Μm Thick GaN Layer Research Articles

Improvement of Optical and Electrical Properties of Indium Tin Oxide Layer of GaN-Based Light-Emitting Diode By Surface Plasmon in Silver Nanoparticles

Recently, GaN-based light-emitting diodes (LEDs) have found many applications, including automobile headlights, full-color displays, traffic light lamps, and solid-state lightings. However, the light extraction efficiency of GaN-based LEDs is limited by the total internal reflection of light at the interface between the LED structure and air caused by the large refractive index difference between the GaN film (n=2.5) and air (n=1). Photons with an incident angle lower than the critical angle of 23° are reflected from the interface, leading to low light extraction efficiency. Although the light extraction efficiency has been increased by using indium tin oxide (ITO) as a transparent conducting layer (TCL), further improvements in the TCL performance and the optical output power are still required to realize high efficiency LEDs. In this study, we demonstrate that the ITO layer with Ag nanoparticles can be used as an optically and electrically high performing TCL for LEDs. The transmittance of the ITO layer was improved by the resonance between incident light in the ITO layer and localized surface plasmons (LSPs) at the ITO-Ag interface. The optical output power and electrical characteristics of LEDs were improved by the Ag nanoparticles deposited on the ITO layer of the LEDs. Figure 1(a) shows a schematic diagram of blue LED with Ag nanoparticles deposited on ITO TCL. The LEDs with a blue emission at 465 nm were grown on a c-plane (0001) patterned sapphire substrate by metalorganic chemical vapor deposition. After growth of a GaN nucleation layer, a 2 μm-thick undoped GaN layer and a 2 μm-thick n-GaN layer were grown at 1025 °C. Then, multiple quantum wells, consisting of five periods of undoped InGaN wells and GaN barriers, were grown at 750 °C, followed by growth of a 200 nm-thick p-GaN layer at 980 °C. To fabricate the LEDs, the p-GaN layer was etched by an inductively coupled plasma etching process to expose the n-GaN layer for an n-type ohmic contact. Then, a 150 nm-thick ITO layer was deposited on p-GaN layer as a TCL by electron-beam evaporation. To investigate the effect of the Ag nanoparticles on the ITO layer, LEDs with Ag nanoparticles deposited on ITO layer were also fabricated. To form Ag nanoparticles, an Ag layer with different film thickness was deposited on the ITO layer by electron-beam evaporation, and then annealed at 500 °C for 1 min in a rapid thermal annealing (RTA) system. This was followed by deposition of Cr/Au on the n-GaN layer to form an n-pad electrode and on the ITO layer to form a p-pad electrode. Figure 1(a) also shows atomic force microscopy (AFM) image of the ITO layer with Ag nanoparticles after RTA processing. The surface of an as-deposited Ag layer on the ITO layer is smooth and featureless. However, the thin Ag layer is transformed into nanoparticles, and the particle size is increased by thermal annealing via the Ostwald ripening process. Figure 1(b) shows the optical output power of the blue LEDs with different density and size of Ag nanoparticle deposited on the ITO TCLs as a function of injection current. As shown in Fig. 1(b), the optical output powers of the LEDs with Ag nanoparticles on top of the ITO TCL are higher than that of the LED with the bare ITO TCL. In particular, the optical output power of an LED with Ag nanoparticles (1 nm-thick Ag layer) is increased by 16% at an injection current of 20 mA compared to that of the LED without Ag nanoparticles and the enhancement is increased with increasing the density of Ag nanoparticles. This improvement is mainly attributed to the increased transmittance of ITO by the resonance of the light with the LSPs in the randomly distributed Ag nanoparticles. The electrical properties of the LEDs are also improved by the enhanced conductivity of the ITO TCL caused by the Ag nanoparticles. These results indicate that the Ag nanoparticles/ITO with high transmittance and low resistivity can be used as a high efficiency TCL for LEDs. Figure 1

High quality crack-free GaN film grown on si (1 1 1) substrate without AlN interlayer

High quality crack-free GaN film has been grown on 2in. n-type Si (111) substrate without AlN interlayers by metalorganic chemical vapor deposition (MOCVD). By using a two-step-pressure growth technique for the AlN buffer layer, we have obtained crack-free 1.8μm thick continuous GaN layer. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) results indicate that the two-step-pressure growth method can improve the crystalline quality of the GaN film by prolonging the three-dimensional (3D) growth process of the GaN layer, Raman spectrum analysis shows that tensile stress in the overlaying GaN layer can be reduced obviously to 0.62GPa, which helps to reduce the possibility of cracking.

