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n-GaP Schottky diode: Its influence on interface state density and relaxation time
Author’s Accepted Manuscript Effect of Au 8+ irradiation on Ni/n-GaP Schottky diode: its influence on interface state density and relaxation time N Shiwakoti, A Bobby, K Asokan, Bobby Antony www.elsevier.com/locate/physb
PII: DOI: Reference:
S0921-4526(16)30471-9 http://dx.doi.org/10.1016/j.physb.2016.10.010 PHYSB309670
To appear in: Physica B: Physics of Condensed Matter Received date: 9 June 2016 Accepted date: 9 October 2016 Cite this article as: N Shiwakoti, A Bobby, K Asokan and Bobby Antony, Effect of Au 8+ irradiation on Ni/n-GaP Schottky diode: its influence on interface state density and relaxation time, Physica B: Physics of Condensed Matter, http://dx.doi.org/10.1016/j.physb.2016.10.010 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effect of Au8+ irradiation on Ni/n-GaP Schottky diode: its influence on interface state density and relaxation time a N Shiwakoti , A Bobbya, K Asokanb, Bobby Antonya* a
Department of Applied Physics, Indian School of Mines, Dhanbad – 826004, INDIA. Inter University Accelerator Centre, Aruna Asaf Ali Marg, New Delhi – 110067, INDIA.
b
*
Corresponding author’s e-mail:[email protected] Abstract The in-situ capacitance-frequency and conductance-frequency measurements of 100 MeV Au8+ swift heavy ion irradiated Ni/n-GaP Schottky structure at a constant bias voltage have been carried out in the frequency range 1 kHz – 1 MHz at room temperature. The interface states density and the relaxation time of the charge carriers have been calculated from Nicollian and Brews method. Various dielectric parameters such as dielectric constant, dielectric loss, loss tangent, series resistance, ac conductivity, real and imaginary parts of electric modulus have been extracted and analyzed under complex permittivity and complex electric modulus formalisms. The capacitance and conductance characteristics are found to exhibit complex behaviors at lower frequency region (1 kHz – 20 kHz) for all the samples. The observed peaks and dips at low frequency region are attributed to the relaxation mechanisms of charge carriers and the interface or dipolar polarization at the interface. The dielectric properties are found to be effectively changed by the ion fluence which is attributed to the variation in interface states density and their relaxation time. Keywords: Gallium phosphide; Schottky diodes; ion irradiation; dielectric properties; negative capacitance
1. Introduction The study on metal-semiconductor (MS) contact or Schottky contacts have become very popular in semiconductor technology due to its promising applications as solid state devices. Its applications in radiation harsh environment as rectifiers, photo-detectors, radiation ion detectors, γ-ray detectors are in intense research work. [1-4]. The efficiency and reliability of these devices on the electric and dielectric properties mainly depend on the interface state density [5-7]. Any alteration in the MS interface density can largely influence the frequency and voltage response of such devices. Ion irradiation has proved to be an efficient technique for manipulating the interface state densities [8-13] in Schottky diodes. Moreover, the energy of ion beam plays a key role in determining the changes brought at the interface state density or in the production of damage distribution. At low impact energies, the projectile ion loses energy by elastic collisions within the target and nearly reaches the interface site. These foreign ions act as impurities and create defects like vacancies or interstitial states. They may act as scattering centers for charge carriers, thus aggravating the device switching speed. Page 1 of 15
However, the swift heavy ion (SHI) loses energy by both elastic and inelastic collisions. The inelastic collision is dominant over the interface, which causes excitation of electronic interface states. These excited electrons may result in ion induced annealing effects, thus reducing the defect density at the interface. Nevertheless, in some literature [10] enhanced diode properties are observed, while in others [11,12] a degraded diode properties were reported, upon SHI irradiation with different fluences. The group III-V binary compounds like GaAs and GaP are found to be more radiation resistant compared to InP and InSb [13]. Among many group III-V phosphide and arsenide compounds, the wide band gap (2.24 eV) gallium phosphide (GaP) based Schottky contacts are in particular of research interest due to their high temperature operation [14]. The degree of damage caused due to ion irradiation on the target material also depends on the fluence dose [10–12,15]. The relative concentration of damage distribution due to ion irradiation with fluence is found to be quite different in phosphide compounds compared to arsenide compounds [18,19]. The AFM study on 100 MeV Fe9+ ion irradiated GaP crystal shows the formation of nano-hillocks on the surface [20]. This surface roughness were found