GaN Nucleation Layer Research Articles

Improved crystalline quality of N-polar GaN epitaxial layers grown with reformed flow-rate-modulation technology

A reformed flow-rate-modulation technology was developed for the metalorganic vapor phase epitaxy (MOVPE) growth of the N-polar GaN epitaxial layers. To improve the crystalline quality of the N-polar GaN epitaxial layers, a GaN nucleation layer was grown at relatively low temperature with carefully-controlled pulsed supply of Ga source and showed diverse morphology with atomic force microscope (AFM). Furthermore, the electrical and optical properties of the grown N-polar GaN epitaxial layers were investigated extensively by means of Hall effect, photoluminescence (PL), and X-ray rocking curve (XRC) measurements. The characterization results revealed that as compared with the N-polar GaN epitaxial layer grown over the conventional GaN nucleation layer which was deposited with continuous supply of both N and Ga sources, the electrical and optical properties of the N-polar GaN epitaxial layer grown with optimized supply of Ga source for the GaN nucleation layer were significantly improved.

Reduced efficiency droop of nonpolar a-plane (11-20) GaN-based light-emitting diodes by vertical injection geometry

Vertical nonpolar a-plane (11-20) InGaN/GaN light-emitting diodes (LEDs) have been demonstrated by using laser lift-off technique. The forward voltage of the a-plane vertical LEDs was 4.3 V at 350 mA, which was reduced by 0.8 V compared to that of the a-plane lateral LEDs. The vertical geometry of the a-plane LEDs produced the higher quantum efficiency with a low efficiency droop and also enhanced the output power by more than 40%, when compared to those of a-plane lateral LEDs. These results can be attributed to the high thermal dissipation as well as uniform current spreading of the vertical geometry of the a-plane LEDs. Furthermore, elimination of the highly defected GaN nucleation layer after removing the sapphire substrates during the fabrication process can also enhance current injection efficiency, followed by the increase in the output power.

RHEED에 의한 GaN, InN 핵생성층의 열처리 효과 분석

GaN and InN epilayers with nucleation layer (LT-buffer) were grown on (0001) sapphire substrates by radio-frequency plasma-assisted molecular beam epitaxy (RF-MBE). As-grown and annealed GaN and InN nucleation layers grown at various growth condition were observed by reflection high-energy electron diffraction (RHEED). When temperature of effusion cell for III source was very low, diffraction pattern with cubic symmetry was observed and zincblende nucleation layer was flattened easily by annealing. As cell temperature increased, LT-GaN and LT-InN showed typical diffraction pattern from wurtzite structure, and FWHM of (10-12) plane decreased remarkably which means much improved crystalline quality. Diffraction pattern was changed to be from streaky to spotty when plasma power was raised from 160 to 220 W because higher plasma power makes more nitrogen adatoms on the surface and suppressed surface mobility of III species. Therefore, though wurtzite nucleation layer was a little hard to be flattened compared to zincblende, higher cell temperature led to easier movement of III surface adatoms and resulted in better crystalline quality of GaN and InN epilayers.

Improved performance of GaN based light emitting diodes with ex-situ sputtered AlN nucleation layers

The crystal quality, electrical and optical properties of GaN based light emitting diodes (LEDs) with ex-situ sputtered physical vapor deposition (PVD) aluminum nitride (AlN) nucleation layers were investigated. It was found that the crystal quality in terms of defect density and x-ray diffraction linewidth was greatly improved in comparison to LEDs with in-situ low temperature GaN nucleation layer. The light output power was 3.7% increased and the reverse bias voltage of leakage current was twice on LEDs with ex-situ PVD AlN nucleation layers. However, larger compressive strain was discovered in LEDs with ex-situ PVD AlN nucleation layers. The study shows the potential and constrain in applying ex-situ PVD AlN nucleation layers to fabricate high quality GaN crystals in various optoelectronics.

