Sourav Adhikary

High-performance, long-wave (∼10.2 μm) InGaAs/GaAs quantum dot infrared photodetector with quaternary In0.21Al0.21Ga0.58As capping

A high-performance InGaAs/GaAs vertical quantum dot infrared photodetector (QDIP) with combined barrier of quaternary In0.21Al0.21Ga0.58As and GaAs was investigated in this study. A dominant long wavelength (∼10.2 μm) response was observed from the device. The device demonstrates large responsivity (2.16 A/W) with narrow spectral-width (Δλ/λ ∼0.14) and high detectivity (1.01 × 1011 cm Hz1/2/W at 0.3 V) at 10.2 μm at 77 K. In addition, the device has also produced a detectivity in the order of 6.4 × 1010 cm Hz1/2/W at 100 K at a bias of 0.2 V, indicating its suitability for high-temperature operations.

Multi-stack InAs/InGaAs sub-monolayer quantum dots infrared photodetectors

We report on the design and performance of multi-stack InAs/InGaAs sub-monolayer (SML) quantum dots (QD) based infrared photodetectors (SML-QDIP). SML-QDIPs are grown with the number of stacks varied from 2 to 6. From detailed radiometric characterization, it is determined that the sample with 4 SML stacks has the best performance. The s-to-p (s/p) polarized spectral response ratio of this device is measured to be 21.7%, which is significantly higher than conventional Stranski-Krastanov quantum dots (∼13%) and quantum wells (∼2.8%). This result makes the SML-QDIP an attractive candidate in applications that require normal incidence.

A multicolor, broadband (5–20 μm), quaternary-capped InAs/GaAs quantum dot infrared photodetector

An InAs/GaAs quantum dot infrared photodetector with strong, multicolor, broadband (5–20 μm) photoresponse is reported. Using a combined quaternary In0.21Al0.21Ga0.58As and GaAs capping that relieves strain and maintains strong carrier confinement, we demonstrate a four color infrared response with peaks in the midwave- (5.7 μm), longwave- (9.0 and 14.5 μm), and far- (17 μm) infrared regions. Narrow spectral widths (7% to 9%) are noted at each of these wavelengths including responsivity value ∼95.3 mA/W at 14.5 μm. Using strain field and multi-band k⋅p theory, we map specific bound-to-bound and bound-to-quasibound transitions to the longwave and midwave responses, respectively.

Sub-monolayer quantum dots in confinement enhanced dots-in-a-well heterostructure

We have investigated optical properties and device performance of sub-monolayer quantum dots infrared photodetector with confinement enhancing (CE) barrier and compared with conventional Stranski-Krastanov quantum dots with a similar design. This quantum dots-in-a-well structure with CE barrier enables higher quantum confinement and increased absorption efficiency due to stronger overlap of wavefunctions between the ground state and the excited state. Normal incidence photoresponse peak is obtained at 7.5 μm with a detectivity of 1.2 × 1011 cm Hz1/2 W−1 and responsivity of 0.5 A/W (77 K, 0.4 V, f/2 optics). Using photoluminescence and spectral response measurements, the bandstructure of the samples were deduced semi-empirically.

Barrier Selection Rules for Quantum Dots-in-a-Well Infrared Photodetector

We report on a systematic study of the effect of barriers on quantum dots-in-a-well infrared photodetectors. Four devices are fabricated and characterized with varying composition for barriers adjacent to quantum dots and away from quantum dots. Effects of these “proximity” and “remote” barriers are studied by comparing photoluminescence, responsivity, dark current, background-limited operating temperature, activation energy, and detectivity. The growth mechanism for a conformal coverage of quantum dots with proximity barriers is described and supported with reflection high-energy electron diffraction and transmission electron microscopy images. It is shown that proximity barriers and remote barriers influence the characteristics of the detector very differently, with increases in proximity barrier energy leading to higher responsivity and lower dark current, while remote barriers reduce the responsivity and dark currents simultaneously. It is demonstrated that confinement enhancing barriers as proximity barriers optimize the SNR at low bias range, suitable for focal plane array applications.

