Patterned Quantum Dot Research Articles

Controlled growth of InGaAs/InGaAsP quantum dots on InP substrates employing diblock copolymer lithography

Selective metalorganic chemical vapor deposition growth with diblock copolymer nanopatterning is utilized to produce InGaAsP(Q1.15 μm)/In0.53Ga0.47As/InGaAsP(Q1.15 μm) and InP/In0.53Ga0.47As/InP quantum dots (QDs) on InP substrates. The QD patterning is prepared by dense nanoscale diblock copolymer lithography followed by pattern-transfer onto a dielectric template mask and reactive ion etching is utilized to form nanosized openings exposing the underlying InGaAsP layer. By varying the In0.53Ga0.47As layer thickness within the QDs, the emission wavelength can be selected within the 1.4–1.6 μm region. Strongest photoluminescence (PL) intensity is observed from QDs employing InP rather than InGaAsP barriers, demonstrating room temperature PL near 1.6 μm.

Improved photoluminescence efficiency of patterned quantum dots incorporating adots-in-the-well structure

InAs quantum dots embedded in InGaAs quantum well (DWELL: dots-in-the-well) structuresgrown on nanopatterned GaAs pyramids and planar GaAs(001) surface are comparativelyinvestigated. Photoluminescence (PL) measurements demonstrate that the DWELL structuregrown on the GaAs pyramids exhibits a broad quantum well PL band (full width at half-maximum∼ 90 meV) and a higher quantum dot emission efficiency than the DWELL structure grown onthe planar GaAs(001) substrate. These properties are attributed to the InGaAs quantumwell with distributed thickness profile on the faceted GaAs pyramids, which introduces atapered energy band structure and enhances carrier capture into the quantum dots.

High density patterned quantum dot arrays fabricated by electron beam lithography and wet chemical etching

We present a quantum dot (QD) fabrication method which allows for the definition of the explicit location and size of an individual QD. We have obtained high optical quality, high density QD arrays utilizing hydrogen silsesquioxane, a negative tone electron beam resist, as a wet etch mask for an underlying quantum well. Linewidths as small as 24meV from a large ensemble of QDs have been demonstrated in photoluminescence measurements at 77K. The magnitude of the experimentally obtained blueshift due to quantum confinement effects is found to be consistent with that predicted by theory.

Optical properties of patterned InAs quantum dot ensembles grown on GaAs nanopyramids

We demonstrate the ability to form either coupled or isolated patterned quantum dot (PQD) ensembles on nanopatterned GaAs pyramidal buffers. The coupled PQD “clusters” consist of close-spaced PQDs with inter-QD spacing of 5nm. The isolated PQD “pairs” are comprised of two PQDs well separated by 110nm. The photoluminescence behavior, measured in integrated intensity, linewidth, and emission peak as a function of excitation intensity and temperature, indicates lateral coupling within the QD clusters and an isolated nature for QD pairs. The ability to tailor PQD formation and subsequent carrier recombination characteristic may prove useful in developing PQD-based devices for optical computing applications.

Ge/Si Quantum Dot Formation From Non-Uniform Cluster Fluxes

The controlled growth of ultra-small Ge/Si quantum dot (QD) nuclei (≈1 nm) suitable for the synthesis of uniform nanopatterns with high surface coverage, is simulated using atom-only and size non-uniform cluster fluxes. It is found that seed nuclei of more uniform sizes are formed when clusters of non-uniform size are deposited. This counter-intuitive result is explained via adatom-nanocluster interactions on Si(100) surfaces. Our results are supported by experimental data on the geometric characteristics of QD patterns synthesized by nanocluster deposition. This is followed by a description of the role of plasmas as non-uniform cluster sources and the impact on surface dynamics. The technique challenges conventional growth modes and is promising for deterministic synthesis of nanodot arrays.

Controlled InAs quantum dot nucleation on faceted nanopatterned pyramids

The selective quantum dot (QD) nucleation on nanofaceted GaAs pyramidal facets is explored. The GaAs pyramids, formed on a SiO2 masked (001) GaAs substrate, are characterized by well-defined equilibrium crystal shapes (ECSs) defined by three crystal plane families including {11n}, {10n}, and (001). Subsequent patterned QD (PQD) nucleation on the GaAs pyramidal facets is highly preferential towards the (11n) planes due to superior energy minimization. The GaAs pyramid ECS and PQDs are examined using high-resolution scanning electron microscopy and room temperature photoluminescence.

Selective GaAs Quantum Dot Array Growth using Dielectric and AlGaAs Masks Pattern-Transferred from Diblock Copolymer

