EP0270392B1 - Electroluminescent display device using amorphous hydrogenated and carbonated silicon - Google Patents

Electroluminescent display device using amorphous hydrogenated and carbonated silicon Download PDF

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EP0270392B1
EP0270392B1 EP87402320A EP87402320A EP0270392B1 EP 0270392 B1 EP0270392 B1 EP 0270392B1 EP 87402320 A EP87402320 A EP 87402320A EP 87402320 A EP87402320 A EP 87402320A EP 0270392 B1 EP0270392 B1 EP 0270392B1
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layer
display device
electroluminescent
photoconductive layer
photoconductive
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EP0270392A1 (en
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Pascal Thioulouse
Ionel Solomon
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France Telecom R&D SA
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    • 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

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  • the present invention relates to an electroluminescent display device using hydrogenated and carbonated amorphous silicon.
  • the device of the invention is not necessarily a memory effect, as will be better understood later, it is all the same this property which is most often sought in practice. We can therefore briefly recall what it consists of. It is said that a display device has a memory effect if its electro-optical characteristic (luminance-voltage curve) has hysteresis. For the same voltage located inside the hysteresis loop, the device could thus have two stable beings: is on or on. Plasma and alternately excited screens have such a characteristic of bistability, which is currently widely used.
  • a memory effect display to display a still image, it is sufficient to apply simultaneously and continuously to the entire screen a so-called maintenance voltage.
  • the latter can be a sinusoidal signal or in the form of slots for example.
  • the form and frequency of this maintenance signal can be chosen independently of the complexity of the screen, in particular the number of lines of display points. There is therefore in principle no limit to the complexity of a memory effect display screen.
  • ACTFEL capacitive coupling
  • the PC-EL memory effect the principle of which is as follows.
  • the photoconductor When the device is in the steady state, the photoconductor is not very conductive and retains a large part of the voltage V applied to the assembly. If one increases V to a value Von such that the voltage present at the terminals of the electroluminescent layer exceeds the electroluminescence threshold, the PC-EL device switches to the on state. The photoconductor is then lit by the electroluminescent structure and goes into the conductive state. The voltage across its terminals drops and this results in an increase in the voltage available for the electroluminescent structure. To switch off a PC-EL device, it suffices to reduce the total voltage V to a value Voff lower than Von: this gives a luminance-voltage characteristic comprising a hysteresis.
  • FIG. 1 It comprises a glass substrate 10 on which is deposited an electrode 12, for example made of ITO (tin and indium oxide), a first dielectric layer 14, an electroluminescent layer 16, for example in ZnS: Mn, a second dielectric layer 18, a photoconductive layer 20 consisting of a stack of layers n + - nn + , made of amorphous silicon hydrogenated a-Si: H and finally an electrode 22, for example of aluminum.
  • the electrodes 12 and 24 are connected to a voltage source 24.
  • the layers PC and EL are thin layers, the thickness of which is of the order of a micron.
  • the n + layers, heavily doped n and of very small thickness have the role of allowing a quasi-ohmic electronic injection into the so-called intrinsic n layer.
  • this quasi-ohmic injection is obtained, it is the electrical and photoconductive characteristics of the intrinsic layer which first determine the behavior of the stack n + - nn + , called here "photoconductive layer", and the memory characteristics. of the PCEL device.
  • Such a structure is simple to produce because it does not require an optical screen or additional etching steps. Furthermore, the current-voltage behavior of the thin layer photoconductor in the dark is highly non-linear and reproducible. The beneficial consequences are that the electrical ignition of the device is always easy, that the hysteresis depends only slightly on the excitation frequency and that the reproducibility of the hysteresis margin from one manufacturing to another is guaranteed. .
  • the inventors have also studied the question of the sensitivity spectrum of a photoconductive material. This spectrum is directly linked to the width of the forbidden band of the material used, its intensity depends on the characteristics of the centers of recombination of the electron-hole pairs generated by photo-excitation (energy depth, effective capture section, trapping time, etc. .). For an optimal PC-EL memory effect, it would be desirable to adapt the spectrum of the photoconductor to that of the electroluminescent structure, so as to improve the optical coupling between the EL and PC layers. But in a system using a-Si: H, there is no possibility of carrying out this adaptation.
  • This material is hydrogenated and carbonated amorphous silicon whose formula is a-Si x C 1-x : H.
  • the invention relates to an electroluminescent display device comprising, on an insulating support an electroluminescent layer and a photoconductive layer, these layers being stacked one on the other, all of these two layers being interposed between two systems of electrodes connected to an electric voltage source allowing the excitation of certain zones of the light-emitting layer, this device being characterized by the fact that the photoconductive layer is made of hydrogenated and carbonated amorphous silicon ⁇ -SixC, _x : H with 1-x between 0.05 and 0.50.
  • the photoconductive layer will consist of an intrinsic layer n of hydrogenated and carbonated amorphous silicon a-Si x C 1-x : H, optionally inserted between two necks. ches n + of quasi-ohmic injection, also in hydrogenated and carbonized amorphous silicon a-Si y C 1-y : H.
  • Photoconductivity drops at short wavelengths (high energies) due to the absorption of radiation in the material.
  • a characteristic of the photoconductivity spectrum of a-SixC 1-x : H is the energy E 04 (in Ev) at which the absorption coefficient a is 10 4 cm -1 . It is this energy which is represented on figure 3,
  • 1-x is preferably between 0.05 and 0.50.
  • the carbon concentration [C] / [C] + [Si] is between 5% and 50%.
  • the emission wavelengths used for the polychrome display range from approximately 450 nm for blue to approximately 640 nm for red.
  • An adaptation to such spectra can be obtained by taking 1-x equal to 0.50 for blue and 0.05 for red, respectively.
  • the photoconductive layer can give an electroluminescent structure an excellent display contrast due to the accompanying "black layer” effect.
  • the photoconductive layer in fact masks the rear aluminum electrodes, absorbs ambient light and prevents its reflection on the electrodes.
  • the principle of using an absorbent layer is naturally known. It is described for example in the publication of the inventors cited above, as well as in the American patent US-A-3,560,784. But in these previous documents, the "black layer” is sometimes a-Si: H, sometimes a dielectric, in other words bodies of composition, therefore of property, given. We cannot therefore play freely on the optical properties of these bodies.
  • the refractive index in a wide range (2.0 to 3.6) by acting on 1-x and optically adapt the black layer to the other layers, for example to the dielectric layer which adjoining and which may be Ta 2 O 5 with index 2.1 or with the electroluminescent layer, which may be ZnS with index 2.35. This minimizes the reflection of ambient light by the photoconductive layer-underlying layer (insulator or electroluminescent layer).
  • a layer of a-Si: H with phosphorus (P) makes it possible to very significantly increase the density of free carriers: up to 10 18 ⁇ 10 19 cm- 3 . It is this high density of free carriers which ensures quasi-ohmic electronic injection into the intrinsic layer.
  • the ohmic conductivity of such layers n + is so high (10 -2, 10 -3 ⁇ -1 cm -1) it causes lateral parasitic conduction phenomena in PCEL materials screens.
  • the incorporation of C into the n + layer makes it possible to significantly reduce the conductivity without significantly modifying the density of free carriers.
  • the incorporation of carbon into the a-Si: H has significant positive effects both for the intrinsic layer n and for the layers n + possibly introduced into the photoconductive structure.
  • the layers of hydrogenated and carbonated amorphous silicon are deposited by glow discharge ("glow discharge") from a mixture of silane (SiH 4 ) and methane (CH 4 ).
  • glow discharge a mixture of silane (SiH 4 ) and methane (CH 4 ).
  • CH 4 mixture content varies in a range from 0 to 60%
  • the carbon 1-x content in the deposited layer varies from 0 to 0.2.
  • FIGS. 2 and 3 make it possible to better specify the experimental conditions to be implemented to obtain certain performances.
  • the abscissa axis corresponds to the concentration C of methane to be used in the methane-silane gas mixture.
  • the ordinate axis corresponds to the refractive index n in FIG. 2 and to the energy E04 of the absorption band of the photoconductor expressed in electron volts in FIG. 3.
  • adjusting 1-x to optimize the DOS, resistivity, photoconductivity, spectrum, index, etc. does not in any way exclude the adjustment of the operating conditions of deposition (temperature of the substrate, power of the plasma, etc.). This adjustment makes it possible to perfect the effects of the presence of carbon, or on the contrary, to compensate for this action. This is the case, for example, when 1-x has been chosen to adjust the photoconductivity spectrum and / or the index and we also want to corrode the DOS, the resistivity and the photoconductivity which result from this choice.
  • n + layers of a-Si 1-y C y : H will be obtained for example by adding to the mixture of [SiH 4 ] - [CH 4 ] an adequate concentration of [PH 3] (typically 0.5%).
  • the invention is applicable to any type of electroluminescent structure in thin layers or based on powder, and with continuous or alternative excitation, this although the example described relates to electroluminescence in thin layers with alternative excitation.

