Lateral Overgrowth Method Research Articles

Improvement of GaN crystalline quality by SiNx layer grown by MOVPE

In this work the mechanism which helps to reduce the dislocation density by deposition of a SiNx interlayer is discussed. It is shown that the dislocation reduction by SiNx interlayer deposition is influenced by dislocation density in the underlying GaN layers. The SiNx interlayer is very effective when the original dislocation density is high, while in the case of lower dislocation density the deposition of SiNx is not effective for crystal quality improvement. Although it is widely accepted that SiNx serves as a barrier for dislocation propagation, similarly to the enhanced lateral overgrowth method, it is shown that after masking the SiNx deposition cannot be the dominant dislocation reduction mechanism. The most probable mechanism is the annihilation of bended neighbouring dislocations during the coalescence of 3D islands. The SiNx layer cannot serve as a barrier for dislocations, since it is probably dissolved during the following GaN growth and dissolved Si atoms are incorporated into the above-grown GaN layer which stimulates the 3D island formation. Then the use of the SiNx interlayer for dislocation reduction is recommended only for the improvement of layers with a high dislocation density. On the other hand, the PL signal was strongly enhanced for both low and high dislocation density structures with the SiNx interlayer, suggesting that the interlayer might help to suppress the nonradiative recombination in subsequent GaN that is not related to the dislocation density, which remained the same. But its origin has to be studied further.

InGaN/GaN light-emitting diode having direct hole injection plugs and its high-current operation.

The light-emitting diode (LED) with an improved hole injection and straightforward process integration is proposed. p-type GaN direct hole injection plugs (DHIPs) are formed on locally etched multiple-quantum wells (MQWs) by epitaxial lateral overgrowth (ELO) method. We confirm that the optical output power is increased up to 23.2% at an operating current density of 100 A/cm2. Furthermore, in order to identify the origin of improvement in optical performance, the transient light decay time and light intensity distribution characteristics were analyzed on the DHIP LED devices. Through the calculation of the electroluminescence (EL) decay time, internal quantum efficiency (IQE) is extracted along with the recombination parameters, which reveals that the DHIPs have a significant effect on enhancement of radiative recombination and reduction of efficiency droop. Furthermore, the mapping PL reveals that the DHIP LED also has a potential to improve the light extraction efficiency by hexagonal pyramid shaped DHIPs.

Dislocation glide in GaN films grown by the lateral-overgrowth method induced by low-energy electron-beam irradiation

The effect of irradiation on the dislocation structure of epitaxial GaN films, grown by the lateral-overgrowth method, is studied using the electron-beam-induced current mode in a scanning electron microscope. Low-energy electron-beam irradiation is found to lead to the gliding basal-plane dislocations even at very low excitation levels. Changes in the relative contrast of two segments of adjacent basal-plane dislocations may also indicate dislocation movement in the prismatic planes.

A partly-contacted epitaxial lateral overgrowth method applied to GaN material.

We have discussed a new crystal epitaxial lateral overgrowth (ELO) method, partly-contacted ELO (PC-ELO) method, of which the overgrowth layer partly-contacts with underlying seed layer. The passage also illustrates special mask structures with and without lithography and provides three essential conditions to achieve the PC-ELO method. What is remarkable in PC-ELO method is that the tilt angle of overgrowth stripes could be eliminated by contacting with seed layer. Moreover, we report an improved monolayer microsphere mask method without lithography of PC-ELO method, which was used to grow GaN. From the results of scanning electron microscopy, cathodoluminescence, x-ray diffraction (XRD), transmission electron microscopy, and atomic force microscope (AFM), overgrowth layer shows no tilt angle relative to the seed layer and high quality coalescence front (with average linear dislocation density <6.4 × 103 cm−1). Wing stripes peak splitting of the XRD rocking curve due to tilt is no longer detectable. After coalescence, surface steps of AFM show rare discontinuities due to the low misorientation of the overgrowth regions.

An InP/Si heterojunction photodiode fabricated by self-aligned corrugated epitaxial lateral overgrowth

