Large area deep ultraviolet light of Al 0.47 Ga 0.53 N/Al 0.56 Ga 0.44 N multi quantum well with carbon nanotube electron beam pumping

Large area deep ultraviolet (DUV) light is generated by carbon nanotube (CNT) cold cathode electron beam (C-beam) irradiation on Al 0.47 Ga 0.53 N/Al 0.56 Ga 0.44 N multi quantum wells (MQWs) anode. We developed areal electron beam (EB) with CNT cold cathode emitters. The CNT emitters on silicon wafer were deposited with an area of 188 mm 2 , and these were vertically aligned and had conical structures. We optimized the C-beam irradiation conditions to effectively excite AlGaN MQWs. When AlGaN MQWs were excited using an anode voltage of 3 kV and an anode current of 0.8 mA, DUV with a wavelength of 278.7 nm was generated in a large area of 303 mm 2 . This DUV area is more than 11 times larger than the light emitting area of conventional EB pumped light sources and UV-LEDs.

Electron beam (EB) pumped deep ultraviolet (DUV) light sources and UV light emitting diodes (UV-LEDs) are emerging as alternative UV lighting technologies of excimer lasers and mercury vapor lamps. UV-LEDs have been extensively studied, but their efficiency is low at sub 280 nm due to DUV absorption and lattice mismatch in AlGaN materials. [1][2][3][4][5][6][7] One of the alternative techniques to solve these problems is EB pumped DUV light source. Many research groups are working on EB pumping technology for the DUV generation. [7][8][9][10][11][12] In 2009, K. Watanabe et al. investigated EB pumping on the hexagonal boron nitride (hBN) powder to obtain DUV emission at a wavelength of 225 nm. 8 Also in 2010, Oto et al. reported EB pumping technique with AlGaN multi quantum wells (MQWs), demonstrating high output power of 100 mW. 9 Since then, AlGaN MQWs have been widely used as fluorescent materials with EB pumping for DUV generation. [10][11][12] Most studies have been performed using thermal electron emitters as an electron source. The conventional technology, in which uses thermal electron source for EB excitation, requires warm-up time to heat a tungsten filament to emit electrons. In the thermal electron sources, the total power efficiency of the device is significantly low due to power consumption during the warm-up time. The electrons are emitted in all directions, 360 degrees throughout the thermal electron source. However, almost all applications require the electrons to be in the surface normal direction of the anode, since electrons of other directions not utilized. Also, it is very difficult to make flat areal EB for large area DUV lighting. On the other hand, since cold cathode does not require warm-up time and it is easy to make large area electron emitters, cold cathodes can be used as various devices of higher efficiency than thermal electron sources.
Carbon nanotube (CNT) emitters have excellent properties and many groups are researching CNT emitters for vacuum nanoelectronics applications. [13][14][15][16] Previously, we used CNT emitters as a cold cathode to generate DUV light with homemade Zn 2 SiO 4 anode. 16 CNT emitters were grown on silicon substrate with direct current plasma enhanced chemical vapor deposition (DC-PECVD). 17 Enhanced electron emission properties are attributed to the vertically aligned structure of CNTs which is due to the growth by DC-PECVD and these CNTs are suitable for electron sources ARTICLE scitation.org/journal/adv of DUV lighting. 16 As a large area EB irradiation technology, we introduce carbon nanotube cold cathode electron beam (C-beam) pumping technique. The DUV generation with EB pumping technology has limitations in lighting area, as of today the DUV light emission area is limited to less than 30 mm 2 . 8,12 C-beam pumping technology can produce a flat areal UV lamp having a large light emitting area. In the previous work, we fabricated a visible lamp with light emitting area of 25.4 cm 2 using C-beam irradiation on anode phosphor. 18 The light emitting areas depend on the structure of C-beam and the area of the CNT emitters. In this letter, we report the fabrication of large area DUV light with Al 0.47 Ga 0.53 N/Al 0.56 Ga 0.44 N MQWs and the C-beam irradiation pumping technique.
