Oriented Attachment: a Path to Columnar Morphology in Chemical

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Oriented Attachment: a Path to Columnar Morphology in Chemical Bath Deposited PbSe Thin Films Tzvi Templeman, Sucheta Sengupta, Nitzan Maman, Eyal Baror, Michael Shandalov, Vladimir Ezersky, Eyal Yahel, Gabby Sarusi, Iris Visoly-Fisher, and Yuval Golan Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01771 • Publication Date (Web): 16 Jan 2018 Downloaded from http://pubs.acs.org on January 16, 2018

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Crystal Growth & Design

Oriented Attachment: A Path to Columnar Morphology in Chemical Bath Deposited PbSe Thin Films Tzvi Templeman,1,2 Sucheta Sengupta,1 Nitzan Maman,1,3 Eyal Bar-Or,1,2 Michael Shandalov,4 Vladimir Ezersky,1 Eyal Yahel,4 Gabby Sarusi,1,5 Iris Visoly-Fisher1,3 and Yuval Golan1,2* 1

Ilse Katz Institute for Nanoscale Science and Technology, Ben-Gurion University of the

Negev, Be'er Sheva 84105, Israel 2

Department of Materials Engineering, Ben-Gurion University of the Negev, Be'er Sheva

84105, Israel 3

Department of Solar Energy and Environmental Physics, Swiss Institute for Dryland

Environmental and Energy Research, Jacob Blaustein Institutes for Desert Research, BenGurion University of the Negev, Midreshet Ben-Gurion 8499000, Israel 4

Department of Physics, Nuclear Research Center Negev, P.O. Box 9001 Be'er Sheva, Israel

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Department of Electro-Optics Engineering, Ben-Gurion University of the Negev, Be'er

Sheva 84105, Israel

Keywords: Columnar microstructure, chemical bath deposition, oriented attachment, lead selenide.

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ACS Paragon Plus Environment

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

Abstract. We have studied columnar PbSe thin films obtained using chemical bath deposition. The columnar microstructure resulted from an oriented attachment growth mechanism, in which nuclei precipitating from solution attached along preferred crystallographic facets to form highly oriented, size quantized columnar grains. This is shown to be an intermediate growth mechanism between the ion-by-ion and cluster growth mechanisms. A structural zone model depicting the active growth mechanisms is presented for the first time for semiconductor thin films deposited from solution. The columnar films showed well-defined twinning relations between neighboring columns, which exhibited 2D quantum confinement, as established by photoluminescence spectroscopy. In addition, anisotropic nano-scale electrical properties were investigated using current sensing AFM, which indicated vertical conductivity, while maintaining quantum confinement. Introduction Thin films with tailored microstructure for desired anisotropic physical properties play an important role in many fields of technology. Applications such as magnetic storage media, optical components (birefringent filters, tunable laser mirrors, linear polarizers, etc.) and gas sensors, all rely on microstructural control for their unique anisotropic physical properties.1–4 The potential for directly grown anisotropic film growth is quite promising as it cuts back on both time and costs of post growth processing, which would otherwise be required. With the emergence of nanotechnology-based applications, there has been increasing interest in complex nanoscale structures and their incorporation into state-of-the-art devices.1–7 Applications relying on nanoscale components require control over the growth mechanisms, achievable by understanding the fundamental processes leading to nano-structured films. Mass production of nanoscale devices will always prefer simplicity and lower costs relative to the conventional, currently available growth methods.

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To date, the most commonly used technique for the growth of structured films is physical vapor deposition (PVD), which originates as far back as 1886.8 Growth of columnar thin films (CTF) has been achieved by controlling the angle of incident vapor flux while maintaining low temperature and pressure. Ordered growth of CTF requires control over the arriving adatoms, namely surface diffusion length, angular distribution and incoming flux angle are all crucial for achieving columnar microstructure.4,7,9,10 Predicting film microstructures based on deposition parameters has been demonstrated by the structural zone model (SZM), established by Thornton for films deposited using PVD.9 Thornton presented a general model describing the effect of growth conditions in PVD, in-which CTF was achieved by maintaining low substrate temperatures and low chamber pressure. Both of these conditions resulted in low adatom mobility, preventing lateral grain growth. However, high values of angle distribution of the incoming vapor flux and adatom angle of incidence might still perturb CTF growth by selfshadowing effects, where the growing columns prevent adatoms from reaching neighboring grains and expanding. Maintaining both low adatom surface mobility and a uniform angular distribution will result in well-ordered CTF. An alternative, simpler and less expensive technique for the growth of columnar thin films is by chemical solution deposition. Chemical solution deposition (CD) is one of the simplest and least expensive methods for the growth of high quality thin films.11–14 In contrast to PVD, CD does not require stringent vacuum systems or maintaining high temperatures, thus, enabling the growth of thin films in a simple and inexpensive manner. Growth bath is prepared by introducing salt precursors of both cations and anions into an aqueous solvent. Once the reaction is initiated, very low solubility product results in precipitation. To attain film growth upon a solid substrate, complexing agents are introduced to prevent uncontrollable, rapid precipitation. Two of the most commonly reported and thoroughly investigated mechanisms of film growth in CD are the ion-by-ion (IBI) growth mechanism and the cluster growth mechanism.11,15 In the 3



