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Electric-field controllable photoluminescence in porous silicon
Solid State Communications, Vol. 86, No. 9, pp. 593-596, 1993. Printed in Great Britain.
0038-1098/93 $6.00 + .00 Pergamon Press Ltd
ELECTRIC-FIELD CONTROLLABLE PHOTOLUMINESCENCE IN POROUS SILICON J.T. Lue, K.Y. Lo,* S.K. Ma, C.L. Chen and C.S. Chang Department of Physics and Department of Electrical Engineering,* National Tsing Hua University, Hsinchu, Taiwan, ROC
(Received 28 October 1992; in revised form 22 December 1992 by H. Kamimura) Intense photoluminescences (PL) at wavelengths near 600nm are observed when either the (100) or (I 1 1) surface of the electrochemically etched silicon wafers are illuminated by the 514.5 nm argon laser line. A fascinating phenomenon has been discovered indicating that the PL intensity can be suppressed exhaustively by applying an electric field parallel to the surface. The PL recovers its intensity very slowly when the bias is taken off, suggesting that the slow relaxation of the accumulate charges inside the porous silicon.
IN THIS LETTER, we are the first to report a peculiarly switching-off phenomenon of the photoluminescence (PL) of porous silicon by an external lateral field. Porous silicon (PS) layers formed electrochemically from crystalline wafers have been shown to emit strongly in the red by UV irradiation [1, 2]. There are many papers concerning the dependence of photoluminescence on the substrate orientations and resistivity, and on the current density and hydrofluoric (HF) acid concentration during electrochemical etching [3]. Electroluminescence from porous silicon has also been explored [4, 5] showing with extremely low quantum efficiency. Diversified proposals for the origin of the PL have been suggested, it can be either due to the hydrogenated amorphous layer formed by hydrofluoric anodization [6], or from quantum confined silicon nanocrystallines embedded in SiO2 layers [7]. Though HF etching is well-known to passivate the silicon surface, leaving hydrogen atoms attached to the silicon dangling bonds near the surface. Unexpectedly, the PL peaks and Raman shifts are not in accordance with those from amorphous silicon. Recent works of Brandt et al. [8] by comparing the similarities of the optical property and the structure of anodically oxidized porous silicon with those of chemically synthesized siloxene (Si603H6) suggest the absorbing or bonding of hetero-atoms or molecules of S i - O - H on the well surface of PS after chemical etching. Presumably, the central frequencies of the PL and Raman lines are determined by the SiO - H bonds attached on the well surface of PS. The blue shift of the PL peaks with the etching time ensures the approvement of quantum size effect.
Occasionally, we find that the PL intensity can be suppressed exhaustively by applying an electric field parallel to the surface. The PL recovers its intensity very slowly (several hours) when the bias is taken off. To demonstrate such a phenomenon, we dwell on the experimental procedures as follows. The unpolished sides of (1 1 1) and (1 0 0) p-type silicon wafers are deposited with silver film by d.c. sputtering for obtaining a good ohmic contact. The PS layers are formed by anodizing these wafers in 48 wt% HF and 98wt% C2HsOH solution with a constant current density of 50 ~ l l 0 M A c m 2 at an etching time of 15 min. The scanning electron micrographs (SEM) of the (1 1 1) and (1 0 0) PS layers are shown in Figs. l(a) and l(b), respectively, which clearly reveal porous structure for the (1 00) surface but not for the (1 1 1) surface. The (1 1 1) surface shows tiny cracks like fish scales. These scales also emit strong electroluminescence by being exposed to the UV light. To detect the PL, the porous silicon is pumped by the 514nm line of the argon laser at room temperature. The d.c. electric bias is applied on the surface through two connecting wires which are indium bonded on the same sides of the PS spot. As shown in Fig. 2, the PL intensity for the (1 1 1) surface decreases as the lateral bias increases, and drops to zero as the bias approaches to 29Vcm -I. When the bias is removed, the PL intensity recovers at a relaxation time of 5 s, and may take one day for fully recovering to its original intensity (see Fig. 3). When the pumped argon laser is turned off, the PL recovering time is faster. To the (1 00) PS surface, the switching-off of the
593
594
ELECTRIC-FIELD CONTROLLABLE PHOTOLUMINESCENCE
Vol. 86, No. 9
2500-
2000-
©
•~
1500-
1000-
c ©
500
0
o
*l,,,,,fllJ""*""""ll"l~l~""l""'l~lPl"l"t""111'"l loo 200 300 400
500
Time (second) Fig. 3. The time dependence of the PL intensity for (1 1 1) PS just after switching off the biasing voltage (29 V in this case).
