A variety of experimental methods have been employed to investigate the physical mechanisms, such as linear optical properties, PL characteristics and transient absorption phenomena. In this chapter, we first introduce the preparation of the used samples. Then, we used a commercial instrument to measure the absorption spectrum and carried out the time-integrated PL system with pulsed laser excitation.
Finally, the complete setup of optically degenerate pump-probe technique was introduced.
3.1 Sample preparation
Our c-axis-oriented ZnO thin films with thickness of 70-nm and 1-μm were grown on the c-plane sapphire substrates by pulsed laser deposition with base vacuum of 2.1×10-8 torrs and working pressure of 1.1×10-7 torrs using a KrF excimer laser for different deposition times. Hall measurements yielded a background electron concentration of 1.68×1018 cm-3 for the 70-nm epifilm and 1.87×1017 cm-3 for the 1-μm epifilm, respectively. The detail growth apparatus and condition can be found in Ref. 43.
Our c-axis oriented ZnO/Zn0.95Mg0.05O multiple quantum well structures (MQWs) was grown on a c-plane sapphire substrate using high-vacuum pulsed laser deposition (PLD) with KrF excimer laser. A ZnMgO buffer layer with thickness of 80 nm was deposited at 600 °C and subsequently annealed at 700 °C. The used MQWs comprised of five periods of Zn0.95Mg0.05O barrier layers and ZnO quantum well layers. From the TEM measurement as shown in Fig. 3-1, the well layer thickness was around 8-39 nm, and the thickness of barrier layer was approximately 115 nm.
Fig. 3-1. The TEM measurement in ZnO/ZnMgO MQWs.
3.2 Measurement for absorption and time-integrated PL spectra
The absorption spectrum was measured at room temperature (RT) by using the spectral photometer (Jasco V-670) with resolution of 0.5 nm. In addition, the time-integrated PL (TIPL) excited by a frequency doubled mode-locked Ti:sapphire laser with central wavelength of 360 nm was recorded by a single grating monochoromator (iHR320) at RT. The TIPL system setup was shown in Fig. 3-2.
Fig. 3-2. System setup of time-integrated photoluminescence (TIPL) at RT.
3.3 Optical pump-probe system
The time-resolved optical pump-probe measurements as shown in Fig. 3-3 were also carried out at RT by utilizing a mode-locked Ti:sapphire laser (Tsunami, Spectral Physics Inc.) with 82 MHz repetition rate. The pulsed laser was equipped with a frequency doubler (Model 3980) to provide the ultraviolet (UV) excitation
wavelength ranging from 360 nm (3.444 eV) to 383 nm (3.238 eV) with pulse width around 150 fs. The UV beam was divided by a beam splitter into a pump beam and a probe beam with intensity ratio larger than 20:1. The probe beam passed through a motorized translation stage in order to control the time delay relative to the pump one.
The pump and the probe beams were focused and overlapped onto the sample surface by a focal lens with focal length of 5 cm. These two beams made a small angle and the beam diameter of pump beam is slightly larger than that of probe beam. Due to large absorption coefficient of 2×105 cm-1 near the band-edge, the transmitted light is too weak to be detected when the sample is thicker than 500 nm; and the reflection type measurement is required. The setup of time-resolved reflection is similar to the previous experimental arrangements [44].
For the purpose of reducing the coherent artifact, resulting from two beams interference if they possess the same polarization as well as distinguish-ability in detection, we kept the polarization of pump and the probe beams orthogonal to each other. Because our c-axis oriented ZnO epitaxial films were grown on the c-plane sapphire substrate, its crystal axis is normal to the surface. The probe beam incident normally to the sample surface has polarization orthogonal to the c-axis. Although there is a small angle (~ 8°) made between the pump beam and the surface normal, the pump beam with TE polarization is also orthogonal to the c-axis. The time-resolved setup in this work is independent of polarization. In order to increase the signal-to-noise ratio, the pump beam was modulated at 1 kHz by a mechanical chopper, and the transmitted probe beam was detected by a photo-diode and measured as a function of time delay by a lock-in amplifier.
Fig. 3-3. Schematic diagrams of transmission and reflection type optical pump-probe measurement techniques.
3.4 Open-aperture Z-scan method
Z-scan is the most popular method due to its high sensitivity, simple experimental setup, and easy alignment [45]. We applied this method in our optical nonlinear absorption measurements.
The nonlinear absorption experiment as shown in Fig. 3-4 was demonstrated by using the standard on-axis open aperture Z-scan technique at RT. The laser pulse was modulated by a mechanical chopper, and then it was divided into the reflected and the transmitted beams by using a beam splitter. The reflected beam was measured by a photo-diode as the reference representing the incident light. The transmitted light was detected by another photo-diode and focused into the sample by a focal lens with focal length of 5 cm. The sample was mounted on a motorized translation stage controlled by the computer and moved along the z-axis with respect to the focus of the lenses. The output signals from these two photo-diodes were connected to a lock-in amplifier to increase the signal-to-noise ratio.
Fig. 3-4. The experimental arrangement for open-aperture Z-scan method.