Internal strain induced significant enhancement of deep ultraviolet light extraction efficiency for AlGaN multiple quantum wells grown by MOCVD

Abstract

In this work, combined analysis of internal strain effects on optical polarization and internal quantum efficiency (IQE) were conducted for the first time. Deep ultraviolet light extraction efficiency of AlGaN multiple quantum wells (MQWs) have been investigated by means of polarization-dependent photoluminescence (PD-PL) and temperature-dependent photoluminescence (TD-PL). With the increase of compressive internal strain applied to the MQWs by an underlying n-AlGaN layer, the degree of polarization (DOP) of the sample was improved from -0.26 to -0.06 leading to significant enhancement of light extraction efficiency (LEE) as the PL intensity increased by 29.2% even though the internal quantum efficiency declined by 7.7%. The results indicated that proper management of the internal compressive strain in AlGaN MQWs can facilitate the transverse electric (TE) mode and suppress the transverse magnetic (TM) mode which could effectively reduce the total internal reflection (TIR) and absorption. This work threw light upon the promising application of compressively strained MQWs to reduce the wave-guide effect and improve the LEE of deep ultraviolet light emitting diodes (DUV LEDs).

Multiple fields manipulation on nitride material structures in ultraviolet light-emitting diodes

“…Highly compressively strained QWs were realized by using AlN bulk or patterned AlN/sapphire as substrates owing to the differences of thermal expansion coefficients and the coalescence process 99 , 100 . In 2018, Long et al proposed a method for increasing the compressive strain of MQWs by inserting an underlying n -AlGaN layer, thereby enhancing the PL intensity 101 . In 2020, Zhang et al adopted multiple alternation cycles of low- and high-temperature growth to modulate the strain state of the AlN template, and the polarization degree of AlGaN QWs effectively increased from 41.5% to 61.9% 102 .…”

On the Luminescence Properties and Surface Passivation Mechanism of III- and N-Polar Nanopillar Ultraviolet Multiple-Quantum-Well Light Emitting Diodes

“…III-nitride based ultraviolet light-emitting-diodes (UV-LEDs) are useful for many applications including UV curing, photo therapy and UV disinfection, due to the direct bandgap property of wurtzite phase III-nitride crystals [1][2][3][4]. However, the extraction efficiency of UV LEDs is relatively low due to the strong internal light reflection and strong transverse magnetic (TM) polarization of the emitted light [5][6][7]. Challenges in light extraction have encouraged research and development of small length scale devices for new applications, like nanostructured LEDs, for example [8][9][10].…”

Deep UV Laser at 249 nm Based on GaN Quantum Wells

“…As the Al composition steps up into the DUV spectrum, the optical polarization of the AlGaN emission gradually switches from transverse electric (TE) to transverse magnetic (TM) due to the rearrangement of the topmost valence sub-bands of AlGaN . Because TM-polarized light cannot be easily extracted from the surface of mesa-type devices, the light extraction efficiency (LEE) of the AlGaN MQW DUV LEDs is severely compromised . The TM mode is also not as desirable for edge-emitting lasers as the TE mode, because the former suffers higher absorption by the p-type layers and metals due to its wider optical profile along the c -axis …”

“…14 Because TM-polarized light cannot be easily extracted from the surface of mesa-type devices, the light extraction efficiency (LEE) of the AlGaN MQW DUV LEDs is severely compromised. 15 The TM mode is also not as desirable for edge-emitting lasers as the TE mode, because the former suffers higher absorption by the p-type layers and metals due to its wider optical profile along the c-axis. 13 Recently, the AlN/GaN MQWs have emerged as a promising alternative structure for UV emission.…”