US20060102870A1 - Infrared detection material and method of production - Google Patents

Infrared detection material and method of production Download PDF

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US20060102870A1
US20060102870A1 US11/252,140 US25214005A US2006102870A1 US 20060102870 A1 US20060102870 A1 US 20060102870A1 US 25214005 A US25214005 A US 25214005A US 2006102870 A1 US2006102870 A1 US 2006102870A1
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thin film
compound
semiconductor thin
copper
film compound
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Jean Viens
Juejun Hu
Fernand Viens
Anuradha Agarwal
Lionel Kimerling
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Massachusetts Institute of Technology
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/0248—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies
    • H01L31/0256—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies characterised by the material
    • H01L31/0264—Inorganic materials
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L27/00—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate
    • H01L27/14—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including semiconductor components sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation
    • H01L27/144—Devices controlled by radiation
    • H01L27/146—Imager structures
    • H01L27/14643—Photodiode arrays; MOS imagers
    • H01L27/14649—Infrared imagers
    • H01L27/14652—Multispectral infrared imagers, having a stacked pixel-element structure, e.g. npn, npnpn or MQW structures
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/0248—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies
    • H01L31/0256—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies characterised by the material
    • H01L31/0264—Inorganic materials
    • H01L31/0272—Selenium or tellurium
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L27/00—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate
    • H01L27/14—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including semiconductor components sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation
    • H01L27/144—Devices controlled by radiation
    • H01L27/146—Imager structures
    • H01L27/14601—Structural or functional details thereof
    • H01L27/14625—Optical elements or arrangements associated with the device
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L29/00—Semiconductor devices specially adapted for rectifying, amplifying, oscillating or switching and having potential barriers; Capacitors or resistors having potential barriers, e.g. a PN-junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof ; Multistep manufacturing processes therefor
    • H01L29/66—Types of semiconductor device ; Multistep manufacturing processes therefor
    • H01L29/68—Types of semiconductor device ; Multistep manufacturing processes therefor controllable by only the electric current supplied, or only the electric potential applied, to an electrode which does not carry the current to be rectified, amplified or switched
    • H01L29/76—Unipolar devices, e.g. field effect transistors
    • H01L29/772—Field effect transistors
    • H01L29/78—Field effect transistors with field effect produced by an insulated gate
    • H01L29/786—Thin film transistors, i.e. transistors with a channel being at least partly a thin film
    • H01L29/78681—Thin film transistors, i.e. transistors with a channel being at least partly a thin film having a semiconductor body comprising AIIIBV or AIIBVI or AIVBVI semiconductor materials, or Se or Te

Definitions

  • the invention relates to a quaternary compound semiconductor comprising arsenic (As), selenium (Se), tellurium (Te) and copper (Cu) atoms, whose covalent atomic arrangement is predominantly amorphous or polycrystalline, and whose electronic properties can be tuned by an adjustment of the molar concentration of tellurium and copper atoms in the said compound, and whose electronic bandgap can be tuned to detect infrared radiation energy.
  • the invention also relates to the method of fabricating thin films of this quaternary compound semiconductor at low cost.
  • infrared semiconductor materials such as In—Ga—As—P, Pd—Te and Hg—Cd—Te crystalline compounds
  • infrared semiconductor materials such as In—Ga—As—P, Pd—Te and Hg—Cd—Te crystalline compounds
  • the large lattice constant mismatches between the substrates and the epitaxial layers cause many defects to be created in crystalline semiconductor compound thin film, with detrimental effects on the material integrity, on the infrared detection performance and on the ability to fabricate infrared detectors or large infrared focal plane arrays at low cost.
  • Some of the major problems facing heteroepitaxial growth of crystalline infrared compounds are the lattice constant mismatch, polarity mismatch and thermal expansion mismatch with the substrate, such that infrared device fabrication typically requires the use of expensive and exotic substrates, which involve severe constraints over the growth size and thickness of the heteroepitaxial layers, and which involve expensive fabrication processes such as molecular beam epitaxy.
  • the development of crystalline infrared compounds for infrared detectors and cameras has been hampered time and again by the costs and constraints involving the heteroepitaxial growth of crystalline infrared compounds.
  • the conductivity increases exponentially with temperature and they have pronounced photoconductivity.
