This program has been supported by the Australian Government through the Australian Renewable Energy Agency (ARENA). Responsibility for the views, information or advice expressed herein is not accepted by the Australian Government.

1. Experimental Setup
<ol>
	<li>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.</li>
	<li>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 &#181;S/cm at a temperature of 20 &#176;C.</li>
	<li>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.</li>
	<li>Switch on the computer and lifetime tester. Open the lifetime tester file on the computer and click the &#39;measure&#39; button to ensure correct communication between computer and the lifetime tester. The light source on the lifetime tester should flash if the computer and tester have been connected correctly.</li>
	<li>Place a halogen lamp inside the fume hood. Position the lamp such that it can illuminate the lifetime tester stage where the sample will be located.</li>
	<li>Connect the halogen lamp to a power source which should be situated outside of the fume hood. When the halogen lamp is switched on, its intensity should be at least 0.02 W/cm2 on the lifetime tester stage.</li>
</ol>
2. Preparing the 15% HF Solution
Note: HF is a dangerous chemical and must be treated with care. It causes slow, sustained, and deep damage to the body following exposure. HF does not readily burn the skin like other acids - rather it absorbs quickly into the skin and causes deep blistering and damage to bones. This means that bones become brittle and blistered as the fluorine reacts with calcium. HF also binds with free calcium which is used in nerve regulation and osmotic cell balance, so binding of free calcium in the body can be fatal. It is paramount that the user follows laboratory safety protocols when using HF, and ensures they know the location of the HF first aid kit and hexafluorine (or calcium gluconate gel).
<ol>
	<li>Place a chemically cleaned round plastic container (H: 55 mm, D: 170 mm) into the fume hood (same as in Section 1). The container must have a clear plastic lid. See Section 6 on how to chemically clean the container prior to use.</li>
	<li>Clear the area around the plastic container in the fume hood so the HF solution can be prepared without any obstructions nearby.</li>
	<li>Apply all personal protection equipment (PPE).</li>
	<li>CAUTION: Add 50 ml of HF to 100 ml DI water (H2O) in the container.</li>
	<li>CAUTION: Add 20 ml of hydrochloric acid (HCl) to the container and mix the H2O : HF : HCl solution using plastic tweezers. Rinse the tweezers thoroughly afterwards.</li>
	<li>CAUTION: Place the lid on the plastic container and allow the solution to settle for 1 hr. During this time, HF fumes will condense on the lid.</li>
	<li>Adequately label the container, and make it clear that it contains HF. 
	Note: The HF solution will last 1-2 months with heavy usage. Therefore there is no need to replace the solution each time a measurement is to be performed.</li>
</ol>
3. Calibration of the Lifetime Tester
<ol>
	<li>Locate at least 6 silicon wafers of known resistivity and conductance. Ideally the resistivity range should span 0.1-100 &#937;-cm.</li>
	<li>On the computer, open the lifetime tester calibration file. Enter the resistivity of each wafer in the table and then click &#39;Update # of wafers&#39;.</li>
	<li>Click the &#39;Get data&#39; button in the calibration file and enter the details of the lifetime tester.</li>
	<li>Apply all necessary PPE.</li>
	<li>CAUTION: Place the plastic container filled with the HF solution on the lifetime tester stage and position it over the inductive coil (blue circle region).</li>
	<li>When prompted to measure the &#39;air voltage&#39; on the computer, measure the voltage of the HF solution by clicking the &#39;Ok&#39; button. In this case, the air has been replaced by the conductance of the HF solution.</li>
	<li>CAUTION: Carefully remove the container filled with HF from the stage and place it on the fume hood bench. Carefully remove the lid.</li>
	<li>CAUTION: When the lid has been removed from the container, immerse the first silicon wafer into the HF solution using plastic tweezers. Place the lid back on the container.</li>
	<li>CAUTION: Place the plastic container back onto the lifetime tester stage and position it over the inductive coil. Ensure the silicon wafer is centered over the coil (blue circle region).</li>
	<li>When prompted to measure the &#39;sample voltage&#39; on the computer, click the &#39;Ok&#39; button once again. Remove the container from the stage and place it on the fume hood bench.</li>
	<li>CAUTION: Carefully remove the silicon wafer from the HF solution using plastic tweezers and rinse the wafer in the dedicated rinse beakers. Do a final rinse under the tap in the fume hood.</li>
	<li>CAUTION: Place the lid back onto the container, and position the container back onto the lifetime tester stage.</li>
	<li>Repeat steps 3.6 to 3.12 until all samples have been measured.</li>
	<li>Once all of the samples have been measured, click the &#39;Fit data&#39; button in the calibration file. This will fit a parabolic curve to the measured data and provide calibration coefficients A, B and C which are specific to this setup.</li>
	<li>On the computer, open the lifetime tester file and click on the &#39;settings&#39; tab. Enter in the new calibration coefficients A, B and C for the HF setup. Save the file under a new name. 
