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Feb 5, 2018 - We demonstrate the first InGaN/GaN blue light emitting diodes (LEDs) on freestanding GaN grown using a Si substrate. Transmission electr...
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InGaN/GaN blue light emitting diodes using freestanding GaN extracted from a Si substrate Moonsang Lee, Mino Yang, keunman Song, and Sungsoo Park ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.7b01453 • Publication Date (Web): 05 Feb 2018 Downloaded from http://pubs.acs.org on February 6, 2018

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ACS Photonics

InGaN/GaN blue light emitting diodes using freestanding GaN extracted from a Si substrate Moonsang Lee,†,* Mino Yang,ǂ Keunman Song,Ŧ,* and Sungsoo Park§, ǁ,* †Korea Basic Science Institute, Daejeon, 169-148, Republic of Korea. ǂKorea Basic Science Institute, Seoul Center, Seoul, 5, Republic of Korea. ŦDevice Development Dept. 2, Technology Development Division, 109, Gwanggyo-ro, Yeongtong-gu, Suwon-Si, Gyeonggi-do, 443-270, Republic of Korea. §Jeonju University, Analytical Laboratory of Advanced Ferroelectric Crystals, Jeonju, 303, Republic of Korea ǁJeonju University, Department of Science Education, Jeonju, 303, Republic of Korea. E-mail: [email protected], [email protected], [email protected]

KEYWORDS: GaN, HVPE, light emitting diodes, efficiency, Si

ABSTRACT: We demonstrate the first InGaN/GaN blue light emitting diodes (LEDs) on freestanding GaN grown using a Si substrate. Transmission electron microscopy and X-ray diffraction analysis revealed that the InGaN/GaN multi quantum wells (MQWs) on freestanding GaN grown using Si substrates have excellent structural properties suitable for high-performance optical devices. Photoluminescence measurements confirm the high crystal quality of the InGaN/GaN MQWs and remarkable emission wavelength uniformity with a standard deviation of 0.68%. Light-current-voltage characteristics indicate that the InGaN/GaN LEDs on freestanding GaN grown using Si substrate exhibits a forward voltage of 3.75 V at a current of 20 mA and rectifying characteristics with very low leakage current and high breakdown voltage. Furthermore, they provide stable blue electroluminescence (λ = 460 nm) with small variation in the emission wavelength of 0.2% over a 2-inch area. The internal quantum efficiency of InGaN/GaN LEDs on freestanding GaN grown using Si substrates is remarkable at ~80%. Despite using Si substrates as the support, the optoelectronic properties of the InGaN/GaN LEDs are outstanding. We believe that the InGaN/GaN LEDs based on freestanding GaN crystals extracted from Si substrates are promising for the development of GaN-based high-performance devices.

Gallium nitride (GaN) has gained significant attention as an important material for modern optoelectronics and microelectronics, owing to its unique physical properties such as a wide bandgap, high thermal conductivity, high breakdown voltages, and excellent thermal and chemical resistance.1-3 Further, GaN-based light emitting diodes (LEDs) are considered basic elements of solidstate lighting aimed at significant energy savings by replacing incandescent and fluorescent bulbs with LED light sources owing to their superior characteristics such as a long-term lifetime, high efficiency, and excellent brightness.4-7 Until recently, GaN-based LEDs, however, are typically fabricated on foreign substrates such as sapphire and SiC owing to a lack of a native substrate. This deteriorates the performance of GaN-based LEDs, due to high dislocation density and internal reflection at the interface between GaN and the substrates.8-10 Indeed, vertical LEDs and flip-chip processes could be the potential solutions. These, however, lead to high costs owing to complex semiconductor processing operations, and a low production yield.11, 12 Overall, the present high prices and questionable LED quality prevent widespread application of these products. To circumvent these problems, the use of freestanding GaN as substrates for GaN-based LEDs could be an appropriate alternative. Although the current commercial freestanding GaN substrates have been grown on Al2O3 or GaAs substrates by hydride vapour phase epitaxy (HVPE), drawbacks such as size limitations, wafer bowing, and high production costs of conventional freestanding GaN crystals have to be overcome for the commercial success of freestanding GaN wafers.13 Currently, there are two main alternative approaches for successfully overcoming the current difficulties. The first option

