current is modulated by differences in the electrical properties of the amorphous and crystalline states. The success of this recording scheme results from the remarkable ability of layered III-VI materials, such as InSe, to maintain useful electrical properties at their surfaces after repeated cycles of amorphization and recrystallization.

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.

Structural and optical properties of pseudomorphic InxGa1−xN alloys

Thick (225 nm) InxGa1−xN layers, grown on 5 μm thick GaN, were found by x-ray diffraction (XRD) measurements to be pseudomorphic up to x=0.114. Transmission electron microscopy showed that no misfit or additional threading dislocations were created at the InxGa1−xN/GaN interface. Composition of the overlayers was determined by Rutherford backscattering spectrometry and correlated to both the a and c lattice constants from XRD. It was found that Vegard’s law is applicable at these compositions, if the biaxial strain is included. Biaxial strain must also be considered to accurately determine the bowing parameter as shown by optical transmission measurements.

Junction leakage in titanium self‐aligned silicide devices

Successful utilization of a titanium self‐aligned silicide (salicide) process for reproducible device fabrication with high yield requires junction leakage due to the silicide process to be minimized. The microstructure and microchemistry of titanium salicide shallow junction diodes were studied and correlated with junction leakage. The direct correlation between junction leakage and junction structure was established by using several analytical techniques. The main cause of large leakage current was found to be a loss of p+/n junction under the titanium silicide layer and formation of titanium silicide/n‐silicon Schottky barrier contact at the perimeter of the diodes. Process parameters for low leakage titanium silicide/p+/n diode fabrication were also established.

Role of ions in electron cyclotron resonance plasma‐enhanced chemical vapor deposition of silicon dioxide

Silicon dioxide films were deposited in an electron cyclotron resonance plasma onto substrates at temperatures ranging from 65 to 200 °C. In order to determine the effect of ions on the deposition, both ion flux and ion energy were investigated. The ratio of ion flux to deposition flux was found to be a significant parameter and this ‘‘flux ratio’’ was varied between 8 and 100. Mean ion energy was investigated at 10, 50, and 75 eV. The silicon dioxide films were characterized by measurement of wet etch rate, density, composition, and stress, and by infrared spectroscopy. It was found that above a flux ratio of about 20, high‐quality SiO2 was deposited whether or not the substrate was thermally floating or at 65–200 °C, indicating that the flux ratio was dominant over the temperature. Use of radio‐frequency bias to increase the mean ion energy to 50 eV or above was effective in producing high‐quality SiO2 when the flux ratio was below 20, but not as effective as using a high flux ratio. Thus high ion flux and low ion energy were found to be useful in producing SiO2 at temperatures as low as 65 °C with properties close to that of thermal silicon dioxide. The role of ions in the deposition process was found to be densification, removal of –OH (or hydrogen) and alteration of Si–O bonding.

Analysis of wafer fusing for 1.3 μm vertical cavity surface emitting lasers

We report low densities of electrically active defects and low optical losses at the wafer fused interface between InP and GaAs. Electron beam induced current analysis shows electrically active defects with an average spacing of 4.5 μm at the interface and significantly lower densities 0.4 μm from the fused interface. Optical measurements of a Fabry–Perot resonator made by fusing an InP epilayer to a GaAs/AlAs mirror demonstrate a 3% increase in mirror transmission after fusing and negligible absorption at the fused interface. Based on these results, we present design considerations for fused surface emitting lasers.

Admittance spectroscopy measurements of band offsets in Si/Si1−xGex/Si heterostructures

Admittance spectroscopy has been used to measure conduction‐ and valence‐band discontinuities in Si/Si1−xGex heterojunctions (0<x<0.45). Most of the band‐gap discontinuity was in the valence band. The measured valence‐band offset increased with increasing Ge concentration in the strained Si1−xGex films, and it decreased when the Si1−xGex layers started to relax. These results indicate that admittance spectroscopy can be used to monitor the electronic properties of transistorlike Si/Si1−xGex/Si heterostructures.

Surface Photovoltage Measurement of Hydrogen‐Treated Si Surfaces

A contactless surface photovoltage technique was used to measure surface-potential barriers resulting from hydrogen termination of silicon surfaces and their evolution as the surfaces gradually oxidized in air at room temperature. Hydrogen termination formed by annealing in a hydrogen ambient was more complete than passivation formed by aqueous HF treatment, In addition, the surface-potential barrier depended on the wafer orientation and ordering of surface terraces by high temperature hydrogen annealing. Oxides grown at room temperature exhibited higher surface-potential barriers than oxides grown at elevated temperatures.

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

The DC performance of Si/Si/sub 1-x/Ge heterojunction bipolar transistors (HBTs) fabricated from epitaxial layers grown by limited reaction processing is presented. The highest gain ( approximately=400) device has a 20-nm, 31%-Ge base heavily doped with boron to a level of 7*10/sup 18/ cm/sup -3/. Measurements of the collector current as a function of temperature yield values of the valence band discontinuity, Delta E/sub v/, for four different Ge compositions. The dependence of Delta E/sub v/ on Si/sub 1-x/Ge/sub x/ layer thickness was also measured and found to decrease as strain relaxation occurred.<<ETX>>