# Computation of Heterojunction Parameters at Low Temperatures in Heterojunctions Comprised of n-Typeβ-FeSi2 Thin Films and p-Type Si(111) Substrates Grown by Radio Frequency Magnetron Sputtering

Authors: Phongsaphak Sittimart; Adison Nopparuchikun; Nathaporn Promros
Journal: Advances in Materials Science and Engineering
 (2017)

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## Abstract

In this study, n-typeβ-FeSi2/p-type Si heterojunctions, inside which n-type β-FeSi2 films were epitaxially grown on p-type Si(111) substrates, were created using radio frequency magnetron sputtering at a substrate temperature of 560°C and Ar pressure of 2.66×10-1 Pa. The heterojunctions were measured for forward and reverse dark current density-voltage curves as a function of temperature ranging from 300 down to 20 K for computation of heterojunction parameters using the thermionic emission (TE) theory and Cheung’s and Norde’s methods. Computation using the TE theory showed that the values of ideality factor (n) were 1.71 at 300 K and 16.83 at 20 K, while the barrier height (ϕb) values were 0.59 eV at 300 K and 0.06 eV at 20 K. Both of the n and ϕb values computed using the TE theory were in agreement with those computed using Cheung’s and Norde’s methods. The values of series resistance (Rs) computed at 300 K and 20 K by Norde’s method were 10.93 Ω and 0.15 MΩ, respectively, which agreed with the Rs values found through computation by Cheung’s method. The dramatic increment of Rs value at low temperatures was likely attributable to the increment of n value at low temperatures.


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## Body

## 1. Introduction

At present, semiconducting iron disiliside (β-FeSi2) has received much attention for utilization in novel low-cost optoelectronic devices in part due to its various attractive physical properties [1–3]. It has been reported that β-FeSi2 can be epitaxially grown on Si with 2–5% lattice mismatches [4]. Furthermore, its composition consists of nontoxic elements (Fe and Si) [5]. More importantly, it possesses high optical absorption coefficients (greater than 105 cm−1 at photon energies above 1.5 eV) [6, 7] and a direct band gap of 0.85 eV corresponding to an optical telecommunication wavelength in the spectral range of near-infrared (NIR) [8, 9].Molecular beam epitaxy [10], ion beam synthesis [11], and reactive deposition epitaxy [12] have previously been utilized for growth of β-FeSi2 thin films. In order to grow β-FeSi2, these methods require a postannealing procedure at temperatures of at least 800°C [13]. Since the annealing procedure accelerates the diffusion of Fe atoms into the Si side, it is a potential cause for fabricated heterojunctions without rectifying action. Consequently, there is the occurrence of deep trap levels in the Si, which could act as leakage centers for the rectifying action [14]. Hence, the growth of β-FeSi2 thin films at low temperatures is preferable. Previously, the authors utilized radio frequency magnetron sputtering (RFMS) for the growth of β-FeSi2 at low argon (Ar) pressure [15]. Such low-pressure sputtering is effective for suppressing the diffused Fe atoms to the Si, whereas enhancing the kinetic energy of the grown species as the increased means free path is effective for β-FeSi2 film growth at low temperatures. The authors confirmed that the β-FeSi2 thin films grown by RFMS were epitaxially grown on p-type Si(111) substrates at a substrate temperature of 560°C. Furthermore, the n-type β-FeSi2/p-type Si heterojunctions were fabricated and subsequently utilized as NIR photodiodes. Their photodetection properties exhibited obvious improvement at low temperatures.As such, the aim of the present study was to further investigate the electrical characteristics of n-typeβ-FeSi2/p-type Si heterojunctions grown using RFMS. Their forward and reverse dark current density-voltage (J-V) curves were measured as a function of temperature ranging from 300 down to 20 K. The values for heterojunction parameters such as ideality factor (n), barrier height (ϕb), and saturation current density (J0) including the series resistance (Rs) were computed using the thermionic emission (TE) theory and Cheung’s and Norde’s methods. Based on the totality of knowledge among the group members, this manuscript represents the first investigation of its kind for heterojunction parameters at low temperatures for n-type β-FeSi2/p-type Si heterojunctions grown by RFMS using the aforementioned theory and methods.


