The GaAs-InGaAsP heterointerfaces formation have been studied and optimized using a direct intermolecular wafer bonding (fusion)of an active region heterostructure on an InP substrate and distributed Bragg reflector heterostructures on GaAs substrates for the fabrication of hybrid heterostructures of long-wave vertical-cavity surface-emitting lasers (VCSEL). The heterostructures were grown by solid-source molecular beam epitaxy. It was shown that in the case of incomplete removal of oxide films during the preparation of the wafers before fusion and/or the presence of adsorbed water on the wafer surfaces, the fused interface contains a large number of amorphous inclusions, most likely related to the III-group oxides. Optimization of the formation regimes of a buried tunnel junction on the surface of the InP-based heterostructure made it possible to reduce the surface roughness down to 1 nm and to ensure the thickness of the GaAs-InGaAsP fused interface < 5 nm, with no dislocations or other extended defects in the region of the fused heterointerfaces. The 1.55 μm-range VCSELs fabricated from the hybrid heterostructures created by using the developed technology demonstrate efficient lasing under continuous wave pumping over a wide temperature range, which indicates the high optical quality of the fused heterointerfaces in the VCSEL heterostructure.
The impact of transverse optical confinement on the static and spectral characteristics of 1.55 µm vertical-cavity surface-emitting lasers (WF-VCSEL) with a buried tunnel junction (BTJ) n++-InGaAs/р++-InAs/р++-InAlGaAs, implemented using molecular-beam epitaxy and wafer fusion. For lasers with a tunnel junction (TJ) etching depth of ~ 15 nm, it was found that the single-mode lasing occurs up to 8 μm BTJ mesa size due to a relatively weak lateral optical confinement, while the effect of a saturable absorber (SA) appears when the BTJ mesa size is less than 7 µm. Enhancing lateral optical confinement by increasing the BTJ etching depth up to ~ 20 nm leads to suppression of the SA effect at the BTJ mesa size of 5–6 µm, but at the same time limits the maximum single-mode optical power. According to the results of the analysis, an increase in the spectral mismatch between the maximum of the gain spectrum of the active region and the resonance wavelength of the WF-VCSEL up to ~35-50 nm will make it possible to suppress the undesirable SA effect in a wide range of the BTJ mesa sizes maintaining the single-mode lasing.
The results of complex studies of static and dynamic performance of 1550 nm-range VCSELs, which were created by direct bonding (wafer fusion technique) InAlGaAs/InP optical cavity wafers with AlGaAs/GaAs distributed reflector wafers grown by molecular beam epitaxy, are presented. The VCSELs with a buried tunnel junction diameter less than 7 μm demonstrated a single-mode lasing with a side-mode suppression ratio more than 40 dB; however, at diameters less than 5 μm, a sharp increase in the threshold current is observed. It is associated to the appearance of a saturable absorber due to penetration of optical mode into the non-pumped regions of the active region. The maximum single-mode output optical power and the–3 dB modulation bandwidth reached 4.5 mW and 8 GHz, respectively, at 20°C. The maximum data rate at 20ºC under non-return-to-zero on-off keying modulation was 23 Gb/s for a short-reach link based on single-mode fiber SMF-28. As the length of the optical link increased up to 2000 m, the maximum data rate dropped to 18 Gbit/s. The main factors affecting the high-speed operation and data transmission range are defined and discussed, and the further ways to overcome themit are proposed.
The analysis of internal optical loss and internal quantum efficiency in 1.3 μm-range InAlGaAsP/AlGaAs a composite n++-InGaAs/р++-InGaAs/р++-InAlGaAs tunnel junction obtained in the frame of molecular-beam epitaxy and wafer fusion technology. The level of internal optical losses in the lasers under study was varied by depositing a dielectric layer on the surface of the output mirror. It is shown that it is possible in principle to achieve low internal optical loss of less than 0.08% and 0.14% per one pass (round-trip) at temperatures of 20°С and 90°С, respectively.
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