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InAsSb heterostructures
Accepted Manuscript Title: Molecular beam epitaxial growth of AlSb/InAsSb heterostructures Author: Yuwei Zhang Yang Zhang Min Guan Lijie Cui Yanbo Li Baoqiang Wang Zhanping Zhu Yiping Zeng PII: DOI: Reference:
S0169-4332(14)01280-X http://dx.doi.org/doi:10.1016/j.apsusc.2014.06.009 APSUSC 28058
To appear in:
APSUSC
Received date: Revised date: Accepted date:
28-9-2013 3-5-2014 3-6-2014
Please cite this article as: Y. Zhang, Y. Zhang, M. Guan, L. Cui, Y. Li, B. Wang, Z. Zhu, Y. Zeng, Molecular beam epitaxial growth of AlSb/InAsSb heterostructures, Applied Surface Science (2014), http://dx.doi.org/10.1016/j.apsusc.2014.06.009 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Molecular beam epitaxial growth of AlSb/InAsSb heterostructures Yuwei Zhang a,b, Yang Zhang a,*, Min Guan a, Lijie Cui a, Yanbo Li a, Baoqiang Wang a, Zhanping Zhu a, and Yiping Zeng a Key Laboratory of Semiconductor Materials Science, Institute of Semiconductors, Chinese Academy of Sciences, Beijing, 100083, People’s Republic of China
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a
Department of Physics, Tsinghua University, Beijing, 100084, People’s Republic of China
*
E-mail: [email protected]
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b
Abstract
AlSb/InAsSb heterostructures have been successfully grown on GaAs substrate by
an
modulated molecular-beam epitaxy (MMBE). New shutter sequence has been presented and room temperature mobility of 16,170 cm2/Vs has been achieved with our
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non-intentionally doped structures. With a view for optimization, we analyze variation of electron mobility induced by growth temperature and InAsSb thickness. By
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increasing growth temperature and thickness of InAsSb, improvement of electron
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mobility has been observed. With our optimized AlSb/InAsSb heterostructures, accurate control of composition in InAsSb alloy and reduced interface mixing have been
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confirmed by X-Ray diffraction and Raman spectroscopy measurements.
Keywords: Molecular-beam epitaxy, Antimonides, Heterostructures, Raman scattering
1. Introduction
InSb, which has the highest electron mobility of all III-V binary compounds [1], has
generated considerable interest for fabrication of high-speed devices, including high electron mobility transistors (HEMTs), terahertz subharmonic mixer and ballistic mesoscopic devices [2-4]. However, due to the large lattice mismatch between InSb and GaAs substrate, growth of high-quality InSb is challenging. To minimize the problem of lattice strain, InAsSb ternary alloy is anticipated to be a substitution of InSb. Owing to the small effective mass, both InAs and InSb have relatively high electron mobility even 1
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at room temperatures. Hence, InAsSb alloys are expected to exhibit superior electron mobility due to their smallest effective mass among all III-V compounds, involving both InAs and InSb [5,6]. Additionally, a combination of InAsSb quantum well with AlSb barrier yields high value of two-dimensional electron gas (2DEG) concentration and
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type-I band alignment to confine both electrons and holes [7]. Therefore AlSb/InAsSb heterostructures have great potential for high-speed applications. However, precise
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control over group-V composition in ternary compounds containing both As and Sb has
been proven to be extremely difficult to achieve in conventional solid-source
us
molecular-beam epitaxy (MBE). Given to such difficulties, the digital alloying technology has often been applied to emulate a conventional grown alloy by building a
an
short period superlattice that exhibits similar basic properties [8].
In this paper, we investigate the shutter sequence dependence of electronic properties
M
during the growth procedure of InAs0.8Sb0.2 ordered alloy using MMBE. New shutter sequence has been presented to reduce intermixing and give accurate control of Sb
d
composition, confirmed by X-ray diffraction and Raman spectroscopy. With a view to optimize the heterostructures for high-speed applications, variations of transport
te
properties with growth temperature and InAs0.8Sb0.2 thickness are examined in
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AlSb/InAs0.8Sb0.2 heterostructures. Electron mobility exceeding 16,000 cm2/Vs is attainable for our InAs0.8Sb0.2 quantum well heterostructures at 300 K.
2. Experiments
All samples are grown on semi-insulating GaAs (001) substrate using an EPI GEN-II
solid-source MBE system. Arsenic and antimony are supplied by valved cracker cells to produce As2 and Sb2, respectively. After desorption of surface oxide, 200 nm GaAs buffer layer is first grown followed by 2 μm AlSb buffer layer. The InAs0.8Sb0.2 layer of 4ML InAs/1ML InSb is grown at a variety of channel thickness and growth temperatures which are measured by the thermocouple. Following the InAsSb layer are AlSb upper barrier and InAs cap layers. The AlSb/InAs0.8Sb0.2 interfaces are forced to be InSb-like by migration-enhanced epitaxy (MEE). It should be noted that our heterostructure is unintentionally doped. A cross section of the material layers for this 2
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design is shown in Fig. 1. During the growth of InAs0.8Sb0.2 layer, two different shutter sequences are employed. In sample A, InAsSb digital alloy is grown according to the shutter sequence in Fig. 2 (a), which is similar to previous report [9], where the shutter of In is kept open while the
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As and Sb shutters are alternately opened and closed. Besides, we present a new shutter sequence shown in Fig. 2 (b) to grow the InAsSb digital alloy of all other samples
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where the shutters of the group-III element (In) and group-V element (As or Sb) are kept open simultaneously for growing InAs and InSb layers and an interruption is
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introduced between their growth.
After the growth, Hall measurement with standard Van Der Pauw configuration is
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carried out to characterize the electronic properties of AlSb/InAs0.8Sb0.2 heterostructures. The alloy composition and crystalline quality are evaluated by double-crystal x-ray
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diffraction (XRD), using a Cu Kα source and (004) reflection. Transmission electron microscopy (TEM) is used to characterize the interfaces between AlSb and InAs0.8Sb0.2 layers and TEM samples are prepared via conventional mechanical thinning procedure
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followed by Ar+ ion milling. Raman scattering is performed in a back-scattering
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geometry to give comparisons among samples differed by growth conditions.
