N. Halder

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.

Vertical ordering and electronic coupling in bilayer nanoscale InAs/GaAs quantum dots separated by a thin spacer layer

The vertical ordering and electronic coupling in bilayer nanoscale InAs/GaAs quantum dots separated by a thin (7-9 nm) spacer layer has been investigated by transmission electron microscopy and photoluminescence measurements. The nanoscale dots are grown by molecular beam epitaxy (MBE) at 0.028 ML s(-1) growth rate. The active dots having higher monolayer coverages showed reduced ordering due to local strain at the growth surface. Also the active dots with increased monolayer coverage is a probable cause of tunneling-assisted carrier transfer between the dot layers.

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.

Effect of InAlGaAs and GaAs combination barrier thickness on the stacking of InAs/GaAs quantum dot heterostructure grown by MBE

We have investigated a novel approach of introducing a combined capping of quaternary alloy (InAlGaAs) and GaAs layer for the realization of stacked quantum dots (QD) heterostructure, in which the InAlGaAs act as a surface-strain-driven phase separation alloy activated by the predeposited InAs QDs. For a heterostructure sample with thin barrier thickness, high resolution transmission electron microscopy (HRTEM) image showed the stacking of QDs only upto the 5th layer and in the upper layers the dots are missing. We presume the stoppage of dot formation is due to, the uneven surface of the InAlGaAs alloy overgrown on the InAs QDs, as a result of the local compositional deviations of the Group-III atoms. In addition, we have noted the increase in dot formation time in the subsequent QD superlattice (SL) of the sample. As the growth rate is constant (0.2ML/s), this indicate that the amount of InAs material required for the formation of the dots in subsequent layer increases with decrease in barrier thickness, which is unreported till date. The barrier thickness has been varied to see its effect on stacking of QDs.

Investigation of structural and optical properties of coupled multilayer InAs/GaAs quantum dots with combinational InAlGaAs/ GaAs capping

The self assembled InAs/GaAs quantum dots (QDs) have been widely researched for their potential use in optoelectronic devices. Most of the studies so far have been confined towards uncoupled QD systems, whereas the coupled multilayer QD systems have remained relatively less unexplored. We have made optical and structural characterization of coupled InAs/GaAs quantum dot heterostructures with varying thicknesses of combination capping of the quaternary InAlGaAs and GaAs by Atomic Force Microscopy (AFM), Double Crystal X-Ray Diffraction (DCXRD) and Raman spectroscopy. The periodic satellite peaks in the XRD rocking curves indicates nice stacking and defect free formation of the dots. In low temperature Raman study we get quite prominent peak of InAs dots with higher thickness of capping layer.

Molecular beam epitaxial growth of GaAs/AlGaAs multiquantum well on germanium substrate

We have addressed Molecular Beam Epitaxial (MBE) growth of GaAs/AlGaAs multi quantum well (MQW) structure on germanium (Ge) substrate. The growth mechanism includes deposition of a low-temperature (350°C) migration enhanced epitaxy (MEE) grown GaAs layer which was overgrown by a thin GaAs layer grown at low-temperature (475°C). The overgrown GaAs layer was annealed multiple times at 600°C during the growth. We believe the MEE layer and this special annealing scheme effectively blocked the dislocations at the GaAs/Ge interface from propagating into the top epitaxial layers. The morphological and optical properties of the samples were studied by Atomic Force Microscopy (AFM), Cross-sectional Transmission Electron Microscopy (XTEM) and Photoluminescence (PL) measurements respectively. The AFM shows a smooth surface with a root mean square (rms) roughness ~0.16nm which is of the same order to a control sample consisting of a similar MQW structure grown on GaAs substrate. Though some defects at the GaAs/Ge interface are clearly visible from XTEM, the TEM image of the QWs show that they are free from structural defects. From the PL spectra, distinctly sharp peaks are clearly visible for the QWs of different widths and the observed PL linewidth is comparable to that of the control sample.

Strain Induced Stoppage of the Emission Peak Blueshift of Annealed Bilayer Quantum Dot Structures Separated by Thin Spacer

We have investigated the effect of post-growth rapid thermal annealing (RTA) at different temperature on two InAs/GaAs bilayer quantum dots samples with different spacer thicknesses (7.5nm and 8.5nm). It is found that when RTA temperature gradually increases, there is usual blue shift of ground state emission peak wavelength for the sample having thinner spacer but for the other sample the emission peak sustains at same peak wavelength position upto a higher annealing temperature. The dots inside the sample with less spacer thickness dissolute much earlier (beyond 700°C annealing temperature) in comparison to the other sample. The structural and optical characterization has been done by cross sectional transmission electron microscope (XTEM) and low temperature photoluminescence (PL) experiments respectively.