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Observation of light-induced current from a porous silicon device
310
Materials Chemistry and Physics, 32 (1992) 310-314
Observation H. Zimmermann,
of light-induced F. H. Cocks
and
current from a porous silicon device
U. Gosele
Deparhnent of Mechanical Engineering and Materials Science, Duke University, Durham NC 27706 (USA) (Received
May 5, 1992)
Abstract Many technological and scientific applications of porous silicon arise from its unique properties, which include photoluminescence and electroluminescence. Here we report for the first time the observation of a photocurrent from a micro-porous silicon device. The wavelength dependence of the photocurrent from the porous silicon device is shown and compared to the photoluminescence spectrum. A high-concentration phosphorus profile between a metal layer and a layer of p-type porous silicon is used to form an electrical contact to the porous p-type silicon. The phosphorus profile results in a n-type porous layer and in a p/n-junction in the porous silicon. The resulting n+/p-structure possesses a much lower electrical resistance than a metal contact directly on p-type porous silicon. The current voltage characteristic of the device shows ohmic current limitation.
Introduction
Experimental
It has been known since the development of the first silicon-based semiconductor devices that a spongy layer of porous silicon will form on singlecrystalline silicon in hydrofluoric acid under an anodic bias [l, 21. However, the main technological interest in this material is at first consisted in its application in producing fully isolated, porous, oxidized silicon layers (FIPOS) [3]. In this context, many studies of the structural [4-71, material [&lo], and interfacial [ll-131 properties of porous silicon have been carried out. Recently, it has been observed that the optical properties of porous silicon differ drastically from those of bulk crystalline silicon [14,15]. From the increased bandgap compared to bulk crystalline silicon, it has been concluded that the formation of porous silicon is associated with a quantum wire effect [14]. Simultaneously, strong room-temperature photoluminescence in the visible range was detected from porous silicon [15]. Light emission during anodization, has also been found [16] and initial examples of electroluminescence were reported [17, 181. However, the quantum efficiency is presently too low for technological application as light emitting diodes. Our intention, therefore, was to investigate whether a photocurrent, the reverse process of electroluminescence, can be generated in a porous silicon device.
In our experiments we used a n+/p-structure. The boron doped substrate had a resistivity between 2 and 8 IR cm. The sheet resistance of the n+-phosphorus layer was 38.2 a/square. The porous silicon layer was grown in a 1:l (98 wt.% ethanol: 48 wt.% hydrofluoric acid) electrolyte. Gallium was rubbed onto the base of the sample in order to obtain a homogeneous current distribution during anodization. A platinum electrode in the ethanol-hydrofluoric-electrolyte served as a cathode during the anodization of the silicon sample. A power supply was used in a constant current mode with a current density of 45 mA crn2. The anodization was performed in a teflon container under room light. The current was applied for 5 minutes and the sample was then rinsed in deionized water. Immediately after drying, the sample was transferred to the vacuum chamber of a sputter-coater, and a semitransparent gold/ palladium (60:40) film of 18 nm thickness was deposited. The sample then was soldered with indium/tin to a contact plate to form an ohmic contact to the substrate. A gold wire with a drop of indium/gallium eutectic was used to contact the gold/palladium spot, which was about 2.5 mm in diameter. Figure 1 shows the structure of the porous silicon device. The diameter of the porous silicon island was approximately 3.5 mm. The thickness of the porous silicon layer can be estimated to be about 12 pm according to [14].
0254-0584/92/$5.00
0 1992 - Elsevier
Sequoia.
All rights reserved
311
A.,
._
Au
+
IniGa 1.0.
d
00
0
0 +
0.8-
poroos reference
0
+
+ 0
+ 0
+
s 0.6. 5
E
o+
0
0
0.4-
?0 +
+ 0.2 -
+t :
+
Fig. 1. The schematic
0.04 b + 300
i
i
structure
of the porous silicon device.
