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WO2009033279A1 - Dispositifs électroluminescents à film mince d'oxyde de zinc - Google Patents

Dispositifs électroluminescents à film mince d'oxyde de zinc Download PDF

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Publication number
WO2009033279A1
WO2009033279A1 PCT/CA2008/001607 CA2008001607W WO2009033279A1 WO 2009033279 A1 WO2009033279 A1 WO 2009033279A1 CA 2008001607 W CA2008001607 W CA 2008001607W WO 2009033279 A1 WO2009033279 A1 WO 2009033279A1
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WIPO (PCT)
Prior art keywords
tfel
layer
accordance
active layer
zinc oxide
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Ceased
Application number
PCT/CA2008/001607
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English (en)
Inventor
Jean-Paul Noel
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Group IV Semiconductor Inc
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Group IV Semiconductor Inc
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    • C—CHEMISTRY; METALLURGY
    • C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
    • C09K—MATERIALS FOR MISCELLANEOUS APPLICATIONS, NOT PROVIDED FOR ELSEWHERE
    • C09K11/00—Luminescent, e.g. electroluminescent, chemiluminescent materials
    • C09K11/08—Luminescent, e.g. electroluminescent, chemiluminescent materials containing inorganic luminescent materials
    • C09K11/64—Luminescent, e.g. electroluminescent, chemiluminescent materials containing inorganic luminescent materials containing aluminium
    • C09K11/641—Chalcogenides
    • C09K11/642—Chalcogenides with zinc or cadmium
    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
    • H05B33/00—Electroluminescent light sources
    • H05B33/12—Light sources with substantially two-dimensional radiating surfaces
    • H05B33/14—Light sources with substantially two-dimensional radiating surfaces characterised by the chemical or physical composition or the arrangement of the electroluminescent material, or by the simultaneous addition of the electroluminescent material in or onto the light source