involves the growth of the LED structure directly on cheap and large Si substrates. This approach was developed recently and transferred to the production line.14 However, the production cost is still high, and the growth of thick buffer layers by metal organic chemical vapour deposition (MOCVD) poses challenges for commercial success. Moreover, the optical and electrical performances of Si-based GaN LEDs are inadequate compared to conventional LEDs.14 Another option is the GaN-on-GaN platform. Recently, the successfully developed ammonothermal method for the growth of GaN crystals has opened new opportunities in the fabrication of nitride-based LEDs and lasers.15 Excellent device performances were reported for these systems. However, the barrier of size limitation of GaN crystals grown by the ammonothermal method prevents large-scale implementation in common with the conventional HVPE-grown freestanding GaN. Here, we propose that growing freestanding GaN wafers on Si substrates can be a possible solution. Meanwhile, it has been impossible to obtain freestanding GaN wafers grown on a Si substrate because of the large lattice mismatch (16.9%) and the difference between the thermal expansion coefficients of GaN (αa = 5.59 × 10-6 /K) and Si (αa = 3.77 × 10-6 /K). These factors impede the growth of freestanding GaN on Si substrates because they lead to formation of cracks and large bowing, owing to a high tensile stress on thick GaN layers grown when the reactor cools from the growth temperature to room temperature after the growth.16-18 Moreover, the reaction between Ga and Si due to the out-diffusion of Si atoms into the GaN layers, namely, the meltback effect, renders it even more difficult to obtain freestanding GaN wafers using Si substrates.19-20 Recently, we fabricated and characterized a 2-inch freestanding GaN crystal, 400 µm in thickness and 5 µm in wafer bowing, grown using Si substrates by

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ACS Photonics

In this paper, we demonstrate and characterize InGaN/GaN blue LEDs based on freestanding GaN extracted from Si substrates, for the first time. The structural and optoelectronic properties of the InGaN/GaN LEDs on freestanding GaN were investigated comprehensively. Though a Si substrate was used as the starting material for the growth of InGaN/GaN LEDs on freestanding GaN, their optoelectronic performance is excellent. We believe that the LEDs based on freestanding GaN wafers with a large diameter fabricated at low cost using Si substrates will open the possibility of replacing the conventional LEDs with these.

■ EXPERIMENTAL METHODS The InGaN/GaN LED structures studied in this work were fabricated using MOCVD (Aixtron G3 2600) on 2-inch HVPE freestanding GaN wafers grown on Si substrates. The fabrication of the InGaN/GaN LEDs is illustrated in Figure 1 and Figure S1, supporting information. Trimethylgallium (TMGa) and ammonia (NH3) were used as source materials for the growth of LED structures. Bicyclopentadienyl magnesium (Cp2Mg) and silane (SiH4) were incorporated as n- and p-type dopants, respectively. Further, triethylgallium (TEGa) and trimethylindium (TMIn) were employed as the Ga and In sources, respectively, for the growth of multi quantum wells (MQWs). The epitaxial structures of InGaN/GaN-based LEDs were grown on 2-inch freestanding GaN wafers and comprised a 3.5-µm-thick n-type GaN grown at 1020 ° C, InGaN well (3 nm), 12-nm-thick MQW (4 pairs) grown at 820

determined by photoluminescence (PL) measurements at 25 °C with 325 nm wavelength excitation using a He–Cd laser. The structural properties of the grown MQW layers in the LED structures were analysed by X-ray diffraction (XRD) performed on a Bruker D8 system using the Cu Kα line (λ = 0.154060 nm) and transmission electron microscopy (TEM) on a FEI F20 microscope with a field emission gun operating at 200 kV. TEM samples for cross-sectional observations were prepared by grinding and mechanical polishing followed by ion milling.

■ RESULTS AND DISCUSSION XRD analysis was employed to investigate the crystal quality and the interfaces of the InGaN/GaN MQWs on freestanding GaN grown using a Si substrate over the entire 2-inch wafer, as shown in Figure 2. 1.2

Normalized Intensity

HVPE, which was achieved by in-situ removal of the Si substrate.21 The structural and optical properties of the freestanding GaN wafers grown by this method were comparable to those grown on other foreign substrates such as sapphire and GaAs.

(002) X-ray rocking curves FWHM on the Center : 115 arcsec FWHM on the Edge : 118 arcsec

1.0 0.8 0.6 0.4 0.2 0.0

°C, and 150 nm-thick p-type GaN grown at 1020 °C. To compare the device characteristics between InGaN/GaN LED fabricated using sapphire and freestanding GaN grown using Si substrates, lateral chips with the same LED structure were fabricated using conventional photolithography, dry etching, and metallization.

