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Radiation-hardness of silicon p–i–n photodiodes operated under illumination by light of different wavelengths
Nuclear Instruments and Methods in Physics Research A 632 (2011) 59–68
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Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Radiation-hardness of silicon p–i–n photodiodes operated under illumination by light of different wavelengths S.J. Moloi a,n, M. McPherson b a b
Department of Physics, P. O. Box 392, University of South Africa, Pretoria 0003, South Africa McPherso Academic Consulting, Post Net Suite 194, Private Bag X 2230, Mafikeng South 2791, South Africa
a r t i c l e in f o
abstract
Article history: Received 17 September 2010 Received in revised form 15 December 2010 Accepted 18 December 2010 Available online 29 December 2010
The photo response of silicon p–i–n photodiodes has been investigated for light of different wavelengths using I–V and C–V techniques. The measurements were carried out prior to and after radiation damage by 1 MeV neutrons. A main indication is that effects due to incident photons are more pronounced at low radiation fluence but they become negligible as the fluence increases. This is due to defect levels induced by the irradiation in the energy gap of the silicon. The number of these levels increases with radiation fluence and they act mainly to recombine photo-generated carriers. This recombination reduces the number of mobile charges and hence the measured current and capacitance of the photodiode. The results show that silicon gets quickly damaged by radiation in the initial stages of the irradiation process but as the fluence increases the material becomes resistant to further damage. We contend that silicon becomes radiation-hard after initial heavy damage by 1 MeV neutrons. & 2011 Elsevier B.V. All rights reserved.
Keywords: Semiconductor Silicon Photodiode Wavelength Current Capacitance
1. Introduction In electrical characterisation of semiconductor diodes, the current–voltage (I–V) and the capacitance–voltage (C–V) techniques are, respectively, used to determine the carrier density in the diode and the doping profile in the substrate material [1–3]. These semiconductor diodes are used in a variety of applications such as detectors for high energy particles [4] or optoelectronic devices for communication [5]. The ability to vary the conductivity of these diodes as a function of temperature or as a function of optical excitation [6] makes them attractive for these applications. However, when the diodes are subjected to high radiation environments, they fail to operate efficiently due to defect levels created by radiation damage in the energy gap of the substrate material [7–9]. Thus, in trying to fabricate radiation-hard detectors, the properties of the defect levels induced in the energy gap and their influence on the diodes have to be investigated. In order to better understand the properties of the defects induced in silicon, the photo response of neutron-irradiated silicon p–i–n photodiodes was investigated for different light intensities [10]. The results obtained then were for infra-red light and were used to explain the effects of light intensity on photodiodes irradiated only up to a very low fluence of 2.5 1014 n cm 2.
n
Corresponding author. E-mail addresses: [email protected], [email protected] (S.J. Moloi), [email protected] (M. McPherson). 0168-9002/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2010.12.180
Since particle detectors will normally be operated under very high radiation environments [11–13], it is required that the effects of light on highly irradiated photodiodes are studied as well. Furthermore, we believe that it is also important to study the photo response of irradiated photodiodes for different wavelengths. Since an increase in light intensity results in an increase in the measured current and capacitance of the photodiodes [10,14–15], it is expected that both current and capacitance will also increase with an increase in the incident light of the wavelengths ranging from 565 to 660 nm. Therefore, a full characterisation of highly irradiated photodiodes for different wavelengths can be used to supplement and expand on the work carried out in [10,14–15] for different light intensities. It is for this reason that this project was carried out. In this work, the current and capacitance response of several silicon p–i–n photodiodes were investigated under illumination by light emitting diodes (LEDs) of different wavelengths (or colours). The measured current and capacitance were found to increase with an increase in wavelength. This increase in the current and capacitance with the wavelength has been found to be more pronounced on the unirradiated photodiode and on the photodiodes that were irradiated to low fluences. This shows that the incident light has a strong effect on these photodiodes. As the fluence increases further, however, the increase of the measured current and capacitance with the wavelength is found to be reduced. This shows that the photodiodes are becoming resistant to the effects of the incident light.
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S.J. Moloi, M. McPherson / Nuclear Instruments and Methods in Physics Research A 632 (2011) 59–68
The results obtained in this work are interpreted in terms of the defect levels that are induced by the irradiation in the energy gap of the silicon. These defect levels increase with the radiation fluence and they act preferentially as recombination centres [16–18] to trap all the photo-generated carriers to decrease the measured current and capacitance and so to increase the resistivity of the material [19]. The induced levels, however, can also generate carriers to increase the dark current and capacitance of the photodiode. These defect levels are generation–recombination (g–r) centres and if they generate and recombine equally they are responsible for the relaxation behaviour of the material [20]. Relaxation material tends to be resistant to radiation damage and has tD b t0 , which is different to lifetime material where tD 5 t0 [21–24]. In these relations tD is the dielectric relaxation time and t0 is the minority carrier recombination lifetime.
2. Experimental details Six p–i–n photodiodes fabricated from high purity, high resistivity silicon were acquired from Hamamatsu with the series number of S3590-08. The active diode area is 1 cm2 and the thickness is 300 mm. Five of the photodiodes were irradiated at the National Energy Corporation of South Africa (NECSA) by 1 MeV neutrons to different fluences in the range from 1013 to 1017 n cm 2. The sixth photodiode was left unirradiated to be used as a control and it was named H00. The five photodiodes irradiated to 1013, 1014, 1015, 1016 and 1017 n cm 2 were named H13, H14, H15, H16 and H17, respectively. Prior to irradiation, I–V and C–V measurements were taken on all six photodiodes in the dark and at 302 K. The temperature was monitored for every photodiode during the measurements. The characteristics of the photodiodes before irradiation were found to be very similar such that we assume that the six photodiodes are one photodiode that has undergone repeated radiation damage. After irradiation the photodiodes were characterised by I–V and C–V measurements in the dark and under illumination at 302 K. The illumination was achieved by use of LEDs of different wavelengths. After several investigations to ensure full illumination, the LEDs were each placed at a distance of 10 mm in front of the photodiodes. The LEDs were positioned at right angles to the centre of the photodiode in such a manner that the maximum light was incident onto the centre of the photodiode. In this way it was possible to assume that the illumination was, as much as possible, over the whole active area of the photodiode. Thus, any differences in the results would only be due to changes in the wavelength of the incident light. The LEDs were powered from a Goodwill GSP-3030 d.c. voltage source, which can supply up to 30 V. The current through the LEDs was kept constant at 20 mA throughout each measurement to ensure constant illumination. The properties of the LEDs used are summarised in Table 1, where some of the values have been extracted from the manufacturer’s data sheet [25]. The colours of the LEDs used are presented in the first column of the table corresponding to their peak emission wavelengths, lLED, as quoted in the manufacturer’s data sheet [25]. The third column is Table 1 Properties of the LEDs that were used to illuminate the photodiodes. Some of the values were extracted from the manufacturer’s data sheet [25]. LED
kLED (nm)
ELED (eV)
VLED (V)
Red Orange Yellow Green
660 610 590 565
1.88182 2.03607 2.10509 2.19823
5 5 5 5
the energy value, ELED, calculated by use of the relation ELED lLED ¼ hc, which is presumed to be the photon energy of the light incident onto the photodiode. In this relation, h is Planck’s constant and c is the speed of light in vacuum. It can be seen that the photon energy emitted by all the LEDs is higher than the energy gap of silicon (¼1.12 eV at 300 K), the substrate material for the photodiodes. Since the photon energy is higher than the energy gap of silicon, it is expected that the incident light will be absorbed throughout the depth of the substrate material to generate electron–hole (e–h) pairs that will be drawn to the electrodes by the applied voltage to contribute to the measured current and capacitance. Even though the measurements for this work were performed at 302 K, it is considered that they are a fair estimate of those carried out at 300 K at which 1.12 eV was evaluated. The I–V measurements were carried out using a Keithley 6487 Picoammeter with a built-in voltage source. The instrument was operated in the remote mode by connecting it to a computer through an RS-232 cable. Thus, the computer was used for control of the picoammeter and for data acquisition. The measurements were carried out for all photodiodes using a sweep mode under reverse bias conditions. In all cases the data were taken up to 100 V in voltage steps of 0.01–0.10 V, of 0.10–1.00 V and finally of 1.00–100 V. The time between measurements was set as 1 s, a time determined after many trials [26] and intended to allow for the reading to settle (or for the photodiode to stabilise). For all measurements, each photodiode and LED were placed in an inhouse, custom-made, high-resistance test fixture for the voltage sweep run. Between measurements, the photodiodes were stored at sub-zero temperatures in a fridge. The C–V measurements in reverse bias were carried out manually using an Agilent 4263B LCR metre. The data were acquired up to 30 V using an external voltage supplied by a Goodwill GSP-3030 d.c. voltage source. The measurements were carried out for the six photodiodes, in the dark and at a frequency of 1 kHz. After several trial measurements [26], a time of 8 s was chosen between readings. This was found to be the most suitable time to allow for the capacitance to settle and thus to allow acquisition of a more realistic value. The photodiodes were mounted onto an external Agilent 16065A test fixture. The illumination conditions as described above (photodiode and LED) were maintained. The LCR metre was zeroed, first as an open circuit and then as a short circuit, prior to taking readings. This was done in order to correct the capacitance of the leads and for any stray capacitance.
