(Invited) Insights into the UV-C Optical Absorption of AlN Substrates Grown by PVT

Abstract

High temperature growth of AlN single crystals by physical vapor transport (PVT) was used to produce 2-inch diameter AlN substrates free of macrodefects and with average dislocation densities below 103 cm-2 [1, 2]. In spite of its high structural quality and uniformity, AlN grown by PVT has been characterized by high below-bandgap optical absorption in the ultraviolet C (UV-C) region, due to the presence of several deep-level, impurity-related absorption bands. UV-C light emitting diodes (LEDs) operating in the 260-280 nm range are being actively pursued for applications such as water purification and surface disinfection. These devices typically require light extraction through the substrate, for which high transparency in the emission wavelength range of interest is critical to high device performance. Adoption of AlN substrates for use in UV-C LEDs thus requires low optical absorption coefficients in the 260-280 nm emission range. However, several groups have identified an absorption band around 265 nm, which is associated with the carbon impurity, in these substrates [3]. Reducing this unwanted absorption band is critical to enhancing light extraction in UV-C LEDs grown on AlN substrates. In earlier work, co-doping with silicon was shown to reduce the unwanted 265 nm absorption band [4]. Formation of a carbon-silicon defect complex with an absorption band at 225 nm was demonstrated by optical absorption and photoluminescence measurements together with density functional theory calculations [4]. In this work, we have grown large diameter boules with varying concentrations of impurities in order to demonstrate 2-inch AlN substrates with low 265 nm absorption coefficients. The optical properties of double-side polished, c-plane substrates were measured using a high performance UV/Vis spectrophotometer (PerkinElmer Lambda 850). Transmittance and absolute reflectance spectra were collected using a 150 mm integrating sphere, and absorption coefficients were calculated with an approximation-free method [5] which accurately accounted for all reflectance losses. The absorption coefficients at 265 nm were correlated with impurity concentrations measured by secondary ion mass spectrometry (SIMS) of the three most prevalent electrically-active impurities, carbon, oxygen, and silicon, for various locations throughout the boule volume. Growth and characterization results of UV-C transparent 2-inch AlN substrates grown by PVT will be presented.