Description

La présente invention a pour objet un dispositif d'affichage électroluminescent utilisant du silicium amorphe hydrogéné et carboné.The present invention relates to an electroluminescent display device using hydrogenated and carbonated amorphous silicon.

Bien que le dispositif de l'invention ne soit pas nécessairement à effet mémoire, comme on le comprendra mieux par la suite, c'est tout de même cette propriété qui est le plus souvent recherchée dans la pratique. On peut donc rappeler brièvement en quoi elle consiste. On dit qu'un dispositif d'affichage est à effet mémoire si sa caractéristique électro-optique (courbe luminance-tension) présence un hystérésis. Pour une même tension située à l'intérieur de la boucle d'hystérésis, le dispositif pourra ainsi avoir deux étants stables: étient ou allumé. Les écrans à plasma et à excitation alternative présentent une telle caractéristique de bistabilité, qui est aujourd'hui couramment exploitée.Although the device of the invention is not necessarily a memory effect, as will be better understood later, it is all the same this property which is most often sought in practice. We can therefore briefly recall what it consists of. It is said that a display device has a memory effect if its electro-optical characteristic (luminance-voltage curve) has hysteresis. For the same voltage located inside the hysteresis loop, the device could thus have two stable beings: is on or on. Plasma and alternately excited screens have such a characteristic of bistability, which is currently widely used.

Les avantages d'un affichage à effet mémoire sont appréciables: pour afficher une image fixe, il suffit d'appliquer simultanément et continûment à tout l'écran une tension dite d'entretien. Cette dernière peut être un signal sinusoïdal ou en forme de créneaux par exemple. Mais surtout, la forme et la fréquence de ce signal d'entretien peuvent être choisies indépendamment de la complexité de l'écran, notamment du nombre de lignes de points d'affichage. Il n'y a donc en principe pas de limite à la complexité d'un écran d'affichage à effet mémoire. Ainsi, on trouve sur le marché des écrans à plasma et à excitation alternative de 1200x1200 pixels.The advantages of a memory effect display are appreciable: to display a still image, it is sufficient to apply simultaneously and continuously to the entire screen a so-called maintenance voltage. The latter can be a sinusoidal signal or in the form of slots for example. But above all, the form and frequency of this maintenance signal can be chosen independently of the complexity of the screen, in particular the number of lines of display points. There is therefore in principle no limit to the complexity of a memory effect display screen. Thus, one finds on the market plasma screens and with alternating excitation of 1200x1200 pixels.

Par ailleurs, la technologie de l'affichage par électroluminescence en couches minces et à couplage capacitif (en abrégé ACTFEL) est maintenant parvenue pratiquement à maturité sur le plan industriel. On peut doter cette technologie d'un effet mémoire dit inhérent mais au prix d'une dégradation sensible des performances électro-optiques. Une méthode plus attrayante consiste à connecter une structure photoconductrice (PC) en série avec une structure électroluminescente (EL) et à coupler optiquement ces deux structures.In addition, the technology of display by thin-film electroluminescence and with capacitive coupling (abbreviated as ACTFEL) has now practically reached maturity on an industrial level. This technology can be endowed with a so-called inherent memory effect, but at the cost of a significant deterioration in electro-optical performance. A more attractive method is to connect a photoconductive structure (PC) in series with an electroluminescent structure (EL) and to optically couple these two structures.

