• PDF / 189,569 Bytes
• 6 Pages / 612 x 792 pts (letter) Page_size
• 48 Downloads / 212 Views

DOWNLOAD

REPORT

---

1066-A04-05

Characterization of Gap Defect States in Hydrogenated Amorphous Silicon Materials Lihong (Heidi) Jiao1, and C. R. Wronski2 1 School of Engineering, Grand Valley State University, Grand Rapids, MI, 49504 2 Electrical Engineering, Pennsylvania State University, University Park, PA, 16802 ABSTRACT An enhanced simulation model based on the carrier recombination through these states was developed to characterize the gap defect states in hydrogenated amorphous silicon materials (aSi:H). The energy dependent density of electron occupied gap states, kN(E), was derived directly from Dual Beam Photoconductivity (DBP) measurements at different bias currents. Through Gaussian de-convolution of kN(E), the energy peaks of the multiple defect states, including both neutral and charged states, were obtained. These energy levels, together with the information on the capture cross sections, were used as known input parameters to self-consistently fit the subgap absorption spectra, the electron mobility-lifetime products over a wide range of generation rates, as well as the energy dependent density of electron occupied gap state spectra. Accurate gap state information was obtained and the nature of the defect states was studied. Simulation results on light degraded hydrogen diluted, protocrystalline a-Si:H show that the density of charged states is 4.5 times that of neutral states. The two states close to the midgap act as effective recombination centers at low generation rates and play key roles in photoconductivity studies. INTRODUCTION Hydrogenated amorphous silicon (a-Si:H) materials have been an interest in many device applications and it is important to understand the nature of the gap defect states in these materials. Attempts have been made in the past to characterize the gap defect states in both asgrown and light degraded states. Extensive analysis has been carried out on the subgap optical absorption spectra obtained from Constant Photocurrent Method (CPM) or Photothermal Deflection Spectroscopy (PDS) [1,2]. However, it is difficult to obtain reliable gap defect state parameters from just the subgap absorption data. Dual Beam Photoconductivity (DBP) measurements at different generation rates along with the mobility-lifetime products, µτ, measurements have allowed detailed analysis to be carried out [3]. Due to the large number of parameters involved in the simulation, it still remained a challenge to obtain accurate information on the gap defect states. Recently, Pearce et al introduced the energy dependent density of electron occupied gap states derived directly from the DBP measurements [4]. In their approach, by first-order approximation, the densities of states in the conduction band were treated as a constant. Taking the derivative of subgap absorption, α (hυ ) , the densities of electron occupied gap states were obtained from the equation kN gap ( E ) = (hν )(d [α (hν )] / dE ) + α (hν ) , where kNgap represents the energy dependent density of electron occupied gap states that pertains to the distribution