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Evidence for a donor-acceptor pair band in CuInSe2
Solid State Communications, Vol. 18, pp. 395—397, 1976.
Pergamon Press.
Printed in Great Britain
EVIDENCE FOR A DONOR—ACCEPTOR PAIR BAND IN CuInSe2 * Phil Won Yu Department of Physics, University of Dayton, Dayton, OH 45469, U.S.A. (Received 7 July 1975 by J. Tauc) The broad-band emission, observed in p-type crystals, was studied as a function of excitation intensity and temperature. The band peak is usually in the energy range of 0.93—0.95 eV at 42°K.The band shifts to higher energy as the excitation intensity of temperature increases. This behavior is consistent with a donor—acceptor pair.band mechanism. The acceptor energy EA is 85 ±2 meV. The acceptors and donors involved in the pair band appear to be Cu and Se vacancies.
INTERIMPURITY-RADIATIVE recombination of electrons bound to donors with holes bound to acceptors may occur for both close and remote impurity pairs. When the pair distance is sufficiently large, the recombination of the pairs is where given by equation hz~’=BA E9and 2/er, Eg the is the band gap, (EA + ED ) + e energy, ressmall, individual pair lines may be observed. When r becomes larger, the emission merges into a broad band. A recent photoluminescence study1 in CuInSe 2 showed large differences between the liquid-nitrogen
PHOTON ENEROY (eV) O~9
C~fl5S~2
//\\
—
aximum and mini urn Se pr: ur:. Tb :pectral features were explained by the presence of an acceptor (EA 40 meV) and a donor (ED 70 meV)2level. in the Generally, the observation of sharp pair lines emission spectrum is direct evidence of the presence of a donor—acceptor pair recombination. When sharp lines are not observed, several alternative indicators are usually employed to identify donor—acceptor pair emission. Such indications are; (1) changes in band shape and band-peak energy as a function of excitation intensity, and (2) band shift to higher energy (generally) with an increase in temperature and eventual dominance of a higher-energy band. In this communication results are presented on the behavior of the low-temperature broadband emission spectrum of p-type CuInSe 2 with change in temperature and excitation intensity. The broad-band emission shifts to higher energy as the excitation intensity or temperature is increased. This feature is consistent with a donon—acceptor pair mechanism for this band. *
Work performed at Aerospace Reasearch Laboratories, Wright—Patterson Air Force Base, under contract No. F33615-72-C-2114. 395
P-~YPS
If\\
~}J \ ~ / \
J I
GROWN
04
004
\ \
002 0.004
I
1.20
I
I
wAVELENGTH(s)
I
I
.50
Fig. 1. Effect of excitation intensity on the band position at 4.2°K.The fractional excitation entensities are shown on each spectrum. Maximum power is 240 mW from the 6471 A line of a Kr laser. Emission intensity decreases the intensity of excitation. Each emission spectrum with has been normalized.
The crystals were melt grown in an excess Se atmo3 sphere, the p-type details with having beenconcentration discussed elsewhere. They were a hole ranging from 2 x 1016 to 7 x 1016 cm3 at room temperature. Luminescence spectra were obtained for the samples at temperatures ranging from 4.2 to l10°K.For fixedtemperature measurements the samples were immersed directly into liquid helium. A Heli—Tran Dewar was used for variable temperatures. A Kr laser operating at the wavelength of 6471 A, with maximum power of
396
EVIDENCE FOR A DONOR—ACCEPTOR PAIR BAND IN CuInSe2
0’
~
Cu tnS2 2 #15-3 P-TYPE 4 2~I<
:
1
02
L~L.e~p(aEp) a.470eV~ 4 .5r,~eV
~
0
-
-
Vol. 18, No.3
as-grown P-type sample having a broad band. It is evident that this emission band moves to lower energies as the excitation intensity is reduced. The half-width of the band also increases with attenuation of the excitation intensity. In Fig. 2 the excitation intensity L is plotted as a function of peak energy E~for the same sample. It is clear that L varies exponentially with E~. i.e., L = L0 exp (aE~), where a isa proportionality constant. The energy change per decade of excitation intensity is 5 meV. This number was observed to vary from sample to sample depending upon the band-peak energy. This effect can be explained by an increase of the Coulomb term in the acceptor—donor pair-energy relation when the increase of excitation intensity favors the closer pairs. A similar peak shift with excitation has
092
093 I
094 I
Al
095
Ep(2V)
Fig. 2. Excitation intensity vs band peak energy. a is a proportionality constant. ~3is the energy change per decade of excitation intensity.
