**3.2. Photodiodes and phototransistors response to successive gamma and neutron irradiation**

In recent few years there have been carried out a number of studies with the aim of observing the behavior of different photovoltaic detectors in terms of gamma and neutron radiation [11– 17]. Most common topics were photodiode, as one of the most used and simplest types of optical sensors. The effect of gamma and neutron radiation on semiconductors is well known and described in the available literature. This chapter will present the results of research of behavior of photovoltaic detector due to successive gamma and neutron radiation. The samples were first exposed to gamma radiation and after 30 days to neutron radiation.

**Figure 5** shows the results of the photodiodes and the phototransistors spectral response measurements before and after gamma and neutron radiation [18].

**Figure 6.** Spectral response of the reverse biased photodiode BPW41N before and after gamma and neutron irradia‐ tion.

**Figure 7.** Spectral response of the reverse biased photodiodeBPW34 before and after gamma and neutron irradiation.

The Impact of Successive Gamma and Neutron Irradiation on Characteristics of PIN Photodiodes and Phototransistors http://dx.doi.org/10.5772/62756 77

**Figure 8.** Spectral response of the reverse biased photodiode SFH203FA before and after gamma and neutron irradia‐ tion.

**Figure 5.** Spectral response of the reverse biased photodiode BP104 before and after gamma and neutron irradiation.

**Figure 6.** Spectral response of the reverse biased photodiode BPW41N before and after gamma and neutron irradia‐

**Figure 7.** Spectral response of the reverse biased photodiodeBPW34 before and after gamma and neutron irradiation.

tion.

76 Radiation Effects in Materials

As it can be seen from Figures 5 to 10, neutron irradiation, applied 30 days after gamma irra‐ diation, at first was deteriorate response and characteristics of photodetectors. However, after 30 days of recovery, there was a partial improvement of the spectral photodetector response and the increasing of photocurrent. The degree of improvement is different for each type of photodetector.

**Figure 9.** Spectral response of phototransistor BPW40 before and after gamma and neutron irradiation.

**Figure 10.** Spectral response of phototransistor LTR4206 before and after gamma and neutron irradiation.

Neutron irradiation, by itself, causes the formation of displacement damage in photodiodes and phototransistors, which leads to the degradation of their electrical characteristics, as is shown in Chapter 3.1 (Figures 2–4). However, if it is applied after the gamma radiation, neutron radiation makes such changes which increasing the efficiency of the recovery process and, as a result, we have improved electrical characteristics. To achieve these effects to be occurred, the concentration of charge carriers must be increased in semiconductor material. Taking previous studies into account [12, 19–21], it can be concluded that the possible cause is tunneling of charge carriers supported by traps and increased generation.

Defects in the material represent traps for the free charge carriers and that can lead to tunneling supported by traps, and this increases the tunneling current at low voltages which are commonly attributed to SILC (*Stress-Induced Leakage Current*) [22–24]. Tunneling supported by traps is a process where particle spend some time trapped in the defect (trap) before tunneling through the barrier (*Trap Asissted Tunneling—TAT*) [20, 21]. This process is caused by inelastic transfer of charge carriers with the help of emission of phonons [21].

Let the electron from the field 1 in **Figure 11** receive enough energy to cross the barrier and came to the area 2. This process undermines the law of conservation of energy for a short period of time determined by Heisenberg's uncertainty principle. Now, if some other electron from the field 2 tunneled in a similar way in a similar time in the area 3, then total number of electrons that are passed from area 1 to area 3 is one. This tunneling is called inelastic tunneling because the excited electron-hole pair occurs, which dissipates after a short time through the interaction carrier-carrier [25].

**Figure 11.** Elastic and inelastic tunnelling through a double barrier [25].

**Figure 10.** Spectral response of phototransistor LTR4206 before and after gamma and neutron irradiation.

tunneling of charge carriers supported by traps and increased generation.

transfer of charge carriers with the help of emission of phonons [21].

carrier-carrier [25].

