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SERVI<;::O DE BIBLIOTECA E INFC'RVA<;::.2i.OH y d ro g e n a te d a m o rp h o u s s ilic o n film s b y 6 0 H z g lo w -d is c h a rg e d e p o s itio n
J. F. Fraoalli, L. Misoguti, A. N. Nakagaito, V. Grivickas,al and V, S. Bagnato
7nstltutoCfeFisica e Quimica de Sao Carlos-USP, Caixa Postal369, /3S6Q.970 sao Carlos SP. Brazil
H. M. Branz
National Renewable Energy Laboratory, Golden, Colorado 80401
(Received 28 September 1992; accepted for publication 11 March 1993)
We deposit hydrogenated amorphous silicon (a-Si:H) in a low-frequency (60 Hz) glow-discharge deposition system. The films show electronic and optical properties nearly equivalent to those of films produced by the conventional radio-frequency (13.56-MHz) glow-discharge technique. The optimal substrate temperature for the low-frequency glow-discharge technique is 150-170 ·C, about 100·C lower than at radio frequency. We report measurements of film properties including dark conductivity, photoconductivity, ambipolar diffusion length, infrared absorption, optical band gap, and deep defect density.
Hydrogenated amorphous silicon (a-SHI) used in commercial electronic, xerographic, and photovoltaic de- vices is normally deposited from a 13.56-MHz radio- frequency glow discharge (rf-GO) in silane gas. However, numerous alternative deposition techniques have been ex- plored in the hope of reducing the deep defect density, improving the stability, increasing the deposition rate, or lowering the deposition temperature of the films.I The
plasma excitation frequency, for example, has been varied from 10 kHz to 300 MHz2-s and dc excitation of the glow
discharge is also used.6 Most recently, Tochitani et al.7
re- ported a Si:H growth in a 50-Hz diode reactor. They found dark conductivity, photoconductivity, and infrared absorp- tion spectra characteristic of device-quality rf-GO. They doped the material and deposited prototype p-i-n solar
cells. In this communication, we describe the properties of high-quality a-Si:H deposited by triode low-frequency glow discharge Of-GO) from silane at 60 Hz. We have previ- ously given a preliminary account of our results. HHere, we
emphasize the dependence of film quality on deposition parameters and characterize the films by a wide variety of optoelectronic techniques.
To assess film quality, we measure dark conductivity
(Ud), photoconductivity (Ul'h)' transient photoconductiv- ity, ambipolar diffusion length (Ld), infrared (lR) absorp-
tion, Urbach energy (Eo), optical band gap (Eg), deep
defect density (Nd), and other properties. We find that our
best If-GO films have optoelectronic properties nearly as good as device-quality rf-GO a-Si:H films. Our optimal If-GO a-Si:H is deposited at a substrate temperature Tj of 150-170 ·C. This is -40·C lower than the temperature found by Tochitani et al. and about 100·C lower than nor-
mally used in rf-GO and dc-GO deposition. Such low dep- osition temperatures may be an advantage when depositing device layers on heat-sensitive materials. The low deposi- tion frequency is also an· advantage. Because 50-60 Hz power is universally available at low cost, our technique
does not require the power conversion equipment used dur- ing rf-GD deposition.
