Morphology of n-type macroporous silicon: doping density

Photoelectrochemical texturization of n-type multicrystalline silicon. R. Tena-Zaera , S. Bastide , C. Lévy-Clément. physica status solidi (a) 2007 ...
0 downloads 0 Views 4MB Size
Download PDF
J. Phys. Chem. 1995,99, 4132-4140

4132

Morphology of n-Type Macroporous Silicon: Doping Density Dependence E. Galun,*jt C. Reuben: S. Matlis,* R. Tenne: and C. LCvy-ClCment* Department of Materials and Inte$aces, and Chemical Services Unit, Weizmann Institute, Rehovot 76100, Israel, and Laboratoire de Physique des Solides de Bellevue, C.N.R.S. UPR 1332, 1, Place Aristide Briand, F-92195 Meudon Cedex, France Received: June 14, 1994; In Final Form: December 28, 1994@

Macroporous silicon film was prepared on n-type Si (100) substrates by anodizing n-type silicon under illumination. The nanoporous film, which is responsible for the visible luminescence, is selectively dissolved by KOH solution exposing the macroporous layer. The dependence of the morphology on doping density and charge passed through the electrode of the macroporous layer is reported here. The doping densities were varied from Nd = loi3to 4 x 10i8/cm3. The morphology was studied using scanning electron microscope (SEM) and atomic force microscope (AFM). A dense superficial pattem of submicron size etch pits was found for the heavily doped samples after 5 1 C/cm2. The lightly doped samples (Nd < 10i6/cm3)do not show any particular morphology before 1 C/cm2. The AFM, which in general has larger resolution than the scanning electron microscope, does not reveal further (secondary) morphological pattem, beneath the ones resolved by the SEM. A comparison with the morphology of photoetched 11-VI compounds is presented. A model is proposed which attempts to address the initiation of the macroporous morphology through nonuniformities in the photoetching current. Using this model a breakdown field for photocurrent multiplication near surface dopant atoms is found for Nd > 10i6/cm3. Furthermore, a saturation of the etch pits density for Nd > 1017/cm3is predicted. Both results are supported by the present experiments.

Introduction While most of the research on luminescing silicon (so-called porous silicon) has been focused on p-type silicon, some effort was devoted to the properties of porous silicon which is obtained from n-type samples, For an excellent review of the general subject, see ref 7. Porous silicon is obtained from bulk p-type Si wafer by electrochemical anodization in HF solutions. Auxiliary illumination, or large reverse bias which leads to breakdown of the junction,8 is needed in the case of n-type Si. The photoexcitation process increases the density of surface holes which carry out the etching process, leading thus to porous silicon. This process, if carried out under controlled conditions, was shown112a to have a beneficial influence on the photoresponse of a Si photoanode in a concentrated HI solution. More recently, a multicrystalline Si photovoltaic cell with a texturized surface, prepared by anodization of the emitter in HF solution, was demonstrated.1ds2b Optimization of this process requires a detailed knowledge of the morphology of the macroporous silicon upon material and process parameters. To distinguish this effect from the well-known photocorrosion of Si, this process has been coined photoelectrochemical etching (PEC-etching). Prior to that, the favorable influence of PECetching on the photoresponse of photoanodes was observed and investigated in quite detail in a number of other semiconductors and in particular in 11-VI compound^^.^^ (for a general review of this subject see ref 11). Some distinct differences in the morphology of porous Si layer were observed between the two substrates (n and p type Si). The morphology of porous Si obtained by anodization in the dark (p and n+ materials) is like a sponge. Perhaps the Department of Materials and Interfaces, Weizmann Institute.

* Chemical Services Unit, Weizmann Institute.

Laboratoire de Physique des Solides de Bellevue. @Abstractpublished in Advance ACS Absrracrs, February 15, 1995.

