Emission Efficiency Dependence on the p-GaN ... - IEEE Xplore

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Dec 1, 2011 - Che-Hao Liao, Chih-Yen Chen, Horng-Shyang Chen, Kuang-Yu Chen, Wei-Lun Chung, Wen-Ming Chang,. Jeng-Jie Huang, Yu-Feng Yao, ...
IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 23, NO. 23, DECEMBER 1, 2011

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Emission Efficiency Dependence on the p-GaN Thickness in a High-Indium InGaN/GaN Quantum-Well Light-Emitting Diode Che-Hao Liao, Chih-Yen Chen, Horng-Shyang Chen, Kuang-Yu Chen, Wei-Lun Chung, Wen-Ming Chang, Jeng-Jie Huang, Yu-Feng Yao, Yean-Woei Kiang, and Chih-Chung Yang

Abstract—The dependencies of quantum-well (QW) internal quantum efficiency (IQE) and device behaviors on the p-layer thickness in a high-indium InGaN/GaN QW light-emitting diode (LED) are demonstrated. During the high-temperature growths of the p-AlGaN and p-GaN layers, the QWs are thermally annealed to increase their IQEs and blue-shift the emission with increasing p-layer thickness. Meanwhile, the quantum-confined Stark effect is enhanced with increasing p-layer thickness to decrease the IQEs and red-shift the emission. Based on the counteraction between the two effects, the maximum IQE and the shortest emission wavelength are observed in a sample with an optimized p-layer thickness, which includes a p-AlGaN layer of 20 nm and a p-GaN layer of 60 nm in thickness under our growth conditions. The fabricated LEDs of different p-GaN thicknesses show the similar variation trends in emission efficiency and wavelength. Index Terms—Internal quantum efficiency (IQE), light-emitting diode (LED), p-GaN, quantum-confined Stark effect, thermal annealing.

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T HAS been shown that postgrowth thermal annealing of an InGaN/GaN quantum well (QW) at a high temperature could cause the changes of indium composition fluctuation and indium-rich cluster structures and hence alter the emission behaviors [1]–[3]. Such a change is particularly prominent when the average indium content in the QW is high. In this regard, the high-temperature postgrowths of the p-AlGaN electron blocking layer and the p-GaN hole transport layer represent a crucial process in determining the emission efficiencies of the InGaN/GaN QWs in a light-emitting diode (LED), particularly an LED in the green or longer-wavelength range. Research efforts have been made to reduce the growth temperature of the p-type layer for minimizing the thermal annealing effect [4], [5]. In this letter, we demonstrate the effects of the high-temperature overgrowths of the p-AlGaN and p-GaN layers on the InGaN/GaN QW emission behaviors in a high-indium LED. Manuscript received June 27, 2011; revised September 07, 2011; accepted September 14, 2011. Date of publication September 22, 2011; date of current version November 04, 2011. This work was supported by the National Science Council, The Republic of China, under Grant NSC 99-2622-E-002-022-CC2, Grant NSC 99-2221-E-002-123-MY3, and Grant NSC 99-2221-E-002-113. The authors are with the Institute of Photonics and Optoelectronics and Department of Electrical Engineering, National Taiwan University, Taipei, 10617 Taiwan (e-mail: [email protected]; [email protected]). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2011.2169243

