In-plane Compressive Strain Research Articles

Spin-phonon interaction increased by compressive strain in antiferromagnetic MnO thin films

MnO thin films with various thicknesses and strains were grown on MgO substrates by pulsed laser deposition technique, then characterized using x-ray diffraction and infrared reflectance spectroscopy. Films grown on (0 0 1)-oriented MgO substrates exhibit homogenous biaxial compressive strain which increases as the film thickness is reduced. For that reason, in paramagnetic phase, the frequency of doubly-degenerate phonon increases with the strain, and splits below Néel temperature TN due to the magnetic-exchange interaction. The phonon splitting in the MnO (0 0 1) films is 20% larger than that of the bulk MnO. Films grown on (1 1 0)-oriented MgO substrates exhibit a huge phonon splitting already at room temperature due to the anisotropic in-plane compressive strain. Below TN, the lower-frequency phonon splits in the IR spectra and the higher-frequency phonon strongly hardens in AFM phase; these features are evidences for a spin-order-induced structural phase transition from tetragonal to a lower symmetry phase. Total phonon splitting is 55 cm−1 in (1 1 0)-oriented MnO film, which is more than twice the value in bulk MnO, but since the splitting is present already in paramagnetic phase, we cannot clearly correlate it with the value of exchange coupling constant. Nevertheless, at least observation of enhanced phonon splitting in strained MnO (0 0 1) films show that the exchange coupling could be enhanced by the compressive strain which supports recent theoretical predictions published by Wan et al (2016 Sci. Rep. 6 22743) and Fischer et al (2009 Phys. Rev. B 80 014408).

Strain modulated magnetocaloric effect in (111) oriented La0.7Sr0.3MnO3–SrRuO3 superlattices

Magnetocaloric effect in (111)-oriented La0.7Sr0.3MnO3-SrRuO3 (LSMO-SRO) superlattices grown with both the stacking orders by reversing the individual layer thickness on (111)-oriented SrTiO3(STO) substrates using the pulsed laser deposition technique has been studied. Pseudomorphic growth with 0.64% in-plane tensile strain in [11 unit cell (u.c.)SRO/3u.c.LSMO]×15 superlattice is favourable for a larger change in entropy (ΔSM) as compared to relaxed growth with in-plane compressive strain in [11u.c.LSMO/3u.c.SRO]×15 superlattice. The reduction of ΔSM in [11u.c.LSMO/3u.c.SRO]×15 could be due to the orientation-dependent in-phase and out-of-phase tilt of the unit cell between ±1° along the 103pc of the {103}pc, which softens the exchange coupling and leads to the faster alignment of the magnetization near the Curie temperature (TC). Stabilization of the orthorhombic phase of LSMO in the superlattices with both stacking orders is evidenced from the existence of anomaly around the TC of LSMO and SRO in the temperature-dependent phonon frequency shifts. Reduction in symmetry of LSMO from the rhombohedral to orthorhombic structure modulates the Mn-O-Mn bond length and angles, which induces the spin reorientations and hence, modifies the electronic and magnetic properties in these LSMO-SRO superlattices. The ΔSM of these superlattices suggest that the strain, magnitude of the magnetic field, volume and magnetization of the ferromagnet can control the magnetocaloric effect. These results will be useful for designing the magnetic entropy based devices to improve renewable energy systems.

Emergent multiferroicity and strain-driven metal-semiconductor transitions in LaMnO3/RMnO3 superlattices (R = Pr, Pm, Sm and Gd).

It is known that rare-earth manganites LnMnO3 with Ln = La to Gd are typical Mott insulators favoring the A-type antiferromagnetic (A-AFM) state. Certainly no ferroelectricity can be possible although the alternatively stacked LnO layers are both polar. Nevertheless, under the inspiration that one plus one is more than two, it is appreciated that by combining two components of this manganite series into a superlattice functionality is added. In this work, we construct a (001)-oriented LaMnO3/RMnO3 (R = Pr, Pm, Sm and Gd) superlattice and investigate the possible emergent ferroelectricity by means of first-principles calculations. It is revealed that the lattice matching in these superlattices may generate lattice distortions to each component based on the scenario of hybrid improper ferroelectricity, resulting in spontaneous ferroelectric polarization, which is larger than the traditional type II Ln'MnO3 (Ln' radius is smaller than that of Gd) polarization. In the meantime, the A-AFM state remains the magnetic ground state of these superlattices. Furthermore, it is predicted that the externally imposed in-plane compressive strain can trigger the semiconductor to half-metal transitions accompanying the A-AFM to ferromagnetic (FM) transitions. The present work sheds light on the possibility to design multiferroic materials and functionality by tailoring artificial superlattices/heterostructures from those non-ferroelectric systems, and to design electronic devices by utilizing the electronic transport properties under epitaxial strain.

