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Creating a n-p junction on a p-type silicon wafer
A n-p junction is a diode and a solar cell
It can also be the start of a n-p-n bipolar transistor or a FET
transistor
Starting with a n-type wafer would create a p-n junction
When creating a doped junction in silicon
There are two situations
•Using a limited source
•Using a infinite source
Each situation has a different mathematical calculation to determine diffusion depth
With an infinite source, the dopant is constantly supplied
Example – phosphorus gas continually supplied
Clean silicon waferP-type
P P P P P P P P P P P P P P P P P P P P P P P P P P P P P
With an limited source, the dopant has a fixed amount
availableExample – spin on phosphorus film
Clean silicon waferP-type
P P P P P P P P P P P P
For the solar cell being fabricated for this lab
The following will apply
Start with a clean p-type silicon wafer
Clean silicon waferP-type
A SiO2 layer is deposited via PECVD on the backside to prevent backside doping
Clean silicon waferP-type
Backside SiO2
Using a liquid n-type phosphorus spin on dopant, a layer of phosphorus is left on
the surface
Clean silicon waferP - type
Backside SiO2
Spin on phosphorus (n) dopant
To avoid contamination, a separate spinner is used for the spin on dopants
This spinner is located in the yellow room closest to the garment change out
room
Spin at 3 KRPM for 20 seconds. Spin is preset for
correct conditions
Wafer held with vacuum.
Vacuum switch is
located on control panel
Liquid dopants are located in the dry box. Use P509 and always check the
expiration date
Use P509 – expiration date
1-30-2012
High temperature tube furnace for 9500C diffusion
Furnace #6 is for phosphorus doping only. Do not cross contaminate apparatus and tweezers. Verify correct temperature before using
After diffusion @ 9500C a n-region is created in the
silicon. Time and temperature determine the depth of the
n doped region
P doped silicon
Backside SiO2
n doped silicon --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
An HF etch removes any residual phosphorus on the
surface and the backside SiO2
P-doped
n doped -------------------------------------------------------------------------------------------------------------------------------------------------
A n/p junction has been formed
P-doped
n doped -------------------------------------------------------------------------------------------------------------------------------------------------
Solid phosphorus sources are available
Liquid POCl3 with a nitrogen carrier gas is also used as a phosphorus dopant. POCl3 will form HCl on exposure to moist air or
water !
To high temperature
furnace tube
Nitrogen carrier gas in
Constant temperature cooler
(200C)
A photo-mask will need to be created using AutoCAD
Due to the current limitations on the tester in the clean room, cell size is limited to a maximum of
10 cm2
Current solar light simulator in Cameron clean room with Keithley
4200 I/V tester @ 1 amp max
Example of a photo-mask transparency
AutoCAD dimension drawing must be submitted for approval
• Dimension Drawing is due in 2 weeks• Any pattern for top side conductor is
acceptable. Multiple top side conductor patterns preferred
• Cell size can vary from 2 cm2 to 10cm2 • Patterns must be in-line to cut with the dicing
saw
A new solar cell tester will be available in EPIC with higher current capability (10 amp)
The new tester will provide the capability to test standard size
industrial cells
125mm x 125mm cell
