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Cathodoluminescence defectoscopy of ZnS and ZnSe crystals
Materials Science and Engineering B91– 92 (2002) 349– 352 www.elsevier.com/locate/mseb
Cathodoluminescence defectoscopy of ZnS and ZnSe crystals M.V. Nazarov Technical Uni6ersity of Moldo6a, Bl6d. Stefan cel Mare 168, MD-2004 Kishine6, Moldo6a
Abstract The quality and reliability of microelectronic materials can be dramatically changed by point and linear defects, which occur in specimens naturally, or which can be deliberately introduced. The control of defect distribution requires a measurement method capable of investigating the defects on a micrometer scale. This article surveys some of the opportunities for the control of defects in solids by using a combination of different cathodoluminescence modes including color cathodoluminescence (CCL) in the scanning electron microscope (SEM) with computer graphics. The method of cathodoluminescence defectoscopy was applied to microcharacterization of commercial wide-band-gap II – VI semiconductors (ZnS, ZnSe) as well as to the thin diffusion layers formed in these materials in the process of annealing. The luminescence topography of the radiative centres distribution at different Al and Bi concentrations was investigated and a model of two-polar and dissociative diffusion of Zn, Al and Bi from the melt to the crystal volume was proposed. © 2002 Published by Elsevier Science B.V. Keywords: Cathodoluminescence; Scanning electron microscopy; Semiconductors; Defects; Diffusion; Annealing
1. Introduction The construction of the devices emitting green, blue and ultraviolet radiation is one of the topical problems of modern electronics. From this point of view the investigation of the luminescence properties of ZnS and ZnSe crystals, doped with impurities causing the desirable color in the whole visible range of the spectrum is a vital problem. However, no single method can supply the necessary information, which is now required by companies involved in material development or product manufacture. The microscopic properties of materials used for electronic device fabrication are often caused by inhomogeneities on a microscopic scale. In particular, the defect distribution (vacancies, dopants) can alter these properties substantially. Therefore, by controlling this distribution, the material properties can be improved. The control of defect distributions requires a measuring method capable to investigate the defects on a micrometer scale. The possibilities of SEM techniques for the measurement of local properties of semiconductor structures and for revealing the physical property inhomogeneities E-mail address: [email protected] (M.V. Nazarov).
in microelectronic structures are discussed. We have chosen some of the traditional and some original modes and image processing to illustrate these possibilities [1,2]. The results of experimental investigations of some semiconductor materials by conventional techniques such as backscattered electrons as well as by new techniques such as color cathodoluminescence (CCL) defectoscopy and others, are presented.
2. Methods and experimental details Experiments were carried out with a commercial ‘JSM-50A’ and a modified SEM ‘Stereoscan’ with an additional attachment for the color CL mode [3,4]. We used a recording system consisting of three multipliers with different light filters (R, G, B) and of a multichannel device connected with photomultipliers. It allows simultaneous transmission of video signals on all channels in any pre-determined range of the spectrum. Images of the surface under study and the corresponding CL emission from the surface were displayed on video monitors. The CL images can be recorded with the total emitted integral (panchromatic) CL as well as with light of fixed spectral wavelength by using a suitable photodetector.
0921-5107/02/$ - see front matter © 2002 Published by Elsevier Science B.V. PII: S 0 9 2 1 - 5 1 0 7 ( 0 1 ) 0 1 0 6 4 - 9
M.V. Nazaro6 / Materials Science and Engineering B91–92 (2002) 349–352
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Fig. 1. CCL images of ZnS:Al (a), 10 − 3 at.% Al; (b), 10 − 2 at.% Al; (c), 10 − 1 at.% Al (original colour).
The combination with the secondary electron image or with the backscattered electrons allows the analytical CL image to be compared with the surface topography of the sample.