Un tel dispositif est décrit par exemple dans l'article de A. H. Kitai et G. J. Wolga intitulé "Hysteretic Thin Film EL Devices Utilizing Optical Coupling of El Output to a Series Photoconductor" publié dans les Comptes Rendus de la Conférence SID 84, pages 255, 256.Such a device is described for example in the article by AH Kitai and GJ Wolga entitled "Hysteretic Thin Film EL Devices Utilizing Optical Coupling of El Output to a Series Photoconductor" published in the Proceedings of the SID Conference 84, pages 255, 256 .

On peut ainsi produit un effet mémoire de type extrinsèque que l'on appellera effet mémoire PC-EL- dont le principe est le suivant. Quand le dispositif est dans l'état étient, le photoconducteur est peu conducteur et retient une partie importante de la tension V appliquée à l'ensemble. Si l'on augmente V jusqu'à une valeur Von telle que la tension présente aux bornes de la couche électroluminescente excède le seuil d'électroluminescence, le dispositif PC-EL bascule dans l'état allumé. Le photoconducteur est alors éclairé par la structure électroluminescente et passe à l'état conducteur. La tension à ses bornes chute et il en résulte une augmentation de la tension disponible pour la structure électroluminescente. Pour éteindre un dispositif PC-EL, il suffit de diminuer la tension totale V jusqu'à une valeur Voff inférieure à Von: on obtient ainsi une caractéristique luminance-tension comportant un hystérésis.It is thus possible to produce a memory effect of the extrinsic type which will be called the PC-EL memory effect, the principle of which is as follows. When the device is in the steady state, the photoconductor is not very conductive and retains a large part of the voltage V applied to the assembly. If one increases V to a value Von such that the voltage present at the terminals of the electroluminescent layer exceeds the electroluminescence threshold, the PC-EL device switches to the on state. The photoconductor is then lit by the electroluminescent structure and goes into the conductive state. The voltage across its terminals drops and this results in an increase in the voltage available for the electroluminescent structure. To switch off a PC-EL device, it suffices to reduce the total voltage V to a value Voff lower than Von: this gives a luminance-voltage characteristic comprising a hysteresis.

Une nouvelle structure PC-EL a été décrite récemment dans le document FR-A-2 574 972 et dans l'article des inventeurs initulé "Monolithic AC-EL Photoconductor Thin Film Structure with Extrinsic Memory by Optical Coupling" et publié dans les Comptes rendus de l'International Display Research Conference de 1985, pages 177-181.A new PC-EL structure was recently described in document FR-A-2 574 972 and in the inventors' article entitled "Monolithic AC-EL Photoconductor Thin Film Structure with Extrinsic Memory by Optical Coupling" and published in the Reviews. from the 1985 International Display Research Conference, pages 177-181.

Cette structure est illustrée sur la figure 1. Elle comprend un substrat de verre 10 sur lequel est déposée une électrode 12, par exemple en ITO (oxyde d'étain et d'indium), une première couche diélectrique 14, une couche électroluminescente 16, par exemple en ZnS:Mn, une seconde couche diélectrique 18, une couche photoconductrice 20 constituée d'un empilement de couches n+-n-n+, en silicium amorphe hydrogéné a-Si:H et enfin une électrode 22, par exemple en aluminium. Les électrodes 12 et 24 sont reliées à une source de tension 24. Dans cette réalisation les couches PC et EL sont des couches minces, dont l'épaisseur est de l'ordre du micron. On rapelle que les couches n+, fortement dopées n et de très faible épaisseur (20 nm typiquement) ont pour rôle de permettre une injection électronique quasi-ohmique dans la couche n dite intrinsèque. Pourvu que cette injection quasi-ohmique soit obtenue, ce sont les caractéristiques électriques et photoconductrices de la couche intrinsèque qui déterminent en premier lieu le comportement de l'empilement n+-n-n+, appelé ici "couche photoconductrice", et les caractéristiques de mémoire du dispositif PCEL.This structure is illustrated in FIG. 1. It comprises a glass substrate 10 on which is deposited an electrode 12, for example made of ITO (tin and indium oxide), a first dielectric layer 14, an electroluminescent layer 16, for example in ZnS: Mn, a second dielectric layer 18, a photoconductive layer 20 consisting of a stack of layers n + - nn + , made of amorphous silicon hydrogenated a-Si: H and finally an electrode 22, for example of aluminum. The electrodes 12 and 24 are connected to a voltage source 24. In this embodiment, the layers PC and EL are thin layers, the thickness of which is of the order of a micron. It is recalled that the n + layers, heavily doped n and of very small thickness (typically 20 nm) have the role of allowing a quasi-ohmic electronic injection into the so-called intrinsic n layer. Provided that this quasi-ohmic injection is obtained, it is the electrical and photoconductive characteristics of the intrinsic layer which first determine the behavior of the stack n + - nn + , called here "photoconductive layer", and the memory characteristics. of the PCEL device.

Une telle structure est simple à réaliser car elle ne nécessite pas d'écran optique ni d'étapes de gravure supplémentaires. Par ailleurs, le comportement courant-tension du photoconducteur en couche mince dans l'obscurité est fortement non- linéaire et reproductible. Les conséquences bénéfiques en sont que l'allumage électrique du dispositif est toujours aisé, que l'hystérésis ne dépend que faiblement de la fréquence d'excitation et que la reproductibilité de la marge d'hystérésis d'une fabrication à l'autre est garantie.Such a structure is simple to produce because it does not require an optical screen or additional etching steps. Furthermore, the current-voltage behavior of the thin layer photoconductor in the dark is highly non-linear and reproducible. The beneficial consequences are that the electrical ignition of the device is always easy, that the hysteresis depends only slightly on the excitation frequency and that the reproducibility of the hysteresis margin from one manufacturing to another is guaranteed. .

