Auger Electron Spectroscopy, Electron Energy Loss Spectroscopy, UV

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Auger Electron Spectroscopy, Electron Energy Loss Spectroscopy, UV Photoelectron Spectroscopy, and Photoluminescence Characterization of In2O3 Associated to the Theoretical Calculations Based on the Generalized Gradient Approximation and Modified Becke Johnson Khadidja Boulenouar,† M’hammed Bouslama,*,† Azzeddine Mokadem,*,†,‡ Sébastien Vizzini,§ Zakia Lounis,† Abdelkader Abdellaoui,† Bachir Reguad,† Mahmoud Bedrouni,† Kheira Hamaida,† Tahar Guenouna,† and Mohamed Ghaffour† †

Laboratoire Matériaux, Ecole Nationale Polytechnique (ENP) d’Oran, BP 1523 Oran Mnaouar, Oran, Algeria Université Hassiba Ben Bouali, Faculté de Technologie, Département de Science et Technologie, Chlef, Algeria § Institut Matériaux Microélectronique Nanosciences de Provence (IM2NP), Avenue Escadrille Normandie Niémen, 13013 Marseille, France ‡

ABSTRACT: The indium oxide In2O3 is among the transparent conducting oxides (TCO) appropriate for solar cells and optoelectronics. The physical properties are based on the electron distribution on the core levels and in the valence band of material. The knowledge of the electron distribution on the different states is fundamental to predict the possibilities of electron transitions. In this respect, we adopt calculations based on the generalized gradient approximation (GGA) and modified Becke Johnson (mBJ) to show the electron state density. We associate to the numerical simulation the experimental analysis techniques auger electron spectroscopy (AES), electron energy loss spectroscopy (EELS), and UV photoelectron spectroscopy (UPS) of great sensitivity to characterize the material surfaces. The analysis technique AES is used for proving the chemical composition of the In2O3 compound through the In-M45N45N45 and O-KLL signals. The energy loss peak at 16.3 eV on the EELS spectra is related to plasmons. The energy losses lower than 16.3 eV are related to interband transitions (ITs). They arise from the hybridation of states (s, p, and d) of indium and (s, p) of oxygen. The energy loss at 19 eV is mainly related to IT transition from the d states of indium in hybridation with a slight contribution of p and s states of indium and oxygen. The calculation is useful to predict the states from which the interband transitions occur. The EELS associated with UPS constitutes powerful techniques to show the energy states of the electron distribution. The irradiation of In2O3 by the UV photons at 320 nm leads to the photoluminescence emission at low energy around 580 nm, appropriate to laser applications.

1. INTRODUCTION The indium oxide In2O3 is an important semiconductor with wide energy gap of 3.7 eV.1−5The transparent aspect constitutes a great advantage for this compound to be applied as an electrode in the photovoltaic systems. This material is also interesting in the optoelectronic field.6−13 In2O3 corresponding to a transparent conducting oxide (TCO) is of great interest because of its transparency properties in the visible and its good electrical conductivity.14 Karazhanov et al. showed the isotropic character of In2O3. The crystal structure of In2O3 is cubic. The optical spectra related to real and imaginary parts of the dielectric function, the absorption coefficient, the refractive index, and the extinction coefficient are isotropic along the crystallographic axes a, b, and c.4 The TCO In2O3 is applied as solar cells, as sensors, as front electrodes, in flat panel displays, in mirrors of automobiles, in smart windows, etc.15−19 The physical properties of materials depend on their chemical composition and their physical structure.20 The electron © XXXX American Chemical Society

distribution is useful to predict the physical properties. On this basis, we use the computational simulation to study In2O3 using the full potential linearized augmented plane wave (FPLAPW) through the Wien2K program.21,22 The electron distribution plays an important role in determining the lattice structure, the density of states (DOS), and the charge density. The theoretical calculation based on the approximations gradient generalized approximation (GGA) and modified Beck Johnson (mBJ) have proved their performance to predict the physical properties of materials.23 In this study, we associate the experimental results related to characterization methods auger electron spectroscopy (AES), electron energy loss spectroscopy (EELS), UV photoelectron spectroscopy (UPS), Received: November 23, 2016 Revised: March 25, 2017

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DOI: 10.1021/acs.jpcc.6b11823 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C and photoluminescence (PL) to theoretical results obtained by GGA and mBJ.24−34

