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SIMS and SEM analysis of In1−x−yAlxGayP LED structure grown on InxGa1−xP graded buffer
Applied Surface Science 252 (2006) 7279–7282 www.elsevier.com/locate/apsusc
SIMS and SEM analysis of In1 x yAlxGayP LED structure grown on InxGa1 xP graded buffer A. Vincze a,*, A. Satka a,b, L. Peternai b, J. Kovac a,b, S. Haseno¨hrl c, M. Vesely b a International Laser Centre, Ilkovicova 3, 81219 Bratislava, Slovakia Department of Microelectronics, Slovak University of Technology, Ilkovicova 3, 81219 Bratislava, Slovakia c Institute of Electrical Engineering, Slovak Academy of Science, Dubravska Cesta 9, 84104, Bratislava, Slovakia b
Received 12 September 2005; accepted 15 February 2006 Available online 2 May 2006
Abstract Composition, grading and doping of In1 x yAlxGayP LED structures grown on the GaP substrate by step-wise graded InxGa1 xP buffer were investigated by employing SIMS and SEM methods. Different amount of Al precursor has been used during MOCVD growth, resulting different Al content and correspondingly different wavelength of the emitted light. The buffer comprised of eight intentionally 300 nm thick InxGa1 xP layers with step increase of In DxIn 3% toward In1 x yAlxGayP LED layers were grown to accommodate relatively high lattice mismatch between In1 x yAlxGayP and GaP. Vertical structure of the samples has been visualised using backscattered electron method of the SEM. From the cleaved edge of samples the layers of different composition were revealed and the thickness of In1 x yAlxGayP and InxGa1 xP layers has been determined. In contrary p–n junction position was determined from SIMS depth profiling of the dopants and estimated only from secondary electron images. The compositional changes in the structures were examined using SIMS depth profiling, from which all the eight different In content steps in InxGa1 xP buffer layers were detected. Composition of the LED layers has been determined from EDS measurements and compared with SIMS depth profiles. From SIMS and EDS measurements quaternary composition of the components Al, In, P and Ga were evaluated and optimized for the growth process. # 2006 Elsevier B.V. All rights reserved. Keywords: LED; InGaP; Buffer; InAlGaP/GaP; SIMS; MOCVD
1. Introduction Recently there has been a renewed interest in GaP in connection with epilayers and heterostructures. The driving force for new devices like more efficient light emitting diodes (LEDs) has stimulated growth investigation of new GaP structures and their alloys, using metal organic chemical vapor deposition (MOCVD). One possible solution for the LED improvement is the direct growth of LED structure on optically transparent GaP substrate, which eliminates the use of additional wafer bonding techniques [1]. The In1 x yAlxGayP quaternary alloy represents the proper material system for LED applications due to wide direct band gap [2,3]. In1 x yAlxGayP is not lattice-matched to GaP substrates and therefore InxGa1 xP
* Corresponding author. Tel.: +421 2 65421575; fax: +421 2 65423244. E-mail address: [email protected] (A. Vincze). 0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2006.02.128
graded buffer layers are necessary for direct growth on GaP substrate. In this way LEDs from green to orange range of visible spectra can be prepared. High quality In1 x yAlxGayP and InxGa1 xP layers are of key importance for LED performance. Above-mentioned large lattice mismatch between GaP substrate and In1 x yAlxGayP epitaxial layers (exhibiting direct band gap structure) requires demanding growth process control [4] and extensive optimization of growth parameters [5]. In our contribution we present results of SIMS and SEM/EDS investigations on In1 x yAlxGayP LED structures grown on InxGa1 xP/GaP graded buffer. 2. Experimental Structures under investigation were grown by IR-heated low-pressure MOCVD (AIXTRON AIX 200) on (1 0 0) GaP:S exactly oriented substrates with epi-ready finalization. Phosphine (PH3) was used as group V source. Group III
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precursors were trimethyilgallium (TMGa), trimethylaluminum (TMAl) and trimethyilindium (TMIn). Palladium diffused hydrogen was used as the carrier gas. First, 300 nm thick GaP:Si buffer layer (n 2 1018 cm 3) was grown, followed by buffer stack of the eight InxGa1 xP layers, 300 nm thick each, with step-wise compositional change of DxIn 0.03 between layers. This stack should accommodate the lattice mismatch of 2.37% between GaP substrate and top InGaP layer [5]. On the top of graded buffer, LED structure comprising of 1000 nm In1 x yAlxGayP:Si and 1000 nm In1 x yAlxGayP:Zn was grown (Fig. 1a). Five LED structures of different Al content (labelled from A to E) were grown consisting of the same buffer structure to set-up altered wavelengths of emitted