hybridized Boron Nitride - ACS Publications

Subscriber access provided by AUSTRALIAN NATIONAL UNIV

Functional Inorganic Materials and Devices

Defect-Mediated In-Plane Electrical Conduction in Few-Layer sp-hybridized Boron Nitride 2

Dong Yeong Kim, Hokyeong Jeong, Jaewon Kim, Nam Han, and Jong Kyu Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b04389 • Publication Date (Web): 27 Apr 2018 Downloaded from http://pubs.acs.org on April 27, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Defect-Mediated In-Plane Electrical Conduction in Few-Layer sp2-hybridized Boron Nitride

Dong Yeong Kim, Hokyeong Jeong, Jaewon Kim, Nam Han and Jong Kyu Kim*

Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang, 37673, Republic of Korea

KEYWORDS. Boron nitride, space-charge-limited conduction, vacancy, grain boundary, doping, metal-organic chemical vapor deposition.

ACS Paragon Plus Environment

1

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 25

ABSTRACT

In-plane electrical conduction in sp2-hybridized boron nitride (sp2-BN) is presented to explore a huge potential of sp2-BN as an active material for electronics and ultraviolet optoelectronics. Systematic investigation on temperature-dependent current-voltage (I-V) characteristics of a few-layer sp2-BN grown by metal-organic vapor phase epitaxy revealed two types of predominant conduction mechanisms, that are Ohmic conduction at the low bias region and space-charge-limited conduction at the high bias region. From the temperature-dependent I-V characteristics, two shallow traps with activation energies of approximately 25 meV and 185 meV are observed. Based on the near-edge X-ray absorption fine-structure spectroscopy, boronboron (B-B) homoelemental bonding from grain boundary and nitrogen vacancy (VN) are proposed as the origin of the shallow traps mediating the in-plane conduction in the sp2-BN layer. In addition, a drastic enhancement in the electrical conductivity was observed with increasing the amount of VN which acts as donors, implying that controlled generation of VN is an alternative and better approach for n-type doping of the sp2-BN film rather than ineffective conventional substitutional doping methods.


2

Page 3 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


INTRODUCTION

sp2-hybridized boron nitride (sp2-BN), especially, hexagonal BN has attracted a great attention as passive layers, such as substrates1-3, dielectrics4,5 and passivation layers6,7, for emerging two-dimensional (2D) materials-based electronic devices due to its excellent dielectric properties and chemical, mechanical, and thermal stability. On the other hand, sp2-BN can be potentially used as an excellent active material for high-temperature electronics or deep ultraviolet optoelectronics by taking full advantages of its wide-gap semiconducting nature with strong light-matter interaction8-12. For exploring such a new and huge potential of active h-BN, it is essential to understand the semiconducting properties of sp2-BN including electrical conduction and to figure out a way to control its conductivity. Nevertheless, many studies have focused on understanding the dielectric properties rather than the semiconducting properties of sp2-BN. Crystalline defects are key features to understand and control the electrical properties of crystals including sp2-BN. The early experimental work showed that the electric conductivity of BN can be also strongly affected by defects by the comparison of two turbostratic BN films grown by sputtering with pure Ar or N2 gases13. More recently, studies on the transport characteristics of h-BN grown by thermal chemical vapor deposition (CVD) that is widely used for synthesis of 2D materials14,15 as well as oxide materials16,17, have been carried out. Spacecharge limited transport was observed in monolayer h-BN on Cu (111) foil grown by thermal CVD method18, and weak p-type Ohmic conduction was reported in 1.7 um-thick h-BN grown by metal-organic chemical vapor deposition (MOCVD)19. The origin of the p-type conductivity of h-BN is suggested to be caused by boron vacancy (VB), in analogous to the acceptor-like Ga


3


Page 4 of 25

vacancy in GaN and acceptor-like Al vacancy in AlN20,21. However, it was not experimentally revealed that what kinds of defects affect the charge transport in sp2-BN, because there have been a lack of systematic studies on such defect-mediated electrical conduction properties of sp2BN. In this study, we present defect-mediated in-plane electrical conduction in wafer-scale fewlayer sp2-BN on 2-inch sapphire substrate grown by MOCVD. Ohmic conduction and spacecharge-limited conduction (SCLC) are found to be mainly responsible for the electrical conduction at low and high applied bias, respectively. Temperature-dependent current-voltage (IV) measurement reveals that two-shallow traps with activation energies of 25 meV and 185 meV are involved in the electrical conductions. Those traps are suggested to originate from boronboron (B-B) homoelemental bondings and nitrogen vacancies (VN), respectively, according to the near X-ray absorption fine structure (NEXAFS) spectroscopy. In addition, a drastic change in the electrical conductivity over 5 orders of magnitude was observed with different amounts of VN. This is the first experimental report to demonstrate that VN can act as a donor. Based on our experimental findings, we propose the controlled generation of VN as a new and effective n-type doping method for sp2-BN, rather than ineffective substitutional doping22 including Si doping method23. These findings can enhance understanding of the defect-mediated electrical conduction characteristics in sp2-BN as well as the way to control it. Also these can help to pave the way for a realization of active sp2-BN-based high-temperature electronics and deep ultraviolet optoelectronics by taking advantages of its large band gap energy, strong light-matter interaction, thermal and chemical stability. sp2-BN films were grown on 2-inch sapphire substrates at the temperature of 1050 °C and the pressure of 30 mbar by a commercial multi-wafer MOCVD system. Pulsed injections of 10 sccm


