InGaN–GaN MQW LEDs With Current Blocking Layer ... - IEEE Xplore

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IEEE ELECTRON DEVICE LETTERS, VOL. 25, NO. 6, JUNE 2004

InGaN–GaN MQW LEDs With Current Blocking Layer Formed by Selective Activation Chia-Ming Lee, Chang-Cheng Chuo, Yu-Chuan Liu, I-Ling Chen, and Jen-Inn Chyi, Senior Member, IEEE

Abstract—Selective activation technique was used to define a semi-insulating current-blocking layer underneath the p-pad of InGaN–GaN multiple-quantum-well light-emitting diodes (LEDs). The output power of the LEDs at 20 mA was increased 10% because less current was injected underneath the opaque p-pad.

demonstrate a selective activation technique to define the area with different conductivity through a metal mask. This technique is applied to InGaN–GaN multiple-quantum-well (MQW) LEDs to form a current-blocking layer so as to enhance the external quantum efficiency.

Index Terms—Current blocking layer, GaN, light-emitting diodes (LEDs), selective activation.

II. EXPERIMENTS

I. INTRODUCTION

H

IGH-BRIGHTNESS GaN-based light-emitting diode (LED) is a subject of great interest because of its vital role in full-color display and lighting applications [1]. Since the successful demonstration of p-type conductivity on magnesium (Mg)-doped GaN, using post-growth low energy electron beam irradiation (LEEBI) [2] or thermal annealing in a nitrogen atmosphere [3], high-brightness blue and green LEDs have become a reality and commercially available for several years. The potential of solid-state light sources for general lighting is also well recognized and has driven the whole industry to improve the output power and quantum efficiency of LEDs. For most of the InGaN-based LEDs, both n- and p-type electrodes are formed on the same side because sapphire substrate is an insulating material [4]. The p-type electrode consists of a thin transparent layer and a thick wire-bonding pad on top. In such a configuration, the thick p-pad meal inevitably blocks part of the light output and reduces the external quantum efficiency. In addition, due to the poor conductivity of p-type GaN layer and not-so-good conductivity of the thin transparent layer, there often exists current crowding effect around the p-pad. This may further reduce the quantum efficiency and device reliability as well. It is well known that Mg-H complexes formed in Mg-doped GaN if hydrogen is present in the growth ambient [5]. This H passivation limits the electrical activity of Mg. An activation process is therefore required to release the hydrogen to obtain p-type conductivity of Mg doped GaN [3]. In this letter, we

Manuscript received February 23, 2004. This work was supported in part by Tekcore Company, Ltd. and the National Science Council of Taiwan, R.O.C. under Contract NSC91-2215-E-008-014, and in part by the Program for Promoting Academic Excellence of Universities, Ministry of Education of Taiwan, R.O.C. under Contract 91-E-FA06-1-4. The review of this letter was arranged by Editor P. Yu. C.-M. Lee, Y.-C. Liu, and J.-I. Chyi are with the Department of Electrical Engineering, National Central University, Chung-Li 32054, Taiwan, R.O.C. C.-C. Chuo is with the Opto-Electronics and System Laboratories, Industrial Technology Research Institute, Hsinchu 300, Taiwan, R.O.C. I.-L. Chen is with the Tekcore Company, Ltd., Nantou 540, Taiwan, R.O.C. Digital Object Identifier 10.1109/LED.2004.829666

The GaN bulk layer and LED samples studied in this work were grown on c-face sapphire by low-pressure metal–organic chemical vapor deposition (MOCVD). Ammonia (NH ), trimethylgallium (TMG), trimethylindium (TMI), biscyclopen), and silane (SiH ) were used as tadienilmagnesium ( precursors and dopants. Two-step growth method was used to obtain GaN films with smooth surface [2], [6]. The substrate temperatures for the low temperature GaN nucleation layer and high temperature GaN layer were 500 C and 1050 C, respectively, while that for the InGaN–GaN MQWs was 780 C. The bulk sample studied in this work is a 2- m-thick Mg-doped layer deposited on a 0.2- m-thick GaN undoped layer. The LED consists of a 4- m-thick n-type GaN layer, five pairs of N-well (3 nm) and GaN barrier (10 nm) MQWs, In Ga and a 0.3- m-thick p-type GaN. The InN mole fraction in the InGaN wells is about 0.1 as determined by X-ray diffraction. Ti–Al (30–300 nm) and Ni–Au (5/7 nm), which was known as a transparent conductive layer (TCL), were used as the n-electrode and p-electrode, respectively. The p-type bonding pad was made of Ti–Au (30–1000 nm). After growth, thermal annealing in nitrogen atmosphere was performed to obtain p-type conductivity of the Mg-doped GaN. The annealing temperature is 725 C, and the annealing time is 15 min. The area, which was intended to be kept semi-insulating, was covered by a layer of metal composed of Ti–Au (10/1000 nm) when performing the activation process. This metal layer was then removed after the activation for device fabrication. The electrical characteristics of the samples were assessed by Van der Pauw Hall effect measurements. After thermal annealing in nitrogen atmosphere, typical hole concentration and resistivity of Mg-doped GaN samples was cm and 2 cm, respectively. However, the area 3 covered by Ti–Au when annealing was found to be semi-insulating. The resistivity of the Mg-doped GaN samples with and without the Ti–Au mask was qualitatively evaluated by measuring the current–voltage (I–V) characteristics between two contact pads as shown in Fig. 1. Apparently, the masked region is much resistive than the unmasked region. As reported by [3], the as-grown Mg-doped GaN exhibits high resistivity unless the sample is annealed at temperatures higher than