radiation. Secondary ion mass spectrometry (SIMS) profiling as well as scanning electron microscopy (SEM) topography and energy dispersive X-ray spectroscopy (EDS) were employed for LED structure characterisation and composition determination. The time of flight-based SIMS instrument (Ion-TOF) with high energy Au+ primary source was employed for LED structure analysis. For structure depth profiling high energy pulsed primary source (25 keV) was combined with low energy sputter guns at 2 keV (Cs+ and O2+) in 458 to sample surface because of low erosion rate of analysing gun. Sputtering ion beam is rastered over 300 mm 300 mm area while the primary beam within 70 mm 70 mm area in the center of sputtered area. In depth profile all eight different In content steps were determined. Field emission SEM LEO 1550 in secondary electron (inlens and SE2 detectors) and backscattered electron (RBSD
detector) modes were used to characterize the surface morphology and to visualize vertical structure of investigated samples. Dynamic SIMS profiling of elemental and molecular ions provides basis for the valuable insight into the specific compositions of the quaternary and ternary alloys as well as structures with ultimate vertical resolution whereas EDS provides a quantitative information and calibrated data of layer composition. 3. Results and discussion SEM cross-sectional view of the sample B in backscattered electron mode is shown in Fig. 1b. Eight individual InxGa1 xP layers are well resolved due to the In composition grading steps in InxGa1 xP layers. The upper In1 x yAlxGayP LED layers were detected as the consequence of Al fraction in the layers. Thickness of InxGa1 xP layers was determined from Fig. 1b and varies in the range of 260–280 nm. The total thickness of both In1 x yAlxGayP layers was determined as 1600 nm, which is slightly less than expected from MOCVD growth data. EDS signals measured on vertical cross-section (Fig. 1b) show basic traces of quaternary elements. In content in the In1 x yAlxGayP layers is nearly constant, and continuously decreases in graded InxGa1 xP buffer towards substrate reaching nearly zero value in GaP substrate. Al signal is detected in LED part of the structure only, and relatively high signal at the top of the structure is artefact of Al sample holder. Ga content shows the grading in the buffer layers and is practically complementary to In, whereas P content in
Fig. 1. (a) Sample structure and (b) cross-sectional SEM view of the sample B and corresponding EDS profile.
A. Vincze et al. / Applied Surface Science 252 (2006) 7279–7282
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Fig. 2. Grading of the In content in the InxGa1 xP layers of sample E in corresponding InP SIMS signal.
Fig. 4. Comparison of SIMS Al signal in B, C, D and E structures.
In1 x yAlxGayP layers is measured slightly higher than in the rest of the structure. Grading of In content in the buffer was more precisely determined from SIMS depth profiles in negative secondary ion polarity of InP-related signal (Fig. 2). All eight different In content steps (DxIn) in InxGa1 xP layers can be resolved. Absolute value of individual steps slightly differ from the projected value of DxIn 0.03 but their mean value remained preserved. In addition, small amount of In in GaP buffer layer has been detected from the semi-logarithmic plots of SIMS depth profiles (Fig. 3). This small value can be attributed to the unintentional doping spike with In in the buffer layer possible during MOCVD process. The profile depths of sputtered structure were verified by Talystep measurements (Taylor Hobson). Signals from overall structure are shown in Fig. 3. The overall LED structure thickness of 1800 nm was determined from Al signal, which is in good agreement with In1 x yAlxGayP layer
thickness determined from SEM investigation (Fig. 1b). Al related signal remains constant across quaternary layers (Fig. 3). P–n junction position at 800 nm was determined from the sharp increase in Si signal, which is also in good agreement with position determined from secondary electron images (not presented here). LED structures including different Al content are compared in Fig. 4. Structure depth profiles were measured and evaluated for both polarities of secondary ions. In and InP signals could be used for In content determination, but their ion yields are also inversely proportional. This In signal can also be used for DxIn grading step evaluation. EDS results of the quaternary structure composition was used for Al signal evaluation from SIMS profiles (Fig. 4). The composition from EDS measurements was determined as follows: structure B, In0.244Al0.24Ga0.516P; structure C, In0.234Al0.14Ga0.626P; structure D, In0.246Al0.012Ga0.744P; structure E, In0.254Al0.002Ga0.736P. 4. Conclusions
Fig. 3. Negative polarity SIMS depth profile, sample B.
