Precise Doping Research Articles

Atom-by-atom fabrication with electron beams

Assembling matter atom-by-atom into functional devices is the ultimate goal of nanotechnology. The possibility of achieving this goal is intrinsically dependent on the ability to visualize matter at the atomic level, induce and control atomic-scale motion, facilitate and direct chemical reactions, and coordinate and guide fabrication processes towards desired structures atom-by-atom. In this Perspective, we summarize recent progress in chemical transformations, material alterations and atomic dynamics studies enabled by the converged, atomic-sized electron beam of an aberration-corrected scanning transmission electron microscope. We discuss how such top-down observations have led to the concept of controllable, beam-induced processes and then of bottom-up, atom-by-atom assembly via electron-beam control. The progress in this field, from electron-beam-induced material transformations to atomically precise doping and multi-atom assembly, is reviewed, as are the associated engineering, theoretical and big-data challenges. Electron beams in a transmission electron microscope can be used to manipulate matter atom-by-atom. This Review surveys the recent advances and discusses how the further development and integration of machine learning, theory, feedback and instrumentation will push the field forward.

Tuning the morphology of chevron-type graphene nanoribbons by choice of annealing temperature

Bottom-up synthesis of graphene nanoribbons (GNRs) by surface-assisted polymerization and cyclodehydrogenation of specifically designed precursor monomers has been shown to yield precise edges and doping. Here we use a precursor monomer containing sulfur atoms to fabricate nanostructures on a Au(111) surface at different annealing temperatures. The nanostructures have distinct configurations, varying from sulfur-doped polymers to sulfur-doped chevron-type GNRs (CGNRs) and, finally, pristine graphene nanoribbons with specific edges of periodic five-member carbon rings. Non-contact atomic force microscopy provides clear evidence for the cleavage of C–S bonds and formation of pristine CGNRs at elevated annealing temperatures. First-principles calculations show that the CGNRs exhibit negative differential resistance.

Reversibility of Graphene Photochlorination

The reversibility of graphene photochemical chlorination was investigated. A high content of Cl is obtained through photochlorination, without changing C hybridization. To accommodate the incorporated Cl, graphene corrugation takes place. However, due to weak bonding, Cl atoms desorb during air exposure and long periods of storage. Chlorination also leads to graphene doping. When Cl is removed, doping decreases and graphene returns to its original morphology. Only a small amount of Cl (Cl/C ∼ 0.1) remains strongly bonded to graphene, most likely at grain edges and defects. Therefore, to maintain a precise doping level, Cl trapping methods are essential. Moreover, Cl removal using laser irradiation can be used to tune doping in micrometric areas, making it a promising technique to be used in applications where different doping levels are needed.

An Efficient Photocatalyst for Fast Reduction of Cr(VI) by Ultra-Trace Silver Enhanced Titania in Aqueous Solution

For the purpose of establishing a simple route to prepare a metal-semiconductor hybrid catalyst efficiently and reduce its cost through precise doping noble metals. In this study, ultra-trace silver doped TiO2 photocatalysts were fabricated via a “green” ultrasonic impregnation-assisted photoreduction strategy in an ethanol system, and its photocatalytic performance was systematically investigated by utilizing Cr(VI) as the model contaminant. A schottky energy barrier was constructed in Ag@TiO2, which served as a recombination center and possessed superior photocatalytic activity for Cr(VI) reduction. The obtained catalysts exhibited a significant e−/h+ separation efficiency which directly led to an obvious photocatalytic property enhancement. Then, the resultant Ag@TiO2 (0.06 wt %, 30 min irradiation) showed about 2.5 times the activity as that of commercial P25 NPs for Cr(VI) degradation. Moreover, after five cycles, it still maintained considerably high catalytic ability (62%). This work provides a deep insight into preparation techniques of metal-semiconductor photocatalyst and broadens their application prospect.

Reaction paths of alane dissociation on the Si(0 0 1) surface

Building on our earlier study, we examine the kinetic barriers to decomposition of alane, AlH3, on the Si(0 0 1) surface, using the nudged elastic band approach within density functional theory. We find that the initial decomposition to AlH with two H atoms on the surface proceeds without a significant barrier. There are several pathways available to lose the final hydrogen, though these present barriers of up to 1 eV. Incorporation is more challenging, with the initial structures less stable in several cases than the starting structures, just as was found for phosphorus. We identify a stable route for Al incorporation following selective surface hydrogen desorption (e.g. by scanning tunneling microscope tip). The overall process parallels PH3, and indicates that atomically precise acceptor doping should be possible.

