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Electroluminescence (EL) from photo-chemically etched silicon
Materials Science and Engineering B69 – 70 (2000) 205 – 209 www.elsevier.com/locate/mseb
Electroluminescence (EL) from photo-chemically etched silicon Naokatsu Yamamoto *, Atsushi Sumiya, Hiroshi Takai Department of Electrical Engineering, Tokyo Denki Uni6ersity, 2 -2 Kanda Nishikicyo, Chiyodaku, Tokyo, 101 -8457, Japan
Abstract Visible luminescence from Si-based materials has been investigated to develop new opto-electronic devices on a Si wafer. In this report, we propose a photo-chemical etching method in order to form a luminescent layer on a Si wafer. A comparison between electroluminescence (EL) and photoluminescence (PL) from the photo-chemically etched silicon is discussed. In the photo-chemical etching method, a Si wafer (100) with resisitivity of 35 – 45 or 0.22 – 0.38 ohm-cm is set at the bottom of a vessel filled with an etchant (HF+H2O2), and a He–Ne laser (633 nm, 18.4mW/cm2) is irradiated onto the surface for 30 min. An Au thin film (thickness 5 nm) is deposited onto the the etched layer, and a Au – Sb (1 wt%) film is deposited on the reverse side of the Si wafer to form ohmic contacts. The EL from the etched layer is observed by applying a voltage to the electrodes( − 30 +30V). PL from the etched layer is measured by He–Cd laser excitation (325 nm). As a result, it is clear that the peak wavelength of EL at forward bias coincides with a peak wavelength of PL. EL spectra at backward bias can be fitted by two gaussian functions, and one of them coincides with a peak wavelength of PL. In addition, the EL from the photo-chemically etched silicon can be explained schematically by an electrical circuit model. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Photo-chemical etching; Electroluminescence(EL); Photoluminescence(PL); Silicon
1. Introduction Silicon-based luminescence materials have a potential for future opto-electronic applications. Porous Silicon (PS) is of great interest, because it exhibits visible bright photoluminescence (PL) at room temperature[1]. Porous Silicon can be formed by an electro-chemical reaction of a single crystal Si wafer in a HF solution. Red or yellow emission from PS is generally observed, and the blue or green light emission has been investigated in order to develop the full colour display devices. Recently, Light Emitting Diode (LED) and electroluminescence(EL) using PS has been investigated[2] while the luminescence efficiency from LEDs is reported to be 0.01 – 0.2%.[3,4] The anodization is a simple method to form PS on single crystal Si. However, it is difficult to form PS on Silicon On Insulator (SOI) structure such as Silicon On Sapphire (SOS), or on multi layered Integrated Circuit (IC), since current flow from the back surface of a Si wafer to electrolyte solutions is required in the anodization method. This becomes an obstacle for applications of the PS for the visible luminescence layer. * Corresponding author. Tel./fax: +81-3-5280-3307. E-mail address: [email protected] (N. Yamamoto)
We report here the formation of visible light emitting layers on Si wafer by a photo-chemical etching method in HF and H2O2 solution. We discuss EL from photochemically etched silicon. Additionally, the PL from the etched layer is measured by He–Cd laser excitation (325 nm), and EL spectra are compared with PL.
2. Experimental methods Fig. 1 shows a schematic set up for the photo-chemical etching method. A single crystalline n-type Si wafer (100) having resistance of 0.22–0.38 or 35–45 ohm-cm was set at the bottom of a vessel filled with a mixture of HF and H2O2 as an oxidant (HF:H2O2 = 100:17). He– Ne laser (633 nm,18.4mW/cm2) was irradiated onto the Si wafer surface through the solution for photo-chemical reaction. A visible luminescence layer was formed selectively in a region the laser irradiated. The photochemical etching time varied from 5 to 40min. PL from the photo-chemically etched layer was observed by the He–Cd laser (325 nm) excitation in the vacuum (under 10 − 2 torr) at RT. An EL device was fabricated by using the luminescence layer which is formed as a result of the photo-chemical etching for 30 min. A thin gold film (with a thickness of approximately 5 nm) was
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deposited on the entire surface of the etched area, then half of the etched surface was covered by a thicker Au film(approximately 100 nm) in order to make an external contact. Au– Sb(1 wt%) film was deposited on the back surface of a silicon substrate. Samples were kept at 290 K in a vacuum using a cryostat system during EL measurement.
