1. Introduction

GaN-based light-emitting diodes (LEDs) are a promising alternative to conventional light sources owing to their high efficiency, long life-time, and environmental friendliness [1–3]. However, for general illumination applications, the efficiency and light output power of state-of-the-art LEDs must be further increased. The vertical structure design based on the laser lift-off (LLO) technique has been widely investigated to improve the light extraction efficiency and to reduce the power consumption [4–6]. In vertical LEDs (V-LEDs), the current spreading ability is superior to that in conventional lateral LEDs, because the current injected by the top contact is spread throughout the n-type GaN, which has a higher conductivity than p-type GaN [7,8]. However, current crowding is still a major obstacle in V-LEDs operated at a high current density under high-power operations, because the small-area top n-contacts and large-backside reflective p-contacts cause current crowding, resulting in the reduction of light output power in V-LEDs. To overcome current crowding in V-LEDs, various kinds of current blocking layers (CBLs), such as an SiO₂ insulating layer, ion implantation, and plasma treatments, have been proposed [9–12]. However, the electrical properties of V-LEDs could be deteriorated by the increased resistivity of p-type GaN during the deposition process of SiO₂ by plasma-enhanced chemical vapor deposition (PECVD), as previously reported [12]. Plasma-treatment-based CBL has disadvantages, for example, it is hard to align at the CBL during the photolithography process for n-electrode deposition.

Here, we demonstrate the novel use of Al₂O₃ CBL materials for the enhancement of light output power and wall-plug efficiency (WPE) in V-LEDs. V-LEDs with Al₂O₃ and SiO₂ CBLs show similar enhancements in light output power (of 22.4% and 21.1% at 500 mA, respectively) as compared to V-LEDs without a CBL. However, V-LEDs with an Al₂O₃ CBL deposited by electron-beam evaporation exhibit a lower forward voltage than V-LEDs with a SiO₂ CBL deposited by PECVD, because the conductivity of p-type GaN is not deteriorated by plasma during CBL deposition. As a result, the WPE of V-LEDs with an Al₂O₃ CBL at 500 mA was improved by 5% as compared to V-LEDs with a SiO₂ CBL.

2. Methods

The InGaN/GaN multiple quantum well (MQW) LED structure was grown by metal-organic chemical vapor deposition on c-plane sapphire substrates, and it consisted of a 500-nm-thick undoped GaN buffer layer, a 4-µm-thick n-type GaN, an InGaN/GaN multiple quantum-well active region, a p-type AlGaN electron blocking layer, and a p-type GaN layer. For the deposition of an Al₂O₃ CBL, the n-electrode structures were patterned on the sample using a photoresist. After HCl treatment, Al₂O₃ (2000 Å) was deposited by electron-beam evaporation under a base pressure of 2 × 10⁻⁶ Torr. After the lift-off process, the sample was treated with piranha solution (H₂SO₄: H₂O₂ = 1:1) [13] and boiling aqua regia (HCl:HNO₃ = 3:1) [14] for 10 min. For comparison, the SiO₂ (2000 Å) CBL deposited by PECVD on the entire p-type GaN was patterned by a combination of photolithography and BOE wet etching. Then, the Ag-based reflective p-type ohmic contact was deposited on the p-type GaN, followed by annealing at 400°C for 2 min in ambient air. The diffusion barrier and Au-Sn bonding layers were deposited, and the sample was bonded to the Si substrate by a thermo-compressive bonding process at 300°C. Subsequently, the LLO process of the sapphire/MQW LED/Si structure was performed using a KrF pulsed excimer laser. After the LLO process, undoped GaN was removed and active regions were defined by inductive plasma etching. Next, the SiO₂ passivation layer and Cr/Au n-type ohmic contact was deposited on the n-type GaN, forming n-side-up V-LEDs on the Si substrate. Finally, the surface roughening of the n-GaN was performed with NaOH-based wet-etching [15]. The light output power of the V-LEDs was measured in an unpackaged (on-wafer) configuration using an integrating sphere. The SpecLED (STR, Inc.) simulation program was used for the calculation of the current distribution in the V-LEDs with and without CBL.