cluster growth mechanism, solid clusters precipitate in solution and diffuse to the substrate, resulting in nanocrystalline films; the IBI mechanism refers to direct reaction and subsequent deposition upon the substrate, resulting in films which are typically characterized with large grain size. However, only a few cases of CD grown CTF microstructures were reported, and the mechanisms leading to them remained un-answered. Investigations on the structural and electro-optical properties of hexagonal CdS films grown using CD were reported by Wenyi et al. who observed columnar morphologies, yet the main focus was placed on film properties, not dealing with the growth mechanism involved.

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Another example of CTF grown by CD

was reported by Shandalov and Golan,17 where columnar growth was shown to be an intermediate microstructure within the film. To the best of our knowledge, the only systematic investigation of CTF using CD was reported for ZnO films. Govender et al. investigated the CD growth conditions required for size controlled ZnO columnar films.5 Control over column size and distribution was presented, but the fundamental growth mechanisms remained unanswered. Interestingly, these authors suggested that either oriented aggregation or dissolution-recrystallization processes facilitated columnar growth. In this work we successfully identified, for the first time, the growth mechanism which gives rise to CTF in CD. The growth mechanism was investigated for PbSe thin films on monocrystalline GaAs, a materials system which has been thoroughly investigated.17–21 Development of Se2- substrate pretreatments enabled reversal of the order of reactant addition, thus increasing film uniformity and process reproducibility.21 Once identified, growth mechanism was investigated by monitoring variations in growth rate and film morphology as a function of growth duration through time interrupted growth studies. By systematically investigating the effect of growth parameters on film microstructure, the conditions required for CTF morphology in CD were established and an empirical model was presented for the growth mechanism and subsequent morphological evolution. Moreover, the columnar PbSe 4


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films presented noticeable and controllable 2D quantum size effects, resulting in enhanced lateral charge scattering while maintaining conduction channels along the column length. Experimental Materials and Chemicals Sodium sulfite (Aldrich, analytical 99.95%), selenium powder (Aldrich, 100 mesh, analytical 95+%), lead acetate trihydrate (Aldrich, analytical 99+%) and potassium hydroxide (Frutarom, analytical) were used without further purification. Single crystal GaAs(100) substrates were purchased from AXT (epi-polished, undoped, ±0.1° miscut). The films were deposited from a solution with a final composition of 30 mM Pb(CH3COO)2·3H2O, 50mM Na2SeSO3 and various concentrations of KOH (complexing agent for Pb) at a final pH > 13. Films were deposited in a thermostatic bath into which a 50 ml Pyrex beaker was placed. The beaker was equipped with a custom built Teflon stage for mounting the substrates. A stock solution of Na2SeSO3 (0.2 M) was prepared by mixing an excess of sodium sulfite (0.5 M) with selenium powder in distilled water and stirring at 90 °C for 1 h. The sodium selenosulfate (Na2SeSO3) solution was filtered in order to remove non-reacted selenium powder. The film deposition method described by Templeman et al. was implemented in this work. 21 Structural, chemical and optical characterization X-Ray Diffraction (XRD) The crystallographic phase and texture of the films were studied by XRD. A Panalytical Empyrean diffractometer equipped with a PIXcel linear detector and monochromator on diffracted beam was used. Data were collected in the 2θ/θ geometry using Cu Kα radiation (λ = 1.5405 Å) at 40 kV and 30 mA. Diffraction scans were taken during 8 minutes in a 2θ range of 20-65° with a step size of ~ 0.033°.

