I. Mukhametzhanov

Independent manipulation of density and size of stress-driven self-assembled quantum dots

A method to independently manipulate the density and size of stress-driven self-assembled quantum dots (QDs) is demonstrated in the InAs/GaAs material system. In bilayer stacks, different InAs deposition amounts in the initial (seed) and second layer are shown to enable independent control, respectively, of the density and the size distribution of the second layer QDs. The approach allows enhancing the average volume and improving the uniformity of InAs QDs, resulting in, respectively, low and room temperature photoluminescence at 1.028 eV (∼1.2 μm) and 0.955 eV (∼1.3 μm) with remarkably narrow linewidths of 25 and 29 meV for 1.74 ML (seed)/3.00 ML InAs stacking.

Normal incidence InAs/AlxGa1−xAs quantum dot infrared photodetectors with undoped active region

We have performed a comprehensive investigation of n-type quantum dot infrared photodetectors (QDIPs) based on InAs/GaAs epitaxical island quantum dots (QDs) grown via the innovative punctuated island growth technique. The structural properties of the QDs were investigated with cross-sectional transmission electron microscopy and atomic force microscopy. The electronic properties of the QDs inserted in QDIP devices were investigated with photoluminescence (PL), PL excitation, and intra- and inter-band photocurrent spectroscopy. The influence of AlGaAs layers inserted into the QDIP active regions on the performance of dark current and inter- and intra-band photocurrent was examined. Initial results on intra-band responsivity and detectivity of these QDIPs at 77 K with undoped active region show promise for application.

Punctuated island growth: An approach to examination and control of quantum dot density, size, and shape evolution

The later stages of the evolution of epitaxical island quantum dots are examined systematically for InAs depositions on GaAs(001) following the conventional continuous deposition mode and an approach introduced here called punctuated island growth (PIG). The comparative study provides clear structural and optical evidence for a change in InAs island shape at a self-limiting lateral size, first reached for depositions ∼2 ML. The PIG approach has also allowed realization of the narrowest reported inhomogeneous linewidth of 23 meV for low temperature photoluminescence from a single layer of binary InAs/GaAs quantum dots.

Self-assembled InAs/GaAs quantum dots studied with excitation dependent cathodoluminescence

We have examined the optical properties of self-assembled InAs quantum dots (QDs) with polarization sensitive and time-resolved cathodoluminescence (CL) techniques. The InAs QDs were formed via self-assembly during molecular beam epitaxial growth of InAs on unpatterned GaAs(001). CL spectra exhibited a two-component line shape whose linewidth, intensity, and peak positions were found to be temperature and excitation dependent. The two components are found to be consistent with state filling of the QDs, resulting in emission involving ground state and excited state excitonic transitions. The luminescence intensities and lineshapes of the QD and wetting layer (WL) excitonic transitions were analyzed with constant excitation and time-resolved CL for various temperatures and excitation levels to study the thermal activation, re-emission, and recombination kinetics of carriers. Thermal quenching of the QD ground state and excited state components in the 105–175 K range is correlated with a rise in the WL emiss...

Nature of Stranski–Krastanow growth of InAs on GaAs(001)

The nature of the two-dimensional (2D) to three-dimensional (3D) morphological transition in the highly strained epitaxy of InAs on GaAs(001) is discussed, based on in situ scanning tunneling microscope and atomic force microscope studies, combined with photoluminescence (PL) and PL excitation spectroscopy results. A re-entrant 2D–3D morphology change is observed, in which quasi-3D (Q3D) clusters appear, disappear, and reappear well in advance of the formation of 3D islands. We suggest that the Q3D clusters may act as a kinetic pathway to 3D island formation, spreading out the 2D–3D transition over a delivery range of ∼0.3 monolayers. Large (>∼50 nm wide) 2D clusters sitting on top of the wetting layer (WL) undergo morphological changes with increasing strain and ultimately lose their material to 3D islands. Small (<20 nm) 2D clusters decorating the WL appear to contribute to the commonly observed redshift of the InAs WL PL peak prior to 3D island formation. A diffusion-limited formation of 3D islands for...

