Radiation producing semiconductor device.

The invention relates to a radiation-generating semiconductor device according to the preamble of claim 1.

LEDs and laser diodes provide, for example, for communication via optical fiber cable the most interesting sources of light. The advantages of having laser diodes compared to LEDs, are a substantially higher external quantum yield (that is, number of emitted photons per electron) and thus better performance, a considerably smaller line width and a defined polarization of the emitted light.

With regard to various technical applications, however, LEDs have a number of significant advantages over laser diodes. The emitted power is less dependent on temperature at LEDs so that the LEDs in practical use are significantly more stable than laser diodes. LEDs can be operated at much higher temperatures and do not require a complex control circuit. In addition, LEDs are much easier to manufacture and have a higher life expectancy.

Currently LEDs in the material system AlGaAs / GaAs are offered by various manufacturers, the (partially, the LEDs in plastic lenses poured) have a more or less directed radiation only when using a lens. Currently, commercially available LEDs with lens are the quantum yield of the order of 1 to 2% and the beam divergence (ie, the full angle of half the radiation maximum, eng. Full angle at half maximum) at about 10 ° to 15 ° with a lens diameter of 4, 7 mm, and at 20 ° to 60 ° when using smaller lens diameters.

Object of the present invention is to provide a simple to produce radiation-emitting semiconductor component, which has a low beam divergence at high quantum yield.

This object is achieved with the semiconductor device having the features of claim 1. Further embodiments result from the dependent claims.

Description will be made of the invention constructed in accordance with semiconductor components, in particular LEDs, on the basis of figures 1 to 8, in each of which an embodiment of this component of the invention is shown in sectional plan. The 9 to 11 each show an inventive device in supervision each with a different arrangement of the emission regions. Fig. 12 is a diagram showing typical directional characteristics.

The structure of the invention is based on a newly discovered emission mechanism on excitation and emission of Oberflächenplasmonpolaritonen. Oberflächenplasmonpolaritonen are transverse electric (TE) or transverse magnetic (TM) surface modes, which can propagate at the interface of two different media. With a suitable periodic structuring the interface, these modes can be excited with electromagnetic waves. Using this emission mechanism, the properties of light emitting diodes (LEDs and laser diodes) fundamentally and dramatically improve. occurring in conventional LEDs loss mechanisms that limit their efficiency, are thus avoided, reducing the losses. At the same time significantly reduced linewidth drastically increase the external quantum efficiency and a directional radiation defined polarization is achieved with a beam divergence of less than 6 °, which can be dispensed with the use of a lens.

The difference from conventional LEDs is in the inventive structure in a periodic structuring of the surface of the semiconductor material, which is manufactured by means of conventional technology. This grid structure can in longitudinal section z. B. have sinusoidal, triangular or rectangular profile, but there are also other surface profiles with lattice structure possible. An on this structuring applied semitransparent metal film (the first contact 5 in Fig. 1) enables the excitation of Oberflächenplasmonpolaritonen on the surface of this metal film.

The metal film may also serve as an ohmic front contact (as shown in Figure 1.); the metal film on the emission of Oberflächenplasmonpolaritonen can also be performed in addition to the ohmic contact metallization. The Oberflächenplasmonpolaritonen are excited by generated during operation of the device in an active zone 3 photons whose H-field vector is oriented parallel to the illustrated in Fig. 1 axis 6. The excited Oberflächenplasmonpolaritonen emit light due to their specific characteristics defined polarization (p-polarization, ie H-field vector parallel to the axis 6) in a dependent on the grating period direction. This emission can therefore be adjusted as desired by varying the grating period. For a vertical emission, the grating period (lattice constant) must be approximately equal to the wavelength to be emitted. Exactly must hold that the grating period and equal to the product of about emitting wavelength √<o>1 + 1 / ε</o> (Ε = dielectric constant of the metal) is, so that it is radiated vertically emitting wavelength. The opening angle of the radiation (beam divergence) is substantially less than in conventional surface-emitting LEDs without periodic structuring of the surface.

Is applied to the metal film in addition a dielectric (the thickness of which is typically not greater than 5 microns) is applied, then the propagation of TE_i- And TM_i-Moden I-th order possible, whereby the number of modes and the maximum index i depends on the thickness of the dielectric: With increasing thickness, the number of modes increases in the order TM_O, TE_O, TM_l, TE_l... And so on, that is spread in addition to TM_O-mode From (in the absence of the dielectric) other modes. In this notation, TM_O-mode Identical to the concept of surface plasmon. The generic term for all modes occurring in the presence of the dielectric (TM_i, TE_i) Is Oberflächenplasmonpolariton.

the TM can at the interface between the metal film and the air_O-mode Spread by p-polarized light (H-field vector parallel to the axis 6) can be excited. Accordingly, the TE may be located at the interface between the additionally applied dielectric and the air_O-mode Propagate by s-polarized light (E-field vector parallel to the axis 6) can be excited. In especially advantageous embodiments of the device according to the invention are primarily the TM_O- That TE_O- And the TM_l-mode Of importance was.

