Microstructural and light emission properties of ZnSnP2 thin film absorber: Study of native defects

Accepted Manuscript Microstructural and light emission properties of ZnSnP2 thin film absorber: Study of native defects S. Mukherjee, T. Maitra, A. Nayak, S. Mukherjee, A. Pradhan, M.K. Mukhopadhyay, B. Satpati, S. Bhunia PII:

S0254-0584(17)30796-4

DOI:

10.1016/j.matchemphys.2017.10.014

Reference:

MAC 20053

To appear in:

Materials Chemistry and Physics

Received Date: 1 March 2017 Revised Date:

22 September 2017

Accepted Date: 6 October 2017

Please cite this article as: S. Mukherjee, T. Maitra, A. Nayak, S. Mukherjee, A. Pradhan, M.K. Mukhopadhyay, B. Satpati, S. Bhunia, Microstructural and light emission properties of ZnSnP2 thin film absorber: Study of native defects, Materials Chemistry and Physics (2017), doi: 10.1016/ j.matchemphys.2017.10.014. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Microstructural and light emission properties of ZnSnP2 thin film absorber: study of native defects S. Mukherjeea, T. Maitraa, A. Nayaka*, S. Mukherjeeb, A. Pradhanb, M. K. Mukhopadhyayb, B. Satpatib, S. Bhuniab Department of Physics, Presidency University, 86/1, College Street, Kolkata 700 073, India

b

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a

Surface Physics and Material Science Division, Saha Institute of Nuclear Physics, Kolkata

700 064, India

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ABSTRACT

Thin films of ZnSnP2 were successfully grown on p-type silicon (001), sapphire and glass substrate by

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e-beam evaporation method. The as-deposited films were characterized using scanning and high resolution transmission electron microscopes, reflectance and transmittance spectroscopy and photoluminescence measurement at low temperature. Polycrystalline nature of the films was verified by high resolution transmission electron microscopy. An optical band gap of 1.71 eV was estimated at

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room temperature. Two broad luminescence bands at 1.529 eV and at 1.634 eV were observed at 15 K. The light emission characteristics of the ZnSnP2 were explained in terms of donor-acceptor pair recombination mechanism. Tin-on-zinc sites and zinc vacancies/zinc-on-tin sites were considered as

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donor and acceptor due to presence of native defects in the films. A schematic band model based on

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the experimental finding was suggested to account for the DA pair recombination.

Keywords: Thin film, Electron-beam evaporation, Solar cell absorber, Direct optical transition, Donor-acceptor pair, Photoluminescence

*

Corresponding author

E-mail address: [email protected]

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ACCEPTED MANUSCRIPT 1. Introduction ZnSnP2, a member of II-IV-V2 group compound semiconductors, has recently attracted much attention as a potential absorbing material for thin film solar cell applications. Recent

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theoretical calculations on the electronic structure of ZnSnP2indicate thatit possesses anorder chalcopyrite structure which can transform to a disordered sphalerite structure at high temperature (7200C) with a variation of room temperature optical band gaps from 1.70 eV to 0.75 eV [1-4]. In the chalcopyrite structure (bandgap =1.70 eV), Zn and Sn occupy on

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specific sites of the fcc lattice, while in the sphalerite structure (bandgap = 0.75 eV) they are

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randomly distributed on fcc sites giving rise to disorder phase. This feature indicates that bandgap of ZnSnP2 could be tuned from 0.75 eV to 1.70 eV by controlling growth temperature and hence the atomic configuration. The grading of band gaps from low to high values could be achieved by growing an initial layer at a higher temperature corresponding to lower band gap and then the subsequent layers are grown at relatively lower temperatures.

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The exact growth temperatures and their relation to the band gap values of ZnSnP2 need to be optimized properly. Thus, the band gap tuning facilitates the fabrication of various homo p-n

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junctions by properly choosing a definite phase of ZnSnP2 and doping. Interestingly, an optical band gap of 1.68 eV, which is close to the optimum band gap (~ 1.50 eV) at the

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Schockley-Quisser limit, has been reported [1, 4]. Moreover, other advantages such as ternary ZnSnP2is low-toxic, the constituent elements for the preparation of ZnSnP2are earth abundant and inexpensiveand high absorption coefficient (> 104 cm-1) above 1.60 eV, find ZnSnP2 a potential alternative for photovoltaic device applications.It has excellent lattice matching with GaAs substrates. Thegrowth of ZnSnP2thin films have been reported previously by coevaporation [5, 6] and in ultrahigh vacuum by molecular beam epitaxy (MBE) [7]. Recently, ZnSnP2 based thin-film solar cell with ZnSnP2 absorber is fabricated by phosphidation method under the variation of Zn/Sn atomic ratio [8]. The solar cell parameters (JSC = 2.63 2

