Epitaxial 4H-silicon carbide (4H-SiC) Schottky barrier diode (SBD) detectors can be suitable for astronomy radiation imaging systems by considering their attractive features such as visible blindness, room temperature operation and radiation hardness^[1–4]. In space environments, the detectors experience displacement damage due to exposure to high-energy gamma irradiation^[4], thereby deteriorating the detector performance. So, studies on gamma irradiation induced displacement damage in the epitaxial 4H-SiC SBDs require special attention for such applications. To explore this, our earlier work^[5] is targeted at analyzing the gamma irradiation effects on 4H-SiC SBDs at a high dose level of 100 Mrad. It is well known that thermal annealing can partially recover the radiation induced damage in the detectors^{[6, 7]}; moreover, this study is important to extend the detector operational lifetime. Therefore, the current work focusses on investigating the annealing impacts on the gamma irradiated 4H-SiC SBDs.

Note that, Ni is a widely used metal for Schottky contact on n-type 4H-SiC epilayers because of its high work function and thermal stability^[8–14]; and also, as-deposited Ti can create an Ohmic contact with a heavily doped (~10¹⁸ cm⁻³) n-type 4H-SiC without any annealing process^[15]. Considering these points, Ni/4H-SiC/(Ti/Au) SBDs on n-type epitaxial 4H-SiC (Ni Schottky contact and Ti based Ohmic contact) are used for the current study. In the literature, thermal annealing effects on non-irradiated 4H-SiC SBDs (i.e. changes in electrical characteristics and trap signatures) are extensively analyzed^{[11, 12, 16–29]}. However, the annealing effects on gamma irradiated 4H-SiC SBD characteristics have not yet been examined. Hence, to understand the annealing impacts at various temperatures, thermal annealing studies are carried out on gamma irradiated (100 Mrad) Ni/4H-SiC SBDs over a wide range of temperatures (from 400 to 1100 °C). The thermal evolution of the deep level defects in the SBDs is investigated by an irreversible single shot capacitance transient technique called thermally stimulated capacitance (TSCAP) spectroscopy^[5]. Furthermore, annealing induced changes in the electrical characteristics of the irradiated SBDs are analyzed.

The Ni/4H-SiC SBDs were fabricated on n-type epitaxial 4H-SiC samples (30 µm epilayer with a doping concentration of 5 × 10¹⁴ cm⁻³) from CREE Inc; the SBD fabrication details are discussed elsewhere^{[5, 30]} and briefly discussed below. Upon the surface cleaning process, Ni (200 nm) was sputtered through a metal mask on the lightly doped epilayer (on the Si-face of 4H-SiC), which acts as a Schottky contact. Similarly, a sputtered Ti(50 nm)/Au(150 nm) bi-metal layer on the entire C-face of 4H-SiC (on highly doped ~10¹⁸cm⁻³ substrate) was chosen for back Ohmic contact. It should be noted that both the top and back contacts of the SBDs were not annealed during the fabrication. The active area of the investigated SBDs is ~0.8 mm². The SBDs were irradiated with ⁶⁰Co-gamma rays up to a dose of 100 Mrad at UGC-DAE CSR, Kolkata centre; the irradiation information can be found elsewhere^[5]. The TSCAP spectrum and electrical characteristics of the Ni/4H-SiC SBDs were measured before and after the gamma irradiation. The experimental procedure for acquiring the TSCAP spectrum is reported in our earlier works^[5,31]. In TSCAP measurements, upon cooling the sample to a low temperature of 150 K, the traps in the SBD were filled by injecting a forward current of 2 mA for 100 s. Thereafter, the sample temperature was increased up to 650 K at a heating rate of 0.12 K/s and subsequently the SBD capacitance versus temperature plot (i.e. TSCAP spectrum) was obtained at the reverse bias voltage of −40 V.

Thermal annealing of gamma irradiated SBDs was carried out by using a CARBOLITE GERO tube furnace (STF 16/450) from a temperature of 400 °C to a high temperature of 1100 °C with a step size of 100 °C in inert gas ambient. From the normal ambient conditions, the samples were loaded into the furnace to the desired annealing temperature. At each temperature, the samples were heated for 30 min duration in argon ambient. After annealing, the samples were unloaded and allowed to cool for some time in room ambience. Consequently, the TSCAP spectrum and electrical properties of the SBDs were measured after the heat treatments.

