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.