§

0022-3654/95/2099-4132$09.00/0

most striking difference between porous Si obtained in the dark and under illumination is, that in n-type PEC-etched Si, two porous films are found. The morphology of the nanoporous silicon film (top surface) is characterized through a network of tangled filaments of a nanometer thickness; some of them exhibit quantum size effects.2c,2dThis nanoporous (NP) film, which appears fibrous, was shown to be the source of the visible luminescence of an anodized silicon wafer.% A macroporous (MP) film with pore sizes in the micron range is observed beneath the NF' silicon film (schematically shown in Figure 1). The MP film can be easily distinguished from the NP film by a selective dissolution of the latter in KOH solution.*S6 The present study is aimed at characterizing the morphology of the MP film. In particular it is found that the morphology of the MP film is very much influenced by the doping density of the substrate. Heavily doped substrates exhibit a much more complicated morphology than previously envisaged.2a The time (charge) evolution of the morphology during the etching process can be described by three consecutive phases (Figure 1). This morphology consists of a macroporous pattem phase 1 (MP1 < 1 C/cm2), which is produced through the initial period of the anodization process for Nd > 10i6/cm3,and a macroporous pattern phases 2 and 3 (MP2 and MP3, respectively), which are formed as the anodization proceeds. In MP2 (1 < MP2 < 3.3 C/cmZ)the initial etch pits coalesce into larger pores (Nd > 10i6/cm3),and initial pores are obtained for Nd < 10i6/cm3.In MP3 ('3.3 C/cm2) large pores with diameter > 1 pm and aspect ratio (length/diameter) > 1 are formed. For Nd < 10i6/cm3they have smooth walls, whereas for Nd > 10i6/cm3side branching of secondary pores are produced along the walls of the pores. 11-VI compounds crystallize in zincblende or wurzite structure which are a superstructure of the diamond lattice, in which silicon crystallizes. Analogies and discrepancies between PEC etching of Si and 11-VI semiconductors are discussed. A mechanism for the initiation step of the pore formation is proposed. 0 1995 American Chemical Society

J. Phys. Chem., Vol. 99, No. 12, 1995 4133

Morphology of n-Type Porous Silicon

Bulk Si

The Macroporous Pattern Heavily Doped

Lightly Doped

MI’I

I

MP2

M1’3

Figure 1. Schematic representation of the porous silicon patterns and the three macroporous phases.

Experimental Section

n-Type Si (100) with five doping densities was used in this study: Nd = 4 X (0.01 Mcm); loi7 (0.1 Mcm); (0.5 Mcm); 2 x 1015 (5 Mcm); and 1013/cm3(1000 Mcm). To the exclusion of the 1017/cm3crystal, which was antimony doped, all crystals were phosphorous doped. Preparation of the porous Si film was described in detail in previous publications.1*2In short, Si crystals were chemically etched with HNO3/HF/acetic acid in the ratio 16:3: 1. They were mounted as electrodes using In-Ga eutectic as the back contact. PEC etching was carried out in 5% HF solution under -0.1 V vs SCE. The light intensity was regulated so that the photocurrent during the process did not exceed 10 mAlcm2. The experimental parameters of the PEC-etching, like potential, HF concentration, light intensity, etc., were carefully selected according to guidelines which are described in detail in previous publications.2- Essentially, one should not go into the regime where electropolishing starts, which is usually beyond (more anodic) the first peak of the current-voltage curve. The position of the peak depends on the experimental parameters mentioned above. Once these parameters have been chosen, PEC-etching can take place in a potential more cathodic of the first peak. Following PECetching, the samples were soaked in 1 M KOH solution until gas bubbling ceased, in order to dissolve the NP film. Scanning electron microscopy (SEM) was performed with Jeol Model 6400. Atomic force microscopy (AFM)was carried out using Nanoscope 11AFM equipped with SiN cantilever scanner with 12 pm2 scan size and 4 pm2 vertical range. Typical forces were 10 nN in air. Prior to each measurement the samples were

soaked for a short time (ca. 1 min) in 48% HF to remove the native oxide.

Results The evolution of the morphology of the PEC-etched electrodes was followed first by SEM analysis. Figure 2 shows SEM micrographs of the Nd = 4 x 10i8/cm3sample after PECetching with 0.25 (a), 0.5 (b), 3.3 (c), and 10 C/cm2(d) of charge and subsequent immersion in KOH solution. SEM analysis showed no particular change in the structure for the samples prior to and after chemical etching. AFM analysis showed some roughening of the surface with amplitudes < 10 nm. However, after PEC-etching and KOH dip a distinct morphology was revealed by the SEM. For 0.25 C/cm2 (Figure 2a), the highest doped sample (4 x 1018/cm3)exhibited some roughening of the surface with etch pits varying in size (< 100 nm) and depth. Upon transfer of 0.5 C/cm2 (Figure 2b), the morphology of the 4 x 101%m3 was found to be well developed. The average size of the etch pits was ca. 0.1 pm (submicron etch pits). This morphology was designated macroporous pattern phase 1 (MP1). The average size of etch pits increased to about 0.2 pm at 1 C/cm2 of charge. When 3.3 C/cm2 of charge were passed through the electrode, coalescence of submicron etch pits started for the heavily doped Si (Figure 2c). A dense pattern of rectangular pits was obtained forming the MP2 pattern. Finally the passage of 10 C/cm2 (Figure 2d) lead to further coalescence of the rectangular etch pits into larger (> 1 pm) rectangular pores (MP3 morphology). Furthermore, by cleaving the crystal and inspecting the cleaved section, formation of new side (lateral) pores (1 p m in diameter) from the main pore was clearly

