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Study on GaN-based light emitting diodes grown on 4-in. Si (111) substrate
Optics Communications 326 (2014) 20–23
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Study on GaN-based light emitting diodes grown on 4-in. Si (111) substrate Mei-Yu Wang a, Guo-An Zhang a, Zhen-Juan Zhang a, Min Shi a, Jing Huang a, You-hua Zhu a,b,n, Takashi Egawa b a b
School of Electronics and Information, Nantong University, Nan Tong 226019, China Research Center for Nano-Device and System, Nagoya Institute of Technology, Nagoya 466-8555, Japan
art ic l e i nf o
a b s t r a c t
Article history: Received 26 December 2013 Received in revised form 21 March 2014 Accepted 1 April 2014 Available online 18 April 2014
GaN-based light emitting diodes (LEDs) grown on 4-in. Si (111) substrate by metal–organic chemical vapor deposition have been systematically characterized. The significantly smooth surface of the sample has been confirmed by an atomic force microscope. In X-ray diffraction measurement, two kinds of fringe peaks were observed, which are believed to originate from the strained-layer superlattice (SLS) and multiple quantum wells. Moreover, from cross-sectional scanning electron microscope images of the sample, it is found that the interfaces of SLS are smooth. The 100-pair AlN/GaN SLS can be employed to modulate the strain between the GaN layer and substrate, resulting in the improvement of GaN crystalline quality. The full width at half maximum of ω-scan of the GaN (0002) diffraction is around 630ʺ. In addition, the device properties have been investigated in detail, and the maximum light output power can reach 2.02 mW with a high saturation injection current of 320 mA. & 2014 Elsevier B.V. All rights reserved.
Keywords: GaN Silicon substrate Light-emitting diodes Metal–organic chemical vapor deposition Light output power
1. Introduction Nowadays, GaN and its related alloys are one of the promising materials for many applications in optoelectronic and microelectronic devices [1,2]. Actually, commercially available GaN-based light-emitting diodes (LEDs) have been fabricated on sapphire, SiC and freestanding GaN substrate. However, low thermal conductivity and insulating characteristic make sapphire less perfect, and large price prevents SiC from being used widely. Meanwhile, the usage of silicon (Si) as a substrate offers many advantages, such as low cost, good thermal conductivity, and the possibility for integration with Si electronics on the same chip [3]. Moreover, the fabrication process of LED on Si is easier than that of LED on sapphire, because one of the ohmic contacts can be made on the backside of the conductive Si substrate. Therefore, compared with the conventional sapphire or SiC substrates, the production of GaN-based LEDs on Si is becoming more attractive. On the other hand, in order to get high quality GaN epilayer grown on Si, some problems, such as cracking, high threading dislocation density (TDD) and cloudy surface morphology [4] contributing to the large mismatches in lattice constants and thermal expansion coefficients (CTE) between the GaN layer and Si substrate, need to be solved. Despite these issues, LEDs grown on Si have been n Corresponding author at: School of Electronics and Information, Nantong University, Nan Tong 226019, China. E-mail address: [email protected] (Y.-h. Zhu).
http://dx.doi.org/10.1016/j.optcom.2014.04.002 0030-4018/& 2014 Elsevier B.V. All rights reserved.
reported by many groups [2,5–9]. Several typical approaches have been previously attempted to control the strain and minimize the cracks, such as using AlN/GaN multilayers with a thin AlN/AlGaN buffer layer [2,3], an insertion of AlGaN/GaN superlattice structure [10], employing a low-temperature AlN or SixNy interlayer [5,11], and so on [7,8,12]. In addition, in the previous reports, the LEDs structures have been grown on 2 or 6 in. Si substrates. Although GaN-based hetero-junction field effect transistors and high electron mobility transistors have been successfully fabricated on 4-in. Si substrate [13,14], only a few studies have been presented on the LEDs grown on the same-size Si substrate [15,16]. It should be noted that the previously reported GaN-based LEDs on Si were often grown with a thin AlN/AlGaN buffer layer and an AlN/GaN or AlGaN/GaN strainedlayer superlattice (SLS) [2–4,10]. All the device performances are not so high, such as light output power with several tens micro-watt, which is due to the unsatisfactory whole epilayer quality, including cracks, high TDD and rough epilayer interface. In this study, we have successfully fabricated the GaN-based LEDs grown on 4-in. Si (111) substrate. In order to improve the GaN crystalline quality, a 100-pair AlN/GaN SLS has been utilized to modulate the strain between the GaN layer and substrate. The total thickness in the LEDs epitaxial structure is over 4.8 μm. The structural properties have been revealed through the corresponding measurements. Also, both the electrical and optical characteristics of the device have been evaluated, and LEDs with more than 2 mW light output power have been realized.
