Nanoscopic evaluation of semiconductor properties by scanning probe microscopies

Microelectron. Reliab., Vol. 36, No. 11/12, pp. 1767-1774, 1996 Copyright © 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0026-2714/96 $15.00 + .00

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N A N O S C O P I C E V A L U A T I O N OF S E M I C O N D U C T O R P R O P E R T I E S BY SCANNING PROBE M I C R O S C O P I E S L. J. BALK, R. HEIDERHOFF, P. KOSCHINSKI, M. MAYWALD Bergische Universit~it - Gesamthochschule Wuppertal Lehrstuhl for Elektronik, Fuhlrottstr. 10, D-42097 Wuppertal

Abstract: By application of new or modified techniques of scanning tunneling and

scanning force microscopes a comprehensive analysis of semiconductor materials and devices becomes feasible in conjunction with a detailed topological correlation. In this manner properties like electrical and thermal conductivity or mechanical properties can be imaged with nanometer resolution as well as local variations of diffusions length and locations of space charge regions. As most of these modes can be applied within one experiment a maximum of information can be obtained. Finally, a combination of scanning force and scanning electron microscope enables a direct comparison with the well-established electron beam methods. Copyright © 1996 Elsevier Science Ltd INTRODUCTION

Semiconducting materials must be characterized, i.e. all material properties as relevant for a charge carrier flow through the material must be evaluated to ensure reliable and efficient operation of devices realized of these materials. Therefore the characterization of materials should not be restricted to solely electrical parameters, on the contrary a more general and comprehensive approach is necessary. In order to obtain a full set of informations additional properties must be determined, like thermal conductivities, important for devices in which local heating may occur, or mechanical properties, which influence for example the interface quality between a semiconductor and an ohmic contact. Due to the complexity and sizes of modern semiconductor systems, which ranges down to the nanometer region, an evaluation of properties with techniques allowing a spatial resolution in the micron range, i.e. scanning electron microscope (SEM) related techniques, will only deliver an average value of the properties, which is no longer satisfactory. In contrast to SEMs Scanning probe microscopes (SPMs) exhibit a spatial resolution in the nanometer range. Therefore SPMs operating in new modes offer an opportunity for an evaluation of all relevant parameters with simultaneous imaging of the surface morphology with extremely high spatial resolution. It is the aim of this paper to report techniques using these modes, to explain the principles of these techniques and to present some results. The report is focused on two main representatives of SPMs, the scanning tunneling microscope (STM) and the scanning force microscope (SFM), since they are nowadays standard equipment for most laboratories and only minor changes are necessary to use them for semiconductor characterization.

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However, even though SPM techniques are most suitable for semiconductor characterization, they exhibit one drawback in comparison to established SEM techniques. It is not possible with SPM to switch from "mesoscopic" resolution for orientation to "nanoscopic" resolution for evaluation with these instruments. It will be shown how this problem can be circumvented by using a combined SEM-SPM microscope.

EVALUATION OF SEMICONDUCTOR PROPERTIES WITH SCANNING PROBE MICROSCOPES Evaluation of Electronic Properties with a SPM

For the purpose of an electronic characterization Vtip= - 48V both, the scanning tunneling microscope and the currentIemission=2 nA scanning force microscope can be used, depending , ~ signa! on the property under investigation. A STM bases on a current flow between a tip operationaland a sample due to tunneling mechanisms at close amplifier proximity between tip and sample. At voltages between tip and sample in the range from 1 V to 10 V, the sample topography among other surface properties can be measured due to the strong currentI I~: I Schottkv[ ~ ! contact distance dependence of the mechanism. If the voltage between tip and sample is raised up to some ten volts, field emission of electrons occurs and the tip can be regarded as an extremely fine source for low energy electrons. These electrons generate electron-hole-pairs (eh-pairs) in the semiconductor. The eh-pairs can then be used for two different kinds Figurel: Experimental set-up used for STM-EBIC measurements of evaluation techniques. Scanning tunneling microscope electron beam induced current (STMEBIC) measurements can be performed if space charge regions are present within the sample or if these regions are at least assumed. Under the influence of internal electrical fields associated with these regions the eh-pairs are separated, which leads to a detectable current (induced current) in an external circuit [1]. With this technique internal potential gradients, 5~rll electrically active defects (charged traps, 2.5~rn dislocations) [2], dopand inhomogeneities 0 [3] or diffusion lengths [4] can be evaluated. Due to the low energy of the Figure2: STM-EBIC micrograph of a AI-Siinjected electrons the generation volume is Schottky-diode very small and therefore a high spatial resolution is achievable. Figure 1 depicts a schematic sketch of the experimental set-up as used for STM-EBIC measurements. The sample is equipped with an ohmic contact and a Schottky-