to decrease with increase in ion fluence [21]. The XTEM images of 593 MeV Au ions irradiated GaP crystal show the formation of discontinuous ion tracks [22]. GaP crystal is known to have stable point defects like Ga vacancy, which may create complex defect traps upon irradiation [20]. The frequency response of dielectric parameters of MS structures may behave strangely due to added defect density. Any defects at the interface create an energy level within the band gap which alters the carrier life time. Although there are few reports regarding the study of ion irradiation on GaP surface and bulk topology [16-21], we can hardly find any reports on SHI influence on MS device characteristics of GaP based structures. In this article, we report the impact of 100 MeV Au8+ ion irradiation on the frequency dependent dielectric properties of Ni/n-GaP Schottky contact under wide fluence range operating over frequencies of 1kHz - 1MHz. 2. Experimental The n-type GaP (111) crystals with carrier concentration of the order of 10 16/cm3 at room temperature (RT) were used to fabricate Ni-GaP Schottky diode. The de-greasing of the crystals was done initially in trichloroethylene, acetone and isopropanol, followed by thorough rinse in 18 MΩ-cm de-ionized water. Subsequently, the crystals were dipped in 10% HCl for few seconds to de-oxidise the surface. Employing thermal evaporation technique, ohmic contact was prepared with 150 nm thick indium (99.99%) on the unpolished crystal surface, keeping the base pressure at 10-6 Torr. The Schottky contacts were made by depositing 200 nm thick nickel (99.99%) as 2 mm dots using e-beam evaporation at 10-7 Torr base pressure. The diodes were irradiated by 100 MeV Au8+ ions at varying fluences ranging from 1 x 1011 ions/cm2 to 1 x 1013 ions/cm2. The in-situ capacitance (C–f) and conductance Page 2 of 15
(G/ω – f) measurements at different frequencies (1kHz – 1MHz) were carried out at each fluence with Agilent Technologies B1500A Semiconductor Device Analyzer. 3. Results and discussion Fig.1 shows the variation of capacitance with frequency for the pristine and the irradiated samples. It is observed that the capacitance for the pristine and the irradiated diode shows complex behavior in the low frequency range (1kHz – 20 kHz). A peculiar negative capacitance (NC) has been observed for all the samples. Such behaviors have also been reported in other MS structures [22-32]. For the pristine sample, two distinct dips are observed at 2 kHz and 4.6 kHz with dip minimum values of 1.28 nF and 2.38 nF respectively. With the increase of frequency, capacitance switches to positive side with a maximum value of 2.64 nF at 6.5 kHz. Beyond this region capacitance decays slowly and finally attains a constant value 1.5 nF at higher frequencies, above 20 kHz. Similar trend in capacitance is also observed for the irradiated samples, with the peak and dip positions being shifted to the lower frequency region.
Fig.1. C-f plot of pristine and irradiated Ni/n-GaP (111) Schottky diodes at RT
Fig. 2 shows the variation of conductance (G/ω) with frequency for the pristine and the irradiated samples. It is observed that the G/ω plot shows dips at around 2 kHz for all the samples. The peaks in the positive side are also observed, which shift towards the lower frequency side with the increase of fluence. The magnitude of the G/ω peak increases with the increase in irradiation fluence. At higher frequencies, the G/ω is found to be independent of fluence dose for the pristine and the irradiated samples.
Page 3 of 15
Fig. 2. G/ω-f plot of pristine and irradiated Ni/n-GaP (111) Schottky diodes at RT
The observed frequency dependent capacitance behavior at low frequency and nearly independent behaviors at higher frequency can be attributed to the particular distribution of the interface states density (Nss). Practically, the Schottky contact and the back ohmic contact are contaminated by various kinds of defects. Such disordered or defective interface may host different types of trap states in the form of interstitials and vacancies with energy levels well within the band gap of the semiconductor. There can be both shallow trap states having energy level above Fermi level and deep trap states having energy level below Fermi level. These trap states can retain the charge carriers in accordance with their life time (τ), commonly referred as relaxation time of the charge carriers. The values of Nss and τ can be estimated by the capacitance and the conductance measurements as described by Nicollian and Brews [40]. In this method, the parallel conductance is measured as a function of frequency at a particular bias voltage expressed as:
( ) (1) where q is the electronic charge and ω is the angular frequency. The measured capacitance and conductance are used to extract the equivalent parallel conductance (Gp) as:
(
)
(2)
where Cox is the oxide capacitance, evaluated from the peak value of the C-V characteristics at 1MHz.
Page 4 of 15
Fig. 3 (a). Gp/ω-f plot of pristine and irradiated Ni/n-GaP (111) Schottky diodes at RT.
