J. Phys. Chem. 1995,99, 15247-15252
15247
Optical and Electronic Transport Properties of Electrodeposited Thallium(II1) Oxide Films Robert A. Van Leeuwen, Chen-Jen Hung, Daniel R. Kammler, and Jay A. Switzer* University of Missouri-Rolla, Department of Chemistry and Graduate Center for Materials Research, Rolla, Missouri 65401 Received: June 2, 1995; In Final Form: August 14, 1 9 9 9
Thallium(1II) oxide is a degenerate n-type semiconductor which can be electrodeposited from aqueous solution at room temperature. Thin films were characterized by transmission and specular reflectance spectroscopy and by four-point resistivity and Hall measurements. Optical parameters were determined by fitting the observed specular reflectance to the Drude equation. Due to the high free carrier concentration, the material reflects strongly in the near-infrared, and the band-to-band optical transitions are shifted by up to 1.1 eV by the Moss-Burstein effect. The optical and electrical properties of the films were a function of the deposition overpotential. Films grown at 44 mV had an intrinsic bandgap of 0.66 eV, resistivity of 2 . 8 ~ ohm-cm, mobility of 27 cm2Ns, and conduction band effective mass of 0.43m. Films grown at 300 mV had an intrinsic band gap of 0.51 eV, resistivity of 7.8 x ohm cm, mobility of 93 c m 2 N s, and conduction band effective mass of 0.29m0. Mobilities measured by contact and optical methods are similar, which shows the optical technique may be used for conditions in which contact methods might fail.
Introduction
Thallium(III) oxide is a highly conducting oxide that has been characterized previously in thin film’ and single crystal2 form to be a degenerate n-type semiconductor. The high conductivity and transparency have been utilized in a high-efficiency solar cell that was fabricated by photoelectrochemically depositing a thin layer of thallium(1II) onto n-type ~ i l i c o n .Due ~ to the high carrier concentration, the oxide has a strong reflectance in the 1300-1500 nm region of the near-infrared that may be important in devices for optical communications, since optical absorption losses are lowest in this wavelength range for optical fibers. An additional incentive for studying the optical and electrical properties of thallium(II1) oxide is that thallium is a component of many high Tc superconductors4 and has been described as a “charge reservoir“ layer in these superconductors? We are particularly interested in the optical and electrical properties of thallium(II1) oxide since we have shown that it is possible to electrodeposit both compositional superlattices based on T1-Pb-06-9 and “defect chemistry” superlattices of thallium(III) oxide.’O A schematic of a defect chemistry superlattice is shown in Figure 1. The nanometer-scale defect chemistry superlattices are grown in a beaker by pulsing the applied potential during deposition. Our previous results suggested that the defect chemistry of the oxide is dependent on the applied overpotential (i.e., applied potential minus equilibrium potential). Oxygen vacancies are favored at high overpotentials and cation interstitials are favored at low overpotentials. These layers are shown in Figure 1 as Tl2O3-, and TlxT1203. The transition from one defect chemistry to another in this nonequilibrium process occurred in the same overpotential range (100-120 mV) in which the rate of the back electron transfer rate became significant.’O Since the layers of the superlattice were as thin as 6.7 nm and difficult to characterize, in the present work we characterize the electrical and optical properties of bulk (Le., 1 p m or thicker) films of the oxide as a function of deposition overpotential. It is important to understand the band structure of these films grown at various overpotentials before we can understand the optical and electrical properties of superlattices
* To whom correspondence @
should be addressed. Abstract published in Advance ACS Abstracts, October 1, 1995.
0022-3654/95/2099- 15247$09.00/0
Figure 1. Schematic of a “defect chemistry” thallium(II1) oxide and superlattice. The alternating layers shown in the figure, Tl,TlzO,, are the oxygen-deficient and the excess thallium layers. The superlattice modulation wavelength, A, is the bilayer thickness.
