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Far infrared absorption in ZnO
Solid State Communications, Vol. 9, pp. 2251—2253, 1971.
Pergamon Press.
Printed in Great Britain
FAR INFRARED ABSORPTION IN ZnO* S. Perkowitz, R.K. Murty-Gutta and A.K. Garrison Physics Department, Emory University, Atlanta, Georgia 30322
(Received 7 September 1971 by N.B. Hannay)
Far infrared absorption measurements on as-grown and Li-doped ZnO 00cm’. A large Drude-type freehave been made in the range 15—2 carrier absorption was observed, as were absorption bands near 115 and 160 cm~. Combined absorption and EPR data on the Li-doped sample were interpreted as showing the presence of traps.
THE OPTICAL properties of ZnO have previously been examined in the middle infrared ‘~ where lattice effects compete with free-carrier effects and dominate the free-carrier contribution for carrier concentrations much below 5 x l0~cm3. In the far infrared, if one may use previous experience with GaAs~6 as a guide, one would expect first, that the free-carrier effects become relatively more important; and second, that the free-carrier absorption can be described by a simple Drude equation. We have carried out absorption measurements in single-crystal ZnO in the range 15—200cmt to test these conjectures. In addition, we have investigated the effect of u.v. radiation on both the far infrared absorption and the electron paramagnetic resonance (EPR) absorption. We examined two samples of ZnO, one asgrown and the other Li-doped in the melt, which were obtained from the 3M Company. Nominal limits for the sample parameters were supplied by 3M. For the as-grown sample at 298°Kthe carrier concentration was quoted as lying in the range 1 x 10~ to 5 x 10~6cm3 and the Hall mobility was given as approximately 150 cm2/Vsec. The Li-doped sample was described as having a *
dark resistivity of 101~fl-cm and a light resistivity, when illuminated by a mercury arc lamp, of 10~fl-cm. These values were obtained by a two-probe measurement and so are probably not very reliable; however, they are so much greater than the as-grown resistivity that it seems reasonable to take the Li-doped sample as heavily compensated. Both samples were in the farm of mechanically polished flat pieces about O.lcm thick and were cut with the c-axis perpendicular to the faces. Fourier transform spectroscopy was used to obtain the far infrared data. The spectrometer was the Grubb Parsons Cube interferometer with a Unicam Golay cell as detector. The light source in the interferometer is a high-pressure mercury arc lamp which emits a significant amount of u.v. radiation. Unpolarized light was used in all the measurements. All spectra were recorded with a resolution of 8 cm~. The absorption coefficient a was calculated ~rorn the measured transmission coefficient T by means of the equation 2
T
=
“~
~
-a~
(1)
Work supported in part by grants from the Alfred
where R is the reflectivity and x is the sample
P. Sloan Foundation and the Research Corporation.
thickness. I? was calculated in a manner which has been described elsewhere. 6 In calculating 2251
2252
FAR INFRARED ABSORPTION IN ZnO
Vol.9, No.24
a=a
‘+0
a
p 2T2) (2) nc(1 + W where the first term depends on lattice properties lat
+
only and is independent of free-carrier behavior. a
30
a
0
a
r
a
a
o
a a
In the second term, which is the free-carrier absorption ai~,n is the refractive index, r is
0
a
a
the electronic scattering time and = 4TTNe 2/m* is the plasma frequency where N is the carrier
a
concentration and m* is the electron effective mass. Because of the heavy compensation, the x~
10
0 £
0
A ‘~
0 0
‘+0
80 ** 120 160 FREQUENCY (1/CM)
200
absorption coefficient for the Li-doped sample should represent a 12~only. Figure 2 is a plot of i\a, the difference between the absorption coefficients for the as-grown and Li-doped samples at 300°K, which is to be compared to ~ A 1 striking feature of this curve is that the 115 cmnot peak has vanished almost completely. This is the case for the 160cm1 band, however.
FIG. 1. Absorption coefficient a as a function of frequency. o, as-grown sample at 300°K; x, Lidoped sample at 300cK; ~, Li-doped sample at 77°K; +, Li-doped sample at 77°K with u.v. filter. The data for the as-grown sample at 77°K is almost identical to that for the Li-doped sample at 77°K and has been omitted for clarity.
30
______________________________________
25 20 CD \
15
~
10
R, we used the following values of the static and high-frequency dielectric constants and transverse optical frequency, as determined by Collins and Kleinman:2 ~ = 8.15, E~ = 4.0 and fT = o-~T/277C = 414cm1.