First-principles studies of spin-phonon coupling in monolayer Cr2Ge2Te6

We perform systematic first-principles calculations to investigate the spin-phonon coupling (SPC) of Cr2Ge2Te6 (CGT) monolayer (ML). It is found that the Eg phonon mode at 211.8 cm^{-1} may have a SPC as large as 3.19 cm^{-1}, as it directly alters the superexchange interaction along the Cr-Te-Cr pathway. Furthermore, the strength of SPC of the CGT ML can be further enhanced by an in-plane compressive strain. These results provide useful insights for the understanding of SPC in novel two-dimensional magnetic semiconductors and may guide the design of spintronic and spin Seebeck materials and devices.

Magnetotransport of SrIrO3 films on (110) DyScO3

Epitaxial perovskite (110) oriented SrIrO3 (SIO) thin films were grown by pulsed laser deposition on (110) oriented DyScO3 (DSO) substrates with various film thickness t (2 nm < t < 50 nm). All the films were produced with stoichiometric composition, orthorhombic phase, and with high crystallinity. The nearly perfect in-plane lattice matching of DSO with respect to SIO and same symmetry result in a full epitaxial in-plane alignment, i.e., the c-axis of DSO and SIO are parallel to each other with only slightly enlarged d110 out-of-plane lattice spacing (+0.38%) due to the small in-plane compressive strain caused by the DSO substrate. Measurements of the magnetoresistance MR were carried out for current flow along the [001] and [1-10] direction of SIO and magnetic field perpendicular to the film plane. MR appears to be distinctly different for both directions. The anisotropy MR001/MR1-10 > 1 increases with decreasing T and is especially pronounced for the thinnest films, which likewise display a hysteretic field dependence below T* ≈ 3 K. The coercive field Hc amounts to 2-5 T. Both, T* and Hc are very similar to the magnetic ordering temperature and coercivity of DSO which strongly suggests substrate-induced mechanism as a reason for the anisotropic magnetotransport in the SIO films.

Configurations evolution of a buckled ribbon in response to out-of-plane loading

Classical Euler buckled ribbon has become the cornerstone of many unprecedented applications such as the recently developed stretchable electronics and mechanical-guided 3D assembly. In practical applications, such Euler buckled ribbon may be subjected to out-of-plane loading. However, previous studies mostly concern about the buckling and post-buckling configurations of ribbons under in-plane compression, but the further mechanical behavior of buckled ribbons in response to out-of-plane loading is rarely involved. In this paper, the mechanical behaviors of Euler buckling ribbons under out-of-plane loading are systemically investigated, and distinct configurations evolution paths were observed, which are dependent on both axial in-plane compressive strain of Euler buckling and out-of-plane loading. An analytical model based on geometrical decomposition method and finite deformation beam theory is proposed to clarify the mechanism of the competitions between different configurations evolution paths, to construct phase diagrams to distinguish different deformation modes, and to draw the corresponding deformed profiles, which agree well with the experiments and finite elemental stimulations. Furthermore, the out-of-plane stiffness of Euler buckled ribbons with unusual characteristics (e.g., negative stiffness and stiffness reduction) is also quantified, which has general utility as an engineering design rule for exploiting Euler buckled ribbons in stretchable electronics, 3D bio-interfaces, mechanical-guided 3D assembly and mechanical metamaterials.

Presence of a purely tetragonal phase in ultrathin BiFeO3 films: Thermodynamics and phase-field simulations

The stability of a purely tetragonal phase relative to the nominal rhombohedral phase in ultrathin BiFeO3 films is investigated using thermodynamics and phase-field simulations. The thermodynamic analysis demonstrates the possible presence of a purely tetragonal state primarily due to the interfacial effect from the constraint of the adjacent layer although the built-in potential and compressive in-plane strain also play a role. Phase-field simulations of the corresponding ultrathin films reveal the coexistence of tetragonal and rhombohedral phases at certain film thickness arising from strain phase separation. It is shown that the piezoelectric coefficient d33 of the two-phase mixture is up to 200% higher than that of the rhombohedral single phase.

