Received Date: January 07, 2016; Accepted Date: February 11, 2016; Published Date: March 15,2016
Citation: Mohammed-Brahim T, Zanat K, Hafdallah A, Aida MS, Mahdjoubi L, et al. (2016) Realization and Characterization of Gold-Microcrystalline Silicon Schottky Diodes. J Electr Electron Syst 5:176. doi:10.4172/2332-0796.1000176
Copyright: © 2016 Mohammed-Brahim T, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
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Thin films of microcrystalline silicon (μc-Si) were deposited by (LPCVD: Low Pression Chemical Vapour Deposition) to realise Au-μcSi Schottky diodes which are powerful devices to determine several μc-Si characteristic parameters which are never obtained with simple layers without serious difficulties. Therefore, good quality Schottky devices have been successfully made and present an excellent reproducibility. The current voltage (I-V) characteristics of these devices have given an ideality factor (n) in the range (1.5-2) and a barrier height Ф(I-V) between 0.63 and 0.97eV. This last parameter was found lower than the barrier height Ф(C-V) deduced from the capacitance-voltage characteristics (C-2-V). This difference is attributed to the existence of insulated interface layer as several other authors.
Thin films; Microcrystalline silicon; Schottky diode
Metal/semiconductor (MS) contacts are frequently used in integrated circuits, e.g. as-gates in MESFETs or MOSFET, in light detectors and as solar cells. Schottky barrier diodes (SBDs) are among the simplest MS contact devices [1-4].
Nowadays, the microcrystalline silicon appears as one of the most promising semiconductors in the technology of solar cells and flat video screens because the control of its elaboration in thin films deposited at low substrate temperature, is not to be any more demonstrated.
This gets it back a more moderate cost. So, the preparation of thin layers of this material at the lowest possible temperature constitutes a challenge around the world.
The purpose of our work is to find the technological parameters of this thin film elaboration to have low coats (μc-Si) layers with the most optimal characteristics for photovoltaic and flat video screens applications. The interest of Shottky device is well known because the metal-semiconductor contact is a powerful device used in the study of the semi conducting materials [5,6]. Indeed, the characterization of such a structure allows to reach physical parameters which cannot be obtained with a simple layer of semiconductor.
The μc-Si films were deposited on N highly doped silicon wafers (5 mΩ cm) of two inches in diameter with a preamble ohmical contact on the back side by Low Pressure Chemical Vapour deposition (LPCVD) [7-9] at the base pressure of 0.9 mbar. The deposition temperature pressure was fixed at 550°C. The films are phosphorus in-situ doped, using silane and phosphine diluted in helium. Several phosphorus concentrations obtained by varying the phosphine/silane gas flow ratio, were undertaken. The μc-Si layers thickness is around 500 nm. These films were then crystallised at 600°C under vacuum condition. The Schottky diodes are then completed by thermal deposition of gold dots of 2 mm diameter and 600 nm thickness.
The characterization of diodes so realized, by Courant-Voltage (IV) and Capacitance-Voltage (C-V) measurements was undertaken. The diodes capacity was measured with the impedance bridge HP4192A at 1 MHz.
where I0 is the saturation current derived from the straight line intercept of ln I axis at zero bias, and ideality factor n is introduced to take into account the deviation of the experimental I–V data from the ideal thermionic model, and the value of ideality factor should be one for an ideal contact. The saturation current I0 is given by
From Eqs. (1) and (2), the ideality factor n and barrier height Fb(I–V) can be written respectively as
where A*, a and ?B0 represent the Richardson constant (120 Acm- 2 K-2 for n-type Si), the contact area and the SBH respectively. From the slope of ln I vs. V curve in Eq (3), the value of ideality factors are calculated. I0 is determined from the intercept of log I vs. V curve on the y-axis. Putting these values of I0 in Eq. (4), the values of SBHs were calculated (Figure 1).
The experimental values of ?B0 and n were determined from equations (3) and (4), respectively, and are shown in Table 1 the diode of better quality is number (D24) because of its higher barrier height and its factor of ideality close to the unit (Table 1).
