WO2006076788A1

WO2006076788A1 - Dark current reduction in metal/a-se/metal structures for application as an x-ray photoconductor layer in digital image detectors

Info

Publication number: WO2006076788A1
Application number: PCT/CA2006/000004
Authority: WO
Inventors: Gueorgui Belev; Safa O. Kasap
Original assignee: University Of Saskatchewan
Priority date: 2005-01-18
Filing date: 2006-01-04
Publication date: 2006-07-27

Abstract

The present invention provides a method for dark current reduction in metal /a-Se/ metal devices, highly suitable for application in high resolution x- ray image detectors for digital mammography is proposed. a-Se is amorphous selenium, stabilized by alloying it with a small amount of As. It is an x-ray photoconductor that is used in an x-ray detector. The method/technology is based mainly on the optimization of the a-Se vacuum deposition process and conditions and is not very sensitive to the composition of the used Se alloy. The detector may be a single layer a-Se or a two-layer a-Se structure, fabricated by the invented processes described herein.

Description

DARK CURRENT REDUCTION IN METAL/A-SE/METAL STRUCTURES

FOR APPLICATION AS AN X-RAY PHOTOCONDUCTOR LAYER IN

DIGITAL IMAGE DETECTORS

CROSS REFERENCE TO RELATED U.S. APPLICATION

This patent application relates to, and claims the priority benefit from, United States Provisional Patent Application Serial No. 60/644,041 filed on January 18, 2005 in English, and which is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to a method for dark current reduction in metal/a-Se/metal structures for application as an x-ray photoconductor layer in digital image detectors.

BACKGROUND OF THE INVENTION

Amorphous selenium (a-Se) is an x-ray sensitive photoconductor. However, for use in a simple metal/a-Se/metal detector, it has to be operated at high electric fields (under large bias voltages). The application of large bias voltages leads to large dark currents which obscure the detection of photocurrents arising from the x-ray radiation that is incident on the detector. For an a-Se detector to be commercially viable, its dark current must be reduced. Thus a major problem of a-Se detectors is the presence of these large dark currents in metal/a-Se/metal based structures which prevent proper x-ray detection and imaging.

Previous attempts to overcome this problem have used multilayers, such as p-i-n structuctures, that are revere biased. Such a device preparation technique is difficult and requires the right dopants in right combinations for the p-layer and n-layers. P represents a p-layer, an a-Se layer that has been doped to have good hole transport but trap electrons: i-Se is an intrinisc photoconducting layer that transports both holes and electrons, and n represents an n-layer that transports electrons but traps holes. The i-layer acts as an x-ray photoconductor, that is, it executes the photoconductive process for generating a current upon absorbing x-rays. The p layer is connected to the negative bias terminal (the device needs an applied voltage), n-layer to the positive bias terminal. The p and n layers are normally thin compared with the i-layer, which is thick. X-rays are absorbed mainly in the i- layer. P-layer and n-layer are a-Se layers that have been alloyed and doped to give those layers their characteristics electronic properties. The n-layer is an alkali (Na or other) doped a-Se alloyed with a few percentages of As for stability. The p-layer is usually a halogen (Cl) doped a-Se, and some As for stability. It may also be an As₂Se₃ layer, which is known to be p-type

(transport holes but trap electrons).

United States Patent No. 6,171 ,643 issued to Polischuk et al is directed to reducing the dark current in a-Se X-ray detectors by the inclusion of buffer layers, or blocking layers. These are specially alloyed and doped layers placed on the top and bottom surfaces of the a-Se layer, between the a-Se and the bias electrodes. The buffer layer between the a-Se x-ray photoconductor and the negative electrode is called the p-layer. This p-layer is a buffer layer, which is normally As₂Se₃. The buffer layer between the positive bias electrode and a-Se is called the n-layer, and it is an a-Se layer that has been doped by an alkaline dopant, and has been alloyed with As to achieve stability against crystallization. This multilayer structure with buffer or blocking layers between a-Se and the bias electrodes, normally metals, reduces the dark current. The buffer layers are much thinner than the a-Se photoconductor layer so that the x-rays are absorbed mainly in the a-Se layer. United States Patent No. 6,642,534 issued to Shima is directed to the reduction of the dark current in an a-Se x-ray detector by using a buffer or blocking layer based on antimony trisulfide (Sb₂S₃). The only fundamental difference between this patent and the Polischuk patent (U.S. Patent No. 6,171 ,643) is that the dark current reduction is achieved by using a Sb₂S₃ layer rather than an As₂Se₃ layer. The difference is therefore in the chemical composition of the blocking or buffer layer that is inserted between the a-Se photoconductor and the positive electrode. While Polischuk et al. use As₂Se₃, Shima et al. use Sb₂S₃. Otherwise both the patents utilize buffer or blocking layers to reduce the dark current in which the blocking layers are of a particular composition that is different than the actual i-layer that absorbs the x-rays and generates the charges. It would be very advantageous to provide a method and structure for dark current reduction in metal/a-se/metal structures for application as an x- ray photoconductor layer in digital image detectors.

