TECHNOLOGY VCSELs
the lasing aperture. Longitudinal mode spacing indicates a 4 µm cavity length, which is sufficient to support a number of longitudinal modes. Multiple modes might account for the higher threshold currents in these devices.
Switching from sapphire to a native substrate increased operating lifetime significantly. However, even with this superior foundation, the threshold current still increased after only 10 minutes of operation. Meanwhile, variations in cavity length are higher than those for the VCSEL formed on sapphire, because lapping and polishing is required to remove native GaN.
Researchers at Nichia and NCTU have continued to refine their GaN VCSELs over the last few years, and other groups have also contributed to the development of this device. Advances at NCTU led to the report of room-temperature, continuous-wave lasing in summer 2010, using a hybrid DBR design on a sapphire substrate. Improvements in performance resulted from: a trimming of ITO thickness to 30 nm to reduce optical loss; the addition of an AlGaN electron- blocking layer to quash carrier overflow; and an improved p-side contact, thanks to the introduction of a 2-nm-thick p+
-InGaN layer between p-GaN and ITO.
Using a similar fabrication process, device geometry and epitaxial bottom DBR as before, but moving to a ten- period InGaN (2.5 nm)/GaN (12.5 nm) quantum well/barrier stack and increasing the pairs of Ta2
O5 and SiO2
layers from eight to ten, helped improve device performance. Emitting at 410 nm, the second-generation VCSEL had a reduced threshold voltage of 6 V and
By switching to non-polar or semi-polar quantum wells, in addition to avoiding or reducing the QCSE, we can ensure that the polarization direction of the lasing mode is always aligned along a given crystal direction. This is not the case for conventional VCSELs, which typically exhibit a random polarization direction.
Figure 4: Standing-wave profile in the cavity region of a typical GaN-based VCSEL, illustrating the alignment of the gain region with a standing wave peak and the lossy ITO with a standing wave null
produced an output power of 37 µW. Threshold current and threshold current density were 9.7 mA and 12.4 kA cm-2
,
and the polarization ratio at this shorter wavelength was only 55 percent.
Researchers at Nichia have also made recent advances, using their fabrication process for violet VCSELs to make blue and green cousins. The blue emitter produced a continuous-wave output of 0.7 mW, while the green equivalent delivered a pulsed output (power not revealed). According to this team, the homogeneity of the lasing spot within the injection aperture is critical to obtaining higher performance. This underscores the importance of effective, lateral- current-spreading layers.
Further contributions to VCSEL development have come from Panasonic Corporation, which has produced violet VCSELs capable of continuous-wave operation using a fabrication process similar to that developed by Nichia. This team, which is currently developing GaN-based VCSEL arrays, has turned to relatively long cavity lengths of typically 6 µm to permit multiple longitudinal modes within the gain spectrum. Merits of this approach include reduced thermal sensitivity and increased uniformity of the elements in the arrays.
Figure 3: Electrically injected GaN-based VCSEL output power vs. time. Closed circles are CW, open circles are pulsed, and squares represent output powers originally reported in arbitrary units (a.u.). Data points are color- coded to the emission wavelength
Non-polar structures Another option for improving VCSEL performance is to turn to non-polar structures, which increase optical gain and lower threshold current density through the elimination of the quantum confined Stark effect (QCSE). A team from the University of California, Santa Barbara, which included myself, pursued this approach. We were the first to demonstrate a GaN-based VCSEL on a nonpolar (m-plane) substrate.
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www.compoundsemiconductor.net January / February 2014
We formed our VCSEL on a free- standing, m-plane GaN substrate. Fabrication involved a band-gap- selective, photo-electrochemical (PEC) undercut etching of an intra-cavity embedded sacrificial layer of InGaN. This enabled bonding and removal of the epitaxial layers from the substrate. Thanks to this approach, we could realise precise cavity length control while employing top and bottom dielectric DBRs and recycling expensive, free- standing GaN substrates. To improve
current spreading, we used a λ/4-layer of ITO that was positioned at a standing- wave null of the cavity to minimise optical loss (see Figure 4).
Violet VCSELs produced in this manner emitted 19 µW and had a threshold current of 70 mA. This high threshold may be due to ITO absorption loss or cracking during the bonding process. Polarization of the lasing mode was observed to be along the a-direction for all devices tested.
The polarization ratio for these devices is close to one (see Figure 5 for an illustration of polarization alignment). Thanks to polarization locking in these non-polar VCSELs, they could someday be used to fabricate large arrays of devices with the same polarization direction. Such emitters may find use in various applications.
Alternative mirrors Further progress of the electrically injected, GaN-based VCSEL was reported in 2012 by a team from EPFL. They formed monolithic devices using highly reflective, defect-free Al0.18
In0.82 N
DBRs that are lattice matched to GaN. A free-standing c-plane GaN substrate provided the foundation for this emitter, which featured a bottom epitaxial DBR and a top dielectric DBR. Such a structure avoids the tricky fabrication steps of the other approaches, which require the removal of the cavity from the substrate or substrate thinning. The price to pay for this is a more challenging epitaxial structure. However, with well- developed epitaxial DBR technology, this
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