Greetings.
I am having trouble achieving ohmic contacts between Aluminium and Germanium that work at low temperatures. My contacts behave ohmically at room temperature, with a relatively high resistance though (8~20 kΩ), but at temperatures around 4K I obtained only noise-like signals and a parabolic IV plot.
The germanium (actually a Ge layer between SiGe layers, all on top of a Si substrate) is nominally undoped, although I have been informed by the person who grew it that due to imperfections in his growing method, the Germanium is actually very lightly p-doped.
My recipe is basically depositing a thin layer of Ti (as glue to prevent the Al from peeling off) followed by an Al layer. I have tried different annealing conditions with temperatures 275 °C, 300 °C, and one sample was mistakenly annealed at 410 °C. The annealing times were 2, 4 and 15 minutes.
All the samples showed similar behaviour and failed to display ohmic IV curves at 4K.
Because the alloying temperature of Al and Ti is lower than for Al and Ge, I thought the TiAl alloy was being achieved first and making difficult the formation of the AlGe alloy. So in a different batch of samples, I deposited only Al, but this was of no help.
My guess is that there is a thin native oxide layer in the contact region, but the annealing temperature is not high enough to break the GeO2 to form AlO2 which was then supposed to be absorbed into the Al bulk, clearing the contact surface. The growth temperature of the Ge was, however, 300 °C , and I was advised by the person who grew it that I should not anneal at higher temperatures and that this was a non-alloy ohmic contact.
Do you have any idea how to solve this problem?
Thank you very much.
Best regards,
Gabriel Gulak Maia
Dear Gabriel,
I would advice you to deposit a P++ layer on the low doped p-layer. Any layer deposited on such heavily doped p++ layer will act as low ohmic contact because of tunneling.
The second solution is that you build an alloying p++ -p junction by alloying aluminum to germanium.
You must remove the buffer titanium layer as it can prevent the eutectic of aluminum and geranium.
Best wishes
GeO2 is very unstable compound. try heating your sample and convert GeO2 into GeO, and then deposit your Aluminium layer..
Dear Gabriel,
I would advice you to deposit a P++ layer on the low doped p-layer. Any layer deposited on such heavily doped p++ layer will act as low ohmic contact because of tunneling.
The second solution is that you build an alloying p++ -p junction by alloying aluminum to germanium.
You must remove the buffer titanium layer as it can prevent the eutectic of aluminum and geranium.
Best wishes
Thanailakis, A., and D. C. Northrop. "Metal-germanium Schottky barriers." Solid-State Electronics 16, no. 12 (1973): 1383-1389.
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