Exponential Doping 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.

Effect of Gradient Doping on Charge Collection Efficiency of EBCMOS Devices

We investigated the effect of different gradient doping methods on the charge collection efficiency of the electron multiplication layer of EBCMOS devices. Exponential doping of the electron multiplication layer can form a built-in electric field in the electron multiplication layer that is favorable for photoelectron transport, so exponential doping instead of uniform doping in the electron multiplication layer of EBCMOS can effectively improve the charge collection efficiency. It is shown that exponential heavy doping on the side of the electron multiplication layer near the dead layer and exponential light doping on the side near the depletion region can improve the built-in electric field structure and increase the lifetime of the multiplied electrons, thereby improving the charge collection efficiency of EBCMOS. The optimized device achieves a charge collection efficiency of 94.48% at an incident electron energy of 4 keV, an electron beam diameter of 20 nm, a dead layer thickness of 60 nm, and a P-type silicon epitaxial layer thickness of 10 μm.

Electronic performance of GaN nanowires cathode under electric field

The built-in electric field of exponential doping can promote the concentration of the photogenerated carrier center to the top surface of the nanowire. The external electric field also can bend the motion trajectory of the emitted electrons toward the collecting side. These field-assisted methods promote the quantum efficiency. In this paper, the emission theory of a single GaN nanowire photocathode is studied for the first time. The effects of height and width of the nanowire, wavelength, intensity of electric field on quantum efficiency of uniformly doped or exponentially doped GaN nanowire photocathodes were explored. It shows that the top of the exponentially doped cathode has a higher quantum efficiency than uniformly doped cathode. With the absence of the field, quantum efficiency of a uniformly doped cathode reaches a maximum value of 55.29% when the width is 150 nm and the wavelength is 220 nm. The form of exponentially doped cathode can generate an internal field. With the internal field, a maximum value rises 56.73% when the height is 900 nm and the wavelength is 230 nm. The theoretical results can direct the preparation of this kind of photocathode.

Energy band adjustment of 808 nm GaAs laser power converters via gradient doping

The gradient doping regions were employed in the emitter layer and the base layer of GaAs based laser power converters (LPCs). Silvaco TCAD was used to numerically simulate the linear gradient doping and exponential gradient doping structure, and analyze the transport process of photogenerated carriers. Energy band adjustment via gradient doping improved the separation and transport efficiency of photogenerated carriers and reduced the total recombination rate of GaAs LPCs. Compared with traditional structure of LPCs, the photoelectric conversion efficiency of LPCs with linear and exponential gradient doping structure were improved from 52.7% to 57.2% and 57.7%, respectively, under 808 nm laser light at the power density of 1 W/cm2.

High-performance GaAs nanowire cathode for photon-enhanced thermionic emission solar converters

GaAs nanowire cathodes with exponential doping and graded Al composition structures are proposed for photon-enhanced thermionic emission (PETE) devices. The conversion efficiency models with these two nanowire cathodes are deduced on the basis of one-dimensional continuity equations. The conversion efficiency as a function of wire length, wire width, Al composition distribution, cathode temperature, emissive surface and back interface recombination velocity are also simulated, respectively. Results show that exponential doping and graded Al composition cathode structures can obviously improve the conversion efficiency of devices through introducing a built-in electric field along the growth direction of nanowire. Besides, the optimum wire length and wire width range are 300–340 nm and 5.9–6.4 μm, respectively. Moreover, wider Al composition range is beneficial to achieve higher conversion efficiency. These simulations provide an interesting attempt to explore the working mechanism of GaAs nano-based PETE devices and are expected to be verified by the experimental results in the future.

Effect of isoelectric point on cheese whey wastewater treatment using a microbial electrochemical system

In this study, a microbial electrochemical system (MES) was employed to investigate the effect of isoelectric point (IEP) on cheese whey wastewater treatment. The experiments were carried out in a bioreactor equipped with a semicircular carbon cloth and stainless steel electrodes as anode and cathode, respectively. The effects of IEP, whey protein concentration, electrical current, and time were studied. The IEP of the whey protein was determined at pH 5.9. The optimum electrical current was obtained at 6 mA for synthetic cheese whey wastewater. The results of rotary exponential doping showed that the third structure of proteins chenges to the second structure at the IEP. The highest protein removal (98%) was obtained at pH 6. The results showed that 76%, 83%, and 98% protein removal were achieved at 2, 4, and 8 h, respectively.

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.