A la suite de ces premiers résultats encourageants, les inventeurs ont poursuivi leurs travaux pour mieux saisir les phénomènes mis en jeu dans de telles structures et pour mieux définir les contraintes imposées par la structure PC-EL. Ils ont pu ainsi éconcer clairement les conditions à satisfaire pour obtenir un dispositif performant:

  • 1. Il est préférable que la couche photoconductrice soit le plus mince possible (épaisseur inférieure à 2 Ilm) de manière à limiter les perturbations qu'elle risque de produire sur la structure électroluminescente sur laquelle elle est déposée. Ces perturbations consistent par exemple en contraintes mécaniques pouvant induire un décollement des couches ou en une mauvaise cicatrisation des claquages électriques intervenant dans la structure électroluminescente.
  • 2. Le fait que la couche photoconductrice doive supporter, dans l'étant éteint, une tension de 30 à 50 Volts appliquée perpendiculairement au plan des couches, associé à la contrainte évoquée ci- avant d'une épaisseur faible, impose à la couche photoconductrice de pouvoir supporter des champs électriques pouvant aller jusqu'à 106 V/ cm. Le matériau doit donc être d'une résistivité élevée.
  • 3. Il convient de fixer la photoconductivité à une valeur suffisamment faible pour annuler pratiquement toute influence de la lumière ambiante sur le fonctionnement du dispositif PC-EL en tirant parti au mieux de l'écart important entre le niveau d'éclairement ambiant (inférieur à 1000 lux typiquement) et celui qui est engendré par la couche électroluminescente (de l'ordre de 20000 lux typiquement).
  • 4. Les mécanismes gouvernant la conduction dans a-Si:H, posent également certains problèmes. Des études théoriques ont été publiées sur ce sujet où il apparaît que le mécanisme de conduction dans des structures n+-n-n+ en a-Si:H est du type "conduction limitée par la charge d'espace" ou SCLC en abrégé (pour Space Charge Limited Conduction). Cela signifie que la conduction dans la couche n dépend bien sûr de la résistivité R de la couche en régime ohmique, mais aussi et surtout de la charge d'espace distribuée dans toute la profondeur de la couche. Dans un article de I. Solomon et al, intitulé "Space-Charge-Limited Conduction for the deter- mination of the midgap density of states in amorphous silicon: Theory and Experiment" publié dans "The American Physical Society", vol. 6, No. 6, pp. 3422-3429, les auteurs ont défini un modèle théorique précis pour le comportement courant-tension (I-V) d'une structure n+-n-n+ en régime de quasi-équilibre (tension appliquée continue). Ils ont aussi déterminé l'influence de la résistivité R et de la densité d'états au quasi-niveau de Fermi appelée (DOS) de la couche n sur la courbe I-V. La formule suivante permet de rendre compote approximativement de cette dépendance:
    Figure imgb0001
    où L est l'épaisseur de la couche n, g(EF) la DOS au quasi-niveau de Fermi et a une constante.
Following these first encouraging results, the inventors continued their work to better understand the phenomena involved in such structures and to better define the constraints imposed by the PC-EL structure. They were thus able to clearly state the conditions to be satisfied in order to obtain an efficient device:
  • 1. It is preferable that the photoconductive layer is as thin as possible (thickness less than 2 μm) so as to limit the disturbances that it risks producing on the electroluminescent structure on which it is deposited. These disturbances consist for example of mechanical stresses which can induce a take-off layers or poor healing of electrical breakdowns occurring in the electroluminescent structure.
  • 2. The fact that the photoconductive layer must withstand, in the extinguished state, a voltage of 30 to 50 volts applied perpendicular to the plane of the layers, associated with the stress mentioned above of a small thickness, imposes on the photoconductive layer to be able to withstand electric fields of up to 10 6 V / cm. The material must therefore be of high resistivity.
  • 3. The photoconductivity should be fixed at a sufficiently low value to cancel practically any influence of ambient light on the functioning of the PC-EL device by making the best use of the large difference between the level of ambient illumination (less than 1000 lux typically) and that generated by the electroluminescent layer (typically around 20 000 lux).
  • 4. The mechanisms governing conduction in a-Si: H also pose certain problems. Theoretical studies have been published on this subject where it appears that the conduction mechanism in n + - nn + structures in a-Si: H is of the type "conduction limited by space charge" or SCLC for short (for Space Charge Limited Conduction). This means that the conduction in layer n depends of course on the resistivity R of the layer in ohmic regime, but also and above all on the space charge distributed throughout the depth of the layer. In an article by I. Solomon et al, entitled "Space-Charge-Limited Conduction for the determination of the midgap density of states in amorphous silicon: Theory and Experiment" published in "The American Physical Society", vol. 6, No. 6, pp. 3422-3429, the authors defined a precise theoretical model for the current-voltage behavior (IV) of an n + - nn + structure in a quasi-equilibrium regime (continuous applied voltage). They also determined the influence of the resistivity R and the density of states at the quasi-Fermi level called (DOS) of the layer n on the curve IV. The following formula makes it possible to roughly compote this dependence:
    Figure imgb0001
     where L is the thickness of the layer n, g (E F ) the DOS at the quasi-Fermi level and has a constant.

De ces résultats théoriques (qui, il faut le souligner, ne concernent en rien l'application aux dispositifs PC-EL), les inventeurs ont déduit que, pour un courant de conduction donné (cas du fonctionnement du PC dans une structure PC-EL à l'état éteint), la tension aux bornes de la structure n+-n-n+ est pratiquement proportionnelle à la DOS, d'une part, et au logarithme de la résistivité d'autre part.From these theoretical results (which, it should be emphasized, in no way concern the application to PC-EL devices), the inventors deduced that, for a given conduction current (case of the operation of the PC in a PC-EL structure in the off state), the voltage across the structure n + - nn + is practically proportional to the DOS, on the one hand, and to the logarithm of the resistivity on the other.

