Diffuse reflectance spectroscopy: An effective tool to probe the defect states in wide band gap semiconducting materials

doi:10.1016/j.mssp.2018.06.025

Materials Science in Semiconductor Processing

Volume 86, 1 November 2018, Pages 151-156

https://doi.org/10.1016/j.mssp.2018.06.025 Get rights and content

Abstract

Optical properties of widely used semiconducting oxides namely; TiO₂, ZnO and ZrO₂ were investigated using diffuse reflectance spectroscopy (DRS). Prior to the optical absorption measurements, the structural purity of these samples was examined using powder x-ray diffraction experiments carried out on Indus-2 synchrotron source. It is observed that all the studied samples are structurally pure. The DRS of all the studied samples show an extra peak much below fundamental band gap. In order to understand the origin of the said low energy peak, the theoretical optical absorption spectra for these samples have been simulated. The simulations were performed using density functional theory, considering, ideal as well as defected systems i.e. by considering vacancy at all possible sites (for TiO₂ we have considered the vacancy at Ti site and also at O site taking in to account surface and bulk effects). It is observed that the simulated optical spectra show very similar feature as that of experimental optical absorption for oxygen vacancy. Photoluminescence spectroscopy further supports the presence of defect states in the studied samples. Thus; it appears that the diffuse reflectance spectroscopy is a useful tool to probe the signature of defects present in the sample.

Introduction

The transition metal oxides have been extensively studied, because of their technical importance and the fundamental interest [1], [2]. There has been increasing interest in the optical properties [3], [4], [5] of functional semiconducting oxide materials for their potential use as a transparent conductive oxide material, solar cell/photovoltaic material etc [2]. The band gap (E_g) of any semiconductor material is an important feature of that material to assess its applicability for the devices. Further for various critical applications, it is important to characterize these materials especially for defect etc., as many of the properties of these materials are controlled by the defects. It is also well known and observed experimental fact that the behavior of the defects is different in different materials, for example, in case of MgO oxygen defects leads to excess electrons which are localized on its surface; but are de-localized in case of TiO₂ [6]. In the case of oxide semiconductor materials the signature of these defect have been reported through valence band spectroscopy [7], Raman spectroscopy [8], photoluminescence spectroscopy [9] etc. Interestingly, these defects leads to an extra states (accepter or donor level) between valence and conduction band [10]. Even though the signature of the defect is visible in above mentioned experimental data but with these techniques it is difficult to predict the site-specific defect i.e. whether the signature is due to Ti defect or oxygen defect in case of TiO₂. This requires a high-resolution imaging in transmission electron microscopy, atomic force microscopy or scanning tunneling microscopy etc.

Recently, optical spectroscopy has been extensively used to probe the electronic excitations in solid samples i.e. to probe the band gap [4], [5], [11]. In case, if any acceptor or donor level is present in the sample due to the point defects/vacancies etc. it may get reflected in the optical absorption spectroscopy. Further, it is well known that the polycrystalline oxide samples have large surface area as compared to that of their single crystal counterparts and hence have more surface defect density. Thus, the possibility of detecting such defect states in polycrystalline samples appears to be higher as compared to their single crystal counterparts.

Keeping above in mind, in the present study, we have systematically investigated three different and widely used polycrystalline transition metal oxides using namely; ZnO, TiO₂ and ZrO₂ using diffuse reflectance spectroscopy. The optical absorption spectra for all the studied powder samples show a peak much below fundamental band gap edge. In order to understand the origin of the above discussed low energy peak, the optical absorption spectra of these samples were simulated using first principle density functional theoretical calculations (DFT) under generalized gradient approximation (GGA) considering 3 × 3 × 2 super cell. All the simulations were performed considering ideal as well as defected structure i.e. by considering vacancy at all possible sites, for example for TiO₂ we have considered the vacancy at Ti site and at O site. It is observed that the simulated optical spectra show very similar feature as that of experimental optical absorption for oxygen vacancy. Thus, it appears that the optical absorption spectroscopy is very useful and sensitive tool to probe the signature of defects present in the sample. It is also observed that oxygen defects are inherently present in poly crystalline oxide samples.

Section snippets

Sample preparation and characterization

High purity samples (99.95% pure) of ZnO, TiO₂, and ZrO₂ were procured from Sigma Aldrich. The structural phase purity of these samples was confirmed through powder x-ray diffraction studies carried out using angle dispersive x-ray diffraction beamline BL12 at Indus-2 synchrotron x-ray source [12], [13], [14].

