Atomic Layer Deposition of p-Type Bi

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Atomic Layer Deposition of p-Type BiS Neha Mahuli, Debabrata Saha, and Shaibal K. Sarkar

J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b12629 • Publication Date (Web): 22 Mar 2017 Downloaded from http://pubs.acs.org on March 30, 2017

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The Journal of Physical Chemistry

Atomic Layer Deposition of p-type Bi2S3 Neha Mahuli1, Debabrata Saha2 and Shaibal K Sarkar2,* 1

Center for Research for Nano Technology and Sciences 2

Department of Energy Science and Engineering.

Indian Institute of Technology Bombay, Powai, 400076 India KEYWORDS: Bismuth Sulfide, Atomic Layer Deposition (ALD), in-situ Quartz Crystal Microbalance (QCM), Electrical Transport

ABSTRACT

Atomic Layer Deposition (ALD) of bismuth sulfide (Bi2S3) is demonstrated by the sequential exposure of bismuth(III)bis(2,2,6,6-tetramethylheptane-3,5-dionate) [Bi(thd)3] and hydrogen sulfide (H2S) at 200°C. The saturated growth rate of 0.34-0.37 Å/cycle is observed via in-situ (quartz crystal microbalance, QCM) and, verified by, ex-situ (X-ray reflectivity, XRR) measurements throughout the ALD temperature window. As-deposited Bi2S3 films are polycrystalline in nature without any preferential orientation. In addition to the direct band gap at ca. 1.56 eV as normally seen in for Bi2S3 we also find an evidence for the indirect band gap at ca. 1.03 eV. Ultraviolet photoelectron spectroscopy (UPS) and Seebeck measurements strongly

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support the degenerate p-type conductivity of the as-grown thin films in contrast to the n-type nature normally found in the literature. Temperature dependent (70-300 K) electrical resistivity measurements show that in the temperature range of 70-100 K, variable range hopping (VRH) is the dominant carrier transport process while above 100 K clear deviation from the VRH transport equation is observed, implying a crossover from localized states to band-like transport process.


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INTRODUCTION: Bismuth Sulfide (Bi2S3) is well explored as technologically important material due to its nontoxic nature with moderately good chemical and environmental stability, high figure of merit, low band gap and tunable electronic properties1,2,3 . The upper valence band of the Bi2S3 consists of the S 3p levels with very small contribution of the Bi 6p levels while the conduction band is predominantly contributed from the Bi 6p level that is hybridized with S 3p character4. Mostly this material is found to be n-type in nature due to predominant sulfur vacancies, similar to many other metal sulfides like Sb2S3, CdS etc. Sulfur defects have shown notable capability to tune the electrical conductivity considerably in a bulk semiconductor4,5. Doping with metal cations like Ag+ 6, Cu+

7,8

and halides8 poses remarkable enhancement in the net carrier concentration. In a

recent article, Biswas et al.1 show n-type nature of the Bi2S3 ingots with an addition of BiCl3. Similarly, it is interesting to find that Sn acts as an amphoteric dopant in Bi2S3; Sn4+ as n-type dopant while Sn2+ as p-type dopant4. Though technologically interesting, p-type Bi2S3 is not widely studied or even less reported apart from very recent report of experimentally obtained ptype ultrathin exfoliated nanosheets of Bi2S39. However reports of CuxBiySz or sometimes referred as Cu3BiS3, found to be p-type; exist in the literature10. Most often the cation deficiency induces p-type conductivity in metal chalcogenides, like CuS11, yet there is no such result that we came across in Bi2S3. Various techniques are explored to deposit Bi2S3 thin films in the literature so far. Although solution routes like chemical bath deposition12,2,13, SILAR3,14 etc. are more commonly practiced, considerable number of reports are presented via chemical vapor deposition (CVD)

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, (MO)-

CVD16,17,18 as well.


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During the last two decades, atomic layer deposition (ALD) has been materialized as one of the promising vacuum based deposition techniques due to its self-limiting growth mechanism19. ALD attracts attention due to its capability to produce uniform, homogeneous, pinhole free and conformal semiconductor coatings even on high aspect ratio structures19. The major drawback of current state of the art vacuum techniques where they typically fail to deliver a very precise thickness control below ca. 10 nm films is overcome by an atomistic precision over angstrom level thickness using ALD19. A variety of deposition chemistry are explored in ALD to deposit metals and compounds20. Binary and ternary metal chalcogenides; predominantly sulfides; are deposited mostly using hydrogen sulfides as the chalcogenide precursor. Lately efforts to grow Bi2S3 via ALD have been investigated by Liu et al21. They report a method to obtain thin films of Bi2S3 by sulfurization of ALD grown Bi2O3 films within the temperature range of 500-700oC. In this paper, we report the atomic layer deposition of p-type Bi2S3 using the sequential exposure of Bi(thd)3 and H2S at 200°C in a custom-built hot wall reactor equipped with in-situ QCM. Self-limiting growth is optimized with an overall growth rate of 0.34-0.37 Å per ALD cycle within the ALD temperature window of 175-250°C. As-deposited films are found to be polycrystalline in nature with high chemical purity. UV-VIS measurements suggest a combination of direct and indirect band gaps. The detailed investigation with photoelectron spectroscopy suggests high density of localized states at or near the Fermi energy. Temperature dependent electrical resistivity measurements are carried out in the temperature range of 70-300 K to gain some insights into the underlying charge transport mechanisms. In the temperature range of 70-100 K, variable range hopping (VRH) transport is found to be the predominant carrier transport process. However, further increment in temperature shows a clear crossover from localized states to band-like transport process.