Effect of barrier thickness on structural, optical, and spectral behaviors of vertically strain coupled InAs/GaAs quantum dot infrared photodetectors

The optical, electrical, and spectral properties of a strain coupled InAs quantum dot detector with a fixed quaternary capping of InAlGaAs and variable GaAs barrier thickness were investigated along with an equivalent uncoupled structure. Self-assembled quantum dots with a multimodal dot size distribution were achieved owing to vertical strain coupling. Strain and electronic coupling were utilized to improve the optical and electrical performance of the fabricated quantum dot infrared photodetector. The peak spectral response was tuned by varying barrier thickness, and a blue shift (almost 1 μm) was observed by increasing the capping thickness from sample A (90 A capping) to E (500 A capping). High responsivity and detectivity (∼1010 cm Hz1/2/W) were observed for all coupled samples as compared to the uncoupled sample. All coupled samples showed high thermal stability in the photoluminescence peak with high-temperature annealing.

Demonstration of high responsivity(~2.16 A/W) and detectivity(~10 11 Jones) in the long wavelength (~10.2μm) from InGaAs/GaAs quantumdot infrared photodetector with quaternary InAlGaAs capping

The Self-assembled InGaAs/GaAs quantum dot infrared detectors (QDIPs) have emerged as a promising technology in many applications such as missile tracking, night vision, medical diagnosis, environmental monitoring etc. On account of the 3-D confinement of carriers in QDs, a number of advantages arise over the QW counterparts. Here we report a quaternary (InAlGaAs) capped In(Ga)As/GaAs QDIP. The samples were grown on a semi-insulating (001) GaAs substrate by solid source molecular beam epitaxy (MBE), and the dots were then capped with a combination of 30A quaternary (In0.21Al0.21Ga0.58As) and 500Å of GaAs layer. Both the QD layer and the combination capping were repeated for 35 periods. The device was fabricated by conventional photolithography, ICP etching and metal evaporation technique. XTEM image of the sample depicted nice stacking of defect free quantum dot layers. The dark current is symmetric both for positive and negative bias with a low dark current density of 4.32x10-6A/cm2 at 77K and 1.6 x10 -3A/cm2 at 200K at a bias of 2V. The high intense peak response observed at 10.2μm, with a very narrow spectral width (▵λ/λ) of 14% (▵λ is the FWHM), is probably due to bound-to-bound transition of carriers in the QDs. A very high responsivity of 2.16 A/W was measured at a bias of -0.40 Volt bias. The highest value of detectivity is measured to be ~1011 cm.Hz1/2/W at a bias of 0.3V.

A novel approach to increase emission wavelength of InAs/GaAs quantum dots by using a quaternary capping layer

In this paper, we present a new approach to obtain large size dots in an MBE grown InAs/GaAs multilayer quantum dot system. This is achieved by adding an InAlGaAs quaternary capping layer in addition to a high growth temperature (590°C) GaAs capping layer with the view to tune the emission wavelength of these QDs towards the 1.3 μm/0.95 eV region important for communication devices. Strain driven migration of In atoms from InAlGaAs alloy to the InAs QDs effectively increases the size of QDs. Microscopic investigations were carried out to study the dot size and morphology in the different layers of the grown samples. Methods to reduce structural defects like threading dislocations in multilayer quantum dot samples are also studied.

Enhancement in Peak Detectivity and Operating Temperature of Strain-Coupled InAs/GaAs Quantum Dot Infrared Photodetectors by Rapid Thermal Annealing

We report the effects of rapid thermal annealing on the optical, structural, and device properties of 30 layer strain-coupled InAs/GaAs quantum dot infrared photodetectors. Stability in the photoluminescence peak is observed for annealing up to 800°C, which has not been previously reported. Cross-sectional transmission electron microscopy images show preservation of quantum dots is observed up to 800°C. Device with total capping thickness of 150 nm annealed at 750°C exhibit a fivefold enhancement in spectral intensity compared to as-grown devices and increase in the temperature of detector operation is observed from 100 to 140 K from the same device. The annealed devices exhibited a single-order enhancement in peak detectivity compared to as-grown quantum dot infrared photodetector.

Multi-spectral InAs/GaAs-based quantum dot infrared photodetector with quaternary (InAlGaAs) capping operates at low bias voltage

The quantum dot infrared photodetector is an emerging technology for advanced imaging. Multi-color imaging technologies are favored as they extend the boundary of applications of the device. We report multi-spectral performance of MBE grown InGaAs/GaAs (device A) and InAs/GaAs (device B) based photodetector with In0.21Al0.21Ga0.58As capping at 77K. Spectral response measurement of device A shows the presence of a strong photoresponse at 10.2μm. Device B exhibits a four color response (5.7, 9.0, 14.5, 17 and 20 μm) over a broad range (5-20μm) at very low bias voltage.