Lateral Overgrowth Research Articles

Defect seeded remote epitaxy of GaAs films on graphene

Remote epitaxy is an emerging materials synthesis technique which employs a 2D interface layer, often graphene, to enable the epitaxial deposition of low defect single crystal films while restricting bonding between the growth layer and the underlying substrate. This allows for the subsequent release of the epitaxial film for integration with other systems and reuse of growth substrates. This approach is applicable to material systems with an ionic component to their bonding, making it notably appealing for III–V alloys, which are a technologically important family of materials. Chemical vapour deposition growth of graphene and wet transfer to a III–V substrate with a polymer handle is a potentially scalable and low cost approach to producing the required growth surface for remote epitaxy of these materials, however, the presence of water promotes the formation of a III–V oxide layer, which degrades the quality of subsequently grown epitaxial films. This work demonstrates the use of an argon ion beam for the controlled introduction of defects in a monolayer graphene interface layer to enable the growth of a single crystal GaAs film by molecular beam epitaxy, despite the presence of a native oxide at the substrate/graphene interface. A hybrid mechanism of defect seeded lateral overgrowth with remote epitaxy contributing the coalescence of the film is indicated. The exfoliation of the GaAs films reveals the presence of defect seeded nucleation sites, highlighting the need to balance the benefits of defect seeding on crystal quality against the requirement for subsequent exfoliation of the film, for future large area development of this approach.

Structural characterization of ELO-GaN film on mask-stripe patterned GaAs (0 0 1) substrate grown by metalorganic vapor phase epitaxy

GaN films were epitaxial laterally overgrown on the [110]-stripe-patterned GaAs (001) substrate by MOVPE. AlGaN intermediate layers and low-temperature GaN buffer layer were inserted in order to prevent structural defects. The epitaxial lateral overgrowth of GaN results in a trapezoidal shape with flat facets. Structural phases and defects distribution were studied by TEM. Selective-area diffraction pattern indicates the mixed phase of GaN and a high density of stacking faults at nucleation. However, with the deposition of dielectric SiNx masks, a lateral overgrowth resulted in nearly free of defects, and the cubic phase transformed into hexagonal despite a cubic template. Moreover, dark-field TEM images were used to demonstrate the structural phase transformation.

Investigation of off-cut angle of sapphire for epitaxial lateral overgrowth of AlN and fabrication of high-quality AlN template

Using a metal–organic vapor phase epitaxy, the epitaxial lateral overgrowth (ELO) of AlN was performed by varying a vicinal off-cut angle from 0.15 to 2.0° of sapphire substrates oriented to the m-axis, i.e., the a-axis of AlN. During overgrowth, step bunching resulted in the efficient conversion of dislocations to elongated voids. The dislocations were terminated at the voids formed over the groove regions and the low and high etch pit density (EPD) regions were observed. The dislocation was reduced by increasing the off-cut angle of the sapphire substrates up to 0.8°; however, low-EPD region became small in the ELO. Thus, we have attempted to achieve a longer growth of the ELO-AlN layer on the sapphire substrate with the off-cut angle of 0.15°. As a result, the EPD decreased with the increasing thickness. The ELO-AlN layer with the thickness of 15.9 µm exhibited the EPD of 2.1 × 106 cm−2 in the low dislocation region.

Dislocation dynamics in α-Ga2O3 micropillars from selective-area epitaxy to epitaxial lateral overgrowth

Epitaxial lateral overgrowth (ELO) is an effective strategy to achieve metastable phased α-Ga2O3 with low dislocation densities, which is desirable for developing ultralow-loss and ultrahigh power devices, whereas the involved dislocation dynamics have not been fully exploited. In this Letter, we investigated the dislocation propagations and reactions in α-Ga2O3 micropillar arrays selectively grown by halide vapor phase epitaxy technique. Screw dislocations in α-Ga2O3 micropillars grown from the selective area epitaxy (SAE) to ELO mode exhibited an independent character with an average density of 4.5 × 106 cm−2 while the edge dislocation density was reduced to 5.3 × 108 cm−2. During the initial SAE process, the α-Ga2O3 hexagonal pyramid is developed with the observed inversion domains within the pillar cores. The successive epitaxial lateral overgrowth ELO facilitates the formation of inclined facets upon the SiO2 mask. Almost complete filtering of the underlying threading dislocation has been demonstrated in the ELO wings. Strong image forces induced by inclined free surfaces drive the propagation and reaction of threading dislocations until annihilation, which is well described by the dislocation-filtering model during the dynamic geometry transition of micropillars. These findings may pave the way for the success of the heteroepitaxy of low dislocation density α-Ga2O3 toward the development of high-performance power devices.

