Screw Dislocation Density Research Articles

Deformation mechanisms for a new medium-Mn steel with 1.1 GPa yield strength and 50% uniform elongation

A new medium-Mn steel was designed to achieve unprecedented tensile properties, with a yield strength beyond 1.1 GPa and a uniform elongation over 50%. The tensile behavior shows a heterogeneous deformation feature, which displays a yield drop followed by a large Lüders band strain and several Portevin-Le Châtelier bands. Multiple strain hardening mechanisms for excellent tensile properties were revealed. Firstly, non-uniform martensite transformation occurs only within a localized deformation band, and initiation and propagation of every localized deformation band need only a small amount of martensite transformation, which can provide a persistent and complete transformation-induced-plasticity effect during a large strain range. Secondly, geometrically necessary dislocations induced from macroscopic strain gradient at the front of localized deformation band and microscopic strain gradient among various phases provide strong heter-deformation-induced hardening. Lastly, martensite formed by displacive shear transformation can inherently generate a high density of mobile screw dislocations, and interstitial C atoms segregated at phase boundaries and enriched in austenite play a vital role in the dislocation multiplication due to the dynamic strain aging effect, and these two effects provide a high density of mobile dislocations for strong strain hardening.

Growth of N-polar In-rich InAlN by metal organic chemical vapor deposition on on- and off-axis sapphire

Metal organic chemical vapor deposition is used to grow N-polar In-rich InAlN layers directly on on- and off-axis (misoriented by 4° towards a plane) c-plane sapphire substrates. During the InAlN growth, trimethylaluminum, ammonia, and the total flow was kept at 4.74 μmol/min, 3 slm and 10 slm, respectively, while trimethylindium (TMIn) flow was selected between 8.42 and 13.48 μmol/min. All samples were grown at about 706 °C however; TMIn flow of 10.95 μmol/min was also tested at the growth temperature of 686 °C. With increasing the TMIn flow, the In molar fraction increased from 0.55 to 0.69, irrespectively of the substrate miscut. For the moderate TMIn flow and In molar fraction of less than 0.63, density of screw and edge dislocations were from 0.3 to 12 ✕ 109 cm-2 and 5 to 7 ✕ 1010 cm-2, respectively, while decisively lower densities appeared on on-axis structures. On the other hand, InAlN surface RMS roughness was from 1 to 8 nm favouring the off-axis substrate and low TMIn flow. Structural and surface deterioration appeared with the highest TMIn flow of 13.48 μmol/min and In molar fraction of 0.69, which hold also for reduced temperature of the growth. Nevertheless, room temperature photoluminescence full-width at half maximum less than 270 meV appeared for all samples, with maxima between 1.9 and 1.5 eV. In-rich N-polar InAlN grown on on-axis sapphire can be used as a buffer layer in new types of heterostructure devices.

High-quality N-polar GaN optimization by multi-step temperature growth process

We report growth optimization of Nitrogen (N)-polar GaN epitaxial layers by hot-wall metal–organic vapor phase epitaxy on 4H-SiC (0001̄) with a misorientation angle of 4° towards the [112̄0] direction. We find that when using a 2-step temperature process for the N-polar GaN growth, step bunching is persistent for a wide range of growth rates (7 nm/min to 49 nm/min) and V/III ratios (251 to 3774). This phenomenon is analyzed in terms of anisotropic step-flow growth and the Ehrlich–Schwöebel barrier, and their effects on the surface step height and step width. The N-polar GaN growth is further optimized by using 3-step and 4-step temperature processes and the layers are compared to those using the 2-step temperature process in terms of surface morphology and defect densities. It is shown that a significantly improved surface morphology with a root mean square of 1.4 nm and with low dislocation densities (screw dislocation density of 2.8 × 108 cm−2 and edge dislocation density of 1.3 × 109 cm−2) can be achieved for 4-step temperature process. The optimized growth conditions allow to overcome the step-bunching problem. The results are further discussed in view of Ga supersaturation and a general growth strategy for high-quality N-polar GaN growth is proposed.