M.-Y. Wang et al. / Optics Communications 326 (2014) 20–23
2. Experimental details A commercial metal–organic chemical vapor deposition (MOCVD) reactor system (Nippon Sanso SR-4000) was used for the epitaxial growth of the InGaN-based multiple-quantum wells (MQWs) LEDs structure. Trimethylgallium (TMGa), trimethylindium (TMIn) and ammonia (NH3) were used as sources for gallium, indium and nitrogen, respectively. Monosilane (SiH4) diluted in hydrogen was used as n-type dopant, and the p-type dopant was bis-cyclopentadienyl magnesium (Cp2Mg). Prior to the growth of LEDs structure, a buffer layer (BL) consisting of a 5 nm n-AlN layer and a 20 nm n-AlGaN layer was grown at 1030 1C. Then, 100-pair AlN/GaN (5/20 nm) SLS layers and a 2 μm-thick n-GaN layer were grown at 1130 1C. Finally, an undoped 10-period MQW consisting of 4 nm InxGa1 xN wells and 8 nm InyGa1 yN barriers at 800 1C, and a 20 nm p-AlGaN layer and a 100 nm p þ -GaN cap layer at 1030 1C were grown successively. In order to confirm the crystalline quality of epitaxial layers, atomic force microscope (AFM), high-resolution X-ray diffraction (HR-XRD) (Philips X'Pert Epitaxy, Cu Kα radiation), and scanning electron microscope (SEM) (Hitachi SU-70, operating voltage of 2.0 kV) measurements were performed. The top-emitting LEDs with a chip size of 500 500 μm2 were fabricated using a standard process [3]. The main LEDs fabrication
Fig. 1. Schematic structure of LEDs in this study. The L.C. and V.C. represent the lateral and vertical conduction forward bias forms, respectively.
21
process included multiple-mask photo-lithography, reactive ion etching, metal evaporation and annealing. Fig. 1 shows the LEDs schematic structure. As shown in Fig. 1, concerning LEDs operation, the lateral and vertical conduction forward bias forms are conducted, which are denoted as L.C. and V.C., respectively, for simplicity of following discussion. The LEDs were characterized using an on-wafer configuration. Current–voltage (I–V) measurement was carried out using a semiconductor parameter analyzer (Agilent 4155C). Electroluminescence (EL) spectra and light output power–current (L–I) were measured using an integrated sphere detector (Otsuka Electronics MPCD-7000) under direct current at room temperature.
3. Results and discussion The sample shows mirror-like surface without any cracks. The AFM morphology images of the sample are shown in Fig. 2. The atomic step-like images indicate that the p þ -GaN top layer has a significantly smooth surface. When the scan areas are 3 3 and 1 1 μm2, the root-mean square roughness of the sample is 0.72 and 0.17 nm, respectively. Fig. 3 shows GaN(0002) XRD 2θ ω scan of the sample, where the strongest peak arises from the GaN layer. At the same time, it can be clearly observed that two kinds of satellite peaks appear on both sides of the major GaN peak, which implies good interface and layer periodicity. Using the equation D¼ nλ/2(sinnth sin0th) [17], the period thickness can be determined. The average period thickness of MQWs can be calculated to be 13.5 nm from the satellite diffraction peaks, as marked by arrow, which is close to the designed value (12 nm). From the other kind of satellite diffraction peak, the period thickness is calculated as 30.8 nm, which is consistent with the designed thickness of SLS (30 nm). Therefore, it can be concluded that one kind of well-defined satellite diffraction peaks to high orders originates from the SLS, and the other one (as marked by arrow) arises from MQWs. Zhang et al. have already reported similar results [3]. However, compared with their data, the relative intensities of the SLS and MQWs in this sample are stronger, which are mainly attributed to the more SLS pairs and higher crystalline quality of MQWs. The inset of Fig. 3 shows the GaN(0002) ω-scan rocking curve of this sample with the full width at half maximum (FWHM) of 626ʺ. The density of screw component threading dislocations is calculated to be 7.9 108/cm2 from the extracted tilt value [18]; it is the same order for the LEDs grown on sapphire. The cross-sectional SEM image of SLS is shown in Fig. 4. The inset of Fig. 4 corresponds to the top
Fig. 2. AFM images of p þ -GaN surface for sample. The scan areas are 3 3 and 1 1 μm2.