Evolution and characteristics of GaN nanowires produced via maskless reactive ion etching

The formation of nanowires (NWs) by reactive ion etching (RIE) of maskless GaN layers was investigated. The morphological, structural and optical characteristics of the NWs were studied and compared to those of the layer they evolve from. It is shown that the NWs are the result of a defect selective etching process. The evolution of density and length with etching time is discussed. Densely packed NWs with a length of more than 1 μm and a diameter of ∼60 nm were obtained by RIE of a ∼2.5 μm thick GaN layer. The NWs are predominantly free of threading dislocations and show an improvement of optical properties compared to their layer counterpart. The production of NWs via a top down process on non-masked group III-nitride layers is assessed to be very promising for photovoltaic applications.

Stress distribution of GaN layer grown on micro-pillar patterned GaN templates

High-resolution Raman mapping of the stress distribution in an etched GaN micro-pillar template and a 5 μm thick GaN layer grown on a micro-pillar patterned GaN template is investigated. Raman mapping of the E2 (high) phonon shows differences in stress between the coalescing boundary, the top surface of the pillar region and around the GaN micro-pillar. Increased compressive stress is observed at the coalescing boundary of two adjacent GaN micro-pillars, when compared to the laterally grown GaN regions. The electron channeling contrast image reveals the reduction of threading dislocation density in the GaN layer grown on the micro-pillar patterned GaN template.

Study on the effects of AlN interlayer in thick GaN grown on 3C-SiC/Si substrates

For epitaxy of GaN on 3C-SiC/Si substrates, optimization of growth temperature of AlN interlayers (ILs) was performed. With a proper growth condition of AlN IL, crack-free 3.5μm thick GaN layer was realized by multiple AlN ILs on 3C-SiC/Si (111) substrate. The distribution of D0X peak position in low temperature cathodoluminescence spectra was mapped to investigate the stress in as-grown wafer. An increase of tensile stress was found in the top GaN layers above AlN ILs. Cross-sectional transmission electron microscopy images confirmed that AlN ILs could induce compressive stress and reduce threading dislocations in the GaN epilayer grown on 3C-SiC/Si. The reduction of dislocations should account for the part of incremental tensile stress revealed by the inhomogeneous distribution of luminescence.

The growth of high-quality and self-separation GaN thick-films by hydride vapor phase epitaxy

About 1.2 mm thick GaN bulk crystals were obtained by combining a pulsed NH 3-flow modulation (PFM) method and a self-separation method of short-shutting NH 3 flow when using hydride vapor phase epitaxy (HVPE). High crystal quality of bulk GaN was evaluated by X-ray rocking curves (XRC) and the full width at half maximum (FWHM) values were 110, 72 and 83 arcsec for (002), (102) and (100) reflection planes, respectively. The PFM method is proved to be effective in reducing cracks and keeping the surface smooth. And the method of short-shutting NH 3 flow can lead to GaN thick layer separate from sapphire substrate when cooling from the high growth temperature. Growth and separation mechanisms were investigated. Two states were found in PFM method. With PFM method modulating between high quality state and low stress state, 300 μm thick GaN layers without cracks were obtained. Study of spontaneous separation mechanism revealed that the separation attributed to formation of voids inside the GaN layer.

