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The principal properties of the CuInGaSe2 (CIGS) crystals were investigated in this work by using temperature-dependent electrical resistivity and Hall effect. CuInGaSe2 (CIGS) crystals were grown successfully with the modified Bridgman method. The electrical conductivity and Hall effect measurements have been explored in the investigated temperature range was between 198 K and 388 K. Parameters for the Hall such as Hall mobility of the material, resistivity, carrier concentration, Hall coefficient and conductivity were determined. The DC electrical conductivity measurements were performed using traditional two–probe technique for CuInGaSe2 (CIGS) crystals.
Also, the activation energies were calculated from studies of DC conductivity. The results of Hall effect measurements of CuInGaSe2 crystals show that all samples were (p-type), the carrier concentration and Hall mobility are unequivocally reliant on the temperature. Results show the sample suits a wide variety of optoelectronic instruments.
Within a world primarily characterized by information technology, today products with sophisticated electrical properties are ubiquitous. Later technological advances in the scaling down of electronic devices or the ever-increasing achievement of displays and information storage devices rely on sophisticated electronic and magnetic components in a basic way.
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Products with superior electrical properties are in very much demand for the realization of information technology's manifold products. Semiconductors have various useful properties that can be used in fields for example, electronics and optoelectronics for the realization of a large number of high performance devices.
The I–III–VI2 family of semiconducting compounds, which incorporates CuIn1-xGaxSe2 (CIGS), has been broadly utilized in photovoltaics since of its numerous points of interest.
Copper indium gallium diselenide (CIGS) is a source of interest for applications in solar cells. CIGS is an alloy of copper indium diselenide (CIS) and copper gallium diselenide (CGS) and the chemical formula CuIn1-xGaxSe2 explains this. CIGS exhibits a direct bandgap, high coefficients of absorption for visible light with wavelengths of up to 105 cm-1, and long-term optoelectronic steadiness. Therefore, the CIGS bandgap energy can be balanced by changing the compositional proportion, Ga/(In + Ga). Within the case of CuInSe2 (CIS) the direct bandgap energy can be tuned from around 1.03 eV to around 2.5 eV within the case of CuGaS2.
For instance, CIGS shows a direct bandgap, high absorption coefficients for visible light with wavelengths of up to around 105 cm−1, and long-term optoelectronic stability. Furthermore, the bandgap energy of CIGS can be tuned by changing the compositional ratio, Ga/(In + Ga). The direct bandgap energy can be tuned from around 1.03 eV in the case of CuInSe2 (CIS) to about 2.5 eV in the case of CuGaS2 [5]. The performance of CIS and CIGS-based photovoltaics can be further enhanced by exploiting their nanostructures, which are unique. They have a tuneable bandgap, large specific surface area, well-defined conducting pathways, and show carrier multiplication.
For characterizing semiconductors, not only the resistivity but also the density and mobility of the charge carrier furthermore, besides, the form of charge carrier (electrons or holes) are of great importance and can be calculated by measuring the Hall effect. Measurements of the Hall effect [7] and resistivity from the most relevant significant strategy to straightforwardly get the charge carrier concentration and mobility of a conductive specimen. Its wide spread use as a simple and rapid routine measuring technique is also obviously based on simplicity of the van der Pauw formalism [8,9], Which doesn't require more precise micro-structuring during sample preparation.
As far as we may be conscious, there are currently no data on the Hall effect or electrical conductivity of CuInGaSe2 or their temperature dependence in the literature. This work thus provides comprehensive data on the basic physical parameters acquired through the calculation of the Hall effect and electrical conductivity and their dependence on temperature. To obtain high quality devices, this examination gives exact and direct data on the carrier density, mobility, energy gap, position of impurity level and type of conductivity.
In our crystal-growth laboratory, CuInGaSe2 (CIGS) crystals were developed using a modified Bridgman method from a stoichiometric melt of sealed starting materials in evacuated (such as 10-6 mbar) and carbon-coated quartz tubes, each with a tip at the edge. Growth and experimental equipment were described in detail elsewhere [10]. Specimens were prepared for measurements with typical 5.7×1.7×1 mm3 rectangular dimensions. By recording the current-voltage characteristics the ohmic nature of the contacts was confirmed.
In a special cryostat with a traditional DC-type measuring device the conductivity and the Hall coefficient were calculated using a compensation process. The Hall voltages were determined by reversing the direction of the current and the magnetic field and by taking the correct averages. The measurements were carried out under vacuum conditions in a cryostat specifically designed for mounting samples between the polar expansions of an electromagnet. The developed cryostat has allowed measurements to be collected in a wide range of temperatures. The magnetic field had been perpendicular to the crystallographic c-direction for the Hall measurements. The applied current direction at all measurements was parallel to the sample's crystallographic c-axis.
The electrical conductivity of CIGS was found to exhibit typical semiconductor behavior, with three distinct regions identified in the temperature-dependent plot. The extrinsic region (198 to 253 K) showed a slow increase in conductivity due to ionized acceptors. The transition region (253 to 303 K) indicated a balanced contribution from charge carrier concentration and mobility. The intrinsic region (above 303 K) displayed a rapid increase in conductivity, suggesting contributions from both electrons and holes. The activation energy and energy gap were calculated, yielding values of 0.14 eV and 0.5 eV, respectively.