## 2. Materials and Methods

β-FeSi2 layers were grown on Si(111) substrates by using FeSi2 alloy targets. Before the growth of β-FeSi2 layers, the native oxide layer on the Si substrates was eliminated by immersion of the substrates in a diluted hydrofluoric acid (HF) solution and rinsing in deionized water. After cleaning, the Si substrates were immediately introduced to the inside of an RFMS system. The pressure inside the chamber was evacuated down to less than 10-4Pa. β-FeSi2 layers with a thickness of 300 nm were grown on p-type Si(111) substrates at a substrate temperature of 560°C for 10 hours. The sputtering pressure was maintained at 2.66 × 10-1Pa under Ar atmosphere, with RF power of 20 W tuned by using an RF automatching system performed for the discharge of Ar gas. In order to form front and back electrodes at room temperature after the growth of the β-FeSi2 layers on Si substrates, Pd and Al were grown using another RFMS system on the front Si substrate in a finger-shaped formation and on the entire back of the β-FeSi2 layer, respectively.The crystallinity for theβ-FeSi2 thin films was studied using X-ray diffraction (XRD; Rigaku RINT-2000/PC), while the surface morphology was investigated by Scanning Electron Microscope (SEM: Hitachi S-4700 Scanning Electron Microscope). The darkJ-V curves as a function of temperature ranging from 300 down to 20 K for the present heterojunctions were measured under forward and reverse bias conditions by using a source meter (Keithley 2400). The thermionic emission (TE) theory and Cheung’s and Norde’s methods were utilized for computation of the heterojunction parameters, including the values ofn, ϕb, J0, and Rs. To evaluate the heterojunction parameters by TE theory, the n value was computed from the slope of the linear part obtained from the plot of ln⁡J versus V, while the ϕb value was computed from the J0 value computed from the straight line intercept of the ln⁡J-V at an applied bias voltage of 0 V. In addition, the group was made aware of the fact that the value of J0 for activation energy (Ea) could be computed from the slope of the Arrhenius plot of ln⁡J0 versus 1000/T. To evaluate the Rs value by Cheung’s method, the relationships of dV/d(ln⁡J)-J and Cheung’s function (H(J)) [16–18] and J were plotted. Subsequently, the value for Rs was computed from the slope of these plots. To affirm the agreement and precision of the values for ϕb and Rs, Norde’s method was utilized, by which the relationship between Norde’s function (F(V)) [16, 17, 19] and V was plotted to compute ϕb as well as Rs from the minimum point of F(V)-V plot.