Strong light-matter coupling in bulk GaN-microcavities with double dielectric mirrors fabricated by two different methods

Two routes for the fabrication of bulk GaN microcavities embedded between two dielectric mirrors are described, and the optical properties of the microcavities thus obtained are compared. In both cases, the GaN active layer is grown by molecular beam epitaxy on (111) Si, allowing use of selective etching to remove the substrate. In the first case, a three period Al0.2Ga0.8N/AlN Bragg mirror followed by a λ/2 GaN cavity are grown directly on the Si. In the second case, a crack-free 2 μm thick GaN layer is grown, and progressively thinned to a final thickness of λ. Both devices work in the strong coupling regime at low temperature, as evidenced by angle-dependent reflectivity or transmission experiments. However, strong light-matter coupling in emission at room temperature is observed only for the second one. This is related to the poor optoelectronic quality of the active layer of the first device, due to its growth only 250 nm above the Si substrate and its related high defect density. The reflectivity spectra of the microcavities are well accounted for by using transfer matrix calculations.

Large area GaN and AlN template substrates fabricated by HVPE

AbstractHydride vapour phase epitaxy (HVPE) is known as fast deposition method to produce both free‐standing GaN substrate materials and thick low‐defect GaN on 2‐inch sapphire and AlN layers on 2‐inch sapphire and conductive silicon carbide (SiC) substrates. These substrate materials of 2‐inch size are available as commercial product. In this paper, we extend the HVPE abilities to new fields including up to 23 μm thick crack‐free AlN layers growth on 3‐inch and 100‐mm SiC substrates (AlN/SiC template substrates) and up to 125 μm thick GaN layers grown on 3‐inch sapphire (GaN/sapphire template substrates). For 21‐23 μm thick AlN layers grown on 100‐mm SiC substrates, the X‐ray rocking curve FWHM was ranged from 200 to 300 arc sec and from 600 to 800 arc sec for the (00.2) and the (10.2) AlN reflections, respectively. For 115‐125 μm thick GaN layers, the X‐ray FWHM values were 120‐150 arcsec and 200‐230arc sec for the (00.2) and the (10.2) GaN reflections, respectively, that corresponds screw dislocation density of (3‐5)×107 cm–2 evaluated based on XRD data. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

GaN layer growth by HVPE on m‐plane sapphire substrates

AbstractSemipolar GaN layers were grown on m‐plane sapphire substrates by HVPE. Insertion of AlxGa1–xN (x ∼ 0.1‐0.6) layer in‐between m‐plane sapphire substrate and GaN layer promoted to improve crystalline quality and to grow of semipolar (11‐22) plane GaN layers. X‐ray diffraction (11‐22) ω‐scan rocking curve FWHM of 298 arcsec was measured for a 30 μm thick (11‐22) GaN layer. Depending on growth conditions, m‐plane GaN layer having micro‐crystallites of other orientations (mainly of (11‐24) plane GaN layer) was also grown. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Optimization of HVPE growth of freestanding c‐plane GaN layers using (100) γ‐LiAlO 2 substrates

Freestanding c-plane GaN substrates are obtained using (100) γ-LiAlO2 substrates due to spontaneous separation during post-growth cool down. It is shown that by growth optimization the structural properties of 200 µm thick freestanding GaN layers can be improved resulting in FWHM values of ω-rocking curves below 400 arcsec for 002 and 302 reflection and threading-dislocation densities below 109 cm–2. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Single crystalline GaN grown on porous Si(111) by MOVPE

AbstractIn this work, GaN growth on porous Si(111) will be reported. The porosity of the substrates was 30% or 50%. In the latter case, various thicknesses, from 0.6 μm to 10 μm, were investigated. The morphology of the GaN surfaces was analyzed by optical interference microscopy. The crystalline quality of the epitaxial layers was characterized by High Resolution X‐Ray Diffraction (HR‐XRD) and cross‐sectional Transmission Electron Microscopy (TEM). A Full Width at Half Maximum (FWHM) of the X‐ray symmetric rocking curve (0002) 2θ – ω scan of 290 arc sec was obtained for a 1 μm thick GaN layer, which is comparable with that of GaN grown on bulk Si(111) substrates. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Spatial variation of optical and structural properties of ELO GaN directly grown on patterned sapphire by HVPE