78 Radiation Effects in Materials

Neutron irradiation, by itself, causes the formation of displacement damage in photodiodes and phototransistors, which leads to the degradation of their electrical characteristics, as is shown in Chapter 3.1 (Figures 2–4). However, if it is applied after the gamma radiation, neutron radiation makes such changes which increasing the efficiency of the recovery process and, as a result, we have improved electrical characteristics. To achieve these effects to be occurred, the concentration of charge carriers must be increased in semiconductor material. Taking previous studies into account [12, 19–21], it can be concluded that the possible cause is

Defects in the material represent traps for the free charge carriers and that can lead to tunneling supported by traps, and this increases the tunneling current at low voltages which are commonly attributed to SILC (*Stress-Induced Leakage Current*) [22–24]. Tunneling supported by traps is a process where particle spend some time trapped in the defect (trap) before tunneling through the barrier (*Trap Asissted Tunneling—TAT*) [20, 21]. This process is caused by inelastic

Let the electron from the field 1 in **Figure 11** receive enough energy to cross the barrier and came to the area 2. This process undermines the law of conservation of energy for a short period of time determined by Heisenberg's uncertainty principle. Now, if some other electron from the field 2 tunneled in a similar way in a similar time in the area 3, then total number of electrons that are passed from area 1 to area 3 is one. This tunneling is called inelastic tunneling because the excited electron-hole pair occurs, which dissipates after a short time through the interaction

Elastic tunneling is the process of tunneling of same electrons from and in the region 2 with preserving of the phase which is why this process is coherent. Elastic tunneling depends on the internal structure of the area between the barrier and the amount and polarization of the applied voltage. Inelastic tunneling is the dominant process in comparison with the elastic tunneling except in the case of low voltage, low temperature, or low density of quantum dot states [26, 27]. Area 2 is also called a virtual state and simultaneous tunneling through this state, co-tunneling.

Shockley–Read–Hall model describes the process of recombination and the generation of charge carriers in a semiconductor with the help of quantum tunneling mechanism [19, 28, 29]. The transition of an electron from the valence band to the conductive band represents the generation of electron-hole pair, because in the valence band, hole remains in the place of electrons which contribute to the current. The reverse process is recombination. In order to be transferred from the valence band into the conductive band electron must have greater energy than the energy gap. As it may be defects in the crystal structure of the semiconductor caused by impurities (or other causes, eg. radiation), it may appear within the bandgap. Such defects are called traps and they represent energy levels that can trap electrons ejected from the valence band [30]. According to Dharival-Rajvanshi's model, traps can be near the edge of the valence and conduction band (*Tail State*) and near the Fermi level on both sides (*Dangling Bond*). In order to move electron from the valence band into the trap, it requires much less energy than for the transition to the conductive band, so the traps actually facilitate the process of gener‐ ation of free carriers. The probability that an electron will fall into the trap and spend some time in it depends on the material, the density of defects in the energy gap, the present electric field, temperature, concentration of electrons in the conduction band, and the concentration of holes in the valence band. Schokley–Read–Hall model assumes one level within a gap where electrons or holes can come, which dynamic is quasi-stationary [25, 31, 32].

When gamma radiation and neutron radiation are acting individually on a photodiode there is, as the final result, an increase in the concentration of recombination centers which, according to Schokley-Read formula [33], result in a reduction of minority charge carriers lifetime:

$$\tau = \frac{1}{\langle c\_n \rangle \cdot N\_\iota} \frac{n\_0 + \delta n + n\_1}{n\_0 + p\_0 + \delta n} + \frac{1}{\langle c\_p \rangle \cdot N\_\iota} \frac{n\_0 + \delta n + p\_1}{n\_0 + p\_0 + \delta p} \tag{1}$$

where *τ* = *τp* = *τn* is the life time of electrons and holes and *Nt* concentration of R-centers (recombination centers which can accept both electrons and holes). The reduction of minority carrier lifetime causes photocurrent decreasing. Previously stated explanation is related to the influence of neutron irradiation on the new, previously non-irradiated photodiodes. However, if we change the initial conditions, i.e. if the photodiode previously has been exposed to gamma radiation, the effects of neutron irradiation will be different. One of the results of gamma radiation are interstitial (PKA), vacancies, and their complexes [34, 35]. Vacancies are also one of the main products of neutron irradiation of the material. When the material, which already contains a number of vacancies, is exposed to the effects of neutron radiation, there is high probability that the defects such as vacancies would be found physically close to each other. When the two vacancies occur next to each other within the grid, they form defective complex called divacancies complex. This complex captures electrons and also can stress the homopolar bonds, which can lead to the termination of the connection. Straining of homopolar connections and its termination can lead to the release of one or two electrons from the defective complex in the conductive band, which results in increased generation.