Fig. 1. The SEM surface morphologies of (a) (1 1 1), and (b) (1 0 0) surfaces of porous silicon.
PL intensity by the lateral bias is much laboured, while it has the same spectra variation with bias as shown in Fig. 2 and will not be duplicated here. As depicted in Fig. 4, the PL intensity diminishes slowly when the applied bias increases toward 30Vcm -1, then it drops to zero at 44 V cm -1. The recovering of the PL for the (1 00) PS is slower than that of(1 1 1) PS. Some samples never recover their PL after being completely switched off. The blue shift in PS comes from the increase of binding energy which is proportional to 1/R 2 by the quantum confinement
2500 25000o
2000 20000 ch
1500
0
X2~
15000
0
~
1ooo
>.., 4~ • ~ 10000 (-
E 5O0
c 5000
6,,ll,~,~
lll,,,~,
,,,,,,,,,
,,,llll~l
50 0
,i
500
Wavelength A Fig. 2. Room-temperature photoluminescence spectra of (1 1 1) PS at various lateral biasing for (a) 0V, (b) 10V, (c) 20V, (d) 22.5V, (e) 25V and (f) 29V, with the electrode distance to be 1 cm. Inset is the geometry of the electrical contacts.
0
0
,,,,,,
'''10
,,,,,,,,,
,,,,,,,,,
20
,'~'"
210
''''
''410
'''""
50
Voltage Fig. 4. The drop of the peak PL intensity with the applied lateral voltage for the (1 1 l) surface of the porous silicon•
ELECTRIC-FIELD CONTROLLABLE PHOTOLUMINESCENCE
Vol. 86, No. 9 1600 -
1200 • c~ k(3 8OO
cG)
C
400
0
, , , , , , , , , i , , , , , , , , , i r , , , , , , , , l ' ' ' ' ' ' ' ' ' b
460
480
500
Wave
520
number
540 ( cm-1
)
Fig. 5. The Raman spectra of the (1 00) PS under 2.41 eV photon excitation at various lateral biasing for (a) 0V, (b) 10V, (c) 20V and (d) 30V, respectively. of a particle size R. In the meantime, we have also measured the Raman shift with the lateral biasing as shown in Fig. 5 which expresses no new outcome by biasing. This Raman line is analysed [9] to be due to the quantum confinement with a characteristic dimension of 3-4 #m and is not suggested to be due to the LO/TO phonon splitting. The switching-off and the slow relaxation of the PL by the lateral bias, could be accounted for by the Stark transitions between the rotational levels within the vibrational states as sketched in Fig. 6. The nanocrystalline silicons might be considered as huge molecules with S i - O - H attached on the surface.
1
b •.
