Crystalline Quality Of GaN Research Articles

Direct high-temperature growth of single-crystalline GaN on ScAlMgO4 substrates by metalorganic chemical vapor deposition

In this note, we demonstrate direct high-temperature growth of single-crystalline GaN on c-plane ScAlMgO4 substrates by metalorganic chemical vapor deposition, without using low-temperature buffers. We found that a trimethylaluminium preflow was crucial to suppress island growth and to achieve uniform mirror-like Ga-polar GaN layers. The preflow time was found to have a direct impact on the crystalline quality of GaN. We also show that thin GaN layers directly grown at high temperature can be used as buffers for the growth of lattice-matched In0.17Ga0.83N layers on ScAlMgO4. The presented results demonstrate the potential of direct growth of GaN on ScAlMgO4.

Influence of growth temperature of p-GaN layer on the characteristics of InGaN/GaN blue light emitting diodes

Optimisation for p-GaN epitaxial growth temperature ranges from 920°C to 1020°C was successfully conducted on InGaN/GaN blue light-emitting diodes (LEDs). These InGaN/GaN blue LEDs have been grown on patterned sapphire substrates (PSS) via metal organic chemical vapour deposition (MOCVD). The significance of p-GaN growth temperature was studied in detail according to the structural and optoelectronic performances of InGaN/GaN blue react to depth mechanism of surface evolution to LEDs grown on PSS. From experimental analysis, increment in the epitaxial growth temperature of p-GaN layer caused the crystalline quality of GaN and structural properties slightly improve. Other than that, there are decrement of RMS roughness and PL FWHM due to the increment in the epitaxial growth temperature of p-GaN layer. However, the epitaxial growth temperature of p-GaN increase led to no improvement of optoelectronic properties of InGaN/GaN blue LEDs. The LEDs with higher epitaxial growth temperature of the p-GaN layer show poorer optoelectronic properties. Therefore, the optimised epitaxial growth temperature of the p-GaN layer is of significant importance in achieving highly efficient InGaN/GaN blue LEDs.Besides that, introducing PSS as the substrate did not demonstrate high impact to achieve better performance LEDs. Further improvement of the p-GaN layer crystalline and structural quality might be required for the optimisation of the optoelectronic properties of InGaN/GaN blue LEDs.

Performance enhancement of InGaN/GaN MQWs grown on SiC substrate with sputtered AlN nucleation layer

InGaN/GaN multiple quantum wells (MQWs) based light-emitting diode (LED) structures grown on silicon carbide (SiC) substrates with and without sputtered AlN nucleation layer (NL) were investigated. High resolution X-ray diffraction rocking curve results indicated an improvement of GaN crystalline quality by using sputtered AlN NL. The surface morphology improvement of the LED structures was characterized by atomic force microscope and the root-mean-square roughness decreased from 14.1 nm to 2.1 nm by using sputtered AlN NL. The obvious enhancement of luminescence property was identified by photoluminescence and cathode luminescence spectra. These results pave the way for potential applications in SiC based microwave power devices and light emitting devices.

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.

Improved crystal quality of nonpolar a-plane GaN based on the nano pattern formed by the annealed thin Ni layer

Abstract A method to improve the nonpolar (11–20) a-plane GaN crystalline quality is investigated. The promoted crystal quality of a-plane GaN is achieved through growing on the a-plane GaN template with nano pattern. For on-axis (11–20) plane X-ray rocking curves, the full width at half maximum values are remarkably reduced from 2834 to 1210 arcsec, 2868 to 780 arcsec along the m-axis and the c-axis directions, respectively. Also, the improved crystalline quality results in the better optical properties according to the photoluminescence and Raman measurements. Moreover, the cross-sectional transmission electron microscopy is also used to study the mechanisms of dislocation reduction. Evidently, this technique is effective in promoting crystal quality of nonpolar a-plane GaN.

