# Long-Wavelength VCSELs: Status and Prospects
## 7 VCSELs Fabricated by Wafer Fusion
One needs to increase the Al content in such QWs up to approximately 18%, which results in the growth of nonradiative defects. The usage of highly tensile-strained InGaAs QWs has been reported in the literature [156; 157]. However, no results have been reported for making 1300 nm WF VCSELs [156]. As an alternative to InAlGaAs QWs used in the active region of WF VCSELs grown by MOVPE [66; 158], we proposed using InGaAs/InAlGaAs superlattices (SLs) [159] for more flexible control of modal gain and transparency of the current density of the active region [10; 67; 160]. When MOVPE is used, it is difficult to realize very sharp heterointerfaces [75]. MBE instead enables atomic-level control of the growth process [3; 30]. We developed MBE growth for 1300-1550 nm range WF VCSELs. In particular, the problem of TJ regrowth was solved using an industrial MBE system based on the original composite n\({}^{++}\)/p\({}^{++}\) In(Al)GaAs TJ. As mentioned above (cf. Section 3), to realize the single-mode lasing of BTJ VSCELs, the topology of the overgrown surface relief as well as the etching depth of TJ can be varied. As result, the optical confinement can be easily estimated within the framework of the effective refractive index [22], using the design height difference of the relief \(\Delta L\). Unlike the MOVPE during MBE overgrowth, it does not show even partial planarization of the surface relief (i.e., the height difference in the overgrown surface morphology is equal to the etching depth in the TJ layers). Etching depths, \(\Delta L\), of less than 15 nm allow [10; 161] to realize weak index-guiding for higher-order transverse modes. As a result, single-mode lasing at large BTJ diameters (up to 8 \(\upmu\)m) for 1300 nm VCSELs is demonstrated [10]. At the same time, the formation of a saturable absorber in the unpumped peripheral parts of the active region at small BTJ sizes (\(<\)7 \(\upmu\)m) is observed, and an abrupt increase in the output power with an increase in the driving current is observed. An increase of \(\Delta L\) to 30 nm results in single-mode emission at an ultra-small BTJ diameter (less than 4 \(\upmu\)m) due to the strong index-guiding [10]. The compromise is based on using an etching depth of about 20-25 nm. As result, single-mode lasing for lasers with BTJ diameters in the range of 4-8 \(\upmu\)m at 20 \({}^{\circ}\)C is found. More than 30 dB side mode suppression ratio (SMSR) is demonstrated for all device sizes. Devices with BTJ diameters less than or equal to 5 \(\upmu\)m show single-mode lasing with more than 40 dB SMSR across the entire operating range of driving currents [162]. According to the modeling, even at a relatively small \(\Delta L\) value, a strong waveguide effect is realized. 6 mW CW single-mode output power was demonstrated for 5 \(\upmu\)m BTJ size devices (cf. Figure 2 [10]). To analyze this result, we estimated the n\({}_{\text{eff}}\) value based on the effective refractive index model [163]. Despite the location of the TJ layers at the node of the electromagnetic field, n\({}_{\text{eff}}\) values exceed 0.025 (for the case of \(\Delta L\)\(\sim\)25 nm at MBE regrowth of TJ) [164]. The estimated n\({}_{\text{eff}}\) value is noticeably larger than that observed for near-infrared oxide-confined (In)AlGaAs/GaAs VCSELs [43]. It should be noted that in hybrid VCSELs (for \(\Delta L\)\(\sim\) 20 nm), the n\({}_{\text{eff}}\) value can reach 0.03 [165], which limits the maximum size of the BTJ mesa to a level of 4-5 \(\upmu\)m, where stable lasing via the fundamental mode can be realized [23]. At the same time, for MOVPE-grown WF-VCSELs (for \(\Delta L\)\(\sim\) 30 nm), single-mode lasing can be obtained for BTJ diameters up to 7 \(\upmu\)m [166], which indicates a decrease in the height difference in the morphology of the overgrown surface. The differential gain of our SL-based active regions is three times larger than for the 1300-nm range InAlGaAs QW-based WF VCSELs grown by MOVPE [152].