Reduction Of Threading Dislocations Research Articles

Threading Dislocation Reduction of Ge by Introducing a SiGe/Ge Superlattice

Ge growth on Si is essential e.g. for photonics device integration (e.g. Ge photodiode) into Si based CMOS platform (1). However, due to 4.2% of lattice mismatch between Ge and Si, misfit dislocations (MD) and threading dislocations (TD) are introduced for strain relaxation after exceeding critical thickness. In order to realize low-darkcurrent Ge photodiode, improvement of crystallinity is important. High quality Ge growth techniques by combining various annealing techniques (at 800~850oC) are reported (2-3). However, because of low melting point of Ge (937oC), the Ge growth process is typically integrated at the last part of front-end process. Therefore, these annealing processes are affecting device parameters of existing devices. In previous paper, we reported Ge/SiGe superlattice (SL) fabrication and found improvement of TD density (TDD) by increasing Si composition of SiGe in the SL (4). Here we report TDD improvement of Ge by introducing the SL without annealing.Ge growth with SiGe/Ge SL is carried out using reduced pressure chemical vapor deposition system. Si(100) wafers are used. After an HF last cleaning, the wafer is loaded into reactor and baked at 1000oC and cooled down to 600oC in H2 and further cooled down to 350oC in N2. After that 100 nm-thick seed Ge is deposited using N2-GeH4 gas. Then the wafer is heated up to 500oC in H2 and Ge/SiGe superlattice is fabricated using H2-GeH4 and H2-SiH4-GeH4 gases, respectively. Furthermore, 2.8 µm thick Ge is deposited at 550oC using H2-GeH4 gas.Thickness is measured by cross section SEM and spectroscopic ellipsometry. TDD is determined by Secco defect etching technique. Strain and relaxation of Ge and SiGe layers are measured by XRD reciprocal space mapping. Distribution of the MD and the TD are measured by cross section TEM.In the case of direct Ge growth on Si (Fig. 1a), MDs are formed at interface between Ge and Si. TDs are spreading into the Ge layer. Some of the TDs are long and extending to surface. These MD and TD formations are due to the lattice mismatch between Ge and Si. On the other hand, in the case of Ge sample grown on ×20 Si0.3Ge0.7 / Ge SL (Fig. 1b), most of MDs are formed at interface between 100 nm-thick Ge buffer and Si substrate. Additionally high density of TDs are observed in the Ge buffer and the SL layer. In the SL layer, lower TDD is observed in the upper part compared to that in the deeper part. At the interface between upper 2.8 µm thick Ge and the SL layer, MD and high density of TDs toward lateral direction is observed indicating relaxation. TDD in the 2.8 µm thick Ge layer seems to be lower compared to that in Fig. 1a).In the case of direct Ge growth on Si, TDD of ~7.5×108 cm-2 is obtained (Fig. 2). The TDD is comparable to previously reported results (2). By depositing 2.8 µm thick Ge on Si0.2Ge0.8 SL, TDD is decreased to ~5.4×108 cm-2. With decreasing Ge concentration of SiGe layer of the SL, decrease of TDD is observed until 40%. However, by replacing SiGe layers to mono-layers of Si, increased TDD is observed, which might be due to too high strain at interface between Si and SiGe.Figure 3 shows TDD of 2.8 µm thick Ge deposited on Si0.2Ge0.8/Ge SL with different Si0.2Ge0.8 and Ge thickness combination without changing periodicity of the SL. Lower TDD is observed when the 2.8 µm thick Ge is deposited on the SL with higher Si0.2Ge0.8 thickness. Possible reason is higher degree of relaxation of the SL layer.In Fig. 4, TDD of 2.8 µm thick Ge deposited on SLs of various lattice parameters are shown. By reducing lateral lattice parameter of the SiGe/Ge SL by increasing SiGe thickness or Si concentration, TDD decreases. The reduction of the TDD is fitting to one line for both cases. These results indicate that higher plastic relaxation of the SL occurs on the Ge buffer due to increased compressive strain of SiGe in the SL, and the TDD reduction of the Ge on the SL may be caused by similar mechanism of reverse graded buffer. By introducing SiGe/Ge SL, multiple effect of the reverse graded buffer may be expected. Reference (1) S. Lischke et al. Proc. IEEE Bipolar/BiCMOS Circuits and Technology Meeting (BCTM) , (2014) 29(2) Y. Yamamoto et al Solid-State Electron. 60 (2011) 2(3) Y. Yamamoto et al. Semicond. Sci. Technol. 33 (2018) 124007(4) Y. Yamamoto et al. Jpn. J. Appl. Phys. 59, SGGK10 (2020) Figure 1

