Highresolution lifetime mapping using modulated freecarrier absorptionStefan W. Glunz and Wilhelm Warta Citation: Journal of Applied Physics 77, 3243 (1995); doi: 10.1063/1.358677 View online: http://dx.doi.org/10.1063/1.358677 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/77/7?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Highresolution absorption mapping with a pu surface impedance method. J. Acoust. Soc. Am. 127, 2003 (2010); 10.1121/1.3385194 Free-carrier absorption and active layer heating in large optical cavity high-power diode lasers J. Appl. Phys. 100, 023104 (2006); 10.1063/1.2212147 Carrier lifetime measurements using free carrier absorption transients. II. Lifetime mapping and effects ofsurface recombination J. Appl. Phys. 84, 284 (1998); 10.1063/1.368025 Measurement of FreeCarrier Lifetimes in GaP by Photoinduced Modulation of Infrared Absorption J. Appl. Phys. 42, 3205 (1971); 10.1063/1.1660708 Transient HighEnergy RadiationInduced FreeCarrier Absorption in Silicon J. Appl. Phys. 40, 3416 (1969); 10.1063/1.1658210

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High-resolution lifetime mapping using modulated free-carrier absorption Stefan W. Glut-& and Wilhelm Warta Fraunhofer-Institute jk Solar Energy Systems (FhG-BE), Oltmannsstrasse 22, D-79100 Freiburg, Germany

(Received 19 July 1994; accepted for publication 15 December 1994)

Modulated free-carrier absorption (MFCA) is introduced as a novel contactless lifetime mapping technique with high spatial resolution. The advantages of MFCA mapping as compared to other methods are discussed. Examples of lifetime maps on multicrystalline silicon are presented, Solar cells fabricated from identical wafers have been investigated using light beam induced current mapping. Excellent agreement between structures obtained in the measured lifetime and in the current maps was observed. In addition, a quantitative comparison with diffusion lengths determined by local internal quantum efficiency evaluations is presented. 0 1995 American Institute of Physics.

I. INTRODUCTION

Lateral spatial homogeneity of silicon quality is an im- portant parameter for semiconductor device performance. In solar cells the minority carrier lifetime is a crucial parameter for obtaining high conversion efficiencies. For multicrystal- line material an integral lifetime characteristic is not suffi- cient; only a lateral lifetime distribution gives the informa- tion necessary for solar cell and crystal growth technology.’

It is possible to distinguish between two principal types of lifetime measurements: (i) microscopically resolved meth- ods, like electron beam induced current (EBIC) and (ii) methods measuring the recombination parameters averaged over an area of a size comparable to or larger than the dif- fusion length of minority charge carriers. While the first group of methods is used to investigate microstructures like precipitates or dislocations,” the latter ones are more suitable to predict the device performance. These methods deliver an integral value for the recombination characteristics within the range of the diffusion length determining the solar cell

performance. The resolution accessible with the method pre- sented in this article represents a reasonable link between the macroscopic device performance, i.e., short circuit current and efficiency, and the internal recombination parameters.

II. METHOD

In the modulated free-carrier absorption (MFCA) method3’4 the free carriers photogenerated in a silicon wafer are optically monitored by the absorption of infrared (IR) light. The excess charge carriers are generated by a sine modulated light source. Due to the lifetime of the carriers a phase shift between the generation and the IR transmission through the sample occurs. This phase shift p is used to calculate the lifetime in the bulk rb and the surface recom- bination velocity S.

The determination of the recombination parameters from the phase shift T is discussed in detail in Refs. 3 and 4. The integral of the excess minority carrier density, AN, is given by

aL,~(S, + aD){(DIL,rr)sinh( W/&r) +S?[cosh( W/.&r) - 11) (~lL,,)(S1+Sz)cosh(WIL,ff)+[(D2/L~~)+S1S,}sinh(WIL,ff)-1

with

J DQ L&f= ___ I+ iwrh’

where 77 is the quantum efficiency, R is the reflectance, Cp is the photon flux density of the laser diode, LY is the absorption coefficient, St, S2 are the surface recombination velocities of the illuminated and the dark surface, respectively, and Leff is the effective diffusion length. The phase shift q is

‘. (3)

For the simple case of S i = S, = 0, Eq. (3) reduces to

@E-mail: glunz@ise.fhg.de

tan t+?= orb . (4)

Since the measured phase shift depends on both S and 71,) additional information about S is needed in order to extract rb. Presently the samples used for MFCA are coated by a low-temperature LPCVD oxide to reduce surface recombination.’

