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Electrical properties of luminescent porous silicon
LUMINESCENCE
Journal of Luminescence 57 (1993) 293—299
JOURNAL OF
Invited paper
Electrical properties of luminescent porous silicon Hideki Koyama and Nobuyoshi Koshida Division of Electronic and Information Engineering, Faculty of Technology, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184, Japan
The relation between the visible luminescence of porous silicon (PS) and its electrical properties has been investigated with an emphasis on carrier transport, photoconductivity, and the effects of an external electric field on the photoluminescence (PL). The PS layers are formed by anodization ofp- or n-type Si wafers in an HF solution in the dark or under illumination. At first, the correlation of the electrical conduction mode with the PL efficiency is shown using a self-supporting PS film as a sample. Next, the photoconduction effects of PS for visible light are characterized for the experimental cells of the form (semitransparent thin Au film/PS/Si substrate/Al contact). Finally, the reversible electrical PL quenching is demonstrated. These results support the hypothesis that the visible luminescence of PS is based on surface-sensitive quantum confinement effects in Si nanocrystallites.
1. Introduction
2. Experimental
To determine the mechanism of visible luminescence of PS [1—3],many characterization studies of the optical, structural, electronic, and interfacial properties of PS have been carried out. However, the relation between the electrical properties and the optical activity of PS has not been clarified, Detailed measurements of the carrier transport in PS, has not been clarified. Detailed measurements of the carrier transport in PS, the photoelectronic response, and the interaction of the electric field and the PL emission would provide useful information for understanding the luminescence mechanism. In addition, the electrical characterization of PS is very important for analyses of the visible electroluminescence (EL) from PS. On the basis of our previous observation of visible EL from PS diodes with a solid state contact [4], the present paper describes the electrical properties of luminescent PS in terms of the temperature dependence of the resistivity, photoconduction characteristics [5], and a field-induced PL quenching [6].
2.1. Sample preparation Nondegenerate p-type or n-type (111) Si wafers (0.018—2OQcm) were cleaned, and then ohmic contacts were formed onto the back side. The PS layers were formed by anodization of these wafers in 20—50% aqueous or ethanoic HF solutions at current densities of 10—100 mA/cm2 for 5—60 mm in the dark or under the illumination by a 500 W tungsten lamp from a distance of 20 cm. The thickness of PS layers is 3—50 ~tm. In some cases, anodized PS samples were illuminated in order to enhance the PL intensity. When it is necessary to evaluate the intrinsic nature of PS without the effects of the Si substrate (such as in the case of measurements of the temperature dependence of the electrical and luminescence properties), self-supporting PS films (20—40 J.tm thick) [7] were prepared by electrochemical separation of anodized PS layers from p-type Si substrates.
2.2. Measurements Correspondence to: Dr. H. Koyama, Division of Electronic and Information Engineering, Faculty of Technology, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184, Japan. 0022-2313/93/S06.O0 © 1993 SSDI 0022-2313(93)EOl l9-L
—
The experiments were carried out on the following subjects.
Elsevier Science Publishers B.V. All rights reserved
294
H. Koyama, N. Koshida / Electrical properties of luminescent porous silicon
(1) Electrical conduction and PL characteristics: Thin Al films were evaporated onto the both sides of self-supporting PS films, and were used as the electrodes. The dark resistivity of PS films along the depth direction was measured as a function of temperature in the range 77—300 K. The temperature dependence of the PL characteristics was also measured for the same PS samples using 325 nm Cd—He laser as an excitation source. The overall spectral resolution of the PL measurement system was less than 5 nm. (2) Photoconduction effect: Anodized PS sampies were immediately transferred into the vacuum chamber, and semitransparent thin Au films were evaporated onto the PS layer surface. The PS layer was excited through the thin Au film by light from a 500 W Xe lamp, and vertical photoconduction was measured under the condition that a positive or negative bias voltage was applied to the Au contact with respect to the Al electrode. The ternperature dependences of the dark- and photocurrents, including the spectral response, were measured as a function of the bias voltage. (3) Electrical effect on the PL characteristics: The effects of the external electric field on the PL characteristics were studied for some PS samples. The experimental cell structure was the same as that used for photoconduction measurements. The PS samples were excited with a 325 nm He—Cd laser through the thin Au film, and the PL spectra and intensity were measured in a N 2 gas atmosphere under the positively or negatively biased condition. To avoid possible changes in the PL characteristics during the measurements, the excitation intensity was adjusted to a minimum level acceptable for the instrument,
3. Results and discussion 3.1. Carrier transport and PL characteristics The dark resistivity of a self-supporting PS film (40 ~smthick) is shown by the dashed curve in Fig. 1 as a function of reciprocal temperature. In the lowand high-temperature regions, the electrical conduction mode of PS shows the hopping type and the thermal activation type, respectively. Near
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Fig. 1. Temperature dependence ofthe resistivity (dashed curve) .
