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.
The authors describe a procedure they believe to be optimum for determining the path of a ray through an atmosphere described by a radially varying refractive index profile on the Earth. The result is exact for patched quadratic profiles inverse problem and is suitable for calculating ray paths in complex profiles. The method also facilitates greatly the inverse problem.
A method for reducing noise in near-IR laser communications has been proposed that relies upon the dual wavelength output of the He-Xe laser having a high level of noise coherence. However, in transmissions through the atmospheric boundary layer, an additional and significant noise component is added by atmospheric scintillation. These scintillations are mainly limited to frequencies of less than 1 kHz and are correlated in the two laser channels to a degree determined by the channel wavelength separation, the transmission range and the severity of the turbulence regime. To analyze the propagation of waves in random media one normally considers the statistics of the field. In the case of small angle forward scattering, which is the case of interest in laser propagation, field moments higher than the fourth are so difficult to solve that no solutions are known outside of the asymptotic weak and strong approximations. An alternative approach is to conduct numerical experiments in which one generates a realization of the random medium (with the desired statistics) and then calculates the wave field. We have numerically modeled the spatial irradiance intensity as a function of range from a point source under turbulence regimes typical of daytime conditions near the Earth’s surface. Simulations were performed for two closely separated channels in the near-IR (1556.5 and 1558.1 nm). We present the results of these simulations together with the implications for the mitigation of atmospheric scintillation noise by common mode rejection.
There is increasing interest in free space optical communications as an alternative to fibre optics and radio frequency communications, particularly in 'last mile' applications and applications with weight and power restrictions e.g. communications with unmanned aerial vehicles. The potential advantages of free space optical communications include: high bandwidth; no licensing issues; smaller, lighter payloads; low probability of intercept; and immunity from interference/jamming. However, propagation through the atmosphere is subject to atmospheric scintillation noise affecting the signal-to-noise ratio (SNR), effectively reducing the range and bandwidth of the communication link. This scintillation is experienced even over relatively short propagation paths and is caused by small temperature variations in the atmosphere, resulting in index of refraction changes. In this paper we present a technique to correct for atmospheric scintillation noise in free space optical communications and laser remote sensing. It uses common mode rejection to remove co-channel noise, where each channel is transmitted on separate, but closely spaced, wavelengths. The signal-to-noise ratio is significantly increased, thereby increasing the range and/or bandwidth of the link. To date, tests have been conducted with analogue audio and video transmissions. This has been successful, with improvements of up to 12dB in SNR having been demonstrated. This has been limited by the current implementation, which is only at prototype stage -- the ultimate achievable improvement in SNR is anticipated to be significantly higher.
This paper will detail investigations into rapid thermal annealing (RTA) treatment of ohmic contacts to reactive ion etch (RIE) damaged p-type GaN. It was found that annealing at moderate temperatures in N2 atmosphere can improve the ohmic nature of contacts to RIE-damaged p-GaN. After chlorine-based RIE treatment of the p-GaN surface the sheet resistance and contact resistivity of the ohmic contact metallisation scheme increased, and the contacts became extremely non-ohmic. After RTA treatment in N2 atmosphere at 550°C, linearity of the I-V curves was substantially improved, and the contact resistivity decreased. This improvement is most likely related to improvements in the metal-GaN interface and/or improvements in the bulk material when protected by the contact metal. Unprotected surfaces were further damaged (manifested as higher sheet resistance) by the annealing procedure.
