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Report
Growth of High Performance Multicrystalline
Silicon; A Literature Review
Author(s)
Arjan Ciftja
Gaute Stokkan
SINTEF Materials and Chemistry
Solar Cell Silicon
2014-07-08
SINTEF Materialer og kjemi
SINTEF Materials and Chemistry
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Postboks 4760 Sluppen
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KEYWORDS:
Silicon
Solar cells
High Performance
Multicrystalline Silicon
Crystal Growth
Report
Growth of High Performance
Multicrystalline Silicon; A Literature Review
VERSION
DATE
1
2014-07-08
AUTHOR(S)
Arjan Ciftja
Gaute Stokkan
CLIENT(S)
CLIENT’S REF.
Research Council of Norway
Bjørn Thorud
PROJECT NO.
NUMBER OF PAGES/APPENDICES:
102005624
25 + Appendices
ABSTRACT
This review on "Growth of High Performance Multicrystalline Silicon (HPMCSi)" is intended as a summary of the state of the art at the beginning of the KPN
project "Impurity Control in High Performance Multicrystalline Silicon". This
project focuses on the role of impurities in this type of material, and a general
understanding of the characteristics of the special material is thought to be
essential for conducting good and relevant research. Although the industrial
production of this material starts to dominate the market, publications are still quite
sparse. Nevertheless, important characteristics, such as grain size, grain growth,
grain boundary types, dislocation density and dislocation cluster characteristics
have been described and discussed. Also conditions during nucleation and growth
and their effect on material properties have been reported in some detail. This
overview should be sufficient to generate a good understanding of HPMC-Si for the
members of the consortium to be able to perform effective and relevant research on
impurity control in this material.
PREPARED BY
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Arjan Ciftja, Gaute Stokkan
CHECKED BY
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Semih Senkader
APPROVED BY
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Eivind Øvrelid
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Document history
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Table of contents
1
Introduction .................................................................................................................................. 4
2
Characteristics of High Performance Multicrystalline Silicon ........................................................... 4
3
The road to HPMCSi ....................................................................................................................... 5
3.1 Standard production of mc-Si ........................................................................................................ 5
3.2 Grain boundaries and dislocations densities in standard multicrystalline silicon. ........................ 6
3.3 Growth control by controlling nucleation properties – dendritic nucleation ............................... 8
3.4 Seeding techniques for mono-like Si growth ............................................................................... 11
3.5 Small grains versus large grains ................................................................................................... 13
4
A closer look at the development of HPMCSi performed at NTU with Taiwanese industry (A+ A4+ qualities) .............................................................................................................................. 13
4.1 The control of growth front ......................................................................................................... 13
4.2 The control of grains and grain boundaries ................................................................................. 15
4.3 Control of nucleation conditions ................................................................................................. 17
5
Methods and techniques for nucleation control ........................................................................... 17
5.1 Chip seeding ................................................................................................................................. 17
5.2 Coating and crucible as nucleation source .................................................................................. 19
6
Control of grain size and defect density during the growth phase ................................................. 20
7
Summary..................................................................................................................................... 21
8
References .................................................................................................................................. 22
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1 Introduction
This review on "Growth of High Performance Multicrystalline Silicon (HPMC-Si)" is intended as a
summary of the state of the art at the beginning of the KPN project Impurity Control in High
Performance Multicrystalline Silicon, which is a competence building project (KPN) sponsored by the
Research Council of Norway, REC Solar, REC Silicon, Steuler Solar and The Quartz Corp. The
research partners in the project are SINTEF, NTNU and IFE. This project focuses on the role of
impurities in this type of material, and a general understanding of the characteristics of the special
material is thought to be essential for conducting good and relevant research, and the report is intended
to serve this purpose.
Photovoltaic solar energy is recognized as one of the most promising future sustainable energy sources.
Multicrystalline silicon solar cells represent one of the most cost effective alternatives, and consequently
has the highest market share. During crystallization of multicrystalline-silicon (mc-Si) ingots, crystal
defects and impurities are introduced from the feedstock, the crucibles and the coating and due to the
process. The impurities and defects interact to reduce the solar cell efficiency. Understanding the effect
of the defects and impurities on the efficiency of silicon solar cells is a major focus of research and
development. This research which aims to increase solar cell efficiency needs to address two factors:
How to minimize the presence of crystal defects and impurities, and how to mitigate their effects. Recent
developments in crystallization technology have shown that it is possible to produce mc-Si with
particularly low defect density in a robust industrial manner. Such silicon is currently termed High
Performance Multicrystalline Silicon (HPMC-Si) and is the topic of this report.