2. RESULTS 2.1. Calculation Process. The calculations of electronic structures are performed with the full-potential linearized augmented plane wave method within the density functional theory22,23 implemented in the package Wien2K.25,35The generalized gradient approximation (GGA)24is used as the exchange-correlation potential. Full relativistic calculations are performed for core states. The scalar approximation is applied for semicore states and valence states. In order to separate core and valence states, the taken energy threshold is of −8.0 Ry with the parameters RMTKmax = 8 and lmax = 10. For the Brillouin zone k-point sampling, we use the Monkhorst−Pack mesh with 9 × 9 × 9 k-points. The radius of muffin tins is taken as 2 and 1.6 units (au) for In and O atoms, respectively, allowing a good precision in the self-consistent calculations. The calculations with Wien2K code are based on the full optimization of crystal lattice parameters and atomic internal parameters. The relaxation phenomenon of atoms is established according to the Hellmann−Feynman forces lower than 1 mRy/au. The crystal lattice parameters are optimized through the fitting of the total energy on the basis of the Murnaghan equation of state.36 2.2. Structural Properties. In2O3 crystallizes in the bixbyite structure (space group Ia3). The geometrical representation is cubic with large unit cell of 40 atoms (see Figure 1).

Figure 2. Total energy as a function of the volume with GGA calculation of In2O3.

Table 1. Lattice Parameter, Bulk Modulus B, Pressure Derivatives B′, and Volume of In2O3 in Comparison with Experimental and Theoretical Results method this work exptl37 exptl38 calcd39

GGA

LDA+U

a (Å)

B (GPa)

B′

V (Å3)

10.2902 10.12 10.118 10.057

150.2609 -

4.81466 -

544.8053 518.2168 517.9096 508.5988

Table 2. Atomic Positions before and after Relaxation

GGA

exptl40

calcd39

atom

In

In

O

W

8b

24d

48e

X Y Z X Y Z X Y Z

0.2500 0.2500 0.2500 0.2500 0.2500 0.2500 0.2500 0.2500 0.2500

0.4645 0 0.2500 0.4665 0 0.2500 0.4659 0 0.2500

0.3918 0.1551 0.3812 0.3901 0.1543 0.3818 0.3899 0.1543 0.3821

Figure 1. In2O3 crystal structure.

From the GGA, we show the variation of the total energy as a function of volume (see Figure 2). The results related to lattice constant a, bulk modulus B, pressure derivative B′ of the bulk modulus, and volume in comparison with other theoretical and experimental data are indicated in Table 1.We note that the value of the lattice constant is in good agreement with the experimental and theoretical results.37,39 Also, the internal parameters (x, y, and z) are in good concordance with the theory39 and the experiments,40 as shown in Table 2. 2.3. Electronic Properties. 2.3.1. Electronic Band Structure. The Brillouin zone of the orthorhombic cell is shown in Figure 3. The electronic band structures within GGA and mBJ calculation of cubic In2O3 along the symmetry lines of the Brillouin zone are shown in Figure 4. The energy scale is taken in eV with the top of the valence band (VB)

Figure 3. Brillouin zone of In2O3.

corresponding to zero. The direct band gap is of 1.39 and 3.5 eV with GGA and mBJ, respectively, at point Γ. These results are in agreement with the ones obtained by Aliabad according to the FP-LAPW method.39We give in Table 3 the values of the calculated and experimental band gaps. 2.3.2. Density of States (DOS). We calculate the density of states (DOS) of In2O3 by GGA and mBJ. We note as shown in Figure 5 that the results obtained by both approximations are in good concordance. There is hybridation of states (s-In; s-O) B


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The Journal of Physical Chemistry C

Figure 4. (a) and (b) Band structure of In2O3 according to the GGA and mBJ calculations, respectively.

indium and oxygen on the wide range −2 to −5.8 eV. However, the electron contribution of (d-In) states becomes important in the energy range −16.7 to −19 eV with a weak contribution of hybridized states (s-In; p-In) and (s-O; p-O). We deduce the prediction of the easy possibility of electron transfer from the upper level (d-In) of indium to the conduction band. 2.3.3. Charge Density. The ionic bond character between indium and oxygen in In2O3 is justified by the charge transfer between these two atomic species, as shown in Figures 6 and 7. There is an increasing charge transfer from In to O atoms because of their difference in the electronegativity, inducing the creation of the dipolar moment which assumes a good chemical and physical stability of the In2O3 compound. 2.3.4. Auger Electron Spectroscopy (AES). We give some indications about the great sensitivity of electron spectroscopy, AES and EELS, to characterize the material surfaces. The spectra are recorded by using a hemispherical spectrometer operating in direct mode with high resolution. The good compromise between the transmission and resolution of the apparatus depends on the pass energy. It is important to give some details about the AES spectrum of In metal in comparison with the In2O3 one. The spectrum related to AES transition InM45N45N45 of In metal is of quasi-atomic type.44−52 The Auger process leads to create two localized holes in Coulomb interaction. However, the signal related to the AES transition In-M45N45N45 appears with narrow full width at half maximum (fwhm) in agreement with other reported results.53,54 We also give for comparison the recorded AES spectrum In-M45N45N45 related to oxidized indium partially to locate the peak related to the chemical bond Inox (see Figure 8). We note that the chemical bond Inox is shifted 4 eV from the main peak related to the chemical bond In−In. The recorded spectrum InM45N45N45 of massive oxidized indium is of band structure type as shown in Figure 9. The signal shape is broader than the one of In metal.55The experimental procedure is based on the use of an electron gun with filament LaB6 which provides an incident electron beam of energy 3 keV focused on an area of 1 mm diameter.