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Field Emission Gun Scanning Electron Microscopy (FEG-SEM) The morphology of the films was observed using an ultrahigh resolution JEOL JSM-7400F FEG-SEM without coating of the surface. The acceleration voltage was 3.5 kV and signal was collected using an in-lens secondary electron detector. Film thickness was measured from freshly cleaved cross sections (X-sec) while surface topography was observed in plan-view. Transmission Electron Microscopy (TEM) Cross sections were prepared by cutting the sample into slices normal to the interface and gluing them together face-to-face using M-Bond 610 adhesive (Allied HighTech Ltd.). The samples were polished with a precision small-angle tripod holder on a series of diamond polishing papers (Allied HighTech Ltd) until a nominal thickness of 30μm was achieved, and glued to a Mo slot grid (1×2 mm2). Final thinning was done by Ar ion milling using a Gatan PIPS-2 precision ion polishing system. Plan-view samples were prepared with a FEI Helios G4 FX FIB using lift-out technique. TEM, high resolution TEM (HRTEM) and selected area electron diffraction (SAED) were carried out using a JEOL JEM-2100F instrument operating at 200kV. The Gatan Digital Micrograph 3 software was used for fast Fourier transform (FFT) analysis of HRTEM lattice images. Analytical TEM (ATEM) investigations including Energy Dispersive Spectroscopy (EDS) analysis in the scanning TEM (STEM) mode were carried out using a JEOL JEM-2100F ATEM operating at 200 kV. Optical properties Photoluminescence (PL) measurements were performed using a Bruker VERTEX 80v Fourier transform infra-red (FTIR) spectrometer in the mid-IR (MIR) and near-IR (NIR). Scans ranged from 8000cm-1 to 1500cm-1 in 64cm-1 steps using a liquid nitrogen cooled InSb high gain detector. The PL module combined with an external lock-in amplifier (Stanford Research model 6


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830) enabled time resolved step scans for the reduction of thermal background. Excitation was achieved by a 100 mW 785 nm laser with a mechanical chopper operating at 2500 Hz. Current sensing atomic force microscopy (CS-AFM) Simultaneous current and topography mapping were performed using an Agilent 5500 AFM operating in contact mode. A Pt/Ir coated Si probe with a tip radius of 30nm (AppNano CONTV-PT) was used with a pre-amplifier of 10nA/V. The substrate was grounded and a positive bias of 0.1V was applied to the tip. Contact force was maintained constant throughout the measurements. Light Scattering (LS) Scattering data were collected from the deposition solutions by using a CSG-3 device (ALV, Langen, Germany). The laser power was 20 mW at the He–Ne laser line (632.8 nm). Averaged scattered intensities were measured by an ALV/LSE 5004 multiple tau digital cross correlator, at scattering angles of 300 and 60° and at temperatures of 25 and 30 °C as a function of reaction duration. The scattering intensities were normalized with respect to blank solvent (water and toluene). Results and Discussion Film growth rates were extracted by plotting changes in film thickness, measured from SEM cross section images, and XRD integrated intensities as a function of growth duration (Figure 1a and 1b). A clear transition in growth rate was observed at ~40min of growth, accompanied by a change in film morphology. At this point, a transition from well-defined nano-columnar growth to bulk-like growth was observed in SEM imaging (Figure 1c-1f). Time-dependent variations in growth rate typically indicate a change in growth mechanism, often due to depleting cation concentrations that lead to increasing complex/cation ratio.11,15,19 It appears that the initial growth rate is lower, even though solution concentrations should be higher at the 7



initial stages of the reaction. Note that the slightly negative slope in Figures 1a and 1b prior to transition indicates a minor decrease in growth rate, and should not be confused with a decrease in overall film thickness due to film dissolution, as at each stage of the deposition the overall film thickness (and corresponding overall XRD reflection intensity) continuously increases. This may be attributed to low effective solution concentration, i.e., lower concentrations of available ions which contribute to film growth. Such behavior is observed when cluster growth is active, where effective concentrations are low due to formation of hydroxide clusters and their subsequent transformation. However, the transition in film morphology, from columnar to bulk-like structures, could originate from film-substrate lattice mismatch, where interface defects are affecting film growth. To verify this point, successive film growth was initiated; if indeed the transition results from depleting cationic solution concentrations, this should be eliminated by replenishing the solution. SEM imaging of films grown from three successive baths, where in each growth was terminated prior to the transition point, are presented in Figure S1a. By implementing this method columnar growth is unaffected, thus confirming the origin of this transition as a direct result of time dependent solution depletion. It is important to note that such successive growth allowed us to obtain nano-columnar films up to one micron in thickness (Figure S1b).

