M.J. CONDENSED MATTER VOLUME 4, NUMBER 1 1 DECEMBER 2001
* chahboun@cemes.fr 4 52 2001 The Moroccan Statistical Physical and Condensed Matter
Society
Study of Au/n- ZnSe contact by ballistic electron emission microscopy
A. Chahboun∗ and I. Zorkani
Laboratoire de la Physique des Solides, Faculté des sciences Dhar Mehraz, BP 1796, Fès-Maroc R. Coratger, F. Ajustron and J. Beauvillain
CEMES/CNRS- BP 4347, 31055 Toulouse 04, FRance
Ballistic Electron Emission Microscopy (BEEM) has been used to characterise the Au/n-ZnSe contact. A mean statistical BEEM threshold of 1.63eV is in good agreement with literature. Metal - Insulator- Semiconductor (MIS) structures are invoked to explain the Schottky barrier height dispersion and the observed shift of BEEM thresholds to higher values.
PACS: 73.30.+y, 61.16.ch, 73.23.Ad, 73.61.Ga I. INTRODUCTION
The recent interest of researchers to study the wide band gap ZnSe compound may be explained by its applications as a short wavelength emitter1,2 and the fabrication of various optoelectronic devices such solar cells3. The aim of these studies is the production of a low and reliable barrier height between metal and semiconductor and therefore, to decrease the energetic coast of ZnSe based devices. Previous results were reported on the Au/ZnSe contacts by different investigation techniques. Various Schottky barrier height (ranging from 0.9 eV to 2.1 eV) were found, with an ascertainment that the fabrication processes were different for each research group5,6,7.
In 1988, Kaiser and Bell4 invent Ballistic Electron Emission Microscopy (BEEM). BEEM permits metal- semiconductor interface characterisation with unequalled lateral resolution (below 2 nm).
In this paper, we report BEEM results on Au/n-ZnSe contact. It is shown that the experimental BEEM spectra for bias voltages ranging from 0 to 2 V can be fitted by the Bell-Kaiser (BK) model7. A BEEM statistical study was realised showing barrier heights ranging from 1.4 eV to 2 eV. The average value is 1.63±0.05 eV which agrees with the results reported by Tarricone et al8 and those predicted by the Schottky model.
Forward macroscopic current voltage characteristics I(V) spectrum realised on these samples give three distinct Schottky barrier height values 0.9, 1.2, and 2.1 eV. The spatial variation of the barrier height is attributed to thin insulating microclusters due to native oxide on the semiconductor surface before gold evaporation. A simultaneous STM and BEEM images recorded show some gold surface domains with zero electron injection between metal and semiconductor. The Metal-Insulator- Semiconductor (MIS) structure is invoked to explain this observation.
II. EXPERIMENTAL DETAILS
In this work the experiments were carried out at room temperature and in ambient atmosphere. The sample
consisted of 0.5 mm diameter Au dots. The gold film is evaporated on n-ZnSe (1.5 1016 cm-3 Indium doped) grown on n+GaAs (doping 1018cm-3). Metal overlayers with a thickness comparable to the mean free path of electrons in the metal (~10 nm) are used for the Schottky barrier formation for this type of experiments. Indium ohmic contact is realised on the back of GaAs. Details of chemical preparation, metal deposition and macroscopic I(V) procedure are described elsewhere9. The BEEM used in this work is a home build setup10. For Ballistic Electron Emission Spectroscopy, a specific electronic device provides a variable bias voltage between tip and sample (VT), and detects the induced collector current (Ic) metal- semiconductor junction. This collector current is detected with a resolution in the picoampere range. To determine the Schottky barrier height ΦSB, the Kaiser-Bell (KB) model[4 has been used. The BEEM spectrum fit yields the barrier height and an energy independent scattering factor R. This factor is proportional to the fraction of injected carriers, which reach the interface and surmount the potential barrier.