2 Characteristics of High Performance Multicrystalline Silicon
Multicrystalline silicon together with monocrystalline silicon represents the basis of today’s
photovoltaic technology. The production of mc-Si modules is set to dominate PV manufacturing during
2014, with p-type multicrystalline Si technology accounting for 62% of all modules produced,
according to the latest NPD Solarbuzz PV Equipment Quarterly report (see Figure 1) [1]. The reason for
this is that multicrystalline silicon offers advantages over monocrystalline silicon with respect to
manufacturing costs and feedstock tolerance. The efficiencies of multicrystalline silicon solar cells are
affected by recombination-active impurity atoms and extended defects such as grain boundaries and
dislocations. Growing large crystals have been related to increasing quality because this reduces the
volume affected by grain boundaries in the silicon matrix [2].
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Figure 1 Market share of solar cells for 2014 [1]
Although the grain boundaries themselves are not particularly harmful to solar cell performance as long
as the grain size remains above a certain threshold [2], they act as concentrators for thermal stress
during the production process and are pronounced sources for dislocations and sub grain boundaries –
defects that are far more harmful to the solar cell performance [3-5]. The idea is that large grains
provide more ideal conditions (similar to monocrystalline silicon) and should give fewer crystal defects.
As it is reported in the literature, dislocation clusters generated from grain boundaries tend to expand
and cover large areas during the crystal growth process [4]. The regions with low performance affect the
performance of the entire device. Furthermore, they limit our efforts to improve the good areas, for
instance by impurity control during crystallization or by gettering [6, 7].
During the last few years the general trend to grow large grains is being replaced by a new strategy
which focuses on generating much smaller grains by controlling the nucleation stage [8]. Through this
new strategy, it is possible to stop the uncontrolled proliferation of dislocation clusters during growth
[9]. The result is a material with much higher macroscopic homogeneity and a different defect structure:
fewer dislocation clusters and sub-grain boundaries, different microscopic dislocation properties, higher
density of grain boundaries and different grain boundary properties [10]. This new, high quality
material, named high performance multicrystalline silicon (HPMC-Si), is already starting to dominate
the market as a commercial product, and the development has been so rapid that it can be characterized
as revolutionary. General characteristics are:
• Much smaller initial grain size than traditional multicrystalline Si, typically in the mm2 range,
• Much higher proportion of random angle grain boundaries and smaller proportion of twins.
• Dislocation clusters appear, but they are not allowed to proliferate uncontrolled and only cover a
limited height.
• General efficiency increases due to material quality only (subject to same cell process as in
traditional mc-Si) of at least 0.5 % absolute.
3 The road to HPMCSi
3.1 Standard production of mc-Si
Multicrystalline silicon (mc-Si) is crystallized in a silicon nitride (Si3N4)-coated quartz crucible by
directional solidification. In the so-called Bridgman technique crystallization is realized by slowly
moving downward the liquid silicon-containing crucible out of the heated hot zone of the process
chamber, as shown in Figure 2. This, and other methods aimed at controlling a unidirectional, vertical
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temperature gradient, are still the main methods used for the fabrication of multicrystalline ingots.
Currently, commercial 270 - 600 kg directional solidification systems are widely used in production and
the systems capable of producing 800–1000 kg ingots are also being developed. The Si3N4 coating
prevents the adhesion of the silicon ingot to the quartz crucible walls.
Figure 2 Conventional Bridgman technique for multicrystalline silicon production [10].
After solidification starts in the bottom region, the crystallization front (the liquid–solid interphase)
moves in a vertical direction upwards and results in a columnar crystal growth. Adjacent wafers
fabricated out of the ingots show nearly identical defect structures (grain boundaries and dislocations).
Common crystallization velocities used in industrial production are in the range of about 0.8-1.5 cm/h.
Mc-Si directionally solidified from a nitride-coated quartz crucible contains many defects, such as grain
boundaries, dislocations, inclusions, and impurities which usually serve as recombination centers for the
light generated electrons and holes and therefore are harmful to the solar cell performance. As a result,
the efficiency of mc-Si solar cells is usually lower than that of mono-crystalline silicon [11]. The main
crystal defects in multicrystalline silicon are grain boundaries and dislocations. The concentrations of
these defects as well as their electrical activity are considered as crucial factors determining the solar
cell efficiency. With respect to the grain boundaries and the grain size, basically smaller grains are
observed at the beginning of the crystallization process at the ingot bottom. With increasing ingot
height, certain grains prevail at the expense of surrounding grains and thus give rise to an increase in the
mean grain size. The mechanisms determining grain growth during directional solidification are not well
described for multicrystalline silicon, but the phenomenon is related to solid liquid interface energies for
different crystal orientations, grain boundary energies and twinning.
Dislocations are thought to be generated at grain boundaries and subsequently multiply during their
motion through the silicon matrix. Such multiplication is mainly driven by the thermal stress generated
during solidification. In addition, the amount of metals in mc-Si is much higher than that in
monocrystalline Si because of the less pure coating and crucible materials [12, 13]; these elements
interact with dislocations and make them severely influence the electrical properties of the crystal.
