Effect Of Strain Relaxation Research Articles

Suppression of mixed-phase areas in highly elongated BiFeO3thin films on NdAlO3substrates

Mixed-phase areas are produced in highly elongated BiFeO${}_{3}$ (BFO) thin films as a consequence of strain relaxation. A (001) neodymium aluminate (NdAlO${}_{3}$; NAO) substrate ($a\ensuremath{\sim}3.747$ \AA{}) prominently suppresses the strain relaxation effect and prevents the formation of mixed-phase regions. This creates a pathway to the thick, quasipure, highly elongated phases required for magnetoelectric applications. We characterize the crystal structure, the interface between film and substrate, the surface morphology, and the ferroelectric domain structure of BFO films on NAO substrates and compare them with those of films on typical lanthanum aluminate substrates. The underlying mechanisms are discussed based on the intriguing nature of phase competition in bismuth ferrite phases using first principles density functional calculations for the misfit strain-dependent total energy.

Role of InGaN Insertion Layer on Nitride-Based Light-Emitting Diodes

In this study, we systematically investigate the effect of InGaN insertion layer (IL) on nitride-based light-emitting diodes. First, a series of samples with different InGaN ILs (Si doping level, thickness) were fabricated and investigated. An optimized condition of the IL was obtained based on current–voltage and electroluminescence measurements. Furthermore, in order to investigate the dominant mechanism for the improved performance of the samples with IL, the optimized sample and a control sample without InGaN IL were compared by means of X-ray diffraction, atomic force microscopy, photoluminescence, injection-current-dependent electroluminescence measurements and infrared camera images. Based on the discussion of these measurement results, we conclude that the performance improvements of samples with InGaN IL are due to both the effects of strain relaxation and better current spreading.

A study of (111) oriented epitaxial thin films of In2O3 on cubic Y-doped ZrO2 by synchrotron-based x-ray diffraction

Abstract

Role of InGaN Insertion Layer on Nitride-Based Light-Emitting Diodes

In this study, we systematically investigate the effect of InGaN insertion layer (IL) on nitride-based light-emitting diodes. First, a series of samples with different InGaN ILs (Si doping level, thickness) were fabricated and investigated. An optimized condition of the IL was obtained based on current–voltage and electroluminescence measurements. Furthermore, in order to investigate the dominant mechanism for the improved performance of the samples with IL, the optimized sample and a control sample without InGaN IL were compared by means of X-ray diffraction, atomic force microscopy, photoluminescence, injection-current-dependent electroluminescence measurements and infrared camera images. Based on the discussion of these measurement results, we conclude that the performance improvements of samples with InGaN IL are due to both the effects of strain relaxation and better current spreading.

Effects of Strains and Defects on the Internal Quantum Efficiency of InGaN/GaN Nanorod Light Emitting Diodes

The internal quantum efficiency of GaN-based nanorod light emitting diode (LED) arrays is determined by the effects of reduced quantum confined Stark effect and sidewall-defect-related non-radiative recombination. Here we report the characterizations of light output of nanorod LED arrays with different rod etching depths. During the definition of nanorods, the effect of strain relaxation is accompanied by the formation of sidewall defects picked up from dry etching. The sample with shallower nanorods possesses fewer defects and thus a higher light output power. On the other hand, the device with longer nanorods has more relaxed strain and smaller efficiency droop. This paper indicates that a shorter nanorod etching depth is preferred for a higher light output. However, the longer nanorod structure has a less severe droop effect and a higher operating current, which may eventually lead to higher optical output if the defects can be properly suppressed.

Investigation of the optical properties of InGaN/GaN nanorods with different indium composition

AbstractIn this paper, the optical properties of our nanorod array structures fabricated from InGaN/GaN multiple quantum wells (MQWs) epiwafers with two different kinds of indium compositions have been investigated. The optical performance of our nanorod samples are significantly enhanced compared to the as‐grown epiwafers, in particular, such improvement is much more pronounced in the nanorod array structure with a higher indium composition. This is mainly attributed to effective strain relaxation as a result of fabrication into nanorod structure, leading to significant reduction in quantum confined Stark effect (QCSE). X‐ray reciprocal space mapping (RSM) measurements have been performed in order to quantitatively evaluate the stain relaxation in the nanorod samples, confirming that the majority of the strain in InGaN/GaN MQWs can be relaxed as a result of fabrication into nanorods. Finally, excitation‐power dependant photoluminescence measurements performed at a low temperature also support the conclusion (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Improving Accuracy and Precision of Strain Analysis by Energy-Filtered Nanobeam Electron Diffraction