Study of Defects and Nano-patterned Substrate Regulation Mechanism in AlN Epilayers.

The high crystal quality and low dislocation densities of aluminum nitride (AlN) grown on flat and nano-patterned sapphire substrate that are synthesized by the metal-organic chemical vapor deposition (MOCVD) method are essential for the realization of high-efficiency deep ultraviolet light-emitting diodes. The micro-strains of 0.18 × 10-3 cm-2 for flat substrate AlN and 0.11 × 10-3 cm-2 for nano-patterned substrate AlN are obtained by X-ray diffractometer (XRD). The screw and edge dislocation densities of samples are determined by XRD and transmission electron microscope (TEM), and the results indicate that the nano-patterned substrates are effective in reducing the threading dislocation density. The mechanism of the variation of the threading dislocation in AlN films grown on flat and nano-patterned substrates is investigated comparatively. The etch pit density (EPD) determined by preferential chemical etching is about 1.04 × 108 cm-2 for AlN grown on a nano-patterned substrate, which is slightly smaller than the results obtained by XRD and TEM investigation. Three types of etch pits with different sizes are all revealed on the AlN surface using the hot KOH etching method.

Hot-Wall MOCVD for High-Quality Homoepitaxy of GaN: Understanding Nucleation and Design of Growth Strategies

Thick GaN layers with a low concentration of defects are the key to enable next-generation vertical power electronic devices. Here, we explore hot-wall metalorganic chemical vapor deposition (MOCVD) for the development of GaN homoepitaxy. We propose a new approach to grow high-quality homoepitaxial GaN in N2-rich carrier gas and at a higher supersaturation as compared to heteroepitaxy. We develop a low-temperature GaN as an optimum nucleation scheme based on the evolution and thermal stability of the GaN surface under different gas compositions and temperatures. Analysis in the framework of nucleation theory of homoepitaxial layers simultaneously grown on GaN templates on SiC and on hydride vapor phase epitaxy GaN substrates is presented. We show that residual strain and screw dislocation densities affect GaN nucleation and subsequent growth leading to distinctively different morphologies of GaN homoepitaxial layers grown on GaN templates and native substrates, respectively. The established comprehensive picture provides a guidance for designing strategies for growth conditions optimization in GaN homoepitaxy. GaN with atomically flat and smooth epilayer surfaces with a root-mean-square roughness value as low as 0.049 nm and low background carbon concentration of 5.3 × 1015 cm–3 has been achieved. It is also shown that there is no generation of additional dislocations during homoepitaxial growth. Thus, our results demonstrate the potential of the hot-wall MOCVD technique to deliver high-quality GaN material for vertical power devices.

Role of low temperature Al(Ga)N interlayers on the polarity and quality control of GaN epitaxy

Polarity control is essential for lateral polarity heterostructures. N-polar GaN films were directly grown on high temperature AlN buffer layer. A thin low temperature AlN interlayer could invert the polarity of subsequent GaN layer to Ga-polarity but a low temperature GaN interlayer with the same condition could not do it rather deteriorate the quality of GaN film. In addition, a relationship between the yellow luminescence band and the surface roughness was discovered. The blue luminescence band was found only on the inverted Ga-polar GaN film and was connected to screw dislocation density.

Influence of a Two-Dimensional Growth Mode on Electrical Properties of the GaN Buffer in an AlGaN/GaN High Electron Mobility Transistor.

An extensive study has been conducted on a series of AlGaN/GaN high electron mobility transistor (HEMT) samples using metalorganic vapour phase epitaxy, to investigate the influence of growth modes for GaN buffer layers on device performance. The unintentional doping concentration and screw dislocation density are significantly lower in the samples grown with our special two-dimensional (2D) growth approach, compared to a widely-used two-step method combining the 2D and 3D growth. The GaN buffer layers grown by the 2D growth approach have achieved an unintentional doping density of 2 × 1014 cm−3, two orders lower than 1016 cm−3 of the GaN samples grown using a conventional two-step method. High-frequency capacitance measurements show that the samples with lower unintentional doping densities have lower buffer leakage and higher breakdown limits. This series of samples have attained sub-nA/mm leakages, a high breakdown limit of 2.5 MV/cm, and a saturation current density of about 1.1 A/mm. It indicates that our special 2D growth approach can effectively lessen the unintentional doping in GaN buffer layers, leading to low buffer leakage and high breakdown limits of GaN/AlGaN HEMTs.

