UHV STM of Cesium on Oxygenated Epitaxial Diamond (100) Films

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Mat. Res. Soc. Symp. Proc. Vol. 509 01998 Materials Research Society

Figure 1. An atomic resolution UHV STM image of a CVD diamond (100) epitaxial film before Cs and 02 deposition, showing a (2xl) dimer reconstruction. respectively. Diborane was introduced at a concentration of 3 ppm relative to hydrogen in order to grow a conducting p-type film. The tungsten filament temperature was 2200 °C. The growth experiment was terminated by first shutting off only the methane flow while maintaining the sample, filament, and H 2 settings for 2 minutes. Then the filament, sample heater and H2 flow were turned off in that order. After growth, Cs was deposited on the surface using an SAES Getters Cs dispenser at a pressure : 10-9 Torr. After the Cs getter was turned off and cooled to room temperature, the diamond surface, also at room temperature, was then exposed to 02 at a pressure of 2x10"7 Torr for 100 seconds (approximately 10 L ). The STM images were obtained in UHV at a pressure ! 10-1 Torr using an Aris 5000 UHV compatible STM from Burleigh Instruments, Inc. Tunneling currents of approximately 0.1 nA and sample bias of ±7V were used to obtain atomic resolution. RESULTS AND DISCUSSION After growth, the epitaxial film appears smooth and transparent to the naked eye. We have found that further investigation with a scanning electron microscope (SEM) shows a featureless surface. Figure 1 shows an atomic resolution UHV STM image of the diamond (100) epitaxial film before Cs and 02 deposition showing a (2xl) dimer reconstruction of the surface [5]. Single steps are observed consisting of atomic planes with dimer rows rotated in the x-y plane 900 relative to the dimer rows in the upper or lower atomic planes. Figure 2 shows a large scale STM image of the same diamond film after Cs and 02 deposition obtained using a negative tip bias with respect to the sample. The sub-monolayer of adsorbates are distributed evenly over the diamond (100) surface and appear as round bright structures in the topographic images. Figures 3(a)-(e) show a series of atomic resolution UHV STM images taken of this same area. The images shown in Figs. 3(b)-(e) were taken one after the other using positive and negative tip voltages with respect to the sample. It is well known that with a negative tip bias, electrons will tunnel from the metal tip into empty electron states of the sample surface. Conversely, with a positive tip bias the electrons will tunnel from the filled electron states of the sample surface to

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Figure 2. Large scale UHV STM image of the diamond (100) film after Cs and 02 deposition. A sub-monolayer of adsorbates is observed with a tip bias of-7V and a tunneling current of 0.1 nA.

Figure 3(a)-(e). STM topographs of diamond (100) after Cs and 02 exposure. Tip bias of ±7v and tunneling currents of 0.1 nA were used. These figures show the adsorbates' dependence on tip polarity. the metal tip [6]. This technique of changing the tip bias from one scan to the next is a very useful way of identifying adsorbates on a semiconductor surface

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