WO1996035236A1

WO1996035236A1 - Detector for indicating ionising radiation and process for its production

Info

Publication number: WO1996035236A1
Application number: PCT/DE1996/000824
Authority: WO
Inventors: Frantisek Dubecky; Juraj Darmo; Pier Giovanni Pelfer; Peter Kordos; Arno Förster
Original assignee: Forschungszentrum Jülich GmbH; Universität Zu Florenz; Elektrotechnicky Ustav Sav
Priority date: 1995-05-05
Filing date: 1996-05-03
Publication date: 1996-11-07
Also published as: DE19680300D2

Abstract

The invention relates to a GaAs detector for radiation detection with a high breakdown voltage and a high degree of absorption for the charge and a better energy resolution and higher radiation resistance. The invention also relates to a process for its production. Here, the GaAs detector contains a monocrystalline coating layer (6) with a high resistance and a trap density of over 1017 cm-3, in which the layer is deposited on at least one side of the GaAs wafer, over the GaAs surface, preferably by epitaxial growth using molecular bunches (MBE) with molecular flow of the Ga and the As at reduced temperature to form a layer which is of non-stoichiometric structure to a high degree and has a maximum thickness of 20 nm.

Description

Ionizing radiation detector and method for its manufacture

The invention relates to a detector for displaying ionizing radiation and a method for its production.

1. Field of the invention

The invention relates to the field of detectors for displaying ionizing radiation by means of semiconductors, in particular a device relating to GaAs materials. The invention also relates to the methods for producing the detector.

2. State of the art

Detectors for the display of ionizing GaAs radiation are the subject of in-depth research, particularly as regards the way they work at room temperature, the increased resistance to radiation and a higher adsorption coefficient for X-rays compared to Si devices. Such detectors have several Areas of application, such as, for example, the field of medicine, metereology, for physical experiments for researching the particles and in the field of nuclear physics, astronomy, for the monitoring of industrial production lines, national security, etc., in particular those fields of application in which Si detectors cannot be used, since they do not have sufficient display power and have low radiation resistance.

Semi-insulating GaAs materials which have not been doped and which have been used with the known method Liquid Encapsulated Czochraiski (LEC) (1-10) are used as the starting material for the production of detectors of ionizing radiation such as GaAs. This material has a high resistance, which is greater than 10 ohm cm at room temperature and leads to an excellent compensation of the flat impurities by deep impurities (deep), which are named EL2, with a concentration of around 10 ^ιe cm ^"3 and an activation energy of 0.75 eV compared to the lower level of the conduction band ^edge .

This material is more suitable for those cases where the resistance to radiation from

Importance is. For various special fields of application, for example in the field of nuclear spectrometry or X-ray radiation, the material GaAs is used in a pure state and is produced using epitaxy technology (5, 10-13). Almost all detectors for displaying radiation which are currently used on a GaAs basis are preferably made with Schottky and ohmic contacts, usually Au or Ti, as described in (1,3,5,6,8 , 10, 11, 15), or sometimes by means of two Schottky contacts on both sides, preferably by means of Au, as described in (14, 15), for metallizing the electrodes. Au is a metal that forms a high Schottky barrier qφ _b , where q is the charge of the

Electrons and φ _{b represents} the height of the Schottky barrier of approx. 0.92 eV compared to the Fermi level in the interface with GaAs (16). For this reason, the level of the deep impurity EL2, to which the compensation can be attributed, is in the vicinity of the interface above the Fermi level, so that complete ionization takes place, and a control is carried out on account of its dominant concentration the thickness of the emptied layer, as shown in (17, 18) and the intensity of the electric field in the active area of the detector.