Before discussing the annealing effects, it is important to know the gamma irradiation induced changes in the Ni/4H-SiC SBD characteristics at a dose of 100 Mrad are briefly described below^[5]:

Two deep level bulk traps such as P1 (E_C – 0.63 eV, Z_1/2) and P2 (E_C – 1.13 eV, EH5) are detected in the non-irradiated SBDs with concentrations in the 10¹³ cm⁻³ range^{[5, 31]}. After gamma irradiation, the trap density for P1 and P2 is increased by an order of magnitude (~10¹⁴ cm⁻³ range), along with the creation of a new trap G420 (E_C – 0.89 eV, RD_1/2) with a density of 3.8 × 10¹³cm^−3[5]. The interface states in the non-irradiated SBDs are distributed over the energies of E_C – 0.4 eV to E_C – 1.04 eV, as determined from the forward I–V characteristics (discussed later)^{[11, 31, 32]}. Surprisingly, no significant changes in the interface state density are found after irradiation. Fig. 1 shows the energy location of the bulk traps (P1, P2 and G420) and the interface states (E_C –0.4 eV to E_C – 1.04 eV) in the energy band diagram of the Ni/4H-SiC SBDs.

In non-irradiated SBDs, inhomogeneous^{[31, 33–35]} Schottky barrier height (SBH) is identified at the Ni/4H-SiC interface with the two different SBHs (SBH1 = 1.18 eV, SBH2 = 1.29 eV) and ideality factors (n₁ = 1.32, n₂ = 1.88). In this condition, the current conduction preferentially occurs through the spatially localized low SBH regions at the Ni/4H–SiC interface, thereby deviating the electrical characteristics from the ideal case. This inhomogeneous nature is also observed in the irradiated SBDs. There are no considerable changes in the SBH (SBH1 = 1.2 eV, SBH2 = 1.3 eV) and ideality factor (n₁ = 1.33, n₂ = 1.9) values after irradiation. Hence, nonideal electrical properties are obtained for Ni/4H-SiC SBDs before and after irradiation. Moreover, a reduction in the forward current is noticed in the SBDs upon exposure to the gamma irradiation (because of the increased series resistance).

The effective doping concentration of 4H-SiC epilayer is decreased (from 5 × 10¹⁴ to 3.8 × 10¹⁴ cm⁻³) after irradiation due to the compensation of the donor dopants by the irradiation produced acceptor-like deep level traps^[5]. The frequency dependent C–V characteristics (capacitance increases with signal frequency) are observed in the SBDs before and after the irradiation. Most importantly, the reverse current is decreased after gamma irradiation; the phenomenon behind this observation is reported in our earlier article^[5].

Fig. 2 shows the TSCAP spectrum of the gamma irradiated Ni/4H-SiC SBDs at different annealing temperatures from 400 to 600 °C. The increasing TSCAP steps (P1, P2 and G420) in Fig. 2 reveal the electron traps in the SBD^[5]. The mid-temperature of the TSCAP step is extracted from the spectrum and the trap activation energy is calculated by ^{[5, 36]}

${E_ {\rm{T}}} = k{T_{1/2}} \ln {\frac{{vk{T_{1/2}}}}{{q\left( {{E_ {\rm{T}}} + 2k{T_{1/2}}} \right)}}} ,$ (1)

where is Boltzmann’s constant, is the escape frequency factor^[5], and is the elementary charge. The same energy location is identified for the traps P1 (E_C – 0.63 eV), G420 (E_C – 0.89 eV), P2 (E_C – 1.13 eV) in the annealed SBDs. The annealing induced changes in the trap concentration are calculated from the TSCAP signal magnitude (step height) as per the following expression^[5,36,37]

${N_ {\rm{T}}} = \frac{{2\left( {{C_2} - {C_1}} \right)}}{{{C_1}}}{N_ {\rm{d}}},$ (2)