and the PL intensity (solid curve) for a self-supporting PS film (40 jim thick) anodized in a solution of HF(50%): ethanol (99.5%) = 1:1. Information on the substrate and the anodization parameters is also indicated.
room temperature, the band conduction via traps is dominant. The presence of traps was also suggested from the current—voltage characteristics [5]: the slope of the log—log plot of dark- and photocurrents versus the applied voltage is about 1.4, which is different from the value expected from the trapfree conduction mode [8]. The resistivity of PS at room temperature is several orders of magnitude higher than the original value of the Si substrate. The PS layer can be regarded as a semi-insulating semiconductor. This high resistivity is interpreted to be the result of a combination of carrier depletion in Si crystallites, low mobility due to scattering at grain boundaries, suppression of surface conduction due to hydrogen termination, and a band gap widening [7,9,10]. Depletion of Si remnants in PS was previously confirmed by impedance spectra analyses of the PS—electrolyte interface [11,12]. The solid curve in Fig. 1 shows the temperature dependence of the PL intensity of the same PS film
H. Koyama. N. Koshida / Electrical properties of luminescent porous silicon
as that used for electrical measurements. The PL intensity shows a maximum at about 200 K. This temperature corresponds to the transition point at which the electrical conduction mode changes from the hopping type to the thermal activation type. It is evident that the decrease in the PL intensity at temperatures beyond 200 K is due to thermal quenching. Although the origin of a decrease in the PL intensity at low temperatures is not clear at present, a possible explanation is an increase in the nonradiative recombination rate through some locaiized states. In any case, an intrinsic correlation between the carrier transport and the PL efficiency implies that both the band structure and the interfacial property of PS are important determining factors in the PL emission process.
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3.2. Photoconduction effect The PS cells exhibit a definite photoconduction effect, almost independent of the polarity of the bias voltage. Fig. 2 shows a typical example of the ternperature dependence of the dark- and photocurrents for a PS layer (40 l.tm thick) cell as a function of the negative bias voltage. At low temperatures, both the dark- and photocurrents tend to show a slight temperature dependence as in the case of hydrogenated Si [13,14]. Fig. 3 showsamorphous the normalized photoconduction spectra of a thin PS (20 J.tm thick) cell at room temperature as a function of the positive bias voltage. The effect of the absorption spectrum of a thin Au film was corrected in order to obtain the net spectral response of PS. The result of Fig. 3 is characterized by the facts that the PS layer behaves as a wide-gap semiconductor sensitive to visible light, and that the peak wavelength shifts toward the lower-energy side with increasing bias voltage, At low temperatures below 200 K, however, the spectral response becomes independent of the bias voltage, and then there is no red shift of the peak energy even at high positive bias voltages, as shown in Fig. 4. As seen from Figs. 1 and 2, the transition temperature of 200 K corresponds to the point at the beginning of the thermal activation-type conduction.
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Fig. 2. Temperature dependence of the dark(filled circles) and photocurrents (open circles) for a thick PS layer cell at different negative bias voltages. The PS layer (40 jim thick) was formed on a p-type Si wafer (8—il acm) by anodization in a 50% HF . 2 . solution at 25 mA/cm ). The illuminated light intensity was 2. 150 mW/cm
For the negative bias, in contrast, the peak wavelength at room temperature was independent of the applied voltage. The spectral response curve in this case is similar to that at 50 V in the positive bias case. These results can be explained by band diagrams shown in Figs. 5(a) and (b). The PS—Si interface is assumed to be a kind of heterojunction. When a bias voltage is applied, the major potential drop is produced across the PS layer because of its extremely high resistivity. For negative bias voltages, electrons generated in the Si substrate cannot contribute to the photoconduction current. In this situation, the intrinsic properties of PS should appear in the spectral
H. Koyama, N. Koshida / Electrical properties of luminescent porous silicon
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response, independent of the magnitude of the bias voltage, as shown in Fig. 5(a). For positive bias voltages, on the other hand, photoexcited electrons in the Si substrate have a chance of contributing to the photoconduction effect. As the bias voltage increases, this additional component becomes predominant over the original photoconduction in the PS layer, and then the overall spectral response should show a red shift, as indicated in Fig. At low temperatures below 200 K, however, the direct contribution of electrons injected into the PS layer to the photoconduction is substantially suppressed by carrier trapping effects. 5(b).