4

Page 5 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


triethylborane (TEB) and 8000 sccm ammonia (NH3) precursors into the MOCVD reactor were used with hydrogen carrier gas in order to suppress parasitic gas-phase pre-reactions between the precursors. The details of sp2-BN growth are provided in Supporting Information S1. MOCVDgrown sp2-BN films on sapphire substrates were characterized by Raman spectroscopy, optical absorbance spectroscopy, and X-ray reflectance (XRR). For both X-ray photoemission spectroscopy (XPS) and NEXAFS spectroscopy using a synchrotron X-ray source, the sp2-BN films grown on a sapphire substrate were transferred onto a conductive silicon substrate to prevent the charging effect by the polymethyl methacrylate (PMMA)-assisted method accompanied with diluted hydrofluoric acid treatment as shown in Supporting Information S2. Transfer process doesn’t seem to have a big influence on the sp2-BN film according to the Raman spectroscopy as well as NEXAFS spectroscopy as shown in Supporting Information S2. Figure 1 shows the structural and the optical properties of the MOCVD-grown BN film. Raman scattering at 1370 cm-1 is observed which corresponds to the E2g in-plane vibration mode of sp2-BN24,25 as shown in Figure 1a. Figure 1b is the optical absorption spectrum of the MOCVD-grown sp2-BN film, exhibiting a strong optical absorption at around 202 nm without any long-wavelength tail. This corresponds to the optical bandgap energy of 5.92 eV (inset of Figure 1b) which is in range of reported values8,24,25. XPS core-level spectra are shown in Figure 1c. Core-level binding energies of B 1s and N 1s states are 190.8 eV and 398.4 eV, respectively, which are consistent with previous results25,26. Thickness of the grown sp2-BN is estimated to be 2.66 nm by XRR measurement as shown in Figure 1d which corresponds to 7~8 atomic layers of sp2-BN. Cross-sectional transmission electron microscopy (TEM) image clearly shows a layered structure of the grown sp2-BN film with approximately 7 atomic layers. (see Supporting Information S3). To identify the crystal structure of multilayer sp2-BN, the stacking order should


5


Page 6 of 25

be studied at the atomic level because sp2-BN film can have AB stacking (hexagonal), ABC stacking (rhombohedral), and random stacking (turbostratic)27,28. Although the crystal structure of BN could not be determined exactly in this work, Figure 1 and X-ray spectroscopies (discussed later) imply that the grown BN film is sp2-hybridized.

Figure 1. Structural and optical properties of MOCVD-grown sp2-BN film. (a) Raman spectrum showing E2g vibration mode of sp2-BN film at 1370 cm-1. (inset) E2g in-plane vibration mode of sp2-BN. (b) Optical absorbance spectrum and (inset) corresponding Tauc’s plot. Measured optical bandgap energy is about 5.9 eV. (c) Core-level X-ray photoemission spectroscopy of sp2BN film for B 1s and N 1s whose binding energies are 190. 8 eV and 398.4 eV, respectively. (d) X-ray reflectance of the sp2-BN film. Film thickness is estimated to be about 2.66 nm.


6

Page 7 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


For the electrical conduction measurement, a series of rectangular Ti/Au (thickness of 20nm/150nm) metal electrodes in the form of a transmission line measurement (TLM) type were fabricated on the as-grown sp2-BN film on sapphire substrate by photolithography, followed by electron beam evaporation of Ti and Au. The deposited metal contact was annealed at 800 °C for 1min under N2 ambient by using a rapid thermal annealing system. No measurable current was observed before the annealing as shown in Supporting Information S4. Electrical conduction measurement was carried out in a chamber under vacuum of ~ 10 mTorr at the various temperatures from 100 K to 500 K. The optical image of the fabricated TLM type sample and schematic of the measurement setup are shown in the Supporting Information S5. Figure 2a shows the temperature-dependent I-V characteristics of the sp2-BN film with the channel length of 3 µm and width of 200 µm. It is a non-linear but symmetric for both positive and negative applied voltages which is like a back-to-back Schottky diode with the same metal contacts. Since the current through the sapphire is in order of few hundreds of fA, (see Supporting Information S6), the observed current-voltage characteristic is mainly attributed to the in-plane electrical conduction in the few-layer sp2-BN film. On the other hand, a potential parasitic factor that could affect the electrical conduction by the diffusion of metal atoms from the metal contacts across the sp2-BN film during the annealing at 800 °Cis excluded due to following two reasons: a drastic decrease of conductivity when annealed at temperature higher than the optimum temperature, and no noticeable conduction in the same device configuration fabricated on the sapphire substrate. (See Supporting Information S7 for a detailed discussion). Figure 2b is the log(I)-log(V) plot of the I-V characteristics measured at the temperatures of 100 K, 250 K, and 500 K. For the low voltage range smaller than 0.5 V (region (i) in the graph), the slopes at all the temperatures are near unity, indicating the Ohmic conduction. However, the


