Huirong Qian

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.

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.

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.

Epitaxial growth and its mechanism of GaN films on nitrided LiGaO2(001) substrates by pulsed laser deposition

High-quality GaN films have been grown on nitrided LiGaO2 substrates by pulsed laser deposition with an in-plane epitaxial relationship of GaN[11−20]//LiGaO2[010]. The surface morphologies and structural properties of the as-grown GaN films are studied in detail by various characterization methods. These characterizations for the as-grown GaN films show excellent crystalline quality with a full-width at half-maximum value of 0.1° and a very smooth surface with a surface root-mean-square roughness of 1.1 nm. There is an interfacial layer existing between GaN films and LiGaO2 substrates with a thickness of 0.9 nm. Furthermore, the nitridation effect on the properties of GaN films and the growth mechanism of GaN films on nitrided LiGaO2 substrates by pulsed laser deposition have also been systemically studied. This work opens up a broad prospect for the growth of high-efficiency GaN-based devices on LiGaO2(001) substrates.

Nitridation effect of the α-Al2O3 substrates on the quality of the GaN films grown by pulsed laser deposition

GaN films have been grown on the nitrided and non-nitrided α-Al2O3 substrates by pulsed laser deposition (PLD). The surface morphologies and structural properties of these as-grown GaN films have been investigated carefully. This reveals that, when the nitridation process is performed on the α-Al2O3 substrates, the surface morphologies and structural properties of the as-grown GaN films grown on the as-nitrided α-Al2O3 substrates are improved dramatically. The effect of nitridation on the properties of GaN films and the growth mechanism of GaN films on nitrided α-Al2O3 substrates by PLD have been carefully studied. An effective approach to achieve high-quality GaN films on α-Al2O3 substrates by PLD is hence presented.

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.

Epitaxial growth and characterization of high-quality aluminum films on sapphire substrates by molecular beam epitaxy

High-quality Al epitaxial films with homogeneous thickness have been epitaxially grown on 2 inch sapphire substrates by molecular beam epitaxy with an in-plane alignment of Al[10]/Al2O3[100]. The as-grown about 200 nm-thick Al (111) films grown at 750 °C show excellent uniform thickness distribution over the whole 2 inch substrate and a very flat Al surface with the surface root-mean-square roughness of 0.6 nm, as well as high crystalline qualities with the Al (111) full width at half maximum as small as 0.05°. There is no interfacial layer existing between as-grown Al epitaxial films and sapphire substrates. Instead, sharp and abrupt Al/Al2O3 hetero-interfaces are achieved. The effects of the growth temperature on the surface morphologies and the crystalline qualities of the as-grown Al epitaxial films have been studied in detail. This achievement of Al epitaxial films is of great importance in the application of Al-based microelectronic devices.

Epitaxial growth of high-quality AlN films on metallic nickel substrates by pulsed laser deposition

Single-crystalline AlN films have been grown on metallic nickel (Ni) substrates by pulsed laser deposition with an in-plane epitaxial relationship of AlN[110]//Ni[10]. The as-grown AlN films reveal that a very smooth AlN film surface with a root-mean-square roughness of 1.0 nm has been obtained, and there is no interfacial layer existing between the AlN films and the Ni (111) substrates. Furthermore, with the increase in the growth temperature, the surface morphologies, crystalline qualities, and interfacial properties of as-grown AlN films gradually deteriorate. The as-grown about 300 nm-thick AlN films are almost fully relaxed only with an in-plane compressive strain of 0.67%. This work has potential in applications of AlN-based devices which require abrupt hetero-interfaces and flat films surfaces.