The Dynamic REd All-sky Monitoring Survey (DREAMS) is a near-infrared fully automated all-sky survey that will be conducted using a custom built 0.5m telescope that will be located at the Australian National University’s Siding Spring Observatory. The telescope feeds 6 individual InGasAs cameras spanning a total field-of-view of 3.8sq.deg using a novel optomechanical design.
Here we present the finished details of the telescope construction and its measured performance as well as details of the survey it will perform of the Southern transient sky.
Type-II superlattice (T2SL) based semiconductors have emerged as a rival to well-established HgCdTe-based IR detectors, promising comparable performance at significantly lower cost. T2SLs are complex nanostructures that exhibit multiple-carrier and highly-anisotropic electronic transport properties, which renders them exceedingly challenging to study experimentally. The lack of reliable experimental data has limited optimisation and modelling efforts, and thus hampered progress. This paper will present a systematic experimental study of electronic transport in InAs/InGaSb T2SLs, by employing world-leading mobility spectrum techniques developed at UWA and state-of-the art T2SL structures from three leading research groups developing infrared detector technologies based on T2SLs.
High performance infrared (IR) sensing and imaging systems require IR optoelectronic detectors that have a high signal-to-noise ratio (SNR) and a fast response time, and that can be readily hybridised to CMOS read-out integrated circuits (ROICs). From a device point of view, this translates to p-n junction photovoltaic detectors based on narrow bandgap semiconductors with a high quantum efficiency (signal) and low dark current (noise). These requirements limit the choice of possible semiconductors to those having an appropriate bandgap that matches the wavelength band of interest combined with a high optical absorption coefficient and a long minority carrier diffusion length, which corresponds to a large mobility-lifetime product for photogenerated minority carriers. Technological constraints and modern clean-room fabrication processes necessitate that IR detector technologies are generally based on thin-film narrow bandgap semiconductors that have been epitaxially grown on lattice-matched wider bandgap IR-transparent substrates. The basic semiconductor material properties have led to InGaAs (in the SWIR up to 1.7 microns), InSb (in the MWIR up to 5 microns), and HgCdTe (in the eSWIR, MWIR and LWIR wavelength bands) being the dominant IR detector technologies for high performance applications. In this paper, the current technological limitations of HgCdTe-based technologies will be discussed with a view towards developing future pathways for the development of next-generation IR imaging arrays having the features of larger imaging array format and smaller pixel pitch, higher pixel yield and operability, higher quantum efficiency (QE), higher operating temperature (HOT), and dramatically lower per-unit cost.
There have been a dramatic increase in the number of optical and radio transient surveys due to astronomical transients such as gravitational waves and gamma ray bursts, however, there have been a limited number of wide-field infrared surveys due to narrow field-of-view and high cost of infrared cameras, we present two new wide-field near-infrared fully automated surveyors; Palomar Gattini-IR and the Dynamic REd All-sky Monitoring Survey (DREAMS). Palomar Gattini-IR, a 25 square degree J-band imager that begun science operations at Palomar Observatory, USA in October 2018; we report on survey strategy as well as telescope and observatory operations and will also providing initial science results. DREAMS is a 3.75 square degree wide-field imager that is planned for Siding Spring Observatory, Australia; we report on the current optical and mechanical design and plans to achieve on-sky results in 2020. DREAMS is on-track to be one of the first astronomical telescopes to use an Indium Galium Arsenide (InGaAs) detector and we report initial on-sky testing results for the selected detector package. DREAMS is also well placed to take advantage and provide near-infrared follow-up of the LSST.
While optical and radio transient surveys have enjoyed a renaissance over the past decade, the dynamic infrared sky remains virtually unexplored from the ground. The infrared is a powerful tool for probing transient events in dusty regions that have high optical extinction, and for detecting the coolest of stars that are bright only at these wavelengths. The fundamental roadblocks in studying the infrared time-domain have been the overwhelmingly bright sky background (250 times brighter than optical) and the narrow field-of-view of infrared cameras (largest is VISTA at 0.6 sq deg). To address these challenges, Palomar Gattini-IR is currently under construction at Palomar Observatory and we propose a further low risk, economical, and agile instrument to be located at Siding Spring Observatory, as well as further instruments which will be located at the high polar regions to take advantage of the low thermal sky emission, particularly in the 2.5 micron region.
