Band gap engineering of n alloyed ga2o3 thin films

Band gap engineering of N alloyed Ga2O3 thin films Band gap engineering of N alloyed Ga2O3 thin films Dongyu Song, , Li Li, , Bingsheng Li, , Yu Sui, and Aidong Shen, Citation AIP Advances 6, 065016 ([.]

Band gap engineering of N-alloyed Ga2O3 thin films , , , , Dongyu Song , Li Li , Bingsheng Li , Yu Sui, and Aidong Shen Citation: AIP Advances 6, 065016 (2016); doi: 10.1063/1.4954720 View online: http://dx.doi.org/10.1063/1.4954720 View Table of Contents: http://aip.scitation.org/toc/adv/6/6 Published by the American Institute of Physics Articles you may be interested in Oxygen deficiency and Sn doping of amorphous Ga2O3 AIP Advances 108, 022107022107 (2016); 10.1063/1.4938473 Structural and optical properties of β-Ga2O3 thin films grown by plasma-assisted molecular beam epitaxy AIP Advances 34, 02L10902L109 (2016); 10.1116/1.4942045 AIP ADVANCES 6, 065016 (2016) Band gap engineering of N-alloyed Ga2O3 thin films Dongyu Song,1,a Li Li,2,a Bingsheng Li,1,b Yu Sui,1 and Aidong Shen3,b Department of Physics, Harbin Institute of Technology (HIT), Harbin 150080, China School of Life Science and Technology, HIT, Harbin 150080, China Department of Electrical Engineering, The City College of New York, New York 10031, USA (Received 20 April 2016; accepted 12 June 2016; published online 20 June 2016) The authors report the tuning of band gap of GaON ternary alloy in a wide range of 2.75 eV The samples were prepared by a two-step nitridation method First, the samples were deposited on 2-inch fused silica substrates by megnetron sputtering with NH3 and Ar gas for 60 minutes Then they were annealed in NH3 ambience at different temperatures The optical band gap energies are calculated from transmittance measurements With the increase of nitridation temperature, the band gap gradually decreases from 4.8 eV to 2.05 eV X-ray diffraction results indicate that as-deposited amorphous samples can crystallize into monoclinic and hexagonal structures after they were annealed in oxygen or ammonia ambience, respectively The narrowing of the band gap is attributed to the enhanced repulsion of N2p -Ga3d orbits and formation of hexagonal structure C 2016 Author(s) All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/) [http://dx.doi.org/10.1063/1.4954720] The band gap is an intrinsic characteristic of semiconductors and plays a central role in modern device physics & technology It governs the operation of semiconductor devices Recently, wide band gap semiconductor materials have attracted much more attention due to their various applications in the short wavelength region, such as ultraviolet (UV) emitter, UV photodetector, transparent electrode, and field effect transistors.1–8 Generally, a tunable band gap is highly desirable because it would allow great flexibility in designing and optimizing the devices There are many successful stories to tune the band gap and extend the range of their applications For example, with band gap engineering, the optoelectronic devices, such as emitter and photodetector, with different operation wavelengths have been realized by utilizing the GaN-based and ZnO-based semiconductor alloys.4,9 In this paper, we will introduce a new N-alloyed Ga2O3 material with widely tunable band gap from 4.8 eV to 2.05 eV Ga2O3 is a wide band gap semiconductor with chemical and physical stabilities It has been demonstrated that Ga2O3 is a promising candidate for solar blind UV photodetector, power devices, and photocatalysis in solar water splitting.10–13 Ga2O3 possesses distinctive characteristics By introducing dopants of Si or Sn, high levels of n-type conductivity have been achieved, with carrier concentrations exceeding 1019 cm−3 and mobilities up to 300 cm2/V.14 Furthermore, very recently, high quality thin film and bulk crystal with large size have been realized, resulting in further improvement of Ga2O3-based device performance.12 As a versatile functional material, Ga2O3 not only has the various device applications by utilizing its intrinsic properties, but also owns the flexibility to form important material systems by alloying with other materials, which have been playing a crucial role in different kinds of device applications As shown in Fig 1, alloying with In2O3 and ZnO, a transparent amorphous-In-Ga-Zn-O (a-IGZO) system for the active channel in transparent thin film transistor has