Polarization Enhanced Charge Transfer: Dual-Band GaN-Based Plasmonic Photodetector

Here, we report a dual-band plasmonic photodetector based on Ga-polar gallium nitride (GaN) for highly sensitive detection of UV and green light. We discover that decoration of Au nanoparticles (NPs) drastically increases the photoelectric responsivities by more than 50 times in comparition to the blank GaN photodetector. The observed behaviors are attributed to polarization enhanced charge transfer of optically excited hot electrons from Au NPs to GaN driven by the strong spontaneous polarization field of Ga-polar GaN. Moreover, defect ionization promoted by localized surface plasmon resonances (LSPRs) is also discussed. This novel type of photodetector may shed light on the design and fabrication of photoelectric devices based on polar semiconductors and microstructural defects.

Optical and Electrical Properties. We investigated the optical and photoelectric properties of the three samples. Figure 3a shows the optical reflectance spectra obtained after calibration with a standard Al foil. The reflectance peaks at 365 nm correspond to the band edge of GaN (E g = 3.4 eV). The presence of Au NPs increases reflectance in the visible region due to enhanced backward scattering by Au NPs. The broad peaks in green light region can be attributed to the dipole mode of LSPRs of Au NPs 25 . The LSPR peak for Au500C is more pronounced due to higher scattering efficiency of larger Au NPs 26 . Figure 3b shows the photoelectric responsivity (PR, defined as photocurrent divided by incident optical power) of the samples from 355 nm to 615 nm. Au300C and Au500C exhibit two responsive peaks in the UV and green light region with greatly enhanced photoelectric responsivities than the blank sample. The enhancement ratios for UV light are ~27 and ~54 times, while enhancement ratios for green light are ~18 and ~64 times. As the Au NPs were obtained by annealing at elevated temperatures, possible effects of thermal treatment should be considered. Thus, we measured the photoelectric response of two blank photodetectors obtained by annealing at 500 °C for 1 h in vacuum. The measured dark currents and photocurrents reveal very little difference from the sample without annealing, which suggests that the defect structures of GaN (grown at 1100 °C) will not be changed by annealing at 500 °C. Figure 3c shows the I-V curves of the blank sample and Au500C under UV and green light illumination, which shows an Ohmic like behavior. The slopes of Au500C in the UV and green light region are ~54 and ~64 times greater, respectively, than those of the blank sample, in agreement with the photoelectric response results, further demonstrating the impact of Au NPs on increased the photocurrents. Figure 3d shows the photocurrentpower curves for the blank sample and Au500C under green light illumination with the optical power varying from 10 μ W to 100 μ W. The photocurrents for the blank sample increase slightly with the optical power, while the photocurrents for Au500C increase drastically and start to saturate after the power reaches 40 μ W. Figure 3e and 3f show the photocurrent-time curves obtained by turning the light sources on and off every 10 s. The photocurrents for Au500C are much greater than that of the blank sample. Notably, the rise/fall times are smaller for UV (2.9 s/6.2 s) than that for green light (7.7 s/8.2 s). The experimental data can be fitted by where τ 1 and τ 2 are the time constants and A 1 and A 2 are the fitting constants for the slow and fast components, respectively. It should be noted that reducing the interdigitated finger spacing will also reduce the response time.
Mechanism for the Dual-Band Detection. For a coplanar photodetector, most of the photocurrent is generated from the near surface region. We perform finite element method simulation (FEM, Comsol Multiphysics) to examine the current distribution for a blank coplanar GaN photodetector with the same geometry and a bias voltage of 0.8 V, identical to the parameters used in our experiments. The resistivity of GaN was set to be 0.05 Ω •cm. As expected, the current density decreases exponentially from the surface to the bottom, as shown in Fig. 4a. An integration of the data reveals that the current in the top 100 nm layer of the GaN is ~7.5 times greater than that in the bottom.  Since Au NPs can concentrate light into the near surface region of a dielectric 28,29 , we perform FEM calculations of the scattering efficiency of a hemispherical Au NP (r = 25 nm). We chose 360 nm and 530 nm as the typical wavelengths for UV and green light, respectively. Figure 4b shows the spherical computation domain surround by a perfectly matched layer (PML). The thickness of the PML is set to be half of the incident wavelength. The maximum mesh size (MMS) in the Au NP is set to 5 nm, while the MMS in other domains is set to 1/10 of the incident wavelength. The light is represented by a plane wave beam transmitted in the negative z direction and polarized along the x axis. The background field was calculated according to the Fresnel law. Due to the symmetric boundary conditions, only half of the domain was calculated. The scattering efficiency for the Au NP in air resembles that of a dipole, characterized by identical forward and backward scattering profiles (Fig. 4c). In contrast, the scattering efficiency for the Au NP on GaN is quite different. The Au NP scatters most incident light forward into GaN (Fig. 4d) due to high density optical modes in GaN because light scattered with high in-plane wave-vectors that are evanescent in air can propagate in GaN 30 . Note that the scattering efficiency for 530 nm is greater than that for 360 nm.
