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β-SiC–based Photovoltaic and Optical Devices
B.S. Richards, A.S. Brown, T. Trupke, R.P. Corkish and M.A. Green
Special Research Centre for Third Generation Photovoltaics
University of New South Wales
Sydney NSW 2052
AUSTRALIA
Telephone:
+61 2 9385 5914
Facsimile:
+61 2 9662 4240
E-mail:
b.richards@unsw.edu.au
Abstract
This paper investigates the potential role of cubic- or beta-silicon carbide (β-SiC) in the future
development of photovoltaics. The bandgap of β-SiC is too wide (Eg = 2.2 eV) for an ideal top cell in
a tandem SiC/Si solar cell. However, it is optimal for an impurity photovoltaic (IPV) cell. Although
IPV cells avoid the current matching problem inherent to tandem structures, the deep impurity level
can provide an additional mechanism for recombination as well as generation. One possible solution
is to fabricate purely optical SiC devices, called up- and down-converters, which convert two lowenergy photons into one high energy photon and vice versa, respectively.
INTRODUCTION
Silicon (Si) has been the dominant material of the photovoltaic (PV) industry for several decades. There are several
reasons for this status. Firstly, silicon is the most abundant element on planet Earth, albeit not in the purified state that
is required for device manufacture. Secondly, silicon PV devices have benefited greatly from the technology and
processes developed by the silicon microelectronics industry. The reason for silicon’s success in microelectronics can
be attributed to its native oxide, silicon dioxide (SiO2), which can be thermally grown and affords excellent passivation
to the abruptly terminated surfaces of the crystal. Today, laboratory-scale silicon solar cells are able to convert sunlight
at efficiencies approaching 25% (Zhao et al. 1998), which is very close to the maximum theoretical performance for
such a solar cell operating under one-sun conditions.
The highest efficiency solar cells that are produced today are complex three- and four-junction tandem devices,
typically made from gallium arsenide (GaAs) and germanium (Ge) based semiconductors achieving conversion
efficiencies of over 30%. Many of these semiconducting materials are either very limited in supply, toxic, or both.
Therefore, while the world’s supply of such materials may be able to satisfy the demands of the space PV industry, they
will never have a great impact on the terrestrial PV market. Therefore, if high-efficiency solar cells are to become
commonplace in tomorrow’s energy market, alternative materials that are abundant and environmentally benign have to
be found. This work explores the potential of beta silicon carbide (β-SiC), also called cubic or 3C-SiC, to satisfy the
requirements of several high efficiency solar cell structures.
WHY CHOOSE β-SiC?
There are several reasons for choosing β-SiC as a future semiconducting material for the PV industry. Firstly, both the
elements of carbon and silicon are very abundant and non-toxic. Secondly, as with silicon, SiO2 can be thermallygrown or deposited in order to passivate the surfaces of the SiC crystal. Thirdly, a large SiC micro- and powerelectronics market is currently under development, and it is anticipated that once again the PV industry may take
advantage of this situation. To date, research has been primarily focused on hexagonal SiC (or α-SiC) for high-power,
high-frequency and high-temperature electronics (Davis et al. 1991). Fourthly, it exhibits excellent electronic
properties including high electron mobility (up to 1000 cm2/Vs) and saturated electron drift velocity (2.5 × 107 cm/s),
both of which are significantly greater than for silicon (a comparison is provided in Table 1). Fifthly, although singlecrystal β-SiC wafers are not expected to be commercially available until mid-2003 (Hoya 2000, Nagasawa 2002),
200 mm-diameter polycrystalline wafers are available today (Sullivan, 2002a) and high-quality epitaxial layers of β-SiC
can be grown on silicon wafers by chemical vapour deposition (CVD) (Shields et al 1993) Sixthly PV devices have
β-SiC–based Photovoltaic and Optical Devices
B.S. Richards et al.
