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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