Near-Infrared Absorption Enhancement of GaAs Photocathode Through “Sandwich” Micro-Nano Structure

Abstract

In this paper, a nano-layered transmission GaAs photocathode structure is proposed. The near-infrared absorption of the photocathode is enhanced by inserting a “sandwich” structure of nano-SiO₂ layer + Si₃N₄ nanopillar array + nano-SiO₂ layer between the cathode optical window and the photocathode. Compared with the flat film structure GaAs photocathode used in the current third-generations image intensifiers, the optical absorption of the optimized “sandwich” structure GaAs photocathode in the near-infrared band has been significantly improved: when the wavelength λ is 868 nm and 896 nm, the optical absorption is increased by 41.69%, 55.08%, respectively. The effects of structural parameters including film thickness and grating filling medium on the light absorption of photocathode are investigated. The results show that the near-infrared light absorption enhancement is the most obvious when Si₃N₄ is selected as the grating filling medium for the current design, and the deposition of SiO₂ film with 10 nm thickness could effectively prevent the damage of Si₃N₄ during bonding with the photocathode. The theoretical analyses offer important guidance in material selection and structural optimization in the grating cathode optical window used in the third-generation image intensifier for improving performance.

1. Introduction

Gallium arsenide negative electron affinity (NEA) photocathodes are the core photoelectric conversion components of third-generation image intensifiers. They enable efficient photoelectron excitation and emission, featuring high quantum efficiency (QE), low dark emission, narrow energy and angular distributions of emitted electrons, long-wavelength tunability, and great potential for wavelength response extension. Owing to these advantages, they are widely applied in spin-polarized electron sources, photodetectors, and low-light-level imaging systems [1,2,3,4,5]. In application scenarios of low-light-level night vision devices, photocathodes typically detect the reflected light of a scene. Taking grassland as a typical example, the reflected spectral energy of night-sky illumination is generally weak in the visible band but concentrated in the near-infrared region [6]. For conventional planar GaAs photocathodes, the intrinsic absorption coefficient and thickness limitations cause a rapid decrease in absorption efficiency in the long-wavelength region, resulting in significantly reduced quantum efficiency. This severely restricts the near-infrared detection capability of photoelectric imaging systems under extremely low illumination.

According to Spicer’s three-step model of photoemission [7], increasing the absorption of incident photons, enhancing photoelectron transport, and reducing the electron emission work function are three main approaches to improving photocathode quantum efficiency. Graded-doping or graded-bandgap structures [8,9,10,11] bend the energy bands and form internal built-in electric fields. These built-in fields facilitate electron transport toward the emitting surface, increasing their diffusion length and enhancing the spectral response of the photocathode. Chen et al. [2] grew a transmission-mode (t-mode) AlGaAs photocathode with a graded Al-composition structure using molecular beam epitaxy (MBE). When the outermost GaAs layer reached nano-scale, the quantum efficiency in the blue-green spectral region increased significantly. This is because nano-scale surface structures exhibit quantum confinement effects, which reduce the surface electron affinity and enhance the electron escape probability, thereby allowing more photoelectrons to be emitted into the vacuum. Enhancing optical absorption is also a common method for improving quantum efficiency. Typical structures include distributed Bragg reflector (DBR) structures and nano-array structures. Wang et al. [12] utilized a DBR to reflect specific wavelengths of transmitted light back into the emission layer to further enhance absorption, thereby improving the response of AlGaAs photocathodes at 532 nm. For t-mode multi-alkali photocathodes used in super-second-generation image intensifiers, a transmissive diffraction grating on the glass window to increase the optical path within the emission layer was proposed [13]. It is reported that the visible light and near-infrared light absorption is effectively enhanced without altering the photocathode thickness. With regard to negative electron affinity (NEA) GaAs photocathodes, existing studies indicate that, for both reflection-mode (r-mode) [14,15,16] and transmission-mode photocathodes [17], the prevailing approach is to nanostructure one of the photocathode layers. By contrast, design strategies that do not modify the intrinsic photocathode structure, similar to those adopted in super-second-generation image intensifiers, have only been reported for nano-scale NEA photocathodes [18].