SUMMARY OF THE INVENTION The present invention provides two types of detector structures. The first is primarily a double structure, an n-layer and an i-layer (n-i structure) in which the required structure has been fabricated differently by using cold deposition to produce the n-layer. The cold deposition refers to the deposition of a-Se from its vapor in a vacuum deposition system on a substrate that is not heated; it is either at room temperature or it is cooled. The n-layer is obtained by changing the deposition process instead of the actual chemical composition of the layer. The second structure is a simple single layer structure that has been fabricated by using a two-step process to achieve a low dark current. The detector structure disclosed herein provides a simple single or two layer structure that is achieved by using cold deposition for the n-layer in contrast to using an alkali dopant. Cold deposition refers to the substrate temperature being kept at room temperature or below during the vacuum deposition of the a-Se layer. Any x-ray detector/sensor which uses a-Se as a photoconductor material requires a structure which reduces the dark current in order to be able to obtain a good signal over the dark signal, the dark signal is the current in the detector without any incident radiation. It is the unwanted current. The desired current is the current (photocurrent) that is generated when the x-ray radiation is incident on the detector. The fabrication process disclosed herein and the resulting detector structure are able to provide a detector with a low dark current. This means the a-Se detector can be made commercially competitive with other detectors.

Thus, in one aspect of the invention there is provided a method of producing a single layer structure for use in a detector of electromagnetic radiation, such as x-rays, comprising: forming an photoconductor layer by thermally evaporating onto an electrode substrate a layer of amorphous selenium (a-Se) alloyed with between about 0.2 to about 1% arsenic (As), with a chlorine content of Cl < 2ppm, the electrode substrate being held at a temperature well below a glass transition temperature of the alloy at a value which is optimal for the a-Se material used; annealing said layer structure at an annealing temperature slightly above the glass transition temperature of the Amorphous selenium (a-Se) alloyed with between about 0.2 to about 1% arsenic (As); and depositing an electrode layer on a top surface of the photoconductor layer.

The present invention also provides a detector for detecting electromagnetic radiation such as x-rays, comprising: a metal electrode substrate to which a positive potential bias is applied; a photoconductor layer produced by thermally evaporating amorphous selenium (a-Se) alloyed with between about 0.2 to about 1 % arsenic (As), with a chlorine content of Cl < 2ppm onto the metal electrode substrate which is held at a substrate temperature below a glass transition temperature of the alloy; and a metal electrode deposited onto a top surface of the photoconductor layer to which a negative bias is applied; and wherein in use the detector is positioned so that x-rays being detected are incident on the metal electrode to which a negative bias potential is applied.

The present invention also provides a method of producing a layered structure for use in a detector of electromagnetic radiation, such as x-rays, comprising: a) forming a hole trapping n-layer by thermally evaporating onto an electrode substrate a layer of amorphous selenium (a-Se) alloyed with between about 0.2 to about 1% arsenic (As), with a chlorine content of Cl < 2ppm, the electrode substrate being held at a temperature below the glass transition temperature of the a-Se alloy; b) forming an photoconductor layer by thermally evaporating onto a electrode substrate a layer of amorphous selenium (a-Se) alloyed with between about 0.2 to about 1 % arsenic (As), with a chlorine content of Cl < 2ppm, the electrode substrate being held at a temperature well below a glass transition temperature of the alloy at a value which is optimal for the a-Se material used; c) annealing said hole trapping layer and said photoconductor layer at an annealing temperature slightly above the glass transition temperature of the starting material alloy; and d) depositing an electrode layer on a top surface of the photoconductor layer.

The present invention also provides a detector for detecting electromagnetic radiation such as x-rays, comprising: a metal electrode substrate; a hole trapping n-layer produced by thermally evaporating onto the electrode substrate a layer of amorphous selenium (a-Se) alloyed with between about 0.2 to about 1 % arsenic (As), with a chlorine content of Cl < 2ppm, the electrode substrate being held at a temperature below the glass transition temperature of the a-Se alloy; a photoconductor layer produced by thermally evaporating amorphous selenium (a-Se) alloyed with 0.2 - 1 % arsenic (As), with a chlorine content of

