InGaN-based light emitting diodes (LEDs) with a top nano-roughened p-GaN surface are fabricated using self-assembled CsCl nano-islands as etch masks. Following formation of hemispherical GaN nano-island arrays, electroluminescence (EL) spectra of roughened LEDs display an obvious redshift due to partial compression release in quantum wells through Inductively Coupled Plasma (ICP) etching. At a 350-mA current, the enhancement of light output power of LEDs subjected to ICP treatment with durations of 50, 150 and 250 sec compared with conventional LED have been determined to be 9.2, 70.6, and 42.3%, respectively. Additionally, the extraction enhancement factor can be further improved by increasing the size of CsCl nano-island. The economic and rapid method puts forward great potential for high performance lighting devices.
© 2011 Optical Society of America
III-nitride wide bandgap light-emitting diodes (LEDs) have recently attracted considerable interest due to their various applications, such as traffic signals, back lighting in liquid crystal display, and white-light LED lighting. However, the majority of applications are currently hindered by the low extraction efficiency and Lambertian-like radiation profile caused by the large difference in the refractive index between LED die and external medium [1,2]. In this case, the critical angle for the light generated in the InGaN/GaN multi-quantum well (MQW) layers to escape is approximately 23°, and only a small fraction of light can be extracted from active region. Most photons are trapped inside the LED device by total internal reflection (TIR) and converted to heat, which limits the external quantum efficiency of conventional LEDs to only a few percent .
To solve this problem, an intensive study has been made, in order to improve the light output power by increasing the light scattering at the GaN/air interface by using a surface-roughening technique. Typically, surface texturing of p-GaN layer is performed by using maskless wet etching [3,4], electron-beam and nanoimprint lithography [5–7] as well as a self-assembled cluster of metal drops [8,9]. However, the roughness obtained using wet etching is not uniform, and thus, leads to a variation in the improvement of light extraction efficiency of the LEDs across the sample. There are still problems in applying electron-beam and nanoimprint lithography to commercial products because the nano-fabrication methods are quite expensive and time consuming. Additionally, the self-assembled metal cluster is formed at higher temperature of 800-900 °C, which may impair InGaN/GaN quantum well and reduce the internal quantum efficiency of LEDs. Therefore, developing a novel cost-effective and reproducible approach for surface texturization of LED chips with large-scale area is urgent.
Most recently, several approaches using self-assembled polystyrene (PS) or/and SiO2 nano-spheres have been employed to improve light extraction efficiency of nitride LEDs [10–12]. In this letter, we report a new method to roughen p-GaN surface using self-assembled CsCl nano-islands as an etch mask and generation of nano-patterns on the p-GaN surface, resulting in substantial performance improvement of InGaN-based LED. The method is economic, controllable and rapid, and may be extensively applied in the roughened LEDs field in the future. In addition, the fabrication process and optical and electrical properties of the roughened LED will be discussed in detail.
The LED samples were grown on c-plane sapphire substrates by means of a Veeco metal-organic chemical vapor deposition (MOCVD) system at a growth pressure of 200 mbar. The LED structures consisted of a 30 nm-thick GaN low-temperature buffer layer, a 1 μm-thick unintentionally doped GaN layer, a 2 μm-thick n-type GaN layer, a 40 nm-thick n-Al0.15Ga0.85N blocking layer, a multiple quantum well (MQW) active layer, a 40 nm-thick p-Al0.15Ga0.85N electron blocking layer, and finally 0.4 μm-thick p-type GaN layer. The MQW active layer consisted of eight periods of 3/12 nm-thick In0.2Ga0.8N/GaN quantum well layers and barrier layers. The LED wafer was thermally treated at 720 °C for 20 min to activate the p-type GaN:Mg layer. A schematic of the roughening p-GaN process was shown in Fig. 1 . First, a CsCl thin layer with a thickness of 80-350 nm was deposited on the p-GaN surface at room temperature by thermal evaporation in vacuum 0.01 Pa using a homemade vacuum equipment. Then, on exposure to water vapor, nanosize hemispherical islands were formed, driven by the size-dependent solubility of CsCl. The average diameter and coverage ratio of CsCl nano-islands can be controlled by varying the CsCl film thickness, relative humidity and developing time. Here, CsCl nano-islands with the size of 150-650 nm were obtained at relative humidity of 40-50% within 20-50 min, following a coverage ratio of 30%. Subsequently, the samples were transported to an ICP chamber and etched with 450 W power, 75 bias power, and 4 mTorr chamber pressure and then dipped into deionized water for 5 min to remove the remaining CsCl islands. Thus, the nanostructures were transferred by ICP dry etching and separate island-shaped p-GaN surface structure was formed, as illustrated in Fig. 1.