Review of Highly Mismatched III-V Heteroepitaxy Growth on (001) Silicon.

Si-based group III-V material enables a multitude of applications and functionalities of the novel optoelectronic integration chips (OEICs) owing to their excellent optoelectronic properties and compatibility with the mature Si CMOS process technology. To achieve high performance OEICs, the crystal quality of the group III-V epitaxial layer plays an extremely vital role. However, there are several challenges for high quality group III-V material growth on Si, such as a large lattice mismatch, highly thermal expansion coefficient difference, and huge dissimilarity between group III-V material and Si, which inevitably leads to the formation of high threading dislocation densities (TDDs) and anti-phase boundaries (APBs). In view of the above-mentioned growth problems, this review details the defects formation and defects suppression methods to grow III-V materials on Si substrate (such as GaAs and InP), so as to give readers a full understanding on the group III-V hetero-epitaxial growth on Si substrates. Based on the previous literature investigation, two main concepts (global growth and selective epitaxial growth (SEG)) were proposed. Besides, we highlight the advanced technologies, such as the miscut substrate, multi-type buffer layer, strain superlattice (SLs), and epitaxial lateral overgrowth (ELO), to decrease the TDDs and APBs. To achieve high performance OEICs, the growth strategy and development trend for group III-V material on Si platform were also emphasized.

Development of core technology for reconfigurable FET based on bulk CMOS

Although silicon-on-insulator (SOI) technologies can help improve the performance of semiconductor devices, they have drawbacks, including high manufacturing cost and low productivity. Dr Dongwoo Suh, Materials and Components Research Division, Electronics and Telecommunications Research Institute, is working to overcome these limitations. His research concerns the development of core technology for reconfigurable field-effect transistors (RFETs) based on bulk complementary metal-oxide-semiconductor (CMOS) and he and his collaborators have developed a new technique that could help advance the field of semiconductor devices. In recent work, Suh and the team have applied an existing technique called epitaxial lateral overgrowth (ELO) to the development of reconfigurable transistors. The researchers were able to form a large, high-quality SOI layer on a silicon wafer by combining ELO with a conventional chemical vapour deposition (CVD) process. By doing this, the team created a growth process of crystalline semiconductor on amorphous substrate with applications to advanced silicon transistors. By combining ELO with reconfigurable transistors and their logic gates the researchers are maximising the potential that RFETs hold for electronics. They believe that their technique can be used in advanced silicon transistors to create more advanced transistors with desirable characteristics.

α-Gallium Oxide Films on Microcavity-Embedded Sapphire Substrates Grown by Mist Chemical Vapor Deposition for High-Breakdown Voltage Schottky Diodes.

α-Gallium oxide, with its large band gap energy, is a promising material for utilization in power devices. Sapphire, which has the same crystal structure as α-Ga2O3, has been used as a substrate for α-Ga2O3 epitaxial growth. However, lattice and thermal expansion coefficient mismatches generate a high density of threading dislocations (TDs) and cracks in films. Here, we demonstrated the growth of α-Ga2O3 films with reduced TD density and residual stress on microcavity-embedded sapphire substrates (MESS). We fabricated the two types of substrates with microcavities: diameters of 1.5 and 2.2 μm, respectively. We confirmed that round conical-shaped cavities with smaller diameters are beneficial for the lateral overgrowth of α-Ga2O3 crystals with lower TD densities by mist chemical vapor deposition. We could obtain crack-free high-crystallinity α-Ga2O3 films on MESS, while the direct growth on a bare sapphire substrate resulted in an α-Ga2O3 film with a number of cracks. TD densities of α-Ga2O3 films on MESS with 1.5 and 2.2 μm cavities were measured to be 1.77 and 6.47 × 108 cm-2, respectively. Furthermore, cavities in MESS were certified to mitigate the residual stress via the redshifted Raman peaks of α-Ga2O3 films. Finally, we fabricated Schottky diodes based on α-Ga2O3 films grown on MESS with 1.5 and 2.2 μm cavities, which exhibited high breakdown voltages of 679 and 532 V, respectively. This research paves the way to fabricating Schottky diodes with high breakdown voltages based on high-quality α-Ga2O3 films.

