DD158444A1 - METHOD FOR PRODUCING A GLASS TYPE TORCH SEMICONDUCTOR ON A SUBSTRATE - Google Patents

Abstract

The invention relates to a method for producing vitreous-amorphous semiconductor layers, wherein this amorphous material has a low electronic density of states in the region of the energy gap and can thus be used in active components of electronics. The object of the invention is the production of cost-effective semiconductor layers which can replace expensive semiconductor layers or disks in electronics and which have sufficiently good or better electronic parameters than the known semiconductor base materials. The invention has for its object to provide a method for producing amorphous semiconductor layers, with a low electronic density of states (<equal to 10 high 17 cm high -3eV high -1 for silicon) i. Ber. d. forbidden zone, i. a low density of defects z. B. is achieved in the form of nichtabgesaettigter Haegender stomarer bonds in the amorphous semiconductor material. According to the invention, the object is achieved by d. Semiconductor material in any form (amorphous or (poly-) crystalline) applied as a thin film on a non-monocrystalline material of the film to be applied, well heat-conductive substrates (substrate) and then irradiated with a short, high-intensity energy pulse so that the pulse energy completely absorbed in the semiconductor film and thereby melted. Due to the short energy pulse, no heating of the substrate by heat conduction occurs during the irradiation, so that after the pulse end a very rapid cooling of the semiconductor layer - quenching - by heat conduction into the substrate material takes place. This creates a thin glassy-amorphous layer on the substrate.

Description

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Field of application of the invention

The invention relates to a method for producing glassy amorphous semiconductor layers, wherein this amorphous material has a low electronic density of states in the region of the energy gap and can thus be used in active components of electronics. '

Characteristic of the known technical solutions

In electronics, semiconductors (in particular silicon) in the form of monocrystalline disks and epitaxial layers are used for active components. The material properties of the Halble.itermaterials used for the components significantly affect their functionality. Deviations from the optimal electronic parameters caused by impurities (eg disturbance levels in the forbidden zone) and microinhomogeneities (eg crystal defects, 'precipitation agglomerates') in the area of the pn junctions impair the fun! tion of the components. Therefore, one is forced to produce a semiconductor material with a nearly ideal crystal structure using very complex processes. "

For certain applications, the price of the semiconductor base material was not critical so far, since, for example, in microelectronics, the price of monocrystalline, high-purity silicon wafers less than 5% is included in the cost of the finished device / K. Tempelhoff, Wissenschaft und Fortschritt 30_ (1980) 74 /.- For other applications, eg. For example, the conversion of sunlight into electrical energy by means of solar cells, i & the manufacture of cheap semiconductor layers, the prerequisite for a broad application / K. Tempelhoff, Wissenschaft: und Fortschritt 30 (1980) 74 /. The production costs of solar cells made of semiconductor base material can be kept very low today with the help of modern technologies _s so that the cost of the semiconductor base material determine the price of the solar cells to gain energy by means of solar cells is thus because of the high; The price of semiconductor silicon has only been limited and economical for very specific applications.

For these reasons, it is currently trying. "Cheap solar cells, for example, _e using polycrystalline silicon layers to produce," Because the grain boundaries' in the polycrystalline semiconductor material such semiconductor layers for other electronic components are not very suitable.

On the other hand, amorphous semiconductors are an interesting material for versatile use in electronics if it is possible to keep electronic density of states in the forbidden zone low / Physics Today 12/1979, p. 20 /. A relatively low density of states in the forbidden zone has hydrogenated amorphous silicon (about 10 'cm eV), which is obtained by deposition from a german or silane gas atmosphere under the influence of a glow discharge. On the basis of this material in particular cheap solar cells are to be developed, whereby at present the quantum yield is still too low, / Electronics 8 (1980) 40 /. In / W. A. Spear, Adv. Phys., 26 (1.977) 811 / a brief overview of this method is given. ,

Another manufacturing method is reactive sputtering in hydrogen / T.D. Moustakas, et. al., Solid State Comm. 22 (1977) 155 /. In the material thus produced, the electronic density of states in the. The advantage of this method is that the semiconductor material can be better doped during the coating process »

In /A.K. Gosh, et. al., J. Appl. Phys. 50 (1979) 34-09 / is reported on the production of hydrogenated amorphous silicon films by electron beam evaporation under the action of an atomic hydrogen jet. The process of ion plating was already in /F.H. Cocks et. al., Appl. Phys. Lett, 26 (1980) 909 / used for the production of hydrogenated amorphous silicon layers.