22
M.-Y. Wang et al. / Optics Communications 326 (2014) 20–23
Fig. 3. HR-XRD in GaN(0002) 2θ ω scan of sample. In this figure, the fringe peaks indicated by the number from 7 to þ6 originate from the SLS layers. And the fringe peaks indicated by the arrow with the number from 3 to þ2 arise from the MQWs layers. The inset shows the GaN(0002) ω-scan rocking curve of the sample.
Fig. 5. EL spectrum of sample at 20 mA with an L.C. injection form at room temperature. The line inside indicates the results of Gaussian fitting. The inset shows the corresponding emission image.
Fig. 6. I–L characteristics of sample for the L.C. and V.C injection forms. The inset shows I–V characteristics of the sample.
Fig. 4. Cross-sectional SEM image for the SLS layers of sample under a magnifications of 35k. The inset corresponds to the top part of the SLS with a magnification of 100k.
part of the SLS with a higher magnification of 100k. The smooth interfaces have been certified, which are consistent with the aforementioned XRD results. Based on the above results, it can be believed that a LEDs structure with high quality AlN/GaN SLS and InGaN-based MQWs grown on Si has been obtained. The buffer layer structure, film quality and thickness are critical for the growth of crack-free GaN on Si (111) substrate [19]. It is worth noting in this study that the whole AlN/GaN SLS layers have acted as one part of the BL. Due to the lattice mismatch between the SLS and GaN, some degree of compressive stress would be generated in the above GaN layer, which could compensate the tensile stress formed due to the CTE mismatch, in particular during the growth cooling period from high temperature to room temperature. Similar explanations have been proposed in Ref [19]. In addition, it is also considered that obtaining maximum compressive stress on the GaN film is significant for the growth of thick crack-free GaN film on the large-size Si substrate [19]. In this study, the method of combining 100-pair AlN/GaN SLS with the bottom AlN and AlGaN as a whole BL has been suggested. Also,
it can be expected that the LEDs with good performance can be realized by this approach. Some further investigations of strain modulation will be carried out in the future. Fig. 5 shows the EL spectrum of this sample at 20 mA with an L. C. injection form at room temperature. The peak position can be obtained to be 488 nm by Gaussian fitting. In addition, as shown in the inset of Fig. 5, one single LED with bare chip exhibits high brightness and uniform emission from the optically active layer. The measured LED output powers as a function of injection current are shown in Fig. 6. In the two different injection cases, it can be clearly seen that first the output power of the LED increases linearly with injection current, then tends to saturate, reaches a maximum value, and finally decreases slightly as the injection current is further increased. Actually, this phenomenon is also related to “efficiency droop” [20], which has been proposed due to several factors, such as electron leakage by polarization mismatch [21], poor hole injection [22], delocalization of carriers [23] and Auger recombination [24]. It should be noted that the maxmium light output power of this sample is 2.02 mW. On the other hand, there is no obvious difference between the L.C. and V.C. in the L–I curve of the LED under 120 mA. However, at the high current, some differences emerge between the two kinds of conduction forms, which is mainly due to the presence of heat induced by the higher resistance of SLS with the AlN and AlGaN BL. It coincides with the I–V results as shown in the inset of Fig. 6. Herein, it is worth noting that the saturation operating current value is 320 and 280 mA for the L.C. and V.C., respectively, which are about twice higher than those values from the other groups [6,8].
