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Surface Termination Control in Chemically Deposited PbS Films: Nucleation and Growth on GaAs(111)A and GaAs(111)B A. Osherov, M. Matmor, N. Froumin, N. Ashkenasy, and Y. Golan* Department of Materials Engineering and the Ilse Katz Institute for Nanoscale Science and Technology, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel
bS Supporting Information ABSTRACT: This study addresses the question of whether chemically deposited PbS thin films grown on GaAs(111) are affected by the oppositely terminated substrate surfaces, gallium terminated GaAs(111)A and arsenic terminated GaAs(111)B. The differences in PbS film deposition pathway in both cases of substrate surface termination were investigated using X-ray photoelectron spectroscopy (XPS), Raman scattering, and contact potential difference (CPD) measurements. The morphology, microstructure, and crystallographic orientation of the films were studied using scanning electron microscopy, X-ray diffraction, and transmission electron microscopy. XPS and CPD measurements indicated that PbS films deposited on oppositely terminated GaAs(111) surfaces possessed corresponding surface terminations, with PbS(111)B obtained on GaAs(111)B and PbS(111)A on GaAs(111)A. Subsequently, different surface oxides were detected by XPS on A and B terminated PbS(111), with lead oxide obtained on PbS(111)A and PbSO3 obtained on PbS(111)B. Moreover, CPD measurements revealed that PbS(111)A shows a 40 mV smaller work function than PbS(111)B surfaces, therefore emphasizing the importance of polarity and surface termination control for heterojunction based electronic devices.
1. INTRODUCTION Surface characteristics are one of the key parameters influencing the performance and degradation of electronic and optoelectronic devices. There are numerous examples showing that distinct physical and chemical behavior can be attributed to distinct termination of semiconductor surfaces.15 Furthermore, occurrence of epitaxial growth frequently depends on substrate polarity.6 For instance, the importance of the physicochemical interactions between the two different InP faces and the chemical species present in the deposition solution were highlighted by Lincot et al., who established that CdS growth is very sensitive to the polarity of the substrate, changing from polycrystalline films obtained on the In-terminated InP(111) face to epitaxial growth on the P-terminated InP(1 1 1) face.7 Likewise, polar characteristics of Ga terminated GaAs(111)A and As terminated GaAs(111)B surfaces were shown to be manifested in differences in surface etching,8,9 formation of Schottky barriers,10 physiochemical reactions,5 surface passivation,1,4,11,12 and epitaxial growth.6,13 Hence, determination of the surface and interfacial layer composition and structure are critical in understanding fundamental properties such as epitaxy, adhesion and electrical behavior of materials. PbS thin films are widely used semiconductors, with applications in infrared sensing and emission devices.1416 Chemical bath deposition (CBD) of PbS films, an advantageous, inexpensive alternative for vapor deposition techniques, was applied for r 2011 American Chemical Society
deposition of PbS films on various types of substrates.1721 Processes occurring at the semiconductor/deposition solution interface such as adsorption of ions, breaking of semiconductor surface bonds and formation of surface compound species are often very complex and may depend on surface termination. Indeed, it has been found that the substrate has pronounced influence on the resulting film morphology and microstructure, thus affecting the physical properties of the material.1821 For instance, PbS films deposited on GaAs(100) substrate showed a well-defined, yet nonconventional orientation relationship with (011)PbS||(100)GaAs and [011]PbS||[011]GaAs.1921 Similarly, PbSe deposited on GaAs exhibited (111)PbSe||(100)GaAs and [11 2]PbSe||[110]GaAs orientation relationships.21,22 Another complexity occurs due to the fact that the bare GaAs surface is usually covered with a few nanometers of native oxide. However, although GaAs surface oxides are usually stable, they can be chemically etched in alkaline or acidic solutions.23 Hence, substrate surface etching during the incubation period, where no significant growth rates can be detected, is unavoidable in the highly alkaline PbS deposition solution.17,24 The deposition solution may remove native oxide from the substrate surface; yet, due to relatively slow etching rate compared to deposition/species Received: May 5, 2011 Revised: July 17, 2011 Published: July 18, 2011 16501