We prepare a-Si:H films using the deposition system shown schematically in the inset of Fig. I (a). The system consists of a stainless-steel chamber diffusion pumped to a base pressure of 10-6_10-7 Torr before each deposition run. Unlike Tochitani et al.•7we use a triode configuration. The two plasma electrodes are stainless-steel screens 15 cm in diameter and about 2 cm apart. Each has an array of 5·mm-diam holes separated by about 1 mm. To these elec- trodes, we apply the 6O-Hz voltage that excites the plasma. We use a transformer to set the voltage slightly above the threshold for extinguishing the plasma. Typically, the ap- plied voltage is 450 V and the plasma current is 4 mA. The substrate and both plasma electrodes are normally at de ground. For some runs, we applied a positive dc-bias volt- age to the lower electrode. Before deposition, the chamber and substrates are cleaned by a plasma of Ar gas. During deposition. electronic-grade SiH4 and H2 gases are intro-
duced in the chamber. We experimented with both low (about 10 sccm) and high (about 100 sccm) gas flow rates. Our typical deposition rate is about 40 A/min. We nor- mally measure the substrate temperature with a thermo- couple attached to the heater block. Calibration experi- ments show that during the deposition, the actual substrate temperature slowly rises to about 20·C higher than this thermocouple. However, in keeping with convention in the field, we report Tj as measured by the thermocouple at- tached to the heater block.
Films are simultaneously deposited on Coming glass, fused quartz, and crystalline Si to permit various charac- terizations. Our best films show good adhesion to all sub- strates and we grow films up to 1.5 pm. The thickness variation is abollt 5% across a 3-cm-long substrate. To investigate the relation between film properties and depo- sition conditions, we vary the partial pressure of SiH., H2
dilution of the SiH., gas-flow rate, dc-bias voltage and Tj• We grow our best films without H2 dilution or de bias and
10-3 0) FLOW HElTER a+LOW 10-4 ..!.~HIGH I "- 1 10-5 t· ''\ E
f
\
0 I. \ c: 10-6 1 • \ I \ >- \ ~ 1 \ > 10-7 I \ f- I \ tTph U ::l / \ a / \ z \ 0 10-8"
u"
"- "- • "-.
"- 10-9:..t--t---~d
...•,-..
---
10-10 DEPOSITION TEMPERATURE 1150 C I 0.1 0.5 1.0 1.5 2.0 PRESSURE (Torr I 10-3 b) FLOW ~LOW 10-4 ~..!.HIGH ~ -JJ--t- - -8 - /~ 0 0 I 10-5 /' / 8 , Ei
i' u ~. c: 10-6;:
•...>-t
> 10-1 •... ~: u :J a ( z 10-6•
0 + u 10-9 CTd 10-10 50 100 150 200 250 TEMPERATURE Ie).
, FIG. I. Dark conductivity and photoconductivity as a function of (a) ;. pressure at the fixedT, of ISO'C and (b) substrate temperature at the , filed pressure ofa.sTorr, The inset is a schematic diagram of the If-GO "~deposition system.
. We obtained device-quality rf-GD films from the U. S.
1National Renewable Energy Laboratory and measure their
;' properties using the same apparatuses and conditions . .These rf-GD results are reported below for purpose of .comparlson.
In Table I, we summarize measured optoelectronic . properties of the 1-llm If-GO a-Si:H prepared at optimal
temperature and pressure. The properties we measure for device-quality rf-GO I-flm films are shown for compari- son. We next discuss these results in detail.
A. Photoconductivity and dark conductivity
Figure 1 shows photoconductivity and dark conductiv- ity of If-GO films. We measured photoconductivities with 2.5 mW /cm2 of 0.633flm illumination from a He-Ne laser. The measurements are performed at room temperature in a coplanar electrode configuration. The data scatter shown is for repeated film depositions at nominally identical condi- tions.
The highest photo-to-dark conductivity ratio is ob- tained only for 0.5 Torr of SiH .• pressure and T$ between 150 and 170 ·C. The low dow rate yields a better ratio than the high flow rate. The other optoelectronic and structural indicators of film quality also indicated the same narrow range of optimal film deposition parameters. The photo-to- dark conductivity ratio of 7.5 X 104 of our If-GO films is about 20% lower than in rf-GO films. The small difference
may simply reflect a slightly higher optical band gap and smaller absorption in the If·GD films. The If-GO films have a room temperature value of CTd=: 10-9(ncm) -I and
a dark conductivity activation energy Ea of about 0.7 eV.