Topological band crossings in epitaxial strained SnTe

Epitaxial SnTe (111) is grown by molecular-beam epitaxy on $\mathrm{B}{\mathrm{i}}_{2}\mathrm{T}{\mathrm{e}}_{3}$ substrates. Structural evaluation indicates that SnTe deviates from cubic due to in-plane compressive strain, which induces significant changes in the electronic band structure. More specifically, a pair of gapless crossings between the two uppermost valence bands occurs in $k$ space along the out-of-plane \ensuremath{\Gamma}Z direction of the Brillouin zone, associated with a band inversion, thus defining topological three-dimensional Dirac nodes. Combined first-principles calculations and angle-resolved photoelectron spectroscopy reveal an overtilted Dirac cone indicating that the crossing is a topological type-III Dirac node at the borders between type-I and type-II Dirac nodes.

Interface structure of Pb(Zr,Ti)O3/MgO(001) epitaxial thin film in early stage of Stranski–Krastanov growth mode

This study investigated structural changes of Pb(Zr,Ti)O3/MgO(001) thin films prepared using MOCVD in the early stage of the Stranski–Krastanov growth mode. The strain state of the interface layer and the island region of PZT changes accompanied by a downward climb motion of misfit dislocations into the PZT/MgO interface with crystal growth. The local strain field around the misfit dislocation core formed in-plane compressive strain in the in-plane direction in the interface layer, which was strained tetragonally, but the island region had a pseudocubic structure. These results suggest that the residual strain induced phase stability modulation as well as Zr/Ti compositional gradient. A proposed model structure involves consequently dislocations with 〈110〉MgO slip on MgO can account for a stable interface without stacking fault. These results indicate the misfit strain-induced modulation of the interface structure and the phase stability in the early stage of the crystal growth on an atomic scale.

(Invited) (Si)GeSn Semiconductors for Integrated Optoelectronics, Quantum Electronics, and More

Sn-containing group IV semiconductors (Si)GeSn represent a versatile platform to implement a variety of Si-compatible photonic, optoelectronic, and photovoltaic devices. This class of semiconductors provides two degrees of freedom, strain and composition, to tailor the band structure and lattice parameter thus enabling a variety of heterostructures and low-dimensional systems on a Si substrate. In this presentation, we will discuss the recent progress in the growth of metastable (Si)GeSn semiconductors. We will highlight the current understanding of their fundamental properties and elucidate the effects of the atomistic-level structure on their optoelectronic performance. The relevance of these semiconductors for Si-compatible mid-infrared optoelectronics and quantum information will also be discussed form both materials and device perspectives. The (Si)GeSn multi-layer heterostructures discussed in this work have a Sn content reaching ~20at.% grown in a chemical vapor deposition (CVD) reactor on 4-inch Silicon wafers, using Monogermane (GeH4) and tin-tetrachloride (SnCl4) precursors, and disilane (Si2H6). By reducing the growth temperature, the Sn content in the alloy is increased, while preserving a high degree of crystal quality for the heterostructure in the topmost Sn-rich layer. No threading dislocations are observed in the upper layers, while misfit and edge dislocations remain mainly confined in the low Sn content layers underneath and in the proximity with the Ge-GeSn interface. By comparing multiple sample series with different strain and composition we demonstrate that the growth rate and the choice of the Ge hydride gas precursor play a limited role in enhancing the incorporation of Sn in the Ge lattice. However, strain minimization and the reduced growth temperature below 350 °C are of paramount importance to achieve Sn-rich GeSn semiconductors.[2,3] Atom probe tomography (APT) measurements will be discussed to address the abruptness of the interfaces in the GeSn multi-layer heterostructure and the composition uniformity.[2] The absence of short-range atomic ordering and Sn precipitates is estimated from the extended statistical APT analyses. Positron annihilation lifetime spectroscopy (PAS) and depth-profiled Doppler broadening measurements will be presented and discussed to elucidate the behavior of point defects in these semiconductors. Based on these analyses, we found that divacancies are the predominant type of point defects in GeSn.[4] Surprisingly, the increase in Sn content in the alloy yields an increase in the concentration of divacancies together with a small reduction in vacancy clusters. The interaction and possible pairing between Sn and vacancies have been proposed to explain the reduced formation of larger vacancy clusters in GeSn with higher Sn content. Photoluminescence (PL) emission studies will be described. In our GeSn with a Sn content of 18at.% the room temperature PL emission was found to be centered at 0.36 eV (i.e. 3.5 μm wavelength).[2] However, the compressive in-plane strain (-1.3 %) in these GeSn layers reduces the directness of the alloy, leading to a higher energy gap value. By releasing the strain down to -0.2% in the 18 at.% Sn layer using a fully-underetched micro-disk geometry, a 50 meV red-shift of the PL emission energy down to 0.31 eV (i.e. 4.0 μm wavelength) is obtained. Moreover, the strained and relaxed PL emission and reflectance measurements ranging from 300K down to 4K will be shown. These observations will be discussed in the light of photocurrent measurements and photodetector performance. Finally, we will also discuss the use of GeSn as a platform to develop highly tensile strained Ge quantum wells (QWs. These low-dimensional systems, made possible by the availability of GeSn templates, create a wealth of opportunities to design and fabricate new quantum devices. For instance, unlike the compressively strained QWs, the top of the valence band is of light hole (LH) type under tensile strain, thus corresponding to a much smaller effective mass and ½ spin. These characteristics, combined with high hole mobility and strong spin-orbit coupling in Ge, make tensile strained Ge QWs more attractive for hole spin qubits. Acknowledgements The authors thank J. Bouchard for the technical support with the CVD system. O.M. acknowledges support from NSERC Canada (Discovery, SPG, and CRD Grants), Canada Research Chairs, Canada Foundation for Innovation, Mitacs, PRIMA Québec, and Defence Canada (Innovation for Defence Excellence and Security, IDEaS).