A study on MOCVD growth window for high quality N-polar GaN for vertical device applications

Lateral N-polar GaN devices have outperformed Ga-polar GaN counterparts in mm-wave applications. A vertical N-polar GaN device structure with higher breakdown voltage is expected to further improve the output power density. This paper is aimed at understanding the growth conditions for such a vertical N-polar GaN device stack. A vertical device stack requires high-quality n-GaN layer with low unintentional dopant concentration and high electron mobility for better ON resistance–breakdown voltage performance. A systematic study by varying reactor temperature, pressure and NH3 flow rate is performed in the metal organic chemical vapor deposition growth of N-polar GaN for vertical device structure. Structural quality of the sample is increased with increase growth temperature and higher NH3 flow rate, yet an increase in surface roughness is observed for highest temperature used. The N-polar GaN grown at the optimized conditions exhibited a x-ray diffraction rocking curve full width half maximum of symmetric (0002) and skew-symmetric (20 ) scans of 358″ and 569″ respectively with a low oxygen and carbon concentration of 6 × 1016 cm−3 and 5 × 1016 cm−3, respectively. The optimized sample exhibited a low screw dislocation density of 2.5 × 108 cm−2 on sapphire substrate. A gradual increase in the bulk mobility of electrons with decrease in screw dislocation density in N-polar GaN is also reported.

Study of residual stress in reactively sputtered epitaxial Si-doped GaN films

Si-doped GaN films were grown epitaxially on c-sapphire by rf magnetron reactive co-sputtering of GaAs and Si at various partial pressures of N2 in Ar–N2 growth atmosphere. Energy dispersive x-ray spectroscopy revealed ∼2 at.% Si in all the films, but the N/Ga ratio decreased substantially as N2 percentage was reduced from 100% to 10%. The lattice parameters (a and c) were obtained independently to determine the in-plane and out-of-plane components of strain in films, which were analyzed to deduce the hydrostatic and biaxial strain contributions. High resolution x-ray diffraction revealed the dominant presence of edge dislocations (∼1012 cm−2) in the films grown at 30%–100% N2, which decreases and becomes comparable to the density of screw dislocations (∼1011 cm−2) at lower N2 percentages. The films grown at 75%–100% N2 displayed compressive biaxial stress along with large hydrostatic strain and micro-strain due to the incorporation of nitrogen at interstitial locations and grain boundaries. Both hydrostatic strain and micro-strain decrease in the films grown below 75% N2, in which, a reversal of biaxial stress to tensile is seen due to the prevalence of in-plane tensile stress generated during coalescence. The decrease of tensile stress in films grown below 30% N2, accompanied by increase in micro-strain and hydrostatic strain are ascribed to Ar incorporation and voided morphology. The presence of Si does not have a significant influence on residual stress of films and appears to be masked by the dominant growth-related intrinsic effects.

200 mm-scale growth of 2D layered GaSe with preferential orientation

In this article, we present a fab-compatible metal–organic chemical vapor deposition growth process, realized in a hydrogen ambience, of two-dimensional (2D) layered GaSe on 200 mm diameter Si(111) wafers. Atomic scale characterization reveals initial stages of growth consisting of passivation of the H–Si (111) surface by a half-monolayer of GaSe, followed by nucleation of 2D-GaSe from the screw dislocations located at the step edges of the substrate. We, thus, demonstrate that by using a Si wafer that is slightly misoriented toward [1̄1̄2], the crystallographic orientation of 2D-GaSe can be step-edge-guided. It results in a coalesced layer that is nearly free from antiphase boundaries. In addition, we propose a sequential process to reduce the density of screw dislocations. This process consists in a subsequent regrowth after partial sublimation of the initially grown GaSe film. The local band bending in GaSe near the antiphase boundaries measured by Kelvin probe force microscopy emphasizes the electrical activity of these defects and the usefulness of having a nearly single-orientation film. Such a low defectivity layer opens up the way toward large-scale integration of 2D-optical transceivers in Si CMOS technology.

