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We report the successful fabrication of a compact deep ultraviolet emission device via a marriage of AlGaN quantum wells and graphene nanoneedle field electron emitters. The device demonstrated a 20-mW deep ultraviolet output power and an approximately 4% power efficiency. The performance of this device may lead toward the realization of an environmentally friendly, convenient and practical deep ultraviolet light source.
©2012 Optical Society of America
1. Introduction
Compact and highly efficient deep ultraviolet (DUV) light sources such as light-emitting diodes (LEDs) and laser diodes are of considerable technological interest as alternatives to large, toxic, low-efficiency excimer lasers and mercury lamps. Wide bandgap materials such as diamond [1], boron nitride [2–4] and III–V nitride semiconductors have potential for the realization of ultraviolet LEDs and laser diodes. Specifically, AlxGa1-xN alloy has attracted significant attention due to its wide emission range, from 210 to 365 nm, which is achieved by changing the molar fraction, x. Additionally, it has the added benefit that it can be produced using the same growth techniques and materials as the widely used InGaN-LED technology [5]. However, the emission efficiency is still low for AlGaN-LEDs with wavelengths of less than 300 nm [6–13] due to the high resistivity of the p-type AlGaN layer as well as the low hole concentration and high density of threading dislocations in AlGaN materials. One promising technique to avoid these problems related to the epitaxial growth of the AlGaN layer is the use of electron beam (EB) excitation. Several groups have proposed a promising alternative—EB-pumped nitride semiconductor light sources [14,15]. As recently reported by Oto et al. [14], DUV emission from EB-pumped AlGaN quantum wells resulted in a high output power of 100 mW and a high power efficiency, exceeding 40%. The physical mechanisms of efficient cathodeluminescence (CL) from AlGaN quantum wells have not yet been clarified; nevertheless, this report broadens the possible applications of AlGaN from electron and hole injection to vacuum nanoelectronics.
The above characteristics (power and efficiency) were examined in a vacuum chamber with a thermal electron source. If the EB excitation is generated by a thermal cathode, the total power efficiency of the device is significantly reduced due to the consumption of electric power at the thermal filament. To avoid electric power losses at the thermal filament, it is necessary to use a field emission cold cathode. The drawback of field emission electron sources for practical use is the requirement of vacuum systems at a low residual pressure of less than 10−7 Pa.
We have achieved stable, high-brightness field emission from a novel nanoneedle cathode using a two-dimensional (2D) graphene sheet structure at a high residual pressure, approximately 10−4 Pa [16,17]. The unsaturated behavior of stable electron emission under high pressure distinguishes graphene nanoneedle (GN) field emitters from other types of field emitters such as carbon nanotubes. The 2D graphene structure enables the high performance of electron field emission due to quantum relativistic effects because the carrier mobility and electron mass have exceptionally large and small values, respectively [18–20]. We have demonstrated the performance of GN cold cathodes via various applications, such as the fabrication of a miniature X-ray tube [21] and a field emitter for a scanning electron microscope (SEM) with a convenient vacuum system [13]. The X-ray tube boasts a long lifetime of approximately 10,000 hours and a stability of better than 1% [21].
Here, we report the successful fabrication of a compact DUV emission device via the combination of a highly efficient CL layer using AlGaN quantum wells and GN field electron emitters. Because of its low current consumption during operation, the device can be handheld and driven by AA dry batteries. The device has a lamp size with a diameter of 30 mm and a length of 60 mm; the power unit has a width of 100 mm, a height of 35 mm, and a length of 150 mm. The device has a 20-mW DUV output power and a power efficiency of approximately 4%. In addition to demonstrating a compact DUV emission device, we propose the physical mechanism behind the CL efficiency of the layer of AlGaN quantum wells by interpreting the experimental results of the CL intensity versus the EB energy. Non-intuitively, excitons generated in the thick, 15 μm AlN layer, which are far away from the thin, 60 nm AlGaN quantum well layer, condense and fall into the quantum wells. This finding is very promising for the fabrication of not only an incoherent light source but also a DUV laser source. Using vacuum nanoelectronics technology, the marriage of III-V nitride semi- conductors and graphene field emitter paves the way toward the realization of a sophisticated DUV light source.