The role of Ga supersaturation on facet formation in the epitaxial lateral overgrowth of GaN

In this paper, facet formation of (0001) {112¯0} {112¯2} facets during epitaxial lateral overgrowth (ELO) of GaN is investigated for different Ga vapor supersaturations. The ELO was conducted via metalorganic chemical vapor deposition on patterned GaN/sapphire templates with SiO2 masks aligned along the ⟨11¯00⟩ direction of GaN. Scanning electron microscopy was used to characterize the cross section shapes of the ELO GaN islands. A correlation of supersaturation, facet formation, and the shape of the ELO GaN islands is found. It is shown that {112¯2} facets are favored under high Ga vapor supersaturation, while {112¯0} facets are favored under low Ga vapor supersaturation. A qualitative model based on Wulff construction and density functional theory calculation is proposed to illustrate the mechanism of the facet formation of the ELO GaN islands.

Study on Nucleation and Growth Mode of GaN on Patterned Graphene by Epitaxial Lateral Overgrowth

Study on Nucleation and Growth Mode of GaN on Patterned Graphene by Epitaxial Lateral Overgrowth

A Self-Disappear-Mask for Epitaxial Lateral Overgrowth of Gan Films

A Self-Disappear-Mask for Epitaxial Lateral Overgrowth of Gan Films

Four-inch high quality crack-free AlN layer grown on a high-temperature annealed AlN template by MOCVD

In this work, based on physical vapor deposition and high-temperature annealing (HTA), the 4-inch crack-free high-quality AlN template is initialized. Benefiting from the crystal recrystallization during the HTA process, the FWHMs of X-ray rocking curves for (002) and (102) planes are encouragingly decreased to 62 and 282 arcsec, respectively. On such an AlN template, an ultra-thin AlN with a thickness of ~700 nm grown by MOCVD shows good quality, thus avoiding the epitaxial lateral overgrowth (ELOG) process in which 3–4 μm AlN is essential to obtain the flat surface and high crystalline quality. The 4-inch scaled wafer provides an avenue to match UVC-LED with the fabrication process of traditional GaN-based blue LED, therefore significantly improving yields and decreasing cost.

Strain-driven phase manipulation of α- and κ-Ga2O3 by nanoepitaxial lateral overgrowth on embedded α-In2O3 submicron dots

Heteroepitaxy of corundum-structured α-Ga2O3 and intriguing ferroelectric κ-Ga2O3 is proven as an alternative strategy to solve current challenges in heat dissipation and large-scale productivity for Ga2O3-based power electronic devices, whereas the fundamental growth dynamics and phase control of metastable Ga2O3 are still far unexplored. In this Letter, we demonstrate the strategy of strain engineering for the in situ phase manipulation of metastable Ga2O3 by embedding α-In2O3 submicrometer dots. Phase transition is modulated by the surface coverage of α-In2O3 due to the competitive growth of κ-Ga2O3 upon α-In2O3 and the homoepitaxy of α-Ga2O3 on the exposed α-Ga2O3 seed region. Upon discrete α-In2O3 submicrometer dots with a low surface coverage, the growth undergoes a nano-scale epitaxial lateral overgrowth mode, in which the selective homoepitaxy of α-Ga2O3 is dominant, and embedded α-In2O3 serves as nano-masks to prevent the threading dislocation propagation into the lateral overgrown Ga2O3 layer. In comparison, κ-Ga2O3 is energetically favorable on the interconnected α-In2O3 submicrometer dots, which are driven by the in-plane tensile strain as probed by the geometric phase analysis of transmission electron microscopy. Phase manipulation by embedded sub micrometer dots allows us to deliver high-quality Ga2O3 with well-defined phases and to conceive advanced devices with ultra-low loss, high frequency, and memorizing functionality.

