Optical Energy Gap of Ti:Al2O3 Single Crystals
Hamdan Hadi Kusuma,
Mohamad Khairi Saidin
The optical absorption spectra of single crystals of titanium doped sapphire (Ti:Al2O3) with two different doping concentration (0.1% wt. and 0.25% wt.) have been recorded in the UV-visible spectroscopy at room temperature. It was observed that both crystals start to show strong absorptions around 325 nm. From this spectrum attempt was made to estimate the values of energy gap and the type of transition occurs in Ti:Al2O3. From the analysis it show that the optical energy gap are 5.57 eV for 0.1% wt. Ti and 5.94 eV for 0.25 wt.% Ti and it a direct band gap transition.
Received: August 24, 2010;
Accepted: February 03, 2011;
Published: February 24, 2011
Sapphire (Al2O3) crystal is an important and widely used
material in todays technology. It was used for optical and electro-optical
applications. Sapphire crystal is being an important technological material
as lasing material in solid state lasers, substrate for micro-electronic, radiation
dosimeter and an insulator (Jheeta et al., 2007).
Doping Al2O3 with foreign ions can be used to modify the
optical properties and makes the system useful for large variation application,
such us tunable solid state laser (Moulton, 1986; Blasse
and Verweij, 1990) or optical waveguides (Townsend
et al., 1990; Crunteanu, et al., 2003;
Pollnau and Romanyuk, 2007). The pure and doped single
crystal Al2O3 has been known as an excellent material
for optics, optoelectronics and laser applications (Mikhailik
et al., 2005). Pure Al2O3 is a durable material
with optical transmission spanning the range from UV to IR. This role has been
taken over by Ti:Al2O3 that is now successfully used as
tunable laser material for near infrared spectra region of 0.7-1.1 μm (Moulton,
1986). It also exhibit a broad absorption band, located in the blue-green
region of the visible spectrum that is associated with phonon-coupled excitation
of the 3d electron of the Ti3+ ions (Macalik
et al., 1992).
The optical transition, optical band gap and band structure of crystalline
and non-crystalline materials can be determined using the optical absorption
spectra. Particularly, measurement of the optical absorption coefficient near
the fundamental absorption edge is a standard method for the investigation of
optically induced electronic transition in many materials. Generally, two types
of optical transition i.e. direct and indirect occur at the absorption edge.
Both of these transitions occur when an electromagnetic wave interacts with
a valence electron and raises it across the energy gap to the conduction band.
Previously, we studied the optical absorption of the Ti:Al2O3
crystals (Kusuma et al., 2010).
The purpose of the study is to present the fundamental optical absorption of Ti:Al2O3 single crystal in the visible and UV region. The optical energy gap of the Ti:Al2O3 single crystals were investigated.
MATERIALS AND METHODS
The Ti:Al2O3 crystals used in this study with doped Ti
of 0.1% wt. (sample A) and 0.25 wt.% (sample B) were purchased from RODITI International
Company (England) in 2008. The samples have both large faces polished to optical
quality. The samples were transparent, free of pores, bubbles and grains. The
absorption spectra were measured by using a Perkin Elmer UV-3101 PC UV-VIS spectrophotometer
in the range 200-800 nm waveband at room temperature. The experimental absorption
spectroscopy calibrated apparatus consists of a halogen lamp used as the light
source for the measurement. The light beam is diffracted by a plane diffraction
gratings attached to a step motor. From the results of absorption measurements
which are used to determine an absolute value of the absorption coefficient
of crystals and the optical band gap was calculated by using absorption spectrum.
RESULTS AND DISCUSSION
The optical absorption spectra of Ti:Al2O3 single crystals
with different doped titanium at room temperature are shown in Fig.
1. The spectrum represents the optical absorption of the Ti:Al2O3
single crystal. Both the samples show two absorption peak (492 and 560 nm)
that has been reported (Yamaga et al., 1994) except
for 374 nm (0.25 wt.%) and 389 nm (0.1 wt.%). The spectra exhibit two wide bands
in the range 400-600 nm, associated with the transitions within different d-levels
of the Ti3+ ions (t2g→eg transition) (Wong
et al., 1995; Lupei et al., 1986). The
main absorption is double structured band with overlapping peaks at ~491 nm and
~562 nm, due to transitions from the 2T2 ground state of
Ti3+ to the 2E excited state. The visible band at ~491 nm
is the crystal filed absorption band and corresponds to intra-configurationally
transition t2g-eg of the d1 configuration in
the octahedral field approximation. The blue-green absorption band of Ti:Al2O3
is due to the vibronically broadened 2T2→2E
transition (Macalik et al., 1992; Sanchez
et al., 1988). The weak infrared absorption band with the peak at 650
nm and the strong UV absorption band below 300 nm are observed in sample. For
0.1 %wt. sample, the abrupt increased in absorption started from 325 nm and show
a shoulder at 234 nm before increase again at 216 nm. As for 0.25 wt.% sample,
the sharp increased started at about the same point but show the abrupt increased
of optical absorption spectra started from 325 nm. The sharp increased from 325
nm for both samples can be attributed to fundamental optical absorption edge for
the Ti:Al2O3 single crystals.
