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FUNDAMENTAL TUNNELING PROCESSES IN MOSa SOLAR CELLS
I. Balberg, J. Hanak, H. Weakliem, E. Gal
To cite this version:
I. Balberg, J. Hanak, H. Weakliem, E. Gal. FUNDAMENTAL TUNNELING PROCESSES IN MOSa SOLAR CELLS. Journal de Physique Colloques, 1981, 42 (C4), pp.C4-459-C4-462.
�10.1051/jphyscol:1981496�. �jpa-00220953�
JOURNAL DE PHYSIQUE
CoZZoque C4, suppZ6ment au nOIO, Tome 42, o c t o b r e 1981 page C4-459
FUNDAMENTAL TUNNELING PROCESSES I N MOS, SOLAR CELLS
7%
I . Balberg, J.J. Hanak, H . A . Weakliem and E . Gal
RCA L a b o r a t o r i e s , P r i n c e t o n , N . J . 08540, U.S.A.
he
Hebrew U n i v e r s i t y o f Jerusalem, J e r u s a l e m 92904, I s r a e lAbstract.- In previous studies of tunneling through a MOSa tunnel junction, where S, was a-Si:H, it was shown that their characteristics resemble those of MOSc devices where Sc was crystalline silicon. In the present work we would like to report a demonstration of fundamental tunneling processes in such tunnel junctions. In particular, the transition from semiconductor controlled regime to tunneling controlled regime can be clearly distinguished. The pre- sent results represent one of the rare cases where a fundamental semiconductor process is demonstrated more clearly in an amorphous semiconductor device than in a crystalline semiconductor device. Following the above findings we have identified tunneling into forbidden-gap states and we got a qualitative map of the state distribution in the forbidden-gap.
Introduction.- As is well known1 a tunnelable oxide in a Schottky-barrier solar cell may yield an increase in the open-circuit voltage of the cell without reduct- ion in its short-circuit current. This is achieved when the oxide thickness, d , in this metal-oxide-semiconductor (MOS) junction is optimized. When the oxide is very thin (d<ZOA) the transport through the junction is controlled by the transport through the semiconductor rather than the tunneling through the oxide. Qualitative- ly the junction behaves as a Schottky barrier.* This behavior is changed continuous- ly as the oxide thickness is increased until at about 30A the current becomes cont- rolled by its exponential decrease with d, i.e., by the tunneling through the oxide.
This situation is maintained with thicker oxides and at dZ70A the currents through the junction are hardly dete~table.~ Since the tunneling takes place between occu- pied and empty states one expects (in an n-type semiconductor) the following tun- neling processes.4 Under low forward bias the electron current, Jcm, from the semi- conductor conduction band to the metal, while for high forward bias, electron cur- rent Jm from the semiconductor valence band into the metal. Under low reverse bias we may expect the saturation electron current-Jcm from the metal into the con- duction band and Jmv from the metal into the valence band, while for high reverse bias we expect the current Jmc from the metal into the conduction band. Another process becomes important when the semiconductor has states in the forbidden gap.
This is the tunneling between the metal and these states, which yields the current Jms
In the semiconductor controlled regime Jcm is so large that it is impossible to apply the forward bias required to obtain J,, and under reverse bias the semicon- ductor is under deep depletion, i.e., it is hard to get the voltage drop across the oxide which is needed to obtain JmC. On the other hand, in the tunneling controlled regime J,, and JmC dominate, yielding the known "conductance This is since the electron density in the metal and in the valence band are much larger than the electron density in the conduction band. It is obvious then that for solar cell
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1981496
JOURNAL DE PHYSIQUE
application one will be interested in the transition region between the semiconduct -or controlled regime and the tunneling controlled regime, so that Jcm will be
attenuated by the oxide while JmV will still be determined by the holes supply in the semiconductor. A similar consideration is applicable for localized level spectroscopy. Here one would like to have an observable Jms which will not be masked by Jcm, -Jcm and Jmv.
In this paper we demonstrate the transition between the above mentioned two regimes and measure Jms between the metal and hydrogenated amorphous silicon, a-Si:H. A comprehensive comparison between our results on MOSa junction, where S, is the amorphous material, and results on MOS,, where S, is crystalline silicon, will be given elsewhere. We note however at this point that there are two advant- ages to the MOSa system which enable a better control of the transition region and consequently the study of each of the fundamental tunneling processes outlined above. The amorphous material can have a high enough resistivity to limit Jcm, and
(since it is a thin film) it limits the width of the deep depletion region. These advantages will be demonstrated below.
The preparation of the junctions and the method of measurement were described previously. The thickness of the oxide was controlled by oxidation time and tem- perature and its thickness has been determined by ellip~ometr~.~'~
Results and Discussion.- The effect of increasing the oxide thickness on a Schottky-like junction is demonstrated in Fig.1. It is seen that for all three
oxides the forward bias conductance which corresponds to Jcm is not
'gm-s, affected by the oxide. On the other
hand, under reverse bias the presence
ES 232-6
J!!L
of the oxide enables, in the deplet- ion mode, a voltage drop across the oxide which is enough to initiate the conductance associated with Jmc. That the rise of Jmc with applied voltage- / D -10 D ID 2 0
V , " ,
V becomes steeper with increasing d is exhibited by the larger values of
:
? .