In some previous studies, increased generation [12] and increased recombination [36, 37] have been observed through the process of electron transfer directly between the defects located close to each other without passing through the conductive belt. This process can be very fast and therefore dominant compared to the Shockley–Read–Hall process. In order to occur the intercenter charge transfer, defects must be physically close to one another. Two irradiation of the same material, such as gamma and neutron, allowing some defects to be close to one another.

The divacancy has three energy levels in the bandgap: a hole trap and two aceptor states. In standard Shockley–Read–Hall theory, current generation in silicon depletion regions is mediated by isolated defect levels in the forbidden bandgap. Generations occurs when a hole is emitted from the defect level into the valence band (i.e. electron captured from it) and an electron is emitted into the conduction band. Each transition occurs with a rate, *en* or *ep*, and is governed by the time constant *τne* ili *τpe*. 1 If several defect levels exist, they are regarded as the sum of the individual components. In coupled defect generation, illustrated in **Figure 12**, an electron is first captured by the donor state in the bottom half of the bandgap. This is an efficient process with time constant *τpe1* being very short hence the fractional occupation of this level is ≈1. The electron can then transfer directly to a higher state in a nearby defect without going

<sup>1</sup> *en = 1/τne* and *ep = 1/τpe*.

via the conduction band. The time constant for this step is denoted *τ1→2*. The final transition to the conduction band then occurs as normal with a time constant *τne2*. The enhancement of the generation rate arises because the large transition from the valence band to the above midgap level is mediated by the donor level. This shortens the time taken for the upper state to become filled and hence increases its fractional occupancy [12, 38].

When gamma radiation and neutron radiation are acting individually on a photodiode there is, as the final result, an increase in the concentration of recombination centers which, according to Schokley-Read formula [33], result in a reduction of minority charge carriers lifetime:

> 01 0 1 0 0 0 0

 d

á ñ× + + á ñ× + + (1)

 d

concentration of R-centers

*n nn n n p c Nn p n c Nn p p*

(recombination centers which can accept both electrons and holes). The reduction of minority carrier lifetime causes photocurrent decreasing. Previously stated explanation is related to the influence of neutron irradiation on the new, previously non-irradiated photodiodes. However, if we change the initial conditions, i.e. if the photodiode previously has been exposed to gamma radiation, the effects of neutron irradiation will be different. One of the results of gamma radiation are interstitial (PKA), vacancies, and their complexes [34, 35]. Vacancies are also one of the main products of neutron irradiation of the material. When the material, which already contains a number of vacancies, is exposed to the effects of neutron radiation, there is high probability that the defects such as vacancies would be found physically close to each other. When the two vacancies occur next to each other within the grid, they form defective complex called divacancies complex. This complex captures electrons and also can stress the homopolar bonds, which can lead to the termination of the connection. Straining of homopolar connections and its termination can lead to the release of one or two electrons from the defective complex

In some previous studies, increased generation [12] and increased recombination [36, 37] have been observed through the process of electron transfer directly between the defects located close to each other without passing through the conductive belt. This process can be very fast and therefore dominant compared to the Shockley–Read–Hall process. In order to occur the intercenter charge transfer, defects must be physically close to one another. Two irradiation of the same material, such as gamma and neutron, allowing some defects to be close to one

The divacancy has three energy levels in the bandgap: a hole trap and two aceptor states. In standard Shockley–Read–Hall theory, current generation in silicon depletion regions is mediated by isolated defect levels in the forbidden bandgap. Generations occurs when a hole is emitted from the defect level into the valence band (i.e. electron captured from it) and an electron is emitted into the conduction band. Each transition occurs with a rate, *en* or *ep*, and is

sum of the individual components. In coupled defect generation, illustrated in **Figure 12**, an electron is first captured by the donor state in the bottom half of the bandgap. This is an efficient process with time constant *τpe1* being very short hence the fractional occupation of this level is ≈1. The electron can then transfer directly to a higher state in a nearby defect without going

If several defect levels exist, they are regarded as the

1

+ + + +

1 1 *n t p t*

d

d

= +

where *τ* = *τp* = *τn* is the life time of electrons and holes and *Nt*

in the conductive band, which results in increased generation.

another.

1

*en = 1/τne* and *ep = 1/τpe*.

governed by the time constant *τne* ili *τpe*.

t

80 Radiation Effects in Materials

**Figure 12.** Schematic diagram of Schokley–Read–Hall theory and intercenter charge transfer generation processes [12].

The enhancement of the fractional occupancy increases the number of electrons generated per unit of time from a defect state and hence increases the photocurrent [33].
