Shuo Zhang

Constant advance in improving the luminous efficacy (η_L) of nitride-based light-emitting diodes (LEDs) plays a critical role for saving measurable amounts of energy. Further development is motivated to approach the efficiency limit for this material system while reducing the costs. In this work, strategies of using thin AlN prebuffer and transitional-refraction-index patterned sapphire substrate (TPSS) were proposed, which pushed up the efficiency of white LEDs (WLEDs). The AlN prebuffer was obtained through physical vapor deposition (PVD) method and TPSS was fabricated by dry-etched periodic silica arrays covered on sapphire. Devices in mass production confirmed that PVD AlN prebuffer was able to improve the light output power (φ_e) of blue LEDs (BLEDs) by 2.53% while increasing the productivity by ~8% through shortening the growth time. Additionally, BLEDs on TPSS exhibited an enhanced top η_ext of 5.65% in contrast to BLEDs on the conventional PSS through Monte Carlo ray-tracing simulation. Consequently, φ_e of BLEDs was experimentally enhanced by 10% at an injected current density (J_in) of 40 A/cm². A peak η_L of 295.2 lm/W at a J_in of 0.9 A/cm² and the representative η_L of 282.4 lm/W at a J_in of 5.6 A/cm² for phosphor-converted WLEDs were achieved at a correlated color temperature of 4592 K.

The heteroepitaxy of nitride semiconductors can be achieved without needing lattice matching between two material systems.

InGaN-based micro-structured light-emitting diodes (µLEDs) play a critical role in the field of full-color display. In this work, selected area growth (SAG) of a micro-pyramid LED array was performed on a 2-inch wafer-scale patterned SiO₂ template (periodicity: 4 µm diameter), by which a uniform periodic µLED array was achieved. The single-element pyramid-shaped LED exhibited 6 equivalent semipolar {1-101} planes and a size of about 5 µm, revealing a good crystalline quality with screw and edge dislocation densities of 8.27 × 10⁷ and 4.49 × 10⁸ cm⁻². Due to the stress–relaxation out of the SAG, the as-built compressive strain was reduced to 0.59 GPa. The µLED array demonstrated a stable emission, confirmed by a small variation of electroluminescence (EL) peak wavelength over a wide range of current density up to 44.89 A/cm², as well as tiny fluctuations (within 1.9 nm) in the EL full width at half maximum. The photoluminescence peak wavelength exhibits a good uniformity throughout the whole wafer with a discrete probability of only 0.25%.

We demonstrate the AlGaN/GaN high electron mobility transistor (HEMT) featured with a cantilever structure. The output performances of the HEMT before and after the cantilever fabrication process are investigated. By interacting with the cantilever, the output characteristics can be directly modulated by external strain. Additionally, the output current density shows an approximately linear relation with applied strain. The cantilever-structured AlGaN/GaN HEMT is very suitable to build a strain-controlled platform for artificial intelligence systems.

The nitride films with high indium (In) composition play a crucial role in the fabrication of In‐rich InGaN‐based optoelectronic devices. However, a major limitation is In incorporation requiring a low temperature during growth at the expense of nitride dissociation. Here, to overcome this limitation, a strain‐modulated growth method, namely the graphene (Gr)‐nanorod (NR) enhanced quasi‐van der Waals epitaxy, is proposed to increase the In composition in InGaN alloy. The lattice transparency of Gr enables constraint of in‐plane orientation of nitride film and epitaxial relationships at the heterointerface. The Gr interlayer together with NRs buffer layer substantially reduces the stress of the GaN film by 74.4%, from 0.9 to 0.23 GPa, and thus increases the In incorporation by 30.7%. The first principles calculations confirm that the release of strain accounts for the dramatic improvement. The photoluminescence peak of multiple quantum wells shifts from 461 to 497 nm and the functionally small‐sized cyan light‐emitting diodes of 7 × 9 mil² are demonstrated. These findings provide an efficient approach for the growth of In‐rich InGaN film and extend the applications of nitrides in advanced optoelectronic, photovoltaic, and thermoelectric devices.

