- Email: [email protected]
Room temperature operating InAsSb-based photovoltaic infrared sensors grown by metalorganic vapor phase epitaxy
Journal of Crystal Growth (xxxx) xxxx–xxxx
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Room temperature operating InAsSb-based photovoltaic infrared sensors grown by metalorganic vapor phase epitaxy ⁎
Ryosuke Hasegawa , Akira Yoshikawa, Tomohiro Morishita, Yoshitaka Moriyasu, Kazuhiro Nagase, Naohiro Kuze UVC project, Asahi Kasei Corporation, 2-1, Samejima, Fuji, Shizuoka 416-8501, Japan
A R T I C L E I N F O
A BS T RAC T
Communicated by: T.F. Kuech
We have developed InAsxSb1−x-based photovoltaic infrared sensors (PVS) for room temperature operation by metalorganic vapor phase epitaxy (MOVPE). To obtain high performance, we improved the crystallinity of the InAs0.12Sb0.88 absorber layer and utilized a Ga0.33In0.67Sb electron barrier layer. An investigation of InAs0.12Sb0.88 growth conditions using a high-quality InSb buffer layer showed that we were able to obtain the smallest full-width at half-maximum (FWHM) of the X-ray diffraction omega rocking curve, 560 arcsec, for a growth temperature of 520°C for a 1 µm thick layer. Moreover, we successfully grew a Ga0.33In0.67Sb barrier layer coherently on an InAs0.12Sb0.88 absorber layer, which is the first report of GayIn1−ySb growth on Sb-rich InAsxSb1−x. An InAsxSb1−x PVS with a responsivity at wavelengths of 8–12 µm was obtained, and estimated detectivity peak at room temperature was approximately 7×107 cm Hz1/2 W−1, which is 1.3 times higher than without a Ga0.33In0.67Sb electron barrier. These results demonstrate that our InAsxSb1−x PVS is a promising device for the 8–12 µm wavelength range at room temperature.
Keywords: A1. Characterization A3. Metalorganic vapor phase epitaxy B1. Antimonides B2. Semiconducting indium compounds B3. Infrared devices
1. Introduction Infrared sensors are promising devices for several applications such as human body detection and gas sensing. Pyroelectric detectors are generally used to detect human body at room temperature. However, these detectors require a metallic package to separate electromagnetic and thermal noise for high sensitivity, which makes it difficult for them to be miniaturized. Moreover, pyroelectric detectors cannot detect a stationary human body because they use the temperature change of pyroelectric material to generate an electrical output. Photon detectors are candidates to detect the stationary human body because they can detect the absolute quantity of infrared irradiation. High-performance miniaturized InSb photovoltaic infrared sensors (PVS) operating at room temperature have been reported [1]. However, to detect a stationary human body with high sensitivity, it is important that the spectral response of photon detectors be consistent with the infrared irradiation spectrum from the human body. The development of photon detectors having a spectral response in the range of 8–14 µm is thus required. InAsxSb1−x hetero-junction photodiodes have been proposed for this [2,3], but insufficient sensitivity was obtained due to poor InAsxSb1−x crystallinity and the Auger recombination process. The large lattice mismatch (7.2% < ∆a/a < 14.6%) between InAsxSb1−x and GaAs substrates makes it difficult to
⁎
obtain a high crystallinity InAsxSb1−x epilayer [4]. Poor crystallinity may decrease the lifetime of photo-excited carriers [5] and results in a low photocurrent (Ip). Moreover, high intrinsic carrier density in the InAsxSb1−x absorber layer causes a high diffusion current and results in decreased resistance at zero bias (R0). Because the signal-to-noise ratio (SNR) is proportional to Ip×R01/2 in photovoltaic mode, a decrease of Ip and R0 results in low SNR. In this work, we grew by metalorganic vapor phase epitaxy (MOVPE) an InAsxSb1−x PVS for room temperature operation. To improve the sensitivity, it is important to improve the InAsxSb1−x crystallinity, as the lifetime of a photo-excited carrier is likely to depend on crystallinity [5]. To reduce the lattice mismatch between InAsxSb1−x and GaAs substrates, we used InSb as a buffer layer. The InSb buffer layer was grown using a two-step growth method that enables us to obtain high crystallinity and electrical transport properties [6]. Moreover, the potential barrier for electrons, formed by inserting a wide band gap layer such as AlInSb, is considered to suppress the diffusion of photo-excited electrons, resulting in an improvement of Ip and R0 [7,8]. However, it is difficult to obtain high quality AlInSb by MOVPE due to C and O incorporation and pre-reactions between Al and Sb sources [9]. Therefore, we inserted a Ga0.33In0.67Sb electron barrier layer as an alternative material in the InSb/InAs0.12Sb0.88/InSb hetero-junction. We successfully grew Ga0.33In0.67Sb coherently on an
Corresponding author. E-mail address: [email protected] (R. Hasegawa).
http://dx.doi.org/10.1016/j.jcrysgro.2016.12.002
0022-0248/ © 2016 Elsevier B.V. All rights reserved.