2. Epitaxial layer structure and optical properties
The left inset of Fig. 1 shows the schematic of the sample’s structure. AlxGa1-xN/AlN samples were grown via high-temperature metal-organic vapor phase epitaxy (HT-MOVPE) on a c-plane sapphire substrate. A high-quality 15-μm AlN layer was initially grown on the sapphire substrate at 1300 °C by HT-MOVPE using trimethylaluminum (TMA) and ammonia as precursors. Next, AlGaN multiple quantum wells (MQWs) were grown by HT-MOVPE by supplying trimethylgallium (TMG) with TMA and ammonia at 1000 °C. The MQWs were composed of 10 periods of alternating AlxGa1-xN/AlN layers with a stoichiometric x of 0.7. The AlxGa1-xN quantum wells and the AlN barrier layers were each 3 nm thick, yielding a 60-nm-thick quantum well region. The dislocation density (DD) of the samples was 1.8 × 108 cm−2, determined by the full widths at half maximum (FWHMs) of X-ray rocking curve (XRC) ω-scans. The DD was calculated using the (0002) and (1012) diffraction peaks and the following formula: ρ = β2/(4.35|b|2), where ρ is the DD, |b| is the magnitude of the Burgers vector, and β is the FWHM of the XRC [22].
Fig. 1 The left inset displays a schematic of an AlxGa1-xN/AlN multiple quantum well structure grown on a c-plane sapphire substrate. The multiple quantum wells were composed of 10 periods of AlxGa1-xN/AlN layers with a stoichiometric x of 0.7. The AlxGa1-xN quantum wells and the AlN barrier layers were each 3 nm thick, yielding a 60-nm-thick quantum well region. The main figure shows a photoluminescence spectrum of the Al0.7Ga0.3N multiple quantum wells obtained at room temperature with an ArF excimer laser. The right inset shows an internal quantum efficiency curve as a function of carrier density obtained by the Shockley-Reed-Hall model. The dislocation density was assumed to be 1.8 × 108 cm−2 based on 2θ/ω-scan X-ray diffraction measurements.
To investigate the quality of the AlGaN/AlN MQW layer, photoluminescence (PL) measurements were performed at room temperature. Figure 1 shows the PL spectra with a peak emission wavelength of 242 nm. There is no emission observed near 210 nm, indicating that the carriers generated in the AlGaN/AlN layers are captured and confined in the Al0.7Ga0.3N quantum wells. Therefore, high internal quantum efficiency (IQE) is expected at room temperature. We characterized the IQE of the MQWs by excitation density-dependent PL measurements using the Shockley-Reed-Hall (SRH) model [23]. The DD of the fabricated samples was 1.8 × 108 cm−2, and the IQE curve as a function of carrier density (excitation power) is shown in the right inset of Fig. 1. For higher density excitation regions, such as that of the ArF excimer laser, the IQE is more than 0.8. The generated carrier density is estimated to be 1018-1019 cm−3 for the EB excitation; therefore, the IQE of the MQWs is higher than 0.6.
3. Fundamental cathodeluminescence properties
High-energy electrons irradiated onto the thin MQW layer penetrate into the AlN layer; therefore, excitation occurs not only at the MQW layer but also at the AlN layer. However, lower energy electrons do not sufficiently penetrate the MQWs. To determine the optimum excitation energy for a given quantum well structure, we simulated numerous electron trajectories in our MQW structure using different acceleration voltages (Va) with a Monte Carlo method (software ‘CASINO’, available at http://www.gel.usher brooke.ca/casino) [24]. Figures 2(a) and 2(b) show contour plots of the electron energy as a function of MQW depth for (a) Va = 3 kV and (b) Va = 10 kV. A beam radius of 10 nm was applied for the simulation. The purple region shows the 60-nm-thick Al0.7Ga0.3N/AlN layer, and the blue color region shows the 15-μm-thick AlN layer. For the MQW layer, a Va of 3 kV (Fig. 2(a)) is a sufficiently high excitation voltage to obtain an efficient CL. Almost none of the electrons at Va = 10 kV attenuate in the MQW layer, but they do attenuate in the AlN layer; therefore, both the intensity and the efficiency are reduced when the acceleration voltage is more than 2-3 kV. For electron energies between 0 and 15 keV, we theoretically analyzed the CL intensity as a function of incident electron energy by quantitatively evaluating the absorbed energy at the MQW layer using the Monte Carlo method; the result of the theoretical curve is shown by the dotted line in Fig. 2(d). The simulation therefore indicates that a Va between 2 and 4 kV is optimum for efficient excitation of the MQW layer.
