Available online at www.sciencedirect.com

SiliconPV: 17-20 April 2011, Freiburg, Germany

Novel noncontact approach to monitoring the field-effect passivation of emitters

Marshall Wilsona*, Alexandre Savtchouka, Jacek Lagowskia, Ferenc Korsosb, Atilla Tothb,

Radovan Kopecekc, Valentin Mihailetchic, Roman Petresc and Tim Boescked

aSemilab SDI LLC, Tampa, FL 3361, USA

bSemilab Zrt, Budapest, Hungary cInternational Solar Energy Research Center-ISC Konstanz,Konstanz, Germany

dBosch Solar Energy AG, Erfurt, Germany

Abstract

We report a comprehensive noncontact approach to measurement of the field-effect passivation of emitters for p and n-type base silicon solar cells. The corona charge-voltage technique, extensively used in silicon IC fabrication lines, is utilized in an automated sequence with two complementary measurements that monitor charge induced changes of recombination in the emitter. Quasi-steady-state microwave photoconductance decay, QSS-µPCD measures decay lifetime τeff, the injection level, Δn and the emitter saturation current, J0. UV and blue ac-surface photovoltage (UV-SPV) gives a passivation indicator analogous to short wavelength quantum efficiency.

Field-effect characteristics of τeff, Seff, J0, and UV-SPV are presented for symmetrical n+ and p+ emitter test wafers that quantify the effect of charge, polarity and differences between n+ and p+ emitters. New findings include field effect hysteresis and charging induced emitter degradation phenomenon analogous to stress induced degradation in MOS devices. © 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of SiliconPV.

Keywords: emitter passivation; field-effect; J0; corona charging.

*Corresponding author. Tel.: +1-813-977-2244; fax: +1-813-977-2455. E-mail address: mwilson@semilabsdi.com.

1876–6102 © 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of SiliconPV 2011.doi:10.1016/j.egypro.2011.06.104

Energy Procedia 8 (2011) 71–77

© 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of SiliconPV 2011.

72 Marshall Wilson et al. / Energy Procedia 8 (2011) 71–77

1. Introduction

Emitter passivation is a significant element in solar cell fabrication. It requires chemical and physical engineering of the silicon-dielectric interface region and dielectric charge. The goal is to optimize two elements; 1- to minimize interface defects that act as recombination centers and to create interface and dielectric defects that are electrically charged and can repel the minority carriers providing the field-effect passivation [1, 2]. In this work we present a metrology solution for monitoring these two critical elements. The effective surface recombination in the emitter (or the emitter saturation current, J0) is measured versus dielectric charge controlled by corona charging in order to determine the emitter field-effect characteristic. The initial dielectric charge is then compared with the optimal charge for the emitter field-effect passivation. Our approach is aimed at development of an automated monitoring sequence for PV developmental needs and off-line manufacturing testing. The methodology adopts production proven techniques used in IC manufacturing [3, 4]. 2. Experimental approach and apparatus The experimental setup is shown schematically in Fig. 1. It includes three noncontact measurement techniques; 1- quasi-steady-state microwave photoconductance decay, QSS-µPCD, lifetime; 2- vibrating Kelvin-probe surface voltage, and 3- short wavelength surface photovoltage, SPV. These measurements are combined in an automated sequence with charge dosing on a dielectric surface by a corona discharge in air with a point corona gun. This allows measurement of charge characteristics for the parameters listed at the bottom of Fig. 1. The QSS-µPCD [5] is a version of bias-light PCD [6]. It measures decay lifetime, τeff, and the injection level Δn, with the capability of scanning steady state generation rate G and corresponding Δn. The steady state illumination is below the nonlinear Auger recombination range. The decay is confirmed to be exponential and independent of laser power. This condition corresponds to a linear total recombination rate and results in actual lifetime rather than differential as confirmed by the integration procedure described in ref [7]. The parameter of primary interest for field-effect passivation is the emitter saturation current, J0, that is determined with the Kane and Swanson method [8] from a simultaneously measured set of τeff and Δn data.

Fig. 1. Multifunction measurement apparatus for automated monitoring of the field-effect passivation.

Corona charging of dielectrics and vibrating Kelvin probe surface voltage measurements are similar to techniques used in noncontact C-V metrology for dielectrics in IC Fab-lines [4]. A corona discharge gun deposits a pulse of ionic charge with controlled dose and polarity on the dielectric surface. After charging, a time resolved measurement of surface voltage V is performed with a Kelvin probe. The

Marshall Wilson et al. / Energy Procedia 8 (2011) 71–77 73

voltage change, , extrapolated to termination of charging, gives the dielectric capacitance, = / . The voltage decay rate ( ) after charging gives the dielectric leakage current

. Assessment of leakage is very important for corona based measurements. Leakage can neutralize the deposited charge before other measurements are performed. This limits the range of net charge in field-effect experiments. Ideally, the limits would be provided by Fowler-Nordheim tunneling. In practice, they may be significantly lower due to leakage via traps. In the present study, the low leakage range of in q/cm2 was -6.5e12 to +3.2e12 instead of -2.1e13 to +1.3e13 FN limit. However, with time resolved monitoring the measurements could be extended to larger range.