Afterwards, the conventional LED and LEDs with nano-roughened surface were fabricated using the standard process with a mesa of 1 × 1 mm2. An indium tin oxide (ITO) layer was deposited on the p-GaN surface as a transparent contact layer (TCL). Cr-Pt-Au (70/30/145 nm) metal layers were deposited on the p-type GaN and n-type GaN surfaces as the contact electrodes. Surface morphology was inspected by using the scanning electronic microscope (SEM, a Hitachi S-4800 apparatus) and atomic force microscope (AFM, a Nanoscope III apparatus operated in the tapping mode) techniques. The light output power, EL spectra, and current-voltage (I-V) curves were studied with an Everfin-PMS50 optical spectrum analyzer.
3. Results and discussion
Figure 2 shows AFM images illustrating the change of the surface morphology of the p-GaN layer before and after the ICP dry etching. As shown in Fig. 2(a), the uniformly distributed hemispherical CsCl islands with an average size of around 450 nm may be observed on the p-GaN surface. The self-assembled mask height and density are approximately 250 nm and 3.5 × 108 cm−2, respectively. The CsCl island size and density can be easily controlled by tuning processing conditions, such as CsCl film thickness, relative humidity and ripening time. Figure 2(b)–2(d) show the topography of nano-roughened surface after ICP etching with time durations of 50, 150 and 250 sec, corresponding to the root mean square (RMS) roughness of 10.8, 22.3 and 26.8 nm, respectively. With the increase of etching time, the diameter of GaN nano-island arrays transferred from CsCl islands is also enlarged due to lateral etching. The provided AFM images confirm that the p-GaN surface is effectively roughened using CsCl islands as the mask. However, when the etching time is increased up to 250 sec responding the GaN etching depth of 150 nm, CsCl islands are almost decomposed, and lots of acicular structures appear on the surface of GaN. Furthermore, the diminished surface roughness of etched p-GaN compared to that of CsCl mask covered also indicates that CsCl has more brittle resistance to ICP etching with respect to p-GaN.
The current-voltage (I-V) curves of conventional and roughened LEDs are shown in Fig. 3(a) . The forward voltages of conventional LED and those etched with 50, 150 and 250 sec at the 20 mA operation current are 5.06, 6.42, 4.67 and 4.66 V, respectively. Usually, nano-roughening facilitates p-type contact, resulting in an ohmic contact by the increase in the contact area of the nano-roughened surface . However, it is also noted that the ICP etching of 50 sec results in a Schottky contact with a high specific resistance between the roughened p-GaN and TCL layers. A similar anomalous phenomenon is also observed for the 25 sec-etched sample. The phenomenon indicates that the ICP damage of p-GaN surface dominates the electrical properties of 50 sec-etched sample and increased contact area of 50 sec-etched sample is inadequate to improve the Ohmic contact. The anomalous behavior is also observed in C. C. Yang’s results  and further investigation is needed to clarify the reason. Figure 3(b) reveals the enhancement of the reverse leakage current together with the increase in the ICP etching time. Except for the LED with 250 sec etching, the leakage currents of LEDs with 50 and 150 sec ICP etching are acceptable, corresponding to 520 and 940 nA, respectively, at a reverse bias voltage of −10 V. The great leakage current observed for the 250 sec-etched LED should be attributed to the ICP etching-induced damages of p-GaN, as shown in the inset of Fig. 3(b).
Figure 4(a) shows the light output power versus injection current (L-I) characteristics of the conventional and roughened LEDs. It is found that the output power of all LEDs increases as we increase the injection current. Under 350-mA current injection, the light output intensity of ICP-etched LEDs for 50, 150 and 250 sec is equal to 9.2%, 70.6% and 42.3%, respectively, in magnitude as compared with that of conventional LED with a flat surface. The enhancement can be attributed to the reduction of the photon extraction path length, which is made possible by nano-roughened LED surface according to Snell’s law . The shape of the rough surface is related to the light extraction efficiency, and hemispherical islands formed by ICP etching of 150 sec exhibit the largest enhancement in the light output power, with the appropriate oblique etching facet. The inset of Fig. 4(a) presents an optical microscope photograph of conventional and 150 sec-etched LEDs at an injection current of 1 mA. The images show that the EL emissions from the nano-roughened LED are brighter and more uniform than those of the conventional LED. Additionally, for the LED etched for 50 sec, the light output power firstly decreases at a higher operating current caused by greater thermal heat generated at the non-ohmic contact layer.