3. Results and Discussion Fig. 2 highlights the dependence of the PL spectrum from the photo-chemically etched layer on the etching time. The peak wavelength shifts towards a shorter
Fig. 3. The etching time dependence of peak wavelength and peak intensity of PL.
Fig. 1. Photo-chemical etching method.
Fig. 2. Dependence of PL spectrum of the etched layer on the etching time.
wavelength and the PL intensity increases in line with the etching time for 30 min, and then decreases. The peak position and the wide Full Width at Half Maximum (FWHM) from these layers are considered to be similar to those from porous silicon.[5,6] Fig. 3 shows the etching time dependence of the peak intensity and the peak wavelength of PL of luminescence layers on the substrates with low resistance. The peak intensity increases with the etching time for 30min. The maximum PL intensity is obtained at 30 min, and then decreases. The PL peak wavelength becomes shorter, and is then saturated with increased etching time. These tendencies can be observed in the case of photo-chemically etched layers formed on the high resistive substrate. The layer etched for 5 min displays orange luminescence with a peak at 650 nm, while the layer etched for 30 min shows strong yellow luminescence with a peak at 625 nm. These luminescences can be observed in daylight except for in instances that are etched for less than 5min. It is considered that the wavelength from photo-chemically etched layers can be controlled by the etching time. Fig. 4 illustrates the dependence of the current density on the voltage applied to a EL device on the low resistive substrate. The schematic structure of an EL device is displayed in the figure. The forward bias is defined when a positive voltage is applied to the etched silicon surface. It is found that the EL device rectifies the current flow. The rectification of the EL device may be a result of the Schottky effect between the gold film and the photo-chemically etched layer. Fig. 5 highlights the dependence of the EL spectrum on the applied forward bias voltage. The etched layer on the low resistive substrate emits luminescence (peak wavelength:
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625 nm) at a voltage as low as+ 5.0V, and intensity increases with increasing applied voltage. This yellow luminescence can be observed by the naked eye in the dark. In addition, it has been confirmed that the whole etched surface covered by the thin gold film emits luminescence. Fig. 6 displays the dependence of the EL spectra on the applied backward bias voltage. The EL can be observed by applying voltageB − 20 V, and the intensity increases with decreasing voltage. It is considered that EL spectra at backward biases are more
Fig. 6. Dependence of EL spectrum on applied backward bias voltage.
Fig. 4. Dependence of current density on applied voltage of the EL device.
Fig. 7. Normalised EL spectra at the forward and backward bias.
Fig. 5. Dependence of EL spectrum on applied forward bias voltage.
complicated than those at forward in Fig. 5. Fig. 7 shows the normalised EL spectra at the forward and backward biases. Both EL spectra from forward and backward biased layers can be fitted by two gaussian functions, each of them has a peak wavelength at 625 nm (peak-1) and 530 nm (peak-2), respectively. It is noted here that the intensity of peak-2 for backward bias is much stronger than in the case of forward bias. Additionally, a peak wavelength(625 nm) of peak-1 coincides with that of PL from the silicon etched for 30
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N. Yamamoto et al. / Materials Science and Engineering B69–70 (2000) 205–209
min in Fig. 2. Coincidence of peak wavelength of PL and EL is also observed when the etched layer is formed on the high resistive substrate. Dependence of the peak wavelength of peak-1 and peak-2 and their intensities on applied forward bias are shown in Fig. 8. It is clear that wavelengths of peak-1 and peak-2 are 625 and 530 nm, respectively, and independent of the forward bias voltage. Intensity of peak-1 increases exponentially with the applied voltage, while intensity of peak-2 increases gradually. Accordingly, it is clear that the intensity ratio of
Fig. 10. EL model of the photo-chemically etched silicon.
Fig. 8. Dependence of peak wavelength and EL intensity on the applied forward bias voltage.
