Yunnong Zhu

Performance improvement of GaN-based light-emitting diodes grown on Si(111) substrates by controlling the reactor pressure for the GaN nucleation layer growth

GaN-based light-emitting diodes (LEDs) have been grown on Si(111) substrates with various reactor pressures for the growth of the GaN nucleation layer (NL) using metal-organic chemical vapor deposition. The influence of the reactor pressure on the GaN NLs and the properties of GaN-based LEDs grown on Si(111) substrates is investigated in detail. It is revealed that crack-free GaN films are grown on the Si(111) substrate. As the reactor pressure for GaN NLs increases from 200 to 600 Torr, the full width at half maximum values of the X-ray diffraction rocking curves for the GaN (0002) and (112) planes decrease from 480 to 351 arcsec, and 868 to 445 arcsec, respectively, and as a result the threading dislocation density is greatly reduced, which is confirmed via the cross-sectional transmission electron microscopy measurement. Subsequently, the relationship between bending and annihilation for dislocations, and the modes for GaN NLs are elucidated. The effect of reactor pressure for the GaN NL growth on the mode of the GaN NL is also systematically studied. Furthermore, the light output power of GaN-based LEDs with GaN NLs grown at a reactor pressure of 500 Torr is greatly improved by 73.66% in comparison to that of GaN-based LEDs with GaN NLs grown at a reactor pressure of 200 Torr. This work provides a new approach for achieving highly-efficient GaN-based LEDs on Si(111) substrates.

A new approach to epitaxially grow high-quality GaN films on Si substrates: the combination of MBE and PLD.

High-quality GaN epitaxial films have been grown on Si substrates with Al buffer layer by the combination of molecular beam epitaxy (MBE) and pulsed laser deposition (PLD) technologies. MBE is used to grow Al buffer layer at first, and then PLD is deployed to grow GaN epitaxial films on the Al buffer layer. The surface morphology, crystalline quality, and interfacial property of as-grown GaN epitaxial films on Si substrates are studied systematically. The as-grown ~300 nm-thick GaN epitaxial films grown at 850 °C with ~30 nm-thick Al buffer layer on Si substrates show high crystalline quality with the full-width at half-maximum (FWHM) for GaN(0002) and GaN(102) X-ray rocking curves of 0.45° and 0.61°, respectively; very flat GaN surface with the root-mean-square surface roughness of 2.5 nm; as well as the sharp and abrupt GaN/AlGaN/Al/Si hetero-interfaces. Furthermore, the corresponding growth mechanism of GaN epitaxial films grown on Si substrates with Al buffer layer by the combination of MBE and PLD is hence studied in depth. This work provides a novel and simple approach for the epitaxial growth of high-quality GaN epitaxial films on Si substrates.

Epitaxial growth of GaN films on lattice-matched ScAlMgO4 substrates

High-quality GaN films have been epitaxially grown on ScAlMgO4 (SCAM) (0001) substrates with an in-plane epitaxial relationship of GaN[1−100]//SCAM[1−100] by pulsed laser deposition (PLD). The effect of laser repetition rate on the qualities of GaN epitaxial films is studied in depth. It is found that as the laser repetition rate increases from 10 to 40 Hz, the qualities of as-grown ∼300 nm-thick GaN epitaxial films increase first and then decrease, and show the optimized values at a laser repetition rate of 30 Hz. The ∼300 nm-thick GaN epitaxial films grown with the laser repetition rate of 30 Hz present very high crystalline quality with full-width at half-maximum values for GaN(0002) and GaN(10−12) X-ray rocking curves of 0.18° and 0.40°, and reveal a very smooth surface with a root-mean-square surface roughness of 1.5 nm. The as-grown GaN films also show an in-plane tensile stress of 0.51 GPa. Meanwhile, cross-sectional transmission electron microscopy confirms the presence of sharp and abrupt GaN/SCAM hetero-interfaces.

Effect of Al evaporation temperature on the properties of Al films grown on sapphire substrates by molecular beam epitaxy

High-quality Al films with an in-plane epitaxial relationship of Al[1−10]//sapphire[1−100] have been epitaxially grown on sapphire substrates by molecular beam epitaxy. The as-grown and ∼200 nm thick Al films prepared at an Al evaporation temperature of 1100 °C were highly crystalline, with a full-width at half-maximum of 180 arcseconds, and had a very smooth surface, with a root mean square roughness of 0.6 nm. There was no interfacial layer between the Al and sapphire. Furthermore, the effect of the Al evaporation temperature on the properties of the as-grown ∼200 nm thick Al films has been studied in detail. This work of achieving high-quality Al films is of great importance for the fabrication of high-performance Al-based devices.

Quality-enhanced AlN epitaxial films grown on Al substrates by two-step growth

Quality-enhanced AlN epitaxial films have been grown on Al substrates by pulsed laser deposition with two-step growth by the combination of low-temperature (LT) and high-temperature (HT) growth. The effect of the HT growth temperature on the interfacial property, surface morphology and crystalline quality of the as-grown AlN epitaxial films is studied in detail. It is found that as the HT growth temperature increases from 450 to 650 °C, the AlN/Al hetero-interfaces of ∼300 nm thick AlN epitaxial films remain sharp and clear, and the surface morphology and crystalline quality of ∼300 nm thick AlN epitaxial films are improved gradually. Especially, the ∼300 nm thick AlN epitaxial films grown at a HT growth temperature of 650 °C show sharp and abrupt AlN/Al hetero-interfaces, very smooth surfaces with a root-mean-square surface roughness of 1.1 nm, and high crystalline quality with full-widths at half-maximum for AlN(0002) and AlN(102) X-ray rocking curves of 0.45° and 0.72°, respectively. The quality-enhanced AlN epitaxial films on Al substrates are of paramount importance for the fabrication of highly-efficient AlN-based devices.

The effect of micelles with random pH-sensitive/hydrophobic structure on the workability, hydration process and microstructure of cement paste

In this study, the workability, hydration rate and microstructure of cement paste in the presence of the micelles prepared by two different diblock copolymers with random pH sensitive/hydrophobic structure (poly(polylactide methacrylate-co-tert-butyl methacrylate)-b-poly(poly(ethylene glycol) methyl ether methacrylate) (P(PLAMA-co-tBMA)-b-PPEGMA) and poly(ethylene glycol) methyl ether-b-(poly-lactic acid-co-poly(β-amino esters)) (MPEG-b-(PLA-co-PAE)) copolymers, respectively) were investigated. In fresh cement paste, the micelles were efficiently adsorbed on cement particles; although the average zeta potential of cement particles was not significantly changed in the presence of the micelles due to their low concentrations, with a high water to cement ratio, the adsorbed micelles altered the zeta potential of partial cement particles, leading to a better workability of cement paste. In hardened cement paste, the micelles retarded the hydration rate of C3S at very early hydration age, but accelerated the hydration rate of cement paste at later stages, resulting in a denser cement matrix and increased mechanical property. The most plausible mechanisms are related to the nucleation effect, molecular structure and adsorption performance of the polymeric micelles.