Semi-polar GaN Materials
First generation GaN materials are subject to large built-in electric fields due to spontaneous and piezoelectric polarization. Devices can only operate at low current densities due to the rapid decrease of efficiency (efficiency 'droop') as the current density increases. As a result, total brightness per given chip area is thus low, hindering LED usage in many high power applications. In addition, long wavelength devices are still absent due to significantly low performance between 520 and 635 nm (green gap), limiting display and illumination applications for the highest spectral sensitivity of the human visual perception of brightness.
Semi-polar gallium nitride (GaN) materials can address these long-standing problems with lower polarization-induced electric fields and reduced blue shift. The reduction of efficient droop can be achieved by increasing the volume of the InGaN active region. Quantum Confined Stark effect (QCSE) is eliminated by reducing polarization fields. Semi-polar materials have also been used to enable high efficiency long wavelength devices such as direct green lasers.
Traditionally, semi-polar materials are produced by the off-axis slicing of bulk form HVPE-grown GaN substrates that are expensive and physically incompatible with mass production. To overcome this obstacle, Saphlux uses an innovative orientation controlled epitaxy (OCE) method to selectively grow semi-polar GaN materials directly on standard sapphire wafers. The C-plane GaN is grown obliquely from the sidewall of trench-etched substrates to produce stacking-fault free semi-polar GaN upon coalescence that can be mass-manufactured.
With Saphlux' recent success in mass-producing such materials, the ‘end of Moore’s law’ could be in sight for conventional c-plane GaN device.