Atomic Evolution Mechanism and Suppression of Edge Threading Dislocations in Nitride Remote Heteroepitaxy.

The majority of dislocations in nitride epilayers are edge threading dislocations (TDs), which diminish the performance of nitride devices. However, it is extremely difficult to reduce the edge TDs due to the lack of available slip systems. Here, we systematically investigate the formation mechanism of edge TDs and find that besides originating at the coalescence boundaries, these dislocations are also closely related to geometrical misfit dislocations at the interface. Based on this understanding, we propose a novel strategy to reduce the edge TD density of the GaN epilayer by nearly 1 order of magnitude via graphene-assisted remote heteroepitaxy. The first-principles calculations confirm that the insertion of graphene dramatically reduces the energy barrier required for interfacial sliding, which promotes a new strain release channel. This work provides a unique approach to directly suppress the formation of edge TDs at the source, thereby facilitating the enhanced performance of photoelectronic and electronic devices.

ß-Ga2O3 Air-Channel Field-Emission Nanodiode with Ultrahigh Current Density and Low Turn-On Voltage.

Field-emission nanodiodes with air-gap channels based on single ß-Ga2O3 nanowires have been investigated in this work. With a gap of ∼50 nm and an asymmetric device structure, the proposed nanodiode achieves good diode characteristics through field emission in air at room temperature. Measurement results show that the nanodiode exhibits an ultrahigh emission current density, a high enhancement factor of >2300, and a low turn-on voltage of 0.46 V. More impressively, the emission current almost keeps constant over a wide range (8 orders of magnitude) of air pressures below 1 atm. Meanwhile, the fluctuation in field-emission current is below 8.7% during long-time monitoring, which is better than the best reported field-emission device based on ß-Ga2O3 nanostructures. All of these results indicate that ß-Ga2O3 air-gapped nanodiodes are promising candidates for vacuum electronics that can also operate in air.

High-quality AFM image acquisition of living cells by modified residual encoder-decoder network.

Atomic force microscope enables ultra-precision imaging of living cells. However, atomic force microscope imaging is a complex and time-consuming process. The obtained images of living cells usually have low resolution and are easily influenced by noise leading to unsatisfactory imaging quality, obstructing the research and analysis based on cell images. Herein, an adaptive attention image reconstruction network based on residual encoder-decoder was proposed, through the combination of deep learning technology and atomic force microscope imaging supporting high-quality cell image acquisition. Compared with other learning-based methods, the proposed network showed higher peak signal-to-noise ratio, higher structural similarity and better image reconstruction performances. In addition, the cell images reconstructed by each method were used for cell recognition, and the cell images reconstructed by the proposed network had the highest cell recognition rate. The proposed network has brought insights into the atomic force microscope-based imaging of living cells and cell image reconstruction, which is of great significance in biological and medical research.

Wafer-Scale Transferrable GaN Enabled by Hexagonal Boron Nitride for Flexible Light-Emitting Diode.

Epitaxy growth and mechanical transfer of high-quality III-nitrides using 2D materials, weakly bonded by van der Waals force, becomes an important technology for semiconductor industry. In this work, wafer-scale transferrable GaN epilayer with low dislocation density is successfully achieved through AlN/h-BN composite buffer layer and its application in flexible InGaN-based light-emitting diodes (LEDs) is demonstrated. Guided by first-principles calculations, the nucleation and bonding mechanism of GaN and AlN on h-BN is presented, and it is confirmed that the adsorption energy of Al atoms on O2 -plasma-treated h-BN is over 1 eV larger than that of Ga atoms. It is found that the introduced high-temperature AlN buffer layer induces sufficient tensile strain during rapid coalescence to compensate the compressive strain generated by the heteromismatch, and a strain-relaxation model for III-nitrides on h-BN is proposed. Eventually, the mechanical exfoliation of single-crystalline GaN film and LED through weak interaction between multilayer h-BN is realized. The flexible free-standing thin-film LED exhibits ≈66% luminescence enhancement with good reliability compared to that before transfer. This work proposes a new approach for the development of flexible semiconductor devices.

