GaAs Emitter Research Articles

Quantum efficiency enhancement of reflective GaAs photocathodes with exponential-doping structure generating a favorable built-in electric field.

The rapid development of GaAs photocathodes has led to an increased focus on the attainment of high quantum efficiency. Three types of exponential-doping structures with a high to low doping concentration distribution from the interior to the surface are proposed for reflective GaAs emission layers. These three structures generate different built-in electric fields that facilitate photoelectron emission. The one-dimensional continuity equations for the increasing, constant, and decreasing types of built-in electric fields are derived, respectively. The electron concentration distribution and quantum efficiency varying with the wavelength are solved numerically by the finite difference method. The simulation results indicate that the quantum efficiency of the GaAs photocathode with the increasing type of built-in electric field is superior to that with the constant built-in electric field, while the GaAs photocathode with the decreasing type of built-in electric field shows the worst performance. Then, the designed GaAs photocathodes with the increasing and constant types of built-in electric fields are grown by metal-organic chemical vapor deposition and activated by cesium-oxygen alternating deposition. The measured spectral response curves show that the quantum efficiency of the GaAs photocathode with the increasing type of built-in electric field is higher in the whole band than that with the constant type of built-in electric field. In addition, the exponential-doping structure generating the increasing type of built-in electric field is beneficial for improving the surface potential barrier and increasing the surface electron escape probability.

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.

Photoemission from GaAs-based photocathodes with multilayer complex structures

To enhance the near-infrared response of photocathodes in the application of image detection, two kinds of multilayer complex structures of GaAs-based photocathodes are developed by changing the structure of the buffer layer underneath the emission layer, wherein one is the graded bandgap structure, and the other is the distributed Bragg reflector (DBR) structure. The theoretical quantum efficiency suitable for the reflection-mode (r-mode) GaAs-based photocathode with these two kinds of buffer structures are deduced based on one-dimensional continuity equations, among which the reflectivity changing with the wavelength of incident light is considered in particular. By comparison of spectral characteristics of photocathodes with the two different structures, and analysis of cathode structure parameters, it is found that the photoelectric performance of the photocathodes with the two structures are quite different, and the structure parameters especially the thickness of GaAs emission layer have a great impact on the spectral characteristic. The quantum efficiency of photocathode with graded bandgap structure is improved due to the introduction of the built-in electric field and the decrement of interface recombination, while for cathode with DBR structure, the quantum efficiency is improved by the multiple reflection of light between two parallel mirrors in the stack of alternating quarter-wave layers of high and low refractive. The theoretical quantum efficiency calculation and analysis would provide theoretical guidance for the better design of GaAs-based photocathodes.

Impact of 1 MeV proton irradiation on InGaAsN solar cells

The impact of 1 MeV proton irradiation on 1.12 eV bandgap InGaAsN solar cells was studied through device and material characterizations. After a 1013 p+cm−2 proton fluence, the photocurrent decreases by 28%, due to the formation of defects in both the GaAs emitter and the InGaAsN absorber. Furthermore, photoluminescence measurements suggest that the proton radiation hardness of InGaAsN increases with the nitrogen.

Effect of Carrier Heating by Intrinsic Stimulated Picosecond Emission in GaAs on a Linear Increase at the Front and the Duration of the Spectral Component of This Emission

Effect of Carrier Heating by Intrinsic Stimulated Picosecond Emission in GaAs on a Linear Increase at the Front and the Duration of the Spectral Component of This Emission

Amplification Lengths of Spectral Components of Intrinsic Stimulated Picosecond Emission. Dependence of the Characteristic Relaxation Time of These Components on Their Amplification Lengths. Relation Between Stimulated and Spontaneous Emission Spectra in GaAs

The high-power picosecond optical pumping of a thin GaAs layer generates intense picosecond stimulated emission. The emission spectrum is a light continuum. Based on the results of previous experimental work, the following is carried out. (a) The amplification lengths of the emission spectral components, i.e., the different distances over which different emission components are amplified in the active medium produced by pumping and (b) the dependences of the characteristic relaxation time of emission components on their gain lengths are estimated. It is shown that the spectrum of the product of the gain length and the gain linearly relates the spectra of the spontaneous and stimulated emission of GaAs. This relation is established upon gain saturation which is evidenced by a dip in the gain spectrum, “burned out” by intense emission.

