GaAlAs Photocathodes Research Articles

Reflection-mode nanostructured GaAlAs photocathode with narrow-band response to 532 nm

The underwater photoelectric detection equipment mainly uses 532 nm laser as the light source, but the corresponding photocathodes like Na2KSbCs, GaAs and GaAsP have a wide spectral response region and are easily affected by other signals. Thereby, GaAlAs are materials worth developing because of their adjustable band gap, which usually is used as a window layer of GaAs-based photocathode. In this paper, the finite difference time domain (FDTD) method is used to carry out nanostructure design simulations. The results show that GaAlAs with Al component of 0.63 can obtain the cutoff wavelength near 532 nm, which is an excellent photocathode material to meet the requirement of narrow-band spectral response of 532 nm laser. Furthermore, the light absorptance can be improved effectively by the quadrangular prism or cylinder nanostructured array prepared on the Ga[Formula: see text]Al[Formula: see text]As emission layer surface, and the maximum light absorptance can reach 96.2% at 532 nm for the cylinder nanostructure array with a height of 900 nm and a base width of 100 nm. Nevertheless, the reflection-mode Ga[Formula: see text]Al[Formula: see text]As photocathode with the quadrangular prism nanostructured array can be slightly influenced with incident angle of light.

Femtosecond ultrafast dynamics study on the photoemission performance of reflection-mode GaAlAs photocathode

The photoemission characteristic of reflection-mode GaAlAs photocathode has been investigated, while a convenient and nondestructive method for evaluating the photocathode performances will be introduced. In this paper, two same reflection-mode photocathode structures with different research methods are adopted. One way is using multi-information measurement system to measure the photoemission characteristic curve of GaAlAs photocathode. Another method is the femtosecond transient reflection spectroscopy. The ultrafast dynamics property of the GaAlAs carrier is studied with the femtosecond transient reflectivity change, and the quantitative calculating is also shown. According to the results of two experimental methods, we analyze some photoemission performance parameters, such as carrier lifetime, diffusion length etc. The results show that the surface electron escape probability P and the drift length LDE are different, while are the same of the back recombination velocity Sv by the two method. In a word, compared with multi-information measurement system, the femtosecond ultrafast dynamics method is more suitable to research on the photoemission characteristic of reflection-mode GaAlAs photocathode which is no damage and accurate.

Femtosecond pump-probe spectroscopy investigation of carrier dynamics in GaAlAs photocathods

Based on the study of GaAs photocathode, the carrier dynamics of reflection-mode GaAlAs photocathode has been investigated. In this paper, the reflection-mode GaAlAs/GaAs photocathode is grown by metal organic chemical vapor deposition (MOCVD) epitaxial technology. The femtosecond pump-probe spectroscopy is performed to measure carrier dynamics in GaAlAs and GaAs photocathods. After laser excitation the surfaces of GaAlAs/GaAs photocathode, the reflectivity of photocathode surface is changed. At the same time, the dynamic processes of the non-equilibrium carrier distribution in semiconductor material is obtained by the transient reflectance spectrum. Compared with GaAs photocathode, the Al element in GaAlAs photocathode is unfavorable to photoemission under some conditions, while it solves the problem that the spectrum response range of binary compounds is not adjustable. The change trend of initial relaxation transient of GaAs photocathode in the 22 ps range is the same as GaAlAs, while the change of GaAs and GaAlAs relaxation state is significantly different with the change of delay time after 22 ps. The results show that the GaAlAs increases band gap due to a significantly slower than GaAs photocathode in initial process. Meanwhile, this paper provides a reference for the preparation of the GaAlAs photocathode that only respond to blue and green light range.

High quantum efficiency transmission-mode GaAlAs photocathode with a nanoscale surface structure

To obtain a high quantum efficiency transmission-mode GaAlAs photocathode, a photocathode with a graded Al composition structure is designed, in which the Al composition of the emission layer is decreased gradually from bulk to surface, and the outermost layer is GaAs. Based on the graded Al composition structure, GaAlAs photocathodes with different thicknesses of GaAs layers are prepared. The experimental results show that the quantum efficiency of the GaAlAs photocathode in blue-green light significantly increased when the thickness of the outermost GaAs achieves a dozen nanometers. The peak quantum efficiency of a GaAlAs photocathode with a nanoscale GaAs layer could achieve 31%, which appears at 620 nm. The nanoscale surface structure has a quantum confinement effect, which is the main factor in the increased quantum efficiency of the GaAlAs photocathode.