M.-Y. Wang et al. / Optics Communications 326 (2014) 20–23
The good optical properties are mainly attributed to the high quality of epitaxial layer and excellent thermal conductivity of Si substrate. Also, we notice that improving the electrical characteristics in the further research is inevitable. As is well known, in terms of the LEDs grown on Si, lower light output power compared with that of sapphire results from several factors, such as the light absorption by Si substrate, the GaN epilayer quality, the structural design and growth parameters [8]. Despite these influence issues, it can be believed that the performance of LEDs grown on Si can be improved further by some new effective approaches. In recent years, our group has suggested that the crack-free thin-film LED epilayer region can be transferred onto a copper carrier using metal-to-metal bonding and the selective lift-off technique [25]. Also, Zhang et al. have proposed that crack-free GaN-based LEDs with embedded electrode structures can be transferred from the Si substrate onto the electroplating copper submount [26]. In the two studies, the optical power has been enhanced by 49% and 122%, correspondingly. 4. Conclusion In summary, GaN-based LEDs grown on 4-in. Si (111) substrate by MOCVD have been studied. The structural, electrical and optical properties have been revealed by the corresponding measurements. The atomic step-like surface and significant smooth interfaces in the AlN/GaN SLS have been confirmed. The LEDs with a maximum output power of 2.02 mW and a high saturation operating current of 320 mA have been demonstrated. Nevertheless, device researchers will pay attention to these clarified important results and some correlations between the device performance and structural property. Also, it would spur them to further improve the performance of LEDs grown on Si substrate.
Acknowledgments This work was supported by the “The Six Top Talents” of Jiangsu Province (Grant no. XCL-013) and the Natural Science Foundation of Nantong University (Grant no. 03080666).
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References [1] S. Nakamura, T. Mukai, M. Senoh, Appl. Phys. Lett. 64 (1994) 1687. [2] T. Egawa, B. Zhang, H. Ishikawa, IEEE Electron Device Lett. 26 (2005) 169. [3] B. Zhang, T. Egawa, H. Ishikawa, Y. Liu, T. Jimbo, Jpn. J. Appl. Phys. 42 (pt2) (2003) L226. [4] H. Ishikawa, G.Y. Zhao, N. Nakada, T. Egawa, T. Jimbo, M. Umeno, Jpn. J. Appl. Phys. 38 (1999) L492. [5] A. Dadgar, M. Poschenrieder, J. Bläsing, K. Fehse, A. Diez, A. Krost, Appl. Phys. Lett. 80 (2002) 3670. [6] J. Li, J.Y. Lin, H.X. Jiang, Appl. Phys. Lett. 88 (2006) 171909. [7] S. Tripathy, V.K.X. Lin, S.L. Teo, A. Dadgar, A. Diez, J. Blä sing, A. Krost, Appl. Phys. Lett. 91 (2007) 231109. [8] B. Zhang, H. Liang, Y. Wang, Z. Feng, K.W. Ng, K.M. Lau, J. Cryst. Growth 298 (2007) 725. [9] C. Mo, W. Fang, Y. Pu, H. Liu, F. Jiang, J. Cryst. Growth 285 (2005) 312. [10] S. Raghvan, J.M. Redwing, J. Appl. Phys. 98 (2005) 023514. [11] A. Dadgar, M. Poschenrieder, O. Contreras, J. Christen, K. Fehse, J. Blä sing, A. Diez, F. Schlze, T. Riemann, F.A. Ponce, A. Krost, Phys. Status Solidi A 192 (2002) 308. [12] Y.H. Zhu, J.C. Zhang, Z.T. Chen, T. Egawa, J. Appl. Phys. 106 (2009) 124506. [13] J.D. Brown, R. Borges, E. Piner, A. Vescan, S. Singhal, R. Therrien, Solid-State Electron. 