HgCdTe has dominated the high performance end of the IR detector market for decades. At present, the fabrication costs
of HgCdTe based advanced infrared devices is relatively high, due to the low yield associated with lattice matched
CdZnTe substrates and a complicated cooling system. One approach to ease this problem is to use a cost effective
alternative substrate, such as Si or GaAs. Recently, GaSb has emerged as a new alternative with better lattice matching.
In addition, implementation of MBE-grown unipolar n-type/barrier/n-type detector structures in the HgCdTe material
system has been recently proposed and studied intensively to enhance the detector operating temperature. The unipolar
nBn photodetector structure can be used to substantially reduce dark current and noise without impeding photocurrent
flow. In this paper, recent progress in MBE growth of HgCdTe infrared material at the University of Western Australia
(UWA) is reported, including MBE growth of HgCdTe on GaSb alternative substrates and growth of HgCdTe nBn
structures.
While optical spectroscopy has shown great promise in a multitude of applications, the cost, size, and fragility of spectrometer instruments have hindered widespread application of the technology. :tvfEMS microspectrometers offer great hope for low-cost, lightweight, and robust spectrometers, paving the way for pervasive use in many fields. In this invited paper, we report on nearly 15 years of development on MEMS spectrometers in our research group, beginning with devices designed for the shortwave infrared (SWIR) and midwave infrared (MWIR), and moving on to our most recent work towards MEMS spectrometers in the visible and near infrared (NIR) as well as the thermal long-wave infrared (LWIR) bands.
Electronic transport parameters in a nominally P+/π/P+ InAs/GaSb type-II superlattice vertical photoconductor
structure for long-wavelength infrared detectors have been characterized employing magnetic field dependent
resistivity and Hall-effect measurements, and high-resolution mobility spectrum analysis. Carrier transport parameters
from both the P+ and nominally π regions were obtained over the 80 to 300K temperature range. At
300 K, the minority carrier electrons in the nominally π region was found to be characterized by a mobility and
concentration of 11,000 cm2/Vs and 1.1×1017 cm-3, respectively. Taking into account our previously reported
room-temperature vertical electron transport parameters,1 the vertical to lateral mobility and carrier concentration
ratios have been determined to be 0.19 and 5.5×10-4 , respectively. A miniband energy gap of 192±8 meV
was estimated from the thermal activation of the minority carrier electrons in the lightly doped InAs/GaSb
superlattice region.
Recent progress in short wavelength infrared MEMS based Fabry-Pérot microspectrometers at The University of
Western Australia is presented. The original monolithic approach has been replaced with a hybrid one due to HgCdTe
restricting the thermal budget and affecting the quality of structural silicon nitride films. The spectral resolution has been
improved by introducing five layer Bragg mirrors and by limiting the electrostatically actuated top mirror bowing and
tilting using stress balancing between layers. In effect the FWHM has been reduced to 30nm at ~2.0μm in comparison to
the ideal theoretical mid-range value of 9nm. Analysis of mirror profiles shows that this difference is a result of
remaining mirror imperfections.
In this work, we report on the measurement of vertical transport parameters in p-doped InAs/GaSb type-II
superlattices for long-wavelength infrared detectors. Variable magnetic eld geometrical magnetoresistance mea-
surements have been employed to extract the vertical transport parameters, since the Hall-eect technique cannot
be employed in the vertical transport conguration. The room-temperature magnetoresistance measurements
were performed employing a kelvin-mode set up, at electric elds not exceeding 25 V/cm and at magnetic eld
intensities up to 12 T. The measured magnetoresistance, shown to exhibit multiple-carrier conduction charac-
teristics, were analyzed using a high-resolution mobility spectrum analysis technique. It is shown that, at room
temperature, the electrical conductivity of the sample is due to four distinct carriers, associated with the major-
ity carrier holes, sidewall inversion layer electrons, and two minority carrier electrons likely associated with two
distinct conduction band levels.
