1 INTRODUCTION

Over the last few decades, GaN has become one of the fascinating compound semiconductor material for researchers in the field of device-fabrication technology due to its unique properties like large and direct energy bandgap, high melting point, high chemical resistance, and good thermal conductivity [1–4]. These properties make GaN a potential material of modern electronic devices for various applications such as amplifiers, dynamic switches, detectors, lasers, solar cells, transistors for terahertz plasma applications, diodes, including the devices that are reliable in radiation-rich environments [1–3, 5–9]. Low-energy ion irradiation is an effective technique for customizing materials and devices containing semiconductors. These ions, however, introduce electrically active defects that modify the properties of semiconductor material and thus alter the characteristics of the devices fabricated on it [6, 10–14]. Another advantage of low-energy ion irradiation is to test the reliability of the semiconductor devices under exposure to radiation at higher altitudes, e.g., for military and space applications [9, 15–17] and their response in radiation environments at high energy physics experiments/accelerator facilities [18–22].

Therefore, studies of the influence of energetic ions on semiconductors and their device properties are primary research areas. In this context, we report the 650-keV Ar2+ ion-irradiation effects on the structural, optical, and electrical properties of Ru|Pt|n-GaN Schottky barrier diodes (SBDs) for various fluences to understand the current transport mechanism for its applications in different fields.

2 EXPERIMENTAL

In the present work, Ru(30 nm)|Pt(20 nm)|n-GaN SBDs were fabricated using a 2-μm thick n-GaN on the Al2O3 (C-plane) substrate. The comprehensive process of fabrication of Ru|Pt|n-GaN SBDs is described in detail in [23]. The Ru|Pt|n-GaN SBDs were irradiated by 650-keV Ar2+ ions using the low-energy ion-beam facility (LEIBF) housed at IUAC, New Delhi, India [24]. During irradiation, the beam current was 1 μA (max), and the ion fluences ranged from 1 × 1013 to 1 × 1015 ions cm–2. The X-ray diffraction (PANalytical: X’pert3 Powder) and photoluminescence (PL-Fluorolog, Xenon lamp, 450 W, Excitation: 350 nm, resolution: 0.3 nm) studies for pristine and irradiated samples were conducted at UGC-NRC School of Physics, Hyderabad. Room-temperature current–voltage (I–V) and capacitance–voltage (C–V) characterization were done using the semiconductor device analyzer (Agilent Technologies B1500A) at IUAC, New Delhi, India. All these electrical measurements were done as per MIL-STD 750 E standards [25].

3 RESULTS AND DISCUSSION

To understand the mechanism of energy loss, ion range, 650-keV argon ion-irradiation distribution, stopping range, non-ionizing energy loss (NIEL), linear energy transfer (LET), and ionization displacement profile were calculated (Table 1) using the stopping and range of ions in matter (SRIM) and transport of ions in matter (TRIM) simulations [26].

Table 1. TRIM Calculations for 650 keV Ar in Ru|Pt|n-GaN SBDs
Full size table

The methodologies followed for damage profile, LET, and NIEL calculations are similar to the details given in [27, 28]. Figure 1 displays the distribution of total ionization of target, and Fig. 2 displays the target’s total displacements as a function of the device’s thickness.

Fig. 1.
figure 1

The distribution of ionization losses in Ru|Pt|n-GaN SBDs for 650-keV Ar.

Full size image
Fig. 2.
figure 2

The distribution of displacement losses in Ru|Pt|n-GaN SBDs for 650-keV Ar.

Full size image

The distributions of ionization losses and displacement losses indicate the maximum damage at the metal-semiconductor (M–S) interface and decrease along with the depth of the semiconductor material. As a consequence, a large number of displacements/vacancies are commonly created due to displacement damage. In the same way, ionization damage produces electron–hole pairs. Higher LET values cause ionization and displacement damage. Table 2 lists the fluence-dependent total ionizing dose (TID) and displacement damage (Dd).

Table 2. Dependence of TID and Dd on the fluence of 650-keV Ar in Ru|Pt|n-GaN SBDs
Full size table

Room-temperature current–voltage I–V characteristics are shown in Fig. 3 for pristine and irradiated SBDs at different fluences: 1 × 1013, 1 × 1014, and 1 × 1015 ions cm–2 of 650-keV Ar2+, respectively.

Fig. 3.
figure 3

I–V plots of pristine and irradiated Ru|Pt|n-GaN SBDs for different fluences of 650-keV Ar2+ beam.

