Our new precursor offers an improved ALD growthWindow for CaS thin films and alloys by using a new calcium amidinate precursor on calcium sulphide and oxide Films.
Calcium is an important element used in a wide variety of multicomponent materials for microelectronics. Calcium oxide has promising applications as an alloy due to its wide band gap of 6.8 eV.1 This compound finds uses with HfO2 for gate-capacitor dielectrics in metal-oxide semiconductor field-effect transistors (MOSFETs), and MgO as an epitaxial gate insulator in GaN-based high-electron mobility transistors (HEMTs).2 Calcium sulphide may also have a role when alloyed with SnS which is a strong visible light absorber. The calcium stabilizes the cubic phase of Sn1-xCaxS.3 There is also potential in calcium’s use in metal fluoride films for dielectric materials which are optically transparent over a wide wavelength range, and for use in optical filters due to a low refractive index.4
For many of these applications, atomic layer deposition (ALD) is the method of choice for depositing uniform thin films of metals, metal oxides and metal chalcogenides on nanostructured substrates. ALD provides a constant film-growth rate within a defined temperature window. This temperature window is important in the selection of precursors for multicomponent thin films. This is especially critical for calcium which is mostly alloyed with other metal oxides and chalcogenides for functional material applications. One of the most widely used Ca precursors, Ca(tmhd)2, has a narrow, high-temperature ALD growth window of 325-400oC for CaS .5 Another precursor, Ca(i-Pr3Cp)2, has a lower temperature range for ALD of CaO films (200-300 oC)6 but not as well adopted. Other types of volatile Ca precursors such as fluoroalkoxides, fluorinated ß-diketonates and aminoboranes are known, but as of now not reported for use in ALD applications.
Strem currently, offers a calcium alkyl amidinate precursor exhibiting excellent thermal stability and volatility. Bis(N,N’-di-i-propylformamidinato)calcium(II) dimer, (99.99%-Ca) PURATREM (20-8200) is a thermally stable liquid at its evaporation temperature. The evaporation rate of this calcium amidinate is about 37 times higher than that of Ca(tmhd)2(20-1000) at 225oC based on isothermal TGA experiments. The observed ALD window for 20-8200 grows CaS films from 150-280oC, about twice as wide and 150 oC lower than the window for Ca(tmhd)2 .7 This improvement in the ALD window should stimulate the study of alloys incorporating calcium as most atomic layer depositions of metal oxides and sulphides are performed below 300oC.
Items mentioned in this blog:
Please see our literature sheet for a full listing of our Metal Amidinates.