In modern nano-scale lithography, an essential role of the source, the illumination, and projection lenses is to deliver the precise amount of energy at a specific wavelength to the photoresist deposited on a wafer surface during exposure. Unfortunately, the source of the most advanced lithography processes may produce unwanted infrared components passing through the illumination and projection lenses and reaching the wafer surface. These infrared residues can cause local heating resulting in deformation of the optical elements and the exposed wafer, thus causing deterioration of the image quality. Some infrared spectrum components are in the band from 2 μm to 12 μm. An infrared detector that can measure only these spectral components of the exposure beam, without being affected by the much more powerful exposure spectral component, is helpful for optics diagnostic purposes and improving imaging quality. In this paper, an ultra-thin uncooled integrable-on-chip linear array infrared detector to measure the band of 2-12 μm infrared radiation is designed and fabricated based on the photovoltaic multiple junction heterostructure from VIGO Photonics, made of a HgCdTe narrow bandgap semiconductor. Features such as zero bias, low noise, and fast response, together with a wide active window, make the detector unique for use in the mid-infrared band. Besides lithography applications, the new detector can be useful in testing, inspection, and equipment using infrared sources such as: He-Ne lasers (0.6 to 4 μm), STEAM lasers (2 to 200 μm), CO2 lasers (5 to 11 μm), InGaAsP lasers (0.8 to 3 μm), and PbSnTe (3 to 20 μm) and PbSnSe (7 to 40 μm) lasers.
A Peltier cooled long wavelength infrared (LWIR ) position sensitive photon detector (PSD) based on the lateral effect is reported for the first time. It is a modified PIN LWIR HgCdTe photodiode forming the tetra lateral PSD of the photosensitive area 1×1 mm2, cooled to 205 K, optimized for the 4-11 µm wavelength band and reverse biased. The position resolution close to 1 µm was achieved with 32x averaging of 100 ns QCL laser pulses of the 10.5 µm wavelength and the 0.3 nJ energy focused on the spot of 240 µm 1/e2 diameter, with the box-car integration times of 1 µs and correlated double sampling.
We report on the quadrant photon HgCdTe detectors optimized for 2-11 μm wavelength spectral range and Peltier or no cooling, and photosensitive area of a quad-cell of 1×1 to 4×4 mm. The devices are fabricated as photoconductors or multiple photovoltaic cells connected in series (PVM). The former are characterized by a relatively uniform photosensitive area. The PVM photovoltaic cells are distributed along the wafer surface, comprising a periodical stripe structure with a period of 20 μm. Within each period, there is an insensitive gap/trench < 9 μm wide between stripe mesas. The resulting spatial quantization error prevents positioning of the beam spot of size close to the period, but becomes negligible for the optimal spot size comparable to a quadrant-cell area. The photoconductors produce 1/f noise with about 10 kHz knee frequency, due to bias necessary for their operation. The PVM photodiodes are typically operated at 0 V bias, so they generate no 1/f noise and operation from DC is enabled. At 230 K, upper corner frequency of 16 to 100 MHz is obtained for photoconductor and 60 to 80 MHz for PVM, normalized detectivity D*~6×107 cm×Hz1/2/W to >1.4×108 cm×Hz1/2/W for photoconductor and >1.7×108 cm·Hz1/2/W for PVM, allowing for position control of the radiation beam with submicron accuracy at 16 MHz, 10.6 μm wavelength of pulsed radiation spot of ~0.8 mm dia at the close-to-maximal input radiation power density in a range of detector linear operation.
In this paper, the results of the initial work on determining the photoelectric properties of graphene detector operating in a photoconductive mode are presented. Graphene is considered as a material for uncooled fast detectors. The investigation has been done by electrical and optical characterization. Two values of substrate resistivity have been used in the project – below 1 Ωcm and higher than 1.6 kΩcm. Measurements of detectors response to short, strong light pulses were conducted. Presented studies show that the use of high resistivity substrates is necessary to prevent capacitive shorting of the signal to the substrate, causing signal losses and increasing response time.
The nominally sharp interfaces in layered HgCdTe heterostructures are affected by interdiffusion for growth at a
temperature of above 300 K. Significant composition and doping grading always occur in layered HgCdTe
heterostructures grown with MOCVD (360°C), LPE (480°C), and ISOVPE (500°C) epitaxial techniques. MBE (170°C) is
the only technique that practically does not introduce significant diffusion grading, but it can be introduced by post
growth processing, especially during dopants activation. The purpose of this paper was to explain how the grading
affects performance of photodetectors operating at near room temperatures (190-300 K). Influence of the growth related
and intentional grading on dark currents and response time was studied with numerical calculations and experiments.
