Technical Insight
Nanowires enhance laser performance in the deep UV
BY SONGRUI ZHAO, XIANHE LIU, AND ZETIAN MI FROM McGILL UNIVERSITY, CANADA
Ultraviolet light plays a crucial role in countless tasks that benefit humanity. Uses include sterilization, water purification, preventing counterfeiting, and the analysis of chemical and biological samples.
Today, the dominant sources of UV emission are mercury lamps, a few types of gas laser, and lasers based on frequency conversion. Unfortunately, all these UV sources tend to suffer from a low efficiency and a large footprint. What's more, their operation requires a toxic gas and produces a significant amount of mercury emission, which contaminates our environment. For example, back in 2005, more than 200 tons of mercury was emitted to air from mercury containing products, such as mercury lamps. Exposure to mercury has a devastating impact, deteriorating the health of millions of people and damaging the developing fetus. Clearly, what the world needs are greener, more efficient UV sources. Thankfully, this is possible by turning to compact compound semiconductor devices.
Given the drawbacks of the incumbent UV sources, it is not surprising that development of AlGaN-based, deep-UV lamps has attracted significant attention during the past decade. AlGaN is an ideal alloy for making a UV laser, as its bandgap can be tuned to deliver emission that spans from 200 nm to 364 nm. However, with the III-N material system, efficient sources are far harder to realise in the deep UV than in the blue, green, and near-UV. External quantum efficiencies of AlGaN quantum well deep-UV LEDs are typically just a few percent. Additional concerns are an operating wavelength for electrically injected quantum-well laser diodes that is limited to 336 nm, and a threshold current density on the order of tens of kA/cm2. These weaknesses stem from material issues: dislocation and defect densities in the AlGaN material system are high, and current conduction in aluminium-rich AlGaN is incredibly inefficient.
Recently, our team at McGill University has tackled these critical challenges with AlGaN nanowire structures, which are grown on a silicon substrate by plasma-assisted MBE under nitrogen-rich conditions. The devices that we form are nearly free of defects and dislocations, and benefit from a tremendous reduction in the formation energy of the magnesium-dopant near the surface of the nanowires. This enhances magnesium-dopant incorporation and increases the efficiency of current conduction.
Another great attribute of our devices is that they incorporate quantum-dot-like nanostructures into core-shell AlGaN nanowire arrays. This leads to a significantly reduced transparency current density, thanks to the three-dimensional quantum confinement of charge carriers. Thanks to this, we have been able to produce electrically injected semiconductor lasers operating in the UV-AII, UV-B, and UV-C bands.
Nanowire lasers
Regardless of the form of laser, it must provide optical feedback, because this holds the key to increasing the population of photons and ensuring lasing. With a conventional Fabry-Pérot laser, feedback is realised with mirror facets and a long gain medium; and in our sub-wavelength scale nanowire arrays, it occurs via a phenomena known as Anderson localization, wherein the recurrent light scattering leads to strong photon confinement (see Figure 1 (a)). Note that the strength of the light localization is governed by the orientation, size, and filling fraction of the nanowire arrays.
In our devices, AlGaN random nanowire arrays are vertically aligned on a silicon substrate (see Figure 1(b) for a typical scanning electron microscopy image). The nanowires also function as a gain medium − it has a very small cavity size compared with that of a Fabry-Perot laser.
Detailed simulations reveal that there is a high probability of forming high-Q cavity nanowire arrays when the average diameter of the nanowire is 60 nm to 75 nm, the fill factor of the array ranges from 15 percent to 55 percent, and the emission wavelength of the source is 290 nm (see Figure 1(c)).
We have also simulated the in-plane electric field component of the optical field (see Figure 1(d), where image size is 2.35 µm x 2.35 µm). These calculations highlight the strong confinement present in randomly arranged nanowire arrays.
Figure 1: (a) Light recurrent scattering in AlGaN nanowire arrays provides the optical feedback required for lasing. (b) Scanning electron microscopy reveals the nature of the self-organized AlGaN nanowires on silicon. (c) The probability of forming cavities with a Q-factor higher than 1,000 for a wavelength of 290 nm, as a function of average nanowire diameter and filling factor. (d) In-plane electric field distribution. (e) The electric field (left axis) and effective refractive index (right axis) as a function of the distance from the substrate, showing the optical confinement along the nanowire vertical direction by the tapered nanowire morphology.
