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Luminescence excitation of donor acceptor pairs in CdTe: Acceptor ground state splitting and associate energy transfers
Journal of Luminescence 24/25 (1981) 159—162 North-Holland Publishing Company
159
LUMINESCENCE EXCITATION OF DONOR ACCEPTOR PAIRS IN CdTe: ACCEPTOR GROUNO STATE SPLITTING AND ASSOCIATE ENERGY TRANSEERS G. NEU. R. LEGROS and Y. MARFAING Lahoratnire de Physique des Solides. CNRS 92190 Meudnn FRANCE
Excitation spectroscopy of lithiuns-indium donor-acceptor pairs in CdTe is used to investigate the inspurity interaction effect on the acceptor ground state. The interaction with a neighbouring donor atom causes a splitting of the degenerate hole state. Two sharp narrow lines, associated with the pair neutralization transition in the ground state sublcvels, are observed up to pair separations of 70A . The splitting can he accounted for by two possible mechanisms : a strain field around the acceptor, induced by the donor, or an electric field gradient, due to the charged donor. Comparison with experiments seems in favour of a Couloiub type perturbation. INTRODUCTION The first experimental evidence of a lifting of tlse acceptor ground state degeneracy in a semiconductor due to the influence of a nearby donor, was obtained by Morgan and Maier [ s ], after a careful analysis of donor-acceptor pairs lines in GaP. This effect, which clearly depends on the radial and angular paranseters of interimpurity separation, was attributed to local strain fields induced by substitutional donors. Valence band perturbations due to the presence of charged defects in the host crystal can result from several sources : size differences between a lattice atom and the inspurity atom and, in polar conspounds, Coulomb interaction between the foreign and the crystal ions, can cause a displacement ofthe lattice atoms around the donor site. This lattice distortion generates strain fields which locally affect the hole states. [ 2 } - lleside strain effects, a perturbation of the bound hole wave functions by the donoracceptor Coulonsb interaction has to15a/a be considered. a result acceptor of the inhomogencity of the quadruply As degenerate level is split into two Coulomb doublets field ofa nearby ionized donor, the 3 ]. The analytical expression of the splitting being analogous in the two cases ; it is generally difficult to discriminate between a strain field effect and a Coulomb field effect. The only experimental work has been done on GaP where discrete donor-acceptor (0-A) pair lines can be observed, which allows to study in detail the impurity interactions. In this paper, we apply excitation spectroscopic techniques to investigate the ground state of shallow acceptors in CdTe, where D-A pair lines are usually not observed. A direct neutralization of ionized pairs by transitions between the inspurity ground states permits to detect the splitting and to study it for various pair separafions. EXPERIMENTAL Excitation spectroscopy measurements are conducted on InCd - LiCd pairs in compensated In doped, CdTe at 1.6K. Excitation light is obtained from a tunable DEOTC dye Laser punsped by the red lines from a Kr Laser. The luminescence is analysed by a double grating spectrometer. The Li acceptor , previously characterized from excitation data [ 4 ], is the dominant residual inspurity. Its ionization energy, deduced by use of the effective mass theory of llALDERESCHI-LIPARI [ 5 is EA = 60.7 sneV. The In donor binding energy is evaluated to F 0 = 14.5 meV. The pair separations RDA associated with a given luminescence energy TSWDUM is then fully determined by the usual relation 2/EEDA (i) = EG - ED ÷e
160
0
Vet
—
:•~, —
1 ciii
Ikomr dccc 001 /Jci/rs in Cd Id
for a direct recoinbinat ion or =
c~/~R.
+
br a recoinhination with L() phonon emission. L~is the hand gap energy and (‘onser~uently. the escitation resonant line at =
is the static dielectric constant of the material. speetroni of a selected pair Inmineseence
two
displays a sharp
-
!1W
+
tidLo
svtueh is associated svitli : ii the direct neutralization of 1)—A pairs svitli separation R ‘I) A
j) and indirect neutralization via LU photon emission of 0—A tairs with separation RVA’ I hese processes are schematically deserihed in Fig. ( I) with an energy diagram analogous to ttie one presented in Ret. [6 I. At low energy side of the 0-A tununeseenee hand ) tidTufl 5 202 I, ~ is large and the prohahility br transitions hetsveen IS levels of impurities is low. [tie RDA pairs are principally neutralized hy non selective excitation processes. [lie resonant excitation line at tw1014+tts~0 energy is then only ohtained front R l)A pairs.
H?
