Xiaoning Hu

Numerical simulation of refractive-microlensed HgCdTe infrared focal plane arrays operating in optical systems

The optoelectronic performance of the mid-wavelength HgCdTe infrared focal plane array (IRFPA) with refractive microlenses integrated on its CdZnTe substrate has been numerically simulated. A reduced light-distribution model based on scalar Kirchhoff diffraction theory was adopted to reveal the true behavior of IRFPAs operating in an optical system under imaging conditions. The pixel crosstalk obtained and the energy-gathering characteristics demonstrated that the microlenses can delay the rise in crosstalk when the image point shifts toward pixel boundaries, and can restrict the major optical absorption process in any case within a narrow region around the pixel center. The dependence of the microlenses’ effects on the systems properties was also analyzed; this showed that intermediate relative aperture and small microlens radius are required for optimized device performance. Simulation results also indicated that for detectors farther from the center of the field of view, the efficacy of microlenses in crosstalk suppression and energy gathering is still maintained, except for a negligible difference in the lateral magnification from an ordinary array without microlenses.

A preliminary study on MBE-grown HgCdTe two-color FPAs

This paper presents the recent progress on the study of device processings at multilayer HgCdTe film for integrated two-color (SWIR/MWIR) n-p-P-P-N detector arrays. The four-layer p-P-P-N heterostructures Hg1-xCdxTe film needed to achieve two color detector arrays was grown by molecular beam epitaxy (MBE) on (211)B oriented GaAs substrates. The secondary ion mass spectroscopy (SIMS) data for the HgCdTe film was obtained. The p-type layer on top of a thin P-type potential barrier layer and the SWIR P-on-N homojunction photodiode formed in-situ during MBE growth using indium impurity doping was processed into the MWIR planar photodiode by selective B+-implantation. The preliminary 256×1 linear arrays of SWIR/MWIR HgCdTe two-color FPAs detector were then achieved by mesa isolation, side-wall passivation and contact metallization. At 78K, the average R0A values of SWIR and MWIR are 3.852×105 Wcm2 and 3.015×102 Wcm2, and the average peak detectivities Dλp* are 1.57×1011cmHz1/2/W and 5.63×1010 cmHz1/2/W respectively. The SWIR photodiode cut-off wavelength is 3.04μm and the MWIR photodiode cut-off wavelength is 5.74μm, quite consistent with the initial device design. The SWIR response spectrum of the two-color detector with a distinct fall-off at shorter wavelength regime was discussed especially.

Injection efficiency of DI and CTIA readout integrated circuit

Advanced hybrid infrared focal plane arrays (IRFPA) consist of a semiconductor photodetector chip which is bump-bonded to a silicon CMOS readout integrated circuit (ROIC) chip generally containing a reset integrator preamp to accumulate detector photon-induced current over a fixed integration time. A number of readout structures have been developed for different system applications and concerns, among which the direct injection (DI) and the capacitor feedback transimpedance amplifier (CTIA) are popularly used. The structures and performances, especially injection efficiency, of DI and CTIA are respectively discussed and compared. The DI enables an ultralow power consumption and has a high injection efficiency at large background photocurrent, while the CTIA has a comparatively higher power dissipation and higher injection efficiency at low background photocurrent.

Large format high SNR SWIR HgCdTe/Si FPA with multiple-choice gain for hyperspectral detection

This paper reports the development of 2000×256 format SWIR HgCdTe/Si FPA with multiple-choice gain (i.e. multiple-choice charge handling capacity) for hyperspectral detection. The spectral resolution is about 8nm. To meet the demands of variable low flux detection within each spectral band in the short wave infrared range, low dark current, low noise, variable conversion gains and high SNR (Signal to Noise Ratio) of FPA are needed. In this paper, we fabricate 512×512 pixel 30μm pitch SWIR HgCdTe diode array on Si by using a novel stress-release construction of HgCdTe chip on Si. Moreover, we design low noise, variable conversion gain and large dynamic range read-out integrated circuit (ROIC) and hybridized the ROIC on the HgCdTe diode array on Si substrate. There are 8-choice gains which can be selected locally according to the incident flux to meet high SNR detection demand. By high-accuracy splicing 4 512×512 HgCdTe/Si FPA we get mosaic 2000×512 FPA, and characterizations have been carried out and reveal that the array dark current densities on an order of 10-10A/cm2, quantum efficiency exceeding 70%, and the operability of 99.5% at operating temperature of around 110K. The SNR of this FPA achieved 120 when illuminated under 5×104photons/pixel.

Optimization of plasma etching of SiO2 as hard mask for HgCdTe dry etching

HgCdTe is one of the dominating materials for infrared detection. To pattern this material, our group has proven the feasibility of SiO2 as a hard mask in dry etching process. In recent years, the SiO2 mask patterned by plasma with an auto-stopping layer of ZnS sandwiched between HgCdTe and SiO2 has been developed by our group. In this article, we will report the optimization of SiO2 etching on HgCdTe. The etching of SiO2 is very mature nowadays. Multiple etching recipes with deferent gas mixtures can be used. We utilized a recipe containing Ar and CHF3. With strictly controlled photolithography, the high aspect-ratio profile of SiO2 was firstly achieved on GaAs substrate. However, the same recipe could not work well on MCT because of the low thermal conductivity of HgCdTe and CdTe, resulting in overheated and deteriorated photoresist. By decreasing the temperature, the photoresist maintained its good profile. A starting table temperature around -5°C worked well enough. And a steep profile was achieved as checked by the SEM. Further decreasing of temperature introduced profile with beveled corner. The process window of the temperature is around 10°C. Reproducibility and uniformity were also confirmed for this recipe.