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EXPERIMENTAL: Bismuth Sulfide (Bi2S3) films are deposited by atomic layer deposition (ALD) in a custombuilt

hot-wall

viscous

flow

reactor

in

the

temperature

range

of

150-300°C.

Bismuth(III)bis(2,2,6,6-tetramethylheptane-3,5-dionate) (Bi(thd)3, from Gelest Inc.) and hydrogen sulfide (H2S) (99.99% purity, from Asia advanced gas, Hong Kong) are the two metal and chalcogen precursors used here. Bi(thd)3; a solid precursor; is heated to a temperature ca. 95°C inside a closed stainless steel container to produce sufficient vapor pressure. High purity nitrogen (N2, 99.999% purity) gas is used as the carrier gas through the overhead assembly to carry the precursor into the reaction chamber during the process. Reactants are carried to the reaction chamber through differentially heated channels. The partial pressures of both the reactants inside the reaction chamber are kept constant as ca. 1.2 ± 0.1 Torr and 1.5 ± 0.1 Torr for Bi(thd)3 and H2S respectively throughout the deposition. The reactor base pressure at all times is maintained at 1 Torr by flowing 200 sccm of N2. Run-time chamber pressures and precursor partial pressures are monitored by capacitance manometer, Baratron™. N2 is used also as a purge gas between successive doses of reactants. The overall dosing sequence is expressed as (n x t1) - t2 - (m x t3) - t4 where n and m represent the number of pulses of Bi(thd)3 and H2S respectively while t1, t3 are the dose times and t2, t4 are the intermediate N2 purge times for Bi(thd)2 and H2S half cycles respectively. The process automation is monitored with the help of LabView. The deposition characteristics are studied with in-situ Quartz Crystal Microbalance (QCM). A polished, gold coated AT cut gold crystal (6 MHz resonant frequency) along with the SQM 160


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(both from Inficon, USA) thickness monitor are used to measure frequency changes during each half of ALD cycle. Mass changes are calculated using Saurbrey equation. A crystal drawer and retainer assembly (from Inficon, USA) is used with a non-conducting silver paste to vacuum seal the sensor. Additionally, 20 sccm N2 flow is maintained through the QCM assembly to prevent unwanted deposition at the back contact. The sensor surface is coated with 200 cycles of ALD grown Al2O3 before starting the experiment. The material growth rate, obtained by in-situ QCM studies, is again verified by ex-situ X-ray reflectivity (XRR) measurements. For all ex-situ measurements films are deposited on Si (111) substrate with native SiO2. Rigaku Smartlab X-ray Diffractometer, equipped with Cu-Kɑ source, is used for XRR measurements. Commercially available Globalfit software is used to simulate the experimentally obtained data. Crystallographic nature of the as-deposited Bi2S3 films is studied by X-ray Diffraction (XRD) and Transmission Electron Microscopy (TEM) characterizations. For XRD measurements, the Bi2S3 films are grown on soda-lime glass substrates. XRD is acquired with the same Rigaku Smartlab X-ray Diffractometer with fixed incident angle (0.5o). The high resolution TEM (HRTEM) images and the small area electron diffraction (SAED) are obtained using Tecnai G2, F30 model from FEI using 300 kV beam energy. Time-of-flight secondary ion mass spectroscopy (TOF-SIMS) and X-ray photoelectron spectroscopy (XPS) are explored to study the chemical composition of the deposited ALD films on Si(111) and glass substrates respectively. PHI TRIFT V nanoTOFTM instrument from ΦULVAC-Physical Electronics, Mn, USA is used to acquire the depth profile in positive SIMS mode with 50 µm x 60 µm of 30 kV Ga analysis gun point while 600 µm x 600 µm of 1 kV Cs sputtering gun crater. As the films grown are comparatively low in thickness, a low sputtering


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speed of 2 sec with as high as 6 analysis frames per cycle are optimized for depth profiling. Electron Spectroscopy for Chemical Analysis (ESCA) of model AXIS Supra by Kratos Analytical, UK (SHIMADZU group) for XPS and Ultra-violet Photoelectron Spectroscopy (UPS) measurements is used. For XPS analysis, Al 600 W X-ray source (λ =1486.6 eV) is used while He-I (21.2 eV) source is used for UPS measurements. Films are sputtered with 2 keV Ar+ for 30 sec before acquiring XPS and UPS measurements to remove any surface oxidation if present. The acquired spectra are calibrated against the carbon peak at 284.6 eV. Curve fitting is carried out with XPS Peak 4.1 software. The total optical transmission is obtained using a Lambda 950 UV-NIR visible spectrometer with an integrated sphere assembly from Perkin-Elmer. Total transmission with reflection correction is used to obtain band gap of Bi2S3 thin films grown on glass substrates. Electrical measurements are performed in four-point probe Van der Pauw geometry using Lakeshore 8404 AC/DC Hall Measurement System with a helium atmosphere-based closed cycle refrigerator. As-deposited Bi2S3 films on glass substrates of size 5 mm x 5 mm with indium contacts at the four corners are used. AC Hall measurement technique is used for reliable extraction of the low Hall voltage signal. An AC magnetic field of frequency 100 mHz with amplitude of 1.2 T is used and the desired ac Hall voltage was measured using a lock-in amplifier.

