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El-Mostafa Bourim,한정인 대한금속·재료학회 2016 ELECTRONIC MATERIALS LETTERS Vol.12 No.1
Size effect of InGaN/GaN multiple quantum well (MQW) blue light emittingdiodes (LEDs), on electrical characteristics in forward bias voltage at highinjection current in light emission regime, is observed to induce a substantialdispersion in the current density and normalized negative capacitance (NC) (i.e.,capacitance per chip area). The correction of normalized NC by considering theLED p-n junction series resistance has been found to be independent of chiparea size with lateral dimensions ranging from 100 μm × 100 μm to 400 μm ×400 μm. This fact, confirms that the inductive effect which is usually behind theNC apparition is homogeneously and uniformly distributed across the entiredevice area and hence the dispersive characteristics are not related to local paths. From the characteristics of NC dependence on temperature, frequency and direct current bias, a mechanism based on theelectrons/holes charge carriers conductivity difference is proposed to be responsible for the transient electron-hole pairrecombination process inducing NC phenomenon. Direct measurement of light emission brightness under modulated frequencydemonstrated that modulated light output evolution follows the same behavioral tendency as measured in NC under alternatingcurrent signal modulation. Thus it is concluded that the NC is valuable information which would be of practical interest inimproving the characteristics and parameters relevant to LED p-n junction internal structure.
El Mostafa Bourim,김동욱 한국물리학회 2013 Current Applied Physics Vol.13 No.3
We investigated the admittance spectra of resistive switching Pt/Nb-doped SrTiO3 single-crystal junctions at different resistance states in air and vacuum. The analyses showed that the carrier lifetime at the traps was largely varied depending on the resistance state, indicating the surface potential modification. The ambient dependence suggested that the charges at the trap states were affected by the oxygen adsorption/desorption at the surface. The conductance spectroscopy method clearly revealed the importance of the interface trap states in the resistive switching behavior.
El-Mostafa Bourim,한정인 대한금속·재료학회 2015 ELECTRONIC MATERIALS LETTERS Vol.11 No.6
Characterizations of InGaN/GaN-quantum wells based LED heterostructure were undertaken by static and dynamic electrical measurements at different temperatures. The analysis of the current-voltage (I-V) characteristics demonstrated different mechanisms involved in the current charge transport in the LED device. Experimental admittance spectra have been investigated in broad frequency range, at various temperature and different direct current biases. A specific extraction of the quantum well conductance, based on Nicollian and Goetzberger’s model related to interface state conductance in Metal-Insulator-Semiconductor structure, has shown the effect of the quantum structure on the electric transport, and hence a correlation between the I-V electrical characteristics and the admittance spectroscopy has revealed the different conduction mechanisms involved in the charge transport in the InGaN/GaN LED. Activation energies and carrier capture velocity obtained from Arrhenius plots, determined from the thermally activated quantum well conductance peaks which are revealed with the used model, have confirmed that quantum well parameters are related to the carrier emission from confined levels in quantum wells.