Actomyosin controls planarity and folding of epithelia in response to compression.

Throughout embryonic development and adult life, epithelia are subjected to compressive deformations. While these have been shown to trigger mechanosensitive responses such as cell extrusion and differentiation, which span tens of minutes, little is known about how epithelia adapt to compression over shorter timescales. Here, using suspended epithelia, we uncover the immediate response of epithelial tissues to the application of in-plane compressive strains (5-80%). We show that fast compression induces tissue buckling followed by actomyosin-dependent tissue flattening that erases the buckle within tens of seconds, in both mono- and multi-layered epithelia. Strikingly, we identify a well-defined limit to this response, so that stable folds form in the tissue when compressive strains exceed a 'buckling threshold' of ~35%. A combination of experiment and modelling shows that this behaviour is orchestrated by adaptation of the actomyosin cytoskeleton as it re-establishes tissue tension following compression. Thus, tissue pre-tension allows epithelia to both buffer against deformation and sets their ability to form and retain folds during morphogenesis.

Real-space exciton distribution in strained-siligraphene g-SiC7

Siligraphene belonging to the family of two-dimensional (2D) materials has great potential in optoelectronics due to its considerable excitonic effects. In this study, the strain effects on the electronic structure and the real-space exciton wave functions of g-SiC7 are investigated using the first-principles calculations based on the ab initio many-body perturbation theory. Alongside the increase (decrease) of the bandgap with compressive (tensile) strain, our results show that the exciton in the siligraphene monolayer under in-plane biaxial compressive strains is much more localized than that in the case of tensile one, leading to the higher and lower exciton binding energies, respectively. Moreover, the π↦π and π↦σ exciton state transition emerges when applying the compressive and tensile strains, respectively. Overall, our study reveals that a desirable way to dissociate the electron-hole coupling and to reduce the electron-hole recombination process is applying “in-plane biaxial tensile strain,” making g-SiC7 an excellent potential functional 2D semiconductor in optoelectronics.

Strain modulated nanostructure patterned AlGaN-based deep ultraviolet multiple-quantum-wells for polarization control and light extraction efficiency enhancement

AlGaN-based deep ultraviolet (DUV) multiple-quantum-wells (MQWs) incorporating strain-modulated nanostructures are proposed, demonstrating enhanced degree of polarization (DOP) and improved light extraction efficiency (LEE). The influence of Al composition and bi-axial strains on the optical behaviors of the DUV-MQWs were carefully examined. Compared with planar DUV-MQWs, strain-modulated nanostructure patterned MQWs show three times higher photoluminescence and increased DOP from −0.43 to −0.16. Moreover, nanostructure patterned DUV-MQWs under compressive strains further illustrate higher DOP and LEE values than those under tensile strains due to more efficient diffraction of the guided modes of the transverse electric (TE) polarized light. Our work demonstrates, for the first time, that a combination of compressive in-plane strain and surface nanostructure show unambiguous advantages in facilitating TE mode emission, thus have great promises in the design and optimization of highly efficient polarized DUV optoelectronic devices.

