Atomic Layer Deposition of Tin Monosulfide Thin Films

Thin film solar cells made from earth‐abundant, non‐toxic materials are needed to replace the current technology that uses Cu(In,Ga)(S,Se)2 and CdTe, which contain scarce and toxic elements. One promising candidate absorber material is tin monosulfide (SnS). In this report, pure, stoichiometric, single‐phase SnS films were obtained by atomic layer deposition (ALD) using the reaction of bis(N,N′‐diisopropylacetamidinato)tin(II) [Sn(MeC(N‐iPr)2)2] and hydrogen sulfide (H2S) at low temperatures (100 to 200 °C). The direct optical band gap of SnS is around 1.3 eV and strong optical absorption (α > 104 cm−1) is observed throughout the visible and near‐infrared spectral regions. The films are p‐type semiconductors with carrier concentration on the order of 1016 cm−3 and hole mobility 0.82–15.3 cm2V−1s−1 in the plane of the films. The electrical properties are anisotropic, with three times higher mobility in the direction through the film, compared to the in‐plane direction.


Introduction
To achieve cost-effective thin film solar cells for large-scale production of solar energy, the absorbing semiconductor material used in the device needs to satisfy many requirements. First, the constituent elements should be inexpensive, non-toxic, and abundant.
Second, to obtain high energy conversion efficiency, the material should have appropriate optical and electrical properties such as a suitable optical band gap, a high optical absorption coefficient, a high quantum yield for the excited carriers, a long carrier diffusion length, and a low recombination velocity. In this aspect, tin monosulfide (SnS) satisfies some of these criteria and thus is a promising candidate as an absorber material for solar cell. Its constituent elements (tin and sulfur) are inexpensive, environmental friendly, and abundant in nature.
This feature gives it an advantage over the current best-developed thin film photovoltaic (PV) cells made from Cu(In,Ga)(S,Se) 2 (CIGS) and CdTe, which contain toxic Cd and the rare elements In, Ga, Se, and Te. Moreover, the binary compound SnS provides much simpler chemistry than multicomponent Cu 2 ZnSnS 4 (CZTS), one of the most promising earthabundant absorber materials, with solar conversion efficiency up to 9.6%. [1] In addition, SnS has proper optical properties for PV application; it has an indirect optical band gap of 1.0-1.1 eV and a high absorption coefficient (greater than 10 4 cm -1 ) above the direct absorption edge at 1.3-1.5 eV. [2][3][4][5][6] SnS crystallizes in an orthorhombic unit cell, with lattice parameters of a = 4.334 Å, b = 11.200 Å, and c = 3.987 Å, [7] which can be viewed as a distorted rocksalt (NaCl) structure. It is composed of double SnS layers perpendicular to the b axis with tin and sulfur atoms covalently bonded within the layers and weak van der Waals bonds between the layers. [8] The double layer held by van der Waals forces in SnS is expected to give a chemically inert surface with few surface states. [9] This defect-tolerant surface might reduce the carrier recombination loss due to defects at p-n junctions and at grain boundaries.
Despite these promising properties, solar cells based on SnS absorbers have not achieved conversion efficiency higher than 1.3%, [3] while theoretically such cells should be able to reach 24% efficiency. [10] This poor performance may be due to defects and/or impurities in SnS layers that result from the preparation methods used to make the films.
Tin monosulfide films have been deposited from various techniques, which can be roughly divided into three categories. The first method involves solution-based techniques such as chemical bath deposition (CBD), [9,11] successive ionic layer adsorption and reaction (SILAR), [12,13] and electrochemical deposition (ECD). [2,5,14,15] A second technique is physical vapor deposition from a SnS target such as thermal evaporation, [16][17][18] RF sputtering, [19] and electron beam evaporation. [20] The last method uses transport by chemical vapors such as chemical spray pyrolysis, [21][22][23] chemical vapor deposition (CVD), [6,[24][25][26][27] and atomic layer deposition (ALD). [28] Due to various oxidation states of tin (0, +2, and +4), traces of other phases (i.e. Sn, Sn 2 S 3 , and SnS 2 ) have often been co-deposited into the films as reported by several authors. [2,16,27,29] To prevent the formation of Sn 2 S 3 and SnS 2 phases, relatively high deposition temperatures are needed in thermal evaporation (above 275 o C) [17] and CVD (above 500 o C) [26,30] . Although pure SnS phase can be obtained at certain substrate temperatures or growth conditions, the chemical composition of the films usually deviates from ideal stoichiometric SnS by as much as 10-20%. For vapor-based deposition techniques such as chemical spray pyrolysis and thermal evaporation, this effect is attributed to the high vapor pressure of sulfur, which evaporates out from the deposited film. [21,31] Nonstoichiometric SnS caused by either Sn +2 vacancies [32,33] or excess tin atoms [6,20] contains deep acceptor states with an activation energy (E a ) in the range between 0.22 and 0.45 eV [6,20,[32][33][34][35][36] depending on the deposition technique. The deep acceptor states, which could act as carrier recombination catalysts, are expected to lower the solar energy conversion efficiency.
In addition to other binary compounds and the non-stoichiometric problem, solution-based deposition methods such as CBD and ECB give films contaminated by oxygen in the forms of SnO 2 or SO 4 -2 . [2,5] Chlorine contamination was also reported in SnS films grown by CVD using tin chloride. [6,24] ALD SnS was reported by Kim et al. using the reaction of tin 2,4-pentanedionate (Sn(acac) 2 ) and hydrogen sulfide (H 2 S). [28] However, the growth rate was relatively low (0.24 Å cycle -1 ), which might be partly because Sn-O bonds (532 kJ mol -1 ) are stronger than Sn-S bonds (464 kJ mol -1 ), [37] resulting in an unfavorable thermodynamic enthalpy change. Optical properties were measured only on a very thin film (23 nm) and crystal structure and electrical properties of the film were not studied.
In this report, we present ALD of pure, stoichiometric, single-phase SnS films from the reaction of bis(N,N'-diisopropylacetamidinato)tin(II) [Sn(MeC(N-i Pr) 2 ) 2 ], referred to as Sn(amd) 2 in this work, and H 2 S at substrate temperatures between 100 and 200 o C. The utilization of a Sn(II), rather than Sn(IV) precursor used in previously reported CVD processes, [6,26,29,30] mitigates possible contamination by Sn 2 S 3 and SnS 2 phases. Moreover, the deposition temperature is relatively low, thus overcoming the problem of sulfur-deficiency due to sulfur re-evaporation. Low temperature growth of SnS also enables the formation of solar cells on thermally sensitive substrates such as flexible polymers. The deposited SnS films were thoroughly characterized by their elemental composition, surface morphology, crystal structure, optical, and electrical properties. These SnS films possess high potential for use in photovoltaic devices.   obtained from a ȕ-diketonate precursor. [28] 2.2 Surface morphology and topography   of SnS and the average grain size calculated from Scherrer formula. [38] The Bragg angle ș and full width at half maximum (FWHM) of each XRD peak were determined by fitting a Gaussian distribution to the experimental values. The shape factor used for the calculation is 0.9 (spherical grain shape). Since the grain shape is not exactly spherical, the calculated values only present rough estimation for comparison.

ALD Growth Behavior
From the intensity ratios shown in Table 1 are mainly from the grain boundaries within the film rather than from the top surface. Thus, the minimum surface energy is to have {010} planes parallel to the grain boundaries, which is perpendicular to the substrate surface due to the nature of columnar structure in this case. In single crystals, the hole mobility along the layer direction is about 10 times higher than that in the perpendicular direction. [39] Therefore, this preferred orientation of having layer planes perpendicular to the substrate is desirable in solar cells since the carrier transport occurs within the layer plane which has higher mobility and is also along the defect tolerant surfaces.

