Electrical Behavior of Pure and Cu Doped Diamondlike Carbon Prepared by Pulsed Laser Deposition
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incorporation of foreign atoms such as Cu into the growing films[6]. Since using of DLC in electronic devices has been investigated, with the demonstration of photo-conductivity and some simple junction devices, the effect of Cu incorporation on the electrical behavior will be of great concern. In the present work, we prepared pure DLC and DLC films that contain various contents of copper atoms by pulsed laser deposition. The films were characterized by Rutherford back scattering spectrometry, visible Raman spectroscopy, and transmission electron microscopy for chemical composition, atomic bonding as well as microstructure information. Electrical measurements using four-probe technique were performed on the films. The temperature dependence of resistivity was studied in order to understand the transport mechanism of the copper containing DLC films. EXPERIMENT We have adopted an ingenious modification of the traditional PLD process in order to incorporate copper atoms into the growing films. Briefly, we covered the graphite target in part with a piece of copper. The copper atoms were incorporated into the growing DLC film during PLD process, with the concentration of copper monitored by changing the geometry of the target. Since the target was spinning during deposition, the focused laser beam would impinge sequentially on graphite and the copper piece to ablate the target materials to form a composite layer. The p-type Si (100) wafers were used as substrates, which were cleaned in acetone and methanol ultrasonic baths followed by HF dip to remove the native oxide layer before loading into the laser deposition chamber. The laser beam source used was KrF pulsed excimer laser (X=248 nm, t,=25 nm) at a repetition rate of 10 Hz, with an energy density close to 3.0 J/cm 2 . All the depositions were conducted at room temperature in a high vacuum exceeding lx l07 torr. Pure DLC films and films with four different Cu contents were deposited for 30 and 40 mins, respectively. The coatings were analyzed using Rutherford back scattering spectrometer for copper contents, and Raman spectroscopy for bonding characteristics. Transmission electron microscopy (TEM) and radial distribution function (RDF) analysis were used to study the microstructures of the films. The TEM was TOPCON002B operated at 200 KV at a point-to-point resolution of 1.8 A at the first Scherzer focus. Four probe electrical measurements and temperature dependent resistivity measurements were performed for the understanding of the electrical properties and transport mechanisms. RESULTS AND DISCUSSION The thickness of the pure DLC films is about 300 nm, and the thickness of the DLC films that contain Cu is within the range of 400-500 nm. Table I list the copper contents of some specimens from RBS measurements and values estimated based on a geometrical consideration. Table I Copper contents in the DLC films (at%) Sample number Cu- I Cu-2 Cu-3 Cu-4 Concentration 1.2 1.4 1.5 2.5 Fig. 1 shows the Raman spectra of pure DLC and DLC+1.4 at% Cu (sample number Cu-2). As is
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