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theta (degree) (a) 1.2

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(102) X-ray rocking curves FWHM on the Center : 79 arcsec FWHM on the Edge : 86 arcsec

1.0 0.8 0.6 0.4 0.2 0.0

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theta (degree) (b)

Figure 1. Schematics of the fabrication procedure of InGaN/GaN LED using freestanding GaN substrates grown using Si substrates. The LED characteristics were evaluated by light-current-voltage (L-I-V) measurements. In addition, the optical properties were

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ACS Photonics GaN substrate grown using a Si substrate is as good as or superior to those of the conventional InGaN/GaN LEDs using freestanding GaN substrates grown using other foreign materials such as sapphire, and GaAs.27 Cross-sectional STEM images of the InGaN/GaN MQWs of LED structures on sapphire and freestanding GaN wafers grown using Si substrates are illustrated in Figure 3. As shown in Figure 3(a), there are structural defects such as threading dislocations for the InGaN/GaN LED on the sapphire substrate. In contrast, such structural defects are not observed in the InGaN/GaN LED on freestanding GaN grown using a Si substrate.

(c) Figure 2. X-ray rocking curves corresponding to (a) (002) reflection, (b) (102) reflection, and (c) a wider 2θ range for InGaN/GaN MQWs on freestanding GaN grown using a Si substrate. The red and blue circles represent the measurement data for the centre and edge of a 2-inch GaN wafer, respectively.

The values of the full width at half maximum (FWHM) of the X-ray rocking curves for (002) and (102) diffractions of InGaN/GaN MQWs on the entire wafer are 115–118 arcsec and 79–86 arcsec, respectively, (Figure 2(a) and 2(b)). This indicates the high crystal quality of the InGaN/GaN LEDs grown on freestanding GaN grown using Si substrates.22 It is interesting to note that the variation in the FWHM values from the centre to edge of the entire wafer is 3 arcsec and 8 arcsec for the (002) and (102) reflections, respectively. It is well known that the FWHM of the (102) X-ray rocking curves reflects the lattice distortion by the threading dislocations with all the components including pure edge, screw, and mixed types.23 A narrow deviation of the FWHM values for the (102) reflection on the entire wafer implies a good uniformity with a lower lattice distortion. We speculate that the low variations in the FWHM values of the X-ray rocking curves could be attributed to the low wafer bowing of the freestanding GaN grown using Si substrates. Moreover, we can clearly observe that the InGaN/GaN MQWs exhibit discrete satellite peaks up to 3rd orders, implying that the interfaces between InGaN/GaN MQWs are discrete and abrupt.24 In addition, we analysed the interface roughness (IRN) of the MQW structures fabricated on the freestanding GaN grown using a Si substrate using the following equation:25, 26     2 / ∆Ө

 

In addition, we could clearly confirm that uniform MQW layers with a flat and abrupt interface were successfully grown on freestanding GaN wafers without any fluctuation in the MQW thickness and generation of dislocations, which is consistent with the results of IRN analysis. It has been reported that the threading dislocation affects the interface roughness of the InGaN/GaN MQWs.29 Dislocations cause a non-uniform distribution of Indium content in the InGaN MQW layers, resulting in structural deterioration due to the segregation of In-rich domains in the InGaN MQW layers. These are closely related to the nonradiative recombination of carriers, leading to the deterioration of the InGaN/GaN LED performance.30-32 Therefore, we can easily expect that the InGaN/GaN MQWs fabricated on freestanding GaN wafers grown using Si substrates consist of highly homogenous InGaN MQWs with the suppression of In decomposition and segregation of In-rich domains in InGaN/GaN MQWs, thereby enabling the realization of highly efficient InGaN/GaN LEDs despite using Si-based materials.

(a)

(1)

where, n is the order of the satellite, Λ and σ/Λ are the period of the satellite peak and interface roughness, respectively. ∆θM denotes the angle distance between adjacent satellite peaks, and Wn and W0 denote the full widths at half maximum of the nth- and zeroth-order peaks, respectively. Equation (1) provides a low IRN of 1.7%, which is in good agreement with previous reports based on freestanding GaN grown on sapphire substrates.27 It is well known that the defects, microstructure, and phase separation in MQWs influence the IRN of MQWs.25, 28 Therefore, these results manifest that the crystal quality and interface uniformity of the InGaN/GaN MQW LEDs’ epitaxial structure on the freestanding

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Figure 3. Cross-sectional STEM images of InGaN/GaN MQW on (a) sapphire and (b) freestanding GaN grown using a Si substrate. The scale bar is 20 nm.