With the measured E,-I.72-1.8 eV this indicates a Fenni
energy near midgap. There is a rapid falloff in film quality for values of TJ above and below the optimal range. For
example, films deposited at 230·C have a greatly reduced
CTph/CTd< 104and Ea=:0.3 eV.
The dependence of photoconductivity on intensity in the range, 1= 1-100 mW/cm1 can be expressed as CTph=:[Y
for both If-GO and rf-GD films. The value of r is compa- rable in the two types of films. This suggests that roughly the same recombination pathways detennine the If·GD and rf·GD steady-state photoconductivities.
B. Near-Infrared absorption
Figure 2 shows the absorption coefficient spectra from which we derive Eo and Nd' The spectra are taken by the
constant photocurrent method (CPM).9 Eo is about 10 me V wider in our If-GO films than in the rf-G 0 films, indicating a somewhat broader valence bandtail state dis- tribution. We calculate the absolute magnitude of Nd by
multiplying the absorptance at 1.2 eV by a calibration fac- tor of 1.9 X 1016cm
-2, as suggested by Smith et af.10How-
ever, measurements in other laboratories on nominally identical rf·GD films suggest that the absolute magnitude of Ndwe detennine may be too high by a factor of 2 or 3
due to calibration error. In any case, the defect optical absorption in the best If-GO film is twice as high as in rf·GD a-Si:l!.
C. DIffusIon length and transIent photoconductIvIty We measure the ambipolar diffusion length by the steady-state photocarrier grating technique. II Ld is about 130 nm for the high flow oflf-GD films. comparable to the value in rf-GD films. It is only about 83 nm in low-flow If-GO films. The measurements were made in the low elec-
If-GO
Units High Dow Low Dow n-GD Error
T, 'C 170 170 240 =10
a"
(Oem)-I O.7X 10-9 1.3X 10-9 1X 10-9 20%a"
activation energy E. eV 0.7 0.6S 0.71 =O.OSaph (Hem)-I 2 X 10-5 I X 10-4 I X 10-4 20% arh/a" 3x 10 4 7.5XI~ lXIa' 40% Y from arh-/f 0.66 0.8S 0.67 =0.05 Eo meV 58 54.5 46 :1:4 N" from CPM em-J 2x 1016 3x 1016 I X 1016 :1:100% L" nm 130 83 130 :1:10
JI. reI. units' I 0.9 :1:20%
{3from ~arh-(t)-tl 1.03 1.05 :1:0.05
E, eV 1.73 1.80 1.75 =0.05
R OJ 0.25 0.065 :1:30%
Hydrogen content at. % 16 12 8 :1:20%
tric field « I kV/cm) and ambipolar regimesl2 in which
Ld is governed by hole transport in the material.
We performed a transient photoconductivity study of our films by using 4 ns laser pulse excitation at A=0.59
J.Lm. In this intensity range « 10 I-lJ/cm2) photoconduc- tivity is a linear function of intensity. We find the early time mobility J.L at the peak of photo response from the expression J.L=flGhvL2/e( I-R)E. Here flG is the mea-
sured conductance, R=0.3 is the assumed reflectivity from surface, hv is the photon energy, L is the length of the photoconductive gap, and E is the pulse energy. We find that in the high-flow If-GO sample. I-l is between 0.1-D.5 cm2jV s, comparable to our rf-GO result. As an indicator
-, -
-
. 10' :. + . " If· GO • rt • GO E : ; t : . u 10' f- Z W U (;: 10' u..
w ~+4 0 ~:. u t z 0 10' t f- Il. a: 0 VI aJ « ~ '0 , 10· • ~'ff," . • • • - : + • . • . t-.,..:
• + \.+.
