# Long-Wavelength VCSELs: Status and Prospects
## 3 Current Confinement and Tunnel Junctions  
Current confinement in InP-based LW VCSELs can be achieved by selective lateral etching of AlInAs-based TJs (so-called air-gap aperture), which is part of the whole TJ [54]. The use of a TJ allows one to inject holes through an n-doped region, thereby reducing the series resistance and free carrier absorption. Further, the BTJ concept was used for current confinement in InP-based LW VCSELs. The active region is embedded here between n-InP and p-AlInAs for effective carrier confinement. Due to the poor electrical, thermal, and optical properties of AlInAs, it is necessary to reduce their thickness [3]. BTJs allow one to reduce the thickness of AlInAs to one-quarter of the emission wavelength and replace it with low-resistance n-InP, yielding the desired cavity length [3]. BTJ for short-cavity VCSELs was first developed by Prof. [PERSON]'s group, whereas regrown tunnel junctions embedded in wafer-fused VCSELs were developed by Prof. [PERSON]'s group [55]. Below, we use BTJ denotation for both types of TJs. The main idea of BTJs for LW VCSEL design is to use a laterally structured tunnel junction inside a p-type layer to:  
* convert most of the current confinement layers in the p-type region from p- to n-type to get lower electric resistance, reducing self-heating;
* use ohmic contacts from two sides to n-type layers with low contact resistance;
* reduce optical losses in n-type layers used at the former p-side of the laser;
* use intra-cavity contacts with low resistance;
* achieve effective lateral current confinement;
* achieve strong lateral optical confinement [56]. The principal structure and electric circuit of BTJs are presented, e.g., in refs. [56; 57]. To realize SM lasing, the topology of the overgrown surface relief and the etching depth of TJs should be considered. The lateral refractive index contrast is qualitatively determined by the height of overgrown TJ (\(\Delta L\)) [3]: \(\Delta n_{\text{eff}}/n_{\text{eff}}=\Delta L/L\), where \(n_{\text{eff}}\) is the effective refractive index and \(L\) is a microcavity length. The BTJ consists here [3] of highly doped p- and n-type (InGaAl)As layers. The BTJ is located on the p-side of the diode. The side walls of the BTJ are formed by selective etching of the n\({}^{+}\)-InGaAs top layer and parts of the bottom p\({}^{+}\) InAlGaAs layers, which contain only a few percent of aluminum [58]. The BTJ is overgrown by a moderately doped n-InP layer. Therefore, when a reverse bias is applied to the BTJ, the p\({}^{+}\)n\({}^{+}\) junction demonstrates ohmic behavior and extremely low resistance due to tunneling. The BTJ is located in a node of the optical field aimed at eliminating free-carrier absorption in the heavily doped TJ layers. p\({}^{+}\)-InGaAlAs/n\({}^{+}\)-In(Al)GaAs TJ can be grown by molecular-beam epitaxy (MBE), chemical beam epitaxy [26; 59], and metalorganic vapor-phase epitaxy (MOVPE) [56; 60; 61]. Using MBE involves an extra hydrogen cleaning procedure [62; 63] based on the RF source in the chamber [64; 65] to deoxidize the oxidized InGaAs surface before TJ regrowth. Growth of InAlGaAs-based BTJs is difficult in industrial MBE systems. An alternative approach is using composite n\({}^{++}\)-InGaAs/p\({}^{++}\)-InGaAs/p\({}^{++}\)-InAlGaAs TJs [10; 66; 67]. A similar approach to using aluminum-free (n\({}^{++}\)-InGaAs/p\({}^{++}\)-InGaAs) TJs was previously presented in ref. [68]. In both cases, solid-source MBE regrowth, including the surface planarization effect, is absent [3; 10; 26; 59; 61; 69] and shows strong index-guiding results as compared to MOVPE regrowth. Additional attention is devoted to doping optimization of the overgrown InP layer at BTJ formation. As previously mentioned for hybrid VCSELs [27], the reduction of overgrown layer doping down to 1 \(\times\) 10\({}^{17}\) cm\({}^{-3}\) allows one to increase the modulation bandwidth by 35% (from 6.7 to 9 GHz).