Mukti M. Rana

High Responsivity

Microbolometers of a-Si0.15Ge0.85O.0.236:H were fabricated by radio frequency (RF) magnetron sputtering. A sandwich layer of silicon nitride-silicon germanium oxide-silicon nitride was used. Surface micromachining was used to fabricate the detectors into a suspended bridge structure. To reduce the 1/f-noise, the detectors were passivated at 250degC in forming gas. A high temperature coefficient of resistance (TCR) of -4.8%/K was obtained at room temperature. Due to observed photo generation below 2.5 mum of wavelength, the responsivity and detectivity measurements were performed with 2.5 mum long pass filter placed in between the infrared light source and detectors. The highest responsivity and detectivity obtained were 1.05times104 V/W and 8.27times106 cm-Hz1/2/W from a 40 mum times 40 mum pixel. Lowest thermal conductance obtained was 4times10-8 while the response time was 3.44 ms.

{\rm a\!-\!Si}_{\rm x}{\rm Ge}_{1-{\rm x}}{\rm O}_{\rm y}:{\rm H}

In this paper, we considered the issues related to modeling and analysis of uncooled infrared detectors noise analysis utilizing pulse train equalization technique. We first developed a mathematical model appropriate for the noise performance study of the detector and then derive the detectivity and noise equivalent temperature difference (NETD) expressions for the predetection, detection, and postdetection noise sources. Our study of the postdetection noise sources was focused on the capacitive transimpedance amplifier unit cell electronics which provides a tight control over the detector bias current and handles a lower background radiation with a superior noise performance. We developed a parametric tradeoff study which provides a guide for optimizing the bolometer noise performance through the adjustment of detector parameters. We also discussed the use of scanning format and time-delay integration technique for improving the performance of multiple arrays of detectors.

Microbolometers

We report the design of an uncooled pyroelectric detector utilizing a nanometer sized mesh to support the micromachined detector. The design had been optimized by using different geometry and electrodes. The thickness, width and dimension of each layer were changed to achieve the lowest thermal conductance. Ca-modified lead titanate (PCT) was employed as the thermometer in the detector. The design and performance of pyroelectric detectors has been conducted by simulating the structure with Intellisuite™. The simulated detector had spider web-like structure with each of the struts (ring) of spider web 100 nm wide. The pyroelectric detectors utilized a NiCr absorber, PCT sensing layer, Ti electrodes, Al2O3 structural layer to obtained lower thermal conductivity between the detector and substrate. The thermal conductance between the sensor and the substrate was found to be as low as 4.57 × 10-9 W/K.

Noise Performance Evaluation of Uncooled Infrared Detectors (June 2009)

Polycrystalline ferroelectric modified lead titanate thin films have been deposited by pulsed laser deposition and annealed for pyroelectric infrared detection. The deposited thin films have been characterized for the temperature dependence of their pyroelectric parameters.

Nanomachined pyroelectric detector with low thermal conductance — Design and concepts

Two dimensional (2D) materials have become a growing subject in the last 15 years mainly due to the isolation of graphene, which created a completely different class of material based on its unique, monolayer design. Since then, various stable materials of few atoms thick are showing emerging capabilities in optical electronics and photonics. Semiconducting monolayers of transition metal dichalcogenides (TMDs) such as MoS2, Mo1-xWxS2, and WS2 exhibit direct electronic band gaps; bulk crystals display indirect band gaps. Interestingly, these 2D materials show significant light interaction over a broad bandwidth ranging from infrared to ultraviolet wavelengths. The materials allow photodetection in this bandwidth without the need of cooling, thus creating new potential for uncooled detection. In this review, we discuss various 2D materials and their interaction with light for photodetection applications.

Pyroelectric modified lead titanate thin films for uncooled infrared detection

Pyroelectric materials show a change in their spontaneous polarization due to the temperature variations. This property makes these materials unique for sensing radiation in the infrared (IR) broad range. Here, we report the deposition and characterization of pyroelectric Calcium Lead Titanate (PCT) thin films for using them to fabricate pyroelectric detectors. PCT films were deposited on both silicon and Si/SiN/Ti/Au substrates at 13 mTorr pressure by 200W Radio Frequency (RF) sputtering in Ar+O2 environment for four hours. Substrates were kept at variable temperatures starting from 550 ºC up to 800 ºC during the deposition. The PCT films were annealed at 550, 600, 650 and 700 ºC in O2 environment for 15 minutes. X-ray diffraction (XRD) results confirm the polycrystalline nature of these films. Energy dispersive spectroscopy (EDS) function of scanning electron microscope (SEM) was done to determine the elemental composition of PCT films. Our EDS result reveals the presence of the elements such as Calcium, Lead, Titanium , and Oxygen in the thin films. Moreover, it shows that the films are stoichiometric (Ca0.43Pb0.57)TiO3 (Ca/Ti=0.5, Pb/Ti=0.66). The film thicknesses were measured using a Dektak model XT profilometer which ranges from ~ 250 to 400 nm. The surface morphology obtained from SEM and atomic force microscopy confirms the crack-free nature of our films as well as their smoothness and low surface roughness. Temperature dependence of capacitance, pyroelectric current, and pyroelectric coeeficient were investigated for different PCT films. Our results show that films deposited at 550ºC and 600 ºC demonstrate better quality and larger values of pyroelectric coefficient. On the other hand, the capacitance fabricated on the PCT films at 550 ºC showed the highest value of pyroelectric current and pyroelectric coefficient which are 14 pA and 50 μC/m2K respectively at higher temperature.

