Low Threading Dislocation Density Research Articles

GaAs Solar Cells Grown Directly on V-Groove Si Substrates.

The direct epitaxial growth of high-quality III-V semiconductors on Si is a challenging materials science problem with a number of applications in optoelectronic devices, such as solar cells and on-chip lasers. We report the reduction of dislocation density in GaAs solar cells grown directly on nanopatterned V-groove Si substrates by metal-organic vapor-phase epitaxy. Starting from a template of GaP on V-groove Si, we achieved a low threading dislocation density (TDD) of 3 × 106 cm-2 in the GaAs by performing thermal cycle annealing of the GaAs followed by growth of InGaAs dislocation filter layers. This approach eliminates the need for a metamorphic buffer to directly integrate low-TDD GaAs on Si. We used these low-TDD GaAs/V-groove Si templates to grow GaAs double heterostructures that had a minority carrier lifetime of 5.7 ns, as measured by time-resolved photoluminescence, a value consistent with the material quality associated with a 20%+ efficient GaAs solar cell. However, front-junction GaAs solar cells grown on these low-TDD substrates produced a conversion efficiency of only 6.6% without an antireflection coating. Electron channeling contrast imaging measurements on this cell showed a high density of misfit dislocations at the interface between the AlInP/GaInP window layer and the GaAs absorber and between the GaAs absorber and the GaInP back surface field (BSF), likely causing a high surface recombination velocity and thus poor performance. We showed that we could reduce (and in the case of the BSF, eliminate) these dislocations by employing an AlGaAs-based window layer and BSF. Compared to GaInP, AlGaAs has dislocation glide properties that are more similar to those of GaAs, resulting in more even threading dislocation glide between layers. AlGaAs passivation improved the external quantum efficiency and open-circuit voltage of the devices, but the overall device performance was still low at an efficiency of 7.7% without an antireflection coating, likely due to cracking in the devices. This work demonstrates a route to high material quality in GaAs grown directly on Si that can be used for the production of III-V/Si optoelectronic devices.

Reducing dislocations for room-temperature continuous-wave InGaAs/AlGaAs multiple quantum well lasers monolithically grown on Si

As dislocation-sensitive light-emitting devices, quantum well lasers monolithically grown on Si substrates operate poorly compared to their counterparts on native ones. In this work, a GaAs/Si virtual substrate with a low threading dislocation density of 8.9 × 106/cm2 was achieved by using a combined dislocation-reducing method. Meanwhile, by using two In-containing strained layer separately inserted into upper and lower AlGaAs cladding layers, the active region was virtually free of gliding dislocations. With the reduced dislocations in the active region, the fabricated Si-based broad-stripe ridged lasers exhibited continuous-wave lasing at room temperature, and a low threshold current density of 1015 A/cm2, high operation temperature of 90 °C as well as a device lifetime of over 87.25 h were achieved. These results demonstrate an experimentally feasible way to optimize a Si-based InGaAs/AlGaAs quantum well laser and further promote the research on monolithic integration.

Exploring Inorganic Flexible Electronics: III‐Nitride Light‐Emitting Diode Epilayers on Wafer‐Scale Exfoliable Mica Substrate

AbstractMica is a promising substrate for flexible photonic applications because it can be fabricated using wafer‐scale multiple exfoliations. However, the large lattice mismatch between mica and III‐nitrides hinders their application in III‐nitride semiconductor devices. In this study, two types of light‐emitting diode (LED) epilayers, blue and green, are epitaxially fabricated on 2‐inch exfoliated mica substrates using sputtered AlN as a buffer layer to address the lattice mismatch. The LED epilayers on mica substrate exhibit excellent single‐crystalline quality with a low threading dislocation density of 2.07 × 109 cm−2 in the GaN region and smooth surface morphology with a root mean square (RMS) roughness of 0.58 nm in a 5 × 5 µm2 scanned area, demonstrating that mica is an excellent platform for III‐nitride semiconductors. Moreover, the structures are reproduced stably on multiple exfoliated mica at the wafer scale, which verified the reusability and reproducibility of III‐nitride/mica in terms of crystallinity, surface morphology, and transparency. Strong electroluminescence of the LED epilayers confirmed the potential of mica in electrically driven flexible opto‐electronics. Therefore, this results demonstrated the potential of mica substrates in III‐nitride semiconductors and provide a novel pathway for revolutionizing the fabrication of III‐nitride‐based flexible devices.

