Article Cite This: ACS Photonics 2019, 6, 1565−1571
pubs.acs.org/journal/apchd5
Electrical Modulation of Narrowband GaN/AlGaN Quantum-Well Photonic Crystal Thermal Emitters in Mid-Wavelength Infrared Dongyeon Daniel Kang,†,# Takuya Inoue,*,‡ Takashi Asano,† and Susumu Noda†,‡ †
Department of Electronic Science and Engineering and ‡Photonics and Electronics Science and Engineering Center, Kyoto University, Kyoto 615-8510, Japan
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S Supporting Information *
ABSTRACT: Narrowband thermal emitters operating in the mid-wavelength infrared (MWIR, 3−8 μm) are important for spectroscopic sensing systems in various fields of research such as chemistry, healthcare, and environmental science. To increase the signal-to-noise ratio in these spectroscopic applications, it is required that only thermal emission in a narrow target wavelength range be modulated electrically while other wavelength components are unmodulated. In addition, an increase of the emitter’s temperature is highly desired for high-power operation in the MWIR. To date, a number of efforts have been put into the demonstration of electrical modulation of thermal emission by using semiconductors, phase-change materials, and graphene. However, these emitters have not achieved selected modulation of narrowband thermal emission in the MWIR at high temperatures. Here, we demonstrate the electrical modulation of a narrowband MWIR thermal emission at high temperatures of up to 500 °C using GaN/AlGaN multiple quantum well (MQW) photonic crystals. Our emitter exhibits a narrowband thermal emission (Q = 40) owing to the combination of intersubband absorption in the MQWs and optical resonances of the photonic crystals, the intensity of which can be electrically modulated at high speed (50 kHz) through the control of the electron density in the MQWs. Our demonstration of electrical modulation of MWIR narrowband thermal emitters at high temperature will accelerate the practical use of narrowband thermal emitters in various spectroscopy applications such as optical gas sensors, including CO2 sensors. KEYWORDS: thermal emission, dynamic control, photonic crystal, nitride quantum well
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without changing the temperature. In the early 2010s, the possibility of the dynamic control of thermal emission without changing temperature was suggested,4,5 and the studies on the external control of emissivity started to be active.6,7 Nevertheless, these studies did not achieve the dynamic control of thermal emission much faster than the conventional temperature-modulation method. In 2014, we reported the first experimental demonstration of dynamic thermal emission control,8 where a narrowband thermal emission spectrum was realized by using photonic crystal resonances and intersubband transitions (ISB-T) in n-type GaAs/AlGaAs multiple quantum wells (MQWs),9,10 and the emissivity (absorptivity) was modulated at a very high frequency (∼600 kHz) by electrically
avelength-selective thermal emitters operating in the mid-wavelength infrared (MWIR, 3−8 μm) region are attractive as compact and energy-efficient optical sources for spectroscopic sensing systems owing to the presence of the intrinsic absorption lines of many industrially important gases such as CO2 (4.2 μm), CO (4.6 μm), CH4 (3.3 μm), and N2O (4.5 μm).1−3 In such applications, the modulation of the thermal emission intensity at high speed for synchronous detection is desired to increase the signal-to-noise ratio. Specifically, it is required that only thermal emission in a narrow target wavelength range can be modulated while other components remain unmodulated for spectroscopic applications. Previously, the modulation of thermal emission was usually performed by temperature modulation of bulky emitters, which are very slow. A solution to this problem is to dynamically manipulate the emissivity (absorptivity) © 2019 American Chemical Society
Received: March 21, 2019 Published: May 13, 2019 1565
DOI: 10.1021/acsphotonics.9b00440 ACS Photonics 2019, 6, 1565−1571
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manipulating the electron density inside the MQWs.8 After this demonstration, many groups have investigated the high-speed electrical control of thermal emission spectra using semiconductors,11 graphene,12,13 phase-change materials,14 and microelectromechanical systems.15 However, the above demonstrations have been performed at relatively low temperatures (250 °C at most) limited by thermal endurance and/or degradation of controllability (e.g., leakage current), which are insufficient for high-power operation in the MWIR range. In addition, the modulated spectra obtained in the above demonstrations (except for our demonstration in 2014) were broadband owing to the broadband material absorption. We note that very fast modulation of thermal emission in the nearinfrared range by temperature modulation of hot (∼2000 K) electrons in graphene coupled with a photonic crystal resonator, which is mainly intended for optical communication applications, has been reported very recently.16 However, broadband free-carrier absorption in graphene inevitably leads to generation of multiple emission peaks and modulation of broadband emission at longer wavelengths. In order to overcome these issues, here, we employ photonic crystals based on GaN/AlGaN MQWs, which maintain good thermal stability and electrical controllability at a high temperature owing to their wide electronic bandgaps,17 and demonstrate electrical high-speed modulation of narrowband thermal emission at a wavelength of 4 μm with well-suppressed background at a temperature as high as 500 °C.