## 3. Results and Discussion

The XRD diffraction pattern forβ-FeSi2 thin films grown using RFMS on Si(111) substrates at a substrate temperature of 560°C and Ar pressure of 2.66 × 10-1Pa is illustrated in Figure 1(a). The XRD pattern demonstrates a weak 404/440 diffraction peak as well as a strong 202/220 diffraction peak. These observed peaks are typical for epitaxial growth of β-FeSi2 thin films on Si(111) substrates. Figure 1(b) shows a pole figure pattern for the β-440/404 peak. The grown β-FeSi2 films demonstrate the appearance of three epitaxial variants that are rotated by an angle of 120° with respect to each other.Figure 1
(a) 2θ-θ XRD pattern for β-FeSi2 thin films grown on Si(111) substrates and (b) a pole figure pattern for the β-440/404 diffraction peak.
(a)
(b)Figure2(a) displays an SEM micrograph for the surface of the β-FeSi2 thin films grown by RFMS on Si(111) substrates. It is clear that the β-FeSi2 thin films consist of a large amount of crystallites on the small side. Figure 2(b) illustrates a cross-sectional SEM micrograph of the β-FeSi2 thin films, which clearly exhibited a sharp interface between the Si substrates and β-FeSi2 thin films. Furthermore, the grown β-FeSi2 thin films were uniform.Figure 2
(a) SEM micrograph of the surface forβ-FeSi2 thin films grown by RFMS on Si(111) substrates and (b) a cross-sectional SEM micrograph of the β-FeSi2 thin films on Si(111) substrate.
(a)
(b)TheJ-V curves for the n-type β-FeSi2/p-type Si heterojunctions grown by RFMS on a logarithmic scale as a function of temperature ranging from 300 down to 20 K under forward and reverse bias conditions in the applied bias voltage range of −1.5 to +1.5 V are displayed in Figure 3. All of the measurements were carried out in the dark. From particular consideration at room temperature, the fabricated heterojunctions exhibited a clear rectifying action similar to conventional p-n abrupt heterojunctions, which is due to the successful formation of a junction between β-FeSi2 thin films and Si substrates using RFMS at a substrate temperature of 560°C. Furthermore, this rectifying action evidently improved at lower temperatures. However, the β-FeSi2/Si heterojunctions had a large leakage current at room temperature, which is likely because of the partially diffused Fe atoms in the depletion region of the Si sides acting as leakage centers for the carriers. This leakage current diminished at low temperatures as well. In addition, theJ-V curves also showed linearity variation in a large voltage range, limited by the Rs effect at a high forward applied bias voltage. From consideration of the region of applied forward bias voltage ≤ 0.2 V, the forward current density exhibited a linear change with the applied bias voltage across the heterojunction. This can be described by means of the TE theory as the following relationship [20–23]:(1)J=J0exp⁡qVnkT-1,where V is the applied bias voltage across the heterojunction, J is the current density, T is the absolute temperature, k is Boltzmann’s constant, q is the electron charge, n is the heterojunction ideality factor, and J0 is the saturation current density, which can be computed from the straight line intercept of the y-axis of ln⁡J-V at an applied bias voltage of 0 V. After the value for J0 is computed, the ϕb value can be computed by the following equation:(2)J0=A∗T2exp⁡-qϕbkT.Based on (1) for the V value higher than 3kT/q, the n value can be computed from the slope of the linear part of ln⁡J-V as the following equation:(3)n=qkTdVdln⁡J=qkT1slope.Figure 3
Semilogarithmic plot of the darkJ-V curves of the present heterojunctions under forward and reverse bias conditions in the voltage range of −1.5 to +1.5 V at temperatures from 300 down to 20 K.Estimation of then value following (3) can indicate the characteristics of the rectifying behavior in a junction. If it equals unity, the junction is ideal. Conversely, the junction is flawed if the value of n increases. In the case of the n value being equal to one, the transportation mechanism across the junction of the carrier is governed by a thermal diffusion process. If the n value is greater than one and lower than or equal to two, the transportation mechanism across the junction of the carrier is dominated by a generation-recombination (G-R) process. Besides, the transportation mechanism of the carrier is probably governed by a carrier tunneling process if the n value is greater than two [22].Figure4 illustrates a plot of n value versus the temperature for the present heterojunctions. By computation using (3), the n value at 300 K was 1.71 and remained nearly constant at a temperature ranging from 300 down to 120 K. As mentioned above, an n value greater than one and