Electrochemical aspects and structure characterization of VA-VIA compound semiconductor Bi2Te3/Sb2Te3 superlattice thin films via electrochemical atomic layer epitaxy

This paper concerns the electrochemical atom-by-atom growth of VA-VIA compound semiconductor thin film superlattice structures using electrochemical atomic layer epitaxy. The combination of the Bi2Te3 and Sb2Te3 programs and Bi2Te3/Sb2Te3 thin film superlattice with 18 periods, where each period involved 21 cycles of Bi2Te3 followed by 21 cycles of Sb2Te3, is reported here. According to the angular distance between the satellite and the Bragg peak, a period of 23 nm for the superlattice was indicated from the X-ray diffraction (XRD) spectrum. An overall composition of Bi 0.25Sb0.16Te0.58, suggesting the 2:3 stoichiometric ratio of total content of Bi and Sb to Te, as expected for the format of the Bi2Te3/Sb2Te3 compound, was further verified by energy dispersive X-ray quantitative analysis. Both field-emission scanning electron microscopy and XRD data indicated the deposit grows by a complex mechanism involving some 3D nucleation and growth in parallel with underpotential deposition. The optical band gap of the deposited superlattice film was determined as 0.15 eV by Fourier transform infrared spectroscopy and depicts an allowed direct type of transition. Raman spectrum observation with annealed and unannealed superlattice sample showed that the LIF mode has presented, suggesting a perfect AB/CB bonding in the superlattice interface.     

Facile assembly of size- and shape-tunable IV-VI nanocrystals into superlattices

In comparison to the previous lengthy approaches, we described a general and simple strategy for engineering the superlattice assembly of IV-VI semiconductor nanocrystals (NCs) with tunable sizes and morphologies. Not only the well-studied spherical NCs but also some special-shaped NCs, such as the quasi-cubic, cubic, truncated octahedral, and octahedral, could self-assemble into well-ordered patterns, as demonstrated in PbS, PbSe, and PbTe. These results extended our proposed model about the configuration of ligand chains in the superlattice assembly. This powerful capability of assembling superlattices was dominated by a heat-treatment process, providing a significant and extensive direction in the engineering of morphology-tunable NC superlattices.     

Growth of Ge nanofilms using electrochemical atomic layer deposition, with a "bait and switch" surface-limited reaction

Ge nanofilms were deposited from aqueous solutions using the electrochemical analog of atomic layer deposition (ALD). Direct electrodeposition of Ge from an aqueous solution is self-limited to a few monolayers, depending on the pH. This report describes an E-ALD process for the growth of Ge films from aqueous solutions. The E-ALD cycle involved inducing a Ge atomic layer to deposit on a Te atomic layer formed on Ge, via underpotential deposition (UPD). The Te atomic layer was then reductively stripped from the deposit, leaving the Ge and completing the cycle. The Te atomic layer was bait for Ge deposition, after which the Te was switched out, reduced to a soluble telluride, leaving the Ge (one "bait and switch" cycle). Deposit thickness was a linear function of the number of cycles. Raman spectra indicated formation of an amorphous Ge film, consistent with the absence of a XRD pattern. Films were more stable and homogeneous when formed on Cu substrates, than on Au, due to a larger hydrogen overpotential, and the corresponding lower tendency to form bubbles.     

Pb deposition on I-coated Au(111). UHV-EC and EC-STM studies

This article concerns the growth of an atomic layer of Pb on the Au(111)( radical3 x radical3)R30 degrees -I structure. The importance of this study lies in the use of Pb underpotential deposition (UPD) as a sacrificial layer in surface-limited redox replacement (SLRR). SLRR reactions are being applied in the formation of metal nanofilms via electrochemical atomic layer deposition (ALD). Pb UPD is a surface-limited reaction, and if it is placed in a solution of ions of a more noble metal, redox replacement can occur, but limited by the amount of Pb present. Pb UPD is a candidate for use as a sacrificial layer for replacement by any more noble element. It has been used by this group for both Cu and Pt nanofilm formation using electrochemical ALD. The I atom layer was intended to facilitate electrochemical annealing during nanofilm growth. Two distinctly different Pb atomic layer structures are reported, studied using in situ scanning tunneling microscopy (STM) with an electrochemical flow cell and ultrahigh vacuum surface analysis combined directly with electrochemical reactions (UHV-EC). Starting with the initial Au(111)( radical3 x radical3)R30 degrees -I, 1/3 monolayer of I on the Au(111) surface, Pb deposition began at approximately 0.1 V. The first Pb UPD structure was observed just below -0.2 V and displayed a (2 x radical3)-rect unit cell, for a structure composed of 1/4 monolayer each of Pb and I. The I atoms fit in Pb 4-fold sites, on the Au(111) surface. The structure was present in domains rotated by 120 degrees. Deposition to -0.4 V resulted in complete loss of the I atoms and formation of a Pb monolayer on the Au(111), which produced a Moiré pattern, due to the Pb and Au lattice mismatch. These structures represent two well-defined starting points for the growth of nanofilms of other more noble elements. It is apparent from these studies that the adsorption of I- on Pb is weak, and it will rinse away. If Pb is used as a sacrificial metal in an electrochemical ALD cycle and adsorbed I atoms are employed for electrochemical annealing, I atoms will need to be applied each cycle.     