  • the invention relates to a quaternary compound semiconductor comprising arsenic (As), selenium (Se), tellurium (Te) and copper (Cu) atoms with adjustable molar concentrations during processing, whose atomic arrangement is predominantly amorphous or polycrystalline, and dominated by covalent chemical bonds between the said atoms.
  • the amorphous nature of the atomic arrangement gives predominance to short range atomic order, eliminating the constraints of lattice constant mismatch and polarity mismatch with the substrate, which opens the way to wide chemical compositional adjustments and to lower-cost deposition processes such as thermal evaporation or sputtering.
  • the flexibility of the chemical formula can be used to adjust the electronic properties of the semiconductor compound.
  • the molar concentration of tellurium and copper atoms in the said quaternary compound is adjusted to modify the density of localized and extended electronic states in the material, and to modify the energy difference between the valence and conduction bands.
  • This compositional adjustment provides a way to modify significantly the properties of the semiconductor compound, such as increasing its electrical conduction by up to 11 orders of magnitude and decreasing its electronic bandgap by 1 order of magnitude, and to tune the material for the detection of infrared light waves having energies between 1.8 eV and 0.15 eV.
  • a thin film of semiconductor compound with As w Se x Te y Cu z chemical formulation is obtained by a thermal co-evaporation of As 2 Se 3 glass and CuTe mineral.
  • the quaternary semiconductor compound comprising arsenic (As), selenium (Se), tellurium (Te) and copper (Cu) atoms can be produced in thin film form by a mixed physical vapor deposition process involving a vapor combination of at least two different vaporized sub-compounds of different enthalpies of evaporation.
  • the co-evaporation process provides a way to adjust easily the molar composition of the semiconductor compound, and to deposit thin films uniformly over a large area, with a wide range of thin film thicknesses, and onto various surfaces.
  • the design is amenable to the production of infrared detectors of tunable infrared detectivity and of various light collecting areas.
  • FIG. 1 is a table showing exemplary compositions of the semiconductor compound, showing large variations of electrical conductivity and bandgap energy upon addition of tellurium and copper;
  • FIG. 2 is a phase diagram of the As w Se x Te y Cu z system of the invention, with x ⁇ 0%, showing the compositional region where glassy atomic arrangement is obtained;
  • FIG. 3 is a diagram of the bandgap energy for varying molar concentration of copper in the compound of composition (As 50 Se 1 Te 49 ) 100-x Cu x and (As 25 Se 1 Te 74 ) 100-x Cu x , where x corresponds to the molar concentration of copper in percent;
  • FIG. 4 is a diagram of the optical absorption spectra ⁇ ( ⁇ ) as function of copper molar concentration in the compound of composition (As 40 Se 30 Te 30 ) 100-x Cu x , where x is the molar concentration of copper in percent;
  • FIG. 5 is a diagram of the optical absorption spectra of thin films of As w Se x Te y Cu z , with y ⁇ 0%, produced by the method of thermal co-evaporation of As 2 Se 3 and CuSe sub-compounds.
  • the invention involves a quaternary compound semiconductor comprising arsenic (As), selenium (Se), tellurium (Te) and copper (Cu) atoms, whose atomic arrangement is predominantly amorphous or polycrystalline, and whose electronic properties can be tuned during processing by an adjustment of the molar concentration of tellurium and copper atoms in the said compound, and whose electronic bandgap can be tuned to detect infrared radiation energy from the near-infrared regime to the long-infrared regime.
  • As arsenic
  • Se selenium
  • Te tellurium
  • Cu copper
  • the conductivity increases exponentially with temperature, and they have strongly pronounced thermoelectric powers, photo-emfs, and photoconductivity.
  • thermoelectric powers, photo-emfs, and photoconductivity There are a wide variety of different compositions having varying semiconductor properties within the family of chalcogenide compounds.
  • arsenic by antimony or bismuth is not accompanied by so appreciable a change in the electrical conductivity; however, it is accompanied by an appreciable reduction of the glass network stability and increased ability for crystallization.
  • the most appreciable increase of the conductivity is observed on going from selenides to tellurides amorphous chalcogenide compounds. This replacement is accompanied by an appreciable reduction of the glass network stability and increased ability for crystallization.
