Minority carrier lifetime in ultrananocrystalline diamond/hydrogenated amorphous carbon composite films

Abstract

Ultrananocrystalline diamond (UNCD)/hydrogenated amorphous carbon (a-C:H) composite (UNCD/a-C:H) films possess the following specific characteristics: (a) the appearance of additional energy levels in diamond bandgap and (b) large absorption coefficients ranging from visible to ultraviolet, both of which might be due to large number of grain boundaries between UNCD grains and those between UNCD grains and a-C:H. Owing to them, UNCD/a-C:H films are expected to be applied to photovoltaics such as UV sensors. Actually thus far, we have fabricated pn heterojunction diodes comprising p-type UNCD/a-C:H films and n-type Si substrates, and confirmed their photovoltaic action. In this study, the minority carrier lifetime, which is an important factor for photovoltaics, was experimentally measured by microwave reflected photoconductivity decay, and it was estimated to be 0.21 and 0.43 µs for UNCD/a-C and UNCD/ a-C:H, respectively. In addition, on the basis of the previous work on the heterojunctions, the effects of hydrogenation on the photovoltaic action of the heterojunctions were studied. The photocurrent apparently increases with an enhancement in the hydrogenation of UNCD/a-C:H films, which might be because dangling bonds in the UNCD/a-C:H films, which act as photogenerated-carrier trap centers, are terminated by hydrogen atoms.

Evidence of hydrogen termination at grain boundaries in ultrananocrystalline diamond/hydrogenated amorphous carbon composite thin films synthesized via coaxial arc plasma

“…These films were deposited with applied voltages of 70, 85, 100, and 115V to the arc gun. All of the films were synthesized under the same preparation conditions with our recent paper [33], and the whole film fabrication process and a principle of controlling hydrogen content by changing an applied pulsed voltage are described in Ref. [33].…”

“…All of the films were synthesized under the same preparation conditions with our recent paper [33], and the whole film fabrication process and a principle of controlling hydrogen content by changing an applied pulsed voltage are described in Ref. [33]. XPS and NEXAFS measurements were performed at beamline 12 of NAOFUMI NISHIKAWA…”

“…Actually to date, p-and n-type conductions of the UNCD / a-C:H films by CAPD have been already realized by doping boron [24,25] and nitrogen [28], respectively in experimental conditions, and pn heterojunction diodes have been also fabricated by depositing these films on n- [24] and p-type [28] single-crystalline Si substrates. In recent research, it has been clarified that hydrogenation would be a key factor of an enhancement of photovoltaic performance in these diodes [26,27,33]. And as a result of the latest study, it was indirectly confirmed through electrical properties of the diodes that hydrogenation would be assisted with a control of applied pulsed voltages to a graphite target equipped with an arc gun, by customizing an apparatus similar to CAPD one in principle [33].…”

“…sp 3 and sp 2 bonds, which largely affect the physical properties of the films. UNCD / a-C and UNCD / a-C:H films synthesized by CAPD have been studied up to the present, by taking the advantages of conventional diamonds, toward the applications not only to hard coatings [18][19][20][21][22] and thermoelectric conversion elements [23], but also to optoelectronic devices such as photovoltaics [24][25][26][27][28][29][30][31][32][33][34][35][36]. As mentioned above, UNCD / a-C:H films prepared by CAPD include more a-C and/or a-C:H fractions and GBs that would act as generation sources of photo-induced carriers, which might be a cause of extremely large light absorption coefficients excessing 10 5 cm −1 in the photon energy range of 3 to 6 eV [15].…”

“…In recent research, it has been clarified that hydrogenation would be a key factor of an enhancement of photovoltaic performance in these diodes [26,27,33]. And as a result of the latest study, it was indirectly confirmed through electrical properties of the diodes that hydrogenation would be assisted with a control of applied pulsed voltages to a graphite target equipped with an arc gun, by customizing an apparatus similar to CAPD one in principle [33]. However, it is unclear how hydrogenation induced with this technique has an effect on the chemical bonds of the films.…”