3.2 Grain boundaries and dislocations densities in standard multicrystalline silicon.
As mentioned before, the grain boundaries and dislocations have a profound effect on the wafer quality
and impact energy conversion efficiency of the mc-Si solar cells. Grain orientations, grain boundaries,
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and the dislocation density are closely related to each other and affect one another. As such, they should
be taken into consideration in conjunction rather than as separate characteristics. Grain boundaries will
lead to very different mechanical and electrical characteristics under different orientations. For example,
so called Σ3 grain boundary is electrically inactive even in contaminated samples, and hence it does not
act as recombination center for the charge carriers. The electrical activity of grain boundaries and
dislocations is determined by their impurity decoration (specifically by transition metals) and strongly
increases with increasing impurity concentrations [14].
A report of grain boundary character in standard multicrystalline silicon is given in [15]. It shows that
grain boundaries are dominated by twins and other Coincidence Site Lattice (CSL) boundaries, as seen
in Figure 3. This is consistent with other reports of grain boundary character [16]. CSL boundaries
separate grains that have a very specific rotation angle between each other (e.g. 60° rotation around a
<111> axis for the Σ3 boundary), which leads to special symmetric patterns on the grain boundary [17,
18] and have generally lower grain boundary energies than grain boundaries without such symmetry,
also called random angle grain boundaries (RAGB).
Figure 3 Distribution of grain boundary types in standard multicrystalline silicon. From [15].
Crystal dislocations are the most efficiency deteriorating defects in solar cell mc-Si. For energetic
reasons dislocationsoften align in "fences" that have the characteristics of grain boundaries separating
very small grains that have nearly the same crystal orientation. Such grain boundaries are called "subgrain boundaries" and have been shown to be very recombination active. The dislocation density can be
experimentally measured by counting micron-size etch pits after appropriate chemical etching steps. It is
a very important parameter which shows a good correlation to the electrical properties of the silicon
wafers, namely wafer lifetime and diffusion length. It is furthermore a characteristic of mc-Si that
dislocation density tends to increase with height inside the ingot [5]. An investigation showing this
tendency is shown in Figure 4. Dislocation density ranges from ~104 cm-2 in good areas to >106 cm-2 in
high density clusters. A good way of representing dislocation density is to quantify the proportion of
area that has a dislocation density above a certain threshold, e.g. 105 cm-2 as is done in Figure 4.
Dislocations are induced and multiplied by thermal stress that is originated from temperature-time
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gradients during crystallization and cooling of the ingot, which depend on crystal orientation. Therefore,
in order to obtain high quality mc-Si wafers for high-efficiency solar cells, grain control during
directional solidification (DS) is very important [5].
Figure 4 Dislocation density as function of height in wafers produced in a pilot scale furnace [5].
In the early days of DS growth of mc-Si, little attention was paid to the initial conditions during
nucleation. The focus was on obtaining as low global thermal stress as possible both during growth
(which is achieved by controlling the solid liquid interface as flat as possible) and during cooling [19],
which is controlled by the time change of thermal gradients. An "anneal" phase at temperatures higher
than 1000°C was often implemented after growth before further cooling to release thermal stress from
the growth phase, but it is very doubtful if this has any proper effect on the dislocation density. Since
grain boundaries have been shown to be profound dislocation sources, obtaining large grains to reduce
grain boundaries was regarded as the most straight forward approach to increase the crystal quality and
the solar cell efficiency. However, as it will be shown later through the report, having small grains with
random orientations might be more advantageous for silicon solar cell efficiencies.
3.3 Growth control by controlling nucleation properties – dendritic nucleation
The focus on the importance of nucleation properties on ingot quality was pioneered by a group at
Tohoku University in Japan lead by Prof. Kazuo Nakajima. The melt growth behavior of mc-Si was
investigated by Fujiwara et al. [3] by using an in situ observation system to obtain information on how
to control the multicrystalline structure during directional growth. Growing a mc-Si ingot with specially
oriented grains was considered as a way to avoid the problems that grain boundaries create in solar cells.
With this idea in mind, the researchers at Tohoku University investigated a technique to induce dendrite
formation at the initial stage of directional solidification which would lead to obtaining such a structure.
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Their attempts were successful and a grain could grow very fast in the direction perpendicular to the
crystal growth direction along the bottom of the crucible, resulting in a large crystal grain as shown in
Figure 5. Implementation of dendrite growth at the initial stage of DS in a silica crucible indicated the
feasibility of this technique to obtain mc-Si ingots with large grains and with specific orientations.
Figure 5 EBSD orientation maps of samples cooled at: (a) 1 K/min and (b) 50 K/min. Reference
direction is shown in color [this is not correct ref].
Figure 6 shows the concept employed by Fujiwara et al. [3] to grow mc-Si ingot using dendrite growth
during casting. According to this approach, dendrite growth is induced along the bottom wall of a
crucible at the initial stage of directional growth by controlling the cooling conditions. Then, the upper
planes of most dendrite grains would result with the same orientation if the degree of undercooling is
equal anywhere in the bottom of the crucible. Crystallization in the vertical direction is subsequently
initiated with the dendrites as nuclei that determine the crystal orientation.