Development of aspect ratio trapping growth of GeSn on Si for midwave infrared applications

The development of all-group IV GeSn materials on Si substrates is of interest for monolithic midwave infrared (MWIR) silicon photonics. Despite their great potential, the growth of high crystalline quality materials is still challenging. The conventional growth of GeSn films on Si displays high densities of dislocations and, hence, large dark currents. We have examined a state-of-the-art lithographically defined aspect ratio trapping (ART) growth approach for threading dislocations (TDs) filtering in GeSn and demonstrated the selective growth in the holes embedded in the SiO2/Si(001) template. While an exceptionally low density of TDs was confirmed by transmission electron microscopy imaging, we find that Sn incorporation is strongly suppressed in the ART-grown structures. Nonetheless, these results provide a solid basis for further efforts to enhance the Sn fraction of the GeSn ART growth.

Reduction of GaAs Buffer Thickness and Its Impact on Epitaxially Integrated III-V Quantum Dot Lasers on a Si Substrate.

Monolithic integration of III-V quantum dot (QD) lasers onto a Si substrate is a scalable and reliable approach for obtaining highly efficient light sources for Si photonics. Recently, a combination of optimized GaAs buffers and QD gain materials resulted in monolithically integrated butt-coupled lasers on Si. However, the use of thick GaAs buffers up to 3 μm not only hinders accurate vertical alignment to the Si optical waveguide but also imposes considerable growth costs and time constraints. Here, for the first time, we demonstrate InAs QD lasers epitaxially grown on a 700 nm thick GaAs/Si template, which is approximately four times thinner than the conventional III-V buffers on Si. The optimized 700 nm GaAs buffer yields a remarkably smooth surface and low threading dislocation density of 4 × 108 cm-2, which is sufficient for QD laser growth. The InAs QD lasers fabricated on these ultrathin templates still lase at room temperature with a threshold current density of 661 A/cm2 and a characteristic temperature of 50 K. We believe that these results are important for the monolithically integrated III-V QD lasers for Si photonics applications.

Well Number Dependence of Internal Quantum Efficiency in AlGaN Quantum Wells on Low‐Dislocation Sputtered AlN Templates

The internal quantum efficiency (IQE) and cathodoluminescence intensity line profile of AlGaN multiple quantum well (MQW) structures on low‐dislocation face‐to‐face annealed sputtered AlN (FFA Sp‐AlN) and on conventional metalorganic vapor‐phase epitaxy‐grown AlN (MOVPE‐AlN) templates are evaluated and the effect of the number of quantum wells (QWs) on the IQE is discussed. The higher IQE in the FFA samples is probably due to the lower threading dislocation (TD) density; however, the IQE also increases with the number of QWs although the TD density remains constant. Effective diffusion length increases with the number of QWs, indicating that the AlGaN MQW layer helps to suppress point defect diffusion, resulting in IQE increase. Furthermore, the segregation of point defects into the TDs and point defect diffusion via the TDs may explain the difference in the IQE improvement rate between the MQWs on the FFA Sp‐AlN and MOVPE‐AlN templates.

High-quality germanium growth on (111)-faceted V-groove silicon by molecular beam epitaxy

High-quality and low-defect-density germanium (Ge) buffer layers on silicon (Si) substrates have long been developed for group IV and III–V devices by suppressing defect propagation during epitaxial growth. This is a crucial step for the development of highly efficient photonic devices on Si substrates. Patterned silicon substrates have increasingly been employed for their ability to restrict and hinder the motion of defects. In this work, we demonstrate the effectiveness of an optimised two-step growth recipe structure on a (111)-faceted V-groove silicon substrate with a 350 nm flat ridge. This strategy successfully reduces the threading dislocation (TD) density while growing a 1 μm Ge buffer layer via molecular beam epitaxy. As a result, a high-quality buffer is produced with a low TD density on the order of 107 cm−2 and a surface roughness below 1 nm.

