Journal of Physics D: Applied Physics

Zakharov, Vasily S; Wang, Xinbing; Zakharov, Sergey V; Zuo, Duluo

doi: 10.1088/1361-6463/ac8ffepmid: N/A

A laser-produced plasma excited by CO2 laser pulses with various durations and energies on liquid tin droplets with diameters of 150 μm and 180 μm is considered. A two-dimensional radiative-magnetohydrodynamic code is used for numerical simulations of multicharged ion plasma radiation and dynamics. The code permits to understand the plasma dynamics self-consistent with radiation transport in non-local equilibrium multicharged ion plasma. Results of simulations for various laser pulse durations and 75 ÷ 600 mJ pulse energies with both Gaussian and experimentally taken temporal profiles are discussed. It is found that if the mass of the target is big enough to provide the plasma flux required (the considered case) a kind of dynamic quasi-stationary plasma flux is formed. In this dynamic quasi-stationary plasma flux, an interlayer of relatively cold tin vapor with mass density of 1 ÷ 2 g cm−3 is formed between the liquid tin droplet and low density plasma of the critical layer. Expanding of the tin vapor from the droplet provides the plasma flux to the critical layer. In critical layer the plasma is heated up and expands faster. In the simulation results with spherical liquid tin target, the conversion efficiency into 2π is of 4% for 30 ns full width half maximum (FWHM) and just slightly lower—of 3.67% for 240 ns FWHM for equal laser intensities of 14 GW cm−2. This slight decay of the in-band extreme ultraviolet (EUV) yield with laser pulse duration is conditioned by an increasing of radiation re-absorption by expanding plasma from the target, as more cold plasma is produced with longer pulse. The calculated angular distributions of in-band EUV emission permit to optimize a collector configuration.

Song, Lihui; Hu, Zechen; Lin, Dehang; Yang, Deren; Yu, Xuegong

doi: 10.1088/1361-6463/ac9066pmid: N/A

Crystalline silicon solar cells are always moving towards ‘high efficiency and low cost’, which requires continuously improving the quality of crystalline silicon materials. Nevertheless, crystalline silicon materials typically contain various kinds of impurities and defects, which act as carrier recombination centers. Therefore these impurities and defects must be well controlled during the solar cell fabrication processes to improve the cell efficiency. Hydrogenation of crystalline silicon is one important method to deactivate these impurities and defects, which is so-called ‘hydrogenation engineering’ in this paper. Hydrogen is widely reported to be able to passivate diverse defects like crystallographic defects, metallic impurities, boron-oxygen related defects and etc, but the effectiveness of hydrogen passivation depends strongly on the processing conditions. Moreover, in this decade, advanced hydrogenation technique has been developed and widely applied in the photovoltaic industry to significantly improve the performance of silicon solar cells. As the research on hydrogenation study has made a significant progress, it is the right time to write a review paper on introducing the state-of-the-art hydrogenation study and its applications in photovoltaic industry. The paper first introduces the fundamental properties of hydrogen in crystalline silicon and then discusses the applications of hydrogen on deactivating/inducing typical defects (e.g. dislocations, grain boundaries, various metallic impurities, boron–oxygen related defects and light and elevated temperature induced degradation defect) in p- and n-type crystalline silicon, respectively. At last, the benefits of hydrogenation engineering on the next-generation silicon solar cells (e.g. tunnel oxide passivated contact (TOPCon) and silicon heterojunction (SHJ) solar cells) are discussed. Overall, it was found that hydrogen can deactivate most of typical defects (sometimes induce defect) in n- and p-type crystalline silicon, leading to a significant efficiency enhancement in passivated emitter rear contact, TOPCon and SHJ solar cells. In conclusion, the paper aims to assist young researchers to better understand hydrogenation research.

Ota, Yuichi; Imura, Masataka; Banal, Ryan G; Koide, Yasuo

doi: 10.1088/1361-6463/ac8fffpmid: N/A

The natural band alignment of BAlN and BGaN alloys was investigated using the atomic solid-state energy scale approach. The band edge positions relative to the vacuum level were determined for BAlN and BGaN alloys, and the band offset values for each heterostructure were estimated. The results suggest that the natural band alignment of BAlN and BGaN alloys behaves according to the common anion rule. Further, the Schottky barrier height (SBH) was calculated based on the results of band alignment for BAlN and BGaN alloys. The predicted SBH values are expected to be an important guideline for boron nitride and its related alloy device design.