The bias voltage between the tip and the Au metal is realised by pasting in one half of the dot a 50µm Copper wire. The collector current Ic vs bias voltage VT curves are measured in the conventional constant current tunnelling mode. The samples investigated in this work are highly resistive. To obtain good signal to noise ratios, the tunnelling current It is held in the (10-20nA) range. The presented spectra are usually an average of fifteen spectra taken in one scan. This sequence is repeated at different locations of the sample.
To carry out simultaneous STM and BEEM images, the tip scans an area under constant bias voltage, higher than the threshold value. Two parameters are simultaneously stored: the variation of the tip z-position yielding the surface topography under constant tunnel current mode and the different values of collector current shown in grey
4 STUDY OF AN/N-ZNSE CONTACT 53 levels which allow for BEEM image of the interface to be
built up.
III. RESULTS
Before BEEM investigations macroscopic I(V) characteristics have been carried out on different dots of the same wafer. Three discrete Schottky barrier height values at 0.9, 1.2 and 2.1 eV have been obtained.
While the forward macroscopic I(V) characteristics give a Schottky barrier height averaged over the contact area (0.5mm in diameter), BEEM allows this barrier height over an area of 2 nm in diameter to be determined. A standard BEEM spectrum for 0.5≤VT≤2.2 V recorded in the homogeneous emitting area is shown in fig. 1 as circle curve.
1.5 2.0 2.5
-10 0 10 20 30 40 50 60 70 80
90 experimental KB fit
Ic(pA)
V(V)
FIG. 1: Typical BEEM spectrum of an Au/ZnSe contact in the (1.2-2.3 V) range (IT=10nA). A fit using the KB model is also shown and yields ΦSB=1.65 eV with R=0.03 eV-1. This type of spectrum is typical and may be obtained on different dots of the same sample. Also shown, as a solid line, the best fit using the KB model. The R parameter of the model was freely adjusted until a good fit of the experimental data is obtained.
About 200 standard spectra have been realised in different locations spaced at least by 300 nm on three different gold dots of the same wafer. Fig. 2 shows a histogram of the results.
1.5 1.6 1.7 1.8 1.9 0
10 20 30 40 50
cumulative count
Schottky barrier height range (eV)
FIG.2: Barrier height histogram deduced from BEEM spectroscopy for three different Au/n-ZnSe diodes of the same wafer. The mean barrier height is 1.63±0.05
FIG.3: Simultaneous record of STM (left) and BEEM (right) images of a 60 nm ×60 nm surface. These images are realised at 2.2 V and 10 nA. The maximum corrugation in the STM image is below 11 nm and the difference between grains and the surrounding surface is about 10 nm. Note a zero injection yield corresponding to the gold grains.
This histogram reveals barrier height dispersion of barrier height ranging from 1.4 eV to 2 eV. The distribution is relatively narrow and symmetric and has an average value of 1.63±0.05 eV.
Surface microscopy of the Au/n-ZnSe surface by STM showed that the deposited Au formed continuous flat grain layers with occasional grain clusters. A scan of these domains shows the absence of any electron injection through the metal-semiconductor interface. Fig.3 gives an example of such domains.
The left image represents the STM topography, while the right one represents the BEEM yield shown in grey level.
Note that when a tunnelling tip scans grains overhanging the surrounding flat granular surface (centre area), an absence of collector current may be evidenced. The conventional spectra obtained in these domains reveal a shift of the BEEM threshold to values around 3eV. Figure 4 shows an average of thirty spectra taken on one of these domains.
54 A. CHAHBOUN AND I ZORKANI 4
2.5 3.0 3.5 4.0 4.5
0 10 20 30 40 50
Ic(pA)
Bias voltage (V)
FIG.4: Experimental data realised on the previous grain cluster, which shows a shift of the BEEM threshold to around 3eV due to a MIS structure.
