Z.W. Liu

A new system for achieving high-quality nonpolar m-plane GaN-based light-emitting diode wafers

High-quality nonpolar m-plane GaN-based light-emitting diode (LED) wafers have been deposited on LiGaO2(100) substrates by a combination of pulsed laser deposition and molecular beam epitaxy technologies. The high-resolution X-ray diffraction measurement reveals that high-quality crystalline nonpolar m-plane GaN films have been achieved on LiGaO2(100) substrates. Scanning electron microscopy and atomic force microscopy reveal the very flat surface with a surface root-mean-square roughness of 1.3 nm for p-GaN in the nonpolar m-plane GaN-based LED wafer grown on LiGaO2(100) substrates. A strong photoluminescence emission peak is observed at 446 nm with a full width at half maximum (FWHM) of 21.2 nm. Meanwhile, the electroluminescence spectra of nonpolar m-plane GaN-based LEDs on LiGaO2(100) substrates show a very slight blue shift in wavelength and is kept constant in FWHM with the increase of current from 20 to 150 mA. At an injection current of 20 mA, the light output power for this nonpolar LED is 30.1 mW with the forward voltage of 2.8 V in a chip size of 300 × 300 μm2. Furthermore, the nonpolar m-plane GaN-based LED on LiGaO2(100) exhibits the best external extraction efficiency value of 50.8%. These results indicate the good optoelectronic properties of nonpolar LEDs grown on LiGaO2(100) substrates. This achievement of nonpolar m-plane GaN-based LEDs on LiGaO2(100) substrates opens up a new possibility for achieving highly-efficient LED devices.

Epitaxial growth of high quality AlN films on metallic aluminum substrates

AlN (0001) epitaxial films have been grown on Al (111) substrates with an in-plane epitaxial relationship of AlN[110]//Al[10] by pulsed laser deposition. The as-grown AlN films grown at 450 °C exhibited a very smooth and flat surface with a surface root-mean-square roughness less than 1.1 nm. There is no interfacial layer existing between AlN films and Al substrates, indicating an abrupt interface. The as-grown ~302 nm thick AlN films are almost fully relaxed only with an in-plane compressive strain of 0.16%. With the increase in growth temperature, the interfacial layer thickness increases, resulting in the degradation in the crystalline quality of the as-grown AlN films. These AlN films are of great interest for the commercial development of AlN-based devices.

Epitaxial growth of 2 inch diameter homogeneous AlN single-crystalline films by pulsed laser deposition

2?inch diameter homogeneous AlN films are epitaxially grown on sapphire substrates by pulsed laser deposition (PLD). By optimizing laser rastering and PLD growth conditions, the 2?inch diameter single-crystalline AlN films exhibit excellent thickness uniformity with root-mean-square (RMS) inhomogeneity less than 4.5% and very smooth surface with RMS roughness less than 1.53?nm. There is a maximum of 1.5?nm thick interfacial layer, if there is any, existing between the as-grown AlN and the pre-nitrided sapphire substrate, and the as-grown AlN films are almost fully relaxed only with a 0.26% in-plane compressive strain. The achievement of high-quality large-scale AlN films with uniform thickness and atomically abrupt interface is of great interest for the commercial development of AlN-based devices, particularly acoustic filters where abrupt heterointerfaces with substrates and flat surfaces for AlN films are highly desired.

Homogeneous epitaxial growth of AlN single-crystalline films on 2 inch-diameter Si (111) substrates by pulsed laser deposition

Homogeneous and crack-free AlN films have been epitaxially grown on 2 inch Si (111) substrates by pulsed laser deposition (PLD). By optimising the laser rastering and PLD growth conditions, the 2 inch-diameter single-crystalline AlN films exhibit excellent thickness uniformity with a root-mean-square (RMS) inhomogeneity of less than 3.6% and a very smooth surface with a RMS roughness of 1.4 nm. There is a 1.5 nm-thick interfacial layer between the as-grown AlN films and Si substrates, which is confirmed by HRTEM and a GIXR simulation, and the as-grown AlN films are almost fully relaxed with only 0.3% in-plane tensile strain. The achievement of high-quality and crack-free AlN films with a uniform thickness and abrupt interface on 2 inch Si (111) substrates is of great interest for AlN-based devices, particularly acoustic filters, where abrupt heterointerfaces with the substrates and flat surfaces of AlN films are highly desired.

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.