been taken as evidence of a donor—acceptor pair band 4 (Al, Ga)P,5 (In. Ga)P,6 GaAs,7 ZnSe,8 and InP.9 in many semiconductors such as GaP, Figure 3 shows the emission spectra obtained using the same excitation intensity but an increase in tempera-
ture to 1 10°Kfor the as-grown p-type crystal. The band remains at the same energy up to
PHOTON ENERGY (eV)
—2~----——--r-r
105
aBS
35°K.With an
increase in temperature, the band shifts to higher energies: with further increase, a second band begins to emerge on the higher-energy side and the first band
CuinS. 2
/
A
*‘S’ P-TYPE,GS GR0%B?~
is responsible for the band; at higher temperature the carriers released thermally from their original sites move to energetically more favorable sites, which reduces the distance r in the pair-energy relation. A similar band shift with temperature for the donor—acceptor pair
z U I—
Cl)
U Cl,
120 Jill
begins to diminish. This can be easily understood under the assumption that a single recombination mechanism
130
WAvELENGmh~)
32•B T.20•K 40 10~ 4
Fig. 3. Effect of temperature on the band. D—A donor—acceptor pair band. F—B free electron to
bound hole to acceptor transition.
study ture onof the the band energy, excitation we7in find and intensity InP.9 that the From broad and the temperahand present dominating Se donor atmosphere and acceptor ateffect 4.2° is due Kofpairs. in to the the crystals radiative recombination under excess of recombinatjon When the second of free band electrons Fig. with 3 grown isholes attributed bound to to the acceptors band was (“free-to-bound” observed in GaAs recombination), the acceptor
ionization energy BA is found to be 85 ±2 meV using the relation h~= Eg BA + kT and Eg(77°K)= 1.060 eV. The energy gap was calculated by means of —
the observed free exciton energy’°1 .042 eV and by
240 mW, was used for excitation. The emission spectra were analyzed by a 3/4-rn grating Czery—Turner spectrometer (11 A/mm) and detected with a roomtemperature PbS cell, The emission spectrum of the as-grown p-type
taking the free exciton binding energy to be 1 8 meV from ~(CdSe” + ZnSe12). The BA was also obtained from a free-to-bound radiative recombination observed at 4.2°K,which yields the same value forEA (details to be discussed elsewhere). The intrinsic defects’3 play
crystals was dominated by a broad band at 4.2°K.The
important roles in determining electrical properties in
peak energies of the band of most samples were in the
I—I1I—V1
range 0.93—0.95 eV. Figure 1 shows spectra taken at 4.2°Kwith changing excitation intensity for a typical
2, as evidenced by maximum and minimum Se heat-treatment. The Cu and Se vacancies are probably acceptors and donors influencing the electrical
Vol. 18, No.3
EVIDENCE FOR A DONOR—ACCEPTOR PAIR BAND IN CuInSe2
properties in CuInSe2. In this work heat treating the crystals in vacuum was observed to influence the broadband peak energy. The Se and Cu vacancies appear to be the donors and acceptors involved in the pair band. In CuInSe2 2/Vsec is oba ratherwhile high electron mobility of~’ cm served, the hole mobility is 9200 cm2/Vsec at room ‘-‘
397
temperature. This indicates that the electron effective mas~ is smaller than the hole mass. Therefore, EA should be larger than ED. Acknowledgement We thank D.C. Reynolds for many helpful discussions. —
REFERENCES 1. 2.
MIGLIORATO P., SHAY J.L., KASPER H.M. & WAGNER S., J. Appi. Phys. 46, 1777 (1975). See for, example, HOPFIELD J.J., THOMAS D.G. & GERSHENSON M., Phys. Rev. Lett. 10, 162 (1963).
3.
YU P.W., FAILE S.P. & PARK Y.S., Appl. Phys. Lett. 26, 384 (1975).
4. 5.
MAEDA K.,J. Phys. Chem. Solids 26, 595 (1965). MERZ J.L. & LYNCH R.T., .1. Appl. Phys. 39, 1988 (1968).
6.
WILLIAMS E.W., ASHFORD A., PORTEOUS P. & WHITE A.M., Solid State Commun. 8, 501 (1970).
7.
DINGLE R.,Phys. Rev. 184, 788 (1969).
8.
DEAN P.J. & MERZ J.L., Phys. Rev. 178, 1310 (1969).
9.
LEITE R.C.C., Phys. Rev. 157, 672 (1967).
10. 11. 12. 13.
The free exciton from the conduction band and the uppermost valence band was observed in the crystals grown in the crystals grown from stoichiometric melt. WHEELER R.G. & DIMMOCK J.O., Phys. Rev. 125, 1805 (1962). SEGALL B. & MARPLE D.TF, in Physics and Chemistry of II— VI Compounds, (edited by AVENS M. & PRENER J.S.), Wiley-Interscience, NY (1967). TELL B., SHAY J.L. & KASPER H.M.,J. App!. Phys. 43, 2469 (1972).
