S. S. Iyer

Growth temperature dependence of interfacial abruptness in Si/Ge heteroepitaxy studied by Raman spectroscopy and medium energy ion scattering

The influence of growth temperature on the interfacial abruptness of strained Ge layers, a few monolayers thick, embedded in Si has been studied using Raman spectroscopy to identify the presence of GeGe and GeSi bonds and medium energy ion scattering to characterize the spatial extent of the layers. Atomically sharp interfaces are observed for growth temperatures just above the crystalline to amorphous transition range, with pseudomorphic growth found for growth temperatures >∼250 °C. Asymmetric mixing of Ge into the Si capping layer occurs during growth at higher temperatures. Significantly less intermixing occurs on annealing after growth, pointing to the role of dynamical processes occurring at the growth front.

A gate-quality dielectric system for SiGe metal-oxide-semiconductor devices

The authors present a high-quality dielectric system for use with Si/sub 1-x/Ge/sub x/ alloys. The system employs plasma-enhanced chemical vapor deposited (PECVD) SiO/sub 2/ on a thin (6-8-nm) layer of pure silicon grown epitaxially on the Si/sub 1-x/Ge/sub x/ layer. The buffer layer and the deposited oxide prevent the accumulation of Ge at the oxide-semiconductor interface and thus keep the interface state density within acceptable limits. The Si cap layer leads to a sequential turn-on of the Si/sub 1-x/Ge/sub x/ channel and the Si cap channel as is clearly observed in the low-temperature C-V curves. The authors show that this dual-channel structure can be designed to suppress the parasitic Si cap channel. The MOS capacitors are also used to extract valence-band offsets.<<ETX>>

Experimental evidence of both interstitial‐ and vacancy‐assisted diffusion of Ge in Si

We present the first experimental identification of the diffusion mechanisms of Ge in Si. Using thermal nitridation reactions to create either excess self‐interstitials or vacancies, it is established that under equilibrium conditions at 1050 °C Ge diffusion takes place by both substitutional‐interstitial interchange and vacancy mechanisms, with comparable contributions from each. If previous conjectures that Ge diffusion in Si is similar to Si self‐diffusion are correct, our findings support the idea that Si self‐diffusion takes place by both interstitial and vacancy mechanisms.

Low‐temperature silicon cleaning via hydrogen passivation and conditions for epitaxy

In low‐temperature epitaxial Si deposition methods such as molecular beam epitaxy (MBE), pre‐epitaxial substrate preparation usually incorporates a high temperature (≳800 °C) step. Elimination of this step is essential to wider applicability of these epitaxial methods. We show that Si(100) wafers exposed to HF vapors in a laboratory ambience are bulk terminated and that such termination is stable in air for several tens of minutes, and in vacuum for several hours. It is possible to obtain good epitaxy, as determined by surface diffraction and transistor characteristics, provided epitaxy is commenced on these bulk‐terminated surfaces. We also give evidence that under certain conditions, bulk‐terminated surfaces are maintained in low‐temperature epitaxy using the method of ultrahigh vacuum chemical vapor deposition.

Molecular beam epitaxy of metastable, diamond structure SnxGe1−x alloys

Single‐phase SnxGe1−x alloys with x up to 0.3 have been grown by molecular beam epitaxy. X‐ray diffraction measurements indicate the layers to have the diamond crystal structure. The metastability of the alloys is apparent as increases in the growth temperature, layer thickness, or Sn composition cause phase separation of the Sn into a noncubic (white or β‐Sn) form. Rutherford backscattering spectrometry and reflection high‐energy electron diffraction measurements indicate that the initial stages of growth are complicated. The first several hundred angstroms of growth are compositionally graded, with the Sn incorporation rate increasing with film thickness. Thereafter, the alloy composition remains constant, determined by flux composition, until a critical thickness for phase separation is reached (≂2000 A for x=0.3).

Formation of stoichiometric SiGe oxide by electron cyclotron resonance plasma

Electron cyclotron resonance (ECR) plasma oxidation of SiGe alloys was investigated at temperatures from room temperature to 500 °C. Both Si and Ge are shown to be fully oxidized, forming SiO2 and GeO2. Auger depth profiling reveals that there is no Ge‐rich SiGe layer after oxidation. With increasing temperature up to 500 °C, the oxide is stoichiometric and it does not lose its GeO2 component. Oxidation has also been carried out at both positive and negative sample bias in order to identify the role of ions, electrons, and neutrals. From biasing experiments negative oxygen ions and atomic neutrals appear to be the major reaction species.

Origin and reduction of interfacial boron spikes in silicon molecular beam epitaxy

An interfacial boron spike is formed during the molecular beam epitaxial growth of Si. We show two possible sources for this unintentional spike. We have found that some boron contamination invariably occurs when silicon surfaces are exposed to air. A greater degree of contamination results when the sample is heated to temperatures greater than 800 °C, as required for creating an atomically clean surface prior to molecular beam epitaxial growth. A source of boron suboxides, internal to the ultrahigh‐vacuum system, was detected by residual gas analysis. While anneals at 1000 °C or greater result in almost complete activation of the B, we observe that for a cleaning regimen at 850 °C, less than 10% of the boron is active. Our results are consistent with the oxidation of the suboxides on oxygen‐contaminated surfaces and their subsequent reduction at higher temperatures by silicon, with the volatization of SiO. Subsequent incorporation is by indiffusion.

Grain-boundary states and hydrogenation of fine-grained polycrystalline silicon films deposited by molecular beams

Abstract The energy distribution of grain-boundary states is determined for polycrystalline silicon films grown under ultra-high vacuum conditions. Conductivity and electron spin resonance measurements on n-type films reveal both exponential bandtails and deep gap states corresponding to disorder-induced gap states and dangling-bond defect levels (D° and D−). The latter are responsible for the pinning of the Fermi level observed at moderate doping. Both experimental techniques agree with a location of the D− level at E C-0.30 eV and the D° level at E C-0.65eV ± 0.05eV. It is shown that a hydrogen-plasma treatment at 500°C reduces the dangling-bond density by an order of magnitude and that it also yields a conduction bandtail twice as steep. The replacement of weak Si-Si bonds by more energetic Si-H bonds would explain the steeper bandtails. This view is supported by absolute measurements by nuclear reaction showing that the hydrogen content exceeds by two orders of magnitude the original dangling-bond den...

Highly activated shallow Ga profiles in silicon obtained by implantation and rapid thermal annealing

The physical and electrical characteristics of gallium (Ga) ion implanted 〈100〉 silicon, annealed for times of the order of seconds to several tens of seconds for temperatures in the range of 550–900 °C, are presented. It is shown that for the chosen doses of 1–6×1015/cm2 and energy of 100 keV highly electrically active p‐type layers (approaching 100%) can be achieved. The highest activation being reached at temperatures below ∼650 °C with no profile distortion. For temperatures in excess of 800 °C this electrical activation decreases and significant profile movement occurs even for times as short as 2 s.

Detection and characterization of individual Ge layers in Si(100) using Raman spectroscopy

The Raman active vibrational modes of single, 1 to 6 layer thick Ge films grown epitaxially on Si(100) and covered by thin layers of Si(100) have been measured. Both the Ge‐Ge vibrations from the interior of the Ge films and the Ge‐Si vibrations at the Ge‐Si interfaces have been observed. These modes and the weak defect activated scattering are used to characterize the Ge layers. These results show that Raman spectroscopy can now be used to directly characterize the properties of buried interfaces at the level of single atomic layers.