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Effects of oxide layers and metals on photoelectric and optical properties of Schottky barrier photodetector
ARTICLE IN PRESS
Renewable Energy 31 (2006) 1493–1503 www.elsevier.com/locate/renene
Effects of oxide layers and metals on photoelectric and optical properties of Schottky barrier photodetector W.F. Mohamada,, A. Abou Hajarb, A.N. Salehc a
College of Engineering, Al Isra’ University, P.O. Box 22 and 33, Al Isra University Post Office, Amman 11622, Jordan b College of Engineering, Aleppo University, Aleppo, Syria c College of Engineering, Mosul University, Mosul, Iraq Received 14 April 2005; accepted 6 December 2005 Available online 7 March 2006
Abstract Recently, a lot of attention has been paid to Schottky barrier photo detectors due to their promising properties and easy of fabrication. Many samples of SB devices prepared by thermal deposition under high vacuum are studied in this research. Different types and thicknesses of oxides were deposited on silicon substrate. Metals of different types and thicknesses were deposited on top of oxides. Variation of photogenerated current, responsivity, quantum efficiency and detectivity as a function of incident light wavelength were measured. It was found that the shape of the curves has two maxima, one was around 500 nm and the other was around 700 nm. Ni (1 0 0)–SiO2–Si structure shows the maximum responsivity at 550 nm and it is equal to 400 mA/W. When comparison was made between devices of different metals, the nickel layer device showed high responsivity at visible region while the aluminum layer device showed high responsivity at near infrared region. Finally, the aluminum layer device showed detectivity higher than nickel layer device. The maximum detectivity of aluminum device was 6.4 1010 cm/Hz W. r 2006 Elsevier Ltd. All rights reserved. Keywords: Schottky barrier photodetector; MOS structures; Photoelectric properties of MOS
Corresponding author.
0960-1481/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2005.12.012
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1. Introduction The principles of photo detector operation depend on the interaction between the light and detector material. Light photons give their energy to the detector material to produce free electron–hole pairs. Collection of these free electron-hole pairs produces an electric signal. There are many types of photo detector; homojunction, hetero junction and Schottky barrier (SB) photo detectors. In recent years much attention has been paid to the SB photo detectors due to their promising properties and easy of fabrication [1–4]. Schottky barriers differ from pn junction structures. The SB has a potential barrier lower than the energy gap of the semiconductor and forming a space charge region on the semiconductor side. The space charge region is responsible for the generation of the carriers and high electric field at the semiconductor side. Thus the thermal emission mechanism is responsible for the current transportation in SB structures. It is found that the photovoltaic responsivity of SB structure was very low due to thermal current transportation mechanisms. This drawback has been rectified by introducing an oxide layer between metal and semiconductor layers [5]. This type of structure (MOS) is a special type of solar cell. When a positive voltage is applied to the metal side (reverse voltage), the energy band in the semiconductor layer will bend and a potential well between the oxide and semiconductor will be formed. The electron–hole pairs generated at the metal side due to the incident light will be stored in the potential well. The collection of this free charge will produce an electrical signal [6]. MOS detectors consist of three layers as shown in Fig. 1. The metal layer should have low series resistance and be transparent to allow light to be absorbed and transmitted through it. This can be achieved by the choice of metals at an optimized thickness. Type and thickness of the metal layer affect the output signal of photodetector. Device responsivity (R) is defined as the ratio of the output power to the input light power. The output power can be current or voltage and can be written as follows [7,8]: R¼
I Ph V ¼ , P P
(1)
where Iph is the photogenerated current, V the output voltage and P the input light power. The spectral responsivity (SR) depends on the light incident wavelength and can be expressed in terms of external quantum efficiency (Z) as follows [9]: SRðlÞ ¼
Z , hc=ql
(2) +V Transparent metal Insulator
P-Substrate
Semiconductor GND Fig 1. MOS-Structure.