10'+-...-,-...-,-...-,-...-,-...-..,..-...-..,..-..,..---j 08 10 1'2 "14 1'6 18 20 22 ENERGY leV}FIO. 2. Constant photocurrent method absorption coefficient spectra for high-flow If-GO and rf-OO films.
of the density of shallow conduction bandtail traps,I3 we measure the decay of photoconductivity from 10 to 400 ns. It follows Gph=t-fJ, with {3= 1.03. Because {3 and J.Lare nearly identical in rf-GO films, we suggest there are similar conduction bandtails.
D.IR s p e c tr u m
Figure 3 shows the IR absorption spectra of optimized low-flow and high-flow If-GO films together with the spec- trum of an rf-GO film. By integrating the wagging mode peak (640 cm - I) and multiplying the proportionality con- stant 1.6X 1019 cm2
,13 we determine the hydrogen content
in the films. The frequency of this 640 em-I vibration does not seem to depend on the preparation conditions. 14,13 The
H content is between 13-20 at. % in high-flow and 10-15 at. % in low-flow If-GO films, but is between 7 and 9 at. % in rf-GO films.
A small band-bending mode (840-890 em -I) peak is observed in our If-GO, as in those of Ref. 7. It appears that isolated SiH2 groups, having a mode at 875 cm - I, are
FIO. 3. IR absorption spectra of rf·OO and If-OO films. Absorption peak positions disc:ussed in text are indicated by dot-duhed lines.
present, rather than polymerized (SiH2)" groups with their scissors modes at 845 and 890 cm-1,14 The If-GO film also shows a small peak at 980 cm-I,which was identified as an
oxygen-related hydrogen complexl4 and suggest some 0
contamination. Within the stretch-mode peak, the ratio of the 2090 cm-1to the 2000 cm-I peak areas, R. is com- monly adopted as a microstructure parameter of a-Si:H, It is a factor of 4 greater in the If-GO film than in the rf-GD film, most likely due to the isolated SiH2 groups.
Finally, the ratio of the stretching to the wagging band : areas in both high- and low-flow If-GO films is up to 70% · larger than in the rf-GD films. There may be an unusually
1 large oscillator strength of the H stretch mode in the If-GD
: films. Shanks et ai.16 previously found that the oscillator ; strength decreases with increasing H content in rf-
sputtered films annealed below 260'C. Here we observe the opposite behavior: higher H content in If-GO films is cor-
related with an increase of the oscillator strength of the
stretch-mode band. It should be noted that Tochitani
. et aJ.7 grew high-quality If-G D films with a ratio of stretch-
:ing to wagging Si-H modes that appears similar to the ratio · observed in rf-GD a-Si:H.
We speculate that the differences between If-GO dep- osition and previous rf-GD and dc-GO deposition results are caused by a low-energy, periodic ion bombardment of : the growing If-GO a-Si:H surface. Ions are basically im- · mobile in discharges of frequencies above about I MHz, ; but bombardment can occur at lower frequencies. For ollr ,
iIf-GO deposition, we are using a three-electrode geometry · similar to that used in proximity dc-GO deposition to pre- ,vent ion bombardment of the sample.6 However, in If-GO,
. the voltage reverses twice per cycle and is near zero for a sizable portion of the cycle. At 10 V, an H+ ion is accel- erated across typical electrode spacings in about IllS and .this time decreases with voltage as V-I12. Consequently,
'for our 60 Hz applied voltage of450 V.some lower-energy ions will reach the substrate while the applied voltage is low. This may cause a periodic etching and deposition at the a-Si:H surface similar to that investigated by Tsuo
'et aJ.17 Because the ion bombardment supplies energy for
the mass transport required 10 prevent formation of coor-
dination defects and microstructure. it may explain tlle low iubstrate temperature that optimizes If-GO of a-Si:H.
IV. CONCLUSIONS
'." To summarize, we deposited high-quality a-Si:l1 by a Hz glow discharge from SiH4 gas. The If-GO films have
r f'
optoelectronic properties nearly as good as those of device· quality rf-GO a-Si:H. Further optimization of If-GO dep- osition parameters should improve the properties still fur- ther by control of ion bombardment.
