Exponential-doping Structure Research Articles

Different built-in electric fields for transmission-mode GaAs photocathodes through doping engineering: Design and modeling

In order to enhance the emission performance of transmission-mode GaAs photocathodes used in optoelectronic field, different exponential doping structures are designed for the GaAs emission layer to generate different types of built-in electric fields. These built-in electric fields include the constant, increasing and decreasing types, which can help photoelectron transport toward the emission surface. By solving the one-dimensional continuity equation using the finite difference method, the electron concentration distribution and quantum efficiency of the three types of exponential doping GaAs photocathodes are derived. Meanwhile, the light absorption distributions in the emission layer are simulated by the finite difference time domain method. By comparing the optical absorption distribution and the electron concentration distribution, it is concluded that, among the three types of exponential doping structures, the exponential doping structure generating the increasing built-in electric field has the most sufficient long-wave light absorption capacity and the strongest photoelectron transport ability, thereby achieving the highest quantum efficiency. This theoretical work can help understand the mechanism of improving the photoemission performance of GaAs photocathodes through doping engineering.

Improvement of quantum efficiency of field-assisted GaN monolayer photocathode

In this paper, in order to explore quantum efficiency of field-assisted GaN monolayer photocathode, uniform-doping and exponential-doping reflection-mode GaN monolayer photocathode model with an external electric field have been established, and quantum efficiency formulas have been derived. We simulate the quantum efficiency of photocathode with uniform-doping and exponential-doping structure under different external electric fields, number of monolayers in emission layer, thickness of buffer layer and interface recombination velocity. The results show that quantum efficiency enhances with increase of external electric field. As the thickness of emission layer increases, the quantum efficiency decline, while as the thickness of buffer layer increases, the quantum efficiency improves. At the same time, an appropriate interface recombination velocity can improve the quantum efficiency of photocathode. When back surface recombination velocity and GaN ML/GaN ML interface recombination velocity are 8 × 106 cm/s and 108 cm/s, respectively, the optimal quantum efficiency can be obtained.

Theoretical analysis and modeling of photoemission ability and photoelectric conversion characteristics of GaAs nanowire cathodes based on photon-enhanced thermionic emission

An axially exponential-doping GaAs nanowire cathode is proposed as the cathode material of photon-enhanced thermionic emission (PETE) device. Two-type theoretical models are respectively developed based on the numerical solution of multi-dimensional continuous equations with the boundary conditions. The quantum efficiency and the conversion efficiency of exponential-doping GaAs nanowire cathode based on PETE mechanism as a function of wire length, wire width, operating temperature, emissive surface recombination velocity and surface electron affinity are simulated and analyzed, respectively. Results show that the PETE devices with exponential-doping cathode can exhibit more superiority than those with uniform-doping structure by comparing the conversion efficiency, which mainly attributes to the built-in electric field induced by the axially exponential-doping structure. In addition, the optimal wire height and wire width for exponential-doping GaAs nanowire cathode are about 6 μm and 300 nm, respectively. Moreover, the emissive surface recombination velocity has an obvious inhibition on the quantum efficiency and conversion efficiency of GaAs nanowire PETE devices. The simulations would provide valuable theoretical guidance to achieve high-performance nano-based PETE devices.

Thermally enhanced external photoelectric emission theoretical study for transmissive exponentially doped GaAs photocathode

Thermally enhanced photoelectric emission (TEPE) is a new proposed mechanism to improve solar cell efficiency at low temperatures (< 400K). In this paper, the mechanism of TEPE phenomenon is discussed based on the transmission-mode GaAs photocathode with exponential-doping structure. Parameters varying with temperature are quantitatively studied in the photoelectric emission process, such as diffusion length, absorption coefficient, escape probability and the drift length. Combining the variation of these parameters by temperature with the quantum efficiency of the transmissive-mode exponential-doping photocathode, the final TEPE quantum efficiency formula is derived, which proves and explains that the temperature has an auxiliary effect on the improvement of the photoemission quantum efficiency in low-temperature environment.

Effect of graded bandgap structure on photoelectric performance of transmission-mode AlxGa1-xAs/GaAs photocathode modules.