In this paper a brief history of HgCdTe based research and development in Australia is presented. It describes some of
the motivation behind decisions made in Australia related to the HgCdTe international research effort, the early stages of
development of HgCdTe materials and device research in Australia, and main research activities in Australia over a
period of twenty five years. Also presented are some of the major achievements and inevitable failures.
This paper reports work on the development of rugged micro-electromechanical systems (MEMS)-based
microspectrometers for real-time applications in agriculture. The devices are electrostatically actuated, first order Fabry-
Perot tuneable optical filters, hybridised with InGaAs photodiode detectors. Tuning range and resolution of the devices
are 1615 nm to 2425 nm and 52 nm (FWHM) at 2000 nm, respectively. To our knowledge, this tuning range is the
largest reported for a MEMS-based Fabry-Perot filter. Three-layer distributed Bragg reflectors are used for the Fabry-
Perot mirrors, and consist of e-beam evaporated layers of germanium - silicon monoxide - germanium. The moveable
mirror also includes two silicon nitride layers that act as the MEMS flexures, stress compensation layers, and as an
encapsulant for the mirror layers. The spectral resolution matches the theoretical expected for the mirror structures used
when the residual bowing of the mirror (~15 nm across a diameter of 70 μm) is included, and can be improved to ~10 nm
if five layer mirrors are used. The out of band rejection is approximately 20 dB. Experimental results show that the
throughput of the device is sufficient to allow transmittance, specular reflectance and diffuse reflectance spectra to be
measured. The primary outstanding issue is wavelength calibration, and is being addressed using a number of
approaches including incorporation of wavelength calibration standards in the hybrid structure and accurate, real-time
measurement of the separation of the two mirrors.
There is an increasing need for infrared spectroscopic instrumentation that is low-cost and extremely robust for
applications in agriculture, environmental monitoring, food science and medicine. This paper describes a MEMS-based
tunable Fabry-Perot filter that can be directly integrated on a detector. The fabrication process is detector independent,
and has been demonstrated on Si as well as one of the most unforgiving detector material systems, HgCdTe. Results are
presented that show that the technology is applicable for coverage of a wide spectral range, with examples of tuning from
~1600nm to ~2300nm and ~3800nm to ~4800nm using voltages <20V with line widths < 100nm and tuning speeds of
50kHz. Modeling shows that the device should be stable to shocks up to 250G. Line widths and tuning speeds can be
significantly improved using different actuator designs and removal of squeezed-film damping effects. The process uses
a maximum process temperature of 125°C, and is therefore compatible with a wide range of detector materials including
Si, Ge, InGaAs, InSb, as well as more specialized detector materials such as InAs quantum dots and InAs/GaSb
superlattices. Work is currently underway to demonstrate application of microspectrometers fabricated using this
technology in real-time testing of soils for agricultural applications.
In this work gated midwave infrared (MWIR) Hg1-xCdxTe photodiodes are used to investigate the physical
origin of 1/f noise generation. Gated photodiodes were fabricated on liquid phase epitaxy p-type HgCdTe MWIR
material with a vacancy doped concentration of 1.6 x 1016cm-3 and x = 0.31. CdTe was thermally deposited and
used as both a passivant for the HgCdTe and a mask for the plasma-based type conversion, and ZnS was used as
an insulator. Fabricated devices show a R0A of 1-5x104&OHgr;cm2 at 77K with zero gate bias. Application of 2V
to the gate improves the R0A by more than two orders of magnitude to 6.0 x 106&OHgr;cm2, which corresponds to the
p-type surface being at transition between depletion and weak inversion. Trap-assisted tunelling (TAT) current
was observed at negative gate biases and reverse junction biases. For gate biases greater than 3V a field-induced
junction breakdown was observed. Gated photodiodes show diffusion limited behaviour at zero bias above 200K,
and TAT, band-to-band tunnelling, and generation-recombination (GR) limited behaviour below, for gate biases
from -8V to 8V. Field-induced junction breakdown current was also observed to be temperature independent.