In this work, a “sandwich” t-mode NEA GaAs photocathode structure consisting of a nano-SiO₂ layer, a Si₃N₄ nanopillar array (NPA), and another nano-SiO₂ layer inserted between the photocathode window was proposed. This configuration aims to enhance near-infrared (800–900 nm) absorption in GaAs photocathode. An optical model based on the finite-difference time-domain (FDTD) method was established, in which the photocathode structure is the same as the material composition and architecture of existing third-generation t-mode NEA GaAs photocathodes. The relationship between structural parameters of the nanopillar array and the 800–900 nm absorption of the photocathode was investigated. By adjusting these parameters, significant absorption enhancement was achieved. Additionally, the effects of film thickness and grating filling material on optical absorption were analyzed. These theoretical analyses that could be verified by the other applicable fields serve as the most important guidance for material selection and structural design of grating-type cathode windows for third-generation image intensifiers.

2. Materials and Methods

The unit cell of the designed “sandwich” nanostructured GaAs photocathode is shown in Figure 1a. The device is mainly comprised of a grating-type cathode glass window and a t-mode GaAs photocathode. The grating-type cathode glass window consists of an anti-halo photoelectric glass substrate, a SiO₂ layer, a hexagonal close-packed Si₃N₄ nanopillar array with its fill layer, and thin SiO₂ nano-layers. The GaAs photocathode stack includes a SiO₂/Si₃N₄ passivation layer, an Al_0.7Ga_0.3As window layer, an Al_0.2Ga_0.8As window layer, and the GaAs emitting layer. The nanopillar array is characterized with period P, pillar diameter D, and pillar height H. Compared with approaches that directly nanostructure the photocathode itself, it is unnecessary for the present design to consider the negative feedback of electron collection efficiency on quantum efficiency [15,19]. Without altering the structure of the GaAs photocathode, the photocathode optical absorption can be controlled by adjusting the structural parameters of the nanopillar array.

Optical modeling and simulations of the GaAs photocathode were conducted based on the finite-difference time-domain (FDTD) method. The simulation region and monitor arrangement are illustrated in Figure 1b. The horizontal boundary conditions of the simulation region were set to periodic, and the vertical boundary conditions were set to a perfectly matched layer (PML). The light source was a normally incident plane wave with a wavelength range of 500–1000 nm. The reflection (R) and transmission (T) were monitored by placing two-dimensional power monitors on the x–y plane in the SiO₂ window and vacuum, respectively. The reflection monitor (R monitor) was placed above the light source, and the transmission monitor (T monitor) was placed below the emission layer. The absorption (A) was calculated by A = 1 − R − T. The refractive indices and extinction coefficients for materials such as Si₃N₄ and GaAs were taken from references [20,21,22,23,24,25,26,27]. For the Si₃N₄ material, the main fitting parameters include fit tolerance 0.03 and max coefficient 6. The simulation mesh was set with mesh accuracy 3, and the nanopillar array region was subjected to mesh refinement, with a step size of 5 nm in the x, y, and z directions; i.e., dx = dy = dz = 5 nm.

3. Results and Discussion

3.1. Optimization of Nanopillar Array Structure

Focusing on achieving high absorption in the near-infrared short-wave range for the GaAs photocathode, the structure of the nanopillar array was firstly optimized. A parameter sweep of the Si₃N₄ nanopillar array structure was performed with a scanning interval of 10 nm, and the optimal structural parameters were determined as P = 680 nm, D = 400 nm, and H = 310 nm. The optical performance of the photocathode with this optimized structure is shown in Figure 2. In comparison with the typical flat film GaAs photocathodes used in third-generation image intensifiers, the nanopillar array photocathode exhibits a significant enhancement in absorption within the near-infrared band. In particular, at wavelengths of 868 nm, 896 nm, and 957 nm, the absorption is increased by 41.69%, 55.08%, and 18.86%, respectively. Correspondingly, Figure 2b shows that the reflection at these wavelengths decreases substantially, while the transmission increases to varying degrees. Since the wavelength λ = 957 nm exceeds the cutoff wavelength of GaAs photocathodes, the enhancement at this wavelength is of limited practical significance and not be discussed further. To investigate the mechanism by which this nanostructure enhances light absorption, the magnetic field intensity distributions at λ = 868 nm and 896 nm were analyzed through the vertical cross-section of the Si₃N₄ nanopillars, as shown in Figure 3. Local magnetic field hot spots are observed to be periodically distributed along the x-axis within the Si₃N₄ nanopillars. This distribution matches the characteristic features of guided-mode resonances excited in waveguide gratings [28], indicating that guided-mode resonance is excited in the nanopillar array structure.

