anomalous dependence of photoluminescence properties on composition x in cd1–xmnxga2s4 mixed...

phys. stat. sol. (c) 3, No. 8, 2915–2918 (2006) / DOI 10.1002/pssc.200669622

© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Anomalous dependence of photoluminescence propertieson composition x in Cd1−xMnxGa2S4 mixed crystals

Hyun Park1, Young-San Kim2, Seung-Cheol Hyun3, Chang-Dae Kim∗4, Moon-Seog Jin5,Duck-Tae Kim6, Kiwan Jang7, Hyung-Gon Kim8, and Wha-Tek Kim9

1 Department of General Education, Mokpo Science College, Mokpo 530-730, Republic of Korea2 Department of Radiological Technology, Mokpo Science College, Mokpo 530-730, Republic of Korea3 Department of Opthalmic Optics, Sunghwa College, Kangjin 527-812, Republic of Korea4 Department of Physics, Mokpo National University, Mokpo 534-729, Republic of Korea5 Department of Physics, Dongshin University, Naju 520-714, Republic of Korea6 Department of Electronics, Dong-A College, Youngam 526-870, Republic of Korea7 Department of Physics, Changwon National University, Changwon 641-773, Republic of Korea8 Department of Elec. Eng., Chosun University Technical College, Gwangju 501-759, Republic of Korea9 Department of Physics, Chonnam National University, Gwangju 500-757, Republic of Korea

Received 24 February 2006, revised 10 April 2006, accepted 9 June 2006Published online 4 September 2006

PACS 78.55.Hx

Photoluminescence spectra of Cd1−xMnxGa2S4 mixed crystals were investigated in the composition regionof 0.00 ≤ x ≤ 1.00 and in the temperature region of 6 to 300 K. The photoluminescence spectrum ofCdGa2S4(x = 0.00) showed two emission bands at 594 nm (2.09 eV) and 460 nm (2.70 eV), whichwere attributed to be due to donor-acceptor pair recombination. The same feature of these bands was alsoobserved for the crystals doped with 5 mol% Mn, excepting a slightly red shift and a decreasing of the half-width of the emission band in the longer wavelength region. For the crystals in the composition region ofx ≤ 0.67, only the emission band in the longer wavelength region were observed. The photoluminescencespectra of MnGa2S4(x = 1.00) showed a broad emission band 675 nm (1.838 eV), which is different fromthat of CdGa2S4. The relative intensity of the main emission band increased in the composition region of0.00 ≤ x ≤ 0.67 with increasing x while decreased in the region of 0.90 ≤ x ≤ 1.00. The anomalouscomposition dependence was connected with an occurrence of a miscibility gap of the crystal structure inthat region.

1 Introduction Cd1−xMnxGa2S4 is a new quaternary solid solution based on CdGa2S4 and MnGa2S4

which are ternary compounds with the general formula AB2C4. The CdGa2S4 is a wide-gap semiconduc-

tor (Eg = 3.5 eV at 300 K) with high photoconductivity and strong luminescence in the visible region [1-3].

Also, MnGa2S4 belongs to the class of materials referred to as semimagnetic semiconductors or diluted

magnetic semiconductors [4, 5]. Therefore, the Cd1−xMnxGa2S4 system is expected to be an interesting

material with semiconducting, as well as magnetic properties for more applications in optoelectronic de-

vices, such as II1−xMxVI (M = Co, Fe, Mn, etc) containing a fraction x of magnetic ions substitutionally

placed on the group II sites. In addition, the possibility of tuning the energy gap and the lattice constants

by means of the Mn content in the system may lead to future applications.

Some works have been done on the series A1−xMnxB2C4 [6-8]. In a previous work [9], we investigated

the composition dependence of the structural and optical properties of Cd1−xMnxGa2S4 mixed crystals.