Noise current, In = &agr;I&bgr;f-0.5 trend was observed above 200pA reverse bias dark current, with &agr; = 3.5 x 10-5
and &bgr; = 0.82, which corresponds to the TAT dominated region. Below 200pA, junction GR current starts to
dominate and this previously mentioned trend for In is no longer observed. Junction GR current was not seen
to be correlated with 1/f noise in these photodiodes.
We have previously developed a SWIR microspectrometer based on monolithic integration of a parallel plate Micro-
Electro-Mechanical Systems (MEMS) optical filter directly with a HgxCd1-xTe-based infrared detector. The primary
technical challenge in achieving the integration of a MEMS Fabry-Perot filter with the HgxCd1-xTe detector is to keep the processing temperature less than 150°C, as the performance of HgxCd1-xTe based photoconductors degrade at higher process temperatures. In this work we present our results to extend the operation into the 3-5 μm (MWIR) wavelength
range. For our preliminary results, the MWIR microspectrometer was based on a hybrid packaging approach, fabricating
the MWIR filter separately from the HgxCd1-xTe detector; however the key process parameters relating to temperature
control were maintained during fabrication of the MWIR filter, ensuring we can migrate this technology into an
integrated solution. Linewidths of 210 nm, switching times of 20 μs and a tuning range of 900 nm have been achieved.
The tuning speed is limited by squeezed film damping due to the physically narrow gap (&lgr;/2) between the Fabry-Perot
mirrors.
We have developed a microspectrometer based on monolithic integration of a Fabry-Perot optical filter directly with a
HgxCd1-xTe-based infrared detector. The tunable Fabry-Perot is created by a parallel plate MEMS fabricated from two
dielectric mirror stacks separated by an initial air gap of 1.4 μm. We have measured linewidths as low as 55 nm,
switching times of 40 μs and a tuning range of 380 nm. However this tuning corresponds to only 42% of the desired
tuning range, from 1.6-2.5 μm (900 nm). The tuning range is limited by a process called "snap down" which occurs
when the MEMS is drive by a voltage source. It can be shown that for a parallel plate snap down occurs at 1/3 the
initial gap; complete tuning across the SWIR band requires a physical deflection of at least 60% of the gap. We have
developed a modified actuator design which allows 60% tuning of the moveable mirror. Further, the method minimizes
actuation-induced stress gradients which can lead to substantial bowing of the mirror and subsequently broad optical
linewidths. We will compare the results of our current microspectrometer with our new extended tuning designs. These
designs are based on Coventorware and analytical mechanical models combined with optical models for the Fabry-
Perot.
Hyperspectral imaging in the infrared bands is traditionally performed using a broad spectral response focal plane array,
integrated in a grating or a Fourier transform spectrometer. This paper describes an approach for miniaturizing a
hyperspectral detection system on a chip by integrating a Micro-Electro-Mechanical-System (MEMS) based tunable
Fabry Perot (FP) filter directly on a photodetector. A readout integrated circuit (ROIC) serves to both integrate the
detector signal as well as to electrically tune the filter across the wavelength band. We report the first such
demonstration of a tunable MEMS filter monolithically integrated on a HgCdTe detector. The filter structures, designed
for operation in the 1.6-2.5 μm wavelength band, were fabricated directly on HgCdTe detectors, both in photoconducting
and high density vertically integrated photodiode (HDVIP) detectors. The HDVIP detectors have an architecture that
permits operation in the standard photodiode mode at low bias voltages (≤0.5V) or in the electron avalanche photodiode
(EAPD) mode with gain at bias voltages of ~20V. In the APD mode gain values of 100 may be achieved at 20 V at 200
K. The FP filter consists of distributed Bragg mirrors formed of Ge-SiO-Ge, a sacrificial spacer layer within the cavity
and a silicon nitride spacer membrane for support. Mirror stacks fabricated on silicon, identical to the structures that will
form the optical cavity, have been characterized to determine the optimum filter characteristics. The measured full width
at half maximum (FWHM) was 34 nm at the center wavelength of 1780 nm with an extinction ratio of 36.6. Fully
integrated filters on HgCdTe photoconductors with a center wavelength of approximately 1950 nm give a FWHM of
approximately 100 nm, and a peak responsivity of approximately 8 × 104 V/W. Initial results for the filters on HDVIP
detectors exhibit FWHM of 140 nm.