Figure 6 Concept of growing mc- Si ingots with large grains . Dendrite growth along the bottom wall of
the crucible is used at the initial stage of directional growth [3].
Let us take a look at closely how such a structure could be formed, as reported by the researchers at
Tohoku University. Mc-Si ingots were grown in 50-mm diameter silica crucibles using a small casting
furnace. An undercooling of approximately ∆T = 10 K is required to induce dendrite growth at the
initial stage of directional growth. First, the bottom part of the crucible was cooled rapidly to induce
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dendrite growth and then the crucible was pulled down at 0.2 mm/min in a temperature gradient zone of
20 K/cm. For comparison, another ingot (reference) was grown by simply pulling the crucible down at
0.2 mm/min in the same temperature gradient zone and the results were compared. Figure 7 shows the
orientation maps of 15 mm2 wafers cut from the bottom, middle and top parts of structure-controlled
(Figure 7 (a)) and reference (Figure 7 (b)) ingots. The (112) plane occupies almost the entire surface of
the wafers and large grains are observed in the bottom part to top part in Figure 7(a), because <110>
dendrites grow along the bottom wall of the crucible in the initial stage. In contrast, many equiaxed
grains with random orientations were observed in the sample shown in Figure 7(b). A sharp vertical
temperature gradient of 20 K/cm was thus used to obtain two very different structures, the difference
between the two being that the rapid cooling leads to a degree of supercooling which, upon nucleation,
initiates dendritic growth in the plane of the crucible (growth upwards is limited due to the temperature
gradient) while a different growth mode dominates for slow cooling where a smaller supercooling is
realized before growth starts from many more nuclei.
Figure 7 EBSP maps of Si wafers cut from: (a) structure-controlled and (b) non-structure-controlled
ingots. [ref]
Stokkan [5] studied nucleation of mc-Si and its correlation to dislocation density in a set of five
solidification tests which was not intentionally nucleation controlled. It was noticed that large grains
develop from the start;also in such material and concluded that these are result of growth from large
planar grains formed on the crucible bottom and not from random nucleation and grain selection in this
region. Low dislocation density was shown to correlate with samples where the nucleated grains had an
orientation likely to result from dendritic growth very close to parallel to the crucible bottom, but large
grains and preferred (as compared to random) growth orientations could also lead to high defect
densities if the orientation deviated from that. The work thus indicates that already in regularly grown
multicrystalline ingots, the dendritic growth in the first phase plays an important role, and the success of
the method depends on the ability to control this growth in a narrow region close to the crucible bottom.
Implementation of the technique developed by Fujiwara et al. in the PV industry was shown to be a
challenge. Structure control of the silicon ingots through initial dendritic growth of grains proved to be
difficult mainly because of the challenges in controlling the thermal conditions over large crucible areas.
Furthermore the thickness and relatively low thermal conductivity of quartz crucibles make it difficult
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establishing a high degree of supercooling in a thin enough layer near the crucible bottom: Reaction heat
from the fast dendritic growth heats up adjacent melt, destroying the necessary vertical thermal gradient
and may disturb both the growth direction of the dendrites and growth in adjacent regions. It is reported
that in industrial environments, the large dendrites are regularly surrounded by regions of smaller grains
[8], and that probably a very tight timing of the nucleation is necessary to achieve the desired
structure[20]. Such tight timing is difficult to achieve over large crucible areas.
The work on nucleation and growth carried out at Tohoku University [3, 21, 22] and other work
following research along the same principles of controlling quality by controlling the nucleation
properties [5, 8, 23, 24] indicated how important this phase is to obtain good quality, low defect
material. Although the currently evolving practice indicates that the dendritic nucleation method is not
the one favored to control this in large industrial systems [8, 10], yet the focus on the importance of
nucleation can be attributed to this work.
3.4 Seeding techniques for mono-like Si growth
The next logical step after realization of the importance of nucleation was elimination of nucleation
altogether by the use of seeded growth to establish the desired crystal orientation. The most
straightforward method is to place seeds of a given direction in the bottom of the crucible, as
demonstrated by Stoddard et al. [25, 26]. The presence of seeds in the bottom of the crucible is helpful
to suppress random nucleation. For these reasons, preservation of seed crystals is significantly important
and therefore, a certain height of the seed crystals must be kept un-melted during the melting process. A
flat with slightly convex seed-melt (s-m) interface is required during the melting process, which
depends on precise control of the thermal field in the silicon [13]. Figure 8 shows images from an ingot
and a wafer grown by this technique. Zhang et al [12] argues that although excellent results can be
produced with this technology, randomness of nucleation will affect the repeatability in industrial
operation which implies that robustness might be questionable.