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where h is the Plank’s constant, c the light velocity, q the electron charge and l the wavelength of the incident light. The spectral responsivity of the device is a measurable quantity and defined as short circuit current density per unit irradiance as a function of wavelength. From the above definition and Eq. (2), the external quantum efficiency (Z) can be defined as the fraction of minority carrier’s generated in the device due to the light application and collected under short circuit current conditions. It can be written as follows [10]: I ph hc Z¼ . (3) P ql The external quantum efficiency can be defined as the ratio of generated electrons to the number of incident photons, while the internal quantum efficiency (g) is the ratio of generated electrons to the number of absorbed photons and can be related to the external quantum efficiency as follows [11]: Z ¼ ð1 RFS Þag,
(4)
where RFS is the reflectivity of the front surface and (a) the absorption coefficient. In order to obtain high quantum efficiency and detectivity, the high reflective surface must be placed at the device rear and make the depletion layer as wide as possible. But small signal detectivity depends on the noise around the device. Noise equivalent power (NEP) is the ratio between applied power to noise which is equal to one at bandwidth of 1 Hz at defined wavelength. The reciprocal of NEP is defined as the detectivity (D) of the device. The specific detectivity (D*) is defined as the product of detectivity multiplied by the detector area (A) and band width frequency (Df) as follows [12]: D ¼ ðADf Þ1=2 =NEP:
(5)
The specific detectivity can be expressed in terms of external quantum efficiency as follows [13]: D ¼
Zl 2hcðj d =qÞ1=2
,
(6)
where jd is the dark current of the device. It is clear that the specific detectivity is decreased as the dark current (or more specific the leakage current) is increased. 2. Samples preparation Many samples were prepared by thermal deposition under high vacuum using Balzer coating unit. The deposition parameters and the sequence of fabrication of MOS devices shown in Fig. 1 are as follows: (a) Aluminum of 2000 A˚ thickness was deposited on the back surface of the silicon wafer to form the back contacts of the device. (b) In second step the prepared samples were prepared in two groups: the first group was left in the air for the native oxide of SiO2 to be grown, while the native oxide was removed in the second group and other oxides were deposited, namely SnO2 and SiO.
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Table 1 Prepared samples for different parameters Notes
Structure
Sample
Without oxide, Ni ¼ 100 A˚ Native oxide, Ni ¼ 50–300 A˚ SnO2 ¼ 22–43 A˚, Ni ¼ 100 A˚ SiO ¼ 36–43 A˚, Ni ¼ 100 A˚ Without oxide, Al 100 A˚ Native oxide, Ni ¼ 50–300 A˚ Native oxide, In ¼ 50–100 A˚
Ni–Si Ni–SiO2–Si Ni–SnO2–Si Ni–SiO–Si Al–Si Al–SiO2–Si In–SiO2–Si
S1 S3 S4 S5 S6 S8 S11
(c) Aluminum or nickel was deposited on top of the oxides with different thicknesses to form front contacts. (d) Annealing process at 350 1C (under vacuum) for one hour was carried out for all samples to support front and back contact formation. All samples prepared in this research for different parameters are shown in Table 1. The substrate was n silicon of 400 mm thickness coated with a thin layer of n+ silicon of 6 mm thicknesses. The samples were circles of 0.2 cm2 area. 3. Results and discussions It was proved in the previous section that the quantum efficiency is directly proportional to the photogenerated current. Consequently, responsivity and detectivity will behave in the same manner. Here the effect of thickness and type of metal on the photodetector behavior as a function of incident light wavelength will be examined. Fig. 2 shows the variation of photogenerated current as a function of incident light wavelength for the fabricated samples (S3). Nickel thickness of the sample was varied from 50 A˚ up to 300 A˚ in 50 A˚ steps. No bias fields were applied on the samples, in other words the devices were working in the photovoltaic mode. It is clear that the photogenerated current increased as the wavelength increased, and reached its maximum value at 600 nm of wavelength. Another current maxima was noticed at 700 nm of wavelength. Beyond 700 nm of wavelength, the current decreased and reached minimum value at 1100 nm wavelength. Also it is noticed that maximum photogenerated current is obtained from the sample of 100 A˚ nickel thickness. It is thought that the degree of light trapping in metal layer is adjusted by varying the thickness upon the device absorbance [13]. When nickel thickness increased more than 100 A˚, photogenerated current was decreased due to high reflection and low transmission of light through thick nickel layer. Reverse voltage of 2 V was applied on the samples and photogenerated current variation was calculated as a function of incident light wavelength as shown in Fig. 3. This means that the devices were working in photoconductor mode. Again two current maxima are noted at the same values of wavelength calculated in the previous figure. It could be concluded from this figure, that the photogenerated current increased by a factor of 1.5 approximately compared with the current calculated in the previous figure. It is known that the total currents in these devices are the summation of dark and photogenerated
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0.016 100 Ang.Ni 50 Agn.Ni 200 Ang.Ni 300 Ang.Ni 250 Ang.Ni
0.014
Photocurrent (mA)
0.012 0.01 0.008 0.006 0.004 0.002 0 400
500
600
700
800
900
1000
1100
Wavelength (nm)
Fig. 2. Variation of photogenerated current vs. wavelength for sample S3, of different nickel thickness.
