Investigation of interface state density between a-Si:H and insulating layers by ESR and photothermal deflection spectroscopy

J O U R N A L OF

NON-CRYSTALLSOLII INE ELSEVIER

Journal of Non-CrystallineSolids 198-200 (1996) 778-781

Section 14. Interfaces and multilayers

Investigation of interface state density between a-Si:H and insulating layers by ESR and photothermal deflection spectroscopy Ikurou Umezu *, Kazuaki Kitamura, Keiji Maeda Department of Materials Science and Technology, Science UniversiO, of Tokyo, Noda, Chiba 278, Japan

Abstract The nature of surface defects in insulating films and interface defects between a-Si:H and insulating layers was investigated by electron spin resonance measurements. Surface state densities in a-Si:H, a-SiNt.y:H and a-SiO2,o were estimated by the thickness dependence of ESR signal intensity. The interface state densities in a-SiNI.7:H/a-Si:H and a-SiO2.o/a-Si:H were estimated by using stacked structures of the thin layers. These results were compared with results of photothermal deflection spectroscopy. The main peak of electron spin resonance absorption in these structures was near g = 2.0055. This value means that the defects in these structures lie at the a-Si:H side of the interface. The results obtained by the electron spin resonance measurements are consistent with our previous report that the interface defects in the top insulator structures are formed by plasma surface reaction.

1. Introduction The interface state density between a-Si:H and insulating layers is important for the application of thin film transistors (TFTs). Electron spin resonance (ESR) and photothermal deflection spectroscopy (PDS) are conventional methods to investigate surface and interface defects [1-6]. Although these methods were carefully compared to clarify the nature of bulk defects [7], reports which compare ESR and PDS measures of surface and interface defects are rare. PDS has an advantage because it can detect

* Corresponding author. Present address: Department of Applied Physics, Faculty of Science, Konan University, 8-9-1 Okamoto, Hikashinada-ku, Kobe 568, Japan. Tel.: +81-78 431 4341; fax: + 81-78 435 2539; e-mail: [email protected]

the energy distribution of the defects. The chemical environment of defect species can be estimated by the chemical shift of g-value of the ESR signal. Therefore, we can investigate the nature of the defect more clearly by comparing these two methods. By using the PDS technique, we have recently suggested that interface state densities in insulating layers on a-Si:H structures are strongly influenced by chemical reactions at the surface of the a-Si:H layer with plasma species [1]. In the present paper, we estimate the surface and bulk defect densities of a-Si:H, aSiNL7:H and a-SiO2. 0 films from the thickness dependence of the ESR signal intensity. Interface state densities between a-Si:H and a-SiNl.v:H or a-SiO2. 0 were measured by preparing stacked structures. These ESR results are compared with PDS measurements to understand the origin of interface defects.

0022-3093/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. Pll S0022-3093(96)00011-7

L Umezu et al./Journal of Non-Co'stalline Solids 198-200 (1996) 778-781

2. Experimental Amorphous silicon and insulating layers were prepared by the conventional P-CVD technique. The gas flow rate was 25 s c c m S i l l 4 and 25 s c c m H 2 for the a-Si:H layer, 5 Sill 4 sccm and 150 sccm NH 3 for the a-SiNI.7:H layer, and 3 sccm Sill 4 and 240 sccm N 2 0 for the a-SiO2 o layer. Total gas pressure and substrate temperature were kept at 0.5 Torr and 250°C, respectively. To investigate the interface defects, we prepared a-SiNl.v:H on a-Si:H (aSiNI.7:H/a-Si:H) and a-SiO2. 0 on a-Si:H (aSiO2.0/a-Si:H) structures. The sample structures used in both measurements were identical except for the substrate. The substrates were vitreous silica and Coming 7059 glass for the ESR and PDS measurements, respectively. The thicknesses of the a-Si:H, a-SiNt.v:H and a-SiO2. 0 layers estimated from deposition rate were 21, 13 and 17 rim, respectively.

