In situ electron-spin-resonance measurements of film growthof hydrogenated amorphous silicon
Satoshi Yamasaki,a) Takahide Umeda,b) Junichi Isoya,c) and Kazunobu Tanakab)
Joint Research Center for Atom Technology (JRCAT), National Institute for Advanced Interdisciplinary
Research (NAIR), 1-1-4, Higashi, Tsukuba, Ibaraki 305, Japan~Received 28 October 1996; accepted for publication 2 January 1997!
In situ electron-spin-resonance ~ESR! measurements of film growth of hydrogenated amorphoussilicon ~a-Si:H! using a remote hydrogen plasma technique have been performed. The Si
dangling-bond signal in a-Si:H during and after deposition has been detected, in addition to thegas-phase ESR signals both of atomic hydrogen and radicals related with silane molecules. Dynamicchanges of the Si dangling-bond signal intensity have been observed in real-time, where the signal
intensity increases with deposition time and decreases after stopping the deposition due to astructural relaxation. High potentiality of in situ ESR techniques for microscopic understanding offilm growth and surface reaction has been demonstrated. © 1997 American Institute of Physics.
@S0003-6951~97!03609-7#
In the field of semiconductor technology, various kindsof processing are utilized to fabricate devices. To get goodquality films, surfaces, and interfaces, various probes such as
infra-red, visible light, electron beam, scanning probes, etc.
have been introduced for monitoring microscopic processesof growing or reacting surfaces. Infra-red reflection absorption spectroscopy,1 spectroscopic ellipsometry,2 scanningprobe microscopy3 etc. provide information on film density,
bonding configuration, surface atomic arrangement, and others. On the other hand, defect centers, whether they are in thebulk or at surfaces and interfaces, play an essential role indetermining final device performances.4 If one can directly
observe the creation and annihilation of defect centers andthose dynamic bonding configurations during film depositionand surface reaction in real-time, it will give much more
important information for the improvement of device performances. Initial attempts to measure defect centers by electron spin resonance ~ESR! technique during film growth
were carried out by Johnson et al.5 for a-Si:H deposition
using the remote plasma system, and they detected ESR signals associated with hydrogen atoms and E8 centers of thesilica-glass tube. However, no Si dangling-bond centers ofa-Si:H ~D centers! could be detected, perhaps, because ofpaucity of deposited material.
We performed in situ ESR measurements during deposition of hydrogenated amorphous silicon (a-Si:H! for a wide
range of deposition conditions and different geometrical relationships between the ESR cavity and the remote plasma
system. We succeeded in depositing a-Si:H film in the ESR
cavity and observing D center signals from the film during
and after the deposition. In this letter, ESR signals not only
from the solid phase but also from the gas phase are presented with a tentative interpretation of their dynamicalchanges.
For in situ ESR measurements of growing a-Si:H films,a remote hydrogen plasma system was used.6 The basic design of the system was adapted for compatibility with theESR apparatus. The sketch of our experimental setup isshown in Fig. 1~a!. The hydrogen plasma discharge was sustained within a microwave cavity ~2.45 GHz!, which is located upward of the x-band ESR cavity. Molecular hydrogenflowing through a high-purity vitreous-silica ~HPVS! tube,outer diameter of 6 mm, was dissociated into atomic hydrogen at the position of the discharge cavity. Silane gas wasmixed with atomic hydrogen in the ESR cavity and the primary reaction takes place there as follows:7
SiH41H→SiH31H2.

~1!

As a result, a thin film of a-Si:H was deposited on the insidewall of an outer HPVS tube having an outer diameter of 10mm, as shown in the figure. The substrate temperature of thissetup was room temperature. ESR measurements were madeusing the BRUKER ESP 300E system.
The distance between the end of the inner tube and theESR cavity was adjusted so that the a-Si:H film was deposited with its thickest point at around the center of the ESR
active region, whose thickness distribution with respect toa!

Permanent address: Electrotechnical Laboratory. Electronic mail:satoshi@jrcat.or.jpb!
Also at: Institute of Materials Science, University of Tsukuba, 1-1-1 Tennoudai, Tsukuba, Ibaraki 305, Japan.
c!
Also at: University of Library and Information Science, 1-2, Kasuga,Tsukuba, Ibaraki 305, Japan.

FIG. 1. ~a! Sketch of experimental setup. ~b! Film thickness as a function ofvertical position, x. The thickest point is at the end of an inner high-purity
vitreous-silica tube. The ESR active region is 0 mm ,x, 10 mm.

Appl. Phys. Lett. 70 (9), 3 March 1997
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FIG. 2. ESR spectra after film deposition. ~a! ESR spectrum of D centers ina-Si:H after deposition with pressure of 5 Torr, silane flow rate of 5 sccm,
hydrogen flow rate of 50 sccm, discharge microwave power of 20 W. The
peak-to-peak width and g-value are 0.7 mT and 2.0055, respectively. ~b!
ESR spectrum for the case of relatively low pressure deposition. Dangling
bond signal in a-Si:H, E8 center ~dangling bond signal of SiO2 ) and hydrogen hyperfine structure with splitting of 7.4 mT related with E8 centers areseen.