__________________________
0 0
Figure 1 shows a as a function of frequency for both samples at 300 and 77°K. The main structures seen in all cases are the absorption peak centered at about 115 cm~and the absorption band which begins at about 160 crn~ and increases with frequency. These bands are not due to freecarrier absorption, which exhibits no large peaks and generally decreases with increasing frequency; nor do they seem to arise from photo-ionization processes, since they do not increase with decreasing temperature. We suggest that these bands represent multiphonon processes.
the data at 300°K, we assume that To the analyze free-carrier absorption is described bY the Drude theory. Then, if the carrier concentration is not too large, the total absorption can be written as6
‘+0
80 FREQUENCY
120
160
200
t1/C~1)
FIG. coefficient Aa at 3000K2. asDifference a functionabsorption of frequency. ~, experimental points; —, least-squares fit using a~.
For frequencies below 160 cm1, ~ provides an excellent fit to the data as the least-squares curve in Fig. 2 shows. From the fit parameters and the value rn* = O.27m 0 measured by Button et al.7 we calculated N and the drift mobility 3 and = ~‘T/7fl~ and obtained N = 4.9 = 2SOcm2/Vsec. The value for xN 10~ fallscm well within the range given by 3M but the optical value for ~ is significantly higher than the nominal Hall mobility. Even if we include all sources of error and assume that the Hall mobility is as large as
Vol.9, No.24
FAR INFRARED ABSORPTION IN ZnO
2/Vsec, the maximum value measured 180 cm by Hutson,8 there is still an unexplained discrepancy of 20% between the Hall and calculated mobilities. This deviation may represent the difference between a Hall and a drift mobility, At 77°K, as Fig. 1 shows, the absorption has decreased sharply and is almost the same for both samples. We interpret this behavior as due to the freezing-out of carriers in the as-grown sample so that in both samples a,~ is smaller than aiat at 77°K. Judging from the values of the absorption coefficients, the carrier concentration at liquid nitrogen temperature is 2 x i0’~cm3 or less. Hutson has observed a similar degree of freezeout in his as-grown ZnO samples. The EPR of the Li-doped sample was observed at 77°K after the sample had been exposed to the u.v. light of the far infrared spectrometer. The four-line EPR spectrum of the Li hole center first described by Kasai9 was obtained, indicating the sample received enough u.v. to create about l0~ centers. However, as is
shown in Fig. 1, the absorption coefficient at 77°K decreased only slightly when measured with a u.v. filter inserted between the spectrometer source and the sample. The decrease corresponded to a loss of 10’s cm3 free carriers at most. We therefore conclude that in the Lidoped sample there are traps other than Li that are effective at 77°Kin lowering the carrier concentration. This result supports the conclusion of Schirmer and Zwingelt° that the electrons causing the yellow luminescence in ZnO do not recombine from the conduction band but from shallow states near it. Our results show that Drude-type free-carrier absorption plays an important role in the far infrared behavior of ZnO but does not completely dominate other effects as is the case in GaAs. Further analysis of the structure at 115 cm~ and 160 cmt would be of interest. Even without a detailed knowledge of these additional effects, however, the sensitivity of the far infrared absorption to carrier concentration can be used together with other techniques as a tool to study electronic behavior in ZnO.
REF ERENCES 1.
THOMAS D.G., J. Phys. Chern. Solids 10, 47 (1959).
2.
COLLINS R.J. and KLEINMAN D.A., I. Phvs. Chen~.Solids 11, 190 (1959).
3.
WEIHER R.L., Phys. Rev. 152, 736 (1966).
4.
PERKOWITZ S., J. app!. Phvs. 40, 3751 (1969).
5.
SOBOTTA H., Phvs. Len. 32A, 4 (1970).
6.
PERKOWITZ S., J. Pi~\s. Chern. Solids, to be published.
i.
BUTTON K.J., COHN D.R. and DREYBRODT W., Bull. ‘un. Phys. Soc. 16, 417 (1971).
8.
HUTSON A.R., Phy~. Rev. 108, 222 (1957).
9.
KASAI P.H., Ph)s. Rev. 130, 989 (1963).
10.
2253
SCHIRMER O.f’. and ZWINGEL D., Solid State Commun. 8, 1559 (1970).
Absorptionsmessungeri wurden im fernen Ultrarot an natürlich gewachsenem und mit Li gedopten ZnO im Bereich von 15 bis 200 cm~durchgefiihrt. Grolie trägerfreie Absorption von der Art der Drude-Absorption wurden beobachtet sowie Absorptionsbänder in der Nähe von 115 und 160cm1. Die aus der Kombination von Absorption und EPR erhaltenen Daten sind durch die Anwesenheit von Traps zu erklären.