7


Page 8 of 25

slopes gradually increase at the region (ii) and become approximately 2 at the voltage range from 15 V to 30 V (region (iii) in the graph) which is related to the space-charge-limited conduction (SCLC)29,30. For the low temperature of 100 K, the current is proportional to V with ~ 2.62 for the voltage from 1.0 V to 1.5 V which is due to trap-filled-limited (TFL) conduction31. Since TFL conduction is only observed at the temperatures of 100K and 150K (not shown in the graph), there would exist a shallow trap whose trap-level is in the order of a few tens of meV. For the temperature of 500 K, the slope becomes larger than 2, thus, the current becomes proportional to V with > 2 as the voltage increases. This also indicates another trap may be involved in the electrical conduction of sp2-BN film, which can be thermally activated at the high temperature and generate additional free-carriers32-34, this will be disused in more detail later.

Figure 2. Temperature-dependent electrical conduction properties of sp2-BN film. (a) I-V characteristics in a linear scale and schematic structure of the fabricated device, and (b) I-V characteristics in a log-log scale for the temperature of 100K, 250K, and 500K. Slopes of the graph gradually increase from 1 to 2 as the voltage increases for all measured temperatures.


8

Page 9 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3a shows the temperature-dependent I-V characteristics near zero bias from -0.4 V to 0.4 V. The linear relation means that the dominant conduction mechanism is Ohmic conduction which is typically observed in dielectric films when a very low bias voltage is applied, and while other carrier transport mechanisms do not have a significant contribution29. Since ohmic conduction is mainly attributed to the thermally excited carriers from defect levels rather than from the valence band in wideband gap semiconductors, the activation energy of the defect can be obtained from the Arrhenius plot of temperature-dependent conductivity (σ) as shown in Figure 3b. Assuming that the current flows uniformly in the sp2-BN film, the conductivity is simply calculated by σ = ⁄ × where, l is the channel length, S is the channel area and R is the resistance of the sp2-BN channel obtained from the slope of the I-V curves shown in Figure 3a. According to the equation showing the temperature-dependence of conductivity29, σ = exp −

where, is the pre-exponential factor, is an activation energy of the defect and is the thermal energy at a given temperature T, the measured data can be fitted into two linear lines with different slopes as shown in Figure 3b. This means that two defects are involved in the electrical conduction, of which activation energies are about 35.6 meV and 193 meV. This observation coincides with the prediction based on the temperature-dependent I-V characteristics of the sp2-BN film plotted in linear scale. As discussed, TFL conduction, which was only observed at the temperatures of 100 K and 150 K, can predict the presence of the shallow trap at the energy of a few tens of meV. The current proportional to V with > 2 at the temperature of 500K also implies the presence of another trap.


9


Page 10 of 25

Figure 3. Electrical conduction in sp2-BN under the small applied voltage. (a) Linear I-V characteristics of sp2-BN film indicating Ohmic conduction, and (b) Arrhenius plot of temperature-dependent conductivity obtained from (a). The temperature-dependent conductivity can fit into two linear lines indicating presence of two defects whose activation energies are about 35.6 meV and 193 meV.

Figure 4a shows the relation between the current and square of the applied voltage. The linear I-V2 relation implies that the dominant conduction mechanism is SCLC following Child’s rule expressed as

I = C ∙

" !

where C is a parameter which is 0.7 for the rectangular geometry used in this study, is the vacuum permittivity, is the dielectric constant of sp2-BN, is the carrier mobility, L is the channel length and W is the channel width29,30. Assuming that the dielectric constant of sp2-BN along the in-plane direction is 6.635, the carrier mobility of the MOCVD-grown few layer sp2-BN


10

Page 11 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


is estimated to be 0.003 cm2/(V·s). The estimated carrier mobility is comparable to that of monolayer h-BN grown on Cu foil which is 0.01 cm2/(V·s) when the dielectric constant is assumed to be 318. On the other hand, the current becomes proportional to V with > 2 as the temperature and voltage increase (see also Supporting Information S8). This is related to free carrier generation from traps in the sp2-BN film. When assuming presence of a single-level trap in a film, SCLC current is given by the following relation considering the ratio of free to trapped carriers, $, I = C ∙

"×$ !

The $ is expressed by $=

%& ' exp − %'

, where %& is the effective density of states in the conduction band, %' is the concentration of traps, and ' is the energy level of the trap29,30. Therefore, the temperature-dependence of the current at a given bias also shows the trap-level involved in the electrical conduction. Figure 4b shows the temperature-dependence of the current at 6 different bias conditions from 5V to 30V. Similar to the conductivity at Ohmic conduction regime, the current at a given bias can be explained by two linear lines indicating two-shallow traps at the energies of approximately 18 meV and 178 meV. The trap-levels are slightly smaller than that obtained through the temperature-dependence of conductivity since it doesn’t consider the barrier lowering by electric field, that is called, the Poole-Frenkel effect. In any case, the temperature-


11


Page 12 of 25

dependent I-V characteristics of sp2-BN clearly show that two shallow traps at the energies of approximately 25 meV and 185 meV affect the electrical conduction in the sp2-BN.

