Vertical cavity surface emitting lasers
Vertical cavity surface emitting laser (VCSEL) structures have been grown by both metal-organic chemical vapour deposition (MOCVD) and molecular beam epitaxy (MBE). These incorporate 3 strained InGaAs / GaAs quantum wells placed resonantly in a two wavelength long optical cavity, formed between AlAs / GaAs quarter wave dielectric reflector stacks through which current is injected. The reflection spectra of these stacks is studied in detail; the effects on the laser threshold gain of absorption due to impurities and of errors in growth are investigated. Methods of disruption of the AlAs / GaAs heterointerfaces have been used to reduce the operating voltage. The completed designs use 200A intermediate layers containing 30 or 50% aluminium or a superlattice graded region simpler than that used in previous designs. The effectiveness acceptor dopants; Be in MBE, C and Zn in MOCVD; is studied also. Modulation doping was employed to reduce the effects of optical absorption. Devices were fabricated into mesas by SiC14 reactive ion etching or defined by proton implant isolation. MBE grown devices were resonant at wavelengths in the range 950 to 1059mn with essentially constant (at —1020nm) eihhi transition energies in the wells. A detailed study of the wavelength variation of threshold current density Jth (X)was made. A minimum of 366A.cnr2 was measured at 1018nm in mesa devices. A similar relation is found for ion-implanted devices but the minimum is increased to 535A.cm-2 by incomplete isolation. Gain calculations, including strain effects, are used to explain the Jth(X) variation. Implanted devices offer superior c.w. performance due to reduced thermal and ohmic resistances. The relative offset between the gain spectrum and cavity resonance was examined for c.w. operation. It was found that carrier thermal effects limit the output power rather than shifts in the offset. The bias voltage of MOCVD grown devices is as low as 1.7V and the threshold current is as low as 764A.cm-2. This is higher than for MBE grown devices because of growth thickness errors and non-optimal alignment of the gain spectrum and cavity mode. The uniformity in emission wavelength is ±1% over 80% of a 2 inch diameter wafer, offering suitability for very large uniform arrays.