Effect of Carrier Heating by the GaAs Picosecond Stimulated Emission on Duration of Emission

The real-time measured envelopes of high-power picosecond optical pumping and of the intrinsic stimulated picosecond emission of a thin GaAs layer are compared. The emission duration excess over the pump duration is explained on the basis of negative feedback between the emission intensity and carrier heating by this emission. The main parameter of this duration excess is the characteristic carrier cooling time slowed down due to carrier heating by emission. The effect of this time on renormalization of the band gap due to the Coulomb interaction of carriers is also noted. The part of the concept of the picosecond dynamics of the intense stimulated emission of GaAs, which has been obtained to date in our works with co-authors, is given in Conclusions.

Research on the emitter thickness optimization of GaInP/GaAs/Ge triple-junction solar cell under space proton irradiation based on TCAD simulation

The degradation of GaInP/GaAs/Ge triple-junction solar cells with different GaAs sub-cell emitter thickness induced by proton irradiation is studied using numerical simulation. The degradation results of triple-junction solar cells with different GaAs emitter thickness have been investigated. The degradation mechanism induced by proton irradiation is analyzed. The results show that the short-circuit current and maximum power increase first and then decrease with the increase in GaAs sub-cell emitter thickness. The degradation of the GaAs sub-cell external quantum efficiency is greater than that of the GaInP sub-cell induced by proton irradiation. Meanwhile, the remaining external quantum efficiency of the GaAs sub-cell first increases and then decreases with the increase in emitter thickness after proton irradiation.

Structural Optimization and Temperature-Dependent Electrical Characterization of GaAs Single-Junction Solar Cells

This paper reports the structural optimization and the temperature-dependent electrical characterization of GaAs single-junction solar cells (SJSCs) based on the p+-n-n+ junction structure. First, the effects of the p+-Al0.9Ga0.1As window layer were investigated using illuminated current density-voltage (LJ−V) and photoreflectance measurements to determine the optimal structure of the SJSC. In addition, the thickness and the doping concentration of different layers, such as the p+-GaAs emitter and the n-GaAs base layers, were optimized, leading to the maximum efficiency (η = 21.53%). These results show that the device performance can be improved further when compared to the world record (η = 28.8%). Second, the capacitance-voltage (C−V) and the dark J−V characteristics of a GaAs SJSC with the optimal structure were investigated over the temperature range from 100 to 300 K. As the temperature was increased, both the junction capacitance at zero voltage (Cj0) and the carrier concentration in the active region (Nd) increased while the depletion width (W) and the built-in potential (Vbi) decreased due to purely thermal effects. Among the SC parameters obtained from the dark J−V curves, the ideality factor (n) decreased from 4.38 to 1.70 and the reverse saturation current density (J0) increased from 6.05 × 10 −12 to 4.26 × 10 −10 A/cm2 with increasing temperature. These values at 300 K were similar to those simulated theoretically and reported previously for high-quality GaAs junction diodes.

Anticorrelation between the Intensity of Stimulated Picosecond Emission in GaAs and the Characteristic Time of Charge-Carrier Cooling

During the powerful picosecond optical pumping, intense stimulated picosecond emission arises in a thin GaAs layer. It is found that, first, the maximum emission intensity decreases as the diameter of the pump beam increases (while the pump energy density is fixed). Second, this dependence anticorrelates with the dependence of the characteristic relaxation time of the emission on the beam diameter. This time is related to the characteristic cooling time of charge carriers, which is slowed due to carrier heating by emission. This leads to the anticorrelation given in the title.

Characterization of a GaAs/GaAsBi pin solar cell

The structural and photovoltaic properties of the GaAs/GaAsBi pin solar cell with GaAs0.983Bi0.017 active layer are investigated by optical and electrical measurement techniques. The bandgap of GaAsBi active layer is determined to be 1.3 eV at room temperature. Current density-voltage (J-V) under AM 1.5G spectrum and spectral response measurements are carried out to determine photovoltaic properties of the solar cell. The presence of a midgap trap levels in GaAsBi active layer is identified by deep level transient spectroscopy (DLTS). J-V characteristics is analysed by using Sah-Noyce-Shockley (SNS) theory which includes the midgap trap found in DLTS measurement. The observed deviation between experimental and calculated J-V results is ascribed for metallic cluster formation at the interface between GaAs emitter and GaAsBi intrinsic active layer. Interface metallic clusters create local Schottky junction between emitter and active layers. The best fit to the experimental J-V characteristic of the solar cell is obtained by considering the presence of GaBi metallic cluster at the interface between GaAs emitter and GaAsBi active layer of the solar cell. We showed that work function of interface clusters have a significant effect on the open circuit voltage and filling factor.

