Versatile Doping Control of Black Phosphorus and Functional Junction

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C: Physical Processes in Nanomaterials and Nanostructures

Versatile Doping Control of Black Phosphorus and Functional Junction Structures Jingyuan Jia, Sumin Jeon, Jaeho Jeon, Jin-Hong Park, and Sungjoo Lee J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b01492 • Publication Date (Web): 02 Apr 2019 Downloaded from http://pubs.acs.org on April 2, 2019

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The Journal of Physical Chemistry

Versatile Doping Control of Black Phosphorus and Functional Junction Structures

Jingyuan Jia1,2, Sumin Jeon1, Jaeho Jeon1, Jin-Hong Park1,3, and Sungjoo Lee1,4,* 1SKKU

Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University (SKKU), Suwon, 440-746, Korea 2School

of Optoelectronic Engineering and Instrumentation Science, Dalian University of Technology, Dalian, 116024, China 3College

of Information and Communication, Sungkyunkwan University (SKKU), Suwon, 440-746, Korea 4Department

of Nano Engineering, College of Engineering, Sungkyunkwan University (SKKU), Suwon, 440-746, Korea *corresponding authors: [email protected]

Abstract We report the versatile doping control of black phosphorus (BP) using an ionic liquid (EMIM:TFSI) as a surface charge acceptor for a wide range of electronic and optoelectronic device applications. BP following EMIM:TFSI doping achieves a high hole density of up to 1013 cm−2, which represents a degenerate doping level. Furthermore, this doping effect can be reversibly removed through immersion in isopropanol, and the electrical performance of BP devices is able to recover and approach the pristine state (cyclic process). Using a selective-area doping technique, both lateral junction structures and self-aligned source/drain BP field effect transistors (FETs) are fabricated through a polymethyl-methacrylate pattern method. The BP lateral junction devices show ideal rectifying behavior. Self-aligned BP FETs with channel lengths as low as 60 nm are presented. Such FETs demonstrate the potential for fabricating nanoscale BP electronic devices.

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1. Introduction Black phosphorus (BP), an emergent two-dimensional material, was recently isolated from layered BP crystals, where each phosphorus atom is covalently bonded to three neighboring atoms to form a puckered orthorhombic structure. BP has a thicknessdependent direct band gap of 0.33–1.8 eV1-3 and the carrier mobility of BP was theoretically calculated to be as high as 20000 cm2/Vs3 and experimentally determined to be 1000 cm2/Vs.2,

3

These impressive properties make layer-structured BP a

promising semiconducting channel material for future nanoscale electronic device applications. Similar to other semiconductor materials, controlled chemical doping is essential for modulating the carrier concentrations and electrical properties of BP to enable the fabrication of various lateral junctions and complex devices. Several doping methods for BP have been proposed using MoO3, CsCO3, benzyl viologen deposition,4, 5

surface-adsorbed K, Cu, Se, and Te as n- and p-type dopants.6-9 These methods have

achieved a limited range of doping levels. Notably, adsorbed atoms on BP have been shown to lack air stability because BP reacts with oxygen and water molecules to form a rough surface containing impurities.3, 10, 11 The existence of other atoms can strengthen this degradation reaction. Therefore, the development of air-stable doping approaches for BP that can achieve high doping levels is critical for further advancement in this field. Furthermore, the tenability and reversibility of BP doping are crucial for constructing high-performance and ultra-sensitive electronics. However, this degenerate and revisable doping subject was only achieved using a MoS2-based technique,12-14 which has not been addressed in previous reports on BP-based devices. In this paper, we present a degenerate and reversible p-doping technique for BP using an ionic liquid (EMIM:TFSI) as a surface charge transfer acceptor. Previously, ionic liquid was used as a solution for liquid exfoliation of BP films15 and as a dielectric material for top-gated BP FET applications16. Here, an ionic liquid is used as a dopant to introduce the p-doping effect. EMIM:TFSI can impose a wide range of charge induction effects on underlying BP materials. After doping, the hole carrier


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concentration of BP can reach up to 1013 cm−2 without impurity injection. Additionally, the degenerate p-doping effect can be easily removed through immersion in an isopropanol (IPA) solution for 5 min, which also reversibly tunes the electrical properties of the BP. This reverse doping process is analyzed through Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and electrical measurements. The BP doping and de-doping process is a cyclic process without any negative impact on performance. Furthermore, BP lateral junction structures and self-aligned source/drain BP field effect transistors (FETs) were fabricated through the selective doping of BP using polymethyl methacrylate (PMMA) as a doping mask. The BP lateral junction devices exhibit rectifying behavior with an ideality factor of 1.1. Self-aligned BP FETs with channel lengths as low as 60 nm were fabricated using a process similar to the conventional CMOS gate-first process, demonstrating the potential for fabricating nanoscale BP devices.

