Nanomechanical gaps between semiconductor quantum wells create electron cavities in much the same way that a microscale gap between two mirrors creates an optical cavity. We have demonstrated this with heterostructures, and we are moving on to 2-D materials to eliminate surface recombination parasitics.
Since the advent of modern epitaxial techniques and the development of semiconductor heterostructures the bandgap and other properties of semiconductors have been determined by materials and quantum size effects introduced during crystal growth. Once a semiconductor is grown, its properties are largely fixed. However, there is a strong duality between the wave behavior of electrons in the band of a solid and photons in an optical cavity. Theory indicates that electron wavefunctions confined by a heterostructure near the surface of a mechanical component will couple coherently across a small gap with another similar component. This occurs in much the same manner that photons will couple across gaps between optical cavities. Here we demonstrate that the relative mechanical position of surface heterostructures can strongly alter effective band-gap of the system in real-time giving scientists a powerful new tool to perform bandgap engineering. Our method provides larger changes than the small perturbations in energy levels that can be achieved with applied heating, stress, and magnetic or electric fields. This concept is shown for interband transitions, but it may have its largest impact on intersubband transitions and 2-D materials, where the energy state tuning will be limited only by thermal broadening at low energies and band offsets at high energies.
|Schematic of collapsed cantilever. A fiber-coupled laser excites photoluminescence in a localized region. Scanning it along the cantilever allows us to probe how the gap affects the electron states.||SEM micrograph in backscatter mode of a 40 μm wide collapsed cantilever. The top of the cantilever, its sidewall and the bottom quantum well surface are marked. The top quantum well is underneath the cantilever and not visible. The air gap vanishes on the right indicating collapse and direct surface contact. Figure b is from a similar structure to the one measured.|
In our experiments we use a micromachined cantilever such as the one in the figure above to adjust the position between two surface heterostructures. One heterostructure is located on the underside of the cantilever, and the other on top of the substrate. Surface forces during etch release cause the cantilever to collapse and form a gap whose width varies with position. In regions where the gap is large, the quantum wells do not noticeably interact with each other, and the electronic structure is that of the individual wells. As the gap decreases, the interaction through the potential barrier strengthens, causing the electron levels to shift; one increases in energy while the other one decreases. A smaller but analogous effect occurs in the valence bands. At the cantilever tip, the wells merge to form a composite well of their combined widths. In essence, one can control the bandgap of surface heterostructure by adjusting the mechanical gaps between them. The figure below compares the tuning range and wavelength region of the proposed method to experimental values in literature.
|The largest change in the energy of electron states occurs between the individual (air gap) and the coupled configuration (tip).||The maximum attainable shift across the cantilever structure depends strongly on the width of the wells. The difference of the emission wavelength is largest in thin quantum wells. However the carrier loss due to surface recombination decreases their usefulness.|
Scanning a fiber-coupled laser along the cantilever allows us to pinpoint specific regions in which to excite photoluminescence (PL). The coupling of quantum states changes across the air gap, which alters the effective bandgap of the semiconductor. Any shifts in PL are therefore indicative of changes in quantum coupling. The figure below illustrates the photoluminescence (PL) spectrum for the indicated position along the cantilever. The initial region - curve 1 - where the cantilever touches the surface has a PL spectrum of a single 400 Å quantum well, that is, two 200 Å wells in contact. The next region is the gap region, where the cantilever transitions from contact to anchor. In measurements 2 and 3 one observes that the increasing air gap alters the energy states of the electrons. Scanning further towards the anchor, however, allowed excited carriers to diffuse from the transition region to the anchor region where they recombine radiatively, hence a second peak develops. Once significant diffusion occurs, the anchor will dominate the luminescence because of its large total area and low interface recombination, which encourages bandfilling. Band filling does not account for the PL shifts in the cantilever transition region near and at the tip where surface recombination sufficiently empties the band. This was explicitly tested in this region by reducing the excitation light intensity in steps from the maximum 1200 W cm-2. No wavelength shifts were seen, indicating band filling was not occurring where the quantum coupling data was taken. At very low intensities, the anchor PL without band filling peaked at 1561 nm. This wavelength was also the peak of the photoluminescence of a surface quantum well underneath the cantilever that had been removed with tape and tested individually. This further verifies that we observed quantum coupling at the tip. The wavelength shift of 22 nm matches well with theoretical calculations.
|The photoluminescence spectrum changes as photoexcitation moves from the tip of the cantilever towards the anchor. At the tip and over much of the gap, quantum coupling is responsible for the shift. Band filling in the anchor accounts for the shift from 1560 nm to 1515 nm.||Optical micrograph of a collapsed 40 μm cantilever. The colour indicates the presence of an air gap as verified by SEM and white light interferometry. The three regions from the first figure are labeled for comparison. The circles indicate the illumination spot for the spectral measurements. Measurement 1 did not noticeably change over the tip. This image is of a similar structure; in the measured structures, the anchor forms an acute triangle.|