5 Experimental Results and Discussion
5.2 Transient Carrier Dynamics of InN
For this work, the samples were measured by optical pump-terahertz probe system as described in 4.2.3. We use 800nm (1.55ev) light to generate photo-electrons in InN-epilayer and LT-NR with energy bandgap about 0.7ev to investigate the transient carrier dynamics. The photo-electrons with excess energy of 0.85ev which is far below the distance between the conduction minimum to the next local minimum are confined within Γ valley without intervalley scattering. We monitor the peak amplitude of THz waveform, T, as a function of delay time between the terahertz and 800nm optical pump pulse. The differential transmission Δ
T T
/ 0 of both samples are plotted in Fig. 5-17 at 1900 pump fluence where is the THz peak amplitude before optical excitation .The peak amplitudes decrease 70% and 25% after excitation for InN-epilayer and LT-NR, respectively, and then gradually recover due to the recombination and trapping events that decrease the number of mobile electron and reduce the photoconductivity. The optical pump pulse absorbed and scattered by the InN-epilayer and LT-NR are about 80% and 95%, respectively, but the change of peak amplitude of LT-NR is much less than that of InN-epilayer. Terahertz absorption by the photo-electrons is because of acceleration of the free carriers generated by optical pump pulse, but the free carriers generated in LT-NR are confined within the nanostructure and have much lower mobility (62cm/ 2
μ J cm T
02/Vs) than that of InN-epilayer that make the terahertz absorption by LT-NR much less than by InN epilayer. The decay curves shown in Fig.5-16 are fit with a sum of two exponential terms [11]:
max 1 1 2
The fitting results are listed in Table 5-1.The fast decay time of LT-NR is about 2.6ps
which is much faster than that of epilayer (30.7ps). We propose that the fast decay time is mainly controlled by trapping events, and trapping of mobile electrons in nanorods occurs mainly at surface and interface and is strongly affected by surface area due to the surface or trap states increase with increasing surface area. For the nanorods morphology, electrons interact more frequently with surface resulting in a fraction of them are captured by surface and the rest of them are scattered. The trapping time (2.6ps) is longer than carrier scattering time (13fs) obtained from fit of Drude-Smith model and therefore electrons can interact with surface many times before being trapped. The much longer decay time of LT-NR is because of free carriers excited in the silicon substrate in which carriers have nanosecond to millisecond life time. The optical pump-terahertz probe results are consistent with previous results of static optical constants characterization that the nanorods morphology causes electrons to be confined within nanostructure and interact with surface by scattering or trapping.
A
1A
2τ
1(ps) τ
2(ps)
Epilayer LT-NR
0.71 0.88
0.29 0.12
193.5 7839
30.7 2.6
Table.5-1 Bi-exponential decay fit of time-solved data shown in Fig.5-19
0 20 40 60 80 100 120 140 160 -0.1
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
Δ T/T
0Delay time(ps)
(a) InN-epilayer
0 20 40 60 80 100 120 140
-0.05 0.00 0.05 0.10 0.15 0.20 0.25 0.30
Δ T/T
0Delay time(ps) (b)
LT-NR
Fig.5-16 Relative differential transmission as a function of pump-probe delay time (open circle) for (a) InN-epilayer and (b) LT-NR comparison with a bi-exponential decay model (solid line)
5.3 Terahertz Emission from InN Surface
For this work, three InN emitters were measured by electro-optic THz system.
The photo-exciting beam is collimated on the samples with a spot size of ~ 2 mm at the angle of incidence of 70˚, which is near the Brewster angle. The emitted THz pulses were detected by free-space electro-optic sampling in a 2-mm-thick ZnTe crystal as a function of delay time with respect to the optical pump pulse. We have also investigated the azimuthal angle dependence of THz emission, which reflects the contribution of optical rectification effect. No significant dependence was observed for all three samples.