Cl < 2ppm onto the metal electrode substrate which is held at a substrate temperature below a glass transition temperature of the alloy; and a metal electrode deposited onto a top surface of the photoconductor layer. In another aspect of the present invention there is provided a detector for detecting electromagnetic radiation such as x-rays, comprising: a metal electrode substrate; a hole trapping n-layer produced by thermally evaporating onto the electrode substrate a layer of amorphous selenium (a-Se) alloyed with between about 0.2 to about 1 % arsenic (As), with a chlorine content of Cl < 2ppm, the electrode substrate being held at a temperature below the glass transition temperature of the a-Se alloy; a photoconductor layer produced by thermally evaporating amorphous selenium (a-Se) alloyed with 0.2 to 1% arsenic (As), with a chlorine content of Cl < 2ppm onto the metal electrode substrate which is held at a substrate temperature below a glass transition temperature of the alloy; an electron trapping p-layer formed by thermally evaporating onto the photoconductor layer an a-Se alloy containing from about 0.2 to about 1 % of As and Cl from about 10ppm to about 20 ppm with the photoconductor layer being held at a temperature above a glass transition temperature of the a-Se alloy; a metal electrode deposited onto a top surface of the electron trapping p- layer.

The a-Se layer alloyed with between about 0.2 to about 1% arsenic (As), with a chlorine content of Cl < 2ppm exhibits very low dark currents, electron mobilities (μ_Θ) in a range of from about 0.002 to about 0.0035 cm² V^"1 s^"1, and τ_e > 350 μs where μ_e is the electron drift mobility, and τ_e is the electron trapping time or lifetime and hole mobilities (μ_h) in a range of from about 0.1 to about 0.13 cm² V^"1 s^"\ and T_h is in a range from about 2 to about 10 μs, where μ_h is the hole drift mobility, and T_h is the hole trapping time or lifetime. The electron mobility (μ_e) in the electron trapping p-layer is in a range between about 0.002 to about 0.003 cm² V^"1 s^"1 and an electron lifetime is about 25 μs. BRIEF DESCRIPTION OF THE DRAWINGS

The x-ray photoconductor layered detector structure for use in digital image detectors produced according to the present invention will now be described, by way of example only, reference being made to the accompanying drawings, in which:

Figure 1 is a cross sectional view of a typical prior art detector structure, specifically a Metal-Photoconductor-Metal pin detector structure for detecting incident x-ray photons which uses an n-layer located between the positively biased electrode and the photoconductor for trapping holes and a p- layer located between the negatively biased electrode and the photoconductor for trapping electrons;

Figure 2 shows a sectional view of an embodiment of a single-layer structure for reducing dark currents produced in accordance with the present invention which does not have the n-layer or p-layer shown in the device of Figure 1 ;

Figure 3 is a sectional view of a first embodiment of a layered structure for reducing dark currents produced in accordance with the present invention having only an n-layer, but no p-layer, located between the positively biased electrode and the photoconductor for trapping holes in which the negatively biased electrode is oriented to receive the radiation being detected; and

Figure 4 shows a sectional view of a second embodiment of a layered structure for reducing dark currents produced in accordance with the present invention having only an n-layer, but no p-layer, located between the positively biased electrode and the photoconductor for trapping holes in which the positively biased electrode is oriented to receive the radiation being detected.

DETAILED DESCRIPTION OF THE INVENTION Definitions As used herein, the term "a-Se" means amorphous selenium. It does not refer to undoped or unalloyed (pure) a-Se but to a-Se which has been stabilized against crystallization by alloying pure a-Se with a small amount of As. Some Cl doping in the parts per million range is sometimes used to obtain favorable electron and hole transport parameters, that is, both holes and electrons are able to drift in this layer when an electric field is applied.

As used herein, the phrase "n-layer" refers to an a-Se layer (or a-Se alloyed with As) that has been doped appropriately, for example with an alkaline metal, that allows the transport of electrons but traps holes. This definition is counterintuitive to the definition normally used in semiconductor science (for example, see S.O. Kasap, Principles of Electronic Materials and Devices, 2nd Edition, McGraw-Hill, 2002). In the a-Se field, n-type refers to a layer that transport electrons but traps holes.

As used herein, the phrase "p-layer" refers to an amorphous semiconductor layer that transport holes but traps electrons. The best known example is amorphous arsenic triselenide (a-As₂Se₃) in which holes can drift but electrons become trapped. The term "intrinsic a-Se" refers to an a-Se layer that is able to transport both holes and electrons; both carriers can drift without being trapped. The term "intrinsic" in normal semiconductor science (for example, see S.O. Kasap, Principles of Electronic Materials and Devices, 2nd Edition, McGraw- Hill, 2002), refers to a semiconductor that has not been doped. The term "intrinsic" as applied to a-Se refers to an a-Se whose electronic properties have not been modified to create an n-type or p-type a-Se but an a-Se layer in which both electrons and holes are able to drift.

