1.
Introduction
As one of the II–VI compounds, cadmium sulphide (CdS) is an important semiconductor with both photovoltaic effect and piezoelectric effect properties. It has been studied for several decades and has been widely used in sensors, optoelectronic devices and memory devices[1–3]. Effective middle IR lasers based on CdS system single crystal have recently drawn more and more attention from researches[4–6].
CdS bulk single crystals have been grown by a few groups using the physical vapor transport (PVT) method, which has been indicated to be an effective method on the growth of II–VI bulk single crystals[7–10]. CdS single crystals with a size of about Φ8 × 10 mm2 and Φ15 × 10 mm2 have been successfully grown using the PVT method from Sankar’s group and Hong’s group, respectively[11,12]. Φ40 × 30 mm2 CdS single crystal has been grown by Cheng et al. in our group using the same method[13]. Φ100 mm CdS single crystals have already been grown[14,15].
CdS single crystal is sensitive to ultraviolet light. So it can be used as an ultraviolet detector. Meanwhile, CdS single crystal has high transmittance in the infrared range from 3.0 to 5.0 μm, and so the crystal is also well known as an infrared window material. However, most of the reports focus on the growth and photocurrent of the crystal. Nowadays, limited studies on the optical properties of the infrared range have been conducted on CdS bulk single crystal.
In this paper, a large size CdS bulk single crystal with Φ55 × 15 mm2 has been successfully grown by the PVT method. The properties of the crystal were designed to be two parts from the top to the bottom by changing the growth conditions. So the crystal had been separated into two parts to be measured. Observed values of the Hall mobility, specific resistivity, and carrier concentration for the two parts of the CdS single crystal were characterized. The top and bottom of the crystal structures were confirmed using X-ray diffraction (XRD). Transmittance of the CdS single crystal with the incident light ranging from 2.5 to 16.0 μm was also investigated. The corresponding absorption coefficient was calculated to analyse its absorption mechanism.
2.
Experimental
Φ55 × 15 mm2 CdS single crystal with wurtzite structure was successfully grown by PVT method. CdS powder with high purity of 99.9999% was weighed and put into a small quartz tube as raw material. A sapphire wafer was placed on top of the tube. Then the small tube was put into a larger one. At last, the larger quartz tube was placed into an electric furnace with five growth zones. After 48 h, a CdS polycrystal crystal would be grown, which was made as a crystal source. Both the source and a CdS seed crystal were put into a quartz tube to grow CdS single crystal in the same five zones electric furnace.
Two different growth conditions were used to grow the CdS single crystal. In order to grow high quality CdS single crystal, the growth rate was very low at the beginning of the growth. Then a high growth rate would be used. The length of the whole CdS single crystal was 15 mm. The properties for the top and bottom of the crystal could be different. Therefore, the crystal was divided into two parts to check the properties. The CdS single crystal with a size of Φ55 × 15 mm2 would be grown for about seven days. Two parts of the crystal were confirmed by the XRD with Cu Kα radiation using the powder of crushed CdS single crystal. The structural quality of the CdS crystal was evaluated by the full-width at half-maximum (FWHM) of the XRD rocking curve peak. Electrical properties were measured using samples with size of 8 × 8 × 0.5 mm3 by the Hall measurement system. The transmittance of the CdS single crystal was measured using a VERTEX 70 Fourier transform infrared spectrometer.
3.
Results and discussion
The as-grown CdS single crystal with a diameter of 55 mm is shown in Fig. 1. The crystal was yellow and almost transparent . The double diffraction X-ray rocking curve of CdS wafer is given in Fig. 2. The FWHM of the XRD rocking curve peak can reflect the structural quality of the CdS crystal. The corresponding (002) XRD FWHM was 60.00 arcsec. The compositions of the bottom and top parts of the crystal were checked by glow discharge mass spectrometry. The impurity compositions of the top and bottom parts were very close and less than 2 ppma, indicating the homogeneity of the whole CdS single crystal. Fig. 3 shows XRD patterns of two crushed crystal powders. It is shown that both the top and bottom of the crystal had the same structure, P63mc, without any second phase. All of the peaks in the two XRD patterns were nearly at the same angle confirming that the lattice structures from the top to the bottom were almost coincident.
onerror="this.onerror=null;this.src='http://www.jos.ac.cn/fileBDTXB/journal/article/jos/2018/12/PIC/18040011-1.jpg'" class="figure_img" id="Figure1"/>
Download
Larger image
PowerPoint slide
Figure1.