been obtained.8 Alloying with Al2O3 or In2O3, a superior transparent conducting electrode with tunable band gap can a These authors equally contributed to this work b Authors to whom correspondence should be addressed Electronic addresses: libingsheng@hit.edu.cn and ashen@ccny cuny.edu 2158-3226/2016/6(6)/065016/6 6, 065016-1 © Author(s) 2016 065016-2 Song et al AIP Advances 6, 065016 (2016) FIG Ga2O3 can be alloyed with other compounds to form important functional materials: amorphous-InZnGaO by alloying In2O3, ZnO and Ga2O3; Ga2-xAlxO3 and Ga2-xInxO3 alloys by alloying with Al2O3 and In2O3, respectively; p-type I-Ga-O2 materials, such as LiGaO2, AgGaO2 and CuGaO2 alloys, by alloying with Li2O, Ag2O or Cu2O compounds, respectively GaON ternary alloy with tunable band gap may be formed if Ga2O3 is alloyed with GaN be realized.15,16 Alloying with I2O (I = Li, Cu, or Ag elements) compounds, natural p-type I-III-O2 oxide system can also be achieved However, most alloying processes are based on the combination of different cations.17,18 On the other hand, alloying with anions has not been widely studied It was well known that GaN, self-evidently, is an important material in the present society If Ga2O3 and GaN, as indicated in Fig 1, are combined to form GaON ternary alloy, as with alloys from AlN & GaN or ZnO & MgO, it is expected that the band gap of GaON can be tuned continuously at least between 3.4 eV (Eg of GaN) and 4.8 eV (Eg of Ga2O3) In this paper, we report the tuning of GaON band gap in a wide range from 2.05 eV to 4.8 eV by introducing nitrogen to replace oxygen in Ga2O3 The tuning of band gap was successfully realized by utilizing a two-step nitridation method X-ray diffraction (XRD) measurements demonstrate that GaON crystalizes with hexagonal phase from the amorphous GaON with the increase of nitridation temperature (NT) Based on the XRD results and the dependence of GaON band gap on the NT, the possible mechanisms for band gap narrowing are discussed The thin film samples were deposited on 2-in fused silica substrates for 60 minutes by employing megnetron sputtering system in which the ceramic Ga2O3 target is loaded The substrates are ultrasonically cleaned in ethanol, acetone, dilute HCl solution, rinsed in deionized water for for each step, then blow-dried with nitrogen gas before being loaded into the growth chamber which has the background pressure of less than 4x10−5 pa After the substrates were heated to the targeted temperatures, NH3 and Ar gases were flown into the chamber as the nitridation gas and sputtering gas, respectively The gas flow ratio of NH3 to Ar is 1:1 and the chamber pressure was Pa After the deposition of the thin films by sputtering, the sample prepared at 600 ◦C are selected and cut into small pieces and then placed into a thermal annealing furnace Thermal nitridation was carried out in an ammonia ambient with different annealing temperatures varying from 600 to 1000 ◦C The crystal structure of thin films was studied by x-ray diffraction (XRD) using an anode x-ray diffractometer with Cu K α radiation of 1.54 Å The transmittance spectra were carried out at room temperature Shown in Fig (left) are the transmittance spectra in the range from 200 nm to 1000 nm for the thin film samples with different NTs The transmittance of a Ga2O3 thin film, annealed in oxygen ambience, is also added in Fig for comparison The transmittances of most samples with absorption edge above GaN line in ultraviolet and visible are highly transparent (above 80%) A steep absorption edge for Ga2O3 film is observed With NT increase, the absorption edge gradually shifts to long wavelength (∼600 nm) Additionally, after the absorption edge striding over the line of GaN, the slope of the absorption edge starts to decrease The decrease of the sloop in the absorption (transmission) spectra is probably due to the increased lattice disorder in GaON with the increase of N composition As shown in Fig 2(right), in order to evaluate the band gap, we employ the characteristic function,(αhv)2 ∝ hv − Eg , to calculate the band gap energy, where hv is the energy of photons, and Eg is the bulk band gap α is the absorption coefficient which can be deduced from the transmittance with the scattering from the film surface being neglected The optical band gap is 065016-3 Song et al AIP Advances 6, 065016 (2016) FIG (left) transmittance spectra of Ga2O3 and GaON