A high lifetime FZ wafer with the same surface prepara- tion was used to extract SLPCvD . The bulk lifetime of the FZ wafer was determined using the photoconductance decay method4.’ with the wafer immersed in hydrothroric acid, known to suppress the surface recombination very effectively.6 With knowledge of the bulk lifetime, SLpcvD can be calculated directly from the effective lifetime measured on the LPCVD-passivated FZ wafer.

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I Signal Reference

FIG. 1. MFCA measurement setup.

With Si = Ss = Smcvo, Eq. (3) is used to determine rb . Since all parameters, including the surface recombination ve- locities S, and S?, are known, it is possible to determine the bulk lifetime uniquely from Eq. (3) and Eq. (l), respectively. In order to reach the highest sensitivity, the frequency f of the generation light (with f= 27ro) was chosen for the aver- age phase shift to be around 45”, because a small shift of the lifetime in this frequency range results in the highest phase variation.

In our setup (Fig. 1) we used a GaAlAs laser diode (X =780 nm) driven by a sine generator for the photogeneration of excess carriers. The IR beam of a HeNe laser (3.4 pm) transmitting the sample is attenuated sinusoidally by the in- tegral carrier modulation. The phase shift is measured with a lock-in amplifier. The frequency reference is taken from the sine wave generator, while the signal channel is connected to a liquid-nitrogen cooled InSb IR detector (A) detecting the modulated IR beam. A fraction of the IR beam is directed toward a second InSb detector (B) monitoring the laser noise. Subtracting this monitor signal from the output of de- tector (A) the signal to noise performance is drastically im- proved. An additional white bias light is applied to define the injection level. With a high-resolution xy table, the sample is moved in small steps (resolution 0.5 prnj through the detect- ing IR beam.

A diameter of the detection spot on the sample of about 200 pm is easily obtained by a single sapphire lens. Apply- ing appropriate beam expansion optics, a spatial resolution in the 10 km range appears to be feasible. Due to the diver- gence of the generation light transmitted to the sample by a glass fiber the generation spot has a diameter of around 1 cm. Thus, the area of photogeneration is much larger than both sample thickness and detection area. This is a major advan- tage of MFCA lifetime mapping, as discussed below. In con- ductance mapping methods like photoconductance decay (PCD) or time resolved microwave conductance (TRMC)

3244 J. Appl. Phys., Vol. 77, No. 7, 1 April 1995

spatial resolution is obtained by reducing the diameter of the generation spot, while the excess carriers are detected over a wider area.

ill. EXPERIMENT

A. Sample preparation

The silicon used was p-type, multicrystalline block cast material with a thickness of 345 pm and a base resistivity of 1 R cm. A low-pressure chemical vapor deposition (LPCVD) oxide was deposited at a temperature of 550 “C to reduce the surface recombination velocity. Due to the low process tem- perature the changes of the recombination parameters in- duced by the thermal stress are expected to be minor. A high- lifetime FZ p-type reference wafer was used to determine the recombination velocity SLpcvn at the LPCVD-oxidized sur- face (see Sec. II). After the lifetime mapping a solar cell with oxide passivated emitter and aluminum back surface field was fabricated and a light beam induced current (LBIC) mapping was performed.

B. Results

In Fig. 2 a high-resolution lifetime map performed with a step size of 200 pm over an area of 30X30 mm (150X 150 points) is shown. The measuring time per point was 1 sec- ond. Shorter measurement times appear to be feasible with minor improvements. On the solar cell fabricated from the same wafer an LBIC mapping (Fig. 3) was performed over the same area as the lifetime map of Fig. 2 (300X300 points on a 30X30 mm area). The LBIC setup operates with a beam spot size of 100 pm. Fast data acquisition allows measure- ments with a rate of 100 points per second using a lock-m amplifier.7 The structures in the current map correlate well with the ones observed in the lifetime map.

In order to compare the lifetimes measured in the start- ing material quantitatively with the diffusion lengths limiting the short circuit current in the solar cell, we have performed measurements of the internal quantum efficiency (IQEj at different spots of 2 mm in diameter. The internal quantum efficiency is defined as the ratio of carriers contributing to the photocurrent to the number of photons absorbed in the cell. By choosing different wavelengths h of monochromatic light the penetration depth Q-I (cw=absorption coefficient of the light) is changed. Plotting the inverse TQE versus the penetration depth (Y-I of light in the near infrared results in a straight 1ine.s The intercept of this linear fit with the x axis gives the negative value of the effective diffusion length L,, (Fig. 4) determined by the diffusion length in the bulk Lb and the surface recombination velocity Shock, but not by the op- tical reflection at the rear side of the cell, due to the wave- length range used for the evaluation of LeE.9>‘o This evalua- tion is very similar to the one of the steady-state surface photovoltage (SPV), see, e.g., Ref. 11. The results for three selected positions (marked in Fig. 3) are given in Table I.

