- Email: [email protected]
Thermal degradation in InGaN quantum wells in violet and blue GaN-based laser diodes
Current Applied Physics 11 (2011) S167eS170
Contents lists available at ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Thermal degradation in InGaN quantum wells in violet and blue GaN-based laser diodes Jihoon Kim a, b, Hyunsoo Kim c, Sung-Nam Lee a, * a
Optoelectronic Materials & Devices Lab., Department of Nano-Optical Engineering, Korea Polytechnic University, Siheung 429-793, Republic of Korea Future Convergence Ceramic Division, Korea Institute of Ceramic Engineering and Technology, Seoul 153-801, Republic of Korea c School of Semiconductor and Chemical Engineering, Semiconductor Physics Research Center, Chonbuk National University, Jeonju 561-756, Republic of Korea b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 October 2010 Received in revised form 24 December 2010 Accepted 9 February 2011 Available online 14 July 2011
The effects of thermal damage in AlInGaN-based light emitting devices, with InxGa1 xN single quantum well (QW) as an active layer, from violet to blue emissions are reported. The intensity of the electroluminescence (EL) of laser diodes (LDs) grown at high temperatures (>1000 C) for p-type layers was drastically decreased above approximately 440 nm. The threshold current and slope efficiency of the LDs were significantly deteriorated with an increase in the lasing wavelength from 405 to 435 nm. From the TEM and AFM measurements, the surface degradation and In phase separation was observed in the blue InGaN QW structure due to the surface migration of adatoms and the spinodal decomposition during the high temperature ramp-up and long growth time for the p-type layers, respectively. Crown Copyright Ó 2011 Published by Elsevier B.V. All rights reserved.
Keywords: GaN Laser diode (LD) Light-emitting diode (LED) Quantum well (QW)
1. Introduction III-nitrides have attracted much attention for application in optoelectronic devices with emission wavelengths ranging from ultraviolet to green light due to their wide bandgap [1e3]. However, some problems remain in achieving high performance light emitting devices with longer wavelengths, such as blue and green light emitting diodes (LEDs) and laser diodes (LDs) [4e6]. Among several issues in film growth, it is well known that the significant difference in the growth temperatures between the GaN and InGaN epilayers leads to the crystal and optical degradations of the InGaN active layer [1e3]. In particular, significant problems can occur in blue and green GaN-based LED/LDs with high In content (>15%) InGaN quantum well (QW) structures. Due to the solid phase immiscibility in the InGaN/GaN material system, the In phase separation is expected to have significant growth problems in the typical epitaxial growth as presented in the theoretical study by Ho and Stringfellow [7]. Evidence of phase separation has been reported previously in polycrystalline InGaN films annealed at temperatures below 700 C by Osamura et al. [8]. For InGaN QWs, the phase separation and formation of In-rich InGaN segregations were also observed via the
* Corresponding author. E-mail address: [email protected] (S.-N. Lee).
annealing process at a temperature of 950 C for 40 h [9]. The interdiffusion of In and Ga in the InGaN/GaN QW structures, accompanied with the lack of phase separation, was also found at high annealing temperatures of 1300e1400 C [10]. Besides, the luminescence from InGaN QWs was proposed to be closely related to the In-rich regions that are quantum dot-like structures as observed using transmission electron microscopy (TEM) [11,12]. These structural degradations of the InGaN active layer should be understood in order to achieve high performance blue/green LD/ LEDs. Therefore, the effects of the thermal degradation on violet and blue InGaN QW LDs were systematically investigated with different In compositions after the in situ thermal ramp-up process and growth of a thick p-type layer at a high temperature of 1030 C.
