Theory of high field carrier transport and impact ionization in ZnO

Abstract

We present a full band Monte Carlo study of high field carrier transport and impact ionization properties of wurtzite ZnO. The proposed model is based on an accurate electronic structure calculated with a nonlocal empirical pseudopotential method and a phonon dispersion determined with density functional theory. The model includes the full details of the lowest eight conduction bands and the top six valence bands derived from the empirical pseudopotential method and a numerically calculated impact ionization transition rate based on a wave-vector dependent dielectric function. The carrierphonon interaction is treated using the rigid pseudoion formalism, thus removing adjustable parameters such as deformation potential coefficients. Electric-field-induced interband transitions are included in the model by the direct solution of the time-dependent multiband Schrödinger equation. The hole ionization coefficient is found to be very low compared to the electron ionization coefficient. The low ratio of hole and electron ionization coefficients k = β /α holds the promise for high speed and low noise avalanche photodetection in the ultraviolet spectral range.

Characterization of Impact Ionization Coefficient of ZnO Based on a p-Si/i-ZnO/n-AZO Avalanche Photodiode

“…We substitute the quantities in Equation (10) with realistic values; then, we acquire that E BR is equal to 4.2 × 10 5 V/cm. The R(3 E g /2) we used is 1.1 × 10 13 s −1 from the reference [ 31 ].…”

“…Based on this analysis, we can draw two important conclusions: first, the impact ionization multiplication happens in the i-ZnO layer; second, this avalanche multiplication is an electron-dominant process, and the contribution from holes can be neglected. Moreover, from the calculated results in the literature [ 31 ], we know that for ZnO, the ionization coefficient of the electrons (α) is much greater than that of the holes (β). This also supports the conclusion that we can omit the influence of the holes and solely consider the electrons’ impact ionization.…”

“…ZnO is also an attractive wide band gap semiconductor that has a lot of advantages such as high exciton binding energy, direct band gap, low cost, ease of fabrication, and environmental friendliness [28][29][30]. Based on a simulation work, for ZnO, the ratio of the impact ionization coefficient of holes to that of electrons is rather low [31]. This has a very appealing merit that may be fully exploited to achieve high-performance, low excess noise APD devices.…”