Study on vertical coupled quantum dots fabrication using selective area growth of nitride nano-pillars

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Quantum dot (QD) structures of wide bandgap GaN and related nitride materials are of great interest in use for coherent quantum devices such as quantum logic gate because the large conduction band offset of 2 eV at the GaN/AlGaN interface provides strong electron confinement in their quantized levels. The energy separation between ground and exited states in the GaN dot of 5nm in a diameter is large as 125 meV. So, we can forcibly oscillate an electron in binary quantum states in the quantum dot using infrared optical sources. In order to realize such quantum logic, single-electron transistor using GaN quantum dots is important and the fabrication process was investigated. The vertical GaN quantum dot was formed in a GaN/AlN heteropillar grown by MBE selective growth method on the SiO2 masked AlGaN substrate at 800Åe. The height of GaN dot with a 100 nm diameter was 5 nm and the barrier thickness was 1 nm. The source and drain electrodes were formed backside of the substrate and top of the dot, respectively. Gate electrodes for transistor operations were buried in SiO2 musk before GaN growth process. Clear reductions in the differential conductance curves due to resonant tunneling through the quantum states in the dot were observed at 6 K. The Charge stability diagram exhibited the periodically Coulomb diamond structures at near zero source-drain voltages regions. The 6 mV of the charging energy corresponds to self-capacitance of 60 aF. Large charging energy will be obtained by future decrease in SiO2 mask diameter to 5 nm. This GaN quantum dots formation technique and the single-electron devices are promising for future quantum logic gate.