Semiconductor Quantum Dots
Quantum dots are small semiconductor structures in a host medium with higher electronic (energy) bandgap. Dot dimensions typically range from 2 to 10 nm or ~10 to 50 atoms. More specifically, quantum dots for laser diodes are self-organized nanostructures that form spontaneously and controllably on a lattice-mismatched III-V substrate during epitaxial growth processes, like MBE or MOCVD. They function by localizing charge carriers, i.e., electrons and holes (excitons), through quantum confinement. This restricts the three translational degrees of freedom and can dramatically enhance useful electronic properties. At the moment, quantum dots are perhaps the best practical example of emerging nanotechnologies. Industrial applications range from optoelectronics and electronics to medical and biological.
AFM image of Quantum Dots
Quantum dots constitute the gain medium in virtually all of Innolume’s semiconductor diode lasers and related products. This is because their composition, discreteness, and 3D carrier confinement provide valuable laser performance advantages over conventional quantum well heterostructures that offer only 1D confinement and exhibit strain limits. Benefits include reduced threshold current, temperature independence, broadened gain spectrum, and low relative intensity noise. Historically such advantages were more theoretical than practical since they were predicted on the basis of idealized quantum dot behavior unperturbed by the host medium and structure. The reality is much more complicated. Full realization of the theoretical advantages has required years of MBE (molecular beam epitaxy) process development, heterostructure engineering, and device optimization at Innolume’s Dortmund, Germany fab.
Today Innolume has achieved the promise of quantum dot nanotechnology for semiconductor lasers, as well as for associated optical devices like gain chips, semiconductor optical amplifiers (SOAs), light emitting diodes (LEDs), superluminescent diodes (SLDs), and single- or multi-mode laser bars. Achievements include: 1) demonstrated temperature independent laser performance from -20º to 90º C; 2) very broad lasing spectra (> 80 nm) with uniform intensity; 3) a unique comb laser diode permitting 10 Gb/s error free modulation of many high power (≥ 5 mW) channels; 4) stable and robust mode-locking at high peak power; and 5) overall laser performance competitive with the best quantum well diode lasers.
Innolume’s preferred compound semiconductor material system is InAs/GaAs, namely, indium arsenide quantum dots in gallium arsenide with aluminum gallium arsenide barriers, all on gallium arsenide substrates. The lasing wavelength window for this system is between 1064 nm and 1320 nm, controlled by quantum dot size, distribution, and indium concentration. Thus, Innolume’s quantum dot lasers fill the wavelength gap between quantum well lasers based on either GaAs (< 1100 nm) or InP (> 1300 nm). Furthermore, despite the relatively low concentration of the discrete gain medium, e.g., compared to continuous quantum well layers, quantum dots enable high power devices with high wall plug efficiency. For example, Innolume’s single-mode lasers in laser bars output 600 mW/laser and multimode laser bars output >8 W/laser, with 40% to 60% efficiencies depending on wavelength and modal details.
Innolume’s technologists have been intimately involved in quantum dot nanotechnology applied to optoelectronics for decades. Virtually all technical staff trained at the world-famous Ioffe Institute in St. Petersburg, Russia under Professor Alferov (2000 Nobel Laureate for laser heterostructure) and his colleagues. They worked all over the globe during the 1990s on MBE and laser technologies, agglomerating in 2003 under the Innolume umbrella in Dortmund to commercialize quantum dot nanotechnology. All phases of fabrication are done there, from MBE to packaging. To further consolidate its technology, in 2006 Innolume bought the only other company with quantum dot laser products, Zia Laser in New Mexico, USA.