Exposure of p-type HgCdTe material to H2-based plasma is known to result in p-to-n conductivity type conversion. While this phenomenon is generally undesirable when aiming to perform physical etching for device delineation and electrical isolation, it can be utilized in a novel process for formation of n-on-p junctions. The properties of this n-type converted material are dependent on the condition of the plasma to which it is exposed. This paper investigates the effect of varying the plasma process parameters in an inductively coupled plasma reactive ion etching (ICPRIE) tool on the carrier transport properties of the p-to-n type converted material. Quantitative mobility spectrum analysis of variable-field Hall and resistivity data has been used to extract the carrier transport properties. In the parameter space investigated, the n-type converted layer carrier transport properties and depth have been found to be most sensitive to the plasma process pressure and temperature. The levels of both RIE and ICP power have also been found to have a significant influence.
In this article the design, fabrication and characterization of micro-Fabry-Perot filters operating in the mid-wavelength infrared range is presented. Using surface micromachining techniques, low temperature silicon nitride based structures with distributed Bragg mirrors made of Ge/SiO/Ge layers have been fabricated and tested, both mechanically and optically. The membrane/mirror deflection has been measured using an optical profilometer and is estimated to be of the order of 800nm with voltage bias up to 17V while still preserving good mirror parallelism. The respective optical transmission peak shifted from 4.5μm to 3.6μm. Without antireflection coating at the back of the silicon substrate ~50% maximum transmission has been measured at the resonance peaks. The FWHM was measured to be 210+/-20nm, which is ~20% larger than estimated theoretically. In agreement with theoretical modeling, after crossing 1/3 of the cavity length, the membrane/mirror structure has been found to enter into an unstable region followed by snap-down to the bottom mirror surface. In order to prevent this detrimental effect, membranes with anti-stiction bumps have been fabricated demonstrating repeatable structure recovery from the stage of full collapse.
Layered Surface Acoustic Wave (SAW) based sensors with: InOx / SiNx / 36° YX LiTaO3 structure were developed for sensing different hydrogen (H2) concentrations between 0.06% (600ppm) and 1% H2 in synthetic air. This paper presents a comparative study of the sensors performances in terms of response time, recovery time and response magnitude as a function of operational temperature. The SAW devices consist of metal interdigitated electrodes fabricated on lithium tantalate (LiTaO3) piezoelectric substrate forming the input and output Interdigital Transducers (IDTs). A 1 μm thick silicon nitride (SiNx) intermediate layer was deposited over these finger pairs, either by Plasma Enhanced Chemical Vapour Deposition (PECVD) or by r.f. magnetron sputtering. A 100 nm thin film of indium oxide (InOx) deposited by r.f. magnetron sputtering provides the selectivity towards hydrogen. The highest sensitivity for the sensor with r.f. magnetron sputtered SiNx intermediate layer was recorded at 190° C, when the frequency shift of 361 KHz for 1% H2 in synthetic air was recorded. However larger responses were obtained for the sensor with the PECVD SiNx intermediate layer at 290° C, when the large frequency shift of 516 KHz was recorded for the same H2 concentration. Microstructural characterization of the InOx and SiNx films by Atomic Force Microscopy (AFM) and X-Ray Photoelectron Spectroscopy (XPS) is also presented.
