DE3205461A1 - Photodiodes having a resonator structure to enhance absorption - Google Patents

Abstract

The invention relates to a photodiode which is provided with additional means for enhancing light absorption and which is suitable for integration with other electronic or electro-optical elements. The invention is based on the concept that, due to reflections at the boundaries of the diode, the light passes through the absorbing path repeatedly and the absorbed light intensity is thus enhanced. An optical resonator enhances the absorbed light intensity in the photodiode at wavelengths with small absorption coefficients to an extent which ensures, or indeed makes possible, the usefulness of the overall integrated circuit in that wavelength region.

Description

Patent application

Photodiode with resonator structure to increase absorption The invention relates to a photodiode with additional means to increase light absorption is provided and designed for integration with other electronic or electro-optical Elements is suitable. The invention is based on the idea that by reflections at the borders of the diode the light passes through the absorbing path several times and the absorbed light intensity is increased in this way.

How semiconductor photodiodes work, converting light signals into electrical ones Converting signals is based on the fact that irradiated light is in the space charge zone of the p-n junction of the diode is absorbed and electron-hole pairs are generated. The absorbed light intensity depends essentially on the absorption coefficient α, which indicates how much intensity is absorbed when traveling a distance becomes (dJ P - α ~ J ~ dl). If the wavelength-dependent oc is small, that is in particular in the area of the band edge of semiconductors the case must be with increasing wavelength the distance traveled by the light in the space charge zone can be increased, to ensure a sufficient generation of electron-hole pairs.

Is α in the wavelength range for which a photodiode is planned becomes so small that the overall length of the diode is technically not increased to the same extent can be to compensate for the small oc, can or must be a different Ha conductor material be used that has a sufficient «in this wavelength range, However, the new semiconductor material often does not have all properties that support its use of the low oc material desirable. Often need its properties must first be examined more closely and # t2II0 technology is less well mastered than that of the known semiconductor, so that the manufacture of integrated circuits is not yet possible.

In optical communications technology, the use of photodiodes is intended only the areas # # 1.1 µm can be expanded. For photodiodes are in this area basically quaternary semiconductor materials as a substitute for silicon, deso band edge at A w 1.1 pm is conceivable. With their application one had to take advantage of the advantages do without the very well-mastered Si technology, in particular the possibility of integrate the photodiode with other circuits for which Si as the base material serves.

The invention was therefore based on the object of the field of application of semiconductor materials for integrable photodiodes with limited overall length to the smallest possible s values. This object is achieved according to the invention solved that the photodiode on a substrate with electronics and / or other electro-optical Elements e.g. a laser is integrated and that the light entry side of the photodiode and the light exit side opposite it is designed as a reflector and both form a symmetrical or asymmetrical optical resonator which, possibly with the help of a voting device, at least for part of the irradiated light is in resonance.

Fig. I is intended to explain the mode of action using the strahlenopcischen Approach.

It is II the core area of the resonator, which is wholly or partly in the space charge zone of a photodiode, I the area in front and III the area behind the resonator. R1 and R2 are the reflectors on the light entry and exit side, which possibly has a finite extent in the z-direction 1 represents an incident one Light wave. If this hits the reflector R, a partial wave 2 is reflected, another partial wave 3 penetrates the Re; gate a. The latter becomes partial again at R2 reflected (4) and partially transmitted into area III (5).

Partial wave 4 is reflected again at R1 (6), but partially arrives also back to area I (partial wave 7). Partial wave 6 can be used in the same way as part wave pursue, etc. The mode of operation of the resonator is based on the fact that the Light often traverses the distance 1, the effective absorption length is thus a multiple of 1 is. The absorbed light intensity is therefore compared when x is small to a diode without a resonator is greatly increased, so that its usability is greater Wavelengths extended. The first reflected partial wave 2 interferes with the waves 7 and so on back into area I and can largely be wiped out will. Even with a large reflection factor of R 1, most of the intensity from the incoming wel into the resonator. The operating state in which the partial wave 2 is reduced most effectively, is called resonance.

For a given length 1 and a given absorption coefficient ot can the R.

