-rays irradiation effects on dielectric properties of Ti/Au/GaAsN Schottky diodes with 1.2%N

A. TEFFAHIa,b , D. HAMRIa , A. DJEGHLOUFa ,M. ABBOUN ABIDa, A. SAIDANEa, N. Al Saqric , J.F. Felixd, M. HENINIe a CaSiCCE Laboratory, ENP-Oran, B.P 1523 El M'Naouar, Oran 31000, Algeria b Ecole Préparatoire Sciences & Techniques, BP 474 Place des Martyres, Alger ,Algerie cDepartment of Physics, College of Science, Box 36, Sultan Qaboos University, Al Khoud 123, Oman dUniversidade de Brasília, Instituto de Física, Núcleo de Física Aplicada, Brasília, DF 70910-900, Brazil eSchool of physics and Astronomy, Nottingham University , NG72RD, United Kingdom aek1976@outlook.com


Abstract:
Dielectric properties of As grown and irradiated Ti /Au/GaAsN Schottky diodes with 1.2%N are investigated using capacitance/conductance-voltage measurements in 90 -290 K temperature range and 50-2000 kHz frequency range. Extracted parameters are interface state density, series resistance, dielectric constant, dielectric loss, tangent loss and ac conductivity. It is shown that exposure to γ-rays irradiation leads to reduction in effective trap density believed to result from radiation-induced traps annulations. An increase in series resistance is attributed to a net doping reduction. Dielectric constant (ε') shows usual step-like transitions with corresponding relaxation peaks in dielectric loss. These peaks shift towards lower temperature as frequency decrease. Temperature dependent ac conductivity followed an Arrhenius relation with activation energy of 153 meV in the 200-290 K temperature range witch correspond to As vacancy. The results indicate that γ-rays irradiation improves the dielectric and electrical properties of the diode due to the defect annealing effect.

Introduction
The search for new high speed devices that resist radiation is required for a number of useful applications. Among these semiconductor materials candidates, dilute nitride gallium arsenide (GaAsN) has been recently proposed (Shafi et al., 2011). Its electrical properties can be improved by adding a small nitrogen fraction, and can be tailored for many applications such as solar cells fabrication (Yamaguchi et al., 2012), infra-red lasers devices (Kondow et al., 1997), terahertz emitters and detectors (Park et al., 2009) and so on. The presence of some percent of nitrogen (N) induces an impressive diminution of forbidden band gap of GaAs semiconductor. Due to large electronegativity of nitrogen and its small covalent radius, forbidden band gap decreases by approximately 0.1eV for each percent of nitrogen (N) in the alloy.
Researchers (Tisch et al., 2002) suggested that this dependence of energy gap on small nitrogen fraction (0<x<5) can be calculated using empirical expression:  (Klangtakai et al., 2015) have also investigated gamma-ray irradiation effects on structural properties of GaAs 1-x N x films (N between 1.9 and 5.1 at%). They found that gamma-ray irradiation causes structural changes including displacement damage and gamma-ray heating.
In our previous work (Teffahi et al., 2016) we have characterized electrical properties of Ti/Au/GaAs 1-x N x Schottky diodes at room temperature using I-V and C/G-V-f techniques, The investigated parameters are ideality factor (n), series resistance (Rs), barrier height (Φ B ), doping concentration (N D ), relaxation time and density of interface states (Nss). We have shown a decrease in measured barrier height and an increase in extracted Ideality factor and the series resistance. The density of interface states distribution increase after radiation due to irradiation-induced defect (Al Saqri et al., 2015).
However, workers (Bobby et al., 2014) have deduced that irradiation with a cumulative dose of 10 Mrad improves electrical characteristics of Au/n-GaAs diodes.
They found a decrease in series resistance and in ideality factor. Researchers (K. M. Yu and W. Walukiewicz, 2002) stated that pulsed laser annealing improves N incorporation in GaN x As 1−x thin films with synthesized films being thermally stable.
All above studies show that electrical properties of GaAsN Schottky diodes are complex and still raise numerous questions related to N concentrations and irradiation effects. In this paper, electrical and dielectric properties of as grown and γ-rays irradiated Au/Ti/GaAsN 1.2%N Schottky diodes are analyzed. The effect of γ-rays on interfaces states density , series resistance, permittivity, dielectric constant, loss factor (tan δ) and ac conductivity are examined over a wide temperature range [90 K-290 K] and frequency range [10 KHz-2 MHz].