alloys formed at different nitridation temperatures (right) Square of the absorption coefficient, α, multiply the photo energy, hv, for the calculation of optical band gaps evaluated by extrapolating the linear part of the curve to intercept the energy axis The dependence of band gap of GaON on the NT is summarized in Fig Apparently, the band gap can be modified significantly and the values have been tuned in a range of from 2.05 eV to 4.8 eV Very recently, it has been demonstrated that Ga2O3 is a good candidate for ultraviolet photodetector in the solar blind wavelength region, which shows large ratio of Iλ∼254/Idark = 104 and high responsibility.19 However, there have been no attempt to extend the operation wavelength to the visible solar blind region (280 nm - 400 nm) due to the limitation of intrinsic band gap (cutting off wavelength < 258 nm) With nitridation of Ga2O3, we realize the band gap tuning, covering the whole visible solar blind region It has been reported that the photoconductive response has been sufficiently enhanced with the introduction of a certain amount of O into GaN to form amorphous GaON.20 This indicates that GaON has the potential application as a photodetector in the visible solar blind wavelength region It has also been reported that Ga2O3-based photocatalyst has a high efficiency for water splitting.13 With band gap being narrowed to the visible light region in N-alloyed Ga2O3, a larger spectral range of the sunlight can be utilized in photocatalyst application Figure shows the θ-2θ XRD patterns of the films with nitridation treatment at different temperatures There is no characteristic diffraction peak from the as-deposited samples, indicating the thin films are amorphous Even with the NT increased to 700 ◦C, the sample is still amorphous The crystallization starts to occur at an NT of 750 ◦C for 120 minutes As shown in Figure 4, at that NT, three peaks with weak intensity start to appear With further increase of NT, new peaks FIG The dependence of the optical band gap of GaON alloys on the nitridation temperatures The band gaps of Ga2O3 and hexagonal GaN are also given for references 065016-4 Song et al AIP Advances 6, 065016 (2016) FIG XRD patterns of GaON films prepared under different nitridation temperature (NT) The NT and time for nitridation are provided in the corresponding curve For comparison, β-Ga2O3 annealed at 1200 ◦C in oxygen ambience is shown (top) Reference patterns for monoclinic β-Ga2O3 and hexagonal GaN are given as vertical lines arise in the larger angle region Additionally, the diffraction intensity of XRD peaks is increased gradually, accompanying by the linewidths narrowing This suggests that the structural quality is improved It is known that Ga2O3 are polymorphism, including α-, β-, γ-, δ-, and ε-phase.21 Among them, β-Ga2O3 with monoclinic crystal structure is the most stable phase We also carried out the annealing process in oxygen ambience for the amorphous Ga2O3 sample at different temperature (detail results will be reported elsewhere) The top of Fig is XRD pattern of the sample annealed at 1200 ◦C in oxygen ambience Comparing with the standard card of β-Ga2O3 (JCPDS: 11-0370), characteristic peaks of typical monoclinic structure have been observed In the nitridation procedure, the crystallization was also observed However, the XRD patterns of nitridized samples are completely different from standard card of β-Ga2O3 and other kinds of Ga2O3 structures This indicates that nitridation procedure results in a new phase We know that GaN is hexagonal crystal structure Carefully comparing with standard XRD pattern of GaN (JCPDS: 74-0243), we found that the diffraction patterns of nitridized samples are very close to GaN Although we cannot rule out the possibility that the phase separation occurred and that GaN inclusions precipitated during the annealing process, we believe it’s less likely.22 The decrease of the band gap of GaON is due to the increased of N composition when the films are undergoing high temperature thermal annealing under nitrogen ambience (NH3) This is also consistent with report on the optical band gap of GaON alloy.23 If more and more crystalline GaN inclusions are formed in the films at higher annealing temperature (as indicated by the increase of XRD peak intensities if we assume these peaks are from GaN, one would expect a decrease of N in the GaON films and an