RESULTS AND DISCUSSION: Bi2S3 ALD is accomplished with the sequential exposures of Bi(thd)3 and H2S at 200°C. Following the other Metal-(thd)x precursor chemistry22, the proposed binary reaction can be written as follows:


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2Bi(thd)3 + 3H2S Bi2S3 + 6H(thd)

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(1)

This overall reaction can be split in two half-cycle reactions as follows: (A)

Bi-SH* + Bi(thd)3 Bi-S-Bi-(thd)2* + H-(thd)

(2)

(B)

Bi-(thd)2* + 2H2S Bi-(SH)2* + 2H-(thd)

(3)

Where, ‘*’ denotes the surface species at the end of each half cycle. The overall deposition chemistry for Bi2S3 deposition using Bi(thd)3 and H2S is depicted in the equation 1. In the first half cycle, represented by equation-2, gaseous Bi(thd)3 undergoes a surface limited reaction resulting in H-(thd) as the byproduct. The above reaction results in a new altered surface species, S-Bi(thd)*. During the second half cycle, similar surface limited reaction with H2S rejuvenates the original thiol terminated surface as depicted by equation-B. Repetition of ABABAB…. combinations lead to a layer-by-layer growth of Bi2S3.

Figure 1. In-situ QCM growth characteristics for Bi2S3 revealing nucleation and linear regimes on Al2O3 surface at 200°C. We employ in-situ QCM to study the above mentioned sequential ABAB.. growth characteristics. Figure 1 represents mass changes acquired by QCM during the first 500 cycles of


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Bi2S3 on Al2O3 surface at 200°C using a sequential pulsing sequence of (5x1-15-1x3-15). The film undergoes a rigorously long nucleation period before entering into the linear growth regime. During the initial few cycles from the nucleation regime, the growth is typically on Al-OH* surface from starting Al2O3 surface. During this period, the H(thd) species tend to react with the unreacted Al-OH* ligands to form Al(thd)* species. These Al(thd)* species are observed to be significantly less reactive to the H2S exposures in the second half cycle and hence act as surface blocking sites for the upcoming reaction cycles. The in-situ FTIR studies with M-(thd)x as the precursor has already been reported in our earlier report22 which clearly indicates the high stability of these bonds during H2S exposures and are seen to desorb only with time. Hence, the longer nucleation periods can be attributed to the contributed effect of the steric hinderences from larger (thd) molecules and the surface poisoning by the re-adsorption of (thd)* species to form stable Al(thd)*. The similar surface poisoning observations have also been reported with M-(acac)x and M-(hfac)x precursors which result in formation of very stable Al(acac)*23,24 and Al(hfac)*25 species.

Figure 2. In-situ QCM mass gain and corresponding precursor exposures versus time for two representative cycles from linear regime.


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Figure 2 represents the close-up of two representative ALD cycles from Bi2S3 linear regime where the transient mass changes and the corresponding reactant partial pressures are represented alongside. As expected the positive mass change is observed during the Bi(thd)3 exposures while a sharp decrease in mass is evident after the H2S dose. The total mass increase during the Bi(thd)3 exposures depicts the net Bi(thd)* adsorption on the surface. Whereas the mass loss upon H2S doses is a resultant of two competing mechanisms, removal of H-thd species from the surface and addition of –SH moieties. From the figure 2, the overall positive mass change during the first half cycle is ∆m1 = 43-46 ng/cm2. The mass loss of ∆m2 = 10-14 ng/cm2 during the second half cycle is majorly dominated by the removal of heavy (thd) ligands. Hence the total mass changes per cycle of ∆m = 32-34 ng/cm2 is observed in the linear regime. The above mass change corresponds to a growth rate of 0.34-0.37 Å/cyl. The quantitative analysis of the in-situ QCM data is then done thoroughly for validation of the binary reaction mechanism shown by equation-2 and equation-3. The proposed equations ascribing the probable surface limiting or ligand exchange reactions allow us to calculate the relative mass changes in an individual step. As is proposed in equation-2, -Bi(thd)2* (m/z = 575) species are added while –H* (m/z = 1) species in the form of H-(thd) are released during the Bi(thd)3 exposure in first half cycle. Similarly, -(SH)2* (m/z = 66) species will replace –(thd)2* (m/z = 368) species in the consecutive H2S half cycle as projected by equation-3. The relative mass change during each half cycle (M1 or M2) is considered equal to Mi - Mb where Mi is the intermediate addition of surface species while Mb is the appropriate exchanged ligand. M0 is the total molar mass of Bi2S3 forming at the end of each cycle. The most probable reaction mechanism is usually determined by evaluating the M0/M1 and M0/M2 ratios during the ALD half cycles. M0/M1 during the first half cycle is observed to be ca. 0.79 and M0/M2 during the


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consecutive half cycle is found to be ca. 2.08 which is within the experimental error boundaries of theoretically calculated ratios (0.89 and 1.75 respectively) validating the proposed binary surface reaction for Bi2S3 growth with the sequential pulsing of Bi(thd)3 and H2S. The characteristic self-saturation behavior is one of the salient features of any ALD mechanism. It implies the formation of a closely spaced monolayer. Upon formation of a single monolayer, any further reactions are inhibited in the same ALD cycle. This ensures the layer-bylayer growth assembly.

