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Doping-induced bandgap tuning of α-Ga2O3 for ultraviolet lighting
Current Applied Physics 17 (2017) 713e716
Contents lists available at ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Doping-induced bandgap tuning of a-Ga2O3 for ultraviolet lighting Minseok Choi a, *, Junwoo Son b a b
Department of Physics, Inha University, Incheon 22212, Republic of Korea Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang 37673, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 September 2016 Received in revised form 8 December 2016 Accepted 21 February 2017
We propose the novel strategy for indirect-to-direct band gap transition of gallium oxide-based semiconductors for ultraviolet lighting device through first-principles calculations using a screened hybrid functional. Our calculations show that the tuning of electronic band gap of a-Ga2O3 is straightforward by adding dopants, which mimics alloy-like system. In order to put the band gap in the energy range of ultraviolet light, Group-III (In, Tl) at the Ga site and Group-V (N, P) or Group-VI (S, Se) at the O site are examined. We find that the most of doped Ga2O3 possess direct or nearly direct band gaps lying in the ultraviolet energy that is essential for optoelectronic devices. © 2017 Elsevier B.V. All rights reserved.
Keywords: First-principles calculation Gallium oxide Doping
1. Introduction GaN-based Group-III nitride semiconductor, i.e. GaN or its alloy with InN and AlN, is remarkable materials system that enables the inaccessible functionality to silicon, such as solid-state lighting and high-power/high-frequency electronics [1,2]. For example, with direct band-gaps ranging from 0.7 eV (InN) through 3.4 eV (GaN) to 6.0 eV (AlN), this material system has demonstrated deep ultraviolet (>~4.1 eV), ultraviolet (>~3.1 eV), blue (~2.7 eV), and green (~2.4 eV) emitters for light-emitting diodes and laser diodes. As shown in Fig. 1, the main advantage of GaN-based materials system over other semiconductors is that GaN-based nitrides can offer the direct band-gap from 0.7 to 6.0 eV by band-gap tuning with adjusting the ratio of Group-III elements (In, Ga, and Al). Such advantage makes it possible to fabricate various optoelectronic devices with GaN via band engineering such as quantum-well structures and two dimensional electron gas. Despite the tremendous success of GaN-based compounds in solid-state lighting, there is still a pressing need to develop new semiconducting materials system for wider band-gap devices due to the growth and device issues in ultraviolet emitter containing high Al content AlxGa1-xN (AlGaN) materials and heterostructures [3]. Recently, monoclinic Ga2O3 (b-Ga2O3) is promising as a fourthgeneration semiconductor material for power devices due to wide
* Corresponding author. E-mail addresses: [email protected] (M. Choi), [email protected] (J. Son). http://dx.doi.org/10.1016/j.cap.2017.02.019 1567-1739/© 2017 Elsevier B.V. All rights reserved.
band-gap (4.8e4.9 eV) and relatively high mobility (300 cm2V 1s 1) originated from dispersive conduction band [4]. Its large band-gap could enable power devices such as field effect transistors [5,6] with high breakdown voltages and low onresistance as well as optical devices such as green light emitting diode [7] and ultraviolet photo detectors [8,9]. In addition, the efforts have been reported to use Ga2O3 substrates of GaN-based light emitting diodes because the substrates have a low resistance and wide band-gap [10]. However, the use of the monoclinic phase would be not a good choice for device applications in a strategic sense because the low symmetry of monoclinic b-Ga2O3 has obstructed the formation of the Ga2O3-based highly-crystalline thin films and heterostructures with commercially available substrates. In this respect, there are three key challenges to overcome for good device performance: 1) the growth of the controlled Ga2O3 semiconductor layers with high crystallinity and purity, 2) the design of the direct band-gap, and 3) the development of the heterostructures to observe highly efficient luminescence. The corundum-structured a-Ga2O3 is therefore attractive since it can be epitaxially obtained on sapphire substrates in contrast to b-Ga2O3 on sapphire (See Fig.2), which is beneficial for the fabrication with other corundum-structures oxides such as Fe2O3 and Cr2O3 that exhibit a variety of optical properties [11]. In addition, the high quality (Ga, In)2O3 alloys are enable using pulsed laser deposition technique [12], possibly due to the fact that Ga2O3 and In2O3 have the same metastable corundum structure. (Ga, In)2O3 alloy films possess a tunable band gap covering the energy of 3.8e5.1 eV by controlling the indium content, which offers various
