나노 크기의 InGaN/GaN 다층양자우물 LED의 제조

화학공학의 이론과 응용 제8권 제1호 2002년

한윤봉

전북대학교 화학공학부

Fabrication of Nano-Sized Multiple Quantum Well InGaN/GaN

Theories and Applications of Chem. Eng., 2002, Vol. 8, No. 1

Light-Emitting Diodes

Yoon Bong Hahn

School of Chemical Engineering and Technology, Semiconductor Physics Research Center, Chonbuk National University, Chonju 561-756, Korea

Electronic mail: ybhahn@moak.chonbuk.ac.kr

I. INTRODUCTION

In recent years, there has been remarkable progress for light-emitting diodes (LEDs) and laser diodes (LDs) by utilizing wide band gap III-nitride semiconductors which have a direct band gap, suitable for blue light-emitting devices. Especially, InGaN/GaN multiple quantum well (MQW) LEDs have attracted more attention because the InGaN active layer emits light by the recombination of the injected electrons and holes into the InGaN.1 Fabrication of these GaN-based optoelectonic devices depends largely on dry etching. Recently, the most significant advancement in dry etching of the III-nitrides is the utilization of high density plasmas such as inductively coupled plasmas (ICPs), in which plasma density (or the ion flux) and ion energy are controlled independently.2-3

In this paper, the fabrication of nano-sized InGaN/GaN MQW LEDs grown by metal-organic chemical vapor deposition is reported in terms of etch rates, surface morphology, and forward and reverse voltages, and plasma-induced damage.

II. EXPERIMENTAL

Nano-sized InxGa1-xN/GaN multiple quantum well LED structures were grown on c-plane sapphire substrate by metal-organic chemical vapor deposition system. Trimethylgallium (TMGa), trimethylindium (TMIn), ammonia (NH3), and silane (SIH4) were used as the precursors of Ga, In, N, and Si, respectively. A GaN nucleation layer of 25 nm thickness was grown on the cleaned substrate at 560 ℃, and a 4-mm-thick GaN:Si (n=3×1018 cm-3) was then grown at 1100 ℃. Figure 1 shows the InGaN/GaN MQW LED structure grown at 750 ℃, having a nano-sized 6-period InGaN(2nm)/GaN(6nm) quantum well structure. A 0.25-mm-thick Mg-doped p-GaN (p = 1 × 1017 cm-3) was grown finally at 1100 ℃ on the top of MQWs.

For fabrication of the InGaN/GaN MQW LED chips, the processing procedures were summarized as: 1) SiO2 film was deposited by PECVD onto the epiwafer as the etch mask before ICP mesa etching, 2) ICP etching was carried out to form mesa structure with Cl2/Ar discharges, 3) Au(5nm)/Ni(7nm) bi-layer for p-ohmic metal was deposited by e-beam evaporation and lift-off, then alloyed at 500 ℃, and 4) Ti(30nm)/Al(70nm) bi-layer for n-type contact was deposited and patterned by lift-off. Etching was performed in a planar-type inductively coupled plasma (ICP) system, in which the ICP source operated at 13.56 MHz. The ion energy was controlled by the radio frequency (rf) chuck power at 13.56 MHz, applied to the backside helium-cooled chuck. For etch rate experiments, samples were patterned with AZ 4330 photoresist, etch depths were obtained from stylus profilometry measurements of etched samples after removal of the photoresist. Surface morphologies and etch depth profiles of the fabricated MQW LEDs were analyzed with atomic force microscopy (AFM) operating in tapping mode with Si tip and scanning electron microscopy (SEM), respectively. The forward and reverse I-V characteristics of the InGaN/GaN MQW LED were measured with a semiconductor parameter analyzer (HP 4145B).

