Structural, Morphological, Optical and Electrical Properties of Zn_(1-x)Cd_xO Solid Solution Grown on a- and r-Plane Sapphire Substrate by MOCV

Afif Fouzri, Mohamed Amine Boukadhaba, Al Housseynou Tauré, Nawfel Sakly, Amor Bchetnia, Vincent Sallet

Group Study of Condensed Matter, CNRS/University of Versailles Saint Quentin en Yvelines, Paris, France.
Laboratory of Physical Chemistry Interfaces, Department of Physics, Faculty of Sciences of Monastir, University of Monastir, Monastir, Tunisia.
Laboratory of Physical Chemistry Materials, USCR “High Resolution X-Ray Diffractometer”, Department of Physics, Faculty of Sciences of Monastir, University of Monastir, Monastir, Tunisia.
Research Unit of Heteroepitaxy and Its Applications, Department of Physics, Faculty of Sciences of Monastir, University of Monastir, Monastir, Tunisia.

DOI: 10.4236/jcpt.2013.31006                

Abstract

Zn_(1-x)Cd_xO films have been grown on (a-plane) and (r-plane) sapphire substrate by metal organic chemical vapor deposition. A maximum cadmium incorporation of 8.5% and 11.2% has been respectively determined for films deposited on a- and r-plane sapphire. The optical transmission spectra and energy band-gap equation established by Makino et al. were used to estimate the cadmium mole fraction of the solid solutions. Structural, morphological and optical properties of these films were examined using high resolution X-ray diffraction (HRXRD), atomic force microscopy (AFM) and room and low temperature photoluminescence (Pl) as Cd incorporation and employed substrate. X-ray diffraction study revealed that all films had wurtzite phase but solid solution grown on a-plane sapphire are polycrystalline with a preferred orientation along the [0001] direction and a-plane film are epitaxially grown on r-plane sapphire. AFM image show significant differences between morphologies depending on orientation sapphire substrate but no significant differences on surface roughness have been found. The near band-edge photoluminescence emission shifts gradually to lower energies as Cd is incorporated and reaches 2.916 eV for the highest Cd content (11.2%) at low temperature (20 K). The room temperature hall mobility decreases with the Cd incorporation but it is larger for Zn_(1-x)Cd_xO grown on r-plane sapphire.

1. Introduction

As a direct wide-band-gap semiconductor, ZnO has received increasing attention due to its potential applicability to optoelectronic devices such as ultraviolet (UV)- light emitting diodes (LEDs) and laser diodes (LDs) [1,2]. Since the first reports on ZnO-based heterostructures the issue of band gap engineering, as a means to control the actual device emission energy, was addressed [3-6]. ZnO has an ability to modulate the band gap to lower level by alloying with CdO [7]. The growth of both solid solutions presents the difficulty of combining materials with different crystalline structures, on the one hand hexagonal ZnO and then cubic CdO. Thus, the achievement of high Cd concentrations represents a challenge for crystal growers, since these growth problems lead to phase separation.

Most of the studies deal with c-plane oriented thin films. However, devices based on [0001]-oriented wurtzite materials are known to present spontaneous and piezoelectric electrostatic fields which spatially separate electrons and holes in the active layers and, thus, limit the device quantum efficiency [8]. Therefore, alternative growth orientations have been recently proposed with the polar [0001] direction within the growth plane [9,10] and quantum wells (QWs) free of electric fields have already been demonstrated [9-11].

In this paper, we analyze the structural, morphological, optical and electrical properties of Zn_(1−x)Cd_xO solid solution grown by metal organic chemical vapor deposition (MO-CVD) on and -plane sapphire. The effect of increasing Cd concentration on the optical properties of the films has been evaluated by photoluminescence (Pl), while high-resolution X-ray diffracttion (HRXRD), atomic force microscopy (AFM) has been used to analyze the structural properties and morphology of Zn_(1−x)Cd_xO layers as function of cadmium concentration. The electrical property was investigated by Van der Pauw Hall measurements at room temperature.

2. Experimental Details

The layer is grown in horizontal MO-CVD reactor at atmospheric pressure under N₂ carrier gas. Diethyl-Zinc (DEZn), Dimethyl-Cadmium (DMCd) and tertiary butanol (ter-butanol) are used as Zn, Cd and oxygen precursors, respectively at a growth temperature of 380˚C. The growth conditions are described elsewhere [12]. With similar growth parameters, thin films of Zn_(1−x)Cd_xO are directly deposited on aand r-plane sapphire substrates from Crystec. The cadmium incorporation is obtained by using different flux ratios between DMCd and DEZn while the DEZn partial pressure is kept constant. The growth parameters of two series of four samples are listed in Table 1. We will note in the following, the first series of solid solution deposited on a-plane sapphire substrate by MSAi and the second series deposited on r-plane sapphire substrate by MSRi, where i is the manipulation number.

