Reduction Of Threading Dislocations Research Articles

On the origin of threading dislocations during epitaxial growth of III-Sb on Si(001): A comprehensive transmission electron tomography and microscopy study

Electron tomography and complementary (scanning) transmission electron microscopy (STEM) are applied to investigate the origin of threading dislocations in the large lattice misfit, heteroepitaxial system of III-Sb on vicinal Si(001). Buried AlSb islands of the initial wetting layer are revealed at the interface toward the substrate in the three-dimensionally reconstructed data. Locations of island coalescence are retrieved from the tomogram. Complementary (S)TEM measurements reveal the location of threading dislocations and the presence of antiphase boundaries at the same specimen area. The number density of threading dislocations emanating from the interface and their distribution are unexpected. It is shown that the presence of threading dislocations is not simply correlated to sites of AlSb-islands coalescence or to the film closure during the transition from a 3D to a 2D growth. Moreover, an interaction with antiphase boundaries is suggested by the presented observations. Consequently, the contemporary notion of threading dislocation formation is refined and, eventually, it is suggested that measures to avoid antiphase domains and such to reduce threading dislocations have to be balanced for future strategies to epitaxially grow sphalerite structure III-V semiconductors on Si or Ge.

Threading dislocation reduction in three-dimensionally grown GaN islands on Si (111) substrate with AlN/AlGaN buffer layers

Three-dimensional GaN island growth without any masks was first introduced under high pressure in metalorganic chemical vapor deposition after the growth of AlN and AlGaN buffer layers on Si (111) substrate, followed by two-dimensional GaN growth to form a continuous GaN film with improvement of the crystalline quality and surface smoothness. X-ray diffraction and cross-sectional scanning transmission electron microscopy analyses show that a high-quality GaN film can be achieved by bending of edge threading dislocations (TDs) and the formation of dislocation half-loops. It is observed that most of edge TDs bend 90° from the growth direction along c-axis, whereas mixed TDs bend about 30° towards the inclined sidewall facets of the islands. Consequently, a 1.2 μm thick GaN epitaxial film with a low threading dislocation density of 2.5 × 108 cm−2 and a smooth surface of 0. 38 nm roughness can be achieved on Si substrate.

ヘテロエピタキシャル成長における歪緩和と貫通転位の低減 : Si(001)基板上の高品質Si_ Ge_x歪緩和層の成長( バルク成長分科会特集 : 結晶成長の科学と技術)

ヘテロエピタキシャル成長における歪緩和と貫通転位の低減 : Si(001)基板上の高品質Si_ Ge_x歪緩和層の成長( バルク成長分科会特集 : 結晶成長の科学と技術)

Non-polar GaN film growth on (0 1 0) gallium oxide substrate by metal organic chemical vapor deposition

Abstract

Defect structure of high temperature hydride vapor phase epitaxy–grown epitaxial (0 0 0 1) AlN/sapphire using growth mode modification process

Defect structures were investigated by transmission electron microscopy for AlN/sapphire (0001) epilayers grown by high temperature hydride vapor phase epitaxy using a growth mode modification process. The defect structures, including threading dislocations, inversion domains, and voids, were analyzed by diffraction contrast, high-resolution imaging, and convergent beam diffraction. AlN film growth was initiated at 1450°C with high V/III ratio for 8min. This was followed by low V/III ratio growth for 12min. The near-interfacial region shows a high density of threading dislocations and inversion domains. Most of these dislocations have Burgers vector b=1/3〈1120〉 and were reduced with the formation of dislocation loops. In the middle range 400nm<h<2μm, dislocations gradually aggregated and reduced to ∼109cm−2. The inversion domains have a shuttle-like shape with staggered boundaries that deviate by ∼±5° from the c axis.Above 2μm thickness, the film consists of isolated threading dislocations with a total density of 8×108cm−2. Most of threading dislocations are either pure edge or mixed dislocations. The threading dislocation reduction in these films is associated with dislocation loops formation and dislocation aggregation-interaction during island growth with high V/III ratio.