  • binary chalcogenide compounds such as As—S and As—Se are relatively low. Because of their low electrical conductivity and large bandgap energy, these amorphous compounds can be more readily regarded as dielectrics than semiconductors.
  • a substantial increase of the conductivity of binary compounds is obtained by introducing into them a third component in the metal group—thallium, copper, silver, etc. The electrical conductivity is then increased by 6-10 orders of magnitude.
  • a concomitant decrease of the bandgap energy is also observed when these metals are introduced into arsenic chalcogenides. With increasing content of metals in the arsenic chalcogenides a transition is observed from compounds having dielectric properties to semiconductor properties. The largest contribution to the increase of the conductivity and to the decrease of the bandgap energy of arsenic chalcogenides is made by thallium and copper.
  • the character of the change of the mechanical, thermal and other properties of the compound when metals are introduced in them depends on the composition and structure of the glassy network structural units produced in the material. Thus, when the large thallium atom is introduced into the glassy network, the mechanical and thermal stability of the arsenic chalcogenide compound decreases substantially. When the smaller copper atom is introduced, on the contrary, it is observed that the structure of the chalcogenide becomes stronger.
  • the semiconducting character of the conductivity is determined by the predominance of covalent bonds in the compound.
  • An increase of the fraction of the ionic component in the chemical bonds lowers both the conductivity and the chemical stability of the glassy network.
  • Elemental copper can be introduced in appreciable amounts in arsenic chalcogenides and alter substantially the structural-chemical makeup of the glassy network, causing an abrupt change of the electrical conductivity and bandgap energy, and also an appreciable increase of chemical stability. This can counteract the reduction of chemical-interaction energy, and therefore the concomitant glassy network instability, of selenides being replaced by tellurides in the arsenic chalcogenides.
  • the introduction of tellurium and copper in the arsenic chalcogenide is beneficialent from a standpoint of increasing the electrical conductivity and decreasing the bandgap energy of the compound; the introduction of copper and selenium in the arsenic chalcogenide is remedient in maintaining a glassy network structural stability.
  • the replacement of arsenic by antimony or bismuth is not accompanied by an appreciable change in the electrical conductivity, but is accompanied by an appreciable reduction of the glass network stability and increased ability for crystallization. Therefore, a compound of chemical formula As—Se—Te—Cu can be designed such as to achieve a combination of good semiconducting properties and stable amorphous atomic structure. It will be appreciated by those skilled in the art of infrared optical detection that the combination of good semiconducting properties and stable structure is a necessary feature of infrared detecting materials.
  • a semiconducting infrared detecting compound that 1) can be produced in thin film form over large areas at reasonable cost; 2) that can be processed monolithically using standard VLSI lithographic techniques; 3) that show substantial electrical conductivity for charge readout upon illumination, and; 4) that is characterized by a bandgap energy, or activation energy, between 0.15 eV and 1.8 eV.
  • the invention consists of a quaternary compound semiconductor thin film comprising arsenic (As), selenium (Se), tellurium (Te) and copper (Cu) atoms that meet these criteria.
  • the As—Se—Te—Cu compound semiconductor can be produced in thin film form at reasonable cost by virtue of its amorphous, or glassy, or polycrystalline atomic arrangement.
  • the disordered structure of the thin film material eliminates the constraints of lattice constant matching and polarity matching with the substrate, which opens the way to flexible chemical compositional adjustments and to lower-cost, large-area, large-thickness deposition processes such as thermal evaporation, thermal co-evaporation, sputtering or co-sputtering, as opposed to more expensive heteroepitaxial crystal growth techniques.
  • the compound is dominated by covalent chemical bonds between the said atoms, and thin films can be deposited on a variety of different substrates, such as silicon, oxide, chalcogenide or polymer substrates, by thermal evaporation or sputtering due to the robust stoichiometry of bonding in the material.
  • the compound is characterized by an atomic arrangement of the arsenic, selenium, tellurium and copper atoms that is predominantly amorphous, glassy or polycrystalline.
  • the atomic arrangement of an amorphous material is often described as a “random close packing”, while the atomic arrangement of a glassy material is often described as a “continuous random network”. In both cases the local environment, i.e., the location of atomic nearest neighbors, of an atom is well-defined, but there is no repeating unit cell or crystalline order throughout the material.
  • the amorphous or glassy state is a state of a solid that has become disordered, the main feature being the absence of long range order.