Table 3. Calculated Band Gap for In2O3 Using GGA and mBJ our study exptl41 exptl42 calcd39 calcd43

methods

Γ−Γ

Γ−H

Γ−N

Γ−P

GGA mBJ

1.39 3.50 3.6 2.7 1.43 1.67

4.40 6.25 4.50 -

3.47 5.25 3.47 -

4.10 5.87 -

LDA+U LDA+U

Figure 5. (a) and (b) Total density of states and partial density of states of In2O3 according to the GGA and mBJ calculation, respectively.

and (p-In; p-O) with (d-In) in the energy range −6 to 0 eV. The electron contribution of (d-In) states is more important near the Fermi level taken as reference level (0 eV). The top of the valence band involves the electrons of (d-In) states localized significantly in the energy range −2 to 0 eV. Beyond −2 eV, the (d-In) states are in hybridation with the states of C


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Figure 6. Charge density contours of In2O3.

(DOS). In the EELS, the measurements of the energy differences between the elastic peak and inelastic peaks correspond to plasmons and ITs. The EELS technique is useful to characterize the material surface of point of view chemical composition and structure. We vary the primary energy Ep from 1000 to 300 eV. For these energies, the characterized thickness is then estimated varying from 30 Å up to 10 Å. As shown in Figure 10, the highest peak of the first EELS spectrum recorded at Ep = 1000 eV is located at 16.3 eV. We note that the intensity of this peak decreases when the primary energy Ep decreases. So, this peak is attributed to bulk plasmons. The peak located at 19 eV always remains visible without change in intensity, although there is variation of the primary energy Ep. This peak is related to interband transition. We note that the interaction phenomenon of electrons−matter depends on several parameters such as the cross section, the surface morphology, the impact point, the energy of the electron beam, the irradiated depth, and so on. It appears that the recorded EELS spectrum at the intermediate energy Ep = 400 eV corresponds to the best conditions of the interaction process electrons−matter. There are peaks at 2.5, 4.4, and 9 eV besides those appearing on the other recorded EELS spectra, 19, 11, and 5 eV, at primary energies Ep lower and higher than 400 eV. All these peaks are attributed to energy losses due to interband transitions. 2.3.6. UV Photoelectron Spectroscopy (UPS). In order to argue the EELS results, we give in Figure 11 the UPS spectrum, recorded with He−I emission of 21.2 eV. The UV spectroscopy is used for determining the electron distribution. That spectroscopy method is complementary to the EELS. The UPS spectrum involves the peaks related to the electron distribution located around the same energies as those appearing on the EELS spectrum of Ep = 400 eV. Such an energy is of a great physical efficiency concerning the electron interaction process with In2O3 in comparison with other energies (higher or lower than 400 eV). Also, the association of UPS and EELS is useful to validate the theoretical results related to the calculated density of states (DOS). We indicate that the optical parameters of the TCO In2O3 are reported by other authors.2,4,56−59 2.3.7. Photoluminescence (PL) Measurements. In order to study the electron transitions of In2O3 with great sensitivity, we adopt the photoluminescence (PL) measurements at low energy using a Horiba iHR550 spectrometer associated with CCD Camera. The In2O3 compound is irradiated by UV laser radiation (320 nm; 30 mWatts). The recorded spectrum shown in Figure 12 indicates the appearance of photoluminescence peak with high intensity, located around 580 nm. Such a wavelength corresponds to energy lower than the one related to gap energy of In2O3. There are energy levels in the gap, near the conduction band displayed by the interband transition

Figure 7. Line plots of the charge density of In2O3.

Figure 8. AES spectrum of In-MNN of (a) In metal and (b) In oxidized partially.