3. Results and discussion
In order to examine the variation of transport properties with different shutter
sequences during the growth of InAs0.8Sb0.2 layer, we first compare the electronic properties between sample A and sample B1 presented in Table 1. A five-second interruption is introduced between the growth of InAs and InSb layers in sample B1. At 300 K, electron mobility of 7,599 cm2/Vs could be achieved in sample A and increases to 16,170 cm2/Vs with shutter sequence employed for sample B1, which is higher than the structures stated in [10]. To further explore the impact of shutter sequence on transport properties, (004) double crystal x-ray diffraction spectra for sample A and sample B1 are plotted in Fig. 3. For sample B1, InAsSb peak is overlapping with AlSb peak, indicating As composition of InAsSb alloy to be 0.8 which is well correlated with the value we designed. For sample A, an additional peak is visible which corresponds to 3
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InAsSb alloy with As mole fraction of 0.7. We attribute the discrepancy of As composition with our designed value to interface mixing where In shutter is kept open with Sb and As shutters alternatively open and closed. As a result, residual Sb present in the growth chamber after the closing of Sb shutter might involve in the following
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growth of InAs, leading to decreased As composition in InAsSb alloy in sample A. Note
that narrower peak width is generally considered to reflect better crystalline quality. The
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full-width at half-maximum (FWHM) of X-ray rocking curve measurements for InAsSb layer is 302 arcsec in sample B1 and 397 arcsec in sample A, indicating our presented
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shutter sequence very useful for improving crystalline quality by minimizing interface mixing and accurately controlling composition in InAsSb alloy. As shown in Fig. 4,
an
atomic force microscopy (AFM) measurement reveals a smooth surface for sample B1 and a rougher surface for sample A. Root-mean-square (rms) roughness over a 10×10
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μm2 area is measured to be 2.26 nm for sample B1 and 20 nm for sample A, in consistent with the superior electron mobility of sample B1 over sample A. It should be
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noted that apart from the InAsSb layer, structural parameters are identical between sample A and sample B1, so it is reasonable to assume that surface roughness
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characterized by AFM originates from fluctuations of the interfaces. According to
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theoretical and experimental result, interface-roughness scattering acts as a main factor to limit the electron mobility, so increased electron mobility in sample B1 is probably due to reduced scattering process from interface roughness [12,13]. To investigate the interface mixing more directly, the cross sectional TEM
micrographs of AlSb/InAs0.8Sb0.2 heterostructures of sample A and sample B1 are shown in Fig. 5, respectively. Flat interfaces between AlSb and InAsSb layers are clearly seen for sample B1 by introducing interruption between growth of InAs and InSb. In contrast, the flatness of upper interface between AlSb and InAsSb layers degraded for sample A, indicating serious intermixing at the interface. XRD measurement results show significant changes in alloy composition and thus reveal that interface mixing occurs at InAs/InSb digital alloy which contribute to degraded flatness in AlSb/InAsSb upper interfaces for sample A. It is concluded that sample B has sharp interfaces both at InAs/InSb and AlSb/InAsSb than that in sample A. It should be noted 4
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that InSb and InAs layer could hardly be distinguished due to their extreme small thickness. Consequently, MMBE has a significant effect in minimizing interface mixing for AlSb/InAsSb heterostructures. In order to further investigate the influence of shutter sequence on transport
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properties and examine interface mixing between InAs and InSb layers in AlSb/InAsSb
heterostructures, sample B2 and sample B3 are grown with shutter sequence shown in
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Fig. 2 (b), whereas different interruptions are introduced between InAs and InSb layers
presented in Table 1. In comparison between sample B1 and sample B2, an increased
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interruption from 5 seconds to 10 seconds after InAs growth yields electron mobility of 15,660 cm2/Vs for sample B2 which approximates to the value of 16,170 cm2/Vs for
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sample B1 at 300 K, indicating the impact of residual As in the growth chamber after InAs growth has little effect on total electron mobility of our AlSb/InAsSb
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heterostructures, which could be attributed to relatively small incorporation coefficient of As2 at the growth temperature of 470 °C. To investigate the incorporation of Sb2 into
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InAs layer, a comparison between sample B2 and sample B3 is presented in Table 1, where different interruptions are introduced after InSb growth but keep interruption
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after InAs growth identical. With a longer interruption after InSb growth, residual Sb in
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the growth chamber is reduced, confirmed by the decrease of reactor base pressure, which yields the electron mobility of 16,050 cm2/Vs for sample B3 at 300 K. The increase of electron mobility from sample B2 to sample B3 could be due to reduced incorporation of Sb2 into InAs layer when the interruption after InAs growth is increased from 5 seconds to 10 seconds, indicating relatively higher incorporation coefficient of Sb2 at the growth temperature of 470 °C which is well correlated to the deviation of Sb composition in InAs0.8Sb0.2 alloy in sample A as presented in Fig. 3. Based on the results above, a five second interruption after both InAs and InSb growth is enough to ensure high electron mobility, so the shutter sequence of sample B1 is selected for following discussions. The growth temperature of InAsSb layer is considered to be one of the most important parameters affecting 2DEG mobility. Serious intermixing would occur to deteriorate electron mobility at high growth temperatures [14]. In order to test the 5
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dependence of electron mobility on growth temperature, sample C grown at 470 ºC and sample D grown at 430 ºC are taken for comparison. These two samples have identical structural parameters other than the growth temperature of InAs0.8Sb0.2 layer. Hall measurements for sample C and sample D at 300 K are summarized in Table 1. Unlike
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previous report [15], electron mobility exceeding 14,000 cm2/Vs is attainable for sample C, compared to sample D of only 5,624 cm2/Vs at 300 K. Several groups have
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investigated the critical temperature for intermixing of anion species and determined
410 ºC for Sb intermixing with InAs and 350 ºC for As intermixing with InSb [16].
