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 K_α). 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 SeO₄^2- in solution. The actual molar ratio of ZnSe to SiO₂ of thin films is about 1/2 of the original molar ratio of Zn²⁺(SeO₄^2-) 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.

References

[1] HUONG N Q, BINMAN J L. Electronic state and optical response of a semiconductor quantum dot lattice embedded in an organic host [J]. Journal of Luminescences , 2000, 87: 333-336.

[2] TOHGE N, ASUKA M, MINAMI T. Sol-gel preparation and optical properties of silica glasses containing Cd and Zn chalcogenide microcrystals[J]. Journal of Non-Crystalline Solids, 1992, 147-148: 652-656.

[3] NESHEVA D, RAPTIS C, LEVI Z. Photoluminescence of CdSe nanocrystals embedded in a SiO_X thin film matrix[J]. Journal of Luminescence, 1999, 82: 233-240.

[4] LIFSHITZ E, DAG I, LITVIN I. Optical properties of CdSe nanoparticle films prepared by chemical deposition and sol-gel methods[J]. Chemical Physics Letters, 1998, 288: 188-196.

[5] KUMARESAN R, ICHIMURA M, ARAI E. Photochemical deposition of ZnSe polycrystalline thin films and their characterization[J]. Thin Solid Films, 2002, 414(1): 25-30.

[6] CHANDRAMOHAN R, MAHALINGAM T, CHU J P, SEBASTIAN P J. Preparation and characterization of electrosynthesized zinc selenide thin films[J]. Journal of New Materials for Electrochemical Systems, 2005, 8(2): 143-148.

[7] VENKATACHALAM S, MANGALARAI D, NARAYANDASS S K, KIM K, YI J. Structure, optical and electrical properties of ZnSe thin films[J]. Physica B: Condensed Matter, 2005, 358(1-4): 27-35.

[8] BELOGOROKHOV A I, BELOGOROHOVA L I, GAVRIOV S. Investigation of properties of porous silicon embedded with ZnSe and CdSe[J]. Journal of Crystal Growth, 1999, 197: 702-706.

[9] YIN S B, CHEN L, HSIEH W F. Fabrication and raman analysis of ZnSe quantum dots in glass matrix thin films by pulsed laser evaporation[J]. Japanese Journal of Applied Physics, 1998, 37: 4154-4157.

[10] LI G M, MASAYUKI N. Preparation and optical properties of sol-gel derived ZnSe crystallites doped in glass films[J]. Journal of Applied Physics, 1994, 75(8): 4276-4278.

[11] LIPOVSKII A A, KOLOBKOVA E V, PETRIKOV V D. Growth and optical properties of CdS_xSe_1-x nanocrystals embedded in a novel phosphate glass[J]. Journal of Materials Science, 1999, 34(4): 887-890.

[12] LIU YUN, REN W, ZHANG L Y, YAO X. New method for making porous SiO₂ thin films[J]. Thin Solid Films, 1999, 353: 124-128.

[13] ASPNES D E, THEETEN J B. Investigation of effective-medium models of microscopic surface roughness by spectroscopic ellipsometry[J]. Phys Rev B, 1979, 20: 3292-3302.

(Edited by YANG Hua)

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