lower than two was suggestive of the transportation mechanism across the junction of the carrier being governed by a G-R process at the β-FeSi2/Si heterojunction interface [24]. The existing defects in the β-FeSi2 layer might create deep energy levels in the bandgap, which could behave as recombination centers. At a temperature of 100 K, the n value was computed at 3.25 and increased to 16.83 at 20 K. When the n value was higher than two, the suggestion is that the transportation mechanism across the junction of the carrier in this range of temperature was governed by a tunneling process [24]. This is likely due to the existing interface states at the interface of the β-FeSi2/Si heterojunctions. The heterojunctions possess a large value forn, which is likely owing to the presence of a flawed contact behavior as well as the result of inhomogeneity and tunneling [25]. When the values of J0 become known, the value for activation energy (Ea) can be computed by the following relation [21, 22, 26]:(4)J0∝expΔEakT.Figure 4
Plot of then value versus temperature.Figure5 illustrates an Arrhenius plot for ln⁡J0 versus 1000/T. From the plot, the J0 value was 5.34 × 10−4 A/cm2 at room temperature and decreased to 9.93 × 10−12 A/cm2 at 20 K. The two regions indicate that there are two values for Ea in the present heterojunctions. The slopes for each region can compute the values of Ea. At 120–300 K, the recombination process is dominant with an Ea value of 0.24 eV, whereas the carrier tunneling process is dominant with an Ea value of 9.18 meV in temperatures below 120 K.Figure 5
Arrhenius plot for logJ0 versus 1000/T. The inset is a plot of the ϕb value versus temperature.After the computation of theJ0 value, the ϕb value can be computed using (2). The inset of Figure 5 shows the plot of ϕb versus temperature. At room temperature, the ϕb value was 0.59 eV. This value decreased to 0.06 eV at 20 K. From the experimental results, the value of n increased and the value of ϕb decreased at low temperatures. This is likely because of barrier height inhomogeneity, which may be attributable to the variation in thickness and composition of the interfacial layer as well as nonuniformity of the interfacial charges [27, 28].There are many methods to acquire the value ofRs. For the current study, the value for Rs was computed by using methods developed by Cheung. The equation by means of Cheung’s method can be expressed as [18, 23, 28, 29](5)dVdln⁡J=ARsJ+nkTq,(6)HJ=ARsJ+nϕb=V-nkTqln⁡JA∗T2,where Rs, ϕb, A∗, and A are series resistance, barrier height, Richardson’s constant of β-FeSi2, and area of heterojunctions, respectively.Based on (5), the relationship of dV/d(ln⁡J)-Jwas plotted on the left axis at (a) 300 K and (b) 20 K, as displayed in Figure 6. From this plot, nkT/q and ARs were computed from the y-axis intercept and slope, respectively. In order to affirm agreement of the value for Rs by using (6), HJ-J on the right axis in Figure 6 was also plotted at (a) 300 K and (b) 20 K. The values for ARs andnϕb were computed from the slope and intercept point of the y-axis, respectively. From the computations, the n values were 1.63 at 300 K and 16.13 at 20 K, while the ϕb values were 0.54 eV at 300 K and 0.05 eV at 20 K. These values agreed with those computed from the TE theory. More importantly, the Rs values computed from the relationship of dV/d(lnJ)-J were 13.05 Ω at 300 K and 0.14 MΩ at 20 K. From theH(J)-J plot, the Rs values were 13.18 Ω at 300 K and 0.14 MΩ at 20 K. These results proved that the Rs values computed from the relationship of dV/d(ln⁡J)-J were approximately equal to those computed from the relationship ofH(J)-J. The increasingRs values at low temperatures were likely due to the increment of n value at low temperatures.Figure 6
Relationship betweendV/d(ln⁡J)-J (left axis) and H(J)-J (right axis) at temperatures of (a) 300 K and (b) 20 K.
(a)
(b)Besides, the current study utilized Norde’s method to compute theRs value. The relationship based on Norde’s method can be derived as [19, 30](7)FV=Vγ-kTqln⁡JA∗T2,where V is bias voltage,J is the voltage-dependent forward current density,T is temperature, and γ is the first integer higher than the n value. Both the ϕb and Rs values could be computed by determining the minimum of F(V)-V in the following equation:(8)ϕb=FV0+V0γ-kTq,where F(V0) is the minimum of F(V) and V0 is the corresponding voltage, and(9)Rs=γ-nkTqJminA,where A is the area of the heterojunctions and Jmin is the corresponding current density.The plots ofF(V)-V at temperatures of 300 K (red line) and 20 K (black line) are displayed in Figure 7. From the plot at 300 K, the values for F(V0) and V0 were 0.58 and 0.05 V, respectively. The values for F(V0) and V0 at 20 K changed to 0.06 and 0.26 V, respectively. Based on the computation using (8), the ϕb values were 0.58 eV at 300 K and 0.07 eV at 20 K. In addition, Rs value computed by (9) was 10.93 Ω at 300 K and enhanced dramatically to 0.15 MΩ at 20 K. The results from computation using Norde’s method were in agreement with those computed by the TE theory and Cheung’s method.Figure 7