Definitions

  • the present invention relates to thin film electro-luminescent device including an active layer structure comprising a direct-bandgap semiconductor material, such as a zinc-oxide (ZnO) or a ZnO alloy, with a dopant for populating the direct bandgap semiconductor material with free-exciton binding centers in concentrations above native defect concentration, and in particular to an active layer structure comprising a seed layer and an overlying layer.
  • a direct-bandgap semiconductor material such as a zinc-oxide (ZnO) or a ZnO alloy
  • ZnS zinc sulphide
  • the emissive phosphor layer interposed between two dielectric layers and operated with electric fields of approximately 2MV/cm, using an ac drive voltage with a frequency of approximately IkHz.
  • the ZnS is activated optically with activator atoms, e.g. manganese (Mn) or rare earth atoms, to achieve intra-atomic transitions by impact excitation, thus creating light in the visible spectrum corresponding to the electron level configuration of the impurity atoms (Mn or rare earth).
  • activator atoms e.g. manganese (Mn) or rare earth atoms
  • Zinc oxide is a multifunctional semiconductor material which has been used in various areas, including phosphors, piezoelectric transducers, surface acoustic wave devices, gas sensors, and varistors. With a band gap of approximately 3.3 eV, ZnO is similar to that of Gallium Nitride (GaN), but with a higher free-exciton binding energy of 60 meV, compared to 25 meV for GaN, thereby favoring efficient free-exciton emission at room temperature. Free-excitons are coupled electron-hole pairs not bound to anything else other than themselves, i.e. they are perfect electric dipoles. In a semiconductor, they are equivalent to efficiently stored potential (light) energy, akin to a "light capacitor”.
  • the high free-exciton binding energy in ZnO means that free-excitons can exist in ZnO at temperatures up to approximately 700 °K, or 430 °C, at which point they begin to "boil” apart and free-exciton recombination can no longer occur. Accordingly, ZnO is a promising material for light emitting devices that are both efficient and practical at room temperature. In comparison, the low free-exciton binding energy in GaN, i.e. 25 meV, results in the free-excitons "boiling" apart at or below room temperature, making GaN unsuitable for free-exciton light emission.
  • ZnO zinc oxide film
  • ITO Indium-tin oxide
  • US$700/kg cf. for indium compared to US$4.00/kg for Zn a transparent conducting oxide
  • Zinc-oxide films have been synthesized by numerous methods, such as metal-organic chemical vapor deposition, molecular beam epitaxy, magnetron sputtering, pulsed laser deposition, atomic layer deposition, spray pyrolysis. Low temperature deposition is required in most flat-panel processes in order to avoid reactive and elemental diffusion of different layers and to protect substrates, such as polymers. Among these methods, ZnO films can be synthesized at temperature as low as 100 °C by metal-organic chemical vapor deposition and atomic layer deposition, and even at room temperature by magnetron sputtering and pulsed laser deposition. The high kinetic energies of growing precursors in the last two methods are believed to play a key role in the realization of low temperature deposition critical to the flat panel display industry.
  • the required material properties for producing ZnO films suitable as an efficient light emitter, as opposed to a TCO, are more stringent, which has hampered the development of ZnO light emitters.
  • the main issue has been the formation of undesirable native defects in ZnO, e.g. vacancies and interstitials of both Zinc and Oxygen atoms, which are deep-level defects that reduce the efficiency of emission at the bandgap energy by trapping the free excitons and substantially reducing the energy of any subsequent radiative emission, or favoring non-radiative emission, i.e. stored bandgap energy is lost to other undesirable pathways such as heat.
  • Reducing (during process) and maintaining (post-process) the undesirable deep-level defect concentration to low values, while simultaneously providing (during process) an appropriate concentration of desirable shallow optical binding centers to prevent the free excitons from migrating to the deep-level defects, are the key elements needed to enable bandgap (or near bandgap) radiative recombination to dominate.
  • An object of the present invention is to overcome the shortcomings of the prior art by providing an electro-luminescent structure comprising a direct-bandgap semiconductor material, such as including zinc oxide, doped with exciton binding centers, while reducing the concentration of deep-level defects and controlling the concentration of shallow optical binding centers.
  • a direct-bandgap semiconductor material such as including zinc oxide, doped with exciton binding centers
  • the present invention relates to a thin film electro-luminescent (TFEL) device comprising:
  • an active layer structure comprised of a seed layer of a polycrystalline direct bandgap semiconductor material doped with exciton binding centers supported by the substrate and forming a template, and an overlying columnar layer of direct bandgap semiconductor doped with exciton binding centers disposed on the template;
  • electrodes for applying an electric field to the active layer.
  • the present invention relates to a method of fabricating a thin film electroluminescent (TFEL) device comprising the steps of:
  • step b) comprises: depositing a seed layer of zinc oxide doped with exciton binding centers forming hexagonal grains; and depositing an overlying layer of zinc oxide doped with exciton binding centers that grows in a columnar structure using the underlying seed layer as a template.
  • Figure Ia is a cross-sectional view of an electro-luminescent device in accordance with the present invention.
  • Figure Ib is a cross-sectional view of an active layer of the EL device of Figure Ia;
  • Figure Ic is a transmission electron micrograph in plan view for a ZnO seed layer
  • FIGS. 2a to 2f illustrate the fabrication method in accordance with the present invention.
  • Figure 3 is a plot of light output vs. wavelength for the device of Figs. Ia and Ib.