Figure 4 illustrates the room-temperature PL mapping and PL spectrum of the InGaN/GaN LED on freestanding GaN grown using Si substrates. A reference LED with the same structure on a sapphire substrate was prepared for comparison. The wavelength map of the LED on a GaN substrate shows an average emission wavelength of 453.7 nm with good uniformity and a standard deviation of 3.11 nm over the entire 2-inch wafer, which is comparable to that of LEDs on a sapphire substrate (not shown). It is noteworthy that the InGaN/GaN LEDs on freestanding GaN substrates exhibits approximately 5-fold enhanced PL intensity compared to the InGaN/GaN LED on sapphire substrate (Figure 4(b)). We speculate that this is due to the high crystal quality of the GaN substrate, although it was grown using a Si substrate as a support. Obviously, the typical bowing value of as-grown freestanding GaN on a sapphire substrate is approximately a few hundreds of micrometres, which can cause large deviation in the emission properties of the InGaN/GaN LEDs, which is undesirable. Considering this, we are strongly convinced that the InGaN/GaN LEDs on the freestanding GaN grown via in-situ removal of the Si substrate shows favourable emission uniformity. We also note that the PL peak position of the InGaN/GaN LEDs on freestanding GaN is red-shifted. This shift could be explained by the enhanced Indium incorporation in InGaN/GaN MQWs on freestanding GaN due to low temperature of substrate surface, which results from the better thermal transport of GaN substrate compared to that on a sapphire substrate.33, 34

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(b) Figure 4. (a) PL wavelength mapping of a 2-inch InGaN/GaN LED fabricated using a freestanding GaN grown using a Si substrate. (b) Room-temperature PL spectrum of InGaN/GaN LEDs on sapphire and freestanding GaN grown using a Si substrate.

Figure 5 shows the light output-current-voltage (L-I-V), electroluminescence spectrum, leakage current, and breakdown voltage of the InGaN/GaN LEDs based on freestanding GaN grown using Si substrates. The InGaN/GaN LEDs on the 2-inch freestanding GaN substrate grown using a Si substrate exhibits an average forward voltage (Vf) of 3.65 V and optical power (Po) of 14.9 mW at an injection current of 20 mA, as illustrated in Figure 5(a). The optical power is superior compared to that of the reference InGaN/GaN LED on a sapphire substrate with Vf of 3.45 V and Po of 12.9 mW at an injection current of 20 mA. Figure 5 (b) shows the EL emission spectrum as a function of increasing injection current. The peak wavelength and FWHM at the injection current of 20 mA are 460 nm and 25.21 nm, respectively. The peak wavelengths are slightly red-shifted, and the linewidths broaden with increasing injection current. This can be attributed to the band shrinkage effect by Joule heating that generated from the power dissipation owing to the non-radiative recombination process.35 The red shift of the peak wavelength in the current range of 0 to 100 mA is ~1.0 nm, and the broadening of the spectrum width is ~1.1 nm in the corresponding current range. These small variations in the peak shift and the spectral width broadening even under a high current at room temperature confirm the excellent emission stability of the InGaN/GaN LEDs fabricated on freestanding GaN grown using a Si substrate. This is comparable or better than state-of-the-art conventional InGaN/GaN LEDs.36 The inset of Figure 5 (b) shows the EL spectrum of InGaN/GaN LEDs on GaN and sapphire substrates. The EL intensity of the InGaN/GaN LED on freestanding GaN is higher by 18% than that of the reference InGaN/GaN LED on sapphire over the entire current range. This value is much smaller than that expected considering an enormous enhancement of the PL intensity in InGaN/GaN LED using freestanding GaN. It has been reported that a considerable amount of the downward light around the blue emission in the homoepitaxial LED is absorbed by the substrate.37 The leakage current and breakdown voltage of InGaN/GaN LED fabricated using sapphire and freestanding GaN extracted from Si substrates were investigated, as shown in Figure 5(c). Both devices clearly represent a rectifying behaviour. InGaN/GaN LED on freestanding GaN grown using a Si substrate exhibited negligible leakage current. In detail, the leakage currents of the InGaN/GaN