Pergamon Press.
Printed in Great Britain
FAR INFRARED ABSORPTION IN ZnO* S. Perkowitz, R.K. Murty-Gutta and A.K. Garrison Physics Department, Emory University, Atlanta, Georgia 30322
(Received 7 September 1971 by N.B. Hannay)
Far infrared absorption measurements on as-grown and Li-doped ZnO 00cm’. A large Drude-type freehave been made in the range 15—2 carrier absorption was observed, as were absorption bands near 115 and 160 cm~. Combined absorption and EPR data on the Li-doped sample were interpreted as showing the presence of traps.
THE OPTICAL properties of ZnO have previously been examined in the middle infrared ‘~ where lattice effects compete with free-carrier effects and dominate the free-carrier contribution for carrier concentrations much below 5 x l0~cm3. In the far infrared, if one may use previous experience with GaAs~6 as a guide, one would expect first, that the free-carrier effects become relatively more important; and second, that the free-carrier absorption can be described by a simple Drude equation. We have carried out absorption measurements in single-crystal ZnO in the range 15—200cmt to test these conjectures. In addition, we have investigated the effect of u.v. radiation on both the far infrared absorption and the electron paramagnetic resonance (EPR) absorption. We examined two samples of ZnO, one asgrown and the other Li-doped in the melt, which were obtained from the 3M Company. Nominal limits for the sample parameters were supplied by 3M. For the as-grown sample at 298°Kthe carrier concentration was quoted as lying in the range 1 x 10~ to 5 x 10~6cm3 and the Hall mobility was given as approximately 150 cm2/Vsec. The Li-doped sample was described as having a *
dark resistivity of 101~fl-cm and a light resistivity, when illuminated by a mercury arc lamp, of 10~fl-cm. These values were obtained by a two-probe measurement and so are probably not very reliable; however, they are so much greater than the as-grown resistivity that it seems reasonable to take the Li-doped sample as heavily compensated. Both samples were in the farm of mechanically polished flat pieces about O.lcm thick and were cut with the c-axis perpendicular to the faces. Fourier transform spectroscopy was used to obtain the far infrared data. The spectrometer was the Grubb Parsons Cube interferometer with a Unicam Golay cell as detector. The light source in the interferometer is a high-pressure mercury arc lamp which emits a significant amount of u.v. radiation. Unpolarized light was used in all the measurements. All spectra were recorded with a resolution of 8 cm~. The absorption coefficient a was calculated ~rorn the measured transmission coefficient T by means of the equation 2
T
=
“~
~
-a~
(1)
Work supported in part by grants from the Alfred
where R is the reflectivity and x is the sample
P. Sloan Foundation and the Research Corporation.
thickness. I? was calculated in a manner which has been described elsewhere. 6 In calculating 2251
2252
FAR INFRARED ABSORPTION IN ZnO
Vol.9, No.24
a=a
‘+0
a
p 2T2) (2) nc(1 + W where the first term depends on lattice properties lat
+
only and is independent of free-carrier behavior. a
30
a
0
a
r
a
a
o
a a
In the second term, which is the free-carrier absorption ai~,n is the refractive index, r is
0
a
a
the electronic scattering time and = 4TTNe 2/m* is the plasma frequency where N is the carrier
a
concentration and m* is the electron effective mass. Because of the heavy compensation, the x~
10
0 £
0
A ‘~
0 0
‘+0
80 ** 120 160 FREQUENCY (1/CM)
200
absorption coefficient for the Li-doped sample should represent a 12~only. Figure 2 is a plot of i\a, the difference between the absorption coefficients for the as-grown and Li-doped samples at 300°K, which is to be compared to ~ A 1 striking feature of this curve is that the 115 cmnot peak has vanished almost completely. This is the case for the 160cm1 band, however.
FIG. 1. Absorption coefficient a as a function of frequency. o, as-grown sample at 300°K; x, Lidoped sample at 300cK; ~, Li-doped sample at 77°K; +, Li-doped sample at 77°K with u.v. filter. The data for the as-grown sample at 77°K is almost identical to that for the Li-doped sample at 77°K and has been omitted for clarity.
30
______________________________________
25 20 CD \
15
~
10
R, we used the following values of the static and high-frequency dielectric constants and transverse optical frequency, as determined by Collins and Kleinman:2 ~ = 8.15, E~ = 4.0 and fT = o-~T/277C = 414cm1.