Experimental and theoretical research on heat cleaning process of GaAlAs photocathodes

During the heat cleaning process of GaAlAs photocathodes, Ga2O3 and Al2O3 occur through chemical reaction. Adsorption models with Ga2O3 and Al2O3 on Ga0.5Al0.5As(001)β2 (2 × 4) phase are built and calculated through first-principles method. A heat cleaning experiment is performed. Combining the experimental and calculation results, heating temperature and chemical reactions at the surface are analyzed. Work function, electron transfer and dipole moment are analyzed, the reason why quantum efficiency of GaAlAs photocathodes are much lower than that of GaAs photocathodes is analyzed. Additionally, band structure and DOS curves are compared.

Study of cesium and oxygen adsorption on surface of GaAlAs photocathode in ultra-high vacuum chamber

The mechanism of cesium (Cs) and oxygen (O) adsorption on GaAlAs photocathode has been investigated. The models of Cs and O adsorption on GaAlAs surface are studied, and the electron affinity changing with Cs coverage on GaAlAs surface is calculated based on Topping model. The experiments of Cs activation on GaAlAs and GaAs are performed, and the Cs, O activation experiments with different heat cleaning temperatures are performed on GaAlAs photocathodes. The photocurrent and spectral response curves in the experiments are measured and analyzed. The results show that the Topping calculation of electron affinity changing with Cs coverage on GaAlAs surface is in consistent with the result of experiment, and the electron affinity nearly reaches the bottom of conduction band after the Cs activation. The GaAlAs phtocathode treated by 700°C heat cleaning could obtain a good photoemission performance after Cs, O activation.

Differences in stability and repeatability between GaAs and GaAlAs photocathodes

For the applications in vacuum photodetectors and photoinjectors, a crucial limiting factor for conventional GaAs photocathodes is the limited lifetime, depending on the Cs–O activation layer vulnerable to the harmful residual gases. In order to develop a type of GaAs-based photocathode with good stability and repeatability, Cs/O activation and multiple recesiation experiments under the same preparation condition were performed on reflection-mode exponential-doped GaAs and GaAlAs photocathodes grown by metalorganic vapor phase epitaxy, and quantum efficiency and photocurrent decay were measured after activation and recesiation. The experimental results show that the photoemission characteristics on cathode degradation and repeatability are different between GaAs and GaAlAs photocathodes. In an unsatisfactory vacuum system, the operational lifetime for GaAlAs photocathode is nearly twice longer than that for GaAs photocathode after Cs/O activation under a high intensity illumination. After multiple recesiations, the quantum efficiency and operational lifetime for GaAlAs photocathode remain nearly unchanged, while those for GaAs photocathode become lower and lower with the increase of recesiation cycles, which reflects the superiority in stability and repeatability for GaAlAs photocathode in contrast to GaAs photocathode operating in the poor vacuum environment.

A density functional theory research on Cs–O activation process of GaAlAs photocathodes

Using plane-wave ultrasoft pseudopotential based on first-principles, the Cs–O activation process of GaAlAs photocathodes is investigated. In the Cs–O alternate activation process, Cs–O unit is inclined to form the Cs-outer structure. After the adsorption of O atom, the additional O atoms weaken the dipole between Cs atoms and β 2 (2 × 4) surface and introduce additional dipole between Cs and O. Dual-dipole model is used to express the variation tendency of dipole moment and work function. During activation process, electrons of Cs atoms offset to the surface, the additional electrons at the surface cause n-type surface state, and result in downwards band bending; at the same time, the vacuum level is gradually lowered, forming two potential barriers. An experiment is performed and the photocurrent curve is well consistent with the calculation results. After Zn doping, the work function near Zn atoms is lower than surrounding, causing the scale-like effect.

Structural Design and Experiment of Narrow-Band Response GaAlAs Photocathodes**Supported by the National Natural Science Foundation of China under Grant Nos 61171042 and 61301023, and the Introducing Talent Scientific Initial Foundation of Nanjing Institute of Technology of China under Grant Nos YKJ201320, YKJ201322, and YKJ201323.

To obtain the peak response at 532 nm, narrow-band response GaAlAs photocathodes with two GaAlAs active layers of different aluminum compositions are designed in consideration of the maximum absorptivity and quantum efficiency. The transmission-mode and the corresponding reflective-mode photocathodes are grown by metalorganic chemical vapor deposition. The results indicate that the peak response and the cut-off wavelength occur at 532 nm for the two kinds of photocathodes respectively. The response of the reflection-mode photocathode is an order of magnitude higher than that of the transmission-mode photocathode, whereas the better growth quality and the thicker second GaAlAs active layer can improve the transmission-mode response.

Influence of Zn doping on the early activation stage of GaAlAs photocathodes: A density functional theory research

Abstract Using plane-wave ultrasoft pseudopotential method based on density functional theory (DFT), the influence of Zn atoms on Cs activation process of NEA GaAlAs photocathodes is researched. The Zn-induced E-Mulliken population and ionicity change are analyzed, the influence of Zn on average dipole moment and work function are researched and the density of state and band structure of Cs adsorbed Ga 0.5 Al 0.5 As (0 0 1)β 2 (2 × 4) reconstruction surface before and after Zn doping are compared. Results show that Zn doped Ga 0.5 Al 0.5 As (0 0 1)β 2 (2 × 4) reconstruction surface shows Negative Electron Affinity (NEA) state when Cs coverage is 0.75 mL.