A n’/p-structure was chosen because metal on porous silicon that has been formed on p-type substrates results in a contact resistance in the 10 Ma-1 EC.! range. Hafnium, which forms a Schottky contact on p-type crystalline silicon [19], as well as gold, which forms a Schottky contact on n-type silicon [20], also showed a similar very high resistance on porous silicon that had been produced using lightly doped p-type silicon. In order to measure the photocurrent as a function of the wavelength of incoming light, the finished device was placed in the sample chamber of a Shimadzu RF 5OOOU spectrofluorometer. A 150 W xenon lamp served as the light source. The excitation slit width determined a bandwidth of 20 nm for the monochromatic light. The photocurrent of the device was measured with a Keithley 177 Microvolt digitai multimeter in the d.c.-current range of 20 PA. The photoluminescence of an identically anodized n+/p-structure also was examined. This spectrum was taken using appro~mately 16 mW of focused 514.5 nm radiation from an argon laser. The current-voltage characteristics of the porous silicon device and of a non-anodized (normal, crystalline) reference sample were measured with a Hewlett-Packard 4145 semiconductor analyzer. The sample was kept in the dark during this measurement.
Results and discussion The dependence of the photocurrent on the photon wavelength was examined for the porous silicon device. Figure 2 shows the resulting spectrum at room temperature. The Au/Pd contact showed a negative voltage compared to the ohmic contact to the p-substrate. For comparison the spectrum of a non-anodized (normal, crystalline) reference sample is also shown. Both spectra were
0
, 500
. 700
, 900
1 )O
Wavelength (nm) Fig. 2. Normalized photocurrent of the porous silicon device (+ : 1,,=3.1 PA) shown in Fig. 1 and of a non-anodized n+preference sample (0: Imax= 73 PA) as a function of light wavelength.
corrected for the spectral intensity distribution of the xenon lamp. The largest photocurrent from the porous silicon device was obtained for a wavelength of 650 nm, whereas the maximum current of the reference sample was observed between 500 and 5.50 nm. The width at half maximum is approximately 270 nm for the porous silicon device and about 380 nm for the reference sample. One possible explanation of the difference in these maxima may be that in the porous silicon there exists a nonradiative relaxation path for electronhole pairs generated by photons of wavelength shorter than appro~ately 600 nm. As another possibility, the loss of a part of the carrier energy and the emission of photoluminescence due to carrier recombination can be mentioned. This process then would possess a much larger probability than the generation of a photocurrent for wavelengths shorter than about 600 nm. A different explanation might be that the electron-hole pairs generated in quantum wires whose diameter corresponds to wavelengths shorter than about 600 nm cannot separate in such thin wires. A slow decrease of the photocurrent with time was observed for the porous silicon device, most likely resulting from an oxygen reaction that is catalyzed by light on the surface of the quantum structures. A decreasing intensity of the photoluminescence has been explained by a light induced reaction with oxygen [21). The phot~urrent decreased by about 10% in 5 minutes for a wavelength of 550 nm. The photoluminescence of an identically anodized n+/p-structure was measured at room temperature (Fig. 3). For the porous n”/p-structure, the m~imum of the photoluminescence at 760 nm occurs at the same wavelength as that of p-
312
I
300
I
500
I
I
I
I
700 900 Wavelength (nm)
Fig. 3. Photoluminescence porous p/n-junction.
spectrum
I
1100
of an identically
anodized
type porous silicon anodized under identical conditions. The intensity of the photoluminescence from the n+/p-structure is comparable to p-type porous silicon within a factor of two. The width at half maximum of the photoluminescence spectrum for the n+/p-sample is approximately 190 nm. Compared to the maximum at 760 nm in the photoluminescence spectrum of Fig. 3, the maximum in the photocurrent is seen to be shifted by 110 nm to shorter wavelengths. According to the observations of Zhang [22], n-type porous silicon contains pores and columns with dimensions in the 0.1-l pm-range, whereas the photoluminescence and the light induced current originates from the p-type porous layer with typical pore and column dimensions in the nm range, which is consistent with the bandgap-widening by about 0.5 eV. The different wavelength of the maxima in photoluminescence and photocurrent possibly can be explained by inhomogeneities in the thickness of the quantum-size columns in the p-type porous layer and a variation in the position of the resulting energy levels. The columns with typical dimensions of 2 to 3 nm may approximately be considered as quantum wires with a square cross section. The energy of the first quantum level in a wire with a square cross section and a side length d increases when the width of the column decreases according to:
to flow through the wires with side length d. Figure 4 schematically shows a likely band structure in the porous silicon device resulting from thickness variations along the quantum size wires. In order to overcome the barriers and to reach the n+layer or the p-substrate, respectively, the carriers have to be generated by light-quanta of a higher energy compared to the photoluminescence from the narrow bandgap regions in the band structure of the p-type porous layer. Based on this explanation the thickness variation of the quantum-size columns can be estimated from the difference in the maxima of the photocurrent and photoluminescence spectra. With the difference in these maxima being 110 nm, a thickness variation of 2 to 3 atomic layers can be thus estimated. It should be noted that this explanation, however, cannot be used to exclude the possibility of molecules such as siloxene [23] at the surface of columns as the source of photoluminescence. The assumption of molecules at the surface as the source of the photoluminescence and a volume effect as source for the light induced current could also explain the difference in the maxima of photocurrent and photoluminescence. Besides the electro-optical and optical characterization of the porous silicon device, electrical measurements have been carried out. The currentvoltage characteristic of the porous silicon device is shown in Fig. 5. For comparison, the currentvoltage characteristic of a non-anodized (normal, crystalline) reference sample is also shown. The reference sample shows the typical forward conducting branch of a silicon diode. In reverse direction, however, the leakage current is rather high due to a steep doping gradient of the n+layer. The forward current of the porous silicon device increases much more slowly than that of the reference device. The forward current of the porous silicon device is proportional to the voltage for voltages exceeding about 0.6 V. This behavior
EC -
PL
Ev
The symbols h, m*, and d denote Planck’s constant, the effective hole or electron mass, and the thickness of the quantum wire, respectively. AE calculated from eqn. (1) determines the energy barrier AL& for holes and LLE, for electrons
h n-layer
p-type porous
p-Substrate
Fig. 4. Variations in bandgap width of the porous silicon device due to variations in the thickness along the quantum size wires.
313
2Qo_I 1 150
3
100
E 8 5
!
50 i
0
‘i
-50
: -5
!
7
0
of the porous silicon device is smaller than that of the reference device. The reduced reverse current of the porous silicon device at voltages larger than 3.0 V probably results from a wider space charge region in the p-porous part of the p/n-junction compared to the p/n-junction of the reference device. This increased space charge region results from the pores in the porous material, which allow the assumption of an effective dielectric constant that is reduced compared to the dielectric constant of crystalline bulk silicon. The linear dependence of the reverse current on the applied voltage, however, has to be considered as specific for the used n+/p-junction, because different junctions showed non-linear behavior [24]. No electroluminescence was observed from the porous silicon device. Probably, the necessary current density was not reached due to the high electrical resistance of the device. In order to lower the resistivity, a shorter anodization time has to be chosen to reduce the thickness of the p-type porous silicon layer.
current
5
Voltage (V)
Fig. 5. Current voltage characteristics of the porous silicon device [(-), solid line] and of a non-anodized reference sample [(- - -), dashed line]. is characteristic for an ohmic-limited current flow. From the slope of the forward current branch, a resistance around 22.2 ka can be calculated. This resistance probably results from the p-type porous part of the device and from its spongy structure of wires and areas with quantum size dimensions in the nm-range. The results of Zhang [22] that n-porous silicon shows pores and columns with dimensions in the 0.1-l pm range lead to this conclusion. An ohmic limitation of the forward current also was observed for voltages larger than about 8 V for a metal/p-type porous silicon device 1181. Compared to metal contacts on p-type porous silicon with resistances higher than 10 Ma, the electrical resistance was improved by at least three orders of magnitude due to a n+-layer between the metal and the p-type porous silicon. This improvement can be explained by a larger contact area between the thick n+-columns and the metal layer than between the thin p-columns and the metal. As a further reason for the improved contact resistance between metal and the n+-columns the possibility of tunneling through a thin energy barrier between the n+-silicon and the metal layer can be suggested. The barrier between lightly doped, very thin p-wires and metal, however, is much wider and tunneling is not likely. The reverse current of the porous silicon device also shows a linear increase with the applied voltage. The slope, however, is much smaller than that of the forward current. A resistance of 120 kfl can be calculated for the reverse direction. The difference in the reverse current branch for the porous silicon device and for the reference device is much smaller than in the forward current branches. For voltages larger than 3.0 V the reverse
Conclusions The following conclusions can be drawn from our results. (i) Light induced current was observed in a porous silicon device. The difference in the wavelength dependence of light-induced current compared to that of photoluminescence is explained by thickness variations along quantum wires. This light induced current decreases with time. (ii) The electrical contact to a p-type porous silicon layer is improved considerably by using a n +-layer between the metal contact and the ptype porous silicon. The current-voltage characteristic of the porous silicon device showed ohmic current limitation in the forward direction. (iii) For a technological application of porous silicon as a light or current source the two major problems, which will have to be overcome, are high resistance and poor long term stability.