A Multiregion Approach to Modeling the Base-Collector Junction Capacitance

The junction capacitance of heterojunction bipolar transistors (HBTs) is commonly modeled based on the theory of the p-n-homojunction with constant doping levels, made more flexible by the introduction of adjustable model parameters. In III–V HBTs, however, the low-doped collector is often not uniform, but contains both material and doping steps used to suppress the collector current blocking and to improve the linearity. Under these circumstances, the classical formulation is not sufficient to model the capacitance. Similarly, the exponential spatial doping dependence in the collector of modern high-speed HBTs may require a more in-depth approach. In this brief, a smooth expression for modeling the junction capacitance of a multiregion layer stack is presented. The accuracy of the new model is demonstrated based on both simulated and measured data.

Theoretical study of the temperature distribution in high power gain fiber of gradient doping

Thermal effect in the gain fiber is one of the main factors which restrict the power improvement of high power fiber amplifiers. Previous studies have shown that the temperature distribution is closely related to the doping concentration along the gain fiber. In order to reduce the maximum temperature of the gain fiber, we propose to use doping concentration varying along the gain fiber as a method to disperse the thermal effect of the fiber laser and improve the laser output power. Based on the rate equation model and thermal conduction model, the thermal distributions and output powers of several different gradient doping gain fibers are simulated in the cases where the output powers are approximately the same. Our study shows that compared with the conventional constant doping gain fiber, linear doping of the rare earth ion along the gain fiber can reduce the maximum temperature of the gain fiber as well as the temperature of the fusion point greatly, thus improving the stabilities of the fusion point and the fiber laser amplifier. In the case of cosinoidal doping, the gain fiber can not only reduce the temperature of the fusion point but also make the temperature have a periodic distribution along the gain fiber, which can suppress the stimulated Brillouin scattering effect effectively. The exponential doping of the gain fiber can also reduce the maximum temperature and the temperature of the fusion point, which is beneficial to the further scaling of the fiber laser output power. At the same time, it can make the gain of the signal light have a uniform distribution along the gain fiber, which suppresses the mode instability effect and improves the output beam quality of the fiber laser. These conclusions also hold true when the pump power changes. Therefore, the gradient doping of the gain fiber proposed in this paper can optimize the temperature distribution along the fiber and improve the stability of the fusion point. Besides, it can improve the beam quality of the output laser and suppress the nonlinear effect and mode instability effect. The results indicate that the gradient doping of the gain fiber is an effective and feasible way to improve the output power of fiber amplifier. Last but not the least, it is possible to produce the gradient doping gain fiber by the laser heated pedestal growth method and the direct nanoparticle deposition technique. The investigation can present a reference for designing the gain fiber in high-power fiber laser systems.

Comparative research for GaAs photocathodes with graded compositional AlxGa1−xAs buffer layer

The graded Al compositional structure design in AlxGa1−xAs buffer layer can well increase the quantum efficiency than the stationary Al compositional structure design, especially at short-wave band. For deep study, the samples of two structures were prepared by metal organic chemical vapor deposition for carrying out the comparative research between surface photovoltage before Cs–O activation and spectral response current after Cs–O activation. Through fitting calculations and analyzes, the mixed structure of graded Al composition in AlxGa1−xAs buffer layer and exponential doping concentration in GaAs active layer cannot only form continuous internal electric field throughout window layer and active layer, but also decrease the misfit dislocations and stacking faults arising from lattice mismatch at the AlxGa1−xAs/GaAs interface. The better interface properties can further decrease the back interface recombination velocity than the standard structure design. Furthermore the continuous internal electric field can well increase the electron diffusion and drift length for active layer and surface escape probability for surface barriers and can also drive more photo-excited electrons at short-wave band in buffer layer to cathode surface.

Comparison of module structure of wideband response GaAs photocathode grown by MBE and MOCVD

In order to compare the structures of GaAs photocathodes grown by molecular beam epitaxy (MBE) and metal–organic chemical vapour deposition (MOCVD), four wideband response exponential doping photocathodes were prepared. Their reflectivity and transmittivity were measured by the spectrophotometer, and three thin layer thicknesses were fitted based on the matrix theory in Thin-film Optics. The comparison of the results indicated that for the GaAs photocathode grown by MBE, only one amendatory layer with the low Al component should be added between the Ga1−xAlxAs window layer and the GaAs active layer in order to the higher fitting accuracy. Opposite occurs for the MOCVD samples. In the case of accurately controlling the layered doping concentration, the material grown by MBE is the optimal, while that grown by MOCVD is suit for the exponential doping situation. These results are available for the material growth and the module preparation of the varied doping transmission-mode GaAs photocathodes.