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Severe intermixing could be induced by increased growth temperature and cause dramatic decrease in electron mobility. To the contrary, our experimental results
an
demonstrate that 470 ºC is more favorable to obtain high electron mobility in relative to 430 °C for AlSb/InAs0.8Sb0.2 heterostructures, where intermixing is reduced by taking
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the new shutter sequences we employed, allowing relatively higher growth temperature for the subsequent AlSb barrier and InAs cap layers.
d
The dependence of transport properties on InAs0.8Sb0.2 thickness is further demonstrated by Fig. 6 where electron mobility and 2DEG concentration are plotted as
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a function of InAsSb thickness at 300 K. The period number of InAs/InSb superlattice is
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increased from 10 to 50. The growth temperature of InAs0.8Sb0.2 layer is fixed at 470 ºC and the shutter sequence is set as Fig. 2 (b) with five seconds interruption. For InAs/InSb superlattice ranging from 10 to 25 periods, electron mobility increases rapidly from approximately 6,000 cm2/Vs to 14,480 cm2/Vs, the trend of which has also been observed in AlInSb/InAsSb heterostructures by Kudo et al. [15]. Interface roughness scattering between AlSb and InAs0.8Sb0.2 layers is considered to be the dominant scattering mechanism in this region [17]. For InAs/InSb superlattice covering the range from 25 to 35 periods, electron mobility increases slowly from 14,480 cm2/Vs to 16,170 cm2/Vs, indicating that electron mobility is still governed by scattering process from interfaces between InAs0.8Sb0.2 and AlSb layers, however, contributions from interface roughness scattering between InAs and InSb layers begin to act. To further increase the InAsSb thickness, a decrease of electron mobility is observed from 16,170 cm2/Vs to 13,390 cm2/Vs, confirming that scattering process from interface 6
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roughness between InAs and InSb layers dominates over that between InAsSb and AlSb layers. Note that our structures are unintentionally doped, where 2DEG concentration remains at a low level. Further increasing electron mobility depends on improving 2DEG concentration through remote doping where Coulomb scattering could be
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minimized by screening effect.
Finally, to gain a more detailed understanding of variation of electron mobility with
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shutter sequence and growth temperature, Raman scattering is measured at room temperature in the background-configuration by using a 514.5 nm Ar+ ion laser. In Fig.
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7, we present Raman scattering spectra for sample A and sample B1 with both AlSb/InAs0.8Sb0.2 interfaces grown to be InSb-like. The data are collected in both and Z(X,Y)
configurations. For both sample A and sample B1, the peak at
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Z(X,X)
338 cm-1 is visible for AlSb-LO mode. InAs-LO mode and InSb-LO mode appear at 233
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cm-1 and 182 cm-1, respectively. The additional mode at 318 cm-1 represents AlSb-TO mode. The appearance of TO mode, which is forbidden in our configuration, could be
d
attributed to scattering from imperfection of AlSb deposition process due to lattice
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mismatch between AlSb buffer layer and GaAs substrate. It should be emphasized that the appearance of a broad band ranging from 100 cm-1 to 200 cm-1 is considered to be
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the disorder-activated longitudinal acoustic (DALA) vibrational mode which originates from alloy disorder or strain disorder in the alloy [18]. The absence of DALA region for sample B1 could be attributed to well defined interfaces between InAs and InSb layers and no occurrence of interface intermixing, indicating that shutter sequence has a significant effect on total electron
mobility
through interface
mixing
for
AlSb/InAs0.8Sb0.2 heterostructures. In contrast, DALA mode is observed in sample A which is due to alloy disorder in InAs/InSb superlattices induced by intermixing between InAs and InSb layers, in consistent with deviation of As compositions confirmed by XRD measurements mentioned above. Temperature dependent electron mobility is also examined by Raman scattering spectra presented in Fig. 8. All peaks, including AlSb-LO mode, AlSb-TO mode, InAs-LO mode, and InSb-LO mode are visible for both sample C and sample D. Calculations have shown that interface mode 7
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density of vibrational states depends on interface roughness [19]. Localization decouples and thereby reducing the energy exchange, allowing the interface mode to vibrate at larger amplitude [20]. Therefore, stronger peaks are expected for rougher interfaces in Raman spectra. It should be noted our InAs/InSb superlattice begins with
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InAs layer and ends to InAs layer as well. Consequently, intensity of InAs-LO mode could be representative of degree of interface roughness of AlSb/InAs0.8Sb0.2 interfaces.
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According to Raman spectra analysis, InAs-LO peak intensity is 12,113 for sample C
(grown at 470 °C) inferior to 14,130 of sample D (grown at 430 °C), indicating that the
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scattering, well correlated with mobility values.
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lower electron mobility in sample D corresponds to more severe interface roughness
4. Conclusion
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In summary, AlSb/InAs0.8Sb0.2 heterostructures have been successfully grown on GaAs substrate by MMBE. New shutter sequence has been presented and proved to be very useful for reducing interface mixing and accurately controlling Sb composition in
d
InAsSb alloy. Room temperature mobility of 16,170 cm2/Vs is attainable with our
te
non-intentionally doped structures. With a view for optimization, we analyze variation
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of electron mobility induced by growth temperature and InAsSb thickness. With increased growth temperature and InAsSb thickness, improvement of electron mobility has been reported. Raman analysis is also demonstrated to further evaluate the crystalline quality of the epi-layers from the perspective of crystal disorder and interface roughness. Despite that further improvement is desirable on transport properties of AlSb/InAsSb heterostructures, it is evident that our structures are promising for high-speed applications.
Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant No. 61204012), Beijing Natural Science Foundation (Grant No. 2112040), and Beijing Nova Program (Grant No. 2010B056). 8
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References [1] W. K. Liu, X. Zhang, W. Ma, J. Winesett, M. B. Santos, J. Vac. Sci. Technol. B 14 (1996) 2339.