3. Results and discussion The results presented here are the current and the capacitance both measured in reverse bias. The current is a measure of the carrier density in the photodiode while the capacitance is a measure of the defect density in the substrate material. It is expected that both the current (carriers or mobile charge) and the capacitance (defects or fixed charge) will increase after the irradiation since the number of defects increases with the irradiation. Here the capacitance measures the doping profile, which is the density of defects created by the irradiation since such defects recombine all available dopants [27]. 3.1. Reverse bias current The measured current of a diode is related to the applied voltage [28] as eV I ¼ Is exp 1 ð1Þ ZkT
S.J. Moloi, M. McPherson / Nuclear Instruments and Methods in Physics Research A 632 (2011) 59–68
where Is is the saturation current given as the negative current since the first term in the equation is negligible in reverse bias. In the equation above, e is the electronic charge, V is the applied voltage, Z is the ideality factor, k is the Boltzman constant and T is the absolute temperature. Under illumination, the total current is given [6] by eV 1 Ii I ¼ Is exp ð2Þ ZkT where Ii is the current due to photo-generated carriers. The light intensity transmitted through a diode,Ft , is related to the absorption coefficient, a [29] as
Ft ¼ F0 expðalÞ
ð3Þ
where F0 is the intensity of a photon incident on the diode and l is the active thickness of the diode. The above relation shows that at a constant l, the light intensity increases exponentially with a decrease in the absorption coefficient. The absorption coefficient varies with the photon wavelength, l, and the energy gap, Eg [30] as g hc ap Eg ð4Þ
l
where h and c are as defined before and g ¼½, 3/2 and 2 for the direct transition, for the vertical forbidden transition and for the indirect transition between extrema situated at different points in the zone, respectively. Eq. (4) holds only for hc=l 4 Eg : The opposite case occurs if the wavelength of the incident photons is greater than 1700 nm, in which case the absorption coefficient of the silicon becomes negligible [31]. This dependence of the absorption coefficient on the wavelength shows that the intensity of photons that have a shorter wavelength is reduced with the depth of penetration into the silicon such that they are absorbed over a short distance. These photons could also be absorbed very close to the surface of the silicon. In either case the result is a lower amount of photo-generated carriers contributing to the measured current. Photons with a long wavelength, on the other hand, are absorbed
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over a long distance into the silicon and this results into a higher amount of photo-generated carriers contributing to the measured current. Thus, at the same applied electric field, short wavelengths will show low current while long wavelengths will show high current. The effect of wavelength is shown in Fig. 1(a) for the unirradiated photodiode, H00 with an arbitrary reference line drawn from 1 mA at 4 mV to 1 mA at 0.15 kV. The measured current increases by more than two orders of magnitude when the photodiode is illuminated. The current measured in the dark is much lower than that measured with the red LED, which is much higher than that measured with the orange, the yellow and the green LEDs. These results show that light from the red LED is absorbed over a longer distance to generate more e–h pairs that contribute to the measured current than that from the other LEDs. Another observation from Fig. 1(a) is that the photocurrent shows saturation up to 1 102 V for all the LEDs indicating that the photocurrent is independent of the voltage. This lack of dependence shows that the electric field is not high enough to draw the charges to opposite electrodes to contribute to the measured current. The profile of the dark current at low voltages differs from that at high voltages. The current increases linearly up to 4 10 2 V, after which some form of saturation is observed up to 1 101 V. The initial increase of the current could be due to thermal generation of carriers when the voltage is low or due to the built-in charge of the silicon material. At voltages higher than 1 101 V an increase in the current shows that the photodiode is attempting to approach breakdown, which means that at this high voltage the electric field is able to separate the e–h pairs to draw them to opposite electrodes to contribute to the measured current. The profile of Fig. 1(a) was used to generate a plot of the current read off at 100 V for all colours as a function of wavelength. This analysis is shown in the logarithmic plot of Fig. 1(b) and was done in order to investigate a variation of the current with the wavelength of the incident light. A linear relation in logarithmic scale is noted with a slope much greater than unity (at 43.1).
10-2 10-3
Red
10-4
Current (A)
10-5
Orange and yellow
10-6 10-7
Reverse current @ 100 V (A)
10
-4
Green
Sample H00 T = 302 K
10
Dark
-8
Green - 565 nm
Sample H00 Linear fit Slope = 43.052 Intercept = 262.393
10-5
10-6
Yellow - 590 nm Orange - 610 nm
10-9
Dark
Red - 660 nm
10-10 10-2
10-1
100 Reverse bias (V)
101
102
10-7 550
600
650
700
Wavelength (nm)
Fig. 1. A variation of the reverse I–V characteristic under illumination with light of different wavelengths at 302 K for H00 (a), and a plot of the reverse current read off at 100 V as a function of the wavelength (b). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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The magnitude of the slope shows that the current varies exponentially with the wavelength. This means that carrier generation is exponentially related to the wavelength of the incident light. Fig. 2 shows the I–V results for H13, the photodiode irradiated to a fluence of 1013 n cm 2, with an arbitrary reference line similar to the one in Fig. 1. The dark current is higher than that of H00 and the profile is found to be positioned closer to the profile of the green light at high voltages. This increase of the dark current is due to an increase in the number of e–h pairs after radiation damage in the silicon. The increase in e–h pairs is due to defect levels that are induced in the energy gap of the silicon [32–34]. The photocurrent for all wavelengths, on the other hand, still shows saturation up to 102 V. It can be noted that only the current generated by the green LED has increased slightly in this photodiode and that it is no longer completely independent of the voltage. The significance of this is that in this photodiode the defects generated by the radiation are not high enough in number to be able to recombine those carriers generated by the green light. Thus, some current is measured so that the trend is both due to those carriers generated by the radiation defects as well as those generated by the incident photons. A plot of the current read off at 100 V as a function of the wavelength for H13 is shown in Fig. 2(b). A linear relation in logarithmic scale is obtained. The slope is found to be 41.2, slightly lower than the 43.1 obtained for H00. This reduction of the slope shows that carrier generation is less exponential indicating that the photocurrent has decreased after irradiation. This is due to the defect levels that are induced by the irradiation in the energy gap. These defects recombine the photo-generated carriers to reduce the amount of carriers that are drawn to the electrodes. This results in a decrease in the measured photocurrent. Since these defects increase with an increase in the radiation fluence [11], it is expected that the measured photocurrent will decrease as the radiation fluence increases further. Fig. 3 shows the I–V characteristic of the photodiodes irradiated to 1014 (a), 1015 (b), 1016 (c) and 1017 n cm 2 (d) all with the
arbitrary reference line as in previous figures. A main observation from plot (a) for photodiode H14 is that the current generated by shorter wavelengths is equal to the dark current. These results show that the properties of this photodiode are independent of the incident light of the shorter wavelengths. Even though the current generated by the red LED is higher than that generated by the other LEDs, it can be seen that the trend has come closer to that of the dark current (or that the red LED current has decreased) for low voltages. This is because the photo-generated carriers are recombined by the radiation-induced defect levels to reduce the photocurrent. In comparison with H13, the dark current in H14 has increased to show that the defect levels can also generate carriers to contribute to the measured current. Thus, these levels can recombine photogenerated carriers to reduce the photocurrent or generate carriers to increase the dark current. Another observation from plot (a) is that the trend for the red LED is different to all the other trends, showing saturation up to 100 V from where the current increases linearly with voltage up to 7 101 V. At voltages higher than 7 101 V the current becomes constant (or is saturated). The trends of the shorter wavelengths, in contrast, show a linear increase of current with voltage up to 7 10 2 V from where saturation (constant current) is observed up to 102 V. Since the number of defect levels increases with an increase in radiation fluence, it is expected that the properties of the photodiodes irradiated to fluences higher than 1014 n cm 2 will be independent for all wavelengths of the incident light. The results for H15, the photodiode irradiated to a fluence of 1015 n cm 2 are shown in plot (b). The trends have come closer together and the photocurrent has decreased for all voltages. A significant observation is that for all wavelengths, the current continues increasing at 102 V. The trend for the red LED at low voltages, however, is independent of the applied voltage to show saturation of the current. Two linear regions with different slopes are observed in the plot for the trends of the shorter wavelengths and dark current. It can be noted that the slope of the linear region
10-2 10-3
Red
10-4
Orange and yellow
Current (A)
10-5 Green
10-6
Dark
10-7
Sample H13 T = 302 K Dark Green - 565 nm
10-8
Reverse current @ 100 V (A)
10-4
Sample H13 Linear fit Slope = 41.207 Intercept = 251.611
10-5
10-6
Yellow - 590 nm Orange - 610 nm Red - 660 nm
10-9 10-10 10-2
10-1
100 Reverse bias (V)
101
102
10-7 550
600 650 Wavelength (nm)
700
Fig. 2. A variation of the reverse I–V characteristic under illumination with light of different wavelengths at 302 K for H13 (a), and a plot of the reverse current read off at 100 V as a function of the wavelength (b). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
10-2
10-2
10-3
10-3
10-4
10-4
10-5
10-5 Current (A)
Current (A)
S.J. Moloi, M. McPherson / Nuclear Instruments and Methods in Physics Research A 632 (2011) 59–68
