Growth Of Semiconductor Thin Films Research Articles

Van der Waals Epitaxy of HgCdTe Thin Films for Flexible Infrared Optoelectronics

AbstractVan der Waals epitaxial (vdW) growth of semiconductor thin films on 2D layered substrates has recently attracted considerable attention since it provides a potential pathway for realizing monolithically integrated devices and flexible devices. In this work, direct growth of epitaxial HgCdTe (111) thin films on 2D layered transparent mica substrates is achieved via molecular beam epitaxy. The full width at half maximum of the ω‐mode X‐ray diffraction peak is measured to be around 306 arc sec. Mid‐wave infrared photoconductors based on the as‐grown HgCdTe thin films have been demonstrated and the self‐heating effect has been evaluated. A peak responsivity at the wavelength of around 3500 nm is measured to be about 110 V W−1 at 80 K and 8 V W−1 at room temperature under a bias of 25 V cm−1. Twinning defects are observed, limiting the crystallinity and mobility‐lifetime product in HgCdTe/mica. Benefiting from the vdW epitaxial growth, an etch‐free layer transfer process for lifting off the HgCdTe from the mica substrate has been demonstrated, leading to large area free‐standing HgCdTe thin films.

Exploration of a large-scale reconstructed structure on GaN(0001) surface by Bayesian optimization

GaN(0001) surfaces with Ga- and H-adsorbates are fundamental stages for epitaxial growth of semiconductor thin films. We explore stable surface structures with a nanometer scale by the density-functional calculations combined with Bayesian optimization and reach a single structure with satisfactorily low mixing enthalpy among hundreds of thousand possible candidate structures. We find that the obtained structure is free from any postulated high symmetry previously introduced by human intuition, satisfies an electron counting rule locally, and shows a complex adsorbate arrangement, reflecting characteristics of nitride semiconductors. The proposed scheme toward a high-resolution surface phase diagram contributes to a more precise design of GaN epitaxial growth conditions, especially the ratio of Ga and H partial pressures.

Fast Organic Vapor Phase Deposition of Thin Films in Light-Emitting Diodes.

Fast deposition of thin films is essential for achieving low-cost, high-throughput phosphorescent organic light-emitting diode (PHOLED) production. In this work, we demonstrate rapid and uniform growth of semiconductor thin films by organic vapor phase deposition (OVPD). A green PHOLED comprising an emission layer (EML) grown at 50 Å/s with bis[2-(2-pyridinyl-N)phenyl-C](acetylacetonato)iridium(III) (Ir(ppy)2(acac)) doped into 4,4'-bis(N-carbazolyl)-1,1'-biphenyl (CBP) exhibits a maximum external quantum efficiency of 20 ± 1%. The morphology, charge transport properties, and radiative efficiency under optical and electrical excitation of the PHOLED EML are investigated as functions of the deposition rate via both experimental and theoretical approaches. The EML shows no evidence for gas phase nucleation of the organic molecules at deposition rates as high as 50 Å/s. However, the roll-off in quantum efficiency at high current progressively increases with deposition rate due to enhanced triplet-polaron annihilation. The roll-off results from accumulation of stress within the PHOLED EML that generates a high density of defect states. The defects, in turn, act as recombination sites for triplets and hole polarons, leading to enhanced triplet-polaron annihilation at high current. We introduce a void nucleation model to describe the film morphology evolution that is observed using electron microscopy.

Atomic mechanism of strong interactions at the graphene/sapphire interface

For atomically thin two-dimensional materials, interfacial effects may dominate the entire response of devices, because most of the atoms are in the interface/surface. Graphene/sapphire has great application in electronic devices and semiconductor thin-film growth, but the nature of this interface is largely unknown. Here we find that the sapphire surface has a strong interaction with some of the carbon atoms in graphene to form a C-O-Al configuration, indicating that the interface interaction is no longer a simple van der Waals interaction. In addition, the structural relaxation of sapphire near the interface is significantly suppressed and very different from that of a bare sapphire surface. Such an interfacial C-O-Al bond is formed during graphene growth at high temperature. Our study provides valuable insights into understanding the electronic structures of graphene on sapphire and remote control of epitaxy growth of thin films by using a graphene–sapphire substrate.

Atomic Layer Epitaxy of Aluminum Nitride: Unraveling the Connection between Hydrogen Plasma and Carbon Contamination.

