Eric W. Jones

Uncooled thermopile infrared detector linear arrays with detectivity greater than 10/sup 9/ cmHz/sup 1/2//W

We have fabricated 63-element linear arrays of micromachined thermopile infrared detectors on silicon substrates. Each detector consists of a suspended silicon nitride membrane with 11 thermocouples of sputtered Bi-Te and Bi-Sb-Te films. At room temperature and under vacuum these detectors exhibit response times of 99 ms, zero frequency D* values of 1.4/spl times/ 10/sup 9/ cmHz/sup 1/2//W and responsivity values of 1100 V/W when viewing a 1000 K blackbody source. The only measured source of noise above 20 mHz is Johnson noise from the detector resistance. These results represent the best performance reported to date for an array of thermopile detectors. A test procedure is described that measures many of the relevant electrical, optical, and thermal properties of the detectors without specialized test structures.

Space science applications of thermopile detector arrays

Thermal detectors, while typically less sensitive than quantum detectors, are useful when the combination of long wavelength signals and relatively high temperature operation makes quantum detectors unsuitable. Thermal detectors are also appropriate in applications requiring flat spectral response over a broad wavelength range. JPL produces thermopile detectors and linear arrays to meet space science requirements in these categories. Thermopile detectors and arrays are currently being fabricated for two space applications. The first is the Mars Climate Sounder (MCS) instrument, to fly on the Mars Reconnaissance Orbiter mission, scheduled to launch in 2005. MCS is an atmospheric limb sounder utilizing nine 21-element thermopile arrays. The second application is the Earth Radiation Budget Suite (ERBS), part of the National Polar Orbiting Environmental Satellite System (NPOESS). This instrument measures upwelling radiation from the earth in the spectral range 0.3-100 μm.

High-performance micromachined thermopile linear arrays

Linear thermopile infrared detector arrays have been produced with D* values as high as 2.2 X 109 cmHz1/2W for 83 ms response times. Typical responsivity is 1000 V/W. This result has been achieved with Bi-Te and Bi-Sb-Te thermoelectric materials on micromachined silicon nitride membranes. Results for several device geometries are described and compared to literature values for Schwartz type thermocouple detectors and for thin film thermopile detectors and arrays. Measurements of responsivity as a function of modulation frequency and wavelength are presented.

Nanostructured surfaces for tuned infrared emission for spectroscopic applications

Thermal emission from heated materials follows the blackbody curve, multiplied by emissivity. Emissivity may be, but is not usually a strong function of wavelength. Ion Optics has developed a variety of surface texturing processes that create specific nano-structures which alter the emissivity in predictable fashion. Random structures produced by ion beam etching create long and/or short wavelength cutoffs. Repeated patterns produced by fine-line lithography, resembling photonic bandgap materials, have large peaks in the emitted spectrum. The central wavelength and bandwidth for lithographic structures can be varied with geometry. FWHM values for ((Delta) (lambda) /(lambda) ) are less than 0.1. These light sources reduce power requirements for applications now using broadband sources with filters, and in some cases entirely eliminate the need for filters.

Tuned Infrared Emission From Lithographically-Defined Silicon Surface Structures

Photonic bandgap structures have received much attention as optical and infrared filters with controllable narrow-band absorbance. There is a need, however, for the same kind of control of the thermal emittance of surfaces for applications ranging from control of radiative heat transfer to gas absorption spectroscopy. We report on the fabrication of photonic bandgap structures on silicon surfaces using standard lithographic techniques. Substrate resistivity varied from n − to n + and in some cases background surface emissivity was suppressed with a high reflectivity coating such as aluminum. We have measured the infrared reflectance and emittance of these patterned surfaces. Peak absorption wavelength and spectral purity (linewidth) correlate with photonic bandgap feature size and spacing as well as surface conductivity. We demonstrate band emission with a sharp short wavelength cut-off from these structures when heated.