Fig. 3(b). Gp/ω-f plot of pristine and irradiated Ni/n-GaP (111) Schottky diodes at RT
The Gp/ω – f plot thus obtained is shown in Fig. 3 (a) [enlarged in Fig. 3 (b)] which shows characteristics peaks and dips notably at three different frequency domains. It indicates that the MS interface hosts continuum distribution of interface states (both shallow and deep states) with varying density and relaxation time. At the peak position of Gp/ω – f plot, the first derivative of Eq. (2) with respect to ωτ should be zero from where the Nss and τ can be obtained as: Nss GP max 0.402qA when 1.98 P , Page 5 of 15
where ωp is the angular frequency at which the Gp/ω peak appears. The calculated values of Nss and τ for three different frequency domains are given in Table 1. Table 1: Values of Nss and τ at different fluences and frequency domains. Sample
Domain I 2
Pristine 1 x 1011 1 x 1012 1 x 1013
Nss (1013 /eVcm ) 0.78 2.34 2.34 0.89
Domain II τ (µS) 25.34 22.28 22.28 23.53
2
Nss (1013 /eVcm ) 2.39 1.88 1.81 0.89
Domain III τ (µS) 15.02 17.25 17.33 19.44
2
Nss (1013 /eV cm ) 2.46 2.39 2.38 2.43
τ (µS) 6.6 9.96 10.28 10.80
As can be seen from the Table 1, the Nss and τ values are altered randomly with the fluence dose. However, on the whole the average τ value is found to increase with the increase of fluence dose. The peaks and the dips in C-f characteristics at low frequency regimes (below 20 kHz) can be thus ascribed to the interface states, as τ ≈ 1/2πf in this range. The excess positive capacitance in the low frequency region (Fig. 1) is basically identified to be due to the Maxwel-Wegner type interfacial polarization and dipolar polarization [28]. However, at higher frequency region, the contribution of the interfacial and dipolar polarizations are insignificant as the interface states do not follow the applied signal and hence the capacitance attains its geometrical value. On the other hand, the observed negative capacitance at low frequency region cannot be explained in terms of such simple mechanisms as it signifies that the interface charge density variation is reversed i.e. the occupancy of electrons below the Fermi level exceeds the occupancy of electrons above the Fermi level. There can be complex physical mechanisms in Schottky structures which will cause the interface states to attain such orientations. According to Wu et al [5], the impact ionization of trap states by hot electrons, resulting in the loss of interface charge at the occupied states below the Fermi level can give rise to NC effect. In contrast, J. Werner et al. [29] and C. H. Champness et al [30] attributes the NC effect at low frequency not to interface states but rather to the injection of excessive minority charge carriers to the bulk from the Schottky contact, which results in out of phase current with the applied voltage. Ershov et al. [31,32] describes the NC effect or inductive behaviour as the inertial delay in charge carrier flow due to the Schottky contact injection. A. K. Jonscher [33] analyzed and interpreted the negative capacitance in terms of transient current response of the device. Dielectric properties with frequency and fluence: The values of the dielectric constant (ε'), dielectric loss (ε"), dielectric loss tangent (tan δ) and real (M') and imaginary (M") parts of the electric modulus have been evaluated from the measured C and G values for the pristine and irradiated samples. In complex permittivity (ε*) formalism, ε* of a Schottky barrier diode (SBD) with a thin insulator layer at the MS interface and can be defined as [35,36], Page 6 of 15
(3) where ε' and ε" are the real and the imaginary parts of the complex permittivity, j is the root of –1, C and G are the capacitance and conductance of the dielectric, Co is the capacitance of the empty capacitor and ω is the angular frequency (ω = 2πf) of the applied electric field. The capacitance of the empty capacitor Co is obtained from, (4) Here ε0 (8.85 x 10-12 F/m) is the permittivity of free space, A is the rectifier contact area and di is the thickness of the interfacial insulator layer. In the strong accumulation region, the maximum capacitance (Cac) of the SBD corresponds to the insulator capacitance. The Cac measured at 1 MHz is used to calculate di from the relation, ( ) where εr is relative permeability, (5.5 for Ga2O3) [37]. Thus, ε' and ε" for various frequencies can be calculated from the measured values of C and G/ω respectively. Fig. (4,5) show the variation of ε' and ε" with frequency for the pristine and the irradiated samples. In all the samples, frequency dispersion of ε' and ε" is observed in the low frequency region and nearly frequency independent behavior in the high frequency region. The dips in ε' and ε" at low frequencies can be attributed to relaxation phenomena of the charge carriers. The inertial delay of charge carriers may result in negative dielectric values. The excess positive ε' and ε" values in the low frequency region can be attributed to the interfacial polarization. At low frequencies the interface dipoles can follow the applied ac field which yields excess dielectric values. However, at higher frequencies, the dipoles have very less time to orient themselves in response to fast switching ac field and hence ε' and ε" attain a constant value independent of frequency. For the irradiated samples the peaks positions of ε' shift towards the lower frequency region indicating the increase in relaxation time of the charge carriers. The relaxation time generally increases if the defect density is increased as the charge carriers have to mediate through more energy levels within the band gap in response to applied field.
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Fig. 4. ε' - f plot of pristine and irradiated Ni/n-GaP (111) Schottky diodes at RT
Fig. 5. ε"- f plot of pristine and irradiated Ni/n-GaP (111) Schottky diodes at RT
The loss tangent (tanδ), which is the measure of energy loss during ac current conduction is calculated as tan δ = ε"/ ε'. The frequency dependence of tanδ is given in Fig.6 for the pristine and the irradiated samples. Several sharp peaks with both negative and positive values are observed at low frequency region. The peak positions depend on many factors such as the interface states density, interfacial oxide layer thickness and the series resistance. The occurrence of several sharp peaks and dips at low frequency region are due to the particular distribution of the interface states which exhibits strong relaxation processes at definite values of frequency. The shifting of peaks and dips towards lower frequency region
Page 8 of 15
after irradiation suggests the increase in the interface states density causing longer relaxation time and thus increase the energy loss.
Fig. 6. Tangent loss versus f plot of pristine and irradiated Ni/n-GaP (111) Schottky diodes at RT
The ac electrical conductivity (σac) of the dielectric material is given by [35]: ( )
(6)
Fig. 7 shows the dependence of σac with frequency for the pristine and irradiated samples. At low frequency region characteristics peaks are observed on both sides, which can be attributed to hoping mechanisms of the charge carriers. With the applied field, the charge carriers are mediated through several trap states, which are well synchronized for low frequency range and hence contributes to ac conductivity. However, at higher frequencies the hopping rate and the applied ac field become out of phase and hence do not contribute to σac.