Electrodeposition of CuInSe2 (CIS) via electrochemical atomic layer deposition (E-ALD)

The growth of stoichiometric CuInSe(2) (CIS) on Au substrates using electrochemical atomic layer deposition (E-ALD) is reported here. Parameters for a ternary E-ALD cycle were investigated and included potentials, step sequence, solution compositions and timing. CIS was also grown by combining cycles for two binary compounds, InSe and Cu(2)Se, using a superlattice sequence. The formation, composition, and crystal structure of each are discussed. Stoichiometric CIS samples were formed using the superlattice sequence by performing 25 periods, each consisting of 3 cycles of InSe and 1 cycle of Cu(2)Se. The deposits were grown using 0.14, -0.7, and -0.65 V for Cu, In, and Se precursor solutions, respectively. XRD patterns displayed peaks consistent with the chalcopyrite phase of CIS, for the as-deposited samples, with the (112) reflection as the most prominent. AFM images of deposits suggested conformal deposition, when compared with corresponding image of the Au on glass substrate.     

Rapid vapor-phase fabrication of organic-inorganic hybrid superlattices with monolayer precision

We report a new layer-by-layer growth method of self-assembled organic multilayer thin films based on gas-phase reactions. In the present molecular layer deposition (MLD) process, alkylsiloxane self-assembled multilayers (SAMs) were grown under vacuum by repeated sequential adsorptions of C=C-terminated alkylsilane and titanium hydroxide. The MLD method is a self- limiting layer-by-layer growth process, and is perfectly compatible with the atomic layer deposition (ALD) method. The SAMs films prepared exhibited good thermal and mechanical stability, and various unique electrical properties. The MLD method, combined with ALD, was applied to the preparation of organic-inorganic hybrid nanolaminate films in the ALD chamber. The organic-inorganic hybrid superlattices were then used as active mediums for two-terminal electrical bistable devices. The advantages of the MLD method with ALD include accurate control of film thickness, large-scale uniformity, highly conformal layering, sharp interfaces, and a vast library of possible materials. The MLD method with ALD is an ideal fabrication technique for various organic-inorganic hybrid superlattices.     

The Effect of Surface Terminations on the Initial Stages of TiO2 Deposition on Functionalized Silicon

As atomic layer deposition (ALD) emerges as a method to fabricate architectures with atomic precision, emphasis is placed on understanding surface reactions and nucleation mechanisms. ALD of titanium dioxide with TiCl₄ and water has been used to investigate deposition processes in general, but the effect of surface termination on the initial TiO₂ nucleation lacks needed mechanistic insights. This work examines the adsorption of TiCl₄ on Cl−, H−, and HO− terminated Si(100) and Si(111) surfaces to elucidate the general role of different surface structures and defect types in manipulating surface reactivity of growth and non-growth substrates. The surface sites and their role in the initial stages of deposition are examined by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). Density functional theory (DFT) computations of the local functionalized silicon surfaces suggest oxygen-containing defects are primary drivers of selectivity loss on these surfaces.     

Layer-by-layer analysis of A4B6 thin-film heterostructures using inductively coupled plasma atomic fluorescence spectrometry (ICP-AFS)

The technique of layer-by-layer analysis of semiconductive A₄B₆ heterostructures using electrochemical etching and inductively coupled plasma atomic fluorescence spectrometry (ICP AFS) is developed. The Norr etching solution and electrochemical oxidation with subsequent dissolution of oxidized layers in acid were used to remove layers of 0.5 to several microns thickness. Pb_1?xEu_xTe/PbTe and Pb_1?xEu_xSe/PbSe heterostructures were analyzed. The Eu content in removed layers was determined by ICP-AFS. The data on Eu depth profiles were used for the evaluation of the Eu diffusion coefficient in this heterostructures to predict the changes of properties along the depth of the structure. The technique can be applied to the analysis of semiconductive heterostructures with different admixtures.     

