Future works

Since the optically pumped lasing action of the two dielectric DBRs VCSEL have been demonstrated, current injection GaN-based VCSELs are desire to be achieve. Recently, electrically driven cw lasing of a GaN-based VCSEL was reported [6.1]. The VCSEL consist of an epitaxially grown GaN/AlN DBR, a GaN cavity with

InGaN/GaN MQWs and a Ta2O5 /SiO2 dielectric DBR. In order to provide high reflectivity, a 29-pairs GaN/AlN was grown in the VCSEL structure of this report.

According to the two dielectric DBRs VCSEL structure, we propose a current injection GaN-based VCSEL structure with two dielectric DBRs that provide high reflectivity for reducing threshold current. Figure 6.1 shows the schematic diagram of the two dielectric DBRs VCSEL structure for electrical driving. The transparent contact on p-type GaN serves as a current spreading layer for improving uniform current distribution. Mg⁺ was implanted into the p-GaN layer before bonding process.

The implanted p-GaN forms an area with high resistivity [6.2] such that the injection current flow through the area that was not implanted. The dash lines in the figure indicate the current path that confined by the implanted area. Therefore the emission apertures of the VCSELs were defined by Mg⁺ implantation process.

Preliminary demonstration of two dielectric DBRs VCSELs

SEM image of the VCSEL device is shown as Fig. 6.2. The n-contact and

dielectric DBR were on the n-GaN surface inside the defined mesa area. The emission aperture located at the center of the DBR could be fund in the image. Figure 6.3 shows the emission patterns of the two BDRs VCSELs under different current injection levels. The light emission was found inside the aperture indicating that Mg⁺ implant area provides an effective current confinement. However non-uniform emission pots were found inside the aperture. The spotty emission could be caused by two possible reasons. The inhomogeneity indium composition in the MQws causes the inhomogeneous emission just as indicated in the previous chapter. Another reason is that the injected current could not spread uniformly over the emission aperture because the diameters of aperture (which defined by implant area) and n-contact did not much well such that the injection current could not spread to the central region of the aperture but flow directly to the p-contact (bonding metal) along the fringe region of the aperture.

There are some issues that need to be improved and noticed for the following works. Since the ITO layer was embedded between the epitaxial GaN-based structure and dielectric DBRs, the ITO layer should be consider as a part of the resonant cavity.

That is, the thickness of the ITO layer should be taken into consideration as designing the cavity length of the two dielectric VCSEL structure. The thickness of the ITO layer has influence on the overlap between ant-node of optical standing field and

MQWs. The thick thickness of the epitaxial structure is another noticeable issue. As mentioned in the previous chapter, in order to protect the MQWs from damaging during LLO, the total thickness of the epitaxial structure is about 4 μm. The thick cavity length results in a high threshold current. Thus the cavity length should be reduced in the future works. Modifying the thickness of the n-GaN by ICP etching after LLO is an alternative method. Because one of the DBRs should be deposited on the n-GaN surface, the surface roughness after ICP etching need to be concerned. The diameter ratio between emission aperture and n-contact aperture is also an important parameter. According to the preliminary result, it is suggested that the diameter of emission aperture should be larger than that of n-contact aperture such that the injected current can go through the central region of the emission aperture.

Figure 6.1 Schematic diagram of the two dielectric DBRs VCSEL structure for electrical driving.

Conductive substrate

Bonding metal Mg⁺ implant area N-contact

Dielectric DBR

InGaN/GaN MQWs

Transparent contact layer

Dielectric DBR

Figure 6.2 SEM image of the two dielectric DBRs VCSEL structure.

Figure 6.3 Emission images of the two dielectric VCSEL structure operated under different injection current.

50 mA

80 mA

References

[6.1] T. C. Lu, C. C. Kao, H. C. Kuo, G. S. Huang, and S. C. Wang: Appl. Phys.

Lett. 92, 141102 (2001).

[6.2] X. A. Cao and S. J. Pearton, G. T. Dang, A. P. Zhang, and F. Ren, R. G.

Wilson, J. M. Van Hove: J. Appl. Phys. 87, 1091

Publication list

[1] C. H. Chiu, T. C. Lu, H. W. Huang, C. F. Lai, C. C. Kao, J. T. Chu, C. C. Yu, H. C.

Kuo, S. C. Wang, C. F. Lin, T. H. Hsueh: “Fabrication of InGaN/GaN nanorod light-emitting diodes with self-assembled Ni metal islands”, Nanotechnology 18, 445201 (2007)

[2] T. C. Lu, T. T. Kao, C. C. Kao, J. T. Chu, K. F. Yeh, L. F Lin, Y. C. Peng, H. W.