already been fabricated from both α-SiC and amorphous silicon carbide (a-SiC). α-SiC solar cells were chosen for use
in space due to the excellent radiation hardness exhibited by all forms of SiC (Raffaelle et al. 2001), while a-SiC layers
are commonly used as a p-type emitter layer in amorphous silicon (a-Si) tandem solar cells due its high bandgap
(Kuwano and Tsuda, 1987). Finally, β-SiC has an indirect bandgap of Eg = 2.2 eV, significantly lower than α-SiC (Eg =
2.8–3.2 eV). While this is greater than the 1.4 eV bandgap of GaAs, the optimum for a single-junction solar cell
operating under AM1.5G conditions, β-SiC shows great promise for several multiple-threshold device structures and
Table 1 Comparison of electronic and optical properties of SiC and other common semiconductors.
Property
Crystal structure
Bandgap (eV at 300 K)
Indirect/Direct Band (I/D)
Max. Operating Temp (°C)
Melting Point (°C) (* = sublimes)
Physical Stability
Electron Mobility @ RT (cm2/V-s)
Hole Mobility @ RT (cm2/V-s)
Breakdown Voltage (106 V/cm)
Thermal Conductivity (W/cm-°C)
Sat. Elec. Drift Vel. (107 cm/s)
Dielectric Constant, K
Si
Diamond
1.1
I
300
1420
Good
1400
600
0.3
1.5
1
11.8
GaAs
Zincblende
1.4
D
460
1238
Fair
8500
400
0.4
0.5
2
12.8
GaP
Zincblende
2.3
I
925
1470
Fair
350
100
_
0.8
_
11.1
α-SiC (6H)
Wurtzite
3.0
I
1310
>1800 *
Excellent
400
50
2.5
4.9
2.0
9.7
β-SiC
Zincblende
2.2
I
873
>1800 *
Excellent
1000
40
4
5.0
2.5
9.7
these will be explored in greater detail in this paper.
The news is not all-good, and several disadvantages exist. β-SiC exhibits a rather low index of refraction (about 2.6 at
1000 nm) making light trapping less effective than with most other semiconductors (Beaucarne et al. 2002). A second
problem is the fact that there appears to be no shallow p-type dopant, which may limit the flexibility in device design
(Beaucarne et al. 2002). Thirdly, whether the low hole mobility will play a limiting role in the design of β-SiC-based
PV devices remains to be seen. While β -SiC possesses outstanding stability, this means that standard PV
manufacturing techniques such as emitter diffusion would need to be performed at temperatures greater than 2000°C
(Davis et al., 1991). Alternative techniques for junction formation could be either changing gases during CVD film
growth, or high temperature ion-implantation (Davis et al., 1991). Table 1 summarises many of the electronic and
optical properties of β-SiC and compares them to other well-known semiconductors (Sze, 1981; Powell and Matus,
Figure 1(a) Refractive index and (b) absorption coefficient of β-SiC (data taken from Harris 1995 and Achachi 1999)
1989; Casady and Johnson, 1996), while Figures 1(a) and (b) plot the refractive index and absorption coefficient of βSiC, respectively.
β-SiC–based Photovoltaic and Optical Devices
B.S. Richards et al.
POSSIBLE β-SiC HIGH-EFFICIENCY PV DEVICE STRUCTURES
1.1
β-SiC/Si Tandem Solar Cells
As relatively thick films of β-SiC can be grown epitaxially
on silicon wafers, the fabrication of a high-quality β-SiC/Si
tandem solar cell would seem to be quite feasible. β-SiC
growth on Si has been reported at temperatures as low as
400 – 620°C using certain deposition techniques (Davis et
al., 1991). A schematic of such a tandem PV device with a
high bandgap solar cell on top of a low bandgap (in our
case β-SiC on Si) solar cell is shown in Figure 2. The βSiC upper cell will most efficiently convert the shorter
wavelength light (UV-blue) into useful current, while the Si
lower cell will effectively absorb red and near-infrared
light. Electrical interconnection between the two cells is
achieved either by mechanical stacking with contacts
between the cells or by a very thin and highly-doped tunnel
junction.
Figure 2 Schematic of a β-SiC/Si tandem solar cell.
