This work was financially supported by the National Key R&#38;D Program of China (No. 2018YFB0406703), the National Natural Science Foundation of China (Nos. 61474109, 61527814, 11474274, 61427901), and the Beijing Natural Science Foundation (No. 4182063)

CAUTION: Several of the chemicals used in these methods are acutely toxic and carcinogenic. Please consult all relevant material safety data sheets (MSDS) before use.
1. Preparation of NPSS by nanoimprint lithography (NIL)
<ol>
	<li>Deposition of SiO2 film
	<ol>
		<li>Wash the 2&#34;&#160;c-plane flat sapphire substrate with ethanol followed by deionized water three times.</li>
		<li>Dry the substrate with a nitrogen gun.</li>
		<li>Deposit 200 nm SiO2 film on the flat sapphire substrate by plasma-enhanced chemical vapor deposition (PECVD) under 300 &#176;C. The deposition rate is 100 nm/min.</li>
	</ol>
	</li>
	<li>Spinning nanoimprint resist
	<ol>
		<li>Wash the sapphire substrate with ethanol followed by deionized water 3x.</li>
		<li>Dry the substrate with a nitrogen gun.</li>
		<li>Spin a 200 nm nanoimprint resist (NIR) TU-2 on the flat sapphire substrate at 3000 r/min for 60 s.</li>
	</ol>
	</li>
	<li>Thermoplastic imprinting
	<ol>
		<li>Place a patterned mold onto the nanoimprint resist polymer film.</li>
		<li>Apply high pressure as 30 bar at 60 &#176;C to heat the sapphire substrate to above the glass transition temperature of the polymer.</li>
		<li>Expose to ultraviolet irradiation for 60 s and maintain for 120 s after turning off the UV source to solidify the NPR TU-2.</li>
		<li>Cool down the sapphire substrate and mold to room temperature (RT).</li>
		<li>Release the mold.</li>
	</ol>
	</li>
	<li>Pattern transfer
	<ol>
		<li>Etch the sapphire substrate exposed from the nano-holes on the NIR by inductive coupled plasma reactive ion etching (ICP-RIE) with BCl3 to transfer the pattern onto the sapphire substrate. The etching power is 700 W and etching time is 3 min.</li>
		<li>Remove the residual NPR TU-2 by O2 plasma etching in a RIE system for 20 s. The etching pressure is 5 mTorr and etching power is 100 W. Finally, the width of the unetched regions is 300 nm and the depth is 400 nm. The period of pattern is 1 &#956;m. 
		NOTE: NIL is not the only way to get NPSS. The NPSS are commercialized and could be bought elsewhere.</li>
	</ol>
	</li>
</ol>
2. APCVD growth of graphene on NPSS
<ol>
	<li>Rinse the NPSS with acetone, ethanol, and deionized water 3x.</li>
	<li>Dry the NPSS with a nitrogen gun.</li>
	<li>Load the NPSS into a three-zone high temperature furnace for long, flat temperature zone. Heat the furnace to 1050 &#176;C and stabilize for 10 min under 500 sccm Ar and 300 sccm H2</li>
	<li>Introduce 30 sccm CH4 into the reaction chamber for the growth of graphene on NPSS for 3 h. After the growth of graphene, switch off the CH4 and naturally cool.</li>
</ol>
3. N2-plasma treatment
<ol>
	<li>Rinse the graphene-NPSS with deionized water.</li>
	<li>Dry the NPSS with a nitrogen gun.</li>
	<li>Etch the graphene-NPSS by N2-plasma with a N2 flow rate of 300 sccm for 30 s and power of 50 W in a reactive ion etching (RIE) chamber.</li>
</ol>
4. MOCVD growth of AlN on graphene-NPSS
<ol>
	<li>Edit the MOCVD recipe for AlN growth and load the graphene-NPSS and its NPSS counterpart into the homemade MOCVD chamber.</li>
	<li>After heating for 12 min, the temperature is stabilized at 1200 &#176;C. Introduce 7000 sccm H2 as ambient, 70 sccm trimethylaluminum (TMAl), and 500 sccm NH3 for the growth of AlN for 2 h.</li>
</ol>
5. MOCVD growth of AlGaN MQWs
<ol>