Thickness of ZnCdO film deposited on aand r-plane sapphire substrate are respectively about 2.8 µm and 2.2 for MSA4 and MSR4 (Figure 1). They clearly show

Table 1. MOCVD growth parameters, energy band gap (E_g) and cadmium concentration at % of Zn_(1−x)Cd_xO solid solutions deposited on aand r-plane sapphire substrate.

the non-uniformity of layers thickness. These samples are characterized by optical transmission measurements in the range 360 - 690 nm using a DR/4000U spectrophotometer which can return either the absorption coefficient or the transmittance in percentage.

HRXRD experiments were performed with D8 discover Bruker AXS diffractometer using CuKa1 radiation at 1.5406 Å and the surface morphology of our film was observed by AFM. All the images were recorded with a in the tapping mode (25˚C, in air). All the measurements were carried out at room temperature. Photolumines Nanoscape III a microscope from digital instruments Inc. cence (Pl) measurements were made for different layers using the 325 nm line of He-Cd laser at room and low

Figure 1. Cross section SEM images of MSA4 and MSR4. Boundaries and average thickness of each layer is reported on each image.

temperature.

3. Results and Discussion

3.1. Cadmium Incorporation

To determine the optical band gap E_g, we have used Tauc et al.’s plot [13] where the absorption coefficient a is a parabolic function of the incident photon energy (E = hn) and optical band gap E_g. This relation is given by:

(1)

where A is function of refractive index of the material, reduced mass and speed of light.

The plot of (aE)² as a function of the energy of incident radiation for the Zn_(1−x)Cd_xO solid solution deposited on a-(MSAi) and r-(MSRi) plane sapphire substrate (i = 1, 2, 3 and 4) has been shown in Figure 2.

The energy band gap is obtained from intercept of the extrapolated linear part of the curve with the energy axis.

As seen in Figure 2, the Zn_(1−x)Cd_xO films shows

Figure 2. Plots of (aE)² as a function of photon energy (E) for the Zn_(1−x)Cd_xO solid solution deposited on a-(MSAi) and r-(MSRi) plane sapphire substrate (i = 1, 2, 3 and 4).

shrinkage in energy gap, which provides supportive evidence that Cd incorporates in ZnO. The cadmium concentrations in these layers deposited on aand r-plane sapphire substrate are determined from energy band gap (E_g) equation established by T. Makino et al. [6], where we have used respectively E_g (x = 0) deduced from sample MSA1 and MSR1. The corresponding values of cadmium concentration at % have been given in Table 1. In our previous work [14], the Cd incorporation in Zn_(1−x) Cd_xO has been shown to be nearly twice as high on yield on aand r-plane as the Cd incorporation yield obtained on c-oriented substrate, indicating Cd incorporation is favored in the non polar orientation.

3.2. Structural Properties

Figure 3 shows the X-ray diffraction (XRD) pattern for ZnCdO grown on aand r-plane sapphire substrate. The 2q - q scan revealed that all the films had wurtzite phase and no indications of any rocksalt phase related to segregate CdO within the layers are detected. The pattern of MSAi (i = 1, 2, 3 and 4) (Figure 3(a)) showed, in addition to substrate peaks and, peaks located at 2q = 31.25˚, 34.32˚, 36.03˚, 72.35˚ and 76.55˚ which are respectively assigned to the peaks, (0002), , (0004) and of layer. The layer peak (000l) intensities are more important than others revealing the presence of a preferred orientation along the direction [0001] that coincides with the orientation of the sapphire substrate. However, in addition to the reflection and its harmonic from the r-plane sapphire substrate, only the ZnCdO reflection and its harmonic were observed (Figure 3(b)) which confirms the a-plane orientation of the layer [12,15-18].

We also note in Figure 3 (right one), a slight shift of layer peaks to small angles in function of x cadmium composition increase. The mosaicity of the film can be characterized by measuring the corresponding w-rocking curve of the layer peak diffraction, which is shown in Figure 4.

The full widths at half maximum (FWHMs) of the layers peak were reported in Table 2. FWHM increases with Cd composition indicating a degradation of the crystalline quality of the layers. For maximum Cd composition obtained, FWHM is about 1.27˚ for sample MSA4 which is higher compared with 0.90˚ for sample MSR4 although the Cd composition in this sample is more important than that of MSA4.

