Amlan Majumdar

Two-color quantum-well infrared photodetector with voltage tunable peaks

A two-color quantum-well infrared photodetector with voltage tunable detection peaks is demonstrated. It is based on electron transfer between two asymmetric coupled quantum wells under an applied bias. At 10 K, the peak detection wavelength is 7.5 μm for positive bias when the electrons reside in one of the wells, and switches to 8.8 μm at a large negative bias when the electrons are transferred to the other well. The electron activation behavior of dark current in this detector depends on both temperature and bias. We observed two distinctive activation energies under negative bias for two different temperature regimes and only one under positive bias. The voltage tunability of the detector and the activation energy are well explained by the calculated energy levels and oscillator strengths of intersubband transitions in the structure.

Voltage tunable two-color infrared detection using semiconductor superlattices

We demonstrate a voltage tunable two-color quantum-well infrared photodetector (QWIP) that consists of multiple periods of two distinct AlGaAs/GaAs superlattices separated by AlGaAs blocking barriers on one side and heavily doped GaAs layers on the other side. The detection peak switches from 9.5 μm under large positive bias to 6 μm under negative bias. The background-limited temperature is 55 K for 9.5 μm detection and 80 K for 6 μm detection. We also demonstrate that the corrugated-QWIP geometry is suitable for coupling normally incident light into the detector.

Binary superlattice quantum-well infrared photodetectors for long-wavelength broadband detection

We have adopted a binary superlattice structure for long-wavelength broadband detection. In this superlattice, the basis contains two unequal wells, with which more energy states are created for broadband absorption. At the same time, responsivity is more uniform within the detection band because of mixing of wave functions from the two wells. This uniform line shape is particularly suitable for spectroscopy applications. The detector is designed to cover the entire 8–14μm long-wavelength atmospheric window. The observed spectral widths are 5.2 and 5.6μm for two nominally identical wafers. The photoresponse spectra from both wafers are nearly unchanged over a wide range of operating bias and temperature. The background-limited temperature is 50K at 2V bias for F∕1.2 optics.

Voltage tunable superlattice infrared detector for mid- and long-wavelength detection

We have designed and fabricated a voltage tunable superlattice (SL) infrared photodetector where the detection wavelength switches from the 3–5μm midwavelength infrared (MWIR) range under negative bias to the 8–12μm long-wavelength infrared (LWIR) range under large positive bias. The structure consists of multiple periods of two different SLs that are separated by undoped blocking barriers on one side and heavily doped layers on the other side. The background-limited temperature with F∕1.2 optics is 110 and 70 K for mid- and long-wavelength detection, respectively. This voltage tunable MWIR/LWIR detector has a performance comparable to those of one-color quantum-well infrared detectors designed for the respective wavelength ranges.

Towards a voltage tunable two-color quantum-well infrared photodetector

Two-color quantum-well infrared photodetectors (QWIPs), based on electron transfer between coupled QWs, suffer from the presence of the shorter wavelength peak at all bias voltages Vb. We investigate this problem by studying the bias dependence of absorption coefficient α and photoconductive gain g as a function of wavelength at 10 K. We fabricate such a detector with a peak wavelength of 8 μm for both bias polarities but a voltage tunable cutoff wavelength (9 μm for Vb>0 and 11 μm for Vb 0 and 10 μm for Vb<0. g has a pronounced peak at 7.8 μm for both bias polarities and determines the line shape of the QWIP spectral responsivity. These results are attributed to insufficient electron transfer between the coupled QWs and to low tunneling probability of the longer wavelength photoelectrons. A modified QWIP structure has...

Light coupling mechanism of quantum grid infrared photodetectors

Rigorous electromagnetic modeling based on a modal solution of the pertinent boundary-value problem was performed to study the light coupling mechanism in quantum grid infrared photodetectors with lamellar patterns. Our theory shows that vertical field component and absorption quantum efficiency η can be strongly enhanced by judiciously adjusting the width w of individual grid lines. This enhancement is further increased if the top of the grid lines is covered by metal, which behave as a collection of dipole scatterers. We have experimentally verified the dipole scattering characteristics with different w, and found that the variation of η agrees very well with the theory. We also found that, as expected, the conductivity of the metal strips affects η significantly due to internal dissipation.