714
M. Choi, J. Son / Current Applied Physics 17 (2017) 713e716
Fig. 1. Band gap and wavelength tuning of nitride and oxide semiconductors with adjusting the ratio of Group-III elements (In, Ga, Al). Note that Ga2O3 materials can be used for ultraviolet (UV) optoelectronics without substituting Al atoms at Ga sites (See purple rectangular). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 2. Crystal structure of Ga2O3: (a) a-phase (R3 cH) and (b) b-phase (C12/m1). The green and red balls correspond to gallium and oxygen, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
optical device applications. In this work, we investigate the electronic structure of a-Ga2O3based oxides by adding several dopants through the screened hybrid functional calculations, allowing an accurate description of band gaps [13,14]. Our goal is to design direct (or nearly direct) band-gap Ga2O3-based alloys, whose band gaps are tunable to the value of 4.5 eV (deep ultraviolet) or smaller, for ultraviolet optoelectronic application. A rational selection of dopants is made based on the literature and a route to get the desired band gap uses the knowledge of conventional nitrides. In gallium nitride, aluminum is considerable to make a larger band gap than that of GaN, while indium is added to make a smaller band gap. In the same manner, indium and thallium are considered to obtain the band gap of gallium oxide lying in the range of ultraviolet energy. The other is doping the anion elements which have the proper valence p-level position with respect to the vacuum and oxygen 2p-level [15].
unitcells containing ten atoms for a-Ga2O3, and the integrations over the Brillouin zone were performed using a 2 2 2 k-point grid. All the lattice parameters and atomic coordinates were fully optimized until the Hellmann-Feynman forces was less than 0.01 eV/Å. As summarized in Table 1, the HSE functional produces the lattice parameters and electronic band gap of bulk a-Ga2O3 which excellently agree with the experimental data, differently from the generalized gradient approximation (GGA) [19] that is typical semilocal functional. The HSE calculations show that the lattice constants a and c are 4.98 Å and 13.45 Å, which are close to the experimental values of 4.98 Å and 13.43 Å, respectively [20]. We find that a-Ga2O3 has an indirect band gap with the value of 4.70 eV, which is similar to the experimental values of 4.7 eV [21] and 4.9 eV [22]. The conduction-band minimum is positioned at the G-point, and the valence-band maximum locates between the
2. Computational details In the calculations, the screened hybrid functional of HeydScuseria-Ernzerhof (HSE) [16], implemented with the projectaugmented wave method [17] in the Vienna Ab-initio Simulation Package code [18] is employed. The mixing parameter in the HSE was set to 25%. The electronic wave functions were expanded in a plane wave basis set with an energy cutoff of 400 eV. The calculations were performed using periodic boundary conditions with
Table 1 Calculated lattice parameter and bandgap of a-Ga2O3 using GGA and HSE functionals. The values in parenthesis are at G-point. Experimental values are included for comparison.
a (Å) c (Å) Eg (eV)
GGA
HSE
Experiment
5.06 13.63 2.44 (2.64)
4.98 13.45 4.70 (4.90)
4.98 13.43 4.7, 4.9
M. Choi, J. Son / Current Applied Physics 17 (2017) 713e716
L- and the G-point. The calculated energy gap at the G-point is 4.91 eV. The difference between the valence-band maximum and the energy at the G-point is only ~0.21 eV, which may be suitable for the transition to the direct band gap.