Figure 1. Schematic of electrically controllable GaN/AlGaN MQW photonic crystals-based thermal emitters.
and without the applied voltages between the p-GaN and nGaN layers are shown in Figure 2b. The MQWs are partially depleted even without applying an external voltage because a large built-in potential (∼3 V) is applied to the MQWs. When a reverse bias is applied, the electrical potential of the MQWs moves further away from the Fermi level, extracting the electrons from the MQWs. When a forward bias is applied, the potential approaches the Fermi level, canceling out the initial depletion. Figure 2c shows the calculated average electron density in ten wells as a function of applied voltage, taking into account the ionization ratio of the donors at room temperature (∼22 °C) and at 500 °C. It can be seen that the electron density of the MQWs becomes zero at a voltage of −20 V at room temperature and −25 V at 500 °C, which leads to extinction of the ISB absorption. We confirm that the electric field necessary for the depletion of the MQWs is smaller than the breakdown electric field of GaN (>3 MV/cm).21 The rightaxis of Figure 2c shows the peak absorption coefficient of the ISB-T calculated by assuming the full-width at half-maximum (fwhm) of 53 meV (it should be noted that the absorption coefficient of the ISB-T is linearly proportional to the electron density in the MQWs when the electron density in the second subband is negligibly small; for details, see Supporting Information). As can be seen, we can control the absorption coefficient of the ISB-T by applying voltages to the p−n junction over the range from 3200 cm−1 to near-zero at 500 °C. Considering the theoretically estimated voltage-dependent ISB absorption shown in Figure 2c, we calculated the emissivity spectrum of the designed photonic crystal thermal emitter by the rigorous coupled wave analysis (RCWA) method. The structural parameters utilized for the triangularlattice photonic crystal structure are a lattice constant of a = 3.0 μm, an air hole radius of r = 0.22a, an air hole depth of h = 0.85 μm, and a slab thickness of t = 1.3 μm. In the calculation, the free-carrier absorptions in the p-GaN layer and n-GaN layer are taken into account by using a simple Drude model, in which the ionization ratios of the acceptors (η) and donors are assumed to be 10% and 100%, respectively. The emissivity spectra with applied voltages of +2 V, 0 V, and −30 V are shown in Figure 2d. When no voltage is applied, a narrowband peak (A) having a peak emissivity of 0.57 is obtained at a wavenumber of 2396 cm−1 (λ ∼ 4.2 μm). The emissivity is decreased by the application of a reverse bias, and increased by the application of a forward bias. Since the ISB absorption becomes extinct when we apply the reverse bias of −30 V in the calculation, the residue of thermal emission at −30 V is that generated by the free-carrier absorption in the p-GaN layer and n-GaN layer. The differences between the spectra at zero and at nonzero voltages are shown in Figure 2e, and indicate that modulation of a single narrowband thermal
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DESIGN AND PRINCIPLE In the previous demonstration,8,9 we utilized ISB-T in n-type GaAs/AlGaAs MQWs to limit the bare absorption spectrum of the material composing the emitter, and formed photonic crystal structures to obtain a narrower absorption/emission spectrum and to enable vertical emission by using the in-plane resonant mode. For the electrical control of the electron density (and thus absorptivity), we utilized a structure in which the MQWs are positioned inside a p−n junction. When we apply a negative voltage to the p−n junction, the electron density and ISB absorption in the MQWs decrease, thus modulating the emissivity of the emitter. However, the electrical controllability deteriorates at high temperatures above 250 °C owing to the increase of thermally excited free-carriers and leakage currents in the p−n junction of GaAs/ AlGaAs systems. Further, realization of ISB-T wavelengths shorter than 5 μm in GaAs/AlGaAs systems is challenging because of the small conduction band offset. Here, we utilize GaN/AlGaN systems in order to overcome these issues. GaN/ AlGaN systems have a large conduction band offset enabling an ISB-T wavelength range up to the near-infrared.18 Such systems are also promising for high-temperature electronics up to 600 °C17 owing to their good thermal stability and wide electronic bandgap (less intrinsic carriers). Figure 1 shows a schematic of the proposed electrically controllable narrowband thermal emitter composed of GaN/AlGaN MQWs and photonic crystals. First, we designed an epitaxial structure, composed of p-GaN (acceptor density: 1.5 × 1019 cm−3), undoped GaN (100 nm), MQWs (63 nm), and n-GaN (donor density: 5 × 1017 cm−3). We employed MQWs consisting of 10 pairs of n-GaN wells (3 nm, dopant density: 6 × 1018 cm−3) and undoped Al0.4Ga0.6N barriers (3 nm) to exhibit ISB-T at a wavelength of 4 μm as shown in Figure 2a.19,20 The calculated electron potentials of the conduction band in the proposed epitaxial structure with 1566
