Jun 14, 2019

High efficiency and output power of near-ultraviolet light-emitting diodes grown on GaN substrate with back-side etching


In this paper, we investigate the differences between optical and electrical properties of near-ultraviolet (NUV) InGaN/GaN multi-quantum well light-emitting diodes (LEDs) grown on GaN substrate with a roughened back-side on the N-face surface of GaN substrate through a chemical wet-etching process, and on pattern sapphire substrate (PSS). Back-side etching-treated NUV-LEDs have larger output power than conventional NUV-LEDs, NUV-LEDs with wider wells and NUV-LEDs grown on PSS. When the NUV-LEDs were operated at a forward current of 20 mA, the output power of back-side etching-treated NUV-LEDs was improved by approximately 100, 106 and 8% compared with that of conventional NUV-LEDs, NUV-LEDs with wider wells and NUV-LEDs grown on PSS, respectively. This larger enhancement results from the improved light extraction that was attributed to the different transmittance because a hexagonal pyramid on the N-face GaN that was etched formed at the stable crystallographic etching planes of the GaN {1011} planes.


Source:IOPscience

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Jun 5, 2019

Dramatic increase in the growth rate of GaN layers grown from Ga2O vapor by epitaxial growth on HVPE-GaN substrates with a well-prepared surface

A GaN growth technique using Ga2O vapor allows high-temperature growth, which results in high-crystalline GaN. In this study, we succeeded in increasing the growth rate up to 180 µm/h, and with this technique, we maintained the crystallinity of the epitaxial layers at the same level as the crystallinity of the seed substrates [the FWHM of the (0002) GaN X-ray rocking curve was 71'']. To achieve this improvement, growth occurred on atomically smooth, damage-free seed substrates prepared by chemical mechanical polishing (CMP), and these substrates were subjected to a subsequent H2 heating process. Moreover, for this growth process, both a high temperature (1200 °C) and a H2/N2 atmosphere were also found to improve crystallinity.


Source:IOPscience

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May 28, 2019

Dependence of thermal stability of GaN on substrate orientation and off-cut

The thermal stability of ±c- and m-plane GaN substrates and its dependence on the m-plane GaN off-cut angle were investigated at and above 1200 °C at atmospheric pressure in flowing N2 with and without added NH3. It was clarified that decomposition of the +c-plane GaN surface is promoted by the addition of NH3, while decomposition of the –c- and m-plane GaN surfaces is suppressed by added NH3. It was found that –c- and m-plane GaN substrates can be heated at 1300 °C with an added NH3 input partial pressure of more than 0.2 atm, which is 100 °C higher than for +c-plane GaN substrates. Nevertheless, decomposition of m-plane GaN substrates became noticeable with increase in the off-cut angle of the substrate. It was revealed that the decomposition was better controlled by using m-plane GaN substrates with an off-cut towards [000overline 1] than towards [0001].


Source:IOPscience

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May 23, 2019

Influence of AlN thickness on strain evolution of GaN layer grown on high-temperature AlN interlayer

The strain evolution of a GaN layer grown on a high-temperature AlN interlayer with varying AlN thickness by metalorganic chemical vapour deposition is investigated. In the growth process, the growth strain changes from compression to tension in the top GaN layer, and the thickness at which the compressive-to-tensile strain transition takes place is strongly influenced by the thickness of the AlN interlayer. It is confirmed from the x-ray diffraction results that the AlN interlayer has a remarkable effect on introducing relative compressive strain to the top GaN layer. The strain transition process during the growth of the top GaN layer can be explained by the threading dislocation inclination in the top GaN layer. Adjusting the AlN interlayer thickness could change the density of the threading dislocations in the top GaN layer and then change the stress evolution during the top GaN layer's growth.



Source:IOPscience

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May 9, 2019

Freestanding GaN slab fabricated on patterned silicon on an insulator substrate

We propose the growth of GaN on patterned silicon-on-insulator (SOI) substrates, i.e. the GaN-on-patterned-SOI technique. The selective growth is suppressed at the low temperature molecular beam epitaxy (MBE) growth, and GaN nanocolumn are thus epitaxially grown on a silicon substrate and a silicon oxide substrate. The GaN slabs grown on the silicon oxide substrate are totally suspended in space by an association of bulk silicon micromachining and buffered HF etching. The photoluminescence and the reflection results suggest that silicon absorption of the emitted light is eliminated for the freestanding GaN slab, and the reflection losses are reduced at the GaN nanocolumn surface. This work provides a promising way to combine SOI technology with the growth of GaN for producing new optical devices.


Source:IOPscience

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Apr 30, 2019

Investigations of leakage current properties in semi-insulating GaN grown on Si(1 1 1) substrate with low-temperature AlN interlayers

In this work, the leakage current properties of semi-insulating GaN (SI-GaN) grown on Si(1 1 1) substrate with a low-temperature (LT) AlN interlayer were studied. Two terminal lateral current–voltage measurements revealed an early conduction phenomenon via buffer layers with an LT-AlN interlayer, in which both p- and n-type conduction phenomena were observed from Hall-effect measurement during temperature changing. It is suggested that the p-type conduction existed in the Si substrate due to the diffusion of Al atoms into the Si substrate during the epitaxial growth. The origin of n-type conduction was two-dimensional electron gas (2DEG) in the LT-AlN/GaN interface, which acted as a conduction channel in SI-GaN. Furthermore, the vertical leakage current measurement and the space charge limited current model were used to analyse the impact of the 2DEG conduction channel on the leakage current properties. It is revealed that leakage current properties are very sensitive to the thickness of the GaN layer above the LT-AlN interlayer (Top-GaN). Increasing the thickness of Top-GaN becomes an effective way to suppress leakage current. Therefore, both strain engineering and leakage current properties are essential factors to be considered in the selection of strain compensation interlayer in the growth of SI-GaN on Si substrate for power switching applications.


Source:IOPscience

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Apr 24, 2019

Characterization and growth mechanism of nonpolar and semipolar GaN layers grown on patterned sapphire substrates

Nonpolar and semipolar GaN layers with markedly improved crystalline quality can be obtained by selective-area growth from the sapphire sidewalls of patterned sapphire substrates (PSSs). In this paper, we review the crystalline qualities of GaN layers grown on PSSs and their growth mechanism. We grew semipolar {1 1 −2 2} and {1 0 −1 1} GaN layers on r- and n-PSSs. The crystalline qualities of the GaN layers grown on the PSSs were higher than those of GaN layers grown directly on heteroepitaxial substrates. To reveal the growth mechanism of GaN layers grown on PSSs, we also grew various nonpolar and semipolar GaN layers such as m-GaN on a-PSS, {1 1 −2 2} GaN on r-PSS, {1 0 − 1  1} GaN on n-PSS, m-GaN on c-PSS and a-GaN on m-PSS. It was found that the nucleation of GaN on the c-plane-like sapphire sidewall results in selective growth from the sapphire sidewall, and nonpolar or semipolar GaN can be obtained. Finally, we demonstrated a light-emitting diode fabricated on a {1 1 −2 2} GaN layer grown on an r-PSS.


Source:IOPscience

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