	<li>Lower the temperature of MOCVD chamber to 1130 &#176;C to grow 20-period AlN (2 nm)/Al0.6Ga0.4N (2 nm) layer superlattice (SL) with periodic changes in TMAl flow to adjust the deposition component. The ambient gas is H2. The mole flow rates of TMAl, TMGa, and NH3 for AlN are 50 sccm, 0 sccm, and 1000 sccm; and for AlGaN are 32 sccm, 7 sccm, and 2,500 sccm, respectively.</li>
	<li>Lower the temperature of MOCVD chamber to 1002 &#176;C and introduce a silicane flow for the growth of a 1.8 &#181;m n-Al0.55Ga0.45N layer. The ambient gas is H2 and concentration of n-type AlGaN is 5 x 1018 cm-3.</li>
	<li>Grow 5-period Al0.6Ga0.4N (3 nm)/Al0.5Ga0.5N (12 nm) MQWs by switching the TMAl from 24 sccm to 14 sccm, and TMGa from 7 sccm to 8 sccm, for each period at 1002 &#176;C. The ambient gas is H2.</li>
	<li>Deposit 50 nm Mg-doped p- Al0.65Ga0.35N electron blocking layer (EBL) at 1002 &#176;C. The mole flow rates of TMAl, TMGa, and NH3 are 40 sccm, 6 sccm, and 2500 sccm. The ambient gas is H2.</li>
	<li>Deposit 30 nm p-Al0.5Ga0.5N cladding layer with NH3 flow of 2500 sccm. The ambient gas is H2.</li>
	<li>Deposit 150 nm p-GaN contact layer with an NH3 flow of 2500 sccm. The ambient gas is H2. The mole flow rates of TMGa and NH3 are 8 sccm and 2500 sccm. The hole concentration of p-AlGaN is 5.4 x 1017 cm-3.</li>
	<li>Lower the temperature of MOCVD chamber to 800 &#176;C and anneal the p-type layers with N2 for 20 min. The ambient gas is N2.</li>
</ol>
6. Fabrication of AlGaN-based DUV-LEDs
<ol>
	<li>Spinning photoresist 4620 on the wafers and lithography. The UV exposure time, developing time, and rinsing time are 8 s, 30 s, and 2 min, respectively.</li>
	<li>ICP etching of p-GaN. The etching power, etching pressure, and etching rate of GaN are 450 W, 4 m Torr, and 5.6 nm/s, respectively.</li>
	<li>Put the sample into acetone at 80 &#176;C for 15 min followed by washing the sample with ethanol and deionized water 3x.</li>
	<li>Spinning negative photoresist NR9 and lithography. The UV exposure time, developing time, and rinsing time are 12 s, 20 s, and 2 min, respectively.</li>
	<li>Wash the sample with acetone, ethanol, and deionized water 3x.</li>
	<li>Deposit Ti/Al/Ti/Au by electron beam (EB) evaporation.</li>
	<li>Spin negative photoresist NR9 and lithography. The UV exposure time, developing time, and rinsing time are 12 s, 20 s, and 2 min, respectively.</li>
	<li>Wash the sample with acetone, ethanol, and deionized water 3x without ultrasonication.</li>
	<li>Deposit Ni/Au by EB evaporation.</li>
	<li>Wash the sample with ethanol and deionized water 3x to clean the sample.</li>
	<li>Deposit 300 nm SiO2 by plasma enhanced chemical vapor deposition (PECVD). The deposition temperature is 300 &#176;C and deposition rate is 100 nm/min.</li>
	<li>Spin photoresist 304 and lithography. The UV exposure time, developing time, and rinsing time are 8 s, 1 min, and 2 min, respectively.</li>
	<li>Immerse the wafers into 23% HF solution for 15 s.</li>
	<li>Wash the sample with ethanol and deionized water 3x and dry with a nitrogen gun.</li>
	<li>Deposit Al/Ti/Au by EB evaporation after photolithography. The photolithography process is the same as that performed in steps 6.4-6.7.</li>
	<li>Wash the sample with ethanol and deionized water 3x.</li>
	<li>Grind and polish the sapphire to 130 &#181;m by mechanical polishing.</li>
	<li>Wash the sample with dewaxing solution and deionized water.</li>
	<li>Cut the whole wafer into pieces of 0.5 mm x 0.5 mm devices with a laser and cut it into chips using a mechanical dicer.</li>
</ol>