||Absorption spectra observed at room temperature for Ti:Al2O3
The absorption spectrum beyond 200 nm cannot be obtained due to limitation
of the instruments. The UV absorption edges for the crystals were observed at
vicinity of 300 nm. The dependent of optical absorption coefficient with the
phonon energy helps to study the band structure and the type of transition of
electrons (Krishnan et al., 2008). The dependence
of absorption on photon energy is analyzed in the high absorption regions to
obtain the detailed information about the energy band gaps (Goksen
et al., 2007). The optical absorption coefficient (α) was calculated
from the transmittance using the following relation:
where, T is the transmittance and d is the thickness of the crystal. The incident
photon energy hv and the optical energy gap Eg are related as in
Eq. 2 (Bang et al., 1996),
where α is the optical absorption coefficient:
In Eq. 2, A is a constant that depends on the transition probability and p is index that characterizes the optical absorption process and it is theoretically equal to 2 or ½ for a indirect or direct allowed transitions band structure, respectively. The dependence of absorption coefficient α for the Ti:Al2O3 single crystals on hv near band edge are shown in Fig. 2 and 3.
In a crystalline or polycrystalline material both direct and indirect optical
transitions are possible depending on the band structure of the material (Tyagi
and Vedeshwar, 2001). Figure 2 shows the calculated room
temperature coefficient α for sample A in the photon energy range 4.77
to 6.20 eV. Figure 3 shows the calculated room temperature
coefficient α for sample B in the photon energy range 5.34 to 6.20 eV.
The analysis of the experimental data for sample A, shows that the absorption
coefficient is proportional to (hv-Eg)p with p = 2 and
½ for range (4.77-5.64) eV and (5.69-6.20) eV, respectively. While, that
the sample B is in the range (5.34-5.69) eV and (5.74-6.20) eV. Insets 1 and
2 for Fig. 2 and 3 display the dependences
(αh v)2 on photon energy hv, respectively. The usual method
of determining band gap is to plot a graph between and
(αh v)2 versus photon energy hv. The linear dependences were
observed for the relations and
(αh v)2 versus hv. Energy gap Eg was calculated by
the extrapolation of the linear part (Krishnan et al.,
2008). By extrapolation to =
0 and (αh v)2 = 0, such plots inset 1 and 2 at Fig.
2 give the values of the optical energy gap Eg for the indirect
and direct band structure to be 4.69 and 5.57 eV, respectively, while plots
at Fig. 3 give the values of the optical energy gap Eg
for the indirect and direct band structure to be 5.38 and 5.94 eV, respectively.
|| The variation of coefficient as a function of photon energy
for the Ti:Al2O3 single crystal with doped Ti: 0.1
wt.% at T = 300 K. Inset 1 and 2 represent the dependences of and
(αh v)2 on photon energy, respectively
||The variation of coefficient as a function of photon energy
for the Ti:Al2O3 single crystal with doped Ti: 0.25
wt.% at T = 300 K. Inset 1 and 2 represent the dependences of and
(αh v)2 on photon energy, respectively
Therefore, it is suggested that the colourless transparent Ti:Al2O3
single crystals have a direct band structure and an optical band gap of 5.57
eV for Ti:Al2O3 with doped Ti: 0.1% and 5.94 eV for Ti:Al2O3
with doped Ti:0.25%. The direct and indirect band gap were found to increase
as the Ti increases and the best for all the experimental points was observed
in the case of (αhv)2 vs. hv plot (Fig. 3).
The analysis of accurate measurements of the optical absorption in Ti:Al2O3 single crystals have shown that this material possesses both direct and indirect band gaps. The phonon assisted indirect transition is indirect allowed. The energies of the phonons have been determined. Further, The single crystals of Ti: Al2O3 have a direct band structure and the optical energy were found 5.57 eV for doped Ti: 0.1% and 5.94 eV for doped Ti: 0.25%.
The authors wish to thank the Ministry of Science, Technology and Innovation for their financial support via Science Fund No. 03-01-06-SF0572. We would also thanks to Universiti Teknologi Malaysia for the support on this project.
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