where is the capacitance at the step starting temperature, is the capacitance at the step ending temperature and is the epilayer doping concentration. The trap concentrations determined at different annealing temperatures are summarized in Table 1. Note that, the TSCAP technique is very insensitive to the detection of interface states in the SBDs^{[5, 36, 37]}. Moreover, the interface trap energies are expected to distribute continuously at the metal/semiconductor interface^[38]; thus they possibly produce significantly longer TSCAP steps (broader spectrum in case of thermally stimulated current) based on their energy distribution range. Here, such a behavior is not observed in the TSCAP spectrum. Accordingly, it is considered that the deep levels P1, G420, and P2 belong to the bulk traps. From Fig. 2, it is noticed that the TSCAP spectrum shifts towards the downside after annealing (discussed later). A little decrease in the trap density for P1 and P2 is observed with the annealing temperature up to 600 °C (see Table 1), as similar to the annealing nature of non-irradiated 4H-SiC SBDs^[31]. However, the trap concentrations for P1 and P2 are still in the range of 10¹⁴ cm⁻³. On the other hand, the concentration of the trap G420 (RD_1/2) is minimized to half of its pre-annealing value (from 0.38 × 10¹⁴ to 0.18 × 10¹⁴ cm⁻³) at 400 °C. Furthermore, the trap G420 has disappeared from the post-annealing TSCAP spectrum acquired at 500 °C (see Fig. 2), which reveals that the trap concentration for G420 is reduced (< 10¹³ cm⁻³) below the detection limit of the TSCAP technique after the heat treatment at 500 °C. This result is contradictory to the observations of Dalibor et al.^[39], where the RD_1/2 is thermally stable even at higher annealing temperatures (up to 1400 °C); but, their SBDs are not gamma irradiated. It is anticipated that the reduced trap density for G420 upon annealing at 500 °Cmay be advantageous in terms of minimizing the charge trapping effects in the real epitaxial 4H-SiC SBD detectors. The TSCAP spectra have not been acquired with the SBDs annealed ≥ 700 °C, so thermal evolution of trap densities cannot be determined from 700 °C.

Fig. 3 displays the forward current-voltage (I_F–V_F) characteristics of the gamma irradiated Ni/4H-SiC SBDs for the annealing temperatures of 400 to 800 °C. The forward current is found to decrease with the annealing temperature. The variations in the forward voltage drop (V_F) at 1 mA, SBH, and ideality factor (n) of the SBDs are determined^[12,31,38] at each annealing temperature and are summarized in Table 1. The changes in the I_F–V_Fcharacteristics (in semi-log scale) of the gamma irradiated SBDs before and after the annealing temperature of 400 °C are plotted in Fig. 4. Two linear regions are perceived in the pre-annealing curve, whereas a single linear region is noticed in the post-annealing plot at 400 °C. Therefore, the double SBH nature^{[31, 33–35]} of the pre-annealed gamma-irradiated SBDs is eliminated after the annealing at 400 °C and the stable SBH of 1.4 eV is also achieved. The forward voltage (V_F) drop at 1 mA is increased to 1.7 V at 400 °C due to the improvement in the SBH (which reduces the thermionic emission current^[38]). Nonetheless, the increase in the V_F at 1 mA is identified (although SBH decreases, see Table 1) along with the degraded ideality factors from 500 °C. So, anomalous I_F–V_Fcharacteristics are obtained starting from the temperature of 500 °C.

To understand the abnormal variations in the post-annealing characteristics, the interface state density (N_SS) distribution in the annealed SBDs is computed^{[11, 31, 32]} from the forward I–V characteristics. In this calculation, the SBH is considered as bias-dependent and subsequently the effective SBH is evaluated by^{[11, 31, 32]}

${\varPhi _ {\rm{eff}}} = {\varPhi _ {\rm{B}}} + \left( {1 - \frac{1}{{n({V_ {\rm{F}}})}}} \right){V_ {\rm{F}}},$ (3)

where is the forward voltage and the bias-dependent ideality factor is expressed by^[11,31,32]

$n({V_ {\rm{F}}}) = \frac{{q{V_ {\rm{F}}}}}{{kT \ln ({I_ {\rm{F}}}/{I_ {\rm{S}}})}},$ (4)

where is the forward current and is the reverse saturation current. The interface trap density in equilibrium with the semiconductor can be written as^{[11, 31, 32]}

${N_ {\rm{SS}}} = \frac{1}{q}\left\{ {\frac{{{\varepsilon _ {\rm{i}}}}}{\delta }\left[ {n({V_ {\rm{F}}}) - 1} \right] - \frac{{{\varepsilon _ {\rm{s}}}}}{W}} \right\},$ (5)

where is the interfacial oxide permittivity, is the oxide layer thickness identified from the high-frequency (1 MHz) C–V, is the permittivity of 4H-SiC, and is the depletion region thickness estimated from the (1/C²)–V properties at 1 MHz. The energy position of the interface states in n-type semiconductor is located with respect to the conduction band bottom , as given by^{[11, 31, 32]}

${E_ {\rm{C}}} - {E_ {\rm{SS}}} = q({\varPhi _ {\rm{eff}}} - {V_ {\rm{F}}}).$ (6)