with thin altemating layers of materials grown at these overpotentials. We also compare the properties of the electrodeposited films to single crystal thallium(1II) oxide and to films deposited by reactive sputtering. Experimental Section
The thallium(1II) oxide films were deposited with overpotentials ranging from 44 to 535 mV from an aqueous solution of 100 mM TWO3 in 1M NaOH (caution: thallium compounds are extremely toxic). A thallium(III) oxide-coated platinum wire with a potential of -204 mV us the saturated calomel electrode served as the reference electrode. The substrates used were tindoped indium oxide (ITO)coated (thickness = 30 nm) glass rotated at 500 rpm in the solution during deposition. The substrates were washed with deionized water prior to deposition. Deposition potentials were controlled with an EG&G Princeton Applied Research (PAR) Model 273A potentiostatlgalvanostat. A constant temperature was maintained at 25 “C during deposition. The films adhered to the substrates very well with no peeling and no apparent pinholes. X-ray diffraction measurements on the films showed that they were polycrystalline but highly textured in the [loo] direction. 0 1995 American Chemical Society
15248 J. Phys. Chem., Vol. 99, No. 41, 1995
Van Leeuwen et al.
E Reflectance TIanrmittance
0.8 indirect sition
transition
Wavelength (nm)
Figure 2. Reflectance and transmittance spectra for a 0.5 pm TlzO3 film deposited at an overpotential of 300 mV as a function of wavelength. Reflectance is plotted as the solid line while transmittance is shown as a dashed line. Oscillations at low wavelengths are interference fringes while the “shoulder” at 1500 nm for the reflectance is a surface plasmon.
Thallium(1II) oxide has the bixbyite crystal structure (space group Zu3) with a lattice parameter which is a function of the deposition overpotential.’o The lattice parameter of a film grown at 44 mV is 10.515 A, while the lattice parameter of a film grown at 530 mV is 10.555 A. Optical transmittance and absolute specular reflectance were measured at wavelengths from I = 300 nm to A = 2800 nm using a Caq 5 spectrophotometer. We used an ITO-coated substrate in the reference beam for transmittance measurements to compensate for the effect of the I T 0 and substrate on the film spectra. Hall measurements were performed on the films by the standard ASTM technique.” The films were masked off and etched with ascorbic acid to the standard shape for Hall measurements as specified by ASTM. The etched films were then cut from the glass substrate with a low-speed diamond saw. Pressure contacts to the films were augmented with thin strips of silver print along the current contacts to ensure uniform current flow along the length of the sample and small dots of silver print on the “ear” pins for good contact. Resistance of the films as a function of temperature was measured with a standard four-point technique. Gold contacts were sputtered onto the films to ensure ohmic behavior. Indiumtipped, spring-loaded pins were pressed onto the gold contacts for connections during cooling. The current was applied across two pins, and the resultant voltage across the other pins was measured several times. The current was then reversed, and the subsequent measurements were averaged with the first. This was repeated for temperatures ranging from 300 to 15 K. Results and Discussion
Figure 2 shows the reflectance and transmittance spectra for a 0.5 pm TI203 film deposited at 300 mV. As can be seen from the figure, the reflectance in the visible range is low but increases rapidly in the near-infrared (NIR) due to the plasma edge of the free electrons. The transmittance for these films is high only in the visible and NIR regions. The NIR cutoff is due to the plasma edge, and the cutoff in the visible is from the absorption band edge. The absorption coefficient, a, was determined fromI2
where t is the thickness and R and T are the reflectance and
k Figure 3. Schematic diagram of the band structure for a degenerate semiconductor. The arrows show two possible optical transitions from the valence band to the conduction band. The transition energy will be displaced upwards by an amount AE, the Moss-Burstein shift, above the bottom of the conduction band due to the high carrier concentration.
