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Field Electron Emission Mechanism in an Ultrathin Multilayer Planar Cold

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Field Electron Emission Mechanism in an Ultrathin Multilayer Planar Cold Field electron emission mechanism in an ultrathin multilayer planar cold cathode Ru-Zhi Wang1* Hui Yan1* Bo Wang1 Xing-Wang Zhang2 Xiao-Yuan Hou3 1 Laboratory of Thin Film Materials, Beijing University of Technology, Beijing 100022, China 2Key Laboratory of Semiconductor Materials Science, Institute of Semiconductors, Chinese Academy of Sciences, P. O. Box 912, Beijing 100083, China 3Surface Physics Laboratory (National Key Laboratory), Fudan University, Shanghai 200433, China Field electron emission from an ultrathin multilayer planar cold cathode (UMPC) including quantum well structure has been both experimentally and theoretically investigated. We found that only tuning the energy levels of UMPC the field electron emission (FE) characteristic can be evidently improved, which is unexplained by the conventional FE mechanism. Field electron emission mechanism dependent on the quantum structure effect, which supplies a favorable location of electron emission and enhances tunneling ability, has been presented to expound the notable amelioration. An approximate formula brought forward can predict the quantum FE enhancement, which the theoretical prediction is close to the experimental result. PACS codes: 79.70.+q; 73.40.Gk; 73.20.At; 02.60.-x; FE current is unstable and may lead to a I. Introduction vacuum breakdown [2]. People tend to The cold cathodes have been attracted consider the planar cold cathodes more and more attentions for its constructed by some wide band-gap important applications in flat-panel semiconductor (WBGS) films [3], which display and some power amplifier [1]. owes to its simple fabrication, easy However, the present cold cathodes do integration, convenient control, and stable emission et al. Nevertheless, its FE not have so good performance that it can be considered as a commercial current density is not high enough and/or application on a large scale. As an the operational voltage is not low enough effective conventional technique, the [1]. Therefore, best of all, it is necessary nanoscale protrusion (or microtip) cold to advance electron emission properties of cathodes have been widely used to the planar cold cathodes for potential lower the threshold voltage enabling device. For the planar cold cathode, there field emission (FE) by utilizing the are presently three feasible mechanisms geometric field enhancement effect; except for the geometric field however, its fabrication processes are enhancement to advance its electron so complicated that make its price too emission properties. One is the Schottky expensive. As for the heartening diode with a negative-electron-affinity carbon nanotubes (CNTs), a too high (NEA) semiconductor surface [4]; the _____________ *To whom correspondence should be addressed: [email protected]; [email protected] 1 second is the composite of the resulted from a favorable emission electric-field enhancement, and location and an advantaged electron Schottky diode with a NEA tunneling due to the quantum structure semiconductor [5]; the last is the effect. Furthermore, we present an surface barrier lowered with an approximate formula to predict the ultrathin wide band-gap semiconductor quantum FE enhancement. layer (UTSC) [6]. For these II. Results and discussion mechanisms, their essential goal is to make electrons more easily tunnel by The inset of figure 1 shows the schematic the reduction of the surface potential drawing of the planar cold cathode barrier. Most recently, it is interestingly structure. The cathode structure was found that the quantum well states and grown by an EPI Gen-II solid-source surface resonance states in ultrathin molecular beam epitaxy (MBE) system on films can be characteristic by the field an n+ type GaAs (001) substrate. After the electron emission (FE) current [7, 8]. native oxide was desorbed at 580 oC The previous theoretical analysis [9-15] under As atmosphere, the substrate was also showed that, due to electron heated up to 600 oC. A 170nm-thick confinement in quantum well, electron Si-doped GaAs buffer layer (n=1×1018 emission from the ultra-thin cold cm-3) was grown initially, followed by the cathodes presents a distinct resonant GaAs/AlAs two-layer planar cold cathode. behavior. Since the tunneling ability of To investigate the effect of the quantum electrons corresponding to resonant structure of the planar cathodes on the states in ultrathin films can be electron emission, two samples, 3nm evidently affected by the quantum GaAs/ 6nmAlAs for sample A and 6nm structure [10], it impliedly suggests an GaAs/3nm AlAsfor sample B, were electron emission mechanism to prepared. For both samples, the growth improve FE characteristic by temperature is 600 oC and the V/III beam modulating quantum structure of equivalent pressure (BEP) ratio used is 20. ultrathin films. In this paper, we The growth rate of GaAs and AlAs is