Appl. Phys. A 70, 403–406 (2000) / Digital Object Identifier (DOI) 10.1007/s003390000409 Applied Physics AMaterialsScience & Processing

Scanning tunnelling microscopy imaging and modification ofhydrogen-passivatedGe(100) surfacesY.F. Lu, Z.H. Mai, W.D. Song, W.K. Chim

Laser Microprocessing Lab, Data Storage Institute and Department of Electrical Engineering, National University of Singapore,10 Kent Ridge Crescent, Singapore, 119260

Received: 1 March 1999/Accepted: 18 September 1999/Published online: 1 March 2000 – Springer-Verlag 2000

Abstract. Scanning tunnelling microscopy (STM) studyand modification of hydrogen (H)-passivatedGe(100) sur-faces have been investigated. Thermal oxidation proced-ures were used to minimise surface roughness.Ge sam-ples were passivated inHF solution after thermal oxidation.STM and atomic force microscope (AFM) imaging showedthat, usingHF etching after thermal oxidation, we can ob-tain a naturalH-passivated topographically and chemicallyflat Ge(100) surface. The root-mean-square (rms) roughnessofaH-passivatedGe(100) surface measured both by STM andAFM is less than2 Å. Electric properties ofH-passivatedGe(100) surfaces were studied by scanning tunnelling spec-troscopy (STS) in nitrogen ambient. STS showed that theH-passivated Ge surfaces were not pinned. Modification onH-passivatedGe(100) surfaces was carried out using STMby applying an electric voltage between the sample and tipin air. Modified features were characterised by STM andAFM imaging. On the H-passivatedGe(100) surfaces, sta-ble, low-voltage, nanometer-scale modified features can beproduced.

PACS: 61.16.Ch; 85.40.Ux; 68.35.Bs

In response to science and technological opportunities ofnanostructures and nanoelectronics, scanning probe lithogra-phy has been studied extensively. One of the scanning probelithographies is nano-oxidation on H-passivated Si surfaces,using scanning probe microscope (SPM) by applying an elec-tric voltage between the tip and sample [1–6]. This SPM-generated oxide can be used for subsequent etching. It hasalso been demonstrated that a reflection mode scanning near-field optical microscope (SNOM) can be used to locally ox-idise an H-passivatedSi surface [7]. However, no study ofSPM-based nano-modification onH-passivatedGe surfaceswas reported.

Gesurfaces have been studied using various methods [8–15]. In SPM-based nanotechnology, modified regions arein the nanometer scale. Compared to the well-known spec-troscopy methods, such as Fourier transform infrared, Raman

scattering, optical time-resolved and photoemission spec-troscopy, STM provides spectroscopic studies in a very smallregion on the sample surface. STM makes it possible torecognise surface topography and electric properties in inter-ested regions in the STM-based nanotechnology. Althoughmany studies on cleanedGe surface using STM and scan-ning tunnelling spectroscopy (STS) have been reported [16–21], knowledge is still lacking for electric properties onH-passivatedGesurfaces.

In this article, we present STM study and modificationof H-passivatedGe surfaces. Thermal oxidation procedureswere used to minimise the surface roughness ofH-passivatedGe surfaces.Ge samples were passivated inHF solutionafter thermal oxidation. Electric properties ofH-passivatedGe(100) surfaces were studied by STS. Modification onH-passivatedGe(100) surfaces was carried out using STM byapplying an electric voltage between the sample and tip in air.Modified features were characterised both by STM and AFMimaging.

1 Experimental

Investigations were performed on a commercial SPM system(Model Autoprobe CP, Park Scientific Instruments). p-typeGe(100) samples with a resistivity of8–12Ω cm were used.The commercial p-typeGesamples have a native oxide layerof 2 to 3 nmand an initial rms roughness of8 Å. In order toobtain smootherGe surfaces, the thermal oxidation proced-ure was used. First a thick oxide layer grew on aGesurfaceusing the thermal oxidation method, and then the oxide layerwas removed byHF solution and a smoother surface was ob-tained.Ge samples were ultrasonically cleaned in acetone.The thermal oxidation was carried out using a hot plate inair. The hot plate has a uniform temperature distribution onthe plate surface. The maximum temperature the hot platesurface can reach is500C. A temperature controller con-trols the hot plate. p-typeGe(100) samples were passivated inHF solution, rinsed in deionised (DI) water and then dried ina continuous nitrogen flow. TheH-passivatedGe(100) sam-ples were mounted on mounting discs using silver painting.

