- Email: [email protected]
GaN multi-quantum wells
Journal of Crystal Growth 247 (2003) 62–68
Effects of barrier growth temperature on the properties of InGaN/GaN multi-quantum wells Sunwoon Kima,*, Kyuhan Leea, Keunseop Parka, Chang-Soo Kimb a b
Department of Materials Science, Optronix Inc., 104-6, Moonji-Dong, Yusung-Gu, Taejon 305-380, South Korea National Research Laboratory, Materials Evaluation Center, Korea Research Institute of Standards and Science, Taejon 305-600, South Korea Received 15 September 2002; accepted 24 September 2002 Communicated by M. Schieber
Abstract The effects of the growth temperature and ambient of GaN quantum barriers on the characteristics of InGaN/GaN multi-quantum wells (MQWs) grown by a thermally pre-cracked ion-supplied metalorganic chemical vapor deposition (TPIS-MOCVD) system were investigated. The improvement of optical, structural properties and surface morphology in the MQWs with increasing the growth temperature of quantum barriers was found. Without a GaN capping layer, there were many pits and the thickness of quantum pair reduced by the thermal etching during the temperature-ramping process. Photoluminescence (PL) peaks showed a blue-shift and double peaks, but relative PL intensity abruptly increased due to the suppression of deep level related defects and smooth surface morphology caused by the increased surface mobility of adatom in the high temperature region. By using a GaN capping layer on the InGaN well layer, the thermal decomposition of the InGaN well layer was suppressed and pits on the surface abruptly reduced. A hydrogen carrier gas for the GaN barrier growth also improved the optical and structural properties of MQWs. r 2002 Elsevier Science B.V. All rights reserved. PACS: 78.55.Cr; 81.05.Ea; 81.10.Bk; 81.15.Gh Keywords: A1. Atomic force microscopy; A1. Surfaces; A1. X-ray diffraction; A3. Metalorganic chemical vapor deposition; B1. Nitride; B3. Light-emitting diodes
1. Introduction High quality InGaN-based quantum wells (QWs) and heterointerfaces are of great importance because of their potential applications in light-emitting diodes (LEDs) and laser diodes *Corresponding author. Tel.: +82-42-861-3701; fax: +8242-861-3703. E-mail address: [email protected] (S. Kim).
(LDs) [1]. A lot of progress, both in the material and device fabrication of this system, has been made recently. Many studies on InGaN-based QW characteristics have been reported with respect to band gap potential inhomogeneity (carrier localization) due to either a large compositional fluctuation of indium in the InGaN epilayers or a strong internal piezoelectric field in the strained InGaN QW structures [2–4]. However, the process of blue and green LDs is often limited by the
0022-0248/03/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 2 ) 0 1 9 4 3 - 7
S. Kim et al. / Journal of Crystal Growth 247 (2003) 62–68
fundamental problems of InGaN. First of all, the low miscibility of InN in GaN [5] leads to a low growth temperature (below 9001C) to obtain a higher indium mole fraction in the InGaN films. However, this lower growth temperature is usually accompanied by poor crystal quality of the epilayer. In the case of the thermal decomposition of ammonia as a nitrogen source, it becomes less efficient with decreasing the temperature due to the high kinetic barrier for breaking N–H bonds. To get over the nitrogen deficiency on the growing surface, an extremely high V/III ratio is required [6]. Otherwise, the In metal droplet formation is observed due to the unstable thermodynamics of ternary InGaN epilayers [7]. Several research groups have investigated the growth conditions of InGaN/GaN multi-quantum wells (MQWs) [8–11]. Keller reported the effects of the growth rate and barrier doping on the properties of InGaN/GaN [8]. Cheong reported how growth interruption affected the structural and optical properties of high indium InGaN/GaN MQWs [9]. With increasing interruption time, the quantum-dot-like region and well thickness decreased due to the indium re-evaporation or the thermal etching effect. Harris showed how the different growth parameters, including the NH3 flow rate, hydrogen flow rate, total reactor flow rate and Trimethylindium (TMIn) flow rate, affected the photoluminescence (PL) properties [10]. Leem showed that indium flow during growth interruption increased the peak wavelength and intensity [11]. The conventional method of growing the quantum barrier and well layers of MQWs in compound semiconductors is by controlling the alkyl source flow into the reactor. This is because the optimized growth conditions of the quantum barrier and well layers, including growth temperature, pressure and ambient, are very similar. However, the growth of MQWs in -nitrides is much more complex. As mentioned above, the InGaN/GaN MQWs growth temperature should be lower than 9001C due to the low dissociation temperature of InN. This lower temperature results in very poor crystal quality of GaN barriers. The poor crystal quality of a GaN barrier layer can be mainly attributed to the three-
63
dimensional growth mode due to the low surface mobility of adatoms in the low growth temperature as well as the increase of nitrogen vacancy due to the low cracking efficiency of ammonia. This fundamental problem must be solved in order that device performance be improved by minimizing the deteriorating effect of a GaN barrier layer. In this work, we investigated the effect of the quantum barrier growth temperature on the optical and the structural properties of InGaN/ GaN MQWs. Especially, we have studied the surface morphology and the thickness reduction of well layers with respect to the quantum barrier growth conditions by using PL, high-resolution X-ray diffraction (HRXRD) and atomic force microscope (AFM) images.