2. Materials and methods 2.1 Preparing the BP films and the ionic liquid doping process. Few-layered BP samples were obtained by cleaving commercially available bulk BP crystals with blue Nitto tape. Following mechanical exfoliation, for further thickness control, an inductively coupled plasma process was used to treat the BP flakes. Ar gas was used to maintain a pressure of 30 mTorr and 350 W of power delivered at 13.56 MHz was applied to a four-turn spiral coil to discharge the high-density plasma. For the doping process, an ionic liquid (EMIM:TFSI) was dropped directly onto the BP films. For the de-doping process, both the BP and dopants (EMIM:TFSI) were immersed into an IPA solution for at least 5 min, followed by rinsing with a fresh IPA solution and drying with N2 gas. 2.2 Characterization of the EMIM:TFSI-doped BP films. The Raman spectra of un-doped BP, doped BP, and de-doped BP were collected from



BP/SiO2/Si, EMIM:TFSI/BP/SiO2/Si, and EMIM:TFSI-removed BP/SiO2/Si samples, respectively, using a laser micro-Raman spectrometer (Kaiser Optical System RAMANRXN1) with an excitation wavelength of 532 nm. The chemical configurations of the three conditions were determined by XPS (ESCA2000). The XPS measurements were performed using Al Kα and Mg Kα X-ray sources. Transmission election microscopy (TEM, JEOL JEM 2100F) measurements were performed by transferring the BP samples onto a copper grid with a carbon mesh. TEM imaging was carried out with an acceleration voltage of 300 kV 2.3 BP device fabrication and measurements. Back-gated BP transistors were fabricated by patterning source/drain electrodes onto the BP/SiO2/Si samples using e-beam lithography, followed by Cr/Au (10/50 nm) deposition in an e-beam evaporator. The diode structures and self-aligned source/drain FETs were fabricated by partially capping the BP surface with PMMA (BP channels were patterned using e-beam lithography) through a mask to facilitate patterned EMIM:TFSI doping. The EMIM:TFSI chemical dopant was then dropped onto the partially passivated BP channels. All electrical measurements were conducted using a Keithley 4200 parameter analyzer under ambient conditions.

3. Results and discussion Fig. 1a presents a schematic illustration of the doping and de-doping processes of BP. Atomically thin BP is prepared through the mechanical exfoliation of bulk BP with blue Nitto tape. The exfoliated BP flakes are then transferred onto a 285-nm SiO2/p++ Si wafer and subjected to soft Ar+ plasma treatment. This process is used to control the BP layer thickness and clean the BP surface, as reported in our previous work.17 The ionic liquid dopant molecule (EMIM:TFSI) used in this work consists of the highdensity positive ion EMIM+ (C6H11N2+) and the negative ion TFSI− (C2F6NO4S2−). After drop-casting the ionic liquid onto the BP surface, the high-density positive ions (EMIM+), with a low molecular weight and high polarizability, are adsorbed onto the


