B.1 Radial pn Junction, Wire Array Solar Cell Fabrication
For convenience, the “state of the art” radial pn junction, wire array solar cell fabrication is presented here in full, from array patterning to sample metallization.
B.1.1 Array Patterning
1. We start with 3”, ∼ 400 μm thick, n-type Si(111) wafers of 0.005 - 0.02 Ω cm resis- tivity, purchased from International Wafer Service with a 300 nm wet thermal oxide.
2. Place the wafer on the spinner and turn on the vacuum. Blow off any dust particles from the wafer surface with a N2gun. Coat the wafer surface with MCC Primer 80/20 (Microchem), wait for 10 s, and then spin dry for 30 s at 3000 rpm (acceleration index ACL = 100).
3. Coat the wafer with S1813 photoresist (Microchem), and then spin at 3000 rpm for 1 min (acceleration index ACL = 100).
4. Cure on a hotplate at 115 ◦C for 2 mins.
5. Photolithographically pattern the resist using the Karl Suss MA 6 mask aligner (aka
”the new mask aligner”). For most of our masks the active area is less than the size of a 3” wafer, therefore to maximize the usable sample area it is best to cleave the sample into 4 - 6 pieces at this stage. Then a pattern can be transferred by exposing for∼ 10 s (exact time will vary with bulb age and should be recalibrated from time to time) in Hard Contact mode.
158
6. Place the sample in MF-319 developer (Microchem) for 60 s. Check with the optical microscope that the pattern has come out correctly, and if not adjust the exposure time accordingly for subsequent patterns.
7. Cure on a hotplate at 115 ◦C for 1 min.
8. After development of the pattern, the oxide within the patterned holes can be removed by immersion of the samples for 3 mins in buffered HF (Transene) (for the 3 μm diameter, 7 μm pitch pattern). It is important to use the slotted wafer holder to minimize resist lift-off / failure during the etch. Using this wafer holder the BHF etches the oxide at ∼2 nm/s.
9. Thermally evaporate 300 nm of either Au (Electronic Space Products International, 3N5 purity), Cu (Alfa Aesar, 5N purity) or Ni (Electronic Space Products Interna- tional, 4N5 purity). Note that the liquid nitrogen cooled stage is required for best results with Ni. Start liquid nitrogen cooling 10 mins before starting to heat the metal to ensure the samples reach a low enough temp. Note also that with the cooled stage and a tooling factor of 148%, the quartz crystal monitor overestimates the thickness of the deposited metal film by∼a factor of 3).
10. Lift off the resist by submerging the sample in acetone and (if necessary) leaving overnight and/or (if necessary) sonication. Rinse sample in acetone, isopropanol, methanol, and finally H2O before drying with nitrogen.
B.1.2 Wire Growth
11. Cleave the patterned samples to the desired size and transfer to the SiCl4 CVD system in Watson 255. Anneal the samples at 1000 ◦C for 20 mins under 1 atm of H2 at a flow rate of 1000 sccm. Wires are then grown under 1 atm of H2 and SiCl4, at flow rates of 1000 sccm and 20 sccm, respectively, for up to 30 mins. Growth rate varies depending upon catalyst, ∼ 2 - 3 μm/min for Au, ∼ 2 - 7 μm/min (or even faster) for Cu and Ni.
B.1.3 Catalyst Removal
12. After a wire array was grown, the first step towards making a device involved removing the metal catalyst particle at the tip as well as the near-surface of the wires. For gold the tip was removed the following way:
(a) 10:1 solution of DI water: 48% HF for 30 s to remove surface oxide on the Si.
(b) 9:1 solution of TFA etchant (Transene): 38% HCl for 20 mins.
(c) 10:1 solution of DI water: 38% HCl for 10 s to rinse.
(d) Rinse in DI water, dry with N2.
For Cu and Ni the following procedure appears to work well:
(a) Buffered HF (Transene) for 30 s.
(b) RCA 2 clean (6:1:1 H2O:H2O2:HCl at 70◦C) for 10 mins.
(c) Buffered HF (Transene) for 30 s.
(d) Rinse in DI water, dry with N2.
13. The Si at the near-surface was then removed by immersion of the sample in 60 wt.%
KOH at 30 ◦C for 30 s. This should remove ∼ 10 nm of the wire surface. Prior to doping, we ensured that the pattern oxide is fully removed (from the front side only) to ensure that the wire emitters were all connected. This was done by immersion in buffered HF (Transene).
B.1.4 Doping: Radial pn Junction Formation
14. The front side of the arrays were then doped n-type by stacking parallel to SiP2O7 diffusion wafers (PH-950, Saint-Gobain Ceramics), introducing to a tube furnace at 750 ◦C, ramping up to 800 ◦C (over about 5 mins), holding at 800 ◦C for 40 mins, and then ramping back down, all under UHP N2 at 5 lpm.
15. The front surface was then exposed to buffered HF for 2 mins.
160
16. The samples were then re-introduced to the tube furnace at 750 ◦C under house N2 bubbled through near boiling H2O, ramped to 855◦C (over about 16 mins), and held at 855◦C for 20 mins, in order to create a high-quality surface oxide to prevent out- diffusion of the dopant atoms. The gas was then changed to dry house N2, and the temperature ramped up to 1100 ◦C and held at this temperature for up to 23 hours in order to “drive in” the dopant atoms. Temperature was then ramped back down to room temperature, and the samples were not removed from the center of the furnace until the temperature there had at least dropped below 750◦C.
17. The front surface was then exposed to buffered HF for 2 mins.
18. The front surface of the samples were then doped p-type by stacking parallel to BN diffusion wafers (BN-975, Saint-Gobain Ceramics), introducing to a tube furnace at 750◦C, ramping up to 950◦C, holding at 950◦C for 40 mins, and then ramping back down, all under UHP N2 at 5 lpm.
19. The front surface was then exposed to buffered HF for 2 mins.
20. This was followed by a low temperature oxidation at 750◦C for 20 mins, under O2 at 5 lpm.
21. The front surface was again exposed to buffered HF for long enough that the oxide was removed (determined by the time it took to change a planar control from hyrdophilic to hydrophobic).
B.1.5 Metallization
22. Finally, oxide was removed from the entirety of the samples with buffered HF, the sample edges were cleaved off to prevent macroscopic shunting, and contact was made to the back surface by immediately rubbing Ga/In onto the back of the sample and the sample then bonded with Ag paste (SPI Supplies) to a piece of stainless steel to which electrical contact could be made with a probe tip.
23. Contact was made to the front surface with a spot of Ga/In and an electrical probe tip.