Emeny, P. D. Buckle, T. Ashley, Phys. Rev. B 77 (2008) 165334.
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[2] J. M. S. Orr, A. M. Gilbertson, M. Fearn, O. W. Croad, C. J. Storey, L. Buckle, M. T.
[3] T. Ashley, A. B. Dean, C. T. Elliott, G. J. Pryce, A. D. Johnson, H. Willis, Appl. Phys.
cr
Lett. 66 (1995) 481.
[4] R. Magno, J. G. Champlain, H. S. Newman, M. G. Ancona, J. C. Culberson, B. R.
us
Bennett, J. B. Boos, D. Park, Appl. Phys. Lett. 92 (2008) 243502.
[5] I. Vurgaftman , J. R. Meyer, L. R. Ram-Mohan, J. Appl. Phys. 89 (2001) 5815.
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[6] S. S. Kizhaev, S. S. Molchanov, N. V. Zotova, E. A. Grebenshchikova, Y. P. Yakolev, E. Hulicius, T. Simecek, K. Melichar, J. Pangrac, Techonol. Phys. Lett. 27 (2001)
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964.
[7] M. J. Yang, B. R. Bennett, M. Fatemi, P. J. Lin-Chung, W. J. Moore, C. H. Yang, J.
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Appl. Phys. 87 (2000) 8192.
[8] R. Kaspi, G. P. Donati, J. Crystal Growth 251 (2003) 515.
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[9] Y. H. Zhang, J. Crystal Growth 150 (1995) 838.
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[10] J. B. Boos, M. J. Yang, B. R. Bennett, D. Park, W. Kruppa, R. Bass, Electron. Lett. 35 (1999) 847.
[11] H. Miyoshi, R. Suzuki, H. Amano, Y. Horikoshi, J. Crystal Growth 237-239 (2002) 1519.
[12] Y. Zhang, Y. Zhang, M. Guan, L. Cui, C. Wang, Y. Zeng, J. Appl. Phys. 114 (2013) 153707.
[13] C. R. Bolognesi, H. Kroemer, J. H. English, Appl. Phys. Lett. 61 (1992) 213. [14] B. P. Tinkham, B. R. Bennett, R. Magno, B. V. Shanabrook, J. B. Boos, J. Vac. Sci. Technol. B 23 (2005) 1441. [15] M. Kudo, T. Mishima, T. Tanaka, J. Vac. Sci. Technol. B 18 (2000) 746. [16] B. R. Bennett, B. V. Shanabrook, M. E. Twigg, J. Appl. Phys. 85 (1999) 2157. [17] Y. Li, Y. Zhang, Y. Zeng, J. Appl. Phys. 109 (2011) 073703. [18] A. Sayari, N. Yahyaoui, A. Meftah, A. Sfaxi, M. Oueslati, J. Lumin. 129 (2009) 9
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105. [19] D. Kerchrakos, J. C. Inkson, Semicond. Sci. Technol. 6 (1991) 155. [20] I. Sela, C. R. Bolognesi, L. A. Samoska, H. Kroemer, Appl. Phys. Lett. 60 (1992)
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3283.
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Fig. 1 Schematic of AlSb/InAs0.8Sb0.2 heterostructure.
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Fig. 2
Schematic time sequence of In, As and Sb shutter positions during the MMBE growth
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of AlSb/InAs0.8Sb0.2, (a) for sample A and (b) for sample B1, sample B2, sample C, and
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sample D.
Fig. 3
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Symmetric omega-2theta scan in the vicinity of the (004) Bragg peaks for GaAs, AlSb,
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and InAsSb for sample A (upper trace) and sample B1 (lower trace).
Fig. 4
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AFM image of sample A (a) and sample B1 (b).
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Fig. 5
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Cross-sectional TEM micrograph of AlSb/InAs0.8Sb0.2 heterostructures for sample B1 (a) and sample A (b).
Fig. 6
Electron mobility and sheet electron concentration of AlSb/InAs0.8Sb0.2 heterostructures as a function of period numbers of InAs/InSb superlattice at 300 K.
Fig. 7 Comparison of Raman scattering spectra between sample A and sample B1 at 300 K with both AlSb/InAs0.8Sb0.2 interfaces grown to be InSb-like. The data are collected in both Z(X,X)
and Z(X,Y)
configurations.
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Fig. 8 Comparison of Raman scattering spectra for AlSb/InAs0.8Sb0.2 heterostructures between
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sample C and sample D at 300 K.
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Table 1. AlSb/InAs0.8Sb0.2 heterostructure samples and their related electron mobility from Hall measurements with Van Der Pauw configuration at 300 K.
