Improved Vertical Carrier Transport for Green III-Nitride LEDs Using (In,Ga)N Alloy Quantum Barriers

Abstract

We report on experimental and simulation-based results using $(\mathrm{In},\mathrm{Ga})\mathrm{N}$ alloy quantum barriers in c-plane green light-emitting diode (LED) structures as a means to improve vertical carrier transport and reduce forward voltage $({V}_{F})$. Three-dimensional device simulations that include random alloy fluctuations are used to understand carrier behavior in a disordered potential. The simulated current density--voltage (J-V) characteristics and modified electron-hole overlap $|{F}_{\mathrm{mod}}{|}^{2}$ indicate that increasing the indium fraction in the $(\mathrm{In},\mathrm{Ga})\mathrm{N}$ quantum barriers leads to a reduced polarization discontinuity at the interface between the quantum barrier and quantum well, thereby reducing ${V}_{F}$ and improving $|{F}_{\mathrm{mod}}{|}^{2}$. Maps of electron and hole current through the device show a relatively homogenous distribution in the $XY$ plane for structures using $\mathrm{Ga}\mathrm{N}$ quantum barriers; in contrast, preferential pathways for vertical transport are identified in structures with $(\mathrm{In},\mathrm{Ga})\mathrm{N}$ barriers as regions of high and low current. A positive correlation between hole (electron) current in the p-side (n-side) barrier and indium fraction reveals that preferential pathways exist in regions of high indium content. Furthermore, a negative correlation between the strain ${\ensuremath{\epsilon}}_{zz}$ and indium fraction shows that high indium content regions have reduced strain-induced piezoelectric polarization in the Z direction due to the mechanical constraint of the surrounding lower indium content regions. Experimentally, multiple quantum well green LEDs with $(\mathrm{In},\mathrm{Ga})\mathrm{N}$ quantum barriers exhibit lower ${V}_{F}$ and blue-shifted wavelengths relative to LEDs with $\mathrm{Ga}\mathrm{N}$ quantum barriers, consistent with simulation data. These results can be used to inform heterostructure design of low ${V}_{F}$, long-wavelength LEDs and provide important insight into the nature of carrier transport in III-nitride alloy materials.