Pergamon Press.
Printed in Great Britain
EVIDENCE FOR A DONOR—ACCEPTOR PAIR BAND IN CuInSe2 * Phil Won Yu Department of Physics, University of Dayton, Dayton, OH 45469, U.S.A. (Received 7 July 1975 by J. Tauc) The broad-band emission, observed in p-type crystals, was studied as a function of excitation intensity and temperature. The band peak is usually in the energy range of 0.93—0.95 eV at 42°K.The band shifts to higher energy as the excitation intensity of temperature increases. This behavior is consistent with a donor—acceptor pair.band mechanism. The acceptor energy EA is 85 ±2 meV. The acceptors and donors involved in the pair band appear to be Cu and Se vacancies.
INTERIMPURITY-RADIATIVE recombination of electrons bound to donors with holes bound to acceptors may occur for both close and remote impurity pairs. When the pair distance is sufficiently large, the recombination of the pairs is where given by equation hz~’=BA E9and 2/er, Eg the is the band gap, (EA + ED ) + e energy, ressmall, individual pair lines may be observed. When r becomes larger, the emission merges into a broad band. A recent photoluminescence study1 in CuInSe 2 showed large differences between the liquid-nitrogen
PHOTON ENEROY (eV) O~9
C~fl5S~2
//\\
—
aximum and mini urn Se pr: ur:. Tb :pectral features were explained by the presence of an acceptor (EA 40 meV) and a donor (ED 70 meV)2level. in the Generally, the observation of sharp pair lines emission spectrum is direct evidence of the presence of a donor—acceptor pair recombination. When sharp lines are not observed, several alternative indicators are usually employed to identify donor—acceptor pair emission. Such indications are; (1) changes in band shape and band-peak energy as a function of excitation intensity, and (2) band shift to higher energy (generally) with an increase in temperature and eventual dominance of a higher-energy band. In this communication results are presented on the behavior of the low-temperature broadband emission spectrum of p-type CuInSe 2 with change in temperature and excitation intensity. The broad-band emission shifts to higher energy as the excitation intensity or temperature is increased. This feature is consistent with a donon—acceptor pair mechanism for this band. *
Work performed at Aerospace Reasearch Laboratories, Wright—Patterson Air Force Base, under contract No. F33615-72-C-2114. 395
P-~YPS
If\\
~}J \ ~ / \
J I
GROWN
04
004
\ \
002 0.004
I
1.20
I
I
wAVELENGTH(s)
I
I
.50
Fig. 1. Effect of excitation intensity on the band position at 4.2°K.The fractional excitation entensities are shown on each spectrum. Maximum power is 240 mW from the 6471 A line of a Kr laser. Emission intensity decreases the intensity of excitation. Each emission spectrum with has been normalized.
The crystals were melt grown in an excess Se atmo3 sphere, the p-type details with having beenconcentration discussed elsewhere. They were a hole ranging from 2 x 1016 to 7 x 1016 cm3 at room temperature. Luminescence spectra were obtained for the samples at temperatures ranging from 4.2 to l10°K.For fixedtemperature measurements the samples were immersed directly into liquid helium. A Heli—Tran Dewar was used for variable temperatures. A Kr laser operating at the wavelength of 6471 A, with maximum power of
396
EVIDENCE FOR A DONOR—ACCEPTOR PAIR BAND IN CuInSe2
0’
~
Cu tnS2 2 #15-3 P-TYPE 4 2~I<
:
1
02
L~L.e~p(aEp) a.470eV~ 4 .5r,~eV
~
0
-
-
Vol. 18, No.3
as-grown P-type sample having a broad band. It is evident that this emission band moves to lower energies as the excitation intensity is reduced. The half-width of the band also increases with attenuation of the excitation intensity. In Fig. 2 the excitation intensity L is plotted as a function of peak energy E~for the same sample. It is clear that L varies exponentially with E~. i.e., L = L0 exp (aE~), where a isa proportionality constant. The energy change per decade of excitation intensity is 5 meV. This number was observed to vary from sample to sample depending upon the band-peak energy. This effect can be explained by an increase of the Coulomb term in the acceptor—donor pair-energy relation when the increase of excitation intensity favors the closer pairs. A similar peak shift with excitation has
092
093 I
094 I
Al
095
Ep(2V)
Fig. 2. Excitation intensity vs band peak energy. a is a proportionality constant. ~3is the energy change per decade of excitation intensity.
