Properties of InAs Lasers

Abstract

Two different recombination mechanisms can be distinguished in the infrared radiation spectra of InAs. The photoluminescence of n-type InAs corresponds to a direct band-to-band (or weak exciton) transition. The photoluminescence of Zn-doped p-type InAs and the injection luminescence of most Zn-diffused diodes correspond to transitions between conduction-band states (not excluding shallow donors for p-type crystals with a heavy donor compensation) and acceptor states about 17 meV above the valence band. The diode emission, therefore, results from injection of electrons into the p-type side of the junction, although band-to-band radiation has also been obtained in some diodes under certain conditions of current density and magnetic field. Present InAs diode lasers which emit near 3.1 μ (band-to-acceptor line) have external quantum efficiencies of about 12% and threshold current densities of about 300 A·cm−2 at 11°K, and they can be operated continuously with currents of several amperes. With magnetic fields of several kG applied perpendicular to the diode current the threshold has been further reduced and the efficiency increased to as much as 25%. This enhancement of emission is shown to be associated with a decrease of injected carrier diffusion in the magnetic field which produces a more compact active region. The threshold increases nearly exponentially with temperature up to 150°K, the highest temperature for laser action at present. The angular spread of the coherent radiation beam in the plane perpendicular to the junction plane is about 30°, which corresponds to a coherently emitting region of 6 μ at the junction. Angles as small as 7° were measured in the plane of the junction. Coherent emission at 3.0 μ corresponding to a band-to-band transition is obtained from n-type (ND=2×1016 cm−3) InAs crystals at 11°K by exciting them with 0.84-μ emission of a GaAs p-n junction laser. The angular spread of the coherent radiation beam in the plane perpendicular to the optically excited plane is 9° which corresponds to a coherently emitting region of 20 μ, over three times larger than the 6 μ for the diode laser.