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Abstract
Vanadium-doped ZnTe (ZnTe:V) crystals 30 mm in diameter and 45 mm in length were grown by the temperature gradient solution growth method. The band gap of as-grown ZnTe:V crystals was estimated to be about 2.22 eV. Infrared spectra exhibit a mean transmittance of 50%-60% in the wavenumber range from 500 cm−1 to 4000 cm−1. Compared with the intrinsic ZnTe crystal, the resistivity of ZnTe:V is increased 6-7 orders of magnitude up to 109 Ω·cm and the carrier concentration reduced from 1014 to 108 cm−3. Accordingly, the THz detection sensitivity is also enhanced by 20%-30%. The improvements on the optical and electrical properties were attributed to the compensation of Zn vacancies by the vanadium element.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Zinc telluride (ZnTe) as a II-VI compound semiconductor with a direct band gap of 2.26 eV has immense potential for a variety of optical and electro-optical devices, such as pure green light emitting diodes (LEDS), laser diodes (LDS), solar cells, microwave and terahertz (THz) devices, etc [1–4]. However, in order to improve the optical and electrical properties of the intrinsic ZnTe, interest has significantly increased in studies of doping crystals. ZnTe crystals doped with P usually have low-resistivity [5, 6], which is suitable for green light-emitting diodes. Cr-doped ZnTe can provide broadly tunable radiation with high efficiency in the mid-IR spectra region (2-3 μm) [7, 8]. Vanadium-doped ZnTe (ZnTe:V) has improved photorefractive response from 0.6 to 1.3 μm [9], which has attractive applications of optical power limiting, holographic interferometry, and optical communicating [10–12].
ZnTe crystals could be grown by the melt method [13, 14], vapor phase method [15, 16] and solution method [17, 18]. However, the melting point (~1295 °C) of ZnTe is higher than the softening temperature of silica (1100-1200 °C), bringing difficulties to grow ZnTe crystals from a melt. The vapor phase method could lower the growth temperature [19], while, the low growth rate limits obtaining large sized crystals. The solution method could have a lower growth temperature by changing the composition of the flux. For example, the growth temperature could be reduced to 1060 °C when the molar ratio of additional flux Te is about 70% [20]. In addition, large-size single crystals are more easily obtained than the vapor phase method.
In this work, ZnTe:V crystals were grown from Te solution by the temperature gradient solution growth (TGSG) method. The phase structure was measured by power X-ray diffraction (XRD). To obtain a further insight on the optical characterization, IR and UV-Vis-NIR spectra were carried out. The electrical properties and carrier concentrations were evaluated at room temperature. Finally, the THz responses were measured by terahertz time domain spectra (THz-TDS) in the range of 0.1-3 THz at room temperature. Based on these results, the differences in optical and electrical properties between intrinsic ZnTe and ZnTe:V crystals were discussed.
2. Experimental section
Te-rich polycrystals were prepared by solid state reaction method. The starting materials are the high purity Zn (7 N) rod, Te (7 N) powder, and V (3 N) wire. According to a molar ratio of Zn:Te = 3:7, the raw materials were put into a carbon-coated quartz ampoule with an inner diameter of 30 mm under 5 × 10−5 Pa. Then, the sealed ampoule was put in a rocking furnace, and the temperature rises gradually to 1100 °C in 15 hours. Then the furnace was rocking slowly for 12 hours to mix the material. After that, the furnace was maintained at 1100 °C for 48 hours to synthesize the Te-rich polycrystals. Finally, it was cooled down to the room temperature for 12 hours. Vanadium element doping in ZnTe polycrystals has two ways. Element vanadium and VTe2 compound were used as the starting materials, which were named ZnTe:VELE and ZnTe:VCOM, respectively. The nominal doping concentration of vanadium is 1%. In addition, VTe2 compound was synthesized at 800 °C by Te power and V wire with a molar ratio of V:Te = 1:2 in our lab.
The synthesized Te-rich polycrystals were used for the growth of ZnTe:V single crystals. The ampoule was put into a two zone furnace to start growth with a temperature gradient of about 10 °C/cm. Initially, the ampoule was placed in the hot zone of the vertical furnace. It was heated gradually up to 1080 °C, and maintained at this temperature for 3 hours for homogenizing. Then, the ampoule was moved to the starting growth position with the tip temperature of 1060 °C. The ampoule was descended along the axis of the furnace with a rate of 0.5 mm/h. After the growth, the furnace was cooled to room temperature at the cooling rate of 5 °C/h.