0.025 100 Ang.Ni 250 Ang.Ni 300 Ang.Ni
Photocurrent (mA)
0.02
0.015
0.01
0.005
0 400
500
600
700
800
900
1000
1100
Wavelength (nm)
Fig. 3. Variation of photogenerated current vs. wavelength for sample S3 at reverse voltage ¼ 2 V.
currents. As the reverse voltage applied on these devices, the capacitance will be decreased, and the depletion layer width will be increased. This effect caused an increase in the electric field which enhances the carrier collection at the inversion layer at the semiconductor side; consequently the photogenerated current will be increased.
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200
100
0 400
500
600
700
800
900
1000
1100
Wavelength (nm)
Fig. 4. Variation of device responsivity vs. wavelength for sample S3 at reverse voltage ¼ 2 V.
Fig. 4 shows the relation between responsivity vs. wavelength for sample S3, when 2 volts of reverse voltage was applied. Maximum responsivity of 400 mA/W was obtained for sample of 100 A˚ nickel at 550 nm wavelength. For other samples of different nickel thicknesses, the responsivity was low and constant for different values of wavelengths. This is because the junction was undefined and the dark current was unstable which affect the photogenerated current. Fig. 5 shows a comparison between the responsivity of different samples (S1, S3, S4 and S5) as a function of wavelength. The responsivity of S1, S3 and S4, increases as the wavelength increases, approaching maximum value at 550 A˚ wavelengths. This shows that SnO2 oxide does not enhance the device operation. The effect of SiO oxide was to reduce the responsivity of the device down to 35 mA/W. This value decreased as the wavelength increases. A simple comparison was made between responsivity of three samples (S3, S11 and S8) of different types of metals as a function of wavelengths and shown in Fig. 6. Nickel was deposited in sample S3, aluminum in sample S8 and indium in sample S11. All samples have the same thickness of metal (100 A˚) and have SiO2 native oxide as an insulated layer. It is interesting to note that the aluminum device (S8) has two maxima one at 550 nm wavelength, the other maxima was in the near infrared region (precisely at 950 nm wavelength). The other interesting point is that the indium device has one maxima for responsivity and it was occurred at 450 nm which is the middle of visible region. The effect of metals on photoelectrical effect of SB photodetector is discussed in the next paragraph when we discuss the device quantum efficiency.
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500 Ni(100)-SiO2-Si (S3) Ni-Si (S1) Ni-SnO2(33)-Si (S4) Ni-SiO(36)-Si (S5)
Response (mA/W)
400
300
200
100
0 400
500
600
700
800
900
1000
1100
Wavelength (nm)
Fig. 5. Variations of resposivity vs. wavelength at reverse voltage ¼ 2 V for S3, S1, S4 and S5 Samples.
Fig. 7 shows the variation of the quantum efficiency as a function of wavelength of photoconductor samples, S1, S3, S4, S5 and S8. The highest quantum efficiency was obtained from Ni–SiO2–Si device at 400 nm and is equal to 75%. While Al–SiO2–Si device gives lower quantum efficiency around 25% at 500 nm. Only 3.5% quantum efficiency was obtained from the Al–Si device at that wavelength. It is obvious now that nickel structure will absorb more light than the aluminum structure. This is due to the atomic weight difference of the two metals. As the atomic weight of metals increased higher thickness was obtained, consequently more photogenerated current could be collected [14]. This means that lighter vapor metals suffer from collisions with residual gases and brought back near the source away from the silicon wafer. While the higher atomic weight vapor materials have higher energy and suffer from less collisions consequently they can travel further. Also the higher energy of the impinging particles with silicon substrate creating scratches or defects enhances the binding energy between the metal and the semiconductor or oxide substrates [15]. In order to know the variation of quantum efficiency with reverse bias voltage a monochromatic laser was used. He–Ni laser of 632 nm wavelength and 1 mW power was
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200
100
0 400
500
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Wavelength (nm)
Fig. 6. Variations of resposivity vs. wavelength at reverse voltage ¼ 2 V for S3, S8 and S11 samples.
80 Ni (100) SiO2-Si (S3) N-Si (S1)
70
Ni (100) SnO2-Si (S4) Ni-SiO(36)-Si (S5)
60
Al (50)-SiO2-Si (S8)
Quantum effeciency %
Al (100)-SiO2-Si (S8)
50
40
30
20
10
0 400
500
600
700
800
900
1000
1100
Wavelength (nm)
Fig. 7. Variations of quantum efficiency vs. wavelength at reverse voltage ¼ 2 V for S1, S3, S4, S5 and S8 samples.
