Understanding Voltage Decay in Semiconductor Thin Films

[yunusx]

Hello!
I am trying to carry out wavelength depended photoconductivity experiment to a semiconductor thin film material. I use keithley 2400 sourcemeter. I do apply voltage to the sample under pressure and read the current. My problem is when i apply a constant voltage to the sample i see that the current slowly decreases. For example, i applied 15 volt to the sample and read the current of 2,45897 microAmp. As i wait i see that the voltage drops continuously and slowly. it becomes 2,45754 three second later and it cahnges almost at this rate. I do the experiment in the ohmic region for that sample. I tried different higher and lower voltages but the situation didnt change. Temperature is constant. i don't know why that effect happens. Could anyone help me about why this occours?
Thanks

 

[yunusx]

No one has an idea? is there a problem about the question?

 

[phyzguy]

It's probably heating up. You say "temperature is constant", but you probably mean that the ambient temperature is constant. As you pass current through the film, you are dissipating power in the film. This power is probably heating the film and causing its conductivity to drop. If this is the case, the current should eventually level out. If you reduce the voltage so less current flows, the effect should be reduced.

This is my best guess.

 

[ZapperZ]

1. What does it mean when you say that you applied voltage to the sample "under pressure"?

2. Have you tried doing this at a different (lower) voltage? Does the current decay at the same rate?

3. What kind of electrical connections do you have with the film?

4. Is the thin film formed on a substrate? If yes, what type of substrate?

5. Have you tried making electrical connections via other points on the film?

6. Have you checked if the power source itself has a problem?

Zz.

 

[yunusx]

1-i mean that the measurement has been made in vacuum, about 10^-2 torr.
2-yes i did try doing it at different voltages, at lower and higher voltages. When i increase the voltage the decay in current is a little faster.
3-I use two contact at the sides of the film. The dimensions of the film is (1cm x 1cm x 6,8 micrometer). I apply the voltage and read the current from the same contacts.
4-Film has been grownth on aglass lamel.
5-Yes i used 4 point electrical connections which are at the corners of the films.
6-I checked the power source. it doesn't have a problem.
My english is not good. I am sorry for missunderstandings.

 

[phyzguy]

I see three things that can cause this:

(1) As I said earlier, it may be heating up.
(2) Your measurement equipment may be heating up.
(3) You may be creating traps in the film or at the surface of the film.

Try the following measurement:

(A) Measure the current. It decays from some value I0 to I0-δ.
(B) Turn the voltage off. Go away for an hour. Have a cup of coffee.
(C) Repeat the measurement. Does it start at I0 or at I0-δ?

If it starts at I0, then I think it is probably a thermal effect. If it starts at I0-δ, then I would suspect traps in the film.

Why does it matter? How about if you just pass current through the film until it stabilizes, then do your measurements? Or, can you just do the measurement at a lower current, where the drift is less?

 

[yunusx]

It doesn't get stabilized and this decay effects results. I will try to do what you said.
Thank you everyone answering.