CHARACTERISATION OF CORONA-CHARGED
OXIDE-PASSIVATED SILICON
Simeon C. Baker-Finch and Keith R. McIntosh
Centre for Sustainable Energy Systems,
The Australian National University, Canberra ACT 0200,
AUSTRALIA
Email: simeon.baker-finch@anu.edu.au
ABSTRACT
Corona charge is used in the photovoltaic industry to help distinguish surface and bulk
recombination. In this study, the deposition of corona charge on Si-SiO2 structures was
investigated. The magnitude and polarity of the charge is directly measured with a
Kelvin probe, while its effect upon surface recombination is inferred from the
measurement of transient photoconductance.
Corona charge is deposited on 5 Ω cm n-type Si passivated with a 20 nm thick oxide at
a constant rate of 4 µC cm-2 min-1 until charge saturation is observed. The charge
density observed at the saturation point is consistent with the onset of oxide breakdown.
The oxide breakdown field is 5.5 MV cm-1 for positive charge and 11.9 MV cm-1 for
negative charge.
Transient photoconductance measurements indicate that the surface recombination is
affected by the corona charge in two ways: (i) the recombination decreases due to the
repulsion of like-polarity carriers from the Si–SiO2 interface, and (ii) the recombination
increases due to interface damage induced by the charge. These two effects were
distinguished by removing the surface charge with isopropanol. These results are
consistent with those presented by Jin et al. (2007).
INTRODUCTION
Corona charging is used in various fields of applied electrostatics (Dascalescu et al.
1999), as a tool for charging electrets (Sessler 1987), for xerography (Sessler 1987,
Günther & Xia 1993), and as a method to maintain device cleanliness (Ohmi et al.
1994). In the field of photovoltaics research, with few exceptions, the use of corona
charge has been limited to novel characterisation methods involving the use of the
charge as a non-permanent, non-invasive means of surface passivation (Schöfthaler et
al. 1994, Schröder 2002). A surface concentration of corona-deposited charges on an
oxidised Si surface is used to influence the carrier concentrations and recombination
velocity in the Si near the Si-SiO2 interface.
The corona charging process is one that depends on the use of an inhomogeneous
electric field in air to produce a discharge of ions that are deposited on the surface of a
sample at atmospheric pressure (Sessler 1987). It involves the placement of the sample
on a grounded metal table beneath a needle to which a potential of several kilovolts is
applied. The resultant high electric field in the vicinity of the needle tip ionises
surrounding air molecules (Schöfthaler et al. 1994, Glunz et al. 1999). Depending on
the polarity of the voltage applied at the source, the predominant ionic species are CO3or H3O+ (Schröder 2002). The electric field forces the ionised air molecules along the
electric field lines toward the sample (Schöfthaler et al. 1994, Schröder 2002).
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Corona charging is generally considered to be a non-destructive method for passivation
of Si surfaces. Recently, however, Stesmans and Afanas’ev (2004) and Jin et al. (2007)
have concluded that damage in the form of defects at the Si-SiO2 interface is incurred
during the process.
In this work, accurate Kelvin probe techniques are used to characterise the magnitude
and lateral uniformity of both positive and negative corona charge as deposited on a
thermal oxide of Si. Photoconductivity decay (PCD) techniques are used to determine
the influence of corona charging upon the passivation of the Si surface, which itself
depends upon the density of interface defects possibly incurred during charging, as well
as the extent to which the charge deposited on the oxide surface repels carriers of likepolarity in the Si away from the Si-SiO2 interface.
ACCURATE KELVIN PROBE MEASUREMENT OF CHARGE DENSITY ON
OXIDE SURFACE
The modern Kelvin probe is a non-contact, non-destructive device that employs a
vibrating capacitor technique to measure the ‘work function difference’ between a
vibrating tip and a conducting sample (Baikie and Estrup 1998). The vibrating tip is a
reference surface that forms the counter electrode of a parallel plate capacitor, whose
other plate is the sample surface. As the tip vibrates, electrons flow back and forth in the
external circuit as an electric field is induced between the capacitor plates. A voltage,
termed the applied or backing potential, denoted Va, is applied to the probe head in
order to compensate for this external electric field. Va is the negative of the contact
potential, Vc.
For accurate derivation of surface charge density Qs from Kelvin probe measurement,
the contributions of charges located at the Si-SiO2 interface, in the oxide, as well as on
the surface of the oxide toward a profile of electric potential must be considered (see
Fig. 1). This is particularly relevant when attempting to harness the increasing accuracy
of Kelvin probe technologies. Previous publications in the field of photovoltaics (eg.
