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Structural properties of Si-doped β-Ga2O3 layers grown by MOVPE
Journal of Crystal Growth ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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Structural properties of Si-doped β-Ga2O3 layers grown by MOVPE D. Gogova n,1, G. Wagner, M. Baldini, M. Schmidbauer, K. Irmscher, R. Schewski, Z. Galazka, M. Albrecht, R. Fornari Leibniz Institute for Crystal Growth, Max-Born-Str. 2, 12489 Berlin, Germany
art ic l e i nf o
Keywords: A1. High resolution X-ray diffraction A3. Organometallic vapor phase epitaxy B1. Oxides B2. Semiconducting materials
a b s t r a c t Thin films of β-Ga2O3 doped with Si are grown on Al2O3 (0001) and β-Ga2O3 (100) substrates by metal organic vapor phase epitaxy. Homoepitaxial growth of single-phase, smooth β-Ga2O3 layers doped with Si, with a dislocation density not exceeding those of the melt grown substrate, was demonstrated. The interplay between growth conditions, structural and electrical properties of the Ga-oxide layers is studied. XRD and HRTEM show that Si-doping in the concentration range 1017–1018 cm 3 does not deteriorate the quality of the Ga-oxide layers compared to undoped ones. The different nature of defects in undoped and doped material is investigated by HRTEM. It is found out that the density of twins and stacking faults on a-planes are an order of magnitude lower in β-Ga2O3:Si than in undoped material, while defects on inclined planes have rarely been observed in β-Ga2O3. & 2013 Elsevier B.V. All rights reserved.
1. Introduction Although known since decades, only recently III-sesquioxides have received attention as a new class of wide-band-gap semiconductors. In the past polycrystalline highly doped In2O3:Sn was actually used as high-conductivity material for transparent electrodes in “smart windows” [1], photovoltaics [2] or as chemical sensors [3]. Nowadays, the research is focusing on single-crystalline oxide layers with low defect densities and semiconducting behavior. The β-Ga2O3 is the most attractive representative of this class of materials due to the large optical band-gap (4.9 eV) and high break-down field (8 MV/cm) promising applications in transparent electronics [4], short wavelength photonics, optoelectronics, etc. We have grown undoped epitaxial films of the thermodynamically stable β-Ga2O3 phase by metal organic vapor phase epitaxy (MOVPE) which turned out to be insulating owing to the large band gap [5]. To make them semiconducting, impurities such as Sn or Si have to be incorporated on Ga sites. A theoretical study predicted that Si is an efficient n-type dopant in β-Ga2O3 [6]. We have chosen to study the behavior of Si dopants since the Si atomic radius is smaller than that of Sn, which should favor its incorporation. In the literature, however, the Si-doping is discussed controversially [7,8], although there is some consensus that Si substitutes Ga in the crystal lattice. Study of bulk β-Ga2O3 grown by the floating zone method [7] evidenced that single crystals
n
Corresponding author. Tel.: þ 49 30 6392 2841. E-mail addresses: [email protected], [email protected] (D. Gogova). 1 On leave from Central Lab of Solar Energy at the Bulgarian Academy of Sciences, 72 Tzarigradsko Chaussee Blvd., 1784 Sofia, Bulgaria.
were contaminated by Si from the residual in the Ga2O3 source powder. The crystals were n-type with electron concentrations in the range from 1016 cm 3 to 1018 cm 3 and the contribution of oxygen vacancies, if any, was considered not to be dominant [7]. The present paper reports on MOVPE growth and characterization study of Si-doped β-Ga2O3 (β-Ga2O3:Si) layers. It aims also at checking the Si-doping potential in the MOVPE process. To the best of our knowledge such an investigation does not exist in the literature. The interplay between growth regimes, structural and electrical properties of Ga-oxide layers will be discussed. 2. Experimental 2.1. Epitaxial growth of Si-doped β-Ga2O3 thin films We have deposited epitaxial thin films of β-Ga2O3 on sapphire (0001) and on native (home-made) β-Ga2O3 (100) substrates [9]. The sapphire and Ga-oxide substrates undergo a thermal treatment before the growth to obtain a damage-free surface with 200 pm high atomic steps. Low-pressure MOVPE with trimethylgallium (TMG) as a source of gallium and water vapors as a source of oxygen were employed for this study. As a silicon source we have taken tetra-ethyl-ortho-silicate (TEOS). The TEOS bubbler temperature was kept at 0 1C and of TMG: 5 and 10 1C. The reactor base pressure in this series of experiments was fixed to 5 and 10 mbar. The flow rate of Ar carrier gas through the water bubbler was in the range 200–750 sccm. For optimal growth of Sidoped layers the deposition temperature of 800 1C was selected. Some growth experiments at higher temperatures (up to 850 1C) were done as well. During the heating up and cooling down stage
0022-0248/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jcrysgro.2013.11.056
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the substrates and the thin films were kept in O2-containing atmosphere to prevent their decomposition. The thickness of the β-Ga2O3:Si layers was varied in the 80–250 nm range. Some additional annealing processes for 1 h at a temperature ranging from 850 to 1050 1C in O2 atmosphere were performed. 2.2. Characterization
3. Results and discussions 3.1. Morphology and compositional study All thin films of β-Ga2O3:Si, deposited at 800 1C were homogeneous, highly transparent and with a smooth surface. A root mean square (RMS) roughness ranging from 0.4 nm to 0.8 nm was evaluated on the basis of the AFM images (not shown here). SEM study was also performed in order to have another independent assessment of the surface morphology. Further increase of the growth temperature to 815, 825 and 850 1C resulted in noncontinuous layers. All films were grown at O2-rich conditions because it had been experimentally observed to improve the crystalline quality. A H2O/Ga molar ratio from 400 to 1000 was employed. By adjustment of TMGa to Si molar ratios from 88 to 2285, a series of β-Ga2O3:Si layers with Si concentration in the range from 4 1017 to 5 1019 cm 3 (determined by SIMS – see Fig. 1) was prepared. Fig. 1 demonstrates the homogeneous Si depth distribution in the grown layers. Fig. 2 displays the concentration of the incorporated Si in dependence on the Si molar flux. Black squares symbolize the experimental points. The red line represents the linear least-square fit (slope ¼ 0.98) to these points in the log-log plot indicating direct proportionality.
