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We report the growth of InGaN/GaN multiple quantum wells blue light-emitting diodes (LEDs) on a silicon (111) substrate with an embedded nanoporous (NP) GaN layer. The NP GaN layer is fabricated by electrochemical etching of n-type GaN on the silicon substrate. The crystalline quality of crack-free GaN grown on the NP GaN layer is remarkably improved and the residual tensile stress is also decreased. The optical output power is increased by 120% at an injection current of 20 mA compared with that of conventional LEDs without a NP GaN layer. The large enhancement of optical output power is attributed to the reduction of threading dislocation, effective scattering of light in the LED, and the suppression of light propagation into the silicon substrate by the NP GaN layer.
© 2016 Optical Society of America
1. Introduction
GaN-based light-emitting diodes (LEDs) have been widely used as a solid-state lighting source in various applications, including traffic signals, large displays, and automobiles [1–5]. Conventional GaN-based epitaxial layers are generally grown on sapphire and SiC substrates. However, the sapphire substrate is an insulator with poor thermal conductivity, and the SiC substrate is very expensive and available only with small diameters [6,7]. Recently, a silicon substrate has attracted great interest for the growth of GaN-based LEDs owing to the cost benefits, large scale manufacturability, matured state of silicon-based technology, and the integration potential of electronic and optical devices [8–12]. Additionally, it is known that the high thermal conductivity of a silicon substrate enables a more effective heat dissipation from the LEDs compared with the use of a sapphire substrate [13]. To grow the c-axis-oriented würtzite crystal structure of GaN (0001), the (111) plane of silicon has been used as the most favored orientation owing to the three-fold surface symmetry. However, growing GaN on the silicon (111) substrate is not straightforward owing to the presence of the lattice mismatch (~17%) and difference in thermal expansion coefficient (~56%) [14,15]. The large lattice mismatch between the silicon (111) substrate and GaN results in a high dislocation density in GaN that would lead to the low performance of LEDs. The difference in thermal expansion coefficient introduces a large tensile stress in GaN during the cooling process after the growth of LEDs, which causes wafer bowing and crack generation. To date, several groups have reported the reduction of threading dislocation, stress mitigation and removal of cracks for the GaN layer on the silicon (111) substrate by using various methods, such as epitaxial lateral overgrowth (ELOG) [16,17], graded AlGaN interlayer growth [18,19], and AlGaN/GaN superlattice growth [20,21]. In particular, the use of a low-temperature AlN (LT-AlN) layer, developed by Krost et al., has been regarded as a novel method to release the stress of GaN [22–24]. However, although the LT-AlN layer can play an important role in mediating the residual tensile stress in GaN, it introduces new dislocation at the upper interface of LT-AlN/GaN owing to the 2.4% lattice constant mismatch [25]. In our previous study, a nanoporous (NP) GaN layer has been demonstrated as an alternative interlayer to increase the internal quantum efficiency (IQE) and light extraction efficiency (LEE) of GaN-based LEDs grown on the sapphire substrate [26]. The NP GaN layer formed by the electrochemical (EC) etching method was found to improve the crystalline quality by annihilating defects and relaxing the compressive stress in regrown GaN on a NP GaN layer owing to the presence of nanopores in the NP GaN layer. However, there are no reports on the NP GaN layer for GaN-based LEDs grown on a silicon (111) substrate.
In this paper, we report an enhanced performance of GaN-based LEDs grown on a silicon (111) substrate by inserting a NP GaN layer. It was found that the NP GaN layer embedded in LEDs not only releases the residual tensile tensile stress in overgrown GaN-based epitaxial layers, but also reduces the threading dislocations without the introduction of cracks on the LED wafers. Furthermore, we demonstrate that the NP GaN layer embedded in LEDs increases the light extraction by changing the light path in LEDs and suppressing the light propagation toward the silicon substrate.
