Zhicheng Xu

Hybrid particle swarm global optimization algorithm for phase diversity phase retrieval.

The core problem of phase diversity phase retrieval (PDPR) is to find suitable optimization algorithms for wave-front sensing of different scales, especially for large-scale wavefront sensing. When dealing with large-scale wave-front sensing, existing gradient-based local optimization algorithms used in PDPR are easily trapped in local minimums near initial positions, and available global optimization algorithms possess low convergence efficiency. We construct a practicable optimization algorithm used in PDPR for large-scale wave-front sensing. This algorithm, named EPSO-BFGS, is a two-step hybrid global optimization algorithm based on the combination of evolutionary particle swarm optimization (EPSO) and the Broyden-Fletcher-Goldfarb-Shanno (BFGS) algorithm. Firstly, EPSO provides global search and obtains a rough global minimum position in limited search steps. Then, BFGS initialized by the rough global minimum position approaches the global minimum with high accuracy and fast convergence speed. Numerical examples testify to the feasibility and reliability of EPSO-BFGS for wave-front sensing of different scales. Two numerical cases also validate the ability of EPSO-BFGS for large-scale wave-front sensing. The effectiveness of EPSO-BFGS is further affirmed by performing a verification experiment.

Evolution of interfacial properties with annealing in InAs/GaSb superlattice probed by infrared photoluminescence

Postgrowth rapid-annealing effects are investigated by infrared photoluminescence (PL) in the InAs/GaSb type-II superlattice (T2SL) with intentional InSb interfaces. The changes in PL energy, linewidth, and integral intensity with temperature indicate that the PL process is dominated by electron–phonon interaction in the InSb-like interfaces and adjacent narrow portions of InAs layers. The interfacial electron level serves as a thermal escape channel for the first miniband electrons and affects the T2SL high-temperature properties. Annealing promotes the interfacial atom exchange and changes the electron thermal escape energy. It transforms the PL-related interfacial regions to InSb1−xAsx at annealing temperatures below 470 °C, activates In/Ga exchange, and transforms the regions to In1−yGaySb at 500 °C. The results indicate that an optimized annealing temperature is crucial for improving the T2SL performance by postgrowth annealing, and infrared PL can serve as an effective criterion for the optimization.

Performance comparison between the InAs-based and GaSb-based type-II superlattice photodiodes for long wavelength infrared detection

In this paper, we report on the characterization of InAs/GaAsSb type-II superlattice long wavelength infrared photodiodes grown on InAs substrates by molecular-beam epitaxy and also present the device performance comparison with the superlattice devices grown on GaSb substrates. These devices with PIN structures had a 100% cutoff wavelength of 10 μm. The dark current density of InAs-based device at -30 mV reverse bias was 4.01 × 10-4 A/cm2 and the resistance-area product at zero bias (R0A) was 36.9 Ωcm2. The dark current density of GaSb-based device is higher more than one order of magnitude than that of InAs-based device. The temperature-dependence and bias-dependence of the dark current are studied experimentally and correlated to the theory. Good agreement was achieved between the measured I-V curves and the simulated ones, and between the experimental and theoretically predicted differential resistance values. Compared with InAs-based superlattice device, the generation-recombination current of GaSb-based device is larger and dominates in a wider temperature range due to shorter carrier lifetime and higher defect density.

InAs-based type-II superlattice long wavelength photodetectors

We report on the performance of long wavelength infrared type-II InAs-based InAs/GaAsSb superlattice photodiodes grown by molecular-beam epitaxy. The detectors had a 100% cutoff wavelength of ~ 9.7 μm and a peak current responsivity of 2.16 A/W at 80 K. The dark current density at -50 mV bias was 6.4×10-4 A/cm2 and the resistance-area product at zero bias (R0A) was 36.9 Ωcm2. The black body detectivity and peak detectivity were 7.5×1010 cm Hz1/2/W and 1.97×1011 cm Hz1/2/W, respectively. The quantum efficiency at 7.6 μm was measured to be ~34%. Good agreement was achieved between the measured I-V curves and the simulated ones, and between the experimental and theoretically predicted differential resistance values. At temperatures exceeding 75 K diffusion currents dominate the device performance. In the temperature range between 65 and 75 K, the performance of the InAs-based SL photodiodes is limited by GR processes. Trap-assisted tunneling current provides a significant contribution at temperatures below 65 K, while coherent tunneling currents are not of importance.

Dark current analysis of long wavelength InAs/GaSb superlattice infrared detector

Dark current characteristics of long wavelength InAs/GaSb superlattice (SL) detectors have been studied in this paper. The long wavelength SL structure consists of periodic 14 monolayers (MLs) InAs and 7 MLs GaSb with 50% cutoff wavelength around 11 μm. Three InAs/GaSb superlattice detectors of PBIN structure were grown at different temperatures. Wet chemical etching was used to define device mesa. SiO2 was used for device passivation to suppress the sidewall leakage current. Electron barriers were inserted between the absorber region and P-type conducting region to reduce the bulk dark current. The detectors grown at 380oC have the lowest dark current densities as 0.01A/cm2 and the best R0A value as 13 Ωcm2. We simulated four main dark current mechanisms. The result shows that the intrinsic carrier density is extracted to be 3.5E15 cm-3 which matches the C-V measurement very well. And the GR and trap assisted tunnel current dominate the dark current of the device due to the large trap densities and short GR lifetimes.

