Epitaxial Lateral Overgrowth Technique Research Articles

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.

Fabrication of a Micron-Scale Three-Dimensional Single Crystal Diamond Channel Using a Micro-Jet Water-Assisted Laser

Two types of a trench with conventional vertical and new reverse-V-shaped cross-sections were fabricated on single crystal diamond (SCD) substrate using a micro-jet water-assisted laser. In addition, a microwave plasma chemical vapor deposition device was used to produce multiple micrometer-sized channels using the epitaxial lateral overgrowth technique. Raman and SEM methods were applied to analyze both types of growth layer characterization. The hollowness of the microchannels was measured using an optical microscope. According to the findings, the epitaxial lateral overgrowth layer of the novel reverse-V-shaped trench produced improved SCD surface morphology and crystal quality.

Ridge-channel AlGaN/GaN normally-off high-electron mobility transistor based on epitaxial lateral overgrowth

A ridge-channel AlGaN/GaN high-electron mobility transistor (HEMT) utilizing selective-area growth and epitaxial lateral overgrowth (ELO) technique is proposed in this work to achieve high-performance normally-off devices. It has a c-plane platform for the source and the drain contacts, and sidewalls of lattice plane for the gate contact. The sidewalls have characteristics of weak polarization and thin barrier, which are advantageous for realizing normally-off operation. Two ridge HEMTs with triangular and trapezoid channel are designed. Theoretical simulation demonstrates a threshold voltage of 0.03 V for the sidewall channel with reduced polarization and barrier thickness, and a threshold voltage of 1.1–1.3 V for the ridge HEMTs assuming no polarization charge in sidewall channel. The ridge-channel device also exhibits high saturation drain current. The ELO-based ridge-channel opens a new way to achieve normally-off AlGaN/GaN HEMT.

Growth of 300-nm-thick epitaxial AlInN films on a semi-relaxed c-plane GaInN template by metalorganic chemical vapor deposition

Metalorganic chemical vapor deposition of approximately 300-nm thick epitaxial AlInN films with different alloy compositions was performed using a semi-relaxed c-plane GaInN template as an underlying substrate. The GaInN template consisted of a Ga0.98In0.02N film on a facet-structured GaN film formed on a c-plane sapphire substrate by the epitaxial lateral overgrowth technique, and its surface was treated with the chemical-mechanical polishing. It was observed that an Al0.835In0.165N film grown with an in-plane tensile strain exhibited a relatively smooth surface whereas an Al0.781In0.219N film grown with an in-plane compressive strain exhibited a granular morphology owing to a columnar polycrystalline structure. This phenomenon was quite similar to that observed for AlInN films grown non GaN/sapphire templates (GaN templates); therefore, it was speculated that the microstructure variation might have been caused by the in-plane compressive strain generated in AlInN films on the GaInN template in the same way as on GaN templates or FS-GaN substrates.

Study on shallow donor-type impurities of GaN epilayer regrown by epitaxial lateral overgrowth technique

p-doped gallium nitride (GaN) regrowth by epitaxial lateral overgrowth using a SiO2 mask is studied. A comparison between SiO2 and Al2O3 masked p-GaN by cathodoluminescence spectroscopy and scanning electron microscopy indicates that donor-type impurities are related to the SiO2 mask. A domain peak of 3.25 eV induced by shallow-donor and acceptor transitions and the dark contrast of obtuse triangles have been detected in SiO2 masked p-type GaN. Secondary ion mass spectroscopy is simultaneously employed for the analysis of SiO2 and Al2O3 masked p-GaN and identifies that the source of donor-type impurities is from Si atoms. Furthermore, the experimental results of cross-sectional microstructures at different regrowth times have been investigated. It is found that the donor-type impurities tend to cluster in semi-polar 112¯2 facets before the coalescence at the bottom of adjacent triangular stripes starts. The explanation for the non-uniform distribution of impurities is that semi-polar 112¯2 facets exhibit more dangling bond densities than the (0001) plane, and the SiO2 mask exposed to the vapor phase would likely introduce more impurities before the coalescence of GaN stripes.

