Muchun Jin

Blue-green reflection-mode GaAlAs photocathodes

In order to obtain the suitable photocathode which could be applicable for the field of ocean exploration, the p-type zinc (Zn)-doped reflection-mode GaAlAs photocathode sample using exponential-doping technique is grown by metal organic chemical vapor deposition, the Al component of GaAlAs emission layer is designed to be 0.63. After the chemical etching, the photocathode samples are heated in vacuum at high-temperature of 650°C and 600°C respectively, the vacuum variation curves during the heat cleaning are measured, which correspond to the desorption of oxides in the surface of GaAlAs emission layer. The (Cs, O) activation for the photocathodes is executed after heat cleaning. Different proportion of Cs and O is performed on the different photocathode samples. The activation photocurrent curves of two samples with different heat cleaning temperature show that the GaAlAs surface treated by higher heat cleaning temperature is more sensitive to the Cs-O adsorption. The photocathode activated with the larger Cs current has a shorter time to reach the first photocurrent peak, and also obtains a bigger final photocurrent peak. According to the measured spectral response curves, it could be found that a suitable heat cleaning temperature and a moderate Cs/O current ratio are very important to prepare high performance GaAlAs photocathode. The prepared reflection-mode GaAlAs photocathodes are response to the blue-green light, and the cut-off wavelength is at about 580 nm.

Spectral response of InGaAs photocathodes with different emission layers.

Three InGaAs photocathode samples with different emission layers were prepared using metal organic chemical vapor deposition and activated by Cs, O. The spectral responsivity curves of the three samples were obtained, and the quantum efficiency formula of the InGaAs photocathodes with multi-sublayers was derived. Results show that the InGaAs samples with thick emission layers have higher spectral responsivity, and the wavelength of the threshold decreases with the decrease of the In component. According to the performance parameters obtained by fitting the quantum efficiency of the experiment, it was found that a higher In component corresponds to a lower electron escape probability. Therefore, it is difficult to prepare InGaAs photocathodes with a high electron escape probability and a long threshold wavelength at the same time. By adding mini transition layers between the sublayers, the interface recombination velocity decreases, and the critical thickness of the sublayers increases. In conclusion, mini transition layers are very important for the preparation of InGaAs photocathodes capable of high performance.

Effect of surface cleaning on spectral response for InGaAs photocathodes

Photocathode surface treatment aims to obtain high sensitivity, where the key point is to acquire an atomically clean surface. Various surface cleaning methods for removing contamination from InGaAs photocathode surfaces were investigated. The atomic compositions of InGaAs photocathode structures and surfaces were measured by x-ray photoelectron spectroscopy and Ar ion sputtering. After surface cleaning, the InGaAs surface is arsenoxide-free, however, a small amount of Ga2O3 and In2O3 still can be found. The 1:1 mixed solution of hydrochloric acid to deionized water followed by thermal annealing at 525°C has been demonstrated to be the best choice in dealing with the surface oxides. After the Cs/O activation, a surface model was proposed where the oxides on the surface will lead to a positive electron affinity, adversely affecting low-energy electrons escaping to the vacuum, which is reflected by the photocurrent curves and the spectral response curves.

Research on quantum efficiency for reflection-mode InGaAs photocathodes with thin emission layer.

In order to understand the photoemission mechanism of the reflection-mode InGaAs photocathode with a thin emission layer, the formula describing reflection-mode quantum efficiency is revised by solving the one-dimensional continuity equation, in which the electrons generated in the GaAs buffer layer are considered. Compared with the conventional formula, the revised formula is proved to be more suitable for the reflection-mode InGaAs photocathode with a thin emission layer. In experiment, the InGaAs sample goes through two-step surface preparation including a wet chemical cleaning process and a heat treatment process. Then the sample is activated by Cs/O and the experimental quantum efficiency curves are measured simultaneously every other hour. The measured results show that the shapes of the quantum efficiency curves degrade with time because of the contamination of residual gases in the vacuum system. All the quantum efficiency curves are well fitted by the revised formula.

Photoemission behaviors of transmission-mode InGaAs photocathode

Based on the studies of the GaAs photocathode, the surface model of the InGaAs photocathode is investigated and the energy distributions of electrons reaching the band bending region, reaching the surface and emitting into vacuum are calculated. We use the quantum efficiency formula to fit the experimental curves, and obtain the performance parameters of the photocathode and the surface barrier parameters. The results show that the electron escape probability is seriously influenced by energy distribution and plays an important role in the research of high quantum efficiency as well. After the theoretical calculation, the energy range of electrons crossing the BBR broaden, the peak of the electron energy distribution shifts forward to low energy, the number of low energy electrons increases obviously; The surface barriers of the InGaAs photocathode is similar to that of the GaAs photocathode. The height of barrier II not only decreases the number of electrons, but also makes the width of electron energy distribution narrow. The prepared transmission-mode InGaAs photocathode contains 20% InAs and 80% GaAs. This combination of InGaAs photocathodes is widely used in the weak light detection field, such as night vision technology, forest fire prevention and harsh climate monitoring.