Engineering the Mechanical Properties of Polymer Networks with Precise Doping of Primary Defects.

Polymer networks are extensively utilized across numerous applications ranging from commodity superabsorbent polymers and coatings to high-performance microelectronics and biomaterials. For many applications, desirable properties are known; however, achieving them has been challenging. Additionally, the accurate prediction of elastic modulus has been a long-standing difficulty owing to the presence of loops. By tuning the prepolymer formulation through precise doping of monomers, specific primary network defects can be programmed into an elastomeric scaffold, without alteration of their resulting chemistry. The addition of these monomers that respond mechanically as primary defects is used both to understand their impact on the resulting mechanical properties of the materials and as a method to engineer the mechanical properties. Indeed, these materials exhibit identical bulk and surface chemistry, yet vastly different mechanical properties. Further, we have adapted the real elastic network theory (RENT) to the case of primary defects in the absence of loops, thus providing new insights into the mechanism for material strength and failure in polymer networks arising from primary network defects, and to accurately predict the elastic modulus of the polymer system. The versatility of the approach we describe and the fundamental knowledge gained from this study can lead to new advancements in the development of novel materials with precisely defined and predictable chemical, physical, and mechanical properties.

(Invited) Transistor Applications Using Vertical III-V Nanowires on Si Platform

III-V compound semiconductor nanowires (NWs) have stimulated interest in recent years because of their unique properties and potential applications as building blocks for nanoscale electronic and photonic devices. For future electronics, inherent issue for low-power LSI is lowering supply voltage of transistor while avoiding physical limit of subthreshold slope (SS = 2.3 k BT/q = 60 mV/dec). Otherwise, this limitation will stop further scaling of the power consumption even if a multi-gate architecture and III-V/Ge channels are implemented. In this talk we report on new tunnel junction based on III-V/Si interface [1,2] which is formed by selective-area growth of III-V nanowires (NWs) on Si [3-5], and demonstrated vertical TFETs with steeper SS[6,7]. This new tunnel junctions can inherently forms abrupt heterojunction regardless of precise doping because the band discontinuity is determined only by the offset of each III-V and Si. Thus, good gate-electrostatic control and depletion-width control for the tunnel transport is defined only by the III-V channel region regardless of degeneration of source materials. For integration of III-V NWs on Si, selective-area growth has no thick buffer layer, thus the III-V NWs/Si interface is able to show unique band structures. The narrow gap III-Vs such as InAs and In0.7Ga0.3As NWs, for instance, exhibits staggered-Type II band structure when the n-type NWs are formed on p-Si substrates regardless precise doping. Thus, we integrated InGaAs NWs on Si substrate by utilizing specific growth sequence to align vertical NWs [6]. Sn was used for n-type dopant, and Zn-pulse doping technique [7] was used to make pseudo intrinsic layer as channel region. The formation of intrinsic layer in such small NW-volume would be very important to induce large internal electrical field at the InGaAs/Si heterojunction. The length of the Zn-pulse doped region correspond to channel-length (200 nm-long in this case). Next, we demonstrate electronic devices using III-V NWs on a Si substrate. Fabricated vertical transistors contained In0.7Ga0.3As NWs with modulation doped layer. We observed n-type FET behavior in ID-VDS and ID-VG characteristics. The performances are threshold voltage ~ 0.25 V, Gm,max = 1.45 µS/µm at VDS = 0.50 V, Ion / Ioff > 106, subthreshold slope, SS = 78 mV/decade. Besides the III-V NW-based vertical FETs, the heterointerface between the III-V NWs and Si can be used as tunnel FETs (TFETs). In this case, we use band discontinuity between III-V NWs and Si substrate for TFETs. The device fabricated by InAs NW on Si substrate showed switching behavior with an average subthreshold slope (SS) of 30 mV/dec, which is much lower than the physical limitation of metal-oxide-semiconductor field-effect transistors (MOSFETs, SS ~ 60 mV/dec.) [8,9]. Reducing the opening diameter of the masked substrate decreased the numbers of misfit dislocations. The TFETs made by using III-V NW/Si heterojunctions demonstrated steep-SS properties at room temperature. One of the key issues in the vertical TFETs is the formation of intrinsic channels inside the tiny NW-body and another is the reduction of NW-diameter. We also discuss several current-boosting technologies for the TFETs. The current-boosted TFET exhibits steep SS and high Ion, and the transconductance efficiency (Gm/ID), which is important parameter for low-power analog circuits, is a thousand times higher than that of conventional MOSFETs [10]. References 1) K. Tomioka and T. Fukui, Appl. Phys. Lett., 98, 083114-1 – 3 (2011). 2) K. Tomioka and T. Fukui, Appl. Phys. Lett., 104, 073507-1 – 4 (2014). 3) K. Tomioka et al., Nano Lett., 8, 3475 – 3480 (2008). 4) K. Tomioka et al., Nanotechnology, 20, 145302 – 145309 (2009) 5) K. Tomioka et al., Nature, 488, 189 – 192 (2012). 6) K. Tomioka et al., Nano Lett., 13, 5822 – 5826 (2013) 7) E. Nakai et al., Jpn. J. Appl. Phys., 54, 015201-1 – 3 (2015). 8) K. Tomioka et al., IEEE IEDM Tech. Dig. 88 – 91 (2013). 9) K. Tomioka et al., IEEE ESSDERC Tech. Dig. 397 – 402 (2016). 10) K. Tomioka et al., paper submitted