Phosphor-free micro-pyramid InGaN-based white light-emitting diode with a high color rendering index on a ß-Ga2O3 substrate.

We demonstrate the InGaN/GaN-based monolithic micro-pyramid white (MPW) vertical LED (VLED) grown on (-201)-oriented ß-Ga2O3 substrate by selective area growth. The transmission electron microscopy (TEM) reveals an almost defect-free GaN pyramid structure on (10-11) sidewalls, including stacked dual-wavelength multi-quantum wells (MQWs). From the electroluminescence (EL) spectra of the fabricated MPW VLED, a white light emission with a high color rendering index (CRI) of 97.4 is achieved. Furthermore, the simulation shows that the light extraction efficiency (LEE) of the MPW VLED is at least 4 times higher compared with the conventional planar LED. These results show that the MPW VLED grown on ß-Ga2O3 has great potential for highly efficient phosphor-free white light emission.

Determining the degree of chromosomal instability in breast cancer cells by atomic force microscopy.

Chromosomal instability (CIN) is a source of genetic variation and is highly linked to the malignance of cancer. Determining the degree of CIN is necessary for understanding the role that it plays in tumor development. There is currently a lack of research on high-resolution characterization of CIN and the relationship between CIN and cell mechanics. Here, a method to determine CIN of breast cancer cells by high resolution imaging with atomic force microscopy (AFM) is explored. The numerical and structural changes of chromosomes in human breast cells (MCF-10A), moderately malignant breast cells (MCF-7) and highly malignant breast cells (MDA-MB-231) were observed and analyzed by AFM. Meanwhile, the nuclei, cytoskeleton and cell mechanics of the three kinds of cells were also investigated. The results showed the differences in CIN between the benign and cancer cells. Also, the degree of structural CIN increased with enhanced malignancy of cancer cells. This was also demonstrated by calculating the probability of micronucleus formation in these three kinds of cells. Meanwhile, we found that the area of the nucleus was related to the number of chromosomes in the nucleus. In addition, reduced or even aggregated actin fibers led to decreased elasticities in MCF-7 and MDA-MB-231 cells. It was found that the rearrangement of actin fibers would affect the nucleus, and then lead to wrong mitosis and CIN. Using AFM to detect chromosomal changes in cells with different malignancy degrees provides a new detection method for the study of cell carcinogenesis with a perspective for targeted therapy of cancer.

Cell recognition based on atomic force microscopy and modified residual neural network.

Cell recognition methods are in high demand in cell biology and medicine, and the method based on atomic force microscopy (AFM) shows a great value in application. The difference in mechanical properties or morphology of cells has been frequently used to detect whether cells are cancerous, but this detection method cannot be a general means for cancer cell detection, and the traditional artificial feature extraction method also has its limitations. In this work, we proposed an analytic method based on the physical properties of cells and deep learning method for recognizing cell types. The residual neural network used for recognition was modified by multi-scale convolutional fusion, attention mechanism and depthwise separable convolution, so as to optimize feature extraction and reduce operation costs. In the method, the collected cells were imaged by AFM, and the processed images were analyzed by the optimized convolutional neural network. The recognition results of two groups of cells (HL-7702 and SMMC-7721, SGC-7901 and GES-1) by this method show that the recognition rate of dataset with the combination of cell surface morphology, adhesion and Young's modulus is higher, and the recognition rate of the dataset with optimal resolution is higher. Our study indicated that the recognition of physical properties of cells using deep learning technology can serve as a universal and effective method for the automated analysis of cell information.

Plasmon-enhanced deep ultraviolet Micro-LED arrays for solar-blind communications.