1: 9.8x10-7 (mol/min)
Concentration (at/cm3)
1022
2: 3.9x10-7 (mol/min)
1021
3: 5.2x10-8 (mol/min)
1020
4: 6.2x10-9 (mol/min) 2 1
1019
3
1018
4
1017 1016 0.00
0.05
0.10
0.15
0.20
0.25
Depth (µm) Fig. 1. SIMS depth profiles of the Si concentration in four β-Ga2O3 samples grown on Al2O3 (0001) with different Si molar flux.
1019
1018
1017 10-9
10-8
10-7
10-6
TEOS flux (mol/min) Fig. 2. Concentration of the incorporated Si in dependence on the Si molar flux.
109 108 10
Intensity (cps)
The thin film surface morphology was characterized by atomic force microscopy (AFM) and scanning electron microscopy (SEM). The Si concentration was determined by secondary ion mass spectrometry (SIMS) using a standard, prepared by Si ion implantation in a β-Ga2O3 bulk crystal. The thickness of the samples was measured by ellipsometry and was confirmed independently by SEM and SIMS. The crystalline quality of the layers, grown on sapphire, was investigated by X-ray diffraction (XRD). Highresolution XRD experiments (HRXRD) were employed in a triplecrystal setup in order to record X-ray reciprocal space maps with an angular resolution of about 11 arcsec for the layers grown homoepitaxially. Transmission electron microscopy (TEM) investigations have been done with an aberration corrected microscope at an acceleration voltage of 300 kV.
1020
Si concentration (at/cm3)
2
7
Al2O3
Al2O3
00.6
00.12
Ga2O3
106
-402
105
Ga2O3
Al2O3
Ga2O3
-603
00.9
-804
104 103 102
x 100
(a)
x 10
(b)
101
(c)
100 30
40
50
60
70
80
90
100
2θ (degrees) Fig. 3. 2θ–ω scans for β-Ga2O3:Si films grown on Al2O3 (0001) with different Si concentrations: (a) 4 1017 cm 3, (b) 3 1018 cm 3 and (c) 5 1019 cm 3.
3.2. Structural study 3.2.1. X-ray investigations of the structure Fig. 3 illustrates three typical XRD spectra (2θ–ω-scans) of β-Ga2O3:Si layers grown at 800 1C on Al2O3 (0001) with different Si doping. The three sharp peaks at 2θ ¼ 41.691, 64.51 and 90.721 can be identified as the 00.6, 00.9 and 00.12 Bragg reflections of the c-plane sapphire, respectively. Additional peaks observed at 2θ ¼37.81, 58.71 and 80.71 (curves a and b) are caused by the epitaxial layers and can be assigned to the 402, 603 and 804 Bragg reflections of the monoclinic modification of Ga-oxide – the β-phase (Ref. [10]). From Fig. 3 it can be stated that there exists a well-defined epitaxial relationship with the hexagonal (0001) Al2O3 and that the β-Ga2O3:Si layers are ( 201)-oriented. It is striking that the width of the 603 Bragg reflection of the βGa2O3 layers is very large whereas the widths of the 402 and 804 Bragg reflections are quite small. This peculiarity is not fully understood yet. We, however, suspect that a high density of stacking faults (SFs), which leads to a large vertical disorder in diffracting lattice planes, is the reason for this characteristic behavior. A similar behavior has been observed in (Na,Bi)TiO3 thin films and could be theoretically described by vertical stacking disorder [11]. The role of SFs will be discussed later in Section 3.2.2. The impact of Si-doping on the structural quality of β-Ga2O3: Si films is obvious in Fig. 3. For Si-doping levels between 4 1017 and 3 1018 cm 3 pronounced β-Ga2O3 Bragg reflections can be observed – (curves a and b) proving that β-Ga2O3:Si films are crystalline. On the other hand, the lack of β-Ga2O3 Bragg peaks
Please cite this article as: D. Gogova, et al., Journal of Crystal Growth (2013), http://dx.doi.org/10.1016/j.jcrysgro.2013.11.056i
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3
104 as-grown Ga O 2 3
Intensity (cps)
103
-402 annealed
102
101 bulk
100 36.5 37.0 37.5 38.0 38.5 39.0 39.5
2 theta (degree) Fig. 4. Positions of the Bragg reflection ( 402) from 2θ–ω scans of β-Ga2O3 layers grown on Al2O3 (0001) with different Si-doping given in the legend.
