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Flexible InGaN-based green light emitting diodes (LEDs) were fabricated by transferring epilayer to a flexible polyimide substrate with laser lift-off (LLO) and double-transfer technologies. We present a method of increasing light output power in flexible LEDs without modifying their epitaxial layers. These improvements are achieved by reducing the quantum-confined Stark effect by reducing piezoelectric polarization that results from compressive stress in the GaN epilayer. The compressive stress is relaxed due to the external stress induced by increasing bending displacement of flexible substrate. The light output power of the flexible LED at an injection current of 150 mA is increased by approximately 42.2%, as the external bending went to the case of effective length of 15 mm. The experimental results demonstrated that applying external tensile stress effectively compensates for the compressive strain and changes the piezoelectric field in the InGaN/GaN MQWs region, thereby increases the probability of radiative recombination.
© 2015 Optical Society of America
1. Introduction
High efficiency InGaN-based blue-green light emitting diodes (LEDs) have attracted considerable attention because of numerous applications, particularly in the areas of LCD screen backlighting and solid state lighting for general illumination. LEDs have replaced conventional fluorescent and incandescent lighting because of their high luminous efficiency, long operating life and energy-savings [1–3]. It is well known that flexible electronics have potential applications in portable electronic devices because they are easily accommodated in pockets or backpacks, are light-weight, easily carried, convenient to use and low cost. However, such high performance InGaN-based LEDs are typically epitaxially grown on sapphire, silicon and silicon carbide, resulting in a discrepancy between the substrate stiffness and the mechanical flexibility desired for the next generation optoelectronic devices. A viable technology for fabricating InGaN-based LEDs onto flexible substrates would enable a variety of applications from curved displays to biomedical, implantable or bio-integrated optoelectronic devices.
On the other hand, large biaxial compressive stress always existed in the InGaN/GaN MQWs of green LEDs grown on sapphire due to a large lattice mismatch between the substrate and InGaN/GaN layers [4]. The difference in thermal expansion coefficients between the GaN film and sapphire substrate facilitates damage when the sample is cooled from the growth temperature to room temperature [5]. This compressive stress will cause energy band gap distortion and charge separation in the quantum wells. The GaN energy band structure distortion strongly affects the LED device performance, such as recombination efficiency, emission peak wavelength, and threshold current [6]. Electrical fields are caused by spontaneous and piezoelectric polarization at the InGaN/GaN hetero-interface, leading to the quantum-confined Stark effect (QCSE), resulting in reduction in internal quantum efficiency (IQE) and efficiency droop [7]. These are the reasons green LED luminous efficiency decreases significantly. The MQWs of green InGaN-based LEDs with an indium composition of more than 20% can suffer from formation defects that lead to low radiative efficiency. Several specific nonrectangular QWs designs have recently been proposed to suppress the charge separation issue, including triangular QWs [8], staggered QWs [9], trapezoidal QWs [10], and dip-shape QWs [11]. These approaches used various methods to engineer the InGaN QWs in order to obtain structures with large optical matrix elements that lead to a significantly improved radiative recombination rate. However, all of these methods were accomplished by modifying the LED epitaxial layers. Another approach based on strain compensation in the InGaN active layer has also been proposed to enhance the IQE of LEDs [12,13]. Strain or stress state knowledge in epitaxial GaN is critical for better understanding the film properties and improving nitride-based devices. Although uni-axial external stress effect on green InGaN/GaN LEDs had been reported [14], the stress existing the epilayer did not be relaxed due to the epilayer with the sapphire substrate. If the sapphire substrate can be removed, the compressive stress of the LED epilayers will be relaxed and it is more useful to study the stress effect on device behavior.
