K. L. Teo

National University of Singapore, Tumasik, Singapore

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Publications (68)151.53 Total impact

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    ABSTRACT: The cyclotron resonance of polarons in Zn 1− x Mg x O/ZnO heterostructures (with 0.15 < x < 0.22 ) was studied by terahertz time-domain spectroscopy. Low-temperature magnetoconductivity spectra of the 2D electron gas at the Zn 1− x Mg x O/ZnO interface determined the polaron density, mass, and scattering rate. The cyclotron mass of 2D polarons was found to increase significantly with magnetic field B from 0.24 me at B = 2 T to 0.37 me at B = 7.5 T. A nonlinear cyclotron frequency with B was also observed for 3D polarons in ZnO. The findings are discussed in the context of polaron mass renormalization driven by the electron-LO-phonon and electron-acoustic phonon interactions.
    Applied Physics Letters 05/2015; 106(20):202103. DOI:10.1063/1.4921469 · 3.52 Impact Factor
  • K. Zheng · J. L. Zhao · K. S. Leck · K. L. Teo · E. G. Yeo · X. W. Sun
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    ABSTRACT: The improved resistive switching performance of TaOx by introducing ZnO was reported in this paper. By co-sputtering, the ZnTaO x device shows better endurance, lower operating voltage and more uniform resistance distribution. The improvement is mainly due to the more abundant intrinsic defects such as oxygen vacancies which facilitates the formation of conductive filaments in the oxide compound.
    ECS Solid State Letters 05/2014; 3(7):Q36-Q39. DOI:10.1149/2.0101407ssl · 0.78 Impact Factor
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    ABSTRACT: In this study, the origin and transport properties of high mobility carriers of two-dimensional electron gas (2DEG) at ZnMgO/ZnO interface have been investigated. It is found that the observed experimental dependence of carrier sheet density of 2DEG on ZnMgO thickness and Mg composition exhibits an excellent agreement with the theoretical calculations based on the surface charge model. It indicates that the mobile electrons in 2DEG are originated from the donor-like surface states. Moreover, the anomalous periodicity of Shubnikov-de Haas oscillations has been observed and the partial spin polarization feature of 2DEG has also been demonstrated, which exhibits the promising development in oxide spintronics applications. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    physica status solidi (c) 08/2013; 10(10-10):1268-1271. DOI:10.1002/pssc.201200927
  • K. Zheng · X.W. Sun · K.L. Teo
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    ABSTRACT: We report a unidirectional bipolar resistive switching in an n-type GaOx/p-type NiOx heterojunction fabricated by magnetron sputtering at room temperature. The resistive switching (RS) of the heterojunction directly relate with the concentration of intrinsic defects in oxide, such as oxygen vacancies and oxygen ions. Under external electric field, these electromigrated defects accumulate at the pn junction interface and modify the interface barrier, forming or rupturing the filamentary paths between n-GaOx and p-NiOx, leading to the switching between Ohmic and diode characteristics of the device.
    Nanoelectronics Conference (INEC), 2013 IEEE 5th International; 08/2013
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    ABSTRACT: A spin-related photocurrent with swirly distribution and anomalous dependence of the total spin-related photocurrent on the incident angle were observed on spin-polarized two-dimensional gas in a Mg0.2Zn0.8O/ZnO heterostructure under illumination of circular polarized light at room temperature. The ferromagnetic two-dimensional Rashba model was adopted to interpret the results. It is demonstrated that a radial spin current induced by the gradient of the spin-polarized electron density is the origin of the anomalousness. This spin current only exists in spin polarized systems.