Factors of R 1 and R 2 in relation to the absorbed light intensity be optimized. However, the functionality of the diode described above also remains obtained when the reflection factors of R I and R 2 are more or less of deviate from the optimal values. The resonator structure is suitable for both application vertical for diodes with light irradiation to the p n transition as also for diodes with light irradiation parallel to the p n junction (diodes with cross irradiation).

The light guidance in one or more of the areas I to III can take place by a waveguide structure according to Figure 2, the leading refractive index transitions possibly caused by different dopings. The choice of Size b and the values of the refractive indices determine the number of guided Light modes in the waveguide in question.

In particular there are the possibilities of the multimode and the single mode waveguide, both of which are suitable for use in the diode with a resonator structure.

The mode of action of a resonator to increase absorption was described in the diploma thesis (July 1981, TU Munich) examined by Reinhard Müller. There were but only considered resonators in which the reflection factors of R 1 and R 2 are the same and both result from refractive index jumps or interference reflect. Furthermore, this work did not address the possibility of integrating a photodiode pointed with resonator structure with other components. The technical efforts which are necessary to produce a good resonator (reflection factors close to 1), are hardly justified for a photodiode as a single element (choosing another Materials).

They are, however, when the light absorption of a photodiode is increased in an integrated circuit by the resonator and thus the usability the entire integrated circuit secured in the desired frequency range or is only made possible.

The photodiode of this invention, the operation of which is described with reference to FIG differs significantly from the one from Jörg Müller's publication (IEEE, J. Solid State Circuits SC 13 (1978), 173-179) known photodiode according to FIG. 3. According to this publication, in a diode according to FIG. 3, both reflectors be designed as a metal mirror. This has the consequence that because of the high transmission losses of metal mirrors only a small, non-reflective or even non-reflective one Irradiation window of width d can be used. The mode of operation of the diode is based therefore not on an extinction of the first reflected part by others Partial waves. However, this is an essential physical one for a diode according to FIG Occurrence.

Furthermore, in the case of a diode according to FIG. 3, the width d must always be chosen in this way because the relation d s w. W ~ tan ocS holds, i.e. the angle <# £ must not b can assume any small values without d becoming too small. A large angle ocl h without countermeasures, large losses at the ends of the diode result. In Fig. 3 leaves the light on the right the diode or its junction.

The novelty of the diode according to Fig.1, however, is that the radiation is not is limited to a window and the angle of the light in the diode is arbitrarily small Itrl can assume, in particular aS x 0, as shown in Fig. 1; barely lateral losses occur. In the diode of this invention, the veri of metal mirrors completely avoided, as is desirable for applications in integrated optics is. It is essential for their mode of operation that c R1 is a low-loss mirror that Even with a high reflection factor, light from I to II and from II to I is sufficient can get.

The reflectors R1 and R2 can be produced in various ways. The e-foldest reflector consists of a simple jump in the refractive index.

This occurs, for example, on the fracture surface of a semiconductor crystal on where the refractive index of the semiconductor material and the subsequent medium E.g. air meet.

Reflectors with higher reflection factors up to almost one can e.g. can be realized by dielectric interference mirrors. These consist of aufei other layers of different refractive indices.

When the resonator is composed entirely or partially of waveguide pieces there is another possibility for the realization of reflectors. C can by structuring the interface (s) between n1 and n2 and / or n1 u a so-called. Grating reflector are formed (Corrugation grating, Bragg -Reflekt Die reflectors R 1 and R 2 by no means have to be based on the same physical mode of operation. For example, R 1 can be an interference mirror and R 2 a Gi reflector. As the photodiode is to be ir grated with other components on a substrate, comes for the light exit side, which does not lie on the edge of the substrate because of the complex an interference mirror Layer-by-layer HE is hardly in question. There you actually become the reflector using Corrugat grating.

If a reflector forms a transition between two waveguides, it occurs in general mode conversion occurs in the case of reflections, i.e. energy is transferred from a of the waveguide converted into other modes. By suitable design of the reflectors mode conversion can be wholly or partially prevented or even avoided wrestles will. For example, if two symmetrical waveguides (n2 = n1) collide, one can Avoid mode conversion as far as possible if the value 22 is the same in both areas n1-n2 is assumed.