3. Results Usually, ESR spectra are obtained as a derivative of the ESR absorption. In this paper, we obtained the ESR absorption by integrating raw spectra obtained in the measurement. The g-values of thick a-Si:H, a-SiN1.7:H and a-SiO2. 0 films were 2.0055, 2.0030 and 2.0013, respectively. The g-values of 2.0030 and 2.0013 are in good agreement with those of the K'-center in a-Si3N 4 and the E'-center in a-SiO 2, respectively [8,9]. There was a subpeak at g = 2.0013 for thin a-Si:H and a-SiN1.7:H films. Because this peak is considered to be due to plasma damage of the substrate, it was subtracted from the signal of a-Si:H and a-SiN1.7:H by peak separation in the subsequent results. Although this peak may also exist in the a-SiOz0 layer, we could not separate it from the a-SiO2, o layer because the g-value of this peak is the same as that of the a-SiOzo layer. Therefore, the results for a-SiO2. 0 include the defects due to the plasma damage of the substrate. The correlation between film thickness and defect density per unit area is shown in Fig. 1. The bulk defect densities determined from the slopes in this figure were 5 × 1016 c m - 3 , 3 × 10 j8 cm -3 and 1 × 1018 cm -s for a-Si:H, a-SiNj.v:H and a-SiO2, o layers, respectively. The surface defect densities estimated from the inter-

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cept at zero thickness were about 1 × 1013 cm 2 for a-Si:H and 0.5 × l013 cm 2 for a-SiNI7:H. The result for a-Si:H is in good agreement with that obtained by the thickness dependence of PDS [2]. The surface state density of the a-SiO2. 0 layer estimated by this method was about 1 × 10 ~3 cm -2. Because this value includes defects in the substrate as mentioned above, the actual surface state density in a-SiO2. 0 should be smaller than this value. Because the stacked structures were used to investigate the interface defects in the present paper, it is necessary to estimate the defect density per unit area of each layer used in the stacked structure. The bulk defect densities per unit area in 21 nm thick a-Si:H, in 13 nm thick a-SiNI.v:H and in 17 nm a-SiO2. 0 are 0.01 × 1013, 0 . 4 × 10 ~3 and 0 . 2 × 1 0 ~3 cm -2, respectively. Since the interface defect density between the insulating layers and a-Si:H is about 1013 cm -2, the bulk defects in the 21 nm thick a-Si:H layer is negligible [1,2]. On the other hand, the bulk defect densities in a-SiNI.v:H and a-SiO2. 0 are not negligible. Although the defect densities per unit area in the a-SiNI.v:H and a-SiO2. 0 layers are almost the same order as those in the a-Si:H layer, we could not observe defects in these insulating layers by PDS [1]. This inability is due to a limitation of the range of the excitation energy (0.8 to 3.0 eV) in our PDS apparatus. The defect absorption in insulating films lies at much higher energies owing to the wide optical gaps. For example, the defect absorption of SiO 2 glass is known to be centered at around 4.8 eV and that of nitrogen rich a-SiN X films is estimated to

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1. Umezu et al. / Journal of Non-Crystalline Solids 198-200 (1996) 778-781

be centered above 2.0 eV [10,11]. This means that PDS measurement is insensitive to defects in these insulating layers within our experimental condition. This feature is favorable for measuring the interface defects in the stacked structure since it is not necessary to take into account the optical absorption of defects which exist in these insulating layers. The results of the ESR measurements of the a-SiN1. 7:H/a-Si:H and a-SiOz.0/a-Si:H structures are shown in Fig. 2. In contrast to the case of the PDS measurement, we must subtract the signal of defects in the a-SiN~.7:H or a-SiO2. 0 layer from these stacked structures to obtain the interface defect density. We prepared the a-SiN1.7:H film exactly the same as that used in the a-SiNLv:H/a-Si:H structure in order to estimate interface defect density in a-SiN1.7:H/aSi:H. The ESR spectrum of this film and the result of subtraction of the signal is shown in Fig. 2(a) as dotted line and broken line, respectively. As mentioned before, the signal of the a-SiO20 film includes the defects in the substrate. Therefore, we can not attempt the same method in the case of a-SiO2.0:H/a-Si:H as has been done for aSiNLv:H/a-Si:H. Thus we tried to subtract the component of the a-SiO20 layer by peak separation. The result of the peak separation is shown in Fig. 2(b). Dotted and broken lines indicate the components of a-SiO 2 and a-Si:H, respectively. The ESR spectra of the interfaces obtained by these methods are shown in Fig. 3. The interface defect densities in a-