10-6 Sample H14 T = 302 K Dark Green - 565 nm Yellow - 590 nm Orange - 610 nm Red - 660 nm
10-7 10-8 10-9
10-6 Sample H15 T = 302 K Dark Green - 565 nm Yellow - 590 nm Orange - 610 nm Red - 660 nm
10-7 10-8 10-9
10-10
10-10 10-2
10-1
100
101
102
10-2
10-1
Reverse bias (V)
10-2
10-2
10-3
10-3
10-4
10-4
10-5
10-5
10-6
Sample H16 T = 302 K Dark Green - 565 nm Yellow - 590 nm Orange - 610 nm Red - 660 nm
10-7 10-8 10-9
100
101
102
Reverse bias (V)
Current (A)
Current (A)
63
10-6 Sample H17 T = 302 K Dark White - 565 nm Green - 590 nm Orange - 610 nm Red - 660 nm
10-7 10-8 10-9
10-10
10-10 10-2
10-1
100
101
102
Reverse bias (V)
10-2
10-1
100
101
102
Reverse bias (V)
Fig. 3. A variation of the reverse I–V characteristic under illumination with light of different wavelengths at 302 K for H14 (a), H15 (b), H16 (c) and H17 (d). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
at voltages lower than 1 10 1 V is higher than the slope of the linear region at high voltages. A high slope at low voltages shows that in H15 the current is due to carriers that are generated by the incident light, by the temperature and by the radiation-induced defect levels. Unlike the current measured for H00, H13 and H14, the current measured for H15 does not show saturation at high voltages for all wavelengths, which indicates the onset of breakdown. This indication of breakdown implies that there are more charges drawn to the electrodes. This in turn shows that at high voltage it is becoming easier to separate the e–h pairs such that any mobile charge gets drawn to the electrodes to contribute to the measured current. The results for H16, the photodiode irradiated to a fluence of 1016 n cm 2 are shown in plot (c). At low voltages the trend for
the red LED shows a slight independence of voltage. At high voltages, however, the current generated at all wavelengths is nearly equal to the dark current. The current increases linearly up to 7 100 V, after which a gentle increase is observed up to 5 101 V. At voltages higher than 5 101 V, the current varies as V 1=2 to show that the photodiode is attempting to approach breakdown. Unlike in the case of photodiode H00 where breakdown is observed for dark current only, in H16 and H15 breakdown is observed for all currents. This means that any mobile charge is being drawn to the respective electrodes to contribute to the measured current. The current for photodiode H17, as shown in plot (d), increases linearly with voltage up to 3 100 V after which a gentle increase is observed up to 7 101 V. At voltages higher than 7 101 V the
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S.J. Moloi, M. McPherson / Nuclear Instruments and Methods in Physics Research A 632 (2011) 59–68
current increases quickly with voltage up to 102 V to signify the onset of breakdown. Similar to H16 and H15, the onset of breakdown in H17 is also observed for all currents. Since the trends for photocurrent are not horizontal as it was observed in H00 and H13 and are found to be grouped together with the trend of the dark current, it can be argued that the effects of light are reduced as the fluence increases. This is due to the defect levels induced in the energy gap of the material by the irradiation. They recombine photo-generated carriers and make the measured current to be independent of the incident photons. From the obtained results it can be seen that the linear region of the trend at low voltages increases with an increase in radiation fluence. As a result, the photodiodes become more Ohmic as the fluence increases. This Ohmic behaviour of the photodiodes has been explained elsewhere [32,35] in terms of the defects that turn silicon into a relaxation material. The behaviour is caused by the g–r centres that are induced in the energy gap of the material by radiation damage [20]. These levels are situated close to the midgap where they interact equally with both bands to maintain the Fermi energy at the intrinsic level [36]. This type of material is known to be resistant to radiation damage. Thus, the results presented here also show that the material becomes radiation-hard after the initial heavy damage by radiation. At high electric fields, however, the Ohmic behaviour is not that well-pronounced to show that the field is able to quickly sweep out carriers. Thus, a large number of carriers are collected at opposite electrodes to induce breakdown. It must be noted here that the charge in the photodiode is due to photo-generated carriers as well as to e–h pairs generated from g–r centres. However, at this fluence the measured current becomes limited by the g–r centre activity such that the photo-generation activity is irrelevant. This is also shown by the photocurrent being equal to the dark current at all voltages. A linear relationship of the reverse current measured at 100 V with wavelength was also obtained for the diodes H14, H15, H16 and H17 as in Figs. 1(b) and 2(b). The slopes obtained were used to generate the plot of Fig. 4. These values of the slopes are shown in Table 2 for all the diodes tested. At low fluences a considerable dependence of the slope on the fluence is observed but from H15 the slope decreases gently with fluence to show that the measured current becomes independent of the incident light. These results
Slope (Reverse current at 100 V versus λ)
45 40
H00 H13 H14
35 30 H15
25 20 15
H16
10 5 H17
0 0.0
2.0x103
4.0x103
6.0x103
8.0x103
1.0x104
Fluence (x 1013n cm-2) Fig. 4. A variation of the slopes (obtained from a current versus wavelength plot) with fluence. The slope decreases as the fluence increases.
Table 2 Slopes obtained from the plots of the reverse current read off at 100 V as a function of the wavelength for unirradiated and irradiated photodiodes. Photodiode
H00
H13
H14
H15
H16
H17
Slope
43.052
41.307
39.701
25.598
14.502
4.285
show that the effects of light are reduced as the material becomes radiation-hard (or relaxation-like) after heavy irradiation. 3.2. Reverse bias capacitance The capacitance was measured as a function of the wavelength of light incident onto the photodiodes. At reverse bias, the capacitance of a semiconductor diode is the depletion capacitance given [6] by 1=2 A ees e0 C¼ Nd ð5Þ 2 ðVbi þVÞ where A is the active area of the photodiode, es is the dielectric constant of the semiconductor,e0 is the dielectric constant of free space, Vbi is the diode built-in voltage and Nd is the dopant density of the semiconductor. The above equation shows that C2pNd for a constant V. Since an increase in light intensity results in an increase in the number of e–h pairs, it is expected that the capacitance will increase with an increase in the wavelength of light incident onto the photodiode for the same voltage. Fig. 5 shows a reverse C–V characteristic of the unirradiated photodiode, H00, in the dark and under illumination by light of different wavelengths. A main observation from plot (a) is that the capacitance measured at short wavelengths (610, 590 and 565 nm) is nearly equal to that measured in the dark for all voltages. Another striking feature is that the profiles of the capacitance measured at these wavelengths differ from that of the red LED (660 nm). The profiles for low wavelengths show a steep fall at low voltages which becomes a gradual fall at higher voltages. The profile for the red LED, on the other hand, is horizontal at voltages lower than 1 101 V to show that the capacitance is nearly independent of the voltage. This lack of dependence is due to the fact that the e–h pairs generated by the red LED are not being drawn out to the electrodes such that there is a large electrical neutral bulk (ENB) as fixed charge is neutralised. However, as the voltage increases these e–h pairs are reduced in the depletion region as they become drawn out and this results in a decrease in the measured capacitance of the photodiode (or an increase in the depletion width). These results show that a high voltage is required to fully deplete the photodiode when it is under illumination by the red LED, and this is shown where the capacitance suddenly falls at about 2.5 101 V. The profiles of plot (a) were used to generate a graph of the capacitance read off at 3 101 V as a function of wavelength and this is shown in plot (b). Like the results obtained from the I–V characteristics, a linear relation of the capacitance with wavelength is observed with a slope much greater than unity (63.8) to confirm that carrier generation is exponentially related to the incident wavelength. It has to be noted that this linear relation is obtained in the logarithmic scale. Thus, the C–V data presented in this section will support the I–V data presented earlier. Fig. 6 shows the C–V results for H13, the photodiode irradiated to a fluence of 1013 n cm 2. As for H00, the capacitance measured at shorter wavelengths is very close to that measured in the dark as shown in plot (a). However, the dark capacitance has increased slightly due to an increase in the number of e–h pairs after radiation damage. The capacitance measured under illumination by the red LED has increased and at low voltages it is independent of the
S.J. Moloi, M. McPherson / Nuclear Instruments and Methods in Physics Research A 632 (2011) 59–68
10-5
65
10-7
10-8
10-6
Capacitance (F)
Sample H00 T = 302 K
10-8
Dark Green - 565 nm Yellow - 590 nm Orange - 610 nm
10-9
Red - 660 nm
Capacitance @ 30 V (F)
10-9 10-7
10-10
10-11
Sample H00
10-12
Linear fit Slope = 63.842
10-10
10-13
10-11
Intercept = 386.499
10-14 100
101
550
600
Reverse bias (V)
650
700
Wavelength (nm)
Fig. 5. A variation of the C–V characteristic under illumination with light of different wavelengths at 302 K for H00 (a), and a plot of the capacitance read off at 30 V as a function of the wavelength (b). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
10-7
10-5
10-8
10-6
Capacitance (F)
10-7
10-8
Capacitance @ 30 V (F)
Sample H13 T = 302 K Dark Green - 565 nm Yellow - 590 nm Orange - 610 nm Red - 660 nm
10-9
Sample H13 Linear fit Slope = 43.182 Intercept = 256.85
10-9
10-10
10-11
10-12 10-10
10-13
10-14
10-11 100
101 Reverse bias (V)
550
600
650
700
Wavelength (nm)
Fig. 6. A variation of the C–V characteristic under illumination with light of different wavelengths at 302 K for H13 (a) and a plot of the capacitance read off at 30 V as a function of the wavelength (b). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
voltage as in the case for H00. The sudden fall in capacitance occurs at a lower voltage of 2 101 V, though. A plot of the capacitance read off at 3 101 V as a function of the wavelength for H13 is shown in plot (b). A linear relation in logarithmic scale is obtained with a slope of 43.2. This slope is lower than the one
evaluated for H00 (63.8) to show that the rate at which the capacitance due to incident photons increases has reduced for H13. As explained before, this reduction of capacitance is due to recombination of photogenerated carriers with radiation-induced defect levels to increase the ENB and so to increase the depletion width.