Please cite this article as: Hasegawa, R., Journal of Crystal Growth (2016), http://dx.doi.org/10.1016/j.jcrysgro.2016.12.002
Journal of Crystal Growth (xxxx) xxxx–xxxx
R. Hasegawa et al.
InAs0.12Sb0.88 absorber layer, which is the first report of GayIn1−ySb growth on Sb rich InAsxSb1−x. InAs0.12Sb0.88 PVS with sensitivity at wavelengths of 8–12 µm was obtained, and estimated detectivity peak at room temperature was approximately 7×107 cm Hz1/2 W−1.
(b)
(a)
p+-InSb (0.5 µm) p+-Ga0.33In0.67Sb (20 nm)
2. Experimental procedures p--InAs0.12Sb0.88 (2 µm)
2.1. Growth conditions study of InAsxSb1−x
InAsxSb1-x (1 µm)
I Sb (1 µm)) InSb
n+-InSb I Sb (1 µm))
Semi-insulating (001) GaAs substrate
Semi-insulating (001) GaAs substrate
InAsxSb1−x was grown on semi-insulating GaAs (001) substrates by using a close-coupled showerhead reactor system. The 1-μm-thick InSb was utilized as a buffer layer, as shown in Fig. 1(a). Trimethylindium (TMIn), trisdimethylaminoantimony (TDMASb), and tertiarybutylarsine (TBAs) were used as In, Sb, and As sources, respectively. The growth pressure was maintained at 100 mbar with purified H2 as a carrier gas. The growth temperature on the substrate surface was measured with a two-wavelength pyrometer. The InSb buffer layer was grown by a previously reported two-step growth method [6]. The 1-μmthick InAsxSb1−x was grown at various growth temperatures and V/III ratios with 2 µm/h growth rate while the growth condition of the InSb buffer layer was constant. The As content varies with V/III and growth temperature [10]. Therefore we changed the As/Sb ratio in order to obtain the same As content at each growth conditions. The film thickness was estimated by X-ray fluorescence analysis. There is a proportional relationship between thickness of InAsxSb1−x and intensity of secondary X-ray from In element. Therefore the intensity of secondary X-ray from In element was measured for these samples and reference sample, and the layer thickness was calculated from them. The crystallinity and As content of InAsxSb1−x film were evaluated by X-ray diffraction measurements and the surface morphology was observed by atomic force microscopy (AFM).
Fig. 1. Schematic diagrams of (a) InAsxSb1−x/InSb and (b) InAs0.12Sb0.88 PVS film structure.
(a) Rocking curve F WHM (arcsec)
1500 Growth temperature= 490oC
1000 000
2.2. Device fabrication and characterization
500
0
5
10 V/III ratio
15
The InAs0.12Sb0.88 PVS film structure is shown in Fig. 1(b). It consists of a 1-μm-thick n+-InSb layer followed by a 2-μm-thick p-InAs0.12Sb0.88 absorber layer, a 20-nm-thick p+-Ga0.33In0.67Sb barrier layer, and a 0.5-μm-thick p+-InSb top-contact layer. The growth pressure was maintained at 100 mbar with purified H2 as a carrier gas. Triethylgallium (TEGa), diethylzinc (DEZn), and diethyltellurium (DETe) were used as Ga source, p-type, and n-type dopant, respectively. Other sources were the same materials as used in the growth conditions study of InAs0.12Sb0.88. The n+-InSb layer was grown by a two-step growth method [6]. The p--InAs0.12Sb0.88 absorber layer and the p+-Ga0.33In0.67Sb barrier layer were grown at the growth temperature of 520°C and V/III ratios of 8 and 5, respectively. Then, the top p+InSb contact layer was grown at the growth temperature of 500 °C and V/III ratio of 5. The growth rate of the InSb and InAs0.12Sb0.88 layers was 2 µm/h. The Zn concentrations of the p- layer and p+ layer were 1×1017 cm−3 and 3×1018 cm−3, respectively. After MOVPE growth, the mesa structure was formed by photolithography and metals were deposited for the n-type and p-type contacts. To confirm the lattice relaxation state of the Ga0.33In0.67Sb barrier layer, XRD reciprocal space mapping measurement was carried out. The output voltage of InAs0.12Sb0.88 PVS was measured by using a 700 K blackbody with light chopping at room temperature. Infrared light was irradiated onto the backside (GaAs substrate side) of the sample. The chopping frequency was 10 Hz. The InAs0.12Sb0.88 PVS was operated in photovoltaic mode. R0 is the averaged value of the resistance, obtained by applying a voltage of ±0.01 V to InAs0.12Sb0.88 PVS. Finally, the detectivity of the InAs0.12Sb0.88 PVS at room temperature was calculated by using output voltage and R0 values [2,3].