The present film quality is sufficient to enable a-Si:H deposition, research and device fabrication withont the use of frequency conversion equipment. The reduced substrate temperature at which the best If-GO films are deposited presents opportunities to use a-Si:H with temperature- sensitive materials and in novel device structures.
The authors thank R. S. Crandall of the National Re- newable Energy Laboratory for supplying rf-GD a-Si:H samples, 1. G. Ferreira for assistance with optical absorp- tion measurements, and Y. S.Tsuo for helpful discussions.
V. G. acknowledges CNPq/RHAE for a research fellow-
ship at IFQSCIUSP.
IFor a review, see Y. S. Tsuo and W. Lull. Appl. Phys. Commun. 10.71
( 1990).
IH. Fujita, H. Handa. M. Nagano, and H. Matsuo, Jpn. J. Appl. Phys. 26. 1112 (t987).
IF. Boulitrop. N. Proust. J. Magarino. E. Criton, J. F. Peray, and M. Dupre, J. Appl. Phys. 58. 3494 (1985).
• A. MalSuda, T. Kaga. H. Tanaka, and K. Tanaka, Jpn. J. Appl. Phy~. 23. LS67 (1984).
\ fI. Cunis. N. Wyrsch. and A. Shah, Electron. Lett. 23. 228 (1987). "D. E. Carlson, C. W. Magee, and J. H. Thomas III, Solar Cells I. 371
( 1980).
JG. Tochitani, M. Shimozuma. and H. Tagashira. J. Appl. Phys. 72, 23~
(1992).
IJ. F. Fragalli. L. Misoguti. A. N. Nakagaito. H. M. Branz. V. Grivic kas, and V. S. Bagnato, Proceedings r.f Workshop on Crystalline and Amorphous Silicon and its Alloys, CNPq·NSF, Campinas, Brazil, May, 1992, p. I\.
9M. Vanevek, J. Kovka. J. Stuchlik. Z. Kozisek, O. Stika, and A. Trivka . Sol. Energy Mater. 8, 411 (1983).
IOZ. E. Smilh. V. Chu. K. Shepard. S. Aljishi. D. Siobodin. J. Kolodzey.
S. Wagner. and T. L. Chu. Appl. Phys. Leu. SO. 1521 (1987).
IID. RiUer, E. Zeldov. and K. Weiser, Appl. Phys. Leu. 49, 79 (1986)
lID. RiUer. E. Zeldov. and K. Weiser. Phys. Rev. B 38,8296 (1988); I. Balberg and S. Z. Weisz. Appl. Phys. 59. 1726 (1991).
IIJ.Kovka. C. E. Nebel. and C. D. Abel. Philos. Mag. B 63. 221 (1991). "G. Lucovsky. J.Yang, S. S. Chao, J. E. Tyler. and W. Czubatyj, Phy~
Rev. 0 28. 3225 (1983).
1\J P. Conde, K. K. Chan, J.M. Blum, and M. Arienzo, J.Appl. Phy~ 71. 3990 (1992).
I"H. Shanks, C. J. Fang. L. Ley, M. Cardona. F. J. Demond. and S Kalbitzer. Phys. Slatus Solidi B 100.43 (1980).
I1S. Tsuo. R. Weil, S. Asher. A. Nelson, Y. Xu. and R. Tsu. in Proc~~d·
ings of the 19th IEEE Photovoltaic Specialists Conference(IEEE. Ne"
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PAINEL
54 I'HIJllIll,,:AIl F: (:AHACTER 17./\<;1\0 Dr. Flur!S'VINOS DE SIL1cIO AMORFO
1lIlII!OI;I-:tIAIJO (n-:-;i :11) POll IlESGARGA LUMINESCENTE A 60 IIz
Lillo rlinogllli, .Io!le F, I·n!l~lllli. Antonio N. Nakagaito,
r
Vyl:nllln~ l;rivi"I'ns (Villlill~ IIl1iv.! p V.S. Bagnato - IFQSC-USP
, ,~
---"'.