Van der Waals epitaxy of GaN-based light-emitting diodes on wet-transferred multilayer graphene film

We experimentally investigated the possibility of using multilayer graphene to solve large mismatch problems between sapphire and nitride and further studied the effects of a multilayer graphene interlayer on the optical and electrical properties of LEDs. For the subsequent growth of 3-µm-thick GaN on AlN, multilayer graphene helps release stress and effectively removes cracks. In addition, multilayer graphene increases the diffraction of the substrate surface as determined from the increase in optical transmittance spectra in the wavelength range of 400–900 nm. Although the crystalline quality of GaN with multilayer graphene is slightly decreased, LEDs grown on multilayer graphene still show a higher output power than those grown on conventional sapphire. The present findings showed that the multilayer graphene layer is attractive as a potential substrate for the epitaxial growth of III–nitride to reduce stress and it could improve back light extraction as a rough layer to increase external quantum efficiency.

Effect of 3C-SiC intermediate layer in GaN—based light emitting diodes grown on Si(111) substrate

GaN-based light emitting diodes (LEDs) have been grown by metalorganic chemical vapor deposition on Si(111) substrate with and without 3C-SiC intermediate layer (IL). Structural property has been characterized by means of atomic force microscope, X-ray diffraction, and transmission electron microscope measurements. It has been revealed that a significant improvement in crystalline quality of GaN and superlattice epitaxial layers can be achieved by using 3C-SiC as IL. Regarding of electrical and optical characteristics, it is clearly observed that the LEDs with its IL have a smaller leakage current and higher light output power comparing with the LEDs without IL. The better performance of LEDs using 3C-SiC IL can be contributed to both of the improvements in epitaxial layers quality and light extraction efficiency. As a consequence, in terms of optical property, a double enhancement of the light output power and external quantum efficiency has been realized.

Epitaxial growth of GaN films on nearly lattice-matched hafnium substrates using a low-temperature growth technique

We demonstrated epitaxial growth of GaN (0001) films on nearly lattice-matched Hf (0001) substrates by using a low-temperature (LT) epitaxial growth technique. High-temperature growth of GaN films results in the formation of polycrystalline films due to significant reaction at GaN/Hf heterointerfaces, while LT-growth allowed us to suppress the interfacial reactions and to obtain epitaxial GaN films on Hf substrates with a GaN112̄0//Hf112̄0 in-plane orientation. LT-grown GaN films can act as buffer layers for GaN growth at high temperatures. The interfacial layer thickness at the LT-GaN/Hf heterointerface was as small as 1 nm, and the sharpness of the contact remained unchanged even after annealing up to approximately 700 °C, which likely accounts for the dramatic improvement in GaN crystalline quality on Hf substrates.

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.

Spatial distribution of crystalline quality in N-type GaN grown on patterned sapphire substrate

The epitaxial layers of n-type GaN were grown on both planar and patterned sapphire substrate (PSS) by metal organic chemical vapor deposition. By comparing the epitaxial layers grown on planar substrate, GaN grown on PSS exhibited many improvements both on surface morphology and crystalline quality according to the characterization of atoms force microscopy, and high resolution X-ray diffraction. Spatially resolved micro-Raman scattering results were performed for mapping the spatial variations in crystalline quality of the n-type GaN grown on PSS. According to the variations on the intensity and the full width at half maximum of GaN E2 (high) peaks, the crystalline quality improvement occurred in the lateral growth regions which correspond to center region of the pyramid patterns. We proposed that the bending of dislocations during the lateral growth plays an important role in the spatial variations of GaN crystalline quality. Cross sectional transmission electron microscope and spatial cathodoluminescence mapping results further supported the explanation of the dislocation inhibition during the growth process of GaN grown on PSS.

Barrier Strain and Carbon Incorporation‐Engineered Performance Improvements for AlGaN/GaN High Electron Mobility Transistors**

