I have made a vertical schottky diode on GaN on GaN substrate. The substrate has a doping of 1018/cm3, while the drift layer was doped to 1016/cm3 during MOCVD growth. After Fabrication of diodes CV analysis indicates whole 30um of the drift layer has depleted. I have wafers with different drift layer thicknesses 2um, 5um and 15um, all of them show similar results of depleted drift layer.
- I would like to know what could have caused this, as the device fabrication process is quite generic.
- Has anyone else measured a depletion width as high as 30 um as most literature suggests depletion width is in 'nm'?
Thanks
Dear Nanyang,
welcome,
I assume that the doping of the drift layer is n-type. What is the voltage range applied in measuring the C-V curve. What is degree of crystallinity of the drift region. Is it single crystal. Did you measure the carrier concentration in the drift region before you deposited the metal.
would you display the I-V curve and the C-V curve.
Are you sure that the measurement you made is correct.?
The relation between the applied reverse voltage Vr and the depletion region width w is:
phi+Vr= qNdw^2/2 epsilon,
where Nd: doping concentration and epsilon is the dielectric constant. Phi is the contact difference of potential. From the above equation one can calculate w for a given material and reverse voltage.
Best wishes
Hi Prof. A.Zekry
Thanks for your reply.
My apologies for improper framing of my question.
Yes, the doping is n type.
CV was measured from -5V to 5V.
It is a single crystal.
No, carrier concentration wasn't measured before metal deposition. The growth team did say the doping was done to 1016/cm3 range.
Please find the I-V and C-V curves attached.
I am confident about my measurements as multiple diodes and wafers (which were grown together) exhibit similar results.
Thanks
Abhinay
Dear Abhinay,
Yes the C-V shows that the reverse capacitance does not show variation with the reverse voltage meaning that the drift region is depleted.
I would like to pay your attention to two things:
- the small cut in voltage of the diode in the forward biasing which points out either a low barrier height or fermi level pinning due to surface state.
- The random variation of the reverse current which means that you did not anneal the device after metal deposition.
I would expect that your devices do not work well because you did not anneal them after metal deposition.
If this is the case if you anneal your devices properly you get ordinary functioning diodes.
Please inform us about any advancements.
Best wishes
Hi Prof A. Zekry
Could you kindly elaborate on what you mean by small cut in voltage.
The device was annealed at 775C.
The random variation in reverse current was probably because near limit measurement of the probe station. (If I dont normalize the reverse current it measures in range of 50-200pA, which is very close to the limit of probe station.)
Could improper annealing lead to completely depleted drift region of 30um? Are there any other situations when such a thick drift region can get depleted?
Thanks
Abhinay
What is the expected barrier height with the metal you used?
As the material is is wide band gap i expected that if we made a linear plot of the I-V curve we can get the the voltage at the on set of the forward conduction of the diode.
I see that for your Schottky diode this voltage is too small pointing out that the junction is anomalous. The other important thing is series resistance dominates the i-v curve at high currents.
Why this high annealing temperature?
Best wishes
Its a Ni based Schottky contact expected barrier height is 1.1eV, measured values ranged 0.77 - 0.84eV(I-V). The ohmic contact on the other hand is Ti based.
The diode was designed to act as a sensor in reverse bias conditions, whose major requirement was very low reverse leakage current and high breakdown. The low doping concentration was meant to sacrifice the higher conductivity for lower leakage.
The annealing temperature mention was for Ti based Ohmic contacts. The reported range for annealing temperatures range between 750C to 850C for Ti based ohmic on GaN. (The schottky contacts haven't been annealed as its been reported to increase reverse leakage current.)
Thanks
Now, Abhiny, it became clear. What i expected is really existing. Your Schottky contact is not annealed. So, you have to anneal the Schottky contact to stabilize it and bring the atoms and the semicondcutor into intimate contact on the atomic level.
You can anneal the diode at relatively low temperature say 450 degree centigrade at under nitrogen atmosphere.
The i-v curve points out lower barrier height than you reported.
Best wishes