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contact, which provides the internal electrical field. Both contacts form a closed circuit via a current amplifier. The induced current is amplified and then displayed on the computer monitor as a grey scale picture. Figure 2 depicts a result obtained by STM-EBIC measurements on a AI-Si-Schottkycontact. The electric fields at the edge of the contact contribute to the induced current whereas the regions underneath and far away from the contact do not lead to a current. This picture demonstrates clearly that a localisation of space charge regions within a material is possible with STM-EBIC with a spatial resolution in the nanometer range.

Scanning tunneling microscope electron beam induced conductivity (STMEBICond) measurements allow access to the

Vtip= -48V I emission=2nA

~""t,,,,,'~conducti~vity

operationalsignal local recombination behaviour of charge :scanningamplifier carriers in the material. Figure 3 shows the ~eting. set-up used for STM-EBICond microscope measurements. Similar to the STM-EBIC method eh-pairs are generated in the semiconductor. These excess charge carriers are now separated by an external electrical field originating from an external voltage VEBIC=4V[ ~ applied to the semiconductor. The carriers modulate the local conductivity of the semiconductor [5]. Depending on the local recombination behaviour of the excess Figure3: Experimental set-up used for STMcharge carriers the current through the EBICond measurements semiconductor is modulated, too. By measuring this current modulation investigations of the recombination behaviour are possible, possibly indicating the existence of e.g. dislocations or other reasons for enhanced recombination. Figure 4a depicts the topography image of a indiumphosphide substrate sample equipped with two ohmic contacts, Figure 4b the corresponding conductivity image. The emission voltage was V = - 4 8 V , the emission current was I=2 nA. The sample was biased with Vb~a., = 4V enabling a current flow through the semiconductor. Figure 4a reveals a almost homogeneous flat surface with a surface roughness of approximately 60 nm, whereas the conductivity image clearly shows areas with different conductivity. The dark areas are indicating an enhanced recombination, and some very small inclusions with strong

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Figure 4: Topography image (a) and conductivity image (b) of an InP-substrate

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recombination are visible probably caused by iron precipitates. This assumption is justified by results from transmission microscopy, which shows similar iron clusters in indiumphosphide substrates. The characterization techniques discussed so far need the generation of eh-pairs in the sample by impinging electrons. Additionally an electrical field is impressed into the semiconductor by the biased tip leading to an unavoidable potential change in the semiconductor. This impressed electrical field can also be used for semiconductor characterization without any emission current. Of course, these techniques can no longer be realized in a STM since the STM needs an emission current for operation. Therefore, a SFM is most suitable for nano-fieldeffect-microscopy. Figure 5 depicts the experimental lock-in 1 set-up as used for nano-fieldeffectmicroscopy. The tip of a SFM is positioned above a semiconductor surface with a voltage applied I feUn ncti°n- I I I between sample and conductive tip. I J I electronicThe distance tip-surface is sufficient •I.._ ",; I controlUnit to avoid any emission of electrons from the tip, neither by tunneling [ (omi'~onal7I gene~t°r2 [ -- - r samp~ processes nor by field emission, and is kept constant during the measurement %h J ,-? • 1~ resistor ~,1 [6]. The electrical field between tip and sample penetrates the sample ,~ dc-source o~ leading to a potential change, i.e. band bending in the semiconductor. This band bending modulates the channel Figure 5: Experimental set-up used for nanowidth for a current flow perpendicular fieldeffect-microscopy to the field, initiated by an applied voltage to the sample. The penetration depth of the field into the semiconductor depends on the electronic properties of the surface. For instance, surface states can become charged or uncharged depending on the surface potential and influence due to their charges, the band bending, too. The sample investigated consists of a layer structure of differently doped MBE-grown lnGaAs (200 nm p-doped layer on n-doped material). A voltage of V = 18mV was applied to 30 nm

