1. Introduction

Flexible electronic and optoelectronic devices have recently attracted substantial attention in wearable electronics, lighting, solar cells, sensors, and biomedical devices [1–3]. Inorganic LEDs (ILEDs) based on GaAs or GaN materials have been intensively studied in application in these areas as a replacement for organic LEDs (OLEDs), due to their advantages in luminance, external quantum efficiency (EQE) and long term stability [4–6]. Although flexi-ILEDs have been successfully demonstrated by multiple bonding and related transfer processes, a high quality flexi-ILED is still difficult to achieve due to the rigidity and brittleness properties of III nitrides [7–9].

To enhance the mechanical stability and increase the flexibility of GaN based LEDs, 3D micro structures, such as micro disks and micro pyramids, have been proposed to replace the traditional thin film structures [10–13]. In most cases, a sacrificial layer or a special insert layer is employed to realize the transfer of the GaN arrays from sapphire substrates onto flexible substrates by chemical etching or mechanical releasing [11, 13]. In this case, the GaN arrays were not directly grown on the sapphire substrate, which increased the defect density to a relatively high level (10¹⁰ cm ⁻²) compared with planar thin film LEDs (10⁷ cm ⁻²) [14, 15]. The selection of the electrode materials, such as In/Au solder, Ag paste and indium tin oxide (ITO) in previous reports, also limited the flexibility of the devices, due to the fragility of the electrode materials [11, 13, 16].

In order to improve the material quality and also increase the flexibility of the device, we tried to fabricate a few millimeter sized super flexi-ILED device based on a pyramid structure grown directly on 3 dimensional (3D) micro patterned sapphire substrates. This pyramid structure was estimated to have an improvement of internal quantum efficiency (IQE) by a factor of 3 compared with planar LEDs due to the reduction of the quantum confined stark effect (QCSE) and the dislocation density, according to our previous studies [17]. Pyramid arrays were transferred face up onto flexible substrates by LLO and dual transfer processes. The technology of print transfer was employed in fabrication of flexi-ILEDs several years ago, whereby an elastomer stamp, typically made of polydimethylsiloxane (PDMS), served as a carrier to transfer arrays of devices from their native substrate onto non-native destination substrates [18]. However, the uneven surface of the pyramid structures increased the fabrication difficulty. Thus, we used semi-solidified PDMS as a temporary holder to increase the adhesive stress between the pyramid structures and the PDMS holder. Dual transfer processes also ensure the front surface of the pyramid is the light emitting surface, which is more efficient than the back side emission [11] in previous studies. Electrical connection was achieved using Ag grid and Ag nanowire, for good current spreading, and high flexibility as well as favorable transmittance. Both theoretical simulation and experiment results proved the feasibility of producing a super flexible device with our method.

2. Experiment

3D patterned substrates with a pattern of 10μm circle windows in 30μm pitch were fabricated by the laser drilling method on SiO₂/sapphire substrate [17]. A low temperature 30nm thick GaN buffer layer was first grown at 580°C on a patterned sapphire substrate by metal organic vapor phase epitaxy (MOVPE). Then the temperature was raised to 1080°C to grow a 4μm low doped (2 × 10¹⁸cm⁻³) n-GaN. A highly doped (2 × 10¹⁹cm⁻³) 5μm n-GaN was grown afterwards, followed by 10 periods of InGaN/GaN MQWs, an un-doped AlGaN electron blocking layer (EBL) (100nm) and an Mg-doped p-GaN layer (200nm). Uniform pyramid micro LEDs with a bottom side length of 15μm were thus obtained on this patterned substrate.

An 180nm thick ITO layer was deposited onto the p-GaN surface of pyramid arrays used as a transparent current spreading layer. The sheet resistance of this ITO layer after annealing at 500 °C for 5min was tested as 7.4 Ω/□ and the transmittance was 88% at a wavelength range of 450nm to 550nm. PDMS was then coated on top of the pyramid array by spin coating in order to fill the gap between the pyramids. Transfer processes (Fig. 1(a-d)) were conducted after plasma cleaning of the PDMS residues on the P-GaN side for 2min with CF₄/O₂ (3:1).

 

Fig. 1 (a-d) Dual transfer processes: (a) bond the pyramid arrays with temporary holder, (b) remove the sapphire substrate by LLO, (c) paste the pyramid arrays onto a flexible substrate and (d) release the temporary holder. (e) Structure of flexible device in disassembly.

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First, a transfer process was implemented to transfer the pyramid arrays from the sapphire substrate onto a temporary holder. Semi-solidified PDMS was used as the temporary holder and bonded to the pyramid array by baking at 90°C for 10min (Fig. 1(a)). After that, LLO was employed to remove the original sapphire substrates (Fig. 1(b)). In the second transfer process, carbon conductive tape, which had been used in flexible device fabrication in a previous study [19], was used as the adhesive layer. Any kind of flexible substrate, such as polymer, thin metal layer or even fabric, can be used as the substrate, since the follow-up procedure is undertaken at a relatively low temperature. PET was chosen as the flexible substrate for demonstration in this paper. The temporary holder was mechanically peeled off after the second transfer process (Fig. 1(d)).