transmittance of the film. Knowledge of the wavelengthdependent absorption coefficient allows for the determination of the optical band gap of our material. Electronic transitions take place from the valence band to the conduction band when the energy of the photon exceeds the band gap energy. These transitions can occur as either a direct or an indirect p r o ~ e s s . ’ ~ A direct transition is one in which only a photon is involved to excite an electron to the conduction band. An indirect transition is a process which involves a photon and also a phonon that is either emitted or absorbed in the excitation event. A schematic of these two processes in a band diagram is shown in Figure 3 for an n-type degenerate semiconductor. The lowest energy transition can be indirect for a degenerate semiconductor even when the conduction band minimum and valence band maximum are located at k = 0. The energy of this transition is dependent on the carrier density, since the optical transition is shifted by the energy difference between the Fermi level and the bottom of the conduction band (Moss-Burstein effect). Direct transitions originate from deeper states in the valence band. Quantum transition probabilities for both these processes are shown in the following equationsI3
where E, is the photon energy, Egd is the direct gap energy, Egi is the indirect gap energy, and Ep is the phonon energy. Therefore, if the result of plotting (UJ~,)~us E, or all2us E, is linear, then we can determine both the type of transition that occurs for TI203 and the energy gap of that transition from the x-intercept of the fit to the linear region of the plot. Figure 4 shows a plot of us E, for a 300 mV film where a has been determined from eq 1. From eq 2, the energy intercept of 2.57 eV from the linear region is the optical direct band gap. Figure 5 shows plotted as a function of E,. This plot displays linear behavior at a different energy region, which shows that in addition to the direct transition, there is also an indirect optical transition. From eq 3, the energy-intercept of 1.65 eV is either the sum or the difference between the indirect band gap and the phonon energies. Normally, the phonon energy is on the order of 0.01 eV, so the intercept is a close
Properties of Thallium(II1) Oxide
J. Phys. Chem., Vol. 99, No. 41, 1995 15249
t
1
t
i
0.0 O
Figure 4. Plot of (~IY,)~ as a function of photon energy, E,, to determine the direct transition from the valence band to conduction band. The energy-intercept of the fit to the linear region gives a direct optical band gap of 2.57 eV.
4001
’
500
E, (ev)
A
/
*
U
1000 1500 2000 Wavelength (nm)
2500
3000
Figure 6. Reflectance plotted us wavelength for a thick 44 mV film. The solid line is the reflectance for the as-deposited film. The dashed and dotted lines are the reflectance of the film after polishing with 0.05 pm alumina powder for 5 and 10 min, respectively. The increase in reflectance after polishing is due to a decrease in surface roughness. The polished film was fit to the Drude equation. c1 = n
2
- k2 c2 = -2nk
(6)
and is used to fit the reflectance of the film using the Fresnel equation,
R=
E, (eV)
Figure 5. Absorption coefficient, a, as determined from the transmittance and reflectance spectra, is shown here plotted as all2us photon energy, E,, for a 300 mV film. The region between 2 and 2.7 eV is linear and indicates an indirect transition. The energy-intercept of the fit to this region yields an indirect gap near 1.65 eV since the phonon energy is small.
approximation to the actual gap. These compare well to band gap measurements of Tl2O3 determined for reactively sputtered films.’ Optical fitting to thick Tl2O3 deposited on IT0 required that we fiist polish the films because of the surface roughness of the films. Figure 6 shows the reflectance of a 44 mV film that has been polished for 5 min periods with a 0.05 pm A1203 slurry. We fit the reflectance spectra of the thick films of Tl2O3 to the Drude equation
where €0 is the zero wavelength (high-frequency) dielectric constant, q is the electron charge, mc is the effective mass of the carriers in the conduction band, N is the carrier concentration, and z is the average scattering time of the canier. The spectra can be fit to three parameters in the Drude equation, EO, AN, and r, using a least squares algorithm.l4 The complex index of refraction is related to the real, 6 1 , and imaginary, €2, parts of the Drude dielectric function by
(7)
Figure 7 shows the fit and experimental reflectance spectra for a 300 mV film. From the three parameters of this fit, the compfex dielectric function and index of refraction can be determined. For this film, the parameters were EO = 6.21, AN = 610 nm, and r = 0.020, while for a sample deposited at 44 mV, the parameters were €0 = 5.53, AN = 595 nm, and r = 0.082. From the values of I’,we can determine the mean time between collisions for the electrons from eq 5 . For the lowoverpotential film it is 3.9 x s while for the high-potential film, z = 1.6 x s. The shorter time between collisions for the low-overpotential film compared to the high-overpotential film is consistent with an increased density of interstitial atoms at low overpotentials. The complex index of refraction is shown in Figure 8 for two different overpotentials: 300 and 44 mV. The wavelength where n and k intersect is the plasma wavelength, A,. This wavelength, along with the surface plasmon wavelength, As, and the wavelength of minimum reflectance, A,, can be determined from the fit parameters byI5 Ap=AN&
where
+ + +
(n - 1)* k2 (n 112 k2
A,=AN&-=i
As=AN&TT
(8)
The plasma wavelengths are 1520 and 1400 nm for the two different overpotentials. The value of Ap is larger for the 300 mV film than for the 44 mV film by 120 nm due to the larger carrier density of the 44 mV film. Note that the imaginary part of the index of refraction, k, is very large in the infrared region and drops rapidly down toward zero near the visible region. In contrast, n is small and almost a constant at large wavelengths but increases rapidly at the plasma wavelength. Values of n for the two different overpotentials differ by up to a factor of 4 in the NIR region. The Hall effect is measured with a modified ASTM technique where the Hall voltage, VH,is measured as a function of applied field. The Hall coefficient, RH,is then determined from the following equation
Van Leeuwen et al.