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Consequently, the spectral curve is almost determined by the intrinsic behavior of PS even at high bias voltages, as shown in Fig. 4. The above explanation is consistent with the previously reported results of optical analyses for PS by transmission and reflectance spectra measurements [7, 10]. According to the determined absorption spectra, the optical penetration lengths in the PS layer under study at wavelengths of 500, 700and900nmareabout5,30and100~.tm,respec-
____________________________________ I I I I 20~m Positive Bias ~ •, 146K • SOy I0 100 -
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Fig. 3. Room-temperature photoconduction spectra of a thin PS layer (20 jim thick) cell as a function of the positive bias voltage. The anodization conditions are the same as in Fig. I.
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Fig. 4. Photoconduction spectra of a thin PS layer cell at 146 K as a function ofthe positive bias voltage. The experimental cell is the same as that used in Fig. 2.
tively. In the wavelength region of interest, therefore, significantincarrier occur due to absorption the Sigeneration substrate should 20 ~.tm below the surface of the device. This provides a quantitative support for the interpretation of the result of Fig. 3. Further support is that the photoconduction spectra for thick PS layer (over 40 l.tm) cells changes only slightly with the positive bias voltage. When the above PS sample was excited by a He—Cd laser, it showed an efficient visible photoluminescence whose spectrum peaks at 700 nm. A significant difference in the peak wavelengths
H. Koyama, N. Koshida / Electrical properties of luminescent porous silicon I
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teristics of PS are presumably caused by a bandgap widening in PS. Although further analyses of the microscopic mechanisms of the optical excitation and the carrier transport in PS are needed, the observed photoelectronic response of PS is reasonable in view of the quantum confinement model in
Fig. 6. PL spectra of a PS sample under various bias conditions. The PS layer (50 ~smthick) was formed on a heavily doped n-type Si wafer by anodization in a solution of HF (50%): 2. The inset is a schematic ethanol (99.5%) = 1:1 at 100 mA/cm illustration of the experimental cell structure.
between the photoconduction spectra and the PL ones is due to a potential fluctuation along the depth direction in the PS layer. In the photoconduction case, above band gap excitation is necessary
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a heavily-doped n-type wafer as a function of the bias voltage Vb. A schematic illustration of the experimental diode structure is also given in this figure. Under the condition of no external bias voltage, this sample exhibits an efficient visible (red) PL. When a positive or negative bias voltage is applied to the Au contact, the PL intensity was
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significantly decreased, and at Vb ±40 V it was almost completely quenched. This optoelectronic effect was reversible; when the bias voltage was turned off, the PL intensity was immediately restored to the original level. The implication is that
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Fig. 7 shows the PL intensity curve versus the bias voltage (filled circles) and the corresponding behavior of the diode current (open and circles). The diode shows a rectifying behavior, it emits a uniform visible EL for high injection currents at positive bias voltages of about 40 V. The PL intensity rapidly decreases with increasing magnitude of the bias voltage in either positive or negative direction, and major quenching is completed in the low-current region. Thus the PL quenching is thought to be due to field-induced carrier
298
H. Koyama, N. Koshida
/ Electrical properties of luminescent porous silicon
sweeping-out effects in the PS bulk. This is a strong indication that the electronic excitation and the subsequent radiative recombination occur in Si nanocrystallites. It is difficult for the molecular luminescence model to explain the observed electrical PL quenching. Similar electrical PL quenching effects have been reported for GaAs quantum wells by several authors [15,16]. Those were explained by the model of carrier leakage through tunneling between adjacent quantum wells. This picture seems to be applicable to PS, because the luminescent PS layer is composed of a large number of Si nanocrystallites surrounded by a thin oxide layer [10,17]. This external electric field should enhance tunneling escape of photoexcited carriers. This explanation is also supported by the experimental fact that the PS samples with inhomogeneous structures (such as PS formed on p-type wafers with high resistivities) show little electrical PL quenching, possibly because of a nonuniform electrical conduction. The electrical PL quenching at room temperature is consistent with the electrical properties of PS (Fig. 1) and with its crystalline electronic structure expected from the optical and interfacial characterizations reported previously [10, 18]. The observation of this phenomena is very important for further development of physical and technological studies.