<ol>
	<li>Sakai, Y., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Demonstration+of+AlGaN-Based+Deep-Ultraviolet+Light-Emitting+Diodes+on+High-Quality+AlN+Templates.">Demonstration of AlGaN-Based Deep-Ultraviolet Light-Emitting Diodes on High-Quality AlN Templates.</a> Jappanese Journal of Applied Physics. 49, 022102 (2010).</li><li>Yun, J., Hirayama, H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Investigation+of+the+light-extraction+efficiency+in+280+nm+AlGaN-based+light-emitting+diodes+having+a+highly+transparent+p-AlGaN+layer.">Investigation of the light-extraction efficiency in 280 nm AlGaN-based light-emitting diodes having a highly transparent p-AlGaN layer.</a> Journal of Applied Physics. 121, 013105 (2017).</li><li>Khan, A., Balakrishnan, K., Katona, T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Ultraviolet+light-emitting+diodes+based+on+group+three+nitrides.">Ultraviolet light-emitting diodes based on group three nitrides.</a> Nature Photonics. 2, 77-84 (2008).</li><li>Balushi, Z. Y. A., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+impact+of+graphene+properties+on+GaN+and+AlN+nucleation.">The impact of graphene properties on GaN and AlN nucleation.</a> Surface Science. 634, 81-88 (2015).</li><li>Motoki, K., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Growth+and+characterization+of+freestanding+GaN+substrates.">Growth and characterization of freestanding GaN substrates.</a> Journal of Crystal Growth. 237, 912-921 (2002).</li><li>Kim, Y., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Remote+epitaxy+through+graphene+enables+two-dimensional+material-based+layer+transfer.">Remote epitaxy through graphene enables two-dimensional material-based layer transfer.</a> Nature. 544, 340-343 (2017).</li><li>Hemmingsson, C., Pozina, G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Optimization+of+low+temperature+GaN+buffer+layers+for+halide+vapor+phase+epitaxy+growth+of+bulk+GaN.">Optimization of low temperature GaN buffer layers for halide vapor phase epitaxy growth of bulk GaN.</a> Journal of Crystal Growth. 366, 61-66 (2013).</li><li>Chen, Z., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=High-Brightness+Blue+Light-Emitting+Diodes+Enabled+by+a+Directly+Grown+Graphene+Buffer+Layer.">High-Brightness Blue Light-Emitting Diodes Enabled by a Directly Grown Graphene Buffer Layer.</a> Advanced Materials. 30, 1801608 (2018).</li><li>Dong, P., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=282-nm+AlGaN-based+deep+ultraviolet+light-emitting+diodes+with+improved+performance+on+nano-patterned+sapphire+substrates.">282-nm AlGaN-based deep ultraviolet light-emitting diodes with improved performance on nano-patterned sapphire substrates.</a> Applied Physics Letters. 102, 241113 (2013).</li><li>Imura, M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Epitaxial+lateral+overgrowth+of+AlN+on+trench-patterned+AlN+layers.">Epitaxial lateral overgrowth of AlN on trench-patterned AlN layers.</a> Journal of Crystal Growth. 298, 257-260 (2007).</li><li>Kueller, V., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Growth+of+AlGaN+and+AlN+on+patterned+AlN/sapphire+templates.">Growth of AlGaN and AlN on patterned AlN/sapphire templates.</a> Journal of Crystal Growth. 315, 200-203 (2011).</li><li>Kneissl, M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Advances+in+group+III-nitride-based+deep+UV+light-emitting+diode+technology.">Advances in group III-nitride-based deep UV light-emitting diode technology.</a> Semiconductor Science &amp; Technology. 26, 014036 (2010).</li><li>Kunook, C., Chul-Ho, L., Gyu-Chul, Y. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Transferable+GaN+layers+grown+on+ZnO-coated+graphene+layers+for+optoelectronic+devices.">Transferable GaN layers grown on ZnO-coated graphene layers for optoelectronic devices.</a> Science. 330, 655-657 (2010).</li><li>Kim, J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Principle+of+direct+van+der+Waals+epitaxy+of+single-crystalline+films+on+epitaxial+graphene.">Principle of direct van der Waals epitaxy of single-crystalline films on epitaxial graphene.</a> Nature Communications. 5, 4836 (2014).</li><li>Han, N., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Improved+heat+dissipation+in+gallium+nitride+light-emitting+diodes+with+embedded+graphene+oxide+pattern.">Improved heat dissipation in gallium nitride light-emitting diodes with embedded graphene oxide pattern.</a> Nature Communications. 4, 1452 (2013).</li><li>Gupta, P., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=MOVPE+growth+of+semipolar+III-nitride+semiconductors+on+CVD+graphene.">MOVPE growth of semipolar III-nitride semiconductors on CVD graphene.</a> Journal of Crystal Growth. 372, 105-108 (2013).</li><li>Heinke, H., Kirchner, V., Einfeldt, S., Hommel, D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=X-ray+diffraction+analysis+of+the+defect+structure+in+epitaxial+GaN.">X-ray diffraction analysis of the defect structure in epitaxial GaN.</a> Appllied Physics Letters. 77, 2145-2147 (2000).</li><li>Lughi, V., Clarke, D. R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Defect+and+Stress+Characterization+of+AlN+Films+by+Raman+Spectroscopy.">Defect and Stress Characterization of AlN Films by Raman Spectroscopy.</a> Appllied Physics Letters. 89, 2653 (2006).</li><li>Li, Y., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Van+der+Waals+epitaxy+of+GaN-based+light-emitting+diodes+on+wet-transferred+multilayer+graphene+film.">Van der Waals epitaxy of GaN-based light-emitting diodes on wet-transferred multilayer graphene film.</a> Jappanese Journal of Applied Physics. 56, 085506 (2017).</li><li>Chang, H., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Graphene-assisted+quasi-van+der+Waals+epitaxy+of+AlN+film+for+ultraviolet+light+emitting+diodes+on+nano-patterned+sapphire+substrate.">Graphene-assisted quasi-van der Waals epitaxy of AlN film for ultraviolet light emitting diodes on nano-patterned sapphire substrate.</a> Applled Physics Letters. 114, 091107 (2019).</li></ol>