IV. DISCUSSION
The mean statistical value (ΦSB = 1.63±0.05 eV) obtained in this work (fig.2) could be compared to the 1.65eV value previously reported by Tarricone et al12. This result evidences a characteristic Fermi level pinning position for the Au/n-ZnSe contact. The dispersion of threshold values could be related to the stoichiometry of ZnSe at the interface. This creates defect states in the gap that cause multi pinning positions of the Fermi level. In the literature13, 14 a multitude of ΦSB values (0.9, 1.2, 1.45, 1.65, 1.8 and 2.1eV) were reported and linked to the different sample preparations. Another argument could be invoked is that samples are not exempt of thin insulating layer at the interface as discussed below and that could locally influence measured barrier height, so this value could be compared with the barrier height calculated from the Schottky model. This model predicts a value ΦSB=φm−χ= 5.10-3.51= 1.59 eV (Where ΦSB is the Schottky barrier height,
φ
m is the metal work function andχ
is the ZnSe electron affinity). To understand the Fermi level pinning at a metal-semiconductor interface, most of the models that deal with this phenomenon, such as the original Schottky model and the metal induced gap- state model, try to relate the barrier height with metal and semiconductor intrinsic properties. Others describe the Schottky barrier formation in terms of interface properties, such defects generated during metal deposition or formation of a chemically reacted layer. Therefore, in referring to the mean statistical value one could think that these Au/n-ZnSe contacts approximately follow the Schottky model. However, the ΦSB values obtained from macroscopic I(V) (0.9, 1.2 and 2.1 eV) have never been detected in our BEEM observations. For the two lower values (0.9 and 1.2 eV), are should keep in mind that macroscopic I(V) is described by the thermionic emission theory which predicts that electrons flow through areas of lowest thresholds. It could be also argued that I(V) and BEEM experiments have not been performed in the same time. Indeed, BEEM spectra were realised some days after I(V) characterisation. As experimentally proved by other authors 5, the diode ageing shifts the barrier height to largevalues. Therefore, higher values are expected from BEEM measurements. For the 2.1 eV threshold, the existence of a thick insulating layer between metal and semiconductor, which shifts the barrier height to upper values may be assumed. This hypothesis could be considered if we think that simultaneous recorded STM and BEEM images (figure3) show domain with zero injection between metal and semiconductor at 2.2 V bias voltage. So 2.1 eV threshold could be compared with BEEM threshold values carried in these domains.
To understand the low or absence electron transmission observed in some spaced surface domains at low bias voltages (see fig.1), several hypothesis could be discussed.
Preferential orientations of the crystallites in the metal film are possible, and might introduce local deviations of the work function that increase the barrier height. This increase of barrier height is of few meV15 in order, and
could not be compared with the difference between the conventional ZnSe barrier height (1.65 V) and 3 V recorded in fig.4. Electron scattering in the gold film could not be invoked because the corrugation between the larger grains and the average gold surface is lower than 8 nm.
However, the electron mean free path in this energy range is about 10 nm16. The gold film thickness effect is therefore negligible and is ill suited to explain these observations. In addition, in all simultaneous STM and BEEM images recorded on Au/n-ZnSe no correlation between the collector current and the surface corrugation was observed.
Previous BEEM experiments performed on Au/Si17 junctions have shown domains where the absence of electron transmission has been interpreted in terms of thick oxide layer at the interface. Therefore, the absence of electron transmission may be due to microscopic metal- insulator-semiconductor (MIS) structures formed in different regions of the contact as has been experimentally measured by a controlled manner18. This phenomenon was previously observed by Kaczer et al19 in metal-oxide- semiconductor (MOS) structures.
This last hypothesis seems more suitable since the presence of thick deposits pocket at the interface such as native oxides or other reacted phases may act as an insulating layer and shifts the barrier height to higher values, which is demonstrated in fig.4.
Spectra taken in domains of no electron injection throughout the metal-semiconductor interface show the threshold shift to values around 3 eV. This large shift is probably due to a Metal Insulator Semiconductor structure (MIS) induced by the sample preparation process as observed for Au/Si and Au/CdTe.
