Xiaochan Li

High-efficiency vertical-structure GaN-based light-emitting diodes on Si substrates

High-quality GaN-based light-emitting diode (LED) wafers have been grown on Si substrates by metal–organic chemical vapor deposition by designing epitaxial structures with AlN/Al0.24Ga0.76N buffer layers and a three-dimensional (3D) GaN layer. The AlN/Al0.24Ga0.76N buffer layers are directly grown on Si substrates to provide the large compressive stress to balance the tensile stress introduced during the cooling process, and the 3D GaN layer is grown on the Al0.24Ga0.76N buffer layer to reduce the dislocation density of GaN epitaxial films. The as-grown GaN-based LED wafers exhibit very high crystalline quality with full-width at half-maximums for GaN(0002) and GaN(10−12) of 300 and 345 arcsec, respectively, and an internal quantum efficiency of ∼80.1%. Afterwards, the LED wafers are fabricated into vertical-structure LED chips with a size of 1 × 1 mm2 by the standard process. The as-prepared vertical-structure LED chips exhibited a light output power of 569 mW with a working voltage and a wall-plug efficiency of 2.82 V and 57.6%, respectively, at a current of 350 mA. These high-efficiency vertical-structure LED chips are expected to find a wide range of applications in solid-state lighting fields.

High-performance nonpolar a-plane GaN-based metal–semiconductor–metal UV photo-detectors fabricated on LaAlO3 substrates

High-performance nonpolar a-plane GaN-based metal–semiconductor–metal (MSM) ultraviolet (UV) photo-detectors are fabricated based on high-quality nonpolar a-plane GaN epitaxial films grown on LaAlO3(100) substrates. By effectively controlling the interfacial reactions between GaN and LaAlO3(100) through low temperature growth by pulsed laser deposition, together with systematically studying the formation mechanism of GaN/LaAlO3 hetero-interfaces by first-principles calculations, high-quality nonpolar a-plane GaN epitaxial films with no interfacial layer are obtained. The as-grown ∼300 nm-thick nonpolar a-plane GaN epitaxial films grown at a low temperature of 450 °C show high crystalline quality with the full-width at half-maximum values of 0.21° and 0.41° for GaN(11−20) and GaN(10−11) X-ray rocking curves, respectively; and a very smooth surface with a root-mean-square surface roughness of 1.2 nm. These high-quality nonpolar a-plane GaN epitaxial films are then fabricated into MSM UV photo-detectors, which reveal a high responsivity of 1.35 A W−1 with a low dark current of 8.2 nA @ 5 V. These are the best values for nonpolar GaN-based MSM UV photo-detectors ever reported. These high-performance nonpolar a-plane GaN-based MSM UV photo-detectors have immense potential for application in UV warning and curing systems and light therapy devices.

Defect-related anisotropic surface micro-structures of nonpolar a-plane GaN epitaxial films

The anisotropic surface etching behavior of nonpolar a-plane GaN (110) epitaxial films, grown by pulsed laser deposition, was investigated experimentally by wet chemical etching. Crystal-orientation dependent face state, induced by anisotropic growth kinetics, is the origination of the anisotropic properties of the a-plane GaN epitaxial films. Defects that propagate into the surface offer initial positions for the etching process. A joint effect of two factors determines the etching-exposed surface morphology, primarily including triangular prisms and pits, thus making wet chemical etching a promising tool for the investigation of defect distribution. Type I1 basal stacking faults and partial dislocations are proven to have a direct connection with etching-exposed triangular prisms and pits, respectively. This study presents a mechanism research from the standpoint of the evolution process of the surface morphology during the etching process and brings insight for further understanding of the anisotropic properties of the nonpolar GaN epitaxial films for the realization of a high polarization light-emission device, which has a broad application in display and backlighting.

Nucleation layer design for growth of a high-quality AlN epitaxial film on a Si(111) substrate

A high-quality AlN epitaxial film has been grown on a Si(111) substrate by metal–organic chemical vapor deposition through designing the AlN nucleation layer. The structure of a low temperature nucleation layer hinders the formation of amorphous SiNx in the AlN/Si heterointerface, reduces the dislocation density, and thereby improves the crystalline quality of the AlN epitaxial film. The influence of nucleation layer temperature on the surface morphology and the crystalline quality of the AlN epitaxial film is also revealed in detail. The AlN epitaxial film with the optimized nucleation layer grown at a temperature of 800 °C shows a sharp AlN/Si heterointerface, with an XRD full-width at half-maximum for AlN(0002) of 0.30°, and a very smooth surface with a root-mean-square surface roughness of 1.9 nm. Meanwhile, the 200 nm-thick AlN epitaxial film is almost fully relaxed with an in-plane tensile strain of only 0.42%. This work provides an effective approach for the growth of high-quality AlN epitaxial films in the application of AlN-based ultraviolet photonic devices and GaN-based optoelectronic devices.

