The effect of low dose γ -irradiation on the optoelectric properties of n-GaN based MSM structure

The room temperature effect of a low dose rate (10 -4 rad (Si)/s) 60 Co γ -irradiation on the structural propertiesand dark current of the GaN-based metal-semiconductor-metal (MSM) structure has been studied. In contrast to previous studies, a non-monotonous dependence of the dark current with the γ -irradiation dose is observed. The intensity and linewidth of the photoluminescence (PL) peaks correlate with the changes in electrical characteristics and eventually degrade after prolonged exposure to the γ -radiation. The abnormal behavior of the MSM structure and particularly its I-V and PL characteristics are explained by considering the carrier transfer mechanism in the localized states. These phenomena are associated with the decrease of shallow dono rs’ density in localized states and the activation of the non-radiative centers as radiation dose increases. These experimental results and the mechanism presented are essential for understanding the interaction of the γ irradiation with n -GaN and for estimation of reliability of GaN-based (opto)electronics in harsh conditions of γ -radiation (space applications, liquidation of consequences of technogenic catastrophes, etc.).


Introduction
Gallium nitride (GaN) and its related alloys such as AlxGa1-xN and InxGa1-xN are widely used in the ultraviolet (UV) and visible spectrum regions, especially as the active region of photodetectors (PD) 1 . III-nitride-based PDs are particularly commonly used in harsh environments such as space applications 2,3 Meanwhile, the wide bandgap (3.4 eV 2 ), high breakdown voltage 4 , thermal and chemical stability, and irradiation resistance introduce GaN as a suitable candidate to use in harsh and irradiated environments. 5,6 Exposure of the GaN layers with electromagnetic irradiation and/or high-energy particles has been shown to lead to the vacancy generation and defects in the crystal structure such as Ga and/or N atoms displacements from their respective lattice sites 7 . This phenomenon generates new energy levels in the bandgap, which act as scattering centers such as donor, acceptor, or recombination centers 1,8 . Therefore, it is crucial to study the irradiation effects on semiconductor devices to understand the behavior of optoelectric devices in irradiation environments and fulfill the materials and design requirements consequently. There are significant differences between low and high dose rate irradiation effects on the semiconductor devices because the dose rate required to achieve maximum damage is different for various devices [9][10][11] . Although some devices are not sensitive to dose-rate below 1 rad (Si)/s, some other ones are significantly affected by dose rate even at 10 -3 rad (Si)/s 12 . It has been shown that the more extensive damages occurred in semiconductors under low dose-rate conditions than that for the higher dose 9,10 . The low dose-rate effects up to 10 -3 rad (Si)/s were reported for linear bipolar transistors in Ref. 12 and it was shown that the effect of the lower dose-rate is larger than that for the higher ones. This paper discusses low dose-rate test results for basic n-GaN-based metalsemiconductor-metal (MSM) structure, presenting data at dose rates as low as 10 -4 rad(Si)/s. To analyze the optoelectronic behavior of the device under radiation, current-voltage (I-V), X-ray diffraction (XRD), scanning electron microscopy (SEM) and photoluminescence (PL) spectroscopy were implemented. To the best of our knowledge, it is the first study of the effect of the low dose rate γ-irradiation on the GaN-based MSM structure and it is believed to be helpful with complying with the MSM structure device requirements in terms of materials and design.

Experiment Details
The gallium nitride epilayer with the thickness of 1.5 μm was grown on a standard c-plane sapphire substrate using a metal-organic vapor phase epitaxy (MOVPE) technique. The GaN epitaxial layer was doped with Si up to a concentration of 10 17 cm -3 . More details about the growth condition can be found elsewhere 13 . To establish electrical connections, a silver paste was used as a Schottky contact in MSM structure, as depicted in Figure 1. For convenience, the characteristics of the MSM structure are summarized in Table 1.  The effects of 60 Co γ-irradiation on the dark current of the MSM structure at room temperature were studied. The 60 Co source with 2μCi activity used in this work and the design of the experiment are shown in Figure 2. Since the γ-60 Co attenuation length in GaN is 1 cm 1 , which is much larger than the thickness of the GaN epilayer (1.5μm), it is considered that the sample is irradiated uniformly in the proposed test setup. The gamma source was located exactly in front of the detector surface to prevent gamma energy loss due to the absorption by air molecules. The current-