AbstractThe improvements in electrical characteristics of AlGaN/GaN high electron mobility transistors (HEMTs) grown using metal‐organic (MO)CVD by engineering structure, barrier strain, and unintentional carbon incorporation, are demonstrated in this work. Both normal HEMT structure (with a high temperature (HT) AlN buffer) and advanced HEMT structure (with a high‐low‐high temperature (HLHT) AlN buffer, and a HT AlN interlayer (IL)) present a breakdown voltage higher than 200 V, while a much smaller breakdown voltage of 17 V is measured on the conventional structure using a low‐temperature GaN buffer. The HT AlN IL inserted in the middle of the conventional HEMT structure introduces a reduction in the tension of the AlGaN barrier, which results in an improvement of the surface morphology (0.46 nm). As a consequence, the two‐dimensional electron gas (2DEG) mobility increases by remarkable 46% (1900 cm2 V−1 s−1). The HLHT AlN buffer, substituting for the HT AlN buffer, leads to the enhancement of GaN crystalline quality, which contributes to the performance improvement for HEMTs. The advanced HEMT, using both an AlN IL and an HLHT AlN buffer, produces increases in the DC maximum drain current by 35.5% (∼680 A mm−1), and in the transconductance by 15% (114 mS mm−1) in comparison with the normal HEMT with an AlN buffer. The very low leakage current in the advanced HEMTs is caused by optimizing the design of the buffer and modifying growth parameters. Lastly, the reduction of AlGaN barrier tensile strain by inserting the HT AlN IL is promising for an improvement in AlGaN/GaN HEMT reliability.

Study on GaN-based light emitting diodes grown on 4-in. Si (111) substrate

GaN-based light emitting diodes (LEDs) grown on 4-in. Si (111) substrate by metal–organic chemical vapor deposition have been systematically characterized. The significantly smooth surface of the sample has been confirmed by an atomic force microscope. In X-ray diffraction measurement, two kinds of fringe peaks were observed, which are believed to originate from the strained-layer superlattice (SLS) and multiple quantum wells. Moreover, from cross-sectional scanning electron microscope images of the sample, it is found that the interfaces of SLS are smooth. The 100-pair AlN/GaN SLS can be employed to modulate the strain between the GaN layer and substrate, resulting in the improvement of GaN crystalline quality. The full width at half maximum of ω-scan of the GaN (0002) diffraction is around 630ʺ. In addition, the device properties have been investigated in detail, and the maximum light output power can reach 2.02mW with a high saturation injection current of 320mA.

Impact of AlN nucleation layer on strain in GaN grown on 4H-SiC substrates

The impact of AlN nucleation layers on the strain evolution in a subsequent GaN layer was investigated by in-situ wafer curvature measurement. It is shown that growth temperature and thickness of the AlN nucleation layer strongly influence the built-in strain and crystalline quality of GaN. A two-step AlN growth procedure leads to a decrease of compressive strain as well as the reduction of the total dislocation density (6–9×108cm−2) in the overgrown GaN layer. TEM analysis reveals different relaxation mechanisms of GaN in v-pits and on flat surfaces of AlN. With a two-step AlN nucleation layer a low wafer bow can be achieved together with a low dislocation density.

GaN growth on (111)Al substrates by CS‐MBE and their chemical lift‐off technique

AbstractThe low‐temperature growth of GaN on (0001) sapphire and (111)Al substrates by compound‐source molecular beam epitaxy (CS‐MBE) with was investigated. Although precise control of the V/III ratio was required for the growth, the near‐bandedge‐emission (NBE)‐dominant photoluminescence spectra of hexagonal GaN layers were observed at RT. The GaN layers were also grown on aluminum substrates with and without nitridation by CS‐MBE. Reflection high‐energy electron diffraction patterns of the layers indicate that the nitridation is effective for GaN growth by CS‐MBE on aluminum substrates. For further improvement of the GaN crystalline quality, the use of epitaxially grown Al layers as the substrates for the growth was proposed. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Growth of GaN and AlGaN on (100) β‐Ga2O3 substrates

AbstractThe crystalline quality of GaN and Al0.08Ga0.92N epitaxial layers on (100) β‐Ga2O3 substrates was significantly improved by the facet‐controlled growth method. The facets were controlled by changing the nitrogen ambient thermal annealing temperature. We demonstrated the high‐crystalline‐quality GaN and Al0.08Ga0.92N on β‐Ga2O3 substrates, which were comparable to GaN and AlGaN on sapphire substrates using low‐temperature buffer layers. This method is useful for the fabrication of vertical‐type ultraviolet (UV) light‐emitting diodes (LEDs) on β‐Ga2O3 substrates. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Influence of V/III Ratio of Low Temperature Grown AlN Interlayer on the Growth of GaN on Si Substrate