4. Conclusions From various measurements of the electrical properties, some important information about the optoelectronic activity of PS has been obtained: (1) At temperatures beyond about 200 K, the carrier transport in PS is dominated by the band conuuction. (2) The intrinsic correlation between the change in the electrical conduction mode and the PL efficiency implies that the band scheme holds for the description of the light emission process in PS. (3) The PS layer is sensitive to visible light, and the characteristic behavior of the photoconduction spectra confirms a wide-bandgap nature of PS. -
-
.
.
.
.
.
(4) The PL emission can reversibly be quenched by the external electric field, presumably because of field-enhanced tunneling escape of excited carriers. These results support our central assumption that the visible luminescence of PS is based on the electronic excitation in Si nanocrystallites with a modified band structure. The observed electrical PL quenching is particularly important, since it is a direct evidence of the quantum confinement model. Complete surface passivation and uniform carrier injection are the key factors for further development of physical and technological studies.
Acknowledgements The authors would like to thank S. Yoshimura, M. Araki, Y. Yamamoto and T. Oguro for their cooperation in the experiments. This work was partially supported by an NHK Grant for Broadcasting Technology, and by a Grant-in-Aid from the Ministry of Education, Science and Culture of Japan.
References [ii
L.T. Canham, AppI. Phys. Lett. 57 (1990) 1046. [2] N. Koshida and H. Koyama, Jpn. J. AppI. Phys. 30 (1991) Li221. [3] A. Bsiesy, J.C. Vial, F. Gaspard, R. Herino, M. Ligeon, F. Muller, R. Romestain, A. Wasiela, A. Halimaoui and G. Bomchil, Surf. Sci. 254 (i99i) 194. [4] N. Koshida and H. Koyama, AppI. Phys. Lett. 60 (1992) 347. [5] N. Koshida, Y. Kiuchi and S. Yoshimura, Proc. ioth Symp. Photoelectron. Image Devices, London, 1991 (lOP, Bristol 1992) p. 377. [6] H. Koyama, T. Oguro and N. Koshida, AppI. Phys. Lett. 62 (1993), in press. [7] H. Koyama, M. Araki, Y. Yamamoto and N. Koshida, Jpn. J. AppI. Phys. 30 (1991) 3606. [8] RH. Bube, Photoelectronic Semiconductors (Cambridge University Press, Cambridge, 1992) p. 15. [9] V. Lehmann and U. Gosele, AppI. Phys. Lett. 58 (1991) 856. [10] N. Koshida and H. Koyama, Mater. Res. Soc. 283 (1993) (in press). [11] H. Koyama and N. Koshida, J. Electrochem. Soc. 138 (1991) 254. [12] N. Koshida and K. Echizenya, J. Electrochem. Soc. 138 (199i) 837.
H. Koyama, N. Koshida
/ Electrical properties of luminescent porous silicon
[13] PG. LeComber and W.E. Spear, Phys. Rev. Lett. 25(1970) 509. [14] WE. Spear, Ri. Loveland and A. Al-Sharbaty, J. NonCryst. Solids iS (i974) 410. [15] E.E. Mendez, G. Bastard, L.L. Chang, L. Esaki, H. Morkoc and R. Fischer, Phys. Rev. B 26 (i982) 7101.
299
[16] J.A. Kash, E.E. Mendez and H. Morkoc, AppI. Phys. Lett. 46 (1985) 173. [17] iC. Vial, A. Bsiesy, F. Gaspard, R. Herino, M. Ligeon, F. Muller, R. Romestain and R.M. Macfarlane, Phys. Rev. B 45 (1992) 14i7i. [18] Y. Uchida, N. Koshida, H. Koyama andY. Yamamoto, to be published.