Figure 4. Space-charge-limited conduction in MOCVD-grown sp2-BN films. (a) Linear I-V2 characteristics indicating space-charge-limited conduction, and (b) Arrhenius-type plot for temperature-dependence of the current at a given applied voltage. Temperature-dependence of current, composed of two linear curves, indicates the presence of two defects at approximately 18 meV and 178 meV.

In analyzing the I-V characteristics of sp2-BN films, it is important to distinguish between the SCLC in the presence of traps and field-enhanced bulk conduction such as the Poole-Frenkel conduction, because they show similar electric field-dependent conductivity at various channel lengths36. For clarifying that the MOCVD-grown sp2-BN film follows the SCLC, I-V characteristics measured at different channel lengths are compared as shown in Supporting Information S9. The current should be identical at the same electric field regardless of the channel length when the Poole-Frenkel conduction is dominant. However, the current not only varies depending on the channel length but also is inversely proportional to the sixth power of


12

Page 13 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the channel length at the same applied bias, which is consistent with the SCLC-type conduction mechanism. While the current should be inversely proportional to the square of the channel length according to the Childs’s raw assuming a trap-free film. The higher order dependence is also possible when the trap density is exponentially distributed depending on the energy level in the sp2-BN film. In order to figure out the chemical origin of the defects in the sp2-BN film, NEXAFS spectroscopy, referring the transition between from core-level states to unoccupied states, was performed by using the synchrotron X-ray source at Pohang Accelerator Laboratories. NEXAFS spectroscopy is very sensitive to the local environment of the atoms, so crystalline defects can be reflected in the spectrum37. Figure 5a shows the NEXAFS B K-edge and N K-edge spectra from the sp2-BN film transferred onto a conductive silicon substrate. In the NEXAFS B K-edge spectrum, the peaks at 192.0 eV and 199.0 eV correspond to the π*, and σ* anti-bonding states of the sp2-hybridized boron, respectively. Similarly, the peaks at 399.7 eV and 406.8 eV in the NEXAFS N K-edge spectrum correspond to the π* and σ* anti-bonding states of sp2-hydrizied nitrogen, respectively38. One remarkable feature is the shoulder peaks at the vicinity of π* signal in the NEXAFS B K-edge spectrum which is clearly shown in Figure 5b.


13


Page 14 of 25

Figure 5. Near-edge X-ray absorption fine-structure (NEXAFS) spectrum. (a) NEXAFS B Kedge and N K-edge spectrum of the sp2-BN film, and (b) NEXAFS B K-edge spectrum showing π* peak with two shoulder peaks. MOCVD-grown sp2-BN includes nitrogen vacancy as well as boron-boron homoelemental bonding which can be present in the grain boundary. The defect forming the trap with the activation energy of approximately 25 meV is presumed to be related to the grain boundaries in the sp2-BN film, because the grain boundaries are likely to include B-B homoelemental bondings39 which is reflected as the low-energy shoulder of π* signal40,41 of the NEXAFS B K-edge spectrum as shown in Figure 5b. In addition, the grain boundaries are known to cause in-gap states near the band edge42. The grain size of MOCVDgrown sp2-BN is only a few nanometer (see TEM image in Supporting Information S10) which is much smaller than the channel length. Localized states of the grain boundaries contribute to space-charge evolution in the channel which can play an important role in determining SCLC conduction43. On the other hand, the high-energy shoulder at around 192.5 eV is attributed to VN38. The B/N ratio of the sp2-BN film estimated by a XPS/ESCA (electron spectroscopy for chemical analysis) using Al Kα X-ray -radiation is over unity as shown in Supporting Information S11 that can also imply the existence of a large amount of VN in the film. Theoretically, VN is predicted to be a shallow-level defect acting as a donor for h-BN44,45. More recent photoluminescence study showed that VN is located at approximately from 100 meV (for multi-layer h-BN) to 400 meV (for monolayer h-BN) below the conduction band edge depending on the number of atomic layers46. Taken together with the observations of this study and previous reports, VN is the most likely candidate for the defect corresponding to the trap with the activation energy of approximately 185 meV. Therefore, VN can affect the electrical conductivity of sp2-BN because