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80 Ni-Si (S1) Ni (100) SiO2-Si (S3)
70
Ni-Sno2-Si (S4) AlSiO2-Si (S8)
Quantum effeciency %
60
50
40
30
20
10
0 0
0.5 0.75 1
2
3 4 5 6 7 Reverse Voltage (V)
8
9
10
Fig. 8. Variations of quantum efficiency vs. reverse voltage at single wavelength of 632 nm for S3, S1, S4 and S8 samples.
used as a monochromatic and single frequency light source. Fig. 8 represents the variation of quantum efficiency with bias voltage for different prepared photoconductor samples (S1, S3, S4 and S8). Sample S3 yields 50% quantum efficiency and does not change with bias voltage, as the field exceeds 0.5 V which is enough to produce an optimum depletion layer for maximum trap carriers. For other samples (Ni–SnO2–Si and Al–SiO2–Si), the quantum efficiency starts to build up slowly as the reverse voltage exceeds 0.5 V and reaches its maximum value at 9 V. It is worth mentioning that the quantum efficiency produced in Fig. 8 is less than that noticed in Fig. 7. This is because the laser light does not cover the full area of the photodetector used, and because of the smaller light absorption coefficient of silicon at 632 nm wavelength. Fig. 9 shows the variation of detectivity as a function of wavelength when 2 V reverse voltage was applied on the samples S1, S2, S3, S6 and S8. The detectivity increased as the wavelength increased and it has two maxima. Detectivity variation with wavelength could be attributed to enabling the wasted portion of the solar spectrum to contribute to the device output [16]. The first maxima (occurred at 500 nm) could be due to absorption of incident photons at the surface far from the effect of the electric field in the space charge region. While the second maxima (occurred at 1000 nm) is due to the optical absorption depth increase as the wavelength increase (band-to-band absorption). Sample S8 produced the highest detectivity around 6.4 1010 Cm/Hz W at 500 nm wavelength. This sample had the lowest dark current compared with the rest samples. From this figure and Eq. (4), it is clear that the dark current is a noise current.
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60
Ni (100) SnO2-Si(S4) Al-SiO2-Si (S8)
D*(*109) Cm/Hz.W
50
40
30
20
10
0 400
500
600
700 800 900 Wavelength (nm)
1000
1100
Fig. 9. Variations of detectivity vs. wavelength at reverse voltage ¼ 2 V for S1, S3, S6 and S8 samples.
4. Conclusions It is concluded from the above results that varying the top contact metal thickness will adjust the degree of light trapping upon the device absorbance. But increasing the metal thickness more than an optimized value will increase the light reflection causing a high decrease in the photogenerated current. It was also noted that applying a reverse voltage across the MS structure will enhance carrier collection in the inversion layer at the semiconductor side; consequently the photogenerated current will be increased. Introducing an oxide layer between metal and semiconductor increases the total absorption depth which in turn utilizes the wasted portion of the solar spectrum. References [1] Singh J. Semiconductor Optoelectronics Physics and Technology. New York: Mc Graw Hill; 1995. [2] Ivanov VG. Quantum efficiency of Schottky photodiode near the long—wavelength edge. Semiconductors 1997;31(6):631. [3] Averin SV. Fast response photodetectors with large active area based on Schottcky—barrier semiconductor structure. Kvantovaya Electronika 1996;23(3):284. [4] Dimitruk NL. Ultraviolet control in Schottky barrier heterostructure with textured interface. Thin Solid Films 2000;364(1–2):280. [5] Pulfrey DL. MIS solar cells: a review. IEEE Trans. Electron Dev 1978;ED-25(11):1308. [6] Liu CW, Lee MH, Kno WS, Hsu BC. A novel photodetector using MOS tunneling structures. IEEE Electron Dev Lett 2000;21(6):307. [7] Seto M, Rochefort C, de Jager S, Hendriks RF, Hooft GW, Mark MB. Low leakage—current MISIM photodetector on silicon with SiO2 barrier-enhancement layer. Appl Phys Lett 1999;75(13):1676. [8] Keyes RJ. Optical and Infrared Detectors. Topics in Applied Physics, vol. 19. Berlin: Springer; 1977.