2. Experimental procedure The epitaxial growth of GaN and InGaN/GaN MQWs on (0 0 0 1) c-plane sapphire wafers was performed in a thermally pre-cracked ion-supplied metalorganic chemical vapor deposition (TPISMOCVD, HANVAC Corp. HR-series) system with a horizontal quartz reactor and an RF heater. High purity ammonia gas was thermally precracked in front of the mixing region to increase the cracking efficiency, resulting in the higher nitrogen source in the growth region. Unlike a normal MOCVD system, V/III ratio was as low as 1000 for GaN growth in this system. The input of the metalorganic source was separated with ammonia gas in front of the growth region to minimize the pre-reaction. After conventional acid cleaning, a wafer was mounted onto the graphite susceptor. At first, the thermal annealing process was carried out at around 1080–11001C. Trimethylgallium (TMGa), trimethylindium (TMIn), and high purity ammonia were used as the source precursors for Ga, In, and N, respectively. A 30 nm thick GaN nucleation layer on the sapphire wafer was grown at 5001C, followed by 2 mm thick GaN epitaxial layer at the elevated temperature of 10801C and the reactor pressure of 300 Torr. The 5 pairs of In0.28Ga0.72N (3 nm)/GaN (7 nm) MQWs were grown on a GaN layer. InGaN quantum well layers were grown in the temperature range of
S. Kim et al. / Journal of Crystal Growth 247 (2003) 62–68
700–7401C at nitrogen ambient and the GaN quantum barrier layers were 840–8801C at nitrogen or hydrogen ambient. A high-resolution X-ray diffraction (HRXRD) measurement was conducted to analyze the quality and In composition of the InGaN/GaN MQWs. The 325 nm line of an He–Cd laser was used as an exciting source for the room temperature PL measurements and layer surfaces were observed using an AFM.
3. Results and discussion We reported [12,13] that the NH3 pre-heater of a TPIS-MOCVD system was useful in high quality GaN and InGaN growth. Even though the V/III ratio was as low as 1000, the mobility of GaN epilayer was over 600 cm2/V s. The typical fullwidth at half-maximum (FWHM) of the double crystal X-ray rocking curves were B300 arcsec for (0 0 2) and B400 arcsec for (1 0 2). The InGaN epilayers also showed high quality and droplet-free surface with high In concentration by using NH3 pre-heating system. On the optimized base-GaN growth conditions, we grew InGaN/GaN MQWs on GaN/sapphire and varied the growth conditions of quantum barrier to investigate the optical and structural characteristics of MQWs. Table 1 and Fig. 1 show the barrier growth conditions and HRXRD peaks of InGaN/GaN MQWs. The quantum well and barrier layers of the KSW 01 sample were grown at 7201C and nitrogen ambient. The quantum barrier of the KSW 02 sample was grown at 8801C without a capping layer and that of the KSW 03 sample was grown with a GaN capping layer to suppress the indium evaporation on the surface of the InGaN well layer during the temperature-ramping process
between the quantum well and barrier layer growth. The thickness of a GaN capping layer was 2–4 monolayers and the interruption time for the temperature-ramping process was 20–40 s. In the case of the KSW 04 sample, the carrier gas of the quantum barrier was changed to hydrogen to increase the GaN crystal quality and the interface morphology. The 0th order peak of MQWs using triple axis diffraction of HRXRD (not shown here) shows the average In concentration of well+barrier layers (QW pairs). The satellite peaks arise from the periodicity of the quantum well superlattice and the thickness of the QW pair was determined from the positions of these satellite peaks [14]. As shown in Fig. 1, average In concentration and the QW pair thickness of the KSW 01 sample were the same as the intended experimental conditions. The calculated thickness from the superlattice periods of the KSW 01 sample was 9.53 nm and the peak position of 0th order was 489 arcsec. Those of the KSW 02 sample were 8.44 nm and 367.2 arcsec, respectively. The
Intensity (a.u.)
64
10
9
10
7
10
5
10
3
10
1
0th 1s t 2n d KSW 04
3rd
KSW 03 KSW 02 KSW 01
10
-1
-7200
-3600
0
3600
Omega (arcsec)
Fig. 1. High-resolution X-ray diffraction results of MQWs with respect to the quantum barrier growth conditions.