III. RESULTS AND DISCUSSION

Figure 2 shows the effect of dc bias (or ion energy) on the forward turn-on (VF) and reverse breakdown (VB) voltages (left), along with the etch rate and the surface roughness (right). VF and VB are the voltages assigned at 20 mA injection current and at 10 mA leakage current, respectively, and they are obtained from the I-V measurements. The etch rate and the surface roughness increased with the ion energy, typical in ICP etching systems. Little effect on the forward voltage was observed below dc biases of - 180 V. As the dc bias was increased from - 180 to - 245 V, VF and VB somewhat increased and decreased, respectively. This is attributed to a substantial increase in surface roughness (Fig. 2, right) and a damage accumulation on the sidewall due to energetic ion bombardment. Figure 3 shows the SEM micrographs of etch profiles of the LED structures at various dc biases. The depth profile became more anisotropic as the dc bias increased, due to the perpendicular nature of the incident ion energies. However, it is seen that etch products are deposited on the sidewall and covered the MQW active layer. The sidewall contamination became worse at higher dc biases probably because of redeposition of etch products on the sidewall.

Figure 4 shows the effect of the ICP source power on the forward voltage and reverse voltage (left), the etch rate and the rms roughness (right). VF decreased slightly up to 800 W with the ICP power, showing a minimal effect on the turn-on voltage, but increased at 1000 W because of the plasma damage and the increase in surface roughness. By contrast, the breakdown voltage showed severe degradation at lower and higher ICP powers. Higher etch rates and more anisotropic etch profiles were obtained at higher ICP powers, but more degradation and rougher surface morphology observed probably due to increased ion scattering as well as more interactions of reactive neutrals with the sidewall of the mesa. In order to improve the electrical properties of the MQW LEDs defected by the plasma-induced damage, annealing was carried out under N2 at 930 ℃ for 30 sec after the ICP mesa etching. The turn-on and breakdown voltages were measured after contact metallization. Although not illustrated, substantial improvements of the electrical properties were obtained with the annealing after the mesa etching.

IV. CONCLUSIONS

화학공학의 이론과 응용 제8권 제1호 2002년

Nano-sized InGaN/GaN MQW LEDs were fabricated using the ICP etching technique. Physical degradation of the sidewall along with the rougher surface morphology deteriorated the forward and reverse voltages at higher dc biases (above - 180 V). Not only higher etch rates and more anisotropic etch profiles, but also more degradation and rougher surface morphology were observed at higher ICP powers due to increased ion scattering as well as more interactions of reactive neutrals with the sidewall of the mesa. The sidewall contamination by etch products on the nano-sized active layer induced a severe degradation of the breakdown voltage. The forward voltage was relatively independent of the pressure up to 20 mTorr, but showed a sudden increase beyond 20 mTorr. The reverse voltage was relatively insensitive to the pressure effect. The experimental results led to a conclusion that the forward turn-on voltage is more sensitive to the surface roughness and the reverse breakdown voltage is strongly affected by the sidewall contamination.

ACKNOWLEDGMENTS

This work was supported by Grant No. R01-2000-00330 from the Basic Research Program of the Korea Science and Engineering Foundation.

REFERENCES

1. S. Nakamura, in GaN and Related Materials, edited by S. J. Pearton (Gordon and Breach, New York, 1997).

2. Y. B. Hahn, D. C. Hays, S. M. Donovan, C. R. Abernathy, J. Han, R. J. Shul, H. Cho, K. B. Jung, and S. J. Pearton, J. Vac. Sci. Technol. A, 17, 763 (1999)

Fig. 1 Nano-sized InGaN/GaN multiple quantum well light-emitting diode structure.

Fig. 2 Effect of dc bias on forward and reverse voltages (left)

and etch rate and surface roughness (right).

-240 V dc        -180 V dc          -140 V dc         -75 V dc

Fig. 3 SEM micrographs of etch profiles of InGaN/GaN MQW LED structures,

etched varying dc bias

화학공학의 이론과 응용 제8권 제1호 2002년

Fig. 4 Effect of ICP power on forward and reverse voltages of InGaN/GaN

LED (left) and etch rate and surface roughness (right).