Effect of GaAs substrate orientation on the growth kinetic of GaN layer grown by MOVPE

We have investigated the kinetic growth of low temperature GaN nucleation layers (LT-GaN) grown on GaAs substrates with different crystalline orientations. GaN nucleation layers were grown by metal organic vapor phase epitaxy (MOVPE) in a temperature range of 500–600 °C on oriented (001), (113), (112) and (111) GaAs substrates. The growth was in-situ monitored by laser reflectometry (LR). Using an optical model, including time-dependent surface roughness and growth rate profiles, simulations were performed to best approach the experimental reflectivity curves. Results are discussed and correlated with ex-situ analyses, such as atomic force microscopy (AFM) and UV-visible reflectance (SR). We show that the GaN nucleation layers growth results the formation of GaN islands whose density and size vary greatly with both growth temperature and substrate orientation. Arrhenius plots of the growth rate for each substrate give values of activation energy varying from 0.20 eV for the (001) orientation to 0.35 eV for the (113) orientation. Using cathodoluminescence (CL), we also show that high temperature (800–900 °C) GaN layers grown on top of the low temperature (550 °C) GaN nucleation layers, grown themselves on the GaAs substrates with different orientations, exhibit cubic or hexagonal phase depending on both growth temperature and substrate orientation.

Comparison of the Performance of Lateral and Vertical InGaN/GaN-Based Light-Emitting Diodes with GaN and AlN Nucleation Layers

We investigated the effects of in situ GaN and sputtered AlN nucleation layers on the output power of GaN-based blue (445 nm) LEDs on patterned sapphire substrate (PSS) and planar sapphire substrates. Both the PSS LEDs and planar LEDs showed the same operation voltages, but the LEDs with the AlN layer had much lower reverse leakage current at −15 V. The LEDs with the AlN layer showed higher external quantum efficiency (EQE) at low current regions than those with the GaN nucleation layer. However, the AlN nucleation layer LEDs experienced a little more severe efficiency droop than did the GaN layer LEDs. It was shown that the AlN nucleation layer samples contained fewer defects than the GaN nucleation layer samples. The Raman spectral results showed that the AlN nucleation layer samples experienced higher compressive stress than the GaN nucleation layer samples. Simulation results also showed that both the AlN nucleation layer and air voids in the GaN nucleation layer samples hardly affected the light extraction efficiency. Based on the electrical, X-ray diffraction (XRD), cathodoluminescence (CL), and Raman results, the electrical behavior of the PSS and planar LEDs are described and discussed.

Conversion Efficiency Improvement of InGaN/GaN Multiple-Quantum-Well Solar Cells With <italic>Ex Situ</italic> AlN Nucleation Layer

The crystal quality, electrical, and optical characteristics of GaN solar cells (SCs) were improved using ex situ AlN nucleation layer. Replacing the in situ GaN nucleation layer with the sputtered ex situ AlN nucleation layer reduced the total dislocation density of GaN layer from $3.7 \times 10^{8}$ to $2.2 \times 10^{8}$ cm $^{-2}$ . The dislocation density reduction of GaN with sputtered ex situ AlN nucleation could suppress the reverse leakage current and the forward recombination current in low forward voltage range of SCs, and thus can increase short-circuit current density ( $J_{\rm sc})$ and open-circuit voltage ( $V_{\rm oc})$ of the SCs. A 1-sun power conversion efficiency ( $\boldsymbol \eta $ %) of SCs with ex situ AlN nucleation (1.89%) showed an enhancement of 26% compared with that of conventional SC (1.50%). Furthermore, the 100-sun $\boldsymbol \eta $ % of SCs with ex situ AlN nucleation (1.97%) showed 18% improvement compared with that of conventional SC (1.67%).

