sih4

MATERIALS CHEM;iTR’RYt?D ELSEVIER

Materials

Chemistry

and Physics

51 ( 1997)

152-156

Characteristics of a-Si:H films prepared by ECR CVD as a function of the H2/SiH4 Moonsang ‘Department

Kang a**, Jaeyeong Kim a, Yongseo Koo a, Taehoon Lim ‘, Inhwan Oh b, Bupju Jeon CTIlhyun Jung ‘, Chul An a of Electronic Engineering, Sognng University, R271, Sinsu-Dong, Mnpn.Ku, Seoul 121-742, South Korea ’ DiGion of Chemical Engineering, Korea Institute of S&we and Technology, Seoul, Somh Korea ’ Departmenr of Chemical Engineering. Dankook lJniversi@, Seed, .%wth Korea

Received 18 December 1996; revised 29 May 1997; accepted 29 May 1997

Abstract

The optical, electrical and structural properties of hydrogenated amorphous silicon films were investigated as a function of the HJSiH, ratio. The films were deposited by electron cyclotron resonance plasma chemical vapor deposition method in the source gas limited and electron flux limited mode. In the source gas limited mode, the properties of amorphous silicon films were improved with increasing deposition rate photoconductivity, hydrogen content increased and optical band gap, full width at half maximum of the Raman spectroscopy and the ratio of the concentration of dihydride to that of monohydride decreased. In the electron flux limited mode, the oprical, electrical and structural properties as well as the deposition rate did not improved any more. The photoconductivity was over lo-’ a- ’ cm-’ when the optical band gap was 1.75 h 1.77 eV, FWHM was below 7.5 cm- ‘, hydrogen content was about 21 at.% and the ratio of dihydride to the monohydride was about 1.5 in the electron flux limited mode. 0 1997 Elsevier Science S.A. Kqwords:

ECR CVD; a-Si:H; Photoconductivity; Electron flux limited mode; Source gas limited mode

1. Introduction Hydrogenated amorphoussilicon (a-Si:H) films havebeen widely applied [ 1] to semiconductordevices, suchas photosensitive devices, solar cells and thin film transistors (TFT). In the conventional radio-frequency (RF) plasma chemical vapor deposition (CVD) [ 21 using SiH, gas, it is difficult

to obtain good characteristics

of amorphous

silicon

films at high deposition rate. The substratesbeing located in the plasma region,

plasma damage

[3]

due to charged par-

ticles with high energy is inevitable. Recently, in order to minimize this problem, the electron cyclotron resonance (ECR) plasmaCVD method [4-91 has been tried for the deposition

of hydrogenated

amorphous

silicon

films.

The

attractive features of the ECR plasmaCVD are as follows. (a) The ECR plasmaCVD processingdoesnot involve electrodes so that contamination due to sputtering of electrodes during plasmaprocessingisnegligible. (b) The ECR plasma CVD processing yields a high degree of electron and ion generations.

(c) The ECR plasma CVD processing

can be

* Corresponding author. Tel.: i- 82 2 706 3401; fax: + 82 2 706 1216; e-mail: kangms96~llownuri.nowcom.co.kr 0254-0584/97/$17.00 0 1997 Elsevier Science S.A. All rights reserved PIZSO254-0584(97)01986-X

operatedat a wide rangeof gaspressures.(d) Plasmaprocessing is a low temperature processing. Therefore, ECR plasmaCVD hasbeenusedto obtain high depositionrate and high quality amorphoussilicon films. In our previous work [lo] we studied the quality of the a-Si:H films with the variation of substratetemperature.The high quality a-Si:H films could be obtained with high substrate temperature where the depositionrate was low. In this paper, the optical and electrical propertiesof ECR CVD produced a-Si:H films are reported to give further insight into the various H2/SiHS ratio affecting the electrical and optical properties of this material. In this case, higher quality a-Si:H films were obtained with lower dilution ratio where the deposition rate was high. This paper alsoreports an investigation of the sourcegaslimited mode and electron flux limited mode [ 41 asa function of the ratio of Hz to SiH,. In order to study Ihe difference

of the electrical

and optical

characteristicsbetweenthe sourcegaslimited andthe electron flux limited mode,we observedphotoconductiviry ( gp), dark conductivity

(ad),

optical band gap (E,,,) , hydrogen

content

( C,), full width at half maximum (FWHM) of the Raman spectroscopyand the ratio of the infrared absorptioncoefficient at 2090 cm- ’ ( cxzosO) to 2000 cm- ’ ( a)2000).