20
(b) 700 Rocking curve FWHM (arcsec)
V/III ratio= 8 650
600
550
500 450
500 550 Growth temperature (oC)
600
Fig. 2. Relations between the InAs0.12Sb0.88 (004) rocking curve FWHM and growth conditions: (a) V/III ratio, (b) growth temperature.
2
Journal of Crystal Growth (xxxx) xxxx–xxxx
R. Hasegawa et al.
Fig. 3. AFM images of InAs0.12Sb0.88 films grown at (a) 490 °C, (b) 510 °C, and (c) 530 °C.
p+-InSb
n+-InSb
8100
Fermi energy
8000
p--InAs0.12Sb0.88
p+-Ga0.33In0.67Sb
0.2 0.1 0
Conduction band
0.0
Qz ×10000 ((rlm)
Relative electron energy (eV)
0.3
-0.1 Valence band
-0.2 -0.3 0.0
0.5
1.0
1.5 2.0 Thickness (µm)
2.5
3.0
3.5
Ga0.33In0.67Sb 7900
7800
InAs0.12Sb0.88
Fig. 4. The energy band diagram of InAs0.12Sb0.88 PVS.
3. Results and discussion 7700 2100
3.1. Growth condition study of InAsxSb1−x First, we prepared InAsxSb1−x films on InSb buffer layers grown on GaAs substrates at the growth temperature of 490 °C and V/III ratio ranging from 5 to 14. The As content of InAsxSb1−x films and the FWHM of XRD (004) omega rocking curve were evaluated. Because InAsxSb1−x film on InSb was some strain-relaxed state, the As content was determined from XRD reciprocal space mapping measurement around the (−1 1 5) plane, resulting in 0.12. Fig. 2(a) shows the FWHM as a function of the V/III ratio. The FWHM was almost constant for the V/III ratio in the range of 7–14 and dramatically increased for V/III=5. Next, InAs0.12Sb0.88 films were grown on InSb buffer layers grown on GaAs substrates at growth temperatures ranging from 490 to 540 °C, and the V/III ratio was kept to 8. The FWHM of the omega rocking curve is shown in Fig. 2(b) as a function of the growth temperature. The smallest FWHM was obtained for the growth temperature in the range of 510–520 °C. Fig. 3 shows the AFM images of the InAs0.12Sb0.88 surface at (a) 490 °C, (b) 510 °C, and (c) 530 °C. The film grown at 490°C had a rough surface due to insufficient adatom migration. For a growth temperature of 510 °C, a smooth surface was obtained and monolayer
2200
2300
2400
Qx ×10000 (rlm) Fig. 5. XRD reciprocal space map around (−115) plane.
high terraces were observed. This trend is similar to the growth temperature dependence of rocking curve FWHM. However, the steps lost their smooth edges, and three dimensional growth was observed on terraces at the growth temperature of 530 °C, indicating the changing growth mode of InAs0.12Sb0.88. From the perspective of crystallinity and surface morphology, we utilized the V/III ratio of 8 and the growth temperature of 520 °C for the growth of the InAs0.12Sb0.88 PVS structure. The FWHM of the InAs0.12Sb0.88 layers was always broader than that of the InSb layer [6]. This is probably due to the presence of dislocations in InAs0.12Sb0.88 layers as well as possible composition fluctuation.