The performance of the β-SiC and Si devices were modelled using the principle of detailed balance, with the
assumption that 0.3% and 1% of the total recombination rate for Si and β-SiC, respectively, was radiative. A change in
air mass results in a different spectral content of the light that reaches the solar cell. For example, high air masses,
typical of sunlight close to dusk or dawn, exhibit a red-shifted spectrum. Performing the calculations for a variety of air
masses is important, because unless the two series-connected cells are generating the same current, a current-mismatch
will exist in the device and efficiency losses will be incurred. This is one of the inherent disadvantages to the tandem
structure with a single front and back contact. One approach that renders the tandem solar cell insensitive to
fluctuations in illumination levels is to introduce a third contact at the interface between the two solar cells, however
this would be difficult to achieve in practice.
Figure 3 plots the resulting energy conversion efficiencies
of a modelled β-SiC/Si tandem cell from air mass 1 to 10.
Also shown are the data for a high-efficiency silicon solar
cell as well as a three-cell tandem device. The latter has an
amorphous silicon (a-Si) middle cell. The performance of
the β-SiC/Si tandem cell at best only equals that of the Si
single-junction cell at AM1. This is because β-SiC absorbs
only the high-energy photons and as the air mass increases
the amount of high-energy photons decreases and the
efficiency goes down. Therefore, the bandgap of β-SiC is
not a good match for a tandem solar cell structure based on
a Si lower cell. In addition, the performance of the βSiC/Si tandem cell drops off rapidly with decreasing light
intensity, whereas the performance of the Si cell actually
increases slightly due to a red-shift in the spectrum. The
performance of a β-SiC tandem structure can be improved
by inserting a middle cell with a bandgap similar to that of
a-Si (Eg = 1.6 eV), which in a much more even distribution
of photons throughout the three cells. In this scenario, the
efficiency increases to 35.8% under AM1.5 illumination.
Due to the high-deposition and processing temperatures for
β-SiC, it may not be feasible to use a-Si in this device.
However the next section will present a method whereby
this desired bandgap can be achieved without an additional
semiconducting material.
Figure 3 Modelled conversion efficiencies of two β-SiC
tandem solar cells compared to a high-efficiency silicon
device. The lines are only a guide to the eye.
β-SiC–based Photovoltaic and Optical Devices
1.2
B.S. Richards et al.
β-SiC/Si Superlattices
One possible method of creating a material with an effective bandgap of 1.6–1.8 eV using only group IV elements is to
use β-SiC/Si superlattices. Many bilayers of very thin material (typically 20–60 bilayers of about 1–5 nm thickness)
could be deposited or grown onto a silicon wafer, substantially modifying the optical and electrical properties. In the
past, such superlattices have commonly used SiO2/Si bilayers (Cho et al. 2001). Recently, our group has begun
investigating the fabrication of β-SiC/Si superlattices by initially depositing the layers as amorphous silicon carbide (aSiC) and amorphous silicon (a-Si) using plasma enhanced CVD. While a-SiC/a-Si superlattices have been
experimented with in the past (Abeles and Tiedje, 1983; Kuwano and Tsuda, 1989), no work has been reported on
crystalline β-SiC/Si superlattices to our knowledge. The main challenge in these experiments is expected to be
crystallising the a-SiC layers. As previously mentioned, the inert nature of SiC requires extremely high processing
temperatures – often higher than the melting point of silicon, in fact. In the literature, crystallisation experiments have
been performed on β-SiC films that have been amorphised by ion implantation radiation. Typically, full recrystallisation
of the β-SiC is not achieved until temperature of 1800°C are reached (Davis et al., 1991).