Figure 3. Mass changes versus dose times of H2S for self-saturation during Bi2S3 deposition. QCM is employed to study the self-saturation behavior during each half cycle of Bi2S3 deposition. Figure 3 represents the net mass change as a function of H2S dose during second half cycle (equation-3) keeping a single pulse of Bi(thd)3 constant. It is clearly depicted that any further growth is constrained beyond 3 sec of H2S dosing. This denotes the saturated surface limited growth of ca. 10 ng/cm2.


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Figure 4. Mass changes versus number of pulses of Bi(thd)3 for self-saturation during Bi2S3 deposition. Similar to the above, the optimum growth characteristics for Bi(thd)3 are investigated as shown in figure 4. Contrary to the above, the number of pulses of Bi(thd)3 are varied ensuring sufficient vapor pressure inside the bubbler. The net dosage of chalcogenide precursor is kept constant at 3 sec during these experiments. The Bi(thd)3 half cycle is observed to saturate with at least 5 consecutive Bi(thd)3 pulses of 1 sec each leading to an overall mass change of 32-34 ng/cm2 per cycle. Considering the density of Bi2S3 as ca. 6.1 g/cm3; as calculated from ex-situ XRR measurements; the overall growth rate per cycle is observed to be 0.34-0.37 Å/cyl. Henceforth, a saturated pulsing sequence of (5,1)-15-(1,3)-15 is pursued for further investigations. The thickness of Bi2S3 films is also kept constant as ca. 30 nm for all the ex-situ experiments.


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Figure 5. A representative ex-situ X-ray Reflectivity scan for ALD grown Bi2S3 on (111) Silicon for 1000 cycles. The growth rate obtained by QCM experiments is further verified by ex-situ XRR measurement. A representative XRR scan for 1000 cycles of Bi2S3 ALD on (111) silicon with native SiO2 is shown in figure 5, depicting growth rate of ca. 0.3 Å/cyl.

Figure 6. Growth rate per cycle versus deposition temperature for ALD grown Bi2S3 via ex-situ XRR measurements. Figure 6 demonstrates the effect of deposition temperature on Bi2S3 growth rate within the temperature range of 125-300°C. Any further decrease in temperature would lead to


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condensation of precursor hence we restrained ourselves to collect any further data points by lowering the temperature beyond 125°C. The characteristic growth rates are found here from XRR measurements. Growth rates within the temperature range of 175-250°C are observed to be fairly constant (ca. 0.3 Å/cycle). Lower growth rates are observed for the deposition temperatures below 175°C and above 250°C. The growth rates in the lower temperature regime are known to be limited by the precursor reactivity whereas increased desorption restricts the growth in the higher temperature range. A similar mechanistic behavior is also seen for other metal sulfides like SnS23, NiS22. The constant growth rate regime within the temperature range of 175–250°C is also termed as ALD temperature window.

Figure 7. XRD pattern for as-grown Bi2S3 on glass via ALD at 200°C. As-deposited films of ca. 30 nm are found to be polycrystalline in nature. Figure 7 represents a representative XRD pattern for as grown Bi2S3 thin film deposited at 200°C. The fine match between the experimental peak data and the JCPDS file no. 00-017-0320 provides unambiguous evidence of formation of orthorhombic Bi2S3 without any preferred orientation. High-resolution transmission electron microscopy (HR-TEM) and their selected area diffraction pattern (SAED) also reciprocate the XRD result (figure S1).


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2.

Figure 8. (a) Survey scan for as-grown ca. 30 nm Bi2S3 film on glass and high resolution scans for (b) Bi 4f and S 2p regions, (c) Bi 4d and (d) S 2s. The elemental compositional analysis is performed with XPS (figure 8a,b) and ToF-SIMS (figure S2) for as-deposited ca. 30 nm films grown on glass substrate. The XPS survey scan (Figure 8a) suggests impurity concentration is beyond detection limit of instrument in bulk of the sample (after Ar+ sputtering) whereas survey scan of as-loaded samples (figure S3) indicate presence of C 1s and O 1s representing surface oxidation as is observed in most of the cases. Figure 8b, 8c and 8d show the high resolution scans of Bi 4f and S 2p, Bi 4d and S 2s core levels respectively. Figure 8b indicates partial overlap of Bi 4f core level peaks with S 2p as expected.


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Peaks at binding energies ca. 158.1 eV and 163.28 eV are assigned to the Bi 4f7/2 and Bi 4f5/2 respectively (spin orbit split of 5.2 eV). Similarly, the doublet at binding energies of 160.3 eV and 161.4 eV corresponds to S 2p3/2 and S 2p1/2 respectively. However, due to the overlap of the Bi and S peak position, it is difficult to quantify the compositional ratio by fitting these peaks. For the same purpose, alternative Bi 4d and S 2s peaks in different energy ranges are selected as shown in figure 8c and 8d respectively. Though Bi 4d and S 2s are less prominent intensity peaks, due to the absence of any overlap within elements it will be much more reliable to use these for elemental quantification. As can be seen from figure 8c, peaks at 441.35 eV and 464.1 eV can be assigned to binding energies of Bi 4d5/2 and Bi 4d3/2 respectively whereas from the figure 8d, peak corresponding to core level of S 2s is observed at 224.9 eV. The compositional ratio of Bi:S of ALD grown samples from Bi 4d and S 2p is found to be 43:57. However, as XPS is highly surface sensitive characterization, it does not always provide bulk composition of the material. The more reliable EDAX analysis is then carried out which indicates Bi:S compositional ratio to be 33:66. Hence we believe that the as-deposited Bi2S3 films are Bi deficient.


