M. L. Green

Totally relaxed GexSi1-x layers with low threading dislocation densities grown on Si substrates

We have grown compositionally graded GexSi1−x layers on Si at 900 °C with both molecular beam epitaxy and rapid thermal chemical vapor deposition techniques. Triple‐crystal x‐ray diffraction reveals that for 0.10<x<0.53, the layers are totally relaxed. GexSi1−x cap layers grown on these graded layers are threading‐dislocation‐free when examined with conventional plan‐view and cross‐sectional transmission electron microscopy. Electron beam induced current images were used to count the low threading dislocation densities, which were 4×105±5×104 cm−2 and 3×106±2×106 cm−2 Eq. 2×106 cm−2 for x=0.23 and x=0.50, respectively. Photoluminescence spectra from the cap layers are identical to photoluminescence from bulk GexSi1−x.

Rapid thermal oxidation of silicon in N2O between 800 and 1200 °C: Incorporated nitrogen and interfacial roughness

Oxynitrides can suppress the diffusion of boron from the polycrystalline silicon gate electrode to the channel region of an ultralarge scale integrated device, and are therefore important potential substrates for thin SiO2 gates. Direct oxynitridation of Si in N2O is a simple and manufacturable N incorporation scheme. We have used rapid thermal oxidation to grow O2‐ and N2O‐oxides of technological importance (∼10 nm thick) in the temperature range 800–1200 °C. Accurate measurements of the N content of the N2O‐oxides were made using nuclear reaction analysis. N content increases linearly with oxidation temperature, but is in general small. A 1000 °C N2O‐oxide contains about 7×1014 N/cm2, or the equivalent of about one monolayer of N on Si (100). Nonetheless, this small amount of N can retard boron penetration through the dielectric by two orders of magnitude as compared to O2‐oxides. The N is contained in a Si‐O‐N phase within about 1.5 nm of the Si/SiO2 interface, and can be pushed away from the interface...

Mechanically and thermally stable Si‐Ge films and heterojunction bipolar transistors grown by rapid thermal chemical vapor deposition at 900 °C

Traditional techniques for growing Si‐Ge layers have centered around low‐temperature growth methods such as molecular‐beam epitaxy and ultrahigh vacuum chemical vapor deposition in order to achieve strain metastability and good growth control. Recognizing that metastable films are probably undesirable in state‐of‐the‐art devices on the basis of reliability considerations, and that in general, crystal perfection increases with increasing deposition temperatures, we have grown mechanically stable Si‐Ge films (i.e., films whose composition and thickness places them on or below the Matthews–Blakeslee mechanical equilibrium curve) at 900 °C by rapid thermal chemical vapor deposition. Although this limits the thickness and the Ge composition range, such films are exactly those required for high‐speed heterojunction bipolar transistors and Si/Si‐Ge superlattices, for example. The 900 °C films contain three orders of magnitude less oxygen than their limited reaction processing counterparts grown at 625 °C. The fi...

Elimination of dislocations in heteroepitaxial MBE and RTCVD Ge x Si 1-x grown on patterned Si substrates

We have grown GexSi1-x (0 <x < 0.20,1000–3000Å thick) on small growth areas etched in the Si substrate. Layers were grown using both molecular beam epitaxy (MBE) at 550° C and rapid thermal chemical vapor deposition (RTCVD) at 900° C. Electron beam induced current images (EBIC) (as well as defect etches and transmission electron microscopy) show that 2800Å-thick, MBE Ge0.19Si0.81 on 70-μm-wide mesas have zerothreading and nearly zero misfit dislocations. The Ge0.19Si{0.81} grown on unpatterned, large areas is heavily dislocated. It is also evident from the images that heterogeneous nucleation of misfit dislocations is dominant in this composition range. 1000Å-thick, RTCVD Ge0.14Si0.86 films deposited on 70 μm-wide mesas are also nearly dislocation-free as shown by EBIC, whereas unpatterned areas are more heavily dislocated. Thus, despite the high growth temperatures, only heterogeneous nucleation of misfit dislocations occurs and patterning is still effective. Photoluminescence spectra from arrays of GeSi on Si mesas show that even when the interface dislocation density on the mesas is high, growth on small areas results in a lower dislocation density than growth on large areas.

Thickness dependence of boron penetration through O/sub 2/- and N/sub 2/O-grown gate oxides and its impact on threshold voltage variation

We report on a quantitative study of boron penetration from p/sup +/ polysilicon through 5- to 8-nm gate dielectrics prepared by rapid thermal oxidation in O/sub 2/ or N/sub 2/O. Using MOS capacitor measurements, we show that boron penetration exponentially increases with decreasing oxide thickness. We successfully describe this behavior with a simple physical model, and then use the model to predict the magnitude of boron penetration, N/sub B/, for thicknesses other than those measured. We find that the minimum t/sub ox/ required to inhibit boron penetration is always 2-4 nm less when N/sub 2/O-grown gate oxides are used in place of O/sub 2/- grown oxides. We also employ the boron penetration model to explore the conditions under which boron-induced threshold voltage variation can become significant in ULSI technologies. Because of the strong dependence of boron penetration on t/sub ox/, incremental variations in oxide thickness result in a large variation in N/sub B/, leading to increased threshold voltage spreading and degraded process control. While the sensitivity of threshold voltage to oxide thickness variation is normally determined by channel doping and the resultant depletion charge, we find that for a nominal thickness of 6 nm, threshold voltage control is further degraded by penetrated boron densities as low as 10/sup 11/ cm/sup -2/.

