K. Sawano

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.

Fabrication of high-quality strain-relaxed thin SiGe layers on ion-implanted Si substrates

We fabricated high-quality strain-relaxed thin SiGe layers by Ar ion implantation into Si substrates before epitaxial growth. The surface of 100-nm-thick Si0.8Ge0.2 layers, the relaxation ratio of which was more than 80%, was found to be very smooth, with a rms roughness of 0.34 nm. Cross-sectional transmission electron microscopy analysis confirmed that strain-relieving dislocations were effectively generated due to the ion-implantation-induced defects and confined in the vicinity of the heterointerface, resulting in a dislocation-free SiGe surface. Moreover, in-plane strain-field fluctuation was found to be largely reduced by this ion implantation method.

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.

Surface smoothing of SiGe strain-relaxed buffer layers by chemical mechanical polishing

We flattened the rough surface of the strain-relaxed Si 0.7 Ge 0.3 buffer layers by chemical mechanical polishing technique and successfully obtained the ultra-smooth surface whose root mean square roughness was less than 1 nm. The regrowth of Si 0.7 Ge 0.3 films on the polished Si 0.7 Ge 0.3 buffer layer was found to keep its surface roughness less than I nm, and efficient photoluminescence with narrow line width was observed from quantum wells grown on the polished buffer layer. This indicates high crystalline quality of the regrown layer with smooth interfaces, which makes it possible to fabricate high-performance SiGe devices with low surface roughness scattering.

Compressive strain dependence of hole mobility in strained Ge channels

The strain dependence of the hole mobility was systematically investigated in the compressively strained Ge channel modulation-doped structure. It was clearly observed that the mobility increases with increasing compressive strain until the strain as high as 1.9%. The highest mobility of 20 800 and 2000cm2∕Vs at 8 K and room temperature, respectively, was obtained for the Ge channel structure grown on the relaxed SiGe buffer layers with Ge composition of 53%. The origins of this mobility increase are speculated to be the reduction of effective mass, suppression of interband phonon scattering, and the increased confinement of the holes in the channel layer.

Mobility enhancement in strained Si modulation-doped structures by chemical mechanical polishing

The strained Si modulation-doped (MOD) structure formed on the strain-relaxed SiGe buffer layer planarized by chemical mechanical polishing (CMP) was found to show significant mobility enhancement. The enhancement reaches a factor of 6 at low temperatures. The backgate dependence as well as temperature dependence of the transport properties of the MOD structure were investigated, and it was suggested that CMP drastically reduced the roughness scattering and increased the mobility of two-dimensional electron gas in the strained Si.

Magnetotransport properties of Ge channels with extremely high compressive strain

Ge channel structures with extremely high compressive strain up to 2.8% were fabricated and their magnetotransport properties were evaluated. It was found that at the same hole density the sample with the higher strain showed the lower hole effective mass and that the compressive strain effectively reduced the effective mass. The Dingle ratios obtained were very high (>5) for all samples, indicating that remote impurity scattering was a dominant scattering mechanism rather than high angle scatterings caused by degradation of the channel layers. This result strongly suggests that Ge channels with extremely high strain are very promising for high performance complementary-metal-oxide-semiconductor applications.

Stacked Ge islands for photovoltaic applications

Abstract Stacked Ge islands formed via the Stranski–Krastanov growth mode were incorporated into the intrinsic layer of Si-based pin diode to improve the performance of the solar cells in the near-infrared regime. The onset of the external quantum efficiency was extended up to around 1.4 mm for the solar cells with stacked Ge islands. The quantum efficiency was found to increase with increasing number of stacking, and the onset of the photocurrent response was in good agreement withroom-temperature photoluminescence energy of the Ge islands. These results manifest that the Ge islands did play a role toincrease the quantum efficiency. Furthermore, a part of electron-hole pairs generated within Ge islands was separated by the internal electric field and contribute to the photocurrent.

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.

Fabrication of p-i-n Si/sub 0.5/Ge/sub 0.5/ photodetectors on SiGe-on-insulator substrates

This study demonstrates the fabrication and evaluation of a Si/sub 0.5/Ge/sub 0.5/ p-i-n photodetector for 1.3 /spl mu/m light detection on SiGe-on-insulator (SGOI) substrates with the Ge content of 0.5. Gas-source-MBE grown SiGe heterostructures on SGOI substrates are promising systems not only for high-speed SiGe hetero-devices, such as strained-Si and strained-Ge MOSFETs, but also for Si-based optoelectronic integrated circuits (OEICs).