Momsang

Kong

et al. /Mnterinls

Chemisrr-y

and Physics

51 (1997)

152-156

153

2. Experiments The purpose [ 1 I] of ECR plasma is to increase the path of electrons in the plasma by applying a magnetic field normal to the electron trajectory, enhancing the probability of ionization. Resonance is achieved when the frequency at which energy is fed to an electron circulating in a magnetic field is equal to the characteristic frequency at which the electron circulates. This circulating motion increases the ratio of ionized to non-ionized species in ECR plasma by three orders of magnitude over that in simple RF plasmas [ 121. The high efficiency in exciting the reactants in ECR plasma allows the deposition of films at room temperature without the need for thermal activation. An Astex- 1000 ECR plasma generator, a home-made CVD reactor (Fig. 1) and a high vacuum system were employed to carry out the experiments. It consisted of two chambers, the plasma chamber and the deposition chamber. The reactants are introduced through two separate inlets; hydrogen molecules are introduced into the plasma chamber, while silane is fed into the deposition chamber. Microwave power is introduced into the plasma chamber through a rectangular waveguide at a frequency of 2.45 GHz. The cylindrical plasma chamber operates as a microwave cavity resonator. The electron cyclotron frequency is controlled by magnetic coils arranged at the periphery of the chamber. At the above microwave frequency resonance is achieved at a magnetic strength of 875 Gauss. A highly activated plasma is then obtained at very low gas pressures. Ions are extracted from the plasma chamber into the deposition chamber and sub2.45 GHz MICROWAVE ECTANGULAR WAVEGUIDE

d

a-Si:H Corning

f

no. 7059

Fig. 2. Mask layout and cross-section measurement sample.

view

of photo/dark

conductivity

jetted to a divergent magnetic field that spreads the plasma stream over the entire wafer. Table 1 shows the deposition conditions of a-Si:H films. The dilution ratio H2/SiH, was 0, l/9, l/6, l/4, l/2, 2, 4, and 9. The total flow rate was 20 standard cubic centimeters per minute (seem) The microwave power was 500 W. The a-Si:H films were prepared on the Coming no. 7059, single crystalline silicon ( 100) wafer and thermal oxide on silicon (SiO,/Si) wafer. Photoconductivity, dark conductivity and optical absorption spectra measurements were performed on the Corning no. 7059 substrate. Infrared (IR) absorption spectra measurements were performed on single crystalline Si wafer. Raman spectroscopy analyses were performed on SiOZ/Si wafer. The photo and dark conductivity of a-Si:H films were measured using coplanar Al electrodes (Fig. 2) deposited by an electron beam evaporation technique on a-Si:H films. L is the distance of the electrodes. W is the width of the electrodes and d is the thickness of the a-Si:H film. The photoconductivity was measured under AM1 ( 100 mW cm-*) illumination [ 131. The infrared absorption coefficient was measured by Fourier transform infrared (FTIR) spectroscopy.

3. Results and discussion Fig. 3 shows the deposition rate of the a-Si:H films as a function of H2/SiH,. As the H,/SiH, ratio decreased from 9 to 0.5 the deposition rate increased. However, at the H,/SiH, Fig. 1. Schematic Table 1 Deposition

conditions

Base pressure Total flow rate HJSiH, ratio Pressure Microwave frequency iMicrowave power Magnetic field Substrate temperature Substrates

diagram

of a-Si:H

of ECR plasma CVD system.

films

lo-‘Torr 20 bccm

0, l/9,

116, 114, I/2,2,4,

9

4.5 mTorr 2.45 GHz

500 w 875 Gauss

0.0

3oo”

Corning

0.1

_

1.0

LO.0

H, I SiH,

no. 70.59, c-Si, Si02 on Si (SiO,/Si) Fig. 3. Deposition

rate as a function

of HJSiH,

ratio.

Moonsong

0.0 0.1

Kang et al. /Mrtrrrinls

1.0

Chemisly

10.0

H, I SiH,

Fig. 3. Photo/dark

conductivity

as a function

ofH,/SiH,.