Suppressing the zincblende-phase inclusions in nitrogen-polar wurtzite-phase InGaN films by pulsed metalorganic chemical vapor deposition

Nitrogen-polar (N-polar) wurtzite (WZ)-phase InGaN films suffer from severe zincblende (ZB)-phase inclusions and thus present a rough surface, which severely restricts its application in GaN-based optoelectronic devices. In this paper, several different pulsed modes were adopted to grow N-polar InGaN films with the purpose of suppressing the ZB-phase inclusions, and their effects on the surface morphology and crystal quality of the InGaN films were studied in detail. Our results demonstrate that the pulsed mode of continuously feeding In source while alternately injecting N and Ga sources can effectively suppress the formation of ZB-phase inclusions, due to the increased mobility of group-III adatoms on the N-polar surface. Benefiting from the suppression of the ZB-phase inclusions, the surface roughness of InGaN films is significantly reduced. Meanwhile, the screw dislocation density of the InGaN decreases from 1.4 ​× ​109 to 6.0 ​× ​108 ​cm−2. This work provides an approach to the growth of high-quality N-polar InGaN films free of ZB-phase inclusions for N-polar InGaN-based devices.

Growth of High Mobility InN Film on Ga‐Polar GaN Substrate by Molecular Beam Epitaxy for Optoelectronic Device Applications

AbstractThe fabrication of high‐speed electronic and communication devices has rapidly grown the demand for high mobility semiconductors. However, their high cost and complex fabrication process make them less attractive for the consumer market and industrial applications. Indium nitride (InN) can be a potential candidate to fulfill industrial requirements due to simple and low‐cost fabrication process as well as unique electronic properties such as narrow direct bandgap and high electron mobility. In this work, 3 µm thick InN epilayer is grown on (0001) gallium nitride (GaN)/Sapphire template under In‐rich conditions with different In/N flux ratios by molecular beam epitaxy. The sharp InN/GaN interface monolayers with the In‐polar growth are observed, which assure the precise control of the growth parameters. The directly probed electron mobility of 3610 cm2 V‐1 s‐1 is measured with an unintentionally doped electron density of 2.24 × 1017 cm‐3. The screw dislocation and edge dislocation densities are calculated to be 2.56 × 108 and 0.92 × 1010 cm‐2, respectively. The step‐flow growth with the average surface roughness of 0.23 nm for 1 × 1 µm2 is confirmed. The high quality and high mobility InN film make it a potential candidate for high‐speed electronic/optoelectronic devices.

Several cracks in a rectangular bar reinforced by a piezoelectric layer subjected to torsion

This paper provides the torsional solution of multiple arbitrarily shaped cracks in an isotropic rectangular cross-section coated by a piezoelectric layer. A uniform electric displacement is applied on the left and right edges of the piezoelectric layer. First, the stress components and the warping function of the rectangular cross-section bar with the piezoelectric coating containing a Volterra-type screw dislocation are found in terms of the screw dislocation density function using Fourier integral transform. The dislocation solution is then reduced to a set of integral equations with a singularity of the Cauchy-type to examine multiple arbitrarily shaped cracks situated in the coated rectangular bar subjected to combining torsion and the forced electric displacement by making use of the distributed dislocation technique (DDT). The numerical solution of the integral equations is presented to calculate stress intensity factors (SIFs) and torsional stiffness. Next, several examples are presented to evaluate the effects of significant parameters on the fracture behavior of the problem. Furthermore, a novel idea is presented to reduce stress intensity factors to a minimum through suitable values of the uniform electric displacement on the left and right edges of the piezoelectric coating.

