Volume 17 Issue 2
Jul.  2019
Article Contents

Joel Díaz-Reyes, Roberto Saúl Castillo-Ojeda, José Eladio Flores-Mena. Characterization of Cd1-xZnxTe (0≤x≤1) Nanolayers Grown by Atomic Layer Deposition on GaSb and GaAs (001) Oriented Substrates[J]. Journal of Electronic Science and Technology, 2019, 17(2): 97-108. doi: 10.11989/JEST.1674-862X.70929112
Citation: Joel Díaz-Reyes, Roberto Saúl Castillo-Ojeda, José Eladio Flores-Mena. Characterization of Cd1-xZnxTe (0≤x≤1) Nanolayers Grown by Atomic Layer Deposition on GaSb and GaAs (001) Oriented Substrates[J]. Journal of Electronic Science and Technology, 2019, 17(2): 97-108. doi: 10.11989/JEST.1674-862X.70929112

Characterization of Cd1-xZnxTe (0≤x≤1) Nanolayers Grown by Atomic Layer Deposition on GaSb and GaAs (001) Oriented Substrates

doi: 10.11989/JEST.1674-862X.70929112
More Information
  • Author Bio:

    Joel Díaz-Reyes was born in the State of Puebla, Mexico in 1961. He received the B.S. and M.S. degrees in physics from Meritorious Autonomous University of Puebla (BUAP), Puebla, Mexico in 1984 and 1987, respectively. He received the Ph.D. degree from the Electrical Engineering Department, CINVESTAV-IPN, Mexico City, Mexico in 1996. Currently he is working with the Center for Research in Applied Biotechnology, Instituto Politécnico Nacional, Tepetitla, Mexico. His research interests include the growth of semiconductor materials and their characterization

    Roberto Saúl Castillo-Ojeda was born in the State of Yucatán, Mexico in 1961. He received the B.S. degree in electric engineering from Technological Institute of Mérida, Mérida, Mexico in 1993. He received the M.S. and Ph.D. degrees from the Department of Electrical Engineering, CINVESTAV-IPN in 1994 and 2001, respectively. He is currently working with Universidad Politécnica de Pachuca, Pachuca, Mexico. His research interests include the growth of semiconductor materials by liquid phase epitaxy (LPE) and metal organic chemical vapour deposition (MOCVD) and their characterization

    José Eladio Flores-Mena was born in the State of Puebla, Mexico in 1959. He received the B.S., M.S., and Ph.D. degrees in physics from BUAP in 1982, 1989, and 2002, respectively. He is currently working with the Faculty of Sciences of the Electronics, Autonomous University of Puebla, Puebla, Mexico. His research interests include dynamic systems and theory of optimal control, robotics, and automation

  • Corresponding author: J. Díaz-Reyes is with the Center for Research in Applied Biotechnology, Instituto Politécnico Nacional, Tepetitla 90700, Mexico (e-mail: joel_diaz_reyes@hotmail.com).
  • Received Date: 2017-09-27
  • Rev Recd Date: 2018-04-26
  • Publish Date: 2019-06-01
  • ZnTe, CdTe, and the ternary alloy CdZnTe are important semiconductor materials used widely for the detection of an important range of electromagnetic radiation as gamma ray and X-ray. Although, recently these materials have acquired renewed importance due to the new explored nanolayer properties of modern devices. In addition, as shown in this work they can be grown using uncomplicated synthesis techniques based on the deposition in vapour phase of the elemental precursors. This work presents the results obtained from the deposition of nanolayers of these materials using the precursor vapour on GaAs and GaSb (001) substrates. This growth technique, extensively known as atomic layer deposition (ALD), allows the layers growth with nanometric dimension. The main results presented in this work are the used growth parameters and the results of the structural characterization of the layers by the means of Raman spectroscopy measurements. Raman scattering shows the peak corresponding to longitudinal optical (LO)-ZnTe, which is weak and slightly redshift in comparison with that reported for the ZnTe bulk at 210 cm–1. For the case of the CdTe nanolayer, Raman spectra presented the LO-CdTe peak, which is indicative of the successful growth of the layer. Its weak and slightly redshift in comparison with that reported for the CdTe bulk can be related with the nanometric characteristic of this layer. The performed high-resolution X-ray diffraction (HR-XRD) measurement allows to study some important characteristics such as the crystallinity of the grown layer. In addition, the HR-XRD measurement suggests that the crystalline quality has dependence on the growth temperature.
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Characterization of Cd1-xZnxTe (0≤x≤1) Nanolayers Grown by Atomic Layer Deposition on GaSb and GaAs (001) Oriented Substrates

doi: 10.11989/JEST.1674-862X.70929112
  • Author Bio:

  • Corresponding author: J. Díaz-Reyes is with the Center for Research in Applied Biotechnology, Instituto Politécnico Nacional, Tepetitla 90700, Mexico (e-mail: joel_diaz_reyes@hotmail.com).

