# Long-Wavelength VCSELs: Status and Prospects
## 7 VCSELs Fabricated by Wafer Fusion  
A research group from the Royal Institute of Technology in Stockholm, Sweden, proposed liquid wafer fusion in NH\({}_{4}\)OH ammonia hydrate at 2 MPa pressure and 650 \({}^{\circ}\)C temperature for 30 min in an H\({}_{2}\) atmosphere [129]. This approach was used for the fusion of AlGaAs/GaAs top DBRs with InGaAsP/InP active regions and bottom InGaAsP/InP DBRs, resulting in 1550 nm spectral range lasers with carrier injection through doped mirrors and oxide-confined current apertures [130]. However, despite the success of wafer fusion, this approach was not further developed, presumably due to the chosen VCSEL geometry. A research group from the University of California, Santa Barbara, USA, proposed to use dry wafer fusion at 2-3 MPa pressure and 630 \({}^{\circ}\)C temperature for 20 min in an H\({}_{2}\) atmosphere [131]. An array of 10 \(\upmu\)m wide, 150-200 nm deep channels with 200-250 \(\upmu\)m intervals was formed on the InP wafer surface. Surface preparation included two stages, namely, the first treatment with C\({}_{2}\)H\({}_{3}\)Cl\({}_{3}\), acetone, and isopropyl alcohol followed by drying with dry nitrogen, treatment in an oxygen plasma to remove any remains of photoresist (if photolithography was used), and chemical treatment in an NH\({}_{4}\)OH solution to remove oxides. The second stage included treatment with ozone to remove all carbon, followed by repeated chemical treatment with NH\({}_{4}\)OH ammonia solution or HF/HCI:H\({}_{2}\)O solution to remove oxide films. After the fusion process was complete, there was either cooling down to 250 \({}^{\circ}\)C at a speed of 4 \({}^{\circ}\)C/min or first lowering the temperature to 500 \({}^{\circ}\)C, which was followed by cooling at a rate of 10 \({}^{\circ}\)C/min. This approach was used both for the single fusion of AlAs/GaAs top DBRs and InGaAsP/ InP active regions with bottom InGaAsP/InP DBRs to fabricate 1300 nm VCSELs [132] and for the double wafer fusion of InGaAsP/InP active regions with top and bottom AlAs/GaAs DBRs to fabricate 1550 nm spectral range VCSELs, followed by chemical removal of the substrates [133]. In the first case, a top dielectric mirror and carrier injection through the bottom of the n-type doped mirror and the top p-type intra-cavity contact were used, whereas in the second case, emission through the substrates and carrier injection through doped mirrors were used. A research group from the Ecole Polytechnique Federale de Lausanne, Switzerland, was most successful in this area. To create edge-emitting lasers, they first used dry wafer fusion at 0.2 MPa pressure and 650 \({}^{\circ}\)C temperature for 30 min in an H\({}_{2}\) atmosphere [134]. An array of 10 \(\upmu\)m wide, 0.2 \(\upmu\)m deep channels with a 250 \(\upmu\)m interval was formed on the InP wafer surface. Surface preparation included cleaning in organic solvents (e.g., acetone and isopropyl alcohol), followed by chemical treatment in diluted HF/HCI acid [135]. Unlike others, pneumatic pressure was applied here. After the fusion process was complete, cooling at a speed of 5 \({}^{\circ}\)C/min followed. This approach was used for double wafer fusion of InGaAsP/InP active regions with top and bottom AlGaAs/GaAs DBRs, followed by chemical removal of the relevant substrates to fabricate 1550 nm VCSELs with carrier injection through doped mirrors and current confinement based on oxide-confined apertures [136]. Using pneumatic pressure leads to a homogeneous wafer across the entire area, and reduced pressure (under 10\({}^{-5}\) mbar) eliminates the discharge channel system being applied previously [137] to prevent the development of macro- and micropores related to the release of liquid reagent or inert gases as well as volatile elements generated upon heating the fusion area. This technology also reduced the probability of 60\({}^{\circ}\)-dislocation creation on the fusion interface due to mutual misorientation of the bonded wafers [135].