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Figure 6: Topography image (a) and nano-fieldeffect-image (b) of a lnGaAs sample

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the ohmic contacts of the p-layer. The current flow through the layer was measured as a voltage drop over a resistor of I k~ by means of a lock-in amplifier. A dc-voltage of 15 V and an accomponent o f 4 Vpp were applied to the tip. Figure 6a shows the topography of the sample, Figure 6b the corresponding nanofieldeffect image. The topography is flat with a surface roughness of 30 nm. In the nanofieldeffect image areas of different current changes can be seen. Dark areas correlate with little current changes AJ = 2.92/2.4, bright areas to strong changes A/= 3.3pA. It can be seen that electronic inhomogeneities of the surface are present and that these inhomogeneities are distributed across the surface in band shaped structures. Another SFM technique is the contact topography current measurement (CCM) technique, in feedback which locally injected carriers from a -~ photodiodes laser metallized SFM tip into the semiconductor are evaluated [7][8]. The tip is brought in direct sc a n n e ~ @ x ' ~ / ~ contact with the semiconductor with a voltage ~ applied between tip and sample or counter [ , , ' v ~ _~U ~ - - ~ S F M tip electrode, respectively. The current flow from the tip into the sample is measured as a voltage drop at a resistor (Figure 7). Specimen damage amplifier bulk contact is avoided by maintaining a minimum applied load due to the feedback loop active in the measurement. By choosing an appropriate Figure T: Experimental set-upforCCM voltage source, both, dc- or ac-measurements in conjunction with the sensitive lock-in technique are possible. An evaluation of electronic bulk properties is possible with CCM, since the carrier flow depends mainly on bulk properties, e.g. doping variations or buried defects, even though surface near structures contribute more to the signal than bulk features. Figure 8a depicts the topography image of a crossection of a InP substrate obtained in CCM measurements. The corresponding CCM micro graph (Figure 8b) clearly shows inhomogeneites of the crossection affecting the current flow through the material.

Figure 8: Topography image (a) and CCM-image (b) ofa crossection in InP MR 36/11/12-N

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Evaluation of thermal Properties with a SPM Variations of local thermal conductivities of semiconductors can directly be imaged with the scanning thermal microscopy (SThM) mode of a SFM [9][10]. By using a heated tip of platinum wire as SFM tip and measuring the change of heat flux from the tip into the sample by a thermal feedback loop which keeps the heat flux into the sample constant while scanning the sample surface in contact mode, thermal properties can be determined. By using a periodic modulation of tip heating sensitive lock-in techniques can be used leading to enhanced scanning thermal microscopy (ESThM). The modulation can be performed by periodic variation of the tip temperature (TMM: temperature modulation mode, Figure 9a) due to an superimposed ac-voltage, i.e. temperature difference, applied to the thermal feedback loop. Figure 9b shows the ESThM measurement result obtained in TMM mode on a s.i. InP substrate. The static temperature difference was 15 K with a modulation amplitude of 20 K at a frequency of 64 kHz. Bright areas correspond to high thermal conductivities and vice versa. The thermal images reveal fairly large thermal structures (200 nm - 300 nm) with no fine structure visible. One possible reason is the generation of the thermal signal in a large sample volume.

a) topography feedback photodiodes . i

sc nner ~

laser

sample feedback

Figure 9: Experimental set-up for ESThM: TMM-mode (a) and ESThM image in TMM mode (b) Evaluation of Mechanical Properties with SPM An evaluation of mechanical properties of semiconductors like elasticities is possible by generating and subsequent detection of acoustical waves in the material, since the wave propagation strongly depends on the mechanical features of the material. The excitation of acoustical waves can be performed by laser heating [11] or electron bombardment [12], both periodically exciting the material. Other approaches uses the SFM as sensor located at the sample surface [13][14] for the waves when they are excited on the backside of the sample. A relative new technique is the scanning probe acoustic microscopy (SPAM), in which the tip in contact mode acts as a local excitation source for acoustical waves. The waves are detected by a piezo electric transducer at the bottom of the sample. If a ac-voltage is applied to the tip scanner a periodic force modulation occurs leading to a generation of acoustic waves in the material at a given frequency. The transducer transforms the mechanical wave into an electrical

