Photoluminescence properties of ZnSe/SiO2 composite thin films prepared by sol-gel method
JIANG Hai-qing(姜海青)1, 2, CHE Jun(车 俊)2, YAO Xi(姚 熹)2
1. School of Technical Physics, Xidian University, Xi’an 710071, China;
Electronic Material Research Laboratory, Key Laboratory of Education Ministry,Xi’an Jiaotong University，Xi’an 710049, China
Received 10 April 2006; accepted 25 April 2006
Abstract:
ZnSe/SiO2 composite thin films was prepared by sol-gel method. XRD results indicate the phase structure of ZnSe particles embedded in ZnSe/SiO2 composite thin films is sphalerite (cubic ZnS). Spectroscopic ellipsometers were used to investigated the dependences of ellipsometric angle with wavelength of ZnSe/SiO2 composite thin films. The optical constant, thickness, porosity and the concentration of ZnSe of ZnSe/SiO2 thin composite films were fitted according to Maxwell-Garnett effective medium theory. The thickness of ZnSe/SiO2 composite thin thin films was also measured through surface profile. The photoluminescence properties of ZnSe/SiO2 thin composite thin films was investigated through fluorescence spectrometer. The photoluminescence results show that the emission peak at 487 nm (2.5 eV) is excited at 395 nm corresponds to the band-to-band emission of sphalerite ZnSe crystal(2.58 eV). The strength free exciton emission and other emission peaks correlating to ZnSe lattice defect were also observed.
Keyword: ZnSe/SiO2 composite thin films; sol-gel method; photoluminescence; optical constant
1 Introduction
In recent years, composite materials have attracted much attention for applications in opto-electronic device. Especially, the Ⅱ-Ⅳ group semiconductor composite thin films, such as CdSe, CdS, CdTe, ZnS nanopaticles embedded in strained system have attracted much more attentions for the fabrication of visible semiconductor laser [1-8].
As one of wide-band gap Ⅱ-Ⅳ group semiconductor, ZnSe has gained much interest for its technological application in blue laser diodes, IR optical window, solar cell and sensors. But there are fewer scientific reports on fabrication and properties of ZnSe/SiO2 composite thin films. YIN et al [9] fabricated the ZnSe quantum dots in glass matrix thin films by pulsed laser evaporation. LI et al [10] prepared the ZnSe crystallites doped in borosilicate glass films by sol-gel method. LIPOVSKII et al [11] successfully prepared the Ⅱ-Ⅳ semiconductor nanocrystallite in a novel phosphate glass.
In this paper the fabrication of ZnSe/SiO2 composite thin films on SiO2/Si(100) substrate through sol-gel method was reported. ZnSe/SiO2 composite thin films were synthesized under carbon monoxide atmosphere. The synthesis technique of ZnSe/SiO2 composite thin films can solve the problems of the emergence of ZnO phase and escape of Se element under H2 reduction condition.
2 Experimental
2.1 Preparation of ZnSe/SiO2 composite thin films
There are two steps of preparation of TEOS solution [12]. Fig.1 shows the flowchart of preparation of TEOS solution and ZnSe/SiO2 composite thin films.
2.2 Characterization
The phase structure of ZnSe/SiO2 thin films was investigated by XRD(Rigaku D/MAX-2400, Cu Kα). The relationship of ellipsometric angle with wavelength of ZnSe/SiO2 thin composite film was investigated through spectroscopic ellipsometers (M-2000UI, J.A.WOOLAM.
Co.Inc). The optical constant, thickness, porosity and concentration of ZnSe in ZnSe/SiO2 composite thin films were fitted according to Maxwell-Garnett effective medium theory(M-G EMT). The thickness of ZnSe/SiO2 composite thin films was also recorded through surface profile(Dektak3 ST). The photoluminescence properties of ZnSe/SiO2 thin composite than film were investigated by fluorescence spectrometer (FLS 920, Edinburgh Instruments Ltd) at room temperature.
Fig.1 Flowchart of preparation of ZnSe/SiO2 composite thin films by sol-gel method
3 Results and discussion
3.1 XRD analysis
The XRD patterns of ZnSe/SiO2 thin films synthesized at 500 ℃under carbon monoxide atmosphere are shown in Fig.2. XRD results indicate that the phase structure of ZnSe is sphalerite (cubic ZnS) and the intensity of diffraction peak of ZnSe phase increases with the increase of reduction time.