Fig. 2 Contour plots of the electron energy as a function of the Al0.7Ga0.3N multiple quantum well depth (the vertical direction) for (a) Va = 3 kV and (b) Va = 10 kV obtained by the Monte Carlo method. The purple region is the 60-nm-thick (10 periods) Al0.7Ga0.3N (3 nm)/AlN (3 nm) multiple quantum well layer, and the blue color region is the 15-μm-thick AlN layer. The radius of the incident electron beam is 10 nm. The percentages (degradation of the red color region) indicate the electron energy with respect to the initial value. (c) Cathodeluminescence spectrum as a function of the excitation energy of the electron beam for 3 keV (green line), 5 keV (blue line), and 10 keV (red line). The main emission peak at approximately 240 nm is the band-edge emission of the multiple quantum well layer, and the peak at approximately 210 nm observed for an excitation energy of 10 keV is the band-edge emission from the AlN layer. The broad emission at approximately 350 nm observed for all excitation energies is the deep level and/or defect state emission of the multiple quantum well layer, and the emission peak near 330 nm observed for an excitation energy of 10 keV is the defect state emission of the AlN layer. (d) Observed cathodeluminescence intensities (red circles: 1 nA, blue circles: 0.1 nA) as a function of incident electron energy. The dotted line is the theoretically obtained cathodeluminescence intensity as a function of incident electron energy, where the cathodeluminescence intensity is assumed to be proportional to the absorbed energy in the multiple quantum wells. The solid line is the theoretically fitted line for the results of the cathodeluminescence intensity as a function of incident electron energy based on the diffusion and trapping processes of excitons generated in the multiple quantum wells.
To confirm the above Monte Carlo simulation, we performed excitation energy-dependent CL spectrum measurements. The CL measurements were performed by using an SEM apparatus equipped with a spectrometer. Figure 2(c) shows the CL spectrum as a function of the excitation energy of the EB. At the lower energy excitation region of Va = 3 kV, the EB penetrated into the MQW layer, and electrons were absorbed; therefore, the band-edge emission of the MQW layer was observed at a peak wavelength of approximately 240 nm. In addition to the band-edge emission, deep level emission and/or defect state emission of the MQWs at approximately 350 nm was observed in the CL spectrum. With a higher excitation energy of 5 keV, the electrons excite the entire 60-nm MQW region; therefore, CL emission occurs not only from the surface layer but also from the interface between the AlN and MQW layers. In this region, the densities of deep level and/or defect states are relatively high compared to that of the surface of the MQWs. The emission peak at approximately 330 nm is due to the defect state emission of the AlN layer. As the excitation energy was increased to 10 keV, the band-edge emission originating from the AlN layer was observed at approximately 210 nm.
The CL intensities (red circles: 1 nA, blue circles: 0.1 nA) are linearly proportional to the electron energy for values below 10 keV, as shown in Fig. 2(d). The CL intensities are linearly proportional to the excitation current. The Monte Carlo simulation prediction is shown by the dotted line for the DUV intensity as a function of the electron excitation energy; however, the CL measurements show that the intensity saturation behavior was observed far beyond the theoretically predicted value of 2 keV, up to 10 keV. The difference between the Monte Carlo simulation (dotted line) and the experimental results (red and blue circles) can be explained by the excitons generated at the AlN layer diffusing into the lower energy MQWs. By introducing the dead voltage due to electron irradiation in a high residual pressure SEM system [25] and by considering the diffusion process of excitons generated at the AlN layer being captured in the MQWs, the CL intensity (I) can be theoretically described as follows: I = α × (V-V0)q × (Ddif)3/Vexc for the (Ddif)3/Vexc<1 region and I = α × (V-V0)q for the (Ddif)3/Vexc≧1 region, where Ddif is the diffusion length of the excitons; Vexc is the excitation region of the electron irradiation; V0 is the dead voltage; q is the power index, which is generally assumed to be unity [25]; and α is a linearly proportional fitting parameter. It is possible to fit the experimental results by using the theoretically derived line, shown in Fig. 2(d) as a solid line, where the dead voltage and diffusion length are assumed to be V0 = 2.2 kV and Ddif = 1.2 μm, respectively.