The graded bandgap AlxGa1-xAs/GaAs photocathode with graded composition and exponential doping structure has shown great potential for improving photoemission capability. In order to better study the performance of transmission-mode AlxGa1-xAs/GaAs photocathode with the complex graded bandgap structure, the experimental optical properties and quantum efficiency are measured by comparison with uniform composition and exponential doping Al0.7Ga0.3As/GaAs photocathode. The theoretical optical properties of the multilayer AlxGa1-xAs/GaAs photocathode modules are calculated by matrix formula on the basis of thin-film optical principles. The effect of cathode thickness and aluminum proportion on optical properties are analyzed by simulation. The results show that these parameters have complicated effects on the optical properties. Different parameters are presented as the changes of peak and valley of the optical property curves. Meanwhile, the emission layer has a significant effect on the absorptivity values of the photocathode modules, which will obviously influence photoemission performance. By using the optical properties via calculation, a better fit of the experimental data with the theoretical model can be achieved, which would make reasonable guidance for further investigation of these complex graded bandgap photoemitters.

Field enhanced GaN photocathode and a proposed implementation method

Varied p-type doping structure has been verified to be efficient to improve the QE of III–V semiconductor photocathode because of the built-in electric field, and the gradient-doping structure is also efficient for GaN photocathode. Here we study the band structure of the exponential-doping GaN photocathode and get the QE formula. Based on the formula, the parameters influencing the QE are analyzed. Due to the limitation of MOCVD method, it is hard to obtain exponential-doping structure GaN photocathode. Here ALD method is proposed to grow exponential-doping GaN material, TMG, NH3, and Cp2Mg are chosen as the Ga, N, and Mg precursor source, respectively. Growth temperature is fixed at the range of 450–550°C, and within this temperature range crystal structure GaN material can be obtained. Finally, to get the exponential-doping structure, the relationship of Ga and Mg cycle is studied, and the formula is given.

Quantum efficiency of transmission-mode AlxGa1−xAs/GaAs photocathodes with graded-composition and exponential-doping structure

A transmission-mode AlxGa1−xAs/GaAs photocathode with the combination of composition-graded AlxGa1−xAs window layer and exponential-doping GaAs emission layer is developed to maximize the cathode performance. The theoretical quantum efficiency model with this complex structure containing twofold built-in electric fields is deduced by solving the one dimensional continuity equations combined with the three-step model. By comparison of spectral characteristics of photocathodes with different composition and doping structures, and through analysis of cathode structure parameters, it is found that the twofold built-in electric fields can effectively improve photoemission performance of AlxGa1−xAs/GaAs photocathode, which is related to Al proportion variation range and thicknesses of window layer and emission layer. The quantum efficiency model would provide theoretical guidance for better design of transmission-mode graded bandgap photocathodes.

Resolution properties of transmission-mode exponential-doping Ga0.37Al0.63As photocathodes.

Using the modulation transfer function obtained by establishing and solving the two-dimensional continuity equation, we have calculated and comparatively analyzed the resolution characteristics of transmission-mode exponential-doping and uniform-doping Ga0.37Al0.63As photocathodes. The calculations show that compared with a uniform-doping Ga0.37Al0.63As photocathode, the exponential-doping structure can significantly improve not only the resolution, but also the quantum efficiency of the photocathode. This improvement is different from the approach for high resolution by reducing the emission layer thickness Te and electron diffusion length LD, or by increasing the recombination velocity of the back-interface, SV, which results in low quantum efficiency. Furthermore, the improvement in resolution and quantum efficiency for the transmission-mode exponential-doping Ga0.37Al0.63As photocathodes is the result of the effect of the built-in electric field on electron transport and lateral diffusion.

Resolution properties of reflection-mode exponential-doping GaAs photocathodes

Resolution properties of reflection-mode exponential-doping and uniform-doping GaAs photocathodes were calculated and comparatively analyzed by using the modulation transfer function obtained by establishing and solving the two-dimensional continuity equation. The calculated results show that compared with the uniform-doping GaAs photocathode, the exponential-doping structure can improve significantly not only the resolution, but also the quantum efficiency of the photocathode. This improvement differs from the approach for achieving high resolution by reducing the emitting layer thickness Te and the electron diffusion length LD or by increasing the recombination velocity of back-interface SV, which results in low quantum efficiency. Furthermore, the improvement of resolution and quantum efficiency of the reflection-mode exponential-doping GaAs photocathode is due the effect of the built-in electric field on electron transport and on lateral diffusion.