715
We investigate change in the band-gap energy by putting Group-III elements such as indium and thallium at the gallium site, with series of content. The examined atomic structure here mimics the alloy samples due to the high content of dopants, and the alloy structure simply is assumed to have the same crystal symmetry of host material, i.e., corundum structure. Like as the nitride semiconductors (Ga,In)N, indium doping, i.e., (Ga,In)2O3 is firstly examined at the Ga site and is monitored how the doped indium affect the electronic structure. Overall, although the content of indium is very high, the shape of conduction-band edge and valence band are little changed. More importantly, we find that an increase of indium content leads to (i) the reduced band gap and (ii) make the band gap nearly direct at the G-point (See Fig. 3). It seems that the former results from mostly the conductionband lowering. Because the position of the conduction band with respect to the vacuum level is much lower in In2O3 than that in Ga2O3, and the energy difference between the valence-band maximum in two oxides is smaller than that between the conduction-band minimum [23]. With 25% of the In content, the calculated band gap is 4.14 eV, which is reduced by 16% than the bulk band gap of 4.70 eV, and the
In content of 50% gives the band gap of 3.55 eV that is 28% smaller than the bulk value. These values are positioned in the energy range of deep ultraviolet or ultraviolet (See Fig. 1). Both cases exhibits the valence-band maximum along the L- and the G-point like as that of the pristine a-Ga2O3, but the energy difference between the valence-band maximum and the valence-band edge at the G-point is quite small (~0.1 eV). In the same sense, we looked at the thallium dopant at the Ga site. Our calculations predict that the thallium doping produces much narrower band gaps, compared to the indium doped cases, with little change in the valence band, but it makes the Ga2O3 bandgap direct at the G-point. With 25% of the Tl content, the calculated band gap is 2.90 eV, lowered by 38% than the bulk band gap of 4.70 eV, and the Tl content of 50% produces a value of 1.89 eV that is 60% smaller than the bulk value. However, due to too much narrowing of the gap, the thallium doping with these contents does not put the band-gap energy in the ultraviolet range, thus we would guide experimentalists to dope thallium less than 25% of the content for ultraviolet application. Moving to anion doping at the oxygen site, Group-V (N, P) or Group-VI elements (S, Se), possessing higher p-levels in energy than O 2p [15], are taken into account. Because the valence-band edge of a-Ga2O3 mainly consist of O 2p orbital, and thus anion dopants, having the higher p-levels, may reduce the band gap by shifting the valence-band edge upward. The concentration of anion dopant corresponds to one of six oxygen sites in the simulation cell. We expect that these are likely to produce a desired band gap for potential ultraviolet optoelectronic devices. Fig. 4(a) and (b) show
Fig. 3. Band structure of Ga2O3 doped with Group-III element ((a), (b) In and (c), (d) Tl) at the Ga site. The valence band maximum is set as zero.
Fig. 4. Band structure of Ga2O3 doped with a Group-V ((a) N, (b) P) or Group-VI ((c) S, (d) Se) element at the O site. The highest electron occupied state is set as zero.
3. Results and discussions
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M. Choi, J. Son / Current Applied Physics 17 (2017) 713e716
that the calculated band structure of nitrogen-doped and phosphorus-doped a-Ga2O3. Nitrogen doping produces the 3.17 eV band-gap that is well positioned in the ultraviolet region, while phosphorus leads to a very small band gap of 1.43 eV that is far from the ultraviolet energy (>~3.1 eV). A noticeable feature is that they produce the direct band gap that is an important factor to make the better performance of light emitting. However, an empty energy level is introduced because nominal valence charge of nitrogen and phosphorus ions is 3 that is larger than 2 of oxygen ion, resulting in the creation of a hole carrier per dopant. Fig. 4(c) and (d) show the band structure for sulfur and selenium doping. Unlike Group-V elements, sulfur and selenium are isoelectric to oxygen, and it is thus expected that they lead to electrically inactive alloy samples without any hole bands. Our calculations indicate that sulfur doping produce a band gap of 2.77 eV, and selenium doping does that of 2.10 eV. Based on the results, we expect that these systems can emit ultraviolet or near ultraviolet light by controlling content.