Plots ofF(V)-V at 20 K (left axis) and at 300 K (right axis).The NIR light detection for n-typeβ-FeSi2/p-type Si heterojunctions formed by RFMS was experimentally demonstrated in a previous study [15]. Their light detection performance was unexpectedly degraded. In order to understand the causes of this degradation, junction parameters such as n and Rs were investigated in the current study. At room temperature, the junctions revealed a large reverse leakage current together with a weak response for NIR light irradiation [15]. From the computation of the junction parameters, the n value was found to be >1 and ≤2 at room temperature. The implication is that the carrier recombination process was dominant in the transportation mechanism across the heterojunction interface [31]. The presence of defects in β-FeSi2 thin films might generate deep energy levels in the bandgap, which could behave as recombination centers [31]. For this reason, the weak response of NIR light was likely due to the recombination of photocarriers at the heterojunction interface, resulting from the presence of defects [32]. Additionally, the estimated Rs value, which was 13.05 Ω at room temperature, might be attributed to the diffusion of Fe atoms into the Si side [33]. The diffused Fe atoms generate deep trap levels in the depletion region, which produce the leakage current in the formed heterojunctions [33, 34]. Moreover, the NIR light detection was spoiled because the photogenerated carriers were trapped in the deep trap levels owing to the diffusion of Fe atoms [33, 34].At low temperatures, the ratio between the photocurrent and dark current was increased, which was probably due to the reduction of the reverse leakage current at low temperatures [15]. This reduction of leakage current is attributable to the decreased carrier densities in the β-FeSi2 films at low temperatures [15, 31, 35]. Unexpectedly, the photocurrent decreased along with the decrease of the leakage current at low temperatures [15]. Based on the computation of the n value, this value increased to be >2 at low temperatures. This might imply that a tunneling process contributed to the carrier transportation mechanism [31]. This is likely owing to the existence of interface states at the heterojunction interface [31]. The existent interface states could behave as a trap center of photogenerated carriers [35], which would be the possible cause for the reduction of photocurrent at low temperatures. Additionally, we consider that the carrier tunneling process implies the appearance of spikes in conduction and valence bands owing to a heterojunction band offset as shown in Figure 8. These spikes prevent the flow of photogenerated carriers at the heterojunction interface. The decreased photocurrent at low temperatures might be because the spikes that appear in conduction and valence bands are higher at low temperatures and they could behave as a barrier for the photogenerated carriers flowing at the heterojunction interface. It is expected that the spikes were enlarged at low temperatures owing to the Fermi levels of Si and β-FeSi2 layers approaching the center of the band gaps at low temperatures. Also, from the computation of the Rs, this parameter increased at low temperatures. This might be attributable to the low mobility of carriers in the β-FeSi2 thin films, which degrades at low temperatures [36].Figure 8
Energy band diagram that indicates the spikes in conduction and valence bands for n-typeβ-FeSi2/p-type Si heterojunctions.


## 4. Conclusions

Measurement and analysis were conducted on n-typeβ-FeSi2/p-type Si heterojunctions grown by RFMS for the darkJ-V curves as a function of temperature. Their heterojunction parameters were systematically computed using the TE theory, Cheung’s method, and Norde’s method. From computation using the TE theory, the value of n was 1.71 at 300 K and increased to 16.83 at 20 K, whereas the ϕb value was 0.59 eV at 300 K and decreased to 0.06 eV at 20 K. The values for both n and ϕb were approximately equal to those computed from Cheung’s and Norde’s methods. The values for Rs computed by Cheung’s method were 13.05 Ω at 300 K and 0.14 MΩ at 20 K, which were approximately equal to those computed using Norde’s method.



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*Source: 6590606-2017-03-02.xml*