Resolution characteristics for reflection-mode exponential-doping GaN photocathode

According to the expression for modulation transfer function obtained by solving the established 2D continuity equation, the resolution characteristics for reflection-mode exponential-doping and uniform-doping GaN photocathodes have been calculated and comparatively analyzed. These calculated results show that the exponential-doping structure can upgrade not only the resolution capability but also the quantum efficiency for a GaN photocathode. The improvement mechanism is different from the approach for high resolution applied by reducing Te and L(D) or increasing S(V), which leads to low quantum efficiency. The main contribution factor of this improvement is that the mechanism that transports electrons toward the NEA surface is facilitated by the built-in electric field formed by this exponential-doping structure, and the corresponding lateral diffusion is reduced.

Effect of exponential doping structure on diffusion length of reflection-mode GaN NEA photocathodes

In order to improve the performance of the photocathode, a kind of photocathode with exponential doping structure is designed and grown; meanwhile two other kinds of photocathodes with uniform doping structure and gradient doping structure are also designed. Through comparing their quantum efficiencies, the exponential doping photocathode has the best quantum efficiency. The inherent mechanism leading to the increase of the diffusion length is analyzed in detail. The diffusion length formula of the photo excited electron in the exponential doping structure is deduced, and the diffusion length in the activation layer is calculated. The result has significance to further study the GaN photocathode.

The High Quantum Efficiency of Exponential-Doping AlGaAs/GaAs Photocathodes Grown by Metalorganic Chemical Vapor Deposition

An exponential-doping structure is successfully applied to the preparation of AlGaAs/GaAs photocathodes through the metalorganic chemical vapor deposition (MOCVD) technique. The experimental results show that the quantum efficiency in the entire waveband region for the exponential-doping photocathodes grown by MOCVD is remarkably enhanced as compared to those grown by molecular beam epitaxy. As a result of the improved built-in electric fields and cathode performance parameters, the photoemission characteristics for the MOCVD-grown transmission-mode and reflection-mode AlGaAs/GaAs photocathodes are different over the wavelength region of interest.

Affection of different doping structure on quantum efficiency and spectrum response of reflection-mode GaN photocathode

In order to analyze the effect of varied doping structure on the quantum efficiency and spectrum response of the photocathode, six kinds of reflective varied doping GaN photocathode samples are designed and grown. The quantum efficiency curves and the spectrum response curves are obtained after these samples are activated. Through the analysis of experiment result, the factors and the reasons that can affect the quantum efficiency and spectrum response are concluded. The experiment result shows that the photocathode with exponential-doping structure has the best spectrum response and the highest quantum efficiency. The experiment result provides suggestions for the study on optimizing design of photocathode doping structure.

Negative Electron Affinity AlGaAs/GaAs Photocathodes with Exponential-Doping Structure

Obtaining photocathodes with high quantum yield has been the focus during the process of photocathode development. With the limitation of basic industrial manufacturing level, the further performance improvement of the negative electron affinity photocathode is subject to the quality of grown material itself. For this reason, according to the band engineering science, we have proposed an exponential-doping structure applied to the active-layer of reflection-mode and transmission-mode AlGaAs/GaAs photocathodes via molecular beam epitaxy technique, to increase the photocathode emission efficiency. A series of theoretical and experimental researches including structure design, material growth, surface cleanness, Cs-O activation and performance evaluation have been carried out to confirm the actual effect of exponential-doping photocathodes. As a result of the built-in electric field, the cathode performance was enhanced for exponential-doping AlGaAs/GaAs photocathodes.

Comparative research on the transmission-mode GaAs photocathodes of exponential-doping structures

Early research has shown that the varied doping structures of the active layer of GaAs photocathodes have been proven to have a higher quantum efficiency than uniform doping structures. On the basis of our early research on the surface photovoltage of GaAs photocathodes, and comparative research before and after activation of reflection-mode GaAs photocathodes, we further the comparative research on transmission-mode GaAs photocathodes. An exponential doping structure is the typical varied doping structure that can form a uniform electric field in the active layer. By solving the one-dimensional diffusion equation for no equilibrium minority carriers of transmission-mode GaAs photocathodes of the exponential doping structure, we can obtain the equations for the surface photovoltage (SPV) curve before activation and the spectral response curve (SRC) after activation. Through experiments and fitting calculations for the designed material, the body-material parameters can be well fitted by the SPV before activation, and proven by the fitting calculation for SRC after activation. Through the comparative research before and after activation, the average surface escape probability (SEP) can also be well fitted. This comparative research method can measure the body parameters and the value of SEP for the transmission-mode GaAs photocathode more exactly than the early method, which only measures the body parameters by SRC after activation. It can also help us to deeply study and exactly measure the parameters of the varied doping structures for transmission-mode GaAs photocathodes, and optimize the Cs-O activation technique in the future.