The series of LED structures consisting of In1 x yAlxGayP p- and n-layers on graded n-InxGa1 xP buffer were grown on nGaP substrates by MOCVD. SIMS and SEM methods were used for detailed characterization of the structures. Compositional dependence of quaternary elements, P, Ga, In and Al was examined across the structure. InP negative secondary ion signal has been found as the most appropriate for In content evaluation. In contrast the In secondary ion signal has higher ion yield despite of worse depth resolution. EDS measurements are helpful as additional tool for verifying the evaluated data. The DxIn step grading has been found to be in the range of 0.023–0.037. Such a InxGa1 xP step graded optically transparent buffer structure was found suitable for light emitting structure preparation based on In1 x yAlxGayP quaternary alloys. The deviation of the layer thicknesses from the nominal values is 10% and 15% for InxGa1 xP and In1 x yAlxGayP
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layers, respectively. The p–n junction position determined from Si doping profiles is in good agreement with the position determined from the SEM observations. The LEDs fabricated from these structures are emitting in green–orange colour region. Acknowledgements The projects of IST-2001-32793, aAV/805/02, VEGA 1/0152/03, 1/0130/03, APVT 51-050602 are kindly acknowledged.
References [1] F.A. Kish, F.M. Steranka, D.C. Defevere, D.A. Vanderwater, K.G. Park, C.P. Kuo, T.D. Osentowski, M.J. Peanasky, J.G. Yu, R.M. Fletcher, D.A. Steigerwald, M.G. Craford, V.M. Robbins, Appl. Phys. Lett. 64 (1994) 2839. [2] Th. Gessmann, E.F. Schubert, J. Appl. Phys. 95 (5) (2004) 2203–2216. [3] M.R. Krames, M. Ochiai-Holcomb, G.E. Ho¨fler, C. Carter-Coman, E.I. Chen, I.H. Tan, P. Grillot, N.F. Gardner, H.C. Chui, H.W. Juang, S.A. Stockman, F.A. Kish, M. Craford, Appl. Phys. Lett. 75 (16) (1999) 2365– 2367. [4] W. Korber, K.W. Benz, J. Cryst. Growth 73 (1985) 179. [5] S. Haseno¨hrl, J. Novak, I. Vavra, A. Satka, J. Cryst. Growth 272 (2004) 633–641.
SIMS and SEM analysis of In1 x yAlxGayP LED structure grown on InxGa1 xP graded buffer A. Vincze a,*, A. Satka a,b, L. Peternai b, J. Kovac a,b, S. Haseno¨hrl c, M. Vesely b a International Laser Centre, Ilkovicova 3, 81219 Bratislava, Slovakia Department of Microelectronics, Slovak University of Technology, Ilkovicova 3, 81219 Bratislava, Slovakia c Institute of Electrical Engineering, Slovak Academy of Science, Dubravska Cesta 9, 84104, Bratislava, Slovakia b
Received 12 September 2005; accepted 15 February 2006 Available online 2 May 2006
Abstract Composition, grading and doping of In1 x yAlxGayP LED structures grown on the GaP substrate by step-wise graded InxGa1 xP buffer were investigated by employing SIMS and SEM methods. Different amount of Al precursor has been used during MOCVD growth, resulting different Al content and correspondingly different wavelength of the emitted light. The buffer comprised of eight intentionally 300 nm thick InxGa1 xP layers with step increase of In DxIn 3% toward In1 x yAlxGayP LED layers were grown to accommodate relatively high lattice mismatch between In1 x yAlxGayP and GaP. Vertical structure of the samples has been visualised using backscattered electron method of the SEM. From the cleaved edge of samples the layers of different composition were revealed and the thickness of In1 x yAlxGayP and InxGa1 xP layers has been determined. In contrary p–n junction position was determined from SIMS depth profiling of the dopants and estimated only from secondary electron images. The compositional changes in the structures were examined using SIMS depth profiling, from which all the eight different In content steps in InxGa1 xP buffer layers were detected. Composition of the LED layers has been determined from EDS measurements and compared with SIMS depth profiles. From SIMS and EDS measurements quaternary composition of the components Al, In, P and Ga were evaluated and optimized for the growth process. # 2006 Elsevier B.V. All rights reserved. Keywords: LED; InGaP; Buffer; InAlGaP/GaP; SIMS; MOCVD
1. Introduction Recently there has been a renewed interest in GaP in connection with epilayers and heterostructures. The driving force for new devices like more efficient light emitting diodes (LEDs) has stimulated growth investigation of new GaP structures and their alloys, using metal organic chemical vapor deposition (MOCVD). One possible solution for the LED improvement is the direct growth of LED structure on optically transparent GaP substrate, which eliminates the use of additional wafer bonding techniques [1]. The In1 x yAlxGayP quaternary alloy represents the proper material system for LED applications due to wide direct band gap [2,3]. In1 x yAlxGayP is not lattice-matched to GaP substrates and therefore InxGa1 xP