3. Results and discussion
3.1. Zinc sulphide Among wide-band-gap II– VI materials, ZnS is finding an increasing number of applications as a host crystal for doping to create luminescence in the blue spectral range. Single crystals of low resistivity (z: 100 V cm − 1) ZnS:Al were annealed in a Bi melt with different Al concentrations for 100 h at 1200 K with subsequent cooling in air. We used Bi in ZnS and ZnSe host crystals despite the greater atomic radius of Bi compared with the radius of both Zn and S. Owing to the little possibility of Bi diffusion we can realize an Al doping from the Bi melt without any change of the stoichiometry of starting crystals. The differences in the luminescence topography of the cleavages subjected to various heat treatments were observed by using the highly sensitive CCL mode. It was established that the annealing in the above mentioned melts changes the impurity-defect composition. Fig. 1 presents CCL images of the cross-section of ZnS crystals annealed in Bi melt with different Al content. The surface regions, which are enriched with dislocation disturbances, exhibit various emission bands of different colors. Their width and color depend on the Al concentration in the Bi melt. At the Al concentration of 10 − 3 at.%, a 100 mm wide blue – violet zone appears near the surface (bright band in Fig. 1(a)). It enlarges to 450 mm at an Al concentration of 10 − 2 at.% (bright bands in Fig. 1(b)). When the Al concentration was augmented up to 10 − 1 at.%, a green band has been observed (also bright band in black– white picture, Fig. 1(c)).
For the quantitative interpretation of the CCL results, the photoluminescence and cathodoluminescence spectra were recorded. The photoluminescence was excited by a N2 laser (u= 337 nm) and the PL spectra are presented in Fig. 2. The blue band centered at 470 nm (Fig. 2(a and b)) can be related to the Al doping. The ions of Al+ 2 , taking a site into zinc sublattice, can also form a complex of (V2Zn− Al+ Zn). In the first case, Al acts as a donor and in the second one it acts as an acceptor. Being in different charge states, they can form the donor–acceptor pairs. Since the zinc solubility in Bi is high, we can suppose that the melt-crystal interface could be a source of Zn vacancies (VZn), and Al diffusion can take place from the melt into this region. So, on the basis of AlZn and VZn, donor –acceptor pairs can − 2− be formed, where Al+ Zn is a donor and VZn or the (VZn + AlZn) complex are acceptors. Apparently, only those of them are displayed, which have not been dissociated at 300 K. The green luminescence (530 nm) of the PL spectrum, observed in the subsurface region, participates in the formation of the long wavelength slope of the blue radiation band. Its contribution increases with Al concentration.
Fig. 2. PL spectra (T=300 K) of ZnS crystals annealed in Bi melt with Al (— , volume; , surface).
M.V. Nazaro6 / Materials Science and Engineering B91–92 (2002) 349–352
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green band we obtain Deff = 7×10 − 10 and 1 × 10 − 10cm2 s − 1, respectively. These values are by a few orders of magnitude lower than the diffusion coefficients of the single components. Simultaneously with the displacement of the native Zn vacancy (VZn), the displacement of the Al atoms occur. As a result, the diffusion becomes of a two-polar type leading to a low value of Deff. Apparently, our experimental results reflect a dissociative diffusion mechanism, i.e. an Al displacement process from one site to another via [Al+ Zn (VZnAl+ Zn)] complexes.
3.2. Zinc selenide
Fig. 3. CL spectra (T= 77 K) of ZnSe crystals annealed in (Bi + 10 − 2 at.% Al) (a), volume, (b), near surface region (X =200 mm), (c), subsurface layer (X= 100 mm).
Fig. 4. CCL image of ZnSe crystals annealed in (Bi + 10 − 2 at.% Al) (original colour).
