Flexible Display..III-Nitride Material Use Possible???

Background

We are very familiar with the concept of miniature display through our smartphones, smartwatches, tablets, e-readers etc. These displays have undergone a steady evolution through the years starting from Liquid Crystal Display(LCD) to LED Backlight to Active Matrix Organic LED(AMOLED) Display.

The next big development in this area is going to be – Flexible Display.

Fig 1. Flexible Displays[3]

Flexible Display is amenable to being folded/bent as opposed to traditional displays. This is possible due to the organic materials used in making these displays.

In spite of the being amenable by nature, organic materials have lower reliability and cannot be processed at higher temperatures[5][6][7].

This lacuna can only be overcome if we could

• Increase the reliability of organic materials (work in progress) or
• Use of inorganic materials to make Flexible Display

The possible use of inorganic materials warrants a hybrid heterostructure grown on two-dimensional (2D) layered materials like Graphene or hexagonal Boron Nitride(hBN)[2][18][19][20].

Accordingly, the article talks about the research carried out by two different groups using graphene as the template for GaN growth

In 2014, a research group at Seoul National Universiy(SNU) grew Gallium nitride(GaN) on graphene using MOCVD paving the way for its adoption in Flexible Displays[1][8].

Similarly, in 2017, a Japanese research group grew crystalline GaN films using graphene buffer on an amorphous substrate(glass) with the use of Pulsed Sputtering Deposition (PSD)[2].

Material Charateristics 

Graphene is a monolayer of carbon atoms arranged in a chicken wire or hexagonal pattern[1][9].

Fig 2. Graphene Monolayer[10]

Graphite(3D) is formed by stacking graphene(2D) which are bonded by weak Van der Waal’s force. A monolayer of graphene was isolated from Graphite in 2004 at University of Manchester by mechanical exfoliation.

Fig 3. Different forms of Carbon [9][10]

Fig 4. Typical GaN LED structure [11]

III-Nitride Semiconductors like GaN and InN are used for making Light Emitting Diodes(LEDs) (along with quantum structures like quantum wells, quantum dots etc.) for covering the entire Visible, Infrared(IR) and UltraViolet(UV) spectrum.

Research is also ongoing to make high power, deep Ultra-Violet(DUV) LEDs using AlGaN or AlN which have applications in disinfection, curing, gas sensing and sterilization[21][22][24].

Due to unavailability of lattice-matched substrates, Nitrides are grown on non-native materials like Sapphire or Silicon Carbide(SiC)[12]. This leads to mismatch and generation of faults like stacking fault, threading dislocation(TDD) etc. at the boundary of GaN/substrate.

Fig 5. Atomic arrangement in growth plane for (0001) GaN layer, grown on sapphire (0001) a mismatch of lattice parameters equal to −0.33 (a) and 0.14 (b). [13][14]

When devices are fabricated on these materials, the faults act as trap sites(for the charge carriers) thereby reducing the output and reliability of the LED[15].

Hence the best option is to grow GaN on materials which are lattice matched to it or have a low mismatch to it. This helps in adopting GaN for applications like flexible displays and devices beyond two terminals like HEMT etc.

III-Nitride for Flexible Display??

Organic materials used in fabricating OLED’s are a choice for making flexible displays. III-Nitride materials  owing to its superior light output, ease of fabrication and reliability can be adopted to make flexible displays if they meet the following criteria:

• Low-temperature growth
• Compatible with low-cost substrates like plastic, glass etc.

Normally, III-Nitride semiconductors like GaN, AlGaN, InGaN etc. are grown using Metal Organic Chemical Vapor Deposition(MOCVD) or Molecular Beam Epitaxy(MBE). Owing to this, the temperature required for a high-quality epitaxial film is in excess of 850^oC [1][2][8].

Also, the substrate(sapphire/SiC) currently utilized for III-Nitride semiconductor growth are crystalline and hence resist any change in shape.

Hence, the substrate of choice should have low lattice mismatch, similar structure and be flexible(bendable) for III-Nitride semiconductor growth.

Considering this, one well-researched material meets the criteria – Graphene

 

Graphene for III-Nitride Growth

As discussed earlier, we are now going to delve into the research work by two groups on their efforts to grow GaN on graphene. The device outcome from the films is similar to GaN on Sapphire or GaN on Silicon Carbide(SiC).

A] Research at Seoul National Universiy(SNU)[2014]

A research group at Seoul National Universiy(SNU) grew Gallium nitride(GaN) on graphene using MOCVD[1][8]. Post-growth, they incorporated a transfer technique which assisted to measure GaN LEDs on an organic(polyimide) substrate.

• Graphene Growth

Graphene films are grown on Cu foil by means of Chemical Vapor Deposition(CVD). The grown films are Multi-Layer Graphene(MLG) which is semi-transparent in nature[8].

• Transfer & Growth

This MLG graphene film is then transferred to a SiO2/Si substrate to act as a template for III-Nitride growth.

To grow the GaN micro-rods, MOCVD reactants were used along with nitrogen as the carrier gas. A 2-μm-thick GaN buffer layer was grown to improve the vertical alignment of the micro-rods.

 Fig 6. The growth of GaN micro-rod post transfer of graphene on SiO2/Si substrate[8]

The micro-rods were not grown selectively. The geometry of the GaN micro-rods depends on the growth time[8].

• Material Characterization

XRD

GaN micro-rods grown on graphene with/without GaN buffer layer are examined using an XRD θ–2θ scan[8]. The range of scan was 20^o-80^o.

GaN with a buffer layer – Two Wurtzite Peaks

GaN with no buffer layer – Multiple Peaks

 Fig 7. GaN Micro-rod without buffer shows multiple peaks v/s two peaks for with buffer in XRD scan[8]

SEM

The FE-SEM image reveals the surface morphology of the GaN buffer layer and GaN micro-rods which is as expected[8].