Page 9 of 15
Fig. 7. σac versus f plot of pristine and irradiated Ni/n-GaP (111) Schottky diodes at RT
The series resistance (Rs) is an important parameter, which strongly influence the electrical and dielectric characteristics of SBD. The frequency dependent Rs can be calculated from the measured values of capacitance and conductance using the Nicollian and Goetzberger method [40]:
(
)
(7)
The variation of series resistance (Rs) with frequency is shown in Fig. 8 for the pristine and irradiated samples. The values of Rs for all the samples are found to decrease with increase in frequency and increases considerably with the increase in irradiation fluence. The increase in Rs values with irradiation dose indicates increased defect density.
Page 10 of 15
Fig. 8. Rs versus f plot of pristine and irradiated Ni/n-GaP (111) Schottky diodes at RT
In order to separate the bulk and the surface phenomenon, the complex permittivity (ε*) data are generally transformed into the complex electric modulus (M*) by using the following relation [6,39], ( ) The real component (M') and the imaginary component (M") of M* are calculated from ε' and ε". Fig. (9,10) show the frequency dependent values of M' and M" of electric modulus M* for the pristine and irradiated samples respectively. It is seen that both M' and M" approach zero for all the samples at low frequency region which indicates that the electrical polarization is suppressed in the low frequency region. With the increase of frequency, M' increases and attains nearly constant value. Also at the intermediate frequency M" exhibits broad peaks for all the samples. Such behaviors can be associated with various relaxation phenomena.
Page 11 of 15
Fig. 9. M' versus f plot of pristine and irradiated Ni/n-GaP (111) Schottky diodes at RT
Fig. 10. M" versus f plot of pristine and irradiated Ni/n-GaP (111) Schottky diodes at RT
4. Conclusions The in-situ capacitance (C–f) and conductance (G/ω – f) measurements of pristine and 100 MeV Au+8 ions irradiated Ni/n-GaP Schottky diodes at constant bias voltage show strong frequency dispersion at low frequency region, which is regarded due to a particular distribution of the interface states. The observed anomalous negative dips in C–f and G/ω – f may be attributed to the inertial delay of charge flow due to the finite retention time of trap states. The values of Nss and the τ were calculated from Nicollian and Brews method. The Page 12 of 15
shifting of the position of capacitance dips towards the low frequency region in the irradiated diode indicates the presence of a new type of deep and shallow trap states with higher relaxation time. The multiple characteristics peaks and dips in Gp/ω – f plot suggests the presence of continuum distribution of the interfaces states at the Ni/n-GaP interface. The relaxation mechanisms of charge carriers across the interface have been reflected in various dielectric parameters. It is seen that the interface charges and interfacial polarization have strong influence on the dielectric properties especially at low frequency region. At lower frequencies, the interface dipoles can follow the applied ac field and yields excess ε' and ε" values. On the other hand, at higher frequencies the dipoles have very less time to orient themselves in response to fast ac field and hence yield only geometrical values. The dielectric characteristics of Ni/n-GaP Schottky diode also show strong dependency on ion fluences. The shifting of ε' peaks towards low frequency region indicates the formation of new types of trap states with longer relaxation time after irradiation. The observed multiple peaks and dips in tanδ –f plot show strong relaxation processes at definite frequencies. The characteristics peaks of σac suggest the transport of charge carriers by hoping mechanisms at lower frequencies. The increase in Rs values for the irradiated diodes shows increase in interface states density. The variation of the electric modulus with frequency can be related with various relaxation phenomena. The SHI irradiation induced alteration of dielectric properties of Ni/n-GaP can be associated with the energy loss mechanisms of 100 MeV Au8+ ions at the interface. Acknowledgements Authors are grateful for the useful discussions with Dr. S Verma (IUAC, New Delhi). The technical support from Mr. S. R Abhilash (IUAC, India) and the members of the Pelletron group are also acknowledged. NS thank ISM, Dhanbad, India for the financial support under ISMJRF scheme and AB thank Department of Science and Technology, India for the financial support under Grant No: SB/FTP/PS-072/2013. References [1] E. Omotoso, W.E. Meyer, F. D. Auret, A.T. Paradzah, M. Diale, S. Coelho et al. Nucl. Instrum. Meth. B. 365 (2015) 264. [2] K. Shinohara, D. C. Regan, Y. Tang, A. L. Corrion, D.F. Brown, J.C. Wong et al. IEEE Trans. Electron Dev. 60 (2013) 2982. [3] A Bobby, N. Shiwakoti, P.M. Sarun, S. Verma, K. Asokan, B. K. Antony. Curr. Appl. Phys. 15, (2015) 1500. [4] A. Bobby, N. Shiwakoti, S. Verma, P.S. Gupta, B.K. Antony. Mat. Sci. Semicon. Proc. 21 (2014) 116. [5] X. Wu, E. S. Yang, H. L. Evans. J. Appl. Phys. 68 (1990) 2845. [6] A. Tataroğlu, İ. Yücedağ, Ş. Altindal. Microelectron. Eng. 85 (2008) 1518 [7] D. E. Yıldız, İ. Dökme. J. Appl. Phys. 110 (2011) 014507. Page 13 of 15
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PII: DOI: Reference:
S0921-4526(16)30471-9 http://dx.doi.org/10.1016/j.physb.2016.10.010 PHYSB309670
To appear in: Physica B: Physics of Condensed Matter Received date: 9 June 2016 Accepted date: 9 October 2016 Cite this article as: N Shiwakoti, A Bobby, K Asokan and Bobby Antony, Effect of Au 8+ irradiation on Ni/n-GaP Schottky diode: its influence on interface state density and relaxation time, Physica B: Physics of Condensed Matter, http://dx.doi.org/10.1016/j.physb.2016.10.010 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effect of Au8+ irradiation on Ni/n-GaP Schottky diode: its influence on interface state density and relaxation time a N Shiwakoti , A Bobbya, K Asokanb, Bobby Antonya* a
Department of Applied Physics, Indian School of Mines, Dhanbad – 826004, INDIA. Inter University Accelerator Centre, Aruna Asaf Ali Marg, New Delhi – 110067, INDIA.
