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Polycrystalline GaAs for large area imaging detectors
Nuclear Instruments and Methods in Physics Research A 466 (2001) 9–13
Polycrystalline GaAs for large area imaging detectors J.C. Bourgoin* ! ! et Het ! erog ! enes, " Laboratoire des Milieux Desordonn es Universite! Pierre et Marie Curie (Paris VI), C.N.R.S., UMR 7603, Tour 22, Case 86, 4 Place Jussieu, F-75252 Paris Cedex 05, France
Abstract Epitaxial layers of GaAs, when thick enough, are now recognized as good candidates for X-ray imaging. In order to overcome the limitation in the size of the substrates for large imaging applications, we are developing a growth technique by chemical reaction which allows to obtain several hundred microns thick polycrystalline layers, i.e. of unlimited dimensions, in few hours. The size and orientation of the grains can be controlled to some extent through the growth conditions. We shall present data concerning optical (IR absorption, luminescence), structural (double X-ray diffraction) and electrical (resistivity, current–voltage characteristics of Schottky barriers) characterizations performed on 300 mm thick non-intentionally doped layers in which the crystallites, of columnar structure, are h1 1 1i oriented and of diameter 70 10 mm. We now aim at controlling the location of the crystallites on the glass substrate in order to get a regular pattern and at resuming a new epitaxy on a polycrystalline surface. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Gallium arsenide; Imaging; X-detector; Polycrystal
1. Introduction The largest efficiency for an X-ray detector is reached with a structure in which the X-photon is directly converted into electron hole pairs, then collected in a high electric field. Such detector requires a thick enough (several hundred microns) high quality (high carrier mobility and long minority carrier lifetime) semiconductor material characterized by a gap of the order of 1.5 eV (to minimize the noise at room temperature and to maximize the number of electron hole pairs created by one photon). Up to now, only CdZnTe alloys fulfill these conditions. Gallium arsenide could also be a candidate [1]. However, thick GaAs layers are not suitable because they can only *Tel.: +33-1-4427-7998; fax: +33-1-4427-7998. E-mail address: [email protected] (J.C. Bourgoin).
be obtained from bulk semi-insulating (SI) Czochralski (Cz) grown ingots which contain a non uniform high concentration of defects [2]. However, recently, 100–600 mm thick epitaxial GaAs layers of good electronic quality have been produced [3,4]. Since GaAs substrates can now be obtained with large dimensions (4 in. diameter) and because a micro-electronic technology exists for this material, GaAs becomes potentially more interesting than CdTe at least for energies below 100 KeV, i.e. in the medicine range. For large imaging, often required in medical applications, attempts are made to use the photoconduction properties of amorphous or microcrystalline layers, which detect visible photons produced by a scintillator (Si) or directly the X-photons (PbO). Unfortunately, such materials are yet far from exhibiting good enough electronic properties.
0168-9002/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 0 8 1 8 - X
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J.C. Bourgoin / Nuclear Instruments and Methods in Physics Research A 466 (2001) 9–13
Here, we present a new type of material, a thick polycrystalline GaAs layer, which could allow to reach large areas while retaining the advantages of structures using charge collection in an electric field. Indeed, if the grain size is large enough compared to the pixel dimension and the structure of the grain is columnar, such a layer could be suitable for the production of large area pixel detectors exhibiting the same performances as those made on epitaxial layer.
2. Growth We have grown GaAs on Si substrates, covered by a native SiO2 layer. The growth is performed with a technique where the rate of transport is limited by the chemical reaction. This technique described in Refs. [5,6], is the only one able to reach growth rates of the order of few mm per minute, i.e. allowing to grow epitaxial layers of several hundreds mm in a matter of hours. It is based on the decomposition of a source material at a temperature T using a reactant, here H2O, producing volatile species which are transported by the pressure gradient onto a substrate where the reverse reaction takes place, its temperature y being lower than T. The growth rate V g is not limited by the transport of the volatile species because the source is placed very close to the substrate (few millimeters in practice), but only by the temperature of the reaction and the partial pressure p of the reactant. Fig. 1 shows the variation of V g versus p for given values of T and y illustrating that for these values a layer 300 mm thick can be obtained typically in 2 hours. Polycrystalline layers (1 cm in diameter) have been grown with thicknesses ranging from 200 to 600 mm, i.e. thick enough to get a self-supported material. Indeed, during cooling down, the difference in the expansion coefficients between GaAs and Si induces the fracture between the substrate and the layer. Apparently, the type and size of the crystallites obtained seem to depend on the growth temperature. They have a random shape and do not exhibit well developed faces, except in a specific temperature range. The layers, whose characterization is described below, have been
Fig. 1. Variation of the growth rate versus the square root of the partial pressure of the reactant (H2O) for source and substrate temperatures of 8508C and 7008C, respectively.
grown for T ¼ 8508C and y¼ 7008C, in which case, as we shall see below, the size and structure of the crystallites are promising for applications.
