Transparent conductive oxide films mixed with gallium oxide nanoparticle/single-walled carbon nanotube layer for deep ultraviolet light-emitting diodes
© Kim et al.; licensee Springer. 2013
Received: 24 September 2013
Accepted: 18 November 2013
Published: 2 December 2013
We propose a transparent conductive oxide electrode scheme of gallium oxide nanoparticle mixed with a single-walled carbon nanotube (Ga2O3 NP/SWNT) layer for deep ultraviolet light-emitting diodes using spin and dipping methods. We investigated the electrical, optical and morphological properties of the Ga2O3 NP/SWNT layers by increasing the thickness of SWNTs via multiple dipping processes. Compared with the undoped Ga2O3 films (current level 9.9 × 10-9 A @ 1 V, transmittance 68% @ 280 nm), the current level flowing in the Ga2O3 NP/SWNT increased by approximately 4 × 105 times and the transmittance improved by 9% after 15 times dip-coating (current level 4 × 10-4 A at 1 V; transmittance 77.0% at 280 nm). These improvements result from both native high transparency of Ga2O3 NPs and high conductivity and effective current spreading of SWNTs.
High-brightness deep ultraviolet light-emitting diodes (UV LEDs) have attracted much attention in areas of air/water sterilization and decontamination, bioagent detection and natural light, identification, UV curing, and biomedical and analytical instrumentation . To date, the maximum external quantum efficiency (EQE) for commercialization of deep UV LEDs is 3% at the wavelength of 280 nm [2, 3]. Various reasons can account for the poor EQE, mainly such as relatively low-resistance ohmic contacts, low hole concentration in p-type AlGaN layer, and the absence of transparent conductive oxides (TCOs) electrode in the deep UV wavelength region [4, 5]. In particular, it is believed that the development of high-performance TCOs electrode in the deep UV region is a key to increase the EQE of UV LEDs. Conventionally, indium tin oxide (ITO), which exhibits high conductance and good transparency in a visible region, has been widely used as the TCOs electrodes in LEDs and solar cells [6, 7]. However, it has an opaque property in the deep UV (<300 nm) region due to a small bandgap (approximately 3.2 eV), and hence, new TCO materials need to be explored for deep UV LEDs. The wide bandgap materials such as SiO2, Si3N4, HfO2 are attractive as TCOs for deep UV LEDs because of their high transmittance in deep UV regions, but it is difficult to provide electrical conductivity into these materials. In the meantime, the gallium oxide with β phase (β-Ga2O3) having a large optical bandgap of 4.9 eV has been reported as a deep-UV TCO material  because its conductivity can be improved by thermal annealing, impurity doping, or incorporating some conducting paths using SWNTs. The Ga2O3 film has also excellent adhesion to GaN surfaces . For example, since undoped Ga2O3 film has insulating properties (i.e., conductivity (σ) <10-9 Ω-1 · Cm-1), it was doped with tin (Sn) atoms to increase the conductivity at the expense of optical transmittance. For 3 mol% Sn-doped Ga2O3 films, the conductivity was increased up to 375 Ω-1 · Cm-1 (42 Ω/square) but the transmittance decreased to approximately 15% in the deep UV region (280 nm) . In order to improve the low optical properties, several groups have reported synthesized TCO layer by wet-based nanoparticles (NPs), such as ITO, indium zinc oxide (IZO), antimony zinc oxide (AZO), antimony tin oxide (ATO), etc. [11–14]. This small particle size (i.e., NPs size), typically <30 nm, guarantees a low light scattering and thus allows a high optical quality of the materials . Unfortunately, even with some improvement of optical properties, these synthesized TCO NP layers still do not satisfy the requirement for deep UV applications due to the added dopants such as Sn, Sb, In, Ga, etc. .
In this work, we propose a TCO electrode scheme of gallium oxide nanoparticle/single-walled carbon nanotube (Ga2O3 NP/SWNT) layer, consisting of undoped Ga2O3 NPs for high transmittance and SWNT for high conductivity, for deep UV LED applications.
The surface morphology of the films was observed by a scanning electron microscope (SEM, Hitachi S-4300, Tokyo, Japan). In order to confirm the electrical properties, the sheet resistance and current-voltage (I-V) characteristics of the Ga2O3 NP/SWNT layer were measured by four-point probe method (CMT-SR1000N digital four-point testing instrument, AIT, Korea) and the semiconductor parameter analyzer (Keithley 4200-SCS, Tokyo, Japan), respectively. The optical transmission was measured using a double beam spectrophotometer (PerkinElmer, Lambda 35, Waltham, MA, USA) in the wavelength range of 280 to 700 nm.
Results and discussion
Consequently, we obtained the most uniform condition after the 6-cycle repetitive coating, as shown in Figure 3f.
We proposed and investigated the electrical and optical properties of undoped Ga2O3 NP layer combined with SWNTs by using the simple spin and dip-coating methods for deep UV LEDs. From the I-V curve characteristics, the Ga2O3 NP/SWNT layer showed a high current level of 0.4 × 10-3 A at 1 V. Compared with the undoped Ga2O3 NP layer, optical transmittance of Ga2O3 NPs/SWNT layer after 15 times of dipping was decreased by only 15% at 280 nm. By adjusting the dipping times in the Ga2O3 NP/SWNT layer, we obtained improved optical transmittance of 77.0% at 280 nm after 15 times of dip-coating processes.
This work was supported by the National Research Foundation of Korea (NRF) Grant funded by the Korean government (No. 2011–0028769).
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