Nitrides Seminar - Burhan Saifaddin, Graduate Student Researcher

5/1/2018 12PM ESB 1001

Deep UV LEDs in the range of 260-280 nm are needed to develop new disinfection and biotechnology applications. UV LEDs are expected to replace Mercury-vapor lamps. However, while low-pressure mercury lamps are ~30% energy efficient, the best reported AlGaN UV LED in the 260-280 nm range is ~6% energy efficient. Moreover, AlGaN-based UV-C LEDs (260-280 nm) remain inefficient compared to InGaN visible LEDs due to UV absorbing p-GaN limiting light extraction, optical polarization, and high threading dislocations. Sapphire, the most popular substrate material for AlGaN LEDs, is transparent and inexpensive but has some disadvantages in epitaxy and device performance. In contrast, SiC has small lattice mismatch with AlN (~1%), similar Wurtzite crystal structure, more chemically stable, and contains no oxygen, which degrades the IQE and compensates holes. Moreover, SiC-based LEDs can be processed to thin-film flip-chip (TFFC) LEDs with a reflective p-contact.

We demonstrate the first thin-film flip-chip (TFFC) ultraviolet LEDs grow on SiC. The TFFC LED power was with 7.8 mW at 95 mA at 278.5 nm grown on AlN/SiC with TDD~1x109 cm-2. The external quantum efficiency (EQE) and power conversion efficiency (PCE) were 1.8% and 0.6%, respectively. The PCE was limited because of the high resistivity of n-AlGaN (Hall electron concentration~ 2x1018 cm-3). The exposed N-Face AlN layer of the TFFC LED is etched in dilute aqueous KOH which generates hexagonal pyramids that significantly improves the light extraction of AlGaN MQWs TE and TM emissions. KOH roughening enhanced the light extraction efficiency (LEE) by 100% and ~180% for UV LEDs with 10 nm p-GaN and 5 nm p-GaN, respectively, without affecting the devices IV characteristics.