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10-5
10-5
Sample H14
10-6
10-6
Sample H15
T = 302 K
T = 302 K
10
Dark
Green - 565 nm
-7
Yellow - 590 nm Orange - 610 nm
10
Red - 660 nm
-8
Capacitance (F)
Capacitance (F)
Dark
10-7
Yellow - 590 nm Orange - 610 nm
10
Red - 660 nm
-8
10-9
10-9
10-10
10-10
100
Green - 565 nm
101
100
101 Reverse bias (V)
10-5
10-5
10-6
10-6
10-7
10-7
10-8
Capacitance (F)
Capacitance (F)
Reverse bias (V)
Sample H16 T = 302 K
Sample H17
10-8
T = 302 K Dark Green - 565 nm
Dark Green - 565 nm
10-9
Yellow - 590 nm
10-9
Orange - 610 nm
Yellow - 590 nm
Red - 660 nm
Orange - 610 nm Red - 660 nm
10-10
100
10-10
101 Reverse bias (V)
100
101 Reverse bias (V)
Fig. 7. A variation of the C–V characteristic under illumination with light of different wavelengths at 302 K for H14 (a), H15 (b), H16 (c) and H17 (d). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 7 shows the C–V characteristic of the photodiodes that were irradiated to 1014 (a), 1015 (b), 1016 (c) and 1017 n cm 2 (d). It can be seen from plot (a) for photodiode H14 that the capacitance measured at shorter wavelengths is still comparable to that measured in the dark. The trends are similar to those of photodiodes H00 and H13. Unlike for photodiodes H00 and H13, at voltages lower than 3 100 V a trend of the red LED follows that of the other LEDs, after which it becomes independent of the applied
voltage up to 1 101 V. The trend for the red LED has shifted closer to that of the dark capacitance showing that the capacitance measured with the red LED has decreased. The reduction in photocapacitance is due to the recombination of photo-generated carriers by radiation-induced defect levels. At voltages higher than 1 101 V the capacitance increases gradually with the applied voltage. A reason for this slight increase of the capacitance at high voltages is currently not known. It might be an indication of the
S.J. Moloi, M. McPherson / Nuclear Instruments and Methods in Physics Research A 632 (2011) 59–68
70 Slope (Reverse bias capacitance at 30 V)
effects of the incident light on the photodiodes just before type inversion of the material [37]. The results for H15, the photodiode irradiated to 1015 n cm 2 are shown in plot (b). The trends have come closer together and the capacitance measured under illumination is equal to that measured in the dark. In comparing the capacitance measured for photodiode H14 with that measured for photodiode H15, one notices that an increase of the capacitance measured in the dark is equal to a decrease of the capacitance measured under illumination by the red LED (10%). This result shows that the radiation-induced defect levels can generate carriers at the same rate at which they can recombine them. This process where the generation rate is equal to the recombination rate can only occur if the defect levels are positioned at the midgap where they interact equally with both bands. Properties and effects of these levels on the material have been explained elsewhere [20] and they were found to be a cause of the relaxation behaviour of the material. The diodes fabricated from this type of material have been found to perform better as radiation-hard detectors [38]. Thus, light can be used to investigate the radiation-hardness of the semiconductor onto which the diode has been fabricated. As the material becomes radiation-hard (or relaxation-like) due to the initial heavy radiation damage, properties of the photodiode become independent of the incident light. The results for H16, the photodiode irradiated to 1016 n cm 2 are shown in plot (c). It was expected that the capacitance would be independent of the light incident onto the photodiode as was shown for the current in plot (c) of Fig. 3. However, the results show that the capacitance varies with the wavelength. Unlike for photodiodes H00, H13, H14 and H15, in the case of H16 a variation of the capacitance is observed for all wavelengths. The trends, on the other hand, are following each other and the capacitance decreases steadily up to a voltage of 2 101 V. At voltages higher than 2 01 V the fall in capacitance is reduced to show that the photodiode is becoming fully depleted. A significance of the dependence of the capacitance on wavelength is that the capacitance is increasing to become of a trend similar to that of the red LED shown for H00 and H13. This occurs as the photodiode attempts to attain a level of full damage. This is the level at which the light has no effect and the material is governed only by the g–r centre activity, which is a limiting factor. The results for H17, the photodiode irradiated to 1017 n cm 2 are shown in plot (d). The trends have come closer together to show that the capacitance measured in the dark is nearly equal to that measured under illumination. The dark capacitance has increased and is found to be independent of the applied voltage at voltages lower than 1 101 V. At voltages higher than 1 101 V the capacitance becomes of negative slope. Unlike for photodiodes H00 and H13 where only a trend of the red LED shows that the capacitance is independent of the voltage, for photodiode H17 the capacitance is independent of the voltage for all wavelengths. Of most importance is that the trend for all wavelengths is different to all the other photodiodes, and is much similar to the trend for the red LED for photodiodes H00 and H13. As stated, this trend indicates that carrier transport in the photodiode is governed only by the g–r centres, which are the cause of the relaxation behaviour of the material. A linear relationship of the capacitance measured at 3 101 V with wavelength was obtained for diodes H14, H15, H16 and H17 as in Figs. 5(b) and 6(b). The slopes obtained were used to generate Fig. 8 and are shown in Table 3 for all the diodes tested. This trend for the capacitance is similar to the one shown in Fig. 4 for the current. A considerable dependence of the slope on fluence is observed at low fluence to show that the incident light has an effect on the measured capacitance. At high fluence the trend becomes horizontal to show that the slope is becoming independent of the fluence. This independence of the slope shows
67
H00
60 50 40
H13 H14
30 20 H16
10
H17 H15
0 2.0x103 4.0x103 6.0x103 8.0x103 1.0x104
0.0
Fluence (x 1013n cm-2) Fig. 8. A variation of the slopes (obtained from a capacitance versus wavelength plot) with fluence. The slope decreases as the fluence increases.
Table 3 Slopes obtained from the plots of the capacitance read off at 30 V as a function of the wavelength for unirradiated and irradiated photodiodes. Photodiode
H00
H13
H14
H15
H16
H17
Slope
63.842
43.182
36.139
6.557
9.929
5.54
that at high fluence the effects of light on the measured capacitance are reduced. Since at high fluence the relaxation behaviour of the diodes is clearly observed, the results show that as the material becomes radiation-hard, the diode capacitance become independent of the incident light. The radiation hardness of silicon due to initial heavy damage has been explained before for a diode irradiated to 1016 n cm 2 [37]. Device behaviour at high fluence or after radiation damage is governed solely by g–r activity.
4. Conclusion The results obtained from this work show that the measured current and capacitance of radiation-damaged photodiodes increase with an increase in the wavelength of light incident onto the photodiodes. These effects tend to be reduced as the fluence increases due to the defect levels induced by the radiation in the energy gap. These levels recombine photo-generated carriers to reduce the photocurrent and the photocapacitance of the photodiode. They also act as generation centres since the dark current and the dark capacitance increase after irradiation. Defect centres that can generate and recombine charge at the same rate have been found to be a cause of the relaxation behaviour of silicon [20]. They stabilise the Fermi level at the intrinsic position where it is not affected by further irradiation [36]. The diodes fabricated from this material have been found to be resistant to radiation damage [38]. Thus, light of different wavelengths can be used to investigate the properties of diodes fabricated on radiation-hard material. After a diode has become a radiation-hard (or relaxation) device, the effects of light on its properties are reduced. A conclusion regarding the radiation-hardness of silicon after initial heavy damage was also reached by Litovchenko et al. [13] based on results obtained from diodes irradiated only to 1016 n cm 2.
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Acknowledgements The first author acknowledges the National Research Foundation (NRF) for a Ph.D. studentship. The second author continues to thank BK Jones for useful discussions. We both thank C. Franklyn at NECSA for the irradiation runs. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
R D Larrabee, Phys. Rev. 121 (1961) 37. R R Senechal, T Basinski, J. Appl. Phys. 39 (1968) 4581. E Borchi, et al., Nucl. Phys. B (Proc. Suppl.) 61B (1998) 481. A D Holland, A D T Short, T Cross, Nucl. Instr. and Meth. A 346 (1994) 366. L T Canham, et al., Appl. Surf. Sci. 102 (1996) 436. D A Neamen, Semiconductor Physics and Devices, second ed., Tom Casson, London, 1997. M Moll, et al., Nucl. Instr. and Meth. A 388 (1997) 335. G Lindstrom, M Moll, E Fretwurst, Nucl. Instr. and Meth. A 426 (1999) 1. J Barthe, Nucl. Instr. and Meth. B 184 (2001) 158. M McPherson, J. Opt. A: Pure Appl. Opt. 7 (2005) S325–S330. M McPherson, Radiat. Eff. Defects Solids 158 (2004) 45. U Biggeri, et al., Nucl. Instr. and Meth. A 409 (1998) 176. P G Litovchenko, et al., Nucl. Instr. and Meth. A 568 (2006) 78. JD Dowell, et al., Nucl. Instr. and Meth. A 424 (1999) 483. MA Cappelletti, et al., Semicond. Sci. Technol. 21 (2006) 346.