Table 1 MQWs growth conditions and HRXRD results Sample number
Barrier growth condition
0th order peak (arcsec)
Well+barrier thickness (nm)
KSW KSW KSW KSW
Constant growth temperature N2 ambient Ramping without capping layer N2 ambient Ramping with capping layer N2 ambient Ramping with capping layer H2 ambient
489 367.2 372.8 453.6
9.53 8.44 10.76 9.66
01 02 03 04
S. Kim et al. / Journal of Crystal Growth 247 (2003) 62–68
Normalized PL Intensity (a.u.)
thickness of QW pair decreased when we increased the barrier growth temperature by using the ramping process. However, the source flow rate and the growth times of the quantum well and barrier were the same for both samples. Thus, the reduction of the thickness indicates that the well or barrier layer was etched as the growth temperature of barrier was varied. We suggest that the InGaN well layer might be etched more easily than the GaN barrier layer. If the annealing temperature is higher than the growth temperature, the surface of semiconductor would decompose and reduce the bulk thickness slightly, but would not alter the bulk composition. In addition, the reduction of the barrier thickness causes the increase of 0th order peak separation, which result from the increase of average In concentration. Therefore, the InGaN well layer was etched while the temperature was ramped up. The thermal etching effect of the InGaN well layer was confirmed by PL results. Fig. 2 shows the room temperature PL spectra of InGaN/GaN MQWs. The PL peak positions of the KSW 02 sample showed blue-shift and double peaks. It has been reported that the lattice mismatch and the difference in the thermal expansion coefficient between the InGaN and the GaN layer can result in a strain in the InGaN layer [15]. Then, the strain in the MQW will induce a piezoelectric field, thus influence the effective band gap. Ramping the barrier growth temperature during MQW growth will reduce the well width, as shown in Table 1,
350
KSW 04 KSW 03
KSW 02 KSW 01
400
450
500
550
600
Wavelength (nm) Fig. 2. Normalized PL peaks of InGaN/GaN MQWs with respect to the quantum barrier growth conditions.
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and thereby reduce a strain [16]. Therefore, the significant blue-shift of the PL peak may be caused by the reduction of a well width and the strain arising due to ramping the barrier growth temperature. The barrier of the KSW 03 sample was grown with a capping layer on the InGaN well layer and ramped up the growth temperature. Compared with the KSW 01 and 02 samples, the thickness of QW pair was increased up to 10.76 nm and the peak separation of 0th order was shorter than that of the KSW 01 sample. In this case, the reduction of average In composition was caused by the increased GaN thickness by a capping layer. The emission wavelength of PL measurement of the KSW 03 sample showed red-shift from the KSW 02 sample, which means that a capping layer effectively suppressed the decomposition of the InGaN quantum well layer. But it was a little bit shorter than that of the KSW 01 sample. This indicates that the temperature-ramping process affects the InGaN well or the interface state between the InGaN well and a capping layer. The barrier of the KSW 04 sample was grown at hydrogen ambient and other growth conditions were the same as the KSW 03. Even though the KSW 04 sample had a GaN capping layer, the thickness of QW pair and the peak separation of 0th order was similar with the KSW 01 sample. This means that the growth rate of the GaN barrier in the hydrogen ambient is lower than in the nitrogen ambient, or a hydrogen carrier gas etches some of a GaN capping layer during the temperature-ramping process. When the barrier is grown using a nitrogen carrier gas, the surface morphology becomes rough, resulting in a higher density of nucleation site at which growth can occur [17]. The growth rate depends on the overall roughness of the surface, thus, it is expected that the growth rate using a nitrogen carrier gas be greater than that using a hydrogen carrier gas. Therefore, the thickness reduction of QW pair of the KSW 04 sample can be explained. The HRXRD peak of the KSW 04 sample showed high peak intensity, sharp shape and high-order satellite peaks, which indicated an improvement of the interface flatness of the MQWs or the GaN barrier crystal quality.
S. Kim et al. / Journal of Crystal Growth 247 (2003) 62–68
66
Relative PL Intensity (a.u.)
Fig. 3 shows the relative PL intensity and Fig. 4 shows the surface images of InGaN/GaN MQWs using AFM. When the quantum well and barrier were grown at the constant temperature (Fig. 4(a)), the surface morphology was rough.
100 80 60 40 20 0 KSW 01
KSW 02
KSW 03
KSW 04
Sample Number
Fig. 3. Relative PL intensity of InGaN/GaN MQWs with respect to the quantum barrier growth conditions.