Fig. 5. X-ray reciprocal space map of a homoepitaxial β-Ga2O3:Si layer on β-Ga2O3 (100) substrate in the vicinity of the 600 reciprocal lattice point. “S” and “L” mark the position of the β-Ga2O3 substrate and layer Bragg reflections, respectively.
indicates amorphization of the films (curve c) when the Si-doping is higher than 5 1019 cm 3. Fig. 4 illustrates the position of the ( 402) reflex of β-Ga2O3 for layers grown at equivalent growth conditions but at different Si-doping concentrations. It is obvious that all positions of the peak ( 402) are shifted to the smaller 2θ, i.e. to a larger out-ofplane lattice parameter, compared to bulk material. Annealing of the samples in O2 leads to shifts of the peak positions to that of bulk, i.e. there is a lattice relaxation. It might be caused by recovery of some of oxygen vacancies and/or reducing of lattice defects during the annealing [5]. Homoepitaxial growth of β-Ga2O3:Si layers on bulk β-Ga2O3 (100) was successfully performed. In Fig. 5 an X-ray reciprocal space map of a β-Ga2O3:Si layer grown on (100) β-Ga2O3 substrate is shown. Besides the strong substrate reflection (marked as “S”) a broad feature (marked as “L”) arising from the β-Ga2O3 layer is observed. The center of mass of this feature deviates from the substrate peak position indicating a larger vertical lattice
Fig. 6. (a) A typical bright-field TEM image of an undoped homoepitaxial β-Ga2O3 layer and (c) of β-Ga2O3:Si layer; (b and d) HRSTEM images of the samples from (a) and (c), respectively.
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4
parameter of the β-Ga2O3 layer as compared to the substrate. This could e.g. be caused by an oxygen deficiency in the β-Ga2O3:Si layer which is known to lead to a larger lattice parameter in perovskite oxides [12]. However, the observed difference in vertical lattice spacing of about 0.8% is rather large and would imply a very large oxygen deficiency. As an alternative explanation also SFs could lead to a change of the mean vertical lattice parameter. More details on homoepitaxy will be reported additionally. Here we will focus our study on the impact of Si on the layer crystalline quality.
3.2.2. TEM investigation of the structure Fig. 6a and c shows typical bright-field TEM images of a Si-doped and an undoped homoepitaxial β-Ga2O3 layers. Both layers are single-crystalline and epitaxially grown but contain high density of planar defects. As can be seen already from the bright-field images, planar defects in undoped layers are limited to the a-plane, while they lie exclusively on inclined planes in β-Ga2O3:Si. A better insight into the nature and atomic structure of these defects comes from the HRSTEM images performed with a high angle annular dark-field detector (Fig. 6b and d). In these images the Ga columns appear bright (the unit cell is inserted into the image). Fig. 6b shows the planar defects in the undoped layer. They are essentially twins and SFs on a-planes that can be described by a c/2 glide reflection of the lattice. In the case of β-Ga2O3:Si layers (Fig. 6c) the defects lie on inclined planes. The contrast pattern originates from a superposition of the two b-projected lattices shifted by 223 pm in a-direction and 216 pm in c-direction. This corresponds to a shift of the lattices along the (20 1) planes on which only octahedral coordinated gallium atoms are placed. The full analysis of the displacement vector of these defects requires analyses in different crystallographic directions and is in progress. It is interesting to note that the density of twins and SFs on a-planes are an order of magnitude lower in β-Ga2O3:Si, while defects on inclined planes have rarely been observed in undoped material. Planar defects on inclined planes have been also observed in In-doped layers (to be reported additionally). Their formation mechanism will be studied by refined analyses of the initial stages of growth.
3.3. Optical transmission and electrical properties study Fig. 7 shows the internal spectral transmission of an as-grown
β-Ga2O3:Si layer on sapphire (Si ¼ 4 1018 cm 3) in comparison
to the transmission of this layer after annealing in oxygen for 1 h at 800 1C and 850 1C, respectively. The spectra are related to the 100
Transmission (%)
80
60 as grown 800 °C, 1 h, O2 850 °C, 1 h, O2
40
4. Conclusions
20
0 200
300
400
500
600
700
transmission of the sapphire substrate yielding approximately the internal transmittance. The distinctly increased transmission after annealing is due to the defect reduction already observed by XRD (Fig. 4). Obviously, the resulting β-Ga2O3:Si layers are highly transparent in the UV-VIS-IR range of the spectrum. Although SIMS shows that Si is incorporated in the films Hall effect measurements performed at RT demonstrate that the resulting material is not electrically conductive. In an attempt to electrically activate the Si-species, the epitaxial layers were annealed in O2 at temperatures from 800 to 1050 1C with steps of 50 1C for 1 h. However, Hall and conductivity measurements carried out on the layers deposited on sapphire, and capacitancevoltage measurements performed on oxide layers grown on native substrates showed that the films were