This results in a small thickness of the emptied zone in the range of μm, a very strong electric field in the contact interface with GaAs and a very low electric field in the almost neutral part of the detector, as can be seen from the calculations (18 , 19). Furthermore, the strong electric field present in the contact point leads to destruction of the barrier with low voltages at the ends of the Detector. A proposal to solve this problem was made by Brozei (13) and was based on the basic idea of reducing the concentration of EL2 in the starting material. It can be said that this method is possible, but it presents considerable problems. Furthermore, the method is time-consuming and costly since it involves the development of a new growth technology for GaAs for use in detectors. The standard embodiment of the contacts on GaAs detectors is not symmetrical, ie there is a small contact on one side (above) and a large contact on the other side (below) usually takes place as large as the entire area of the detector . This form of formation of the contacts of the detector is not optimal since the distribution of the electric field scatters very strongly and for this reason the resolution of the detector decreases. Furthermore, the leakage current for the transport of the charge carriers via inactive parts of the detector increases. In practice, only insulating layers made of silicon oxide or silicon nitride (2, 10, 20) are used to cover the free surface of the GaAs, ie the surface on which no metallization of the contacts is provided.

In the case of GaAs, mainly for areas of application in which a high resistance to large radiation is required, the resistance to radiation influences of this layer is particularly important. Compared to the known prior art, it can be said that measurements of the breakdown voltage have been carried out to date in the GaAs radiation detectors as previously carried out corresponded to approximately one volt per 1 μm of the thickness of the detector (21) and that the best value determined for the absorption capacity of the charge was close to 75% and the resolution in energy is greater than 10% for a detector with a thickness of approximately 200 μm, provided that the α particles are indicated with an energy around MeV (10). The resistance to radiation of the GaAs detectors that have been manufactured to date is limited by the resistance to radiation of the layers of insulating material that are used to cover the surfaces, such as SiO ₂ or SiN ₄ , for example. These usually have a resistance to radiation which is lower than that of the GaAs materials due to the decomposition of the insulating material, due to the introduction of charges which take place during the interactive process with the ionizing particles (22).

It is a GaAs radiation detector, the features of which have been significantly improved, using sandwich electrodes which are advantageously arranged symmetrically and in a mirror image on both sides of the GaAs wafer. The free surface of the pane is covered at least on one side with a monocrystalline semi-insulating layer made of GaAs, and this layer has a concentration of trapping points which is greater than 10 cm ^" . At least one of the electrodes arranged forms one Energy lock, the amount of which is opposite the interface with GaAs is less than 1.1 times the energy difference between the lowest level of the conduction band edge and the Fermi level in the semiconductor. The GaAs, at least on one side of the pane, is advantageously removed by means of etching technology to a depth of greater than 60 μm in the zones where the metallization for the electrodes is deposited.

4. Description of the invention

^ 10

The above invention relates generally to a GaAs detector for radiation display with a higher value for the breakdown voltage and a higher degree of absorption for the charge as well as a better resolution

15 in energy and a greater resistance to radiation. The invention further relates to a method for its production.

The GaAs detector includes:

20 a) a monocrystalline cover layer with high resistance (LTG) with a trap density greater than 10 ¹⁷ cm ^"3 , the layer at least on one side of the GaAs wafer, above the GaAs surface, in advance

25 Zugter due to epitaxial growth by means of molecular bundles (MBE) with molecular flows of Ga and As at a reduced temperature of the disc to form a layer which is not highly stoichiometric, and a maximum thickness

30 of 20 nm is deposited. b) electrodes which are preferably arranged symmetrically and in a mirror-image manner with a metal coating which is arranged on at least one side of the GaAs pane on the windows from which the LTG layer has been removed with the aid of an etching technique.

c) A metallization of the electrodes on at least one side of the GaAs wafer, which preferably forms an energy barrier, the height of which in the interface with the GaAs is less than 1.1 times the energy difference between the lowest level of the conduction band edge and the Fermi level in the semiconductor is.

d) GaAs is preferably removed under

Use of an etching technique to a depth of more than 60 μm of the etching trench for the electrodes before the metal layer is applied.

