Suzhou Institute of Nano-Tech and Nano-Bionics (SINANO) of Chinese Academy of Sciences has improved green light internal quantum efficiency (IQE) from indium gallium nitride (InGaN) quantum wells (QWs) by increasing the miscut angle of the gallium nitride template to encourage step-flow growth. This study was published in Applied Physics Letters. 

The researchers claimed that step-flow QWs have not been achieved up to now for green light-emitting InGaN. Instead, green InGaN QWs tend to be formed from two-dimensional (2D) island growth. Partly motivated by the desire for improved green light performance in laser diodes (LDs) for pico-projectors and displays, they included the fabrication of laser diodes with reduced threshold current in this work.

High-quality step-flow growth for 27%-indium-content green QWs was encouraged by increasing the miscut angle of the GaN/sapphire template from 0.20° to 0.48°. This reduced the step terrace width from~80nm to~30nm, respectively, and decreased the distance that adatoms had to diffuse on the surface before being incorporated into the step growth front.  

The lower growth temperatures needed for high-indium-content InGaN reduced the diffusion distance of adatoms. Low-temperature growth with insufficient miscut led to two-dimensional island or even three-dimensional growth, which can be indicated through increased surface roughness.

Green InGaN 4-period multiple QWs (MQWs) had x-ray diffraction (0002) peak full widths at half maximum (FWHMs) of 351arcsec and 381arcsec for 0.48° and 0.20° miscuts, respectively. The narrower peak suggested sharper well/barrier interfaces from the higher crystal quality achieved through step-flow growth.

Room-temperature photoluminescence (PL) peaks were observed at 2.313eV and 2.309eV for 2D island and step-flow MQWs, respectively. It is noted that the FWHMs of green MQWs with step-flow morphology are smaller than that of green MQWs with 2D island morphology at all temperatures, being 131 meV and 166 meV at room temperature, respectively.

"The smaller FWHM value indicates better potential uniformity, which is beneficial for the improvement of the peak gain of green laser diodes and is believed to result partially from the sharper well/barrier interface, consistent with the XRD result. The indium composition homogeneity may also have improved as a result of the step-flow morphology, but this requires further study,” the researchers said.

Comparing the PL at 10K and room temperature, the internal quantum efficiencies (IQEs) were estimated at 30% and 15% for the step-flow and 2D island samples, respectively. The researchers attributed the IQE improvement for step-flow growth to “better material quality and fewer non-radiative recombination centers in the step-flow MQW sample”.

Temperature-dependent PL analysis suggested that a kind of non-radiative recombination centers with an activation energy of 18.8 meV could be eliminated, resulting into significant increase of IQE.

Laser diodes fabricated from these materials demonstrated the benefit of step-flow growth with reduced threshold current at 300mA, compared with 500mA for the 2D island laser diodes. The researchers attribute this reduction to the improved IQE for step-flow material.

Figure: Schematic of 2D island InGaN QW growth on miscut template with large terraces. (Image by SINANO)