5. Les inventeurs se sont également penchés sur la question du spectre de sensibilité d'un matériau photoconducteur. Ce spectre est directement lié à la largeur de la bande interdite du matériau utilisé em son intensité dépend des caractéristiques des centres de recombinaison des paires électrons-trous engendrées par photo- excitation (profondeur en énergie, section efficace de capture, temps de dépiégeage, etc.). Pour un effet mémoire PC-EL optimal, il serait souhaitable d'adapter le spectre du photoconducteur à celui de la structure électroluminescente, de manière à améliorer le couplage optique entre les couches EL et PC. Mais dans un système utilisant a-Si:H, il n'y a aucune possibilité d'effectuer cette adaptation.5. The inventors have also studied the question of the sensitivity spectrum of a photoconductive material. This spectrum is directly linked to the width of the forbidden band of the material used, its intensity depends on the characteristics of the centers of recombination of the electron-hole pairs generated by photo-excitation (energy depth, effective capture section, trapping time, etc. .). For an optimal PC-EL memory effect, it would be desirable to adapt the spectrum of the photoconductor to that of the electroluminescent structure, so as to improve the optical coupling between the EL and PC layers. But in a system using a-Si: H, there is no possibility of carrying out this adaptation.

En résumé, les inventeurs, s'appuyant tant sur leurs travaux personnels que sur certaines études théoriques effectuées sur a-Si:H, ont pu poser clairement certains problèmes recontrés pour les dispositifs électroluminescents à photoconducteur. Le choix du matériau photoconducteur devrait permettre de commander à des valeurs appropriées:-

  • la résistivité,
  • la densité d'états au quasi-niveau de Fermi,
  • la photoconductivité,
  • le spectre de la couche photoconductrice.
In summary, the inventors, relying both on their personal work and on certain theoretical studies carried out on a-Si: H, were able to clearly pose certain problems encountered for electroluminescent photoconductive devices. The choice of photoconductive material should make it possible to order at appropriate values: -
  • resistivity,
  • the density of states at the quasi-Fermi level,
  • photoconductivity,
  • the spectrum of the photoconductive layer.

Or, les matériaux conventionnels comme CdS ou CdSe ou a-Si:H ne peuvent pas satisfaire à toutes ces conditions de manière reproductible, n'étant intrinsèquement pas suffisamment résistif et trop photoconducteur en général. Pour réaliser la structure PC-EL décrite dans la dernière référence par exemple, les inventeurs avaient dû déposer la couche de a-Si:H sur un substrat maintenu à une température relativement élevée; entre 350° et 400°C. Mais alors la reproducibilité de caractéristiques comme la densité d'états et la résistivité, posait des problèmes considérables, du fait de la très forte dépendance de celles-ci vis- à-vis de la température de dépôt dans cette gamme.However, conventional materials such as CdS or CdSe or a-Si: H cannot satisfy all of these conditions in a reproducible manner, being intrinsically not sufficiently resistive and too photoconductive in general. To produce the PC-EL structure described in the last reference, for example, the inventors had to deposit the layer of a-Si: H on a substrate maintained at a relatively high temperature; between 350 ° and 400 ° C. But then the reproducibility of characteristics such as the density of states and the resistivity, posed considerable problems, because of the very strong dependence of these vis-à-vis the deposition temperature in this range.

Ayant pu poser ainsi clairement le problème, les inventeurs ont trouvé un matériau qui permet de remplir toutes (on tout au moins un grand nombre de) ces conditions. Ce matériau est le silicium amorphe hydrogéné et carboné dont la formule est a-SixC1-x:H.Having thus been able to clearly state the problem, the inventors have found a material which makes it possible to fulfill all (we at least a large number of) these conditions. This material is hydrogenated and carbonated amorphous silicon whose formula is a-Si x C 1-x : H.

De façon plus précise, l'invention a pour objet un dispositif d'affichage électroluminescent comprenant, sur un support isolant une couche électroluminescente et une couche photoconductrice, ces couches étant empilées l'une sur l'autre, l'ensemble de ces deux couches étant intercalé entre deux systèmes d'électrodes connectées à une source de tension électrique permettant l'excitation de certaines zones de la couche électroluminescente, ce dispositif étant caractérisé par le fait que la couche photoconductrice est en silicium amorphe hydrogéné et carboné α-SixC,_x:H avec 1-x compris entre 0,05 et 0,50.More specifically, the invention relates to an electroluminescent display device comprising, on an insulating support an electroluminescent layer and a photoconductive layer, these layers being stacked one on the other, all of these two layers being interposed between two systems of electrodes connected to an electric voltage source allowing the excitation of certain zones of the light-emitting layer, this device being characterized by the fact that the photoconductive layer is made of hydrogenated and carbonated amorphous silicon α-SixC, _x : H with 1-x between 0.05 and 0.50.

Plus précisément, la couche photoconductrice sera constituée d'une couche intrinsèque n de silicium amorphe hydrogéné et carboné a-SixC1-x:H, insérée éventuellement entre deux couches n+ d'injection quasi-ohmique, aussi en silicium amorphe hydrogéné et carboné a-SiyC1-y:H.More precisely, the photoconductive layer will consist of an intrinsic layer n of hydrogenated and carbonated amorphous silicon a-Si x C 1-x : H, optionally inserted between two necks. ches n + of quasi-ohmic injection, also in hydrogenated and carbonized amorphous silicon a-Si y C 1-y : H.

De toute façon, l'invention sera mieux comprise à la lumière de la descripton qui va suivre, faite en référence à des dessins annexés sur lesquels:

  • la figure 1, déjà décrite, représente une coupe d'un dispositif d'affichage électroluminescent à couche photoconductrice,
  • la figure 2 est une courbe montrant les variations de l'indice de réfraction de la couche photoconductrice en fonction de la concentration en méthane dans le mélange gazeux utilisé pour déposer a-SixC1-x:H,
  • la figure 3 représente la variation du spectre de cette même couche en fonction de cette même concentration.
In any case, the invention will be better understood in the light of the description which follows, made with reference to the appended drawings in which:
  • FIG. 1, already described, represents a section of an electroluminescent display device with a photoconductive layer,
  • FIG. 2 is a curve showing the variations in the refractive index of the photoconductive layer as a function of the methane concentration in the gas mixture used to deposit a-Si x C 1-x : H,
  • FIG. 3 represents the variation of the spectrum of this same layer as a function of this same concentration.

