Fig. 1. (a) Lattice diagram of hexagonal structure and origination scheme of polarization. (b), (c) Polarization variation under tensile and compressive strain. (d) Origination of interface charges due to polarization effect. (e), (f) Charge distribution and energy band diagram of AlGaN heterostructure. (g), (h) Designed devices SA and SB in simulations.

Fig. 2. Simulated energy-band diagram, electric field, and carrier concentration in dark and on illumination for devices (a)–(c) SA and (d)–(f) SB, respectively.

Fig. 3. Simulated (a) IV curves in dark and on illumination and (b) spectral responses under different biases for devices SA and SB (semilog scale in the inset).

Fig. 4. (a) Schematic epitaxial structures of devices SA and SB. (b) Temperature and reflectance curves during the growth procedure of devices SA and SB (inset). (c), (d) XRD and XRC results of the wafers for devices SA and SB. (e) Absorption spectra of the wafers for devices SA and SB. (f) Structure diagram and electrode optical image of the fabricated devices.

Fig. 5. (a) IV curves in dark and on illumination for devices SA and SB. (b), (c) Band diagrams of devices SB and SA in dark. (d) Spectral responses of devices SA and SB (inset). (e), (f) Band diagrams of devices SB and SA on illumination. (g) Response time of devices SA and SB as a function of load resistance. Inset: Normalized transient spectra at 10  kΩ. (h) Schematic circuit to measure the transient spectra.

Fig. 6. (a) Band alignment and carrier distribution with high work function metal to adjust the carrier concentration. (b) Structure diagram and electrode optical image of device SC. (c) IV curves of device SC in dark and on illumination. (d) Spectral response of device SC under different biases.

Fig. 7. Transmittance spectrum of the Ni/Au semitransparent film.

Fig. 8. Schematic plan-view of the electrodes of devices (a) SA, SB, and (b) SC.

Fig. 9. (a) C−V measurement results of the wafer for devices SA and SC. (b) Ohmic check for Hall measurements at room temperature. (c) Temperature-dependent Hall measurement results of the wafer for devices SA and SC.