Fig. 5-17~5-19 shows the time-domain waveforms and their corresponding amplitude spectrums of THz emission for LT-NR, HT-NR, and the InN epilayer, respectively. Each sample is excited at the laser fluence of 720μ J/cm2. The bandwidth of each emitter is up to 2.5THz. The dependence of peak amplitude of THz emission on the pump power is plotted in Fig. 5-20. The peak THz amplitudes of HT-NR and InN film increase by about twice as the pump power increases by as much as ten times, while that of LT-NR increases at least six times. Excited by laser pulses at 1 mJ/cm2, the THz emission from LT-NR is about three times stronger than that from InN thin film and HT-NR.
THz radiation from the narrow bandgap semiconductor is generated by the ultrafast-laser-driven accelerated carriers formed in a shallow surface area. Therefore, efficient THz emission from these materials is closely related to the near-surface characteristics of the materials, including their morphology, point defects, and the effective surface area. For InN film, since the surface accumulation layer is typically very thin (~ 10 nm) compared to the penetration depth of the laser pulse [29], its contribution to the total THz radiation is negligible compared to the photo-Dember
effect [16] [14]. Moreover, for n-type InN with a high carrier concentration, the direction of surface field is opposite to the photo-Dember field and consequently reduces the total magnitude of THz radiation. Room-temperature polarized Raman spectroscopic studies [27] showed that the concentrations of free carriers in nanorods are one order of magnitude higher than that in the InN film, suggesting that there are a considerable amount of structural defects in the LT-NR. Further, Room-temperature photoluminescence (PL) signals of HT-NR and LT-NR are about one to two orders of magnitude lower than that of the InN film [27]. This phenomenon has been attributed to strong surface electron accumulation effect, which screens photocarriers and in turn reduces the radiative recombination in InN nanorods.
Nanorods have drastically increased effective surface area compared to the film and every surface exposed to the photo-excitation pulses may participate in absorption.
In order to investigate the relation between the surface area and optical absorption, we studied the reflectance of each sample. Measured nearly-normal-incident reflectance in Fig. 5-21 demonstrates that over the whole range of excitation level, absorption in nanorods (~ 95 %) is much larger than that in the film (~ 80 %). And it may correspond to the enhanced optical absorption in nanorods by increased effective surface area. Here one must notice that HT-NR absorbs as much of excitation energy as LT-NR, while THz radiation from HT-NR in Fig. 5-20 is much lower than that from LT-NR. This inefficient conversion of optical absorption to THz emission in HT-NR may be understood by the nanorod-size dependence of THz emission. Since the penetration depth of the laser pulse in the nanorod is limited by its geometrical shape, the surface-to-volume ratio becomes the crucial factor for the optical absorption and THz emission.
The radius of large-size nanorods formed in both LT- and HT-NR is about 65 nm.
This is smaller than the penetration depth of the laser pulse, but still larger than the surface accumulation layer. Since the surface field is opposite in direction to the photo-Dember field, if we exclude the accumulation layer (~ 10 nm) from the total volume of nanorods, THz radiation by the photo-Dember field is mainly generated in the inner volume with the radius of ~ 55 nm. Meanwhile, the ultra-small nanorods in HT-NR have much smaller radius (~ 30 nm) and their effective volume of THz radiation is as small as the inner volume with the radius of 20 nm. Under the assumption that THz emission from surface accumulation layer is negligible because of screening, we can roughly calculate the effective volume of THz emission by the nanorods solely due to the photo-Dember effect. Comparing with LT-NR, the number of large-size nanorods of HT-NR reduces by ~ 40 % (see FE-SEM images in Fig. 3-2).
On the other hand, there is about three times more number of ultra-small nanorods with small effective volume closely packing the space between large-size nanorods.
Although there are much more nanorods including both large-size and ultra-small rods, the total effective volume of THz emission for HT-NR is about twice smaller than that for LT-NR. As the excitation power increases, the absorption and THz emission in LT-NR increase accordingly due to its large effective volume. In contrast, the increased absorption in the ultra-small nanorods of HT-NR may not be effectively converted into the THz emission due to the decreased effective volume.