the end of the inner tube is shown in Fig. 1~b!. The positionof x50 mm is referred to at the end of inner tube. A typical
ESR spectrum after deposition is shown as curve ~a! in Fig.
2. The spin density is around 1018 cm23 . The present deposition condition was with a gas pressure of 5 Torr, a silane
flow rate of 5 standard cubic centimeter per minute ~SCCM!,
a hydrogen flow rate of 50 SCCM, and a discharge microwave power of 20 W. The spectrum ~a! is identified as a D
center in a-Si:H because its peak-to-peak width and g value
were found to be 0.7 mT and 2.0055, respectively. This isquite different from the report of Johnson et al.3 They observed spectra of the E8 center associated with a dangling
bond of SiO2 ~Ref. 8! as well as a hydrogen hyperfine structure with a splitting of 7.4 mT related with E8 centers,9,10 and
speculated that those signals originated from the film resulting from a reaction between silane–hydrogen plasma andoxygen extracted from a HPVS tube. This difference between the present data and those of Johnson et al. might becaused by the difference of plasma conditions, although intheir paper, detailed plasma conditions are not given. Actually, we observed almost the same signals as theirs using aplasma condition of much lower gas pressure ~, 1 Torr!,
which is also shown as curve ~b! in Fig. 2. Even for this case,D centers were observed in addition to the signal related with
E8 centers. This kind of spectrum was also observed from thesystem after exposure to the hydrogen plasma in the absenceof silane flow.
During deposition of the a-Si:H film, two kinds of signals were observed, which are shown in Fig. 3; a doublet
hyperfine structure of atomic hydrogen, in ~a!,11 and a single
peak related with silane gas in ~b!. Both signals are thought
to originate from gas-phase, because they are seen only during deposition and their intensity and shape are influenced byplasma conditions. The latter signal intensity increases withan increase of silane gas-flow rate and line shape seems to bedominated by pressure broadening. This signal could be related with SiH3 radicals, because in this remote plasma system SiH3 is the main radical of plasma reaction, although weFIG. 3. ESR spectra during deposition ~a! a doublet hyperfine structure of
atomic hydrogen, and ~b! a single peak of spin centers related with silanegas. ~c! The second derivative curve of ESR absorption during deposition
being the in-phase 2v component of a modulation. This signal was clearlyobserved even after stopping deposition and was confirmed to be identical tothat of the D centers.

need more intensive research for the final identification.
Another independent structure is also seen at the positionof g52.0055 superposed on the large gas-phase signal ofFig. 3~b!, as is marked by a circle in the figure. To see thedetailed shape we took a second derivative curve of absorption being the in-phase 2v component of a modulation,
which is shown in Fig. 3~c!. This signal was clearly observed
even after stopping deposition and was confirmed to be identical to that of the D centers. Since the signal intensity isincreased with the increase of deposition time ~which isshown in Fig. 4! the ESR signal is considered to originate
mainly from the bulk region. After stopping the plasma, along-lasting decay of D center signal was observed invacuum, as shown in the figure. According to a tentative

FIG. 4. D center ESR intensity as a function of elapsed time during andafter film deposition. During deposition the signal intensity increases withan increase of time, while it decreases after stopping deposition.

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analysis, the decay curve comprises fast and slow components. It is speculated that the slow decay is related withstructural relaxations in a bulk region depending on the depth
from the surface and the fast one is due to a relaxation in thevicinity of the surface. Details of the post growth signal intensity decay will be published in the near future.
For this measurement a film with high spin density wasdeposited to observe the dynamic change of D centers. Evenfor a device-quarily a-Si:H sample after deposition it is wellknown that there exists a high spin density region around asurface by ;1012 cm22 . Since the typical area of this system
is ;3 cm2 , this spin number is in a detectable range for ESR
measurements, although the noise level during deposition islarger than that of plasma-free conditions.
During the growth of a-Si:H film a variety of chemical
reactions are expected to occur at the growing surface
such as [Si-~s!1SiH3 ~g!→[Si–SiH3 ~s!, [Si–H~s!1H~g!
→[Si-~s!1H2 ~g!,
[Si–H~s!1[Si–H~s!→[Si-Si[~s!
1H2 ~g!, [Si~s!1[Si-~s! →[Si–Si[~s!, etc., where ~g!
and ~s! denote ‘‘gas-phase’’ and ‘‘surface,’’ respectively,
and [Si-~s! represents a D center at the surface. For thesesurface processes, hyperfine interactions between D centersand hydrogen atoms and/or other interactions between
Dcenters should be involved. However, as is seen in Fig.
3~c!, no discernible characteristic signals except D centers ina-Si:H have been observed. Reasons why such signals asrelated with hyperfine and/or electron spin–spin interactions
are absent might be the fast reaction time of surface processes, because a fast reaction causes lifetime broadening of
the ESR spectra, and/or paucity of their density. In order toclarify this, similar experiments at low substrate temperatures will be done in the future.
In conclusion, the ESR signal associated with D centers
has been observed from a growing a-Si:H film in addition to

the gas-phase signals related with silane and hydrogen. This
experiment opens up the possibility of directly observing thedynamic process of defect creation or annihilation, alongwith its detailed bonding configuration, on the growing surface as well as in the bulk region undergoing structural relaxation. One can also discuss the microscopic reaction processes between the gas phase and the surface.
The authors thank A. Matsuda, G. Ganguly, and J.
Hanna for valuable discussions. This work, partly supported
by NEDO, was performed in the Joint Research Center for
Atom Technology ~JRCAT! under the joint research agreement between the National Institute for Advanced Interdisciplinary Research ~NAIR! and the Angstrom Technology
Partnership ~ATP!.

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