IN THIS THESIS we have investigated a dispersive charge transport in hydrogenated amorphous silicon. The employed experimental technique is time-resolved photocurrent measurement: time-of-flight technique. The time-of-flight photocurrent transient signals are very sensitive to temperature, applied electric field and gap state distribution.
It was found that the dispersion parameters obtained from the photocurrent transient signals were temperature and field dependent for the both electron and hole. The electron current transient reveals a transition from non-dispersive to dispersive below room temperature. But the hole current transient maintains dispersive transport behaviours. The both transient signals exhibit more dispersion at lower electric field. These show that the dispersive charge transport is caused by the multiple trapping process. It was observed that the drift mobility of the electron revealed thermally activated behaviour with activation energy of 0.17 eV and was electric field-independent in the wide temperature range of 180~340 K. Time-resolved activation energy has been obtained from the temperature dependence of the transient photocurrent measured at fixed time after short light flash on sample. The observed activation energy was 0.17 eV and remained unchanged for different fixed measurement times. On the other hand the hole mobility activation energy was field-dependent and increased with decreasing electric field. These results are explained by the multiple trapping processes of the charge carriers with band tail states on the base of the assumption of the double exponential distributions.
The effects of the band tail state distribution on the transient charge transport have been studied by the post-treatments of the sample such as annealing and light soaking. It was observed that the both treatments resulted in the distortion of the current transient signal and reduced the mobility-lifetime product. However it was found that the both was responsible for different mechanisms of the charge transport. It was observed that the electron drift mobility, dispersion parameters was decreased by the annealing while the mobility activation energy remained unchanged. These can be explained by the gradual broadening of the exponential band tail due to the annealing process. On the other hand it was observed that the light soaking has little influence on the drift mobility, as expected in the only increase of the deep gap states.
본 연구에서는 Time-of-Flight 방법을 사용하여 수소화된 비정질 규소의 전하 분산수송의 과정을 고찰하였다. 전자와 양공의 과도적 전류신호들은 일반적으로 온도, 전기장밑 금지대내의 상태밀도분포의 의존성을 갖는 것이 관측되었으며 전하 분산수송은 다음과 같이 설명할 수 있었다.
1. 전하 분산수송은 multiple trapping 과정에 의해서 일어난다.
2. 전자 분산수송은 전도대의 잉여전자들이 전도대밑 0.17eV에 위치하며, 비교적 좁은 에너지 범위에 분포한 trapping states들과의 상호작용에 의해서 일어난다.
3. 양공 분산수송은 전도대의 잉여양공들과 상호작용을 하는 효과적 트랩핑 레벨 (effective trapping level)이 점진적으로 깊은 밴드 끝 상태 (band tail states) 로 이동하므로서 일어난다.
4. 전하 분산수송은 밴드 끝 상태를 이중 지수함수로 가정하여 설명할 수 있었다.
5. 열처리와 광유도효과 실험으로부터 금지대 내의 상태들중 밴드끝 상태들만이 전하 분산수송에 관여함을 알 수 있었다.