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ACS Photonics LEDs on freestanding GaN and sapphire at a reverse voltage of −5 V are −5.5×10-11 A, and −7.9×10-8 A, respectively. No apparent breakdown is observed for the InGaN/GaN LED on freestanding GaN grown using a Si substrate. Meanwhile, the breakdown phenomenon for the LED on a sapphire substrate occurred at approximately −35 V. Despite the fact that a Si substrate can cause the deterioration of the structural properties of InGaN/GaN LEDs, such as the propagation of threading dislocations and large wafer bowing, InGaN/GaN LEDs on freestanding GaN grown using a Si substrate show excellent optical and electrical characteristics. We ascribe this to the good structural properties of freestanding GaN grown using Si substrates, which originates from relaxed residual strain and high crystal quality via inclined threading dislocations.21 Consequently, we believe that this led to the improved optoelectronic performance of the InGaN/GaN LED grown Sibased freestanding GaN substrate. (c) Figure 5. (a) L-I-V curves for InGaN/GaN LEDs on sapphire (diamond) and freestanding GaN grown using a Si substrate (circle). The blue circles represent the light output power, and the red circles represent the voltage at various driving currents. (b) Electroluminescence vs. the injection current for an InGaN/GaN LED on freestanding GaN grown using a Si substrate. The inset displays the EL of the InGaN/GaN LED on sapphire vs. the injection current. (c) Reverse bias voltage-current curves of InGaN/GaN LEDs on sapphire and freestanding GaN. The inset represents the leakage current of InGaN/GaN LEDs vs. the reverse bias voltage of InGaN/GaN LEDs on sapphire and freestanding GaN.

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To investigate the internal quantum efficiency (IQE, ɳIQE), we performed temperature-dependent PL measurements for the InGaN/GaN LEDs on a sapphire substrate and a freestanding GaN grown using a Si substrate (Figure 6). The IQE value was extracted by dividing the PL intensity at each temperature by that at 7 K under 30 mW excitation power, assuming that the PL intensity at 7 K to be 100% and normalizing the others with respect to the maximum efficiency at 7 K. For the InGaN/GaN LED on freestanding GaN, the IQE decreases with an increase in the temperature and then saturates at 100 K. This is attributed to the gradual increase in the non-radiative recombination rate with increasing temperature. The IQE at room temperature saturated at ~80%. In contrast, for the InGaN/GaN LED on sapphire, the IQE is initially ~60% that of the freestanding GaN grown using a Si substrate at 7 K and abruptly declines at 70 K. At room temperature, the IQE saturated at ~8%. Although the InGaN/GaN LEDs fabricated in this work were basically grown on Si substrates, these remarkable IQE characteristics are comparable to those previously reported using a sapphire substrate.38-40

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* E-mail: [email protected]. Tel: +82-42-865-3519. * E-mail: [email protected]. Tel: +82-31-546-6328. Fax: +82-31-546-6309 (K. M. Song) * E-mail: [email protected]. Tel: +82-63-220-2281. Fax: +82-63220-2053 (S. Park)

Notes The authors declare no conflict of interest.

■ ACKNOWLEDGEMENTS Figure 6. Internal quantum efficiencies of InGaN/GaN LEDs on sapphire (red) and freestanding GaN extracted from a Si substrate (blue).

■ CONCLUSIONS

■ REFERENCES

InGaN/GaN LEDs were fabricated on freestanding GaN grown using a Si substrate as the support. X-ray rocking diffraction and TEM measurements reveal that InGaN/GaN MQWs with low lattice distortion, and sharp and abrupt interfaces were grown, which is an appropriate structure for high-performing InGaN/GaN LEDs. L-I-V characteristics showed a forward voltage of 3.75 V at a current of 20 mA and rectifying characteristics with very low leakage current and high breakdown voltage. Further, the LEDs provide a stable and uniform blue emission at 460 nm over the entire 2-inch emission area. Furthermore, an excellent IQE of ~80% for InGaN/GaN LEDs based on freestanding GaN extracted from a Si substrate is demonstrated. InGaN/GaN blue LEDs on freestanding GaN crystals grown using Si substrates exhibited remarkable optoelectronic properties without the negative effects due to the Si used as the supporting material. We expect that the InGaN/GaN LEDs based on freestanding GaN grown using Si substrates will open further opportunities for the development of GaN-based optoelectronic devices, thereby allowing new designs of the InGaN/GaN LED to facilitate cheap and large-scale production.

■ ASSOCIATED CONTENT Supporting information The supporting Information is available free of charge on the ACS publications website at DOI : Figure S1. Figure S2.

■ AUTHOR INFORMATION

This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF2016R1C1B1013667).

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For Table of Contents Use Only Names of authors: Moonsang Lee,†,* Mino Yang,ǂ Keunman Song,Ŧ,* and Sungsoo Park§, ǁ,* A brief synopsis: This figures describe how to fabricate InGaN/GaN LEDs on freestanding GaN grown using Si substrates and their opto-electrical properties.

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