In our nanowire lasers, vertical optical confinement results from the effective refractive index variations along the nanowire growth direction. Due to the tapered nanowire morphology, the effective refractive index reaches a minimum at the nanowire bottom, and has a maximum value at the nanowire top (see Figure 1 (e)). Consequently, phonons are strongly confined near the nanowire laser active region. This is beneficial, reducing optical leakage through the underlying silicon substrate.
Strain benefits
Due to the large surface-to-volume ratio and the resulting highly efficient strain relaxation, nanowires offer several advantages, including direct growth on foreign substrates, such as silicon, low defect and dislocation densities, and smaller polarization fields, compared to conventional planar structures. Often, however, this has failed to lead to impressive optical emission efficiencies, due to significant non-radiative surface recombination.
To address this challenge, we have developed AlGaN core-shell nanowire heterostructures that suppress non-radiative surface recombination with a wide bandgap, aluminium-rich AlGaN shell. The structure of our single AlGaN nanowire consists, along the growth direction, of n-GaN, n-AlGaN, i-AlGaN, p-AlGaN, and p-GaN (see Figure 2(a)).
The presence of an aluminium-rich AlGaN shell is clearly revealed with energy dispersive X-ray spectrometry (see Figure 2 (b)). The aluminium-rich AlGaN shell is spontaneously formed during epitaxial growth, due to aluminium adatoms undergoing slower migration than those of gallium. The resulting increase in the aluminium content in the near-surface region is beneficial, leading to a strong carrier confinement, which slashes non-radiative surface recombination and boosts the carrier injection efficiency into the active region.
Fabrication of electrically injected deep-UV lasers is not easy, so some groups have developed optically pumped AlGaN quantum well lasers in the UV-B (280 nm to 315 nm) and UV-C (200 nm to 280 nm) bands. Threshold powers are of the order of
0.1 MW/cm2 to 1 MW/cm2; and due to the large bandgap and the large effective mass of electrons and holes in AlGaN, to reach transparency, the carrier densities must be of the order of 1019 cm-3 or higher "“ that is, the carrier density must reach this value to realise the onset of population inversion in conventional AlGaN quantum wells.
Turning to three-dimensionally quantum-confined nanostructures, such as quantum dots, enables a dramatic reduction in the transparency carrier concentration "“ and ultimately, the lasing threshold. However, prior to our work, there were no demonstrations of AlGaN quantum dot lasers in the deep UV.
Figure 2: (a) Schematic of AlGaN core-shell nanowire heterostructures. (b) Energy-dispersive X-ray spectrometry (EDXS) line scans along the radial direction of the active region indicate the presence of aluminium-rich, AlGaN shells. The red line in the inset denotes where EDXS line scans were performed. (c) High-resolution scanning tunnelling electron microscopy reveals highly localized compositional modulations of alternating gallium-rich(brighter)/aluminium-rich(darker) planes at the atomic-scale. (d) The gallium-map (displayed in temperature-scale) and concurrently acquired annular dark-field (ADF) signal from electron energy loss spectroscopy − spectral imaging (EELS-SI) at atomic-resolution, showing a direct correspondence between the local increases in gallium-signal with the ADF signal within single atomic-planes.
Our success stems from discovering a new mechanism for forming quantum-dot-like nanostructures in nearly defect-free AlGaN nanowire arrays. According to detailed scanning transmission electron microscopy images, our aluminium-rich AlGaN nanowires feature extensive gallium-rich nanoclusters (see Figure 2(c)). They are between 0.25 nm and 2 nm high (height measured in nanowire axial direction, which is the growth direction), and 2 nm to 5 nm wide (the nanowire radial dimension).
Electron energy-loss spectroscopy enables elemental mapping of the gallium atoms in this structure (see Figure 2(d), which also show a corresponding high-resolution, dark-field scanning tunnelling electron microscopy image). This spectroscopic technique provides an estimate of the localized variation in the gallium concentration of 5-10 atomic percent. The formation of quantum-dot-like nanostructures in the AlGaN nanowires is directly related to the interplay between the chemical ordering and the non-uniform incorporation of gallium and aluminium adatoms at the nanowire growth front.
Conduction challenges
One of the greatest challenges associated with the fabrication of an electrically pumped AlGaN laser in the mid or deep UV is the realisation of sufficient current conduction. There is a tendency for this to be extremely poor, due to the large activation energies for silicon and magnesium dopants, and significant carbon impurity incorporation in the conventional MOCVD growth process. Making matters worse, defects, such as nitrogen vacancies, can further compensate holes in magnesium-doped AlGaN.
