Patent Publication Number: US-2010117059-A1

Title: Light modulation comprising si-ge quantum well layers

Description:
The invention relates to light modulation in Si—Ge quantum well layers at wavelengths suitable for fiberoptics- communications. 
     While the role of silicon as the major material for electronics is well known, its application to optoelectronics and photonics has been less evident. The reason for this shortcoming lies in the nature of its electronic band structure, especially its indirect energy gap, as a result of which it exhibits inferior optoelectronic properties in comparison with many compound semiconductors, such as for example GaAs, InP and their alloys from which semiconductor lasers, detectors and modulators are usually made. 
     The indirect energy gap of Si has so far precluded its use as a laser material, with the exception of the recently demonstrated Raman laser, requiring optical pumping. Silicon&#39;s applications to detectors and modulators for optical communications purposes are hindered less by the nature of its gap but rather by its size, making it impossible to absorb light at the relevant wavelengths of 1.3 and 1.55 μm. In order to make Si suitable for such applications it therefore needs to be combined with other materials. While monolithic integration of compound semiconductor optoelectronic and silicon electronic functionalities would be the most desirable form of this combination, this approach has so far been hampered by materials compatibility issues (for a review on GaAs integration, see for example Mat. Res. Soc. Symp. Proc. 116 (1989), the content of which is incorporated herein by reference). 
     Germanium on the other hand is a material largely compatible with Si processing, and therefore much easier to incorporate into a Si technology, as shown for example in U.S. Pat. No. 5,006,912 to Smith et al., the content of which is incorporated herein by reference. Integrated SiGe/Si optoelectronic integrated circuits have in fact been proposed (see for example U.S. Pat. No. 6,784,466 to Chu et al., the content of which is incorporated herein by reference). 
     The application of SiGe/Si heterostructures to optoelectronic devices integrated on Si substrates is facilitated by the favorable band structure of Ge with a direct transition at the Γ-point with an energy of 0.8 eV, not far above the indirect fundamental gap of 0.66 eV. This, together with the miscibility of Si and Ge over the whole concentration range, has led to a number of proposals for device applications. Photodetectors made from epitaxial Ge layers on Si substrates have been proposed for example by Wada et al., in U.S. Pat. No. 6,812,495, the content of which is incorporated herein by reference. Optical modulators based on the Franz-Keldysh effect, in which the absorption edge is shifted in the presence of an electric field, have been proposed by Kimerling et al., in U.S. Pat. No. 2003/0138178, the content of which is incorporated herein by reference. Other concepts make use of the quantum-confined Stark Effect in SiGe quantum wells (see for example U.S. Pat. No. 2006/0124919 to Harris et al., the content of which is incorporated herein by reference). 
     It is a common feature of all prior art that optoelectronic devices have been fabricated from material epitaxially deposited by either molecular beam epitaxy (MBE) or chemical vapour deposition (CVD). It is a further common feature that optoelectronic SiGe devices suitable for operation at wavelengths of 1.3 and 1.55 μm need to be composed of Ge-rich layers, since the energies of indirect and direct band gaps rise rapidly with decreasing Ge-content in SiGe alloys. For example the energy of the direct gap at the Γ-point of a Si 1−x Ge x  alloy corresponds to a wavelength of 1.3 μm at a Ge-content of approximately x=0.95. At lower Ge-contents light with a wavelength of 1.3 μm can no longer induce a direct transition, and is therefore not efficiently absorbed. As a result, detectors and modulators made of Si-rich material require light to travel long distances or may no longer be applicable to wavelengths of 1.3 and 1.55 μm at all. 
     Unfortunately, the high Ge-contents necessary for optoelectronic devices of the kind considered above makes their fabrication by MBE or CVD cumbersome. The reason is that low growth temperatures need to be used in order to control the epitaxial growth, where especially CVD, considered to be the main production technique, becomes inherently slow (see for example see for example U.S. Pat. No. 5,659,187 to Legoues et al. and Yu-Hsuan Kuo et al., Nature 437 (2005) pp. 1334-1336, the contents of which are incorporated herein by reference). 
     A prior art technique providing fast epitaxial growth at low substrate temperatures is low-energy plasma-enhanced chemical vapour deposition (LEPECVD), which previously was applied to the fabrication of electronic SiGe material (see for example Int. Pat. Nos. WO 03/044839A2 to von Känel, and WO2004085717A1, the contents of which are incorporated herein by reference). 
     It is therefore an objective of the present invention to provide an optical modulation structure offering a sufficient optical band gap for light modulation in fiberoptics communication and being manufactured efficiently. 