,,.:=v:2a: o
l
-
,
v=O J= 0 nonradiafive decay
B-
595
Within each electronic band, there exist large spaced vibrational levels and finely spaced rotational levels specified by quantum numbers v and J respectively. The dangling bonds of the nanocrystalline Si introduce free spins to PS and is approved by the electron spin resonance spectrum. The molecules in the ground state of the quantum confined nanocrystals supposing to be in the spin singlet S0A state can be pumped optically to the excited Sib state which then decay non-radiatively to the Stn level. Finally, the molecules decay spontaneously to the S0a state with an emission of visible fluorescences. Since Slb and Soa embrace many allowable levels, the fluorescence bandwidth is rather broad. The molecules in the S0~ state, afterward, will relax to the ground state to obviate the reabsorption of the emitted light. The switching-off of the PL in PS can be succinctly addressed as follows. Applying a lateral bias, the injection of electrons will drive the molecules from S0~ to Soa resulting from Stark effect. For rigid linear molecules, the rotational transition energy is [101
E.t = h[BJ(J-F l) - D j 2 ( j + 1)21,
(1)
where J is the sum of rotational and spin quantum numbers and D = 4 B 3 / w 2. The value of B, for diatomic molecules in a state of vibrational quantum number v is
By = Be - Wo(V + 1/2),
(2)
where B e is the electronic equilibrium value, w0 is the fundamental vibrational frequency. Applying an electric field ~ to linear molecules with a permanent dipole # implies a Stark transition from the ground to the rotational excited states with a transition probability proportional to
[
I(JKMj I ~U l J'KMj,)I] 2
3M)[SJ 2 + (6J+ 5) - 4J(J+ 2)I = h2 o2 [J(S+ 2)(2S- l)(2S+ I)(2J+ 3)(ZS+ 5)]B for J # O, 8 /.Z2~2 15h 2u02
for J = 0,
(3)
I _
_
•
A-
Stark effect
-v=l
J=l
: v:O J : 0
Fig. 6. The suggested transition energy bands for the photoluminescence of porous silicon.
where M s is the component of J along the linear direction of the molecule, the selection rule is J - J ' = 1, and v0 is the transition energy at zero field. The transition probabilities for molecules in S0A to the S0a state, therefore, will be proportional to the square of the electric field. When the population of molecules in Sod is large enough, the PL will be
596
ELECTRIC-FIELD CONTROLLABLE PHOTOLUMINESCENCE
switched off due to the depopulation of the ground SoA and the reabsorption of the emitted PL by molecules in S0a. Turning off the bias, the S0~ is depopulated and the PL will be recovered. Since PS has a rather high resistivity, the trapped charges in the porous silicon can hardly be removed and the Stark effect sustains fairly long even switching off the bias. As shown in Fig. 1 the (1 00) PS is much more plausible to trap the carriers within the well bottoms and the structure is much weaker to be broken by bias than the (1 1 1) PS. This can fully elucidate the reason for the slower recovery of the PL in (1 0 0) PS than that in (1 1 1) PS. In conclusion, we have notably demonstrated the switching-off of the photoluminescence of the fluorescence lower states. These properties may be in favour of ensuing Stark transition in PS and yielding weighty applications in opto-electronic devices.
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
Acknowledgement - This work was supported by the National Science Council of the Republic of China.
Vol. 86, No. 9
L.T. Canham, Appl. Phys. Lett. 57, 1046 (1990). V. Lehmann & U. G6sele, Appl. Phys. Lett. 58, 856 (1991). A. Halimaoui, C. Ovles & G. Bomchil, Appl. Phys. Lett. 59, 15 (1991). N. Koshida & H. Koyama, Jpn J. Sppl. Phys. 30, L1221 (1991). N. Koshida & H. Koyama, Appl. Phys. Lett. 60, 20 (1992). C. Pickering, M.I.J. Beale, D.J. Robbins, P.J. Pearson & R. Greef, J. Phys. C: Solid State Phys. 17, 6534 (1984). J.C. Vial, A. Bsiesy, F. Gaspard, R. Herino, M. Ligeon, F. Muller & R. Romestain, Phys. Rev. B45, 14 171 (1992). M.S. Brandt, H.D. Fuchs, M. Stutzmann, J. Weber & M. Cardona, Solid State Commun. 81, 307 (1992). Z. Sue, P.P. Leong, I.P. Herman, G.S. Higashi & H. Temkin, Appl. Phys. Lett. 60, 2086 (1992). W. Gordy, W.V. Smith & R.F. Tranbarulo, Microwave Spectroscopy, pp. 94-159, Wiley, New York (1953).