Bourim, El Mostafa,Kim, Yoonjung,Kim, Dong-Wook The Electrochemical Society 2014 ECS journal of solid state science and technology Vol.3 No.7
<P>In this study, we investigated the resistive switching (RS) behavior of Pt/Nb-doped SrTiO<SUB>3</SUB> (Pt/Nb:STO) single-crystal junctions in air and vacuum. We performed steady-state electrical characterizations: the direct current (DC) current–voltage relationship and relaxation current–time dependence under an applied voltage step. The ideality factor of the junction suggested the existence of interface states and tunneling current. We observed that the relaxation current followed the Curie–von Schweidler law; electrical conduction was dominated by a space-charge-limited current based on charge recombination at the interface states. The dynamic electric response was obtained using an alternating current (AC) conductance technique. The carrier lifetime at the interface traps was largely dependent on the resistance state and the ambient environment. Thus, surface potential modification by charge capture/release at the interface traps played a crucial role in the RS of our junctions. Additionally, the ambient effect showed that oxygen desorption (adsorption) at the Nb:STO surface increased (decreased) the interface state density. Finally, an RS model based on interface states in Pt/Nb:STO was proposed.</P>
Lee, Joonmyoung,Bourim, El Mostafa,Shin, Dongku,Lee, Jong-Sook,Seong, Dong-jun,Park, Jubong,Lee, Wootae,Chang, Man,Jung, Seungjae,Shin, Jungho,Hwang, Hyunsang Elsevier 2010 CURRENT APPLIED PHYSICS Vol.10 No.1
<P><B>Abstract</B></P><P>To analyze the switching mechanism of Nb doped SrTiO<SUB>3</SUB> (Nb:STO) single crystal in a high resistive state (HRS) and low resistive state (LRS), we performed a complex impedance spectroscopy in the frequency domain. We demonstrated the domination of the oxygen vacancies to the resistive switching. Based on the impedance spectroscopy in the HRS and LRS, we concluded that the origin of resistive switching is due to the combination of Schottky junction and a generation of conduction electron from oxygen vacancies. The calculation of activation energies in each resistance state has been performed comparatively, which proposed that a first ionization of oxygen vacancies is responsible for the switching.</P>
Investigation of State Stability of Low-Resistance State in Resistive Memory
Jubong Park,Minseok Jo,Bourim, El Mostafa,Jaesik Yoon,Dong-Jun Seong,Joonmyoung Lee,Wootae Lee,Hyunsang Hwang IEEE 2010 IEEE electron device letters Vol.31 No.5
<P>We investigated the state stability of the low-resistance state (LRS) in a resistive switching memory having a Pt/Cu:MoO<I>x</I>/GdO<I>x</I>/Pt structure. Various resistance values of LRS were accurately controlled using an external load resistor connected in series with the resistive memory device. We found that the retention time decreased with an increase in the resistance of LRS. We performed accelerating tests for resistance transition from a low- to a high-resistance state under temperatures ranging from 200°C to 250°C. A predicted resistance of LRS for a ten-year retention period at 85°C was determined based on the Arrhenius law.</P>
권민지,이은송이,Ahrum Sohn,El Mostafa Bourim,김동욱 한국물리학회 2010 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.57 No.6
We investigated the resistive switching behaviors of metal/oxide junctions consisting of Pt electrodes and Nb-doped SrTiO3(001) single crystals. The doping level affected the resistive switching ratio and the transport mechanism (thermionic emission for low doping and thermionic field emission for high doping). Pulse-mode switching experiments showed that an increase in the interface electric field by several times could enhance the switching speed by hundreds of times. The dependence of the retention time on the doping ratio was also examined. All the results suggested that ionic migration and carrier trapping could explain the resistive switching characteristics.
Asymmetric bipolar resistive switching in solution-processed Pt/TiO<sub>2</sub>/W devices
Biju, Kuyyadi P,Liu, XinJun,Bourim, El Mostafa,Kim, Insung,Jung, Seungjae,Siddik, Manzar,Lee, Joonmyoung,Hwang, Hyunsang Institute of Physics [etc.] 2010 Journal of Physics. D, Applied Physics Vol.43 No.49
<P>The resistive switching characteristics of Pt/TiO<SUB>2</SUB>/W devices in a submicrometre via-hole structure are investigated. TiO<SUB>2</SUB> film is grown by the sol–gel spin coating technique. The device exhibits reversible and reproducible bistable resistive switching with a rectifying effect. The Schottky contact at the Pt/TiO<SUB>2</SUB> interface limits electron injection under reverse bias resulting in a rectification ratio of >60 at 2 V in the low-resistance state. The switching mechanism in our device can be interpreted as an anion migration-induced redox reaction at the tungsten bottom electrode (W). The rectifying effect can significantly reduce the sneak path current in a crossbar array and provide a feasible way to achieve high memory density.</P>