We can neglect the re-absorption of the band-edge emission of the AlN layer because the IQE of the AlN layer is relatively low, on the order of 5% at room temperature [26]. Therefore, we consider that the highly efficient CL from the AlGaN MQW layer [14] originates from the exciton-diffusion and exciton-trapping processes. The discrepancy in the spectra between the ArF excitation PL measurements and the EB excitation CL measurements is due to the difference between the respective absorption coefficients; the absorption coefficient of a 193-nm PL excitation is large, on the order of 106 cm−1 [27,28]. Therefore, the PL emission probes the high-quality surface region of the MQW layer, whereas the CL emission probes the surface and the interfaces of the MQW layer, where the deep level emission is large compared to the surface region. This emission spectrum change as a function of depth was also quantitatively observed in the excitation energy dependence of the CL measurements. The intensity of the deep level emission is lower than that of the MQW emission for lower excitation energies.
4. Electron microscopy images of a graphene nanoneedle field emitter
In addition to the importance of achieving a highly efficient CL layer to construct a compact DUV emission device, it is also important to fabricate a field electron emitter with stable emission under high residual pressure. Among many types of field emitters, the 2D GN structure is one of the most promising candidates for the field emission of electrons because it has many advantages, including operation at high residual pressures on the order of 10−4 Pa, a long lifetime on the order of 10,000 hours, and a high brightness on the order of 1013 Asr−1m−2 [16,17,21]. Therefore, we believe that the GN structure is suitable as a convenient EB excitation source for the fabrication of a compact DUV emission device.
The GN field emitters were fabricated via hydrogen plasma etching of a carbon rod in a microwave plasma chemical vapor deposition chamber [21]. A transmission electron microscopy (TEM) image of a nanoneedle is shown in Fig. 3(a) . The typical aspect ratio of the nanoneedle is on the order of 1,000, and the radius of curvature in the top region of the needle is less than 5 nm. The small radius and high aspect ratio make the nanoneedle suitable as a field electron emission cathode. A high-resolution TEM image of the nanoneedle is shown in Fig. 3(b), where a lattice fringe pattern is shown to extend from the bottom to the top of the needle. Based on the lattice fringe and diffraction patterns (c-axis) shown in Fig. 3(c), the top region of the nanoneedle consists of bilayers or trilayers of 2D graphene sheets with an interplanar spacing of 0.36 nm. This spacing is larger than that of the hexagonal graphite structure (0.34 nm), which indicates that the c-axis lattice is relaxed. Another diffraction pattern (a-axis), whose direction is orthogonal to the interplanar direction, is observed. Based on the distance of the a-axis diffraction patterns, we can determine the atomic level of spacing to be 0.21 nm. This value corresponds to the (010) plane spacing of the six-member ring in the graphene sheets. The interaction between the graphene sheets at the top region is weakened due to the larger interplanar lattice spacing; thus, the GNs possess 2D graphene characteristics instead of 3D graphite characteristics. In addition to the weakened interplanar interaction, the GN containing lattice fringes from the bottom to the top demonstrates promising field emission capabilities. Due to quantum relativistic effects, the carrier mobility and electron mass have exceptionally large (μ = 15,000 cm2 V−1 s−1) and small (0.007 me, me: free electron mass) values, respectively [18–20].
Fig. 3 (a) Transmission electron microscopy image of a single graphene nanoneedle. (b) High-resolution transmission electron microscopy image of a single graphene nanoneedle. (c) Selected area electron diffraction pattern. Based on the spacing of the c-axis diffraction patterns, the structure of the needle was determined to be a two-dimensional graphene sheet with an interplanar spacing of 0.36 nm. Based on the spacing of the a-axis diffraction pattern, the (010) plane spacing based on a six-member ring was determined to be 0.21 nm.
