Detection of Trace Amounts of Oxygen Possibly Owing to Polaronic Nature of SrTiO 3 . Toru Hara

1. Detection of Trace Amounts of Oxygen Possibly Owing to Polaronic Nature of SrTiO3. Toru Hara

2. The detection of trace amounts of oxygen is gaining ever-increasing attention since semiconductor manufacturers have faced process drift and yield losses attributed to this gas phase form of contamination. It has been shown that n-type semiconductive dielectric SrTiO3-based thin films are promising candidates for monitoring trace levels of oxygen by the in situ monitoring of semiconductor manufacturing processes. [Published Papers] 1. Toru Hara et al, Jpn. J. Appl. Phys. 47 (2008) 7486. 2. Toru Hara et al., Mater. Sci. Eng. B 161 (2009)142. 3. Toru Hara et al., Sens. Actuators, B136 (2009) 489. 4. Toru Hara et al., Jpn. J. Appl. Phys. 48 (2009) 09KA17. 5. Toru Hara et al., J. Ceram. Soc. Jpn. 118 (2010) 300. 6. Toru Hara et al., Jpn. J. Appl. Phys 49 (2010) 041104. 7. Toru Hara et al., Jpn. J. Appl. Phys. 49 (2010) 09MA15.

3. Sensitivity to Oxidative Gases of Conventional s-Orbital Systems Reach Critical Limit. (Ref) Q. H. Li et al., Appl. Phys. Lett., 85 (2004) 6389

4. Imperfect Perovskite-type d-Orbital System Exhibits Relatively High Sensitivity to Oxidative Gases. (Ref.) F. Rettig et al., Sens. Actuators B123 (2007) 413. SnO2 WO3 Isotropic s orbital Anisotropic d orbital

5. Do Perfect Perovskites Exhibit Higher Sensitivity to Oxidative Gases?

6. Polarons in SrTiO3 Large Polaron in Highly Electron-doped SrTiO3 Small Polaron in Insulative SrTiO3 Large Polaron (Nearly Small Polaron) Mixed [Large (Nearly Small) and Small) Polarons (Ref.) J. L. M. van Mechelen et al, Phys. Rev. Lett. 100 (2008) 226403. (Ref.) H. P. R. Frederikse et al., Bull. Am. Phys. Soc. 11 (1966) 108. (Ref.) R. P. Feynman et al., Phys. Rev. 127 (1962) 1004. (Ref.) D. M. Eagles, Phys. Status Solidi B 48 (1971) 407; D. M. Eagles, P. Lalousis, J. Phys. C 17 (1984) 655. D. M. Eagles, Phys. Rev. 145 (1966) 645. D. M. Eagles, Phys. Rev. 181 (1969) 1278. D. M. Eagles, J. Phys. Chem. Solids 26 (1965) 672.

7. Size-Modulation of Polarons in SrTiO3 (Ref.) W. Huybrechts, et al., Solid State Commun. 17 (1975) 401; J. P. Boyeaux et al., J. Phys. C 12 (1979) 545. (Ref.) M. Itohet al., Phys. Rev. Lett. 82 (1999) 3540.

8. Original Idea The neutralization of electric charges upon oxygen absorption is achieved not only by electron supply from a sensing material but by ionic polarization in the material. The electrical conduction in polar materials can be controlled by polarons. In general, the conductance is high in the presence of large polarons, whereas it is low in the presence of small polarons. Therefore, it is expected that the sensitivity of the sensor will be improved by changing the conductance using a dielectric in which both small and large polarons coexist, such as SrTiO3.

9. Verification of Original Idea (PLD Epitaxial Thin Film) SrTiO3 is sensitive to oxygen even at very low concentrations, at which semiconductive behavior cannot be effective to detect trace amounts of oxygen.

10. Verification of Original Idea (PLD Epitaxial Thin Film) Sr(Ti0.995Cr0.005)O3 (2.5 nm thick) / SrTiO3(001) Rms = 0.27 nm, Ra = 0.20 nm

11. Verification of Original Idea (PLD Epitaxial Thin Film) SrTiO3 exhibited a high sensitivity to oxygen. In contrast, BaTiO3 exhibited a low sensitivity and a high resistance. Polaronic Mechanism Semiconductive Mechanism

12. Verification of Original Idea (PLD Epitaxial Thin Film) Not suｒprisingly, the (002) peak of homoepitaxially grown Sr(Ti0.995Cr0.005)O3 film (50 nm thick) cannot be discriminnated from the (002) peak of the substrate. The (002) peak of heteroepitaxially grown Ba(Ti0.995Cr0.005)O3 film (50nm) thick) can be observed. Sr(Ti0.995Cr0.005)O3 (50 nm thick) / SrTiO3(001) Ba(Ti0.995Cr0.005)O3 (50 nm thick) / SrTiO3(001)

13. Verification of Original Idea (PLD Epitaxial Thin Film) The carrier conduction in ferroelectric BaTiO3 is mainly controlled using small polarons. Therefore, the effect of oxygen adsorption on electrical conductivity is weak in BaTiO3. In contrast, paraelectric SrTiO3 can have the axial tetragonal distortion only when oxygen is adsorbed onto the surface. Therefore, oxygen adsorption can modulate the mobility of carrier electrons at SrTiO3 surface.

14. Verification of Original Idea (PLD Epitaxial Thin Film) With increasing Ba ratio, the electrical resistance was increased probably owing to the decrease in the polaron size. Furthermore, the sensitivity to oxygen of ferroelectric materials were deteriorated.

