Chapter 6 Potential barriers and current-blocking structures induced by V-shaped
6.6 Conclusions
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The effects of V-shaped pits on the performance of LEDs were studied. The TEM images indicate that these V-shaped pits initiated at threading dislocations and, thus, enclosed the pits. Additionally, the quantum wells grown on the sidewalls of V-shaped pits were thinner than those grown on normal regions.
Due to the quantum confinement effect, the wells surrounding the threading dislocations had a larger effective band gap compared with that of normal wells grown of the (0001) plane. Accordingly, a potential barrier formed at the interface between these two wells. This barrier is beneficial to internal quantum efficiency since electron-hole pairs in normal wells cannot overcome the barrier to nonradiative dislocations. The two emission peaks in PL spectra confirmed the existence of these two effective band gaps. The energy of the barrier in the sample, based on the energy difference of these two PL peaks, was 0.54eV, which is sufficient for blocking electron-hole pairs since it is much larger than thermal energy KT, even at room temperature. The merit of this barrier is also demonstrated by the weak dependence of main peak intensity on temperature compared with that of the high-energy peak. Furthermore, the EL spectra were studied. Since the injection mechanism of electron-hole pairs by electrical current differs from that of optical excitation, an unintentionally formed current-blocking structure formed on the V-shaped pit was found by comparing the EL and PL spectra. This blocking structure effectively suppresses current flowing into the dislocations at the centers of V-shaped pits. In light of the possible paths from electron-hole to dislocations, since both are blocked either by a potential barrier in a well or current blocking structure upon a V-shaped pit, the drawbacks arising from dislocations are reduced, and the internal quantum efficiencies of nitride LEDs are improved by these V-shaped pits. However, the details of the blocking mechanism need further investigations.
96
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98
Fig. 6.1 Sample structure InGaN
p-GaN p-AlGaN MQWs x 9
LT n-GaN n-GaN u-GaN Nucleation layer Sapphire substrate
99
Fig. 6.2 (a) Cross-sectional TEM image of the LED structure. (b) The magnified image of a V-shaped pit. The well grown on the sidewall is narrower than that on the c-plane
100
Fig. 6.3 The Cross-sectional TEM images of the MQWs on normal plane
101
Fig. 6.4 (a) These V-shaped pits have a six-wall structure with the walls corresponding to the six symmetrical {1011} planes. [7] (b) The void associated with V-shaped pits can be filled by p-type layers [9]
102
Fig. 6.5 (a) Cross-sectional TEM image of the LED structure. (b) Magnified image of a V-shaped pit. The dashed line shows one quantum well, and A, B, and C identify three different points in the well
(b)
20nm A B
C
n - GaN LT n++ - GaN
MQWs p - AlGaN
p - GaN
200nm (a)
V-pit
V-pit
103
Fig. 6.6 (a) The PL spectra measured at different temperatures. (b) The log scale of the PL spectra
104
wavelength (nm)
350 400 450 500 550
P L i n te n s ity ( a .u .)
intensities and intensity ratio of the high-energy peak and main peak in (a)105
Fig. 6.8 Band diagram of the well along the dashed line (Fig. 1(b)). A, B, and C denote different points and correspond to A, B, and C in Fig. 1(b), respectively.
The solid lines denote the conduction band and valence band edges, while dashed lines denote the ground-states of electrons and holes. The electrons and holes are denoted by solid and empty circles, respectively
E
h0dislocation defects
A B C
hv hv hv
E
e0valence band conduction
band
well on (0001) well on sidewall
of V-shaped pit
106
2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6
300K
30K
30K 40K 60K 80K 100K 120K 140K 160K 180K 200K 220K 240K 260K 280K 300K
Intensity (a.u.)
Energy (eV)
Fig. 6.9 The EL spectra of the device measured under 20mA injection at different temperatures
107
350 400 450 500 550 600
100
350 400 450 500 550 600
100 different range of temperatures. (a) 60K - 160K (b) 170K – 300K
108
2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 10-3
10-2 10-1 100
normalized Intensity (a.u.)
Energy (eV)
PL 50K EL 50K
2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 10-3
10-2 10-1 100
normalized Intensity (a.u.)
Energy (eV)
PL 140K EL 140K
109
2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6
10-3 10-2 10-1 100
normalized Intensity (a.u.)
Energy (eV)
PL 300K EL 300K
Fig. 6.11 Normalization of the PL and EL spectra at (a) 50K (b) 140K (c) 300K
110 high-energy peak and main peak in (a)
111
Publish list Refereed Papers
1. Y. S. Wang, N. C. Chen, C. Y. Lu, and J. F. Chen, “Optical joint density of states in InGaN/GaN-based multiple-quantum-well light-emitting diodes ”, Physica B. 406, 4300 (2011). (SCI)
2. M. C. Hsieh, J. F. Wang, Y. S. Wang, C. H. Yang, Ross C. C. Chen, C. H.
Chiang, Y. F. Chen, and J. F. Chen, “Role of the N-related localized states in the electron emission properties of a GaAsN quantum well”, J. Appl. Phys.