4. Conclusions In summary, we performed first-principles calculations for gallium oxide-based materials which have direct or nearly direct band-gaps in the energy range of ultraviolet light. In order to reasonable description of electronic structures, a screened hybrid was used in the calculations. By doping Group-III (In, Tl) at Ga site and Group-V (N, P) and Group-VI elements (S, Se) at the O site, we theoretically identified the band gap of gallium oxide can be tuned to put in the energy range of ultraviolet light. The proposed Ga2O3based semiconductors have potential to resolve entire problems in GaN ultraviolet optoelectronics and enable us to add oxides as a new class of a semiconductor materials system.
Acknowledgement M.C. was supported by Basic Science Research Programs through the National Research Foundation of Korea (NRF2015R1C1A1A02037595) and by Inha University Research Grant (INHA-53352). J.S. was supported by Basic Science Research Programs through the National Research Foundation of Korea (NRF2014R1A1A1035950). References [1] S. Pimputkar, J.S. Speck, S.P. DenBaars, S. Nakamura, Nat. Photonics 3 (2009) 180. [2] J.S. Speck, S.F. Chichibu, MRS Bull. 34 (2009) 304. [3] A. Khan, K. Balakrishnan, T. Katona, Nat. Photonic 2 (2008) 77. [4] S. Fujita, Jpn. J. Appl. Phys. 54 (2015) 030101. [5] K. Sasaki, A. Kuramata, T. Masui, E.G. Villora, K. Shimamura, S. Yamakoshi, Appl. Phys. Express 5 (2012) 035502. [6] M. Higashiwaki, K. Sasaki, A. Kuramata, T. Masui, S. Yamakoshi, Appl. Phys. Lett. 100 (2012) 013504. [7] L. Binet, D. Gourier, J. Phys. Chem. Solids 59 (1998) 1241. [8] Y. Kokubun, K. Miura, F. Endo, S. Nakagomi, Appl. Phys. Lett. 90 (2007) 031912. [9] T. Oshima, T. Okuno, N. Arai, N. Suzuki, S. Oshira, S. Fujita, Appl. Phys. Express 1 (2008) 011202. [10] K. Shimamura, E.G. Villora, K. Domen, K. Yui, K. Aoki, N. Ichinose, Jpn. J. Appl. Phys. 44 (2005) L7. [11] D. Shinohara, S. Fujita, Jpn. J. Appl. Phys. 47 (2008) 7311. [12] F. Zhang, H. Jan, K. Saito, T. Tanaka, M. Nishio, T. Nagaoka, M. Arita, Q. Guo, Thin Solid Films 578 (2015) 1. [13] S. Park, B. Lee, S.H. Jeon, S. Han, Curr. Appl. Phys. 11 (2011) S337. [14] B. Lee, C. Lee, C.S. Hwang, S. Han, Curr. Appl. Phys. 11 (2011) S293. [15] Y. Yan, S.-H. Wei, Phys. Status Solidi b 245 (2008) 641. [16] J. Heyd, G.E. Scuseria, M.J. Ernzerhof, Chem. Phys. 118 (2003) 8207. €chl, Phys. Rev. B 50 (1994) 17953. [17] P.E. Blo [18] G. Kresse, J. Hafner, Phys. Rev. B 47 (1993) 558. [19] J.P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 78 (1997) 1396. [20] M. Marezio, J.P. Remeika, J. Chem. Phys. 46 (1967) 1862. [21] G. Schmitz, P. Gassmann, R. Franchy, J. Appl. Phys. 83 (1998) 2533. [22] S. Schamm, G. Zanchi, Ultramiscroscopy 96 (2003) 559. [23] H. Peelaers, D. Steiauf, J.B. Varley, A. Janotti, C.G. Van de Walle, Phys. Rev. B 92 (2015) 085206.
