Effective Mg Incorporation in CdMgO Alloy on Quartz Substrate Grown by Plasma -assisted MBE A. Adhikari1 A. Wierzbicka1 A. Lysak1 P. Sybilski1 B. S. Witkowski1 and E . Przezdziecka1

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Effective Mg Incorporation in CdMgO Alloy on Quartz Substrate

Grown by Plasma-assisted MBE

A. Adhikari*,1, A. Wierzbicka1, A. Lysak1, P. Sybilski1, B. S. Witkowski1, and E. Przezdziecka1

1Institute of Physics, Polish Academy of Sciences, Al. Lotnikow 32/46, Warsaw, Poland, 02-668

Email: adhikari@ifpan.edu.pl

Abstract

The development of CdMgO ternary alloy with a single cubic phase is challenging but meaningful

work for technological advancement. In this work, we have grown a series of Cd1-xMgxO ternary

random alloys with various Mg concentrations (x = 0, 30, 32, 45, and 55%) on quartz substrate by

plasma-assisted molecular beam epitaxy (PA-MBE) technique. The structural investigations of

alloys were performed using the X-ray diffraction (XRD) technique. The decreases in average

crystallite size and lattice parameters were observed with an increase in Mg content in the alloys.

XRD analysis confirms a single cubic phase is obtained for alloy compositions. The elemental and

morphological studies were carried out using energy dispersive x-ray (EDX) spectroscopy and

atomic force microscope (AFM) technique, respectively. The optical investigation was carried out

using UV-Vis spectroscopy. The optical bandgaps were estimated using the Tauc relation and it

was varied from 2.34 eV to 3.47 eV by varying the Mg content from zero to 55% in the alloys.

The Urbach energy increases from 112 meV to 350 meV which suggests a more disordered

localized state with an increase in Mg incorporation in the alloys.

Keywords: oxides, ternary alloys, molecular beam epitaxy, urbach energy

1. Introduction

The oxide semiconductors have gained more attention among the other semiconductor

materials because of their potential applications in optoelectronic industries like flat panel displays

(FPDs) [1], thin-film transistors (TFTs) [2], photodetectors [3,4], LEDs [5], sensors [6,7], etc.

Metal oxide semiconductors that have a large bandgap energy value, have a better tolerance against

high voltage. Apart from that, these are relatively stable in an atmospheric environment which can

make use of the fabrication process easy [8]. Among the oxide semiconductors, group-II oxides

receive increasing interest as they share similar features with group-III nitrides which could be

very promising for semiconductor industries. From the early 90’s research on wurtzite ZnO as an

alternative to the GaN system has started and still is going on because of its wide direct band gap

of 3.37 eV and large exciton binding energy of 60 meV which is higher than in GaN (binding

energy 25 meV) [9]. The rise in research not only about the basic properties of ZnO but also on

the related group II-VI semiconductor oxides like MgO and CdO which can be combined to form

heteroepitaxial device structure. The large bandgap tunability, stable structure, and ease of

fabrication process make the ZnO-CdO-MgO system a possible alternative to nitrides [10].

Among these metal-semiconductor oxides, CdO is considered to be a promising material

for photovoltaic applications due to its unique features like stable cubic rock-salt (RS) structure,

high transparency, high exciton binding energy of 75 meV, a high dielectric constant of 21.9, and

high electron mobility of 180 cm2V-1s-1 [11]. However, a relatively small direct bandgap of 2.3 eV

along with two indirect bandgaps of 1.2 eV and 0.8 eV [12] at room temperature is a key problem

that restricts its uses in a shorter wavelength regime. On the other hand, MgO is a direct bandgap

(7.5 eV) material with a high exciton binding energy of 80 meV [13] that crystallizes in the cubic

rocksalt structure. Thus, MgO can be combined with CdO by the meaning of alloying to form

various heterostructures such as ternary alloys, CdO/MgO superlattice structures (SLs), etc. which

can be useful for a variety of optoelectronic devices over a wide spectral range. Previously Chen

et al. [14] studied the change in the electronic structure of CdMgO alloys on glass substrates

prepared by radio frequency magnetron sputtering technique. Guia et al. studied Mg-rich to Cd-

rich CdMgO alloys on a sapphire substrate prepared by the metal-organic chemical vapor

deposition (MOCVD) technique and reported a phase separation at Cd concentration above 27%