2. Experimental details
The epitaxial layers of GaN-based LEDs were grown on a 2-inch silicon (111) substrate by using metal organic chemical vapor deposition (MOCVD). After thermal treatment in a hydrogen flow to remove the native oxide at 1050 °C for 10 min, a 100 nm-thick AlN and 3-step graded-AlGaN buffer layers were grown to protect the silicon melt-back etching and reduce the tensile stress in the epitaxial layers, respectively. A 1.2 μm-thick undoped GaN layer, which includes two pairs of 12 nm-thick low-temperature AlN layers, was then grown to generate the incoherent bonding for the crack reduction. A 500 nm-thick n-type GaN layer with a silicon doping concentration of 6 × 1018 cm−3 was subsequently deposited at a pressure of 200 torr. The 500 nm-thick n-type GaN layer was electrochemically etched to obtain the NP GaN layer in 0.2 M oxalic acid electrolyte at room temperature for 10 min using a platinum rod as the cathode. The porosity of NP GaN layer was controlled by varying the applied voltages during EC etching of the n-type GaN layer. A 500 nm-thick undoped GaN layer was grown on NP GaN. Finally, a 2.0 μm-thick n-type GaN layer and six periods of InGaN/GaN (2.3/7.7 nm) multiple quantum wells (MQWs) were grown, followed by the growth of a 200 nm-thick p-type GaN layer.
3. Results and discussion
Figure 1(a) shows a photograph image of the crack-free GaN-based LEDs epitaxial layer grown on a 2-inch silicon (111) wafer. The NP GaN layer was fabricated by EC etching at an applied voltage of 15 V. The 4.7 μm-thick GaN epitaxial layer with the embedded NP GaN layer, which is characterized by a gray color on the silicon (111) wafer, shows a mirror-like uniform surface without cracks. Figure 1(b) shows the cross-sectional scanning electron microscopy (SEM) image of a GaN-based LED grown on the silicon (111) substrate. The NP GaN layer is embedded between the LED epitaxial layer and the Al(Ga)N-based buffer layer grown on a silicon (111) substrate. The Al(Ga)N-based buffer layer includes the 100 nm-thick AlN, 3-step graded-AlGaN buffer layers, and the 1.2 μm-thick undoped GaN layer. The sphere-like nanopores in the NP GaN layer have an average diameter of 250 ± 137 nm and these are evenly distributed in the 500 nm-thick NP GaN layer, as shown in the inset of Fig. 1(b). The cross-sectional SEM images show that the fraction of nanopores in the NP GaN layer is 33.4%.
Fig. 1 (a) Photograph image of an LED epitaxial layer with and without the nanoporous (NP) GaN grown on a 2-inch silicon (111) substrate. (b) Cross-sectional SEM image of a GaN-based LED epitaxial layer with the NP GaN layer. The inset shows an enlarged SEM image of nanopores in the NP GaN layer.
To characterize the crystalline quality and residual stress of regrown GaN on the NP GaN layer, the omega-scan on the diffraction peak and E2-high phonon mode were measured by x-ray diffraction (XRD) and Raman spectroscopy, respectively, as shown in Fig. 2. The full width at half maximum (FWHM) of (0002) and (10-12) planes typically represents the crystalline imperfection originating from the densities of screw (ρs) and edge (ρe) dislocations in the epitaxial layer, respectively [27,28]. The ρs and ρe dislocation densities are evaluated by [28]:
where β(0002) and β(10-12) are the FWHM of (0002) and (10-12) planes, bc and ba stand for the Burgers vector lengths equated to c- and a-axial lattice constants, respectively. The FWHM value of the (0002) and (10-12) diffraction peak of regrown GaN is notably decreased from 671 to 429 arcsec and from 793 to 670 arcsec, respectively, by insertion of a NP GaN layer, as shown in Fig. 2(a). The ρs and ρe are calculated as 2.93 × 109 cm−2 and 1.37 × 109 cm−2 for regrown GaN without NP GaN and 1.20 × 109 cm−2 and 9.78 × 108 cm−2 for regrown GaN with NP GaN. This result indicates that the crystalline quality of regrown GaN on the NP GaN layer is improved. Figure 2(b) shows the frequency changes of the E2-high and A1 longitudinal optical (LO) phonon modes on the Raman spectra of regrown GaN on the NP GaN layer. The frequencies of E2-high and A1 (LO) phonons shift when a würtzite GaN crystal is biaxially deformed [29,30]. The biaxial stress can be calculated by the shift of the frequency of the E2-high phonon mode peak using the following equation [30]:where ωγ and ωo represent the E2-high phonon mode peak of the regrown GaN with and without the NP GaN layer, respectively. A proportionality factor Kγ of 4.2 cm−1 GPa−1 is used for hexagonal GaN [30,31]. The peak position for regrown GaN with and without the NP GaN layer appears at 563.2 and 561.0 cm−1, respectively, while the frequency of the E2-high phonon mode for the fully relaxed bulk GaN is observed at 568 cm−1 [30]. The red-shift from 568 cm−1 of the E2-high phonon peak means that the residual tensile stress still remains in both epitaxial layers grown on the silicon (111) substrate. The value of residual tensile stress in the regrown GaN with and without NP GaN layer is calculated to be 1.143 and 1.667 GPa, respectively. Therefore, the blue-shift of 2.2 cm−1 from that of the regrown GaN without the NP GaN layer clearly shows that the tensile stress is