Journal of Luminescence 57 (1993) 293—299
JOURNAL OF
Invited paper
Electrical properties of luminescent porous silicon Hideki Koyama and Nobuyoshi Koshida Division of Electronic and Information Engineering, Faculty of Technology, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184, Japan
The relation between the visible luminescence of porous silicon (PS) and its electrical properties has been investigated with an emphasis on carrier transport, photoconductivity, and the effects of an external electric field on the photoluminescence (PL). The PS layers are formed by anodization ofp- or n-type Si wafers in an HF solution in the dark or under illumination. At first, the correlation of the electrical conduction mode with the PL efficiency is shown using a self-supporting PS film as a sample. Next, the photoconduction effects of PS for visible light are characterized for the experimental cells of the form (semitransparent thin Au film/PS/Si substrate/Al contact). Finally, the reversible electrical PL quenching is demonstrated. These results support the hypothesis that the visible luminescence of PS is based on surface-sensitive quantum confinement effects in Si nanocrystallites.
1. Introduction
2. Experimental
To determine the mechanism of visible luminescence of PS [1—3],many characterization studies of the optical, structural, electronic, and interfacial properties of PS have been carried out. However, the relation between the electrical properties and the optical activity of PS has not been clarified, Detailed measurements of the carrier transport in PS, has not been clarified. Detailed measurements of the carrier transport in PS, the photoelectronic response, and the interaction of the electric field and the PL emission would provide useful information for understanding the luminescence mechanism. In addition, the electrical characterization of PS is very important for analyses of the visible electroluminescence (EL) from PS. On the basis of our previous observation of visible EL from PS diodes with a solid state contact [4], the present paper describes the electrical properties of luminescent PS in terms of the temperature dependence of the resistivity, photoconduction characteristics [5], and a field-induced PL quenching [6].
2.1. Sample preparation Nondegenerate p-type or n-type (111) Si wafers (0.018—2OQcm) were cleaned, and then ohmic contacts were formed onto the back side. The PS layers were formed by anodization of these wafers in 20—50% aqueous or ethanoic HF solutions at current densities of 10—100 mA/cm2 for 5—60 mm in the dark or under the illumination by a 500 W tungsten lamp from a distance of 20 cm. The thickness of PS layers is 3—50 ~tm. In some cases, anodized PS samples were illuminated in order to enhance the PL intensity. When it is necessary to evaluate the intrinsic nature of PS without the effects of the Si substrate (such as in the case of measurements of the temperature dependence of the electrical and luminescence properties), self-supporting PS films (20—40 J.tm thick) [7] were prepared by electrochemical separation of anodized PS layers from p-type Si substrates.
2.2. Measurements Correspondence to: Dr. H. Koyama, Division of Electronic and Information Engineering, Faculty of Technology, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184, Japan. 0022-2313/93/S06.O0 © 1993 SSDI 0022-2313(93)EOl l9-L
—
The experiments were carried out on the following subjects.
Elsevier Science Publishers B.V. All rights reserved
294
H. Koyama, N. Koshida / Electrical properties of luminescent porous silicon
(1) Electrical conduction and PL characteristics: Thin Al films were evaporated onto the both sides of self-supporting PS films, and were used as the electrodes. The dark resistivity of PS films along the depth direction was measured as a function of temperature in the range 77—300 K. The temperature dependence of the PL characteristics was also measured for the same PS samples using 325 nm Cd—He laser as an excitation source. The overall spectral resolution of the PL measurement system was less than 5 nm. (2) Photoconduction effect: Anodized PS sampies were immediately transferred into the vacuum chamber, and semitransparent thin Au films were evaporated onto the PS layer surface. The PS layer was excited through the thin Au film by light from a 500 W Xe lamp, and vertical photoconduction was measured under the condition that a positive or negative bias voltage was applied to the Au contact with respect to the Al electrode. The ternperature dependences of the dark- and photocurrents, including the spectral response, were measured as a function of the bias voltage. (3) Electrical effect on the PL characteristics: The effects of the external electric field on the PL characteristics were studied for some PS samples. The experimental cell structure was the same as that used for photoconduction measurements. The PS samples were excited with a 325 nm He—Cd laser through the thin Au film, and the PL spectra and intensity were measured in a N 2 gas atmosphere under the positively or negatively biased condition. To avoid possible changes in the PL characteristics during the measurements, the excitation intensity was adjusted to a minimum level acceptable for the instrument,
3. Results and discussion 3.1. Carrier transport and PL characteristics The dark resistivity of a self-supporting PS film (40 ~smthick) is shown by the dashed curve in Fig. 1 as a function of reciprocal temperature. In the lowand high-temperature regions, the electrical conduction mode of PS shows the hopping type and the thermal activation type, respectively. Near
TEMPERATURE (K) 300 200 150 100 1 .2
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Fig. 1. Temperature dependence ofthe resistivity (dashed curve) .