A low temperature MEMS process integrated with an infrared detector technology has been developed. The integrated microsystem is capable of electrically selecting narrow wavelength bands in the range from 1.6 to 2.5 μm within the short-wavelength infrared (SWIR) region of the electromagnetic spectrum. The integrated fabrication process is compatible with two-dimensional infrared focal plane array technology. The demonstration prototypes consist of both HgCdTe SWIR photoconductive as well as high density vertically integrated photodiode (HDVIP®) detectors, two distributed Bragg mirrors formed of Ge-SiO-Ge, an air-gap optical cavity, and a silicon nitride membrane for structural support. The tuning spectrum from fabricated MEMS filters on photoconductive detectors indicates a wide tuning range and high percentage transmission. Tuning is achieved with a voltage of only 7.5 V, and the FWHM ranged from 95-105 nm over a tuning range of 2.2 μm to 1.85 μm. The same MEMS filters, though unreleased, and with the sacrificial layer within the optical cavity, have been fabricated on planarised SWIR HDVIP® photodiodes with FWHM of less than 60 nm centred at a wavelength of approximately 1.8 μm. Finite element modelling of various geometries for the silicon nitride membrane will also be presented. The modelling is used to optimize the filter geometry in terms of fill factor, mirror displacement versus applied voltage, and membrane bowing.
A monolithically integrated low temperature MEMS and HgCdTe infrared detector technology has been implemented and characterised. The MEMS-based optical filter, integrated with an infrared detector, selects narrow wavelength bands in the range from 1.6 to 2.5 μm within the short-wavelength infrared (SWIR) region of the electromagnetic spectrum. The entire fabrication process is compatible with two-dimensional infrared focal plane array technology. The fabricated device consists of an HgCdTe SWIR photoconductor, two distributed Bragg mirrors formed of Ge-SiO-Ge, a sacrificial spacer layer within the cavity, which is then removed to leave an air-gap, and a silicon nitride membrane for structural support. The tuning spectrum from fabricated MEMS filters on photoconductive detectors shows a wide tuning range and high percentage transmission is achieved with a tuning voltage of only 7.5 V. The FWHM ranged from 95-105 nm over a tuning range of 2.2 μm to 1.85 μm. Finite element modelling of various geometries for the silicon nitride membrane will also be presented. The modelling is used to determine the best geometry in terms of fill factor, voltage displacement prediction and membrane bowing.
Thin-film MEMS are essential to realization of intelligent integrated microsystems. Of critical importance in such microsystems is the determination and control of mechanical properties in the thin films used for construction of the MEMS, which can be the decisive factor in the realization and subsequent performance, reliability, and long-term stability of the system. In future microsystems the need to fabricate MEMS on temperature sensitive, non-standard substrates will be of particular importance. In this work, mechanical properties of low-temperature (50-300°C) plasma-enhanced chemical vapour deposited silicon nitride thin films have been investigated using depth sensing indentation. Young’s modulus, E, and hardness, H, values obtained for the examined film/substrate bilayers were found to vary asymptotically from the thin film properties for shallow indents to the substrate properties for deep indents. A simple empirical formulation is shown to relate E and H obtained for the film/substrate bilayers to corresponding material properties of the constituent materials via a power-law relation. The temperature of the deposition process was found to strongly influence the thin film mechanical properties. Values of E ~ 150-160GPa and H ~ 14-15GPa were observed for depositions above 225°C. Decreasing the deposition temperature initially caused a moderate and linear decrease in E and H parameters, which was followed by an abrupt decrease in E and H once the deposition temperature was lowered below 100°C, such that E ~ 50GPa and H ~ 3.5GPa at a deposition temperature of 50°C.
Two experimental techniques have been investigated to examine residual stress in low temperature plasma enhanced chemical vapour deposited (PECVD) SiNx thin films: one that measures the stress induced substrate curvature, and the other that takes advantage of the stress induced deformation of freestanding diagnostic microstructures. A general linear dependence of residual stress on deposition temperature is observed, with the magnitude of stress changing linearly from circa 300MPa tensile stress to circa 600MPa compressive stress as the deposition temperature is decreased from 300°C to 100°C. However, the results deviate from the linear dependence by a different degree for both measurement techniques at successively lower deposition temperatures. The stress values obtained via the substrate curvature method deviate from the linear dependence for deposition temperatures below 200°C, whereas the values obtained via the diagnostic microstructures method deviate from the linear dependence for deposition temperatures below 100°C. Stress uniformity over the deposition area is also investigated.