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BP surface and form an electric double layer (high-density dipoles), which induces the accumulation of high-density hole carriers on the underlying BP surface.18 Following this doping process, the ionic liquid (including BP) is dissolved in an IPA solution and the positive ions (EMIM+) de-adsorb from the interface. Therefore the accumulation of electrons dissipates and the doping effect disappears. Fig. 1b presents the Raman spectra collected from three different samples: un-doped BP, doped BP, and de-doped BP (BP after immersion in IPA for 10 min to remove doping). The spectrum of the undoped BP exhibits a single peak at 366 cm−1 (A1g, shown in Fig. S1) corresponding to out-of-plane vibrations and two conventional peaks at 443 cm−1 and 471 cm−1 (B2g and A2g, respectively) corresponding to in-plane vibrations.3 The doped BP displays red shifts (~1.3 cm−1) in the B2g and A2g peaks based on an increase in hole carrier density, which is consistent with previous reports on the Raman peak changes observed in doped two-dimensional materials.19-21 No shift can be observed in the A1g peak. The de-doped BP sample shows the same A1g, B2g, and A2g peak positions as the un-doped sample (Fig. S1), indicating that the p-doping effect can be completely reversed through IPA immersion. To intuitively visualize the Raman peak position changes after doping, the A2g peak position in the Raman mapping spectrum is presented in Fig. 1c. One can see a clear color distribution difference between the un-doped and doped samples, indicating uniform doping of BP with EMIM:TFSI. Fig. 1d presents the XPS measurements for the three different BP films (un-doped (black), doped (blue), and dedoped (red)). The P 2p peaks obtained for the doped BP are shifted toward lower values compared to the peaks obtained for the un-doped BP. Specifically, P 2p1/2 is shifted from 130.2 to 129.8 eV and P 2p3/2 is shifted from 129.2 to 128.8 eV. The downshift in the XPS peaks can be attributed to the doping effects, which shifted the Fermi level toward the valence band edge (p-doping). Both P 2p1/2 and P 2p3/2 of the de-doped BP returned to their original positions are nearly identical to those of the un-doped BP. To analyze the doping mechanism, the N and F elements, which originate from the ionic liquid, were also investigated for the de-doped BP sample, as shown in in Fig. S2a–b. One can see that there are no N or F peaks, implying that EMIM:TFSI does not react with BP to form phosphorus compounds and that there is no ion introduction into the



material. Additionally, for typical BP XPS measurements, a broad peak at ~135 eV is attributed to phosphate species because many phosphorus compounds exhibit peaks in the range of 134–136 eV.22,

23

In our results, the phosphorus compound peak is

negligible for the three different conditions, indicating that no impurities are injected into the BP during the surface charge transfer process. TEM images of un-doped BP and de-doped BP are presented in Fig. 1e and 1f, respectively, to highlight the physical and crystal changes that occur during and after the doping process. The surface morphology and crystal structure of the BP are consistent with the TEM results, confirming that the BP after doping and de-doping has a high-quality structure with no noticeable impurities, which agrees with the results for the un-doped BP film. Fig. 1e and 1f also show the same lattice fringes of 0.16 nm and 0.22 nm, which are ascribed to the (002) and (200) planes of the BP crystal (selected area electron diffraction (SAED) images revealing the crystal orientation are presented in Fig. S3), respectively. To study the degenerate doping effect of EMIM:TFSI on the transport properties of BP, back-gated devices consisting of few-layered (10 nm) BP were first fabricated and studied for three conditions, namely un-doped, doped, and de-doped. The fabrication process begins with the exfoliation and transfer of BP on a SiO2(285 nm)/Si substrate using an adhesive tape. Source/drain metal contacts are then formed by electron beam lithography, metal evaporation, and lift-off processes. The doping method using EMIM:TFSI to fabricate few-layered BP FETs is simple and straightforward. The EMIM:TFSI is directly deposited on top of the channel material and electrical measurements are performed. A schematic diagram of this process is presented in Fig. 2a. Fig. 2b presents the typical forward transfer characteristics evolution (Vg sweep from −50 to 80 V) on a logarithmic scale for un-doped BP devices at drain voltages of 0.1 V and 0.01 V. The initial transfer curve without doping represents p-type dominant ambipolar transport behavior. The output Id-Vd characteristics in Fig. 2c reveal a linear dependence of Id on Vd, indicating ohmic contact formation. Fig. 2d presents the transfer characteristics of the same BP device after doping with EMIM:TFSI (doped