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[9] Mohamad WF. Calculation of light trapping and internal quantum efficiency of in-doped silicon (n) structure. Renew Energy J UK 2002;28(2):311. [10] Keevers MJ, Green MA. Efficiency improvements of silicon solar cells by the impurity photovoltaic effect. Appl Phys J 1994;75:4022. [11] Goldberg YG. Topical review: semiconductor near-ultraviolet photo electronics. Semicond Sci Technol 1999;14:41. [12] Laqua K, Schradar B, Moore DS, Vo-Dinh T. Detection of radiation. Part XI, IUPAC Recommendations, 1995. [13] Keevers MJ, A Green M. Absorption edge of silicon from solar cell spectral response measurement. Appl Phys Lett 1995;66(2):174. [14] Mohamad WF, Atallah FS. The effect of evaporation parameters on deposition rate of metals deposited under low pressures. Rafidian J Sci Iraq 1996;17(2):108. [15] Kiviasi RT. Optical properties of obliquely evaporated aluminum films. This Solid 1983;97:153. [16] Keevers MJ, et al. Screening of optical dopants in silicon solar cells for improved infrared response. In: Proceedings of 13th European Communities Photovoltaic Solar Energy conference, Nice, France, 1995, p. 1215.
Renewable Energy 31 (2006) 1493–1503 www.elsevier.com/locate/renene
Effects of oxide layers and metals on photoelectric and optical properties of Schottky barrier photodetector W.F. Mohamada,, A. Abou Hajarb, A.N. Salehc a
College of Engineering, Al Isra’ University, P.O. Box 22 and 33, Al Isra University Post Office, Amman 11622, Jordan b College of Engineering, Aleppo University, Aleppo, Syria c College of Engineering, Mosul University, Mosul, Iraq Received 14 April 2005; accepted 6 December 2005 Available online 7 March 2006
Abstract Recently, a lot of attention has been paid to Schottky barrier photo detectors due to their promising properties and easy of fabrication. Many samples of SB devices prepared by thermal deposition under high vacuum are studied in this research. Different types and thicknesses of oxides were deposited on silicon substrate. Metals of different types and thicknesses were deposited on top of oxides. Variation of photogenerated current, responsivity, quantum efficiency and detectivity as a function of incident light wavelength were measured. It was found that the shape of the curves has two maxima, one was around 500 nm and the other was around 700 nm. Ni (1 0 0)–SiO2–Si structure shows the maximum responsivity at 550 nm and it is equal to 400 mA/W. When comparison was made between devices of different metals, the nickel layer device showed high responsivity at visible region while the aluminum layer device showed high responsivity at near infrared region. Finally, the aluminum layer device showed detectivity higher than nickel layer device. The maximum detectivity of aluminum device was 6.4 1010 cm/Hz W. r 2006 Elsevier Ltd. All rights reserved. Keywords: Schottky barrier photodetector; MOS structures; Photoelectric properties of MOS
Corresponding author.
0960-1481/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2005.12.012
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1. Introduction The principles of photo detector operation depend on the interaction between the light and detector material. Light photons give their energy to the detector material to produce free electron–hole pairs. Collection of these free electron-hole pairs produces an electric signal. There are many types of photo detector; homojunction, hetero junction and Schottky barrier (SB) photo detectors. In recent years much attention has been paid to the SB photo detectors due to their promising properties and easy of fabrication [1–4]. Schottky barriers differ from pn junction structures. The SB has a potential barrier lower than the energy gap of the semiconductor and forming a space charge region on the semiconductor side. The space charge region is responsible for the generation of the carriers and high electric field at the semiconductor side. Thus the thermal emission mechanism is responsible for the current transportation in SB structures. It is found that the photovoltaic responsivity of SB structure was very low due to thermal current transportation mechanisms. This drawback has been rectified by introducing an oxide layer between metal and semiconductor layers [5]. This type of structure (MOS) is a special type of solar cell. When a positive voltage is applied to the metal side (reverse voltage), the energy band in the semiconductor layer will bend and a potential well between the oxide and semiconductor will be formed. The electron–hole pairs generated at the metal side due to the incident light will be stored in the potential well. The collection of this free charge will produce an electrical signal [6]. MOS detectors consist of three layers as shown in Fig. 1. The metal layer should have low series resistance and be transparent to allow light to be absorbed and transmitted through it. This can be achieved by the choice of metals at an optimized thickness. Type and thickness of the metal layer affect the output signal of photodetector. Device responsivity (R) is defined as the ratio of the output power to the input light power. The output power can be current or voltage and can be written as follows [7,8]: R¼
I Ph V ¼ , P P
(1)
where Iph is the photogenerated current, V the output voltage and P the input light power. The spectral responsivity (SR) depends on the light incident wavelength and can be expressed in terms of external quantum efficiency (Z) as follows [9]: SRðlÞ ¼
Z , hc=ql
(2) +V Transparent metal Insulator
P-Substrate
Semiconductor GND Fig 1. MOS-Structure.
