Influence of high-excited light intensity

The PL conditions as mentioned above are excited by low excitation light intensity. At low excitation light intensity (low density regime in Fig. 2-10), the PL properties are determined by single electron-hole pairs, either in the exciton states or in the continuum. Higher excitation intensity (intermediate density regime in Fig.

2-10) makes more excitons; such condition would lead to the exciton inelastic scattering processes and form the biexciton. The scattering processes may lead to a collision-broadening of the exciton resonances and to the appearance of new luminescence bands, to an excitation-induced increase of absorption, to bleaching or to optical amplification, i.e., to gain or negative absorption depending on the excitation conditions. If we pump the sample even harder, we leave the intermediate and arrive at the high density regime in Fig. 2-6, where the excitons lose their identity

as individual quasiparticles and where a new collective phase is formed which is known as the electron-hole plasma (EHP).

Fig. 2-10 The general scenario for many-particle effects in semiconductors⁴⁴

1. Scattering Processes

In the inelastic scattering processes, an exciton is scattered into a higher excited state, while another is scattered on the photon-like part of the polariton dispersion and leaves the sample with high probability as a luminescence photon, when this photon-like particle hits the surface of the sample. This process is shown schematically in Fig. 2-11 and the photons emit in such a process have energies En

given by Ref. 28

n kT E

E E

_{n ex b}^ex

2 3 1 1₂⎟−

⎠

⎜ ⎞

⎝⎛ −

−

= , (2-7)

and kT is the thermal energy. The resulting emission bands are usually called P-bands with an index given by n.

energy

Fig.2-11 Schematic representation of the inelastic exciton-exciton scattering processes.⁴⁴

2. Electron-Hole Plasma

In this high density regime, the density of electron-hole pairs np is at least in parts of the excited volume so high that their average distance is comparable to or smaller than their Bohr radius, i.e., we reach a “critical density” in an EHP, given

to a first approximation by . We can no longer say that a certain electron is bound to a certain hole; instead, we have the new collective EHP phase. The transition to an EHP is connected with very strong changes of the electronic excitations and the optical properties of semiconductors.

c

np 3 ^c_p ≈1

Bn a

Chapter 3 Experiment Details

3.1 Sample Preparation

3.1.1 Substrate preparation

Silicon (100) wafer and a-plane sapphire were used as the substrate for the growth of ZnO-based nanostructures. Before the surface treatment, the substrates were cut into an area of 10^╳5 mm² for the nanostructures growth. Then these substrates were cleaned by using the following steps:

(1) Rinsed in D. I. water by a supersonic oscillator in 20 min.

(2) Rinsed in ACE (Acetone) solution by a supersonic oscillator in 20 min.

(3) Rinsed in D.I water by a supersonic oscillator in 20 min.

(4) Dried with N2 gas.

(5) Baked at 200°C on hot plate.

After the surface treatment, the substrates were placed in an alumina boat filled with the growth metal and ready to grow ZnO-based nanostructures.

3.1.2 Growth of ZnO and ZnMgO nanostructures

The substrate was placed on the alumina boat filled with pure Zn powder (99.9999 %) or mixture of Zn and Mg (99.6 %) powders. The vertical distance between the metal source and the sample was about 3~5 mm. Then the alumina boat which carried the substrate was inserted into a quartz tube. This quartz tube was placed inside a furnace, with the center of the alumina boat positioned at the center of the furnace and the substrates placed downstream of growth metal powder (Fig. 3-1). The quartz tube was evacuated to a pressure below 2╳10^-2torr using

a flow rate of 200 sccm and pressure controlled at 50 sccm. The temperature of the furnace was increased to 570⁰C~700⁰C at a rate of 20-30⁰C min^-1 and then kept at fixed temperature for 30~60 min under an O2 flow at about 2 sccm. After the furnace was cooled to the room temperature, dark gray-white material was obtained on the surface of the substrates.

Fig. 3-1 Thermal vapor transport system

3.2 Scanning Electron Microscope (SEM) system

The morphology of ZnO-based nanostructures were observed by the Field Emission Gun Scanning Electron Microscopy (FEG-SEM) [JEOL JSM-6500F]. The accelerated voltage is 0.5-30kV and the magnification is 20-300k times, as shown in Fig. 3-2.