Practical infrared devices with controlled grading were grown with programmed MOCVD and characterized. The studies
revealed interesting properties of the N+pP+ devices with graded interfaces. Controlled grading minimizes Auger,
Shockley-Read and tunnel currents, increases responsivity and linearity range. The grading is also important for high
frequency performance of the devices.
Sensitive and broadband detection of MWIR and LWIR radiation with any wavelength within the 2 to 16 μm
spectral range and bandwidth from DC to GHz range is reported. Recent efforts have been concentrated on the extension
of useful spectrum range above 13 micrometers. This was achieved with improved architecture of the active element, use
of monolithic optical immersion technology, enhanced absorption of radiation, dedicated electronics, series connection
of small cells and applying more efficient Peltier coolers.
We have developed various types of photodetectors operating without cryocooling. Initially, the devices were mostly
used for uncooled detection of CO2 laser radiation. Over the years the performance and speed of response has been
steadily improved. At present the uncooled or Peltier cooled photodetectors can be used for sensitive and fast response
detection in the MWIR and LWIR spectral range. The devices have found important applications in IR spectrometry,
quantum cascade laser based gas analyzers, laser radiation alerters and many other IR systems. Recent efforts were
concentrated on the extension of useful spectral range to >13 μm, as required for its application in FTIR spectrometers.
This was achieved with improved design of the active elements, use of monolithic optical immersion technology,
enhanced absorption of radiation, dedicated electronics, series connection of small cells in series, and last but not least,
applying more efficient Peltier coolers. Practical devices are based on the complex HgCdTe heterostructures grown on
GaAs substrates with MOCVD technique with immersion lens formed by micromachining in the GaAs substrates. The
results are very encouraging. The devices cooled with miniature 4 stage Peltier coolers mounted in TO-8 style housings
show significant response at wavelength exceeding 16 μm.
We report fast and sensitive long (10 μm) wavelength photodetectors operating at near room temperature. The
devices are based on HgCdTe multilayer heterostructures grown by MOCVD on (211) and (111) GaAs substrates.
Device-quality heterostructures are obtained without any post growth anneal. The recent improvements of MOCVD
growth were: optimized design of the device architecture to increase speed of response, better IMP growth
parameters selection taking into account interdiffusion time changes during growth, stoichiometry control during
growth by the layer anneal at metal rich vapors during each IMP cycle, precursor delivery to the growth zone
monitored with IR gas analyzer, additional metal-rich vapor anneal at the end of growth and passivation of detector
structures with wide gap HgCdTe overgrowth deposition. Monolithic optical immersion of the detectors to GaAs
microlenses has been applied in purpose to improve performance and reduce RC time constant. The response time of
the devices have been characterized using 10μm quantum cascade laser, fast oscilloscope with suitable
transimpedance amplifier as a function of detector design, temperature and bias. Detectivity of the best
thermoelectrically cooled optically immersed photodiodes approaches 1⋅1010 cmHz1/2/W at ≈10 μm wavelength. The
response time of small area decreases with reverse bias to response achieving <100 ps with weak reverse bias.
We report here the recent progress at VIGO/MUT (Military University of Technology) MOCVD Laboratory in the growth of Hg1-xCdxTe multilayer heterostructures for various types of uncooled infrared devices. The detectors are optimized for any wavelength within 1-12 μm spectral range. Hg1-xCdxTe growth with interdiffused multilayer process (IMP) technique has been improved. The total flow of the carrier gas was optimized to improve lateral uniformity of the composition and doping. The parasitic transient stages between the CdTe and HgTe phases were reduced to reasonable minimum. As a result, we were able to grow layers with homogeneous composition and doping, characterized by steep interfaces. The additional benefits were improved morphology, reduced dislocation density, and minimized consumption of precursors. The other issues addressed in this work were growth of heavy As-doped low-x and heavy Idoped high-x materials. Special modification to IMP process has been applied for in-situ control of stoichiometry. To maintain low vacancy concentration, special growth finish procedure has been developed. No post-growth thermal anneal was necessary for device-quality material. The MOCVD grown heterostructures have been successfully used for advanced uncooled infrared photodetectors such as multiple heterojunction photodiodes, multicolor and specially shaped spectral response multiabsorber devices.