Structural characterization of a spin-assisted colloid-polyelectrolyte assembly: stratified multilayer thin films

The assembly of polyelectrolytes and gold nanoparticles yields stratified multilayers with very low roughness and high structural perfection. The films are prepared by spin-assisted layer-by-layer self-assembly (LbL) and are characterized by X-ray reflectivity (XRR), UV-vis spectroscopy, atomic force microscopy (AFM), and transmission electron microscopy (TEM). Typical structures have four repeat units, each of which consists of eight double layers (DL) of poly(sodium 4-styrenesulfonate)/poly(allylamine hydrochloride), one monolayer of gold nanoparticles (10 nm diameter), and another layer of poly(allylamine hydrochloride). XRR scans show small-angle Bragg peaks up to seventh order, evidencing the highly stratified structure. Pronounced Kiessig fringes indicate a low global roughness, which is confirmed by local AFM measurements. TEM images corroborate the layered structure in the growth direction and nicely show the distinct separation of the individual particle layers. An AFM study reveals the lateral gold particle distribution within one individual particle layer. Interestingly, the spin-assisted deposition of polyelectrolytes reduces the roughness induced by the particle layers, leading to self-healing of roughness defects and a rather perfect stratification.     

Layer-by-layer analysis of A4B6 thin-film heterostructures using inductively coupled plasma atomic fluorescence spectrometry (ICP-AFS)

The technique of layer-by-layer analysis of semiconductive A₄B₆ heterostructures using electrochemical etching and inductively coupled plasma atomic fluorescence spectrometry (ICP AFS) is developed. The Norr etching solution and electrochemical oxidation with subsequent dissolution of oxidized layers in acid were used to remove layers of 0.5 to several microns thickness. Pb_1–xEu_xTe/PbTe and Pb_1–xEu_xSe/PbSe heterostructures were analyzed. The Eu content in removed layers was determined by ICP-AFS. The data on Eu depth profiles were used for the evaluation of the Eu diffusion coefficient in this heterostructures to predict the changes of properties along the depth of the structure. The technique can be applied to the analysis of semiconductive heterostructures with different admixtures.     

窄带隙IV-VI族半导体PbTe(111)的表面氧化及氧的热脱附机理

利用X射线光电子能谱(XPS)、扫描隧道显微镜(STM)以及低能电子衍射(LEED),对PbTe(111)薄膜的表面氧化及氧的热脱附机理进行了研究.结果表明:PbTe(111)薄膜经500VAr+轰击加上250℃高温退火循环处理,可得到呈(1×1)周期性排列的清洁表面.将此清洁表面暴露于大气两天后,表面被氧化形成了PbO2、PbO和TeO2,氧化层的厚度大于2个单原子层(ML),与清洁PbTe(111)表面相比,被氧化的PbTe(111)表面的Te3d5/2与Pb4f7/2芯态谱峰的面积比明显减小,表明被氧化的PbTe(111)表面是富Pb的.在热脱附处理过程中,PbO2和TeO2的芯态峰消失,且O1s芯态峰的强度迅速减弱,表明加热处理不仅使PbO2和TeO2发生了分解,同时也使氧发生了脱附,但PbO即使在350℃退火仍吸附于PbTe(111)表面.     

CdTe electrodeposition on InP(100) via electrochemical atomic layer epitaxy (EC-ALE): studies using UHV-EC

The II-VI compound semiconductor CdTe was electrodeposited on InP(100) surfaces using electrochemical atomic layer epitaxy (EC-ALE). CdTe was deposited on a Te-modified InP(100) surface using this atomic layer by atomic layer methodology. The deposit started with formation of an atomic layer of Te on the InP(100) surface, as Cd was observed not to form an underpotential deposition (UPD) layer on InP(100), although it was found to UPD on Te atomic layers. On the In-terminated 'clean' InP(100) surface, Te was deposited at -0.80 V from a 0.1 mM solution of TeO2, resulting in formation of a Te atomic layer and some small amount of bulk Te. The excess bulk Te was then removed by reduction in blank solution at -0.90 V, leaving a Te atomic layer. Given the presences of the Te atomic layer, it was then possible to form an atomic layer of Cd by UPD at -0.58 V to complete the formation of a CdTe monolayer by EC-ALE. That cycle was then repeated to demonstrate the applicability of the cycle to the formation of CdTe nanofilms. Auger spectra recorded after the first three cycles of CdTe deposition on InP(100) were consistent with the layer-by-layer CdTe growth. It is interesting to note that Cd did not form a UPD deposit on the In-terminated InP(100) surface and only formed Cd clusters at an overpotential. This issue is probably related to the inability of the Cd and In to form a stable surface compound.     

