High temperature operation of 760 nm vertical-cavity surface-emitting lasers investigated using photomodulated reflectance wafer measurements and temperature-dependent device studies

The wafer of a 760 nm vertical-cavity surface-emitting laser (VCSEL), designed for oxygen sensing up to high temperatures, is investigated using photomodulated reflectance (PR). By varying the angle of incidence, the VCSEL cavity mode (CM) wavelength is tuned through the positions of two excitonic quantum well (QW) transitions. The PR is also measured over a large temperature range to determine when the QW ground-state transition is tuned with the CM. When tuned, the QW/CM PR lineshape becomes anti-symmetric, as predicted by theory. This occurs at 388 K, where the CM and QW wavelengths coincide at 760.7 nm. It is also observed that when tuned, the CM width measured in the reflectance spectrum is maximised. Temperature dependent device studies are also conducted on a 760 nm edge-emitting laser containing a similar active region as the VCSEL. It is found that up to 250 K the device behaves ideally, with the threshold current being entirely due to radiative recombination. However, as the temperature increases, electron leakage into the indirect X-minima of the barrier and cladding layers becomes increasingly significant. At 300 K, approximately 25% of the threshold current is found to be attributed to electron leakage and this increases to 85% at 388 K. The activation energy for this leakage process is determined to be 255±5 meV, indicating that electron escape from the QWs into the X-minima of the barrier and/or cladding layers is chiefly responsible for the device´s poor thermal stability. These results suggest that VCSELs containing this active region are likely to suffer significantly from carrier leakage effects.