4 9 2001 The Moroccan Statistical Physical and Condensed Matter Society Au/n-Si(100) contact homogeneity studied by direct and reverse ballistic electron
emission microscopy and spectroscopy A. Chahboun and I. Zorkani
L. P. S., Faculté des Sciences Dhar Mehraz, BP 1796 – FES (Atlas), Maroc R. Coratger , F. Ajustron and J. Beauvillain
CEMES/CNRS-BP 4347-31055 Toulouse cedex 04- France
The Au/n-Si(100) contact has been studied using reverse ballistic electron emission microscopy and spectroscopy.
Two types of localised collector currents have been observed; one, positive corresponding to electron injection into Si, and the other, negative, associated with hole injection into the semiconductor. The comparative trial of BEEM and reverse BEEM images from the same area shows this difference to be linked to the interface structure. Effects of surface roughness on the observed contrasts are also discussed.
PACS: 61.16P, 73.40N, 68.20
I. INTRODUCTION
In numerous modern microelectronic applications, metal-semiconductor contact plays an important role in many modern microelectronic applications. Understanding the physics of electron transport and the growth morphology of these structures is of great importance when attempting to optimize their electronic properties.
In 1988, Bell and Kaiser [1] successfully introduced Ballistic Electron Emission Microscopy (BEEM) for measurement of the Schottky barrier height and its spatial variations at the buried metal-semiconductor interface.
This technique relies on the Scanning Tunnelling Microscopy (STM) [2] tip used as a ballistic electron (or hole) source across the vacuum barrier into and across a thin metal base region deposited on a semiconductor.
Typically, metal thickness is comparable to the electron mean free path in the bulk material. Then, a significant fraction of electrons may be collected in the third electrode consisting of the semiconductor substrate. Electron transverse momentum conservation implies a narrow area to be probed at the interface and gives a great resolution to this technique (below 2nm).
FIG.1: Schematic diagram of BEEM system
Most BEEM studies reported in the literature involve an electron injection from the STM tip to the metal surface. In a previous work, Bell et al [3] showed that on Au/n-Si contacts, hole injection gives collector currents with the same positive sign as electron injection. On the other hand, Ludeke et al [4] have observed a negative current under reverse bias conditions on Cr/n-GaP contacts.
In this paper, we report the study of Au/n-Si(100) contact by using electron or hole injection. Reverse BEEM (RBEEM) characteristics highlight two behaviours. A collector current of identical polarity, as observed in a conventional BEEM experiment, accounted for by an Auger phenomenon and an opposite polarity current accounted for by hole extraction from the Si valence band.
Correlation between BEEM and RBEEM images from the same area shows that, the positive collector current obtained at negative bias voltage is observed in areas where the conventional BEEM current is uniform at positive bias voltage. Negative collector currents obtained at negative bias voltage are observed in areas where the forward BEEM current is absent. Metal-insulator- semiconductor (MIS) structures at the interface account for these observations. In addition, the effects of surface roughness on the injection geometry are invoked to account for the absence of collector current in BEEM and positive current in RBEEM experiments.
II. EXPERIMENTAL SETUP
Au/n-Si (100) devices were examined at room temperature, in the surrounding air. The sample consisted of 0.5 mm diameter Au dots with a thickness of about 8 nm. The gold film was evaporated on n doped (1015 cm-3) Si(100) surfaces . The Si(100) substrate was HF cleaned before being inserted into the preparation chamber, where vacuum was kept in the 10-7 - 10-8 torr range. The ohmic contact is achieved by eutectic In/Ga on the back of the sample. The BEEM apparatus used in this experiment is a home-built setup already described elsewhere [5]. For ballistic electron emission microscopy, a specific electronic setup provides a variable bias voltage between tip and sample (VT), and detects the induced collector current (Ic) variations with a resolution in the picoampere range. The positive or negative bias voltage was applied to the metal surface by pasting a 50µm diameter copper wire on the dot edge. The collector current Ic vs bias voltage VT pA
STM tip
nA
Vt
Au Si
curves were measured in the conventional constant current mode with tunnelling currents from 1 to 10 nA. During spectrum acquisition, the STM tip was located at the same place over the sample. To obtain STM and BEEM images simultaneously, the tip scans an area under a constant bias voltage, higher than the threshold value. Two parameters are simultaneously stored: the tip height variation yielding surface topography under constant tunnel current mode, and the different values of collector current shown in grey levels that allow for the BEEM image to be built.
III. BEEM TECHNIQUE
BEEM allows determination of buried metal/semiconductor interfaces with high resolution.
