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

Summary

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

Abstract

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.

Introduction

AlN and AlGaN are the most essential materials in DUV-LEDs^1,2, 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 accumulation^5,6, 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 templates^9,10,11. 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 μm) 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 growth^14,15. 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 µm, 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.

Protocol

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)

1. Deposition of SiO₂ film
   1. Wash the 2" c-plane flat sapphire substrate with ethanol followed by deionized water three times.
   2. Dry the substrate with a nitrogen gun.
   3. Deposit 200 nm SiO₂ film on the flat sapphire substrate by plasma-enhanced chemical vapor deposition (PECVD) under 300 °C. The deposition rate is 100 nm/min.
2. Spinning nanoimprint resist
   1. Wash the sapphire substrate with ethanol followed by deionized water 3x.
   2. Dry the substrate with a nitrogen gun.
   3. Spin a 200 nm nanoimprint resist (NIR) TU-2 on the flat sapphire substrate at 3000 r/min for 60 s.
3. Thermoplastic imprinting
   1. Place a patterned mold onto the nanoimprint resist polymer film.
   2. Apply high pressure as 30 bar at 60 °C to heat the sapphire substrate to above the glass transition temperature of the polymer.
   3. Expose to ultraviolet irradiation for 60 s and maintain for 120 s after turning off the UV source to solidify the NPR TU-2.
   4. Cool down the sapphire substrate and mold to room temperature (RT).
   5. Release the mold.
4. Pattern transfer
   1. 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.
   2. 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 μm.
      NOTE: NIL is not the only way to get NPSS. The NPSS are commercialized and could be bought elsewhere.

2. APCVD growth of graphene on NPSS

1. Rinse the NPSS with acetone, ethanol, and deionized water 3x.
2. Dry the NPSS with a nitrogen gun.
3. Load the NPSS into a three-zone high temperature furnace for long, flat temperature zone. Heat the furnace to 1050 °C and stabilize for 10 min under 500 sccm Ar and 300 sccm H₂
4. 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.

3. N₂-plasma treatment

1. Rinse the graphene-NPSS with deionized water.
2. Dry the NPSS with a nitrogen gun.
3. 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.

4. MOCVD growth of AlN on graphene-NPSS

1. Edit the MOCVD recipe for AlN growth and load the graphene-NPSS and its NPSS counterpart into the homemade MOCVD chamber.
2. After heating for 12 min, the temperature is stabilized at 1200 °C. Introduce 7000 sccm H₂ as ambient, 70 sccm trimethylaluminum (TMAl), and 500 sccm NH₃ for the growth of AlN for 2 h.

5. MOCVD growth of AlGaN MQWs

1. Lower the temperature of MOCVD chamber to 1130 °C to grow 20-period AlN (2 nm)/Al_0.6Ga_0.4N (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.
2. Lower the temperature of MOCVD chamber to 1002 °C and introduce a silicane flow for the growth of a 1.8 µm n-Al_0.55Ga_0.45N layer. The ambient gas is H₂ and concentration of n-type AlGaN is 5 x 10¹⁸ cm^-3.
3. Grow 5-period Al_0.6Ga_0.4N (3 nm)/Al_0.5Ga_0.5N (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 °C. The ambient gas is H₂.
4. Deposit 50 nm Mg-doped p- Al_0.65Ga_0.35N electron blocking layer (EBL) at 1002 °C. The mole flow rates of TMAl, TMGa, and NH₃ are 40 sccm, 6 sccm, and 2500 sccm. The ambient gas is H₂.
5. Deposit 30 nm p-Al_0.5Ga_0.5N cladding layer with NH₃ flow of 2500 sccm. The ambient gas is H₂.
6. 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.
7. Lower the temperature of MOCVD chamber to 800 °C and anneal the p-type layers with N₂ for 20 min. The ambient gas is N₂.

6. Fabrication of AlGaN-based DUV-LEDs

1. 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.
2. 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.
3. Put the sample into acetone at 80 °C for 15 min followed by washing the sample with ethanol and deionized water 3x.
4. Spinning negative photoresist NR9 and lithography. The UV exposure time, developing time, and rinsing time are 12 s, 20 s, and 2 min, respectively.
5. Wash the sample with acetone, ethanol, and deionized water 3x.
6. Deposit Ti/Al/Ti/Au by electron beam (EB) evaporation.
7. Spin negative photoresist NR9 and lithography. The UV exposure time, developing time, and rinsing time are 12 s, 20 s, and 2 min, respectively.
8. Wash the sample with acetone, ethanol, and deionized water 3x without ultrasonication.
9. Deposit Ni/Au by EB evaporation.
10. Wash the sample with ethanol and deionized water 3x to clean the sample.
11. Deposit 300 nm SiO₂ by plasma enhanced chemical vapor deposition (PECVD). The deposition temperature is 300 °C and deposition rate is 100 nm/min.
12. Spin photoresist 304 and lithography. The UV exposure time, developing time, and rinsing time are 8 s, 1 min, and 2 min, respectively.
13. Immerse the wafers into 23% HF solution for 15 s.
14. Wash the sample with ethanol and deionized water 3x and dry with a nitrogen gun.
15. Deposit Al/Ti/Au by EB evaporation after photolithography. The photolithography process is the same as that performed in steps 6.4-6.7.
16. Wash the sample with ethanol and deionized water 3x.
17. Grind and polish the sapphire to 130 µm by mechanical polishing.
18. Wash the sample with dewaxing solution and deionized water.
19. 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.

Results

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...

Discussion

As shown in Figure 1A, the NPSS prepared by the NIL technique illustrates the nano-concave cone patterns with 400 nm depth, 1 μm 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...

Materials

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