In this work, the influence of the V/III ratios of the low temperature (LT) AlN interlayer on GaN grown on Si<111> substrate have been investigated. It was found that V/III ratio of LT-AlN interlayer is another important growth parameter, which the crystalline quality of GaN was strongly dependent on. By optimizing the V/III ratio of LT-AlN, the high quality crack-free GaN epilayer with lower dislocation density on Si substrate have been obtained. We attribute it to the different LT-AlN surface morphology originated from optimizing the V/III ratio. Different V/III ratios could result in different GaN growth rates in vertical and lateral direction, which could effectively prevent the threading dislocation from penetrating through the LT-AlN interlayer.

Influence of V/III Ratio of Low Temperature Grown AlN Interlayer on the Growth of GaN on Si&lt;111&gt; Substrate

In this work, the influence of the V/III ratios of the low temperature (LT) AlN interlayer on GaN grown on Si<111> substrate have been investigated. It was found that V/III ratio of LT-AlN interlayer is another important growth parameter, which the crystalline quality of GaN was strongly dependent on. By optimizing the V/III ratio of LT-AlN, the high quality crack-free GaN epilayer with lower dislocation density on Si substrate have been obtained. We attribute it to the different LT-AlN surface morphology originated from optimizing the V/III ratio. Different V/III ratios could result in different GaN growth rates in vertical and lateral direction, which could effectively prevent the threading dislocation from penetrating through the LT-AlN interlayer.

Improvement of a-plane GaN crystalline quality by overgrowth of in situ etched GaN template

This study demonstrates improvement of crystalline quality of a-plane GaN by growing it on a porous GaN template fabricated by in situ etching of a first GaN film using hydrogen and ammonia gases at 1150 °C in a metal organic chemical vapor deposition (MOCVD) reactor. Photoluminescence (PL) and high-resolution X-ray diffraction (HR-XRD) measurements show that the crystalline quality of the GaN film re-grown on the porous GaN template was superior to the quality of the initially grown GaN film. This study demonstrates a simple, short procedure for growing high quality a-GaN using a single MOCVD tool without ex situ processes.

Demonstration on GaN-based light-emitting diodes grown on 3C-SiC/Si(111)

GaN-based light-emitting diodes (LEDs) grown on template of 3C-SiC/Si(111) were demonstrated. The structural properties have been investigated systematically by means of atomic force microscopy, x-ray diffraction, and transmission electron microscopy. It is found that the intermediate layer (IL) of 3C-SiC leads to not only a significant improvement in the crystalline quality of GaN, but also better interfaces between the buffer layer and the initial layers of strained-layer superlattice. The device properties were also evaluated using the measurements of current-voltage, electroluminescence, and light output power-current. Compared to conventional LEDs that do not contain 3C-SiC IL, the device with IL exhibits enhanced output power by more than 200% at an injection current of 20 mA, and the operating voltage is slightly increased from 3.7 to 3.9 V. These results indicate that using 3C-SiC as IL is one of the promising approaches to improve the performance of LEDs on silicon.

Growth of GaNAs films with As 2 source in atomic hydrogen-assisted molecular beam epitaxy

We have investigated the effect of using As 2 source instead of a more commonly used As 4 source on the crystalline quality of GaN x As 1− x thin films grown by atomic hydrogen-assisted molecular beam epitaxy. Improved structural and optical properties of GaNAs thin films were obtained by using As 2 source. The nitrogen atoms were incorporated into GaAs at a more stable rate under As 2 flux than As 4 flux, and a two-dimensional nucleation growth mode was promoted for growth of GaN x As 1− x with As 2 source. As a consequence, the surface roughness measured for a 500 nm-thick Ga 0.008N 0.992As sample grown with As 2 flux was 1–2 monolayers, which was three times more smoother than that for As 4 sample. The photoluminescense measurements showed an improved potential fluctuation of 78.1 meV and twice the intensity at room temperature for As 2 sample.