RESULt S Luminescence excitation (LI-..) spectra of Li-In pairs are t tshown two svlien sharp peaks tahelled t and in(2 Fig. are (2). ohsersed
___________________________________
15 ~
I
2
A
D
5
415
5
the energy difterence hetween luminescence and excitation eqnats an optical ptinnon. They appear upon a background signal related to non selective pair neutralization. WIute the resonant tine intensity remains nearly constant, the haekgrnund associated with the ROA pair
LiM~
5 ~
~LO
Innuneseenee increases when the selected lonuneseenee energy raises up to S — 2A — 2 Line (I) is precisely located at the energy (1) . and line (2) at The 270 = ~WLUv + ~d20 = ~ + ~ + A where A is a
fita,~
~
+
.
~
I
p
.
p
__________________________________
function of the interimpurity distance. These
Pair Separation
‘
~DA
lines (I) and (2) are attrihuted to the direct neutraheation of RI-iA pairs, the final state of 51=. (1 the hole being one of the two ground state sublevels. At I .0K the hole excited to - ttie higher suhtevel is rapidly thermahzed to the lower suhlevet before
reeonihinalion to
.
When
+tisj LOS
A IS 2
20
is
split the
—E 0
—2 A
relation
+e/ch 0
12) is niodified
and heroines
—~A(h) 0
2
a
where R0 is slightly different from R’DA. The energies of the excitation peaks are ~(1,2) 270
=
2
—2
0
—2
A
+
D
e/d0
± A(s )/‘ 0
n
(1)
Ci. -Vi’u ci uf
A
/ Donor acceptor puim in
e=eoA 790
I I
161
[nell
800
810
fcli’e
I
F
2
~
—
‘:
R=62A
1~
E
~
10 20 30 ENERGY DIFFERENCE BETWEEN LUMINESCENCE AND EXCITATION )meV[ Fig.
1,52
1,54
l,ss
‘hüj
1
lEVI
Fig. (3)
(2)
An other experimental nsethod to study the A 15 splitting is to use the selective pair luminescence (SPL) technique [6,7) the pair luminescence spectra are recorded with the excitation light kept at a tixed energy t~~70 . For simplicity, we suppose that the splitting energy A depends only on the magnitude of the pair separahon. Two groups of 0-A pairs with interiuspurity distance Ra and Rb can then be neutralized by the incident light, according to the sublevel occupied by the hole in the final state. Ra and Rb are defined by to and
BXO
=
2
0
—
2
A
—
E
0
+
u/eB
a
—
~A(F ( 2 a
(3)
= 2~ — E2— E~ + e2/Rb + ~A(Bb) (6) The two SPL spectra which are reproduced in Fig. 3, are excited at an energy corresponding to the direct neutralization of pairs with mean interimpurity distances of 62 A and 80 A. respectively. The LO phonon replica of the excitation line is split into lines (a) and (b) when the b2A 0-A pairs are
excited. These lines are associated with the Ra and Rb pair recomhination. Since A is very small when R’DA = 80 A, the lines (a) and (b) are not resolved in this ease. D1SCUSSION The results of the measurements by L.L. of the acceptor ground state splitting are reported in Fig. 4. For pair separations varying between 50 and 70 A, A is nearly proportional to ( t/R’DA)a within experimental errors. Strain field effecta. After Ref. [1), when the acceptor Bohr radius is small compared with the impurity separation, the expression for A beconses 3) (A + B p2(~))h/2 (~) A/2 = D’u (v/o
162
Ci. \ in et al.
Donor aci -ep (or pair.v iii (die
where A and B are nsaterial constants and depend on the deformation potenhial constants of the valence band Vu and D’u. We use Vu -1.05 eV and Vu = -4,3 eV as niean values in li-VI compounds [8). V is proporhional to the displacements of ttie nearest neighbors of the donors. Q (R) depends upon the direction of 0-A pairs in the lattice. For pair separations larger than 50 A an isotropic mean value Q 0.26 can he used [9). For a splitting A = 1meV. the corresponding 0-A pair separation is 22 A in GaP and 50 A in CdTe. 1Ius long range interaction in CdTe leads to a 3large value for he the which is to quantihv Vl)’u eVA eVX~ in GaP. The compared to VD’u 55 10.5
0,8 ~
—
~
4
—
—
deduced displacements (~dhX) of the nearest neighbors of the donor are too large in CdTe to be accepted. Electric Jield el/errs. At ter Ref. [2), the expression ot the splitting is similar to Eq. (7 =
•‘/O)(2./FH(%’+2.’~-5I~
I 60
0~24Q
~
..