InGaSb Defect Filter Layer to Improve Performance of GaSb Solar Cells Grown on GaAs Substrates

The reduction of the threading dislocation density in metamorphic GaSb grown on GaAs substrates through the use of InGaSb defect filter layers has been investigated. More specifically, we study the effects of strain and thickness on the ability of a InGaSb defect filter layer to reduce threading dislocations in GaSb solar cells grown on GaAs substrates. The strain between the GaSb metamorphic layer on GaAs substrate (99.5% relaxed) and the InGaSb defect filter layer is varied by changing the indium composition in the InGaSb layer. It is demonstrated that an InGaSb defect filter layer with 0.6% strain is more effective for blocking threading dislocations compared with higher-strain layers, resulting in improved short-circuit current (Jsc) and open-circuit voltage (Voc) for the metamorphic GaSb solar cell. The optimization of the defect filter layer involves varying the thickness of the layer to achieve the lowest possible threading dislocation density. This also takes into account the critical thickness of the InGaSb layer on GaSb to avoid generation of threading dislocations from the InGaSb layer itself. It is shown that adding an In0.11Ga0.89Sb defect filter layer with thickness of 250 nm and 0.6% strain beneath a GaSb solar cell grown on a GaAs substrate improves Voc from 0.1 V to 0.16 V and Jsc from 19.7 mA/cm2 to 24.7 mA/cm2.

Defect filtering for thermal expansion induced dislocations in III–V lasers on silicon

Epitaxially integrated III–V semiconductor lasers for silicon photonics have the potential to dramatically transform information networks, but currently, dislocations limit performance and reliability even in defect-tolerant InAs quantum dot (QD)-based lasers. Despite being below the critical thickness, QD layers in these devices contain previously unexplained misfit dislocations, which facilitate non-radiative recombination. We demonstrate here that these misfit dislocations form during post-growth cooldown due to the combined effects of (1) thermal-expansion mismatch between the III–V layers and silicon and (2) mechanical hardening in the active region. By incorporating an additional sub-critical thickness, indium-alloyed “misfit dislocation trapping layer,” we leverage these mechanical hardening effects to our advantage, displacing 95% of misfit dislocations from the QD layer in model structures. Unlike conventional dislocation mitigation strategies, the trapping layer reduces neither the number of threading dislocations nor the number of misfit dislocations. It simply shifts the position of misfit dislocations away from the QD layer, reducing the defects' impact on luminescence. In full lasers, adding a misfit dislocation trapping layer both above and below the QD active region displaces misfit dislocations and substantially improves performance: we measure a twofold reduction in lasing threshold currents and a greater than threefold increase in output power. Our results suggest that devices employing both traditional threading dislocation reduction techniques and optimized misfit dislocation trapping layers may finally lead to fully integrated, commercially viable silicon-based photonic integrated circuits.