3
Journal of Crystal Growth (xxxx) xxxx–xxxx
R. Hasegawa et al.
the 7–8 µm wavelength range and cut-off wavelength was around 12 µm, as is to be expected from the band gap of InAs0.12Sb0.88. The detectivity peak was approximately 7×107 cm Hz1/2 W−1 at room temperature, which is 1.3 times higher than that without the Ga0.33In0.67Sb barrier layer. This result demonstrates that the Ga0.33In0.67Sb barrier layer grown coherently on the InAs0.12Sb0.88 absorber layer suppresses the diffusion of photo-exited electrons, resulting in improvement of detectivity. 4. Conclusion We reported the room temperature operation of an InAs0.12Sb0.88 photovoltaic infrared sensor prepared by MOVPE. An investigation of the InAs0.12Sb0.88 growth conditions using a high-quality InSb buffer layer revealed 560 arcsec of FWHM for a thickness of 1 µm. The Ga0.33In0.67Sb barrier layer was inserted between a p--InAs0.12Sb0.88 layer and a p+-InSb layer to form a potential barrier for photo-excited electrons. An InAs0.12Sb0.88 PVS with sensitivity in the wavelength range of 8–12 µm was obtained, and the estimated detectivity peak was approximately 7×107 cm Hz1/2 W−1 at room temperature. These results indicate that our InAs0.12Sb0.88 PVS is suitable for infrared detection at the 8–12 µm wavelength range.
Fig. 6. The calculated detectivity spectrum of InAs0.12Sb0.88 PVS at room temperature.
3.2. Device fabrication and characterization
Acknowledgments
We prepared an InAs0.12Sb0.88 PVS structure on semi-insulating GaAs (001) substrates using the growth conditions determined most favorable in 3.1 for InAs0.12Sb0.88. The energy band diagram of the InAs0.12Sb0.88 PVS is shown in Fig. 4. The potential barrier for electrons was formed by inserting the Ga0.33In0.67Sb barrier layer between the p--InAs0.12Sb0.88 and p+-InSb layers. XRD reciprocal space mapping measurement around the (−1 1 5) plane was carried out to confirm the state of the lattice relaxation of the Ga0.33In0.67Sb barrier layer. As the dashed line in Fig. 5 shows, the Qx value of the Ga0.33In0.67Sb barrier layer was almost the same as that of the InAs0.12Sb0.88 absorber layer. This indicates that the Ga0.33In0.67Sb barrier layer was fully lattice-matched to the InAs0.12Sb0.88 absorber layer. Finally, we evaluated the spectral responsivity and R0 value and calculated the Johnson-noise limited detectivity at room temperature to investigate the effect of the Ga0.33In0.67Sb barrier layer. Fig. 6 shows the calculated Johnson-noise limited detectivity of InAs0.12Sb0.88 PVS as a function of wavelength. The detectivity peak was observed around
The authors would like to thank H. Goto, H. Geka, E.G. Camargo, and N. Motokawa for discussions and advice. References [1] K. Ueno, E.G. Camargo, T. Katsumata, H. Goto, N. Kuze, Y. Kanagawa, K. Kakimoto, Jpn. J. Appl. Phys. 52 (2013) 092202. [2] J. Kim, S. Kim, D. Wu, J. Wojkowski, J. Xu, J. Piotrowski, E. Bigan, M. Razeghi, Appl. Phys. Lett. 67 (1995) 2645. [3] J.D. Kim, M. Razeghi, Opto-Electron. Rev. 6 (1998) 217. [4] M.Y. Yen, R. People, K.W. Wecht, Appl. Phys. Lett. 64 (1988) 952. [5] E. Michel, H. Mohseni, J.D. Kim, J. Wojkowski, J. Sandven, J. Xu, M. Razeghi, Appl. Phys. Lett. 71 (1997) 1071. [6] A. Yoshikawa, Y. Moriyasu, N. Kuze, J. Cryst. Growth 414 (2015) 110. [7] N. Kuze, E.G. Camargo, K. Ueno, Y. Kawakami, Y. Moriyasu, K. Nagase, M. Sato, K. Ishibashi, M. Ozaki, Phys. Status Solidi (C) 3 (3) (2006) 431. [8] N. Kuze, E.G. Camargo, K. Ueno, T. Morishita, M. Sato, M. Kurihara, H. Endo, K. Ishibashi, J. Cryst. Growth 301–302 (2007) 997–1000. [9] R.M. Biefeld, A.A. Allerman, K.C. Baucom, J. Electron. Mater. 27 (1998) L43. [10] T. Krug, L. Botha, P. Shamba, T.R. Baisitse, A. Venter, J.A.A. Engelbrecht, J.R. Botha, J. Cryst. Growth 298 (2007) 163–167.