relaxed by 0.524 GPa in the regrown GaN on the NP GaN layer. The A1 (LO) phonon peak also shows a blue-shift from 645.3 to 647.0 cm−1, as shown in the inset of Fig. 2(b), indicating that the stress is further relieved by the NP GaN layer in the same manner as that of the E2-high phonon. According to the result of our previous work [26], the stress relief mechanism of the NP GaN layer can be attributed to the shape deformation or atomic restructuring of nanopores in NP GaN. The nanopores in NP GaN formed by EC etching initially undergo shape deformation at high temperatures during the regrowth of LED epitaxial layer on NP GaN. It is also noteworthy to mention that the restructured nanopores partially separate the atomic arrangement between subsequent layers and the underlying buffer layer. Therefore it is believed that nanopores in NP GaN not only block the propagation of strain through the LED epitaxial layer via partial separation of adjacent strained layers by nanopores but also play a crucial role in reducing the stress between the Al(Ga)N-based buffer layer and the LED epitaxial layer by shape deformation or atomic restructuring process.Fig. 2 (a) XRD omega-scan for the symmetric (0002) plane of regrown GaN on the silicon (111) substrate with and without NP GaN. The inset shows XRD omega scans for the asymmetric (10-12) plane of regrown GaN on the silicon (111) substrate with and without NP GaN. (b) Room-temperature Raman spectra of regrown GaN with and without the NP GaN layer. The inset shows room-temperature Raman spectra of the A1 (LO) mode peak.
To explore the properties of LEDs grown on the NP GaN layer, room-temperature and temperature-dependent photoluminescence (TD-PL) measurements were performed on the LED samples, as shown in Fig. 3. The LED with and without the NP GaN layer are denoted as the NP LED and reference LED, respectively. The PL spectra were measured using a He-Cd laser (λ = 325 nm) with an excitation laser power of 20 mW. As shown in Fig. 3(a), the PL intensity of the NP LED is much higher than that of the reference LED and the peak wavelength is blue-shifted by 10.8 nm, from 479.9 nm of the reference LED to 469.1 nm of the NP LED. Since the NP GaN layer releases the residual tensile stress in the regrown GaN on NP GaN by atomic arrangement of nanopores at high growth temperature and the physical separation of Al(Ga)N-based buffer layer and the regrown GaN, the indium incorporation in the InGaN layer of MQWs can be decreased and a blue shift of PL emission wavelength is observed in NP LED. Additionally, the Fabry-Perot fringe pattern on the spectrum of the NP LED, which arises from the interference of light waves owing to multiple reflections at the GaN/air and GaN/silicon interfaces, is reduced by insertion of the NP GaN layer. The integrated PL intensity of the NP LED is increased by 2.29 times compared with that of the reference LED. The large increase in the PL intensity can be attributed to the improved crystalline quality by the NP GaN layer. To further examine the effect of the NP GaN layer on the optical properties of MQWs, the two-channel Arrhenius plot of TD-PL spectra was constructed, as shown in Fig. 3(b). The integrated PL intensity of MQWs can be described by the following equation [32,33]:
where I(T) is the normalized integrated PL intensity of the MQWs, I0 is a scaling factor, C1 and C2 are constants related to the density of non-radiative recombination centers, and kB is Boltzmann’s constant. E1 is an energy barrier against carrier loss into dislocations or other non-radiative centers in MQWs in the high-temperature regime and E2 is the energy barrier against the carrier hoping process from a shallow localized state to a deeper state which relates to the thermal quenching at the low-temperature regime [32]. For the purpose of clear comparison of data, the graphs of normalized integrated PL intensity and Arrhenius plot for the NP LED were arbitrarily upshifted by changing the scaling factor in Fig. 3(b). The calculated values of E1 and E2 from Eq. (4) are 140 and 12.5 meV for the reference LED and 180 and 15.5 meV for the NP LED. Additionally, the constants C1 and C2 are 32 and 2.0 for the reference LED and 21 and 1.8 for the NP LED. These results reveal that the NP GaN layer embedded in LEDs increases the radiative recombination in MQWs by the enhanced carrier capturing in localized states, owing to the high energy barrier of the radiative centers, and by the improved crystalline quality of MQWs owing to the low defect density such as mixed and screw-type dislocations. The IQE of the reference LED and NP LED, which is estimated from Fig. 3(b) assuming that the IQE is 100% at a low temperature of 10 K [32], is 43.6 and 51.2%, respectively. The increase in the IQE of the NP LED indicates that the potential energy fluctuations in MQWs is increased with the high crystalline quality and this leads to the increase of the radiative recombination rate of NP LEDs.Fig. 3 (a) Room-temperature PL spectra of the GaN-based LED with and without NP GaN layer (denoted as the NP LED and reference LED, respectively). (b) Arrhenius plots of the reference LED and NP LED.