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BP) at drain voltages of 0.1 V and 0.01 V. The characteristic curve is significantly different compared to the Id-Vg curve for the un-doped case. The gate dependence of the current is significantly diminished in the Vg range of −80–80 V. The Id-Vd curves for the doped BP in the Vg range of −50–50 V were also measured. The results are presented in Fig. 2e. In contrast to the result in Fig. 2c, the Id-Vd curves for the doped BP device show negligible changes with gate voltage modification, which confirms the monotonicity of the back gate. The air stability of this doping process is presented in Fig. S4. Although the ionic liquid is hydroscopic,24 little amount of water can be transported onto the BP surface. Therefore, the performance of doped-BP and the partially-doped (junction) BP device have much better stability in air than un-doped BP. However, it remains necessary to store both the doped and partially-doped BP devices in vacuum to improve their stability. The sheet resistance (Rs) of BP in different temperature ranges before and after doping was also investigated. The results are presented in Fig. 2f. The Rs of the doped BP is as low as ~5 K𝛺/𝑠𝑞, which is more than one order of magnitude smaller than that of the un-doped BP. This result is consistent with the Hall measurement result (Fig. 2g). The Hall measurements were performed using two opposing contacts oriented perpendicular to the drain current path (Fig. 2f inset). The carrier density can be calculated using the expression nH = IB / eVH, where nH is the carrier concentration, B is the vertical magnetic field, VH is the horizontal voltage, and I is the current through the BP channel. As shown in Fig. 2g, the hole concentration increases from 3 × 1011 to 3 × 1013 cm−2 and the conductance increases from 40 to 300 μS after doping. Hole concentration and conductance were also measured for the de-doped BP device. Both values recover to approach original values of the un-doped BP device. To investigate doping reversibility and electric performance in greater detail, the Id-Vg characteristics were compared between the un-doped, doped, and de-doped devices, as shown in Fig. 2h. Following immersion in IPA, the current level decreases monotonically over time, the gate dependence of the channel gradually



reappears, and the on-off ratio recovers from 1 to more than 104. After 5 min of immersion, as immersion time increases, the threshold voltage exhibits a relatively small shift and a slightly lower Id-off value. Eventually, the values approach those of the original un-doped state. When a doped sample is immersed in IPA, the dopant molecules gradually desorb from the surface and dissolve into the IPA, and the device properties of the BP FET return to their original states. This phenomenon represents a unique feature of the surface charge transfer doping process of BP. This degenerate doping process can be both reverse and repeated. Such doping and de-doping processes can be repeated many times with no significant impact on electrical performance. Fig. 3a presents the Id-Vg transfer curve of a BP FET during repeated doping and de-doping processes. In each step, the BP FET is completely doped and de-doped. The drain currents and on-off ratios over the repeated steps are summarized in Fig. 3b. Even after more than four cycles of repeated doping and de-doping, the degradation in the drain current and on-off behavior is nearly negligible, demonstrating the excellent stability of the doping and de-doping processes. Consequently, we can conclude that dopants and doping properties can be removed using proper solvents, which presents a potential method for selective doping, where surface doping can be controlled by using a patterning mask. To fabricate and study BP lateral junction structures by selective doping, single BP flakes were aligned using electron beam lithography (EBL). Their left ends were then exposed to EMIM:TFSI and their right ends were passivated by PMMA with three Cr/Au (10/50 nm) contacts located on both edges and the middle of the channel, as shown in Fig. 4a. Fig. 4b presents the back-gated transfer characteristics (Id-Vg characteristics) of the EMIM:TFSI-doped (①-② electrode) and un-doped (PMMApassivated ②-③ electrode) ends of the same channel. The un-doped channel (②-③ electrode) exhibits clear ambipolar Id-Vg semiconductor transfer behavior, with a minimum current of 5 nA at Vd = 0.1 V. The drain current, which is strongly modulated by Vg, sweeps from negative to positive voltages. Id varies from 10 A at Vg = −30 V to