In the embodiment according to Fig. 1, on an n-doped substrate 1 in succession one above the other grown an n-doped first confinement layer 2, a p-type active layer 3 and a p-doped second confinement layer 4. The substrate 1 and the active layer 3 are, for. example, GaAs, the confinement layers 2, 4 Al_0.3ga_0.7As. Are drawn further a first contact 5 and a second contact 7 for applying the operating current. The signs of the doping can be reversed, and there are other common for LEDs or laser diodes material systems in question. Essential for the assembly of the invention is a grid-like structure of the surface of the second confinement layer 4, which in turn can be composed of several component layers. This periodic structure of the surface, without the excitation and emission of Oberflächenplasmonpolaritonen is not possible transfers to the thin layered applied first contact 5 made of metal. On its surface, that interface with the air, the surface plasmons are excited to be in a value determined by the energy and momentum equations angle that depends on the grating period, radiated. Instead of the rectangular longitudinal section profile of the individual grid bars as shown in FIG. 1, a wavy, z. B. sinusoidal grating profile as shown in FIG. 2 or a symmetrical or asymmetrical triangular grating profile as shown in FIG. 3 comes in question. Essential is the<u>periodic</u> Structuring.

For the effectiveness of the excitation and emission of surface plasmons, the thickness d4 of the second confinement layer 4, the grating period Lg (lattice constant), the grating height h, ie the height of the applied surface structuring (amplitude of the difference in layer thickness) of the second confinement layer 4 and the ratio of grating period Lg to grid height h crucial. The distance between the active layer 3 from the metal surface (that is, the interface metal / air) of the first contact 5 (d4 + d5, wherein D5 is the thickness of the first contact 5) for the degree of coupling between the photon 3 produced in this active layer, and the excited surface plasmons on the metal surface significantly. By reducing the distance (d4 + d5) of the active layer 3 of the metal surface therefore can be increased, the efficiency of light-emitting diode. Advantageous dimensions are: thickness d4 of the second confinement layer 4 more than 3 order, grating height h than 1 .mu.m, the thickness d5 of the first contact 5 more than 2 microns.

Fig. 4 shows an embodiment in which the surface of the second confinement layer 4 is structured with a cross grating. The radiation is then carried out with two mutually perpendicular directions of polarization. Thus, the quantum yield can be increased further. Fig. 5 shows an embodiment with a reciprocal to the cross-grating shown in Fig. 4 grid. Fig. 6 has a cylinder through which are arranged periodically, (so-called quantum dots) structured surface. It is essential that the surface of the second confinement layer 4 is periodically structured in two mutually perpendicular directions. The term mesh is to be interpreted in a general sense. Just as the grid structures with rectangular profile illustrated in FIGS trellis come with sinusoidal or triangular transverse or longitudinal sections in question.

Fig. 7 shows an application to a laser diode. In this structure, a strip-shaped first contact 5 is applied to a lattice structure of the surface of the second confinement layer 4th This strip-shaped first contact 5 causes a lateral boundary (earnings guidance) of the generated in the active layer 3 radiation that is reflected between the resonator end. 8 The between these resonator end 8, which may additionally be mirrored, reflected photons stimulate Oberflächenplasmonpolaritonen on at the free, that is adjacent to the air top of the first contact. 5 This Oberflächenplasmonpolaritonen decay with emission of radiation emitted extremely focused because of the narrow linewidth of the laser radiation. The coupling degree is determined by the distance between the active layer 3 from the provided with the metal strip surface of the second confinement layer 4 and may be depending on the distance (about 0.1 to 2 microns) varied as desired.

In a laser diode, which is constructed in accordance with the present invention, each of the above-mentioned lattice structure comes into consideration. In Fig. 8 a corresponding laser diode is shown with approximately sinusoidal grating profile. The first contact 5 forming metal strip is provided with a dielectric layer 10 for the excitation of TE modes. The resonator end are provided with mirrors. 9 Even when a laser diode different grating profiles are possible, wherein in each case a periodic structure in the longitudinal direction of the laser is essential. The embodiment shown in FIG. 7 or FIG. 8 can be an arrangement of a plurality of coupled together laser (laser diode array) extend.