__________________________
0 0
Figure 1 shows a as a function of frequency for both samples at 300 and 77°K. The main structures seen in all cases are the absorption peak centered at about 115 cm~and the absorption band which begins at about 160 crn~ and increases with frequency. These bands are not due to freecarrier absorption, which exhibits no large peaks and generally decreases with increasing frequency; nor do they seem to arise from photo-ionization processes, since they do not increase with decreasing temperature. We suggest that these bands represent multiphonon processes.
the data at 300°K, we assume that To the analyze free-carrier absorption is described bY the Drude theory. Then, if the carrier concentration is not too large, the total absorption can be written as6
‘+0
80 FREQUENCY
120
160
200
t1/C~1)
FIG. coefficient Aa at 3000K2. asDifference a functionabsorption of frequency. ~, experimental points; —, least-squares fit using a~.
For frequencies below 160 cm1, ~ provides an excellent fit to the data as the least-squares curve in Fig. 2 shows. From the fit parameters and the value rn* = O.27m 0 measured by Button et al.7 we calculated N and the drift mobility 3 and = ~‘T/7fl~ and obtained N = 4.9 = 2SOcm2/Vsec. The value for xN 10~ fallscm well within the range given by 3M but the optical value for ~ is significantly higher than the nominal Hall mobility. Even if we include all sources of error and assume that the Hall mobility is as large as
Vol.9, No.24
FAR INFRARED ABSORPTION IN ZnO
2/Vsec, the maximum value measured 180 cm by Hutson,8 there is still an unexplained discrepancy of 20% between the Hall and calculated mobilities. This deviation may represent the difference between a Hall and a drift mobility, At 77°K, as Fig. 1 shows, the absorption has decreased sharply and is almost the same for both samples. We interpret this behavior as due to the freezing-out of carriers in the as-grown sample so that in both samples a,~ is smaller than aiat at 77°K. Judging from the values of the absorption coefficients, the carrier concentration at liquid nitrogen temperature is 2 x i0’~cm3 or less. Hutson has observed a similar degree of freezeout in his as-grown ZnO samples. The EPR of the Li-doped sample was observed at 77°K after the sample had been exposed to the u.v. light of the far infrared spectrometer. The four-line EPR spectrum of the Li hole center first described by Kasai9 was obtained, indicating the sample received enough u.v. to create about l0~ centers. However, as is
shown in Fig. 1, the absorption coefficient at 77°K decreased only slightly when measured with a u.v. filter inserted between the spectrometer source and the sample. The decrease corresponded to a loss of 10’s cm3 free carriers at most. We therefore conclude that in the Lidoped sample there are traps other than Li that are effective at 77°Kin lowering the carrier concentration. This result supports the conclusion of Schirmer and Zwingelt° that the electrons causing the yellow luminescence in ZnO do not recombine from the conduction band but from shallow states near it. Our results show that Drude-type free-carrier absorption plays an important role in the far infrared behavior of ZnO but does not completely dominate other effects as is the case in GaAs. Further analysis of the structure at 115 cm~ and 160 cmt would be of interest. Even without a detailed knowledge of these additional effects, however, the sensitivity of the far infrared absorption to carrier concentration can be used together with other techniques as a tool to study electronic behavior in ZnO.
REF ERENCES 1.
THOMAS D.G., J. Phys. Chern. Solids 10, 47 (1959).
2.
COLLINS R.J. and KLEINMAN D.A., I. Phvs. Chen~.Solids 11, 190 (1959).
3.
WEIHER R.L., Phys. Rev. 152, 736 (1966).
4.
PERKOWITZ S., J. app!. Phvs. 40, 3751 (1969).
5.
SOBOTTA H., Phvs. Len. 32A, 4 (1970).
6.
PERKOWITZ S., J. Pi~\s. Chern. Solids, to be published.
i.
BUTTON K.J., COHN D.R. and DREYBRODT W., Bull. ‘un. Phys. Soc. 16, 417 (1971).
8.
HUTSON A.R., Phy~. Rev. 108, 222 (1957).
9.
KASAI P.H., Ph)s. Rev. 130, 989 (1963).
10.
2253
SCHIRMER O.f’. and ZWINGEL D., Solid State Commun. 8, 1559 (1970).
Absorptionsmessungeri wurden im fernen Ultrarot an natürlich gewachsenem und mit Li gedopten ZnO im Bereich von 15 bis 200 cm~durchgefiihrt. Grolie trägerfreie Absorption von der Art der Drude-Absorption wurden beobachtet sowie Absorptionsbänder in der Nähe von 115 und 160cm1. Die aus der Kombination von Absorption und EPR erhaltenen Daten sind durch die Anwesenheit von Traps zu erklären.