The thin-film InGaN-based LED (TF-LED) was prepared using the laser lift-off (LLO) process and wafer bonding technique [15,16]. The TF-LED is one of several high potential light-emitting devices that promise higher power operation and high light extraction due to their excellent thermal dissipation and surface random roughening. However, the conventional TF-LED has been widely used on sapphire substrates to Si or metal substrates for their enhanced high current operating capabilities. Studies on flexible LEDs have received great attention due to many recent achievements. Several flexible LEDs fabrication approaches on flexible substrates such as a polyethylene terephthalate (PET) film, a flexible stainless steel substrate and a polyimide (PI) substrate [17–20]. In this study, in order to reduce the polarization fields and enhance light output power in flexible LED performance, an effective method is presented to relax the compressive strain in InGaN/GaN MQW LEDs by externally straining the LED epilayers. Thin film InGaN-based green LEDs were fabricated by combination of a LLO process to remove the sapphire substrate and using a simple transferring process to transfer LEDs epilayer onto a flexible polyimide substrate. The PI substrate has many advantages because it is biodegradable, low-cost, flexible and light-weight. The electrical and optical characteristics of InGaN-based green LEDs on a flexible polyimide substrate under external bending stress were analyzed after the LLO and transfer processes.
2. Experimental
Flexible InGaN-based green LED fabrication is schematically illustrated in Fig. 1. The InGaN/GaN MQW green LED epitaxial structures emitting at λ ∼530 nm were grown on c-plane sapphire by metalorganic chemical vapor deposition system. The epitaxial structure consists of a 30 nm-thick GaN buffer layer, a 3 μm-thick undoped GaN layer, a 2 μm-thick Si-doped n-GaN layer, a multi-quantum wells active region consisting of seven periods of 3 nm-thick In0.22Ga0.78N well layers and 10 nm-thick GaN barrier layers, a 20 nm-thick p-Al0.1Ga0.9N electron blocking layer, and a 0.2 μm-thick Mg-doped p-GaN layer. To obtain flexible LED devices, the LEDs were fabricated into a mesa size of 450 μm × 450 μm by conventional photolithography and etching processes. It is worthy to mention that the epilayer with LED structure was only mesa etched to n-type GaN layer and did not be isolated. An indium-tin-oxide film with 200-nm thickness was deposited onto the p-GaN as a transparent conductive layer by electron beam evaporator, and Cr/Au was deposited as n- and p-type electrodes, designated as the regular sapphire-based LED (R-LED), as shown in Fig. 1(a). Subsequently, the LED was mounted onto a temporary glass substrate by adhesive layer and the sapphire substrate was removed using LLO technique, as shown in Figs. 1(b)-1(c). To enhance the light-extraction of LEDs, a 4M NaOH solution was used to roughen the n-GaN surface of the LEDs at 60 °C for 4 min [Fig. 1(d)]. After, the top side (n-side) of the inverted LED was then bonded to a flexible PI substrate, as shown in Fig. 1(e). Finally, the temporary substrate was removed and the LEDs epilayer was transferred to the PI substrate. Figure 1(f) illustrates a cross-section of the completed flexible green LED structure on a polyimide substrate. The inset of Fig. 1(f) showed the LEDs being transferred to the PI substrate and presenting flat. Moreover, the flat PI substrate can also be bendable.
Fig. 1 Flex-LED process flowchart: (a) Sapphire-based LED device, (b) Bonding to temporary substrate, (c) LLO sapphire, (d) Removing u-GaN and roughening n-GaN layer, (e) Glue bond on PI/glass substrate, (f) Completed Flex-LED structure and photograph. The inset of (f) shows the LEDs epilayers being transferred to the PI substrate before and after bending.
In order to study the external stress effect on the LEDs performance, the PI substrate with TF-LEDs epilayer was further attached to the Cu metal with 200 um thickness. The effective bending length (Le) of the flexible substrate as an indicator for external bending stresses [18] and shown in Fig. 2. The flexible LED located at the center (or top) of the copper plate substrate was used to evaluate the stress effect on the LED characteristics. As the flexible LED without and with 10 and 15 mm Le, designated as the non-bending flexible LED (NF-LED), flexible LED-10 (F-LED-10) and flexible LED-15 (F-LED-15), respectively. The current-voltage (I-V) characteristics of four LEDs were measured at room temperature by using an Agilent 4155B semiconductor parameter analyzer. The light output power and electroluminescence (EL) spectra of four LEDs were measured using an integrating sphere detector (CAS 140B, Instrument Systems). The Raman spectrum was measured using a 488 nm argon-ion laser with a monochromator (Horiba Jobin-Yvon LabRam HR800 UV-vis μ-Raman).
