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Hydrogen in Ga2O3
Hydrogen in Ga2O3
9
Michael Stavola*, W. Beall Fowler*, Ying Qin*, Philip Weiser*, Stephen Pearton† *Department of Physics, Lehigh University, Bethlehem, PA, United States, †Department of Materials Science and Engineering, University of Florida, Gainesville, FL, United States
Chapter Outline 9.1 Introduction 191 9.2 Hydrogen in the transparent conducting oxides ZnO, SnO2, and In2O3 9.3 Hydrogen in β-Ga2O3 195 9.3.1 9.3.2 9.3.3 9.3.4
192
Theory 195 Thermal stability of deuterium in Ga2O3 196 Muon spin resonance 196 Vibrational properties of H in Ga2O3 196
9.4 Conclusion 205 Acknowlegments 206 References 206
9.1
Introduction
Semiconductors with bandgaps larger than the 3.4 eV bandgap of GaN are emerging as a new class of ultrawide-bandgap (UWBG) electronic materials [1–5]. In spite of the promising applications that are possible for UWBG materials, an understanding of their fundamental properties is at an early stage of development. The focus of this chapter is the hydrogen impurity and its interactions with other defects in β-Ga2O3, a transparent conducting oxide with an ultrawide bandgap of 4.9 eV [6–9]. (It is the most thermally stable monoclinic β phase of Ga2O3 to which we refer throughout this chapter.) The UWBG semiconductors show promise for device applications with dramatically improved performance. The large bandgap of Ga2O3 leads to a theoretical breakdown field of 8 MV/cm [3, 4, 9, 10]. The Baliga figure of merit for Ga2O3 (the figure of merit for power devices) is at least four times larger than those of GaN and 4H-SiC. An experimental breakdown field of 3.8 MV/cm for Ga2O3 has already been achieved in metal-oxide-semiconductor field-effect transistors and is higher than the critical field strengths of GaN and SiC [11]. The ultrawide bandgap of Ga2O3 also opens up opportunities for optoelectronic devices in the deep UV and for devices that can operate in harsh environments. An advantage of Ga2O3 over the wide-bandgap nitrides is the availability of native single-crystal substrates [4]. Bulk Ga2O3 can be grown by the floating zone [12], Gallium Oxide. https://doi.org/10.1016/B978-0-12-814521-0.00009-9 © 2019 Elsevier Inc. All rights reserved.
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Czochralski [13], vertical Bridgeman [14], and edge-defined film-fed growth (EFG) methods [15]. With the EFG method, it has been possible to produce high-quality, 400 -diameter, Ga2O3 wafers. Ga2O3 shows unintentional n-type conductivity [16]. While the conventional wisdom has been that O vacancies can cause this conductivity, theory finds that O vacancies are not shallow donors in Ga2O3 and suggests that H can be a cause of n-type behavior [17]. This chapter is a survey of the properties of hydrogen in Ga2O3. These properties are discussed in the context of hydrogen’s behavior in other transparent conducting oxides.
9.2
Hydrogen in the transparent conducting oxides ZnO, SnO2, and In2O3
The passivation of dopants and defects in conventional semiconductors such as Si, GaAs, and GaN by hydrogen is well known and has an impact on technology that is widely recognized [18]. However, hydrogen in semiconducting oxides is dramatically different [19]. Recent theory has reinvigorated interest in hydrogen as a possible source of conductivity [17, 20–25] and has drawn attention to work performed in the 1950s where hydrogen in ZnO was found to give rise to shallow donors [26, 27]. The properties of hydrogen in ZnO, SnO2, and In2O3 are surveyed briefly here so that the behavior of hydrogen in Ga2O3 can be compared and contrasted with the behavior of hydrogen in other oxides that have been studied recently. Two defects have been predicted to be shallow donors in several oxide hosts: interstitial hydrogen, Hi, and hydrogen trapped at an oxygen vacancy, HO [17, 20–25]. Hi forms strong O-H bonds with stretching frequencies above 3000 cm1. HO gives rise to a novel multicenter bond with a much lower vibrational frequency. The muon is a positively charged particle with 1/9 the mass of a proton that mimics the properties of hydrogen. The spectroscopy of implanted muons has been widely used to investigate the properties of hydrogen in solids [28, 29]. Implanted muons have been found to form shallow donors in several oxide hosts, suggesting that the behavior of hydrogen will be similar. Hydrogen also interacts with cation vacancies in transparent conducting oxides [30] and can also form H2 molecules [31, 32]. ZnO: ZnO has the hexagonal wurtzite structure and a bandgap of 3.4 eV [19]. Hydrogen in ZnO gives rise to both Hi and HO shallow donors whose properties have been investigated extensively by theory and experiment. Muon spin resonance measurements showed that implanted muons form shallow donors in ZnO [33], so a similar result is expected for hydrogen. An O-H vibrational line that is polarized along the c-axis of ZnO was found at 3611 cm1 [34]. The 3611 cm1 line was found to be marginally stable at room temperature and was assigned to the Hi shallow donor [35–37]. When Hi is annealed away at temperatures near 150°C, hydrogen then forms interstitial H2 molecules that provide a reservoir of hydrogen in the sample that can be converted back to Hi by thermal annealing [31, 32].
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193
ZnO also contains a more thermally stable hydrogen shallow donor that is annealed away at 500°C [35–37] and that theory has predicted is due to HO [22]. The HO center in ZnO has been found to form a novel multicenter bond where H is weakly bonded to four Zn neighbors [22]. The vibrational mode predicted for HO in ZnO lies in a region where ZnO is strongly absorbing. In this case, photothermal ionization spectroscopy (PTIS) was used to detect the vibrational lines of HO at 742 and 792 cm1 that were assigned to the A1 and E modes of the C3V center [38]. The Hi and HO shallow donors in ZnO have electronic transitions that also have been studied by photoluminescence, Raman, and PTIS measurements [37]. Infrared (IR) lines at 3312.2 and 3349.6 cm1 have been assigned to the O-H stretching modes of the VZnH2 center in ZnO [34]. IR lines at 3303 and 3321 cm1 have been assigned to the doubly degenerate and singly degenerate O-H stretching modes, respectively, of the VZnH3 center in ZnO [39]. SnO2: SnO2 has the rutile structure and a direct bandgap of 3.6 eV [19]. Early experimental work showed that annealing SnO2 in a hydrogen-containing ambient gives rise to strong n-type conductivity [40]. Muon spin resonance experiments have found that muons implanted into SnO2 form shallow donors [41]. Interstitial hydrogen and hydrogen at an oxygen vacancy have been predicted by theory to be donors in SnO2 [23, 42]. Hi was predicted to be mobile near room temperature, whereas HO was predicted to be more thermally stable. Recent experiments confirm that annealing SnO2 single crystals in an H2 ambient increases their conductivity and also gives rise to several O-H vibrational lines with distinctive polarization properties [42, 43]. Two hydrogen shallow donors have been discovered, one that is marginally stable at room temperature and a second donor that is stable up to 650°C [43]. A vibrational line at 3156.1 cm1 was assigned to the less stable Hi center. The more stable donor has properties consistent with the HO center predicted by theory [23]. The vibrational modes of several additional O-H centers in SnO2 have also been discovered [42, 43]. Structures with H bound to a Sn vacancy (VSn) [42] or with H bound to a Sn interstitial (ISn) [44] have been proposed. The polarization properties of the vibrational lines determined the O-H bond angles for two of these additional O-H centers and support their assignment to structures with H bonded to an interstitial defect, possibly ISn [44]. Other lines remain unassigned and could be due to structures involving VSn. The electrically active, hydrogen-shallow-donor centers have been found to interact with the additional centers that involve hydrogen complexed with native defects [42, 43]. These defects can be interconverted from one to another by thermal treatments and can give rise to unexpected changes in the conductivity of SnO2 samples that contain H upon annealing. In2O3: In2O3 has the cubic bixbyite structure with a conventional unit cell that contains 80 atoms [19, 45]. The oxygen sites are all equivalent, and there are two inequivalent In sites [24]. Recent theoretical and experimental works find that hydrogen can be an important shallow donor in In2O3. Muon-spin-resonance experiments have found that implanted muons form shallow donors in In2O3 [41]. In2O3 thin films containing hydrogen show n-type conductivity with high mobility that has attracted
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attention for solar-cell applications [46, 47]. Furthermore, theory finds that Hi and HO are shallow donors [24]. Annealing In2O3 single crystals in an H2 ambient at temperatures near 450°C has been found to produce a thin conducting layer near the sample surface with a thickness near 100 μm and a carrier concentration of 1019 cm3 [48]. Infrared absorption experiments have found that substantial free-carrier absorption and several O-H vibrational lines are produced by hydrogenation treatments. The broad absorption due to free carriers that increases as the frequency decreases is shown in Fig. 9.1A. The O-H absorption lines produced by the hydrogenation of In2O3 are shown in Fig. 9.1B. In annealing experiments, the O-H vibrational line at 3306 cm1 was found to be correlated with the free-carrier absorption and was assigned to the interstitial hydrogen shallow-donor center that is responsible for the hydrogen-related conductivity [48]. Unlike ZnO and SnO2 where HO is the dominant hydrogen shallow donor, Hi has been found to be the dominant H shallow donor in In2O3. Several additional O-H lines near 3400 cm1 were produced by annealing In2O3 in a hydrogen ambient [48]. These lines have been suggested to be due to defect complexes with H trapped by indium vacancies (VIn).