Glunz et al. 1999) have neglected the contributions of all charges other than Qs, so that
Qs = Vc
ε ox
d ox
……………………………………………………….. (1)
where dox is the thickness of the oxide and εox is the dielectric permittivity of the oxide.
In this work, all contributors to the profile of electric potential, Ψ(x), shown in Fig. 1
are included in the analysis. That is, we define
⎛ d f / 2 ⎞⎞
ε ⎛
⎟⎟ ⎟
…………………………...
Qs = ox ⎜⎜Vc − φ ms − ΨSi − Q f ⎜⎜
⎟
d ox ⎝
ε
⎝ ox ⎠ ⎠
(2)
in dark conditions, and where ΨSi is the potential in the Si at the Si-SiO2 interface, Qf is
a density of fixed charge in the oxide, located at a distance df/2 from the Si-SiO2
interface and φms is the metal-silicon work function difference defined by Sze (2002) as
φ ms = φ m − χ + (Ec − E f ,Si ) …………………………………….… (3)
[
]
where φm is the work function of the Kelvin probe metal tip, χ is the electron affinity of
Si, Ec is the conduction band energy level far from the Si surface, and Ef, Si is the Si
Fermi level.
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φms can be determined from known values and semiconductor properties. In the
following experimental work, φm is assumed to be 5.1 eV. Values available in the
literature range between 4.83 and 5.45 eV (Sachtler et al. 1966, Rivière 1966). Also, the
quantity of Qf and an approximation of df are often known (Aberle 1999). Depending on
the processing conditions, the value of Qf is between 5×1010 and 2×1011 cm-2; usually, df
≤ 2 nm (Aberle 1999).
Girisch et al. (1988) describe a means of numerical calculation of ΨSi assuming that the
electron and hole quasi-Fermi levels are constant throughout the surface space charge
region. Note that with changing Qs, ΨSi is non-constant.
The solution of Equation 2 for known (measured) Vc requires the iteration of Qs and
parallel calculation of ΨSi for each iteration. As the value of Qs approaches the solution,
the potential on the Kelvin probe tip (Qcontact in Fig. 1) converges to zero. In the
following experimental work, a satisfactory value of Qs is found when |Qcontact| is less
than 1 × 1015 cm-2.
The technique proposed here is required for accurate estimation of Qs for Vc > -0.3 V.
At values of Vc between -0.3 V and 1 V (-0.1 < Qs < 0.03 μC cm-2), ΨSi varies
nonlinearly, and its magnitude is non-negligible, so the simplified relationship is not
valid. Furthermore, Qs is overestimated for Vc > 1 V (a constant offset of ~0.3 μC cm-2
between the two solutions is observed).
Fig. 1: The energy bands, electric field and electric potential profile in Si, oxide, probe
tip system during Kelvin probe measurement. The potential gradient across the air gap
is flat when Va compensates for the external electric field generated by the combination
of charges QSi, Qit, Qf and Qs.
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MAGNITUDE AND UNIFORMITY OF CORONA CHARGE ON OXIDEPASSIVATED SILICON
Sample Preparation and Procedure
The samples were fabricated from 4-5 Ω cm n-type Si wafers. They underwent the
following processing steps: TMAH etch for 20 minutes at 85 °C; RCA clean; dry
oxidation for 40 minutes in O2 at 950 °C; 30 minute in-situ anneal in N2 at the same
temperature (resultant oxide thickness of 15 - 25 nm); anneal in forming gas at 400
degrees for 30 minutes. Wafers were cleaved into quarters. The thin layer of SiO2 was
removed from the rear surface of the silicon via HF fuming and ~80 nm of aluminium
was evaporated onto the bare Si surface.
Corona charging was carried out with a conventional setup. A voltage of ±10 kV was
applied to the source needle. The sample was left in the chamber for a period of time
ranging from zero seconds to five minutes - the oxide surface became charged with a
layer of static charge. In order to measure the uniformity of this layer of deposited
charge across the surface of the sample, five test points were chosen on the sample
surface, each at a distance of 0 to 25 mm from the sample centroid. The sample centroid
was aligned directly below the tip of the corona needle.
After each interval, a Kelvin probe was used to make 100 consecutive measurements of
Va (thus Vc), at five test points. The mean was taken for the remaining analysis. The
value of surface charge density Qs was determined for each value of Vc according to the
calculation process described above. Qit was calculated from the integral of states across
the band gap, where all states (a density of 5×1010 cm-2 eV-1) were donor-like, and the
energy-dependent capture cross sections of electrons and holes were those of Aberle et
al. (1992). The resultant value of Qit was 2×10-8 C cm-2. Note that the calculation
process is fairly insensitive to this charge. Qf was set at 5 × 1010 cm-2 and df at 2 nm.