This article deals with uncertainty in the analysis of strain in silicon nanoscale structures and devices using nanobeam electron diffraction (NBED). Specimen and instrument related errors and instabilities and their effects on NBED analysis are addressed using a nanopatterned ultrathin strained silicon layer directly on oxide as a model system. We demonstrate that zero-loss filtering significantly improves the NBED precision by decreasing the diffuse background in the diffraction patterns. To minimize the systematic deviations the acquired data were verified through a reliability test and then calibrated. Furthermore, the effect of strain relaxation by specimen preparation using a FIB is estimated by comparing profiles, which were acquired by analyzing slices of strained structures in a 220-nm-thick region of the sample (invasive preparation) and the entire strained nanostructures, which are embedded in a thicker region of the same sample (noninvasive preparation). Together with the random deviation, the corresponding systematic shift results in a total deviation of ∼1 × 10(-3) for NBED analyses, which is employed to estimate the measurement uncertainty in the thinner sample region. In contrast, the strain in the thick sample region is not affected by the preparation; the systematic shift reduces to a minimum, which improves the total deviation by ∼50%.

Optical studies of GaN nanocolumns containing InGaN quantum disks and the effect of strain relaxation on the carrier distribution

AbstractThe optical properties of GaN nanocolumns containing pairs of InxGa1–xN (x ∼ 0.1) quantum disks (QDisks) have been experimentally measured. Strain simulations reveal an inhomogeneous distribution of strain, and furthermore, a relaxation between successive QDisks along the growth direction of the column. Although this inhomogeneous distribution of strain may indicate a lateral separation of carriers in the QDisk, the decay lifetime of the emission suggests far less carrier separation. The quantum confined stark effect is found to create an inhomogeneous distribution of charge within the QDisks along the growth direction of the rod. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Self‐assembled GaN nanostructures by dry etching and their optical properties

AbstractThe influence of process parameters and template characteristics on the morphology and optical properties of self‐assembled nanostructures produced by dry etching from GaN/sapphire templates was investigated. It was found that the chemical component in the etching gas–chlorine supports the formation of nanostructures and that a minimum amount of RF and ICP power (i.e., energy) is necessary. Additionally the crystalline imperfection and the doping concentration of the template greatly affect the nanostructure morphology. All nanostructures exhibit higher luminescence intensity than the templates they are fabricated from and exhibit a red‐shift of the luminescence with respect to the epilayer. The results indicate effective strain relaxation as well as more efficient absorption of the incoming photons the smaller the nanostructures become.

Monodisperse (In, Ga)N insertions in catalyst-free-grown GaN(0001) nanowires

Vertical stacks of (In, Ga)N insertions in GaN nanowires are grown by molecular beamepitaxy. The chemical composition and strain within the structure are probed by acombination of high-resolution x-ray diffraction, transmission electron microscopy, andgeometrical phase analysis. The (In, Ga)N insertions are coherently strained.Finite-element simulations strongly support an effective strain relaxation mechanism of thesurrounding GaN matrix due to the nanowire geometry, leading to high-quality (In,Ga)N/GaN nanowire heterostructures. An intense green photoluminescence emission isobserved and attributed to an inter-well transition between the stacked (In, Ga)Ninsertions.

Strain control of magnetic anisotropy in (Ga,Mn)As microbars

We present an experimental and theoretical study of magnetocrystalline anisotropies in arrays of bars patterned lithographically into (Ga,Mn)As epilayers grown under compressive lattice strain. Structural properties of the (Ga,Mn)As microbars are investigated by high-resolution x-ray diffraction measurements. The experimental data, showing strong strain relaxation effects, are in good agreement with finite element simulations. Magnetization measurements are performed using a superconducting quantum interference device to study the control of magnetic anisotropy in (Ga,Mn)As by the lithographically induced strain relaxation of the microbars. Microscopic theoretical modeling of the anisotropy is performed based on the mean-field kinetic-exchange model of the ferromagnetic spin-orbit coupled band structure of (Ga,Mn)As. Based on the overall agreement between experimental data and theoretical modeling, we conclude that the micropatterning induced anisotropies are of magnetocrystalline, spin-orbit coupling origin.