High-quality homoepitaxial silicon films deposited by rapid thermal chemical vapor deposition

Homoepitaxial Si films have been deposited by rapid thermal chemical vapor deposition (RTCVD), a growth technique based on the combination of rapid thermal annealing lamps and a chemical vapor deposition chamber. The low thermal mass of the system allows the substrate to be heated and cooled rapidly, and to be held at temperature for short periods (seconds) of time, thereby allowing the growth of thin films. Si films have been grown epitaxially at temperatures between 600 and 900 °C. At 800 °C growth temperature, films with total C and O impurities less than 20 ppm, and defect densities less than 102 cm−2, have been grown. Finally, RTCVD and molecular‐beam epitaxy have been compared with respect to the growth of Si‐based structures.

EFFECT OF INCORPORATED NITROGEN ON THE KINETICS OF THIN RAPID THERMAL N2O OXIDES

We have grown ∼10 nm O2 and N2O‐oxides on Si(100) by RTO (rapid thermal oxidation) over the temperature range 800–1200 °C. Although the growth rates of both oxides exhibit Arrhenius behavior over the entire temperature range, the N2O‐oxides exhibit a large change in the Arrhenius preexponential factor for oxidation temperatures greater than 1000 °C. Above this temperature, N2O‐oxides grow a factor of 5 slower than O2 oxides. Below this temperature, N2O‐oxide growth rates approach those of O2‐oxides. This growth rate inflection can be explained in terms of N incorporation, which increases with increasing oxidation temperature. The equivalent of one monolayer of N coverage is achieved at about 1000 °C, coincident with the inflection. The incorporated N retards the linear growth of the thin N2O‐oxides either by occupying oxidation reaction sites or inhibiting transport of oxidant species to the vicinity of the interface.

Growth temperature dependence of the Si(001)/SiO2 interface width

The growth temperature dependence of the thin thermally oxidized Si(001)/SiO2 interface width was studied using synchrotron x‐ray diffraction. Nine samples with oxide thickness of about 100 A were studied, with growth temperatures ranging from 800 to 1200 °C. The oxides were prepared by rapid thermal oxidation. We found that interfacial roughness decreases linearly with increasing growth temperature, with a measured interface width of 2.84 A for the sample grown at 800 °C, and 1.76 A when grown at 1200 °C.

Initial growth studies of silicon oxynitrides in a N2O environment

We have investigated the initial growth of silicon oxynitride films on a clean Si(100) single crystal in a N2O ambient under ultrahigh vacuum conditions using Auger electron spectroscopy and nuclear reaction analysis. Variations in the growth parameters, e.g., exposure, N2O pressure and sample temperature, have been systematically investigated. Nitrogen incorporated in the oxynitride film is distributed in a region close to the film/substrate interface and most nitrogen is incorporated within a film thickness of ∼2.5 nm. These studies find an important application to the semiconductor industry with regard to possible new high quality gate oxide materials.

Oxygen and carbon incorporation in low temperature epitaxial si films grown by rapid thermal chemical vapor deposition (RTCVD)

Using SIMS analysis, we have measured oxygen and carbon concentrations in epitaxial Si films grown between 550 and 900° C. The films were grown by rapid thermal chemical vapor deposition from SiH4 as well as several different SiH2Cl2 sources. We have found that at low deposition temperatures (∼750° C or lower), oxygen incorporation is first dictated by source gas impurities and then by residual chamber gases. For the case of SiH2Cl2, which can have substantial oxygen content due to its reactivity with H2O, oxygen concentrations of about 1020 cm-3 are typical at low deposition temperatures. SiH4, however, can be obtained in higher purity, and oxygen concentrations of 1018 cm-3 can be realized at low temperatures. At higher deposition temperatures (750-900° C), SiO volatilizes, leaving the films grown from all sources with low oxygen concentrations, typically less than 5 × 1017 cm-3. Carbon incorporation is much less of a problem since it is present to a lesser extent both in the chamber background and in the source gases. Carbon levels less than or equal to 1018 cm-3 can be obtained at all deposition temperatures greater than about 650° C. The performance ofp/n junctions is shown to degrade significantly for junctions grown below 850° C. We conclude that for growth of long lifetime Si films in the temperature range <800° C, that low residual H2O partial pressures (<10-10 Torr) are desired. Therefore, CVD chambers should be loadlocked and also capable of base pressures as low as about 10-9 Torr.