ACCEPTED MANUSCRIPT mA/cm2, VOC -=3.7 mV, FF = 27.2%, and conversion efficiency = 0.0027%) have been measured using near-stoichiometric ZnSnP2 as the absorber layer. In the study, the authors found two different current areas. Low-current area is attributed to the presence of Zn3P2 secondary phase forming the shunt paths while the high-current area (0.014 cm2) with

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efficiency = 0.021%, JSC = 5.03 mA/cm2is obtained due the formation of pure ZnSnP2 phase. The energy band gap of ZnSnP2 thin film prepared by phosphidation method was reported as 1.38 eV indicating the growth of sphalerite ZnSnP2 structure with appropriate variation of

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Zn/Sn atomic ratio. Nakatsuka et al [9, 10] studied the J–V characteristics of the heterojunction solar cell [Al/AZO/ ZnO/CdS/ZnSnP2/Mo]. The performance of the fabricated cell

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was found quite low (conversion efficiency is 0.087%). Necessary modifications of the cell structure to achieve better performances such as improvement of the resistance of the heterointerfaces and the use of appropriate buffer materials have been suggested. Though some limited studies have been made by several researchers on the preparation of

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crystalline bulk material and thinepitaxial layer, detailed understanding of optical and electrical properties, the nature of native defects responsible top-type conductivity in ZnSnP2

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are still obscure. In ZnSnP2 thin film, zinc vacancies or zinc-on-tin sites are considered as acceptor which is responsible for p-type conductivity. Defects introduce localized levels in the energy gap of

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ZnSnP2 and other compound semiconductors through which they control solar cell device performance, efficiency and reliability [11]. Native defects such as vacancies, self-interstitials, and antisite defects often act as unintended dopants or compensate intentionally introduced dopants of these materials [12]. In addition, native defects as well as contaminant impurities also limit the efficiency of light-emission in ZnSnP2 and related materials [13]. Thus, it is of great significance to study the nature and behaviour of such defects under different conditions.

Reports on e-beam deposited thin films of ZnSnP2 and their structural and optical properties are not available in the literature till date. In this communication, thin film of

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ACCEPTED MANUSCRIPT ZnSnP2 deposited on silicon (001), sapphire and glass substrates by electron beam (e-beam) evaporation technique at elevated temperatures and the interesting findings on the structural, optical and photoluminescence properties at low temperatures are reported. The crystallinity and optical band gap of the films have been studied with the standard methods. The emission

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characteristics and their close relation to the intrinsic defects in the ZnSnP2films are

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discussed.

2. Materials and experiment

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The source materials used in the deposition of ZnSnP2 thin films were prepared by the direct chemical reaction of the constituent elements (Zn, Sn, P). Raw materials of Zn granules (99.9999% Sigma Aldhrich), Sn granules (99% merck) and red Phosphorus nano powder (99.999%Sigma Aldhrich) were sealed in a quartz ampoule of inner diameter of 11.6 mm

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and outer diameter 13.6 mm under a vacuum (10-3mbar). For the preparation of ZnSnP2 bulk crystal, the pseudo- binary phase diagram of the Sn-ZnP2 system [14] was considered and theampoule sealing was accomplished under excess Sn(with 89.9 mole% Sn concentration)

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condition [2, 15]. The quartz ampoule was placed properly in a custom built vertical single zone tube furnace. Temperature of the furnace was stepped up to 5000C with a ramp rate of

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40C per minute. Temperature of the ampoule was kept fixed for two hours at 5000C. Then temperature was once again increased to 6000C with a ramp rate 50C per minute and kept fixed for one hour. Finally the temperature is further moved up to 7000C with a ramp rate 50C per minute and kept constant for 24 hours. Finally, the ampoule was rapidly cooled to room temperature. Bulk crystal obtained after this process was treated by 0.1M HCl to remove the excess Sn and the greyish precipitate was collected in powder form. The powder ZnSnP2 material thus obtained was characterized by powder X-Ray Diffraction (XRD, Rigaku Smart