increase of band gap towards the value of Ga2O3, which is just opposite to what we observed Thus, we concluded that GaON ternary alloy with single phase hexagonal structure has been formed Since the diameter of oxygen and nitrogen is almost the same to each other, which results in the lattice constant of hexagonal GaON is close to the GaN It is very interesting that a large tunability of band gap for the GaON alloy can be achieved The huge spectral span of 2.75 eV (4.8 eV-2.05 eV) is much larger than the expected value of 1.4 eV 065016-5 Song et al AIP Advances 6, 065016 (2016) (4.8eV Ga2O3 - 3.4eVGa N ) As shown in Fig 3, the dependence of band gap on NT can be roughly divided into region I and II Region I is NT ≤ 700◦C, in which the band gap gradually decreases with the increase of NT However, with NT ≥ 750oC (region II), the rate of band gap narrowing increases rapidly and deviate from the apparently expected trend, indicated by the dotted line which is extrapolated from the experimental results in region I Before further theoretical calculations are performed to study the band gap of the material, we tentative attribute the decrease of band gap to the increase of N composition in GaON The decrease of the band gap to a value less than the GaN band gap at elevated annealing temperatures indicate the existence of a very large energy gap bowing in the material system Such large bowings have been observed in other material system such as (ZnO)1-x(GaN)x and (ZnO)1-x(AlN)x.24–27 Additionally, for N-alloyed Ga2O3, the effects of phase transition may need to be considered The introduction of N into Ga2O3 will lift up the valence band Theoretical calculation suggests that the repulsion of p and d orbits shifts the valence band maximum upward without affecting the conduction band minimum.28 A possible explanation of narrower band gap of Ga2O3 is that the enhanced p-d repulsion, provide by Ga3d and N2p, can push the valence band maximum upward in Ga2O3 This mechanics plays a dominant role in the region I On the other hand, in region II, there is another reason for the band gap narrowing Note that the change of band gap becomes apparent with NT at 750 ◦C where the new phase of hexagonal structure just appears With the improvement of hexagonal crystalline structure at higher NT, the value of band gap even approaches 2.05 eV Because of the band gap of amorphous sample is consistent with that of β-Ga2O3, the unit cell of amorphous samples may be very similar to monoclinic structure.29 In contrast to the formation of monoclinic phase in oxygen annealing, a new phase with hexagonal structure was formed with nitridation treatment This implies that introduction of nitrogen into Ga2O3 induces the phase transition from monoclinic to hexagonal structure, which may be contributed to band gap narrowing Note that the band gaps of Ga2O3-based alloys are dependent on the crystal structures For example, the band gaps of CuGaO2 are 1.47 eV and 3.53 eV with hexagonal and rhombohedral delafossite crystal structure, respectively.30,31 In summary, the band gap of GaON has been tuned in a wide range from 4.8 eV to 2.05 eV continually with a two-step nitridation method performed on as-deposited amorphous thin films With the increase of NT, the amorphous samples crystallized into GaON ternary alloys with hexagonal structure The reasons for band gap narrowing in N-alloyed Ga2O3 are connected with N2p Ga3d orbital repulsion and the formation of hexagonal phase ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (11474076) and the Postdoctoral Science-Research Developmental Foundation of Heilongjiang Province(Grant No LBH-Q13082) Shin-ichiro Inoue, Tamari Naoki, Toru Kinoshita, Toshiyuki Obata, and Hiroyuki Yanagi, APPLIED PHYSICS LETTERS 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ADVANCES 6, 065016 (2016) Band gap engineering of N- alloyed Ga2O3 thin films Dongyu Song,1,a Li Li,2,a Bingsheng Li,1,b Yu Sui,1 and Aidong Shen3,b Department of Physics, Harbin Institute of Technology... decrease of band gap to the increase of N composition in GaON The decrease of the band gap to a value less than the GaN band gap at elevated annealing temperatures indicate the existence of a very... or ZnO & MgO, it is expected that the band gap of GaON can be tuned continuously at least between 3.4 eV (Eg of GaN) and 4.8 eV (Eg of Ga2O3) In this paper, we report the tuning of GaON band gap