AbstractTo reach the full theoretical potential advantages of ideal Quantum Dots (QDs) for diode lasers and photodetectors, elimination of the wetting layer, which is inherent to self-assembled QDs of Stranski-Krastnow (SK) growth mode, and achieving a uniform mono-modal QD size distribution is needed. The SK QD approach is complicated by the randomness of the QD size distribution and inherent presence of the wetting layer. These factors have been experimentally identified as the underlying cause for low optical gain and high temperature sensitivity in diode lasers which result from carrier leakage out of the QDs into the wetting layer.An alternate approach to QD formation is the use of nanopatterning with diblock copolymers combined with selective MOCVD growth. We utilize cylinder-forming PS-b-PMMA which have the ability of preserving the hole size through the pattern transfer procedures. The combination of diblock copolymer lithography with selective MOCVD growth of the QDs could lead to a higher degree of control over QD shape, size uniformity, and composition over the self-assembly process. Since the SK self-assembly process is not employed, the problematic wetting layer states are eliminated and improved optical gain can be expected. Control over the QD height, shape, and strain, also allows for the design of increased energy spacing between ground and excited QD states and hence a wider control or selection of the emission wavelengths. Since the QD strain is decoupled from the size, the process also has potential for achieving longer wavelength emission compared with SK QDs. On a GaAs substrate, hexagonally arranged uniform size QD arrays have been fabricated. The QD patterning is prepared by dense nanoscale diblock copolymer lithography, which consists of perpendicularly ordered cylindrical domains of polystyrene-block-poly(methylmethacrylate) (PS-b-PMMA) matrix. To transfer polymer patterns to QD arrays, two mask materials have been taken under consideration. Firstly, to characterize the profile and distribution of the QDs, a dielectric template mask was utilized and the polymer patterning is transferred on it. After the pattern transfer to the dielectric and subsequent removal of the polymer, single crystal GaAs QDs are selectively grown by MOCVD. SEM images indicate that the QD density is larger than 5×10^10/cm2, comparable to SK growth mode and the size distribution peaks at a 12nm diameter. Finally, to grow QDs and subsequently cover the QDs in situ, an AlxGa1-xAs template mask is being investigated. After the pattern transfer to a 15nm thick AlGaAs template layer using RIE, the native oxide on the patterned AlGaAs surface acts as a selective growth mask for InAs QDs. After deposition of the QDs, capping layers can be grown on QDs without removing the sample from the reactor. We are currently investigating optimal growth conditions for covering the QDs and characterizing the photoluminescence. University of Wisconsin -Madison gratefully acknowledges support from the ARO MURI W911NF-05-1-0262 (Dr. John Prater) and NSF NSEC DMR-0425880.

Selective MOCVD growth of single-crystal dense GaAs quantum dot array using cylinder-forming diblock copolymers

In order to realize the theoretical advantages of ideal quantum dots (QDs) (i.e., complete three-dimensional carrier confinement), an alternate approach to self-assembly has been investigated for elimination of the wetting layer and achieving a uniform mono-modal QD size distribution. On a GaAs substrate, hexagonally arranged uniform-sized QD arrays have been fabricated. The QD patterning is prepared by dense nanoscale diblock copolymer lithography, which consists of perpendicularly ordered cylindrical domains of polystyrene- block-poly(methylmethacrylate) (PS- b-PMMA) matrix. The polymer patterns are transferred to a dielectric template mask and then single-crystal GaAs QDs are selectively grown on the patterned substrate by metalorganic chemical vapor deposition (MOCVD). Single-crystal QDs are confirmed by X-ray diffraction with a large area detector. Scanning electron mocroscope images indicate that the QD density is larger than 5×10 10/cm 2, comparable to Stranski–Krastnow growth mode. Using the technique of diblock copolymer lithography followed by the selective MOCVD growth of the QDs, a higher degree of control of QD shape, size uniformity, and composition is expected.

Spectral and threshold performance of patterned quantum dot lasers

Semiconductor quantum dots have been widely researched as a means of improving the performance of optoelectronic devices. Self-assembly has been the dominant method of fabricating quantum dots because of its relative ease compared to more explicit techniques. We have developed a method for fabricating quantum dots in a more explicit manner using electron beam lithography and selective-area metal-organic chemical vapor deposition crystal growth. By eliminating the dependence on strain-driven self-assembly, we can avoid the size distribution and resulting inhomogeneously broadened emission spectrum associated with self-assembled quantum dot ensembles. We report on the threshold and spectral properties of patterned quantum dot lasers. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Regrowth of self-assembled InAs quantum dots on nanohole and stripe templates

Self-assembled InAs quantum dots (QDs) are grown on two types of templates by molecular beam epitaxy (MBE). A nanohole template is prepared by first growing InAs QDs on a GaAs substrate by a standard MBE process, then capping the QDs with a thin lattice-mismatched layer of GaAs. Subsequent annealing of the nanohole template results in a stripe template. We are able to transfer the patterns of self-assembled QDs onto these two types of templates: regrowth on the nanohole template results in uniform QDs situated in the nanoholes, while regrowth on the stripe template results in chains of QDs aligned along the stripes. Our results imply that self-assembled QDs can be grown onto in-situ prepared patterned substrates, with limited degree of randomness.

A single-electron transistor made from a cadmium selenide nanocrystal

The techniques of colloidal chemistry permit the routine creation of semiconductor nanocrystals, whose dimensions are much smaller than those that can be realized using lithographic techniques. The sizes of such nanocrystals can be varied systematically to study quantum size effects or to make novel electronic or optical materials with tailored properties. Preliminary studies of both the electrical and optical properties of individual nanocrystals have been performed recently. These studies show clearly that a single excess charge on a nanocrystal can markedly influence its properties. Here we present measurements of electrical transport in a single-electron transistor made from a colloidal nanocrystal of cadmium selenide. This device structure enables the number of charge carriers on the nanocrystal to be tuned directly, and so permits the measurement of the energy required for adding successive charge carriers. Such measurements are invaluable in understanding the energy-level spectra of small electronic systems, as has been shown by similar studies of lithographically patterned quantum dots and small metallic grains.