Photon Factory Activity Report 2005 #23Part B (2006)
Surface and Interface

7B/2003G146,2005G091

Relationship between strain and electronic structureof Ge nanoislands on a Si(111)-7×7 surface
Masahiko SUZUKI, Ryota NEGISHI, Yukichi SHIGETA*
Field of Nanoscience and Technology
International Graduate School of Arts and Sciences,
Yokohama City University,Seto, Kanazawa-ku, Yokohama 236-0027, Japan.

Energy calculation
We also calculated electronic structure for the adatom
on the Ge island. To simplify the calculation, we chose acluster forming a T4 site as shown in Fig. 1(a). In thecalculation, we fixed the first-layer atoms at the ideal
sites of the Si crystal to reproduce the situation on the Sisubstrate. The energies of the electronic states werecalculated for a Si and a Ge cluster with an adatom atvarious heights in the range of 0.08–0.26 nm from thefirst layer. The energy calculations were performed basedon the density functional method within the generalized
gradient approximation using the Becke3LYP functional[2,3] and the 6-311G basis functions by Gaussian 03[4].
The relations between the height of the adatom and theenergy of the DB state for the Si and Ge cluster are
shown in Fig. 1(b).
For the whole adatom height range, the calculated
electron density of the highest occupied molecular orbital(HOMO) was preferentially distributed around theadatom. Therefore, the HOMO state is the DB state at theadatom. The energy of the HOMO state decreases withthe adatom height almost linearly at the rate of −9.4
eV/nm for the Si cluster and −8.2 eV/nm for the Ge
cluster while their total energies reach their minimums at0.185 nm for the Si and 0.193 nm for the Ge from the firstlayer, respectively.

-4-4.5
Energy (eV)Introduction
Germanium quantum dots formed on the Si substrate
have been attracted for their potential applications as Sibased quantum electronic and photo electronic devices. In
such a hetero-epitaxial system, a key phenomenon is thechange in the electronic structure due to the strain causedby the lattice mismatch between Si and Ge. To control theproperty, it is very important to understand the relation
between the strain and the electronic structure for thesystem, synthetically.
In this report, we show a relation between theelectronic structure and the the relaxation structure of Genanoislands on a Si(111)-7×7 surface[1]. Since therelaxation structure depends on the size of Ge nanoislands,
we fabricated the Ge nanoislands a uniform size on theSi(111) substrate and investigate the electronic structureby the angle resolved ultraviolet photoelectron
spectroscopy (ARUPS).

(b)
Ge5H7
Si5H7-5-5.5-6-6.5

0.09 0.12 0.15 0.18 0.21 0.24
Height (nm)Fig.1. (a) Model cluster of energy calculation. Thecluster forms a T4 site and all the Si (or Ge) atoms
except for the adatom were fixed on the ideal sitesof the Si bulk crystal. (b) Relationships between theheight of the adatom and the energy of the DB state
for the Si and Ge clusters.

Experiment
The STM measurements were performed in an-8
ultrahigh-vacuum STM chamber (base pressure: 2 × 10Pa) equipped with reflection high-energy electron
diffraction (RHEED) and an electron beam evaporation
source. STM images were observed with an STM. TheARUPS measurements were performed by usingsynchrotron radiation on the beam line BL-7B. The
experimental chamber, whose base pressure is about 6 ×-810 Pa, is equipped with an angle-resolved photoelectron
spectrometer, an evaporation source, a quartz crystal
thickness monitor and a sample manipulator with acryostat.

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