5. Deep ultraviolet emission device structure and cathode lifetime
Figure 4(a) shows a schematic of a diode-type field emission DUV tube composed of a GN cold cathode and an Al/AlGaN/AlN/sapphire DUV emission target. The DUV glass tube was evacuated by a turbomolecular pump to a base pressure of 10−7 Pa, and then, the glass tube was chipped off. For DUV emission, a negative voltage of 3-10 kV was applied to the cathode. Electrons accelerated at this voltage were irradiated onto the Al/AlGaN/AlN/sapphire target. A 30-nm-thick Al metal back layer was deposited onto the AlGaN MQW layer in order to (i) reduce the anode resistivity, (ii) reflect the backscattering component of DUV emission frontward, and (iii) reduce the build-up potential due to a high density of electron irradiation in vacuum.
Fig. 4 (a) The deep ultraviolet emission tube is composed of a graphene nanoneedle cold cathode and an Al/AlGaN/AlN/sapphire deep ultraviolet emission target. The electrons emitted from the graphene nanoneedle cold cathode excite the AlGaN multiple quantum well layer, causing the multiple quantum wells to emit deep ultraviolet fluorescence. This deep ultraviolet emission is extracted through a sapphire window for which the back of the substrate (output side) is roughened by a mechanical process to enhance the deep ultraviolet emission extraction efficiency. (b) Lifetime test of the graphene nanoneedle cold cathode at a constant current of 500 μA. No degradation in the anode bias voltage is observed, and the lifetime of the graphene nanoneedle cold cathode exceeds 5,000 hours.
The electrons emitted from the GN cathode excited the AlGaN MQWs, causing the MQW layer to emit DUV fluorescence. The DUV emission was extracted through the back of the substrate (output side), where the sapphire window was roughened by a mechanical process to avoid total reflection at the substrate/air interface. We confirmed that the roughened surface enhanced the DUV light extraction efficiency by 50%. The total DUV power through the sapphire substrate was measured by using a thermal optical power meter featuring a flat response over a wide wavelength range (190 nm to 25 μm) and a resolution of 0.2 mW.
Figure 4(b) shows the lifetime test results of the GN cathode conducted at a constant current of 500 μA. We observed no degradation in the anode bias voltage in maintaining the constant current, and the lifetime of the GN exceeded 5,000 hours. The lifetime testing of the field emitter is currently being investigated and it is on target to exceed 10,000 hours. Both a high emission current of approximately 1 mA and a lifetime of more than 10,000 hours make this device practical for commercial use.
6. Device appearance and operation state
Based on laboratory tests for the AlGaN MQW CL layer, the operation condition of the electric power supply requires a cathode voltage of 3-10 kV and a cathode current on the order of 100 μA. The cathode current of the device is low; therefore, it is possible to construct a handheld DUV light source with a small power source. Figure 5(a) shows a photograph of a battery-driven DUV light source. The power supply operates at a cathode voltage range of 0-10 kV and a cathode current range of 0-500 μA. The power is obtained by using DC-DC converters driven by AA dry batteries in series. The dimensions of the DUV tube and the power supply are 30φ × 60 mm and 100 mm × 35 mm × 150 mm, respectively. The total weight of the DUV light source with the power supply is approximately 650 g, which is suitable for portable applications such as purification and sterilization.
Fig. 5 (a) Photograph of a battery-driven deep ultraviolet emission device. The power supply operated at a cathode voltage range of 0-10 kV and a cathode current range of 0-500 μA, which is obtained by using DC-DC converters driven with AA dry batteries in series. The dimensions of the deep ultraviolet tube and the power supply are 30φ × 60 mm and 100 mm × 35 mm × 150 mm, respectively, and the total weight of the deep ultraviolet light source with the power supply is approximately 650 g. (b) Photoluminescence image of a Y2O3:Eu phosphor irradiated by deep ultraviolet emission under a cathode voltage of 7.5 kV and a cathode current of 80 μA. The inset is a photoluminescence-excitation spectrum of the Y2O3:Eu phosphor, where absorption occurs from the shorter excitation wavelengths of less than 300 nm. The obtained power and efficiency are approximately 20 mW and 4%, respectively.