Due to the lower surface mobility of adsorbed species at the lower growth temperature, the terrace width between steps decreased and the surface roughened. As the growth temperature of the GaN barrier increased, the surface morphology became smooth due to the increased surface mobility of adsorbed species. However, there were many pits on the surface (Fig. 4(b)). It seems that pits can be generated by the thermal decomposition of the InGaN well layer during the temperature-ramping process. This kind of a defect causes the leakage current path in LEDs. In addition, room temperature PL measurement showed double peaks, which were caused by the pit formation and the In composition fluctuation by the temperature-ramping process. But PL intensity of the KSW 02 abruptly increased. The low temperature GaN showed the intense yellow luminescence which was related with the formation of defects such as dislocations, grain boundary, gallium vacancy and its related complex [18]. Stephenson
Fig. 4. AFM images of InGaN/GaN MQWs surface morphology: (a) KSW 01, (b) KSW 02, (c) KSW 03, and (d) KSW 04.
S. Kim et al. / Journal of Crystal Growth 247 (2003) 62–68
et al. showed that the growth mode could be changed from a three-dimentional type at low temperature, through a layer-by-layer type, to a step growth mode at high temperatures [19]. Therefore, the high temperature GaN barrier suppresses the deep level related defects in optical properties of MQWs and the stacking faults region through the increased surface mobility of adatoms on the growing surface. By using a GaN capping layer on the InGaN well layer, the number and size of the pit formation on the surface abruptly reduced (Fig. 4(c)). As mentioned above, a GaN capping layer effectively suppressed the decomposition of the InGaN well layer, resulting in the improvement of the surface morphology and the PL intensity. In the case of the KSW 04 sample, the surface morphology was nearly pit-free and smooth. Therefore, the reason why the relative PL and HRXRD peak intensity increased in hydrogen ambient is considered like this. As shown in Fig. 4(d), smooth and pit-free surface morphology means that the interface between the InGaN well and the GaN barrier layer is flat, which improves the quantum confinement and prevents the defect formation which is related with non-radiative recombination centers in the interface region. In addition, the etching effect of a hydrogen carrier gas in high temperature is higher than that of a nitrogen carrier gas [20]. Therefore, the GaN capping layer of the KSW 04 may be partially etched during the temperature-ramping process and its thickness may be thinner than that of the KSW 03. As the thickness of the GaN capping layer was increased above 1 nm, the PL peak intensity corresponding to the emission from the MQW was significantly decreased [21]. This implies that thick GaN cap layers are not effective in preventing the decomposition of the MQW structure. In our work, the optimum capping thickness is another reason of the PL intensity improvement of the KSW 04. These results show that the electrical, optical, and structural properties of the MQWs in LEDs can be significantly improved by increasing the growth temperature of quantum barrier layers and inserting the optimum capping layer in MQWs.
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4. Conclusions We investigated the effect of the GaN barrier growth temperature and a capping layer on the characteristics of InGaN/GaN MQWs using a TPIS-MOCVD system. The temperature-ramping process showed an improvement of optical and structural properties of the MQWs, which was attributed to the suppression of deep level related defects and the smooth and flat interface caused by the increased surface mobility of adatom. A GaN capping layer must be inserted on the InGaN well layer to prevent the thermal decomposition of the quantum well and hydrogen was better than nitrogen as a carrier gas for a high quality quantum barrier of MQWs.
References [1] S. Nakamura, G. Fasol, The Blue Laser Diode, Springer, Berlin, 1997. [2] T. Wang, H. Saeki, J. Bai, T. Shirahama, M. Lachab, S. Sakai, P. Eliseev, Appl. Phys. Lett. 76 (2000) 1737. [3] Mee-Yi Ryu, Young Jun Yu, Phil Won Yu, Eun-Joo Shin, Joo In Lee, Sung Kyu Yu, Eun Soon Oh, Ok Hyun Nam, Chul Soo Sone, Yong Jo Park, J. Korean Phys. Soc. 37 (2000) 989. [4] T. Wang, J. Bai, S. Sakai, J. Crystal Growth 224 (2001) 5. [5] I.H. Ho, Stringfellow, Appl. Phys. Lett. 69 (1996) 2701. [6] J.C. Harris, H. Brisset, Takao Someya, Yasuhiko Arakawa, Jpn. J. Appl. Phys. 38 (1999) 2613. [7] S. Keller, B.P. Keller, D. Kapolnek, U.K. Mishra, S.P. DenBaars, I.K. Shmagin, R.M. Kolbas, S. Krishnankutty, J. Crystal Growth 170 (1997) 349. [8] S. Keller, S.F. Chichibu, M.S. Minsky, E. Hu, U.K. Mishra, S.P. DenBaars, J. Crystal Growth 195 (1998) 258. [9] M.G. Cheong, F.J. Choi, C.S. Kim, H.S. Yoon, C.H. Hong, E.K. Suh, H.J. Lee, H.K. Cho, J.Y. Lee, J. Korean Phys. Soc. 38 (2001) 701. [10] J.C. Harris, H. Brisset, T. Someya, Y. Arakawa, Jpn. J. Appl. Phys. 38 (1999) 2613. [11] Shi-Jong Leem, Min Hong Kim, Johngoen Shin, Yoonho Choi, Jichai Jeong, Jpn. J. Appl. Phys. 40 (2001) L371. [12] H.J. Kim, S.Y. Kwon, S.G. Yim, H.S. Na, B. Kee, E.J. Yoon, J.H. Kim, S.H. Park, H.S. Jeon, S.W. Kim, J.H. Seo, K.S. Park, Presented at the ICNS-4, A4.2, Denver, Colorado, 2001 (unpublished). [13] Sunwoon Kim, Junho Seo, Kyuhan Lee, Haeseok Lee, Keunseop Park, Chang-Soo Kim, Mater. Res. Soc. Proc. 722 (2002) K7.13.1. [14] Tzu-Chi Wen, Wei-I Lee, Jpn. J. Appl. Phys. 40 (2001) 5302.
