Saumya Sengupta

Presentation and experimental validation of a model for the effect of thermal annealing on the photoluminescence of self-assembled InAs/GaAs quantum dots

We present a model for the effect of thermal annealing on a single-layer InAs/GaAs quantum dot (QD) heterostructure and study the corresponding variation in full photoluminescence (PL) spectrum. In/Ga interdiffusion due to annealing is modeled by Fickian diffusion and the Schrodinger equation is solved separately for electrons and holes to obtain ground state PL peaks of the heterostructure at different annealing temperatures. We theoretically examine the decrease in strain effects and carrier confinement potentials with annealing. PL spectra of the entire ensemble of QDs, annealed at different temperatures, are calculated from a lognormal distribution of QD heights derived from experimental atomic force microscopy (AFM) data. Results from our calculations, which illustrate the blueshift in emission wavelength and linewidth variation in PL with annealing, are in excellent agreement with experimental PL observations on the same samples. This highlights the potential of the model to assist in precisely engi...

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.

Comprehensive study on molecular beam epitaxy-grown InAs sub-monolayer quantum dots with different capping combinations

Here the authors report a comprehensive study on InAs sub-monolayer quantum dots with different capping layers. After performing systematic optimization of InAs deposition and GaAs thickness, they grew three samples, namely A, B and C, using solid-state molecular beam epitaxy with identical architecture but different capping materials (2 nm of GaAs, InGaAs-GaAs, and InAlGaAs-GaAs, respectively). Photoluminescence emission peaks due to the ground state transition from the dots were observed at 898, 917, and 867 nm for samples A, B, and C, respectively. Narrow full-width half-maxima (19–32 meV) of the emission peaks indicates high uniformity of dot size distribution. Using the conventional Arrhenius plot, the authors calculated the thermal activation energies from temperature-dependent photoluminescence experiment for samples A, B, and C as 49, 112, and 109 meV, respectively. To complete the study, single-pixel photodetectors were fabricated from samples A, B, and C and temperature-dependent dark current va...

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.

Comparison of single-layer and bilayer InAs/GaAs quantum dots with a higher InAs coverage

Epitaxially grown self-assembled InAs quantum dots (QDs) have found applications in optoelectronics. Efforts are being made to obtain efficient quantum-dot lasers operating at longer telecommunication wavelengths, specifically 1.3 μm and 1.55 μm. This requires narrow emission linewidth from the quantum dots at these wavelengths. In InAs/GaAs single layer quantum dot (SQD) structure, higher InAs monolayer coverage for the QDs gives rise to larger dots emitting at longer wavelengths but results in inhomogeneous dot-size distribution. The bilayer quantum dot (BQD) can be used as an alternative to SQDs, which can emit at longer wavelengths (1.229 μm at 8 K) with significantly narrow linewidth (∼16.7 meV). Here, we compare the properties of single layer and bilayer quantum dots grown with higher InAs monolayer coverage. In the BQD structure, only the top QD layer is covered with increased (3.2 ML) InAs monolayer coverage. The emission line width of our BQD sample is found to be insensitive towards post growth treatments.

Optimization of the Number of Stacks in the Submonolayer Quantum Dot Heterostructure for Infrared Photodetectors

We studied the optical, electrical, and spectral properties of InAs submonolayer quantum dot infrared photodetectors with different number of stacks. Three samples with 4, 6, and 8 dot stacks were grown by molecular beam epitaxy under identical conditions. Increasing the number of stacks results in a gradual shift in the photoluminescence ground-state transition energy of the samples from 1.195 to 1.111 eV. Cross-sectional transmission electron microscopy images confirm increase in dot size with increasing number of stacks from 4 to 8. Samples with 4 and 6 stacks measured moderately uniform dot size distribution and with further increasing the number of stacks 4 to 8 variations in dot sizes along with improper dot size formation were observed. The activation energy of the samples was measured by both optical and electrical methods increase with increasing number of dots. All photodetectors exhibit a photocurrent peak in the range of 7.3-7.8 μm at 77 K at an applied bias of -1 V. Highest peak responsivity value of 0.04523 A/W at 77 K was observed from the 6 stacked sample, which was highest among the three samples. It also exhibited highest detectivity of 5E9 Jones with lowest noise current density among the others. The sample with 6 dot stacks is the best as it exhibited lowest dark current density of 6.1 (10-7 A/cm2 and highest operating temperature of 110 K).