Full size image

The thermionic emission (TE) property of the device is extracted from the I–V characteristics of the M–S contacts using the relation [29]:

$$I = AA{\text{*}}{{T}^{2}}\exp \left( { - \frac{{q{{\varphi }_{{\text{B}}}}}}{{kT}}} \right)\exp \left( { - \frac{{q(V - I{{R}_{S}})}}{{nkT}}} \right),$$
(1)

where A* is the effective Richardson constant, A is the diode area, RS is the series resistance, n is the ideality factor, φB is the Schottky barrier height, and the other terms have their usual meaning.

Employing the Cheung and Cheung’s method [30], the electrical parameters RS, n, and φB are calculated using the following relations:

$$\frac{{dV}}{{d(\ln I)}} = I{{R}_{S}} + n\left( {\frac{{kT}}{q}} \right),$$
(2)
$$H(I) = V - n\frac{{kT}}{q}\ln \left( {\frac{I}{{AA{\text{*}}{{T}^{2}}}}} \right),$$
(3a)
$$H(I) = n{{\varphi }_{{\text{B}}}} + I{{R}_{S}}.$$
(3b)

The extracted values of n, RS, and φB from the above functions are given in Table 3.

Table 3. The values of n, φB and RS from Cheung and Cheung’s method [23] and Norde method [24] of Ru|Pt|n-GaN SBDs for different fluences of 650-keV Ar2+ beam
Full size table

The values of RS obtained from Eq. (2) (Fig. 4a) and from Eq. (3b) (Fig. 4b) are nearly equal, which gives the uniformity of the method.

Fig. 4.
figure 4

(a) The dV/dlnI vs. I plot and (b) The H(I) plot, of pristine and irradiated Ru|Pt|n-GaN SBDs for different fluences of 650-keV Ar beam.

Full size image

The Norde method [31] is used in the determination of large values of RS of diodes, which involves the F(V) function given as

$$F(V) = \frac{V}{2} - \frac{{kT}}{q}\ln \left( {\frac{{I(V)}}{{AA{\text{*}}{{T}^{2}}}}} \right).$$
(4)

The RS and φB are related as

$${{R}_{S}} = \frac{{kT}}{{q{{I}_{0}}}}.$$
(5)

The minimum of the function F(V) from the plot (Fig. 5) gives the values of F(V0), V0, which is related to minimal current I0

$${{\varphi }_{{\text{B}}}} = F({{V}_{0}}) + \frac{V}{2} - \frac{{kT}}{q}.$$
(6)
Fig. 5.
figure 5

The F(V) plot of pristine and irradiated Ru|Pt|n-GaN SBDs for different fluences of 650-keV Ar2+ beam.

Full size image

The values of n, RS, and barrier φB extracted from Norde’s method for different fluences of 650-keV Ar2+ ion irradiation are shown in Table 3.

The C–V measurements for pristine and irradiated Ru|Pt|n-GaN SBDs are done at a frequency of 1 MHz. Figure 6 demonstrates the I/C 2 reliance on the applied voltage of Ru|Pt|n-GaN SBDs.

Fig. 6.
figure 6

C–2 vs. V plot of pristine and irradiated Ru|Pt|n-GaN SBDs for different fluences of 650-keV Ar2+ beam.

Full size image

From the slopes of linear fits to the experimental I/C 2 vs. V plots, the effective concentration of donor atoms Nd is calculated using relation [32]:

$$\frac{1}{{{{C}^{2}}}} = \frac{2}{{{{A}^{2}}q{{\varepsilon }_{0}}{{\varepsilon }_{r}}{{N}_{{\text{d}}}}}}\left( {{{V}_{{{\text{bi}}}}} - \frac{{kT}}{q} - V} \right),$$
(7)

where A is the area of the Schottky contact, Nd is the effective dopant concentration, Vbi is the built-in potential, and V is the applied reverse bias voltage, while the other terms have their usual meaning.

And the value of φB is calculated using the relation

$${{\varphi }_{{\text{B}}}} = {{V}_{{{\text{bi}}}}} + \frac{{kT}}{q}\ln \left( {\frac{{{{N}_{{\text{c}}}}}}{{{{N}_{{\text{d}}}}}}} \right),$$
(8)

where Nc is the density of states in the conduction band.

The extracted values of φB and Nd from C–V analysis are given in Table 4.