b
*
Corresponding author’s e-mail:[email protected] Abstract The in-situ capacitance-frequency and conductance-frequency measurements of 100 MeV Au8+ swift heavy ion irradiated Ni/n-GaP Schottky structure at a constant bias voltage have been carried out in the frequency range 1 kHz – 1 MHz at room temperature. The interface states density and the relaxation time of the charge carriers have been calculated from Nicollian and Brews method. Various dielectric parameters such as dielectric constant, dielectric loss, loss tangent, series resistance, ac conductivity, real and imaginary parts of electric modulus have been extracted and analyzed under complex permittivity and complex electric modulus formalisms. The capacitance and conductance characteristics are found to exhibit complex behaviors at lower frequency region (1 kHz – 20 kHz) for all the samples. The observed peaks and dips at low frequency region are attributed to the relaxation mechanisms of charge carriers and the interface or dipolar polarization at the interface. The dielectric properties are found to be effectively changed by the ion fluence which is attributed to the variation in interface states density and their relaxation time. Keywords: Gallium phosphide; Schottky diodes; ion irradiation; dielectric properties; negative capacitance
1. Introduction The study on metal-semiconductor (MS) contact or Schottky contacts have become very popular in semiconductor technology due to its promising applications as solid state devices. Its applications in radiation harsh environment as rectifiers, photo-detectors, radiation ion detectors, γ-ray detectors are in intense research work. [1-4]. The efficiency and reliability of these devices on the electric and dielectric properties mainly depend on the interface state density [5-7]. Any alteration in the MS interface density can largely influence the frequency and voltage response of such devices. Ion irradiation has proved to be an efficient technique for manipulating the interface state densities [8-13] in Schottky diodes. Moreover, the energy of ion beam plays a key role in determining the changes brought at the interface state density or in the production of damage distribution. At low impact energies, the projectile ion loses energy by elastic collisions within the target and nearly reaches the interface site. These foreign ions act as impurities and create defects like vacancies or interstitial states. They may act as scattering centers for charge carriers, thus aggravating the device switching speed. Page 1 of 15
However, the swift heavy ion (SHI) loses energy by both elastic and inelastic collisions. The inelastic collision is dominant over the interface, which causes excitation of electronic interface states. These excited electrons may result in ion induced annealing effects, thus reducing the defect density at the interface. Nevertheless, in some literature [10] enhanced diode properties are observed, while in others [11,12] a degraded diode properties were reported, upon SHI irradiation with different fluences. The group III-V binary compounds like GaAs and GaP are found to be more radiation resistant compared to InP and InSb [13]. Among many group III-V phosphide and arsenide compounds, the wide band gap (2.24 eV) gallium phosphide (GaP) based Schottky contacts are in particular of research interest due to their high temperature operation [14]. The degree of damage caused due to ion irradiation on the target material also depends on the fluence dose [10–12,15]. The relative concentration of damage distribution due to ion irradiation with fluence is found to be quite different in phosphide compounds compared to arsenide compounds [18,19]. The AFM study on 100 MeV Fe9+ ion irradiated GaP crystal shows the formation of nano-hillocks on the surface [20]. This surface roughness were found to decrease with increase in ion fluence [21]. The XTEM images of 593 MeV Au ions irradiated GaP crystal show the formation of discontinuous ion tracks [22]. GaP crystal is known to have stable point defects like Ga vacancy, which may create complex defect traps upon irradiation [20]. The frequency response of dielectric parameters of MS structures may behave strangely due to added defect density. Any defects at the interface create an energy level within the band gap which alters the carrier life time. Although there are few reports regarding the study of ion irradiation on GaP surface and bulk topology [16-21], we can hardly find any reports