- Email: [email protected]
Photoluminescence of AlGaAs alloy grown by LP-MOVPE at different temperatures using TBA in N2 ambient
Materials Science in Semiconductor Processing 4 (2001) 651–654
Photoluminescence of AlGaAs alloy grown by LP-MOVPE at different temperatures using TBA in N2 ambient X.H. Tang*, J.Y. Zhu, Y.C. Chan Photonics Research Group, School of Electrical and Electronic Engineering Block S2, Nanyang Technological University, Nanyang Avenue, Singapore 639798, Singapore
Abstract High quality of AlxGa1 xAs alloys have been grown by the LP-MOVPE using tertiary-butyl arsine as group V precursor in 100% nitrogen ambient. The photoluminescence (PL) properties of Al0.25Ga0.75As alloy grown at different temperature have been studied. The PL peak emission intensity of the samples increases with the substrate growth temperature initially and saturated at the growth temperature 7601C. The emission intensity decreases when further increase the temperature. This is attributed to the oxygen content in the samples. r 2002 Published by Elsevier Science Ltd. Keywords: Photoluminescence; LP-MOCVD; AlGaAs; TBA
1. Introduction Metalorganic chemical vapor-phase epitaxy (MOVPE) for the growth of III–V semiconductors has been a rapidly developing technology. Now, MOVPE is recognized as a key technology for manufacturing optoelectronic devices due to its high-quality growth, high throughput, short downtimes, and scalability to large reactors. Nevertheless, there is the safety concern of the process. Conventional MOVPE is performed in hydrogen ambient, which is an explosive and a safety hazard, and highly toxic hydrides: AsH3 and PH3, as group V-precursors. To solve this problem, safer alternative group V metalorganics: tertiary-butyl arsine (TBA) and tertiary-butyl phosphine (TBP), have been used in MOVPE growths for fabrication of semiconductor devices [1,2]. The inert gas nitrogen has also been demonstrated an excellent replacement of the explosive hydrogen as the carrier gas in MOVPE for the growth of GaAs and InP-based materials [3,4]. The combination of inert N2 carrier with TBA and TBP has been used by Keiper and others in MOCVD to grow InP and InGaAs [5]. But in their growths H2 was still used for the MO*Corresponding author. Fax: +65-792-0515. E-mail address: [email protected] (X.H. Tang).
sources lines. The actual nitrogen content in their MOVPE reactor was about 90%. In this paper, we report the LP-MOVPE growths of high-quality AlGaAs alloys using TBA as group V precursor and in 100% N2 ambient. Their photoluminescence (PL) performance with different substrate growth temperature has been evaluated.
2. Experimental procedure All the samples were grown in a low-pressure horizontal MOCVD reactor (Aixtron, AIX 200). To receive the uniform deposition gas foil rotation of the susceptor was used in the system and the rotation rate was set at about 100 rpm. The precursors used in the growths were TBA, TMGa and TMAl. The nitrogen carrier gas was purified by a SAES Monotorr phase II Getter column and the dew point was kept below 1101C. The pressure for all the growths were set at 20 mbar and V/III ratio was kept at 17.5. The group III TMGa and TMAl source flow rate was kept at 4.42 10 5 and 1.11 10 5 mol/min, respectively. IR heat lamps were used for heating up the reactor. GaAs epiready (1 0 0) semi-insulation substrates were used for all the growths.
1369-8001/01/$ - see front matter r 2002 Published by Elsevier Science Ltd. PII: S 1 3 6 9 - 8 0 0 1 ( 0 2 ) 0 0 0 3 6 - 7
652
X.H. Tang et al. / Materials Science in Semiconductor Processing 4 (2001) 651–654
To study the temperature dependence of the growth, a 1.5 mm thick Al0.25Ga0.75As layer was grown at different temperatures while all the other growth conditions kept unchanged for all the growths. Before starting the AlGaAs layer growth, a 250 nm thick GaAs buffer layer was first grown with the temperature changed linearly from 6501C to 7501C and then a thin AlAs etching stop layer of 50 nm was grown at 7501C. After that, the growth temperature was set and the AlGaAs layer growth commenced. On top of the AlGaAs layer, a 100 nm GaAs cap layer was grown. The epilayer thickness was measured using an optical microscope with the magnification of 500 . The growth rate of the AlGaAs epilayers under our growth conditions increased from 1.1 to 1.53 mm/h when the growth temperature increase from 6501C to 7801C.