     The present invention comprises optical modulators in compressively strained Si 1−1 Ge x  quantum wells with Ge-contents x chosen in a range such that the direct Γ 25′ -Γ 2′  transition, also denoted as Γ 8   + -Γ 7   −  transition in the double-group representation, lies below the Γ 25′ -Γ 15  transition. Modulation is based on a plurality of physical effects, such as the quantum-confined Stark effect (QCSE), exciton quenching or band filling by hole injection, the Franz-Keldysh effect, or thermal modulation of the band structure, or thermal modulation of the index of refraction and absorption coefficient via modulation of the carrier temperature. A preferred method of providing such structures is by growing single or multiple quantum wells onto relaxed SiGe buffer layers by low-energy plasma-enhanced chemical vapour deposition (LEPECVD). 
     According to one aspect of the present invention LEPECVD provides a method for growing strain-compensated Si 1−y Ge y /Si 1−x Ge x /Si 1−y′ Ge y′  quantum wells onto relaxed SiGe buffer layers acting as pseudosubstrates, where x&gt;y, y′, and y and y′ may vary along the growth direction, preferably y and y′ may increase along the growth direction. 
     According to another aspect of the present invention LEPECVD provides a method for fabricating single or multiple quantum well structures incorporating doped layers underneath the active layers. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows the band structures of pure Si and Ge; 
         FIG. 2  shows a multiple quantum well structure; 
         FIG. 3  shows two possible profiles of the Ge content in the active layer structure; 
         FIG. 4  is a reciprocal space map of a Ge/SiGe multiple quantum well structure on a relaxed graded SiGe alloy layer; 
         FIG. 5  shows reciprocal space maps of pseudosubstrates comprising a constant-composition SiGe alloy layer; 
         FIG. 6  shows a modulator structure with Schottky contact on the top; 
         FIG. 7  shows absorption spectra of a Ge/SiGe multiple quantum well (MQW) structure grown on relaxed graded SiGe alloy layer; 
         FIG. 8  shows absorption spectra of a Ge/SiGe MQW device for various applied voltages; and 
         FIG. 9  shows a modulator structure with an integrated heater element. 
     
    
    
     The invention can best be appreciated by noting that upon alloying Si and Ge the lowest energy direct transition at the Γ-point occurs from the valence band Γ 25′  to the σ 2′  conduction band (see  FIG. 1 ), except for Ge contents below about 30%. For any optoelectronic material of interest for fiberoptic communications (λ=1.3 or 1.5 μm) the interest of the present invention to active Si 1−x Ge x  layers ranges with Ge contents above about x=0.3. 
     In one embodiment of the invention shown in  FIG. 2  an active single or multiple quantum well structure  200 , consisting of n well layers  204 ,  204 ′ and Si 1−y Ge y  barrier layers  202 ,  206 ;  202 ′,  206 ′, with x&gt;y is grown onto a strain relaxed SiGe buffer layer  100  acting as a pseudosubstrate. Preferably, the average Ge content of the active quantum well structure  200  is chosen to be close to or equal to the Ge content x f  at the top of the buffer layer, such as to provide partial or complete strain compensation. The barrier layers  202 ,  206 ;  202 ′,  206 ′ and well layers  204 ,  204 ′ may be doped or undoped. The pseudosubstrate  100  may be comprised of a Si substrate  102 , a doped or undoped Si 1−x′ Ge x′  alloy layer  104  with a graded Ge content, whereby x′ can be established in the range of the value for x f . Following layer  104  another doped or undoped layer  106  is grown with a constant final Ge content x f , determining the lattice parameter of the pseudosubstrate. Next a boron-doped layer  108  is grown, followed by an undoped layer  110 , both at a Ge content of x f . Following the active layer structure  200  a cap layer  300  is grown, which may be undoped or doped with donor impurities, and which preferably has a composition of Si 1−xf Ge xf . 
     The complete layer sequence  100 - 300  is preferably grown by LEPECVD, wherein growth time of the pseudosubstrate  100  can be minimized by choosing dense-plasma conditions offering high deposition rates, while active layer structures  200  are deposited at low rates by reducing the plasma density. The actual Ge profile in active layer structures  200  can be chosen to have a plurality of shapes, examples of which are specified in  FIG. 3 . 
     In one embodiment of the invention the active layer structure  200  is obtained by changing the Ge content in a step-wise fashion, as shown in  FIG. 3(   a ). 
     In another embodiment of the invention the Ge profile in the quantum well layer(s)  204  of active layer structure  200  is chosen to have a parabolic shape, as shown in  FIG. 3(   b ). 
     In yet another embodiment of the invention the Ge profile in the quantum well layer(s),  204  and in the barrier layers  202 ,  206  of active layer structure  200  is chosen to have a sinusoidal shape, as shown in  FIG. 3(   c ). 