600 2
0.3
as treated
500 Absorbance
300
0.2 400
1 400 0.1
500 0
300
600
as treated 0.0
2000
3000
4000
3300
3400
–1
Frequency (cm )
(A)
(B)
Fig. 9.1 A selection of IR absorption spectra (T ¼ 4.2 K) for an In2O3 sample that initially had been hydrogenated by an anneal (30 min) in an H2 ambient at 500°C. The sample was then annealed sequentially in flowing He at the temperatures shown in °C. (A) The absorption due to free carriers. (B) The IR absorption lines in the O-H stretching region. These spectra were baseline corrected to remove the contribution from free carriers.
Hydrogen in Ga2O3
195
Hydrogen in β-Ga2O3
9.3
β-Ga2O3 has a complex monoclinic crystal structure (Fig. 9.2A) [49, 50]. There are two inequivalent Ga sites and three inequivalent oxygen sites. Ga(1) and Ga(2) are tetrahedrally and octahedrally coordinated, respectively. O(1) and O(2) are threefold coordinated and O(3) is fourfold coordinated.
9.3.1 Theory Ga2O3 shows unintentional n-type conductivity [16]. While the conventional wisdom has been that O vacancies can cause this conductivity, theory finds that O vacancies are not shallow donors in Ga2O3 and suggests that H can be a cause of n-type behavior [17]. Both Hi and HO have been predicted to be shallow donors [17]. Given the number of possible Ga and O sites in Ga2O3, there are several possible configurations for Hi and HO centers. Hydrogen can affect the electrical properties of Ga2O3 by acting as a shallow donor or by compensating other defects, such as the gallium vacancy, that act as deep acceptors [17, 30]. The Ga vacancy has low formation energy and is predicted to interact with H to produce VGa-H defects with high thermal stability [30]. The interactions of H with VGa in β-Ga2O3 are predicted to give rise to large configurational relaxations of the VGa defect [30, 51].
[1 0 2] direction
a Light perpendicular to (–2 0 1) plane b
c
[0 1 0] direction out of plane
(A)
(B)
Fig. 9.2 (A) The unit cell of β-Ga2O3. The inequivalent sites in this and subsequent figures are color coded as follows: Ga(1), purple; Ga(2), dark green; O(1), red; O(2), yellow; O(3), light green. (B) Experimental orientation of the samples used in our experiments. These and subsequent figures were constructed using MOLDRAW (P. Ugliengo, Torino 2006, available at http://www.moldraw.unito.it/) and POV-Ray (http://povray.org).
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Gallium Oxide
9.3.2 Thermal stability of deuterium in Ga2O3 Deuterium was introduced into the (201) face of β-Ga2O3 by plasma exposure (up to 270°C) and by ion implantation (100 keV, 1015 cm2) to investigate its thermal stability [52, 53]. Secondary ion mass spectrometry (SIMS) experiments showed that the out-diffusion of deuterium from samples treated in a plasma occurs at a lower temperature than for samples deuterated by ion implantation. [Deuterium was used in these experiments because the detection limit for D by SIMS is approximately three decades lower than for H because of the small natural isotopic abundance of D (0.0156%).] For samples deuterated by plasma exposure, the out-diffusion of D from the sample surface was proposed to be limited by the formation of D2 molecules and occurred at an annealing temperature of 500°C [53]. For samples deuterated by ion implantation, the out-diffusion of D was proposed to by limited by the interaction of D with implantation damage and occurred at an annealing temperature of 650°C [53]. Simulations of the in-diffusion and out-diffusion behaviors of D for both plasma-treated and ion-implanted samples produced satisfactory fits to the data.
9.3.3 Muon spin resonance When muons were implanted into powdered samples of β-Ga2O3, they were found to give rise to a shallow donor center with a binding energy between 15 and 30 meV and a ˚ [54]. These results are consistent with the muon forming a shallow Bohr radius of 20 A effective mass-like defect in β-Ga2O3. A subsequent report for muons implanted into single-crystal specimens of Ga2O3 below room temperature resolved two neutral centers, Mu1 and Mu2, with electron binding energies of 7 and 16 meV [55]. The earlier measurements [54] in powder samples were suggested to be an unresolved average of the Mu1 and Mu2 centers. At reduced temperatures, several metastable states for the muon were observed. At temperatures above 620 K, thermalized muons occupy a diffusively mobile state that shows ground state behavior [55]. An activation energy for Mu+ diffusion of 1.65 eV was inferred from the observed hop rate for this state.
9.3.4 Vibrational properties of H in Ga2O3 Hydrogen-containing defects in Ga2O3 have been investigated and microscopic properties have been determined through studies of their O-H vibrational modes [56]. Properties of the broad IR absorption that can arise from free carriers were investigated to determine whether hydrogen centers give rise to conductivity [57, 58]. Furthermore, the polarization properties of the hydrogen vibrational modes in ZnO and SnO2, for example, have provided valuable information about the structures of the defects that can form [34, 43, 44]. Similarly, the polarization properties of the vibration modes of O-H centers in Ga2O3, when combined with theory, help to identify defect structures [51].
Hydrogen in Ga2O3
197
9.3.4.1 Vibrational spectroscopy Fig. 9.2B shows the sample orientation used in these experiments. H and D were introduced into Ga2O3 by annealing n-type, single-crystal substrates purchased from MTI Corporation in an H2 or D2 ambient at temperatures above 800°C. Fig. 9.3A and B show that O-H and O-D vibrational lines are produced at 3437.0 and 2546.4 cm1, respectively, by the introduction of H and D. These lines are strongly polarized and are seen for the polarization with electric vector E // [102] but not for the polarization with electric vector E // [010]. Because this sample had a (201) face, the polarization with E // [201] could not be thoroughly investigated. These results do not preclude the possibility of absorption in the direction of the incident light (E // [201]), which would not be observed. Experiments are ongoing to pursue this further. The O-H and O-D lines shown in Fig. 9.3, along with a few additional lines, can also be produced by the implantation of protons or deuterons at room temperature [51]. Fig. 9.4 shows results for a Ga2O3 sample purchased from the Tamura Corporation that was implanted at room temperature with deuterons with multiple doses and energies up to 280 keV to produce a deuterated layer, 1200 nm thick, with a deuterium concentration of approximately 1020 cm3. Introducing deuterium into Ga2O3 by the implantation of deuterons produces the 2546 cm1 line that corresponds to the
Fig. 9.3 Polarized IR absorption spectra obtained at 10 K for the hydrogenated, deuterated, and codoped samples from MTI.
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Gallium Oxide
Fig. 9.4 Polarized IR absorption spectra obtained at 10 K for a Ga2O3 sample from the Tamura Corp. that had been implanted at room temperature with deuterons. Effects of annealing at 400°C and 500°C are shown. Upper (blue) spectra were obtained with E // [102], the lower null spectra (red) with E // [010].
