Lasers have comparable or lower lasing thresholds than those on higher cost AlN substrates

III-nitride deep-ultraviolet (DUV) lasers are expected to have numerous applications such as high-density optical storage, non-line-of-sight (NLOS) communication and biochemical identification. However, the state-of-the-art III-nitride DUV lasers have been grown on free-standing AlN substrates that are lower in dislocation density but significantly higher in cost and smaller in size, as compared to sapphire substrates.

Recent work on AlGaN multiple quantum well (MQW) laser grown on sapphire carried by a collaborative team led by Xiao-Hang Li and Russell D. Dupuis at Georgia Institute of Technology, has shown that it is possible to grow deep UV lasers with comparable or lower lasing thresholds on the sapphire substrates versus those on the AlN substrates.

The study, published in Applied Physics Letters, is claimed to be the first demonstration of deep UV lasers grown on the sapphire with lasing wavelength shorter than 250 nm. The lasers in this study also exhibits lower or comparable lasing thresholds compared to the state-of-the-art lasers grown on the AlN substrates at similar wavelengths. The results indicate that it is possible to grow with high quality AlN/sapphire templates;  optimised AlGaN MQW growth condition, effectively creating an active region with high internal quantum efficiency leading to low threshold stimulated emission.

The laser structure was grown on 2-inch diameter c-plane sapphire substrates in a 3x2-inch low-pressure MOCVD reactor with a close-coupled showerhead configuration. The structure first comprises a 3.5µm AlN template layer deposited on the sapphire substrate, followed by an AlGaN graded layer and five periods of AlGaN MQWs grown pseudomorphically on the template layer. 

Subsequent to the growth, the wafer was cleaved into Fabry-Perot laser bars assisted by laser or hand scribing from the back side of the sapphire substrates. The laser bars were later characterised by a 193-nm excimer laser for optical pumping at room temperature. Stimulated emission was observed at wavelengths of 256 nm and 249 nm with thresholds of 61 kW/cm² and 95 kW/cm² at room temperature, respectively.

An important question arising from this work, is whether one can overcome the challenges of high quality p-doping in high-Al AlGaN materials. To date, the shortest wavelength of electrically-driven III-nitride laser is 336 nm. Based on this study, further investigation is necessary to realise electrically-driven III-nitride lasers operating in the deep UV spectral range.

This is a very brief summary of the paper, 'Low-threshold stimulated emission at 249nm and 256nm from AlGaN-based multiple-quantum-well lasers grown on sapphire substrates', from Applied Physics Letters 105, 141106 (2014).

AngelTech Live III: Join us on 12 April 2021!

AngelTech Live III will be broadcast on 12 April 2021, 10am BST, rebroadcast on 14 April (10am CTT) and 16 April (10am PST) and will feature online versions of the market-leading physical events: CS International and PIC International PLUS a brand new Silicon Semiconductor International Track!

Thanks to the great diversity of the semiconductor industry, we are always chasing new markets and developing a range of exciting technologies.

2021 is no different. Over the last few months interest in deep-UV LEDs has rocketed, due to its capability to disinfect and sanitise areas and combat Covid-19. We shall consider a roadmap for this device, along with technologies for boosting its output.

We shall also look at microLEDs, a display with many wonderful attributes, identifying processes for handling the mass transfer of tiny emitters that hold the key to commercialisation of this technology.

We shall also discuss electrification of transportation, underpinned by wide bandgap power electronics and supported by blue lasers that are ideal for processing copper.

Additional areas we will cover include the development of GaN ICs, to improve the reach of power electronics; the great strides that have been made with gallium oxide; and a look at new materials, such as cubic GaN and AlScN.

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