IV. DISCUSSION

A. Comparison between MFCA, LBIC, and IQE results

The lifetime map (Fig. 2) and the current map (Fig. 3) exhibit a very good structural correlation, representing the

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-0 5 IO 15 20 25

x Cmml FIG. 2. Lifetime map of a multicrystalline sample,

grain structure of the multicrystalline material. Due to the different light reflections of the different grains it is possible to recognize the measured structure-s also optically on the solar cell. The differential measurement technique used in

the MFCA method assures that the varying optical reflection of the surface does not influence the determination of the lifetime.

It is a very interesting question to what extent the vol-

- (A)

-0)

0 5 10 15 20 25 30

x rmr-4 FIG. 3. LBIC map of the solar cell processed from sample of Fig. 2.

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3

-100 0 100

Penetration Depth [pm]

200

FIG. 4. Inverse internal quantum efficiency piotted versus penetration depth for the three diierent spots marked in Fig. 3; symbols: measured values, solid lines: extrapolated Linear fits; intercept with x-axis=-effective diffu- ston length L,, .

lume recombination present in the starting material is changed by the solar cell processing. As a first approach to this problem, we have compared the volume lifetime in the starting material with the bulk diffusion length in the solar cell at several positions. From the internal quantum effi- ciency IQE (see Sec. II) the volume diffusion length Lb is readily extracted if the rear surface recombination velocity at the Al-BSF, Shack , is known.“,”

Shack was determined to be 2500 cm/s by photocurrent and voltage decay (PCVD) on a solar cell processed with the same rear surface treatment on FZ material. The PCVD method combines the information coming from the decay of the short circuit current and the open circuit voltage of a solar cell after switching off a generation light very fast. The experimental setup is described in Ref. 13. Using a material with a high bulk lifetime like high purity FZ silicon, the value of the surface recombination velocity Sbacck of the Al- BSF can be obtained with high accuracy. This method will be described in detail elsewhere.14

Although the illumination spot size in the spectral re- sponse measurement was about 2 mm wide, quite reasonable fitting curves are calculated using single values for the vol- ume diffusion length (solid lines in Fig. 4). The resulting bulk diffusion lengths and the corresponding lifetimes (cal- culated with a diffusion constant of 28 cm”/s) are given in Table I.

TABLE I. Effective diffusion lengths L,, measured at the positions of the solar cell marked in Fig. 3, calculated base diffusion lengths Lb, and volume lifetimes 76 (see Sec. IV).

&,,,=2500 cm/s)

Position Tb (d

(A) 202 200 14.3 03 149 140 7.0 ((3 65 62 1.4

FIG. 5. Comparison of measurement geometry of the MFCA and the TRMC method.

Comparing these lifetimes with the ones measured with MFCA on the starting material, it is found that for this spe- cial solar cell process and the material used the volume life- times tend to deteriorate. Within the large grains with life- times above 20 ps in the MFCA map a decrease of about 30% is observed j-position (A)]. The polycrystalline areas with starting lifetimes of only l-3 ,us do not change signifi- cantly [position (C)l. The few grains with intermediate life- times show a high degradation of about a factor of 2 [posi- tion (B)]. A detailed and comprehensive comparison of MFCA and LBIC maps for different materials is in prepara- tion.

B. Comparison between different lifetime mapping methods

The MFCA method is different in principle from other measurement techniques used conventionally. First, MFCA is a purely optical method providing a signal which is directly proportional to the integral carrier density over a wide range. Thus, there is no limitation in the dopant concentration of the silicon material like there is, e.g., for TRMC.t’

Another important feature is the relation between gen- eration and detection volume, as is demonstrated in Fig. 5. While, e.g., in conductance methods, like the time-resolved microwave conductance TRMC, the spatial resolution is ob- tained by focusing the generation spot onto a large detection area, the MFCA uses a small area detection beam. Photoge- neration is carried out homogeneously over a large area es- tabfishing an excess carrier density influenced by the inho- mogeneous recombination in the same way as in a solar cell.

In the conventional detection geometry [case [a) in Fig. 51 several problems may arise if the diameter of the light spot generating the excess carriers is in the range of the sample thickness or of the investigated structures. A principal problem arises from the fact that the evaluation is oversim- plified by a one-dimensional differential equation. Only if the illuminating spot has a diameter of more than five times the sample thickness is this simplification valid. Additionally, the TRMC shows high sensitivity on nonuniform lateral con- ductivity distributions, caused by local carrier generation. This crucial effect on measured microwave reflection tran- sients is discussed in detail in Ref. 16. Another problem oc- curs due to the high radial excess carrier gradient around a highly localized photogeneration. This gradient causes an en- hanced carrier diffusion into neighboring regions with differ- ent recombination parameters. This problem is emphasized, if, for example, the small illumination spot is located in a

3246 J. Appl. Phys., Vol. 77, No. 7, 1 April 1995 S. W. Glunz and W. Warta [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

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high Iifetime area, directly beside a low lifetime region. For a large illumination area with a small detection cylinder as in the MFCA setup this problem is not occurring, because the neighboring regions are illuminated as well. Thus, the excess carrier distribution is determined by the local recombination completely equivalent to the case of a homogeneously illu- minated solar cell.

