🚀 go-pugleaf

In article <1992Sep28.071622.3040@trl.oz.au> jdell@trl.OZ.AU (John Dell - Photonics) writes:
>
>Anyway, the problem (at last) is that we need to measure lowish temperatures
>now (of the order of 300-500 C) and we would like some recommendations of how
>to do this using optical pyrometry.  What have been peoples' experiences, how
>do you calibrate the system, what products are available, how accurate
>are they, how precise are they, do you look at the block or the semiconductor,
>have you had to worry about the window (ie do you need a sapphire window
>or will a standard window do and simply change the effective emissivity), 
>do you need to heat the window to prevent condensation of As, how do you
>limit the field of view .....? 
>
	I'll tell you what I did as a grad student at the U. of
Illinois.  The moly blocks can be cleaned with conc. HCl.  The wafers
themselves should first be degreased thoroughly using several rinses of
TCA (trichloroethane) followed by acetone and methanol and then DI
water.  Then they should be etched using a warm solution of
H2SO4:H202:H20 (I think).  This can be found in the literature in
several articles by Al Cho (Bell Labs) or several other people.  I can
find the references if you want to though I doubt that, working in MBE,
you would be unaware of them.  
After etching the wafers, they are
mounted as usual with In solder on the moly  blocks.  Be *extremely*
careful and patient with this.  Use the edge of a pair of tweezers to
slide the wafer around on the block to ensure that the backside of the
wafer is uniformly coated with the In.  One of the biggest causes of
temperature variations is that the In does not coat the backsides of the
wafer uniformly.
	Finally, get yourself a RHEED setup.  Once you get a properly
As-stabilized surface, you are ready to go.  Typically, we heated the
wafer to about 590 degrees C or so to get rid of the oxides, waited to
get the As-stabilized surface RHEED pattern (with the As oven shutter
open, of course), opened the Ga shutter and proceeded.
	We never had a problem with the pyrometer, though it got
increasingly off target (never more than 10 or 20 degrees) as the
window started getting coated with As and Sb.  Eventually, we got to the
point where we monitored oxide desorption and growth initiation almost
solely by RHEED and chamber pressure and used temperature to ensure that
we were roughly at 580-590 which if I remember right is optimum of
growing GaAs. 
	If you are having problems with temp variations, I would look at
your In-wafer interface first, and think of getting RHEED next.
	Boy, this stuff brings back a lot of memories.  It is not often
that you see compound semiconductor types posting to the net!
 

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                                  | "I am not a satanist!  I find it hard
Sam Sivakumar                     | enough to conjure myself out of bed
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                		  | the devil!"  --  Ozzy Osbourne