2. Experimental In0.15Ga0.85N QW structures were prepared to clarify the thermal damage during the temperature ramp-up process, which consists of the temperature ramp-up to a high temperature of 1030 C and a cool-down to room temperature, without the growth of p-type layers using metalorganic chemical vapor deposition (MOCVD). In addition, the LD structures including clad, waveguide, QWs, and electron blocking layer (EBL) were grown on GaN/sapphire templates in order to investigate the thermal degradation of the InxGa1 xN QW in LD structures during high growth temperature of
1567-1739/$ e see front matter Crown Copyright Ó 2011 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2011.07.024
S168
J. Kim et al. / Current Applied Physics 11 (2011) S167eS170
Fig. 1. The EL of the GaN-based LDs and the PL intensities InGaN QW structures with different In content as a function of spontaneous emission wavelength.
p-type layers [4,5]. The active layer was composed of 25 Å InxGa1 xN wells separated by 100 Å InyGa1 yN barriers, where the x and y were chosen to be 8e15% and 2%, respectively, to achieve the emission wavelength from violet to blue. To control the In content in the InGaN well, the growth temperature of the InGaN QW was changed from 790 C to 700 C. For the EBL between the InGaN QW and p-type GaN, p-type AlGaN/GaN multi-quantum barriers were employed to effectively suppress the electron overflow to the p-
Fig. 2. The light output power of the GaN-based LD with lasing wavelengths of 405 and 435 nm plotted as a function of the injection current.
type layers [4]. The growth temperature of the p-type layers was 1030 C. Electroluminescence (EL) analyses were performed by forming In dots as n-type and p-type contact metals on the LD wafers. Additionally, the ridge waveguide stripe was produced via the LD chip fabrication process. The cavity length and width of the ridge stripe were 600 mm and 2.0 mm, respectively. The lasing
Fig. 3. (a) and (b) AFM surface morphologies and (c) SEM cross section image of In0.15Ga0.85N QW without the temperature ramp-up process. After the temperature ramp-up process, (b) and (c) show AFM surface morphology and SEM image of In0.15Ga0.85N QW region, respectively.
J. Kim et al. / Current Applied Physics 11 (2011) S167eS170
performances were evaluated under cw conditions at room temperature as the ridge waveguide structures. The surface morphologies of the InGaN QW were analyzed using an atomic force microscope (AFM). Then, the interfacial and crystal qualities of InGaN QW were evaluated via high resolution transmission electron microscope (HR-TEM) and scanning electron microscope (SEM). 3. Results and discussion Fig. 1 shows the EL intensities of the GaN-based LDs and the PL intensities of the InGaN QWs with different In contents as a function of the emission wavelength. The EL intensity of the LDs with p-type layers grown at a temperature of 1030 C was significantly decreased with an increase in the emission wavelength above 440 nm. In particular, the EL intensity of the violet LD with an emission wavelength of 395 nm was three orders higher than that of the blue LD with a wavelength of 453 nm. However, the PL intensity of the InGaN QW grown on the GaN/ sapphire template slightly decreased by increasing the emission wavelength up to 455 nm due to the intrinsic material degradation of the InGaN with a high In content, such as the In phase segregation and generation of point defects [7,11,12]. Both results imply that the optical quality of the InGaN active layer could be maintained up to the blue region (w450 nm) by removing the ptype layers grown at a high temperature. In general, a high growth temperature is required to achieve high quality p-type layers that consist of EBL, p-GaN waveguide, p-AlGaN cladding layer, and p-contact in the LD structures [13]. It is suggested that the InGaN active layer would be exposed to a high temperature, resulting in the acceleration of the diffusion-induced In phase segregation in the InGaN active region. Therefore, it can be assumed that the structural and optical degradation of the InGaN QW would be significant due to the growth of the p-type layer in the LD structure. Fig. 2 shows the light output power of the GaN-based LDs with lasing wavelengths of 405 nm and 435 nm as a function of the injection current. The threshold current and slope efficiency of the InGaN LDs with 405 nm were 35 mA and 1.0 W/A, respectively, in cw conditions at room temperature. However, the lasing performances of the LDs with a blue emission wavelength of 435 nm were significantly deteriorated. The threshold current