4134 J. Phys. Chem., Vol. 99, No. 12, 1995

Galun et al.

Figure 2. SEM micrographs of anodized Si, Nd = 4 x lOI8/cm3, (100) oriented, PEC-etched with 0.25 (a), 0.5 (b), 3.3 (c), and 10 C/cm2 (d) of charge.

observed in MP3. The evolution of the morphology with time (charge passed) was not very different for the 1017/cm3sample, although the density of pores was somewhat smaller in this case (vide infra). It was further confirmed that the above morphologies were produced under illumination, only. In the range of potentials used in the present work (-0.1 V vs SCE and below) very low current was observed in the dark, and hence no morphology could be developed without illumination. It is well established1a2” that in highly doped Si (n+ > 1018/cm3)a substantial (anodic) dark current flows under such conditions which leads to the formation of a distinct morphology. Furthermore, no etch pits were obtained for samples PEC-etched under a potential close to flat-band conditions (- -0.4 V vs SCE).2v6b These results shows that space charge electric field has a paramount effect on the morphology of the PEC-etched silicon. Similar series of SEM micrographs for the 10%m3 sample is shown in Figure 3. No pattern could be discerned before 1 C/cm2 of charge (MPl), in the lowly doped samples. Only after relatively large amount of charge, passed through the silicon electrode, the morphology of the pores is seen. Irregularly distributed pores with round-like shape (MP2) and pore diameter of ca. 1 pm were obtained after 3.3 C/cm2 (Figure 3a). These pores grew larger and deeper with PEC-etching time and yielded the MP3 morphology as shown in Figure 3b, after 10 C/cm2. After 50 C/cm2 of charge, all samples exhibited a well developed MP3 morphology as shown in Figure 4 for the samples with (a) 4 x 10l8and (b) 1.2 x 1015/cm3(top view of the MP film is shown in the upper micrographs and cross-section in the lower ones). Furthermore, SEM analysis of PEC-etched samples cleaved along the (110) axis revealed conical pores

with side pores formed at the walls and MP1-like morphology near the surface for the 4 x 1018/cm3 sample (Figure 4a). Contrarily, large pores with much higher aspect ratio and no side pores were observed for the 1015/cm3(Figure 4b) sample. By comparing parts b and c of Figure 2 it was concluded that, for heavily doped Si crystals, the superficial dimensions of the etch pits increased very fast in the early stages of the process when the MP1 morphology transformed into the MP2 morphology through coalescence of pores. On the other hand, the lateral etching on the semiconductor surface continued in a rather slow rate, after the size of the pores reached some kind of a critical dimension of about 2 pm in MP2. The case of the lightly doped material, low surface density of pores appears during MP2 above 1 C/cm2, and almost no coalescence of pores occurred. The morphology of the MP3 is characterized by deepening of the pores without branching. Figure 5 shows a series of SEM pictures of n-type Si after 3.3 C/cm2 charge and for different doping densities. Clearly, the etch pit density increases with the doping density but not in a linear fashion. Such dependence could be verified also after shorter PEC-etching (e.g., 1 C/cm2), but the quality of the SEM micrographs for samples with Nd < 1017/cm3was rather poor. Electrochemically treated electrodes are not expected to have perfectly smooth surfaces. The resolution of the SEM is limited to ca. 0.1 pm. It was important, however, to verify that there is no secondary pattern in the macroporous silicon below the range accessible for the SEM. To that end a similar study was undertaken, using the AFM. It is well established that AFM can attain subangstrom resolution on atomically flat surfaces. However, texturized surfaces prepared by PEC etching can not be easily studied by this technique. Particularly, the vertical

J. Phys. Chem., Vol. 99, No. 12, 1995 4135

Morphology of n-Type Porous Silicon

found to be 5 x lo9 and 4 x 109/cm2for the 4 x 10l8 and 1017/cm3samples after 0.5 C/cm2 and 2 x lo9 and 1.5 x lo9/ cm2 after 1 C/cm2, respectively. Thus the etch pit density obtained from the SEM analysis is somewhat larger than that derived from the AFM analysis.

