X. N. Hu

Analysis of temperature dependence of dark current mechanisms for long-wavelength HgCdTe photovoltaic infrared detectors

Resistance-voltage curves of n-on-p Hg1−хCdxTe long-wavelength infrared photodiodes forming 128-element array are measured in the temperature range of 40–150 K. Experimentally obtained characteristics are fitted by the simultaneous-mode nonlinear fitting program. The dark current mechanisms induced by diffusion, generation recombination, trap-assisted tunneling, band-to-band tunneling, and series resistance effect are included in the physical model for R-V curve fitting. Six characteristic parameters as function of temperature are extracted from measured R-V curves. The characteristics of extracted current components at low temperatures indicate significant contributions from tunneling effects, which is the dominant leakage current mechanism for reverse bias greater than approximately 50 mV. The Hg-vacancy-induced acceptor trap tends to invert to donor type at higher temperature, typically larger than 120 K, while it can maintain stable at the temperature of 60–40 K. The stable temperature of ion-implanta...

Parameter determination from resistance-voltage curve for long-wavelength HgCdTe photodiode

A data-processing approach has been developed to obtain device parameters from resistance-voltage (R-V) curves measured on long-wavelength HgCdTe n-on-p photodiodes. The physical model used for R-V curve fitting includes the dark current mechanisms induced by diffusion, generation recombination, trap-assisted tunneling, and band-to-band tunneling. Moreover, the series resistance effect is also taken into account. Six parameters, which include the dopant density Nd in the n region, the ratio of mobility to lifetime of electrons μn∕τn in the p region, the effective lifetime τ0 in the depletion region, the relative energy position of trap level Et∕Eg and its density Nt in the depletion region, and the series resistance Rs, can be extracted from measured R-V curves. The fitting procedure has been presented in detail and the error ranges of the extracted parameters have been discussed. By fitting to the R-V characteristics of three long-wavelength devices with different Cd compositions, the applicability of ou...

Modulation of the two-photon absorption by electric fields in HgCdTe photodiode

We demonstrate the tunability of the two-photon absorption (TPA) coefficient by adjusting the electric field in a HgCdTe (MCT) photodiode with cutoff wavelength of 5.2μm. The TPA coefficient was measured by using a picosecond pulsed laser with wavelength of 7.92μm. An enhancement of the TPA coefficient occurs in the space charge region of the MCT pn junction, which can be attributed to the Franz–Keldysh effect induced by the built-in electric field. By applying a reverse bias to intensify the built-in field, the TPA coefficient is found to be further enhanced by a factor of 18.9. This electric field dependence of the TPA coefficient has been fairly interpreted by the pn junction model with the Franz–Keldysh effect included.

Study on optimizing the performance of infrared detectors using material chip technology

Using material chip technology, large area n-on-p planar junctions with various boron implantation doses have been fabricated on an Hg1−xCdxTe (x = 0.291) film, similarly grown using molecular beam epitaxy, for the mid-infrared wavelength range. The current–voltage characteristics of p–n junctions have been measured on a cold stage at 77 K and an optimized ion implantation dose has been discovered. Zero bias resistance of all the junctions has been calculated and shows distinct dependence upon the boron implantation doses.

Investigations on a Multiple Mask Technique to Depress Processing-Induced Damage of ICP-Etched HgCdTe Trenches

A multiple mask technique, integrating patterned silicon dioxide (SiO2) film over patterned thick photoresist (PR) film, has been investigated as a method to perform mesa etching for device delineation and electrical isolation of mercury cadmium telluride (HgCdTe) third-generation infrared focal-plane arrays. The multiple mask technique was achieved by standard thick PR photolithography, SiO2 film deposition to cover the thick PR patterned film, and etching the SiO2 film at the bottom region after another photolithography process. The dynamic resistance in the zero-bias and low-reverse-bias regions of HgCdTe photodiode arrays isolated by inductively coupled plasma (ICP) etching with the multiple mask of patterned SiO2 and patterned thick PR film underneath was improved one- to twofold compared with a simple mask of patterned SiO2. It is suggested that the multiple mask technique is capable of maintaining high etching selectivity while reducing the side-wall processing-induced damage of ICP-etched HgCdTe trenches. The results show that the multiple mask technique is readily available and shows great promise for etching HgCdTe mesa arrays.