not-conductive. One reason for that behavior could be the formation of SiO2 as a second phase in the β-Ga2O3 (not measurable by XRD). This is very likely thinking of the large affinity of Si to oxygen. In this way the Si is passivated and not electrically active. Another reason for the low conductivity of our β-Ga2O3:Si films could come from the compensation of the Si-donors by dislocations and SFs. Further investigations of the complexes formed by Si in the β-Ga2O3 lattice by X-ray photoelectron spectroscopy and positron annihilation lifetime spectroscopy are in progress. Based on DFT, Varley et al. [6] calculated the formation energies and charge state transition levels for oxygen vacancies and donor impurities in β-Ga2O3. They found that Si, Ge, Sn, and H behave as shallow donors. The ionization energy of the isolated Si donor, determined by Irmscher et al. [9], is 36 meV and at RT nearly all donors should be ionized. Why is the electrical activity of the Si-species so poor and why are we unable to observe free electrons? Unfortunately the literature does not help much in this regard. For example, Takakura et al. [8] investigated Si-doping in β-Ga2O3 films deposited by RF magnetron co-sputtering of Gaoxide and Si targets. The Si incorporated in the films was determined by EDX to be from 0 to 13%. They found out that the conductivity of the β-Ga2O3:Si films does not increase with Si-doping and ascribed this experimental evidence to an increase of the β-Ga2O3:Si band gap. However, we do not observe any change in the optical band gap for the Si-doping range typical for semiconductor doping. Sasaki et al. [13] reported on Si-doping by ion implantation of bulk crystals synthesized by the floating zone method. For Si ¼1– 5 1019 cm 3, an activation efficiency of above 60% was obtained after annealing in N2 at a temperature of 900–1000 1C. Our experiments with MOVPE β-Ga2O3:Si, doped in the same range, as-grown and exposed to annealing in O2 or N2, however, provided non-conductive material. At this stage we are inclined to believe that this may derive from formation of some complexes of structural defects with unidentified yet impurities that passivate the Si donor. Theoretical studies have shown that the formation of Ga-vacancies at oxygen-rich conditions (like ours) may have even negative formation energy and carbon on oxygen site may behave as a double acceptor. More investigations are being performed to highlight these aspects.
800
Wavelength (nm) Fig. 7. Spectral transmission of a β-Ga2O3:Si (Si ¼ 4 1018 cm 3) layer on sapphire for the as-grown state and after annealing in oxygen for 1 h at 800 1C and 850 1C, respectively.
We demonstrated that monoclinic β-phase Ga2O3 thin films doped with Si can be grown on Al2O3 (0001) substrates by MOVPE. The β-Ga2O3:Si films are ( 201)-oriented and exhibit an epitaxial relationship with the hexagonal substrate. SIMS investigations showed that Si may be incorporated up to about 5 1019 cm 3 but Hall and CV measurements confirmed a very low conductivity of both as-grown and annealed films. We are presently working to understand whether the bad electrical activity of Si is due to
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passivation of the incorporated Si atoms, to misplacement within the β-Ga2O3 matrix or rather to compensation by structural and point defects. Homoepitaxial growth of single phase, smooth β-Ga2O3:Si layers is demonstrated. It has been discovered that the Si incorporation changes the type of the defect structures. Acknowledgments This work was supported by Grant no SAW-2012-IKZ-2 from the Leibniz-Gemeinschaft. We thank R. Grueneberg for the technical assistance and A. Kwasniewski for the XRD measurements. References [1] D. Gogova, L. -K. Thomas, B. Camin, Thin Solid Films 517 (11) (2009) 3326. [2] J. Herrero, C. Guillén, Thin Solid Films 451–452 (2004) 630. [3] P. Sowti Khiabani, E. Marzbanrad, C. Zamani, R. Riahifar, B. Raissi, Sens. Actuators B: Chem. 166–167 (2012) 128.
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[4] J. Wager, Science 300 (5623) (2003) 1245. [5] G. Wagner, M. Baldini, D. Gogova, M. Schmidbauer, R. Schewski, M. Albrecht, Z. Galazka, D. Klimm, R. Fornari, Phys. Status Solidi A, 1–7 (2013) / http://dx.doi. org/10.1002/pssa.201330092A. [6] J.B. Varley, J.R. Weber, A. Janotti, C.G. Van de Walle, Appl. Phys. Lett. 97 (2010) 142106. [7] E.G. Víllora, K. Shimamura, Y. Yoshikawa, T. Ujiie, K. Aoki, Appl. Phys. Lett. 92 (2008) 202120. [8] K. Takakura, S. Funasaki, I. Tsunoda, H. Ohyama, D. Takeuchi, T. Nakashima, M. Shibuya, K. Murakami, E. Simoen, C. Claeys, Phys. B: Condens. Matter 407 (2012) 2900. [9] K. Irmscher, Z. Galazka, M. Pietsch, R. Uecker, R. Fornari, J. Appl. Phys. 110 (2011) 063720. [10] Joint Committee on Powder Diffraction Standards (JCPDS, PDF No. 43043-1012 and 041-1103). [11] V. Kopp, V. Kaganer, J. Schwarzkopf, F. Waidick, T. Remmele, A. Kwasniewski, M. Schmidbauer, Acta Crystallogr. A 68 (2012) 148–155. [12] C.M. Brooks, L. Fitting Kourkoutis, T. Heeg, J. Schubert, D.A. Muller, D.G. Schlom, Appl. Phys. Lett. 94 (2009) 162905. [13] K. Sasaki, M. Higashiwaki, A. Kuramata, T. Masui, S. Yamakoshi, Appl. Phys. Express 6 (2013) 086502.