Strain relaxation and phonon confinement in self-assembled InAsSb/InP (001) quantum dashes: Effect of deposition thickness and composition

This paper presents a study on the strain relaxation and phonon confinement effect in InAsSb/InP quantum dashes (QDashes). The phonon mode with a frequency between that of InAs-like longitudinal optical mode and that of InP transverse optical mode is determined to be originated from InAsSb QDashes. Despite the small height of the QDashes, their phonon frequency is found to be mainly determined by the strain relaxation in the dashes. With increasing InAsSb deposition thickness and Sb composition in InAsSb dashes, the phonon mode shows an upward shift of its frequency due to the increased compressive strain.

Compensation effect and differential capacitance analysis of electronic energy band structure in relaxed InAs quantum dots

The use of a differential capacitance technique for analyzing the effect of strain relaxation on the electronic energy band structure in relaxed InAs self-assembled quantum dots (QDs) is presented. Strain relaxation is shown to induce a deep defect state and compensate the ionized impurity in the bottom GaAs layer, leading to a double depletion width and a long emission time. An expression of capacitance at different frequency and voltage is derived for analyzing the experimental data. It has been shown that the relationship between the low-frequency and high-frequency capacitances can be well explained by a Schottky depletion model with a compensated concentration in the bottom GaAs layer. A simple expression is presented to account for the modulation of the free electrons in the top GaAs layer. This capacitance analysis shows a long low-energy tail for the electron ground state, suggesting not very uniform strain relaxation. The results of this study illustrate a carrier compensation effect of the defect state on the electronic energy band structure near the QDs.

Homobuffer thickness effect on the background electron carrier concentration of epitaxial ZnO thin films

Epitaxial ZnO thin films were grown on r-plane sapphire substrates using plasma-assisted molecular-beam epitaxy. ZnO homobuffer layers grown at a lower temperature were introduced to improve the crystallinity of the top ZnO thin films. Thicker homobuffer layers lead to better crystallinity of the subsequent epitaxial ZnO thin films due to the strain relaxation effect. Residual background electron carrier concentration in these undoped ZnO thin films first decreases, then increases as the buffer layer thickness increases from ∼1 to 30 nm, with a minimum electron concentration of ∼1×1017 cm−3 occurring in ZnO homobuffer of ∼5 nm. These results demonstrate that the optimized ZnO homobuffer thickness to achieve both good ZnO crystallinity and low residual electron concentration is determined by the relative electron carrier concentration ratios and mobility ratios between the buffer and epi-ZnO layers.

Strain relaxation effect on electronic properties of compressively strained InGaAs/InP vertically stacked multiple quantum wires

Electronic properties of compressively strained InGaAs/InP vertically stacked multiple quantum wires were investigated using an six-band strain-dependent k⋅p Hamiltonian. The strain tensor ϵyy (ϵxx) is found to relax from its initial strain. The amount of relaxation is dependent on the number of wire layers in the vertical stack and increases with the number of wire layers. The interband transition energy also decreases with the number of wire layers. This is mainly attributed to the decrease in the conduction band energy because subband energies in the valence band are nearly independent of the strain. The matrix element is shown to slightly decrease with increasing number of the wire layer in the vertical stack.

Study of Light Emission Enhancement in Nanostructured InGaN/GaN Quantum Wells

Recently, InGaN/GaN quantum wells with different nanostructures such as nanoholes and nanorods have been proposed to enhance the light emitting efficiency. This paper calculates the influence of nanostructures to the strain and band profile of the quantum well. The effects of strain relaxation and surface states are analyzed, which could possibly influence the diode emission properties. Our calculation results show that the strain relaxation and the surface state pinning play important roles in enhancing the light emission, reducing the quantum confined Stark effect, and causing the blue shift of the spectrum. Our calculation results provide useful information in analyzing emission properties of nanohole arrays and similar structures.

