Negative electron affinity (NEA) GaAs photocathodes have attracted a wide scope of interest because of their high quantum efficiency and low dark emission. Traditionally, fabrication of GaAs photocathodes has taken two approaches: molecular beam epitaxy (MBE) and metal–organic chemical vapor deposition (MOCVD). Understanding the difference between these two methods in terms of device performance can help guide future device development. While past research has indicated that photocathodes grown by MOCVD generally have better spectral response and quantum efficiency, these reports are all based on steady-state analysis and measurement. There has been little prior work comparing the dynamic response of devices fabricated with different technologies. In this presentation, we report a comparative study of the ultrafast response of two gradient-doped GaAs photocathodes fabricated using two different methods, viz. MBE and MOCVD. Our approach is based on femtosecond pump-probe reflectometry (PPR), which measures the transient reflectivity of these devices upon optical excitation by femtosecond pulses. Preliminary PPR result shows that carrier build-up near photocathode surface in the MOCVD device is more efficient compared to the MBE device. A carrier-diffusion model is used to analyze photoelectron transport, accumulation, and decay in the active layer. Experiment-theory comparisons indicate a bi-exponential nature of free-electron population decay near device surface. Excellent agreement between theoretical predictions and measured data not only validates the numerical model but also allows various device parameters to be evaluated quantitatively.
Using first-principle calculations, we compare the quantum efficiency and stability of Cs-GaN planar model and Cs-GaN nanowire model. The results show that the work function of GaN nanowire photocathodes decreases continuously with the increase of θCs, the “Cs-kill” phenomenon disappears, resulting in a lower work function (1.76 eV) than the conventional GaN planar photocathodes (1.82 eV). However, we find that the nanowire GaN photocathodes had a lower stability by calculating the adsorption energy. In addition, the surface atomic structures of both kinds of photocathodes are almost identical, which account for the similarity of their best adsorption sites. Our study is helpful to the growth of GaN nanowire materials in the future and can be used to guide the improvements of GaN-based equipment photoelectric efficiency.
Significant advance has been made over the last decade in the development of broadband optoelectronic devices based on novel technologies such as 2D materials, metamaterials, plasmonics, negative electron affinity photoemission, etc. Understanding carrier dynamics in such devices, especially carrier relaxation and transportation near device surfaces, requires time-resolved, broadband reflective spectroscopy with femtosecond temporal resolutions. Femtosecond pump-probe reflectivity measurement (PPRM) has long been used to study carrier dynamics in semiconductor devices. However, conventional PPRM lacks the necessary bandwidth and the ability to make spectroscopic measurement. In this presentation, we report the demonstration of wavelength-resolved transient reflectivity measurement using a ultrabroad-band few-cycle pump-probe system. The system allows device transient reflectivity to be mapped onto a two-dimensional space formed by time and wavelength, providing a comprehensive characterization of ultrafast carrier dynamics. Preliminary results based on a GaAs substrate and GaAs/AlGaAs layered structures have offered interesting insights into device dynamics that otherwise would not be clear. These results demonstrate the feasibility of performing wavelength-resolved transient reflectivity measurement and the effectiveness of this technique in characterizing broadband optoelectronic devices.
Negative electron affinity (NEA) photocathodes have attracted a lot of interest over the last two decades due to their high quantum efficiency and low dark emission, which are desirable for night vision and other low-light applications. Recently, gradient-doping technique has shown promise to significantly improve the quantum yield of GaAs/AlGaAs heterojunction photocathodes by assisting electron diffusion toward the surface. In the present work, femtosecond pumpprobe transient reflectivity measurement has been used to study the ultrafast carrier dynamics in NEA GaAs/AlGaAs photocathodes. The research focuses on the comparison between a traditional, uniform-doped structure (1.7 μm p-GaAs (1×1019 cm-3) / 0.7 μm p-Al0.57Ga0.43As (3×1018 cm-3) / si-GaAs substrate) and a gradient-doped structure (0.1 μm pGaAs (1×1018 cm-3) / 1.2 μm p-Al0.63Ga0.37As (doping level gradually changes from 1×1018 cm-3 to 1×1019 cm-3) / 0.5 μm p-GaAlAs (1×1019 cm-3) / si-GaAs substrate). Our result indicates that gradient doping not only leads to more efficient electron transportation but also results in better electron accumulation (i.e. higher concentration and longer lifetime) near device surface, a feature well-suited for photocathodes. Moreover, we have shown that pump-probe transient reflectivity measurement is able to offer a direct picture of electron diffusion inside NEA photocathodes, which can be of significant importance to device development.
II-VI colloidal quantum dots (QDs) are ideal for optical sensors thanks to their high fluorescent brightness and good size uniformity. However, embedding colloidal QDs into a glass matrix with the standard sol-gel process leads to the QDs being damaged by the acid catalyst. Here, we report an acid-free sol-gel technique, which proves to be both simple and effective in fabricating silica glass thin films embedded with commercial II-VI colloidal QDs. Octadecylamine ligands are used as a bifunctional aid to not only stabilize the QDs in solution, but also assist the formation of the SiO2 gel. We demonstrate that high-quality QD-embedded glass thin films can be developed with this technique, and our fluorescent tests indicate that, except for a small blueshift in the emission spectrum, the QDs are very well preserved through the sol-gel process. This method offers a fast and low-cost path towards thin-film QD sensors with good mechanical and thermal stabilities, which are desirable for applications involving highly focused laser beams, such as ultrafast nanophotonics.