3437 cm1 O-H line along with several weaker lines at 2518, 2592, and 2632 cm1. All of these lines are seen for the polarization with E // [102]. Ga2O3 samples can be prepared that contain both H and D to test whether the defects that have been seen contain more than one hydrogen atom. (A defect that contains two H atoms, for example, will have an isotopic sibling that contains both H and D with distinctive vibrational properties that allow it to be identified.) The spectra shown in Fig. 9.3C and D reveal that Ga2O3 samples prepared by annealing in a mixture of both H2 and D2 do, in fact, show additional new vibrational lines at 3438.2 cm1 for O-H and 2547.1 cm1 for O-D [51]. These new lines are a signature of a defect that contains two identical H or D atoms. To explain the spectra shown in Fig. 9.3, the defect that contains two identical H (or D) atoms must have two coupled O-H (or O-D) modes, the first of which is IR active and the second of which is IR inactive. For a defect that contains both H and D, the O-H and O-D modes of the defect become dynamically decoupled and give rise to new vibrational lines for the decoupled oscillators. Furthermore, because the coupled and decoupled modes lie close in frequency, the coupling of the two O-H (or O-D) oscillators must be weak.
9.3.4.2 Evidence for a “hidden hydrogen” species An especially effective way to introduce H into Ga2O3 and to produce the 3437 cm1 center has been found to involve a two-step annealing process. First, a Ga2O3 sample was annealed in an H2 ambient at a temperature of 800°C or greater. Immediately following this annealing treatment, it was found that the 3437 cm1 line could be either weak or absent [as is shown in Fig. 9.5, spectrum (i)]. The second step was an
Hydrogen in Ga2O3
199
Fig. 9.5 IR absorbance spectra for a Ga2O3 sample that had received a two-step annealing treatment to introduce hydrogen. Spectrum (i) was measured for a sample annealed in an H2 ambient for 1 h at 900°C. Spectrum (ii) is for the same sample after a subsequent anneal at 400°C in flowing N2.
annealing treatment at a lower temperature in an inert ambient. Fig. 9.5, spectrum (ii), shows that a second annealing treatment at 400 °C in flowing N2 produced the 3437 cm1 O-H vibrational line. These results show that H is introduced into Ga2O3 by the first anneal at T > 800°C in a form that does not give rise to an observable O-H line. A second anneal at a temperature near 400°C transforms this hidden reservoir of H into the defect that gives rise to the 3437 cm1 center. (Samples that had been deuterated by annealing in a D2 ambient also showed a “hidden D” species that gave rise to a strong 2546 cm1 line in a similar two-step annealing process.) The first anneal in an H2 ambient at high temperature (T 800°C) charges the sample with hydrogen. The second anneal at 400°C produces the 3437 cm1 center. There are several possibilities for the hidden form of hydrogen that is produced by annealing in H2 gas at elevated temperatures. (i) There could be O-H centers with transition moments oriented primarily along the [201] direction. Such a defect would not be visible for the polarizations of the probing light that have been used in the experiments described above and, therefore, would not have been seen. (ii) Hydrogen in Ga2O3 could be introduced in the form of interstitial H2 molecules whose vibrational modes are not IR active. The introduction of hidden H2 was discovered to be important in ZnO where H2 acted as a source and sink for H in defect reaction [31, 32]. (iii) The hidden-hydrogen defect could be the HO shallow donor predicted by theory [17] that has a low vibrational frequency that appears in the spectral region where oxides are highly absorbing. An example of this possibility is provided by hydrogen trapped at an oxygen vacancy in ZnO [22, 38]. Ga2O3 samples hydrogenated (or deuterated) by the ion implantation of protons (or deuterons) also show interesting hydrogen reactions upon annealing at 400°C in an
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Gallium Oxide
inert ambient. Fig. 9.5 shows that annealing a sample at 400°C that had been implanted with D increases the strength of the 2546 cm1 line by roughly 4 times, suggesting the presence of the same hidden species that is formed in Ga2O3 samples annealed in an H2 ambient. Data shown in Fig. 9.6 support the suggestion that an H-related shallow donor can be a source of “hidden” hydrogen in Ga2O3 samples. Spectrum (i) in Fig. 9.6A shows the free-carrier absorption for a Ga2O3 sample that was annealed in an H2 ambient at 1000°C. This annealing treatment does not produce the 3437 cm1 O-H line. Spectrum (ii) shows that the free-carrier absorption is reduced by a subsequent anneal at 400°C in an inert ambient. Furthermore, this second annealing treatment also produces the 3437 cm1 line [spectrum (ii) in Fig. 9.6A and B]. These results suggest that an H-related shallow donor introduced by the anneal in H2 is converted into the 3437 cm1 O-H center by a subsequent anneal at 400°C in an inert ambient. Spectrum (iii) shows that further annealing at 1000°C in an inert ambient removes the 3437 cm1 center from the sample. The spectra labeled (iv) in Fig. 9.6A and B show differences of spectra (i) and (ii) in the same panels. These difference spectra emphasize contributions arising from H rather than from other donors (presumably Sn or Si) that can be present. The broad absorption shown in Fig. 9.6A, spectrum (iv), that increases at low frequency is due to the free-carrier absorption that is due to hydrogen in the sample. The downward going IR line shown in Fig. 9.6A and B, spectrum (iv), is due to the 3437 cm1 center that grows in during the second anneal at 400°C in an inert ambient.
Fig. 9.6 IR absorption spectra (T ¼ 77 K) for a Ga2O3 sample from Tamura Corp. that initially had been hydrogenated by annealing at 1000°C. This sample was subsequently annealed at in Ar at the temperatures indicated. (A) Shows the absorption due to free carriers and (B) shows the 3437 cm1 O-H vibrational line that corresponds to the free-carrier data shown in (A).
Hydrogen in Ga2O3
201
The data in Fig. 9.6 suggest that the Hi or HO shallow donors predicted by theory are good candidates for the reservoir of hidden hydrogen that can be produced by annealing a Ga2O3 sample in an H2 ambient. However, these results do not eliminate other possibilities (like H2) that may also provide a reservoir of hydrogen in our samples that are difficult to observe directly.
9.3.4.3 Theory of defect structures and their vibrational properties There are a variety of defects that can exist in this complex structure [59–62], and hydrogen can interact with many of them. Therefore, there are many possible configurations for interstitial H and for H complexed with native defects such as O or Ga vacancies. The total absence of any O-H vibrational absorption in the [010] direction is remarkable and provides severe constraints regarding the possible nature and structure of the observed defects. (Analysis of the dichroism associated with monoclinic Ga2O3 reveals [63, 64] that its effect will be negligibly small in these experiments.) Furthermore, as noted, the line shifts associated with codoping with H and D further narrow the possible candidates for the observed defects. While qualitative chemical arguments can be used to suggest approximate O-H axes, detailed calculations are needed to provide, with some confidence, the structures and properties of possible defect configurations. Such calculations have been carried out using the CRYSTAL06 code [65], choosing density functional theory (DFT) with a gradient-corrected approximation to the exchange-correlation functional (Becke’s B3LYP hybrid potential [66] with 20% exact exchange and Lee-Yang-Parr correlation [67]). This approach has been used successfully for defect calculations in a number of similar systems [43, 48, 68–73]. Calculations were carried out in fully relaxed periodic supercells containing 80, 120, or 160 host atoms, plus one or more H impurities, with lattice constants computed for the relaxed perfect crystal. A 2 2 2 k-point mesh of Monkhorst-Pack type [74] was used. The SCF convergence criterion was 107 Ha except for vibrational calculations, where 1010 Ha was used. Gaussian basis functions were of the type 311p(1) for H [75], 8411 for O [76], and 864111d(41) for Ga [77]. Charge states for the defect calculations are based on atomic ions; e.g., the charge state for one H at a Ga vacancy is (2). Harmonic and anharmonic vibrational frequencies may be calculated by CRYSTAL06. A number of potential defect configurations were investigated, beginning with interstitial H. Fig. 9.7 shows how Hi could attach to one of two opposing sites on O(1) or O(2), or one of four quasitetrahedral sites on O(3). Several of these have their O-H dipole perpendicular to [010] and therefore bear consideration as candidates for single O-H defects. A second candidate as a host for O-H dipoles is a Ga vacancy, for which there is both experimental [78] and theoretical [30] support. While there is theoretical [30, 51] evidence that Ga(1) vacancies are energetically favored over Ga(2) vacancies, both types are possible candidates for H traps. When atomic relaxation is taken into account, there are three inequivalent Ga(1) vacancy configurations [30, 62] to consider. One is the simple vacancy, with four “dangling bonds.” The other two arise from
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[1 0 2] direction
Incident light perpendicular to (–2 0 1) plane
[0 1 0] direction out of plane
Fig. 9.7 Possible hydrogen interstitial sites (in blue) in an unrelaxed Ga2O3 lattice.