Wafers with the dimension of 5 × 5 × 2 mm3 of <110> direction were sliced from the as-grown crystals. Then, they were mechanically and chemically polished to obtain a flat and smooth surface. The concentrations of doping elements were measured by Optima 8300 ICP OES. The phase structure of the Te-rich polycrystals and the as-grown ZnTe:V crystals were examined by a RICOH D/max2500 power X-ray diffraction. The transmittance spectra were performed at room temperature using Shimadzu UV-3150 UV-Visible-NIR and Nicolet Nexus Fourier transform infrared spectrometer. The bulk resistivity was obtained from the I-V curves (Agilent 4155C) at room temperature. Hall tests were conducted using a homemade optoelectronic measuring system with a magnetic field of 5000 Gs. The THz spectra were measured by TAS7400SP THz-TDS system in the range of 0.1-3 THz at room temperature.
3. Results and discussion
3.1 Crystal growth
Three ingots with dimensions of Φ30 mm × 45 mm were grown by TGSG method. One was intrinsic ZnTe crystal and the other two were ZnTe:VELE and ZnTe:VCOM crystals. Figure 1(a) shows an as-grown ZnTe:VCOM ingot after polishing under intense incandescent light. The ingot consists of ZnTe:V single crystal, the transition region of Te and ZnTe:V, and the pure Te phase region, respectively. Figure 1(b) shows a circular slice with 2 mm thickness cut perpendicular to the growth direction from the ingot. Two obvious grains are identified by the homogeneous transparency under intense light. The cracks, twins and significant Te-rich secondary particles were not observed in the ingot and slices, which indicates the good quality of the ingot. Figure 1(c) shows an X-ray rocking curve of the <110> ZnTe:V wafer, the full width at half maximum (FWHM) about 70 arcsec and good symmetry of the rocking curve also demonstrates the good quality of the ZnTe:V ingot. ICP analysis of the sample from the top and tail of ZnTe:VELE and ZnTe:VCOM crystals showed the doping concentrations of vanadium were 10-20 ppm. The similar low doping concentration of ZnTe:VELE and ZnTe:VCOM crystals is mainly resulted by the limited solubility of the V dopants.
Fig. 1 ZnTe:VCOM crystal (a) As-grown ingot with dimension of Φ30 mm × 45 mm under intense incandescent light, (b) wafer sliced perpendicular to the growth direction, (c) X-ray rocking curve of <110> ZnTe:V crystal. The inset in (c) is the <110> ZnTe:V wafer.
3.2 X-ray diffraction patterns
The powder XRD patterns of the pre-growth (Te-rich) ZnTe:V polycrystals and the as-grown ZnTe:V crystals are shown in Fig. 2. The peaks of ZnTe and elemental Te are found in the pre-growth ZnTe:V polycrystals, which are consistent with the standard XRD patterns of ZnTe (JCPDS 15-0746) and Te (JCPDS 36-1452). For as-grown ZnTe:V crystals, only the ZnTe peaks appeared. The peaks of vanadium and the related compounds were not observed in both pre-growth ZnTe:V polycrystals and ZnTe:V crystals.
Fig. 2 Power X-ray diffraction patterns of Te-rich polycrystals and as-grown ZnTe:V crystal, respectively.