Irreversible damage was avoided during LLO and dual transfer processes, due to the increased mechanical stability of the pyramid structure. The GaN pyramid arrays remain intact without any cracks after LLO, as Fig. 2(a) shows. Figure 2(b) indicates that the pyramid array still keeps completely free of damage after it was transferred onto the PET substrates. Even though cracks appear in the thin film between GaN pyramids, the device performance is not influenced by this, since the separated pyramid was charged independently. A complete and regular pyramid array can be transferred onto flexible substrates, using a semi-solidified PDMS as the temporary holder. The results imply that the bonding stress between the PDMS holder and the pyramid array is large enough to prevent the damage caused by LLO and the stress is small enough for the mechanical exfoliation of the pyramid arrays in the second transfer process.

 

Fig. 2 SEM images of (a) backside of pyramid arrays after LLO, (b) topside of pyramid arrays without PDMS filler after transferring it onto flexible substrate, (c) topside of the device after the deposition of the Ag grid and (d) annealed Ag nanowire network spin coated on the pyramid arrays.

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The last step is the transparent electrode fabrication. 20μm to 30μm length Ag nanowires with a mass fraction of 0.1% were spin coated onto the pyramid arrays followed by the deposition of a 250nm thick Ag grid. The transmittance of the Ag nanowires was 83% and the sheet resistance was 24.5Ω/□. The Ag grids were obtained by photolithography, metal sputtering and lift off processes, with a grid width of 120μm and grid length of 1000μm, as shown in Fig. 2(c). In addition, thermal annealing at 180°C for 20min enables an enhancement of the electrical conductivity by melting and fusion of the Ag nanowires [20], forming a connected Ag network as shown in Fig. 2(d). The Ag grid facilitates the long range current spreading, while Ag nanowire ensures a good transparent and mechanical flexibility.

The final flexible devices consist of pyramid arrays embedded in soft polymer, with a transparent electrode on top of the pyramid, a flexible substrate holding pyramid arrays and a backside electrode connecting the pyramid arrays and substrate, as illustrated by Fig. 1(e).

3. Results and discussion

5 × 5 mm² green flexible devices were obtained based on pyramid arrays using the fabrication processes described above. The devices were folded through different radius of curvature (R), namely 8mm, 4mm, 1mm and even 0.5mm. Figure 3 (a) and (b) show the light emission that was obtained from 5 × 5 mm² green flexible LEDs at R = 1mm and R = 0.5mm. Due to the super flexibility of our structure, the device can be mechanically peeled off from the PET substrate without damage and can be pasted on anywhere. Moreover, it still can be cut into a specific shape after the fabrication of the device. A 7 × 5 mm² blue flexible LED was cut into T-shape using scissors, as Fig. 3(c) demonstrates. Mechanical damage occurred during the shaping process, causing a non-uniform light emission. This can be further improved by replacing the use of scissors with a more precise cutting method, laser dicing for example, or by processing the cutting procedure before the fabrication of transparent electrode.

 

Fig. 3 Optical image of the green flexi-ILEDs at radius of curvature of (a) 1mm, (b) 0.5mm and (c) shaping a 7 × 5 mm² blue flexi-ILED with a pair of scissors. The inset of (b) is the SEM image of the device with a radius of curvature of 0.5mm and the inset of (c) is the lighting image of the T shape blue LED.

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Figure 4(a) shows the room temperature electroluminescence (EL) spectra of the GaN pyramid LEDs at various currents in the range of 10mA to 100mA. At the current of 20mA, the dominant EL peak was observed at 532.5 nm. However, as the current was increased to 80mA, the EL peak blue-shifted toward 521 nm. The observed blue shift in EL peak position values presumably resulted from non-uniform indium compositions and thicknesses of the QW layers formed on the multifaceted n-GaN micro structures [13, 21, 22]. Additionally, a slightly red shift appeared for 100mA due to the increased junction temperature. Electroluminescence (EL) spectra for green LEDs was also evaluated under each bending condition accompanied by the photoluminescence (PL) spectra of the pyramid arrays as a reference, as Fig. 4(b) shows. A slight red-shift of the wavelength peaks appears when comparing the EL with PL. The fluctuation of the EL peak wavelength is negligibly small when the bending radius is larger than 1mm and the red shift appears when continually reducing the bending radius. The red shift of the emission peak results from an increase in junction temperature during electrical injection or under extreme bending conditions.

 

Fig. 4 (a) Room temperature EL spectra at various applied currents and (b) Room temperature EL spectra under various bending conditions accompanied by PL spectra of the pyramid arrays.