Contents lists available at ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Doping-induced bandgap tuning of a-Ga2O3 for ultraviolet lighting Minseok Choi a, *, Junwoo Son b a b
Department of Physics, Inha University, Incheon 22212, Republic of Korea Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang 37673, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 September 2016 Received in revised form 8 December 2016 Accepted 21 February 2017
We propose the novel strategy for indirect-to-direct band gap transition of gallium oxide-based semiconductors for ultraviolet lighting device through first-principles calculations using a screened hybrid functional. Our calculations show that the tuning of electronic band gap of a-Ga2O3 is straightforward by adding dopants, which mimics alloy-like system. In order to put the band gap in the energy range of ultraviolet light, Group-III (In, Tl) at the Ga site and Group-V (N, P) or Group-VI (S, Se) at the O site are examined. We find that the most of doped Ga2O3 possess direct or nearly direct band gaps lying in the ultraviolet energy that is essential for optoelectronic devices. © 2017 Elsevier B.V. All rights reserved.
Keywords: First-principles calculation Gallium oxide Doping
1. Introduction GaN-based Group-III nitride semiconductor, i.e. GaN or its alloy with InN and AlN, is remarkable materials system that enables the inaccessible functionality to silicon, such as solid-state lighting and high-power/high-frequency electronics [1,2]. For example, with direct band-gaps ranging from 0.7 eV (InN) through 3.4 eV (GaN) to 6.0 eV (AlN), this material system has demonstrated deep ultraviolet (>~4.1 eV), ultraviolet (>~3.1 eV), blue (~2.7 eV), and green (~2.4 eV) emitters for light-emitting diodes and laser diodes. As shown in Fig. 1, the main advantage of GaN-based materials system over other semiconductors is that GaN-based nitrides can offer the direct band-gap from 0.7 to 6.0 eV by band-gap tuning with adjusting the ratio of Group-III elements (In, Ga, and Al). Such advantage makes it possible to fabricate various optoelectronic devices with GaN via band engineering such as quantum-well structures and two dimensional electron gas. Despite the tremendous success of GaN-based compounds in solid-state lighting, there is still a pressing need to develop new semiconducting materials system for wider band-gap devices due to the growth and device issues in ultraviolet emitter containing high Al content AlxGa1-xN (AlGaN) materials and heterostructures [3]. Recently, monoclinic Ga2O3 (b-Ga2O3) is promising as a fourthgeneration semiconductor material for power devices due to wide
* Corresponding author. E-mail addresses: [email protected] (M. Choi), [email protected] (J. Son). http://dx.doi.org/10.1016/j.cap.2017.02.019 1567-1739/© 2017 Elsevier B.V. All rights reserved.
band-gap (4.8e4.9 eV) and relatively high mobility (300 cm2V 1s 1) originated from dispersive conduction band [4]. Its large band-gap could enable power devices such as field effect transistors [5,6] with high breakdown voltages and low onresistance as well as optical devices such as green light emitting diode [7] and ultraviolet photo detectors [8,9]. In addition, the efforts have been reported to use Ga2O3 substrates of GaN-based light emitting diodes because the substrates have a low resistance and wide band-gap [10]. However, the use of the monoclinic phase would be not a good choice for device applications in a strategic sense because the low symmetry of monoclinic b-Ga2O3 has obstructed the formation of the Ga2O3-based highly-crystalline thin films and heterostructures with commercially available substrates. In this respect, there are three key challenges to overcome for good device performance: 1) the growth of the controlled Ga2O3 semiconductor layers with high crystallinity and purity, 2) the design of the direct band-gap, and 3) the development of the heterostructures to observe highly efficient luminescence. The corundum-structured a-Ga2O3 is therefore attractive since it can be epitaxially obtained on sapphire substrates in contrast to b-Ga2O3 on sapphire (See Fig.2), which is beneficial for the fabrication with other corundum-structures oxides such as Fe2O3 and Cr2O3 that exhibit a variety of optical properties [11]. In addition, the high quality (Ga, In)2O3 alloys are enable using pulsed laser deposition technique [12], possibly due to the fact that Ga2O3 and In2O3 have the same metastable corundum structure. (Ga, In)2O3 alloy films possess a tunable band gap covering the energy of 3.8e5.1 eV by controlling the indium content, which offers various