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Figure 9. Total optical transmission data after reflection correction and (inset) absorption coefficient data for the same curve for Bi2S3 thin films on glass. Optical properties of the as-deposited Bi2S3 films (ca. 30 nm) are investigated with UV-VIS spectroscopy under transmission mode with reflection correction within the wavelength range 250 - 1800 nm. For the transmission correction we used T+R=1; considering the reflection from both front and back surfaces. In all such measurements, there are invariably some losses and hence we normalize the spectra to 100% where there is no absorption. This helps to measure the absorption onset rather more accurately. The raw transmittance and reflectance data are given in the figure S4 while total optical transmission and the corresponding absorption coefficient for the representative sample are as shown in figure 9 and 9(inset) respectively. It is worth mentioning here that the as-deposited films show significantly high absorption coefficient (> 3-4 x 105 cm-1). We then plot (ɑhν)n versus hν (Tauc plot) to determine the band-gap of as-deposited Bi2S3 thin films as shown in the Figure S5 and S6. The experimentally determined transmission spectra after reflection correction revealed a possibility of existence of indirect transition along with the direct band gap. The direct band gap (n=2) is found to be ca. 1.56 eV whereas presence of an indirect band gap at ca. 1.03 eV is also clearly observed. Though Bi2S3 reports so far have not emphasized on such existence, certain other materials like Si26, Ge27 and Ge1-ySny28 have shown similar optical band gap nature where existence of direct as well as indirect band gap is observed simultaneously. But conveniently a strong optical absorption (> 105 cm-1) across the solar spectrum mainly throughout the visible range for ALD grown Bi2S3 films makes it a potential photovoltaic absorber.


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Figure 10. Expanded UPS spectrum for the valence band onset for as-deposited Bi2S3 films. Figure 10 shows the expanded valence band onset spectrum of the Bi2S3 film as obtained by the UPS measurement. The photo emission intensity has been given in logarithmic scale to have a better illustration of the weak density of state feature at or near the Fermi energy. In UPS spectrum, valence band maximum (EVBM) with respect to the Fermi level (EF) position i.e., (EF – EVBM) is usually given by the intersection point of tangents to the leading edge of valence band onset and the Fermi level (B.E. = 0 eV). Interestingly, as can be seen from the figure 10, EF – EVBM is observed to be ca. 0.1 eV which signifies the surplus hole carriers at the valence band. The weak metal-like Fermi edge emission as evident from the step-like increase in the emission intensity at zero binding energy clearly indicates that the Fermi level is pinned very close to the valence band maximum. The unavoidable potential fluctuations due to the positional and energetic disorders plausibly give rise to the increased valence band tailing which might have resulted in the observed Fermi level pinning in the UPS spectrum. At room temperature, electrons from the valence band can easily jump into these localized tail states, leaving behind holes and making the acceptor states negatively charged which indeed contribute in the UPS emission. Such carrier activation process renders the film heavily p-type conducting at room


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temperature that has also been confirmed by the Seebeck effect measurement (figure S7). A similar observation of such small valence band onset has also been made by Clark et al9 in a recently published Bi2S3 article. Room temperature and temperature dependent electrical characterizations have been carried out for as-grown Bi2S3 thin films of ca. 30 nm grown on glass substrate. The room temperature resistivity, Hall mobility and carrier density of the film are measured to be ca. 9.82 Ω-cm ,ca. 0.1 cm2/V.s and ca. 6.8 x 1018 cm-3 respectively. The as-grown material is found to be p-type, also confirmed through the Seebeck effect measurement (figure S7) and from the valence band spectroscopy measurement as discussed in figure 10 earlier. The high hole density plausibly arises due to the formation of intrinsic shallow acceptor type point defects. Chemical compositional analysis of the films as discussed above reveals the formation of sub-stoichiometric films which corresponds to a chemical formula Bi1.5S3 (from EDAX measurement). It implies the formation of Bi vacancies which may act as the native ptype point defects in the as-grown films. Even though the material exhibits sufficiently high carrier density, the low Hall mobility ca. 0.1 cm2/V.s renders the film highly resistive. The low carrier mobility can be attributed to strengthened carrier scattering from the ionized impurities, grain boundaries and other micro-structural disorders in the film. The high degree of static disorder in as-grown Bi2S3 film is indeed confirmed by its polycrystalline nature without any preferred orientation and the pronounced sub-gap optical absorption as have been observed in the XRD pattern (figure 7) and UV-VIS optical absorption spectrum (figure 9) respectively. Figure 11 shows temperature dependent resistivity, ρ(T), characteristics of as grown Bi2S3 film. As depicted in the figure 11(a), the resistivity value increases rapidly at lower temperature ranges (T < 100 K) and reaches substantially high value (ca. 5.14 x 105 Ω-cm) at 70 K whereas,


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the resistivity of the sample is beyond measurement limit of the system below 70 K. It is worth mentioning here that throughout the measurement temperature range (70-300 K), electrical contacts on the sample surface are ohmic in nature. The resistivity curve shows two interesting features: (a) high value of relative resistivity ratio [ρr = ρ (70 K)/ρ (300 K)] (ca. 5 x 104) and (b) diverging ρ(T) curve when extrapolated to T0 K. A metal is defined as an electronic system which has zero activation energy and therefore it exhibits a finite value of resistivity at T0 K. Metals also exhibit weak temperature dependence of electrical resistivity. On the contrary, insulators exhibit diverging resistivity behavior at T0 K because of the freeze out of all the carrier activation processes and therefore, a strong temperature dependence of resistivity is usually observed which is indeed depicted by the Bi2S3 film. Thus, we elucidate that the Bi2S3 film lies on the insulating side of the metal-to-insulator transition (MIT)29,30,31.