and the PL intensity (solid curve) for a self-supporting PS film (40 jim thick) anodized in a solution of HF(50%): ethanol (99.5%) = 1:1. Information on the substrate and the anodization parameters is also indicated.
room temperature, the band conduction via traps is dominant. The presence of traps was also suggested from the current—voltage characteristics [5]: the slope of the log—log plot of dark- and photocurrents versus the applied voltage is about 1.4, which is different from the value expected from the trapfree conduction mode [8]. The resistivity of PS at room temperature is several orders of magnitude higher than the original value of the Si substrate. The PS layer can be regarded as a semi-insulating semiconductor. This high resistivity is interpreted to be the result of a combination of carrier depletion in Si crystallites, low mobility due to scattering at grain boundaries, suppression of surface conduction due to hydrogen termination, and a band gap widening [7,9,10]. Depletion of Si remnants in PS was previously confirmed by impedance spectra analyses of the PS—electrolyte interface [11,12]. The solid curve in Fig. 1 shows the temperature dependence of the PL intensity of the same PS film
H. Koyama. N. Koshida / Electrical properties of luminescent porous silicon
as that used for electrical measurements. The PL intensity shows a maximum at about 200 K. This temperature corresponds to the transition point at which the electrical conduction mode changes from the hopping type to the thermal activation type. It is evident that the decrease in the PL intensity at temperatures beyond 200 K is due to thermal quenching. Although the origin of a decrease in the PL intensity at low temperatures is not clear at present, a possible explanation is an increase in the nonradiative recombination rate through some locaiized states. In any case, an intrinsic correlation between the carrier transport and the PL efficiency implies that both the band structure and the interfacial property of PS are important determining factors in the PL emission process.
1000
295
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3.2. Photoconduction effect The PS cells exhibit a definite photoconduction effect, almost independent of the polarity of the bias voltage. Fig. 2 shows a typical example of the ternperature dependence of the dark- and photocurrents for a PS layer (40 l.tm thick) cell as a function of the negative bias voltage. At low temperatures, both the dark- and photocurrents tend to show a slight temperature dependence as in the case of hydrogenated Si [13,14]. Fig. 3 showsamorphous the normalized photoconduction spectra of a thin PS (20 J.tm thick) cell at room temperature as a function of the positive bias voltage. The effect of the absorption spectrum of a thin Au film was corrected in order to obtain the net spectral response of PS. The result of Fig. 3 is characterized by the facts that the PS layer behaves as a wide-gap semiconductor sensitive to visible light, and that the peak wavelength shifts toward the lower-energy side with increasing bias voltage, At low temperatures below 200 K, however, the spectral response becomes independent of the bias voltage, and then there is no red shift of the peak energy even at high positive bias voltages, as shown in Fig. 4. As seen from Figs. 1 and 2, the transition temperature of 200 K corresponds to the point at the beginning of the thermal activation-type conduction.
-
001
I
I
0
5 10 3/ T
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Fig. 2. Temperature dependence of the dark(filled circles) and photocurrents (open circles) for a thick PS layer cell at different negative bias voltages. The PS layer (40 jim thick) was formed on a p-type Si wafer (8—il acm) by anodization in a 50% HF . 2 . solution at 25 mA/cm ). The illuminated light intensity was 2. 150 mW/cm
For the negative bias, in contrast, the peak wavelength at room temperature was independent of the applied voltage. The spectral response curve in this case is similar to that at 50 V in the positive bias case. These results can be explained by band diagrams shown in Figs. 5(a) and (b). The PS—Si interface is assumed to be a kind of heterojunction. When a bias voltage is applied, the major potential drop is produced across the PS layer because of its extremely high resistivity. For negative bias voltages, electrons generated in the Si substrate cannot contribute to the photoconduction current. In this situation, the intrinsic properties of PS should appear in the spectral
H. Koyama, N. Koshida / Electrical properties of luminescent porous silicon
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response, independent of the magnitude of the bias voltage, as shown in Fig. 5(a). For positive bias voltages, on the other hand, photoexcited electrons in the Si substrate have a chance of contributing to the photoconduction effect. As the bias voltage increases, this additional component becomes predominant over the original photoconduction in the PS layer, and then the overall spectral response should show a red shift, as indicated in Fig. At low temperatures below 200 K, however, the direct contribution of electrons injected into the PS layer to the photoconduction is substantially suppressed by carrier trapping effects. 5(b).