Please cite this article as: D. Gogova, et al., Journal of Crystal Growth (2013), http://dx.doi.org/10.1016/j.jcrysgro.2013.11.056i
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Structural properties of Si-doped β-Ga2O3 layers grown by MOVPE D. Gogova n,1, G. Wagner, M. Baldini, M. Schmidbauer, K. Irmscher, R. Schewski, Z. Galazka, M. Albrecht, R. Fornari Leibniz Institute for Crystal Growth, Max-Born-Str. 2, 12489 Berlin, Germany
art ic l e i nf o
Keywords: A1. High resolution X-ray diffraction A3. Organometallic vapor phase epitaxy B1. Oxides B2. Semiconducting materials
a b s t r a c t Thin films of β-Ga2O3 doped with Si are grown on Al2O3 (0001) and β-Ga2O3 (100) substrates by metal organic vapor phase epitaxy. Homoepitaxial growth of single-phase, smooth β-Ga2O3 layers doped with Si, with a dislocation density not exceeding those of the melt grown substrate, was demonstrated. The interplay between growth conditions, structural and electrical properties of the Ga-oxide layers is studied. XRD and HRTEM show that Si-doping in the concentration range 1017–1018 cm 3 does not deteriorate the quality of the Ga-oxide layers compared to undoped ones. The different nature of defects in undoped and doped material is investigated by HRTEM. It is found out that the density of twins and stacking faults on a-planes are an order of magnitude lower in β-Ga2O3:Si than in undoped material, while defects on inclined planes have rarely been observed in β-Ga2O3. & 2013 Elsevier B.V. All rights reserved.
1. Introduction Although known since decades, only recently III-sesquioxides have received attention as a new class of wide-band-gap semiconductors. In the past polycrystalline highly doped In2O3:Sn was actually used as high-conductivity material for transparent electrodes in “smart windows” [1], photovoltaics [2] or as chemical sensors [3]. Nowadays, the research is focusing on single-crystalline oxide layers with low defect densities and semiconducting behavior. The β-Ga2O3 is the most attractive representative of this class of materials due to the large optical band-gap (4.9 eV) and high break-down field (8 MV/cm) promising applications in transparent electronics [4], short wavelength photonics, optoelectronics, etc. We have grown undoped epitaxial films of the thermodynamically stable β-Ga2O3 phase by metal organic vapor phase epitaxy (MOVPE) which turned out to be insulating owing to the large band gap [5]. To make them semiconducting, impurities such as Sn or Si have to be incorporated on Ga sites. A theoretical study predicted that Si is an efficient n-type dopant in β-Ga2O3 [6]. We have chosen to study the behavior of Si dopants since the Si atomic radius is smaller than that of Sn, which should favor its incorporation. In the literature, however, the Si-doping is discussed controversially [7,8], although there is some consensus that Si substitutes Ga in the crystal lattice. Study of bulk β-Ga2O3 grown by the floating zone method [7] evidenced that single crystals
n
Corresponding author. Tel.: þ 49 30 6392 2841. E-mail addresses: [email protected], [email protected] (D. Gogova). 1 On leave from Central Lab of Solar Energy at the Bulgarian Academy of Sciences, 72 Tzarigradsko Chaussee Blvd., 1784 Sofia, Bulgaria.
were contaminated by Si from the residual in the Ga2O3 source powder. The crystals were n-type with electron concentrations in the range from 1016 cm 3 to 1018 cm 3 and the contribution of oxygen vacancies, if any, was considered not to be dominant [7]. The present paper reports on MOVPE growth and characterization study of Si-doped β-Ga2O3 (β-Ga2O3:Si) layers. It aims also at checking the Si-doping potential in the MOVPE process. To the best of our knowledge such an investigation does not exist in the literature. The interplay between growth regimes, structural and electrical properties of Ga-oxide layers will be discussed. 2. Experimental 2.1. Epitaxial growth of Si-doped β-Ga2O3 thin films We have deposited epitaxial thin films of β-Ga2O3 on sapphire (0001) and on native (home-made) β-Ga2O3 (100) substrates [9]. The sapphire and Ga-oxide substrates undergo a thermal treatment before the growth to obtain a damage-free surface with 200 pm high atomic steps. Low-pressure MOVPE with trimethylgallium (TMG) as a source of gallium and water vapors as a source of oxygen were employed for this study. As a silicon source we have taken tetra-ethyl-ortho-silicate (TEOS). The TEOS bubbler temperature was kept at 0 1C and of TMG: 5 and 10 1C. The reactor base pressure in this series of experiments was fixed to 5 and 10 mbar. The flow rate of Ar carrier gas through the water bubbler was in the range 200–750 sccm. For optimal growth of Sidoped layers the deposition temperature of 800 1C was selected. Some growth experiments at higher temperatures (up to 850 1C) were done as well. During the heating up and cooling down stage
0022-0248/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jcrysgro.2013.11.056
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the substrates and the thin films were kept in O2-containing atmosphere to prevent their decomposition. The thickness of the β-Ga2O3:Si layers was varied in the 80–250 nm range. Some additional annealing processes for 1 h at a temperature ranging from 850 to 1050 1C in O2 atmosphere were performed. 2.2. Characterization
3. Results and discussions 3.1. Morphology and compositional study All thin films of β-Ga2O3:Si, deposited at 800 1C were homogeneous, highly transparent and with a smooth surface. A root mean square (RMS) roughness ranging from 0.4 nm to 0.8 nm was evaluated on the basis of the AFM images (not shown here). SEM study was also performed in order to have another independent assessment of the surface morphology. Further increase of the growth temperature to 815, 825 and 850 1C resulted in noncontinuous layers. All films were grown at O2-rich conditions because it had been experimentally observed to improve the crystalline quality. A H2O/Ga molar ratio from 400 to 1000 was employed. By adjustment of TMGa to Si molar ratios from 88 to 2285, a series of β-Ga2O3:Si layers with Si concentration in the range from 4 1017 to 5 1019 cm 3 (determined by SIMS – see Fig. 1) was prepared. Fig. 1 demonstrates the homogeneous Si depth distribution in the grown layers. Fig. 2 displays the concentration of the incorporated Si in dependence on the Si molar flux. Black squares symbolize the experimental points. The red line represents the linear least-square fit (slope ¼ 0.98) to these points in the log-log plot indicating direct proportionality.