[15,16]. Przezdziecka et al. studied quasi alloys short-period {CdO/MgO} SLs grown on sapphire

substrate by plasma-assisted molecular beam epitaxy (PA-MBE) technique and found out that the

bandgap of SLs can be varied from 2.6 eV to 6 eV by changing the CdO sublayer thick-ness at a

constant MgO layer thickness [17,18]. Recently, Adhikari et al. reported CdxMg1-xO ternary alloys

on m- and c-plane sapphire substrate grown by PA-MBE technique. They have found that the

direct bandgap can be varied from 2.42 eV to 5.5 eV by changing Mg content from 0 to 96% [19].

For the design of optoelectronic devices, optical coatings, and the manufacture of solar

cells, a limited number of suitable materials are available. Few theoretical articles have been

reported in recent years that indicate the possible tunability of the energy band gap of CdMgO and

stability of the cubic rock-salt structure across a full range of magnesium which can prove useful

from a practical point of view [20]. The development of new oxide-based devices requires

improving our knowledge about ternary alloys like CdMgO. Previously, there have been some

studies for quasi-ternary alloys based on ZnO/MgO [21,22] and ZnO/CdO [23,24] structures where

composition inhomogeneity can be overcome by growing superlattice structures. However, there

is still limited work being carried out on CdO-MgO alloys [25–27]. In this paper, we have exploited

CdMgO layers deposited on quartz substrate by plasma-assisted MBE technique. In particular, in

the MBE method obtained layers are hydrogen-free. In distinguishing from other methods e.g.:

MOCVD where metalorganic precursors are the source of the hydrogen and carbon contamination

in the samples, the number of contaminations in the MBE method is low. Both the substrates and

the growth method influence the crystallographic and optoelectrical parameters of the

semiconductors [28–30]. In particular, crystalline substrates can impose a preferential layer

orientation, as it was observed for CdMgO layers on differently oriented sapphire substrates [19].

It is, therefore, reasonable to characterize CdMgO samples obtained by different methods grown

on different substrates including amorphous substrates. We have chosen the quartz substrate as a

cheaper alternative to the sapphire substrate for our study. Moreover, the amorphous substrate does

not dictate preferential orientation in CdMgO films by the orientation of the substrate. The forced

orientation on amorphous substrates is related only to the thermodynamics of growth.

Simultaneously, the use of a transparent quartz substrate allows for a relatively easy study of the

energy gap of the layers by the optical transmission method. The Mg content in the alloys was

varied by changing the growth condition e.g.: Mg and Cd fluxes ratio. Structural, morphological,

and optical investigations of these samples were carried out in order to deeper characterize CdMgO

ternary material.

2. Experimental Methods

A Riber Compact 21B MBE system equipped with Mg and Cd effusion cells, and

radiofrequency oxygen plasma cells were used to grow CdMgO random alloys on a quartz

substrate. As the sources, Mg (6N purity, from PUREMAT technologies) and Cd (6N purity, from

JX Nippon Mining & Metal Corporation) ingots were used. The substrates were cleaned with

acetone and next with isopropanol for 20 minutes followed by deionized water in order to remove

organic contaminations and next dried with pure 5N N2 gas. During the growth process, the Cd

and Mg fluxes were controlled by varying the effusion cell's temperatures. The temperature of the

Cd effusion cell was fixed at 380oC whereas, the Mg effusion cell temperature was varied from

TMg= 500oC, 510oC, 520oC, and 530oC for samples S2 to S4 respectively. The growth process was

carried out at 360oC measured by thermocouple with a constant oxygen flow of 2.5 ml/min at a

fixed 400 W radio frequency power of the oxygen plasma.