Significant enhancements of dielectric and magnetic properties in Bi(Fe1−xMgx)O3−x/2 induced by oxygen vacancies

Bi(Fe1?xMgx)O3?x/2 (x?=?0?10%) ceramics were synthesized by high-energy ball milling and solid-state reaction. It was found that a small amount of Mg doping leads to a dramatic enhancement in dielectric permittivity (?two orders of magnitude), along with an apparent improvement in ferromagnetism. The observed significant enhanced dielectric properties may be interpreted by the Maxwell?Wagner relaxation in association with internal barrier layer capacitance. The ferromagnetism can be ascribed to the creation of unbalanced Fe3+ spins and relative long-range coupling mediated by the oxygen vacancies trapped localized electrons.

Synthesis of homogeneous and high-quality GaN films on Cu(111) substrates by pulsed laser deposition

GaN films were grown on Cu(111) substrates by growing an AlN buffer layer with an in-plane alignment of GaN[11−20]//AlN[11−20]//Cu[1−10] using pulsed laser deposition. It is found that by optimizing the laser rastering program and the epitaxial growth temperature, the thickness homogeneities, surface morphologies and structural properties of the GaN films can be greatly improved. Especially, the as-grown GaN films, grown at 750 °C with the optimized laser rastering program, exhibit excellent thickness uniformity with a root-mean-square (RMS) thickness inhomogeneity of less than 2.8%, and a very smooth and flat surface with a surface RMS roughness of 2.3 nm. The as-grown ~102 nm thick GaN films are almost fully relaxed with an in-plane compressive strain of only ~0.53%. No interfacial layer exists between the AlN buffer layer and the GaN film. Furthermore, with an increase in growth temperature from 550 to 750 °C, the surface morphologies and structural properties of the as-grown ~102 nm thick GaN films are improved significantly. The homogeneous and high-quality GaN films produced offer a broad prospect for future applications of GaN-based devices on Cu substrates.

Deposition of nonpolar m-plane InGaN/GaN multiple quantum wells on LiGaO2(100) substrates

High-quality nonpolar m-plane InGaN/GaN multiple quantum wells (MQWs) have been deposited on LiGaO2(100) substrates by the combination of pulsed laser deposition (PLD) and molecular beam epitaxy (MBE) technologies. This work opens up a new prospect for achieving high-efficiency nonpolar m-plane GaN-based devices.

Interfacial reaction control and its mechanism of AlN epitaxial films grown on Si(111) substrates by pulsed laser deposition.

High-quality AlN epitaxial films have been grown on Si substrates by pulsed laser deposition (PLD) by effective control of the interfacial reactions between AlN films and Si substrates. The surface morphology, crystalline quality and interfacial property of as-grown AlN/Si hetero-interfaces obtained by PLD have been systemically studied. It is found that the amorphous SiAlN interfacial layer is formed during high temperature growth, which is ascribed to the serious interfacial reactions between Si atoms diffused from the substrates and the AlN plasmas produced by the pulsed laser when ablating the AlN target during the high temperature growth. On the contrary, abrupt and sharp AlN/Si hetero-interfaces can be achieved by effectively controlling the interfacial reactions at suitable growth temperature. The mechanisms for the evolution of interfacial layer from the amorphous SiAlN layer to the abrupt and sharp AlN/Si hetero-interfaces by PLD are hence proposed. This work of obtaining the abrupt interfaces and the flat surfaces for AlN films grown by PLD is of paramount importance for the application of high-quality AlN-based devices on Si substrates.

Epitaxial growth mechanism of pulsed laser deposited AlN films on Si (111) substrates

The epitaxial growth mechanism and causes of dislocation formation in AlN films on a Si substrate by pulsed laser deposition (PLD) are comprehensively proposed. Due to the high energetic effect and pulsed effect of PLD, the epitaxial process of AlN film growth on Si (111) by PLD is a two-dimensional layer-by-layer growth regime in relation to the strain–relaxation mechanisms. Three PLD sub-stages of the epitaxial process for AlN films on a Si substrate have been suggested and interpreted in detail. Under optimum growth conditions, the film exhibits 1.5 nm-thick single-crystalline AlN interfacial layers with a high density of dislocations, rather than an amorphous SiNx layer. On the contrary, severe interfacial reaction and an AlSiN layer were found at the AlN/Si (111) interface under non-optimal growth conditions, which is derived from the interfacial interdiffusion and penetration between active Si atoms and AlN species, resulting in a high density of dislocations and defects at the AlN/Si (111) interface.