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The Journal of Physical Chemistry C adsorption rate, the efficiency of oxide removal is unclear. As a result, the surface may be covered with a layer of elemental arsenic/gallium, as well as with different possible residual oxides.25 For instance, Shandalov et al. established the presence of an ultrathin and discontinuous layer of amorphous Ga2O3 at the interface formed during CBD of PbSe on GaAs(100).24 Evidentially these oxides may affect the electronic properties of the interface, e.g., to produce high density of surface states that pin the surface Fermi level within the band gap of the semiconductor.26 Furthermore, adsorption of atoms, molecules, and ions may induce charge transfer that may affect the final electronic structure of the modified surface. For example, it has been reported that adsorption of sulfur atoms from sulfide solutions resulted in considerable reduction of surface recombination velocity and improved the performance of electronic devices.2628 The dependences of these effects on the specific interface crystallographic orientation is, however, unclear. Lead oxide, lead carbonate, and lead hydroxide were found by X-ray photoelectron spectroscopy (XPS) to be present at the surface of galena, the natural mineral form of PbS.29,30 No oxidized sulfur species were reported to be found at the surface in most of these investigations. Yet, the cleavage plane of galena used in these studies was not indicated. Since the {111} plane in Fm3m type structures is a unique plane comprised of a single type of ions, apparent dissimilarities in the chemical and electronic behavior of the Æ111æ oriented surfaces are expected. The polarity of Æ111æ oriented crystals is often determined by convergent beam electron diffraction (CBED) studies or anisotropic etch procedures. However, given that Fm3m is a centrosymmetric crystal structure, polarity determination using CBED is unfeasible in this case due to lack of differences in intensity obtained along the Æ111æ and directions. A large variety of chemicals can be used for etching of lead chalcogenides,31 although recipes for anisotropic etch of PbS have not been reported to date. Furthermore, the sensitivity of the Kelvin probe technique to changes in surface electronic properties, which is done by measuring the contact potential differences (CPD),32 may provide information on the effect of surface termination on the resulting electronic properties. Indeed, previous studies have shown correlation between the work function and the consequent electron affinity of semiconductors and metals to various surface treatments33 and different polarities.34 Here we present a detailed photoelectron emission study that emphasizes the role of GaAs(111) termination in the formation of bonds with adsorbates (Pb or S ions) at the semiconductor/ solution interface and the influence of the high alkalinity of the solution on the GaAs surface during the incubation period for different surface/liquid interfaces. We show that the polarity of the GaAs(111) substrate strongly affects the adsorption processes occurring during various stages of the PbS deposition. In accordance with the polarity, differences in the nucleation and growth of PbS on GaAs(111)A and GaAs(111)B faces are demonstrated. In particular, correlation between the surface polarities of PbS and GaAs was observed, subsequently affecting surface electrical properties, which are essential in designing electronic and optoelectronic devices.
