Responsivity Study of Diamond X-ray Monitors with Contact

Nitrogen doped ultrananocrystalline diamond (nUNCD) grown on the surface of a CVD single crystal diamond is tested at various beamlines covering an x-ray photon energy range of 200eV to 28 keV. The nUNCD has much lower x-ray absorption than metal contacts and is designed to improve the performance of our device. The responsivity of nUNCD diamond x-ray detector is compared with the conventional platinum coated diamond x-ray beam position monitor and the results are presented in this paper. INTRODUCTION The ultrananocrystalline diamond (UNCD) thin films exhibit similar physical properties to single crystal diamond in the aspects of mechanical, thermal and resistance to radiation damage. Current UNCD thin films are prepared by microware plasma chemical vapor deposition (MPCVD) or hot filament chemical vapor deposition (HFCVD) with mixed H2/CH4 or Ar/CH4 plasma chemistries. Nitrogen is introduced to UNCD thin film to improve its electrical conductivity to semi-metallic level, and therefore can be considered as a replacement of metal to function as an electrode in various applications like biomedical devices, biosensors, and in our case the xray monitors. [1, 2] In this paper, a diamond device prepared with nitrogen doped ultrananocrystalline diamond (nUNCD) on both sides as electrodes is studied and results are compared with the device fabricated with traditional platinum contact. EXPERIMENTAL In order to improve the performance of the diamond xray monitor, nUNCD layers of 200 nm ~ 500 nm are grown on a 4 mm × 4 mm × 0.3 mm electronic grade ([N] ~ppb) single crystal CVD diamond as contact electrodes at Argonne National Laboratory (ANL). X-ray white beam topography and birefringence images were recorded for this sample prior to device fabrication. X-ray topography was performed at Beamline X19C, National Synchrotron Light Source (NSLS) and birefringence images were taken using a polarized microscope. 2D current maps and calibrated responsivity vs photon energy of this diamond were collected at various beamlines in NSLS, covering a photon energy range of 0.2~28keV. Beamline U3C provides monochromatic beam of photon energy from 0.2~1 keV, while beamline X8A provides x-ray from 1~6.5 keV and beamline X15A from 6.5~28 keV, with an intensity ranged from 10~10 W/mm. [3] Measurements at U3C are performed in a vacuum level of 10torr and at X8A in a vacuum of 10 torr due to the high absorption rate of air for low energy photons, and is in air at X15A. Incident xray power is calibrated using silicon photodiodes at both U3C and X8A (with thickness of 25 μm and 52 μm respectively), while air-filled ionization chamber is used at X15A. Various biases in the saturated range of either DC level or in the form of square wave with adjustable magnitude, frequency and duty cycle, were applied on tested diamond plates to assure full collection of desired charge carriers (positive biases for hole collection and negative biases for electron collection). RESULTS AND DISCUSSION Figure 1 shows the compared results of the 2 imaging techniques. The nUNCD contact is completely transparent under the white beam at X19C (Fig.1a). The dark circled area in Fig.1b indicates the nUNCD contacts on both sides of the diamond. Prominent contrast of slip bands in diamond single crystal was observed in both topography and birefringence images. Figure 1: (a) X-ray topography and (b) birefringence images of the nUNCD coated diamond. 2D current maps of this diamond are collected at various beamlines at different energies. The values of xray attenuation length of diamond (the depth into the diamond measured along the surface normal where the ____________________________________________ *Work was supported by U.S. Department of Energy under grants KC0407-ALSJNT-I0013 and DE-FG02_08ER41547. Use of the National Synchrotron Light Source, Brookhaven National Laboratory, was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC0298CH10886. #mgaowei @bnl.gov nUNCD intensity of x-rays falls to 1/e of its value at the surface) at these energies are listed in Table 1. According to x-ray database CXRO, a minimum energy of 1350eV is needed for a photon to travel all the way through a 300 μm thick diamond. [4] Since it is difficult to control the growing process of the nUNCD layer, the thickness of this contact may vary in a range of 200nm to 500nm. Current maps collected at the listed energies and representative results of these measurements are presented in Fig.2. Both positive and negative biases were applied with duty cycles for electron detrapping. [5] Table 1: Diamond X-ray attenuation length at various energies from CXRO X-Ray Database [4] Beamline Photon Energy (eV) X-ray attenuation length (μm)