Nitrogen-related dopant and defect states in CVD diamond E. Rohrer, C. F. O. Graeff, R. Janssen, C. E. Nebel, and M. Stutzmann Walter Schottky Institut, Technische Universita ¨t Mu ¨nchen, Am Coulombwall, D-85748 Garching, Germany H. Gu ¨ ttler and R. Zachai Materials Research, Daimler Benz AG, D-89013 Ulm, Germany Received 31 October 1995; revised manuscript received 11 April 1996 Subbandgap absorption of chemical-vapor-deposition diamond films, with nitrogen contents varying from 10 to 132 ppm has been explored by the constant-photoconductivity method CPM, photothermal-deflection spectroscopy PDS and electron spin resonance ESR. The spectra measured by PDS increase monotonically and are structureless with increasing photon energies indicating absorption due to amorphous carbon and graphite. The CPM data show distinct features, with absorption bands at h  =1.6, 4.0, and 4.7 eV in the nominally undoped film, and 2.4 and 4.7 eV in nitrogen-rich layers respectively. The CPM spectra of the doped films are comparable to photoconductivity data of synthetic Ib diamond. The defect densities involved increase with increasing nitrogen content. From ESR, a vacancy-related defect density ( g =2.0028) is deduced. Para- magnetic nitrogen ( g =2.0024) can be detected in the high-quality CVD layer or by illuminating the nitrogen- rich samples with photon energies larger than the band gap. S0163-18299609535-5 I. INTRODUCTION The unique properties of diamond have attracted consid- erable interest, especially after the discovery that low-cost and large-area diamond films can be grown by the chemical- vapor-deposition CVD technique. 1,2,3 Presently, one of the main applications of CVD diamond is as a substrate for high- power electronics due to its excellent thermal conductivity. Recent improvement of film quality suggests that active elec- tronic devices for high-temperature or high-power operation can be realized in the near future. However, a variety of problems still have to be solved. Due to the microcrystalline structure of CVD diamond, grain boundaries, and surface and bulk defect states dominate the optical and electronic properties. Especially amorphous and graphitic carbon at surfaces and interfaces affect the dark conductivity as well as the optical absorption. Spectroscopy of bulk defect states, regarding their distributions and densities in the band gap, is therefore difficult and data available in the literature is still limited. N -type doping has up to now not been understood. Nitro- gen is a possible donor in diamond with lower formation energy than phosphorus. 4,5 Experiments with synthetic and natural diamond, however, show that in addition to the deep donor level at 1.7 eV Ref. 29 below the conduction band, a variety of nitrogen-related defects are created that dominate transport and recombination of carriers. For successful pro- duction of electronic garde material, exploration of nitrogen- related effects is important. Excellent techniques for the in- vestigation of band-gap states are photothermal-deflection spectroscopy PDS and the constant-photocurrent method CPM. 6,7 PDS experiments detect spectrally resolved states that absorb light, while CPM is sensitive to absorption events that generate mobile carriers. CPM is therefore much less sensitive to surface than to bulk absorption. The application of both techniques then allows us a study of both surface and bulk defect densities. For an investigation of nitrogen-related doping and defect effects, electron-spin-resonance ESR ex- periments have also been applied. A series of CVD diamond films with nitrogen content varying between 10 and 132 ppm will be discussed in the following. II. EXPERIMENT The diamond films were grown at T =750 °C on silicon in a standard CVD system described in Ref. 8, using H 2 and CH 4 as the main gas sources. To study the effect of nitrogen on the film properties, the nitrogen content in the gas phase was varied from 0 to 4% N 2 /CH 4 . Elastic-recoil-detection experiments 9 show that these films contain 10–132 ppm ni- trogen see Table I. The intentionally doped films show nei- ther oriented nor textured crystal growth and appear nearly black in color. Sample thicknesses varied between 7 and 9 m. Even the nominally undoped sample had 10 ppm nitro- gen in the layer of thickness d =55 m. This diamond film is highly oriented, 100 textured with typical 10-m grain size. Photoconductivity measurements with planar contacts on top of the diamond sample coplanar contact configuration, photothermal-deflection spectroscopy, and electron-spin- resonance measurements were performed using free-standing TABLE I. Nitrogen content in the gas phase compared to the fraction incorporated in the diamond film measured by ERD Ref. 9. N 2 /CH 4 % N/C ppm 0 10 0.3 25 1.13 35 2.67 100 4 132 PHYSICAL REVIEW B 15 SEPTEMBER 1996-I VOLUME 54, NUMBER 11 54 0163-1829/96/5411/78747/$10.00 7874 © 1996 The American Physical Society