Research on hexagonal boron nitride (hBN) has been intensified recently due to the application of hBN as a promising system of single-photon emitters. To date, the single photon origin remains under debate even though many experiments and theoretical calculations have been performed. We have measured the pressure-dependent photoluminescence (PL) spectra of hBN flakes at low temperatures by using a diamond anvil cell device. The absolute values of the pressure coefficients of discrete PL emission lines are all below 15 meV/GPa, which is much lower than the pressure-induced 36 meV/GPa redshift rate of the hBN bandgap. These PL emission lines originate from atom-like localized defect levels confined within the bandgap of the hBN flakes. Interestingly, the experimental results of the pressure-dependent PL emission lines present three different types of pressure responses corresponding to a redshift (negative pressure coefficient), a blueshift (positive pressure coefficient), or even a sign change from negative to positive. Density functional theory calculations indicate the existence of competition between the intralayer and interlayer interaction contributions, which leads to the different pressure-dependent behaviors of the PL peak shift.
which are environmentally friendly and enable portability and high efficiency. Up to date, great progress has been made on the UVC light-emitting diodes (LEDs) by using active regions of AlGaN multiple quantum wells (MQWs) .[8-14] However, the optical output power of current UVC LEDs drops significantly as the light emission wavelength gets shorter. Those LEDs suffer from poor hole injection efficiency in high-Al-content p-type AlGaN, low internal quantum efficiency (IQE) caused by large-lattice-mismatch heteroepitaxy, and strong quantum-confined Stark effect (QCSE), as well as the absorption by the nontransparent GaN contact layers. [15][16][17] A promising approach that dramatically improves the light output power is electron-beam (e-beam) pumping, especially for the short-wavelength UVC spectral range. [3,[18][19][20][21][22][23][24] This approach allows one to bypass the need for p-type or n-type injection layers and, thus, can largely increase the carrier injection efficiency. This provides a unique advantage over conventional LEDs at UVC range, since the p-type doping for high-Al-content AlGaN is High-output-power electron-beam (e-beam) pumped deep ultraviolet (DUV) light sources, operating at 230-270 nm, are achieved by adjusting the well thickness of binary ultrathin GaN/AlN multiple quantum wells. These structures are fabricated on high-quality thermally annealed AlN templates by metal-organic chemical vapor deposition. Owing to the reduced dislocation density, large electron-hole overlap, and efficient carrier injection by e-beam, the DUV light sources demonstrate high output powers of 24.8, 122.5, and 178.8 mW at central wavelengths of 232, 244, and 267 nm, respectively. Further growth optimization and employing an e-gun with increased beam current lead to a record output power of ≈2.2 W at emission wavelength of ≈260 nm, the key wavelength for water sterilization. This work manifests the practical levels of high-output-power DUV light sources operated by using e-beam pumping method. The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adom.201801763.Solid-state deep ultraviolet (DUV) optoelectronic devices in the spectral range of 200-280 nm, i.e., ultraviolet-C (UVC), have attracted much attention for their wide applications in sterilization, medical treatment, security, solar-blind photodetection, and so on. [1][2][3][4][5][6][7] Currently, Al(Ga)N material system is the most promising candidate for solid-state UVC light sources
Quantum technologies require robust and photostable single-photon emitters. Here, room temperature operated single-photon emissions from isolated defects in aluminum nitride (AlN) films are reported. AlN films were grown on nanopatterned sapphire substrates by metal organic chemical vapor deposition. The observed emission lines range from visible to near-infrared, with highly linear polarization characteristics. The temperature-dependent line width increase shows T3 or single-exponential behavior. First-principle calculations based on density functional theory show that point defect species, such as antisite nitrogen vacancy complex (NAlVN) and divacancy (VAlVN) complexes, are considered to be an important physical origin of observed emission lines ranging from approximately 550 to 1000 nm. The results provide a new platform for on-chip quantum sources.
Fabricating single-crystalline gallium nitride (GaN)-based devices on a Si(100) substrate, which is compatible with the mainstream complementary metal-oxide-semiconductor circuits, is a prerequisite for next-generation high-performance electronics and optoelectronics. However, the direct epitaxy of single-crystalline GaN on a Si(100) substrate remains challenging due to the asymmetric surface domains of Si(100), which can lead to polycrystalline GaN with a two-domain structure. Here, by utilizing singlecrystalline graphene as a buffer layer, the epitaxy of a single-crystalline GaN film on a Si(100) substrate is demonstrated. The in situ treatment of graphene with NH 3 can generate sp 3 CN bonds, which then triggers the nucleation of nitrides. The one-atom-thick single-crystalline graphene provides an in-plane driving force to align all GaN domains to form a single crystal. The nucleation mechanisms and domain evolutions are further clarified by surface science exploration and first-principle calculations. This work lays the foundation for the integration of GaN-based devices into Si-based integrated circuits and also broadens the choice for the epitaxy of nitrides on unconventional amorphous or flexible substrates.
In this paper, we propose and demonstrate a convenient and flexible approach for preparation large-area of photonic crystals (PhCs) structures on the GaN-based LED chip. The highly-ordered porous anodic alumina (AAO) with pitch of wavelength scale was adopted as a selective dry etching mask for PhCs-pattern transfer. The PhCs with different pore depths were simultaneously formed on the entire surfaces of GaN-based LED chip including ITO, GaN surrounding contacts and the sidewall of the mesa by one-step reactive ion etching (RIE). The light output power improvement of PhCs-based GaN LED was achieved as high as 94% compared to that of the conventional GaN-based LED.
The performance of nitride-based LEDs was improved by inserting dual stage and step stage InGaN/GaN strain relief layer (SRL) between the active layer and n-GaN template. The influences of step stage InGaN/GaN SRL on the structure, electrical and optical characteristics of GaN-based LEDs were investigated. The analysis of strain effect on recombination rate based k·p method indicated 12.5% reduction of strain in InGaN/GaN MQWs by inserting SRL with step stage InGaN/GaN structures. The surface morphology was improved and a smaller blue shift in the electroluminescence (EL) spectral with increasing injection current was observed for LEDs with step stage SRL compared with conventional LEDs. The output power of LEDs operating at 20 mA was about 15.3 mW, increased by more than 108% by using step stage InGaN/GaN SRL, which shows great potential of such InGaN/GaN SRL in modulating InGaN/GaN MQWs optical properties based on its strain relief function.
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