Radiation may be emitted from those areas of the surface of the semiconductor material, in which a periodic structure of this surface is provided with a semitransparent metal film. In the described embodiments, each of the first contact 5 served as an ohmic contact for the connection of the operating current and as this semitransparent metal film to generate Oberflächenplasmonpolaritonen. Instead of this simple embodiments, it is also possible to form the metal contact in places as semi-transparent metal film and the rest as a correspondingly thicker applied ohmic contact. An emission then takes place in the area where the semi-transparent metal film is deposited on the periodic structure (lattice-like structure) of the second confinement layer 4th This periodic structure in turn does not need to cover the entire free surface of the second confinement layer 4th It is therefore possible to limit by limiting this periodic structuring the area of ​​the radiation emission. One may additionally applied dielectric layer 10 also does not need the entire metal film or the whole covered with this metal film patterned to cover the area, but may be limited to portions of this range.

In Fig. 9 to 11 which provided for the radiation top three embodiments are illustrated in plan view. There are each laser diode with mirrors 9, whose top has a shown in the figures by a rung-like hatching periodic structuring. In this top side in each case, a metal layer is applied which comprises a semi-transparent metal film 50 on this lattice-like structure and an optionally applied thicker ohmic contact 51st The area in which a dielectric layer 10 is also applied to the top of the metal film is shown with a dashed boundary. In Fig. 9, the range of radiation emission is limited to a toy in the center of the component window. This area is also completely covered with the dielectric 10th In Fig. 10, the emission of radiation over the entire length of the laser is effected, while the dielectric layer 10 is applied only in a partial area. In FIG. 11, the periodic structure is applied over the whole area, but the semi-transparent metal film 50 is located only in a strip-shaped region, while the ohmic contact 51 an adjacent portion of the surface covered. Also in the embodiment of FIG. 11, a dielectric layer may be applied in an arbitrary area. In this way, a further scope for the arrangement of the radiation emission and intended for electrical contact areas.

For the semi-transparent metal film, particularly the metals gold, silver and aluminum hydroxide; it is, in principle, any metal can be used. The limiting condition that the frequency of the electron plasma of the metal must be greater than the frequency of the radiation to be emitted is fulfilled in all practical cases for any metals.

FIG. 12 is a diagram showing the far-field pattern at a wavelength of 867 nm. A prepared without structured surface LED has no use of a lens, the far-field pattern corresponding to curve c in Fig. 12. The emission is virtually non-directional and completely unpolarized. In a structured LED according to the present invention, the axis 6 is about excitation and emission of surface plasmons, the proportion of H-field vector along greatly amplified decoupled and radiated extremely focused. The resultant far-field pattern in a lattice constant with the 850 nm modulated LED is shown as curve a in FIG. 12. The emission takes place without use of a lens with a beam divergence of less than 6 ° to an angle of +/- 1.3 °. The far-field pattern of the plane perpendicular to the polarization component is equal to an unstructured LED and is therefore represented by curve c. Although only the light of one polarization component is decoupled amplified the overall external quantum efficiency at the same time the linewidth in the emission maximum (+/- increased to 1.51% in this embodiment compared to the conventional unstructured LED 51%, 1.3 ° dropped from the major axis) by 26% to 27.5 nm. The far-field pattern of the grating period 1024 nm modulated LED is represented by curve b and demonstrates over the period of the grating surface almost infinitely adjustable radiation.

Advantages of the LED according to the invention are, in particular an improvement of the efficiency and the emission characteristic. Structured surface according to the present invention, LEDs radiate without using active optical components from extremely focused. The beam divergence been thus achieved is far below that of conventional LEDs, which require the bargain bulky lenses to emit directed.

The quantum efficiency of LEDs with surface structuring is dramatically increased since the conventional LED own and usually unavoidable and Fresnel reflection losses are reduced. On the one hand can namely also photons whose emission angle is greater than about 16 ° and which are normally totally reflected in the substrate, excite surface plasmons and are coupled via these surface plasmons as radiation. On the other hand, the proportion of reflected back to the LED / air interface photons drops sharply, as in the surface plasmon resonance, the reflection is minimal. Photons that excite surface plasmons, can thus be coupled with virtually no loss of the substrate ideally.

Due to the narrow excitation line of Oberflächenplasmonpolaritonen the line width decreases in the maximum of the emitted intensity significantly.

The structure of the invention is not limited to the material system AlGaAs / GaAs. It can be generally applied to light-emitting diodes and laser diodes any emission wavelengths and material composition. Since the attenuation of the Oberflächenplasmonpolaritonen decreases with increasing wavelength, the excitation and emission mechanism of Oberflächenplasmonpolaritonen especially in the infrared range is particularly effective. It must be adapted to the wavelength, only the grating period (lattice constant).

Since the excitation efficiency of Oberflächenplasmonpolaritonen is determined by generated photons in the active layer primarily by the distance between this active layer of the periodically structured surface, the efficiency of the emission mechanism is largely independent of the type of LED or laser diode. Therefore, the emission process can by Oberflächenplasmonpolaritonen generally find application in constructions with homo-, hetero- or double hetero junctions.