ZnSe is considered as perspective material for optoelectronic devices, in particular LEDs and lasers emitting in the blue range. ZnSe crystals annealed in Bi melt or Bi + 10 − 2 at.% Al were studied. Annealing was carried out at the same conditions as for ZnS. From the CL spectra (Fig. 3), one can see that in the crystal volume the main luminescence band is centered at 461 nm. This band is usually attributed to the edge emission of ZnSe. Near the surface, the long wavelength band 610 nm becomes predominant. The luminescence-topography distribution is presented in Fig. 4. The appearance of characteristic CL bands at 461, 560, 625 nm (Fig. 5(a)) allow to display the distribution of the respective luminescence centres and their redistribution after annealing. An advantage of the applied method is the possibility to register the spectra in the integral regime from the whole surface and, in the local mode, from about 1 mm large regions. It is very important to study the thin diffusion layers or inhomogeneous areas. In particular, some histograms recorded for different spots, presented in Fig. 6. It is shown that the red–orange luminescence also exists in the cracks, where Al diffusion takes place. The luminescence at 560 nm is most probably attributed to neutral associative pairs (VZnDZn). If Al
Fig. 5. CL spectrum (T=77 K) with three characteristic bands from the volume of a ZnSe crystal annealed in (Bi +10 − 2 at.% Al).
The observed experimental data allows to make a supposition about the nature of the radiation centres. The effective diffusion coefficient (Deff) was estimated from the width of the colored zones. For the blue and
Fig. 6. Histogram of CL intensities of characteristic bands from ZnSe in the points 1 – 3, as marked in Fig. 4.
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serves as donor, it forms an associative centre (V−− Zn Al+ Zn). For the samples annealed in Bi melt without Al, the red band is centered at 625 nm (1.98 eV). Its origin is an optical transitions between negatively charged + − associative pairs and neighboring defects (V−− Zn AlZn) . After annealing in Bi with Al, the maximum is shifted to 610 nm (2.032 eV) and it can be considered as a superposition of the 560 and 625 nm bands. Different concentrations of VZn and Al can change the contribution of associative pairs as well as the formation of neutral and negatively charged complexes. As a result of such quasi chemical reactions, the formation of these complexes apparently takes place according to the following scheme: ZnZn + Al + Zn(melt)+AlZn +e−
(1)
Uncombined electrons (e−) are captured by neutral Zn vacancies, whose concentration is high in subsurface region: VZn +2e− V−−Zn
(2)
The disturbed equilibrium can be restored by Zn diffusion from the Bi melt: ZnZn Zn(melt)+VZn
(3)
Therefore, one double charged Zn vacancy is formed at two positively charged Al ions. So, our assumption is in a good accordance with the experiment. From the obtained data, one can estimate the effective diffusion coefficient, which is estimated to be Deff = 1.2× 10 − 10 cm2 s − 1. An equation for the temperature dependence of the Al diffusion coefficient was also obtained as:
Deff =2.9× 10 − 3exp −
1.73 eV kT
(5)
The high value of the activation energy of the Al atoms (ED =1.73 eV) confirms our suggestion about the vacancy nature of the diffusion mechanism and about the dominant influence of luminescence centres
made of neutral and charged associative pairs in the ZnSe sublattice.
4. Conclusions The method of cathodoluminescence defectoscopy is a perspective tool for the investigation of opto-electronic materials. The proposed method can be used both for checking the material state and studying the characteristics of dislocations and point defects. The combined investigations of diffusion processes in ZnS and ZnSe crystals, subjected to the thermal treatments, allowed us to establish the nature of the formed luminescence centres on the basis of VZn and AlZn, their interaction, and the mechanism of their diffusion: dissociative in the subsurface layers and two-polar in the crystal volume. Acknowledgements The author thanks Dr Obyden S.K. for technical assistance during the SEM experiments and Dr Korotkov V.A., Dr Sobolevskaja R.L., Dr Sushkevich K.D. for annealing the samples and for constructive comments. References [1] S.K. Obyden, P.V. Ivannikov, G.V. Saparin, Color cathodoluminescence display in the SEM of deep relief surfaces, Scanning 19 (1997) 533. [2] Nazarov M.V, Nazarova T.A. Cathodoluminescence defectoscopy. European Microscopy and Analysis. 1995 pp. 21 –23. [3] G.V. Saparin, S.K. Obyden, Colour display of video information in scanning electron microscopy. Principles and applications to physics, geology, soil science, biology and medicine, Scanning 10 (1988) 87. [4] Saparin G.V., Obyden S.K. Colour in the microworld: real colour cathodoluminescence mode in scanning electron microscopy. European Microscopy and Analysis 1993 pp. 7 – 9.