14

Page 15 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


of its small activation energy. Thus, controlled generation of VN can be an effective doping method. For verifying the doping effect of VN, the electrical conduction properties of three sp2BN films grown under different conditions during MOCVD growth were compared as shown in Figure 6. The sp2-BN #2 has been grown under the condition introduced earlier in the paper, and is the sample that has been discussed so far. The other sp2-BN films were grown under the same conditions with sp2-BN #2 except that the reactor pressure was changed to 400 mbar for sp2-BN #1 and the carrier gas was changed to nitrogen gas for sp2-BN #3. As the growth pressure increases, VN decreases while the grain boundary in sp2-BN increases due to the enhanced chemical reaction rate and nucleation formation rate47. Meanwhile, hydrogen carrier gas is necessary to enhance the crystallinity of MOCVD-grown sp2-BN27,48, so both VN and grain boundary increase when nitrogen is used as carrier gas. More detailed information on the growth condition and the structural properties of the sp2-BN films are discussed in Supporting Information S12. Figure 6a shows the NEXAFS B K-edge spectra of the sp2-BN films for the photon energy from 189.5 eV to 194.5 eV. Looking at the change of high-energy shoulder of π* peak, the sp2-BN #3 has the largest amount of VN, and the sp2-BN #1 has the least amount. Figure 6b is the I-V characteristics of the sp2-BN films at the temperature of 500 K with the channel length of 6.0 µm. This shows that SCLC is the dominant conduction mechanism for all of the sp2-BN films (I ∝ , with m ≥ 2), and conduction levels are dramatically enhanced by increasing VN. Under the same applied voltage, the current through the sp2-BN #3 is more than 5 orders higher than that through the sp2-BN #1. The I-V characteristics of the sp2-BN film at room temperature show the same trend with those at 500K as shown in Supporting Information S13.Therefore, VN can increase the conductivity of the sp2-BN film like a dopant.


15


Page 16 of 25

Figure 6. Electrical conduction tuning by nitrogen vacancy. (a) NEXAFS B K-edge spectrum for three sp2-BN films. Difference in high-energy shoulder shows thee sp2-BN films have different amount of nitrogen vacancy, and (b) I-V characteristics of three sp2-BN films measured for channel length of 6 µm at the temperature of 500K.

Carrier type of the MOCVD-grown sp2-BN films is probably electron, thus, they are n-type semiconductors based on the following three aspects: At first, it is generally known that VN acts as a donor based on the first-principle calculations44,45. Secondly, larger current flows with the Ti/Au contact compared to the Ni/Au contact as shown in Supporting Information S14. Since Ti has smaller work function (~4.3 eV) than Ni (~5.2eV), allowing larger current for n-type semiconductors. In addition, Ti/Au and Ni/Au contacts are widely used as n-type and p-type ohmic contacts for III-nitrides semiconductors, respectively. The formation of Ti-based Ohmic contact on III-nitrides semiconductors is attributed to the formation Ti nitrides after thermal annealing that generates VN 49, which can occur between Ti and sp2-BN. A similar phenomenon can occur between Ti and sp2-BN. Thirdly, the sp2-BN film shows n-type sensing behavior to the


16

Page 17 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


NO2 gas which means current decreases when NO2 is exposed, as shown in Supporting Information S15. Since NO2 is a well-known oxidizing gas capturing electron from the material, n-type sensing behavior infers electrical transport depends on the electron density. Therefore, VN can act as a donor providing an electron in sp2-BN film. In general, substitutional doping is widely employed to control both the amount and the type of conductivity of a semiconductor. However, it was theoretically predicted to be very ineffective for n-type doping for h-BN. Activation energy of silicon substituting boron (SiB) in hBN is 1.19 eV which is too high to employ Si as a donor of h-BN. In addition, other substitutional dopants including SN, CB, GeB, SnB, SeN, and TeN, were also found to have very high activation energies (>1 eV) because they induce deep and localized impurity states22. More recently, the activation energy of Si dopant was experimentally determined as 1.2 eV which is consistent with the theoretical prediction23. Meanwhile, VN, revealed to be a donor theoretically and experimentally, shows much smaller activation energy (~185meV) comparing to the Si dopant. Therefore, VN generation in sp2-BN in a controlled way can be an alternative approach for n-type doping of sp2-BN to replace conventional substitutional doping. VN generation can be controlled in the MCOVD growth of sp2-BN. It was reported that formation of VN is enhanced by decreasing reactor pressure47 and by using nitrogen carrier gas rather than hydrogen carrier gas49. Therefore, it is possible to control the generation of VN by regulating the reactor pressure and adjusting the fraction of hydrogen gas and nitrogen gas used as a carrier gas in the MCOVD growth.


17


Page 18 of 25

CONCLUSION

In conclusion, in-plane electrical conduction characteristics of sp2-BN were studied to demonstrate the possibility of sp2-BN as an active material for electronics and deep ultraviolet optoelectronics. In this work, the defect-mediated in-plane electrical conduction property in a few layer sp2-BN was reported for the first time. The temperature-dependent I-V characteristics revealed that there are two shallow traps originating from B-B homoelemental bonding from the grain boundary and the nitrogen vacancy according to the NEXAFS spectroscopy. The activation energies of the two shallow defects were estimated to be approximately 25 meV and 185 meV, respectively. Furthermore, it was also shown that the conductivity of the sp2-BN film can be intentionally adjusted by controlling the amount of VN. Since activation energy of VN (~185 meV) is much smaller than that of conventional Si dopant (~1200 meV), controlled generation of VN can be a new and better approach for obtaining n-type sp2-BN than conventional substitutional doping. We believe that the understanding the defect-mediated electrical conduction characteristics in sp2-BN and a way to control it proposed in our study can pave the way for a realization of active sp2-BN-based high-temperature electronics and deep ultraviolet optoelectronics