and slope efficiency of LDs were 118 mA and <0.5 W/A, respectively. In addition, the lasing performance could not be obtained for the GaN-based LD with a blue emission wavelength of more than 440 nm in these experiments. These results are consistent with the drastic reduction of the EL intensity with the increasing emission wavelength shown in Fig. 1. Thus, it is believed that the optical deterioration of the LD structure is primarily caused by the thermal degradation of the InGaN active layer with a high In content. For the thermal degradations of the blue InGaN active layer in the LD structures, two causes are deduced: one is the short time thermal degradation during the temperature ramp-up process used to grow the p-type layer just after the growth of InGaN QW; the other is the long time thermal damage during the p-type layers growth at a high temperature of 1030 C. Fig. 3(a) and (b) shows the AFM surface morphologies (2 2 mm2) of the blue In0.15Ga0.85N QW structures without and with the high temperature ramp-up treatment, respectively. As shown in Fig. 3(a), the surface morphology of the blue InGaN QW structure showed a spiral surface morphology that is a typical surface structure of InGaN epilayers. However, the surface morphology of the blue InGaN QW was modified from a spiral to a step-like surface structure after the temperature ramp-up and annealing process (30 s) at 1030 C [6]. This significant
S169
modification of the InGaN surface structure can be explained by the In and Ga atoms being desorbed, re-deposited, diffused, and migrated on the surface of the InGaN QW under NH3 and N2 atmospheres in the MOCVD reactor. Fig. 3(c) and 3(d) shows the SEM images of the InGaN QW without and with the temperature ramp-up process, respectively. There are no In segregations in the InGaN QW regions in both samples. From the AFM and SEM analyses, it is believed that only the temperature ramp-up process after the InGaN QW growth can affect the surface modification, rather than the In phase separation, in the InGaN regions. Fig. 4(a) and (b) shows the cross section HR-TEM images of the GaN-based LDs with different In compositions of 8% and 15% in the InGaN QWs, respectively. The interface quality of the violet In0.08Ga0.92N QWs LD was much better than that of the blue In0.15Ga0.85N QW LD. Although the growth temperature of the players for both samples was 1030 C, it was found that the In phase segregations were significantly developed at the In0.15Ga0.85N QW
Fig. 4. HR-TEM cross section images of (a) In0.08Ga0.92N and (b) In0.15Ga0.85N QW LD with high temperature p-type layers at a high growth temperature of 1030 C.
S170
J. Kim et al. / Current Applied Physics 11 (2011) S167eS170
region in the blue LD as shown in Fig. 4(b). It implied that the In phase separation and interfacial deterioration were caused by the interdiffusion of the In and Ga atoms in the InGaN QW with a high In content during the growth of the thick p-type layers at a high temperature of 1030 C. In addition, there was no In phase segregation in the In0.08Ga0.92N QW region. From these results, it is assumed that the In phase segregation would be significantly affected by the In composition in the InGaN QWs as well as the growth temperature of the p-type layers. Therefore, it is suggested that the high temperature process is a critical issue in achieving blue and green GaN-based LD structures with higher In compositions (>15%) in InGaN QWs. 4. Conclusion Two types of significant degradation processes were observed for InGaN QW structures with different In compositions in violet and blue LDs. The first significant process was the temperature ramp-up process for the growth of the p-type layer just after the growth of the InGaN QW, which results in a drastic surface modification of the InGaN QWs due to the desorption, migration, and surface diffusion of the In, Ga, and N atoms. The second significant degradation process was the high temperature p-type layer growth that results from the interdiffusion of the In atoms between the well and barrier by introducing a high thermal energy for a long time, which results in the In phase separation in the InGaN active layer. Therefore, it is suggested that high performance blue LDs can be achieved by suppressing the thermal degradation of the InGaN QW region.
Acknowledgment This research was supported by the Basic Science Research Program (Grant no. 2010-0011815) and Core Corporation Research Program (Grant no. 2010-0026523) through the National Research Foundation of Korea (NRF) funded by the Korean Ministry of Education, Science and Technology (MEST).