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Figure 2. (a) Energy potential of the GaN/AlGaN MQWs calculated by using Schrödinger’s equation (black line). Black dashed lines and red solid lines show energy levels and probability density functions, respectively. (b) Calculated energy potentials of the designed epitaxial structure with/ without applied voltages to the p−n junction. (c) Calculated average electron density (left y-axis) and the corresponding absorption coefficient of the ISB-T (right y-axis) in the MQWs as a function of applied voltage. (d) Theoretical emissivity spectra in the designed thermal emitter with applied voltages of +2 V, 0 V, and −30 V. (e) Difference between the spectra with and without applied voltages. The hole and electron densities are assumed to be 1.5 × 1018 cm−3 (ionization ratio η of 10%) and 5 × 1017 cm−3, respectively.
Figure 3. (a) Measurement setup of ISB absorption using multipass-geometry while applying voltages. (b) TM-polarized transmittance of the fabricated sample containing GaN/AlGaN MQWs inside p−n junction with applied voltages from +3 V to −30 V. (c) Changes in absorbance due to the applied voltages. (d) Peak absorption coefficients of the ISB absorption as a function of the applied voltage.
reverse bias and vice versa. In Figure 3c, the difference in the absorption spectra with and without applied voltages is presented as a change in the absorbance (−log10(I/I0)), where the modulation of the narrowband Lorentzian-shape absorption peak is confirmed at a wavelength of 4 μm, the fwhm of which is 428 cm−1. Figure 3d shows the peak absorption coefficient of ISB-T as a function of the applied voltage, which is estimated by fitting the measured spectra with the spectra calculated by using the RCWA method considering the optical path including multiple reflections and absorptions in the epitaxial structure. The peak absorption coefficient of ISB absorption changes from 2100 cm−1 at +3 V to near-zero at −30 V (it should be noted that broadband free-carrier absorption in the p-GaN/n-GaN layers cannot be evaluated in this experiment, which becomes more dominant than the ISB absorption at reverse voltages larger than 25 V). The absorption coefficient change obtained in the experiment
emission peak can be realized while almost completely suppressing modulation at other wavelengths.
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EXPERIMENTAL SECTION To experimentally evaluate the electrical controllability of the bare absorption spectrum of the above-designed GaN/AlGaN MQWs prior to fabricating the photonic crystal structure, an epitaxial wafer was grown on a Si (111) substrate and electrodes were attached to both the p-layer and n-layer by implementing a mesa-structure. We measured the transmittance of TM-polarized light incident on the 30°-angled edge face at room temperature by using Fourier-transform infrared spectroscopy (FTIR) while applying voltages, in a multipass geometry (see Figure 3a) to enhance the interaction between the incident light and the MQWs. The result is shown in Figure 3b, where an absorption dip around a wavelength of 4 μm originating from ISB-T decreases when we apply a 1567
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agrees with the calculation result shown in Figure 2c (dashed line). The required reverse voltage for complete depletion of the MQWs (30 V) in the experiment is a little larger than that in the calculation (20 V), probably because the ohmic contacts of the electrodes were not perfect at room temperature, causing a decrease of the effectively applied voltage to the MQWs owing to the contact resistance. Based on the above investigation, we fabricated the designed photonic crystal thermal emitters that can be electrically modulated. Details of the fabrication methods are provided in Methods section. Figure 4a,b shows optical and SEM
Figure 5. (a) Schematic image of the measurement setup for thermal emission spectra. (b) Thermal emission intensity and (c) Emissivity spectra at applied voltages of +5 V, 0 V, and −30 V. (d) Difference between the thermal emission intensity spectra at 0 V and nonzero applied voltages.