A 275 µm thick GaN layer was directly grown on the SiO2-prepatterned sapphire in a home-built vertical hydride vapour phase epitaxy (HVPE) reactor. The variation of optical and structure characteristics were microscopically identified using spatially resolved cathodeluminescence and micro-Raman spectroscopy in a cross section of the thick film. The D 0 XA line with the FWHM of 5.1 meV and etch-pit density of 9 × 106 cm−2 illustrated high crystalline quality of the thick GaN epitaxial layer. Optically active regions appeared above the SiO2 masks and disappeared abruptly due to the tapered inversion domains at 210–230 µm thickness. The crystalline quality was improved by increasing the thickness of the GaN/sapphire interface, but the region with a distance of 2 µm from the top surface revealed relatively low quality due to degenerate surface reconstruction by residual gas reaction. The x-ray rocking curve for the symmetric (0 0 2) and asymmetric (1 0 2) reflections also showed good quality and a small wing tilt of the epitaxial lateral overgrowth (ELO) GaN.

GaN micromachined FBAR structures for microwave applications

Abstract This paper presents the manufacturing of GaN membrane supported F-BAR structures. The 2.2 μm thick GaN layer was grown using MOCVD techniques on a high-resistivity 〈111〉-oriented silicon substrate. Conventional contact lithography, electron-gun Ti/Au evaporation and lift-off techniques were used to define top-side metallization of the the FBAR structures. Bulk micromachining techniques were used for the release of the GaN membrane. The bottom-side metallization of the micromachined structure was obtained by means of sputtered gold. S-parameter measurements have shown a pronounced resonance around 1.2 GHz. The extracted value of acoustic velocity is in good agreement with those reported by other authors on materials fabricated by other methods. The demonstrated FBAR function in epitaxially grown GaN layers opens new avenues for a low-cost monolithic integration with GaN-based electronics and sensing devices.

Si diffusion in epitaxial GaN

AbstractThe aim of this work is to determine diffusion coefficient of silicon in gallium nitride under different annealing conditions. Silicon was implanted into 3 µm thick GaN layer deposited on sapphire substrate with MOVPE (Metalorganic Vapour Phase Epitaxy) method. Implantation was performed at room temperature to a dose of 6×1015/cm2 at energy of 100 keV. The samples were annealed under 1 atm nitrogen pressure at 900–1200 °C with or without an AlN protective layer. The extent of diffusion of the implanted species was characterized using the SIMS (Secondary Ion Mass Spectrometry) technique. The resultant concentration profiles are characterized with two diffusion fronts: slow diffused Si atoms at high concentration and fast diffused Si atoms at low concentration. We ascribe the first mechanism as to originate from diffusion in the crystal bulk (substitutional migration), while the second one – from diffusion via grain boundaries/dislocations. Si diffusion coefficient in a crystal was found to be dependent on annealing conditions, in contrast to diffusion via grain boundary. Calculated diffusion coefficient parameters for annealing with and without AlN layer amount respectively to: DO = 6.5×10–11 cm2/s, Ea = 0.89 eV and DO = 9.1×10–8 cm2/s, Ea = 1.55 eV. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Depth dependence of defect density and stress in GaN grown on SiC

We report high resolution x-ray diffraction studies of the relaxation of elastic strain in GaN grown on SiC(0001). The GaN layers were grown with thickness ranging from 0.29to30μm. High level of residual elastic strain was found in thin (0.29to0.73μm thick) GaN layers. This correlates with low density of threading screw dislocations of 1-2×107cm−2, observed in a surface layer formed over a defective nucleation layer. Stress was found to be very close to what is expected from thermal expansion mismatch between the GaN and SiC. A model based on generation and diffusion of point defects accounts for these observations.

