Due to the difficulty to obtain high quality ZnCdO, many fundamental optical properties of ZnCdO are still not fully understood. In this thesis,the optical characteristics of the ZnCdO thin film grown by plasma-assisted MBE were investigated. The temperature-dependent PL and time-resolved PL (TRPL) were used to study the thermal-activated carrier transfer dynamics of ZnCdO.
5
Fig. 1-3 (a) Structure of ZnO
Fig. 1-3 (b) Structure of CdO
6
Fig. 1-2 Optical and structural properties of CdyZn1-yO and MgxZn1-xO alloy films mapped out in a plane of a-axis length and room-temperature band gap energy (Ref. 7)
7
Fig. 1-3 Obtained films, either type A, B or C, against partial pressures of Zn and Cd. (Ref. 8)
8
Fig. 1-4 Integrated CL spectra at T=4 K. The Cd concentrations and peak positions are summarized in the inset. (Ref. 9)
9
Chapter 2 Experiments
In this chapter, the experimental techniques used in this thesis are described, including molecular beam epitaxy, photoluminescence, transmittance, and time-resolved photoluminescence.
2.1 Molecular beam epitaxy (MBE) system
The SVT Associates molecular beam expitaxy (MBE) system is shown in Figure 2-1. It consists of a vertical growth chamber with ten sources, a load-lock chamber, and analytical equipment.
Currently, seven solid sources Zinc (Zn), Manganese (Mn), Cadmium (Cd), Magnesium (Mg), Chromium (Cr), Selenium (Se), Tellurium (Te), and Zinc Chloride (ZnCl2), and two gas sources, Oxygen (O2) and Nitrogen (N2), are operated each source has its own shutter to control the growth time. There is a main-shutter between sources and the substrate to avoid the unintentional evaporation before growth.
The load-lock chamber used to transfer substrate from air to the growth chamber and was maintained at 5×10-8 torr by using a dry rotary pump and a turbo molecular pump. The growth chamber was further pumped down to a base
10
pressure of 8×10-10 torr. The reflection high-energy electron diffraction (RHEED) system is also set up in the growth chamber. It is an invaluable tool to determine different aspects of the deposition layer. Morphological data of the surface may be interpreted from the spot and line pattern, which appear on the phosphor screen display during growth.
ZnO and ZnCdO thin films were grown on c-plane sapphire substrates. The group-II atoms were produced in conventional effusion cells by evaporating elemental Zn (6N), and Cd (6N). The group-VI atoms were supplied by into the growth chamber it was desorbed at 850 oC and treated in oxygen plasma, which is expected to produce an oxygen terminated Al2O3 surface. In order to reduce the lattice mismatch between ZnCdO and Al2O3, we grow a 70 nm-thick ZnO at 650 oC as the buffer layer. The structure of our samples was shown in Figure 2-2 (a), (b). ZnO and ZnCdO epilayers with thickness of 480 nm were then grown at 450 oC. After growth, the optical characterizations of the samples
11
were analyzed by photoluminescence, transmittance and time-resolved photoluminescence at various temperatures.
2.2 Photoluminescence (PL) system
To study optical properties of semiconductors, laser beams with photon energy higher than the band gap energy of the semiconductor are usually used to excite electrons from the valence band to the conduction band and leave holes in the valence band. The excited (electrons/holes) relaxed to (bottom of the conduction band/top of the valence band) through the carrier-phonon interaction.
Electrons and holes form exciton by Coulomb interaction. Electrons and holes, Excitons, recombination could emit photons and be detected by photoluminescence (PL) spectroscopy.
The experimental set-up for the photoluminescence measurement is shown in Figure 2-3. PL measurements were performed by using the 325 nm line of a He-Cd laser. The incident beam was focused by a lens (f=10 cm). The PL emission from sample was collected by second lens (f=10 cm) and focused by the third lens (f=30 cm) to the spectrometer. The signal was dispersed by an Ihr550 spectrometer and detected by liquid nitrogen cooled charge-coupled device (CCD). The spectrometer was controlled by a computer, which was used
12
to store and plot the collected data. For temperature-dependent PL measurements, samples were cooled in a closed-cycle refrigerator system. The temperature was varied from 10 K to 300 K.