Indium-doped SnO2 nanobelts for high-performance transparent and flexible photosensors by a facile assembly

Flexible and transparent electronics is the emerging future technology for optoelectronic devices. Recently, it has attracted considerable attention from the research community due to its prodigious commercial applications. Herein, we report highly flexible and transparent ultraviolet photosensors based on indium-doped tin oxide nanobelts with enhanced simultaneous photosensitivity and recovery speed, compared to pure SnO2 nanobelts. Optoelectronic properties of the nanobelt photosensors have been found to be strongly related to the indium doping concentration and grain size of the nanobelts. A facile assembly method has been used to prepare the well-aligned nanobelt device for UV photosensors. Excellent flexible properties of the nanobelts have been explored, which show a superior response during bending tests and almost maintain their properties after 300 bending cycles. The enhanced photosensitivity of about 70 times that of undoped SnO2 nanobelts along with a highly enhanced recovery speed of less than 1.75 s have been achieved by the precise doping of In3+ into SnO2 lattice nanobelts. All these results show that our prepared photosensors demonstrate superior mechanical, electrical, and optical properties for their use in flexible and transparent electronics.

Chemically Tailoring the Dopant Emission in Manganese‐Doped CsPbCl3 Perovskite Nanocrystals

AbstractDoping in perovskite nanocrystals adopts different mechanistic approach in comparison to widely established doping in chalcogenide quantum dots. The fast formation of perovskites makes the dopant insertions more competitive and challenging. Introducing alkylamine hydrochloride (RNH3Cl) as a promoting reagent, precise controlled doping of MnII in CsPbCl3 perovskite nanocrystals is reported. Simply, by changing the amount of RNH3Cl, the Mn incorporation and subsequent tuning in the excitonic as well as Mn d–d emission intensities are tailored. Investigations suggested that RNH3Cl acted as the chlorinating source, controlled the size, and also helps in increasing the number of particles. This provided more opportunity for Mn ions to take part in reaction and occupied the appropriate lattice positions. Carrying out several reactions with varying reaction parameters, the doping conditions are optimized and the role of the promoting reagent for both doped and undoped systems are compared.

Chemically Tailoring the Dopant Emission in Manganese-Doped CsPbCl3 Perovskite Nanocrystals.

Doping in perovskite nanocrystals adopts different mechanistic approach in comparison to widely established doping in chalcogenide quantum dots. The fast formation of perovskites makes the dopant insertions more competitive and challenging. Introducing alkylamine hydrochloride (RNH3 Cl) as a promoting reagent, precise controlled doping of MnII in CsPbCl3 perovskite nanocrystals is reported. Simply, by changing the amount of RNH3 Cl, the Mn incorporation and subsequent tuning in the excitonic as well as Mn d-d emission intensities are tailored. Investigations suggested that RNH3 Cl acted as the chlorinating source, controlled the size, and also helps in increasing the number of particles. This provided more opportunity for Mn ions to take part in reaction and occupied the appropriate lattice positions. Carrying out several reactions with varying reaction parameters, the doping conditions are optimized and the role of the promoting reagent for both doped and undoped systems are compared.

