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Negative electron affinity GaAs wire-array photocathodes have been fabricated by reactive ion etching and inductively coupled plasma etching of bulk GaAs material followed by Cs-O activation. Scanning electron microscope has revealed that the thus obtained high-density GaAs wire arrays had high periodicity, large height, and good morphology. Photoluminescence spectra indicated the wire arrays were of good crystalline quality and free from any obvious damage. Compared to the original GaAs wafer, the photoluminescence peak positions of the wire arrays were somewhat red-shifted, which may be attributed to the temperature effect and strain relaxation. The wire-array structures showed significantly reduced light reflection compared with the original wafer due to the excellent light-trapping effect. Cs-O activation experiments of the GaAs wire arrays have been performed to reveal the effect of incident angle on quantum efficiency. The results show that maximum quantum efficiency was obtained at about 30°. Given these unique electrical and optical properties, a GaAs wire-array photocathode is an attractive alternative to its planar-structured counterpart.
© 2016 Optical Society of America
1. Introduction
The widespread use of negative electron affinity (NEA) gallium arsenide (GaAs) photocathodes during the last half century, encompassing many areas of research, provides ample testimony of the advantageous performance characteristics of this material. These include high quantum efficiency, high brightness, low dark current, good long-wavelength response, and narrow energy spread [1–5]. Recently, with the rapid development of space detection, high-resolution night-vision imaging, next-generation electron accelerators, low-energy electron microscopes, and electron beam lithography, an ever-pressing demand has arisen for photocathodes with higher sensitivity, wider spectral response range, higher emission current density, and higher spin polarization. Thus, some new GaAs-based photoemission material structures, such as graded doping/band-gap [6–8], strained [9], and superlattice structures [10–12], have been proposed to increase the quantum efficiency or spin polarization of GaAs-based photocathodes. However, all of these new photoemission materials rely on GaAs-based epitaxial thin films as the active layers of photocathodes. Thin-film planar photoemission materials have good quality epitaxial crystals and a uniform surface, but they are not without drawbacks. In GaAs photocathodes with planar geometry, the photoelectrons can only be emitted when they have been transported to the cathode surface. This scenario increases the chance of recombination and decreases the spectral response. In addition, the reflectivity over the wavelength region of interest for GaAs photocathodes with planar geometry is greater than 30%, which drastically affects the absorption of photons.
Recent advances in nanofabrication have enabled the utilization of nanostructured semiconductors, such as Si wire arrays, in photovoltaic and optoelectronic devices [13–15]. Semiconductor wire-array geometry is an attractive alternative to planar geometry because it offers both long optical paths for efficient light absorption and short transport distances so as to ensure collection of the photo-generated charge carriers before they undergo recombination [13–20]. Wire arrays have large surface area, high aspect ratio, and an intrinsic antireflection effect (which increases light absorption), and specifically designed arrays have the ability to direct light absorption [16]. In addition, due to the small radii of the constituent wires, the photoelectron transport distance to the emissive wire surface is significantly reduced, thus favoring photoelectron emission. These properties of wire arrays exactly overcome the deficiencies of traditional planar geometry photocathodes in terms of photon absorption and electron transport, making them very attractive for photoemission. In this work, we have fabricated and characterized NEA GaAs wire-array photocathodes, and have found some interesting properties that are completely different from those of their planar structure counterparts.
2. Experiment details
GaAs wire arrays can be fabricated by either bottom-up (growth) methods [21,22] or top-down (etching) methods [23,24]. Wire arrays obtained by growth methods have low dislocation and defect densities, but are limited by drawbacks such as uncontrolled growth orientation, inhomogeneity in diameter, and high cost. In this work, the top-down etching method, reactive ion etching and inductively coupled plasma etching (ICP-RIE), is used to fabricate GaAs wire arrays [25]. ICP-RIE has the significant advantage of using a coil RF power source to ionize the gas and another platen RF power to accelerate the ions toward the sample surface where etching takes place through ion bombardment. By increasing the thickness of the mask layer and reducing that of the photoresist layer, we have been able to prevent the top stripping encountered in previous studies [25], providing a suitable route to GaAs wire arrays.
