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To date, semiconductor light emitters have been developed to exceed 50% efficiency across the entire visible region. [6,7] However, the performance of AlGaNbased ultraviolet light-emitting diodes (LEDs) has been severely limited by the extremely inefficient strain-induced polarization fields and prohibitively large defect densities. Various approaches, including sputtered AlN nucleation layer, polarization doping, patterned sapphire substrate (PSS), AlGaN superlattices, and non-/ semipolars have been studied to improve the efficiency of LEDs in the ultraviolet region. [1,[8][9][10][11][12] Due to low magnesium (Mg)doping efficiency and large Mg activation energy, [13][14][15] AlGaN-based LEDs usually exhibit very high threshold voltages in the deep ultraviolet range. [13,16] Furthermore, the lattice mismatch between AlGaN materials and the substrate materials induces high dislocations and stacking faults with thermal stress in the Al-rich AlGaN epitaxial layers. [14] The AlGaN multiple quantum wells (MQWs) grown along a polar [0001] orientation suffer from inherent spontaneous and piezoelectric polarizations. The total polarization fields result in the spatial separation of the electron and hole wave functions and thus in restricting the radiative recombination efficiency, known as the quantum-confined Stark effect (QCSE). [17,18] As a result, because of the poor material quality and the polarization-induced electric field, it is still difficult to achieve internal quantum efficiency (IQE) in AlGaN epitaxial structures. This poor internal quantum efficiency eventually leads to extremely low output power of AlGaN-based LEDs operating in the UVA-C bands.Such critical issues can be potentially addressed by employing 1D nanowire structures and nonpolar core-shell heterostructures. [1,12] GaN-based nanowire structures have been intensively studied in the past decade. [19][20][21] Recent reports have shown that such nanowires can drastically reduce the dislocation densities due to efficient surface strain relaxation, and also can significantly enhance the p-type current conduction due to reduced Mg-dopant formation energy in p-Al(Ga)N structures. [22,23] With the use of nonpolar structures grown along m-plane direction in GaN structure, the improved light output power and electrical performance have been demonstrated in InGaN-based quantum well LEDs with the absence of polarization. [12,24,25] Therefore, it is expected that the incorporation of the nonpolar core-shell and the nanowire structures can significantly improve the internal quantum efficiency, p-contact Highly efficient nonpolar AlGaN nanowire ultraviolet light-emitting diode is developed, wherein core-shell AlGaN multiple quantum well layers are incorporated in the nonpolar active regions. It is confirmed that the core-shell light-emitting diode (LED) heterostructures are uniformly grown on the nonpolar surfaces of hexagonal GaN nanowires by metalorganic chemical vapor deposition (MOCVD) technique. At room temperature, the nearly defect-free core-shell AlGaN...

Aluminum gallium nitride (Al x Ga 1−x N) alloy films and nanostructures have attracted extensive research attention for ultraviolet (UV) and deep ultraviolet optoelectronic applications. However, the morphology-controlled growth of high-quality Al x Ga 1−x N quasi one-dimensional nanostructures has been limited by the complex multicomponent phase diagram and inhomogeneous composition distribution. Here, we demonstrated the growth of Si-doped ntype compositionally uniform Al 0.45 Ga 0.55 N nanorods employing a metal organic chemical vapor deposition (MOCVD) technique for the application in UV-C photodetectors. A two-step growth process, namely, growth of undoped GaN seeds and subsequent growth of n-AlGaN nanorods over GaN seeds, has been developed. Various characterization techniques have been used to study the crystalline quality, orientation, and optical properties of the realized nanorods. Field emission scanning electron microscopy revealed a uniform distribution of vertically aligned n-AlGaN nanorods over the GaN seeds. X-ray diffraction studies showed that the grown nanorods are preferentially (0002) oriented with hexagonal crystal structure. High-resolution transmission electron microscopy images indicated the nanorods are single crystalline in nature, without any significant crystalline defects and dislocations. Cathodoluminescence spectra of AlGaN nanorods displayed a strong band edge excitonic emission peak at 276 nm at 77 K and shifted to lower energy as the temperature increased to 300 K. The photocurrent current (I p ) of the fabricated photoconductive device was significantly higher in the UV region (250−276 nm) compared to the corresponding dark current. The photocurrent displayed a nonlinear power density (P)-dependent characteristics (I p ∝ P 0.64 ). The photoresponsivity and sensitivity of the fabricated photodetector were estimated to be ∼115 mA/W and ∼64%, respectively, in the UV-C region.

The superior photoconductive behavior of a simple, cost-effective n-GaN nanorod (NR)-graphene hybrid device structure is demonstrated for the first time. The proposed hybrid structure was synthesized on a Si (111) substrate using the high-quality graphene transfer method and the relatively low-temperature metal-organic chemical vapor deposition (MOCVD) process with a high V/III ratio to protect the graphene layer from thermal damage during the growth of n-GaN nanorods. Defect-free n-GaN NRs were grown on a highly ordered graphene monolayer on Si without forming any metal-catalyst or droplet seeds. The prominent existence of the undamaged monolayer graphene even after the growth of highly dense n-GaN NRs, as determined using Raman spectroscopy and high-resolution transmission electron microscopy (HR-TEM), facilitated the excellent transport of the generated charge carriers through the photoconductive channel. The highly matched n-GaN NR-graphene hybrid structure exhibited enhancement in the photocurrent along with increased sensitivity and photoresponsivity, which were attributed to the extremely low carrier trap density in the photoconductive channel.

Compositionally uniform high crystalline quality Al0.45Ga0.55N nanoflowers have been synthesized by MOCVD for application in high-performance ultraviolet-C photodetectors. The photodetectors fabricated with nanoflowers displayed a photoresponsivity of ∼0.72 A W−1 along with a sensitivity of ∼40% at 2 V bias in the ultraviolet-C region.