Discussion The etching process was shown to be limited to the pore tips and the pore length increased with time (charge), while lateral growth ceased after a critical size has been r e a ~ h e d . ~ ~A* ~ , ~ ~ J ~ few models were forwarded in order to explain the morphology of the MP film. Beale and co-workers observed17 that the resistance of the porous silicon film, obtained in the dark (p type), is similar to that of an intrinsic material and concluded that the space charge field is depleted of majority carriers, between the walls of the pores, but not in the bottom of the pore where the electrochemical-etching process proceeds. ZhanglSa and Shen et al.15bshowed that the wall thickness in n-type material is determined by the depletion of majority carriers, Le., through the condition d 2W. Searson and coworkers attributed the pore generation to the focussing of the electric field at the pore tip.15c The etching process in this case was done in the dark and under relatively high bias, ca. 10 V. Lehmann and F0116 used a backside illumination of the Si wafer, which enhanced the current focussing at the pore tip, thereby producing deep pores. Using KOH treatment for preconditioning of the front surface they found that the separation between the pores was determined by the ratio of the applied photocurrent (I) to the photocurrent at the first maximum (Zp,l). In this way wall thicknesses of down to d = 2W/10 were observed. This observation casts some doubt on the necessity to obey the condition d = 2W in the initiation of pore formation (MP1). Alternatively, the wall thickness was shown to be determined by the supply of photogenerated carriers, i.e., the photocurrent used for the etching process.6 Another that the pore walls are passivated through chemical means, remains to be verified. It is believed that the driving force for the initiation of the MP1 film and the initiation of the side pores along the main pores of highly doped silicon (MP3) is associated with the dopant atoms near the semiconductor surface. It was previously suggested that nonuniform microfields exist around the dopant atoms nearest to the semiconductor/electrolyte i n t e r f a ~ e . ~ ? ' ~ Alternatively, nonuniform microfields near the semiconductor surface were attributed to a trapped (localized) positive charge formed in the first instant of the photocorrosion at the interface.20 Table 1 shows the variation of a number of relevant parameters as a function of the doping density of the silicon wafer. W is the width of the space charge layer: W = (2€€0v&Nd)''~; LD is the Debye screening length for the charged donors nearest to the interface: LD = (EcokT/q2Nd)1/2;a is the average distance of the first donor layer from the surface: a = l/(2Nd1I3);o is the surface density of the first donor layer: 0 = Nd213; n = W/(2a) is the average number of donor layers within the space charge layer. Here v b is the band bending at the interface (1 V); E = 11.9 is the dielectric constant; Nd is the donor density. An important result which comes out of this calculation is that the Debye screening length, LD,is larger than the mean distance of the first donor atoms, a, up to Nd = 10i7/cm3. For higher doping densities the reverse is correct (i.e., LD < a). Therefore the positive charge of the donors, which reside at a distance larger than LD from the surface is shielded, and hence these donors are not expected to have any influence on the etch pit pattern for Nd > 17/cm3. In this case the area density of donors which contribute to the nonuniform current is o = Nd213U/W,

-

3

Figure 3. SEM micrographs of anodized Si, Nd = 10i6/cm3, (100) oriented, PEC-etched with 3.3 (a) and 10 C/cm2 (b) of charge.