Abstract: ZnTe, CdTe, and the ternary alloy CdZnTe are important semiconductor materials used widely for the detection of an important range of electromagnetic radiation as gamma ray and X-ray. Although, recently these materials have acquired renewed importance due to the new explored nanolayer properties of modern devices. In addition, as shown in this work they can be grown using uncomplicated synthesis techniques based on the deposition in vapour phase of the elemental precursors. This work presents the results obtained from the deposition of nanolayers of these materials using the precursor vapour on GaAs and GaSb (001) substrates. This growth technique, extensively known as atomic layer deposition (ALD), allows the layers growth with nanometric dimension. The main results presented in this work are the used growth parameters and the results of the structural characterization of the layers by the means of Raman spectroscopy measurements. Raman scattering shows the peak corresponding to longitudinal optical (LO)-ZnTe, which is weak and slightly redshift in comparison with that reported for the ZnTe bulk at 210 cm–1. For the case of the CdTe nanolayer, Raman spectra presented the LO-CdTe peak, which is indicative of the successful growth of the layer. Its weak and slightly redshift in comparison with that reported for the CdTe bulk can be related with the nanometric characteristic of this layer. The performed high-resolution X-ray diffraction (HR-XRD) measurement allows to study some important characteristics such as the crystallinity of the grown layer. In addition, the HR-XRD measurement suggests that the crystalline quality has dependence on the growth temperature.

Joel Díaz-Reyes, Roberto Saúl Castillo-Ojeda, José Eladio Flores-Mena. Characterization of Cd1-xZnxTe (0≤x≤1) Nanolayers Grown by Atomic Layer Deposition on GaSb and GaAs (001) Oriented Substrates[J]. Journal of Electronic Science and Technology, 2019, 17(2): 97-108. doi: 10.11989/JEST.1674-862X.70929112
Citation: Joel Díaz-Reyes, Roberto Saúl Castillo-Ojeda, José Eladio Flores-Mena. Characterization of Cd1-xZnxTe (0≤x≤1) Nanolayers Grown by Atomic Layer Deposition on GaSb and GaAs (001) Oriented Substrates[J]. Journal of Electronic Science and Technology, 2019, 17(2): 97-108. doi: 10.11989/JEST.1674-862X.70929112
    • With the renewed interest of the properties of nanostructures and nanoelectronics, materials have been studied extensively, such as CdTe, ZnTe, and also the ternary alloy in the nanometric form. In addition, the combination of these materials was explored, as is the case when these materials are grown in materials that have shown to have wide applications in the field of optoelectronics, such as GaAs and GaSb substrates. The main goal is to take advantage of the properties of ZnTe and CdTe, and in addition take advantage of the optical properties of GaAs and others[1]-[5]. The CZT (CdZnTe) band gap energy can be modulated from 1.45 eV to 2.25 eV. This property is used for designing a large variety of radiation detectors[6]-[8]. In addition, the layers with nanometric dimension can be obtained with relatively not complex growth techniques such as the case of the known as atomic layer deposition (ALD), which can be implemented by using high purified hydrogen combined with a vapour phase deposition of the precursors on the substrate. So, it is not only an interesting issue but also a relatively inexpensive way of obtaining nanolayers for future optoelectronic devices fabrication. As is known the ALD growth regime can be accomplished with several growth techniques as molecular beam epitaxy (MBE), metal organic chemical vapour deposition (MOCVD), and metal organic molecular beam epitaxy (MOMBE)[9]-[13], but one of the simplest is the one used in this work. From the pointed out above, it is evident the importance of the growth and study of the nanolayers on III-V substrates obtained with this vapour phase deposition system.

