J. E. Turner

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>>

Effects of strain on boron diffusion in Si and Si1−xGex

Boron diffusion in in situ doped Si1−xGex and Si, subjected to inert‐ambient furnace annealing at 800 °C, was investigated. For Si1−xGex, the effect of biaxial compressive and tensile strain on boron diffusion was studied using Si1−xGex layers with a constant Ge content (x≊0.10 and x≊0.20) grown epitaxially on various relaxed Si1−yGey (0≤y≤0.20) substrates. Boron diffusion is primarily a function of Ge content, x in the Si1−xGex layers, and does not show a strong dependence on macroscopic biaxial strain. For Si, the effect of biaxial tension was investigated using relaxed Si1−yGey layers as substrate templates for epitaxial Si layers. As in Si1−xGex, boron diffusion in Si does not depend strongly on biaxial strain.

Comparison of boron diffusion in Si and strained Si1−xGex epitaxial layers

We have investigated boron diffusion in Si and strained Si1−xGex, in situ doped, epitaxial layers. During inert ambient annealing at 860 °C, boron diffusion is observed to be slower in Si0.83Ge0.17 than in Si for boron concentration levels between 5×1016 and 2.5×1019 cm−3. Computer simulations of the measured boron profiles for annealed samples indicate that the effective boron diffusivity Deff in Si0.83Ge0.17 is approximately an order of magnitude lower than that in Si. This disparity is found to increase with increasing boron concentration.

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.

Composition non-uniformities in selective area growth of GaInAs on InP grown by OMVPE

Selective area epitaxial growth of Ga0.47In0.53As on InP substrates patterned with silicon nitride was done by low pressure organometallic vapor phase epitaxy. Good surface morphology and clean side walls of the epitaxial layers were obtained in most of the areas of selective GalnAs growth. However, both GaAs incorporation and InAs incorporation increased near the edges of the selective growth areas due to the extra flux of Gacontaining and In-containing species migrating on the surface of silicon nitride. The increase in InAs incorporation was found at a higher rate when the adjacent silicon nitride area was large, hence, cross-hatching appeared near the edges. A characteristic length of adjacent silicon nitride seemed to be connected with the enhanced InAs incorporation, which was about 40μm at 600°. The non-uniformities in composition appeared in all wafers grown in the temperature range between 570 and 650°.

A high-speed bipolar technology featuring self-aligned single-poly base and submicrometer emitter contacts

An experimental bipolar transistor structure with self-aligned base-emitter contacts formed using one polysilicon layer is presented with geometries and frequency performance comparable to those of double-polysilicon structures. This structure, called STRIPE (self-aligned trench-isolated polysilicon electrodes), provides a 0.2- mu m emitter-base polysilicon contact separation. A 0.4- mu m emitter width is achieved with conventional 0.8- mu m optical lithography. Scaling of the emitter width of 0.3 mu m has been performed with minimal degradation of device performance, and scaling of the emitter width pattern to 0.2 mu m has been demonstrated. These dimensions are the smallest achieved in single-polysilicon structures with polysilicon base contacts and are comparable to those achieved in double-polysilicon structures. The STRIPE structure has been used to fabricate transistors with f/sub t/ as high as 33.8 GHz.<<ETX>>

Kinetics of selective epitaxial deposition of Si1−xGex

The kinetics of selective deposition of epitaxial Si1−xGex layers in an atmospheric‐pressure reactor have been examined. Adding HCl to the SiH2Cl2/GeH4/H2 system decreases the Si component of the deposition rate more than the Ge component, increasing the Ge fraction in the deposited layer. HCl addition also decreases boron incorporation. When deposited selectively on oxide‐patterned wafers, lateral transport of the depositing materials causes the deposition rate and the Ge fraction to be higher in small patterns than in large patterns and also to be higher near the edges of patterns than at their centers.

EFFECT OF OXYGEN ON MINORITY-CARRIER LIFETIME AND RECOMBINATION CURRENTS IN SI1-XGEX HETEROSTRUCTURE DEVICES

A p+‐i‐n diode structure is presented which is suitable for determining the recombination lifetime in thin Si1−xGex layers grown on Si. Electrical measurements and computer simulations are used to extract carrier lifetimes in Si1−xGex layers with various oxygen concentrations. The minority‐carrier lifetime increases dramatically as the oxygen concentration in the Si1−xGex decreases from 3×1020 to less than 3×1017 cm−3. Lifetimes extracted from the p+‐i‐n diodes are consistent with those obtained from measurements on heterojunction bipolar transistors with high oxygen concentrations in the Si1−xGex base.

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.

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.