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

(Invited) Neutron Irradiation Effect on GaN-Based Materials

For the predominant property, GaN becomes a promising material for ultraviolet/ blue light-emitting diode, solar blind ultraviolet photodetector, radiation resistant particle detectors, and high power high temperature electronic devices. Because of the radiation hardness of GaN, those devices have the potential to be used in outer space and other radiation environment. Irradiation induced defects play the main role in the reliability of GaN based devices. Recently, a strong effort by several groups was devoted to the analyses of electrical and optical properties of defects and their interaction by irradiating GaN with electrons, ions, neutrons and protons [1–4], which is very important for the devices operating at extreme harsh environment. Positron annihilation spectroscopy (PAS) with a slow-positron beam is an efficient tool for the investigation of open-volume defects such as vacancies in semiconductors, at which they get trapped. In a vacancy the electron density is decreased which makes the positron lifetime longer and narrows the positron-electron momentum distribution. The latter can be observed by recording the Doppler broadening of the energy spectrum of the annihilation radiation. Especially the momentum distribution of the core electrons is sensitive to the chemical environment of an annihilation site. However, in the case of light elements such as O or N the annihilation probability with the core electrons is negligible. The samples used in the present work were grown on C-plane sapphire substrates by metal organic chemical vapor deposition (MOCVD) with unintentionally doped and Si doped. A 20 nm low temperature GaN nucleation layer was initially grown, and 4 μm GaN film grown at 1050°C was followed. The sample was cut into small pieces with an area of about 10×10 mm2, and irradiated with 2MeV proton from heavy ion accelerator. The incident fluence is 1014, 1015, or 1016 cm-2, respectively. Positron annihilation spectroscopy, XRD, and Raman spectra were measured. Irradiation induced defect is vacancies which increased with incident proton. XRD result shows that the crystal-order was kept after proton irradiation. Raman spectra of the Si doped sample shows a “carrier removal” effect but there is not any change in the other spectra.

Effect of interface nucleation time of the GaN nucleation layer on the crystal quality of GaN film

In this paper, the influences of the growth time of low-temperature (LT) GaN nucleation layer on the crystal quality and optical properties of GaN film are investigated. It is found that the optimal LT nucleation layer growth time can effectively reduce the crystal defects and is favorable to forming the annihilation of dislocations. GaN films are grown on c-plane sapphire substrates by metal-organic chemical vapor deposition. Crystal quality and optical properties are characterized by atomic force microscopy (AFM), scanning electron microscopy (SEM), high-resolution X-ray diffraction (HRXRD), and photoluminescence spectra, respectively. In the AFM images, the island density decreases as growth time increases, while the size of island becomes larger and the uniformity of island size deteriorates as growth time increases, leading to the phenomenon that the number of interfaces formed during the nucleation island coalescence, first decrease and then increase as detected by SEM, which also induces the screw dislocation density and edge dislocation density to first decrease and then increase as measured by HRXRD. This first-decrease-and-then-increase variation trend is consistent with the first-increase-and-then-decrease variation trend of the ratio of the band edge emission peak intensity to the yellow luminescence peak intensity tested by photoluminescence, which is confirmed by HRXRD. It is shown that GaN islands with different sizes and densities could lead to different mechanisms of dislocation evolution, thereby forming GaN epitaxial layers with different dislocation densities and optical properties. Through controlling the nucleation time, GaN films with the smallest dislocation density could be obtained.

Performance improvement of GaN-based light-emitting diodes grown on Si(111) substrates by controlling the reactor pressure for the GaN nucleation layer growth

Highly-efficient GaN-based LEDs on Si(111) substrates were achieved using MOCVD with a suitable reactor pressure for the GaN nucleation layer growth.

Efficiency improvement of GaN-based ultraviolet light-emitting diodes with reactive plasma deposited AlN nucleation layer on patterned sapphire substrate.

The flip chip ultraviolet light-emitting diodes (FC UV-LEDs) with a wavelength of 365 nm are developed with the ex situ reactive plasma deposited (RPD) AlN nucleation layer on patterned sapphire substrate (PSS) by an atmospheric pressure metal-organic chemical vapor deposition (AP MOCVD). The ex situ RPD AlN nucleation layer can significantly reduce dislocation density and thus improve the crystal quality of the GaN epitaxial layers. Utilizing high-resolution X-ray diffraction, the full width at half maximum of the rocking curve shows that the crystalline quality of the epitaxial layer with the (RPD) AlN nucleation layer is better than that with the low-temperature GaN (LT-GaN) nucleation layer. The threading dislocation density (TDD) is estimated by transmission electron microscopy (TEM), which shows the reduction from 6.8 × 107 cm−2 to 2.6 × 107 cm−2. Furthermore, the light output power (LOP) of the LEDs with the RPD AlN nucleation layer has been improved up to 30 % at a forward current of 350 mA compared to that of the LEDs grown on PSS with conventional LT-GaN nucleation layer.