Table 4. The values of φB and Nd of Ru|Pt|n-GaN SBDs for different fluences of 650-keV Ar2+ beam
Full size table

Tables 3 and 4 summarizes the values of n, RS, and φB using the Cheung and Cheung method, Norde method, and capacitance–voltage measurement analysis. With higher fluences of 650-keV Ar2+ ion irradiation, RS increases owing to the evolution of various defect levels in the band gap [33, 34]. Moreover, the increased value of RS is due to the formation of a damaged layer in the semiconductor material along the ion path [18, 35]. The decrease in the φB after a fluence of 1 × 1013 ions cm–2 is a prominent reason of rise in the reverse saturation current. At lower fluence of 1 × 1013 ions cm–2, the reverse current decreases nearly ten times as compared of pristine. It may be due to that after argon ion irradiation, more donors were introduced to the GaN surface. Whereas for highest fluence of 1 × 1015 ions cm–2, the reverse current increases, which may be due to the formation of deep energy levels in the middle of the forbidden gap that acts as generation–recombination (G–R) centers. The present study shows that the process of tunneling due to ion-induced defects has significantly altered the mechanism of charge transport in irradiated Ru|Pt|n-GaN SBDs [27, 28, 36]. After initial irradiation fluences, the value of n is observed to rise, which can be due to the increased contribution of tunnelling current. Whereas the drop in φB values after a fluence of 1 × 1013 ions cm–2 is a prominent cause for the drastic increase in the reverse saturation current. TRIM simulations predict ionization and target damage for 650-keV Ar ion irradiation as 442.6 and 14.92 keV/ion. Due to the lower value of nuclear energy loss Sn, they cause less displacement damage than ionization across the interface. This additionally prompts a noteworthy decrease in the values of φB. Whereas with higher fluence, several non-trivial effects can be observed because of defects caused by avalanche overlapping [14]. The elastic collision produces defects such as vacancies and interstitials in the semiconductor material. The combination or agglomeration of these defects leads to complex and stable defect structures [14]. These defects have associated deep levels within the semiconductor band gap and act as traps for free carriers, which reduces their concentration. The Schottky diode’s decrease in capacitance means an increase in the depletion width of the semiconductor [14].

In addition, I–V attributes were drawn in the logarithmic scale for the pristine and 650-keV Ar2+ ion irradiated SBDs to explore the dominant current conduction mechanism in the forward-bias region, as shown in Fig. 7.

Fig. 7.
figure 7

The logV vs. logI plot of pristine and irradiated Ru|Pt|n-GaN SBDs for different fluences of 650-keV Ar2+ beam.

Full size image

The current in the region with forward bias (logI vs. logV plots) is given as [14, 37]

$$I \propto {{V}^{p}},$$
(9)

where p is the slope of different linear regions and shows the various current conduction mechanisms of the forward-biased Ru|Pt|n-GaN SBDs.

logI vs. logV plots (Fig. 7) are found to have three (I, II, and III) linear regions with different slopes shown in Table 5.

Table 5. The slope from the logV vs. logI plot of Ru|Pt|n-GaN SBDs for different fluences of 650-keV Ar2+ beam
Full size table

These linear regions demonstrate the existence of fluctuating conduction mechanisms. For pristine, the slope value for Region I is about one corresponding to the ohmic behavior, which may be due to the expansion of thermally-produced carriers. Whereas, the slope value close to 2 in irradiated samples demonstrates the space-charge-limited current with discrete trapping levels [38, 39].

Figure 8 shows the results of the reverse current conduction in SBDs after 650-keV Ar2+ ion irradiation to classify the major current transport mechanism at M–S junction between Poole–Frenkel emission (PFE) or Schottky emission (SE) mechanism [40, 41].

Fig. 8.
figure 8

The IR vs. V1/2 plot of pristine and irradiated Ru|Pt|n-GaN SBDs for different fluences of 650-keV Ar2+ beam.

Full size image

When dominated by PFE mechanism, the reverse current is given by

$${{I}_{{\text{R}}}} \propto {{I}_{0}}\exp \left( {\frac{{{{\beta }_{{{\text{PF}}}}}{{V}^{{1/2}}}}}{{kT{{d}^{{1/2}}}}}} \right),$$
(10)

and for SE mechanism, current is given by

$${{I}_{{\text{R}}}} \propto AA{\text{*}}\exp \left( { - \frac{{{{\varphi }_{{\text{B}}}}}}{{kT}}} \right)\exp \left( {\frac{{{{\beta }_{{{\text{SE}}}}}{{V}^{{1/2}}}}}{{kT{{d}^{{1/2}}}}}} \right).$$
(11)

The theoretical βPF and βSE values are given by

$${{\beta }_{{{\text{PF}}}}} = 2{{\beta }_{{{\text{SE}}}}} = {{\left( {\frac{{{{q}^{2}}}}{{\pi {{\varepsilon }_{0}}{{\varepsilon }_{r}}}}} \right)}^{{1/2}}}.$$
(12)

For Ru|Pt|n-GaN SBDs, the theoretical values of field-lowering coefficients are estimated to be 2.54 × 10–5 eV m1/2 V–1/2 (βPF) and 1.27 × 10–5 eV m1/2 V–1/2 (βSE), respectively.

The findings has shown that the reverse current conduction mechanism is mainly due to SE in Region III, and also increased at higher fluences (Table 6).

