AVS2001 Session SS4-ThP: Semiconductor Morphology Poster Session

Thursday, November 1, 2001 5:30 PM in Room 134/135
Thursday Afternoon

Time Period ThP Sessions | Topic SS Sessions | Time Periods | Topics | AVS2001 Schedule

SS4-ThP-1 Surface Stress of Thin Epitaxial CaF2 Films on Si(111)
P. Zahl, A. Klust, M. Bierkandt, J. Wollschläger (Universität Hannover, Germany); M. Horn- von Hoegen (Universität GH Essen, Germany)
In this paper we present the first elastic properties characterisation of ultrathin epitaxial grown calcium fluoride films (thickness up to five molecular layers) using SSIOD (Surface Stress Induced Optical Defection1). This experimental technique will be explained and demonstrated by Hydrogen ad-/desorption on Si(111). From the large difference in thermal expansion coefficients2 between CaF2 and Si a change in volume misfit from 0.5% at room temperature up to 2.5% at 1000 K takes place. The analysis of stress increase during growth at different temperatures (e.g. misfits) allows estimating the stress at the Si/CaF2 interface. A change of the surface reconstruction at approx. 900K between 1 and 3 TL is detectable by SSIOD. Furthermore, the measurements are showing, that although at 600K a relaxed and stress free thin films can be grown, a volume lattice misfit of approx 1.5% exists.

1 P. Zahl, "Oberflächenspannung auf Si(111): Heteroepitaxie von Ge und CaF2, Adsorption von H und Sb", Logos Verlag Berlin 2000, ISBN 3-89722-571-9
2 M. A. Olmstead, in: Thin Films: Heteroepitaxial Systems, Chapter: "Heteroepitaxy of Strongly Disparate Materials: From Chemisorption to Epitaxy in CaF2/Si(111)", pp. 211-266. World Scientific Publishing, Singapore, 1999
3 Ohmi, S. K. Tsutsui, S. Furukawa; Jpn. J. Appl. Phys., 33:1121, 1994
4 J. Wollschläger, A. Klust, H. Pietsch; Appl.Surf.Sci., 123/124:496-500, 1998.

SS4-ThP-2 STM Investigation of Sn,Pb/Si(111)-(3x3) Phase at RT and LT
H. Morikawa, K. Horikoshi, S. Hasegawa (University of Tokyo, Japan)
Pb,Sn/Ge(111) surfaces are well known systems for their 'phase transitions' from room-tenperature(RT) √3x√3 phases to low-temperature(LT) 3x3 phases. However, relatively few investigations have been made for the Pb, Sn/Si(111) systems.1 We have made STM studies for these surfaces both at RT and LT (70K). Although a long-range ordered 3x3 phase does not appear in STM images at LT in the Sn/Si system, we could see an apparent 3x3 domains for the Pb/Si system around defects. Only in the case of Pb/Si, defects aligns to form the 3x3 periodicity. In these systems, defects are important for inducing the 3x3 order. We investigated the defect-defect correlation for both sytems, in relation with the 'Defect Density Wave'on the Sn/Si(111).2 Furthermore we found √3x√3(RT) and 3x3(LT) islands for Pb/Si, which was not seen in the Sn case.

1K Horikoshi, et al,Phys.Rev.B 60,13287 (1999)
2L Ottaviano,et al,Phys.Rev.Lett.86,1809(2001).

SS4-ThP-3 High-Temperature Phase Transitions on the Si(111) Surface
C. Jeon (Sung Kyun Kwan University, Korea); C.C. Hwang, K.-J. Kim, T.-H. Kang, B. Kim (Pohang Accelerator Laboratory, Korea); C.Y. Park (Sung Kyun Kwan University, Korea)
Surface melting of semiconductors has been widely studied up to now. Recent theoretical1and experimental2 results suggest that the Si(111) surface undergoes an incomplete melting at high temperature below the bulk melting point (1680 K). In this work, angle resolved ultra-violet photoemission spectroscopy was performed to investigate the phase transitions on the Si(111) surface with rising temperature from room temperature (RT) up to 1600 K. By analyzing of the integrated emission intensity at the Fermi level, we observed a change of metallic character as a function of temperature.3 As previously reported, the Si(111) 7x7 surface has metallic nature at RT. A linear increase with temperature and an abrupt rise of the intensity at the critical temperature (Tc=1100K) is observed. At the same time, the Si(111) surface structure transforms from the (7x7)-DAS to the high temperature 1x1 structure. At 1470 K, the decrease in its metallicity shows the presence of another phase transition taking place at about 200 K below the bulk melting point. Based on these results, we'll discuss the origin of the two phase transitions on the Si(111) surface at elevated temperatures.