Formation and characterization of Sb2Te3 nanofilms on Pt by electrochemical atomic layer epitaxy

A nanocrystalline Sb2Te3 VA-VIA group compound thin film was grown via the route of electrochemical atomic layer epitaxy (ECALE) in this work for the first time. The electrochemical behavior of Te and Sb on Pt, Te on Sb-covered Pt, and Sb on Te-covered Pt was studied by methods of cyclic voltammetry, anode potentiodynamic scanning, and coulometry. A steady deposition of the Sb2Te3 compound could be attained after negatively stepped adjusting of the UPD potentials of Sb and Te on Pt in each of the first 40 depositing cycles. The structure of the deposit was proven to be the Sb2Te3 compound by X-ray diffraction. The 2:3 stoichiometric ratio of Sb to Te was verified by EDX quantitative analysis, which is consistent with the result of coulometric analysis. A nanocystalline microstructure was observed for the Sb2Te3 deposits, and the average grain size is about 20 nm. Cross-sectional SEM observation shows an interface layer about 19 nm in thickness sandwiched between the Sb2Te3 nanocrystalline deposit and the Pt substrate surface. The optical band gap of the deposited Sb2Te3 film was determined as 0.42 eV by FTIR spectroscopy and it is blueshifted in comparison with that of the bulk Sb2Te3 single crystal because of its nanocrystalline microstructure.     

Electrochemical formation of a III–V compound semiconductor superlattice: InAs/InSb

We report on the use of electrochemical atomic-layer epitaxy (EC-ALE) to grow thin-films of the III–V compounds InAs, InSb, and an InAs_xSb_1−x superlattice. EC-ALE is a method for forming compound semiconductors with improved control, compared to other electrodeposition methodologies. It involves the use of surface limited reactions to form deposits an atomic layer at a time, in a cycle. The EC-ALE cycle uses underpotential deposition (upd) to form atomic layers of each of the component elements. One cycle ideally produces one monolayer (ML) of the desired compound. Studies to optimize the InAs cycle are reported, specifically the dependence on the In and As deposition potentials. These studies show that the potentials must be adjusted for each of the first 25 or more cycles, as a contact potential between the Au substrate and the growing semiconductor develops. After deposition of this initial ‘buffer layer’, steady state conditions are reached, and the same potentials can be used without change, for the remaining cycles. The formation of InSb has also been investigated, and the EC-ALE growth of InSb deposits is reported for the first time. Due to a 6% lattice mismatch, and a less than fully optimized cycle, the InSb deposits on Au appear composed of 70 nm particles. By combining the InAs and InSb programs, a superlattice was formed with 41 periods, where each period involved ten cycles of InAs followed by ten cycles of InSb. X-ray diffraction (XRD) indicated a period of 5.5 nm, whereas a 7.4 nm period was expected, based on 1 ML/cycle and the (111) interplanar spacing, derived from the lattice constants for InAs and InSb. Given the stoichiometry of the resulting deposit, and the shorter periodicity observed, it appears that 1 ML/cycle of InAs was formed, while only a 1/2 ML/cycle of InSb was obtained. IR absorption measurements indicate that the deposit was red shifted relative to the lower bandgap compound, InSb (0.17 eV), which is consistent with a type II superlattice. If an alloy had been formed, the bandgap should have been a linear function of the bandgaps and relative mole fractions of InAs and InSb, or about 0.31 eV, twice the observed bandgap.     

PbTe colloidal nanocrystals: synthesis, characterization, and multiple exciton generation

We report an alternative synthesis and the first optical characterization of colloidal PbTe nanocrystals (NCs). We have synthesized spherical PbTe NCs having a size distribution as low as 7%, ranging in diameter from 2.6 to 8.3 nm, with first exciton transitions tuned from 1009 to 2054 nm. The syntheses of colloidal cubic-like PbSe and PbTe NCs using a PbO "one-pot" approach are also reported. The photoluminescence quantum yield of PbTe spherical NCs was measured to be as high as 52 +/- 2%. We also report the first known observation of efficient multiple exciton generation (MEG) from single photons absorbed in PbTe NCs. Finally, we report calculated longitudinal and transverse Bohr radii for PbS, PbSe, and PbTe NCs to account for electronic band anisotropy. This is followed by a comparison of the differences in the electronic band structure and optical properties of these lead salts.     

