WO2009003848A1 - Defect-based silicon laser structure - Google Patents

Abstract

Semiconductor laser diode of the VCSEL type having a silicon substrate, a silicon or silicon/germanium laser cavity which extends from a main surface of the silicon substrate into the interior of the substrate and has, as resonator end surfaces, a first mirror layer on the substrate surface intended for light emission and a buried second mirror layer in the silicon substrate; a crystalline silicon or silicon/germanium amplification region which is arranged in the laser cavity and contains structural defects of its crystal lattice and, located at the latter, a multiplicity of electronic multilevel systems which can be excited, by means of charge carrier injection of a respectively suitable charge carrier density into the active region, to spontaneously emit light and, as a whole, into a population inversion state which is suitable for optically amplifying the light by means of stimulated emission; and having silicon or silicon/germanium semiconductor regions which are laterally adjacent to the amplification region on opposite sides, have opposite conductivity, are doped in a suitable manner for charge carrier injection into the amplification region and are provided with electrically conductive contacts for applying an operating voltage which is suitable for charge carrier injection.

Description

 Defect-based silicon laser structure

The invention relates to a semiconductor laser diode. In particular, the present invention relates to a silicon-based semiconductor laser diode of VCSEL (Vertical Cavity Surface Emitting Laser) type, that is, a surface emitting laser diode having a vertical laser cavity.

WO 2006/1 17389 A1 discloses a displacement-based silicon-based light emitter. In these light emitters, the light emission is caused by certain emissions of the known silicon D-luminescence band. Luminescence with light wavelengths in the range of 1, 3 microns or 1, 55 microns can be conveyed by selective adjustment of rotation and tilt angle of the lattice structures of two adjacent silicon layers. The D-luminescence band (also referred to as D-band luminescence) is then caused by electronic multilevel systems, which are strongly localized in the region of a dislocation network at the interface between the two silicon layers, cf. T. Sekiguchi, S. Ito, A. Kanai, "Cathodoluminescence study on the tilt and twist boundaries in bonded silicon wafers", Materials Science and Engineering B 91-92 (2002), pages 244-247. In WO 2006/117389 A1 it is stated that the light emitter described there can also be designed as a laser diode. It is further described that a resonator can be produced either by mirroring the substrate edges or alternatively by mirroring the substrate surfaces. The latter corresponds to a VCSEL structure.

It would be desirable to improve the laser structure known from WO 2006/117389 A1 with regard to the intensity of its light emission.

The technical problem underlying the present invention is to provide a silicon semiconductor laser diode having light emission caused by defects, which has an increased light emission.

The technical problem is solved by a semiconductor laser diode according to claim 1. More specifically, it is a VCSEL type semiconductor laser diode

a silicon substrate,

a laser cavity made of silicon or silicon germanium, which extends from a main surface of the silicon substrate into the interior of the substrate and has, as resonator end faces, a first mirror layer on the substrate surface intended for light emission and a buried second mirror layer in the silicon substrate;

- A arranged in the laser cavity gain region of crystalline silicon or silicon-germanium containing structural defects of its crystal lattice and to this contains a plurality of electronic multi-level systems, which are excitable by carrier injection of a suitable charge carrier density in the active region - the spontaneous emission of light and as a whole into a population inversion state suitable for optically amplifying the light by stimulated emission;

and with

lateral and opposite conductive semiconductor regions of silicon or silicon germanium, which are suitably doped for the charge carrier injection into the amplification region and provided with electrically conductive contacts for applying an operating voltage suitable for the charge carrier injection.

With the semiconductor laser diode according to the invention it is possible to exploit defect-based luminescence of silicon or silicon germanium to form a semiconductor laser to stimulated emission with greater light intensity. For this purpose, in cooperation, on the one hand, a lateral charge carrier injection caused by the laser structure into the amplification area during operation of the diode and, on the other hand, the second mirror layer embedded in the substrate contribute to this.

Embodiments of the semiconductor laser diode according to the invention will be described below. The additional features of the embodiments may be combined to form further embodiments of the semiconductor laser diode of the invention, unless explicitly described as alternatives.

The structural defects used for optical amplification of the light in the gain region of the semiconductor laser diode may be different in alternative embodiments. In one embodiment, these are dislocations in the silicon crystal lattice. In another, alternative embodiment, the structural defects are so-called rod-shaped defects (rod like defects). This type of defect is known to have radiant transitions between electronic states that occur at - A -

Room temperature lead to a luminescence in the range of about 0.75 to 0.9 eV, with a maximum at 0.85 eV.