Acknowledgements The authors are grateful to P. Enquist from Research Triangle Institute for photoluminescence measurements. They also would like to thank B. Faust from the School of the Environment at Duke University for the possibility to use their spectrofluorometer. One of the authors (H.Z) was financially supported by the Alexander von Hum-
314
boldt-Foundation. Financial support by the Mobil Foundation is also acknowledged.
References 1 A. Uhlir, Bell Syst. Technol. J., 35 (1956) 333. 2 D. R. Turner, J. Electrochem. Sot., 105 (1958) 402. 3 K. Imai and H. Unno, IEEE Trans. Electron Device, ED-31 (1984) 297. 4 M. I. J. Beale, J. D. Benjamin, M. J. Uren, N. G. Chew and A. G. Cullis, J. Cryst. Growth, 73 (1985) 622. 5 T. Unagami and M. Seki, J. Electrochem. Sot., 125 (1978) 1339. 6 K. Barla, R. Herino, G. Bomchil, J. C. Pfister and A. Freund, J. Ciyst. Growth, 68 (1984) 727. 7 H. Sugiyama and 0. Nittono, J. Ctyst. Growth, 103 (1990) 156. 8 Y. Arita and Y. Sunohara, J. Electrochem. Sot., 124 (1977) 285.
9 R. W. Hardeman, M. I. J. Beale, D. B. Gasson, J. M. Keen, C. Pickering and D. J. Robbins, Su$ Sci., 152/153 (1985) 1051. 10 Y. Kato, T. Ito and A. Hiraki, Jpn. J. Appl. Phys., 27 (1988) L1406. 11 R. Herino, P. Jan and G. Bomchil, J. Electrochem. Sot., 132 (1985) 2513. 12 T. Ito and A. Hiraki, Jpn. J. Appl. Phys., 26 (1987) 1219. 13 T. Ito, T. Yasumatsu, H. Watabe and A. Hiraki, Jan. J. A&. __ Phys., ‘29 (1990) L201. 14 V. Lehmann and U. Gosele, Appl. Phys. Lett., 58 (1991) 856. 15 L. T. Canham, Appl. Phys. Lett., 57 (1990) 1046. 16 A. Halimaoui, C. Oules and G. Bomchil, Appl. Phys. Lett., 59 (1991) 304. 17 A. Richter, P. Steiner, F. Kozbwski and W. Lang, IEEE Electron Device Lett., 12 (1991) 691. 18 N. Koshida and H. Koyatna, Abpl. Phys. Lett., 60 (1992) 347. 19 A. N. Saxena. ADDI. Phvs. Lett.. 19 (1971) 71. z%I S. M. Sze, Phykci if Semkonducior Devices; Wiley, New York, 1981, p. 291. 21 Z. Y. Xu, M. Gal and M. Gross, Appl. Phys. Lett., 60 (1992) 1375. 22 X. G. Zhang, J. Electrochem. Sot., 138 (1991) 3750. 23 M. S. Brandt, H. D. Fuchs, M. Stutzmann, J. Weber and M. Cardona, Solid State Commun., 81 (1992) 307. 24 H. Zimmermann and P. Enquist, in Appl. Phys. Lett.
Materials Chemistry and Physics, 32 (1992) 310-314
Observation H. Zimmermann,
of light-induced F. H. Cocks
and
current from a porous silicon device
U. Gosele
Deparhnent of Mechanical Engineering and Materials Science, Duke University, Durham NC 27706 (USA) (Received
May 5, 1992)
Abstract Many technological and scientific applications of porous silicon arise from its unique properties, which include photoluminescence and electroluminescence. Here we report for the first time the observation of a photocurrent from a micro-porous silicon device. The wavelength dependence of the photocurrent from the porous silicon device is shown and compared to the photoluminescence spectrum. A high-concentration phosphorus profile between a metal layer and a layer of p-type porous silicon is used to form an electrical contact to the porous p-type silicon. The phosphorus profile results in a n-type porous layer and in a p/n-junction in the porous silicon. The resulting n+/p-structure possesses a much lower electrical resistance than a metal contact directly on p-type porous silicon. The current voltage characteristic of the device shows ohmic current limitation.