In a rough approximation, the ionic character of ZnSe, may suggest that the Schottky model is appropriate to describe this compound. However, recent works have demonstrated that barrier height is independent of the metal15. Our results are in good agreement with the fact that the most appropriate model for Schottky barrier formation is the defect model associated with surface and bulk imperfections. Indeed, deep levels due to defects pin the
4 STUDY OF AN/N-ZNSE CONTACT 55 Fermi level to discrete values within the band gap in a
similar manner to the defect model of Spicer et al16. Indeed, in this work specific deep levels in the gap of the ZnSe are reported in agreement with pinning Fermi level positions given by other searchers.
V. CONCLUSION
In this paper, Schottky barrier height measurements of Au/n-ZnSe performed by the BEEM technique are
presented. The average value is consistent with previous results reported by other authors. Large domains where no collector current is observed are thought to result from MIS structures that shift the threshold to high values. These results on Au/n-ZnSe demonstrate that BEEM is a powerful technique for determining the effects of interface states and localised defects.
VI. ACKNOWLEDGMENTS The authors would like to think Dharmadasa is thinkedfor providing the n-ZnSe samples
Bibliography:
[1] R. M. Park, M. B. Troffer, C. M. Rouleau, J. M.
Depuydt and M. A. Haasse, Appl. Phys. Lett. 57, 2127 (1990).
[2]M. A. Haasse, J. Qiu, J. M. Deuyt and H. Cheng, Appl. Phys. Lett. 59, 1272 (1991).
[3]Y. Fan, J. Han, L. He, J. Saraie, R. L. Gunshor, H.
Hagerott, H. Jeon, A. V. Nurmikko, G. C. Hux and N.
Otsuka, Appl. Phys. Lett. 61, 3160 (1992).
[4]L. D. Bell and W. J. Kaiser, Phys. Rev. Lett. 61, 2368 (1988).
[5] R. Coratger, C Girardin, J. Beauvillain, I. M.
Dharmadasa, A. P. Samanthilaka, J. E. F. Frost, K. A.
Prior and B. C. Cavenett, J. Appl. Phys. 81, 7870 (1997).
[6]I. M. Dharmadasa, C. J. Blomfield, R. Coratger, F.
Ajustron, J. Beauvillain, J. Simpson, K. A. Prior and B.
C. Cavanett, Mat. Sci. Technol. 12, 86 (1996).
[7]B. A. Morgan, K. M. Ring, K.L. Kavanagh, A. L.
Talin, R. S. Williams, T. Yasuda, T. Yasui and Y.
Segawa, J. Appl. Phys. 79, 1532 (1996).
[8] L. Tarricone, Rev. Phys. Appl. 15, 1617 (1980).
[9]I. M. Dharmadasa, C. J. Blomfield, G. E. Gregory, B.
C. Cavanett, K. A. Prior and J. Simpson, Surf. Interface Anal. 21, 718 (1994).
[10]R. Coratger, J. Beauvillain, F. Ajustron, J. C. Lacaze and C. Tremollieres, Rev. Sci. Instrum. 62, 830 (1991).
[11]R. Coratger, F. Ajustron and J. Beauvillain, J. Phys.
III France 3, 2211 (1993).
[12]M. S. Tyagi and S. N. Arora, Phys. Status. Solidi (a)32, 165 (1975).
[13]D. D. Nedeoglo, Dang Hue Lam and A. V.
Simashkevich, Phys. Status. Solidi. (a)44, 83 (1977).
[14]C. S. Wang and B. M. Klein, Phys. Rev. B24, 3393 (1981).
[15]I. M. Dharmadasa, J. M. Thorton and R. H.
Williams, Appl. Phys. Lett. 54, 137(1989)
[16]W. E. Spicer, I. Lindau, P. Skeath, C. Y. Cu and P.
W. Chye, Phys. Rev. Lett. 44, 420 (1980).