Effect of Cs adsorption on the photoemission performance of GaAlAs photocathode.

The effect of Cs adsorption on the photoemission performance of a reflection-mode GaAlAs photocathode in an ultrahigh vacuum chamber has been investigated. The experiments for Cs/O activation, multiple recaesiation, and degradation are performed on a GaAlAs photocathode. Meanwhile, the Cs/O activated and recaesiated photocurrent curves, degraded photocurrent curves, and spectral response curves are measured and analyzed. Besides, the performance parameters of the photocathodes are obtained by using the formula to fit with the experimental quantum efficiency curves. The results show that the Cs atoms not only make the atomically clean surface form the negative electron affinity, but also make the degraded photocathode recover to a good level. The quantum efficiency and the lifetime of GaAlAs photocathode become lower with increasing the recaesiation times.

Quantum efficiency study of the sensitive to blue–green light transmission-mode GaAlAs photocathode

The quantum efficiency of the blue–green transmission-mode GaAlAs photocathode has been studied. Two transmission-mode GaAlAs photocathodes with different structures are prepared, and the quantum efficiency curves are measured. We use the quantum efficiency formula to fit the experimental curves, and obtain the performance parameters of photocathodes such as the electron diffusion length, the back interface recombination velocity, and the surface electron escape probability. The effects of the Al compositions, the thickness of emission layer, and the electron diffusion length on quantum efficiency are investigated. The results show that both of transmission-mode GaAlAs photocathodes are sensitive to the blue–green light. The peak quantum efficiency of one photocathode appears at about 565nm, while that of another photocathode appears at about 535nm. The Al composition of emission layer plays a major role on the peak position of quantum efficiency of transmission-mode GaAlAs photocathode. Besides, the thickness of emission layer and the Al composition of window layer also have a large influence on the quantum efficiency.

Recesiation of GaAlAs photocathodes in an ultrahigh vacuum system

To seek a photocathode with good stability and repeatability in an ultrahigh vacuum system, activation and recesiation experiments are carried out on reflection-mode GaAlAs and GaAs photocathodes grown by metalorganic chemical vapor deposition, and the spectral response curves and photocurrent decay curves are measured after Cs/O activation and recesiation. Experimental results show that the photocurrent decay lifetime for GaAlAs photocathode illuminated by white light with an intensity of 100 lx is longer than that for GaAs photocathode after Cs/O activation and recesiation under ultrahigh vacuum condition. Moreover, GaAlAs photocathode exhibits a coincident blue-green response capability and a photocurrent decay lifetime after multiple recesiations, reflecting the superiority in stability and repeatability for GaAlAs photocathode operating in the vacuum system, and may provide an experimental guidance for exploring marine vacuum detectors and vacuum electron sources.

Roles of cesium and oxides in the processing of gallium aluminum arsenide photocathodes

The roles of cesium (Cs) and oxides in the processing of gallium aluminum arsenide (GaAlAs) photocathodes have been investigated. The Cs/O activation, Cs/O reactivation and degradation experiments are performed on GaAlAs photocathode. The activated photocurrent, degraded photocurrent and quantum efficiency curves are measured using a self-developed multi-information measurement system. We use the quantum efficiency formula to fit the experimental quantum efficiency curves, and obtain the performance parameters of the photocathode. The results show that the oxide gases play an important role in the degradation of the photocathode, and the Cs atoms could react with the oxides on the degraded surface and the residual oxide gases. Besides, the quantum efficiency of the degraded GaAlAs photocathode could be restored to a good level after Cs/O reactivation.

Comparison between exponential-doping reflection-mode GaAlAs and GaAs photocathodes

A reflection-mode GaAlAs photocathode and a reflection-mode GaAs photocathode using exponential-doping technique are prepared by metal organic chemical vapor deposition, and the Al content of GaAlAs emission layer is 0.63. The two photocathodes are activated in an ultra-high vacuum system, and the spectral response curves are measured after activation. The quantum efficiency formula for exponential-doping reflection-mode photocathode is used to fit the experimental curves of the two photocathodes respectively, and the effects of some performance parameters on photoemission are analyzed, such as electron diffusion and drift length, back-interface recombination velocity, surface electron escape probability, etc. The results show that the Al content of the GaAlAs photocathode plays a bad role in the photoemission compared with that the GaAs photocathode, but it solves the problem that the GaAs photocathode cannot be well used in the area of detecting the narrow wavelength light due to the broad spectral response. The reflection-mode GaAlAs photocathode prepared is responsive to the blue and green light.