3. Results and discussion

The schematic illustration of a V-LED (1.1 × 1.1 mm²) fabricated with an Al₂O₃ CBL is shown in Fig. 1(a) . The Al₂O₃ CBL was deposited on p-type GaN using an n-electrode pattern, because the current is predominantly injected through the small n-electrode. Figure 1(b) shows the optical microscopy and electroluminescence (EL) images of V-LEDs with an Al₂O₃ CBL. The LLO process was successfully performed without any cracks, indicating that the adhesions between the GaN substrate, Al₂O₃, and reflective p-contact are good. There was no light emission at the Al₂O₃ CBL, as shown in Fig. 1(b), because the Al₂O₃ CBL was successfully embedded between the p-type GaN and reflective p-contact. The calculated current density distributions (Fig. 1(c)) and profiles (Fig. 1(d)) in the active region clearly show that the current injection through the p-electrode was effectively suppressed by the CBL embedded between the p-type GaN and reflective p-contact. Subsequently, current crowding was reduced over the entire active region.

 

Fig. 1 (a) Schematic illustration of V-LEDs fabricated with an Al₂O₃ CBL. (b) Optical microscopy image of V-LEDs with an Al₂O₃ CBL after laser lift-off (left) and EL image at 5 mA (right). The n-electrode was not deposited. (c) Simulated current density distribution in the active region with and without CBL at 500 mA. (d) Current density profiles along the dotted line from Fig. 1(c).

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The CBL materials should have high light transmittance to prevent the CBL embedded between the p-type GaN and reflective p-type ohmic contact from absorbing the light from the active region. The Al₂O₃ was chosen owing to its large band-gap (7~9.5 eV) [16], as compared to other electron-beam evaporable films such as TiO₂ [17] and Ta₂O₅ [18]. The light transmittance spectra of Al₂O₃ and SiO₂ CBLs as a function of wavelength are shown in Fig. 2 . For the measurement of transmittance, the Al₂O₃ and SiO₂ films were deposited on a glass substrate. The Al₂O₃ CBL showed a high transmittance of over 95% at visible wavelengths owing to the large band-gap, which is comparable to that of the SiO₂ CBL. This result indicates that Al₂O₃ is a favorable CBL material, because the Al₂O₃ CBL does not severely absorb light from the active region.

 

Fig. 2 Light transmittance spectra of an Al₂O₃ (2000 Å) CBL deposited by electron-beam deposition and a SiO₂ (2000 Å) CBL deposited by plasma-enhanced chemical vapor deposition.

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Corrosion resistance to acidic solutions such as piranha solution or aqua regia is a key requirement for CBL materials. The adhesion between the reflective p-contact and GaN substrate is very important for completing the LLO process without the occurrence of cracks [19]. Therefore, a surface cleaning process to remove residual photoresist and organics using piranha solution is essential before the deposition of the reflective p-contact [13]. Furthermore, the aqua regia treatment is required to remove the surface oxide layer in order to produce good ohmic characteristics of the contact and the consequently low forward voltage of V-LEDs [14]. Fig. 3 shows the depth profiles of an Al₂O₃ CBL before and after the piranha solution and aqua regia treatments. There are no significant changes in the height of the Al₂O₃ CBL after the treatments, indicating that the Al₂O₃ CBL has good corrosion resistance.

 

Fig. 3 Depth profiles of an Al₂O₃ (2000 Å) CBL before and after piranha solution and aqua regia treatments.

