Session We-A2

a-Si/a-Ge: Alloys and Clathrates II

Chair: Jun Xu, Nanjing University

We-A2.1 10:40–11:00

Germanium Thin-Film on Glass: Amorphous-to-Nanocrystalline Phase

A. R. Middya*, Swati Ray, S. C. De, and A. K. Barua

Energy Research Unit, Indian Association for Cultivation of Science, Jadavpur, Kolkata - 700 032, India

*Present Address: Silicon Solar, Inc., Fremont, CA 94536

In this report, we shall discuss about physics of formation of new microstructure of germanium thin-film at the phase boundary of amorphous-to-nanocrystalline state. We systametically investigated growth of hydrogenated amorphous germanium (a-Ge:H) as well as germanium (Ge) thin-film at the phase boundary of amorphous-to-nanocrystalline developed by plasma-enhanced chemical vapor deposition (PECVD) technique. We used germane (GeH4) and hydrogen (H2) as the source gases. We found a particular regime of PECVD process, we could develop amorphous-to-nanocrystalline germanium thin-film that shows conductivity ~0.13 Scm–1 and dark conductivity activation energy ~0.2 eV. The resistivity (1/σ) value reported earlier for amororphous-to-nanocrystalline germanium is lower than that of undoped crystalline germanium wafer (c-Ge). We used transmission electron microscopy (TEM) to investigate the nanostructure of amorphous-to-nanocrystalline germanium thin-film. We found first hallow ring typical of amorphous phase, however, few minutes letter, we started observing diffraction rings {(111), (220) and (311)} corresponding to crystalline phase of germanium and we observed spotted ring, corresponds to symmetry exists within crystalline phase of germanium. We studied systematically using Raman spectroscopy, how amorphous-to-nanocrystalline phase transition takes place by varying laser intensity as well as time of illumination. The results of this systematic investigation will be presented. We discovered a classic regime of germanium thin-film microstructure, never reported before, where electronic transport path can be easier than the case of undoped crystalline germanium (c-Ge) wafer. Our invention strongly suggests thin-film form of matter will open up new horizon of materials structure and consequently optical, mechanical, electronic and magnetic properties.

We-A2.2 11:00–11:20

Low Temperature Formation of Crystalline Si/Ge Heterostructures by Plasma Enhanced CVD in Combination with Ni-NDs Seeding Nucleation

Yimin Lu (1), Katsunori Makihara (1), Daichi Takeuchi (1), Kouhei Sakaike (2), Muneki Akazawa (2), Mitsuhisa Ikeda (2), Seiichiro Higashi (2) and Seiichi Miyazaki (1)

1. Graduate School of Engineering, Nagoya University, Nagoya 464-8601, Japan

2. Advanced Sciences of Matter, Hiroshima University, Higashi-Hiroshima 739-8527, Japan

Highly-crystallized Si/Ge tandem structures have attracted much attention because of their potential application to thin film solar cells and transistors. In the high-rate growth of crystalline Si, Ge and their alloy films on non-crystalline substrates by plasma enhanced CVD, to suppress the formation of an amorphous incubation layer is one of major concerns. In our previous work, we have demonstrated that an introduction of the Ni-NDs is quite effective to enhance Ge crystalline nucleation and subsequent formation of a highly-crystallized Ge layer. In this work, we extended our research to form highly-crystallized Si/Ge heterostructures from VHF-ICP of SiH4 just after of GeH4 by means of Ni-NDs as seed materials.