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Figure 1. Growth rate as a function of reaction duration extracted from (a) X-section SEM thickness measurements and (b) XRD integrated intensities. X-section and plan-view SEM imaging of films deposited at 25ºC and a [OH-] of 1.2M for a duration of (c,e) 10 min and (d,f) 60 min.

Transitions in CD growth mechanisms have been studied in-situ using LS by Shandalov et al. 19 and Sengupta et al. 22 To date, this method has been implemented to monitor cluster to IBI transitions. The nature of cluster growth mechanism involves light scattering precipitants, whose scattering intensity strongly depends on the particle size and particle concentration. 9



Particles smaller than incident beam wavelength should give rise to isotropic Rayleigh scattering (under ~100nm), thus remain independent of scattering angle; larger particles (forgoing less scattering) are detected at smaller angles. Once transition to IBI occurs the solution becomes translucent, thus sharply decreasing scattering intensities. Thus, LS can effectively determine the transition point between the mechanisms, which can be further correlated with the structural characterization of the films obtained from a time interrupted growth series. In-situ LS measurements were performed on deposition solutions maintained at 25 ºC and 30 ºC (Figure 2a, 2b), increasing deposition temperature decreases transition time, thus, by measuring both temperatures, information regarding particle size can be gained.

Figure 2.Time resolved LS data obtained from reaction solutions (pH 13.95) at two different scattering angles (a) 30º and (b) 60º.

A time interrupted series of samples was examined in the SEM to identify transition points (Figure S2). Films grown at 30 ºC undergo transition at ~15min, and those grown at 25 ºC show a transition at ~ 50min of growth. High angle scattering does not reveal the dependence of the transition point on the deposition temperature, indicating that larger particles are dominant during the columnar growth stage, as opposed to the cluster mechanism where smaller particle size dominated the scattering behavior.19,22 Once transition occurs, the solution 10


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becomes translucent signifying that IBI mechanism has become dominant. This behavior is quite similar to previous reports on cluster to IBI transitions; however, in cluster mechanism high angle scattering dominates due to small particle size. Analysis of the growth mechanism during cluster growth was performed by Gorer et al., who observed spherical lead hydroxide nano-particles at varying stages of conversion to PbSe. Once fully converted, the particles maintained spherical morphologies.15 To further investigate the growth mechanism, solution aliquots were removed 30 seconds after reaction initiation, deposited onto gold mesh grids and dried under nitrogen. These samples were analyzed using TEM imaging, which revealed chain networks composed of smaller cube-like particles of ~10 nm (Figure 3); both SAED and EDS analyses confirmed the particles to be single phase PbSe. At this initial stage, no crystallographic ordering was observed between adjacent cubic particles as verified from HRTEM imaging and FFT analysis, indicating that the particles are randomly attaching to form larger, chain-like networks.

Figure 3. (a) High angle annular dark field STEM and (b) HRTEM micrographs from particles removed 30 sec after reaction initiation. Solution pH was 13.95 and solution temperature was 25 ºC.

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This sampling method was repeated several minutes into the reaction, once columnar growth is established in the films, at ~2 min intervals from reaction initiation. Large agglomerates of ~400nm (Figure 4a and 4b) of single phase rock salt PbSe cubes (ICDD#050592) were identified using SAED, which agrees well with our LS data which showed strong scattering from large particles at low scattering angles. Closer examination of HRTEM micrographs showed that each larger cube is composed of smaller (~10nm) oriented nano-cubes attached to each other along {100} facets with some degree of rotational misalignment, indicated by the presence of rotational moiré fringes (arrow in Figure 4c); Once again, FFT analysis confirmed the presence of single phase PbSe (Figure 4). Contrary to cluster growth mechanism, where the lead hydroxide nano-clusters remain spherical even when fully converted to PbSe, the current mechanism presents direct and uniform nucleation of PbSe nanocubes, as rocksalt lead chalcogenides are indeed expected to form cube shaped precipitants, by favoring the lower surface energy of {100} facets.23 This behavior, where particles arrange along preferred crystallographic facets during growth, has been termed oriented attachment (OA), a non-classical crystallization mechanism, which often takes place in cases of nanocrystalline precipitation. Driven by reduction in surface energy, colliding particles attach and rotate to maintain higher structural coherence, leading to a low-energy configuration.24,25 As has been stated above, some degree of misalignment between adjacent particles was observed in the HRTEM images (Figure 4c and 4d), which disappeared after exposure to the electron beam as shown in the corresponding FFT patterns (Figure 4e and 4f).

