Elements of AlGaN-Based Light Emitters

Doctoral thesis, 2013

The III-nitrides have enabled a range of optoelectronic devices and associated applications of great industrial and societal importance. However, the full potential of the III-nitrides remains to be explored. In this thesis, different important elements of AlGaN-based light emitters have been developed to allow for improvements of deepultraviolet (DUV) light emitting diodes (LEDs), blue vertical cavity surface emitting lasers (VCSELs), and near-infrared (NIR) quantum cascade lasers (QCLs). AlGaN is unique among the wide-bandgap semiconductors in that both p- and n-type conductivity can be achieved. However, effective p-type doping remains difficult, in particular for high-Al content AlGaN. Here we report on progress towards lower resistivity Mg-doped Al0.85Ga0.15N, which will benefit the development of DUV-LEDs. A resistivity of 7 kΩ⋅cm was achieved. We also report on the use of transferred double layer metal-free graphene as a transparent contact on p-GaN for uniform current injection, which could benefit the development of surface emitting LEDs and VCSELs emitting in the blue-green. The graphene transparent contact was shown to momentarily sustain a current density of 1 kA/cm2, which is close to the threshold current density of state-of-the-art blue VCSELs. The large conduction band offset of AlN/GaN quantum wells may enable the wavelength range of QCLs to be extend to the NIR, potentially even covering the telecom wavelength of 1550 nm where a QCL could provide e.g. chirp free modulation. A particular challenge for AlGaN-based short wavelength QCLs is the design of a low loss waveguide that also allows for efficient current injection and extraction. We have therefore developed two such waveguide designs, one employing a dielectric cladding and an off-center metal contact in a ridge configuration and a second employing a ridge waveguid with a ZnO upper cladding and an AlN lower cladding for mode confinement, and investigated their performance characteristics. We also show, both experimentally and theoretically, that the temperature dependence of the intersubband transition energy in AlN/GaN QWs designed for short wavelength absorption/emission is very weak (15 µeV/K), which suggests that AlGaN-based telecom QCLs could operate without active temperature control.