    Applied Physics Letters 05/2013; 102(19). DOI:10.1063/1.4805079 · 3.52 Impact Factor
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    ABSTRACT: We report a unidirectional bipolar resistive switching in an n-type GaOx/p-type NiOx heterojunction fabricated by magnetron sputtering at room temperature. The resistive switching behavior coincides with the switching between Ohmic conduction (low resistance) and rectifying behavior (high resistance) of the heterojunction diode. Under external electric field, electromigrated intrinsic defects, such as oxygen vacancies and oxygen ions, accumulate at the pn junction interface and modify the interface barrier, forming or rupturing the filamentary paths between n-GaOx and p-NiOx, leading to the switching between Ohmic and diode characteristics of the device. The device shows good endurance, retention performance, and scaling capability, signaling the potential of a diode-structured resistive switching device for non-volatile memory applications.
    Applied Physics Letters 10/2012; 101(14). DOI:10.1063/1.4757761 · 3.52 Impact Factor
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    ABSTRACT: We report on the high mobility wide electron slabs with enhanced correlation effects by tailoring the polarization effects in a functionally graded ZnMgO/ZnO heterostructures. The characteristics of three-dimensional (3D) spreading electrons are evidenced by the capacitance-voltage profiling and the quantization of 3D Fermi surface in magneto-transport measurements. Despite the weak spin-orbit interaction, such electron slabs are spin-polarized with a large zero-field spin splitting energy, which is induced by the carrier-mediated ferromagnetism. Our results suggest that the vast majority of electrons are localized at the surface magnetic moment which does not allow spin manipulations, and only in the region visited by the itinerant carriers that the ferromagnetic exchange interactions via coupling to the surface local moments contribute to the spin transport. The host ferromagnetism is likely due to the formation of Zn cation vacancies on the surface regime induced by the stabilization mechanism and strain-relaxation in ZnMgO polar ionic surface.
    Scientific Reports 07/2012; 2:533. DOI:10.1038/srep00533 · 5.58 Impact Factor
  • K. Han · N. Tang · J. D. Ye · J. X. Duan · Y. C. Liu · K. L. Teo · B. Shen
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    ABSTRACT: Quantum transport properties of two-dimensional electron gas (2DEG) in undoped MgxZn1−xO/ZnO heterostructures grown by metal organic vapor phase epitaxy have been investigated. A large zero-field spin-splitting energy more than 15 meV in the 2DEG is determined at 1.6 K. Meanwhile, ferromagnetism is observed in the heterostructures. The findings reveal that the 2DEG is spin polarized at zero magnetic fields. It is believed that the exchange interaction between the itinerant electrons in the two-dimensional channel and the magnetic polarons in the MgxZn1−xO barrier around the interface results in the spin polarization of the 2DEG.
    Applied Physics Letters 05/2012; 100(19). DOI:10.1063/1.4711775 · 3.52 Impact Factor
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    ABSTRACT: AlGaN/GaN MOS-HEMTs on a silicon platform were realized using a CMOS-compatible gold-free process. Process modules commonly used in CMOS were used, including gate stack formation, etching modules, etc. Ron of 3 mΩ.cm2 was obtained. Breakdown voltage VBR of 800 V was achieved, the highest for LGD below 10 µm for AlGaN/GaN/Si MOS-HEMTs fabricated using a gold-free process. The devices could be generally useful for cost-competitive power switching circuits with supply voltage in the range of several hundred volts.
    01/2012; DOI:10.1109/VLSI-TSA.2012.6210119
  • Applied Physics Letters 01/2012; 101(14):143110-5. · 3.52 Impact Factor
  • J.D. Ye · S.T. Lim · S.L. Gu · H.H. Tan · C. Jagadish · K.L. Teo
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    ABSTRACT: Magneto-transport studies on ZnMgO/ZnO heterostructures grown by MOVPE technique have been performed. The features of spin-polarization of 2DEG and intrinsic ferromagnetism of heterostructure were observed at 1.5 K and the related physical mechanisms are discussed.