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M McPherson, Physica B 344 (2004) 52. M Petasecca, F Moscatelli, GU Pignatel, Nucl. Instr. and Meth. A 546 (2005) 291. MD Osborne, PR Hobson, SJ Watts, IEEE Trans. Electron. Devices 47 (2000) 529. S J Moloi, M McPherson, Physica B 404 (2009) 2251. B K Jones, J Santana, M McPherson, Nucl. Instr. and Meth. A 395 (1997) 81. W van Roosbroeck, Phys. Rev. 123 (1961) 474. H J Queisser 1972 In: P.N. Robson (Ed.), Solid State Devices, IOP Conference Series, vol. 15, p. 145. M Ilegems, H J Queisser, Phys. Rev. B 12 (1975) 1443. N M Haegel, Appl. Phys. A 53 (1991) 1. RS components /www.rssouthafrica.comS 2007 (accessed Feb. 2007). S.J. Moloi, M.Sc. Thesis, University. of KwaZulu-Natal, Durban, South Africa, 2005. M McPherson, Nucl. Instr. and Meth. A 488 (2002) 100. S M Sze, Physics of Semiconductor Devices, second ed., Wiley, New York, 1981. B G Streetman, Solid State Electronic Devices, third ed., Prentice Hall, London, 1990. B Sapoval, C Hermann, Physics of Semiconductors, Springer-Verlag, New York, 1993. A Bar-Lev, Semiconductors and Electronic Devices, third ed., Prentice Hall, New York, 1993. B K Jones, J Santana, M McPherson, Solid State Commun. 105 (1998) 547. HN Becker, AH Johnston, IEEE Trans. Nucl. Sci. 51 (2004) 3572. MA Cappelletti, AP Cedola, E.L Peltzer y Blanca, Semicond. Sci. Technol. 23 (2008) 025007. S J Moloi, M McPherson, Physica B 404 (2009) 3922. V N Brudnyi, S N Grinyaev, V E Stepanov, Physica B 212 (1995) 429. D Pitzl, et al., Nucl. Instr. and Meth. A 311 (1992) 98. B K Jones, M McPherson, Semicond. Sci. Technol. 14 (1999) 667.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Radiation-hardness of silicon p–i–n photodiodes operated under illumination by light of different wavelengths S.J. Moloi a,n, M. McPherson b a b
Department of Physics, P. O. Box 392, University of South Africa, Pretoria 0003, South Africa McPherso Academic Consulting, Post Net Suite 194, Private Bag X 2230, Mafikeng South 2791, South Africa
a r t i c l e in f o
abstract
Article history: Received 17 September 2010 Received in revised form 15 December 2010 Accepted 18 December 2010 Available online 29 December 2010
The photo response of silicon p–i–n photodiodes has been investigated for light of different wavelengths using I–V and C–V techniques. The measurements were carried out prior to and after radiation damage by 1 MeV neutrons. A main indication is that effects due to incident photons are more pronounced at low radiation fluence but they become negligible as the fluence increases. This is due to defect levels induced by the irradiation in the energy gap of the silicon. The number of these levels increases with radiation fluence and they act mainly to recombine photo-generated carriers. This recombination reduces the number of mobile charges and hence the measured current and capacitance of the photodiode. The results show that silicon gets quickly damaged by radiation in the initial stages of the irradiation process but as the fluence increases the material becomes resistant to further damage. We contend that silicon becomes radiation-hard after initial heavy damage by 1 MeV neutrons. & 2011 Elsevier B.V. All rights reserved.
Keywords: Semiconductor Silicon Photodiode Wavelength Current Capacitance
1. Introduction In electrical characterisation of semiconductor diodes, the current–voltage (I–V) and the capacitance–voltage (C–V) techniques are, respectively, used to determine the carrier density in the diode and the doping profile in the substrate material [1–3]. These semiconductor diodes are used in a variety of applications such as detectors for high energy particles [4] or optoelectronic devices for communication [5]. The ability to vary the conductivity of these diodes as a function of temperature or as a function of optical excitation [6] makes them attractive for these applications. However, when the diodes are subjected to high radiation environments, they fail to operate efficiently due to defect levels created by radiation damage in the energy gap of the substrate material [7–9]. Thus, in trying to fabricate radiation-hard detectors, the properties of the defect levels induced in the energy gap and their influence on the diodes have to be investigated. In order to better understand the properties of the defects induced in silicon, the photo response of neutron-irradiated silicon p–i–n photodiodes was investigated for different light intensities [10]. The results obtained then were for infra-red light and were used to explain the effects of light intensity on photodiodes irradiated only up to a very low fluence of 2.5 1014 n cm 2.
n
Corresponding author. E-mail addresses: [email protected], [email protected] (S.J. Moloi), [email protected] (M. McPherson). 0168-9002/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2010.12.180
Since particle detectors will normally be operated under very high radiation environments [11–13], it is required that the effects of light on highly irradiated photodiodes are studied as well. Furthermore, we believe that it is also important to study the photo response of irradiated photodiodes for different wavelengths. Since an increase in light intensity results in an increase in the measured current and capacitance of the photodiodes [10,14–15], it is expected that both current and capacitance will also increase with an increase in the incident light of the wavelengths ranging from 565 to 660 nm. Therefore, a full characterisation of highly irradiated photodiodes for different wavelengths can be used to supplement and expand on the work carried out in [10,14–15] for different light intensities. It is for this reason that this project was carried out. In this work, the current and capacitance response of several silicon p–i–n photodiodes were investigated under illumination by light emitting diodes (LEDs) of different wavelengths (or colours). The measured current and capacitance were found to increase with an increase in wavelength. This increase in the current and capacitance with the wavelength has been found to be more pronounced on the unirradiated photodiode and on the photodiodes that were irradiated to low fluences. This shows that the incident light has a strong effect on these photodiodes. As the fluence increases further, however, the increase of the measured current and capacitance with the wavelength is found to be reduced. This shows that the photodiodes are becoming resistant to the effects of the incident light.
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The results obtained in this work are interpreted in terms of the defect levels that are induced by the irradiation in the energy gap of the silicon. These defect levels increase with the radiation fluence and they act preferentially as recombination centres [16–18] to trap all the photo-generated carriers to decrease the measured current and capacitance and so to increase the resistivity of the material [19]. The induced levels, however, can also generate carriers to increase the dark current and capacitance of the photodiode. These defect levels are generation–recombination (g–r) centres and if they generate and recombine equally they are responsible for the relaxation behaviour of the material [20]. Relaxation material tends to be resistant to radiation damage and has tD b t0 , which is different to lifetime material where tD 5 t0 [21–24]. In these relations tD is the dielectric relaxation time and t0 is the minority carrier recombination lifetime.
2. Experimental details Six p–i–n photodiodes fabricated from high purity, high resistivity silicon were acquired from Hamamatsu with the series number of S3590-08. The active diode area is 1 cm2 and the thickness is 300 mm. Five of the photodiodes were irradiated at the National Energy Corporation of South Africa (NECSA) by 1 MeV neutrons to different fluences in the range from 1013 to 1017 n cm 2. The sixth photodiode was left unirradiated to be used as a control and it was named H00. The five photodiodes irradiated to 1013, 1014, 1015, 1016 and 1017 n cm 2 were named H13, H14, H15, H16 and H17, respectively. Prior to irradiation, I–V and C–V measurements were taken on all six photodiodes in the dark and at 302 K. The temperature was monitored for every photodiode during the measurements. The characteristics of the photodiodes before irradiation were found to be very similar such that we assume that the six photodiodes are one photodiode that has undergone repeated radiation damage. After irradiation the photodiodes were characterised by I–V and C–V measurements in the dark and under illumination at 302 K. The illumination was achieved by use of LEDs of different wavelengths. After several investigations to ensure full illumination, the LEDs were each placed at a distance of 10 mm in front of the photodiodes. The LEDs were positioned at right angles to the centre of the photodiode in such a manner that the maximum light was incident onto the centre of the photodiode. In this way it was possible to assume that the illumination was, as much as possible, over the whole active area of the photodiode. Thus, any differences in the results would only be due to changes in the wavelength of the incident light. The LEDs were powered from a Goodwill GSP-3030 d.c. voltage source, which can supply up to 30 V. The current through the LEDs was kept constant at 20 mA throughout each measurement to ensure constant illumination. The properties of the LEDs used are summarised in Table 1, where some of the values have been extracted from the manufacturer’s data sheet [25]. The colours of the LEDs used are presented in the first column of the table corresponding to their peak emission wavelengths, lLED, as quoted in the manufacturer’s data sheet [25]. The third column is Table 1 Properties of the LEDs that were used to illuminate the photodiodes. Some of the values were extracted from the manufacturer’s data sheet [25]. LED
kLED (nm)
ELED (eV)
VLED (V)
Red Orange Yellow Green
660 610 590 565
1.88182 2.03607 2.10509 2.19823
5 5 5 5
the energy value, ELED, calculated by use of the relation ELED lLED ¼ hc, which is presumed to be the photon energy of the light incident onto the photodiode. In this relation, h is Planck’s constant and c is the speed of light in vacuum. It can be seen that the photon energy emitted by all the LEDs is higher than the energy gap of silicon (¼1.12 eV at 300 K), the substrate material for the photodiodes. Since the photon energy is higher than the energy gap of silicon, it is expected that the incident light will be absorbed throughout the depth of the substrate material to generate electron–hole (e–h) pairs that will be drawn to the electrodes by the applied voltage to contribute to the measured current and capacitance. Even though the measurements for this work were performed at 302 K, it is considered that they are a fair estimate of those carried out at 300 K at which 1.12 eV was evaluated. The I–V measurements were carried out using a Keithley 6487 Picoammeter with a built-in voltage source. The instrument was operated in the remote mode by connecting it to a computer through an RS-232 cable. Thus, the computer was used for control of the picoammeter and for data acquisition. The measurements were carried out for all photodiodes using a sweep mode under reverse bias conditions. In all cases the data were taken up to 100 V in voltage steps of 0.01–0.10 V, of 0.10–1.00 V and finally of 1.00–100 V. The time between measurements was set as 1 s, a time determined after many trials [26] and intended to allow for the reading to settle (or for the photodiode to stabilise). For all measurements, each photodiode and LED were placed in an inhouse, custom-made, high-resistance test fixture for the voltage sweep run. Between measurements, the photodiodes were stored at sub-zero temperatures in a fridge. The C–V measurements in reverse bias were carried out manually using an Agilent 4263B LCR metre. The data were acquired up to 30 V using an external voltage supplied by a Goodwill GSP-3030 d.c. voltage source. The measurements were carried out for the six photodiodes, in the dark and at a frequency of 1 kHz. After several trial measurements [26], a time of 8 s was chosen between readings. This was found to be the most suitable time to allow for the capacitance to settle and thus to allow acquisition of a more realistic value. The photodiodes were mounted onto an external Agilent 16065A test fixture. The illumination conditions as described above (photodiode and LED) were maintained. The LCR metre was zeroed, first as an open circuit and then as a short circuit, prior to taking readings. This was done in order to correct the capacitance of the leads and for any stray capacitance.