     The combination of fast pseudosubstrate growth and slow active layer growth yields active quantum well structures fully strained to the underlying pseudosubstrate, as shown in  FIG. 4  for a strain-compensated active layer structure  200  having the step-like Ge profile of  FIG. 3(   b ).  FIG. 4  is a X-ray reciprocal space map in the vicinity of an asymmetric &lt;224&gt; reflection, showing that the pseudosubstrate  100  graded to a final Ge content of 70% is fully relaxed, while the active layer structure, comprising tensile-strained Si 0.45 Ge 0.55  layers and compressively strained Ge layers, is coherent with the pseudosubstrate. Similar strain compensated quantum well structures have been obtained on pseudosubstrates final Ge contents x f  of 80 and 90%. 
     In another embodiment the pseudosubstrate comprises a Si 1−x′ Ge x′  buffer layer with a constant Ge content. This has the advantage of smoother surfaces since the surface cross-hatch normally present on graded buffer layers is absent in this case. According to the present invention Ge-rich Si 1−x′ Ge x′  buffer layers deposited by LEPECVD at constant Ge content x′ are fully strain relaxed, even in the absence of a post-growth anneal. This can be seen in the X-ray reciprocal space map of  FIG. 5  for buffer layers with Ge contents of 70, 80 and 90%. The layers were epitaxially grown on Si(001) at a substrate temperature of 520° C. As can be seen from  FIG. 5 , the mosaic structure of the x′=70% sample is most pronounced, indicating that the crystal quality deteriorates with decreasing Ge content. The quality can, however, be improved by post-growth annealing. 
     In a preferred embodiment of the invention, a boron doped layer  108  followed by an undoped spacer layer  110  is grown before the active layer structure  200 . According to the invention boron segregation into the active layer structure  200  can be prevented by employing the following means. First the substrate temperature is decreased to at least 550° C. during growth of buffer layers  104  and  106 . In a second step, the plasma density is lowered by about a factor of about ten before the boron doped layer  108 . This has been shown to be effective in preventing dopant segregation induced by ion bombardment. In a third step the boron doped layer  108  is grown at reduced plasma density, and preferably reduced temperature to below 520° C., by introducing a diborane containing gas to the deposition chamber. Boron segregation can further be minimized by admixing a flux of hydrogen gas during growth of doped layer  108  and subsequent undoped layer  110 , whereby the hydrogen flux is preferably chosen to be larger than the flux of the doping gas. For example for the structure of  FIG. 4  the hydrogen flux was twice as large as the flux provided by the mass flow controller introducing the doping gas. 
     In one embodiment of the invention a quantum well structure, such as one of those shown in  FIG. 3 , is provided with a top electrode  400 , as shown in  FIG. 6  in cross-section. Electrode  400  may be a Schottky junction or an n-doped semiconductor layer, such as poly-silicon doped with donor atoms or an n-doped epitaxial SiGe layer or a metal-insulator junction, or an ohmic contact. The device of  FIG. 6  can be fabricated in a plurality of modifications, such as to allow light to enter either through the top, or through the substrate, or from the side in case of a waveguide configuration. 
       FIG. 7  shows absorption spectra for temperatures between 20 and 300 K, obtained on a MQW structure coherent with a pseudosubstrate  100 , graded to a final Ge content x f =0.9. Here, the absorption has been deduced from experimental transmission data through illumination from the top. 
     The corresponding absorption spectra, obtained on a device fabricated according to one of the embodiments of  FIG. 6 , with a Schottky barrier contact  400  on top of the quantum well structure, can be seen in  FIG. 8 . Here, the absorption, as obtained from photocurrent spectroscopy at a temperature of 17 K, is depicted on the left hand side for various applied voltages across the device. The shift of the absorption edge, corresponding to the transition from the lowest confined hole state HH1 to the lowest confined electron state E1at the Γ-point, derived from the Γ 2 , band in  FIG. 1 , is shown on the right hand side of  FIG. 8  as a function of electric field. The shift of the absorption edge can be seen to be quadratic in the electric field. 
     In another embodiment of the invention the QW structure of  FIG. 2  is illuminated by an external light source, such as a solid state laser, providing photons which are absorbed by the QW. Band filling by electron-hole pairs generated by this source may lead to phase space filling or quenching of the excitons, thereby leading to a modulation of the optical absorption. In this embodiment electrical contacts may not be needed. An electric field applied across the device may, however, help in extracting minority carriers, and thus making the device faster. 
     In another embodiment of the invention the QW structure of  FIG. 2  is illuminated by an external light source, such as a solid state laser, providing photons which are not absorbed by the QW. Here, the electric field present in the light source replaces the field applied by the contacts in  FIG. 6 , leading to a modulation of the absorption through the optical Stark effect. 
     In yet another embodiment of the invention the top contact  400  of  FIG. 6  is replaced by a heater element  500  as shown in  FIG. 9 . In this embodiment the poor heat conduction of SiGe alloys is used to modulate the temperature of the QW structure  200 . For this reason, a thick buffer layer is preferably used as the pseudosubstrate  100 . A heater element integrated on the QW structure allows for fast modulation of its band structure, thereby giving rise to a modulation of the direct optical transition energies at the Γ-point.