3.2 Spectroscopic ellipsometers measurement
To interpret the spectroscopic ellipsometric experimental results of ZnSe/SiO2 composite thin films with low volume fractions of ZnSe semiconductor,
MGEMT[13] was used to calculate the optical constant of ZnSe/SiO2 composite thin films. The relationship between extinction coefficient, refractive index and
Fig.2 XRD patterns of ZnSe/SiO2 composite thin films synthesized at 500 ℃ under carbon monoxide atmosphere for different time (a) 20 min; (b) 15 min
wavelength are shown in Fig.3. From Fig.3 it can be seen that the extinction coefficient and refractive index increase with the increase of molar ratio of ZnSe to SiO2. The extinction coefficient of pure SiO2 thin films will be invariable at the wavelength range of 300-1650 nm. The extinction coefficient of ZnSe/SiO2 composite thin films increases at wavelength less than 800 nm and sharply increases at wavelength less than 500 nm. These results indicates the absorption of ZnSe/SiO2 composite thin films increases when wavelength is less than 800 nm. In Fig.3(a), there exist five different absorption regions which correspond to 800-500 nm, 500-458 nm, 458-441 nm, 441-350 nm, 350-300 nm. In 800-500 nm region, the extinction coefficient of ZnSe/SiO2 composite thin films is slightly larger than that of pure SiO2 films for the lattice defect of sphalerite ZnSe crystal, such as 
,
,
which induce the complicated absorption mechanism including free excited absorption and lattice defect absorption. A sharply absorption in 500-458 nm for the existence of sphalerite ZnSe crystal corresponds to the band-to-
band absorption. There is a relative smooth curve in 458-441 nm without specially absorption. In 441-350 nm, 350-300 nm, there are the emergence of wurtzite ZnSe and ZnO which sharply increase the absorption of ZnSe/SiO2 thin films. The refractive index curve also reveals the existent of ZnSe crystal. In 500-458 nm, 440-458 nm region, the refractive index of samples shows a sharply change. The fitting results of the molar ratio of ZnSe:SiO2, porosity and thickness of ZnSe/SiO2 composite thin films according to M-G EMT are shown in Table 1. The results reveal that the actual ratio of ZnSe to SiO2 increases with the increase of original concentration of Zn2+ and SeO42- in solution. The actual molar ratio of ZnSe to SiO2 of thin films is about 1/2 of the original molar ratio of Zn2+(SeO42-) to SiO2 in solution. The porosity of ZnSe/SiO2 composite thin films is 38%-22%, which is consistent to every sample concerning the experimental error. The thickness of single layer ZnSe/SiO2 composite thin films increases with the increase of the molar ratio of ZnSe to SiO2. The thickness of single layer pure SiO2 thin films is 277.7 nm and the thickness of single layer ZnSe/SiO2 thin films is no less than 300 nm.
3.3 Photoluminescence properties
The photoluminescence(PL) properties of ZnSe/
SiO2 composite thin films and ZnSe polycrystal were investigated by fluorescence spectrometer at room temperature. Fig.4 shows emission and excited spectra of ZnSe polycrystal. As shown in Fig.4(a), it is obvious that a relative weak emission peak at 487 nm which responds to the band-to-band emission at 2.58 eV for sphalerite ZnSe(cubic ZnS). The reason of weak emission peak at 487 nm is the self-absorption effect and nonradiative recombination of semiconductor which will decrease the outer quantum yield ratio of ZnSe semiconductor. In Fig.4(a), in the long wavelength region, there are four peaks at 503, 571, 577 and 586 nm. The emission peak at 503 nm arises from the free exciton radiative recombination of an electron and a hole. The emission peaks at 571, 577 and 586 nm are assigned to lattice defect, such as 
,, and other impurity energy level. Fig.4(b) shows the excitation spectrum responding to 487 nm emission peak. The result shows the excitation wavelength responding to emission peak at 487 nm appears at 395 nm.
Fig.5 shows PL spectra of ZnSe/SiO2 composite thin films was excited at 395 nm. As shown in Fig.5, there are several emission peaks at 487, 507, 571, 577, 586 nm, respectively. Every peak is similar to the PL spectrum of ZnSe polycrystal. The intensity of PL
Fig.3 Relation between optical constant and wavelength: (a) Extinction coefficient; (b) Refractive index
Table 1 Fitting results of different single layer films according to M-G effective medium theory
Fig.4 PL spectrum and excitation spectrum of ZnSe polycrystal at 300 K: (a) Emission spectrum; (b) Excitation spectrum
Fig.5 PL spectra of ZnSe/SiO2 films excited at 395 nm (300 K): (a) Five layers 30% ZnSe/SiO2 films; (b) Five layers 25% ZnSe/SiO2 films; (c) Five layers 20% ZnSe/SiO2 films; (d) Five layers 10% ZnSe/SiO2 films; (e) Five layers 15% ZnSe/SiO2 films; (f) Five layers pure porous SiO2 films; (g)SiO2 /Si(100) substrate
spectrum increases with the increase of ZnSe/SiO2 molar ratio. Compared to the PL spectrum of ZnSe crystal, the intensity of peak at 487 nm decreases and the intensity of the other emission peaks increase. It is obvious that the emission peaks at 487, 507, 571, 577, 586 nm do not appear in PL spectra of sample(f) and sample(g), which means the emission peaks aforementioned are not aroused by SiO2 films and SiO2/Si(100) substrate.
4 Conclusions
ZnSe/SiO2 composite thin films was prepared by the sol-gel method. XRD indicates the ZnSe embedded in the composite thin films is sphalerite. Spectroscopic ellipsometers and M-G EMT were used to measure and to evaluate the optical experimental results, which can provide relatively accurate information of ZnSe/SiO2 composite thin films. The PL spectrum of ZnSe/SiO2 composite thin films reveales that the emission peak at 487 nm corresponds to the band-to-band radiative emission of sphalerite ZnSe. The intensive free exciton radiative recombination and other lattice defect radiative emission were also observed.
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Foundation item: Project (2002CB613305) supported by the National Basic Research Program; project supported by the International Cooperation Research Project of Chinese-Israel of Ministry Education of China
Corresponding author: JIANG Hai-qing; Tel: +86-29-82668679; Fax: +86-29-82668794; E-mail: jianyhaiqing@vip.sina.com, hqjiang@mail.xidian.edu.cn