* Corresponding author. Tel.: +421 2 65421575; fax: +421 2 65423244. E-mail address: [email protected] (A. Vincze). 0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2006.02.128
graded buffer layers are necessary for direct growth on GaP substrate. In this way LEDs from green to orange range of visible spectra can be prepared. High quality In1 x yAlxGayP and InxGa1 xP layers are of key importance for LED performance. Above-mentioned large lattice mismatch between GaP substrate and In1 x yAlxGayP epitaxial layers (exhibiting direct band gap structure) requires demanding growth process control [4] and extensive optimization of growth parameters [5]. In our contribution we present results of SIMS and SEM/EDS investigations on In1 x yAlxGayP LED structures grown on InxGa1 xP/GaP graded buffer. 2. Experimental Structures under investigation were grown by IR-heated low-pressure MOCVD (AIXTRON AIX 200) on (1 0 0) GaP:S exactly oriented substrates with epi-ready finalization. Phosphine (PH3) was used as group V source. Group III
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precursors were trimethyilgallium (TMGa), trimethylaluminum (TMAl) and trimethyilindium (TMIn). Palladium diffused hydrogen was used as the carrier gas. First, 300 nm thick GaP:Si buffer layer (n 2 1018 cm 3) was grown, followed by buffer stack of the eight InxGa1 xP layers, 300 nm thick each, with step-wise compositional change of DxIn 0.03 between layers. This stack should accommodate the lattice mismatch of 2.37% between GaP substrate and top InGaP layer [5]. On the top of graded buffer, LED structure comprising of 1000 nm In1 x yAlxGayP:Si and 1000 nm In1 x yAlxGayP:Zn was grown (Fig. 1a). Five LED structures of different Al content (labelled from A to E) were grown consisting of the same buffer structure to set-up altered wavelengths of emitted radiation. Secondary ion mass spectrometry (SIMS) profiling as well as scanning electron microscopy (SEM) topography and energy dispersive X-ray spectroscopy (EDS) were employed for LED structure characterisation and composition determination. The time of flight-based SIMS instrument (Ion-TOF) with high energy Au+ primary source was employed for LED structure analysis. For structure depth profiling high energy pulsed primary source (25 keV) was combined with low energy sputter guns at 2 keV (Cs+ and O2+) in 458 to sample surface because of low erosion rate of analysing gun. Sputtering ion beam is rastered over 300 mm 300 mm area while the primary beam within 70 mm 70 mm area in the center of sputtered area. In depth profile all eight different In content steps were determined. Field emission SEM LEO 1550 in secondary electron (inlens and SE2 detectors) and backscattered electron (RBSD
detector) modes were used to characterize the surface morphology and to visualize vertical structure of investigated samples. Dynamic SIMS profiling of elemental and molecular ions provides basis for the valuable insight into the specific compositions of the quaternary and ternary alloys as well as structures with ultimate vertical resolution whereas EDS provides a quantitative information and calibrated data of layer composition. 3. Results and discussion SEM cross-sectional view of the sample B in backscattered electron mode is shown in Fig. 1b. Eight individual InxGa1 xP layers are well resolved due to the In composition grading steps in InxGa1 xP layers. The upper In1 x yAlxGayP LED layers were detected as the consequence of Al fraction in the layers. Thickness of InxGa1 xP layers was determined from Fig. 1b and varies in the range of 260–280 nm. The total thickness of both In1 x yAlxGayP layers was determined as 1600 nm, which is slightly less than expected from MOCVD growth data. EDS signals measured on vertical cross-section (Fig. 1b) show basic traces of quaternary elements. In content in the In1 x yAlxGayP layers is nearly constant, and continuously decreases in graded InxGa1 xP buffer towards substrate reaching nearly zero value in GaP substrate. Al signal is detected in LED part of the structure only, and relatively high signal at the top of the structure is artefact of Al sample holder. Ga content shows the grading in the buffer layers and is practically complementary to In, whereas P content in
Fig. 1. (a) Sample structure and (b) cross-sectional SEM view of the sample B and corresponding EDS profile.