3.3 UV-Vis-NIR transmission spectra
Figure 3 demonstrates the typical UV-Vis-NIR transmittance spectra of ZnTe:VELE, ZnTe:VCOM and intrinsic ZnTe from 200 nm to 1500 nm. It is suggested that the absorption edge lays in the spectral range 550-555 nm. The band gap (Eg) of the as-grown crystals can be calculated from the Tauc law [21]
where hν is the incident photon energy, α is the absorption coefficient, A is a constant that depends on the electronic transition probability, and the exponent m is a parameter, dependent on the type of electronic transition responsible for the absorption. For ideal direct band gap semiconductor material ZnTe, the value of m is ½. By extrapolating the linear portion of the curves of (αhν)2 vs. hν to zero absorption, the obtained values of Eg are 2.218 eV, 2.220 eV, and 2.223 eV for ZnTe:VELE, ZnTe:VCOM and intrinsic ZnTe crystals, respectively, which are in good agreement with the previous reported of ZnTe [22]. Although vanadium was introduced into ZnTe crystals, the absorption edge tailing effect caused by the shallow level defects did not appear in the near band edge region. In other words, deep level in the band was introduced by V doping, which is similar to the vanadium in CdTe [23].Fig. 3 Typical UV-Vis-NIR transmittance spectra of ZnTe:VELE, ZnTe:VCOM and intrinsic ZnTe crystals at room temperature. The inset is the Tauc plot of (αhν)2 vs hν.
3.4 IR transmission spectra
Figure 4 shows the IR transmittance spectra of ZnTe:VELE and ZnTe:VCOM wafers, and compared with the intrinsic ZnTe wafers over the wavenumber range from 500 cm−1 to 4000 cm−1. The curves show that the mean transmittance is about 50%-60%. Generally, the IR transmittance T of the intrinsic ZnTe crystal is given by [24]
where d is the thickness of the sample, α is the absorption coefficient, and R is the reflectivity could be defined asThe refractive index n of ZnTe is about 2.7 at the room temperature throughout the 2.5-20 μm range [25,26]. R is almost independent of wavelength with the value of 0.21. To simplify Eq. (2), the transmittance is determined only by the absorption coefficient α for ZnTe:V and intrinsic ZnTe wafers with the thickness of d = 2 mm. And the absorption coefficient α could be given by [27]where N is the carrier concentration, λ is the wavelength, is the effective mass, is the mobility and is the permittivity of vacuum. Therefore, the absorption coefficient varies proportionally to λ2 and N. Sen et al. [28] and Li et al. [29] have reported the free carrier absorption is the major factor for the decrease of the transmittance of intrinsic ZnTe wafers at low wavenumber region. As shown in Fig. 4, the IR transmittance of the intrinsic ZnTe wafers decreased with the wavenumber from 2000 cm−1 to 500 cm−1. However, the transmittance of ZnTe:V increased about 6-10% in this region. The enhanced transmittance indicates the decreasing carrier concentration of ZnTe:V crystals.3.5 Resistivity
The electrical properties usually affected by the dopants. Figure 5 reveals the current-voltage (I-V) curves of the as-grown V-doped and intrinsic ZnTe crystals. The resistivity of intrinsic ZnTe is 450-500 Ω·cm, which is similar to the previous reported results [30, 31]. While the resistivity of ZnTe:V is up to 1.39-2.53 × 109 Ω·cm, which is 6-7 orders higher than that of intrinsic ZnTe. The carrier concentration and the Hall mobility were evaluated by Hall effect at room temperature. The results are summarized in Table 1. All as-grown V-doped and intrinsic ZnTe crystals are p-type conductivity. The carrier concentration and mobility of the intrinsic ZnTe crystal are (1-2) × 1014 cm−3 and 20-25 cm2V−1s−1, respectively, which are slightly lower than the reported results [18]. For V-doped ZnTe crystals, the mobility is 32.9 cm2V−1s−1 and 17.3 cm2V−1s−1 for ZnTe:VELE and ZnTe:VCOM crystals, respectively, which is similar to the intrinsic ZnTe. However, the carrier concentrations are significantly decreased to 1.63 × 108 cm−3and 2.93 × 108 cm−3 for ZnTe:VELE and ZnTe:VCOM crystals, respectively, which is 6-7 orders lower than that of intrinsic ZnTe.
Table 1. Electrical properties of ZnTe:VELE, ZnTe:VCOM and intrinsic ZnTe crystal.