3. Results and discussion The results presented here are the current and the capacitance both measured in reverse bias. The current is a measure of the carrier density in the photodiode while the capacitance is a measure of the defect density in the substrate material. It is expected that both the current (carriers or mobile charge) and the capacitance (defects or fixed charge) will increase after the irradiation since the number of defects increases with the irradiation. Here the capacitance measures the doping profile, which is the density of defects created by the irradiation since such defects recombine all available dopants [27]. 3.1. Reverse bias current The measured current of a diode is related to the applied voltage [28] as eV I ¼ Is exp 1 ð1Þ ZkT
S.J. Moloi, M. McPherson / Nuclear Instruments and Methods in Physics Research A 632 (2011) 59–68
where Is is the saturation current given as the negative current since the first term in the equation is negligible in reverse bias. In the equation above, e is the electronic charge, V is the applied voltage, Z is the ideality factor, k is the Boltzman constant and T is the absolute temperature. Under illumination, the total current is given [6] by eV 1 Ii I ¼ Is exp ð2Þ ZkT where Ii is the current due to photo-generated carriers. The light intensity transmitted through a diode,Ft , is related to the absorption coefficient, a [29] as
Ft ¼ F0 expðalÞ
ð3Þ
where F0 is the intensity of a photon incident on the diode and l is the active thickness of the diode. The above relation shows that at a constant l, the light intensity increases exponentially with a decrease in the absorption coefficient. The absorption coefficient varies with the photon wavelength, l, and the energy gap, Eg [30] as g hc ap Eg ð4Þ
l
where h and c are as defined before and g ¼½, 3/2 and 2 for the direct transition, for the vertical forbidden transition and for the indirect transition between extrema situated at different points in the zone, respectively. Eq. (4) holds only for hc=l 4 Eg : The opposite case occurs if the wavelength of the incident photons is greater than 1700 nm, in which case the absorption coefficient of the silicon becomes negligible [31]. This dependence of the absorption coefficient on the wavelength shows that the intensity of photons that have a shorter wavelength is reduced with the depth of penetration into the silicon such that they are absorbed over a short distance. These photons could also be absorbed very close to the surface of the silicon. In either case the result is a lower amount of photo-generated carriers contributing to the measured current. Photons with a long wavelength, on the other hand, are absorbed
61
over a long distance into the silicon and this results into a higher amount of photo-generated carriers contributing to the measured current. Thus, at the same applied electric field, short wavelengths will show low current while long wavelengths will show high current. The effect of wavelength is shown in Fig. 1(a) for the unirradiated photodiode, H00 with an arbitrary reference line drawn from 1 mA at 4 mV to 1 mA at 0.15 kV. The measured current increases by more than two orders of magnitude when the photodiode is illuminated. The current measured in the dark is much lower than that measured with the red LED, which is much higher than that measured with the orange, the yellow and the green LEDs. These results show that light from the red LED is absorbed over a longer distance to generate more e–h pairs that contribute to the measured current than that from the other LEDs. Another observation from Fig. 1(a) is that the photocurrent shows saturation up to 1 102 V for all the LEDs indicating that the photocurrent is independent of the voltage. This lack of dependence shows that the electric field is not high enough to draw the charges to opposite electrodes to contribute to the measured current. The profile of the dark current at low voltages differs from that at high voltages. The current increases linearly up to 4 10 2 V, after which some form of saturation is observed up to 1 101 V. The initial increase of the current could be due to thermal generation of carriers when the voltage is low or due to the built-in charge of the silicon material. At voltages higher than 1 101 V an increase in the current shows that the photodiode is attempting to approach breakdown, which means that at this high voltage the electric field is able to separate the e–h pairs to draw them to opposite electrodes to contribute to the measured current. The profile of Fig. 1(a) was used to generate a plot of the current read off at 100 V for all colours as a function of wavelength. This analysis is shown in the logarithmic plot of Fig. 1(b) and was done in order to investigate a variation of the current with the wavelength of the incident light. A linear relation in logarithmic scale is noted with a slope much greater than unity (at 43.1).
10-2 10-3
Red
10-4
Current (A)
10-5
Orange and yellow
10-6 10-7
Reverse current @ 100 V (A)
10
-4
Green
Sample H00 T = 302 K
10
Dark
-8
Green - 565 nm
Sample H00 Linear fit Slope = 43.052 Intercept = 262.393
10-5
10-6
Yellow - 590 nm Orange - 610 nm
10-9
Dark
Red - 660 nm
10-10 10-2
10-1
100 Reverse bias (V)
101
102
10-7 550
600
650
700
Wavelength (nm)
Fig. 1. A variation of the reverse I–V characteristic under illumination with light of different wavelengths at 302 K for H00 (a), and a plot of the reverse current read off at 100 V as a function of the wavelength (b). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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The magnitude of the slope shows that the current varies exponentially with the wavelength. This means that carrier generation is exponentially related to the wavelength of the incident light. Fig. 2 shows the I–V results for H13, the photodiode irradiated to a fluence of 1013 n cm 2, with an arbitrary reference line similar to the one in Fig. 1. The dark current is higher than that of H00 and the profile is found to be positioned closer to the profile of the green light at high voltages. This increase of the dark current is due to an increase in the number of e–h pairs after radiation damage in the silicon. The increase in e–h pairs is due to defect levels that are induced in the energy gap of the silicon [32–34]. The photocurrent for all wavelengths, on the other hand, still shows saturation up to 102 V. It can be noted that only the current generated by the green LED has increased slightly in this photodiode and that it is no longer completely independent of the voltage. The significance of this is that in this photodiode the defects generated by the radiation are not high enough in number to be able to recombine those carriers generated by the green light. Thus, some current is measured so that the trend is both due to those carriers generated by the radiation defects as well as those generated by the incident photons. A plot of the current read off at 100 V as a function of the wavelength for H13 is shown in Fig. 2(b). A linear relation in logarithmic scale is obtained. The slope is found to be 41.2, slightly lower than the 43.1 obtained for H00. This reduction of the slope shows that carrier generation is less exponential indicating that the photocurrent has decreased after irradiation. This is due to the defect levels that are induced by the irradiation in the energy gap. These defects recombine the photo-generated carriers to reduce the amount of carriers that are drawn to the electrodes. This results in a decrease in the measured photocurrent. Since these defects increase with an increase in the radiation fluence [11], it is expected that the measured photocurrent will decrease as the radiation fluence increases further. Fig. 3 shows the I–V characteristic of the photodiodes irradiated to 1014 (a), 1015 (b), 1016 (c) and 1017 n cm 2 (d) all with the
arbitrary reference line as in previous figures. A main observation from plot (a) for photodiode H14 is that the current generated by shorter wavelengths is equal to the dark current. These results show that the properties of this photodiode are independent of the incident light of the shorter wavelengths. Even though the current generated by the red LED is higher than that generated by the other LEDs, it can be seen that the trend has come closer to that of the dark current (or that the red LED current has decreased) for low voltages. This is because the photo-generated carriers are recombined by the radiation-induced defect levels to reduce the photocurrent. In comparison with H13, the dark current in H14 has increased to show that the defect levels can also generate carriers to contribute to the measured current. Thus, these levels can recombine photogenerated carriers to reduce the photocurrent or generate carriers to increase the dark current. Another observation from plot (a) is that the trend for the red LED is different to all the other trends, showing saturation up to 100 V from where the current increases linearly with voltage up to 7 101 V. At voltages higher than 7 101 V the current becomes constant (or is saturated). The trends of the shorter wavelengths, in contrast, show a linear increase of current with voltage up to 7 10 2 V from where saturation (constant current) is observed up to 102 V. Since the number of defect levels increases with an increase in radiation fluence, it is expected that the properties of the photodiodes irradiated to fluences higher than 1014 n cm 2 will be independent for all wavelengths of the incident light. The results for H15, the photodiode irradiated to a fluence of 1015 n cm 2 are shown in plot (b). The trends have come closer together and the photocurrent has decreased for all voltages. A significant observation is that for all wavelengths, the current continues increasing at 102 V. The trend for the red LED at low voltages, however, is independent of the applied voltage to show saturation of the current. Two linear regions with different slopes are observed in the plot for the trends of the shorter wavelengths and dark current. It can be noted that the slope of the linear region
10-2 10-3
Red
10-4
Orange and yellow
Current (A)
10-5 Green
10-6
Dark
10-7
Sample H13 T = 302 K Dark Green - 565 nm
10-8
Reverse current @ 100 V (A)
10-4
Sample H13 Linear fit Slope = 41.207 Intercept = 251.611
10-5
10-6
Yellow - 590 nm Orange - 610 nm Red - 660 nm
10-9 10-10 10-2
10-1
100 Reverse bias (V)
101
102
10-7 550
600 650 Wavelength (nm)
700
Fig. 2. A variation of the reverse I–V characteristic under illumination with light of different wavelengths at 302 K for H13 (a), and a plot of the reverse current read off at 100 V as a function of the wavelength (b). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
10-2
10-2
10-3
10-3
10-4
10-4
10-5
10-5 Current (A)
Current (A)
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10-6 Sample H14 T = 302 K Dark Green - 565 nm Yellow - 590 nm Orange - 610 nm Red - 660 nm