2. EXPERIMENTAL SECTION 2.1. Basic Materials and Chemicals. Thiourea (ACS analytical >99.0% Aldrich reagent), lead nitrate (Aldrich, analytical 99.99+%), and sodium hydroxide AR were used without further
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purification. Single crystal GaAs(111)A and (111)B substrates (AXT, one side polished, undoped, ( 0.1 miscut) were used. 2.2. Chemical Bath Deposition. PbS films were deposited from solution with final composition of 1 mM Pb(NO3)2, 5.7 mM CS(NH2)2 and 146 mM NaOH at final 12 < pH < 13. Fresh stock solutions of 17.5 mM Pb(NO3)2, 1 M CS(NH2)2 and 57 mM NaOH were prepared. The deposition solution was prepared by stirring 10 mL of 57 mM NaOH solution with 25 mL distilled water followed by slow addition of 2 mL 17.5 mM Pb(NO3)2. Finally, 2 mL of 1M CS(NH2)2 was added with additional stirring. Films growth was carried out at 20 C for various periods of time. 2.3. Anisotropic Etch Procedure. Identification of PbS(111) film surface termination was obtained by anisotropic etch in solution containing 0.5 M KOH and 0.2 M K3[Fe(CN)6], for various periods of time.31 2.4. Characterization Methods. 2.4.1. High Resolution Scanning Electron Microscopy (HR-SEM). The morphology of the films was observed using a JEOL 7400F field emission gun SEM without coating the samples. Secondary electrons were used to obtain the topography images. Acceleration voltages ranged from 2 to 5 kV. 2.4.2. 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 thin wedge was formed. The sample was then glued to a Cu slot grid (1 2 mm2) and final thinning was done by Ar ion milling using a Gatan model 691 precision ion polishing system. TEM imaging was carried out using a JEOL 2010 instrument operating at 200 keV. Films thickness was measured from TEM cross sections. Selected area electron diffraction patterns were obtained from both the PbS films and the GaAs substrate. 2.4.3. X-ray Photoelectron Spectroscopy (XPS). XPS spectra were measured using an ESCALAB 250 spectrometer with monochromatic Al X-ray source (excitation energy 1486.6 eV) at base pressure of 1 109 mbar. General elemental survey and high-resolution spectra of selected elements were recorded. Energy calibration was performed according to the position of the C 1s line (285 eV). Initially XPS spectra of the bare (untreated) GaAs(111)A and GaAs(111)B were recorded. Afterward the samples were treated with NaOH in order to examine the influence of NaOH on the bare GaAs substrates. Thereafter, the substrates were subjected to treatment in basic NaOH solution containing Pb2+ ions. Finally, thiourea was added as source of S2 ions and XPS spectra were recorded after 5 min and 1 h of PbS deposition. 2.4.4. Raman Scattering (RS). Measurements were performed on a Jobin-Yvon LabRam HR 800 micro-Raman system, equipped with a liquid-nitrogen-cooled detector and a 50 microscope objective lens. HeNe (633 nm) and Ar (514 nm) lasers were used for excitation. Most measurements were taken using a 600 lines/mm grating and a microscope confocal hole setting of 100 μm, corresponding to a resolution of about 4 cm1. The incident intensity on the sample was attenuated using neutral density filters. Preparation of samples was as described for XPS measurements. 2.4.5. Contact Potential Difference (CPD). CPD measurements were conducted using a Kelvin probe (Besocke Delta-PHi, Germany) in a dark Faraday cage under ambient conditions. The 16502
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Figure 1. (a) Schematic presentation of the research question: whether GaAs(111) termination has influence on the growth and termination of chemically deposited thin PbS films. (b) XRD pattern indicating Æ111æ orientation of the PbS films deposited on GaAs(111)A. (c) Cross-sectional TEM image of a PbS film deposited on GaAs(111)A. (d) Selected area electron diffraction pattern at the interface indicating [110] PbS zone axis parallel to [110] GaAs zone axis. Similar diffraction patterns were observed on GaAs(111)B. (e) Indexing for the diffraction pattern shown in (d).
electrical potential was measured relative to an Au reference electrode. The samples were grounded by a conductive tip that was pressed against the sample surface. The reported values are an average of measurements made on three samples for each wafer, and at least two measurements on each sample.
