3.1 Changes in forward voltage drop

The defect energy levels introduced by electron irradiation in semiconductor materials affect electrical properties of a device. The forward voltage drop of the samples were measured, and the results are shown in Fig. 1. The V_F of the irradiated samples increased with the irradiation fluence, and the EB energy, indicating the beam energy effects.

Fig. 1.Full-screen View

The forward voltage drop V_F as z function of the irradiation fluence Ф_n, at electron beam energies of 0.5–5,0 MeV.

3.2 Changes in minority carrier lifetime

The exact locations and concentrations of deep energy levels in a semiconductor have a significant impact on minority carrier lifetime of the device. To ensure that the concentration of the deep level defects introduced by EB irradiation is sufficient to carry out DLTS measurements, we measured the minority carrier lifetime of the devices.

Fig. 2. shows the minority carrier lifetime of the devices irradiated under different EB energies, as a function of the irradiation fluence. The experimental data were averaged for each group. The results show that at 0.5 MeV, the minority carrier lifetime changed little, while it was reported that 0.5 MeV was a lattice atom displacement threshold [16]. This may be because that the diode packaging in this work was thicker, or the electron beam density in this work was lower. At 1.5, 2.0 and 5.0 MeV, the minority carrier lifetime decreased with increasing fluences, and the amplitude increased with EB energy. We calculated the reciprocal of the minority carrier lifetime. As shown in Fig.2(b), the τ⁻¹ has a good linear relationship with the irradiation fluence Ф_n. For radiation damage in semiconductors, according Ref.[1], the minority carrier lifetime is related to irradiation fluence by 1/ τ =1/τ₀ + kΦ_n, where τ₀ and τ are the minority carrier lifetime before and after irradiation, respectively; k is the irradiation damage coefficient; and Ф_n is the irradiation fluence. From the slopes in Fig.2(b), we have k = (4.78±1.24)×10⁻¹⁶ at 1.5 MeV, (5.06±0.96)×10⁻¹⁵ at 2.0 MeV and (4.96±0.40)×10⁻¹⁴ at 5.0 MeV, i.e. the degradation rate of minority carriers increases with the beam energy.

Fig. 2.Full-screen View

The minority carrier lifetime τ vs. irradiation fluence Ф_n (a) at different EB energies, and the fitting curves (b) of Ф_n and τ⁻¹.

3.3 Changes in defect energy levels

The principle and formula of DLTS analysis in Refs.[17, 18] were used. We used the rate window method, using liquid nitrogen to complete a slow temperature scan from low to high temperatures. During this period the sample was held under an appropriate reverse bias voltage of 15 V, then a periodic forward pulse of 1 V was superimposed on the bias to check the capacitance in a fixed time interval. The rate window was t₂/t₁ =2, and the control did not show deep energy levels. Because the minority carrier lifetime changes were the most significant at 2.0 MeV or 5.0 MeV, we chose the two sets of samples for the DLTS measurements. The results are shown in Fig. 3.

Fig. 3.Full-screen View

(Color online) Deep level transient spectroscopy of the minority and minority carriers from the samples irradiated by 2.0 and 5.0 MeV electron beams.

The characteristics of the DLTS include the energy level position E_T relative to the conduction or valence energy level (i.e., E_c-E_T or E_T-E_v), the energy level concentration N_T, and the capture cross section σ_T. The higher is the spectral peak, the greater is the energy level concentration; the larger the X axis value is, the deeper is the energy level. While a spectral peak is a minority carrier trap, a valley-like spectral peak is a majority carrier trap. The detailed information about the relationship between DLTS spectra and irradiated defects can be found in Refs. [16, 19].

In n-Si, the majority carrier defects trap electrons, while the minority carrier defects trap holes. The filling and emission process of electrons and holes in the defect energy levels can cause changes in the junction capacitance, which yield the DLTS signal. The majority carrier level can also be observed in the minority carrier energy spectrum. However, measuring the majority carrier level in such a way does not yield accurate results because of the compensation between majority and minority carrier levels. Therefore, a separate measurement was conducted to measure the majority carrier level spectrum.

In Fig.3, at 2.0 MeV and Ф_n=3.3×10¹⁴ /cm², and at 5.0 MeV and Ф_n=5.5×10¹³ /cm², the spectra had four minority carrier defect energy levels (H₁–H₄) and four majority carrier defect energy levels (E₁–E₄). The number of energy levels differed with the other irradiation conditions. The specific parameters of each energy level are shown in Table 1. Among the parameters, the position of the energy levels H₁–H₄ are expressed as E_T−E_v, while E₁–E₄ are expressed as E_c−E_T (where E_c is the conduction band energy, E_v is the valence band energy, and E_T is the measured energy level); N_T denotes the energy level concentration (i.e., the concentration of lattice defects). The possible formation reason and characteristics of the defect levels were discovered by comparing our results with those in Refs. [16, 20]. Not all parameters for H₁ are given in Table 1 as the sample-cooling liquid nitrogen is 77 K.

The results show that the defect energy levels introduced by electron irradiation may decrease, disappear and move under certain conditions, in addition to appearing when the energy and fluence reaches a certain threshold and increasing with the fluence. The changes in each energy level can be categorized in six ways as follows:

(1) Appearance of spectral peaks. At 2.0 MeV and 3.3×10¹⁴ /cm², the energy levels H₁, H₂, H₃, E₂ and E₄ appeared. This demonstrates that during the irradiation, some defect energy levels require this fluence threshold value in addition to the energy already required to achieve the material displacement threshold [21].