	Note: The HF setup only requires a calibration every 6 months. Therefore there is no need to calibrate the setup each time a measurement is to be performed.</li>
</ol>
4. Wet Chemical Treatment of Silicon Wafers Prior to Measuring
<ol>
	<li>Prepare standard clean 1 (SC 1).
	<ol>
		<li>CAUTION: In an alkaline allocated fume hood, add 185 ml of ammonium hydroxide (NH4OH) to 1,295 ml of DI water in a 2 L glass beaker.</li>
		<li>CAUTION: Place the beaker onto a hot plate and heat the H2O : NH4OH solution to a temperature of ~50 &#176;C. Use a watch glass to cover the beaker.</li>
		<li>CAUTION: Once the H2O : NH4OH solution has reached a temperature of ~50 &#176;C, add 185 ml of hydrogen peroxide (H2O2) and continue to heat until the temperature reaches ~75 &#176;C. This H2O : NH4OH : H2O2 solution is known as SC 1. 
		Note: SC 1 should be changed daily to effectively clean silicon wafers.</li>
	</ol>
	</li>
	<li>Prepare standard clean 2 (SC 2).
	<ol>
		<li>CAUTION: In an acid allocated fume hood, add 185 ml of HCl to 1,295 ml of DI water in a 2 L glass beaker.</li>
		<li>CAUTION: Place the beaker onto a hot plate and heat the H2O : HCl solution to a temperature of ~50 &#176;C. Use a watch glass to cover the beaker.</li>
		<li>CAUTION: Once the H2O : HCl solution has reached a temperature of ~50 &#176;C, add 185 ml of H2O2 and continue to heat until the temperature reaches ~75 &#176;C. This H2O : HCl:H2O2 solution is known as SC 2. 
		Note: Change SC 2 daily to effectively clean silicon wafers.</li>
	</ol>
	</li>
	<li>Prepare the silicon etching solution.
	<ol>
		<li>In an alkaline allocated fume hood, add 1,600 ml of tetramethylammonium hydroxide (TMAH) to a chemically clean 2 L glass beaker. Please see Section 6 on how to chemically clean the beaker prior to use.</li>
		<li>Place the beaker onto a hot plate and heat the TMAH solution to a temperature of ~85 &#176;C. Use a watch glass to cover the beaker. 
		Note: The TMAH solution will not need changing until it starts to crystallize, i.e., after 1 month.</li>
	</ol>
	</li>
	<li>Perform the wet chemical treatment.
	<ol>
		<li>Load samples into a quartz cradle and place them in a communal HF solution in the lab. The concentration is not critical.</li>
		<li>After ~10 sec or once the samples become hydrophobic (pull dry), remove the cradle from the HF solution and rinse using 3 beakers filled with DI water.</li>
		<li>Transport the cradle of wafers to the fume hood where SC 1 has been prepared. When SC 1 has stabilized at ~75 &#176;C, slowly immerse the cradle of wafers into SC 1.</li>
		<li>Clean the wafers in SC 1 for 10 min.</li>
		<li>After 10 min has elapsed, remove the cradle of wafers from SC 1 and rinse them using three 2 L glass beakers filled with DI water. Chemically clean these glass beakers prior to filling them with DI water. Please see Section 6 on how to chemically clean the beakers.</li>
		<li>After rinsing, place the samples into a HF solution which is dedicated only for post SC 1 processing. The concentration is not critical.</li>
		<li>After ~10 sec or once the samples become hydrophobic (pull dry), remove the cradle from the HF solution and rinse using 3 beakers filled with DI water. Use the same beakers as in step 4.4.5.</li>
		<li>Transport the cradle of wafers to the fume hood where SC 2 has been prepared. When SC 2 has stabilized at ~75 &#176;C, slowly immerse the cradle of wafers into SC 2.</li>
		<li>Clean the wafers in SC 2 for 10 min.</li>
		<li>After 10 min has elapsed, remove the cradle of wafers from SC 2 and rinse them using 3 beakers filled with DI water. Use the same beakers as in step 4.4.5. 
		Note: The cradle of wafers could be stored in one of the rinse beakers filled with DI water until the following day.</li>
		<li>After rinsing, place the samples into a HF solution which is dedicated only for post SC 2 processing. The concentration is not critical.</li>
		<li>After ~10 sec or once the samples become hydrophobic (pull dry), remove the cradle from the HF solution and rinse using 3 beakers filled with DI water. Use the same beakers as in step 4.4.5.</li>
		<li>Transport the cradle of wafers to the fume hood where the TMAH solution has been prepared. When the TMAH solution has stabilized at ~85 &#176;C, slowly immerse the cradle of wafers into the TMAH solution.</li>
		<li>Etch the wafers in TMAH for 5 min. This will remove ~5 &#181;m of silicon.</li>
		<li>After 5 min has elapsed, remove the cradle of wafers from the TMAH solution and rinse them using 3 beakers filled with DI water. Use the same beakers as in step 4.4.5. More than 3 rinses might be required as TMAH is quite &#39;sticky&#39;.</li>
		<li>Transport the cradle of wafers in DI water to the fume hood where the experiment has been setup. See section 1.1.</li>
		<li>Measure the prepared silicon wafers within 2 hr after step 4.4.16.</li>
	</ol>
	</li>
</ol>
5. Measurement Procedure
<ol>
	<li>Apply all necessary PPE.</li>
	<li>Fill two 2 L plastic beakers with DI water and place them in the fume hood. These will be used for rinsing the silicon samples post measurement.</li>
	<li>Place plastic tweezers into an empty 500 ml plastic beaker and place inside the fume hood near the rinse beakers. These tweezers will be used to handle the silicon samples.</li>
	<li>On the computer, open the lifetime tester file. Make sure the file contains the correct calibration coefficients for the HF measurement setup. See section 3.</li>