a large shift of a neighboring Ga to a site midway between two resulting halfvacancies. These three configurations are shown and labeled in Fig. 9.8. Varley et al. [30] and Kyrtsos et al. [62] find a shifted configuration to be lower in energy than the unshifted, while Deak et al. [61] find the opposite. Recent results [51] as well favor the stability of the shifted configurations, with the Ga(1)21 configuration more stable than the Ga(1)23 configuration. A search for single-hydrogen sites associated with Ga vacancies such that the O-H axis has no [010] component leads to a number of possibilities. Two inequivalent H locations for the Ga(2) vacancy satisfy this condition [Fig. 9.9A]. There are two inequivalent H sites for the unshifted Ga(1) vacancy that could qualify [Fig. 9.9B] and also two pairs of equivalent Ga(1)21 sites [Fig. 9.10A and B]. Two equivalent H locations for the shifted Ga(1)23 site are candidates (Fig. 9.11).
Shifted Ga(1)23
g
g
Shifted Ga(1)21
Ga(1) vacancy
(A)
(B)
(C)
Fig. 9.8 Possible Ga(1) vacancy sites: (A), unrelaxed; (B), neighboring Ga(1) shifted to site with O(2) and O(3) neighbors [Ga(1)23]; (C) neighboring Ga(1) shifted to site with O(2) and O(1) neighbors [Ga(1)21].
[1 0 2] direction H
H
H H
Incident light perpendicular to (–2 0 1) plane
(A)
[0 1 0] direction out of plane
(B)
Fig. 9.9 (A) Ga(2) vacancy site plus two inequivalent H sites with no (010) O-H projection. (B) Unshifted Ga(1) vacancy site plus two inequivalent H sites with no (010) O-H projection.
[1 0 2 ] direction H H
H
H
Incident light perpendicular to (–2 0 1) plane
(A)
(B)
[0 1 0] direction out of plane
Fig. 9.10 (A, B) Two relaxed configurations of Ga(1)21, each with two equivalent H sites with no (010) O-H projection.
D dipole moment = 0
[1 0 2] direction
Incident light perpendicular to (–2 0 1) plane D dipole moment ¹ 0 [0 1 0] direction out of plane
Fig. 9.11 Relaxed configuration of Ga(1)23 plus two equivalent H sites with no (010) O-H projection. Shown also are the stretch modes for the corresponding two O-H defect, one with zero change in dipole moment and the other with nonzero change in dipole moment.
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The emergence of new lines in the region of the main O-D and O-H lines upon codoping with H and D is, as noted, the signature of a defect that involves two hydrogens. But because these two hydrogens generate only one observed line for H or D, one of the two O-H stretch modes is unobservable. Furthermore, the small shift associated with HD means that the individual O-H components are weakly coupled. In order for a stretch mode involving two O-H partners to have zero dipole moment, the two O-H’s must be symmetry equivalent and either parallel or antiparallel. The configurations shown in Figs. 9.10 and 9.11, which are associated with a shifted Ga(1) vacancy, may be reconsidered, now with both H sites occupied, in which case they do satisfy this condition. (Those shown in Fig. 9.9 are not symmetry equivalent and so are not candidates.) However, the configuration of Fig. 9.10A has only a modest (102) component and that of Fig. 9.10B lacks a (102) projection. Shifted Ga(1)23 plus two H, shown in Fig. 9.11, is the most likely candidate for the 3437 cm1 line. It has a large (102) component and also has the lowest energy (by 0.6 eV) of all configurations investigated, including a number that do not satisfy the experimental polarization constraints. Furthermore, the large distance between the individual O-H components leads to weak coupling between them.
9.3.4.4 Additional IR lines More recent experiments have revealed several additional IR lines that may be due to some of the other defect structures that have been predicted by theory. The focus is on O-D centers because their IR lines are detected with higher signal to noise ratio than for the corresponding O-H centers. The 2547 cm1 vibrational line is the dominant feature seen in a Ga2O3 sample deuterated in a D2 ambient. However, when such a sample was annealed at elevated temperature (T > 900 °C) to remove H or D and then retreated in D2, a few additional IR lines were produced. Fig. 9.12 shows O-D lines at 2584 and 2632 cm1 that were seen
Fig. 9.12 Polarized IR absorption spectra measured at 77 K for a Ga2O3 sample from the Tamura Corp. that had been repeatedly annealed in a D2 ambient.
Hydrogen in Ga2O3
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(in addition to the 2547 cm1 line) for the polarization with E // [102] for a Ga2O3 sample that had been repeatedly treated in D2 at 900°C. [The 2632 cm1 line was also seen in a sample implanted with deuterons (Fig. 9.4)]. It is not yet certain whether the additional lines that enter upon heat treatment involve more than one H. If not, one or more of the configurations shown in Figs. 9.8–9.11 with only one H, including hydrogen interstitials, may well be responsible for these lines. Work is ongoing by both experiment and theory that promises to identify additional hydrogen defect structures.
9.4
Conclusion
Properties of the hydrogen impurity in Ga2O3 have been discussed and compared with hydrogen’s behavior in other oxides. SIMS measurements show that hydrogen forms thermally stable defects in Ga2O3 [52, 53]. Muon spin resonance finds that implanted muons form shallow donors in Ga2O3 [17], and H is suggested to behave similarly [55]. Theory predicts that Hi and HO centers are shallow donors in Ga2O3 [17] and that the VGa deep acceptor traps H to produce a defect with low formation energy and high thermal stability [30]. Vibrational spectroscopy and its analysis by theory are being applied to Ga2O3 to provide insight into the structures and properties of the defects that hydrogen can form [51]. Interstitial hydrogen is prominent in ZnO, SnO2, and In2O3 single crystals that had been hydrogenated by annealing in an H2 ambient where it behaves as a shallow donor with a strong O-H stretching line with a vibrational frequency >3000 cm1 [34, 42, 43, 48]. On the contrary, no definitive evidence has yet been found for a Hi center in Ga2O3 that had been hydrogenated by annealing in H2 or by ion implantation [51]. The O-H vibrational lines of complexes of hydrogen with native defects have been observed for ZnO [34, 39], SnO2 [42, 43], and In2O3 [48]. The structures and vibrational properties of these defects have been predicted by theory [30]. Similarly, a strong O-H vibrational line at 3437 cm1 is produced in Ga2O3 by hydrogen introduced either by annealing in an H2 ambient or by proton implantation [51]. Theory has investigated the structures and vibrational properties of several possible VGa–Hn complexes in Ga2O3. The observed polarization properties of the 3437 cm1 line and its behavior in samples that contained both H and D suggest the assignment of the 3437 cm1 line to a particular VGa-2H structure (Fig. 9.11) [51]. ZnO [31, 32] and SnO2 [42, 43] have been found to contain “hidden” hydrogen that does not give rise to prominent O-H vibrational lines. Both interstitial H2 and HO are species that do not give rise to O-H absorption but that can provide a reservoir of H in a sample. When Ga2O3 is annealed in an H2 ambient, a hydrogen species not seen by IR absorption is produced. A subsequent anneal in an inert ambient at 400 °C converts this “hidden” species into the VGa-2H center with a strong O-H line at 3437 cm1. Both H2 and HO are candidates for the reservoir of hidden H in Ga2O3 as are hydrogen centers that could have their transition moments along a [201] direction. While progress has been made by experiment and theory toward determining the behavior of hydrogen in Ga2O3, much remains to be done before the properties of H in
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Ga2O3 are understood at a level that is typical of other semiconducting oxides such as ZnO, SnO2, and In2O3.
Acknowlegments The work at Lehigh University was supported by NSF Grant No. DMR 1160756 and the Sigma Xi Grants-in-Aid of Research program. The work at UF is partially supported by HDTRA1-17-1-0011. The project or effort depicted is sponsored by the Department of the Defense, Defense Threat Reduction Agency. The content of the information does not necessarily reflect the position or the policy of the federal government, and no official endorsement should be inferred.