Figure 11. (a) Temperature dependent resistivity curve in logarithmic scale with respect to measurement temperature and (b) activation energy as a function of measurement temperature for ALD grown Bi2S3 thin films. In the insulating regime, carrier transport at relatively higher temperatures (close to the room temperature) presumably occur using activated carriers in the extended states. This mechanism


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of transport produces temperature dependence of the resistivity in the form of ρ(T) = ρ0 exp(Ea/kT), where Ea is the carrier activation energy. Therefore, in this temperature range, localized state conduction using impurity levels (or band tails) is practically masked by the bandlike transport process. However, as temperature decreases, freeze out of different thermal activation processes renders band-like transport less important. Hence, at sufficiently low temperatures, charge transport is dominated by phonon assisted hopping conduction process in which charge carriers jump from one occupied state to an

unoccupied

impurity

state.

The

generalized hopping transport process can be mathematically expressed as [ρ(T) = ρ0 exp(T0/T)p] where, p value determines the characteristic of the activation process. The resistivity curve in the entire temperature range cannot be fitted by considering a single activation energy which imply multiple thermal activation mechanisms are involved in the carrier transport process. The ρ(T) curve of the Bi2S3 film in this temperature range shows a linear dependence with (1/T)1/4, as shown in figure S9. Such behavior primarily indicates that Mott variable range hopping (VRH) [ρM(T) = ρM0 exp(TM/T)1/4] is the dominant charge transport mechanism29,30. In order to unambiguously address the low temperature hopping conduction mechanism and its temperature range, we have plotted reduced activation energy ѡ, [ѡ = -d(lnρ)/d(lnT)] as a function of measurement temperature T, figure 11b. Considering the general form of the temperature dependence of VRH [ρ(T) = ρ0 exp(T0/T)p], the reduced activation energy can be written as ln ѡ(T) = A-plnT where, A = plnT0+ln p 29,31. The reduced activation energy plot shows a negative slope in the entire temperature range (ca. 70-300 K) which further confirms that the sample is in the insulating side of the MIT29,31,32. As


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can be seen, ln ѡ(T) vs ln T plot shows a linear behavior up to temperature ca. 100 K. The least square fitting of the linear portion of the plot provides p as ca. 0.25 which implies Mott VRH as the dominant transport mechanism only in this temperature range32,33. It is worth mentioning here that at temperatures T ≥ 100 K, Hall voltage of the sample is measured quite reliably using ac magnetic field. However at T < 100K definite Hall voltage signal cannot be measured. Thus, VRH does not likely to be the dominant carrier transport mechanism in the entire range of the measurement temperature. This observation further implies that electrical transport at T ≥ 100 K preferentially occurs through the extended states32,33. Thus at ca. 100 K possible crossover from localized state hopping conduction to diffusive band-like transport occurs in these films. The vertical arrow in figure 11(b) indicates the deviation from Mott VRH equation at T ~ 100K. At T < 100 K, a significant carrier freeze out effect is seen in the as deposited Bi2S3 film that might have resulted in low hole density in the valence band. Therefore, carrier transport (at T < 100 K) is dominated by phonon-assisted variable range hopping process between localized states with energies lying in the vicinity of the Fermi level (EF ± ɛ) and without involving valence band. On the contrary, the resistivity curve at T > 100 K cannot be fitted by considering single activation energy, implying different thermal activation mechanisms are involved in the carrier transport process. CONCLUSION: Atomic layer deposition (ALD) of Bismuth Sulfide (Bi2S3) is investigated using Bi(thd)3 and H2S as the metal and chalcogen precursors respectively in a custom built hot wall laminar flow reactor. Detailed growth mechanism study involving in-situ QCM and ex-situ XRR measurements reveal considerably longer nucleation periods followed by linear growth within


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the ALD temperature window of 175-250°C with a saturated growth rate of 0.34-0.37Å per ALD cycle. Structural analysis confirms a polycrystalline, non-preferential Bi2S3 growth. Optical bandgaps of ca. 1.56 eV (direct) and ca. 1.03 eV (indirect) are measured. The as-deposited films are found to be degenerate p-type with carrier concentration of ca. 6.8 x 1018 cm-3 at room temperature. Between 70-100K the hole transport is dominated by variable range hopping while at higher temperature, band-like transport dominates.

