Browsing by Author "Zhang X."
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Item Open Access Beyond the 'East-West' dichotomy: Global variation in cultural models of selfhood(American Psychological Association Inc., 2016) Vignoles, V. L.; Owe, E.; Becker, M.; Smith, P. B.; Easterbrook, M. J.; Brown, R.; González, R.; Didier, N.; Carrasco, D.; Cadena, M. P.; Lay, S.; Schwartz, S. J.; Rosiers, S. E. D.; Villamar, J. A.; Gavreliuc, A.; Zinkeng, M.; Kreuzbauer, R.; Baguma, P.; Martin, M.; Tatarko, A.; Herman, G.; de Sauvage, I.; Courtois, M.; Gardarsdóttir, R. B.; Harb, C.; Gallo, I. S.; Gil, P. P.; Clemares, R. L.; Campara, G.; Nizharadze, G.; Macapagal, M. E. J.; Jalal, B.; Bourguignon, D.; Zhang, J.; Lv, S.; Chybicka, A.; Yuki, M.; Zhang X.; Espinosa, A.; Valk, A.; Abuhamdeh, S.; Amponsah, B.; Özgen E.; Guner, E. Ü.; Yamakoglu, N.; Chobthamkit, P.; Pyszczynski, T.; Kesebir, P.; Trujillo, E. V.; Balanta, P.; Ayala, B. C.; Koller, S. H.; Jaafar, J. L.; Gausel, N.; Fischer, R.; Milfont, T. L.; Kusdil, E.; Çağlar, S.; Aldhafri, S.; Ferreira, M. C.; Mekonnen, K. H.; Wang, Q.; Fülöp, M.; Torres, A.; Camino, L.; Lemos, F. C. S.; Fritsche, I.; Möller, B.; Regalia, C.; Manzi, C.; Brambilla, M.; Bond, M. H.Markus and Kitayama's (1991) theory of independent and interdependent self-construals had a major influence on social, personality, and developmental psychology by highlighting the role of culture in psychological processes. However, research has relied excessively on contrasts between North American and East Asian samples, and commonly used self-report measures of independence and interdependence frequently fail to show predicted cultural differences. We revisited the conceptualization and measurement of independent and interdependent self-construals in 2 large-scale multinational surveys, using improved methods for cross-cultural research. We developed (Study 1: N = 2924 students in 16 nations) and validated across cultures (Study 2: N = 7279 adults from 55 cultural groups in 33 nations) a new 7-dimensional model of self-reported ways of being independent or interdependent. Patterns of global variation support some of Markus and Kitayama's predictions, but a simple contrast between independence and interdependence does not adequately capture the diverse models of selfhood that prevail in different world regions. Cultural groups emphasize different ways of being both independent and interdependent, depending on individualism-collectivism, national socioeconomic development, and religious heritage. Our 7-dimensional model will allow future researchers to test more accurately the implications of cultural models of selfhood for psychological processes in diverse ecocultural contexts. © 2016 American Psychological Association.Item Open Access Comparative study of field-dependent carrier dynamics and emission kinetics of InGaN/GaN light-emitting diodes grown on (11 2-2) semipolar versus (0001) polar planes(AIP Publishing, 2014) Ji Y.; Liu W.; Erdem, T.; Chen R.; Tan S.T.; Zhang Z.-H.; Ju, Z.; Zhang X.; Sun, H.; Sun, X. W.; Zhao Y.; DenBaars, S. P.; Nakamura, S.; Demir, Hilmi VolkanThe characteristics of electroluminescence (EL) and photoluminescence (PL) emission from GaN light-emitting diodes (LEDs) grown on (11 (2) over bar2) semipolar plane and (0001) polar plane have been comparatively investigated. Through different bias-dependent shifting trends observed from the PL and time-resolved PL spectra (TRPL) for the two types of LEDs, the carrier dynamics within the multiple quantum wells (MQWs) region is systematically analyzed and the distinct field-dependent emission kinetics are revealed. Moreover, the polarization induced internal electric field has been deduced for each of the LEDs. The relatively stable emission behavior observed in the semipolar LED is attributed to the smaller polarization induced internal electric field. The study provides meaningful insight for the design of quantum well (QW) structures with high radiative recombination rates.Item Open Access Contextualism as an important facet of individualism-collectivism: personhood beliefs across 37 national groups(Sage Publications, Inc., 2013) Owe, E.; Vignoles, V. L.; Becker, M.; Brown, R.; Smith, P .B.; Lee, S. W. S.; Easterbrook, M.; Gadre, T.; Zhang X.; Gheorghiu, M.; Baguma, P.; Tatarko, A.; Aldhafri, S.; Zinkeng, M.; Schwartz, S. J.; Des