In view of the fact that the electronic density of states in the forbidden zone of hydrogenated amorphous silicon is still relatively high, for use in solar cells or thin film transistors, a significant disadvantage is that this material becomes unstable at about 350 ^° C., because then the hydrogen is out the material diffused out.

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An improvement in the. Thermal stability seems to occur with the production of fluoridated amorphous-silicon in / H. Matsumura.-et. al., Appl. Phys. Lett. 36 (1980) 439 / and / Ή. Van Dong and C. Thuault, Phys. Lett. 5A £ (1980) 229 zu'sein / managed, since it is found that fluorine can reduce the density of electronic states in the forbidden zone as well as hydrogen and fluoridated amorphous silicon its properties even after prolonged annealing at 600 ⁰ C scarcely changes. The electronic density of states in the forbidden zone of fluoridated amorphous silicon, however, as well as in hydrogenated amorphous silicon for many applications (eg, the microelectronics) is still too high. Here better amorphous semiconductor material (silicon) is needed with a nearly ideal atomic order. For its production, no solutions have yet been disclosed.

Object of the invention

The object of the invention is the production of cost-effective semiconductor layers which can replace expensive semiconductor layers or disks in electronics and which have sufficiently good or better electronic parameters than the known semiconductor base materials.

Presentation of the invention

The invention has for its object to provide a method for producing amorphous semiconductor layers, with a low electronic density of states (<: 10 ' cm ~ eV for silicon) in the forbidden zone, ie a low density of defects z. B. in the form of nich'tabgesättigter suspended atomic bonds in the amorphous semiconductor material is achieved. According to the invention, the object is achieved in that the semiconductor material is applied in some form (amorphous or (poly-) crystalline as a thin film to a non-monocrystalline material of the film to be applied existing, good heat-conductive pad (substrate) and then with a short, high-intensity energy pulse is irradiated so that the pulse energy

it is melted. Due to the short energy pulse, there is no heating of the substrate by thermal conduction during the irradiation, so that after the pulse end a very rapid cooling of the semiconductor layer - a quenching - by heat conduction into the substrate material. This creates a thin glassy-amorphous layer on the substrate. Thicker vitreous-amorphous layers are produced by repeatedly depositing thin films of the semiconductor material below the transition temperature amorphously (poly) crystalline on the already amorphized semiconductor layer and then converting them again into the amorphous phase by means of a corresponding energetic impulse, "Thicker consumed ones Semiconductor films can not be amorphized by an energy pulse glassy because for thick molten films, the cooling rate is too low and thus the material solidifies (poly) crystalline. By glassy-amorphous semiconductors are meant atomic configurations in which the ones of crystalline state. known Nahordnung is largely fulfilled (no or only a few nichtabgesättigte hanging bonds) "but by disturbed bond angle'dark remote order" The pad should 'be such that it has the best possible thermal conductivity-and not of monocrystalline material of the applied film consists. If its surface is amorphous or has a crystal structure and / or lattice constant that deviates from the crystal structure and lattice constant of the deposited semiconductor material, it does not act as a nucleation agent for the solidifying semiconductor material. In the absence of nuclei, thin liquid semiconductor layers solidify amorphously even at lower cooling rates compared to the case where nuclei are present. Such crystallization nuclei are 'z. B ", when polycrystalline material of the film to be applied is used as a base, ie, material that does not have the properties required above. Vorteiltiafterweise also this ia can be used cheap material when it is irradiated so by a correspondingly short power pulse before the application of "the first semiconductor film is that only a surface layer of smallest thickness is melted, which then ν!" Λ "t \ tr \ 4 - A m ^ _ qm ^ -n ^ V * ai ^ σ "H £ 3 Τ * Τ * Ή