    • The layers were grown on the (001) oriented GaSb and GaAs substrates. The substrates preparation was as usual: Degreased with organic solvents and oxide elimination using HCl followed by chemical polishing. In the case of the GaAs substrate, the used etching solution was H2SO4:H2O2:H2O (5:1:1) while for the GaSb substrate was used a solution based on tartaric acid, hydrofluoric acid, and hydrogen peroxide[14]. After chemical etching, the substrates were rinsed in deionized water and then dried by blowing nitrogen on their surfaces. As the next step, the substrates were introduced into the growth chamber and were placed side-by-side on a graphite rule. In order to guarantee the same growth experimental conditions on both GaSb and GaAs substrates, both substrates were exposed simultaneously at the same growth run. This was accomplished by cutting rectangular substrates of 0.5 cm×1.0 cm and placing them side-by-side to form a 1.0 cm×1.0 cm substrate. In this way, the possible factors unintentionally introduced by variations in the growth processes were eliminated, assuring the same growth experimental conditions for the epitaxial layers on both substrates and allowing the direct comparison between the layers grown on the GaSb and GaAs substrates. The experiments were carried out using a high purity Pd-diffused hydrogen flow of 300 ml/min at atmospheric pressure. The growth system used in the experiments has a horizontal geometry; the body of the reactor is made of a quartz tube, while the suceptor is made of high purity graphite. The substrates were exposed alternatively to the precursor vapour by sliding the graphite rule containing them. The elemental precursor vapour arrived on the substrate surface from little quartz tubes where these precursors were contained. In this way both of them, the anion flux and the cation flux, saturated the growth surface one after the other making the process self-regulated[15],[16]. All, the graphite suceptor, substrates, and precursor sources, were maintained at the same temperature during the growth process. The graphite rule with the substrate was coupled to a stepper motor controlled by a computer program. This motor was located outside of the growth chamber. A computer program allowed sequencing the layer growth and provided the desired exposure time. This system allowed growing samples with more than 800 layers. The procedure employed for growing the CZT structure, in this case, was based on the use of a cation source prepared making a mixture of Cd and Zn, combined by weights. Different weight combinations gave different molar composition in the final grown layer. In order to study the characteristics of the grown epitaxial layer with temperatures and thicknesses, high-resolution X-ray diffraction (HR-XRD) measurement was performed. All diffractograms were acquired using a Philips analytical diffractometer. The parameters used for scanning were CuKα1 line, λ=1.540597 Å, Δλ/λ=2×10–4, and 0.10 s as time per step, and the diffraction plane was (004) with a continuous mode scan type. All the used substrates were cleaved from the same epi-ready GaSb wafer and in the same way for the case of the GaAs substrate. For assessing the thickness of the grown ZnTe layers, ellipsometry measurement was performed using a Gaertner ellipsometer L117 with a He-Ne laser and λ=632.8 nm, by varying the incident angles and obtaining the extinction parameters. Raman scattering experiment was performed at room temperature using the 6328 Å line of a He-Ne laser at normal incidence for excitation. The laser light was focused in a circular spot of 6 μm on diameter using a 50×(numerical aperture=0.9) microscope objective. The nominal laser power used in the measurement was 20 mW. Care was taken to avoid the heating of the sample inadvertently to the point of changing its Raman spectrum. Scattered light was analysed using a micro-Raman system (Lambram model of Dilor), a holographic notch filter made by Kaiser Optical System, Inc. (model super Notch-Plus), a 256×1024-pixel charge couple device (CCD) used as the detector cooled to 140 K using liquid nitrogen, and two interchangeable gratings (600 g/mm and 1800 g/mm). Typical spectrum acquisition time was limited to 60 s to minimize the sample heating effects discussed above. Absolute spectral feature position calibration better than 0.5 cm–1 was performed using the observed position of the Si peak, which was shifted by 521.2 cm–1 from the excitation line.