References [1] S.N. Lee, J.K. Son, H.S. Paek, T. Sakong, W. Lee, K.H. Kim, E. Yoon, J.Y. Kim, Y.H. Cho, Y.J. Lee, S.S. Kim, D.Y. Noh, O.H. Nam, Y. Park, J. Cryst. Growth 275 (2005) e1041. [2] D.H. Yoon, K.S. Lee, J.B. Yoo, T.Y. Seong, Jpn. J. Appl. Phys. 41 (2002) 1253. [3] S. Keller, B.P. Keller, D. Kapolnek, U.K. Mishra, S.P. DenBaars, I.K. Shmagin, R.M. Kolbas, S. Krishnankutty, J. Cryst. Growth 170 (1997) 349. [4] S.N. Lee, S.Y. Cho, H.Y. Ryu, J.K. Son, H.S. Paek, T. Sakong, T. Jang, K.K. Choi, K.H. Ha, M.H. Yang, O.H. Nam, Y. Park, Appl. Phys. Lett. 88 (2006) 111101. [5] S.N. Lee, H.Y. Ryu, H.S. Paek, J.K. Son, Y.J. Sung, K.S. Kim, H.K. Kim, H. Kim, T. Jang, K.H. Ha, O.H. Nam, Y. Park, IEEE Elect. Device Lett. 29 (2008) 870. [6] S.N. Lee, H.S. Paek, J.K. Son, T. Sakong, Y.N. Kwon, O.H. Nam, Y. Park, Phys. Stat. Sol. C 4 (2007) 141. [7] I.H. Ho, G.B. Stringfellow, Appl. Phys. Lett. 69 (1996) 2701. [8] K. Osamura, S. Naka, Y. Murakami, J. Appl. Phys 46 (1975) 3432. [9] M.D. McCluskey, L.T. Romano, B.S. Krusor, D.P. Bour, N.M. Johnson, S. Brennan, Appl. Phys. Lett. 72 (1998) 1730. [10] M.D. McCluskey, L.T. Romano, B.S. Krusor, N.M. Johnson, T. Suski, J. Jun, Appl. Phys. Lett. 73 (1998) 1281. [11] Y. Narukawa, Y. Kawakami, M. Funano, S. Fujita, S. Nakamura, Appl. Phys. Lett. 70 (1997) 981. [12] C.-C. Chuo, C.-M. Lee, T.-E. Nee, J.-I. Chyi, Appl. Phys. Lett. 76 (2000) 3902. [13] S.N. Lee, T. Jang, J.K. Son, H.S. Paek, T. Sakong, E. Yoon, O.H. Nam, Y. Park, J. Cryst. Growth 287 (2006) 554.
Contents lists available at ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Thermal degradation in InGaN quantum wells in violet and blue GaN-based laser diodes Jihoon Kim a, b, Hyunsoo Kim c, Sung-Nam Lee a, * a
Optoelectronic Materials & Devices Lab., Department of Nano-Optical Engineering, Korea Polytechnic University, Siheung 429-793, Republic of Korea Future Convergence Ceramic Division, Korea Institute of Ceramic Engineering and Technology, Seoul 153-801, Republic of Korea c School of Semiconductor and Chemical Engineering, Semiconductor Physics Research Center, Chonbuk National University, Jeonju 561-756, Republic of Korea b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 October 2010 Received in revised form 24 December 2010 Accepted 9 February 2011 Available online 14 July 2011
The effects of thermal damage in AlInGaN-based light emitting devices, with InxGa1 xN single quantum well (QW) as an active layer, from violet to blue emissions are reported. The intensity of the electroluminescence (EL) of laser diodes (LDs) grown at high temperatures (>1000 C) for p-type layers was drastically decreased above approximately 440 nm. The threshold current and slope efficiency of the LDs were significantly deteriorated with an increase in the lasing wavelength from 405 to 435 nm. From the TEM and AFM measurements, the surface degradation and In phase separation was observed in the blue InGaN QW structure due to the surface migration of adatoms and the spinodal decomposition during the high temperature ramp-up and long growth time for the p-type layers, respectively. Crown Copyright Ó 2011 Published by Elsevier B.V. All rights reserved.