15250 J. Phys. Chem., Vol. 99, No. 41, 1995
0.8
- -
8
0.6
-
8
0.4
-
2
fit
0
exp
1
500
I000
1500
2000
2500
3000 Applied Field (Oe)
Figure 9. Hall voltage for a 1 pm film deposited at a 300 mV overpotential as a function of applied field. The current used was 5 mA. Since the slope is negative, the majority carriers are electrons. From the slope of the line, the canier density was determined to be n = 8.7 x 1020cm-3.
- n --k 3.0 2.5 -
#Y
3.5
//
r
4 E
2.0
N //
TABLE 1: Room Temperature Electronic Properties for Electrodeposited Films of T1203 Grown at a Range of Dewsition Overpotential@ resistivity mobility overpotential carrier density @&2cm) (cmW s) ") ( x 1020cm-j)
-
535
. I
J
400 300 220 190 117 90 44
-
1.0 -
1.5
0.5
-
n nV."
a
500
1000
1500
2000
2500
3000
Wavelength (nm)
Figure 8. Plot of the real and imaginary parts of the index of refraction, n and k, us wavelength for films grown at two different overpotentials: 44 and 300 mV. The solid lines represent the real part, n, while the dashed lines are the imaginary part, k.
(9) where t is the thickness of the film, B is the applied magnetic field, and I is the current applied through the film. The measured Hall voltage has contributions from both the Tl2O3 and the underlying I T 0 film. The fraction of the signal due to the Tl203 can be deconvolved from the measured value usingI6
where y = (dlp2 /&PI) while p1, p2 and d ~d2, are the layer resistivities and thicknesses of the IT0 and Tl203, respectively. After the determination of the Hall coefficient of the T l 2 0 3 , the carrier concentration and mobility can be determined from
where p~ is the Hall mobility and r is a scale factor close to 1 for high magnetic fields or high carrier concentrations. Typical values of y2 are about so the last term in eq 10 can be neglected and the error in the measured Hall voltage from the I T 0 layer is less than 4%.
8.6 8.3 8.7 9.5 9.8 9.5 10.7
11.0
86 80 78 75 14 14 130 285
100 94 93 88 86 89 45
27
All films were grown to a Faradaic thickness of 1 pm.
Figure 9 shows the measured Hall voltage as a function of applied field for a 1 p m film deposited at 300 mV. From the slope of this plot, we determined the carrier concentration to be 8.7 x lozo~ m - ~The . negative slope of the line indicates that the majority carriers are electrons. In conjunction with fourpoint resistivity measurements, we can determine the Hall electron mobility, p~ = RH/^. Values for the mobility vary from less than 30 c m 2 N s for the low overpotential to 100 cm2N s for the high-potential films. These values are tabulated in Table 1 along with carrier densities and resistivities for 1 pm films deposited with a range of overpotentials from 44 to 535 mV. The films deposited at high overpotentials are electrically very similar to the as-grown single crystals analyzed by Wirtz.I7 Those single crystals had a resistivity of 75 pQ-cm and a carrier concentration of (7-8) x lozo ~ m - ~The . high carrier concentration and low resistivity have been attributed to the defect chemistry of thallium oxide. The donors are either oxygen deficiencies or ion interstitials. It has been shown that there is up to 5% excess thallium in the films electrodeposited at overpotentials of less than 120 mV.'O This suggests that the decrease in mobility and increase in resistivity of the 44 mV overpotential film compared to the 300 mV film is due to the interstitial thallium ions. Using the optical measurement of the plasma wavelength and the carrier density from the Hall measurements, we are able to determine the effective electron mass, m,, from eq 5. For our 300 mV sample we determined the mass to be 0.29m0 and 0.43m0 for the 44 mV film, where m, is the electron rest mass. Widely differing values for the effective electron mass have been previously determined optically by Geserich and thermodynamically by Wirtz to be 0.32 and 0.82, respectively. Our measurements agree with the work by Geserich. The effective
J. Phys. Chem., Vol. 99, No. 41, 1995 15251
Properties of Thallium(II1) Oxide
TABLE 2: Optical and Band Structure Properties for Electrodeposited T1203 Films Deposited at Two Different Overpotentials: 44 and 300 mVa overpotential (mV) i,(nm) N x 10Z0(~m-3)t x (s) mJm, E,i(eV) EBd (eV) Ef(eV) Eg(eV) mdm, pH(cm*Ns) po,,(cm2Ns) 300 44 a
1520 1400
8.7 13.5
16.2 3.85
0.29 0.43
1.65 1.70
2.57 2.75
1.15 1.04
0.50 0.66
0.37 0.44
98 11
98 16
The mobilities measured by Hall and optical methods are p~ and pOpt, respectively.