GaN (1.5 μm)
Sapphire (500 μm c-plane) Ag Ag S=3-6 mm +2 V -2 V voltage (I-V) characteristics were measured at room temperature with a -2 V to +2 V range using the Palmsens EmStat2 system. The total radiation dose by the 60 Co source on a MSM structure was calculated using the following equation below: Two similar MSM structures were considered in the test procedure: MSM structure-I and MSM structure-II. The MSM structure-I is irradiated for 2 hours (equivalent to 0.72 rad (Si)) and then followed by three steps each lasting for 30 minutes (equivalent to 0.18 rad (Si) at each step). The electrical measurements were promptly performed after each step. At the end of the last step, the sample was annealed for 24 hours at room temperature, and the electrical current was measured again. To study the dependence of the irradiation time on the I-V characteristics, the MSM structure-II was irradiated in four steps (30 minutes irradiation time at each step) and then irradiated additionally for 2 hours in a row. The I-V characteristics were measured upon each step. Table 2 shows the experiment process and parameters. To evaluate the γ irradiation-induced changes on the crystalline structure of the active layer, the X-ray diffraction (XRD) patterns of the thin films were recorded before and after the γ-irradiation. For XRD analysis, a high-resolution X-ray diffraction spectroscopy (XRD Model: PANalytical X'pert PRO) with Cu-Kα radiation of 1.54 ˚A wavelength was applied. The XRD data consisted of 2θ-ω symmetric (002) scan and the corresponding rocking curves (ω-scans) providing information on any changes in the corresponding interplanar spacings (effectively, lattice parameters) and dislocation densities, respectively. To examine the surface morphology, a Vega2 Tescan scanning electron microscope (SEM) was used. Photoluminescence spectroscopy (PL) analysis was carried out using PerkinElmerLS55 (equipped with Xe lamp) at an excitation wavelength of 250 nm, to analyze the optical behavior of the pristine and γ-irradiated samples. During the measurements, the samples were held in dark conditions to minimize the disturbance from the environment.

I. Radiation effect on n-GaN epilayer structure
High-resolution X-ray diffraction spectroscopy consisted of 2θ-ω symmetric (002) scanwere performed, and dislocation density was calculated as shown in Table  3. in Fig. 3 and therefore the calculated dislocation density is slightly higher for the irradiated sample, which indicates the dislocation density development after the γirradiation 14 . Similar results were obtained for the skew-symmetric reflectance.
Assuming a random distribution of dislocations when a Gaussian-like shape of the diffraction profile is observed, the dislocation density ρ can be determined using the equation (2) 15,16 : Where b is Burger's vector and integral breadth, β, is related to the FWHM peak width, H, by The structural parameters calculated from the XRD data for GaN epilayer are listed in Table 3 for the conditions before and after γ-irradiation.  To understand the γ-irradiation effects on the surface morphology of MSM structure-II, the scanning electron microscope (SEM) images were recorded. Figure  4 depicts the MSM structure SEM images before and after irradiations. There was no significant deformation at a low dose, which was expected based on previous observations 17 .