peak-1 to peak-2 becomes larger by increasing the forward bias voltage. On the other hand, both intensities of peak-1 and peak-2 increase exponentially when the layer is backward biased as shown in Fig. 9. This result means that intensity ratio of peak-2 to peak-1 is almost constant (:45%) under these backward biased conditions. In order to investigate the difference of intensity ratio between forward and backward biased EL spectra, a new EL model is proposed as shown in Fig. 10. In this model, Diode D0 and resistor Rk are connected serially to luminescence elements(l1, l2). The Diode D0 represents the Schottky rectifier shown in Fig. 4. Since only DC bias is applied to EL devices, capacitors are neglected. l1 and l2 are assumed to be luminescence sources which consist of nano crystalline silicons with different size which affects the wavelength of luminescence. Thus, it is assumed that l1 and l2 emit luminescences at 625 nm(peak-1) and 530 nm(peak-2), respectively. Parallel diodes D are connected to resistor R1 and R2. Here, R2 \ R1 must be satisfied. In the case of forward bias, current flows through the D0, l1 and R1, resulting in an emission of 625 nm. Since R2 \ R1, however, luminescence from l2 is hardly observed. On the other hand, current flows through both l1 and l2, so that luminescence from them can be obtained. These considerations can explain the experimental results displayed in Fig. 7.
4. Conclusion Fig. 9. Dependence of peak wavelength and EL intensity on the applied backward bias voltage.
In this report, we have demonstrated the EL from photo-chemically etched silicon. The etched layer
N. Yamamoto et al. / Materials Science and Engineering B69–70 (2000) 205–209
emits EL at a voltage as low as +5.0V. In addition, it was confirmed that EL intensity increases with increasing forward and backward bias voltage. A peak wavelength of EL obtained by forward bias voltage coincides with that of PL excited by He – Cd laser. However, an additional peak of EL is found in the case of the backward bias. It is believed that EL characteristics of photo-chemically etched layers can be explained by the electrical circuit model.
.
209
References [1] [2] [3] [4] [5]
L.T. Canham, Appl. Phys. Lett. 57 (1990) 1046. N. Koshida, H. Koyama, Appl. Phys. Lett. 60 (1992) 347. P. Steiner, F. Kozlowski, W. Lang, Appl. Phys. Lett. 62 (1993) 2700. N. Llalic, J. Linnros, J. Appl. Phys. 80 (1996) 5971. S. Shin, K. Jung, T.Y. Hsieh, J. Sarathy, J.C. Campbell, D.L. Kwong, Appl. Phys. Lett. 60 (15) (1992) 1863. [6] M. Schoisswohl, J.L. Cantin, H.J. vonBardeleben, Appl. Phys. Lett. 66 (26) (1995) 3660.
Electroluminescence (EL) from photo-chemically etched silicon Naokatsu Yamamoto *, Atsushi Sumiya, Hiroshi Takai Department of Electrical Engineering, Tokyo Denki Uni6ersity, 2 -2 Kanda Nishikicyo, Chiyodaku, Tokyo, 101 -8457, Japan
Abstract Visible luminescence from Si-based materials has been investigated to develop new opto-electronic devices on a Si wafer. In this report, we propose a photo-chemical etching method in order to form a luminescent layer on a Si wafer. A comparison between electroluminescence (EL) and photoluminescence (PL) from the photo-chemically etched silicon is discussed. In the photo-chemical etching method, a Si wafer (100) with resisitivity of 35 – 45 or 0.22 – 0.38 ohm-cm is set at the bottom of a vessel filled with an etchant (HF+H2O2), and a He–Ne laser (633 nm, 18.4mW/cm2) is irradiated onto the surface for 30 min. An Au thin film (thickness 5 nm) is deposited onto the the etched layer, and a Au – Sb (1 wt%) film is deposited on the reverse side of the Si wafer to form ohmic contacts. The EL from the etched layer is observed by applying a voltage to the electrodes( − 30 +30V). PL from the etched layer is measured by He–Cd laser excitation (325 nm). As a result, it is clear that the peak wavelength of EL at forward bias coincides with a peak wavelength of PL. EL spectra at backward bias can be fitted by two gaussian functions, and one of them coincides with a peak wavelength of PL. In addition, the EL from the photo-chemically etched silicon can be explained schematically by an electrical circuit model. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Photo-chemical etching; Electroluminescence(EL); Photoluminescence(PL); Silicon
1. Introduction Silicon-based luminescence materials have a potential for future opto-electronic applications. Porous Silicon (PS) is of great interest, because it exhibits visible bright photoluminescence (PL) at room temperature[1]. Porous Silicon can be formed by an electro-chemical reaction of a single crystal Si wafer in a HF solution. Red or yellow emission from PS is generally observed, and the blue or green light emission has been investigated in order to develop the full colour display devices. Recently, Light Emitting Diode (LED) and electroluminescence(EL) using PS has been investigated[2] while the luminescence efficiency from LEDs is reported to be 0.01 – 0.2%.[3,4] The anodization is a simple method to form PS on single crystal Si. However, it is difficult to form PS on Silicon On Insulator (SOI) structure such as Silicon On Sapphire (SOS), or on multi layered Integrated Circuit (IC), since current flow from the back surface of a Si wafer to electrolyte solutions is required in the anodization method. This becomes an obstacle for applications of the PS for the visible luminescence layer. * Corresponding author. Tel./fax: +81-3-5280-3307. E-mail address: [email protected] (N. Yamamoto)
We report here the formation of visible light emitting layers on Si wafer by a photo-chemical etching method in HF and H2O2 solution. We discuss EL from photochemically etched silicon. Additionally, the PL from the etched layer is measured by He–Cd laser excitation (325 nm), and EL spectra are compared with PL.