When incident light interacts with the grating, multiple diffraction orders are generated. Guided-mode resonance occurs when the wave vector of a diffracted order matches the phase of the guided-mode wave vector, causing lateral propagation along the grating surface (the x-y plane) and confining the energy within the waveguide layer [29,30]. During propagation, the guided-mode experiences leakage due to the periodic perturbation of the grating. The wave leaking upwards undergoes destructive interference with the reflected wave, and the wave leaking downwards undergoes constructive interference with the transmitted wave. This results in a decrease in reflection and an increase in transmission. Consequently, the optical energy is efficiently directed toward the transmission side and absorbed by the underlying photocathode layers, thereby enhancing the absorption of the photocathode. In the subwavelength regime, only the zeroth-order diffracted light can propagate to the far field, while higher-order diffracted waves become evanescent. Thus, energy is exchanged solely between the zeroth-order reflection and transmission channels, leading to sharp reflection or transmission peaks at the resonance wavelengths. To further investigate this effect, the extinction coefficient of the absorbing material in the model was adjusted to avoid absorption, enabling observation of the intrinsic reflection and transmission characteristics of the nanostructure. At λ = 868 nm and 896 nm, the refractive index of GaAs is 3.54, and only the extinction coefficient k of GaAs is nonzero. For GaAs, the refractive index n was set to 3.54 and the extinction coefficient k was set to 0. The simulated optical response under these conditions is shown in Figure 4. Transmission and reflection peaks appear at 860 nm and 905 nm, respectively, further confirming the presence of guided-mode resonance. The slight deviation from 868 nm and 896 nm may be attributed to approximations in the material refractive index and extinction coefficient.

3.2. Influence of Varied Nanopillar Array Structure on Photocathode Absorption

During the fabrication of nanostructures, the structural parameters generally show a gap from the theoretical design due to inevitable errors introduced by lithography and etching. To determine the acceptable tolerance range, the effects of each structural parameter of the nanopillar array on the photocathode absorption were investigated based on the modelling optimized structure. Figure 5 shows the absorption spectra of photocathodes with different array periods. As the nanopillar period increases, the number, position, and peak values of the absorption peaks all change. Specifically, at λ = 868 nm and 896 nm, the peak absorption first increases and then decreases as the period increases, with a large amplitude of change. Existing studies on the influence of waveguide structural parameters on guided-mode resonance mainly focus on single-layer or double-layer waveguide grating structures. However, the nanostructured photocathode model in this work is a multilayer waveguide grating structure. The influence of the nanostructure parameters on photocathode absorption differs from the trends predicted by the conventional guided-mode resonance theory. This difference could be attributed to the mutual influence between guided-mode resonance and effects such as thin-film interference. The effects of nanopillar diameter and height on the photocathode absorption are shown in Figure 6, and they exhibit similar trends. Taking diameter as an example, the peak absorption at λ = 868 nm and 896 nm first increases and then decreases as the diameter increases. At λ = 868 nm, the peak varies slowly with diameter, resulting in a wider bandwidth for the peak absorption–diameter curve. At λ = 896 nm, the variation is faster and the curve is narrower.

3.3. Influence of Refractive Index of the Grating Filling Material on Photocathode Absorption

Keeping the structural parameters unchanged, the effect of the filling material’s refractive index on the photocathode absorption was investigated. The filling material Si₃N₄ (n ~ 2.06) was modified to other dielectrics including Al₂O₃ (n ~ 1.68), HfO₂ (n ~ 1.89), Ta₂O₅ (n ~ 2.13), and TiO₂ (n ~ 2.5), and the comparative results are shown in Figure 7. The refractive index has a significant regulatory effect on the photocathode’s light absorption, with clear performance differences among the various grating filling materials. In the visible light band and parts of the near-infrared short-wave region (λ < 850 nm), the absorption of the grating photocathodes with all filling materials is lower than that of the flat film photocathode. When λ is over 850 nm, however, the grating structure photocathodes in all groups exhibit several absorption peaks, showing a distinct enhancement compared to the flat film photocathode. Furthermore, the peak positions and values of the absorption peaks vary among the different filling materials. According to the optical absorptivity in the near-infrared short-wave region and considering the cut-off wavelength of the GaAs photocathode, the integral absorptivity of the photocathode in the 850–950 nm band under different filling materials is calculated (

Table 1. Integrated absorptivity of photocathodes in 850–950 nm band with different grating filling materials.