ratio below 0.5 the depositionratedid not increasedanymore. The depositionmode, in the region H,/SiH, ratio below 0.5, is the electron flux limited mode and the other is the source gaslimited mode.In the sourcegaslimited mode,deposition rate increasedwhen the SiH, gasincreased. Fig. 4 shows photo and dark conductivity of the a-Si:H films asa function of H2/SiH,. The dark conductivity values were in the range of lo-” w lo-l0 R-’ cm)-‘. These are smaller than those obtained by the conventional RF plasma CVD method. In order to find out the contamination which affect the dark conductivity during the film deposition, secondary ion massspectrometry (SIMS) measurementswere used. Oxygen, nitrogen and carbon were as low as those obtained by conventional plasmaCVD and any metalswere not detected in the a-Si:H films. In the source gas limited mode, the photoconductivity increasedwith decreasingHz/ SiH, ratio and dark conductivity values did not change. In the electron flux limited mode, photo and dark conductivity values did not change. Photoconductivity was about 10m5fi2-i cm-‘, and sensitivity (log(o,lrrr,)) was about h 5.5 in the electron flux limited mode. Figs. 3 and 4 are replotted in Fig. 5. In the source gas limited mode, the photoconductivity increasedwith increasing depositionrate. This relationshipof the film quality to the deposition rate (RQD) is positive [4]. While someof the optical properties of a-Si:H films have been reported [ 3-51, the relation between the optical properties and deposition rates have not beenextensively studied.Kobayashi et al. [ 3] obtainednegative RQD at the different substratetemperature and positive RQD at the different microwave power by ECR CVD. ECR CVD has also been applied to prepare a-Si:H films by Zhang et al. [4]. A positive RQD wasobtainedwhen changing the dilution ratio of SiHj with He (He/SiH,). On the contrary, the RQD was negative in the dependenceon the H, dilution (H,/SiH,). With increasing H2/SiH, ratio the photoconductivity increases and the deposition rate decreases.Hishikawa et al. [ 5 ] alsoreported the relationship of the film quality to the deposition rate as a function of substratetemperature,gas pressureand SiH, flow rate. But the data of their report wasnot enoughto get the exact result as a function of SiH, flow rate. In our study, we obtained positive RQD at the dilution ratio H2/SiH1. In the electron

and Physics 51 (1997)

152-156

flux limited mode,photoconductivity anddepositionrate did not change.From Fig. 5, it is found that the optical and electrical properties areimproved with increasingdepositionrate in the sourcegaslimited mode. The optical absorptioncoefficient ( cu) of the a-Si:H films were decided Fromtransmittanceandreflectancespectra The optical band gap of a-Si:H films on glasssubstratewasdetermined [ 141 from the straight-line intercept of a (n&l,) “’ versus hv curve at a relatively high absorption region (cr> lo4 cn- ‘). cyis absorption coefficient, hrl is photon energy and ??is the real part of the refractive index. The (n&v) “’ plot has better linearity than the (&v) “’ plot. The well-known ( ah Y) “’ plot is less suited for detailed discussionof the optical band gap than the cube root plot, becausethe plot includes a large ambiguity in the optical band gap. The optical band gap decreasedwith decreasing H21SiH4 ratio in the source gas limited mode as shown in Fig. 6. In the electron flux limited mode,optical bandgap did not change.In our previous work 1IO], the optical band gap decreasedwith increasingthe optical properties.The hydrogen content can be determined [ 15] from the wagging mode at 640 cm-’ becausethe wagging mode absorption is proportional to the total hydrogen content independent of the bonding configuration. Absorption at 840.890 cm- ’ andfor the stretching modes, on the contrary, is dependenton the bonding configuration. We alsousedmethodof Langford et al. [ IS] to determine hydrogen content becausethe most commonly used method of Brodsky, Cardona, and Cuomo IEJ

r

Electron

flux limited

Source

1E-8 e 0

10

10

gas limited

30

40

50

Deposition rate (nm/min) Fig. 5. Photoconductivity

as a function

rate.

30

1.90 Electron

nur I

Fig. 6. Optical ratio.

of deposition

band gap and hydrogen

limited

content

as a function

of HZ/Sill,

Moonsntrg Kmg et al. /Mnre~Yals

Cizemist~

leadsto significant errors in absorption coefficient, particularly in films of lessthan 1 pm thick. The hydrogen content decreasedwith increasingHJSiI& in the sourcegaslimited mode asshown in Fig. 6. In Fig. 7, the ratio of the infrared absorptioncoefficient at 2090 cm- ’ to 2000 cm- ’ and FWHM are shown asa function of the H2/SiH4 ratio. Ramanspectroscopyrevealed that no microcrystalline or polycrystalline silicon phaseswas present, based on the absence of a Raman peak near 520 cm-‘. Only the broad amorphous silicon peak at 480 cm-’ was detected. The absorption coefficient at 2090 cm-’ is SiH, bonding mode and 2000 cm:’ is SiH bonding mode [ 3,161. As the H2/SiH, ratio was decreased, the ratio of c+~~,,/cy2woand FWHM decreasedin the source 85,

,4.0 Electron

, Source

flux

gas

Fig. 7. FWHM and (Y~~~XI/CY.. ,w as a function of H2/SiH., ratio.

and Physics 51 (1997)

