power density at 1.2 W cm-'. .... 131 W. Paul, A.J. Lewis, G.A.N. Connell and T.D. Moustakas, Solid State. Commw., 20 ... 161 R.C. Ross and R. Messier, .I. Appl.
ELSEVIER
Materials
Chemistry
and Physics 47 ( 1997) 97-100
Materials Science Communication
Electrical and optical properties of phosphorus-doped a-SiC:H thin films M.S. Aida, M. Ghrieb Instirut de Physique, Received
22 February
Universite’
de Constantine,
1995; revised 24 January
25000 Constantine, 1996; accepted
Algeria
14February 1996
Abstract Films of a-SiC:H have been prepared by sputtering of a silicon target with a small disc of triphenylphosphine (TPP) placed over thecentral region of the target. The triphenylphosphine is used as the feedstock of carbon and phosphorus. The optical and electrical characterizations reveal a reduction in both the optical gap and the activation energy and an increase in the dark conductivity for the obtained a-SiC:H film, as compared with films deposited without triphenylphosphine. This suggests that triphenylphosphine can be used as a dopant for preparation of a-SiC:H films. Keywords:
Amorphous
silicon carbide: Phosphorus;
Thin films
1. Introduction The deposition of amorphous silicon thin films by the sputtering method allows the possibility of introducing impurities in the deposited films by using an appropriate feed gas. Earlier, the introduction of a mixture of hydrogen and argon in the deposition chamber was used to produce a-Si:H films, where the hydrogen insertion reduces the density of gap states. Since successful substitutional doping was discovered in a-Si:H films [ 1,2], the incorporation of impurities in amorphous silicon thin films has been an active area of research. Earlier attempts were made towards phosphorus or boron doping in an atmosphere containing PH, or B2H6, as demonstrated by the Harvard group [ 31, During the last few years, a large number of amorphous alloys, namely a-Si:N, a-Si:Sn, a-Si:Cl, a-Si:C and a-SiC:H, have emerged and found a large number of potential applications. The work reported in this paper deals with a method of doping amorphous silicon carbide thin films by triphenylphosphine, P( C,H,) (TPP), in order to prepare phosphorusdoped amorphous silicon carbide thin films.
Ar/H, gas atmosphere. The target-substrate spacing was fixed at 5 cm. The partial pressure of H2 and the total pressure of the gas mixture were respectively 3.75 and 75 mtorr. Prior to the gas-mixture introduction, the deposition chamber was pre-evacuated to a pressure of the order of 10m7 torr. The substrate temperature was maintained at 200°C and the r.f. power density at 1.2 W cm-‘. The thicknesses of the films were of the order of 0.2 km. The deposition rate was of the order of 25 A min-‘. Infrared spectroscopy was performed on films deposited on intrinsic double-side polished monocrystalline silicon substrates. The opticai gap Eg was determined as usual by means of the Taut plot [ 41, according to the equation (dw)
=B(hv-Es)2
where hv is the energy of the incoming photons. The PTproducts were derived from the photoconductivity measurements at 600 nm and the activation energies E, from the temperature dependence of the dark conductivity measured on a coplanar structure.
3. Results and discussion 2. Film preparation
and experimental
details
Films were prepared by r.f. sputtering. A disc with 1 cm diameter prepared from powders of triphenylphosphine was placed over the central region of a silicon target with a 15 cm diameter. The latter and the T’TP disc were co-sputtered in an 0254-0584/97/$17.00
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Fig. 1 shows the IR absorption spectrum in the range 700 to 3000 cm-‘. The carbon-originated absorption bands are present and in particular the broad peak centred at 780 cm- ‘, coming from the rocking or wagging vibration of the Si-C, the Si-CH3, C-H, and C-H1 vibrations occurring at 960,
98
M.S. Aida, M. Ghrieb
Wave
Fig. 1. Infrared
absorption
Number
/ Materials
Chemistry
and Physics 47 (1997) 97-100
( cm-‘1
spectrum
for TPP-doped
film.
10"; :
0 Fig. 3. The Taut plot of doped and undoped
photor!
energy
(eV)
Fig. 2. The optical absorption coefficient variation vs. the photon energy for doped and undoped films together with the Harvard group result for PH,doped film.
2900 and 2860 cm-‘, respectively.‘The.peak at 2100 cm-’ is assigned to Si-HZ stretching vibration or clusters of Si-H [S] . However, as observed in the intrinsic a-Si:H films [ 6,7], classical 2000 cm-’ absorption due to the Si-H stretching is absent. In the case of amorphous-silicon-based alloys, it has been generally recognized that the absorption peaks shift towards a higher wave number [ 81. This phenomenon has been observed more clearly in a-SiC:H sputtered films, where the 2000 cm-’ peak shifted towards 2100 cm-’ [9]. This shift is due to the carbon environment of the Si-H bonds. This effect of environment has also been observed in amorphous silicon nitride films [ 101. As shown in Fig. 2 and Table 1, the doping of silicon thin films with triphenylphosphine changes the optical absorption shape and shifts the absorption edge to lower energy. The absorption shape of the obtained a-SiC:H film is almost comTable 1 Sample preparation
conditions
films.