on SHI influence on MS device characteristics of GaP based structures. In this article, we report the impact of 100 MeV Au8+ ion irradiation on the frequency dependent dielectric properties of Ni/n-GaP Schottky contact under wide fluence range operating over frequencies of 1kHz - 1MHz. 2. Experimental The n-type GaP (111) crystals with carrier concentration of the order of 10 16/cm3 at room temperature (RT) were used to fabricate Ni-GaP Schottky diode. The de-greasing of the crystals was done initially in trichloroethylene, acetone and isopropanol, followed by thorough rinse in 18 MΩ-cm de-ionized water. Subsequently, the crystals were dipped in 10% HCl for few seconds to de-oxidise the surface. Employing thermal evaporation technique, ohmic contact was prepared with 150 nm thick indium (99.99%) on the unpolished crystal surface, keeping the base pressure at 10-6 Torr. The Schottky contacts were made by depositing 200 nm thick nickel (99.99%) as 2 mm dots using e-beam evaporation at 10-7 Torr base pressure. The diodes were irradiated by 100 MeV Au8+ ions at varying fluences ranging from 1 x 1011 ions/cm2 to 1 x 1013 ions/cm2. The in-situ capacitance (C–f) and conductance Page 2 of 15
(G/ω – f) measurements at different frequencies (1kHz – 1MHz) were carried out at each fluence with Agilent Technologies B1500A Semiconductor Device Analyzer. 3. Results and discussion Fig.1 shows the variation of capacitance with frequency for the pristine and the irradiated samples. It is observed that the capacitance for the pristine and the irradiated diode shows complex behavior in the low frequency range (1kHz – 20 kHz). A peculiar negative capacitance (NC) has been observed for all the samples. Such behaviors have also been reported in other MS structures [22-32]. For the pristine sample, two distinct dips are observed at 2 kHz and 4.6 kHz with dip minimum values of 1.28 nF and 2.38 nF respectively. With the increase of frequency, capacitance switches to positive side with a maximum value of 2.64 nF at 6.5 kHz. Beyond this region capacitance decays slowly and finally attains a constant value 1.5 nF at higher frequencies, above 20 kHz. Similar trend in capacitance is also observed for the irradiated samples, with the peak and dip positions being shifted to the lower frequency region.
Fig.1. C-f plot of pristine and irradiated Ni/n-GaP (111) Schottky diodes at RT
Fig. 2 shows the variation of conductance (G/ω) with frequency for the pristine and the irradiated samples. It is observed that the G/ω plot shows dips at around 2 kHz for all the samples. The peaks in the positive side are also observed, which shift towards the lower frequency side with the increase of fluence. The magnitude of the G/ω peak increases with the increase in irradiation fluence. At higher frequencies, the G/ω is found to be independent of fluence dose for the pristine and the irradiated samples.
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Fig. 2. G/ω-f plot of pristine and irradiated Ni/n-GaP (111) Schottky diodes at RT
The observed frequency dependent capacitance behavior at low frequency and nearly independent behaviors at higher frequency can be attributed to the particular distribution of the interface states density (Nss). Practically, the Schottky contact and the back ohmic contact are contaminated by various kinds of defects. Such disordered or defective interface may host different types of trap states in the form of interstitials and vacancies with energy levels well within the band gap of the semiconductor. There can be both shallow trap states having energy level above Fermi level and deep trap states having energy level below Fermi level. These trap states can retain the charge carriers in accordance with their life time (τ), commonly referred as relaxation time of the charge carriers. The values of Nss and τ can be estimated by the capacitance and the conductance measurements as described by Nicollian and Brews [40]. In this method, the parallel conductance is measured as a function of frequency at a particular bias voltage expressed as:
( ) (1) where q is the electronic charge and ω is the angular frequency. The measured capacitance and conductance are used to extract the equivalent parallel conductance (Gp) as:
(
)
(2)
where Cox is the oxide capacitance, evaluated from the peak value of the C-V characteristics at 1MHz.
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Fig. 3 (a). Gp/ω-f plot of pristine and irradiated Ni/n-GaP (111) Schottky diodes at RT.