"Hole" and "electron mobilities" refer to the drift mobilities of holes and electrons as measured by the conventional time-of-flight transient photoconductivity technique.

Hole and electron lifetimes refer to hole and electron deep trapping times in which deep trapping refers to the loss of carriers from conduction due to their capture by defects, impurities, other centers etc. as measured by the conventional time-of-flight transient photoconductivity technique under low fields or interrupted field time-of-flight transient photoconductivity techniques which will be known to those skilled in the art.

Referring to Figure 1 , with reference to a prior art photoconductor structure shown generally at 10 the current practice for dark current reduction in a-Se x-ray photoconductors relies on the insertion of a thin hole-trapping layer 12, that is, an n-layer (holes are trapped but electrons can move) between electrode 16 and the actual a- Se photoconductor layer 20, and a thin electron trapping layer 14, called a p-layer (electrons are trapped but holes can move) between the metal electrode 18 and the actual a- Se photoconductor layer 20. The a-Se photoconductor layer 20 between the n- layer 12 and p-layer 14 is called an Mayer because it transports both holes and electrons. The hole trapping layer, the n-layer 12, is of greater importance, since the dark current is mainly caused by hole injection from the positively biased metal electrode 16.

Currently hole trapping layers such as layer 12 are produced by suitably doping the a-Se layer with alkaline metals such as, for example, Na. The doping is achieved by coevaporating a-Se and Na, the latter from a Na dispenser. Normally a-Se is alloyed with more than usual As to prevent the crystallization of Na doped a-Se layer. When a voltage is applied to the detector structure in Figure 1 , electrons from the negative electrode become trapped in the p-layer 14. This p-layer effectively becomes the negative terminal. Holes supplied by the positive terminal become trapped in the n- layer 12. This n-layer becomes effectively a positive terminal.

Absorption of X-rays in the photoconductor layer, that is in the intrinsic a-Se layer 20, generates electron hole pairs. Electrons move toward the positive electrode 16, and holes towards the negative electrode 18. Electrons and holes drift under the applied field produced in the i-layer 20 by the application of voltage source V across the i-layer 20.

I. Single layer a-Se photoconductors with very low dark currents for negative bias

Referring to Figure 2, a major advantage of the invention disclosed herein is that it can be used for dark current reduction in negatively biased a- Se photoconductor layers without the use of the usual hole trapping n-layer 12 in Figure 1 between the metal electrode 16 and the photoconductor i-layer 20. This is very important for high resolution x-ray detectors (small pixel sizes), working with negative bias because the trapping in the hole blocking n-layer and related effects can cause significant deterioration in detector performance. The invention is also important for X-ray imaging with a detector in which the pixels are read fast, as in fluoroscopy. The production of device 40 in Figure 2 using single layer a-Se photoconductor layer 20 with low dark current involves two processing steps. In the first step, a-Se alloyed with between about 0.2 to about 1 % As, no or very little Cl (< 2ppm), is thermally evaporated onto a cold substrate (the substrate temperature is kept well below the glass transition temperature of the used alloy at a value which is optimal for the a-Se material used). The substrate temperature may be increased gradually during the deposition process but it must start from a temperature below the glass transition temperature.

The second step involves annealing in vacuum of the whole structure at mild conditions (annealing temperature is around or slightly above the glass transition temperature of the starting material). The resulting a-Se layer has very low dark currents, good electron transport (μ_e = 0.002 - 0.0035 cm² V^"1 s^" ¹, τ_e >350 μs,, where μ_e is the electron drift mobility and τ_e is the electron trapping time or lifetime) and reasonable hole transport (μ_h = 0.1 - 0.13 cm² V^" ¹ s^"1, Th = 2 - 10 μs, where μ_h is the hole drift mobility and X_h is the hole trapping time or lifetime) which makes it an ideal candidate to work as a photoconductor layer in negatively biased direct conversion x-ray detectors for digital mammography (low x-ray energy application).

As mentioned above, when the photoconductor i-layer 20 is deposited as above in a special two step process, disclosed herein, there is no need to insert blocking layers 12 and 14. In particular, there is no need to insert n- layer 12 between the i-Se layer 20 and the positive bias electrode 16, which blocks hole injection. In addition, there is no need to insert p-layer 14 between the i-layer 20 and the negative bias electrode 18. Consequently the structure is a simple single layer detector, as in Figure 2, with a small dark current. The thickness of the photoconductor 20 in Figure 2 depends on the x-ray detector application but it is typically in the 10 micron to 1000 micron range. Without being limited to any theory, the inventors believe that the low dark current is probably due to the fact that surface states of the bottom surface of the cold deposited Se layer are modified in a way which reduces hole injection from the bottom (substrate) electrode.