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For STM/STS study and STM modification in our in-vestigations, a pre-cut tungsten tip was used. The tip wasconnected to the ground and sample bias provided the tun-nelling voltage and modification voltage. STM imaging ofaH-passivatedGe(100) surface was taken in constant-currentmode. A H-passivatedGe (100) surface was studied topo-graphically and electronically using AFM and STM. STSstudies were used to examine the electric properties ofH-passivatedGe(100) surfaces.I −V curves were obtainedfrom the feedback condition: the tip was brought into feed-back to make sure a certain tip–sample distance, then thefeedback loop was disabled, and the sample voltage wasramped. After I −V measurements the feedback was en-abled. The tip–sample distance can be changed by means ofvarying the setpoint of tunnelling current.

2 Results and discussion

Thermal oxidation processing of aGe(100) sample was firststudied prior to the STM and AFM studies. Thickness of anoxide layer was measured using a surface profiler with a ver-tical resolution of5 Å . Oxide layers were etched in50% HFto form a step for the thickness measurement. STM imagingwas used to examine the etched surface. If the oxide layer isremoved completely, the surfaces should be terminated by hy-drogen and STM imaging would show a smooth and stablesurface. In our experiment, the oxide layer on aGe samplewas etched in50% HF for 30 sand rinsed in DI water for30 s.The above procedures were repeated three times. The last stepfor etching is to dip the sample in50% HF for 10 s. Finally thesample was dried in a continuous nitrogen flow. STM imagingshowed that a thick oxide layer can be removed effectivelyusing the above-mentioned procedures.

H-passivatedGe(100) surfaces were studies using bothSTM and AFM. Thermal oxidation was carried out at thetemperature of500C. Thermal oxide layers with differ-ent thickness were prepared with different oxidation time.After thermal oxidation,Ge samples were passivated usingthe above-mentioned procedures. The rms roughness of anH-passivatedGe(100) surface was measured over a1×1µmarea. The dependence of rms roughness of aH-passivatedGesurface on oxide thickness is illustrated in Fig. 1. A thickoxide layer by thermal oxidation is helpful for reducing thesurface roughness. The rms roughness can be effectively re-duced to less than2 Å. When the thickness of the oxide layeris more than60 nm, no further reduction on surface roughnesscan be obtained.

I −V measurements of aH-passivatedGe surface wereperformed in nitrogen ambient. STS experiments were imme-diately performed in the scanned area, so that the tip heightfrom the surface is determined by the STM scanning condi-tion. I −V curves for different tip–sample spaces were ob-tained on the as-prepared samples. The results are shown inFig. 2. As the tip is brought close to the surface, the elec-tric field increases. This has an increased effect on the bandbending as also observed by Kaiser [22].I −V curves ofH-passivatedGe(100) surfaces keep growing smoothly andexhibit no significant structure. The parabolic shape resultsmainly from the monotonic increase of tunnelling rate withapplied voltage due to reduction of the average tunnel-barrierheight. Therefore, theH-passivatedGe(100) surfaces were

Fig. 1. The relationship between surface roughness ofH-passivatedGe (100) surfaces and thickness of the covered oxide layers. The oxidelayers grew at the temperature of500C. The rms roughness was measuredby AFM

Fig. 2. I −V curves for different tip–sample distances. The tip–sample dis-tances were determined by the scanning condition. At fixed tunnellingvoltage, the larger the tunnelling current, the smaller the tip–sample dis-tance

not pinned. TheI −V curves in Fig. 2 are very similar tothat of a MIS structure with a very thin insulator layer [23].We discuss theI −V curves in terms of the MIS model in-cluding the effect of the band bending and the work functiondifference betweenGe and W. The work function ofW is4.5 eV, which is very close to that of a p-typeGe samplewith a resistivity of8–12Ω cm. At thermal equilibrium, thep-typeGe surface is in flat band because the work functiondifference between the tip and sample is almost zero. In theregion of the positive sample bias corresponding to the ac-cumulation region, the dominant current is due to electronstunnelling from theW tip to the valence band, and the currentincreases monotonically with increasing the positive samplebias. The region of the negative sample bias around0 V isthe depletion region. The region of the large negative samplebias is the weak-inversion region. The differences between