	<li>In the lifetime tester file, select the &#39;Transient&#39; mode. Enter the details of the silicon wafer to be measured, for example, thickness, resistivity and dopant type.</li>
	<li>CAUTION: Place the container (lid on) filled with HF onto the stage of the lifetime tester and center it over the inductive coil (blue circle region). Let the solution settle for 1 min.</li>
	<li>On the computer, click on the &#39;Zero instrument&#39; button in the lifetime tester file. This will measure the voltage of the solution. 
	Note: Over time, the solution voltage will decrease because the composition of the solution changes with time. Irrespective of this, the HF solution does not show any degradation in its passivating quality, which is why the solution can be used for 1-2 months without being changed.</li>
	<li>CAUTION: Carefully remove the container from the lifetime tester stage and place it on the fume hood bench.</li>
	<li>CAUTION: Carefully remove the lid from the container filled with the HF solution. If there is some condensation on the lid, carefully rinse the lid in DI water using the tap in the fume hood.</li>
	<li>CAUTION: Carefully immerse the first silicon wafer into the HF solution. Using the plastic tweezers, lightly press down on the silicon wafer to ensure it is sitting on the bottom of the container.</li>
	<li>Rinse the tweezers and place them back into the empty 500 ml plastic beaker.</li>
	<li>CAUTION: Carefully place the lid back on the container and then place the container on the lifetime tester stage. Ensure the silicon wafer is positioned over the inductive coil (blue circle region).</li>
	<li>Rinse and dry your gloves. Remove them before operating the computer.</li>
	<li>If the fluorescent light in the fume hood is on, it must be switched off before a measurement is to be conducted. Minimum light is required during the measurement.</li>
	<li>Switch the halogen lamp on, and ensure it is illuminating the silicon wafer through the lid of the plastic container. Time for 1 min (the exact time is not critical).</li>
	<li>During the illumination period, click the &#39;Measure&#39; button in the lifetime file on the computer. A small window labeled &#39;Taking data&#39; will appear.</li>
	<li>In the &#39;Taking data&#39; window, type in a name for the file. Under &#39;sample averaging&#39; select 10.</li>
	<li>When the silicon wafer has been illuminated for 1 min, switch the halogen lamp off and immediately click the &#39;Average&#39; button in the &#39;Taking data&#39; window. The lifetime tester will flash 10 times and average the lifetime measurements. 
	Note: Once the halogen lamp is switched off, the passivation of the silicon wafer will start to degrade, and thus it is important to measure immediately after illumination to achieve the best results.</li>
	<li>When the averaging is complete, click the &#39;Ok&#39; button in the &#39;Taking data&#39; window.</li>
	<li>If another measurement is required, repeat steps 5.15-5.18.</li>
	<li>CAUTION: Once the measurement is complete, remove the container from the lifetime tester stage and place it on the fume hood bench.</li>
	<li>CAUTION: Carefully remove the lid from the container filled with the HF solution. If there is some condensation on the lid, carefully rinse the lid in DI water using the tap in the fume hood.</li>
	<li>CAUTION: Carefully remove the silicon wafer from the HF solution using plastic tweezers and rinse the wafer in the dedicated rinse beakers. Do a final rinse under the tap in the fume hood.</li>
	<li>If more samples are to be measured, repeat steps 5.10-5.23.</li>
	<li>Once all samples have been measured, ensure the lid is placed on the container and store the HF solution in the fume hood. Make sure the container is adequately labeled.</li>
	<li>Rinse all beakers and tweezers and store them in their allocated spots in the laboratory.</li>
	<li>Allow the lifetime tester to remain in the fume hood for 1 hr after use and then remove it from the fume hood.</li>
</ol>
6. Chemical Cleaning of Beakers and Containers
<ol>
	<li>In an alkaline allocated fume hood, prepare an SC 1 solution as outlined in sections 4.1.1-4.1.4.</li>
	<li>To chemically clean beakers and containers, pour the SC 1 solution into the beakers/containers. The beakers can only be cleaned one at a time.</li>
	<li>Let the solution clean the beaker/container for 10 min. If the beaker/container is plastic, it cannot be placed on a hot plate, and therefore the SC 1 solution will cool during the clean. This will not affect the cleaning process.</li>
	<li>After 10 min, pour the SC 1 solution into another beaker/container if they require cleaning also. Otherwise let the SC 1 solution cool in a large beaker. Once cooled, SC 1 can be poured down the sink with running water.</li>
	<li>Rinse SC 1 cleaned beaker/container in DI water.</li>
	<li>In an acid allocated fume hood, prepare an SC 2 solution as outlined in sections 4.2.1-4.2.4.</li>
	<li>Pour the SC 2 solution into beaker/container from step 6.5. Allow the solution to clean the beaker/container for 10 min.</li>
	<li>After 10 min, pour the SC 2 solution into another beaker/container if they require cleaning also. Otherwise let the SC 2 solution cool in a large beaker. Once cooled, SC 2 can be poured down the sink with running water.</li>
	<li>Rinse SC 2 cleaned beaker/container in DI water. The beaker/container is now chemically cleaned.</li>
</ol>