The GaAs (100) samples used for etching were typically 1 cm2 p-type substrates. Vertically aligned GaAs wire arrays were fabricated by the following procedures. Firstly, an SiO2 layer of thickness 2.2 μm was deposited on the GaAs (100) substrate by PECVD (System 100, Oxford Instruments, Oxford, UK). Secondly, a 1.6 μm layer of AZ5214 photoresist was spun onto the SiO2 layer to allow exposure and transfer of the grating pattern. The sample was then subjected to RIE (Tegal 903e) to transfer the pattern to the SiO2 hard mask, thereby exposing the etching window. Thereafter, an optimized ICP (Oxford Plasmalab System 100 ICP180) process was carried out to obtain the GaAs wire array. The photoresist was subsequently cleaned with acetone, and the SiO2 mask layer was removed by treatment with buffered oxide etchant (BOE). Ultimately, high-quality GaAs wire arrays with high density, smooth surfaces, and straight sidewalls were fabricated. The height of the wires depended on the ICP etching time and the wire pattern.
Next, to obtain a favorable electron emission surface (NEA state) for the GaAs wire array, a very thin Cs/O layer had to be deposited on the top and sidewall surfaces of the wires. The Cs-O activation experiments on the GaAs wire-array photocathodes were performed in an ultra-high vacuum system (UHV, base pressure ≤ 3.3 × 10−8 Pa) connected to a spectral response measuring instrument [26]. Prior to activation, samples were subjected to chemical cleaning and heat treatment [26,27]. Heat cleaning of the GaAs wire arrays was performed in UHV by annealing (at 873 K) until no traces of contamination remained on the wire surfaces. The surfaces of the GaAs wire arrays were subsequently activated to the NEA state by the co-adsorption of cesium and oxygen [27,28].
The morphology of the GaAs wires was observed by means of a field-emission scanning electron microscope (SEM) (NOVA NANOSEM 450). Photoluminescence (PL) spectra of the GaAs wire arrays were measured with a LabRAM HR 800 micro-PL system with a 532 nm/6 mW laser. Reflectance spectra were measured by means of a Perkin-Elmer Lambda 750 UV/Vis/NIR spectrometer at an incident angle of 0° to analyze the optical properties of the GaAs wire arrays. The quantum efficiencies of the GaAs wire-array photocathodes were measured in situ by a spectral response measuring instrument. The experimental error in quantum efficiency measurements is less than 2%. The error is mainly caused by fluctuations of the optical power and the photocurrent error. All experiments were performed at room temperature.
3. Results and discussion
Figure 1 shows SEM images of as-grown GaAs wire arrays fabricated by ICP-RIE, which indicate that the high-density GaAs wire arrays were vertically well aligned. In Fig. 1(a) and (c), the wires are seen to be 3 and 5 μm in diameter, separated by periods of 6 and 10 μm, respectively, as designed. As shown in Fig. 1(b) and (d), the GaAs wire-array structures exhibited a uniform cylindrical shape with smooth surfaces and straight sidewalls, which was due to the optimization of the ICP-RIE method. The heights of the wires in Fig. 1(b) and (d) are 8.5 and 14.46 μm, respectively. It can be concluded that, utilizing the ICP-RIE technique, p-type GaAs wafers were successfully etched to form high-density wire arrays with high periodicity, large height, and good morphology.
Fig. 1 Top view ((a) and (c)) and side view ((b) and (d)) SEM images of GaAs wire arrays fabricated by ICP etching for 20 min ((a) and (b)) and 40 min ((c) and (d)).
Herein, we define the key parameters as the period structure of the square lattice P, the wire diameter D, and the wire height L. In this work, samples S1, S2, and S3 denote D/P = 0.5, D = 2 μm, 3 μm, and 5 μm, and L = 10.16 μm, 13.51 μm, and 14.46 μm, respectively; S4 and S5 are samples with D = 5 μm, D/P = 0.33 and 0.25, and L = 19.4 μm and 20.5 μm, respectively.