Annihilation of Threading Dislocations in Strain-Relaxed Si1-XGex Layer By Hydrogen Ion Implantation

According to the international technology roadmap for semiconductor (ITRS), the development of MOSFETs based on the strain technology is necessary for the next generation technology beyond 10-nm C-MOSFETs. By being applied the strain in the channel of MOSFET, the electron mobility can be enhanced results in the higher current in MOS channel at the fixed gate voltage and oxide thickness. However, due to the lattice mismatch of 4.2 % between Si and Ge, many threading dislocations are generated during the epitaxial growth, resulted in increasing the leakage current of the C-MOSFETs. Currently, the threading dislocation density of 3-µm-thick X-at% (>30-at%) graded Si1-xGex layer is higher than 105 cm-2, which is too high level to be applied in mass production because of degrading the productivity and the performance of the C-MOSFETs. In this study, for that reason, to reduce threading dislocations in the strain-relaxed Si1-xGex layer, we investigated how the hydrogen ion implantation annihilated the threading dislocations in strain-relaxed Si1-xGex layer/Si (100) substrate. First, a relaxed Si1-xGex layer was grown on a Si (100) substrate by epitaxial growth. Second, hydrogen ions were implanted onto the interface between the strain-relaxed Si1-xGex layer and the Si substrate. After ion implantation, the sample was annealed at 800oC for 30 min in a nitrogen ambient. For the case of hydrogen ion implantation with a dose of 5ⅹ1015 cm-2, it was confirmed that the cross-hatch pattern density and the surface roughness was decreased and threading dislocations near the interface were not observed in hydrogen ion-implanted Si1-xGex/Si wafer as shown in Fig. 1. In addition, as shown in Fig. 2, by using in-situ real-time high-resolution TEM, it was confirmed that a threading dislocation in hydrogen ion-implanted Si0.7Ge0.3 layer/Si substrate was annihilated with hydrogen-related defect while the TEM specimen was being annealed at 600oC for 30 min in the heating holder. Therefore, we present the formation mechanism of threading dislocations in a strain-relaxed Si1-xGex layer/Si (100) substrate and the annihilation mechanism of how the threading dislocations are annihilated by hydrogen ion implantation.* This work was financially supported by the Brain Korea 21 Plus Program in 2016 and SiWEDS (Silicon Wafer Engineering and Defect Science). Reference [1] J. G. Park et al., Mat. Sci. Eng. B 134 142-153 (2006) [2] S. Takagi, Strained-Si CMOS Technology in Advanced Gate Stacks for High-Mobility Semiconductors, Springer Berlin Heidelberg 1 (2007) Figure 1

Effect of curved graphene oxide in a GaN light-emitting-diode for improving heat dissipation with a patterned sapphire substrate

AbstactThis article reports on the development of a reduced graphene oxide (rGO)-embedded light-emitting diode (LED) on a patterned sapphire substrate (PSS), for improving heat dissipation and reducing threading dislocations. The prototype device fabrication involves the generation of scalable curved graphene oxide microscale patterns on a PSS, followed by thermal reduction and epitaxial lateral overgrowth of GaN in a metal–organic chemical vapor deposition system with a one-step process. Using the forward voltage method, the junction temperature Tj of the rGO-embedded LED was found to be reduced by about 17 °C from the ∼62 °C of the LED grown without the rGO buffer layer. Temperature-dependent light output power and chip surface temperature measurements were also performed and the results are discussed.

Grouped and Multistep Nanoheteroepitaxy: Toward High-Quality GaN on Quasi-Periodic Nano-Mask.

A novel nanoheteroepitaxy method, namely, the grouped and multistep nanoheteroepitaxy (GM-NHE), is proposed to attain a high-quality gallium nitride (GaN) epilayer by metal-organic vapor phase epitaxy. This method combines the effects of sub-100 nm nucleation and multistep lateral growth by using a low-cost but unique carbon nanotube mask, which consists of nanoscale growth windows with a quasi-periodic 2D fill factor. It is found that GM-NHE can facilely reduce threading dislocation density (TDD) and modulate residual stress on foreign substrate without any regrowth. As a result, high-quality GaN epilayer is produced with homogeneously low TDD of 4.51 × 10(7) cm(-2) and 2D-modulated stress, and the performance of the subsequent 410 nm near-ultraviolet light-emitting diode is greatly boosted. In this way, with the facile fabrication of nanomask and the one-off epitaxy procedure, GaN epilayer is prominently improved with the assistance of nanotechnology, which demonstrates great application potential for high-efficiency TDD-sensitive optoelectronic and electronic devices.

High-Performance Ultraviolet 385-nm GaN-Based LEDs With Embedded Nanoscale Air Voids Produced Through Atomic Layer Deposition and Al2O3Passivation

This letter developed the high-performance GaN-based light-emitting diodes (LEDs) for ultraviolet (UV) light emission at 385 nm with the light output power enhanced by up to 52% compared with that of the conventional UV-LEDs. The high-power-efficiency UV-LEDs include sidewall-passivated Al2O3 dielectric nanoscale air voids, which were created using the atomic layer deposition method and act as intermediate media for improving the internal quantum efficiency, reducing threading dislocation, and relaxing strain. The fabricated air void nanostructure also enhanced the light extraction efficiency by $\sim 32.7$ %, resulting in a high-power UV light-emitting device.