Epitaxial lateral overgrowth of r-plane α-Ga2O3 with stripe masks along ⟨1¯21¯⟩

We demonstrated the epitaxial lateral overgrowth (ELO) of (1¯012) (r-plane) α-Ga2O3 using a striped mask pattern along ⟨1¯21¯0⟩. α-Ga2O3 stripes with an asymmetric cross-sectional shape were formed selectively on the windows at the initial growth stage. They grew vertically and laterally to coalesce with each other, and a compact film was obtained. The film surface exhibited wave-like morphology with macro-scale inclined terraces and steps because of the asymmetric cross-sectional stripe shape. The macrosteps moved laterally like traveling waves as the growth proceeded. Transmission electron microscopy revealed that a domain on a window grew toward the inclined c-axis direction to cover the adjacent domain after the coalescence. As a result, the dislocations, which propagated into the α-Ga2O3 stripe from the seed layer through the window, bent toward the c axis and concentrated in a narrow area. This concentration should enhance the pair annihilation. Therefore, the dislocation density was markedly reduced on the top surface including the areas above the windows and coalesced boundaries in contrast to the cases of conventional c-, a-, and m-plane ELO.

Visualization of threading dislocations in an α-Ga2O3 epilayer by HCl gas etching

Threading dislocations in a heteroepitaxial α-Ga2O3 film was visualized as etch pits on the surface. We found that etch pits were formed on a c-plane α-Ga2O3 epilayer by HCl gas etching. The epilayer was prepared by using epitaxial lateral overgrowth technique with a stripe mask pattern. The etch pit density was very high in the window region, and much lower in the laterally grown area on the mask. A line of etch pits was observed at a coalesced boundary. Thus, the etch pit density had a clear correlation with the dislocation density. The correspondence between the etch pits and dislocations was confirmed by cross-sectional bright- and dark-field transmission electron microscopy (TEM). This gas-etching technique can clarify the distribution of dislocations in a wide area, which cannot be explored effectively by TEM.

Effect of a Lateral Overgrowth Process on the Strain Evolution of AlN Films Grown on a Nanopatterned Sapphire Substrate for Ultraviolet‐C Light‐Emitting Diode Applications

Strain evolution of AlN grown on hole‐type (HT) and pillar‐type (PT) nanopatterned sapphire substrates (NPSSs) during the epitaxial lateral overgrowth process is comparably studied. It is found that, after complete coalescence, the residual strain of AlN on HT‐NPSS is tensile, whereas AlN on PT‐NPSS is almost strain free. The different strain states are associated with different behaviors of crystal merging during the lateral overgrowth process, depending on the type of pattern. Then, ultraviolet‐C (UVC) light‐emitting diode (LED) wafers are grown on two types of AlN templates with the distinctive strain states. It is confirmed that AlN on HT‐NPSS with tensile strain offers the advantage of producing a high‐quality AlGaN layer with a fairly flat surface, which is the key point for fabricating high‐performance UVC LEDs.

Confined lateral epitaxial overgrowth of InGaAs: Mechanisms and electronic properties

Template-assisted selective area growth techniques have gained popularity for their ability to grow epitaxial materials in prefabricated dielectric templates. Confined epitaxial lateral overgrowth (CELO) is one such technique that uses dielectric templates to define the geometry of the grown nanostructures. Two terminal low-temperature magneto-transport measurements were used to determine electronic properties. For doped In0.53Ga0.47As CELO nanostructures, we observe Shubnikov–De Hass oscillations in the longitudinal magnetoresistance and utilize these to estimate effective mass, carrier density, and mobilities. This analysis both reveals the presence of defects in these nanostructures and material variabilities between growth runs. Electron beam lithography and contact deposition for transport measurements were enabled by parasitic growth removal. In the future, this approach can enable other material systems to be explored for confined lateral epitaxy, improve material quality, and investigate a variety of quantum transport phenomenon in such nanoscale devices.