Localized surface plasmon resonance (LSPR)-enhanced deep ultraviolet (DUV) Micro-light emitting diodes (Micro-LEDs) using Al nanotriangle arrays (NTAs) are reported for improving the -3 dB modulation bandwidth. Through self-assembled nanospheres, the high-density Al NTAs arrays are transferred into the designated p-AlGaN region of the Micro-LEDs, realizing the effect of LSPR coupling. A 2.5-fold enhancement in photoluminescence (PL) intensity is demonstrated. Combined with the PL intensity ratio at 300 K and 10 K, internal quantum efficiency (IQE) may be increased about 15-20% by the plasmonic effect and the carrier lifetime decreases from 1.15 ns to 0.82 ns, suggesting that LSPR accelerates the spontaneous emission rate. Resulting from the improvement of the IQE, the electroluminescence intensity of Micro-LED arrays with LSPR is obviously increased. Meanwhile, the -3 dB bandwidth of 6 × 6 Micro-LED arrays is increased from 180 MHz to 300 MHz at a current density of 200 A/cm2. A potential way is proposed to further increase both the IQE and the modulation bandwidth of DUV Micro-LEDs.

Use of Atomic Force Microscopy in UVB-Induced Chromosome Damage Provides Important Bioinformation for Cell Damage Assessment.

The chromosomal structure derived from UVB-stimulated HaCaT cells was detected by atomic force microscopy (AFM) to evaluate the effect of UVB irradiation. The results showed that the higher the UVB irradiation dose, the more the cells that had chromosome aberration. At the same time, different representative types of chromosome structural aberrations were investigated. We also revealed damage to both DNA and cells under the corresponding irradiation doses. It was found that the degree of DNA damage was directly proportional to the irradiation dose. The mechanical properties of cells were also changed after UVB irradiation, suggesting that cells experienced a series of chain reactions from inside to outside after irradiation. The high-resolution imaging of chromosome structures by AFM after UVB irradiation enables us to relate the damage between chromosomes, DNA, and cells caused by UVB irradiation and provides specific information on genetic effects.

Atomic Mechanism of Strain Alleviation and Dislocation Reduction in Highly Mismatched Remote Heteroepitaxy Using a Graphene Interlayer.

Remote heteroepitaxy is known to yield semiconductor films with better quality. However, the atomic mechanisms in systems with large mismatches are still unclear. Herein, low-strain single-crystalline nitride films are achieved on highly mismatched (∼16.3%) sapphire via graphene-assisted remote heteroepitaxy. Because of a weaker interface potential, the in-plane compressive strain at the interface releases by 30%, and dislocations are prevented. Meanwhile, the lattice distortions in the epilayer disappear when the structure climbs over the atomic steps on substrates because graphene renders the steps smooth. In this way, the density of edge dislocations in as-grown nitride films reduces to the same level as that of the screw dislocations, which is rarely observed in heteroepitaxy. Further, the indium composition in InxGa1-xN/GaN multiquantum wells increases to ∼32%, enabling the fabrication of a yellow light-emitting diode. This study demonstrates the advantages of remote heteroepitaxy for bandgap tuning and opens opportunities for photoelectronic and electronic applications.

Atomic-Scale Mechanism of Spontaneous Polarity Inversion in AlN on Nonpolar Sapphire Substrate Grown by MOCVD.

The performance of nitride devices is strongly affected by their polarity. Understanding the polarity determination and evolution mechanism of polar wurtzite nitrides on nonpolar substrates is therefore critically important. This work confirms that the polarity of AlN on sapphire prepared by metal-organic chemical vapor deposition is not inherited from the nitrides/sapphire interface as widely accepted, instead, experiences a spontaneous polarity inversion during the growth. It is found that at the initial growth stage, the interface favors the nitrogen-polarity, rather than the widely accepted metal-polarity or randomly coexisting. However, the polarity subsequently converts into the metal-polar situation, at first locally then expanding into the whole area, driven by the anisotropy of surface energies, which results in universally existing inherent inverse grain boundaries. Furthermore, vertical two-dimensional electron accumulation originating from the lattice symmetry breaking at the inverse grain boundary is first revealed. This work identifies another cause of high-density defects in nitride epilayers, besides lattice mismatch induced dislocations. These findings also offer new insights into atomic structure and determination mechanism of polarity in nitrides, providing clues for its manipulation toward the novel hetero-polarity devices.

Continuous Single-Crystalline GaN Film Grown on WS2 -Glass Wafer.