Fig. 3-2 SEM system

3.3 Transmission electron microscope (TEM) system

The detailed structure of ZnO-based nanostructures can be found by TEM. The TEM images were taken by JEOL JEM-2000FX transmission electron microscope (TEM) operated at 200 keV. The prepared nanowires were firstly suspended in alcohol by supersonic jolt and then the suspension was moved to the copper grid for observation, as shown in Fig. 3-3.

Fig. 3-3 TEM system

3.4 X-ray diffraction

After growing the nanostructures, the crystalline structures of the as-grown ZnO nanowires were analyzed by using Philips PW1700 X-ray diffractometer (XRD) with Cu Ka radiation (λ=1.5405 Å). The maximum voltage of the system is 40 kV with a maximum current 40 mA. The scanning step is 0.020 and scanning rate is 40 /min, as shown in Fig. 3-4.

Fig. 3-4 XRD system

3.5 Raman system

The optical characterization was analyzed with Raman measurement. A 5mW Ar-ion laser with an incident wavelength of 488 nm was used as the excitation source for the Raman spectroscopy. The scattered light was collected using backscattering geometry and detected by the SPEX 1877 triplemate equipped with liquid nitrogen cooled CCD. Measurements were carried out at room temperature, as shown in Fig.

3-5.

Fig. 3-5 Raman system

3.6 Photoluminescence (PL) system

For cw PL measurement, we used a He–Cd laser (325nm) [Kimmon IK5552R-F]

as the excitation source; for pulsed pumping, we used the thirdharmonic of Nd:YVO4

laser (355 nm) [JDS uniphase CDRH Power Chip Nanolaser] with pulse width of

~500 ps and repetition rate of 1KHz. The emission light was dispersed by a TRIAX-320 spectrometer and detected by a UV-sensitive photomultiplier tube. As shown in Fig. 3-6 and Fig. 3-7.

He-Cd laser (325 nm)

M1

M2 Sample holder

M3 (Focus lens)

M4 (Focus lens) Triax 320

Fig. 3-6 PL system

Pulse laser (355 nm)

Sample holder M5 (Focus lens)

Triax 320

Collecting fiber

Fig. 3-7 High power pumping PL system

Chapter 4 Results and Discussion

4.1 Structural and optical properties of saw-like ZnO nanostructures

4.1.1 Growth of ZnO nanosaws on Si substrate

Synthesis of ZnO nanostructure was achieved in a simple vapor transport process⁴⁵. First, a very thin layer of Au was deposited on a (100)-silicon substrate.

A mixture of pure zinc powder (99.9999%) and Mg3N2 powder (99.6%) with a weight ratio of 10:1 was placed in a ceramic boat as the starting materials. The boat was positioned in the center of the quartz furnace tube and the substrate was placed 5 cm downstream from the mixed powder. After the high-purity argon gas was infused into the system with a flow rate of 200 sccm, the furnace temperature was increased to 700°C and kept at this temperature for 60 min under an O2 flow at about 2 sccm.

After the reaction is complete, the system was cooled to the ambient temperature and a gray-white colored product was found deposited on the substrate.

4.1.2 Structural properties and growth mechanism of ZnO nanosaws

As shown in Figure 4-1(a), the typical SEM micrograph clearly reveals that a large quantity of one dimension nanostructure was formed on the Si substrate. A high magnification SEM image is shown in Fig. 4-1(b)-(c). The saw-shape nanostructure, with one side flat and the other side with teeth, is the most dominant morphology. Typical length of nanosaws exceeds several tens of µm with sharp teeth in a saw having lengthof about 1-5 µm and diameter ranging about 100 nm.

elements, with no other detectable elements.

0 1 2 3 4 5

O

Counts (a.u.)

Energy (keV)

Zn

Figure 4-1 (a) The SEM image of the ZnO nanosaws, (b)-(c) magnified SEM image of the ZnO nanosaws, and (d) The EDX of the ZnO nanosaws.

Figure 4-2 shows the XRD pattern of the sample. All diffraction peaks and their relative intensities coincide with the JCPDS card no. 36-1451. Therefore, the products are the hexagonal ZnO phase with random orientation and distribution.

The lattice constants calculated according to the following equations to be a=3.246 Å and c=5.2 Å,:

(d)

2

where h, k, l are Miller exponents, and

λ

=1.54056Å and θ are X-ray wavelength and Bragg angle, respectively.