3. Characterization 3.1. Structural characterization Because the grown layer can be separated from the substrate, the interface layer has been observed with an optical microscope. The image (see Fig. 2) shows clearly the existence of hexagonal structures
Fig. 2. Observation of the grains by optical microscopy on the nucleation face.
J.C. Bourgoin / Nuclear Instruments and Methods in Physics Research A 466 (2001) 9–13
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Fig. 3. Double diffraction spectra recorded on the growth face (a) and the nucleation face (b).
having similar sizes of 70 10 mm. This clearly indicates that the crystallites have a h1 1 1i orientation perpendicular to the substrate surface. Below 7008C, the orientation of the grains tends to be randomly distributed. This observation is confirmed by double X-ray diffraction which demonstrates that the most intense line in the spectrum (see Fig. 3) corresponds to the h1 1 1i orientation. The crystallites have a columnar structure since the DDX spectrum performed on the top of the layer also indicates that the h1 1 1i orientation still dominates. The other peaks on the spectrum are associated with small crystallites in the interface regions, which become larger as the width of the layer increases.
voltage (below 20 V) the resistivity at room temperature is of the order of 105 Ocm in nonintentionally doped layers, i.e. grown from a SI Cz GaAs substrate. The temperature dependence of this resistivity indicates (see Fig. 4) the existence of
3.2. Electrical characterization Resistivity measurements have been performed with the electric field perpendicular to the layer, i.e. in the h1 1 1i direction, along a grain. At low
Fig. 4. Temperature dependence of the resistivity recorded with the electric field in the h1 1 1i direction.
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J.C. Bourgoin / Nuclear Instruments and Methods in Physics Research A 466 (2001) 9–13
a donor level close to the conduction band (40 MeV). Below 250 K, the resistivity is practically temperature independent. Capacitance–voltage (C–V) measurements have been performed on Schottky barriers (of area 0.8 mm2) deposited by gold evaporation through a mask. The C2 (V) plot is linear and indicates that the residual doping level is in the range 1013–1014 cm3. Schottky barrier measurements confirm the existence of shallow level defects since the capacitance increases when the frequency decreases below 103 Hz (Fig. 5). The change in capacitance allows to evaluate their concentration (of the order of 1016 cm3). 3.3. Optical characterization We have compared the photoluminescence and IR absorption spectra of the layer with that of a conventional epitaxial layer. For both layers, the surfaces were not specially prepared, i.e. not covered by a GaAlAs window preventing surface recombination. Photoluminescence spectra of these two layers, recorded at 80 K, are characterized by a peak at 8250 cm1 (see Fig. 6). The width at half maximum of this peak is equal to 100 cm1 for the polycrystalline layer. The peak of the crystalline layer, characterized by a width of 50 cm1, exhibits the classical fine structure not seen in the polycrystalline layer. The peak amplitude of the polycrystalline layer is two times smaller than that of the crystalline one indicating that the minority
Fig. 5. Frequency dependence of a 0.8 mm2 Schottky barrier measured at room temperature.
carrier lifetime is of the same order of magnitude in both layers. As to the transmittance in the IR region, monitored at 5 K, it is very similar to that of a SI Cz material (see Fig. 7), indicating it originates from the columnar crystalline regions. Only, the transmitted signal is very low because of light diffusion in the grain boundaries and at the surface.
4. Conclusion Polycrystalline layers of GaAs with specific size and orientation of the grains can be grown, opening the possibility to make large area devices. These devices could be pixel X-ray detectors because we have demonstrated that several hundred micron thick polycrystalline layers can be obtained easily. The material inside the grains exhibiting electronic and optical properties similar to that of a monocrystalline material should be suitable for the fabrication of such detectors. The grain size can be made large enough to accommodate one pixel adapted to medicine applications. Because it is possible to obtain grains of the
Fig. 6. Comparison of the luminescence spectra of a polycrystalline layer (a) and of a monocrystalline epitaxial layer (b).