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where h is the Plank’s constant, c the light velocity, q the electron charge and l the wavelength of the incident light. The spectral responsivity of the device is a measurable quantity and defined as short circuit current density per unit irradiance as a function of wavelength. From the above definition and Eq. (2), the external quantum efficiency (Z) can be defined as the fraction of minority carrier’s generated in the device due to the light application and collected under short circuit current conditions. It can be written as follows [10]: I ph hc Z¼ . (3) P ql The external quantum efficiency can be defined as the ratio of generated electrons to the number of incident photons, while the internal quantum efficiency (g) is the ratio of generated electrons to the number of absorbed photons and can be related to the external quantum efficiency as follows [11]: Z ¼ ð1 RFS Þag,
(4)
where RFS is the reflectivity of the front surface and (a) the absorption coefficient. In order to obtain high quantum efficiency and detectivity, the high reflective surface must be placed at the device rear and make the depletion layer as wide as possible. But small signal detectivity depends on the noise around the device. Noise equivalent power (NEP) is the ratio between applied power to noise which is equal to one at bandwidth of 1 Hz at defined wavelength. The reciprocal of NEP is defined as the detectivity (D) of the device. The specific detectivity (D*) is defined as the product of detectivity multiplied by the detector area (A) and band width frequency (Df) as follows [12]: D ¼ ðADf Þ1=2 =NEP:
(5)
The specific detectivity can be expressed in terms of external quantum efficiency as follows [13]: D ¼
Zl 2hcðj d =qÞ1=2
,
(6)
where jd is the dark current of the device. It is clear that the specific detectivity is decreased as the dark current (or more specific the leakage current) is increased. 2. Samples preparation Many samples were prepared by thermal deposition under high vacuum using Balzer coating unit. The deposition parameters and the sequence of fabrication of MOS devices shown in Fig. 1 are as follows: (a) Aluminum of 2000 A˚ thickness was deposited on the back surface of the silicon wafer to form the back contacts of the device. (b) In second step the prepared samples were prepared in two groups: the first group was left in the air for the native oxide of SiO2 to be grown, while the native oxide was removed in the second group and other oxides were deposited, namely SnO2 and SiO.
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Table 1 Prepared samples for different parameters Notes
Structure
Sample
Without oxide, Ni ¼ 100 A˚ Native oxide, Ni ¼ 50–300 A˚ SnO2 ¼ 22–43 A˚, Ni ¼ 100 A˚ SiO ¼ 36–43 A˚, Ni ¼ 100 A˚ Without oxide, Al 100 A˚ Native oxide, Ni ¼ 50–300 A˚ Native oxide, In ¼ 50–100 A˚
Ni–Si Ni–SiO2–Si Ni–SnO2–Si Ni–SiO–Si Al–Si Al–SiO2–Si In–SiO2–Si
S1 S3 S4 S5 S6 S8 S11
(c) Aluminum or nickel was deposited on top of the oxides with different thicknesses to form front contacts. (d) Annealing process at 350 1C (under vacuum) for one hour was carried out for all samples to support front and back contact formation. All samples prepared in this research for different parameters are shown in Table 1. The substrate was n silicon of 400 mm thickness coated with a thin layer of n+ silicon of 6 mm thicknesses. The samples were circles of 0.2 cm2 area. 3. Results and discussions It was proved in the previous section that the quantum efficiency is directly proportional to the photogenerated current. Consequently, responsivity and detectivity will behave in the same manner. Here the effect of thickness and type of metal on the photodetector behavior as a function of incident light wavelength will be examined. Fig. 2 shows the variation of photogenerated current as a function of incident light wavelength for the fabricated samples (S3). Nickel thickness of the sample was varied from 50 A˚ up to 300 A˚ in 50 A˚ steps. No bias fields were applied on the samples, in other words the devices were working in the photovoltaic mode. It is clear that the photogenerated current increased as the wavelength increased, and reached its maximum value at 600 nm of wavelength. Another current maxima was noticed at 700 nm of wavelength. Beyond 700 nm of wavelength, the current decreased and reached minimum value at 1100 nm wavelength. Also it is noticed that maximum photogenerated current is obtained from the sample of 100 A˚ nickel thickness. It is thought that the degree of light trapping in metal layer is adjusted by varying the thickness upon the device absorbance [13]. When nickel thickness increased more than 100 A˚, photogenerated current was decreased due to high reflection and low transmission of light through thick nickel layer. Reverse voltage of 2 V was applied on the samples and photogenerated current variation was calculated as a function of incident light wavelength as shown in Fig. 3. This means that the devices were working in photoconductor mode. Again two current maxima are noted at the same values of wavelength calculated in the previous figure. It could be concluded from this figure, that the photogenerated current increased by a factor of 1.5 approximately compared with the current calculated in the previous figure. It is known that the total currents in these devices are the summation of dark and photogenerated
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Photocurrent (mA)
0.012 0.01 0.008 0.006 0.004 0.002 0 400
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600
700
800
900
1000
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Wavelength (nm)
Fig. 2. Variation of photogenerated current vs. wavelength for sample S3, of different nickel thickness.