Before the introduction in the analysis chamber, the sample was cleaned chemically by acetone, ethanol, and deionized water. The AES displays the cleaned state of In2O3 obtained after argon ion bombardment of 1000 eV energy and of 2.5 μA current focused on an area of 1 cm diameter. The recorded AES signals correspond only to transitions In-MNN and O-KLL of In2O3 without any appearance of other signals due to impurities. 2.3.5. Electron Energy Loss Spectroscopy (EELS). The incident electron beam with primary energy varying between 1000 and 300 eV is focused on an area of 1 mm diameter of target. The inelastic backscattered electrons are collected by the hemispherical analyzer operating in direct mode N(E). The recorded spectra are commented on the basis of energy losses due to plasmons and interband transitions (ITs). We associate the EELS and the AES to the calculated density of states D


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Figure 9. In-MNN AES spectrum of In2O3.

around 2.5 eV shown on the EELS spectrum recorded at Ep = 400 eV. Such an emission phenomenon is of great importance in the laser emission applications of In2O3. The PL measurement also allows us to justify the conductive character of the In2O3, very appropriate to the conversion of solar energy. We note that the TCO In2O3 is of optical transparency in the visible region and of high electrical conductivity, as reported by King,60 showing the conductor character due to accumulation of electrons at the surface to create states near the bottom of the conduction band because of oxygen vacancies or punctual defect (emplacement of In atoms in interstitial sites). Our theoretical results of the calculated density of state (DOS) are in agreement with those reported by Walsh et al. so that the valence band edge for In2O3 is significantly closer to the bottom of the conduction band.2 The calculation of the band structure does not involve the states due to defects in the band gap. So, we do not associate the calculated band structure to PL experimental measurements. The In 2O3 sample was cleaned by argon ion bombardment. It does not involve impurities as shown by the AES. The AES signals shown in Figure 9 correspond only to InMNN and O-KLL transitions related to indium and oxygen of In2O3.

3. CONCLUSION We adopt the AES to characterize the In2O3 compound through the electron transition In-M45N45N45 of band structure type. The AES is a powerful technique to display the chemical bond indium−oxygen in In2O3 in comparison with the indium−metal one of quasiatomic type. The discrimination between the peaks related to chemical bonds (In−In) and (In− O) is 4 eV. The association of experimental techniques EELS and UPS is of great importance to show the electron distribution in In2O3 as a function of the energy. The EELS peaks related to energy losses due to interband transition are in good agreement with those characterizing the electron distribution appearing on the UPS spectrum. The comparison between the calculated results and those obtained experimentally by EELS and UPS reveals the hybridation character in the energy range 3−11 eV of states (s, p, and d) of indium and (s, p) of oxygen. We note that the states (d) of indium are of large influence near the Fermi level. The energy at 19 eV is mainly

Figure 10. EELS spectra of In2O3 recorded at different primary energies Ep.

E


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Figure 11. UPS spectrum of In2O3 obtained by He−I (21.2 eV) emission.

iHR550 and hemispherical spectrometers so used are of great sensitivity to confirm the presence of states in the gap, near the conduction band. The PL measurements are also of great interest to show the importance of In2O3 in the laser emission. Furthermore, they justify the conductive character of In2O3 for the conversion of solar energy.

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AUTHOR INFORMATION

Corresponding Authors

*(M.B.) E-mail: [email protected]. *(A.M.) E-mail: [email protected]. ORCID

Azzeddine Mokadem: 0000-0001-9104-5201 Notes

The authors declare no competing financial interest. Figure 12. PL spectrum of In2O3 obtained by UV laser radiation of 320 nm.

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ACKNOWLEDGMENTS

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REFERENCES

We are grateful to HORIBA for the high performance of their iHR550 spectrometer with camera CCD of a great sensitivity operating in the photoluminescence measurement. We express to them our acknowledgements for their technological contribution, allowing the development of the scientific research.

related to IT transition from the (d) state of indium in hybridation with a slight contribution of (p and s) states of indium and oxygen. So, there is a complementarity between the numerical simulation and the EELS and UPS techniques to exhibit the states of the electron distribution from which the possibilities of interband transitions occur. The recorded PL spectrum of irradiated In2O3 by UV laser radiation (320 nm; 30 mWatts) allows the appearance of a photoluminescence peak around 580 nm corresponding to an energy lower than the one related to the gap energy of In2O3. That emission is due to defects introduced in the gap near the conduction band as oxygen vacancies or location of indium in the interstitial sites of the In2O3 lattice. Such states are revealed by the interband transition (2.5 eV) on the EELS spectrum (Ep = 400 eV) and by the last peak appearing on the UPS spectrum at 2.4 eV. The optical properties of In2O3 are justified experimentally by the photoluminescence due to electron distribution near the bottom of the conduction band. These electron states are displayed by EELS and UPS. The numerical simulation based on the calculation of the density of states (DOS) constitutes a good means to identify the nature of states susceptible to contributing to the process of interband transitions. The

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Auger Electron Spectroscopy, Electron Energy Loss Spectroscopy, UV

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