sequence
InAs0.8Sb0.2
after InAs
after InSb
temperature
thickness
(s)
(s)
(°C)
(nm)
Fig. 2 (a)
0
0
470
B1
Fig. 2 (b)
5
5
470
B2
Fig. 2 (b)
10
5
B3
Fig. 2 (b)
10
10
C
Fig. 2 (b)
5
5
D
Fig. 2 (b)
5
5
mobility 300K
(cm2/Vs)
21.5
7599
21.5
16170
470
21.5
15660
470
21.5
16050
470
15.4
14480
430
15.4
5624
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te
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an
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A
Electron
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Shutter
Growth
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Sample
Interruption Interruption
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Fig. 1
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cr
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Fig. 2
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M
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cr
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Fig. 3
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cr
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Fig. 4 (a)
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cr
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Fig. 4 (b)
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AlSb
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Fig. 5 (a)
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AlSb
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InAsSb
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Fig. 5 (b)
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AlSb
M
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InAsSb
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AlSb
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cr
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Fig. 6
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Fig. 7
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Fig. 8
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Highlights:
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AlSb/InAsSb heterostructures have been successfully grown by MBE with electron mobility exceeding 16000 cm2/Vs. We present a new shutter sequence to reduce interface mixing and accurate control of Sb composition in InAsSb alloy Higher temperature could be applied for InAsSb growth with good crystalline quality, allowing relatively higher growth temperature for the subsequent AlSb barrier. Raman spectra is used to examine interface mixing between AlSb and InAsSb layers
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S0169-4332(14)01280-X http://dx.doi.org/doi:10.1016/j.apsusc.2014.06.009 APSUSC 28058
To appear in:
APSUSC
Received date: Revised date: Accepted date:
28-9-2013 3-5-2014 3-6-2014
Please cite this article as: Y. Zhang, Y. Zhang, M. Guan, L. Cui, Y. Li, B. Wang, Z. Zhu, Y. Zeng, Molecular beam epitaxial growth of AlSb/InAsSb heterostructures, Applied Surface Science (2014), http://dx.doi.org/10.1016/j.apsusc.2014.06.009 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Molecular beam epitaxial growth of AlSb/InAsSb heterostructures Yuwei Zhang a,b, Yang Zhang a,*, Min Guan a, Lijie Cui a, Yanbo Li a, Baoqiang Wang a, Zhanping Zhu a, and Yiping Zeng a Key Laboratory of Semiconductor Materials Science, Institute of Semiconductors, Chinese Academy of Sciences, Beijing, 100083, People’s Republic of China
ip t
a
Department of Physics, Tsinghua University, Beijing, 100084, People’s Republic of China
*
E-mail: [email protected]
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cr
b
Abstract
AlSb/InAsSb heterostructures have been successfully grown on GaAs substrate by
an
modulated molecular-beam epitaxy (MMBE). New shutter sequence has been presented and room temperature mobility of 16,170 cm2/Vs has been achieved with our
M
non-intentionally doped structures. With a view for optimization, we analyze variation of electron mobility induced by growth temperature and InAsSb thickness. By
d
increasing growth temperature and thickness of InAsSb, improvement of electron
te
mobility has been observed. With our optimized AlSb/InAsSb heterostructures, accurate control of composition in InAsSb alloy and reduced interface mixing have been
Ac ce p
confirmed by X-Ray diffraction and Raman spectroscopy measurements.
Keywords: Molecular-beam epitaxy, Antimonides, Heterostructures, Raman scattering
1. Introduction
InSb, which has the highest electron mobility of all III-V binary compounds [1], has
generated considerable interest for fabrication of high-speed devices, including high electron mobility transistors (HEMTs), terahertz subharmonic mixer and ballistic mesoscopic devices [2-4]. However, due to the large lattice mismatch between InSb and GaAs substrate, growth of high-quality InSb is challenging. To minimize the problem of lattice strain, InAsSb ternary alloy is anticipated to be a substitution of InSb. Owing to the small effective mass, both InAs and InSb have relatively high electron mobility even 1
Page 1 of 24
at room temperatures. Hence, InAsSb alloys are expected to exhibit superior electron mobility due to their smallest effective mass among all III-V compounds, involving both InAs and InSb [5,6]. Additionally, a combination of InAsSb quantum well with AlSb barrier yields high value of two-dimensional electron gas (2DEG) concentration and
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type-I band alignment to confine both electrons and holes [7]. Therefore AlSb/InAsSb heterostructures have great potential for high-speed applications. However, precise
cr
control over group-V composition in ternary compounds containing both As and Sb has
been proven to be extremely difficult to achieve in conventional solid-source
us
molecular-beam epitaxy (MBE). Given to such difficulties, the digital alloying technology has often been applied to emulate a conventional grown alloy by building a
an
short period superlattice that exhibits similar basic properties [8].
In this paper, we investigate the shutter sequence dependence of electronic properties
M
during the growth procedure of InAs0.8Sb0.2 ordered alloy using MMBE. New shutter sequence has been presented to reduce intermixing and give accurate control of Sb
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composition, confirmed by X-ray diffraction and Raman spectroscopy. With a view to optimize the heterostructures for high-speed applications, variations of transport
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properties with growth temperature and InAs0.8Sb0.2 thickness are examined in
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AlSb/InAs0.8Sb0.2 heterostructures. Electron mobility exceeding 16,000 cm2/Vs is attainable for our InAs0.8Sb0.2 quantum well heterostructures at 300 K.
2. Experiments
All samples are grown on semi-insulating GaAs (001) substrate using an EPI GEN-II
solid-source MBE system. Arsenic and antimony are supplied by valved cracker cells to produce As2 and Sb2, respectively. After desorption of surface oxide, 200 nm GaAs buffer layer is first grown followed by 2 μm AlSb buffer layer. The InAs0.8Sb0.2 layer of 4ML InAs/1ML InSb is grown at a variety of channel thickness and growth temperatures which are measured by the thermocouple. Following the InAsSb layer are AlSb upper barrier and InAs cap layers. The AlSb/InAs0.8Sb0.2 interfaces are forced to be InSb-like by migration-enhanced epitaxy (MEE). It should be noted that our heterostructure is unintentionally doped. A cross section of the material layers for this 2
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design is shown in Fig. 1. During the growth of InAs0.8Sb0.2 layer, two different shutter sequences are employed. In sample A, InAsSb digital alloy is grown according to the shutter sequence in Fig. 2 (a), which is similar to previous report [9], where the shutter of In is kept open while the
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As and Sb shutters are alternately opened and closed. Besides, we present a new shutter sequence shown in Fig. 2 (b) to grow the InAsSb digital alloy of all other samples
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where the shutters of the group-III element (In) and group-V element (As or Sb) are kept open simultaneously for growing InAs and InSb layers and an interruption is
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introduced between their growth.
After the growth, Hall measurement with standard Van Der Pauw configuration is
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carried out to characterize the electronic properties of AlSb/InAs0.8Sb0.2 heterostructures. The alloy composition and crystalline quality are evaluated by double-crystal x-ray
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diffraction (XRD), using a Cu Kα source and (004) reflection. Transmission electron microscopy (TEM) is used to characterize the interfaces between AlSb and InAs0.8Sb0.2 layers and TEM samples are prepared via conventional mechanical thinning procedure
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followed by Ar+ ion milling. Raman scattering is performed in a back-scattering
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geometry to give comparisons among samples differed by growth conditions.
