Optical-Resonance-Enhanced Photoemission from Nanostructured $\mathrm{Ga}\mathrm{As}$ Photocathodes

A negative electron affinity photocathode based on $\mathrm{Ga}\mathrm{As}$ nanopillar-array (NPA) Mie-type resonators is demonstrated and significant quantum efficiency enhancement is observed. Nanophotonic resonance-assisted photoelectron emission into vacuum is investigated, indicating an enhanced density of optical states due to increased light concentration and increased electron emission area. For visible wavelengths, the Mie resonances in $\mathrm{Ga}\mathrm{As}$ NPA reduce light reflectivity to less than 6% compared to a typical value >35% and result in maximum quantum efficiency 3.5 times greater than a $\mathrm{Ga}\mathrm{As}$ wafer photocathode without the NPA structure. Comprehensive simulations and experimental studies are presented.

Figure 1

Hexagonal $\mathrm{Ga}\mathrm{As}$ NPA resonator photocathode. (a) Device structure. (b) Schematics of the fabrication process. (c) Ultrahigh vacuum chamber used for the NEA activation and QE measurement. (d) Time evolution of photocurrent during a typical “yo-yo” activation for a $\mathrm{Ga}\mathrm{As}$ NPA photocathode together with a photograph of one sample mounted to a holder for installation in the ultrahigh vacuum test chamber.

Figure 2

Photoemission model of the $\mathrm{Ga}\mathrm{As}$ NPA resonator photocathode. (a) The electronic band structure and photoemission process of the NEA photocathode based on Spicer's model, with letters A, B, and C signifying the processes of photoelectron excitation, electron transportation to surface, and escape from the surface into vacuum, respectively. (c) and (d) are the photoemission processes of $\mathrm{Ga}\mathrm{As}$ NPA and flat wafer photocathodes, respectively. The red color regions denote areas where light absorption is greatest and the blue arrows indicate the trajectories of photoemitted electrons. (d) TCAD setup for analyzing the photoelectric properties of the $\mathrm{Ga}\mathrm{As}$ NPA photocathode, with the electron density distributions [elec._density (cm⁻³)] on the three-dimensional surfaces (left) and on the vertical center cross section (i.e., the cross-section plane that contains the pillar axis of symmetry) of $\mathrm{Ga}\mathrm{As}$ NPA are illustrated (right), respectively. Note the two color bars are independent and not scalable to each other.

Figure 3

Simulated light absorption spectra in $\mathrm{Ga}\mathrm{As}$ NPA resonators. Absorption percentage indicated by color scale in all figures: (a) Dependence of η_a spectra upon diameter D when P = 600 nm and H = 750 nm. The black dots M1 (D = 120 nm) and M2 (D = 260 nm) correspond to wavelength and geometry combinations used in (b). (b) Normalized electric-magnetic field profiles (|E|-|H|, color) and field lines (white) in vertical crosscuts through the center of the nanopillars M1 (MD-ED resonance at 625 nm) and M2 (MQ-EQ resonance at 610 nm), respectively. (c) Dependence of η_a (color) spectra upon height H for dipole NPA resonator when P = 600 nm and D = 120 nm. (d) Dependence of η_a (color) spectra upon lattice period P for dipole NPA resonator when D = 120 nm and H = 750 nm.

Figure 4

Surface morphology and reflectance spectra of $\mathrm{Ga}\mathrm{As}$ NPA fabricated by SCIL on $\mathrm{Ga}\mathrm{As}$ substrate. (a) SEM image of the NPA on $\mathrm{Ga}\mathrm{As}$ substrate. (b) Measured (solid line) and calculated (dash line) reflectances of $\mathrm{Ga}\mathrm{As}$ NPA samples S1 (D = 120 nm, H = 350 nm, P = 600 nm), S2 (D = 260 nm, H = 750 nm, P = 600 nm), and S3 (D = 280 nm, H = 750 nm, P = 600 nm). The reflectance of a $\mathrm{Ga}\mathrm{As}$ flat wafer W1 is also measured and shown in (b).

Figure 5

Photoemission performances of the $\mathrm{Ga}\mathrm{As}$ NPA and flat wafer photocathodes. (a) Measured QE spectra of the $\mathrm{Ga}\mathrm{As}$ NPA samples S1 (D = 120 nm, H = 350 nm, P = 600 nm), S2 (D = 260 nm, H = 750 nm, P = 600 nm), S3 (D = 280 nm, H = 750 nm, P = 600 nm), and $\mathrm{Ga}\mathrm{As}$ flat wafer samples W1 (flat wafer exposed to air in this work), W2 (epiready flat wafer reported previously [15]). (b) Dependence of photocurrent and QE on operational time for $\mathrm{Ga}\mathrm{As}$ NPA sample S2 under the incident light wavelength of 630 nm and power of 15 µW.

Figure 6

Analysis of the $\mathrm{Ga}\mathrm{As}$ NPA photocathodes S1 and S2 by fitting the two-valleys model to the measured QE. (a) and (b) are the QE spectra of the samples S1 and S2. (c) The fitted escape probabilities of electrons emitted into vacuum from the Γ (P_Γ) and L (P_L) valleys. (d) and (e) are the calculated light absorption in $\mathrm{Ga}\mathrm{As}$ NPA S1 and S2 and the flat wafer. (f) Distribution of photoelectron generation rate in (pairs cm⁻³ s⁻¹) within the vertical cross sections of the samples S1 and S2 in the plane parallel to the E field of the linearly polarized incident light (light intensity is 1 W cm⁻²) at the resonance wavelength.