We report a graphene-based transparent conductive electrode for use in ultraviolet (UV) GaN light emitting diodes (LEDs). A few-layer graphene (FLG) layer was mechanically deposited. UV light at a peak wavelength of 368nm was successfully emitted by the FLG layer as transparent contact to p-GaN. The emission of UV light through the thin graphene layer was brighter than through the thick graphene layer. The thickness of the graphene layer was characterized by micro-Raman spectroscopy. Our results indicate that this novel graphene-based transparent conductive electrode holds great promise for use in UV optoelectronics for which conventional ITO is less transparent than graphene.
© 2010 OSA
Graphene is a 2-dimensional carbon material which consists of a hexagonal array of carbon atoms. Graphene has recently been reported to be a promising material for use in nanoelectronics, sensors and optoelectronics due to its superior properties, which include its high thermal conductivity (~5000 W/m·K), fast carrier mobility (> 21,000 cm2/Vs at room temperature), high transparency (> 80%) and good mechanical stability [1–3]. Highly transparent graphene layers could be used as transparent conductive electrodes for solar cells and LEDs [4–6]. Indium tin oxide (ITO) is widely used as a transparent conductive layer in solar cells and LEDs. However, ITO is a very expensive material, unstable in acids or bases, and has poor transparency in the UV region. Weber et al. examined and compared the transmittance spectra of a graphene layer and ITO from 200nm to 800nm . The transmittance of ITO in the UV region is much lower than that of graphene, indicating that a graphene layer would be an ideal replacement for ITO in UV LED applications. In this regard, graphene is an ideal alternative because of the extraordinary properties described above. Jo et al. fabricated GaN-based blue LEDs with patterned multi-layer graphene (MLG) as the transparent conducting graphene electrodes using a chemical vapor deposition (CVD) technique . They showed the possibility of using graphene transparent conductive electrodes in LEDs although the method was very complex. The extremely high thermal conductivity of graphene is one of the key properties of this material for use in fabricating the transparent conductive electrodes of high brightness and high power LEDs. One of the critical issues in high power LEDs is the high operating temperature albeit in a highly localized area . However, the localized peak temperature of high power LEDs can better distributed by using graphene-based transparent conductive electrodes due to its extremely high thermal conductivity.
Graphene layers have been fabricated using various methods including high temperature vacuum annealing of SiC, the simple mechanical exfoliation method (i.e. the scotch-tape method), CVD and the reduction of graphene oxide [9–13]. In this study, the scotch-tape method was used to deposit the graphene layers with a variety of thickness on UV GaN-based LEDs. Since the mechanical cleavage can supply high quality FLG layers with various thickness, the electroluminescence (EL) properties can be easily compared from thin to thick FLG layers. Higher efficiency UV LEDs are needed for several applications including use in water purification systems, white illumination and optical equipment [14,15].
2. Experimental details
The UV LED was deposited in a metal-organic chemical vapor deposition system. The growth initiated with a 100nm AlN layer grown at 1010°C and 50 Torr. Subsequently, the temperature and pressure was raised to 1025°C and 150 Torr. The remaining structure consisted of 2μm GaN:Si, a AlGaN/GaN/AlGaN 8nm/5nm/8nm single quantum well (SQW), and a 200nm GaN:Mg contact layer. The FLG layer was fabricated by the simple mechanical exfoliation of highly ordered pyrolytic graphite (HOPG: ZYA grade). The graphene layer was deposited on the GaN-based UV LED with GaN SQW. The thickness of graphene obtained by the scotch method was measured by micro-Raman spectroscopy. Raman spectra of the graphene layer were collected using a 514nm Ar-ion laser. The laser power at the sample was 0.5mW. The accumulation time was 60 seconds. The micro-Raman measurements were obtained using a backscattering geometry with a JY LabRam HR equiped with a liquid-nitrogen cooled CCD detector. The back contact (Ti/Au: 40nm/100nm) was deposited by e-beam evaporator since the SiC substrate was conducting. Then, the sample was annealed by RTA at 450°C for 40 seconds in N2 condition. The emission properties were obtained using both an optical microscope and EL spectra, which were measured using an optical fiber that was connected to an Ocean Optics 2000 + . The current-voltage (I-V) data from the LEDs were obtained using Agilent 4155C parameter analyzer.