Cathodoluminescence defectoscopy of ZnS and ZnSe crystals M.V. Nazarov Technical Uni6ersity of Moldo6a, Bl6d. Stefan cel Mare 168, MD-2004 Kishine6, Moldo6a
Abstract The quality and reliability of microelectronic materials can be dramatically changed by point and linear defects, which occur in specimens naturally, or which can be deliberately introduced. The control of defect distribution requires a measurement method capable of investigating the defects on a micrometer scale. This article surveys some of the opportunities for the control of defects in solids by using a combination of different cathodoluminescence modes including color cathodoluminescence (CCL) in the scanning electron microscope (SEM) with computer graphics. The method of cathodoluminescence defectoscopy was applied to microcharacterization of commercial wide-band-gap II – VI semiconductors (ZnS, ZnSe) as well as to the thin diffusion layers formed in these materials in the process of annealing. The luminescence topography of the radiative centres distribution at different Al and Bi concentrations was investigated and a model of two-polar and dissociative diffusion of Zn, Al and Bi from the melt to the crystal volume was proposed. © 2002 Published by Elsevier Science B.V. Keywords: Cathodoluminescence; Scanning electron microscopy; Semiconductors; Defects; Diffusion; Annealing
1. Introduction The construction of the devices emitting green, blue and ultraviolet radiation is one of the topical problems of modern electronics. From this point of view the investigation of the luminescence properties of ZnS and ZnSe crystals, doped with impurities causing the desirable color in the whole visible range of the spectrum is a vital problem. However, no single method can supply the necessary information, which is now required by companies involved in material development or product manufacture. The microscopic properties of materials used for electronic device fabrication are often caused by inhomogeneities on a microscopic scale. In particular, the defect distribution (vacancies, dopants) can alter these properties substantially. Therefore, by controlling this distribution, the material properties can be improved. The control of defect distributions requires a measuring method capable to investigate the defects on a micrometer scale. The possibilities of SEM techniques for the measurement of local properties of semiconductor structures and for revealing the physical property inhomogeneities E-mail address: [email protected] (M.V. Nazarov).
in microelectronic structures are discussed. We have chosen some of the traditional and some original modes and image processing to illustrate these possibilities [1,2]. The results of experimental investigations of some semiconductor materials by conventional techniques such as backscattered electrons as well as by new techniques such as color cathodoluminescence (CCL) defectoscopy and others, are presented.
2. Methods and experimental details Experiments were carried out with a commercial ‘JSM-50A’ and a modified SEM ‘Stereoscan’ with an additional attachment for the color CL mode [3,4]. We used a recording system consisting of three multipliers with different light filters (R, G, B) and of a multichannel device connected with photomultipliers. It allows simultaneous transmission of video signals on all channels in any pre-determined range of the spectrum. Images of the surface under study and the corresponding CL emission from the surface were displayed on video monitors. The CL images can be recorded with the total emitted integral (panchromatic) CL as well as with light of fixed spectral wavelength by using a suitable photodetector.
0921-5107/02/$ - see front matter © 2002 Published by Elsevier Science B.V. PII: S 0 9 2 1 - 5 1 0 7 ( 0 1 ) 0 1 0 6 4 - 9
M.V. Nazaro6 / Materials Science and Engineering B91–92 (2002) 349–352
350
Fig. 1. CCL images of ZnS:Al (a), 10 − 3 at.% Al; (b), 10 − 2 at.% Al; (c), 10 − 1 at.% Al (original colour).
The combination with the secondary electron image or with the backscattered electrons allows the analytical CL image to be compared with the surface topography of the sample.
