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DOPING EFFECTS IN TRANSPORT PROPERTIES IN a-Si p-n JUNCTIONS
Wu Yan, Y. Marfaing, J. Dixmier
To cite this version:
Wu Yan, Y. Marfaing, J. Dixmier. DOPING EFFECTS IN TRANSPORT PROPERTIES IN a-Si p-n JUNCTIONS. Journal de Physique Colloques, 1981, 42 (C4), pp.C4-515-C4-518.
�10.1051/jphyscol:19814110�. �jpa-00220727�
CoZZoque supptgment au nOIO, Tome 4 2 , octobre 1981
DOPING EFFECTS
IN
TRANSPORT PROPERTIESI N
a-Si p-n JUNCTIONSWu Zhong yan*, Y. Marfaing and J. Dixmier
Laboratofre de Physique des SoZides, CNRS, 92190 BeZZevue, France
Abstract. The electric transport mechanisms in a-Si:Hjunctions have been studied as a function of impurity doping level. At low doping current is limited by generation-recombination in the depletion layer. At high doping (10-3 R-lx cm-l on the junction sides) tunnel current are observed with an effective tunneling gap of 0.8 eV. The differential conductance around the origine is 3.10-1~-1cm-~.
Introduction. While the transport properties of amorphous silicon has received a considerable attention fewer studies have been devoted to the current mechanisms in p-n (1) or p-i-n junctions ( 2 ) ( 3 ) . However these mechanisms represent a
determining factor for the operation of a-Si solar cells. Furthermore the concept of stacked multispectral cells has been recently extended to amorphous Si-based materials ( 4 ) (5). In such structures connexions between photo-active junctions are assumed to be realized with "tunnel" p+
-
n+ diodes. In that perspective it appears useful to study the influence of doping on the behaviour of p-n junctions up to the eventual limiting case of tunneling.Experimental. a-Si films of various doping were deposited by cathodic sputtering in Ar-H2 atmosphere (usually 10% ~~).~ubstrate temperature was 250°C. Before
fabricating junctions single films with p, n, p+ or n+ conductivity type were studied separately. The corresponding temperature dependence of conductivity is shown in Fig. 1. Crystalline 0.5 n.cm p and 0.01 n.cm n silicon targets were used but the concentration of the B or P dopants measured by SIMS was 3 to 5 times lower in the p or n a-Si films. Consequently medium conductivity values were obtained in
Fig. 1 Conductivity temperature dependence of diversely doped a-Si films.
*permanent address : Ceramic Institute of ShangaT, China.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19814110
JOURNAL DE PHYSIQUE
agreement with previous results ( 6 ) . In order to get the more conductive and n+
films elemental A1 or Sb were added onto the targets and CO-sputtered.
Conductivities as high as in doped glow discharge prepared amorphous silicon were arrived at (7).
Junctions were fabricated by successive deposition of 2000
1
thick p+ and n films or p+ and n+ films (in that case 30 % H2 in ~r). Contacts were made with Cr-Sb on the n-type side and Au on the p-type side. The electric characteristic of a p+-n junction is presented in Fig. 2. The forward bias branch has the expected exponential-like variation limited above 0.5 Volt by series resistance. TheFig. 2 Current-voltage characteristic of a p+ - n junction (0.07 cm2 surface area).
ideality factor at room temperature is around 2. The variation of the pre-
exponential current .Io with temperature is given in Fig. 3, from which an activation energy of 1 eV is deduced. ~ l l these facts are indicative of current limitation by
Fig. 3 Preexponential forward current as a function of temperature for two a-Si junctions.
T~ N 10-l0 sec, a very short value related to the high band gap density of states in that material.
Fig. 4 shows the electric characteristic of a p+
-
n+ junction at two temperatures. Note the quasi-synrmetrlcal behaviour and the small temperature dependence. The latter is also illustrated in Fig. 3 where Ln (J,) appears to vary linearly with temperature. At room temperature the differential conductance around the origine is about 3 X 10-1 n-l cm-2.Current transport in p+
-
nf junction. It can be accounted for in both bias directions by tunneling between conduction and valence band tail states. A general expression for the current is (9) :I, = K
vP
E~ exp (-
a et3l2/~) (1)where V is the applied voltage, E is the junction electr'c field, et is the energy
* 5
gap overcome by tunneling and a is given by : a = (m ) /2@ where m* is the reduced effective mass and q is the electronic charge. Under reverse bias
conditions, the tunneling energy gap is constant and represents the energy distance between the onsets of tail states. Giving to E its maximum value EM = 2(Vi
-
v)/w,where V is the built in voltage and W the depletion width it can be easily shown that for V <<Vi one has (9) :
This linear variation as a function of V is verified in Fig. 5. From this plot one deduces (introducing WO = 200 A measured at V = 0, and Vi = 1.4 Volt) a B 3/2 = 3.107 volt cm-l. This value can be com ared with the theoretical evafuation made for crystalline Si(9) : 4.8 X 107, and be associated with an effective tunneling energy gap of 0.8 eV.
l"
1 l. S7 m-= v (volts)
Fig. 4 Current voltage curve of a p+
-
n+ junction (0.07 cm2 area).d (In I t ) d
(lnV)
-
I I
0.1
0.15V (volts)
Fig. 5 Voltage dependence of the reverse current for several tunnel junctions.
JOURNAL DE PHYSIQUE
Under forward bias conditions the current can be regarded as an excess tunnel current through the tunneling gap (1) (10) : e t = E
-
qv-
6,-
6p, whereE is the energy band gap and S,, 6 are the Fermi level pogitions on the n and p s ~ d e s g respectively. The current vo?tage curve is exponential. The zero voltage extrapolation varies with temperature as :
From the measured linear xariation shown in Fig. 2 one deduces d(Eg
-
6,-
~ ~ ) f d T =-
3.6 X 10- eVfK, a reasonable value for the band gap temperature coefficient.
Thus the voltage and temperature dependences are in agreement with the classical expression for the tunneling probability. However the absolute value of the current, measured by the conductance around zero voltage, is ten orders of magnitude higher than the theoretical estimation for direct band tunneling (11).
Such a discrepancy also affects the results of (l). Obviously amorphous semiconductors deserve a particular theoretical treatment as tunneling occurs between tail states and the energyfwave vector representation is not adequate.
Conclusion. In amorphous silicon p+
-
n+ junctions tunnel currents are observed the analysis of which leads to an effective tunneling gap (0.8 e ~ ) about half the optical band gap. The conductance around the origine corresponds to a voltage drop of 20 mV for a current density of 10 mAx cm-2, values compatible with the use in amorphousnultispectral cells.References
.
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2
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( 3 )
FEUSON
D.E. and WRONSKI C.R., J. Electron. Lett. 6 (1977) 95.( 4 ) MARFAING Y., 2nd EC. Photovoltaic Solar Energy ~onierence Berlin 1979,
Reidel (~ordrecht) p. 424.
(5) DALAL V.L. and FAGEN E.A., 14th IEEE Photovoltaic Specialist Conference 1980, p. 1066.
(6) NGUYEN VAN DONG and TRAN QUOC HAI, Phys. Stat. Sol. (b)
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(1978) 555.( 7 ) SPEAR W.E. and Le COMBER P.G.,Philosoph. Mag.
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(1976) 935.( 8 ) SAH C.T., NOYCE R.N. and SHOCKLEY W., Proc. IRE
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