4
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Room temperature operating InAsSb-based photovoltaic infrared sensors grown by metalorganic vapor phase epitaxy ⁎
Ryosuke Hasegawa , Akira Yoshikawa, Tomohiro Morishita, Yoshitaka Moriyasu, Kazuhiro Nagase, Naohiro Kuze UVC project, Asahi Kasei Corporation, 2-1, Samejima, Fuji, Shizuoka 416-8501, Japan
A R T I C L E I N F O
A BS T RAC T
Communicated by: T.F. Kuech
We have developed InAsxSb1−x-based photovoltaic infrared sensors (PVS) for room temperature operation by metalorganic vapor phase epitaxy (MOVPE). To obtain high performance, we improved the crystallinity of the InAs0.12Sb0.88 absorber layer and utilized a Ga0.33In0.67Sb electron barrier layer. An investigation of InAs0.12Sb0.88 growth conditions using a high-quality InSb buffer layer showed that we were able to obtain the smallest full-width at half-maximum (FWHM) of the X-ray diffraction omega rocking curve, 560 arcsec, for a growth temperature of 520°C for a 1 µm thick layer. Moreover, we successfully grew a Ga0.33In0.67Sb barrier layer coherently on an InAs0.12Sb0.88 absorber layer, which is the first report of GayIn1−ySb growth on Sb-rich InAsxSb1−x. An InAsxSb1−x PVS with a responsivity at wavelengths of 8–12 µm was obtained, and estimated detectivity peak at room temperature was approximately 7×107 cm Hz1/2 W−1, which is 1.3 times higher than without a Ga0.33In0.67Sb electron barrier. These results demonstrate that our InAsxSb1−x PVS is a promising device for the 8–12 µm wavelength range at room temperature.
Keywords: A1. Characterization A3. Metalorganic vapor phase epitaxy B1. Antimonides B2. Semiconducting indium compounds B3. Infrared devices
1. Introduction Infrared sensors are promising devices for several applications such as human body detection and gas sensing. Pyroelectric detectors are generally used to detect human body at room temperature. However, these detectors require a metallic package to separate electromagnetic and thermal noise for high sensitivity, which makes it difficult for them to be miniaturized. Moreover, pyroelectric detectors cannot detect a stationary human body because they use the temperature change of pyroelectric material to generate an electrical output. Photon detectors are candidates to detect the stationary human body because they can detect the absolute quantity of infrared irradiation. High-performance miniaturized InSb photovoltaic infrared sensors (PVS) operating at room temperature have been reported [1]. However, to detect a stationary human body with high sensitivity, it is important that the spectral response of photon detectors be consistent with the infrared irradiation spectrum from the human body. The development of photon detectors having a spectral response in the range of 8–14 µm is thus required. InAsxSb1−x hetero-junction photodiodes have been proposed for this [2,3], but insufficient sensitivity was obtained due to poor InAsxSb1−x crystallinity and the Auger recombination process. The large lattice mismatch (7.2% < ∆a/a < 14.6%) between InAsxSb1−x and GaAs substrates makes it difficult to
⁎
obtain a high crystallinity InAsxSb1−x epilayer [4]. Poor crystallinity may decrease the lifetime of photo-excited carriers [5] and results in a low photocurrent (Ip). Moreover, high intrinsic carrier density in the InAsxSb1−x absorber layer causes a high diffusion current and results in decreased resistance at zero bias (R0). Because the signal-to-noise ratio (SNR) is proportional to Ip×R01/2 in photovoltaic mode, a decrease of Ip and R0 results in low SNR. In this work, we grew by metalorganic vapor phase epitaxy (MOVPE) an InAsxSb1−x PVS for room temperature operation. To improve the sensitivity, it is important to improve the InAsxSb1−x crystallinity, as the lifetime of a photo-excited carrier is likely to depend on crystallinity [5]. To reduce the lattice mismatch between InAsxSb1−x and GaAs substrates, we used InSb as a buffer layer. The InSb buffer layer was grown using a two-step growth method that enables us to obtain high crystallinity and electrical transport properties [6]. Moreover, the potential barrier for electrons, formed by inserting a wide band gap layer such as AlInSb, is considered to suppress the diffusion of photo-excited electrons, resulting in an improvement of Ip and R0 [7,8]. However, it is difficult to obtain high quality AlInSb by MOVPE due to C and O incorporation and pre-reactions between Al and Sb sources [9]. Therefore, we inserted a Ga0.33In0.67Sb electron barrier layer as an alternative material in the InSb/InAs0.12Sb0.88/InSb hetero-junction. We successfully grew Ga0.33In0.67Sb coherently on an
Corresponding author. E-mail address: [email protected] (R. Hasegawa).
http://dx.doi.org/10.1016/j.jcrysgro.2016.12.002
0022-0248/ © 2016 Elsevier B.V. All rights reserved.
