Researchers Improve Crystal Quality of AlGaN

Researchers at Changshu Institute of Technology and Southeast University and in China have improved the crystal quality of a-plane aluminum gallium nitride (AlGaN). The AlGaN was created with Al content of 60% to 68 %. The researchers fabricated the material with metal-organic chemical vapor deposition (MOCVD) on r-plane sapphire [Jianguo Zhao et al, Appl. Phys. Express, vol10, p011002, 2017].

The team’s method used separate pulses of nitrogen and aluminum precursors to avoid parasitic reactions that reduce aluminum incorporation in the crystal structure. High-Al-content AlGaN is used for deep UV LEDs with wavelengths under 300nm which can kill micro-organisms in water and air for disinfection and purification.
Presently the efficiency of DUV LEDs is only a few percent. Charge polarization effects are known to reduce electron-hole overlap and hinder recombination into photons (called the quantum-confined Stark effect).

The researcher carried out the a-plane AlGaN growth at 40Torr low pressure. They tested various AlGaN layer growth processes including continuous (sample A), pulsed-flow (B), and two-way pulsed-flow (C).
The sample B method used pulses in the ammonia flow only, for both the AlN buffer and the AlGaN epilayer. They grew Sample C with pulses of ammonia and TMAl to suppress parasitic gas-phase reactions between the components while using a continuous flow TMGa. The precursor pulses in all cases lasted 6 seconds.

Top-view surface SEM micrographs for samples  A,  B and C with white arrows pointing in c-direction; and cross-sectional SEM micrographs for samples (d)A  and (e)C.

Top-view surface SEM micrographs for samples  A,  B and C with white arrows pointing in c-direction; and cross-sectional SEM micrographs for samples (d)A  and (e)C.

X-ray analysis suggested that the Al content of the AlGaN epilayers was 60%, 66% and 68% for samples A to C, respectively. The researchers said that the higher Al content in samples B and C is likely due to suppression of the parasitic gas-phase reactions between ammonia and TMAl, enhancing Al incorporation. Sample C was found to have much-reduced roughness with large defect-free areas.

The team explained that two-way pulsed-flow growth technology which made the reduced crystal defects possible gives “…more time for the Al adatoms to migrate to their appropriate lattice sites under an Al-rich growth condition.”

Sample C also revealed increased transmittance of light, especially when the wavelength was longer than 400nm. The peak enhancement was 36.4% in the 200-400nm range.