II. Convenient Method For Production Of Hole Trapping Layers In Multilayer A-Se Photoconductor Structures.

While the present invention allows one to produce the simple metal- photoconductor-metal layered structure without the need for hole and electron blocking layers as discussed above, the cold deposition technique disclosed herein may also be used for the production of hole trapping n-layers 12 in the layered structure 10 shown in Figure 1. Similarly, Figure 3 shows a cross sectional view of an embodiment of a layered structure 22 for reducing dark currents produced in accordance with the present invention. In the device 22 shown the Figure 3, the p-layer 14 of Figure 1 is absent and the compositions of the i-layer 20 (Figure 1) and 26 (Figure 3) are the same. The n-layers 12 and 24 are generated by having this layer deposited onto a cold substrate, that is, the substrate temperature is kept below the glass transition temperature. When a voltage is applied across the device, holes supplied by the positive terminal to the n-layer 12, 24 become trapped in this layer. The n- layers 12, 24 act effectively as the positive terminal. Because the hole detrapping time is long in the n-layers 12 and 24, the dark current is reduced. The x-rays are absorbed in the i-layer 20, 26, and generate electrons and holes in these layers. Holes drift to the terminals 18, 28, and electrons to the effective positive terminal 24. Holes are collected by the layer 12 (Figure 1 ) and layer 24 (Figure 3), and hence by the metal electrodes 16 and 30 respectively.

The procedure for producing the n-layers 12, 24 is similar to the one described above but generally a lower substrate temperature is used in this case. A low substrate temperature means that the temperature of the substrate during the vacuum deposition process is kept below the glass transition temperature of the a-Se alloy. The amorphous selenium (a-Se) alloyed with 0.2 - 1 % As, no or very little Cl (< 2ppm), is thermally evaporated onto the cold electrode substrate 30 (Figure 3). The resulting n-layer 24 produced according to the method disclosed herein has very similar hole trapping properties to the ones produced from Na doped a-Se and has a number of advantages which include better electron transport, in that the electron mobility is about the same but the electron lifetime is at least four (4) times longer than in Na doped n-layer. Further, the cold deposited hole trapping layers 24 exhibit long term stability, whereas in previously made layers Na tends to migrate (diffuse) in the structure and causes a-Se to crystallize over a long time. When the starting material is properly chosen, the blocking layer and the thick photoconductor layer can be deposited from one and the same starting a-Se alloy by simply changing the substrate temperature during the deposition process thereby providing a simpler technology. The thickness of the hole trapping n-layer depends on the exact x-ray detector application but it is typically in the range 1 micron to 100 micron. The n-layer is normally thinner than the i-layer.

After deposition of the n-layer, the photoconducting i-layer 26 is then deposited onto the n-layer 24. The method is the same as for device 40 in Figure 2 described above, namely a-Se alloyed with 0.2 - 1 % As, no or very little Cl (< 2ppm), is thermally evaporated onto the n-layer 24 cold substrate

(the substrate temperature is kept below the glass transition temperature of the used alloy at a value which is optimal for the a-Se material used). The substrate temperature may be increased during the deposition but it should start from a temperature below the glass transition temperature. While Figures 1 and 3 show device structures in which the radiation receiving electrode has been biased negatively, Figure 4 shows a layered device at 32 having the same structure as layered structure 22 in Figure 3 but with positive bias applied to the radiation receiving electrode 28. The n-layer 24 (Figure 4), made in accordance with the present invention, in which it is cold deposited, and is always inserted between the intrinsic a-Se layer 26 and the positively biased electrode (electrode layer 28 in device 32 in Figure 4 and electrode layer 30 in device 22 in Figure 3). The principle of operation of the layered detector in 32 is the same as that in device 22. The only difference is that the radiation receiving electrode 28 in device 32 of Figure 4 is biased positively and that instead of collecting electrons (negative charge) on the pixel capacitor, electrode 30 of device 32 collects holes (positive charge) on the pixel electrode.

In case of an x-ray detector using negative bias on the radiation receiving electrode as in Figure 3, the hole trapping layer, that is the n-layer 24, can be made thinner whereby the bottom surface of the cold deposited layer and the hole trapping in that layer act together to block the hole injection from the metal electrode, and thereby reduce the dark current.

III. Production of electron trapping layers

According to the present invention, an electron trapping p-layer (p-layer 14 in Figure 1 ) can be made by changing the deposition conditions. The inventors have discovered that combination of high boat temperature (> 280 ⁰C) and substrate temperature well above the glass transition temperature for certain Se alloys containing from about 0.2 to about 1 % of As and relatively high amount of Cl (10ppm - 20 ppm) will result in films with very poor electron transport. The electron mobility in such films was found to be 0.002-0.003 cm²

V¹ s^'1 and the electron lifetime was about 25 μs. The hole mobility was between 0.11 and 0.13 cm² V^"1 s^"1 and the hole lifetime was above 30 μs. These charge transport measurements show that the carrier motilities in electron trapping layers (p-layer 14) produced from specially doped Se alloys are approximately the same, and electron lifetime is only 3-4 times shorter.

