Nanocathodoluminescence reveals the mitigation of the Stark shift in InGaN quantum wells by silicon doping

Abstract

InGaN quantum wells (QWs) show high internal quantum efficiencies over the ultraviolet to green spectrum and in white light emitting diodes (LEDs). However a persistent challenge to the development of higher efficiency devices is the strong polarisation field across the across the QWs along the polar axis. The polarisation induced internal electric fields cause the spatial separation of the electron and hole wavefunctions in the QWs, known as the quantum confined Stark effect (QCSE). It has been proposed that the internal electric field can be suppressed by silicon doping the quantum barriers (QBs) [1]. Moreover, Kim et al . have theoretically shown that the device efficiency may be improved by variations in the silicon dopant concentration through the QWs [2]. To confirm the simulated properties though, it is crucial to resolve the spectral properties of individual QWs. In this study, nano‐cathodoluminescence (nanoCL) reveals for the first time the spectral properties of individual InGaN QWs in high efficiency LEDs and the influence of silicon doping on the emission properties [3]. A silicon doped layer at 5×10 18 cm ‐3 is included immediately prior to the growth of the 1 st QW and the QBs between the QWs are subsequently doped to 1×10 18 cm ‐3 (sample A). Two further multiple QW InGaN/GaN structures were also grown for reference with QB doping levels of 1×10 18 cm ‐3 (sample B) and 1×10 17 cm ‐3 or less (sample C). NanoCL reveals variations in the emission wavelength that directly correlate with individual QWs. With QB doping greater than 1×10 18 cm ‐3 , there is a continuous blue shift in the emission wavelength of each of the subsequently grown QWs. The inclusion of a higher doped layer immediately prior to the growth of the 1 st QW in the LED structure leads to a blue shift unique to the 1 st QW. The experimental variations in the emission wavelengths were reproduced by Schrödinger‐Poisson simulations. The blue shift in emission wavelength through the QWs due to QB doping is found to be caused by screening of the internal electric fields. The reduction in the emission wavelength of the first grown QW due to the higher doped layer is also found to be the result of screening of the internal electric field. The mitigation of the QCSE and consequently stronger overlap of the electron and hole wavefunction, thus should result in an increase in the radiative recombination. NanoCL thus may serve as an experimental approach to study and refine the design of future optoelectronic nanostructures, including the effects from doping and lead to improvements in device efficiency and functionality.