The curve shows the typical behavior of the semiconductors and consists of three regions. The first region ranges from 198 to 253 K and represents the extrinsic region. In this region, due to the release of ionized acceptors and their transition from impurity level, the conductivity was observed to increase slowly in the low-temperature range. In the region of low temperatures the relation between the temperature and electrical conductivity can be given as follows:
= o exp (– Ea / 2 kBT) (1)
where σo is the pre–exponential factor and ΔΕa is the ionisation energy of the acceptors. Using this formula, the ionisation energy ΔΕa is 0.14 eV. The value of σ at 300 K is 0.016 Ω-1 m-1. The second region reflects the transition region, where both the concentration of the charge carrier and its mobility regulate the action of σ. This region covers between 253 and 303 K. In the high-temperature range above 303 K a rapid linear increase in the conductivity is observed. This finding shows that, in the high-temperature range, both electrons and holes contribute to conduction.
This temperature range dependence follows the relation given below:
= o exp (– Eg / 2 kBT) (2)
The Eg is the width of the energy gap = 0.5 eV using this formula, as determined from the slope of the formula above. Not consistent in the literature with the optical band gap values (over a range of 1.04–1.67 eV) [11], the photon energy hν must be equal to or larger than the energy gap 𝐸𝑔. The value of σ at 300 K is 0.017 Ω-1 m-1.
Because the Hall effect can accurately assess the carrier density and semiconductor carrier mobility, we carried out this measurement at the same temperature. Between 198 K and 388 K the variety of the Hall coefficient versus, temperature can be analyzed in Fig. 2. This behavior is in line with the three regions in the conductivity curve observed. The findings show that the sign of the CuInGaSe2 Hall coefficient is positive over the entire investigation range. This finding implies that the compound is a semiconductor of the p-type, in reasonable agreement with previous studies [12]. At room temperature, the Hall coefficient was calculated as 7.150 x 10-3 m3/C.
The energy gap and the energy of ionization can be calculated from the Hall data by drawing the relationship between Ln RH T3/2 and 103/T (Fig. 3) according the equation:
RH T 3/2 = c1 exp (– Eg / 2 kBT) (3).
The forbidden gap width was measured in the temperature area where the conductivity is primarily intrinsic to be ΔΕg = 0.73 eV. The acceptor center depth was calculated from the area where the conductivity derived mainly from impurity atoms, and was found to be 0.15 eV.
The measuring curve leads to the following conclusions: the three curve regions suggest that extrinsic conduction occurs between 198 and 253 K, while intrinsic conduction occurs between 283 and 388 K, and the transition region is between 253 and 283 K.
The present research investigated the effect of temperature on free carrier mobility due to the importance of mobility data in the field of solids, in particular for semiconductors. This method allows one to pick up understanding into the scattering mechanism of the charge carrier. The Hall measurements and the electrical conductivity data were combined to test the Hall mobility temperature dependence. Fig. 4 displays μ variability as a function of temperature.
From the graph one can infer that the μ versus T general plot can be divided into two regions. In the low-temperature region corresponding to extrinsic conductivity and mobility, μ tends to rise with rising temperature, reaching a maximum value of 1.24 x 10-4 m2 / V s for 303 K and following a power law, μH ≈ T1.74. Mobility in the high-temperature area decreases with increasing temperature according to the μH ≈ T-6.7. This dependence suggests that the mechanism responsible for mobility activity within the high temperature range is phonon scattering.
From these results, it appears that the value of the exponent n in the relationship between H α Tn and the 1.5 value expected by theory (μ α T1.5) is consistent with those obtained in other semiconductors for impurity and lattice scattering. The Hall mobility behaviour of semiconducting quaternary compounds, with its higher resistivity [13], might be because of an adjustment in the transport mechanism between localized states either within the energy gap or in regions at the edge of the conduction band at the top of the valence band. Be that as it may, the variety of mobility with temperature in these semiconductors has not been recently announced, so there is as yet deficient experimental information to illuminate that behaviour. Additional work needs to be done to establish the temperature mobility behaviour before any definitive conclusions can be drawn from this form of measurement.
The charge carriers concentration was calculated using the Hall coefficient data relation: n = 1 / RHe, where n is the electron concentration and e is the electron charge.
The relation can be applied to describe the temperature dependence of the charge carrier concentration within the intrinsic conduction region. This relation enables us to calculate the energy gap. The determined energy gap width from this relation is 1.11 eV, and the value of approves of that obtained from fig. 1 and 3. In addition, the carrier concentration is calculated at room temperature to be 2.57 x108 m-3. A further calculation of the holes diffusion coefficient gives an amount of 32.4× 10-7 m2 / Sec.
Assuming the effective mass for holes is equal to the rest mass and using the value for the mobility of the hole at room temperature, the mean free time was 1.29×10-12 S. In addition, the diffusion length of the holes in the CuInGaSe2 specimen was measured at 46.44×10-17m.
A modified Bridgman technique was used to manufacture CuInGaSe2 crystals. The electrical conductivity and Hall effect was measured over a broad temperature range (198-388 K) under vacuum. All measurements were taken in a specially built cryostat, under vacuum conditions. The measurements of the Hall coefficient indicate a conductivity of the form p. The energy gap and acceptor level depth were established. The experimental data allowed us to determine the carrier concentration, mobility, diffusive coefficient, diffusive length and relaxation time of the primary carriers.
Electrical Properties of CIGS Crystals: Conductivity and Hall Effect Analysis. (2024, Feb 18). Retrieved from https://studymoose.com/document/electrical-properties-of-cigs-crystals-conductivity-and-hall-effect-analysis
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