1 A. Natori, T. Suzuki, and H. Yasunaga, Suf. Sci. 367, 56 (1996)
2 Y. Fukaya and Y. Shigeta, Phys. Rev. Lett. 85, 5150 (2000)
3 I. Stich, R. Car, and M. Parrinello, Phys. Rev. Lett. 63, 2240 (1989)

SS4-ThP-4 Thermal Decay of Isolated Single Silicon Mounds on the Si(100)-(2x1) Surface
S. Nishida, A. Ichimiya (Nagoya University, Japan)
We have studied the decay process of a silicon mound on the Si(100)-(2x1) surface at 500°C. The mound has been fabricated by an STM tip. The shape of the mound is a quadrangular pyramid with facets of regular array of steps that have double layer height. For the all step, the dimer rows are perpendicular to the step edges, so called the DB step. Just after fabrication, the pyramid begins to decay layer-by-layer. During the decay, area of the bottom single layer of the pyramid is grown toward the dimer row direction. The layer width does not change toward perpendicular to the dimer rows. The area of the topmost layer of the pyramid decreases toward perpendicular to the dimer rows, because the both SB step edges are pinned by the step edges of the lower ones. Therefore the DB step edges remain at the each step edge, and move scarcely. But only the SB steps at the bottom layer move due to attachment and detachment of atoms which are detached from the upper layers. The decay process of a single layer mound on the Si(100) surface is very different from that of the topmost layer of the pyramid. The SB steps fluctuate greatly. Roughness of the SB steps changes periodically. The aspect ratio of the mound oscillates between about 2.5 and 1.5. The decay rate of a single layer mound varies with each mound. The rate is affected by the defect of the surface. The increase of the defect ratio causes the decrease of the decay rate.
SS4-ThP-5 Growth of Cu on Si(5 5 12)
P.H. Woodworth, J.C. Moore, J.L. Skrobiszewski, A.A. Baski (Virginia Commonwealth University)
We have used scanning tunneling microscopy (STM) to study the growth behavior of Cu on the row-like surface of Si(5 5 12). Our prior work concerning noble metal growth (Au, Ag) on this surface has shown a variety of row-like overlayer structures and Au-stabilized facet planes.1 This study compares and contrasts the growth behavior of Cu to its counterparts. At very low coverage (<0.1 ML) and moderate annealing temperatures (600°C), Cu induces faceting of the (5 5 12) surface to the nearby (337) and (5 5 13) planes, neither of which is a stable clean Si surface. Faceting to (337) has also been observed for low-coverage Au deposition, indicating a similarity between these two metals. In contrast to both Au and Ag growth, however, Cu is not seen to form row-like overlayer structures on the basal Si(5 5 12) surface. At higher coverages up to one monolayer, Cu causes the surface to form sawtooth facets comprised of (113) and (111) planes, where an ordered (113)2x2 reconstruction is observed. Interestingly, a large number of domain boundaries form between the 2x2 regions, resulting in a "weave-like" pattern of short rows. The stabilization of the (113) plane by metal deposition on (5 5 12) has also been observed for Au, Ag, and Sn deposition, but the 2x2 reconstruction is unique to Cu. Results for Cu deposition on (113) to produce the 2x2 phase will also be presented.