In addition, experience with SiO2/Si superlattices has demonstrated that the crystallisation temperatures required for aSi layers are significantly greater as the thickness becomes less than 3 nm (Grom, 2001). Our hypothesis is that the a-Si
will crystallise first, and subsequently, each a-SiC layer will be sandwiched between two microcrystalline silicon (µcSi) layers. It is known that the growth of β-SiC is favoured over α-SiC when using silicon substrates (Davis et al.,
1991) and since the heat of formation of β-SiC is lower than α-SiC it is anticipated that the outer µc-Si layers will assist
in the crystallisation of the a-SiC layers. With SiO2/Si superlattices, the SiO2 is an amorphous material and does not
provide a seeded crystallographic preference (Grom, 2001), however it is anticipated that with two materials that exhibit
a crystalline phase the growth of the β-SiC polycrystalline film will be supported on the Si surface. Crystallisation
techniques to be examined include solid phase crystallisation, rapid thermal processing, and laser-based processing.
1.3
β-SiC Impurity Photovoltaic (IPV) Devices
The idea behind the impurity photovoltaic (IPV) effect is to use sub-bandgap photons to enhance solar cell performance
(Wolf, 1960). By doping the host lattice with an impurity, it is possible to generate electron-hole pairs via a two-step
process, whereby an electron is first excited to an impurity state within the bandgap and subsequently into the
conduction band. The maximum performance of an IPV solar cell with a single defect state in the bandgap operating
under AM1.5 global illumination has been predicted to be 48.9% (Brown and Green, 2001). While the debate still
continues as to whether the introduction of a defects will lead to an efficiency increase in practice, or merely provide
another recombination mechanism, researchers agree on one thing – that the host lattice must be a wide bandgap
semiconductor.
The ideal bandgap for an IPV solar cell has been shown to lie in the range Eg = 2.0–2.5 eV, with an ideal impurity level
of one-third of the bandgap, either above the valence band or below the conduction band (Beaucarne et al. 2002). It is
currently not known whether a direct bandgap semiconductor IPV device would exhibit an advantage over an indirect
material. Therefore, we have begun investigating the potential of β-SiC, whose bandgap lies in the middle of the ideal
range, as an IPV host material (Beaucarne et al. 2002). The lack of a shallow p-type dopant in β-SiC was mentioned
previously, however it is fortuitous that boron lies at a trap level of ET – Ev = 0.735 eV (Davis et al., 1991). β-SiC has
shown limited potential in the field of optoelectronics, and the first blue LEDs were made of β-SiC. This evidence of
radiative recombination occurring in β-SiC is significant for IPV devices, as radiative processes are known to be
generally weak in indirect semiconductors. Furthermore, recombination from the boron energy level is also known to
occur radiatively (Beaucarne et al. 2002). The ideal impurity concentration has been estimated to lie at levels just
above that of the background doping of the β-SiC (Suzuki et al., 1977). The maximum conversion efficiency predicted
for this scenario (B-impurity in a β-SiC solar cell) is 38%, assuming that the impurity is 100% radiatively efficient
(Beaucarne et al. 2002). Strikingly, if this criterion is relaxed to 0.5% radiative efficiency, an IPV solar cell with a
conversion efficiency of greater than 30% can still be achieved (Beaucarne et al. 2002).
1.4
β-SiC-based Up- and Down-Converters
Another way to reduce thermalisation and transmission losses in a solar cell is the conversion of the incident solar
spectrum to other photon energies that can be more efficiently converted by the solar cell. This conversion consists of
up-conversion of low energy photons or down-conversion of high-energy photons. An ideal up-converter emits one
high energy photon for every two low energy photons absorbed while a down converter emits two low energy photons
β-SiC–based Photovoltaic and Optical Devices
B.S. Richards et al.
for every high-energy photon absorbed. As they have no external contacts, up- and down-converters are purely optical
devices. The attraction for PV is the potential ease at which such a device could be applied to the front (downconverter) or rear (up-converter) surface of an existing solar cell. The theory behind up- and down-conversion is
detailed elsewhere in the literature (Trupke et al., 2002a; Trupke et al., 2002b).
1.4.1 Up-Conversion
The requirements for the most efficient up-converter are similar to those for an IPV device – a material with a bandgap
of Eg = 2.0 – 2.5 eV and an impurity level lying at approximately one-third of the bandgap. As with the IPV scenario,
β-SiC with a boron impurity at 0.74eV is almost ideal. It is anticipated that the light will be up-converted during two
intermediate band transitions, followed by the band-to-band recombination that results in the emission of a high-energy
photon. Naturally, as with IPV devices, the additional impurity level also provides an additional path for a two-step
recombination process to occur.