Figure 8 Ingots with large grains: (a) side view of an ingot with the size of 156 mm x 300 mm and (b)
top view of a 156 x 156 mm2 wafer covering 95% single grain [12].
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Investigations by Lan et al. [8] on defect evolution in seeded vs. parasitic regions indicated that the
growth rate of the dislocation clusters is lower inside the seeded area compared to the mc-Si growth
parts. Figure 9shows the development of the defect area for the wafers from the seeded and unseeded
regions where one could observe that the growth of dislocations appears slower inside the seeded area.
This figure illustrates the challenges connected to defect generation in seeded growth: Although some
parts of the material have very high quality, other parts are prone to development of large dislocation
clusters. It quickly turned out that this approach had several challenges attached to it:
• Parasitic multicrystalline silicon growth from crucible side-walls [27]
• Increased extent of bottom poor quality material (red zone)[28]
• Increased cost due to seeds and longer cycle time
• High defect levels from junction sites and adjacent to multicrystalline regions [29].
While the first of these appear to be solvable by controlling the growth profile, the other three appear
much more difficult to solve in an industrial manner. Industry focus on this technology has currently
diminished greatly although research is still focused by several institutes, notably that carried out at
INES[28].
Figure 9 Grain structures (upper row) and defect mappings (lower row) of the wafers from seeded
growth at that different ingot positions; the solid line indicates the seeded area.
Figure 10 Comparison of defect developments in the wafers from the seeded and unseeded areas. The
defect distribution of A++ wafers (see below) is also included for comparison.
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3.5 Small grains versus large grains
Given the challenges with defect formation in mono-seeded systems, the importance of achieving large
grains with controlled grain boundaries for producing low defect/high quality material is questionable.
Although several works indicate a minimal defect development in systems where large grains of
favorable orientation appear, this conclusion from researching smaller systems was confronted
statistically by the relevant work performed in larger systems, and reported in the work performed by
National Taiwan University in collaboration with Taiwanese industry [8, 10]. This work gave statistical
evidence that the other extreme grain configuration, i.e. small grains with random orientation, was not
so disadvantageous considering dislocation density. As a result, several producers began implementing
nucleation schemes that favored this configuration. This technology proved easier to control and
producers have improved this product to a degree that it is now commonly known as High Performance
Multicrystalline Silicon (HPMCSi). It is important to note that different producers have different
approaches to achieving this quality and that the label refers to a product with a certain characteristic
rather than resulting from a specific process. And the characteristics are: Small grain size, high
proportion of random angle grain boundaries, low density of dislocation clusters, and a mechanism by
which dislocation clusters that appear at the start of the crystallization process disappear afterwards at a
higher level in the ingot. In other words, dislocations are not allowed to multiply and thus, affect the
quality of the silicon from the bottom to the top of the ingot.
4 A closer look at the development of HPMCSi performed at NTU with Taiwanese
industry (A+ - A4+ qualities)
Since the nomenclature (HPMC-Si) and the first focus on the method originated at National Taiwan
University and Taiwanese industry, the following chapter is a summary of the results published from their
development. A clarification of the naming convention "A+, A++, A3+ and A4+ wafers" will be given
below.
4.1 The control of growth front
The shape of the growth front (solid / liquid interface) during directional solidification is important and
affects grain growth. It is one of the factors that determine whether grains will grow inward or outward
in the silicon ingot. Thus, for a convex growth front, the grains will grow outward and the grain size can
get bigger easily if no new grains generate from the existing grains. On the contrary, for a concave
growth front the grains will grow inward. In this case, new grains might crystallize from the side wall of
the crucible and equiaxed grains might develop near the end as the columnar grains collide [8]. Since
the solidification front is perpendicular to the heat flux, the easiest way to get a convex front during DS
of silicon is to insulate the crucible to avoid the heat loss from the sidewall [8] or have an active heating
from the side. The first approach was employed by Lan et al. to control the grain growth by controlling
the growth front. In their work, Lan et al. used an industrial-furnace (GT DSS 450) to cast silicon ingots
with size of 84 cm x 84 cm x 36 cm and with a mass of up to 580–600 Kg. Applying alumina felt
around the crucible assured for better insulation. Hence, the heat loss from the side walls of the crucible
was reduced making it thus possible to direct the heat flow into the cooling pad beneath the crucible
The results obtained are shown through Figure 11 and Figure 12. Figure 11 shows the cross section of the
ingots grown from three different hot zones, where the first one is the standard hot zone referred as
default hot zone (a) where no special insulation was applied to, and the other two are hot zones that
result from different configurations of the insulation around the crucible, hot zones I(b) and II(c),
respectively. Hot zone II has the best crucible insulation near the growth front. As shown by the grain
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structure in Figure 11(a), with the default hot zone, the interface results very concave, and the grains
tend to grow inward. On the other hand, for the improved hot zones, with a better crucible insulation or
less radial heat loss from the interface (see Figure 11(b) and(c)), the concavity is greatly reduced, and
grains near the center tend to grow slightly outward. Particularly, the grains from the side wall are
significantly reduced for the case in Figure 11(c) [7].