In order to clarify the origin of the increased resistivities and reduced carrier concentrations of ZnTe:V, the intrinsic point defects in ZnTe crystal are discussed. The low resistivity of pure ZnTe is commonly attributed to the existence of zinc vacancy (VZn) due to the high vapor pressure of Zn element at high temperature. The concentration of intrinsic point defects (VZn) in Te-rich ZnTe crystal could be calculated by the partial pressure of Zn (PZn). The Zn and Te equilibrium vapor pressure at certain temperature in ZnTe crystals could be defined as the following relations:
where PZn and PTe2 are the partial pressure of Zn and Te2, and K is the equilibrium constant. Jordan et al. [32] reported that the minimum partial pressure of Zn and Te2 is 0.02 atm and 0.01 atm at 1300 K, respectively. Therefore, the concentration of the point defects (VZn) is approximately 1.13 × 1017 cm−3 calculated by PZn, which is slightly lower than the doping concentration of the vanadium about 1018 cm−3 (~15 ppm). Therefore, VZn can be occupied by V element to reduce the free carrier concentration. However, the actual doping concentration of vanadium is much lower than the nominal concentration, which is mainly ascribed to the low solubility of V in ZnTe. The solubility is determined by the structural incompatibility between monoclinic VTe2 and cubic ZnTe. In addition, vanadium element in ZnTe:V is regard as a deep donor [33, 34], which can also pin the fermi level near the mid of the band gap, resulting the higher resistivity.3.6 THz spectra
High resistivity ZnTe crystals could be used as the terahertz wave emitter or sensors. Figure 6(a) and 6(b) reveal the THz detection and transmission frequency spectra obtained using 2 mm <110> ZnTe and ZnTe:VCOM crystals. The maximum amplitude of the detection signal of ZnTe:V and intrinsic ZnTe crystals locates at ~0.58 THz. However, the detection sensitivity is enhanced 20%-30% for ZnTe:V crystal. The transmission frequency spectra shows the amplitude of ZnTe:V crystal is higher than intrinsic ZnTe, demonstrating the transmission of ZnTe:V is approximately 10% higher than the intrinsic ZnTe (~60%) at low frequency region, as shown in Fig. 6(c). There exists an absorption band around 1.7 THz caused by the transverse TA(X) phonon in Fig. 6(a) and Fig. 6(c) [35, 36]. Fig. 6(d) shows the refractive index which was calculated through the THz transmission spectra [37]. It is obvious that the refractive index of ZnTe:V crystal is rising from 2.7 to 3.3 as the frequency vary from 0.3 to 3 THz, slightly higher than the intrinsic ZnTe from 2.2 to 3.2.
Fig. 6 THz properties of intrinsic ZnTe and ZnTe:VCOM crystals (a) THz detection frequency spectra; (b) THz transmission frequency spectra; (c) The transmission at 0.3-3 THz; (d) Refractive index at 0.3-3 THz.
Harrel et al. [38] and Ku et al. [39] have reported that the saturated THz conversion efficiency of ZnTe crystals mainly depends on the free carrier absorption. The transmitted terahertz wave amplitude through ZnTe crystal could be estimated from the equation [40]
where R is the reflectivity of THz field at the air-ZnTe interface, n is the index of the reflection at THz frequencies, ρ is the resistivity, and L is the thickness of crystal. Therefore, compared to the intrinsic ZnTe, the lower carrier concentration of ZnTe:V indicates higher resistivity, which resulted in the higher transmission and THz efficiency.4. Conclusions
High quality of ZnTe:V crystals were grown by TGSG technique using vanadium element and VTe2 compound as dopants, and characterized by optical and electrical methods. ICP analysis showed that the concentration of vanadium was 10-20 ppm. The band gap of ZnTe:V crystals is similar to the intrinsic ZnTe, with the value ~2.22eV. However, the IR spectra shows the transmittance is about 6-10% higher than intrinsic ZnTe at 500-2000 cm−1, which is attributed to the less free carrier absorption. This is in good agreement with the high resistivity ZnTe:V crystal (~109 Ω·cm). Furthermore, the weaker free carrier absorption determines that the THz detection sensitivity of ZnTe:V is enhanced by 20-30% compared to the intrinsic ZnTe.
Disclosures
The authors declare that there are no conflicts of interest related to this article.
Funding
National Natural Science Foundations of China (Nos. U1631116 and 51202197); National Key Research and Development Program of China (2016YFE0115200); Natural Science Basic Research Plan in Shaanxi Province of China (2016KJXX-09 and 2017KW-029); Fundamental Research Funds for the Central Universities (3102017zy057).
Acknowledgements
We thank Mr Yi Zhang from DAHENG New Epoch Tech, INC, for helping the THz measurement.
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