Layer Growth Techniques Research Articles

Electrostatic Design of Polar Metal-Organic Framework Thin Films.

In recent years, optical and electronic properties of metal–organic frameworks (MOFs) have increasingly shifted into the focus of interest of the scientific community. Here, we discuss a strategy for conveniently tuning these properties through electrostatic design. More specifically, based on quantum-mechanical simulations, we suggest an approach for creating a gradient of the electrostatic potential within a MOF thin film, exploiting collective electrostatic effects. With a suitable orientation of polar apical linkers, the resulting non-centrosymmetric packing results in an energy staircase of the frontier electronic states reminiscent of the situation in a pin-photodiode. The observed one dimensional gradient of the electrostatic potential causes a closure of the global energy gap and also shifts core-level energies by an amount equaling the size of the original band gap. The realization of such assemblies could be based on so-called pillared layer MOFs fabricated in an oriented fashion on a solid substrate employing layer by layer growth techniques. In this context, the simulations provide guidelines regarding the design of the polar apical linker molecules that would allow the realization of MOF thin films with the (vast majority of the) molecular dipole moments pointing in the same direction.

Study of heterostructures with a combined In(Ga)As/GaAs quantum dot/quantum well layer and a Mn δ layer

Using high-resolution transmission electron microscopy and photoelectric spectroscopy methods, the effect of Mn δ layer embedding and GaAs coating layer growth techniques in structures with In(Ga)As/GaAs quantum dots and wells on their structural and optoelectronic characteristics is studied. It is shown that the low-temperature GaAs coating layer in a structure with a Mn δ layer is structurally inhomogeneous and can cause a decrease in the quantum-dot photosensitivity.

Phosphor-Free Monolithic White-Light LED

Phosphor-free monolithic InGaN-based white-light LED has the advantages of simpler device process and potentially higher efficiency. Several techniques have been developed for implementing such white-light LEDs. Among them, the key issue is the growth of a high-quality high-indium InGaN/GaN quantum well (QW). An underlying InGaN layer growth technique is introduced for enhancing the crystal quality of a high-indium QW. To demonstrate the superior properties of a QW grown with this technique, a green LED is fabricated based on the underlying growth technique to compare with another LED of the same emission wavelength based on the conventional growth method. Then, the underlying growth technique is used to grow three yellow-emitting QWs of high efficiency. The yellow photons mix with blue light from an overgrown blue-emitting QW to produce white light. The improved properties of the phosphor-free monolithic white-light LED are discussed in detail.

SELF-LIMITING GROWTH OF TITANIUM SILICATE AND EFFECTS OF THERMAL ANNEALING ON THE ELECTRICAL PROPERTIES OF TITANIUM SILICATE/SiO2

Titanium silicate thin films were deposited using self-limiting atomic layer growth technique. Grown films showed smooth film surface morphology. As deposited, 8 nm-thick film surface showed an RMS value of 0.43 nm and annealed film showed a smoother surface having RMS of 0.2 nm. Electrical properties of titanium silicate/ SiO 2 bilayer were investigated using capacitance–voltage (C–V) and leakage current-voltage (I–V) measurements. The grown films showed high dielectric properties with low impurity contents and low leakage currents. Upon annealing at 800°C, capacitance slightly decreased, which is likely due to the interfacial reactions. Generally thermal annealing decreased hysteresis in C–V results.

Characterizations of SiC/SiO<sub>2</sub> Interface Quality Toward High Power MOSFETs Realization