Table 1: Electrical parameters of diffÃ©rent Schottky diodes Au-(Âµc-Si).
At the same time as the (I-V) characterization, we led the study of the (C-V) characteristics. These allow us to reach the donors concentration Nd as well as the potential of diffusion Vbi. This last parameter offers the possibility to measure again the barrier height (?B(C-V)) which we can compare with the barrier height (?B (I-V).
Examples of curves C-2 = f V are represented on the Figure 2. These curves are raised between (-1 and 1Volt). They present several more or less linear parties.
The first party situated on both sides by the origin of the tensions, allows us to reach the value of Nd (cm -3) which corresponds to the constant doping of the layer. The extrapolation of the same right towards the axes of the tensions gives access to the value of Vint from which is deducted the barrier height ΦB(C-V) = Vint + (EC-EF)/q + KT/q) . The values of these important parameters for the control of the technological process are collected in Table 2.
|Diodes||Nd (cm-3)||Vbi (V)||Vint (V)||EC-EF (V)||?(C-V) (V)||?(I-V)Â (V)||? (V)||d (Ã…)|
Table 2: Electrical parameters of Au-Âµc-Si Schottky diodes deducted from (I-V) and (C-V) characteristics.
The second party which is in the substrate side presents a light uniform curvature which we likened to a right. This one has a weaker slope indicating that the concentration of the donors is bigger. Indeed, the values of the concentration of doping determined in this region for the most part of diodes are situated between 1016 and 1019 cm-3. These values suggest that the width of the layer of microcrystalline silicon depleted by the reverse polarization reached the substrate because effectively the silicon wafers used as substrate have a resistivity of 5 10-3 Ωcm to which corresponds, according to the bibliographical data, a concentration of doping Nd of the order of 1019 cm-3.
Besides, the results summarized in Table 2 show that the values of the barrier calculated by the method C-2-V, are generally, more higher than those determined by the method (I-V). We notice that the difference Δ = ?B (C-V)-?B (I-V) is rather big to be attributed to the errors of the measures. According to Goodman [1Besides, the results summarized in Table 2 show that the values of the barrier calculated by the method C-2-V, are generally, more higher than those determined by the method (I-V). We notice that the difference Δ = ?B (C-V)-?B (I-V) is rather big to be attributed to the errors of the measures. According to Goodman , Δ is attributed to the existence of an insulating interfacial layer which leads to a higher value of ?B (C-V). The examination of the values of the width δ of the interfacial layer shows that the thickness of oxide is 20 nm for the diode D23 and 2 nm for the diode D24. These thicknesses are acceptable because their order of height is compatible with the thickness of the layer μc-Siwhich is of the order of 400 nm.3], Δ is attributed to the existence of an insulating interfacial layer which leads to a higher value of ?B (C-V). The examination of the values of the width δ of the interfacial layer shows that the thickness of oxide is 20 nm for the diode D23 and 2 nm for the diode D24. These thicknesses are acceptable because their order of height is compatible with the thickness of the layer μc-Siwhich is of the order of 400 nm.
Thin films of microcrystalline silicon (μc-Si), made by (LPCVD: Low Pression Chemical Vapour Deposition) were performed to realise Au-μcSi Schottky diodes to use this powerful device to determine other μc-Si characteristic parameters which are never obtained with simple layers without serious difficulties.
In fact, good quality (Au-μcSi) Schottky devices have been successfully made and present an excellent reproducibility. The current voltage (I-V) characteristics of these devices have given a rectifying factor (Fq) greater than 500 at 1 Volt, an ideality factor (n) in the range (1.5-2) and a barrier height ?(I-V) between 0.63 and 0.97 eV with a mean value of 0.70 eV which is close to those obtained with monocristalline silicon. These last parameters were found lower than those of barrier heights ?(C-V) deduced from the capacitance-voltage characteristics (C-2-V). This difference is attributed to the existence of an insulating interface layer as several other authors. Elsewhere, from these last characteristics, the depth profile of doping concentration in μcSi layers was computed and used with acuteness to measure again the μcSi thickness by depleting the whole films during the capacitance versus voltage measurement.
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