Free and bound excitonic effects in Al0.5Ga0.5N/Al0.35Ga0.65N MQWs with different Si-doping levels in the well layers.

Free exciton (FX) and bound exciton (BX) in Al0.5Ga0.5N/Al0.35Ga0.65N multiple quantum wells (MQWs) with different Si-doping levels in the well layers are investigated by photoluminescence (PL) spectra. Low temperature (10 K) PL spectra identify a large binding energy of 87.4 meV for the BX in undoped sample, and 63.6 meV for the BX in Si-doped (2 × 1018 cm−3) sample. They are attributed to O-bound and Si-bound excitons, respectively. The large binding energies of BX are assumed to originate from the strong quantum confinement in the quantum wells, which also leads to a stronger FX PL peak intensity in comparison with BX at 10 K. Si-doping is found to suppress the FX quenching by reducing threading dislocation density (TDD) in the well layers, leading to a significant improvement of IQE from 33.7% to 45%.

Dramatic reduction of dislocations on a GaN point seed crystal by coalescence of bunched steps during Na-flux growth

In our study, we found that threading dislocation density (TDD) in GaN crystals naturally reduced from ~109cm−2 in a seed to less than ~103cm−2, just by using the small-sized seed called a “point seed”. However, the mechanism of the dramatic reduction was unclear. In order to reveal the mechanism of this substantial reduction of TDD, we investigated the relationship between TDD and the crystal habit during the growth. Cathodoluminescence (CL) and scanning electron microscopy (SEM) images showed that TDD was dramatically reduced after the c face became small (<50×50μm2) in the habit-change process caused by changes of supersaturation during growth, in which bunched steps growing from the edge of the c face coalesced at the center. It is thought that the shrinking of the c face in the growth process enabled the coalescence of bunched steps, which led to the gathering of threading dislocations (TDs), and resulted in the dramatic reduction of TDD. We concluded that the natural reduction of TDs was caused by coalescence of bunched steps, which easily occurs in during the Na-flux growth on small-sized “point seeds”, and which allowed fabrication of low-dislocation-density GaN wafers.

Extenuation of Stress and Defects in GaN Films Grown on a Metal–Organic Chemical Vapor Deposition-GaN/c-Sapphire Substrate by Plasma-Assisted Molecular Beam Epitaxy

We investigated curbing the defects and stress/strain in epitaxially grown crystalline GaN films on a metal–organic chemical vapor deposition-GaN/c-sapphire (MGcS) template by using plasma-assisted molecular beam epitaxy and demonstrated the impact of growth temperature on their structural, morphological, and optical properties. An in-plane compressive stress having a minimum value of 0.34 GPa has been investigated by vibrational spectroscopy. This alleviated stress was attributed to a less pitted and smoother surface morphology along with reduced threading dislocation densities. Moreover, photoluminescence measurements explicate reduced yellow band emissions relative to near-band edge emission for the film grown under optimum growth conditions. The stress-relaxed and defect-free crystalline GaN film can further be utilized for tremendous optoelectronic and photonic based applications.

Direct growth of GaN layer on carbon nanotube-graphene hybrid structure and its application for light emitting diodes.

We report the growth of high-quality GaN layer on single-walled carbon nanotubes (SWCNTs) and graphene hybrid structure (CGH) as intermediate layer between GaN and sapphire substrate by metal-organic chemical vapor deposition (MOCVD) and fabrication of light emitting diodes (LEDs) using them. The SWCNTs on graphene act as nucleation seeds, resulting in the formation of kink bonds along SWCNTs with the basal plane of the substrate. In the x-ray diffraction, Raman and photoluminescence spectra, high crystalline quality of GaN layer grown on CGH/sapphire was observed due to the reduced threading dislocation and efficient relaxation of residual compressive strain caused by lateral overgrowth process. When applied to the LED structure, the current-voltage characteristics and electroluminescence (EL) performance exhibit that blue LEDs fabricated on CGH/sapphire well-operate at high injection currents and uniformly emit over the whole emission area. We expect that CGH can be applied for the epitaxial growth of GaN on various substrates such as Si and MgO, which can be a great advantage in electrical and thermal properties of optical devices fabricated on them.

