Development of aluminum gallium nitride-based emitters in the form of graded-index separate confinement heterostructure (GRINSCH)

The development of ultraviolet semiconductor emitters (LEDs and lasers) will enable a large number of industrial and medical applications. AlGaN alloys are ideally suited for the development of such devices since their energy gap can be tuned from the near UV (365 nm) to deep UV (200 nm). However, the doping of such materials n- and p-type is difficult. Another problem is the generally poor light extraction efficiency from both UV and visible LEDs. This research addressed the first problem by developing UV emitters in the form of graded-index-separate-confinement-heterostructure (GRINSCH). In these device the active region is embedded in two compositionally graded wave guiding layers. Due to the polar nature of nitride semiconductors these compositionally graded AlGaN films are doped p- or n-type if the grading changes from high to low concentration or from low to high concentration respectively. Thus, a p-n junction is automatically formed without the incorporation of dopants. The polarization induced doping level in these structures was calculated to be 1018cm-3 for the p- and n-sides. A number of devices, whose active region is either 75 nm Al0.72Ga0.28N bulk film or multiple QWs have been grown on 6H-SiC substrates by Molecular-Beam Epitaxy (MBE) and investigated. The emission properties of these structures were investigated by cathodoluminescence (CL) and by measuring their optical gain. A maximum net modal gain in excess of 80 cm-1 was measured with an optical gain threshold of 14 µJ / cm2. Some of these structures, emitting in the near UV, were also electrically pumped. The second problem was addressed by incorporating dielectric (TiO2) photonic crystals on the phosphor plates of white LEDs in order to increase the light extraction efficiency upon illumination with blue LEDs. The two-dimensional (2D) hexagonal-lattice of TiO2 photonic crystal was formed by e-beam lithography on low-scattering (Y1-xCex)3Al5O12 (YAG:Ce) ceramic phosphor plates. Yellow light extraction enhancement by a factor of 4.4 was achieved with a 2D photonic crystal nano-cylinders having diameter 430 nm, lattice constant of 580 nm and height of 350 nm. Simulations using a three-dimensional finite difference time domain are consistent with our measured data

Exploiting smallest error to calibrate non-linearity in SAR ADCs

This paper presents a statistics-optimised organisation technique to achieve better element matching in Successive Approximation Register (SAR) Analog-to-Digital Converter (ADC) in smart sensor systems. We demonstrate the proposed technique ability to achieve a significant improvement of around 23 dB on Spurious Free Dynamic Range (SFDR) of the ADC than the conventional, testing with a capacitor mismatch σu = 0.2% in a 14 bit SAR ADC system. For the static performance, the max root mean square (rms) value of differential nonlinearity (DNL) reduces from 1.63 to 0.20 LSB and the max rms value of integral nonlinearity (INL) reduces from 2.10 to 0.21 LSB. In addition, it is demonstrated that by applying grouping optimisation and strategy optimisation, the performance boosting on SFDR can be effectively achieved. Such great improvement on the resolution of the ADC only requires an off-line pre-processing digital part

Revealing microstructure and dislocation behavior in BAlN/AlGaN heterostructures

We reveal the microstructure and dislocation behavior in 20-pair B0.14Al0.86N/Al0.70Ga0.30N multiple-stack heterostructures (MSHs) exhibiting an increasing dislocation density along the c-axis, which is attributed to the continuous generation of dislocations (edge and mixed-type) within the individual B0.14Al0.86N layers. At the MSH interfaces, the threading dislocations were accompanied by a string of V-shape pits extending to the surface, leading to interface roughening and the formation of surface columnar features. Strain maps indicated an approximately 1.5% tensile strain and 1% compressive strain in the B0.14Al0.86N and Al0.70Ga0.30N layers, respectively. Twin structures were observed, and the MSH eventually changed from monocrystalline to polycrystalline.Acknowledgments The KAUST authors acknowledge the support of the GCC Research Program REP=1=3189-01-01, Baseline BAS=1=1664-01-01, and Equipment BAS=1=1664-01-07. The work at QU was supported by the GCC Research Program GCC-2017-007. The work at the Georgia Institute of Technology was supported in part by DARPA under Grant No. W911NF-15-1-0026 and NSF under Grant No. DMR-1410874. R.D.D. acknowledges the additional support of the Steve W. Chaddick Endowed Chair in Electro-Optics and Georgia Research Alliance