Phase purity
Mathew et al. reported traces of Sn 2 S 3 and SnS 2 in the SnS films grown by pulsed electrodeposition that cannot be detected by XRD but can be observed by Raman spectroscopy. [2] To confirm the phase purity, Raman spectra were taken of the ALD SnS films grown at between 120 and 300 o C. Figure 6 shows the Raman peaks of ALD SnS detected at 68, 94, 162, 191, 219, and 288 cm -1 , all of which match well with those from single crystal SnS observed at 40,49,70,85,95,164,192,218, and 290 cm -1 . [40] This result confirms that the films grown at 120-300 o C are pure SnS without any contamination from SnS 2 and Sn 2 S 3 , which have their strongest Raman peaks at around 312 and 307 cm -1 , respectively.
Due to the insensitivity of XRD and Raman spectroscopy to amorphous phases, transmission electron microscopy (TEM) was used to look for amorphous phase between crystallite grains. Figure 7 shows TEM micrograph and selected area electron diffraction (SAED) of a 25 nm-thick SnS film deposited at 120 o C. These results show that ALD SnS films do not contain significant amounts of amorphous phase despite being deposited at relatively low temperature. The utilization of a Sn(II) precursor rather than a Sn(IV) precursor [6,26,29,30] broadens the narrow substrate temperature window reported previously and provides only pure SnS over a wider range of temperature.

Stoichiometry
All the as-deposited films grown between 100 and 200 o C were measured to be stoichiometric SnS to within ±1%, which is the sensitivity limit of the RBS analysis shown in  [4] However, using the same deposition condition, stoichiometric SnS can be obtained only for films thicker than 1 μm; the atomic ratio of Sn to S increases almost linearly from 1.0 at 1.2 μm-thick to 1.1 at 100 nm-thick. [31] Such a thickness effect on film composition has not been observed in our ALD SnS films; all ALD SnS films deposited below 200 o C in the thickness range of 50-370 nm are stoichiometric SnS with tin to sulfur atomic ratio of 1.00 ± 0.01. This stoichimetric SnS is expected to provide a low density of native point defects and Sn 2+ vacancies in the material. The density of the ALD film, calculated from atomic area density (by RBS) and film thickness (by SEM), is about 4.6 g cm -3 . This value is 88% of the bulk density of SnS (5.2 g cm -3 ), which is relatively high for thin polycrystalline films.

Elemental purity
The incorporation of carbon is sometimes found in films deposited from a metal organic precursor due to incomplete dissociation of organic ligands from the metal center at low growth temperature or precursor decomposition at high growth temperature.