Figure 11 Longitudinal grain structures from (a) default hot zones; (b) hot zone I; (c) hot zone II.
The axial distributions of the grain size from different hot zones are shown in Figure 12. As it can be
seen from the results in Figure 12, the grain size in all cases still decreases with height. In their report,
the decrease of the grain size was attributed to the generation of new grains during growth. More
importantly, the high defect area, in terms of the area having the dislocation density larger than 1 x
106/cm2, also increases rapidly with the growth distance, even in Hot zone II. Near the end of the
growth, the defect area is greater than 80 cm2 (more than 30%). This indicates strong multiplication of
dislocations along the ingot height. Despite the high defect density, the efficiency of cells produced
from ingots made in the improved hot zone increased by 0.3 % absolute, from 15.5 – 15.9% to 15.8 –
16.1 %, from wafer improvements only. The naming convention A+ was chosen to signify this
improvement of 0.3 %.
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Figure 12: Grain size distributions from different hot zones.
4.2 The control of grains and grain boundaries
In order to reduce the growth rate of dislocations and the formation of sub-grains, the control of
nucleation and the initial grain growth could be crucial. According to the nucleation theory, a certain
undercooling is required for nucleation, and a higher cooling rate leads to a higher undercooling and
nucleation rate. As a common practice in casting, the higher nucleation rate generates more grains, and
as a result, the grain size becomes smaller. The opposite is true for dendritic casting method, where due
to the faster <112> dendrite growth, bigger grains can be obtained. This idea was examined first in a
laboratory scale DS furnace and then in industrial-scale furnace as described in the previous section.
The high undercooling in the industrial-scale furnace was difficult due to the much lower cooling rate as
compared with the one in the lab-scale furnace. Nevertheless, the results obtained were similar to the
lab-scale results. One main finding, according to the authors, is that increasing grain size with height, or
grain growth, is important to reduce the growth rate of dislocations. Dislocation density in the ingot
with grain growth appears lower than in the one without. The dislocation mappings of the wafers from
different heights of the ingots are shown in Figure 13.
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Figure 13: Defect mapping of the wafers at different positions of the ingots having two growth
conditions for nucleation and initial growth control having: (a) grain size decreased with height (A+);
(b) grain size increased with height (A++).
Where grain growth was achieved, the high-dislocation area was smaller than 25 cm2. This
improvement again led to an efficiency improvement of ~0.3 %, from 16.7 to 17 %, leading to the
naming convention A++ for the wafers where grain growth was achieved. In Yang et al. [10] it is shown
that this type of ingots was in fact a result from controlling the supercooling to a very narrow window
where the supercooling is high enough that many grains are nucleated and allowed to grow, but not so
high that dendritic growth is initiated, as shown in Figure 14.
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Figure 14: Three typical grain structures from different undercoolings: (a) low undercooling, (b)
medium undercooling, and (c) high undercooling [10].
4.3 Control of nucleation conditions
The next development towards stable production of HPMC-Si was achieved by controlling the nucleation
conditions. The method employed was described as using "an incubation layer containing a nucleation agent"
[10]. Although a nucleation agent may be thought of as a foreign medium which locally reduces the barrier
for nucleation (silicon carbide and beta-silicon nitride have been discussed), it is likely that this refers simply
to the method known as "chip seeding" where the nuclei themselves are present at the crucible bottom in the
form of small silicon particles. The method will be described in more details below. The efficiency increase
was measured to 17.4 % and the wafers are called A+++ wafers.
The development also lead to a fourth class, A4+ wafers (17.8%), and this improvement is described as
being a result of use of purified coating and crucible materials.
5 Methods and techniques for nucleation control
5.1 Chip seeding
The common name of this method, "chip seeding" is based on the type of silicon used to define the
initial growth, which is in the form of small particles, often in form of the smallest fraction of silicon
from regular Siemens produced feedstock, called "chips". Other types of particles, such as granules from
a Fluidized Bed Reactor (FBR) can also be used. Development of grain structures from such seeds has
been investigated in the industry and some results are reported in the literature. Wong et al. [30]
reported the results on three mc-Si ingots of 70 mm in diameter that were cast in silica crucibles
insulated by alumina felt to better control the solidification front. Spherical silicon beads of 0.92 mm in
diameter were used as the seed layer (about 20 mm in height)), before silicon small chips were placed
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inside the crucible. Before solidification started, silicon raw material was melted leaving about 5–10
mm of silicon seed layer at the bottom. The ingots were cast at three different solidification speeds, 1
cm/h (similar to normal industrial growth), 5 cm/h, and 20 cm/h and the longitudinal cross sections of
the grown ingots are shown in Figure 15 for comparison. Columnar grains grow upward from the silicon
beads. New grains, however, seem to generate from the side walls of the crucible growing inwards. An
interesting observation made was that some grains were terminated suddenly by other grains because of
their tilted growth orientation.