8 0 (A)
-
IS)
where the values of the distance K and of the constants A’ and B’ are unfortunately unknown. However. the long range interaction observed in CdTe can he associated to the large donor Bohr radius. This the dielectric screening of the Coulomb field by the bound electron is less imtortant in CdTe tItan in GaP. An important consequence of the acceptor ground state splitting is that the reeombination transition of a pair of given Ri overlaps with the direct excitation of another pair having a slightly larger interinspunty distance R 2. By thus process, excitation energy can be transfered from close to distant pairs leading to a broadening on the ugh energy side of the L.E. peaks . In this way. the characteristic slowing down shape of the excitation spectrum of close pairs observed just above the resonant condition = ta Lil’,l can he explained. CONCLUSION L.E. and SPL techniques allow us to reveal the splitting of the Li acceptor ground state in CdTe for 1)-A pair separations larger than 50 A. Experimental data seems not to he accounted for by local strain fields surrounding the donors. C’oulonibic type perturbation is more likely. Similar experiments have been conducted on ZnSe [10) where a splitting of the 2S state has been observed, these experiments should be extended to other semicondsictors having various Bohr radii. With more data, a definite explanation ofthe origin of the acceptor state splitting could he obtained.
REFERENCES [11 TN. MORGAN and H. MAtER. Phys. Rev. Lett. 27, 1200)1971) 2) T.N. MORGAN, in Proceedings of the XI Internahional Conference on the Physics of Semiconductors, WARSAW (1972) ~3) V.G. BARU, V.A. PETROV and V.B. SANDOMIRSKII. Soy. Phys. Senucond. 9. 1344 (1976) 14) G. NEU, Y.MARFA1NG, R. LEGROS, R. TRIBOULET and L. SVOR. J. of Luminescence 21. 293 (1980) [51 A. BALDERESCI-il and NO. LIPAR!. Phys. Rev. B9, 1525 (1974) 161 H. TEWS, H. VENGHAUS and P.J. DEAN. Phys.Rev. B19, 5178 (1979) [71 S. NAKASHtMA, T. HATTOTI and Y. YAMAGUCHI. Solid. State Conimun. 25. 137 (1978) 181 0. KRANZER. J. Phys. (Solid state phys.) 6,2977(1973) [9J G, NEU. Thesis, University Pet M Curie, Paris 1981 (unpublished) Ito) H. TEWS and G. NEU. To be published in Phys. Rev. B.
159
LUMINESCENCE EXCITATION OF DONOR ACCEPTOR PAIRS IN CdTe: ACCEPTOR GROUNO STATE SPLITTING AND ASSOCIATE ENERGY TRANSEERS G. NEU. R. LEGROS and Y. MARFAING Lahoratnire de Physique des Solides. CNRS 92190 Meudnn FRANCE
Excitation spectroscopy of lithiuns-indium donor-acceptor pairs in CdTe is used to investigate the inspurity interaction effect on the acceptor ground state. The interaction with a neighbouring donor atom causes a splitting of the degenerate hole state. Two sharp narrow lines, associated with the pair neutralization transition in the ground state sublcvels, are observed up to pair separations of 70A . The splitting can he accounted for by two possible mechanisms : a strain field around the acceptor, induced by the donor, or an electric field gradient, due to the charged donor. Comparison with experiments seems in favour of a Couloiub type perturbation. INTRODUCTION The first experimental evidence of a lifting of tlse acceptor ground state degeneracy in a semiconductor due to the influence of a nearby donor, was obtained by Morgan and Maier [ s ], after a careful analysis of donor-acceptor pairs lines in GaP. This effect, which clearly depends on the radial and angular paranseters of interimpurity separation, was attributed to local strain fields induced by substitutional donors. Valence band perturbations due to the presence of charged defects in the host crystal can result from several sources : size differences between a lattice atom and the inspurity atom and, in polar conspounds, Coulomb interaction between the foreign and the crystal ions, can cause a displacement ofthe lattice atoms around the donor site. This lattice distortion generates strain fields which locally affect the hole states. [ 2 } - lleside strain effects, a perturbation of the bound hole wave functions by the donoracceptor Coulonsb interaction has to15a/a be considered. a result acceptor of the inhomogencity of the quadruply As degenerate level is split into two Coulomb doublets field ofa nearby ionized donor, the 3 ]. The analytical expression of the splitting being analogous in the two cases ; it is generally difficult to discriminate between a strain field effect and a Coulomb field effect. The only experimental work has been done on GaP where discrete donor-acceptor (0-A) pair lines can be observed, which allows to study in detail the impurity interactions. In this paper, we apply excitation spectroscopic techniques to investigate the ground state of shallow acceptors in CdTe, where D-A pair lines are usually not observed. A direct neutralization of ionized pairs by transitions between the inspurity ground states permits to detect the splitting and to study it for various pair separafions. EXPERIMENTAL Excitation spectroscopy measurements are conducted on InCd - LiCd pairs in compensated In doped, CdTe at 1.6K. Excitation light is obtained from a tunable DEOTC dye Laser punsped by the red lines from a Kr Laser. The luminescence is analysed by a double grating spectrometer. The Li acceptor , previously characterized from excitation data [ 4 ], is the dominant residual inspurity. Its ionization energy, deduced by use of the effective mass theory of llALDERESCHI-LIPARI [ 5 is EA = 60.7 sneV. The In donor binding energy is evaluated to F 0 = 14.5 meV. The pair separations RDA associated with a given luminescence energy TSWDUM is then fully determined by the usual relation 2/EEDA (i) = EG - ED ÷e
160
0
Vet
—
:•~, —
1 ciii
Ikomr dccc 001 /Jci/rs in Cd Id
for a direct recoinbinat ion or =
c~/~R.