  • the present invention relates to a new class of thin film electro-luminescent (TFEL) devices using a direct-bandgap semiconductor material, such zinc oxide (ZnO) or a ZnO alloy, e.g. with beryllium, cadmium and magnesium, as the emissive layer, which has been designed, fabricated and operated to produce near-band-edge ultra-violet (UV) emission of high spectral purity, e.g. exhibiting peak emission at ⁇ 385nm (3.22eV) and a peak width of - 200meV at half maximum, and a radiometric power of ⁇ 10 ⁇ W/cm 2 .
  • a direct-bandgap semiconductor material such zinc oxide (ZnO) or a ZnO alloy, e.g. with beryllium, cadmium and magnesium
  • UV near-band-edge ultra-violet
  • the UV emission mechanism from ZnO is excitonic in nature, and not related to intra-atomic transitions caused by impact excitation, as in traditional TFEL. Since impact excitation (with relatively low cross-section) is not involved in the present ZnO TFEL devices, and the radiative lifetime of excitons in ZnO is only 300 p.s., compared with approximately 2 ms for Mn and rare earth atoms in ZnS, the near-band emission from ZnO TFEL devices has the potential to achieve significantly higher optical output power and higher power conversion efficiency than traditional ZnS-based TFEL devices. Moreover, the high binding energy, i.e.
  • free excitons are electric dipoles, their center-of-mass motion is not affected by electric fields within the device structure.
  • the center-of-mass motion of free excitons is governed instead by local energy gradients formed by alloy bandgap fluctuations and gradations (whether they are engineered or naturally-occurring), surface states, and points defects, such as donor or acceptor impurities, which can capture and bind the electron or the hole comprising the free exciton with a binding energy less than the energy required to dissociate the free exciton, e.g. approximately 60meV for ZnO.
  • the emission mechanism of a high-field TFEL is excitonic, it should therefore exhibit inherently higher optical output power and power conversion efficiencies. Compare the linear processes: [ZnO impact ionization - ⁇ > ZnO exciton emission] versus [ZnS impact ionization -> hot electron impact excitation of Mn or rare earth atoms] for the traditional TFEL approach.
  • the high binding energy of ZnO excitons e.g. approximately 60meV, or 430 0 C thermal equivalent, also permits efficient TFEL operation well above room temperature, which is in contrast to other direct-gap semiconductors, such as GaN with a binding energy of 25meV, or 20 °C, and ZnS with a binding energy of 35 meV, or 137 0 C, in which excitons, even if they are formed with higher probability in a lower-field environment, could not exist due to dissociation at high temperature.
  • direct-gap semiconductors such as GaN with a binding energy of 25meV, or 20 °C, and ZnS with a binding energy of 35 meV, or 137 0 C
  • an electro-luminescent device 1 in accordance with the present invention includes a conducting substrate 2, preferably comprising silicon, with a metal contact layer 3 supported on one side thereof.
  • An additional dielectric layer (not shown) can be disposed between the active layer 4 and the electrode layer 7.
  • the light emitting wells 9 are isolated from the conducting portions of the substrate 2 by field dielectric regions 11 disposed directly below the metal contacts 8. By placing the field dielectric regions 11 below the metal contacts 8, there is no current injection directly under the metal contacts 8 as the underlying thick field oxide regions 11 , represents a barrier to current flow. Accordingly, an optically active region of the active layer structure 4, wherein any current injection via the transparent electrode layer 7 contributes to the generation of light, is confined only to the light emitting well 9, between the field dielectric regions 11. Doc No: 135-10 PCT
  • the dielectric layer 6 is 1 ⁇ m thick and comprised of silicon dioxide (SiO 2 ), but other dielectric layers and thicknesses, e.g. between 2 nm and 10 ⁇ m are feasible.
  • Silicon nitride (Si 3 N 4 ) prepared by low pressure chemical vapor deposition, is more suitable than SiO 2 due to the lower diffusion constant of Zn, thereby reducing void formation at the ZnO-dielectric interface due to high temperature processing; however, aluminum oxide, yttrium oxide, and hafnium oxide are some other possibilities for the dielectric layer 6.
  • the dielectric layer 6 protects the active layer 4 from the electrodes 3 and 7 and from the conductive substrate 2, and has the ability to change the electron energy distribution of hot electrons passing therethrough, i.e. cooling the electrons.
  • a reflective layer 12 can be provided between the substrate 2 and the contact layer 3, if the substrate 2 is transparent, or between the substrate 2 and the dielectric layer 6, to reflect light back through the active layer 4 and out the top of the well 9, as shown by arrow 13, to ensure maximum light emission efficiency of the device 1.
  • the active layer 4 is doped with exciton binding centers between 0.001 at% and 30 at%, preferably 0.1 to 1 atomic percent, and most preferably 0.4 atomic percent, in order to provide optical binding centers to the free excitons when they are formed.
  • the exciton binding centers prevent free excitons from diffusing toward and recombining at native defect centers, e.g. Zn and O vacancies and interstitials, which are known to be in relatively high equilibrium concentrations even in good-quality ZnO due to the high bandgap energy.
  • the exciton binding centers are one or more of the elements selected from the group consisting of boron, aluminum, gallium, indium, thallium, nitrogen, phosphorous, arsenic, antimony, and bismuth, but preferably aluminum as herein described.
  • the device 1, illustrated without the electrode structures includes the active layer 4 comprised of a stack made up of 1) a relatively thin seed layer 16, having a thickness of between 5 nm and 200 nm, of a direct-bandgap semiconductor material, such as a zinc-oxide (ZnO) or a ZnO alloy, doped with exciton binding centers, such as aluminum, deposited, e.g. by a spin-on process, to form hexagonal grains, and 2) an overlying layer 18 of the direct-bandgap semiconductor material, e.g. zinc oxide, doped with exciton binding centers, e.g. aluminum, e.g. deposited by a sputter process.