10-7 10-8 10-9
10-6 Sample H15 T = 302 K Dark Green - 565 nm Yellow - 590 nm Orange - 610 nm Red - 660 nm
10-7 10-8 10-9
10-10
10-10 10-2
10-1
100
101
102
10-2
10-1
Reverse bias (V)
10-2
10-2
10-3
10-3
10-4
10-4
10-5
10-5
10-6
Sample H16 T = 302 K Dark Green - 565 nm Yellow - 590 nm Orange - 610 nm Red - 660 nm
10-7 10-8 10-9
100
101
102
Reverse bias (V)
Current (A)
Current (A)
63
10-6 Sample H17 T = 302 K Dark White - 565 nm Green - 590 nm Orange - 610 nm Red - 660 nm
10-7 10-8 10-9
10-10
10-10 10-2
10-1
100
101
102
Reverse bias (V)
10-2
10-1
100
101
102
Reverse bias (V)
Fig. 3. A variation of the reverse I–V characteristic under illumination with light of different wavelengths at 302 K for H14 (a), H15 (b), H16 (c) and H17 (d). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
at voltages lower than 1 10 1 V is higher than the slope of the linear region at high voltages. A high slope at low voltages shows that in H15 the current is due to carriers that are generated by the incident light, by the temperature and by the radiation-induced defect levels. Unlike the current measured for H00, H13 and H14, the current measured for H15 does not show saturation at high voltages for all wavelengths, which indicates the onset of breakdown. This indication of breakdown implies that there are more charges drawn to the electrodes. This in turn shows that at high voltage it is becoming easier to separate the e–h pairs such that any mobile charge gets drawn to the electrodes to contribute to the measured current. The results for H16, the photodiode irradiated to a fluence of 1016 n cm 2 are shown in plot (c). At low voltages the trend for
the red LED shows a slight independence of voltage. At high voltages, however, the current generated at all wavelengths is nearly equal to the dark current. The current increases linearly up to 7 100 V, after which a gentle increase is observed up to 5 101 V. At voltages higher than 5 101 V, the current varies as V 1=2 to show that the photodiode is attempting to approach breakdown. Unlike in the case of photodiode H00 where breakdown is observed for dark current only, in H16 and H15 breakdown is observed for all currents. This means that any mobile charge is being drawn to the respective electrodes to contribute to the measured current. The current for photodiode H17, as shown in plot (d), increases linearly with voltage up to 3 100 V after which a gentle increase is observed up to 7 101 V. At voltages higher than 7 101 V the
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current increases quickly with voltage up to 102 V to signify the onset of breakdown. Similar to H16 and H15, the onset of breakdown in H17 is also observed for all currents. Since the trends for photocurrent are not horizontal as it was observed in H00 and H13 and are found to be grouped together with the trend of the dark current, it can be argued that the effects of light are reduced as the fluence increases. This is due to the defect levels induced in the energy gap of the material by the irradiation. They recombine photo-generated carriers and make the measured current to be independent of the incident photons. From the obtained results it can be seen that the linear region of the trend at low voltages increases with an increase in radiation fluence. As a result, the photodiodes become more Ohmic as the fluence increases. This Ohmic behaviour of the photodiodes has been explained elsewhere [32,35] in terms of the defects that turn silicon into a relaxation material. The behaviour is caused by the g–r centres that are induced in the energy gap of the material by radiation damage [20]. These levels are situated close to the midgap where they interact equally with both bands to maintain the Fermi energy at the intrinsic level [36]. This type of material is known to be resistant to radiation damage. Thus, the results presented here also show that the material becomes radiation-hard after the initial heavy damage by radiation. At high electric fields, however, the Ohmic behaviour is not that well-pronounced to show that the field is able to quickly sweep out carriers. Thus, a large number of carriers are collected at opposite electrodes to induce breakdown. It must be noted here that the charge in the photodiode is due to photo-generated carriers as well as to e–h pairs generated from g–r centres. However, at this fluence the measured current becomes limited by the g–r centre activity such that the photo-generation activity is irrelevant. This is also shown by the photocurrent being equal to the dark current at all voltages. A linear relationship of the reverse current measured at 100 V with wavelength was also obtained for the diodes H14, H15, H16 and H17 as in Figs. 1(b) and 2(b). The slopes obtained were used to generate the plot of Fig. 4. These values of the slopes are shown in Table 2 for all the diodes tested. At low fluences a considerable dependence of the slope on the fluence is observed but from H15 the slope decreases gently with fluence to show that the measured current becomes independent of the incident light. These results
Slope (Reverse current at 100 V versus λ)
45 40
H00 H13 H14
35 30 H15
25 20 15
H16
10 5 H17
0 0.0
2.0x103
4.0x103
6.0x103
8.0x103
1.0x104
Fluence (x 1013n cm-2) Fig. 4. A variation of the slopes (obtained from a current versus wavelength plot) with fluence. The slope decreases as the fluence increases.
Table 2 Slopes obtained from the plots of the reverse current read off at 100 V as a function of the wavelength for unirradiated and irradiated photodiodes. Photodiode
H00
H13
H14
H15
H16
H17
Slope
43.052
41.307
39.701
25.598
14.502
4.285
show that the effects of light are reduced as the material becomes radiation-hard (or relaxation-like) after heavy irradiation. 3.2. Reverse bias capacitance The capacitance was measured as a function of the wavelength of light incident onto the photodiodes. At reverse bias, the capacitance of a semiconductor diode is the depletion capacitance given [6] by 1=2 A ees e0 C¼ Nd ð5Þ 2 ðVbi þVÞ where A is the active area of the photodiode, es is the dielectric constant of the semiconductor,e0 is the dielectric constant of free space, Vbi is the diode built-in voltage and Nd is the dopant density of the semiconductor. The above equation shows that C2pNd for a constant V. Since an increase in light intensity results in an increase in the number of e–h pairs, it is expected that the capacitance will increase with an increase in the wavelength of light incident onto the photodiode for the same voltage. Fig. 5 shows a reverse C–V characteristic of the unirradiated photodiode, H00, in the dark and under illumination by light of different wavelengths. A main observation from plot (a) is that the capacitance measured at short wavelengths (610, 590 and 565 nm) is nearly equal to that measured in the dark for all voltages. Another striking feature is that the profiles of the capacitance measured at these wavelengths differ from that of the red LED (660 nm). The profiles for low wavelengths show a steep fall at low voltages which becomes a gradual fall at higher voltages. The profile for the red LED, on the other hand, is horizontal at voltages lower than 1 101 V to show that the capacitance is nearly independent of the voltage. This lack of dependence is due to the fact that the e–h pairs generated by the red LED are not being drawn out to the electrodes such that there is a large electrical neutral bulk (ENB) as fixed charge is neutralised. However, as the voltage increases these e–h pairs are reduced in the depletion region as they become drawn out and this results in a decrease in the measured capacitance of the photodiode (or an increase in the depletion width). These results show that a high voltage is required to fully deplete the photodiode when it is under illumination by the red LED, and this is shown where the capacitance suddenly falls at about 2.5 101 V. The profiles of plot (a) were used to generate a graph of the capacitance read off at 3 101 V as a function of wavelength and this is shown in plot (b). Like the results obtained from the I–V characteristics, a linear relation of the capacitance with wavelength is observed with a slope much greater than unity (63.8) to confirm that carrier generation is exponentially related to the incident wavelength. It has to be noted that this linear relation is obtained in the logarithmic scale. Thus, the C–V data presented in this section will support the I–V data presented earlier. Fig. 6 shows the C–V results for H13, the photodiode irradiated to a fluence of 1013 n cm 2. As for H00, the capacitance measured at shorter wavelengths is very close to that measured in the dark as shown in plot (a). However, the dark capacitance has increased slightly due to an increase in the number of e–h pairs after radiation damage. The capacitance measured under illumination by the red LED has increased and at low voltages it is independent of the
S.J. Moloi, M. McPherson / Nuclear Instruments and Methods in Physics Research A 632 (2011) 59–68
10-5
65
10-7
10-8
10-6
Capacitance (F)
Sample H00 T = 302 K
10-8
Dark Green - 565 nm Yellow - 590 nm Orange - 610 nm
10-9
Red - 660 nm
Capacitance @ 30 V (F)
10-9 10-7
10-10
10-11
Sample H00
10-12
Linear fit Slope = 63.842
10-10
10-13
10-11
Intercept = 386.499
10-14 100
101
550
600
Reverse bias (V)
650
700
Wavelength (nm)
Fig. 5. A variation of the C–V characteristic under illumination with light of different wavelengths at 302 K for H00 (a), and a plot of the capacitance read off at 30 V as a function of the wavelength (b). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
10-7
10-5
10-8
10-6
Capacitance (F)
10-7
10-8
Capacitance @ 30 V (F)
Sample H13 T = 302 K Dark Green - 565 nm Yellow - 590 nm Orange - 610 nm Red - 660 nm
10-9
Sample H13 Linear fit Slope = 43.182 Intercept = 256.85
10-9
10-10
10-11
10-12 10-10
10-13
10-14
10-11 100
101 Reverse bias (V)
550
600
650
700
Wavelength (nm)
Fig. 6. A variation of the C–V characteristic under illumination with light of different wavelengths at 302 K for H13 (a) and a plot of the capacitance read off at 30 V as a function of the wavelength (b). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
voltage as in the case for H00. The sudden fall in capacitance occurs at a lower voltage of 2 101 V, though. A plot of the capacitance read off at 3 101 V as a function of the wavelength for H13 is shown in plot (b). A linear relation in logarithmic scale is obtained with a slope of 43.2. This slope is lower than the one
evaluated for H00 (63.8) to show that the rate at which the capacitance due to incident photons increases has reduced for H13. As explained before, this reduction of capacitance is due to recombination of photogenerated carriers with radiation-induced defect levels to increase the ENB and so to increase the depletion width.