Keywords: GaN Laser diode (LD) Light-emitting diode (LED) Quantum well (QW)
1. Introduction III-nitrides have attracted much attention for application in optoelectronic devices with emission wavelengths ranging from ultraviolet to green light due to their wide bandgap [1e3]. However, some problems remain in achieving high performance light emitting devices with longer wavelengths, such as blue and green light emitting diodes (LEDs) and laser diodes (LDs) [4e6]. Among several issues in film growth, it is well known that the significant difference in the growth temperatures between the GaN and InGaN epilayers leads to the crystal and optical degradations of the InGaN active layer [1e3]. In particular, significant problems can occur in blue and green GaN-based LED/LDs with high In content (>15%) InGaN quantum well (QW) structures. Due to the solid phase immiscibility in the InGaN/GaN material system, the In phase separation is expected to have significant growth problems in the typical epitaxial growth as presented in the theoretical study by Ho and Stringfellow [7]. Evidence of phase separation has been reported previously in polycrystalline InGaN films annealed at temperatures below 700 C by Osamura et al. [8]. For InGaN QWs, the phase separation and formation of In-rich InGaN segregations were also observed via the
* Corresponding author. E-mail address: [email protected] (S.-N. Lee).
annealing process at a temperature of 950 C for 40 h [9]. The interdiffusion of In and Ga in the InGaN/GaN QW structures, accompanied with the lack of phase separation, was also found at high annealing temperatures of 1300e1400 C [10]. Besides, the luminescence from InGaN QWs was proposed to be closely related to the In-rich regions that are quantum dot-like structures as observed using transmission electron microscopy (TEM) [11,12]. These structural degradations of the InGaN active layer should be understood in order to achieve high performance blue/green LD/ LEDs. Therefore, the effects of the thermal degradation on violet and blue InGaN QW LDs were systematically investigated with different In compositions after the in situ thermal ramp-up process and growth of a thick p-type layer at a high temperature of 1030 C.
2. Experimental In0.15Ga0.85N QW structures were prepared to clarify the thermal damage during the temperature ramp-up process, which consists of the temperature ramp-up to a high temperature of 1030 C and a cool-down to room temperature, without the growth of p-type layers using metalorganic chemical vapor deposition (MOCVD). In addition, the LD structures including clad, waveguide, QWs, and electron blocking layer (EBL) were grown on GaN/sapphire templates in order to investigate the thermal degradation of the InxGa1 xN QW in LD structures during high growth temperature of
1567-1739/$ e see front matter Crown Copyright Ó 2011 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2011.07.024
S168
J. Kim et al. / Current Applied Physics 11 (2011) S167eS170
Fig. 1. The EL of the GaN-based LDs and the PL intensities InGaN QW structures with different In content as a function of spontaneous emission wavelength.
p-type layers [4,5]. The active layer was composed of 25 Å InxGa1 xN wells separated by 100 Å InyGa1 yN barriers, where the x and y were chosen to be 8e15% and 2%, respectively, to achieve the emission wavelength from violet to blue. To control the In content in the InGaN well, the growth temperature of the InGaN QW was changed from 790 C to 700 C. For the EBL between the InGaN QW and p-type GaN, p-type AlGaN/GaN multi-quantum barriers were employed to effectively suppress the electron overflow to the p-
Fig. 2. The light output power of the GaN-based LD with lasing wavelengths of 405 and 435 nm plotted as a function of the injection current.
type layers [4]. The growth temperature of the p-type layers was 1030 C. Electroluminescence (EL) analyses were performed by forming In dots as n-type and p-type contact metals on the LD wafers. Additionally, the ridge waveguide stripe was produced via the LD chip fabrication process. The cavity length and width of the ridge stripe were 600 mm and 2.0 mm, respectively. The lasing
Fig. 3. (a) and (b) AFM surface morphologies and (c) SEM cross section image of In0.15Ga0.85N QW without the temperature ramp-up process. After the temperature ramp-up process, (b) and (c) show AFM surface morphology and SEM image of In0.15Ga0.85N QW region, respectively.