    • ZnTe layers were grown in a wide range of temperatures; however, ZnTe layers with a shiny mirror-like surface should be grown at the temperatures between 370 °C and 410 °C on both GaSb and GaAs substrates. In the case of the GaAs substrate, at the temperatures higher than 410 °C the layer surface was deteriorated and showed a hazy appearance, but on the GaSb substrate the layer surface remained shiny even at the temperatures near at 420 °C. In other experiments series, the exposure time to the Zn and Te vapour sources was explored to determine the shortest exposure time that could be used for growing these layers. From these experiments, it was found that for the exposure time less than 2.5 s there was no growth on the GaAs substrate, while in the case of the GaSb substrate the shortest time was found to be 1.5 s for 385 °C. This difference in the exposure time indicates that the growth kinetic is different for the GaSb substrate compared with the GaAs substrate. Additionally, the exposure time is not determined by the transport of the reactants on the growth surface. After the experiments with the exposure time, it was adopted 3.0 s for growing the following samples and 3 s for the interruption time, as other researchers have reported[17]. The ZnTe layer thicknesses were evaluated by ellipsometry measurement. The normalized results are summarized in Fig. 1, where the presented layers were grown using different exposition cycles in an interval from 300 to 900. The horizontal scale corresponds to the growth temperatures used in the experiments. It can be observed that the average value of the thickness per cycle is around 0.3 nm that is the value corresponding to a monolayer of ZnTe. This indicates that the growth regime is the one known as ALD. As mentioned earlier, this technique leads to very good control on the layer thickness due to the tight dependence of the final thickness with the performed growth cycles.

      Figure 1.  ZnTe thickness measurements showing that the growth rate was the corresponding to a monolayer per growth cycle, for both used substrates.

    • In order to explore the sample’s surface morphology, atomic force microscope (AFM) topography images were acquired, the results of these ones are illustrated in Fig. 2. In some cases, the AFM images present some structures as artifacts due to the tip adherence and contamination. The vertical scale is presented in nanometres, it can be observed in the case of the 3D images that the maximum height is around 20 nm in the analysed area which was 2.0 µm×2.0 µm. As can be seen in the surface images presented corresponding to the grown layers of the ZnTe compound, these are highly uniform in their surface morphology. The measured roughness was 32.37 nm in terms of the average roughness and 39.69 nm as the root mean square.

      Figure 2.  AFM topography images on ZnTe epilayers grown by ALD regime: (a) typical ZnTe depth profile image grown on GaAs, (b) ZnTe same zone, (c) ZnTe 3D topography, and (d) the height distribution of ZnTe sample.

    • As a result of HR-XRD measurement, the peak corresponding to the (004) diffraction plane of ZnTe was identified and investigated. In Fig. 3, the thin peak about 33° corresponds to the (004) diffraction plane from the GaAs substrate, which was used as a reference for the ZnTe layer peak position adjustment and its comparison. The wide peaks at the left of it correspond to the (004) diffraction planes from the epitaxial layers, their intensities were amplified by 200 times due to the weaknesses of these peaks as a consequence of the thin thickness involved on the diffraction. The full width at half maximum (FWHM) of the diffraction peaks was very wide (600 arcsec to 800 arcsec) indicating a highly distorted lattice due mainly to mosaicity[18]. Additionally, in this figure as is expected it can be observed that the intensities of the peaks increase with the thickness of the layers. Moreover, as the layer thickness increases, the peak position shifts away from the peak corresponding to the GaAs substrate towards the position that must have the correspondence to the ZnTe in bulk. This effect could be attributed to the relaxation of the layers and the increase of dislocation defects in the interface, due to the fact that for highly dislocated layers, the peak position is determined by the mean distortion of the lattice[19]. For the samples whose diffractograms are showed in Fig. 3, the calculated thicknesses are ranged between 135 nm to 270 nm. In the case of the ZnTe layers grown on GaSb, it was not possible to resolve clearly the ZnTe diffraction peak from the one corresponding to the GaSb substrate. This is a consequence of the smaller lattice mismatch between the ZnTe layers and the GaSb substrate, since the lattice constants of the two materials are very close, aGaSb=6.0959 Å and aZnTe=6.1034 Å. In addition, the interface defects and the nanometric thickness of the ZnTe layers are related to the width of the diffraction peak[20],[21]. The described above is shown in Fig. 4, which presents an X-ray diffraction (XRD) curve of a typical 125 nm thick ZnTe layer that was obtained by performing 410 growth cycles on the GaSb substrate. In Fig. 4, as a result of the deconvolution treatment, the peak corresponding to the ZnTe layer near the dominant peak can be observed due to the (004) diffraction plane of the GaSb substrate. Compared with the GaSb substrate peak, the great peak width of the layer and its weakness are evident.

      Figure 3.  Diffractograms of ZnTe on GaAs where peak position shift is related with the increase of interface dislocation.

      Figure 4.  Diffraction curves of the ZnTe (004) peak compared with the substrate peak: (a) GaSb and (b) GaAs.

