n-Type doping of diamond and the device applications

S. Koizumi

Advanced Materials Laboratory, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan.

n-Type diamond thin films were successfully formed by optimizing the growth condition of chemical vapor deposited diamond under phosphorus doping to obtain better crystalline perfection. Phosphorus atoms can be incorporated in {111} oriented surface during the growth and the concentration can be controlled within the range of 10¹⁷ ~ 5×10¹⁹ cm^-3. Phosphorus forms a donor level at 0.6 eV below the conduction band minimum that is confirmed by Hall measurements, photocurrent and infrared absorption (FTIR). The maximum electron mobility is 400 cm² V^-1 s^-1 at room temperature obtained from lightly doped samples (3x10¹⁷ cm^-3). Cathodoluminescence of phosphorus doped films show strong bound exciton luminescence at 5.19 eV at low temperatures and free exciton luminescence (5.27 eV) can be observed even at room temperature. As a next step, we have studied pn -junction of diamond by stacking boron doped p-type layer and phosphorus doped n-type layer. The pn -junction has shown good rectification characteristics with the ratio of 10¹⁰ at ± 10 V. The diode characteristics and the behavior of pn -junction interface have been characterized by temperature dependent IV and CV measurements. By forward bias operation, the diode shows ultraviolet light emission at 235 nm that attributes free exciton luminescence. The diode also shows photoelectric response against ultraviolet light with the wavelength shorter than 225 nm that corresponds the band-gap of diamond. It is expected these characteristics of diamond pn -junction will be applied for the next generation opto-electrical devices.