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a minimum of 5 nA at Vg = 4 V, and increases to several hundred nanoamperes for larger Vg values. This indicates that EMIM:TFSI doping effects can be completely prevented by PMMA passivation. For the EMIM:TFSI-doped end (①-② electrode), the device clearly exhibits much higher current with negligible gate control (degenerate p-doping, consistent with Fig. 2d). Next, we measured the electrical performance of a homogeneous BP lateral junction (①-③ electrode, half doped and half un-doped). Fig. 4c presents the Id-Vd curve for the BP lateral junction without any applied gate voltage. By comparing this curve to the curve before doping (①-③ electrode before doping), asymmetric and rectifying behaviors can be observed. One can see that the obtained forward/reverse current ratio is enhanced from 1 to 20. These asymmetric and rectifying behaviors are caused by the hole carrier concentration difference between the two sections (exposed region and PMMA-passivated region). The concentrations differ by a factor of approximately 50 (Hall measurement from Fig. 2g). Therefore, the hole current exhibits significant differences under different drain voltage directions, as shown in Fig. 4d. The Id-Vd curve for the BP lateral junction exhibits an ideality factor of approximately 1.1, indicating ideal diode characteristics, which can be attributed to the homogeneous lateral structure with no impurities or traps at the interface. Based on selective doping and the above lateral junction structure, we studied a new approach to fabricating self-aligned BP FET structures (Fig. 5a). A multi-layer BP FET with a channel length of several micrometers was fabricated with Cr/Au contacts. To construct a self-aligned source/drain structure, the BP channel was first coated with a PMMA layer, followed by EBL treatment to passivate the BP channel under the PMMA layer in the middle and form highly-doped source/drain regions (exposed regions) next to the channel on both sides, resulting in a p+/i/p+ structure, which is similar to conventionally fabricated p-MOSFETs. The effective channel length can be modulated by tuning the PMMA layer length. Fig. 5b presents the Id-Vd curves for the metal source/drain BP FET and dopedsource/drain FET. A significant drain current increase can be attributed to the reduction



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of the effective channel length. The channel length is approximately 3 μm prior to doping and only 200 nm following source/drain region doping. It should also be noted that for the doped-source/drain FET with a 200-nm channel length, a super-linear increase in Id can be observed when Vd > ~0.9V based on the avalanche effect under a strong electric field (~ 45 kV/cm). The charge carrier multiplication factor (M) of the 200-nm self-aligned BP device was calculated from the Id-Vd characteristic curve as M = I/Is, where I is the channel material current and Is is the saturation current prior to avalanche occurrence. However, Is was set to the current measured at the critical point of avalanche because attaining saturation is not practical for BP devices. The absence of saturation in BP devices can be attributed to carrier impact ionization occurring preferentially before saturation at lower voltages. Furthermore, M can also be expressed as follows25: 1

M= 1―

,

𝑉 𝑛 𝑉𝑏

( )

(1)

where n is an index corresponding to the ionization rate and Vb is a fitting parameter for the 200-nm-channel-length BP device. Equation (1) can also be written in a different form as follows:

(

1

)

ln 1 ― 𝑀 = 𝑛 × (𝑙𝑛(𝑉) ― 𝑙𝑛(𝑉𝑏))

(2)

Fig. 5c presents the M values of the 200-nm-channel-length BP device calculated from I/Is. The linear relationship between ln (1 − 1/M) and ln (V) clearly indicates avalanche effect occurrence for applied voltages over 1 V. By linearly fitting M using equation (2), as indicated by the red line in Fig. 5c, the values of n and Vb can be retrieved. The value of n is 3.8 and Vb is over 1.5 V. Fig. 5d presents the transfer characteristics of self-aligned BP FETs with different channel lengths (6 μm, 200 nm, 100 nm, and 60 nm), which were obtained through PMMA layer patterning. All measurements were performed at a drain voltage of 0.1 V. The increase in the off current and decrease in the on-off current as the channel length decreases from 6 μm to 60 nm can be attributed


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to reduced gate controllability caused by the weakening of the drain-induced barrier and enhanced thermal-assisted tunneling, which implies that further detailed investigations and structural improvements are required for the successful implementation of nanoscale BP transistors.

4. Conclusion We performed strong p-doping (hole density of approximately 1013 cm−2) of BP crystals based on surface charge transfer at the interfaces between ionic liquid dopant (EMIM:TFSI) molecules and BP to form surface dipoles. Raman spectroscopy, XPS, and TEM measurements confirmed the presence of doping effects in BP with no damage to its crystal structure. Further electrical characterization, including device I-V behavior analysis and Hall measurements, confirmed the modulation of carrier density and conductance. It was also demonstrated that BP doped in this manner can be dedoped to recover its original properties through simple immersion in IPA. Cyclic BP property modulation based on cyclic doping and de-doping processes was also demonstrated. Based on the versatile doping control of BP and a partial doping method using a PMMA patterning approach, homogeneous BP lateral junction structures were fabricated. These structures exhibited ideal rectifying characteristics. Additionally, a self-aligned source/drain BP FET was successfully fabricated and avalanche effects on Id-Vd characteristics were analyzed based on a short-channel-length BP transistor. The results of this analysis provide a solid foundation for further scientific studies on nanoscale BP devices.