(2) Disappearance of spectral peaks. At 5.0 MeV and 5.5×10¹³ /cm², the energy levels H₁, H₂, and E₁ appeared. This is possibly because that the EB energy and fluence reached the highest threshold value of the samples, and the semiconductor materials were compensated especially at low temperatures. The concentration of deep defect energy levels is too high, so that free electrons and holes jump to the energy levels the closest to the mid-gap at low temperatures, such as H₄, E₃, and E₄. This leads to very low number of free electrons and holes in the conduction and valence energy levels, and thus the filling and firing processes cannot be completed in the shallow energy levels, preventing them from being scanned using DLTS, and the spectra are distorted at low temperatures.

(3) Increase of peak height. The energy levels H₄, E₁ and E₃ at 2.0 MeV, and the energy levels H₄ and E₄ at 5.0 MeV, increased with the fluence (e.g., the H₄ level concentration increased from 6.30×10¹¹ to 7.70×10¹² cm⁻³ at 2.0 MeV, and from 1.34×10¹³ to 2.98×10¹³ cm⁻³ at 5.0 MeV). With the increased total number of electrons injected into the semiconductor per unit time, the collision probability and frequency of the incident electrons with the lattice atoms increased, hence the increase of defect concentration.

(4) Decrease of peak value. At 5.0 MeV, the peak height of energy level E₂ decreased slightly (from 1.19×10¹² to 9.10×10¹¹ cm⁻³). Possibly, this is because that the radiation-induced defects were unstable, and were prone to annealing effects as the temperature increased. Although the environmental heat dissipation was increased, lattice atom vibrations caused by energy exchange between the electrons and lattice atoms were still intense. Therefore, the EB irradiation process can be regarded as a short annealing process in the crystal lattice [22], hence a decreased concentration of unstable defects. Also, the same as the disappearance of spectral peaks, another reason may be the excessive radiation caused spectral distortion at low temperatures.

(5) Little change in peak value. At 5.0 MeV, the height of energy level E₃ changed little. This shows that some energy levels reached concentration saturation under certain irradiation conditions and no longer increased by with the fluence.

(6) Peak position shift. At 2.0 MeV, energy levels H₄ and E₁ shifted to the left (the energy level became shallower). At 5.0 MeV, energy levels H₄, E₂, E₃ and E₄ shifted to the left, but energy level H₃ shifted right finally (the level became deeper).

In the DLTS spectra, different types of energy levels can compensate each other. When that happens, the minority and majority carrier peaks appear at the same or at a similar position. Two peaks mutually compensating each other leads to a visible peak reduction. The same type of energy levels can be superimposed on each other. If several minority or majority carrier peaks appear at the same or at similar positions, they will overlap and form a new peak. The left-shifting of peak position as the fluence increases may have two explanations:

(1) The energy levels are each composed of a few energy levels that are close in position: as the concentration of the shallower energy levels increases, the position of entire energy level shifts left.

(2) Owing to the increase in the concentration of defect energy levels, the trapping ability is enhanced for both electrons and holes. The filling and emission processes of the electrons and holes in the energy level will occur at lower temperatures, hence the left-shift of energy level.

At 5.0 MeV, the peak of energy level H₃ shifted left first and then right. This may be due to a new energy level emerged at a deeper position at 3.3×10¹⁴ /cm². The new energy level and the original level H₃ superimposed on each other, resulting in the position shift.

The concentrations of defect levels change with the irradiation conditions, while the compensation and superposition effects between different energy levels change the positions of the peak, e.g., the position of energy level H₄ ranged from E_c-0.355 to E_c-0.241 eV (a difference of 0.114 eV). This may be one of the reasons why the energy level positions always fluctuated within a certain range in previous studies.

In EB irradiation of semiconductors, the electrons enter the lattice and impact the lattice atoms, leaving the original positions of the atoms to enter the gap. Vacancy and interstitial atoms will be formed, i.e. the Frenkel defects. These defects can form more complex defects with impurities in the semiconductors, such as oxygen vacancy pairs, phosphorus vacancy pairs, and double vacancy pairs. If the energy levels of the defects lie within the forbidden band, they will act as a compound center, compounding carriers. To visualize the position of each energy level clearly we used band diagrams. As an example, the band diagram for a 2.0 MeV electron beam with a fluence of 3.3×10¹⁴ e/cm² is shown in Fig. 4.

Fig. 4.Full-screen View

Energy band diagram at 2.0 MeV and 3.3×10¹⁴ /cm².

The majority carrier levels E₁–E₄ positions (E_c−E_T) are acceptor levels, which capture electrons; while the minority carrier levels H₁–H₄ (E_T−E_v) are donor levels, which capture holes. The efficiency of lifetime control is related to the position, concentration, and capture cross section of the defect energy levels. The effect of defect energy levels near the mid-gap on carrier recombination is more obvious than those of the shallow levels. In this work, the E₃, E₄, and H₄ levels were located the closest to the mid-gap, and their concentrations were much higher, so these defect energy levels caused a rapid decrease in the minority carrier lifetime. This is consistent with our results on the minority carrier lifetime.