Huang, H. C. Kuo, S. C. Wang: “GaN-based high-Q vertical-cavity light-emitting diodes”, IEEE Elec. Devi. Lett. 28, 884 (2007)

[3] S. C. Wang, T. C. Lu, C. C. Kao, J. T. Chu, G. S. Huang, H. C. Kuo, S. W. Chen, T.

T. Kao, J. R. Chen, L. F. Lin: “Optically pumped GaN-based vertical cavity surface emitting lasers: Technology and characteristics”, Japa. J. of Appl. Phys. 46, 5397 (2007)

[4] H. G. Chen, N. F. Hsu, J. T. Chu, H. H. Yao, T. C. Lu, H. C. Kuo, S. C. Wang:

“Strong ultraviolet emission from InGaN/AlGaN multiple quantum well grown by multi-step process”, Japa. J. of Appl. Phys. 46, 2574 (2007)

[5] C. C. Kao, H. C. Kuo, K. F. Yeh, J. T. Chu, W. L. Peng, H. W. Huang, T. C. Lu, S.

C. Wang: “Light-output enhancement of nano-roughened GaN laser lift-off light-emitting diodes formed by ICP dry etching”, IEEE Phot. Tech. Lett. 19, 849 (2007)

[6] H. W. Huang, C. C. Kao, J. T. Chu, W. C. Wang, T. C. Lu, H. C. Kuo, S. C. Wang, C. C. Yu, S. Y. Kuo: “Investigation of InGaN/GaN light emitting diodes with nano-roughened surface by excimer laser etching method”, Mate. Scie. and Engi. B 136,182 (2007)

[7] J. W. Shi, H. Y. Huang, C. K. Wang, J. K. Sheu, W. C. Lai, Y-S. Wu, C. H. Chen, J.

T. Chu, H. C. Kuo, W. P. Lin, T. H. Yang, J. I. Chyi: “Phosphor-free GaN-based transverse junction light emitting diodes for the generation of white light “, IEEE Phot.

Tech. Lett. 18, 2593 (2006)

[8] H. W. Huang, C. C. Kao, J. T. Chu, W. D. Liang, H. C. Kuo, S. C. Wang, C. C. Yu:

“Improvement of InGaN/GaN light emitting diode performance with a

nano-roughened p-GaN surface by excimer laser-irradiation”, Mate. Chem. and Phys.

99, 414 (2006)

[9] J. T. Chu, T. C. Lu, M. You, B. J. Su, C. C. Kao, H. C. Kuo, S. C. Wang:

“Emission characteristics of optically pumped GaN-based vertical-cavity surface-emitting lasers”, Appl. Phys. Lett. 89, 121112 (2006)

[10] H. W. Huang, J. T. Chu, T. H. Hsueh, M. C. Ou-Yang, H. C. Kuo, S. C. Wang:

“Fabrication and photoluminescence of InGaN-based nanorods fabricated by plasma etching with nanoscale nickel metal islands”, J. of Vacu. Scie. & Tech. B 24 1909 (2006)

[11] H. W. Huang, H. C. Kuo, J. T. Chu, C. F. Lai, C. C. Kao, T. C. Lu, S. C. Wang, R.

J. Tsai, C. C. Yu, C. F. Lin: “Nitride-based LEDs with nano-scale textured sidewalls using natural lithography”, Nanotechnology 17, 2998 (2006)

[12] H. W. Huang, J. T. Chu, C. C. Kao, T. H. Hsueh, T. C. Lu, H. C. Kuo, S. C.

Wang, C. C. Yu, S. Y. Kuo: “Enhanced light output in InGaN/GaN light emitting diodes with excimer laser etching surfaces”, Japa. J. of Appl. Phys. 45 (4B): 3442 (2006)

[13] Y. C. Peng, C. C. Kao, H. W. Huang, J.T. Chu, T. C. Lu, H. C. Kuo, S. C. Wang, C. C. Yu: “Fabrication and characteristics of GaN-based microcavity light-emitting diodes with high reflectivity AIN/GaN distributed Bragg reflectors”, Japa. J. of Appl.

Phys. 45, 3446 (2006)

[14] J. T. Chu, T. C. Lu, H. H. Yao, C.C. Kao, W. D. Liang, J. Y. Tsai, H. C. Kuo, S.

C. Wang: “Room-temperature operation of optically pumped blue-violet GaN-based vertical-cavity surface-emitting lasers fabricated by laser lift-off”, Japa. J. of Appl.

Phys. 45 (4A): 2556 (2006)

[15] C.C. Kao, T.C. Lu, H.W. Huang, J.T. Chu, Y.C. Peng, H.H. Yao, J.Y. Tsai, T.T.