The requirements for energy pulses for amorphizing polycrystalline substrate surfaces are particularly high, d. H. they must be absorbed in a thin surface layer and be shorter than energy pulses that serve to amorphize subsequently deposited semiconductor films. The base should be a good heat conductor, so that it does not come to a heat accumulation under the applied heated semiconductor film in the cooling phase. Good heat conduction in the underlay results in a high cooling rate of the molten semiconductor film. With the same pulse duration, it is therefore possible to amorphize thicker semiconductor layers by using better thermally conductive underlays. On the other hand, by using highly thermally conductive substrates, the necessary cooling rate for a glassy-amorphous solidification is achieved even for relatively long energy pulses. The pad may have electrically insulating, semiconducting or metallic behavior. The energy pulse (eg laser light pulse, pulse of accelerated electrons or ions) must be such that the energy is essentially absorbed in the last deposited semiconductor layer (which can be achieved, for example, by the choice of a suitable photon, electron or Ion energy can be achieved) that the absorbed energy sufficient to melt the last applied semiconductor layer and that the pulse duration is short enough for the required rapid quenching of the short-term molten semiconductor layer.

The thickness of the semiconductor film, which is applied to the well-heat-conductive substrate or the already amorphized semiconductor layer, must be such that it can be melted with an energy pulse, without on the surface of the semiconductor film, the boiling temperature of the semiconductor material is achieved. On the other hand, the semiconductor film must be so thin that the critical cooling rate for. a glassy-amorphous solidification: is exceeded. Assuming surface absorption of the energy, the cooling rate and the maximum amorphous layer thickness are essentially a function of the energy pulse duration.

The application of the semiconductor material to the substrate or an already glassy-amorphized semiconductor layer "in the form of an amorphous or CiDoly- kristai

(eg, by deposition from a gas atmosphere (eg, silane), by vapor deposition, by reactive sputtering, by ion plating). "The temperature of a semiconductor layer already amorphous in glassy form may be increased by the application of another semiconductor film Transition temperature amorphous (poly) crystalline, because by crystallizing the glassy-amorphous material disturbing grain boundaries arise.

With the invention, heterostructure structures can advantageously be produced by successively applying and irradiating semiconductor layers of different materials. It should be noted that semiconductor material with the higher melting temperature must always be applied before the lower melting and irradiated. So must z. B. a glassy-amorphous silicon layer (melting temperature T " ^x = 14-12 ⁰ C) in front of a glassy amorphous germanium layer (T. = 93 ° C) are generated because in a reverse order by die.hohe melting temperature of the silicon and the underlying germanium is melted and it comes to the mixing of the semiconductor materials. Furthermore, it should be noted that the energy density of the pulse to. be adapted material treated, ie z _e as a germanium layer requires less energy density as a uniformly thick silicon layer or "at the same energy density, a thicker layer of germanium glassy amorphized v / ground. Utilizing the invention can also advantageously doped p-. and η-conductive glassy-amorphous semiconductor layers can be prepared by directly or p-type η-conductive semiconductor material on the substrate or an already by known methods (eg silane in the silane gaseous hydrides such as BpHg or PH ^ are added) existing glassy-amorphous layer separates. During the amorphization process, the dopants become the resulting atomic configuration. built-in and thus, electrically activated. When a thick glassy amorphous semiconductor layer is formed by repeatedly applying and amorphizing thin semiconductor films, each of these films can be made either p-type, η-type or undoped. The conductivity type of a semiconductor film may also be determined by other doping methods ₅ such as, for example, ion implantation,