mass for conduction electrons of other conducting oxides such as Zn0I2 (m, * 0.32) and IT018 (m, * 0.35) is also very similar to the effective mass we determine for Tl2O3. The Fermi level is located in the conduction band due to the very high carrier density, so the optically measured band gap can be much larger than the actual band gap, which is only from the top of the valence band to the bottom of the conduction band. This is a manifestation of the Pauli exclusion principle. From the effective mass and carrier density, we can determine the Fermi energy level within the conduction band, assuming the band has a parabolic shape from19
1.00-
.-b ’J
.E 0.95-
8
LL
3= 0.90E
z0
0.85-
10 Temperature (K)
where N is the carrier density, k is Boltzmann’s constant, h is Planck’s constant, T i s the temperature in Kelvin, and Efis the Fermi energy as defined from the bottom of the conduction band. We can then calculate the actual band gap from the difference between the indirect optical band gap and the Fermi energy. From the results of this calculation, the reduced mass, l/m, = llm, UmC,and therefore the valence band mass, m,, can be determined from the direct optical band gap and the MossBurstein shift,2o AE, which is the energy difference between the optically measured gap and actual band gap.
+
The values for these calculations are shown in Table 2 for the two thick films deposited at 44 and 300 mV. The effective mass of the film deposited at low overpotentials is almost 50% larger than the high-overpotential film. Actual band gaps determined for the two overpotentials differ by about 0.15 eV with the film having the higher carrier density being larger. Usually the band gap of a highly doped material decreases as the carrier concentration increases.’* The opposite effect in our films can be attributed to a change in the lattice parameter as determined by X-ray analysis and a possible shifting in the atomic positions caused by the increased number of interstitials affecting the band structure of the low-overpotential film. Resistance vs temperature of a 300 mV film is shown in Figure 10. Again, there is an effect of the current flowing through the IT0 underlayer in addition to the film itself. Simple calculations assuming uniform current flow through two films in parallel show that the effect is very small. For a typical case of a 1 p m film with a resistivity of 100 pS2 cm, the error in the measurement of the resistance is less than 3%. Unlike most typical semiconductors with low canier densities, the resistivity of Tl2O3 decreases with decreasing temperature, similar to a metal; however, it is not linear. At low temperatures, the resistivity flattens out due to impurity scattering effects. It is also possible to calculate the carrier mobility from the optical parameter, z, and the effective mass from the following equation12
Figure 10. Normalized resistivity us temperature for a 44 mV film that is 1pm thick. The resistivity decreases with decreasing temperature but does not have the same linear dependence as a metal. At a temperature less than 50 K, the resistivity is independent of temperature due to impurity scattering effects. Popt
=
(kb
Values for the mobility using both contact and optical measurements are included in Table 2. The optical mobilities of 98 and 16 agree well with the mobilities determined from resistivity measurements, 98 and 1 1 cm2 N s for the 300 and 44 mV films, respectively. Figure 11 is a plot of the reflectivity calculated from the Drude equation for varying values of the mobility. Other parameters used to calculate the spectra were kept constant at N = 3.4 x 1020~ m - m, ~ ,= 0.3m0,EO = 4,and ,Ip = 2000 nm. As can be seen from the figure, the “steepness” of the plasma edge is very sensitive to the electron mobility. For systems deposited on conducting substrates or materials in which grain boundaries prevent reliable measurements, optical determination of the mobility would be a good method for determining the carrier mobilities of films where contact measurements might otherwise fail.