  • a substance in the amorphous or glassy state has short range order within the limits of one or several unit cells, in the range 1 nm to 10 nm. Beyond the unit cell the order is not maintained, each unit cell differs from the preceding one in its position and orientation.
  • the atomic arrangement of a polycrystalline material is described by local units of ordered crystalline domains or grains, in the range 10 nm to 100 nm, of various shapes and orientations, separated by disordered boundaries between the said units.
  • the compound is characterized by an atomic long range order broken by random network units, or by grain boundaries between ordered units.
  • the As—Se—Te—Cu thin film compound semiconductor can be processed monolithically using standard VLSI lithographic techniques, such a contact stepper lithography, photoresist patterning, wet chemical etching using hydroxide solutions, or dry chemical etching using fluorine gases.
  • VLSI lithographic techniques such as a contact stepper lithography, photoresist patterning, wet chemical etching using hydroxide solutions, or dry chemical etching using fluorine gases.
  • the post-deposition processing is not limited to these particular processing techniques.
  • the electronic properties of the As—Se—Te—Cu thin film compound semiconductor can be tuned during the deposition process by an adjustment of the molar concentration of tellurium and copper atoms in the said compound.
  • the electronic properties include principally the electrical conductivity and the bandgap energy.
  • the electrical conductivity of a material refers to the propensity of charges within the material, e.g. electrons, to be delocalized and bounded to no specific atoms or trapping defects such that the charges are free to flow upon application of a driving electrical potential.
  • the bandgap energy refers to the energy difference between the valence band and the conduction band of the material, which corresponds to the energy barrier that must be overcome for a valence state to be activated to a conduction state.
  • the electrical conductivity can be increased by several orders of magnitude, and the bandgap energy tuned to detect infrared radiation energy from the near-infrared regime to the mid-infrared regime to the long-infrared regime.
  • the disorder represented by deviations in the bond lengths and bond angles and bond defects upon adjustment of the molar concentration of tellurium and copper atoms in the compound during processing broadens the electron distribution states, induces electron interaction that splits the valence states into bonding and anti-bonding levels (resulting in a modification of the bandgap energy), and causes electron localization as well as strong scattering of electrons upon application of an electric potential.
  • the three principal features of the structure of amorphous or glassy or polycrystalline semiconductors are the short range order of the network, the long range disorder and the significant presence of coordination defects.
  • the preservation of short range order results in a similar overall electronic structure of an amorphous material compared to the equivalent ordered crystal.
  • the abrupt band edge of a crystal are replaced by a broadened tail of states extending into the bandgap, called bandtail, which originates from the deviations of the bond length and angle arising from the long range structural disorder.
  • the bandtails are important because electronic transport occurs at the band edge.
  • Electronic states deep within the bandgap arise from departures from ideal network, such as coordination defects. These defects determine many electronic properties by controlling trapping and recombination of electronic charges.
  • the electronic structure of an amorphous semiconductor comprises the bands (extended states), the bandtails (localized states) and the defect states (localized states) in the bandgap.
  • the structure and the electrical conduction mechanism such that 1) the structural short range order determines the extended state conduction and the bandgap energy (i.e., usual conduction mechanism in intrinsic semiconductors), 2) the structural deviations of the bond length and angle determines the bandtail conduction (i.e., hopping conduction mechanism from localized to extended states) and 3) the departures from ideal network, such as coordination defects, determine the hopping conduction mechanism at the Fermi energy (i.e. tunneling of states deep within the bandgap).
  • the structural short range order determines the extended state conduction and the bandgap energy (i.e., usual conduction mechanism in intrinsic semiconductors)
  • the structural deviations of the bond length and angle determines the bandtail conduction (i.e., hopping conduction mechanism from localized to extended states)
  • the departures from ideal network such as coordination defects
  • the electronic properties of the As—Se—Te—Cu thin film compound semiconductor can be tuned during the deposition process by an adjustment of the molar concentration of tellurium and copper atoms in the said compound by virtue of modifications of the short range order, deviations of the bond length and angle, and changes in the density of coordination defects, which result in changes in the bandgap energy, in the extended state conduction, in the bandtail conduction and in the hopping conduction at the Fermi level.
  • the listed electrical conduction represents a sum of the abovementioned conduction mechanisms.