2. Experimental methods Fig. 1 shows a schematic set up for the photo-chemical etching method. A single crystalline n-type Si wafer (100) having resistance of 0.22–0.38 or 35–45 ohm-cm was set at the bottom of a vessel filled with a mixture of HF and H2O2 as an oxidant (HF:H2O2 = 100:17). He– Ne laser (633 nm,18.4mW/cm2) was irradiated onto the Si wafer surface through the solution for photo-chemical reaction. A visible luminescence layer was formed selectively in a region the laser irradiated. The photochemical etching time varied from 5 to 40min. PL from the photo-chemically etched layer was observed by the He–Cd laser (325 nm) excitation in the vacuum (under 10 − 2 torr) at RT. An EL device was fabricated by using the luminescence layer which is formed as a result of the photo-chemical etching for 30 min. A thin gold film (with a thickness of approximately 5 nm) was
0921-5107/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 9 9 ) 0 0 4 0 5 - 5
N. Yamamoto et al. / Materials Science and Engineering B69–70 (2000) 205–209
206
deposited on the entire surface of the etched area, then half of the etched surface was covered by a thicker Au film(approximately 100 nm) in order to make an external contact. Au– Sb(1 wt%) film was deposited on the back surface of a silicon substrate. Samples were kept at 290 K in a vacuum using a cryostat system during EL measurement.
3. Results and Discussion Fig. 2 highlights the dependence of the PL spectrum from the photo-chemically etched layer on the etching time. The peak wavelength shifts towards a shorter
Fig. 3. The etching time dependence of peak wavelength and peak intensity of PL.
Fig. 1. Photo-chemical etching method.
Fig. 2. Dependence of PL spectrum of the etched layer on the etching time.
wavelength and the PL intensity increases in line with the etching time for 30 min, and then decreases. The peak position and the wide Full Width at Half Maximum (FWHM) from these layers are considered to be similar to those from porous silicon.[5,6] Fig. 3 shows the etching time dependence of the peak intensity and the peak wavelength of PL of luminescence layers on the substrates with low resistance. The peak intensity increases with the etching time for 30min. The maximum PL intensity is obtained at 30 min, and then decreases. The PL peak wavelength becomes shorter, and is then saturated with increased etching time. These tendencies can be observed in the case of photo-chemically etched layers formed on the high resistive substrate. The layer etched for 5 min displays orange luminescence with a peak at 650 nm, while the layer etched for 30 min shows strong yellow luminescence with a peak at 625 nm. These luminescences can be observed in daylight except for in instances that are etched for less than 5min. It is considered that the wavelength from photo-chemically etched layers can be controlled by the etching time. Fig. 4 illustrates the dependence of the current density on the voltage applied to a EL device on the low resistive substrate. The schematic structure of an EL device is displayed in the figure. The forward bias is defined when a positive voltage is applied to the etched silicon surface. It is found that the EL device rectifies the current flow. The rectification of the EL device may be a result of the Schottky effect between the gold film and the photo-chemically etched layer. Fig. 5 highlights the dependence of the EL spectrum on the applied forward bias voltage. The etched layer on the low resistive substrate emits luminescence (peak wavelength:
N. Yamamoto et al. / Materials Science and Engineering B69–70 (2000) 205–209
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625 nm) at a voltage as low as+ 5.0V, and intensity increases with increasing applied voltage. This yellow luminescence can be observed by the naked eye in the dark. In addition, it has been confirmed that the whole etched surface covered by the thin gold film emits luminescence. Fig. 6 displays the dependence of the EL spectra on the applied backward bias voltage. The EL can be observed by applying voltageB − 20 V, and the intensity increases with decreasing voltage. It is considered that EL spectra at backward biases are more
Fig. 6. Dependence of EL spectrum on applied backward bias voltage.