714
M. Choi, J. Son / Current Applied Physics 17 (2017) 713e716
Fig. 1. Band gap and wavelength tuning of nitride and oxide semiconductors with adjusting the ratio of Group-III elements (In, Ga, Al). Note that Ga2O3 materials can be used for ultraviolet (UV) optoelectronics without substituting Al atoms at Ga sites (See purple rectangular). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 2. Crystal structure of Ga2O3: (a) a-phase (R3 cH) and (b) b-phase (C12/m1). The green and red balls correspond to gallium and oxygen, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
optical device applications. In this work, we investigate the electronic structure of a-Ga2O3based oxides by adding several dopants through the screened hybrid functional calculations, allowing an accurate description of band gaps [13,14]. Our goal is to design direct (or nearly direct) band-gap Ga2O3-based alloys, whose band gaps are tunable to the value of 4.5 eV (deep ultraviolet) or smaller, for ultraviolet optoelectronic application. A rational selection of dopants is made based on the literature and a route to get the desired band gap uses the knowledge of conventional nitrides. In gallium nitride, aluminum is considerable to make a larger band gap than that of GaN, while indium is added to make a smaller band gap. In the same manner, indium and thallium are considered to obtain the band gap of gallium oxide lying in the range of ultraviolet energy. The other is doping the anion elements which have the proper valence p-level position with respect to the vacuum and oxygen 2p-level [15].
unitcells containing ten atoms for a-Ga2O3, and the integrations over the Brillouin zone were performed using a 2 2 2 k-point grid. All the lattice parameters and atomic coordinates were fully optimized until the Hellmann-Feynman forces was less than 0.01 eV/Å. As summarized in Table 1, the HSE functional produces the lattice parameters and electronic band gap of bulk a-Ga2O3 which excellently agree with the experimental data, differently from the generalized gradient approximation (GGA) [19] that is typical semilocal functional. The HSE calculations show that the lattice constants a and c are 4.98 Å and 13.45 Å, which are close to the experimental values of 4.98 Å and 13.43 Å, respectively [20]. We find that a-Ga2O3 has an indirect band gap with the value of 4.70 eV, which is similar to the experimental values of 4.7 eV [21] and 4.9 eV [22]. The conduction-band minimum is positioned at the G-point, and the valence-band maximum locates between the
2. Computational details In the calculations, the screened hybrid functional of HeydScuseria-Ernzerhof (HSE) [16], implemented with the projectaugmented wave method [17] in the Vienna Ab-initio Simulation Package code [18] is employed. The mixing parameter in the HSE was set to 25%. The electronic wave functions were expanded in a plane wave basis set with an energy cutoff of 400 eV. The calculations were performed using periodic boundary conditions with
Table 1 Calculated lattice parameter and bandgap of a-Ga2O3 using GGA and HSE functionals. The values in parenthesis are at G-point. Experimental values are included for comparison.
a (Å) c (Å) Eg (eV)
GGA
HSE
Experiment
5.06 13.63 2.44 (2.64)
4.98 13.45 4.70 (4.90)
4.98 13.43 4.7, 4.9
M. Choi, J. Son / Current Applied Physics 17 (2017) 713e716
L- and the G-point. The calculated energy gap at the G-point is 4.91 eV. The difference between the valence-band maximum and the energy at the G-point is only ~0.21 eV, which may be suitable for the transition to the direct band gap.