Threading Dislocation Reduction of Ge by Introducing a SiGe/Ge Superlattice

The influence of introducing a SiGe / Ge superlattice (SL) between Ge layers and Si substrate for the sake of the reduction of the threading dislocation density (TDD) without additional annealing is investigated. In the case of 2.8 μm thick Ge directly grown on Si, the TDD at the surface is 7.6×10-8 cm-2. A slight TDD reduction is observed by introducing a Si0.2Ge0.8 / Ge SL between the Si substrate and the Ge layer. By inserting 5, 10 and 20 cycles of Si0.2Ge0.8 / Ge, the TDD is reduced to 7.1×10-8, 5.9×10,-8 and 5.3×10-8 cm-2, respectively. The lateral lattice parameters of these SLs are ~5.656Å, which is a smaller value compared to that of bulk Ge, indicating plastic relaxation by misfit dislocation (MD) formation. Further TDD reduction is realized with increasing Si concentration in the SiGe / Ge SL without changing the cycle of the SL. However, surface roughening due to pit formation occurs if the Si concentration in the SL is higher than 50% because of increased strain at the interfaces between SiGe and Ge. With increasing SiGe and Ge thickness ratio in the SL layer and maintaining periodicity and cycles, the TDD is reduced to 2.8×10-8 cm-2 without degrading the surface roughness. This improvement is related to a relaxation of the SiGe/Ge SL by plastic deformation.

A Pathway to Thin GaAs Virtual Substrate on On‐Axis Si (001) with Ultralow Threading Dislocation Density

With recent developments in high‐speed and high‐power electronics and Si‐based photonic integration, the concept of monolithic III–V/Si integration through epitaxial methods is gaining momentum. However, the performance and reliability of epitaxially grown devices are still limited by defects in the semiconductor material, especially the threading dislocation density (TDD). Herein, a novel “asymmetric step‐graded filter” structure grown by molecular beam epitaxy (MBE) is proposed based on a systematic study of the commonly used techniques for threading dislocation reduction for high‐quality GaAs on Si (001) growth. The proposed structure greatly enhances the plastic relaxation in the filter layers. A surface TDD lower than 2 × 106 cm−2 is achieved with a total buffer thickness of only 2.55 μm. This provides a clear pathway to further reduce defect density down to the theoretical limit in the 105 cm−2 regime with a thin buffer structure.

Optical and interfacial properties of epitaxially fused GaInP/Si heterojunction

This work investigates the optical and interfacial properties of epitaxially fused direct GaInP/Si heterojunctions realized by the corrugated epitaxial lateral overgrowth (CELOG) approach. To provide a broad analysis of the above heterojunction, photoluminescence (PL), cathodoluminescence (CL), Raman, and high-resolution transmission electron microscopy (TEM) were employed in this study. The enhanced luminescence intensity was observed in the direct GaInP/Si heterojunction in the cross-sectional CL because of the reduced defect density in the CELOG GaInP. The spatial resolution dependent PL and CL spectra of GaInP on Si yielded the composition variation of GaInP arising from the anisotropic growth behavior of CELOG. The Ga composition, x, in GaxIn1−xP/Si at the interface deduced from the lattice constant measured by TEM has a good agreement with the results of PL and CL. Low thermal and lattice mismatch strain in CELOG GaInP on Si were revealed by the Raman spectra. TEM investigation further revealed the atomic structure of some planar defects in CELOG GaInP over Si. It is confirmed that although a thin atomic disorder was observed on the surface of Si substrate, an epitaxially fused GaInP/Si heterojunction with a reduced threading dislocation density of ∼6.4 × 107 cm−2 in comparison to ∼4.8 × 108 cm−2 in the InP seed on Si has been successfully fabricated by the CELOG technique despite about 4% lattice mismatch between GaInP and Si. The findings of this study demonstrate the great potential of the CELOG technique for promoting monolithic integration of III-V/Si-based optoelectronics.

Improving performance of semipolar (202¯1) light emitting diodes through reduction of threading dislocations by AlGaN/GaN superlattice interlayer

Abstract We report reducing the density of threading dislocations (TDs) in stacking-fault-free semipolar (20 2 ¯ 1 ) GaN grown on sapphire by inserting an AlGaN/GaN superlattice (SL). We have studied the influence of AlGaN/GaN SL on the reduction of TDs and found that the density of TDs decreases by more than a factor of three with increasing the number of AlGaN/GaN SL pairs up to 10. Furthermore, blue light emitting diodes (LEDs) have been grown on semipolar (20 2 ¯ 1 ) GaN/sapphire templates with inserted 10 pairs of AlGaN/GaN SL showing doubled light output power in comparison with the LEDs grown without AlGaN/GaN SL.