Figure 4 shows a schematic diagram of how a β-SiC solar
cell/up-converter stack might look. The up-converter is
placed behind a solar cell of a similar bandgap, and would
convert two photons (with energy between 0.74–1.46 eV
and 1.46–2.20 eV, respectively) that were not absorbed on
the first pass through the cell up to a total energy greater
than the 2.2 eV bandgap. This high-energy photon has then
the opportunity to be absorbed and collected by the solar
cell. As the up-converter is not in electrical contact with the
solar cell, contacting the solar cell is best achieved by using
a bifacial solar cell and then bringing the up-converting
layer into good mechanical and optical contact at the rear.
Ideally, a reflector is added to the rear of the up-converter to
prevent light escaping out the rear of the device.
Figure 4 Schematic of a β-SiC solar cell/up-converter.
In the scenario presented here, part of the emitted high-energy light will be unavoidably re-absorbed by the upconverter as it possesses the same bandgap as the solar cell. Therefore, it may be either be advantageous to use a
material with a fractionally higher bandgap for the up-converter. One method of increasing the bandgap of β-SiC
would be to use polycrystalline material, which has a bandgap of up to 2.65 eV (Sullivan, 2002b). An alternative
method is to use rare-earth (RE) atoms in an insulating host. RE atoms exhibit well-defined energy levels that are
relatively independent from the host environment, and many of the transitions within the RE atom’s electron shell are
known be radiatively efficient. Our work in this area is beginning by examining the potential of boron-doped β-SiC for
up-conversion purposes.
1.4.2 Down-Conversion
A diagram of a β-SiC down-converter on top of a Si solar
cell is shown in Figure 5. High energy (>2.2 eV) light is
absorbed by the β-SiC down-converter, and two low energy
photons (~1.1eV) are re-emitted. These low energy
photons, along with all other wavelengths that were not
absorbed by the down-converter, can then be converted into
useful current by the silicon solar cell.
The requirements for an ideal down-converter are that the
bandgap should be twice that of the solar cell to which it
will be applied, and that a radiatively efficient mid-gap
impurity must be present. β-SiC possesses a bandgap twice
that of silicon, however there are currently no known deeplevel impurities that reside at about 1.1 eV in β-SiC. For
this reason, the role of RE impurities will also be
investigated.
Figure 5 Schematic of a β-SiC down-converter and Si
solar cell.
CONCLUSIONS
We have presented the case for β-SiC becoming one of the likely candidates in the future of photovoltaics. This is due
β-SiC–based Photovoltaic and Optical Devices
B.S. Richards et al.
efficiency solar cell structures, the IPV and tandem cell, were examined. Boron lies at the ideal level in β-SiC for an
IPV solar cell, and a conversion efficiency of up to 38% is predicted. It was determined that the bandgap of β-SiC is
too wide for an ideal two-cell tandem. An efficiency of 36% could be expected from a three-cell device involving βSiC and Si as the top and bottom cells, with a 1.6 eV middle cell. The potential of bandgap engineering using β-Si/Si
superlattices to obtain an intermediary bandgap was discussed. Finally, the use of β-SiC as a host material for an upand down-converter was presented.
ACKNOWLEDGMENTS
The Special Research Centre for Third Generation Photovoltaics is supported by the Australian Research Council’s
Special Research Centres scheme.
REFERENCES
Abeles B. and Tiedje T. (1983) Amorphous semiconductor superlattices, Phys. Rev. Lett. 51, p.2003.
Achachi S. (1999) Optical constants of crystalline and amorphous semiconductors: numerical data and graphical information, Kluwer Academic,
pp.63-72.
Beaucarne G., Brown A.S., Keevers M.J., Corkish R. and Green M.A. (2002) The Impurity Photovoltaic (IPV) effect in wide bandgap
semiconductors: An opportunity for very high efficiency solar cells?, Prog. in Photovoltaics 10, pp.345-353.