The operating state of the resonance can by, appropriate choice of the ratio Wavelength to light path between R1 and R2 can be achieved. By temperature changes During operation, the length 1 of the diode can increase due to thermal expansion change, whereby the resonance condition is no longer fulfilled.

This can be counteracted by voting facilities.

For the tuning of the diode it is, for example, conceivable to use a To connect control element for the mechanical tension on the resonator according to FIG. This control element can consist of a piezo element or a magnetoscriktiven Elemex1L exist.

A change in length of the diode can be achieved by applying an electrical Spazut1ng l1 to be counteracted on the control element. A change in tension on the Reg # 1 element there is a compression due to a change in the mechanical tension ## d ## r elongation and / or curvature of the diode result. Both effects influence. ### 'i the light path in the diode and can be used for tuning. With a radiated normal frequency (pilot frequency) can be determined whether the diode is in resonance and, if necessary, a vote can be initiated.

Another possibility of tuning the diode is obtained when in the arrangement of FIG. 4 the control element is a Peltier element. With this can the temperature of the diode and thus its length are adjusted by heating or cooling that the resonator is in resonance.

The effect of the resonance has the consequence that the absorption at wavelengths or frequency change of the incident light within the bandwidth atll den half the maximum value. For diodes with resonators one can use vacuum wavelength with an optimal choice of the reflection factors of Rl and R2 in relation to maximum absorption Half widths between half and 20 i obtained.

One possible application of a photodiode with a resonator structure is z. 8th.

the wavelength range around 1.1 for Si photodiodes. A Si photodiode With a resonator structure, apart from the extended wavelength range, it would also have the advantage of that they are on a Si substrate with electronics and other electro-optical devices could be combined into an integrated circuit. This could e.g. happen that the diode with the associated Mirror last Step an integrated circuit is made by adding appropriately doped Si layers to a conventionally manufactured circuit (1100 ° C diffusion) with ~ cold "epitaxy (600 0C atomic strat epitaxy).

The integration of the Si-based photodiode is only an example here cited. It can also be done with other semiconductor materials, in particular with those that are suitable for the construction of lasers.

Claims (9)

Claims: photodiode with additional funds to increase the light absorption is provided, characterized in that the photodiode is on a substrate with electronics and / or other electro-optical elements e.g. Laser is integrated and that the light entry side of the photodiode and the one opposite it Light exit side are designed as a reflector and both are symmetrical or asymmetrical optical resonator, which, possibly with the help of a Tuning device, at least for part of the incident light in resonance is. 2. Photodiode according to claim 1, characterized in that the mode of operation at least one reflector wholly or partially on the effect of a center reflector (Corrugation Grating) is based. 3. Photodiode according to claim 1, characterized in that the reflector on the light exit side is essentially a citter reflector and that the We kweise the reflector on the light entry side either on the Principle of a citter reflector is based or on another physical one Principle, such as the effect of refractive index jumps or interference mirrors. 4. Photodiode according to one or more of claims I to 3, characterized characterized in that the tuning is carried out by regulating the mechanical tension on the resonator by means of the piezo effect or the magnetostrictive effect and / or takes place by a thermal influence, for example by a Peltier element. 5. Photodiode according to one or more of claims 1 to 4, characterized characterized in that the reflectors of the light entry and / or exit side at least act on part of the incoming or outgoing light. 6. Photodiode according to one or more of claims 1 to 5, characterized characterized in that the lateral light guide in the photodiode at least partially adopts a waveguide structure that guides one or more light modes. 7. Photodiode according to one or more of claims 1 to 6, characterized characterized in that the reflectors are designed so that occurring diodes conversions can be wholly or partially prevented or at least reduced. 8. Photodiode according to one or more of claims 1 to 7, characterized gt.kQfl; izLi (: l net that the substrate is silicon and / or a quiternire compound is. 9. Production of the photodiode according to one or more of the claims I to 8 on Si substrate, the electronic one already produced in the usual way Contains switching elements (e.g. transistors), characterized in that an atomic beam epitaxy process is used, the temperature of which is well below the diffusion temperature of the doping is in the integrated electronic circuit.