Guan, Sitao; Wang, Yixian; Wu, Jingbo; Lyu, Yangyang; Zhang, Zhiyong; Chen, Jian; Wang, Huabing; Wu, Xinglong

doi: 10.1088/1361-6463/ac8ffdpmid: N/A

High-efficiency terahertz (THz) emission and detection are of great interest because of their promising applications in high-speed communications, biomedicine, and imaging. A previous study has achieved efficient room-temperature THz emission at ∼360 GHz by green-light exciting the lattice symmetric stretching vibrations of ZnO nanoplates self-assembled into ZnO microspheres (MSs). Herein, we explore resonant THz radiation of this kind of ZnO MSs under around 360 GHz excitation. A Fabry–Perot resonant cavity is designed and used to obtain the resonant THz signal. Compared to the case without the ZnO MSs, the THz output powers are enhanced by 1.5 and 3.2 times under two excitations of 356.1 and 375.8 GHz with an input power of 6.5 mW, respectively. Furthermore, it is shown that when a wide frequency THz wave irradiates on the ZnO MSs in the cavity, the output THz signal strength shows an obvious variation with frequency and can thus be utilized to detect the presence of some THz waves with specific frequencies. This work indicates that such self-assembled MSs can not only radiate the enhanced THz waves via a resonator, but also effectively apperceive some specific THz signals as a detector.

Karpenkov, A Yu; Skokov, K P; Dunaeva, G G; Semenova, E M; Lyakhova, M B; Pastushenkov, Yu G

doi: 10.1088/1361-6463/ac90d2pmid: N/A

The performance of sintered permanent magnets with nucleation-type coercivity mechanism is largely governed by the magnetic state of thin surface layers of constituent grains, and a deeper insight into magnetization–demagnetization processes occurring in the shell part of the grains is very important for further improvement of hard magnetic materials. In this work, we used Nd2Fe14B and SmCo5 single crystals as model objects. By applying magneto-optical Kerr microscopy and conventional magnetometry, we compare the magnetization–demagnetization processes occurring in the thin surface layer and in the volume of both single crystals. We show that upon magnetization along the c-axis, the volume of the single crystals saturates in the field, rigorously determined by demagnetization factor of the bulk sample, whereas in the surface layer a magnetic domain structure exists up to 1.88 T in Nd2Fe14B and 1.19 T in SmCo5 regardless of their bulk demagnetization factors. This means that the surface layer with orientation perpendicular to c-axis magnetizes as a thin magnetic film and has an effective demagnetization factor Neff∼ 1. We also show that this effect can be reproduced in the framework of conventional finite element method modeling but the analytical solution of this problem still needs to be found. We believe that our findings can be useful for understanding of the formation of a high coercive state in nucleation-type permanent magnets, where the phenomenological concept of the large effective demagnetization factor Neffplays an important role.

Dong, Yujie; Sun, Xiyu; Li, Yan; Liu, Yi

doi: 10.1088/1361-6463/ac8f56pmid: N/A

Terahertz (THz) metasurfaces have potential research value in high-quality molecular imaging, high-speed broadband communication, biology, etc. In this paper, thermally tunable full space metasurface was proposed and analyzed by using Finite Difference Time Domain method based on Pancharatnam–Berry phase modulation and InSb temperature characteristic. At 220 K, a metasurface composed of InSb elements can simultaneously achieve efficient transmission and reflection of the incident circularly polarized light at 0.8 THz and 1.15 THz, respectively. On the contrary, at 360 K, the metasurface absorbs all of the incident terahertz waves, essentially turning off the incident beam. In addition, dynamically tunable metalenses were proposed and used to generate focused vortex light. The proposed metasurface provides a potential direction for developing an efficiency-tunable PB terahertz device in the future.

Rasskazov, Ilia L; Sonwalkar, Nishikant; Scott Carney, P

doi: 10.1088/1361-6463/ac8ffcpmid: N/A

We suggest a strategy for designing regular 2D arrays of nanoholes (NHs) in metal films with far-field scattering properties similar to that of regular 2D arrays of nanodisks (NDs) with the same periodicity. Full-wave simulations for perfectly conducting, Ag and Au NDs and respectively designed arrays of NHs demonstrate a minor difference between far-field properties either at wavelengths corresponding to Wood–Rayleigh anomalies of the arrays or in a broad wavelength range, depending on the array periodicity and sizes of NDs (NHs). Our results have broad implications in plasmon-enhanced-driven applications, including optoelectronic and photovoltaic devices, where the NH arrays are preferable to be fabricated for nano-structured optics.