3. RESULTS AND DISCUSSION This study addresses the fundamental question of whether GaAs(111) substrate termination has influence on the growth and polarity of chemically deposited thin PbS films (Figure 1a). In order to answer this question, the following structural, chemical, and electrical characterization tools were used: X-ray diffraction and cross-sectional TEM were applied for characterization of crystallographic orientation relations and microstructure, XPS and Raman spectroscopy were used for defining the chemical nature of the PbS surface and substrate interface during the different stages of PbS growth. CPD was applied for electrical characterization. Furthermore, an anisotropic etch procedure was developed for rapid and unequivocal differentiation between the two opposite surface terminations of PbS (111). 3.1. Structural Characterization. The microstructure and crystallographic orientation of thin PbS films grown on A and B terminated GaAs(111) surfaces were characterized using XRD and cross-sectional TEM (Figure 1). The results shown in Figure 1be confirmed the single crystal nature of the films as previously reported for PbS/GaAs(100).6,19 The following orientation relations {111}PbS||{111}GaAs and Æ110æPbS||Æ110æGaAs were obtained (Figure 1). No substantial differences in the film microstructure, morphology, or apparent crystallographic orientation
were observed when the GaAs(111)A substrates were replaced with GaAs(111)B. 3.2. Physico-Chemical Characterization of PbS Film Surfaces. Despite the apparent similarity observed in TEM, films deposited on GaAs(111)A and GaAs(111)B possessed considerable differences in surface composition, as revealed by X-ray photoelectron spectroscopy (XPS). Dissimilar oxidation tendencies of 300 nm thick PbS(111) layers deposited on GaAs(111)A and GaAs(111)B are demonstrated in Figure 2, which shows Pb4f and S2p photoelectron emission spectra. Two distinct components can be resolved from the deconvolution of the spectral envelope of the Pb4f peak in Figure 2, panels a and b. The peak at 137.4 eV is associated with PbS photoelectron emission.35 The higher energy peak is clearly more pronounced for PbS/GaAs(111)B compared with PbS/ GaAs(111)A, indicating a different surface composition. This peak which appears at 137.9 eV in the case of PbS/GaAs(111)A and at 138.4 eV in the case of PbS/GaAs(111)B can be attributed to different forms of oxidized lead species such as PbO36 for the former and such as PbS2O337 or PbSO337 for the latter. Furthermore, this chemical state can also be assigned to Pb(OH)235,38 that was reported to be formed on the surface of metal oxides and metal sulfides in aqueous solution, rendering them hydrophilic.39,40 However, the existence of Pb(OH)2 in the UHV environment is questioned since it is thought to exist only in aqueous solutions.41,42 The corresponding S2p spectra shown in Figure 2, panels c and d further elucidate the origin of the differences observed in the photoelectron emission values of the oxidized lead species for GaAs(111)A and GaAs(111)B. The S2p peak at 160.2 eV is characteristic for the PbS bond.43 However, films deposited on GaAs(111)B exhibit an additional chemical 16503
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Figure 3. CPD of 300 nm thick PbS films grown on GaAs (111)A (left) and GaAs(111)B (right). These CPD measurements are directly correlated with the work function, Φ, of the PbS films. Bottom: Schematic illustrations of the band diagram shown below highlighting the different contributions of the effective affinity, χ*, to the Φ of the films for an n-type semiconductor. EG is the PbS(111) band gap, EF is its Fermi level, χ is the bulk electron affinity, and VL is the local vacuum level.
Figure 2. Pb4f photoelectron emission spectra for 300 nm thick PbS films on (a) GaAs(111)A and (b) GaAs(111)B, and S2p photoelectron emission spectra for 300 nm thick PbS films on (c) GaAs(111)A and (d) GaAs (111)B.
