[en] 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 LED’s 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. (paper)

[en] 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. (review)

[en] A new structure of p-GaN/InGaN heterojunction has been proposed to enhance hole injection for blue GaN-based light-emitting diodes (LEDs). It is demonstrated by the simulation results that a p-GaN (50 nm)/In_0.05Ga_0.95N (150 nm) heterojunction can make a 25% and 10% increment of hole and electron concentration in the active region, respectively, finally resulting in a 55% improvement on the LED’s radiative recombination intensity. The simulation also reveals that the efficiency droop is alleviated from 32.9% to 21.7% at the current density of 100 A cm⁻². The enhanced hole injection is mainly attributed to the increased average background hole concentration of the area between the p-AlGaN electron blocking layer (EBL) to the p-GaN/InGaN heterojunction. The increasing potential barrier of the conduction band, resulting from the introduction of p-GaN/InGaN heterojunction, would also weaken electron leakage and is favorable to the LED’s luminous performance. The experimental results show that the wall-plug efficiency (WPE) of the p-GaN/InGaN LED increases by 26.0% at the injection current of 75 mA, in spite of the increasing electric resistance, which impairs the improvement of the LED’s performance from the enhanced hole injection. The structure of the p-GaN/InGaN heterojunction is novel in the field of p-type region design, and is a simple but effective way to promote the LED’s performance, which is very promising for application in further high-performance LED fabrication. (paper)

[en] 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 films on Si substrates by PLD, and of great interest for the fabrication of AlN-based devices

[en] High-quality GaAs films have been epitaxially grown on Si (111) substrates by inserting an In_xGa_1−xAs interlayer with proper In composition by molecular beam epitaxy (MBE). The effect of In_xGa_1−xAs (0 < x < 0.2) interlayers on the properties of GaAs films grown on Si (111) substrates by MBE has been studied in detailed. Due to the high compressive strain between InGaAs and Si, InGaAs undergoes partial strain relaxation. Unstrained InGaAs has a larger lattice constant than GaAs. Therefore, a thin InGaAs layer with proper In composition may adopt a close lattice constant with that of GaAs, which is beneficial to the growth of high-quality GaAs epilayer on top. It is found that the proper In composition in In_xGa_1−xAs interlayer of 10% is beneficial to obtaining high-quality GaAs films, which, on the one hand, greatly compensates the misfit stress between GaAs film and Si substrate, and on the other hand, suppresses the formation of multiple twin during the heteroepitaxial growth of GaAs film. However, when the In composition does not reach the proper value (∼10%), the In_xGa_1−xAs adopts a lower strain relaxation and undergoes a lattice constant smaller than unstrained GaAs, and therefore introduces compressive stress to GaAs grown on top. When In composition exceeds the proper value, the In_xGa_1−xAs will adopt a higher strain relaxation and undergoes a lattice constant larger than unstrained GaAs, and therefore introduces tensile stress to GaAs grown on top. As a result, In_xGa_1−xAs interlayers with improper In composition introduces enlarged misfit stress to GaAs epilayers grown on top, and deteriorates the quality of GaAs epilayers. This work demonstrates a simple but effective method to grow high-quality GaAs epilayers and brings up a broad prospect for the application of GaAs-based optoelectronic devices on Si substrates

[en] GaN-based light-emitting diodes (LEDs) on Si substrates are promising to replace conventional lamps due to the advantages of energy-saving and low-cost of LEDs grown on large-size Si substrates. However, high-density dislocations and cracks of GaN epitaxial films are usually formed that limit the further development and application of GaN-based LEDs. To circumvent the issues, the step-graded AlGaN buffer layers are carefully designed to grow GaN epitaxial films on Si substrates. The mechanisms of dislocations and stresses for GaN epitaxial films controlled by step-graded AlGaN buffer layers are also investigated by analyzing dislocations evolution and stresses relaxation at the hetero-interfaces. Afterwards, 3.0 μm-thick high-quality GaN epitaxial films grown on Si substrates have been obtained, and high-quality GaN-based LED wafers are obtained accordingly with small full-width at half-maximums (FWHMs) for GaN(0002) and GaN(10–12) X-ray rocking curves of 272 and 297 arcsec, respectively. The corresponding vertical-structure LED chips reveal high-performance with a high light output power of 592 mW and a small working voltage of 2.77 V @ 456 nm, at a current of 350 mA. This work provides an effective approach for the growth of high-quality crack-free GaN epitaxial films on Si substrates for the fabrication of high-performance GaN-based devices.