46 (2002) 1535. [14] S. Tan, T. Suzue, S.L. Selvaraj, T. Egawa, Jpn. J. Appl. Phys. 48 (2009) 111002. [15] Y. Zhu, A. Watanabe, L. Lu, Z. Chen, T. Egawa, Jpn. J. Appl. Phys. 50 (2011) 04DG08. [16] Y. Zhu, A. Watanabe, L. Lu, Z. Chen, T. Egawa, Jpn. J. Appl. Phys. 51 (2012) 01AG02. [17] I.H. Kim, H.S. Park, Y.I. Park, T. Kim, Appl. Phys. Lett. 73 (1998) 1634. [18] Z.T. Chen, K. Xu, L.P. Guo, H. Zhang, G.Y. Zhang, J. Cryst. Growth 294 (2006) 156. [19] K. Lin, E. Chang, Y. Hsiao, W. Huang, T. Doug, J. Maa, S. Hus, C. Lee, Appl. Phys. Lett. 91 (2007) 222111. [20] D.S. Shin, D.P. Han, J.Y. Oh, J.I. Shim, Appl. Phys. Lett. 100 (2012) 153506. [21] M.H. Kim, M.F. Schubert, Q. Dai, J.K. Kim, E.F. Schubert, J. Piprek, Y. Park, Appl. Phys. Lett. 91 (2007) 183507. [22] I.V. Rozhansky, D.A. Zakheim, Semiconductors 40 (2006) 839. [23] Y. Yang, X.A. Cao, C. Yan, IEEE Trans. Electron Devices 55 (2008) 1771. [24] J. Iveland, L. Martinelli, J. Peretti, J.S. Speck, C. Weisbuch, Phys. Rev. Lett. 110 (2013) 177406. [25] B. Zhang, T. Egawa, H. Ishikawa, Y. Liu, T. Jimbo, Appl. Phys. Lett. 86 (2005) 071113. [26] T.F. Chen, Y.Q. Wang, P. Xiang, R.H. Luo, M.G. Liu, W.M. Yang, Y. Ren, Z.Y. He, Y. B. Yang, W.J. Chen, X.R. Zhang, Z.S. Wu, Y. Liu, B.J. Zhang, Appl. Phys. Lett. 100 (2012) 241112.
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Study on GaN-based light emitting diodes grown on 4-in. Si (111) substrate Mei-Yu Wang a, Guo-An Zhang a, Zhen-Juan Zhang a, Min Shi a, Jing Huang a, You-hua Zhu a,b,n, Takashi Egawa b a b
School of Electronics and Information, Nantong University, Nan Tong 226019, China Research Center for Nano-Device and System, Nagoya Institute of Technology, Nagoya 466-8555, Japan
art ic l e i nf o
a b s t r a c t
Article history: Received 26 December 2013 Received in revised form 21 March 2014 Accepted 1 April 2014 Available online 18 April 2014
GaN-based light emitting diodes (LEDs) grown on 4-in. Si (111) substrate by metal–organic chemical vapor deposition have been systematically characterized. The significantly smooth surface of the sample has been confirmed by an atomic force microscope. In X-ray diffraction measurement, two kinds of fringe peaks were observed, which are believed to originate from the strained-layer superlattice (SLS) and multiple quantum wells. Moreover, from cross-sectional scanning electron microscope images of the sample, it is found that the interfaces of SLS are smooth. The 100-pair AlN/GaN SLS can be employed to modulate the strain between the GaN layer and substrate, resulting in the improvement of GaN crystalline quality. The full width at half maximum of ω-scan of the GaN (0002) diffraction is around 630ʺ. In addition, the device properties have been investigated in detail, and the maximum light output power can reach 2.02 mW with a high saturation injection current of 320 mA. & 2014 Elsevier B.V. All rights reserved.
