Electron field emission measurements from boron-doped CVD diamond on tantalum

Brazilian Journal of Physics, vol. 33, no. 1, March, 2003 94 Electron Field Emission Measurements from Boron-Doped CVD Diamond on Tantalum J. A. N. Gonçalves, G. M. Sandonato, Laboratório Associado de Plasma, Instituto Nacional de Pesquisas Espaciais Caixa Postal 515, CEP 12201-970, São José dos Campos, SP, Brazil and K. Iha Instituto Tecnológico de Aeronáutica, Departamento de Quı́mica 12228-900, São José dos Campos, SP, Brazil Received on 12 March, 2002. Revised version received on 2 August, 2002 Boron-doped polycrystaline diamond films grown by hot-filament-assisted chemical vapor deposition were studied with ultraviolet photoemission spectroscopy (UPS), Raman spectroscopy, X-ray diffractometry and current voltage measurements. The UPS measurement shows that the work function () without electric field is about 3.9 eV . The field-emission current-voltage measurements indicate a threshold voltage ranging from 8.97x106 to 9.64x106 V=m and a work function () about 0.3 eV . These results show that boron doped diamond films exhibit a negative electron affinity in high electric field. I Introduction II Experimental details Diamond possesses unique semiconductor properties, such as wide bandgap, high breakdown voltage, and both high electron and hole mobilities [1]. These properties make diamond attractive for application in high-frequency and highpower electronics. Boron doped diamond films have been prepared by various chemical vapor deposition (CVD) methods and some works have provided information on the structural, optical an electrical properties of the films [2-8]. Boron doped diamond is a p-type semiconductor, and it is the only dopant which has been successfully and reproducible used to prepare semiconductor diamond. Because of its small size, the boron atom is easily incorporated in the diamond lattice. The introduction of boron atoms would affect the structure and properties of the diamond films, so it is important to understand the nature of boron atoms in the diamond films for preparing semiconductor diamond. In the present work, polycrystalline diamond films with high boron concentration were prepared on tantalum substrate by hot filament CVD method. The boron was introduced by B2 O3 in methanol solution. Scanning electron microscopy (SEM) and x-rays diffractometry (XDR) were employed to study the morphology and structure of these films. Raman spectroscopy, currentvoltage characteristics IvsV and ultraviolet photoemission spectroscopy (UPS) were used to investigate their optical and electrical properties. ( ) The diamond films were grown on monocrystalline tantalum (111) by hot filament CVD in a 10 cm diameter and 30 cm high cylindrical quartz reactor. Boron was introduced in the reactor by flowing hydrogen through the B 2 O3 methanol solution. The substrates were previously cleaned with acetone and scratched by a diamond paste (grain size of 1m) in order to promote a better nucleation of the diamond films. The diamond films were grown at 800 o C from 0.5 H 2 /CH4 mixture at a total pressure of 50 T orr. Boron oxide in methanol solution was used with 20000 ppm B/C ratio. A spectroscope Renishaw model RM 3000 was used for Raman spectroscopic investigations with an air cooled 785 nm laser diode (with an output power of 17 Mw) for excitation of the samples. The diffractograms were obtained using a spectrometer Phillips PW 1840 X-ray with a solid state goniometer, and equipped with CuKa (1.5406) cathode and nickel filter. The control parameters for the samples investigated are shown in Table 1. % Table 1. X-ray parameters for the sample investigated. Power Step size  Number of steps Time per steps s Scan speed =s Receiving slite (2 ) () (2 ) 40 KV e 250 mA 0.020 5500 1.00 0.02 0.2 J.A.N. Gonçalves, G.M. Sandonato, and K. Iha 95 ( ) 550 500 450 Intensity [AU] The first type of electron emission measurement employed was UPS wherein the ultraviolet light is incident on the conduction band [9]. Electrons with sufficient energy to overcome the electron affinity of the material are emitted into the vacuum. The second type of electron emission measurement was current-voltage characteristics I  V . The I  V measurements were taken at two distances using an experimental setup specially developed to this purpose. 400 350 300 250 200 150 III Results 100 800 The surface of the diamond film was observed by scanning electron microscopy (SEM). Figure 1 shows SEM micrograph of surface morphologies for boron doped diamond deposited on tantalum. Crystal facets are observed with a thickness of 15m and grain sizes are 2 m. Both thickness and grain sizes were obtained by SEM microscopy. 1000 1200 1400 1600 1800 -1 Wavenumber [cm ] Figure 2. Raman spectroscopy of diamond films grown with B/C of 20000 ppm. 12000 Ta (110) 10000 Ta (211) Intesity, UA 8000 TaC (200) 6000 Ta (200) Diam (111) TaC (111) 4000 Ta (310) Ta (220) TaC (331) TaC (420) TaC (220) Diam (220) Diam (311) Ta (422) 2000 0 0 20 40 60 80 100 120 2q (degree) Figure 1. Scanning Electron Microscopy (SEM) of diamond film grown with B/C of 20000 ppm. Since Raman spectroscopy is a nondestructive method, it is commonly used in characterization of the structure and quality of synthesized diamond films [10]. A drastic change of Raman spectra of diamond film with high level of boron was observed. The results reveal that the peak intensity at 1332 cm 1 , corresponding to the transversal mode (related to the sp3 bound) of the diamond, decreases as the boron concentration increases [11]. Nevertheless, a wide band arises around 1220 cm 1 and increases as the boron concentration is increased, as shown in Fig. 2. The same sample was investigated by x-ray diffractometry and the peaks related to the tantalum, tantalum carbide and diamond are shown in Figure 3. Although the tantalum substrate has no any preferential growing direction, it can be noted that the diamond peak of the plane (111) is more intense than the peak of the plane (220). This indicates that the growing process occurs preferentially at the plane (111), otherwise the orientation of the diamond films should be in the plane (110), i.e., in the transversal plane to the plane (111). Figure 3. Typical XDR diffractogram of diamond film on tantalum. UPS measurements were performed in the Linköping University, in an ultra high vacuum chamber (base pressure below 2x10 10 T orr). The sample excitation in this system was provided by 21.5 eV light a helium resonance discharge lamp, and a hemispherical analyzer was used to measure the energy spectrum of the photoemitted electrons. The UPS system is described in more details elsewhere [12]. The work function measured was 3.9eV for the boron doped diamond film. Field emission measurements were obtained within high vacuum chamber (background pressure bellow 10 5 T orr) and the measurements were performed in the characterization system. The sample was placed and he (...truncated)