High‐Temperature Annealing and Patterned AlN/Sapphire Interfaces

Using the example of epitaxial lateral overgrowth of AlN on trench‐patterned AlN/sapphire templates, the impact of introducing a high‐temperature annealing step into the process chain is investigated. Covering the open surfaces of sapphire trench sidewalls with a thin layer of AlN is found to be necessary to preserve the trench shape during annealing. Both the influence of annealing temperature and annealing duration are investigated. To avoid the deformation of the AlN/sapphire interface during annealing, the annealing duration or annealing temperature must be low enough. Annealing for 1 h at 1730 °C is found to allow for the lowest threading dislocation density of 3.5 × 108 cm−2 in the subsequently grown AlN, while maintaining an uncracked smooth surface over the entire 2 in. wafer. Transmission electron microscopy study confirms the defect reduction by high‐temperature annealing and reveals an additional strain relaxation mechanism by accumulation of horizontal dislocation lines at the interface between annealed and nonannealed AlN. By applying a second annealing step, the dislocation density can be further reduced to 2.5 × 108 cm−2.

Sub- μ m features patterned with laser interference lithography for the epitaxial lateral overgrowth of α-Ga2O3 via mist chemical vapor deposition

The low growth rate of mist chemical vapor deposition normally requires a long growth time to achieve coalescence in the epitaxial lateral overgrowth of α-Ga2O3 thin films on sapphire substrates. To address this issue, sub-μm features were patterned using laser interference lithography. Periodical stripes with a ∼590-nm pitch allowed the overgrowth of crack-free, void-free, and continuous thin films, while typical growth conditions using a low carrier gas flow rate and a low Ga precursor concentration were maintained. Coalescence was achieved even with a short growth time of <30 min and a low film thickness of <500 nm. Transmittance and x-ray diffraction spectra show that the film was predominantly in α-phase. Transmission electron microscopy (TEM) images reveal cup-top-like α-Ga2O3 regions of low dislocation density on the SiOx mask. Selected area electron diffraction and high-resolution TEM analyses confirm that an α-Ga2O3 layer was formed even on the top of the SiOx mask. Interestingly, the dislocations formed on the window areas did not bend toward the center of the masks; rather, a dislocation bending outward from the center was observed. This suggests the occurrence of early coalescence and/or atomic rearrangement.

Effect of MOVPE growth conditions on AlN films on annealed sputtered AlN templates with nano-striped patterns

The metalorganic vapor phase epitaxial (MOVPE) growth of AlN films was performed on face-to-face annealed sputtered AlN templates (FFA Sp-AlN) having a nano-striped pattern with a period of 300 nm and depth of 120 nm. Then, the effect of MOVPE growth conditions on the crystallinity was elucidated. The faceted structure clearly changed with the growth temperature (Tg), following the same trend as the epitaxial lateral overgrowth of GaN film on micro-patterned GaN. High Tg levels enhanced the lateral growth, and thus with a high Tg of 1300 °C, the coalescence in the early growth stage resulted in AlN films with coalesced surfaces and a thickness of 1 µm. For the AlN film grown at a Tg of 1300 °C, the threading dislocation density (TDD) was estimated to be 6.0 × 108 cm−2. This value proved that coalesced AlN films on nano-patterned FFA Sp-AlN templates with low TDDs could be achieved with the appropriate MOVPE growth conditions. The compressive strain in the AlN film on nano-patterned FFA Sp-AlN was lower than that of the film grown on FFA Sp-AlN without a pattern.

Completely merged multi-color asymmetric pyramid with suppressed stress and defect density

The selective area growth (SAG) technique enables the creation of multi-color micro-structures. However, the substrate pattern to obtain multi-color and mass transfer of these micro-structures are complicated to achieve. Moreover, high dislocation density as well as large residual stress in c-plane quantum wells still remain challenges. Here, an improved SAG strategy was proposed to achieve completely merged asymmetric pyramids on closely connected deep-concave holes via the lateral overgrowth of adjacent pyramids. These asymmetric pyramids were directly grown on the sapphire substrate, making mass transfer be possible. Growth rate distinction appears at the different positions of the overgrowth region, resulting in a multi-wavelength emission. More importantly, the residual stress and the dislocation density can be suppressed by utilizing the deep-hole pattern and lateral overgrowth, contributing to a lower piezoelectric polarization field and a higher spontaneous emission rate. These ultrafast and high-efficient multi-color micro-LEDs are suitable for the application in high-resolution display, flexible devices, and high-speed visible-light-communication.