Between the STM tip and metal surface, we know that electrons contribute to tunnelling current It. This is completely different for the collector current Ic. On the one hand, these electrons should have a sufficient energy to overcome the potential step and to be collected in the semiconductor (eV>ΦSB). On the other hand, limitations on transverse momentum conservation lead to reflection of electron whose transverse momentum component K// (i.e.
parallel to the interface) is too large (with + ⊥
=K K Kρelectron ρ ρ
// ). This effect could be taken into account by calculating the critical angle θc between the incident electron path and the normal to the interface. This angle is given by the following relation [6]:
eV E V m mt c
F SB +
= ( − )
2 .
sin φ
θ
where mt is the electron effective mass parallel to the interface within the semiconductor, m is the electron mass, EF is the Fermi energy and V the tip-sample voltage.
FIG.2: Different possibilities for electron injection at the metal-semiconductor interface. Path 1 corresponds to an electron refracted in the semiconductor because its K// is relatively small. Electron 2 (K// is too large and θ> θc) and 3 (multiple interactions) remain in the semiconductor.
For silicon in which mt is equal 0.1 m, we obtain a critical angle of 5.5° for V=1.5 V. This angle is therefore relatively small and explain the resolution of this new microscopy: for an Au thickness of 10 nm, the resolution at the interface is 1.7 nm. Each spectrum gives the local
electronic characteristic of the metal/semiconductor interface with this high resolution, the same properties apply to the BEEM images.
Interpretation of the experimental spectra has been furnished by Kaiser and Bell in their first paper on BEEM.
This one dimensional theoretical model has been later improved by two dimensional calculations. The authors have used the formalism, introduced by Simmons in 1963 for tunnelling between planar electrodes [7], to describe both tunnelling and collector current. Hence, the collector current may be written as:
∫ ⊥
∞ ∫
= RC D E ⊥ f E dE dE Ic
E
E min
max
0 ( ) //
) (
where C is a constant, E⊥ and E// are the energy perpendicular and parallel to the interface respectively, f(E) is the Fermi-Dirac function and D(E⊥) is the well known expression for the transmission probability of a square barrier. Emin and Emax are given by the energy and momentum matching conditions at the interface. R is a measure of attenuation due to scattering in the metal layer.
In the calculations, the collector current only depends on two parameters: the Schottky barrier height ΦSB and R. We have used this formalism to fit the experimental data in the case of low bias voltages, i.e. at the most 0.5 V above the threshold.
IV. RESULTS
Fig.3 shows simultaneous STM and BEEM images of an Au/n-Si(100) junction at a positive bias voltage of 1.3 V with IT = 3 nA.
Thus, electrons are injected from the STM tip to the gold layer. The STM image (left) shows the classical granular structure of the gold layer with a maximum corrugation of approximately 15 nm. For the BEEM image (right), light to dark areas correspond to a collector current from 0 pA (dark), to 70 pA (white). This image reveals various domains (A, B and C) with well-defined values of BEEM current, the edges of which follow surface topographic features such as grains.
FIG.3: Simultaneous STM (left) and BEEM (right) images recorded at a tunnelling current of IT = 3nA and a bias voltage of VT = 1.3 V. Three domains (A, B and C) corresponding to three different interface behaviours can be seen.
STM tip
N 1
3 2
metal
semiconductor
Fig.4a is a typical spectrum obtained on a uniform emitting are a such as A. The theoretical fit using the Kaiser-Bell (KB) [6] model is also shown and gives a barrier height of 0.78 eV.
B and C domains show no collector current at a tunnelling bias voltage of 1.3 V. Increasing the voltage to 3 V, some electrons can be detected in the semiconductor on domains.
Fig.4b gives an example of such phenomenon and shows a spectrum giving a threshold value of 2.75 V.
Fig.5 shows the result obtained on the same area as in Fig.3 with a negative bias voltage of –1.3 V and the same absolute value of tunnelling current (electrons injected from the gold layer to the STM tip).
FIG.4a : BEEM spectrum recorded on a A type domain, showing electron injection into the semiconductor. The KB model fits perfectly the experimental spectrum, yielding a Schottky barrier of ΦSB=0.78 eV.
On the RBEEM image, the collector current ranges from 13 pA (white) observed in the A domains, to –390 pA (dark) observed in the C domains. Thus, with a negative bias voltage, electrons or holes can be injected into the semiconductor, according to the tip position above the metal surface. In the A domains, in which a significant BEEM current is measured under positive bias, electron injection into the semiconductor is also observed under reverse bias. On the other hand, the areas (B) in which a zero direct BEEM current is detected, displays a weak positive collector current (about 10 pA), which demonstrates that electron injection occurs in these domains.
FIG.4b : BEEM Spectrum obtained on a C type domain, with a threshold of 2.75 eV.
C domains show no direct BEEM current, but exhibit negative currents under reverse bias corresponding to hole injection.