Keywords: GaN Silicon substrate Light-emitting diodes Metal–organic chemical vapor deposition Light output power
1. Introduction Nowadays, GaN and its related alloys are one of the promising materials for many applications in optoelectronic and microelectronic devices [1,2]. Actually, commercially available GaN-based light-emitting diodes (LEDs) have been fabricated on sapphire, SiC and freestanding GaN substrate. However, low thermal conductivity and insulating characteristic make sapphire less perfect, and large price prevents SiC from being used widely. Meanwhile, the usage of silicon (Si) as a substrate offers many advantages, such as low cost, good thermal conductivity, and the possibility for integration with Si electronics on the same chip [3]. Moreover, the fabrication process of LED on Si is easier than that of LED on sapphire, because one of the ohmic contacts can be made on the backside of the conductive Si substrate. Therefore, compared with the conventional sapphire or SiC substrates, the production of GaN-based LEDs on Si is becoming more attractive. On the other hand, in order to get high quality GaN epilayer grown on Si, some problems, such as cracking, high threading dislocation density (TDD) and cloudy surface morphology [4] contributing to the large mismatches in lattice constants and thermal expansion coefficients (CTE) between the GaN layer and Si substrate, need to be solved. Despite these issues, LEDs grown on Si have been n Corresponding author at: School of Electronics and Information, Nantong University, Nan Tong 226019, China. E-mail address: [email protected] (Y.-h. Zhu).
http://dx.doi.org/10.1016/j.optcom.2014.04.002 0030-4018/& 2014 Elsevier B.V. All rights reserved.
reported by many groups [2,5–9]. Several typical approaches have been previously attempted to control the strain and minimize the cracks, such as using AlN/GaN multilayers with a thin AlN/AlGaN buffer layer [2,3], an insertion of AlGaN/GaN superlattice structure [10], employing a low-temperature AlN or SixNy interlayer [5,11], and so on [7,8,12]. In addition, in the previous reports, the LEDs structures have been grown on 2 or 6 in. Si substrates. Although GaN-based hetero-junction field effect transistors and high electron mobility transistors have been successfully fabricated on 4-in. Si substrate [13,14], only a few studies have been presented on the LEDs grown on the same-size Si substrate [15,16]. It should be noted that the previously reported GaN-based LEDs on Si were often grown with a thin AlN/AlGaN buffer layer and an AlN/GaN or AlGaN/GaN strainedlayer superlattice (SLS) [2–4,10]. All the device performances are not so high, such as light output power with several tens micro-watt, which is due to the unsatisfactory whole epilayer quality, including cracks, high TDD and rough epilayer interface. In this study, we have successfully fabricated the GaN-based LEDs grown on 4-in. Si (111) substrate. In order to improve the GaN crystalline quality, a 100-pair AlN/GaN SLS has been utilized to modulate the strain between the GaN layer and substrate. The total thickness in the LEDs epitaxial structure is over 4.8 μm. The structural properties have been revealed through the corresponding measurements. Also, both the electrical and optical characteristics of the device have been evaluated, and LEDs with more than 2 mW light output power have been realized.
M.-Y. Wang et al. / Optics Communications 326 (2014) 20–23
2. Experimental details A commercial metal–organic chemical vapor deposition (MOCVD) reactor system (Nippon Sanso SR-4000) was used for the epitaxial growth of the InGaN-based multiple-quantum wells (MQWs) LEDs structure. Trimethylgallium (TMGa), trimethylindium (TMIn) and ammonia (NH3) were used as sources for gallium, indium and nitrogen, respectively. Monosilane (SiH4) diluted in hydrogen was used as n-type dopant, and the p-type dopant was bis-cyclopentadienyl magnesium (Cp2Mg). Prior to the growth of LEDs structure, a buffer layer (BL) consisting of a 5 nm n-AlN layer and a 20 nm n-AlGaN layer was grown at 1030 1C. Then, 100-pair AlN/GaN (5/20 nm) SLS layers and a 2 μm-thick n-GaN layer were grown at 1130 1C. Finally, an undoped 10-period MQW consisting of 4 nm InxGa1 xN wells and 8 nm InyGa1 yN barriers at 800 1C, and a 20 nm p-AlGaN layer and a 100 nm p þ -GaN cap layer at 1030 1C were grown successively. In order to confirm the crystalline quality of epitaxial layers, atomic force microscope (AFM), high-resolution X-ray diffraction (HR-XRD) (Philips X'Pert Epitaxy, Cu Kα radiation), and scanning electron microscope (SEM) (Hitachi SU-70, operating voltage of 2.0 kV) measurements were performed. The top-emitting LEDs with a chip size of 500 500 μm2 were fabricated using a standard process [3]. The main LEDs fabrication
Fig. 1. Schematic structure of LEDs in this study. The L.C. and V.C. represent the lateral and vertical conduction forward bias forms, respectively.