Figure 4. (a) Optical and (b) SEM micrographs of the fabricated thermal emitter.
cm−1) shown in Figure 2d). Selective modulation of a narrow emission peak is greatly beneficial in a synchronized spectroscopic system because a narrowband absorption line of a target analyte (material) can be detected, avoiding disturbances caused by other materials without using narrowband filters. It should be noted that the measured emissivities are smaller than the calculated emissivities: the peak emissivity without applying a voltage in the experiment was 0.46 (at 4.1 μm, Q = 40) while in the calculation it was 0.56 (at 4.2 μm, Q = 110). A possible reason for the decrease in the emissivity is spectral broadening owing to inhomogeneous fluctuations of the structural parameters of the photonic crystals such as the lattice constant, thickness of the slab, and radii and depths of the air-holes. For example, if a fluctuation of the lattice constant with a standard deviation of σ = 0.004a occurs, the theoretical spectrum is broadened to have the same peak emissivity as in the experiment. This implies that by suppressing the fabrication imperfections and, consequently, the spectral broadening, the peak emissivity change observed (Δε = 0.15) in the experiment can be increased. The emissivity change observed (Δε = 0.15) can be increased also by reducing the residual emission at the reverse bias of −30 V. This residual emission was caused by the freecarriers in the p-GaN layer as described in the design section, which varies depending on the hole density (note that when we assume that the ionization ratio of Mg dopant is 10%, the small residual emissivity (∼0.1) and large emissivity change (>0.4) are expected even considering the spectral broadening). To confirm this, we measured the thermal emission spectra of another device (a = 2.8 μm, r = 0.22a, h = 0.85 μm, t = 1.1 μm) by varying the temperature from 300 to 500 °C because the hole density in p-GaN is known to be sensitive to temperature owing to high ionization energy of Mg dopants.21,24 Figure 6a shows the peak emissivity change as a function of the applied voltage. The emissivity at −25 V indicates the residual emission, which varies from 0.19 to 0.28
micrographs, respectively, of the fabricated thermal emitter. We employed a mesa-structure to apply a voltage to a p−n junction to suppress the surface leakage currents via the sidewalls, which were formed by plasma etching techniques. The photonic crystal membrane with an area of 1.8 × 1.8 mm2 was formed inside of the mesa area. We utilized Ni/Au alloy covered with Ti/Pt/Au as the electrodes on p-GaN and Ti/Al/ Ni/Au alloy as the electrodes on n-GaN which allow ohmic contacts with good thermal tolerance.22,23 We implemented a comb-shaped contact with 0.8 mm intervals on p-GaN in order to decrease the in-plane resistance of the p-GaN layer and thereby increase the modulation speed. Next, we measured the thermal emission spectra of the fabricated device while applying a voltage. A schematic of the measurement system is shown in Figure 5a. The thermal emitter (a = 3.0 μm, r = 0.22a, h = 0.85 μm, t = 1.3 μm) was heated to 500 °C by an external heater, and thermal emission radiated in the vertical direction (as shown in the figure) was collected by a lens with an NA = 0.052 (corresponding to a collection half angle of 3°) and characterized using FTIR while we applied varying DC voltages to the device. The measured thermal emission spectra are shown in Figure 5b,c as the thermal emission intensity spectra and the emissivity spectra, respectively. The peak emissivity, occurring at a wavelength of 4.1 μm (2460 cm−1) and indicated by “A” in Figure 5c, can be varied from 0.36 to 0.51 by changing the applied voltage from −30 V to +5 V. The measurement of the thermal emission around 4.2 μm was difficult due to absorption by atmospheric CO2. The difference between the thermal emission spectra at 0 V and nonzero applied voltages is shown in Figure 5d. We have realized selective modulation of a narrow thermal emission peak at 4.1 μm (Q = 40), while modulation of other wavelengths is well suppressed (the smaller modulation peak at 4.4 μm, indicated by “B”, is considered to correspond to the two peaks in the theoretical spectrum at around 4.35 μm (2300 1568
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Figure 6. (a) Experimental and (b,c) theoretical peak emissivity as a function of the applied voltage at temperatures over the range 300−500 °C. The numbers in parentheses in legends in (b) and (c) indicate the theoretical ionization ratio η of the acceptor densities in p-GaN of 1.5 × 1019 cm−3 and 0.3 × 1019 cm−3, respectively. Inhomogeneous fluctuations of the lattice constant with a standard deviation of σ = 0.008a were assumed in the calculations of both (b) and (c).