Investigating various metal contacts for p-type delafossite α-CuGaO2 to fabricate ultraviolet photodetector

Abstract Delafossite semiconductors have attracted substantial attention in the field of electro-optics owing to their unique properties and availability of p-type materials that are applicable for solar cells, photocatalysts, photodetectors (PDs) and p-type transparent conductive oxides (TCOs). The CuGaO2 (CGO), as one of the most promising p-type delafossite materials, has appealing electrical and optical properties. In this work, we are able to synthesize CGO with different phases by adopting solid-state reaction route using sputtering followed by heat treatment at different temperatures. By examining the structural properties of CGO thin films, we found that the pure delafossite phase appears at the annealing temperature of 900 °C. While at lower temperatures, delafossite phase can be observed, but along with spinel phase. Furthermore, their structural and physical characterizations indicate an improvement of material-quality at temperatures higher than 600 °C. Thereafter, we fabricated a CGO-based ultraviolet-PD (UV-PD) with a metal–semiconductor-metal (MSM) configuration which exhibits a remarkable performance compared to the other CGO-based UV-PDs and have also investigated the effect of metal contacts on the device performance. We demonstrate that UV-PD with the employment of Cu as the electrical contact shows a Schottky behavior with a responsivity of 29 mA/W with a short response time of 1.8 and 5.9 s for rise and decay times, respectively. In contrast, the UV-PD with Ag electrode has shown an improved responsivity of about 85 mA/W with a slower rise/decay time of 12.2/12.8 s. Our work sheds light on the development of p-type delafossite semiconductor for possible optoelectronics application of the future

Single peak deep ultraviolet emission and high internal quantum efficiency in AlGaN quantum wells grown on large miscut sapphire substrates

Single peak deep ultraviolet emission and high internal quantum efficiency in AlGaN quantum wells grown on large miscut sapphire substrate

Structural properties, crystal quality and growth modes of MOCVD-grown AlN with TMAl pretreatment of sapphire substrate

The growth of high quality AlN epitaxial films relies on precise control of the initial growth stages. In this work, we examined the influence of the trimethylaluminum (TMAl) pretreatment of sapphire substrates on the structural properties, crystal quality and growth modes of heteroepitaxial AlN films on (0 0 0 1) sapphire substrates. Without the pretreatment, the AlN films nucleated on the smooth surface but exhibited mixed crystallographic Al- (N-) polarity, resulting in rough AlN film surfaces. With increasing the pretreatment time from 1 to 5 s, the N-polarity started to be impeded. However, small islands were formed on sapphire surface due to the decompostion of TMAl. As a result, small voids became noticeable at the nucleation layer (NL) because the growth started as quasi three-dimensional (3D) but transformed to 2D mode as the film grew thicker and got coalesced, leading to smoother and Al-polar films. On the other hand, longer pretreatment time of 40 s formed large 3D islands on sapphire, and thus initiated a 3D-growth mode of the AlN film, generating Al-polar AlN nanocolumns with different facets, which resulted into rougher film surfaces. The epitaxial growth modes and their correlation with the AlN film crystal quality under different TMAl pretreatments are also discussed.The KAUST authors would like to acknowledge the support of GCC Research Program REP/1/3189-01-01, Baseline BAS/1/1664-01-01, and Equipment BAS/1/1664-01-07. The work at QU was supported by GCC Research Program GCC-2017-007. The work at Georgia Institute of Technology was supported in part by DARPA under grant W911NF-15-1-0026 and NSF under grant DMR-1410874. RDD acknowledges the additional support of the Steve W Chaddick Endowed Chair in Electro-Optics and Georgia Research Alliance.Scopu