J.C. Bourgoin / Nuclear Instruments and Methods in Physics Research A 466 (2001) 9–13
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Fig. 7. IR transmittance recorded at 5 K of a 600 mm thick polycrystalline layer. The peak at 3500 cm1 is due to water, the layer having been rinsed and insufficiently heated prior to the measurement.
same size, the polycrystalline layer shows a regular pattern. Hence, to make pixel detectors only remains to initiate the growth of each grain at a specific location over the substrate. Another possible way to explore is to grow individual crystallites over a Si substrate in which patterns of the correct size and shape have been etched.
References [1] C.M. Buttar, Nucl. Instr. and Meth. A 395 (1997) 1. [2] M. Rogalla, M. Fiederle, R. Irsigler, T. Frœmunchen, J.W. Chen, D.G. Ebling, P. Hug, K. Runge, S. Lauxtermann, J. Ludwig, T. Schmid, R. Geppert, S. Joost, in: P.G. Pelfer, J. Ludwig, K. Runge, H.S. Rupprecht (Eds.), Gallium Arsenide and Related Compounds, World Scientific, London, 1996, p. 63.
[3] J.C. Bourgoin, A new GaAs material for X-ray imaging, First International Workshop on Radiation Imaging Detectors, Sundsvall (Sweden), Nucl. Instr. and Meth. A 460 (2001) 159. [4] N. de Angelis, J.C. Bourgoin, K. Smith, R. Bates, C. Whitehill, A. Meikle, M. Hammadi, Potential of thick GaAs epitaxial layers for pixel detectors, E-MRS 11th International Workshop on Room Temperature Semiconductor X- and Gamma-Ray Detectors and Associated Electronics, Vienna, Austria, 1999, Nucl. Instr. and Meth. A 458 (2001) 344. [5] L. El Mir, M. Gandouzi, M. Hammadi, H. Samic, J.C. Bourgoin, Compound semiconductor growth by chemical reaction, Curr. Top. Cryst. Growth Res., Res. Trends 5 (1999) 131. [6] J.C. Bourgoin, H. Samic-Sahinpasic, Growth by close space vapor transport, in: M.R. Brozel, G.E. Stillman (Eds.), Properties of GaAs, 3rd Edition, INSPEC, EMIS Data Reviews Series No. 16, London, 1996, pp. 639–642.
SECTION I.
Polycrystalline GaAs for large area imaging detectors J.C. Bourgoin* ! ! et Het ! erog ! enes, " Laboratoire des Milieux Desordonn es Universite! Pierre et Marie Curie (Paris VI), C.N.R.S., UMR 7603, Tour 22, Case 86, 4 Place Jussieu, F-75252 Paris Cedex 05, France
Abstract Epitaxial layers of GaAs, when thick enough, are now recognized as good candidates for X-ray imaging. In order to overcome the limitation in the size of the substrates for large imaging applications, we are developing a growth technique by chemical reaction which allows to obtain several hundred microns thick polycrystalline layers, i.e. of unlimited dimensions, in few hours. The size and orientation of the grains can be controlled to some extent through the growth conditions. We shall present data concerning optical (IR absorption, luminescence), structural (double X-ray diffraction) and electrical (resistivity, current–voltage characteristics of Schottky barriers) characterizations performed on 300 mm thick non-intentionally doped layers in which the crystallites, of columnar structure, are h1 1 1i oriented and of diameter 70 10 mm. We now aim at controlling the location of the crystallites on the glass substrate in order to get a regular pattern and at resuming a new epitaxy on a polycrystalline surface. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Gallium arsenide; Imaging; X-detector; Polycrystal
1. Introduction The largest efficiency for an X-ray detector is reached with a structure in which the X-photon is directly converted into electron hole pairs, then collected in a high electric field. Such detector requires a thick enough (several hundred microns) high quality (high carrier mobility and long minority carrier lifetime) semiconductor material characterized by a gap of the order of 1.5 eV (to minimize the noise at room temperature and to maximize the number of electron hole pairs created by one photon). Up to now, only CdZnTe alloys fulfill these conditions. Gallium arsenide could also be a candidate [1]. However, thick GaAs layers are not suitable because they can only *Tel.: +33-1-4427-7998; fax: +33-1-4427-7998. E-mail address: [email protected] (J.C. Bourgoin).
be obtained from bulk semi-insulating (SI) Czochralski (Cz) grown ingots which contain a non uniform high concentration of defects [2]. However, recently, 100–600 mm thick epitaxial GaAs layers of good electronic quality have been produced [3,4]. Since GaAs substrates can now be obtained with large dimensions (4 in. diameter) and because a micro-electronic technology exists for this material, GaAs becomes potentially more interesting than CdTe at least for energies below 100 KeV, i.e. in the medicine range. For large imaging, often required in medical applications, attempts are made to use the photoconduction properties of amorphous or microcrystalline layers, which detect visible photons produced by a scintillator (Si) or directly the X-photons (PbO). Unfortunately, such materials are yet far from exhibiting good enough electronic properties.