parable with that of phosphorus-doped sputtered amorphous silicon observed by the Harvard group [ 1 I], as reported in Fig. 2. The optical gap Es is determined from the Taut plot (Fig. 3). The reduction of the optical gap in our doped film seems to be in contradiction with the carbon presence. It is generally found that carbon incorporation in an a-Si:H film network increases the optical gap. However, it is well recognized that the silicon forms the fourfold coordinated network, while the carbon forms both the threefold (graphite-like) and the fourfold (diamond-like) coordinations. The sputtering method yields films with more graphitically coordinated carbon atoms [ 12,131 than the glow discharge method. The graphitic component in a-Si:C:Hfilms increases also when hydrogen is eliminated from the film network. This occurs upon sample annealing [ 14,15], high deposition temperature [ 161 or a condition with a high ion bombardment of the growing film. Our deposition condition is similar to the last case. The energy of Ar ions striking the substrate can reach 20 eV and more [ 171, The increase in the graphitic coordination of the carbon leads to a reduced optical gap of about 20% of a-SiC:H films [ 16,18 ] . The same feature has been observed by Shimada et al. [ 12], They have prepared amorphous a-Si& -3. *H by simultaneous r.f. reactive sputtering of silicon and graphite. They inferred, from the measurement of the optical gap as a function of x, that when x varies from 0.6 to 0, besides the increase of threefold graphitic coordination content in their films, the optical gaps are reduced from 2.2 to 1.4 eV. Morever, one can speculate that the optical gap of a-SiC:H alloys increases with the density
and results
Sample
pr., Pw
Ts C’C)
4 (eV)
cdark (a cm-‘)
4 (ev)
p7 (cm’ V-‘)
Undoped Doped
200 200
200 200
1.8 1.61
1.5x10+ 2x 1o-5
0.76 0.40
2x lo-7 7X1W6
M.S. Aida. M. Ghrieb
/Materials
Chemistry
10;
103/ T
(K)-1
Fig. 4. The temperature dependence of the dark conductivity for doped and undoped films with comparison with the Harvard group result forPH-,-doped film.
of bonds stonger than Si-Si, such as StC, C-C, Si-H and C-H. Indeed, this is the the situation in the caseof dominant fourfold coordination in the network. Hencethe reduction of the optical gap in our film can be due to the graphite-like coordination of the carbon, However, it can be alsoassigned to the phosphorusincorporation in the film network. The influence of the latter on the optical gap is weaker than with boron. At high phosphorusdoping level, the absorptionedge is shifted by about 40 meV [ 191 towards a lower energy. In our case the shift is about 20 meV. This difference may originate from the carbon presence,the low doping level or both. The variation of the dark conductivity versus 1/T for n-aSiC:H, comparedto a-Si:H usedasreference, is represented in Fig. 4. There is an increaseby about four orders of magnitude in the dark conductivity. The increasein dark coiductivity may originate from the phosphorusincorporation, as generally approved. However, it may alsooriginate from the threefolded carbon presence,sinceit hasbeenobserved[ 201 that the dark conductivity of a-SiC:H films is increasedby about more than one order of magnitude with increasing carbon graphitic coordination in the films network. As seen in Table 1, doping our films with triphenylphosphine increasesthe product pr and reducesthe activation energy from 0.7 to 0.4 eV. Since the latter is the difference EC- EF, one can deduce that triphenylphosphine doping moves the Fermi level towards the conduction band edge. Then, we point out that relative to the undoped a-Si:H films, the variations of optical and electrical propertiesof the obtaineda-SiC:H have the sametrendsasphosphorus-doped amorphoussilicon. This might be explainedby the combined role of phosphorusand carbon presencein graphitic coordination. However, the interpretation of the role of phosphorus is somewhatcomplicated becauseof the imprecise knowledgeabout the coordination adoptedby the latter in the environment of silicon, hydrogen and carbon.
and Physics 47 (1997) 97-100
99
In a recent study on silicon-basedalloys [ 211, someevidenceof nitrogen and oxygen acting asdonorsin a-Si:H has beenfound, whereascarbon appearsto have no effect on the electrical properties of the material when incorporated in a small amount. The sameconclusionwas drawn by Solomon et al. [22]. They inferred that between0 and 20% of carbon in amorphoussilicon thin films, the semiconductingproperties of the material remainunchanged.This might be another explanation of the similarity of our results to thoseobtained in phosphorus-dopedamorphoussilicon. According to this, the carbon content in our doped films could be small; however, the XPS analysisof our films revealsa carbon concentration of the order of 30%. Indeed, sputtering of triphenylphosphine, which is the samefeedstock of carbon and phosphorus,doesnot enable the independentcontrol of carbonandphosphorusconcentration in the film network. It is worth mentioning that the sublimation temperature of triphenylphosphine at atmospheric pressureis 2OO”C,while, at the working pressure,10M7torr, the sublimation temperatureis around the ambient temperature. Hence, during the film growth, we have two sourcesof carbon and phosphorus:first, the sputtered atoms from the TPP disk and secondlythe dissociatedspeciesfrom the sublimate gas.
4. Conclusions In conclusion, we have proposeda method for preparation of phosphorus-dopeda-SiC:H thin films. It consistsof the cosputtering of a triphenylphosphinedisk and a silicon target. The former actsasa sourceof phosphorus,carbon andhydrogen. The IR spectroscopyanalysisdisplays the carbon-originated absorption. However, both electrical and optical measurementsshowed significant changeswith triphenylphosphinedoping. The properties of the obtained material are almost comparableto that reported in n-type amorphous silicon thin films depositedby sputtering of a silicon target in a PH3atmosphere.This is dueto the threefold coordination adoptedby the carbon.Thus, we suggestthat doping a-SiC:H thin films with triphenylphosphinemight be possible.This method may be interestingfrom the theoretical point of view and can find applications in structures such as p-i-n solar cells. Further transport and optical measurementsare needed for better doping control and understanding.They are under investigation.
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