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ACCEPTED MANUSCRIPT Lab) and used as source material for vacuum growth of the thin film by e-beam evaporation technique. P-type Si (001) wafer, Sapphire and glass were used as substrates for the film deposition. Substrate temperature was kept at 2500C during the growth process. Growth rate and the film thickness observed by the quartz crystal monitor were about 0.10 nm/s and

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43nm, respectively. Hot probe measurement of the as-grown films showed p-type conductivity. Surface morphology of the films was investigated with a field emission scanning electron microscopy (FE-SEM, Quanta 200 FEG). High resolution transmission

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electron microscopy (HRTEM, Tecnai G2 F30 S-TWIN) was used to study the microstructure and crytallinity of the films deposited on Si (001) substrate in cross-sectional mode. The

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reflectance (R) and transmittance (T) measurements of the deposited thin film grown on glass substrate were performed with the help of Perkin-Elmer lambda-750 spectrophotometer equipped with 150 mm integrating sphere assembly.Film grown on the sapphire substrate was used for low temperature (15K-200K) photoluminescence (PL) measurement. PL

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measurement was carried out using 488 nm Ar-Ne excitation laser sources.

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3. Result and discussion

Fig.1, represents the XRD pattern of the bulk ZnSnP2 material. All diffraction peaks

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corresponding to (112), (004), (024), (116), (224), (008), (136) and (028) planes of the chalcopyrite structure of ZnSnP2 are indexed [JCPDS File: 73-0396].The scanning electron microscopic (SEM) image of the film is shown in Fig.2 (a). The surface morphology is uniform and the average grain size is about 20 nm. Fig. 2(b) shows high resolution cross sectional TEM image of the film. It is clearly observed that the film is not of uniform composition throughout the area. A series of dark spots arranged in a line is seen, which is little off-centered towards the surface of the film. A magnified view of one of these spots,

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ACCEPTED MANUSCRIPT shown in Fig. 2(c), indicate the growth of nearly uniform crystallites (10 nm X 20 nm). An inter-planar spacing of 0.2825 nm corresponding to the (004) plane of ZnSnP2 was obtained from the high resolution image. In addition to the large crystallites arranged in the line, we also observed relatively smaller crystallites scattered across the film. The selected area

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electron diffraction pattern shown in Fig. 2(d) depicts a ring with spotty features superimposed on it. The ring pattern appears because of random orientation of the lattice planes of one crystallite with respect to another and also due to contribution from the smaller

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crystallites. The non-uniform distribution of intensity of the ring is due to strong spotty pattern from the larger crystallites. The inter-planar spacing calculated from the diameter of

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the ring of the SAED pattern (0.2850 nm) matches closely with that obtained from high resolution image. The Grazing Incidence X-ray Diffraction (GIXRD) for 43 nm film (Fig.3), however exhibit peaks at 2ϴ = 44.500 and 63.450 corresponding to the diffraction planes (024) and (008) of chalcopyrite ZnSnP2 structure. To understand the preferential alignment of

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the bigger crystallites along the line, we undertook detailed elemental distribution study using energy dispersive X-ray (EDX) analysis (Fig. 4) in the TEM across the film. Fig. 4(a) shows the high annular dark field image (HADF) of the cross section of the film. Fig 4(b) shows the

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EDX spectra across the region marked by yellow rectangle of Fig. 4(a), which clearly shows

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peaks due to Zn, Sn and P. The average stoichiometry ratio of Zn:Sn:P, calculated from the integrated intensities and taking care of the sensitivity factors, was found to be within the desired range of 1:1:2 to the accuracy of ~± 10 % as is the case for determination of composition from EDX analysis. The elemental mapping along a line shown by the orange line in Fig. 4(a) is shown in the plot of Fig. 4(c). Along with the traces for Zn, Sn and P, the figure also shows elemental distribution corresponding to Si which is the substrate. The traces corresponding to Zn and P gradually increases till the interface of the film with the substrate, while that of Sn shows a peak at a distance of about ~20 nm from the edge of the film, which

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the film and the substrate. The larger crystallites probably grew at the expense of the smaller ones during the film deposition following the mechanism of Oswald ripening. The arrangement of the ripened crystallites occurred in a line due to a fix thickness of the film.