0 2 4 6 8 10 12 -0.0002
0.0000 0.0002 0.0004 0.0006 0.0008
E- Fi el d( a. u. )
Delay time(ps)
LT-NR
(a)
0.0 0.4 0.8 1.2 1.6 2.0 2.4 2.8
0.00000 0.00001 0.00002 0.00003 0.00004 0.00005
A m p litu d e (a .u .)
Frequency(THz)
LT-NR
(b)
Fig.5-17 THz (a) time-domain waveform and (b) corresponding amplitude spectrum generated from LT-NR
0 2 4 6 8 10 12 -0.0002
-0.0001 0.0000 0.0001 0.0002 0.0003
E- Fi el d( a. u. )
Delay time(ps)
HT-NR
(a)
0.0 0.4 0.8 1.2 1.6 2.0 2.4 2.8
0.000000 0.000005 0.000010 0.000015 0.000020 0.000025
Ampl it ude( a. u. )
Frequency(THz)
HT-NR
(b)
Fig.5-18 THz (a) time-domain waveform and (b) corresponding amplitude spectrum generated from HT-NR
0 2 4 6 8 10 12 -0.0002
-0.0001 0.0000 0.0001 0.0002 0.0003
E- Fi el d(a. u. )
Delay time(ps)
InN-epilayer
0.0 0.4 0.8 1.2 1.6 2.0 2.4 2.8
0.000000 0.000005 0.000010 0.000015 0.000020
A m plitude (a .u.)
Frequency(THz)
InN-epilayer
Fig.5-19 THz (a) time-domain waveform and (b) corresponding amplitude spectrum generated from InN-epilayer
200 400 600 800 1000 100
200 300 400 500 600 700
Peak amplitude(a.u.)
Pump Fluence (
μJ/cm 2 ) LT
Film HT
Fig.5-20 Peak amplitude of THz emission for InN film (solid squares), HT- (open circles) and LT-NR (solid circles) as a function of laser pump power
300 450 600 750 900 1050
0.00 0.04 0.08 0.12 0.16 0.20
R e flect iv it y
Pump fluence ( μ J/cm 2 ) Film
LT HT
Fig. 5-21 Optical reflectivities in HT-NR (open circles), LT-NR (solid circles), and InN-epilayer (solid squares) as a function of excitation energy. Optical absorption in InN film is about 80 %, while that in nanorods is about 95 %.
6. Conclusions and Future Work
6.1 Conclusions
In this thesis, we have investigated physical parameters of the InN and GaN films and their nanorods in THz range. The simple-Drude model was used to describe the epitaxial films, while the Drude-Smith model for InN nanorods. The mobility and the carrier density of GaN and InN films obtained from the THz time-domain spectroscopy are consistent with those from the Hall effect measurement.
Non-Drude-like behavior of GaN and InN nanorods might be attributed to localization or backscattering of carriers within the nanorods. In optical pump-terahertz probe study, we have investigated the transient carrier dynamics of InN-epilayer and its nanorods. The reduced photoconductivity of InN nanorods in comparison with epilayer has been attributed to the small mobility due to localization of electrons, and the faster recovering time of photoconductivity is because of the increased defect and trap states due to the nanorods morphology.
For THz emission study, we demonstrated that the enhancement of THz emission from InN nanorods. The “screened” photo-Dember effect is found to be the main THz emission mechanism in InN nanorods. The enhancement of THz emission is closely related to the surface-to-volume ratio of the nanorods. Nanorods with the radius smaller or comparable to the thickness of the accumulation layer do not contribute significantly to THz emission.
6.2 Future Work
Nanostructured materials, such as GaN nanorods and ZnO nanowires, have the abundant potentials in the optoelectronic applications. The study of these wide bandgap semiconductors are limited by the lack of an intense light source with the photon energy larger than their bandgaps. Currently, we are working on the
sum-frequency generation of 400nm and 800nm for better understanding of these materials.
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