Acknowledgments This research was supported by the Pioneer Research Center Program through the National Research Foundation of Korea funded by the Ministry of Science, ICT & Future Planning (2014M3C1A3053024), and Basic Science Research Program through the National Research Foundation of Korea funded by the Korean government (MSIP)



(grant

numbers:

2018R1D1A1A09081931,

2015M3A7B7045496,

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and

2017R1A4A1015400).

Supporting Information Raman spectra measurements for three conditions: un-doped, doped, and de-doped BP; XPS spectra measurements of de-doped BP; SAED patterns of un-doped and de-doped BP; Electrical measurements of Id-Vg transfer characteristic of un-doped BP，doped BP and doped-source/drain BP devices with air-exposure time dependence are available in supporting information.

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Electrodes, Nanotechology., 2012, 23, 344013. 21. Kang, D. H.; Shim, J.; Jang, S. K.; Jeon, J.; Jeon, M. H.; Yeom, G. Y.; Jung, W. S.; Jang, Y. H.; Lee, S. Park, J. H. Controllable Nondegenerate P-type Doping of Tungsten Diselenide by Octadecyltrichlorosilane, ACS Nano, 2015, 9, 1099-1107. 22. Wood, J. D.; Wells, S. A.; Jariwala, D.; Chen, K. S.; Cho, E.; K. V.; Sangwan; Liu, X.; Lauhon, L. J.;

Marks, T. J.; Hersam, M. C. Effective Passivation of Exfoliated

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Fig. 1. BP doping and de-doping process. (a) Schematic diagram of the EMIM:TFSI doping and de-doping process for BP. (b) Raman spectra of the same BP film under three different conditions: un-doped, doped, and de-doped. (c) A2g peak center Raman mapping images of un-doped BP and doped BP. (d) XPS spectra (P 2p) of the BP films under three different conditions: un-doped, doped, and de-doped. (e), (f) Highresolution TEM images of un-doped and de-doped reverse conditions, respectively.

Fig. 2. Performances of un-doped and doped BP FETs. (a) Schematic diagram of the EMIM:TFSI doping and de-doping process for a BP FET. (b) Id-Vg transfer


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characteristics of an un-doped BP FET when Vd = 0.1 and 0.01 V. (c) Id-Vd transfer characteristics of the same BP FET in (b) with Vg ranging from −50 V to 50 V. (d) IdVg transfer characteristics of doped BP FET when Vd = 0.1 and 0.01 V. (e) Id-Vd transfer characteristics of the same BP FET in (d) with Vg ranging from −50 V to 50 V. (f) Sheet resistance measurements of the same BP FET for two different conditions: undoped and doped. (g) Carrier concentration and conductance based on Hall measurements for three different conditions: un-doped, doped, and de-doped. (h) Transfer characteristics of an EMIM:TFSI-doped BP FET as a function of time after immersion in IPA. As the dopants dissolve in the IPA solution, the Id-Vg curves approach their original states.

Fig. 3. Cyclic doping and de-doping of a BP FET. (a) The Id-Vg transfer characteristics of the same BP FET over several cycles of doping and de-doping processes. (b) On-off ratio and on-current summary over several cycles of BP doping and de-doping processes.



Fig. 4. Homogeneous BP lateral junction structure formation. (a) Schematic diagram of BP lateral junction formation. (b) Id-Vg transfer characteristics of the doped region (①② in (a)) and un-doped region (②-③ in (a), PMMA passivated) in the same channel. (c), (d) Id-Vd characteristics and energy band diagrams, respectively, of the ①-③ channel in (a) when Vg = 0 V for two conditions: un-doped and doped. (e) Log-scale Id-Vd characteristic curve of BP lateral junction (used to calculate the ideality factor).


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Fig. 5. Self-aligned BP FET formation. (a) Schematic diagram of the BP self-aligned source/drain FET. (b) Id-Vd characteristics of the structure in (a) for two conditions: undoped (L= 3 μm) and doped (L= 200 nm). (c) Calculated avalanche multiplication factor (M) as a function of bias. (d) Id-Vg transfer characteristics of self-aligned BP FETs with different channel lengths: 60 nm, 100 nm, 200 nm, and 6 μm.



Table of Contents Graphic


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Versatile Doping Control of Black Phosphorus and Functional Junction

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