21
process included multiple-mask photo-lithography, reactive ion etching, metal evaporation and annealing. Fig. 1 shows the LEDs schematic structure. As shown in Fig. 1, concerning LEDs operation, the lateral and vertical conduction forward bias forms are conducted, which are denoted as L.C. and V.C., respectively, for simplicity of following discussion. The LEDs were characterized using an on-wafer configuration. Current–voltage (I–V) measurement was carried out using a semiconductor parameter analyzer (Agilent 4155C). Electroluminescence (EL) spectra and light output power–current (L–I) were measured using an integrated sphere detector (Otsuka Electronics MPCD-7000) under direct current at room temperature.
3. Results and discussion The sample shows mirror-like surface without any cracks. The AFM morphology images of the sample are shown in Fig. 2. The atomic step-like images indicate that the p þ -GaN top layer has a significantly smooth surface. When the scan areas are 3 3 and 1 1 μm2, the root-mean square roughness of the sample is 0.72 and 0.17 nm, respectively. Fig. 3 shows GaN(0002) XRD 2θ ω scan of the sample, where the strongest peak arises from the GaN layer. At the same time, it can be clearly observed that two kinds of satellite peaks appear on both sides of the major GaN peak, which implies good interface and layer periodicity. Using the equation D¼ nλ/2(sinnth sin0th) [17], the period thickness can be determined. The average period thickness of MQWs can be calculated to be 13.5 nm from the satellite diffraction peaks, as marked by arrow, which is close to the designed value (12 nm). From the other kind of satellite diffraction peak, the period thickness is calculated as 30.8 nm, which is consistent with the designed thickness of SLS (30 nm). Therefore, it can be concluded that one kind of well-defined satellite diffraction peaks to high orders originates from the SLS, and the other one (as marked by arrow) arises from MQWs. Zhang et al. have already reported similar results [3]. However, compared with their data, the relative intensities of the SLS and MQWs in this sample are stronger, which are mainly attributed to the more SLS pairs and higher crystalline quality of MQWs. The inset of Fig. 3 shows the GaN(0002) ω-scan rocking curve of this sample with the full width at half maximum (FWHM) of 626ʺ. The density of screw component threading dislocations is calculated to be 7.9 108/cm2 from the extracted tilt value [18]; it is the same order for the LEDs grown on sapphire. The cross-sectional SEM image of SLS is shown in Fig. 4. The inset of Fig. 4 corresponds to the top
Fig. 2. AFM images of p þ -GaN surface for sample. The scan areas are 3 3 and 1 1 μm2.
22
M.-Y. Wang et al. / Optics Communications 326 (2014) 20–23
Fig. 3. HR-XRD in GaN(0002) 2θ ω scan of sample. In this figure, the fringe peaks indicated by the number from 7 to þ6 originate from the SLS layers. And the fringe peaks indicated by the arrow with the number from 3 to þ2 arise from the MQWs layers. The inset shows the GaN(0002) ω-scan rocking curve of the sample.
Fig. 5. EL spectrum of sample at 20 mA with an L.C. injection form at room temperature. The line inside indicates the results of Gaussian fitting. The inset shows the corresponding emission image.
Fig. 6. I–L characteristics of sample for the L.C. and V.C injection forms. The inset shows I–V characteristics of the sample.