AUTHOR INFORMATION Corresponding Author: Email: [email protected]

ACKNOWLEDGMENT This article is based upon work supported under the US-India Partnership to Advance Clean Energy-Research (PACE-R) for the Solar Energy Research Institute for India and the United States (SERIIUS), funded jointly by the U.S. Department of Energy (Office of Science, Office of Basic Energy Sciences, and Energy Efficiency and Renewable Energy, Solar Energy Technology Program, under Subcontract DE-AC36-08GO28308 to the National Renewable Energy Laboratory, Golden, Colorado) and the Government of India, through the Department of Science and Technology under Subcontract IUSSTF/JCERDC-SERIIUS/2012 dated 22nd November 2012. The authors also thank the Sophisticated Analytical Instrument Facility (SAIF) at IIT Bombay for material characterizations.


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REFERENCES 1.

Biswas, K.; Zhao, L.-D.; Kanatzidis, M. G., Tellurium-Free Thermoelectric: The

Anisotropic n-Type Semiconductor Bi2S3. Adv. Energy Mater. 2012, 2 (6), 634-638. 2.

Liu, W.; Guo, C. F.; Yao, M.; Lan, Y.; Zhang, H.; Zhang, Q.; Chen, S.; Opeil, C. P.; Ren,

Z., Bi2S3 Nanonetwork as Precursor for Improved Thermoelectric Performance. Nano Energy 2014, 4, 113-122. 3.

Zumeta-Dubé, I.; Ruiz-Ruiz, V.-F.; Díaz, D.; Rodil-Posadas, S.; Zeinert, A., TiO2

Sensitization with Bi2S3 Quantum Dots: The Inconvenience of Sodium Ions in the Deposition Procedure. The Journal of Physical Chemistry C 2014, 118 (22), 11495-11504. 4.

Chmielowski, R.; Péré, D.; Bera, C.; Opahle, I.; Xie, W.; Jacob, S.; Capet, F.; Roussel,

P.; Weidenkaff, A.; Madsen, G. K. H.; Dennler, G., Theoretical and experimental investigations of the thermoelectric properties of Bi2S3. Journal of Applied Physics 2015, 117 (12), 125103. 5.

Mizoguchi, H.; Hosono, H.; Ueda, N.; Kawazoe, H., Preparation and electrical properties

of Bi2S3 whiskers. Journal of Applied Physics 1995, 78 (2), 1376-1378. 6.

Ge, Z.-H.; Zhang, B.-P.; Yu, Y.-Q.; Shang, P.-P., Fabrication and properties of

Bi2−xAg3xS3 thermoelectric polycrystals. Journal of Alloys and Compounds 2012, 514, 205209. 7.

Ge, Z.-H.; Zhang, B.-P.; Liu, Y.; Li, J.-F., Nanostructured Bi2-xCuxS3 Bulk Materials

with Enhanced Thermoelectric Performance. Phys. Chem. Chem. Phys. 2012, 14 (13), 44754481.


24

Page 25 of 29

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


8.

Liu, Z.; Pei, Y.; Geng, H.; Zhou, J.; Meng, X.; Cai, W.; Liu, W.; Sui, J., Enhanced

Thermoelectric Performance of Bi2S3 by Synergistical Action of Bromine Substitution and Copper Nanoparticles. Nano Energy 2015, 13, 554-562. 9.

Clark, R. M.; Kotsakidis, J. C.; Weber, B.; Berean, K. J.; Carey, B. J.; Field, M. R.;

Khan, H.; Ou, J. Z.; Ahmed, T.; Harrison, C. J.; Cole, I. S.; Latham, K.; Kalantar-zadeh, K.; Daeneke, T., Exfoliation of Quasi-Stratified Bi2S3 Crystals into Micron-Scale Ultrathin Corrugated Nanosheets. Chem. Mater. 2016, 28 (24), 8942-8950. 10. Kumar, M.; Persson, C., Cu3BiS3 as a Potential Photovoltaic Absorber with High Optical Efficiency. Appl. Phys. Lett. 2013, 102 (6), 062109. 11. Lou, Y.; Chen, X.; Samia, A. C.; Burda, C., Femtosecond Spectroscopic Investigation of the Carrier Lifetimes in Digenite Quantum Dots and Discrimination of the Electron and Hole Dynamics via Ultrafast Interfacial Electron Transfer. J. Phys. Chem. B 2003, 107 (45), 1243112437. 12. Rincón, M. E.; Sánchez, M.; George, P. J.; Sánchez, A.; Nair, P. K., Comparison of the Properties of Bismuth Sulfide Thin Films Prepared by Thermal Evaporation and Chemical Bath Deposition. J. Solid State Chem. 1998, 136 (2), 167-174. 13. Chao, J.; Xing, S.; Zhao, J.; Qin, C.; Duan, D.; Zhao, Y.; He, Q., Bismuth Sulfide Nanoflowers as High Performance Near-Infrared Laser Detectors and Visible-Light-Driven Photocatalysts. RSC Adv. 2016, 6 (60), 55676-55681. 14. Lin, Y.-C.; Lee, M.-W., Bi2S3 Liquid-Junction Semiconductor-Sensitized SnO2 Solar Cells. J. Electrochem. Soc. 2014, 161 (1), H1-H5.