3. Results and discussion
3.1. Zinc sulphide Among wide-band-gap II– VI materials, ZnS is finding an increasing number of applications as a host crystal for doping to create luminescence in the blue spectral range. Single crystals of low resistivity (z: 100 V cm − 1) ZnS:Al were annealed in a Bi melt with different Al concentrations for 100 h at 1200 K with subsequent cooling in air. We used Bi in ZnS and ZnSe host crystals despite the greater atomic radius of Bi compared with the radius of both Zn and S. Owing to the little possibility of Bi diffusion we can realize an Al doping from the Bi melt without any change of the stoichiometry of starting crystals. The differences in the luminescence topography of the cleavages subjected to various heat treatments were observed by using the highly sensitive CCL mode. It was established that the annealing in the above mentioned melts changes the impurity-defect composition. Fig. 1 presents CCL images of the cross-section of ZnS crystals annealed in Bi melt with different Al content. The surface regions, which are enriched with dislocation disturbances, exhibit various emission bands of different colors. Their width and color depend on the Al concentration in the Bi melt. At the Al concentration of 10 − 3 at.%, a 100 mm wide blue – violet zone appears near the surface (bright band in Fig. 1(a)). It enlarges to 450 mm at an Al concentration of 10 − 2 at.% (bright bands in Fig. 1(b)). When the Al concentration was augmented up to 10 − 1 at.%, a green band has been observed (also bright band in black– white picture, Fig. 1(c)).
For the quantitative interpretation of the CCL results, the photoluminescence and cathodoluminescence spectra were recorded. The photoluminescence was excited by a N2 laser (u= 337 nm) and the PL spectra are presented in Fig. 2. The blue band centered at 470 nm (Fig. 2(a and b)) can be related to the Al doping. The ions of Al+ 2 , taking a site into zinc sublattice, can also form a complex of (V2Zn− Al+ Zn). In the first case, Al acts as a donor and in the second one it acts as an acceptor. Being in different charge states, they can form the donor–acceptor pairs. Since the zinc solubility in Bi is high, we can suppose that the melt-crystal interface could be a source of Zn vacancies (VZn), and Al diffusion can take place from the melt into this region. So, on the basis of AlZn and VZn, donor –acceptor pairs can − 2− be formed, where Al+ Zn is a donor and VZn or the (VZn + AlZn) complex are acceptors. Apparently, only those of them are displayed, which have not been dissociated at 300 K. The green luminescence (530 nm) of the PL spectrum, observed in the subsurface region, participates in the formation of the long wavelength slope of the blue radiation band. Its contribution increases with Al concentration.
Fig. 2. PL spectra (T=300 K) of ZnS crystals annealed in Bi melt with Al (— , volume; , surface).
M.V. Nazaro6 / Materials Science and Engineering B91–92 (2002) 349–352
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green band we obtain Deff = 7×10 − 10 and 1 × 10 − 10cm2 s − 1, respectively. These values are by a few orders of magnitude lower than the diffusion coefficients of the single components. Simultaneously with the displacement of the native Zn vacancy (VZn), the displacement of the Al atoms occur. As a result, the diffusion becomes of a two-polar type leading to a low value of Deff. Apparently, our experimental results reflect a dissociative diffusion mechanism, i.e. an Al displacement process from one site to another via [Al+ Zn (VZnAl+ Zn)] complexes.
3.2. Zinc selenide
Fig. 3. CL spectra (T= 77 K) of ZnSe crystals annealed in (Bi + 10 − 2 at.% Al) (a), volume, (b), near surface region (X =200 mm), (c), subsurface layer (X= 100 mm).
Fig. 4. CCL image of ZnSe crystals annealed in (Bi + 10 − 2 at.% Al) (original colour).
ZnSe is considered as perspective material for optoelectronic devices, in particular LEDs and lasers emitting in the blue range. ZnSe crystals annealed in Bi melt or Bi + 10 − 2 at.% Al were studied. Annealing was carried out at the same conditions as for ZnS. From the CL spectra (Fig. 3), one can see that in the crystal volume the main luminescence band is centered at 461 nm. This band is usually attributed to the edge emission of ZnSe. Near the surface, the long wavelength band 610 nm becomes predominant. The luminescence-topography distribution is presented in Fig. 4. The appearance of characteristic CL bands at 461, 560, 625 nm (Fig. 5(a)) allow to display the distribution of the respective luminescence centres and their redistribution after annealing. An advantage of the applied method is the possibility to register the spectra in the integral regime from the whole surface and, in the local mode, from about 1 mm large regions. It is very important to study the thin diffusion layers or inhomogeneous areas. In particular, some histograms recorded for different spots, presented in Fig. 6. It is shown that the red–orange luminescence also exists in the cracks, where Al diffusion takes place. The luminescence at 560 nm is most probably attributed to neutral associative pairs (VZnDZn). If Al
Fig. 5. CL spectrum (T=77 K) with three characteristic bands from the volume of a ZnSe crystal annealed in (Bi +10 − 2 at.% Al).