Introduction
Experimental
It has been known since the development of the first silicon-based semiconductor devices that a spongy layer of porous silicon will form on singlecrystalline silicon in hydrofluoric acid under an anodic bias [l, 21. However, the main technological interest in this material is at first consisted in its application in producing fully isolated, porous, oxidized silicon layers (FIPOS) [3]. In this context, many studies of the structural [4-71, material [&lo], and interfacial [ll-131 properties of porous silicon have been carried out. Recently, it has been observed that the optical properties of porous silicon differ drastically from those of bulk crystalline silicon [14,15]. From the increased bandgap compared to bulk crystalline silicon, it has been concluded that the formation of porous silicon is associated with a quantum wire effect [14]. Simultaneously, strong room-temperature photoluminescence in the visible range was detected from porous silicon [15]. Light emission during anodization, has also been found [16] and initial examples of electroluminescence were reported [17, 181. However, the quantum efficiency is presently too low for technological application as light emitting diodes. Our intention, therefore, was to investigate whether a photocurrent, the reverse process of electroluminescence, can be generated in a porous silicon device.
In our experiments we used a n+/p-structure. The boron doped substrate had a resistivity between 2 and 8 IR cm. The sheet resistance of the n+-phosphorus layer was 38.2 a/square. The porous silicon layer was grown in a 1:l (98 wt.% ethanol: 48 wt.% hydrofluoric acid) electrolyte. Gallium was rubbed onto the base of the sample in order to obtain a homogeneous current distribution during anodization. A platinum electrode in the ethanol-hydrofluoric-electrolyte served as a cathode during the anodization of the silicon sample. A power supply was used in a constant current mode with a current density of 45 mA crn2. The anodization was performed in a teflon container under room light. The current was applied for 5 minutes and the sample was then rinsed in deionized water. Immediately after drying, the sample was transferred to the vacuum chamber of a sputter-coater, and a semitransparent gold/ palladium (60:40) film of 18 nm thickness was deposited. The sample then was soldered with indium/tin to a contact plate to form an ohmic contact to the substrate. A gold wire with a drop of indium/gallium eutectic was used to contact the gold/palladium spot, which was about 2.5 mm in diameter. Figure 1 shows the structure of the porous silicon device. The diameter of the porous silicon island was approximately 3.5 mm. The thickness of the porous silicon layer can be estimated to be about 12 pm according to [14].
0254-0584/92/$5.00
0 1992 - Elsevier
Sequoia.
All rights reserved
311
A.,
._
Au
+
IniGa 1.0.
d
00
0
0 +
0.8-
poroos reference
0
+
+ 0
+ 0
+
s 0.6. 5
E
o+
0
0
0.4-
?0 +
+ 0.2 -
+t :
+
Fig. 1. The schematic
0.04 b + 300
i
i
structure
of the porous silicon device.
A n’/p-structure was chosen because metal on porous silicon that has been formed on p-type substrates results in a contact resistance in the 10 Ma-1 EC.! range. Hafnium, which forms a Schottky contact on p-type crystalline silicon [19], as well as gold, which forms a Schottky contact on n-type silicon [20], also showed a similar very high resistance on porous silicon that had been produced using lightly doped p-type silicon. In order to measure the photocurrent as a function of the wavelength of incoming light, the finished device was placed in the sample chamber of a Shimadzu RF 5OOOU spectrofluorometer. A 150 W xenon lamp served as the light source. The excitation slit width determined a bandwidth of 20 nm for the monochromatic light. The photocurrent of the device was measured with a Keithley 177 Microvolt digitai multimeter in the d.c.-current range of 20 PA. The photoluminescence of an identically anodized n+/p-structure also was examined. This spectrum was taken using appro~mately 16 mW of focused 514.5 nm radiation from an argon laser. The current-voltage characteristics of the porous silicon device and of a non-anodized (normal, crystalline) reference sample were measured with a Hewlett-Packard 4145 semiconductor analyzer. The sample was kept in the dark during this measurement.