These effects can also be observed on the RBEEM spectra carried out in these different domains. Fig.6a is a RBEEM spectrum obtained from the A domain. The positive value of collector current proves electron injection under reverse bias. The threshold value derived from the KB fit is -0.9 V.
As suggested by RBEEM imaging, the collector current sign may change depending on the STM tip location on the surface. Fig.6b presents an RBEEM spectrum obtained from a C domain. The negative collector current sign shows that hole injection becomes the dominant mechanism of carrier transport through the interface. In some cases, the collector current saturates at a bias voltage of about –1.5 V, as illustrated in Fig.6c, meaning that another current component of opposite sign adds to majority hole injection.
V. DISCUSSION
BEEM spectroscopy highlights the carriers transport properties, while BEEM imaging allows these phenomena to be localised. On the A domain, electrons are always injected into the semiconductor irrespective of bias. Most BEEM spectra obtained in the direct bias regime on these domains are well-fitted by the KB model and yield an average threshold of 0.78 eV (Fig.3a), this is in good agreement with previously reported results [6]. On the other hand, the spectra with a threshold of about 2.75 eV (Fig.4b) account for the fact that some areas, appear dark in the BEEM image at low bias voltage (C domains in Fig.3) despite their low corrugation (below 3nm). These effects have been assumed to be linked to the presence of metal-insulator-semiconductor (MIS) structures [8,9,10]
that shift the local barrier height to higher values.
However, in some dark domains, a significant BEEM current could not be detected even at high voltages. This cannot be due to a gold thickness effect (this issue has already been discussed [8]) but may be associated with a thicker oxide layer that prevents electron transport through the structure. In RBEEM, the A domains, where a significant direct BEEM current is detected, present a uniform positive current, resulting from electron injection through the junction. This is also shown by the spectra (Fig.6a) recorded on these areas. In this case, the KB model does not yield an accurate fit near the threshold.
This indicates that this model describes more accurately electron transport than hole injection and transport [11].
This already reported positive collector current in RBEEM, can be accounted for by an Auger mediated phenomenon [3,12,13]. A hot hole generated in the metal by electron extraction, recombines with an electron near the Fermi level via Auger process, with the excess energy transferred to a second electron. With an adequate energy, this electron may overcome the potential barrier and be collected in the semiconductor.
0,6 0,9 1,2 1,5 1,8
0 20 40 60 80 100
fit
experimental data Ic (pA)
VT(V)
2,0 2,5 3,0 3,5
0 20 40 60 80 100 120
Ic(pA)
VT(V)
FIG.5: Simultaneous STM (left) and BEEM (right) images recorded in RBEEM at VT =-1.3 V and IT =3 nA. Electron injection into the semiconductor is observed on the A and B domains. Electron extraction is revealed in the C domain.
For type A domains, electron injection is, therefore, the dominant mechanism whatever the bias. Usually, this behaviour is observed in large areas and represents the major phenomenon.
For B domains, the collector current is zero in the direct BEEM measurements and about 10 pA in RBEEM. Then, only electrons created inside the gold layer via an Auger process (RBEEM) can be collected in the semiconductor.
On the other hand, those injected by the STM tip (BEEM) never reach the semiconductor. This suggests that surface effects can modify injection geometry or induce electron scattering which prevents current detection under forward bias. As these domains generally show high surface gradients and high corrugations (15nm in Fig.3), it is assumed that electrons reach the interface at angles beyond the acceptance cone or encounter multiple scattering in the gold layer whose thickness in this case is larger than the electron maen-free-path [11,14,15] in this energy range.
On low corrugated C domains, the BEEM current is always zero, while the RBEEM current is negative (Fig.5).
Therefore, it is assumed that these phenomena are induced by a thin insulator layer between metal and semiconductor that prevents electron injection at low voltage and shifts the barrier height to high values. Hole injection observed in these domains in RBEEM could be explained by defect- induced states below the Fermi level that enable hot holes to travel through them, as suggested by Sumiya et al [16].
FIG.6a :Typical RBEEM spectrum recorded on A domain.
Note that the KB model does not accurately fit the experimental points near
threshold.
FIG.6b : RBEEM spectrum recorded on a C domain, showing hole injection from metal to semiconductor.
These negative currents may reach significant values (a few hundred picoamperes). They correspond to hole injection from the metal to the semiconductor due to electron extraction from the Si valence band to the metal [4]. The hole injection barrier height measured in the spectra carried out in these area (Fig.6a) is about 0.8 eV.