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9
Michael Stavola*, W. Beall Fowler*, Ying Qin*, Philip Weiser*, Stephen Pearton† *Department of Physics, Lehigh University, Bethlehem, PA, United States, †Department of Materials Science and Engineering, University of Florida, Gainesville, FL, United States
Chapter Outline 9.1 Introduction 191 9.2 Hydrogen in the transparent conducting oxides ZnO, SnO2, and In2O3 9.3 Hydrogen in β-Ga2O3 195 9.3.1 9.3.2 9.3.3 9.3.4
192
Theory 195 Thermal stability of deuterium in Ga2O3 196 Muon spin resonance 196 Vibrational properties of H in Ga2O3 196
9.4 Conclusion 205 Acknowlegments 206 References 206
9.1
Introduction
Semiconductors with bandgaps larger than the 3.4 eV bandgap of GaN are emerging as a new class of ultrawide-bandgap (UWBG) electronic materials [1–5]. In spite of the promising applications that are possible for UWBG materials, an understanding of their fundamental properties is at an early stage of development. The focus of this chapter is the hydrogen impurity and its interactions with other defects in β-Ga2O3, a transparent conducting oxide with an ultrawide bandgap of 4.9 eV [6–9]. (It is the most thermally stable monoclinic β phase of Ga2O3 to which we refer throughout this chapter.) The UWBG semiconductors show promise for device applications with dramatically improved performance. The large bandgap of Ga2O3 leads to a theoretical breakdown field of 8 MV/cm [3, 4, 9, 10]. The Baliga figure of merit for Ga2O3 (the figure of merit for power devices) is at least four times larger than those of GaN and 4H-SiC. An experimental breakdown field of 3.8 MV/cm for Ga2O3 has already been achieved in metal-oxide-semiconductor field-effect transistors and is higher than the critical field strengths of GaN and SiC [11]. The ultrawide bandgap of Ga2O3 also opens up opportunities for optoelectronic devices in the deep UV and for devices that can operate in harsh environments. An advantage of Ga2O3 over the wide-bandgap nitrides is the availability of native single-crystal substrates [4]. Bulk Ga2O3 can be grown by the floating zone [12], Gallium Oxide. https://doi.org/10.1016/B978-0-12-814521-0.00009-9 © 2019 Elsevier Inc. All rights reserved.
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Czochralski [13], vertical Bridgeman [14], and edge-defined film-fed growth (EFG) methods [15]. With the EFG method, it has been possible to produce high-quality, 400 -diameter, Ga2O3 wafers. Ga2O3 shows unintentional n-type conductivity [16]. While the conventional wisdom has been that O vacancies can cause this conductivity, theory finds that O vacancies are not shallow donors in Ga2O3 and suggests that H can be a cause of n-type behavior [17]. This chapter is a survey of the properties of hydrogen in Ga2O3. These properties are discussed in the context of hydrogen’s behavior in other transparent conducting oxides.
9.2
Hydrogen in the transparent conducting oxides ZnO, SnO2, and In2O3
The passivation of dopants and defects in conventional semiconductors such as Si, GaAs, and GaN by hydrogen is well known and has an impact on technology that is widely recognized [18]. However, hydrogen in semiconducting oxides is dramatically different [19]. Recent theory has reinvigorated interest in hydrogen as a possible source of conductivity [17, 20–25] and has drawn attention to work performed in the 1950s where hydrogen in ZnO was found to give rise to shallow donors [26, 27]. The properties of hydrogen in ZnO, SnO2, and In2O3 are surveyed briefly here so that the behavior of hydrogen in Ga2O3 can be compared and contrasted with the behavior of hydrogen in other oxides that have been studied recently. Two defects have been predicted to be shallow donors in several oxide hosts: interstitial hydrogen, Hi, and hydrogen trapped at an oxygen vacancy, HO [17, 20–25]. Hi forms strong O-H bonds with stretching frequencies above 3000 cm1. HO gives rise to a novel multicenter bond with a much lower vibrational frequency. The muon is a positively charged particle with 1/9 the mass of a proton that mimics the properties of hydrogen. The spectroscopy of implanted muons has been widely used to investigate the properties of hydrogen in solids [28, 29]. Implanted muons have been found to form shallow donors in several oxide hosts, suggesting that the behavior of hydrogen will be similar. Hydrogen also interacts with cation vacancies in transparent conducting oxides [30] and can also form H2 molecules [31, 32]. ZnO: ZnO has the hexagonal wurtzite structure and a bandgap of 3.4 eV [19]. Hydrogen in ZnO gives rise to both Hi and HO shallow donors whose properties have been investigated extensively by theory and experiment. Muon spin resonance measurements showed that implanted muons form shallow donors in ZnO [33], so a similar result is expected for hydrogen. An O-H vibrational line that is polarized along the c-axis of ZnO was found at 3611 cm1 [34]. The 3611 cm1 line was found to be marginally stable at room temperature and was assigned to the Hi shallow donor [35–37]. When Hi is annealed away at temperatures near 150°C, hydrogen then forms interstitial H2 molecules that provide a reservoir of hydrogen in the sample that can be converted back to Hi by thermal annealing [31, 32].
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ZnO also contains a more thermally stable hydrogen shallow donor that is annealed away at 500°C [35–37] and that theory has predicted is due to HO [22]. The HO center in ZnO has been found to form a novel multicenter bond where H is weakly bonded to four Zn neighbors [22]. The vibrational mode predicted for HO in ZnO lies in a region where ZnO is strongly absorbing. In this case, photothermal ionization spectroscopy (PTIS) was used to detect the vibrational lines of HO at 742 and 792 cm1 that were assigned to the A1 and E modes of the C3V center [38]. The Hi and HO shallow donors in ZnO have electronic transitions that also have been studied by photoluminescence, Raman, and PTIS measurements [37]. Infrared (IR) lines at 3312.2 and 3349.6 cm1 have been assigned to the O-H stretching modes of the VZnH2 center in ZnO [34]. IR lines at 3303 and 3321 cm1 have been assigned to the doubly degenerate and singly degenerate O-H stretching modes, respectively, of the VZnH3 center in ZnO [39]. SnO2: SnO2 has the rutile structure and a direct bandgap of 3.6 eV [19]. Early experimental work showed that annealing SnO2 in a hydrogen-containing ambient gives rise to strong n-type conductivity [40]. Muon spin resonance experiments have found that muons implanted into SnO2 form shallow donors [41]. Interstitial hydrogen and hydrogen at an oxygen vacancy have been predicted by theory to be donors in SnO2 [23, 42]. Hi was predicted to be mobile near room temperature, whereas HO was predicted to be more thermally stable. Recent experiments confirm that annealing SnO2 single crystals in an H2 ambient increases their conductivity and also gives rise to several O-H vibrational lines with distinctive polarization properties [42, 43]. Two hydrogen shallow donors have been discovered, one that is marginally stable at room temperature and a second donor that is stable up to 650°C [43]. A vibrational line at 3156.1 cm1 was assigned to the less stable Hi center. The more stable donor has properties consistent with the HO center predicted by theory [23]. The vibrational modes of several additional O-H centers in SnO2 have also been discovered [42, 43]. Structures with H bound to a Sn vacancy (VSn) [42] or with H bound to a Sn interstitial (ISn) [44] have been proposed. The polarization properties of the vibrational lines determined the O-H bond angles for two of these additional O-H centers and support their assignment to structures with H bonded to an interstitial defect, possibly ISn [44]. Other lines remain unassigned and could be due to structures involving VSn. The electrically active, hydrogen-shallow-donor centers have been found to interact with the additional centers that involve hydrogen complexed with native defects [42, 43]. These defects can be interconverted from one to another by thermal treatments and can give rise to unexpected changes in the conductivity of SnO2 samples that contain H upon annealing. In2O3: In2O3 has the cubic bixbyite structure with a conventional unit cell that contains 80 atoms [19, 45]. The oxygen sites are all equivalent, and there are two inequivalent In sites [24]. Recent theoretical and experimental works find that hydrogen can be an important shallow donor in In2O3. Muon-spin-resonance experiments have found that implanted muons form shallow donors in In2O3 [41]. In2O3 thin films containing hydrogen show n-type conductivity with high mobility that has attracted
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attention for solar-cell applications [46, 47]. Furthermore, theory finds that Hi and HO are shallow donors [24]. Annealing In2O3 single crystals in an H2 ambient at temperatures near 450°C has been found to produce a thin conducting layer near the sample surface with a thickness near 100 μm and a carrier concentration of 1019 cm3 [48]. Infrared absorption experiments have found that substantial free-carrier absorption and several O-H vibrational lines are produced by hydrogenation treatments. The broad absorption due to free carriers that increases as the frequency decreases is shown in Fig. 9.1A. The O-H absorption lines produced by the hydrogenation of In2O3 are shown in Fig. 9.1B. In annealing experiments, the O-H vibrational line at 3306 cm1 was found to be correlated with the free-carrier absorption and was assigned to the interstitial hydrogen shallow-donor center that is responsible for the hydrogen-related conductivity [48]. Unlike ZnO and SnO2 where HO is the dominant hydrogen shallow donor, Hi has been found to be the dominant H shallow donor in In2O3. Several additional O-H lines near 3400 cm1 were produced by annealing In2O3 in a hydrogen ambient [48]. These lines have been suggested to be due to defect complexes with H trapped by indium vacancies (VIn).
