Xi Han

Comparison of valence band x-ray photoelectron spectrum between Al-N-codoped and N-doped ZnO films

The valence band structures of Al-N-codoped [ZnO:(Al, N)] and N-doped (ZnO:N) ZnO films were studied by normal and soft x-ray photoelectron spectroscopy. The valence-band maximum of ZnO:(Al, N) shifts up to Fermi energy level by about 300 meV compared with that of ZnO:N. Such a shift can be attributed to the existence of a kind of Al-N in ZnO:(Al, N), as supported by core level XPS spectra and comparison of modified Auger parameters. Al-N increased the relative quantity of Zn-N in ZnO:(Al, N), while N-N decreased that of Zn-N in ZnO:N. (c) 2006 American Institute of Physics.

Very high quantum efficiency in InAs/GaSb superlattice for very long wavelength detection with cutoff of 21 μm

The authors report the dependence of the quantum efficiency on beryllium concentration in the active region of type-II InAs/GaSb superlattice infrared detector with a cutoff wavelength around 21 μm. It is found that the quantum efficiency and responsivity show a clear delineation in comparison to the doping concentration. The quantum efficiency is further improved by gradually doping in the absorbing region. At 77 K, the 50% cutoff wavelength of the VLWIR detector is 18 μm, and the R0A is kept at a stable value of 6 Ω cm2. Different beryllium concentration leads to an increase of an average quantum efficiency in the 8–15 μm window from 35% to 55% with a π-region thickness of 3.0 μm, for Ubias = −0.3 V, and no anti-reflection coating. As for a further result, the quantum efficiency reaches at a maximum value of 66% by gradually doping in the absorbing region with the peak detectivity of 3.33 × 1010 cm Hz1/2/W at 15 μm.

Etching mask optimization of InAs/GaSb superlattice mid-wavelength infared 640 × 512 focal plane array

In this paper we focused on the mask technology of inductively coupled plasma (ICP) etching for the mesa fabrication of infrared focal plane arrays (FPA). By using the SiO2 mask, the mesa has higher graphics transfer accuracy and creates less micro-ripples in sidewalls. Comparing the IV characterization of detectors by using two different masks, the detector using the SiO2 hard mask has the of , while the detector using the photoresist mask has the of in 77 K. After that we focused on the method of removing the remaining SiO2 after mesa etching. The dry ICP etching and chemical buffer oxide etcher (BOE) based on HF and NH4F are used in this part. Detectors using BOE only have closer to that using the combining method, but it leads to gaps on mesas because of the corrosion on AlSb layer by BOE. We finally choose the combining method and fabricated the 640× 512 FPA. The FPA with cutoff wavelength of 4.8 m has the average of and the average detectivity of at 77 K. The FPA has good uniformity with the bad dots rate of 1.21% and the noise equivalent temperature difference (NEDT) of 22.9 mK operating at 77 K.

Significantly extended cutoff wavelength of very long-wave infrared detectors based on InAs/GaSb/InSb/GaSb superlattices

The authors report significant tunability of the bandgap in very long-wave infrared (VLWIR) InAs/GaSb/InSb/GaSb superlattices. Calculations using the empirical tight binding method have shown the flexibility in tuning the energy levels of the valence band by inserting a thin InSb layer in the middle of the GaSb layer of a normal type-II binary InAs/GaSb superlattice. Through the experimental realization of several barrier structure photodiodes with 15 ML InAs/7 ML GaSb active region, the cutoff wavelength was extended from 14.5 μm to 18.2 μm by inserting 0.6 ML InSb at different locations in GaSb layer. The agreement between the theoretical predictions and the experimental measurement suggests a way to exploit this advantage for the realization of very long-wave infrared detection without increasing the thickness of InAs layer. At 77 K, the quantum efficiency of a very long-wave detector with the cutoff wavelength of 16.9 μm reached at a maximum value of 30%, and the R0A is kept at a stable value of 10 Ω ...

Low Crosstalk Three-Color Infrared Detector by Controlling the Minority Carriers Type of InAs/GaSb Superlattices for Middle-Long and Very-Long Wavelength*

We report a type-II InAs/GaSb superlattice three-color infrared detector for mid-wave (MW), long-wave (LW), and very long-wave (VLW) detections. The detector structure consists of three contacts of NIPIN architecture for MW and LW detections, and hetero-junction NIP architecture for VLW detection. It is found that the spectral crosstalks can be significantly reduced by controlling the minority carriers transport via doping beryllium in the two active regions of NIPIN section. The crosstalk detection at MW, LW, and VLW signals are achieved by selecting the bias voltages on the device. At 77K, the cutoff wavelengths of the three-color detection are 5.3 μm (at 0 mV), 14 μm (at 300 mV) and 19 μm (at −20 mV) with the detectivities of 4.6×1011 cmHz1/2 W−1, 2.3 × 1010 cmHz1/2W−1, and 1.0×1010 cmHz1/2W−1 for MW, LW and VLW. The crosstalks of the MW channel, LW channel, and VLW channel are almost 0, 0.25, and 0.6, respectively.

Atomic Mechanism of Interfacial-Controlled Quantum Efficiency and Charge Migration in InAs/GaSb Superlattice

A series of systematic electron microscopy imaging evidence are illustrated to prove that a high-quality interface is vital for enhancing quantum efficiency from 23 to 50% effectively, because improved crystal quality of each layer can suppress the disordered atom arrangement and enhance the carrier lifetime via decreasing the overall residual strain. The distribution width of charge rises and then falls as bias increasing, revealing the existence of an optimum operating voltage, which could be attributed to the proper energy band bending. Our results provide new insights into the understanding of the association between macro-property and microstructure of the superlattice system.

Extended-wavelength InGaAsSb infrared unipolar barrier detectors

We presented an extended-wavelength InGaAsSb infrared unipolar barrier detector working at room temperature. The detector with GaSb lattice-matched InGaAsSb absorb layer and AlGaAsSb unipolar barrier can achieve high material quality and low dark current. The dark current density was 2.29×10-5 A/cm2 at 0 bias at 77K. At room temperature the dark current at 0 bias was 4×10-3 A/cm2 and the R0A is high to 44 Ω · cm2. We fabricated the cone arrays in the InGaAsSb absorb layer to reduce the reflection of the radiation and extend the spectrum response to visible area. The extended-wavelength detector had the response from the wavelength of 0.4 μm. Further experiment showed the cone arrays also reduced the dark current of the detector at room temperature.

Insight into the split and asymmetry of charge distribution in biased M-structure superlattice

The charge distribution in real space of an insertion variant based on an InAs/GaSb superlattice for an infrared detector is illustrated by in situ electron microscopy. The localization split of positive charge can be directly observed in the InAs/GaSb/AlSb/GaSb superlattice (M-structure) rather than in the InAs/GaSb superlattice. With the applied bias increasing from 0 to 4.5 V, the double peaks of positive charge density become asymmetrical gradually, with the peak integral ratio ranging from 1.13 to 2.54. Simultaneously, the negative charges move along the direction of the negative electric field. Without inserting the AlSb layer, the charge inversion occurs in both the hole wells and the electron wells of the InAs/GaSb superlattice under high bias. Such a discrepancy between the M-structure superlattice and the traditional superlattice suggests an effective reduction of tunneling probability of the M-structure design. Our result is of great help to understand the carrier immigration mechanism of the s...