  • FIG. 2 is a phase diagram of the As w Se x Te y Cu z system, with x ⁇ 0%, showing the compositional region where glassy atomic arrangement is obtained (outlined rounded dots). The arrows point to the chemical compositions suitable for near-infrared detection and mid-infrared detection.
  • a glassy structure is generally obtained at about 60% of tellurium concentration and less than 30% copper concentration. It can be seen that, with increased copper concentration, the glassy regime is widened and the structure becomes amenable to a glassy atomic arrangement.
  • Additional exemplary embodiments of the invention include a quaternary compound in the As w Se x Te y Cu z chemical system, with a range of w, x, y, and z molar compositions encompassed by a glassy regime similar to the one shown in FIG. 2 , and produced in thin film solid-state form.
  • the impurity atom, or group of impurity atoms can be any atoms in the periodic table that establish covalent chemical bonding with the As w Se x Te y Cu z compound, such as Group III atoms of the periodic table (B, Al, Ga, In, Tl), or Group IV atoms of the periodic table (C, Si, Ge, Sn, Pb), or Group V atoms of the periodic table (N, P, Sb, Bi), or Group VI atoms of the periodic table (O, Po), or metal atoms in Group II of the periodic table (such as Ag, Au, Zn, Cd, Hg, etc.).
  • the bandgap energy of the quaternary compound semiconductor can be tuned during thin film deposition process by an adjustment of the molar concentration of tellurium and copper atoms in the said compound.
  • FIG. 3 shows that, in the exemplary systems of (As 50 Se 1 Te 49 ) 100-x Cu x (squares) and (As 25 Se 1 Te 74 ) 100-x Cu x (circles), the bandgap of the thin film can be adjusted, or tuned, by a significant amount with a change of copper molar concentration.
  • the bandgap energy can be adjusted from 0.9 eV ((As 50 Se 1 Te 49 ) 100 Cu 0 ) to 0.2 eV ((As 25 Se 1 Te 74 ) 75 Cu 25 ) by changing the copper molar concentration from ⁇ 0% to ⁇ 25% respectively, which is commensurate with infrared radiation energies of wavelength from about 1300 nm (near-infrared) to about 6000 nm (long-infrared), respectively.
  • the bandgap energy can be tuned to shorter infrared wavelengths (higher Eg) by increasing the selenium-to-tellurium concentration ratio, or to longer infrared wavelengths (lower Eg) by decreasing the arsenic-to-tellurium concentration ratio.
  • the range of bandgap energies allowable within the glassy regime of the As w Se x Te y Cu z system has an uppermost limit of about 1.8 eV (tellurium poor and copper poor compositions) to a lowermost limit of about 0.15 eV (tellurium rich and copper rich compositions), corresponding to infrared wavelengths from about 700 nm (near-infrared regime) to about 8000 nm (long-infrared regime).
  • the spectral measurement of the optical absorption ⁇ ( ⁇ ) of the material, near the absorption edge, can reveal with good precision the bandgap energy Eg of the material.
  • the spectral measurement of the optical absorption ⁇ ( ⁇ ) of the material can also reveal the optical frequency range at which infrared radiation is absorbed for the application of these semiconductor materials as infrared light detectors.
  • the As 40 Se 30 Te 30 matrix is known to be a stable glassy material, and the enrichment of this matrix with copper such as to form a (As 40 Se 30 Te 30 ) 100-x Cu x compound is not followed by thin film crystallization.
  • the bandgap energy of the As—Se—Te—Cu thin film compound semiconductor can be tuned during the deposition process by an adjustment of the molar concentration of tellurium and copper atoms in the said compound.
  • the adjustment of the molar concentration of tellurium and copper atoms modifies the bandgap energy and the optical absorption spectrum ⁇ ( ⁇ ) of the compound, which shifts the optical frequency of the optical absorption edge.
  • the spectral response of photo-generated increase of electrical conductivity is shifted in optical frequency upon adjustment of the molar concentration of tellurium and copper atoms in the said compound.
  • the spectral response of photoconductivity is shifted to longer infrared wavelengths upon enrichment of tellurium and/or copper in the As—Se—Te—Cu compound.
  • the thin film deposition process relies on a mass-transport-limited deposition technique so that all locations of the substrate surface are supplied with an equal flux of reactant species.