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Typical I-V characteristics of the pyramid LEDs in various bending conditions were also tested, as Fig. 5 illustrates. The threshold voltage of our structure is larger than commercialized non-flexible LEDs, due to the relatively small ohmic contact area. The high threshold voltage effect has also appeared in previous studies of flexible devices based on pyramid or micro disk structures [10–13]. The I-V curve remains constant at a radius of curvature of 8mm and 4mm. The current shows a slight decrease at R = 1mm. However, a drastic decrease of the forward current is observed at a radius of curvature of 0.5mm. The series resistance (R_S) of the device, which is calculated by dV/dI, is 47Ω before bending. The R_S doesn’t show a noticeable increase until the bending radius of curvature reaches 0.5 mm, which results in a R_S of 80Ω.

 

Fig. 5 I-V curves of the pyramid LEDs under various bending conditions.

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In order to estimate the reason for the performance reduction, three-dimensional finite element simulation was used to calculate the stress and strain distribution in the pyramid arrays and metal electrode during the bending process. Either the cracks appearing in the GaN material or the fracture of the metal electrode can result in the device break down.

Inner stress and strain distribution for the pyramid structure as well as a reference thin film structure under the same extreme condition of bending were calculated first. In order to simplify the simulation, only the pyramid arrays and the gap filler were considered, since other parts would not have a strong influence on the stress distribution. The pyramid model contained a 5 × 3 pyramid array embedded in PDMS. The length, pitch size and inclination angle of the pyramid were set as 15μm, 30μm and 61°, to be consistent with the experimental results. As a comparison, the reference model contained a 6μm thick, 120μm length and 90μm width GaN film structure. The thickness of the thin film structure obtained under the same growth condition of the pyramid structure is 6um and this value is also the commercialized choice of the LED structure. The right side short edge was fixed during the simulation, and a displacement load was applied on the other side. The load value was calculated by the bending degree.

The simulation results of the stress and strain distribution in Fig. 6 are all calculated under a curvature radius of 0.5mm. Figure 6(a) is the simulation results of the stress distribution for the pyramid model, in which the PDMS part is hidden. Figure 6(b) depicts the strain distribution of the centerline section plane. For comparison, the stress distribution results of reference under the same curvature radius are shown in Fig. 6(c). These simulation results indicate that the maximum tensile stress of the pyramid structure under a 0.5mm curvature is only 4.18e⁷Pa, far less than the theoretical ultimate tensile strength of GaN (0.4GaP [23]). In contrast, the maximum tensile stress of the thin film structure is 2.02e¹⁰ Pa, which is far beyond its ultimate tensile strength. Actually, the maximum stress for the pyramid structure is exhibited at the connection point of two pyramids, which can be significantly reduced by separating the connected pyramids. The reason for using a close packed structure is to reach a maximum lighting area. The mechanical stability of a GaN pyramid not only benefits from the structure stability of the pyramid itself, but also takes advantage of a highly elastic material, namely PDMS. The tensile strain was concentrated in this elastic gap filler due to the large differences in the Young’s modulus value. This effect has also been verified by Miyoung Kim, et al [11].

 

Fig. 6 Simulation results of (a) stress distribution of pyramid array, (b) strain distribution of pyramid array, (c) stress distribution of reference thin film, (d) stress distribution of Ag grid above PDMS filler and (e) SEM image of the device after bending.

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The simulation results indicate that the pyramid structure can bear an extremely large bending without cracks. Thus, the reason for performance degradation under the bending condition of R = 0.5mm is not the appearance of cracks in the GaN material but the cracks at the junction of the Ag grid, as Fig. 6(e) illustrates. These cracks cause a block in the current spreading and a reduction of the effective lighting area. Cracks prefer to appear at the junction of Ag grid due to the stress concentration effect at the corner. The simulation result also proves this stress concentration effect, as Fig. 6(d) depicts. At a curvature of 0.5mm, the maximum tensile stress of Ag grid was beyond the theoretical ultimate tensile stress of Ag (1.4e⁸Pa [24]) at the corner of the cross. The crack extends from the corner to the center, resulting in the destruction of the Ag grid and eventually the performance reduction of the device. The electrode break down can be reduced by replacing Ag grid with strip electrode.

4. Conclusion

We have successfully obtained flexible LED devices based on high quality pyramid arrays grown directly on sapphire substrate, which are suitable for super flexibility device fabrication due to the high mechanical stability of the pyramid structure. The elastic gap filler is also beneficial to improve the flexibility of the device by bearing most of the strain during bending processes. 5 × 5 mm² green pyramid LEDs (7 × 5 mm² blue LEDs) were fabricated and folded under multiple bending conditions. No significant degradation of the electrical properties appeared until a curvature radius of 0.5 mm was reached. The reason for the performance reduction is the appearance of cracks at the junction of the Ag grid, which can be improved by replacing the Ag grid with a strip electrode. This method is applicable for LEDs in any shape, size and color and even has no limitation in the choice of substrates. Substrate replacement and size shaping can be performed even after the accomplishment of the device fabrication. This technology offers intriguing possibilities for the development of large scale portable displays and other optoelectronic devices. The super deformability also breads the unlimited potential of the device applied in smart lighting.