110, 103709 (2011) (SCI)
3. M. C. Hsieh, J. F. Wang, Y. S. Wang, C. H. Yang, C. H. Chiang, and J. F.
Chen, “Electron emission properties of GaAsN/GaAs quantum well containing N-related localized states: the influence of illuminance”, Jpn. J.
Applied Phys. 51, 02BJ12 (2012). (SCI)
4. N. C. Chen, W. C. Lien, Y. K. Yang, C. Shen, Y. S. Wang, and J. F. Chen,
“Spectral shape and broadening of emission from AlGaInP light-emitting diodes ”, J. Appl. Phys. 106, 074514 (2009). (SCI)
5. N.C. Chen, Y.N. Wang, Y.S. Wang, W.C. Lien, and Y.C. Chen, “Damage of light-emitting diodes induced by high reverse-bias stress”, J. Crystal Growth 311, 994, (2009). (SCI)
6. J. F. Chen, C. H. Yang, R. M. Hsu, and Y. S. Wang, “Influence of thermal annealing on the electron emission of InAs quantum dots containing a misfit defect state”, J. Appl. Phys. 105, 063705 (2009). (SCI)
7. N.C. Chen, W.C. Lien, Y.S. Wang, and H.H. Liu, “Capacitance-Voltage and Current-Voltage Measurements of Nitride Light-Emitting Diodes”, IEEE T Electron Dev. 54, 3223 (2007). (SCI)
Conference papers
a. International conference
112
1. Y. S. Wang, N. C. Chen, M. C. Hsieh, J. B. Huang, L. S. Hong, and J.
F.Chen. “Elucidating the electrical characteristics of an inversion domain boundary in p-type GaN of light-emitting diodes” p-7-5 2011 Internal Conference on Solid State Devices and Materials, Nagoya, Japan, 28~30, September, 2011.
2. Y. S. Wang, N. C. Chen, and J. F. Chen, “Diffusion-controlled effects of luminescent efficiency in InGaN/GaN light-emitting diodes”, 2700-P0-6, International Quantum Electronics Conference and Conference on Lasers and Electro-Optics, IQEC/CLEO Pacific Rim 2011, Sydney, Australia, August 28 - September 1, 2011.
3. N. C. Chen, W. C. Lien, Y. K. Yang, Y. S. Wang, and J. F. Chen, “GaN grown on boron-implanted silicon (111) substrates”, G2-7, 2011 IEEE Internal NanoElectronics Conference, Chang Gung Univ., Taoyuan, Taiwan June 21-24, 2011.
4. Y. K. Yang, Y. S. Wang, J. W. Chiu, and N. C. Chen, “Heat flow properties analysis of light emitting diodes luminaire”, G11-3, 2011 IEEE Internal NanoElectronics Conference, Chang Gung Univ., Taoyuan, Taiwan June 21-24, 2011.
5. Y. C. Huang, Y. S. Wang, W. J. Wang, and N. C. Chen, “AlGaInP LEDs Reliability Dependence on Different Mg Doping Concentration”, 4700-P0-7, International Quantum Electronics Conference and Conference on Lasers and Electro-Optics, IQEC/CLEO Pacific Rim 2011, Sydney, Australia, August 28 - September 1, 2011.
6. Y.S. Wang, C. H. Chiang, W. C. Lien, Y. K. Yang, J.B. Huang, L. S. Hong, P. H. Ho, N. C. Chen, and J. F. Chen, “Effects of inversion domain in p-type GaN of Nitride light-emitting diodes”，GB16, 2009 International Electron Devices and Materials Symposia, Chang Gung Univ., Taoyuan, Taiwan 19~20, November, 2009.
113
7. N. C. Chen, W. C. Lien, Y. K. Yang, Y. S. Wang, J. F. Chen, T. Y. Chen, and P. H. Ho, “GaN grown on silicon (111) substrates of transferred surface morphology”, GB16, 2009 International Electron Devices and Materials Symposia, Chang Gung Univ., Taoyuan, Taiwan 19~20, November, 2009.
8. Y.N. Wang, Y.S. Wang, W.C. Lien, Y.C. Chen and N.C. Chen, “Damage of light-emitting diodes induced by high reverse-bias stress”, 106, The 4th Asian Conference on Crystal Growth and Crystal Technology, Sendai, Japan, 21-24th, May, 2008
b. Domestic conference
9. C. Y. Lu, Y. S. Wang, W. C. Lien, and N. C. Chen, “Density of States and Blue Shift of Nitride LED”, OPT8-P-022, Optics and Photonics Taiwan 2010, Southern Taiwan University, 3-4, December, 2010.
10. Y. C. Huang, Y. S. Wang, and N. C. Chen, “AlGaInP LEDs Reliability Dependence on Different Mg Doping Concentration”, OPT8-P-023, Optics and Photonics Taiwan 2010, Southern Taiwan University, 3-4, December, 2010.
11. 王 俞 授 、 陳 乃 權 、 張 本 秀 ， ”Multipurpose-Mask for LED Device Analysis-Use Sheet resistance & Contact resistance”，2005年物 理年會.
12. 王俞授、王宥楠、施權峰、張本秀、陳乃權，”多功能MASK與LED物性量 測分析”，第三屆台塑關係企業應用技術研討會, p. C-08, 2003.