Introduction to Infrared Detectors and Quantum Dots

The majority of objects, those with a temperature between 100 and 400 K, emit strong electromagnetic radiation in the infrared region, especially in 1–14 µm region, which includes short-wavelength infrared (SWIR, ~1.0–3.0 µm), medium-wavelength infrared (MWIR, ~3.0–5.0 µm), long-wavelength infrared (LWIR, ~8.0–14.0 µm) and some part of very-long infrared (VLWIR, ~14.0–100.0 µm). MWIR and LWIR detectors are widely used today in a variety of imaging and video-graphic applications, in fields such as spectroscopy, night vision, thermal imaging, health science, and space research and defence. Different types of IR detectors are based on various semiconductor materials, such as Si, InAs1−x Sb x , Pb1−x Sn x Te, and Hg1−x Cd x Te. To overcome limitations in extending the detection wavelength in longer wavelength region the idea of intersubband transition based photodetectors has been introduced. The spacing between different electronics subbands (a few tenths to hundreds of meV) allows emission or detection of a broad range of IR radiation. Quantum mechanical properties dictate that if any material is scaled down to very small dimension both the conduction and valence band can be split into a number of intersubband energy levels. The dimension of the bulk can be reduced to form different nanostructures, such as quantum wells (QWs), quantum wires and quantum dots (QDs). QDs confine the carriers in all three directions, which results in a complete delta-like DOS in the different energy levels. In recent past MBE grown III–V semiconductors based quantum dots infrared photodectors (QDIPs) have emerged as a potential candidate in the field of MWIR and LWIR imaging technology. Their 3-D carrier confinement provides intrinsic sensitivity to normal incidence radiation, lower dark current and a long excited-state lifetime compared to quantum well infrared photodetectors (QWIPs).

Structural, Optical and Spectral Characterization of Single-Layer QDIPs

In this chapter, we have investigated the effect of growth pause on structural, optical and spectral properties of InAs/GaAs QD materials. Introduction of growth pause or ripening time changes the morphology of the QDs by altering effective epitaxial strain during the growth of QDs. Initially, we grew single-layer QD samples, with another QD layer on the top of the surface for structural characterization. Sample sets with two different InAs growth rates (0.032 and 0.197 ML/s) were grown on (100)-oriented GaAs substrates. Three samples, with 0, 25 and 50 s growth pause, were grown with each of the two growth rates, keeping all other growth parameters constant. We have examined the change of their optical and structural properties with different duration of growth pause. For device fabrication, we grew 10 mutually uncoupled QD layers sandwiched between Si-doped thick GaAs contact layers. In this case, the InAs dots were grown at 520 °C with a growth rate of 0.1 MLs−1. Growth pauses of 0, 25 and 50 s were introduced for samples A, B and C, respectively. Finally, single-pixel photodetector devices were fabricated from as-grown A, B and C samples with standard fabrication procedures.

Optical and Spectral Characterization of Sub-monolayer QDIPs

In this chapter, we have explored the properties of an unconventional type of quantum dots, namely sub-monolayer (SML) quantum. We have performed a systematic study to optimize different growth parameters and have investigated structural and optical properties of the materials. We have successfully demonstrated high 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 wave-functions 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 band structure of the samples was deduced semi-empirically.

Structural and Optical Characterization of Bilayer QD Heterostructures

Efforts are being made to obtain efficient quantum dot heterostructures which possess excellent uniformity in size distribution as well as capable to extend the emission wavelength to technologically useful telecommunication wavelengths, specifically 1.3 and 1.55 μm. In InAs/GaAs single-layer quantum dot (SQD) structure, higher InAs monolayer coverage for the QDs gives rise to larger dots emitting at longer wavelengths but results in inhomogeneous dot-size distribution. The bilayer quantum dots (BQDs) can be used as an alternative to SQDs, which can emit at longer wavelengths (1.229 μm at 8 K) with significantly narrow linewidth (~16.7 meV) owing vertical ordering and electronic coupling between the two layers of dots separated by a thin (7–9 nm) spacer layer. Morphological and optical properties of bilayer InAs/GaAs quantum dot heterostructure are investigated. As compared to the similar single-layer quantum dot (SQD) structure, the bilayer quantum dot (BQD) structure showed a more uniform spatial distribution and increased size homogeneity of the dots. It also exhibited longer wavelength photoluminescence (PL) emission at room temperature, with the peak at a wavelength (1.34 μm) in the infrared communication window. In an interesting study, the emission linewidth of our BQD sample is found to be insensitive towards post-growth treatments due to the strain interaction between the layers of dots.