Atomistic control over the growth of semiconductor thin films, such as aluminum nitride, is a long-sought goal in materials physics. One promising approach is plasma-assisted atomic layer epitaxy, in which separate reactant precursors are employed to grow the cation and anion layers in alternating deposition steps. The use of a plasma during the growth-most often a hydrogen plasma-is now routine and generally considered critical, but the precise role of the plasma is not well-understood. We propose a theoretical atomistic model and elucidate its consequences using analytical rate equations, density functional theory, and kinetic Monte Carlo statistical simulations. We show that using a plasma has two important consequences, one beneficial and one detrimental. The plasma produces atomic hydrogen in the gas phase, which is important for removing methyl radicals left over from the aluminum precursor molecules. However, atomic hydrogen also leads to atomic carbon on the surface and, moreover, opens a channel for trapping these carbon atoms as impurities in the subsurface region, where they remain as unwanted contaminants. Understanding this dual role leads us to propose a solution for the carbon contamination problem which leaves the main benefit of the plasma largely unaffected.

Order and Epitaxy during Electrochemical Layer By Layer Growth of Semiconductor Thin Films

Electrochemical Atomic Layer Epitaxy (ECALE), also known as Electrochemical Atomic Layer Deposition (E-ALD) [1], is a method based on Under Potential Deposition (UPD) which allows the layer by layer growth of compounds on single crystal substrates.In the ECALE method [1] a compound can be deposited on a substrate by an alternate UPD of the elements that form the compound. UPD is a surface- limited phenomenon, so the electrodeposition in UPD conditions is generally limited to one atomic layer. ECALE has been demonstrated to be a very effective method for the production of thin films of semiconductor compounds with high grade of crystallinity [2-3]. UPD processes on single crystals are substrate dependent, thus different structures of the deposited layer should be expected on substrates with different orientation. Substrate orientation dependency was shown in a previous investigation on the deposition of CdS films on Ag(111), Ag(110) and Ag(100) [2]. Aim of the present work was the in-situ characterization of the growth process of CdS thin films on Ag(111). In-situ structural characterization of thin films is generally a rather complex task as it requires high brilliance x-ray sources given the small thickness of the film, the high background of the electrochemical environment and the attenuation due to the presence of the solvent. Nevertheless synchrotron light sources can provide such an high photon flux to perform this kind of experiment. Surface X-ray diffraction was used to identify the crystallographic structure and to investigate the epitaxial order on the surface. In-situ characterization of the growth process was performed on ID03 beamline at ESRF (Grenoble) using an electrochemical flow cell specifically designed for this purpose. Even if it's well established that the sequential deposition of UPD layers of Cd and S on Ag(111) results in an ordered multilayer structure with lattice parameters comparable to the one of the wurtzite CdS structure [3], several aspects of the growth process were not yet fully understood. In-situ experiments allowed to record structural changes of the in-plane order and details of the out-of-plane relaxation of the film that could not be observed before. One of the most interesting fact observed during the growth process was the dependency of the epitaxial order on the potential used for the UPD deposition of Cd. In figure X-ray reflectivity curve measured during CdS growth on Ag(111). [1] Gregory and J. L. Stickney, J. Electroanal. Chem. 300, 1991, 543[2] F. Carla', et al., J. Phys. Chem. C, 12, 6132 (2014)[3] M.L. Foresti, et al. Electrochimica Acta, 51, 25, 5532 (2006) Figure 1

Highly oriented lateral growth of SiGe thin films on glass induced by constitutional undercooling

Enhanced lateral growth of semiconductor thin films on glass was investigated. The unidirectional growth of silicon (Si) was achieved by introducing continuous-wave laser lateral crystallization. Alloying germanium (Ge) with Si led to highly oriented lateral growth (HOLG) of SiGe thin films. The HOLG and Ge segregation in these films were explained by the constitutional undercooling model. Quasi-single crystals were grown when the Ge mole fraction was 0.3. Using a single scan of the laser on SiGe causes the amorphous phase to become crystal, which tends to decrease the stability of growth. This growth stability was improved by performing multiple scans, where the growth was dominated by regrowth from crystal to crystal.

Universal fluctuations in the growth of semiconductor thin films

Scaling of surface fluctuations of polycrystalline CdTe/Si(100) films grown by hot wall epitaxy are studied. The growth exponent of surface roughness and the dynamic exponent of the auto-correlation function in the mound growth regime agree with the values of the Kardar-Parisi-Zhang (KPZ) class. The scaled distributions of heights, local roughness, and extremal heights show remarkable collapse with those of the KPZ class, giving the first experimental observation of KPZ distributions in $2+1$ dimensions. Deviations from KPZ values in the long-time estimates of dynamic and roughness exponents are explained by spurious effects of multi-peaked coalescing mounds and by effects of grain shapes. Thus, this scheme for investigating universality classes of growing films advances over the simple comparison of scaling exponents.

A Study on the Computer Simulation for the Fractal Growth of Semiconductor Thin Films

This paper is based on theoretical methods to study the computer simulation and analysis of the growth of semiconductor thin films. First, according to the traditional theory of thin-film growth, the relationship between the growth morphology and the physical parameters are discussed. Then, fractal theory has been applied to improve the diffusion-limited aggregation (DLA) model. And the simulations of the two-dimensional and three-dimensional thin-film growth are proposed. A computer program of the simulation of the thin-film growth is developed with help of MATLAB. Finally, the results of the simulation of the thin-film growth have been analyzed by the fractal dimension and multifractal spectra. The results of this paper can be applied to the dynamic simulation of nanometer thin-film growth, and an effective simulation tool is to provide the semiconductor process.