Phase-controlled epitaxial lateral overgrowth of α-Ga2O3 by halide vapor phase epitaxy

We investigated the effect of mask materials on the epitaxial lateral overgrowth (ELO) characteristics of α-Ga2O3 and developed an ELO technique that can markedly suppress abnormal growth and cracking to enable long-term growth on a wide mask, which is necessary to effectively improve the crystal quality. A conventional SiOx mask provided excellent growth selectivity with only a thin amorphous GaOx layer on the mask between α-Ga2O3 islands. However, the amorphous layer transformed into β-Ga2O3, which disrupted the long-term growth of α-Ga2O3. Amorphous deposition was avoided by using a TiOx mask. When a TiOx mask was used, κ-Ga2O3 deposited on the mask, and no transformation to β-Ga2O3 was observed. α-Ga2O3 islands with a diameter as large as approximately 25 μm were grown by the developed ELO technique using a TiOx mask. Transmission electron microscopy observation did not detect dislocations in the outermost part of the islands.

GaN-based Vertical-Cavity Surface-Emitting Lasers Incorporating Dielectric Distributed Bragg Reflectors

This paper reviews past research and the current state-of-the-art concerning gallium nitride-based vertical-cavity surface-emitting lasers (GaN-VCSELs) incorporating distributed Bragg reflectors (DBRs). This paper reviews structures developed during the early stages of research into these devices, covering both major categories of GaN-based VCSELs: hybrid-DBR and all-dielectric-DBR. Although both types exhibited satisfactory performance during continuous-wave (CW) operation in conjunction with current injection as early as 2008, GaN-VCSELs have not yet been mass produced for several reasons. These include the difficulty in controlling the thicknesses of nitride semiconductor layers in hybrid-DBR type devices and issues related to the cavity dimensions in all-dielectric-DBR units. Two novel all-dielectric GaN-based VCSEL concepts based on different structures are examined herein. In one, the device incorporates dielectric DBRs at both ends of the cavity, with one DBR embedded in n-type GaN grown using the epitaxial lateral overgrowth technique. The other concept incorporates a curved mirror fabricated on (000-1) GaN. Both designs are intended to mitigate challenges regarding industrial-scale processing that are related to the difficulty in controlling the cavity length, which have thus far prevented practical applications of all-dielectric GaN-based VCSELs.

Growth and Fabrication of High External Quantum Efficiency AlGaN-Based Deep Ultraviolet Light-Emitting Diode Grown on Pattern Si Substrate

Growing III-V semiconductor materials on Si substrates for opto-electronic applications is challenging because their high lattice mismatch and different thermal expansion coefficients cause the epitaxial layers to have low quality. Here we report the growth of a high-quality AlN template on a micro-circle-patterned Si substrate by using NH3 pulsed-flow multilayer AlN growth and epitaxial lateral overgrowth techniques. Then, we fabricated and characterized a deep-ultraviolet light-emitting diode (UV-LED) device using this AlN/patterned Si. By using standard lithography and inductively coupled plasma etching, the Si substrate was prepared with very high pattern density and was made deep enough to grow a thick AlN template with high crystal quality and very few threading dislocations, allowing for further re-growth of the deep UV-LED device. And by combining a transparent p-AlGaN contact layer, an electron blocking layer and using this high quality AlN template: a deep UV-LED device fabricated and showed a strong single sharp electroluminescence (EL) peak at 325 nm and achieved an external quantum efficiency (EQE) of about 0.03%, for a deep UV-LED grown on Si substrate.

Electrically driven, highly efficient three-dimensional GaN-based light emitting diodes fabricated by self-aligned twofold epitaxial lateral overgrowth

Improvements in the overall efficiency and significant reduction in the efficiency droop are observed in three-dimensional (3D) GaN truncated pyramid structures fabricated with air void and a SiO2 layer. This 3D structure was fabricated using a self-aligned twofold epitaxial lateral overgrowth technique, which improved both the internal quantum efficiency and the light extraction efficiency. In addition, a reduced leakage current was observed due to the effective suppression of threading dislocations. While this study focuses primarily on the blue emission wavelength region, this approach can also be applied to overcome the efficiency degradation problem in the ultraviolet, green, and red emission regions.

