Role of Oxygen Incorporation in High Temperature Annealed AlGaN

Abstract

Aluminum gallium nitride (AlGaN) based ultraviolet light emitting diodes (UV LEDs) have a variety of applications in areas of biological sensing/detection, optical data communication, medical treatments, and sterilization. However, it is challenging to increase the efficiency of UV LEDs emitting in the UVB and UVC spectral range. [1] One of the approaches to achieve this goal is to develop AlGaN-based UV LEDs on low threading dislocation density (TDD) AlGaN layers.For UV LEDs, AlGaN layers with an Al mole fraction x Al above 0.5 are required to avoid absorption of the UV light generated in the active region of the LEDs. [2][3][4][5] However, it is challenging to reduce the TDD in AlGaN layers. Without improvements in the growth and fabrication process, relaxed or partially relaxed AlGaN layers with x Al > 0.5 usually exhibit a TDD of more than mid-10 9 cm À2 . [6] This is associated with the lattice mismatch between AlGaN and AlN that becomes larger with increasing Ga content. Moreover, relaxation of the AlGaN when growing the layers above the Matthews-Blakeslee critical thickness can lead to surface roughening [7] and generation of new dislocations. [8] Low dislocation density AlGaN layers with the absence of relaxation can be obtained by pseudomorphic growth of the layers on bulk AlN substrates. Nonetheless, pseudomorphic growth is limited by x Al and layer thickness as reported by. [8] For example, pseudomorphic growth of n-Al 0.6 Ga 0.4 N can be achieved with a thickness of up to 0.5 μm, while for n-Al 0.7 Ga 0.3 N, the thickness is up to 1.0 μm. Dalmau et al. found that a 0.4 μm thick Al 0.65 Ga 0.35 N layer already shows 8% of relaxation. [9] In a recent work, [10] pseudomorphic growth was demonstrated for thicker Al 0.6 Ga 0.4 N layers with thicknesses between 0.95 and 3.5 μm. However, these layers suffered from high compressive stress of 3-4 GPa, which could lead to detrimental relaxation in subsequently grown layers with higher Ga content (x Al < 0.6) and huge wafer bow. The latter results in non-uniform growth of the active region of devices and problems in the LED chip process. In the same report, [10] the authors attempted to relieve the stress in the layers by introducing two types of buffer layers, that is, with continuously graded composition and stepwise changed composition, neither of which was successful. From this result, a large barrier for the nucleation of misfit dislocation segments in Al-rich AlGaN layers was concluded. Hence, AlGaN strain relaxation on AlN is challenging especially when growing on low dislocation density AlN buffer layers.At present, UV-transparent bulk AlN substrates are expensive and their availability is still limited. Therefore, AlGaN layers are widely grown on AlN/sapphire templates. A thick fully strained Al 0.8 Ga 0.2 N layer with a TDD of %5 Â 10 8 cm À2 was obtained by growing the layer on maskless epitaxial lateral overgrowth (ELO) AlN/sapphire template. [6,11] In contrast, the TDD was higher

High‐Temperature Annealing of Si‐Doped AlGaN

“…However, HTA of doped AlGaN leads to a reduction in dislocation density compared to undoped samples with comparable Al mole fractions (Figure 6c). [17] A key issue related to the HTA of AlGaN is to maintain the UV transparency of the material. Based on our previous observation, HTA can lead to a strong absorption band in the UVB spectral range.…”

“…Based on our previous observation, HTA can lead to a strong absorption band in the UVB spectral range. [16,17] Figure 7 shows the transmission measurements for all AlGaN layers before and after HTA. It is worth noting that the measurement setup has a relatively low acceptance angle and thus, stray light losses at the unpolished sapphire backside of the samples occur, leading to a reduction of the measured transparency.…”

“…c) Comparison of TDD for HTA of undoped AlGaN as published in ref. [17] with HTA of Si‐doped AlGaN as described in this work.…”

“…[ 16 ] Recently, we have discovered that oxygen from the surface of a sapphire cover wafer during face‐to‐face HTA can be incorporated into AlGaN with x Al between 0.55 and 0.93. [ 17 ] It was assumed that the oxygen incorporation during HTA introduces vacancies in the AlGaN layer, leading to further TD inclination, hence, TDD reduction, and strain relaxation. Moreover, the oxygen helped to maintain the UV transparency of the HTA‐AlGaN.…”

“…Figure 6. a) Strain relaxation and b) in-plane lattice constant, a of undoped and Si-doped AlGaN layers with different IV/III ratios toward AlN underlayer before and after HTA. c) Comparison of TDD for HTA of undoped AlGaN as published in ref [17]. with HTA of Si-doped AlGaN as described in this work.…”