# Vertical-Type Ni/GaN UV Photodetectors Fabricated on Free-Standing GaN Substrates
## 3 Results and Discussion
The surface electron density obtained from TFE model was higher than the carrier density inside the GaN drift layer extracted by \(C\)-\(V\) measurement and dependent on different Schottky contacts [11]. The higher surface electron density is attributed to the unintentional donors at the metal/semiconductor interfaces during the Schottky contact deposition. The difference by using different Schottky contact is from the inhomogeneous distribution of the electrons by the local variations of electric field [11;14]. The highly doped surface layer could shrink the depletion width and increase the current tunneling probability at the reverse voltage. Correspondingly, a relatively lower SBH of 0.9 eV in device-A was obtained, compared to that of device-B. The photo-current of device-A under the UV light (350 nm with a power of 1.32 mW/cm\({}^{2}\)) illumination is shown in Figure 1b. Compared to the dark current on the order of \(10^{-11}\) A at the applied voltage of \(-5\) V, the photocurrent at 350 nm illumination is greatly increased. A signal-to-noise ratio of \(1.6\times 10^{4}\) at \(-1\) V and \(4\times 10^{3}\) at \(-5\) V are obtained. Through the carrier transport analysis with different models as above, it is found that the photocurrent is driven by the TFE model, in which the tunneling of the photocurrent may be induced by UV-inspired defects [4]. The spectral responsivity of the device-A from 630 to 300 nm at different bias voltages is displayed in Figure 2a. We note that, the transparence of the 5 nm-thick Ni Schottky contact is typically higher than 60% for the UV light [15]. The Schottky-type photodetector exhibits a distinct UV response with a cut off wavelength at 370 nm, which is in the visible-blind region. The peak responsivity is located at 350 nm with the value of 0.16 A/W and an external high quantum efficiency (QE) of 56.4% at \(-5\) V. Such values are higher than those in the GaN-based photodetector with W, IrO\({}_{2}\), ITO transparent electrodes [16; 17; 18]. If assuming the shot noise is contributed mainly by the dark current, the specific detectivity (\(D^{*}\)) can be given by [19; 20]:  
\[D^{*}=R/(2 qI_{d})^{1/2} \tag{9}\]  
where \(R\) is the responsivity, \(q\) the elementary charge of electron and \(J_{d}\) the dark current density. The \(D^{*}\) is calculated to be more than \(10^{12}\) Jones at the applied voltage of \(-5\) V. The specific detectivity is much higher than that in W/GaN UV detectors, benefiting from low noise level. Low TDD and high Schottky barrier height contribute to the low leakage [16]. The discrimination ratio between the UV and visible light is extracted in Figure 2b. At \(-5\) V bias, a high spectra selectivity of 1530 is obtained between 350 and 630 nm. It is noted that this ratio is still more than 900 without the applied voltage, indicating a good self-powered capability of the Schottky-type photodetector. A comparison of detector properties from device-A in present study and the literatures are listed in \[J=J_{s}\left[exp\left(\frac{q(V-IR)}{nkT}\right)-1\right] \tag{1}\]0 It is found that UV detector with semi-transparent Ni showed better performance than those with W or Indium tin oxide (ITO) electrodes. Low TDD and high barrier height can suppress the leakage current well, resulting in high specific detectivity and high discrimination ratio. An enhancement of UV responsivity originated from the improved quantum efficiency. \[J=J_{s}\left[exp\left(\frac{q(V-IR)}{nkT}\right)-1\right] \tag{1}\]1 (**a**) Spectral responsivity of detector-A under different bias voltages. (**b**) Bias voltage dependent UV-to-visible (350/630 nm) discrimination ratio. The time response characteristics of the device-A are investigated by a mechanical chopping method, in which the 350 nm light was switched on and off alternately. The irradiance power was kept at 1.32 mW/cm\({}^{2}\). As shown in Figure 3a, during several UV on/off cycles, the device showed nearly identical response and stable photo to dark current ratios.