3. Results and discussion
Figure 1 shows a schematic diagram of UV LEDs on a SiC substrate with transparent conductive graphene electrode. The optical images of the graphene contact on an UV LED before and after applying the forward bias were presented in Fig. 1. The conductive SiC substrate allowed a vertical conduction path through the UV LED with bias applied to the bottom metal and top graphene layer. Mechanical exfoliation of graphene is known to yield graphene with a range of thicknesses. It is well known that an optical microscopy image can quickly locate single or FLG as was done in Fig. 2(a) . The thicknesses of the graphene layers, shown in Fig. 2(a), was confirmed by micro-Raman measurements. The circular scratch at the center of graphene layer was a mark that was formed by probe tip when the bias was applied. The Raman spectra shown in Fig. 2(b) and 2(c) were measured at three different points. The thick layer was used as a pad for the probes. The two most dominant peaks in the Raman spectra of the graphene layers were the G peak at ~1580cm−1 and the G’ peak at ~2700cm−1 . The thickness of the graphene layer can be determined by the feature of the G’ peak in the Raman spectra [16,17]. Figure 2(b) and 2(c) shows the Raman spectra of point 1, 2 and 3. The region at point 1 in Fig. 2(a) and the feature of the G’ peak at point 1 in Fig. 2(c) represent a typical HOPG. The region at point 2 and 3 were more transparent than that of point 1 in Fig. 2(a), where point 2 and 3 was determined to consist of 4 layer and 2 layers of graphene, respectively, by the feature in the G’ peak (Fig. 2(c)).
Figures 3(a) and 3(b) are optical images from same graphene with and without bias. The graphene layer in Figs. 3(a) and 3(b) are same as the graphene layer in Fig. 2(a). In Fig. 3(b), UV light in the graphene layer was emitted at a light input power of 30mW. The brightness of the EL emission in graphene layers varied according to the differences in thickness of the graphene layers which affected the UV light transmittance. The brightness of the region consisting of 2 layers of graphene was much higher than that of the region consisting of 4 layers of graphene and HOPG. Figure 3(c) shows the EL spectra of UV LEDs with graphene-based electrodes. The position of the intense peak in the EL spectra was about 368nm. The EL intensity was relatively small due to the small emission area. Figure 3(d) shows the current-voltage (I-V) characteristic of the LEDs. The current was measured by applying a voltage to the thick layer in the graphene-based transparent conductive electrodes shown in Fig. 2(a). Typical diode characteristics are presented in the I-V data. The forward voltage at an injection current of 1mA was about 26.5V.
To the best of our knowledge, this is the first report of graphene layer for use as a transparent conductive electrode in GaN-based UV LEDs. This approach can remove efficiency limitations for UV LEDs. Specifically, graphene-based transparent conductive electrodes have ultrahigh thermal conductivity and better UV transmission compared to ITO or the standard Ni/Au metallization. Furthermore, ITO with the typical thickness of 100~200nm has low thermal conductivity (11~12 W/m·K). By sharp contrast, graphene layer which has very good thermal conductivity (~5000 W/m·K) even employs a very thin layer, e.g., 1~2nm. The common design for a Ni/Au metallization consists of a grid to improve current spreading in p-GaN, which has a relatively high resistivity. The tradeoff is that the Ni/Au layer absorbs a significant percentage of the light reaching the surface of the LED. A graphene-based contact can minimized or eliminate this photon blocking effect in addition to the increase in long-term stability and reliability owing to the improved heat distribution through the graphene contact.
Graphene containing 2 layers and 4 layers was successfully fabricated using the standard scotch method as transparent conductive layer to the p-contact layer in GaN UV-LED. The thickness of the graphene layer was measured by micro-Raman spectroscopy. Based on the optical images and EL spectra, we demonstrated that graphene is an effective transparent conductive electrode for UV LEDs.
The research at Korea University was supported by New & Renewable Energy R&D program (R0903962) under the Ministry of Knowledge Economy, Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Minstry of Education, Science and Technology(2010-0008242) and BK21 program. Research at the US Naval Research Lab was supported by the Office of Naval Research.
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