Origin of the parasitic luminescence of 235 nm UVC LEDs grown on different AlN templates

AlN layers annealed at high temperatures offer low threading dislocation densities of mid 108 cm−2 and are therefore increasingly used as base layers in ultraviolet (UV) light emitting diode (LED) heterostructure growth. These LEDs, just like those grown on conventional metalorganic vapor phase epitaxy (MOVPE) AlN templates, often suffer from long-wavelength parasitic luminescence. In this work, luminescence properties of far-UVC LED heterostructures grown on MOVPE-AlN/sapphire templates and high-temperature annealed AlN/sapphire templates are compared. To investigate the origin of parasitic emission with high spatial resolution, cross section scanning transmission electron microscopy was combined with cathodoluminescence measurements. As a result, the main origin of the parasitic luminescence band centered at 3.5 eV (354 nm) for the heterostructure grown on annealed AlN is assigned to point defects related to oxygen in the AlN template layer. The defect band centered at 3.0 eV (413 nm) for the heterostructure grown on MOVPE-AlN was found to be related to self-compensating VAl-Si point defect complexes in the n-AlGaN layer and oxygen incorporation close to the AlN/sapphire interface. The results also suggest that the type of AlN template determines the kind of parasitic luminescence from the n-AlGaN layer.

Sub-10 μm-Thick Ge Thin Film Fabrication from Bulk-Ge Substrates via a Wet Etching Method.

Low-defect density Ge thin films are crucial for studying the impact of defect density on the performance limits of Ge-based optical devices (optical detectors, LEDs, and lasers). Ge thinning is also important for Ge-based multijunction solar cells. In this work, Ge wet etching using three acidic H2O2 solutions (HF, HCl, and H2SO4) was studied in terms of etching rate, surface morphology, and surface roughness. HCl-H2O2-H2O (1:1:5) was demonstrated to wet-etch 535 μm-thick bulk-Ge substrates to 4.1 μm with a corresponding RMS surface roughness of 10 nm, which was the thinnest Ge film from bulk-Ge via a wet etching method to the best of our knowledge. The good quality of pre-etched bulk-Ge was preserved, and the low threading dislocation density of 6000-7000 cm-2 was maintained after the etching process. This approach provides an inexpensive and convenient way for accurate Ge substrate thinning in applications such as multijunction solar cells and sub-10 μm-thick Ge thin film preparation, which enables future studies of low-defect density Ge-based devices such as photodetectors, LEDs, and lasers.

Reducing GaN crystal dislocations through lateral growth on uneven seed crystal surfaces using the Na-flux method

In recent years, we have achieved low threading dislocation density in GaN wafers by using the Na-flux multi-point seed technique. However, the resulting wafers exhibit regions of high dislocation density, exceeding 105 cm−2 at the coalescence boundary where pyramidal crystals merge. In this study, we discovered that annealing seed crystals at 900 °C generated an uneven surface with approximately 50 μm of GaN decomposition, and growing GaN on the thus-generated uneven surfaces induced lateral growth composed of facets. We then investigated the effect of the uneven surface on dislocation reduction and found that the average threading dislocation density of the grown crystal was reduced from cm−2 in the seed crystal to cm−2. We confirmed that the reduction in threading dislocation density was due to the termination of dislocations by means of inclusions, and to mergers or annihilation as they encountered one another during facet growth.

Introduction of dislocation filtering with different ammonia flows in low-temperature grown AlN (< 1200 °C)

High crystalline quality of AlN layers are commonly grown at temperatures above 1200 °C. However, this work revealed that introducing a rough surface of AlN layer, defined as dislocation filtering (DF) herein, can improve the crystalline quality of 1.5 μm thick AlN layers grown at 1175 °C via metal organic vapor phase epitaxy technique. With 7 slm of ammonia flow during the DF growth, the lowest threading dislocation density (TDD) of ∼2.9 × 109 cm−2 was obtained. The result was associated to the formation of 3D growths which become reasonably smaller, denser, and more uniformly distributed with increasing ammonia flow up to 7 slm. The 3D growths were suggested to promote lateral growth that can increase the possibility for TD inclination and hence, mutual annihilation. The 3D growths characteristic disappeared when the ammonia flow was 10 slm. The optimized AlN layer that was obtained with 7 slm of ammonia flow exhibited an atomically smooth surface with a few deep trenches and improved transparency. Meanwhile, the calculated in-plane tensile strain for the AlN layers grown on DF with 1–7 slm of ammonia was about 3 times higher than the AlN layer without DF. However, no evidence of microcracks on the surface was observed.

