Scholarly article on topic 'Shallow B-implanted Emitters with Laser Overdoping from AlOx Passivating Layers'

Shallow B-implanted Emitters with Laser Overdoping from AlOx Passivating Layers Academic research paper on "Materials engineering"

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Energy Procedia
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{"n-type PERT solar cells" / AlOx / "laser doping" / "selective emitter"}

Abstract of research paper on Materials engineering, author of scientific article — Thibaut Desrues, Coralie Lorfeuvre, Samuel Gall, Yannick Veschetti

Abstract A simple process for the fabrication of selective emitter structures on n-PERT cells is investigated, using shallow Boron emitters obtained by ion implantation. By tuning the emitter doping process parameters, J0e values as low as 10 fA.cm-2 have been obtained with highly resistive profiles. Laser overdoping processes from AlOx passivating layers are tested on these profiles to locally increase the emitter conductivity and allow better contact properties. Through this process the emitter sheet resistance and doping profile may be locally controlled with a limited impact on the J0e values.

Academic research paper on topic "Shallow B-implanted Emitters with Laser Overdoping from AlOx Passivating Layers"

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Energy Procedia 77 (2015) 291 - 295

5th International Conference on Silicon Photovoltaics, SiliconPV 2015

Shallow B-implanted emitters with laser overdoping from AlOx

passivating layers

Thibaut Desruesa*, Coralie Lorfeuvrea, Samuel Gallb, Yannick Veschettia

aCEA, LITEN, INES, 50 av. du Lac Léman, BP332, F-73370 Le Bourget du Lac, France bCEA Tech Pays de la Loire, TechnoCampus EMC2, Z.I. du Chaffault, F-44 340 Bouguenais, France

Abstract

A simple process for the fabrication of selective emitter structures on n-PERT cells is investigated, using shallow Boron emitters obtained by ion implantation. By tuning the emitter doping process parameters, J0e values as low as 10 fA.cm-2 have been obtained with highly resistive profiles. Laser overdoping processes from AlOx passivating layers are tested on these profiles to locally increase the emitter conductivity and allow better contact properties. Through this process the emitter sheet resistance and doping profile may be locally controlled with a limited impact on the J0e values.

© 2015TheAuthors. Publishedby Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/4.0/).

Peer review by the scientific conference committee of SiliconPV 2015 under responsibility of PSE AG Keywords: n-type PERT solar cells; AlOx; laser doping; selective emitter

1. Introduction

N-type silicon substrates show several advantages to produce very high-performance silicon solar cells, as shown recently by many groups [1-3]. Bifacial PERT (Passivated Emitter Rear Totally diffused) structures involve a simple process, which can be more easily implemented in mass production [4]. Amongst the various approaches for reducing the cost of these solar cells, the use of ion implantation (I2) to create the emitter and BSF regions is one of the most promising. Many studies have shown that this technique not only reduces considerably the number of process steps, but also allows to reach very low emitter saturation current densities (J0e) [5-7]. To further increase

* Corresponding author. Tel.: +33 479 792 877. E-mail address: thibaut.desrues@cea.fr

1876-6102 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Peer review by the scientific conference committee of SiliconPV 2015 under responsibility of PSE AG doi: 10.1016/j.egypro.2015.07.041

the efficiency of n-PERT cells, selective emitter structure may be used to reduce contact recombination (J0,met) while increasing current collection. However such devices require more patterning steps than conventional n-PERT cells [8]. We study in this paper the fabrication of lightly doped Boron emitters through I2 process with laser overdoping from AlOx passivating layers, in order to fabricate selectively doped devices (Fig.1). We first analysed the impact of I2 processes on shallow B-emitters to reach the lowest current saturation densities. The possibility to fire the AlOx layer with laser irradiation and locally decrease the sheet resistance of the emitter is also investigated.

2. Experimental

Industrial 156x156mm2 n-type pseudo-square Cz wafers were implanted with Boron ions on a VSEA "SolionTM" implanter after a wet chemical polishing step. Implantation doses between 1E14 and 1.17E15 at.cm-2 with implantation energies of 8 keV to 10 keV were used. The wafers were then annealed at 950°C or 1050°C with a step of dry oxidation. The emitter sheet resistance (RSheet) values were measured using a 4-point probe after the annealing step. After an HF dip, 4nm or 8nm thick AlOx layers were deposited by plasma ALD technique at 250°C on both sides, as well as 75nm thick undoped hydrogenated silicon nitride (SiNx:H) layers deposited by PECVD at 450°C. Samples underwent a firing step in an IR belt furnace at 800°C to simulate a typical screen printing process. The effective lifetime (and J0e values) of the symmetrical samples were measured using an Inductively Coupled Photo-Conductance Decay (IC-PCD) technique [9]. For the laser doping study the wafers were irradiated with a 15 ps UV laser emitting at 355nm and high frequency (80MHz). Doping process was evaluated through 4-probes sheet resistance and Electro-Chemical Voltage (ECV) measurements using a NH4F electrolyte and a ring surface of 0.101 cm2.