The optical properties of the periodic GaAs wire arrays were investigated by micro-PL measurements. Figure 2 shows a comparison of the PL spectra of GaAs wire arrays (diameters of 2 and 5 μm) and a GaAs wafer. It is noteworthy that the integral intensities of the band edge emissions of the wires in S1, S3, S4, and S5 were enhanced by factors of 1.31, 1.91, 1.94, and 1.78, respectively, compared to those of a polished GaAs wafer. The increase in PL spectral intensity can be mainly attributed to increased illumination light absorption in a GaAs wire-array structure, but it is also associated with other factors, such as the surface characteristics of the wires. The PL spectral parameters for the different samples are listed in Table 1. The full-widths at half-maximum (FWHM) of the peaks were 44-48 nm for the wires and 51 nm for the wafer. The results indicated that, after fabrication into microstructures, the GaAs wire arrays with different patterns were of good crystalline quality and free from any obvious damage due to the ICP etching process.
Fig. 2 PL spectra of GaAs wire arrays with (a) identical D/P but different diameters and (b) identical diameters but different D/P.
Table 1. PL spectral parameters for different samples.
The PL spectral peaks of these samples are located at 887−894 and 878 nm for the wire arrays and wafer, respectively. Compared to the GaAs wafer, the peak positions for the wire arrays are somewhat red-shifted. This red-shift becomes more obvious with decreasing diameter of the wires. The red-shift of the emission from nanowires (diameter 200 nm) fabricated by the ICP etching method is due to strain relaxation [29], whereas the blue-shift of the emission from nanowires (diameter 10-50 nm) fabricated by growth methods is due to radial quantum confinement [30,31]. However, the diameters of the wires in this work were 2−5 μm, and the shift in the PL peaks for microwires is independent of quantum effects. The red-shift for the microwires may be attributed to the temperature increase when the intense laser focuses on their ends and strain relaxation caused by ICP etching. The band gap of GaAs materials decreases with increasing temperature. The temperature increase of the GaAs wafer was smaller than that of the wire arrays due to faster heat dissipation from the former, resulting in a smaller PL peak red-shift. The effect is more pronounced for small diameter wires. When the laser intensity was reduced by 99%, such that the temperature effect for the wires could be neglected, we observed that the PL position for the wires was hypsochromically shifted by about 3 nm, but was still at about 8 nm longer wavelength than the PL position for the wafer. Therefore, beside the temperature effect, strain relaxation may also be responsible for the red-shift in PL from the microwires.
The reflectances of the GaAs wire arrays and of the original wafer were measured by means of a spectrometer, as shown in Fig. 3. As can be seen, the wire-array structures showed significantly reduced light reflection compared with the original wafer with a mirror-like surface. Figure 3(a) shows that the reflectance of S2 (D = 3 μm) was lower than that of S1 (D = 2 μm), but when the diameter was further increased to 5 μm (S3), the reflectance was increased. Figure 3(b) shows that the reflectance increased with increasing gap between the wires (decreasing D/P). This was due to insufficient absorption by the wire-array layer; the incident light reached the wire−substrate interface, resulting in notable reflection from the substrate. From these results, we can conclude that there is an optimal structure for wire arrays to minimize reflectance, which is determined by the trade-off between reflection enhancement and transmission suppression.
Fig. 3 Reflectances of wire arrays and the original wafer: (a) wires with the same D/P (0.5) but different diameters (2, 3, or 5 μm); (b) wires with the same D (5 μm) but different D/P (0.5, 0.33, or 0.25).
The GaAs wire array S3 (D = 5 μm, D/P = 0.5, L = 14.46 μm) was activated to the NEA state on its top and sidewall surfaces by co-adsorption of cesium and oxygen. Photocurrents were then measured in situ as a function of incident angle and wavelength by means of a spectral response measuring instrument. The GaAs sample was fixed on a sample holder connected to a magnetic manipulator. Transfer of the GaAs sample between different chambers was accomplished with translational and rotational magnetic manipulators. The magnetic manipulator could be rotated against an angle scale to change the angle of the incident light. The uncertainty associated with measurement of the incident angle θ was ± 1°. Figure 4 shows the quantum efficiency calculated from the photocurrents using the following equation (Eq. (1)):
where h is Planck’s constant, λ is the wavelength of incident light, I(λ,θ) is the photocurrent of the wire cathode, and P0 is the incident light power at an incident angle of 0°. The actual incident light power P is dependent on the incident angle, P = P0 × cos(θ).Fig. 4 Quantum efficiency of wire-array photocathodes for different incident angles as a function of incident light wavelength. In this work, we define the incident angle as 0° = normal incidence.