The nature of alkanethiol self-assembled monolayer adsorption on sputtered gold substrates

A detailed study of the self-assembly and coverage by 1-nonanethiol of sputtered Au surfaces using molecular resolution atomic force microscopy (AFM) and scanning tunneling microscopy (STM) is presented. The monolayer self-assembles on a smooth Au surface composed predominantly of [111] oriented grains. The domains of the alkanethiol monolayer are observed with sizes typically of 5-25 nm, and multiple molecular domains can exist within one Au grain. STM imaging shows that the (4 x 2) superlattice structure is observed as a (3 x 2) structure when imaged under noncontact AFM conditions. The 1-nonanethiol molecules reside in the threefold hollow sites of the Au[111] lattice and aligned along its [112] lattice vectors. The self-assembled monolayer (SAM) contains many nonuniformities such as pinholes, domain boundaries, and monatomic depressions which are present in the Au surface prior to SAM adsorption. The detailed observations demonstrate limitations to the application of 1-nonanethiol as a resist in atomic nanolithography experiments to feature sizes of approximately 20 nm.     

Pure rotational spectra of PbSe and PbTe: potential function, Born-Oppenheimer breakdown, field shift effect and magnetic shielding

The pure rotational spectra of 41 isotopic species of PbSe and PbTe have been measured in their X 1Sigma+ electronic state with a resonator pulsed-jet Fourier transform microwave spectrometer. The molecules were prepared by laser ablation of suitable target rods and stabilised in supersonic jets of noble gas. Global multi-isotopologue analyses yielded spectroscopic Dunham parameters Y01, Y11, Y21, Y31, Y02, and Y12 for both species, as well as effective Born-Oppenheimer breakdown (BOB) coefficients delta01 for Pb, Se and Te. Unusual large values of the BOB parameters for Pb have been rationalized in terms of finite nuclear size (field shift) effect. A direct fit of the same data sets to an appropriate radial Hamiltonian yielded analytic potential energy functions and BOB radial functions for the X 1Sigma+ electronic state of both PbSe and PbTe. Additionally, the magnetic hyperfine interactions produced by the uneven mass number A nuclei 207Pb, 77Se, 123Te, and 125Te were observed, yielding first determinations of the corresponding nuclear spin-rotation coupling constants.     

LB技术组装聚合物／有机染料多层膜的光致发光和电致发光

半导体超晶格材料由于其特殊的光电性质引起人们的关注,So和Tokito等人用分子束沉积的方式制备了有机及有机无机复合的超晶格材料器件,但分子沉积中有机小分子染料的结晶影响薄膜的质量及器件的寿命。最近,我们用两亲性聚合物分子成功地将有机染料分子组装在聚合物LB膜中,有效地限制有机小分子染料结晶,实验证明此多层LB膜具有超晶格结构和较好的稳定性及发光特性。为制备多量子阱结构器件提供了一种新方法。 两亲性聚合物(ES)是由环氧氯丙烷、乙二胺和硬脂酸共聚而成。将染料分子四苯基丁二烯(TPB)与ES相混合的氯仿溶液(TPB与ES质量比为2:10)分散在去离子水亚相表面,在20mN/m的膜压下将其转移到单晶硅片上。用小角X射线衍射实验观察其多层结构,在小角衍射区存在一个Bragg衍射峰(图1),根据Bragg衍射方程得到其层状周期结构为5.8nm。考虑到ES的分子尺寸,我们认为每一周期结构是由两层聚合物的LB膜组成。由于TPB分子是疏水的,通过分子间的相互作用,TPB分子可能镶嵌或吸附在ES的疏水脂肪链中,与无机半导体超晶格结构对照,聚合物ES的亲水网络由于是绝缘材料,带隙很宽相当于能垒,而镶嵌有TPB的疏水层则相当于势阱,这就很可能形成聚合物/有机染料的超晶格结构。考察其发光特性(图2)时,发现聚合物/有机染     

Atomic Layer Deposition of Cobalt Phosphide for Efficient Water Splitting

Transition‐metal phosphides (TMP) prepared by atomic layer deposition (ALD) are reported for the first time. Ultrathin Co‐P films were deposited by using PH₃ plasma as the phosphorus source and an extra H₂ plasma step to remove excess P in the growing films. The optimized ALD process proceeded by self‐limited layer‐by‐layer growth, and the deposited Co‐P films were highly pure and smooth. The Co‐P films deposited via ALD exhibited better electrochemical and photoelectrochemical hydrogen evolution reaction (HER) activities than similar Co‐P films prepared by the traditional post‐phosphorization method. Moreover, the deposition of ultrathin Co‐P films on periodic trenches was demonstrated, which highlights the broad and promising potential application of this ALD process for a conformal coating of TMP films on complex three‐dimensional (3D) architectures.