Kao, H.C. Kuo, S.C. Wang, C.F. Lin: ” The lasing characteristics of GaN-based vertical-cavity surface-emitting laser with AlN-GaN and Ta2O5-SiO2 distributed Bragg reflectors”, IEEE Photo. Tech. Lett. 18, 877 (2006)

[16] Y. A. Chang, J. T. Chu, C. T. Ko, H. C. Kuo, C. F. Lin, S. C. Wang: “MOCVD growth of highly strained 1.3 mu m InGaAs : Sb/GaAs vertical cavity surface emitting

laser”, J. of Crys. Grow. 287, 550 (2006)

[17] J. T. Chu, C. C.Kao, H. W.Huang, W. D.Liang, C. F. Chu, T. C. Lu, Kuo H C, S.

C. Wang: “Effects of different n-electrode patterns on optical characteristics of large-area p-side-down InGaN light-emitting diodes fabricated by laser lift-off”, Japa.

J. of Appl. Phys. 44 7910 (2005)

[18] H. C. Kuo, Y. H. Chang, Y. A. Chang, F. I. Lai, J. T. Chu, M. N. Tsai, S. C. Wang:

“Single-mode 1.27-mu m InGaAs : Sb-GaAs-GaAsP quantum well vertical cavity surface emitting lasers”, IEEE J. of Sele. Topi. in Quan. Elec. 11, 121 (2005)

[19] H. W. Huang, J. T. Chu, C. C. Kao, T. H. Hseuh, T. C. Lu, H. C. Kuo, S. C.

Wang, C. C. Yu: “Enhanced light output of an InGaN/GaN light emitting diode with a nano-roughened p-GaN surface”, Nanotechnology 16, 1844 (2005)

[20] C. C. Kao, Y. C. Peng, H. H. Yao, J. Y. Tsai, Y. H. Chang, J. T. Chu, H. W.

Huang, T. T. Kao, T. C. Lu, H. C. Kuo, S. C. Wang, C. F. Lin: “Fabrication and performance of blue GaN-based vertical-cavity surface emitting laser employing AlN/GaN and Ta2O5/SiO2 distributed Bragg reflector”, Appl. Phys. Lett. 87,081105 (2005)

[21] J. T. Chu, H. W. Huang, C. C. Kao, W. D. Liang, F. I. Lai,C. F. Chu, H. C. Kuo, S. C. Wang: “Fabrication of large-area GaN-based light-emitting diodes on Cu substrate” , Japa. J. of Appl. Phys. 44, 2509 (2005)

[22] Y. H. Chang, H. C. Kuo, Y. A. Chang, J. T. Chu, M. N. Tsai, S. C. Wang: “10 Gbps InGaAs : Sb-G-aAs-GaAsP quantum well vertical cavity surface emitting lasers with 1.27 mu m emission wavelengths”, Japa. J. of Appl. Phys. 44, 2556 (2005)

[23] H. W. Huang, C. C. Kao, J. T. Chu, H. C. Kuo, S. C. Wang, C. C. Yu:

“Improvement of InGaN-GaN light-emitting diode performance with a nano-roughened p-GaN surface”, IEEE Phot. Tech. Lett. 17, 983 (2005)

[24] C. C. Kao, H. W. Huang, J. T. Chu, Y. C. Peng, Y. L. Hsieh, C. Y. Luo, S. C.

Wang, C. C. Yu, C. F. Lin: “Light-output enhancement in a nitride-based light-emitting diode with 22 degrees undercut sidewalls”, IEEE Phot. Tech. Lett. 17, 19 (2005)

[25] H. W. Huang, C. C. Kao, T. H. Hsueh, C. C. Yu, C. F. Lin, J. T. Chu, H. C. Kuo, S. C. Wang: “Fabrication of GaN-based nanorod light emitting diodes using self-assemble nickel nano-mask and inductively coupled plasma reactive ion etching”, Mate. Scie. and Engi. B 113, 125 (2004)

[26] J. T. Chu, H. C. Kuo, C. C. Kao, H.W. Huang, C. F. Chu,C. F. Lin, and S. C.

Wang: “Fabrication of p-side down GaN vertical light emitting diodes on copper substrates by laser lift-off”, physica status solidi (c) 1, 2413 (2004)

[27] H. W. Huang, C. C. Kao, J. T. Chu, H. C. Kuo, S. C. Wang, C. C. Yu, C. F. Lin:

“Investigation of GaN LED with Be-implanted Mg-doped GaN layer”, Mate. Scie.

and Engi. 113, 19 (2004)

[28] C. F. Chu , F. I. Lai , J.T. Chu , C. C. Yu , C. F. Lin , H. C. Kuo , S. C. Wang:

“Study of GaN light-emitting diodes fabricated by laser lift-off technique,” J. of Appl. Phys. 95, 3916 (2004)

[29] T. C. Lu, J. Y. Tsai, J. T. Chu, Y. S. Chang, S. C. Wang: “InP/InGaAlAs distributed Bragg reflectors grown by low-pressure metal organic chemical vapor deposition” J. of Crys. Grow. 250, 305 (2003)