    Optoelectronic and Microelectronic Materials & Devices (COMMAD), 2012 Conference on; 01/2012
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    ABSTRACT: The effect of in situ vacuum anneal (VA) and silane (SiH4) treatment on the electrical characteristics of AlGaN/GaN metal-oxide-semiconductor high-electron mobility transistors was investigated. Native Ga-O bonds on the AlGaN surface can be completely removed by this in situ passivation technique, which was confirmed by x-ray photoelectron spectroscopy. In situ VA and SiH4 passivation also reduced the device gate leakage current. This is attributed to the suppression of trap-assisted tunneling current through the HfAlO gate dielectric. Saturation drain current for devices with in situ VA and SiH4 passivation was also improved, which is due to increased two-dimensional electron gas density. In addition, devices with in situ VA and SiH4 passivation achieved an Ion/Ioff ratio of around 106 and a subthreshold swing of less than 100 mV/decade.
    Applied Physics Letters 08/2011; 99(9). DOI:10.1063/1.3633104 · 3.52 Impact Factor
  • S. T. Lim · J. F. Bi · Lu Hui · K. L. Teo
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    ABSTRACT: We present the magnetotransport studies of Ge1−xMnxTe ferromagnetic semiconductor under hydrostatic pressure. The investigation of the normal and Hall resistivities provide an insight to the dependence of carrier concentration, mobility, and magnetic properties on pressure. Our results reveal that the application of pressure changes the band structure, which can be explained by a two valence band model. We observe the enhancement and reduction of Curie temperature within a pressure range of 0–24 kbar. Analysis within the framework of the Ruderman–Kittel–Kasuya–Yosida model allows us to identify the factors in controlling the Tc, in which the exchange interaction plays a predominant role in the formation of ferromagnetic phase.
    Journal of Applied Physics 07/2011; 110(2). DOI:10.1063/1.3610499 · 2.19 Impact Factor
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    K. Zheng · X.W. Sun · J.L. Zhao · Y. Wang · H.Y. Yu · H.V. Demir · K.L. Teo
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    ABSTRACT: We report an indium-free transparent resistive switching random access memory device based on GZO-Ga<sub>2</sub>O<sub>3</sub>-ZnO-Ga<sub>2</sub>O<sub>3</sub> -GZO structure by metal-organic chemical vapor deposition. The memory device shows good transmittance in the visible region and bipolar resistive switching behavior with good cycling characteristics and retention time under room temperature. The conduction and resistive switching mechanism was discussed based on filament theory.
    IEEE Electron Device Letters 07/2011; 32(6-32):797 - 799. DOI:10.1109/LED.2011.2126017 · 3.02 Impact Factor
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    ABSTRACT: The device physics of AlGaN/GaN metal-oxide-semiconductor high-electron mobility transistor (MOS-HEMT) with localized stress introduced by a diamond-like carbon (DLC) liner or encapsulation layer was investigated. DLC film with high intrinsic compressive stress (~6 GPa) formed over AlGaN/GaN MOS-HEMTs contributed local compressive stress in the channel region. This was found to reduce the two-dimensional electron gas (2-DEG) density in the channel, leading to a positive threshold voltage shift. Transconductance and drain current at a given gate overdrive were also improved. On the other hand, the DLC introduced local tensile stress in the region between the gate and source/drain contacts, leading to a localized increase in 2-DEG density, giving reduced series resistance. The results of this work are expected to be useful for strain engineering of AlGaN/GaN MOS-HEMTs.
    Applied Physics Letters 05/2011; 98(18). DOI:10.1063/1.3584856 · 3.52 Impact Factor
  • S. T. Lim · J. F. Bi · K. L. Teo · T. Liew
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    ABSTRACT: We have investigated the magnetic and magnetotransport properties of Ge1-xMnxTe (x = 0.1) grown by molecular-beam epitaxy. Our results show that the sample exhibits two ferromagnetic transition temperatures at TC = 34 K and TC* = 100 K. We infer that Tc is a long-range ferromagnetic ordering in view of sufficient carriers generating uniform ferromagnetism, whereas TC* is a short-range ferromagnetic ordering due to ferromagnetic clusters. The temperature dependence of the resistivity ρ(T) curve exhibits a shallow minimum near TC. The upturn of ρ(T) toward the low temperature (T < TC) is well described by a weak-localization model, whereas in the high-temperature regime (T > TC), the phonon scattering dominates.