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[15] Sang Kee Shee, Y.H.Kwon, J.B. Lam, G.H. Gainer, G.H. Park, S.J. Hwang, B.D. Little, J.J. Song, J. Crystal Growth 221 (2000) 373. [16] T. Takeuchi, C. Wetzel, S. Yamaguchi, H. Sakai, H. Amano, I. Akasaki, Appl. Phys. Lett. 73 (1998) 1691. [17] N. Duxbury, P. Dawson, U. Bangert, E.J. Thrush, W. Van Der Stricht, K. Jacobs, I. Moerman, Phys. Status Solidi (b) 216 (1999) 355. [18] J. Neugebauer, C.G. Van de Walle, Appl. Phys. Lett. 69 (1996) 503.
[19] G.B. Stephenson, J.A. Eastman, C. Thompson, O. Auciello, L.J. Thompson, A. Munkholm, P. Fini, S.P. DenBaars, J.S. Speck, Appl. Phys. Lett. 74 (1999) 3326. [20] Lisa Sugiura, Mariko Suzuki, Johji Nishio, Kazuhiko Itaya, Yoshihiro Kokubun, Masayuki Ishikawa, Jpn. J. Appl. Phys. 37 (1998) 3878. [21] Hiroyasu Ishikawa, Naoyuki Nakada, Masayoshi Mori, Guan-Yuan Zhao, Takashi Egawa, Takashi Jimbo, Masayoshi Umeno, Jpn. J. Appl. Phys. 40 (2001) L1170.
Effects of barrier growth temperature on the properties of InGaN/GaN multi-quantum wells Sunwoon Kima,*, Kyuhan Leea, Keunseop Parka, Chang-Soo Kimb a b
Department of Materials Science, Optronix Inc., 104-6, Moonji-Dong, Yusung-Gu, Taejon 305-380, South Korea National Research Laboratory, Materials Evaluation Center, Korea Research Institute of Standards and Science, Taejon 305-600, South Korea Received 15 September 2002; accepted 24 September 2002 Communicated by M. Schieber
Abstract The effects of the growth temperature and ambient of GaN quantum barriers on the characteristics of InGaN/GaN multi-quantum wells (MQWs) grown by a thermally pre-cracked ion-supplied metalorganic chemical vapor deposition (TPIS-MOCVD) system were investigated. The improvement of optical, structural properties and surface morphology in the MQWs with increasing the growth temperature of quantum barriers was found. Without a GaN capping layer, there were many pits and the thickness of quantum pair reduced by the thermal etching during the temperature-ramping process. Photoluminescence (PL) peaks showed a blue-shift and double peaks, but relative PL intensity abruptly increased due to the suppression of deep level related defects and smooth surface morphology caused by the increased surface mobility of adatom in the high temperature region. By using a GaN capping layer on the InGaN well layer, the thermal decomposition of the InGaN well layer was suppressed and pits on the surface abruptly reduced. A hydrogen carrier gas for the GaN barrier growth also improved the optical and structural properties of MQWs. r 2002 Elsevier Science B.V. All rights reserved. PACS: 78.55.Cr; 81.05.Ea; 81.10.Bk; 81.15.Gh Keywords: A1. Atomic force microscopy; A1. Surfaces; A1. X-ray diffraction; A3. Metalorganic chemical vapor deposition; B1. Nitride; B3. Light-emitting diodes
1. Introduction High quality InGaN-based quantum wells (QWs) and heterointerfaces are of great importance because of their potential applications in light-emitting diodes (LEDs) and laser diodes *Corresponding author. Tel.: +82-42-861-3701; fax: +8242-861-3703. E-mail address: [email protected] (S. Kim).