Use of 2D materials as buffer layers has prospects in nitride epitaxy on symmetry mismatched substrates. However, the control of lattice arrangement via 2D materials at the heterointerface presents certain challenges. In this study, the epitaxy of single-crystalline GaN film on WS2 -glass wafer is successfully performed by using the strong polarity of WS2 buffer layer and its perfectly matching lattice geometry with GaN. Furthermore, this study reveals that the first interfacial nitrogen layer plays a crucial role in the well-constructed interface by sharing electrons with both Ga and S atoms, enabling the single-crystalline stress-free GaN, as well as a violet light-emitting diode. This study paves a way for the heterogeneous integration of semiconductors and creates opportunities to break through the design and performance limitations, which are induced by substrate restriction, of the devices.

GaN-based parallel micro-light-emitting diode arrays with dual-wavelength InxGa1-xN/GaN MQWs for visible light communication.

The dual-wavelength InxGa1-xN/GaN micro light emitting diode (Micro-LED) arrays are fabricated by flip-chip parallel connection. It is noted that the Micro-LED arrays with smaller diameter present considerably bigger light output power density (LOPD). For all Micro-LEDs, the LOPD increases continuously with increasing injection current density until it "turns over". It also can be observed that the maximum value of LOPD is determined by the blue quantum well (QW) for the broad area LED. In comparison, the green peak intensity dominates the change of LOPD in the Micro-LEDs. In addition, the enhancement of the green peak intensity value for the Micro-LEDs are considered as a consequence of the combined effects of the reduction in the quantum-confined Stark effect (QCSE) and the crowding effect, high LEE as well as geometric shape. Moreover, -3dB modulation bandwidths of the four different kinds of Micro-LEDs increase with the decrease of the device diameter in the same injected current density, higher than that of the broad area LED. The -3dB modulation bandwidth of the 60 µm Micro-LED shows 1.4 times enhancement compared to that of the broad area LED under the current density of 300 mA/cm2. Evidently, the dual-wavelength InxGa1-xN/GaN Micro-LEDs have great potential in both solid-state lighting (SSL) and the visible light communication (VLC) in the future fabrication.

Transparent dual-band ultraviolet photodetector based on graphene/p-GaN/AlGaN heterojunction.

Versatile applications have driven a desire for dual-band detection that enables seeing objects in multiple wavebands through a single photodetector. In this paper, a concept of using graphene/p-GaN Schottky heterojunction on top of a regular AlGaN-based p-i-n mesa photodiode is reported for achieving solar-/visible-blind dual-band (275 nm and 365 nm) ultraviolet photodetector with high performance. The highly transparent graphene in the front side and the polished sapphire substrate at the back side allows both top illumination and back illumination for the dual band detection. A system limit dark current of 1×10-9 A/cm2 at a negative bias voltage up to -10 V has been achieved, while the maximum detectivity obtained from the detection wavebands of interests at 275 nm and 365 nm are ∼ 9.0 ×1012 cm·Hz1/2/W at -7.5 V and ∼8.0 × 1011 cm·Hz1/2/W at +10 V, respectively. Interestingly, this new type of photodetector is dual-functional, capable of working as either photodiode or photoconductor, when switched by simply adjusting the regimes of bias voltage applied on the devices. By selecting proper bias, the device operation mode would switch between a high-speed photodiode and a high-gain photoconductor. The device exhibits a minimum rise time of ∼210 µs when working as a photodiode and a maximum responsivity of 300 A/W at 6 µW/cm2 when working as a photoconductor. This dual band and multi-functional design would greatly extend the utility of detectors based on nitrides.

Three dimensional truncated-hexagonal-pyramid vertical InGaN-based white light emitting diodes based on ß-Ga2O3.