0.025 100 Ang.Ni 250 Ang.Ni 300 Ang.Ni
Photocurrent (mA)
0.02
0.015
0.01
0.005
0 400
500
600
700
800
900
1000
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Wavelength (nm)
Fig. 3. Variation of photogenerated current vs. wavelength for sample S3 at reverse voltage ¼ 2 V.
currents. As the reverse voltage applied on these devices, the capacitance will be decreased, and the depletion layer width will be increased. This effect caused an increase in the electric field which enhances the carrier collection at the inversion layer at the semiconductor side; consequently the photogenerated current will be increased.
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Fig. 4. Variation of device responsivity vs. wavelength for sample S3 at reverse voltage ¼ 2 V.
Fig. 4 shows the relation between responsivity vs. wavelength for sample S3, when 2 volts of reverse voltage was applied. Maximum responsivity of 400 mA/W was obtained for sample of 100 A˚ nickel at 550 nm wavelength. For other samples of different nickel thicknesses, the responsivity was low and constant for different values of wavelengths. This is because the junction was undefined and the dark current was unstable which affect the photogenerated current. Fig. 5 shows a comparison between the responsivity of different samples (S1, S3, S4 and S5) as a function of wavelength. The responsivity of S1, S3 and S4, increases as the wavelength increases, approaching maximum value at 550 A˚ wavelengths. This shows that SnO2 oxide does not enhance the device operation. The effect of SiO oxide was to reduce the responsivity of the device down to 35 mA/W. This value decreased as the wavelength increases. A simple comparison was made between responsivity of three samples (S3, S11 and S8) of different types of metals as a function of wavelengths and shown in Fig. 6. Nickel was deposited in sample S3, aluminum in sample S8 and indium in sample S11. All samples have the same thickness of metal (100 A˚) and have SiO2 native oxide as an insulated layer. It is interesting to note that the aluminum device (S8) has two maxima one at 550 nm wavelength, the other maxima was in the near infrared region (precisely at 950 nm wavelength). The other interesting point is that the indium device has one maxima for responsivity and it was occurred at 450 nm which is the middle of visible region. The effect of metals on photoelectrical effect of SB photodetector is discussed in the next paragraph when we discuss the device quantum efficiency.
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Fig. 5. Variations of resposivity vs. wavelength at reverse voltage ¼ 2 V for S3, S1, S4 and S5 Samples.
Fig. 7 shows the variation of the quantum efficiency as a function of wavelength of photoconductor samples, S1, S3, S4, S5 and S8. The highest quantum efficiency was obtained from Ni–SiO2–Si device at 400 nm and is equal to 75%. While Al–SiO2–Si device gives lower quantum efficiency around 25% at 500 nm. Only 3.5% quantum efficiency was obtained from the Al–Si device at that wavelength. It is obvious now that nickel structure will absorb more light than the aluminum structure. This is due to the atomic weight difference of the two metals. As the atomic weight of metals increased higher thickness was obtained, consequently more photogenerated current could be collected [14]. This means that lighter vapor metals suffer from collisions with residual gases and brought back near the source away from the silicon wafer. While the higher atomic weight vapor materials have higher energy and suffer from less collisions consequently they can travel further. Also the higher energy of the impinging particles with silicon substrate creating scratches or defects enhances the binding energy between the metal and the semiconductor or oxide substrates [15]. In order to know the variation of quantum efficiency with reverse bias voltage a monochromatic laser was used. He–Ni laser of 632 nm wavelength and 1 mW power was
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Fig. 6. Variations of resposivity vs. wavelength at reverse voltage ¼ 2 V for S3, S8 and S11 samples.
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Fig. 7. Variations of quantum efficiency vs. wavelength at reverse voltage ¼ 2 V for S1, S3, S4, S5 and S8 samples.