Vacuum bake-out for out-gassing is a required process in packaging of devices which are designed to operate at cryogenic temperatures. This process may be problematic for HgCdTe devices, even at relatively low temperatures, since the material is sensitive to heat induced changes. The effect of vacuum baking on HgCdTe photodiode characteristics and performance is investigated through I-V and spectral responsivity measurements. The photodiodes were fabricated on LPE grown HgCdTe on lattice matched CdZnTe substrates. The surface was passivated with thermally evaporated CdTe and the p-n junction was formed by plasma induced p-to-n type conversion. The I-V characteristics of the devices were then measured and the devices were baked under vacuum for varying times at 80°C. This simulates the vacuum bakeout required in vacuum packaging of the devices in cryogenic dewars. The results indicate that the vacuum baking process can significantly modify the I-V characteristics of the photodiodes. There is an initial improvement in device characteristics after a 6 hour bake at 80°C, with a five times increase in the zero-bias dynamic resistance. Further baking sees the dynamic resistance decrease slightly. An insight into the mechanisms and parameters that are affected by the vacuum baking process is also gained by studying the I-V characterisics of the fabricated photodiodes before and after baking. It is observed that tunneling dark currents are significantly reduced after baking.
Current infra-red detectors are limited to detect broad windows in
transmission. By adding Fabry-Perot filtering to these detectors
multi- and hyper-spectral detectors can be fabricated. However,
filtering will reduce the signal available to the detector. In
order to decrease the noise (thereby increasing the signal to
noise ratio), the detector can be moved into the resonant cavity
of the filter. The design of the mirrors is changed by placing the
detector with the resonant cavity. Materials for the design of a
resonant cavity enhanced mercury cadmium telluride detector are
investigated in this paper.
Silicon nitride membrane based MEMS technologies are used in numerous optical micro-systems for many practical applications. In recent years significant effort has been invested into miniaturization of present spectrometers leading to development of compact systems based on linear detector arrays. However, further miniaturization requires the application of MEMS technology. MEMS based micro-Fabry-Perot cavity structures with a flexible mirrors operating in the visible and near infrared range have already been demonstrated. Extension of the technology to mid- (MWIR-3μm to 5μm) and long-wave infrared (LWIR - 8μm to 12μm) seems to be a natural development. However, this requires much larger deflection of the movable mirror of the Fabry-Perot cavity: ~1μm for 3μm to 5μm and ~2μm for 8μm to 12μm wavelengths range. Consequently, precise control of the intrinsic stress in the silicon nitride support film is needed. Our experiments show that suitable silicon nitride properties can be obtained by carefully controlling the process parameters during plasma enhanced CVD growth. In addition to the material requirements, the mechanical structure of the flexible mirror is also important. In order to optimize the mirror structure/shape, finite element analysis was undertaken. The results show that for certain silicon nitride support shapes and thicknesses, mirror deflections needed for both the MWIR and LWIR wavelength regions are possible.
While differential Hall measurements are a standard approach to determination of junction depth in multi-layer semiconductors, significantly more information can be obtained from magnetic field dependent differential Hall measurements. When such measurements are treated using Quantitative Mobility Spectrum Analysis (QMSA), detailed depth resolved profiling of both carrier concentrations and mobilities can be achieved, giving important data directly related to potential device performance. The doping profile is obtained by performing a series of etch-back experiments with magnetic field dependent Hall measurements performed between the etching steps. This technique is illustrated on a number of vacancy and gold doped Hg1-xCdxTe p-type epilayers, which have been partly or wholly converted to n-type by a reactive ion etching (RIE) process. The QMSA analysis reveals that there are several electron species present in the layers as well as the original p-type carrier. The electron species have been identified as low mobility surface electrons, and high and low mobility electrons located at various depths through the epilayer. It also indicates that the p-to-n conversion depths range from less than 0.5micrometers for vacancy doped Hg0.7Cd0.3Te material, to more than 17micrometers for Au-doped Hg0.8Cd0.2Te for the same type conversion conditions.