J. Kim et al. / Current Applied Physics 11 (2011) S167eS170
performances were evaluated under cw conditions at room temperature as the ridge waveguide structures. The surface morphologies of the InGaN QW were analyzed using an atomic force microscope (AFM). Then, the interfacial and crystal qualities of InGaN QW were evaluated via high resolution transmission electron microscope (HR-TEM) and scanning electron microscope (SEM). 3. Results and discussion Fig. 1 shows the EL intensities of the GaN-based LDs and the PL intensities of the InGaN QWs with different In contents as a function of the emission wavelength. The EL intensity of the LDs with p-type layers grown at a temperature of 1030 C was significantly decreased with an increase in the emission wavelength above 440 nm. In particular, the EL intensity of the violet LD with an emission wavelength of 395 nm was three orders higher than that of the blue LD with a wavelength of 453 nm. However, the PL intensity of the InGaN QW grown on the GaN/ sapphire template slightly decreased by increasing the emission wavelength up to 455 nm due to the intrinsic material degradation of the InGaN with a high In content, such as the In phase segregation and generation of point defects [7,11,12]. Both results imply that the optical quality of the InGaN active layer could be maintained up to the blue region (w450 nm) by removing the ptype layers grown at a high temperature. In general, a high growth temperature is required to achieve high quality p-type layers that consist of EBL, p-GaN waveguide, p-AlGaN cladding layer, and p-contact in the LD structures [13]. It is suggested that the InGaN active layer would be exposed to a high temperature, resulting in the acceleration of the diffusion-induced In phase segregation in the InGaN active region. Therefore, it can be assumed that the structural and optical degradation of the InGaN QW would be significant due to the growth of the p-type layer in the LD structure. Fig. 2 shows the light output power of the GaN-based LDs with lasing wavelengths of 405 nm and 435 nm as a function of the injection current. The threshold current and slope efficiency of the InGaN LDs with 405 nm were 35 mA and 1.0 W/A, respectively, in cw conditions at room temperature. However, the lasing performances of the LDs with a blue emission wavelength of 435 nm were significantly deteriorated. The threshold current and slope efficiency of LDs were 118 mA and <0.5 W/A, respectively. In addition, the lasing performance could not be obtained for the GaN-based LD with a blue emission wavelength of more than 440 nm in these experiments. These results are consistent with the drastic reduction of the EL intensity with the increasing emission wavelength shown in Fig. 1. Thus, it is believed that the optical deterioration of the LD structure is primarily caused by the thermal degradation of the InGaN active layer with a high In content. For the thermal degradations of the blue InGaN active layer in the LD structures, two causes are deduced: one is the short time thermal degradation during the temperature ramp-up process used to grow the p-type layer just after the growth of InGaN QW; the other is the long time thermal damage during the p-type layers growth at a high temperature of 1030 C. Fig. 3(a) and (b) shows the AFM surface morphologies (2 2 mm2) of the blue In0.15Ga0.85N QW structures without and with the high temperature ramp-up treatment, respectively. As shown in Fig. 3(a), the surface morphology of the blue InGaN QW structure showed a spiral surface morphology that is a typical surface structure of InGaN epilayers. However, the surface morphology of the blue InGaN QW was modified from a spiral to a step-like surface structure after the temperature ramp-up and annealing process (30 s) at 1030 C [6]. This significant
S169
modification of the InGaN surface structure can be explained by the In and Ga atoms being desorbed, re-deposited, diffused, and migrated on the surface of the InGaN QW under NH3 and N2 atmospheres in the MOCVD reactor. Fig. 3(c) and 3(d) shows the SEM images of the InGaN QW without and with the temperature ramp-up process, respectively. There are no In segregations in the InGaN QW regions in both samples. From the AFM and SEM analyses, it is believed that only the temperature ramp-up process after the InGaN QW growth can affect the surface modification, rather than the In phase separation, in the InGaN regions. Fig. 4(a) and (b) shows the cross section HR-TEM images of the GaN-based LDs with different In compositions of 8% and 15% in the InGaN QWs, respectively. The interface quality of the violet In0.08Ga0.92N QWs LD was much better than that of the blue In0.15Ga0.85N QW LD. Although the growth temperature of the players for both samples was 1030 C, it was found that the In phase segregations were significantly developed at the In0.15Ga0.85N QW
Fig. 4. HR-TEM cross section images of (a) In0.08Ga0.92N and (b) In0.15Ga0.85N QW LD with high temperature p-type layers at a high growth temperature of 1030 C.