Experimental details
As previously described (Teffahi et al., 2016), Capacitance/Conductance-Voltage (C/G-V-T) measurements were carried out using an Agilent LCR meter (E4982a). Device temperature was changed using an ARS Closed Cycle Cryostat.

Results and discussions
1.2% N content Au/Ti/GaAsN Schottky diodes are dc biased from -2 to 0.8V with an added small ac sine signal at 1 MHz to allow deep traps to capture and release electrons. Figure (1) shows capacitance variation against bias voltage before and after irradiation at some preselected temperature. Capacitance decreased after irradiation.
This is attributed to a decrease in donor concentration and to a change in dielectric constant at metal semiconductor interface after gamma irradiation (Shiwakoti et al., 2017). Capacitance also decreased with decreasing temperature due to a continuous distribution of density of interface states (Nss) and change in series resistance (Bobby et al., 2016).
Figure (2) shows conductance variation bias voltage before and after irradiation at some preselected temperature. Conductance increases with increasing voltage and with decreasing temperature for both samples. However, its value decreased after gamma irradiation. This effect can be attributed to a decrease in net ionized doping concentration (Behle and Zuleeg, 1972;Karataş et al., 2005;Uğurel et al., 2008).
Series resistance temperature-voltage dependence in Figure (3) is obtained from C-V-T and G-V-T measurements according to (Bachir Bouiadjra et al., 2014;Nicollian et al., 1982): where C m and G m are measured capacitance and conductance and ω=2πf is the angular frequency. Series resistance was found strongly dependant on temperature and irradiation. Notice that series resistance has a peak for all diodes with peaks shifting towards positive voltage region. Rs temperature dependence is attributed to activation of interface states (Teffahi et al., 2016). Series resistance of irradiated samples has increased due to the decrease of carriers' mobility and carriers' removal effect (Behle and Zuleeg, 1972).
Obtained series resistance values are used to correct measured capacitance and conductance-voltage values. Therefore, measured capacitance and conductance values were corrected by eliminating the effect of Rs in order to obtain the real capacitance Cc and conductance Gc values. Corrected capacitance Cc and conductance Gc values were calculated using (Hill and Coleman, 1980): Hill-Coleman method (Hill and Coleman, 1980) was used to find interface state density using: where A is rectifier contact area, ω is angular frequency, C m and (G m / ω) max are capacitance and conductance at high forward voltage and C ox is native insulator layer capacitance given by (Nicollian et al., 1982): The inset plot of Figure (3) shows active interface states density change against temperature for both as grown and γ-rays irradiated Schottky diodes. Active interface states densities increase with increasing temperature since interface states near the conduction band are more sensitive at high temperatures. These interface state density profiles show the presence of greater density of active localized interface taps in as grown than irradiated samples. Calculated values at 270 K are found to be about 6.04×10 14 eV -1 .cm -2 and 7.77 ×10 12 eV -1 .cm -2 for as grown and irradiated samples, respectively. Gamma rays are found to have an annealing effect that reduces interface states density (AURET et al., 1993;Goodman et al., 1994;Shafi et al., 2011).
The quality of interface contact and its temperature and frequency dependence can also be described in term of dielectric dispersion. Dielectric properties such as dielectric constant, dielectric loss and ac conductivity help to shed some light on 50 kGy γ-rays irradiation effects (Dokme and Altindal, 2011).