Boosted ultraviolet electroluminescence of InGaN/AlGaN quantum structures grown on high-index contrast patterned sapphire with silica array

Epitaxially grown high crystalline quality InGaN/AlGaN multiple quantum structures on patterned sapphire with silica array (PSSA) have been successfully demonstrated. In comparison to conventional epilayers grown on patterned sapphire substrate (PSS), we observed a reduced threading dislocation density in the films grown on PSSA, attributing to the preferable vertical growth mode and reduced misfit at the coalescence boundary. Furthermore, a significant enhancement in light extraction efficiency can be achieved from InGaN/AlGaN-based ultraviolet light-emitting diodes (UVLEDs) built on PSSA owing to the large refractive index contrast between the epilayers and PSSA. More photons can escape from the top and bottom of the device, which was confirmed by numerically modeling light propagation within such device architecture. Benefiting from the reduced threading dislocation density and enhanced light extraction efficiency, the external quantum efficiency (EQE) of the fabricated UVLEDs on PSSA was more than two times higher than that of devices on the conventional flat sapphire substrate and it was additionally enhanced by 26.1% in comparison to the devices fabricated on PSS under an injection current of 150 mA. Therefore, the successful demonstration of UVLED on PSSA provides an unprecedented opportunity in achieving higher electroluminescence performance in future solid-state UV light sources.

Influence of the growth interface shape on the defect characteristics in the facet region of 4H-SiC single crystals

Two 75mm 4H-SiC single crystals are grown by the physical vapor transport (PVT) technique, using different insulation materials. The insulation material of higher thermal conductivity led to an increased radial temperature gradient. The evolution of the growth front was monitored using the in-situ computed tomography (CT). A slightly bent growth interface and a bigger facet are formed during the growth applying a lower radial temperature gradient while a smaller facet and steeper crystal flanks are formed in the case of the larger radial temperature gradient. Micropipes are deflected laterally by large surface steps on the steep crystal flanks and a reduction of threading edge dislocations by 60% is revealed by KOH defect etching.

Low-Threshold Epitaxially Grown 1.3-μm InAs Quantum Dot Lasers on Patterned (001) Si

A three-fold reduction of threshold current, with a minimum threshold current density of 286 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , a maximum operating temperature of 80 °C, and a maximum 3-dB bandwidth of 5.8 GHz was achieved for 1.3- <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">μ</i> m InAs quantum dot lasers on patterned, on-axis (001) Si. This was enabled by the reduced threading dislocation density (from 7 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> to 3 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−2</sup> ), and optimized probe design. The patterned Si produced antiphase domain free material in the coalesced GaAs buffer layer with reduced misfit/threading dislocation nucleation, without the use of Ge/GaP buffers or substrate miscut. Utilizing aspect ratio trapping, cyclic thermal annealing, and dislocation filter layers, high-quality III–V on Si devices were grown, demonstrating the compelling advantages of this patterned Si template for a monolithic Si photonics integration platform.

Optical and interface properties of direct InP/Si heterojunction formed by corrugated epitaxial lateral overgrowth

We fabricate and study direct InP/Si heterojunction by corrugated epitaxial lateral overgrowth (CELOG). The crystalline quality and depth-dependent charge carrier dynamics of InP/Si heterojunction are assessed by characterizing the cross-section of grown layer by low-temperature cathodoluminescence, time-resolved photoluminescence and transmission electron microscopy. Compared to the defective seed InP layer on Si, higher intensity band edge emission in cathodoluminescence spectra and enhanced carrier lifetime of InP are observed above the CELOG InP/Si interface despite large lattice mismatch, which are attributed to the reduced threading dislocation density realized by the CELOG method.