0168-9002/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 0 8 1 8 - X
SECTION I.
10
J.C. Bourgoin / Nuclear Instruments and Methods in Physics Research A 466 (2001) 9–13
Here, we present a new type of material, a thick polycrystalline GaAs layer, which could allow to reach large areas while retaining the advantages of structures using charge collection in an electric field. Indeed, if the grain size is large enough compared to the pixel dimension and the structure of the grain is columnar, such a layer could be suitable for the production of large area pixel detectors exhibiting the same performances as those made on epitaxial layer.
2. Growth We have grown GaAs on Si substrates, covered by a native SiO2 layer. The growth is performed with a technique where the rate of transport is limited by the chemical reaction. This technique described in Refs. [5,6], is the only one able to reach growth rates of the order of few mm per minute, i.e. allowing to grow epitaxial layers of several hundreds mm in a matter of hours. It is based on the decomposition of a source material at a temperature T using a reactant, here H2O, producing volatile species which are transported by the pressure gradient onto a substrate where the reverse reaction takes place, its temperature y being lower than T. The growth rate V g is not limited by the transport of the volatile species because the source is placed very close to the substrate (few millimeters in practice), but only by the temperature of the reaction and the partial pressure p of the reactant. Fig. 1 shows the variation of V g versus p for given values of T and y illustrating that for these values a layer 300 mm thick can be obtained typically in 2 hours. Polycrystalline layers (1 cm in diameter) have been grown with thicknesses ranging from 200 to 600 mm, i.e. thick enough to get a self-supported material. Indeed, during cooling down, the difference in the expansion coefficients between GaAs and Si induces the fracture between the substrate and the layer. Apparently, the type and size of the crystallites obtained seem to depend on the growth temperature. They have a random shape and do not exhibit well developed faces, except in a specific temperature range. The layers, whose characterization is described below, have been
Fig. 1. Variation of the growth rate versus the square root of the partial pressure of the reactant (H2O) for source and substrate temperatures of 8508C and 7008C, respectively.
grown for T ¼ 8508C and y¼ 7008C, in which case, as we shall see below, the size and structure of the crystallites are promising for applications.
3. Characterization 3.1. Structural characterization Because the grown layer can be separated from the substrate, the interface layer has been observed with an optical microscope. The image (see Fig. 2) shows clearly the existence of hexagonal structures
Fig. 2. Observation of the grains by optical microscopy on the nucleation face.
J.C. Bourgoin / Nuclear Instruments and Methods in Physics Research A 466 (2001) 9–13
11
Fig. 3. Double diffraction spectra recorded on the growth face (a) and the nucleation face (b).
having similar sizes of 70 10 mm. This clearly indicates that the crystallites have a h1 1 1i orientation perpendicular to the substrate surface. Below 7008C, the orientation of the grains tends to be randomly distributed. This observation is confirmed by double X-ray diffraction which demonstrates that the most intense line in the spectrum (see Fig. 3) corresponds to the h1 1 1i orientation. The crystallites have a columnar structure since the DDX spectrum performed on the top of the layer also indicates that the h1 1 1i orientation still dominates. The other peaks on the spectrum are associated with small crystallites in the interface regions, which become larger as the width of the layer increases.
voltage (below 20 V) the resistivity at room temperature is of the order of 105 Ocm in nonintentionally doped layers, i.e. grown from a SI Cz GaAs substrate. The temperature dependence of this resistivity indicates (see Fig. 4) the existence of
3.2. Electrical characterization Resistivity measurements have been performed with the electric field perpendicular to the layer, i.e. in the h1 1 1i direction, along a grain. At low
Fig. 4. Temperature dependence of the resistivity recorded with the electric field in the h1 1 1i direction.
SECTION I.
