Mobility Gaps of Hydrogenated Amorphous Silicon Related to Hydrogen Concentration and Its Influence on Electrical Performance

Abstract

This paper presents a comprehensive study of hydrogenated amorphous silicon (a-Si)-based detectors, utilizing electrical characterization, Raman spectroscopy, photoemission, and inverse photoemission techniques. The unique properties of a-Si have sparked interest in its application for radiation detection in both physics and medicine. Although amorphous silicon (a-Si) is inherently a highly defective material, hydrogenation significantly reduces defect density, enabling its use in radiation detector devices. Spectroscopic measurements provide insights into the intricate relationship between the structure and electronic properties of a-Si, enhancing our understanding of how specific configurations, such as the choice of substrate, can markedly influence detector performance. In this study, we compare the performance of a-Si detectors deposited on two different substrates: crystalline silicon (c-Si) and flexible Kapton. Our findings suggest that detectors deposited on Kapton exhibit reduced sensitivity, despite having comparable noise and leakage current levels to those on crystalline silicon. We hypothesize that this discrepancy may be attributed to the substrate material, differences in film morphology, and/or the alignment of energy levels. Further measurements are planned to substantiate these hypotheses.

Keywords: PECVD; Raman; amorphous hydrogenated silicon; flexible substrate; hydrogen bonding; inverse photoemission; photoemission; radiation detector; simulation; thin film.

Figure 1

Side view of the p-i-n diodes: (a) deposited on crystalline silicon and (b) deposited on Kapton. Not to scale.

Figure 2

Reverse leakage current comparison for two sensor types: one deposited on c-Si (blue) and the other on Kapton (orange), with leakage normalized to the detector’s sensitive volume.

Figure 3

Example of acquired current as a function of time for a known dose rate. The signal is consistently stable over time, with a very low noise level (S/N greater than 10).

Figure 4

Response curves of the sensor at varying dose rates: blue lines correspond to detectors on crystalline silicon, and orange lines correspond to detectors on Kapton.

Figure 5

Simulated structure: (a) device deposited on Kapton; (b) device on p-doped c-Si.

Figure 6

Simulated current–time signal when the carrier is generated in both the a-Si:H and c-Si layers (black), only in a-Si:H (red), and only in c-Si (blue).

Figure 7

Raman spectra of a-Si:H film on c-Si (a) and on Kapton (b) substrate. Crystalline silicon spectrum is shown for comparison.

Figure 8

(a) Raman spectra in the frequency area of the stretching vibrations of the hydride Si-H and polyhydride bonds of a-Si:H film on crystalline silicon (cSi) and Kapton. The same region in the bare crystalline silicon spectrum is shown for comparison. The smoothed lines are also shown; the top spectrum with the indication of the LSM and HSM is presented for comparison (ref. [22]). (b) Schematic representation of (1) monohydride, LSM (2) dihydride, and (3) dangling bonds near the HSM voids.

Figure 9

Photoemission Si2p core level deconvolution for a-Si:H device samples: top: a-Si:H/c-Si; bottom: a-Si:H/Kapton.

Figure 10

Schematic diagram of distribution of energy states in a-Si:H. Defect states are represented by two equal Gaussian distributions.

Figure 11

Combined photoemission and inverse photoemission measurements of the Kapton (green) and c-Si samples (red). The lines correspond to the smoothed curves. The value of the transport gap is calculated by the mobility edges of the valence band (photoemission) and conduction band (inverse photoemission) as obtained by the standard graphical method.