A. Vincze et al. / Applied Surface Science 252 (2006) 7279–7282
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Fig. 2. Grading of the In content in the InxGa1 xP layers of sample E in corresponding InP SIMS signal.
Fig. 4. Comparison of SIMS Al signal in B, C, D and E structures.
In1 x yAlxGayP layers is measured slightly higher than in the rest of the structure. Grading of In content in the buffer was more precisely determined from SIMS depth profiles in negative secondary ion polarity of InP-related signal (Fig. 2). All eight different In content steps (DxIn) in InxGa1 xP layers can be resolved. Absolute value of individual steps slightly differ from the projected value of DxIn 0.03 but their mean value remained preserved. In addition, small amount of In in GaP buffer layer has been detected from the semi-logarithmic plots of SIMS depth profiles (Fig. 3). This small value can be attributed to the unintentional doping spike with In in the buffer layer possible during MOCVD process. The profile depths of sputtered structure were verified by Talystep measurements (Taylor Hobson). Signals from overall structure are shown in Fig. 3. The overall LED structure thickness of 1800 nm was determined from Al signal, which is in good agreement with In1 x yAlxGayP layer
thickness determined from SEM investigation (Fig. 1b). Al related signal remains constant across quaternary layers (Fig. 3). P–n junction position at 800 nm was determined from the sharp increase in Si signal, which is also in good agreement with position determined from secondary electron images (not presented here). LED structures including different Al content are compared in Fig. 4. Structure depth profiles were measured and evaluated for both polarities of secondary ions. In and InP signals could be used for In content determination, but their ion yields are also inversely proportional. This In signal can also be used for DxIn grading step evaluation. EDS results of the quaternary structure composition was used for Al signal evaluation from SIMS profiles (Fig. 4). The composition from EDS measurements was determined as follows: structure B, In0.244Al0.24Ga0.516P; structure C, In0.234Al0.14Ga0.626P; structure D, In0.246Al0.012Ga0.744P; structure E, In0.254Al0.002Ga0.736P. 4. Conclusions
Fig. 3. Negative polarity SIMS depth profile, sample B.
The series of LED structures consisting of In1 x yAlxGayP p- and n-layers on graded n-InxGa1 xP buffer were grown on nGaP substrates by MOCVD. SIMS and SEM methods were used for detailed characterization of the structures. Compositional dependence of quaternary elements, P, Ga, In and Al was examined across the structure. InP negative secondary ion signal has been found as the most appropriate for In content evaluation. In contrast the In secondary ion signal has higher ion yield despite of worse depth resolution. EDS measurements are helpful as additional tool for verifying the evaluated data. The DxIn step grading has been found to be in the range of 0.023–0.037. Such a InxGa1 xP step graded optically transparent buffer structure was found suitable for light emitting structure preparation based on In1 x yAlxGayP quaternary alloys. The deviation of the layer thicknesses from the nominal values is 10% and 15% for InxGa1 xP and In1 x yAlxGayP
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layers, respectively. The p–n junction position determined from Si doping profiles is in good agreement with the position determined from the SEM observations. The LEDs fabricated from these structures are emitting in green–orange colour region. Acknowledgements The projects of IST-2001-32793, aAV/805/02, VEGA 1/0152/03, 1/0130/03, APVT 51-050602 are kindly acknowledged.
References [1] F.A. Kish, F.M. Steranka, D.C. Defevere, D.A. Vanderwater, K.G. Park, C.P. Kuo, T.D. Osentowski, M.J. Peanasky, J.G. Yu, R.M. Fletcher, D.A. Steigerwald, M.G. Craford, V.M. Robbins, Appl. Phys. Lett. 64 (1994) 2839. [2] Th. Gessmann, E.F. Schubert, J. Appl. Phys. 95 (5) (2004) 2203–2216. [3] M.R. Krames, M. Ochiai-Holcomb, G.E. Ho¨fler, C. Carter-Coman, E.I. Chen, I.H. Tan, P. Grillot, N.F. Gardner, H.C. Chui, H.W. Juang, S.A. Stockman, F.A. Kish, M. Craford, Appl. Phys. Lett. 75 (16) (1999) 2365– 2367. [4] W. Korber, K.W. Benz, J. Cryst. Growth 73 (1985) 179. [5] S. Haseno¨hrl, J. Novak, I. Vavra, A. Satka, J. Cryst. Growth 272 (2004) 633–641.