Due to their wide band-gaps and excellent chemical & mechanical stability, III-nitride semiconductors play key roles in a wide range of applications, such as for general illumination, 5G mobile communications, visible light wireless communication (Li-Fi), high frequency and high temperature electronics, etc. Very recently there is an increasing demand for developing emitters and photodetectors with III-nitride materials for environmental protection, water purification, medical instrumentation, non-line-of-sight communications, etc. Moreover, the growth of III-nitride devices on industry-compatible silicon substrates exhibit many advantages in terms of wafer costs, scalability, silicon technology compatibility and silicon photonics where the integration of III-nitride emitters/photodetectors and electronics can serve as a platform for the fabrication of photonic and electronic integrated circuits.
Compared with the present III-nitride optoelectronics grown on c-plane substrates, non-polar GaN intrinsically exhibits many major advantages such as zero polarisation effects leading to a higher efficiency and a faster response speed. However, the crucial challenge is due to the crystal quality of non-polar GaN on industrial-compatible substrates which is far from satisfactory. In the last decade our research group at the University of Sheffield has developed a number of cost-effective overgrowth approaches for semi-polar and non-polar GaN on sapphire substrates by using high temperature (HT) AlN buffer technology on regularly arrayed micro-rod templates that lead to a step-change in crystal quality. Due to the advantages mentioned above these approaches are progressed further towards compatibility with silicon substrates. However, Ga melt-back etching and cracking are the main challenges due to a high temperature which GaN growth requires. Furthermore, it is almost impossible to grow non-polar GaN on planar silicon substrates.
In order to address these challenging issues, the project has developed and then has well-established a two-step approach to achieving high crystal quality non-polar ("11" "2" ̅"0" ) GaN on patterned (110) silicon substrates by means of a combination of a HT-AlN buffer and selective MOVPE overgrowth techniques, where gallium melt-back etching has been eliminated and a step-change in crystal quality grown has been achieved. Detailed x-ray diffraction and PL measurements have been performed in order to characterise the non-polar GaN, confirming that both the dislocation density and the density of basal stacking faults (BSFs) have been significantly reduced, in particular, BSFs which have been reduced to an almost invisible level. The detailed optical properties have also been investigated.
On such high crystal quality ("11" "2" ̅"0" ) non-polar GaN with a micro-stripe configuration on the patterned (110) silicon, an InGaN/GaN multiple quantum wells (MQW) structure has been grown, leading to multiple emissions from different facets (polar and non-polar facets, where indium incorporation rates are different leading to a difference in indium content in InGaN). Advanced optical characterisation has been conducted by means of using confocal PL and cathodoluminescence (CL) measurements.
A non-polar GaN metal-semiconductor-metal (MSM) photo-detector (PD) with an ultra-high responsivity and an ultra-fast response speed in the ultraviolet spectral region, which was fabricated on such non-polar ("11" "2" ̅"0" ) GaN stripe arrays with a step-change in crystal quality. The non-polar GaN MSM PD exhibits a responsivity of ~7E2 A/W at 1 V bias and ~1.2E4 A/W at 5 V bias both under 360 nm ultraviolet illumination, which are more than 20 times higher and 4 orders of magnitude higher compared with the current state-of-the-art, respectively. The non-polar GaN MSM-PD displays a rise-time and a fall-time of 66 µs and 43 µs, respectively, which are three orders of magnitude faster compared with the current state-of-the-art.
We have also applied the HT-AlN buffer technique in the growth of GaN electronics on c-plane sapphire in order to explore a new approach toward the intrinsic limits of GaN electronics from the perspective of epitaxial growth. By using a novel two-dimensional growth mode benefiting from the HT-AlN buffer technology, which is different from the classic two-step growth approach, our high-electron-mobility transistors (HEMTs) demonstrate an extremely high breakdown field of 2.5 MV/cm approaching the theoretical limit of GaN (3 MV/cm) and an extremely low off-state buffer leakage of 1 nA/mm at a bias of up to 1000 V. Furthermore, such HEMTs also exhibit an excellent figure-of-merit (Vbr2/Ron,sp) of 5.13 × 108 V2/Ω·cm2.