A ~2.0 nm-thick Ni film was deposited on a quartz substrate and followed by VHF H2-plasma exposure without any external heating to form Ni-NDs. Formation of Ni-NDs with an areal density as high as ~1011cm-2 was confirmed by AFM. Subsequently, ~70 nm thick Ge films were grown from a decomposition of H2-diluted GeH4 in a VHF-ICP setup at 250°C. In Raman scattering spectra measured from the film surface side, characteristic signals peaked at ~300 cm-1 due to TO phonons in crystalline Ge confirm the formation of Ge crystalline network, where the crystallinity determined as a ratio of the integrated intensity of the crystalline phase to disordered one was over 70%. In addition, no detectable incubation layer is formed as confirmed from the backside Raman scattering measurements through the quartz substrate. The Ge film growth was followed by VHF glow discharge decomposition of SiH4 at 250°C and the evolution of crystalline Si layer was also examined. For the sample after deposition of ~30 nm thick Si:H, a sharp feature peaked at ~510 cm-1 originating from the c-Si TO phonon mode and relatively broad signals peaked at ~400 cm-1 due to structurally relaxed Si1–xGex network were clearly observed. Notice that, with further growth of Si:H up to ~90 nm in thickness, an improvement of the crystallinity from 52% to 57% was observed but the Si1–xGex signals were hardly detectable. This implies compositional intermixing in the early stage of Si:H deposition. To get an insight into the growth of crystalline Si network, changes in surface morphology with progressive film growth were examined by AFM. AFM images for Ge:H films shows the formation of crystalline grains with an average diameter of 42nm and with an almost the same density with underlying Ni-NDs, implying that the underlying Ni-NDs initiate crystalline nucleation. On the other hand, with the deposition of ~30 nm thick Si:H, the grain density was increased by a factor of ~4, in which the formation of small grains with an average diameter of ~25 nm was observed. Further deposition of Si:H promoted the grain growth and the average grain size was increased up to ~70 nm for 90 nm thick Si:H films accompanied with grain coalescence. These results suggest that the crystalline grain formation in the early stages play a role on release in a large lattice mismatch between c-Si and c-Ge.

Keywords: Ni-Nanodots, Si/Ge tandem structure, Heterostructures

We-A2.3 11:20–11:40

Preferential Crystal Growth of Germanium by Solid Phase Crystallization

Mikuri Kanai, Tomoaki Yamaguchi , Yuji Kojima, and Masao Isomura

Course of Electrical and Electronic System, Graduate School of Engineering, Tokai University, 4-1-1 Kitakaname, Hiratsuka, Kanagawa 259-1292, Japan

We proposed crystalline germanium (Ge) films as bottom cell materials for multi-junction thin film solar cells. We have investigated to prepare epitaxial growth thin film Ge by solid phase crystallization (SPC) from amorphous Ge (a-Ge) films on single crystal silicon (c-Si) substrates as seed layers. We successfully obtained preferential crystalline growth following the orientation of c-Si substrate at SPC temperatures from 400°C to 475°C when the a-Ge films prepared by electron beam (EB) evaporation. However, the preparation method of a-Ge films affects the preferential crystalline growth, and the a-Ge films prepared by Knudsen cell (K-cell) evaporation cause random crystalline growth. In this report, we study the characteristic of a-Ge films as a precursor of SPC, and the crystalline structure of the preferentially grown c-Ge.

In the a-Ge films by the K-cell evaporation, the impurity contamination is quite serious. Oxygen (O) and nitrogen (N) concentrations are in the order of 1021 atoms/cm3 and 1020 atoms/cm3, respectively. However, the O and N concentrations are reduced by one and two orders of magnitude, respectively, by coating amorphous Si (a-Si) cap layers to prevent the impurity diffusion and the preferential crystalline growth is promoted. It was confirmed that these impurities diffuse from the surface of samples in the air and the random crystalline growth is caused by such high impurity concentrations extended to the c-Si substrates interface.

In the case of the EB evaporation, the O and N concentrations are also high near the surface but decrease from 1021 atoms/cm3 to 1018 atoms/cm3 and from 1019 atoms/cm3 to 1018 atoms/cm3, respectively, while going to deeper positions. The impurity diffusion from the surface is relatively slow compared with the K-cell evaporation, so the impurity atoms do not reach the substrate. The preferential crystalline growth occurs probably because of low impurity concentrations near the Si substrate where the crystalline growth is initiated.

Reflective index of the K-cell a-Ge is lower than that of the EB a-Ge and it is suggested that density of the K-cell a-Ge is lower than that of the EB a-Ge. It is thought that the diffusion of impurities is enhanced because porous structure due to its low density.

In regard to the preferentially grown c-Ge films with low impurity contamination, the locking curve measurements of X-ray diffraction indicate much ambiguity of crystalline orientation, and micro-grain structures of 30 nm size or less are observed by field emission-scanning electron microscope. The c-Ge films are supposed to have micro-domain structures with the same orientation. The micro-domain structures are probably caused by many crystal nucleuses formed on c-Si substrates, and the crystalline growth in horizontal direction is disturbed by other crystal growth.

Future subjects are to suppress the impurity inclusion and to control the crystal nucleation for promoting lateral crystalline growth.

Keywords: germanium, solid phase crystallization, impurity