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Solid State Electrochem. 4 (1), 42-51 (1999).</li><li>Kolasinski, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+mechanism+of+Si+etching+in+fluoride+solutions.">The mechanism of Si etching in fluoride solutions.</a> Phys. Chem. Chem. Phys. 5 (6), 1270-1278 (2003).</li><li>Trucks, G. W., Raghavachari, K., Higashi, G. S., Chabal, Y. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Mechanism+of+HF+etching+of+silicon+surfaces:+A+theoretical+understanding+of+hydrogen+passivation.">Mechanism of HF etching of silicon surfaces: A theoretical understanding of hydrogen passivation.</a> Phys. Rev. Lett. 65 (4), 504-507 (1990).</li><li>Zhang, X. G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Electrochemistry of silicon and its oxide. , (2001).</li></ol>

The authors have nothing to disclose.

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 min17,18,19. 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...

High lifetime (&#62;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 velocities (Seff) of &#60; 5 cm/sec, and thus effectively inhibit surface recombination. These are, silicon nitride (SiNx:H)2, aluminum oxide (Al2O3)3 and amorphous silicon (a-Si:H)4. The deposition and annealing temperatures (~400 &#176;C) of these dielectric films are considered to be low enough not to permanently deactivate the recombination activity of the grown-in defects. Examples of this are the iron-boron5 and boron oxygen6 defects. However, recently it was found that vacancy-oxygen and vacancy-phosphorus defects in n-type Czochralski (Cz) silicon can be completely deactivated at temperatures of 250-350 &#176;C7,8. Similarly a defect in float-zone (FZ) p-type silicon was found to deactivate at ~250 &#176;C9. Therefore, conventional passivation techniques such as plasma enhanced chemical vapor deposition (PECVD) and atomic layer deposition (ALD) may not be suitable for inhibiting surface recombination to examine grown-in bulk defects. Furthermore, SiNx:H and a-Si:H films have been shown to deactivate bulk silicon defects through hydrogenation10,11. Therefore to examine the recombination activity of grown-in defects, a RT surface passivation technique would be ideal. Wet chemical surface passivation fulfils this requirement.
In the 1990s Horanyi et al. demonstrated that immersion of silicon wafers in iodine-ethanol (I-E) solutions provides a means to passivate silicon wafers, achieving Seff &#60; 10 cm/sec12. In 2007 Meier et al. showed that iodine-methanol (I-M) solutions can reduce the surface recombination to 7 cm/sec13, while in 2009 Chhabra et al. demonstrated that Seff of 5 cm/sec can be attained by immersing silicon wafers in quinhydrone-methanol (Q-M) solutions14,15. Despite the excellent surface passivation achieved by I-E, I-M and Q-M solutions, they do not provide adequate surface passivation (Seff &#60;5 cm/sec) to measure the bulk lifetime of high purity silicon wafers.
Another means to achieve a high level of surface passivation is by immersing silicon wafers in HF acid. The notion of using HF to passivate silicon wafers was first introduced by Yablonavitch et al. in 1986, who demonstrated a record low Seff of 0.25&#177;0.5 cm/sec16. Although excellent surface passivation was attained on high resistivity wafers, we have found the technique to be non-repeatable, thus adding a large uncertainty to the lifetime measurement. Therefore to limit the uncertainty by consistently achieving a very low Seff (~1 cm/sec), we have developed a new HF passivation technique that incorporates three critical steps, (i) chemically cleaning and etching of silicon wafers, (ii) immersion in a 15% HF solution and (iii) illumination for 1 min17,18. This procedure is both simple and time efficient in comparison to the traditional PECVD and ALD deposition techniques listed above.

<table>
	<tbody>
		<tr>
			<td>Hydrofluoric acid (48%)</td>
			<td>Merck Millipore,&#160; &#160;http://www.merckmillipore.com/AU/en/product/Hydrofluoric-acid-48%25,MDA_CHEM-100334</td>
			<td>1003340500</td>
			<td>Harmful and toxic. Any supplier could be used provided the chemical is Analytical Reagent (AR) grade.</td>
		</tr>
		<tr>
			<td>Hydrochloric acid 32%, AR</td>
			<td>ACI Labscan, http://www.rcilabscan.com/modules/productview.php?product_id=1985</td>
			<td>107209</td>
			<td>Harmful and toxic. Any supplier could be used provided the chemical is Analytical Reagent (AR) grade.</td>
		</tr>
		<tr>
			<td>Ammonia (30%) Solution AR</td>
			<td>Chem-supply, https://www.chemsupply.com.au/aa005-500m</td>
			<td>AA005</td>
			<td>Harmful and toxic. Any supplier could be used provided the chemical is Analytical Reagent (AR) grade.</td>
		</tr>
		<tr>
			<td>Hydrogen Peroxide (30%)</td>
			<td>Merck Millipore, http://www.merckmillipore.com/AU/en/product/Hydrogen-peroxide-30%25,MDA_CHEM-107209</td>
			<td>1072092500</td>
			<td>Harmful and toxic. Any supplier could be used provided the chemical is Analytical Reagent (AR) grade.</td>
		</tr>
		<tr>
			<td>Tetramethylammonium hydroxide (25% in H2O)</td>
			<td>J.T Baker, https://us.vwr.com/store/catalog/product.jsp?product_id=4562992</td>
			<td>5879-03</td>
			<td>Harmful and toxic. Any supplier could be used provided the chemical is Analytical Reagent (AR) grade.</td>
		</tr>
		<tr>
			<td></td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>640 mL round plastic container</td>
			<td>Sistema, http://sistemaplastics.com/products/klip-it-round/640ml-round</td>
			<td>N/A</td>
			<td>This is a good container for storing the 15% HF solution in.</td>
		</tr>
		<tr>
			<td>WCT-120 lifetime tester</td>
			<td>Sinton Instruments, http://www.sintoninstruments.com/Sinton-Instruments-WCT-120.html</td>
			<td>N/A</td>
			<td></td>
		</tr>
		<tr>
			<td>Dell workstation with Microsoft Office Pro, Data acquisition card and software including Sinton Software under existing license.</td>
			<td>Sinton Instruments, http://www.sintoninstruments.com</td>
			<td>N/A</td>
			<td></td>
		</tr>
		<tr>
			<td>Halogen optical lamp, ELH 300W, 120V</td>
			<td>OSRAM Sylvania, http://www.sylvania.com/en-us/products/halogen/Pages/default.aspx</td>
			<td>54776</td>
			<td>Any equivalent lamp could be used.</td>
		</tr>
		<tr>
			<td>Voltage power source</td>
			<td>Home made power supply</td>
			<td>N/A</td>
			<td>Any power supply could be used provided it can produce up to 90 Volts and 1-5 Amps.</td>
		</tr>
		<tr>
			<td>Conductivity meter</td>
			<td>WTW, http://www.wtw.de/uploads/media/US_L_07_Cond_038_049_I_02.pdf</td>
			<td>LF330</td>
			<td></td>
		</tr>
	</tbody>
</table>

light enhanced hydrofluoric acid passivation: a sensitive technique for detecting bulk silicon defects

A procedure to measure the bulk lifetime (&#62;100 &#181;sec) 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&#160;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 (&#60;1012 cm-3).