La photoconductivité chute aux courtes longueurs d'onde (énergies élevées) du fait de l'absorption du rayonnement dans le matériau. Une caractéristique du spectre de photoconductivité du a-SixC1-x:H est l'énergie E04 (en Ev) à laquelle le coefficient d'absorption a vaut 104cm-1. C'est cette énergie qui est représentée sur la figure 3,Photoconductivity drops at short wavelengths (high energies) due to the absorption of radiation in the material. A characteristic of the photoconductivity spectrum of a-SixC 1-x : H is the energy E 04 (in Ev) at which the absorption coefficient a is 10 4 cm -1 . It is this energy which is represented on figure 3,

Selon l'invention, 1-x est de préférence compris entre 0,05 et 0,50. Autrement dit, la concentration en carbone [C]/[C]+[Si] est comprise entre 5% et 50%.According to the invention, 1-x is preferably between 0.05 and 0.50. In other words, the carbon concentration [C] / [C] + [Si] is between 5% and 50%.

Le choix plus précis de 1-x dans cette plage dépend des buts poursuivis et des applications envisagées. On peut définir 4 plages différentes, comprises dans la fourchette 0,05-0,50:

  • 1. Si 1-x est augmenté de 0 à 0,35, la DOS et R augmentent, avec un optimum situé autour de 0,10.
  • 2. Si 1-x est augmenté de 0,10 à 0,35, la photoconductivité diminue; un optimum peut être défini en fonction du point précédent.
  • 3. Si 1-x est augmenté de 0 à 0,50, le spectre de sensibilité de la photoconductivité se déplace:E04 va de 1,9 à 2,7 Ev.
  • 4. Si 1-x est augmenté de 0 à 0,40, l'indice de réfraction diminue de 3,6 à environ 2,0.
The more precise choice of 1-x in this range depends on the aims pursued and the applications envisaged. We can define 4 different ranges, included in the range 0.05-0.50:
  • 1. If 1-x is increased from 0 to 0.35, the DOS and R increase, with an optimum located around 0.10.
  • 2. If 1-x is increased from 0.10 to 0.35, the photoconductivity decreases; an optimum can be defined according to the previous point.
  • 3. If 1-x is increased from 0 to 0.50, the sensitivity spectrum of the photoconductivity shifts: E 04 goes from 1.9 to 2.7 Ev.
  • 4. If 1-x is increased from 0 to 0.40, the refractive index decreases from 3.6 to about 2.0.

On comprend qu'on possède ainsi, selon l'invention, un degré de liberté supplémentaire (la concentration en carbone) pour ajuster certaines caractéristiques, ce que ne permettrait pas le a-Si:H.It is understood that there is thus, according to the invention, an additional degree of freedom (the carbon concentration) to adjust certain characteristics, which would not allow a-Si: H.

A titre d'exemple, en prenant 1-x=0,10, les inventeurs ont obtenu de manière reproductible une conductivité dans l'obscurité située dans la gamme allant de 10-11 à 10-10Ω-1cm-1 et une DOS située dans la gamme allant de 30 à 40.1016 eV-1 cm-3. Ces caractéristiques ont permis l'obtention de marges d'hystérésis supérieures à 25V à 1 kHz de fréquence d'excitation.By way of example, by taking 1-x = 0.10, the inventors have reproducibly obtained a conductivity in the dark situated in the range from 10-11 to 10-10 Ω -1 cm -1 and a DOS located in the range from 30 to 40.10 16 eV- 1 cm- 3 . These characteristics made it possible to obtain hysteresis margins greater than 25V at 1 kHz of excitation frequency.

En ce qui concerne l'adaptation du spectre du photoconducteur, on sait que les longueurs d'onde d'émission utilisées pour l'affichage polychrome vont d'environ 450 nm pour le bleu à environ 640 nm pour le rouge. On peut obtenir une adaptation à de tels spectres en prenant 1-x égal respectivement à 0,50 pour le bleu et 0,05 pour le rouge.With regard to the adaptation of the spectrum of the photoconductor, it is known that the emission wavelengths used for the polychrome display range from approximately 450 nm for blue to approximately 640 nm for red. An adaptation to such spectra can be obtained by taking 1-x equal to 0.50 for blue and 0.05 for red, respectively.

Outre l'effet mémoire qu'elle procure dans un dispositif PC-EL, la couche photoconductrice peut donner à une structure électroluminescente un contraste d'affichage excellent en raison de l'effet de "couche noire" qui l'accompagne. La couche photoconductrice masque en effet les électrodes arrière en aluminium, absorbe la lumière ambiante et empêche la réflexion de celle-ci sur les électrodes. Une application de l'invention est donc la réalisation de dispositifs d'affichage électroluminescent à contraste élevé, sans effet mémoire.In addition to the memory effect it provides in a PC-EL device, the photoconductive layer can give an electroluminescent structure an excellent display contrast due to the accompanying "black layer" effect. The photoconductive layer in fact masks the rear aluminum electrodes, absorbs ambient light and prevents its reflection on the electrodes. An application of the invention is therefore the production of high contrast electroluminescent display devices, without memory effect.