The effect of 60Co gamma-irradiation on the device characteristics of Al0.35Ga0.65N-GaN heterojunction field effect transistors (HFET) has been investigated using DC and geometrical magnetoresistance measurements. Cumulative gamma-ray doses up to 20 Mrad(Si) are shown to induce drain current degradation, negative threshold voltage shifts and reverse gate leakage current degradation. Analysis of drain magneto-conductance characteristics measured at 80 K indicated an increase in two-dimensional electron gas (2DEG) sheet concentration with accumulated radiation dose. More importantly, the 2DEG mobility-concentration characteristics are noted to remain aproximately constant for total gamma-radiation doses up to 20 Mrad(Si), indicating that the areal density of radiation-induced defects at the heterointerface is likely to be negligible. The threshold voltage shifts are therefore attributable to the introduction of relatively shallow radiation-induced defects in the AlGaN barrier region and/or to defects introduced at the gate-barrier interface. Although the drain conductance characteristics manifested similar degradation trends at 80 and 300 K, the 2DEG parameters obtained at 300 K exhibited significant scatter with increasing dose, possibly a manifestation of device instabilities induced by radiation-induced surface defects in the ungated access region near the edge of the gate. Device failure due to severe gate leakage and loss of gate control over the 2DEG charge, occurred after a total dose of 30 Mrad(Si).
We discuss investigations into a contactless UV-enhanced wet etching technique for GaN. The technique utilizes the oxidising agent potassium persulfate to consume photogenerated electrons, thus avoiding the need for an electrical contact to an external cathode. The etch rate is strongly dependent on illumination intensity and uniformity and on the pH of the KOH solution, as is the roughness of the etched surface. The implementation of a dual illumination scheme whereby an additional UVC lamp was used to illuminate only the solution and not the wafer, resulted in an increased etch rate and smoother etched surface. Finally, the ohmic nature of contacts deposited on n-type GaN that had been etched in this manner was found to be improved compared to contacts on the unetched surface.
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.
Transmission measurements were taken through the atmosphere over surf at Scripps Institute of Oceanography
Pier, La Jolla, USA, during January and February 1996, as a part of a trial of the Electro-optic Propagation
Assessment in Coastal Environments (EOPACE) campaign. Simultaneous measurements in the 3-5um and 8-
l2um infra-red bands were taken using a Mercury-Cadmium-Telluride (MCT) and Indium Antimonide (InSb)
sandwiched detector. The transmissometer was designed so that the image of the source at the receiver is many
times the detector size. In this way, the effect of beam wander and detector non-uniformity are negligible.
Measurements are relative in the sense that there was no attempt to calibrate transmission to a refened-standard
source like a black-body or to use transmission codes. Meteorological and aerosol data were also collected
over the path. It was found that the coastal environment does significantly influence the transmission of infrared
radiation even over a path as short as a few hundred metres.
This paper reviews the development and present status of mid-wavelength infrared (MWIR) HgCdTe photodiode technology in Australia. MWIR n-on-p photodiodes have been produced by both mercury in-diffusion and by boron ion-implantation on Hg-vacancy doped p-type HgCdTe. The photodiodes are planar structures passivated with thermally evaporated ZnS. High performance MWIR photodiodes have been developed with RoA product of 2 X 106 (Omega) cm2 at 77 K. Recent work has focused on developing novel passivation processes based on anodic sulphurdization in which thermally evaporated ZnS is subsequently anodized in a non-aqueous sodium sulphide solution. Preliminary results suggest that this improves the insulating properties of thermally evaporated ZnS layers. The leakage currents, as measured through a 1 mm2 MIS structure at a 1 V bias at 77 K, for the anodized ZnS layer are in the sub-picoamp range for all devices fabricated, whereas for the non-anodized layer the leakage currents are typically much larger by up to 3 - 4 orders of magnitude.
A specialized transmissometer has been installed at a location exposed to open ocean near Perth, Western Australia. The pathlength is 3 km and is 1.5 m above high water. The objective is to conduct atmospheric transmission measurements within the marine surface layer in the 3 - 5 and 8 - 12 micrometer transmission windows to quantify the degradation in transmission as a function of sea and meteorological conditions. A further experiment is being undertaken to examine the statistical properties of rapid fluctuations in the intensity of the transmitted beams.