New Relaxation Mechanism Enabling High-Quality, Laterally Grown Ge on Si

We demonstrate a novel growth technique, metal-catalyzed, lateral epitaxial growth, to grow Ge films laterally on Si with reduced threading dislocation densities (TDD). In contrast to traditional planar film growth on Si, this technique starts with a small crystal nucleation at a specific position on Si followed by Ge lateral film growth. In plan-view and cross-sectional transmission electron microscopy micrographs, high-density threading dislocations were found to be present in the initial growth areas, while the lateral overgrowth areas demonstrated substantially reduced TDD or even defect-free areas. Furthermore, the X-ray diffraction results showed that the films were fully relaxed and mostly pure Ge. Full relaxation and low TDD Ge films are two surprising results given that the growth occurred at a low temperature (375 °C), which cannot be understood with the current misfit dislocation nucleation and glide model of film relaxation. For these reasons, we suggest the presence of a new relaxation mechan...

High quality GaN buffer layer by isoelectronic doping and its application to 365 nm InGaN/AlGaN ultraviolet light-emitting diodes

The isoelectronic doping in semiconductors is an effective method for improving device performance. Here we have investigated effects of isoelectronically Al-doped GaN buffer layer on optical and electrical properties of 365 nm InGaN/AlGaN ultraviolet light-emitting diodes (UV LEDs) grown by metal-organic chemical vapor deposition (MOCVD) on patterned sapphire substrate. In situ reflectance measurements revealed that the transition time from three-dimensional (3D) grain to step-flow two-dimensional (2D) coalesced growth mode was extended in the isoelectronically Al-doped GaN buffer layer as compared to undoped GaN buffer layer. The improved crystal quality, in terms of threading dislocation reduction, of the isoelectronically Al-doped GaN buffer layer was determined by cross sectional transmission electron microscopy studies and showed that the screw-type threading dislocation density was reduced. The light output power of UV LED was enhanced by 7.6% by incorporating optimum isoelectronic Al doping concentration in GaN buffer layer.

Thermophysics Simulation of Laser Recrystallization of High-Ge-Content SiGe on Si Substrate

The high-Ge-content SiGe material on the Si substrate can be applied not only to electronic devices but also to optical devices and is one of the focuses of research and development in the field. However, due to the 4.2% lattice mismatch between Si and Ge, the epitaxial growth of the high-Ge-content SiGe epitaxial layer directly on the Si substrate has a high defect density, which will seriously affect the subsequent device performance. Laser recrystallization technique is a fast and low-cost method to effectively reduce threading dislocation density (TDD) in epitaxial high-Ge-content SiGe films on Si. In this paper, by means of finite element numerical simulation, a 808 nm laser recrystallization thermal physics model of a high-Ge-content SiGe film (for example, Si0.2Ge0.8) on a Si substrate was established (temperature distribution physical model of Si0.2Ge0.8 epitaxial layer under different laser power, Si0.2Ge0.8 epitaxial layer thickness, and initial temperature). The results of this paper can provide important technical support for the preparation of high-quality high-Ge-content SiGe epilayers on Si substrates by laser recrystallization.

Design of a 1.55 µm nanoscale laser array on v-groove patterned Si substrates

A 1.55 µm nanoscale laser array is proposed and designed on v-groove patterned Si (0 0 1) substrates. The laser array is designed by taking full advantage of the periodic groove structures of the patterned substrates. The periodic groove structures separated by a SiO2 mask can effectively reduce threading dislocation density in the active region and provide a strong lateral optical confinement for the laser array structure. The optical mode distributions of the single laser are investigated in detail, and the optical confinement factor is calculated to optimize the structure parameters of the laser array. An optimal value of 9.3% for the optical confinement factor is obtained. The analysis of threshold conditions shows that the laser array can achieve lasing with a cavity length of about 1 mm for an internal loss of 40 cm−1.