Le principe de l'utilisation d'une couche absorbante est naturellement connu. Il est décrit par exemple dans la publication des inventeurs citée plus haut, ainsi que dans le brevet américain US-A-3,560,784. Mais dans ces documents antérieurs, la "couche noire" est tantôt du a-Si:H, tantôt un diélectrique, autrement dit des corps de composition, donc de propriété, donnée. On ne peut donc pas jouer librement sur les propriétés optiques de ces corps. Dans l'invention, on peut fixer l'indice de réfraction dans une gamme étendue (2,0 à 3,6) en agissant sur 1-x et adapter optiquement la couche noire aux autres couches, par exemple à la couche diélectrique qui la jouxte et qui peut être du Ta2O5 d'indice 2,1 ou à la couche électroluminescente, qui peut être du ZnS d'indice 2,35. On minimise ainsi les réflexions de la lumière ambiante par le dioptre couche photoconductrice-couche sous-jacente (isolant ou couche électroluminescente).The principle of using an absorbent layer is naturally known. It is described for example in the publication of the inventors cited above, as well as in the American patent US-A-3,560,784. But in these previous documents, the "black layer" is sometimes a-Si: H, sometimes a dielectric, in other words bodies of composition, therefore of property, given. We cannot therefore play freely on the optical properties of these bodies. In the invention, one can fix the refractive index in a wide range (2.0 to 3.6) by acting on 1-x and optically adapt the black layer to the other layers, for example to the dielectric layer which adjoining and which may be Ta 2 O 5 with index 2.1 or with the electroluminescent layer, which may be ZnS with index 2.35. This minimizes the reflection of ambient light by the photoconductive layer-underlying layer (insulator or electroluminescent layer).

Pour ce qui concerne les couches n+, l'incorporation de C a certains avantages spécifiques que l'on décrit ci-après.With regard to the n + layers, the incorporation of C has certain specific advantages which are described below.

Le dopage d'une couche de a-Si:H au phosphore (P) permet d'augmenter très sensiblement la densité des porteurs libres: jusqu'à 1018―1019 cm-3. C'est cette densité élevée de porteurs libres qui assure une injection électronique quasi-ohmique dans la couche intrinsèque. Cependant, la conductivité ohmique de telles couches n+ est si élevée (10-2, à 10-3Ω-1 cm-1) qu'elle provoque des phénomènes de conduction latérale parasites dans les écrans PCEL materials. L'incorporation de C dans la couche n+ permet d'en diminuer significativement la conductivité sans modifier sensiblement la densité de porteures libres. Ainsi, par exemple, avec un mélange de 50% [SiH4]―50%[CH4] (teneur y en C dans la couche de =0,14), on réduit la conductivité ohmique de la couche n+ dopée P à 10-5―10-6Ω-1. cm-1 typiquement et les effets électriques parasites dans les écrans PCEL matriciels disparaissent.Doping a layer of a-Si: H with phosphorus (P) makes it possible to very significantly increase the density of free carriers: up to 10 18 ―10 19 cm- 3 . It is this high density of free carriers which ensures quasi-ohmic electronic injection into the intrinsic layer. However, the ohmic conductivity of such layers n + is so high (10 -2, 10 -3 Ω -1 cm -1) it causes lateral parasitic conduction phenomena in PCEL materials screens. The incorporation of C into the n + layer makes it possible to significantly reduce the conductivity without significantly modifying the density of free carriers. Thus, for example, with a mixture of 50% [SiH 4 ] ―50% [CH 4 ] (y C content in the layer = 0.14), the ohmic conductivity of the n + P doped layer is reduced to 10 -5 ―10 -6 Ω -1 . cm- 1 typically and the parasitic electrical effects in the matrix PCEL screens disappear.

En résumé, l'incorporation de carbone dans le a-Si:H a des effets positifs importants à la fois pour la couche intrinsèque n et pour les couches n+ éventuellement introduites dans la structure photoconductrice.In summary, the incorporation of carbon into the a-Si: H has significant positive effects both for the intrinsic layer n and for the layers n + possibly introduced into the photoconductive structure.

Pour ce qui est des conditions pratiques à utiliser pour mettre en oeuvre l'invention, on peut se référer à deux articles: le premier de M.P. Schmidt et al intitulé "Physics of Low Density-Of-States a-Si1-xCx films" publié dans Journal of Non-Crystalline Solids 77 and 78, pp. 849-852; le second de M.P. Schmidt et al intitulé "Influence of Carbon incorporation in amorphous hydrogenated silicon" publié dans "Philosophical Magazine" B, 1985, vol. 51, n° 6, pp. 581-589.As regards the practical conditions to be used for implementing the invention, reference may be made to two articles: the first by MP Schmidt et al entitled "Physics of Low Density-Of-States a-Si 1-x C x films "published in Journal of Non-Crystalline Solids 77 and 78, pp. 849-852; the second by MP Schmidt et al entitled "Influence of Carbon incorporation in amorphous hydrogena ted silicon "published in" Philosophical Magazine "B, 1985, vol. 51, n ° 6, pp. 581-589.

Dans la technique décrite dans ces documents, les couches de silicium amorphe hydrogéné et carboné sont déposées par décharge luminescente ("glow discharge") à partir d'un mélange de silane (SiH4) et de méthane (CH4). Lorsque la teneur du mélange en CH4 varie dans une gamme allant de 0 à 60%, la teneur en carbone 1-x dans la couche déposée varie de 0 à 0,2. En faisant varier la teneur en CH4 du mélange gazeux, on obtient, de manière reproductible, des valeurs de conductivité dans le noir et de DOS s'étendant sur des gammes aussi étendues que 10-6―10-13 (Ω cm)-1 pour la conductivité et 2.1015-1018 cm-3 eV-1 pour la DOS. C'est grâce à cette propriété que les inventeurs ont pu, en choisissant une teneur de 35% en CH4 obtenir une valeur d'environ 0,10 pour 1-x et obtenir de manière reproductible une conductivité de la couche photoconductrie de 10-10Ω-1 cm-1 et une DOS d'environ 4.1017eV-1cm-3. Dans des travaux récents, la gamme d'étude de la concentration en CH4 dans le mélange gazeux SiH4-CH4 a été étendue à 95% et il a éte possible d'obtenir par cette méthode des valeurs de 1-x supérieures à 0,5 dont dépassant les exigences de l'invention.In the technique described in these documents, the layers of hydrogenated and carbonated amorphous silicon are deposited by glow discharge ("glow discharge") from a mixture of silane (SiH 4 ) and methane (CH 4 ). When the CH 4 mixture content varies in a range from 0 to 60%, the carbon 1-x content in the deposited layer varies from 0 to 0.2. By varying the CH 4 content of the gas mixture, we obtain, in a reproducible manner, values of conductivity in the dark and of DOS extending over ranges as wide as 10 -6 ―10 -13 (Ω cm) - 1 for conductivity and 2.10 15 - 10 18 cm - 3 eV - 1 for DOS. It is thanks to this property that the inventors were able, by choosing a content of 35% CH 4, to obtain a value of approximately 0.10 for 1-x and to reproducibly obtain a conductivity of the photoconductive layer of 10 - 10 Ω -1 cm- 1 and a DOS of approximately 4.10 17 eV -1 cm -3 . In recent works, the range of study of the concentration of CH4 in the gas mixture SiH4-CH4 has been extended to 95% and it has been possible to obtain by this method values of 1-x greater than 0.5 of which exceeding the requirements of the invention.

