Epitaxial Growth of Oxide Films Using Thermal Laser Epitaxy

 

The synthesis of ultra-clean oxide films and multilayers with well-controlled composition and crystal structure is key for future scientific studies and applications. Thermal Laser Epitaxy (TLE) is a novel epitaxial growth technique that fulfills these requirements by enabling the evaporation from ultrapure elemental sources in corrosive atmospheres from
10-11 hPa (XHV) to almost atmospheric pressure. TLE uses continuous-wave lasers for both substrate heating and thermally evaporating source materials, thereby allowing the application of MBE growth modes to numerous oxides that are impossible to grow by MBE.

  

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

 

In a first exploration to investigate the possibility to grow oxide films by TLE, a wide variety of oxide films has been successfully grown on Si (100) substrates by laser evaporating pure elemental sources in oxygen-ozone mixtures. An example that illustrates the successful growth is given in Fig. 1, which 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. In our first study, more than 15 different oxides have been grown by TLE, including Sc2O3, TiO2, NiO, CuO, ZnO, ZrO2, Nb2O5, MoO3, HfO2, and RuO2. Oxides of multivalent transition metals have even been grown in different oxidation states. From an elemental V source, for example, films of either V2O3, VO2, or V2O5 were grown by tuning the oxygen pressure.      

     

Figure 2:  XRD (left) ω-2θ scan and (right) φ scan of TLE-grown epitaxial RuO2 (110) on MgO (100). TLE readily forms RuO2 epitaxial films on laser-heated MgO (100) substrates, illustrating the advantages of TLE for the epitaxy of refractory metal oxides.  
 

Furthermore, we have demonstrated the epitaxial growth of oxide films using laser-heated substrates. This success is illustrated by Fig. 2, which shows XRD ω-2θ and φ scans of a TLE-grown epitaxial RuO2 (110) film on a MgO (100) substrate. Ru has a very low vapour pressure and is difficult to oxidize, which makes RuO2 films notoriously difficult to grow by MBE. Notwithstanding, such films were 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 been grown in addition. These results reveal that TLE opens up new possibilities for the epitaxial growth of ultrahigh-purity oxide films and heterostructures.

 

For more information on TLE see here.

For more information on oxide film growth by TLE see here and here.

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