An n-InP/p-Si heterojunction photodiode fabricated by corrugated epitaxial lateral overgrowth (CELOG) method is presented. N-InP/p-Si heterojunction has been achieved from a suitable pattern containing circular shaped openings in a triangular lattice on the InP seed layer on p-Si substrate and subsequent CELOG of completely coalesced n-InP. To avoid current path through the seed layer in the final photodiode, semi-insulating InP:Fe was grown with adequate thickness prior to n-InP growth in a low pressure hydride vapor phase epitaxy reactor. The n-InP/p-Si heterointerface was analyzed by scanning electron microscopy and Raman spectroscopy. Room temperature cross-sectional photoluminescence (PL) mapping illustrates the defect reduction effect in InP grown on Si by CELOG method. The InP PL intensity measured above the InP/Si heterojunction is comparable to that of InP grown on a native planar substrate indicating low interface defect density of CELOG InP despite of 8% lattice mismatch with Si. The processed n-InP/p-Si heterojunction photodiodes show diode characteristics from the current-voltage (I-V) measurements with a dark current density of 0.324 mA/cm2 at a reverse voltage of −1 V. Under the illumination of AM1.5 conditions, the InP/Si heterojunction photodiode exhibited photovoltaic effect with an open circuit voltage of 180 mV, a short circuit current density of 1.89 mA/cm2, an external quantum efficiency of 4.3%, and an internal quantum efficiency of 6.4%. This demonstration of epitaxially grown InP/Si heterojunction photodiode will open the door for low cost and high efficiency solar cells and photonic integration of III-Vs on silicon.

Embedding nano‐pillar arrays into InGaN light‐emitting diodes

AbstractThe embedded nano‐pillar light‐emitting diodes are fabricated via nanosphere lithography in combination with epitaxial lateral overgrowth method, followed by standard micro‐fabrication process. Apart from output power enhancement due to diffraction and scattering effect from the embedded nano‐pillars, the device is demonstrated to have insignificant spectral blue shift with increasing injection currents, which is dependent on the built‐in piezoelectric field that is partially suppressed by strain relaxation of nanostructured quantum wells. (© 2014 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Highly stable blue-emission in semipolar (11-22) InGaN/GaN multi-quantum well light-emitting diode

We demonstrate highly stable blue-emission on the semipolar (11-22) InGaN/GaN multi-quantum well light-emitting diode (LED) that has been prepared by the direct lateral-overgrowth method. The LED clearly displays bright blue-emission (λ ∼ 441 nm) with the reduced quantum-confined Stark effect. The variation of the emission-wavelength takes place only at the small injection-current range (<10 mA), and the net variation is less than 1.3 nm for wide driving-current ranges (5–100 mA). After the stabilization of light emission, the optical power is linearly increased with increasing the driving-current (1–1.8 mW). Additionally, the LED exhibits the stable light-polarization characteristics.

X-ray probe of GaN thin films grown on InGaN compliant substrates

GaN thin films grown on InGaN compliant substrates were characterized by several X-ray technologies: X-ray reciprocal space mapping (RSM), grazing incidence X-ray diffraction (GIXRD), and X-ray photoemission spectrum (XPS). Narrow Lorentz broadening and stress free state were observed for GaN grown on InGaN compliant substrate, while mosaic structure and large tensile stress were observed at the presence of residual indium atoms. RSM disclosed the mosaicity, and the GIXRD was conducted to investigate the depth dependences of crystal quality and strain states. XPS depth profile of indium contents indicated that residual indium atoms deteriorated the crystal quality of GaN not only by producing lattice mismatch at the interface of InGaN and GaN but also by diffusing into GaN overlayers. Accordingly, two solutions were proposed to improve the efficiency of self-patterned lateral epitaxial overgrowth method. This research goes a further step in resolving the urgent substrate problem in GaN fabrication.

Gallium Nitride Crystals Grown by Hydride Vapor Phase Epitaxy with Dislocation Reduction Mechanism

The growth of high-crystalline-quality GaN by hydride vapor phase epitaxy, focusing on dislocation density reduction, was studied, and a facet-initiated epitaxial lateral overgrowth (FIELO) method has been proposed. In this method, a GaN layer develops by the overgrowth of facet structures generated from narrow channels opened in a SiO2 mask formed on a GaN template. The threading dislocations changed their propagation direction, bending in the horizontal direction, due to the facet structure. To reduce the number of threading dislocations through the channels, we propose a novel FIELO method with nanometer-size channels. The dislocation density was found to decrease to 4.5 × 107 cm−2 for a 20-μm-thick GaN layer. We also developed random-island FIELO, in which micrometer-size small islands are initially formed on a TiC buffer layer by low-temperature growth of GaN on a sapphire substrate. Under higher growth temperatures that follow, the growth of these small islands changes to three-dimensional (3D) growth with facet planes. The 3D growth was maintained for a thickness of approximately 100–200 μm. As a result, the dislocation density was reduced to ∼3 × 106 cm−2. Freestanding 2- and 4-inch-diameter GaN substrates were also successfully fabricated by using the random-island FIELO method.