3. Results and discussion
In order to examine the variation of transport properties with different shutter
sequences during the growth of InAs0.8Sb0.2 layer, we first compare the electronic properties between sample A and sample B1 presented in Table 1. A five-second interruption is introduced between the growth of InAs and InSb layers in sample B1. At 300 K, electron mobility of 7,599 cm2/Vs could be achieved in sample A and increases to 16,170 cm2/Vs with shutter sequence employed for sample B1, which is higher than the structures stated in [10]. To further explore the impact of shutter sequence on transport properties, (004) double crystal x-ray diffraction spectra for sample A and sample B1 are plotted in Fig. 3. For sample B1, InAsSb peak is overlapping with AlSb peak, indicating As composition of InAsSb alloy to be 0.8 which is well correlated with the value we designed. For sample A, an additional peak is visible which corresponds to 3
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InAsSb alloy with As mole fraction of 0.7. We attribute the discrepancy of As composition with our designed value to interface mixing where In shutter is kept open with Sb and As shutters alternatively open and closed. As a result, residual Sb present in the growth chamber after the closing of Sb shutter might involve in the following
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growth of InAs, leading to decreased As composition in InAsSb alloy in sample A. Note
that narrower peak width is generally considered to reflect better crystalline quality. The
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full-width at half-maximum (FWHM) of X-ray rocking curve measurements for InAsSb layer is 302 arcsec in sample B1 and 397 arcsec in sample A, indicating our presented
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shutter sequence very useful for improving crystalline quality by minimizing interface mixing and accurately controlling composition in InAsSb alloy. As shown in Fig. 4,
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atomic force microscopy (AFM) measurement reveals a smooth surface for sample B1 and a rougher surface for sample A. Root-mean-square (rms) roughness over a 10×10
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μm2 area is measured to be 2.26 nm for sample B1 and 20 nm for sample A, in consistent with the superior electron mobility of sample B1 over sample A. It should be
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noted that apart from the InAsSb layer, structural parameters are identical between sample A and sample B1, so it is reasonable to assume that surface roughness
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characterized by AFM originates from fluctuations of the interfaces. According to
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theoretical and experimental result, interface-roughness scattering acts as a main factor to limit the electron mobility, so increased electron mobility in sample B1 is probably due to reduced scattering process from interface roughness [12,13]. To investigate the interface mixing more directly, the cross sectional TEM
micrographs of AlSb/InAs0.8Sb0.2 heterostructures of sample A and sample B1 are shown in Fig. 5, respectively. Flat interfaces between AlSb and InAsSb layers are clearly seen for sample B1 by introducing interruption between growth of InAs and InSb. In contrast, the flatness of upper interface between AlSb and InAsSb layers degraded for sample A, indicating serious intermixing at the interface. XRD measurement results show significant changes in alloy composition and thus reveal that interface mixing occurs at InAs/InSb digital alloy which contribute to degraded flatness in AlSb/InAsSb upper interfaces for sample A. It is concluded that sample B has sharp interfaces both at InAs/InSb and AlSb/InAsSb than that in sample A. It should be noted 4
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that InSb and InAs layer could hardly be distinguished due to their extreme small thickness. Consequently, MMBE has a significant effect in minimizing interface mixing for AlSb/InAsSb heterostructures. In order to further investigate the influence of shutter sequence on transport
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properties and examine interface mixing between InAs and InSb layers in AlSb/InAsSb
heterostructures, sample B2 and sample B3 are grown with shutter sequence shown in
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Fig. 2 (b), whereas different interruptions are introduced between InAs and InSb layers
presented in Table 1. In comparison between sample B1 and sample B2, an increased
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interruption from 5 seconds to 10 seconds after InAs growth yields electron mobility of 15,660 cm2/Vs for sample B2 which approximates to the value of 16,170 cm2/Vs for
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sample B1 at 300 K, indicating the impact of residual As in the growth chamber after InAs growth has little effect on total electron mobility of our AlSb/InAsSb
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heterostructures, which could be attributed to relatively small incorporation coefficient of As2 at the growth temperature of 470 °C. To investigate the incorporation of Sb2 into
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InAs layer, a comparison between sample B2 and sample B3 is presented in Table 1, where different interruptions are introduced after InSb growth but keep interruption
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after InAs growth identical. With a longer interruption after InSb growth, residual Sb in
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the growth chamber is reduced, confirmed by the decrease of reactor base pressure, which yields the electron mobility of 16,050 cm2/Vs for sample B3 at 300 K. The increase of electron mobility from sample B2 to sample B3 could be due to reduced incorporation of Sb2 into InAs layer when the interruption after InAs growth is increased from 5 seconds to 10 seconds, indicating relatively higher incorporation coefficient of Sb2 at the growth temperature of 470 °C which is well correlated to the deviation of Sb composition in InAs0.8Sb0.2 alloy in sample A as presented in Fig. 3. Based on the results above, a five second interruption after both InAs and InSb growth is enough to ensure high electron mobility, so the shutter sequence of sample B1 is selected for following discussions. The growth temperature of InAsSb layer is considered to be one of the most important parameters affecting 2DEG mobility. Serious intermixing would occur to deteriorate electron mobility at high growth temperatures [14]. In order to test the 5
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dependence of electron mobility on growth temperature, sample C grown at 470 ºC and sample D grown at 430 ºC are taken for comparison. These two samples have identical structural parameters other than the growth temperature of InAs0.8Sb0.2 layer. Hall measurements for sample C and sample D at 300 K are summarized in Table 1. Unlike
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previous report [15], electron mobility exceeding 14,000 cm2/Vs is attainable for sample C, compared to sample D of only 5,624 cm2/Vs at 300 K. Several groups have
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investigated the critical temperature for intermixing of anion species and determined
410 ºC for Sb intermixing with InAs and 350 ºC for As intermixing with InSb [16].