V. CONCLUSION

Modulated free-carrier absorption (MFCA) is estab- lished as a contactless mapping method for minority carrier lifetimes in starting material for silicon solar cells. The value of the local lifetime obtained in a region of inhomogeneous recombination reflects the same superposition of the local recombination properties as in the solar cell. This holds true also for a resolution below sample thickness and diffusion length. Another distinct advantage of MFCA as compared to, e.g., transient microwave retiection is that using the MFCA method, wafers with high dopant concentrations can also be measured without any problem.

The significance of the obtained lifetime distribution for the solar cell performance is approved. The structures ob- served in the lifetime map correlate well with the current distribution on a solar cell processed from the identical wa- fer. The quantitative change of the local lifetime values mea- sured with MFCA as compared to the ones obtained from an evaluation of the local internal quantum efficiency of the solar cell appears to depend on the local quality of the start- ing material. Contactless MFCA measurements can be per- formed in any stage during wafer processing prior to the final metallization. Thus, we have demonstrated that MFCA mea- surements constitute a valuable new tool for the investigation of the influence of different process sequences on multicrys- talline material.

ACKNOWLEDGMENTS

This work was supported by the German Federal Minis- try for Research and Technology under Contract No.

0329306A. The help of H. Lautenschlager, E. Demesmaeker, R. Schindler, and J. Knobloch in sample preparation and E. Schaeffer in measurements is highly appreciated. The au- thors would like to thank W. Wetthng for fruitful discussions and his steady encouragement of this work.

‘W. Warta, S. W. Glunz, A. B. Sproul, H. Lautenschlager, I. Reis, and R. Schindler, in Solid State Phenomena: Polycrystalline Semicondtrctors III, edited by H P. Strunk, .I. H. Werner, B. Fortin, and 0. Bonnaud (Scitec, Switzerland, 1994), Vol. 37-38, p. 415.

*J. Palm, J. Appl. Phys. 74,69 (1993). ‘F. Sanii, R. J. Schwartz. R. E Pierret, and W. M. Au, in Proceedings of the

20th IEEE Photovoltaic Specialists Conference, Las Vegas, 1988 (IEEE, New York, 1989). p. 575.

4S. W. Glunz, A. B. Sproul, W. Warta, and W. Wettling, J. Appl. Phys. 75, 1611 (1994).

“D. E. Kane and R. M. Swanson, in Proceedings of the 18th IEEE Photo- voltaic Specialists Conference, Las Vegas, 1985 (IEEE, New York, 1985), p. 578.

“E. Yablonovitch, D. L. Allara, C. C. Chang, T. Gmitter, and T. B. Bright, Phys. Rev. I&t. 57, 249 (1986).

7B. Wagner, thesis, Technical University of Darmstadt, 1989. sE. D. Stokes and T. L. Chu, Appl. Phys. Lett. 30, 625 (1977). 9P. A. Basore, in Proceedings of the 23rd IEEE Photovoltaic Specialists

Conference, Louisville, 1993 (IEEE, New York, 1993). “W. Warta, in Proceedings of the 11th. E. C. Photovoltaic Solar Energy

Conference, Montreux, 1992, edited by L. Guimar’zies er al. (Harwood Academic. Switzerland, 1993), p. 469.

“J. Lagowski, A. M. Kontkiewicz. L. Jastrzebski, and P. Edelman, Appl. Phys. Lett. 63, 2402 (1993).

‘*M. Hirsch, U. Rau, and J. H. Werner, Solid State Electron. (in press). ‘“W. Warta, R. Bergmann, and B. Vo,S, in Proceedings of the 8th E. C.

Photovoltaic Solar Energy Conference, Florence, 1988, edited by I. So- lomon, BEquer, and P. Helm (Kluwer Academic, Netherlands, 1988), p, 1416.

l4 W. Warta (unpublished). 15Paul A. Basore and Barry R. Hansen, in Proceedings of the 2Zst IEEE

Photovoltaic Specialists Conference, Orlando, 1990 (IEEE, New York, 1990), p. 374.

t6F P Giles, R. J. Schwartz, and J. L. Gray, in Proceedings of the 23rd IEEE . . Photovoltaic Specialists Conference, Louisville, 1993, p. 299.

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