Targeting Telecoms With Nanowires
Nanowires lasers could provide the telecom industry with a widely tuneable source that is easy to integrate with silicon photonics. BY GUOQIANG ZHANG FROM NTT CORPORATION
Almost all devices produced by our industry are formed from compound semiconductor heterostructures. By controlling the composition, doping and thickness of every layer within an epitaxial structure, we are able to produce a wide range of devices with excellent characteristics.
One such device is the edge-emitting InP-based laser. Generating sales of billions of dollars, this class of laser, which operates in the spectral domain spanning 1.2 µm to 1.6 µm, is a key ingredient in optical communications networks.
To produce these telecom lasers, engineers load InP substrates into MOCVD reactors and deposit an epitaxial stack containing layers of InP and InGaAs(P). It is a material combination that has its pros and cons - by adjusting the composition, the designer can adjust the wavelength of the laser's emission, targeting absorption minima in optical fibres; but there are mismatches in lattice and thermal expansion coefficient between InP and its related alloys, and this hampers further integration into functional systems and platforms.
Figure 1. (a) Bandgap and lattice constant of III-V compound semiconductors. InP/InAs can cover a wide spectral range, including the telecom band. (b) Lattice-matching constraints are far less severe in InP/InAs heterostructure nanowires connected by a single indium particle, than in conventional heterostructures. In these nanowires, the quantum confinement effect along the axial direction allows the real bandgap of the InAs layer to be modulated by the thickness.
At NTT, Japan, we are developing a technology that is not held back by these issues. To break the shackles we are heading in a new dimension - rather than working with planar material, we adopt a nanowire structure that is far better at accommodating strain. This opens up new opportunities for heterostructures and functional devices.
By targeting the 1.2 µm to 1.6 µm domain with our nanowire lasers, as well as providing potential sources for data transmission through fibre, we have the opportunity to turn to mature silicon photonics platforms for integration - that's because silicon is transparent at telecom wavelengths.
We form our telecom-band single-nanowire lasers from InP/InAs quantum heterostructure nanowires. The pairing of InP and InAs can create quantum heterostructure with a wide spectral range that includes the telecom band, thanks to a substantial quantum confinement in an InAs quantum disk along the axial direction (see Figure 1).
In planar structures, growing InAs layers on InP tends to result in a high density of dislocations in the InAs active layers, due to significant lattice mismatch - it is 3.1 percent (see Figures 2 (a)-(c)). This mismatch makes it extremely challenging to form dislocation-free InAs active layers with excellent optical properties.
It's a very different state of affairs with the nanowire structure. Due to its nanoscale diameter, its crystalline lattice can be deformed laterally, relaxing the strain induced by lattice mismatch (see Figure 2 (d)). A dislocation-free active region results, with the nanowire heterostructure enduring strain caused by the large lattice mismatch.
Figure 2. Strain relaxation for crystal materials with lattice mismatch varies between conventional heterostructures and nanowires. (a) Lattices of crystalline materials before epitaxy. (b) Coherent growth by elastic deformation of the epi-layer lattice. (c) Incoherent growth (strain relaxation by mismatch dislocations). (d) Coherent growth by elastic deformation of both lattices. In contrast to the film structure, the lattice in the nanowire structure can deform much more along the radial direction because of its microscale or nanoscale diameter. Thanks to this, nanowires can endure higher strain induced by lattice mismatch.
The most common approach for nanowire synthesis is bottom-up, vapour-liquid-solid growth. It offers high controllability regarding composition, doping, the heterostructure architecture and diameter (see Figure 3).
Researchers have used gold-particle-catalysed vapour-liquid-solid growth to fabricate various nanowires, including those based on GaAs and InP. Using gold is not ideal, because it is incompatible with the mainstream CMOS process and consequently hinders integration of optically active III-V nanowires with silicon technology. Far better alternatives for producing compound semiconductor nanowires by the vapour-liquid-solid growth approach are particle catalysts based on either group III or V elements - and out of these two the former is more suitable.
Figure 3. Schematic diagram of vapour-liquid-solid growth mode. InAs and InP nanowires can be grown by the self-catalysed vapour-liquid-solid mode using indium particles.
We have established a growth technology for making InP/InAs heterostructure nanowires by the self-catalysed vapour-liquid-solid mode. This approach provides high controllability, allowing us to insert multiple InAs quantum discs that emit in the telecommunication domain into our InP nanowires (see Figure 4).
The high quality of our structures is evident in high-angle annular dark-field scanning transmission electron microscopy, which reveals the multiple quantum disc layers along the  direction (see Figure 4 (c)). Our structures result from a vapour-liquid-solid growth mode that is dominant along the axial direction. Thanks to the low growth temperature, negligible growth occurs in the radial direction via the conventional film growth mode - this is the uncatalyzed mode. With all these factors at play, there is high growth controllability, ensuring homogeneity of the gain medium throughout the entire multiple quantum disc region.
Imaging our nanowires with high-resolution scanning tunnelling electron microscope allows us to directly visualize the stacking of the InP/InAs heterostructure and the interface traits (see Figures 4 (d) and (e)). These images highlight a dislocation-free InAs/InP interface, despite the 3.1 percent lattice mismatch between InP and InAs. When InAs is grown on InP, the interface is atomically abrupt (see Figure 4(c)); and when InP is added on InAs, a gradient layer results, due to the strong reservoir effect of arsenic in indium catalyst particles. Due to coherent interfaces within the InP/InAs multiple quantum disc structure, there is relatively high strain, stemming from the lattice mismatch. As the InP layers are much thicker than the InAs discs, there is compressive strain in the latter.