Inductively coupled plasma etching of HgCdTe IRFPAs detectors at cryogenic temperature

To fabricate various advanced structures with HgCdTe material, the Inductively Coupled Plasma enhanced Reactive Ion Etching system is indispensable. However, due to low damage threshold and complicated behaviors of mercury in HgCdTe, the lattice damage and induced electrical conversion is very common. According to the diffusion model during etching period, the mercury interstitials, however, may not diffuse deep into the material at cryogenic temperature. In this report, ICP etching of HgCdTe at cryogenic temperature was implemented. The etching system with cryogenic assembly is provided by Oxford Instrument. The sample table was cooled down to 123K with liquid nitrogen. The mask of SiO2 with a contact layer of ZnS functioned well at this temperature. The selectivity and etching velocity maintained the same as reported in the etching of room temperature. Smooth and clean surfaces and profiles were achieved with an optimized recipe.

Transverse magnetoresistance of metal interfaces with Hg1−xCdxTe

A magnetic field perpendicular to a two-terminal semiconductor will increase the sheet resistance under the contact as well as that of the noncontact region. A method using magnetoresistance is presented in this article to study the carrier’s behavior at the metal–Hg1−xCdxTe interface, in which we consider the disruption of the interface by the deposition of metal. Experimental results show that an inversion layer exists between p-Hg0.783Co0.217Te and the deposited Sn/Au.

Modeling of LWIR nBn HgCdTe photodetector

The nBn structure with an electron barrier sandwiched by n-type cap and absorber layers was predicted to suppress the Shockley-Read-Hall (SRH) generation-recombination processes and surface leakage. The MCT nBn structure has been studied by several groups to implement high operating temperature (HOT) device. In this report, the numerical analysis of the Hg1-xCdxTe nBn device in LWIR region (x=0.225) is performed utilizing Crosslight APSYS. The detector performance characterized by dark current, photo-current and detectivity is optimized by adjusting structural parameters such as Cd component and doping of each layer under various biases. Among the parameters, the trade-off between ΔEc and ΔEv is most intensively affected by Cd component of the barrier which was modified carefully and accomplished firstly. Furthermore, the effect of the trap density and trap energy level on the device performance is also investigated especially according to the processing techniques. At 110K, the optimized detectivity of the LWIR MCT nBn device reaches 7.5×1010 cmHz1/2/W in this report, comparable with that of the DLPH device (7.6×1010 cmHz1/2/W). The novel nBn HgCdTe structure is potentially valuable in LWIR region since the controllable p-doping issue is circumvented and passivation process is simplified.

Effects of hetero-related bulk-traps on photoresponse for long-wavelength HgCdTe infrared photodiode

Effects of hetero-related bulk traps on photoresponse for long-wavelength Hg1−xCdxTe infrared photodiodes have been numerically studied. The model involves a generalized approach, taking into account absorption coefficient, trap-assisted and band-to-band tunneling recombination mechanism, and bulk traps distributions associated with misfit dislocations present in the GaAs-substrate/Hg1−xCdxTe hetero-structure. The characteristic x-dependent material parameters, used in the simulations, such as donor concentration, trap density and level, and minority lifetime, are extracted by the simultaneous-mode nonlinear fitting procedure.

Study on parameters extraction from the dark characteristics of LW HgCdTe photodiode

An data-processing method has been developed to obtain the device parameters from the resistance-voltage (R-V) characteristics measured in long-wavelength HgCdTe photodiode.