Optical properties
The absorption coefficient (Į) and the optical band gap (E g ) of the film were determined from the transmittance (T) and reflectance (R) measurements. The absorption coefficient was determined based on the following formula to minimize the interference effects in the absorption of the film; [41] ߙ ൌ where d is film thickness, T is total transmittance, R is total reflectance, and R' is a single reflectance from the film-substrate interface. Due to an unknown refractive index of SnS film in the absorbing region, R' cannot be determined. However, a good estimation of Į can be obtained from the following approximation; this phenomenon has been assigned to the effect of the degree of the film crystallinity. [31,32,42] In addition to the degree of crystallinity, the band gap shift can also stem from the change in the crystal orientation. In principle, layered structure compounds are highly anisotropic and thus can have different optical properties in different crystallographic orientations. HgI 2 film has been shown to switch its preferred crystal plane parallel to the substrate from (102) to (002) when the film thickness reaches ~1.45 μm while its optical band gap shifts from 1.94 to 2.25 eV for the directions perpendicular to the (102) and (002) planes, respectively. [43] In single crystal SnS, the lowest allowed optical energy gaps along the a and c crystallographic axes are 1.60 and 1.34 eV respectively, as determined by electroreflectance and optical transmission. [44] The smallest energy gap is, however, reported to be an indirect transition at 1.07 eV. [45] Makinistian and Albanesi employed an ab initio density-functional theory with a FP-LAPW method to calculate the electronic band structure and the optical spectra of SnS. They reported that although SnS has an isotropic indirect energy band gap at 1.16 eV, the absorption coefficients along three principal crystallographic axes are highly anisotropic; [46] the absorption coefficient along the c axis has a sharp increase from 10 1 to 10 4 cm -1 at 1.21 eV. In contrast, the absorption coefficient along the a and b axes are quite similar and they have one sharp increase from 10 1 to 10 3 cm -1 at 1.20 eV and another sudden rise from 10 3 to 10 5 cm -1 at 1.54 eV.
The smallest absorption coefficient that T and R measurements can detect from the films with thickness less than 500 nm is around 10 2 cm -1 . Below this Į value, the absorption for a film with thickness below 500 nm is less than ~0.005 = (ͳ െ ݁ ିଵாଶൈହாି ) and therefore reliable measurement is difficult to achieve. Since the absorption coefficient close to the optical band gap E g at 1.1-1.2 eV is below 10 2 cm -1 , [45,46] it may not be detected from our samples with T and R measurements. Also, interference effects must be included to analyze optical data for these nearly transparent regions of the spectrum. The optical band gaps that

Electrical properties
Electrical properties of SnS films were determined by Hall measurement using the Van der Pauw method and are summarized in Table 1 D)). Normally, the resistivity decreases as the film gets thicker, due to less carrier scattering at the grain boundaries as the grain size increases. However, SnS is a highly anisotropic material. As mentioned earlier, the hole mobility along the layer direction is about 10 times higher than that in the perpendicular direction in single crystals. [39] Thus, in addition to grain boundary scattering, the crystal orientation is also an important factor that contributes to the electrical Interestingly, in a Au/SnS/Au structure, the resistivity of a 370 nm-thick SnS film deposited at 120 o C (sample (C) in Table 1) measured in vertical direction through the film was 60 ·cm, which is roughly three times lower than that in lateral direction (176 ·cm).

Conclusions
Tin monosulfide thin films were deposited by ALD using the reaction of bis (

Experimental procedure
The tin precursor, bis(N,N'-diisopropylacetamidinato)tin(II) was prepared according to the synthetic route depicted in Figure 11, which was adopted from the work by Lim et al. [47] A solution of methyllithium ( SnS thin films were deposited in a custom-built hot-wall ALD reactor [48] from the reaction of Sn(amd) 2 and H 2 S. The tin precursor was kept at a constant temperature of 95 o C which gives vapor pressure of 0.64 Torr, as measured by a capacitance manometer. A gas mixture of 4% H 2 S in N 2 was used as the source of sulfur. Purified N 2 gas was used for assisting the delivery of Sn(amd) 2 vapor from the precursor container to the deposition zone and also for purging between each precursor pulse. The Sn precursor and H 2 S injection was done by using stop-flow ALD mode. Detailed explanation of the stop-flow ALD mode and the valve operation procedure can be found elsewhere. [49,50] The exposures from each single dose of Sn(amd) 2 and H 2 S were estimated to be 0.50 and 0.36 Torr·s, respectively. By changing the number of tin precursor and H 2 S doses, reproducible variations in the total precursor exposure were achieved. The N 2 purge time between each precursor pulse was set to be 10s, which is sufficient to remove the excess precursor and reaction by-products. The cycles of sequential alternation between Sn(amd) 2 and H 2 S were repeated until the desired film thickness was reached.
Surface morphology and topology of the films were examined by using field-emission scanning electron microscopy (FESEM, Zeiss, Ultra-55). The film thickness was determined using a combination of cross-sectional SEM and X-ray fluorescence spectroscopy (XRF,         500 nm 500 nm