In another alternative embodiment of the semiconductor laser diode are the structural defects that interact with oxygen precipitates. Also, oxygen precipitates in silicon lead to a light emission, which manifests itself in an emission band which has a maximum at 0.8 eV at a temperature of 80 K.

In a further alternative embodiment, the structural defects are the structural defects responsible for one of the known D-band emissions in silicon. The exact nature of these defects is still under discussion in the professional world. The D-band luminescence shows, in particular, narrow and light-intensive luminescence peaks, which are denoted by D1 to D4 and can each also occur alone or in groups with different intensities. Details on this are described in WO 2006/1 17 389 A1. It has been shown that with the laser structure according to the invention, said defect structures can be excited to stimulated emission, whereby a laser activity in the spectral range is achieved, which is particularly interesting for optoelectronic applications. Because in the spectral ranges at 1, 3 and 1, 55 micrometers optical wavelength optical fibers known to have a minimum of their attenuation, so that a light transmission over long distances is possible. So far, however, no semiconductor lasers that can be integrated into advanced silicon technology have come to market maturity.

In one embodiment, the gain region of the semiconductor laser diode is in the shape of a cylinder. The two oppositely conductive semiconductor regions are realized as follows: A first semiconductor region of these oppositely conductive semiconductor regions is likewise cylindrical, but with a smaller radius than the amplification region. This cylindrical first semiconductor region is arranged in the cylinder of the amplification region. It extends from the surface-side center of the reinforcing region in the direction of direction of the substrate interior. The extension extends at least along a portion of the central axis of the cylinder formed by the reinforcing region.

The mirror layers can either be designed as a Bragg reflector structure. Alternatively, the first or second mirror layer may also be a silicon layer containing oxygen precipitates.

In preferred exemplary embodiments, the silicon substrate is designed as a silicon-on-insulator (SOI) substrate to increase the charge carrier confinement, that is to say to reduce the range of motion of the charge carriers to a region that is possibly limited to the gain region. The SOI substrate has a silicon layer containing the gain region toward the surface and a buried insulator layer adjacent to the substrate interior adjacent to the gain region. In this embodiment, the buried second mirror layer is arranged in a silicon layer adjoining the insulator layer to the substrate interior.

Embodiments of the semiconductor laser diode according to the invention are also described in the dependent claims.

Hereinafter, various embodiments of the semiconductor laser diode will be explained with reference to the figures. Show it:

Figure 1 is a schematic cross-sectional view of a first embodiment of a laser diode according to the invention;

Figure 2 is a schematic plan view of the laser diode of Figure 1;

Figure 3 is a schematic cross-sectional view of a second embodiment of a laser diode according to the invention;

Figure 4 is a schematic cross-sectional view of a third embodiment of a laser diode according to the invention. FIG. 1 shows a schematic cross-sectional view of a first exemplary embodiment of a laser diode 100. The laser diode 100 is shown in FIG. 1 in a detail which does not cover the entire width and depth of the component.

The laser diode 100 has a silicon substrate 102, which is manufactured according to one of the production methods for crystalline (bulk) silicon customary in semiconductor technology. By subsequent processing of the substrate, a volume with dislocations, rod-like defects, oxygen precipitates or the like has developed in an amplification area 104. The defects can be generated locally with well-known techniques such as ion implantation and / or thermal treatment.

The gain region 104 is viewed in the vertical direction, which is shown in FIG. 1 as a z-direction, in a laser cavity 106 which is delimited by two mirror layers 108 and 110. The mirror layer 108 is formed on the surface of the silicon substrate 102. The mirror layer 110 is embedded in the substrate at a distance from the reinforcing region.

The extension of the laser cavity 106 in the vertical direction is selected according to a desired emission wavelength of the laser light to be imitated. A high efficiency of the laser diode is achieved if the expansion of the laser cavity in ensures a resonance peak in the spectral range of the maximum gain of the gain region 104. This can be considered according to the used amplification mechanism in the design of the laser diode accordingly.

The mirror layers 108 and 110 in the present embodiment are Bragg reflectors.

The p-type gain region is in the lateral direction, which is also referred to as x-direction in FIG. 1, between an n-conductive region 12 and a p-conductive region 1 14 arranged. The reinforcing region 104 is approximately cylindrical. In its center extends along the cylinder axis, the likewise cylindrical n-type conductive region 1 12, the depth extent and radius, however, are smaller than that of the gain region 104. The reinforcing region 104 is annular in the lateral direction of the p-conductive substrate region 1 14 surrounded. Also, the substrate region 116 disposed between the gain region 104 and the buried mirror layer 110, which forms part of the laser cavity 106, is p-doped.