Figure 15: Longitudinal cross section of ingots at different pulling speeds: (a) 10 mm/h; (b) 50 mm/h;
(c) 200 mm/h; the dashed line indicates the initial melt/solid interface [30].
Although grains initially seem to have random orientations, as the authors noted, going from the bottom
to the top of the ingot the {112} grain would dominate. In the other ingots grown with 5 cm/h and 20
cm/h, the dominating grain was {111} instead. The grain size increases with ingot height for all ingots
and the increase is more pronounced at higher pulling speed. Analysis of grain boundaries that was
carried out showed that the percentage of non-Σ grain boundaries is high at the beginning, ca. 60–70%,
but then decreases slowly to about 45% near the end of the growth. The percentage of Σ3 grain
boundaries on the other hand appears to be only about 20–25% at the beginning, but towards the top of
the ingot their percentage increases to about 40–45% (compare with Figure 4). The coherent Σ3 grain
boundaries increases faster as the pulling speed increases, and for the ingot grown at 20 cm/h, the
coherent Σ3 grain boundaries increase to more than 40% within 10 mm of growth height. The authors
attribute this effect to the increase of undercooling in the groove of grain boundaries for twin nucleation.
Material quality such as dislocation density was however not studied for in this paper.
Other reports of chip seeding on a more industrial scale are given by Zhu et al. and Huang. et al. (same
work but different details) [31, 32]. The thickness of the seed layer is ~20 mm, and it is extremely
important to achieve a near flat solid liquid interface as first growth commences in order to avoid
complete melting of any seed region. The red zone at the bottom is thicker than in conventional growth
(see Figure 16), which may be a result of impurities diffusing into the seeds during melting, that have
not been subjected to directional solidification refining. Similar problems exist for seeded growth of
quasi-monocrystalline material also. The quality in the remaining part of the ingot is however superior
in the seeded ingot.
There are reports from industry [33] that granule seeded ingots are "better" than chip seeded ingots; it is
not clear what "better" means, if it is in terms of efficiency numbers, grain size (assuming smaller is
better) or dislocation density. A 12 kg ingot produced at NTNU/SINTEF where half the ingot was
seeded with granules and half with chips, confirms that smaller grain size results from the granules [34]
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than the chips. It is furthermore reported in the literature that a tight size distribution of the granules is
important, again not defined if this refers to efficiency, dislocations or grain size. However, the actual
size of the granules does not seem to be essential. This observation may also be linked to the difference
between granules and chips, since the size in contact with the melt surface may vary a lot more for
randomly shaped chips. A possible explanation linking grain size and size distribution of the granules is
that in the case of a tight distribution of seeding granules, all particles will have equal possibilities to act
as nuclei which would result with many small grains grown simultaneously from the bottom of the
crucible. And if we consider the other case where the distribution of the granules is not so tight, one
might think that particles larger than the rest of the material may nucleate grains with a preferred
configuration for growth and as a result the grain size distribution will be affected by these fewer
individual nuclei.
Figure 16: Comparison of lifetime of seeded and non-seeded grown ingots, from [31]
5.2 Coating and crucible as nucleation source
The only report on growth of non-seeded HPMC-Si in the public literature is that of Yang. et al [10] stating
that it is important to control the overall supercooling to a very narrow window: High enough that several
nuclei are allowed to grow beyond a critical size, in a lateral direction and form starting points for columnar
growth during directional solidification, but not so high that large dendrites are allowed to dominate the
initial growth.
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This method has the obvious advantage over chip seeding that all impurities incorporated in the feedstock
during melting will be subjected to refining by directional solidification, and a situation as shown Figure 16
can be avoided. On the other hand, it is important that all parts of the crucible bottom area go through the
same conditions that allow for small grain development. Important parameters to control are vertical and
horizontal thermal gradients as well as time dependence of the thermal gradient (pulling speed). Certain
furnace configurations may be more favorable for achieving this than others; this is related both to the
physical configuration of the heater vs. the cooler, and the flexibility of the cooler to control the temperature
over a large area.
Since nucleation originates on particles in the coating (the exact nature of the particles is a point of
investigation [35]), it is expected that both chemical nature and structure of the coating can be controlling
factors for the nucleation. It is suggested that α-Si3N4 will not be able to induce epitaxial growth of silicon,
while β-Si3N4 resulting from the dissolution of coating at high temperature and re-precipitation at lower
temperature may instead be the nucleation agent [35]. Since the coating is oxidized during the firing,
presence of silicon oxide or silicon oxy-nitride is also expected to influence the contact between coating and
melt. Increased oxygen content leads to decreased wettability [36]. However, experiments in industrial
systems have not shown any effect on nucleation properties when firing time was increased from 4 to 8 h
[10]. Lastly, the morphology of the coating is expected to influence the nucleation, and roughened coating or
crucible have been reported (but not published) to be effective to allow many nuclei to grow One method
that is used is to cover the crucible bottom with a layer of quartz chips prior to coating to achieve a macroroughness of the coating. No systematic study of the nature of the roughness necessary to achieve good
nucleation conditions has been reported so far; since silicon nitride particles form agglomerates during
coating, a macroscopically rough surface may be microscopically smooth etc.
