The GaP is certainly decomposing during sputtering. Sputtering is
knocking individual atoms off the target with Argon atoms. However the
relative vapor pressures aren't relevant to the problem here. If a Ga
atom is sputtered and its trajectory reaches your substrate there should
be deposition of the atom and not evaporation.

The issue here is the different atomic weights of Ga and P, (69.72 and
30.97 respectively) relative to Argon atom (39.95) which is used to
sputter. Also, heavier elements have more electrons to result in
inelastic collisions and reduce sputtering efficiency, if I remember
correctly. You may be getting a much lower flux of Ga than P upon
initial sputtering. I am guessing that the wafer you are using as a
target is Ga rich. Continued sputtering might make it Ga rich enough
that your depositions balance out. It would be interesting to see what
would be the composition after you ran some conditioning wafers and then
redeposited and remeasured your composition. Perhaps a higher power
might cause the Ga to sputter with higher relative flux to P, but you
would still have some imbalance.

Evap methods for compound materials involve conditioning the alloy
target until the composition of the melted puddle is skewed so that its
relative rates of evaporation match the desired alloy composition. So if
you have A/B alloy. And A has twice the vapor pressure as B, then you
run the target until the e-beam puddle was 1/3 A and 2/3 B and the vapor
pressure was balanced and as the target melted the puddle would keep a
constant composition. Actually, it would be much more complicated if the
vapor species were not monoatomic but perhaps A2 or something and
include some AB. But the same principle would apply as conditioning the
target until a balanced flux. Russel J. Hill, in the 1976 Airco Temescal
book explains alloy deposition in detail.

Ed

-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Andrew Sarangan
Sent: Wednesday, January 28, 2009 10:22 PM
To: General MEMS discussion
Subject: [mems-talk] Gallium Phosphide Thin Film Deposition

Does anyone have experience depositing amorphous GaP films by sputtering
or evap? I am using a crystalline GaP wafer as the sputtering target in
an RF magnetron system, but the films have too much phosphorous (EDAX
analysis).  My theory is that GaP is decomposing during deposition, and
since Ga has a higher vapor pressure than P it is producing a P-rich
film. I tried depositing at an extremely low rate to reduce
decomposition, but the results were the same. I would appreciate anyone
with experience in this.