The n-conductive region 112 is provided with a metal contact 118, which can be supplied via a feed line 120 with an operating voltage. Also, the p-conductive region 1 14, 1 16 is contacted by means of an annular contact structure 122, which can be supplied by means of a supply line 124 with voltage.

The structure of the contacts is clearly visible in the plan view shown in FIG. An insulator region 126 separates the contact 122 from the lead 120. The insulator layer 126 also extends outside the annular contact structure on the surface of the semiconductor laser diode 100 as seen in the plan view of FIG.

Figures 3 and 4 show alternative embodiments of semiconductor laser diodes which differ from the semiconductor laser diode 100 of Figures 1 and 2 in that an SOI substrate is used as the substrate. In FIGS. 3 and 4, reference numerals corresponding to those of FIG. 1 are used. Only differing from the embodiment of Figure 1 structural elements are provided with new reference numerals. Modified, but in their function relevant elements of the laser diode of Fig. 1 corresponding structural elements are indicated by a high line.

The semiconductor laser diode 300 of FIG. 3 differs from the semiconductor laser diode 100 in that a buried oxide layer 130 is arranged in a depth region adjoining the lower edge of the gain region 104. Between the lower edge of the buried oxide layer 130 and the trenched mirror layer 110 is the correspondingly reduced in thickness p-type silicon region 1 16th

The laser diode 400 of FIG. 4 differs from the laser diode 300 of FIG. 3 in that the buried oxide layer directly adjoins the n-conductive region 112 in the depth direction.

Both alternative laser diodes 300 and 400, which are based on an SOI substrate, have the advantage during operation of the laser diode that the range of motion of the charge carriers injected into the amplification area by the applied operating voltage is reduced in the depth direction, so that no charge carriers can diffuse into the substrate region 16 , This increases the efficiency of the laser diode in comparison with the embodiment of Figure 1.

In another exemplary embodiment, which is not shown here, the buried oxide layer (it is also possible to use an insulator material other than silicon dioxide) itself is used as a mirror layer, so that the formation of a separate, separate mirror layer in the substrate can be dispensed with. This additionally simplifies the structure.

Claims

claims 1. semiconductor laser diode of the VCSEL type with a silicon substrate, a laser cavity made of silicon or silicon germanium, which extends into the substrate interior starting from a main surface of the silicon substrate and has, as resonator end faces, a first mirror layer on the substrate surface intended for light emission and a buried second mirror layer in the silicon substrate; a reinforcement region made of crystalline silicon or silicon germanium arranged in the laser cavity, which contains structural defects of its crystal lattice and a multitude of electronic multilevel systems which can be excited by charge carrier injection of respectively suitable charge carrier density into the active region for the spontaneous emission of light and as a whole in a population inversion state suitable for optically amplifying the light by stimulated emission; and with lateral and opposite conductive semiconductor regions of silicon or silicon germanium, which are suitably doped for the charge carrier injection into the gain region and with electrically conductive contacts for applying a provided for the carrier injection suitable operating voltage. 2. A semiconductor laser diode according to claim 1, wherein the structural defects are dislocations. A semiconductor laser diode according to claim 1, wherein said structural defects are rod-shaped defects. 4. The semiconductor laser diode according to claim 1, wherein the structural defects are filled with oxygen precipitates. The semiconductor laser diode of claim 1, wherein the structural defects are the structural defects responsible for one of the known D-band emissions in the silicon. A semiconductor laser diode according to any one of the preceding claims, wherein the gain region is in the shape of a cylinder and wherein a first semiconductor region of the oppositely conductive semiconductor regions is also cylindrical but of smaller radius than the gain region and extends from the surface side center of the gain region Direction of the substrate interior extends at least along a portion of the central axis of the cylinder formed by the reinforcing region. 7. A semiconductor laser diode according to claim 1, wherein the first or second mirror layer forms a Bragg reflector structure. 8. The semiconductor laser diode according to claim 1, wherein the first or second mirror layer is a silicon layer containing oxygen precipitates. 9. The semiconductor laser diode according to claim 1, wherein the silicon substrate is a silicon on insulator substrate facing the substrate surface Containing reinforcing region-containing silicon layer and the substrate interior adjacent to the reinforcing region includes a buried insulator layer, wherein the buried second mirror layer is disposed in a direction of the substrate interior to the insulator layer adjacent silicon layer.