Fig.3.2.2 Structure of photoconductive antenna and silicon lens
is pump beam and the other is probe beam. The pump beam is focused on the photoconductive antenna by objective lens. A 5volt AC bias with frequency of 1KHz is applied to the emitter antenna to accelerate the carriers excited by the pump pulse by a function generator. Then, the terahertz ray radiated from photoconductive antenna and we used a parabolic mirror to collect the THz beam. We focus the THz beam on the sample and collect the transmittance beam by another parabolic mirror.
Finally, we also focus the transmittance THz beam on the detector which contact with the lock-in Amplifier. Second, the probe beam path through the delay stage is used to adjust the delay compare with the pump beam.
The probe beam finally go into the antenna detector. By using the controller to make the different delay of pump beam and probe beam, we can record the signal as a function of delay by the lock-in amplifier.
Figure 3.2.3 show the THz time domain waveform of air with one step of delay in 10micrometer and total of 512 steps. The corresponding power spectrum is also as shown. In time domain waveform, we can see there are still many noises after main signal and their corresponding power spectrum show many absorption peaks. This is contributed to the water vapor absorption [22]. Water is almost perfectly transparent to visible light but great absorption in THz region which is shown in Figure 3.3.
In order to avoid water vapor absorption, the entire THz beam is located in a closed acrylic box which is purged with nitrogen gas to decrease the water vapor. At purged about 30minutes, the environmental humidity decrease from about 60％ to under 5％. The Thz time-domain waveform and their corresponding power spectrum under the humidity of
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
Fig.3.2.3 THz (a)time-domain waveform (b)corresponding power spectrum in the air
about 5％ are shown in Figure 3.4. From this figure, the time-domain waveform become smoother than before and the peak value of signal is larger. The absorption peaks in power spectrum also disappear and seem a useful range from 0.2 to 3THz in the air with low humidity.
Fig.3.3 The visible and UV spectra of liquid water (http://www1.lsbu.ac.uk/water/vibrat.html#comp)
0 5 10 15 20 25 30 35
-0.0005 0.0000 0.0005 0.0010 0.0015
E-field
time delay
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 1E-16
1E-15 1E-14 1E-13 1E-12 1E-11 1E-10 1E-9 1E-8
Power as MSA
frequency(THz)
Fig.3.4 THz (a)time-domain waveform (b)corresponding power spectrum in the air with humidity under 5％
3.2 Fourier transform infrared spectroscopy
(The Fourier Transform Infrared Spectrometer : VERTEX 70v)
Infrared light emitted from a source is directed into an interferometer, which modulates the light. After the interferometer the light passes through the sample compartment (and also the sample) and is then focused onto the detector. The signal measured by the detector is called the interferogram.
Fig. 3.5 The setup of Fourier transform infrared spectrometer
3.3 Sample preparation
3.3.1 ITO nanorod (from Prof. Yu Peichen)
ITO nanoculumn is grown under the following conditions:
1. Deposition angle=700 2. Vacuum pressure=10-4 torr
3. ITO growth on high resistivity silicon substrate 4. Substrate temperature=240oC
5. With nitrogen flux
6. Using the commercial target composition is 95 wt% In2O3 + 5 wt%
SnO2
Figure 3.6 shows the epitaxial structure of a conventional, single-junction GaAs/AlGaAs solar cell[23]. After the standard fabrication process, the ITO-nanocolumn structure was deposited onto the p-type Al0.8Ga0.2As window layer using glancing-angle e-beam deposition, followed by a post annealing process at 3500C for 25 minutes to improve the transmittance. The fabricated device is schematically illustrated in right of Figure 3.6 Glancing-angle deposition has been employed for preparing microscale and nanoscale porous materials based on nucleation formation and self shadowing effect. However, the characteristic ITO nanocolumn structure seen in this work is rather unique, where the formation involves either catalyst-free or self-catalyzed
vapor–liquid–solid (VLS) growth assisted by the introduced nitrogen.
The substrate is tilted at a deposition angle of 700 with respect to the incident vapor flux, where the chamber pressure is controlled at 1.33 10-2 Pa [2].
Figure 3.7.1 is the picture of chamber taken from camera and Figure 3.7.2 is schematic drawing of chamber：
Fig.3.6 (a) Epitaxial structure of a single-junction GaAs solar cell. (b) Schematic of a GaAs solar cell fabricated employing ITO
nanocolumns[2]
as the conductive AR coating.
C H A M B E R
ψ
Holder
ITO Vacuum rod
Pump
θ Normal Line
Fig.3.7.2 View of the chamber Fig 3.7.1 The picture of chamber
The substrate we used is high resistivity Silicon wafer. With the higher resistivity, the absorption of substrate will become lower. Figure 3.8 show the position of ITO rod grown on Silicon wafer. In section 2.2, we need the reference to determine the properties of sample. Here, the corresponding reference is on the Neighbor side of the sample. It make sure that the reference we used is approximatively same to the substrate of ITO rod.
Figure 3.9 shown the optical beam transmit the sample and reference.
In calculation of section 2.2, the reference require approximatively same to the substrate of sample. Because the Silicon wafer is very thick compare with ITO nanorod, this phenomenon plays an important role in our experiment.
Ref(722nm)
536nm
323nm
ITO rod grown on here
525μm
High resistivity Silicon wafer
Fig.3.8 ITO rod grown on Silicon wafer 687nm
722nm
Ref(687nm)
Ref(536nm)
Ref(323nm)
3.3.2 SEM results
(Scanning electron microscope: JEOL 7000F 20X~200000X)
SEM is type of electron microscope that images the sample surface by scanning it with a high energy beam of electrons. The electrons interact with the atoms cause the sample producing signals that contain information about the surface of our sample.