Strain relaxation effect by nanotexturing InGaN/GaN multiple quantum well

The relaxation of lattice-mismatched strain by deep postetching was systematically investigated for InGaN/GaN multiple quantum wells (MQWs). A planar heterojunction wafer, which included an In0.21Ga0.79N (3.2 nm)/GaN (14.8 nm) MQW, was etched by inductively coupled plasma dry etching, to fabricate high-density nanopillar, nanostripe, and nanohole arrays. The etching depth was 570 nm for all nanostructures. The diameter of the nanopillars was varied from 50 to 300 nm, then the mesa stripe width of the nanostripes and the diameter of the nanoholes were varied from 100 nm to 440 nm and 50 nm to 310 nm, respectively. The effect of strain relaxation on various optical properties was investigated. For example, in an array of nanopillars with diameter 130 nm and interval 250 nm, a large blueshift in the photoluminescence (PL) emission peak from 510 nm (as-grown) to 459 nm occurred at room temperature (RT). PL internal quantum efficiency (defined by the ratio of PL integral intensity at 300 K to that at 4.2 K) was enhanced from 34% (as-grown) to 60%, and the PL decay time at 4.2 K was reduced from 22 ns (as-grown) to 4.2 ns. These results clearly indicate the reduction of lattice-mismatched strain by postetching, which enhanced strain reduction with decreasing nanopillar diameter down to a diameter of 130 nm, where the strain reduction became saturated. The dependence of RT-PL decay time on nanopillar diameter was measured, and the surface nonradiative recombination velocity was estimated to be 5.8×102 cm/s. This relatively slow rate indicates a little etching damage.

Residual order in the thermal oxide of a fully strained SiGe alloy on Si

Introduction Oxidation of SiGe alloy has been a target of research for both fundamental and technological reasons, as in the case of Si oxidation. It has been expected as the fabrication process for the gate oxide of SiGe channel metal oxide semiconductor field effect transistors (MOSFETs), which allows higher carrier mobility than that of Si channel MOSFETs [1]. In recent years, it has also been employed in the fabrication of SiGe-oninsulator and Ge-on-insulator substrates [2]. These substrates are employed to realize high-speed MOSFETs with channel materials of strained Si, SiGe, and Ge. However, the oxidation mechanism of SiGe alloy is still open to question, due to the complexity caused by lattice strain and diffusion of Ge atoms. The most characteristic phenomenon of SiGe oxidation is that Ge atoms are ejected from the interface between the surface oxide and the SiGe layer without the incorporation of Ge atoms into the oxide layer. Ge atoms accumulate at the SiO2/SiGe interface and diffuse into the SiGe layer [1,2]. In the case of a strained SiGe layer epitaxially grown on a Si substrate, the effects of strain relaxation and defect generation during the oxidation process must be taken into account. It is important to investigate the atomic structure of the thermally oxidized layer, because it allows us to understand what really happens at the oxidized interface. In the case of the thermally oxidized layer of Si, the Si atoms in the oxide layer still maintain order, which originates from the diamond structure of the parent Si crystal, although the structure appears to be amorphous at a glance. This provides an actual picture of the oxidation process and explains the almost perfect properties obtained by electric measurements of the SiO2/Si interface. In this study, the residual order in the thermal oxide layer on a fully strained SiGe alloy was investigated.

Effect of strain relaxation of oxidation-treated SiGe epitaxial thin films and its nanomechanical characteristics

In this study, we examined the effect of high-temperature oxidation treatment on the SiGe epitaxial thin films deposited on Si substrates. The X-ray diffraction (XRD), atomic force microscopy (AFM), and nanoindentation techniques were employed to investigate the crystallographic structure, surface roughness, and hardness ( H) of the SiGe thin films, respectively. The high-temperature oxidation treatment led to Ge pileup at the surface of the SiGe thin films. In addition, strain relaxation occurred through the propagation of misfit dislocations and could be observed through the cross-hatch pattern (800–900 °C) and SiGe islands (1000 °C) at the surface of the SiGe thin films. Subsequent hardness ( H) measurement on the SiGe thin films by continuous penetration depth method indicated that the phenomenon of Ge pileup caused a slightly reduced H (below 50 nm penetration depth), while relaxation-induced defects caused an enhanced H (above 50 nm penetration depth). This reveals the influence of composition and defects on the structure strength of high-temperature oxidation-treated SiGe thin films.

Thermal runaway and optical efficiency in InAs/GaAs quantum dot lasers

In contrast to quantum well lasers, we show thermal runaway effects are prominent for quantum dot (QD) lasers emitting at 1300 nm. In addition to the surface states at the cleaved facet and the effect of strain relaxation, which can couple confined states with surface states, the exposed relaxed QDs at the cleaved facet can themselves contribute to extra nonradiative surface recombination, aggravating the facet heating problem. Notable improvement in optical efficiency after the formation of nonabsorbing mirrors by a laser annealing technique highlights the significance of the thermal runaway problem for 1300 nm QD lasers, and demonstrates an effective postfabrication solution.