Figure 4-2 The XRD pattern of the ZnO nanosaws

Figure 4-3(a) shows the TEM image of the sharp teeth of the nanosaws. The width of the teeth is nearly 50 nm. The double image of the nanowires is due to overlapping the one on top of another. The selected-area electron diffraction (SAED) pattern is displayed in Fig. 4-3(b). The appearance of diffraction spot (0002) confirms that teeth of a nanosaw growth along the c-axis direction (along [0001]).

The discrete diffraction spots indicate that the ZnO nanosaws is single crystalline

is known that the growth direction of several ZnO nanostructures have been identified in the literature7~8,30,46~47. Figure 4-3(c) shows an HRTEM image taken from the side of teeth of a nanosaw, which indicates the perfect lattice structure of the nanosaw and the d-spacing between any two neighboring lattice fringes is 0.52 nm and 0.28 nm which match that of [0001] direction and [0110] direction of the wurtzite ZnO, respectively. Schematic models on the geometrical shape and the corresponding crystallographic facets are given in Fig. 4-3(d).

The growth mechanism of the nanosaws is likely the self-catalyst process, which originates from Zn or ZnOx clusters⁴⁸. The possible growth procedure for typical ZnO nanosaw is as follow. The Zn powder turns into Zn vapor during the evaporation duration and forms Zn or ZnOx liquid droplets as the nucleation site of the ZnO nanostructures. Then the solid ZnO particle separates from the droplets, and ZnO nanoribbon formed first. As the reaction process proceeded with Zn vapor turned into Zn or ZnOx liquid droplets, and deposited on the new nucleation sites on the ZnO nanoribbon. Finally, it is known to us that [0001] is the fastest growth direction in the only grew along one side of ZnO nanostructures, including the comb-structure^7-8,47-48 and it corresponds to the formation of ZnO nanosaws.

(d)

Figure4-3 (a)TEM image of the ZnO nanosaws. (b)SAED of the ZnO branches.(c)HRTEM image of the branches of the ZnO nanosaws.(d) Schematic model of the nanosaw

4.1.3 Optical properties of ZnO nanosaws

The typical Raman spectrum of the ZnO nanosaws is shown in Fig. 4-4.

The Raman peaks at 378 and 436 cm^-1 originate from vibration modes of A1(TO) (transverse optical, TO) and E2(high), respectively. The higher frequency of the Raman spectrum was observed that the longitudinal optical (LO) phonon modes are two polar symmetric modes with A1(LO) and E1(LO). The peaks of A1(LO) and E1(LO) were fitted to the spectrum in the insert of Fig. 4-4, which are at 574 and 583 cm^-1, respectively. The result is consistent with the values of ZnO powder (or bulk crystal)⁴⁹. The E2(high) mode corresponds to band characteristic of wurtzite phase.

The appearance of the E1(LO) peak has been attributed to the formation of oxygen vacancy and interstitial zinc atom.

The PL spectra of ZnO nanosaws measured at room temperature are shown in Fig. 4-5. Under cw laser excitation, the spectrum shows a strong UV emission peak having photon energy about 3.22 eV with FWHM~ 150 meV (Wavelength=385 nm, FWHM~ 18 nm) and weak green band having a broad feature in the range of 1.9 eV~2.8 eV (wavelength=440 nm~650nm). This sharp UV emission peak is attributed to the near band edge emission^45,50. As comparison with ZnO bulk crystal (photon energy of free exciton is 3.28 eV), slight shift of the emission peak toward the lower energy in ZnO nanosaws results from a mixture of free exciton and other impurity-related transitions⁵⁰ or the effect of the thermal energy caused by laser heating. The emission of broad peak is the deep-level emission, which is attributed to the oxygen vacancy. The strong UV emission and weak green emission in PL spectra indicate that the ZnO nanosaws have a good crystal quality with few oxygen vacancies.

400 450 500 550 600 650

550 560 570 580 590 600

LO phonon mode peak E1(LO)

A1(LO)

Intensity(a.u.)

Raman shift (cm^-1)

E ₂ (high)

Raman shift (cm ^-1 )

In te ns ity(a .u .)

A ₁ (TO)

Fig. 4-4 The Raman spectra of the ZnO nanosaws

2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4

P L inte ns ity (a .u.)

Photon Energy(eV)

Fig. 4-5 The PL spectra of the ZnO nanosaws.