Threading dislocation reduction in a GaN film with a buffer layer grown at an intermediate temperature

Remarkable reduction of the threading dislocation (TD) density has been achieved by inserting a GaN layer grown at an intermediate temperature (900 °C) (IT-GaN layer), just prior to the growth of GaN at 1040 °C by using a hydride vapor phase epitaxy. The variation in the dislocation density variation along the growth direction was observed by using cathodoluminescence (CL) and transmission electron microscopy (TEM). A cross-sectional CL image revealed that the reduction of the TD density happened during the growth of IT-GaN layer. The TEM measurement provided the proof that the TD reduction could be ascribed to the masking of the TD by stacking faults in the IT-GaN layer.

Epitaxial growth of AlN films on sapphire via a multilayer structure adopting a low- and high-temperature alternation technique

Epitaxial growth of AlN films on c-sapphire using a multilayer structure has been investigated by metal–organic chemical vapor deposition adopting multiple alternation cycles of low- and high-temperature (LT–HT) growth. It is found that the surface morphology and crystal quality can be greatly improved using three alternation cycles with X-ray diffraction ω-scan full width at half maximum values of 311 and 548 arcsec for the (0002) and (10−12) peaks, respectively, which are induced by the alternation of the three-dimensional (3D) and two-dimensional (2D) growth modes caused by the LT–HT process. The first 3D–2D cycle is found to play a major role in threading dislocation reduction, while the second and third cycles mainly account for tensile stress relaxation.

Improved carrier mobility of InAlN/GaN heterostructure grown on cone-shaped patterned sapphire substrate

InAlN/GaN heterostructures were grown on both patterned and planar sapphire substrates by metal–organic chemical vapor deposition. Hall measurements showed that both samples possessed similar two-dimensional electron gas densities, whereas the electron mobility of the heterostructure grown on a patterned substrate was evidently superior. The enhanced mobility was attributed to the reduced threading dislocation density and improved channel-barrier interface morphology. Furthermore, Raman measurement results showed that there was less residual compressive strain in the GaN grown on the pattered sapphire substrate, which could be the reason for the morphology improvement. Consequently, the patterned sapphire substrate is very promising for further improving the performance of high-electron-mobility transistors.

Invited; Synchrotron Radiation Based X-ray Microdiffraction of Advanced Semiconductor Materials

X-ray diffraction (XRD) has been recognized as a well-established method by which lattice structures of crystalline materials can be clarified precisely. High penetrating power of X-ray allows us to reveal inner structures and buried interfaces usually implemented in materials and devices. Conventional XRD with a typical probe size of over 100 μm clarifies only averaged structures having good statistics without any untypical ones, known as ensemble average[1]. However, in general, the property and the performance of materials and devices become governed by the localized specific structures when miniaturized to a nanometer scale. In such a case, an X-ray beam with a probe size much smaller than the conventional one is truly required.X-ray microdiffraction (XRMD) is one of the most effective methods to establish both the advantages of XRD analysis and high special resolution to exceed the ensemble average analysis. The state-of-the-art XRMD system at the beam line of the synchrotron radiation facility, Super Photon ring 8 GeV (SPring-8), in Japan, realizes an X-ray probe with a size of a few hundred nanometer [2]. Thus XRMD gives us precise and rich information especially on the crystalline texture and the local strain distribution on a submicron scale.1. Crystallinity of Ge thin linesLine-shaped Ge films with nanometer-sized widths on Si substrates are promising for applying to next generation three dimensional high-mobility channels in metal-oxide-semiconductor field effect transistors. One of the concerns is the introduction of threading dislocations in the Ge films; 60° dislocations critically alter the crystallinity of Ge films. Aspect ratio trapping with the use of selective epitaxy of Ge on SiO2-patterned Si substrates is one of effective ways to reduce threading dislocations in Ge thin line channels [3]. However, there is insufficient understanding of strain relaxation and local structures in the Ge thin lines with such anisotropic geometry.XRMD has been employed to investigate the crystallinity of the line-shaped Ge films with line widths of 100-1000 nm [4]. It has been revealed that crystal domains with a size of about 100 nm in the Ge thin lines were tilted at different angles within the range of 0.6 degree. The tilt angle range was larger in thinner Ge lines after the growth and reduced by annealing for all line widths. The domaining was found to be mostly caused by the introduced 60° dislocations. The formation of a single large domain with a specific tilt angle was also achieved near the surfaces of Ge thin lines with widths of 100 and 200 nm, which was attributed to the suppression of threading dislocation formation by the SiO2side walls.2. Local strain mapping for AlN thick filmsAlN with a 6.2-eV-wide bandgap is potentially prospective as a substrate material for deep ultraviolet optoelectronics devices. Due to the lack of bulk AlN crystals, thick film growth with hydride vapor phase epitaxy is known to be promising for the production of large scale AlN wafers. Crystalline quality of AlN films directly grown on α-Al2O3substrates is poor with high dislocation density and residual strain due to lattice and thermal mismatches. In contrast, epitaxial lateral overgrowth using trench-patterned templates was proven to be effective in obtaining high-quality films [5].Local strain distribution in thick AlN films grown on a periodic trench-patterned AlN/α-Al2O3template (see Fig. 1(a)) has been investigated by XRMD [2]. Figure 1(a) reveals nanometer-scale voids that form tunnels in the AlN film and are periodically arranged at 6-µm-intervals corresponding to the periodicity of trench pattern. XRMD experiment with the X-ray incidence into the cross-sectional plane of sample (see Fig. 1(b)) was conducted for taking reciprocal space maps (RSMs) from the two-dimensional (2D) scan area at 1- and 2-μm intervals in the X and Z axes, respectively. 2D distribution maps of strains along the Z and X directions obtained from the RSMs are shown in Figs. 2(a) and 2(b), respectively. It should be noticed that both strains are considerably relaxed around the void region, as illustrated in Fig. 2(c). This result demonstrates that XRMD clearly discloses the local strain distribution in the thick AlN film and the voids are effective in relaxing the strain induced during the growth process.

