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GaInP multiple quantum wells light-emitting diode wafers
ARTICLE IN PRESS
Microelectronics Journal 39 (2008) 70–73 www.elsevier.com/locate/mejo
The influence of thermal annealing to the characteristics of AlGaInP/GaInP multiple quantum wells light-emitting diode wafers Shuti Li, Guanghan Fan, Huiqing Sun, Shuwen Zheng Institute of Opto-electronic Materials and Technology, South China Normal University, Guangzhou 510631, PR China Received 8 August 2007; accepted 27 October 2007
Abstract The influence of thermal annealing to the characteristics of AlGaInP/GaInP multiple quantum wells (MQWs) light-emitting diode wafers was studied by means of electrochemical capacitance–voltage (ECV) and photoluminescence (PL). Compared with the sample unannealed, the hole carrier concentration of p-GaP layer increased from 5.5 1018 to 6.5 1018 cm 3, and the hole carrier concentration of p-AlGaInP layer increased from 6.0 1017 to 1.1 1018 cm 3, after wafer was annealed at 460 1C for 15 min in nitrogen. The hole carrier concentrations of p-GaP layers and p-AlGaInP layers did not obviously change when the annealing temperature varied from 460 to 700 1C. However, after the sample was annealed under 780 1C for 15 min, the hole carrier concentration of p-GaP layer and p-AlGaInP layer decreased to 8 1017 and 1.7 1017 cm 3, respectively. At the same time, the diffusion of Mg atoms was observed. r 2007 Elsevier Ltd. All rights reserved. PACS: 81.15.Gh; 78.55.Cr; 61.72.Vv Keywords: Semiconductors; Thin films; Epitaxial growth; Electrochemical measurements; Electrical properties
1. Introduction Significant advancements in the light output performance of visible light-emitting diodes (LEDs) and lightemitting lasers (LDs) have been demonstrated using the AlGaInP alloy system [1–6]. Some investigations indicated that the properties of AlGaInP alloy system could be influenced by rapid thermal annealing [7–11]. After rapid thermal annealing at 875 1C for as short as only 1 s, obvious reduction in the concentration of several deep-level traps of bulk GaInP and GaInP/AlGaInP multiple quantum wells (MQWs) have been reported [7–8]. The intensity of the photoluminescence (PL) peak for GaInP/ AlGaInP single QW also can be increased after only 1 s annealing in the temperature range from 800 to 900 1C. This improvement could be associated with the possibility that annealing treatment removes nonradiative recombination centers from the QWs [10]. In the present contribution, Corresponding author. Tel.: +86 20 85212667x601; fax: +86 20 85210809. E-mail address: [email protected] (S. Li).
0026-2692/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.mejo.2007.10.013
we report the influence of thermal annealing to the characteristics of AlGaInP/GaInP MQWs LED wafers. The results shown that the increase in the hole carrier concentrations of p-GaP layer and p-AlGaInP layer can be obtained with much lower thermal annealing temperature than the reported [7–11]. The Mg diffusion behavior in MQWs was observed by thermal annealing at 780 1C. 2. Experimental details All samples in the present study were grown by EMCORE GS/3200 low-pressure metal–organic chemical vapor deposition (LP-MOCVD). Si-doped GaAs substrates were cut 151 off the (1 0 0) plane toward the [0 1 1] direction. The source materials were TMGa, TMIn, TMAl, AsH3, PH3, respectively. CP2Mg and SiH4 were used for p- and n-type doping reagents. The growth temperature was generally between 620 and 720 1C. Fig. 1 shows the schematic diagram of the AlGaInP/ GaInP MQWs LED wafers in thermal annealing experiments. The layout of the wafers include the following layers: one 0.5-mm-thick n-GaAs buffer layer with carrier
ARTICLE IN PRESS S. Li et al. / Microelectronics Journal 39 (2008) 70–73
71
p-GaP
MQWs n-(Al0.7Ga0.3)In0.5P n-GaAs buffer
F
Intensity (arb. units)
p-(Al0.7Ga0.3)In0.5P
n-GaAs
3. Results and discussion Figs. 2 and 3 show the room temperature PL spectra of six samples before and after annealing. The annealing temperatures of samples are listed in Fig. 3. The peak wavelength of these samples varied from 624 to 626 nm. It shows clearly that the peak wavelength and the PL FWHM values were not affected much by the annealing process. When the annealing temperature was lower than 700 1C, there were no obvious changes for the PL intensity. However, the PL intensity double increased when the annealing temperature reached 780 1C. This indicates that the PL intensity of GaInP/AlGaInP MQWs can be enhanced only if the annealing temperature is high enough. Fig. 4 shows the distribution of carrier concentrations of three samples. The annealing temperatures are listed. Compared with the sample unannealed, the carrier concentration distribution of samples B and F changed after annealing. We choose the values in depth of 1.0 and 5.2 mm as the representative carrier concentrations of p-GaP layer and p-(Al0.7Ga0.3)In0.5P layer, since the
D C B
Fig. 1. The schematic structure of AlGaInP LED wafers.