DUV light with Al 0.47 Ga 0.53 N/Al 0.56 Ga 0.44 N MQWs are generated with C-beam of triode structure. Fig. 1 shows schematic of UV lighting system with C-beam pumping. The triode structure of Cbeam irradiation is composed of cathode, gate, and anode as shown in Fig. 1(a). C-beam consists of a cathode as an electron source and a gate for electron extraction. As a cold cathode, the vertically aligned high aspect ratio CNT structures are needed for high field enhancement and low threshold voltage for electron emission. The fabricated CNT emitters are shown in the Fig. 1(b), with a height of 40 µm, a tip apex diameter of 60 nm and a CNT bottom contact diameter of 3 µm. CNT emitters are grown on Si wafer by an area of 188 mm 2 and are separated by 30 µm distance to reduce the field screening effect. The gate electrode was used to extract the electrons from the CNT emitters. The distance between the gate and the CNT emitters was kept at 250 µm by using ceramic insulator. The electron emission characteristics of C-beam is shown in the Fig. 1(c). The cathode current is the current of electrons emitted by the CNT emitters, a key element of the C-beam, and the anode current is the amount of electrons reaching the anode after passing through gate electrode. The gate current is the amount of electrons absorbed in the gate which is also called as the gate leakage current. At the same cathode current, low gate current and high electron transmission rate would result in higher anode current. Also, the power efficiency of the DUV light would be enhanced. In this study, we applied 5 kV at the anode and 1.05 kV at the gate then we obtained the anode current of 1.53 mA. Under this driving condition, we obtained an electron transmission rate of 88.5%, which 11.5% is the cathode current leak through the gate electrode.
The AlGaN MQWs anode was fabricated on c-plane sapphire substrate through high-temperature metal-organic chemical vapor deposition (HT-MOCVD) system. First, the low temperature AlN buffer layer with 25 nm was grown on on-axis sapphire substrate. Then, the 2.2 µm AlN main layer was grown on the AlN buffer layer. AlGaN/AlN superlattice layers (SLs) and n-AlGaN layer were inserted to control the crack and the dislocations associated at the interface between AlN layer and the active region. Finally, the active region consisted of five period Al 0.47 Ga 0.53 N/Al 0.56 Ga 0.44 N was grown on n-AlGaN layer. The detailed growth conditions are described in Refs. 5 and 19.
The shapes of CNT emitters were analyzed using a scanning electron microscope (SEM, Hitachi S-4700). The thin film characteristics of AlGaN MQWs were confirmed by reciprocal space mapping (RSM) measurement using high-resolution X-ray diffraction (HR-XRD, Panalytical X'Pert Pro) and photoluminescence (PL) measurement using a 213nm pulsed laser (CrysLaS Laser Systems, Q-Series). Avaspec-ULS2048 UV spectrometer (Avantes) was used to identify DUV light generated by C-beam pumping. We used Agilent 34401A, Keithley 248 and Spellman SL1200 systems for power supply and current measurements.
The bandgap energies of GaN and AIN are 3.42 and 6.20 eV at room temperature respectively, and the bowing parameter is b = 0.98 eV. Calculated by substituting x = 0.47 used in MQWs in Equation (1), theoretically, the band gap energy of MQWs is 4.48 eV. It could be confirmed that the MQWs is fabricated as designed with a value similar to the band gap energy of the peak wavelength obtained by the PL measurement, 4.51 eV.