The observed experimental data allows to make a supposition about the nature of the radiation centres. The effective diffusion coefficient (Deff) was estimated from the width of the colored zones. For the blue and
Fig. 6. Histogram of CL intensities of characteristic bands from ZnSe in the points 1 – 3, as marked in Fig. 4.
M.V. Nazaro6 / Materials Science and Engineering B91–92 (2002) 349–352
352
serves as donor, it forms an associative centre (V−− Zn Al+ Zn). For the samples annealed in Bi melt without Al, the red band is centered at 625 nm (1.98 eV). Its origin is an optical transitions between negatively charged + − associative pairs and neighboring defects (V−− Zn AlZn) . After annealing in Bi with Al, the maximum is shifted to 610 nm (2.032 eV) and it can be considered as a superposition of the 560 and 625 nm bands. Different concentrations of VZn and Al can change the contribution of associative pairs as well as the formation of neutral and negatively charged complexes. As a result of such quasi chemical reactions, the formation of these complexes apparently takes place according to the following scheme: ZnZn + Al + Zn(melt)+AlZn +e−
(1)
Uncombined electrons (e−) are captured by neutral Zn vacancies, whose concentration is high in subsurface region: VZn +2e− V−−Zn
(2)
The disturbed equilibrium can be restored by Zn diffusion from the Bi melt: ZnZn Zn(melt)+VZn
(3)
Therefore, one double charged Zn vacancy is formed at two positively charged Al ions. So, our assumption is in a good accordance with the experiment. From the obtained data, one can estimate the effective diffusion coefficient, which is estimated to be Deff = 1.2× 10 − 10 cm2 s − 1. An equation for the temperature dependence of the Al diffusion coefficient was also obtained as:
Deff =2.9× 10 − 3exp −
1.73 eV kT
(5)
The high value of the activation energy of the Al atoms (ED =1.73 eV) confirms our suggestion about the vacancy nature of the diffusion mechanism and about the dominant influence of luminescence centres
made of neutral and charged associative pairs in the ZnSe sublattice.
4. Conclusions The method of cathodoluminescence defectoscopy is a perspective tool for the investigation of opto-electronic materials. The proposed method can be used both for checking the material state and studying the characteristics of dislocations and point defects. The combined investigations of diffusion processes in ZnS and ZnSe crystals, subjected to the thermal treatments, allowed us to establish the nature of the formed luminescence centres on the basis of VZn and AlZn, their interaction, and the mechanism of their diffusion: dissociative in the subsurface layers and two-polar in the crystal volume. Acknowledgements The author thanks Dr Obyden S.K. for technical assistance during the SEM experiments and Dr Korotkov V.A., Dr Sobolevskaja R.L., Dr Sushkevich K.D. for annealing the samples and for constructive comments. References [1] S.K. Obyden, P.V. Ivannikov, G.V. Saparin, Color cathodoluminescence display in the SEM of deep relief surfaces, Scanning 19 (1997) 533. [2] Nazarov M.V, Nazarova T.A. Cathodoluminescence defectoscopy. European Microscopy and Analysis. 1995 pp. 21 –23. [3] G.V. Saparin, S.K. Obyden, Colour display of video information in scanning electron microscopy. Principles and applications to physics, geology, soil science, biology and medicine, Scanning 10 (1988) 87. [4] Saparin G.V., Obyden S.K. Colour in the microworld: real colour cathodoluminescence mode in scanning electron microscopy. European Microscopy and Analysis 1993 pp. 7 – 9.