1: 9.8x10-7 (mol/min)
Concentration (at/cm3)
1022
2: 3.9x10-7 (mol/min)
1021
3: 5.2x10-8 (mol/min)
1020
4: 6.2x10-9 (mol/min) 2 1
1019
3
1018
4
1017 1016 0.00
0.05
0.10
0.15
0.20
0.25
Depth (µm) Fig. 1. SIMS depth profiles of the Si concentration in four β-Ga2O3 samples grown on Al2O3 (0001) with different Si molar flux.
1019
1018
1017 10-9
10-8
10-7
10-6
TEOS flux (mol/min) Fig. 2. Concentration of the incorporated Si in dependence on the Si molar flux.
109 108 10
Intensity (cps)
The thin film surface morphology was characterized by atomic force microscopy (AFM) and scanning electron microscopy (SEM). The Si concentration was determined by secondary ion mass spectrometry (SIMS) using a standard, prepared by Si ion implantation in a β-Ga2O3 bulk crystal. The thickness of the samples was measured by ellipsometry and was confirmed independently by SEM and SIMS. The crystalline quality of the layers, grown on sapphire, was investigated by X-ray diffraction (XRD). Highresolution XRD experiments (HRXRD) were employed in a triplecrystal setup in order to record X-ray reciprocal space maps with an angular resolution of about 11 arcsec for the layers grown homoepitaxially. Transmission electron microscopy (TEM) investigations have been done with an aberration corrected microscope at an acceleration voltage of 300 kV.
1020
Si concentration (at/cm3)
2
7
Al2O3
Al2O3
00.6
00.12
Ga2O3
106
-402
105
Ga2O3
Al2O3
Ga2O3
-603
00.9
-804
104 103 102
x 100
(a)
x 10
(b)
101
(c)
100 30
40
50
60
70
80
90
100
2θ (degrees) Fig. 3. 2θ–ω scans for β-Ga2O3:Si films grown on Al2O3 (0001) with different Si concentrations: (a) 4 1017 cm 3, (b) 3 1018 cm 3 and (c) 5 1019 cm 3.
3.2. Structural study 3.2.1. X-ray investigations of the structure Fig. 3 illustrates three typical XRD spectra (2θ–ω-scans) of β-Ga2O3:Si layers grown at 800 1C on Al2O3 (0001) with different Si doping. The three sharp peaks at 2θ ¼ 41.691, 64.51 and 90.721 can be identified as the 00.6, 00.9 and 00.12 Bragg reflections of the c-plane sapphire, respectively. Additional peaks observed at 2θ ¼37.81, 58.71 and 80.71 (curves a and b) are caused by the epitaxial layers and can be assigned to the 402, 603 and 804 Bragg reflections of the monoclinic modification of Ga-oxide – the β-phase (Ref. [10]). From Fig. 3 it can be stated that there exists a well-defined epitaxial relationship with the hexagonal (0001) Al2O3 and that the β-Ga2O3:Si layers are ( 201)-oriented. It is striking that the width of the 603 Bragg reflection of the βGa2O3 layers is very large whereas the widths of the 402 and 804 Bragg reflections are quite small. This peculiarity is not fully understood yet. We, however, suspect that a high density of stacking faults (SFs), which leads to a large vertical disorder in diffracting lattice planes, is the reason for this characteristic behavior. A similar behavior has been observed in (Na,Bi)TiO3 thin films and could be theoretically described by vertical stacking disorder [11]. The role of SFs will be discussed later in Section 3.2.2. The impact of Si-doping on the structural quality of β-Ga2O3: Si films is obvious in Fig. 3. For Si-doping levels between 4 1017 and 3 1018 cm 3 pronounced β-Ga2O3 Bragg reflections can be observed – (curves a and b) proving that β-Ga2O3:Si films are crystalline. On the other hand, the lack of β-Ga2O3 Bragg peaks
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3
104 as-grown Ga O 2 3
Intensity (cps)
103
-402 annealed
102
101 bulk
100 36.5 37.0 37.5 38.0 38.5 39.0 39.5
2 theta (degree) Fig. 4. Positions of the Bragg reflection ( 402) from 2θ–ω scans of β-Ga2O3 layers grown on Al2O3 (0001) with different Si-doping given in the legend.