0038-1098/93 $6.00 + .00 Pergamon Press Ltd
ELECTRIC-FIELD CONTROLLABLE PHOTOLUMINESCENCE IN POROUS SILICON J.T. Lue, K.Y. Lo,* S.K. Ma, C.L. Chen and C.S. Chang Department of Physics and Department of Electrical Engineering,* National Tsing Hua University, Hsinchu, Taiwan, ROC
(Received 28 October 1992; in revised form 22 December 1992 by H. Kamimura) Intense photoluminescences (PL) at wavelengths near 600nm are observed when either the (100) or (I 1 1) surface of the electrochemically etched silicon wafers are illuminated by the 514.5 nm argon laser line. A fascinating phenomenon has been discovered indicating that the PL intensity can be suppressed exhaustively by applying an electric field parallel to the surface. The PL recovers its intensity very slowly when the bias is taken off, suggesting that the slow relaxation of the accumulate charges inside the porous silicon.
IN THIS LETTER, we are the first to report a peculiarly switching-off phenomenon of the photoluminescence (PL) of porous silicon by an external lateral field. Porous silicon (PS) layers formed electrochemically from crystalline wafers have been shown to emit strongly in the red by UV irradiation [1, 2]. There are many papers concerning the dependence of photoluminescence on the substrate orientations and resistivity, and on the current density and hydrofluoric (HF) acid concentration during electrochemical etching [3]. Electroluminescence from porous silicon has also been explored [4, 5] showing with extremely low quantum efficiency. Diversified proposals for the origin of the PL have been suggested, it can be either due to the hydrogenated amorphous layer formed by hydrofluoric anodization [6], or from quantum confined silicon nanocrystallines embedded in SiO2 layers [7]. Though HF etching is well-known to passivate the silicon surface, leaving hydrogen atoms attached to the silicon dangling bonds near the surface. Unexpectedly, the PL peaks and Raman shifts are not in accordance with those from amorphous silicon. Recent works of Brandt et al. [8] by comparing the similarities of the optical property and the structure of anodically oxidized porous silicon with those of chemically synthesized siloxene (Si603H6) suggest the absorbing or bonding of hetero-atoms or molecules of S i - O - H on the well surface of PS after chemical etching. Presumably, the central frequencies of the PL and Raman lines are determined by the SiO - H bonds attached on the well surface of PS. The blue shift of the PL peaks with the etching time ensures the approvement of quantum size effect.
Occasionally, we find that the PL intensity can be suppressed exhaustively by applying an electric field parallel to the surface. The PL recovers its intensity very slowly (several hours) when the bias is taken off. To demonstrate such a phenomenon, we dwell on the experimental procedures as follows. The unpolished sides of (1 1 1) and (1 0 0) p-type silicon wafers are deposited with silver film by d.c. sputtering for obtaining a good ohmic contact. The PS layers are formed by anodizing these wafers in 48 wt% HF and 98wt% C2HsOH solution with a constant current density of 50 ~ l l 0 M A c m 2 at an etching time of 15 min. The scanning electron micrographs (SEM) of the (1 1 1) and (1 0 0) PS layers are shown in Figs. l(a) and l(b), respectively, which clearly reveal porous structure for the (1 00) surface but not for the (1 1 1) surface. The (1 1 1) surface shows tiny cracks like fish scales. These scales also emit strong electroluminescence by being exposed to the UV light. To detect the PL, the porous silicon is pumped by the 514nm line of the argon laser at room temperature. The d.c. electric bias is applied on the surface through two connecting wires which are indium bonded on the same sides of the PS spot. As shown in Fig. 2, the PL intensity for the (1 1 1) surface decreases as the lateral bias increases, and drops to zero as the bias approaches to 29Vcm -I. When the bias is removed, the PL intensity recovers at a relaxation time of 5 s, and may take one day for fully recovering to its original intensity (see Fig. 3). When the pumped argon laser is turned off, the PL recovering time is faster. To the (1 00) PS surface, the switching-off of the
593
594
ELECTRIC-FIELD CONTROLLABLE PHOTOLUMINESCENCE
Vol. 86, No. 9
2500-
2000-
©
•~
1500-
1000-
c ©
500
0
o
*l,,,,,fllJ""*""""ll"l~l~""l""'l~lPl"l"t""111'"l loo 200 300 400
500
Time (second) Fig. 3. The time dependence of the PL intensity for (1 1 1) PS just after switching off the biasing voltage (29 V in this case).