    • Back scattering configuration was used to obtain the Raman spectra of the (100) GaSb and GaAs substrates that are shown in Fig. 5. GaSb Raman spectrum presents a dominant band at 237 cm−1 that is associated to longitudinal optical (LO) phonon frequency of GaSb, and the other is related to metallic tellurium observed at 274 cm−1, which was found experimentally. The weak shoulder on the low-frequency side of the GaSb-LO band at 229 cm−1 is the GaSb-transverse optical (TO) mode, in principle forbidden in this experimental geometry. Its weak intensity in the Raman spectrum indicates that the crystalline quality of the Te-doped GaSb substrate is very good. Similarly, the GaAs substrate only shows a Raman peak at 293 cm−1 that is associated with the GaAs-LO mode. Raman scattering measurement was performed to the substrates in order to have a reference for the clear identification of the peaks of the grown films. For assessing the quality of the ZnTe grown layers, Raman spectroscopy measurement was performed. The obtained results are shown in Fig. 6. It is noteworthy that the first- and second-order Raman dispersion was obtained of the grown samples. The TO-ZnTe mode was forbidden in the experimental measurement configuration for the (001) substrate orientation, which becomes active by the breakdown of the selection rules in the backscattering configuration. This breakdown is attributed to the generation of structural defects as misfit dislocation in the layer-substrate interface and other crystallinity problems originated from compositional fluctuations and by elastically scattering from ionized doping impurities. Fig. 6 shows the Raman spectra of the samples grown on the GaSb and GaAs substrates. Fig. 6 (a) illustrates the deconvolution of two Raman spectra, which present the first- and second-order Raman scattering of ZnTe, corresponding to the frequencies that are shown in Table 1. In these ones, there are two dominant peaks, associated with the tellurium related bands around 123 cm−1 and 141 cm−1[22]-[24]. The discussion of Raman scattering will be divided into two parts, corresponding to the Raman of the first and second orders.

      Figure 5.  Raman spectra of the substrates used in the growth of ZnTe nanofilms.

      Figure 6.  Raman spectra of the samples grown on GaAs and GaSb substrates: (a) ZnTe Raman spectra of the samples with different thicknesses and substrates and (b) deconvolution of the measured Raman spectra into individual components.

      Order RamanMode assignationGaAsGaSb
      FirstTe2O-related126128
      Te-related144146
      LO-ZnTe208208
      Second2TA(L)-ZnTe9696
      2TA(X)-ZnTe105106
      (W1+W2)-ZnTe159159
      (TO(X)+TA(X))-ZnTe226228
      (LO(X)+TA(X))-ZnTe238238
      2LA(X)-ZnTe273275

      Table 1.  Frequencies of active Raman lines detected for the ZnTe samples grown on the two different substrates

    • ZnTe has a crystal structure of the body-centered cubic (BCC) zinc blende type with two atoms per unit cell. Thus, it would expect three degenerate optical modes of vibrations at the centre of the Brillouin zone (q=0); however, the binding is partially ionic and in this case Poulet[25] has shown that the macroscopic electric field associated with the longitudinal mode of vibration increases its frequency above that of the transverse mode of vibration. The first-order Raman spectrum consists of two bands: The highest in frequency is associated to the LO branch and the lowest is the doubly degenerate TO branch. Thus, the peak observed at 208 cm–1 corresponds to the ZnTe LO phonon[26]-[29]. The Raman spectra exhibit peaks related to the substrate characteristics. For the ZnTe Raman spectra of the layers grown on the GaSb substrate, three dominant peaks between 90 cm–1 and 150 cm–1 can be observed, which are associated with Te. As in the case of the samples grown on the GaAs substrate, the peak at 208 cm–1 corresponds to LO-ZnTe[27],[30]. In addition, as can be observed in Fig. 6 (b), the weakness of the peak corresponding to LO-ZnTe is slightly displacement in comparison with the reported for the ZnTe bulk, possibly due to the layers stress and the nanometric nature of these layers, as has been reported by other authors[24].