In this paper, we report on the anomalous variations of the photoluminescence spectra with composition

∗ Corresponding author: e-mail: kimcd@mokpo.ac.kr, Phone: +82 61 450 2324, Fax: +82 61 453 6738

© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

2916 H. Park et al.: Anomalous dependence of PL properties on composition x in Cd1–xMnxGa2S4

© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.pss-c.com

x of Cd1−xMnxGa2S4 mixed crystals in the region of 0.00 ≤ x ≤ 1.00, together with the anomalous

composition dependence of the band gap and lattice constants [9].

2 Experimental Cd1−xMnxGa2S4 mixed crystals were prepared by using the directional melt tech-

nique in the region of 0.00 ≤ x ≤ 1.00 [9]. The quartz tubes used for the crystal growth were coated

with carbon to prevent the manganese of the constituent elements from reacting with the quartz tubes at

high temperatures. Stoichiometric amounts of the constituent elements were weighted out and sealed in

a quartz tube which was evacuated to a pressure of 2 × 10−6 mmHg. In order to obtain crystals with a

stoichiometric composition, we added excess sulphur of 10 mol%. The ampoule was placed into a hori-

zontal rotating furnace and then slowly heated to 1000 ◦C for about 10 days. Before heating the ampoule

to that temperature, we had to keep it at about 600 ◦C for 3 days in order to avoid an explosion due to the

high pressure produced before the chemical reactions. In order to get homogeneous crystals, we rotated

the ampoule at a speed of 4 rpm during the synthesis procedure.

The composition of the grown crystals were determined by means of inductively coupled plasma mass

spectroscopy (ICPS) and electron probe microanalysis (EPMA). Optical absorption spectra were obtained

at room temperature by using a UV-VIS-NIR spectrophotometer (Hitachi, U-3501) with a 60-mm integrat-

ing sphere. Photoluminescence spectra were measured with a system consisting of a double monochroma-

tor (Spex 1403, f = 0.85 m), a photomultiplier tube (RCA, C31034), and a cryogenic system (APD, SH-4).

The crystals were excited with the 325 nm line of a He–Cd laser (LiConix, 3650N).

3 Results and discussion We reported on the composition dependence of the optical energy gaps at 300

K for the Cd1−xMnxGa2S4 mixed crystals in a previous paper [9]. The results showed that the composition

dependence is divided into two parts; i.e., the optical energy gap varies downward bowingly in the region

of 0.00 ≤ x ≤ 0.67 while it varies linearly in the region of 0.90 ≤ x ≤ 1.00. The anomalous composition

dependence in the region of 0.00 ≤ x ≤ 0.67 is considered to be originated from a peculiar behavior of

the Mn transition metals of the constituent atoms in the crystals. Also, a discontinuity in the composition

dependence of the energy gaps exists in the region of 0.67 ≤ x ≤ 0.90. This corresponded to an occurrence

of a miscibility gap of the crystal structure in that region.

Fig. 1 Photoluminescence spectra of CdGa2S

4

crystals for x = 0.00 in the Cd1−xMnxGa2S4 in the

temperature region of 6 to 300 K.

Fig. 2 Photoluminescence spectra of 5 mol% Mn-

doped CdGa2S4 crystals in the temperature region

of 6 to 300 K.

phys. stat. sol. (c) 3, No. 8 (2006) 2917

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Fig. 3 Photoluminescence spectra of

Cd0.65Mn0.35Ga2S4 crystals for x = 0.35 in

the Cd1−xMnxGa2S4 in the temperature region of 6

to 300 K.

Fig. 4 Photoluminescence spectra of MnGa2S4

crystals for x = 1.00 in the Cd1−xMnxGa2S4 in the

temperature region of 6 to 300 K.

We now discuss on the photoluminescence properties with composition x of the Cd1−xMnxGa2S4

mixed crystals. The photoluminescence spectra were measured in the temperature region of 6 to 300

K and in the wavelength region of 350 to 850 nm. Figure 1 represents the temperature dependence of the

photoluminescence spectra in the temperature region of 6 to 300 K of CdGa2S4 crystals, which are the end

member of x = 0.00 in the Cd1−xMnxGa2S4 mixed crystals. As shown in Fig. 1, the photoluminescence

spectrum at 6 K consists of a strong and broad emission band with the maximum at 594 nm (2.09 eV)

and a relatively weak broad emission band with the maximum at 460 nm (2.70 eV). These emission bands

show a thermal quenching of an abrupt decreasing in the photoluminescence intensity with increasing the

ambient temperature of the crystals, and disappear at 300 K. Considering the characteristics of the photo-

luminescence spectra and the band gaps of CdGa2S4 crystals, the emission bands are considered to be due

to donor-acceptor pair recombination between deep levels in the forbidden band gap of the crystals [10].