  • a direct-bandgap semiconductor material such as a zinc-oxide (ZnO) or a ZnO alloy
  • exciton binding centers such as aluminum, deposited, e.g. by a spin-on process, to form hexagonal grains
  • the overlying layer 18 has a thickness greater than the seed layer 16, e.g. more than 5 nm to 800 nm, and limited only by film stress and adhesion considerations, which grows in a columnar structure using the underlying seed layer 16 as a template.
  • the micrograph, of Figure Ic clearly shows the hexagonal nature of the ZnO seed layer 16.
  • the dark grains are ones in which the crystal axis is a few degrees off normal, which causes the electron diffraction condition during imaging to be changed.
  • the parallel lines seen in some grains is a type of crystal defect called "twinning", which is further proof of the single crystal nature of the individual grains in the seed layer 16.
  • the process will work only for a seeded structure, since a columnar structure cannot be "annealed” into existence when the entire stack 4 has already been deposited.
  • the key is the columns are formed as-grown, due to the presence of the seed layer 16 having atomic steps, following a high temperature anneal, e.g. 900 0 C and 1200 0 C.
  • the columnar structure of the active layer stack 4 is essential to efficient device operation.
  • the device 1 works by generating a plurality of electrons by impact ionization to start the process of exciton formation. If the linear path of electron travel within a single column is interrupted with grain boundaries, such as for a non-columnar structure, then the energetic electrons will encounter defects at the boundaries, reducing efficiency. In other words, the single path traced by an electron, and the hundreds of new electrons it generates by impact ionization, should remain in single crystal material, i.e. one column, throughout its acceleration in the high electric field, which can only be achieved with a columnar structure 18 in the active layer 4.
  • the electrode layer 7 is preferably a transparent conducting oxide (TCO) comprised of zinc oxide doped with aluminum (ZnO: Al), which is deposited by sputtering at temperatures less than approximately 400°C so as to retain its electrical conductivity.
  • the transparent electrode layer 7 can be other TCO 's, such as indium tin oxide (ITO).
  • ITO indium tin oxide
  • the high electron concentration provided by the TCO provides a significant source of electrons to initiate impact ionization in the active layer 4 when the field strength reaches threshold during bipolar operation.
  • the contact layer 3 and the metal contacts 8 are preferably comprised of aluminum, and are approximately 0.5 ⁇ m thick with a sheet resistance and specific contact resistance of approximately 40 ⁇ /D and 3E-4 ⁇ cm 2 , respectively.
  • a process for manufacturing the device 1 of Figure Ia which emits UV light 13, includes first providing the substrate 2, and then depositing a layer of field dielectric material 21 thereon (Figure 2a). In the next step, a portion of the field dielectric layer 21 is removed forming the field dielectric regions 11 and creating the device well area 9 ( Figure 2b). The deposition and removal steps for the field dielectric layer 21 can be replaced by a single step involving deposition of separate field dielectric regions 11. Then the dielectric layer 6, the active layer 4 and the electrode layer 7 are deposited in sequence, as in Figures 2c, 2d and 2e, respectively.
  • Si 3 N 4 Silicon nitride (Si 3 N 4 ) or silicon dioxide (SiO 2 ) can be used for the dielectric layer 6; however, Si 3 N 4 , prepared by low pressure chemical vapor deposition, is the preferred method, due to the lower diffusion constant of Zn, thereby reducing void formation at the ZnO-dielectric interface due to high temperature processing.
  • Other deposition methods include plasma-enhanced chemical vapor deposition, sputtering, and e-beam evaporation.
  • the field dielectric regions 11 and the dielectric layer 21 can be omitted, if desired.
  • the seed layer 16, in the active layer stack 4, is ideally a polycrystalline direct bandgap semiconductor, e.g. ZnO, layer doped with exciton binding centers, e.g. Al, deposited by a spin-on process to form hexagonal grains, and the overlying, e.g. ZnO: Al, layer 18 is deposited using a sputter process that grows in a columnar structure using the underlying seed layer 16 as a template.
  • a polycrystalline direct bandgap semiconductor e.g. ZnO
  • layer doped with exciton binding centers e.g. Al
  • the overlying, e.g. ZnO: Al, layer 18 is deposited using a sputter process that grows in a columnar structure using the underlying seed layer 16 as a template.
  • the process of seeded ZnO columnar growth during sputtering of the overlying layer 18 can occur without intentional wafer heating in the sputter deposition equipment, i.e. the process occurs at or near room temperature or approximately 20 0 C (between 15 0 C and 25 0 C).
  • High-temperature annealing e.g. between 900 0 C and 1200 0 C, following deposition of active layer 4, produces single-crystal columns with the level of crystalline quality required to obtain efficient electron-hole pair generation during impact ionization, with subsequent exciton formation and recombination, as shown in the TFEL device output in Figure 3.
  • the transparent conducting oxide (TCO) layer 7 is also ZnO: Al deposited onto the active layer 4 by sputtering, but not subsequently annealed at high temperature, so as to retain its electrical conductivity.
  • the anneal temperature of the ZnO: Al TCO layer is less than 400 0 C.
  • the contact layer 3 and the upper metal contacts 8 are mounted on the substrate 2 and TCO layer 7, respectively.
  • the second dielectric layer can be deposited between the active layer 4 and the electrode layer 7, if desired.
  • Figure 3 illustrates an electro-luminescent (EL) spectrum (semi-log scale) of a TFEL device 1 operated with a 200 kHz 150V square wave drive.
  • the area of the device well 9 was 4.4 x 10 "4 cm 2 .
  • a radiometric power output of approximately 10 ⁇ W/cm 2 was measured using a calibrated UV source (deuterium) as a reference.
  • the intensity ratio of the near-band-edge emission (387nm peak wavelength) to native defect emission in the middle of the bandgap is > 80:1, indicating high spectral purity.

