SiGe Quantum Cascade Lasers
Quantum cascade (QC) devices based on SiGe heterostructures are being studied to develop a Si-based laser in the mid-infrared spectral range. In QC lasers, the light is generated by intersubband optical transitions between the quantized states. Intersubband transitions are by nature direct, and hence a QC laser based on the indirect Si and Ge material systems is a conceivable avenue towards the realization of an efficient light emitter based on Si.
Recently, we have realized the first electroluminescence signal originating from intersubband transitions in a SiGe heterostructure. Possible applications of SiGe QC lasers include free space communication within the atmospheric window or the detection and trace analysis of the organic materials.
SiGe QC structures are deposited by low temperature MBE, typically at 300°C, on SiGe pseudosubstrates. CVD-grown SiGe and CMP treated pseudosubstrates were obtained from STMicroelectronics. These substrates contain a SiGe buffer composed of a graded and a section of constant Ge concentration (30-50%). Typical QCL structures contains 15-30 periods (cascades), each containing a 10-30 layer Si/SiGe layers with thicknesses between 5 – 30 Angstroem. The thicknesses and concentrations are chosen in a way that the entire structure is strain-symmetrized with respect to the pseudosubstrates, thus inhibiting relaxation processes during growth.
Figure 1a depicts the valence band alignment of a single cascade of a so called bound-to-continuum (BtC) QC structure. Here, holes tunnel trough the injector barrier into the upper heavy hole (HH) state where they radiatively drop into the lower HH miniband, emitting a photon at 170 meV. Subsequently, the holes quickly decay into lower lying energy states and are then efficiently transferred into the next cascade. Figure 1b represents a scanning electron micrograph of a device processed by optical lithography and chemical etching techniques. Here, we use a rich waveguide design to drive the current through the device and to couple out the light at the cleaved edges. Figure 1c shows the current density behavior in dependence of the applied voltage. A high threshold current up to 7 kA/cm2 can be achieved, which is sufficient to operate a quantum cascade laser. The nonlinear current density behavior for low voltages is attributed to a misalignment of the injector level with the upper HH state. Electroluminescence measurements have been carried out, which are seen in figure 1d. The spectrum shows a fully TM-polarized peak at 170 meV, which is in good agreement with the theoretical prediction. However, no lasing is achieved for the BtC design, which is most likely due to a short upper state lifetime, owing to interdispersed light hole (LH) states between the upper and lower laser level.
In order to push up the upper state lifetime, we are in the process of growing and characterizing a novel QC structure, based upon compressively and tensile strained SiGe barriers and quantum wells (xGe ~ 0.2 – 0.6), grown strain-symmetrized upon a SiGe virtual substrate. The main advantage of the novel QC structure with respect to the BtC design lies in the fact that there are no interdispersed states between upper and lower laser level. In order to assess the impact of interdispersed states upon the upper state lifetime, pump and probe measurements on SiGe test structures with and without interdispersed states have been carried out, The experiments yield a substantial increase of the upper state lifetime from 0.5 ps up to 24 ps in the absence of interdispersed states is seen. Such novel QC structures have been grown by MBE, and the samples show a pristine structural quality. Optical investigations are being accomplished at the moment. We are confident that such novel QC structures without interdispersed states between upper and lower laser level indicates a possible route towards the realization of a silicon-based quantum cascade laser.
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