Investigation of MOVPE-grown zincblende GaN nucleation layers on 3C-SiC/Si substrates

Cubic zincblende (zb-)GaN nucleation layers (NLs) grown by MOVPE on 3C-SiC/Si substrates were studied to determine their optimal thickness for subsequent zb-GaN epilayer growth. The layers were characterised by atomic force microscopy, X-ray diffraction and scanning transmission electron microscopy. The as-grown NLs, with nominal thicknesses varying from 3 nm to 44 nm, consist of small grains which are elongated in the [1 −1 0] direction, and cover the underlying SiC surface almost entirely. Thermal annealing of the NLs by heating in a H2/NH3 atmosphere to the elevated epilayer growth temperature reduces the substrate coverage of the films that are less than 22 nm thick, due to both material desorption and the ripening of islands. The compressive biaxial in-plane strain of the NLs reduces with increasing NL thickness to the value of relaxed GaN for a thickness of 44 nm. Both the as-grown and annealed NLs are crystalline and have high zincblende phase purity, but contain defects including misfit dislocations and stacking faults. The zb-GaN epilayers grown on the thinnest NLs show an enhanced fraction of the wurtzite phase, most likely formed by nucleation on the exposed substrate surface at elevated temperature, thus dictating the minimum NL thickness for phase-pure zb-GaN epilayer growth.

Growth temperature and relaxation of lattice strain in epitaxial Pt films exhibiting diffraction fringes

We fabricated Pt epitaxial films on SrTiO3(111) using a dc-sputtering method. The surface structure, the surface morphology, and the electrochemical properties are discussed. Reflection high-energy electron diffraction (RHEED) observation reveals that epitaxial films are obtained at substrate temperatures higher than 450 °C. The RHEED patterns exhibit clear streaks up to the 1st order Laue zone, which indicates the high crystallinity and the atomically smooth surface. Oscillation in the intensity of the X-ray diffraction pattern is observed near the Pt-111 peak. It is caused by multiple scattering, showing the perfection in the lattice plane stacking along the out-of-plane direction. Analysis using a dynamic theory of diffraction shows that the film is subjected to a compressive in-plane lattice strain. The in-plane lattice parameter decreases by approximately 0.3%. The oscillation amplitude decreases at 600 °C, which suggests that the lattice relaxation occurs caused by misfit dislocation.

Strain tuned electronic and magnetic anisotropy in SrTiO3 (110) surface

In this work, the density functional theory (DFT) calculations are employed to investigate the effect of in-plane strain on the electronic and magnetic anisotropy of SrTiO-terminated SrTiO3 (110) surface. The two-dimensional electron gas (2DEG) at the surface comes from the surface Ti 3d orbitals and exhibits anisotropic transport properties. The ordering of the lowest conduction band states can be tuned by in-plane strain. In addition, the in-plane strain can effectively modify and reduce the electron effective mass of the SrTiO3 (110) surface, and the effect of the in-plane strain on the two directions of [001] and [11¯0] is different. The magnetic moment of the unstrained surface Ti atom is 0.342 μB. The magnetic moment increases monotonically with the increasing of in-plane tensile strain and decreases quickly with increasing of the in-plane compressive strain. Importantly, the in-plane strain can change the magnetic moment of the system as well as its magnetic anisotropy energy and direction of easy magnetization axis. In-plane tensile strain causes a switch of the magnetic easy axis between parallel and perpendicular to the STO (110) surface.

Short range biaxial strain relief mechanism within epitaxially grown BiFeO3

Lattice mismatch-induced biaxial strain effect on the crystal structure and growth mechanism is investigated for the BiFeO3 films grown on La0.6Sr0.4MnO3/SrTiO3 and YAlO3 substrates. Nano-beam electron diffraction, structure factor calculation and x-ray reciprocal space mapping unambiguously confirm that the crystal structure within both of the BiFeO3 thin films is rhombohedral by showing the rhombohedral signature Bragg’s reflections. Further investigation with atomic resolution scanning transmission electron microscopy reveals that while the ~1.0% of the lattice mismatch found in the BiFeO3 grown on La0.6Sr0.4MnO3/SrTiO3 is exerted as biaxial in-plane compressive strain with atomistically coherent interface, the ~6.8% of the lattice mismatch found in the BiFeO3 grown on YAlO3 is relaxed at the interface by introducing dislocations. The present result demonstrates the importance of: (1) identification of the epitaxial relationship between BFO and its substrate material to quantitatively evaluate the amount of the lattice strain within BFO film and (2) the atomistically coherent BFO/substrate interface for the lattice mismatch to exert the lattice strain.

