T. T. Sheng

Pseudomorphic growth of GexSi1−x on silicon by molecular beam epitaxy

GexSi1−x layers are grown on Si substrates over the full range of alloy compositions at temperatures from 400–750u2009°C by means of molecular beam epitaxy. At a given growth temperature films grow in a smooth, two‐dimensional manner up to a critical germanium fraction xc. Beyond xc growth is rough. xc increases from 0.1 at 750u2009°C to 1.0 at ∼550u2009°C. Rutherford ion backscattering measurements indicate good crystallinity over a wide range of growth conditions. Transmission electron microscopy reveals that in thin films, the lattice mismatch between the GexSi1−x and Si layers can be accommodated by lattice distortion rather than by misfit dislocation formation. This pseudomorphic growth condition can persist to alloy thicknesses as large as l/4 μm.

Platinum silicide formation under ultrahigh vacuum and controlled impurity ambients

The thin‐film Pt‐Si silicide system has been investigated under clean, well controlled conditions. A (UHV) ultrahigh vacuum apparatus for this study is described which allows preparation of atomically clean Si surfaces and subsequent evaporation of Pt films in vacuo 10−9 Torr. Samples are annealed and characterized with in situ (RBS) Rutherford backscattering and (AES) Auger electron spectroscopy and later examined with (TEM) transmission electron microscopy and (SEM) scanning electron microscopy techniques. Impurity‐free Pt films deposited on clean, room‐temperature Si substrates react initially to form Pt2Si and then PtSi with diffusivities one to three orders of magnitude higher than previously reported in the 200–325u2009°C temperature range. However, the observed activation energies of 1.3±0.2 eV and 1.5±0.2 eV are in reasonable agreement with previous reports. No differences in PtSi‐Si interface width, PtSi surface character, growth rates, and phase growth progression are observed between extremely clea...

Interface and surface structure of epitaxial NiSi2 films

Epitaxial NiSi2 films have been grown on (100) and (111) Si. Interface and surface structures have been examined by Rutherford backscattering and channeling, transmission electron microscopy, and low‐energy electron diffraction. The (111) interface is remarkably flat, whereas the (100) interface has {111} facets. The NiSi2 (111) surface has a bulklike periodicity parallel to the surface, whereas the (100) surface exhibits reconstruction periodicity.

Electrical conduction and breakdown in oxides of polycrystalline silicon and their correlation with interface texture

The electrical properties of oxidized polysilicon (poly‐oxide) were measured for samples of various thicknesses, grown from polysilicon which had been doped at temperatures of 900–1000u2009°C, and oxidized in steam or dry oxygen at temperatures of 850–1050u2009°C. The electrical conduction can be explained in terms of Fowler‐Nordheim tunneling at sites of roughness of the polysilicon/poly‐oxide interface, and in terms of deep electron traps near that interface, in agreement with previously published results. We have found additional evidence for such electron traps. We find that higher doping and oxidation temperatures tend to yield oxides with higher breakdown fields, although there is considerable variation among different sets of samples. We observe no significant differences in the polysilicon/poly‐oxide interface texture by cross‐sectional transmission electron microscopy. However, we find a strong correlation between the applied field necessary for a significant leakage current through the poly‐oxide and th...

Formation and thermal stability of CoSi2 on polycrystalline Si

The high temperature stability of CoSi2/polycrystalline‐Si/SiO2 structures has been explored by techniques of Rutherford backscattering, scanning, and transmission electron microscopy. Results indicate that the silicide formed by reacting 400 A of Co with 3300 A of poly‐Si is metallurgically stable up to 30 min at 900u2009°C. A 30‐min 1000u2009°C sinter causes intermixing of the silicide/Si phase, although the SiO2 layer is not visibly damaged. More severe forming‐gas anneals result in Si loss from the metallization and a complete destruction of the multilayered geometry. This Si loss can be prevented by sintering in a partially oxidizing ambient, which forms an encapsulating SiO2 surface layer. The stability of co‐sputtered films on Si is critically dependent on the as‐deposited Si/Co ratio. Si/Co2 are both unstable for temperature excursions in excess of 800u2009°C. In contrast, Si/Co≂2 composites form stable co‐sputtered silicides with thermal characteristics similar to those of thin Co on polycrystalline Si.