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&#160;(while immersed in HF), as shown in Figure 1</st...

Watch this Scientific Journal Video about Light Enhanced Hydrofluoric Acid Passivation: A Sensitive Technique for Detecting Bulk Silicon Defects at JoVE.com

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

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.

A procedure to measure the bulk lifetime (&#62;100 &#181;sec) of silicon wafers by temporarily attaining a very high level of surface passivation when ...

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9.2K Views.  Australian National University. 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.

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

Polycrystalline Silicon Thin-film Solar cells with Plasmonic-enhanced Light-trapping

One of major approaches to cheaper solar cells is reducing the amount of semiconductor material used for their fabrication and making cells thinner. To compensate for lower light absorption such physically thin devices have to incorporate light-trapping which increases their optical thickness. Light scattering by textured surfaces is a common technique but it cannot be universally applied to all solar cell technologies. Some cells, for example those made of evaporated silicon, are planar as produced and they require an alternative light-trapping means suitable for planar devices. Metal nanoparticles formed on planar silicon cell surface and capable of light scattering due to surface plasmon resonance is an effective approach.
The paper presents a fabrication procedure of evaporated polycrystalline silicon solar cells with plasmonic light-trapping and demonstrates how the cell quantum efficiency improves due to presence of metal nanoparticles.
To fabricate the cells a film consisting of alternative boron and phosphorous doped silicon layers is deposited on glass substrate by electron beam evaporation. An Initially amorphous film is crystallised and electronic defects are mitigated by annealing and hydrogen passivation. Metal grid contacts are applied to the layers of opposite polarity to extract electricity generated by the cell. Typically, such a ~2 &mu;m thick cell has a short-circuit current density (Jsc) of 14-16 mA/cm2, which can be increased up to 17-18 mA/cm2 (~25% higher) after application of a simple diffuse back reflector made of a white paint.
To implement plasmonic light-trapping a silver nanoparticle array is formed on the metallised cell silicon surface. A precursor silver film is deposited on the cell by thermal evaporation and annealed at 23&deg;C to form silver nanoparticles. Nanoparticle size and coverage, which affect plasmonic light-scattering, can be tuned for enhanced cell performance by varying the precursor film thickness and its annealing conditions. An optimised nanoparticle array alone results in cell Jsc enhancement of about 28%, similar to the effect of the diffuse reflector. The photocurrent can be further increased by coating the nanoparticles by a low refractive index dielectric, like MgF2, and applying the diffused reflector. The complete plasmonic cell structure comprises the polycrystalline silicon film, a silver nanoparticle array, a layer of MgF2, and a diffuse reflector. The Jsc for such cell is 21-23 mA/cm2, up to 45% higher than Jsc of the original cell without light-trapping or ~25% higher than Jsc for the cell with the diffuse reflector only.
Introduction
Light-trapping in silicon solar cells is commonly achieved via light scattering at textured interfaces. Scattered light travels through a cell at oblique angles for a longer distance and when such angles exceed the critical angle at the cell interfaces the light is permanently trapped in the cell by total internal reflection (Animation 1: Light-trapping). Although this scheme works well for most solar cells, there are developing technologies where ultra-thin Si layers are produced planar (e.g. layer-transfer technologies and epitaxial c-Si layers) 1 and or when such layers are not compatible with textures substrates (e.g. evaporated silicon) 2. For such originally planar Si layer alternative light trapping approaches, such as diffuse white paint reflector 3, silicon plasma texturing 4 or high refractive index nanoparticle reflector 5 have been suggested.
Metal nanoparticles can effectively scatter incident light into a higher refractive index material, like silicon, due to the surface plasmon resonance effect 6. They also can be easily formed on the planar silicon cell surface thus offering a light-trapping approach alternative to texturing. For a nanoparticle located at the air-silicon interface the scattered light fraction coupled into silicon exceeds 95% and a large faction of that light is scattered at angles above critical providing nearly ideal light-trapping condition (Animation 2: Plasmons on NP). The resonance can be tuned to the wavelength region, which is most important for a particular cell material and design, by varying the nanoparticle average size, surface coverage and local dielectric environment 6,7. Theoretical design principles of plasmonic nanoparticle solar cells have been suggested 8. In practice, Ag nanoparticle array is an ideal light-trapping partner for poly-Si thin-film solar cells because most of these design principle are naturally met. The simplest way of forming nanoparticles by thermal annealing of a thin precursor Ag film results in a random array with a relatively wide size and shape distribution, which is particularly suitable for light-trapping because such an array has a wide resonance peak, covering the wavelength range of 700-900 nm, important for poly-Si solar cell performance. The nanoparticle array can only be located on the rear poly-Si cell surface thus avoiding destructive interference between incident and scattered light which occurs for front-located nanoparticles 9. Moreover, poly-Si thin-film cells do not requires a passivating layer and the flat base-shaped nanoparticles (that naturally result from thermal annealing of a metal film) can be directly placed on silicon further increases plasmonic scattering efficiency due to surface plasmon-polariton resonance 10.
The cell with the plasmonic nanoparticle array as described above can have a photocurrent about 28% higher than the original cell. However, the array still transmits a significant amount of light which escapes through the rear of the cell and does not contribute into the current. This loss can be mitigated by adding a rear reflector to allow catching transmitted light and re-directing it back to the cell. Providing sufficient distance between the reflector and the nanoparticles (a few hundred nanometers) the reflected light will then experience one more plasmonic scattering event while passing through the nanoparticle array on re-entering the cell and the reflector itself can be made diffuse - both effects further facilitating light scattering and hence light-trapping. Importantly, the Ag nanoparticles have to be encapsulated with an inert and low refractive index dielectric, like MgF2 or SiO2, from the rear reflector to avoid mechanical and chemical damage 7. Low refractive index for this cladding layer is required to maintain a high coupling fraction into silicon and larger scattering angles, which are ensured by the high optical contrast between the media on both sides of the nanoparticle, silicon and dielectric 6. The photocurrent of the plasmonic cell with the diffuse rear reflector can be up to 45% higher than the current of the original cell or up to 25% higher than the current of an equivalent cell with the diffuse reflector only.