A Self-Assembled Double-Island Buffer Structure for Improving GaN-Based Materials Grown on Si Substrates

This paper reports a new method to reduce threading dislocation density in GaN epilayers grown on (111) Si substrates by metal-organic chemical vapor deposition. This method utilizes an in- situ SiNx mask to produce a self-assembled double-island structure, which effectively reduces threading dislocation density from 9.6 × 109 cm−2 to 2.6 × 109 cm−2 without using any lithographic and regrowth processes. InGaN light-emitting diodes fabricated on the double-island buffer layer exhibit nearly 20% improvement on the optical output power as well as less energy localization effect, compared to those without the double-island buffer layer. The mechanisms of double-island formation as well as dislocation reduction are proposed based on transmission electron microscopy investigations.

Low dislocation density AlGaN epilayers by epitaxial overgrowth of patterned templates

We present an epitaxial overgrowth process for reducing threading dislocations in AlxGa1−xN over the entire compositional range. This process avoids the use of UV-absorbing GaN layers and results in a spatially uniform defect reduction which eliminates the need for precise alignment of devices to low-defect areas of the wafer. Using the described overgrowth process, we demonstrate Al0.3Ga0.7N and Al0.6Ga0.4N epilayers with a dislocation density of 2×108cm−2 and 5×108cm−2 respectively, rendering them suitable as templates for deep-UV bottom-emitting LEDs and laser diodes. The process involves patterning of submicron-wide-stripes, less than 1μm in height, into an AlGaN/AlN/sapphire template and subsequent regrowth of a 5–10μm thick AlGaN epilayer. The sub-micron width of the mesa allows for bending of threading dislocations that would continue to thread vertically through wider mesas. Utilizing 1.3mm-thick sapphire substrates (3× thicker than commonly used), epilayer cracking from regrowth is eliminated and wafer bow over a 2-in. diameter substrate is reduced to less than 15μm. We observed a 7× increase in photoluminescence intensity from GaN–AlGaN multi-quantum well structures emitting at 340nm and a 15× increase in electroluminescence from laser diode heterostructures when grown on patterned Al0.3Ga0.7N templates.

Improving the quality of GaN epilayer by preparing a novel patterned sapphire substrate

GaN epilayer was grown on a new polyhedral patterned sapphire substrate (new PSS) by metal–organic chemical vapor deposition. The new PSS was prepared by combining the dry etching technique and wet etching technique. The X-ray diffraction indicated that the full width at half maximum values of (0002) and (\(10\overline{1}2\)) diffraction peaks in the GaN epilayer grown on the new PSS were evidently smaller than that in the GaN epilayer grown on the normal treated PSS. The improvement of GaN quality was attributed to the reduction of threading dislocations (TDs) in GaN epilayer, and the mechanism of the reduction of TDs was analyzed. The influence of the new PSS on the optical properties as well as the residual stress in GaN epilayer was also discussed.