Therefore, using the processes disclosed herein, the PIN device 10 of Figure 1 may be produced by appropriately changing the deposition conditions, without having to change the composition. Alternatively, simplified detector structures may be made with two layers, an i-layer with n-layer (i-n) as shown in Figures 3 and 4. The simplest layered x-ray detector structure which may be made according to the present invention is shown in Figure 2 which uses just a metal/i-layer/metal structure. The thickness of the p-layer depends on the exact x-ray detector application but it is typically in the range from about 1 micron to about 100 microns. The n-layer is normally thinner than the i-layer.

Any cold deposited a-Se based layer (with such elements as As, Te etc. or other but with Se as the major component) can be used to block the injection of holes from the positively biased electrode in Metal/a-Se- Alloy/Metal type detector.

As used herein, the terms "comprises", "comprising", "including" and "includes" are to be construed as being inclusive and open ended, and not exclusive. Specifically, when used in this specification including claims, the terms "comprises", "comprising", "including" and "includes" and variations thereof mean the specified features, steps or components are included. These terms are not to be interpreted to exclude the presence of other features, steps or components. The foregoing description of the preferred embodiments of the invention has been presented to illustrate the principles of the invention and not to limit the invention to the particular embodiment illustrated. It is intended that the scope of the invention be defined by all of the embodiments encompassed within the following claims and their equivalents.

Claims

THEREFORE WHAT IS CLAIMED IS:

1. A method of producing a single layer structure for use in a detector of electromagnetic radiation, such as x-rays, comprising: forming an photoconductor layer by thermally evaporating onto an electrode substrate a layer of amorphous selenium (a-Se) alloyed with between about 0.2 to about 1 % arsenic (As), with a chlorine content of Cl < 2ppm, the electrode substrate being held at a temperature well below a glass transition temperature of the alloy at a value which is optimal for the a-Se material used; annealing said layer structure at an annealing temperature slightly above the glass transition temperature of the Amorphous selenium (a-Se) alloyed with between about 0.2 to about 1% arsenic (As); and depositing an electrode layer on a top surface of the photoconductor layer.

2. The method according to claim 1 wherein the a-Se layer alloyed with between about 0.2 to about 1% arsenic (As), with a chlorine content of Cl < 2ppm exhibits very low dark currents, electron mobilities (μ_e) in a range of from about 0.002 to about 0.0035 cm² V^"1 s^~\ and τ_e ≥ 350 μs where μ_e is the electron drift mobility, and τ_e is the electron trapping time or lifetime and hole mobilities (μ_h) in a range of from about 0.1 to about 0.13 cm² V¹ s^"1, and T_h is in a range from about 2 to about 10 μs, where μ_h is the hole drift mobility, and t_h is the hole trapping time or lifetime.

3. The method according to claims 1 or 2 wherein a thickness of the photoconductor layer is in a range from about 10 microns to about 1000 microns.

4. A detector for detecting electromagnetic radiation such as x-rays, comprising: a metal electrode substrate to which a positive potential bias is applied; a photoconductor layer produced by thermally evaporating amorphous selenium (a-Se) alloyed with between about 0.2 to about 1 % arsenic (As), with a chlorine content of Cl < 2ppm onto the metal electrode substrate which is held at a substrate temperature below a glass transition temperature of the alloy; and a metal electrode deposited onto a top surface of the photoconductor layer to which a negative bias is applied; and wherein in use the detector is positioned so that x-rays being detected are incident on the metal electrode to which a negative bias potential is applied.

5. The detector according to claim 4 wherein the a-Se layer alloyed with between about 0.2 to about 1 % arsenic (As), with a chlorine content of Cl < 2ppm exhibits very low dark currents, electron mobilities (μ_e) in a range of from about 0.002 to about 0.0035 cm² V^"1 s^"1, and τ_e > 350 μs where μ_e is the electron drift mobility, and τ_e is the electron trapping time or lifetime and hole mobilities (μ_h) in a range of from about 0.1 to about 0.13 cm² V¹ s^"1, and % is in a range from about 2 to about 10 μs, where μ_h is the hole drift mobility, and T_h is the hole trapping time or lifetime.

6. The detector according to claim 4 or 5 wherein a thickness of the photoconductor layer is in a range from about 10 microns to about 1000 microns.