Fig. 1. The SEM surface morphologies of (a) (1 1 1), and (b) (1 0 0) surfaces of porous silicon.
PL intensity by the lateral bias is much laboured, while it has the same spectra variation with bias as shown in Fig. 2 and will not be duplicated here. As depicted in Fig. 4, the PL intensity diminishes slowly when the applied bias increases toward 30Vcm -1, then it drops to zero at 44 V cm -1. The recovering of the PL for the (1 00) PS is slower than that of(1 1 1) PS. Some samples never recover their PL after being completely switched off. The blue shift in PS comes from the increase of binding energy which is proportional to 1/R 2 by the quantum confinement
2500 25000o
2000 20000 ch
1500
0
X2~
15000
0
~
1ooo
>.., 4~ • ~ 10000 (-
E 5O0
c 5000
6,,ll,~,~
lll,,,~,
,,,,,,,,,
,,,llll~l
50 0
,i
500
Wavelength A Fig. 2. Room-temperature photoluminescence spectra of (1 1 1) PS at various lateral biasing for (a) 0V, (b) 10V, (c) 20V, (d) 22.5V, (e) 25V and (f) 29V, with the electrode distance to be 1 cm. Inset is the geometry of the electrical contacts.
0
0
,,,,,,
'''10
,,,,,,,,,
,,,,,,,,,
20
,'~'"
210
''''
''410
'''""
50
Voltage Fig. 4. The drop of the peak PL intensity with the applied lateral voltage for the (1 1 l) surface of the porous silicon•
ELECTRIC-FIELD CONTROLLABLE PHOTOLUMINESCENCE
Vol. 86, No. 9 1600 -
1200 • c~ k(3 8OO
cG)
C
400
0
, , , , , , , , , i , , , , , , , , , i r , , , , , , , , l ' ' ' ' ' ' ' ' ' b
460
480
500
Wave
520
number
540 ( cm-1
)
Fig. 5. The Raman spectra of the (1 00) PS under 2.41 eV photon excitation at various lateral biasing for (a) 0V, (b) 10V, (c) 20V and (d) 30V, respectively. of a particle size R. In the meantime, we have also measured the Raman shift with the lateral biasing as shown in Fig. 5 which expresses no new outcome by biasing. This Raman line is analysed [9] to be due to the quantum confinement with a characteristic dimension of 3-4 #m and is not suggested to be due to the LO/TO phonon splitting. The switching-off and the slow relaxation of the PL by the lateral bias, could be accounted for by the Stark transitions between the rotational levels within the vibrational states as sketched in Fig. 6. The nanocrystalline silicons might be considered as huge molecules with S i - O - H attached on the surface.
1
b •.