To overcome this challenge we have systematically investigated magnesium dopant incorporation in AlN nanowires from a theoretical and an experimental standpoint (see Figure 3 for the result of a first-principle calculation of aluminium-substitutional magnesium formation energy as a function of doping position in AlN). Our calculations show that magnesium-dopant formation energy is significantly reduced in the near-surface region of the nanowires. This implies that magnesium-dopant incorporation in nanowire structures will be significantly higher than that in conventional planar structures. What's more, experiments show that growth under nitrogen-rich conditions can suppress the formation of defect-related nitrogen vacancies.
Figure 3: Ab-initio calculation of magnesium doping into AlN nanowires. (a) Illustration of the structure used for calculation. (b) Aluminium-substitutional magnesium formation energy along the nanowire radial direction. Index "1" indicates the surface. As this number increases, it denotes the position towards the bulk region of nanowires. It is seen that the near-surface region has much lower formation energy than in the bulk region.
Another key finding is that the primary p-type conduction in AlN nanowires is through the magnesium impurity band. That is good news, because it enables extremely efficient, nearly temperature-invariant current conduction, which was not previously possible in Al(Ga)N epilayers. The activation energy in magnesium-doped AlN nanowires is just 20-30 meV, compared with 500-600 meV for magnesium-doped AlN epilayers.
Armed with these insights, we have demonstrated the first AlN nanowire LEDs. These 300 µm by 300 µm chips have a forward voltage of only around 7 V at 20 mA. This very promising current-voltage characteristic highlights the efficient current conduction in AlN nanostructures, and lays a great foundation for the fabrication of electrically injected AlGaN deep UV lasers.
We produce these lasers with standard optical lithography and metallization techniques. Fabrication of conventional, large-area nanowire devices often involves the use of polymers for surface passivation and planarization. However, they are not suitable for our devices, because they are strong absorber of deep UV light. So we use a relatively thin p-metal contact, which is directly deposited on the top surface of the AlGaN nanowires at a tilting angle (see the inset of Figure 4(a)).
Our device exhibits excellent current-voltage characteristics (see Figure 4(a)), and it can produce a CW output at 6K (Figure 4(b)). As the injection current increases, a sharp lasing peak appears around 334 nm, superimposed on a background emission (see Figure 3). A clear lasing threshold occurs at around 10 A/cm2, a current density that is several orders of magnitude lower than that of a conventional AlGaN quantum well laser operating at 336 nm. We attribute this ultralow threshold to the excellent carrier confinement and the high quality of the optical cavity.
Figure 4: (a) Current-voltage characteristics of a 334 nm AlGaN nanowire laser, with the inset showing the fabricated device. (b) The lasing spectra measured at 6 K from 7.7 A/cm2 to 22 A/cm2. (c) Integrated intensity as a function of injection current.
Figure 5: (a) The emission spectra of a 289 nm AlGaN nanowire laser measured under continuous-wave operation at 10 µA (below threshold) and 80 µA (above threshold) at room temperature. (b) The integrated electroluminescence intensity as a function of injection current. The inset shows the plot in a logarithmic scale. (c) Emission spectra of a 262 nm AlGaN nanowire laser measured at an injection of 50 µA (above threshold) and 7 µA (below threshold) at 77 K.
Tuning the alloy composition enables the production of electrically injected AlGaN nanowire lasers in the UV-B and UV-C bands. For our 289 nm laser, a broad emission spectrum is produced at a low injection current, while a clear threshold occurs at 30 µA, and by 80 µA a sharp lasing peak has emerged (see Figures 5 (a) and 5 (b)). Plotting the light output on a logarithmic scale reveals a clear S-shape dependence, which corresponds to a transition from linear spontaneous emission to super-linear amplified spontaneous emission and then on to linear lasing emission (see the inset of Figure 5(b)). This behaviour provides unambiguous evidence for lasing.
Lasers at even shorter wavelengths can be produced with our nanowire laser technology. We have realised a 262 nm laser, which produces a sharp lasing peak when the current is ramped from 7 µA to 50 µA and the device is held at cryogenic temperatures. The threshold is 20 µA.
Our AlGaN nanowire laser technology offers a new route to the realization of electrically injected, deep-UV lasers. It could underpin the next revolution in photonics, which will see efficient, small-scale, solid-state UV light sources replacing conventional mercury lamps. Such progress can slash mercury emissions to air, soil and water. Our novel nanowire sources also promise more efficient, greener, manufacturing processes for sterilization, security, and medical diagnostics. Great opportunities lie ahead for the electrically injected, deep UV nanowire laser.
Emission from the nanowire lasers can be as short as 262 nm.