Optical and photocurrent spectroscopy studies of inter- and intra-band transitions in size-tailored InAs/GaAs quantum dots

Combined inter- and intra-band spectroscopy studies are presented on structurally well-characterized InAs/GaAs(001) self-assembled quantum dots grown via conventional continuous deposition and the innovative punctuated island growth approach. Temperature and power dependent photoluminescence (PL) and PL Excitation (PLE) on these remarkably uniform quantum dot based samples (with typical PL linewidth 25 meV), reveal details of size-dependent electronic structure. These studies are complemented with systematic near- and middle-infrared photocurrent spectroscopy for interband through electron intra-band transitions as a function of temperature and applied electric field.

Resonant Raman scattering in self-organized InAs/GaAs quantum dots

The exciton-phonon coupling in self-organized InAs/GaAs quantum dots (QDs) is investigated under resonant excitation of the ground-state transition. First- and second-order phonon sidebands of the TO (30.3 meV) and LO (33.2 meV) modes of the strained InAs QDs as well as an interface (35.9 meV) mode are resolved. Huang–Rhys factors of 0.012, 0.026, and 0.006, respectively, indicate enhanced polar exciton-phonon coupling in such strained low-symmetry QDs. Time-resolved measurements support the local character of the phonon modes.

InAs/AlxGa1-xAs quantum dot infrared photodetectors with undoped active region

We report on a study of n-type quantum dot (QD) infrared photodetectors (QDIPs) based on InAs/GaAs QDs grown via an innovative technique called punctuated island growth. The electronic structure of the QDs inserted in QDIP devices is comprehensively investigated with photoluminescence, and intra- and interband photocurrent spectroscopy. The influence of AlGaAs layers inserted in active regions on the performance of the QDIPs is examined. Bias and temperature dependence of intraband photoresponse of QDIPs with undoped active region is examined. Initial results on intraband photoresponsivity and detectivity of these QDIPs at 77 K are reported.

On the atomistic and kinetic nature of strained epitaxy and formation of coherent 3D island quantum ☐es

Some remarkable recent results of the in situ scanning tunneling microscope (STM) and atomic force microscope (AFM) studies of InAs coherent 3D island initiation and evolution on GaAs(001) are presented in the larger context of the field of strained epitaxy. The role of nano and meso scale mesas in manipulating stress/strain is illustrated through examples of growths on in situ, purely growth control prepared stripe and square mesas with linear dimensions as small as ∼ 40 nm. A set of basic kinetic processes and their strain dependence is identified and suggested to form a good core that has the potential for providing a unified framework for understanding strained epitaxy ranging from low misfits to high misfits.

Dark current reduction and operational wavelength shift in normal incidence InAs-GaAs QDIPs through the introduction of AlGaAs layers in the active region of the detector

Self-assembled quantum dots (SAQDs) are an attractive alternative to quantum wells (QWs) for near to long-wavelength infrared photodetector applications. Due to the 3D carrier confinement and lack of symmetry-imposed selection rules, SAQDs are intrinsically sensitive to normal incidence photoexcitation and predicted to have lower dark current and higher sensitivity compared to QW intersubband photodetectors (Ryzhii, 1996), though the filling factor is substantially less for QDs than that for QWs. Initial reported results on normal-incidence QD intersubband photodetectors (e.g. Pan et al, 1998) offer encouragement for further investigations. In this report, we demonstrate the ability to reduce the dark current and to shift the operational wavelength of InAs-GaAs QD infrared photodetectors (QDIPs) through the introduction of AlGaAs barriers in the active region of the detector structure. AlGaAs layers are placed between the QD layers to act as blocking barriers for the dark current contribution through the region between the QDs.