Tuning Amorphous Selenium Composition with Tellurium to Improve Quantum Efficiency at Long Wavelengths and High Applied Fields

Amorphous selenium (a-Se) is a large-area compatible photoconductor that has received significant attention toward the development of UV and X-ray detectors for a wide range of applications in medical imaging, life science, high-energy physics, and nuclear radiation detection. A subset of applications require detection of photons with spectral coverage from UV to infrared wavelengths. In this work, we present a systematic study utilizing density functional theory simulations and experimental studies to investigate optical and electrical properties of a-Se alloyed with tellurium (Te). We report hole and electron mobilities and conversion efficiencies for a-Se1–xTex (x = 0, 0.03, 0.05, 0.08) devices as a function of applied field, along with band gaps and comparisons to previous studies. For the first time, these values are reported at high electric field (>10 V/μm), demonstrating recovery of quantum efficiency in Se–Te alloys. A comparison to the Onsager model for a-Se demonstrates the strong field dependence in the thermalization length and expands on the role of defect states in device performance.

: Images of thin films and an a-Se 0.92 Te 0.08 device used in this study.
All films underwent SEM-EDS to determine the composition and ensure homogeneity of the Se-Te distribution. Compositional maps for the thin films can be found in Figure S2.
The thicknesses of the thin films were found by cross-sectional SEM; thicknesses of the Se-Te layer of devices were found by profilometry. Values of Se and Te content and thickness can be found in Table S1. An amorphous structure is important to the performance of the devices and ensures proper characterization of properties. X-ray diffraction was carried out on all films; Figure   S3 shows scans of stabilized a-Se and a-Se 0.92 Te 0.08 , representative of all samples. The lack of sharp peaks and broad distribution around 25 • is indicative of amorphous behavior.

Photothermal deflection spectroscopy (PDS)
PDS is a high sensitivity technique for evaluating the non-radiative optical transitions below the band edge of a semiconducting material, down to 10 cm −1 . In the setup utilized in this work, a diagram of which can be found in Figure S4, the film is suspended in a liquid with a high gradient of index of refraction with respect to changes in temperature and pumped with a modulated (5 Hz) monochromatic beam (0.6-3 eV) normal to the sample surface. A HeNe probe laser is aligned adjacent and parallel to the sample surface; as the sample is excited and relaxes non-radiatively due to the modulated pump beam, the probe laser is deflected by the changing index of refraction of the suspension medium -in this case, Fluornert FC-72 (C 6 F 14 ). This deflection is measured using a photodetector combined with a knife edge, measuring the change in the intensity of the laser incident on the detector. This signal, V sig , combined with a reference of the pump intensity, V ref , is used to calculate the absorption coefficient by with where d is the thickness of the film, and C norm is a scaling constant determined by transmission, T , from UV-Vis spectroscopy at a wavelength just above the band edge. Mobility, µ, is then calculated as where L is the thickness of the photoconductive layer and V A is the applied voltage across that layer. Figure S6 gives an example of a typical TOF signal and the fits used to determine the transit time. A description of this system may also be found in.

DOS & IPR Plots
Density of states (DOS) and inverse participation ratio (IPR) simulations were used to predict the optical and mobility gaps of a-Se 1−x Te x alloys. Several iterations for each composition were performed to find an average value and error. One iteration from each composition can be seen in Figure S7; general trends in DOS and IPR values are consistent across itera-S6 tions, though locations of the defect states may vary. Figure S7: A DOS and IPR plot for each composition evaluated in simulations. Multiple iterations for each alloy were performed; plots are of one iteration, but show similar trends in DOS and IPR values across iterations.