    • In the second-order scattering process, the energy of the scattered photons differs from the energy of the incident photons by the energy of two vibrational quanta. The two phonons involved can be from the same branch (overtones) or from different branches (sums or differences). The wave vector of the phonon involved is usually orders-of-magnitude larger than that of the photons and conservation of crystal momentum. Therefore it is required that the wave vectors of the two phonons are equal and opposite in sign. Thus, the two phonons involved, although they may be from different branches of the spectrum, must come from the same point in the zone. This of course still allows two phonon scattering from any point in the zone and the scattered spectrum will consist of a continuous background with peaks corresponding to critical points in the two-phonon density of states[31].

      The critical points in the zinc blende phonon spectrum[32] are at the points Γ(0, 0, 0), X(1, 0, 0), L(1/2, 1/2, 1/2), and W(l, 1/2, 0) in the Brillouin zone. Birman[32] has worked out the selection rules for the second-order Raman scattering from these critical points in zinc blende structures. Fig. 6 observes the features of the second-order spectrum and Table 1 contains a list of the frequencies corresponding to these features. Column 2 of Table 1 gives the mode assignments of these features. The features below 160 cm–1 could be due to the combinations of transverse acoustic phonons (sums and overtones) or to differences such as LA(X)-TA(X).

      In these experiments, it was observed that the width of all the features below 160 cm–1 decreased at the same rate with increasing temperatures, and in addition was strongly polarized. On the basis of these observations the low-energy features were assigned to overtones and sums of transverse acoustic phonons.

    • In the case of CdTe samples, these were grown on the (100) oriented GaAs and GaSb substrates as in the ZnTe case, although the range of growth temperatures for the CdTe deposition was slightly different. Temperatures higher than 400 °C were necessary for the CdTe growth. As in the ZnTe case, the exposure time under the vapour sources was selected as 3 s. Figs. 7 and 8 show the XRD curves corresponding to the (004) CdTe diffraction plane for the samples obtained with different growth temperatures and different substrates. All the presented samples have the same number of grown cycles which is directly related with the layer thickness. The sample labelled as CdTe29 was grown at 405 °C, which is close to the minimum temperature required for the layer growth. As shown Fig. 7, the (004) plane diffraction peak presents a shift toward angles corresponding to lower lattice constants, which is indicative of stress accumulation. In comparison, the sample grown at 415 °C shows a shift corresponding to the increase of the lattice constant, which suggests that the stress produced by the lattice constants difference between the layer and substrate has been relaxed due to the dislocation and defects generation mechanism, favoured by the increased used growth temperature. In addition, the defects propagated from the substrate and the interface layer-substrate.

      Figure 7.  Influence of the growth temperature of CdTe/GaAs on the diffraction peaks position.

      Figure 8.  Influence of the growth temperature of CdTe/GaSb on the diffraction peaks position.

    • Fig. 9 shows the Raman spectra for CdTe nanolayers excited by 532 nm wavelength light. Two Raman spectra were fitted with Lorentzian lines (black circles) that are shown in Fig. 9 (b). The peaks at 95 cm–1 to 98 cm–1, 120 cm–1 to 125 cm–1, and 140 cm–1 to 143 cm–1 have been identified from trigonal Te[31]. The 120 cm–1 to 125 cm–1 peak is due to a phonon with A1 symmetry, and the 95 cm–1 to 98 cm–1 and 140 cm–1 to 143 cm–1 peaks are originated from phonons with an E symmetry[32]. However, the 140 cm–1 to 143 cm–1 peak could also be assigned to the TO phonon in CdTe, because it is very close to the reported one[31]-[37]. According to the Raman studies of CdTe[34],[38], the peak located at 157 cm–1 to 160 cm–1 can be assigned to the LO phonon of CdTe, in spite of a difference from 171 cm–1 and 167 cm–1 reported by Islam et al.[39] and Amirtharaj et al.[34], respectively. To the best of our knowledge, there have been no reports of the assignment for the broad band located at 265 cm–1 to 270 cm–1, which probably originates from the combination bands and one tentatively assigns them to the overtones of E and A1 modes in Te. The 295 cm–1 peak could be assigned at LO-GaAs. The peak at 321 cm–1 is associated with the phononic replica of the LO mode of the CdTe[40]. The results of the measurement are summarized in Table 2. A high concentration of tellurium exists in the as-prepared CdTe nanolayers, as shown by the Raman study[41]. Similar phenomena were reported in the CdTe film synthesized through the liquid-phase epitaxy method[39] as well as the surface of single crystal CdTe in bulk form[34]. One possible explanation could be that the free energy of Te is smaller than that of CdTe and thus Te crystals can be easily formed during the synthesis of CdTe. However, further study is still needed to confirm this one. One implication of it is that the surface property of the nanolayers may not be as expected. Isolated elemental domains (Te) are formed or precipitated on the surface of nanolayers; it may affect their applications in optoelectronic devices since its elemental properties are different from its compound properties.