Crystallization of II-VI semiconductor compounds forming long microcrystalline linear assemblies

In this work we report the formation of long microcrystalline linear self-assemblies observed during the thin film growth of several II-VI compounds. Polycrystalline CdTe, CdS, CdCO3, and nanocrystalline CdTe:Al thin films were prepared on glass substrates by different deposition techniques. In order to observe these crystalline formations in the polycrystalline materials, the thin film growth was suspended before the grains reached to form a continuous layer. The chains of semiconductor crystals were observed among many isolated and randomly distributed grains. Since CdTe, CdTe:Al, CdS and CdCO3 are not ferroelectric and/or ferromagnetic materials, the relevant problem would be to explain what is the mechanism through which the grains are held together to form linear chains. It is well known that some nanocrystalline materials form rods and wires by means of electrostatic forces. This occurs in polar semiconductors, where it is assumed that the attraction forces between surface polar faces of the small crystals are the responsible for the chains formation. Since there are not too many mechanisms responsible for the attraction we assume that a dipolar interaction is the force that originates the formation of chain-like grain clusters. The study of this property can be useful for the understanding of nucleation processes in the growth of semiconductor thin films.

A high-vacuum deposition system for in situ and real-time electrical characterization of organic thin-film transistors

We present a home-built high-vacuum system for performing organic semiconductor thin-film growth and its electrical characterization during deposition (real-time) or after deposition (in situ). Since the environment conditions remain unchanged during the deposition and electrical characterization process, a direct correlation between growth mode and electrical properties of thin film can be obtained. Deposition rate and substrate temperature can be systematically set in the range 0.1-10 ML∕min and RT-150 °C, respectively. The sample-holder configuration allows the simultaneous electrical monitoring of up to five organic thin-film transistors (OTFTs). The OTFTs parameters such as charge carrier mobility μ, threshold voltage V(TH), and the on-off ratio I(on)∕I(off) are studied as a function of the semiconductor thickness, with a submonolayer accuracy. Design, operation, and performance of the setup are detailed. As an example, the in situ and real-time electrical characterization of pentacene TFTs is reported.

Simultaneous in situ measurements of x-ray reflectivity and optical spectroscopy during organic semiconductor thin film growth

Simultaneous in situ real-time measurements of x-ray reflectivity and differential reflectance spectroscopy were conducted during deposition of perfluorinated copper-phthalocyanine thin films on SiO2/Si. We found a continuous spectral change coinciding with structural changes from submonolayer coverage, to standing βbilayer-phase and to β-phase for thicker films. This combined measurement enables us to study the relationship between structural and optical properties of organic semiconductor thin films.

Epitaxial growth of semiconductor thin films on metals in the halogenation process. Atomic structure of copper iodide on the Cu(110) surface

The atomic structure of thin (7–20 A) copper iodide layers formed on the Cu(110) surface during a chemical reaction with molecular iodine in ultrahigh vacuum has been studied with scanning tunneling microscopy. A double stripe superstructure with an average period of 90–100 A was found on the surface of CuI. The structural model is proposed for the copper iodide surface taking into account the contraction of the CuI lattice and the formation of striped domain walls.

Real-time observation of the energy band diagram during microcrystalline silicon p–i interface formation

We have performed real-time infrared attenuated total reflection spectroscopy to characterize free-carrier absorption during microcrystalline silicon (μc-Si:H) p–i interface formation. In the μc-Si:H i-layer growth on the p layer, a free-carrier concentration in the p layer showed a large variation, in accordance with a built-in potential present at the p/i interface. We demonstrate that real-time observation of the energy band diagram is possible during semiconductor thin-film growth by employing free carriers in a doped layer as a probe.

Growth of Semiconductors Thin Films by Radio Frequency Sputtering with Two Phases: GaInNAs and GaAs Nanocrystals

We present results on the growth of GaInNAs semiconductor thin films which present evidence of a nanocrystalline GaAs background. The films were grown using the radio frequency sputtering deposition technique in a working gas atmosphere mixture of argon and nitrogen. X-ray diffractograms show evidence of the presence of a phase of GaInNAs material with high N concentration, and of a phase of GaAs material with crystallite sizes in the order of nanometers. From the absorption spectra measured by the photoacoustic technique we show that we have obtained the GaInNAs alloys with variable band gap energies, but also that this alloy is embedded in a background of nanocrystalline GaAs material. We discuss the nature of this formation, the quantum confinement effects evidenced from the absorption spectra, and the grain sizes obtained from the use of a spherical quantum dot model.