Annealing behavior of ion‐implanted Fe in InP

The annealing behavior of implanted Fe+ in InP is studied using secondary ion mass spectrometry (SIMS) and transmission electron microscopy (TEM). Dual implants (275 keV, 1.25×1014 cm−2 and 400 keV, 1.25×1014 cm2) were performed at room temperature (RT) and at 200u2009°C and then annealed at 725u2009°C for one hour. TEM reveals a 3100‐A amorphous region in the unannealed RT implant. Significant defect production is observed in this sample at the amorphous‐crystalline interface following the anneal. SIMS reveals an Fe pileup at this interface. No such pileup is observed in the samples implanted at 200u2009°C. The data also suggest an Fe diffusion constant which is lower than typically reported in the literature. The results are contrasted with the SIMS study by M. Gauneau, H. L’Haridon, A. Rupert, and M. Salvi [J. Appl. Phys. 53, 6823 (1982)].

Cosputtered molybdenum silicides on thermal SiO2

The silicides of molybdenum have been formed by cosputtering mixtures of molybdenum and silicon on oxidized silicon wafers. Alloys with as‐deposited Mo/Si nominal atomic ratios of 0.25 to 4 were sintered in a hydrogen ambient in the temperature range of 900–1150u2009°C. The formation of the silicide in these films was followed by the use of sheet resistance, by x‐ray diffraction and stress measuring techniques, and by the use of transmission electron microscopy. The resistivity was found to decrease with increasing molybdenum content, the lowest being ∼60 μu2009Ωu2009cm. The stress was highest in films richer in the intermetallic MoSi2. The coexistence of the three phases Mo3Si, Mo3Si2, and MoSi2 led to lower stresses due to void formation in the film. A comparison of the resistivity and the sintering behavior in Mo‐Si films has been made with that observed in Ta‐Si and Ti‐Si films. The role of the oxygen contamination has been emphasized as one of the rate controlling factors in the silicide formation.

Ultra-fast (0.5- mu m) CMOS circuits in fully depleted SOI films

CMOS dual-modulus, divide by 128/129, prescaler circuits were built in thin Si films on SIMOX (separation by implantation of oxygen) wafers. They operated at 6.2 GHz, which is 50% faster than control circuits built in bulk Si. Detailed electrical characterization of individual n- and p-channel transistors was performed. The capacitances for the n and p diodes were also measured. Using these data in circuit simulations, it was determined that the gain in speed was primarily due to the decrease in the parasitic capacitances, in particular that of the source/drain junctions. Also measured were the ring-oscillator delay times, with a minimum delay per stage of 34 ps. >

Junction leakage studies in rapid thermal annealed diodes

A detailed and comprehensive study of As‐implanted Si annealed using incoherent tungsten radiation has been perfomed. The study emphasized the leakage currrent results, correlating them with the junction depths obtained from Rutherford backscattering measurements, and the residual damage observed in transmission electron microscopy (TEM). Sheet resistance measurements, as well as comparisons with wafers annealed in conventional furnaces, were also made. It was found that higher temperatures and shorter times resulted in lower leakage currents for given junction depths. We also found that the absence of residual dislocations in TE studies of annealed wafers was not a sufficient indication of low junction leakage.

Kinetics of arsenic activation and clustering in high dose implanted silicon

We have observed a metastable state in activation of high dose arsenic implants into silicon. This state is marked by a local minimum in the sheet resistance which increases to a local maximum as the anneal time increases. Detailed sheet resistance data as well as Rutherford backscattering and transmission electron microscopy indicate that the initial minimum in sheet resistance is due to activation of arsenic atoms by occupying substitutional sites as the amorphous silicon recrystallizes. The subsequent clustering of arsenic atoms is the cause for the increase in the sheet resistance while the final drop is associated with the lateral diffusion of arsenic. We have studied this effect in the 850–1100u2009°C temperature range, and have obtained an activation energy of 1.1 eV for the clustering of arsenic atoms.