+
br a recoinhination with L() phonon emission. L~is the hand gap energy and (‘onser~uently. the escitation resonant line at =
is the static dielectric constant of the material. speetroni of a selected pair Inmineseence
two
displays a sharp
-
!1W
+
tidLo
svtueh is associated svitli : ii the direct neutralization of 1)—A pairs svitli separation R ‘I) A
j) and indirect neutralization via LU photon emission of 0—A tairs with separation RVA’ I hese processes are schematically deserihed in Fig. ( I) with an energy diagram analogous to ttie one presented in Ret. [6 I. At low energy side of the 0-A tununeseenee hand ) tidTufl 5 202 I, ~ is large and the prohahility br transitions hetsveen IS levels of impurities is low. [tie RDA pairs are principally neutralized hy non selective excitation processes. [lie resonant excitation line at tw1014+tts~0 energy is then only ohtained front R l)A pairs.
H?
RESULt S Luminescence excitation (LI-..) spectra of Li-In pairs are t tshown two svlien sharp peaks tahelled t and in(2 Fig. are (2). ohsersed
___________________________________
15 ~
I
2
A
D
5
415
5
the energy difterence hetween luminescence and excitation eqnats an optical ptinnon. They appear upon a background signal related to non selective pair neutralization. WIute the resonant tine intensity remains nearly constant, the haekgrnund associated with the ROA pair
LiM~
5 ~
~LO
Innuneseenee increases when the selected lonuneseenee energy raises up to S — 2A — 2 Line (I) is precisely located at the energy (1) . and line (2) at The 270 = ~WLUv + ~d20 = ~ + ~ + A where A is a
fita,~
~
+
.
~
I
p
.
p
__________________________________
function of the interimpurity distance. These
Pair Separation
‘
~DA
lines (I) and (2) are attrihuted to the direct neutraheation of RI-iA pairs, the final state of 51=. (1 the hole being one of the two ground state sublevels. At I .0K the hole excited to - ttie higher suhtevel is rapidly thermahzed to the lower suhlevet before
reeonihinalion to
.
When
+tisj LOS
A IS 2
20
is
split the
—E 0
—2 A
relation
+e/ch 0
12) is niodified
and heroines
—~A(h) 0
2
a
where R0 is slightly different from R’DA. The energies of the excitation peaks are ~(1,2) 270
=
2
—2
0
—2
A
+
D
e/d0
± A(s )/‘ 0
n
(1)
Ci. -Vi’u ci uf
A
/ Donor acceptor puim in
e=eoA 790
I I
161
[nell
800
810
fcli’e
I
F
2
~
—
‘:
R=62A
1~
E
~
10 20 30 ENERGY DIFFERENCE BETWEEN LUMINESCENCE AND EXCITATION )meV[ Fig.