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J.C. Bourgoin / Nuclear Instruments and Methods in Physics Research A 466 (2001) 9–13
a donor level close to the conduction band (40 MeV). Below 250 K, the resistivity is practically temperature independent. Capacitance–voltage (C–V) measurements have been performed on Schottky barriers (of area 0.8 mm2) deposited by gold evaporation through a mask. The C2 (V) plot is linear and indicates that the residual doping level is in the range 1013–1014 cm3. Schottky barrier measurements confirm the existence of shallow level defects since the capacitance increases when the frequency decreases below 103 Hz (Fig. 5). The change in capacitance allows to evaluate their concentration (of the order of 1016 cm3). 3.3. Optical characterization We have compared the photoluminescence and IR absorption spectra of the layer with that of a conventional epitaxial layer. For both layers, the surfaces were not specially prepared, i.e. not covered by a GaAlAs window preventing surface recombination. Photoluminescence spectra of these two layers, recorded at 80 K, are characterized by a peak at 8250 cm1 (see Fig. 6). The width at half maximum of this peak is equal to 100 cm1 for the polycrystalline layer. The peak of the crystalline layer, characterized by a width of 50 cm1, exhibits the classical fine structure not seen in the polycrystalline layer. The peak amplitude of the polycrystalline layer is two times smaller than that of the crystalline one indicating that the minority
Fig. 5. Frequency dependence of a 0.8 mm2 Schottky barrier measured at room temperature.
carrier lifetime is of the same order of magnitude in both layers. As to the transmittance in the IR region, monitored at 5 K, it is very similar to that of a SI Cz material (see Fig. 7), indicating it originates from the columnar crystalline regions. Only, the transmitted signal is very low because of light diffusion in the grain boundaries and at the surface.
4. Conclusion Polycrystalline layers of GaAs with specific size and orientation of the grains can be grown, opening the possibility to make large area devices. These devices could be pixel X-ray detectors because we have demonstrated that several hundred micron thick polycrystalline layers can be obtained easily. The material inside the grains exhibiting electronic and optical properties similar to that of a monocrystalline material should be suitable for the fabrication of such detectors. The grain size can be made large enough to accommodate one pixel adapted to medicine applications. Because it is possible to obtain grains of the
Fig. 6. Comparison of the luminescence spectra of a polycrystalline layer (a) and of a monocrystalline epitaxial layer (b).
J.C. Bourgoin / Nuclear Instruments and Methods in Physics Research A 466 (2001) 9–13
13
Fig. 7. IR transmittance recorded at 5 K of a 600 mm thick polycrystalline layer. The peak at 3500 cm1 is due to water, the layer having been rinsed and insufficiently heated prior to the measurement.
same size, the polycrystalline layer shows a regular pattern. Hence, to make pixel detectors only remains to initiate the growth of each grain at a specific location over the substrate. Another possible way to explore is to grow individual crystallites over a Si substrate in which patterns of the correct size and shape have been etched.
References [1] C.M. Buttar, Nucl. Instr. and Meth. A 395 (1997) 1. [2] M. Rogalla, M. Fiederle, R. Irsigler, T. Frœmunchen, J.W. Chen, D.G. Ebling, P. Hug, K. Runge, S. Lauxtermann, J. Ludwig, T. Schmid, R. Geppert, S. Joost, in: P.G. Pelfer, J. Ludwig, K. Runge, H.S. Rupprecht (Eds.), Gallium Arsenide and Related Compounds, World Scientific, London, 1996, p. 63.
[3] J.C. Bourgoin, A new GaAs material for X-ray imaging, First International Workshop on Radiation Imaging Detectors, Sundsvall (Sweden), Nucl. Instr. and Meth. A 460 (2001) 159. [4] N. de Angelis, J.C. Bourgoin, K. Smith, R. Bates, C. Whitehill, A. Meikle, M. Hammadi, Potential of thick GaAs epitaxial layers for pixel detectors, E-MRS 11th International Workshop on Room Temperature Semiconductor X- and Gamma-Ray Detectors and Associated Electronics, Vienna, Austria, 1999, Nucl. Instr. and Meth. A 458 (2001) 344. [5] L. El Mir, M. Gandouzi, M. Hammadi, H. Samic, J.C. Bourgoin, Compound semiconductor growth by chemical reaction, Curr. Top. Cryst. Growth Res., Res. Trends 5 (1999) 131. [6] J.C. Bourgoin, H. Samic-Sahinpasic, Growth by close space vapor transport, in: M.R. Brozel, G.E. Stillman (Eds.), Properties of GaAs, 3rd Edition, INSPEC, EMIS Data Reviews Series No. 16, London, 1996, pp. 639–642.
SECTION I.