6 Control of grain size and defect density during the growth phase
The following factors may be employed to limit dislocation density evolution in the ingot during
crystallization [9]:
•
•
•
Reduced possibility of dislocation generation because of fewer generation sites;
Reduced driving force for dislocation multiplication by plastic deformation;
Increased probability of elimination of clusters by grain selection, termination at special grain
boundaries, twinning etc.
As mentioned above, grain boundaries are a source of dislocations. Therefore, one could expect the
density of dislocations generated by the grain boundaries to increase with the number of grain
boundaries per unit volume. This was the logic of favoring the growth of large grains versus small ones
after all. In other words, we would prefer to minimize the number of dislocation generating grain
boundaries per unit volume. However, dislocations are seldom observed to be generated at random
angle grain boundaries since these boundaries have an amorphous structure at high temperature.
Although such boundaries may be rather recombination active [37], they are not problematic from
dislocation generation perspective. Furthermore although dislocation generating boundaries may also be
present in high number in small grained material [9], their strength as sources depends on how much
stress is concentrated on them. It is possible that in small grained material with a high proportion of
random angle grain boundaries and with an amorphous structure, at high temperature, the global stress
may diminish through stress release by sliding grain boundaries. If this happens, one should expect the
existing sources to be less active, or in other words, to generate fewer dislocations.
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Obviously, the most appraisable effect of small grain structures is the blocking of propagation of the
generated dislocation clusters along the ingot height. The mechanism by which the dislocations clusters
terminate at a certain point during crystallization has been identified. Random angle grain boundaries,
which due to their amorphous nature operate as free surfaces may physically terminate a dislocation.
Studies of single clusters and the global trends indicate that this is an important mechanism [9].
Thus it can be summarized that three different mechanisms may contribute to low dislocation density in
small grained material vs larger grains:
• Random angle grain boundaries do not act as dislocation sources;
• Dislocation clusters that are generated may also be terminated at random angle grain boundaries,
which the clusters are likely to interact with quite fast in a small grained material;
• Global stress may be released by the amorphous structure of random angle grain boundaries so
that grain boundaries that do act as sources are less active. This effect may also lead to less
severe multiplication.
Since the type and density of grain boundaries appear to be so essential in controlling the evolution of
defect clusters, a very interesting topic of research is grain selection and how to control growth
properties to optimize this. A large host of works have addressed the topics of grain selection and
twinning [38-41]. Wong et al. [30] discusses their results in the view of these theories and argue that
regular grain selection due to various growth rate of various orientations should lead to dominance of
grains of low interfacial energy, such as (111) and (211). This is in accordance with a model developed
by Fujiwara for low growth rates [38] (as observed, shown above), while twinning on the other hand
leads to a complete shift in orientation of grains, which are not necessarily of low interfacial energy.
Duffar et al. [40] discusses grain competition in view of the preferred orientation of the grain boundary
due to faceting of the groove between growing grains, while Stokkan shows experimental evidence that
twinning tends to originate at grain boundary tri-junctions and argue that changes in grain boundary
energy may be a determining factor that initiate twinning [41].
As a side step, it can be noted that Huang and Zhu [31, 32] argues that the mechanism of epitaxial
growth of silicon on a foreign particle will introduce misfit dislocations that are the source of all further
dislocations during the growth phase, thus explaining the difference between their chip seeded and nonseeded material. This explanation however is not very trustworthy, because it is well known that even in
regular multicrystalline silicon, the majority of the material is nearly dislocation free in the very bottom
of the ingot, while dislocation clusters quickly evolve very locally at grain boundaries. Furthermore, it
has also been shown that nucleation on coating can also lead to material of similar or superior quality
(see paragraph 5.2), and it is clear that dislocation development is much more affected by grain and
grain boundary characteristics as well as thermal stresses.
It is clear that the control of grain properties during growth of HPMC-Si is a still not well understood,
and a very challenging topic, well beyond the scope of this review.
7 Summary
In this report, published literature about material properties, nucleation and growth of High Performance
Multicrystalline Silicon is reviewed. Although the material is industrially dominant, publications are
few and sparse yet. However, important characteristics, such as grain size, grain growth, grain boundary
types, dislocation density and dislocation cluster characteristics have been described and discussed. Also
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conditions during nucleation and growth and their effect on material properties have been discussed in
some detail. This overview should be sufficient to generate a good understanding of HPMC-Si for the
members of the consortium to be able to perform effective and relevant research on impurity control in
this material.
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