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Fig. 8. Variations of quantum efficiency vs. reverse voltage at single wavelength of 632 nm for S3, S1, S4 and S8 samples.
used as a monochromatic and single frequency light source. Fig. 8 represents the variation of quantum efficiency with bias voltage for different prepared photoconductor samples (S1, S3, S4 and S8). Sample S3 yields 50% quantum efficiency and does not change with bias voltage, as the field exceeds 0.5 V which is enough to produce an optimum depletion layer for maximum trap carriers. For other samples (Ni–SnO2–Si and Al–SiO2–Si), the quantum efficiency starts to build up slowly as the reverse voltage exceeds 0.5 V and reaches its maximum value at 9 V. It is worth mentioning that the quantum efficiency produced in Fig. 8 is less than that noticed in Fig. 7. This is because the laser light does not cover the full area of the photodetector used, and because of the smaller light absorption coefficient of silicon at 632 nm wavelength. Fig. 9 shows the variation of detectivity as a function of wavelength when 2 V reverse voltage was applied on the samples S1, S2, S3, S6 and S8. The detectivity increased as the wavelength increased and it has two maxima. Detectivity variation with wavelength could be attributed to enabling the wasted portion of the solar spectrum to contribute to the device output [16]. The first maxima (occurred at 500 nm) could be due to absorption of incident photons at the surface far from the effect of the electric field in the space charge region. While the second maxima (occurred at 1000 nm) is due to the optical absorption depth increase as the wavelength increase (band-to-band absorption). Sample S8 produced the highest detectivity around 6.4 1010 Cm/Hz W at 500 nm wavelength. This sample had the lowest dark current compared with the rest samples. From this figure and Eq. (4), it is clear that the dark current is a noise current.
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Fig. 9. Variations of detectivity vs. wavelength at reverse voltage ¼ 2 V for S1, S3, S6 and S8 samples.
4. Conclusions It is concluded from the above results that varying the top contact metal thickness will adjust the degree of light trapping upon the device absorbance. But increasing the metal thickness more than an optimized value will increase the light reflection causing a high decrease in the photogenerated current. It was also noted that applying a reverse voltage across the MS structure will enhance carrier collection in the inversion layer at the semiconductor side; consequently the photogenerated current will be increased. Introducing an oxide layer between metal and semiconductor increases the total absorption depth which in turn utilizes the wasted portion of the solar spectrum. References [1] Singh J. Semiconductor Optoelectronics Physics and Technology. New York: Mc Graw Hill; 1995. [2] Ivanov VG. Quantum efficiency of Schottky photodiode near the long—wavelength edge. Semiconductors 1997;31(6):631. [3] Averin SV. Fast response photodetectors with large active area based on Schottcky—barrier semiconductor structure. Kvantovaya Electronika 1996;23(3):284. [4] Dimitruk NL. Ultraviolet control in Schottky barrier heterostructure with textured interface. Thin Solid Films 2000;364(1–2):280. [5] Pulfrey DL. MIS solar cells: a review. IEEE Trans. Electron Dev 1978;ED-25(11):1308. [6] Liu CW, Lee MH, Kno WS, Hsu BC. A novel photodetector using MOS tunneling structures. IEEE Electron Dev Lett 2000;21(6):307. [7] Seto M, Rochefort C, de Jager S, Hendriks RF, Hooft GW, Mark MB. Low leakage—current MISIM photodetector on silicon with SiO2 barrier-enhancement layer. Appl Phys Lett 1999;75(13):1676. [8] Keyes RJ. Optical and Infrared Detectors. Topics in Applied Physics, vol. 19. Berlin: Springer; 1977.
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[9] Mohamad WF. Calculation of light trapping and internal quantum efficiency of in-doped silicon (n) structure. Renew Energy J UK 2002;28(2):311. [10] Keevers MJ, Green MA. Efficiency improvements of silicon solar cells by the impurity photovoltaic effect. Appl Phys J 1994;75:4022. [11] Goldberg YG. Topical review: semiconductor near-ultraviolet photo electronics. Semicond Sci Technol 1999;14:41. [12] Laqua K, Schradar B, Moore DS, Vo-Dinh T. Detection of radiation. Part XI, IUPAC Recommendations, 1995. [13] Keevers MJ, A Green M. Absorption edge of silicon from solar cell spectral response measurement. Appl Phys Lett 1995;66(2):174. [14] Mohamad WF, Atallah FS. The effect of evaporation parameters on deposition rate of metals deposited under low pressures. Rafidian J Sci Iraq 1996;17(2):108. [15] Kiviasi RT. Optical properties of obliquely evaporated aluminum films. This Solid 1983;97:153. [16] Keevers MJ, et al. Screening of optical dopants in silicon solar cells for improved infrared response. In: Proceedings of 13th European Communities Photovoltaic Solar Energy conference, Nice, France, 1995, p. 1215.