Please cite this article as: Hasegawa, R., Journal of Crystal Growth (2016), http://dx.doi.org/10.1016/j.jcrysgro.2016.12.002
Journal of Crystal Growth (xxxx) xxxx–xxxx
R. Hasegawa et al.
InAs0.12Sb0.88 absorber layer, which is the first report of GayIn1−ySb growth on Sb rich InAsxSb1−x. InAs0.12Sb0.88 PVS with sensitivity at wavelengths of 8–12 µm was obtained, and estimated detectivity peak at room temperature was approximately 7×107 cm Hz1/2 W−1.
(b)
(a)
p+-InSb (0.5 µm) p+-Ga0.33In0.67Sb (20 nm)
2. Experimental procedures p--InAs0.12Sb0.88 (2 µm)
2.1. Growth conditions study of InAsxSb1−x
InAsxSb1-x (1 µm)
I Sb (1 µm)) InSb
n+-InSb I Sb (1 µm))
Semi-insulating (001) GaAs substrate
Semi-insulating (001) GaAs substrate
InAsxSb1−x was grown on semi-insulating GaAs (001) substrates by using a close-coupled showerhead reactor system. The 1-μm-thick InSb was utilized as a buffer layer, as shown in Fig. 1(a). Trimethylindium (TMIn), trisdimethylaminoantimony (TDMASb), and tertiarybutylarsine (TBAs) were used as In, Sb, and As sources, respectively. The growth pressure was maintained at 100 mbar with purified H2 as a carrier gas. The growth temperature on the substrate surface was measured with a two-wavelength pyrometer. The InSb buffer layer was grown by a previously reported two-step growth method [6]. The 1-μmthick InAsxSb1−x was grown at various growth temperatures and V/III ratios with 2 µm/h growth rate while the growth condition of the InSb buffer layer was constant. The As content varies with V/III and growth temperature [10]. Therefore we changed the As/Sb ratio in order to obtain the same As content at each growth conditions. The film thickness was estimated by X-ray fluorescence analysis. There is a proportional relationship between thickness of InAsxSb1−x and intensity of secondary X-ray from In element. Therefore the intensity of secondary X-ray from In element was measured for these samples and reference sample, and the layer thickness was calculated from them. The crystallinity and As content of InAsxSb1−x film were evaluated by X-ray diffraction measurements and the surface morphology was observed by atomic force microscopy (AFM).
Fig. 1. Schematic diagrams of (a) InAsxSb1−x/InSb and (b) InAs0.12Sb0.88 PVS film structure.
(a) Rocking curve F WHM (arcsec)
1500 Growth temperature= 490oC
1000 000
2.2. Device fabrication and characterization
500
0
5
10 V/III ratio
15
The InAs0.12Sb0.88 PVS film structure is shown in Fig. 1(b). It consists of a 1-μm-thick n+-InSb layer followed by a 2-μm-thick p-InAs0.12Sb0.88 absorber layer, a 20-nm-thick p+-Ga0.33In0.67Sb barrier layer, and a 0.5-μm-thick p+-InSb top-contact layer. The growth pressure was maintained at 100 mbar with purified H2 as a carrier gas. Triethylgallium (TEGa), diethylzinc (DEZn), and diethyltellurium (DETe) were used as Ga source, p-type, and n-type dopant, respectively. Other sources were the same materials as used in the growth conditions study of InAs0.12Sb0.88. The n+-InSb layer was grown by a two-step growth method [6]. The p--InAs0.12Sb0.88 absorber layer and the p+-Ga0.33In0.67Sb barrier layer were grown at the growth temperature of 520°C and V/III ratios of 8 and 5, respectively. Then, the top p+InSb contact layer was grown at the growth temperature of 500 °C and V/III ratio of 5. The growth rate of the InSb and InAs0.12Sb0.88 layers was 2 µm/h. The Zn concentrations of the p- layer and p+ layer were 1×1017 cm−3 and 3×1018 cm−3, respectively. After MOVPE growth, the mesa structure was formed by photolithography and metals were deposited for the n-type and p-type contacts. To confirm the lattice relaxation state of the Ga0.33In0.67Sb barrier layer, XRD reciprocal space mapping measurement was carried out. The output voltage of InAs0.12Sb0.88 PVS was measured by using a 700 K blackbody with light chopping at room temperature. Infrared light was irradiated onto the backside (GaAs substrate side) of the sample. The chopping frequency was 10 Hz. The InAs0.12Sb0.88 PVS was operated in photovoltaic mode. R0 is the averaged value of the resistance, obtained by applying a voltage of ±0.01 V to InAs0.12Sb0.88 PVS. Finally, the detectivity of the InAs0.12Sb0.88 PVS at room temperature was calculated by using output voltage and R0 values [2,3].