The authors have nothing to disclose.

As shown in Figure 1A, the NPSS prepared by the NIL technique illustrates the nano-concave cone patterns with 400 nm depth, 1 &#956;m period of pattern, and 300 nm width of the unetched regions. After the APCVD growth of graphene layer, the graphene-NPSS is shown in Figure 1B. The significant increased D peak of N-plasma treated graphene in Raman spectra Figure 1C demonstrates the i...

AlN and AlGaN are the most essential materials in DUV-LEDs1,2, which have been widely used in various fields such as sterilization, polymer curing, biochemical detection, non-line-of-sight communication, and special lighting3. Due to the lack of intrinsic substrates, AlN heteroepitaxy on sapphire substrates by MOCVD has become the most common technical route4. However, the large lattice mismatch between AlN and sapphire substrate leads to stress accumulation5,6, high density dislocations, and stacking faults7. Thus, the internal quantum efficiency of LEDs are reduced8. In recent decades, using patterned sapphire as substrates (PSS) to induce AlN epitaxial lateral overgrowth (ELO) has been proposed to solve this problem. In addition, great progress has been made in the growth of AlN templates9,10,11. However, with a high surface adhesion coefficient and bonding energy (2.88 eV for AlN), Al atoms have low atomic surface mobility, and the growth of AlN tends to have a three-dimensional island growth mode12. Thus, the epitaxial growth of AlN films on NPSS is difficult and requires higher coalescence thickness (over 3 &#956;m) than that on flat sapphire substrates, which causes longer growth time and requires high costs9.
Recently, graphene shows great potential for use as a buffer layer for AlN growth due to its hexagonal arrangement of sp2 hybridized carbon atoms13. In addition, the quasi-van der Waals epitaxy (QvdWE) of AlN on graphene may reduce the mismatch effect and has paved a new way for AlN growth14,15. To increase the chemical reactivity of graphene, Chen et al. used N2-plasma treated graphene as a buffer layer and determined the QvdWE of high quality AlN and GaN films8, which demonstrates the utilization of graphene as a buffer layer.
Combining the N2-plasma treated graphene technic with commercial NPSS substrates, this protocol presents a new method for quick growth and coalescence of AlN on a graphene-NPSS substrate. The completely coalesce thickness of AlN on graphene-NPSS is confirmed to be less than 1 &#181;m, and the epitaxial AlN layers are of high quality and stress-released. This method paves a new way for AlN template mass production and shows great potential in the application of AlGaN-based DUV-LEDs.

<table>
	<tbody>
		<tr>
			<td>Acetone,99.5%</td>
			<td>Bei Jing Tong Guang Fine Chemicals company</td>
			<td>1090</td>
			<td></td>
		</tr>
		<tr>
			<td>APCVD</td>
			<td>Linderberg</td>
			<td>Blue M</td>
			<td></td>
		</tr>
		<tr>
			<td>EB</td>
			<td>AST</td>
			<td>Peva-600E</td>
			<td></td>
		</tr>
		<tr>
			<td>Ethonal,99.7%</td>
			<td>Bei Jing Tong Guang Fine Chemicals company</td>
			<td>1170</td>
			<td></td>
		</tr>
		<tr>
			<td>HF,40%</td>
			<td>Beijing Chemical Works</td>
			<td>1789</td>
			<td></td>
		</tr>
		<tr>
			<td>ICP-RIE</td>
			<td>AST</td>
			<td>Cirie-200</td>
			<td></td>
		</tr>
		<tr>
			<td>MOCVD</td>
			<td>VEECO</td>
			<td>P125</td>
			<td></td>
		</tr>
		<tr>
			<td>PECVD</td>
			<td>Oerlikon</td>
			<td>790+</td>
			<td></td>
		</tr>
		<tr>
			<td>Phosphate,85%</td>
			<td>Beijing Chemical Works</td>
			<td>1805</td>
			<td></td>
		</tr>
		<tr>
			<td>Sulfuric acid,98%</td>
			<td>Beijing Chemical Works</td>
			<td>10343</td>
			<td></td>
		</tr>
	</tbody>
</table>

graphene-assisted quasi-van der waals epitaxy of aln film on nano-patterned sapphire substrate for ultraviolet light emitting diodes

This protocol demonstrates a method for graphene-assisted quick growth and coalescence of AlN on nano-pattened sapphire substrate (NPSS). Graphene layers are directly grown on NPSS using catalyst-free atmospheric-pressure chemical vapor deposition (APCVD). By applying nitrogen reactive ion etching (RIE) plasma treatment, defects are introduced into the graphene film to enhance chemical reactivity. During metal-organic chemical vapor deposition (MOCVD) growth of AlN, this N-plasma treated graphene buffer enables AlN quick growth, and coalescence on NPSS is confirmed by cross-sectional scanning electron microscopy (SEM). The high quality of AlN on graphene-NPSS is then evaluated by X-ray rocking curves (XRCs) with narrow (0002) and (10-12) full width at half-maximum (FWHM) as 267.2 arcsec and 503.4 arcsec, respectively. Compared to bare NPSS, AlN growth on graphene-NPSS shows significant reduction of residual stress from 0.87 GPa to 0.25 Gpa, based on Raman measurements. Followed by AlGaN multiple quantum wells (MQWS) growth on graphene-NPSS, AlGaN-based deep ultraviolet light-emitting-diodes (DUV LEDs) are fabricated. The fabricated DUV-LEDs also demonstrate obvious, enhanced luminescence performance. This work provides a new solution for the growth of high quality AlN and fabrication of high performance DUV-LEDs using a shorter process and less costs.

