The interest in nanoscale materials stems from the fact that new properties are acquired at this length scale and, equally important, that these properties * To whom correspondence should be addressed. Phone, 404-8940292; fax, 404-894-0294; e-mail, mostafa.el-sayed@ chemistry.gatech.edu. † Case Western Reserve UniversitysMillis 2258. ‡ Phone, 216-368-5918; fax, 216-368-3006; e-mail, burda@case.edu. § Georgia Institute of Technology. 1025 Chem. Rev. 2005, 105, 1025−1102

Chemistry and properties of nanocrystals of different shapes.

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.

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Structural properties of self-organized semiconductor nanostructures

Fundamentally secure quantum cryptography has still not seen widespread application owing to the difficulty of generating single photons on demand. Semiconductor quantum-dot structures have recently shown great promise as practical single-photon sources, and devices with integrated optical cavities and electrical-carrier injection have already been demonstrated. However, a significant obstacle for the application of commonly used III–V quantum dots to quantum-information-processing schemes is the requirement of liquid-helium cryogenic temperatures. Epitaxially grown gallium nitride quantum dots embedded in aluminium nitride have the potential for operation at much higher temperatures. Here, we report triggered single-photon emission from gallium nitride quantum dots at temperatures up to 200 K, a temperature easily reachable with thermo-electric cooling. Gallium nitride quantum dots also open a new wavelength region in the blue and near-ultraviolet portions of the spectrum for single-photon sources.

A gallium nitride single-photon source operating at 200 K.

We demonstrate triggered single photon emission at room temperature from a site-controlled III-nitride quantum dot embedded in a nanowire. Moreover, we reveal a remarkable temperature insensitivity of the single photon statistics, and a g(2)[0] value at 300 K of just 0.13. The combination of using high-quality, small, site-controlled quantum dots with a wide-bandgap material system is crucial for providing both sufficient exciton confinement and an emission spectrum with minimal contamination in order to enable room temperature operation. Arrays of such single photon emitters will be useful for room-temperature quantum information processing applications such as on-chip quantum communication.

Room-Temperature Triggered Single Photon Emission from a III-Nitride Site-Controlled Nanowire Quantum Dot

The Blue Laser Diode

We have successfully grown nanometer-scale InGaN self-assembled quantum dots (QDs) on a GaN surface without any surfactants, using atmospheric-pressure metalorganic chemical vapor deposition. Atomic force microscopy shows that the average diameter of InGaN QDs is as small as 8.4 nm. Next, we have investigated the dependence of the QDs properties on the growth conditions: the amount of InGaN deposited and the growth temperature. Moreover, we have investigated the optical property of InGaN QDs, so that the strong emission was seen at 2.86 eV at room temperature.

Nanometer-scale InGaN self-assembled quantum dots grown by metalorganic chemical vapor deposition

Microscopic photoluminescence spectra were measured for self-assembled InGaN quantum dots (QDs) grown by metalorganic chemical vapor deposition. A thin aluminum layer with 400 nm square apertures was formed on the sample surface to reduce the number of QDs measured. We observed very sharp peaks whose spectral linewidths were typically 170 μeV at 3.5 K, the linewidth being limited by spectral resolution. Such sharp lines were not observed in similar experiments on a reference sample having single InGaN quantum well structure. These experimental results suggest that excitons are strongly confined in our InGaN QD structure.

Narrow photoluminescence peaks from localized states in InGaN quantum dot structures

We have fabricated InGaN quantum dot (QD) structures on hexagonal pyramids of GaN, using metalorganic chemical vapor deposition with selective growth. Intense photoluminescence was observed from the sample at room temperature. To directly observe the emitting areas, microphotoluminescence intensity images with a spatial resolution of a few hundred nanometers were used. The images show the emission was only from the tops of the hexagonal pyramids. The width of the emitting areas is about 300 nm, which is comparable to the spatial resolution of the images. Such a narrow width of emission areas indicates that InGaN QDs are formed on the tops of pyramids.

Selective growth of InGaN quantum dot structures and their microphotoluminescence at room temperature

GaN self-assembled quantum dots (QDs) with high quality and high density have been grown by low-pressure metalorganic chemical vapor deposition under very low V/III ratios. In depositing over a critical thickness of four monolayer GaN, we observed a transition from two-dimensional to three-dimensional growth mode. The density of the QDs could be changed between 109 and 1010 cm−2. The typical diameter and height of the QDs were 20 and 2 nm, respectively. The size of the QDs was controlled to a considerable extent by changing the growth temperature and V/III ratio. Moreover, we observed two photoluminescence peaks from both the QDs and the wetting layer at room temperature. This result clearly demonstrates that the GaN QDs were formed with the Stranski–Krastanow growth mode.

High-density and size-controlled GaN self-assembled quantum dots grown by metalorganic chemical vapor deposition

We have demonstrated laser action of an InGaN self-assembled quantum dot (QD) laser by optical pumping. We have grown the laser structure with the In0.2Ga0.8N QDs embedded in the active layer, using atmospheric-pressure metalorganic chemical vapor deposition. A clear threshold was observed in the relation between the excitation and emission intensity at room temperature. Above the threshold, the width of the emission peak was below 0.1 nm (resolution limit), and the emission was strongly polarized in the transverse electric mode. These results indicate that lasing oscillation in the InGaN self-assembled QD laser has been achieved at room temperature.

K. Tachibana

Papers

Nanometer-scale InGaN self-assembled quantum dots grown by metalorganic chemical vapor deposition

Narrow photoluminescence peaks from localized states in InGaN quantum dot structures

Selective growth of InGaN quantum dot structures and their microphotoluminescence at room temperature

High-density and size-controlled GaN self-assembled quantum dots grown by metalorganic chemical vapor deposition

Room-temperature lasing oscillation in an InGaN self-assembled quantum dot laser