Polycrystalline silicon thin-film solar cells on glass are fabricated by deposition of boron and phosphorous doped silicon layers followed by crystallisation, defect passivation and metallisation. Plasmonic light-trapping is introduced by forming Ag nanoparticles on the silicon cell surface capped with a diffused reflector resulting in ~45% photocurrent enhancement.

One of major approaches to cheaper solar cells is reducing the amount of semiconductor material used for their fabrication and making cells thinner. To ...

Synthesis of Phase-shift Nanoemulsions with Narrow Size Distributions for Acoustic Droplet Vaporization and Bubble-enhanced Ultrasound-mediated Ablation

High-intensity focused ultrasound (HIFU) is used clinically to thermally ablate tumors. To enhance localized heating and improve thermal ablation in tumors, lipid-coated perfluorocarbon droplets have been developed which can be vaporized by HIFU. The vasculature in many tumors is abnormally leaky due to their rapid growth, and nanoparticles are able to penetrate the fenestrations and passively accumulate within tumors. Thus, controlling the size of the droplets can result in better accumulation within tumors. In this report, the preparation of stable droplets in a phase-shift nanoemulsion (PSNE) with a narrow size distribution is described. PSNE were synthesized by sonicating a lipid solution in the presence of liquid perfluorocarbon. A narrow size distribution was obtained by extruding the PSNE multiple times using filters with pore sizes of 100 or 200 nm. The size distribution was measured over a 7-day period using dynamic light scattering. Polyacrylamide hydrogels containing PSNE were prepared for in vitro experiments. PSNE droplets in the hydrogels were vaporized with ultrasound and the resulting bubbles enhanced localized heating. Vaporized PSNE enables more rapid heating and also reduces the ultrasound intensity needed for thermal ablation. Thus, PSNE is expected to enhance thermal ablation in tumors, potentially improving therapeutic outcomes of HIFU-mediated thermal ablation treatments.

Phase-shift nanoemulsions (PSNE) can be vaporized using high intensity focused ultrasound to enhance localized heating and improve thermal ablation in tumors. In this report, the preparation of stable PSNE with a narrow size distribution is described. Furthermore, the impact of vaporized PSNE on ultrasound-mediated ablation is demonstrated in tissue-mimicking phantoms.

High-intensity  focused ultrasound (HIFU) is used clinically to thermally ablate tumors. To  enhance localized heating and improve thermal ablation in ...

Monolayer Contact Doping of Silicon Surfaces and Nanowires Using Organophosphorus Compounds

Monolayer Contact Doping (MLCD) is a simple method for doping of surfaces and nanostructures1. MLCD results in the formation of highly controlled, ultra shallow and sharp doping profiles at the nanometer scale. In MLCD process the dopant source is a monolayer containing dopant atoms.
In this article a detailed procedure for surface doping of silicon substrate as well as silicon nanowires is demonstrated. Phosphorus dopant source was formed using tetraethyl methylenediphosphonate monolayer on a silicon substrate. This monolayer containing substrate was brought to contact with a pristine intrinsic silicon target substrate and annealed while in contact. Sheet resistance of the target substrate was measured using 4 point probe. Intrinsic silicon nanowires were synthesized by chemical vapor deposition (CVD) process using a vapor-liquid-solid (VLS) mechanism; gold nanoparticles were used as catalyst for nanowire growth. The nanowires were suspended in ethanol by mild sonication. This suspension was used to dropcast the nanowires on silicon substrate with a silicon nitride dielectric top layer. These nanowires were doped with phosphorus in similar manner as used for the intrinsic silicon wafer. Standard photolithography process was used to fabricate metal electrodes for the formation of nanowire based field effect transistor (NW-FET). The electrical properties of a representative nanowire device were measured by a semiconductor device analyzer and a probe station.

A detailed procedure for surface doping of Silicon interfaces is provided. The ultra-shallow surface doping is demonstrated by using phosphorus containing monolayers and rapid annealing process. The method can be used for doping of macroscopic area surfaces as well as nanostructures.

Monolayer Contact Doping (MLCD) is a simple method for doping of surfaces and nanostructures1. MLCD results in the formation of highly controlled, ultra ...