(Invited) Application of Synchrotron X-Ray Topography Techniques to the Analysis of Defect Microstructures in Bulk GaN Substrates and Epilayers for Power Devices

Wide bandgap semiconductor gallium nitride (GaN), along with silicon carbide (SiC) are replacing silicon materials in the applications for power electronic devices. While SiC-based power technologies are based on bulk SiC substrates, GaN-based power technologies have until recently been based on GaN layers grown on foreign substrates such as sapphire, SiC and Si substrates. To achieve the full potential of GaN power devices, devices on bulk GaN substrates are desirable. In this review, the use of synchrotron X-ray topography techniques to evaluate defects and strain in bulk GaN substrates for vertical device development is presented. Ray tracing simulations is shown to effectively identify the Burgers vectors of all dislocations on X-ray topographs from GaN materials. Ammonothermal-grown GaN substrate wafers show the best quality among all the wafers. These wafers, which are free of basal plane dislocations (BPDs), have low curvature and threading mixed dislocations (TMDs) dominant among the TDs. However, certain ammonothermal substrates contain growth sectors with different impurity concentrations that are likely inherited from prior seed expansion growth runs. Patterned hydride vapor phase epitaxy (HVPE) GaN reveal a starkly heterogeneous distribution of dislocations with large areas containing low threading dislocation densities in between a grid of strain centers with higher threading dislocation densities and BPDs. These substrates also contain growth sector boundaries separating core regions growing in <11-22> directions and surrounding regions growing along [0001]. HVPE GaN substrates are characterized by high lattice distortion and dislocation densities of the order of 105-106 cm-2, which is much higher than 103-104 cm-2 of ammonothermal samples and dislocation-free areas in the patterned HVPE samples. Epitaxial growth and other processes involving heat treatment (including annealing) does not nucleate new defects but can result in interactions of dislocations (threading screw dislocations(TSDs)/threading mixed dislocations (TMDs)/threading edge dislocations (TEDs)) with point defects leading to changes in dislocation configurations. Mg-ion implantation for p-type doping does not change the distribution of dislocation but introduces lattice damage. By Rocking curve Analysis by Dynamical Simulation (RADS) analysis of X-ray rocking curves, the depth profile of strain can be obtained and correlated with doping profile. During annealing the implanted samples to heal lattice damage and activate the Mg ions, proper capping is necessary to limit loss of matter and introduction of additional lattice distortion. However, high pressure annealing has been shown to eliminate the need for capping. The effect on microstructure from other selectively area doping techniques such as etch and regrowth, diffusion and neutron transmutation will also be discussed.

The growth of low-threading-dislocation-density GaAs buffer layers on Si substrates

Monolithic integration of III–V optoelectronic devices on Si platform is gaining momentum, since it enables advantages of low cost, less complexity and high yield for mass production. With the aim of achieving advances in monolithic integration, the challenges associated with lattice mismatch between III–V layers and Si substrates must be overcome, as a low density of threading dislocations (TDs) is a prerequisite for the robustness of the integrated devices. In this paper, we have investigated and compare different types of dislocation filter layers (DFLs) from InGaAs asymmetric step-graded buffer layer (ASG), InGaAs/GaAs strained-layer superlattices, and quaternary alloy InAlGaAs ASG, on the functionality of reducing TD density (TDD) for GaAs buffer layers on Si. Compared to other DFLs, the sample with InAlGaAs ASG buffer layer shows the lowest average TDD value and roughness, while the decrease of TDD in the sample with InAlGaAs ASG buffer layer can be understood in terms of the hardening agent role of aluminium in the InAlGaAs ASG. By further optimising the InAlGaAs ASG through thermal cyclic annealing, we successfully demonstrate a low surface TDD of 6.3 ± 0.1 × 106 cm−2 for a 2 μm GaAs/InAlGaAs ASG buffer layer grown on Si. These results could provide a thin buffer design for monolithic integration of various III–V devices on Si substrates.