600 2
0.3
as treated
500 Absorbance
300
0.2 400
1 400 0.1
500 0
300
600
as treated 0.0
2000
3000
4000
3300
3400
–1
Frequency (cm )
(A)
(B)
Fig. 9.1 A selection of IR absorption spectra (T ¼ 4.2 K) for an In2O3 sample that initially had been hydrogenated by an anneal (30 min) in an H2 ambient at 500°C. The sample was then annealed sequentially in flowing He at the temperatures shown in °C. (A) The absorption due to free carriers. (B) The IR absorption lines in the O-H stretching region. These spectra were baseline corrected to remove the contribution from free carriers.
Hydrogen in Ga2O3
195
Hydrogen in β-Ga2O3
9.3
β-Ga2O3 has a complex monoclinic crystal structure (Fig. 9.2A) [49, 50]. There are two inequivalent Ga sites and three inequivalent oxygen sites. Ga(1) and Ga(2) are tetrahedrally and octahedrally coordinated, respectively. O(1) and O(2) are threefold coordinated and O(3) is fourfold coordinated.
9.3.1 Theory Ga2O3 shows unintentional n-type conductivity [16]. While the conventional wisdom has been that O vacancies can cause this conductivity, theory finds that O vacancies are not shallow donors in Ga2O3 and suggests that H can be a cause of n-type behavior [17]. Both Hi and HO have been predicted to be shallow donors [17]. Given the number of possible Ga and O sites in Ga2O3, there are several possible configurations for Hi and HO centers. Hydrogen can affect the electrical properties of Ga2O3 by acting as a shallow donor or by compensating other defects, such as the gallium vacancy, that act as deep acceptors [17, 30]. The Ga vacancy has low formation energy and is predicted to interact with H to produce VGa-H defects with high thermal stability [30]. The interactions of H with VGa in β-Ga2O3 are predicted to give rise to large configurational relaxations of the VGa defect [30, 51].
[1 0 2] direction
a Light perpendicular to (–2 0 1) plane b
c
[0 1 0] direction out of plane
(A)
(B)
Fig. 9.2 (A) The unit cell of β-Ga2O3. The inequivalent sites in this and subsequent figures are color coded as follows: Ga(1), purple; Ga(2), dark green; O(1), red; O(2), yellow; O(3), light green. (B) Experimental orientation of the samples used in our experiments. These and subsequent figures were constructed using MOLDRAW (P. Ugliengo, Torino 2006, available at http://www.moldraw.unito.it/) and POV-Ray (http://povray.org).
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Gallium Oxide
9.3.2 Thermal stability of deuterium in Ga2O3 Deuterium was introduced into the (201) face of β-Ga2O3 by plasma exposure (up to 270°C) and by ion implantation (100 keV, 1015 cm2) to investigate its thermal stability [52, 53]. Secondary ion mass spectrometry (SIMS) experiments showed that the out-diffusion of deuterium from samples treated in a plasma occurs at a lower temperature than for samples deuterated by ion implantation. [Deuterium was used in these experiments because the detection limit for D by SIMS is approximately three decades lower than for H because of the small natural isotopic abundance of D (0.0156%).] For samples deuterated by plasma exposure, the out-diffusion of D from the sample surface was proposed to be limited by the formation of D2 molecules and occurred at an annealing temperature of 500°C [53]. For samples deuterated by ion implantation, the out-diffusion of D was proposed to by limited by the interaction of D with implantation damage and occurred at an annealing temperature of 650°C [53]. Simulations of the in-diffusion and out-diffusion behaviors of D for both plasma-treated and ion-implanted samples produced satisfactory fits to the data.
9.3.3 Muon spin resonance When muons were implanted into powdered samples of β-Ga2O3, they were found to give rise to a shallow donor center with a binding energy between 15 and 30 meV and a ˚ [54]. These results are consistent with the muon forming a shallow Bohr radius of 20 A effective mass-like defect in β-Ga2O3. A subsequent report for muons implanted into single-crystal specimens of Ga2O3 below room temperature resolved two neutral centers, Mu1 and Mu2, with electron binding energies of 7 and 16 meV [55]. The earlier measurements [54] in powder samples were suggested to be an unresolved average of the Mu1 and Mu2 centers. At reduced temperatures, several metastable states for the muon were observed. At temperatures above 620 K, thermalized muons occupy a diffusively mobile state that shows ground state behavior [55]. An activation energy for Mu+ diffusion of 1.65 eV was inferred from the observed hop rate for this state.
9.3.4 Vibrational properties of H in Ga2O3 Hydrogen-containing defects in Ga2O3 have been investigated and microscopic properties have been determined through studies of their O-H vibrational modes [56]. Properties of the broad IR absorption that can arise from free carriers were investigated to determine whether hydrogen centers give rise to conductivity [57, 58]. Furthermore, the polarization properties of the hydrogen vibrational modes in ZnO and SnO2, for example, have provided valuable information about the structures of the defects that can form [34, 43, 44]. Similarly, the polarization properties of the vibration modes of O-H centers in Ga2O3, when combined with theory, help to identify defect structures [51].
Hydrogen in Ga2O3
197
9.3.4.1 Vibrational spectroscopy Fig. 9.2B shows the sample orientation used in these experiments. H and D were introduced into Ga2O3 by annealing n-type, single-crystal substrates purchased from MTI Corporation in an H2 or D2 ambient at temperatures above 800°C. Fig. 9.3A and B show that O-H and O-D vibrational lines are produced at 3437.0 and 2546.4 cm1, respectively, by the introduction of H and D. These lines are strongly polarized and are seen for the polarization with electric vector E // [102] but not for the polarization with electric vector E // [010]. Because this sample had a (201) face, the polarization with E // [201] could not be thoroughly investigated. These results do not preclude the possibility of absorption in the direction of the incident light (E // [201]), which would not be observed. Experiments are ongoing to pursue this further. The O-H and O-D lines shown in Fig. 9.3, along with a few additional lines, can also be produced by the implantation of protons or deuterons at room temperature [51]. Fig. 9.4 shows results for a Ga2O3 sample purchased from the Tamura Corporation that was implanted at room temperature with deuterons with multiple doses and energies up to 280 keV to produce a deuterated layer, 1200 nm thick, with a deuterium concentration of approximately 1020 cm3. Introducing deuterium into Ga2O3 by the implantation of deuterons produces the 2546 cm1 line that corresponds to the
Fig. 9.3 Polarized IR absorption spectra obtained at 10 K for the hydrogenated, deuterated, and codoped samples from MTI.
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Gallium Oxide
Fig. 9.4 Polarized IR absorption spectra obtained at 10 K for a Ga2O3 sample from the Tamura Corp. that had been implanted at room temperature with deuterons. Effects of annealing at 400°C and 500°C are shown. Upper (blue) spectra were obtained with E // [102], the lower null spectra (red) with E // [010].