The performance of uncooled photodetectors operating in the middle and long wavelength spectral range is limited by the noise originated from thermal generation and recombination processes in semiconductors. The noise level exponentially increases with decreasing band gap of the semiconductor. Therefore, the uncooled short wavelength devices are characterized by good performance while the long wavelength ones are much less sensitive. The consequence is very poor performance of long wavelength devices at short wavelength range. We report here two-lead multilayer photoconductors that operate over a wide spectral band with performance improved by a large factor at short wavelength ranges. The devices consist of several stacked active regions (absorbers) with their outputs connected in parallel so the resulting output signal current is the sum of the signals generated at all active regions. Due to a high photoelectric gain in the wider gap absorbers and low thermal generation and recombination, the devices offer significantly better performance at short wavelengths while the long wavelength response remains essentially unaffected. The practical devices have been obtained using complex Hg1-xCdxTe heterostructures grown on CdTe or GaAs substrates by ISOVPE, MOCVD or combination of the two epitaxial techniques. An example is an uncooled photoconductor operating up to 11μm, with response at 0.9-4μm increased by ≈3 orders of magnitude in comparison to the conventional 11 μm device.
Resonant cavity enhanced photodetectors are promising candidates for applications in high-speed optical communications due to their high quantum efficiency and large bandwidth. This is a consequence of placing the thin absorber of the photodetector inside a Fabry-Perot microcavity so the absorption could be enhanced by recycling the photons with resonance wavelength.
The performance of uncooled resonant cavity enhanced InGaAs/InAlAs photovoltaic devices operating near 1.55 μm has been studied both theoretically and experimentally. The analyses include two different types of structures with cavity end mirrors made of semiconducting and metallic reflectors as well as semiconducting and hybrid (dielectric Si3N4/SiO2 + metal) Bragg reflectors. Optimization of the device design includes: absorption layer thickness, position of absorption layer within the cavity and number of layers in distributed Bragg reflectors.
Dependence of absorption on wavelength and incidence angle are discussed. Various issues related to applications of resonance cavity enhanced photodiodes in optical systems are considered.
Practical devices with metallic and hybrid mirrors were fabricated by molecular beam epitaxy and by microwave-compatible processing. A properly designed device of this type has potential for subpicosecond response time.
Recent progress at VIGO/MUT (Military University of Technology) MOCVD Laboratory in the growth of Hg1-xCdxTe (HgCdTe) multilayer heterostructures on GaAs/CdTe substrates for uncooled infrared photodetectors is presented. The optimum conditions for the growth of single layers and complex multilayer heterostructures have been established. One of the crucial stages of HgCdTe epitaxy is CdTe nucleation on GaAs substrate. Successful composite substrates have been obtained with suitable substrate preparation, liner and susceptor treatment, proper control of background fluxes and appropriate nucleation conditions. The other critical stage is the interdiffused multilayer process (IMP). The growth of device-quality HgCdTe heterostructures requires complete homogenization of CdTe-HgTe pairs preserving at the same time suitable sharpness of composition and doping profiles. This requires for IMP pairs to be very thin and grown in a short time. Arsenic and iodine have been used for acceptor and donor doping. Suitable growth conditions and post growth anneal is essential for stable and reproducible doping. In situ anneal seems to be sufficient for iodine doping at any required level. In contrast, efficient As doping with near 100% activation requires ex situ anneal at near saturated mercury vapors. As a result we are able to grow multilayer fully doped (100) and (111) heterostructures for various infrared devices including photoconductors, photoelectromagnetic and photovoltaic detectors. The present generation of uncooled long wavelength infrared devices is based on multijunction photovoltaic devices. Near-BLIP performance is possible at ≈ 230 K with optical immersion. These devices are especially promising as 7.8-9.5-μm detectors, indicating the potential for achieving detectivities above 109 cmHz1/2/W.