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Severe intermixing could be induced by increased growth temperature and cause dramatic decrease in electron mobility. To the contrary, our experimental results
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demonstrate that 470 ºC is more favorable to obtain high electron mobility in relative to 430 °C for AlSb/InAs0.8Sb0.2 heterostructures, where intermixing is reduced by taking
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the new shutter sequences we employed, allowing relatively higher growth temperature for the subsequent AlSb barrier and InAs cap layers.
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The dependence of transport properties on InAs0.8Sb0.2 thickness is further demonstrated by Fig. 6 where electron mobility and 2DEG concentration are plotted as
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a function of InAsSb thickness at 300 K. The period number of InAs/InSb superlattice is
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increased from 10 to 50. The growth temperature of InAs0.8Sb0.2 layer is fixed at 470 ºC and the shutter sequence is set as Fig. 2 (b) with five seconds interruption. For InAs/InSb superlattice ranging from 10 to 25 periods, electron mobility increases rapidly from approximately 6,000 cm2/Vs to 14,480 cm2/Vs, the trend of which has also been observed in AlInSb/InAsSb heterostructures by Kudo et al. [15]. Interface roughness scattering between AlSb and InAs0.8Sb0.2 layers is considered to be the dominant scattering mechanism in this region [17]. For InAs/InSb superlattice covering the range from 25 to 35 periods, electron mobility increases slowly from 14,480 cm2/Vs to 16,170 cm2/Vs, indicating that electron mobility is still governed by scattering process from interfaces between InAs0.8Sb0.2 and AlSb layers, however, contributions from interface roughness scattering between InAs and InSb layers begin to act. To further increase the InAsSb thickness, a decrease of electron mobility is observed from 16,170 cm2/Vs to 13,390 cm2/Vs, confirming that scattering process from interface 6
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roughness between InAs and InSb layers dominates over that between InAsSb and AlSb layers. Note that our structures are unintentionally doped, where 2DEG concentration remains at a low level. Further increasing electron mobility depends on improving 2DEG concentration through remote doping where Coulomb scattering could be
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minimized by screening effect.
Finally, to gain a more detailed understanding of variation of electron mobility with
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shutter sequence and growth temperature, Raman scattering is measured at room temperature in the background-configuration by using a 514.5 nm Ar+ ion laser. In Fig.
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7, we present Raman scattering spectra for sample A and sample B1 with both AlSb/InAs0.8Sb0.2 interfaces grown to be InSb-like. The data are collected in both and Z(X,Y)
configurations. For both sample A and sample B1, the peak at
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Z(X,X)
338 cm-1 is visible for AlSb-LO mode. InAs-LO mode and InSb-LO mode appear at 233
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cm-1 and 182 cm-1, respectively. The additional mode at 318 cm-1 represents AlSb-TO mode. The appearance of TO mode, which is forbidden in our configuration, could be
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attributed to scattering from imperfection of AlSb deposition process due to lattice
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mismatch between AlSb buffer layer and GaAs substrate. It should be emphasized that the appearance of a broad band ranging from 100 cm-1 to 200 cm-1 is considered to be
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the disorder-activated longitudinal acoustic (DALA) vibrational mode which originates from alloy disorder or strain disorder in the alloy [18]. The absence of DALA region for sample B1 could be attributed to well defined interfaces between InAs and InSb layers and no occurrence of interface intermixing, indicating that shutter sequence has a significant effect on total electron
mobility
through interface
mixing
for
AlSb/InAs0.8Sb0.2 heterostructures. In contrast, DALA mode is observed in sample A which is due to alloy disorder in InAs/InSb superlattices induced by intermixing between InAs and InSb layers, in consistent with deviation of As compositions confirmed by XRD measurements mentioned above. Temperature dependent electron mobility is also examined by Raman scattering spectra presented in Fig. 8. All peaks, including AlSb-LO mode, AlSb-TO mode, InAs-LO mode, and InSb-LO mode are visible for both sample C and sample D. Calculations have shown that interface mode 7
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density of vibrational states depends on interface roughness [19]. Localization decouples and thereby reducing the energy exchange, allowing the interface mode to vibrate at larger amplitude [20]. Therefore, stronger peaks are expected for rougher interfaces in Raman spectra. It should be noted our InAs/InSb superlattice begins with
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InAs layer and ends to InAs layer as well. Consequently, intensity of InAs-LO mode could be representative of degree of interface roughness of AlSb/InAs0.8Sb0.2 interfaces.
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According to Raman spectra analysis, InAs-LO peak intensity is 12,113 for sample C
(grown at 470 °C) inferior to 14,130 of sample D (grown at 430 °C), indicating that the
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scattering, well correlated with mobility values.
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lower electron mobility in sample D corresponds to more severe interface roughness
4. Conclusion
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In summary, AlSb/InAs0.8Sb0.2 heterostructures have been successfully grown on GaAs substrate by MMBE. New shutter sequence has been presented and proved to be very useful for reducing interface mixing and accurately controlling Sb composition in
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InAsSb alloy. Room temperature mobility of 16,170 cm2/Vs is attainable with our
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non-intentionally doped structures. With a view for optimization, we analyze variation
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of electron mobility induced by growth temperature and InAsSb thickness. With increased growth temperature and InAsSb thickness, improvement of electron mobility has been reported. Raman analysis is also demonstrated to further evaluate the crystalline quality of the epi-layers from the perspective of crystal disorder and interface roughness. Despite that further improvement is desirable on transport properties of AlSb/InAsSb heterostructures, it is evident that our structures are promising for high-speed applications.
Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant No. 61204012), Beijing Natural Science Foundation (Grant No. 2112040), and Beijing Nova Program (Grant No. 2010B056). 8
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References [1] W. K. Liu, X. Zhang, W. Ma, J. Winesett, M. B. Santos, J. Vac. Sci. Technol. B 14 (1996) 2339.
Emeny, P. D. Buckle, T. Ashley, Phys. Rev. B 77 (2008) 165334.