Scanning electron microscopy (SEM) images, X-ray diffraction rocking curves (XRC), Raman spectra, transmission electron microscopy (TEM) images, and electroluminescence (EL) spectrum were collected for the epitaxial AlN film (Figure 1, Figure 2) and AlGaN-based DUV-LEDs (Figure 3). The SEM and TEM are used to determine the morphology of the AlN on graphene-NPSS. XRD and Raman are used to calculate the dislocation densities and the r...

Watch this Scientific Journal Video about Graphene-Assisted Quasi-van der Waals Epitaxy of AlN Film on Nano-Patterned Sapphire Substrate for Ultraviolet Light Emitting Diodes at JoVE.com

Graphene-Assisted Quasi-van der Waals Epitaxy of AlN Film on Nano-Patterned Sapphire Substrate for Ultraviolet Light Emitting Diodes

A protocol for graphene-assisted growth of high-quality AlN films on nano-patterned sapphire substrate is presented.

This protocol demonstrates a method for graphene-assisted quick growth and coalescence of AlN on nano-pattened sapphire substrate (NPSS). Graphene layers ...

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7.1K Views.  Chinese Academy of Sciences. A protocol for graphene-assisted growth of high-quality AlN films on nano-patterned sapphire substrate is presented.

Video: Graphene-Assisted Quasi-van der Waals Epitaxy of AlN Film on Nano-Patterned Sapphire Substrate for Ultraviolet Light Emitting Diodes

Polycrystalline Silicon Thin-film Solar cells with Plasmonic-enhanced Light-trapping

One of major approaches to cheaper solar cells is reducing the amount of semiconductor material used for their fabrication and making cells thinner. To compensate for lower light absorption such physically thin devices have to incorporate light-trapping which increases their optical thickness. Light scattering by textured surfaces is a common technique but it cannot be universally applied to all solar cell technologies. Some cells, for example those made of evaporated silicon, are planar as produced and they require an alternative light-trapping means suitable for planar devices. Metal nanoparticles formed on planar silicon cell surface and capable of light scattering due to surface plasmon resonance is an effective approach.
The paper presents a fabrication procedure of evaporated polycrystalline silicon solar cells with plasmonic light-trapping and demonstrates how the cell quantum efficiency improves due to presence of metal nanoparticles.
To fabricate the cells a film consisting of alternative boron and phosphorous doped silicon layers is deposited on glass substrate by electron beam evaporation. An Initially amorphous film is crystallised and electronic defects are mitigated by annealing and hydrogen passivation. Metal grid contacts are applied to the layers of opposite polarity to extract electricity generated by the cell. Typically, such a ~2 &mu;m thick cell has a short-circuit current density (Jsc) of 14-16 mA/cm2, which can be increased up to 17-18 mA/cm2 (~25% higher) after application of a simple diffuse back reflector made of a white paint.
To implement plasmonic light-trapping a silver nanoparticle array is formed on the metallised cell silicon surface. A precursor silver film is deposited on the cell by thermal evaporation and annealed at 23&deg;C to form silver nanoparticles. Nanoparticle size and coverage, which affect plasmonic light-scattering, can be tuned for enhanced cell performance by varying the precursor film thickness and its annealing conditions. An optimised nanoparticle array alone results in cell Jsc enhancement of about 28%, similar to the effect of the diffuse reflector. The photocurrent can be further increased by coating the nanoparticles by a low refractive index dielectric, like MgF2, and applying the diffused reflector. The complete plasmonic cell structure comprises the polycrystalline silicon film, a silver nanoparticle array, a layer of MgF2, and a diffuse reflector. The Jsc for such cell is 21-23 mA/cm2, up to 45% higher than Jsc of the original cell without light-trapping or ~25% higher than Jsc for the cell with the diffuse reflector only.
Introduction
Light-trapping in silicon solar cells is commonly achieved via light scattering at textured interfaces. Scattered light travels through a cell at oblique angles for a longer distance and when such angles exceed the critical angle at the cell interfaces the light is permanently trapped in the cell by total internal reflection (Animation 1: Light-trapping). Although this scheme works well for most solar cells, there are developing technologies where ultra-thin Si layers are produced planar (e.g. layer-transfer technologies and epitaxial c-Si layers) 1 and or when such layers are not compatible with textures substrates (e.g. evaporated silicon) 2. For such originally planar Si layer alternative light trapping approaches, such as diffuse white paint reflector 3, silicon plasma texturing 4 or high refractive index nanoparticle reflector 5 have been suggested.
Metal nanoparticles can effectively scatter incident light into a higher refractive index material, like silicon, due to the surface plasmon resonance effect 6. They also can be easily formed on the planar silicon cell surface thus offering a light-trapping approach alternative to texturing. For a nanoparticle located at the air-silicon interface the scattered light fraction coupled into silicon exceeds 95% and a large faction of that light is scattered at angles above critical providing nearly ideal light-trapping condition (Animation 2: Plasmons on NP). The resonance can be tuned to the wavelength region, which is most important for a particular cell material and design, by varying the nanoparticle average size, surface coverage and local dielectric environment 6,7. Theoretical design principles of plasmonic nanoparticle solar cells have been suggested 8. In practice, Ag nanoparticle array is an ideal light-trapping partner for poly-Si thin-film solar cells because most of these design principle are naturally met. The simplest way of forming nanoparticles by thermal annealing of a thin precursor Ag film results in a random array with a relatively wide size and shape distribution, which is particularly suitable for light-trapping because such an array has a wide resonance peak, covering the wavelength range of 700-900 nm, important for poly-Si solar cell performance. The nanoparticle array can only be located on the rear poly-Si cell surface thus avoiding destructive interference between incident and scattered light which occurs for front-located nanoparticles 9. Moreover, poly-Si thin-film cells do not requires a passivating layer and the flat base-shaped nanoparticles (that naturally result from thermal annealing of a metal film) can be directly placed on silicon further increases plasmonic scattering efficiency due to surface plasmon-polariton resonance 10.
The cell with the plasmonic nanoparticle array as described above can have a photocurrent about 28% higher than the original cell. However, the array still transmits a significant amount of light which escapes through the rear of the cell and does not contribute into the current. This loss can be mitigated by adding a rear reflector to allow catching transmitted light and re-directing it back to the cell. Providing sufficient distance between the reflector and the nanoparticles (a few hundred nanometers) the reflected light will then experience one more plasmonic scattering event while passing through the nanoparticle array on re-entering the cell and the reflector itself can be made diffuse - both effects further facilitating light scattering and hence light-trapping. Importantly, the Ag nanoparticles have to be encapsulated with an inert and low refractive index dielectric, like MgF2 or SiO2, from the rear reflector to avoid mechanical and chemical damage 7. Low refractive index for this cladding layer is required to maintain a high coupling fraction into silicon and larger scattering angles, which are ensured by the high optical contrast between the media on both sides of the nanoparticle, silicon and dielectric 6. The photocurrent of the plasmonic cell with the diffuse rear reflector can be up to 45% higher than the current of the original cell or up to 25% higher than the current of an equivalent cell with the diffuse reflector only.

