Negative Luminescent Devices Research Articles

MCT-on-Silicon Negative Luminescence Devices with High Efficiency

We used an InSb radiometric thermal imager to characterize the performance of 1″ × 1″ negative luminescent (NL) arrays. The devices grown on both CdZnTe (two arrays) and silicon (three arrays) as substrates have cut-off wavelengths ranging from 5.3 µm to 6.0 µm. The reverse-bias saturation current densities range from 0.3 A cm−2 (λco = 5.3 µm) to 1 A cm−2 (λco = 6.0 µm). The apparent array temperatures decrease by 37.9 K to 42.8 K under reverse bias, which corresponds to external NL efficiencies of 80–85%. Most of the inefficiency results from the non-ideal AR coating, whose reflectivity is ≈15% when weighted over the black body and atmospheric transmission spectra. It is highly encouraging that both the electrical and NL properties are slightly superior for the devices grown on silicon substrates.

HgCdTe negative luminescence devices with high internal and external efficiencies in the midinfrared

The authors report the fabrication and characterization of mid-IR HgCdTe negative luminescence (NL) devices grown on silicon substrates. Sensitive optical modulation measurements of the 5mm square array with ∼5.4μm cutoff and single-layer antireflection (AR) coating yield an internal NL efficiency of 98% at room temperature. This is the highest ever reported, and represents a factor-of-50 suppression of the blackbody emission. The corresponding external NL efficiency of 86% is consistent with the internal efficiency and the spectrally weighted reflectivity of 15%, which could be improved substantially through the incorporation of a state-of-the-art multilayer AR coating.

HgCdTe negative luminescence devices for cold shielding and other applications

Negative luminescence (NL) refers to the suppression of infrared blackbody emission, and hence an apparent temperature reduction, due to free carrier extraction from a reverse-biased p-n junction. A number of applications are envisioned for NL devices, including cold shielding of background-limited uncooled and cryogenic focal-plane arrays, dynamic nonuniformity correction for ir imaging, and ir scene simulation. High-performance NL devices have recently been demonstrated. For example, a HgCdTe/CdZnTe photodiode with 4.8-µm cutoff wavelength achieved an internal NL efficiency of 95% at room temperature. This means that the blackbody emission was suppressed by a factor of 20 and that the apparent temperature of the device surface decreased by 60 K. The corresponding reverse-bias saturation current density was 0.11 A/cm2. Even HgCdTe devices (λco=5.3 µm) grown on large-area silicon substrates with substantial lattice mismatch displayed 88% internal NL efficiency and saturation current densities no larger than 1.3 A/cm2. These results indicate a clear path toward a negative-luminescence device technology that is efficient, operates at low power, and is inexpensive.

Negative luminescence from large-area HgCdTe photodiode arrays with 4.8-6.0-/spl mu/m cutoff wavelengths

We demonstrate the substantial suppression of infrared (IR) blackbody emission from HgCdTe photodiode arrays with cutoff wavelengths of 4.8, 5.5, and 6.0 /spl mu/m. At room temperature, a reverse bias induces internal negative luminescence (NL) efficiencies of 95%, 93%, and 88%, respectively, which correspond to apparent cooling of the surface temperatures by 60, 59, and 49 K. Reverse-bias saturation current densities for the three devices were 0.11, 0.7, and 32 A/cm/sup 2/. Measurement and analysis of the material transmission characteristics indicate that the small residual inefficiencies may be limited by a parasitic absorption process. The 4.8 and 5.5 /spl mu/m photodiodes were fabricated into 18 /spl times/ 2 arrays with total areas of 5 mm /spl times/ 5 mm. In both cases, all 36 array elements were fully operable, and had similar electrical and NL properties. Effective fill factors were /spl ap/100%, since carrier diffusion led to the extraction of carriers from regions between the elements. These results show that efficient, low-power NL devices with active areas in the square-inch range are now feasible for such applications as the cold shielding of infrared focal-plane arrays.