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Consequently, the spectral curve is almost determined by the intrinsic behavior of PS even at high bias voltages, as shown in Fig. 4. The above explanation is consistent with the previously reported results of optical analyses for PS by transmission and reflectance spectra measurements [7, 10]. According to the determined absorption spectra, the optical penetration lengths in the PS layer under study at wavelengths of 500, 700and900nmareabout5,30and100~.tm,respec-
____________________________________ I I I I 20~m Positive Bias ~ •, 146K • SOy I0 100 -
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Fig. 4. Photoconduction spectra of a thin PS layer cell at 146 K as a function ofthe positive bias voltage. The experimental cell is the same as that used in Fig. 2.
tively. In the wavelength region of interest, therefore, significantincarrier occur due to absorption the Sigeneration substrate should 20 ~.tm below the surface of the device. This provides a quantitative support for the interpretation of the result of Fig. 3. Further support is that the photoconduction spectra for thick PS layer (over 40 l.tm) cells changes only slightly with the positive bias voltage. When the above PS sample was excited by a He—Cd laser, it showed an efficient visible photoluminescence whose spectrum peaks at 700 nm. A significant difference in the peak wavelengths
H. Koyama, N. Koshida / Electrical properties of luminescent porous silicon I
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in order to ensure the transport of photogenerated carriers between two electrodes.
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-
_______________________________ 400
500
600
700
800
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WAVELENGTH (nm)
teristics of PS are presumably caused by a bandgap widening in PS. Although further analyses of the microscopic mechanisms of the optical excitation and the carrier transport in PS are needed, the observed photoelectronic response of PS is reasonable in view of the quantum confinement model in
Fig. 6. PL spectra of a PS sample under various bias conditions. The PS layer (50 ~smthick) was formed on a heavily doped n-type Si wafer by anodization in a solution of HF (50%): 2. The inset is a schematic ethanol (99.5%) = 1:1 at 100 mA/cm illustration of the experimental cell structure.
between the photoconduction spectra and the PL ones is due to a potential fluctuation along the depth direction in the PS layer. In the photoconduction case, above band gap excitation is necessary
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this PL due to some quenching electric is not fielddue effect. to thermal effects, but
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a heavily-doped n-type wafer as a function of the bias voltage Vb. A schematic illustration of the experimental diode structure is also given in this figure. Under the condition of no external bias voltage, this sample exhibits an efficient visible (red) PL. When a positive or negative bias voltage is applied to the Au contact, the PL intensity was
~
i_-
2
Fig. 6 shows the room-temperature PL spectra of the PS layer formed by photoanodization on
significantly decreased, and at Vb ±40 V it was almost completely quenched. This optoelectronic effect was reversible; when the bias voltage was turned off, the PL intensity was immediately restored to the original level. The implication is that
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VOLTAGE (V) Fig. 7. Voltage dependence of the PL intensity (filled circles) and the current density (open circles) for a PS sample. The experimental cell is the same as that in Fig. 6.
=
Fig. 7 shows the PL intensity curve versus the bias voltage (filled circles) and the corresponding behavior of the diode current (open and circles). The diode shows a rectifying behavior, it emits a uniform visible EL for high injection currents at positive bias voltages of about 40 V. The PL intensity rapidly decreases with increasing magnitude of the bias voltage in either positive or negative direction, and major quenching is completed in the low-current region. Thus the PL quenching is thought to be due to field-induced carrier
298
H. Koyama, N. Koshida
/ Electrical properties of luminescent porous silicon
sweeping-out effects in the PS bulk. This is a strong indication that the electronic excitation and the subsequent radiative recombination occur in Si nanocrystallites. It is difficult for the molecular luminescence model to explain the observed electrical PL quenching. Similar electrical PL quenching effects have been reported for GaAs quantum wells by several authors [15,16]. Those were explained by the model of carrier leakage through tunneling between adjacent quantum wells. This picture seems to be applicable to PS, because the luminescent PS layer is composed of a large number of Si nanocrystallites surrounded by a thin oxide layer [10,17]. This external electric field should enhance tunneling escape of photoexcited carriers. This explanation is also supported by the experimental fact that the PS samples with inhomogeneous structures (such as PS formed on p-type wafers with high resistivities) show little electrical PL quenching, possibly because of a nonuniform electrical conduction. The electrical PL quenching at room temperature is consistent with the electrical properties of PS (Fig. 1) and with its crystalline electronic structure expected from the optical and interfacial characterizations reported previously [10, 18]. The observation of this phenomena is very important for further development of physical and technological studies.