18

Page 19 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publication website at DIO:

MOCVD growth of BN film. Transfer procedure. Cross-sectional transmission electron microscopy. Metal contact annealing effect. Device scheme and measurement setup. Electrical conduction of sapphire substrate. Effect of metal diffusion on electrical conduction. Spacecharge-limited conduction at high temperature. Comparison between space-charge-limited conduction and Poole-Frenkel conduction. In-plane transmission electron microscopy. B/N ratio of sp2-BN film. Structural properties of various sp2-BN films. I-V characteristics of sp2-BN films at room temperature. Contact scheme dependent I-V characteristics. Sensing behavior of sp2-BN to the NO2 gas(PDF)

AUTHOR INFORMATION Corresponding Author * Jong Kyu Kim, E-mail: [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.


19


Page 20 of 25

Funding Sources This work was supported by Samsung Research Funding Center of Samsung Electronics under Project Number SRFC-MA1401-10, and by the Brain Korea 21 PLUS project for Center for Creative Industrial Materials (F14SN02D1707).


20

Page 21 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


REFERENCES (1) Dean, C. R.; Young, A. F.; Meric, I.; Lee, C.; Wang, L.; Sorgenfrei, S.; Watanabe, K.; Taniguchi, T.; Kim, P.; Shepard, K. L.; Hone, J. Boron Nitride Substrates for High-Quality Graphene Electronics. Nat. Nanotech. 2010, 5, 722-726. (2) Bresnehan, M. S.; Hollander, M. J.; Wetherington, M.; Wang, K.; Miyagi, T.; Pastir, G.; Snyder, D. W.; Gengler, J. J.; Voevodin, A. A.; Mitchel, W. C.; Robinson, J. A. Prospects of Direct Growth Boron Nitride Films as Substrates for Graphene Electronics. J. Mater. Res. 2014, 29, 459-471. (3) Kim, S. M.; Hsu, A.; Park, M. H.; Chae, S. H.; Yun, S. J.; Lee. J. S.; Cho, D. -H.; Fang, W.; Lee, C.; Palacios, T.; Dresselhaus, M.; Kim, K. K.; Lee. Y. H.; Kong, J. Synthesis of Large-Area Multilayer Hexagonal Boron Nitride for High Material Performance. Nat. Commun. 2015, 6, 8662. (4) Kim, K. K.; Hsu, A.; Jia, X.; Kim, S. M.; Shi, Y.; Dresselhaus, M.; Palacios, T.; Kong, J. Synthesis and Characterization of Hexagonal Boron Nitride Film as a Dielectric Layer for Graphene Devices. ACS Nano 2012, 6, 8583-8590. (5) Jang, S. K.; Youn, J.; Song, Y. J.; Lee, S.; Synthesis and Characterization of Hexagonal Boron Nitride as a Gate Dielectric. Sci. Rep. 2016, 3, 30449. (6) Liu, Z.; Gong, Y.; Zhou, W.; Ma, L.; Yu, J.; Idrobo, J. C.; Jung, J.; MacDonald, A. H.; Vajtai, R.; Lou, J.; Ajayan, P. M. Ultrathin High-Temperature Oxidation-Resistant Coatings of Hexagonal Boron Nitride. Nat. Commun. 2013, 4, 2541. (7) Shanmugam, M.; Jain, N.; Jacobs-Gedrim, R.; Xu, Y.; Yu, B. Layered Insulator Hexagonal Boron Nitride for Surface Passivation in Quantum Dot Solar Cell. Appl. Phys. Lett. 2013, 103, 243904. (8) Li, X.; Jordan, M. B.; Ayari, T.; Sundaram, S.; Gmili, Y. E.; Alam, S.; Alam, M.; Patriarche, G.; Voss, P. L.; Salvestrini, J. P.; Ougazzaden, A. Flexible Metal-Semiconductor Device Prototype on Wafer-Scale Thick Boron Nitride Layers Grown by MOVPE. Sci. Rep. 2017, 7, 786. (9) Doan, T. C.; Li, J.; Lin, J. Y.; Jiang, H. X. Growth and Device Processing of Hexagonal Boron Nitride Epilayers for Thermal Neutron and Deep Ultraviolet Detectors. AIP Adv. 2016, 6, 075213. (10) Kubota, Y.; Watanabe, K.; Tsuda, O.; Taniguchi. T. Deep Ultraviolet Light-Emitting Hexagonal Boron Nitride Synthesized at Atmospheric Pressure. Science 2007, 317, 932-934. (11) Watanabe, K.; Taniguchi, T.; Kanda, H. Direct-Bandgap Properties and Evidence for Ultraviolet Lasing of Hexagonal Boron Nitride Single Crystal. Nat. Mat. 2004, 3, 404-409. (12) Dahal, R.; Li, J.; Majety, S.; Pantha, B. N.; Cao, X. K.; Lin, J. Y.; Jiang, H. X. Epitaxially Grown Semiconducting Hexagonal Boron Nitride as a Deep Ultraviolet Photonic Material. Appl. Phys. Lett. 2011, 98, 211110.