state for sulfur at 168 eV that is not present in the case of PbS/ GaAs(111)A. This state is likely to be associated with a more oxidized form of PbS such as PbSO3.44 Thus, the results suggest that PbS films deposited on oppositely terminated GaAs(111) faces exhibit different surface reactivity. In addition to chemical reactivity, differences in the surface dipolar behavior of the films grown on GaAs(111)A and GaAs(111)B were observed, manifested by changes in the work function, Φ, of each sample. This was monitored by measuring the CPD between the sample and a gold reference electrode using the Kelvin probe technique.32,45 The CPD reflects the work function of the surface Φ which is defined as the electron affinity χ plus the energy difference between the conduction band minimum and the Fermi level. The CPD data shown in Figure 3 show that the films grown on GaAs(111)B possess 40 mV higher CPD (and hence higher Φ). This difference can be attributed to differences in the effective electron affinity (χ*) at the surface due to the differences in the surface atom termination layer. Neglecting the band bending, which is expected to be small for this narrow bandgap semiconductor (EG = 0.41 eV),46 these differences in Φ are due to a decrease (increase) in χ* for A (B) terminated surface, respectively. Hence, negative dipole (defined when the negative pole is pointing toward the surface) can be assigned to Pb terminated surfaces (PbS(111)A), leading to a decrease in the semiconductor effective affinity (χ*) and thus, effectively, a smaller Φ. For PbS(111)B terminated films, the surface is terminated by sulfur atoms leading to a positive dipole and thus a larger χ* is inferred. In order to further highlight the differences between the two opposite faces of PbS(111), an anisotropic etching procedure
Figure 4. HR-SEM micrographs of as deposited and etched PbS(111) films. (a) PbS(111)B as deposited, (b) PbS(111)B after 1 min etch, (c) PbS(111)B after 3 min etch, (d) PbS(111)A as deposited, (e) PbS(111)A after 1 min etch, and (f) PbS(111)A after 3 min etch. Scale bars equal 300 nm.
was developed and applied. PbS etch was conducted in KOH solution containing potassium ferricyanide, that serves as an oxidizer. Anisotropic etch for PbS(111) face identification was obtained using a solution containing 0.5 M KOH and 0.2 M K3[Fe(CN)6]. The surface topography of as-deposited PbS(111)B and PbS(111)A films are shown in the HR-SEM micrographs in Figure 4, panels a and d, respectively. Etch pits were identified on sulfide terminated PbS(111)B face after 1 min etch (Figure 4b), whereas the Pb terminated PbS(111)A face showed fewer and smaller etch pits (Figure 4e). Further increase in the etch pit density (EPD) for PbS(111)B was observed after a 3 min etch for both samples, however they appeared to be much shallower for PbS(111)A as can be seen in Figure 4, panels c and f. The etch pits densities as estimated using ImageJ software are ca. 8% for the image shown in Figure 4b and ca. 4% for image shown in Figure 4e, defined as EPD area divided by total area. 16504
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Figure 5. As3p/Pb4f and Ga3d/Pb5d photoelectron emission spectra and Voigt function fits during various stages of PbS deposition on GaAs(111)A (columns (I) and (III)) and GaAs(111)B (columns (II) and (IV)). (a) Bare, as received substrate, (b) NaOH treated substrate, (c) NaOH treated substrate with presence of Pb2+ ions, (d) PbS films deposited for 5 min, and (e) PbS films deposited for 1 h and sputtered for oxide removal. Each Voigt function fit includes both the As 3p3/2 and As 3p5/2, Pb 4f5/2 and Pb 4f7/2, and Pb 5d3/2 and Pb 5d5/2 components of each signal.
However, this is only an estimation due to differences in etch pit dimensions and geometries, subgrain boundaries, etc. The following dissolution reaction has been proposed for etching of PbS with potassium ferricyanide:31 PbS þ 8½FeIII ðCNÞ63 þ 11OH a HPbO2 þ 8½FeII ðCNÞ6 4 þ SO4 2 þ 5H2 O Since FeIII(CN)6 serves as a Pb oxidizer, one may expect more homogeneous etching of lead terminated PbS(111). Indeed, more homogeneous etch is indicated by smaller EPD (Figure 4e). However, since a mixture of lead and sulfur oxides is likely to be present on the films surface (Figure 2), etching proceeds via oxide dissolution and therefore suggests a more complex reaction. 3.3. Monitoring of Nucleation and Growth. Differences between PbS growth on GaAs(111)A and GaAs(111)B were resolved by monitoring the evolution of chemical (XPS, Raman scattering) and electrical (CPD) characteristics during PbS nucleation and growth. XPS spectra were recorded during different stages of PbS deposition process. Figure 5 illustrates the evolution of chemical species in the GaAs/PbS system as a result of the following sequence of surface treatments: treatment in aqueous NaOH solution, treatment in aqueous NaOH solution in the presence of Pb2+ ions, and sequential PbS deposition for 5 min and 1 h at 20 C. Each treatment was carried out under the same experimental conditions on GaAs(111)A and GaAs(111)B surfaces for comparison. Due to overlapping binding energies for As3p and Pb4f (130150 eV) and for Ga3d and Pb5d (1634 eV), both spectral regions of Pb were analyzed. Differences in the nucleation and growth of PbS on GaAs(111) in accordance with the polarity were clearly elucidated.
