E. Hall

Epitaxially-stacked multiple-active-region 1.55μm lasers for increased differential efficiency

Semiconductor lasers emitting at 1.55 μm with external differential efficiencies >1 have been created by monolithically connecting several active regions in series within a single optical waveguide. This is accomplished by epitaxially stacking a number of p–i–n multiquantum well active regions with intermediate n++–p++ back diodes, which enable the entire terminal current to flow through each active region stages in series. Such lasers should also improve the impedance match as well as provide for low-noise, high-efficiency microwave links.

1.55-/spl mu/m InP-lattice-matched VCSELs with AlGaAsSb-AlAsSb DBRs

We review the design, fabrication, and characterization of 1.55-/spl mu/m lattice-matched vertical-cavity surface-emitting lasers, operating continuous wave up to 88/spl deg/C. For one embodiment, the threshold current is 800 /spl mu/A, the differential quantum efficiency is 23%, and the maximum output power is more than 1 mW at 20/spl deg/C and 110 /spl mu/W at 80/spl deg/C. The basic structure consists of AlAsSb-AlGaAsSb mirrors, which provide both high reflectivity and an InP-lattice-matched structure. The quaternary mirrors have poor electrical and thermal conductivities, which can raise the device temperature. However, a double-intracavity-contacted structure along with thick n-type InP cladding layers circumvents these drawbacks and finally leads to an excellent performance. The measured voltage and thermal impedances are much lower for the intracavity-contacted device than an air-post structure in which current is injected through the Sb-based quaternary mirror. The structure utilizes an undercut aperture for current and optical confinement. The aperture reduces scattering loss at the etched mirror and contributes to high differential efficiency and low threshold current density.

88 °C, continuous-wave operation of apertured, intracavity contacted, 1.55 μm vertical-cavity surface-emitting lasers

We demonstrate a lattice-matched 1.55 μm vertical-cavity surface-emitting laser operating continuous wave up to 88 °C. The laser employs AlAsSb-based mirrors, which provide high reflectivity and lattice matching to InP. The poor electrical and thermal conductivity of these mirrors is circumvented by utilizing an InP double-intracavity contacted structure. Benefits of the intracavity contacts are addressed by comparing the characteristics with the alternative contact scheme where current is injected through the Sb-based mirrors. Current and optical confinement is provided by an undercut aperture. The device shows a threshold current of 800 μA, a differential efficiency of 23%, and a maximum output power of over 1 mW at 20 °C.

Near-room-temperature continuous-wave operation of multiple-active-region 1.55 μm vertical-cavity lasers with high differential efficiency

We present completely monolithic, single-step grown, bipolar cascade vertical-cavity surface-emitting lasers at 1.55 μm with a greater-than-unity differential quantum efficiency. A typical device had a threshold current density of 1 kA/cm2, a threshold voltage of 3.2 V, and demonstrated continuous wave operation up to 8 °C. Devices smaller than 10 μm in diameter lased single mode. Active regions in our device were epitaxially stacked in three stages. This technique of multiple-active regions enabled the greater-than-unity differential quantum efficiency operation, which is essential in constructing high-efficiency microwave optical links with gain. We report the device characteristics and a model on the scaling properties of active region stacking in multiple-active-region vertical-cavity lasers.

Improved electrical and thermal properties of InP-AlGaAsSb Bragg mirrors for long-wavelength vertical-cavity lasers

The electrical, thermal and optical properties of n-doped InP-AlGaAsSb 1.5-/spl mu/m Bragg mirrors are reported. A voltage of 10 mV per pair at 1 kA/cm/sup 2/ has been obtained in these mirrors, due to a low conduction band offset. This record electrical performance, combined with a large refractive index contrast (n/sub H//n/sub L/=1.135) and improved thermal properties, makes the combination very promising for long wavelength vertical cavity surface emitting lasers.

Accurate control of Sb composition in AlGaAsSb alloys on InP substrates by molecular beam epitaxy

Abstract The incorporation rates of Sb2 and As2 species are measured by group-V induced oscillations of reflection high-energy electron diffraction in molecular beam epitaxy. These measurements allow the accurate control and reproducibility of group-V composition in the AlGaAsSb system. Using the calibrations of Sb2 incorporation rate we have grown GaAsSb, AlAsSb, AlGaAsSb layers and AlGaAsSb/AlAsSb distributed Bragg reflectors lattice matched on InP substrates. Very intense room-temperature photoluminescence signals of these mirrors demonstrate the good optical quality obtained with this simple method.

Selectively etched undercut apertures in AlAsSb-based VCSELs

Apertures were formed in single-growth, AsSb-based, long-wavelength (1.55 /spl mu/m) vertical-cavity surface-emitting lasers by laterally etching the active region. The materials contrast between the AlAsSb-based mirrors and the AlInGaAs-based active region leads to a high selectivity for the etch, allowing long apertures to be formed with minimal etching of the mirrors. Lasers showing reduced threshold currents and increased efficiencies were demonstrated using these apertures.

InP-based all-epitaxial 1.3-μm VCSELs with selectively etched AlInAs apertures and Sb-based DBRs

We report, for the first time, InP-based all-epitaxially grown 1.3-/spl mu/m vertical-cavity surface-emitting lasers with lattice-matched Sb-based distributed Bragg reflectors and AlInAs etched apertures. The minimum threshold current and voltage under pulsed operation were 3 mA and 2.0 V, respectively. The thermal impedance was as low as 1.2 K/mW without heat sinking. Implementation of the AlInAs etched aperture was quite effective in improving the injection efficiency and reducing the internal loss, resulting in improved differential efficiency.

Molecular-beam epitaxy growth of high-quality active regions with strained InxGa1−xAs quantum wells and lattice-matched AlxGayIn(1−x−y)As barriers using submonolayer superlattices

Submonolayer superlattices (SMS) of Ga0.47In0.53As/InAs/Ga0.47In0.52As and Ga0.47In0.53As/Al0.48In0.52As were used for the growth of strained quantum wells (QWs) and lattice-matched barriers, respectively, in the 1.55 μm active region. QWs grown with different compressive strains show excellent room-temperature photoluminescence, demonstrating the versatility of this technique. State-of-the-art current thresholds are reported for the broad-area lasers fabricated using the SMS active region.

Improved composition control of digitally grown AlAsSb lattice-matched to InP

The growth of AlAsSb lattice-matched to InP as both an analog alloy and a digital alloy is examined. The digital alloy, consisting of alternating layers of AlAs and AlSb with a total period <10 A, has a much weaker dependence on arsenic and antimony beam flux variations, resulting in more reproducible composition control and reliable lattice-matching.