? ;;p1 G for V<O. As the tunneling regime is approached one observes the con- ductance well and only the remnants of the Schottky-like conductance can be observed in the G-V characterist- ics. This case is demonstrated in Fig.2. The structure near V=O is due
0 8 2 .
v , v , to the increase of Jcm with forward
bias which is followed by its limit-
or-->
,
ation due to the series resistance of the semiconductor bulk. This limit- ation enables the increase in bias up to the voltage required to observe J,
. Here,in the phosphorous-doped material there are enough carries to yield a relatively large Jcm. In un-
0 -10 D 10 1 0
V , " , doped a-Si:H where the carrier con-
centration is low one usually ob-
serves a "square conductance ~ e 1 1 " ~ ' ~ without a structure in the forward
fig.1. Conductance voltage character- bias conductance. The structure ob- istic of a MOSa solar cell where S, was served then in the **conductance well1*
phosphorus doped (PH~/s~HI,=~x~o-~) hydro- for reverse bias4 *' must be
genated amorphous silicon. The three junct- identified as due to Jms and not with ions used differ by their oxide thickness: Jcm as in Fig-2. In the following we (a) ~ O A , @) 1 5 ~ and (c) 20A. m e positive present the I-V characteristics of a applied voltage indicates forward bias. MOSa junction and indicate the re-
lation between the observations and the known facts about the electronic structure of a-Si:H. For this purpose we used undoped a-Si:H. Typical results of such a junction are shown in Fig.3. In these characteristics there are some conspicuous features to be noted. These are the different voltage dependences of the positive (for V>O) and negative (for V<O] currents, and the structure around -0.6V. These features were found in the I-V characteristics of other a-Si:H materials4 which were prepared using different glow-discharge systems. The structure becomes more appar- ent in the G-V characteristics and it has been shown7 to be associated with a uni- versal peak in the density of states of a-Si:H which lies about 0.45 eV below the conduction band edge E,.
G ( p m h o )
I
Fig.2. The conduct- ance-voltage char- acteristic of phos- phorus doped
( P H , / S ~ H ~ = ~ O - ~ ) a-Si:H. The voltage convention is that for V>O the metal of the MOSa structure is positive. The oxide thickness was 35A.
Fig.3. I-V characteristic of an undoped a-Si:H MOSa tunnel junction, where the oxide thick- ness was 40A. (For negative voltage the current is negative) The voltage at which the metal Fermi level is opposite the con- duction band edge,EF,and the voltage at which thls level is opposite the valence band edge, E,, are indicated in the figure.
The problem of interpreting tunnel currents in terms of states density is not yet settled. However, it is clear that the simple WKBJ approximation,which predicts that the band-to-band tunneling current does not reflect the density of ~ t a t e s , ~ N(E), is not adequateg and relations between the tunnel current I and N(E) can be worked out.lO-l2 In particular, one would expect that tunneling from a band to local- ized states will yield a larger current than tunneling into extended states since for the localized states the electron wave vector is not a good quantum nwnber.1°
Indeed,experimentally,impurity bands have shown upl1-l3 by peaks in the G-V tunnel- ing characteristics when the impurity concentration has exceeded 5 ~ 1 0 ~ ~ c r n - ~ . In view
C4-462 JOURNAL DE PHYSIQUE
of this background we shall interpret our I-V characteristics using only the well accepted qualitative understanding of the I-N(E) relationship namely, that the current is a monotonically increasing function of the density of states.
The first stage in the interpretation of the above characteristic involves the determination of the relation between the applied voltage scale in Fig.3 and the energy scale of the forbidden gap. The procedure used for doing this for the junction under consideration was explained previously. '6 Consequently, we indicate in Fig. 3 the bias associated oith the onset of Jmc (Vpc) and the bias associated with the onset of Jvm (VzEV).
Using this procedure we can now re-examine the I-V characteristic with respect to the forbidden gap energy scale. This is done in Fig.4 where we have translated the applied bias scale to the energy-gap scale. Applying now the I-N(E) relationship mentioned above we may conclude the following: (a) the tail of the conduction band edge is much sharper than the tail of the valence band edge. (b) The density of states below the valence band edge is lower than the density of states above the conduction band edge. (c) Taking the band gap as Eg=1.6 eV the density of states is lowest between E,-0.5 eV and Ec-0.8 eV. (d) There is structure in N(E) at Ec-0.4eV.
These observations are consistent with previous experimental data14 and theoretical predictions. l 5 In particular, comparison with field effect results16 given by the dashed curve in Fig.4 shows a good agreement between the qualitative features of the state distribution as suggested by the two methods.
Fig.). The results of Fig.3,but with the applied voltage scale replaced by the electron energy scale (solid curve). Also shown is the state distribution deriv- ed in Ref.17 from field effect measurements (dashed curve).
designates the Fermi level of a-Si:H.
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