1. Introduction

The external quantum efficiency (EQE) of any light-emitting diode (LED) is determined not only by internal quantum efficiency but also by the light extraction efficiency (LEE) of photons emitted from the active layer. Photons escape from the high-refractive-index semiconductor material into the lower-refractive-index surrounding material, such as air or resin [1]. Much effort has been made to overcome significant photon loss caused by total internal reflection inside the LED. A large critical angle or rough surface is necessary to enhance the escape probability of photons generated in the active layer of LEDs. Although the refractive index of a semiconductor cannot be changed, the light output can be enhanced by roughening the semiconductor surface. Angular randomization of photons can be achieved through surface scattering from the roughened top surface of LEDs. Therefore, roughening the surface of LEDs can be used to overcome the total internal reflection of light inside these. There are several methods to roughen the top surface of GaN-based LEDs. One is through an etching process; however, this method easily alters the surface state of the p-GaN layer, thereby degrading the electrical properties of the device [2]. In comparison, the process of controlling growth conditions to obtain rough surfaces is superior to the etching method. The application of naturally textured surface V-shaped pits as well as of truncated micropyramids originating from the change of growth conditions during the growth of the p-GaN top contact layer are two well-known approaches [3,4]. The naturally formed texture on the p-GaN surface can lessen the reflection of internal light at the GaN/air interface and enhance LEE. A similar concept has been applied to chip sidewalls. As a result, more photons can escape from LEDs with textured sidewall facets compared with conventional LEDs with flat sidewall facets [5]. Recently, Lee et al. have fabricated GaN-based LEDs with GaN sidewall deflectors by plasma etching to enhance LEE [6]. The sidewall deflector resulted in sidewalls with partially oblique facets for LEDs. These allowed more photons to escape from the sidewalls compared with conventional LEDs with sidewall facets perpendicular to the substrate [7]. In this study, we demonstrate another approach aimed at achieving oblique sidewall facets (OSFs) in the periphery of GaN-based LEDs for the improvement of LEE. The epitaxial layer structures of InGaN/GaN blue LEDs are selectively grown on n-GaN templates. The templates are prepared using metal-organic vapor-phase epitaxy (MOVPE) with SiO₂ patterns to form OSFs. In contrast to previous reports, the SiO₂ mask layer in this study is patterned on an n-GaN template layer rather than on the surface of a sapphire substrate. In addition to enhancing LEE, this approach has other advantages, including its simple processing and low cost. The detailed processing procedures and related results, including the electrical and optical properties of the fabricated LEDs, are discussed.

2. Experiments

Samples used in this study were grown on c-face (0001) 2-inch sapphire substrates in a vertical metal-organic vapor-phase epitaxy(MOVPE) reactor. Before the growth of LED structures, n-type GaN epitaxial layers, including a 1 μm thick undoped GaN layer and a 2 μm thick Si-doped n-GaN layer, were grown on sapphire substrates as templates for the subsequent regrowth process. The carrier concentration of the n-GaN template layer was around 8 × 10¹⁸/cm³. The SiO₂ layer was deposited on the n-GaN template layer and then selectively etched using hydroflouric solution to form a series of patterns on the templates. Afterwards, templates with and without the patterned SiO₂ layer were respectively loaded into the MOVPE reactor to regrow the InGaN/GaN-based layers. These included a 1.7 μm thick Si-doped n-GaN layer grown at 1,000 °C, a 10-pair In_0.3Ga_0.7N/GaN MQW structure grown at 750 °C, a 0.05 μm thick Mg-doped p-Al_0.15Ga_0.85N electron blocking layer grown at 1,000 °C, and a 0.1 μm thick Mg-doped p-GaN top contact layer also grown at 1,000 °C. Finally, a heavily Si-doped short-period superlattice (SPS) structure was grown on the p-GaN contact layer [8]. For the InGaN/GaN MQW active region, each pair consisted of a 3 nm thick In_0.27Ga_0.73N well layer and a 17 nm thick GaN barrier layer.

Figures 1(a) and 1(b) show the schematic diagram of the device structure (labeled LED-I) and the detailed epitaxial layer structure, respectively. For comparative analysis, LEDs without OSFs were also prepared and labeled as LED-II. For the device process of LED-II, wafers were partially dry etched until the n-GaN layer was exposed. The dry etching procedure was not necessary for LED-I because its n-GaN layer was exposed only if the SiO₂ mask layer was removed using the simple wet etching method (HF solution). Current-voltage (I-V) measurements were performed at room temperature using an HP4156C semiconductor parameter analyzer.

 

Fig. 1 (a) Schematic structure of LED-I and photon paths in LED-I; (b) detailed epitaxial layer structure; (c) typical tilted-angle-view SEM image of LED-I; (d) typical SEM image of LED-I after the formation of Cr/Au electrodes; (e) enlarged tilted-angle-view SEM image taken on the regrown area of LED-I after the removal of SiO₂ mask layer; and (f) cross-section view SEM image of LED-I.