152-156

155

gaslimited mode. In the electron gaslimited mode, FWHM decreasedbut the ratio of ~~~~~~~~~~~ did not change. The photoconductivity and photoluminescence intensity are known to decreaseas the ratio of concentration of monohydride to that of dihydride increases[ 171. Monohydride in aSi:H films becomesthe dominant bonding mode and the dangling bonds areprobably decreasedwith decreasingH,/ SiH, ratio in the sourcegaslimited mode. In order to find out the condition of the high photoconductivity, all data are plotted photoconductivity versus Eopt, FWHM. CHand cy2090/~zo00 asshownin Fig. 8. In Fig. 8(a), as the optical band gap decreasedthe photoconductivity increased.The photoconductivity wasabout lO-‘s2-’ cm-’ when the optical band gap was 1.75* 1.77 eV. The dark conductivity valueswere in the range of 10-l ’ h 1O- ‘” flR- ’ cm-‘. These values are smaller than those obtained by the conventional RF plasmaCVD method. Fig. 8(b) showsthe photoconductivity against the FWHM. We obtained high photoconductivity at lower (below 75) FWHM asshown in Fig. 8 (b) The photoconductivity increasedwith decreasing the number of the dangling bonds. In Fig. 8(c), the high photoconductivity was obtained when the hydrogen content was about 21 at.%. In Fig. 8 (d), ~2090/cy20~o and photoconductivity areshown. As the ~~~~~~~~~~~ decreased,photoconductivity increased.When the ratio of the dihydride to the monohydride was below 1.5, high photoconductivity was obtained. From these studies we may conclude that the deposition rate, E,,,, FWHM, CH and hydrogen-bond configuration

IE-3 lE-4

I(a)

.;’ s v 8 w

IE-8 lE-9

Dark

l -e

lE-10 lE-11 IE-12’

1.76

1.80

1.84

1.88

68

’ 70

’ 72

Optical band gap (eV)

’ 74

FWHM

’ 76

’ 78

’ 80

J

82

(cm)-’

IE-3

7 5 5 P g P ‘J 8 w

1E-4

b

IE-5

-

1E-6

-

lE-7

-

lE-8

-

lE-9

-

lE-10

-

lE-8 Dark l

w

v-0

.

lE-It 9

12

Hydrogen

15

t

18

21

24

Dark

1.4

1.6

1.8

2.0

2.2

content (at. %) a2090 1 a2004 Fig.8. Conductivity versus opticalband gap (a), FWHM (b), hydrogen content (c) and az,,90/cuzuw(d),

156

Moonsnng

Kung et al. /Marerinls

Chen~isrq

(SiH,/SiH) influences the properties of photoconductivity of a-Si:H films. The properties of a-Si:H films as a function of H,/SiH, ratio are as follows. (a) In Figs. 4, 6 and 7, the properties of photoconductivity do not improve any more in the electron flux limited mode. On the contrary, the dark conductivity does not change in both modes. In the source gas limited mode, the photoconductivity increased with decreasing E,,,, FWHM, cx-~09u/~2000and increasing the CH. (b) In Fig. 5, the deposition rate affects the optical and electrical properties of a-Si:H films in the source gas limited mode. That is, the relationship of the photoconductivity to the deposition rate is positive. (c) The optical and electrical properties are affected by the Eopr, Ctl, a,090/~200U than by FWHM as shown in Fig. 7. FWHM decreased and the other parameters ( vP, EoptTCH, ~~~~~~~~~~~~did not change in the electron flux limited mode. We cannot determine which effect is most dominant for photoconductivity. Further studies of the relations the film quality and FWHM are required. (d) The photoconductivity increases with decreasing Eopt, C, as shown in Fig. 8. MM, Q+X / ~moo and increasing The properties of a-Si:H films have improved when the films contain a low number of dangling bonds and enough hydrogen which is configured to the monohydrogen bond.

4. Conclusions We have studied the characteristics of the a-Si:H films in the source gas limited mode and the electron flux limited mode as a function of H,/SiH, ratio using ECR plasma CVD method. In the source gas limited mode, the properties of aSi:H films are improved with increasing deposition rate. With an increase of deposition rate, the photoconductivity increased to 10m5 R-’ cm-’ and sensitivity increased to 5.5. The Eopr, FWHM and L~~,,~~/Q,~~ decreased with increasing deposition rate. That is, the properties of the a-Si:H films prepared by ECR CVD using H,/SiH, are improved with increasing deposition rate in the source gas limited mode. In the electron flux limited mode, the properties of the a-Si:H

and Physics 51 (I 997) 152-156

films do not improve any more. The high photoconductivity was obtained over IO-’ s1-l cm-’ when the Enpt was I .75 - 1.77 eV, FWHM was below 75 cm- ‘, CE+was about 21 at.% and the a,,,,/ar,, was about 1.5.

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