Effects of the AlN nucleation layer thickness on the crystal structures of an AlN epilayer grown on the 6H-SiC substrate

ABSTRACTThe influence of the LT-AlN(NL) growth times on the mosaic structure parameters of the AlN layer grown on the LT-AlN(NL)/6H-SiC structures as well as the dislocation densities and the strain behaviours in the AlN epilayers has been investigated using XRD measurements. The growth times of the LT-AlN(NL) were changed to 0, 60, 120, 180, and 240 s. We observed that the mosaic structure parameters of the AlN epilayers were slightly affected by the LT-AlN(NL) growth times. However, the dislocation densities in the AlN layer are affected by the growth times of the LT-AlN(NL) layer. The highest edge dislocation density (5.48 × 1010 ± 2.3 × 109 cm−2) was measured for the sample in which 120 s grown LT-AlN(NL) was used. On the other hand, highest screw type dislocation density (1.21 × 1010 ± 1.7 × 109 cm−2) measured in the sample E that contains 240 s growth LT-AlN(NL). The strain calculation results show that the samples without LT-AlN(NL) suffered maximum compressive in-plane strain (−10.9 × 10−3 ± 1.8 × 10−4), which can be suppressed by increasing the LT-AlN(NL) growth times. The out-of-plane strain also has a compressive character and its values increase with LT-AlN(NL) growth times between 60 and 180 s. Same out-of-plane strain values were measured for the LT-AlN(NL) growth times of 180 and 240 s. Furthermore, the form of the biaxial stress in the AlN epilayer changed from compressive to tensile when the LT-AlN(NL) growth times were greater than 120 s.

Tuning of electronic and optical properties in ZnX (X=O, S, Se and Te) monolayer: Hybrid functional calculations

The structural, electronic and optical properties of graphene-like ZnX (X=O, S, Se and Te) are investigated in the framework of the density functional theory. Calculating strain energy exhibits that all compound has an asymmetric behavior with respect to applied biaxial strain. The electronic results indicate that the electronic properties of the considered layered compounds such as energy gap and gap direction can be tuned using exerting biaxial in-plane compressive and tensile strains. It has been shown that both compressive and tensile strains decrease the energy gap of ZnO monolayer. However, for the other compounds, ZnS, ZnSe and ZnTe, the energy gap increases by applying compressive strain while it decreases under tensile strains, respectively. The band gap direction changes by imposing different types strains. The optical results exhibit red shift and blue shift in the optical absorption spectrum for ZnO and ZnS monolayers by exerting tensile and compressive strains, respectively. Our obtained results suggest that these wide gap semiconductors can be good candidate for optoelectronic nano-base device.

Effect of strain relaxation on performance of InGaN/GaN green LEDs grown on 4-inch sapphire substrate with sputtered AlN nucleation layer

Here we demonstrate high-brightness InGaN/GaN green light emitting diodes (LEDs) with in-situ low-temperature GaN (LT-GaN) nucleation layer (NL) and ex-situ sputtered AlN NL on 4-inch patterned sapphire substrate. Compared to green LEDs on LT-GaN (19 nm)/sapphire template, green LEDs on sputtered AlN (19 nm)/template has better crystal quality while larger in-plane compressive strain. As a result, the external quantum efficiency (EQE) of green LEDs on sputtered AlN (19 nm)/sapphire template is lower than that of green LEDs on LT-GaN (19 nm)/sapphire template due to strain-induced quantum-confined Stark effect (QCSE). We show that the in-plane compressive strain of green LEDs on sputtered AlN/sapphire templates can be manipulated by changing thickness of the sputtered AlN NL. As the thickness of sputtered AlN NL changes from 19 nm to 40 nm, the green LED on sputtered AlN (33 nm)/sapphire template exhibits the lowest in-plane compressive stress and the highest EQE. At 20 A/cm2, the EQE of 526 nm green LEDs on sputtered AlN (33 nm)/sapphire template is 36.4%, about 6.1% larger than that of the green LED on LT-GaN (19 nm)/sapphire template. Our experimental data suggest that high-efficiency green LEDs can be realized by growing InGaN/GaN multiple quantum wells (MQWs) on sputtered AlN/sapphire template with reduced in-plane compressive strain and improved crystal quality.