UV–VIS spectroscopy

The optical absorption spectra of prepared samples have been measured using diffuse reflectance spectroscopy (DRS) [4], [5] measurements. DRS experiments have been performed in the

Results

Fig. 1 shows the experimental diffuse reflectance spectra (DRS) for (a) TiO₂ (b) ZnO and (c) ZrO₂. The obtained DRS has been converted into its corresponding absorption spectra by using the following Kubelka–Munk and Tauc relations [24], [25], [26], $F (R_{\infty}) = \frac{{(1 - R_{\infty})}^{2}}{2 R_{\infty}},$ where; $F (R_{\infty})$ is the Kubelka–Munk function ( $R_{\infty} = R_{s a m p l e} / R_{s \tan d r e d}$ ). The Kubelka–Munk function, which is proportional to the absorption coefficient (α), can be written as- $F (R_{\infty}) \propto α \propto \frac{{(h υ - E_{g})}^{1 / n}}{h υ},$ ${(α h v)}^{n} = A (h v - E_{g}) .$

Inset of each figure shows a

Discussions

In order to further understand the origin of observed low energy peak in optical absorption spectra, we have carefully analyzed the effect of oxygen defect on the overall charge states of titanium in the presence of oxygen vacancy. In the present study, we are only considering the oxygen vacancy, as our experimental data is matching only with that of the simulated spectra having oxygen defect. Further in the case of TiO₂ it is now well accepted that the oxygen vacancy is more likely to be the

Conclusion

In conclusion we have investigated the origin of low energy state in the optical absorption spectra of wide band semiconductor materials present much below fundamental band edge. From the experimental and simulated studies, it appears that the signature of various types of defects is reflected in optical absorption spectroscopy. Importantly optical absorption spectroscopy appears to be very powerful tool to probe the signature of type of vacancies present in the wide band gap semiconductor

Acknowledgments

The authors sincerely thank SIC IIT Indore for providing basic experimental infrastructure. Authors would like to thanks Dr. Vipul Singh for PL measurements. Authors sincerely thank Dr. Satya S. Bulusu and Dr. Sanjay Singh for scientific discussion. Mr. Vikash Mishra, M. Kamal Warshi and Mr. Anil Kumar sincerely thank Ministry of Human Resource Development (MHRD), Government of India, for providing financial support through teaching assistantship at IIT Indore. Ms. Aanchal Sati thanks CSIR

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Diffuse reflectance spectroscopy: An effective tool to probe the defect states in wide band gap semiconducting materials

Abstract

Introduction

Section snippets

Sample preparation and characterization

UV–VIS spectroscopy

Results

Discussions

Conclusion

Acknowledgments

Solid State Commun.

Comput. Phys. Commun.

Surf. Sci.

J. Phys. Chem. Solids

Comput. Phys. Commun.

Comput. Phys. Commun.

Perovskite oxides: preparation, characterizations, and applications in heterogeneous catalysis

ACS Catal.

Structure, physical properties, and applications of SrRuO3 thin films

Rev. Mod. Phys.

Fe doped LaGaO3: good white light emitters

RSC Adv.

Electronic and optical properties of BaTiO3 across tetragonal to cubic phase transition: an experimental and theoretical investigation

J. Appl. Phys.

Strain control of Urbach energy in Cr-doped PrFeO3

Appl. Phys. A

Defects at Oxide Surfaces

Probing defect sites on CeO2 nanocrystals with well-defined surface planes by raman spectroscopy and O2 adsorption

Langmuir

Blue-light emission at room temperature from Ar+-irradiated SrTiO3

Nat. Mater.

Electronic structure of a neutral oxygen vacancy in SrTiO3

Phys. Rev. B

Band structure and optical transitions in LaFeO 3: theory and experiment

J. Phys. Condens. Matter

Large dielectric permittivity and possible correlation between magnetic and dielectric properties in bulk BaFeO3−δ

Appl. Phys. Lett.

Room temperature magnetodielectric studies on Mn-doped LaGaO3

Mater. Res. Express

Observation of large dielectric permittivity and dielectric relaxation phenomenon in Mn-doped lanthanum gallate

RSC Adv.

Inhomogeneous electron gas

Phys. Rev.

Hybrid exchange-correlation energy functionals for strongly correlated electrons: applications to transition-metal monoxides

Phys. Rev. B