We then estimate the photocurrent enhancement ratio by integrating the energy density (Fig. 4d) over the near surface region multiplied by the current density curve (Fig. 4a) under the assumption of a 100% photon-to-electron conversion efficiency for both samples with and without Au NPs. Our calculation reveals that the Au NPs can increase the photoelectric responsivity by 4.4 and 3.1 times for 360 nm and 530 nm, respectively, both of which are smaller than the measured values. A literature review reveals that plasmonic photodetectors fabricated on non-polar semiconductors usually exhibit low photoelectric responsivity, in particular reduced responsivities in the UV region [31][32][33] . Consequently, we propose that the enhanced photoelectric response could originate from high yield injection of optically excited hot electrons 34 by the polarization field of GaN. The hot electrons come from interband transition of the filled d-band electrons of Au NPs (with its upper edge ~ 2.3 eV below the Fermi level) upon UV illumination, as shown in Fig. 5a.
This new principle based on polarization enhanced charge transfer was verified experimentally on N-polar GaN due to the difference in the direction and magnitude of the electric field from Ga-polar GaN 12 . The N-polar GaN used in our study were free standing wafers with similar doping concentrations and conductance to the Ga-polar GaN. A blank sample and a sample covered with Au NPs obtained under the same experimental procedures as Au500C were selected to illustrate the results (abbreviated as N-Blk and N-Au500C, respectively).  Figure 5b shows the photoelectric response curves for both samples. The photoelectric responsivity of N-Au500C to UV light is ~4 times of the blank sample, much smaller than the enhancement ratio for the Ga-polar sample. We also estimate the built-in electric field, E bi , for the Au/GaN interface by using the classical model for Schottky barrier 35 given by E bi = Φ B /W, where Φ B is the Schottky barrier height and W is the width of the depletion layer. E bi was calculated to be < 10 5 V/cm, much smaller than the spontaneous polarization field of GaN (~10 7 V/cm). Therefore, we believe that the polarization field does play a significant role in the collection of hot electrons, similar to hot carrier collection in metal-insulator-semiconductor (MIS) heterostructures 36 .
The enhanced photoelectric responsivity for green light is different from that for UV light. Besides the polarization enhanced charge transfer, the saturation of photocurrent (Fig. 3d) suggests the charge carriers originate from the ionized defects, as shown in Fig. 5c. Microstructural defects, such as point defects, impurities, and their complexes, are usually abundant in GaN wafers [37][38][39] . Their presence breaks the local crystallographic symmetry, creates additional energy levels within the band gap, and enables optical absorption and emission in the visible region. Figure 5d shows a typical PL spectrum of the blank GaN sample characterized by a broad green band. The green band is closely linked to the gallium vacancy (V Ga ) and its complex with substitutional oxygen (V Ga − O N ) 40,41 , which are the dominant defects in n-type GaN 42 . The 30 nm difference can be attributed to defect ionization, as discussed above. Note that only forward scattered light with energy greater than the defect levels can promote defect ionization which contributes to the photoelectric response.
We use the classical theory for a coplanar photoconductor 43 to calculate the carrier generation rate (G) of the plasmonic photodetector. For an optical power of 40 μ W at 525 nm, the number of photons that reach the photodetector per second is 1.06 × 10 14 , corresponding to ~6.6 × 10 9 /cm 2 for a carrier lifetime of ~10 μ s 42,44 greater than the defect density (~1.6 × 10 9 /cm 2 ) for our samples. Therefore, we expect that all V Ga and V Ga − O N in the top surface region of the photodetector have been ionized, resulting in an external quantum efficiency (EQE) of ~27.6% for Au500C, possibly resulted from coupling between LSPRs and the defect levels in a way similar to the

Conclusion
We have developed a dual band plasmonic photodetector on GaN for detection of UV and green light, respectively. We discover that Au NPs covered on Ga-polar GaN can drastically enhance the photoelectric responsivities by ~50 and ~60 times in response to UV and green light, respectively. The enhancement ratios are much greater than the values obtained by FEM simulation, 4.4 and 3.1 times for UV and green lights, respectively, on the photodetectors with the same geometry. A comparative study of similar photodetectors on N-polar GaN reveals a reduced enhancement ratio of ~4. It is concluded that polarity plays an essential role on the measured photocurrent and Ga-polar surface can greatly increase collection efficiency of hot electrons in plasmonic nanostructures upon optical excitation. Possible coupling between LSPR and microstructural defects are also discussed. This novel design principle offers a new avenue for adopting polar semiconductors to high performance optoelectronic devices.