X-Ray Characterization of Nanostructured Semiconductor Short-Period Superlattices

ABSTRACTSpontaneous lateral composition modulation during semiconductor thin film growth offers a particularly versatile and cost-effective approach to manufacture nanoscale devices. Recent experimental and theoretical studies have revealed that regular lateral composition modulation can be achieved via MBE growth of the so-called short-period superlattices and can be optimized via appropriate control of the global strain, substrate surface, and processing conditions. To characterize this phenomenon, we used synchrotron x-ray scattering to identify the interfacial morphology and laterally modulated composition profile of nearly strain-balanced InAs/AlAs short-period superlattices. Our results were compared with a theoretical model. It is shown that the lateral composition modulation is predominately caused by a vertically correlated morphlogical undulation of the superlattice layers.

Application of Supersonic Molecular Jets in Semiconductor Thin Film Growth

The flux and the incident kinetic energy are the most important deposition variables in thin film growth processes. By changing these variables, one can, in principle, alter the reaction pathways and the rate at which they occur and produce a different material than under thermody-namic equilibrium conditions. The significance of supersonic molecular jets stems from the fact that both, the flux and the incident kinetic energy of neutral growth species, can be varied independently. The number of studies that are exploring these advantages in a wide range of materials systems is growing rapidly. In this article the application of supersonic molecular jets in semiconductor thin film growth is reviewed. The effects of both the superthermal incident kinetic energy and the flux on the growth and properties of elemental and compound semiconductors are examined.

Kinetic Pathways to Strain Relaxation in the Si-Ge System

The strain-induced transition of a planar film to a three-dimensional island morphology is presently a significant issue in the growth of semiconductor thin films. Strain-induced roughening can be problematic in the fabrication of coherently strained device structures where it is important to understand the early stages of the transition to avoid or suppress three-dimensional (3D) growth. On the other hand, the strain-driven transition is beneficial for the self-assembly of quantum dots where it is necessary to control the size distribution and self-organizing behavior of the islands. In both cases, it is clearly important to identify and understand the kinetic pathways to island formation. From a more basic perspective, the strain-induced transition of epitaxial films allows us to study in detail the interplay between elastic stresses and surface energy in a carefully controlled experimental environment. One would therefore hope that the lessons learned from model semiconductor systems will be of relevance to understanding related phenomena in other areas of materials science and physical metallurgy.Ihe strain-induced two-dimensional (2D)-to-3D transition in the Si-Ge system is manifested by a rich variety of observed surface morphologies. In the case of pure Ge on Si(OO1), the 4% misfit strain induces the formation of so-called hut clusters with curious elongated shapes. Such islands form almost immediately after the deposition of a wetting layer. In the case of lower misfit alloys, a more gentle ripple morphology can result that develops far from the interface. A general trend in all of the experiments is the decreasing size of typical morphological features with increasing misfit stress. In this article, largely guided by our experimental results, we adopt a nucleation and growth description of the 2D-to-3D transition. This approach appears particularly well-suited to explaining the wide spectrum of morphological development present in the Si-Ge system.

Surfactant mediated crystal growth of semiconductors.

The influence of surfactants on semiconductor thin film growth is studied by means of a mesoscopic model combined with first principles calculations. We introduce a new kinetic mechanism that explains how surfactants induce layer-by-layer growth. The experimentally observed high density of 2D islands is a natural consequence of the chemical passivation of step edges, as well as flat surfaces, by the surfactant. In heteroepitaxial growth, we take strain effects into account, which leads to layer-by-layer growth at low temperatures and three-dimensional growth at high temperatures, in agreement with experiments.

Some Computer Simulations of Semiconductor Thin Film Growth and Strain Relaxation in a Unified Atomistic and Kinetic Model

AbstractAn overview is provided of an evolving atomistic and kinetic model of semiconductor growth that unifies the main features of strain relaxation in low and high lattice misfit heteroepitaxy. The model reveals a kinetic pathway for dislocation formation during growth with little or no energy cost at low misfits, thus providing a way out of the longstanding dilemma of too high dislocation nucleation energies predicted by classical theories of the equilibrium behaviour of a fixed number of particles at low misfits. The essential kinetic processes underlying the model are identified on the basis of comparison of the predictions of kinetic Monte-Carlo simulations of growth with real-time or in-situ data obtained in such experiments as reflection high-energy electron diffraction (RHEED) and scanning probe microscopy (SPM). Relative significance of these atomistic kinetic processes is shown to naturally lead to strain relaxation via defect initiation at low misfits while maintaining smooth surface morphology or at high misfits change to 3-dimensional morphology while initially maintaining coherence. The potential role of steps in providing sources for defect formation is examined through molecular dynamics simulations of Ge overlayers on Si (001) stepped surfaces.