Fig. 5 depicts the distribution profile of the interface state density (N_SS) as a function of E_C – E_SS for the annealed (400 – 700 °C) SBDs. Like non-irradiated 4H-SiC SBDs^{[11, 31, 32]}, exponential increase in the N_SS is noticed towards the conduction band edge for the annealed SBDs. The N_SS is significantly reduced in the energy range of E_C – 0.4 eV to E_C – 0.63 eV after the heat treatment at 400 °C. Thus, the observed enhancement in the SBH at 400 °C reveals that the interface states responsible for the dual SBH nature of pre-annealed SBDs is almost removed at 400 °C. In contrast, from 500 °C, the N_SS is found to rise with the annealing temperature. At 700 °C, the N_SS is dramatically increased to 3.35 × 10¹³ eV⁻¹ cm⁻² (at E_C – 1.0 eV) and 2.57 × 10¹³ eV⁻¹ cm⁻² (E_C – 0.4 eV) from the values at 500 °C of 4 × 10¹² and 8.7 × 10¹² eV⁻¹ cm⁻². It is worth recalling that the trap G420 has disappeared from the post-annealing TSCAP at 500 °C and no significant modifications in the trap concentration for P1 and P2 are noted up to 600 °C. So, the reduction in the trap density of G420 (< 10¹³cm⁻³) at 500 °C has no effect on the electrical characteristics of the SBDs. As a result, the irregular changes in the I_F–V_F characteristics noticed from 500 °C are ascribed to the annealing induced increase in the N_SS.

The annealing (400–700 °C) induced changes in the reverse current–voltage (I_R–V_R) characteristics of the gamma irradiated Ni/4H-SiC SBDs are shown in Fig. 6. A substantial reduction in the reverse current is noticed at 400 °C due to the improvement in the SBH. Overall, the electrical parameters are improved upon the heat treatment at 400 °C, similar to the non-irradiated Ni/4H-SiC SBDs^{[11, 12, 31]} (with Ti based Ohmic contact). However, beginning from the temperature of 500 °C, the reverse current is found to rise with the annealing due to the degraded SBH and the increased N_SS. So, the reverse I–V characteristics of the gamma irradiated SBDs deteriorate on or above 500 °C as analogous to the forward I–V properties. These results suggest that a number of new interface traps are created after annealing ≥ 500 °C and they again form an inhomogeneous Ni/4H-SiC Schottky barrier interface, thereby decreasing the SBH. Accordingly, the reverse current conduction may take place via the spatially localized low SBH section and hence the reverse current increases from 500 °C.

Fig. 7 shows the (1/C²)–V characteristics at 1 MHz of the gamma irradiated Ni/4H-SiC SBDs after the heat treatments (400–700 °C). It is noticed that 1/C² increases (capacitance decreases) with the annealing temperature. Hence, the annealing induced decrease in the capacitance is the reason for the downward movement of the post-annealing TSCAP spectrum seen in Fig. 2. The (1/C²)–V characteristics at different signal frequencies (1 kHz to 1 MHz) for the SBDs at the annealing temperatures 400 and 500 °C are shown in Fig. 8 and in the inset. Similar to the pre-annealing case, frequency dependent (1/C²)–V characteristics are obtained at 400 °C. It should be noted that the computed^{[31, 40]} thermal emission time constant ( 1 ms) of the bulk traps P1, P2 and G420 is much higher than the C–V measurement time at the lowest signal frequency of 1 kHz. Therefore, the observed frequency dependence is not due to the bulk traps; it might be caused by the charge states at the Ni/4H-SiC interface. According to the calculated thermal emission time constant of the interface traps^{[31, 40]}, it is estimated that the interface traps located between the energies viz. E_C–0.4 eV to E_C–0.515 eV can affect the C–V measurements at the frequency of 1 kHz and the interface traps near to the energy E_C–0.4 eV can include their contribution to the measured capacitance up to 100 kHz. Thus, the frequency dependent C–V characteristics may be produced by the interface trap energies E_C–0.4 eV to E_C–0.515 eV.