Flame Experiments at the Advanced Light Source: New Insights into Soot Formation Processes

The following experimental protocols and the accompanying video are concerned with the flame experiments that are performed at the Chemical Dynamics Beamline of the Advanced Light Source (ALS) of the Lawrence Berkeley National Laboratory1-4. This video demonstrates how the complex chemical structures of laboratory-based model flames are analyzed using flame-sampling mass spectrometry with tunable synchrotron-generated vacuum-ultraviolet (VUV) radiation. This experimental approach combines isomer-resolving capabilities with high sensitivity and a large dynamic range5,6. The first part of the video describes experiments involving burner-stabilized, reduced-pressure (20-80 mbar) laminar premixed flames. A small hydrocarbon fuel was used for the selected flame to demonstrate the general experimental approach. It is shown how species&#8217; profiles are acquired as a function of distance from the burner surface and how the tunability of the VUV photon energy is used advantageously to identify many combustion intermediates based on their ionization energies. For example, this technique has been used to study gas-phase aspects of the soot-formation processes, and the video shows how the resonance-stabilized radicals, such as C3H3, C3H5, and i-C4H5, are identified as important intermediates7. The work has been focused on soot formation processes, and, from the chemical point of view, this process is very intriguing because chemical structures containing millions of carbon atoms are assembled from a fuel molecule possessing only a few carbon atoms in just milliseconds. The second part of the video highlights a new experiment, in which an opposed-flow diffusion flame and synchrotron-based aerosol mass spectrometry are used to study the chemical composition of the combustion-generated soot particles4. The experimental results indicate that the widely accepted H-abstraction-C2H2-addition (HACA) mechanism is not the sole molecular growth process responsible for the formation of the observed large polycyclic aromatic hydrocarbons (PAHs).

Gas sampling from laboratory-scale flames with online analysis of all species by mass spectrometry is a powerful method to investigate the complex mixture of chemical compounds occurring during combustion processes. Coupled with tunable soft ionization via synchrotron-generated vacuum-ultraviolet radiation, this technique provides isomer-resolved information and potentially fragment-free mass spectra.

The following experimental protocols and the accompanying video are concerned with the flame experiments that are performed at the Chemical Dynamics ...

Scalable Nanohelices for Predictive Studies and Enhanced 3D Visualization

Spring-like materials are ubiquitous in nature and of interest in nanotechnology for energy harvesting, hydrogen storage, and biological sensing applications.&#160; For predictive simulations, it has become increasingly important to be able to model the structure of nanohelices accurately.&#160; To study the effect of local structure on the properties of these complex geometries one must develop realistic models.&#160; To date, software packages are rather limited in creating atomistic helical models.&#160; This work focuses on producing atomistic models of silica glass (SiO2) nanoribbons and nanosprings for molecular dynamics (MD) simulations. Using an MD model of &#8220;bulk&#8221; silica glass, two computational procedures to precisely create the shape of nanoribbons and nanosprings are presented.&#160; The first method employs the AWK programming language and open-source software to effectively carve various shapes of silica nanoribbons from the initial bulk model, using desired dimensions and parametric equations to define a helix.&#160; With this method, accurate atomistic silica nanoribbons can be generated for a range of pitch values and dimensions.&#160; The second method involves a more robust code which allows flexibility in modeling nanohelical structures.&#160; This approach utilizes a C++ code particularly written to implement pre-screening methods as well as the mathematical equations for a helix, resulting in greater precision and efficiency when creating nanospring models.&#160; Using these codes, well-defined and scalable nanoribbons and nanosprings suited for atomistic simulations can be effectively created.&#160; An added value in both open-source codes is that they can be adapted to reproduce different helical structures, independent of material.&#160; In addition, a MATLAB graphical user interface (GUI) is used to enhance learning through visualization and interaction for a general user with the atomistic helical structures.&#160; One application of these methods is the recent study of nanohelices via MD simulations for mechanical energy harvesting purposes.

Accurate modeling of nanohelical structures is important for predictive simulation studies leading to novel nanotechnology applications.&#160; Currently, software packages and codes are limited in creating atomistic helical models.&#160; We present two procedures designed to create atomistic nanohelical models for simulations, and a graphical interface to enhance research through visualization.

Spring-like materials are ubiquitous in nature and of interest in nanotechnology for energy harvesting, hydrogen storage, and biological sensing ...

Silicon Metal-oxide-semiconductor Quantum Dots for Single-electron Pumping

As mass-produced silicon transistors have reached the nano-scale, their behavior and performances are increasingly affected, and often deteriorated, by quantum mechanical effects such as tunneling through single dopants, scattering via interface defects, and discrete trap charge states. However, progress in silicon technology has shown that these phenomena can be harnessed and exploited for a new class of quantum-based electronics. Among others, multi-layer-gated silicon metal-oxide-semiconductor (MOS) technology can be used to control single charge or spin confined in electrostatically-defined quantum dots (QD). These QD-based devices are an excellent platform for quantum computing applications and, recently, it has been demonstrated that they can also be used as single-electron pumps, which are accurate sources of quantized current for metrological purposes. Here, we discuss in detail the fabrication protocol for silicon MOS QDs which is relevant to both quantum computing and quantum metrology applications. Moreover, we describe characterization methods to test the integrity of the devices after fabrication. Finally, we give a brief description of the measurement set-up used for charge pumping experiments and show representative results of electric current quantization.

The fabrication process and experimental characterization techniques relevant to single-electron pumps based on silicon metal-oxide-semiconductor quantum dots are discussed.

As mass-produced silicon transistors have reached the nano-scale, their behavior and performances are increasingly affected, and often deteriorated, by ...

