QUANTUM DOTS: Columnar dots achieve low-threshold lasing

Researchers at Fujitsu Laboratories Ltd. (Atsugi, Japan) have achieved continuous-wave (CW) lasing from a new type of columnar-shaped self-organized quantum dot. The closely stacked dots demonstrate room-temperature lasing at a record threshold current of 20 mA with a current density of 250 A/cm².

The main feature of a quantum-dot laser is the potential reduction of threshold current caused by enhanced quantum effects and the reduced volume of the active region. Quantum-dot arrays made of these lasers could be used in optical systems that require ultralow power consumption, such as optical interconnections for telecommunications and optical computing.

More than three years ago, Fujitsu researchers reported the first current-injection laser oscillation from indium gallium arsenide (InGaAs) quantum dots at 80 K. The dots, which emitted at 1300 nm, were grown on gallium arsenide (GaAs) substrates by atomic-layer epitaxy and demonstrated the requisite three-dimensional quantum confinement.

Last year, the researchers achieved room-temperature lasing at 1020 nm, observing discontinuous jumps of lasing wavelength from a high-order subband to a lower-order subband with increasing dot layers and with decreasing cavity loss. This result opened the possibility of laser oscillation from quantum dots at the ground state.

The InGaAs quantum dots were grown in the Stranski-Krastanov (S-K) mode at 510°C. They were 20 nm in diameter and approximately 5 nm in height, and their aerial coverage on the substrate was approximately 10%. Three types of active layers were grown in which the number of quantum-dot layers varied from one to three, forming a 2.5-µm-wide ridge structure with cavity lengths of 300, 600, and 900 µm.

Columnar-shaped variation

The researchers now have achieved low-threshold CW lasing (1.17 µm) of a new type of self-organized dots that are columnar-shaped. Using molecular-beam epitaxy, they closely stacked the S-K dots in the growth direction with monolayer-thick intermediate layers (see figure). Highly lattice-mismatched epitaxial growth allowed the re searchers to fabricate high-optical-quality and high-density microcrystals.

A dot laser with cleaved facets operated at a threshold current of 20 mA at 25°C, an output power of 42 mW, and current density per single-dot layer of 250 A/cm2. Fujitsu researchers note that the threshold current is the best yet obtained for a quantum-dot laser and is the result of improvements in uniformity of dot arrays and their optical quality.

The columnar-shaped dots were grown by alternating InAs and GaAs on an 001 n-GaAs substrate at 510°C. Researchers note that the width of the photoluminescence spectra (measured at 300 K) is far better than that of their previous single S-K dots. And the photoluminescence intensity is equal to or better than the single S-K dots and more than one-thousandth that of their previous closely stacked dots.

Meanwhile, researchers at the nearby Nippon Telegraph and Telephone (NTT) Basic Research Laboratory (Atsugi) have a different approach. Instead of growing the dots on the usual 001 face of the GaAs substrate, they chose the 311B substrate. The result is a remarkably ordered array of spontaneously formed disk-shaped InGaAs platelets of nanometer size.

Each disk is 120 nm across and lases at approximately 800 nm at room temperature. The size of such structures is now as small as twice the exiton Bohr radius. Packing two exitons into one dot with complete confinement in all three dimensions enhances the interaction between them.

NTT believes these exciton effects dominate the optical-gain properties in quantum-dot lasers. Its task now—as with the Fujitsu researchers—includes further improving the dot geometry and ordering of dot arrays.