A number of detectors for displaying the GaAs radiation, which was produced in accordance with the present invention, had a better display property for the GaAs display than the devices described, which were produced from halobolating metal disks. The breakdown voltage is approx. 2 V per μm of the thickness of the pane due to the metallization of the electrodes, which forms an energy source at the interface with GaAs, in such a way that the main plane which creates the compensation EL2 is not above the Fermi level comes because of a band bending in the interface; this fact prevents the complete ionization of this plane. This leads to a substantial reduction in the electric field in the interface and also to the creation of a wider, empty zone. This also has positive effects on the distribution of the electric field in the detector. On the basis of these improvements, an improved performance with regard to the absorption of the charge for displaying the α-particles with an energy of 5.48 MeV of Am ²⁴¹ and between 85 and 93% for a detector with a thickness of 200 μm is achieved. Due to the mirror-symmetrical arrangement of the electrodes and the use of the cover layer, an energy resolution is achieved for the α-particles which is less than 6%. This can be seen in a more uniform distribution of the electrical field in the volume of the detector, and also in a reduction in the leakage currents in the interface and on the surface. Furthermore, the resistance to radiation of the GaAs detector is no longer dependent on the radiation resistance of the cover layer as in the case of SiO ₂ or Si ₃ N ₄ , but only on the radiation resistance of the original GaAs material. This is due to the use of a cover layer made of the same GaAs material as the basic material.

The invention is explained in more detail below with the aid of an exemplary embodiment. Show it: Figure 1: Cross section for each of the four possible embodiments for a GaAs detector according to the invention;

FIG. 2: the dependence of the current on the polarization voltage when the surface is treated differently, also with a cover according to the preceding invention;

FIG. 3 shows the dependence of the current on the polarization voltage measured on GaAs detectors using panes which were produced by three different manufacturers according to the invention;

FIG. 4: spectrum of α-particles which were recognized by the GaAs detector and which were manufactured in accordance with the above invention;

FIG. 5: spectrum of the α particles with three different types of energy, which are displayed by a Si display device and by a GaAs detector according to the present invention;

6. Detailed description of some embodiments of the

Invention.

Figure la shows in cross section the basic principle for producing a GaAs detector according to the invention. A layer made of a monocrystalline homoepitaxial material LTG was placed over the side 3 of the GaAs wafer, which is semiconducting and undoped (USI) (5)

(6) with an impurity density of 10 19 cm-3 and a thickness (d6) of approx. 500 nm with a resistance of approx. 10 ^'3 ohm cm at room temperature by means of MBE with molecular fluxes of Ga and As. This layer essentially covers the entire disk made of GaAs on one side of the polished side. The pane with the LTG layer was treated thermally in an As atmosphere at a temperature of 600 ° C. for 10 minutes. According to this method, the resistance of the layer takes on values which are greater than 10 ohm cm at room temperature and the trapping point density is reduced

Another preferred method for producing the GaAs cover layer LTG, in accordance with the above invention, method which can be carried out more easily, quickly and cost-effectively, continues to use the MBE with molecular flows Ga and As at a temperature of 380 ° C. for the thicker pane. The resistance of the layer is approximately 10 ⁶ ohm cm at room temperature and the trapping point density corresponds to approximately 10 ¹⁸ cm ^"3. This layer does not require any subsequent thermal treatment in an As atmosphere and under high pressure as an additional technological step .

In the LTG layer made of GaAs, which has been applied (6), window-like openings (7) are introduced using standardized photolithographic techniques. The layer (6) was made from these openings using etching techniques and using a Solution based on sulfuric acid removed. Over the freshly stripped layer are under a high vacuum metallization (8,9) evaporated, this in preparation for the electrodes (1,2). The combination W + Ti / Au and Ti + Pt / Au are used as metallization for the electrodes (8,9).