Figure 4(b) depicts the dependence of the EL peak wavelength on the current density for the above-discussed LEDs. As illustrated, all samples first exhibit a slight blue-shift trend in the peak emission wavelength and then a red-shift as the injection current increases. The blue-shift behavior is due to the band-filling effect and screening effect in a piezoelectric field quantum well . On the other hand, the red-shift can be attributed to the excess heat generated from the power dissipation that causes the obvious thermal effect and starts to influence the recombination process. Ultimately, LED with 50 sec-etched shows the largest peak shift of 6.3 nm due to poor ohmic contact when the injection current is raised from 10 to 500 mA. Nevertheless, the peak shift is only 1 nm for that for 150 sec etching, attributed to the reduction of heat generation caused by photon self-absorption in MQW. At a same dc current of 20 mA, the peak wavelengths of the LEDs without and with ICP etching for 50, 150, and 250 sec were found to be 457.3, 458.0, 459.0 and 458.5 nm, respectively. Besides, the peak wavelengths of etched LEDs are obviously redshifted compared to that of conventional LED, which is due to the partial relaxation of compressive strain in the MQW layers through the formation of hemispherical islands structure by ICP etching . Further more, the full width at half-maximum (FWHM) of EL peak for these LEDs is plotted as a function of the injection current. With the increase in the current, the FWHM can be seen to increase gradually, for all LEDs. Likewise, the heating reduction effect causes the smaller broadening of EL peak in the case of the 150 sec-etched LED in comparison with the other LEDs. Here, the more pronounced EL peak broadening is observed for the LED with 250 sec etching, originating in the strong leakage current due to ICP damage of p-GaN.
To investigate the influence of surface roughness on light output performance of LED, we also measured the light output radiation patterns of conventional and roughened LEDs at a driving current of 200 mA. Here, the chips were Au-wire bonded and loaded on an aluminium leaded chip carrier without epoxy encapsulation. As shown in Fig. 5 , the emission patterns from the roughened LEDs show omnidirectional enhancement in the overall integrated intensity with almost same view angles of 125-130° compared to conventional LED. Hence, the improvement in light extraction efficiency is considered as a consequence of higher photons scattering efficiency from ICP-etching roughness of p-GaN due to angular randomization or a scrambling of the photons. Especially, the LED etched for 150 sec displays much higher light intensity in the oblique directions at 50° and 120°. The radiation profile should be highly related to the inclined sidewall angle and arrangement of hemispherical nano-islands on the LED surface, similarly to the way the SiO2 microrod array effects on light propagation out from LED MQWs along the specific azimuth angles .
Finally, we further investigate the dependence of light extraction efficiency (LEE) on the size of the CsCl nano-islands under the optimum 150 sec ICP etching at an injection current of 350 mA. To be compared, the fill factor of CsCl nano-islands on the surface are constrained as 30% for different sizes islands. As shown in Fig. 6 , the enhancement factor monotonically increases with the size of nano-islands and reaches a value of 77.9% when the island size is about 650 nm. The nano-islands on the p-GaN surface behave as a layer with a gradient index profile, instead of a layer with an effective refractive index . The device behavior, as the nano-islands size increases, implies a gradually changing gradient of the refractive index profile, which is critical to the enhancement of extraction efficiency. In the case, the bigger CsCl hemispherical nano-islands contribute to increase the sidewall area, and then give the photons higher probabilities to escape from the LED structure. The present results strongly suggest that nano-island surface structure controlled precisely the size and depth with CsCl islands plays an important role in improving the electrical properties and the light-extraction efficiency of LED devices. The rapid and economic method is fully comparable to the better report in the literature  and illustrates unique advantages and promising potential.
In summary, using self-assembled CsCl nano-islands as an ICP etching mask, we present a simple, low-cost and efficient method to improve extraction efficiency of InGaN/GaN LEDs. By optimizing the etching conditions, the electrical performances of roughened LEDs have not been significantly degraded by ICP etching. The hemispherical nanostructure obviously improves the probability of photons to escape from the LEDs due to the angular diffraction, resulting in an increase of nearly 70% in the light extraction efficiency of the LED at 350 mA injection current. With the increase of CsCl nano-island size, the extraction enhancement factor can be further improved, up to 77.9% for 650 nm island. Furthermore, the EL spectra of nano-roughened LEDs show a clear redshift compared with that of conventional LED due to partial release of the compressive strain in the InGaN quantum wells.
This work was supported by the National Natural Sciences Foundation of China under Grant No. 60806001 and 50972144, National High Technology Program of China under Grant No. 2009AA03A198 and the Knowledge Innovation Program of the Chinese Academy of Sciences under Grant No. ISCAS2008T03.
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