The same feature of these bands is also observed for the crystals doped with 5 mol% Mn, as can be seen

in Fig. 2. However, the emission band in the longer wavelength region shows a slightly red shift and a

decreasing of the half-width of the band. Also, the emission band exists at 300 K, although there exists its

thermal quenching. Figure 3 shows the temperature dependence of the photoluminescence spectra for the

Cd0.65Mn0.35Ga2S4 crystals of x = 0.35 in the Cd1−xMnxGa2S4. The temperature dependent behavior

of the emission band shows a similarity to the case of the CdGa2S4:Mn(5mol%), except there exists no

emission band in the shorter wavelength region. This observation is also appeared for the crystals in the

composition region of x ≤ 0.67. Figure 4 represents the temperature dependence of the photolumines-

cence spectra of MnGa2S4 crystals. The photoluminescence spectra exhibits a broad emission band with

the maximum at 675 nm (1.838 eV), which is different from the emission band observed in the composi-

tion region of x ≤ 0.67. It is observed that there is a fluctuation of the peak position with temperature.

Comparing the energy of 1.838 eV of the emission band to that of 2.79 eV [9] of the band gap at 300 K,

the emission band seems to be related with deep levels in the forbidden band gap of the MnGa2S4.

Figures 5 and 6 show the composition dependence of the main emission band for the Cd1−xMnxGa2S4

crystals at 300 K. The relative intensity of the emission band in the composition region of 0.00 ≤ x ≤ 0.67increases with increasing x, while the emission band in the region of 0.90 ≤ x ≤ 1.00 decreases. This

© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.pss-c.com

Fig. 5 Composition dependence of the main emis-

sion band of the Cd1−xMnxGa2S4 in the composi-

tion region of 0.00 ≤ x ≤ 0.67 at 300 K.

Fig. 6 Composition dependence of the main emis-

sion band of the Cd1−xMnxGa2S4 in the composi-

tion region of 0.90 ≤ x ≤ 1.00 at 300 K.

anomalous composition dependence of the emission band corresponds to an occurrence of a miscibility

gap of the crystal structure in that region.

4 Conclusions In summary, photoluminescence spectra of Cd1−xMnxGa2S4 mixed crystals were in-

vestigated in the composition region of 0.00 ≤ x ≤ 1.00 and in the temperature region of 6 to 300 K.

The photoluminescence spectra of CdGa2S4(x = 0.00) showed two emission bands at 594 nm (2.09 eV)

and 460 nm (2.70 eV), which were attributed to be due to donor-acceptor pair recombination. The same

feature of these bands was also observed for the crystals doped with 5 mol% Mn, excepting a slightly red

shift and a decreasing of the half-width of the emission band at 594 nm. The photoluminescence spectra

of MnGa2S4(x = 1.00) showed a broad emission band 675 nm (1.838 eV), which is different from that

of CdGa2S4. The relative intensity of the main emission band increased in the composition region of

0.00 ≤ x ≤ 0.67 with increasing x while decreased in the region of 0.90 ≤ x ≤ 1.00. It is considered that

the anomalous composition dependence of the photoluminescence spectra may originate from a peculiar

behavior of the Mn transition metals of the constituent element of the mixed crystals, and also that it is

connected with an occurrence of a miscibility gap of the crystal structure.

Acknowledgements This work was supported by a Korea Research Foundation Grant (KRF-2000-005-Y00071).

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2918 H. Park et al.: Anomalous dependence of PL properties on composition x in Cd1–xMnxGa2S4