The performance enhancement that can be achieved by using wider bandgap Hg1-xCdxTe as a capping layer for long wavelength infrared Hg1-xCdxTe photoconductors has been studied using both theoretical and experimental results. In the theoretical section it is shown that in the presence of a suitable energy barrier between the Hg1-xCdxTe infrared absorbing layer and the overlaying passivation layer, the high surface recombination rate which is usually present at the semiconductor/passivant interface does not have a significant effect on device performance. The energy barrier, which repels photogenerated minority carries from the semiconductor surface, is introduced by employing a Hg1-xCdxTe wafer which consists of a wider bandgap layer grown on a long wavelength infrared absorbing layer. A device model is presented which allows the responsivity to be calculated by taking into account surface recombination at both the front and back interfaces, thickness of capping and absorbing layers, recombination at the heterointerface, and variations in equilibrium carrier concentration. Experimental results are presented for x equals 0.22 n-type Hg1-xCdxTe conventional single layer photoconductors, and for heterostructure photoconductors consisting of an absorbing layer of x equals 0.22 capped by an n-type layer of x equals 0.31. Using the derived model, a heterointerface recombination velocity of 250 cm/s is extracted for the two layer photoconductor. Experimental results indicate that photoconductors which include a wider bandgap capping layer give a responsivity improvement of more than five times that of a conventional single layer device. Furthermore, heterostructure photoconductors are shown to be insensitive to the condition of the semiconductor/passivant interface.
The performance of Hg1-xCdxTe long wavelength infrared (LWIR) photoconductors is strongly dependent on the semiconductor surface conditions and contact characteristics. In this paper we review these effects in relation to obtaining an optimum device technology suitable for use in two-dimensional infrared focal plane arrays (IRFPAs) based on the fabrication of high performance LWIR photoconductors on epitaxially grown Hg1-xCdxTe. Although the proposed design can be applied to a variety of epitaxially grown Hg1-xCdxTe material, for optimum performance the starting Hg1-xCdxTe semiconductor consists of epitaxially grown heterostructure layers in which a two-dimensional mosaic of lateral design photoconductors are fabricated. The heterostructure layer provides high performance devices at greatly reduced power dissipation levels, while the unique design allows for the high density integration of photoconductors in a two-dimensional array geometry with high fill factor. The proposed photoconductor array with n+ blocking contacts has been experimentally verified in a 3 X 3 array format with all elements in the array exhibiting background limited infrared photodetector (BLIP) performance at 80 K. Performance issues such as response uniformity, pixel yield, fill factor, crosstalk, power dissipation, detector impedance, array architecture, and maximum array size are discussed in relation to the suitability of the proposed photoconductor structure for use in IRFPA modules. It is found that in many cases the proposed photoconductor technology has the potential to deliver significant advantages, such as higher yield, higher fill factor, better uniformity, less crosstalk, and larger potential array size, in comparison to an IRFPA design based on photovoltaic technology.
This study is concerned with the temperature dependent noise and responsivity performance of n-type x equals 0.32 Hg1-xCdxTe photoconductors. The fundamental noise sources that ultimately limit the specific detectivity, D(lambda )*, at temperatures T > 80 K are identified and correlated with the experimental material parameters of the device. A device model is presented for the responsivity and noise voltage which takes into account surface effects such as surface recombination and accumulation layer shunting. The model is in good agreement with the experimental values over the full temperature range from 80 to 300 K. Using a combination of experimental results and model calculations, the optimum device thickness and free electron concentration are presented which maximize D(lambda )* for a given operating temperature. Furthermore, it is shown that under ideal conditions it is possible to achieve background limited performance at temperatures up to 210 K. Experimental results are presented for responsivity, noise voltage, semiconductor surface charge density and D(lambda )* for an n-type x equals 0.32 Hg1-xCdxTe photoconductive detector as a function of temperature in the range 80 to 300 K. For a signal wavelength of 4 micrometer and a 40 degree field of view, a background limited D(lambda )* of 3.8 X 1011 cmHz1/2W-1 was obtained for temperatures up to 180 K, while D(lambda )* of 1.4 X 1011 and 2 X 109 cmHz1/2W-1 were measured at 200 K and 300 K, respectively.