(LDs) [1]. A lot of progress, both in the material and device fabrication of this system, has been made recently. Many studies on InGaN-based QW characteristics have been reported with respect to band gap potential inhomogeneity (carrier localization) due to either a large compositional fluctuation of indium in the InGaN epilayers or a strong internal piezoelectric field in the strained InGaN QW structures [2–4]. However, the process of blue and green LDs is often limited by the
0022-0248/03/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 2 ) 0 1 9 4 3 - 7
S. Kim et al. / Journal of Crystal Growth 247 (2003) 62–68
fundamental problems of InGaN. First of all, the low miscibility of InN in GaN [5] leads to a low growth temperature (below 9001C) to obtain a higher indium mole fraction in the InGaN films. However, this lower growth temperature is usually accompanied by poor crystal quality of the epilayer. In the case of the thermal decomposition of ammonia as a nitrogen source, it becomes less efficient with decreasing the temperature due to the high kinetic barrier for breaking N–H bonds. To get over the nitrogen deficiency on the growing surface, an extremely high V/III ratio is required [6]. Otherwise, the In metal droplet formation is observed due to the unstable thermodynamics of ternary InGaN epilayers [7]. Several research groups have investigated the growth conditions of InGaN/GaN multi-quantum wells (MQWs) [8–11]. Keller reported the effects of the growth rate and barrier doping on the properties of InGaN/GaN [8]. Cheong reported how growth interruption affected the structural and optical properties of high indium InGaN/GaN MQWs [9]. With increasing interruption time, the quantum-dot-like region and well thickness decreased due to the indium re-evaporation or the thermal etching effect. Harris showed how the different growth parameters, including the NH3 flow rate, hydrogen flow rate, total reactor flow rate and Trimethylindium (TMIn) flow rate, affected the photoluminescence (PL) properties [10]. Leem showed that indium flow during growth interruption increased the peak wavelength and intensity [11]. The conventional method of growing the quantum barrier and well layers of MQWs in compound semiconductors is by controlling the alkyl source flow into the reactor. This is because the optimized growth conditions of the quantum barrier and well layers, including growth temperature, pressure and ambient, are very similar. However, the growth of MQWs in -nitrides is much more complex. As mentioned above, the InGaN/GaN MQWs growth temperature should be lower than 9001C due to the low dissociation temperature of InN. This lower temperature results in very poor crystal quality of GaN barriers. The poor crystal quality of a GaN barrier layer can be mainly attributed to the three-
63
dimensional growth mode due to the low surface mobility of adatoms in the low growth temperature as well as the increase of nitrogen vacancy due to the low cracking efficiency of ammonia. This fundamental problem must be solved in order that device performance be improved by minimizing the deteriorating effect of a GaN barrier layer. In this work, we investigated the effect of the quantum barrier growth temperature on the optical and the structural properties of InGaN/ GaN MQWs. Especially, we have studied the surface morphology and the thickness reduction of well layers with respect to the quantum barrier growth conditions by using PL, high-resolution X-ray diffraction (HRXRD) and atomic force microscope (AFM) images.
2. Experimental procedure The epitaxial growth of GaN and InGaN/GaN MQWs on (0 0 0 1) c-plane sapphire wafers was performed in a thermally pre-cracked ion-supplied metalorganic chemical vapor deposition (TPISMOCVD, HANVAC Corp. HR-series) system with a horizontal quartz reactor and an RF heater. High purity ammonia gas was thermally precracked in front of the mixing region to increase the cracking efficiency, resulting in the higher nitrogen source in the growth region. Unlike a normal MOCVD system, V/III ratio was as low as 1000 for GaN growth in this system. The input of the metalorganic source was separated with ammonia gas in front of the growth region to minimize the pre-reaction. After conventional acid cleaning, a wafer was mounted onto the graphite susceptor. At first, the thermal annealing process was carried out at around 1080–11001C. Trimethylgallium (TMGa), trimethylindium (TMIn), and high purity ammonia were used as the source precursors for Ga, In, and N, respectively. A 30 nm thick GaN nucleation layer on the sapphire wafer was grown at 5001C, followed by 2 mm thick GaN epitaxial layer at the elevated temperature of 10801C and the reactor pressure of 300 Torr. The 5 pairs of In0.28Ga0.72N (3 nm)/GaN (7 nm) MQWs were grown on a GaN layer. InGaN quantum well layers were grown in the temperature range of
S. Kim et al. / Journal of Crystal Growth 247 (2003) 62–68
700–7401C at nitrogen ambient and the GaN quantum barrier layers were 840–8801C at nitrogen or hydrogen ambient. A high-resolution X-ray diffraction (HRXRD) measurement was conducted to analyze the quality and In composition of the InGaN/GaN MQWs. The 325 nm line of an He–Cd laser was used as an exciting source for the room temperature PL measurements and layer surfaces were observed using an AFM.
