Understanding the carrier dynamics in III-V semiconductor photocathodes is of great importance to the development of many novel optoelectronic systems such as low-light imagers and coherent x-ray sources. Past experimental effort to study ultrafast response of GaAs photocathodes has relied exclusively on post-photoemission measurement, i.e. via characterization of the temporal properties of emitted electron bunches. Such an indirect approach is not only difficult to achieve experimentally but also liable to sample surface conditions when assessing key carrier dynamics such as photoelectron diffusion and decay. In this report, we present a pre-photoemission investigation of photoelectron transport and population decay in a GaAs/AlGaAs photocathode based on pump-probe transient reflectometry. By measuring the ultrafast behaviors of reflectivity after the excitation of a femtosecond optical pulse, we are able to evaluate the transient evolution of photoelectron population near the device surface, which provides a direct picture of electron diffusion and decay. A carrier diffusion model shows excellent agreement with experiment. Experiment-theory comparisons also suggests a diffusion coefficient much greater than some of the post-photoemission predictions. In addition, the free-electron population decay near the surface is found to be of bi-exponential nature. Possible physical mechanisms underlying these decay lifetimes are discussed.

Pre-Emission Study of Photoelectron Dynamics in a GaAs/AlGaAs Photocathode

We report here a general theory describing photoelectron transportation dynamics in GaAs semiconductor photocathodes. Gradient doping is incorporated in the model through the inclusion of directional carrier drift. The time-evolution of electron concentration in the active layer upon the injection of an excitation pulse is solved both numerically and analytically. The predictions of the model are compared with experiments via carrier-induced transient reflectivity change, which is measured for gradient-doped and uniform-doped photocathodes using femtosecond pump-probe reflectometry. Excellent agreement is found between the experiments and the theory, leading to the characterization of key device parameters such as diffusion constant and electron decay rates. Comparisons are also made between uniform doping and gradient doping for their characteristics in photoelectron transportation. Doping gradient is found to be able to accelerate electron accumulation on the device surface. These results offer new insights into the dynamics of III-V photocathodes and potentially open a new avenue toward experimental characterization of device parameters.

Photoelectron Transportation Dynamics in GaAs Photocathodes

We report here a general theory describing photoelectron transportation dynamics in GaAs semiconductor photocathodes. Gradient doping is incorporated into the model through the inclusion of directional carrier drift. The time-evolution of electron concentration in the active layer upon the injection of an excitation pulse is solved both numerically and analytically. The predictions of the model are compared with experiments via a carrier-induced transient reflectivity change, which is measured for gradient-doped and uniform-doped photocathodes using femtosecond pump–probe reflectometry. An excellent agreement is found between the experiments and the theory, leading to the characterization of key device parameters, such as diffusion constant and electron decay rates. Comparisons are also made between uniform doping and gradient doping for their characteristics in photoelectron transportation. Doping gradient is found to be able to accelerate electron accumulation on the device surface. These results offer new insights into the dynamics of III–V photocathodes and potentially open a new avenue toward experimental characterization of device parameters.

Photoelectron transportation dynamics in GaAs photocathodes

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.

Pump-probe study of ultrafast response of GaAs photocathodes grown by MOCVD and MBE

The effective light‐harvesting performance of graded compositional AlxGa1−xN nano‐cone arrays photocathode for ultraviolet detector—A numerical investigation and simulation

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.

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

Vacuum-semiconductor hybrid photodetector is a new kind of photoelectric detecting device. In this paper, the basic structure and principle of electron bombarded avalanche diode hybrid photodetector are introduced. Then a sample of electron bombarded silicon avalanche diode hybrid photodetector is successfully fabricated. The results show that the response range of the photodetector is 300nm-800nm, and the electron bombarded gain is more than 600 times under the high voltage of -8000V. The breakdown voltage of silicon avalanche diode avalanche is about -202V. The dark current of device under linear avalanche mode with gain equals 30 is about 3.2nA. The total gain of the tube after electron bombarded gain and avalanche gain are cascaded can be up to 1.5 × 104.

Study on operational characteristics of Electron-Bombarded Silicon Avalanche Diode (EBSAD) hybrid photodetector

The present invention relates to a blue-green sensitive transmission-type GaAlAs cathode of Al compositional gradient gradual change which is sensitive to blue-green light. The cathode is formed by a Corning 7056# glass substrate, a SiO2 protection layer, a Si3N4 anti-reflection layer, a Ga1-x1Alx1As window layer with Al constant composition, a Ga1-x2Alx2As emission layer with Al compositional gradient gradual change and a Cs/O activation layer arranged from down to up. The blue-green sensitive transmission-type GaAlAs cathode of Al compositional gradient gradual change prepares the blue-green sensitive negative electron affinity transmission-type GaAlAs photoelectric cathode based on the Ga1-xAlxAs ternary compound Al/Ga composition control technology, the III-V group compound material epitaxy technology, the photoelectric cathode module preparation technology and the ultrahigh vacuum laser technology, combines an electron multiplier to form a blue-green detector, and can be applied to the fields of sea detection, seabed imaging and the like.

Blue-green sensitive transmission-type GaAlAs cathode of Al compositional gradient gradual change

The invention discloses a silicon nanocrystal/graphene wide-spectrum photodetector and a preparation method thereof. The structure comprises a back electrode, a substrate, a silicon nanocrystal layer,a graphene layer and a front electrode in order from bottom to top. The back electrode is a Ti/Au/Ti electrode. The substrate is heavily doped with n-type Si. The silicon nanocrystal layer is spin-coated with hydrogen-silsesquioxane and annealed at high temperature. The graphene layer is single-layer graphene, which is acquired by transferring vapor-deposited Au. The front electrode is a Ti/Au electrode. The photodetector provided by the invention can respond to ultraviolet, visible and near-infrared optical spectrum ranges, achieves signal doubling through the photo-switching effect betweenthe silicon nanocrystal layer and the graphene layer, has a simple preparation process, and is compatible with the CMOS process.

Chen Xinlong

Papers

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

Study on operational characteristics of Electron-Bombarded Silicon Avalanche Diode (EBSAD) hybrid photodetector

Blue-green sensitive transmission-type GaAlAs cathode of Al compositional gradient gradual change

Silicon nanocrystal/graphene wide-spectrum photodetector and preparation method thereof