The Impact of AlN Templates on Strain Relaxation Mechanisms during the MOVPE Growth of UVB‐LED Structures

Abstract

Strain relaxation mechanisms in AlGaN based light emitting diodes emitting in the ultraviolet B spectral range (UVB‐LEDs) grown on different AlN/sapphire templates are analyzed by combining in situ reflectivity and curvature data with transmission electron microscopy. In particular, the impact of dislocation density, surface morphology, and lattice constant of the AlN/sapphire templates is studied. For nonannealed AlN/templates with threading dislocation densities (TDDs) of 4 × 109 and 3 × 109 cm−2 and different surface morphologies strain relaxation takes place mostly by conventional ways, such as inclination of threading dislocation lines and formation of horizontal dislocation bands. In contrast, a TDD reduction down to 1 × 109 cm−2 as well as a reduction of the lattice constant of high temperature annealed AlN template leads to drastic changes in the structure of subsequently grown AlGaN layers, e.g., to transformation to helical dislocations and enhanced surface enlargement by formation of macrofacets. For the growth of strongly compressively strained AlGaN layers for UVB‐LEDs the relaxation mechanism is strongly influenced by the absolute values of TDD and the lattice constant of the AlN templates and is less influenced by their surface morphology.

Radiative Recombination and Carrier Injection Efficiencies in 265 nm Deep Ultraviolet Light‐Emitting Diodes Grown on AlN/Sapphire Templates with Different Defect Densities

“…Upon cooling to RT, the different thermal expansion coefficients of AlN and sapphire result in an increase of ε. [36][37][38] The compressive strain implies a decrease of the a-lattice constant of AlN and thus can be quantified by HR-XRD measurements and a comparison with the a-lattice constant of bulk-AlN (a ¼ 0.3111 nm). [39] For the AlN layer of the ELO template, an a-lattice constant of a ¼ 0.31105 nm is found, leading to ε ¼ À0.02%.…”

“…Especially in case of layers with low TDD, already minor increase of the compressive strain can lead to the formation of new relaxation paths. [36,40] As a consequence, misfit dislocations can be introduced in the AlN and AlGaN layers by the formation of dislocation half-loops (DHLs). [36,40,41] The DHLs have an in-plane misfit component with one TD aligned in the growth direction on each side, as shown in Figure 5.…”

“…[36,40] As a consequence, misfit dislocations can be introduced in the AlN and AlGaN layers by the formation of dislocation half-loops (DHLs). [36,40,41] The DHLs have an in-plane misfit component with one TD aligned in the growth direction on each side, as shown in Figure 5. By interacting with each other, the threading components of the individual halfloops can annihilate and form DHLs of an irregular shape.…”

Toward High-Performance AlGaN-Based UV-B LEDs: Engineering of the Strain Relaxation Process

“…The reported maximum light output power (LOP) of 20 × 20 mil 2 UV-B chips at 304 nm was 57.2 mW under an operating current of 800 mA, with 17% degradation after 1000 h of operation . The major bottlenecks lie in proper strain management and light extraction. − For AlGaN grown on AlN bulk/templates, the huge misfit strain will trigger the strain relaxation inducing: (1) the generation of misfit dislocations (MDs) which enhance nonradiative recombination; (2) severe phase separation and surface roughness causing local current leakage; − (3) increasing point defects contributing to the nonradiative recombination and current leakage. ,, Although the strain relaxation of AlGaN-based LEDs has been reported widely, − the relaxation mechanisms including the critical condition and triggering sequence for high Al-content AlGaN have been less than decisive. Clarifying the relaxation mechanisms and engineering the misfit strain during AlGaN growth is crucial to defining the final structural quality, which is a vital step toward performance improvement of AlGaN-based UV-B LEDs and potential for practical applications.…”

Role of Oxygen Incorporation in High Temperature Annealed AlGaN

“…[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 .…”