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Figure 4. (a) Bright field TEM micrograph and (b) SAED of particles obtained 3 min after reaction initiation; diffraction was taken from the region marked in red in “a”. Solution pH was 13.95 and temperature was 25 ºC. HRTEM imaging of the region marked by the small yellow circle in “a” (c) before and (d) after 1min exposure to the electron beam. FFT analysis taken from (e) image “c” and (f) image “d”. 13



We assume the thermal energy supplied from electron irradiation in the TEM provides sufficient mobility to finalize the OA process. Similar behavior has been previously reported for other systems as well.24 In ZnO, OA occurs along the polar [00.1] direction of the Wurtzite structure, and has been attributed to the attraction between opposing surface polarities, overcoming the repelling double layer barrier present between adjacent particles.5 PbSe {100} facets are non-polar and attraction between adjacent particles should have presented weaker forces. However, in the current case it appears that the high concentration of rapidly forming PbSe nanocube precipitants facilitates the OA mechanism. To understand how OA leads to columnar film growth, detailed cross-sectional HRTEM and SAED analyses were performed on the films. Film microstructure was confirmed to be a columnar “match-stick” morphology which resulted from inhibited growth in lateral x-y dimensions and enhanced growth along z (Figure 5a). Examining the film-substrate interface supports the correlation between growth rate and mechanism, as each column base is in direct contact with the substrate, i.e., no intermediate growth mechanisms (such as the simple cluster mechanism, which resulted in an intermediate layer of rounded, randomly orientedcolloids as reported previously by Shandalov and Golan

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) were observed SAED performed at several

locations reaffirmed XRD indications of film (nano-column) growth along the [111] direction (Figure S3c and S3d, seen also in FFT insets in Fig. 5 including in 5b). Due to the nanocrystalline nature of the films, artifacts such as double diffraction and moiré fringes were commonly observed, complicating analysis of the film-substrate orientation relationship.

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Figure 5. X-TEM analysis of columnar films grown at 25 ºC from a solution of 0.9M [OH-] for 30 min. (a) Bright field TEM film overview. (b) HRTEM micrograph of neighboring columns with their corresponding FFT analysis depicted in (c) and (d), respectively. Regions of interest for FFT analysis are shown in b. 15



However, careful analysis of the SAED data combined with FFT performed on HRTEM micrographs, taken from alternating columns, established film-substrate orientation relationship (Figure s3) as well as orientation relationship between adjacent columns (Figure 5b). PbSe films grown on GaAs(100) single crystal substrates show a non-conventional orientation relationship with the PbSe(111) parallel to the (100) oriented substrate surface as previously shown by Shandalov et al.13,17,21,26 However, analysis of the crystallographic directions parallel to the electron beam direction, also defined as the zone axis (ZA) of the electron diffraction pattern, shows that the in-plane orientation relationship is not the straightforward case described for this system by Osherov et al. where [110]PbSe coincides with [110]GaAs (we note that in cross-section samples the in-plane directions of the film and substrate coincide with the zone axes).13 Based on the analysis performed here, it appears that the columnar microstructure results from twinning defects which continue throughout the film, showing either matrix [1̅10]PbSe or twinned [11̅0]PbSe in-plane orientation. Based on the above results we can now describe the OA growth leading to columnar microstructure. As illustrated in Figure 6a, heterogeneous nucleation upon the substrate proceeds via IBI, forming three-point pyramids which are mirrored by 180º upon the (111)PbSe twinning plane.27 The homogeneously nucleated solution precipitants (PbSe nano-cubes) adhere to exposed {100} pyramidal facets through the OA mechanism. The illustration depicts the ZA and emphasizes that the matrix-twin interfaces seen here are not simple planes perpendicular to the substrate but rather a jagged boundary which results from the attachment of cubes with alternating orientations. A closer examination of twin boundaries between neighboring columns (such as the boundary analyzed in Figure 5) indeed reveals they are not perfectly linear but rather “zigzagging” along the interface as observed from the HRTEM micrograph presented in Figure 6b. Further confirmation can be obtained from HRSEM plan-view imaging of the columnar growth stage 16


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(Figure 6c), where particles are clearly seen attaching at the film surface. However, pure OA would be expected to result in a highly porous film due to misalignment of the adhering particles between adjacent columns. Moreover, as stated before, OA growth does not explain the initial epitaxial nucleation upon the substrate nor the twinning; therefore, we conclude that both OA and IBI are proceeding simultaneously as showed by the dashed lines in Figure 6a. Once the growth mechanism was established, we proceeded to the task of mapping the parameter regime which results in OA growth, as described below.