Fig. 3(b). Gp/ω-f plot of pristine and irradiated Ni/n-GaP (111) Schottky diodes at RT
The Gp/ω – f plot thus obtained is shown in Fig. 3 (a) [enlarged in Fig. 3 (b)] which shows characteristics peaks and dips notably at three different frequency domains. It indicates that the MS interface hosts continuum distribution of interface states (both shallow and deep states) with varying density and relaxation time. At the peak position of Gp/ω – f plot, the first derivative of Eq. (2) with respect to ωτ should be zero from where the Nss and τ can be obtained as: Nss GP max 0.402qA when 1.98 P , Page 5 of 15
where ωp is the angular frequency at which the Gp/ω peak appears. The calculated values of Nss and τ for three different frequency domains are given in Table 1. Table 1: Values of Nss and τ at different fluences and frequency domains. Sample
Domain I 2
Pristine 1 x 1011 1 x 1012 1 x 1013
Nss (1013 /eVcm ) 0.78 2.34 2.34 0.89
Domain II τ (µS) 25.34 22.28 22.28 23.53
2
Nss (1013 /eVcm ) 2.39 1.88 1.81 0.89
Domain III τ (µS) 15.02 17.25 17.33 19.44
2
Nss (1013 /eV cm ) 2.46 2.39 2.38 2.43
τ (µS) 6.6 9.96 10.28 10.80
As can be seen from the Table 1, the Nss and τ values are altered randomly with the fluence dose. However, on the whole the average τ value is found to increase with the increase of fluence dose. The peaks and the dips in C-f characteristics at low frequency regimes (below 20 kHz) can be thus ascribed to the interface states, as τ ≈ 1/2πf in this range. The excess positive capacitance in the low frequency region (Fig. 1) is basically identified to be due to the Maxwel-Wegner type interfacial polarization and dipolar polarization [28]. However, at higher frequency region, the contribution of the interfacial and dipolar polarizations are insignificant as the interface states do not follow the applied signal and hence the capacitance attains its geometrical value. On the other hand, the observed negative capacitance at low frequency region cannot be explained in terms of such simple mechanisms as it signifies that the interface charge density variation is reversed i.e. the occupancy of electrons below the Fermi level exceeds the occupancy of electrons above the Fermi level. There can be complex physical mechanisms in Schottky structures which will cause the interface states to attain such orientations. According to Wu et al [5], the impact ionization of trap states by hot electrons, resulting in the loss of interface charge at the occupied states below the Fermi level can give rise to NC effect. In contrast, J. Werner et al. [29] and C. H. Champness et al [30] attributes the NC effect at low frequency not to interface states but rather to the injection of excessive minority charge carriers to the bulk from the Schottky contact, which results in out of phase current with the applied voltage. Ershov et al. [31,32] describes the NC effect or inductive behaviour as the inertial delay in charge carrier flow due to the Schottky contact injection. A. K. Jonscher [33] analyzed and interpreted the negative capacitance in terms of transient current response of the device. Dielectric properties with frequency and fluence: The values of the dielectric constant (ε'), dielectric loss (ε"), dielectric loss tangent (tan δ) and real (M') and imaginary (M") parts of the electric modulus have been evaluated from the measured C and G values for the pristine and irradiated samples. In complex permittivity (ε*) formalism, ε* of a Schottky barrier diode (SBD) with a thin insulator layer at the MS interface and can be defined as [35,36], Page 6 of 15
(3) where ε' and ε" are the real and the imaginary parts of the complex permittivity, j is the root of –1, C and G are the capacitance and conductance of the dielectric, Co is the capacitance of the empty capacitor and ω is the angular frequency (ω = 2πf) of the applied electric field. The capacitance of the empty capacitor Co is obtained from, (4) Here ε0 (8.85 x 10-12 F/m) is the permittivity of free space, A is the rectifier contact area and di is the thickness of the interfacial insulator layer. In the strong accumulation region, the maximum capacitance (Cac) of the SBD corresponds to the insulator capacitance. The Cac measured at 1 MHz is used to calculate di from the relation, ( ) where εr is relative permeability, (5.5 for Ga2O3) [37]. Thus, ε' and ε" for various frequencies can be calculated from the measured values of C and G/ω respectively. Fig. (4,5) show the variation of ε' and ε" with frequency for the pristine and the irradiated samples. In all the samples, frequency dispersion of ε' and ε" is observed in the low frequency region and nearly frequency independent behavior in the high frequency region. The dips in ε' and ε" at low frequencies can be attributed to relaxation phenomena of the charge carriers. The inertial delay of charge carriers may result in negative dielectric values. The excess positive ε' and ε" values in the low frequency region can be attributed to the interfacial polarization. At low frequencies the interface dipoles can follow the applied ac field which yields excess dielectric values. However, at higher frequencies, the dipoles have very less time to orient themselves in response to fast switching ac field and hence ε' and ε" attain a constant value independent of frequency. For the irradiated samples the peaks positions of ε' shift towards the lower frequency region indicating the increase in relaxation time of the charge carriers. The relaxation time generally increases if the defect density is increased as the charge carriers have to mediate through more energy levels within the band gap in response to applied field.
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Fig. 4. ε' - f plot of pristine and irradiated Ni/n-GaP (111) Schottky diodes at RT
Fig. 5. ε"- f plot of pristine and irradiated Ni/n-GaP (111) Schottky diodes at RT
The loss tangent (tanδ), which is the measure of energy loss during ac current conduction is calculated as tan δ = ε"/ ε'. The frequency dependence of tanδ is given in Fig.6 for the pristine and the irradiated samples. Several sharp peaks with both negative and positive values are observed at low frequency region. The peak positions depend on many factors such as the interface states density, interfacial oxide layer thickness and the series resistance. The occurrence of several sharp peaks and dips at low frequency region are due to the particular distribution of the interface states which exhibits strong relaxation processes at definite values of frequency. The shifting of peaks and dips towards lower frequency region
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after irradiation suggests the increase in the interface states density causing longer relaxation time and thus increase the energy loss.
Fig. 6. Tangent loss versus f plot of pristine and irradiated Ni/n-GaP (111) Schottky diodes at RT
The ac electrical conductivity (σac) of the dielectric material is given by [35]: ( )
(6)
Fig. 7 shows the dependence of σac with frequency for the pristine and irradiated samples. At low frequency region characteristics peaks are observed on both sides, which can be attributed to hoping mechanisms of the charge carriers. With the applied field, the charge carriers are mediated through several trap states, which are well synchronized for low frequency range and hence contributes to ac conductivity. However, at higher frequencies the hopping rate and the applied ac field become out of phase and hence do not contribute to σac.