1,52
1,54
l,ss
‘hüj
1
lEVI
Fig. (3)
(2)
An other experimental nsethod to study the A 15 splitting is to use the selective pair luminescence (SPL) technique [6,7) the pair luminescence spectra are recorded with the excitation light kept at a tixed energy t~~70 . For simplicity, we suppose that the splitting energy A depends only on the magnitude of the pair separahon. Two groups of 0-A pairs with interiuspurity distance Ra and Rb can then be neutralized by the incident light, according to the sublevel occupied by the hole in the final state. Ra and Rb are defined by to and
BXO
=
2
0
—
2
A
—
E
0
+
u/eB
a
—
~A(F ( 2 a
(3)
= 2~ — E2— E~ + e2/Rb + ~A(Bb) (6) The two SPL spectra which are reproduced in Fig. 3, are excited at an energy corresponding to the direct neutralization of pairs with mean interimpurity distances of 62 A and 80 A. respectively. The LO phonon replica of the excitation line is split into lines (a) and (b) when the b2A 0-A pairs are
excited. These lines are associated with the Ra and Rb pair recomhination. Since A is very small when R’DA = 80 A, the lines (a) and (b) are not resolved in this ease. D1SCUSSION The results of the measurements by L.L. of the acceptor ground state splitting are reported in Fig. 4. For pair separations varying between 50 and 70 A, A is nearly proportional to ( t/R’DA)a within experimental errors. Strain field effecta. After Ref. [1), when the acceptor Bohr radius is small compared with the impurity separation, the expression for A beconses 3) (A + B p2(~))h/2 (~) A/2 = D’u (v/o
162
Ci. \ in et al.
Donor aci -ep (or pair.v iii (die
where A and B are nsaterial constants and depend on the deformation potenhial constants of the valence band Vu and D’u. We use Vu -1.05 eV and Vu = -4,3 eV as niean values in li-VI compounds [8). V is proporhional to the displacements of ttie nearest neighbors of the donors. Q (R) depends upon the direction of 0-A pairs in the lattice. For pair separations larger than 50 A an isotropic mean value Q 0.26 can he used [9). For a splitting A = 1meV. the corresponding 0-A pair separation is 22 A in GaP and 50 A in CdTe. 1Ius long range interaction in CdTe leads to a 3large value for he the which is to quantihv Vl)’u eVA eVX~ in GaP. The compared to VD’u 55 10.5
0,8 ~
—
~
4
—
—
deduced displacements (~dhX) of the nearest neighbors of the donor are too large in CdTe to be accepted. Electric Jield el/errs. At ter Ref. [2), the expression ot the splitting is similar to Eq. (7 =
•‘/O)(2./FH(%’+2.’~-5I~
I 60
0~24Q
~
..
8 0 (A)
-
IS)
where the values of the distance K and of the constants A’ and B’ are unfortunately unknown. However. the long range interaction observed in CdTe can he associated to the large donor Bohr radius. This the dielectric screening of the Coulomb field by the bound electron is less imtortant in CdTe tItan in GaP. An important consequence of the acceptor ground state splitting is that the reeombination transition of a pair of given Ri overlaps with the direct excitation of another pair having a slightly larger interinspunty distance R 2. By thus process, excitation energy can be transfered from close to distant pairs leading to a broadening on the ugh energy side of the L.E. peaks . In this way. the characteristic slowing down shape of the excitation spectrum of close pairs observed just above the resonant condition = ta Lil’,l can he explained. CONCLUSION L.E. and SPL techniques allow us to reveal the splitting of the Li acceptor ground state in CdTe for 1)-A pair separations larger than 50 A. Experimental data seems not to he accounted for by local strain fields surrounding the donors. C’oulonibic type perturbation is more likely. Similar experiments have been conducted on ZnSe [10) where a splitting of the 2S state has been observed, these experiments should be extended to other semicondsictors having various Bohr radii. With more data, a definite explanation ofthe origin of the acceptor state splitting could he obtained.
REFERENCES [11 TN. MORGAN and H. MAtER. Phys. Rev. Lett. 27, 1200)1971) 2) T.N. MORGAN, in Proceedings of the XI Internahional Conference on the Physics of Semiconductors, WARSAW (1972) ~3) V.G. BARU, V.A. PETROV and V.B. SANDOMIRSKII. Soy. Phys. Senucond. 9. 1344 (1976) 14) G. NEU, Y.MARFA1NG, R. LEGROS, R. TRIBOULET and L. SVOR. J. of Luminescence 21. 293 (1980) [51 A. BALDERESCI-il and NO. LIPAR!. Phys. Rev. B9, 1525 (1974) 161 H. TEWS, H. VENGHAUS and P.J. DEAN. Phys.Rev. B19, 5178 (1979) [71 S. NAKASHtMA, T. HATTOTI and Y. YAMAGUCHI. Solid. State Conimun. 25. 137 (1978) 181 0. KRANZER. J. Phys. (Solid state phys.) 6,2977(1973) [9J G, NEU. Thesis, University Pet M Curie, Paris 1981 (unpublished) Ito) H. TEWS and G. NEU. To be published in Phys. Rev. B.