S170
J. Kim et al. / Current Applied Physics 11 (2011) S167eS170
region in the blue LD as shown in Fig. 4(b). It implied that the In phase separation and interfacial deterioration were caused by the interdiffusion of the In and Ga atoms in the InGaN QW with a high In content during the growth of the thick p-type layers at a high temperature of 1030 C. In addition, there was no In phase segregation in the In0.08Ga0.92N QW region. From these results, it is assumed that the In phase segregation would be significantly affected by the In composition in the InGaN QWs as well as the growth temperature of the p-type layers. Therefore, it is suggested that the high temperature process is a critical issue in achieving blue and green GaN-based LD structures with higher In compositions (>15%) in InGaN QWs. 4. Conclusion Two types of significant degradation processes were observed for InGaN QW structures with different In compositions in violet and blue LDs. The first significant process was the temperature ramp-up process for the growth of the p-type layer just after the growth of the InGaN QW, which results in a drastic surface modification of the InGaN QWs due to the desorption, migration, and surface diffusion of the In, Ga, and N atoms. The second significant degradation process was the high temperature p-type layer growth that results from the interdiffusion of the In atoms between the well and barrier by introducing a high thermal energy for a long time, which results in the In phase separation in the InGaN active layer. Therefore, it is suggested that high performance blue LDs can be achieved by suppressing the thermal degradation of the InGaN QW region.
Acknowledgment This research was supported by the Basic Science Research Program (Grant no. 2010-0011815) and Core Corporation Research Program (Grant no. 2010-0026523) through the National Research Foundation of Korea (NRF) funded by the Korean Ministry of Education, Science and Technology (MEST).
References [1] S.N. Lee, J.K. Son, H.S. Paek, T. Sakong, W. Lee, K.H. Kim, E. Yoon, J.Y. Kim, Y.H. Cho, Y.J. Lee, S.S. Kim, D.Y. Noh, O.H. Nam, Y. Park, J. Cryst. Growth 275 (2005) e1041. [2] D.H. Yoon, K.S. Lee, J.B. Yoo, T.Y. Seong, Jpn. J. Appl. Phys. 41 (2002) 1253. [3] S. Keller, B.P. Keller, D. Kapolnek, U.K. Mishra, S.P. DenBaars, I.K. Shmagin, R.M. Kolbas, S. Krishnankutty, J. Cryst. Growth 170 (1997) 349. [4] S.N. Lee, S.Y. Cho, H.Y. Ryu, J.K. Son, H.S. Paek, T. Sakong, T. Jang, K.K. Choi, K.H. Ha, M.H. Yang, O.H. Nam, Y. Park, Appl. Phys. Lett. 88 (2006) 111101. [5] S.N. Lee, H.Y. Ryu, H.S. Paek, J.K. Son, Y.J. Sung, K.S. Kim, H.K. Kim, H. Kim, T. Jang, K.H. Ha, O.H. Nam, Y. Park, IEEE Elect. Device Lett. 29 (2008) 870. [6] S.N. Lee, H.S. Paek, J.K. Son, T. Sakong, Y.N. Kwon, O.H. Nam, Y. Park, Phys. Stat. Sol. C 4 (2007) 141. [7] I.H. Ho, G.B. Stringfellow, Appl. Phys. Lett. 69 (1996) 2701. [8] K. Osamura, S. Naka, Y. Murakami, J. Appl. Phys 46 (1975) 3432. [9] M.D. McCluskey, L.T. Romano, B.S. Krusor, D.P. Bour, N.M. Johnson, S. Brennan, Appl. Phys. Lett. 72 (1998) 1730. [10] M.D. McCluskey, L.T. Romano, B.S. Krusor, N.M. Johnson, T. Suski, J. Jun, Appl. Phys. Lett. 73 (1998) 1281. [11] Y. Narukawa, Y. Kawakami, M. Funano, S. Fujita, S. Nakamura, Appl. Phys. Lett. 70 (1997) 981. [12] C.-C. Chuo, C.-M. Lee, T.-E. Nee, J.-I. Chyi, Appl. Phys. Lett. 76 (2000) 3902. [13] S.N. Lee, T. Jang, J.K. Son, H.S. Paek, T. Sakong, E. Yoon, O.H. Nam, Y. Park, J. Cryst. Growth 287 (2006) 554.