range of the scanner is limited, and hence the deeper pores cannot be accurately probed. Moreover the typical radii of commercially available tips are not less than 30 nm. The convolution between the tip and surface asperities leads to artificial imaging of steep walls at the pore. The reported analysis was carried out with due care to these considerations. Figure 6 shows AFM images of a few samples with different doping densities after 1 C/cm2. Contrast intensification of the images was performed using a low pass filter, which eliminated the ripples in the contours of the pores. The roughness of the surfaces varied with the doping density in a manner similar to that indicated by the SEM pictures (Figure 5). This study showed no evidence for a secondary pattern on PEC etched Si surfaces. To put things on a more quantitative basis, using the AFM images, the number of maxima along arbitrary direction were counted. The average of four lines for each sample is reported. Many more scans were counted in this way, which produced similar results, but are not reported in detail here. The density of etch pits was obtained from the square of this value. Figure 7 shows the dependence of etch pit density on the area doping density s = Nd213,along with the change in the pore density as a function of charge passed. Thus the etch pit density decreased by more than one order of magnitude when the doping density decreased from 1017to 10%m3. Also, a dramatic time (charge) dependence is represented in this curve. At 1 C/cm2 charge and below, the heavily doped samples exhibit the MP1 morphology with a pore density approaching 109/cm2. After 3.3 C/cm2 all the samples exhibit the MP2 morphology with a pore density of ca. 108/cm2and below. A similar analysis was done also using the SEM images. The density of etch pits was

Galun et al.

4136 J. Phys. Chem., Vol. 99, No. 12, 1995

Figure 4. SEM micrographs of PEC etched (50 C/cm2), (100) oriented Si, with two different doping densities: (a) Nd = 4 x l O ’ * / ~ m -and ~ (b) Nd = 1.2 x 10i5/cm3(top view of the MP film is shown in the upper micrographs and cross-section in the lower ones, respectively).

TABLE 1: Definitions of the Various Phases in the Evolution of the Macroporous Si Morphology macroporous phase

charge passed (C/cm2)

phase I (MP1) phase I1 (MP2) phase I11 (MP3)

10i7/cm3and above (after 0.5 C/cm2), while it is obtained in 11-VI compounds instantaneously even for Nd -= 10i6/cm3. The morphology of both IIVIS and Si seem to be independent on the doping atom (P and Sb for the case of Si; C1, In, and Ga for the case of CdTegb9l1) used for the (n-type) doping. This fact supports the above model, since the dopant atom is expected to influence the etching pattern through its electric field, only. The nonuniform part of the photocurrent is influenced by the absorption of light and the diffusion length of minority carriers. The larger is the absorption coefficient (a)and the smaller is the diffusion length (b); the larger is the nonuniform contribution to the photocurrent. Silicon, being an indirect bandgap material, exhibits small absorption coefficient in the visible (ca. 103/cm)and large diffusion length of minority carriers (ca. 2 x cm). On the contrary, 11-VI compounds exhibit large absorption coefficient (ca. 1@/cm) and small diffusion length ( IOv4 cm).

Morphology of n-Type Porous Silicon

J. Phys. Chem., Vol. 99, No. 12, 1995 4139

doping density ( ~ m - ~ ) 1oi5 1 1017

1 lo9 I

n

8 IOX --

?'

E

0

A

6 IO8

-

4 10'

-

I

I

-

1 0l8

-I

+3.3ClcmA2 IC/cniA2

.Y M

2

8

-

-

0 1o9

10"'

10"

I OI2

area dopant density (cm-2) Figure 7. Dependence of etch pits density on the area doping density for various.charges.

Figure 8. Schematic representation of the model describing the contribution to the PEC-etching photocurrent from the various layers near the semiconductor surface: a is the distance of the f r s t layer of donors.

Thus

The large difference in this factor for the two kinds of semiconductors may explain the difference between the behavior of the two semiconductors with respect to PEC-etching, as pointed out above, Le., that Si exhibits the MP morphology only after 1 C/cm2 of charge, compared with 0.1 C/cm2 for the IIVIS and in higher doping densities (Lp decreases with an increase of Nd). Physically this difference amounts to a smaller nonuniform and larger uniform PEC-etching currents for the