In Ga N ∕ Ga N multi-quantum-well structures on (111)-oriented bonded silicon-on-insulator substrates

We report growth of InGaN∕GaN multiple quantum wells (MQWs) on (111)-oriented bonded silicon-on-insulator (SOI) substrates by metalorganic chemical vapor deposition (MOCVD). Prior to MOCVD growth of MQWs, about a 1.2μm thick GaN layer was deposited on SOI substrate with a high-temperature transitional buffer layer. The growth conditions were tuned to realize blue-green emission peaks centered around 420–495nm from such MQWs on SOI. X-ray diffraction, atomic force microscopy, scanning electronic microscopy, and photoluminescence techniques were used to characterize these MQWs. Such an approach to realize multicolor light-emitting layers on SOI substrates is suitable for the integration of InGaN∕GaN-based optoelectronic structures on SOI-based micro-optoelectromechanical systems and sensors.

Reactor and growth process optimization for growth of thick GaN layers on sapphire substrates by HVPE

In total, 120 μm thick GaN layers without cracks have been grown on 2 in sapphire substrates by hydride vapor phase epitaxy. This has been achieved by optimization of the flow patterns in the reactor based on 3D process modelling, choice of the growth parameters especially the carrier gas composition and the usage of suitable GaN/sapphire templates. An important finding is that an H 2 content of around 50% in the N 2 carrier yields the lowest crack density.

Optical and Structural Characteristics of Virtually Unstrained Bulk-Like GaN

Bulk-like GaN with high structural and optical quality has been attained by hydride vapor-phase epitaxy (HVPE). The as-grown 330 µm-thick GaN layer was separated from the sapphire substrate by a laser-induced lift-off process. The full width at half maximum values of the X-ray diffraction (XRD) ω-scans of the free-standing material are 96 and 129 arcsec for the (1 0 -1 4) and (0 0 0 2) reflection, respectively, which rank among the smallest values published so far for free-standing HVPE-GaN. The dislocation density determined by plan-view TEM images is 1–2×107 cm-2. Positron annihilation spectroscopy studies show that the concentration of Ga vacancy related defects is about 1.5×1016 cm-3. The high-resolution XRD, photoluminescence, µ-Raman, and infrared spectroscopic ellipsometry measurements consistently prove that the free-standing material is of high crystalline quality and virtually strain-free. Therefore it is suitable to serve as a substrate for stress-free growth of high-quality III–nitrides based device heterostructures.

Metal Organic Vapor Phase Epitaxy of ZnO on GaN/Si(111) Using Tertiary-Butanol as O-Precursor

High-quality ZnO was grown by metal organic vapor phase epitaxy on 1.3 µm thick GaN layers on Si(111) using dimethylzinc and tertiary-butanol as precursors. The variation of the growth temperature shows a strong correlation with the microstructure as observed by atomic force and scanning electron microscopy. With increasing growth temperature we find an increasing size of ZnO crystallites and a transition from 3D to 2D growth. Moreover, increasing the growth temperature leads to a reduction of tensile stress in the ZnO as observed by X-ray diffraction. In highly spatially resolved cathodoluminescence measurements we observe narrow (A0,X) luminescence from the ZnO surface and a strong donator correlated luminescence at macroscopic defects.

High‐temperature operation of an AlGaN/GaN HFET on a Si substrate using a thin GaN film

An AlGaN/GaN heterostructure on a Si substrate was grown using metalorganic chemical vapor deposition (MOCVD) to fabricate a heterojunction field effect transistor (HFET). A homogenous buffer layer was obtained at 1093 K; a 0.5 μm-thick GaN layer was also obtained without cracking in a 2-inch wafer. We fabricated an HFET using an undoped Al0.3Ga0.7N (20 nm)/undoped GaN (450 nm)/Al0.05Ga0.95N buffer (100 nm) on a p-type Si (111) substrate. The gate length and width were 2 μm and 150 μm, respectively. The distance between the source and drain was 10 μm. The ohmic electrode material was Al/Ti/Au and the Schottky electrode material was Pt/Au. The Schottky breakdown voltage of the gate and the source was over 100 V. It was confirmed that this HFET could be operated at 573 K for over 100 h. We thus demonstrated a high-temperature operation of an AlGaN/GaN HFET on a Si substrate using a very thin film of less than 600 nm. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)