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LEE et al.: InGaN–GaN MQW LEDs WITH CURRENT BLOCKING LAYER FORMED BY SELECTIVE ACTIVATION

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Fig. 3. P-pad metallization schemes for the LEDs studied. The solid line shows the current path, and the dotted line shows the light-emission path. Fig. 1. I–V characteristics of the Mg-doped GaN samples activated with and without Ti–Au mask.

Fig. 2. Luminescence photographs of a fully activated LED and a selectively activated LED with striped selective-activation patterns. Fig. 4. I–V curves of sample A, B, C, and D. Shown in the inset are the optical output power versus injection current characteristics of LEDs.

700 C in nitrogen ambient. Since the maximum temperature in the device fabrication processes is around 500 C, this technique is readily applicable to our LED processes for quantum efficiency improvement. There were four samples prepared on two different epi-wafers. Sample A and B were fabricated on the same epi-wafer, and sample C and D were fabricated on the other one. III. RESULTS AND DISCUSSION Fig. 2 shows the photographs of a fully activated LED and a selectively activated LED under operation. Obviously, the light comes out from all over the mesa of the fully activated LED, while the selectively activated LED striped pattern, which is consistent with the mask, is observed. Note that TCL is used for both LEDs. This result demonstrates the feasibility of the selective activation process. Fig. 3 shows the p-pad metallization schemes for sample A and B. Sample A and B are LEDs with mesa fully covered by TCL, while sample C and D are LEDs with a TCL window under the p-pad. For comparison, sample A and C are fully activated and sample B and D are selectively activated leaving the area under the p-pad semi-insulating. In this figure, the solid line and the dotted line shows the expected current path and the light-emission path, respectively. As we can see, without current blocking, sample A is expected to be less efficient because part

of the light is blocked by the p-pad. Sample C is a typical structure used to alleviate the aforementioned effect. Direct contact of Ti–Au p-pad metal on p-type GaN through the TCL window reduces current injection into the p-pad area because Ti–Au is a worse ohmic contact than the TCL. Fig. 4 shows the I–V characteristics of sample A, B, C, and D. Shown in inset are the corresponding light output-current ( - ) curves. The operation voltage at 20 mA of sample A and B is 3.4 and 3.45 V, respectively, and the resistance at 20 mA of sample A and B is 20.8 and 22.8 , respectively. The increase of resistance of about 10% for sample B is in good agreement with the conduction area of these two devices. The light intensity is increased by about 14% for sample B. This intensity improvement can be attributed to the current blocking by the selective activation process. Reduced current crowding effect, which enhances internal quantum efficiency, might also contribute to the intensity improvement through this process. The operation voltage at 20 mA of sample C and D is 3.23 and 3.26 V, respectively, and the resistance at 20 mA is 14.2 and 14.9 , respectively. The light intensity of sample D is about 8.4% higher than that of sample C. This indicates that the direct contact approach is not as effective as the selective activation one because there’s still some current injected to the area underneath the p-pad.

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IEEE ELECTRON DEVICE LETTERS, VOL. 25, NO. 6, JUNE 2004

IV. CONCLUSION We have disclosed a selective activation process for p-type GaN. This technique is successfully demonstrated on InGaN LEDs to form current blocking layer underneath the p-pad. The output light intensity of the LEDs is improved by this process, as compared to the conventional approach, which utilizes a TCL window for current blocking.

ACKNOWLEDGMENT The authors would like to acknowledge the support of the Optical Sciences Center, National Central University.

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