Fabrication of Efficient Blue Light-Emitting Diodes with InGaN/GaN Triangular Multiple Quantum Wells. Abstract

Transcription

1 Fabrication of Efficient Blue Light-Emitting Diodes with InGaN/GaN Triangular Multiple Quantum Wells R. J. Choi, H. W. Shim 2, E. K. Suh 2, H. J. Lee 2, and Y. B. Hahn,2, *. School of Chemical Engineering and Technology, Chonbuk National University, Chonju , Korea 2. Department of Semiconductor Science and Technology, and Semiconductor Physics Research Center, Chonbuk National University, Chonju , Korea Abstract Light-emitting diodes (LEDs) with InGaN/GaN triangular multiple quantum wells (MQWs) were fabricated by low-pressure metalorganic chemical vapour deposition and their electrical and optical properties were investigated. The LEDs with triangular QWs showed a lower operation voltage and a higher light output power than those with the rectangular MQW LEDs. Photoluminescene peak energy of MQWs with triangular structure was independent of temperature. Electroluminescene spectrum of the triangular-qw-based LEDs also showed that the peak energy is nearly independent of the injection current. I. Introduction In recent years, there has been significant progress for the commercial production of lightemitting diodes (LEDs) and laser diodes (LDs) by utilizing wide band gap -nitride semiconductors. -4 Especially InGaN alloy is very important for applications of -nitride materials in LEDs and LDs, because the alloy constitutes the active region in the form of quantum well (QW) and emits light by the recombination of electrons and holes injected into the InGaN. GaN and AlGaN are usually adopted for the barrier of QW in accordance with the emitting spectrum range of interest. Most commonly used QW structure is a rectangular-type, where In in the well layer of InGaN is generally kept at constant composition. In such a case, spatial indirect recombination is expected due to internal piezoelectric field, and thereby poor light emission. However, the emission efficiency is in fact extraordinarily high, which is probably attributed to fluctuation of In content or formation of quantum dots (QDs) in QW. 5,6 Due to high In-phase, whether it is In-composition fluctuation or QDs, the band edges in the well are believed to have oppositely directed triangular-like shape in the conduction and valence band edges. As a result, a spatial direct band gap lower than the indirect gap induced by the internal field would be induced. To improve the emission efficiency or suppress the piezoelectric field effect, we examined a triangular-type QW structure, which is a graded index separate confinement structure heterostructure. Such a structure was first investigated in quantum well lasers of GaAs/AlGaAs, and resulted in highly efficient, very low threshold lasers, capable of very high power operation. 7,8 However, little work has been reported on the applications of the triangular-type QW structure for GaN-based LEDs or LDs. In this paper, we report the electrical and optical properties of the LEDs fabricated by using multiple InGaN/GaN triangular QWs. The triangular band structure in the QW was obtained by modulating In composition in the InGaN well. Their characteristics were compared with those of the LEDs having a rectangular QW structure in terms of I-V characteristics, output power, and EL spectrum. * Corresponding author: ybhahn@moak.chonbuk.ac.kr