Ultra-high thermal stability InAs/GaAs quantum dot lasers grown on on-axis Si (001) with a record-high continuous-wave operating temperature of 150 °C.

Direct epitaxial growth of group III-V light sources with excellently thermal performance on silicon photonics chips promises low-cost, low-power-consumption, high-performance photonic integrated circuits. Here, we report on the achievement of ultra-high thermal stability 1.3 µm InAs/GaAs quantum dot (QD) lasers directly grown on an on-axis Si (001) with a record-high continuous-wave (CW) operating temperature of 150 °C. A GaAs buffer layer with a low threading dislocation density (TDD) of 4.3 × 106 cm-2 was first deposited using an optimized three-step growth method by molecular beam epitaxy. Then, an eight-layer QD laser structure with p-type modulation doping to enhance the temperature stability of the device was subsequently grown on the low TDD Si-based GaAs buffer layer. It is shown that the QD laser exhibits the ultra-high temperature stability with a characteristic temperature T0=∞ and T1=∞ in the wide temperature range of 10-75 °C and 10-140 °C, respectively. Moreover, a maximum CW operating temperature of up to 150 °C and a pulsed operating temperature of up to 160 °C are achieved for the QD laser. In addition, the QD laser shows a high CW saturation power of 50 mW at 85 °C and 19 mW at 125 °C, respectively.

Metamorphic front- and rear-junction 1.7 eV GaInP solar cells with high open-circuit voltage

1.7 eV absorber materials are important in high-performance III-V multi-junction solar cells. Here, we show that rapid thermal annealing significantly improves the carrier lifetimes in lightly doped metamorphic 1.7 eV Ga0.37In0.63P (MM GaInP) grown on GaAs by molecular beam epitaxy. A low threading dislocation density of ∼6 × 105 cm−2 was achieved through the use of an InxGa1-xAs graded buffer. Annealing enables minority carrier lifetimes in 1.7 eV p- and n-GaInP of 3.9 and 28 ns, respectively, both of which are higher than annealed lattice-matched 1.9 eV Ga0.51In0.49P (LM GaInP) grown in the same chamber. With the benefit of annealing, MM 1.7 eV GaInP front-junction solar cells show a high peak internal quantum efficiency of 94.3% and an open-circuit voltage (VOC) of 1.17 V. We also demonstrate the first MM 1.7 eV GaInP rear-heterojunction cells with high fill factor and VOC of 85% and 1.21 V, respectively, by taking advantage of long carrier lifetime while eliminating majority carrier blocking. The high VOC values for the 1.7 eV GaInP cells presented in this work indicate promising tolerance to threading dislocations in MM GaInP.

(Invited) Application of Synchrotron X-Ray Topography Techniques to the Analysis of Defect Microstructures in Bulk GaN Substrates and Epilayers for Power Devices

The study of defects and strain in GaN substrates and epilayers for vertical device development using synchrotron X-ray topography techniques is presented. Synchrotron monochromatic beam X-ray topography (SMBXT) studies show that ammonothermal-grown GaN substrate wafers have the lowest dislocation densities while patterned hydride vapor phase epitaxy (HVPE) GaN reveal a starkly heterogeneous distribution of dislocations with large areas containing low threading dislocation densities and HVPE GaN substrates contain high dislocation densities. Epitaxial growth does not nucleate new defects but interactions of dislocations with point defects can cause changes in dislocation configurations. Rocking curve Analysis by Dynamical Simulation (RADS) is shown to be effective in analysis of implantation by correlating the depth profile of strain with the doping profile. Removal of lattice damage induced by implantation requires annealing with proper capping or under high pressures to limit loss of material and lattice distortion. Inductively coupled plasma (ICP) etching may introduce strain in GaN material that can be partially recovered by TBCl etching. Diffusion doping does not introduce new strains in GaN material while neutron transmutation doping at low levels can lead to curing of defects while higher levels lead to irradiation damage and nucleation of new defects.