Growth and crystal phase transformation of ε-Ga2O3 grown on 4H–SiC by MOCVD

Ga2O3 thin films were hetero-epitaxially grown on 4H–SiC substrates by metal-organic chemical vapor deposition (MOCVD) under various growth conditions. The growth process and crystal quality of the ε-Ga2O3 thin films grown on C-face and Si-face of 4H–SiC substrates were compared. The ε-Ga2O3 film grown on a Si-face substrate shows better performance in improving crystal quality compared to the case of growth on a C-face. A ε-Ga2O3 thin film grown at 665 °C with an H2O flow rate of 200 sccm was thermally annealed at 900 °C for 10 min in an oxygen atmosphere for a crystal phase transformation to a β-Ga2O3 thin film. The RMS roughness of the transformed β-Ga2O3 thin film was 9.023 nm while the RMS value of directly grown was 129.1 nm. The estimated screw and edge dislocation density of the ε-Ga2O3 thin film were 1.41 × 1013 cm−2 and 4.92 × 1012 cm−2 respectively. Transformation of the crystal phase from ε-Ga2O3 to β-Ga2O3 can be considered as an effective method in hetero-epitaxial growth of Ga2O3 on SiC substrate.

A structural analysis of ultrathin barrier (In)AlN/GaN heterostructures for GaN‐based high‐frequency power electronics

Metal–organic chemical vapor deposition (MOCVD) is one of the best growth methods for GaN‐based materials as well‐known. GaN‐based materials with very quality are grown the MOCVD, so we used this growth technique to grow InAlN/GaN and AlN/GaN heterostructures in this study. The structural and surface properties of ultrathin barrier AlN/GaN and InAlN/GaN heterostructures are studied by X‐ray diffraction (XRD) and atomic force microscopy (AFM) measurements. Screw, edge, and total dislocation densities for the grown samples have been calculated by using XRD results. The lowest dislocation density is found to be 1.69 × 108 cm−2 for Sample B with a lattice‐matched In0.17Al0.83N barrier. The crystal quality of the studied samples is determined using (002) symmetric and (102) asymmetric diffractions of the GaN material. In terms of the surface roughness, although reference sample has a lower value as 0.27 nm of root mean square values (RMS), Sample A with 4‐nm AlN barrier layer exhibits the highest rough surface as 1.52 nm of RMS. The structural quality of the studied samples is significantly affected by the barrier layer thickness. The obtained structural properties of the samples are very important for potential applications like high‐electron mobility transistors (HEMTs).

Structural, electrical, and luminescence properties of (0001) ZnO epitaxial layers grown on c-GaN/sapphire templates by pulsed laser deposition technique

A systematic study of growth, structural, electrical, and luminescence properties of zinc oxide (ZnO) layers grown on c-oriented GaN/sapphire templates by the pulsed laser deposition technique is carried out. A thorough high-resolution x-ray diffraction study reveals that c-ZnO films with high crystalline quality can be grown under certain growth conditions. Screw and edge dislocation densities in these films are found to be as low as 7×108 and 3×1010cm−2, respectively. All layers are found to be unintentionally n-type with ∼1019cm−3 electron concentration and mobility as high as ∼50 cm2 V−1 s−1. Temperature and excitation intensity dependent photoluminescence (PL) studies as functions of the growth conditions are carried out to identify the transition processes behind various luminescence features found in these samples. At low temperatures, PL spectra are marked by sharp neutral donor bound excitonic transitions, their phonon replicas, and two broad luminescence bands at 2.2 and 2.9 eV. These broad bands are attributed to transitions from the (2+/0) oxygen vacancy (VO) and (2+/+/0) zinc-interstitial (Zni) levels, respectively, to the valence band. Thermal energy needed to depopulate these defects is found to be 11 and 385 meV, respectively, for the (2+/0) VO and (2+/+/0) Zni levels. Low temperature PL spectra for the samples grown with relatively high oxygen pressures are featured by the Zn-vacancy (VZn) related neutral acceptor bound excitonic transition, its phonon replicas, and a broad band at 2.75 eV. This band diminishes with increasing temperature and, instead, another broad feature appears at ∼2.1 eV. Our study attributes the 2.75 eV band to transition from the conduction band to (0/−) VZn levels and the 2.1 eV feature to the transition between (−/2−) VZn levels and the valence band. It has been found that all the defect related features can be minimized by adjusting the growth conditions.