Epitaxial horizontal nanowires (NWs) have attracted much attention due to their easily large-scale integration. From the reported literature, epitaxial growth is usually driven by minimization of strain between NW and substrate, which governs the growth along with specific crystallographic orientation. Here, we report the first homoepitaxial growth of horizontal GaN NWs from a surface-directed vapor−liquid−solid growth method. The NWs grow along with six symmetry-equivalent 〈1-100〉 ( m -axis) directions, exhibiting a random 60°/120° kinked configuration. Owing to homoepitaxial growth, strain could be eliminated. From the obtained results, we suggest that the formation the horizontal NWs, and their growth direction /orientation is not directly related to the strain minimization. A general rule based on the epitaxial relationship and potential low-index growth orientation is proposed for understanding the arrangement of epitaxial horizontal NWs. It is deduced that kinking of the horizontal NWs was attributed to unintentional guided growth determined by the roughness of the substrates’ surface. This study provides an insight for a better understanding of the evolution of epitaxial horizontal NWs, especially for the growth direction/orientation.

A gallium nitride (GaN) nanowire (NW) UVA photodetector with high responsivity was reported. The GaN NW was grown by horizontal hydride vapor phase epitaxy. The NW morphology is proved tunable via different growth conditions. The axial and radial growths of GaN NWs were investigated through vapor–liquid–solid and vapor–solid mixed growth models. Besides, NWs with different morphologies exhibit different growth crystal orientations, which depend on the flow rate of HCl. NWs with smaller diameters show better optical properties and crystalline quality. More importantly, the UVA detector fabricated by a single NW exhibits excellent responsivity of 4.35 × 10⁴–1.06 × 10⁵ A/W and external quantum efficiency of 1.48 × 10⁷%–3.6 × 10⁷% under different light power densities. The high responsivity and low production cost make the GaN NW UVA detector extremely attractive for several applications, such as fire sensing and missile and rocket warning.

The growth of nitride on large-size and low-cost amorphous substrates has attracted considerable attention for applications in large-scale optoelectronic devices. In this paper, we reported the growth of GaN-based light-emitting diodes (LEDs) on amorphous SiO₂ substrate with the use of nanorods and graphene buffer layers by metal organic chemical vapor deposition (MOCVD). The effect of different growth parameters on the morphology and vertical-to-lateral aspect ratio of nanorods was discussed by analyzing growth kinetics. Furthermore, we tuned nanorod coalescence to obtain continuous GaN films with a blue-LED structure by adjusting growth conditions. The GaN films exhibited a hexagonal wurtzite structure and aligned c-axis orientation demonstrated by X-ray diffractometer (XRD), Raman, and transmission electron microscopy (TEM) results. Finally, five-pair InGaN/GaN multi-quantum-wells (MQWs) were grown. The photoluminescence (PL) showed an intense emission peak at 475 nm, and the current–voltage (I-V) curve shows a rectifying behavior with a turn-on voltage of 5.7 V. This work provides a promising fabrication method for the large-area and low-cost GaN-based devices on amorphous substrates and opens up the further possibility of nitride integration with Si (100) complementary metal oxide semiconductor (CMOS) electronics.

To achieve high quality lighting and visible light communication (VLC) simultaneously, GaN based white light emitting diodes (WLEDs) oriented for lighting in VLC has attracted great interest. However, the overall bandwidth of conventional phosphor converted WLEDs is limited by the long lifetime of phosphor, the slow Stokes transfer process, the resistance-capacitance (RC) time delay, and the quantum-confined Stark effect (QCSE). Here by adopting a self-assembled InGaN quantum dots (QDs) structure, we have fabricated phosphor-free single chip WLEDs with tunable correlated color temperature (CCT, from 1600 K to 6000 K), a broadband spectrum, a moderate color rendering index (CRI) of 75, and a significantly improved modulation bandwidth (maximum of 150 MHz) at a low current density of 72    A / cm 2 . The broadband spectrum and high modulation bandwidth are ascribed to the capture of carriers by different localized states of InGaN QDs with alleviative QCSE as compared to the traditional InGaN/GaN quantum well (QW) structures. We believe the approach reported in this work will find its potential application in GaN WLEDs and advance the development of semiconductor lighting-communication integration.