The flat-band-voltage (Vfb) in metal/insulator (SiN or SiO₂)/n-GaN capacitors has been investigated focusing on an annealing temperature for post-deposit-annealing (PDA). We have found that the Vfb shifts to the negative bias direction as increasing the annealing temperature. Generally, a Vfb shift is known to be proportional to both the spatial trapped and interfacial charge density. If this shift was due to a change of traps distributed in an insulator bulk, PDA should cause a change of an insulator property. Refractive index of as-deposited SiN was 1.98 and it turned to 1.96 by annealing at 600 °C. This change exhibits that the annealing treatment pushes SiN to N-rich side slightly and N-rich SiN turns the Vfb shift to a positive direction. Thus, it is considerable that the negative shift of Vfb is due to the trapped charge mainly exists at interface but SiN bulk.

Estimated activation energies of a trap related to the Vfb shift are around 160 meV for SiN and 130 meV for SiO₂. The difference of these energies is small. This implies the each reaction of SiN/GaN and SiO₂/GaN interface is promoted by a same mechanism. In addition, these energies are too small compared with the formation energy of SiN, SiO₂, or GaN. This result suggests that the PDA does not produce new defect by breaking a bond for the Vfb shift. At the interface, a vacancy defect and an interstitial defect might form a dipole by Coulomb potential. In a deposit process of insulator, each dipole is faced at random direction because the process temperature is room temperature. It is considered PDA might work to face the random direction of dipoles to the same direction. As a result, they affect to the Vfb shift .

We also performed Kelvin force microscopy measurement to observe a charged state of SiN. The measured potential of 600 °C annealed SiN is around 1 eV lower in electron energy scale than as-deposited SiN one. The sign of this is reasonable to shift the Vfb for negative direction.

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