Hi everyone,
I made the graphene field effect transistor and want to observe the current-voltage characteristic under the effect of the back gate voltage. The back gate is highly p-doped and the dielectric layer is 300 nm of SiO2. So when I applied source-drain voltage is 0.05V and change the backgate voltage, I did not observe the Dirac point of graphene, where the channel resistance is maximum. The resistance is only changed from 200 Ohm at Vg=0 V, to 600 Ohm at Vg=80V and 150 Ohm at Vg=-80V. The trend is increase continuously without V-shape. So I do not know what is wrong with my sample. Does anyone have any suggestion?
Seems you got a p-type semiconductor response. May be due chemical alteration of graphene.
Dear Oleg,
Do you have any experience to get rid of this problem?
I do not want to waste my sample.
Thank you
Well one thing is clear, looking at the values mentioned by you here: that your graphene is largely p-doped. it could be due to multiple reasons: humidity, substrate interaction and so on. Many things can be suggested to get rid of this problem (mainly to be able to observe the Dirac point): first one would be try measuring the device in a vacuum chamber (removes adsorbates like humidity from its surface), bringing back the graphene to its 'original'-like situation. Other thing to note (and what you have not mentioned here) is what is your contact metal? Some metals end up doping the graphene more p- depending upon their work funtion difference w.r.t. graphene, compared to others. Hope this helps.
Dear Himadri,
Thank you for your answer.
I deposited Cr/Au on graphene to make source-drain and apply voltage by Ni needles. I always store my sample in small vacuum chamber to get rid of humidity, but the lab where I did measurement is not equipped with big vacuum chamber. Should I anneal sample and measure in ambient condition?
Well, then my friend, I guess its the highly p-doped Si substrate to blame. If possible, I would recommend the following 3 things: (1) make a new device on another (less p-doped) substrate or so, it will definitely be different; (2) try measuring at low temperature, this could bring the Dirac point in view, however, I must caution that the data may be hard to interpret; (3) annealing, if you go for that, would certainly alter the device properties and your device after annealing may not be the same but you should certainly try this (easiest to do than above 2). Alternately, you could also try current annealing (rather than thermal annealing in a furnace) the device first and then do those measurements that you really want.
Good luck.
Hi,
I have tried with vacuum and current annealing but they cannot help to recover the neutrality of graphene. I personally think that is permanent doping so preparing new device is the only choice.
Do you have any suggestion about the temperature and time for annealing for new FET?
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