semiconductor interfaces and heterojunctions

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Applied Surface North-Holland

Science

70/71

(1993) 386-390

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applied surface science

Ballistic electron emission microscopy of metal/semiconductor interfaces and heterojunctions R.H. Williams Department of Physics and Astronomy, University of Wales College of CardifJ P.O. Box 913, Cardiff CF2 3YB, UK Received

24 August

1992; accepted

for publication

13 October

1992

Ballistic electron emission microscopy is rapidly maturing as a metiod to probe electrical with good energy resolution and a lateral resolution of a few tens of A or less. We illustrate concentrating on the Au/GaAs, Au/CdTe and InAs/GaAs interfaces.

1. Introduction

Electrical barriers at semiconductor interfaces govern the behaviour of a large number of electronic device structures. Thus, band discontinuities at heterojunction confine the carriers in quantum well structures, whereas Schottky barriers at contacts determine the behaviour of metal-semiconductor diodes. The ability to control such barriers is an important objective, the realisation of which is depepdent on achieving a full understanding of the physics of barrier formation at semiconductor boundaries. Several models have been put forward in an attempt to account for the systematic variation of Schottky barriers and semiconductor band offsets [1,2]. The so-called “defect” model has found some support and the key role played by metalinduced gap states (MIGS) is now well appreciated. In the latter model conduction band discontinuities can be estimated simply by aligning the calculated charge neutrality levels of the semiconductors on either side of the interface, and Schottky barrier heights may be estimated in an analogous way. Relatively recently, very careful experimental investigations have revealed a correlation between the precise values of Schottky barrier heights and the interface crystallography for systems such as Nisi,--Si [3] and Pb-Si 141, 0169-4332/93/$06.00

0 1993 - Elsevier

Science

Publishers

barriers at semiconductor interfaces the application of the technique by

leading to the concept of an extrinsic charge neutrality level. Current investigations are directed at gaining a thorough understanding of the interdependence of the crystallography and the electrical barriers at semiconductor interfaces. Both Schottky barrier heights and band discontinuities are usually measured by large-area techniques such as I-V, C-V’ and photoresponse. Yet, in view of the arguments presented above, it is clear that techniques which can probe the barriers on a scale closer to the atomic scale would be most valuable. Ballistic electron microscopy (BEEM) has made such investigations at least partly possible [5,6].

2. The technique The principle of BEEM, first described by Kaiser and Bell [5] in 1988, is illustrated in fig. 1. The tip of the scanning tunnelling microscope is separated from a thin metal overlayer on the semiconductor. The tip-metal bias generates the tunnel current and a fraction of the electrons entering the metal base reach the metal/ semiconductor boundary. However, these can only enter the semiconductor provided the tip-metal bias is sufficient and the threshold value of the collector current-tip voltage plot yields the bar-

B.V. All rights reserved

R.H. Williams / BEEM of metal /semiconductor interfaces and heterojunctions

EF

------

‘/

’ f,,,,

< TIP

(b)

Fig. 1. BEEM

/A BASE COLLECTOR

n

mechanism (a) without bias and (b) with tipbase bias. After ref. [5].

rier height. Typical plots for the Au/CdTe interface, for various values of the tunnel current, are shown in fig. 2 [7]. The threshold value here is easily estimated, and can be probed laterally simply by moving the STM tip. A good theoretical description of BEEM has been given by Bell and Kaiser [6] and a somewhat more refined account by Ludeke et al. [8]. The former assumed that the tunnelling process between tip and base ca be described by the Simmons planar process. They also assumed conser-

,

I

0

0.1

0.8 Tunnellmg

I

1.2 bus

Fig. 2. Collector current as a function tunnel currents for the Au/CdTe

(VI of bias for various interface.

387

vation of energy and conservation of the parallel component of momentum as the electron crosses the metal/semiconductor boundary. This last restriction leads to a very high laterDal resolutio$, predicted to be of the order of 10 A for a 100 A layer of Au on silicon. The treatment also indicates that the collector current should be proportional to (V- &,)’ near threshold, where 4,, is the barrier height and I’ the bias. Ludeke et al. [8] considered more extensively both the inelastic scattering of electrons in the base and the quantum-mechanical reflection of carriers at the metal/ semiconductor boundary. They concluded that a (V - &,I 5/2 functional form might be more appropriate. A more complete description of the whole process has been given by Stiles and Hamann [9] for the NiSi,/Si coherent interface. Phonon scattering effects have also been considered by Lee and Schowalter [lo].