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the doping is expediently carried out in front of the glassy amorphizer ', since the subsequent irradiation at the same time a good electrical activity of the. Foreign atoms are reached. If the substrate is metallic and a good ohmic contact between the first produced glassy-amorphous semiconductor film and the metallic substrate is required, this contact can according to the invention by alloying or compound formation (eg silicidation) between the material of the substrate and the applied Semiconductor material in a process step with the Amorph! Tion of the first applied semiconductor film are prepared by the fact that the melting temperature of the substrate is below or at. The melting temperature of the baked semiconductor film and that the irradiation takes place so that by the energy pulse for the amorphization and a thin Metal film is melted at the interface pad semiconductor film. For alloying with silicon z. B. Al minium, but also other low-melting metallic Unterla conditions in question. ,

If the alloy or compound formation of the first generated glassy amorphous semiconductor film to be avoided with the pad or in a metallic substrate a rectifying boundary surface such as a Schottky barrier are generated, the pad must have a melting temperature well above the melting temperature of the applied semiconductor material is located. For the semiconductor material silicon for molybdenum, but also other refractory metals come into question.

embodiments

The invention will be explained below with reference to five embodiments. ..... , 1. An embodiment of the invention is that a glassy-amorphous silicon film for use in de; Electronics is produced. For this purpose, first an approximately 50 mm thick silicon layer is evaporated on a metal substrate. The metal substrate should z. B. made of steel whose surface was refined by a 0.5 / thick molybdenum layer.

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By irradiation with the fourth harmonic of a Q-switched neodymium laser (wavelength λ = 265 nm) with a pulse duration of less than 10 nsec and with an energy density of at most 1 Ws / cm, the vapor-deposited silicon layer is amorphized glass-like. By repeated vapor deposition of about 50 nm thick silicon layers on the already glassy-amorphized layer with each subsequent irradiation with the above-mentioned laser pulse, a glassy-amorphous silicon layer of 0.1 to 2 / produced by thickness. Vapor deposition of the silicon layer on already glassy-amorphized silicon substrate temperature must remain amorphous-crystalline below the glass transition temperature (about 600 ⁰ C for silicon). By doping the silicon during the vapor deposition process by known methods, the glassy amorphous layer can be given a basic doping (p- or η-type). Since the layer contains no grain boundaries such as polycrystalline silicon and has a low density of electronic states in the forbidden zone, it is suitable as a base material for microelectronics, d, h. bipolar and unipolar components can be incorporated in them * It should be noted that in the manufacture of electronic components, the transition temperature amorphous-crystalline may not be exceeded for a long time, ie, only low-temperature processes such as "B. the ion implantation are used. The healing of the implantation damage in the glassy-amorphous structure and the incorporation of the implanted ions into the structure can be achieved by re-melting the surface after ion implantation by the above-mentioned laser pulse and allowed to solidify glassy-amorphous or even before amorphizing the last .dampften layer implanted the impurities ,, ·. '

Instead of a metal substrate can be used for the applied glassy amorphous silicon layer, a substrate consisting of polycrystalline silicon. This material has good thermal conductivity and is i. a. cheap"

The substrate surface is advantageously pretreated by transferring a surface layer of the smallest thickness into the glassy-amorphous state by irradiation with the fourth harmonic of a Q-switched neodymium laser with an energy density of at most 0.5 Ws / cm and a particularly short pulse duration of about 1 nsec. As a result, nuclei are eliminated for a single-crystal solidification of an applied, briefly molten layer and it can now as described above thicker applied silicon films (about 50 nm) are glassy amorphized by longer laser pulses. With the aid of the glass-like amorphous silicon layers according to the invention, an alternative to the SOS (Silicon on Sapphire) technique can also be realized. In the SOS technique, a monocrystalline silicon layer is grown on a suitable insulating substrate (sapphire), which substrate must be of high crystal quality and therefore very expensive. In the production of a glassy amorphous silicon layer on an insulating substrate However, it is only required that the pad has a good thermal conductivity and that their melting temperature is above that of silicon. The pad can, for. Example of BeO or AIpOo crystals of low quality or polycrystalline materials of these compounds, in particular, the well-known AlgO, - and offer BeO pure oxide ceramics. Beryllium and aluminum oxide have a relatively good thermal conductivity as insulators (Ag ₀ Q ~ 200 W / mK, A _A1 q ^ 30 W / ml, while silicon dioxide only Ag _i0 . ^ 1 W / mK). The other further processing of the layer, taking into account the above-made restriction analogous to the SOS technology. A glassy-amorphous silicon layer on a substrate kani method described above are also advantageously prepared by known methods. Films deposited from hydrogen i si erated or fluoridated amorphous silicon and are then each glass-like amorphized. Hydrogen or fluorine can lead to an improvement in the electrical properties of the layer.