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Figs.5(a) and 6(a)show the typical diffuse reflectance (R) and transmittance (T)

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spectra in the wavelength region (300-2000 nm) of the ZnSnP2 thin films on glass and sapphire substrates respectively. Efforts have been taken to eliminate the effect of substrate in the diffuse reflectance and transmittance measurements of ZnSnP2 thin films. For the reflectance measurement, a black paper has been inserted behind the transparent glass and sapphire substrates to prevent reflectance from the substrate-air interface. On the other hand,

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transmittance of the film-substrate structure has been obtained by eliminating the absorption due to pure substrate. The R and T data have been utilized to accurately evaluate the

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coefficient of absorption coefficient ( )using the following relationship [16], T = (100 − R) e

,

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where, d is the thickness of the film. Fig. 5(b) shows the variation of absorption coefficient of e-beam deposited ZnSnP2 thinfilms with photon energy (0.5 – 3.0 eV). The optical band gap of the film has been calculated by plotting ( ℎν) versus the energy (hν) in eV and extrapolating the linear part of the spectrum, ( ℎν) = f(hν), to zero. A direct band gap of 1.71 eV of the ZnSnP2 thin film (43 nm thickness) deposited at 2500C is estimated, which is similar to the recent theoretical calculation (~ 1.71 eV) using modified Becke-Johnson (mBJ) potential [17]. A similar value of bang gap has been estimated for the ZnSnP2 films grown on

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ACCEPTED MANUSCRIPT sapphire substrates as exhibited in Fig. 6(b). This result also agrees quite well with the other theoretically predicted value (1.68 eV) for the ordered chalcopyrite ZnSnP2 structure and previously determined experimental value [4]. However, both theZnSnP2 films exhibit excess optical absorption and considerable band tailing in the low energy region of the spectrum. It

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should be noted that on the low energy side of the absorption edge in the case of thin films, the excess absorption usually causing an exponential tail occurs between the various energy states developed near the band edges due to the presence of impurity and defects. The

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variation of absorption coefficient with photon energy in this low energy region often follows

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Urbach’s rule [18] and usually disturb the band edge related transition.

The temperature (15–200K) and excitation power (13.66-77.9 mW) dependent PL measurements were performed to study the origin of recombination process in ZnSnP2filmsgrown on sapphire substrates at 2500C. Temperature dependent PL spectra are shown in Fig.7 (a). The overall PL spectra of the sample are non-symmetrical and consisted

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of two broad and overlapped peaks. The first peak appeared at 1.529 eV at 15 K which is red shifted and decreased in intensity with increase in temperature. The other peak (1.643 eV, at

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15 K) which is much broader with compared to the first peak also shows similar behaviour (Fig. 7(b)). However, the second peak began to quench as the temperature raised above 100

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K. Considering the nature of the emission band of the peaks, integrated PL intensity has been plotted in each case with inverse temperature in Fig.8 and fitted with the two-channel Arrhenius equation [19]:

I =

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where, A and B are different process rate parameter, ET1 and ET2 are two different activation energies at low and high temperature regions respectively. Fitted parameters are (ET1= 10 8

ACCEPTED MANUSCRIPT meV and ET2= 56 meV) and ET1= 8 meV and ET2= 30 meV) for first and second peaks respectively, indicating the presence of two thermally activated recombination centres in each case. As the transmittance and reflectance measurements provide the optical band gap, Eg = 1.71 eV at room temperature, the emission bands centered around 1.529 eV cannot be

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considered as band to band transition in ZnSnP2. The variation of the emission peak energy, Emax(T), with temperature (T), has been also attempted to fit using Varshni model:

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%2 !"#$ (%) = ! (0) − '+%,

The fitting procedure yielded the values of fitting parameters for the first peak, E (0) = 1.531 +

eV⁄K , ' = 56.10 K), but completely disagreed with the band-edge

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eV, ( = 1.92 X10

related transition. However, an idea regarding the variation of band gap with temperature is obtained. Therefore, it is concluded that the transitions are related to the donor- acceptor pair (DAP) transition due to presence of native defects in the ZnSnP2 films. The experimental

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studies [2] and first-principles study of point defects in ZnSnP2 [16] show that the most likely donors are Sn-on-Zn sites [SnZn] and acceptors are Zn-on-Sn sites [ZnSn] and Zn vacancies

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[VZn]. The DAP recombination process can be explained using the following relation [21, 22]

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with the emission peak maxima, Emax, as !"#$ = !1 − (!2 + !3 ) +