715
We investigate change in the band-gap energy by putting Group-III elements such as indium and thallium at the gallium site, with series of content. The examined atomic structure here mimics the alloy samples due to the high content of dopants, and the alloy structure simply is assumed to have the same crystal symmetry of host material, i.e., corundum structure. Like as the nitride semiconductors (Ga,In)N, indium doping, i.e., (Ga,In)2O3 is firstly examined at the Ga site and is monitored how the doped indium affect the electronic structure. Overall, although the content of indium is very high, the shape of conduction-band edge and valence band are little changed. More importantly, we find that an increase of indium content leads to (i) the reduced band gap and (ii) make the band gap nearly direct at the G-point (See Fig. 3). It seems that the former results from mostly the conductionband lowering. Because the position of the conduction band with respect to the vacuum level is much lower in In2O3 than that in Ga2O3, and the energy difference between the valence-band maximum in two oxides is smaller than that between the conduction-band minimum [23]. With 25% of the In content, the calculated band gap is 4.14 eV, which is reduced by 16% than the bulk band gap of 4.70 eV, and the
In content of 50% gives the band gap of 3.55 eV that is 28% smaller than the bulk value. These values are positioned in the energy range of deep ultraviolet or ultraviolet (See Fig. 1). Both cases exhibits the valence-band maximum along the L- and the G-point like as that of the pristine a-Ga2O3, but the energy difference between the valence-band maximum and the valence-band edge at the G-point is quite small (~0.1 eV). In the same sense, we looked at the thallium dopant at the Ga site. Our calculations predict that the thallium doping produces much narrower band gaps, compared to the indium doped cases, with little change in the valence band, but it makes the Ga2O3 bandgap direct at the G-point. With 25% of the Tl content, the calculated band gap is 2.90 eV, lowered by 38% than the bulk band gap of 4.70 eV, and the Tl content of 50% produces a value of 1.89 eV that is 60% smaller than the bulk value. However, due to too much narrowing of the gap, the thallium doping with these contents does not put the band-gap energy in the ultraviolet range, thus we would guide experimentalists to dope thallium less than 25% of the content for ultraviolet application. Moving to anion doping at the oxygen site, Group-V (N, P) or Group-VI elements (S, Se), possessing higher p-levels in energy than O 2p [15], are taken into account. Because the valence-band edge of a-Ga2O3 mainly consist of O 2p orbital, and thus anion dopants, having the higher p-levels, may reduce the band gap by shifting the valence-band edge upward. The concentration of anion dopant corresponds to one of six oxygen sites in the simulation cell. We expect that these are likely to produce a desired band gap for potential ultraviolet optoelectronic devices. Fig. 4(a) and (b) show
Fig. 3. Band structure of Ga2O3 doped with Group-III element ((a), (b) In and (c), (d) Tl) at the Ga site. The valence band maximum is set as zero.
Fig. 4. Band structure of Ga2O3 doped with a Group-V ((a) N, (b) P) or Group-VI ((c) S, (d) Se) element at the O site. The highest electron occupied state is set as zero.
3. Results and discussions
716
M. Choi, J. Son / Current Applied Physics 17 (2017) 713e716
that the calculated band structure of nitrogen-doped and phosphorus-doped a-Ga2O3. Nitrogen doping produces the 3.17 eV band-gap that is well positioned in the ultraviolet region, while phosphorus leads to a very small band gap of 1.43 eV that is far from the ultraviolet energy (>~3.1 eV). A noticeable feature is that they produce the direct band gap that is an important factor to make the better performance of light emitting. However, an empty energy level is introduced because nominal valence charge of nitrogen and phosphorus ions is 3 that is larger than 2 of oxygen ion, resulting in the creation of a hole carrier per dopant. Fig. 4(c) and (d) show the band structure for sulfur and selenium doping. Unlike Group-V elements, sulfur and selenium are isoelectric to oxygen, and it is thus expected that they lead to electrically inactive alloy samples without any hole bands. Our calculations indicate that sulfur doping produce a band gap of 2.77 eV, and selenium doping does that of 2.10 eV. Based on the results, we expect that these systems can emit ultraviolet or near ultraviolet light by controlling content.
4. Conclusions In summary, we performed first-principles calculations for gallium oxide-based materials which have direct or nearly direct band-gaps in the energy range of ultraviolet light. In order to reasonable description of electronic structures, a screened hybrid was used in the calculations. By doping Group-III (In, Tl) at Ga site and Group-V (N, P) and Group-VI elements (S, Se) at the O site, we theoretically identified the band gap of gallium oxide can be tuned to put in the energy range of ultraviolet light. The proposed Ga2O3based semiconductors have potential to resolve entire problems in GaN ultraviolet optoelectronics and enable us to add oxides as a new class of a semiconductor materials system.
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