Fig. 5. X-ray reciprocal space map of a homoepitaxial β-Ga2O3:Si layer on β-Ga2O3 (100) substrate in the vicinity of the 600 reciprocal lattice point. “S” and “L” mark the position of the β-Ga2O3 substrate and layer Bragg reflections, respectively.
indicates amorphization of the films (curve c) when the Si-doping is higher than 5 1019 cm 3. Fig. 4 illustrates the position of the ( 402) reflex of β-Ga2O3 for layers grown at equivalent growth conditions but at different Si-doping concentrations. It is obvious that all positions of the peak ( 402) are shifted to the smaller 2θ, i.e. to a larger out-ofplane lattice parameter, compared to bulk material. Annealing of the samples in O2 leads to shifts of the peak positions to that of bulk, i.e. there is a lattice relaxation. It might be caused by recovery of some of oxygen vacancies and/or reducing of lattice defects during the annealing [5]. Homoepitaxial growth of β-Ga2O3:Si layers on bulk β-Ga2O3 (100) was successfully performed. In Fig. 5 an X-ray reciprocal space map of a β-Ga2O3:Si layer grown on (100) β-Ga2O3 substrate is shown. Besides the strong substrate reflection (marked as “S”) a broad feature (marked as “L”) arising from the β-Ga2O3 layer is observed. The center of mass of this feature deviates from the substrate peak position indicating a larger vertical lattice
Fig. 6. (a) A typical bright-field TEM image of an undoped homoepitaxial β-Ga2O3 layer and (c) of β-Ga2O3:Si layer; (b and d) HRSTEM images of the samples from (a) and (c), respectively.
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parameter of the β-Ga2O3 layer as compared to the substrate. This could e.g. be caused by an oxygen deficiency in the β-Ga2O3:Si layer which is known to lead to a larger lattice parameter in perovskite oxides [12]. However, the observed difference in vertical lattice spacing of about 0.8% is rather large and would imply a very large oxygen deficiency. As an alternative explanation also SFs could lead to a change of the mean vertical lattice parameter. More details on homoepitaxy will be reported additionally. Here we will focus our study on the impact of Si on the layer crystalline quality.
3.2.2. TEM investigation of the structure Fig. 6a and c shows typical bright-field TEM images of a Si-doped and an undoped homoepitaxial β-Ga2O3 layers. Both layers are single-crystalline and epitaxially grown but contain high density of planar defects. As can be seen already from the bright-field images, planar defects in undoped layers are limited to the a-plane, while they lie exclusively on inclined planes in β-Ga2O3:Si. A better insight into the nature and atomic structure of these defects comes from the HRSTEM images performed with a high angle annular dark-field detector (Fig. 6b and d). In these images the Ga columns appear bright (the unit cell is inserted into the image). Fig. 6b shows the planar defects in the undoped layer. They are essentially twins and SFs on a-planes that can be described by a c/2 glide reflection of the lattice. In the case of β-Ga2O3:Si layers (Fig. 6c) the defects lie on inclined planes. The contrast pattern originates from a superposition of the two b-projected lattices shifted by 223 pm in a-direction and 216 pm in c-direction. This corresponds to a shift of the lattices along the (20 1) planes on which only octahedral coordinated gallium atoms are placed. The full analysis of the displacement vector of these defects requires analyses in different crystallographic directions and is in progress. It is interesting to note that the density of twins and SFs on a-planes are an order of magnitude lower in β-Ga2O3:Si, while defects on inclined planes have rarely been observed in undoped material. Planar defects on inclined planes have been also observed in In-doped layers (to be reported additionally). Their formation mechanism will be studied by refined analyses of the initial stages of growth.
3.3. Optical transmission and electrical properties study Fig. 7 shows the internal spectral transmission of an as-grown
β-Ga2O3:Si layer on sapphire (Si ¼ 4 1018 cm 3) in comparison
to the transmission of this layer after annealing in oxygen for 1 h at 800 1C and 850 1C, respectively. The spectra are related to the 100
Transmission (%)
80
60 as grown 800 °C, 1 h, O2 850 °C, 1 h, O2
40
4. Conclusions
20
0 200
300
400
500
600
700
transmission of the sapphire substrate yielding approximately the internal transmittance. The distinctly increased transmission after annealing is due to the defect reduction already observed by XRD (Fig. 4). Obviously, the resulting β-Ga2O3:Si layers are highly transparent in the UV-VIS-IR range of the spectrum. Although SIMS shows that Si is incorporated in the films Hall effect measurements performed at RT demonstrate that the resulting material is not electrically conductive. In an attempt to electrically activate the Si-species, the epitaxial layers were annealed in O2 at temperatures from 800 to 1050 1C with steps of 50 1C for 1 h. However, Hall and conductivity measurements carried out on the layers deposited on sapphire, and capacitancevoltage measurements performed on oxide layers grown on native substrates showed that the films were not-conductive. One reason for that behavior could be the formation