Wang, Dan; Mao, Zhangsong; Ye, Zhen; Cai, Yahui; Li, Yun; He, Yongning; Qi, Kangcheng; Xu, Yanan; Jia, Qingqing

doi: 10.1088/1361-6463/ac86dfpmid: N/A

Alumina ceramics used in microwave systems are susceptible to the multiplication of secondary electron emission on the surface due to the influence of resonance between electrons and the radiofrequency electric field, and a detrimental multipactor effect may therefore be triggered. For the alumina-loaded microwave components, it is essential to achieve low secondary electron yield (SEY) on the inserted alumina surfaces to mitigate multipactor. In this work, to achieve an ultralow SEY surface of alumina, two recognized low-SEY treatments were combined. For the primary SEY suppression, a series of microstructures were fabricated on the alumina surfaces with varied porosity and aspect ratio at the hundred-micrometer scale by infrared laser etching. The microstructure with 52.14% porosity and 1.78 aspect ratio showed an excellent low-SEY property, which could suppress the SEY peak value (δm) of alumina from 2.46 to 1.00. For the secondary SEY suppression, the SEY dependence of TiN coating on sputtering parameters was studied, and the lowest δmof 1.19 was achieved when the gas flow ratio of Ar:N2 was 15:7.5. Thereafter, by depositing TiN ceramic coating onto the laser-etched porous samples, an ultralow SEY, with δmof 0.69, was achieved on the alumina surfaces. The simulation work revealed the impact of dielectric surface charge on electron multiplication and revealed a mechanism of using low-SEY surfaces to inhibit multipactor. Some coaxial filters filled with alumina were fabricated for verification; the results revealed that the multipactor threshold increased from 125 W to 425 W after applying the TiN-coated porous alumina, and to 650 W after treating another multipactor-sensitive area with the same low-SEY process. This work developed an advisable method to sharply reduce SEY, which is of great significance for the multipactor mitigation of alumina-loaded microwave components.

Wang, Fu-Feng; Meng, Tian-Hang; Yu, Da-Ren; Ning, Zhong-Xi; Zhu, Xi-Ming

doi: 10.1088/1361-6463/ac90cepmid: N/A

Low-power electric propulsion system has high application prospects. The matching small orifice cathode has become a research hotspot. Through the long-life-test, it is found that the small orifice cathode will also have serious erosion, which seriously affects the reliability of the cathode. This kind of erosion is caused by high-energy ion sputtering, which is usually observed only on the large orifice cathode. There is no clear mechanism research on the cause of such high-energy ions in the small orifice cathode plume region. This paper finds that when the energy of the cathode is mainly concentrated at low frequencies, energetic ions much higher than the discharge voltage will be generated in the cathode plume area. The measurement results of the ion energy distribution in the cathode plume area show that the energy of the ions generated near the keeper increase as the cathode oscillation intensifies. It is found that the oscillation phases of low-energy ions and high-energy ions are inconsistent by measuring the changes of different energy ions during the cathode oscillation period. The oscillation of low-energy ions is in the same phase with the cathode current, while that of high-energy ions is in the same phase with the discharge voltage. This means that the previously neglected oscillation of the cathode at the frequency of tens of kHz can also produce high-energy ions. Therefore, inhibiting the oscillation of this frequency band is of great significance to extend the life of the cathode.

Lee, Seungjun; Lee, Jimo; Nam, Woojin; Yun, Gunsu

doi: 10.1088/1361-6463/ac9000pmid: N/A

Hydroxyl radical (OH•) plays an important role in advanced oxidation processes (AOPs), which are employed to decompose organic pollutants in wastewater treatment. OH• is predominantly produced in AOPs for wastewater treatment via ultraviolet photolysis of hydrogen peroxide (H2O2) or ozone, which is a costly and difficult process. This paper introduces an enhanced OH• production method based on microwave-driven atmospheric pressure plasma with negatively biased water. Fluorescence analysis using terephthalic acid and 2-hydroxyterephthalic acid showed that the OH• concentration in a DC coupled plasma-treated water (PTW) can be increased by 1–2 orders of magnitude compared to the case with microwave plasma only. In addition, we found that there exists an optimal concentration of H2O2 in PTW for the ideal production of OH•. As a test case of AOPs, an Fe(III)-ethylenediaminetetraacetic acid solution containing H2O2 was treated with a DC coupled plasma for 10 min, and more than 80% decomposition was recorded.