3. Contacts to semiconductors Kaiser et al. [ll] and Fowell et al. [12] have carried out detailed investigations of the Au/Si and Au/GaAs interface using BEEM. Fowell et al. [12] made diodes by depositing Au onto MBE-grown GaAs(100) in situ, in ultra-high vacuum, thus avoiding interface contamination. The exceptionally high quality diodes gave well defined thresholds with uniform values over large areas. A plot of the square root of collector current as a function of tip-base bias is shown in fig. 3. Here there appear to be three linear regions suggesting three thresholds, one at 0.82 eV and the others displaced to higher energies by around 0.28 and 0.41 eV. The origin of this structure is believed to be related to the band structure of GaAs with the thresholds reflecting the onset of electron injection at the I, L and X conduction band critical points, respectively. The possibility of observing band structure features by BEEM is important as it may lead to the possibility of probing strain at interfaces on a microscopic scale as considered later. Inhomogeneous interfaces may also be readily studied by BEEM. Fig. 4 shows the lateral variation of the threshold voltages for the Au/CdTe

R.H. Williams / BEEM of metal /semiconductor

388

interfaces and heterojunctions

Fig. 5. BEEM

micrograph for th: Au/qdTe region of 400 AX 400 A.

interface,

for a

lieved to be associated with excess Te and the interface. These highly inhomogenous contacts contrast strongly with the highly ideal Au/GaAs interfaces made in situ in ultra-high vacuum. 1.2

1.0

16

1L

610s (VI Fig. 3. Square

root of collector current as a function for the Au/GaAs interface.

of bias

interface, for the case where the CdTe surface was chemically etched using Br in methanol before deposition of the contact [7]. The interface is clearly highly non-uniform. This is seen more clearly in fig. 5 where the collector current as fixed bias is shown as a function of position. The dark regions represent high currents and are be-

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0.704

-200

-100

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100

x position Fig. 4. Variation

of threshold Au/CdTe

200

(A)

voltage with interface.

position

for the

4. BEEM of heterojunctions The microscopic variation of electrical barriers at heterojunctions is of significant interest in solid-state device structures. Fowell et al. [13] extended the BEEM technique to probe such heterojunctions directly, concentrating initially on the InAs-GaAs combination. The n-InAs/GaAs layers were grown by MBE with the InAs being degenerately n-type (4 X 10” cmP3) so that the structure can be considered as analogous to a metal-semiconductor contact. The barrier under consideration is then that between the Fermi level in the InAs and the conduction band minimum in the GaAs at the interface; this approximates to the conduction band discontinuity AE,. Fig. 6 shows the square root of collector current plot for several values of the tunnel current. Here again there exists a well defined threshold which readily allows the barrier height and its lateral variation to be probed. The value of 0.63 eV for the barrier is in excellent agreement with that measured from thick-layer I-I/ investigations and yields a value of 0.72 eV for AE,. Figs. 7a and 7b show the topography and BEEM current plots respectively from the same

R.H. UUliams / BEEM of metal /semiconductor

r

1.0

-L 0.4

0.6

0.6

Tip Fig. 6. Square

Bias

root of current against interface.

1.0

1.2

1.4

(Volts) bias for the InAs/GaAs

region, nearly 600 A X 600 A. For the BEEM investigations the InAs gas 100 A thick and was coated by a further 100 A of Au. It is clear from the topographic image of fig. 7a that the overlayers have formed clusters and indeed to a large extent the BEEM current simply reflects the variable thickness associated with the clustering. This

Fig. 7. (a) STM topographic

plot for Au on InAs on GaAs.