Table 6. The experimental values of βPF and βSE of Ru|Pt|n-GaN for different fluences of 650-keV Ar2+ beam
Full size table

It can be concluded from the aforementioned conduction mechanism curves that the modifications for the Ru|Pt|n-GaN SBDs are mainly due to the high density of structural deformities or trap levels caused by 650-keV Ar2+ ion irradiation [37]. It confirms that the trapping/detrapping of charge carriers takes place predominantly at diode M–S interfaces [14, 42].

Characterizations of the room-temperature photoluminescence (PL) have been investigated for the optically dynamic interface imperfection states and their advancement during 650-keV Ar2+ ion irradiation. Figure 9 displays the deconvoluted PL spectrum of the pristine sample (inset) and the comparative PL spectrum of pristine and 650-keV Ar2+ ion irradiated Ru|Pt|n-GaN SBDs for different fluences.

Fig. 9.
figure 9

The PL spectra of pristine and irradiated Ru|Pt|n-GaN SBDs for different fluences of 650 keV Ar2+ beam (inset: deconvoluted PL spectrum of pre-radiated pristine sample).

Full size image

The deconvoluted PL spectrum of pre-radiated pristine sample reveals emission peaks at near UV (352.8 nm), blue luminescence {BL (431.8 nm}, cyan luminescence {CL (505.8 nm)}, green luminescence {GL (531.2 nm)}, and yellow luminescence {YL (572.9 nm} [43, 44] as shown in Fig. 9 inset. These peaks may be due to Ga|N vacancies or deep-level impurities in the pristine sample [39, 40]. GL and YL intensities decrease with 650-keV Ar2+ ion irradiation, and its shifts to shorter wavelengths may be due to the charge carrier’s recombination, induced geometrical distortions, and formation of defects state, which serve as recombination traps [14, 45, 46]. In addition, the evolution of BL (440.9, 446.9, and 463.7 nm) with the highest fluence (1 × 10–15 ions cm–2) may be due to the further defect formations [14, 47, 48]. These donor-nitrogen vacancies transition indicates that after argon ion irradiation, more donors were introduced to the GaN surface. The intensity of YL decreases for high fluence, which contributes to the introduction of many traps for gallium vacancies [14, 47, 48]. The results of PL are consistent with I–V and C–V data, reconfirming the observation that donor-nitrogen vacancies were introduced to the GaN surface at lower fluences.

In order to investigate structural modifications at the interface of Ru|Pt|n-GaN SBDs due to 650-keV Ar2+ ion irradiation, XRD peaks studies (Fig. 10) were performed, spanning 2θ angles from 25° to 95°.

Fig. 10.
figure 10

The XRD pattern of pristine and irradiated Ru|Pt|n-GaN SBDs for different fluences of 650-keV Ar2+ beam.

Full size image

There are distinct diffraction peaks of GaN (0002), GaN (0004), GaN (0001), Al2O3 (0006), GaN (0003), Al2O3 (0009), and Si (202) [48, 49] in the pristine sample. After 650-keV Ar2+ ion irradiation, peaks of GaN (0002) and GaN (0004) have been found to be broadened, and it can be due to formations of structural damage and associated strains [49, 50].

4 CONCLUSIONS

Room-temperature studies on the current transport properties of Ru|Pt|n-GaN SBDs were investigated for pristine and irradiated SBDs for different fluences; 1 × 1013, 1 × 1014, and 1 × 1015 ions cm–2 of 650 keV Ar2+. The electrical parameters n, RS, and φB were obtained from the thermionic emission (TE) model, Norde’s method, and C–V technique. The value of n increases with higher fluences, due to multiple transport mechanisms activation. The value of φB increases with a fluence of 1 × 1013 ions cm–2 and decreases further at higher fluences. The value of RS increases for higher fluences, which may be due to contributions from various current transport mechanisms, including defect-assisted tunneling (DAT) and G–R along with TE. Ion irradiation mainly reduces carrier mobility and carrier concentration and enhances the RS, as can also be seen in the analysis. As confirmation by SRIM and TRIM calculations, Ar ion irradiation has introduced a large number of displacements and vacancies and these defects increase the RS by reducing carriers concentration. Moreover, ion irradiation shows the significant contribution of tunneling currents as seen from higher forward currents than the low reverse currents up to a fluence of 1 × 1013 ions cm–2. This implies that due to high nuclear energy losses more displacement damage is caused in the lattice, leading to a huge contribution from DAT currents.

The XRD pattern shows that ion irradiation induces broadening of the peaks of GaN (0002), and GaN (0004) because of structural damage. The intensities of green and yellow emissions decrease with ion irradiation and also shift towards shorter wavelengths because of the recombination of chargers, instigated geometrical bends, and arrangement of imperfections state, which enhances the recombination traps.