A
600
620 640 Wavelength (nm)
660
Fig. 2. Photoluminescence spectra of AlGaInP LED wafers before annealing.
F×1/2 F
Intensity (arb. units)
concentration of about 5 1017 cm 3, one 0.5-mm-thick n-(Al0.7Ga0.3)In0.5P cladding layer with carrier concentration of about 3 1017 cm 3, one 10 periods of 10.0-nmthick (Al0.3Ga0.7)In0.5P/5.0-nm-thick Ga0.5In0.5P MQWs active region, one 0.5-mm-thick p-(Al0.7Ga0.3)In0.5P cladding layer with carrier concentration of about 6 1017 cm 3, and one 4.8-mm-thick p-GaP current spreading layer with carrier concentration of about 5 1018 cm 3. After the growth process was finished, PH3 and H2 were continuously injected into the reactor until the temperature is below 200 1C in order to depress the GaP layer decomposition. To ensure the same initial conditions, we chose one finely prepared wafer and cut it into several pieces. Each one of those pieces was annealed at different temperature ranging from 380 to 780 1C for 15 min in nitrogen. The electrical properties were measured by Bio-Rad PN4300 electrochemical capacitance–voltage (ECV) equipment. The PL properties were characterized by Philips PLM-100 YAG laser (532 nm) with 2 W/cm2 excitation density.
E
780°C
E
700°C
D
620°C
C
540°C
B
460°C
A
600
380°C
620 640 Wavelength (nm)
660
Fig. 3. Photoluminescence spectra of AlGaInP LED wafers after different annealing temperatures.
thickness of the p-(Al0.7Ga0.3)In0.5P layer and the p-GaP layer is about 0.5 and 4.8 mm, respectively. Fig. 5 shows the hole carrier concentrations of GaP layer and AlGaInP layer under different annealing temperature. For the case of AlGaInP layer, the hole carrier concentration of the unannealed sample was 6.0 1017 cm 3. The hole carrier concentration of sample A annealed at 380 1C did not change much. However, it increased to 1.1 1018 cm 3 after the sample annealed at 460 1C, and then the hole carrier concentration keeps almost unchanged as the temperature increased to 700 1C. After the temperature reached 780 1C, it suddenly decreased to 1.7 1017 cm 3. Similar trend also was observed for the case of GaP layer. The hole carrier concentration of GaP layers increased from 5.5 1018 to 6.5 1018 cm 3 after annealed at 460 1C
ARTICLE IN PRESS S. Li et al. / Microelectronics Journal 39 (2008) 70–73
72
1E19 unannealed
Carrier concentration (cm−3)
1E18
1E19 B 460°C
1E18
1E19 F
1E18 780°C
0.0
1.0
2.0 3.0 4.0 Depth (µm)
5.0
6.0
Fig. 4. The distribution of carrier concentrations of AlGaInP LED wafers under different annealing temperatures measured by ECV.