Les figures 2 et 3 permettent de mieux préciser les conditions expérimentales à mettre en oeuvre pour obtenir certaines performances. Sur ces figures, l'axe des abscisses correspond à la concentration C de méthane à utiliser dans le mélange gazeux méthane-silane. Autrement dit

Figure imgb0002
FIGS. 2 and 3 make it possible to better specify the experimental conditions to be implemented to obtain certain performances. In these figures, the abscissa axis corresponds to the concentration C of methane to be used in the methane-silane gas mixture. In other words
Figure imgb0002

L'axe des ordonnées correspond à l'indice de réfraction n sur la figure 2 et à l'énergie E04 de la bande d'absorption du photoconducteur exprimée en électrons-volts sur la figure 3.The ordinate axis corresponds to the refractive index n in FIG. 2 and to the energy E04 of the absorption band of the photoconductor expressed in electron volts in FIG. 3.

Ces courbes correspondent à une température du substrat comprise entre 250 et 290°C.These curves correspond to a temperature of the substrate of between 250 and 290 ° C.

Naturellement, l'ajustement de 1-x pour optimiser la DOS, la résistivité, la photoconductivité, le spectre, l'indice, etc. n'exclut en aucune manière le réglage des conditions opératoires de dépôt (température du substrat, puissance du plasma, etc.). Ce réglage permet de parfaire les effets de la présence de carbone, ou au contraire, de compenser cette action. C'est le cas par exemple lorsqu'on a choisi 1-x pour ajuster le spectre de photoconductivité et/ou l'indice et qu'on veut vorriger en plus la DOS, la résistivité et la photoconductivité qui résultent de ce choix.Naturally, adjusting 1-x to optimize the DOS, resistivity, photoconductivity, spectrum, index, etc. does not in any way exclude the adjustment of the operating conditions of deposition (temperature of the substrate, power of the plasma, etc.). This adjustment makes it possible to perfect the effects of the presence of carbon, or on the contrary, to compensate for this action. This is the case, for example, when 1-x has been chosen to adjust the photoconductivity spectrum and / or the index and we also want to corrode the DOS, the resistivity and the photoconductivity which result from this choice.

L'action sur 1-x peut donc avantageusement être combinée aux adjustements classiques sur les conditions opératoires pour satisfaire simultanément à toutes les conditions énoncées plus haut.The action on 1-x can therefore advantageously be combined with conventional adjustments to the operating conditions to simultaneously satisfy all of the conditions set out above.

Les couches n+ de a-Si1-yCy:H seront obtenues par exemple en ajoutant au mélange de [SiH4]-[CH4] une concentration adéquate de [PH3] (0,5% typiquement).The n + layers of a-Si 1-y C y : H will be obtained for example by adding to the mixture of [SiH 4 ] - [CH 4 ] an adequate concentration of [PH 3] (typically 0.5%).

L'invention est applicable à tout type de structure électroluminescente en couches minces ou à base de poudre, et à excitation continue ou alternative, ceci bien que l'exemple décrit concerne l'électroluminescence en couches minces à excitation alternative.The invention is applicable to any type of electroluminescent structure in thin layers or based on powder, and with continuous or alternative excitation, this although the example described relates to electroluminescence in thin layers with alternative excitation.

Claims (6)

1. Electroluminescent display device comprising on an insulating support (10) an electroluminescent layer (16) and a photoconductive layer (20), a said layer being stacked on one another, the two layers being inserted between two systems of electrodes (12, 22) connected to a voltage supply source (24) permitting the excitation of certain zones of the electroluminescent layer, said device being characterized in that the photoconductive layer is of hydrogenated and carbonated amorphous silicon a-SixC1-x:H, in which 1-x is between 0.05 and 0.50.
2. Display device according to Claim 1, characterized in that 1-x is between 0.10 and 0.35.
3. Display device according to Claim 2, characterized in that the thickness of the photoconductive layer is below 2 micrometres and that the device has a PC-EL memory effect.
4. Display device according to Claim 1, characterized in that 1-x is chosen in one of the following ranges 0.05 to 0.15, 0.10 to 0.35 and 0.25 to 0.50, the sensitivity spectrum of the photoconductive layer then being adapted to the light spectrum emitted by the electroluminescent layer, said spectrum corresponding to one of the three primary colours red, green and blue respectively and the PC-EL memory effect can then be produced in each colour.
5. Display device according to Claim 1, characterized in that 1-x is between 0.20 and 0.50.
6. Display device according to Claim 5, characterized in that 1-x is also chosen in such a way that the resistivity of the photoconductive layer exceeds 1011Ω · cm, that the photoconductivity of said layer is substantially zero and that its refractive index is below 3, the photoconductive layer then serving as an absorbing layer permitting a high contrast display.
EP87402320A 1986-10-23 1987-10-16 Electroluminescent display device using amorphous hydrogenated and carbonated silicon Expired - Lifetime EP0270392B1 (en)

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