High-temperature-grown buffer layer boosts electron mobility in epitaxial La-doped BaSnO3/SrZrO3 heterostructures

Abstract

By inserting a SrZrO 3 buffer layer between the film and the substrate, we demonstrate a significant reduction of the threading dislocation density with an associated improvement of the electron mobility in La:BaSnO 3 films. A room temperature mobility of 140 cm 2 V −1 s −1 is achieved for 25-nm-thick films without any postgrowth treatment. The density of threading dislocations is only 4.9 × 10 9 cm −2 for buffered films prepared on (110) TbScO 3 substrates by pulsed laser deposition.Transparent conducting oxides (TCOs) have attracted attention due to their unique properties and applications in electronic devices such as transparent displays and transistors. 1,2 Recently, the transparent perovskite La:BaSnO 3 has gained interest as a novel TCO due to its high mobility at room temperature (RT). 3,4 Single crystals of La:BaSnO 3 have been reported to have RT mobilities as high as 320 cm 2 V −1 s −1 (mobile carrier concentration n = 8 × 10 19 cm −3 ). 4 At doping concentrations above 10 19 cm −3 , La:BaSnO 3 has the second highest RT electron mobility among TCOs and other oxide single crystals 4-6 , exceeded only by CdO. 7 The high RT mobility in La:BaSnO 3 has been attributed to the small electron-phonon interaction and small electron effective mass (m * e = 0.19 m 0 ) arising from the large dispersion of the conduction band comprised of Sn 5s orbitals. 8-10 However, to fully explore the potential of La:BaSnO 3 , thin films with high mobilities are also required.Recently, several efforts to achieve high carrier mobility in La:BaSnO 3 films have been reported. 11-18 Nevertheless, as compared to bulk single crystals, even the best La:BaSnO 3 epitaxial films show a reduced mobility (≤ 183 cm 2 V −1 s −1 ), 18 which specifically has been attributed to scattering from charge defects, such as threading dislocations (TDs) 8,18-20 and, more generally, to small carrier relaxation times. 8,9 TDs form for a large lattice mismatch between film and substrate, and extend perpendicularly through the films. The obvious solution to reduce TDs in La:BaSnO 3 films is to use latticematched substrates, but unfortunately none exists. The substrate with the closest lattice match that is commercially available is PrScO 3 , mismatched by −2.3%. 21 With such a high mismatch only thin commensurate layers of BaSnO 3 can be grown; at a thickness of 32 nm the reported BaSnO 3 films are almost fully relaxed and contain high densities of TDs. 19 To reduce the dislocation density in the La:BaSnO 3 film, we explored the insertion of an undoped BaSnO 3 a) Corresponding author: p.ngabonziza@fkf.mpg.de buffer layer at the interface between the substrate (TbScO 3 ) and the La:BaSnO 3 film. As the thickness of the BaSnO 3 layer increases, the TD density decreases as the threading component of dislocations annihilate each other, leaving behind a network of misfit dislocations. This method is known and has already led to the highest mobilities in BaSnO 3 films to date; but even for thick buffer layers (330 nm), the remaining TD density is still 1.2 × 10 1...

Oxide Two‐Dimensional Electron Gas with High Mobility at Room‐Temperature

“…This is because the PLD grown BSO film was annealed at high temperature which has been shown to cause annihilation of threading dislocations. [ 21,28 ] The dislocation density of the BSO layer grown on the BSO pseudo‐substrate is very similar to that of the BSO pseudo‐substrate (Figure 4d). This provides less charge trapping and scattering, consistent with the increased carrier density and mobility at room temperature for the LSO/BSO heterostructure grown on the BSO pseudo‐substrate.…”

“…One such structural defect is the high density of dislocations in typical BSO thin films (more than 10 9 cm –2 ). [ 19 , 20 , 21 , 22 ] These threading dislocations originate from the large lattice mismatch between BSO and commercially available perovskite substrates (ranging from −5.2% (SrTiO 3 ) to −2.4% (PrScO 3 )). Because of such defects, the mobility of electrons produced by La doped thin films of BaSnO 3 [ 19 ] has not yet exceeded that of La‐doped BaSnO 3 bulk single crystals.…”

One-step epitaxy of high-mobility La-doped BaSnO3 films by high-pressure magnetron sputtering

“…24 On the other hand, inserting insulating buffer layers between LBSO films and substrates provides another route to effectively reduce TD density. 33,41,42,44 The strategy of post-annealing was also adopted to increase the room temperature mobility after deposition. 34,37,39 In particular, oxygen vacancies (VO) induced by high-temperature annealing under wet H2 atmosphere or vacuum can accelerate the movement of dislocations and promote lateral grain growth in LBSO films, leading to an increment of room-temperature mobility up to 122 cm 2 V -1 s -1 .…”

Combined Spectroscopy and Electrical Characterization of La:BaSnO$_\text{3}$ Thin Films and Heterostructures

“…However, the reported µ e in La:BaSnO 3 thin films have only reached a maximum value of 183 cm 2 V −1 s −1 (n 1.2 × 10 20 cm −3 ) for epitaxial films grown by molecular beam epitaxy (MBE) [33]. Other growth techniques resulted in the following electron mobilities: 140 cm 2 V −1 s −1 (n 5.2 × 10 20 cm −3 ) for pulsed laser deposition (PLD) [27], 121 cm 2 V −1 s −1 (n 4.0 × 10 20 cm −3 ) for high-pressure magnetron sputtering [36], and 53 cm 2 V −1 s −1 (n 2.0 × 10 20 cm −3 ) for chemical solution deposition [39]. Various strategies to improve the mobility in La:BaSnO 3 epitaxial films have been explored.…”