1 A.A. Baski, K.M. Jones, K.M. Saoud, Ultramicroscopy 86 23 (2001).

SS4-ThP-6 LEED I-V Curve Analysis of Adsorption Structures of Ba on Si(001) Surface
T. Urano, Y. Takeda, S. Hongo (Kobe University, Japan)
Ba atoms adsorbed Si(001) surfaces show 2x3, c(2x6), 2x1 and 2x4 superstructures at submonolayer coverage, successively.1,2 These structures have been examined by LEED I-V curve analysis. The genetic search algorithm proposed by Doll and Van Hove3 has been operated to find a global optimum structure efficiently. After that the automated Tensor LEED analysis is carried out to refine the structure more precisely. Several structure models have been proposed by other researchers.4 In our previous LEED observation using a single domain Si(001)2x1 substrate, the direction of the two-fold periodicity of the 2x4 and 2x3 structures was determined to be parallel to the Si dimer.1 For the 2x3 structure, several models having Ba atoms of 1/6 monolayer(ML) and 1/3 ML, which is consistent with the UPS results showing the existence of dangling bond and dimer bond states, have been considered. The results of STM observations5 have been also considered. At the moment, though the R-factor value is not small enough, the model in which Ba atoms sit on the 3rd layer substrate Si atoms is most suitable. For the 2x1 and 2x4 structures, an amount of Ba atoms in the models are 1/2 and 1 ML, respectively. These results will be shown in the conference.

1Y.Takeda et.al.,Surf.Sci.402-404(1998)692.
2X.Hu et.al.,Surf.Sci.426(1999)69.
3R.Doll and M.A.Van Hove, Surf.Sci.355(1996)L393.
4W.C.Fan and A.Ignatiev, Surf.Sci.253(1991)297.
5K.Ojima, submitted to Surf.Sci.

