Beams of electrons and ions are now fairly routinely focused to dimensions in the nanometer range. Since the beams can be used to locally alter material at the point where they are incident on a surface, they represent direct nanofabrication tools. The authors will focus here on direct fabrication rather than lithography, which is indirect in that it uses the intermediary of resist. In the case of both ions and electrons, material addition or removal can be achieved using precursor gases. In addition ions can also alter material by sputtering (milling), by damage, or by implantation. Many material removal and deposition processes employing precursor gases have been developed for numerous practical applications, such as mask repair, circuit restructuring and repair, and sample sectioning. The authors will also discuss structures that are made for research purposes or for demonstration of the processing capabilities. In many cases the minimum dimensions at which these processes can be realized are considerably larger than the beam diameters. The atomic level mechanisms responsible for the precursor gas activation have not been studied in detail in many cases. The authors will review the state of the art and level of understanding of direct ion and electron beam fabrication and point out some of the unsolved problems.

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Gas-assisted focused electron beam and ion beam processing and fabrication

Instabilities in semiconductor heterostructure growth can be exploited for the self-organized formation of nanostructures, allowing for carrier confinement in all three spatial dimensions. Beside the description of various growth modes, the experimental characterization of structural properties, such as size and shape, chemical composition, and strain distribution is presented. The authors discuss the calculation of strain fields, which play an important role in the formation of such nanostructures and also influence their structural and optoelectronic properties. Several specific materials systems are surveyed together with important applications.

/pdf/structural-properties-of-self-organized-semiconductor-2w9pzxvcuh.pdf

Structural properties of self-organized semiconductor nanostructures

Recently, strongly modulated photonic crystals, fabricated by the state-of-the-art semiconductor nanofabrication process, have realized various novel optical properties. This paper describes the way in which they differ from other optical media, and clarifies what they can do. In particular, three important issues are considered: light confinement, frequency dispersion and spatial dispersion. First, I describe the latest status and impact of ultra-strong light confinement in a wavelength-cubic volume achieved in photonic crystals. Second, the extreme reduction in the speed of light is reported, which was achieved as a result of frequency dispersion management. Third, strange negative refraction in photonic crystals is introduced, which results from their unique spatial dispersion, and it is clarified how this leads to perfect imaging. The last two sections are devoted to applications of these novel properties. First, I report the fact that strong light confinement and huge light–matter interaction enhancement make strongly modulated photonic crystals promising for on-chip all-optical processing, and present several examples including all-optical switches/memories and optical logics. As a second application, it is shown that the strong light confinement and slow light in strongly modulated photonic crystals enable the adiabatic tuning of light, which leads to various novel ways of controlling light, such as adiabatic frequency conversion, efficient optomechanics systems, photon memories and photons pinning.

https://iopscience.iop.org/article/10.1088/0034-4885/73/9/096501/pdf

Manipulating light with strongly modulated photonic crystals

Vertical-cavity electrically driven lasers with three GaInAs
quantum wells and diameters of several μm exhibit room-temperature pulsed current thresholds as low as 1.3mA with 958 nm output wavelength.

Low-Threshold Electrically Pumps Vertical-Cavity Surface-Emitting Microlasers

High density arrays of chromium (Cr) and layered gold/chromium (Au/Cr) nanodots and nanoholes in metal films were fabricated by evaporation onto nanoporous templates produced by the self-assembly of poly(styrene-b-methyl methacrylate) (P(S-b-MMA)) diblock copolymers. The cylindrical microdomains of the asymmetric block copolymer were oriented normal to the surface by balancing interfacial interactions of the blocks with the substrate. By selectively removing either the minor or major component, nanoporous films of PS or nanoscopic posts of PS could be produced. Thus, a template, comprising an array of hexagonally packed pores in a PS matrix or PS posts, was easily fabricated. Evaporation of Cr and Au onto the template, followed by sonication and UV degradation of the PS, left Cr or Au/Cr nanodots or a nanoporous metallic film.