Rosiers, S. E.; Villamar, J. A.; Mekonnen, K. H.; Regalia, C.; Manzi, C.; Brambilla, M.; Kusdil, E.; Çağlar, S.; Gavreliuc, A.; Martin, M.; Jianxin, Z.; Lv, S.; Fischer, R.; Milfont, T. L.; Torres, A.; Camino, L.; Kreuzbauer, R.; Gausel, N.; Buitendach, J. H.; Lemos, F. C. S.; Fritsche, I.; Möller, B.; Harb, C.; Valk, A.; Espinosa, A.; Jaafar, J. L.; Yuki, M.; Ferreira, M. C.; Chobthamkit, P.; Fülöp, M.; Chybicka, A.; Wang, Q.; Bond, M. H.; González, R.; Didier, N.; Carrasco, D.; Cadena, M. P.; Lay, S.; Gardarsdóttir, R. B.; Nizharadze, G.; Pyszczynski, T.; Kesebir, P.; Herman, G.; Sauvage, Isabelle de; Courtois, M.; Bourguignon, D.; Özgen, E.; Güner Ü. E.; Yamakoğlu, N.; Abuhamdeh, S.; Mogaji, A.; Macapagal, M. E. J.; Koller, S. H.; Amponsah, B.; Misra, G.; Kapur, P.; Trujillo, E. V.; Balanta, P.; Ayala, B. C.; Gallo, I. S.; Gil, P. P.; Clemares, R. L.; Campara, G.; Jalal, B.Beliefs about personhood are understood to be a defining feature of individualism-collectivism (I-C), but they have been insufficiently explored, given the emphasis of research on values and self-construals. We propose the construct of contextualism, referring to beliefs about the importance of context in understanding people, as a facet of cultural collectivism. A brief measure was developed and refined across 19 nations (Study 1: N = 5,241), showing good psychometric properties for cross-cultural use and correlating well at the nation level with other supposed facets and indicators of I-C. In Study 2 (N = 8,652), nation-level contextualism predicted ingroup favoritism, corruption, and differential trust of ingroup and outgroup members, while controlling for other facets of I-C, across 35 nations. We conclude that contextualism is an important part of cultural collectivism. This highlights the importance of beliefs alongside values and self-representations and contributes to a wider understanding of cultural processes. © The Author(s) 2013.Item Open Access Decoupling contact and mirror: an effective way to improve the reflector for flip-chip InGaN/GaN-based light-emitting diodes(Institute of Physics Publishing, 2016) Zhu B.; Liu W.; Lu S.; Zhang, Y.; Hasanov N.; Zhang X.; Ji Y.; Zhang Z.-H.; Tan S.T.; Liu, H.; Demir, Hilmi VolkanIn the conventional fabrication process of the widely-adopted Ni/Ag/Ti/Au reflector for InGaN/GaN-based flip-chip light-emitting diodes (LEDs), the contact and the mirror are entangled together with contrary processing conditions which set constraints to the device performance severely. Here we first report the concept and its effectiveness of decoupling the contact formation and the mirror construction. The ohmic contact is first formed by depositing and annealing an extremely thin layer of Ni/Ag on top of p-GaN. The mirror construction is then carried out by depositing thick layer of Ag/Ti/Au without any annealing. Compared with the conventional fabrication method of the reflector, by which the whole stack of Ni/Ag/Ti/Au is deposited and annealed together, the optical output power is improved by more than 70% at 350 mA without compromising the electrical performance. The mechanism of decoupling the contact and the mirror is analyzed with the assistance of contactless sheet resistance measurement and secondary ion mass spectrometry (SIMS) depth profile analysis. © 2016 IOP Publishing Ltd.Item Open Access High-efficiency and low-loss gallium nitride dielectric metasurfaces for nanophotonics at visible wavelengths(American Institute of Physics Inc., 2017) Emani, N. K.; Khaidarov, E.; Paniagua-Domínguez, R.; Fu, Y. H.; Valuckas, V.; Lu S.; Zhang X.; Tan S.T.; Demir, Hilmi Volkan; Kuznetsov, A. I.The dielectric nanophotonics research community is currently exploring transparent material platforms (e.g., TiO2, Si3N4, and GaP) to realize compact high efficiency optical devices at visible wavelengths. Efficient visible-light operation is key to integrating atomic quantum systems for future quantum computing. Gallium nitride (GaN), a III-V semiconductor which is highly transparent at visible wavelengths, is a promising material choice for active, nonlinear, and quantum nanophotonic applications. Here, we present the design and experimental realization of high efficiency beam deflecting and polarization beam splitting metasurfaces consisting of GaN nanostructures etched on the GaN epitaxial substrate itself. We demonstrate a polarization insensitive beam deflecting metasurface with 64% and 90% absolute and relative efficiencies. Further, a polarization beam splitter with an extinction ratio of 8.6/1 (6.2/1) and a transmission of 73% (67%) for p-polarization (s-polarization) is implemented to demonstrate the broad functionality that can be realized on this platform. The metasurfaces in our work exhibit a broadband response in the blue wavelength range of 430-470 nm. This nanophotonic platform of GaN shows the way to off- and