4.1.4 Random lasing in ZnO nanosaws at room temperature

In Fig. 4-6(a) shows excitation intensity dependence of emission spectra under pulse laser excitation. Under low excitation intensity, the spectrum is composed of a single broad peak of amplified spontaneous emission at 3.2 eV with FWHM~115 meV. As increasing the excitation intensity, the emission peak becomes narrower due to the preferential amplification at wavelength close to the maximum of the gain spectrum. When the excitation intensity exceeded a threshold, Ith=0.96 MW/cm² in Fig. 4-6(b), several narrow peaks emerged in the emission spectra at around 3.18 eV. The linewidth of these peaks was less than 4 meV.

When the pumping intensity increased further, more sharp peaks appeared. The spectrally emission intensity as a function of the excitation intensity is shown in Fig.

4-6(b). Above a certain pump threshold, the intensity of the multiple sharp peaks increases much more rapidly with the pump intensity. These results indicate a clear evidence of lasing action. Note that the position of PL emission peak redshifts with increasing pumping intensity, which is caused by the bandgap renormalization.⁵¹

(a)

3.0 3.1 3.2 3.3

PL intensity(a.u.)

Photon Energy (eV) Increasing

pumping density

0.0 0.5 1.0 1.5 2.0 2.5

Intergrated intensity(a.u.)

Excitation density(MW/cm2) Ith=0.96 MW/cm2

(b)

Fig. 4-6 (a) The emission intensity of ZnO nanosaw versus the pumping intensity. (b) The threshold behavior of the emission intensity with increasing excitation density..

In general, the lasing action in ZnO nanostructures has two kind of lasing mechanism. The first type of lasing with the Fabry-Perot (FP) optical cavity arises from the structure of individual nanostructures, such as nanowire and nanoribbon^18,27,52. These single-crystal nanostructure of ZnO is a gain medium by itself, which is provided by a natural optical cavity formed between the two-end facets and natural optical waveguide so that the lasing action occur in nanostructures can be observed clearly. Another lasing mechanism is called “random lasing”, that the emission light is strongly scattered in gain media, and a close-loop path can be formed through multiple scattering. These loop could serve as ring cavity for light^51,53-57. Following the two possible lasing behavior in morphology of ZnO nanosaws (see Fig.

4-1) and optical pumping PL spectra (Fig. 4-6(a)), and canvassing the details further.

The ZnO nanosaws show feature of resonant modes. The longitudinal mode spacing of the lasing modes can be determined by the equation, Δλ=λ²/(2nL), where L is the cavity length of the laser, n is the refractive index of ZnO (n=2.45), and λ is the resonant wavelength ~390 nm (photon energy = 3.18 eV). For a mode spacing between the closest longitudinal modes is about 0.65 nm, the cavity length is expected to be ~50μm. In spite of the ZnO nanosaw with length of about 50μm might exist, the lasing action from FP mode is less likely to occur in ZnO nanosaws. The natural PF cavity formed in ZnO nanostructure needs both good crystalline and small internal loss^52,55. The morphology of ZnO nanosaw is so rough that strong light scattering occurs at the interfaces of nanosaw. Therefore; it is much more probable that the emitted light is scattered and feedback as a resonant cavity among the ZnO nanosaws.

As a result of light scattering by the randomly distributed ZnO nanosaws, we can attribute the lasing behavior as random lasing action among the ZnO nanosaws.

Light is amplified while following a closed random path provided that it satisfies

scattering mean free path, the emitted light is strongly scattered in ZnO nanosaws.

The closed-loop paths can be formed through multiple scattering. These loops act as ring cavities, namely, the ring cavities are “self-formed” by reduplicating scattering.

To verify our laser cavities are “self-formed” by strong light scattering, we measured emission spectra at different excitation areas when the excitation intensity is fixed at 1.07 MW/cm². As shown in Fig. 4-7(a), before the excitation area reaches a threshold value, the scattered light has not yet gathered sufficient in a certain close loop and thus no sharp peak emerges. When the excitation area has exceeded the threshold area (Ath~1.5 x 10^-4 cm²), a closed path can be formed through multiple scattering in larger excitation area and the lasing action starts. The schematic diagram of the formation of the closed-loop path through multiple scattering in disorder ZnO nanosaws is shown in the inset of Fig. 4-7(a). Sharp peaks with linewidth of less than 0.6 nm (5 meV) were observed provided that laser cavity can be formed for suitable excitation area. The dependence of threshold excitation area (Ath) on the excitation density is shown in Fig. 4-7(b). As the excitation density increases, the threshold of excitation area clearly decreases. It indicates that the scattered photon acquires more gain under stronger pump density before it experiences successive scattering, and thus less optical path or smaller excitation area is required for laser action to take place. Therefore, random cavity can be formed more effectively under higher pump density. Figure 4-8 shows the emission spectra at different observation angle θ, which is defined from the sample surface. The black curves (a) and (b) correspond toθ=75° and θ=45° and short dash lines express the auxiliary lines of emission peaks in Fig. 4-8(a). It was quite obvious that different emission spectra can be observed at different angles. This is due to formation of different laser cavities by multiple scattering and thus different gain lengths for different cavity modes. It leads to different resonant photon energy in different

output direction so that different lasing spectra were observed at different angles.