Infrared negative luminescent devices and higher operating temperature detectors

Infrared LEDs and negative luminescent devices, where less light is emitted than in equilibrium, have been attracting an increasing amount of interest recently. They have a variety of applications, including as a ‘source’ of IR radiation for gas sensing; radiation shielding for, and non-uniformity correction of, high sensitivity staring infrared detectors; and dynamic infrared scene projection. Similarly, infrared (IR) detectors are used in arrays for thermal imaging and, discretely, in applications such as gas sensing. Multi-layer heterostructure epitaxy enables the growth of both types of device using designs in which the electronic processes can be precisely controlled and techniques such as carrier exclusion and extraction can be implemented. This enables detectors to be made which offer good performance at higher than normal operating temperatures, and efficient negative luminescent devices to be made which simulate a range of effective temperatures whilst operating uncooled. In both cases, however, additional performance benefits can be achieved by integrating optical concentrators around the diodes to reduce the volume of semiconductor material, and so minimise the thermally activated generation-recombination processes which compete with radiative mechanisms. The integrated concentrators are in the form of Winston cones, which can be formed using an iterative dry etch process involving methane/hydrogen and oxygen. We present results on negative luminescence in the mid- and long-IR wavebands, from devices made from indium antimonide and mercury cadmium telluride, where the aim is sizes greater than 1 cm×1 cm . We also discuss progress on, and the potential for, operating temperature and/or sensitivity improvement of detectors, where very high-performance imaging is anticipated from systems which require no mechanical cooling.

InGaAsSb negative luminescent devices with built-in cavities emitting at [formula omitted

An As 2S 3 fiber coupled to an InGaAsSb photodiode was used to record the radiation distribution over the emitting surface in InGaAsSb episide-down-bonded negative luminescence devices (λ=3.9 μm) . Emission spectra were recorded under forward and reverse bias and both were modulated by a Fabry–Perot resonator formed by the anode contact and emitting InAs surface in 45-μm thick diodes. The results show that the current/emission distribution crowds in the vicinity of the contact under forward bias, while a uniform current/emission distribution over the emitting surface is seen under reverse bias.

Large-area IR negative luminescent devices

Negative luminescent (NL) devices, which to an IR observer appear colder than they actually are, have a wide range of possible applications, including for use as IR sources in gas-sensing systems and as thermal radiation shields in IR cameras. Additionally, these devices can be used as calibration sources for very large IR focal plane arrays and have many potential advantages over conventional calibration sources, including high-speed operation (for multi-point correction) and low power consumption. For many of these applications a large area ∼1cm2 device which displays an, as large as possible, apparent temperature range is required. However, significant currents are required to reduce the carrier concentrations to the levels needed for maximum dynamic range. The authors have therefore used a novel micromachining technique to fabricate integrated optical concentrators in InSb/InAlSb and HgCdTe NL devices. Smaller area diodes can then be used to achieve the same absorption (e.g. for InSb an area reduction of 16 is possible) and the required currents are thus reduced. Recent results are presented from a large area ∼0.862 medium wavelength (MW) device incorporating integrated optical concentrators. The reverse saturation current of the device was measured to be ∼2.3 A/cm2, which is significantly smaller than the value of ∼9 A/cm2 reported previously for similar devices without optical concentrators. In addition, the large apparent temperature drop of the device ∼20K for a detector of cut-off 6.5µm when under reverse bias, coupled with its large size, mean that this device would be appropriate for use as a calibration source for IR imagers.

Micromachined optical concentrators for IR negative luminescent devices

Negative luminescent (NL) devices, which to an IR observer appear colder than they actually are, have a wide range of possible applications, including use as thermal radiation shields in IR cameras, and as IR sources in gas-sensing systems. For many of these applications a large area (>1 cm2) device which displays as large as possible apparent temperature range is required. However, under reverse bias, significant currents are required to reduce the carrier concentrations to the levels needed for maximum possible absorption. We have therefore used a novel micromachining technique to fabricate integrated optical concentrators in InSb/InAlSb and HgCdTe NL devices. Smaller area diodes can then be used to achieve the same absorption (e.g. for InSb an area reduction of 16 is possible) and the required currents are thus reduced. To fabricate the concentrators, spherical resist masks are first produced, which are ∼10 μm high and ∼53 μm wide, by resist reflow at 120°C. Inductively coupled plasma (ICP) etching is then used to etch alternately the resist mask and the semiconductor, with oxygen and methane/hydrogen respectively, producing concentrators with almost parabolic profiles. Currently, the concentrators are typically 30 μm high, with a top diameter of ∼15 μm. Continuing optimization of the process to reach the theoretical limits of optical gain is described.

Negative luminescence and its applications

The application of nonequilibrium transport techniques to narrowgap semiconductor materials has produced practical, negativeluminescent devices. These structures apparently contravene Kirchhoff's l...

Applications of negative luminescence

The application of non-equilibrium transport techniques to narrow-gap semiconductor materials has produced practical, negative luminescent devices. These structures contravene Kirchhoff's law because they will absorb infrared radiation in a limited band without emitting it. They have many potentially important applications which include toxic gas sensing, cold shields for IR detectors, radiometric reference sources and dynamic infrared scene projectors.