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Where, ED and EA are donor and acceptor activation energies, respectively with their separation (RDA), Eg is the energy band gap and 7 is the dielectric constant of ZnSnP2 material.Since the PL data are not available at room temperature, the recombination processeshave analyzed at 15 K, using the value of band gap, Eg (15 K) = 1.761 eV, has been approximately determined from the Varshni model. We presumably considered that the donor and acceptor levels are due to [SnZn] and [VZn], respectively and RDAis very large compared 9

ACCEPTED MANUSCRIPT to the unit cell dimension andZn-Sn bond length (~ 0.401 nm). A value of (ED+EA) = 232 meV is obtained. Thus, a deep donor level, ED = (232- ET1= 232-56) meV = 176 meV from the conduction band (CB) is determined with an acceptor level, EA = ET2 = 56 meV from the valence band (VB). A schematic band model is given as inset in Fig.6 for clarifying the

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radiative recombination process for the first peak. Miyauchi et al [2] explained their PL emission band at about 1.54 eV considering DA pair transition due to similar defects for the bulk ZnSnP2 crystals grown by solution growth and normal freezing methods. However, in

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our model the band gap energy has been considered to correlate to assign the actual donor or

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acceptor levels.

To explain the origin of the second PL emission peak appeared at 1.643 eV at 15 K, donors at [SnZn] sites (ED=176 meV)and acceptors at [ZnSn] sites (ED = ET2= 30 meV)are considered. A calculation similar to the above provides DA pair separation, i.e., RDA≈ 4.4 nm.The value of RDA is roughly 11 times the Sn-Zn bond length [20] and 4 times the unit cell

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dimension (a=0.5651 nm and c=1.1302 nm) of the ZnSnP2 cell [23], indicating that both the PL peaks are originated from the DA pair transition though their separations are different. In

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the above calculation, we have used the dielectric constant (7 = 8.2) value for chalcopyrite ZnSnP2 [24].It is noteworthy to mention that the activation energy (ET1=8-10 meV) at the low

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temperature region (Fig. 8) is related to the escaping of charge carriers from the band tail states. Our absorption spectrum (Figs. 5-6) shows considerable band tailing at the low energy region (< 1.4 eV).

We have also investigated the variation of integrated intensity of PL emission bands

withexcitation power at 15K (exhibited in Fig.9(a)).It is observed that the PL emission intensity increases with excitation power up to 31 mW and then saturates at the high values of laser power. On the other hand, the PL peaks do not exhibit any appreciable peak shifts with excitation power. A very small blue shift of ~ 4 meV is observed between the emission peaks 10

ACCEPTED MANUSCRIPT measured at 13.66 and 70.90 mW, respectively.These observations are indicative of the donor (D)-acceptor (A) pair related recombination processof distant pairs [25]. Fig. 9(b) shows the typical variation of PL intensity for both the PL emission bands with laser power. The nature of variation indicates that two emission peaks of different origin have similar in quenching

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character.

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4. Conclusion

Successful growth of polycrystalline ZnSnP2 thin films on p-type Si (001), sapphire and glass

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substrates by e-beam evaporation technique is reported. Selected area electron diffraction pattern justifies the polycrystalline nature of the grown structure (Si/ZnSnP2). Estimation of bandgap from the measured optical reflectance and transmittance spectra at room temperature is found to be 1.71eV. The absorption coefficient of the film is about 106 cm-1 above 1.6 eV of photon energy. In low temperature PL measurement two broad emission peaks have been observed at 1.529 eV and 1.643 eV

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respectively. DA pair recombination model is considered to explain the luminescence spectra of the ZnSnP2 films. Antisite defects (SnZn-acceptor, ZnSn-donor) and vacancies (VZn) present in the ZnSnP2

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are suggested as native defects and mainly responsible for the light emission property.

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Acknowledgement

The authors (SM and AN) acknowledge the financial support received from the University Grant Commission (UGC File No. 43-400/2014(SR)), New Delhi and Department of Science and Technology (DST), Government of India for providing equipment utilization facilities under FIST grant. The author (AN) also acknowledges the FRPDF grant received from the Presidency University, Kolkata for carrying out the research work.

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ACCEPTED MANUSCRIPT References [1] David O. Scanlon, Aron Walsh, Appl. Phys. Lett. 100 (2012) 251911-1-251911-3. [2] K. Miyauchi, T. Minemura, K. Nakatani, H. Nakanishi, M. Sugiyama, S. Shirakata, Phys.