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signal, which is amplified by a lock-in topography amplifier for optimum signal-to-noise feedback ph0todi0des laser ratio (Figure 10). Of course, conventional SFM acoustical measurements are also possible with this set-up by exchanging the function of excitation source and SFM tip detector. I z-modulation As an example of SPAM investigations a GaAs tunneling diode has sample been analysed. Figure l la depicts the amplifier topography of the sample showing the piezoelectrictransducer diode, Figure ! lb depicts the corresponding and simultaneously measured SPAM magnitude image. This Figure 10: Experimental set-up for SPAM image clearly shows an area of contact delamination, i.e. areas without direct contact between the semiconductor and top metallization. EVALUATION OF SEMICONDUCTOR COMBINED MICROSCOPE

PROPERTIES

WITH

A

SEM-SPM

One drawback of SPMs in material and device characterization is the restricted area of view of these instruments. The piezoelectric scanner used for tip positioning does not allow a simple "switching" from a mesoscopic view to a nansoscopic view, the first necessary for orientation, the latter necessary for high spatially resolved measurements. To overcome this problem and to allow a comparison with established scanning electron microscope (SEM) measurements a SEM-SPM combined microscope has been developed. This hybrid instrument now allows two kinds of measurements: the SEM can be used for exact positioning of e.g. a SFM tip on the sample and subsequently all the SFM based measurement techniques explained before can be used, or, and this is a new application ofa SFM, the metallized tip can be used as a nanometric, free positionable Schottky-contact on the semiconductor surface. With this set-up established electron beam induced current (EBIC) measurements are possible with an extremely high spatial resolution. See ref. [15] for details. Figure 12 illustrates the set-up and the results obtained with the combined instrument by

Figure 11: Topography image (a) and SPAM image (b) of a GaAs tunneling diode

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beam~lanking

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Figure 12: SEM-SPM hybrid system (left) and EBIC measurements on diamond (right) using the tip as a scanning Schottky-contact on diamond under homogeneous illumination of the sample by electrons provided by the SEM. Investigations under different bias voltages applied to the tip (e.g. 14V) allow a high spatially resolved determination of ohmic-, p-, and ntype regions of the semiconductor. CONCLUSIONS It was the intention of this paper to give an overview of state of the art techniques for material characterization with scanning probe microscopes. It was demonstrated that only by minor changes of conventional SPMs several material properties, like electronic, thermal, or mechanical properties can be investigated with an extremely high spatial resolution. By combining these techniques a comprehensive analysis of semiconductor materials is possible, which can be improved in efficiency if a SEM-SPM hybrid system is used. However, all these techniques do not provide any quantitative information. This is not a problem in principle, it can be solved if the understanding of the underlying signal formation mechanisms of the various techniques is growing. References

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15

H.J. Leamy, J. Appl. Phys. 53(6) R5 l-R80, (1982) G. Weber, S. Dietrich, M. Hiihne, H. Alexander, Inst. Phys. Conf. Set. No. 100, 749 (1989) C. Frigeri, Inst. Phys. Conf. Ser. No. 87, 745 (1987) C.J. Wu, D. B. Wittry. J. AppL Phys. 49(5), 2827 (1978) C. Munukata, Japan. J. Appl.Phys. 6, 963 (1967) D. Sarid, Scanning Force Microscopy, Oxford Press,New York (1991) M. Maywald, R. E. Stephan, L. J. Balk, Microelectronic Engineering 24, 99 (1994) L.J. Balk, M. Maywald, Material Science and Engineering B24, 203 (1994) R.B. Dinwiddie, R. J. Pylkki, P. E. West, Thermal Conductivity 22, 668. Technomic Publishing Co, Lancaster, Basel) (1994) M. Maywald, R. J. Pylkki, L. J. Balk, Scanning Microscopy 8 (2),181 (1994) G. Li et al., Springer Series in Optical Science 69, 384 (1992) K. Kaufmann, et al., J.Phys.D: Appl. Phys. 27, 2401 (1994) M. Radmacher et al., Biophys. J. Vol.64, 735 (1993) U. Rabe, W. Arnold, Ann.Physik 3, 589 (1994) H Heiderhoff, R. M. Cramer, L. J Balk. Inst. Phys. Conf. Ser.No. 149, 189, (1996)