3437 cm1 O-H line along with several weaker lines at 2518, 2592, and 2632 cm1. All of these lines are seen for the polarization with E // [102]. Ga2O3 samples can be prepared that contain both H and D to test whether the defects that have been seen contain more than one hydrogen atom. (A defect that contains two H atoms, for example, will have an isotopic sibling that contains both H and D with distinctive vibrational properties that allow it to be identified.) The spectra shown in Fig. 9.3C and D reveal that Ga2O3 samples prepared by annealing in a mixture of both H2 and D2 do, in fact, show additional new vibrational lines at 3438.2 cm1 for O-H and 2547.1 cm1 for O-D [51]. These new lines are a signature of a defect that contains two identical H or D atoms. To explain the spectra shown in Fig. 9.3, the defect that contains two identical H (or D) atoms must have two coupled O-H (or O-D) modes, the first of which is IR active and the second of which is IR inactive. For a defect that contains both H and D, the O-H and O-D modes of the defect become dynamically decoupled and give rise to new vibrational lines for the decoupled oscillators. Furthermore, because the coupled and decoupled modes lie close in frequency, the coupling of the two O-H (or O-D) oscillators must be weak.
9.3.4.2 Evidence for a “hidden hydrogen” species An especially effective way to introduce H into Ga2O3 and to produce the 3437 cm1 center has been found to involve a two-step annealing process. First, a Ga2O3 sample was annealed in an H2 ambient at a temperature of 800°C or greater. Immediately following this annealing treatment, it was found that the 3437 cm1 line could be either weak or absent [as is shown in Fig. 9.5, spectrum (i)]. The second step was an
Hydrogen in Ga2O3
199
Fig. 9.5 IR absorbance spectra for a Ga2O3 sample that had received a two-step annealing treatment to introduce hydrogen. Spectrum (i) was measured for a sample annealed in an H2 ambient for 1 h at 900°C. Spectrum (ii) is for the same sample after a subsequent anneal at 400°C in flowing N2.
annealing treatment at a lower temperature in an inert ambient. Fig. 9.5, spectrum (ii), shows that a second annealing treatment at 400 °C in flowing N2 produced the 3437 cm1 O-H vibrational line. These results show that H is introduced into Ga2O3 by the first anneal at T > 800°C in a form that does not give rise to an observable O-H line. A second anneal at a temperature near 400°C transforms this hidden reservoir of H into the defect that gives rise to the 3437 cm1 center. (Samples that had been deuterated by annealing in a D2 ambient also showed a “hidden D” species that gave rise to a strong 2546 cm1 line in a similar two-step annealing process.) The first anneal in an H2 ambient at high temperature (T 800°C) charges the sample with hydrogen. The second anneal at 400°C produces the 3437 cm1 center. There are several possibilities for the hidden form of hydrogen that is produced by annealing in H2 gas at elevated temperatures. (i) There could be O-H centers with transition moments oriented primarily along the [201] direction. Such a defect would not be visible for the polarizations of the probing light that have been used in the experiments described above and, therefore, would not have been seen. (ii) Hydrogen in Ga2O3 could be introduced in the form of interstitial H2 molecules whose vibrational modes are not IR active. The introduction of hidden H2 was discovered to be important in ZnO where H2 acted as a source and sink for H in defect reaction [31, 32]. (iii) The hidden-hydrogen defect could be the HO shallow donor predicted by theory [17] that has a low vibrational frequency that appears in the spectral region where oxides are highly absorbing. An example of this possibility is provided by hydrogen trapped at an oxygen vacancy in ZnO [22, 38]. Ga2O3 samples hydrogenated (or deuterated) by the ion implantation of protons (or deuterons) also show interesting hydrogen reactions upon annealing at 400°C in an
200
Gallium Oxide
inert ambient. Fig. 9.5 shows that annealing a sample at 400°C that had been implanted with D increases the strength of the 2546 cm1 line by roughly 4 times, suggesting the presence of the same hidden species that is formed in Ga2O3 samples annealed in an H2 ambient. Data shown in Fig. 9.6 support the suggestion that an H-related shallow donor can be a source of “hidden” hydrogen in Ga2O3 samples. Spectrum (i) in Fig. 9.6A shows the free-carrier absorption for a Ga2O3 sample that was annealed in an H2 ambient at 1000°C. This annealing treatment does not produce the 3437 cm1 O-H line. Spectrum (ii) shows that the free-carrier absorption is reduced by a subsequent anneal at 400°C in an inert ambient. Furthermore, this second annealing treatment also produces the 3437 cm1 line [spectrum (ii) in Fig. 9.6A and B]. These results suggest that an H-related shallow donor introduced by the anneal in H2 is converted into the 3437 cm1 O-H center by a subsequent anneal at 400°C in an inert ambient. Spectrum (iii) shows that further annealing at 1000°C in an inert ambient removes the 3437 cm1 center from the sample. The spectra labeled (iv) in Fig. 9.6A and B show differences of spectra (i) and (ii) in the same panels. These difference spectra emphasize contributions arising from H rather than from other donors (presumably Sn or Si) that can be present. The broad absorption shown in Fig. 9.6A, spectrum (iv), that increases at low frequency is due to the free-carrier absorption that is due to hydrogen in the sample. The downward going IR line shown in Fig. 9.6A and B, spectrum (iv), is due to the 3437 cm1 center that grows in during the second anneal at 400°C in an inert ambient.
Fig. 9.6 IR absorption spectra (T ¼ 77 K) for a Ga2O3 sample from Tamura Corp. that initially had been hydrogenated by annealing at 1000°C. This sample was subsequently annealed at in Ar at the temperatures indicated. (A) Shows the absorption due to free carriers and (B) shows the 3437 cm1 O-H vibrational line that corresponds to the free-carrier data shown in (A).
Hydrogen in Ga2O3
201
The data in Fig. 9.6 suggest that the Hi or HO shallow donors predicted by theory are good candidates for the reservoir of hidden hydrogen that can be produced by annealing a Ga2O3 sample in an H2 ambient. However, these results do not eliminate other possibilities (like H2) that may also provide a reservoir of hydrogen in our samples that are difficult to observe directly.
9.3.4.3 Theory of defect structures and their vibrational properties There are a variety of defects that can exist in this complex structure [59–62], and hydrogen can interact with many of them. Therefore, there are many possible configurations for interstitial H and for H complexed with native defects such as O or Ga vacancies. The total absence of any O-H vibrational absorption in the [010] direction is remarkable and provides severe constraints regarding the possible nature and structure of the observed defects. (Analysis of the dichroism associated with monoclinic Ga2O3 reveals [63, 64] that its effect will be negligibly small in these experiments.) Furthermore, as noted, the line shifts associated with codoping with H and D further narrow the possible candidates for the observed defects. While qualitative chemical arguments can be used to suggest approximate O-H axes, detailed calculations are needed to provide, with some confidence, the structures and properties of possible defect configurations. Such calculations have been carried out using the CRYSTAL06 code [65], choosing density functional theory (DFT) with a gradient-corrected approximation to the exchange-correlation functional (Becke’s B3LYP hybrid potential [66] with 20% exact exchange and Lee-Yang-Parr correlation [67]). This approach has been used successfully for defect calculations in a number of similar systems [43, 48, 68–73]. Calculations were carried out in fully relaxed periodic supercells containing 80, 120, or 160 host atoms, plus one or more H impurities, with lattice constants computed for the relaxed perfect crystal. A 2 2 2 k-point mesh of Monkhorst-Pack type [74] was used. The SCF convergence criterion was 107 Ha except for vibrational calculations, where 1010 Ha was used. Gaussian basis functions were of the type 311p(1) for H [75], 8411 for O [76], and 864111d(41) for Ga [77]. Charge states for the defect calculations are based on atomic ions; e.g., the charge state for one H at a Ga vacancy is (2). Harmonic and anharmonic vibrational frequencies may be calculated by CRYSTAL06. A number of potential defect configurations were investigated, beginning with interstitial H. Fig. 9.7 shows how Hi could attach to one of two opposing sites on O(1) or O(2), or one of four quasitetrahedral sites on O(3). Several of these have their O-H dipole perpendicular to [010] and therefore bear consideration as candidates for single O-H defects. A second candidate as a host for O-H dipoles is a Ga vacancy, for which there is both experimental [78] and theoretical [30] support. While there is theoretical [30, 51] evidence that Ga(1) vacancies are energetically favored over Ga(2) vacancies, both types are possible candidates for H traps. When atomic relaxation is taken into account, there are three inequivalent Ga(1) vacancy configurations [30, 62] to consider. One is the simple vacancy, with four “dangling bonds.” The other two arise from
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Gallium Oxide
[1 0 2] direction
Incident light perpendicular to (–2 0 1) plane
[0 1 0] direction out of plane
Fig. 9.7 Possible hydrogen interstitial sites (in blue) in an unrelaxed Ga2O3 lattice.
a large shift of a neighboring Ga to a site midway between two resulting halfvacancies. These three configurations are shown and labeled in Fig. 9.8. Varley et al. [30] and Kyrtsos et al. [62] find a shifted configuration to be lower in energy than the unshifted, while Deak et al. [61] find the opposite. Recent results [51] as well favor the stability of the shifted configurations, with the Ga(1)21 configuration more stable than the Ga(1)23 configuration. A search for single-hydrogen sites associated with Ga vacancies such that the O-H axis has no [010] component leads to a number of possibilities. Two inequivalent H locations for the Ga(2) vacancy satisfy this condition [Fig. 9.9A]. There are two inequivalent H sites for the unshifted Ga(1) vacancy that could qualify [Fig. 9.9B] and also two pairs of equivalent Ga(1)21 sites [Fig. 9.10A and B]. Two equivalent H locations for the shifted Ga(1)23 site are candidates (Fig. 9.11).
