Growth of Oxide Films by Thermal Laser Epitaxy

 

The new physics and technological applications possible with oxide based heterostructures demand for the synthesis of ultra-clean oxide materials with a controlled atomic structure and composition. Thermal Laser Epitaxy (TLE) is a new epitaxy technique fulfilling these requirements by enabling the evaporation from ultrapure elemental sources in corrosive atmospheres from XHV to almost atmospheric pressure. TLE uses continuous-wave lasers for heating a substrate and for thermally evaporating source materials, thereby allowing the application of MBE growth modes to a dramatically wider range of oxides.

For more information on TLE:
Thermal Laser Epitaxy (
https://www.fkf.mpg.de/6892181/72_Thermal_Laser_Epitaxy_Braun)

An exploratory spectrum of oxide films has been successfully grown on Si (100) substrates by laser evaporating pure elemental sources in oxygen-ozone atmospheres. Fig. 1 shows a grazing-incidence x-ray diffraction (XRD) pattern and a cross-sectional scanning electron microscope (SEM) image of a TLE-grown TiO2 film with a mixture of rutile and anatase phases. Within this study, more than 15 different oxides have been grown by TLE, including Sc2O3, TiO2, NiO, CuO, ZnO, ZrO2, Nb2O5, MoO3, HfO2 and RuO2. Multivalent transition metal sources may form oxides in different oxidation states, for example, an elemental V source forms V2O3, VO2, and V2O5 with increasing oxidizing gas pressure. 

Figure 1: (left) Grazing incidence XRD pattern and (right) cross-sectional SEM image of a TLE-grown TiO2 film on Si (100) in a mixture of the rutile and anatase phases. The polycrystalline TiO2 film grows in a columnar structure on the unheated silicon substrate. 
 

We have also demonstrated the epitaxial growth of oxide films on laser-heated oxide substrates. Fig. 2 shows XRD ω-2θ and φ scans of a TLE-grown epitaxial RuO2 (110) film on a MgO (100) substrate. Despite Ru having a very low vapor pressure and being difficult to oxidize, RuO2 may be readily grown by TLE. This shows the advantages of TLE in particular for synthesizing the oxides of refractory metals in a strongly oxidizing atmosphere. TLE-grown epitaxial NiO (111) and VO2 (020) films on c-plane sapphire substrates have also been grown. These results demonstrate that TLE can open up new possibilities for the epitaxial growth of ultrahigh purity oxide heterostructures.

Figure 2:  XRD (top) ω-2θ scan and (bottom) φ scan of TLE-grown epitaxial RuO2 (110) on MgO (100). TLE readily forms RuO2 epitaxial films on laser-heated MgO(100) substrates, which shows the particular advantages of TLE for the epitaxy of oxides of the refractory metals.
 

For more information on oxide film growth and epitaxy by TLE:

APL Materials 9, 081105 (2021); https://doi.org/10.1063/5.0055237

Journal of Vacuum Science & Technology A 39, 053406 (2021);
https://doi.org/10.1116/6.0001177

Go to Editor View