,~OPTICAL PROPERTIES OF AMORPHOUS SILICON MEASURED USING Z-SCAN
TECHNIQUE
KOLENDA, .J.; GRIYICI<AS, V.
Vilniu.f Unll'emty· Vilniu.
MISOGUTI, L.j FRAGALLI, .J. F.; ZILIO, S. C.;
I
BAGNATO. V. S.
IFQSC, US?· Suo Car/o$
Thc invcstigations of amorphous hiorogenatco silicon (ilrSi:lI) in recent time have attract many attention Jue 1.0 the possible <Ipplications of this layers in op- tical devices. The technology of a-Si:II, comparcd to the cristallinc semiconductors. is inexpensive and there is at prcscnt time thc possibility to producc the largc surface area of the a-Si:II thin films of good optical qua- lity. The involving of this material in devices, based on nonlinear optics elrects predicts the estimation of no- nlinear optical propert.ies of a-Si:lI. In many works the time-resolved photo induced absortion (PA) ami light induced grating (LlG) methods was used to estimate photoinduced changes of the optical dielectric function and carrier dynamics in amorphous silicon films. In this letter we employ z-scan technique (ZST) to investigate the third order nonlincar susccptibility of a-Si:II films produced by glow-discharge tcchniquc. The excitation of the samples was performcd using light pulses produ- ced by Ith6G dye laser pumped with second harmonic light pulses of actively mode-locked Nd:YAG laser. We consider the change of absortion coefficient caused by carriers above optical gap absortion. The infiuense of the overlapping effects (interfcrence effect and thermal drect) 011cxperimcntill rC'sults is discussed.
FOTOLUMINESCENCIA EM LIGAS DE
CARDETO DE SILICIO AMORFO
IIIDROGENADO
l\lAGALllAES, C. S. DI::; ALVARI::Z, F.
IFGlI' /UNIC" AlP
A alta dilui ••iio dc hidrogenio na mistura de mct.ano e silana l'lurante 0 proccsso de dcposi<;ao por "glow dis-
charge" permite produzir Iigas de Carbeto de Silicio amorfo hidrogenado com baixa densidade de cst ados (1016cm-J) e bai..xa cauda de Urbach (5()"60meV) (I). Estuuamos este material atravcs da tccnica de fotolu- minescencia (PL), utilizando 11IT1laser de Argonio na linha 5145A para excitaA;iio. Os resultados obtidos fo- ram comparados com aquelcs do Silicio amorfo e Car- beto de Silicio fabricados de modo convencional, isto e, sem dilui ••ao de hidrogenio. Observamos a i7K que 0 comportamento da largura ua banda PL, relacionada a recombinal;ao cauda-cauda, escala com a cauda de Ur- bach. Esta similaridade nos leva a pensar que a largura cla banda PL rellete a largura das caudas das bandas de valencia e eondu<;iio.
Assumindo que a dependencia da efieiencia PL em
func;ao cia temperatura segue a relac;ao (2) [(I/yr) _1]-1
=
Yo exp(-T/Tt)onde YL
e
a eficiencia PL eTL a inclinac;iio da cauda dbanda, esta sendo em torno de 25K para a-Si:II. Veril
camos que
°
n08SO material se comporta mais proxim mente ao Silicio amorfo do que 0 convencional. Para nosso material obtivemos TL ::::50K, e para 0 convel cional em torno de 1021(.Finalmente, resultados da dependencia da PL com intcnsidade de excita~ao e com a energia de excita~a, sao apresentados e discutidos.
(l)Alvarez F.,Sebastiani M., Pozzili F., Fiorini P. all Evangelisti F., 19\)2, J. Appl. Phys., 71(1), 267.
(2)R.A.Street, "Hydrogenated amorphous silicon