TABLE I SAMPLE PARAMETERS AND MEASUREMENT RESULTS. RT AND LT REPRESENT THE CONDITIONS OF 300 K AND 10 K, RESPECTIVELY

The effects include the thermal annealing process of the QWs to change their indium variation structures [6] and the increase of piezoelectric field in the QWs to alter their quantum-confined Stark effects (QCSEs) [7], [8]. By overgrowing the p-layers of different thicknesses in a series of LED sample, we observe that the internal quantum efficiency (IQE) of the QWs first increases with increasing p-layer thickness and then decreases with a thicker p-layer. The measurement results of temperature-dependent and excitation power-dependent photoluminescence (PL) are used for demonstrating the effects of thermal annealing and QCSE variations. Also, the fabricated LEDs show the consistent variations of electroluminescence (EL) behavior. Six samples (samples A–F) are prepared with metal–organic chemical vapor deposition (MOCVD, Aixtron CCS3x2”FT) on c-plane sapphire substrate. In each of the samples, after the - m n-GaN deposition (at 1100 C), five InGaN/GaN QW periods with the growth temperatures (thicknesses) of the InGaN wells and GaN barriers at 670 and 820 C (3 and 12 nm), respectively, are grown. In sample F, the MOCVD growth stops at the top GaN barrier layer. In sample E, a 20-nm p-Al Ga N layer is added on the top of the QW structure. Then, in samples D, C, B, and A, a 30, 60, 120, and 180-nm p-GaN layer, respectively, are added on the top of the p-Al Ga N layer. All the p-Al Ga N and p-GaN layers are grown at 960 C, which is the lowest growth temperature for cm in p-GaN maintaining a hole concentration of in using our MOCVD reactor. No p-GaN activation process was applied to any sample until LED is to be fabricated. The sample parameters and measurement results are summarized in Table I. PL measurement was performed with top (p-side) excitation and top PL emission monitoring. It was excited by a 406-nm InGaN laser diode (excitation power at 6 mW in temperaturedependent measurement). Fig. 1 shows the PL spectral peak positions as functions of temperature for the six samples. Although

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Fig. 1. Spectral peak positions versus temperature of the six samples.

Fig. 2. Integrated PL intensities versus temperature of the six samples.

it is not very clear in some of them, S-shaped temperature-dependent variations of the spectral peak positions can be seen in the six samples. The S-shaped variation has been widely used for identifying the carrier localization behavior in an InGaN layer due to indium composition fluctuation and indium-rich clustering [6], [9], [10]. Fig. 2 shows the normalized integrated PL intensities as functions of temperature for the six samples. In each sample, the ratio of the intensity at 300 K over that at 10 K is used to represent the IQE of its QWs. The IQEs of samples A–F are listed in Table I. Here, the IQE is increased from 4.2% in the structure of bare QWs (sample F) to 7.9% when the 20-nm p-AlGaN layer is added (sample E). As the p-GaN layer is added to the sample, the IQE is further increased to 8.1 (sample D) and 13.8 (sample C)% with the p-GaN thicknesses at 30 and 60 nm, respectively. However, when the p-GaN layer thickness is further increased to 120 and 180 nm, the IQEs are reduced to 11.6 (sample B) and 9.1 (sample A)%, respectively. To confirm the thermal annealing effect, we use five pieces of sample F for thermal annealing with the temperatures and durations after QW growth the same as the growth conditions of samples A-E in another MOCVD reactor (Veeco p75) to produce samples FA-FE, respectively. The PL measurement results of samples FA-FE are also shown in Table I. Here, the IQE increases monotonically with increasing annealing duration, indicating the significant thermal annealing effects on the

IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 23, NO. 23, DECEMBER 1, 2011

