M. LOCHMARTI et al.: High Phosphorus Gettering Efficiency in Polycrystalline Si 379

phys. stat. sol. (a) 151, 379 (1995)

Subject classification: 72.20; 72.40; S5.11

Laboratoire de Physique et Applications des Semiconducteurs (PHASE) , UPR du CNRS no 292, Strasbourg I ) (a) and Laboratoire de Physique des Mathriaux, Faculth de Sciences, Rabat (b)

High Phosphorus Gettering Efficiency in Polycrystalline Silicon by Optimisation of Classical Thermal Annealing Conditions

BY M. LOGHMARTI (a, b), K. MAHFOUD (a), J. KOPP (a), J. C. MULLER (a), and D. SAYAH (b)

(Received May 11, 1995; in revised form June 22, 1995)

The external gettering effect by phosphorus diffusion is used to improve the minority carrier diffusion length of polycrystalline silicon. Large grain silicon wafers are exposed to a POCI, source at different temperatures and durations and are afterwards post-annealed in a conventional diffusion furnace in a pure argon atmosphere. The influence of the starting and quenching temperatures of the post-annealing step on the gettering efficiency is investigated. The optimisation of the thermal annealing cycle parameters, i.e. the determination of the best combination of starting temperature of post-annealing, heating and cooling rates, post-annealing temperature and duration, pull-out temperature of the wafers, and POC1, diffusion conditions, results in a large improvement of the minority carrier diffusion length in comparison to the starting material. A further advantage of this post-annealing is the improvement of the homogeneity of the active phosphorus distribution and of the electrical properties.

Um die Minoritatsladungstragerdiffusionslange von polykristallinem Silizium zu verbessern, wird der externe Gettereffekt benutzt. GroBkorniges Siliziummaterial wird mit Hilfe einer POC1,-Quelle verschieden lange und bei verschiedenen Temperaturen dotiert. Die Proben erhalten anschlieRend unter reiner Argonatmosphare in einem konventionellen Diffusionsofen einen sogenannten ,,post- anneal". Dabei wird der EinfluR der Anfangs- und Endtemperaturen (Temperatur beim Herausziehen der Proben) des ,,post-annealing"-Schritts auf die Getterwirkung untersucht. Die Optimierung der einzelnen Parameter des thermischen ,,annealing"-Ablaufs, d. h. die Bestimmung der besten Kombi- nation der Anfangstemperatur des ,,post-annealing", Aufheiz- und Abkiihlraten, ,,post-annealing''- Temperatur und Zeitdauer, Endtemperatur der Silizium-Scheiben sowie POC1,-Diffusionsbedingun- gen, ergibt eine gravierende Verbesserung der Minoritatsladungstragerdiffusionslinge im Vergleich zum Ausgangsmaterial. Ein zusatzlicher, positiver Effekt dieses ,,post-annealing" ist die iiber die Probenflache verteilte Homogenitatssteigerung der aktiven Phosphor-Verteilung und der elektrischen Eigenschaften.

1. Introduction

The major disadvantage of polycrystalline silicon is the fact that it contains a lot of defects, which can be classified in specific terms of stress, structural defects, and impurities. If such defects and impurities are electrically active as minority carrier traps, they reduce the excess carrier lifetime T and therefore the effective minority carrier diffusion length L,, which is one of the most important parameters of a solar cell and influences strongly the efficiency of such devices.

I) BP 20, F-67037 Strasbourg Cedex 2, France.

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380 M. LOCHMARTI, K. MAHFOUD, J. KOPP, J. C. MULLER, and D. SAYAH

Structural defects are grain boundaries and dislocations. The grains in multicrystalline materials have an average size of 1 to over 10 mm2 and a dislocation density in the range of lo3 to lo7 cm-'.

Beside the structural defects electrically active impurities can also impact the performance of a solar cell. Nearly in all polycrystalline materials oxygen and carbon are present, depending on the growth technique applied, i.e. oxygen in materials processed in contact with a quartz and carbon with a graphite container. But also metallic impurities such as Fe, Cu, etc. play an important role as minority carrier traps. Furthermore, due to the low absorption coefficient of silicon, a wafer thickness of about 200 pm is required to get sufficient light absorption. Therefore the goal for all process steps is to maintain or enhance the starting minority carrier diffusion length L,, which is usually in the order of less than 50 up to 200 pm, and to reduce the above-mentioned disadvantages of the starting material [l to 31.