7. The detector according to claims 4, 5 or 6 including a storage capacitor electrically connected to the electrode to which a positive bias potential is applied.

8. A method of producing a layered structure for use in a detector of electromagnetic radiation, such as x-rays, comprising: a) forming a hole trapping n-layer by thermally evaporating onto an electrode substrate a layer of amorphous selenium (a-Se) alloyed with between about 0.2 to about 1 % arsenic (As), with a chlorine content of Cl < 2ppm, the electrode substrate being held at a temperature below the glass transition temperature of the a-Se alloy; b) forming an photoconductor layer by thermally evaporating onto a electrode substrate a layer of amorphous selenium (a-Se) alloyed with between about 0.2 to about 1 % arsenic (As), with a chlorine content of Cl < 2ppm, the electrode substrate being held at a temperature well below a glass transition temperature of the alloy at a value which is optimal for the a-Se material used; c) annealing said hole trapping layer and said photoconductor layer at an annealing temperature slightly above the glass transition temperature of the starting material alloy; and d) depositing an electrode layer on a top surface of the photoconductor layer.

9. The method according to claim 8 wherein the a-Se layer alloyed with between about 0.2 to about 1 % arsenic (As), with a chlorine content of Cl < 2ppm exhibits very low dark currents, electron mobilities (μ_e) in a range of from about 0.002 to about 0.0035 cm². V^"1 s^"\ and τ_e > 350 μs where μ_e is the electron drift mobility, and τ_e is the electron trapping time or lifetime and hole mobilities (μ_h) in a range of from about 0.1 to about 0.13 cm² V^"1 s^"1, and X_h is in a range from about 2 to about 10 μs, where μ_h is the hole drift mobility, and X_h is the hole trapping time or lifetime.

10. The method according to claim 8 or 9 wherein a thickness of the photoconductor layer is in a range from about 10 microns to about 1000 microns.

11. The method according to claim 8, 9 or 10 wherein a thickness of the hole trapping n-layer is in a range from about 1 micron to about 100 microns.

12. A method of producing a layered structure for use in a detector of electromagnetic radiation, such as x-rays, comprising: a) forming a hole trapping n-layer by thermally evaporating onto an electrode substrate a layer of amorphous selenium (a-Se) alloyed with between about 0.2 to about 1% arsenic (As), with a chlorine content of Cl < 2ppm, the electrode substrate being held at a temperature below the glass transition temperature of the a-Se alloy; b) annealing said hole trapping n-layer at an annealing temperature slightly above the glass transition temperature of the starting alloy material; c) forming an photoconductor layer by thermally evaporating onto the hole trapping n-layer a layer of amorphous selenium (a-Se) alloyed with between about 0.2 to about 1% arsenic (As), with a chlorine content of Cl < 2ppm, the electrode substrate being held at a temperature usually below the glass transition temperature of the alloy at a value which is optimal for the a- Se material used; and d) depositing an electrode layer on a top surface of the photoconductor layer.

13. The method according to claim 12 wherein the a-Se layer alloyed with between about 0.2 to about 1% arsenic (As), with a chlorine content of Cl < 2ppm exhibits very low dark currents, electron mobilities (μ_e) in a range of from about 0.002 to about 0.0035 cm² V^"1 s^"\ and τ_e > 350 μs where μ_e is the electron drift mobility, and τ_e is the electron trapping time or lifetime and hole mobilities (μ_h) in a range of from about 0.1 to about 0.13 cm² V^"1 s^"1, and T_h is in a range from about 2 to about 10 μs, where μ_h is the hole drift mobility, and th is the hole trapping time or lifetime.

14. The method according to claim 12 or 13 wherein a thickness of the photoconductor layer is in a range from about 10 microns to about 1000 microns.

15. The method according to claim 12, 13 or 14 wherein a thickness of the hole trapping n-layer is in a range from about 1 micron to about 100 microns.

16. A detector for detecting electromagnetic radiation such as x-rays, comprising: a metal electrode substrate; a hole trapping n-layer produced by thermally evaporating onto the electrode substrate a layer of amorphous selenium (a-Se) alloyed with between about 0.2 to about 1% arsenic (As), with a chlorine content of Cl < 2ppm, the electrode substrate being held at a temperature below the glass transition temperature of the a-Se alloy; a photoconductor layer produced by thermally evaporating amorphous selenium (a-Se) alloyed with 0.2 - 1 % arsenic (As), with a chlorine content of Cl < 2ppm onto the metal electrode substrate which is held at a substrate temperature below a glass transition temperature of the alloy; and a metal electrode deposited onto a top surface of the photoconductor layer.