microstructures of the InGaN/GaN QWs to enhance the emission efficiency. Also, a longer thermal annealing duration results in a shorter PL spectral peak wavelength. Although we use the same temperature parameters as the original growth conditions for thermal annealing, the post annealing process is not as effective as the in situ operation. Hence, the IQE of each postannealed sample is lower than that of the corresponding sample of A-E. It is noted that after the QW growth, the total indium content in a QW of a sample is fixed and is expected to be the same among samples A-F. The trend of increasing IQE with increasing post thermal annealing duration is attributed to the changes of the indium-rich clustering structures in the QWs during the thermal annealing process such that carrier localization becomes stronger. A stronger carrier localization results in a lower nonradiative carrier recombination rate and hence a higher IQE [6], [9], [10]. The increase of p-layer thickness in an LED can enhance the piezoelectric fields and hence the QCSEs in its QWs [7]. Although other factors, such as the indium-rich clustering structure, may also influence the results, the strength of the QCSE can be estimated through the carrier screening effect in an excitation power-dependent PL measurement at room temperature. By increasing the excitation power, the screening effect reduces the QW potential tilt and blue-shifts its emission. In the seventh row of Table I, we show the spectral blue shift ranges of various samples when the PL excitation power is increased from 1 to 10 mW. Here, the spectral shift range increases first in samples C-F and then decreases in samples A and B with increasing p-layer thickness. In samples A and B, because of the strong indium-rich clustering behaviors, strong localizations of both electron and hole lead to weaker carrier screening effects such that the spectral blue shift ranges are reduced in the excitation power-dependent PL measurement even though they still have strong QCSEs [6]. Because the QCSE in a QW can reduce its IQE, the increasing trend of IQE due to the thermal annealing effect is balanced and reversed by the increasing trend of the QCSE when the p-layer thickness is increased. In this situation, the maximum IQE is observed in sample C. This counteraction also explains the variation trend of PL spectral peak. The blue-shift trend of the thermal annealing effect is balanced by the red-shift trend of the QCSE, resulting in the shortest emission wavelength in sample C. In samples FA-FE, without the overgrown p-layers, the thermal annealing process does not significantly change the QCSEs in their QWs. Their spectral shift ranges are maintained at the similar levels to that of sample F. The epitaxial structures of samples A-D are used to fabricate standard LEDs of 300 300 m in mesa size after they are annealed at 820 C for 15 min to activate p-AlGaN and p-GaN. Fig. 3 shows the output intensity variations with injection current ( – curves) of the four LED samples. Among them, the output intensity variation trend is consistent with that of the QW IQE. The insert of Fig. 3 shows the output spectra at 40 mA in injection current of the four LED samples. The normalized integrated EL intensities of LED samples A-D are 0.36, 0.77, 1, and 0.03, respectively (see Table I). Here, one can also see that the EL spectral width is the smallest in sample C, followed by B, A, and then D. As shown in Table I, the variation trend of the EL spectral peak wavelength in LED samples A-D at 40 mA in

LIAO et al.: EMISSION EFFICIENCY DEPENDENCE ON THE p-GaN THICKNESS IN A HIGH-INDIUM InGaN/GaN QW LED

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In summary, we have demonstrated that the p-layer thickness was a crucial parameter in determining the emission efficiency and wavelength of a high-indium InGaN/GaN QW LED. During the growth of the p-layer, the QWs were thermally annealed to enhance their IQEs. However, this effect was balanced by the enhanced QCSE due to the increase of p-layer thickness. The performances of the fabricated LEDs showed the similar variation trends in emission efficiency and wavelength. The results indicate that the p-layer thickness needs to be carefully designed for maximizing the efficiency of a high-indium LED. Because both the thermal annealing effect and QCSE are strong only when the indium content is high, the reported p-layer dependence must be weak in a blue LED. Fig. 3. – curves of LED samples A–D. Inset: output spectra at 40 mA in injection current of LED samples A–D.

Fig. 4.



curves of LED samples A–D.

injection current is consistent with that of PL measurement. The EL spectral peak wavelength is longer than that of PL in each sample due to the enhanced potential tilts in QWs with the application of forward-biased voltage [7]. Fig. 4 shows the variations of current versus applied voltage ( – curves) of the four LED samples. Because of the high indium contents in our samples, their turn-on voltages are higher than that of a standard blue LED. From the slopes of the – curves, one can estimate the device resistances to give 76.3, 50.9, 42.9, and 124.3 for LED samples A-D, respectively. Because of the low hole concentration in p-GaN, it is usually believed that a thicker p-GaN layer for more effective current spreading can lead to lower device resistance. However, our results indicate that when the p-GaN layer is thicker than 60 nm, the device resistance is increased. The variation of device resistance with p-layer thickness deserves further investigation.

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