Yunhao Lin

GaN-based light-emitting diodes on various substrates: a critical review.

GaN and related III-nitrides have attracted considerable attention as promising materials for application in optoelectronic devices, in particular, light-emitting diodes (LEDs). At present, sapphire is still the most popular commercial substrate for epitaxial growth of GaN-based LEDs. However, due to its relatively large lattice mismatch with GaN and low thermal conductivity, sapphire is not the most ideal substrate for GaN-based LEDs. Therefore, in order to obtain high-performance and high-power LEDs with relatively low cost, unconventional substrates, which are of low lattice mismatch with GaN, high thermal conductivity and low cost, have been tried as substitutes for sapphire. As a matter of fact, it is not easy to obtain high-quality III-nitride films on those substrates for various reasons. However, by developing a variety of techniques, distincts progress has been made during the past decade, with high-performance LEDs being successfully achieved on these unconventional substrates. This review focuses on state-of-the-art high-performance GaN-based LED materials and devices on unconventional substrates. The issues involved in the growth of GaN-based LED structures on each type of unconventional substrate are outlined, and the fundamental physics behind these issues is detailed. The corresponding solutions for III-nitride growth, defect control, and chip processing for each type of unconventional substrate are discussed in depth, together with a brief introduction to some newly developed techniques in order to realize LED structures on unconventional substrates. This is very useful for understanding the progress in this field of physics. In this review, we also speculate on the prospects for LEDs on unconventional substrates.

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.

Highly-efficient GaN-based light-emitting diode wafers on La0.3Sr1.7AlTaO6 substrates

Highly-efficient GaN-based light-emitting diode (LED) wafers have been grown on La0.3Sr1.7AlTaO6 (LSAT) substrates by radio-frequency molecular beam epitaxy (RF-MBE) with optimized growth conditions. The structural properties, surface morphologies, and optoelectronic properties of as-prepared GaN-based LED wafers on LSAT substrates have been characterized in detail. The characterizations have revealed that the full-width at half-maximums (FWHMs) for X-ray rocking curves of GaN(0002) and GaN(10-12) are 190.1 and 210.2 arcsec, respectively, indicating that high crystalline quality GaN films have been obtained. The scanning electron microscopy and atomic force microscopy measurements have shown the very smooth p-GaN surface with the surface root-mean-square (RMS) roughness of 1.3 nm. The measurements of low-temperature and room-temperature photoluminescence help to calculate the internal quantum efficiency of 79.0%. The as-grown GaN-based LED wafers have been made into LED chips with the size of 300 × 300 μm2 by the standard process. The forward voltage, the light output power and the external quantum efficiency for LED chips are 19.6 W, 2.78 V, and 40.2%, respectively, at a current of 20 mA. These results reveal the high optoelectronic properties of GaN-based LEDs on LSAT substrates. This work brings up a broad future application of 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.

Influence of In content in InGaN barriers on crystalline quality and carrier transport of GaN-based light-emitting diodes

The influence of In content in InGaN barriers on the crystalline quality and carrier transport of GaN-based light-emitting diodes (LEDs) is studied by numerical and experimental investigations. The optimal In content of InGaN barriers is hence obtained. It is found that carrier concentration and crystalline quality degradation are a pair of opposite influential factors as In content increases. In content of 1.2% is optimal because it is the balance value at which a huge gain of carrier concentration is achieved without crystalline degradation. In content of 1.2% in InGaN barriers leads to a remarkable enhancement in both the light output power and external quantum efficiency (EQE) of LEDs. In such cases, the LEDs light output power and the EQE increase by 15.4% and 10.3% at a current of 70 mA, respectively. This work demonstrates the possibility of achieving high-performance LEDs with an aggravated efficiency droop, and is of great interest for the commercial development of GaN-based LEDs.

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.