Figure 6. (a) 3D illustration of the OA mechanism leading to columnar film growth. (b) a 2D cross-section viewed from the direction. Plan-view and X-sec imaging of a columnar PbSe film grown at 25 ºC and a [OH-] of 0.9M. (c) X-TEM micrograph of adjacent column

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interfaces (30 min film growth). (d) HRSEM plan-view imaging after 2min of growth. (e) HRTEM plan-view micrograph after 30min of film growth.

Studying the growth conditions leading to columnar growth indicated a direct dependence on the deposition temperature and the complex/cation ratio. Lower ratios resulted in pure cluster growth while higher ratios lead to pure IBI growth. Gorer et al. reported on the effect of solution complex/cation ratio and temperature on the dominant growth mechanisms.15 They showed that low ratios, or equivalently, lower temperatures resulted in cluster growth as solid metal hydroxides are formed in solution and are slowly transformed into PbSe (true also for additional material systems).11,15,28,29 Increasing the complex/cation ratio (or temperature) limits the formation of solid hydroxides and growth proceeds through the IBI mechanism. However, in their study they focused on the classical cluster and IBI deposition mechanisms, and did not address the intermediate regime leading to columnar growth via OA. Here we have monitored film growth and the resulting microstructure as a function of pH and temperature in order to identify the conditions required for pure cluster, OA and IBI growth. As has already been stated, at low [OH-]/[Pb2+] ratios solid oxide clusters are formed due to Pb(OH)2 precipitation which are then converted to PbSe clusters by reaction with free selenide anions. 30,31 Increasing this ratio leads to the formation of Pb(OH)− If cation solution 3 complexes.

concentration is high enough for saturation, direct precipitation of nano-cubic PbSe occurs which are subsequently deposited through OA. During growth, the cation concentrations are continuously depleted and once a critical point is reached where precipitation is no longer favorable, the PbSe particles may re-dissolve, giving rise to pure IBI mechanism. This behavior well-explains the results presented above, in which negative growth rates were observed prior to transition (Figure 1a and 1b) and the non-abrupt transition in LS data (Figure 2a). Furthermore, it explains why replenishing solution concentrations negate mechanism 18


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transitions, and re-affirms the distinct zones previously observed in CD PbSe films by Shandalov et al.

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, where the different mechanisms often took place at different stages of

growth of the same film. Mapping of the growth mechanism as a function of temperature and complex/cation ratio growth parameters was performed by monitoring post-growth film morphology combined with XRD analysis of the PbSe||GaAs(100) system. Based on the results a SZM was constructed and is presented in Figure 7.

Figure 7. Structural zone model (SZM) for PbSe on GaAs(100) as a function of growth temperature and [OH-].

The transition points between the three distinct mechanisms were determined; more importantly, within the OA regimes, gradual tuning of columnar width can now be performed. 19



To demonstrate 2D quantum confinement, the direct band gap radiative emission was measured using PL as a function of film columnar width. The results are presented in Figure 8a; a clear transition towards higher band gap energies can be seen with decreasing columnar width, which is dependent on deposition temperature. These results are quite interesting as twin-boundaries in semiconductors have been subject to extensive theoretical studies.32,33

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Figure 8. (a) PL spectra from PbSe films grown for 10 min using a [OH-] of 1.2M at varying deposition temperatures. (b-f) SEM plan-view images of the films. (g) CSAFM plan-view measurement performed on the film grown at 30ºC.