350000
The crystal quality and the Al composition of the epitaxial layers have been checked using the highresolution double crystal X-ray diffraction. Fig. 1 shows the X-ray diffraction results of the samples grown at different temperature. The Al composition of all the epitaxial layers was 0.25%. When the substrate temperature changed, the Al composition of the epitaxial layer does not change. The X-ray diffraction spectrum FWHM from the epitaxial layer is similar to that of the GaAs substrate. This shows that the high crystal quality of the epitaxial layers have been received when the growth substrate temperature change from 6501C to 7801C. All the samples have defect-free surface morphology.
˚ ˚ ˚
Sample #72 (700 C) #76 (750 C) #74 (650 C)
300000 Diff. intensity (a.u)
3. Results and discussion
Subs
250000
Al 0.27Ga 0.73As
200000 150000 100000 50000 0
32.85 32.90 32.95 33.00 33.05 33.10 33.15 33.20 33.25 33.30 Diff. ang le Fig. 1. X-ray diffraction results of the AlxGa1 xAs/GaAs epitaxy samples grown by LP-MOVPE using TBA in N2 ambient.
3 dB Optical fiber coupler
SPEX
Objective lens Ar+ laser
Monochromator Chopper
P M T
sample
Power supplier Chopper controller
Ref.
Lock-in Amplifier
High voltage Computer
Fig. 2. Set up diagram of the PL measurement.
X.H. Tang et al. / Materials Science in Semiconductor Processing 4 (2001) 651–654
Fig. 3 shows the PL spectrum of the Al0.25Ga0.75As samples grown at different temperatures. The peak emission wavelength does not change when the growth substrate temperature changed. However, the PL emission peak intensity increase when the growth temperature increased. The PL emission intensity of the sample increases slowly when the growth temperature increased from 6501C to 7001C, and then it increases very fast when the temperature increased from 7001C to 7501C. The emission intensity saturated at the growth temperature of about 7651C and start to decrease when the temperature increase to 7801C. Fig. 4 shows the peak emission intensity of the samples grown at different temperatures. This has been attributed to the oxygen that cause the low PL emission of the Al0.25Ga0.75As [6]
The PL set up used in our measurements is shown in Fig. 2. An Ar+ laser with the wavelength of 514 nm was used as the exciting light and chopped with the frequency of 240 Hz. To reduce the system error during the measurement, the laser beam was coupled into an optical fiber through a 10 objective lens. The sample was mounted at the other end of the fiber. The pump laser light through the fiber shined on the sample directly. The output laser power from the fiber was kept at 10 mW. The PL signal emitted from the sample was coupled back into the same fiber and transmitted to the monochromater through a 3 dB optical fiber coupler. For the low-temperature (77 K) PL measurement, the whole sample with the fiber was immersed into a liquid nitrogen Dewar. 25000
˚ ˚ ˚ ˚
Sample #74 (650 C) #72 (700 C) #77 (725 C) #76 (750 C) #101 (765 C) #100 (780 C)
20000 PL intens ity (a.u)
653
15000
˚ ˚
10000
5000
0 620
640
660
680
700
720
740
760
Wavelength (nm) Fig. 3. PL emission spectrum of the AlxGa1 xAs/GaAs samples grown at different temperature.
4.0 2.0x104
1.5x104
3.0 2.5
1.0x104
2.0 5.0x103
PL Peak Intensity (a.u)
Oxygen Content (a.u)
3.5
1.5 0.0
1.0 640
660
680
700
720
740
760
780
800
˚
Growth Temperatute ( C) Fig. 4. PL peak intensity and the EDX results of oxygen content of the AlxGa1 xAs/GaAs samples with different growth temperature.