Shifted Ga(1)23
g
g
Shifted Ga(1)21
Ga(1) vacancy
(A)
(B)
(C)
Fig. 9.8 Possible Ga(1) vacancy sites: (A), unrelaxed; (B), neighboring Ga(1) shifted to site with O(2) and O(3) neighbors [Ga(1)23]; (C) neighboring Ga(1) shifted to site with O(2) and O(1) neighbors [Ga(1)21].
[1 0 2] direction H
H
H H
Incident light perpendicular to (–2 0 1) plane
(A)
[0 1 0] direction out of plane
(B)
Fig. 9.9 (A) Ga(2) vacancy site plus two inequivalent H sites with no (010) O-H projection. (B) Unshifted Ga(1) vacancy site plus two inequivalent H sites with no (010) O-H projection.
[1 0 2 ] direction H H
H
H
Incident light perpendicular to (–2 0 1) plane
(A)
(B)
[0 1 0] direction out of plane
Fig. 9.10 (A, B) Two relaxed configurations of Ga(1)21, each with two equivalent H sites with no (010) O-H projection.
D dipole moment = 0
[1 0 2] direction
Incident light perpendicular to (–2 0 1) plane D dipole moment ¹ 0 [0 1 0] direction out of plane
Fig. 9.11 Relaxed configuration of Ga(1)23 plus two equivalent H sites with no (010) O-H projection. Shown also are the stretch modes for the corresponding two O-H defect, one with zero change in dipole moment and the other with nonzero change in dipole moment.
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Gallium Oxide
The emergence of new lines in the region of the main O-D and O-H lines upon codoping with H and D is, as noted, the signature of a defect that involves two hydrogens. But because these two hydrogens generate only one observed line for H or D, one of the two O-H stretch modes is unobservable. Furthermore, the small shift associated with HD means that the individual O-H components are weakly coupled. In order for a stretch mode involving two O-H partners to have zero dipole moment, the two O-H’s must be symmetry equivalent and either parallel or antiparallel. The configurations shown in Figs. 9.10 and 9.11, which are associated with a shifted Ga(1) vacancy, may be reconsidered, now with both H sites occupied, in which case they do satisfy this condition. (Those shown in Fig. 9.9 are not symmetry equivalent and so are not candidates.) However, the configuration of Fig. 9.10A has only a modest (102) component and that of Fig. 9.10B lacks a (102) projection. Shifted Ga(1)23 plus two H, shown in Fig. 9.11, is the most likely candidate for the 3437 cm1 line. It has a large (102) component and also has the lowest energy (by 0.6 eV) of all configurations investigated, including a number that do not satisfy the experimental polarization constraints. Furthermore, the large distance between the individual O-H components leads to weak coupling between them.
9.3.4.4 Additional IR lines More recent experiments have revealed several additional IR lines that may be due to some of the other defect structures that have been predicted by theory. The focus is on O-D centers because their IR lines are detected with higher signal to noise ratio than for the corresponding O-H centers. The 2547 cm1 vibrational line is the dominant feature seen in a Ga2O3 sample deuterated in a D2 ambient. However, when such a sample was annealed at elevated temperature (T > 900 °C) to remove H or D and then retreated in D2, a few additional IR lines were produced. Fig. 9.12 shows O-D lines at 2584 and 2632 cm1 that were seen
Fig. 9.12 Polarized IR absorption spectra measured at 77 K for a Ga2O3 sample from the Tamura Corp. that had been repeatedly annealed in a D2 ambient.
Hydrogen in Ga2O3
205
(in addition to the 2547 cm1 line) for the polarization with E // [102] for a Ga2O3 sample that had been repeatedly treated in D2 at 900°C. [The 2632 cm1 line was also seen in a sample implanted with deuterons (Fig. 9.4)]. It is not yet certain whether the additional lines that enter upon heat treatment involve more than one H. If not, one or more of the configurations shown in Figs. 9.8–9.11 with only one H, including hydrogen interstitials, may well be responsible for these lines. Work is ongoing by both experiment and theory that promises to identify additional hydrogen defect structures.
9.4
Conclusion
Properties of the hydrogen impurity in Ga2O3 have been discussed and compared with hydrogen’s behavior in other oxides. SIMS measurements show that hydrogen forms thermally stable defects in Ga2O3 [52, 53]. Muon spin resonance finds that implanted muons form shallow donors in Ga2O3 [17], and H is suggested to behave similarly [55]. Theory predicts that Hi and HO centers are shallow donors in Ga2O3 [17] and that the VGa deep acceptor traps H to produce a defect with low formation energy and high thermal stability [30]. Vibrational spectroscopy and its analysis by theory are being applied to Ga2O3 to provide insight into the structures and properties of the defects that hydrogen can form [51]. Interstitial hydrogen is prominent in ZnO, SnO2, and In2O3 single crystals that had been hydrogenated by annealing in an H2 ambient where it behaves as a shallow donor with a strong O-H stretching line with a vibrational frequency >3000 cm1 [34, 42, 43, 48]. On the contrary, no definitive evidence has yet been found for a Hi center in Ga2O3 that had been hydrogenated by annealing in H2 or by ion implantation [51]. The O-H vibrational lines of complexes of hydrogen with native defects have been observed for ZnO [34, 39], SnO2 [42, 43], and In2O3 [48]. The structures and vibrational properties of these defects have been predicted by theory [30]. Similarly, a strong O-H vibrational line at 3437 cm1 is produced in Ga2O3 by hydrogen introduced either by annealing in an H2 ambient or by proton implantation [51]. Theory has investigated the structures and vibrational properties of several possible VGa–Hn complexes in Ga2O3. The observed polarization properties of the 3437 cm1 line and its behavior in samples that contained both H and D suggest the assignment of the 3437 cm1 line to a particular VGa-2H structure (Fig. 9.11) [51]. ZnO [31, 32] and SnO2 [42, 43] have been found to contain “hidden” hydrogen that does not give rise to prominent O-H vibrational lines. Both interstitial H2 and HO are species that do not give rise to O-H absorption but that can provide a reservoir of H in a sample. When Ga2O3 is annealed in an H2 ambient, a hydrogen species not seen by IR absorption is produced. A subsequent anneal in an inert ambient at 400 °C converts this “hidden” species into the VGa-2H center with a strong O-H line at 3437 cm1. Both H2 and HO are candidates for the reservoir of hidden H in Ga2O3 as are hydrogen centers that could have their transition moments along a [201] direction. While progress has been made by experiment and theory toward determining the behavior of hydrogen in Ga2O3, much remains to be done before the properties of H in
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Gallium Oxide
Ga2O3 are understood at a level that is typical of other semiconducting oxides such as ZnO, SnO2, and In2O3.
Acknowlegments The work at Lehigh University was supported by NSF Grant No. DMR 1160756 and the Sigma Xi Grants-in-Aid of Research program. The work at UF is partially supported by HDTRA1-17-1-0011. The project or effort depicted is sponsored by the Department of the Defense, Defense Threat Reduction Agency. The content of the information does not necessarily reflect the position or the policy of the federal government, and no official endorsement should be inferred.
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