II. Radiation effect on I-V characteristic of n-GaN MSM structure
Dark current as noise is one of the key parameters in explaining the quality of a device. Actually, the decrease or increase of dark current indicates the improvement or destruction of an electronic component. As mentioned, we measured the dark current. Because the structure of the MSM detector was almost symmetrical, the electric current in the positive and negative bias became almost symmetrical. The pristine MSM structures I and II had dark current amount 19.78μA and 0.61μA in +2 volt, respectively. To better see the rate of change of dark current in terms of voltage, we plotted the logarithmic form of the current.
To evaluate the effects of γ-irradiation on the electronic characterization of pristine and irradiated Ag/n-GaN MSM structure, I-V characteristics of samples are compared, as shown in the following figures (Fig. 5 -Fig. 7). In Figure 5, the MSM structure-I was exposed for 2 hours (equivalent to 0.72 rad (Si)) in the first step, followed by three exposure steps as described earlier. The dark current increased to 61.65 μA in +2 volt in first step. An initial current enhancement at the first step followed by a gradual decrease upon gamma dose increasing in the next shorter steps, as shown in the inset picture for 1.4 V. This observation is not similar to the earlier observations for the higher doses 1,18-21 . Thereafter, the sample left for 24 hours and then repeated the I-V test. As depicted in Figure 6, although there is an indication of current reduction, it did not go back to the values observed for the pristine sample. Therefore, one can conclude that the accumulated dose effect on the sample remained, and therefore no full curing occurred after 24 hours of room temperature annealing.
Sample MSM structure-II, was initially irradiated for 30 minutes (for comparison with the sample MSM structur-I, which was initially irradiated for 2 hours), then similar to the sample MSM structure-I, followed by another three 30 min irradiation steps. The dark current increased to 1.28 μA in +2 volt in first step. Similar to MSM structure-I, current enhancement was observed, followed by the gradual decrease in the next steps, as shown in Fig. 7(a). Ultimately, sample MSM structure-II was irradiated for 2 hours and observed the current elevation, approximately 200 times, as a consequence, as illustrated in Figure 7(b). The dark current increased to 120.88 μA in +2 volt in this step.
These observations proving that the electrical behavior of semiconductor structures is dependent on the gamma irradiation dose/duration, even at very low doses, but these dependencies need to be understood in detail.  To explain the abnormal behavior of these MSM structures, the carrier transfer mechanism in the localized states can be suggested. The theory of carrier localization is generally used to justify the high efficiency of III-N light-emitting diodes (LEDs) and some other related, despite the high density of defects [22][23][24][25] . This behavior of carriers in localized states has also been recognized in organic materials with a large number of defects 26 . The localized states are minimum energy levels induced by potential fluctuations due to cluster and/or interstitial atoms in III-V semiconductors and depends on the dopant type, dopant density, semiconductor thickness, material composition, and processing conditions of crystal growth 22 . In the III-V LEDs, carriers are generated by excitation power and stay at the minimum energy levels of localized states (within quantum wells), prevented from being traped by the defects. This means that the emission efficiency is less affected by a large number of defects 14,27,28 . In Figure 8, the schematic diagram indicating the possible mechanism of the carriers transfer in the localized states is shown. The localized states can be observed by spectroscopy techniques such as cathodeluminescence (CL), photoluminescence (PL) spectroscopy, and low energy electron-excited nano luminescence (LEEN) by varying temperature or excitation power density 29,30 . Here, the carrier transfer mechanism in the localized states is suggested to explain the transfer mechanism of carriers by γ-irradiation in n-GaN MSM structure. By exposure, the detector's to the gamma radiation, high energy γ-photons produced carriers which we call its γ-photocarriers.γ-photocarriers density in the GaN epilayer can be varied, which can affect the electrical current when a driving voltage is applied, as depicted in Figures 5 and 7. Initial exposure causing the γphotocarriers in the extended states to achieve energy to overcome the barriers and relax into the localized states, which will continue until saturation of the localized states [31][32][33] . Therefore a smaller fraction of carriers would be involved in the voltage induced current, resulting in the decreasing of the current measured by I-V, as depicted in Figure 5 and 7(a). When the MSM structure continuously irradiated for 2 hours, a higher gamma dose would be causing the generation of more γphotocarriers. In continuous irradiation, the rate of the carriers' generation is higher in comparison with the rate of carriers' recombination, localization, and/or carriers trapped in defects. These delocalized carriers would involve in the current observed in I-V characteristics. As a result, the measured current shows a significant increase, as shown in Figure 7(b) (and (5)). The PL spectroscopy analysis was performed to study the optoelectronic performance of the samples before and after 1.44 rad(Si) irradiation in more details. For comparison, all peaks were fitted using the Gaussian model in MATLAB software (R2014a, V8.3), as depicted in Table 4. Figure 9 depicts the PL spectra before and after 1.44 rad (Si) irradiation. These graphs contain a near band-edge emission (NBE) -(peak1), violet emission peak centered at 3.149 eV -(peak2) -and a yellow luminescence (YL). The peak2 can be associated with the excitons bound to defects, c-axis screw dislocations Ga and N vacancies, deep level impurities, and amorphous phases 34,35 .
To have more insight into the effect of γ-irradiation on PL, the integrated intensities and linewidths of the PL emissions were compared before and after irradiation. For comparison, all peaks were fitted using the Gaussian model in MATLAB software (R2014a, V8.3), as depicted in Table 4. Figure 9 depicts the PL spectra before and after 1.44 rad (Si) irradiation. These graphs contain a near bandedge emission (NBE) -(peak1), violet emission peak centred at 3.149 eV -(peak2) -and a yellow luminescence (YL). The probability of non-radiative recombination is higher for delocalized carriers, which causes the PL intensity quenching, consequently. The reduction of linewidth and integrated intensity of the peak2 after the irradiation can be again explained by the carriers' delocalization and the decrease of shallow donors' density. Thus the rate of non-radiative recombination in shallow states increases in comparison with radiative recombination through the localized states. Another possible interpretation is the activation of non-radiative centers close to the shallow states after radiation 14,36 . In summary, after irradiation, carriers can be trapped by defects, recombine non-radiatively, therefore reducing the carrier distribution in extended states and hence, narrowing the PL linewidth. Additionally, the role of regular thermalization of carriers at room temperature can be ignored because the depth of the localized states is large enough 37 . Moreover, the depth has not changed after irradiation. The depth is equal to 187±5 meV and calculated from the formula below: Depth of the localized states from PL emissions= Emission Energy peak2 -Emission Energy peak1

Conclusion
In summary, the effect of room temperature 60 Co γ-irradiation at a very low dose rate (0.0001 rad(Si)/s) on the behavior and I-V characteristic of MSM structure based on the n-GaN epitaxial layer has been investigated. In contrast to earlier observations, by increasing the gamma dose, an initial increase of the dark current followed by its gradual decrease has been observed. This abnormal behavior is explained here by considering the carrier transfer mechanism in localized states. To prove the correctness of this model, XRD, SEM and, PL analyses were performed. These analyses confirmed that behavior.
We found that in low doses, the radiation has not requisite intensity to destroy the structure of the semiconductor. However, the high energy of radiation is remarkably effective on the carriers to transfer in the semiconductor. The radiation causes a change in the conductivity of the semiconductor and its effect on electric current. Therefore, even in low doses, the response of semiconductors to radiation is significant. This research helps in understanding the effect of 60 Co γ-irradiation at a very low dose on the electronic transference in n-GaN epilayers, which are used in numerous radiation environment applications nowadays.
This work is an innitial study to investigate the effect of radiation on the carrier transfer mechanism in localized states. More research can help us to understand the behavior of semiconductors, especially III-Nitride, and could lead to more applied results in the future.