654
X.H. Tang et al. / Materials Science in Semiconductor Processing 4 (2001) 651–654
samples grown at low temperature. Increasing the temperature will increase the Al suboxide adsorption to the surface. This improves the PL emission of the Al0.25Ga0.75As samples. The relative oxygen content of the samples grown at different temperature was measured by using a JEOL JSM-5600LU SEM plus the OXFORD ISIS EDX system. As shown in Fig. 4, the oxygen content in the sample decreases when the growth temperature increase from 6501C to about 7501C. The oxygen in the sample grown at 6501C is about 3 times of that in the sample grown at 7501C. When the growth temperature reached to 7801C, the oxygen in the AlGaAs epilayer start to increase again. This result agrees with the PL performance of the samples.
decrease when the growth temperature increased further. This has been attributed to the oxygen content variation in the samples when the growth temperature was changed.
Acknowledgements The authors would like to thank Mr. Yuan Gaoqiang for his help in the X-RD measurement of all the samples. This project was supported by the National Science and Technology Board (NSTB), Singapore.
References 4. Conclusions High-quality defects-free AlGaAs alloys have been grown via LP-MOVEP using TBA in 100% nitrogen ambient with the substrate temperature varied from 6501C to 7801C. It has been observed that the PL emission intensity of the AlGaAs epitaxial layers increased when the growth temperature was raised. The PL intensity reached its maximum when the temperature increased to 7651C in our experiment. After that the PL intensity of the AlGaAs material starts to
[1] Chan PT, Shu HS, Hsu CC. Appl Phys Lett 1995;67:1715–7. [2] Yang YF, Hsu CC, Yang ES. IEEE Electron Dev Lett 1996;17:363–5. [3] Hardtdegen H, Hollfelfer M, Ungermanns Chr, Wirtz K, Carius R, Guggi D, Luth H. Inst Phys Conf Ser 1993;136:625. [4] Roehle H, Schroeter H. Eighth International Conference on Indium Phosphide and Related Materials, 1996. p. 478–81. [5] Keiper D, Westphalen R, Landgren G. J Cryst Growth 1999;204:256–62. [6] Stringfellow GB. Academic Press, Inc., 1989. p. 298.
Photoluminescence of AlGaAs alloy grown by LP-MOVPE at different temperatures using TBA in N2 ambient X.H. Tang*, J.Y. Zhu, Y.C. Chan Photonics Research Group, School of Electrical and Electronic Engineering Block S2, Nanyang Technological University, Nanyang Avenue, Singapore 639798, Singapore
Abstract High quality of AlxGa1 xAs alloys have been grown by the LP-MOVPE using tertiary-butyl arsine as group V precursor in 100% nitrogen ambient. The photoluminescence (PL) properties of Al0.25Ga0.75As alloy grown at different temperature have been studied. The PL peak emission intensity of the samples increases with the substrate growth temperature initially and saturated at the growth temperature 7601C. The emission intensity decreases when further increase the temperature. This is attributed to the oxygen content in the samples. r 2002 Published by Elsevier Science Ltd. Keywords: Photoluminescence; LP-MOCVD; AlGaAs; TBA
1. Introduction Metalorganic chemical vapor-phase epitaxy (MOVPE) for the growth of III–V semiconductors has been a rapidly developing technology. Now, MOVPE is recognized as a key technology for manufacturing optoelectronic devices due to its high-quality growth, high throughput, short downtimes, and scalability to large reactors. Nevertheless, there is the safety concern of the process. Conventional MOVPE is performed in hydrogen ambient, which is an explosive and a safety hazard, and highly toxic hydrides: AsH3 and PH3, as group V-precursors. To solve this problem, safer alternative group V metalorganics: tertiary-butyl arsine (TBA) and tertiary-butyl phosphine (TBP), have been used in MOVPE growths for fabrication of semiconductor devices [1,2]. The inert gas nitrogen has also been demonstrated an excellent replacement of the explosive hydrogen as the carrier gas in MOVPE for the growth of GaAs and InP-based materials [3,4]. The combination of inert N2 carrier with TBA and TBP has been used by Keiper and others in MOCVD to grow InP and InGaAs [5]. But in their growths H2 was still used for the MO*Corresponding author. Fax: +65-792-0515. E-mail address: [email protected] (X.H. Tang).
sources lines. The actual nitrogen content in their MOVPE reactor was about 90%. In this paper, we report the LP-MOVPE growths of high-quality AlGaAs alloys using TBA as group V precursor and in 100% N2 ambient. Their photoluminescence (PL) performance with different substrate growth temperature has been evaluated.
