Utilizing the wet oxidation process, high reflectivity GaAs/oxide DBRs and 780 nm VCSELs with oxide apertures are fabricated. Low-threshold 780 nm vertical-cavity surface-emitting lasers with $Al_{0.11}Ga_{0.89}As$ four quantum wells are realized using aluminum oxide apertures. Our fabrication process requires only single-step mask alignment. The 3.4-㎛ square aperture laser exhibits low threshold current of 200μA which is more than an order of magnitude smaller than the previous values obtained at 780 nm. Single mode peak output power is 1.1 mW. This 3.4-㎛ laser is found to operate in the fundamental transverse mode over all operating current range. The 7.6-㎛ square laser shows peak output power of 2.7 mW. By observing the threshold current change with electrical pumping pulse width, it is verified experimentally that the VCSEL with oxide aperture feel index guiding effect by low refractive index of the oxide. In addition, 1×8 VCSEL arrays are fabricated and they show uniform threshold current of 5 % deviations and maximum optical output power inside an array. The VCSEL of 0.6-mA threshold current has the $f_3dB$ of 5 GHz.
A measured finesse value of ∼390 is observed in non-absorbing $Al_xO_y$/AlGaAs cavities. The nonabsorbing cavity consisting of a bottom $Al_xO_y$/AlGaAs DBR, an $Al_xO_y$ spacer layer and a top $Al_xO_y$/AlGaAs DBR is prepared by wet oxidation process. The measured resonance linewidth agrees well with that of calculations indicating very small overall losses in the cavity. The wet oxidation process does not seem to degrade the interface of the epitaxial layers significantly. The lower bound of maximum achievable reflectivity from the $Al_xO_y$ DBR is estimated to be ＞ 99.95 %, assuming average interface roughness of about 0.6nm. The maximum achievable finesse of this type of cavity is expected to be larger than that of the all-epitaxial counterpart assuming the same roughness.
Conditions of stable wet oxidation processes of AlAs are established. Good thermal contact of the oxidation wafer is found to be critical for repeatable oxidation. Vertical uniformity between each AlAs layers on a wafer is improved under $N_2$ flow less than 1.0 l/min. Refractive index of 1.5 and thickness contraction ratio of 95 % is found by reflectivity measurement.