Theoretically, hole barrier height is given by :
[ΦhSB= Eg-ΦSB =0.32 eV], where Eg is the energy gap and ΦSB the electron barrier height. As in forward bias, the presence of a thin insulator layer between metal and semiconductor may also explain the difference between measured and expected barrier height. In some RBEEM spectra, a saturation in the experimental spectrum at about -1.5 V is observed, followed by a current decrease at high voltages (Fig.6c).
Ludeke et al [4] also reported this phenomenon on Cr/GaP contacts with RBEEM currents becoming positive at high voltages. This change in the current derivative could be due to hole impact ionisation in the semiconductor, in which a hote hole created by electron extraction loses its energy by creating one or more
-1,4 -1,2 -1,0 -0,8 -0,6 0 10 20 30 40
Ic(pA) experimental
fit
VT(V)
-2,0 -1,5 -1,0 -0,5
-200 -150 -100 -50 0 VT(V)
Ic(pA)
electron- hole pairs separate in the electric field of the depletion region and contribute to a positive current component similar to electron injection in the BEEM spectra.
consistent with the expected threshold for pair generation, given by Eth= Eg+ΦhSB= 1.44 eV [17,18]. Auger electrons created in the metal at high voltages can also contribute to the current, because the probability of tunnelling transmission through the oxide layer increases with energy.
VI. CONCLUSION
The results obtained in this work suggest that the Au/n-Si(100) contact may exhibit various behaviours depending on surface roughness and interlayer thickness between metal and semiconductor. Direct electron injection in BEEM and RBEEM via an Auger mechanism has been observed for ideal Au/n-Si contacts. No collector current in direct BEEM and hole injection in RBEEM can be attributed to MIS structure. In some spectra, Auger mediated electron injection and hole injection have been observed to compete at high voltages.
FIG.6c : RBEEM spectrum recorded in C domains showing hole injection and a saturation phenomenon near -1.5 V.
In this case, this contribution is subtractive, since it contains the current component due to the extracted electron and causes the net current to diminish. According
The effects of surface roughness and injection geometry induce different behaviours depending on whether electrons are injected by the STM tip or produced inside the metal layer through an Auger process.
to our experimental results (Fig.6c), this mechanism becomes effective near saturation at -1.5 V, this being
Bibliography:
[1] W. J. Kaiser and L. D. Bell, Phys. Rev. Lett. 60, 1406 (1988).
[2] G. Binning, H. Rohrer, Ch. Gerber and E. Weibel, Appl. Phys. Lett. 40, 178 (1982).
[3] L. D. Bell, M. H. Hecht, W. J. Kaiser and L. C. Davis, Phys. Rev. Lett. 64, 2679 (1990).
[4] R. Ludeke, J. Vac. Sci. Technol. A11(4), 786 (1993).
[5] R. Coratger, C. Girardin, J. Beauvillain, I. M.
Dharmadasa, A. P. Samantilleke, J. E. F. Frost, K. A. Prior and B. C. Cavanett, J. Appl. Phys. 81(12), 7870 (1997).
[6] L. D. Bell, W. J. Kaiser, Phys. Rev. Lett. 61, 2368 (1988).
[7] J. G. Simmons, J. Appl. Phys. 34, 2581 (1963).
[8] A. Chahboun, R. Coratger, F. Ajustron, J. Beauvillain, I. M. Dharmadasa and A. P. Samantilleke, J. Appl. Phys, 87 (5), 2422 (2000)
[9] B. Kaczer and J. P. Pelz, J. Vac. Sci. Technol. B14(4), 2864 (1996).
[10] H. J. Wen, R. Ludeke and A. Schenk, J. Vac. Sci.
Technol. B 16 (14), 2296 (1998).
[11] L. D. Bell, W. J. Kaiser, M. H. Hecht and L. D. Davis, J. Vac. Sci. Technol. B9(2), 594 (1991).
[12] R. Ludeke and M. Preitsch, J. Vac. Sci. Technol. A9, 885 (1991).
[13] R. Coratger, F. Ajustron and J. Beauvillain, Microsc.
Microanal. Microstruct. 5, 31 (1994).
[14] R. Coratger, C. Girardin, R. Pechou, F. Ajustron and J. Beauvillain, Eur. Phys. J. AP 5, 237 (1999).
[15] M. Prietsch and R. Ludeke, Phys. Rev. Lett. 66, 2511 (1991).
[16] T. Sumiya, K. Honda, T. Miura and S.-I. Tanaka, J.
Appl. Phys. 85(2), 941 (1999).
[17] A. Bauer and R. Ludeke, J. Vac. Sci. Technol. B12, 2667 (1994).
[18] R. C. Alig, C. Bloom and C. W. Struck, Phys. Rev.
B22, 5565 (1980).
-1,6 -1,4 -1,2 -1,0 -0,8 -0,6
-100 -50 0
Ic(pA)
Vt(V)
