Photoemission study of the interfacial formation process and reactions between Al and hydrogenated amorphous Si

Surface Science Letters North-Holland

243 (1991) Lll-L45

Surface Science Letters

Photoe~ssion study of the interfacial formation process and reactions between Al and hydrogenated amorphous Si * Z.T. Zhong, D.W. Wang ‘, X.B. Liao, S.M. Mou, Y. Fan and C.F. Li Iastitlrteof Semiconductors and Laboratory for Surface Physics, Academia Sinica, Beijing 100083, People’s Rep. of China Received

19 April 1990; accepted

for publication

7 September

1990

Using photoemission spectroscopy and Auger electron spectroscopy, the interfacial formation process and the reactions between Al and hydrogenated amorphous Si are probed, and annealing behaviors of the Al/a-Si : H system are investigated as well. It is found that a ~~~irn~nsion~ growth of Al metal clusters which includes reacted Al and non-reacted metal Al occurs at the initial Al deposition time, reacted AI and Si alloyed layers exist in the Al/a-Si : H interface, and non-reacted Al makes layer-by-layer growth forming a metal Al layer on the sample surface. The interfacial reactions and element interdiffusion of Al/a-Si : H are promoted under the vacuum annealing.

The investigation of the metal/a-Si: H interface is very important for basic physical understanding and application development. There are but a few previous studies on the interface for a metal on amorphous Si (l-6]. Moreover, these studies have focused on the crystallization process and electrical properties. Tsai et al. [S] have investigated interfacial reactions of Au films on a-Si : H using interference enhanced Raman scattering. They have found that Si diffuses into the Au film near room temperature and that crystalline Si islands grow dendritically after being annealing above 150 ’ C. In this Letter, the interfacial reactions and the formation process of Al on a-Si : H are investigated by using X-ray photoemission spectroscopy (XPS) for the first time, from which important and new information is obtained. Besides, the interface interdiffusion of Al/a-Si : H was studied by Auger electron spec-

* The Project Supported by National Nature Science Foundation of China, ’ Radio and Electronics Department, Tsinghua University, Beijing 10084, People’s Rep. of China.

0039~6028/91/$03.50

0 1991 - Elsevier Science Publishers

troscopy (AES). In particular, we have studied the interfacial formation process at the initial stage of Al deposition on a-Si : H substrate under UHV condition and have investigated the influence of vacuum thermal annealing on interdiffusion and interfacial reaction. The interfacial formation process of Al on crystalline Si has also been studied [7-lo]. Owing to the differences in physical structure, the differences of composition and electron state between amorphous and crystalline Si, the formation process of interface for metals on amorphous Si will be different from metals on crystalline Si.

2. Expertmental procedures The a-Si : H films were produced in a homemade equipment with a system of three chambers. The a-Si : H films on the glass were prepared at 250 ‘C in a 30 mW/cm’ power density RF discharge of 99.99% pure SiH4 gas. The glass substrates (Corning 7059) were mounted onto the sample stage, which is electrically grounded and placed at a distance of 4 cm from the RF plate. The base pressure prior to deposition and the

B.V. (North-Holland)

r

pressure during the deposition were 1 x lo-’ Torr and - 0.7 Torr, respectively. The deposition rate of the a-5 : H film for a 10 seem SiH, flow rate c controlled by a mass flow controller was - 2 A/s. These deposited films contain about 8% hydrogen and the thickness of the film layers is 6000 A. Photoemission experiments at the initial stage of interfacial formation of Al/a-%: H were performed on a VG Scientific MICROLAB MKII spectrometer which contains both analysis and preparation chambers with base pressures of 2 X 10 “I Torr. After the a-Si: H deposition. these wafers were immediately loaded into the preparation chamber in UHV, XPS spectra from the a-Si : H surface show that the contamination of oxygen and carbon was below the X-ray photoemission limit. Aluminum was evaporated on the a-Si : H surface at room temperature from a tungsten basket which had been pre-outgased. The deposition rate of Al was low (about 1 A/mm). The photoemission measurements reported here were obtained with an analyzer pass energy of 20 eV and exciting radiation monochromatized Mg Ka X-ray (hv = 1253.6 eV). The intensity of XPS peaks was determined by measuring peak areas. After the measurements at the initial stage of the interfacial formation, the deposited sample Al/a-Si : H was annealed at various temperatures for 10 min in the UHV preparation chamber, at the same time photoemission measurements were perforn~ed in situ in the analysis chamber. The experiments for depth distribution of the composition were done by using a PHI-610 Auger electron spectrometer with pressures I 5 X lo-“’ Torr. The incident electron beam had an energy of 3 keV and current of 3 PA. The samples of Al/aSi : H were formed by evaporating Al onto a-Si : H using the molecular beam epitaxial (MBE) growth technique under ultrahigh vacuum. The thickness of the Al film was 500 A. The thermal annealing treatment of the samples was carried out in an ultrahigh vacuum (I lo-’ Torr) for 10 min.