17. The detector according to claim 16 wherein the a-Se layer alloyed with between about 0.2 to about 1% arsenic (As), with a chlorine content of Cl < 2ppm exhibits very low dark currents, electron mobilities (μ_e) in a range of from about 0.002 to about 0.0035 cm² V^"1 s^'1, and τ_Θ > 350 μs where μ_e is the electron drift mobility, and τ_e is the electron trapping time or lifetime and hole mobilities (μh) in a range of from about 0.1 to about 0.13 cm² V^"1 s^"1, and X_h is in a range from about 2 to about 10 μs, where μ_h is the hole drift mobility, and τ_h is the hole trapping time or lifetime.

18. The detector according to claim 16 or 17 wherein in use a positive potential bias is applied to said metal electrode substrate, a negative potential bias is applied to said metal electrode, and the detector is positioned so that x-rays being detected are incident on said metal electrode to which a negative bias is applied.

19. The detector according to claim 16, 17 or 18 including a storage capacitor electrically connected to the metal electrode substrate to which a positive bias potential is applied.

20. The detector according to claim 16 or 17 wherein in use a positive potential bias is applied to said metal electrode substrate, a negative potential bias is applied to said metal electrode, and the detector is positioned so that x-rays being detected are incident on said metal electrode substrate to which a positive bias is applied.

21. The detector according to claim 20 including a storage capacitor electrically connected to the metal electrode to which a negative bias potential is applied.

22. The detector according to claim 16, 17, 18, 19, 20 or 21 wherein a thickness of the photoconductor layer is in a range from about 10 microns to about 1000 microns.

23. The detector according to claim 16, 17, 18, 19. 20 or 21 wherein a thickness of the hole trapping n-layer is in a range from about 1 micron to about 100 microns.

24. A detector for detecting electromagnetic radiation such as x-rays, comprising: a metal electrode substrate; a hole trapping n-layer produced by thermally evaporating onto the electrode substrate a layer of amorphous selenium (a-Se) alloyed with between about 0.2 to about 1% arsenic (As), with a chlorine content of Cl < 2ppm, the electrode substrate being held at a temperature below the glass transition temperature of the a-Se alloy; a photoconductor layer produced by thermally evaporating amorphous selenium (a-Se) alloyed with 0.2 - 1 % arsenic (As), with a chlorine content of Cl < 2ppm onto the metal electrode substrate which is held at a substrate temperature below a glass transition temperature of the alloy; an electron trapping p-layer formed by thermally evaporating onto the photoconductor layer an a-Se alloy containing from about 0.2 to about 1 % of As and Cl from about 10ppm to about 20 ppm with the photoconductor layer being held at a temperature above a glass transition temperature of the a-Se alloy; a metal electrode deposited onto a top surface of the electron trapping p- layer.

25. The device according to claim 24 wherein the a-Se layer alloyed with between about 0.2 to about 1% arsenic (As), with a chlorine content of Cl < 2ppm exhibits very low dark currents, electron mobilities (μ_e) in a range of from about 0.002 to about 0.0035 cm² V^"1 s^"\ and τ_e > 350 μs where μ_e is the electron drift mobility, and τ_e is the electron trapping time or lifetime and hole mobilities (μ_h) in a range of from about 0.1 to about 0.13 cm² V^"1 s^"1, and T_h is in a range from about 2 to about 10 μs, where μ_h is the hole drift mobility, and T_h is the hole trapping time or lifetime.

26. The device according to claim 24 wherein the electron trapping p-layer is formed by evaporating the a-Se from a boat held at a temperature > 280 ⁰C.

27. The method according to claim 24 or 25 wherein an electron mobility (μ_e) in the electron trapping p-layer is in a range between about 0.002 to about 0.003 cm² V^"1 s^"1 and an electron lifetime is about 25 μs.

28. The detector according to claim 24, 25 or 26 wherein in use a positive potential bias is applied to said metal electrode substrate, a negative potential bias is applied to said metal electrode, and the detector is positioned so that x-rays being detected are incident on said metal electrode to which a negative bias is applied.

29. The detector according to claim 28 including a storage capacitor electrically connected to the metal electrode substrate to which a positive bias potential is applied.

30. The detector according to claim 24 wherein in use a positive potential bias is applied to said metal electrode substrate, a negative potential bias is applied to said metal electrode, and the detector is positioned so that x-rays being detected are incident on said metal electrode substrate to which a positive bias is applied.

31. The detector according to claim 30 including a storage capacitor electrically connected to the metal electrode to which a negative bias potential is applied.

32. The detector according to claim 24, 25, 26, 27, 28 29, 30 or 31 wherein a thickness of the photoconductor layer is in a range from about 10 microns to about 1000 microns, and wherein a thickness of the hole trapping n-layer and the electron trapping p-layer are in a range from about 1 micron to about 100 microns.