MOVPE growth of AlN and AlGaN films on N-polar annealed and sputtered AlN templates

We performed metalorganic vapor phase epitaxy (MOVPE) growth of AlN (MOVPE-AlN) and AlGaN films on N-polar face-to-face annealed and sputtered AlN templates (N-polar FFA Sp-AlN) on sapphire substrates. The off-cut angle of the sapphire substrates was varied from 0.2° to 6.0°. The effects of the off-cut angle on the surface morphology and crystallinity of the N-polar MOVPE-AlN and AlGaN were elucidated. The results showed that the crystallinities of N-polar FFA Sp-AlN and MOVPE-AlN were independent of the off-cut angle. The threading dislocation density (TDD) was estimated to be approximately 5.0 × 108cm−2 by using values of the X-ray rocking curve full-width at half maximum. In comparison, the density of the hillocks on the MOVPE-AlN surfaces was suppressed, and the surface flatness was improved by increasing the off-cut angle of the sapphire substrate. The crystallinity of the AlGaN was improved by increasing the off-cut angle. Our results prove that N-polar AlN and AlGaN films with smooth surfaces and low TDDs could be achieved by MOVPE on N-polar FFA Sp-AlN with a large substrate off-cut angle of 6.0°.

Ethylene for carbon doping of GaN by atmospheric pressure metal organic chemical vapor deposition

The carbon incorporation properties of GaN from ethylene and trimethylgallium precursors was studied by atmospheric pressure metal organic chemical vapor deposition. It is found that V-III ratio was the main restriction factor for carbon incorporation. Carbon concentration could be regulated effectively one order of magnitude by tuning V-III ratio for C2H4 doped GaN, and would be enhanced at relatively low V-III ratio for self-doped GaN. Low threading dislocation density and atomic flat surface of C2H4 doped GaN films was kept even if carbon doping concentration of 9E18/cm3 compared with UID-GaN and a high breakdown voltage of 2278 V at leakage current of 10µA was obtained in C2H4 doped GaN buffer layer.

285 nm AlGaN-BASED DEEP-ULTRAVIOLET LED WITH HIGH INTERNAL QUANTUM EFFICIENCY: COMPUTATIONAL DESIGN

In this paper, the systematic computational design process of AlGaN-based multiple quantum-well (QW) deep-ultraviolet (DUV) light-emitting diode (LED) grown on sapphire (Al2O3) substrate was investigated. An optimization was held to increase internal quantum efficiency (IQE) handling the LED parameters such as doping percentage of the n- and the p-type layers of these devices. The structure parameters of the best design were determined through a customized genetic algorithm integrated into the nanostructure quantum electronic simulation (nextnano). As a determining factor, IQE was obtained to be 24% for the devised 285 nm LED. It has been demonstrated that this result can be increased up to a remarkably high value of 70% by a low threading dislocation density (TDD) and reduced Auger recombination. In addition, the operation input power and potential difference were successfully kept below 0.1 W/mm2 and 5.05 V, respectively.

Comparison of Beyond 1 GHz C-Plane Freestanding and Sapphire-Substrate GaN-Based micro-LEDs for High-Speed Visible Light Communication

With ever-growing demand for 6G networks technology, visible light communication (VLC) as a vital component of 6G has challenging requirement for superior performance of light source. Herein, 20 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula> m blue micro-LED fabricated on 2-inch <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</i> -plane GaN freestanding substrate with high bandwidth over 1 GHz was first demonstrated, and the systematic comparisons to sapphire-substrate micro-LED from the epitaxial materials characterization to optoelectronic properties as well as communication performance were further conducted. GaN-substrate LED wafer has high crystal quality with lower threading dislocation density (TDD), which is two orders of magnitude lower than that of sapphire-substrate LED wafer. The much lower TDD of GaN-substrate LED wafer and better heat dissipation of GaN substrate offer GaN-substrate micro-LED great advantages in optoelectronic properties as well as communication performance. GaN-substrate micro-LED exhibited higher light output power (LOP) especially at high operating current, which is improved by <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\sim$</tex-math></inline-formula> 21% at 96 mA compared to that of sapphire substrate. Also, GaN-substrate micro-LED achieved a relatively higher bandwidth of 1.282 GHz while maintaining higher LOP with lower operating voltage. And a higher data rate of 4.48 Gbps under 1 m free space link was obtained by GaN-substrate micro-LED, proving it being a promising candidate in high-speed VLC.