It should be noted that the metallization with W (S), which is used here as the upper electrode (1), determines an energy level of the barrier in the interface with the GaAs of approximately 0.65 eV, which is approximately the energy of the Fermi Levels in undoped remi-isolating GaAs compared to the lower conductor band. In other words, this metallization of the electrode keeps the guiding band and the valence band flat in the interface with the GaAs. It should be pointed out that other metallizations also have this property. In addition to this, a suitable technology for treating the surface before the vapor deposition process or an additional thermal treatment of the metallization layer that has already been vapor-deposited can also be carried out with the purpose of reducing the energy barrier in the interface of the medium as necessary to achieve tallization with the GaAs.

The preparation of the detector described according to FIG. 1a is that which is most similar to the general embodiment. The difference is in the application of a better covering with a layer of GaAs (6) and in the metallization for the upper contact (8). This preparation of the GaAs display device already remarkably improves its functional parameters.

FIG. 1b essentially shows the same technological process that has already been described for FIG. 1 a in accordance with the above invention. The only difference is that the electrodes of the detector (1, 2) are arranged in mirror image symmetry on both sides (3, 4) of the GaAs disc (5). For the manufacture of the detector, a photolithographic process is used which works on two sides. In this embodiment, the electric field is arranged much more evenly between the electrodes of the detector (1, 2), essentially in accordance with the symmetrical arrangement, which leads to a substantial improvement in the resolution of the detector and to a lowering of the Leakage current leads over the inactive parts of the construction volume of the detector.

FIG. 1c shows essentially the same technological method according to the above invention as already described above, with the difference that the cover layers (6, 6 ') are deposited on both sides of the GaAs wafer (5) and with the provision of window-like openings which are arranged in mirror image and symmetrically (1, 1 '), the cover layer (6, 6') being exposed using a pickling process. This exemplary embodiment suppresses the leakage currents of the interface and the surface as much as possible, which improve the resolution of the detector and the noise parameters.

FIG. 1d shows an embodiment of the detector like the device according to the invention described in FIG. 1b, with the difference that on one side, in the present case on the lower side (4) of the GaAs wafer (5), with an initial thickness of 300 μm, The GaAs is removed in recesses for the lower electrodes (2) using a pickling technique (10) to a depth of approx. 100 μm (h), a combined dry pickling process with ions (RIE) in the plasma CC1 ₂ F ₂ together with a wet pickling process is carried out as a final treatment with a solution based on sulfuric acid, before the metallization (9) for the lower electrode is evaporated.

It should be noted that the RIE pickling process is of essential importance for achieving a sufficiently high anisotropy in the pickling process. The results obtained show that with optimized pickling processes a higher anisotropy ratio of 15: 1 can be achieved when using RIE with a flatness of the walls of the pickled recesses that is acceptable. This embodiment is particularly suitable for the case in which series production of the detector with very thin active parts is to be possible. Furthermore, the electrical field in this exemplary embodiment is considerably more homogeneous and the leakage currents from the interface and surface are due to suppresses the special embodiment of the detector, which makes it possible to dispense with an additional cover layer on the underside. As a standardized final process, the component of the detector is completely encased by a further insulating layer consisting, for example, of SiO ₂ , this layer serves mainly as protection against mechanical damage and against further external influences.

Figure 2 shows the dependence of the current on the

Voltage of the GaAs detectors, which are built on the 250 μm thick disc of a single product. The geometry of the electrodes and the metallization were applied via three different treatment processes of the surface: without a further protective layer (1A) with an insulating protective layer consisting of Si ₃ N ₄ (IB) and finally with a covering layer according to the above invention (IC) to be applied. It can be concluded that the lowest leakage current occurs in the case (IC) (the current is approximately two orders of magnitude smaller than in the case of detectors 1A or 1B). The embodiment of the detectors according to FIG. 1a is selected here.