A Simple Route to Metal Nanodots and Nanoporous Metal Films

A nanometer-scale site-control technique for individual InAs quantum dots (QDs) has been developed by using scanning tunneling microscope (STM) -assisted nanolithography and self-organizing molecular-beam epitaxy. We find that nanometer-scale deposits can be created on a GaAs surface by applying voltage and current pulses between the surface and a tungsten probe of the STM, and that they act as “nanomasks” on which GaAs does not grow directly. Accordingly, subsequent thin GaAs growth produces GaAs nanoholes above the deposits. By supplying 1.1 ML InAs on this surface, QDs are self-organized at the hole sites, while hardly any undesirable Stranski–Krastanov QDs are formed in the flat surface region. Using this technique with nanometer precision, a QD pair with 45 nm pitch is fabricated.

Site-controlled self-organization of individual InAs quantum dots by scanning tunneling probe-assisted nanolithography

We studied a site-control technique for InAs quantum dots (QDs) on GaAs substrates using a combination of in situ electron-beam (EB) lithography and self-organized molecular-beam epitaxy. In small, shallow holes formed on prepatterned mesa structures by EB writing and Cl2 gas etching, QDs were selectively formed, without any formation on the flat region between the patterned holes. The density of the QDs in each hole was dependent on the hole depth, indicating that atomic steps on the GaAs surfaces act as migration barriers to In adatoms. In an array of holes including 5–6 monolayer steps, a single QD was arranged in each hole.

Site-controlled InAs single quantum-dot structures on GaAs surfaces patterned by in situ electron-beam lithography

We studied a site-control method for self-organized InAs quantum dots on GaAs substrates by a combination of in situ electron-beam (EB) lithography and molecular-beam epitaxy (MBE) using an ultrahigh-vacuum multichamber system. Small and shallow holes were patterned on a MBE-grown GaAs (001) surface by in situ EB writing and Cl2-gas etching. When more than a 1.4 monolayer of InAs was applied to the patterned surface, In(Ga)As dots were preferentially self-organized in the holes while dot formation around the holes was sufficiently suppressed. When we further increased the amount of InAs, the dots enlarged remarkably, presumably due to a stress-relaxation effect.

Site control of self-organized InAs dots on GaAs substrates by in situ electron-beam lithography and molecular-beam epitaxy

The optical nonlinear properties of self-assembled InAs/GaAs quantum dots (QDs) were experimentally verified by means of transient absorption measurements. A saturation pulse energy Ps of 13 fJ/μm2 and an absorption recovery time τr of 55 ps were obtained from transmission bleaching and pump/probe measurements for a waveguide sample with ten-layer-stacked QDs. An absorption saturation intensity Is of 2.5×104 W/cm2, calculated from Ps and τr, was found. The saturation pulse energy is up to an order of magnitude smaller than, or at least comparable with, the reported values for excitons in quantum wells of III–V compound semiconductors. The dipole length, as calculated from the absorption cross section, is of the same order as the lattice constant of the InAs QDs. The results are expected to experimentally verify that QDs show a delta-function-like density of states.

Optical nonlinear properties of InAs quantum dots by means of transient absorption measurements

In situ site-control techniques for self-organized InAs quantum dots (QDs) have been developed using an electron beam (EB) and a scanning tunneling microscope (STM) probe combined with molecular beam epitaxy. In the in situ EB-assisted process, InAs dots are preferentially formed in shallow, sub-μm-size GaAs holes with the InAs supply. We find that the specific slope of a hole acts as a favorable site for dot formation. In the in situ STM probe-assisted process, the size and pitch of the holes are considerably reduced into nanoscale. InAs QDs are then self-organized only at the hole sites due to strain-induced selective nucleation. Using this process, two- and three-dimensional QD arrays are fabricated with nanometer precision.

Shigeru Kohmoto

Papers

Site-controlled self-organization of individual InAs quantum dots by scanning tunneling probe-assisted nanolithography

Site-controlled InAs single quantum-dot structures on GaAs surfaces patterned by in situ electron-beam lithography

Site control of self-organized InAs dots on GaAs substrates by in situ electron-beam lithography and molecular-beam epitaxy

Optical nonlinear properties of InAs quantum dots by means of transient absorption measurements

Site-controlled self-organization of InAs quantum dots