The As3p core level of bare (unprocessed) GaAs(111) wafers shown in Figure 5, panels I(a) and II(a), clearly indicates two oxidation states, one at 139.7 eV which corresponds to As in the GaAs compound and the other at 141.2 eV which can be assigned to coexistence of As2O3 and As2O5.47 The Ga3d core level shown in Figure 5, panels III(a) and IV(a), of the unprocessed surface also shows two oxidation states. The broad oxide peak in the Ga3d region (20.2 eV) can be assigned to a mixture of Ga2O3 and GaO2,12 whereas the component at lower binding energy (19.1 eV) corresponds to the Ga bond in GaAs. It can be seen that both As and Ga are somewhat more oxidized on bare GaAs(111)B wafers. In order to achieve spontaneous PbS precipitation the reactions are carried out in highly alkaline solutions. Particularly, NaOH has a dual and complex role in the deposition process; on the one hand NaOH serves as a complexing agent, thus inhibiting deposition rate, while on the other hand high alkalinity of the deposition solution is essential to promote thiourea decomposition, thus accelerating deposition rate.19 Oxide removal as a result of the NaOH treatment is clearly indicated by disappearance of the oxide component from the spectra as shown in the Ga3d binding energies region in Figure 5 III(b) for the A face and by decrease in the extent of oxidized form on the B face as indicated in Figure 5 IV(b). This is further supported for the As3p binding energies region in Figure 5, panels I(b), II(b), and IV(b), respectively. The results indicate that the highly alkaline NaOH solution totally removes GaO2/Ga2O3 and As2O3/As2O5 components from the GaAs(111)A surface. Furthermore, the spectral envelope of As3p (Figure 5 I(b)) for the PbS/GaAs(111)A sample is most likely to be deconvoluted into two components, one assigned to AsGa bond and the other, shifted by 2.5 eV toward lower energies, which was assigned to elemental arsenic As0 component. Less effective oxide removal is observed 16505
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The Journal of Physical Chemistry C on GaAs(111)B (Figure 5, panels II(b) and IV(b)); in addition to the AsGa component and elemental arsenic As0 component with a chemical shift of 2.35 eV toward lower energies, an AsO components was detected. Notably, the reduction to As0 was observed to occur to higher extent on GaAs(111)B. It is reasonable to assume that the majority of the elemental arsenic found on the etched surface originated from the native oxide layer and not from bulk GaAs.23 We note that, taking into account the large concentration of OH in solution, this component can also be assigned to arsenic hydroxides like As(OH)3.25 Furthermore, the broadness of the peak assigned to residual gallium oxide on the GaAs(111)B face in Figure 5 IV(b) (green curve) suggests that it originates from a mixture of gallium oxides with different oxidation states of Ga. Hence, our XPS results demonstrate that during the incubation period, NaOH has an additional important role as an etchant of substrate surface oxides. Indeed, examination of Pourbaix