Synergistic Impacts of Electrolyte Adsorption on the Thermoelectric Properties of Single-Walled Carbon Nanotubes.

Single-walled carbon nanotubes are promising candidates for light-weight and flexible energy materials. Recently, the thermoelectric properties of single-walled carbon nanotubes have been dramatically improved by ionic liquid addition; however, controlling factors remain unsolved. Here the thermoelectric properties of single-walled carbon nanotubes enhanced by electrolytes are investigated. Complementary characterization with absorption, Raman, and X-ray photoelectron spectroscopy reveals that shallow hole doping plays a partial role in the enhanced electrical conductivity. The molecular factors controlling the thermoelectric properties of carbon nanotubes are systematically investigated in terms of the ionic functionalities of ionic liquids. It is revealed that appropriate ionic liquids show a synergistic enhancement in conductivity and the Seebeck coefficient. The discovery of significantly precise doping enables the generation of thermoelectric power factor exceeding 460 µW m-1 K-2 .

Cathodoluminescnece study of pn‐junctions in widegap materials using cross sectional polishing

Precise doping control is of critical importance in power device fabrication using widegap materials. In this study, we characterize pn‐junctions in GaN and SiC by cathodoluminescence. To improve spatial resolution, the specimens were polished using cross‐sectional polisher. GaN specimens were successfully characterized with line profile of CL spectra across the cross‐section. On the other hand, CL data could not be used straightforwardly for characterization of pn‐junctions in the case of SiC.

Modulation‐Doped Multiple Quantum Wells of Aligned Single‐Wall Carbon Nanotubes

Heterojunctions, quantum wells, and superlattices with precise doping profiles are behind today's electronic and photonic devices based on III–V compound semiconductors such as GaAs. Currently, there is considerable interest in constructing similar artificial 3D architectures with tailored electrical and optical properties by using van der Waals junctions of low‐dimensional materials. In this study, the authors have fabricated a novel structure consisting of multiple thin (≈20 nm) layers of aligned single‐wall carbon nanotubes with dopants inserted between the layers. This “modulation‐doped” multiple‐quantum‐well structure acts as a terahertz polarizer with an ultra‐broadband working frequency range (from ≈0.2 to ≈200 THz), a high extinction ratio (20 dB from ≈0.2 to 1 THz), and a low insertion loss (<2.5 dB from ≈0.2 to 200 THz). The individual carbon nanotube films—highly aligned, densely packed, and large (2 in. in diameter)—were produced using vacuum filtration and then stacked together in the presence of dopants. This simple, robust, and cost‐effective method is applicable to the fabrication of a variety of devices relying on macroscopically 1D properties of aligned carbon nanotube assemblies.

Negative differential resistance in new structures based on graphene nanoribbons

Six new structures based on graphene nanoribbons are proposed, all of which show negative differential resistance (NDR) in their I---V characteristics. Electron transfer in these structures is based on intraband tunneling, interband resonant tunneling, or simple interband tunneling. The I---V characteristics of the structures are investigated using a $$\mathrm {\pi }$$ź-orbital tight-binding approach and the nonequilibrium Green's function formalism. Atomically precise doping with boron and/or nitrogen impurities as well as careful selection of nanoribbon width have been used to achieve desired energy-band structures. The introduced structures are found to offer good flexibility to fulfill circuit requirements in terms of peak/valley voltages and currents as well as high speed. In these new structures, the peak current ranges from 5.5 to 1300 nA, the peak voltage from 24 to 115 mV, the peak---valley ratio at room temperature from 34 to 8582, and the NDR width from 40 to 180 mV. Effects of different temperatures are also explored, and the results reported.