Results and discussion The dependence of the photocurrent on the photon wavelength was examined for the porous silicon device. Figure 2 shows the resulting spectrum at room temperature. The Au/Pd contact showed a negative voltage compared to the ohmic contact to the p-substrate. For comparison the spectrum of a non-anodized (normal, crystalline) reference sample is also shown. Both spectra were
0
, 500
. 700
, 900
1 )O
Wavelength (nm) Fig. 2. Normalized photocurrent of the porous silicon device (+ : 1,,=3.1 PA) shown in Fig. 1 and of a non-anodized n+preference sample (0: Imax= 73 PA) as a function of light wavelength.
corrected for the spectral intensity distribution of the xenon lamp. The largest photocurrent from the porous silicon device was obtained for a wavelength of 650 nm, whereas the maximum current of the reference sample was observed between 500 and 5.50 nm. The width at half maximum is approximately 270 nm for the porous silicon device and about 380 nm for the reference sample. One possible explanation of the difference in these maxima may be that in the porous silicon there exists a nonradiative relaxation path for electronhole pairs generated by photons of wavelength shorter than appro~ately 600 nm. As another possibility, the loss of a part of the carrier energy and the emission of photoluminescence due to carrier recombination can be mentioned. This process then would possess a much larger probability than the generation of a photocurrent for wavelengths shorter than about 600 nm. A different explanation might be that the electron-hole pairs generated in quantum wires whose diameter corresponds to wavelengths shorter than about 600 nm cannot separate in such thin wires. A slow decrease of the photocurrent with time was observed for the porous silicon device, most likely resulting from an oxygen reaction that is catalyzed by light on the surface of the quantum structures. A decreasing intensity of the photoluminescence has been explained by a light induced reaction with oxygen [21). The phot~urrent decreased by about 10% in 5 minutes for a wavelength of 550 nm. The photoluminescence of an identically anodized n+/p-structure was measured at room temperature (Fig. 3). For the porous n”/p-structure, the m~imum of the photoluminescence at 760 nm occurs at the same wavelength as that of p-
312
I
300
I
500
I
I
I
I
700 900 Wavelength (nm)
Fig. 3. Photoluminescence porous p/n-junction.
spectrum
I
1100
of an identically
anodized
type porous silicon anodized under identical conditions. The intensity of the photoluminescence from the n+/p-structure is comparable to p-type porous silicon within a factor of two. The width at half maximum of the photoluminescence spectrum for the n+/p-sample is approximately 190 nm. Compared to the maximum at 760 nm in the photoluminescence spectrum of Fig. 3, the maximum in the photocurrent is seen to be shifted by 110 nm to shorter wavelengths. According to the observations of Zhang [22], n-type porous silicon contains pores and columns with dimensions in the 0.1-l pm-range, whereas the photoluminescence and the light induced current originates from the p-type porous layer with typical pore and column dimensions in the nm range, which is consistent with the bandgap-widening by about 0.5 eV. The different wavelength of the maxima in photoluminescence and photocurrent possibly can be explained by inhomogeneities in the thickness of the quantum-size columns in the p-type porous layer and a variation in the position of the resulting energy levels. The columns with typical dimensions of 2 to 3 nm may approximately be considered as quantum wires with a square cross section. The energy of the first quantum level in a wire with a square cross section and a side length d increases when the width of the column decreases according to:
to flow through the wires with side length d. Figure 4 schematically shows a likely band structure in the porous silicon device resulting from thickness variations along the quantum size wires. In order to overcome the barriers and to reach the n+layer or the p-substrate, respectively, the carriers have to be generated by light-quanta of a higher energy compared to the photoluminescence from the narrow bandgap regions in the band structure of the p-type porous layer. Based on this explanation the thickness variation of the quantum-size columns can be estimated from the difference in the maxima of the photocurrent and photoluminescence spectra. With the difference in these maxima being 110 nm, a thickness variation of 2 to 3 atomic layers can be thus estimated. It should be noted that this explanation, however, cannot be used to exclude the possibility of molecules such as siloxene [23] at the surface of columns as the source of photoluminescence. The assumption of molecules at the surface as the source of the photoluminescence and a volume effect as source for the light induced current could also explain the difference in the maxima of photocurrent and photoluminescence. Besides the electro-optical and optical characterization of the porous silicon device, electrical measurements have been carried out. The currentvoltage characteristic of the porous silicon device is shown in Fig. 5. For comparison, the currentvoltage characteristic of a non-anodized (normal, crystalline) reference sample is also shown. The reference sample shows the typical forward conducting branch of a silicon diode. In reverse direction, however, the leakage current is rather high due to a steep doping gradient of the n+layer. The forward current of the porous silicon device increases much more slowly than that of the reference device. The forward current of the porous silicon device is proportional to the voltage for voltages exceeding about 0.6 V. This behavior
EC -
PL
Ev
The symbols h, m*, and d denote Planck’s constant, the effective hole or electron mass, and the thickness of the quantum wire, respectively. AE calculated from eqn. (1) determines the energy barrier AL& for holes and LLE, for electrons
h n-layer
p-type porous
p-Substrate
Fig. 4. Variations in bandgap width of the porous silicon device due to variations in the thickness along the quantum size wires.