Crystal quality improvement of sputtered AlN film on sapphire substrate by high-temperature annealing

Thermal annealing process of sputtered AlN films with different thicknesses on sapphire substrates was studied in N2 ambient. Crystal quality of these AlN films was significantly improved after 1400–1700 °C annealing. For the 200-nm AlN film, the full width at half maximum (FWHM) of the (0002)-plane X-ray rocking curve (XRC) was improved from 157.3 to 103.6 arcsec after 1650 °C annealing, while the FWHM of the (10 $$\bar {1}$$ 2)-plane XRC was significantly improved from 3890 to 353.1 arcsec. Transmission electron microscopy further confirmed that the crystal quality improvement was related to solid-phase reaction at high temperature, allowing columnar structure AlN to recrystallize and thus reducing threading dislocations in the films. Surface morphology with high-density AlN islands transformed to step-and-terrace morphology. X-ray photoelectron spectroscopy measurement also indicated that C and O impurities were decreased by the thermal annealing process. Film strain changed from tensile to compressive stress after annealing. The optical transmissivities of these AlN films did not decrease after annealing, and the optical transmissivity band edge slopes increased, providing further evidence of improved crystal quality.

Reducing threading dislocation density in GaSb photovoltaic devices on GaAs by using AlSb dislocation filtering layers

GaSb based photovoltaic devices have been demonstrated on GaAs substrates by inducing interfacial array of 90° misfit dislocations. Despite the beneficial qualities of the highly stable 90° misfit dislocation, there is a significant density of residual threading dislocations in the GaSb layer, resulting in the degradation of the electrical performance of such photovoltaic cells compared to lattice matched devices. We aim to reduce threading dislocation density by optimizing growth temperature and by using an AlSb dislocation filtering layer. The growth temperature optimization results in a reduction of the threading dislocation density to 1.3 × 108 cm−2. Adding an AlSb dislocation filtering layer further improves the electrical performance of the GaSb solar cells by reducing the threading dislocation density to 4 × 107 cm−2. A comparison between the experimental data and theoretical calculation confirms that the recombination in dislocation centers is a dominant loss mechanism in GaSb solar cell grown on GaAs substrate.

Threading dislocation reduction in InN grown with in situ surface modification by radical beam irradiation

In this study, the applicability of in situ surface modification by radical beam irradiation method to reduce threading dislocation density in InN was investigated. The dislocation behavior, crystallographic quality, electrical properties, and photoluminescence properties of InN grown with this method were studied. Transmission electron microscopy observation revealed that dislocation density in the InN layer reduced by a factor of 3 in certain regions and showed good agreement with the results from photoluminescence spectroscopy. If this technology is established, high-quality InN with low threading dislocation density can be achieved with a very simple and repeatable growth process.

Enhanced Optical Output Power of Blue Light-Emitting Diode Grown on Sapphire Substrate with Patterned Distributed Bragg Reflector

We demonstrate InGaN-based blue light-emitting diodes (LEDs) with embedded SiO2/SiNx distributed Bragg reflectors (DBRs). Five pairs of circular SiO2/SiNx DBRs were fabricated on a sapphire substrate by depositing multiple dielectric layers and patterning. At an injection current of 20 mA, the optical output power of the LEDs with circular DBRs was 33% higher than that of conventional LEDs without DBRs. The enhancement of optical output power is attributed to the improved light extraction efficiency and internal quantum efficiency of an LED caused by inserting the highly reflective circular DBRs, which increase the reflection of light from the substrate and increase the internal quantum efficiency by reducing threading dislocations in the GaN epilayer.

Strain relaxation in convex-graded InxAl1-xAs (x = 0.05–0.79) metamorphic buffer layers grown by molecular beam epitaxy on GaAs(001)

This paper presents a study of structural properties of InGaAs/InAlAs quantum well (QW) heterostructures with convex-graded InxAl1-xAs (x = 0.05–0.79) metamorphic buffer layers (MBLs) grown by molecular beam epitaxy on GaAs substrates. Mechanisms of elastic strain relaxation in the convex-graded MBLs were studied by the X-ray reciprocal space mapping combined with the data of spatially-resolved selected area electron diffraction implemented in a transmission electron microscope. The strain relaxation degree was approximated for the structures with different values of an In step-back. Strong contribution of the strain relaxation via lattice tilt in addition to the formation of the misfit dislocations has been observed for the convex-graded InAlAs MBL, which results in a reduced threading dislocation density in the QW region as compared to a linear-graded MBL.