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The radiant flux of the V-LEDs with and without CBL increased as the injection current was raised, as shown in Fig. 4(a) . However, the rate of increase was smaller in the V-LEDs without CBL due to the current-crowding effect. The enhancement of the light output power in V-LEDs with Al₂O₃ and SiO₂ CBLs was 22.4% and 21.1%, respectively, at 500 mA, showing a similar effect. The current-voltage (I-V) characteristics of V-LEDs with and without CBL are shown in Fig. 4(b). There were no significant changes in forward and reverse leakage currents. However, the forward voltages of V-LEDs were increased with both Al₂O₃ and SiO₂ CBLs. This could be attributed to the decrease in the region of the p-type ohmic contact for hole injection because of the CBL. Here, it is noteworthy that V-LEDs with an Al₂O₃ CBL show lower forward voltages than those with a SiO₂ CBL.

 

Fig. 4 (a) Light output power of V-LEDs with and without CBL as a function of injection current. (b) Current-voltage (I-V) characteristics of V-LEDs with and without CBL. Inset table displays the forward voltage of the V-LEDs at 20, 350, and 500 mA.

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Figure 5 displays the change in wall-plug efficiency (WPE) of V-LEDs with respect to the injection current, showing the efficiency droop behavior of V-LEDs. The WPE was calculated by dividing the measured radiant flux by the power consumption, P = V × I. At a low injection current, the WPE of V-LEDs with a CBL was lower than that of V-LEDs without a CBL because of the higher forward voltage (Fig. 4(b)), and the two types of V-LEDs with a CBL emitted nearly the same radiant flux (Fig. 4(a)). The efficiency droop was significantly decreased for the V-LEDs with a CBL, and the difference in WPE was further increased with the injection current. Furthermore, the V-LEDs with an Al₂O₃ CBL show a higher WPE than V-LEDs with a SiO₂ CBL.

 

Fig. 5 Efficiency droop behavior of V-LEDs: Wall-plug efficiency of V-LEDs with and without CBL as a function of injection current.

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The enhancement of the WPE in V-LEDs with an Al₂O₃ CBL is attributed to not only the increase in light output power by improved current spreading but also the low forward voltage. The current spreading length L_s is given by following equation:

where ρ is the resistivity, t is the thickness of the current spreading layer, n_ideal is the diode ideality factor, J₀ is the current density, and kT is the thermal energy [20]. In V-LEDs, the current could be spread out over the entire p-n junction area at a low injection current because of the low resistivity and thickness of the n-type GaN layer. Therefore, V-LEDs with and without a CBL show similar light output power up to a 150 mA injection current (Fig. 4(a)). However, the current spreading length can decrease with a further increase in the injection current, and subsequently, the current tends to flow directly from the n-electrode (Fig. 1(c) and 1(d)), resulting in a decrease in the light output power. When the Al₂O₃ or SiO₂ CBLs were embedded between the p-type GaN layer and the p-type ohmic contact, the light output power at a high injection current was increased (Fig. 4(a)) because of uniform current spreading as well as less light absorption at the opaque n-electrode. However, the increase in the forward voltage in V-LEDs with a CBL is inevitable, as shown in Fig. 4(b), because the hole injection from the p-type ohmic contact to the p-type GaN layer could be prohibited by the insulating CBL. For the SiO₂ CBL deposited by PECVD, the conductivity of p-type GaN could be degraded by plasma, leading to a further increase in the forward voltage. However, for the Al₂O₃ CBL, there could be no plasma damage on the p-type GaN during its deposition process using electron-beam deposition. Thus, V-LEDs with an Al₂O₃ CBL show a lower forward voltage as compared to those with a SiO₂ CBL (Fig. 4(b)), and they have a consequently improved WPE (Fig. 5).

4. Conclusions

The fabrication and characterization of V-LEDs using the novel Al₂O₃ CBL with improved current spreading and WPE were successfully demonstrated. The V-LEDs with Al₂O₃ and SiO₂ CBLs show a similar enhancement of light output power as compared to V-LEDs without a CBL. However, the increase in the forward voltage in V-LEDs with an Al₂O₃ CBL was smaller than that in V-LEDs with a SiO₂ CBL owing to the lack of plasma damage on p-type GaN, resulting in the improvement of WPE. These experimental results show that Al₂O₃ CBLs are very effective for improving the light output power and wall-plug efficiency of V-LEDs.