Carrier concentration (cm−3)
1E19 A
B
C
D
E
C
D
E
unannealed GaP Layer AlGaInP layer B
1E18 A F unannealed F
300
400 500 600 700 Annealing temperature (°C)
carrier concentrations of sample B and the sample unannealed rapidly decreased and electron carriers were measured at the depth about 5.4 mm. It means that the measurement position reach the interface of p-AlGaInP layer and undoped MQWs. However, the hole carrier concentration of sample F decreased slowly around 5.4 mm. The hole carriers can be measured at the depth about 5.9 mm. It meant that the Mg diffusion happened when the thermal annealing temperature reached to 780 1C. Compared with the PL intensity of samples annealed lower than 780 1C, the PL intensity of sample F double increased. Jalonen et al. [9] reported that the PL peak intensity of GaInP/AlGaInP single QW increased when it was annealed in the temperature range from 800 to 900 1C for 1 s. They believed that the improvement could be associated with the possibility that annealing removed nonradiative recombination centers from the QWs. We consider that Mg diffusion in the MQWs region in high annealing temperature should also be taken into account. 4. Conclusions In summary, the influence of thermal annealing to the characteristics of AlGaInP/GaInP multiple quantum wells light-emitting diode wafers was studied. The hole carrier concentrations of p-GaP layer and p-AlGaInP layer obviously increased after the samples was annealed under 460 1C for 15 min in nitrogen. The hole carrier concentrations of p-GaP layers and p-AlGaInP layers did not obviously change when the annealing temperature ranges from 460 to 700 1C. However, after the sample was annealed under 780 1C for 15 min, the hole carrier concentrations of GaP layer and AlGaInP layer largely decreased. At the same time, the Mg diffusion was accelerated and the undoped AlGaInP/GaInP MQWs changed to p-type conductance, which probably contributed to the increase of PL intensity of AlGaInP/GaInP MQWs.
800
Fig. 5. The hole carrier concentrations of GaP layer and AlGaInP layer in AlGaInP LED wafer under different annealing temperatures.
and keeps that value until 700 1C, then suddenly decreased to 8.0 1017 cm 3 after annealed at 780 1C. The ECV results indicate that the hole carrier concentrations of p-GaP layer and p-AlGaInP layer are influenced by the thermal annealing temperature. The carrier concentrations of p-GaP layer and p-AlGaInP layer could be largely increased by annealing when the thermal annealing temperature is appropriate. However, the electrical properties of p-GaP layer and p-AlGaInP layer would be deteriorated if the annealing temperature is too high. It also can be seen from Fig. 4 that the distribution of carrier concentrations of sample F is largely different from that of samples B and the unannealed sample. The hole
Acknowledgments This work was supported by the National Nature Science Foundation of China (Grant no. 50602018), the Nature Science Foundation of Guangdong province (Grant no. 06025083), and the Science and Technology Program of Guangdong Province (Grant no. 2006A10802001). References [1] K.H. Huang, J.G. Yu, C.P. Kuo, R.M. Fletcher, T.D. Osentowski, L.J. Stinson, M.G. Craford, A.S.H. Liao, Appl. Phys. Lett. 61 (1992) 1045. [2] 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. [3] H. Sugawara, M. Ishikawa, Jpn. J. Appl. Phys. 34 (Part 2) (1995) 1458. [4] C.-Y. Lee, M.-C. Wu, W. Lin, J. Cryst. Growth 200 (1999) 382.
ARTICLE IN PRESS S. Li et al. / Microelectronics Journal 39 (2008) 70–73 [5] Y. Hosokawa, W. Nabekura, T. Hoshina, R. Takeuchi, K. Sakaue, T. Udagawa, J. Cryst. Growth 221 (2000) 652. [6] W.C. Peng, Y.C.S. Wu, Appl. Phys. Lett. 84 (2004) 1841. [7] J. Dekker, A. Tukiainen, N. Xiang, S. Orsila, M. Saarinen, M. Toivonen, M. Pessa, N. Tkachenko, H. Lemmetyinen, J. Appl. Phys. 86 (1999) 3709. [8] M. Guina, J. Dekker, A. Tukiainen, S. Orsila, M. Saarinen, M. Dumitrescu, P. Sipila, P. Savolainen, M. Pessa, J. Appl. Phys. 89 (2001) 1151.
73
[9] M. Jalonen, M. Toivonen, P. Savolainen, J. Ko¨nga¨s, M. Pessa, Appl. Phys. Lett. 71 (1997) 479. [10] A. Tukiainen, J. Dekker, T. Leinonen, M. Pessa, Mater. Sci. Eng. B 91–92 (2002) 389. [11] S. Cassette, S.L. Delage, E. Chartier, D. Floriot, M.A. Poisson, J.C. Garcia, C. Grattepain, J. Mimila Arroyo, R. Plana, S.W. Bland, Mater. Sci. Eng. B 80 (2001) 279.