  • the thin film deposition process relies on physical vapor deposition (PVD), which comprises the well know techniques of thermal evaporation, electron-beam evaporation and ion sputtering.
  • PVD physical vapor deposition
  • Thermal evaporation is generated by heating in vacuum a solid source above its melting temperature using electrically-heated crucibles, and the condensation of the vapors to a solid thin film on the substrate surface.
  • Electron-beam evaporation is generated by heating in vacuum a solid source above its melting temperature using a high-energy electron beam incident on the solid source, and the condensation of the vapors to a solid thin film on the substrate surface.
  • the ion sputtering process involves the ejection of surface atoms from an electrode surface by momentum transfer from the bombarding ions to the electrode surface atoms. The generated vapor of electrode material is then condensed on the substrate.
  • the thin film deposition process can also rely on a physical vapor deposition involving vapor-phase chemical reactions, such as vapor adsorption and desorption at the thin film, encountered typically in chemical vapor deposition, which is defined as the formation of a non-volatile solid film on a substrate by the reaction of vapor-phase chemicals (reactants) that contain the required precursor constituents of arsenic, selenium, tellurium and copper.
  • vapor-phase chemical reactions such as vapor adsorption and desorption at the thin film, encountered typically in chemical vapor deposition, which is defined as the formation of a non-volatile solid film on a substrate by the reaction of vapor-phase chemicals (reactants) that contain the required precursor constituents of arsenic, selenium, tellurium and copper.
  • This problem comes from the different covalent bond energies between the As, Se, Te and Cu atoms in the compound, and from the different bond coordination numbers between the As, Se, Te and Cu atoms in the compound.
  • the arsenic is usually 3-fold bond coordinated
  • the selenium and tellurium are usually 2-fold bond coordinated
  • copper is usually 1-fold bond coordinated, all the said covalent bonds having various different energies, which results in different bond detachment energies within the network unit of the glass structure.
  • the lower-bond-energy and lower-coordination-number atoms may break away first, and be released as vapor first, and the As w Se x Te y Cu z compound may generate a non-stochiometric vapor as the glassy network units are broken, and condensed into a non-stochiometric thin film of different glassy network units onto the substrate. It has been observed in such glasses that tellurium elements tend to vaporize first, and arsenic and copper elements tend to vaporize last. The resulting As w Se x Te y Cu z thin film is usually tellurium-rich, and arsenic-poor, and copper-poor, as compared to the initial As w Se x Te y Cu z bulk source.
  • the deposition process of the quaternary As w Se x Te y Cu z semiconductor compound relies on a mixed physical vapor deposition technique.
  • the mixed physical vapor deposition technique consists of co-evaporating, or co-sputtering, a combination of at least two different solid sources having different enthalpies of evaporation.
  • Sub-compounds of low bond energies are evaporated or sputtered independently at low temperature or low ions energy, while other sub-compounds of higher bond energies are evaporated or sputtered independently at higher temperature or higher ions energy.
  • sub-compounds of low enthalpies of evaporation are evaporated or sputtered independently at low temperature or low ions energy, while other sub-compounds of higher enthalpies of evaporation are evaporated or sputtered independently at higher temperature or higher ions energy.
  • the enthalpy of evaporation is the thermodynamic energy required for a material at a characteristic pressure and temperature to attain first-order phase transition from solid/liquid to vapor. The enthalpy of evaporation depends on the bond energy and bond configuration.
  • the deposition process relies on a mixed physical vapor deposition technique comprising at least two different solid sources that vaporize stoichiometrically.
  • the sub-compounds must have chemical compositions comprising one, two or three of either arsenic, selenium, tellurium or copper atoms, and must vaporize stoichiometrically; these sub-compounds are generally found to have different enthalpies of evaporation.
  • Each sub-compound vaporizations is controlled to a desired vaporization rate, up to a deposition rate of 100 Angstrom per second, and at least two vapors resulting from the said vaporizations of said at least two sub-compounds will combine, and re-create an overall vapor composed of arsenic, selenium, tellurium and copper atoms with molar concentration matching the desired As w Se x Te y Cu z compound.
  • This overall vapor composed of arsenic, selenium, tellurium and copper atoms with molar concentration matching the desired As w Se x Te y Cu z compound will condense on a substrate to form a solid-state thin film of As w Se x Te y Cu z compound.