21


Page 22 of 25

(13) Nose, K.; Yang, H. S.; Oba, H.; Yoshida, T. Defect-Induced Electronic Conduction of tBN Thin Films. Diamond Relat. Mater. 2005, 14, 1960-1963. (14) Zhang, Y.; Zhang, L.; Zhou, C. Review of Chemical Vapor Deposition of Graphene and Related Applications. Acc. Chem. Res. 2013, 45, 2329-2339. (15) Yu, J.; Li, J.; Zhang, W.; Chang, H. Synthesis of High Quality Two-Dimensional Materials via Chemical Vapor Deposition. Chem. Sci. 2015, 6, 6705. (16) Zang, Z.; Nakamura, A.; Temmyo, J. Single Cuprous Oxide Films Synthesized by Radical Oxidation at Low Temperature for PV Application. Opt. Expr. 2013, 21, 11448-11456. (17) Zang, Z.; Nakamura, A.; Temmyo, J. Nitrogen Doping in Cuprous Oxide Films Synthesized by Radical Oxidation at Low Temperature. Mater. Lett. 2013, 92, 118-191. (18) Mahvash, F.; Paradis, E.; Drouin, D.; Szkopek, T.; Siaj, M. Space-Charge Limited Transport in Large-Area Monolayer Hexagonal Boron Nitride. Nano Lett. 2015, 15, 2263-2268. (19) Doan, T. C.; Li, J.; Lin, J. Y.; Jiang, H. X. Charge Carrier Transport Properties in Layer Structured Hexagonal Boron Nitride. AIP Adv. 2014, 4, 107126. (20) Nam, K. B.; Nakarmi, M. L.; Lin, J. Y.; Jiang, H. X. Deep Impurity Transitions Involving Cation Vacancies and Complexes in AlGaN Alloys. Appl. Phys. Lett. 2005, 86, 222108. (21) Nepal, N.; Nakarmi, M. L.; Lin, J. Y.; Jiang, H. X. Photoluminescence Studies of Impurity Transitions in AlGaN Alloys. Appl. Phys. Lett. 2006, 89, 092107. (22) Oba, F.; Togo, A.; Tanaka, I.; Watanabe, K.; Taniguchi, T. Doping of Hexagonal Boron Nitride via Intercalation: A Theoretical Prediction. Phys. Rev. B 2010, 81, 075125. (23) Majety, S.; Doan, T. C.; Li, J.; Lin, J. Y.; Jiang, H. X. Electrical Transport Properties of SiDoped Hexagonal Boron Nitride Epilayers. AIP Adv. 2016, 3, 122116. (24) Gorbachev, R. V.; Riaz, I.; Nair, R. R.; Jalil, R.; Britnell, L.; Belle, B. D.; Hill, E. W.; Novoselov, K. S.; Watanabe, K.; Taniguchi, T.; Geim, A. K.; Blake, P. Hunting for Monolayer Boron Nitride: Optical and Raman Signatures. Small 2011, 7, 465-468. (25) Song, L.; Ci, L.; Lu, H.; Sorokin, P. B.; Jin, C.; Ni, J.; Kvashnin, A. G.; Kvashnin, D. G.; Lou, J.; Yakobson, B. I.; Ajayan, P. M. Large Scale Growth and Characterization of Atomic Hexagonal Boron Nitride Layers. Nano Lett. 2010, 10, 3209-3215. (26) Shi, Y.; Hamsen, C.; Jia, X.; Kim, K. K.; Reina, A.; Hofmann, M.; Hsu, A. L.; Zhang, K.; Li, H.; Juang, Z. -Y.; Dresselhaus, M. S.; Li, L. -J.; Kong, J. Synthesis of Few-Layer Hexagonal Boron Nitride Thin Film by Chemical Vapor Deposition, Nano Lett. 2010, 10, 4134-4139. (27) Chubarov, M.; Pedersen, H.; Högberg, H.; Jensen, J.; Henry, A. Growth of High Quality Epitaxial Rhombohedral Boron Nitride. Cryst. Growth Des. 2012, 12, 3215-3220. (28) Sutter, P.; Lahiri, J.; Zahl, P.; Wang, B.; Sutter, E. Scalable Synthesis of Uniform FewLayer Hexagonal Boron Nitride Dielectric Films. Nano Lett. 2013, 13, 276-281.