diagrams for elemental Ga48 and As49 shows that gallium oxide Ga2O3 and arsenious anhydride As2O3 are unstable at pH 12 and only soluble species of gallium oxides such as HGaO32 and arsenic oxides such as HAsO42 and AsO2 are expected to exist under these conditions. No chemical shifts which can be assigned to As or Ga oxides were found in the spectra after treatment in alkaline solution containing Pb2+ (Figure 5, panels IcIVc), suggesting that the presence of Pb2+ ions in alkaline solution promotes etching of gallium and arsenic oxides. However, it should be noted that spectral overlap between Pb4f and As3p peaks and between Ga3d and Pb5d peaks hinder unambiguous conclusions. Best fitting of the As3p spectra for the PbS/GaAs(111)A and PbS/ GaAs(111)B systems in this case (Figure 5, panels I(c) and II(c)) were achieved with three components. In addition to the AsGa component at 139.4 eV, the components at 138.6 and 138.1 eV respectively may be attributed to PbAs bond and some form of lead oxide compound such as PbO36,38 or Pb(OH)2.35,38 The higher intensity of the PbO peak in the case of growth on GaAs(111)B face indicates preferential adsorption of Pb2+ (Figure 5 IIc). Further support to the preferential Pb2+ adsorption on the GaAs(111)B face is given by the changes in the Ga3d core level spectra after exposure to Pb2+ containing solution (Figure 5, panels III(c) and IV(c)). The spectra exhibit a welldefined component in addition to the GaAs component, with a higher intensity for the PbO peak in case of GaAs(111)B. Our data indicates a small contribution of AsPb bond to the Pb4f and As3p spectral region (Figure 5 II(c)). However, since it is reasonable to assume that AsPb component is overlapping with the AsGa from the GaAs compound, this peak positioning may be somewhat ambiguous. As expected, further lead adsorption and evolution of a PbS chemical bond are indicated in Figure 5, panels (d-e) with increasing films thickness. The lower intensity of the AsGa peak in the case of deposition on GaAs(111)B (Figure 5 II(d)) suggests higher Pb coverage of the surface. The Pb4f peak at 137 eV (Figure 5, panels I(d) and II(d)) and Pb5d peak at 18.2 eV (Figure 5, panels III(d) and IV(d)) can be attributed to compound PbS. The XPS spectra in Figure 5, panels IIV(e) were obtained for ∼300 nm thick PbS films deposited for 1 h and sputtered for oxide removal. Comparison of the Pb4f binding energy region before (Figure 2, panels a and b) and after sputtering (Figure 5, panels I(e) and II(e)), indicates that the oxidized species observed in Figure 2, panels a and b, originates from the sample surface. Thus, an additional chemical state is observed for sulfur at 168 eV in the case of PbS/GaAs(111)B (Figure 2d), which is not present in case of PbS/GaAs(111)A
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Figure 6. Raman spectra of (I) GaAs(111)A and (II) GaAs(111)B after various surface treatments. Spectra were recorded using 633 nm laser excitation on (a) bare, as received substrate, (b) NaOH treated substrate, and (c) NaOH treated substrate with presence of Pb2+ ions.