 Fig 8. FE-SEM of GaN Micro-rod grown with buffer[8]

TEM

TEM images of the sidewall Multi-Quantum Well(MQW)(shown below) of the GaN micro-rod LEDs indicate growth comparable to baseline substrates like Sapphire/SiC[8].

Fig 9. TEM of the GaN Micro-rod sidewall Multi-Quantum Well(MQW) of the GaN micro-rod [8]

• Electrical Characterization

As the main objective of GaN growth on graphene is for flexible display application, let us try to understand the electrical/optical output. Before doing the measurements, devices need to be fabricated on the grown films

Device Fabrication[8]

1. Post GaN micro-rod LED growth, the cavity between LEDs is filled using an insulating layer – polyimide
2. Top layer of  GaN micro-rod LEDs is opened using oxygen plasma etching 
3. Ni/Au used to make ohmic contact to p-GaN
4. SiO2/Si substrate removed using wet(BOE) etching
5. A Ti/Au is deposited and Ag is added at the bottom of LEDs to provide a reliable end for current injection
6. Coaxial GaN micro-rod LEDs were then transferred onto a polyimide substrate

 Fig 10. Device Fabrication process for GaN Micro-LED[8]

The emitted light from the fabricated LED’s despite the transfer process demonstrated reliable operation.

 Fig 11. Light Emission from a device fabricated on GaN Micro-LED[8]

This operation continued even when the polyimide substrate was bent to change the radius of curvature. A radius of curvature of 6 mm for a 20-mm wide substrate under deformation(bent) does not cause the EL intensity & EL peak wavelength to change over 1000 cycles[8].

 Fig 12.  EL spectra for GaN Micro-LED at bending radii of ∞, 6, and 4 mm[8]

The methodology adopted by SNU researchers has a potential to integrate inorganic substrates with organic materials for future flexible displays. The requirement for a low thermal budget is accomplished by the introduction of a transfer technique.

 

B] Research at University of Tokyo[2017]

A different approach was used by a research group at the University of Tokyo. Pulsed Sputtering Deposition(PSD) is used for III-Nitride film growth on glass using graphene as a buffer layer. In this technique, there is no transfer involved post growth of GaN film[2].

• Graphene Growth

Graphene is grown on Ni sheets by Chemical Vapor Deposition(CVD) and subsequently transferred onto thermally oxidized SiO2 on Si substrates.

• GaN Growth

1. Flexible substrates like glass, plastic are amorphous in nature. For GaN growth on a glass substrate, a crystalline buffer layer should be introduced between GaN and glass
2. PSD used for deposition enhances surface migration of the precursors thereby reducing the epitaxial growth temperature. The maximum temperature needed by PSD is 480^oC in contrast to MOCVD for the growth of GaN[2]

• Material Characterization

SEM

The films grown by PSD were characterized using scanning electron microscopy (SEM)[2]. For comparative study, two set of films were grown

GaN film grown on SiO2/Si – Rough Surface, small grains & size of several 100 nm  

GaN film grown on Graphene Buffer/SiO2/Si – Smooth Surface 

 Fig 13. SEM image of GaN film grown on an amorphous SiO2 substrate with & without graphene  buffer[2]

XRD

The GaN film using a graphene buffer layer were scanned using XRD 2θ/ω plot. Diffraction peaks around 26.5^o & 34.5^o originate from graphene {0002} and GaN {0002}[2].

This shows that the crystal quality if the GaN film is improved by introducing the graphene buffer layer. 

 Fig 14. XRD of GaN film grown on graphene buffer/SiO2[2]

The GaN film which is grown using PSD consists of wurtzite and zincblende phase when grown on graphene at 750^oC. Wurtzite Phase versus growth temperature is plotted for the GaN films[2].

Aluminum Nitride(AlN) interlayer/reaction blocking layer before GaN growth on graphene to ward off an interfacial reaction between GaN and graphene.

 Fig 15. The proportion of phase in GaN v/s growth temperature[2]

• Electrical Characterization

Material characterization for GaN film growth on graphene buffer is to be followed by electrical characterization. Before doing the measurements, devices need to be fabricated on the grown films

Device Fabrication[2]

1. AlN/graphene/amorphous SiO2 was the base structure 
2. n-type GaN layers were grown on the AlN interlayers
3. Multiple quantum wells (MQWs) of InGaN/GaN and Mg-doped p-type GaN layers were grown on top
4. Deposition of Pd/Au and In electrodes as ohmic contacts on the p- and n-type GaN surfaces

 

 Fig 16. LED on Glass[2]

Electroluminescence (EL) measurements of the fabricated devices measured with various injection currents (2.1-10.8 mA)[2]. Blue/red LED’s grown by varying the In proportion were also characterized.

 Fig 17. EL Spectra of green LED structure with graphene buffer layer[2]

Conclusion

Considering the process flow, characterization and reliability,  III-Nitride semiconductors are a material of choice for displays, lighting etc. Taking this point logically ahead, research groups and industry are trying to extend the use of III-Nitride in low thermal budget scenarios ie. III-Nitride on a low-cost substrate.

In this article, both the research groups have used graphene to grow GaN. LEDs made from GaN on graphene is comparable to that grown on Sapphire or SiC.

Innovative approaches can help to achieve the temperature requirement permitting the usage of low-cost large area substrates like glass. Pulsed Sputter Deposition(PSD) is an adapted sputter technique which is innovative and can be scaled up without any additional cost.

The next challenge will be to bring in polymeric substrates like plastic etc. These materials also support roll-to-roll processing & manufacturing like glass.

 

If you want to share your opinion kindly do so in the comments section or email me at u2d2tech@gmail.com.

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