3. Results and discussion We reported [12,13] that the NH3 pre-heater of a TPIS-MOCVD system was useful in high quality GaN and InGaN growth. Even though the V/III ratio was as low as 1000, the mobility of GaN epilayer was over 600 cm2/V s. The typical fullwidth at half-maximum (FWHM) of the double crystal X-ray rocking curves were B300 arcsec for (0 0 2) and B400 arcsec for (1 0 2). The InGaN epilayers also showed high quality and droplet-free surface with high In concentration by using NH3 pre-heating system. On the optimized base-GaN growth conditions, we grew InGaN/GaN MQWs on GaN/sapphire and varied the growth conditions of quantum barrier to investigate the optical and structural characteristics of MQWs. Table 1 and Fig. 1 show the barrier growth conditions and HRXRD peaks of InGaN/GaN MQWs. The quantum well and barrier layers of the KSW 01 sample were grown at 7201C and nitrogen ambient. The quantum barrier of the KSW 02 sample was grown at 8801C without a capping layer and that of the KSW 03 sample was grown with a GaN capping layer to suppress the indium evaporation on the surface of the InGaN well layer during the temperature-ramping process
between the quantum well and barrier layer growth. The thickness of a GaN capping layer was 2–4 monolayers and the interruption time for the temperature-ramping process was 20–40 s. In the case of the KSW 04 sample, the carrier gas of the quantum barrier was changed to hydrogen to increase the GaN crystal quality and the interface morphology. The 0th order peak of MQWs using triple axis diffraction of HRXRD (not shown here) shows the average In concentration of well+barrier layers (QW pairs). The satellite peaks arise from the periodicity of the quantum well superlattice and the thickness of the QW pair was determined from the positions of these satellite peaks [14]. As shown in Fig. 1, average In concentration and the QW pair thickness of the KSW 01 sample were the same as the intended experimental conditions. The calculated thickness from the superlattice periods of the KSW 01 sample was 9.53 nm and the peak position of 0th order was 489 arcsec. Those of the KSW 02 sample were 8.44 nm and 367.2 arcsec, respectively. The
Intensity (a.u.)
64
10
9
10
7
10
5
10
3
10
1
0th 1s t 2n d KSW 04
3rd
KSW 03 KSW 02 KSW 01
10
-1
-7200
-3600
0
3600
Omega (arcsec)
Fig. 1. High-resolution X-ray diffraction results of MQWs with respect to the quantum barrier growth conditions.
Table 1 MQWs growth conditions and HRXRD results Sample number
Barrier growth condition
0th order peak (arcsec)
Well+barrier thickness (nm)
KSW KSW KSW KSW
Constant growth temperature N2 ambient Ramping without capping layer N2 ambient Ramping with capping layer N2 ambient Ramping with capping layer H2 ambient
489 367.2 372.8 453.6
9.53 8.44 10.76 9.66
01 02 03 04
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Normalized PL Intensity (a.u.)
thickness of QW pair decreased when we increased the barrier growth temperature by using the ramping process. However, the source flow rate and the growth times of the quantum well and barrier were the same for both samples. Thus, the reduction of the thickness indicates that the well or barrier layer was etched as the growth temperature of barrier was varied. We suggest that the InGaN well layer might be etched more easily than the GaN barrier layer. If the annealing temperature is higher than the growth temperature, the surface of semiconductor would decompose and reduce the bulk thickness slightly, but would not alter the bulk composition. In addition, the reduction of the barrier thickness causes the increase of 0th order peak separation, which result from the increase of average In concentration. Therefore, the InGaN well layer was etched while the temperature was ramped up. The thermal etching effect of the InGaN well layer was confirmed by PL results. Fig. 2 shows the room temperature PL spectra of InGaN/GaN MQWs. The PL peak positions of the KSW 02 sample showed blue-shift and double peaks. It has been reported that the lattice mismatch and the difference in the thermal expansion coefficient between the InGaN and the GaN layer can result in a strain in the InGaN layer [15]. Then, the strain in the MQW will induce a piezoelectric field, thus influence the effective band gap. Ramping the barrier growth temperature during MQW growth will reduce the well width, as shown in Table 1,
350
KSW 04 KSW 03
KSW 02 KSW 01
400
450
500
550
600
Wavelength (nm) Fig. 2. Normalized PL peaks of InGaN/GaN MQWs with respect to the quantum barrier growth conditions.
