Unraveling the enhanced Oxygen Vacancy Formation in Complex Oxides during Annealing and Growth

Abstract

The reduction of oxides during annealing and growth in low pressure processes is a widely known problem. We hence investigate the influence of mere annealing and of growth in vacuum systems to shed light on the reasons behind the reduction of perovskites. When comparing the existing literature regarding the reduction of the perovskite model material SrTiO3 it is conspicuous that one finds different oxygen pressures required to achieve reduction for vacuum annealing and for chemically controlled reducing atmospheres. The unraveling of this discrepancy is of high interest for low pressure physical vapor depositions of thin films heterostructures to gain further understanding of the reduction of the SrTiO3. For thermal annealing, our results prove the attached measurement devices (mass spectrometer/ cold cathode gauge) to be primarily responsible for the reduction of SrTiO3 in the deposition chamber by shifting the thermodynamic equilibrium to a more reducing atmosphere. We investigated the impact of our findings on the pulsed laser deposition growth at low pressure for LaAlO3/SrTiO3. During deposition the reduction triggered by the presence of the laser plume dominates and the impact of the measurement devices plays a minor role. During post annealing a complete reoxidization of samples is inhibited by an insufficient supply of oxygen.

SrTiO3 termination control: a method to tailor the oxygen exchange kinetics

“…[22] An increase of the SrO-termination to 50% results in an increase of the sheet resistance, to about 10 4 Ω at room temperature and about 10 3 Ω below 50 K. This is the typical temperature dependency of the sheet resistance for crystalline LAO/STO dominated by 2DEG conductivity. [11,14,22,[37][38][39][40][41][42][63][64][65] Increasing the SrO-termination further to 100 % did, in agreement with observations reported in literature, [42,43] result in insulating samples, whose sheet resistance is above the measurement limit (10 8 Ω). temperature and STO termination.…”

“…We have previously shown that the growth of crystalline LAO at oxygen pressures ≤ 10 −3 mbar results in a shift from 2DEG conductivity to bulk conductivity, when quenching the sample immediately after growth. [11,22] The appearance of bulk conductivity in crystalline LAO/STO can be explained by the incorporation of oxygen vacancies in the STO bulk, which contribute electrons to the conduction band. During its low pressure growth, LAO sucks oxygen from the underlying STO substrate resulting in the formation of oxygen vacancies in the STO.…”

“…Figure 3 shows that we can utilize the termination control of the STO substrate to tailor the sheet resistance in LAO/STO heterostructures. As the sheet resistance of both, crystalline LAO/STO grown in reducing conditions and amorphous LAO/STO, is defined by the oxygen vacancies [11,14,22,[36][37][38][39][40][41][42][63][64][65][66], we have successfully tailored their incorporation. This also explains the shift from bulk dominated to interface dominated conductivity for crystalline LAO, when increasing the SrO-termination to 50% (Figure 3 a)).…”

“…[9,10] The central role of oxygen vacancies for all STO applications has resulted in intense research efforts to understand and control their formation. [11][12][13][14][15][16][17][18][19][20][21][22] Apart from the classic influence factors temperature and pressure [19][20][21] it was found that e.g. UV radiation [12,[23][24][25][26][27] plays a crucial role.…”

Stability and Decomposition of Perovskite-Type Titanates upon High-Temperature Reduction

“…This implies that great care has to be taken when comparing thermal reduction treatments on ternary perovskite‐type titanates performed in different vacuum systems. The local oxygen partial pressure can differ significantly from the expectation not only depending on ionizing measurement devices but also on the materials used in the system, e.g., when Ti or Si is employed as getter material, overlayer or substrate the possibility of incongruent sublimation should be kept in mind. Sublimation effects may also play a role when local reduction at sufficient high temperature is performed by ion beams since O tends to be preferentially sputtered, or by electron irradiation or X‐rays (radiolytic damage).…”