06 chapter 2

6&4P78& TWO
REVIEW
OF a-SiGe FILMS

CHAPTEK TWO 12
11.1: Historical Background
For over two decades or so, hydrogenated amorphous silicon germanium
(a-SiGe:H) alloy films have been of much interest as a promising material for the
production of solar cells. Great efforts have been made to improve the optoelectronic
properties of a-SiGe:H alloy films, desired as a small energy band gap component of
multi-junction solar cells and other devices employing amorphous hydrogenated
semiconductors[l-6]. Also a-SiGe:H is widely used for the production of multi-
junction solar cell, since the optical energy band gap (Eopt) can be easily tuned to
match the wide range of the solar radiation spectrum. It has been reported that with
increasing germanium content (CGe) the Eopt of these alloys shifts to lower energies.
But the relationship between each property of the alloy film has not yet been well
understood compared with relatively detailed comprehension that exists for
hydrogenated amorphous silicon (a-Si:H)[7,8]. This was mainly due to the complexity
caused by the addition of another component germanium (Ge) to a-Si:H. The actual
composition of a-SiGe:H alloy film has been found to depend strongly on the detailed
deposition conditions. Therefore, optoelectronic and structural properties of a-SiGe:H
alloy films deposited under various conditions have been studied using quite number
of investigations. Most of these studies have dealt with hydrogenated samples
deposited by glow discharge technique[2,9-16]. Besides the enhanced optical
absorption for longer wavelengths, the electronic properties which are relevant for
photovoltaic applications are strongly deteriorated by alloying Ge to a-Si:H. These
properties are like Urbach energy, defect density, and microstructural factor increase
whereas the ambipolar diffusion length decreases with a reduced £^,[17-21]. In spite
of the reduction in Eopt, a-SiGe:H alloy film based diodes have been successively
applied as a bottom cells in stacked solar cells[22]. The electronic quality of a-SiGe:H
alloy films has been improved by optimizing deposition conditions in both diode- or
triode- glow discharge reactors[23-25]. Therefore, in a conventional radio frequency
(RF) glow discharge technique, deposition of a-SiGe:H alloy films with different CGe
and narrow Eopt were carried out using silane (SiKU) and germane (GeKU) as a source
gas mixtures. But the results of these films indicate poor optoelectronic properties
compared to those of the best a-Si.H films deposited from glow discharge[26]. The

CHAPTER TWO 13
deterioration of the optoelectronic properties of a-SiGe:H alloy films was attributed to
the preferential attachment of hydrogen (H2) atoms to silicon (Si) rather than Ge[27],
which in turn leads to the formation of a considerable number of dangling bonds on
the Ge atoms. In addition, an increase of the dihydride (Si-H2) groups has been
observed and negatively affects the material properties via the increased structural
disorder[28]. Several attempts have been made to overcome this problem by using
different source gas mixtures either by diluting SiFU and GeFL* with hydrogen (H2) or
by an introduction of fluorine (F) in addition to H2 as a bond terminator[29-33]. Some
improvement in the material quality has been reported in the literature of the
fluorinated material [34]. Figure (1) shows the schematic diagram of a typical mixture
of the source gases utilized in the plasma deposition of a-SiGe:H alloy films. These
gases are diluted either with hydrogen or hydrogen and fluorine to deposit
hydrogenated or hydrogenated and fluorinated SiGe alloy films.
In 1983, Nozawa et al.[29] have discovered that deposition of a-SiGe.H alloy
films by glow discharge from gas mixtures of fluorine and hydrogen (SiF4 + GeF4 +
H2) improved the photoconductivity {aPh) of alloys of EopX « 1.5 eV by about an order
of magnitude. This was with a speculation that F might be a suitable bond terminator
in the alloys than in unalloyed Si[35]. Thereafter, Mackenzie et al.[3] have established
the existence of significant differences in a-SiGe:H alloy films microstructure when
they are made from a source gas mixture of SiF4 and GeF4 diluted with hydrogen
rather from a mixture of SiH4 and GeH4. However, deterioration in optoelectronic
properties of Ge-rich alloys of a-SiGe:H has been attributed to the changes in the
band-structure and an increased nonuniformity of the film microstructure[3]. In
general development of a-SiGe:H alloy material revealed a common feature which
shows sharp deterioration in the photovoltaic quality of the material of Eopt < 1.5 eV.
Because of that several attempts have been made to improve the material quality using
various techniques. Zeman et al.[36] have found that hydrogen dilution technique was
of major importance to grow structurally dense material with highly improved
optoelectronic properties suitable for device applications. The main objective of the
above techniques is to energize the heavy and sticky Ge radicals in order to reduce
inhomogeneity.

Fig. 1 -Schematic diagram of the typical source gas
mixtures of Si and Ge volatile compounds utilized
in the plasma deposition of SiGe alloy films.

CHAPTER TWO 14
Although a lot of progress has been made as far as the important technological
aspects of a-SiGe:H alloys are concerned, many basic properties of these materials are
still not well understood. This is not surprising if one considers the large sample space
spanned by the different compositions, preparation methods, deposition conditions,
and doping levels. Several attempts have been made in the literature to explain the
degradation of the film quality with the deposition conditions. The interpretations are
based on the existence of various structural and compositional inhomogeneities[37-
39]. The structural inhomogeneity was represented by the heterogeneous network
while compositional inhomogeneity lead to two types of film microstructure
associated with clustered hydrogen and Ge concentration fluctuations. Matsuda[40]
have introduced that the network structure of a film is principally determined by its
surface growing process and low density films are likely to grow if the surface
diffusion coefficients of the adsorbed radicals are too small to find out their
energetically(fkvorabie^sites. According to this argument the structure of a-SiGe:H
alloys can be discussed in terms of the surface diffusion coefficient of adsorbed
radicals.
Various studies on the optoelectronic, structural, and spin properties of
a-SiGe:H alloys deposited by a glow discharge of SiHt, GeH*, and H2 gas mixtures
were carried out using different characterization techniques over a wide range of
deposition conditions. The most commonly used techniques to evaluate the material
properties are the measurements of dark conductivity (a</), photoconductivity (aph) and
subband gap absorption. Ray et al.[41] have observed the increase in oph of the plasma
enhanced chemical vapor deposited (PE-CVD) a-SiGe:H alloy films when they are
deposited at slow rates by controlling the gas flow rates of SLH4 and GeTLt, and
chamber ^pressure. Also they have reported that for Ge-rich films with Eopt < 1.5 eV
low deposition rate and hydrogen dilution of the gas mixture are necessary to improve
the material quality. Using the super chamber method, Haku et al.[42] have obtained a
high photosensitivity {aph^o^) of 106 for an a-SiGe:H alloy films containing low
impurity concentration. Recently Xu et al.[43] have succeeded to fabricate high quality
narrow band gap a-Sio.5sGeo.42:H alloy films by controlling the bonded hydrogen