An Experimental and Theoretical Study of Semiconducting Silicides and
Germanides for Silicon Based Electro-Optic Applications
Terry Golding
Wayne Holland
Arup Neogi
Department of Physics and Materials Science
University of North Texas
Denton, Texas, 76203
Draft White Paper
Executive Summary
We propose a program to investigate the synthesis, materials, and electro-optical
properties of semiconducting silicides and germanides – specifically FeSi2-xGex, FeMSi2
(M = Os, Cr, W) and FeMSi2-xGex.
FeSi2 has recently been shown to be optically active in the technologically important 1.3 1.5m wavelength range. Twelve other silicides have also been identified to be
semiconducting, with bandgaps ranging from 0.07 to 2.3eV. Theoretical work also
suggests that related germanides will also be semiconducting.
The aim of the program is the ‘holy grail’ for silicon integrated optoelectronic devices –
to synthesize silicon based alloys that are optically active, i.e. possess direct bandgaps thus enabling silicon based bandgap engineering schemes, and silicon based
optoelectronic devices.
The experimental program will be assisted by theoretical studies of ab initio densityfunctional studies to determine suitable alloy compositions and elements for alloying.
The materials will be synthesized by molecular beam epitaxy and by ion beam
impantation techniques. Alloy composition, crystalline structure, phase, and strain will
be studied using x-ray diffraction and Raman spectroscopy and the bandstructure of the
alloys will be determined by photoluminescence, FTIR and temperature dependent
magnetostransport studies.
Introduction
The necessity for direct and tunable bandgap semiconductors, such as the II-VI and III-V
compounds for opto-electronics and the dominance of Si based microelectronic circuits
has resulted in considerable efforts over the past three decades to monolithically integrate
these compounds with Si. However, this approach has met with limited success at best,
and it is becoming increasingly evident that such a task may be technologically
unworkable. An alternative approach may be to explore further the properties of
semiconducting silicides. Thisapproach has been made all the more attractive by the
success of FeSi2 for electro-optical applications, having a direct band gap of 0.83-0.87 eV
at room temperature. For this reason it is by far the most studied. The silicides have
high thermal stability, ability to withstand chemicals normally encountered during
fabrication process, and the ability to form a passivating layer of SiO2 by thermal
oxidation fulfill many requirements imposed by fabrication technology and device
compatibility. While the vast majority of silicides are metallic (several for example
CoSi2, NiSi2, and TiSi2, have been employed as interconnects and gates in MOS
structures in silicon integrated circuits) ‘there are however ~ 13 silicides that are
semiconducting with bandgaps ranging from 0.07-0.12eV (hexagonal MoSi2, WSi2 and
ReSi2) to 2.3eV (Os2Si3). Of these FeSi2 and OsSi2 have isocrystal structures, possibly
permitting bandgap engineered schemes similar to the II-VI and III-V systems as are
Ru2Si3 and Os2Si3. Of these four FeSi2 and OsSi2 have the same crystal structure and
fesi2 has been studied extensively.
For epitaxial growth and controllable alloying possibilities we must have the same crystal
type and stoiciometry between any two binaries. This leads us to the following possible
combinations.
FeSi2 and OsSi2.
Space group Cmca
CrSi2, MoSi2, and WSi2.
Space group P6222
Ru2Si3 and Os2Si3.
Space group Pbcn
Germanides. Much less studies. This is what is known.
Know Os2Si3 and Os2Ge3 are both Pbcn as is Ru2Si3 and Ru2Ge3.
Os2Si3 and Os2Ge3 is not determined if they are misible.
Ru2Si3 and Ru2Ge3 are misible.
Os2Ge3 has bandgap 0.87 eV (quasidirect)
OsGe2 – OsGe2
Ternary Silicides
Ternary silicide based compounds with the metal or silicon atoms partially substituted by
other metals or germanium respectively.
Ternary compounds in this couple are most attractive for energy gap engineering.
However, they have not been synthesized or investigated yet.
They are attractive fo flexible regulation of the fundamental electronic properties of
silicides and for epitaxial matching of semiconducting silicide films.
I
1
II
III
IV
V
VI
VII
VIII
H
He
2
Li
Be
3 Na
Mg Mg2Si
CaF2
Orthorhombic
0.78eV
4 K
Ca
Cu
5 Rb
B
Al
Sc
Zn
Sr
Ag
6 Cs
Ga
Cd
Au
Ge
Sn
Pb
Ac
Lr
Ku
Ne
Ar
Cr
CrSi2
CrSi2
0.35eV
Mn
MnSi2-x
0.7eV
Fe
FeSi2
FeSi2
0.78eV
Mo
MoSi2
CrSi2
0.07eV
Tc
Ru
Ru2Si3
Ru2Si3
0.8eV
Te
W
WSi2
CrSi2
0.07eV
Bi
I
Ni
Rh
Pd
Xe
Re
ReSi1.75
0.12eV
Po
Os
[OsSi
0.34eV]
[Os2Si3
Ru2Si3
2.3eV]
[OsSi2
FeSi2
1.8eV]
At
Ns
Table I { adapted from [Borisenko (2000)]}: Listing of all the known semiconducting
silicides with bandgap and structure type. Note: The group VI silicides all have the same
crystal and stociometry type, CrSi2. FeSi2 and OsSi2, and Ru2Si3 Os2Si3.
Systematic study of the ternary silicides is at its very beginning.
Germanides
All of the related germanides are also semiconductors.While a priori estimated possibilities to form ternary silicides by substituting
silicon atoms withgermanium look rather simple, only a few have been experimentally obtained and investigated.
Crystal structure
b-FeSi2
The crystal structure of FeSi2 is a base centered orthorhombic in the D(18,2h) (Cmca) space group having 48 atoms per unit cell and
with lattice parameters a =0.9863 nm, b = 0.7884 nm, and c = 0.7791 nm. The unit cell has two equivalent Fe sites, each occupied by
8 atoms as well as two inequivalent Si sites with 16 atoms in each.
OsSi2
Co
Kr
Sb
Ta
Lu
Hg
Ra
F
As
Nb
Hf
O
P
V
Zr
In
La
N
Si
Ti
Y
Ba
BaSi2
EuGe2
1.3eV
7 Fr
C
Ir
Ir3Si5
1.2eV
Rn
Materials Synthesis
Materials Characterization
Theoretical Program