Frank Mehnke, of the Institute of Solid State Physics at Technische Universität Berlin, will present the following seminar:

“Exploring the wavelength limits of AlGaN-based deep UV LEDs”

Date and time: Friday, June 14 from 2-3 pm

Location: Van Leer Building, Room C240

Abstract:

Applications such as gas sensing (NO: λ = 226 nm, NH3: λ = 217 nm) would benefit from the development of light emitting diodes (LEDs) in the deep ultraviolet (UV) spectral region below 230 nm. However, the fabrication of such short wavelength LEDs is very demanding as a strong decrease in emission power and external quantum efficiency (EQE) is observed with decreasing emission wavelength. This behavior is typically ascribed to a decrease in radiative recombination efficiency (RRE), a decrease in carrier injection efficiency (CIE), as well as a decrease in light extraction efficiency (LEE). Additionally, the wall plug efficiency is further hampered due to high operation voltages of such devices. 

In this talk, we will discuss systematic variations of the heterostructure design in order to achieve short emission wavelength LEDs. Multiple quantum well (MQW) active regions as well as full LED heterostructures have been grown by metalorganic vapor phase epitaxy on (0001) oriented ELO AlN/sapphire. The LEDs were fabricated using Pd-based p-contacts and V/Al-based n-contacts. In order to maximize the spectral power needed for applications, a high aluminum mole fraction x > 0.9 is needed within the AlxGa1-xN:Si current spreading layer to avoid partial absorption of the MQW emission. However, when increasing the aluminum mole fraction, the sheet resistivity and the voltage drop at the n-electrodes increase drastically around x = 0.8. This induces a conductivity-transparency dilemma which will be analyzed. 

Additionally, the influence of the emission wavelength on the electro-optical properties of LEDs emitting between 263 nm and 217 nm has been investigated on fully transparent AlxGa1-xN:Si current spreading layers in order to determine the main contributions causing the observed reduction in EQE. For ray-tracing simulations of the LEE we determined the in-plane degree of polarization of the emitted light (P = (ITE - ITM)/(ITE + ITM)) by polarization resolved photoluminescence and electroluminescence measurements. A reduction from P = +0.9 for emission at 263 nm to P = -0.8 for emission at 220 nm was observed. This is in good agreement with k ∙ p calculations and results in a reduced LEE from 4% to 1.5% in the same wavelength range. Additionally, the RRE was estimated by temperature dependent photoluminescence measurements revealing a reduction from ~16% for emission above 248 nm to ~0.5% for emission below 223 nm. By measuring the LED emission power, a reduction from 0.56 mW to 0.15 μW was observed for emission at 263 nm and 217 nm, respectively. This enables the quantification of the main contributions to the reduced EQE with decreasing emission wavelength as shown in Fig. 1.

Biosketch

Frank Mehnke received the Diploma degree in physics in 2011 and his doctorate in natural sciences in 2017, both from the Technische Universität Berlin, Germany. In 2014, he visited Linköping University, Sweden, for a short-term research stay investigating the electronic properties of the silicon donor in high aluminum mole fraction AlGaN. Until May 2019, he worked as postdoctoral researcher at the Institute of Solid State Physics, Technische Universität Berlin. His research interests include the growth and electro-optical and structural characterization of wide-bandgap III-nitride semiconductors. In particular, he focused on the development of extremely short emission wavelength ultraviolet light emitting diodes.