Si as compared with the 11-VIS. The one to two orders of magnitude difference between the surface density of dopants and 'the initially observed etch pit density can be explained perhaps by the rapid coalescence of the tiny etch pits which are produced during PEC-etching. However, a more detailed study of the initial stage of the PEC-etching, using mostly AFM is necessary to clarify this discrepancy. The morphology of the topmost layer of PEC-etched 11-VI compounds has not been investigated in detail. This film contains, in general, the oxidation products of the semiconductor, i.e., S (Se, Te) and their oxides. Whether this film consists of nanocrystallites, analogous to the nanoporous Si film, is not clear at this time. In conclusion, a systematic study of the morphology of macroporous silicon, obtained through PEC-etching is presented for various doping densities. Marked differences were observed between the morphologies of highly and lightly doped Si, especially in the initialization period of the PEC etching. It is suggested that this kind of morphology, previously found in II-VI compound semiconductors after PEC-etching, stems from nonuniform photocurrents, which emanate from the first layer of dopants near the surface. Acknowledgment. Discussions of the AFM results with Dr. H. Jungblut of the Hahn-Meitner Institute (Berlin) are deeply appreciated. We are grateful to the Ministere des Affaires Etrangkres (France) and the Israeli Ministry of Science and Arts for the support trough Arc en Ciel (Keshet) project. This work was partially supported by a CEE contract (HCM-CHRX-CT920059),by Pirsem-CNRS through Concerted Research Action on multicrystalline silicon, and by a grant (to R.T.) from the Israeli Academy of Sciences. References and Notes (1) (a) Mvy-CIBment, C.; Lagoubi, A.; Neumann-Spallart, M.; Rodot, M.; Tenne, R. J. Electrochem. SOC. 1991, 138, L69. (b) Uvy-CICment, C.; Lagoubi, A.; Tenne, R.; Neumann-Spallart, M. Electrochem. Acta 1992, 37, 877. ( c ) LCvy-ClCment, C.; Lagoubi, A.; Ballutaud, D.; Ozanam, F.;

4140 J. Phys. Chem., Vol. 99, No. 12, 1995 Chazalviel, J.-N.; Neumann-Spallart, M. Appl. Surf. Sci. 1993,65/66,408. (d) Bastide, S.; Cuniot, M.; Williams, P.; Nam, L. Q.; Sarti, D.; LRvyCltment, C. Abstract H42, supplementary abstract to 10th Conference on Photochemical Conversion Storage Solar Energy (IPS-IO),Interlaken, 1994 (see also Abstract H5, pp 395-396 in the same book of abstract) and Proceedings of the 12th EC Photovoltaic Conference, Amsterdam, Harwocd Academic Publishers: 1994. (2) (a) Uvy-Clbment, C.; Lagoubi, A,; Tomkiewicz, M. J. Electrochem. SOC.1994, 141, 958. (b) Lagoubi, A,; Neumann-Spallart, M.; Bastide, S.; Ltvy-Cltment, C. Proceedings of the 1Ith EC Photovoltaic Conference, Montreux; Guimaraes, L., Palz, W., De-Reyef, C., Kiess, H., Helm, P., Hanvood Academic Publ.: 1993; p 250. (c) Albu-Yaron, A.; Bastide, S.; Maurice, J. L.; Ltvy-Cltment, C. J . Lum. 1993, 57, 67. (d) Albu-Yaron, A.; Bastide, S.; Bouchet, D.; Brun, N.; Colliex, C.; Uvy-Cltment, C. J . Phys. Fr. 1994, 4, 1181. (e) Galun, E.; Tenne, R.; Lagoubi, A.; LtvyCltment, C. J . Lum. 1993, 57, 125. (3) Bomchil, G.; Halimaoui, A,; Herino, R. Microelectronic Eng. 1988, 8, 293. (4) Tsai, C.; Li, K.-H.; Campbell, J. C.; Hance, B. K.; Arendt, M. F.; White, J. M.; Yau, S.-L.; Bard, A. J. J. Electronic Mater. 1992, 21, 995. (5) Tsuo, Y. S.; Xiao, Y.; Heben, M. J.; Wu, X.; Pem, F. J.; Deb, S. K. 23rd IEEE Photovoltaic Special Conference, Louisville 1993; and references therein. (6) (a) Lehmann, V.; FO11, H. J . Electrochem. Soc. 1990, 137, 653. (b) Lehmann, V. J. Electrochem. Soc. 1993, 140, 2836. (c) FOll, H. Appl. Phys. A 1991, 53, 8. (d) Lehmann, V.; FO11, H. J . Electrochem. Soc. 1988, 135, 2831. (7) Smith, R. L.; Collins, S.D. J . Appl. Phys. 1992, 71, R1. (8) (a) Theunissen, M. J. J.; Appels, J. A.; Verkuylen, W. H. C. G. J . Electrochem. Soc. 1970, 117,959. (b) Theunissen, M. J. J. J . Electrochem. Soc. 1972, 119, 351. (9) (a) Tenne, R.; Hcdes, G. Appl. Phys. Lett. 1980,37,428. (b) Tenne, R.; Hodes, G. SUI$ Sei. 1983, 135, 453. (10) Galun, E.; Hodes, G.; Peisach, M.; Muranevich, E.; Tenne, R. J . Cryst. Growth 1992, 117, 666. ( 1 1) Tenne, R. In Semiconductor Microprocessing; Lewerenz, H. J., Campbell, S. A., Eds.; John Wiley & Sons, Ltd.: 1994. (12) Tajima, M. Appl. Phys. Lett. 1978, 32, 719. (13) McColley, P.; Lightowlers, E. C. Semicond. Sci. Tech. 1987, 2, 157. (14) Kosai, K.; Gershenzon, M. Phys. Rev. B 1974, 9, 723. (15) (a) Zhang, X. G. J . Electrochem. SOC.1991, 138, 3750. (b) Shen, W. M.; Tomkiewicz, M.; Ltvy-Clement, C. J . Appl. Phys., in press. (c)