Figure 7. (a) Modulation spectrum at a modulation frequency of 1 kHz, determined by time-resolved measurement. (b) Measured frequency characteristic of the fabricated thermal emitter. The power has been normalized by that measured at 100 Hz. The dashed line indicates the 3 dB cutoff frequency.
as the temperature is changed from 300 to 450 °C (the significant decrease of emissivity change at 500 °C in this device was possibly caused by leakage current of the p−n junction, and we excluded this data from the following discussion). For comparison, we calculated the emissivity as a function of the applied voltage considering the temperature dependence of the ionization ratio and the hole mobility in pGaN (for details, see Supporting Information). The result is shown in Figure 6b, where inhomogeneous fluctuations of the lattice constants with a standard deviation of σ = 0.008a were assumed to explain the observed peak broadening of this device. The theoretical result qualitatively agrees with the experimental result, indicating that an increase of residual emission with temperature can be explained by an increase of absorption in p-GaN. Hence, we can conclude that the residual emission in the experiment when we applied a large reverse voltage (>25 V) was mainly generated by the holes in the pGaN layer. Accordingly, we can expect that the emissivity modulation amplitude at high temperatures can be increased by reducing the free-carrier absorption in the p-GaN layer as suggested by Figure 6c (by reducing the acceptor density to 0.3 × 1019 cm−3, residue of emissivity decreases to 0.05). We note that the decrease of the sheet carrier density in the p-GaN and n-GaN layers makes the response to thermal emission slower. Therefore, there is a trade-off between the increase of the modulation speed and the modulation amplitude. Finally, we demonstrated high-speed modulation of thermal emission by applying AC voltages. Here, we utilized a device (a = 2.9 μm, r = 0.28a, h = 1.0 μm, t = 1.6 μm) which had a thicker p-layer (tp = 0.7 μm) with the same acceptor density instead of the device (tp = 0.5 μm) that achieved an emissivity
modulation amplitude (Δε) of 0.15 by applying DC voltages in Figure 5d (we could not characterize the frequency characteristics of the latter device due to the accidental corruption of the device). Narrowband (Q ∼ 50) modulation spectrum with an emissivity modulation amplitude of 0.05 was obtained in this device by the application of DC voltages at 500 °C. Figure 7a shows the modulation spectrum at 500 °C determined by a time-resolved measurement (using the step scan function of FT/IR-6600 FTIR Spectrometer, Jasco International Co., Ltd.) applying a 1 kHz rectangular wave alternating between +5 V and −20 V. In the figure, we can confirm that the modulation of the thermal emission is only obtained in a narrowband range for high-speed operation just as it was for DC operation. Moreover, we evaluated the frequency response of the modulation. For the measurement, we measured the intensity of thermal emission in the vertical direction using a HgCdTe detector and a lock-in amplifier by changing the frequency of the alternating signal applied to the emitter. The measured 3 dB cutoff frequency of the device was 50 kHz as shown in Figure 7b. In other words, an emissivity modulation amplitude of 0.05 was realized in the frequency range from 100 Hz (∼DC) to 10 kHz and it decreased by half at 50 kHz. From the result of Figure 7b, we can also estimate the 3 dB cutoff frequency of the device which realized larger emissivity modulation (Δε = 0.15) in Figure 5d to be 40 kHz, considering the fact that the modulation speed is determined by the in-plane resistances of the p-GaN/n-GaN layers, and the calculated in-plane resistance of the latter device is 20% larger than that of the former device. By increasing the sheet carrier density in the p-GaN and n-GaN layers, even faster modulation of thermal emission can be realized; however, the amplitude of 1569
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thermal tolerance. A membrane structure, in which the Si substrate underneath the photonic crystal area was removed, was realized by mechanical thinning and plasma etching.
the emissivity modulation decreases. Nevertheless, we can achieve both high-speed and large-amplitude modulation by improving the electrode design to decrease the resistance and the capacitance of our device. For instance, the modulation speed can be increased by a factor of 3 by reducing the distance between the electrodes from 0.8 mm to 0.4 mm.
