Design, modeling, and optimization principles for GaAs/ AlGaAs heterojunction interfacial workfunction internal photoemission (HEIWIP) infrared detectors for a broad spectral region are presented. Both n-type and p-type detectors with a single emitter or multiemitters, grown on doped and undoped substrates are considered. It is shown that the absorption, and therefore responsivity, can be increased by optimizing the device design. Both the position and the strength of the responsivity peaks can be tailored by varying device parameters such as doping and the thickness. By utilizing a resonant cavity architecture, the effect of a buffer layer on the response is discussed. Model results, which are in good agreement with the experimental results, predict an optimized design for a detector with a peak response of 9 A / W at 26 m with a zero response threshold wavelength 0 = 100 m. For a 0 =15 m HEIWIP detector, background limited performance temperature (BLIP temperature), for 180°field of view (FOV) is expected around 80 K. For a 0 =70 m optimized design, a highly doped n-type substrate could increase the peak detectivity from 1.7ϫ 10 10 to 3.4ϫ 10 10 Jones at a FOV= 180°operated at temperatures below T Ͻ T BLIP =13 K. Intrinsic response times on the order of picoseconds are expected for these detectors.
An uncooled microbolometer focal plane array (FPA) has been developed and used for imaging of objects illuminated by monochromatic coherent radiation of a free electron laser tunable in the range of 1.25–2.5THz. A sensitivity threshold of 1.3×10−3W∕cm2 was obtained for the FPA with a homemade absolute interferometric power meter. Videos up to 90frames∕s were recorded in both transmission and reflection/scattering modes. When objects were illuminated by laser radiation scattered by a rough metal surface, speckled images were observed. Good quality terahertz images were achieved through the fast rotation of the scatterer.
Free hole absorption in doped Al x Ga 1−x As films, grown by molecular-beam epitaxy on semi-insulating GaAs substrates, was investigated. Free carrier absorption for three different hole concentrations with the same Al fraction and for two different Al fractions with the same doping concentration was studied. Experimental absorption coefficients were obtained from the data using a model that includes multiple reflections in the substrate wafer. In the 100-400 m range, ͑3,5,8͒ ϫ 10 18 cm −3 Be-doped Al 0.01 Ga 0.99 As films have absorption coefficients of ϳ͑3,3.5,5͒ ϫ 10 3 cm −1 , respectively, where the magnitude of the absorption is found to be almost independent of the wavelength. This allows replacing doped GaAs emitters in heterojunction interfacial work function internal photoemission far-infrared (HEIWIP) detectors with pAl x Ga 1−x As layers with x Ͻ 0.017 facilitating the extension of the threshold wavelength of HEIWIP detectors beyond the 92 m limit due to the practical Al fraction growth limit of 0.005 in molecular-beam epitaxy.
Results are reported on Heterojunction Interfacial Workfunction Internal Photoemission ͑HEIWIP͒ detectors designed for operation up to 20 m. The peak response of 100 mA/W at 12.5 m with a D* of 2ϫ10 11 Jones was observed with a cutoff wavelength of ϳ20 m. The BLIP temperature for the devices was 40 K at 1.5 V bias. While the peak response remained almost constant ͑ϳ95 mA/W͒ up to 40 K, the D* reduced to 5ϫ10 9 Jones due to the increased dark current. The response increased with doping while the dark current did not change significantly. Hence, higher responsivity and D* can be expected for designs with higher doping. Designs utilizing increased reflection from the bottom contact are suggested to improve the resonant cavity enhancement for optimizing the detectors, which should lead to higher D* and BLIP temperature.
Articles you may be interested inImproved performance of HgCdTe infrared detector focal plane arrays by modulating light field based on photonic crystal structure Study on the quantum efficiency of resonant cavity enhanced GaAs far-infrared detectors J. Appl. Phys. 91, 5538 (2002); 10.1063/1.1465513 Current-voltage analysis of a tunneling emitter-undoped single quantum well infrared photodetectorResults are reported on p-GaAs homojunction interfacial work function internal photoemission far infrared ͑HIWIP FIR͒ detectors with a ϳ10 19 cm Ϫ3 carbon doped single emitter and a barrier layer for three different barrier thicknesses. A remarkably high quantum efficiency with low dark current and an increased responsivity were observed for devices with 1-, 0.1-, and 4-m-thick barrier regions. The dark current densities for these structures are on the order of 1 -10 A/cm 2 at 4.2 K, corresponding to a high dynamic resistance compared with previous HIWIP FIR detectors. A detector with a barrier thickness of 1 m had a peak responsivity of 18.6 A/W, a peak detectivity D*ϭ9ϫ10 11 cmͱHz/W, and a quantum efficiency of 40% at a wavelength of 58 m under a reverse bias measured at 4.2 K. Cutoff wavelengths of these detectors vary with bias and are around 70 m as expected. The main features of the absorption and responsivity spectra are well described based on a model incorporating free carrier absorption, hot hole transport, and emission over the barrier.
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