    Journal of Applied Physics 03/2011; 109(7):07C314-07C314-3. DOI:10.1063/1.3548829 · 2.19 Impact Factor
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    ABSTRACT: We report the formation of two-dimensional electron gas (2DEG) at the Zn1−xMgxO/ZnO interface grown by metal-organic vapor phase epitaxy on sapphire substrates. The existence of the 2DEG is confirmed by the observation of Shubnikov–de Haas oscillations and the integer quantum Hall effect. In particular, the Zn0.8Mg0.2O/ZnO heterostructure shows a high Hall mobility of 2138 cm2/V s with a carrier sheet density of 3.51×1012 cm−2 at 1.4 K. We attribute the origin of 2DEG to be the donor states on ZnMgO surface. The dependence of carrier sheet density of 2DEG on ZnMgO layer thickness and Mg composition (x) are also investigated.
    Applied Physics Letters 09/2010; 97(11):111908-111908-3. DOI:10.1063/1.3489101 · 3.52 Impact Factor
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    H. Lu · J. F. Bi · K. L. Teo · T. Liew · T. C. Chong
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    ABSTRACT: We report the antiferromagnetic (AFM) thickness dependence study of zinc-blende CrTe/ZnTe/MnTe layered structure grown on GaAs (100) by low temperature molecular beam epitaxy. The coercivity (H<sub>C</sub>) and exchange bias field (H<sub>E</sub>) show a nonmonotonic dependence on the AFM thickness. Both (H<sub>C</sub>) and H<sub>E</sub> exhibit a maximum value at the AFM thickness of ∼15 nm . With decreasing AFM thickness, we found that the blocking temperature drops from 70 (Néel temperature of bulk MnTe) to 22 K, while the Curie temperature is found to increase from 100 to 190 K. Additionally, a vertical magnetization shift is observed in the M-H loop, which can be attributed to the presence of frozen ferromagnetic spins at the interface.
    Journal of Applied Physics 06/2010; 107(9-107):09D717 - 09D717-3. DOI:10.1063/1.3340510 · 2.19 Impact Factor
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    H. Lu · J. F. Bi · K. L. Teo · T. Liew · T. C. Chong
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    ABSTRACT: We present the results on the exchange coupling in the zinc-blende CrTe/spacer/MnTe structure with ZnTe acting as a spacer layer (SL). The magnetic hysteresis (M-H) loops show significant shifts in both horizontal and vertical directions. A nonmonotonic variation of the exchange bias field (H<sub>E</sub>) and coercivity (H<sub>C</sub>) with the SL thickness is observed. The Curie temperature decreases as the SL thickness increases. The maximum of H<sub>E</sub> and H<sub>C</sub> values occur at a SL thickness of 12 ML (monolayer). Strikingly, the exchange coupling remains noticeable even the thickness of SL reaches 24 ML. The exchange bias field and vertical shift are discussed on the basis of frozen ferromagnetic moment model.
    Journal of Applied Physics 06/2010; DOI:10.1063/1.3352578 · 2.19 Impact Factor
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    ABSTRACT: n -ZnO/ n -GaAs heterostructured light-emitting diodes have been fabricated by a low-cost ultrasonic spray pyrolysis technique. Nanoscale interface analysis was carried out with scanning transmission electron microscopy. An ~ 8.6-nm-thick amorphous GaAsZnInO was found in the n -ZnO/ n -GaAs interface. A strong and broad white electroluminescence band centered at ~ 525 nm and a weak near-infrared emission peaked at ~ 815 nm were observed when n -GaAs was positively biased. The 815-nm emission is believed to be related to the interface layer, and the 525-nm emission is assigned to the recombination of electrons from conduction band to deep-level holes in the ZnO layer.
    IEEE Transactions on Electron Devices 02/2010; DOI:10.1109/TED.2009.2034497 · 2.36 Impact Factor