3. Results and discussion The formation of and reactions at the interface between AI and a-Si : H have a great influence on

90 80 100 BINDING ENERGY (eV) Fig. 1. Core level photoemission from SiZp and Al2p of a qwzimen Al/a-E3 : H at room temperature for some Al deposition times.

the electrical properties. The XPS technique effectively gives us the chemical bond information about any interfacial reaction. Typical photoemission spectra from Si2p and Al 2p core levels for some Al depositions on aSi : H are shown in fig. 1. We have observed that the intensity of Al peaks (peak areas) increases, whereas that of Si peaks decreases with the deposition time. It is interesting to note that every peak of aluminum may be decomposed in two peaks. due to Al atoms bonding to Si. For example, the dotted lines of the topmost curve in fig. 1 illustrate this result, i.e. the Al 2p peak consists of Al(A) and Al(B) peaks. The binding energies of the Al(A) and Al(B) peaks are 72.90 and 74.63 eV, which correspond to that of metal Al and %-bonded Al, respectively. We have found that when Al is evaporated on the a-Si : H surface, interdiffusion between most of deposited Al and Si has been made and the Al-S1 bond which results in the existence of larger the Al(B) peak is formed. In addition, we have observed that the core level of Si2p is shifted to lower binding energy with deposition time. This also demonstrates the presence of a chemical reaction between Al and a-Si : H. All the Al-2p peaks in fig. 1 have been decomposed in to Al(A) and Al(B) peaks. and the inten-

2. I. Zhong et al. / Investigation of interfacial reactions and formation process of Al on a-Si : H

sity of every Al(A), Al(B) and Si2p peak (peak areas) has been measured as well. In fig. 2, we have plotted the Al(A) and Al(B) to Si2p XPS peak intensity ratio as a function of deposition time. The Al(A) and Al(B) curves show the evolution for metal Al (non-reacted) and Al (reacted) on a-Si : H surface, respectively, with Al deposition time. In addition, fig. 2 shows that the intensity of the Al(B) peaks is larger than that of the Al(A) peaks for all deposition times, i.e., most of deposited Al have a chemical reaction with a-Si : H. It can easily be seen that a break in the curve of XPS Al/Si intensity ratio versus time occurs at a certain deposition time (- 37 min), i.e., there exists a critical Al film thickness for both curves of Al(A) and Al(B). For a deposition time which is shorter than the critical one, the Al/Si intensity ratio increases slowly with the deposition time. This is characteristic for a three-dimensional growth of Al clusters. The clusters include metal Al and reacted Al (Al-S1 alloy). Exceeding the critical deposition time, a second deposition stage starts and the Al/Si intensity ratio increases rapidly for both curves. This is evidence for Al-S1 interdiffusion that forms a uniform Al-S1 alloy and a metal Al layer-by-layer growth. The microstructure and high density of defects in a-Si : H have been studied previously [ll-151. Messier and Ross [13] pointed out that the evolution of the island topology with film thickness leads to a surface relief pattern that is indicative

0

IO

20 30 DEPOSITION

40 so TIME (min.)

60

Fig. 2. XPS Al(A) and Al(B) to Si2p peak intensity ratio versus deposition time indicated by (0) and by (0) respectively.

Al cluster

metal

Al

(a)

metal AI

(b) a-Si:H Fig. 3. Cross sectional diagrams describing the initial formation process and reactions of interface between Al and aSi : H. (a) The first deposition stage: evaporated Al atoms tend to fall into the gap to form an Al-cluster which includes metal Al and reacted Al (Al-% alloy). (b) The second deposition stage: Al and Si interdiffusion to form uniform Al-Si alloyed region and metal Al layer.