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Stat. Sol. C6 (2009) 1116-1119. [3] L.M. Peter, Philos. Trans. R. Soc. London, Ser. A 369 (2011) 1840-1856.

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[4] P. St-Jean, G.A. Seryogin, S. Francoer, Appl. Phys. Lett. 96 (2009) 231913-1–231913-3. [5] H.Y. Shin, P.K. Ajmera, Mater. Lett. 8 (1989) 464-467.

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[6] P. K. Ajmera, H. Y. Shin, B. Zamanian, Solar Cells 21 (1987) 291 - 299. [7] B. Lita, M. Beck, R. S. Goldman, Appl. Phys. Lett. 77 (2000) 2894-2896. [8] N. Yuzawa, J. Chantana, S. Nakatsuka, Y. Nose, T. Minemoto, Current Appl. Phys. 17 (2017) 557-564.

(2016) 1-4.

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[9] S. Nakatsuka, N. Yuzawa, J. Chantana, T. Minemoto, Y. Nose, Phys. Stat. Sol. A,

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[10] S. Nakatsuka, Y. Nose, Y. Shirai, J. Appl. Phys. 119 (2016) 193107-1-193107-5. [11] S. T. Pantelides, Y. Puzyrev, X. Shen, T. Roy, S. Das Gupta, B. R. Tuttle, D. M.

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Fleetwood, R. D. Schrimpf, Microelectron.Eng. 90, (2012) 3-8. [12] A. García and J. E. Northrup, Phys. Rev. Lett. 74, (1995) 1131-1134. [13] A. Alkauskas, Q. Yan, C. G. Van de Walle, Phys. Rev. B90, (2014) 075202-1-07520217. [14] S. Nakatsuka, H. Nakamoto, Y. Nose, T. Uda, Y. Shira, Phys. Stat. Sol. C12 (2015) 520523.

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ACCEPTED MANUSCRIPT [15] K. Nakatani, T. Minemura, K. Miyauchi, K. Fukabori, H. Nakanishi, M. Sugiyama, S. Shirakata, Jpn. J. Appl. Phys. 47 (2008) 5342-5344. [16] S. Ebraheem, A. El-Saied, Mater. Sci. Application 4 (2013) 324-329

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[17] F. Tran, P. Blaha, Phys. Rev. Lett. 102 (2009) 226401-1-226401-4. [18] R.A. Smith, Semiconductors, Cambridge University Press, Cambridge (1986).

[19] W. Stadler, D.M. Hofmann, H.C. Alt, T. Muschik, B.K. Meyer, Phys. Rev. B51 (1995)

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10619 -10630.

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[20] Yu Kumagai, M. Choi, Y. Nose, F. Oba, Phys. Rev. B90 (2014) 125202-1-125202-12. [21] A. Nayak, D.R. Rao, Appl. Phys. Lett. 63 (1993) 592-593.

[22] R.C.C. Lite, A.E. Digiovani, Phys. Rev. 153 (1963) 841-843.

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[23] A. Nayak, D. R. Rao, Optical Materials 1 (1992) 85-89.

[24] M. Rubensteind, R.W. Ure Jr, J. Phys.Chem. Solids 29 (1968) 551-555.

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[25] R. Dingle, Phys. Rev. 184 (1969) 788-796.

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ACCEPTED MANUSCRIPT Figure Captions: Fig.1. X-ray diffraction pattern of ZnSnP2 bulk material Fig.2(a) Scanning Electron Micrograph of ZnSnP2 thin film. (b) Cross sectional view of Transmission Electron Micrograph and (c) Lattice image of ZnSnP2 thin film deposited on

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silicon (001). (d) Selected area diffraction pattern. Fig. 3. GIXRD spectra of ZnSnP2 thin film on Si (001) and sapphire substrate

Fig.4. (a) TEM dark field image (cross-sectional view), (b) EDX spectra and (c) depth profile

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of ZnSnP2 thin film on Si (001) substrate.

Fig.5(a). Transmittance and reflectance spectra of ZnSnP2 thin film on glass substrate.

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Fig.5(b). (αhν)2 versus hν plot for the calculation of band gap and optical absorption spectra (inset) of ZnSnP2 thin film on glass substrate.

Fig.6(a). Transmittance and reflectance spectra of ZnSnP2 thin film on sapphire substrate. Fig.6(b). (αhν)2 versus hν plot for the calculation of band gap and optical absorption spectra

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(inset) of ZnSnP2 thin film on sapphire substrate.

