Why does the diode behave like a short circuit?

[kbjee]

I have a diode where the mesa structure has layers as follows-

material 1(P+)
thin region (10nm) of material 2(P-) different badgap
material 1(N-)
material 1 (N+).

Overall it is a PIN structure. All materials are narrow bandgap, in mid IR region (~300meV).
When I measure I-V, I get a more-or-less straight line. The resistance is tens of ohms. The device behaves as if there is no P-N junction. I have earlier worked with a similar structure without the thin region and that gives good R0A values.

Do you have any idea why this might happen? I had some ideas but looks like those are not the right reasons according to further experimentation.

 

[IRstuff]

Maybe you blew the junction

TTFN

FAQ731-376

 

[kbjee]

That was a possibility. But I fabricated the devices again on a different sample and retested them with same results. So it is not for just one sample which I accidentally blew up.

 

[IRstuff]

How do you know you fabricated them correctly?

TTFN

FAQ731-376

 

[kbjee]

I have fabricated similar devices (exactly same procedure) before many times. So there is no reason to suspect that there is something wrong with the procedure. Of course, there can be something wrong in one particular run, but then I have made several batches of this particular device and all of them are bad.

 

[Skogsgurra]

To sum up:

1. You did not destroy it when testing.

2. The device is fabricated the same way that other working devices were fabricated. So, it does work.

Conclusion: Nothing wrong with anything. Is that what you are telling us? What do your colleagues say? Or are you doing this alone, in your basement?

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[IRstuff]

3. Bad measurements; did you try it with a known, good, diode?

TTFN

FAQ731-376

 

[kbjee]

I am saying there is nothing wrong with the standard procedure of fabrication. Also, I did not burn the devices while testing. That does not mean "Nothing wrong with anything".

Here are a few other things -
1> The doping profiles were not right during the structure growth creating no P-N junction. (This is not the case. I have done a SIMS profiling and the dopants are present. There is no reason to believe that the dopants are not activated.)
2> There is something wrong at the device surface. Without proper passivation, low bandgap materials can be very leaky. However I think the resistance is too low even for an unpassivated diode.
3> The material has too many defects. I plan to do XRD/TEM to check that.
4> Is there something fundamentally wrong with the structure? Can the thin layer lead to too high tunneling or in some other way disrupt the P-N junction behaviour? An initial analysis suggests it is not the case but I am not sure.

 

[kbjee]

Yeah, I checked the measurement with a standard diode. That is not a problem.

 

[IRstuff]

How big is the diode area?

What are the doping levels? Unless you have degenerate layers, you can't get tunneling. Even then, a standard IV test would reveal the negative resistance regime.

TTFN

FAQ731-376

 

[kbjee]

1e18
5e16(10nm layer)
3e17

Note that these are low bandgap materials.

 

[IRstuff]

True, my experience is strictly silicon.

Can you do a spreading resistance profile to determine the finished profiles?

TTFN

FAQ731-376

 

[kbjee]

I cannot do a spreading resistance profile at the moment. However, I doubt if it can give me the doping profile description at a resolution of few nanometers. Moreover, I know from SIMS that the dopants are there. It is very unlikely that they are not activated.
I would be interested to know if anyone has come across a diode (silicon or any material) which behaves like a short but is NOT burnt. Any other reason that might have rendered the diode behave like a short.

Thanks IRstuff and Skogsgurra for your replies. I appreciate your thinking through the problem.

 

[IRstuff]

How is the contact made? Silicon is prone to aluminum spiking through the junction, but that's usually from either bad crystal or excessive current.



TTFN

FAQ731-376

 

[IRstuff]

Moreover, I know from SIMS that the dopants are there. It is very unlikely that they are not activated.

That wasn't the point. Inactivated dopants result in intrinsic behavior. The SRA would tell you if your lightly doped regions are overrun, or if you have metal contamination.

TTFN

FAQ731-376

 

[kbjee]

The contact was deposited by evaporating Ti/Au.
Thanks for clarifying the SRA point. So you are suggesting that the metal might have diffused into the junction.

 

[IRstuff]

Gold usually, and even silicon sometimes, requires a contact barrier to prevent diffusion into junctions, at least, with silicon processing. I would suspect that other materials, being poorer crystals than silicon, would have similar, if not worse, problems with diffusion.

TTFN

FAQ731-376

 

[Skogsgurra]

I am still wondering. Do you not have colleagues to ask? This kind of activity is not something you do all by yourself in your basement (if you are not a very rich and very excentric person, which you do not seem to be - at least not very excentric). What do your colleagues say about this?

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[kbjee]

I am neither rich nor eccentric. Rather, quite on the contrary I do not have many people to discuss the problem. I am trying out other options.

 

[Skogsgurra]

Fair enough. And somewhat impressing.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...