Shizhong Zhou

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.

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.

Study of defects in LED epitaxial layers grown on the optimized hemispherical patterned sapphire substrates

In this work, we focus on the study of defects in GaN grown on an optimized hemispherical patterned sapphire substrate (PSS). It is demonstrated that the proposed patterns can on the one hand induce the formation of stacking faults, and on the other hand, reduce the strain caused by thermal misfit and lattice misfit. Consequently, the optimized hemispherical patterns work successfully for both the reduction in the number of dislocations spreading to multiple quantum wells and the improvement in surface morphology. The dominant mechanism of defect multiplication and the effects of optimized hemispherical patterns in terms of materials science and device technology are elucidated.

Enhance Light Emitting Diode Light Extraction Efficiency by an Optimized Spherical Cap-Shaped Patterned Sapphire Substrate

This work has proposed a new way to optimize the spherical cap-shaped patterned sapphire substrate (PSS) for highly efficient GaN-based light emitting diodes (LEDs), which has been compared with the hemisphere patterned one. This pattern is achieved by changing the height of hemispherical units on the basis of hemispherical PSS. The height, the distance and the radius of the spherical cap-shaped unit are subsequently optimised by optical simulation. It is revealed that this optimised spherical cap-shaped PSS can enhance light extraction yield of LEDs by over 10% compared with LEDs grown on the optimal hemispherical PSS. The effectiveness of this spherical cap-shaped PSS has been demonstrated by subsequent crystal growth and characterization of LED wafers, and therefore sheds light on a further improvement on LED efficacy by the design of novel patterns for the application of PSS technology.

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.

Near-infrared wavelength-dependent nonlinear transmittance tailoring in glass ceramics containing Er3+:LaF3 nanocrystals

Nonlinear optical (NLO) effects originating from materials doped with rare-earth ions possess colossal potential for application in all-optical switches. However, among previous studies, Er3+ ion-doped glass ceramics (GCs) with remarkable NLO features have been investigated with respect to optical modulation applications by tailoring their nonlinear transmittance upon excitation at various near-infrared (NIR) wavelengths, which might prove to be a simple way of achieving “on–off” optical modulation in future all-optical switches. Here, we present the first observation of tailorable nonlinear transmittance in germanate oxyfluoride GCs containing Er3+:LaF3 nanocrystals, manipulated by excitation at 808, 980, and 1550 nm, which is consistent with the results from theoretical calculations and simulations. Furthermore, we conduct experimental investigation and analysis related to energy level transitions and dynamical evolution, indicating that these intriguing NLO features can be attributed to the differentiation between excited state absorption accompanied by up-conversion luminescence and stimulated emission processes during excitation at discrepant NIR wavelengths. Importantly, bidirectional optical switching for the “on–off” toggle effect has been successfully demonstrated by selectively tailoring the nonlinear transmittance of the single Er3+-doped GCs. This tailorable NLO behavior of Er3+-doped GCs, which is dependent on excitation at different NIR wavelengths, might provide a versatile strategy for the development of next-generation bidirectional all-optical switches.

Growth evolution of AlN films on silicon (111) substrates by pulsed laser deposition

AlN films with various thicknesses have been grown on Si(111) substrates by pulsed laser deposition (PLD). The surface morphology and structural property of the as-grown AlN films have been investigated carefully to comprehensively explore the epitaxial behavior. The ∼2 nm-thick AlN film initially grown on Si substrate exhibits an atomically flat surface with a root-mean-square surface roughness of 0.23 nm. As the thickness increases, AlN grains gradually grow larger, causing a relatively rough surface. The surface morphology of ∼120 nm-thick AlN film indicates that AlN islands coalesce together and eventually form AlN layers. The decreasing growth rate from 240 to 180 nm/h is a direct evidence that the growth mode of AlN films grown on Si substrates by PLD changes from the islands growth to the layer growth. The evolution of AlN films throughout the growth is studied deeply, and its corresponding growth mechanism is hence proposed. These results are instructional for the growth of high-quality nitride fi...