Low Threshold Light Emission from Reverse-Rib n+ Ge Cavity Made by P Diffusion

Strain-tensile n+ Ge is an enabler of monolithically integrated light sources on a Si chip. The challenges are (1) high lasing thresholds and (2) n+ doping methods. (1) The threshold currents reported for electrically pumped Ge lasers are 280 and 510 kA/cm2 [1, 2]; much higher than III-V lasers (less than 1 kA/cm2) and even theoretical prediction of the Ge laser (~6 kA/cm2) [3]. (2) Ge can be used as photodetectors (PDs) and modulators (MODs), which need different in-depth dopant profiles. Ge epitaxy cannot control in-plane P concentration locally unless otherwise one performs multiple epitaxy. In the present study we tried to reduce the threshold using specific epi-structures and to diffuse Phosphorus (P) to make n+ Ge, and successfully observed a threshold in light emission using P diffusion for the first time. We have reported that a reverse-rib structure can be made using the epitaxial lateral overgrowth (ELO) technique [4], and can effectively isolate light modes from Si substrate. It further reduces light scattering at the Ge/SiO2 sidewall formed by dry etching as shown in Fig. 1 [5], some of which should decrease the optical net gain. In addition, our group reported that the Ge/Si interfaces if located in the pn junction enhance significantly non-radiative recombination [6]. Thus, n+ Ge was grown on n+ Si. P diffusion was employed to achieve 1 × 1019 P/cm3 both in the reverse-rib and the blanket Ge epilayers using a well-controlled diffusion method [7]. Optical pumping was used to excite the structure. 40-nm thick SiO2 was formed by thermal oxidation on an n+ Si wafer and patterned in line-and-space shape along [110] orientation by electron beam lithography followed by a wet etching. Epitaxial Ge layers were grown on the wafer using ultra high vacuum chemical vapor deposition (UHVCVD) at 700 ˚C. The epilayers were 1 µm thick at reverse-rib areas, while 1.8 µm thick at blanket areas. P diffusion was performed at 850 ˚C for 5 minutes, which achieves uniform P concentration of 1 × 1019 /cm3 through these Ge epilayers. Schematic of the measurement system is shown in Fig. 2. A continuous wave YAG:Nd laser (wavelength: 1.07 µm, power: ~kW/cm2) was used for optical pumping and a Ge PD was used to measure the relation of light emission and optical pumping. The effective input power density should be actually smaller than the set value since the reverse-rib Ge epilayer (300 µm wide and 3 mm long) is wider than the pumping laser (30 µm wide and 4 mm long), causing out-diffusion of photo-excited minority carriers (holes) to non-pumped areas. The measurements were carried out at room temperature. Figure 3 shows a cross sectional TEM image of the as-grown reverse-rib Ge. The air semi-cylinders are observed over the SiO2 masks (invisible in this image) since Ge will not grow on SiO2 in this condition. The air semi-cylinders are 150 nm high and 450 nm wide, which is big enough to isolate light modes from the Si substrate according to a mode solver simulation. Raman scattering spectroscopy reveals 0.17 % tensile strain at the top surface of the reverse-rib Ge while there should be strain relaxation around the air semi-cylinders. The results of the optical pumping measurements are summarized in Fig. 4. Light emission from the reverse-rib Ge shows a sharp threshold at the input power density of 8 kW/cm2 (set value). The results can be understood as either amplified spontaneous emission (ASE) or lasing. We will soon report the spectra to identify the results. Assuming lasing, the threshold of 8 kW/cm2 is very small compared to the value reported (30 kW/cm2) [8].

Flexible GaN Light‐Emitting Diodes Using GaN Microdisks Epitaxial Laterally Overgrown on Graphene Dots

The epitaxial lateral overgrowth (ELOG) of GaN microdisks on graphene microdots and the fabrication of flexible light-emitting diodes (LEDs) using these microdisks is reported. An ELOG technique with only patterned graphene microdots is used, without any growth mask. The discrete micro-LED arrays are transferred onto Cu foil by a simple lift-off technique, which works reliably under various bending conditions.

Electron traps as major recombination centers in n-GaN films grown by metalorganic chemical vapor deposition

For a group of n-GaN films grown by metalorganic chemical vapor deposition (MOCVD) using both straight MOCVD and epitaxial lateral overgrowth techniques (ELOG proper or pendeo overgrowth), the spectra of deep traps were measured by deep-level transient spectroscopy (DLTS) with electrical or optical injection (ODLTS). The results were compared with diffusion length measurement results obtained from electron-beam-induced current experiments. The results strongly indicate that deep electron traps near Ec − 0.56 eV could be the major recombination centers determining the diffusion length values in pendeo samples.

Bridging the "green gap" of LEDs: giant light output enhancement and directional control of LEDs via embedded nano-void photonic crystals.