Reactive ion etching (RIE) of HgCdTe using CH4:H2 is known to generate p- to n-type conversion in both intrinsically doped and extrinsically doped p-type HgCdTe. The use of RIE to form n-on-p junctions in planar diodes has a number of advantages including state of the art diode performance, high uniformity, passivation of the junction at the surface throughout processing, the possibility of the formation of deep junctions, and removal of any need for high temperature processing after junction formation. However, it has long been believed that H2 based plasma junction formation techniques will be long-term unstable. Initial results are presented indicating that surface passivation plays a major role in determining the stability of planar junctions formed using H2 based RIE. Comparisons of ZnS and CdTe passivation for n-on p-junctions formed on x approximately 0.3 Hg1-xCdxTe show dramatic differences in 2 to 3 hour, 80 degrees C bake stability tests. Diodes fabricated using either passivant initially exhibit R0A performance close to the theoretical limit, but are degraded after a 2 hour, 80 degrees C bake. Diodes with CdTe passivation have moderate performance as fabricated, but exhibit improvement rather than degradation after 3 hour, 80 degrees C bake. Such results indicate that planar junctions formed using H2 based RIE may offer a viable technology for low cost, highly uniform, large area IR detector arrays if passivation issues are satisfactorily resolved. Finally, a dual layer ZnS/CdTe passivation process is introduced which results in bake-stable devices after a 17 hour, 80 degrees C bake.
The current-voltage characteristics measured over a wide temperature range are reported for HgCdTe mid-wavelength IR n-on-p photodiodes fabricated using a novel junction formation technology. The planar homojunction device junctions were formed on LPE grown vacancy doped HgCdTe using a reactive ion etching (RIE) plasma induced conversion process. The zero bias dynamic resistance - junction area product, RoA, was 4.6 X 107 (Omega) .cm2 at 80K an is comparable to the best planar diodes reported using conventional and significantly more complicated ion implantation junction formation technology. Arrhenius plots of RoA exhibit an activation energy equal to the bandgap, Eg, and show that the diodes are diffusion limited for temperatures >= 130K. In order to further compare this junction formation technology to other techniques, a series of temperature dependent 1/f noise measurements were performed. Form this study the activation energy for 1/f noise in the region where the diodes are diffusion limited was found to be 0.7Eg. Energies close to this value have previously been associated with Hg vacancies in HgCdTe. These results are similar to those obtained from high quality HgCdTe photodiodes fabricated using mature ion implantation technology. However, the plasma based technology used in this work is significantly less complex and does not require any high temperature annealing steps.
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 application of HgCdTe as an infrared sensor material requires an accurate knowledge of the electrical parameters which are usually obtained from measurements of both the Hall coefficient and electrical conductivity as a function of temperature and magnetic field. Interpretation of the experimental data is difficult due to the unique band structure of HgCdTe and due to the presence of defects, surface layers, influence of the substrate, as well as localized non-uniformities. All of these effects can lead to 'anomalous' magnetic field dependence of the Hall data. In this paper we present the state of the art analysis of magnetic- field dependent Hall data including the recently developed quantitative mobility spectrum analysis (QMSA). This technique requires no prior assumptions to be made as to carrier type or number, and gives quantitatively accurate results for each carrier concentration and mobility. Of particular interest is a comparative study of the results obtained from QMSA with those obtained from previously developed techniques such as the Beck and Anderson mobility spectrum (MS) analysis and the recently described hybrid approach proposed by Meyer et al. which combines MS analysis with a multi-carrier fitting (MCF) procedure (MS + MCF). This comparison is illustrated with the analysis being performed on experimental Hall measurements from an epitaxial HgCdTe film which displayed 'anomalous' Hall characteristics.
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