20
(b) 700 Rocking curve FWHM (arcsec)
V/III ratio= 8 650
600
550
500 450
500 550 Growth temperature (oC)
600
Fig. 2. Relations between the InAs0.12Sb0.88 (004) rocking curve FWHM and growth conditions: (a) V/III ratio, (b) growth temperature.
2
Journal of Crystal Growth (xxxx) xxxx–xxxx
R. Hasegawa et al.
Fig. 3. AFM images of InAs0.12Sb0.88 films grown at (a) 490 °C, (b) 510 °C, and (c) 530 °C.
p+-InSb
n+-InSb
8100
Fermi energy
8000
p--InAs0.12Sb0.88
p+-Ga0.33In0.67Sb
0.2 0.1 0
Conduction band
0.0
Qz ×10000 ((rlm)
Relative electron energy (eV)
0.3
-0.1 Valence band
-0.2 -0.3 0.0
0.5
1.0
1.5 2.0 Thickness (µm)
2.5
3.0
3.5
Ga0.33In0.67Sb 7900
7800
InAs0.12Sb0.88
Fig. 4. The energy band diagram of InAs0.12Sb0.88 PVS.
3. Results and discussion 7700 2100
3.1. Growth condition study of InAsxSb1−x First, we prepared InAsxSb1−x films on InSb buffer layers grown on GaAs substrates at the growth temperature of 490 °C and V/III ratio ranging from 5 to 14. The As content of InAsxSb1−x films and the FWHM of XRD (004) omega rocking curve were evaluated. Because InAsxSb1−x film on InSb was some strain-relaxed state, the As content was determined from XRD reciprocal space mapping measurement around the (−1 1 5) plane, resulting in 0.12. Fig. 2(a) shows the FWHM as a function of the V/III ratio. The FWHM was almost constant for the V/III ratio in the range of 7–14 and dramatically increased for V/III=5. Next, InAs0.12Sb0.88 films were grown on InSb buffer layers grown on GaAs substrates at growth temperatures ranging from 490 to 540 °C, and the V/III ratio was kept to 8. The FWHM of the omega rocking curve is shown in Fig. 2(b) as a function of the growth temperature. The smallest FWHM was obtained for the growth temperature in the range of 510–520 °C. Fig. 3 shows the AFM images of the InAs0.12Sb0.88 surface at (a) 490 °C, (b) 510 °C, and (c) 530 °C. The film grown at 490°C had a rough surface due to insufficient adatom migration. For a growth temperature of 510 °C, a smooth surface was obtained and monolayer
2200
2300
2400
Qx ×10000 (rlm) Fig. 5. XRD reciprocal space map around (−115) plane.
high terraces were observed. This trend is similar to the growth temperature dependence of rocking curve FWHM. However, the steps lost their smooth edges, and three dimensional growth was observed on terraces at the growth temperature of 530 °C, indicating the changing growth mode of InAs0.12Sb0.88. From the perspective of crystallinity and surface morphology, we utilized the V/III ratio of 8 and the growth temperature of 520 °C for the growth of the InAs0.12Sb0.88 PVS structure. The FWHM of the InAs0.12Sb0.88 layers was always broader than that of the InSb layer [6]. This is probably due to the presence of dislocations in InAs0.12Sb0.88 layers as well as possible composition fluctuation.
