Juha Tommila

Nanoimprint fabrication of gold nanocones with ~10 nm tips for enhanced optical interactions

We show that nanoimprint lithography combined with electron-beam evaporation provides a cost-efficient, rapid, and reproducible method to fabricate conical nanostructures with very sharp tips on flat surfaces in high volumes. We demonstrate the method by preparing a wafer-scale array of gold nanocones with an average tip radius of 5 nm. Strong local fields at the tips enhance the second-harmonic generation by over 2 orders of magnitude compared with a nonsharp reference.

Large array of single, site-controlled InAs quantum dots fabricated by UV-nanoimprint lithography and molecular beam epitaxy

We present the growth of single, site-controlled InAs quantum dots on GaAs templates using UV-nanoimprint lithography and molecular beam epitaxy. A large quantum dot array with a period of 1.5 µm was achieved. Single quantum dots were studied by steady-state and time-resolved micro-photoluminescence experiments. We obtained single exciton emission with a linewidth of 45 µeV. In time-resolved experiments, we observed decay times of about 670 ps. Our results underline the potential of nanoimprint lithography and molecular beam epitaxy to create large-scale, single quantum dot arrays.

Structural and optical properties of InAs quantum dot chains grown on nanoimprint lithography structured GaAs with different pattern orientations

We use large-scale UV nanoimprint lithography prepatterned GaAs substrates for site-controlled growth of InAs quantum dot chains by molecular beam epitaxy. We demonstrate simultaneous fabrication of quantum dot chains with high optical quality along four different crystal orientations, [011], [011¯], [010], and [001]. We show that the [011¯], [010], and [001]-oriented quantum dot chains not only have similar morphology but also experience similar in-plane optical anisotropy, which tends to align along the axis of the quantum dot chain. Our optical and structural results show that InAs quantum dot chains could be a potential platform for nanophotonic waveguiding and integrated circuits.

Soft stamp ultraviolet-nanoimprint lithography for fabrication of laser diodes

We investigate a novel nanofabrication process called soft ultraviolet (UV) nanoimprint lithography (NIL), for nanopatterning of compound semiconductors. We use flexible stamps with three layers and analyze their performance with wafers composed of III-V semiconductors. The developed stamp configuration is in many ways advantageous for the fabrication of precise gratings for various applications in photonics. We describe how to handle the deformation in both lateral and vertical directions by tuning the softness of the stamp and using a two step imprint process. As an application of the UV-NIL, we demonstrate a fabrication process for a laterally corrugated distributed feedback laser. Our laser fabrication process is free from regrowth and therefore easily adaptable to various material compositions and emission wavelengths. Because of the cost-effective full-wafer NIL, these lasers are attractive in various applications where low-cost, single-mode laser diodes are required. Our development work improves the design freedom of the NIL fabrication process of the laser diodes and improves the quality of the transferred patterns. To the best in our knowledge, this is the first demonstration of a single-mode laser diode fabricated by soft UV-NIL.

Structural characterization of InAs quantum dot chains grown by molecular beam epitaxy on nanoimprint lithography patterned GaAs(100)

We combine nanoimprint lithography and molecular beam epitaxy for the site-controlled growth of InAs quantum dot chains on GaAs(100) substrates. We study the influence of quantum dot growth temperature and regrowth buffer thickness on the formation of the quantum dot chains. In particular, we show that by carefully tuning the growth conditions we can achieve equal quantum dot densities and photoluminescence ground state peak wavelengths for quantum dot chains grown on patterns oriented along the [011], [01 ̄1], [011] and [001] directions. Furthermore, we identify the crystal facets that form the sidewalls of the grooves in the differently oriented patterns after capping and show that the existence of (411)A sidewalls causes reduction of the QD density as well as sidewall roughening.

Soft stamp UV-nanoimprint lithography for fabrication of laser diodes

In this paper, we investigate a novel nanofabrication process called soft UV nanoimprint lithography, for nanopatterning of compound semiconductors. We use flexible stamps with three layers and analyze their performance with wafers composed of III-V semiconductors. The developed stamp configuration is in many ways advantageous for the fabrication of precise gratings for various applications in photonics. We describe how to handle the deformation in both lateral and vertical directions by tuning the softness of the stamp and using a two step imprint process. As an application of the UV-NIL, we demonstrate a fabrication process for a laterally corrugated distributed feedback laser. Our laser fabrication process is free from regrowth and therefore easily adaptable to various material compositions and emission wavelengths. Due to the cost effective full wafer NIL, these lasers are attractive in various applications where low cost, single-mode laser diodes are required. Our development work improves the design freedom of the NIL fabrication process of the laser diodes, and improves the quality of the transferred patterns. To the best of our knowledge, this is the first demonstration of a single-mode laser diode fabricated by soft UV-NIL.

Cavity-enhanced single photon emission from site-controlled In(Ga)As quantum dots fabricated using nanoimprint lithography

We report on the emission dynamics of single In(Ga)As quantum dots formed in etched GaAs pits and integrated into micropillar cavities. The site-controlled quantum dots were fabricated by molecular beam epitaxy on nanoimprint lithography patterned GaAs(001) surfaces. Triggered single photon emission confirmed by photon autocorrelation measurements is demonstrated. Time-resolved photoluminescence experiments clearly show an effect of the cavity on the spontaneous emission rate of the quantum dot.

Lasing in microdisks of ultrasmall diameter

It is demonstrated by calculations and experimental results that room-temperature lasing can be obtained at the ground-state optical transition of InAs/InGaAs/GaAs quantum dots in optical microcavities with a record-small diameter of 1.5 μm. In 1-μm cavities, lasing occurs at the wavelength of one of the whispering-gallery modes within the band corresponding to the first excited-state optical transition.

The influence of temperature on the photoluminescence properties of single InAs quantum dots grown on patterned GaAs

We report the temperature-dependent photoluminescence of single site-controlled and self-assembled InAs quantum dots. We have used nanoimprint lithography for patterning GaAs(100) templates and molecular beam epitaxy for quantum dot deposition. We show that the influence of the temperature on the photoluminescence properties is similar for quantum dots on etched nanopatterns and randomly positioned quantum dots on planar surfaces. The photoluminescence properties indicate that the prepatterning does not degrade the radiative recombination rate for the site-controlled quantum dots.

High quality InP nanopyramidal frusta on Si

Nanosized octagonal pyramidal frusta of indium phosphide were selectively grown at circular hole openings on a silicon dioxide mask deposited on indium phosphide and indium phosphide pre-coated silicon substrates. The eight facets of the frusta were determined to be {111} and {110} truncated by a top (100) facet. The size of the top flat surface can be controlled by the diameter of the openings in the mask and the separation between them. The limited height of the frusta is attributed to kinetically controlled selective growth on the (100) top surface. Independent analyses with photoluminescence, cathodoluminescence and scanning spreading resistance measurements confirm certain doping enrichment in the frustum facets. This is understood to be due to crystallographic orientation dependent dopant incorporation. The blue shift from the respective spectra is the result of this enrichment exhibiting the Burstein–Moss effect. Very bright panchromatic cathodoluminescence images indicate that the top surfaces of the frusta are free from dislocations. The good optical and morphological quality of the nanopyramidal frusta indicates that the fabrication method is very attractive for the growth of site-, shape-, and number-controlled semiconductor quantum dot structures on silicon for nanophotonic applications.