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[2] J. M. S. Orr, A. M. Gilbertson, M. Fearn, O. W. Croad, C. J. Storey, L. Buckle, M. T.
[3] T. Ashley, A. B. Dean, C. T. Elliott, G. J. Pryce, A. D. Johnson, H. Willis, Appl. Phys.
cr
Lett. 66 (1995) 481.
[4] R. Magno, J. G. Champlain, H. S. Newman, M. G. Ancona, J. C. Culberson, B. R.
us
Bennett, J. B. Boos, D. Park, Appl. Phys. Lett. 92 (2008) 243502.
[5] I. Vurgaftman , J. R. Meyer, L. R. Ram-Mohan, J. Appl. Phys. 89 (2001) 5815.
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[6] S. S. Kizhaev, S. S. Molchanov, N. V. Zotova, E. A. Grebenshchikova, Y. P. Yakolev, E. Hulicius, T. Simecek, K. Melichar, J. Pangrac, Techonol. Phys. Lett. 27 (2001)
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964.
[7] M. J. Yang, B. R. Bennett, M. Fatemi, P. J. Lin-Chung, W. J. Moore, C. H. Yang, J.
d
Appl. Phys. 87 (2000) 8192.
[8] R. Kaspi, G. P. Donati, J. Crystal Growth 251 (2003) 515.
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[9] Y. H. Zhang, J. Crystal Growth 150 (1995) 838.
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[10] J. B. Boos, M. J. Yang, B. R. Bennett, D. Park, W. Kruppa, R. Bass, Electron. Lett. 35 (1999) 847.
[11] H. Miyoshi, R. Suzuki, H. Amano, Y. Horikoshi, J. Crystal Growth 237-239 (2002) 1519.
[12] Y. Zhang, Y. Zhang, M. Guan, L. Cui, C. Wang, Y. Zeng, J. Appl. Phys. 114 (2013) 153707.
[13] C. R. Bolognesi, H. Kroemer, J. H. English, Appl. Phys. Lett. 61 (1992) 213. [14] B. P. Tinkham, B. R. Bennett, R. Magno, B. V. Shanabrook, J. B. Boos, J. Vac. Sci. Technol. B 23 (2005) 1441. [15] M. Kudo, T. Mishima, T. Tanaka, J. Vac. Sci. Technol. B 18 (2000) 746. [16] B. R. Bennett, B. V. Shanabrook, M. E. Twigg, J. Appl. Phys. 85 (1999) 2157. [17] Y. Li, Y. Zhang, Y. Zeng, J. Appl. Phys. 109 (2011) 073703. [18] A. Sayari, N. Yahyaoui, A. Meftah, A. Sfaxi, M. Oueslati, J. Lumin. 129 (2009) 9
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105. [19] D. Kerchrakos, J. C. Inkson, Semicond. Sci. Technol. 6 (1991) 155. [20] I. Sela, C. R. Bolognesi, L. A. Samoska, H. Kroemer, Appl. Phys. Lett. 60 (1992)
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Fig. 1 Schematic of AlSb/InAs0.8Sb0.2 heterostructure.
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Fig. 2
Schematic time sequence of In, As and Sb shutter positions during the MMBE growth
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of AlSb/InAs0.8Sb0.2, (a) for sample A and (b) for sample B1, sample B2, sample C, and
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sample D.
Fig. 3
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Symmetric omega-2theta scan in the vicinity of the (004) Bragg peaks for GaAs, AlSb,
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and InAsSb for sample A (upper trace) and sample B1 (lower trace).
Fig. 4
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AFM image of sample A (a) and sample B1 (b).
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Fig. 5
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Cross-sectional TEM micrograph of AlSb/InAs0.8Sb0.2 heterostructures for sample B1 (a) and sample A (b).
Fig. 6
Electron mobility and sheet electron concentration of AlSb/InAs0.8Sb0.2 heterostructures as a function of period numbers of InAs/InSb superlattice at 300 K.
Fig. 7 Comparison of Raman scattering spectra between sample A and sample B1 at 300 K with both AlSb/InAs0.8Sb0.2 interfaces grown to be InSb-like. The data are collected in both Z(X,X)
and Z(X,Y)
configurations.
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Fig. 8 Comparison of Raman scattering spectra for AlSb/InAs0.8Sb0.2 heterostructures between
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sample C and sample D at 300 K.
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Table 1. AlSb/InAs0.8Sb0.2 heterostructure samples and their related electron mobility from Hall measurements with Van Der Pauw configuration at 300 K.
sequence
InAs0.8Sb0.2
after InAs
after InSb
temperature
thickness
(s)
(s)
(°C)
(nm)
Fig. 2 (a)
0
0
470
B1
Fig. 2 (b)
5
5
470
B2
Fig. 2 (b)
10
5
B3
Fig. 2 (b)
10
10
C
Fig. 2 (b)
5
5
D
Fig. 2 (b)
5
5
mobility 300K
(cm2/Vs)
21.5
7599
21.5
16170
470
21.5
15660
470
21.5
16050
470
15.4
14480
430
15.4
5624
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A
Electron
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Shutter
Growth
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Sample
Interruption Interruption
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Fig. 1
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Fig. 2
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Fig. 3
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cr
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Fig. 4 (a)
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Fig. 4 (b)
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AlSb
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Fig. 5 (a)
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AlSb
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InAsSb
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Fig. 5 (b)
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AlSb
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InAsSb
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AlSb
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cr
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Fig. 6
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Fig. 7
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Fig. 8
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Highlights:
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AlSb/InAsSb heterostructures have been successfully grown by MBE with electron mobility exceeding 16000 cm2/Vs. We present a new shutter sequence to reduce interface mixing and accurate control of Sb composition in InAsSb alloy Higher temperature could be applied for InAsSb growth with good crystalline quality, allowing relatively higher growth temperature for the subsequent AlSb barrier. Raman spectra is used to examine interface mixing between AlSb and InAsSb layers
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