One possibility for a simultaneous production of a glass-like amorphous silicon layer and an alloy layer between a metallic substrate and the amorphous silicon for the purpose of ohmic contact formation according to the invention is that z. B. on a steel support, a thin platinum layer (50 ... 500 nm thick) is vapor-deposited and that on this platinum layer, a thin silicon layer (20 ... 100 nm thick) vapor-deposited and then with a very short (<1O ns) and intense (0.1 ₀ .. 1 J / cm) short-wave laser pulse is treated. It comes between the vapor-deposited platinum layer and the silicon layer for alloy formation, the alloyed layer consists of different suicides. As in Example 1, further glassy amorphized layers are applied to this first glassy-amorphized layer and the substrate, e.g. B. further processed to a solar cell. The laserinduzier-th formation of the alloy layer has the advantage that the manufacture of the otherwise to ensure a good ohmic contact to the base necessary, strong ~ n ⁺ -doped silicon layer is saved (see Example 3).

The production of a solar cell using the invention takes place z. Example, in that a thin silicon layer (0.03 to 0.1 / um thick) on a metallic substrate, z _e B «steel, is deposited. This silicon layer is then by ion implantation z. B. heavily doped with arsenic ions. With a very short (^ 10 ns) and intense (0.1

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· .. 1 J / cm) short-wave laser pulse, the applied silicon layer is amorphized, wherein the injected doping atoms are simultaneously electrically activated, so that 'an n ⁺ -conductive glassy-amorphous layer is formed. This heavily doped layer produces an ohmic contact with the metallic substrate. On the already glassy-amorphized silicon layer, thin undoped silicon films are repeatedly applied and then each again glass-amorphized with a corresponding laser pulse. If the glassy-amorphized silicon layer has reached a total thickness of 0.2 to 2 axb , then below the conversion temperature . amorphous-crystalline a thin translucent pia-

  amorphous silicon forms a Schottky barrier. To complete the solar cell, an antireflection layer and a grid contact are applied. A solar cell based on Ge, GaAs and other semiconductors can be produced using suitable doping atoms and laser parameters according to the same method.

4. The preparation of a vidicle using the invention may, for. Example, take place in that a very thin, transparent and electrically conductive layer is applied with the function of an electrode on a good thermal conductivity transparent base plate (eg AlgO ^). This layer must also be thermally stable and block the injection of holes into the silicon, it may, for. B. consist of SnO ₂ . Now thin (^ 50 nm) silicon layers are applied as in Example 1 and according to the invention glassy-amorphized until the total thickness of the glassy-amorphized layer 1 c. »2 / um. Subsequently, a porous SboS-3 layer of about 0.1 / um thickness is applied, which acts as an electron stopper and prevents electron injection into the silicon. By the use of the invention according to

, put glassy-amorphous silicon layer instead of also for. Vidikon's known hydrogenated amorphous silicon can further improve the quality of the photoconductor and thus the television picture quality. If the image acquisition is not to be performed in the visible, but in another spectral range, then another vitrified-amorphized semiconductor material ("germanium") can be used as a potol conductor. "

5. Using the invention, heterostructure structures consisting of layers of various glassy amorphized semiconductors can be prepared. Hetero transitions are used, for example, in heterophotodiodes. It is usually used that the semiconductor with the wider forbidden zone E ₁ for photons with W <E is permeable, ie forms a so-called front window. This has the advantage that the reaction of the radiation takes place deep inside the component, the influence of the surface is small. An electron Defektelöktrnn GpriAT.qhi nn oT »f λ! ff-i-viir. ο -, -, - ρ Αππ A ~ ™ -ü ^^^ j- ^.

