Investigation of the electrical properties of ZnS and ZnS:Mn Pellets was done in this study using the Van der Pauw and Two Probe techniques. These pellets were fabricated from powders produced by Sol-gel process, a low cost method in synthesizing semiconductors in powder form as well as in thin films. Results show that the current-voltage properties of the samples behave linearly in both characterization methods. The conductivity of each sample increased when exposed to UV with λ = 365nm = 3.4eV and show sensitive response to gamma radiation using Co60 with E = 1.17MeV. Calculations show that the conductivity of the samples are in the range of 10-9 to 10-3 S∙cm-1. Most samples show expected conductivity type which is n-type for ZnS and p-type for ZnS:Mn.
Zinc Sulfide is a wide band gap semiconductor, thus is useful in conditions involving high temperature operation. But since natural ZnS mineral is not of good quality in terms of its electrical and other properties, various ways of obtaining a synthetic ZnS were developed. Most of these processes require a vacuum condition and the use of high technology equipment, which means producing one, is very expensive. Nevertheless, ZnS had been synthesized and studied in bulk and thin film for a long time. It has been used all along for its wide and direct band gap by doping it with various elements. Such materials have applications in luminescent devices, light emitters, phosphors, optical sensors etc (Karar et.al, 2004).
Axman (2004) reported that a good Mn-doped ZnS has been successfully produced, enhancing its properties, but excess Mn doping results to quenching thus lowering the electrical properties due to irregularity of the crystalinity of ZnS. This study will investigate the electrical properties of ZnS and ZnS:Mn synthesized by a low cost method called the Sol-gel process. This detailed investigation of the produced samples was done using van der Pauw and two probe techniques.
ZnS and ZnS:Mn semiconductor in powder form were produced by Sol-gel process. Then pellets with 1.305cm diameter were fabricated out of the semiconductor powders using a hydraulic press. The pellets were cut into small squares for characterization and were marked as sample 1 to 12. Thin copper wires were silver pasted into the four corners of each ZnS and ZnS:Mn sample.
The electrical characterization was done under dark condition using van der Pauw technique to get the I-V properties together with the resistivity & conductivity of the samples.
Van der Pauw technique is commonly used in characterizing semiconductors with or without high symmetry in its geometry as long as the thickness is known and is uniform. In this study it was used to measure the resistivity/conductivity and to observe the I-V properties of the samples under dark condition (BhattacharjeeB et.al, 2002). The Set up is shown in Figure1.
Using Hall effect measurements the conductivity type, charge carrier density, and Hall mobility of each sample were taken.
The Hall Effect principle states that when a current-carrying conductor is placed into a magnetic field, a Lorentz force is exerted on the current. This force disturbs the current distribution, resulting in a potential difference (voltage) across the output. This measured voltage drop is the Hall voltage VH (Ubale et.al., 2007).
Further investigation was done using the two probe method to observe the I-V behavior of the samples under dark condition, UV illumination and under radiation. This is to qualitatively observe the response of the samples to a given stimulus.
Resistivity, sheet resistance and the conductivity of the samples are obtained using the Van der Pauw configuration of the four probe method in dark condition at room temperature. Two characteristic resistances are acquired from each of the samples and from these data other electrical properties are calculated. Table 1 shows the calculated electrical properties of the samples.
Using Hall effect measurements, the conductivity type of each sample was obtained. Most samples show expected conductivity type. Samples 3, 8 and 11 shows an unexpected n-type conductivity which is may be due to Mn interstitial on ZnS.
Through a qualitative test using two probe method, it has been observed that the produced samples are photoconductive since most of the samples show increase of conductivity when exposed to UV (λ = 365nm = 3.4eV). On the other hand Co60 exposure, with energy of 1.17MeV, may enhance or diminish the conductivity of the samples.
|1||ZnS||7.07 E02||1.65 E04||1.41 E-03||p|
|2||ZnS:Mn||1.76 E07||4.87 E08||5.70 E-08||p|
|3||ZnS:Mn||4.17 E08||5.48 E09||2.40 E-09||n|
|4||ZnS:Mn||7.90 E05||1.14 E07||1.27 E-06||p|
|5||ZnS||6.54 E02||1.63 E07||1.53 E-03||n|
|6||ZnS:Mn||9.63 E05||2.35 E07||1.04 E-06||p|
|7||ZnS:Mn||2.97 E03||4.87 E04||3.36 E-04||p|
|8||ZnS:Mn||4.54 E06||8.11 E06||2.20 E-06||n|
|9||ZnS||4.80 E06||1.09 E06||2.08 E-07||n|
|10||ZnS:Mn||1.28 E07||1.28 E06||1.82 E-06||p|
|11||ZnS:Mn||5.30 E07||5.30 E07||3.43 E-07||n|
|12||ZnS:Mn||3.10 E07||3.10 E07||6.21 E-07||p|
This table shows the resistivity in Ω cm (ρ), sheet resistance in Ω (Rs), conductivity in S/cm (σ) and the conductivity type of the samples. The values here are not exact due to the uncertainties in accuracy and precision of the equipment used. These uncertainties are of the same range with the obtained values.
ZnS and ZnS:Mn semiconductors in pellets were successfully fabricated with about 1cm diameter. Through a widely accepted characterizing method, the electrical properties of these semiconductor pellets were carefully investigated.
From the results, it is concluded the most samples show a good photoconductive response since their resistances lowered when exposed to UV (λ = 365nm = 3.4eV) and Co60 (E = 1.17MeV) radiation. Thus these samples may be of good use in opto-electronics application and when perfected as phosphors but optical analysis and photoluminescence are suggested for confirmation and to give specific and detailed application of the sample.
A diode may also be created out of the samples since n-type ZnS and p-type ZnS:Mn semiconductors were produced.
N. Karar, et. Al. (2004) Preperties of nanocrystalline ZnS:Mn, Journal of Crystal Growth 298 585-589, National Physics Laboratory, New Delhi 110012, India
Y. Axman. (2004) Manganese Doped ZnS NanoParticles: Sythesis, Particle Sizing and Optical Properties, These No 3029, Ecole Polytechnique Federale de Lausanne, Deilom chemikerin, Universitat Ulm, Allemagne et de nationalite allemande
BhattacharjeeB., et.al., (2004) Sythesis and Characterization of Sol-Gel derived ZnS:Mn2+Nanocrystallites embedded in a Silica Matrix, Bull. Mater. Sci., Vol 25, No. 3, June 2002, pp 175-180
A.U. Ubale, et., al. (2007) Size Dependent OpTical Characteristics of Chemically Deposited Nanostructured ZnS thinfilms. Bull Mater Sci., Vol. 30, No. 2, April 2007, pp 147-151