Figure 5(b) shows the PL image of a Y2O3:Eu phosphor irradiated by DUV emission from our device under a cathode voltage of 7.5 kV and a cathode current of 80 μA. The device shows the same DUV emission characteristics as those observed by using an SEM apparatus discussed in the section 3. The PL-excitation spectrum of this phosphor, displayed in the inset of Fig. 5(b), shows that the 610-nm PL originates from the shorter excitation wavelengths of less than 300 nm, which is direct evidence of 240-nm DUV emission. The obtained maximum power and efficiency of this DUV device are approximately 20 mW (7.5 kV, 80 μA) and 3-4%, respectively. Both of these values are higher than those reported for 250 nm-DUV AlGaN light-emitting diodes [10].
7. Discussion and conclusion
Based on the discussion in the literature about energy loss processes of CL [25], the following losses can be considered: (i) backscattering (reflection) of the incident EB; (ii) the minimum ionization energy (secondary electron production energy) of the bandgap, which is correlated with the stopping power [29]; (iii) the Stokes shift between the bandgaps of the AlN and MQW layers; and (iv) the dead layer at the surface region. First, the backscattering process causes energy losses for the Al0.7Ga0.3N MQW layer, and the backscattering is correlated with the average atomic number. By using Tomlin’s relation between the backscattering coefficient (ηB) and the atomic number (Z), ηB = 1/6 × (lnZ-1.5) [30], for the Al0.7Ga0.3N MQW layer, the average atomic number is 12.7 and almost 17% of the incident EB is reflected. Second, in order to create an electron-hole pair by EB irradiation, the energy of the electron (Ei) must be γ times larger than the energy of the bandgap (Eg); in other words, Ei = γEg, where γ depends on the crystal lattice structure and ranges from 2 to 4 [31]. For example, γ is 3.5 for group IV semiconductors and 2 for I-VII compounds. The value of γ for the Al0.7Ga0.3N MQW layer is not yet clearly understood; however, here, we use γ = 2 in order to estimate the maximum efficiency of the CL process. Third, the energy loss due to the Stokes shift (ηs) is estimated to be 13% based on the energy difference between the Al0.7Ga0.3N MQW layer (5.16 eV) and the AlN layer (5.9 eV). The energy loss due to the dead voltage can be ignored because it can be reduced by introducing a metal back process. The total theoretical efficiency (ηt) of EB irradiation onto the Al0.7Ga0.3N MQW layer can be expressed as ηt = ηint × (1-ηb) × (1/γ) × (1-ηs), where ηint was determined to be 0.7 by our IQE measurements as a function of EB excitation density (1018-1019 cm−3), as shown in Fig. 1. By substituting ηint = 0.7, ηb = 0.17, ηs = 0.13, and γ = 2, ηt becomes 0.25, assuming that the light extraction efficiency is 1. We consider that the difference between the theoretical efficiency and the experimental efficiency originates from the light extraction efficiency. Therefore, the total maximum efficiency of EB excitation for the MQW layer can be increased from 4% to 25% by increasing the light extraction efficiency. To obtain much higher power efficiency more than 25%, it is necessary to increase the excitation density and to avoid multi-reflection and re-absorption of the DUV emission inside the AlN/AlGaN/Al layers.
In summary, we have demonstrated the successful fabrication of a compact deep ultraviolet emission device via the combination of a highly efficient AlGaN-quantum-well cathodeluminescence layer and graphene nanoneedle field electron emitters. We obtained a 20-mW deep ultraviolet output power and an approximately 4% power efficiency from the device. The marriage of III-V nitride semiconductors and graphene field emitters will open up new photochemical and biotechnological applications such as sterilization, modification of chemical substances, and photolithography.
Acknowledgment
We thank T. Nakamura, Y. Onizuka (Onizuka Glass Co. Ltd.) for technical assistance, and Y. Honda (Nagoya University) for helpful discussions.
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