Irradiance spatial non-uniformity correction in large-format infrared focal plane array measurement

The format of infrared focal plane arrays (IRFPAs) is continuously increasing, and the spatial non-uniformity in the irradiance affects the result of large-format IRFPAs’ responsivity measurement. When placed in the test system, different pixels on the IRFPA receive different radiation flux due to the specific geometric setup of the dewar, resulting signal response uniformity. This is not the inherent characteristics of the IRFPA and thus should be corrected. The existing methods of correcting irradiance spatial non-uniformity simply consider the solid angle subtended by the aperture of the cold shield viewing from the center of a pixel. However, these methods are only applicable for the case of an infinite blackbody behind the cold shield. This paper presents a more comprehensive and appropriate correction, taking account of the specific geometric setup of the dewar, especially a dewar window of finite size. The relative irradiance received by different pixels on an IRFPA, as a correction factor, is simulated directly through matrix calculations. We apply the FOV correction profile to a 640 x 512 HgCdTe IRFPA with a pitch of 25μm, finding that responsivity ratio of the peripheral pixels to the central pixels on the IRFPA has changed from an original value of 88.0% to 96.4% after correction. This method has been proven to be useful and effective in obtaining more accurate description of the IRFPA performance for further analysis.

Modeling on current-voltage characteristics of HgCdTe photodiodes in forward bias region

Current-voltage (I-V) characteristics of HgCdTe photodiodes in the forward bias region have been modeled on account of mechanisms including diffusion and recombination currents, metal-semiconductor (M-S) contact and constant series resistance. Moreover, a data processing approach has been developed to obtain valuable physical parameters from measured I-V curves. This model and algorithm have also been verified to be available and promising by the fitting results on device parameters of HgCdTe photodiodes.

Crosstalk suppressing design of GaAs microlenses integrated on HgCdTe infrared focal plane arrays

Crosstalk suppressing design of dielectric GaAs microlenses integrated on traditional HgCdTe infrared focal plane arrays (IRFPAs) is presented in this paper, by exploiting the finite difference time domain (FDTD) technique. Responsive photocurrent and crosstalk between most adjacent IR detector pixels have been numerically simulated using Crosslight TCAD commercial software. An optimal curvature of GaAs microlenses has been achieved by maximizing its ability to focus the incoming infrared plane wave at a specific point near the interface of GaAs substrate and HgCdTe absorber layer.

Surface treatment effects on the I-V characteristics of HgCdTe LW infrared photovoltaic detectors

The weakness of HgCdTe materials is a weak Hg–Te bond, which results in bulk, surface and interface instabilities. Usual surface preparation methods for bulk HgCdTe wafers are chemical etch with bromine-in-methanol (Br-MeOH). Because the bromine etch forms Te-enrich HgCdTe surface due to the depletion of Hg and Cd. This Te-enrich surface layer is easily oxidized when exposed to air or acids, and this native oxide is one of the main reasons degrading the passivation properties. Besides the surface of the material will have more damages during the ion implantation, which is used to process a junction. In this way, appropriate surface passivation is required and is very important for HgCdTe photovoltaic detectors. This paper presents the different surface treatment methods on the I-V characteristics of HgCdTe infrared photovoltaic detectors. The results of the experiments show that the performance of the diodes with surface treatment by the bromine-in-ethanol etching and lactic acid in glycol solution oxidation is better than other diodes, which have no surface treatment or only bromine-in-ethanol etching. So it proves that the process of surface treatment in appropriate methods can improve the quality of surface passivation.

N2 atmosphere annealing effect on HgCdTe electrical damage induced by ICP etching process

N2 atmosphere annealing process to recover ICP etching induced damage on p type mercury vacancy HgCdTe film has been exploited in this paper. ICP etching and N2 atmosphere annealing processes were carried out on a series of Hgvacancy-doping p-type HgCdTe samples. The carrier transport and lifetime properties of these samples were characterized by Hall measurement and microwave reflectance method respectively. P-to-n electrical damage of the surface HgCdTe film induced by the ICP etching process was deduced from the polarity inversion of Hall coefficient. The carrier transport and lifetime properties were similar to those of the non-etched samples, indicating that the surface HgCdTe electrical damage was recovered partially by annealing at 210°C for 2 hours. I-V and R-V characteristics curves of photodiodes fabricated on the etched and N2 atmosphere annealing processed MCT samples were also comparable to those of photodiodes on the non-etched MCT samples in the following experiments. These results show that N2 atmosphere annealing process is a readily available and promising recovering technique for HgCdTe ICP etching induced damage.