of large voids, and the surface relief size is - 1000 A, with the additional cauliflower-like structure seen for roughened substrates. The island structure is characterized by lateral dimensions in the range 25-300 A with a distribution peaked around 100 A [12]. From the experimental results of the photoemission spectra and structure feature of hydrogenated amorphous Si, we can describe the initial formation process of Al/a-Si : H interface as shown in fig. 3. Owing to the presence of the island or cauliflower-like structure at an a-Si : H surface evaporated Al atoms fall into the gap between the incomplete coalescence of island-like structures and form Al clusters. The gaps of aSi : H surface are relative to voids. It is likely that the gaps are similar to a columnar microstructure originating from imperfect coalescence. In fig. 3a the observed three-dimensional growth of Al cluster at the initial first stage of Al reposition on the a-Si : H surface is energetically favored. Owing to Al exhibiting reactive features corresponding to strong chemical bonding on Si and high density of defects existing in hydrogenated amorphous Si, most of deposited Al can easily diffuse into a-Si : H and break the Si-H and Si-Si bond to form a Al-Si bond. Evidently, the Al-cluster includes reacted Al (Al-Si bonds) and metal Al as shown in fig. 3a. For the second deposition stage (exceeding

Z. I. Zhong et al. / Investigation

of interfactal reactions and formation

the critical deposition time), Al-Si interdiffusion and chemical reaction form an uniform AI-Si alloyed region, at the same time non-reacted AI makes layer-by-layer growth form metal Al layer on the sample surface as shown in fig. 3b. It is clear that the interfacial formation and reactions of Al on hydrogenated amorphous Si are not identical to those of Al on crystalline Si (the growth of Al islands onto a uniform interfacial layer [9]), due to differences in the physical structure of amorphous and crystalline Si. We now look at the evolution of the Al 2p core level photoemission spectra of the sample Al/aSi : H for various annealing temperatures in the UHV preparation chamber, as shown in fig. 4. We have observed that the intensity of the Al(B) peak increases, whereas that of the Al(A) peak decreases with annealing temperature. After the sample has been annealed to 470” C, the Al(A) peak becomes a shoulder of the Al2p peak at lower binding energies. It is shown that the interfacial reactions of Al/a-% : H are enhanced by increasing the annealing temperature. This forms an intermixed Al-Si alloy. The depth distribution of the interface composition for MBE Al/a-Si : H under various annealing temperatures are shown in fig. 5. Evidently there is no oxygen and carbon composition in the interface region. This also demonstrates the absence of surface contamination. The thickness of these Al layers and the

AL-2p peaks

Al(B) /I

proces.r

0

ofAl

on a-SI

: II

6 12 18 24 SPUTTER TIMElm!n 1

30

Fig. 5. The depth distribution of interface composition Al/a-Si : H under non-annealing (a). annealing at 35O’C and at 500°C (c). The sputtering rate of Ar ions was A/mix

of (b) - 5

sputtering rate are 500 A and 35 k/min, respectively. The interface of MBE Al/a-Si: H without annealing exhibits interdiffusion between Al and Si (fig. Sa). When the MBE Al/a-Si : H sample is annealed at 500 o C for 10 min in UHV, the interdiffusion of Al and Si becomes drastic and the interface region is broadened as shown in fig. 5~. Because of the release of the hydrogen in the a-Si network and the break lug of the Si-H bonds under the vacuum annealing, the interfacial reactions and element interdiffusion of Al/a-Si : H are promoted. Therefore. at elevated temperatures ( - 500” C) the Schottky barrier junction is destroyed and becomes an ohmic contact for Al/ a-Si : fi (161.

4. Conclusions 70 15 80 BINDING ENERGY(eV)

Fig. 4. A12p core level photoemission spectra of the sample Al/a% : H for various annealing temperatures.

Using photoemission spectroscopy electron spectroscopy, we have found

and Auger evidence for

ZI. Zhong et al. / Investigation of interfacial reactions and formation process of Al on a-Si : H

the existence of interdiffusion between Al and Si and Al-Si alloyed region is formed in the structure of an Al film on aSi : H. It is interesting to note that a three-dimensional growth of Al metal clusters, which includes reacted Al (Al-Si bonds) and metal Al, occurs at the initial formation of the interface for Al/a-Si : H, and after exceeding the critical Al deposition time, a reacted Al and Si alloy layer is formed, while non-reacted Al makes layer-by-layer growth form metal Al layer on the sample surface. Owing to the presence of a microstructure and the high density of defects in hydrogenated amorphous Si, differences exist between the interaction of Al with crystalline and hydrogenated amorphous Si. In addition, we have carried out vacuum annealing experiments on interface of Al/a-Si : H. This results indicates that the interdiffusion and the chemical reaction of Al/ a-Si : H interface are enhanced by annealing.

Acknowledgments We would like to thank Professor Kong Guanglin for stimulating discussions. We also would be grateful to Cui Yude, Zhang Libao and Duan Lihong for technical support regarding the AES and XPS works.

References VI S.R. Herd, P. Chaudhari

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