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opposite side of the pn junction, thereby the generation of minority carriers in the front layer and thus the diffusion portion of Photοstromes can be completely suppressed.

A hetero-layer structure for a photodiode can be produced using the invention by, for. B. on a good heat-conducting transparent substrate such as Al ₂ Oo a thin transparent electrically conductive film is applied with a high melting temperature, which may for example consist of an approximately 5 μ thick platinum film or a thicker tin oxide. 20 μm thick amorphous or polycrystalline silicon layers are deposited three times on the electrically conductive layer and then each with the fourth harmonic of a Q-switched Nd-YAG laser with a pulse duration of about 1 nsec and with an energy density of max.

times 1 V / s / cm amorphized glassy. The silicon layers may, for. Be deposited by the known silane process from the gas phase "The applied silicon is prepared by known methods' during the deposition process with n-doping impurities such. B "phosphorus, which is electrically activated in Amorphisierungsproze'ß. On the 60 nm thick, η-type glassy-amorphized silicon layer, are now three times applied about 40 m thick germanium layers and then each amorphized glass-like with the above-described laser pulse. The germanium is during the deposition process with p-doped impurities such. B. gallium, which is electrically activated by the Amorphisierungsprozeß.gleichfalls. Subsequently, an electrically good conductive layer is applied to the glassy amino-germanium layer. ·. ·

The conversion temperature must amorphous-crystalline (ca "600 ⁰ C for Si, about 300 ⁰ G Ge) are non-exceeded in any procedural-.rensschritten except when Amorphisierungsprozeß itself .. It must also be noted that the semiconductor material is higher with the Melting temperature (here silicon with 1412 ⁰ C) in front of the lower melting semiconductor material (here germanium with 937 ⁰ C) is applied and glassy amorphous.isie.rt.

Claims (5)

 Invention claim. A process for producing a vitreous-amorphous semiconductor layer on a substrate, characterized in that one or more thin films of semiconductor material are sequentially deposited on a substrate which has good thermal conductivity and has a surface which is not of monocrystalline material to be applied Film consists, and that each applied semiconductor film is irradiated with a short and intense energy pulse so that the energy in the film completely absorbed and thus it is briefly melted, each applied to a semiconductor film already produced further thin film after the solidification of the previous film at a temperature below the transformation temperature amorphous (poly-) , is applied crystalline. 2. The method according to item 1, characterized in that as a substrate polycrystalline material of the applied first semiconductor film is used and that prior to the application of this film .A correspondingly short energy pulse, the pad is irradiated so that only one. Surfaces-   Layer of the smallest thickness is melted, which then solidifies glassy-amorphous. 3. The method according to item 1, characterized in that successively applied and irradiated semiconductor layers of different materials. 4. The method according to item 1 or 2, characterized in that the respectively last applied semiconductor layer during , of the deposition process or thereafter, but before irradiation, is doped with a conventional technique wherein the temperature of the beautifully glassy-amorphized semiconductor substrate remains amorphous (poly) crystalline below the temperature of the transducing. ·. 5. The method according to item 1, characterized in that the semiconductor layer is applied to a metallic substrate whose melting temperature is below or at the melting temperature is applied to the applied semiconductor layer and that the irradiation takes place in such a way that a thin metal film is also melted at the surface of the substrate layer by means of the amorphization energy pulse. Method according to item 1, characterized in that the thin semiconductor layer is applied to a metallic substrate whose melting temperature is so far above the melting temperature of the semiconductor material that it is not melted by the irradiation.