interfaces and heterojunctions

389

behaviour was also observed by Ludeke et al. [81 for metals on Gap. However, there is further contrast in the BEEM micrograph and it is worth considering the origin of this a little further. There exists a substantial lattice mismatch between InAs and GaAs and for thicknesses of InAs in excess of around 2 ML it is anticipated that an array of misfit dislocations will form at the interface. It is of interest to question whether such dislocations might lead to contrast in BEEM associated perhaps with additional scattering, and whether additional dislocation-induced dipoles and strain fields might alter the electrical barriers locally. No contrast associated with a dislocation array has yet been recognised in the BEEM current micrographs. However, in hindsight, this is perhaps not surprising. The detailed nature of the InAs/GaAs interface is in fact very complex and inhomogeneous as clearly demonstrated in the TEM studies of Zhang et al. [14]. By using wedge-shaped samples Zhang et al. showed that the interface is rough with island-like regions of an indium-gallium-arsenide alloy and with craters extending into the GaAs. The scale of the islanding was similar to that associated with the fine-structure in the micrograph of fig. 7b. We conclude, therefore, that much of the detail in the BEEM current micrographs of the InAs/GaAs interface reflects the morphology and roughness of the interface. While it is comforting to note that the technique does detect these features it is also clear that systems with superior interfaces will be required in order to probe

(b) BEEM current 591 Ax 591 A.

plot for the region

corresponding

to (a). The area is

R.H. Williams / BEEM of metal /semiconductor

390

interfaces and heterojunctions

possible effects associated with misfit dislocations. Future investigations will address these issues.

described in the paper, in particular Angela Fowell, Bernard Richardson, Tony Cafolla, Tiehan Shen and Simon Heghoyan.

5. Conclusions

References

and future directions

Ballistic electron emission microscopy is now establishing itself as a most useful technique to probe electrical barriers at buried interfaces with microscopic resolution. It is being used also to probe phenomena such as alloy clustering in semiconducting layers and hot electron transport in solids. It has the potential for exploring band structure effects in ultra-small areas, thus yielding the possibility for probing strain variations perhaps associated with defects. The investigation of electrical barriers associated with both electrons and holes has been realised. Quite recently BEEM has been applied in the transmission mode, utilising small areas of thinned samples [15]. The possibility of applying BEEM in a light-emitting mode is also under consideration. All in all this newly established technique is providing a fertile area for research over the next few years.

Acknowledgements

The authors wishes to thank all his coworkers and students who have contributed to the studies

111E.H.

Rhoderick and R.H. Williams, in: Metal-Semiconductor Contacts (Oxford University Press, Oxford, 1988). 121C.C. Matthai and R.H. Williams, in: Physics and Technology of Heterojunctions Devices, Eds. D.V. Morgan and R.H. Williams (Peregrinus, 1991). [31 R. Tung, Phys. Rev. Lett. 52 (1984) 461. [41 D.R. Heslinga, H.H. Weitering, D.P. van der Werf, T.M. Klapwijk and T. Hibma, Phys. Rev. Lett. 64 (1990) 1589. 151W.J. Kaiser and L.D. Bell, Phys. Rev. Lett. 60 (1988) 1406. 161L.D. Bell and W.J. Kaiser, Phys. Rev. Lett. 61 (1988) 2368. and T.-H. [71 A.E. Fowell, R.H. Williams, B.E. Richardson Shen, Semicond. Sci. Technol. 5 (1990) 348. [81 R. Ludeke, M. Prietsch and A. Samsavar, J. Vat. Sci. Technol. B 9 (1991) 2342. Phys. Rev. B 40 (1989) 191 M.D. Stiles and D.R. Hamann, 1349. [lOI E.Y. Lee and L.J. Schowalter, in press. [ll] W.J. Kaiser, L.D. Bell, M.H. Hecht and F.J. Grunthaner, J. Vat. Sci. Technol. B 7 (1989) 945. [12] A.E. Fowell, R.H. Williams, B.E. Richardson, A.A. Cafolla, D.I. Westwood and D.A. Woolf, J. Vat. Sci. Technol. B 9 (1991) 581. [13] A.E. Fowell, A.A. Cafolla, B.E. Richardson, T.-H. Shen, M. Elliott, D.I. Westwood and R.H. Williams, Appl. Surf. Sci. 56-58 (1992) 622. [14] X. Zhang, D.W. Pashley, J.H. Neave, J. Zhang and B.A. Joyce, J. Cryst. Growth 121 (1992) 381. [15] W.J. Kaiser, private communication.