Fig.7(a). Temperature variation of photoluminescence (PL) spectra of ZnSnP2 thin film. Fig.7(b). De-convoluted PL spectrum recorded at 15 K.

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Fig.8. Intensity of of peak-1 and peak-2 as a function of reciprocal temperature. Continuous lines are fitted curves using two-channel Arrhenius’s equation.

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Fig.9(a). Intensity variation of PL emission with excitation power. Fig.9(b). Intensity of of peak-1 and peak-2 as a function of excitation power.

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2.5

3.0

3.5

TE D

Energy (eV)

EP

5.0x1012

0.0

AC C

(α ν)2 eV 2cm-2 αhν

1.5x1013

0.5

Figure-5(b)

1.0

1.5

2.0

2.5

Energy (eV)

3.0

3.5

ACCEPTED MANUSCRIPT

20

0 400

AC C

EP

10

Figure-6(a)

30

TE D

30

800

10 1200 Energy (eV)

1600

2000

Reflectance (%)

SC

40

50 40

M AN U

50

20

60

Sapphire/ZnSnP2

T R

60

Transmittance (%)

RI PT

70

ACCEPTED MANUSCRIPT

1.6x106

Sapphire/ZnSnP2

RI PT

2x1013

Sapphire/ZnSnP2

SC

α (cm-1)

1x1013

M AN U

1.2x106

1.0x106 0.5

1.0

1.5

2.0

2.5

3.0

3.5

EP

TE D

Energy (eV)

0 0.5

AC C

(αhν ν)2 (eV2 cm-2)

1.4x106

Figure-6(b)

1.0

1.5

2.0

Energy (eV)

2.5

3.0

3.5

ACCEPTED MANUSCRIPT

3

6.0x10

3

4.0x10

3

2.0x10

3

0.0

SC

8.0x10

M AN U

4

TE D

1.0x10

(a)

1.4

Figure-7 (a)

15K 30K 50K 70K 100K 120K 150K 175K 200K

EP

1.2x10

4

RI PT

4

AC C

Emission Intensity (a.u.)

1.4x10

1.5 1.6 1.7 Energy (eV)

1.8

ACCEPTED MANUSCRIPT

4

1.2x10

(b)

SC

4

3

8.0x10

3

3

2.0x10

0.0

EP

3

Fitted Line (1.529 eV) Fitted Line (1.643 eV)

TE D

6.0x10 4.0x10

Fitted Line

M AN U

1.0x10

AC C

Emission Intensity (a.u.)

15 K (Expt.)

RI PT

4

1.4x10

1.3 1.4 1.5 1.6 1.7 1.8 1.9 Energy (eV) Figure-7(b)

3.9

Peak-1

3.8

At 15 K

3.6

176 meV

D 1.761 eV

3.2 3.0

56 meV

A

2.8 0.00

0.02

0.04 -1

0.06

EP

1/T (K )

Figure-8

3.7 3.6

TE D

3.4

Peak-2

3.8

AC C

Peak Intensity (a.u.)

4.0

M AIntensity Peak NU (A.u.) SC RI PT

ACCEPTED MANUSCRIPT

0.08

0.00

0.02

0.04 -1

1/T (K )

0.06

0.08

4

3x10

4

2x10

4

1x10

4

0

1 .3

Figure-9 (a)

SC

4

M AN U

4x10

mW mW mW mW mW

TE D

4

13.66 13.71 31.90 53.90 70.90

EP

5x10

(a)

AC C

Emission Intensity (a.u.)

6x10

RI PT

ACCEPTED MANUSCRIPT

1 .4

1 .5

1 .6

1 .7

E n e rg y (e V )

1 .8

ACCEPTED MANUSCRIPT

4

RI PT

10

SC M AN U 10

TE D

3

AC C

EP

PL Intensity (a.u.)

(b)

Figure-9 (b)

20

40

Power (mW)

60 80 100

ACCEPTED MANUSCRIPT

Thin ZnSnP2 films are deposited by e-beam evaporation technique for the first time

•

HRTEM indicates polycrystalline nature of the film with (004) preferred orientation

•

An optical band gap 1.71 eV and two PL bands (1.529, 1.634 eV) at 15K are detected

•

DA pair transition is plausibly responsible for the light emission in ZnSnP2 film

•

Possible intrinsic point defects SnZn-donor and VZn/ZnSn-acceptor are suggested.

AC C

EP

TE D

M AN U

SC

RI PT

•