There are several reasons for periodic electronic scattering in semiconductors, among them low and high angle grain boundaries, periodic stacking faults and scattering due to alternating 21



crystal phases in polytypic systems such as CdS, ZnS and SiC. Such studies are motivated by the ability to control anisotropic electrical and optical properties combined with 2D tuning of the material band gap.6,32,34 In the case of incoherent low and high angle grain boundaries, the presence of dangling bonds can introduce intraband states which result in recombination, thus, adversely affecting device efficiency. However, the current thin film system contains alternating stacking faults, e.g. twinning defects, which are lattice matched at the interfaces, but due to alternating Bloch wave function, the boundaries still perform lateral scattering which in the nanocrystalline regime (under 48nm for PbSe) results in strong quantum confinement.32 To demonstrate anisotropic charge transport, CSAFM was used to measure the vertical currents at nanoscale lateral resolution. The results clearly indicated vertical charge transport along column length and significantly lower conductance at grain boundaries, possibly explained by lateral charge scattering at twin boundary interfaces (Figure 8g). Evaluating average columnar width from the SEM plan-view imaging (Figure 8e) does not resolve the origin of quantum confinement effects in the films. However, structural investigations in the TEM and examination of film conduction channels using CSAFM reveals inner columnar sub-domains. These domains are widely observed in our CSAFM measurements, resulting from alternating twin and matrix boundaries performing vertical charge scattering. Comparing the topography and conductivity signals (Figures S4a and S4b) provided evidence that we are not dealing with topography related artifacts in the CSAFM. Further verification for this behavior can also be seen from HRTEM plan-view imaging (Figure 6d), showing alternating twin and matrix boundaries. These columnar sub-domains well-explain the 2D quantum confinement effects observed. The abundancy of reports including a recent and thorough review by A. Barranco et al. 35 emphasize the advantage of incorporating such films in electronic and photonic devices, as they describe current state of the art applications and the need for anisotropic electro-optical material properties.36–38 In this regard, the simple and inexpensive path described here for the 22


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growth of CTFs, with noticeable in-plane charge scattering combined with control over the material band gap, appears to be very promising. One of the most exciting applications of such films is for nanomaterials based upconversion devices. Coupling of the CTF described in this work with an efficient organic light emitting device would benefit from improved image integrity due to carrier scattering at the column boundaries, while maintaining high conductivity along the long axis of the nanocolumns. By taking advantage of quantum confinement in the xy plane, the mid-infrared activity of bulk PbSe is adjusted to the technologically-important short wave infrared range (SWIR), which has major advantages for night vision applications.39,40

Conclusions Bottom-up growth of tunable-sized columnar thin films was achieved using CD. Control over columnar film microstructures results from an OA growth mechanism, which we show here for the first time to be active in CD as an intermediate regime between pure cluster and pure IBI film growth. Based on the results, a structural model was established for depicting the conditions required for the varying growth mechanisms in CD PbSe in the form of a modified SZM.9. Based on the SZM we have demonstrated growth of nano-columnar PbSe films with varying column width, which in turn, resulted in 2D quantum confinement and tunable band gaps, as measured using PL. Nanoscale current sensing was performed to gain insight regarding future implementation of such films and confirmed longitudinal conduction while maintaining lateral insulation between adjacent columns, which are promising features for minimizing lateral charge carrier diffusion in shortwave-IR (SWIR) and MIR sensor devices. Accordingly, our current focus is on the integration of the films into direct SWIR-to-visible up-conversion devices.

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ASSOCIATED CONTENT Supporting Information This material is available free of charge on the ACS Publications website at DOI: SEM X-section, SEM plan view, SAED pattern indexing, CS-AFM (Figures S1−S4) AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Notes The authors declare no competing financial interest. Acknowledgements Expert assistance from Dr. D. Mogilyanski (XRD), Dr. S. Hazan (FTIR), Dr. Dror Horovitz and Dr. L. Li (FIB sample preparation) is gratefully acknowledged. Special thanks to Luna BenArush for graphic assistance in Fig. 7. This work has been partially supported by the Pazy Foundation and by the Israel Science Foundation under Grant no. 156/14.

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For Table of Contents Use Only

Oriented Attachment: A Path to Columnar Morphology in Chemical Bath Deposited PbSe Thin Films Tzvi Templeman, Sucheta Sengupta, Nitzan Maman, Eyal Bar-Or, Michael Shandalov, Vladimir Ezersky, Eyal Yahel, Gabby Sarusi, Iris Visoly-Fisher and Yuval Golan*

Synopsis Nano-columnar thin films active in the short-wave infrared range are of considerable interest for device applications. We present a structural zone model for PbSe thin films deposited from solution, and highlight the mechanism which governs the development of nano-columnar morphology. The nano-columnar films display anisotropic electrical properties along with noticeable quantum size effects.

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Oriented Attachment: a Path to Columnar Morphology in Chemical

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