D. B. Noble

Reduction in misfit dislocation density by the selective growth of Si1−xGex/Si in small areas

Si1−xGex and Si layers have been grown selectively in the exposed Si regions on oxide‐patterned 〈100〉 oriented Si wafers using the chemical vapor deposition technique limited reaction processing. Misfit dislocation spacings at the heterointerface were measured using plan‐view transmission electron microscopy in conjunction with a large‐area thinning technique which allows for examination of 100–150 μm diameter areas. The dislocation density is reduced by at least a factor of 20 for small areas (lateral dimensions: tens of microns) bounded by oxide isolation when compared to adjacent large areas (millimeters) which are uninterrupted by the patterned oxide. The ability to selectively grow Si1−xGex on patterned wafers and the area‐dependent reduction in dislocation density in as‐grown films may be important considerations for future device applications using Si1−xGex strained layers.

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.

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>>

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.

Thermal stability of Si/Si1−xGex/Si heterojunction bipolar transistor structures grown by limited reaction processing

The thermal stability of Si/500‐A‐thick Si0.77Ge0.23 bilayers grown on Si by limited reaction processing is studied as a function of Si capping layer thickness. After annealing for 4 min at 850 °C, misfit dislocation spacings increase monotonically with cap thickness from 0.5 μm for an uncapped film to greater than 50 μm for a layer with a 500‐A‐thick cap. Thus, an epitaxial Si cap of sufficient thickness prevents significant misfit dislocation formation during this anneal. Experimental observations are reported which indicate that the Si cap enhances thermal stability by inhibiting both dislocation nucleation and propagation. These results are very encouraging since they suggest that high‐temperature processing of Si/Si1−xGex device structures may be possible without significant misfit dislocation formation.

The effect of oxygen on the thermal stability of Si1−xGex strained layers

The thermal stability of Si1−xGex strained layers containing 2×1020 oxygen atoms/cm3 is compared with that of similar layers (same Ge fraction and film thickness) containing more than two orders of magnitude less oxygen. For the layers with high oxygen levels, no misfit dislocations were found in films as thick as two times the theoretical equilibrium critical thickness, after annealing at 850 °C for 4 min. In contrast, dislocations were found in the layers with low oxygen levels at thicknesses very near the equilibrium critical thickness after the same anneal. X‐ray measurements of lattice constants in high and low oxygen films of similar Ge content indicate that oxygen does not substantially change the amount of strain in the layers. Oxygen appears to impede the kinetics of dislocation formation.

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>>

Electrical and structural properties of diodes fabricated in thick, selectively deposited Si/Si/sub 1-x/Ge/sub x/ epitaxial layers

The characteristics of diodes fabricated in thick Si/sub 1-x/Ge/sub x/ layers formed by selective epitaxial deposition have been examined by DC electrical measurements, transmission electron microscopy, and X-ray topography. Because depositing in restricted areas limits the propagation of misfit dislocations in thick layers, a lower misfit dislocation density is found in small-area deposited regions. Similarly, diodes fabricated in small deposited regions have more ideal forward characteristics than diodes fabricated in large regions.<<ETX>>

Dependence of misfit dislocation velocities upon growth technique and oxygen content in strained GexSi1−x/Si(100) heterostructures

Misfit dislocation velocities in strained GexSi1−x/Si(100) heterostructures are compared for layers grown by molecular beam epitaxy and limited reaction processing. We demonstrate that velocities are substantially lower in structures with oxygen concentrations ∼1020 cm−3 compared to layers with oxygen concentrations ∼1018 cm−3. For layers with the lower oxygen concentration, the sample growth technique does not appear to be a significant factor affecting misfit dislocation velocity.