Green LEDs do not show the same level of performance as their blue and red cousins, greatly hindering the solid-state lighting development, which is the so-called "green gap". In this work, nano-void photonic crystals (NVPCs) were fabricated to embed within the GaN/InGaN green LEDs by using epitaxial lateral overgrowth (ELO) and nano-sphere lithography techniques. The NVPCs act as an efficient scattering back-reflector to outcouple the guided and downward photons, which not only boost the light extraction efficiency of LEDs with an enhancement of 78% but also collimate the view angle of LEDs from 131.5° to 114.0°. This could be because of the highly scattering nature of NVPCs which reduce the interference giving rise to Fabry-Perot resonance. Moreover, due to the threading dislocation suppression and strain relief by the NVPCs, the internal quantum efficiency was increased by 25% and droop behavior was reduced from 37.4% to 25.9%. The enhancement of light output power can be achieved as high as 151% at a driving current of 350 mA. Giant light output enhancement and directional control via NVPCs point the way towards a promising avenue of solid-state lighting.

Direct Growth and Controlled Coalescence of Thick AlN Template on Micro-circle Patterned Si Substrate

High-density micro-circle patterned Si substrates were successfully fabricated for the direct overgrowth of thick AlN templates by using NH3 pulsed-flow multilayer AlN growth and epitaxial lateral overgrowth techniques. The experimental results show that an 8-μm-thick AlN template was grown at a very high growth rate on the substrates. The AlN template had full widths at half maximum of 0.23° and 0.37° for the (002) and (102) reflection planes in X-ray diffraction rocking curves. Atomic force microscopy and transmission electron microscopy confirmed that the roughness of the surface was low (3.5 nm) and the dislocation density was very low (1.5 × 108 cm−2 (screw), 3.7 × 108 (edge) cm−2).

Growth of very large InN microcrystals by molecular beam epitaxy using epitaxial lateral overgrowth

Very thick InN (∼40 μm) was grown by molecular beam epitaxy using the epitaxial lateral overgrowth (ELO) technique. In some regions, the ELO of InN was observed as expected, indicating an important step toward fabricating quasi-bulk InN substrates. Interestingly, most parts of the sample consist of large flat-topped microcrystals and well-faceted microstructures. This is likely due to local growth condition variations during ELO, which is supported by an experiment where ELO of InN was performed on a substrate with various stripe mask patterns. TEM characterization of a flat top InN microcrystal revealed few stacking faults and only related threading dislocations. Defect-free small faceted microcrystals were also observed. The thick InN crystals show a narrow photoluminescence spectrum with a peak at 0.679 eV and linewidth of 16.8 meV at 4 K.

Growth of InP directly on Si by corrugated epitaxial lateral overgrowth

In an attempt to achieve an InP–Si heterointerface, a new and generic method, the corrugated epitaxial lateral overgrowth (CELOG) technique in a hydride vapor phase epitaxy reactor, was studied. An InP seed layer on Si (0 0 1) was patterned into closely spaced etched mesa stripes, revealing the Si surface in between them. The surface with the mesa stripes resembles a corrugated surface. The top and sidewalls of the mesa stripes were then covered by a SiO2 mask after which the line openings on top of the mesa stripes were patterned. Growth of InP was performed on this corrugated surface. It is shown that growth of InP emerges selectively from the openings and not on the exposed silicon surface, but gradually spreads laterally to create a direct interface with the silicon, hence the name CELOG. We study the growth behavior using growth parameters. The lateral growth is bounded by high index boundary planes of {3 3 1} and {2 1 1}. The atomic arrangement of these planes, crystallographic orientation dependent dopant incorporation and gas phase supersaturation are shown to affect the extent of lateral growth. A lateral to vertical growth rate ratio as large as 3.6 is achieved. X-ray diffraction studies confirm substantial crystalline quality improvement of the CELOG InP compared to the InP seed layer. Transmission electron microscopy studies reveal the formation of a direct InP–Si heterointerface by CELOG without threading dislocations. While CELOG is shown to avoid dislocations that could arise due to the large lattice mismatch (8%) between InP and Si, staking faults could be seen in the layer. These are probably created by the surface roughness of the Si surface or SiO2 mask which in turn would have been a consequence of the initial process treatments. The direct InP–Si heterointerface can find applications in high efficiency and cost-effective Si based III–V semiconductor multijunction solar cells and optoelectronics integration.