Figure 4 presents a very interesting phenomenon; with increasing incident angle, a peak appears in the quantum efficiency at about 30°, whereas the quantum efficiency for a wafer photocathode, as shown in Fig. 5, decreases with increasing incident angle. When the GaAs wire arrays were illuminated with an incident angle of 0°, only the top surfaces of the wires absorbed the light. Because reflection, absorption, and refraction between different wires are related to the incident angle of the light, the absorption of the GaAs wires increased with increasing incident angle, which can be attributed to an increase in absorption on their sidewall surfaces. However, when the angle reaches a certain value, the sidewall surface absorption of a wire gradually decreases because of the increased shading from other wires. These special properties of wire-array cathodes differentiate them from their wafer counterparts.
Fig. 5 Angle-resolved quantum efficiency of wire-array and wafer photocathodes under illumination with a 700 nm light. The quantum efficiency of the wire-array photocathode is assumed to be equal to that of the wafer photocathode at an incident angle of 0°.
With the increase in wavelength, the quantum efficiency in Fig. 4 slightly changes at an identical incident angle, which suggests a balance between photon absorption and electron transport. Considering that the absorption coefficient decreases abruptly near 870 nm, the quantum efficiency drops dramatically at this cutoff wavelength. The quantum efficiency at normal incidence (0°) is lower than those at other angles because of the vertical orientation of the wires. A theoretical treatment of a photoemission model based on ray-tracing techniques was adopted to calculate the spectral sensitivities of the GaAs wire-array photocathodes. The results revealed photocurrent and quantum efficiency (spectral response) peaks for the wire-array photocathodes with increasing incident angle [32]. The experimental results were in good agreement with the theoretical results.
Compared with a planar wafer cathode, the absorption of light and the escape of photoelectrons are completely different for the wire-array cathodes. Therefore, the response of the wafer cathodes should be quite different from that of the wire cathodes. The angle-resolved spectral responses of the wafer and wire-array cathodes per effective unit area are shown in Fig. 5. We assume that the quantum efficiency of wire-array photocathodes is equal to that of a wafer photocathode at an incident angle of 0°. Because the actual light power illuminated on a wafer cathode surface decreases with increasing incident angle, the quantum efficiency of the wafer cathode decreases accordingly. However, for wire-array cathodes, the quantum efficiency is maximized at about 30°. Overall, the quantum efficiency of wire-array cathodes is higher than that of the original wafer cathodes, due to both long optical paths and short transport distances in the wire arrays.
4. Conclusions
In conclusion, a novel photocathode based on GaAs wire arrays has been successfully fabricated. Well-ordered GaAs wire arrays have been observed by SEM. Analysis of the PL and reflection spectra has revealed that, compared with the GaAs wafer structure, the PL peak positions of wire arrays are somewhat red-shifted and the wire arrays show an excellent light-trapping effect. As a result, they can absorb incident photons more efficiently, leading to an enhanced spectral response. Cs-O activation experiments of the GaAs wire arrays have been performed to reveal the effect of incident angle on spectral response. With increasing incident angle, the quantum efficiency of the wire arrays first increased and then decreased. This is a unique feature of wire-array photocathodes. Given these unique electrical and optical properties, a well-ordered GaAs wire-array photocathode is an attractive alternative to its planar-structured counterpart, and has good potential applications in parallel electron beam lithography and high performance vacuum electron sources. However, although this work has achieved some promising preliminary results, there is still much scope for improving the performance of these wire-type photocathodes, such as improving the NEA activation processes, designing new wire-array structures (depth-graded doping and depth-graded composition), reducing wire diameter, and achieving independent control of photoemission for each wire.
Acknowledgments
This work was supported by the National Natural Science Foundation of China, China (Grant Nos. 61261009, 61301023), the Foundation of Training Academic and Technical Leaders for Main Majors of Jiangxi Province, China (Grant No. 20142BCB22006), the Key Program of Science and Technology Research of Ministry of Education, China (Grant No. 212090), and the Natural Science Foundation of Jiangxi Province, China (Grant No. 20133ACB20005).
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