(Invited) Recent Progress in Vertical Si/III-V Tunnel FETs: From Fundamentals to Current-Boosting Technology

Inherent issue for low-power LSI is lowering supply voltage of transistor while beating physical limit of subthreshold slope (SS = 2.3 k BT/q = 60 mV/dec) [1]. Otherwise, this limitation will stop further scaling of the power consumption even if a multi-gate architecture and III-V/Ge channels are implemented [2-4]. The steep-slope transistors such as tunnel FETs (TFETs) and impact ionization FETs have therefore been proposed to overcome the limitation and to reduce supply voltage to sub-0.5 V in future LSI. The tunneling transistors are most promising switching devices because of possibility for lower SS than 60 mV/dec. and better compatibility with conventional integration process. However, there are some difficulties in decreasing SS while increasing tunneling current as high as that of conventional FETs. Recently, we reported on new tunnel junction based on III-V/Si interface [4,5] which is formed by selective-area growth of III-V nanowires (NWs) on Si [6-8], and demonstrated vertical TFETs with steeper SS [9,10]. This new tunnel junctions can inherently forms abrupt heterojunction regardless of precise doping because the band discontinuity is determined only by the offset of each III-V and Si. Thus, good gate-electrostatic control and depletion-width control for the tunnel transport is defined only by the III-V channel region regardless of degeneration of source materials. Here we report on recent progress in the vertical TFETs using III-V NW/Si heterojunction and discuss some current-boosting technology such as strain and other technique. As for integration of III-V NWs on Si, selective-area growth has no thick buffer layer, thus the III-V NWs/Si interface is able to show unique band structures. The narrow gap III-Vs such as InAs and In0.7Ga0.3As NWs, for instance, exhibits staggered-Type II band structure when the n-type NWs are formed on p-Si substrates regardless precise doping [4,5]. Thus, we integrated InGaAs NWs on Si substrate by utilizing specific growth sequence to align vertical NWs [8]. Sn was used for n-type dopant, and Zn-pulse doping technique [10] was used to make pseudo intrinsic layer as channel region. The formation of intrinsic layer in such small NW-volume would be very important to induce large internal electrical field at the InGaAs/Si heterojunction. The length of the Zn-pulse doped region correspond to channel-length (200 nm-ling in this case). The vertical TFET with a single vertically aligned n+-InGaAs/Zn-pulse doped InGaAs axial NW on a p-Si substrate was firstly demonstrated. The device process was reported elsewhere [4,5]. The channel length was 200 nm. In the TFET using the bare InGaAs NW, the tunneling current under revise bias was modulated by gate bias with SS of 80 – 90 mV/dec, and the ION was 2.7 nA/µm at VDS = 0.50 V. With decreasing VDS, the SS become steeper than 60 mV/dec. at low drain current region. The minimum SS was around 50 mV/dec. at VDS= 0.10 V. As for current boosting technology, channel length scaling was characterized [5]. The ION was 100 times enhanced with decreasing the channel length to 50 nm, and the SS decreased to 30 mV/dec. at VDS = 0.10 V. This indicated that the drain end should be close to the tunnel junction width. Next, strain effect was characterized by using InGaAs/InP core-shell nanowire structure. Switching behavior with a SS of 42 mV/dec was obtained under reverse bias direction. This switching characterization appeared at as low as VDS of 10 mV. The SS of the TFET exhibits steeper SS behavior under various VDS. The ION for InGaAs/InP core-shell NW/Si TFET was 10 times higher than that of the bare InGaAs NW/Si TFET. The benefit of InP shell layer is improvement of SS and enhancement of ION. The InP shell layer act as passivation effect on the semiconductor/oxide interface. TEM analysis revealed the large compressive strain was induced at the near InGaAs NW/Si, which ascribes that the uniaxial compressive strain decrease effective band gap of InGaAs/Si while enhancing tunneling probability. Reduction in effective band gap of tunnel junction can partly enhance the tunneling current, but the tunneling leakage at source/channel region was increased degrading SS. Thus, further additional technology to enhance the tunneling current will be discussed.

Ultralow-Noise Atomic-Scale Structures for Quantum Circuitry in Silicon

The atomically precise doping of silicon with phosphorus (Si:P) using scanning tunneling microscopy (STM) promises ultimate miniaturization of field effect transistors. The one-dimensional (1D) Si:P nanowires are of particular interest, retaining exceptional conductivity down to the atomic scale, and are predicted as interconnects for a scalable silicon-based quantum computer. Here, we show that ultrathin Si:P nanowires form one of the most-stable electrical conductors, with the phenomenological Hooge parameter of low-frequency noise being as low as ≈10(-8) at 4.2 K, nearly 3 orders of magnitude lower than even carbon-nanotube-based 1D conductors. A in-built isolation from the surface charge fluctuations due to encapsulation of the wires within the epitaxial Si matrix is the dominant cause for the observed suppression of noise. Apart from quantum information technology, our results confirm the promising prospects for precision-doped Si:P structures in atomic-scale circuitry for the 11 nm technology node and beyond.