When a periodic electric field E is applied to a dielectric, a charge displacement D is created (Fröhlich, 1949;Sagadevan and Sundaram, 2014) with: is the complex dielectric that describes the drop in electric field strength * through dielectric due to inhomogeneity of interface. It can be expressed as: Where ( ) and ( ) are the real and imaginary parts of permittivity. ' '' The relative permittivity of a dielectric is calculated from measured capacitance and ' conductance by taking: 1.2%N content, calculated from measured impedance data using equations (9) and (10), are shown in Figures (4) and (5) (Strzalkowski et al., 1976). Dielectric constant decreases after irradiation. At 250 K, for example, dielectric constant is about 6.51 for as grown and 5.28 for gamma irradiated Schottky diodes. Gamma irradiation decreases existing disorder in as grown diodes and improves interface quality by annealing interface traps (Bobby et al., 2016). Dependence of dielectric constant on frequency is shown only for as grown Ti/Au/GaAsN Schottky diodes and not on γ-rays irradiated ones. There is a decrease in dielectric constant with increasing frequency. Such behavior can be explained by the fact that when frequency is raised, interfacial dipoles have less time to orient themselves in alternating field direction (Fröhlich, 1949).
Dielectric loss is proportional to energy loss in dielectrics. It presents two relaxation peaks at 200 K and 270 K, Figure (5). These peaks correspond to polarization effect of interface states. Interfacial polarization appears when electric field produces a separation of mobile positive and negative charges (Mustafaeva et al., 2009). These peaks shift towards lower temperature as frequency decreases. This is due to the contribution of interface states that cannot follow the ac signal at high frequencies (Swamy et al., 2016). Joule heat may be produced by electrons and ions drift current in Schottky contact witch dissipate part of energy as heat in dielectric. Figure (6) shows loss tangent temperature dependence for various frequencies. Loss tangent increase with increasing temperature; at high temperature more charge participate to conduction. After irradiation a reduction in charge yields a decrease in thermal loss.
Hence, the decrease in loss tangent of irradiated diodes is interpreted on basis of 'annealing effect' of γ-rays. N incorporation decreases the activation energy of dielectric relaxation, thus the dielectric plots shift to lower temperature. Figure (7) shows ac conductivity σ ac temperature dependence for various frequencies.
σ ac shows two peaks with different intensities around 200 K and 270 K. Both peaks are slightly shifted towards lower temperatures when frequency decreases. After irradiation, σ ac decreased due to donors' concentration reduction. σ ac also decreased with decreasing temperature and applied frequency. This is due to charges freezing effect. From ac conductivity evolution with temperature, it is possible to plot Arrhenius function (El Sayed, 2014;Maldzius et al., 2010): where σ 0 is pre-exponential factor, is activation energy and kT is thermal energy.
From the linear part of the inset plot of Figure  of the electrode polarization. Up 150 k the increase with increasing temperature M " due to charge polarization effects. Peaks were observed around 200 k, these peaks decrease with decreasing frequency and shift toward low temperature. These peaks correspond to relaxation process, the density of dipoles contributing to relaxation decrease with decreasing temperature (Shiwakoti et al., 2017). The presence of the peaks in irradiated diode at 270 k it might be interpreted by the traps generation.

Conclusion
Electrical and dielectric properties of as grown and gamma ray irradiated Ti/Au/GaAsN Schottky diodes with 1.2%N have been investigated in 90-290 K temperature range and 50-2000 KHz frequency range. Interface state density, series resistance, dielectric constant, dielectric loss, tangent loss and ac conductivity were extracted. It is shown that these parameters are strongly dependant on both temperature and frequency. Dielectric constant (ε') shows usual step-like transitions with corresponding relaxation peaks in dielectric loss. These peaks shift towards lower temperature as frequency decrease due to space charge polarization. The decrease in dielectric constant and loss tangent of irradiated diodes is interpreted on the basis of annealing effect of γ-rays. Gamma irradiation decrease existing disorder in as grown diodes and makes order at interface by annealing interface traps.