Characterization of nonpolar a-plane InGaN/GaN multiple quantum well using double nanopillar SiO2 mask

The characteristics of nonpolar a-plane () GaN (a-GaN) grown using single and double nanopillar SiO2 masks were investigated. The two nanopillar SiO2 masks were directly fabricated on an r-plane sapphire substrate and a-GaN by the epitaxial lateral overgrowth (ELOG) technique. Through the use of the single and double nanopillar SiO2 masks, the crystalline quality and optical properties of a-GaN were markedly improved because of the nanoscale ELOG effect and a number of voids in the single and double nanopillar SiO2 mask areas in comparison with the planar sample. The submicron pit densities of the planar, single, and double nanopillar mask samples were ∼2 × 109, ∼7 × 108, and ∼4 × 108 cm−2, respectively. The internal quantum efficiency (IQE) values at room temperature of three-period InGaN/GaN multiple quantum wells (MQWs) grown using the planar, single, and double nanopillar masks were 45, 60, and 68% at a carrier concentration of 1.0 × 1018 cm−3, respectively.

Solar‐blind AlxGa1–xN MSM photodetectors on patterned AlN/sapphire templates with 0.4 < x < 1

AbstractSolar‐blind Schottky‐type metal‐semiconductor‐metal (MSM) photodetectors (PDs) based on AlxGa1–xN absorber layers with x varying between 0.4 and 1 are investigated. The impact of the epitaxial lateral overgrowth (ELO) technique on stripe patterned AlN/sapphire templates is evaluated by comparing devices on ELO as well as planar AlN/sapphire templates. Device characteristics have been found to depend on the orientation of the finger‐electrodes with respect to the stripe pattern. The responsivity of ELO PDs with finger‐electrodes perpendicular to the stripes is higher than that for the parallel case or for PDs on planar templates. For x ≥ 0.9 an enhancement of about 50% and for x = 0.4 of more than two orders of magnitude due to photoconductive gain are observed. The orientation dependence is explained by a non‐uniform dislocation density on the ELO template, involving channels of low dislocation density. Additionally, the Al/Ga incorporation is found to be non‐uniform, particularly for low values of x. Due to the absence of photoconductive gain for x ≥ 0.9, the gain observed for x = 0.4 using ELO templates with perpendicular finger orientation is attributed to compositional modulation of the absorber layer. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Non‐polar (11$ \bar 2 $0) InGaN quantum dots with short exciton lifetimes grown by metal‐organic vapour phase epitaxy

AbstractInGaN quantum dot (QD) structures have been grown by metal organic vapour phase epitaxy (MOVPE) on non‐polar (11‐20) GaN surfaces by employing an anneal step in nitrogen immediately after the growth of the InGaN. Here, we compare the growth of such structures on pseudo‐substrates grown using an epitaxial lateral overgrowth (ELOG) technique and on pseudo‐substrates grown using a simpler in situ SiNx interlayer. The less complex defect reduction approach results in a significantly higher defect density, but does not detrimentally effect the QD formation. For both types of pseudo‐substrate, sharp peaks with resolution limited widths are observed in both cathodoluminescence at 9 K and micro‐photoluminescence at 4.2 K. The QDs demonstrate significantly reduced exciton lifetimes compared to structures grown on c‐plane, which has advantages for possible applications in single photon sources. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Excitonic recombination in epitaxial lateral overgrown AlN on sapphire

Excitonic emission in heteroepitaxially grown aluminum nitride (AlN) with reduced defect density due to the epitaxial lateral overgrowth (ELO) of patterned AlN/sapphire templates has been investigated by photoluminescence spectroscopy and compared to AlN/sapphire and homoepitaxially grown AlN. The ELO sample exhibits small linewidths of the free exciton and two different bound exciton emission bands. The free exciton emission energy is shifted by 58.5 meV with respect to unstrained homoepitaxially grown AlN attributed to compressive strain. A donor bound exciton D0X with an exciton localization energy of 13.0–13.5 meV is dominating in the photoluminescence spectra of ELO AlN/sapphire. This D0X does not show strong phonon replica and is dominant at elevated temperatures in ELO AlN/sapphire. The optical quality of heteroepitaxial AlN is significantly improved using the ELO technique and therefore suitable for high efficiency ultraviolet light emitters.