Fig. 4. Dependence of current density on applied voltage of the EL device.
Fig. 7. Normalised EL spectra at the forward and backward bias.
Fig. 5. Dependence of EL spectrum on applied forward bias voltage.
complicated than those at forward in Fig. 5. Fig. 7 shows the normalised EL spectra at the forward and backward biases. Both EL spectra from forward and backward biased layers can be fitted by two gaussian functions, each of them has a peak wavelength at 625 nm (peak-1) and 530 nm (peak-2), respectively. It is noted here that the intensity of peak-2 for backward bias is much stronger than in the case of forward bias. Additionally, a peak wavelength(625 nm) of peak-1 coincides with that of PL from the silicon etched for 30
208
N. Yamamoto et al. / Materials Science and Engineering B69–70 (2000) 205–209
min in Fig. 2. Coincidence of peak wavelength of PL and EL is also observed when the etched layer is formed on the high resistive substrate. Dependence of the peak wavelength of peak-1 and peak-2 and their intensities on applied forward bias are shown in Fig. 8. It is clear that wavelengths of peak-1 and peak-2 are 625 and 530 nm, respectively, and independent of the forward bias voltage. Intensity of peak-1 increases exponentially with the applied voltage, while intensity of peak-2 increases gradually. Accordingly, it is clear that the intensity ratio of
Fig. 10. EL model of the photo-chemically etched silicon.
Fig. 8. Dependence of peak wavelength and EL intensity on the applied forward bias voltage.
peak-1 to peak-2 becomes larger by increasing the forward bias voltage. On the other hand, both intensities of peak-1 and peak-2 increase exponentially when the layer is backward biased as shown in Fig. 9. This result means that intensity ratio of peak-2 to peak-1 is almost constant (:45%) under these backward biased conditions. In order to investigate the difference of intensity ratio between forward and backward biased EL spectra, a new EL model is proposed as shown in Fig. 10. In this model, Diode D0 and resistor Rk are connected serially to luminescence elements(l1, l2). The Diode D0 represents the Schottky rectifier shown in Fig. 4. Since only DC bias is applied to EL devices, capacitors are neglected. l1 and l2 are assumed to be luminescence sources which consist of nano crystalline silicons with different size which affects the wavelength of luminescence. Thus, it is assumed that l1 and l2 emit luminescences at 625 nm(peak-1) and 530 nm(peak-2), respectively. Parallel diodes D are connected to resistor R1 and R2. Here, R2 \ R1 must be satisfied. In the case of forward bias, current flows through the D0, l1 and R1, resulting in an emission of 625 nm. Since R2 \ R1, however, luminescence from l2 is hardly observed. On the other hand, current flows through both l1 and l2, so that luminescence from them can be obtained. These considerations can explain the experimental results displayed in Fig. 7.
4. Conclusion Fig. 9. Dependence of peak wavelength and EL intensity on the applied backward bias voltage.
In this report, we have demonstrated the EL from photo-chemically etched silicon. The etched layer
N. Yamamoto et al. / Materials Science and Engineering B69–70 (2000) 205–209
emits EL at a voltage as low as +5.0V. In addition, it was confirmed that EL intensity increases with increasing forward and backward bias voltage. A peak wavelength of EL obtained by forward bias voltage coincides with that of PL excited by He – Cd laser. However, an additional peak of EL is found in the case of the backward bias. It is believed that EL characteristics of photo-chemically etched layers can be explained by the electrical circuit model.
.
209
References [1] [2] [3] [4] [5]
L.T. Canham, Appl. Phys. Lett. 57 (1990) 1046. N. Koshida, H. Koyama, Appl. Phys. Lett. 60 (1992) 347. P. Steiner, F. Kozlowski, W. Lang, Appl. Phys. Lett. 62 (1993) 2700. N. Llalic, J. Linnros, J. Appl. Phys. 80 (1996) 5971. S. Shin, K. Jung, T.Y. Hsieh, J. Sarathy, J.C. Campbell, D.L. Kwong, Appl. Phys. Lett. 60 (15) (1992) 1863. [6] M. Schoisswohl, J.L. Cantin, H.J. vonBardeleben, Appl. Phys. Lett. 66 (26) (1995) 3660.