2. Experimental procedure All the samples were grown in a low-pressure horizontal MOCVD reactor (Aixtron, AIX 200). To receive the uniform deposition gas foil rotation of the susceptor was used in the system and the rotation rate was set at about 100 rpm. The precursors used in the growths were TBA, TMGa and TMAl. The nitrogen carrier gas was purified by a SAES Monotorr phase II Getter column and the dew point was kept below 1101C. The pressure for all the growths were set at 20 mbar and V/III ratio was kept at 17.5. The group III TMGa and TMAl source flow rate was kept at 4.42 10 5 and 1.11 10 5 mol/min, respectively. IR heat lamps were used for heating up the reactor. GaAs epiready (1 0 0) semi-insulation substrates were used for all the growths.
1369-8001/01/$ - see front matter r 2002 Published by Elsevier Science Ltd. PII: S 1 3 6 9 - 8 0 0 1 ( 0 2 ) 0 0 0 3 6 - 7
652
X.H. Tang et al. / Materials Science in Semiconductor Processing 4 (2001) 651–654
To study the temperature dependence of the growth, a 1.5 mm thick Al0.25Ga0.75As layer was grown at different temperatures while all the other growth conditions kept unchanged for all the growths. Before starting the AlGaAs layer growth, a 250 nm thick GaAs buffer layer was first grown with the temperature changed linearly from 6501C to 7501C and then a thin AlAs etching stop layer of 50 nm was grown at 7501C. After that, the growth temperature was set and the AlGaAs layer growth commenced. On top of the AlGaAs layer, a 100 nm GaAs cap layer was grown. The epilayer thickness was measured using an optical microscope with the magnification of 500 . The growth rate of the AlGaAs epilayers under our growth conditions increased from 1.1 to 1.53 mm/h when the growth temperature increase from 6501C to 7801C.
350000
The crystal quality and the Al composition of the epitaxial layers have been checked using the highresolution double crystal X-ray diffraction. Fig. 1 shows the X-ray diffraction results of the samples grown at different temperature. The Al composition of all the epitaxial layers was 0.25%. When the substrate temperature changed, the Al composition of the epitaxial layer does not change. The X-ray diffraction spectrum FWHM from the epitaxial layer is similar to that of the GaAs substrate. This shows that the high crystal quality of the epitaxial layers have been received when the growth substrate temperature change from 6501C to 7801C. All the samples have defect-free surface morphology.
˚ ˚ ˚
Sample #72 (700 C) #76 (750 C) #74 (650 C)
300000 Diff. intensity (a.u)
3. Results and discussion
Subs
250000
Al 0.27Ga 0.73As
200000 150000 100000 50000 0
32.85 32.90 32.95 33.00 33.05 33.10 33.15 33.20 33.25 33.30 Diff. ang le Fig. 1. X-ray diffraction results of the AlxGa1 xAs/GaAs epitaxy samples grown by LP-MOVPE using TBA in N2 ambient.
3 dB Optical fiber coupler
SPEX
Objective lens Ar+ laser
Monochromator Chopper
P M T
sample
Power supplier Chopper controller
Ref.
Lock-in Amplifier
High voltage Computer
Fig. 2. Set up diagram of the PL measurement.