313
2Qo_I 1 150
3
100
E 8 5
!
50 i
0
‘i
-50
: -5
!
7
0
of the porous silicon device is smaller than that of the reference device. The reduced reverse current of the porous silicon device at voltages larger than 3.0 V probably results from a wider space charge region in the p-porous part of the p/n-junction compared to the p/n-junction of the reference device. This increased space charge region results from the pores in the porous material, which allow the assumption of an effective dielectric constant that is reduced compared to the dielectric constant of crystalline bulk silicon. The linear dependence of the reverse current on the applied voltage, however, has to be considered as specific for the used n+/p-junction, because different junctions showed non-linear behavior [24]. No electroluminescence was observed from the porous silicon device. Probably, the necessary current density was not reached due to the high electrical resistance of the device. In order to lower the resistivity, a shorter anodization time has to be chosen to reduce the thickness of the p-type porous silicon layer.
current
5
Voltage (V)
Fig. 5. Current voltage characteristics of the porous silicon device [(-), solid line] and of a non-anodized reference sample [(- - -), dashed line]. is characteristic for an ohmic-limited current flow. From the slope of the forward current branch, a resistance around 22.2 ka can be calculated. This resistance probably results from the p-type porous part of the device and from its spongy structure of wires and areas with quantum size dimensions in the nm-range. The results of Zhang [22] that n-porous silicon shows pores and columns with dimensions in the 0.1-l pm range lead to this conclusion. An ohmic limitation of the forward current also was observed for voltages larger than about 8 V for a metal/p-type porous silicon device 1181. Compared to metal contacts on p-type porous silicon with resistances higher than 10 Ma, the electrical resistance was improved by at least three orders of magnitude due to a n+-layer between the metal and the p-type porous silicon. This improvement can be explained by a larger contact area between the thick n+-columns and the metal layer than between the thin p-columns and the metal. As a further reason for the improved contact resistance between metal and the n+-columns the possibility of tunneling through a thin energy barrier between the n+-silicon and the metal layer can be suggested. The barrier between lightly doped, very thin p-wires and metal, however, is much wider and tunneling is not likely. The reverse current of the porous silicon device also shows a linear increase with the applied voltage. The slope, however, is much smaller than that of the forward current. A resistance of 120 kfl can be calculated for the reverse direction. The difference in the reverse current branch for the porous silicon device and for the reference device is much smaller than in the forward current branches. For voltages larger than 3.0 V the reverse
Conclusions The following conclusions can be drawn from our results. (i) Light induced current was observed in a porous silicon device. The difference in the wavelength dependence of light-induced current compared to that of photoluminescence is explained by thickness variations along quantum wires. This light induced current decreases with time. (ii) The electrical contact to a p-type porous silicon layer is improved considerably by using a n +-layer between the metal contact and the ptype porous silicon. The current-voltage characteristic of the porous silicon device showed ohmic current limitation in the forward direction. (iii) For a technological application of porous silicon as a light or current source the two major problems, which will have to be overcome, are high resistance and poor long term stability.
Acknowledgements The authors are grateful to P. Enquist from Research Triangle Institute for photoluminescence measurements. They also would like to thank B. Faust from the School of the Environment at Duke University for the possibility to use their spectrofluorometer. One of the authors (H.Z) was financially supported by the Alexander von Hum-
314
boldt-Foundation. Financial support by the Mobil Foundation is also acknowledged.
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