Effective mass enhancement of two-dimensional electrons in a one-dimensional superlattice potential

We report effective mass enhancement of two-dimensional (2D) electrons in an atomically precise one-dimensional AlGaAs/GaAs superlattice potential fabricated by the cleaved-edge overgrowth technique. Magnetotransport measurements reveal that the mobility of the 2D electrons increases with electron density n2D. At low densities (n2D∼1.3×1011u200acm−2), the effective mass m* of the 2D electrons is 0.14u200ame, where me is the free-electron mass. This value of 0.14u200ame is twice the effective mass of electrons in GaAs (0.067u200ame). m* increases with n2D and is ∼0.16u200ame at 2.7×1011u200acm−2. We explain these results based on the formation of energy minibands along the superlattice direction. The electron scattering time τ is calculated from the mobility and effective mass data. At 0.3 K, τ increases with n2D from 5 to 11 ps as n2D is varied from 1.1×1011 to 2.7×1011u200acm−2.

Temperature dependence of electron transfer in coupled quantum wells

We report on the temperature dependence of electron transfer between coupled quantum wells in a voltage tunable two-color quantum-well infrared photodetector (QWIP). The detection peak of this QWIP switches from 7.1 μm under positive bias to 8.6 μm under negative bias for temperatures T⩽40u2009K. For T⩾40u2009K, the 7.1 μm peak is present under both bias polarities and increases significantly with T while the 8.6 μm peak decreases correspondingly. We determine the temperature dependence of electron densities in the two QWs from the detector absorption spectra that are deduced using corrugated QWIPs and find that electron transfer is efficient only when thermionic emission is not significant.

Electron transfer in voltage tunable two-color infrared photodetectors

Two-color quantum-well infrared photodetectors (QWIPs) that are based on electron transfer between coupled QWs suffer from the presence of the shorter wavelength peak at all bias voltages. We investigate this problem in such detectors with 50 or 200 A AlGaAs barriers between the QW pair. We deduce the absorption coefficient α and photoconductive gain g of the detectors with 50 A barriers using corrugated QWIPs with different corrugation periods. We find that α has a number of small peaks in its spectrum but its value remains almost constant between 0.1 and 0.2 μm−1 in the 6–12 μm range for most experimental conditions. The wavelength dependence of g, which always has a pronounced peak at the shorter detection wavelength, determines the responsivity line shape. These results are attributed to insufficient electron transfer between the coupled QWs and to low tunneling probability of the longer wavelength photoelectrons. A comparison of measured responsivity and calculated absorption spectrum of the detector...

High-responsivity high-gain In/sub 0.53/Ga/sub 0.47/As-InP quantum-well infrared photodetectors grown using metal-organic vapor phase epitaxy

We report the growth and fabrication of bound-to-bound In/sub 0.53/Ga/sub 0.47/As-InP quantum-well infrared photodetectors using metal-organic vapor phase epitaxy. These detectors have a peak detection wavelength of 8.5 /spl mu/m. The peak responsivities are extremely large with R/sub pk/=6.9 A/W at bias voltage V/sub b/=3.4 V and temperature T=10 K. These large responsivities arise from large detector gain that was found to be g/sub n/=82 at V/sub b/=3.8 V from dark current noise measurements at T=77 K and g/sub p/=18.4 at V/sub b/=3.4 V from photoresponse data at T=10 K. The background-limited temperature with F/1.2 optics is T/sub BLIP/=65 K for 0<V/sub b//spl les/3.4 V. The highest value of peak detectivity is D/sub /spl lambda///sup */=5.4/spl times/10/sup 9/ cm/spl radic/Hz/W at V/sub b/=2.9 V and T=77 K, while for T<T/sub BLIP/=65 K, the background-limited detectivity, which exhibits negligible bias dependence, is D/sub BLIP//sup */=3/spl times/10/sup 10/ cm/spl radic/Hz/W.