Long-wavelength infrared doping-spike PtSi detector

By incorporating a 1-nm-thick p+ doping spike at the PtSi/Si interface, we have successfully demonstrated extended cutoff wavelengths of PtSi Schottky infrared detectors in the long wavelength infrared (LWIR) regime for the first time. The extended cutoff wavelengths resulted from the combined effects of an increased electric field near the silicide/Si interface due to the p+ doping spike and the Schottky image force. The p+ doping spikes were grown by molecular beam epitaxy at 450 degree(s)C using elemental boron as the dopant source, with doping concentrations ranging from 5 X 1019 to 2 X 1020 cm-3. Transmission electron microscopy indicated good crystalline quality of the doping spikes. The cutoff wavelengths were shown to increase with increasing doping concentrations of the p+ spikes. Thermionic emission dark current characteristics were observed and photoresponse in the LWIR regime was demonstrated.

Long-wavelength stacked Si1-x Gex/Si heterojunction-internal-photoemission infrared detectors

Utilizing the low temperature silicon molecular beam epitaxy (MBE) growth of degenerately doped SiGe layers on Si, long wavelength stacked SiGe/Si heterojunction internal photoemission (HIP) infrared detectors with multiple SiGe/Si layers have been fabricated and demonstrated. The detector structure consists of several periods of degenerately boron doped thin (20 cm-3) has been achieved and high crystalline quality multiple SiGe/Si layers have been obtained. For the experiment several stacked Si0.7Ge0.3/Si HIP detectors with various SiGe layer thickness and doping concentration have been fabricated. The detectors have exhibited strong infrared absorption and near ideal thermionic-emission dark current characteristics. For the stacked Si0.7Ge0.3/Si HIP detectors with [B] equals 4 X 1020 cm-3, strong photoresponse at wavelengths ranging 2 to 20 micrometers has been measured. The effects of doping concentration on the detector optical and electrical characteristics have been studied. Using the measured quantum efficiency and dark current data, detectivity (D(lambda )*) of detectors has been estimated.

MEMS incandescent light source

A MEMS-based, low-power, incandescent light source is being developed. This light source is fabricated using three bonded chips. The bottom chip consists of a reflector on Silicon, the middle chip contains a Tungsten filament bonded to Silicon and the top layer is a transparent window. A 25-micrometers -thick spiral filament is fabricated in Tungsten using lithography and wet- etching. A proof-of-concept device has been fabricated and tested in a vacuum chamber. Results indicate that the filament is electrically heated to approximately 2650 K. The power required to drive the proof-of-concept spiral filament to incandescence is 1.25 W. The emitted optical power is expected to be approximately 1.0 W with the spectral peak at 1.1 micrometers . The micromachining techniques used to fabricate this light source can be applied to other MEMS devices.

Si1-xGex/Si heterojunction internal photoemission long-wavelength infrared detector

Long wavelength Si1-xGex/Si heterojunction internal photoemission (HIP) infrared detectors have been successfully demonstrated utilizing the growth of degenerately boron doped Si1-xGex layers on Si. Recently, Si0.7Ge0.3/Si HIP detectors with either a Si1-xGex single layer or a Si1-xGex/Si multi-layer have been demonstrated with cutoff wavelengths out to 23 micrometers . Near-ideal thermionic emission dark current characteristics were measured and the electrical potential barriers were determined by the Richardson plot. A photoresponse model, similar to the modified Fowler Equation has been developed for the Si1-xGex/Si HIP infrared detector at wavelengths corresponding to photon energies less than the Fermi energy. The optical potential barriers, the corresponding cutoff wavelengths, and the emission coefficients, C1, for the HIP detectors have been determined from the measured spectral responses using the photoresponse model. Similar optical and thermal potential barriers were obtained.

Advanced Si IR detectors using molecular beam epitaxy

SiGe/Si heterojunction internal photoemission (HIP) long wavelength infrared (LWIR) detectors have been fabricated by MBE. The SiGe/Si HIP detector offers a tailorable spectral response in the long wavelength infrared regime by varying the SiGe/Si heterojunction barrier. Degenerately doped p(+) SiGe layers were grown using elemental boron, as the dopant source allows a low growth temperature. Good crystalline quality was achieved for boron-doped SiGe due to the reduced growth temperature. The dark current density of the boron-doped HIP detectors was found to be thermionic emission limited. HIP detectors with a 0.066 eV were fabricated and characterized using activation energy analysis, corresponding to a 18 micron cutoff wavelength. Photoresponse of the detectors at wavelengths ranging from 2 to 12 microns has been characterized with corresponding quantum efficiencies of 5 - 0.1 percent.