Front and back side of pyrometer

Pyrometer is basically a thermometer which can work in distance to measure the temperature of a surface. It is a touch-free thermometer.

The temperature measurement comes from the thermal radiation emitted from the measured surface, as it has its finger print in the range of spectrum.

Normally, the temperature measured in the growth chamber of MBE is measured by pyrometer, not the temperature set in the heater via computer. So much bias information will be given by that set temperature as many factors such as sample holder type, distance between coil and the sample and contamination affect the difference between the real temperature and the set temperature.

That is why pyrometer comes in handy. Beside we can measure the temperature directly on the surface, consideration of sample holder type can be neglected and distance between coil-sample is not something to be very worried about, since the measurement is actually done in the surface of the sample. The only thing need to be remembered is the contamination of the window where the pyrometer is looking at.

In the first setup, the pyrometer is looking at the sample surface directly with only heated viewport in between of them. The heated viewport is a window where the pyrometer observes the sample surface, and this window is heated to certain degree so that deposited metal can be evaporated as soon as possible, i.e. no metal accumulates in the viewport that can deviate temperature measurement.

First pyrometer setup
First pyrometer setup

In the second setup, I notice there is no such kind of controller that can increase temperature in viewport. I am actually do not know how they manage to clean the deposited metal. Maybe during opening schedule? Anyway, I made such a mistake when I tried to use the pyrometer. They have fixed pyrometer with a dial on the right hand side with a sign of “裏パイロ” and “表パイロ” that you can turn 360 deg. At the beginning, I did not really understand what “裏パイロ=back pyro” and “表パイロ=front pyro” actually mean because there is only one pyrometer in that growth chamber. During the past two weeks, sometimes I helped on recording the temperature by using this device, which basically just turning 180 deg to open the pyrometer and another 180 deg to close the pyrometer. Everything went well until last Wednesday.

As usual, I turned the dial 180 deg and my very first time looking at the target which is far off to the left of the sample. So, what I did was shifting the position of pyrometer to the right. After I did that, the recorded temperature was lower than the normal one in the previous growth (with the same set growth temperature). Then I asked help and they have fixed it for me. Again, after they left, I turned the dial 180 deg and observed the same thing again. Once more, I got very wrong temperature reading. Finally, one friend came to the lab and explained to me that the dial actually controls the side of the mirror where the pyrometer is looking at. The wrong side is “表パイロ” where lot of Ga and other metal contamination on it. That is why I got incorrect temperature value. In the other hand, “裏パイロ” has nice mirror surface with minimal surface contamination.

Second type of pyrometer setup, the correct one. The contamination on 表パイロ is illustrated with circles
Second type of pyrometer setup, the correct one. The contamination on 表パイロ is illustrated with circles

What a day. I almost ruined my friend’s data!