Polycrystalline silicon thin-film solar cells on glass are fabricated by deposition of boron and phosphorous doped silicon layers followed by crystallisation, defect passivation and metallisation. Plasmonic light-trapping is introduced by forming Ag nanoparticles on the silicon cell surface capped with a diffused reflector resulting in ~45% photocurrent enhancement.

One of major approaches to cheaper solar cells is reducing the amount of semiconductor material used for their fabrication and making cells thinner. To ...

Fabrication of White Light-emitting Electrochemical Cells with Stable Emission from Exciplexes

The authors present an approach for fabricating stable white light emission from polymer light-emitting electrochemical cells (PLECs) having an active layer which consists of blue-fluorescent poly(9,9-di-n-dodecylfluorenyl-2,7-diyl) (PFD) and &#960;-conjugated triphenylamine molecules. This white light emission originates from exciplexes formed between PFD and amines in electronically excited states. A device containing PFD, 4,4&#39;,4&#39;&#39;-tris[2-naphthyl(phenyl)amino]triphenylamine (2-TNATA), Poly(ethylene oxide) and K2CF3SO3 showed white light emission with Commission internationale de l&#39;&#233;clairage (CIE) coordinates of (0.33, 0.43) and a Color Rendering Index (CRI) of Ra = 73 at an applied voltage of 3.5 V. Constant voltage measurements showed that the CIE coordinates of (0.27, 0.37), Ra of 67, and the emission color observed immediately after application of a voltage of 5 V were nearly unchanged and stable after 300 sec.

The authors present a method for fabricating stable white-light-emitting electrochemical cells utilizing emission from exciplexes formed between a blue-emitting fluorene polymer and aromatic amines.

The authors present an approach for fabricating stable white light emission from polymer light-emitting electrochemical cells (PLECs) having an active ...

Plasma-assisted Molecular Beam Epitaxy of N-polar InAlN-barrier High-electron-mobility Transistors

Plasma-assisted molecular beam epitaxy is well suited for the epitaxial growth of III-nitride thin films and heterostructures with smooth, abrupt interfaces required for high-quality high-electron-mobility transistors (HEMTs). A procedure is presented for the growth of N-polar InAlN HEMTs, including wafer preparation and growth of buffer layers, the InAlN barrier layer, AlN and GaN interlayers and the GaN channel. Critical issues at each step of the process are identified, such as avoiding Ga accumulation in the GaN buffer, the role of temperature on InAlN compositional homogeneity, and the use of Ga flux during the AlN interlayer and the interrupt prior to GaN channel growth. Compositionally homogeneous N-polar InAlN thin films are demonstrated with surface root-mean-squared roughness as low as 0.19 nm and InAlN-based HEMT structures are reported having mobility as high as 1,750 cm2/V&#8729;sec for devices with a sheet charge density of 1.7 x 1013 cm-2.

Molecular beam epitaxy is used to grow N-polar InAlN-barrier high-electron-mobility transistors (HEMTs). Control of the wafer preparation, layer growth conditions and epitaxial structure results in smooth, compositionally homogeneous InAlN layers and HEMTs with mobility as high as 1,750 cm2/V&#8729;sec.

