J. Drewery

High efficiency neutron sensitive amorphous silicon pixel detectors

A multi-layer a-Si:H based thermal neutron detector was designed, fabricated and simulated by Monte Carlo method. The detector consists of two a-Si:H pin detectors prepared by plasma enhanced chemical vapor deposition (PECVD) and interfaced with coated layers of Gd, as a thermal neutron converter. Simulation results indicate that a detector consisting of 2 Gd films with thicknesses of 2 and 4 /spl mu/m, sandwiched properly with two layers of sufficiently thick (/spl sim/30 /spl mu/m) amorphous silicon diodes, has the optimum parameters. The detectors have an intrinsic efficiency of about 42% at a threshold setting of 7000 electrons, with an expected average signal size of /spl sim/12000 electrons which is well above the noise. This efficiency will be further increased to nearly 63%, if we use Gd with 50% enrichment in /sup 157/Gd. We can fabricate position sensitive detectors with spatial resolution of 300 /spl mu/m with gamma sensitivity of /spl sim/1/spl times/10/sup -5/. These detectors are highly radiation resistant and are good candidates for use in various application, where high efficiency, high resolution, gamma insensitive position sensitive neutron detectors are needed. >

Amorphous silicon pixel layers with cesium iodide converters for medical radiography

We describe the properties of evaporated layers of cesium iodide (thallium activated) deposited on substrates that enable easy coupling to amorphous silicon pixel arrays. The CsI(Tl) layers range in thickness from 65 to 220 /spl mu/m. We used the two-boat evaporator system to deposit CsI(Tl) layers. This system ensures the formation of the scintillator film with homogenous thallium concentration which is essential for optimizing the scintillation light emission efficiency. The Tl concentration was kept to 0.1-0.2 mole percent for the highest light output. Temperature annealing can affect the microstructure as well as light output of the CsI(Tl) film. 200-360/spl deg/C temperature annealing can increase the light output by a factor of two. The amorphous silicon pixel arrays are p-i-n diodes approximately 1 /spl mu/m thick with transparent electrodes to enable them to detect the scintillation light produced by X-rays incident on the CsI(Tl). Digital radiography requires a good spatial resolution. This is accomplished by making the detector pixel size less than 50 /spl mu/m. The light emission from the CsI(Tl) is collimated by techniques involving the deposition process on patterned substrates. We have measured MTF of greater than 12 line pairs per mm at the 10% level. >

X-ray and charged particle detection with CsI(Tl) layer coupled to a Si:H photodiode layers

A compact real-time X-ray and charged-particle imager with digitized position output can be built either by coupling a fast scintillator to a photodiode array or by forming one on a photodiode array directly. CsI(Tl) layers 100-1000- mu m thick were evaporated on glass substrates from a crystal CsI(Tl). When coupled to a crystalline Si or amorphous silicon (a-Si:H) photodiode and exposed to calibrated X-ray pulses, their light yields and speed were found to be comparable to those of a crystal CsI(Tl). Single beta particle detection was demonstrated with this combination. The light spread inside evaporated CsI(Tl) was suppressed by its columnar structure. Scintillation detection gives much larger signals than direct X-ray detection due to the increased energy deposition in the detector material. Fabrication of monolithic-type X-ray sensors consisting of CsI+a-Si:H photodiodes is discussed. >

Amorphous silicon position sensitive neutron detector

An investigation of the possibility of using a-Si:H diode coated with an appropriate converter as a position-sensitive neutron detector is presented. Monte Carlo simulation predicts that, using a Gd film approximately 2- mu m thick, coated on a sufficiently thick amorphous silicon n-i-p diode, one can achieve a neutron detection efficiency of 25%. The experimental results presented give an average signal size of about 12000 e/sup -/ per neutron interaction, which is well above the noise and is in good agreement with the expected values. One can also fabricate pixel detectors with an element size as small as 300 mu m and still register a count rate of 2200 events/sec in a typical neutron flux of about 10/sup 7/ n/cm/sup 2/ per second. The fact that these detectors are not sensitive to gamma rays and show excellent radiation hardness makes them good candidates for use in applications such as neutron imaging, neutron crystallography, and neutron scattering.<<ETX>>

Enhanced columnar structure in CsI layer by substrate patterning

Columnar structure in evaporated can be controlled by patterning substrates as well as varying evaporation conditions. Mesh-patterned substrates with various dimensions were created by spin-coating polyimide on glass or amorphous silicon substrates and defining patterns with a standard photolithography technique. CsI(Tl) layers 200-1000 mu m were evaporated. Scintillation properties of these evaporated layers, such as light yield and speed, were equivalent to those of the source materials. The spatial resolution of X-ray detectors consisting of these layers and a linear array of Si photodiodes was evaluated by exposing them to a 25- mu m narrow beam of X-rays. The results obtained with 200- mu m-thick CsI layers coupled to a linear photodiode array with 20-dots/mm resolution showed that the spatial resolution of CsI(Tl) evaporated on patterned substrates was about 75 mu m full width at half maximum (FWHM) whereas that of CsI(T) on flat substrates was about 220 mu m FWHM. Micrographs taken by scanning electron microscopy revealed that these layers retained the well-defined columnar structure originating from substrate patterns.<<ETX>>