Microelectronics Journal 39 (2008) 70–73 www.elsevier.com/locate/mejo
The influence of thermal annealing to the characteristics of AlGaInP/GaInP multiple quantum wells light-emitting diode wafers Shuti Li, Guanghan Fan, Huiqing Sun, Shuwen Zheng Institute of Opto-electronic Materials and Technology, South China Normal University, Guangzhou 510631, PR China Received 8 August 2007; accepted 27 October 2007
Abstract The influence of thermal annealing to the characteristics of AlGaInP/GaInP multiple quantum wells (MQWs) light-emitting diode wafers was studied by means of electrochemical capacitance–voltage (ECV) and photoluminescence (PL). Compared with the sample unannealed, the hole carrier concentration of p-GaP layer increased from 5.5 1018 to 6.5 1018 cm 3, and the hole carrier concentration of p-AlGaInP layer increased from 6.0 1017 to 1.1 1018 cm 3, after wafer was annealed at 460 1C for 15 min in nitrogen. The hole carrier concentrations of p-GaP layers and p-AlGaInP layers did not obviously change when the annealing temperature varied from 460 to 700 1C. However, after the sample was annealed under 780 1C for 15 min, the hole carrier concentration of p-GaP layer and p-AlGaInP layer decreased to 8 1017 and 1.7 1017 cm 3, respectively. At the same time, the diffusion of Mg atoms was observed. r 2007 Elsevier Ltd. All rights reserved. PACS: 81.15.Gh; 78.55.Cr; 61.72.Vv Keywords: Semiconductors; Thin films; Epitaxial growth; Electrochemical measurements; Electrical properties
1. Introduction Significant advancements in the light output performance of visible light-emitting diodes (LEDs) and lightemitting lasers (LDs) have been demonstrated using the AlGaInP alloy system [1–6]. Some investigations indicated that the properties of AlGaInP alloy system could be influenced by rapid thermal annealing [7–11]. After rapid thermal annealing at 875 1C for as short as only 1 s, obvious reduction in the concentration of several deep-level traps of bulk GaInP and GaInP/AlGaInP multiple quantum wells (MQWs) have been reported [7–8]. The intensity of the photoluminescence (PL) peak for GaInP/ AlGaInP single QW also can be increased after only 1 s annealing in the temperature range from 800 to 900 1C. This improvement could be associated with the possibility that annealing treatment removes nonradiative recombination centers from the QWs [10]. In the present contribution, Corresponding author. Tel.: +86 20 85212667x601; fax: +86 20 85210809. E-mail address: [email protected] (S. Li).
0026-2692/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.mejo.2007.10.013
we report the influence of thermal annealing to the characteristics of AlGaInP/GaInP MQWs LED wafers. The results shown that the increase in the hole carrier concentrations of p-GaP layer and p-AlGaInP layer can be obtained with much lower thermal annealing temperature than the reported [7–11]. The Mg diffusion behavior in MQWs was observed by thermal annealing at 780 1C. 2. Experimental details All samples in the present study were grown by EMCORE GS/3200 low-pressure metal–organic chemical vapor deposition (LP-MOCVD). Si-doped GaAs substrates were cut 151 off the (1 0 0) plane toward the [0 1 1] direction. The source materials were TMGa, TMIn, TMAl, AsH3, PH3, respectively. CP2Mg and SiH4 were used for p- and n-type doping reagents. The growth temperature was generally between 620 and 720 1C. Fig. 1 shows the schematic diagram of the AlGaInP/ GaInP MQWs LED wafers in thermal annealing experiments. The layout of the wafers include the following layers: one 0.5-mm-thick n-GaAs buffer layer with carrier
ARTICLE IN PRESS S. Li et al. / Microelectronics Journal 39 (2008) 70–73
71
p-GaP
MQWs n-(Al0.7Ga0.3)In0.5P n-GaAs buffer
F
Intensity (arb. units)
p-(Al0.7Ga0.3)In0.5P
n-GaAs
3. Results and discussion Figs. 2 and 3 show the room temperature PL spectra of six samples before and after annealing. The annealing temperatures of samples are listed in Fig. 3. The peak wavelength of these samples varied from 624 to 626 nm. It shows clearly that the peak wavelength and the PL FWHM values were not affected much by the annealing process. When the annealing temperature was lower than 700 1C, there were no obvious changes for the PL intensity. However, the PL intensity double increased when the annealing temperature reached 780 1C. This indicates that the PL intensity of GaInP/AlGaInP MQWs can be enhanced only if the annealing temperature is high enough. Fig. 4 shows the distribution of carrier concentrations of three samples. The annealing temperatures are listed. Compared with the sample unannealed, the carrier concentration distribution of samples B and F changed after annealing. We choose the values in depth of 1.0 and 5.2 mm as the representative carrier concentrations of p-GaP layer and p-(Al0.7Ga0.3)In0.5P layer, since the
D C B
Fig. 1. The schematic structure of AlGaInP LED wafers.