Results and Discussion
Fig. 2 illustrates the measured values of Qs at each of five measurement points for a
range of corona charging times and for both charge polarities. The mean, minimum and
maximum of these five measurements is summarised as a function of time in Fig. 3.
Fig. 2: Measured Qs values at various distances from the centroid. The sample was
subject to corona charging at +10 kV (left) or -10 kV (right) potential for the period of
time shown. Lines represent the parabolic least squares fit curve for each data set.
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Fig. 3: Mean (solid line), maximum and minimum (dashed lines) Qs as a function of
corona charging time for both positive and negative corona needle potentials.
A quantitative summary of the observed uniformity of deposited surface charge is
provided in Fig. 4. Percentage nonuniformity of surface charge (∆Qs) is defined as
⎛Q
− Qs ,min ⎞
⎟ ……………………………………… (4)
ΔQs = 100⎜⎜ s ,max
⎟
Q
s , max
⎠
⎝
where Qs, max and Qs, min are the maximum and minimum charge densities, respectively,
of the values of surface charge measured across the sample after each charging interval.
Fig. 4: Charge nonuniformity, ∆Qs, for a range of corona charging intervals.
Corona arrangements that do not feature a gate electrode (such as the one used in this
work) are generally expected to deliver a current of ionised air particles with a bellshaped distribution (Sessler 1987). This work suggests that after one minute of
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charging, uniformity improves so that no point within a two centre radius of the sample
centroid is characterised by a surface charge that is more than 10% different from any
other point within that radius.
Regardless of charge polarity, a limit is observed on the maximum magnitude of surface
charge that can be deposited using the corona discharge chamber (as shown in Fig. 3).
For positive charge, this limit is around 1.9 µC cm-2, and is observed across the sample
after 30-40 seconds charging time. In the negative case, the limit is around 4.1 µC cm-2,
and is reached after 60 seconds charging time. After the limit is reached, no more
surface charge can be deposited by corona charging. As a result, for longer charging
time, the initially bell-shaped/parabolic distribution of charge becomes more uniform as
the outside areas of the sample (where charge deposition rate is less) reach the charge
limit, and ∆Qs decreases (see Fig. 4).
The observed charge limit is attributed to oxide breakdown, which occurs at field
strengths of between 5 and 15 MV cm-1 (O’Shea et al. 1995, Weinberg et al. 1976). At
the observed limit of positive charging, the field appearing across a 20 nm thick oxide is
approximately 5.5 MV cm-1. The equivalent field for the negative charge limit has a
magnitude of approximately 11.9 MV cm-1. It is plausible that the oxide breakdown
field differs depending on surface charge polarity. Note that a median oxide thickness of
20 nm was assumed in order to calculate Qs and to approximate the breakdown field
strengths (the processing steps involved in the sample preparation may have led to a
range of oxide thicknesses of between 15 and 25 nm).
The mean rate of charge deposition is approximately 4 µC cm-2 min-1 during the prebreakdown linear phase for both positive and negative charging.
Further investigation of the limitation on deposited charge for samples with a range of
thicknesses is necessary to confirm that oxide breakdown is responsible for the
prohibition of further charge deposition.
CORONA-DEPOSITED SURFACE CHARGE AND EFFECTIVE CARRIER
LIFETIME
Sample Preparation and Procedure
Samples were prepared as for the experimental work described above with the following
difference: the rear oxide was not removed via HF fuming and the rear surface was not
coated with Al.
Two identical samples were subjected to an identical series of charge intervals under
opposite corona needle potentials. After each charging interval, a measurement of
effective lifetime, τeff, was achieved using the PCD technique (Kane and Swanson
1985). Finally, IPA solution was used to remove the surface charge from the samples
(Ohmi et al. 1994), and PCD measurement was once again undertaken.
The PCD or transient lifetime technique involves the measurement of the decay of lightgenerated electron-hole pairs as a function of time. The technique used in this work is
described by Kane and Swanson (1985) and involves the measurement of sheet
photoconductivity over time by a contactless method. It employs the principle of
inductive coupling - the time-varying sheet conductivity is transformed into an excess
carrier density. Then, τeff is determined
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τ eff (Δn ) =
− Δn
. ……………………………………………….. (5)
dΔn / dt
This effective lifetime represents the fundamental mode of carrier decay, and depends
upon two separate components (Sproul 1994)
1
1 1
= + , ……………………………………………………. (6)
τ eff
τb
τs
where τb is the carrier lifetime in the bulk, and τs is the carrier lifetime near the surface.