(Color online) CdS single crystal with size of Φ55 × 15 mm2.
onerror="this.onerror=null;this.src='http://www.jos.ac.cn/fileBDTXB/journal/article/jos/2018/12/PIC/18040011-3.jpg'" class="figure_img" id="Figure3"/>
Download
Larger image
PowerPoint slide
Figure3.
(Color online) X-ray diffraction patterns of the crushed CdS crystal powders.
onerror="this.onerror=null;this.src='http://www.jos.ac.cn/fileBDTXB/journal/article/jos/2018/12/PIC/18040011-2.jpg'" class="figure_img" id="Figure2"/>
Download
Larger image
PowerPoint slide
Figure2.
Double diffraction X-ray rocking curve of CdS wafer.
As shown in Fig. 4, two wafers with a size of Φ55 mm were cut from both the top and the bottom of the as-grown crystal. Both of the wafers were polished precisely. So, the surfaces of the wafer reflect light, as shown in Fig. 4. Then each wafer was separated into 9 pieces with a size of 8 × 8 × 0.5 mm3. The Hall mobility, specific resistivity, and carrier concentration of the above two wafers were measured to evaluate the electric quality of the single crystal.
onerror="this.onerror=null;this.src='http://www.jos.ac.cn/fileBDTXB/journal/article/jos/2018/12/PIC/18040011-4.jpg'" class="figure_img" id="Figure4"/>
Download
Larger image
PowerPoint slide
Figure4.
(Color online) (a) Measuring points and (b) samples of the CdS crystal.
As shown in Fig. 5, 18 CdS samples from the top and bottom parts were measured. The specific resistivity of the top wafer was varied from 0.71 to 0.85 Ω·cm. Correspondingly, the specific resistivity of the bottom wafer was varied from 0.34 to 0.38 Ω·cm, which was lower than the value of the top wafer. The carrier concentration of the bottom part was almost twice higher than those of the top part of the crystal. The Hall mobilities of the top and bottom crystal wafers were nearly the same and all the values were higher than 250 cm2/(V·s). As mentioned above, the growth rate was very low at the growth beginning by improving the seed temperature. Then the high growth rate would be used through reducing the temperature. An excess of Cd was used in the CdS growth system. High Cd pressure under high temperature inhibited the dissociation reaction of CdS, then reduced the content of S. Therefore, S vacancies in the bottom part were more than that in the top part of the CdS crystal. Correspondingly, the carrier concentration of the bottom part was higher than that of the top part. The specific resistivity of the bottom part was lower than that of the top part.
onerror="this.onerror=null;this.src='http://www.jos.ac.cn/fileBDTXB/journal/article/jos/2018/12/PIC/18040011-5.jpg'" class="figure_img" id="Figure5"/>
Download
Larger image
PowerPoint slide
Figure5.
(Color online) (a) Specific resistivity, (b) Hall mobility, and (c) carrier concentration for the top and bottom wafers of the CdS single crystal. (d) Product of specific resistivity, Hall mobility and carrier concentration.
The relationship of specific resistivity, Hall mobility, and carrier concentration can be described as the formula:
$ ho mu Nq = 1,$  | (1) |
where ρ is the specific resistivity, μ is the Hall mobility, and N is the carrier concentration. The q is the quantity of electric charge. As shown in Eq. (1), the product of specific resistivity, Hall mobility, carrier concentration, and quantity of electric charge for the CdS single crystal was almost equal to one, indicating the accuracy of electric data of both the top and the bottom of the crystal.
