Y. Shiraki

Ultrahigh room-temperature hole Hall and effective mobility in Si0.3Ge0.7/Ge/Si0.3Ge0.7 heterostructures

We have obtained ultrahigh room-temperature (RT) hole Hall and effective mobility in Si0.3Ge0.7/Ge/Si0.3Ge0.7 heterostructures with very small parallel conduction. Reducing parallel conduction was achieved by employing Sb doping in Si0.3Ge0.7 buffer layers, which drastically increased RT hole Hall mobility up to 2100 cm2/Vu200as in the strained Ge channel modulation-doped structures and improved device characteristics of the p-type metal–oxide–semiconductor field-effect transistors with the strained Ge channel. The peak effective mobility reached to 2700 cm2/Vu200as at RT, which was much higher than the bulk Ge drift mobility.

In-plane strain fluctuation in strained-Si/SiGe heterostructures

In-plane strain fluctuation in the strained-Si/relaxed-SiGe heterostructure was studied by micro-Raman spectroscopy. It was found that misfit dislocation, which is necessarily induced by strain relaxation of SiGe buffer layers, caused micrometer-scale inhomogeneous strain field in the strained-Si layer as well as SiGe buffer, which may degrade device performance. After annealing, the fluctuation was found to be enhanced due to partial strain relaxation of strained Si, particularly in the region where tensile strain was relatively high before annealing. From homoepitaxial growth of SiGe on planarized SiGe buffer layers, it was confirmed that the growth rate also fluctuated laterally, in correspondence with the in-plane strain variation.

Extremely high room-temperature two-dimensional hole gas mobility in Ge/Si0.33Ge0.67/Si(001) p-type modulation-doped heterostructures

To extract the room-temperature drift mobility and sheet carrier density of two-dimensional hole gas (2DHG) that form in Ge strained channels of various thicknesses in Ge/Si0.33Ge0.67/Si(001) p-type modulation-doped heterostructures, the magnetic field dependences of the magnetoresistance and Hall resistance at temperature of 295 K were measured and the technique of maximum entropy mobility spectrum analysis was applied. This technique allows a unique determination of mobility and sheet carrier density of each group of carriers present in parallel conducting multilayers semiconductor heterostructures. Extremely high room-temperature drift mobility (at sheet carrier density) of 2DHG 2940 cm2 V–1 s–1 (5.11×1011 cm–2) was obtained in a sample with a 20 nm thick Ge strained channel.

Surface Planarization of Strain-Relaxed SiGe Buffer Layers by CMP and Post Cleaning

Surface planarization of strain-relaxed SiGe buffer layers by chemical mechanical polishing (CMP), particularly the influence of a post-CMP cleaning process which is indispensable after CMP, on the surface morphology of SiGe buffer layers was investigated. It was found that the cleaning tended to enhance the surface roughness due to the etching effect that increased with increasing cleaning temperature. The etching effect was suppressed by optimizing cleaning reagents, and the ultrasmooth surfaces of SiGe buffer layers with Ge contents of 30 to 70% were obtained, irrespective to growth methods. The root mean square roughness reached 0.4 to 0.6 nm, which was the lowest value that was ever obtained.

Enhancement of Strain Relaxation of SiGe Thin Layers by Pre-Ion-Implantation into Si Substrates

Almost fully relaxed thin SiGe buffer layers that are essential for the growth of strained SiGe heterostructures are obtained by Ar ion implantation into Si substrates before SiGe molecular beam epitaxy (MBE) growth. The strain-relaxation ratio of the 100-nm-thick Si0.73Ge0.27 layers, which is much thinner than that of the conventional method, on the Si substrates implanted under appropriate conditions exceeds 90%. Strain relaxation is found to strongly depend on ion dose and energy, suggesting the role of implantation-induced defects in strain relaxation.

Optical properties of strain-balanced SiGe planar microcavities with Ge dots on Si substrates

SiGe microcavities with Ge dots were fabricated by employing strain-balanced SiGe/Si Bragg reflectors, and it was observed that photoluminescence from Ge dots embedded in the microcavity structure was significantly modulated due to the cavity effect. The characteristic luminescence of the microcavity was observed up to 200 K, and the thermal activation energy of the luminescence was largely improved compared with that of cavities with quantum wells.

Relaxation enhancement of SiGe thin layers by ion implantation into Si substrates

Strain-relaxed SiGe buffer layers (SiGe virtual substrates) are of great importance for fabricating such strained SiGe high-speed hetero-devices as strained Si channel MOSFETs. To obtain full relaxation of the SiGe films grown on Si substrates by the conventional graded buffer method, however, the thickness of the SiGe films generally has to be beyond several hundred nm. To overcome this problem, a low temperature (LT) method, where defects in the LT Si buffer layer act as dislocation sources and enhance the relaxation of the overgrown SiGe film, was invented. Here we propose a new method to introduce defects in Si substrates by ion implantation, which enhances the relaxation of SiGe films during growth of the SiGe films on ion implanted Si substrates. This ion implantation method may be superior to the LT method because its controllability of defects is much higher than that of LT method and it is applicable to the CVD method in which the LT growth cannot be performed.

Optical Properties of Strain-Balanced Si0.73Ge0.27 Planar Microcavities on Si Substrates.

To realize of Si-based optical devices, SiGe/Si distributed Bragg reflectors (DBRs) with strain-balanced structures were grown on relaxed SiGe virtual substrates, and SiGe microcavities with high qualities were fabricated. Strain balance of the SiGe/Si DBRs was confirmed by X-ray diffraction measurements and Raman spectroscopy, and a record reflectivity of 90% was achieved in a 38.5-pair sample. Spectral changes of photoluminescence due to the microcavity were observed as the excitation power was increased. Moreover, SiGe microcavities with top Si/CaF2 DBRs showed a drastic intensity enhancement and a significant narrowing of spectral width was observed.

Carrier activation process in As+ implanted relaxed Si1−xGex alloys

Abstract We have systematically studied the solubility limit and the activation temperature of As implanted in relaxed Si1−xGex (x=0–0.3) alloys. The solubility limit of As in Si1−xGex alloys at an annealing temperature of 600°C was found to decrease monotonically with increasing Ge content. The carrier activation temperature showed anomalous dependence on Ge content, and increased first and decreased with Ge content above x=0.1. The presence of inter-atomic strain might explain this behavior, even though Si1−xGex alloys are macroscopically unstrained.