In this Letter, we describe the fabrication of three dimensional (3D) truncated-hexagonal-pyramid (THP) vertical light emitting diodes (VLEDs) with white emission grown on ß-Ga2O3 substrate. In the 3D n-GaN layer, it is noted that the longitudinal growth rate of the 3D n-GaN layer increases as the flow rate of N2 decreases and H2 increases. Moreover, the 3D THP VLED can effectively suppress the quantum-confined Stark effect (QCSE) compared with planar VLEDs due to the semipolar facets and strain relaxation. Thus, the internal quantum efficiency (IQE) of the 3D THP VLED has been doubled and the V-shaped pits have been greatly reduced. In particular, the 3D THP VLED enables multi-wavelength emission (448.0 nm and 498.5 nm) and also shows better light extraction efficiency (LEE), which presents an effective way for the realization of phosphor-free white LED devices.

Strain modulated luminescence in green InGaN/GaN multiple quantum wells with microwire array by piezo-phototronic effect.

We have demonstrated piezo-phototronic enhanced modulation in green InGaN/GaN multiple quantum well (MQW) light-emitting diodes (LEDs) with a microwire array (MWA) structure. It is found that an a-axis oriented MWA structure induces more c-axis compressive strain than a flat structure when a convex bending strain is applied. Moreover, the photoluminescence (PL) intensity exhibits a tendency to increase first and then decrease under the enhanced compressive strain. Specifically, light intensity reaches a maximum of about 123% accompanied by 1.1-nm blueshift, and the carrier lifetime comes to the minimum simultaneously. The enhanced luminescence characteristics are attributed to strain-induced interface polarized charges, which modulate the built-in field in InGaN/GaN MQWs and could promote the radiative recombination of carriers. This work opens a pathway to drastically improve InGaN-based long-wavelength micro-LEDs with highly efficient piezo-phototronic modulation.

Fundamental linewidth of an AlN microcavity Raman laser.

Raman lasing can be a promising way to generate highly coherent chip-based lasers, especially in high-quality (high-Q) crystalline microcavities. Here, we measure the fundamental linewidth of a stimulated Raman laser in an aluminum nitride (AlN)-on-sapphire microcavity with a record Q-factor up to 3.7 million. An inverse relationship between fundamental linewidth and emission power is observed. A limit of the fundamental linewidth, independent of Q-factor, due to Raman-pump-induced Kerr parametric oscillation is derived.

Graphene-Nanorod Enhanced Quasi-Van Der Waals Epitaxy for High Indium Composition Nitride Films.

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.

Effects of remote sediment phosphor plates on high power laser-based white light sources.

Phosphor-converted blue laser diodes are regarded as the next-generation high-brightness solid-state lighting sources. However, it is difficult to obtain white light with high angular color uniformity due to the Gaussian distribution of the laser light sources. Meanwhile, laser excitation power density of the light source is high, which would bring serious heating effects to the phosphor layers. In this study, a strategy has been proposed to solve the problem by using remote sediment phosphor plates. In detail, we have compared the effects of remote sediment/non-sediment phosphor plates to the phosphor-converted blue laser diodes on the overall light output characteristics, angular optical distribution properties, as well as their thermal performance. The emission from sediment phosphor samples has been found more divergent, and angular deviation in the correlated color temperature of the emitted light could be greatly reduced from 1486 to 294 K, yet with only 5% luminous flux loss, as compared to non-sediment phosphor samples. Most importantly, the sediment phosphor sample pushes the power damage threshold up to 588.1 W/cm2 (non-sediment sample: 512.3 W/cm2). Our work has demonstrated the sediment phosphor plates would ameliorate the angular color uniformity for the laser-based lighting source, while extending its lifespan with improved thermal stability.

Self-frequency shift of AlN-on-sapphire Kerr solitons.

We study the self-frequency shift of continuously pumped Kerr solitons in AlN-on-sapphire microcavities with Raman gain bandwidths narrower than the cavity free-spectral range. Solitons are generated in ∼230GHz microcavities via high-order mode dispersion engineering. The dependence of the self-frequency shift on soliton pulse width is measured and differs from amorphous material microcavities. Our measurement and simulation reveal the impact of frequency detuning between the cavity resonances and Raman gain peaks, as well as the importance of all three Raman gain peaks. The interplay between the Raman effect and dispersive wave recoil and a potential quiet point are also observed.