Control of interfacial reactions for the growth of high-quality AlN epitaxial films on Cu(111) substrates

High-quality AlN epitaxial films have been epitaxially grown on Cu(111) substrates by pulsed laser deposition (PLD) through effectively controlling the interfacial reactions between AlN epitaxial films and Cu substrates. The interfacial properties of the as-grown AlN/Cu hetero-interfaces and their formation mechanisms have been systemically studied. A 2.1 nm-thick CuxAl1−xN interfacial layer is formed in AlN/Cu hetero-structures at a high growth temperature of 600 °C, while abrupt and sharp AlN/Cu hetero-interfaces with no interfacial layer are achieved by effectively controlling the interfacial reactions at a low growth temperature of 450 °C. The as-grown ∼300 nm-thick AlN epitaxial films grown at 450 °C show very smooth surfaces with a root-mean-square surface roughness of 1.2 nm and high crystalline quality with full-width at half-maximum values of X-ray rocking curves for AlN(0002) and AlN(10−12) of 0.7° and 0.8°, respectively. Meanwhile, the residual stress in the as-grown AlN epitaxial films is also well controlled through low temperature growth with a residual compressive stress of 0.30 GPa. These high-quality AlN epitaxial films are of paramount importance for the commercial development of high-performance AlN-based optoelectronic devices.

High-performance vertical GaN-based near-ultraviolet light-emitting diodes on Si substrates

High-performance vertical GaN-based near-ultraviolet (UV) light-emitting diodes (LEDs) on Si substrates with an electroluminescence emission wavelength of 395 nm have been fabricated by designing epitaxial structures to reduce the dislocation density and enhance the electron confinement and hole injection. By designing the epitaxial structures with a continuously Al-composition-graded AlGaN interlayer between an Al0.30Ga0.70N layer and an Al0.15Ga0.85N layer, the dislocation density in epitaxial films has been greatly reduced, and high-quality GaN epitaxial films grown on Si substrates with full-widths at half-maximum for GaN(0002) and GaN(10−12) X-ray rocking curves of 260 and 280 arcsec, respectively, have been obtained. Furthermore, by applying an electron blocking layer with 8 periods of AlInGaN/GaN superlattices, both electron confinement and hole injection have been enhanced accordingly. High-performance vertical GaN-based 395 nm UV LED chips show a high light output power of 535 mW and a low forward voltage of 3.10 V at a current of 350 mA, corresponding to a high wall-plug efficiency of 49.3%, which are the best values for GaN-based 395 nm UV LEDs ever reported. These high-performance near-UV LED chips find application in medical curing, lighting, etc.

Influence of Oxidation Temperature and Exposure Time on Surface and Tribological Properties of Ti-6Al-4V Alloy After Surface Rolling

Surface modification layers (oxide layer and oxygen diffusion region) were manufactured on Ti-6Al-4V alloy substrate by combining multi-pass surface rolling with thermal oxidation. Firstly, multi-pass surface rolling at the load of 750xa0N was taken to improve the microstructure and properties of material surface layer. Then thermal oxidation was conducted at the temperature of 700, 750, 800, and 850xa0°C for 1, 1.5, and 2xa0h duration, respectively. Some microscopic analysis methods (SEM, EDS and XRD) were used to characterize the surface-modified layer of the samples. The microstructure, surface morphology and microhardness dispersion region of surface-modified layers provided by the different conditions treated samples were compared. Therefore, the effects of the exposure oxidation temperature and time on the microstructure and properties of surface-modified layers were studied on the basis of multi-pass surface rolling. The friction and wear properties of the different conditions treated samples were analyzed in dry friction conditions. It could be found that the properties and wear resistance of surface-modified layer is gradually increased with the increase of oxidation temperature and time. The predominant wear mechanism is the adhesive and abrasive wear under dry friction conditions, as well as the delamination of oxide film and tribolayer.