  • Such co-deposition process is done usually in a vacuum chamber at a residual pressure of less than 1 millitorr, or preferably less than 10 ⁇ 5 torr. It is known that As w Se x Te y Cu z thin films adhere well to a variety of substrates, such as metallic, semiconductor, ceramic, dielectric or polymer substrates. Due to limitations in the purity of the sub-compounds, and the cleanliness of the vacuum chamber, the overall vapor combination may contain impurity atoms of elemental nature different than arsenic, selenium, tellurium, or copper.
  • the co-deposition process preferably involves thermal evaporation and/or electron-beam evaporation and/or ion sputtering of at least two sub-compounds of the following chemical composition categories: arsenic-selenide, arsenic-telluride, arsenic-selenide-telluride, copper-selenide, copper-telluride, pure selenium, pure tellurium, and pure copper.
  • compositions encompassed by these categories are known to vaporize stoichiometrically and with different enthalpies of evaporation; exemplary chemical compositions are As 40 Se 60 , As 40 Te 60 , As 40 Se 60-x Te x ⁇ , CuSe, CuTe, Cu 2 Se, Cu 2 Te, pure Se, pure Te, pure Cu, etc.
  • the method of producing thin films of As w Se x Te y Cu z by co-deposition is not limited to these particular sub-compound chemical compositions. The main requirement is that the sub-compounds must have chemical compositions comprising one, two or three of either arsenic, selenium, tellurium or copper atoms, and must vaporize stoichiometrically.
  • the at least two sub-compounds can be all vaporized by thermal evaporation, or all vaporized by electron-beam evaporation, or all vaporized by ion sputtering, or a combination of thermal evaporation and/or electron-beam evaporation and/or ion sputtering can be used.
  • a preferred method of adjusting the molar concentration of tellurium and copper in the As w Se x Te y Cu z thin film semiconductor compound comprises the control of the respective vaporization rates of the sub-compounds and the adjustment of the geometric arrangement of the sub-compound vapor source inside the vacuum chamber with respect to the substrate.
  • a preferred method of producing an As w Se x Te y Cu z thin film semiconductor compound involves the thermal evaporation of at least two commercially-available sub-compounds of the following chemical composition categories: arsenic-selenide, arsenic-telluride, arsenic-selenide-telluride, copper-selenide, copper-telluride, pure selenium, pure tellurium, and pure copper.
  • FIG. 5 is a diagram of the optical absorption spectra of exemplary 1 ⁇ m thin films of As w Se x Te y Cu z , with y ⁇ 0%, produced by the method of co-deposition of As 2 Se 3 and CuSe sub-compounds on a silicon substrate.
  • the As 2 Se 3 and CuSe sub-compounds have been vaporized by thermal evaporation at relative vaporization rates of 100-to-0, 92-to-8 and 70-to-30, in order to obtain final thin film compositions of (As 2 Se 3 ) 100 (CuSe) 0 , (As 2 Se 3 ) 92 (CuSe) 8 , and (As 2 Se 3 ) 70 (CuSe) 30 .
  • the optical band edge is shifted to longer infrared wavelengths by increasing the copper molar concentration, as explained previously.
  • Other exemplary co-depositions can be performed with stable glass sub-compounds As 2 Se 3 and As 40 Se 30 Te 30 , and binary alloys CuSe and CuTe, in order to obtain final glassy thin film compositions of (As 2 Se 3 ) 100-x (CuSe) x , (As 2 Se 3 ) 100-x (CuTe) x , (As 40 Se 30 Te 30 ) 100-x (CuSe) x or (As 40 Se 30 Te 30 ) 100-x (CuTe) x .
  • the co-deposition process can be followed by a thermal treatment of the thin film, such as a thermal annealing at the glass transition temperature of the compound, in order to modify the free volume in the glassy thin film, or consolidate the material, or activate polycrystallization of the thin film, or activate migration of copper in the material.
  • a thermal treatment of the thin film such as a thermal annealing at the glass transition temperature of the compound, in order to modify the free volume in the glassy thin film, or consolidate the material, or activate polycrystallization of the thin film, or activate migration of copper in the material.
  • thermal treatment can be used to improve electric mobility of the thin film compound.

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