Plasma-assisted molecular beam epitaxy is well suited for the epitaxial growth of III-nitride thin films and heterostructures with smooth, abrupt ...

Production and Characterization of Vacuum Deposited Organic Light Emitting Diodes

A method for producing simple and efficient thermally-activated delayed fluorescence organic light-emitting diodes (OLEDs) based on guest-host or exciplex donor-acceptor emitters is presented. With a step-by-step procedure, readers will be able to repeat and produce OLED devices based on simple organic emitters. A patterning procedure allowing the creation of personalized indium tin oxide (ITO) shape is shown. This is followed by the evaporation of all layers, encapsulation and characterization of each individual device. The end goal is to present a procedure that will give the opportunity to repeat the information presented in cited publication but also using different compounds and structures in order to prepare efficient OLEDs.

A protocol for the production of simple structured organic light-emitting diodes (OLEDs) is presented.

A method for producing simple and efficient thermally-activated delayed fluorescence organic light-emitting diodes (OLEDs) based on guest-host or exciplex ...

A Fabrication and Measurement Method for a Flexible Ferroelectric Element Based on Van Der Waals Heteroepitaxy

Flexible non-volatile memories have received much attention as they are applicable for portable smart electronic device in the future, relying on high-density data storage and low-power consumption capabilities. However, the high-quality oxide based nonvolatile memory on flexible substrates is often constrained by the material characteristics and the inevitable high-temperature fabrication process. In this paper, a protocol is proposed to directly grow an epitaxial yet flexible lead zirconium titanate memory element on muscovite mica. The versatile deposition technique and measurement method enable the fabrication of flexible yet single-crystalline non-volatile memory elements necessary for the next generation of smart devices.

In this paper, we present a protocol to directly grow an epitaxial yet flexible lead zirconium titanate memory element on muscovite mica.

Flexible non-volatile memories have received much attention as they are applicable for portable smart electronic device in the future, relying on ...

Enhanced Electron Injection and Exciton Confinement for Pure Blue Quantum-Dot Light-Emitting Diodes by Introducing Partially Oxidized Aluminum Cathode

Stable and efficient red (R), green (G), and blue (B) light sources based on solution-processed quantum dots (QDs) play important roles in next-generation displays and solid-state lighting technologies. The brightness and efficiency of blue QDs-based light-emitting diodes (LEDs) remain inferior to their red and green counterparts, due to the inherently unfavorable energy levels of different colors of light. To solve these problems, a device structure should be designed to balance the injection holes and electrons into the emissive QD layer. Herein, through a simple autoxidation strategy, pure blue QD-LEDs which are highly bright and efficient are demonstrated, with a structure of ITO/PEDOT:PSS/Poly-TPD/QDs/Al:Al2O3. The autoxidized Al:Al2O3 cathode can effectively balance the injected charges and enhance radiative recombination without introducing an additional electron transport layer (ETL). As a result, high color-saturated blue QD-LEDs are achieved with a maximum luminance over 13,000 cd m-2, and a maximum current efficiency of 1.15 cd A-1. The easily controlled autoxidation procedure paves the way for achieving high-performance blue QD-LEDs.

A protocol is presented for fabricating high-performance, pure blue ZnCdS/ZnS-based quantum dots light-emitting diodes by employing an autoxidized aluminum cathode.

Stable and efficient red (R), green (G), and blue (B) light sources based on solution-processed quantum dots (QDs) play important roles in next-generation ...

Bulk and Thin Film Synthesis of Compositionally Variant Entropy-stabilized Oxides

Here, we present a procedure for the synthesis of bulk and thin film multicomponent (Mg0.25(1-x)CoxNi0.25(1-x)Cu0.25(1-x)Zn0.25(1-x))O (Co variant) and (Mg0.25(1-x)Co0.25(1-x)Ni0.25(1-x)CuxZn0.25(1-x))O (Cu variant) entropy-stabilized oxides. Phase pure and chemically homogeneous (Mg0.25(1-x)CoxNi0.25(1-x)Cu0.25(1-x)Zn0.25(1-x))O (x = 0.20, 0.27, 0.33) and (Mg0.25(1-x)Co0.25(1-x)Ni0.25(1-x)CuxZn0.25(1-x))O (x = 0.11, 0.27) ceramic pellets are synthesized and used in the deposition of ultra-high quality, phase pure, single crystalline thin films of the target stoichiometry. A detailed methodology for the deposition of smooth, chemically homogeneous, entropy-stabilized oxide thin films by pulsed laser deposition on (001)-oriented MgO substrates is described. The phase and crystallinity of bulk and thin film materials are confirmed using X-ray diffraction. Composition and chemical homogeneity are confirmed by X-ray photoelectron spectroscopy and energy dispersive X-ray spectroscopy. The surface topography of thin films is measured with scanning probe microscopy. The synthesis of high quality, single crystalline, entropy-stabilized oxide thin films enables the study of interface, size, strain, and disorder effects on the properties in this new class of highly disordered oxide materials.