In this work, τb is assumed to remain constant, and variation of τeff is attributed to
surface factors (ie. density of interface defects, carrier concentrations in the Si near the
Si-SiO2 interface). During experiment, τeff is determined at ∆n = 1×1015 cm-2.
Results and Discussion
Positive and negative corona needle potentials proved to influence τeff differently, in
accordance with Fig. 5. The lifetime improvement gained in the first stage of corona
charging is lost more rapidly in the case of negative applied charge. Furthermore, once
rinsed with IPA solution to remove all surface charge, a previously negatively-charged
sample displays a significantly decreased τeff when compared to its initial charge-free
value. A previously positively-charged sample does not experience such a large lifetime
reduction. Jin et al. (2007) report a similar observation. Figure 5 indicates that for short
enough charging intervals, lifetime improvement via field-effect passivation of the Si
surfaces is achieved regardless of charge polarity. With an oxide thickness of 50 nm on
10-23 Ω cm p-type samples with a 400 Ω/□ surface-passivating phosphorus diffusion,
Jin et al. (2007) did not observe improvement in τeff by negative charging until charging
interval was increased to around 60 seconds. Given that the oxide used in this work is
considerably thinner, and that there is no surface diffusion, it is reasonable that
improvement in τeff is observed immediately (ie. for a charging time below 5 s). Indeed,
small amounts of surface charge are likely to have a more pronounced impact on the
samples used here.
Fig. 5: τeff dependence upon corona charging time.
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Certainly, after charging and charge removal with IPA solution, τeff is decreased when
compared to initial (zero charging time) τeff values. Permanent damage at the Si-SiO2
interface (as observed by Jin et al. (2007)) is suspected. Further work will consider the
impact of oxide breakdown (as observed during the charge characterisation process
described above) upon the generation of interface defects during corona charging.
SYNTHESIS OF KELVIN PROBE AND PCD RESULTS
The results of Kelvin probe characterisation are synthesised with the photoconductivity
decay measurements in Fig 6. The τeff measured by PCD after a certain period of corona
charging is plotted against the Qs measured by the Kelvin probe after the same charging
period. The relationship between Qs and τeff is compared to the ideal ‘U curve’
relationship between surface charge and τeff. The ideal curve was derived via numerical
calculation of ΨSi (following the technique of Girisch et al. (1988) as described above)
for varying Qs and with the constant input values for Qit, Qf and df defined in the
experimental procedure for charge quantification.
Fig. 6: Synthesis of PCD and Kelvin probe results for positive and negative corona
charging as compared to the ideal relationship.
Damage incurred at the interface during corona charging is thought to account for the
degradation in carrier lifetime observed for larger values of Qs. Particularly once
dielectric breakdown has been reached, interface damage leads to rapid lifetime decay.
This is particularly noticeable for negative charging in Figure 6. Lifetime degradation
may indeed be attributable to permanent damage at the Si-SiO2 interface incurred during
oxide breakdown.
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CONCLUSIONS
For 5 Ω cm n-type samples with an oxide thickness of ~20 nm, a surface charge density
of 2 µC cm-2 (equivalent to a breakdown field of 5.5 MV cm–1) or 4 µC cm-2 (equivalent
to a breakdown field of 11.9 MV cm-1) is the highest surface voltage achieved by
positive or negative corona charging, respectively. The magnitude and lateral uniformity
of deposited charge was accurately determined using a Kelvin probe coupled with an
appropriate calculation of the profile of the electric potential in the Si and oxide. Charge
deposited in a single-needle corona discharge chamber is more uniform for charge
intervals of increased length.
PCD measurements indicate that surface recombination decreases when a small amount
of corona charge is deposited onto the oxide surface. For longer charging intervals, the
recombination increases due to interface damage induced during charging. Interface
damage is more severe during negative charging.
Processes occurring during the dielectric breakdown of SiO2 on oxide-passivated n-type
silicon may be responsible for permanent damage occurring at the Si-SiO2 interface.
ACKNOWLEDGEMENTS
The authors would like to thank Mr Andrew Thomson, Ms Wendy Jellet and Ms Nina
De Caritat for regular discussion and assistance in sample preparation.
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ISES-AP - 3rd International Solar Energy Society Conference – Asia Pacific Region (ISES-AP-08)
Incorporating the 46th ANZSES Conference
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BRIEF BIOGRAPHY OF PRESENTER
Simeon is a final year student of Engineering and Arts at the Australian National
University. He has just completed his honours thesis under the supervision of Dr Keith
McIntosh at the Centre for Sustainable Energy Systems. This work involved the
inclusion of corona charge for improved surface passivation of photovoltaic solar cells.
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