薄膜生长技术Pulsed Laser Deposition

Figure 1 Figure 2 In Pulsed Laser Deposition (PLD) an eximer laser strikes a target which is made of the material that is being grown, as shown in Figure 1. The target's material is ablated resulting in a plume of ionic and neutral species which travel to the substrate and deposit on the surface (See Figure 2). The eximer laser is pulsed resulting in fractions of an atomic layer being deposited with each pulse. Typical laser repetition rates are 0.1 - 10 Hz with an elevated substrate temperature of 600 - 1000?C. PLD is extremely useful due to the simplicity of the technique. Complex materials, such as SrTi03 and La(Sr,Mn)03

Our deposition chamber is unique due to the fact that it is located at the Cornell High Energy Synchrotron Source (CHESS). In-Situ and in real time, we monitor the surface using x-ray reflectivity. Information on the surface roughness can be obtained with millisecond time resolution.

Additionally we can use this technique to count the number of monolayers that we have deposited with extreme precision.

Complex Oxides have a perovskite structure (Figure

3) and exhibit a wide range of desirable materials properties including:

? High Tc Superconductors ? High k Dielectrics

? Piezoelectrics and ferroelectrics

? Ferromagnetics

? Colossal Magnetoresistance

Figure 3

In fig. a above, Atomic Force Microscopy (AFM) was performed after deposition of homoepitaxial SrTiO3. Since the growth was stopped at 1/4 monolayer coverage, the surface has unit cell high islands still present. The inset shows the starting substrate surface. To show the correlation between nearest neighbor islands, a Fast Fourier Transform (FFT) of the image was taken (fig. b). The diffuse ring around the center of the image is due to these in-plane correlations. A line out of the boxed region is shown in fig. c, where the average distance between islands is inversely proportional to the value of Q at the diffuse peak (L=2π/Q).

The same information contained in the FFT above of the surface can be obtained from x-ray diffuse scattering measurements. The advantage of using x-ray scattering is that it can be performed in-situ, to allow for surface kinetic processes to be studied in real time.

The above figure show the experimental setup for diffuse scattering measurements. If islands are present on the sample surface, the diffracted beam will give rise to the diffuse lobes shown in the Q|| direction. By taking these images during growth, the surface kinetic can be monitored in-situ. For a more in depth description of the experiment see this paper.