2.1. Fabrication of shallow B-implanted emitters

We used the I2 process to reach thin and well controlled doping profiles. As shown in Fig. 2a, RSheet values ranging from 100 to 800 Ohms/sq. are obtained, with various depths and peak concentrations (not shown here). The AlOx/SiNx:H stack used in this study is very efficient to passivate the c-Si surface. Using 8nm AlOx thickness, we achieve implied VOC values above 730 mV on undoped wafers. Onto a lightly doped emitter, the J0e values increase as expected with the doping level (Fig. 2b). However for the most highly resistive profiles, J0e levels still below 20 fA.cm-2 are obtained. Emitter profiles having higher RSheet values compatible with a standard cells metallization process show J0e values about 50 fA.cm-2 and more. Using AlOx as passivating layer has two advantages for our purpose. Firstly, due to its negative charges it induces an inversion layer in n-type substrates and may therefore decreases the sheet resistance of shallow emitters [10]. Secondly, the Al atoms contained in this material may be used as dopant source for laser irradiation [11].

<o 500 E

J= 400 o

200 100 0

10KeV_Anneal 1050°C 10KeV_Anneal 950°C 8KeV_Anneal 1050°C 8KeV Anneal 950°C

Dose [at/cm2]

■ 10 KeV_Anneal 1050°C

• 10 KeV_Anneal 950°C

A 8 KeV_Anneal 1050°C

▼ 8 KeV Anneal 950°C

Dose [at/cm2]

Fig. 2. (a): Sheet resistance values obtained for different doses and annealing temperatures. (b): Related J0e values extracted from IC-PCD measurements.

2.2. Laser overdoping from AlOxpassivation layers

To study the effectiveness of Al doping from the AlOx layer, substrates with a shallow emitter have been used (10keV, 2E14 at.cm-2, 1050°C, 410 Ohms/sq.). On those substrates 1cm2 squares have been totally irradiated with different scanning speeds to measure the sheet resistance after laser treatment (Fig. 3). To investigate the effect of laser processes on the surface passivation level, some wafers have been irradiated only on 1% of their surface and lifetime measurements have been performed to extract J0e values. By changing the scanning speed, various energy ranges are reachable with the laser treatment. RSheet values between 378 to 106 Ohms/sq. from the initially B-doped 410 Ohms/sq. are obtained, indicating an actual doping from Al atoms. Slightly higher RSheet are obtained by using a thinner AlOx layer (4nm), probably due to the lowest dopant quantity deposited with those stacks. With deeper profiles, J0e values increase above 50 fA.cm-2, showing the surface degradation after laser doping. Thermal treatments may therefore be needed to heal these degradations and improve the surface passivation in the ablated areas. ECV profiles shown in Fig. 4 indicate that not only the profile depth, but also the peak concentration, can be changed with such a process. This can be of great interest for the doping profile optimization to reach the best contact properties (low contact resistivity and J0,met values).

g" 300

— 200

■ ■

■ * a - •• • 1 ■ •

M _ • • • • * _

■ 1 ■ .......1

40 3 >

• AIO, 8nm - Ri

AICL 4nm - Ro

tr AIO, 8nm - J„,

1 10 Scanning speed [m.s 1]

Fig. 3. Sheet resistance and J0e values obtained for different laser scanning speeds and AlOx layers thickness.

1 1 1 1 1 • 1 1 1 i i i 1 i i i ! i i i 1 i i i —Init (410 Ohms/sq.)

—■— v = 3m.s"1 (147 Ohms/sq.) .

# N —a— v = 1m.s"1 (106 Ohms/sq.)

V* • • • • • • ...... i . ......1.........

0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 1,1 1,2 Depth (ijm)

Fig. 4. ECV profiles obtained for different laser treatments and an AlOx layer thickness of 8 nm.

2.3. Conclusion

The possibility to fabricate n-PERT cells with selective doping was investigated, using shallow B-emitters obtained by ion implantation and laser overdoping from AlOx passivating layers. Through this processes, various emitter doping profiles may be obtained and used for the cells structure optimization. Shallow profiles from I2 process show very low J0 values, whereas more conductive regions are obtained after laser irradiation of the AlOx/SiNx:H stack. Improvements of this process are possible using thicker AlOx layers to obtain lower RSheet values as well as better surface passivation levels. Further investigations will include SIMS measurements for these different profiles to confirm the effectiveness of Al-doping, as well as contact resistivity and J0,met characterizations.

Acknowledgements

This study has received support from the State Program "Investment for the Future" bearing the reference (ANR -10- IEED -0003).

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