NIMTE And KAUST Team Enhance UV Luminescence
Despite the demand for more compact, environmentally-friendly and reliable semiconductor UV sources (LEDs and lasers), the performance of such devices has remained limited. Conventional UVLED/laser structures face tremendous challenges and new schemes to achieve high efficiency UV emitters through the design of novel structures are needed.
Researchers from the Ningbo Institute of Materials Technology and Engineering (NIMTE) at the Chinese Academy of Sciences led by Wei Guo, and King Abdullah University of Science and Technology (KAUST) led by Haiding Sun, think they have a promising new approach that could pave the way to new functional material systems and next-generation high-efficiency UV light sources.
The work, published in Advanced Functional Materials, shows for the first time that polarity control offers another degree of freedom in tuning structural and optical properties of AlGaN MQWs.
They synthesised AlGaN/GaN multiple-quantum-wells (MQWs) based on a lateral-polarity-structure (LPS), consisting of µm-scale III-polar and N-polar domains side-by-side on a sapphire substrate. They have found that with this structure they can achieve a photoluminescence enhancement as large as six times together with increased internal-quantum-efficiency (IQE) in the N-polar domains compared to that in the III-polar domains.
They showed that the thickness variation and zig-zag shape of the quantum wells in the N-polar domains result in local compositional inhomogeneities while improving the light extraction of the MQWs. Carriers are laterally confined within the sub-micron N-polar domains, which is in good agreement with proposed band diagrams of the LPS.
The proposed novel UV-LED structure consists of both vertical MQWs and lateral LPS, in which case carriers can flow three-dimensionally and recombine more efficiently. This was demonstrated by the spatially-resolved IQE map, that N-polar domains exhibit much higher IQE values than III-polar domains, even though the threading dislocation (TD) densities are similar between two adjacent domains.
Figure a) shows photoluminescence spectra of the LPS MQWs and pure III-polar MQW. The small peak located at 386 nm is from the second harmonic of 193 nm Ar-F laser. Space-resolved PL intensity mapping of the LPS samples with periodicity of 2 μm and 6 μm are shown in b) and c) respectively. The colour bar represents the integrated emission intensity from the MQWs. Top view SEM image of the LPS with 6 µm periodicity is shown in d) and space-resolved relative IQE distributions at the same position as the SEM image are given in e).
In all, the newly design LPS-based MQWs exhibit: enhanced luminescence intensity compared with conventional uniform polar MQWs; higher IQE due to smaller carrier diffusion length and reduced quantum-confined-stark-effect (QCSE) in N-polar domains and at the domain boundaries; and reduced strains and inhibition of thin film cracking due to compensation of compressive strains in N-polar domains and tensile strains in III-polar domains.
'Lateral-Polarity-Structure of AlGaN Quantum Wells: A Promising Approach for Enhancing the Ultraviolet Luminescence' by Wei Guo et al; Advanced Functional Materials, 2018, 1802395