2 II. Experimental The samples were grown on c-plane sapphire substrates by a low-pressure metal-organic chemical vapor deposition (MOCVD) system. Trimethylgallium (TMGa), trimethylindium (TMIn), ammonia (NH 3 ), and silane (SIH 4 ) were used as the precursors of Ga, In, N, and Si, respectively. Before growing the nitride films, the substrates loaded into the reactor were thermally cleaned in hydrogen atmosphere at 00 o C for 0 min. A GaN nucleation layer of 25 nm thickness was grown on the cleaned substrate at 560 o C, and a 3-µm-thick GaN:Si was then grown at 30 o C. Figure (left) shows the In x Ga -x N/GaN MQW LED structure grown on the n-gan at 795 o C, having a 5-period InGaN(2 nm)/gan(6 nm) quantum well structure. The triangular- or rectangular-type MQWs of InGaN/GaN were grown by changing In composition with time during the growth as shown in Fig. (right). A 0.25-µm-thick Mg-doped p-gan was grown finally at 00 o C on the top of MQWs. The dopant source was cyclopentadienyl-magnesium (Cp 2 Mg). For fabrication of the InGaN/GaN MQW LED chips, the processing procedures were summarized as: ) SiO 2 film was deposited by PECVD onto the epiwafer as the etch mask before ICP mesa etching, 2) inductively coupled plasma (ICP) etching with Cl 2 /Ar was carried out to form a mesa structure, 3) Au(6 nm)/ni(6 nm) bi-layer for transparent layer was deposited on the p-gan by e-beam evaporation and lift-off, 4) Ti(30 nm)/al(70 nm) bi-layer for n-type contact was deposited and patterned by lift-off, and 5) A Ni/Au (30 nm/ 00 nm) bilayer was deposited as the p-type electrode. These metal contacts were annealed at 500 for 20 seconds under air ambient. The size of LED chip was µm 2. Details of the ICP etching for mesa structure are available elsewhwere. 9 The output power and currentvoltage (I-V) characteristics were measured at room temperature using an HP 455A semiconductor parameter analyser. III. Results and Discussion Figure 2 shows the I-V characteristics (top) and output power vs. injection current (bottom) of the InGaN/GaN MQW LEDs, compared with the triangular and rectangular QW structures, respectively. The LEDs with triangular QWs showed a lower operation voltage of about 3.34 V than that of rectangular QWs (3.7 V) at 20 ma injection current (top). Also, the triangular-type MQW LEDs showed a higher light output power than the rectangular MQW LEDs (bottom). We believe that this is attributed to the formation of densely and uniformly distributed QDs in triangular QW structures, as shown in a plan-view of TEM image (Fig. 3). The QD sizes are in a range of nm, much smaller than those in rectangular QWs (~ 00 nm). 0 It is known that QDs in the InGaN/GaN QWs are generally formed at higher In compositions (> 5 %). As the In content is graded in the course of growing the triangular QW structure, QDs in the triangular become smaller than those in the rectangular QW. It is also interesting to see that the QDs are surrounded by dislocation line, implicating an interaction between the dislocation and the nucleus of QD formation. This result indicates that we can control the size and the distribution of quantum dots in the InGaN/GaN QW system by grading the In composition. Figure 4 shows the EL spectra as a function of injection current of the InGaN/GaN MQW LEDs measured at room temperature. The triangular MQW LEDs showed higher intensities and narrower line widths than those of rectangular MQW LEDs. This is presumably due to the size distribution and/or compositional fluctuation of QDs. Above all, the peak energy is almost independent of the injection current in the triangular-qw-based LEDs as shown in Fig. 4. By contrast, the energy in the rectangular-qw-based-led exhibits slight blue shift with increasing the injection current. The blue shift of the EL of the rectangular MQW LEDs with increasing injection current is attributed to the quantum confined stark effect (QCSE), resulted from piezoelectric fields induced by the lattice mismatch.,2 2