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10-5
10-5
Sample H14
10-6
10-6
Sample H15
T = 302 K
T = 302 K
10
Dark
Green - 565 nm
-7
Yellow - 590 nm Orange - 610 nm
10
Red - 660 nm
-8
Capacitance (F)
Capacitance (F)
Dark
10-7
Yellow - 590 nm Orange - 610 nm
10
Red - 660 nm
-8
10-9
10-9
10-10
10-10
100
Green - 565 nm
101
100
101 Reverse bias (V)
10-5
10-5
10-6
10-6
10-7
10-7
10-8
Capacitance (F)
Capacitance (F)
Reverse bias (V)
Sample H16 T = 302 K
Sample H17
10-8
T = 302 K Dark Green - 565 nm
Dark Green - 565 nm
10-9
Yellow - 590 nm
10-9
Orange - 610 nm
Yellow - 590 nm
Red - 660 nm
Orange - 610 nm Red - 660 nm
10-10
100
10-10
101 Reverse bias (V)
100
101 Reverse bias (V)
Fig. 7. A variation of the C–V characteristic under illumination with light of different wavelengths at 302 K for H14 (a), H15 (b), H16 (c) and H17 (d). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 7 shows the C–V characteristic of the photodiodes that were irradiated to 1014 (a), 1015 (b), 1016 (c) and 1017 n cm 2 (d). It can be seen from plot (a) for photodiode H14 that the capacitance measured at shorter wavelengths is still comparable to that measured in the dark. The trends are similar to those of photodiodes H00 and H13. Unlike for photodiodes H00 and H13, at voltages lower than 3 100 V a trend of the red LED follows that of the other LEDs, after which it becomes independent of the applied
voltage up to 1 101 V. The trend for the red LED has shifted closer to that of the dark capacitance showing that the capacitance measured with the red LED has decreased. The reduction in photocapacitance is due to the recombination of photo-generated carriers by radiation-induced defect levels. At voltages higher than 1 101 V the capacitance increases gradually with the applied voltage. A reason for this slight increase of the capacitance at high voltages is currently not known. It might be an indication of the
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70 Slope (Reverse bias capacitance at 30 V)
effects of the incident light on the photodiodes just before type inversion of the material [37]. The results for H15, the photodiode irradiated to 1015 n cm 2 are shown in plot (b). The trends have come closer together and the capacitance measured under illumination is equal to that measured in the dark. In comparing the capacitance measured for photodiode H14 with that measured for photodiode H15, one notices that an increase of the capacitance measured in the dark is equal to a decrease of the capacitance measured under illumination by the red LED (10%). This result shows that the radiation-induced defect levels can generate carriers at the same rate at which they can recombine them. This process where the generation rate is equal to the recombination rate can only occur if the defect levels are positioned at the midgap where they interact equally with both bands. Properties and effects of these levels on the material have been explained elsewhere [20] and they were found to be a cause of the relaxation behaviour of the material. The diodes fabricated from this type of material have been found to perform better as radiation-hard detectors [38]. Thus, light can be used to investigate the radiation-hardness of the semiconductor onto which the diode has been fabricated. As the material becomes radiation-hard (or relaxation-like) due to the initial heavy radiation damage, properties of the photodiode become independent of the incident light. The results for H16, the photodiode irradiated to 1016 n cm 2 are shown in plot (c). It was expected that the capacitance would be independent of the light incident onto the photodiode as was shown for the current in plot (c) of Fig. 3. However, the results show that the capacitance varies with the wavelength. Unlike for photodiodes H00, H13, H14 and H15, in the case of H16 a variation of the capacitance is observed for all wavelengths. The trends, on the other hand, are following each other and the capacitance decreases steadily up to a voltage of 2 101 V. At voltages higher than 2 01 V the fall in capacitance is reduced to show that the photodiode is becoming fully depleted. A significance of the dependence of the capacitance on wavelength is that the capacitance is increasing to become of a trend similar to that of the red LED shown for H00 and H13. This occurs as the photodiode attempts to attain a level of full damage. This is the level at which the light has no effect and the material is governed only by the g–r centre activity, which is a limiting factor. The results for H17, the photodiode irradiated to 1017 n cm 2 are shown in plot (d). The trends have come closer together to show that the capacitance measured in the dark is nearly equal to that measured under illumination. The dark capacitance has increased and is found to be independent of the applied voltage at voltages lower than 1 101 V. At voltages higher than 1 101 V the capacitance becomes of negative slope. Unlike for photodiodes H00 and H13 where only a trend of the red LED shows that the capacitance is independent of the voltage, for photodiode H17 the capacitance is independent of the voltage for all wavelengths. Of most importance is that the trend for all wavelengths is different to all the other photodiodes, and is much similar to the trend for the red LED for photodiodes H00 and H13. As stated, this trend indicates that carrier transport in the photodiode is governed only by the g–r centres, which are the cause of the relaxation behaviour of the material. A linear relationship of the capacitance measured at 3 101 V with wavelength was obtained for diodes H14, H15, H16 and H17 as in Figs. 5(b) and 6(b). The slopes obtained were used to generate Fig. 8 and are shown in Table 3 for all the diodes tested. This trend for the capacitance is similar to the one shown in Fig. 4 for the current. A considerable dependence of the slope on fluence is observed at low fluence to show that the incident light has an effect on the measured capacitance. At high fluence the trend becomes horizontal to show that the slope is becoming independent of the fluence. This independence of the slope shows
67
H00
60 50 40
H13 H14
30 20 H16
10
H17 H15
0 2.0x103 4.0x103 6.0x103 8.0x103 1.0x104
0.0
Fluence (x 1013n cm-2) Fig. 8. A variation of the slopes (obtained from a capacitance versus wavelength plot) with fluence. The slope decreases as the fluence increases.
Table 3 Slopes obtained from the plots of the capacitance read off at 30 V as a function of the wavelength for unirradiated and irradiated photodiodes. Photodiode
H00
H13
H14
H15
H16
H17
Slope
63.842
43.182
36.139
6.557
9.929
5.54
that at high fluence the effects of light on the measured capacitance are reduced. Since at high fluence the relaxation behaviour of the diodes is clearly observed, the results show that as the material becomes radiation-hard, the diode capacitance become independent of the incident light. The radiation hardness of silicon due to initial heavy damage has been explained before for a diode irradiated to 1016 n cm 2 [37]. Device behaviour at high fluence or after radiation damage is governed solely by g–r activity.
4. Conclusion The results obtained from this work show that the measured current and capacitance of radiation-damaged photodiodes increase with an increase in the wavelength of light incident onto the photodiodes. These effects tend to be reduced as the fluence increases due to the defect levels induced by the radiation in the energy gap. These levels recombine photo-generated carriers to reduce the photocurrent and the photocapacitance of the photodiode. They also act as generation centres since the dark current and the dark capacitance increase after irradiation. Defect centres that can generate and recombine charge at the same rate have been found to be a cause of the relaxation behaviour of silicon [20]. They stabilise the Fermi level at the intrinsic position where it is not affected by further irradiation [36]. The diodes fabricated from this material have been found to be resistant to radiation damage [38]. Thus, light of different wavelengths can be used to investigate the properties of diodes fabricated on radiation-hard material. After a diode has become a radiation-hard (or relaxation) device, the effects of light on its properties are reduced. A conclusion regarding the radiation-hardness of silicon after initial heavy damage was also reached by Litovchenko et al. [13] based on results obtained from diodes irradiated only to 1016 n cm 2.
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Acknowledgements The first author acknowledges the National Research Foundation (NRF) for a Ph.D. studentship. The second author continues to thank BK Jones for useful discussions. We both thank C. Franklyn at NECSA for the irradiation runs. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
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