Interface design and properties in InAs/GaSb type-II superlattices grown by molecular beam epitaxy

In this paper we reported our systematic studies on InSb interface growth in InAs/GaSb SLs structure. Two typical interfaces growth mode, migration-enhanced epitaxy (MEE) and conventional molecular beam epitaxy (MBE), were designed for the 12 ML InAs/12 ML GaSb SLs material and the detail properties were discussion by the experimental measurement and simulation analysis. Our results indicated that the surface of SLs sample with the InSb interface layers grown by MEE method shows smaller RMS both on the 2 μm x 2 μm and 50 μm x 50 μm scan area by AFM measurement, and its PL intensity is about 1.3 times stronger than that of SLs sample grown by MBE. Besides, the MEE samples had significant As composition in InSb interface layers which was extracted by the HRXRD fitting.

High operation temperature mid-wavelength interband cascade infrared photodetectors grown on InAs substrate

In recent years, interband cascade detectors (ICIP) based on typer-II superlattice have shown great performance potential at high operation temperature. In this paper, we report our studies on mid-infrared interband cascade photodetectors first grown on InAs substrate. We examined the photo-generated carriers’ transport in ICIP structures by comparing three detectors grown on InAs substrate. The 2-stages ICIP device has demonstrated a high quantum efficiency around 20% at room temperature. The dark current density of the 2-stages ICIP device at -0.05V is as low as 1 nA at 80K, 1 mA at 150K, which is comparable to the state of art PIN superlattice photodetectors with similar cutoff wavelength. The Johnson-noise limited D* reaches 1.64×1014cm.Hz1/2/W at 3.65 μm and 80K, and 4.1×1010cm.Hz1/2/W at 3.8 μm and 200K. The 300 K background limited infrared performance (BLIP) operation temperature is estimated to be over 140 K.

High performance InAs/GaSb superlattice long wavelength photodetectors based on barrier enhanced structures

The barrier enhanced InAs/GaSb long wavelength photodetectors were designed and demonstrated in this paper. A PBIN detector with an electron barrier inserted between P type contactor and absorption region show significantly improved electrical performances compared to a PIN structure. The RmaxA product of the PBIN detector was measured to be 104 Ωcm2 at 80K and 7360 Ωcm2 at 50K. Temperature dependent measurements show that the tunneling currents dominate the dark current below 50K, the generation-recombination (GR) currents dominate from 50K to 90K, and the diffusion current dominate over 90K. The PBIN structure benefits from a lower electric field in the absorption region and therefore, suppressed the tunnel currents and GR currents. To improve the quantum efficiency, Be-doping was employed to convert the conductivity of the long wavelength SL structure, the PN junction moves away from the B-I hetrostructure to the π-N interface, which loses the barrier effect. Therefore, the hole barrier was needed to form a PBπBN structure. In this paper, hole barrier was designed without Al element to form a PBπBN structure. The RmaxA product of the PBπBN detector was measured to be 77 Ωcm2 and the dark current density under -0.05V bias was measured to be 8.8×10-4A/cm2 at 80K. The peak current responsivity at 9.8 μm was 2.15A/W and the quantum efficiency was 26.7%.

Studies on abrupt and gradual band gap hole barriers in InAs/GaSb superlattice long wavelength photodetectors

The barrier enhanced InAs/GaSb long wavelength photodetectors were proved to have better performance. Our previous work showed a PBIN detector with an electron barrier inserted show significantly improved electrical performances compared to a PIN structure. To improve the quantum efficiency, Be-doping was employed to convert the conductivity of the long wavelength SL structure, the PN junction moves away from the B-I hetrostructure to the π-N interface which loses the barrier effect. Therefore, the hole barrier was needed to form a PBπBN structure. In this paper, both the abrupt and gradual hole barrier was designed without Al element to form a PBπBN structure. The gradual hole barrier was optimized to avoid the blocking of photo generated current, maximized the quantum efficiency. The RmaxA product of the PBπBN detector was measured to be 77 Ωcm2 and the dark current density under -0.05V bias was measured to be 8.8×10-4A/cm2 at 80K. The quantum efficiency of gradual hole barrier detector was measured to be 27.2% at 10.6 μm and the quantum efficiency was slowly decreased under reverse bias. The result shows the gradual hole barrier efficiently eliminate the peak barrier in the electron band. The peak detectivity of this graded detector is calculated to be 9.46×1010cm.Hz1/2.W-1 at 10.6 μm.

Molecular beam epitaxy growth of high quality InAs/GaSb type-II superlattices for long wavelength infrared detection

InAs/GaSb superlattices are excellent candidates for the third-generation long-wave infrared and very-long-wave infrared photodetectors due to their special energy structure and theoretical advantages. To realize their inherent potential, however, superlattice materials with low defect density and improved device characteristics must be demonstrated. Here we report on the demonstration of highperformance PBπN photodiodes based on type-II InAs/GaSb superlattices with full cut-off wavelength ~ 13.0 μm operating at 77 K. Samples with migration-enhanced epitaxy for interface layers were grown by molecular beam epitaxy on GaSb substrates and characterized by high-resolution X-ray diffraction and atomic force microscopy. The FWHM of the 1st-order X-ray diffraction satellite peak of the absorption layers was only 21.6. The average roughness from AFM on a 2×2 μm2 scan area was less than 0.15 nm. Optical and electrical measurements of the photodiodes revealed high uniformity of the type-II superlattice materials. Across the wafer, the detector structure showed a full cut-off wavelength of 13.0 μm at 77 K. The dark current density at -50 mV was 5.1×10-4 A/cm2 and the maximum resistance-area product (RmaxA) was 128.5 Ω cm2.