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S Supporting Information *
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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsphotonics.9b00440.
CONCLUSIONS In summary, we have experimentally demonstrated the electrical control of the emissivity spectrum in narrowband GaN/AlGaN photonic crystal thermal emitters operating in the MWIR region, capable of high-temperature operation. Employing an epitaxial structure composed of GaN/AlGaN MQWs sandwiched by a p−n junction, we have realized electrical modulation of ISB absorption spectra at a wavelength of 4 μm. We have fabricated electrically controllable GaN/ AlGaN photonic crystal thermal emitters and achieved a narrowband (Q ∼ 40) emissivity modulation with DC voltages at a high temperature of 500 °C, the emissivity change of which was 0.15 at a wavelength of 4 μm. Moreover, we have investigated the temperature dependence of the emissivity modulation in the thermal emitter, and have revealed that the amplitude of emissivity modulation can be further increased by reducing the free-carrier absorption of the p-GaN layers. Furthermore, we have demonstrated the high-speed modulation of narrowband thermal emission with the 3 dB cutoff at a frequency of 50 kHz by using another emitter exhibiting an emissivity modulation amplitude of 0.05. This modulation of narrowband thermal emission is expected to contribute to the improvement of the selectivity and sensitivity in spectroscopy applications and the further development of narrowband thermal emitter applications in the MWIR region.
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ASSOCIATED CONTENT
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Additional details of the free-carrier absorption in the p−n junction and the ISB absorption, which were utilized for the calculations of Figures 2 and 6, and the coupled-mode analysis of emissivity considering both ISB absorption and free-carrier absorption (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Dongyeon Daniel Kang: 0000-0001-7778-3195 Takuya Inoue: 0000-0002-8206-8206 Present Address
# Institute for Quantum Computing, Department of Electrical and Computer Engineering, University of Waterloo, Waterloo, Ontario, N2L 3G1, Canada
Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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METHOD
ACKNOWLEDGMENTS This work was mainly supported by a Grant-in-Aid for Scientific Research (17H06125, 17K14665) from the Japan Society for the Promotion of Science (JSPS), and partially supported by NEDO (P16011) and JST CREST (JPMJCR17N3), Japan.
Numerical Calculation. To calculate the electronic state of the epitaxial structure containing the GaN/AlGaN MQWs inside the p−n junction, we used the commercial simulation software Nextnano++ (nextnano GmbH),25 which calculates the Poisson equation, drift-diffusion current equation, and Schrödinger’s equation, self-consistently. In the calculation, the spontaneous polarization, piezoelectric polarization, and deformation potential were taken into account. The lattice constant of the MQWs (a-axis) was assumed to be matched to that of free-standing GaN. The ionization energies of n-type and p-type dopants were assumed to be 20 and 170 meV, respectively. Sample Preparation. An epitaxial wafer including GaN/ AlGaN MQWs and a p−n junction was grown on Si (111) substrate by MOCVD. Photonic crystal patterns and a mesastructure were formed by using EB lithography and photolithography, respectively, where GaN/AlGaN layers were etched by chlorine-gas-based ICP-RIE with a SiO2 mask. The Mg-doped p-GaN was activated by annealing for 5 min in a N2/O2 ambient atmosphere using an RTA apparatus at 700 °C. After surface treatment using boiling NaOH, electrodes for p-GaN [Ni (10 nm)/Au (10 nm)] and for n-GaN [Ti (20 nm)/Al (20 nm)/Ni (50 nm)/Au (300 nm)] were formed by using a lift-off technique with photolithography and EB evaporation deposition. They were annealed at 500 and 700 °C for 5 min in N2/O2 and N2 ambient atmospheres, respectively. Then, the electrode for p-GaN was covered by Ti (10 nm)/Pt (40 nm)/Au (300 nm) in order to improve the
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REFERENCES
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ACS Photonics
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DOI: 10.1021/acsphotonics.9b00440 ACS Photonics 2019, 6, 1565−1571