This behavior is similar to the previous studies on lasing action in ZnO particles and nanorod^56,58.

Furthermore, we measured the polarization dependence of the emission when the excitation intensity is tuned about 2.4 Ith. The emission spectra of maximum of polarized emission intensity (Imax) and minimum of polarized emission intensity (Imin) for the peak at 3.17 eV are shown in Fig. 4-9(a) that correspond to the polarized angles at 40˚ and 130˚, respectively. It reveals that the lasing peaks do not always have large degree of polarization, (Imax-Imin)/(Imax+Imin) as the solid curve of Fig.

4.9(b). And the dashed curve expresses the degree of polarization for polarizer angle rotated 40˚, which corresponds to emission spectra of polarized angles at 80˚ and 170˚.

Clearly, the lasing peaks have not only different degrees of polarization but also preferable polarization at different directions. The inset of Fig. 4-9(b) shows the emission intensity (Ilasing as square dots) of sharp lasing peak at photon energy of 3.17 eV and the smooth spontaneous emission (Ispon as triangles) as a function of polarization angle which can be well fet with square of the cosine function.

However, the lasing emission is partially polarized and has the larger degree of polarization (~0.2) than that of spontaneous emission equal to 0.03. Hence, we conjecture polarization of random lasing may be related to the morphology of nanostructure. According to the report by Cao et al.⁵⁴ and Yu et al.,⁵⁵ the polarization of lasing emission is concerned with the orientations of ZnO film and of ZnO nanorod. Furthermore, the spontaneous emission of ZnO nanowire had been found prefer to polarize perpendicular to c-axis⁵² and the degree of polarization of lasing quickly increases with increasing excitation density.⁵⁹ It is reasonable that lasing emission of ZnO nanosaw possesses partial polarization. Because the teeth of

strong light scattering at teeth interface, the scattered light should have preferable polarization, in turn, light, which is amplified while it follows a closed-loop random path, will be partially polarized. However, individual laser cavity has individual polarization so that we observed different degrees of polarization for different lasing peaks shown in Fig. 4-9(b) is mainly due to cavities formed by different closed-loop random paths.

3.0 3.1 3.2 3.3

P L in te ns ity (a .u .)

Photon Energy (eV)

Increasing excitation area

Ath=1.5x10-4 cm2

0.6 0.7 0.8 0.9 1.0 1.1

0 4 8 12 16

A

th

x10

-4

(c m

2

)

Excitation Density (MW/cm

²

)

(a)

(b)

Figure 4-7 (a) the dependence of excitation area on emission spectra of the ZnO nanosaws at Ip = 1.1 MW/cm², the inset is a schematic diagram showing the formation of the closed-loop path for light through multiple scattering in the sample. (b) the threshold excitation area versus excitation density.

3.10 3.15 3.20 3.25 3.30 3.35 (b)

Sample

θ=45

^ο

θ=75

^ο

Emission

P L inte ns ity (a .u .)

Photon Energy(eV) θ

Pump

(a)

Figure 4-8 Emission spectra of observation angles at θ=75° and θ=45° with θ being defined from the sample surface. The excitation power density is 1.65 MW/cm² and the excitation area is 7.1╳10^-4 cm². The inset is a schematic diagram showing the experimental configuration.

3.12 3.14 3.16 3.18 3.20 3.22 3.24 3.26

I max

I min

PL in ten s ity (a.u .)

Photon Energy(eV)

3.12 3.14 3.16 3.18 3.20 3.22 3.24 3.26 0.0

Figure 4-9 (a) the maximum (Imax) and the minimum (Imin) emission spectra of ZnO nanosaws which corresponding to the polarization angles of 40^o and 130^o. (b) The degree of polarization of (a) as the solid curve and the dash curve for that relatively rotated by 40^o. The inset of (b) is the degree of polarization as a function of polarization angle for lasing peak at 3.17 eV (squares) and smooth

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