Material	Integrated Absorptivity in 850–950 nm Band
Al2O3	25.35%
HfO2	24.09%
Si3N4	31.18%
Ta2O5	25.68%
TiO2	22.97%
Flat film	18.54%

), and it is concluded that Si₃N₄ is the best grating filling material for optical absorption enhancement.

3.4. Influence of SiO₂ Thickness on Photocathode Absorption

Considering the requirements of practical fabrication processes, in order to prevent the Si₃N₄ from being damaged during the bonding of the photocathode optical window with the GaAs epitaxial wafer, a SiO₂ thin film is deposited on the surface of Si₃N₄ filling material. This allows the bonding process to occur between two SiO₂ surfaces, improving material compatibility and process adaptability. Therefore, the influence of the SiO₂ film thickness (d₁) on the optical absorption was investigated, and the results are shown in Figure 8. As the film thickness increases, the absorption peak at λ = 868 nm decreases, while the peak at λ = 896 nm first increases and then decreases. Based on the simulation results, the integrated absorption in the near-infrared region indicates that the system achieves the optimum absorption and protection from damage when the SiO₂ film thickness is 180 nm. Since the GaAs epitaxial wafer used in third-generation image intensifiers already contains a native 170 nm thick SiO₂ layer on its surface, a 10 nm thick SiO₂ film can be deposited on the Si₃N₄ during grating fabrication. This configuration not only achieves the optimal optical absorption but also protects the Si₃N₄ filling material from structural damage during the bonding process.

3.5. Discussion

This design focuses on the GaAs photocathode used in third-generation image intensifiers. Without altering the original photocathode structure, enhanced optical absorption in specific wavelength bands is achieved. Meanwhile, micro-nano fabrication processes are taken into consideration, and corresponding investigations and optimizations are conducted to facilitate practical fabrication and production. At present, the commonly adopted approach for nanostructured NEA GaAs photocathodes is to directly pattern the photocathode surface. Taking a Mie-type nanopillar array reflective GaAs photocathode [15] as an example, the generated photoelectrons are highly localized inside the nanopillars, mainly near the top surfaces and sidewalls. However, not all of these photoelectrons can be effectively transported and emitted into the vacuum. In the absence of space-charge effects and external electromagnetic fields, only electrons emitted within a limited angular range can be collected and thus contribute positively to the quantum efficiency. Electrons emitted outside this angular range are likely to be trapped by the nanopillar sidewalls (e.g., laterally emitted electrons), which leads to a negative impact on the QE. In contrast, the proposed design preserves the planar structure of the GaAs photocathode. As a result, issues related to electron collection efficiency and its negative feedback on the quantum efficiency do not need to be considered.

4. Conclusions

This study proposes a nanostructured t-mode GaAs NEA photocathode in which a Si₃N₄ nanopillar array grating is employed to enhance optical absorption while preserving the original photocathode structure and thereby improve photoemission performance. By reproducing the composition and configuration of the t-mode GaAs NEA photocathodes used in existing third-generation image intensifiers, the optimized structural parameters of nanopillar array as P = 680 nm, D = 400 nm, and H = 310 nm for the modelled photocathode structure are achieved. The significant absorption enhancement in the near-infrared region, especially at characteristic wavelengths of 868 nm and 896 nm, are realized, showing the absorption increases by 41.69% and 55.08% in comparison with a typical flat film GaAs photocathode. Fully considering the micro-nano fabrication processes in practice, the structural parameters including the period, diameter, and height of the nanopillar array are adjusted to investigate the influence on optical absorption. It is shown that the period should be focused on more in controlling the optimum value. According to the comparison with the other grating filling materials, Si₃N₄ is indeed the most effective filling material for enhancing near-infrared absorption with the designed structure parameters due to the suitable refractive index. During grating fabrication with Si₃N₄ as the filling material, a 10 nm SiO₂ layer may be deposited on the Si₃N₄ surface to protect the Si₃N₄ from structural damage during the bonding process without negatively impacting the light absorption performance. In future work, grating-type cathode optical window samples will be fabricated according to the optimized parameters and bonded to GaAs photocathodes.