We have used high resolution X-ray diffraction on symmetric and asymmetric reflections which enables us to precisely measure layer lattice parameters. The lattice parameters of Zn_(1−x)Cd_xO deposited on aand r-plane sapphire substrate are listed in Table 3. The experimental

(a)(b)

Figure 3. 2q - q scan pattern of Zn_(1−x)Cd_xO solid solution deposited on (a) a-(MSAi) and (b) r-(MSRi) plane sapphire substrate (i = 1, 2, 3 and 4).

errors of lattice parameter are estimated at 0.003 Å. The a-, c-axis lengths determined by HRXRD, the ratio c/a and the cell volume are plotted as functions of Cd concentration in Figures 5(a)-(d) respectively. Corresponding parameters of bulk ZnO (a₀ = 3.2495 Å and c₀ = 5.2062 Å) [19] are also represented in dashed line and the solid lines are the linear fit to the corresponding experimental values.

The increase of lattice constant is due to the fact that the covalent radius of Cd²⁺ (1.48 Å) is larger than that of Zn²⁺ (1.25 Å) and therefore the substitution of Zn²⁺ ions by Cd²⁺ induces a lattice expansion [20,21]. We note a practically linear variation of lattice parameters, c/a ratio and cell volume according to Cd composition. We note that the variation as function of x cadmium content of layer lattice parameter a deposited on a-plane sapphire

Table 2. FWHMs of layer Zn_(1−x)Cd_xO peak deposited on (a) aand (b) r-plane sapphire substrate for each cadmium concentration at % obtained.

Table 3. Lattice parameters in Zn_(1−x)Cd_xO solid solutions deposited on aand r-plane sapphire, c/a ratio and cell volume as the function of cadmium concentration at %.

substrate is nearly twice as great as than that deposited on r-plane. On the contrary, we find that the evolution as function of x cadmium content of layer lattice parameter c deposited on r-plane is just over 4 times larger than that deposited on a-plane. This difference is clearly seen in Figure 5(c) where the ratio c/a decreases as function of x for samples MSAi whereas it increase for MSRi. But the increase in cell volume of the two samples series as cadmium x content is close. At high cadmium incorporation, the cell volume varied respectively by 1.7% and 2.33% for MSA4 and MSR4 from that of bulk ZnO. For the layer MSA4, this variation is in good agreement with the value (1.8%) obtained by Zûñiga-Pérez et al. [12] for the same x cadmium incorporation but the solid solution is deposited on r-plane sapphire substrate. In case of MSR4, the percentage variation 2.33% is close to that calculated (2.4%) by the quadratic fit dependence estab-

(a)(b)

Figure 4. w-rocking curve of layer peak diffraction deposited on (a) aand (b) r-plane sapphire substrate for each cadmium concentration at % obtained.

lished in reference [12].

So the maximum attained cell volume variation of 2.33% is achieved before phase separation occurs (x_max = 11.2%), whereas it was of 1.8% for polycrystalline Zn_(1−x)Cd_xO film and respectively 1.7% and 0.9% for c-oriented layer grown on ZnO and c-plane sapphire substrate [22].

In order to compare the effect of Cd incorporation along [hkil] direction, we define the strain in the Zn_(1−x)Cd_xO layer as:

(2)

where is the periodicity along the [hkil] direction of the ZnCdO film and bulk ZnO [18]. Therefore and by considering the epitaxial relationships of ZnO on r-plane [12,18,19,23-27]:

(a)(b)(c)(d)

Figure 5. Cadmium content x dependence of (a) a-axis lattice length, (b) c-axis lattice length, (c) the ratio c/a and (d) the cell volume. In all cases the dashed line represents the corresponding parameters of bulk ZnO [19]. In all case, the solid lines are the linear fit to the corresponding experimental values.

(3)

the deformation out-off the growth plane, i.e. in the direction is:

(4)

and that in the growth plane are: and

(5)

a₀ and c₀ are lattice parameters of ZnO completely relaxed (bulk) [19]. Figure 6 shows the strain in MSRi layers as function of x cadmium content. The strain

Figure 6. Epitaxial strain and of ZnCdO/r-plane sapphire as a function of cadmium content x. the solid lines are the linear fit to the corresponding experimental values.

shows a linear dependence on Cd content. The slope of the fitting curves of strain parallel to [0001] is found to be 3.5 times larger than that parallel to, which indicates that the Cd incorporation induces greater lattice deformation along [0001] than. In our case, the deformations and are identical because we could not quantify any difference between the periodicity along both direction and, if it exists, for experimental errors.

3.3. Surface Morphology

Figure 7 shows the film morphology of Zn_(1−x)Cd_xO grown on aand r-plane sapphire substrate (scan area 5 µm ´ 5 µm) as function of cadmium content.

Conflicts of Interest

The authors declare no conflicts of interest.

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