      Order RamanMode assignationGaAsGaSb
      FirstTe-related9895
      Te2O-related125120
      TO-CdTe143140
      LO-CdTe157160
      UI189
      LO-GaAs295
      2LO-CdTe321
      Second(E+A1)-CdTe270264

      Table 2.  Frequency of active Raman lines detected for the CdTe samples grown on the two different substrates

      Figure 9.  Raman spectra for CdTe nanolayers: (a) with different thicknesses and substrate and (b) the decomposition of the measured spectra into individual components for two samples grown on different substrates where the circles lines are their respective fitting.

    • After successfully obtaining the ZnTe and CdTe layers, it was proceeded to grow the layers of Cd1-xZnxTe ternary alloy, which was on the (100) oriented GaAs substrate. The used temperature range was between 400 °C and 425 °C. In order to grow the ternary alloy layers, a precursor was used as a solid source prepared by weighing the corresponding amounts of zinc and cadmium and mixing them. The procedure for preparing the precursor source was accomplished by combining Zn and Cd in different amounts which then were melted into a liquid forming at high temperatures. The liquid was then cooled rapidly for its solidification to form a new solid. The resulting solid was used as the cation precursor source. Its vapour pressure was controlled by controlling the temperature. The samples studied in this work and presented in Fig. 8 used the combinations of Zn and Cd weights presented in Table 3, in which, the column labelled by “x in the grown layer” represents the final molar composition of the solid Cd1-xZnxTe layers. The molar composition of these layers were evaluated by using the Vergard’s law a(x)=6.103x+6.482(1–x), so, the Zn molar fraction x was determined by the observed D-values obtained from the positions of the respective (004) diffraction peaks. The results of the measurement are summarized in Table 3, from which it can be observed that with the increase of the molar concentration x of the cation solid source, the molar concentration in the final grown solid layer was increased. The obtained concentrations using these three different solutions were: For the sample labelled as T1 was 92.5%, for the sample T5 was 90.9%, and for the sample T7 was 89.7%. From these results, it can be concluded that different molar concentrations of Zn in the layers of the ternary alloy can be obtained by preparing the solid precursor source with different Zn and Cd combinations. In the presented samples, the Zn molar composition was varied from 83.2% to 43.4%, but the variation of the molar composition of the grown layer was from 92.5% to 89.7%, respectively. FWHM of these peaks is above 3600 arcsecs, which can be considered large and attributed to mosaicity, combination with the nanometric dimensions of layers, and the generation of misfit defects.

      SampleSolid sources composition (%)x in the grown layer (%)
      ZnCd
      T183.216.892.5
      T558.042.090.0
      T743.456.689.7

      Table 3.  Cation source composition used for growing the ternary Cd1-xZnxTe

    • This work reported the successful growth of ZnTe, CdTe, and CdZnTe on both GaAs and GaSb substrates by ALD regime as shown by the thickness measurement performed by ellipsometry. Additionally, it was found that the growth kinetic of the binary compounds was different for each substrate, which influenced the growth time necessary to obtain mirror-like nanofilms. CdTe and the ternary alloy Cd1–xZnxTe were grown successfully by the same growth method, by preparing a mixture of cadmium and zinc, melting them, and freezing quickly, so it was possible to form a solid to be used as a cation source and to grow the ternary alloy. Raman spectroscopy and HR-XRD were used in order to assess the crystalline quality of the grown layers. From the measurement it was possible to observe, from the position and width of the peaks, the formation of defects due to the relaxation mechanism in the interface between the layer and substrate. In the case of the ternary alloy, it was shown that the molar fraction x of the grown layer was varied by modifying the composition of the solid used as the cation source. Although, from the presented results it can be appreciated that the relation of the molar composition of the final layer is not linear with the composition of the used cation source. As in the case of the ZnTe, CdTe, and Cd1–xZnxTe samples, XRD showed the dependence of the (004) diffraction peak position with temperatures and sample thicknesses, which can be attributed to the lattice relaxation produced by the increase of defects and the propagation of the defects from the substrates.

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