Stimulated two-photon emission in bulk CdSe

While two-photon emission processes are firmly established in atomic physics, their observation and use in semiconductor physics remains elusive. Here, we experimentally investigate stimulated two-photon emission in photoexcited bulk CdSe and identify requirements for the observation of stimulated two-photon emission. In particular, this process requires population inversion as well as two-photon transition energies close to the bandgap energy. In any regime investigated in the present study, net optical gain is not achieved, as the free-carrier absorption intrinsically linked to the photoexcitation completely masks the two-photon gain. The results are well in line with a recent study on nondegenerate versions of two-photon emission in GaAs and place clear limits for the practical use of two-photon emission in optically excited semiconductors.

Intense THz emission in high quality MBE-grown GaAs film with a thin n-doped buffer

Intense terahertz (THz) emission in high quality GaAs film upon ultrafast excitation is demonstrated. Results showed that high quality GaAs grown by molecular beam epitaxy with a thin n-doped buffer can rival existing intense bare semiconductor THz surface emitters. The incorporation of a 0.2 µm n-GaAs buffer proved effective in enhancing the THz emission of GaAs by 281% and 295% in reflection and transmission THz time-domain excitation geometries, respectively. The GaAs film was of high crystallinity with or without the n-doped buffer layer as confirmed from X-ray diffraction and Raman scattering. The similar crystalline quality of the two samples was further exemplified by their comparable built-in field strength as measured by photoreflectance spectroscopy. The distinguishable difference in GaAs with and without the doped buffer was observed via low temperature photoluminescence (PL) spectroscopy. The GaAs film with the n-doped buffer exhibited intense GaAs PL while the GaAs film without the n-doped buffer exhibited prominent carbon impurity-related PL. THz enhancement was inferred to be due to the decrease in shallow defects in GaAs with n-doped buffer.

Temperature dependence of THz emission and junction electric field of GaAs–AlGaAs modulation-doped heterostructures with different i-AlGaAs spacer layer thicknesses

Photocarrier dynamics in GaAs/AlGaAs modulation-doped heterostructures (MDH) having i-AlGaAs spacer layers of different thicknesses were investigated using temperature-dependent terahertz time-domain spectroscopy (THz-TDS) and photoreflectance spectroscopy. In particular, results are discussed in the framework of the temperature dependence of the heterojunction electric field and photocarrier velocity for two i-AlGaAs spacer layer thickness values. The junction electric field, THz emission intensity and bandwidth of the MDH samples all decrease as temperature decreases. In contrast, the THz emission intensity and bandwidth of a reference bulk undoped GaAs does not significantly vary with temperature. These results imply that THz emission of MDH’s originates primarily from carrier drift due to the GaAs/AlGaAs junction electric field. A general decrease in the THz emission bandwidth of the MDH’s is attributed to a decrease in carrier velocity at lower temperatures, presumably due to the weaker electric field. Moreover, the MDH sample with thinner spacer layer exhibited a higher junction electric field. This work demonstrates the study of temperature-dependent photocarrier transport and junction electric field measurements. The results may provide useful insights in the design of MDH-based devices.

Structural and luminescent characteristics of porous GaAs

In this paper, we present the results of structural and photoluminescence (PL) studies on porous layers produced on a heavily p-doped (100) GaAs wafer by electrochemical anodic etching. The evolution of porous GaAs structure as a function of layer preparation conditions were investigated by x-ray diffraction (XRD). We show that porous layers have the same crystallographic orientation as the substrate from which they are originated. We also report a relative XRD peak separation indicative of a variation of the lattice parameter of the porous layer. The photoluminescence of porous layers were characterized by a narrow emission band in the visible region attributed to the confinement effect in GaAs nanocrystals having a reduced size distribution. Chemical treatment in Sodium Carbonate solution was shown to enhance both the crystalline quality and the intensity ratio of visible to infrared emission of porous GaAs.