Additional measurements of surface passivation were performed using “ultimate” SPV, a version that enables simultaneous monitoring of two SPV signals corresponding to different light wavelengths using slightly shifted light modulation frequencies. For emitter structures the SPV signal represents the junction photovoltage rather than the surface barrier photovoltage. In the present configuration it was measured using a transparent capacitive mesh electrode 0.25mm above the wafer surface. Modulated blue (425nm) and UV (375nm) beams were used and the SPV signal ratio UV/Blue provided a passivation indicator analogous to short wavelength quantum efficiency QE. Ideally the ratio is close to 1. Increasing surface recombination decreases the ratio. Charge-SPV monitoring of emitters was proposed in ref [9]. Symmetrical n+/p/n+ and p+/n/p+ emitter test wafers used in this study were fabricated on Cz silicon substrates using industrial solar cell manufacturing procedures at ISC Konstanz. The passivation dielectric was a stack of interfacial (1.5 to 2)nm SiO2 and about a 60nm antireflection SiNx film. Firing was the final process step. 3. Results and Discussion The as-measured data that illustrate the effect of corona charging on the effective lifetime for p+/n/p+

emitter test structure are given in Fig. 2. The effect of charging is very pronounced in spite of the fact that charging was done only on one surface. The maximum on the τeff vs. intensity curve increases from 270 s to 380 s for negative charge and decreases to 120 s for positive charge. The beneficial effect of negative charge and detrimental effect of positive charge are consistent with the polarity of the field-effect passivation for p+ emitter [1, 2]. The descending τeff range in Fig. 2 corresponds to recombination in the emitter. As discussed in ref [5] it is used for J0 measurement.

Fig. 2. The effect of corona charge on the effective lifetime dependence on QSS light intensity

for p+/n/p+ emitter structure passivated with 1.6nm interfacial SiO2 and 600Å SiNx.

50

100

150

200

250

300

350

400

0 0.5 1 1.5 2

ττef

f(μμ

s)

QSS Light Intensity. (suns)

Negative Qc

Positive Qc

Initial- 6e12 q/cm2

+ 2e12 q/cm2

74 Marshall Wilson et al. / Energy Procedia 8 (2011) 71–77

1.0E-13

1.5E-13

2.0E-13

2.5E-13

3.0E-13

3.5E-13

4.0E-13

-1.2E+13 -6.0E+12 0.0E+00 6.0E+12

J 0[A

/cm

2 ]

Charge, QC [q/cm2]

n+ on p

p+ on n

For n+ emitters the polarity of field-effect passivation is reversed compared to p+ emitters i.e. the recombination in emitter is decreased by positive charge and it is increased by negative charge. The results in Fig. 3 illustrate such a case using the charge effect on the emitter saturation current for a n+/p/n+ test structure. The plots of 1/τeff vs. for different corona charge levels show a charge-induced shift of 1/τeff values. In addition, slope of the lines change (the slope is a measure of ).

Fig. 3. Results of 1/τeff vs. illustrating the determination of for different for a n+/p/n+ emitter structure.

The charge characteristics of for n+ and p+ emitters are shown in Fig. 4. They correspond to the results in Fig. 2 and Fig. 3, respectively. It is seen that in both cases the initial condition ( ) is not optimal considering the field-effect passivation. Especially for the p+ emitter, the saturation current could be decreased by dielectric processing that gives higher negative dielectric charge.

Fig. 4. Charge characteristics of obtained with corona charging and QSS- PCD.

The SPV results of field-effect passivation are illustrated in Fig. 5 for p+ and n+ emitters. The SPV measurements were carried out in the same charging-measuring sequence before QSS- PCD measurements. In response to charging, the signal ratio changes about 4 to 6 times. This is a very strong effect that exceeds by 3 orders of magnitude the resolution of the SPV technique.