3. Results and discussion
Raman spectra is shown in Fig. 3(a) to investigate the stress state of the four LED samples. The biaxial stress levels effect in GaN with the measured Raman shift (Δω) shows the following relationship: Δω = 6.2σGaN [21]. The stress state of the GaN epilayer can be obtained directly using this relationship from the shift in the Raman Stokes signal peak. It is conventionally known that the σGaN>0 (σGaN<0) factor is related to tensile (compressive) stress. With respect to the stress-free GaN E2-high phonon mode peak at 567.5 cm−1 used as the reference value to determine the stress change in the GaN epilayer, the peak R-LED, NF-LED, F-LED-15 and F-LED-10 positions were at 568.2, 567.6, 567.0, and 566.4 cm−1, respectively. It can be found that the Raman peak position of the transferred GaN epilayer (NF-LED) has a redshift to the short wavenumber, when compared with the R-LED sample. The redshift of the Raman peak position implies stress state compressive release in the GaN epilayer. This stress is caused by the difference in thermal expansion coefficients between the GaN film and the sapphire substrate. The stress is accommodated during cooling from the growth temperature. With the increasing bending displacement of flexible substrate samples, a larger redshift in the Raman peak position was observed. The results indicate that the curvature of the flexible LED subjected to a tensile strain in F-LED-15 and F-LED-10 samples.
Fig. 3 (a) Raman spectra of the R-LED, NF-LED, F-LED-15 and F-LED-10. (b) Schematic band structure of F-LED before and after applying external tensile stress. (c) Built-in stress as a function of the flexible LED bowed curvature.
Moreover, it was worthy to mention that there exists two peaks (566.4 cm−1 and 567.6 cm−1) in the F-LED-10 sample. This is an important characteristic of energy separation in the GaN epilayer that is induced by external stress [14,22]. It can be inferred that the larger curvature from external force caused the GaN epilayer to increase to another stress state. The F-LED-10 band diagram before and after applying external tensile stress is schematically shown in Fig. 3(b). Note that the Raman shift results from the transition of a hole from the light-hole band to the heavy-hole band, the observed low-frequency mode (566.4 cm−1) was induced by the biaxial strain splitting energy. By using the relationship (Δω = 6.2σGaN), the stress state of the GaN epilayer can be obtained directly from the shift in the Raman peak position. In F-LED-10 sample, the characteristic mode (566.4 cm−1) resulted from larger external stress was selected to calculate.
Figure 3(c) shows the Raman peak position of four samples and the corresponding residual mechanical stress in the grown GaN layer depending on the substrates and device curvature. The Raman peak position of red- and blue shifts due to compressive stress (GaN on sapphire) and tensile stress (GaN on flexible substrate) were carried out. After calculation, it can be found that the R-LED presented a compressive stress in GaN epilayers, which is estimated to be −0.163 GPa. The tensile stresses for F-LED-15 and F-LED-10 are estimated to be 0.116 and 0.256 GPa, respectively. It has been found that the residual mechanical stress increases as the Le of the flexible substrate is gradually decrease from 20 to 10 mm. It is worth mentioning that the NF-LED stress is very low (−0.023 GPa). This suggests that the LED stress after two epilayer transfers and before external bending force is almost released.