The transmittance of the CdS single crystal in the range from 2.5 to 16.0 μm has been investigated in Fig. 6. The highest theoretical transmittance of the single crystal was 72.3% at 2.5 μm. The transmittances in the range of 2.5 to 4.5 μm for both top and bottom CdS single crystal were measured to be higher than 70.0%, making this single crystal a significant candidate for infrared window materials. The transmittance of the bottom crystal became lower than the top crystal from 4.0 μm.
onerror="this.onerror=null;this.src='http://www.jos.ac.cn/fileBDTXB/journal/article/jos/2018/12/PIC/18040011-6.jpg'" class="figure_img" id="Figure6"/>
Download
Larger image
PowerPoint slide
Figure6.
(Color online) Transmittance and absorption coefficient in the range of 2.5 to 16.0 μm for the CdS single crystal.
The optical transmittance, T, can be obtained from the general formula[16–18],
$T = frac{{{{(1 - R)}^2}exp ( - alpha d)}}{{1 - {R^2}exp ( - 2alpha d)}},$  | (2) |
where d is the sample thickness and α is absorption coefficient. The reflectivity R is given by,
$R = frac{{{{(n - 1)}^2}}}{{{{(n + 1)}^2}}},$  | (3) |
where n is refractive index, which is related with λ. Therefore, the absorption coefficient, α, can be calculated from Eq. (4)[19],
$alpha = - 1frac{1}{d}ln left({left{ {left[frac{{{{(1 - R)}^2}}}{{2T{R^2}}} ight]^2} + frac{1}{{{R^2}}} ight} ^{frac{1}{2}}} - frac{{{{(1 - R)}^2}}}{{2T{R^2}}} ight),$  | (4) |
since both R and d are known.
If there is no light absorption (
The absorption coefficient α and wavelength λ can be described using a power law function of the form:
$alpha = k{lambda ^m},$  | (5) |
where k is a constant, and the exponent, m, is characteristic of the particular kind of absorption mechanism[17]. The different exponents m indicate different absorption mechanisms in relative IR wavelengths. There is a linear relation between
As shown in Fig. 7, the variation tendencies of the absorption coefficient αfor the top and bottom CdS single crystal were essentially the same. From 2.5 to 4.0 μm, the measured optical transmittance T was very close to the theoretical transmittance of the CdS single crystal, indicating the low absorption. The T of the top and bottom crystal wafers were almost the same in this range. Thus, the absorption coefficient from 4.0 to 16.0 μm was used to calculate the m.
onerror="this.onerror=null;this.src='http://www.jos.ac.cn/fileBDTXB/journal/article/jos/2018/12/PIC/18040011-7.jpg'" class="figure_img" id="Figure7"/>
Download
Larger image
PowerPoint slide
Figure7.
(Color online) Relationship of lg α and lg λ.
As shown in Fig. 7, green and purple lines were the fitting lines and the value m could be calculated to be 2.3 and 2.7 for the top and bottom crystal wafers, respectively. The value mwas between 2.0 to 3.0, indicating the free carrier absorption[18, 20]. Correspondingly, the difference of m implies the variation of carrier concentration between the top and bottom of the CdS single crystal. The carrier concentration for the bottom crystal wafer caused by S vacancies was higher than that for the top crystal wafer, which resulted in the higher absorption coefficient and higher m.
4.
Conclusions
In summary, we successfully grew large size (Φ55 × 15 mm2) CdS single crystal using the PVT method. The crystal structure and quality were evaluated by XRD, showing a good consistency of the ingot from the top to the bottom. Moreover, the whole crystal was divided into two parts to check the electric properties, and its specific resistivity was less than 1 Ω·cm. The transmittance in the range of 2.5 to 4.5 μm for the whole CdS single crystal was measured to be higher than 70%. The transmittance of the bottom part was measured to be lower than the values of the top parts, due to the higher free carrier absorption resulting from the different growth conditions.
Acknowledgements
We thank Yanhui Dong and Jing Li from the Centre of Processing and the Center of material characterization for their help. We also thanks Jian Wang.