Marshall Wilson et al. / Energy Procedia 8 (2011) 71–77 75

0.00

0.25

0.50

0.75

1.00

-1.2E+13 -6.0E+12 0.0E+00 6.0E+12 1.2E+13Charge, QC (q/cm2)

SPV

Rat

io (a

rb. u

nits

) n+ emitter on p

p+ emitter on n

Charge injection region

The SPV results are consistent with the behavior in Fig. 4. High SPV ratio that corresponds to good passivation occurs at negative charge for p+ emitters and positive charge for n+ emitters. It shall be noted that short wavelength SPV selectively measures only the top surface. The effective lifetime contains contributions from both surfaces and the bulk. This may contribute to stronger SPV charge dependence. By analogy to IQE, the short wavelength SPV is expected to be controlled by the effective surface recombination. As we already noted, the charge induced changes of 1/τeff are due entirely to changes of the effective recombination of the emitter. In Fig. 6 we show results of Seff = /2τeff at 1 sun measured at the same sites and at the same charge-measure cycle as SPV data in Fig. 5. It is seen that the increase of the SPV UV/Blue ratio corresponds to a decrease in Seff and vice versa. This behavior is seen for both n+ and p+ emitters. The insert in Fig. 6 presents a SPV ratio – Seff correlation plot in arbitrary units. It is seen that the SPV ratio – Seff correlation seems is good for both n+ and p+ emitters.

We have found that the field-effect passivation exhibits hysteresis that is manifested by characteristic hysteresis loops for all parameters sensitive to the recombination in the emitter. We illustrate this behaviour using the vs. charge hysteresis curve in Fig. 7 for a p+/n/p+ emitter structure.

0

40

80

120

160

200

-1.2E+13 -6.0E+12 0.0E+00 6.0E+12 1.2E+13Charge, QC (q/cm2)

S eff

(cm

/s)

n+ emitter on pp+ emitteron n

Charge injection region

SPV Ratio (arb. units)

S eff

Fig. 5. Charge-SPV passivation characteristics for a p+ and n+ emitters. Increase of SPV ratio corresponds to decreasing of recombination in emitter.

Fig. 6. Seff dependence on the net corona charge, obtained from eff at 1 sun. Insert shows the SPV ratio – Seff correlation.

76 Marshall Wilson et al. / Energy Procedia 8 (2011) 71–77

Fig. 7. field-effect hysteresis loop for a p+ emitter. is the net charge on dielectric surface.

It is seen in Fig. 7 that the hysteresis effect shifts from the initial value depending upon direction of charging. Return from negative charge corresponds to degraded emitter with higher . This is reversed by positive charging. Return to zero after positive charge produces value lower than the initial one. By analogy to bias-induced phenomena in MOS structures the behavior may involve recharging of dielectric traps (similar to that in flash memory) and/or the field-induced interface traps.

4. Conclusion

An automated corona charging-measurement sequence opens new possibilities for the monitoring of field-effect emitter passivation. Corona charging plus QSS-µPCD applies very well to emitter test structures. The novel corona-SPV approach correlates well with and Seff. SPV is highly sensitive and it is applicable also to final cells. The metrology is extendable to wafer mapping of field-effect passivation. References

[1] S.W. Glunz, D. Biro, S. Rein, and W. Warta, “Field-Effect Passivation of the SiO2-Si Interface”, Journal of Applied Physics, 86 (1), 683-691 (1999). [2] J. Benick, B. Hoex, M.C.M. van de Sanden, W.M.M. Kessels, O. Schultz and s.W. Glunz, “High Efficiency n-type Si Solar Cells on Al2O3 – Passivated Boron Emitters”, Appl. Phys. Lett. 92, 253504 (2008). [3] D.K. Schroder, “Semiconductor Material and Device Characterization”, (Wiley-Interscience, Hoboken, New Jersey 2006). [4] M. Wilson et al., “The Present Status and Recent Advancements in Corona-Kelvin Non-contact Electrical Metrology of Dielectrics for IC-manufacturing”, ECS Transactions, 3 (3) 3-24 (2006). [5] M. Wilson et al., “QSS-µPCD Measurement of Lifetime in Silicon PV Wafers: Advantages and New Applications”, to be published in 2011 Silicon PV conference proceedings, April 17-20, 2011 Freiberg, Germany. [6] A. Aberle and J. Schmidt, “On data analysis of light-biased photoconductance decay measurements”, Appl. Phys. 79, no. 3, pp. 1491-96, Feb. 1996. [7] J. Schmidt, “Measurement of Differential and Actual Recombination Parameters on Crystalline Silicon Wafers”, IEEE Trans. Electron Devices, vol. 46, no. 10, pp. 2018-25, Oct. 1999. [8] D. Kane and R. Swanson, “Measurement of the Emitter Saturation Current by a Contactless Photoconductivity Decay Method”, Proc of the 18th IEEE Photovoltaic Specialist Conference, pp. 578-583, 1985.

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[9] T. Boescke “Charge-SPV” 2010, unpublished.