(Figure 2c). This arises from the oxidized sulfur species on the sulfur terminated surface, further indicating that PbS(111) films grown on GaAs(111)A possess lead oxide termination, whereas PbS(111) films grown on GaAs(111)B possess sulfur containing oxide termination. Raman scattering highlights further differences between the two types of GaAs substrates upon NaOH treatment (Figure 6). Two peaks are clearly seen in the spectra (Figure 6, panels I(a) and II(a)). The peak marked L (286.6 cm1) arises from scattering from the coupled phonon-plasmon modes in the bulk where free carriers exist. The LO peak (292.5 cm1) is attributed to the surface depletion layer.50 It can be seen from the large L/LO intensity ratio that the contribution of the bulk crystal is dominant at these excitation conditions.26,50 Figure 6 I(b) indicates that alkaline surface treatment of GaAs(111)A resulted in change in the balance between L and LO peaks, while the GaAs(111)B surface was not affected by the alkaline treatment in the same manner (Figure 6 II(b)). The Raman spectra in Figure 6, panels I(c) and II(c), obtained from samples after alkaline surface treatment in the presence of Pb2+ ions further support these observations; however influence of lead addition to the alkaline solution is not observed in the spectra, suggesting physical adsorption of Pb2+. Furthermore, preferential adsorption of Pb(OH)2 on GaAs(111)B was indicated by appearance of a characteristic PbOH vibration mode at 1077.7 cm1 (see Figure S1 in the Supporting Information). The change of the balance between L and LO peaks in the case of GaAs(111)A can be correlated with reduction in surface band bending.26 Furthermore, slight shifts in the wavenumber values of the L peak of the GaAa(111)B to higher values imply slight changes in the bonding type, supporting the XPS results. Differences in the surface work function of films deposited on GaAs (111)A and GaAs (111)B upon sequential addition of the deposition reagents, according to the PbS deposition process, were monitored through changes in the CPD. As a result of NaOH treatment, an increase in the CPD value was observed for both GaAs(111)A and GaAs(111)B faces. This increase may be attributed to oxide removal from the GaAs surface and to formation of As(OH)3, as indicated by XPS results by assigning the peak at 143.2 eV in Figure 5, panels I(b) and II(b), to As(OH)3, as discussed above. The smaller increase (by ∼50 mV) in the CPD in the case of GaAs(111)A face (Figure 7 I (b)) can be attributed to the reduction in the surface band bending as 16506
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’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
Figure 7. CPD during different stages of PbS film deposition on (I) GaAs(111)A and (II) GaAs(111)B (a) bare (as received) substrate, (b) NaOH treated substrate, (c) NaOH treated substrate with presence of Pb2+ ions, and (d) PbS films deposited for 1 h (same data as in Figure 3).
suggested by Raman scattering (Figure 6). We note that the larger error bar in case of GaAs(111)B (Figure 7 II(b)) may indicate higher surface reactivity of As terminated surface relatively to Ga terminated surface after oxide removal. Surfaces treated with alkaline solution containing Pb did not show significant further change in work function within the resolution of the measurement (Figure 7, panels I(c) and II(c)), therefore hinting on the physical nature of Pb2+ adsorption and confirming the XPS results (Figure 5, panels I(c) and II(c)). Further increase in the CPD after the PbS deposition reflects differences in the work function of GaAs and PbS (Figure 7, panels I(d) and II(d)). As discussed above, the larger CPD resulted from the growth of PbS(111)B on GaAs(111)B and PbS(111)A on GaAs(111)A.
’ SUMMARY In this study we demonstrate that different termination of GaAs(111) surfaces induces different termination in chemically deposited PbS films, with PbS(111)B obtained on GaAs(111)B and PbS(111)A on GaAs(111)A. The different surface terminations affect both the GaAs/PbS interface and the PbS surface compositions. Hence, it was shown that the high solution alkalinity promotes etch of the GaAs surface oxides during the incubation period, with oxide removal being more pronounced on GaAs(111)A than on GaAs(111)B surfaces. Furthermore, preferential lead absorption is obtained on the more reactive GaAs(111)B face. These differences result in differences in pathways of PbS nucleation and growth on oppositely terminated GaAs(111). However no substantial differences in the films microstructure, morphology, and apparent crystallographic orientation exist. Nevertheless, the PbS(111)B surface is more oxidized with PbSO3. Lead oxide is obtained on the less oxidized PbS(111)A. The different terminations induces different surface physical properties, such as work function, with the work function of PbS(111)A being 40 mV smaller than the work function of PbS(111)B. Consequently, the surface dipole polarity reversal can change the behavior of electronic devices fabricated on the differently terminated PbS surfaces. ’ ASSOCIATED CONTENT
bS
Supporting Information. Raman spectra of GaAs(111)B surface after NaOH containing Pb2+ treatment showing characteristic PbOH vibration mode at 1077.7 cm1. This material is available free of charge via the Internet at http://pubs.acs.org.
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