The structural investigations of these ternary alloys were performed using a PANalytical

X’Pert Pro MRD, X-ray diffraction technique. Cu anode used for CuKα1 radiation of 1.5406Å

wavelength. The diffractometer is equipped with a Ge analyzer and a 2-bounce Ge (200) hybrid

monochromator. Elemental compositions in the alloys were investigated using a Hitachi SU-70

scanning electron microscope equipped with Thermo Scientific energy-dispersive X-ray (EDX)

spectrometer. The excitation energy was set at 6 keV, which allowed for effective excitation and

measurement of the composition of magnesium and cadmium, and at the same time minimized the

generation of a signal from the substrate that could disturb the measurement. The change in growth

parameter (i.e., Mg effusion cell temperature) results in, different Mg content Cd1-xMgxO ternary

alloys. The surface morphology was studied using Atomic force microscopy (AFM) in tapping

mode (Bruker Dimension Icon model). The optical investigations of these alloys were performed

at room temperature using a Carry 5000 UV-Vis/NIR spectrophotometer equipped with a PbS

detector from Agilent Technologies in a 200-600 nm wavelength range.

3. Results and Discussions

3.1. Structural property

Figure 1(a) shows the structural properties of CdMgO alloys on quartz substrate using X-

ray diffraction (XRD) 2θ scan over a range from 20o to 100o. XRD results confirmed that all the

samples were cubic rocksalt (RS) structures without any additional impurity phases. For pure CdO

(S1), the 111 and 200 diffraction peaks are identified from the diffraction angle at about 33o and

39o respectively-indicates the polycrystalline character of the structure. These identified reflections

are to originate from the cubic RS structure of CdO and are in good consistency with X-ray

diffraction JCPDF card no. 652908. Furthermore, there are no additional peaks observed co-related

to the quartz substrate in XRD analysis. XRD peak positions of pure CdO and MgO with the cubic

structure are marked with a dotted line in Fig. 1(a). A systematic shift of (111) and (200) diffraction

peaks towards a higher diffraction angle have been observed with an increase in Mg content in the

alloy (Fig. 1(b)). In case of layers obtained by pulsed laser deposition (PLD) technique on quartz,

the films grown at low-temperature show preferred orientation along (111) direction, while the

films grown at high temperature have preferred orientation along (200) direction [31]. The two

orientations 111 and 200 were reported in a temperature range of about 400ºC. Our analysis

indicated that preferential crystallographic orientation was also correlated with Mg concentration

in the alloy. The areal comparison of 111 and 200 diffraction peaks is shown in Fig. 1(c). Due to

the increase of the Mg content, the contribution from the 111 orientation increases compared to

200 orientation which reveals that the preferential orientation is strongly influenced by the Mg

content in the alloys. Previously, Adhikari et al. [19] reported the presence of double XRD peaks

in CdxMg1-xO film on m- and c-plane sapphire grown by plasma-assisted MBE technique. It was

evidence of the existence of regions with two different concentrations in the CdMgO ternary

layers. Their study suggested that both growth conditions and the orientation of substrates play an

important role in the homogeneity of the CdMgO layer. However, in this study, no such behavior

has been observed. It was observed that the FWHM of XRD CdMgO peaks on quartz substrates

are much higher than it was reported for sapphire substrates. The observed broadening of the

diffraction peak with an increase in Mg content in the alloy, clearly indicates that the incorporation

of Mg ion into the CdO lattice can be not fully homogenous. Moreover, the noisy diffraction peak

suggests the not-so-good crystallinity of the prepared alloy on the quartz substrate compared to

previously studied samples on sapphire substrates [19].

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EffectiveMgIncorporationinCdMgOAlloyonQuartzSubstrateGrownbyPlasma-assistedMBEA.Adhikari*,1,A.Wierzbicka1,A.Lysak1,P.Sybilski1,B.S.Witkowski1,andE.Przezdziecka11InstituteofPhysics,PolishAcademyofSciences,Al.Lotnikow32/46,Warsaw,Poland,02-668Email:adhikari@ifpan.edu.plAbstractThedevelopmentofCdMgOter...

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Effective Mg Incorporation in CdMgO Alloy on Quartz Substrate Grown by Plasma -assisted MBE A. Adhikari1 A. Wierzbicka1 A. Lysak1 P. Sybilski1 B. S. Witkowski1 and E . Przezdziecka1

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