Brown A.S. and Green M.A. (2001) A new look at the impurity photovoltaic effect, 17th European Photovoltaic Solar Energy Conference, Munich,
pp.250-253.
Casady J.B. and Johnson R.W. (1996) Status of silicon carbide (SiC) as a wide-bandgap semiconductor for high temperature applications: A review,
Solid-State Electronics 39(10), pp.1409-1422.
Cho E.-C., Widenborg P., Xia J., Aberle A.G., Moon D.-S., Kim D.-S. and Lee S.H. (2001), Antireflection and surface passivation behaviour of
SiO2/Si/SiO2 quantum wells on silicon, Proc. of 12th International Photovoltaic Science and Engineering Conference, Kore, June 2001.
Davis R.F., Kelner G., Shur M., Palmour J.W. and Edmond J.A. (1991) Thin film deposition and microelectronic and optoelectronic device
fabrication and characterization in monocrystalline alpha and beta silicon carbide, Proc. of the IEEE 79, 677-701.
Green M.A. (2001) Multiple band and impurity photovoltaic solar cells: general theory and comparison to tandem cells, Prog. Photovoltaics 9,
pp.137-144.
Grom G.F. (2001) Nanocrystalline silicon/silicon oxide superlattices: Fabrication, characterization and applications in nano-flash memories, PhD
Thesis, University of Rochester, New York, U.S.A.
Harris G.L. (1995) Properties of Silicon Carbide, INSPEC, section 2.1, p.16.
Hoya Co., Japan (2000) http://www.hoya.co.jp/eng/news/index5.html
Kuwano Y. and Tsuda S. (1989) Amorphous silicon solar cells using a-SiC materials, in Amorphous and Crystalline Silicon Carbide Harris G.L. and
Yang C.Y.-W. (Eds), Springer Proc. Physics 34, pp.167-178.
Nagasawa H., Private communication with BSR, Hoya Co. (Japan), July 2002.
Powell J.A. and Matus L.G. (1989) Recent Developments in SiC (USA), in Amorphous and Crystalline Silicon Carbide Harris G.L. and Yang C.Y.-W.
(Eds), Springer Proc. Physics 34, p.3.
Raffaelle R.P., Kurinec S., Scheiman D., Jenkins P. and Bailey S.G (2001) Silicon carbide solar cells for space applications, 17th European
Photovoltaic Solar Energy Conference, Munich, pp.281-283.
Shields V.B., Fekade K. and Spencer M.G. (1993) Near-equilibrium Growth of Thick, High Quality Beta Silicon Carbide by Sublimation, Appl. Phys.
Lett. 62, 1919.
Sullivan Datanite (2002a), http://www.datanite.com/physical.htm
Sullivan T. (2002b), Private communications with BSR, Sullivan Datanite (USA), July 2002.
Suzuki A., Matsunami H., Tanaka T.(1977) Photoluminescence due to Al, Ga, and B acceptors in 4H-, 6H-, and 3C-SiC grown from a Si melt, J.
Electrochem. Soc. 124, 241-246.
Sze S.M. (1981) Physics of semiconductor devices, 2nd edition, Wiley.
Trupke T., Green M.A., Würfel P. Improving the efficiency of solar cells by up-conversion of sub-band-gap light, J. Appl. Phys., (accepted for
publication).
Trupke T., Green M.A., Würfel P. (2002) Improving the efficiency of solar cells by down-conversion of high-energy photons, J. Appl. Phys. 92,
p.1668.
Wolf M. (1960) Limitations and possibilities for improvement of photovoltaic solar energy converters, Part I: Considerations for the earth’s surface
operation, Proc. of IRE 48, pp. 1246-1263.
Würfel P. (2000) Physik der Solarzellen, Spektrum Akademischer Verlag, p.185.
Zhao J., Wang. A., Green M.A. and Ferrazza F. (1998) 19.8% efficient “honeycomb” textured multicrystalline and 24.4% monocrystalline silicon
solar cells. Appl. Phys. Letters 73(14), 1991.