David W. Treat

AlGaInP single quantum well laser diodes

The properties and low pressure organometallic vapor phase epitaxy of GaxIn1-xP/(AlGa)0.5In0.5P quantum well (QW) laser diode heterostructures with Al0.5In0.5P cladding layers, and having a wavelength of 614 < (lambda) < 690 nm, are described. At longer wavelengths ((lambda) > 660 nm), threshold current densities under 200 A/cm2 and efficiencies greater than 75% result from a biaxially compressed GaInP QW active region. Although short wavelength laser performance is diminished by the poor electron confinement afforded by AlGaInP heterostructures, good 630 nm band performance, and extension into the 610 nm band, is achieved with strained, single QW active regions.

New diode wavelengths for pumping solid state lasers

High-power laser-diode arrays have been demonstrated to be viable pump sources for solid- state lasers. The diode bars (fill factor of 0.7) were bonded to silicon microchannel heatsinks for high-average-power operation. Over 12 W of cw output power was achieved from a one cm AlGaInP tensile-strained single-quantum-well laser diode bar. At 690 nm, a compressively strained single-quantum-well laser-diode array produced 360 W/cm2 per emitting aperture under cw operation, and 2.85 kW of pulsed power from a 3.8 cm2 emitting- aperture array. InGaAs strained single-quantum-well laser diodes emitting at 900 nm produced 2.8 kW pulsed power from a 4.4 cm2 emitting-aperture array.

High power, 8.5 W cw, visible laser diodes

We report recent results in high power visible diode lasers operating in both the 680 nm band and the 630 nm band. Continuous wave (CW) output powers in excess of 1 W from a 100 p.m aperture and 8.5Wfrom a monolithic 8 mm bar have been obtained in the 680 nm band. At 633 nm, 900 mW have been measured from a 100 jim wide aperture and 3 W from a 1 cm bar. We also discuss the temperature and length dependence of the threshold current density and external efficiency.

High-power visible semiconductor lasers

High power visible semiconductor laser diodes are reported at wavelengths ranging from 620 nm to 690 nm. Broad area laser diodes exhibit peak cw output powers of 3.8 W from a 250 micrometers aperture at 688 nm and > 1 W cw at 636 nm from a 100 micrometers aperture. Monolithic 1 cm arrays with a 24% filling factor provide output powers of 30 W cw at 687 nm. Single mode lasers in the 620 nm wavelength band emit > 50 mW and operate at temperatures up to 80 degree(s)C.

Integration of red, infrared, and blue light sources by wafer fusion

In this work, we demonstrate fusion of GaAs-based laser structures to GaN-based light-emitting diode (LED) heterostructures. Successful operation of red and infrared lasers fused to functioning GaN LEDs is achieved. A single heterostructure consisting of AlGaInAs/AlGaAs quantum well (QW) and GaInP/AlGaInP QW laser diode structures was grown by low-pressure organometallic vapor phase epitaxy (OMVPE) on GaAs substrates. The GaN LED structure was grown by OMVPE on an A-face sapphire substrate. The heterostructures were fused at 650 degrees Celsius in an H2 ambient, while under uniaxial pressure. To fabricate the lasers, the GaAs substrate was selectively etched, leaving the red and infrared QW laser stack structure on GaN. Ridge waveguide QW lasers and GaN LEDs were fabricated with the fused epilayers. Infrared, AlGaInAs QW lasers (4 X 500 micrometer), operated with a threshold current (Ith) of 40 mA and external differential quantum efficiency ((eta) d) of 11.5%/facet at about 821 nm. Red, GaInP QW lasers (4 X 500 micrometer), operated with a Ith of 118 mA and (eta) d of 18.7%/facet at about 660 nm. The adjacent InGaN/GaN LED emitted at 446 nm.

TE/TM cross-polarization laser diodes using tensile-strained quantum wells

Polarization characteristics of TE/TM cross-polarization semiconductor laser diodes are discussed in this paper. Broad area lasers fabricated from tensile strained In0.5+(delta )Ga0.5-(delta )P/(AlGa)0.5In0.5P quantum well laser structures oscillate in TE/TM dual polarizations. Polarization dominance changes from TE to TM as the cavity length of the laser is increased from 250 micrometers to 650 micrometers. The polarization-dependent gain property of a tensile-strained quantum well laser is analyzed from a simple theoretical model. In a slightly tensile strain quantum well, where light-hole and heavy-hole ground states are nearly degenerate in the valence band due to the strain and quantization effect, gain is provided for TM and TE modes simultaneously, and the two mode gain curves cross at certain injection level. Polarization switching is made possible by changing the threshold gain of the laser. The threshold gain dependent polarization switching is utilized to fabricate closely spaced independently-addressable dual beam cross polarization lasers. Results on 650 nm broad area dual beam cross polarization laser are presented. For dual polarization infrared lasers, a dual quantum well structure in which gains for TE and TM modes are provided by lattice-matched and tensile-strained quantum wells separately is designed. Eight-hundred-thirty-five nm broad area laser fabricated from a GaAs and GaAs0.95P0.05 dual quantum well structure oscillating in TE/TM dual polarizations is demonstrated.

Densely packed surface-emitting laser arrays for printing applications

We report on our efforts to develop a laser printbar consisting of a very dense array of independently addressable laterally-oxidized top-emitting VCSELs. In order to maintain wafer planarity for easy electrical routing, the buried oxidation layer in our structure is accessed through small via holes instead of a more typical mesa etch. Unlike most VCSELs, our devices utilize transparent indium-tin- oxide top contacts that allow for a more compact device design. The 200-element array we fabricated has a linear density of one device every 3 micrometers .

Laterally oxidized vertical-cavity lasers with stable polarization

We disclose a method of eliminating the polarization instability in laterally-oxidized vertical-cavity surface- emitting lasers. By employing an appropriately-shaped device aperture, we are able to make the lasers operate in a single polarization direction through their entire L-I curve.

Visible laser diodes for printing

The utilization of visible laser diodes for laser printing is discussed. First, the characteristics of a multiple- element array of single-mode, individually-addressed red (AlGaInP) laser diodes is described. The benefit of shorter- wavelength blue lasers is then evaluated. Finally, towards the realization of a blue laser diode, we describe results for AlGaInN and its heterostructures, which have been grown by OMVPE and characterized, including electrical injection and optical pumping of InGaN/AlGaN heterostructures.

Dual-wavelength laser by selective intermixing of GaAs/AlGaAs quantum wells

The longer-wavelength quantum well in an AlGaAs/GaAs asymmetric dual quantum well laser structure was selectively removed by localized intermixing. High Si-doping on each side of the longer-wavelength well caused intermixing during an anneal under a SiNx cap, while leaving the other nearby well intact. During an anneal under an exposed GaAs surface layer, both quantum wells remained intact. By patterning the surface with alternating SiNx and exposed GaAs, the longer-wavelength quantum well was selectively intermixed under the SiNx. Integrated broad area lasers were fabricated with threshold current density and external quantum efficiency of 260 A/cm2 and 30%/facet at a wavelength of 751 nm in capped regions and 195 A/cm2, 32%/facet at 824 nm in the uncapped regions. This technique can be used to fabricate close spacing multi-wavelength laser arrays.