Ultraviolet Light‐Emitting Diode Using Nonpolar AlGaN Core–Shell Nanowire Heterostructures

Abstract

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...

AlGaN Nanowires for Ultraviolet Light-Emitting: Recent Progress, Challenges, and Prospects

“…Recently, single AlGaN nanowire UV LEDs using lateral quantum wells have also been reported [106]. Such single nanowire LED structures are grown by MOCVD on Si substrate.…”

“…Compared with the EL spectral linewidth of devices using ensemble nanowires (~18 nm) [36,37], the single nanowire LED using lateral nonpolar quantum wells possesses a narrower linewidth (~11 nm), allowing the examination of more detailed spectral characteristics. The EQE of such nonpolar single nanowire LEDs is further derived to be ~3% [106].…”

“…(d) EL spectra under different injection currents. Inset: ELspectra of an axial nanowire UV LED for a comparison purpose (see the main text)[106].…”

UV Emission from GaN Wires with m-Plane Core–Shell GaN/AlGaN Multiple Quantum Wells

“…Core-shell systems have been intensively studied in the case of InGaN/GaN MQWs for the development of wire-based LEDs in the blue-green spectral range. [18][19][20] Focusing on wires for UV emission grown by MOVPE, we can distinguish core-shell structures with three types of inner core: AlN 10-12 , AlxGa1-xN (x<0.35) 13, 14 and GaN [15][16][17] .…”

“…On the contrary, wires with GaN core allow efficient carrier transport and EL from core-shell AlGaN-based heterostructures has already been reported: emission at 365 nm has been observed in core-shell n-i-p AlGaN heterostructures 16 and peak emission at 318 nm has been attained with core-shell AlGaN/AlGaN MQWs grown around a GaN core. 17 These studies represented the first demonstrations of UV-LEDs based on wires grown by MOVPE, but tuning the emission down to the UV-A range with core-shell GaN wires remains challenging using the MOVPE approach. A first difficulty is to limit the UV absorption in the GaN core, but GaN can be eliminated by in situ etching with an H2/NH3 gas mixture.…”