Analysis of the relation between spectral response and absorptivity of GaAs photocathode

In order to study the relation between spectral response and absorptivity of GaAs photocathode, two kinds of GaAs photocathodes are prepared by molecular beam epitaxy (MBE) and metal-organic chemical vapor deposition (MOCVD), respectively. The samples grown by the MBE include varying doping GaAs photocathodes with different values of emission layer thickness from A to E. The thickness of GaAs emission layer is 1.6 μm or 2 μm. The Al component is 0.5 or 0.63. The samples grown by the MOCVD include varying doping or various component GaAs photocathodes with different values of emission layer thickness and different window layer components from F to J. The thickness values of GaAs emission layer are 1.4 μm, 1.6 μm or 1.8 μm, respectively. The Al component is 0.7 or varies from 0.9 to 0. The doping concentration of the GaAs emission layer is divided into 8 sections between 1×1018 cm-3 and 1×1019 cm-3. The experimental spectral response curves for all samples are obtained by the optical spectrum analyzer. And the experimental reflectivity and transmittivity curves are measured by the ultraviolet visible near infrared spectrohootometer. Based on the law of energy conservation, the absorptivity curves are obtained according to the experimental reflectivity and transmittivity. In the same coordinate system, both the curves are obtained by unitary processing according to the max. A similar surface barrier can be given by dividing the normalized absorptivity by the normalized spectral response, and those are termed the similar I barrier and the similar Ⅱ barrier, respectively. The results indicate that for both the GaAs photocathodes, the experimental spectral response curves both tend to move to the infrared band compared with the experimental absorptivity curves. The average energy differences between absorptivity and spectral response are calculated to be 0.3101 eV for the MBE sample, and 0.3025 eV for the MOCVD sample, respectively. The red-shifts of the photocathodes grown by MBE are a bit bigger than those of the photocathodes grown by MOCVD. In the shortwave region, the absorptivity is very large, but the spectral response cuts off nearby 500 nm. In the visible wavelength region, the peak position of the spectral response curve shifts toward the infrared band for several hundred meV in comparison with the absorptivity curve. In the near infrared region, a red shift of several meV appears at the cut-off position of the spectral response curve in comparison with the absorptivity curve. The results have the guiding significance for improving the photoemission performance of wide-spectrum GaAs photocathode by optimizing the optical performance.

Self-synchronization of the modulation of energy-levels population with electrons in GaAs induced by picosecond pulses of probe radiation and intrinsic stimulated emission

Picosecond optical pumping leads to the initiation of intrinsic picosecond stimulated emission in GaAs. As was established previously, due to the interaction of pulses of probe radiation with those of intrinsic emission, the dependence of the absorption α of the probe pulse on its delay τ with respect to the pump pulse is modulated with oscillations. It is found that the oscillatory dependences α(τ) have a similar shape only in the case of certain combinations of energies of the interacting pulses. As a result, it is assumed that the above interaction is, in fact, a synchronization of modulations (formed by pulses) of charge-carrier populations at energy levels; this synchronization occurs in the direction of the reconstruction of detailed equilibrium. The real-time picosecond self-modulation of the absorption α is measured for the first time. The characteristics of this self-modulation as well as absorption α and intrinsic emission self-modulation characteristics measured previously by correlation methods are now accounted for by the concept of synchronization.

Low-frequency noise properties of p-type GaAs/AlGaAs heterojunction detectors

We have measured and analyzed, at different temperatures and bias voltages, the dark noise spectra of GaAs/AlGaAs heterojunction infrared photodetectors, where a highly doped GaAs emitter is sandwiched between two AlGaAs barriers. The noise and gain mechanisms associated with the carrier transport are investigated, and it is shown that a lower noise spectral density is observed for a device with a flat barrier, and thicker emitter. Despite the lower noise power spectral density of flat barrier device, comparison of the dark and photocurrent noise gain between flat and graded barrier samples confirmed that the escape probability of carriers (or detectivity) is enhanced by grading the barrier. The grading suppresses recombination owing to the higher momentum of carriers in the barrier. Optimizing the emitter thickness of the graded barrier to enhance the absorption efficiency, and increase the escape probability and lower the dark current, enhances the specific detectivity of devices.

Study of Photofield Emission in GaAs Using Kronig-Penney Model

Study of Photofield Emission in GaAs Using Kronig-Penney Model

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.