High conductivity transparent <i>n</i>-InAlAs contact layers by InGaAs inserts

We propose a novel design of improved conductivity transparent n-contact layers for use in InAlAs/InGaAs high speed and/or high-power photodiodes for the 1.3 and 1.55 μm telecom wavelengths grown on InP by all-arsenic epitaxy. The compound contact layers consist of an InAlAs matrix with thin inserts of InGaAs, which act as high conductivity channels, doped uniformly throughout. We show that the conductivity in the compound layers of the 25 nm InGaAs/100 nm InAlAs structure is increased by as much as five times over that of n-InAlAs and reaches 1600 Sm/cm, accompanied by an improvement in the layer surface morphology that extends the useful range of doping levels. Due to the Moss–Burstein shift of the absorption edge, the strongly doped compound layers are transparent at least to a wavelength of 1.2 μm with a total of only 100 nm of InGaAs used in a 500 nm thick compound layer. Test diodes manufactured using the compound layer n-contacts demonstrate diode series resistances of about 5 Ω, which shows the usefulness of this design. The enhanced conductivity n-contact layers may also find application in other devices grown by all-arsenic epitaxy.