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SiNI.7:H/a-Si:H, native-oxide/a-Si:H and aSiO2.0/a-Si:H estimated by these spectra are about 0.5 x 10 j3, 1 × 10 ~3 and 2 × 1013 c m - 2 , respectively. These values are consistent with those obtained by the PDS measurement [1].

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Here, it is noteworthy that the main peak of ESR absorption in Figs. 2 and 3 is near g = 2.0055. This value indicates that the interface defects do not lie on the insulator side of the interface but lie on the a-Si:H side. This result is consistent with our previous report that the interface defects in a-SiO2.0/aSi:H structures are due to the surface reaction of the a-Si:H layer with the source gas [1]. The origin of the large surface defect density in a-Si:H compared with a-SiNI.v:H/a-Si:H is surface oxidation in air [2,4]. Both plasma and native oxidation processes increase the interface defect density. Although the precise reaction of surface defect formation is not clear, we discuss the defect formation process from a viewpoint of the bond energy. The bond formation energies of Si-Si, Si-N, Si-O, Si-H, N - H and O - H are 42, 81, 88, 70, 93 and 110 kcal/mol, respectively [12]. Because the reactivity of oxygen is very large, oxygen breaks surface Si-Si bonds to form Si-O bonds and defects should be formed during this reaction. Because the O - H bond is stronger than the Si-H bond, there is a possibility of breaking passivating Si-H bonds and forming O - H bonds accompanied by the formation of Si

1. Umezu et al. / Journal of Non-C~stalline Solids 198-200 (1996) 778 781

dangling bonds. Consequently, Si dangling bonds are formed at the surface of a-Si:H during oxidation. On the other hand, the S i - N bond is weaker than the S i - O bond. Therefore, the reactivity of the N atom with the S i - S i bond is weaker than that of the O atom. Furthermore, the N - H bond is also weaker than the O - H bond. Bond breaking reactions of S i - S i and S i - H by radicals derived from NH 3 are less effective compared with O atoms. Therefore, the interface defect density in the a-SiO2.0/a-Si:H structure is much larger than that in the a-SiNL7 : H / a - S i : H structure. The small defect density at the insulating layer side of the interface is due to the absence of plasma defect reactions. Precursors for depositing a-SiN~. 7 :H and a-SiO2. 0 layers from the present starting gases are SiHm(NH2),, and SiHm(OH),, with m + n < 4, respectively [13,14]. The interface defect density at the insulator side is smaller than that at the a-Si:H side because these precursors already have strong S i - N or S i - O bonds, and bond breaking reactions at the interface is minimal.

5. Conclusion The surface and bulk defect densities in a-Si:H, a-SiNL7 :H and a-SiO2. 0 were estimated by the thickness dependence of ESR. The surface state density of a-Si:H obtained by ESR was in good agreement with PDS measurements. The interface state densities in a-SiN~.v:H/a-Si:H, n a t i v e - o x i d e / a - S i : H and aSiO2.0:H/a-Si:H estimated by ESR were qualitatively consistent with the results of PDS. The main peak of the ESR absorption in these structures was near g = 2.0055. This means that the defects in these

781

structures lie at a-Si:H side of the interface. This result is consistent with our previous report that the interface defects in the top insulator structure is formed by plasma surface reactions in the first stage of deposition.

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