on-chip nonlinear and quantum photonic devices working efficiently at blue emission wavelengths common to many atomic quantum emitters such as Ca+ and Sr+ ions.Item Open Access Improving hole injection efficiency by manipulating the hole transport mechanism through p-type electron blocking layer engineering(Optical Society of America, 2014) Zhang, Zi-Hui; Ju, Z.; Liu W.; Tan S.T.; Ji Y.; Kyaw, Z.; Zhang X.; Hasanov N.; Sun, X. W.; Demir, Hilmi VolkanThe p-type AlGaN electron blocking layer (EBL) is widely used in InGaN/GaN light-emitting diodes (LEDs) for electron overflow suppression. However, a typical EBL also reduces the hole injection efficiency, because holes have to climb over the energy barrier generated at the p-AlGaN/p-GaN interface before entering the quantum wells. In this work, to address this problem, we report the enhancement of hole injection efficiency by manipulating the hole transport mechanism through insertion of a thin GaN layer of 1 nm into the p-AlGaN EBL and propose an AlGaN/GaN/AlGaN-type EBL outperforming conventional AlGaN EBLs. Here, the position of the inserted thin GaN layer relative to the p-GaN region is found to be the key to enhancing the hole injection efficiency. InGaN/ GaN LEDs with the proposed p-type AlGaN/GaN/AlGaN EBL have demonstrated substantially higher optical output power and external quantum efficiency.Item Open Access Integromic analysis of genetic variation and gene expression identifies networks for cardiovascular disease phenotypes(Lippincott Williams & Wilkins, 2015) Yao, C.; Chen, B. H.; Joehanes, R.; Otlu, B.; Zhang X.; Liu, C.; Huan, T.; Tastan, O.; Cupples, L. A.; Meigs, J. B.; Fox, C. S.; Freedman, J. E.; Courchesne, P.; O'Donnell, C. J.; Munson, P. J.; Keles, S.; Levy, D.BACKGROUND - : Cardiovascular disease (CVD) reflects a highly coordinated complex of traits. Although genome-wide association studies have reported numerous single nucleotide polymorphisms (SNPs) to be associated with CVD, the role of most of these variants in disease processes remains unknown. METHODS AND RESULTS - : We built a CVD network using 1512 SNPs associated with 21 CVD traits in genome-wide association studies (at P≤5×10) and cross-linked different traits by virtue of their shared SNP associations. We then explored whole blood gene expression in relation to these SNPs in 5257 participants in the Framingham Heart Study. At a false discovery rate <0.05, we identified 370 cis-expression quantitative trait loci (eQTLs; SNPs associated with altered expression of nearby genes) and 44 trans-eQTLs (SNPs associated with altered expression of remote genes). The eQTL network revealed 13 CVD-related modules. Searching for association of eQTL genes with CVD risk factors (lipids, blood pressure, fasting blood glucose, and body mass index) in the same individuals, we found examples in which the expression of eQTL genes was significantly associated with these CVD phenotypes. In addition, mediation tests suggested that a subset of SNPs previously associated with CVD phenotypes in genome-wide association studies may exert their function by altering expression of eQTL genes (eg, LDLR and PCSK7), which in turn may promote interindividual variation in phenotypes. CONCLUSIONS - : Using a network approach to analyze CVD traits, we identified complex networks of SNP-phenotype and SNP-transcript connections. Integrating the CVD network with phenotypic data, we identified biological pathways that may provide insights into potential drug targets for treatment or prevention of CVD.Item Open Access Low thermal-mass LEDs: Size effect and limits(Optical Society of American (OSA), 2014) Lu, S.; Liu W.; Zhang, Z.-H.; Tan, S.T.; Ju, Z.; Ji, Y.; Zhang X.; Zhang, Y.; Zhu, B.; Kyaw, Z.; Hasanov, N.; Sun X.W.; Demir, Hilmi VolkanIn this work, low thermal-mass LEDs (LTM-LEDs) were developed and demonstrated in flip-chip configuration, studying both experimentally and theoretically the enhanced electrical and optical characteristics and the limits. LTM-LED chips in 25 × 25 μm2, 50 × 50 μm2, 100 × 100 μm2 and 200 × 200 μm2 mesa sizes were fabricated and comparatively investigated. Here it was revealed that both the electrical and optical properties are improved by the decreasing chip size due to the reduced thermal mass. With a smaller chip size (from 200 μm to 50 μm), the device generally presents higher current density against the bias and higher power density against the current density. However, the 25 × 25 μm2 device behaves differently, limited by the fabrication margin limit of 10 μm. The underneath mechanisms of these observations are uncovered, and furthermore, based on the device model, it is proven that for a specific flip-chip fabrication process, the ideal size for LTM-LEDs with optimal power density performance can be identified. ©2014 Optical Society of AmericaItem Open Access On the origin of the electron blocking effect by an n-type AlGaN electron blocking layer(AIP Publishing, 2014) Zhang Z.