2.3 Transmittance system
The experimental set-up for transmittance measurement is shown in Figure 2-4. A broadband xenon lamp was used as an excitation source. The incident beam was focused on the sample by a lens (f=10 cm). The transmittance beam from the sample was collected to the entrance slit by lens L2 (f=10 cm) and L3 (f=30 cm). The transmittance spectra were analyzed by the Ihr550 spectrometer and detected by liquid nitrogen cooled charge-coupled device (CCD).
2.4 Time-resolved photoluminescence (TRPL) system
TRPL system was used to study the decay dynamics of excitons. The experimental setup of TRPL system is similar to the PL system and shown in Figure 2-5. The GaN diode laser with 50 ps pulses and a repetition rate of 40 MHz at a wavelength of 377 nm was used as an excitation source. The peak power of the pulse was estimated to be below 0.1 mW. The laser beam was focused on the sample by a lens (f=10 cm). The combination lenses guide the
13
signal to the iHR550 spectrometer, which was equipped with a high-speed photomultiplier tube to detect the signal. The signal was further analyzed by a computer. The overall temporal resolution of the setup was about 300 ps.
14
Fig. 2-1 SVT Associates molecular beam expitaxy (MBE) system Growth chamber Introduction
chamber chamber
cells RHEED
15
(a)
(b)
Fig. 2-2 (a) Schematic structure of ZnO thin film, (b) Schematic structure of ZnCdO thin film
Sapphire ZnO ZnCdO Sapphire
ZnO ZnO
16
Fig.2-3 Schematic diagram of the photoluminescence (PL) spectra setup
17
Fig.2-4 Schematic diagram of the transmittance spectra setup
18
Fig.2-5 Schematic diagram of the TRPL system setup
19
Chapter 3 Results and discussion
3.1 Low –temperature photoluminescence (PL) and transmittance
The PL and transmittance spectra of ZnO and ZnCdO at 10 K are shown in Figure 3-1(a), (b), respectively. Sharp near-band-edge emissions of ZnO at about 3.361 eV were observed, as shown in Figure 3-1(a). The emission peaks at 3.361 and 3.366 eV were assigned to the excitons bound to neutral donors10. Comparing with the absorption edge of the transmittance spectrum, the peak at 3.377 eV is attributed to the free A excitons (FXA)11. When Cd atoms were introduced into ZnO, the PL peak becomes broad and the energy position shifts to 3.185 eV. Fine structures of bound excitons could not be resolved. The PL peak position of ZnCdO can be written as
EPL = (3.35-9.19x+8.14x2) eV, (1)
according to the results of Gruber et al.14 The Cd composition of our sample is 2
%. Additionally, the PL emission profile of ZnCdO is not symmetry due to the existence of localized state at lower energy. The FXA state of ZnCdO was 3.265 eV, which was determined from the absorption edge of the transmittance spectra.
3.2 Temperature-dependent PL and transmittance
In order to further understand optical properties of ZnCdO epilayer, the
20
temperature dependent PL spectra were studied. In comparison, the temperature dependent PL spectra of ZnO were also carried out, as shown in Figure 3-2(a).
Both DX and FXA in ZnO shift to lower energy and the line-width broadens with the increasing temperature. The broadening of PL band is related to the carrier-phonon interaction. Additionally, because carriers are thermally activated, FXA dominates at higher temperature. In case of ZnCdO epilayer, as shown in Figure 3-2(b), only one peak, P1, was observed at low temperature. The line shape of P1 is highly asymmetric. The lower energy shoulder consists of another emission band, which dominates the P1 emission band at higher temperature.
When T >140 K, additional peaks, P2 and P3, become visible. The intensity of P2 increases and exceeds P1 when T > 240 K. The similar optical properties were observed by Yang et al.15 in ZnSe1-xTex (x=0.01) epilayers, as shown in Figure 3-3. In the case of ZnSe1-xTex at a low temperature of 10 K, the X/Te is not observed and the PL of the Ten-bound excitons (X/Ten) is very pronounced.
