Research Review: Novel Re-growth Sidesteps AlGaAs Exposure
Researchers at the University of Sheffield, UK, have developed a technique for making GaAs-based lasers that circumvents regrowth on exposed AlGaAs surfaces. Avoiding re-growth on this ternary is highly desirable, because it is difficult to adequately planarize AlGaAs without degrading the corrugated GaAs grating needed to form single-wavelength, distributed feedback lasers.
The UK team fabricates its quantum well lasers by re-growth of AlGaAs on patterned InGaP. Although this is not the only way to shun re-growth on exposed AlGaAs, it has several major advantages over two rival techniques developed several years ago.
One of these alternatives - which is associated with laterally loss-coupled gratings and aluminum-free structures - involves the formation of a metal or etched grating after the etching of a laser ridge. Low yields result, prohibiting volume manufacturing. InGaP overgrowth on patterned GaAs is the other alternative. But this hampers design freedom, because InGaP is only lattice-matched at one stoichiometry.
The team from Sheffield avoids all these issues by employing an InGaP/GaAs grating layer in conjunction with an AlGaAs cladding. This cladding is spared from exposure to air during patterning, etch and re-growth of the grating, thanks to the combination of selective etching and the insertion of a protective GaAs layer beneath the InGaP. A further benefit of this novel approach is the opportunity to place the grating as far as 450 nm from the active layer, due to the high refractive index contrast between InGaP and GaAs. This should improve reliability and performance, because the re-growth interface can be located within the heavily doped p-material and well away from the active region.
Laser fabrication began with MOCVD growth of device epistructures on (110) n-doped substrates. The planar wafer, which features two InGaAs quantum wells, was patterned with a 148 nm period grating that had been defined in PMMA by e-beam lithography. A combination of inductivelycoupled plasma and wet etching removed GaAs and GaInP layers, and after cleaning in dilute hydrofluoric acid, high-quality GaAs and AlGaAs layers were deposited on top of the structure.
Ridge lasers with a 3 μm width and a 1 mmlong cavity produced 7.9 mW at a 62 mA drive current. The corresponding threshold current density was 2 kA/cm2, and the dominant, 1006 nm mode had a side-mode suppression ratio of 30 dB.
B.J. Stevens et al. Electron. Lett. 46 1076 (2010)