of SiO2 as a second phase in the β-Ga2O3 (not measurable by XRD). This is very likely thinking of the large affinity of Si to oxygen. In this way the Si is passivated and not electrically active. Another reason for the low conductivity of our β-Ga2O3:Si films could come from the compensation of the Si-donors by dislocations and SFs. Further investigations of the complexes formed by Si in the β-Ga2O3 lattice by X-ray photoelectron spectroscopy and positron annihilation lifetime spectroscopy are in progress. Based on DFT, Varley et al. [6] calculated the formation energies and charge state transition levels for oxygen vacancies and donor impurities in β-Ga2O3. They found that Si, Ge, Sn, and H behave as shallow donors. The ionization energy of the isolated Si donor, determined by Irmscher et al. [9], is 36 meV and at RT nearly all donors should be ionized. Why is the electrical activity of the Si-species so poor and why are we unable to observe free electrons? Unfortunately the literature does not help much in this regard. For example, Takakura et al. [8] investigated Si-doping in β-Ga2O3 films deposited by RF magnetron co-sputtering of Gaoxide and Si targets. The Si incorporated in the films was determined by EDX to be from 0 to 13%. They found out that the conductivity of the β-Ga2O3:Si films does not increase with Si-doping and ascribed this experimental evidence to an increase of the β-Ga2O3:Si band gap. However, we do not observe any change in the optical band gap for the Si-doping range typical for semiconductor doping. Sasaki et al. [13] reported on Si-doping by ion implantation of bulk crystals synthesized by the floating zone method. For Si ¼1– 5 1019 cm 3, an activation efficiency of above 60% was obtained after annealing in N2 at a temperature of 900–1000 1C. Our experiments with MOVPE β-Ga2O3:Si, doped in the same range, as-grown and exposed to annealing in O2 or N2, however, provided non-conductive material. At this stage we are inclined to believe that this may derive from formation of some complexes of structural defects with unidentified yet impurities that passivate the Si donor. Theoretical studies have shown that the formation of Ga-vacancies at oxygen-rich conditions (like ours) may have even negative formation energy and carbon on oxygen site may behave as a double acceptor. More investigations are being performed to highlight these aspects.
800
Wavelength (nm) Fig. 7. Spectral transmission of a β-Ga2O3:Si (Si ¼ 4 1018 cm 3) layer on sapphire for the as-grown state and after annealing in oxygen for 1 h at 800 1C and 850 1C, respectively.
We demonstrated that monoclinic β-phase Ga2O3 thin films doped with Si can be grown on Al2O3 (0001) substrates by MOVPE. The β-Ga2O3:Si films are ( 201)-oriented and exhibit an epitaxial relationship with the hexagonal substrate. SIMS investigations showed that Si may be incorporated up to about 5 1019 cm 3 but Hall and CV measurements confirmed a very low conductivity of both as-grown and annealed films. We are presently working to understand whether the bad electrical activity of Si is due to
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passivation of the incorporated Si atoms, to misplacement within the β-Ga2O3 matrix or rather to compensation by structural and point defects. Homoepitaxial growth of single phase, smooth β-Ga2O3:Si layers is demonstrated. It has been discovered that the Si incorporation changes the type of the defect structures. Acknowledgments This work was supported by Grant no SAW-2012-IKZ-2 from the Leibniz-Gemeinschaft. We thank R. Grueneberg for the technical assistance and A. Kwasniewski for the XRD measurements. References [1] D. Gogova, L. -K. Thomas, B. Camin, Thin Solid Films 517 (11) (2009) 3326. [2] J. Herrero, C. Guillén, Thin Solid Films 451–452 (2004) 630. [3] P. Sowti Khiabani, E. Marzbanrad, C. Zamani, R. Riahifar, B. Raissi, Sens. Actuators B: Chem. 166–167 (2012) 128.
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[4] J. Wager, Science 300 (5623) (2003) 1245. [5] G. Wagner, M. Baldini, D. Gogova, M. Schmidbauer, R. Schewski, M. Albrecht, Z. Galazka, D. Klimm, R. Fornari, Phys. Status Solidi A, 1–7 (2013) / http://dx.doi. org/10.1002/pssa.201330092A. [6] J.B. Varley, J.R. Weber, A. Janotti, C.G. Van de Walle, Appl. Phys. Lett. 97 (2010) 142106. [7] E.G. Víllora, K. Shimamura, Y. Yoshikawa, T. Ujiie, K. Aoki, Appl. Phys. Lett. 92 (2008) 202120. [8] K. Takakura, S. Funasaki, I. Tsunoda, H. Ohyama, D. Takeuchi, T. Nakashima, M. Shibuya, K. Murakami, E. Simoen, C. Claeys, Phys. B: Condens. Matter 407 (2012) 2900. [9] K. Irmscher, Z. Galazka, M. Pietsch, R. Uecker, R. Fornari, J. Appl. Phys. 110 (2011) 063720. [10] Joint Committee on Powder Diffraction Standards (JCPDS, PDF No. 43043-1012 and 041-1103). [11] V. Kopp, V. Kaganer, J. Schwarzkopf, F. Waidick, T. Remmele, A. Kwasniewski, M. Schmidbauer, Acta Crystallogr. A 68 (2012) 148–155. [12] C.M. Brooks, L. Fitting Kourkoutis, T. Heeg, J. Schubert, D.A. Muller, D.G. Schlom, Appl. Phys. Lett. 94 (2009) 162905. [13] K. Sasaki, M. Higashiwaki, A. Kuramata, T. Masui, S. Yamakoshi, Appl. Phys. Express 6 (2013) 086502.
Please cite this article as: D. Gogova, et al., Journal of Crystal Growth (2013), http://dx.doi.org/10.1016/j.jcrysgro.2013.11.056i