demonstrate the field electron emission 1ML/s. The surface of both samples was mechanism by examining FE from an characterized to be atomically smooth by ultralthin 6nmGaAs/3nmAlAs or atomic force microscope (AFM) over the 3nmGaAs/6nmAlAs two-layer planar whole surface of the cathode. Electron cold cathode. We found that by only field emission experiments were modulating the quantum structure the performed in an ultrahigh vacuum FE current is evidently enhanced, and chamber with a base pressure better than the threshold voltage is also reduced, 4×10-9 Torr. The current density-field which can not be explained by the characteristics were measured at room previous FE mechanism. Ulteriorly temperature using a diode structure with a theoretical analysis based on the low resistivity (0.02 Ω cm) silicon wafer quantum self-consistent scheme [14] as the anode. The sample and the silicon show the amelioration of electron were separated by two pieces of round emission from ultra-thin multilayer glass fibre as a spacer. The typical planar cold cathodes (UMPC) is diameter of the round glass fiber was 2 14μm measured by a precision gauge 2 3/ 2 2 ⎛ Aγ ⎞ Bφ with an accuracy of ±0.5μm. ln()J / F = ln⎜ ⎟ − (1) ⎝ φ ⎠ γF where J is current density, χ is the potential drop, φ is the height of surface potential barrier, and A and B are constants, respectively. Here, the potential distribution (Fig. 1) can be calculated by the self-consistent scheme of solving the Poisson’s equation [14], and the relative experimental parameters are selected from the handbook [17] in these calculations. The surface potential barriers φ from the calculated energy band in Fig.1 have the Fig.1 (Color online) Calculated energy same shape and height when the same band diagram of two UMPCs structure field applied to two UMPCs. It means that, with the field of 0.065V/nm applied for an electron in GaAs quantum well with the identical incident energy, there As shown in Fig.2, the current will be the same emission ability from the densities versus electric filed (J-E) surface barrier. From the inset of Fig.2, it curves are evaluated to show field Bφ 3 / 2 emission characteristics for two UMPC can be easily found that the slope γ samples with different quantum structures (see Fig.1). A obvious of the two F-N plots is almost identical. distinction is demonstrated in J-E These results naturally lead to an equal λ curves which can be more clearly and then an exactly same theoretical FN found in the Fowler-Nordheim (F-N) curve for two UMPCs by Eq. (1). plot (see the inset of Fig. 2) that the Obviously, the theoretical analysis will be current densities are enhanced to about very conflictive with the experiment 16 times by the quantum structure measurement in Fig.2. Therefore, electron effect. In addition, if the threshold emission from a UMPC can not be voltage is defined at an emission explained by the conventional field current density of 0.1μA/cm2, the emission mechanism in which the threshold voltage decreases from reduction of the surface potential barrier 56V/μm to 43V/μm. These results is considered as the most important factor indicate the novel effect of quantum to improve field emission from structure on FE characteristic of the semiconductor surfaces [4-6] or an UMPC. For an ideal semiconductor UMPC. The present experiment may film with atomically smooth surface, if suggest field electron emission considered the potential drop (barrier mechanism being independent of the field) χ is linearly determined by the surface potential barrier. What is the origination of the remarkably enhanced field density, then χ = γF , the F-N field emission current? There may be equation can be simplified as [16]: some useful indexes [7-15] to help 3 understand the effect of the quantum structure for the UMPC. However, all previous reports, even in our theoretical investigation [14] assumed the enhancement on field emission current should be mainly benefited from the surface barrier lowered. It is necessary to explore the essence of electron emission by analyzing the effect of quantum structure in an UMPC. Fig.3 (Color online) Resonant transmission in two UMPCs structures with the field of 0.065 V/nm applied. 4πqm k T −(E −E )/ k T J = t B T (E )ln[1+ e x F B ]dE = J J (E )dE = J J h3 ∫∫x x 0 x x 0 T (2) and −(Ex −EF ) / kBT J (E x ) = T (E x )ln[1+ e ] = T (E x )λ(E x ) . Fig.2 (Color online) Electron emission (3) current in two UMPCs structures (the 3 where J 0 = 4/π qmtB k T h , which is inset is the corresponding F-N plots). mainly determined by the electron As shown in Fig.1, the band structure effective transverse mass mt , JT is defined have been calculated out by the by the tunneling factor of the FE structure, self-consistent scheme including the which is the integral of the field emission band bending and the more realistic energy distribution (FEED) J(Ex), q unit image potential [14], and it is obvious charge, kB Boltzmann’s constant, T the that there are very different for the temperature, h Plank’s constant, and EF energy band structure of two UMPCs.
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