3
Journal of Crystal Growth (xxxx) xxxx–xxxx
R. Hasegawa et al.
the 7–8 µm wavelength range and cut-off wavelength was around 12 µm, as is to be expected from the band gap of InAs0.12Sb0.88. The detectivity peak was approximately 7×107 cm Hz1/2 W−1 at room temperature, which is 1.3 times higher than that without the Ga0.33In0.67Sb barrier layer. This result demonstrates that the Ga0.33In0.67Sb barrier layer grown coherently on the InAs0.12Sb0.88 absorber layer suppresses the diffusion of photo-exited electrons, resulting in improvement of detectivity. 4. Conclusion We reported the room temperature operation of an InAs0.12Sb0.88 photovoltaic infrared sensor prepared by MOVPE. An investigation of the InAs0.12Sb0.88 growth conditions using a high-quality InSb buffer layer revealed 560 arcsec of FWHM for a thickness of 1 µm. The Ga0.33In0.67Sb barrier layer was inserted between a p--InAs0.12Sb0.88 layer and a p+-InSb layer to form a potential barrier for photo-excited electrons. An InAs0.12Sb0.88 PVS with sensitivity in the wavelength range of 8–12 µm was obtained, and the estimated detectivity peak was approximately 7×107 cm Hz1/2 W−1 at room temperature. These results indicate that our InAs0.12Sb0.88 PVS is suitable for infrared detection at the 8–12 µm wavelength range.
Fig. 6. The calculated detectivity spectrum of InAs0.12Sb0.88 PVS at room temperature.
3.2. Device fabrication and characterization
Acknowledgments
We prepared an InAs0.12Sb0.88 PVS structure on semi-insulating GaAs (001) substrates using the growth conditions determined most favorable in 3.1 for InAs0.12Sb0.88. The energy band diagram of the InAs0.12Sb0.88 PVS is shown in Fig. 4. The potential barrier for electrons was formed by inserting the Ga0.33In0.67Sb barrier layer between the p--InAs0.12Sb0.88 and p+-InSb layers. XRD reciprocal space mapping measurement around the (−1 1 5) plane was carried out to confirm the state of the lattice relaxation of the Ga0.33In0.67Sb barrier layer. As the dashed line in Fig. 5 shows, the Qx value of the Ga0.33In0.67Sb barrier layer was almost the same as that of the InAs0.12Sb0.88 absorber layer. This indicates that the Ga0.33In0.67Sb barrier layer was fully lattice-matched to the InAs0.12Sb0.88 absorber layer. Finally, we evaluated the spectral responsivity and R0 value and calculated the Johnson-noise limited detectivity at room temperature to investigate the effect of the Ga0.33In0.67Sb barrier layer. Fig. 6 shows the calculated Johnson-noise limited detectivity of InAs0.12Sb0.88 PVS as a function of wavelength. The detectivity peak was observed around
The authors would like to thank H. Goto, H. Geka, E.G. Camargo, and N. Motokawa for discussions and advice. References [1] K. Ueno, E.G. Camargo, T. Katsumata, H. Goto, N. Kuze, Y. Kanagawa, K. Kakimoto, Jpn. J. Appl. Phys. 52 (2013) 092202. [2] J. Kim, S. Kim, D. Wu, J. Wojkowski, J. Xu, J. Piotrowski, E. Bigan, M. Razeghi, Appl. Phys. Lett. 67 (1995) 2645. [3] J.D. Kim, M. Razeghi, Opto-Electron. Rev. 6 (1998) 217. [4] M.Y. Yen, R. People, K.W. Wecht, Appl. Phys. Lett. 64 (1988) 952. [5] E. Michel, H. Mohseni, J.D. Kim, J. Wojkowski, J. Sandven, J. Xu, M. Razeghi, Appl. Phys. Lett. 71 (1997) 1071. [6] A. Yoshikawa, Y. Moriyasu, N. Kuze, J. Cryst. Growth 414 (2015) 110. [7] N. Kuze, E.G. Camargo, K. Ueno, Y. Kawakami, Y. Moriyasu, K. Nagase, M. Sato, K. Ishibashi, M. Ozaki, Phys. Status Solidi (C) 3 (3) (2006) 431. [8] N. Kuze, E.G. Camargo, K. Ueno, T. Morishita, M. Sato, M. Kurihara, H. Endo, K. Ishibashi, J. Cryst. Growth 301–302 (2007) 997–1000. [9] R.M. Biefeld, A.A. Allerman, K.C. Baucom, J. Electron. Mater. 27 (1998) L43. [10] T. Krug, L. Botha, P. Shamba, T.R. Baisitse, A. Venter, J.A.A. Engelbrecht, J.R. Botha, J. Cryst. Growth 298 (2007) 163–167.
4