22

Page 23 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(29) Simmons, J. G. Conduction in Thin Dielectric films. J. Phys. D: Appl. Phys. 1971, 4, 613657. (30) Chiu, F. -C. A Review on Conduction Mechanisms in Dielectric Films. Adv. Mater. Sci. Eng. 2014, 2014, 578168. (31) Lampert, M. A. Simplified Theory of Space-Charge-Limited Currents in an Insulator with Traps. Phys. Rev. 1956, 103, 1648-1656. (32) Lampert, M. A. Volume-Controlled Current Injection in Insulators. Rep. Prog. Phys. 1964, 27, 329-367. (33) Mathur, V. K.; Dahiya, R. P. Space-Charge-Limited Currents in Insulators Containing Traps Distributed in Energy. Solid State. Electron. 1974, 17, 61-70. (34) Gravano. S.; Hassan, A. K.; Gould, R. D. Effects of Annealing on the Trap Distribution of Cobalt Phthalocyanine Thin Films. Int. J. Electronics 1991, 70, 477-484. (35) Ohba, N.; Miwa, K.; Nagasako, N.; Fukumoto, A. First-Principles Study on Structural, Dielectric, and Dynamical Properties for Three BN Polytypes. Phys. Rev. B 2001, 63, 115207. (36) Kumar, S.; Arora, R.; Kumar, A. High-Field Conduction in a-Se80Te20 and a-Se80Te10M10 (M=Ag, Cd or Sb). Physica B 1993, 183, 172-178. (37) Hemraj-Benny, T.; Banerjee, S.; Sambasivan, S.; Balasubramanian, M.; Fischer, D. A.; Eres, G.; Puretzky, A. A.; Geohegan, D. B.; Lowndes, D. H.; Han, W.; Misewich, J. A.; Wong, S. S. Near-Edge X-ray Absorption Fine Structure Spectroscopy as a Tool for Investigating Nanomaterials. Small 2006, 2, 26-35. (38) Jiménez, I.; Jankowski, A. F.; Terminello, L. J.; Sutherland, D. G. J.; Carlisle, J. A.; Doll, G. L.; Tong, W. M.; Shuh, D. K.; Himpsel, F. J.; Core-Level Photoabsorption Study of Defects and Metastable Bonding Configurations in Boron Nitride. Phys. Rev. B 1997, 55, 12025-12037. (39) Liu, Y.; Zou, X.; Yakobson, B. I. Dislocations and Grain Boundaries in Two-Dimensional Boron Nitride. ACS Nano 2012, 6, 7053-7058. (40) Li, D.; Bancroft, G. M.; Fleet, M. E. B K-Edge XANES of Crystalline and Amorphous Inorganic Materials. J. Electron. Spectrosc. Relat. Phenom. 1996, 79, 71-73. (41) Petravic, M.; Peter, R.; Kavre, I.; Li, L. H.; Chen, Y.; Fan, L. -J.; Yang, Y. -W. Decoration of Nitrogen Vacancies by Oxygen Atoms in Boron Nitride Nanotubes. Phys. Chem. Chem. Phys. 2010, 12, 15349-14353. (42) Li, Q.; Zou, X.; Liu, M.; Sun, J.; Gao, Y.; Qi, Y.; Zhou, X.; Yakobson, B. I.; Zhang, Y.; Liu, Z. Grain Boundary Structures and Electronic Properties of Hexagonal Boron Nitride on Cu(111). Nano Lett. 2015, 15, 5804-5810. (43) Kalita, P. KR.; Sarma, B. K.; Das. H. L. Space Charge Limited Conduction in CdSe Thin Films. Bull. Mater. Sci. 2003, 26, 613-617.


23


Page 24 of 25

(44) Orellana, W.; ChaCham, H. Stability of Native Defects in Hexagonal and Cubic Boron Nitride. Phys. Rev. B 2001, 63, 125205. (45) Yin, L. -C.; Chen, H. -M.; Saito, R. Triangle Defect of Hexagonal Boron Nitride Atomic Layer: Density Functional Theory Calculations. Phys. Rev. B 2010, 81, 153407. (46) Du, X. Z.; Uddin, M. R.; Li, J.; Lin, J. Y.; Jiang, H. X. Layer Number Dependent Optical Properties of Multilayer Hexagonal BN Epilayer. Appl. Phys. Lett. 2017, 110, 092102. (47) Kim, D. Y.; Han, N.; Jeong, H.; Kim, J.; Hwang, S.; Song, K.; Cho, S. -Y.; Kim, J. K. Pressure-Dependent Growth of Wafer-Scale Few-layer h-BN by Metal-Organic Chemical Vapor Deposition. Cryst. Growth Des. 2017, 17, 2569-2575. (48) Kim, D. Y.; Han, N.; Jeong, H.; Kim, J.; Hwang, S.; Kim, J. K. Role of Hydrogen Carrier Gas on the Growth of Few Layer Hexagonal Boron Nitrides by Metal-Organic Chemical Vapor Deposition. AIP Adv. 2017, 7, 045116. (49) Dobos. L.; Pécz, B.; Tóth, L.; Horváth. Z. E.; Tóth, A.; Horváth. E.; Beaumont, B.; Bougrioua, Z. Metal Contacts to n-GaN. Appl. Surf. Sci. 2006, 253, 655-661.


24

Page 25 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Abstract graphic


25

hybridized Boron Nitride - ACS Publications

Recommend Documents