Integration of Light Trapping Silver Nanostructures in Hydrogenated Microcrystalline Silicon Solar Cells by Transfer Printing

One of the potential applications of metal nanostructures is light trapping in solar cells, where unique optical properties of nanosized metals, commonly known as plasmonic effects, play an important role. Research in this field has, however, been impeded owing to the difficulty of fabricating devices containing the desired functional metal nanostructures. In order to provide a viable strategy to this issue, we herein show a transfer printing-based approach that allows the quick and low-cost integration of designed metal nanostructures with a variety of device architectures, including solar cells. Nanopillar poly(dimethylsiloxane) (PDMS) stamps were fabricated from a commercially available nanohole plastic film as a master mold. On this nanopatterned PDMS stamps, Ag films were deposited, which were then transfer-printed onto block copolymer (binding layer)-coated hydrogenated microcrystalline Si (&#181;c-Si:H) surface to afford ordered Ag nanodisk structures. It was confirmed that the resulting Ag nanodisk-incorporated &#181;c-Si:H solar cells show higher performances compared to a cell without the transfer-printed Ag nanodisks, thanks to plasmonic light trapping effect derived from the Ag nanodisks. Because of the simplicity and versatility, further device application would also be feasible thorough this approach.

A viable transfer printing-based methodology to introduce plasmonic metal nanostructures in solar cells is described. Using nanopillar poly(dimethylsiloxane) stamps, an Ag-based ordered nanodisk array was integrated with standard hydrogenated microcrystalline Si solar cells, which led to improved device performances due to plasmonic light trapping.

One of the potential applications of metal nanostructures is light trapping in solar cells, where unique optical properties of nanosized metals, commonly ...

Printing Fabrication of Bulk Heterojunction Solar Cells and In Situ Morphology Characterization

Polymer-based materials hold promise as low-cost, flexible efficient photovoltaic devices. Most laboratory efforts to achieve high performance devices have used devices prepared by spin coating, a process that is not amenable to large-scale fabrication. This mismatch in device fabrication makes it difficult to translate quantitative results obtained in the laboratory to the commercial level, making optimization difficult. Using a mini-slot die coater, this mismatch can be resolved by translating the commercial process to the laboratory and characterizing the structure formation in the active layer of the device in real time and in situ as films are coated onto a substrate. The evolution of the morphology was characterized under different conditions, allowing us to propose a mechanism by which the structures form and grow. This mini-slot die coater offers a simple, convenient, material efficient route by which the morphology in the active layer can be optimized under industrially relevant conditions. The goal of this protocol is to show experimental details of how a solar cell device is fabricated using a mini-slot die coater and technical details of running in situ structure characterization using the mini-slot die coater.

Printing Fabrication of Bulk Heterojunction Solar Cells and In Situ Morphology Characterization

Here, we present a protocol to fabricate organic thin film solar cells using a mini-slot die coater and related in-line structure characterizations using synchrotron scattering techniques.

Polymer-based materials hold promise as low-cost, flexible efficient photovoltaic devices. Most laboratory efforts to achieve high performance devices ...

Fabrication and Operation of Acoustofluidic Devices Supporting Bulk Acoustic Standing Waves for Sheathless Focusing of Particles

Acoustophoresis refers to the displacement of suspended objects in response to directional forces from sound energy. Given that the suspended objects must be smaller than the incident wavelength of sound and the width of the fluidic channels are typically tens to hundreds of micrometers across, acoustofluidic devices typically use ultrasonic waves generated from a piezoelectric transducer pulsating at high frequencies (in the megahertz range). At characteristic frequencies that depend on the geometry of the device, it is possible to induce the formation of standing waves that can focus particles along desired fluidic streamlines within a bulk flow. Here, we describe a method for the fabrication of acoustophoretic devices from common materials and clean room equipment. We show representative results for the focusing of particles with positive or negative acoustic contrast factors, which move towards the pressure nodes or antinodes of the standing waves, respectively. These devices offer enormous practical utility for precisely positioning large numbers of microscopic entities (e.g., cells) in stationary or flowing fluids for applications ranging from cytometry to assembly.

Acoustofluidic devices use ultrasonic waves within microfluidic channels to manipulate, concentrate and isolate suspended micro and nanoscopic entities. This protocol describes the fabrication and operation of such a device supporting bulk acoustic standing waves to focus particles in a central streamline without the aid of sheath fluids.

Acoustophoresis refers to the displacement of suspended objects in response to directional forces from sound energy. Given that the suspended objects must ...

Measurement of Quantum Interference in a Silicon Ring Resonator Photon Source

Silicon photonic chips have the potential to realize complex integrated quantum information processing circuits, including photon sources, qubit manipulation, and integrated single-photon detectors. Here, we present the key aspects of preparing and testing a silicon photonic quantum chip with an integrated photon source and two-photon interferometer. The most important aspect of an integrated quantum circuit is minimizing loss so that all of the generated photons are detected with the highest possible fidelity. Here, we describe how to perform low-loss edge coupling by using an ultra-high numerical aperture fiber to closely match the mode of the silicon waveguides. By using an optimized fusion splicing recipe, the UHNA fiber is seamlessly interfaced with a standard single-mode fiber. This low-loss coupling allows the measurement of high-fidelity photon production in an integrated silicon ring resonator and the subsequent two-photon interference of the produced photons in a closely integrated Mach-Zehnder interferometer. This paper describes the essential procedures for the preparation and characterization of high-performance and scalable silicon quantum photonic circuits.

Silicon photonic chips have the potential to realize complex integrated quantum systems. Presented here is a method for preparing and testing a silicon photonic chip for quantum measurements.