Figure 4. InP/InAs multi-quantum-disc (MQD) nanowires. (a) Schematic diagram of a MQD heterostructure nanowire and a magnified view of the heterostructure highlighting the InP/InAs MQD structure. The InAs layer is indicated in red. (b) Scanning electron microscopy images (tilt: 38º) of InP/InAs MQD nanowires grown on an InP (111)B substrate. (c) High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM ) images of an InP/InAs MQD nanowire taken along the  zone axis. The nanowire contains 400 units of InP/InAs heterostructure. ((d) and (e)). Aberration-corrected HAADF-STEM images of InP/InAs heterostructures taken along the  zone axis. The horizontal white arrow indicates the growth direction. The thicknesses of the InAs layer and the InP barrier layer are 9.0±1 nm and 25.6±1 nm, respectively.
We turned to micro-photoluminescence to characterise the optical properties of our nanowires. When they are mechanically dispersed onto foreign substrates, we can remove the indium particles at their tips to create a (111)-facet mirror - and thus a Fabry-Pérot cavity. This structure has intrinsic Fabry-Pérot resonance modes with a sufficiently large mode index in a wide diameter range.
Optically pumping our nanowires leads to lasing behaviour. Measuring the photoluminescence of numerous individual nanowires reveals a broad spontaneous emission spectrum, with a peak wavelength around 1.57 µm to 1.59 µm. When pumped with an excitation power of 2.15 mJcm-2, a spike centred at 1573 nm appears. Plotting the light input-output curve by varying the excitation power uncovers a strong transition from spontaneous to stimulated emission.
Figure 5. (a) Scanning electron microscopy image of a nanowire mechanically dispersed onto a gold-film-covered SiO2-on-silicon substrate. The white arrow indicates a removed indium particle. (b) Photoluminescence spectrum of a single nanowire under a pump laser power of 0.83 mJ•cm-2. (c) Photoluminescence spectra of the nanowire with increasing pump power, revealing spectral narrowing.
One of the merits of our nanowire lasers is that their wavelength can be tuned over a wide range, such as the entire telecom band. By tuning the thickness of single InAs quantum discs over a wide range, we are able to modulate the photon emission energy. To adjust the thickness of our discs, we vary the flow rate of the source materials (see Figure 6). Based on the results realised by this approach, our nanowires promise to provide modulation of the lasing wavelength over a broad range. We can stretch as far as 2.6 µm into the infrared by forming pure InAs nanowires with a wurtzite crystalline structure (note that there are no quantum confinement effects in these structures).
Figure 6. Nanowires allow a tuning of the laser wavelength range in the telecom band. Spectra are offset for clarity. The laser wavelength is modulated by the growth parameters, such as flow rates of the metal-organic sources. Thus a broad wavelength range is covered in the whole telecom band, including the two technologically important telecom-band windows centred around 1.3 µm and 1.55 µm.
So far, we have described optically pumped devices. However, many applications, including photonic integration, demand current-injection light emitters. It is incredibly challenging to realise current-injection lasing from single nanowires, because making a direct electrical contact to them deteriorates light confinement. However, we have taken important steps towards this goal, demonstrating LEDs with p-i-n-structured nanowires that emit in the telecom range.
Fabrication of these current-injection emitters began with the growth of an InP/InAs p-i-n structure, using the self-catalysed vapour-liquid-solid mode. Growth on InP (111)B substrates enabled vertically aligned nanowires (see Figure 7). They feature an InAs active layer as thin as around 9 nm. As that's significantly below the Bohr radius of bulk crystalline InAs, which is around 34 nm, this ensures quantum confinement along the axial direction.
Figure 7. Morphological and structural analysis of InP/InAs heterostructure nanowires. (a) and (b) Cross-sectional scanning electron microscopy images of InP/InAs nanowires. The inset shows the p-i-n structure of these nanowires. (c) and (d), high-angle annular dark-field scanning transmission electron microscopy images of multi-quantum-disc active region between zinc-doped and sulphur-doped InP segments. The MQD active region consisting of 10 InAs quantum discs. (e) A diagram of the nanowire LED device. (Figure reprinted with permission from IOP publishing).
To form our novel LEDs, we began by embedding these nanowires in transparent insulating benzocyclobutene. Subsequent reactive-ion etching then removed this organic from the surface, exposed the nanowire tips, and ultimately paved the way to adding electrical contacts. Depositing AuZn metal onto the p-InP substrate and indium-tin-oxide onto the n-InP nanowire segment created electrical contacts for this device.
We have investigated the optoelectronic characteristics of our nanowire LEDs. The current-voltage (I-V) behaviour mirrors that of conventional LEDs (see Figure 8). To observe the electroluminescence of single nanowires, we carried out micro-electroluminescence measurements at room temperature. One can see the luminescence from nanowires within an area of just 380 µm by 475 µm (see Figure 8 (c)). In general, a single bright spot corresponds to a single nanowire LED. As we increased the bias, more and more nanowires exhibited a continuously increasing electroluminescence intensity.