Recently, a monolithic integration of GaN-based LEDs and GaN power high-electron-mobility transistors (HEMTs) or other electronic devices have been emerged as smart lighting applications [12,34]. However, when the GaN-based LEDs are grown on the silicon substrate, the light output power of the LEDs is decreased by the absorption of light in the silicon substrate [13]. To investigate the loss of the optical output power in the silicon substrate, the finite-difference time-domain (FDTD) simulation was performed by using the LED Utility software (RSoft Design Group Inc.). A single-dipole source with a peak wavelength of 450 nm was used in the simulation and the light intensity was measured by detectors located at the upper (in ambient air) and the bottom (in ambient silicon) positions. The NP GaN layer was considered as a GaN layer having a random array of nanopores. Figure 4(a) shows the simulated electric-field propagation in the reference LED and NP LED structures. The distributed intensity of the electric fields in the LEDs clearly shows that the trajectory and intensity of light are changed by the NP GaN layer at the GaN/NP GaN interface. Figures 4(b) and 4(c) shows the light intensity, measured by the upper and bottom detectors, of the reference LED and NP LED as a function of the propagated length of light (ct), where c and t are the velocity of light and time, respectively. The LEE from the FDTD simulation results is calculated to be 38.1% for reference LED and 50.3% for NP LED. The light intensity of the NP LED measured by the upper detector is increased by 32.6% compared with that of the reference LED. In addition, the light intensity in the silicon substrate (measured by the bottom detector) is reduced by 44.6%. This result indicates that NP GaN with lower refractive index than GaN effectively increases the total internal reflection for more light to escape from NP the LED and suppresses the light propagation into the silicon substrate.
Fig. 4 (a) Simulated electric-field contour map of the reference LED and NP LED. (b) Simulated electric field intensities from the upper (in ambient air) and bottom (in ambient silicon) detectors in the reference LED, and (c) simulated electric field intensities from the upper (in ambient air) and bottom (in ambient silicon) detectors in NP LED.
Figure 5(a) shows the current-voltage (I-V) characteristics of the reference LED and NP LED. The forward voltage of the reference LED and NP LED is 4.12 V and 4.05 V at 20 mA, respectively. The reverse currents for both the LEDs are similar to −13.1 μA at −10 V. Additionally, the series resistance, estimated from the I-V curve of the NP LED, is 37.1 Ω, which is slightly lower than the 41.9 Ω of the reference LED. These results indicate that the decrease in the defects such as threading dislocation and the reduced band gap due to the decrease of indium composition in MQWs would reduce the series resistance and forward voltage [35], improving the I-V properties of NP LEDs. Figure 5(b) shows that the optical output power of the NP LED is increased by 120% compared with that of the reference LED at 100 mA. The large increase in optical output power is attributed to the reduced defect density, the enhanced carrier recombination rate in the localized states of the MQWs, the effective light extraction by nanopores in the NP GaN layer, and the reduced light propagation into the silicon substrate.
Fig. 5 (a) I-V characteristics of LEDs with and without the NP GaN layer. (b) Optical output power of LEDs with and without the NP GaN layer as a function of injection current.
4. Conclusion
In conclusion, we demonstrate high performance GaN-based LEDs on the silicon (111) substrate by insertion of the NP GaN layer. The NP GaN layer, fabricated by the EC etching process, improves the structural and optical properties of the LEDs without the introduction of any cracks on the 2-inch wafer. The optical output power of the NP LED is increased by 120% at 100 mA, compared with that of the reference LED. This improvement is attributed to the increased IQE by the high crystalline quality of GaN-based LED epitaxial layer and the increased carrier confinement of the localized states of the MQWs, the increased LEE by the light scattering with the nanopores in the NP GaN layer, and the suppression of light propagation into the silicon substrate.
Acknowledgment
This work was supported by funding from the ILJIN LED Co., Ltd. and the National Research Foundation of Korea (NRF) funded by the Ministry of Science.
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