Precise and Accurate Doping of Nanoporous Silica Gel for the Synthesis of Trace Element Microanalytical Reference Materials

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Intrinsic "Vacancy Point Defect" Induced Electrochemiluminescence from Coreless Supertetrahedral Chalcogenide Nanocluster.

A deep understanding of distinct functional differences of various defects in semiconductor materials is conducive to effectively control and rationally tune defect-induced functionalities. However, such research goals remain a substantial challenge due to great difficulties in identifying the defect types and distinguishing their own roles, especially when various defects coexist in bulk or nanoscale material. Hereby, we subtly selected a molecular-type semiconductor material as structural mode composed of supertetrahedral chalcogenide Cd-In-S nanoclusters (NCs) with intrinsic vacancy point defect at the core site and antisite point defects at the surface of supertetrahedron and successfully established the correlation of those point defects with their own electrochemiluminescence (ECL) behaviors. The multichannel ECL properties were recorded, and the corresponding reaction mechanisms were also proposed. The predominant radiation recombination path of ECL emission peak at 585 nm was significantly distinguished from asymmetrically broad PL emission with a peak at 490 nm. In addition, the ECL performance of the coreless supertetrahedral chalcogenide nanocluster can be modulated by atomically precise doping of monomanganese ion at the core vacant site. A relatively high ECL efficiency of 2.1% was also gained. Actually, this is the first investigation of ECL behavior of semiconductor materials based on supertetrahedral chalcogenide nanocluster in aqueous solution. Current research may open up a new avenue to probe the roles of various different defects with defined composition and position in the NC. The versatile and bright ECL properties of Cd-In-S NC combined with tunable ECL potential and ECL peak suggest that the new kind of NC-based ECL material may hold great promising for its potential applications in electrochemical analysis, sensing, and imaging.

Doping graphene thin films with metallic nanoparticles: Experiment and theory

Precise doping of graphene at nanoscale resolution is vital for a number of applications in nanoelectronics and sustainable energy. Although large metallic contacts are presumed to move the Fermi level of graphene above or below the Dirac point, little has been done to study these effects when graphene is in contact with nanoscale metallic objects of specific sizes and concentrations and to investigate if such phenomena are associated with some forms of doping as in conventional semiconductors. As a case study we determine here the local effect of copper nanoparticles (Cu-NPs) on the Fermi energy of graphene domains in large-area graphene thin films. Tight-binding calculations corroborate our Kelvin-probe force microscopy experiments indicating that the Fermi level shifts in the presence of Cu-NPs (corresponding to 0.2 eV at 20% graphene area coverage by Cu) which break the electron-hole symmetry of graphene due to its weak Van der Waals interactions with Cu even in the absence of chemical bonding and charge transfer, in contrast to previous predictions for large and flat metallic contacts.

Comprehensive 2D-carrier profiling of low-doping region by high-sensitivity scanning spreading resistance microscopy (SSRM) for power device applications

Superjunction (SJ) MOSFETs with low on-resistance and high sustain voltage are widely used as main switching power devices. For the p/n-pillars of SJ-power devices, precise doping at low-doping region below 1016cm−3 concentrations is required, and thus high-sensitivity 2D-carrier profiling of the pillars is indispensable where conventional SCM is insufficient. Previously, we developed the high-vacuum SSRM enabling high-spatial resolution and site-specific 2D-carrier profiling.In this study, we investigated comprehensively the feasibility of applying SSRM to SJ-power devices at low doping below 1016cm−3, with both SJ-diodes and low-doping references. The bias dependence of SSRM was analyzed on SJ-diodes and was compared with T-CAD simulations, and both the p- and the n-pillars demonstrate Schottky-like behavior between the probe and the sample. Consequently, the pn-junction delineation also moved with applied bias. We also performed SSRM on reference-staircase structures with low-doping layers down to 1014cm−3 of p, n and p/n types, and comparison with SIMS and SRP confirmed the high sensitivity of SSRM. The Schottky contact of the probe-sample was found to be pronounced at low-doping region, particularly p-type doped region. Therefore, the bias polarity should be taken into account to obtain correct information at the low-doping region.