Light Enhanced Hydrofluoric Acid Passivation: A Sensitive Technique for Detecting Bulk Silicon Defects

Summary

A RT liquid surface passivation technique to investigate the recombination activity of bulk silicon defects is described. For the technique to be successful, three critical steps are required: (i) chemical cleaning and etching of silicon, (ii) immersion of silicon in 15% hydrofluoric acid and (iii) illumination for 1 min.

Abstract

A procedure to measure the bulk lifetime (>100 µsec) of silicon wafers by temporarily attaining a very high level of surface passivation when immersing the wafers in hydrofluoric acid (HF) is presented. By this procedure three critical steps are required to attain the bulk lifetime. Firstly, prior to immersing silicon wafers into HF, they are chemically cleaned and subsequently etched in 25% tetramethylammonium hydroxide. Secondly, the chemically treated wafers are then placed into a large plastic container filled with a mixture of HF and hydrochloric acid, and then centered over an inductive coil for photoconductance (PC) measurements. Thirdly, to inhibit surface recombination and measure the bulk lifetime, the wafers are illuminated at 0.2 suns for 1 min using a halogen lamp, the illumination is switched off, and a PC measurement is immediately taken. By this procedure, the characteristics of bulk silicon defects can be accurately determined. Furthermore, it is anticipated that a sensitive RT surface passivation technique will be imperative for examining bulk silicon defects when their concentration is low (<10¹² cm^-3).

Introduction

High lifetime (>1 msec) monocrystalline silicon is becoming ever more important for high efficiency solar cells. Understanding the recombination characteristics of embedded impurities has been, and remains an important topic. One of the most widely used techniques to examine the recombination activity of grown-in defects is by a photoconductance method¹. By this technique it is often difficult to completely separate surface from bulk recombination, thus making it difficult to examine the recombination characteristics of grown-in defects. Fortunately there exist several dielectric films which can achieve very low effective surface recombination velocitie....

Protocol

1. Experimental Setup

1. Locate a suitable fume hood for the measurement technique, and remove any irrelevant equipment to allow better air flow and reduce cluttering. Do not use any chemicals other than hydrofluoric acid (HF) in the fume hood.
2. Test the quality of the deionized (DI) water from the tap within the fume hood using a conductivity meter. Ensure that the DI water has a conductance of at most 0.055 µS/cm at a temperature of 20 °C.
3. Place a minority carrier lifetime tester into the fume hood. Connect the cables to a computer, which is situated on a table outside of the fume hood.
4. Switch on the c....

Results

Figure 1a shows a schematic and Figure 1b shows a photograph of the experimental setup. When a silicon wafer is immersed into the HF solution, subsequently placed onto the lifetime tester stage and a measurement is performed (before illumination), a lifetime curve which is limited by surface recombination will result, as shown by the blue triangles in Figure 2. However, when the sample is illuminated for 1 min (while immersed in HF), as shown in Figure 1.......

Discussion

The successful implementation of the bulk silicon lifetime measurement technique described above is based on three critical steps, (i) chemically cleaning and etching the silicon wafers, (ii) immersion in a 15% HF solution and (iii) illumination for 1 min^17,18,19. Without these steps, the bulk lifetime cannot be measured with any certainty.

As the measurement technique is conducted at RT, the surface passivation quality is highly susceptible to surface contamination (metals, organic.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Hydrofluoric acid (48%)	Merck Millipore,   http://www.merckmillipore.com/AU/en/product/Hydrofluoric-acid-48%25,MDA_CHEM-100334	1003340500	Harmful and toxic. Any supplier could be used provided the chemical is Analytical Reagent (AR) grade.
Hydrochloric acid 32%, AR	ACI Labscan, http://www.rcilabscan.com/modules/productview.php?product_id=1985	107209	Harmful and toxic. Any supplier could be used provided the chemical is Analytical Reagent (AR) grade.
Ammonia (30%) Solution AR	Chem-supply, https://www.chemsupply.com.au/aa005-500m	AA005	Harmful and toxic. Any supplier could be used provided the chemical is Analytical Reagent (AR) grade.
Hydrogen Peroxide (30%)	Merck Millipore, http://www.merckmillipore.com/AU/en/product/Hydrogen-peroxide-30%25,MDA_CHEM-107209	1072092500	Harmful and toxic. Any supplier could be used provided the chemical is Analytical Reagent (AR) grade.
Tetramethylammonium hydroxide (25% in H2O)	J.T Baker, https://us.vwr.com/store/catalog/product.jsp?product_id=4562992	5879-03	Harmful and toxic. Any supplier could be used provided the chemical is Analytical Reagent (AR) grade.
640 ml round plastic container	Sistema, http://sistemaplastics.com/products/klip-it-round/640ml-round		This is a good container for storing the 15% HF solution in.
WCT-120 lifetime tester	Sinton Instruments, http://www.sintoninstruments.com/Sinton-Instruments-WCT-120.html
Dell workstation with Microsoft Office Pro, Data acquisition card and software including Sinton Software under existing license.	Sinton Instruments, http://www.sintoninstruments.com
Halogen optical lamp, ELH 300 W, 120 V	OSRAM Sylvania, http://www.sylvania.com/en-us/products/halogen/Pages/default.aspx	54776	Any equivalent lamp could be used.
Voltage power source	Home made power supply		Any power supply could be used provided it can produce up to 90 Volts and 1-5 Amps.
Conductivity meter	WTW, http://www.wtw.de/uploads/media/US_L_07_Cond_038_049_I_02.pdf	LF330