The energy states of X/Te and X/Ten cluster becomes observable as the temperature was increased. Thus, the PL peaks of P1, P2, and P3 can be attributed to the emissions from X/Cd, X/Cdn, and X/Cdn cluster, respectively. In order to understand the properties of X/Cd and X/Cdn in ZnCdO, the temperature-dependent transmittance spectra of the ZnCdO were investigated as
21
shown in Figure 3-4. The absorption edge shifts monotonically to lower energy as the temperature increases. The peak positions of the FXA, X/Cd, and X/Cdn
emission as a function of temperature were plotted in Figure 3-5(b).
For ZnO, as shown in Figure 3-5(a), the energy positions of FXA and DX monotonically decrease as the temperature increases. The red-shift of these peaks with increasing temperature obeys the Vashinis' formula16, which is writing as
Eg(T) = Eg(0)-αT2/(β+T), (2)
where Eg(0) is the band-gap energy at T=0 K, and α and β are the corresponding thermal coefficients. On the other hand, for ZnCdO, as shown in Figure 3-5(b), only the trace of FXA and X/Cd obey the Vashinis' prediction. The PL spectra of X/Cdn and X/Cd emissions are more complicated, because the X/Cdn and X/Cd emissions are attributed to the localized states emissions due to high fluctuation in Cd composition.
22
3.3 Power dependent PL study of ZnCdO
Figure 3-6 shows the power dependent PL spectra at 220K for ZnCdO. The intensity of PL emission is enhanced with increasing excitation power. Under an excitation power of 65 W/cm2, the emission energy of X/Cdn cluster and X/Cd are at 2.782 and 3.160 eV, respectively. As the excitation power is increased, the X/Cdn cluster exhibits energy blue-shift and the X/Cdn state dominates the spectrum. This result implies the density of state for X/Cdn is much higher than the X/Cdn cluster. The increasing excitation density saturates the lower energy states. As a result, X/Cdn state dominates the emission.
23
3.4 Time resolve photoluminescence (TRPL) spectra of ZnCdO
In order to further study the optical properties of localized states, we performed TRPL measurements at 10 K and 100 K, as shown in Figure 3-7, to demonstrate the origin of the X/Cdn radiative recombination. The carrier recombination time is found to decrease with the increasing temperature. At 10K, although the recombination times of the lower energy states are faster, the dominant emission intensity is from X/Cdn due to the higher density of states. As temperature is raised to 100K, whole emission bands shift to lower energy due to the decreasing energy band and the thermal activation of higher energy states to lower energy states. Therefore, the recombination time of lower energy states become longer.
24
3.25 3.30 3.35 3.40 3.45 FXA
Photon energy (eV)
P L i n te n si ty ( ar b . u n it .)
DX
h
2.8 3.0 3.2 3.4
Photon energy (eV)
P L i n te n si ty ( ar b . u n it .)
h
FXA
(a) (b)
Fig. 3-1 (a) PL and transmittance spectrum of the ZnO thin film at 10K (b) PL and transmittance spectrum of the ZnCdO thin film at 10K
25
Fig. 3-2 (a) PL spectra of the ZnO alloy at various temperatures.
(b) PL spectra of the ZnCdO alloy at various temperatures.
26
Fig. 3-3 Temperature-dependent photoluminescence spectra from ZnSe1-xTex
(x=0.01) epilayer. (ref 15)
27
2.8 3.0 3.2 3.4
h
Photon energy (eV)
300K 10K
Fig. 3-4 Temperature dependent transmittance spectra of ZnCdO.
28
Fig. 3-5. The traces of X/Cd, X/Cdn, FXA and ZnO peak at various temperatures.
The solid line is a Varshni’s fit.