The Time Response of Exponential Doping NEA InGaAs Photocathode Applied to near Infrared Streak Cameras

An exponential doping NEA InGaAs photocathode is theoretically proposed to apply in the near infrared streak camera. The photocathode time response is calculated and analyzed by using a photoelectron non-steady method. The numerical results show that the excited electrons in the InGaAs active layer is accelerated due to the built-in electric field induced by the exponential doping structure, which shortens the transport time of minority carriers in the photocathode and thus, the time response is greatly improved. In addition, the exponential doping InGaAs photocathode possesses time response of less than 10 picoseconds and near-infrared quantum efficiency of 10%.

Comparative research for transmission-mode GaAs photocathodes of different doping structures on surface photovoltage

In order to well study the internal body performance for transmission-mode GaAs photocathode of different varied doping structures, two GaAs photocathodes of exponential doping structure and gradient doping structure were designed respectively. Because surface photovoltage spectrum has close relation with the internal properties of GaAs photocathodes, the connection between surface photovoltage and internal electronic field was well discussed through deduction and calculation. The difference of two structures and the value of internal electronic energy were exactly calculated and verified by experiments. The internal band bending energy could form an internal electronic field with the same direction, which could help the photo-excited electrons to move toward surface barrier layer. This research shows a better method to well study the varied doping structures for GaAs photocathode materials and will help to improve the growth structure for transmission-mode GaAs photocathode module in the future.

Effect of exponential-doping structure on quantum yield of transmission-mode GaAs photocathodes

The quantum yield formulas for exponential-doping and uniform-doping transmission-mode GaAs photocathodes are modified by adding a shortwave constraint factor to the photoelectron generation function in a one-dimensional continuity equation. Based on the modified transmission-mode quantum yield formulas, the experimental exponential-doping and uniform-doping transmission-mode quantum yield curves are respectively fitted, and the fitted curves are consistent well with the experimental curves. In addition, the fitted cathode performance parameters indicate that as compared with the uniform-doping photocathode, the exponential-doping photocathode can obtain a higher cathode performance because of the built-in electric field. The exponential-doping structure can effectively increase the quantum yield of transmission-mode photocathode.

Influence of exponential-doping structure on photoemission capability of transmission-mode GaAs photocathodes

In order to verify the actual effect of an exponential-doping structure on cathode performance, an exponential-doping structure has been applied to the preparation of the transmission-mode GaAs photocathode via molecular beam epitaxy technique. Compared with the uniform-doping photocathode, the activation and spectral response results show that the exponential-doping photocathode can achieve a higher photoemission capability. In addition, based on the revised uniform-doping and exponential-doping transmission-mode quantum yield equations, the cathode performance parameters such as electron average transport length and electron escape probability of the exponential-doping photocathode are obtained, which are greater than those of the uniform-doping one. The improvement in the cathode performance is attributed to the built-in electric field arising from this special doping structure, which effectively increases the electron transport efficiency and escape probability.

Influence of exponential doping structure on the performance of GaAs photocathodes

Obtaining higher quantum efficiency and more stable GaAs photocathodes has been an important developmental direction in the investigation of GaAs photocathodes. One significant approach to this problem is to improve the electron diffusion length. We put forward and investigate an exponential doping mode GaAs photocathode. It was proved by theoretical and experimental results that, because the exponential doping structure is in favor of forming a directional constant built-in electric field, the electron diffusion and drift length of the cathode material can accordingly be enhanced. The mathematical expression of the electron diffusion and drift length L(DE) of an exponential doping photocathode was deduced, and the relationship between the doping coefficient and the electron diffusion and drift length is made certain. This investigation contributes to the understanding of varied doping GaAs photocathodes and provides guidance to optimize the doping structure of GaAs photocathodes for higher quantum efficiency.