A
600
620 640 Wavelength (nm)
660
Fig. 2. Photoluminescence spectra of AlGaInP LED wafers before annealing.
F×1/2 F
Intensity (arb. units)
concentration of about 5 1017 cm 3, one 0.5-mm-thick n-(Al0.7Ga0.3)In0.5P cladding layer with carrier concentration of about 3 1017 cm 3, one 10 periods of 10.0-nmthick (Al0.3Ga0.7)In0.5P/5.0-nm-thick Ga0.5In0.5P MQWs active region, one 0.5-mm-thick p-(Al0.7Ga0.3)In0.5P cladding layer with carrier concentration of about 6 1017 cm 3, and one 4.8-mm-thick p-GaP current spreading layer with carrier concentration of about 5 1018 cm 3. After the growth process was finished, PH3 and H2 were continuously injected into the reactor until the temperature is below 200 1C in order to depress the GaP layer decomposition. To ensure the same initial conditions, we chose one finely prepared wafer and cut it into several pieces. Each one of those pieces was annealed at different temperature ranging from 380 to 780 1C for 15 min in nitrogen. The electrical properties were measured by Bio-Rad PN4300 electrochemical capacitance–voltage (ECV) equipment. The PL properties were characterized by Philips PLM-100 YAG laser (532 nm) with 2 W/cm2 excitation density.
E
780°C
E
700°C
D
620°C
C
540°C
B
460°C
A
600
380°C
620 640 Wavelength (nm)
660
Fig. 3. Photoluminescence spectra of AlGaInP LED wafers after different annealing temperatures.
thickness of the p-(Al0.7Ga0.3)In0.5P layer and the p-GaP layer is about 0.5 and 4.8 mm, respectively. Fig. 5 shows the hole carrier concentrations of GaP layer and AlGaInP layer under different annealing temperature. For the case of AlGaInP layer, the hole carrier concentration of the unannealed sample was 6.0 1017 cm 3. The hole carrier concentration of sample A annealed at 380 1C did not change much. However, it increased to 1.1 1018 cm 3 after the sample annealed at 460 1C, and then the hole carrier concentration keeps almost unchanged as the temperature increased to 700 1C. After the temperature reached 780 1C, it suddenly decreased to 1.7 1017 cm 3. Similar trend also was observed for the case of GaP layer. The hole carrier concentration of GaP layers increased from 5.5 1018 to 6.5 1018 cm 3 after annealed at 460 1C
ARTICLE IN PRESS S. Li et al. / Microelectronics Journal 39 (2008) 70–73
72
1E19 unannealed
Carrier concentration (cm−3)
1E18
1E19 B 460°C
1E18
1E19 F
1E18 780°C
0.0
1.0
2.0 3.0 4.0 Depth (µm)
5.0
6.0
Fig. 4. The distribution of carrier concentrations of AlGaInP LED wafers under different annealing temperatures measured by ECV.
