Owing to the wide bandgap and high-mobility of GaN compared to that of Si, GaN power devices are expected to perform low on-resistance at high withstand voltage. However, due to the presence of defects in the crystal, GaN epi-wafers contain lots of traps sites which results in malfunction during the device operation; current collapse and so on [1]. As the defects are distributed in the whole epi-layer, it is difficult to extract the defect density profile from cyclic voltammetry. In this presentation, we propose a method to extract defect density profile in the GaN epitaxial wafer.

When a DC voltage (V) is applied to an Ohmic electrode connected to 2DEG at AlGaN/GaN interface, the entire epi-layer will depleted (t=0+) and a charge of Q_cap (=C_capV) will appear at both electrodes. At steady state (t=∞), the depletion layer shrinks back depending on the active defect concentration. The thickness of the depletion layer (z) can be extracted by the charges (Q_dep), which is the integral of current until the steady state is reached using the C_dep =Q_depV^-1 relation. Since we have the relation between V and z, the defect density profile at the position x, ρ(x), can be extracted using eq.1. Where q and ε are the elementary charge and the dielectric constant of GaN, respectively.

Figure 2 demonstrates the defect density profile extracted with the proposed method on AlGaN/GaN/buffer structure on a Si substrate. Here, buffer was used to reduce defect density. A distinctive change in the defect profile can be seen at x=4μm, which corresponds to the position where the epitaxial process has been changed.

In summary, a novel method to extract the defect density profile in the GaN epi-wafer has been proposed.

[1] T. Hashizume, etc., IEICE technical report, 104 (2004).