15. Verification of Original Idea (PLD Epitaxial Thin Film) The carrier density increases in donor-doped SrTiO3. It can shield the interaction between adsorbed oxygen and ionic polarization in SrTiO3, resulting in the deterioration of the sensitivity to oxygen. Acceptors can lower the resistance, since the acceptors are charge-compensated with oxygen vacancies, which can generate shallow donor levels. It is assumed that the cause of the difference between Sr(Ti,Cr)O3 and Sr(Ti,Fe)O3 is the difference in the electronic configuration between Cr3+ [(t2)3] and Fe3+ [(t2)3(e)2].

16. Verification of Original Idea (PLD Epitaxial Thin Film) A FeO6 octahedron can have an axial tetragonal distortion even without adsorbed oxygen, since a partially occupied e-type electron orbital can be directed toward a positively charged oxygen vacancy. Such a distortion leads to the generation of small polarons. Therefore, the effect of oxygen adsorption on electrical conductivity is weak in Sr(Ti,Fe)O3. In contrast, CrO6 can have the axial tetragonal distortion only when oxygen is adsorbed onto the surface, since there is no partially occupied e-type electron orbital. Therefore, oxygen adsorption can modulate the mobility of carrier electrons at Sr(Ti,Cr)O3 surface.

17. Verification of Original Idea (PLD Epitaxial Thin Film)

18. Feasibility Study for Mass Production (ALD Highly Oriented Thin Film) Although Pulsed-Laser Deposition (PLD) is ideal for basic research since epitaxially grown films can be obtained by choosing appropriate substrates, it is not feasible for mass production since the use of plumes prevents the deposition of materials the entire surface of wafers, on which devices are formed. Atomic-Layer Deposition (ALD) for SrTiO3 [A. Kosola et al., Appl. Surf. Sci. 211 (2003) 102.] and other oxides [H. Seim et al., J. Mater. Chem. 7 (1997) 449; M. Nieminen et al., J. Mater. Chem. 11 (2001) 3148; J. Päiväsaari et al., J. Mater. Chem. 12 (2002) 1828.] has been established and found suitable for mass production. Sr(thd)2 monolayer → SrO Ti(O-i-Pr)4 monolayer → TiO2 Anneal SrTiO3

19. Feasibility Study for Mass Production (ALD Highly Oriented Thin Film) A 20 nm thick layer of SrTiO3 was deposited onto a c-sapphire substrate at 573 K using an ALD system (Beneq Oy, TFS 500). During the ALD of SrTiO3, Sr(thd)2 (thd = 2,2,6,6-tetramethyl-3,5-heptanedinate) as the source material of strontium and Ti(O-i-Pr)4 (O-i-Pr = isopropoxide) as the source material of titanium were alternately introduced into the ALD reactor using nitrogen as the carrier and purging gases. Ozone and water vapor were used as oxygen sources after the introduction of Sr(thd)2 and Ti(O-i-Pr)4, respectively.

20. Feasibility Study for Mass Production (ALD Highly Oriented Thin Film) Into ALD-SrTiO3 thin film, carrier electrons were doped by a post-ALD annealing in N2. Immobile holes were auto-doped by strontium vacancies, which were generated beneath the topmost layer by the surface segregation of SrO. Oxygen vacancy (VO2+) doping Strontium Vacancy (VSr2-) doping EC EC Electron Doping 0.035 - 0.085 eV 0.300 - 1.000 eV Hole Doping EV EV (Ref.) O. N. Tufte et al., Phys. Rev. 155 (1967) 796; C. Lee et al., Phys. Rev. B 3 (1971) 2525. (Ref.) C. Lee et al., Phys. Rev. B 3 (1971) 2525; Y. Aiura et al.,Surf. Sci. 515 (2002) 61; H. Tanaka et al., Jpn. J. Appl. Phys. 32 (1993) 1405.

21. Feasibility Study for Mass Production (ALD Highly Oriented Thin Film) Sensor response was improved by increasing post-ALD annealing temperature and time, by which the coverage of SrO-based surface segregations was decreased.

22. Feasibility Study for Mass Production (ALD Highly Oriented Thin Film) Sensor response was improved by increasing post-ALD annealing temperature and time, by which the coverage of SrO-based surface segregations was decreased. Annealed at 700 ℃ in N2 for 30 min Annealed at 800 ℃ in N2 for 120 min (Ref.) B. Psiuk et al., Appl. Phys. A89 (2007) 451; Q. -H. Wu et al., Mater. Lett. 59 (2005) 1980; H. Wei et al., J. Electroceram. 8 (2002) 221.

23. Feasibility Study for Mass Production (ALD Highly Oriented Thin Film) Sensor response was improved by increasing post-ALD annealing temperature and time, by which the coverage of SrO-based surface segregations with water and/or hydroxide adsorbates was decreased. Annealed at 700 ℃ in N2 for 30 min Annealed at 800 ℃ in N2 for 120 min Water and/or hydroxide adsorbates (Ref.) Q. -H. Wu et al., Mater. Lett. 59 (2005) 1980.

24. Feasibility Study for Mass Production (ALD Highly Oriented Thin Film) H+-doped SrO1+δ can be p-type semiconductor. Therefore, highly resistive p-n junction can be generated. Annealed at 700 ℃ in N2 for 30 min Annealed at 800 ℃ in N2 for 120 min p-type SrO1+δ・nH2O (H+ doped) Depletion layer n-type SrTiO3 Alkali-ion-doped SrO can be p-type semiconductor (highly insulative). (Ref.) A. Lichanot et al., J. Phys. Chem. Solids 59 (1998) 1119.

25. Feasibility Study for Mass Production (ALD Highly Oriented Thin Film) Sensor response was improved by applying an electric field for prolonged time at a low oxygen concentration, by which the interfacial resistance can be decreased owing to the pileup of oxygen vacancies.

26. Feasibility Study for Mass Production (ALD Highly Oriented Thin Film) Sensor response was improved by increasing UV-light irradiation intensity, by which oxygen desorption is accelerated.