指数掺杂GaAs阴极光子增强热电子发射型太阳能转换器转化效率理论计算

Photon-enhanced thermionic emission solar energy converter is a new highly efficient solar energy utilization technologies.An exponential doping GaAs material was presented as the cathode of photon-enhanced thermionic emission solar converter.The conversion efficiencies of the photonenhanced thermionic emission solar devices with exponential doping GaAs were theoretically analyzed by using the energy-balance and diffusion-drift-emission models.The results show that exponential doping GaAs prominently enhances photon-enhanced thermionic emission conversion,and the efficiency of photon-enhanced thermionic emission solar converters with exponential doping GaAs cathodes is a monotonically increasing function of sun concentration and a monotonically decreasing function of cathode surface recombination velocities.The efficiency of the converter can reach 30%at a flux concentration larger than 200suns and surface recombination velocities maintained at fewer than 104 cm/s.

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.

Memory effect and magnetic relaxation in Ca3Co2O6 and the doped compounds

We report memory effect and magnetic relaxation in spin-chain compounds Ca3Co2O6, Ca3Co2−xTxO6 (T=Mn, Fe, Al, and Mg) with x=0.02 and 0.04, and Ca3−xAxCo2O6 (A=Sr and Na) with x=0.06 and 0.12. The memory effect serves as an evidence of spin-glass-like freezing at low temperatures. The time-dependent magnetization exhibits a nearly exponential variation with time for initial relaxation and is not much influenced by chemical doping. With time goes on, the magnetization follows the stretched exponential function and doping effect gradually appears with uncertainty. In comparison, the influence from Fe-doping is dramatic and possible reason is proposed within the geometrical frustration scenario.

Surface depletion effects in semiconducting nanowires having a non-uniform radial doping profile

Doping profile has a significant effect on nanowire (NW) electrostatics, an effect that is expected to influence NW contact and transport properties. Herein, the electrostatic potential of nanowires (NWs) of non-uniform radial doping is calculated by two means: depletion approximation and a numerical calculation. Two profiles are considered: linear and exponential, corresponding to shallow and abrupt distributions; the results are compared to planar systems with similar doping profiles, and to uniformly doped NW systems. For a given average doping distribution, a non-uniform doping profile results in significantly lower carrier concentrations, an effect which intensifies with doping non-uniformity. Furthermore, in some cases, band diagrams obtained for the exponential doping profile vary greatly from any uniform doping, indicating that unique properties are expected for such NWs.

Quantum efficiency conversion from the reflection-mode GaAs photocathode to the transmission-mode one

In the article, the transmission-mode negative electron affinity GaAs photocathode is compared with the reflection-mode one in structure, working principle and the quantum efficiency equation. The key point to establish relation between the two modes is that the active-layer is in full accord and the biggest difference is that the incident direction of photons is inverse. Based on the relation and difference above, we obtain the electron escape probability P and the spectrum absorption coefficient α of the reflection-mode GaAs photocathode by multilayer film matrix theory. The quantum efficiency of the transmission-mode GaAs photocathode can be estimated through the coefficients P and α of the reflection-mode one. Two groups of samples are analyzed, the one group contains uniform doping reflection-mode and transmission-mode GaAs photocathodes and the other contains exponential doping ones. Through the analysis and curve fitting, we find that the conversion difference of the uniform-doping is about 11.22% while that of the exponential-doping is about 5.46%. Another four groups of samples with exponential-doping active layer are experimentalized and the differences of them are all less than 6%. The results prove the suggested method is feasible and effective and it is more suitable for the conversion of GaAs photocathode with exponential-doping active layer.

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 on GaAs photocathodes before and after activation

Obtaining higher quantum efficiency and more stability has been an important developing direction in the investigation of GaAs photocathodes. By solving the one-dimensional diffusion equation for no-equilibrium minority carriers of reflection-mode GaAs photocathode materials, we can get the equations of the surface photovoltage curve before activation and the spectral response curve after activation for uniform and exponential doping GaAs materials. Through experiments and fitting calculations for two doping structural materials designed by us, the parameters of the body materials are exactly measured by the surface photovoltage curves, and the curves for surface escape probability are also accurately fitted by the comparative research before and after activation. The differences for the fitting results of two doping structures are also well analyzed. This comparative research can form a closed-loop research for GaAs photocathodes and will help us to deeply study the varied doping structures and optimize Cs-O activation technology for GaAs photocathodes in the future.

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.