Selective growth of GaN on slope cone-shaped patterned sapphire substrate

Cone-shaped patterned sapphire substrate was prepared by inductively coupled plasma etching and GaN nucleation layer was grown on it by metal-organic chemical vapor deposition. A selective growth of GaN nucleation layer was found on the slope of the cone-shaped patterned sapphire substrat, and the distribution morphology of GaN had significantly changed after it was recrystallized. GaN selective growth and redistribution were analyzed by investigating the distribution of crystallographic planes on the cone surface and the atom array of specific planes at atom level.

Sapphire Substrate Geometrical Effects on III-N Blue LED Material Properties

Solid-state lighting modules based upon III-N light-emitting diodes (LEDs) are now progressing towards widespread adoption in residential and commercial applications. LED manufacturers are driving die performance and yield upwards while reducing cost. However, color binning due to the sensitivity of the human eye response, which can resolve differences in dominant wavelength within a 2 nm range, remains one of the primary yield and cost limiters. As we continue to improve and understand growth process-related reproducibility, we have observed that some of the within-wafer and wafer-to-wafer characteristics cannot be explained through known physical phenomena from the epitaxy, and are more likely related to substrate properties.In this presentation, we will analyze a set of experiments designed to understand the effect of (0001) sapphire c-plane substrate physical dimensions (thickness, diameter, bow/warp/TTV, edge beveling) and crystal properties (misorientation, surface roughness) on uniformity and reproducibility of constituent LED layers. Our initial studies have found that precisely controlled crystal misorientation (offcut) towards the m-plane is a significant factor influencing wafer-to-wafer peak wavelength variation, GaN surface roughness, and p-doping levels. Quantitative analysis of these results with respect to variation in typical substrate manufacturing capabilities will be presented; the trends are in agreement with the materials characteristics explored in our previous studies [1, 2]. Warp and bow are critical parameters for control over GaN material characteristics. Furthermore, tight control over the total thickness variation (TTV) is needed to obtain highly uniform, repeatable sub-micron dimensions used to create current patterned substrate (PSS) wafers. [1] D. Lu, D.I. Florescu, D.S. Lee, V. Merai, J.C. Ramer, A. Parekh, and E.A. Armour, “Sapphire substrate misorientation effects on GaN nucleation layer properties”, J. Crystal Growth 272:353, (2004).[2] D. Lu, D.I. Florescu, D.S. Lee, V. Merai, A. Parekh, J.C. Ramer, S.P. Guo, and E. Armour, “Advanced characterization studies of sapphire substrate misorientation effects on GaN-based LED device development”, Phys. Stat. Sol.(a) 200:71, (2003).

Whispering gallery modes in GaN microdisks, microrods and nanorods grown by MOVPE

AbstractSelf‐assembled GaN microdisks, micro‐ and nanorods were grown by a mask‐free and self‐catalyzed metal‐organic vapor phase epitaxy method. The morphology of the structures is analyzed by scanning electron microscopy. All rods have a hexagonal shape with smooth sidewall facets and sharp edges. Depending on the deposition of a GaN nucleation layer prior to growth of GaN rod structures the diameter of the rods can be adjusted. Nanorods with diameters down to 400 nm as well as microrods and disks with diameters up to 5 µm can be achieved. The optical properties of the rods are investigated utilizing a combined scanning electron microscopy and cathodoluminescence system. Electron beam excitiation of GaN rods leads to a typical GaN spectrum showing GaN near band edge emission at 369 nm and broad yellow defect luminescence at 560 nm. Enhanced emission is located at the top of the rod. By fixing the focussed electron beam at a sidewall position of a single GaN rod, whispering gallery modes are observed in the cathodoluminescence spectrum. Depending on the diameter of the micro‐/nanorods the spectral position of the modes as well as the spectral distance between the modes can be adjusted. Quality factors up to 475 are measured. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Correlation between the residual stress and the density of threading dislocations in GaN layers grown by hydride vapor phase epitaxy