-H.; Ji Y.; Liu W.; Tan S.T.; Kyaw, Z.; Ju, Z.; Zhang X.; Hasanov N.; Lu S.; Zhang, Y.; Zhu B.; Sun, X. W.; Demir, Hilmi VolkanIn this work, the origin of electron blocking effect of n-type Al 0.25Ga0.75N electron blocking layer (EBL) for c+ InGaN/GaN light-emitting diodes has been investigated through dual-wavelength emission method. It is found that the strong polarization induced electric field within the n-EBL reduces the thermal velocity and correspondingly the mean free path of the hot electrons. As a result, the electron capture efficiency of the multiple quantum wells is enhanced, which significantly reduces the electron overflow from the active region and increases the radiative recombination rate with holes. © 2014 AIP Publishing LLC.Item Open Access P-doping-free InGaN/GaN light-emitting diode driven by three-dimensional hole gas(2013) Zhang, Z.-H.; Tiam Tan, S.; Kyaw, Z.; Liu W.; Ji, Y.; Ju, Z.; Zhang X.; Wei Sun X.; Volkan Demir H.Here, GaN/AlxGa1-xN heterostructures with a graded AlN composition, completely lacking external p-doping, are designed and grown using metal-organic-chemical-vapour deposition (MOCVD) system to realize three-dimensional hole gas (3DHG). The existence of the 3DHG is confirmed by capacitance-voltage measurements. Based on this design, a p-doping-free InGaN/GaN light-emitting diode (LED) driven by the 3DHG is proposed and grown using MOCVD. The electroluminescence, which is attributed to the radiative recombination of injected electrons and holes in InGaN/GaN quantum wells, is observed from the fabricated p-doping-free devices. These results suggest that the 3DHG can be an alternative hole source for InGaN/GaN LEDs besides common Mg dopants. © 2013 AIP Publishing LLC.Item Open Access Polarization self-screening in [0001] oriented InGaN/GaN light-emitting diodes for improving the electron injection efficiency(AIP Publishing, 2014) Zhang Z.-H.; Liu W.; Ju, Z.; Tan S.T.; Ji Y.; Zhang X.; Wang, L.; Kyaw, Z.; Sun, X. W.; Demir, Hilmi VolkanInGaN/GaN light-emitting diodes (LEDs) grown along the [0001] orientation inherit very strong polarization induced electric fields. This results in a reduced effective conduction band barrier height for the p-type AlGaN electron blocking layer (EBL) and makes the electron blocking effect relatively ineffective and the electron injection efficiency drops. Here, we show the concept of polarization self-screening for improving the electron injection efficiency. In this work, the proposed polarization self-screening effect was studied and proven through growing a p-type EBL with AlN composition partially graded along the [0001] orientation, which induces the bulk polarization charges. These bulk polarization charges are utilized to effectively self-screen the positive polarization induced interface charges located at the interface between the EBL and the last quantum barrier when designed properly. Using this approach, the electron leakage is suppressed and the LED performance is enhanced significantly.Item Open Access Self-screening of the quantum confined Stark effect by the polarization induced bulk charges in the quantum barriers(AIP Publishing, 2014) Zhang Z.-H.; Li, W.; Ju, Z.; Tan S.T.; Ji Y.; Kyaw, Z.; Zhang X.; Wang, L.; Sun, X. W.; Demir, Hilmi VolkanInGaN/GaN light-emitting diodes (LEDs) grown along the polar orientations significantly suffer from the quantum confined Stark effect (QCSE) caused by the strong polarization induced electric field in the quantum wells, which is a fundamental problem intrinsic to the III-nitrides. Here, we show that the QCSE is self-screened by the polarization induced bulk charges enabled by designing quantum barriers. The InN composition of the InGaN quantum barrier graded along the growth orientation opportunely generates the polarization induced bulk charges in the quantum barrier, which well compensate the polarization induced interface charges, thus avoiding the electric field in the quantum wells. Consequently, the optical output power and the external quantum efficiency are substantially improved for the LEDs. The ability to self-screen the QCSE using polarization induced bulk charges opens up new possibilities for device engineering of III-nitrides not only in LEDs but also in other optoelectronic devices.