The synthesis of high quality bulk and thin film (Mg0.25(1-x)CoxNi0.25(1-x)Cu0.25(1-x)Zn0.25(1-x))O and (Mg0.25(1-x)Co0.25(1-x)Ni0.25(1-x)CuxZn0.25(1-x))O entropy-stabilized oxides is presented.

Here, we present a procedure for the synthesis of bulk and thin film multicomponent (Mg0.25(1-x)CoxNi0.25(1-x)Cu0.25(1-x)Zn0.25(1-x))O (Co variant) and ...

Film Control to Study Contributions of Waves to Droplet Impact Dynamics on Thin Flowing Liquid Films

Droplet impact is a very common phenomenon in nature and attracts attention due to its aesthetic fascination and wide-ranging applications. Previous studies on flowing liquid films have neglected the contributions of spatial structures of waves to the impact outcome, while this has recently been shown to have a significant influence on the drop impact dynamics. In this report, we outline a step-by-step procedure to investigate the effect of periodic inlet forcing of a flowing liquid film leading to the production of spatiotemporally regular wave structures on drop impact dynamics. A function generator in connection with a solenoid valve is used to excite these spatiotemporally regular wave structures on the film surface while the impact dynamics of uniform-sized droplets are captured using a high-speed camera. Three distinct regions are then studied; viz. the capillary wave region preceding the large wave peak, the flat film region, and the wave hump region. The effects of important dimensionless quantities such as film Reynolds, drop Weber and Ohnesorge numbers parameterized by the film flow rate, drop speed, and drop size are also examined. Our results show interesting, hitherto undiscovered dynamics brought about by this application of film inlet forcing of the flowing film for both low and high inertia drops.

A protocol to study the contributions of waves to droplet impact dynamics on flowing liquid films is presented.

Droplet impact is a very common phenomenon in nature and attracts attention due to its aesthetic fascination and wide-ranging applications. Previous ...

Synchrotron X-ray Microdiffraction and Fluorescence Imaging of Mineral and Rock Samples

In this report, we describe a detailed procedure for acquiring and processing&#160;x-ray microfluorescence (&#956;XRF), and Laue and powder microdiffraction two-dimensional (2D) maps at beamline 12.3.2 of the Advanced Light Source (ALS), Lawrence Berkeley National Laboratory. Measurements can be performed on any sample that is less than 10 cm x 10 cm x 5 cm, with a flat exposed surface. The experimental geometry is calibrated using standard materials (elemental standards for XRF, and crystalline samples such as Si, quartz, or Al2O3 for diffraction). Samples are aligned to the focal point of the x-ray microbeam, and raster scans are performed, where each pixel of a map corresponds to one measurement, e.g., one XRF spectrum or one diffraction pattern. The data are then processed using the in-house developed software XMAS, which outputs text files, where each row corresponds to a pixel position. Representative data from moissanite and an olive snail shell are presented to demonstrate data quality, collection, and analysis strategies.

We describe a beamline setup meant to carry out rapid two-dimensional x-ray fluorescence and x-ray microdiffraction mapping of single crystal or powder samples using either Laue (polychromatic radiation) or powder (monochromatic radiation) diffraction. The resulting maps give information about strain, orientation, phase distribution, and plastic deformation.

In this report, we describe a detailed procedure for acquiring and processing&#160;x-ray microfluorescence (&#956;XRF), and Laue and powder ...

Fabrication of Schottky Diodes on Zn-polar BeMgZnO/ZnO Heterostructure Grown by Plasma-assisted Molecular Beam Epitaxy

Heterostructure field effect transistors (HFETs) utilizing a two dimensional electron gas (2DEG) channel have a great potential for high speed device applications. Zinc oxide (ZnO), a semiconductor with a wide bandgap (3.4 eV) and high electron saturation velocity has gained a great deal of attention as an attractive material for high speed devices. Efficient gate modulation, however, requires high-quality Schottky contacts on the barrier layer. In this article, we present our Schottky diode fabrication procedure on Zn-polar BeMgZnO/ZnO heterostructure with high density 2DEG which is achieved through strain modulation and incorporation of a few percent Be into the MgZnO-based barrier during growth by molecular beam epitaxy (MBE). To achieve high crystalline quality, nearly lattice-matched high-resistivity GaN templates grown by metal-organic chemical vapor deposition (MOCVD) are used as the substrate for the subsequent MBE growth of the oxide layers. To obtain the requisite Zn-polarity, careful surface treatment of GaN templates and control over the VI/II ratio during the growth of low temperature ZnO nucleation layer are utilized. Ti/Au electrodes serve as Ohmic contacts, and Ag electrodes deposited on the O2 plasma pretreated BeMgZnO surface are used for Schottky contacts.

Attainment of high-quality Schottky contacts is imperative for achieving efficient gate modulation in heterostructure field effect transistors (HFETs). We present the fabrication methodology and characteristics of Schottky diodes on Zn-polar BeMgZnO/ZnO heterostructures with high-density two dimensional electron gas (2DEG), grown by plasma-assisted molecular beam epitaxy on GaN templates.

Heterostructure field effect transistors (HFETs) utilizing a two dimensional electron gas (2DEG) channel have a great potential for high speed device ...