Fast refractive microlenses are increasingly important as optical concentrators for uncooled infrared photodetectors. They are used in purpose to improve performance and speed of response. Refractive microlenses formed directly onto semiconductor materials draw much attention because they facilitate monolithic integration with active element of infrared photodetector. Gallium arsenide due to its superior optical and mechanical properties is a material of choice for fabrication of microlenses. We have developed process for fabrication of GaAs refractive microlenses monolithically integrated with InGaAs and HgCdTe photodetectors, both as single element devices and two-dimensional arrays. Specially designed machine tool has been used for preparation of relatively large single spherical GaAs microlenses with 0.5 mm-10 mm diameter. The microlens-detector arrays were prepared using a combination of ion milling and wet chemical etching. The typical process involves one photolithography, one ion milling and one or two chemical etching steps. More advanced procedures have also been proposed to improve quality of the lenses. The lenses can be optimized as optical concentrators for IR photodetectors with circular, square, rectangular and other geometries. This process is especially convenient for fabrication of lenses with size less than 50 μm, but larger lenses with size exceeding 300 μm can be prepared as well with some modifications of the fabrication procedures.
The paper reports on the design and fabrication of LPE-grown (formula available in paper) heterojunction photodetectors operating in the 2-2.4 micrometers wavelength region. Experiments on LPE growth of high-x- content quaternaries as well as optimization of device processing has been carried out. LPE growth at Tapproximately equals 530DEGC enabled obtaining lattice matched heterostructures with 19% indium in the active layer In (formula available in paper) and photodetectors with (lambda) co=2.25micrometers . By increasing the temperature of epitaxial growth to 590DEGC In (formula available in paper)heterostructures (with 23%indium content suitable for photodetectors with (lambda) co=2.35 micrometers have been obtained. Mesa-type photodiodes were fabricated by RIE in Ccl (formula available in paper) plasma and passivated electrochemically in (formula available in paper). These devices are characterized by differential resistance up to (formula available in paper) and the detectivity in the range (formula available in paper), in dependence on the photodiode active area cutoff wavelength.
We report an advanced Hg1-xCdxTe photovoltaic detector based on monolithic Hg1-xCdxTe heterostructure with 3-dimensional architecture. It consists of a narrow gap, p-type Hg1-xCdxTe small size (approximately equals 10x10x7 micrometers ) absorber of infrared radiation buried in a graded gap Hg1-xCdxTe layer surrounding absorber and heterojunction contacts obtained by selective doping of the graded gap Hg1-xCdxTe layer surrounding the absorber region. The heterostructure is passivated with a ZnS layer and coated with contact metallization to nPLU and p-type regions. The device is supplied with 50x50 micrometers immersion microlens formed directly in the CdZnTe substrate. These two layers also play a role of a mirror that improves quantum efficiency for weakly absorbed infrared radiation. In addition, the mirror eliminates backside incidence of thermal radiation, which prevents generation of dark current. The design of the device is optimized to achieve the best compromise between requirements of good absorption and collection efficiency; low thermal generation; and low parasitic impedance. Test devices have been prepared using the modified isothermal vapor phase epitaxy of Hg1-xCdxTe on profiled CdZnTe substrates, negative epitaxy of Hg1-xCdxTe to widen band gap of surface regions, selective doping, multiple chemical etching and ion milling, vacuum deposition of dielectric and metal layers.
We report here the use of isothermal vapor phase epitaxy to grow 3D Hg1-xCdxTe heterostructures for photoconductive, photovoltaic and photoelectromagnetic infrared detectors operated at near room temperatures. A reusable two-zone atmospheric pressure growth system has been developed.the system makes it possible not only to grow epilayers but also to perform in situ other processes such as high temperature annealing to control the compositional grading, the low temperature annealing for reduction of native acceptor concentration, and doping with foreign impurities. The required various composition profiles have been theoretically predicted and then implemented changing the temperature and mercury pressure during growth and subsequent thermal treatment. In addition, post-growth etching, substrate shaping, selective epitaxy, and negative epitaxy have been used to achieve 3D band gap profiles. The photoconductors were based on lightly p-type doped epilayers. Low diffusion length, weak absorption of radiation and a very low junction resistance makes it difficult to obtain useful performance of longwavelength photovoltaic devices operating at near room temperature. This was overcome with development of multiple heterojunction photovoltaic devices in which short elements were connected in series. To improve the performance of any type of heterostructure photodetector, monolithic optical immersion has been used. Detectivities as high as 1 X 108 cmHz1/2/W and 1 X 109 cmHz1/2/W were obtained at (lambda) equals micrometers and temperatures of 300 K and 220 K, respectively.
The p+-n homojunctions were formed by arsenic diffusion in the HgCdTe monocrystals and epilayers. Photodiode performance was established by measuring the current-voltage and spectral response characteristics. LWIR photodiodes are background limited at 77 K. MWIR photodiodes effectively operate at elevated temperature around 200 K and exhibit near BLIP performance when optical immersion is used.
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