29
Fig. 3-6. Power dependent spectra of ZnCdO
30
2.8 2.9 3.0 3.1 3.2
0 1 2 3
Photon energy (eV)
T im e (n s)
10 K
2.8 2.9 3.0 3.1 3.2
0 1 2
Photon energy (eV)
T im e (n s)
100 K
(a)
(b)
Fig. 3-7 Temporal evolution of the PL spectra at (a) 10 K, (b) 100 K.
31
Chapter 4 Conclusion
Zn0.98Cd0.02O epilayer was grown grown by plasma-assisted molecular beam epitaxy. The PL measurements show that Zn0.98Cd0.02O exhibits strong emissions from localized states. The carriers transferred from shallow localized states to the deeper localized states were evidenced by temperature dependent PL measurements. The TRPL measurements show that the recombination times of the localized states are about a few nano-seconds. The thermal-activated carrier transfer processes in ZnCdO result in the increasing recombination time for the lower energy localized states.
32
References
1. S. Nakamura, S. Pearton, and G. Fasol, The Blue Laser Diode, 2nd ed.
(Springer, New York, 1997 )
2. D. J. Tomas, J. Phys. Chem. Solids 15, 86 (1960)
3. A. Tsukazaki, A. Ohtomo, T. Onuma, M. Ohtani, T. Makino, M. Sumiya, K.
Ohtani, S. F. Chichibu, S. Fuke, Y. Segawa, H. Ohtani, H. Koinuma, M.
Kawasaki, Nature materials 4, 42 (2005)
4. J. Ishihara, A. Nakamura, S. Shigemori, T. Aoki, and J. Temmyo, Appl. Phys.
Lett. 89, 091914 (2006).
5. Z. K. Tang, G. K. L. Wong, P. Yu, M. Kawasaki, A. Ohtomo, H. Koinuma, and Y. Segawa, Appl. Phys. Lett. 72, 3270 (1998)
6. C. Sravani, K. T. R. Reddy and P. J. Reddy, Matter. Lett. 15, 356 (1993) 7. T. Makino, Y. Segawa, M. Kawasaki, A. Ohtomo, R. Shiroki, K. Tamura, T.
Yasuda, H. Koinuma, Appl. Phys. Lett. 78, 9 (2001)
8. K. Sakurai, T. Kubo, D. Kajita, T. Tanabe, H. Takasu, S. Fujita, S. Fujita, Jpn.
J. Appl. Phys. 39, L1146-L1148 (2000)
9. F. Bertam, S. Giemsch, D. Forster, J. Christen, R. Kling, C. Kirchner, A. Waag, Appl. Phys. Lett. 88, 061915 (2006)
10. Teke, A., Ozgur, U., Dogan, S., Gu, X., Morkoc, H., Nemeth, B., Nause, J.
33
and Everitt, H. O., Physical Review B: Condensed Matter, 70, 195207 (2004) 11. Muth, J.F., Kolbas, R.M., Sharma, A.K., Oktyabrsky, S. and Narayan, J.
Journal of Applied Physics, 85, 7884 (1999)
12. K. P. O’Donnell, R. W. Martin. P. G. Millleton, Phys. Rev. Lett. 82, 237.
(1999)
13. Hong Seong Kang, Sung Hoon Lim, Jae Won Kim, Hyun Woo Chang, Gun Hee Kim, Jong-Hoon Kim, Sang Yeol Lee, Y. Li, Jang-Sik Lee,
J.K. Lee, M.A. Nastasi, S.A. Crooker, Q.X. Jia, J. Cryst. Growth, 70-73, 287 (2002).
14. Th. Gruber, C. Kirchner, R. Kling, F. Reuss, A. Waag, F. Bertram, D. Forster, J. Christen, and M. Schreck, Appl. Phys. Lett. 83, 3290 (2003).
15. C. S. Yang, D. Y. Hong, C. Y. Lin, W. C. Chou, C. S. Ro, W. Y. Uen, W. H.
Lan, and S. L. Tu, Journal of Applied Physics, 83, 5 (1998) 16. Y. P. Varshni, Physica 34, 149 (1967)