been taken as evidence of a donor—acceptor pair band 4 (Al, Ga)P,5 (In. Ga)P,6 GaAs,7 ZnSe,8 and InP.9 in many semiconductors such as GaP, Figure 3 shows the emission spectra obtained using the same excitation intensity but an increase in tempera-
ture to 1 10°Kfor the as-grown p-type crystal. The band remains at the same energy up to
PHOTON ENERGY (eV)
—2~----——--r-r
105
aBS
35°K.With an
increase in temperature, the band shifts to higher energies: with further increase, a second band begins to emerge on the higher-energy side and the first band
CuinS. 2
/
A
*‘S’ P-TYPE,GS GR0%B?~
is responsible for the band; at higher temperature the carriers released thermally from their original sites move to energetically more favorable sites, which reduces the distance r in the pair-energy relation. A similar band shift with temperature for the donor—acceptor pair
z U I—
Cl)
U Cl,
120 Jill
begins to diminish. This can be easily understood under the assumption that a single recombination mechanism
130
WAvELENGmh~)
32•B T.20•K 40 10~ 4
Fig. 3. Effect of temperature on the band. D—A donor—acceptor pair band. F—B free electron to
bound hole to acceptor transition.
study ture onof the the band energy, excitation we7in find and intensity InP.9 that the From broad and the temperahand present dominating Se donor atmosphere and acceptor ateffect 4.2° is due Kofpairs. in to the the crystals radiative recombination under excess of recombinatjon When the second of free band electrons Fig. with 3 grown isholes attributed bound to to the acceptors band was (“free-to-bound” observed in GaAs recombination), the acceptor
ionization energy BA is found to be 85 ±2 meV using the relation h~= Eg BA + kT and Eg(77°K)= 1.060 eV. The energy gap was calculated by means of —
the observed free exciton energy’°1 .042 eV and by
240 mW, was used for excitation. The emission spectra were analyzed by a 3/4-rn grating Czery—Turner spectrometer (11 A/mm) and detected with a roomtemperature PbS cell, The emission spectrum of the as-grown p-type
taking the free exciton binding energy to be 1 8 meV from ~(CdSe” + ZnSe12). The BA was also obtained from a free-to-bound radiative recombination observed at 4.2°K,which yields the same value forEA (details to be discussed elsewhere). The intrinsic defects’3 play
crystals was dominated by a broad band at 4.2°K.The
important roles in determining electrical properties in
peak energies of the band of most samples were in the
I—I1I—V1
range 0.93—0.95 eV. Figure 1 shows spectra taken at 4.2°Kwith changing excitation intensity for a typical
2, as evidenced by maximum and minimum Se heat-treatment. The Cu and Se vacancies are probably acceptors and donors influencing the electrical
Vol. 18, No.3
EVIDENCE FOR A DONOR—ACCEPTOR PAIR BAND IN CuInSe2
properties in CuInSe2. In this work heat treating the crystals in vacuum was observed to influence the broadband peak energy. The Se and Cu vacancies appear to be the donors and acceptors involved in the pair band. In CuInSe2 2/Vsec is oba ratherwhile high electron mobility of~’ cm served, the hole mobility is 9200 cm2/Vsec at room ‘-‘
397
temperature. This indicates that the electron effective mas~ is smaller than the hole mass. Therefore, EA should be larger than ED. Acknowledgement We thank D.C. Reynolds for many helpful discussions. —
REFERENCES 1. 2.
MIGLIORATO P., SHAY J.L., KASPER H.M. & WAGNER S., J. Appi. Phys. 46, 1777 (1975). See for, example, HOPFIELD J.J., THOMAS D.G. & GERSHENSON M., Phys. Rev. Lett. 10, 162 (1963).
3.
YU P.W., FAILE S.P. & PARK Y.S., Appl. Phys. Lett. 26, 384 (1975).
4. 5.
MAEDA K.,J. Phys. Chem. Solids 26, 595 (1965). MERZ J.L. & LYNCH R.T., .1. Appl. Phys. 39, 1988 (1968).
6.
WILLIAMS E.W., ASHFORD A., PORTEOUS P. & WHITE A.M., Solid State Commun. 8, 501 (1970).
7.
DINGLE R.,Phys. Rev. 184, 788 (1969).
8.
DEAN P.J. & MERZ J.L., Phys. Rev. 178, 1310 (1969).
9.
LEITE R.C.C., Phys. Rev. 157, 672 (1967).
10. 11. 12. 13.
The free exciton from the conduction band and the uppermost valence band was observed in the crystals grown in the crystals grown from stoichiometric melt. WHEELER R.G. & DIMMOCK J.O., Phys. Rev. 125, 1805 (1962). SEGALL B. & MARPLE D.TF, in Physics and Chemistry of II— VI Compounds, (edited by AVENS M. & PRENER J.S.), Wiley-Interscience, NY (1967). TELL B., SHAY J.L. & KASPER H.M.,J. App!. Phys. 43, 2469 (1972).