UC Santa Barbara

Despite the advantages of growing III-nitrides on semipolar planes, challenges still remain for achieving long visible wavelength emission from InGaN layers. The growth of high indium content InGaN required for long wavelength emission is difficult to achieve. First, high indium content InGaN has a large lattice mismatch with GaN, and the large stress in strained InGaN layers acts as a driving force for relaxation. Second, high indium content InGaN layers require low growth temperatures or fast growth rates, which lead to decreased adatom diffusion and desorption and can result in increased impurity concentrations, a breakdown of surface morphology, and growth errors. Third, subsequent high temperature growth steps have been shown to degrade high indium content InGaN layers.

We report device designs in which an electrically injected blue light-emitting diode (LED) optically pumps quantum wells (QWs) with long wavelength emission. Optically pumping offers several advantages over electrically injecting QWs for long wavelength emission. Optically pumped QWs do not have to be confined within a p-n junction, and carrier transport is not a concern. Thus, thick GaN barriers can be incorporated between multiple InGaN QWs to manage stress. Optically pumping long wavelength emitting QWs also eliminates high temperature steps that degrade high indium content InGaN but are required when growing p-GaN for an LED structure. Additionally, by eliminating electrical injection, the doping profile can instead be engineered to affect the emission wavelength.

A device that monolithically integrates a blue LED and optically pumped QWs for long wavelength emission can be optimized to emit white light. This is an alternative to white light created using blue or violet III-nitride LEDs or laser diodes to pump powdered phosphors that emit yellow or red wavelengths. In addition, white light created by nonpolar or semipolar InGaN QWs with varying bandgaps offers the benefit that the emitted light is optically polarized, compared to the unpolarized emission that results from c-plane LEDs, powdered phosphors, and scattered light. This is of significant interest because polarized light has unique applications in, for example, backlighting liquid-crystal displays.

We present demonstrations of electrically injected and optically pumped III-nitride device designs for polarized white light emission. A first device monolithically incorporated a blue (202 ̅1 ̅ ) LED and yellow optically pumped (202 ̅1) QWs. This device produced polarized white light emission with peaks at 440 nm and 560 nm from the electrically injected and optically pumped QWs, respectively, and an optical polarization ratio of 0.40. A second device monolithically incorporated a blue (202 ̅1 ̅ ) LED and optically pumped (202 ̅1) QWs for long wavelength emission, where the doping profile was intentionally engineered to red-shift the emission of one of the optically pumped QWs by creating a built-in electric field that acted in the same direction as the polarization-induced electric field in the QW. This device produced polarized white light emission with a peaks at 450 nm from the electrically injected QW and at 520 nm and 590 nm from the optically pumped QWs, which were grown in n-i-n and p-i-n structures, respectively. The optical polarization ratio was 0.30. A third device was grown on (202 ̅1) using a tunnel junction to incorporate optically pumped QWs for long wavelength emission above an electrically injected blue LED. Use of NH3 molecular beam epitaxy enabled the growth of the tunnel junction in this device, while use of metalorganic chemical vapor deposition enabled the growth of InGaN with high radiative efficiency. By increasing the ratio of yellow to blue emission, future devices can be used to produce polarized white light. Our initial device produced emission peaks at 450 nm and 560 nm from the electrically injected and optically pumped QWs, respectively. The optical polarization ratio was 0.28. Overall, using electrically injected and optically pumped III-nitrides devices, we have demonstrated the first devices with polarized white light emission.