One method to remove impurities is gettering [4]. Gettering means the transfer of unwanted impurities from the active area of a solar cell or another device to an unimportant region. One effective gettering mechanism among many others to remove metallic impurities is the exposure of a silicon wafer to a POCl, source [5]. This gettering mechanism requires special steps depending on each other. First, the electrically active impurities have to be placed into solid solution in the crystal, second, they have to be mobile, and third, gettering sites have to be provided, which are in unimportant regions of the device and can capture the impurities.

In this investigation we have focused our attention to the post-thermal treatment of multicrystalline silicon wafers previously exposed to a POC1, source. We are examining the influence of the starting and quenching (pulling out) temperatures and the heating and cooling rates (of the post-annealing step). We have obtained an important improvement of the minority carrier diffusion length by optimising the process parameters due to the gettering effect of heavy phosphorus diffusion.

2. Experiment

Our studies were carried out on 2.5 x 2.5 cm2 p-type polycrystalline silicon with a thickness of about 320 pm and a specific resistivity of 0.8 Rcm. All samples were initially exposed to a POCl, source at different temperatures and durations, to form an n'pn' structure in a conventional furnace made by Photowatt (France). The samples before annealing were etched in H F and rinsed in deionised water. Those prepared wafers were subjected to a special thermal cycle in a conventional diffusion furnace (see Fig. 1). Four technical parameters were investigated: a) the starting temperature, which was varied from 28 up to 900 "C, b) the quenching temperature, which is the temperature when samples are pulled out of the furnace, c) the plateau temperature and the plateau time for the post-annealing step, which ranges from 800 to 1000 "C and 1 to 180 min, respectively, d) the phosphorus exposure. The heating (10 K/min) and cooling rate (2 K/min) for every thermal cycle was constant and all treatments were carried out in an argon atmosphere.

To characterise the samples we measured the diffusion length L, of the initial (not annealed) and post-annealed wafers by the surface photovoltage technique [6]. For the starting material we obtained 35 to 45 pm diffusion length, which confirms the low quality of the polycrystalline material.

High Phosphorus Gettering Efficiency in Polycrystalline Silicon

180

1 160

h

140 v

E 120 W

5 100 2

P Fr

80:

60

40

38 1

A A A A ~ ~ A

A

- 0 . 0 0 . 0

- 0

A A . 1 m m . . m m 0 0

- Post-annealing temperature=900~ . Post annealing duration=45 min

POCI,: 800°C/80 min - 0 POCI,: 825"C/30 min

A POCI,: q50°C/20 min I I

t T a !

DURATlON - Fig. 1. Illustration of the temperature cycle in the furnace. T, starting temperature, temperature, plateau temperature

quenching

3. Results and Discussion

3.1 Effect of starting temperature

Fig. 2 shows the influence of the starting temperature on the minority carrier diffusion length. The starting temperature was varied from 28 to 900 "C. During this investigation the plateau (annealing) temperature and duration were maintained at 900 "C for 45 min.

26*

382 M. LOGHMARTI, K. MAHFOUD, J. KOPP, J. C. MULLER, and D. SAYAH

Several samples with three starting conditions of POC1, pre-diffusion (800 OCj80 min, 825 "C/30 min, 850 "C/20 min) were used to see the effect of the starting temperature. Fig. 2 can be subdivided into two sectors. The limit of both sectors is at 700 "C.

Below 700 "C a great improvement of the minority carrier diffusion length L, is achieved in comparison to the virgin one of about 40 pm. It is obviously seen that the diffusion length is nearly constant up to the starting temperature of 700 "C for the three initial pre-diffusion conditions. This result can probably be explained by the high generation of gettering sites well adapted to most of the metallic and other impurities.

Beside this, a great difference between the pre-diffusion conditions can be observed. The highest improvement of the minority carrier diffusion length L, is obtained for the case named (850 T / 2 0 min) in Fig. 2. This can be explained by the increase of phosphorus concentration of the n + -doped layer, which is correlated to the SIMS measurement results obtained from the n + pn' samples. The surface concentration was estimated to 2 x cm-, (POCl, 850 "C/20 min) down to less than 5 x 10,' cm-, (POCl, 800 "C/80 min). In the case of (850 "C/20 min) the solubility limit is reached, so that precipitates are present to enhance the gettering effect, while for 800 OC/80 min the phosphorus diffusion is insufficient to achieve a high enough phosphorus concentration. Nevertheless in all three pre-diffusion cases the minority carrier diffusion length L, is much higher than the initial one of about 40 pm. Our results are comparable to those found by Perichaud and Martinuzzi [7].