4. Conclusions From various measurements of the electrical properties, some important information about the optoelectronic activity of PS has been obtained: (1) At temperatures beyond about 200 K, the carrier transport in PS is dominated by the band conuuction. (2) The intrinsic correlation between the change in the electrical conduction mode and the PL efficiency implies that the band scheme holds for the description of the light emission process in PS. (3) The PS layer is sensitive to visible light, and the characteristic behavior of the photoconduction spectra confirms a wide-bandgap nature of PS. -
-
.
.
.
.
.
(4) The PL emission can reversibly be quenched by the external electric field, presumably because of field-enhanced tunneling escape of excited carriers. These results support our central assumption that the visible luminescence of PS is based on the electronic excitation in Si nanocrystallites with a modified band structure. The observed electrical PL quenching is particularly important, since it is a direct evidence of the quantum confinement model. Complete surface passivation and uniform carrier injection are the key factors for further development of physical and technological studies.
Acknowledgements The authors would like to thank S. Yoshimura, M. Araki, Y. Yamamoto and T. Oguro for their cooperation in the experiments. This work was partially supported by an NHK Grant for Broadcasting Technology, and by a Grant-in-Aid from the Ministry of Education, Science and Culture of Japan.
References [ii
L.T. Canham, AppI. Phys. Lett. 57 (1990) 1046. [2] N. Koshida and H. Koyama, Jpn. J. AppI. Phys. 30 (1991) Li221. [3] A. Bsiesy, J.C. Vial, F. Gaspard, R. Herino, M. Ligeon, F. Muller, R. Romestain, A. Wasiela, A. Halimaoui and G. Bomchil, Surf. Sci. 254 (i99i) 194. [4] N. Koshida and H. Koyama, AppI. Phys. Lett. 60 (1992) 347. [5] N. Koshida, Y. Kiuchi and S. Yoshimura, Proc. ioth Symp. Photoelectron. Image Devices, London, 1991 (lOP, Bristol 1992) p. 377. [6] H. Koyama, T. Oguro and N. Koshida, AppI. Phys. Lett. 62 (1993), in press. [7] H. Koyama, M. Araki, Y. Yamamoto and N. Koshida, Jpn. J. AppI. Phys. 30 (1991) 3606. [8] RH. Bube, Photoelectronic Semiconductors (Cambridge University Press, Cambridge, 1992) p. 15. [9] V. Lehmann and U. Gosele, AppI. Phys. Lett. 58 (1991) 856. [10] N. Koshida and H. Koyama, Mater. Res. Soc. 283 (1993) (in press). [11] H. Koyama and N. Koshida, J. Electrochem. Soc. 138 (1991) 254. [12] N. Koshida and K. Echizenya, J. Electrochem. Soc. 138 (199i) 837.
H. Koyama, N. Koshida
/ Electrical properties of luminescent porous silicon
[13] PG. LeComber and W.E. Spear, Phys. Rev. Lett. 25(1970) 509. [14] WE. Spear, Ri. Loveland and A. Al-Sharbaty, J. NonCryst. Solids iS (i974) 410. [15] E.E. Mendez, G. Bastard, L.L. Chang, L. Esaki, H. Morkoc and R. Fischer, Phys. Rev. B 26 (i982) 7101.
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[16] J.A. Kash, E.E. Mendez and H. Morkoc, AppI. Phys. Lett. 46 (1985) 173. [17] iC. Vial, A. Bsiesy, F. Gaspard, R. Herino, M. Ligeon, F. Muller, R. Romestain and R.M. Macfarlane, Phys. Rev. B 45 (1992) 14i7i. [18] Y. Uchida, N. Koshida, H. Koyama andY. Yamamoto, to be published.