Figure 3 shows the dependence of the current on the

Voltage of the GaAs detectors, according to the above invention for an exemplary embodiment, which is shown in FIG. 1a, with the same geometry and metalization of the contacts, using GaAs disks, from three manufacturers and with the same Thickness were generated. As can be seen, the breakdown voltage for the detector (1) is approximately 450 V and for detectors 2 and 3 the voltage is above 500 V. These results lead to an average value of the breakdown voltage which is greater than 2 V per 1 μm is the thickness of the detector and twice as large as the value that can be found in the literature.

FIG. 4 shows the particle spectrum α with an energy of 5.48 MeV Am 247, which was detected by a GaAs detector according to the above invention in an embodiment according to FIG. 1b with a disk thickness of the GaAs of 200 μm. The resolution of the detector is below 5% FWHM, and the effectiveness of recording the load corresponds to approx. 90% at a polarization voltage of 100 V compared to a Si detector. It should be noted that these results are among the best ever published values for undoped remi-isolating GaAs with a thickness of 200 μm.

FIG. 5 shows the spectrum of the α particles with the various energies 5.15 MeV of Pu ²³⁹ , 4.8 MeV of Am ²⁴¹ and 5.795 MeV of Cm ⁴⁴ , which were detected with a detector Si and with a detector according to GaAs of the above invention. Here the same geometry and the same arrangement according to FIG. 4 were chosen. Bibliography

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Claims

Patent claims

1. Detector for ionizing radiation from GaAs and process for its production with the following process steps:

a) Formation of a monocrystalline layer with high homo-epitaxial resistance made of GaAs, which is to a high degree not stoichiometric, with at least an excess of As corresponding to 0.3%, the concentration of trapping sites greater than 10 ¹⁷ cm ^"3 and a thickness greater than 20 nm over a substrate made of GaAs,

b) formation of a window-like opening in the layer with the recess for depositing the metallization for the electrodes, the layer being removed using pickling techniques,

c) formation of a metallization for the electrode in the window-like recesses of the layer which are provided on the surface of the GaAs, d) Creation of a metallization for the electrode on the surface opposite the GaAs.

2. The method according to claim 1, in which a layer with a high resistance value is formed on the GaAs surface, using the epi-taxy with molecular bundles with molecular flows of Ga and As at a temperature of the GaAs.

Disc between 150 ° C and 350 ° C and thermal treatment in an As atmosphere at a temperature greater than 450 ° C.

3. The method according to claim 2, wherein the layer with high resistance is formed at a temperature of the disc between 320 ° C and 450 ° C without subsequent thermal treatment.

4. Detector according to claim 1, wherein at least one of the electrodes introduced forms an energy barrier, the height of which is less than 1.1 times the difference in energy between the lowest level of the Conductor band and the Fermi level in the GaAs corresponds.

5. Detector according to claims 1 and 4 and the method according to claim 2 or 3, wherein the layer is arranged only on one side of the disk and an electrode with a large area is arranged on the opposite side of the GaAs disk ,

6. Detector according to claim 1 and 4 and method according to claim 2 or 3, wherein the layer is arranged only on one side of the disc and the arrangement of the electrodes in mirror image, symmetrical with appropriate geometry and the best shape according to 75% on the two Side of the disk GaAs is arranged.

7. Detector according to claim 1 and 4 and method according to claim 2 or 3, wherein the layer is arranged on both opposite sides of the disk and the arrangement of the electrodes is mirror-image, symmetrical with a corresponding shape and with the best geometry in accordance with 75% is formed by the window-like openings in the receptacles by electrodes, the layer being removed by pickling.

8. Detector according to patent claims 1, 4 and 6 and method according to patent claim 2 or 3, wherein the GaAs by means of a pickling process to a depth of greater than 60 μm in the receptacle of the electrode prior to its arrangement on at least one side of the disk GaAs Will get removed.

9. Detector according to patent claims 1, 4 and 7 and method according to claim 2 or 3, wherein the GaAs is removed by means of pickling to a depth which is greater than 60 μm in the recess of the electrode before its arrangement at least one side of the GaAs disc.