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Fig. 7. σac versus f plot of pristine and irradiated Ni/n-GaP (111) Schottky diodes at RT
The series resistance (Rs) is an important parameter, which strongly influence the electrical and dielectric characteristics of SBD. The frequency dependent Rs can be calculated from the measured values of capacitance and conductance using the Nicollian and Goetzberger method [40]:
(
)
(7)
The variation of series resistance (Rs) with frequency is shown in Fig. 8 for the pristine and irradiated samples. The values of Rs for all the samples are found to decrease with increase in frequency and increases considerably with the increase in irradiation fluence. The increase in Rs values with irradiation dose indicates increased defect density.
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Fig. 8. Rs versus f plot of pristine and irradiated Ni/n-GaP (111) Schottky diodes at RT
In order to separate the bulk and the surface phenomenon, the complex permittivity (ε*) data are generally transformed into the complex electric modulus (M*) by using the following relation [6,39], ( ) The real component (M') and the imaginary component (M") of M* are calculated from ε' and ε". Fig. (9,10) show the frequency dependent values of M' and M" of electric modulus M* for the pristine and irradiated samples respectively. It is seen that both M' and M" approach zero for all the samples at low frequency region which indicates that the electrical polarization is suppressed in the low frequency region. With the increase of frequency, M' increases and attains nearly constant value. Also at the intermediate frequency M" exhibits broad peaks for all the samples. Such behaviors can be associated with various relaxation phenomena.
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Fig. 9. M' versus f plot of pristine and irradiated Ni/n-GaP (111) Schottky diodes at RT
Fig. 10. M" versus f plot of pristine and irradiated Ni/n-GaP (111) Schottky diodes at RT
4. Conclusions The in-situ capacitance (C–f) and conductance (G/ω – f) measurements of pristine and 100 MeV Au+8 ions irradiated Ni/n-GaP Schottky diodes at constant bias voltage show strong frequency dispersion at low frequency region, which is regarded due to a particular distribution of the interface states. The observed anomalous negative dips in C–f and G/ω – f may be attributed to the inertial delay of charge flow due to the finite retention time of trap states. The values of Nss and the τ were calculated from Nicollian and Brews method. The Page 12 of 15
shifting of the position of capacitance dips towards the low frequency region in the irradiated diode indicates the presence of a new type of deep and shallow trap states with higher relaxation time. The multiple characteristics peaks and dips in Gp/ω – f plot suggests the presence of continuum distribution of the interfaces states at the Ni/n-GaP interface. The relaxation mechanisms of charge carriers across the interface have been reflected in various dielectric parameters. It is seen that the interface charges and interfacial polarization have strong influence on the dielectric properties especially at low frequency region. At lower frequencies, the interface dipoles can follow the applied ac field and yields excess ε' and ε" values. On the other hand, at higher frequencies the dipoles have very less time to orient themselves in response to fast ac field and hence yield only geometrical values. The dielectric characteristics of Ni/n-GaP Schottky diode also show strong dependency on ion fluences. The shifting of ε' peaks towards low frequency region indicates the formation of new types of trap states with longer relaxation time after irradiation. The observed multiple peaks and dips in tanδ –f plot show strong relaxation processes at definite frequencies. The characteristics peaks of σac suggest the transport of charge carriers by hoping mechanisms at lower frequencies. The increase in Rs values for the irradiated diodes shows increase in interface states density. The variation of the electric modulus with frequency can be related with various relaxation phenomena. The SHI irradiation induced alteration of dielectric properties of Ni/n-GaP can be associated with the energy loss mechanisms of 100 MeV Au8+ ions at the interface. Acknowledgements Authors are grateful for the useful discussions with Dr. S Verma (IUAC, New Delhi). The technical support from Mr. S. R Abhilash (IUAC, India) and the members of the Pelletron group are also acknowledged. NS thank ISM, Dhanbad, India for the financial support under ISMJRF scheme and AB thank Department of Science and Technology, India for the financial support under Grant No: SB/FTP/PS-072/2013. References [1] E. Omotoso, W.E. Meyer, F. D. Auret, A.T. Paradzah, M. Diale, S. Coelho et al. Nucl. Instrum. Meth. B. 365 (2015) 264. [2] K. Shinohara, D. C. Regan, Y. Tang, A. L. Corrion, D.F. Brown, J.C. Wong et al. IEEE Trans. Electron Dev. 60 (2013) 2982. [3] A Bobby, N. Shiwakoti, P.M. Sarun, S. Verma, K. Asokan, B. K. Antony. Curr. Appl. Phys. 15, (2015) 1500. [4] A. Bobby, N. Shiwakoti, S. Verma, P.S. Gupta, B.K. Antony. Mat. Sci. Semicon. Proc. 21 (2014) 116. [5] X. Wu, E. S. Yang, H. L. Evans. J. Appl. Phys. 68 (1990) 2845. [6] A. Tataroğlu, İ. Yücedağ, Ş. Altindal. Microelectron. Eng. 85 (2008) 1518 [7] D. E. Yıldız, İ. Dökme. J. Appl. Phys. 110 (2011) 014507. Page 13 of 15
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