3 Although not illustrated, from PL spectra measured as a function of temperature, we observed the PL peak energy of MQWs with triangular structure was almost independent of temperature. This indicates that we achieved high quality InGaN films having densely and uniformly distributed QDs using triangular QW structure for the active layer. We also observed very bright and uniform light emission from the triangular MQW LEDs at a low injection current, but spatially non-uniform emission from the rectangular MQW LEDs. These results lead to a conclusion that QD engineering by changing the QW structure is a feasible tool to improve the electrical and optical properties of InGaN/GaN MQW LEDs or LDs. However, to further elucidate the properties of InGaN/GaN MQW LEDs with the triangular quantum well structure, more detailed analyses including an aging test are underway. IV. Summary and Conclusions In summary, InGaN/GaN MQW LED structures with triangular quantum wells were investigated and compared with rectangular ones in terms of electrical and optical properties. The LEDs with triangular QWs showed a lower operation voltage and a higher light output power than those with the rectangular MQW LEDs. This is presumably due to the formation of densely and uniformly distributed QDs in triangular QW structures, which are much smaller than those in rectangular QWs. EL spectrum as a function of injection current showed that the peak energy is nearly independent of the injection current in the triangular-qw-based LEDs. PL peak energy of MQWs with triangular structure was also independent of temperature, indicating that high quality InGaN films having densely and uniformly distributed QDs with the triangular QW structure for the active layer were obtained. References. S. Nakamura, M. Senoh, S. Nagahama, N. Iwasa, T. Yamada, H. Kiyoku, Y. Sugimoto, T. Kozaki, H. Umemoto, M. Sano, and K. Chocho, Jpn. J. Appl. Phys., Part 2, 36, L568 (997). 2. P. A. Crowell, D. K. Yong, S. Keller, E. L. Hu, and D. D. Awschalom, Appl. Phys. Lett., 72, 927 (998). 3. P. Perlin, C. Kisielowski, V. Iota, B. A. Weinstein, L. Mattos, N. A. Shapiro, J. Kruger, E. R. Weber, and J. Yang, Appl. Phys. Lett. 73, 2778 (998). 4. T. Wang, J. Bai, and S. Sakai, Appl. Phys. Lett. 78, 267 (200). 5. Y. Arakawa, T. Someya, and K. Tachibana, Proc. Int. Workshop on Nitride Semiconductor, IPAP Conf. Series, 403 (2000). 6. S. Nakamura, Jpn. Soc. Appl. Phys. International No., 5 (2000). 7. W. T. Tsang, Appl. Phys. Lett., 40, 27 (982). 8. W. T. Tsang, M. C. Wu, T. Tanbun-Ek, R. A. Logan, S. N. G. Chu, and A. M. Sergent, Appl. Phys. Lett., 57, 2065 (990). 9. Y. B. Hahn, R. J. Choi, J. H. Hong, H. J. Park, H. J. Lee, J. Appl. Phys., 92, 89 (2002). 0. M. S. Jeong, J. Y. Kim, Y. W. Kim, J. O. White, E. K. Suh, C. H. Hong, and H. J. Lee, Appl. Phys. Lett., 79, 976 (200).. J. S. IM, H. Kollmer, J. Off, A. Sohmer, F. Scholz, and A. Hangleiter, Phys. Rev. B 57, R9435 (998). 2. T. Mukai, M. Yamada, and S. Nakamura, Jpn. J. Appl. Phys., Vol. 37, L358 (998). 3

4 p-electrode Transparent layer GaN: Mg, ~ 250 nm GaN: Si, ~ 3000 nm In -x Ga x N/GaN MQWs n-electrode GaN In -x Ga x N/GaN GaN CB Sapphire GaN nucleation layer VB Figure. The schematic illustration of InGaN/GaN multiple quantum well LED structure and triangular quantum well structure grown by grading In composition with time Current (ma) Output power (mw) Foward voltage (V) Injection current (ma) Figure 2. I-V characteristics (top) and output power vs. injection current (bottom) of InGaN/GaN MQW LEDs, compared with the triangular and rectangular QW structures, respectively. 4

5 Figure 3. Plan-view TEM image of InGaN/GaN triangular quantum well. EL Intensity (arb. units) nm 467 nm T= 20 I=5 ma I=0 ma I=20 ma I=40 ma I=60 ma I=80 ma Wavelength (nm) EL Intensity (arb. units) nm 479 nm T= 20 I=5 ma I=0 ma I=20 ma I=40 ma I=60 ma I=80 ma Wavelength (nm) Figure 4. EL spectrum of the InGaN/GaN MQW LED as a function of injection current at 20 o C with triangular and rectangular quantum wells. 5