To efficiently excite AlGaN MQWs when using the C-beam pumping technology, proper penetration depth should be studied with variable anode voltage (V A ). Numerous researchers have obtained appropriate penetration depths with various V A through simulations and experiments. [8][9][10][11][12] The penetration depth with V A values depend on the electron beam source and anode structure, but V A below 10 kV is required when the thickness of active layer is less than 100 nm. 10,11 In order to perform C-beam pumping on the Al 0.47 Ga 0.53 N/Al 0.56 Ga 0.44 N MQWs anode, the penetration depth of electrons according to V A was investigated. Fig. 3 shows UV luminescence characteristics with various V A . The anode current (I A ) is fixed at 0.2 mA to reduce anode heating. Fig. 3(a) shows the UV spectra with various V A . When V A is 3 kV, a single peak with a wavelength of 278.7 nm is observed. As the V A increased, the UV intensity was slightly enhanced with the increased input power. When V A increases more than 4 kV, new emission peak is observed near the wavelength of 320 nm. Fig. 3(b) shows the peak wavelength and the irradiance with various V A . The peak wavelength increases with V A up-to 5kV, after which becomes saturated. This is because of the change in peak wavelength due to an increase in peak intensity at a wavelength of about 320 nm, not due to a change in peak at a wavelength of 278.7 nm. We fixed the V A at 3 kV to produce a single peak DUV light similar with the PL spectra in Fig. 2(b). Fig. 4 shows the characteristics of UV emission with various I A . Fig. 4(a) shows that a single peak with a wavelength of 278.7 nm occurs regardless of the change in I A . The peak wavelength and FWHM with I A are shown in Fig. 4(b), and the irradiance variation are shown in Fig. 4(c). Even change in I A , the peak wavelength and FWHM is maintained at about 278.7 nm and 27 nm, respectively. However, the irradiance as shown in Fig. 4(c) increases with I A . When I A is greater than 0.8 mA, the irradiance is saturated to 0.11 mW/cm 2 . When compared to Fig. 3, the peak at the wavelength near 320 nm was not detected in Fig. 4 at V A of 3 kV with I A up to 1.0 mA. In Fig. 3, we found 320 nm UV peak at the V A of 4 kV and I A of 0.2 mA. In this driving condition, the power delivered on the anode is 0.8 watt. However, in the case of V A of 3 kV, we could not find 320 nm UV peak up-to anode power of 3 watt. From this comparison, the origin of the 320 nm peak is not due to heat effect of the power but the UV peak near the 320 nm wavelength had appeared from defect states in MQWs.
Based on these studies, we tried to make flat areal DUV light with a large light emitting area. Fig. 5 shows the light emission image generated by C-beam pumping on AlGaN MQWs. In order to generate a single peak with a wavelength of 278.7 nm, the C-beam irradiation condition was maintained at 3 kV of V A and 0.8 mA of I A . As can be easily seen from the eye, we found visible light of blue color and the emission area is well-overlapped with UV emission area as confirmed by a UV spectrometer. The blue light comes from cathodoluminescence by the excitation of defect states in MQWs with C-beam irradiation. The UV light emission area of 303 mm 2 was obtained by image analysis using the image J program, as shown in Fig. 5(b). The UV lighting area is 1.6 times larger than CNT emitter area of 188mm 2 . S. I. Inoue et al. fabricated DUV-LEDs with mesa size of 0.35 mm 2 . 6 In the case of EB pumping, K. Watanabe et al. obtained the highest emission area of 27.2 mm 2 using EB pumping on the hBN powder. 8 The DUV light emitting area of ∼303 mm 2 we obtained using C-beam pumping is 11 times larger than the previous studies.
In conclusion, large area DUV light was successfully obtained by using C-beam irradiation pumping technique on the Al 0.47 Ga 0.53 N/Al 0.56 Ga 0.44 N MQWs anode. We fabricated 188 mm 2 CNT emitters on Si wafers to obtain large area DUV light. The C-beam shows high electron transmittance through the gate electrode of 88.5%. Al 0.47 Ga 0.53 N/Al 0.56 Ga 0.44 N MQWs, which were

ARTICLE
scitation.org/journal/adv prepared by HT-MOCVD on c-plane sapphire substrate. With optimization of anode bias and current, we obtained a large area DUV light of 303 mm 2 with a wavelength of 278.7 nm. At this time, C-beam was irradiated at the V A of 3 kV and I A of 0.8 mA. Largearea DUV light sources using C-beam pumping open up the potential of UV devices used in a variety of fields such as disinfection, deodorization, photo catalysis, and gas detection.
This work was supported by Radiation Technology R&D program through the National Research Foundation of Korea funded by the Ministry of Science and ICT (NRF-2017M2A2A4A01020658) and the BK21 Plus Program (future-oriented innovative brain raising type, 21A20130000018) funded by the National Research Foundation of Korea (NRF).