Fig. 4. Cross-sectional SEM image for the SLS layers of sample under a magnifications of 35k. The inset corresponds to the top part of the SLS with a magnification of 100k.
part of the SLS with a higher magnification of 100k. The smooth interfaces have been certified, which are consistent with the aforementioned XRD results. Based on the above results, it can be believed that a LEDs structure with high quality AlN/GaN SLS and InGaN-based MQWs grown on Si has been obtained. The buffer layer structure, film quality and thickness are critical for the growth of crack-free GaN on Si (111) substrate [19]. It is worth noting in this study that the whole AlN/GaN SLS layers have acted as one part of the BL. Due to the lattice mismatch between the SLS and GaN, some degree of compressive stress would be generated in the above GaN layer, which could compensate the tensile stress formed due to the CTE mismatch, in particular during the growth cooling period from high temperature to room temperature. Similar explanations have been proposed in Ref [19]. In addition, it is also considered that obtaining maximum compressive stress on the GaN film is significant for the growth of thick crack-free GaN film on the large-size Si substrate [19]. In this study, the method of combining 100-pair AlN/GaN SLS with the bottom AlN and AlGaN as a whole BL has been suggested. Also,
it can be expected that the LEDs with good performance can be realized by this approach. Some further investigations of strain modulation will be carried out in the future. Fig. 5 shows the EL spectrum of this sample at 20 mA with an L. C. injection form at room temperature. The peak position can be obtained to be 488 nm by Gaussian fitting. In addition, as shown in the inset of Fig. 5, one single LED with bare chip exhibits high brightness and uniform emission from the optically active layer. The measured LED output powers as a function of injection current are shown in Fig. 6. In the two different injection cases, it can be clearly seen that first the output power of the LED increases linearly with injection current, then tends to saturate, reaches a maximum value, and finally decreases slightly as the injection current is further increased. Actually, this phenomenon is also related to “efficiency droop” [20], which has been proposed due to several factors, such as electron leakage by polarization mismatch [21], poor hole injection [22], delocalization of carriers [23] and Auger recombination [24]. It should be noted that the maxmium light output power of this sample is 2.02 mW. On the other hand, there is no obvious difference between the L.C. and V.C. in the L–I curve of the LED under 120 mA. However, at the high current, some differences emerge between the two kinds of conduction forms, which is mainly due to the presence of heat induced by the higher resistance of SLS with the AlN and AlGaN BL. It coincides with the I–V results as shown in the inset of Fig. 6. Herein, it is worth noting that the saturation operating current value is 320 and 280 mA for the L.C. and V.C., respectively, which are about twice higher than those values from the other groups [6,8].
M.-Y. Wang et al. / Optics Communications 326 (2014) 20–23
The good optical properties are mainly attributed to the high quality of epitaxial layer and excellent thermal conductivity of Si substrate. Also, we notice that improving the electrical characteristics in the further research is inevitable. As is well known, in terms of the LEDs grown on Si, lower light output power compared with that of sapphire results from several factors, such as the light absorption by Si substrate, the GaN epilayer quality, the structural design and growth parameters [8]. Despite these influence issues, it can be believed that the performance of LEDs grown on Si can be improved further by some new effective approaches. In recent years, our group has suggested that the crack-free thin-film LED epilayer region can be transferred onto a copper carrier using metal-to-metal bonding and the selective lift-off technique [25]. Also, Zhang et al. have proposed that crack-free GaN-based LEDs with embedded electrode structures can be transferred from the Si substrate onto the electroplating copper submount [26]. In the two studies, the optical power has been enhanced by 49% and 122%, correspondingly. 4. Conclusion In summary, GaN-based LEDs grown on 4-in. Si (111) substrate by MOCVD have been studied. The structural, electrical and optical properties have been revealed by the corresponding measurements. The atomic step-like surface and significant smooth interfaces in the AlN/GaN SLS have been confirmed. The LEDs with a maximum output power of 2.02 mW and a high saturation operating current of 320 mA have been demonstrated. Nevertheless, device researchers will pay attention to these clarified important results and some correlations between the device performance and structural property. Also, it would spur them to further improve the performance of LEDs grown on Si substrate.
Acknowledgments This work was supported by the “The Six Top Talents” of Jiangsu Province (Grant no. XCL-013) and the Natural Science Foundation of Nantong University (Grant no. 03080666).
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