Measurements of 1/f noise in A-Si:H pin diodes and thin-film-transistors

We measured the equivalent noise charge of a-Si:H pin diodes (5 {approximately} 45{mu}m i-layer) with a pulse shaping time of 2.5 {mu}sec under reverse biases up to 30 V/{mu}m and analyzed it as a four component noise source. The frequency spectra of 1/f noise on the soft-breakdown region and of the Nyquist noise from contact resistance of diodes were measured. Using the conversion equations for a CR-RC shaper, we identified the contact resistance noise and the 1/f noise as the main noise sources in the low bias and high bias regions respectively. The 1/f noise of a-Si:H TFTs with channel length of 15 {mu}m was measured to be the dominant component up to {approximately}100kHz for both saturation and linear regions. 15 refs., 7 figs.

Improved electrical and transport characteristics of amorphous silicon by enriching with microcrystalline silicon

The authors have deposited n-i-p diodes with microcrystalline intrinsic layers for radiation detection applications. The diodes show interesting electrical characteristics which have not been reported before. From TOF measurement for their best samples, the authors obtained {mu}{sub e} values which are about 3 times larger than their standard a-Si:H. for {mu}{tau} values approximately a factor of 2 improvement was observed. The N*{sub D} values derived from hole-onset measurements show lower ionized dangling bond density than normal a-Si:H material. The authors propose a simple model which can very well explain the experimental results.

Utilization of a-Si:H Switching Diodes for Signal Readout from a-Si:H Pixel Detectors

Two-dimensional arrays of amorphous silicon photodiodes can be used as position-sensitive radiation detectors when they are coupled to an appropriate phosphor. We have developed signal readout schemes from amorphous silicon photodiode arrays utilizing one or two switching diodes attached to each pixel photodiode. Individual cells and prototype arrays of amorphous silicon photodiodes with single- and double-diode switching readout were fabricated and tested. A charge storage time and a readout time were measured. The measurement results were analyzed by simple circuit theory.

Thick (∼ 50 μm) amorphous silicon p-i-n diodes for direct detection of minimum ionizing particles

Thick (∼ 50 μm) amorphous silicon (a-Si:H) p-i-n diodes of device quality are made by helium dilution of the process gas and heat treatment for application to minimum ionizing particle detection. Dilution of SiH4 with He decreased the dangling bond density and increased the deposition rate. The internal stress, which causes substrate bending and delamination, was reduced by a factor of 4 to ∼ 90 MPa when deposited at low (150°C) temperature. The electronic quality of the a-Si:H film was somewhat degraded when grown at a low temperature, but could be mostly recovered by subsequent annealing at 160°C. By this technique 50 μm thick n-i-p diodes were made without significant substrate bending, and the electronic properties, such as electron mobility and ionized dangling bond density, were suitable for detecting minimum ionizing particles. Diode readouts and integrated amplifiers for pixel arrays are also described.

Defect Equilibration and Intrinsic Stress in Undoped Hydrogenated Amorphous Silicon

Relaxation data for the thermal equilibrium defect densities in undoped a-Si:H are obtained by time-of-flight (TOF) measurement in the temperature range of 160° to 250° C. The internal stress in the material is also measured. The mobility-lifetime product of electrons (µτ) increases from 0.50×10-7 to its equilibrium value of 2.24×10-7 cm2/V during the 160° C annealing. The equilibrium value of µτ is equivalent to the spin density (N s) of 1.12×1015 cm-3. The N s curves have a minimun value just before their equilibrium. The time dependence of the N s relaxation follows a two-term stretched exponential form which corresponds to two metastable states, and each relaxation time is activated with activation energies of 1.10 to 1.20 eV. The thermal equilibrium N s increases with temperature with an activation energy of 0.20 to 0.30 eV. The data for the second annealing at 160° C after the first long annealings at 200° and 250° C also follows the two-term stretched exponential form derived from the first annealing data. The result suggests the presence of a multivalley energy configuration diagram at metastable states. The drift mobility of electrons (µ) increases slightly compared with the µτ changes, and no stress change is observed during the various annealing steps. It is concluded that the structural change is much smaller than the change in metastable-state densities during annealing in the temperature range of 160° to 250° C.