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and thereby reduce a strain [16]. Therefore, the significant blue-shift of the PL peak may be caused by the reduction of a well width and the strain arising due to ramping the barrier growth temperature. The barrier of the KSW 03 sample was grown with a capping layer on the InGaN well layer and ramped up the growth temperature. Compared with the KSW 01 and 02 samples, the thickness of QW pair was increased up to 10.76 nm and the peak separation of 0th order was shorter than that of the KSW 01 sample. In this case, the reduction of average In composition was caused by the increased GaN thickness by a capping layer. The emission wavelength of PL measurement of the KSW 03 sample showed red-shift from the KSW 02 sample, which means that a capping layer effectively suppressed the decomposition of the InGaN quantum well layer. But it was a little bit shorter than that of the KSW 01 sample. This indicates that the temperature-ramping process affects the InGaN well or the interface state between the InGaN well and a capping layer. The barrier of the KSW 04 sample was grown at hydrogen ambient and other growth conditions were the same as the KSW 03. Even though the KSW 04 sample had a GaN capping layer, the thickness of QW pair and the peak separation of 0th order was similar with the KSW 01 sample. This means that the growth rate of the GaN barrier in the hydrogen ambient is lower than in the nitrogen ambient, or a hydrogen carrier gas etches some of a GaN capping layer during the temperature-ramping process. When the barrier is grown using a nitrogen carrier gas, the surface morphology becomes rough, resulting in a higher density of nucleation site at which growth can occur [17]. The growth rate depends on the overall roughness of the surface, thus, it is expected that the growth rate using a nitrogen carrier gas be greater than that using a hydrogen carrier gas. Therefore, the thickness reduction of QW pair of the KSW 04 sample can be explained. The HRXRD peak of the KSW 04 sample showed high peak intensity, sharp shape and high-order satellite peaks, which indicated an improvement of the interface flatness of the MQWs or the GaN barrier crystal quality.
S. Kim et al. / Journal of Crystal Growth 247 (2003) 62–68
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Relative PL Intensity (a.u.)
Fig. 3 shows the relative PL intensity and Fig. 4 shows the surface images of InGaN/GaN MQWs using AFM. When the quantum well and barrier were grown at the constant temperature (Fig. 4(a)), the surface morphology was rough.
100 80 60 40 20 0 KSW 01
KSW 02
KSW 03
KSW 04
Sample Number
Fig. 3. Relative PL intensity of InGaN/GaN MQWs with respect to the quantum barrier growth conditions.
Due to the lower surface mobility of adsorbed species at the lower growth temperature, the terrace width between steps decreased and the surface roughened. As the growth temperature of the GaN barrier increased, the surface morphology became smooth due to the increased surface mobility of adsorbed species. However, there were many pits on the surface (Fig. 4(b)). It seems that pits can be generated by the thermal decomposition of the InGaN well layer during the temperature-ramping process. This kind of a defect causes the leakage current path in LEDs. In addition, room temperature PL measurement showed double peaks, which were caused by the pit formation and the In composition fluctuation by the temperature-ramping process. But PL intensity of the KSW 02 abruptly increased. The low temperature GaN showed the intense yellow luminescence which was related with the formation of defects such as dislocations, grain boundary, gallium vacancy and its related complex [18]. Stephenson
Fig. 4. AFM images of InGaN/GaN MQWs surface morphology: (a) KSW 01, (b) KSW 02, (c) KSW 03, and (d) KSW 04.
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et al. showed that the growth mode could be changed from a three-dimentional type at low temperature, through a layer-by-layer type, to a step growth mode at high temperatures [19]. Therefore, the high temperature GaN barrier suppresses the deep level related defects in optical properties of MQWs and the stacking faults region through the increased surface mobility of adatoms on the growing surface. By using a GaN capping layer on the InGaN well layer, the number and size of the pit formation on the surface abruptly reduced (Fig. 4(c)). As mentioned above, a GaN capping layer effectively suppressed the decomposition of the InGaN well layer, resulting in the improvement of the surface morphology and the PL intensity. In the case of the KSW 04 sample, the surface morphology was nearly pit-free and smooth. Therefore, the reason why the relative PL and HRXRD peak intensity increased in hydrogen ambient is considered like this. As shown in Fig. 4(d), smooth and pit-free surface morphology means that the interface between the InGaN well and the GaN barrier layer is flat, which improves the quantum confinement and prevents the defect formation which is related with non-radiative recombination centers in the interface region. In addition, the etching effect of a hydrogen carrier gas in high temperature is higher than that of a nitrogen carrier gas [20]. Therefore, the GaN capping layer of the KSW 04 may be partially etched during the temperature-ramping process and its thickness may be thinner than that of the KSW 03. As the thickness of the GaN capping layer was increased above 1 nm, the PL peak intensity corresponding to the emission from the MQW was significantly decreased [21]. This implies that thick GaN cap layers are not effective in preventing the decomposition of the MQW structure. In our work, the optimum capping thickness is another reason of the PL intensity improvement of the KSW 04. These results show that the electrical, optical, and structural properties of the MQWs in LEDs can be significantly improved by increasing the growth temperature of quantum barrier layers and inserting the optimum capping layer in MQWs.
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4. Conclusions We investigated the effect of the GaN barrier growth temperature and a capping layer on the characteristics of InGaN/GaN MQWs using a TPIS-MOCVD system. The temperature-ramping process showed an improvement of optical and structural properties of the MQWs, which was attributed to the suppression of deep level related defects and the smooth and flat interface caused by the increased surface mobility of adatom. A GaN capping layer must be inserted on the InGaN well layer to prevent the thermal decomposition of the quantum well and hydrogen was better than nitrogen as a carrier gas for a high quality quantum barrier of MQWs.
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