To explain this high gettering efficiency further, different mechanisms in the literature could be pointed out, i.e. the enhanced solubility of metallic impurities, which act as acceptors, and their enhanced pairing with phosphorus within the highly n+-doped layer [8, 91. The solubility of metallic impurities such as Au and Cu is increased with the doping concentration through enhanced metal-solid solubility by Fermi level and ion pairing. Compound formation between metal and phosphorus atoms also enhances the solubility of some metallic impurities as Au. It has been reported that Au,P, and Cu,P, compounds are observed in highly n+-doped silicon [lo, 111. Ourmazd and Schroter [12] have found that Nisi, particles are closely associated with the SIP particles and are most frequently observed in regions containing a high density of Sip particles. They proposed that phosphorus causes Sip particle formation, which leads to a large emission of silicon interstitials, due to a volume expansion. This high improvement of L, suggests that the heavy phosphorus doping dissolves metal precipitates, which is in agreement with Cerofolini et al.'s results [ 131. The post-thermal annealing of heavily phosphorus-doped polycrystalline silicon is required in order to make the segregation more efficient and to move metal impurities into the phosphorus-doped n+-region. A maximum gettering efficiency results from the association of higher generation of gettering sites and higher phosphorus concentration.

Above 700 "C a strong decrease of the diffusion length with increasing starting temperature was observed. For all three initial pre-diffusion conditions we observed the same qualitative decrease in L,. This could be due to the absence of the deactivation of some trapping states, so that the gettering effect vanishes. But the nature of these trapping states is not fully understood and identified up to this moment.

3.2 Effect of quenching temperature

Fig. 3 shows the influence of the quenching temperature on the minority carrier diffusion length. The quenching temperature was varied from 900 down to 28 "C and the plateau

High Phosphorus Gettering Efficiency in Polycrystalline Silicon 383

m s 120

Y 2

60

m =

. Post-annealing temperature=900°C Post-annealin duration= 45 min

m PO&: 800°C/80 min 0 POCI,: 825"C/30 min

m A . A .

A POCI,: 85OoC/20 rnin 40 I I I I I

0 200 400 600 800 1000 QUENCHING TEMPERATURE ("C) -

Fig. 3. Effect of quenching temperature on the diffusion length of POCI, diffused polycrystalline silicon

temperature was maintained at 900 "C for 45 min. Again Fig. 3 can be subdivided like Fig. 2 into two sectors, one below and the other above 700 "C. We found the same tendencies as in Fig. 2. The minority carrier diffusion length was strongly improved for quenching temperatures below 700 "C and decreases above this limit.

Again a difference for the three starting conditions of POC1, pre-diffusion is obtained. The best gettering efficiency is found for the condition named 850 "C/20 min. The behaviour of the curves seen in Fig. 3 can be explained in such a way that after a high temperature step the metallic impurities are in supersaturation. The "fast" diffusing particles (i.e. Co, Cu, Ni) have enough time to precipitate and to form inactive complexes, whereas the "slow" diffusing ones (i.e. Mo, Ti, Cr, Fe) are frozen in electrically active sites during the cooling down step. The notation "slow" and "fast" in this case depends on the quenching temperature and the cooling speed and duration, respectively.

3.3 Effect of post-annealing temperature and duration

The next parameter for this investigation is the plateau (post-annealing) temperature and duration. Fig. 4 shows the dependence of the minority carrier diffusion length for some typical examples of post-annealing temperature and duration. The post-annealing tem- perature was varied from 800 up to 1000 "C and the duration from 1 to 180 min, respectively. The gettering efficiency depends on temperature and duration of the post-annealing. The tendency in the diffusion length improvement is for all cases the same, but the highest values are received at 950 "C for 180 min. An explanation to this fact is the common extrinsic gettering process by phosphorus diffusion, which is well documented [14, 151. Recently, Kang and Schroder [16] have proposed a gettering model, which involves two rate limiting steps: 1. the released diffusion of metallic impurities, which determines the lower temperature limit during gettering, and 2. the segregation model, which governs the