Galun et al. Searson, P. C.; Macaulay, J. M.; Ross, F. M. J . Appl. Phys. 1992, 72, 253. (16) Searson, P. C.; Prokes, S. M.; Glembocki, 0. J. J . Electrochem. SOC.1993, 140, 3327. (17) Beale, M. I. J.; Benjamin, J. D.; Uren, M. J.; Chew, N. G.; Cullis, A. G. J . Crystallogr. Growth 1985, 73, 622. (18) Unagami, T. J . Electrochem. SOC.1980, 127, 476. (19) Dubin, V. M. Su@. Sei. 1992, 274, 82. (20) Allongue, Ph.; Blonkowski, S. Electrochimica Acta 1993,38, 889. (21) Girtner, W. W. Phys. Rev. 1959, 116, 84. (22) Green, M. A. High Ej’iciency Silicon Solar Cells;Trans Tech Publications: Switzerland, 1987; p 226. (23) (a) Green, M. A. High Efficiency Silicon Solar Cells; Trans Tech Publications: Switzerland, 1987; p 58. (b) Sze, S. M. Physics of Semiconductor Devices; John Wiley & Sons: New York, 1981; p 44. (24) Smith, P.; Inoue, M.; Frey, J. Appl. Phys. Lett. 1980, 37, 797. (25) Jacoboi, C.; Canali, C.; Ottaviani, G.; Quaranta, A. A. Solide State Electron. 1977, 20, 77. (26) (a) Matsumura, M.; Morrison, S. R. J . Electroanal. Chem. 1983, 144, 113; 1983, 147, 157. (b) Lewerenz, H. J.; Stumper, J.; Peter, L. M. Phys. Rev. Lett. 1988, 61, 1989. (27) (a) Canham, L. T.; Blackmore, G. W. Mat. Res. SOC.Symp. Proc. Vol. 1992,256,63. (b) Morazzani, V.; Grosman, A,; Ortega, C.; Rigo, S.; Siejka, J. Nuc. Instruments Methods Phys. Res. B 1994, 85, 287. (28) (a) Ltvy-Cltment, C.; Lagoubi, A,; Ballutaud, D.; Ozanam, F.; Chazalviel, J.-N.; Neumann-Spallart, M. Su@. Sei. 1993, 65/66, 408. (b) Williams, P.; Boutry-Forveille, A.; Ltvy-Cltment, C., to be published. (29) von Bardeleben, H. J.; Ortega, C.; Grosman, A.; Morazzani, V.; Siejka, J.; Stievenard, D. J . Lum. 1993, 57, 301. (30) Palm, H.; Arbes, M.; Schulz, M. Phys. Rev. Lett. 1993, 71, 2224. (31) Cimino, R.; Giarante, A.; Hom, K.; Pedio, M. Phys. Rev. Lett., in press. (32) Kirby, S. K.; Ting, D. Z.-Y.; McGill, T. Phys. Rev. B 1994, 48, 15237. (33) Zheng, J. F.; Liu, X.; Newman, N.; Weber, E. R.; Ogletree, D. F.; Salmeron, M. Phys. Rev. Lett. 1994, 72, 1490. (34) Em&,H.; Tjerkstra, R. W.; Cheung, C. W.; Vanmaekelbergh, D. Proceedings of the 10th Intemational Conference on Photochemical Conversion Storage Solar Energy (IPS-IO), Interlaken, 1994; p 221.

An.

JP941464K