Conclusions Thallium@) oxide is a highly conducting metal oxide which can be electrodeposited from aqueous solution at room tem-
perature. Because the material is an n-type degenerate semiconductor with a large concentration of free carriers, it exhibits a sharp plasma resonance in the near-infrared and is strongly reflective at wavelengths exceeding the plasma wavelength. Since the plasma wavelengths in this study were in the 14001500 nm range of the near-infrared, these materials may have utility in devices for optical communications. Losses in optical fibers are lowest in this wavelength range. We showed previously that thallium(II1) oxide films grown at low overpotentials had cation interstitials, and films grown at higher overpotential had oxygen vacancies. The lower mobility of the films grown at low overpotentials, measured by both contact and optical methods, is consistent with scattering from charged interstitials.
15252 J. Phys. Chem., Vol. 99, No. 41, 1995
Van Leeuwen et al. Naval Research Grants N00014-91-J-1499 and N00014-941-0917 and the University of Missouri Research Board.
0.8
References and Notes (1) (2) 269. (3) (4)
Wavelength (nm)
Figure 11. Reflectivity calculated from the Drude model as a function of wavelength for a range of mobilities from 1000 c m Z Ns to 1 c m 2 N s. Other parameters used for the calculation are m, = 0.3m0, EO = 4, and & = 2000 nm, and N = 3.4 x 1020~ m - The ~ . “steepness” of the plasma edge is quite sensitive to the electron mobility.
We have shown that the shape of the reflectance spectrum is very sensitive to the carrier mobility and that the values measured by contact and optical methods are consistent. The carrier mobility for materials with resistive grain boundaries or on conducting substrates may be determined optically where contact methods would be unreliable. Acknowledgment. This work was supported in part by National Science Foundation Grant DMR-9202872, Office of
Geserich, H. P. Phys. Status. Solidi. 1968, 25, 741. Wirtz, G. P.; Yu, C. J.; Doser, R. W. J. Am. Ceram. SOC. 1981,64,
Switzer, J. A. J. Electrochem. SOC.1986, 133, 722. Sheng, Z. Z.; Hermann, A. M. Nature 1988, 55, 332. (5) Cava, R. J. Science 1990, 247, 656. (6) Switzer, J. A.; Shane, M. J.: Phillius, R. J. Science 1990. 247.444. (7) Switzer, J. A.; Raffaelle, R. P.; Phillips, R. J.; Hung, C.4.; Golden, T. D. Science 1992, 258, 1918. (8) Switzer, J. A.; Golden, T. D. Adv. Mater. 1993, 5, 474. (9) Switzer, J. A.; Phillips, R. J.; Golden, T. D. Appl. Phys. Lett. 1995, 66, 819. (10) Switzer, J. A.; Hung, C.-J.; Breyfogle, B. E.; Shumsky, M. G.; Van Leeuwen, R. A.; Golden, T. D. Science 1994, 264, 1573. (1 1) Annual Book of ASTM Standards; American Society for Testing and Materials: Philadelphia, PA, 1986; F76-86. (12) Pankove, J. I. Optical Processes in Semiconductors; Dover: New York, 1975. (13) Bube, R. H. Electrons in Solids; Academic: San Diego, 1988. (14) Bevington, P. R. Data Reduction and Error Analysis for the Physical Sciences; McGraw Hill: New York, 1969. (15) Kostlin, H.; Jost, R.; Lems, W. Phys. Status. Solidi. A 1975, 29, 87. (16) Mindt, W. J. Electrochem. SOC.1969, 116, 1076. (17) Shukla, V. N.; Wirtz, G. P. J. Am. Ceram. SOC.1977, 60, 253. (18) Gupta, L.; Mansingh, A.; Srivastava, P. K. Thin Solid Films 1989, 176, 33. (19) Fistul, V. I. Heauily Doped Semiconductors; Plenum: New York, 1969. (20) Burstein, E. Phys. Rev. 1953, 93, 632.
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