UGaN growth on laser patterned sapphire substrate: quality feature

AbstractLaser patterning technique as a flexible tool to grow high quality undoped gallium nitride (uGaN) on sapphire wafers is presented. A series of stripes were formed on sapphire wafer surface using Nd:YAG laser (100 kHz, 266 nm, 30 ps pulse) with variable pulse power (40‐670 mW), laser spot velocity over the wafer (1‐46 mm/s) and distance between the stripes (4‐15 μm). Stripes width was ∼2‐5 μm. After laser patterning sapphire wafers were cleaned using aqua regia and etched using sulphuric and phosphoric acid solution. SEM images after wet chemical etching show less surface roughness and open sapphire crystalline planes compared to surfaces treated with laser. Undoped GaN was grown by metal‐organic chemical vapour deposition (MOCVD). Used III/IV ratio: 2400, reactor pressure: 150 mBar, temperature: 1065 °C. SEM and AFM results show reduction of dislocation density in grown gallium nitride layer. The growing of this layer was performed using epitaxial lateral overgrowth (ELO) method on laser patterned sapphire substrates. Our observation revealed that the optimal laser power for sapphire substrate patterning for ELO GaN growth is 70‐300 mW. Moreover, we demonstrated intermediate GaN lateral growth states. (© 2012 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Improved emission efficiency of a-plane GaN light emitting diodes with silica nano-spheres integrated into a-plane GaN buffer layer

A simple and inexpensive technique to improve the emission efficiency of nonpolar a-plane light emitting diodes (LEDs) is proposed. The 3-dimensional growth nature of a-plane GaN was utilized to form the regrowth template of a-plane GaN. Subsequently, the controlled integration of silica nano-spheres (CIS) into the regrowth template is performed to improve the crystal quality of a-plane GaN by epitaxial lateral overgrowth method. In addition, the CIS improves light extraction by the scattering process. The light output power from the CIS a-plane GaN LEDs showed 130%–150% increase compared to that of LED without silica nano-spheres.

Chemical Lift-Off of Blue Light-Emitting Diodes Grown on Sapphire Substrate with an Oxide-Patterned Sacrificial Layer

A high quality GaN epilayer was successfully grown on a sapphire substrate with patterned SiO2 as a sacrificial structure utilizing the direct heteroepitaxial lateral overgrowth method. The light-emitting diode (LED) structure was subsequently grown on the GaN epilayer and fabricated using a chemical lift-off process. After SiO2 was removed with a HF solution, the LEDs with Cu substrate could be separated from the sapphire substrate due to an increase in the GaN/sapphire interfacial strain. Similar forward voltage and output power characteristics were obtained for vertical LEDs fabricated using chemical lift-off and laser lift-off processes.

Nanoindentation of laterally overgrown epitaxial gallium nitride

Nanoindentation has been used to investigate and compare the mechanical properties of GaN grown by the lateral epitaxial overgrowth (LEO) method and the defective seed region prepared by metalorganic chemical vapour deposition. Common modulus of elasticity values (∼230 GPa) and hardness values (∼19 GPa) were found for both materials. The GaN response to nanoindentation was found to be purely elastic for low indentation loads with the onset of plasticity being marked by discontinuities or “pop-in” events in the indenter load-penetration curves. The maximum shear stress under the indenter at pop-in events for LEO GaN corresponds well with the critical shear stress necessary for homogeneous dislocation nucleation, indicating that the defects in this region are too sparse and do not aid in dislocation nucleation.

Efficient blocking of planar defects by prismatic stacking faults in semipolar (112¯2)-GaN layers on m-sapphire by epitaxial lateral overgrowth

For the next-generation solid state lighting, the production of high quality semipolar (112¯2) GaN layers on sapphire obtained using asymmetric epitaxial lateral overgrowth (ELO) method has been investigated. This type of ELO leads to efficient blocking of the basal stacking faults (BSFs) in the bulk, and enables the formation of nondefective layers at the surface. The BSFs terminate due to generation of prismatic stacking faults along a well defined boundary. The corresponding intensity of GaN band edge photoluminescence emission is increased by more than four orders of magnitude in comparison to that from semipolar templates.

Growth of High Quality GaN by Hydride Vapor Phase Epitaxy

Hydride vapor phase epitaxy (HVPE) is currently used as a practical method for preparing GaN substrates. However, the reduction of dislocation density and the minimization of wafer curvature are dispensable from device application point of view. To reduce the dislocation density, the authors proposed facet -initiated epitaxial lateral overgrowth (FIELO) method. This method makes it possible to bend and reduce threading dislocations in GaN by forming GaN facet structures near the initial growth stage on foreign substrate, such as sapphire. In addition to the use of stripe-type mask pattern in the FIELO method, a novel FIELO method starting from random-islands having facet sidewalls of GaN formed at relatively low temperature is studied. It is shown that this method is superior to the conventional FIELO for reducing both of dislocations at the hetero-interface and the curvature of freestanding GaN crystal.