The decrease in the 1/C² viewed in Fig. 8 with the signal frequency indicates the presence of acceptor-like interface traps in the SBDs. As a result, including the contribution of acceptor-like interface traps, reduced capacitance value (increased 1/C²) is obtained at low signal frequencies. Consequently, the measured capacitance increases with the signal frequency, as noted from Fig. 8. To minimize the interface state’s charge contribution, the annealing induced changes in the effective doping concentration are evaluated from the (1/C²) –V characteristics at the high signal frequency of 1 MHz. It is identified from Fig. 7 that the effective doping concentration is found to decrease with the annealing temperature (see Table 1). The effective doping concentration is reduced to 3.16 × 10¹⁴ cm⁻³ at 400 °C. Since the concentration of the deep level defects is not increased after the annealing, the donor concentration in the 4H-SiC epilayer must not be affected by them. In fact, the effective doping concentration is underestimated after annealing due to the contribution of the acceptor-like interface states. Strong frequency dependent (1/C²)–V characteristics perceived at 500 °C (see the inset of Fig. 8) also reveal the creation of the acceptor-like interface traps after annealing. Accordingly, the effective doping concentration at 700 °C is undervalued (~1 × 10¹⁴cm⁻³) by about four times its pre-annealing value.

Fig. 9 shows the C–V characteristics of the gamma irradiated Ni/4H-SiC SBDs for the heat treatments ≥ 800 °C (up to 1100 °C). A minor variation in the capacitance is noted with the bias voltage. However, these capacitance values are in the order of the diode geometrical capacitance^{[31, 38]}, by considering the SBD active area (~0.8 mm²) and the total thickness (~395 µm) including the substrate and the epitaxial layer. Moreover, similar C–V properties are noticed at all the signal frequencies (1 kHz to 1 MHz). Because of these peculiar C–V characteristics, the changes in the effective doping concentration are not obtainable from 800 °C.

Fig. 10 shows the typical I–V characteristics (−20 to 20 V) of the gamma irradiated Ni/4H-SiC SBDs at the annealing temperature of 950 °C. Very low forward current (0.54 nA at 5 V and 1.74 µA at 10 V) is obtained at 950 °C. Furthermore, undesirable electrical parameters (V_F at 1 mA > ~4 V, SBH < 1 eV, and n > 8) are identified for the SBDs annealed ≥ 800 °C (see Table 1). So, the I–V and C–V characteristics state that the rectifying properties of the SBDs have vanished from the annealing temperature of 800 °C. It is reported^[41–43] that the Ni Schottky contact on n-type 4H-SiC epilayers can change into an Ohmic contact around the annealing temperature of 950 °C. Here, Ohmic nature is not found in the I–V characteristics (asymmetrical I–V in Fig. 10) at 950 °C (even up to 1100 °C). Generally, the Ni/4H-SiC Schottky contact is expected to improve after the annealing temperatures of 500–700 °C due to the creation of nickel silicide at the interface^{[8, 40, 44, 45]}. In this work, considerable deterioration in the electrical characteristics is observed from the temperature of 500 °C. As a result, the reason for the disappearance of the SBD rectifying behavior may be associated with the deterioration in the back contact properties (Ti/Au bilayer with 4H-SiC substrate) at elevated temperatures. From the work of Kim et al.^[46], it is noted that 800 °C is not the eutectic temperature for Ti/4H-SiC interface. However, it is clear that the rectifying property of the Ni/4H-SiC SBDs (with Ti based Ohmic contact) has vanished from 800 °C. Studies are underway to understand the physical mechanism responsible for the disappearance of the SBD rectification at ~800 °C.

Thermal annealing induced changes in the gamma irradiated Ni/4H-SiC SBD properties are investigated over a wide range of temperatures 400–1100 °C. The concentration of the trap at E_C–0.89 eV is reduced (< 10¹³cm⁻³) below the detection limit of the TSCAP technique at the annealing temperature of 500 °C; the reduced trap density for G420 upon annealing at 500 °C may be beneficial in terms of minimizing the charge trapping effects in the real epitaxial 4H-SiC SBD detectors. On the other hand, no considerable changes in the trap density for E_C–0.63 eV and E_C–1.13 eV are identified up to 600 °C. Like non-irradiated Ni/4H-SiC SBDs (with Ti based Ohmic contact), the electrical characteristics are improved at 400 °C with the disappearance of dual SBH nature and the reduction in the interface trap density. Hence, the optimum annealing temperature for these kinds of Ni/4H-SiC SBD structures is ~400 °C. Nevertheless, from 500 °C, the electrical characteristics are found to degrade with the annealing temperature due to the increase in the interface state density. Reasonable electrical properties are noted up to 600 °C and finally, the SBD rectification has disappeared from 800 °C.

Acknowledgment

The authors would like to thank CSIR-CEERI for providing facilities for SBD fabrication. We also would like to thank Dr. Abhijit Saha, UGC-DAE CSR, Kolkata, for the gamma irradiation of SBDs.