The low channel mobility in N-MOS 4H-SiC transistor is a major key issue for the development of power devices with satisfactory on state characteristics. Previous works have demonstrated that this low channel mobility is due to high interface state density (Dit) near the conduction band edge. Furthermore, the realization of SiC MOSFETs sustaining high reverse field necessitates thick epitaxial layer growth. An important thickness (> 30 gm) unfortunately involves important surface roughness which may result in a high interface trap density (Dit) and surface potential fluctuation (sigma(s)) at the SiC/SiO2 interface. In this study, we focus on SiO2/SiC MOS interface quality characterization as a function of process conditions and material properties (dopant type, thick layer growth technique). Investigations of the oxide quality on thick layers grown by CVD and PVT has been realized using CV under UV lightening and GV techniques. We evidenced that the Dit value (between 10(10) cm(-2).eV(-1) and 9x10(10) cm(-2).eV(-1) from 0.9 Ev to 0.2 eV below Ec) and sigma(s) value (60 mV) were slightly lower for thick PVT layers. A discrepancy in the Dit values obtained from C-V and G-V measurements is attributed to the large surface potential standard deviation. Results from an original oxide growth process using a deposited sacrificial silicon layer under UHV conditions are also presented.

Structural properties of MOVPE GaN layers grown by a new multi-buffer aproach

GaN undoped layers of good morphology, good crystallinity and electrical properties were grown on c-plane sapphire substrates by the atmospheric pressure MOVPE technique using a new multi-buffer growth approach. A suitable buffer layer growth technique was worked out which enabled growth of GaN layers with properties superior to those grown in a conventional process scheme. Additional buffer layers, deposited with increasing temperature and increasing V/III molar ratio, were inserted between the low temperature buffer layer and the high temperature GaN layer grown on it. The c and a lattice constants of the high temperature GaN overgrown layer were evaluated from X-ray data. The layer mosaicity and c-lattice parameter variation were determined. The relationship between c and a lattice parameters and the second buffer layer growth scheme has been studied. The effect of second buffer layer growth conditions, buffer layer annealing time as well as the influence of V/III molar ratio during the high temperature GaN deposition on the crystalline and electrical properties of overgrown GaN epitaxial layers are presented. Characterization includes surface morphology examination by SEM and Nomarski optical microscope, X-ray diffraction and C-V measurements.

Surface Chemistry for Atomic Layer Growth

Atomic layer controlled film growth is an important technological and scientific goal that is closely tied to many issues in surface chemistry. This article first reviews the basic concepts of atomic layer growth using molecular precursors and binary reaction sequence chemistry. Many examples are given for the various films that have been grown using this atomic layer growth technique. The paradigms for atomic layer epitaxy (ALE) and atomic layer processing (ALP) are then discussed in terms of self-limiting surface reactions. Recent investigations of the surface chemistry of SiO2 and Al2O3 ALP and GaAs ALE are examined and used to illustrate the possible mechanisms of atomic layer growth. Subsequently, the characteristics of film deposition using atomic layer growth techniques are explored using recent examples for Al2O3 ALP. The structure of the deposited films is also reviewed using results from previous Al2O3 deposition investigations. This article then concludes by discussing possible complications to...

Growth from high-temperature solutions effected by chemical potential gradients

The process of crystallization from a thin solution zone under conditions whereby the driving force of growth is the difference in the chemical potentials of the substrate and source has been investigated experimentally in the Si- Al system. It has been shown that significant growth rates can be achieved in compositions with polycrystal and stressed crystal sources. This allows use of the process as the basis for crystal and epitaxial layer growth techniques. Experimental parameter dependencies support the theoretical expressions for the growth rate. The difference in the chemical potential of silicon atoms in single crystals and polycrystals has been determined. The reasons for the occurence of this difference are discussed.

The high temperature interaction between molybdenum and graphite

This paper describes a series of experiments elucidating the interaction of molybdenum and graphite at temperatures between 2000 and 3000° C. The kinetics of the reaction were studied by a layer growth technique, perpendicular and parallel to the plane of deposition of a layered pyrocarbon. The variation of contact angle with temperature was measured optically. The behaviour of Mo wire embedded in natural graphite powder was studied. The electron microprobe was used to determine diffusion of Mo in C and C in Mo. Electron scanning was shown to be a powerful tool for following the movement of Mo particles on graphite surfaces. A model and related mechanism are proposed to explain these results. Additional work is reported to compare the above behaviour with the reaction of Mo with glassy carbon and other metals with pyrocarbon.