Internal quantum efficiency and internal loss of ultraviolet laser diodes on the low dislocation density AlGaN underlying layer

AbstractWe fabricated and evaluated GaN/AlGaN multi‐quantum wells (MQWs) and ultraviolet laser diodes (UV LDs) on high and low dislocation density underlying layers by epitaxial lateral overgrowth (ELO) method. We analyzed the internal quantum efficiency (IQE) versus carrier concentration characteristics quantitatively by excitation intensity dependent photoluminescence method. The IQE of the MQWs on the ELO AlGaN is 75% when the carrier density is 1x1019 cm‐3. We demonstrated the UV LD on the ELO AlGaN. However, the UV LD on flat AlGaN did not operate. Also, we investigated the internal loss (αi) and the IQE multiplied the injection efficiency by changing the reflectivity of the facets. The results showed that the internal loss is 6 cm‐1, and the IQE multiplied by the injection efficiency is 18% (© 2011 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Semi‐polar nitride surfaces and heterostructures

AbstractThis paper reviews semi‐polar GaN surfaces, of interest for light emitting devices, from both theoretical and experimental perspectives. Theoretical results on polarization charges at InGaN/GaN heterointerfaces and In incorporation into InGaN films are presented for polar (0001), semi‐polar (1122) and non‐polar (1100) surfaces. Specific features of semi‐polar InGaN/GaN structures are emphasized which can be beneficial for improving optical and transport properties of quantum‐well‐based light emitting devices. The analysis favours semi‐polar surfaces such as the (1122) surface as growth plane for long‐wavelength light emitters. Therefore, the experimental sections emphasize progress towards long‐wavelength LEDs and lasers by growth of InGaN/AlGaN/GaN(1122) heterostructures on large‐area GaN(1122)/m‐sapphire templates. The current status of such templates as grown by hydride vapour phase epitaxy is presented. The implementation of an epitaxial lateral overgrowth method on such templates to improve device performances is demonstrated.

Reduction of threading dislocations in hydrothermally grown ZnO films by lateral epitaxial overgrowth

Epitaxial ZnO films have been hydrothermally grown in water at 90 °C on MgAl 2O 4 (111) substrates. A lateral epitaxial overgrowth (LEO) method was utilized to reduce threading dislocations at the boundaries of crystal mosaic in the epitaxial films. In the optimized ZnO LEO with a ratio of window to wing (3:10), dislocations arose from two regions, i.e. unmasked window and coalescence of masked wings while they were seldom present in the wing area. The average density of dislocation was significantly reduced from 1.4 × 10 9 to 2.3 × 10 8 cm − 2 . A double LEO process was applied for further dislocation reduction with the 3 μm-wide unmasked region centered on the coalesced wings region from the first LEO. The average density of dislocation was found not to decrease by another order of magnitude but only to 1 × 10 8 cm − 2 because new threading dislocations were created at the coalescence of wings from the first LEO.

Aqueous lateral epitaxy overgrowth of ZnO on (0001) GaN at 90 °C: Part I. Increasing the critical thickness

Thick, epitaxial ZnO thin films have been grown on (0001) GaN buffered Al 2O 3 substrates using an aqueous solution at 90 °C. Films with improved structural, optical and electrical characteristics, were grown using a lateral epitaxial overgrowth (LEO) method. Different photoresist masks were used to enable LEO. The masks included linear windows and two different hexagonal arrays of circular windows. Films that exceeded a critical thickness mechanically failed through buckling, consistent with the large compressive stresses expected due to the mismatch of the ZnO lattice with the underlying GaN substrate. It was shown that improved mechanical stability could be achieved using the LEO method. Without LEO, a film thickness no greater than 4 µm could be grown without buckling. The critical thickness could be increased to 10 µm using linear windows, whereas a critical thickness of 50 μm was achieved with one array of circular windows, and 80 µm for a second array. The two different arrays of circular windows differed relative to their orientation on the substrate. It was also shown that the critical thickness increased with increasing distance between the growth windows. Optical transmission, micro-photoluminescence and Hall Effect measurements showed that the LEO method also results in improved optoelectronic properties.

Recent Progress in GaN‐Based Light‐Emitting Diodes

AbstractIn the last few years the GaN‐based white light‐emitting diode (LED) has been remarkable as a commercially available solid‐state light source. To increase the luminescence power, we studied GaN LED epitaxial materials. First, a special maskless V‐grooved c‐plane sapphire was fabricated, a GaN lateral epitaxial overgrowth method on this substrate was developed, and consequently GaN films are obtained with low dislocation densities and an increased light‐emitting efficiency (because of the enhanced reflection from the V‐grooved plane). Furthermore, anomalous tunneling‐assisted carrier transfer in an asymmetrically coupled InGaN/GaN quantum well structure was studied. A new quantum well structure using this effect is designed to enhance the luminescent efficiency of the LED to ∼72%. Finally, a single‐chip phosphor‐free white LED is fabricated, a stable white light is emitted for currents from 20 to 60 mA, which makes the LED chip suitable for lighting applications.