Review of optical properties of two-dimensional transition metal dichalcogenides

Uncooled infrared detectors are utilized in various radiometric devices and cameras because of their low cost, light weight and performance. A pyroelectric detector is a class of uncooled infrared detector whose polarization changes with change in temperature. Infrared radiation from objects falls on top of the sensing layer of the pyroelectric detector and the absorbed radiation causes the temperature of the sensing layer to change. This work describes the deposition and characterization of AlxNy thin films for using them as pyroelectric detector’s sensing material. To test the sensitivity of infrared detection or pyroelectric effect of AlxNy thin films, capacitors of various sizes were fabricated. The diameter of the electrodes for capacitor used during testing of the device was 1100 μm while the distances between these two electrodes was 1100 μm. On a 3-inch diameter cleaned silicon wafer, 100 nm thick AlxNy thin films were deposited by radio frequency (RF) sputtering from an Al target in Ar: N2 environment. On top of this, a 100-nm thick Au layer was deposited and lifted off by using conventional photo lithography to form the electrodes of capacitors. All the layers were deposited by RF sputtering at room temperature. The thin film samples were annealed at 700 °C in N2 environment for 10 minutes. X-ray diffraction showed the films are poly-crystalline with peaks in (100), (002) and (101) directions. When the temperature varied between 303 K to 353 K, the pyroelectric coefficient was increased from 8.60 × 10-9 C/m2K to 3.76 × 10-8C/m2K with a room temperature pyroelectric coefficient value of 8.60×10-9C/m2K. The non-annealed films were found to be transparent between the wavelengths of 600 nm to 3000 nm. The refraction coefficient was found to be varied between 2.0 and 2.2 while the extinction coefficient was found to be zero. The optical bandgap determined using Tauc’s equation was 1.65 eV.

Calcium lead titanate thin films for pyroelectric detector application

This work presents the deposition and characterization of AlxNy thin films for using them as pyroelectric detector material. To test the pyroelectric effect, capacitors with Au electrodes were fabricated. The diameter of the electrodes for capacitor used was 1100 μm while the distances between these two electrodes was 2200 μm. On a 3- inch diameter cleaned silicon wafer a 100-nm thick AlxNy films were deposited using an Al target and Ar:N2 = 1:1 flow and 5 mTorr chamber pressure. Finally, a 100-nm thick Au layer was deposited and lifted off by using conventional photo lithography to form the electrodes of capacitors. All the layers were deposited by radio frequency sputtering at room temperature. The AlxNy thin films were annealed at 700 0C in N2 environment for 10 minutes. X-ray diffraction showed that the films are poly-crystalline with peaks in (100), (002) and (101) directions. The pyroelectric current increased from 3.38 × 10-14 A at 303 K to 1.75 × 10-13 at 353 K. When the temperature varied between 303 K to 353 K the pyroelectric coefficient was increased from 8.60 × 10-9 C/m2K to 3.76 × 10-8 C/m2K while the loss tangent remains almost constant to ~1.5 × 10-5 when the temperature was varied in the same range.

Properties of reactively sputtered AlxNy thin films for pyroelectric detectors

Pyroelectric detectors are the class of thermal detectors which change their spontaneous polarization when there is a change in temperature. The change in the spontaneous polarization occurs due to the absorption of infrared radiation which eventually produces a voltage. This work demonstrates the deposition and characterization of calcium modified lead titatante (Pb1-xCaxTiO3, PCT) thin films for using them as materials of pyroelectric thermal detectors. The PCT thin films were sputtered using an RF sputter system in Ar:O2 environment at room temperature. The thin films were grown on Au electrode. The capacitance was formed by using Au electrodes on top of PCT thin films which were fabricated by sputtering and liftoff. The PCT films were annealed at 450, 500, 550 and 600 °C in O2 environment for 15 minutes. Energy dispersive spectroscopy was done to determine the atomic composition of PCT films. Variations of capacitance, pyroelectric voltage, loss tangent and pyroelectric current between the temperature range 303 K to 353 K were determined. The PCT films were annealed at 550 °C showed the highest value of pyroelectric current and pyroelectric coefficient of 2.45 × 10-12 A and 1.99 μC/m2K respectively at room temperature. The loss tangent did not change much with temperature for all the PCT samples.

Radio frequency sputtered AlxNy thin films for thermal detectors

Microbolometer arrays are the most used technology in thermal infrared imaging. Recent progress in materials and fabrication techniques for these devices have sparked much competition. Vanadium oxide (VOx) has been and is currently the most used material for commercial use of bolometers, followed by amorphous silicon (a-Si). However, other silicon derivatives, such as silicon-germanium (a-SiGe, poly-SiGe, and a-GexSi1-xOy) have shown promise in the recent years. Extensive research is performed to search for different bolometer materials that combine performance, lowcost, and convenience for uncooled thermal infrared imaging applications. In this review article, we discuss materials derived from VOx and Si and their fabrication process used in microbolometers, as well as important figures of merit such as temperature coefficient of resistance, responsivity, detectivity and resistivity.