X.H. Tang et al. / Materials Science in Semiconductor Processing 4 (2001) 651–654
Fig. 3 shows the PL spectrum of the Al0.25Ga0.75As samples grown at different temperatures. The peak emission wavelength does not change when the growth substrate temperature changed. However, the PL emission peak intensity increase when the growth temperature increased. The PL emission intensity of the sample increases slowly when the growth temperature increased from 6501C to 7001C, and then it increases very fast when the temperature increased from 7001C to 7501C. The emission intensity saturated at the growth temperature of about 7651C and start to decrease when the temperature increase to 7801C. Fig. 4 shows the peak emission intensity of the samples grown at different temperatures. This has been attributed to the oxygen that cause the low PL emission of the Al0.25Ga0.75As [6]
The PL set up used in our measurements is shown in Fig. 2. An Ar+ laser with the wavelength of 514 nm was used as the exciting light and chopped with the frequency of 240 Hz. To reduce the system error during the measurement, the laser beam was coupled into an optical fiber through a 10 objective lens. The sample was mounted at the other end of the fiber. The pump laser light through the fiber shined on the sample directly. The output laser power from the fiber was kept at 10 mW. The PL signal emitted from the sample was coupled back into the same fiber and transmitted to the monochromater through a 3 dB optical fiber coupler. For the low-temperature (77 K) PL measurement, the whole sample with the fiber was immersed into a liquid nitrogen Dewar. 25000
˚ ˚ ˚ ˚
Sample #74 (650 C) #72 (700 C) #77 (725 C) #76 (750 C) #101 (765 C) #100 (780 C)
20000 PL intens ity (a.u)
653
15000
˚ ˚
10000
5000
0 620
640
660
680
700
720
740
760
Wavelength (nm) Fig. 3. PL emission spectrum of the AlxGa1 xAs/GaAs samples grown at different temperature.
4.0 2.0x104
1.5x104
3.0 2.5
1.0x104
2.0 5.0x103
PL Peak Intensity (a.u)
Oxygen Content (a.u)
3.5
1.5 0.0
1.0 640
660
680
700
720
740
760
780
800
˚
Growth Temperatute ( C) Fig. 4. PL peak intensity and the EDX results of oxygen content of the AlxGa1 xAs/GaAs samples with different growth temperature.
654
X.H. Tang et al. / Materials Science in Semiconductor Processing 4 (2001) 651–654
samples grown at low temperature. Increasing the temperature will increase the Al suboxide adsorption to the surface. This improves the PL emission of the Al0.25Ga0.75As samples. The relative oxygen content of the samples grown at different temperature was measured by using a JEOL JSM-5600LU SEM plus the OXFORD ISIS EDX system. As shown in Fig. 4, the oxygen content in the sample decreases when the growth temperature increase from 6501C to about 7501C. The oxygen in the sample grown at 6501C is about 3 times of that in the sample grown at 7501C. When the growth temperature reached to 7801C, the oxygen in the AlGaAs epilayer start to increase again. This result agrees with the PL performance of the samples.
decrease when the growth temperature increased further. This has been attributed to the oxygen content variation in the samples when the growth temperature was changed.
Acknowledgements The authors would like to thank Mr. Yuan Gaoqiang for his help in the X-RD measurement of all the samples. This project was supported by the National Science and Technology Board (NSTB), Singapore.
References 4. Conclusions High-quality defects-free AlGaAs alloys have been grown via LP-MOVEP using TBA in 100% nitrogen ambient with the substrate temperature varied from 6501C to 7801C. It has been observed that the PL emission intensity of the AlGaAs epitaxial layers increased when the growth temperature was raised. The PL intensity reached its maximum when the temperature increased to 7651C in our experiment. After that the PL intensity of the AlGaAs material starts to
[1] Chan PT, Shu HS, Hsu CC. Appl Phys Lett 1995;67:1715–7. [2] Yang YF, Hsu CC, Yang ES. IEEE Electron Dev Lett 1996;17:363–5. [3] Hardtdegen H, Hollfelfer M, Ungermanns Chr, Wirtz K, Carius R, Guggi D, Luth H. Inst Phys Conf Ser 1993;136:625. [4] Roehle H, Schroeter H. Eighth International Conference on Indium Phosphide and Related Materials, 1996. p. 478–81. [5] Keiper D, Westphalen R, Landgren G. J Cryst Growth 1999;204:256–62. [6] Stringfellow GB. Academic Press, Inc., 1989. p. 298.