SS4-ThP-7 High Resolution AES Mapping and TEM Study of Cu(In,Ga)Se2 Thin Film Growth
C.L. Perkins, Y. Yan, K.M. Jones, R. Noufi (National Renewable Energy Laboratory)
The chalcopyrite Cu(In,Ga)Se2 (CIGS) shows promise as an absorber layer in thin polycrystalline solar cells, however, many details of this complicated material's growth processes remain unclear. At one point during the so called "three stage" CIGS growth process used at the National Renewable Energy Laboratory (NREL) and elsewhere, the growing film is subjected to a copper rich flux in a selenium ambient. Previous workers have postulated that during this stage there exists a thin film of liquid CuxSe at the outermost portion of the CIGS, and that this layer acts as a reservoir of copper as well as a layer in which rapid mass transport is possible. In this paper we present TEM and high resolution AES mapping data taken on samples that had their growth interrupted at a stage when CuxSe was expected to be present. The AES maps show CIGS grains which are highly enriched in copper relative to the rest of the CIGS film, and that these same areas contain almost no indium, results consistent with the presence of CuxSe. Small area diffraction analysis and energy dispersive spectroscopy (EDS) performed on these same samples independently confirm the presence of CuxSe at the surface of growing CIGS films. The implications of these results are discussed in terms of the efficiencies of CIGS photovoltaic devices.
SS4-ThP-8 Surface Structures of SiC(0001) Surfaces and Oxygen Adsorption Effects
T. Aoyama, A. Ichimiya (Nagoya University, Japan); Y. Hisada, S. Mukainakano (Denso Corporation, Japan)
Surface structures of SiC(0001) √3x√3 R30° and 3x3 surfaces have been studied by rocking curves of reflection high energy electron diffraction (RHEED) intensities and auger electron spectroscopy (AES). The √3x√3 reconstruction is observed by annealing the Si pre-deposited specimen in a Si flux at 1000°C for 5 min. The 3x3 reconstruction is observed by successive annealing the √3x√3 surface in a Si flux at 940°C for 5 min. It is obserbed on the auger spectrums of √3x√3 and 3x3 surfaces that Si LVV peak of the √3x√3 is located at kinetic energy of 87 eV and that of the 3x3 surface is located at 91 eV. From the peak to peak ratio of Si LVV to C KLL of these surfaces, it is considered that the 3x3 is Si-rich surface (the ratio: 3) and the √3x√3 surface is Si-poor (the ratio: 0.6). From the structural analysis with RHEED dynamical calculations, it is concluded that the √3x√3 surface is terminated with C trimers on T4 or H3 sites and the 3x3 surface is terminated with Si pyramidal clusters. Oxygen adsorption effects on the 3x3 surface have been also studied by RHEED and AES. After 105 L (1.5x10-4 Torr, 15 min) oxygen exposure at 1000°C on the 3x3 surface, the RHEED pattern changes to 1x1 periodicity and the peak to peak ratio of Si LVV to C KLL and Si LVV to O KLL becomes 0.5 and 0.8, respectively. It is estimated that about 1/3 ML of oxygen atoms are adsorbed on the surface. From the AES results, it is considered that the oxygen atoms are located on embedded sites.
SS4-ThP-9 Surface Morphology of CuInS2 Thin Films
A. Azuma, Y. Akaki, H. Komaki, T. Ikari (Miyazaki University, Japan)
Solar cell techniques using I-III-VI2 chalcopyrite semiconductors have made rapid progress for the solution of environmental and resources problems. Among chalcopyrite semiconductors, CuInS2 may be the most promising material for photovoltaic applications due to the bandgap energy of 1.5 eV which perfectly matches the solar spectrum for energy conversion. However, the physical properties of CuInS2 are almost unknown.In this work, the CuInS2 thin films were grown by vacuum evaporation method. The source material was CuInS2 compound material grown by Hot-Press method. Using substrate was glass and growth temperature is room temperature. After the vacuum evaporation, the sample was annealing under Ar or air atmospheres from 200 to 500 °C. The samples were examined by X-ray diffraction (XRD), electron probe microanalysis (EPMA), scanning electron microscopy (SEM) and optical absorption measurements.The XRD spectra indicate that poly CuInS2 films are successful grown on glass substrate until 300 °C. CuIn5S8 and Cu2S are grown with increasing the annealing temperatures. The EPMA results are in good agreement with the stoichiometry compositions of the CuInS2, CuIn5S8 and Cu2S films. Surface Morphology of the samples is drastically changed. A number of the spots increase with increasing the annealing temperatures. Furthermore, absorption coefficients decrease with increasing the temperatures.
SS4-ThP-10 Atomic Resolution Imaging of Si(100)1x1:2H Dihydride Surface with Non-contact AFM
S. Araragi, Y. Sugawara, S. Morita (Osaka University, Japan)
Noncontact atomic force microscopy (NC-AFM) is a useful tool which has an atomic resolution applicable to many fields such as surface science. It is very important to understand the interaction on hydrogen atoms and silicon surface, when we deepen the knowledge as the surface treatment and semiconductor materials of a nano scale. Therefore, the hydrogen terminated silicon surfaces have been actively studied as the most important fields of surface physics. In the previous experiment, we succeeded in the NC-AFM imaging of individual hydrogen atom on the Si(100)2x1:H monohydride surface. Thus, the NC-AFM is suitable for the observation of the hydrogen terminated silicon surface. In this paper, we investigate how the NC-AFM images the Si(100)1x1:2H dihydride surface. As a force sensor, we used a clean silicon tip. The image with the 1x1 structure was observed when the tip was relatively far from the sample. When the distance between the tip and the sample became closer, the 2x1 structure where the bright line and the dark line were alternately located was observed. Furthermore, when the tip became closer, the 1x1 structure was again appeared. It is considered that the attractive interactions between the tip and the sample changed with the change of the distance between the tip and the sample. When the tip is relatively far from the sample, the 1x1 structure of the original dihydride surface is imaged because of less the attractive interaction. However, when the tip approaches the sample, the attractive interaction between the tip and the sample becomes stronger and hence the structure of dihydride surface changes. It turns out that the Si(100)1x1:2H dihydride surface changes by the attractive interaction between the tip and the sample in NC-AFM.
Time Period ThP Sessions | Topic SS Sessions | Time Periods | Topics | AVS2001 Schedule