Carrier concentration (cm−3)
1E19 A
B
C
D
E
C
D
E
unannealed GaP Layer AlGaInP layer B
1E18 A F unannealed F
300
400 500 600 700 Annealing temperature (°C)
carrier concentrations of sample B and the sample unannealed rapidly decreased and electron carriers were measured at the depth about 5.4 mm. It means that the measurement position reach the interface of p-AlGaInP layer and undoped MQWs. However, the hole carrier concentration of sample F decreased slowly around 5.4 mm. The hole carriers can be measured at the depth about 5.9 mm. It meant that the Mg diffusion happened when the thermal annealing temperature reached to 780 1C. Compared with the PL intensity of samples annealed lower than 780 1C, the PL intensity of sample F double increased. Jalonen et al. [9] reported that the PL peak intensity of GaInP/AlGaInP single QW increased when it was annealed in the temperature range from 800 to 900 1C for 1 s. They believed that the improvement could be associated with the possibility that annealing removed nonradiative recombination centers from the QWs. We consider that Mg diffusion in the MQWs region in high annealing temperature should also be taken into account. 4. Conclusions In summary, the influence of thermal annealing to the characteristics of AlGaInP/GaInP multiple quantum wells light-emitting diode wafers was studied. The hole carrier concentrations of p-GaP layer and p-AlGaInP layer obviously increased after the samples was annealed under 460 1C for 15 min in nitrogen. The hole carrier concentrations of p-GaP layers and p-AlGaInP layers did not obviously change when the annealing temperature ranges from 460 to 700 1C. However, after the sample was annealed under 780 1C for 15 min, the hole carrier concentrations of GaP layer and AlGaInP layer largely decreased. At the same time, the Mg diffusion was accelerated and the undoped AlGaInP/GaInP MQWs changed to p-type conductance, which probably contributed to the increase of PL intensity of AlGaInP/GaInP MQWs.
800
Fig. 5. The hole carrier concentrations of GaP layer and AlGaInP layer in AlGaInP LED wafer under different annealing temperatures.
and keeps that value until 700 1C, then suddenly decreased to 8.0 1017 cm 3 after annealed at 780 1C. The ECV results indicate that the hole carrier concentrations of p-GaP layer and p-AlGaInP layer are influenced by the thermal annealing temperature. The carrier concentrations of p-GaP layer and p-AlGaInP layer could be largely increased by annealing when the thermal annealing temperature is appropriate. However, the electrical properties of p-GaP layer and p-AlGaInP layer would be deteriorated if the annealing temperature is too high. It also can be seen from Fig. 4 that the distribution of carrier concentrations of sample F is largely different from that of samples B and the unannealed sample. The hole
Acknowledgments This work was supported by the National Nature Science Foundation of China (Grant no. 50602018), the Nature Science Foundation of Guangdong province (Grant no. 06025083), and the Science and Technology Program of Guangdong Province (Grant no. 2006A10802001). References [1] K.H. Huang, J.G. Yu, C.P. Kuo, R.M. Fletcher, T.D. Osentowski, L.J. Stinson, M.G. Craford, A.S.H. Liao, Appl. Phys. Lett. 61 (1992) 1045. [2] 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. [3] H. Sugawara, M. Ishikawa, Jpn. J. Appl. Phys. 34 (Part 2) (1995) 1458. [4] C.-Y. Lee, M.-C. Wu, W. Lin, J. Cryst. Growth 200 (1999) 382.
ARTICLE IN PRESS S. Li et al. / Microelectronics Journal 39 (2008) 70–73 [5] Y. Hosokawa, W. Nabekura, T. Hoshina, R. Takeuchi, K. Sakaue, T. Udagawa, J. Cryst. Growth 221 (2000) 652. [6] W.C. Peng, Y.C.S. Wu, Appl. Phys. Lett. 84 (2004) 1841. [7] J. Dekker, A. Tukiainen, N. Xiang, S. Orsila, M. Saarinen, M. Toivonen, M. Pessa, N. Tkachenko, H. Lemmetyinen, J. Appl. Phys. 86 (1999) 3709. [8] M. Guina, J. Dekker, A. Tukiainen, S. Orsila, M. Saarinen, M. Dumitrescu, P. Sipila, P. Savolainen, M. Pessa, J. Appl. Phys. 89 (2001) 1151.
73
[9] M. Jalonen, M. Toivonen, P. Savolainen, J. Ko¨nga¨s, M. Pessa, Appl. Phys. Lett. 71 (1997) 479. [10] A. Tukiainen, J. Dekker, T. Leinonen, M. Pessa, Mater. Sci. Eng. B 91–92 (2002) 389. [11] S. Cassette, S.L. Delage, E. Chartier, D. Floriot, M.A. Poisson, J.C. Garcia, C. Grattepain, J. Mimila Arroyo, R. Plana, S.W. Bland, Mater. Sci. Eng. B 80 (2001) 279.