,,.:=v:2a: o
l
-
,
v=O J= 0 nonradiafive decay
B-
595
Within each electronic band, there exist large spaced vibrational levels and finely spaced rotational levels specified by quantum numbers v and J respectively. The dangling bonds of the nanocrystalline Si introduce free spins to PS and is approved by the electron spin resonance spectrum. The molecules in the ground state of the quantum confined nanocrystals supposing to be in the spin singlet S0A state can be pumped optically to the excited Sib state which then decay non-radiatively to the Stn level. Finally, the molecules decay spontaneously to the S0a state with an emission of visible fluorescences. Since Slb and Soa embrace many allowable levels, the fluorescence bandwidth is rather broad. The molecules in the S0~ state, afterward, will relax to the ground state to obviate the reabsorption of the emitted light. The switching-off of the PL in PS can be succinctly addressed as follows. Applying a lateral bias, the injection of electrons will drive the molecules from S0~ to Soa resulting from Stark effect. For rigid linear molecules, the rotational transition energy is [101
E.t = h[BJ(J-F l) - D j 2 ( j + 1)21,
(1)
where J is the sum of rotational and spin quantum numbers and D = 4 B 3 / w 2. The value of B, for diatomic molecules in a state of vibrational quantum number v is
By = Be - Wo(V + 1/2),
(2)
where B e is the electronic equilibrium value, w0 is the fundamental vibrational frequency. Applying an electric field ~ to linear molecules with a permanent dipole # implies a Stark transition from the ground to the rotational excited states with a transition probability proportional to
[
I(JKMj I ~U l J'KMj,)I] 2
3M)[SJ 2 + (6J+ 5) - 4J(J+ 2)I = h2 o2 [J(S+ 2)(2S- l)(2S+ I)(2J+ 3)(ZS+ 5)]B for J # O, 8 /.Z2~2 15h 2u02
for J = 0,
(3)
I _
_
•
A-
Stark effect
-v=l
J=l
: v:O J : 0
Fig. 6. The suggested transition energy bands for the photoluminescence of porous silicon.
where M s is the component of J along the linear direction of the molecule, the selection rule is J - J ' = 1, and v0 is the transition energy at zero field. The transition probabilities for molecules in S0A to the S0a state, therefore, will be proportional to the square of the electric field. When the population of molecules in Sod is large enough, the PL will be
596
ELECTRIC-FIELD CONTROLLABLE PHOTOLUMINESCENCE
switched off due to the depopulation of the ground SoA and the reabsorption of the emitted PL by molecules in S0a. Turning off the bias, the S0~ is depopulated and the PL will be recovered. Since PS has a rather high resistivity, the trapped charges in the porous silicon can hardly be removed and the Stark effect sustains fairly long even switching off the bias. As shown in Fig. 1 the (1 00) PS is much more plausible to trap the carriers within the well bottoms and the structure is much weaker to be broken by bias than the (1 1 1) PS. This can fully elucidate the reason for the slower recovery of the PL in (1 0 0) PS than that in (1 1 1) PS. In conclusion, we have notably demonstrated the switching-off of the photoluminescence of the fluorescence lower states. These properties may be in favour of ensuing Stark transition in PS and yielding weighty applications in opto-electronic devices.
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
Acknowledgement - This work was supported by the National Science Council of the Republic of China.
Vol. 86, No. 9
L.T. Canham, Appl. Phys. Lett. 57, 1046 (1990). V. Lehmann & U. G6sele, Appl. Phys. Lett. 58, 856 (1991). A. Halimaoui, C. Ovles & G. Bomchil, Appl. Phys. Lett. 59, 15 (1991). N. Koshida & H. Koyama, Jpn J. Sppl. Phys. 30, L1221 (1991). N. Koshida & H. Koyama, Appl. Phys. Lett. 60, 20 (1992). C. Pickering, M.I.J. Beale, D.J. Robbins, P.J. Pearson & R. Greef, J. Phys. C: Solid State Phys. 17, 6534 (1984). J.C. Vial, A. Bsiesy, F. Gaspard, R. Herino, M. Ligeon, F. Muller & R. Romestain, Phys. Rev. B45, 14 171 (1992). M.S. Brandt, H.D. Fuchs, M. Stutzmann, J. Weber & M. Cardona, Solid State Commun. 81, 307 (1992). Z. Sue, P.P. Leong, I.P. Herman, G.S. Higashi & H. Temkin, Appl. Phys. Lett. 60, 2086 (1992). W. Gordy, W.V. Smith & R.F. Tranbarulo, Microwave Spectroscopy, pp. 94-159, Wiley, New York (1953).