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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 26 of 29

15. Lu, F.; Li, R.; Li, Y.; Huo, N.; Yang, J.; Li, Y.; Li, B.; Yang, S.; Wei, Z.; Li, J., Improving the Field‐Effect Performance of Bi2S3 Single Nanowires by an Asymmetric Device Fabrication. ChemPhysChem 2015, 16 (1), 99-103. 16. Monteiro, O. C.; Trindade, T.; Park, J. H.; O'Brien, P., The LP-MOCVD of CdS/Bi2S3 Bilayers Using Single-Molecule Precursors. Mater. Lett. 2004, 58 (1–2), 119-122. 17. Waters, J.; Crouch, D.; Raftery, J.; O'Brien, P., Deposition of Bismuth Chalcogenide Thin Films Using Novel Single-Source Precursors by Metal-Organic Chemical Vapor Deposition. Chem. Mater. 2004, 16 (17), 3289-3298. 18. Monteiro, O. C.; Trindade, T.; Park, J. H.; O'Brien, P., The Use of Bismuth(III) Dithiocarbamato Complexes as Precursors for the Low‐Pressure MOCVD of Bi2S3. Chem. Vap. Deposition 2000, 6 (5), 230-232. 19. George, S. M.; Ott, A. W.; Klaus, J. W., Surface Chemistry for Atomic Layer Growth. J. Phys. Chem. 1996, 100 (31), 13121-13131. 20. Miikkulainen, V.; Leskelä, M.; Ritala, M.; Puurunen, R. L., Crystallinity of Inorganic Films Grown by Atomic Layer Deposition: Overview and general trends. J. Appl. Phys. 2013, 113 (2), 021301. 21. Liu, H. F.; Antwi, K. K. A.; Wang, Y. D.; Ong, L. T.; Chua, S. J.; Chi, D. Z., Atomic Layer Deposition of Crystalline Bi2O3 Thin Films and Their Conversion into Bi2S3 by Thermal Vapor Sulfurization. RSC Adv. 2014, 4 (102), 58724-58731. 22. Mahuli, N.; Sarkar, S. K., Atomic Layer Deposition of NiS and Its Application as Cathode Material in Dye Sensitized Solar Cell. J. Vac. Sci. Technol., A 2016, 34 (1), 01A142.


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Page 27 of 29

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


23. Kim, J. Y.; George, S. M., Tin Monosulfide Thin Films Grown by Atomic Layer Deposition Using Tin 2,4-Pentanedionate and Hydrogen Sulfide. J. Phys. Chem. C 2010, 114 (41), 17597-17603. 24. Sarkar, S. K.; Kim, J. Y.; Goldstein, D. N.; Neale, N. R.; Zhu, K.; Elliott, C. M.; Frank, A. J.; George, S. M., In2S3 Atomic Layer Deposition and Its Application as a Sensitizer on TiO2 Nanotube Arrays for Solar Energy Conversion. J. Phys. Chem. C 2010, 114 (17), 8032-8039. 25. Goldstein, D. N.; George, S. M., Enhancing the Nucleation of Palladium Atomic Layer Deposition on Al2O3 Using Trimethylaluminum to Prevent Surface Poisoning by Reaction Products. Appl. Phys. Lett. 2009, 95 (14), 143106. 26. Hybertsen, M. S.; Louie, S. G., First-Principles Theory of Quasiparticles: Calculation of Band Gaps in Semiconductors and Insulators. Phys. Rev. Lett. 1985, 55 (13), 1418-1421. 27. Kersauson, M. d.; Jakomin, R.; Kurdi, M. E.; Beaudoin, G.; Zerounian, N.; Aniel, F.; Sauvage, S.; Sagnes, I.; Boucaud, P., Direct and Indirect Band Gap Room Temperature Electroluminescence of Ge Diodes. J. Appl. Phys. 2010, 108 (2), 023105. 28. Jiang, L.; Gallagher, J. D.; Senaratne, C. L.; Aoki, T.; Mathews, J.; Kouvetakis, J.; Menéndez, J., Compositional Dependence of the Direct and Indirect Band Gaps in Ge 1− y Sn y Alloys from Room Temperature Photoluminescence: Implications for the Indirect to Direct Gap Crossover in Intrinsic and n -type Materials. Semicond. Sci. Technol. 2014, 29 (11), 115028. 29. Vavro, J.; Kikkawa, J. M.; Fischer, J. E., Metal-Insulator Transition in Doped SingleWall Carbon Nanotubes. Phs. Rev. B 2005, 71 (15), 155410. 30. Mott, N. F., Conduction in Non-Crystalline Materials. Clarendon Press, Oxford: 1993.


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31. Saha, D.; Misra, P.; Joshi, M. P.; Kukreja, L. M., UV Light Induced Insulator-Metal Transition in Ultra-Thin ZnO/TiOx Stacked Layer Grown by Atomic Layer Deposition. J. Appl. Phys. 2016, 120 (8), 085704. 32. Wang, S.; Ha, M.; Manno, M.; Daniel Frisbie, C.; Leighton, C., Hopping Transport and the Hall Effect Near the Insulator-Metal Transition in Electrochemically Gated Poly(3hexylthiophene) Transistors. Nat. Commun. 2012, 3. 33. Nomura, K.; Kamiya, T.; Ohta, H.; Ueda, K.; Hirano, M.; Hosono, H., Carrier Transport in Transparent Oxide Semiconductor with Intrinsic Structural Randomness Probed Using SingleCrystalline InGaO3(ZnO)5 Films. Appl. Phys. Lett. 2004, 85 (11), 1993-1995.


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199x188mm (150 x 150 DPI)


Atomic Layer Deposition of p-Type Bi

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