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  • Chemical & Material Sciences (AREA)
  • Inorganic Chemistry (AREA)
  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
  • Organic Chemistry (AREA)
  • Electroluminescent Light Sources (AREA)

Abstract

L'invention concerne un dispositif électroluminescent à film mince (TFEL) comprenant une couche active constituée d'un matériau semi-conducteur à bande d'énergie interdite directe, par exemple d'oxyde de zinc, dopé avec des centres liant les excitons, tels que l'aluminium, en petites quantités, par exemple de 0,001 % atomique à 30,0 % atomique. Les centres liant les excitons empêchent les excitons libres, créés par ionisation par choc, de diffuser vers les centres de défaut natifs et de se recombiner avec ceux-ci. Pour obtenir une structure en colonnes, on dépose d'abord une couche de germes polycristallins pour obtenir une matrice, puis on dépose une couche superposée formant des colonnes en suivant la matrice.
PCT/CA2008/001607 2007-09-11 2008-09-11 Dispositifs électroluminescents à film mince d'oxyde de zinc Ceased WO2009033279A1 (fr)

Applications Claiming Priority (2)

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US97137307P 2007-09-11 2007-09-11
US60/971,373 2007-09-11

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WO2009033279A1 true WO2009033279A1 (fr) 2009-03-19

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Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5721562A (en) * 1991-08-20 1998-02-24 Fuji Electric Co., Ltd. Electroluminescent display device including a columnar crystal structure insulating film
JP2000068560A (ja) * 1998-08-24 2000-03-03 Casio Comput Co Ltd 電界発光素子
EP0818943B1 (fr) * 1996-07-09 2005-02-23 Sony Corporation Dispositif électroluminescent
CA2635307A1 (fr) * 2005-12-28 2007-07-05 Group Iv Semiconductor Inc. Structure de pixels pour dispositif electroluminescent a semi-conducteur
CA2635303A1 (fr) * 2005-12-28 2007-07-05 Group Iv Semiconductor Inc. Structure technique pour emetteurs de lumiere a l'etat solide

Patent Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5721562A (en) * 1991-08-20 1998-02-24 Fuji Electric Co., Ltd. Electroluminescent display device including a columnar crystal structure insulating film
EP0818943B1 (fr) * 1996-07-09 2005-02-23 Sony Corporation Dispositif électroluminescent
JP2000068560A (ja) * 1998-08-24 2000-03-03 Casio Comput Co Ltd 電界発光素子
CA2635307A1 (fr) * 2005-12-28 2007-07-05 Group Iv Semiconductor Inc. Structure de pixels pour dispositif electroluminescent a semi-conducteur
CA2635303A1 (fr) * 2005-12-28 2007-07-05 Group Iv Semiconductor Inc. Structure technique pour emetteurs de lumiere a l'etat solide

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