384 M. LOGHMARTI, K . MAHFOUD, J. KOPP, J. C. MULLER, and D. SAYAH

250

i 200

v 3 E 3 B 2 100

F4 n

150

z

Fr

50

-m-POCI,: 825"C/30 min+post-annealing 1 min -0-POCI,: 825"C/30 min+post-annealing 15 min -A--POCI,: 825"C/30 min+post-annealing 45 min -.-POCI,: 825OCi30 min+post-annealing 180 min -+-Virgin silicon + post-annealing 180 min

(Imtial value of Ln: 40 pm) t

*-- I I I I +

800 850 900 950 1000 POST- ANNEALING TEMPERATURE ("C)

Fig. 4. Effect of post-thermal annealing temperature and duration on gettering efficiency of POCI, (825 T / 3 0 min) pre-diffused polycrystalline silicon

gettering above the optimum temperature. In our case, the critical temperature, given for the highest gettering efficiency, ranges from 900 to 950 "C.

Measured values for unprocessed (without phosphorus doping) samples of the same starting material were also introduced in this figure. The annealing duration for these virgin samples was 45 min. Towards higher annealing temperatures a slight decrease of the minority carrier diffusion length is observed. This reduction is probably due to the activation of the residual metallic impurities, which degraded the volume of the grains or their boundaries. Nevertheless the gettering efficiency of phosphorus doping is clearly demonstrated by comparison to undoped samples.

3.4 Effect of POCI, diffusion temperature

In the foregoing chapters we have already seen an influence on the diffusion length due to the pre-diffusion conditions. Fig. 5 shows the experimental results for the variation of the POCl, diffusion temperature. Different samples were exposed to a POC1, source at 800,850 and 900 "C for 10 min. Afterwards they were post-annealed at temperatures between 800 and 1000 "C for 180 min. For all curves the tendency is the same. The maximum in diffusion length is reached around a post-annealing temperature of 950 "C. Above this temperature, a decrease was observed, which could be due to induced structural defects. However, it is also seen that increasing the POCl, pre-diffusion temperature causes the minority carrier diffusion length to increase. Our results show clearly that the optimal phosphorus concentration to obtain a high gettering effect is about 7 x 1020 cm-, corresponding to a POCl, temperature of 900 "C for 10 min.

In order to increase the statistical significance and to get more information about the lateral homogeneity of the wafer, a lot of similarly processed samples and many positions

High Phosphorus Gettering Efficiency in Polycrystalline Silicon 385

800 850 900 950 1000 POST-ANNEALING TEMPERATURE ("C) -

Fig. 5. Effect of POCI, pre-diffusion temperature on the diffusion length after post-thermal annealing at different temperatures and a duration of 180 min

I . I . I . I . 1 . I . L . I . I

20 40 60 80 100 120 140 160 180

DIFFUSION LENGTH (pm) - Fig. 6. Distribution of diffusion length in P-diffused polycrystalline silicon before and after post- annealing (post-annealing temperature and duration 900 "C/45 min, POCI, diffusion temperature and duration 850 "C/20 min)

386 M. LOGHMARTI et al.: High Phosphorus Gettering Efficiency in Polycrystalline Si

of each wafer were measured. So we obtained a mapping for the homogeneity of the electrical parameters of the used material. In Fig. 6 we show the effect of the post-thermal annealing on the redistribution of the bulk diffusion length. We observe that the standard deviation (6) is reduced after the thermal treatment, confirming that the homogeneity of the doping and the distribution of bulk diffusion length values are better in such bad polycrystalline material after this post-annealing treatment.

4. Conclusion

In this paper we have demonstrated that a great improvement of low quality polycrystalline silicon for solar cell application can be achieved by a suitable choice of post-thermal annealing and POC1, pre-diffusion conditions. An increase of the bulk diffusion length from 40 to about 200 pm was achieved, if the starting and quenching (pull out) temperatures of the post-annealing treatment are below 700 "C, while the heating and cooling rates are fixed at about 10 and 2 K/min, respectively. The post-annealing (plateau) temperature and duration of the cycle were found to be best in the range of 950 "C and 180 min, respectively. Another important parameter was the POCl, pre-diffusion temperature and duration. The highest improvement was achieved at a POC1, diffusion temperature of 900 "C for 10 min. Applying this procedure, a bad polycrystalline silicon wafer with 40 pm minority carrier diffusion length can be improved by phosphorus gettering to a material which has a diffusion length close to that of monocrystalline silicon.

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