The correlation between the residual stress and the density of threading dislocations was investigated in polar GaN layers that were grown by using hydride vapor phase epitaxy (HVPE) on three different GaN templates. The first template type was GaN grown on sapphire by metal-organic vapor phase epitaxy. The second template type was a closed GaN nucleation layer grown on sapphire by HVPE. The third template type was a non-closed GaN nucleation layer grown by HVPE, which formed isolated pyramids on the sapphire surface. The residual stress was determined using the combination of micro-Raman spectroscopy and modified sin2ψ method. The interplanar spacings needed for the sin2ψ method were obtained from the reciprocal space maps that were measured using high-resolution X-ray diffraction. The density of threading dislocations was concluded from the broadening of the reciprocal lattice points that was measured using high-resolution X-ray diffraction as well. The fitting of the reciprocal space maps allowed the character of the threading dislocations to be described quantitatively in terms of the fractions of edge and screw dislocations. It was found that the threading dislocation density increases with increasing compressive residual stress. Furthermore, the dislocation density and the residual stress decrease with increasing thickness of the GaN layers. The edge component of the threading dislocations was dominant in all samples. Still, some differences in the character of the dislocations were observed for different templates.

Evaluation of AlGaN/GaN heterostructures properties by QMSA and AFM techniques

Atomic force microscopy and Quantitative Mobility Spectrum Analysis (QMSA) were applied for characterization and evaluation of the quality of AlGaN/GaN heterostructures. The structural uniformity, growth mode and electrical properties of the heterostructures were determined. The obtained results indicated that the time of growth of the low temperature GaN nucleation layer influenced the morphology and electrical properties of the AlGaN/GaN heterostructure.

GaN-Based LEDs With an HT-AlN Nucleation Layer Prepared on Patterned Sapphire Substrate

We report the growth and fabrication of GaN-based light-emitting diodes (LEDs) with a high-temperature (HT) AlN nucleation on patterned sapphire substrate. It was found that the undercut sidewalls were only formed for the HT-AlN LED through defect selective etching. At 1-A current injection, the output power of the LED with HT-AlN nucleation was 12% higher than that of an LED with a conventional low temperature GaN nucleation layer.

Improved crystal quality of a-plane GaN with high- temperature 3-dimensional GaN buffer layers deposited by using metal-organic chemical vapor deposition

a-plane GaN on r-plane sapphire substrates suffers from high density defects and rough surfaces. To obtain pit-free a-plane GaN by metal-organic chemical vapor deposition, we intentionally grew high-temperature (HT) 3-dimensional (3D) GaN buffer layers on a GaN nucleation layer. The effects of the HT 3D GaN buffer layers on crystal quality and the surface morphology of a-plane GaN were studied. The insertion of a 3D GaN buffer layer with an optimum thickness was found to be an effective method to obtain pit-free a-plane GaN with improved crystalline quality on r-plane sapphire substrates. An a-plane GaN light emitting diode (LED) at an emission wavelength around 480 nm with negligible peak shift was successfully fabricated.

Nucleation of GaN on sapphire substrates at intermediate temperatures by Hydride Vapor Phase Epitaxy

AbstractA novel approach to deposit GaN layers directly on a sapphire substrate by Hydride Vapor Phase Epitaxy is presented. The two‐step deposition process includes the growth of GaN nucleation layers at intermediate temperatures in the range of 750 – 900 °C and subsequent high‐temperature overgrowth at about 1040 °C. Closed and non‐closed nucleation layers with a thickness of up to 2 μm were produced and characterized by scanning and transmission electron microscopy, micro‐Raman spectroscopy and X‐ray diffraction. A growth temperature of 780 °C is found to be optimal with respect to density and size distribution of nucleation islands. Raman measurements performed on the nucleation layers reveal nearly zero residual stress indicating effective stress relaxation on cooling down from growth temperature. The results of first overgrowth experiments demonstrate the possibility to grow 10 μm thick, crack‐free GaN layers of high crystalline quality on the nucleation layers. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)