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3. Results and discussions

Figure 1(b) shows the typical tilted-view SEM image of LEDs that have been selectively grown on an n-GaN template (LED-I). According to the energy dispersive X-ray spectroscopy analysis (data not shown here), only gallium could be observed on the surface of the SiO₂ mask layer. This indicates that GaN-based layers cannot be deposited on the SiO₂ layer. The method demonstrates excellent growth selectivity for the GaN film. Figure 1(c) shows a typical SEM image of LED-I after the formation of Cr/Au electrodes. Figures 1(d) and 1(e) show the enlarged tilted-angle and cross-sectional views of the SEM images, respectively. These were taken from the regrown area of LED-I after the removal of the SiO₂ mask layer. The periphery of LED-I includes several oblique facets in the mesa. The facets contain {$10\overline{1}\overline{1}$} planes [9,10]. The measured angles of regrown GaN sidewall facets were approximately 62 degrees inclined to the c-plane surface. These oblique facets with respect to the plane of (0001) were initiated on the edges of the SiO₂ and n-GaN layers during the regrowth process. In addition, the formation of oblique facets can be attributed to the higher growth rate in the vertical direction [0001] compared with the inclined lateral facets [9,10]. According to the aforementioned results, one can anticipate that LEDs with these types of OSFs will have higher LEE than LED-II with its vertical sidewall facets. To evaluate the effect of naturally formed OSFs on the LEE of LEDs, light outputs of the bare-chip LEDs were measured using an integral sphere. The device fabrication processes began with the deposition of ITO on top of the regrown mesa area to form the transparent contact layer [4,7]. The Cr/Au bilayer metals were then deposited on the masked n⁺-GaN template area and the ITO layer to form the anode electrode and the n-type Ohmic contact (i.e., cathode), respectively [11] [Figs. 1(a) and 1(c)]. The dimension of the LED chips used in this study was 340 × 340 μm².

Figure 2 shows the respective typical light output-current (L-I) characteristics of LED-I and LED-II in bare-chip form. All LEDs have an emission wavelength of around 460 nm when a DC of 20 mA is applied. The total light output of the devices were measured using an integrating sphere to allow light, which was emitted in all directions from the LEDs, to be collected. The results in Fig. 2 show that by utilizing the selective growth process to form the OSFs, the light output in LED-I significantly improved by 21% in terms of magnitude compared with that in LED-II. The enhancement of light output in LED-I can be attributed to the reduction of the re-absorption probability of the photons by the active layers. This assumption can be indirectly supported by the measurements of the beam patterns of the studied LEDs. As shown in the inset of Fig. 2, the typical beam patterns of LED-I and LED-II were determined at a DC driving current of 20 mA. The figure clearly shows that LED-I displays a wider beam pattern due to the fact that the percentage of photons extracted from the sidewall facets of LED-I is higher than that of LED-II, resulting in wider beam patterns observed in the former. For a conventional GaN-based LED, the majority of light emitted from an isotropic source incident on the GaN/air interfaces, including the four sidewall facets and top surface, can undergo total internal reflection or Fresnel reflection, passing small amounts of light through the GaN/air interfaces into the air. The escape probability of photons emitted from a textured semiconductor surface is higher than that from a smooth semiconductor surface [1]. In principle, the critical angle (θ_c) of blue light impinging at the top GaN/air interface is around 23.5 degrees. Therefore, a LED-II growing on sapphire with a specular surface and four vertical sidewall facets can result in waveguide modes that trap most photons inside the LED when the incident angle of photons is less than the θ_c. This allows only a few photons to out-couple outside the LED from the sidewall facets if they are not completely absorbed by the material. However, for LED-I, the oblique facets can play the role of a guided light deflector and facilitate multiple chances for photons to escape from the LED sidewall facets. This allows more photons to be extracted from the OSFs, resulting in a wider beam pattern. This means that the average path length of photons in LED-I is shorter than that in LED-II before the photons escaped from the LEDs. Therefore, the enhancement of light output in LED-I can be attributed to the reduction of the internal absorption losses.

 

Fig. 2 Typical light output-current (L-I) characteristics of LED-I and LED-II with bare-chip form; the inset shows the typical beam patterns taken from LED-I and LED-II. These LEDs were all bonded on the TO 66.

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Considering the electrical properties of LED-I, Si and/or O atoms may be able to diffuse in the n-GaN template layer and result in a heavily doped GaN surface layer during the regrowth process at temperatures of 750–1,000 °C [12,13]. In this study, the non-alloyed Cr/Au metal contacts on the dry-etched surface and masked template n⁺-GaN layers all display linear I-V characteristics and exhibit specific contact resistances (ρ_c) of approximately 3 × 10⁻⁴ and 1 × 10⁻⁴ Ω cm², respectively. Figure 3 shows the I-V characteristics of LEDs with oblique (LED-I) and vertical (LED-II) sidewall facets. The forward voltages (V_f) measured at 20 mA are approximately 3.10 and 3.12 V for LED-I and LED-II, respectively. The series resistances (R_s) determined through the extraction of dynamic resistance (Fig. 3) are almost the same in both LED-I and LED-II. This result can be attributed to the fact that the ρ_c value of the Cr/Au contacts on the etched GaN surface is comparable with that deposited on the SiO₂ masked n-GaN template layer.

 

Fig. 3 I-V and dynamic resistance characteristics of the LED-I and LED-II.

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4. Conclusions

In summary, this research has demonstrated the electrical and optical properties of GaN-based LEDs with naturally formed oblique facets by applying a selective regrowth method. GaN-based LEDs with oblique facets exhibited a 21% enhancement in light output when a DC of 20 mA was applied. The enhancement can be attributed to the reduction of the photon extraction path length and internal absorption losses. Although the ρ_c value of the Cr/Au contacts deposited on the SiO₂ masked n-GaN template layer was slightly lower than that of the Cr/Au contacts on the etched GaN surface, the typical series resistance and the 20 mA-driven forward voltage of LED-I were almost the same as those of LED-II. Thus, the selective regrowth process via the SiO₂ layer masked on the n-GaN template layer is not expected to degrade the electrical properties of LED-I.