Dislocation density control of GaN epitaxial film and its photodetector

At present, GaN-based ultraviolet photodetectors (UV PDs) is confronted with the challenge of high dark current and low responsivity. In this work, we designed and prepared high-performance GaN-based UV PDs on the sapphire substrates by the combination of low-temperature pulsed laser deposition (LT-PLD) and high-temperature metal-organic chemical vapor deposition growth (HT-MOCVD). The as-grown GaN film shows high-crystalline with edge dislocation density of 6.50 × 107 cm−2 and screw dislocation density of 1.92 × 108 cm−2, respectively. Furthermore, the optimized electrode structure has been well designed to enhance the carrier transport characteristics in GaN-based UV PDs, which improves the performance of GaN-based UV PDs accordingly. Based on the high-quality GaN film and the optimized electrode structure, high-performance GaN-based UV PDs are fabricated, which reveals the very high responsivity and small dark current of 1.4 A/W and 4.8 nA@5V, respectively. These high-performance GaN-based UV PDs shed light on the potential application in UV warning.

Effect of hydrogen on evolution of deformation microstructure in low-carbon steel with ferrite microstructure

In this study, the deformation microstructure of hydrogen-charged ferritic-pearlitic 2Mn-0.1C steel was characterized using SEM-BSE, SEM-EBSD, TEM, and neutron diffraction. The microscopic mechanism of hydrogen-related quasi-cleavage fracture along the {011} planes was also discussed. It was found that hydrogen increased the relative velocity of screw dislocations to edge dislocations, leading to a tangled dislocation morphology, even at the initial stage of deformation (e = 3%). In addition, the density of screw dislocations at the later stage of deformation (e = 20%) increased in the presence of hydrogen. Based on the experimental results, it is proposed that a high density of vacancies accumulated along {011} slip planes by jog-dragging of screw dislocations, and coalescence of the accumulated vacancies led to the hydrogen-related quasi-cleavage fracture along the {011} slip planes.

The role of AlN thickness in MOCVD growth of N-polar GaN

The influence of the thickness of aluminum nitride (AlN) on the surface morphology, crystalline quality, and polarity of N-polar gallium nitride (GaN) grown by metal organic chemical vapor deposition (MOCVD) is investigated in this study. With a medium temperature (1000 ℃) AlN buffer layer of 150 nm, a hexagonal-defect-free N-polar GaN thin film with low dislocation densities (4.2 × 108 cm-2 for screw dislocation density and 3.7 × 109 cm-2 for edge dislocation density) is obtained on a 2-in. vicinal sapphire substrate (c off m plane 2°). With the reduction of the AlN thickness to 80 nm, the N-polar GaN surface becomes rough and the crystalline quality is exacerbated. While the thickness of AlN is increased to 200 nm, the polarity of GaN is changed to Ga-polar. The transmission electron microscopy (TEM) results reveal an undulated interface between GaN and AlN with a large density of pyramidal defects which may contribute to the polarity inversion of GaN. The controllable polarity inversion through changing the thickness of AlN provides a promising method for the polarity engineering of nitride-based devices.

The influence of residual GaN on two-step-grown GaN on sapphire

In this paper, it is found that the residual GaN has a great impact on the two-step-grown GaN on sapphire substrates. The samples which are grown with a conventional system baking process present a higher dislocation density than those without the baking process. It is found that, without baking, residual GaN accumulate in the MOCVD reactor, decompose, form the nucleation site on the sapphire substrate during the process of high-temperature surface treatment, and thus influence the initial nucleation process of GaN layer growth. Based on this understanding, a novel growth method to improve the quality and stability of two-step-grown GaN on sapphire is proposed, in which the TMGa and NH3 were added in the high-temperature surface treatment process. With this method, as low as 3.71 × 107 cm−3 and 1.94 × 108 cm−3 for screw and edge dislocation density of GaN epitaxial films were achieved on sapphire.