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Fig. 3a–c.Modification on aH-passivatedGe (100) surface:a STM imageof a 2×2 oxide dot array;b STM image of a square oxide layer;c AFMimage of the square oxide layer inb

the band bending for positive and negative sample bias causeasymmetric shapes in these two regions. Therefore, theI −Vcharacteristics of the doping condition can be obtained fromthe H-terminated sample. Although it is difficult to estimatethe dopant concentrations quantitatively from the STM re-sults, theI −V curve can be distinguished between the p-typeand n-type samples.

Surface modification was performed immediately afterSTM imaging in the scanned area. A pre-cut tungsten tip wasused. Before surface modification, the sample surface was ex-amined by STM imaging in air to make sure that the surface ispassivated properly and the surface roughness is small. STMimaging was performed at an applied tip–sample voltage of0.4 V and a tunnelling current of0.1 nA. During surface mod-ification, the feedback inz movement is disabled and the tipheight from the surface is determined by the STM scanningcondition. This prevents the tip touching the sample surface.Figure 3a is an STM image of a 2×2 oxide dot array cre-ated at a modifying voltage of3 V with a duration of0.1 s.During the modification, other parameters were kept the sameas those in imaging. STM images were taken using the sametip after modification. No time effect was observed duringSTM imaging of the oxide structures within the time scaleof our experiments. These dots are with a size of approxi-mately50 nm, and with an apparent depth of approximately3 nm. The shape of dots depends on the geometry of thetip. Figure 3b is a STM image of a square oxide layer cre-ated at the sample voltage of3 V. Since STM may containa convolution of electronic and topographic information, analternative technique, based on a different contrast mechan-ism, is required to interpret the images. Although the squarelayer appeared as a depressed region in the STM image, theyappear as a protruding region in an AFM image of the sameregion, as shown in Fig. 3c.

The dependence of apparent depth of oxide on samplevoltage is shown in Fig. 4. We can see that there is an obvi-ous threshold for negative and positive sample voltage. Thethreshold should be larger than the sample bias for STMimaging. The curve has a linear relationship between the ap-parent depth and applied voltage. The thresholds for nega-tive sample voltage and positive sample voltage are different.

Fig. 4. The dependence of apparent depth on modification voltage

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This is because of the different mechanism for these twocases [24, 25]. Modifications at positive sample voltages andnegative sample voltages are due to anodic and cathodic oxi-dation, respectively. Figure 4 shows a large apparent depth athigh modification voltages. In order to avoid the tip–samplecontact, the tip–sample spacing must be larger than the sumof the real oxide height and the apparent depth. In our experi-ments, the sum of apparent depth and the real oxide height ismuch larger than the typical separation between an STM tipand sample during normal operation. This precludes STM inthe tunnelling regime but rather indicates an electrochemicalprocess [26].

3 Conclusions

STM study and modification ofH-passivatedGe(100) sur-faces have been presented. A simple method for minimis-ing roughness ofH-passivatedGe surfaces was proposed.Growth of an oxide layer by thermal oxidation is helpful toreduce the surface roughness. After thermal oxidation,Gesamples were passivated in a50% HF solution. A cyclicprocedure can be used to properly passivate aGe(100) sam-ple with a thick oxide layer. STM and AFM imaging showthat using the above-mentioned procedures, the rms rough-ness of aH-passivatedGe(100) surface can be reduced to lessthan2 Å. On the passivatedGe (100) surface, we have pro-duced stable, low-voltage, nanometer-scale chemically modi-fied features in air. The features have been characterised usingSTM and AFM. We have shown thatGecan be used for STM-based nanotechnology.

Acknowledgements.The authors thank Ms. Koh Hwee Lin and Mr. GohYeow Whatt for their kind help in installation of the SPM system andother facilities in our experiments. This work was supported by the NationalUniversity of Singapore under grant No. RP3972692.

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