Stress modulates the energy gap of GaN films and, consequently, the optical properties [23]. By controlling the stress state in the transferred GaN thin film, it can be changed the built-in piezoelectric field and alleviate the QCSE to further improve the lighting performance. As shown in Fig. 4, the electroluminescence (EL) spectra emission peak wavelengths for the LED samples at various driving currents were performed to explore the substrate influence on bowing curvature upon the optical properties. The emission peak of all flexible LED samples shifted to a shorter wavelength compared with that of R-LED as the injected current increased from 20 to 100 mA. These findings clearly indicate that the mechanical stress introduced by the wafer bowing may change the piezoelectric field in the InGaN/GaN MQW active region and modify the energy band profile. The same phenomenon had been discovered by ref [14] although their results were resulted from conventional type LED (with sapphire substrate) applied the external stress. Thus, the blue-shift in peak wavelength and energy are attributed to the increase in effective band gap due to the decrease in piezoelectric field in InGaN/GaN MQWs. However, a stronger redshift occurred in the F-LED sample spectra when the current exceeded 100 mA because of the Joule heating effect resulting from the poor thermal conductivity of PI substrates. As the bending length of flexible LEDs increased to 15 mm, the EL intensity increased and the peak wavelength was blue-shifted. This is because the external tensile stress could relax the compressive stress in InGaN/GaN MQW LEDs epilayers, reducing the QCSE by reducing the piezoelectric polarization. Therefore, the electron and hole wave function overlap and the probability of radiation recombination are simultaneously increased, resulting in increased IQE. Further increase in curvature caused a reduction in EL intensity (F-LED-10 sample), thus extremely large stress values can produce cracks in these films. On the other hand, the emission wavelengths of F-LEDs (531 nm for F-LED-15, 529 nm for F-LED-10 @20 mA) are shorter than that (532 nm) of NF-LED. This suggests that spontaneous polarization still exists in the NF-LED even when the sapphire substrate is removed. Thus, further external tensile stress can be used to compensate for the spontaneous polarization.
Fig. 4 EL peak wavelengths of R-LED, NF-LED, F-LED-15 and F-LED-10 as a function of the injection current.
For the R-LED, NF-LED and F-LED, the forward voltages were about 2.3 V at an injection current of 20 mA. The leakage currents of the R-LED, NF-LED and F-LED at the reverse voltage of 5 V were about 0.2 μA. This indicates that the electrical properties of the LED devices were not degraded during the fabrication process and bending test. Figure 5 shows the light output power and wall plug efficiency (WPE) of the R-LED, NF-LED, F-LED-15 and F-LED-10 as a function of injection current dependence on the various bending lengths. The light output powers of the R-LED, NF-LED, F-LED-15 and F-LED-10 were 37, 42.6, 52.6 and 38.9 mW, respectively, at an injection current of 150 mA. Compared with the R-LED, the light output power of the NF-LED, F-LED-15 and F-LED-10 were improved by 15.1%, 42.2% and 5.1% at 150 mA, respectively. In addition, the WPE values at an injection current of 150 mA of the R-LED, NF-LED, F-LED-15 and F-LED-10 were estimated to be 8.7%, 9.52%, 12.07% and 8.93%, respectively. For one thing, the light output power enhancement factor was increased due to the double-sided LED roughening leading to a stronger scattering effect in the guided light and greater light extraction out of the LEDs; for another, it is clear that the enhanced light output power of the F-LED-15 exposed to external tensile stress was more improved. This can be attributed to the compressive strain relaxation in the InGaN/GaN MQWs by mechanical bending. The F-LED-15 manifestly displayed better light output. The obtained result is consistent with that of B. Ryu et al report [14]. On the other hand, the light output power of F-LED-10 was only slightly increased at high injection current. This poor performance could be attributed to the excessive bending stresses caused by adhesive layer cracking between the copper plate and the Flex-LED, which increased the injection current and generated considerable heat in the LED.
Fig. 5 Light output power and WPE against the injection current for the R-LED, NF-LED, F-LED-15 and F-LED-10.
4. Conclusion
In conclusion, we investigated InGaN-based green LEDs fabrication onto a flexible polyimide substrate using LLO and glue-bonding techniques. Optimizing the external stress in flexible LEDs by changing their curvatures may allow for built-in piezoelectric polarization control and QCSE relaxation to improve the light output power. The F-LED-15 shows improvement in light output power up to 42.2% at an injection current of 150 mA, as the external bending went to the effective length case of 15 mm. The improved light output power and emission efficiency of the Flex-LED can be attributed to compressive strain relaxation in the InGaN/GaN MQWs by changing the external bending in the effective length case. These flexible LED research results may provide some suggestions for improved fabricating technology and device configuration design for the next generation flexible lighting applications.
Acknowledgment
This work was supported by the Ministry of Science and Technology (MOST) and Hsin Chu Science Park, Taiwan under Grants MOST 102-2221-E-005-071-MY3, 103A13 and 104A23, respectively.
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