Growth by molecular beam epitaxy and properties of inclined GaN nanowires on Si(001) substrate

Influence of Si Substrate Preparation Procedure on Polarity of Self-Assembled GaN Nanowires on Si(111): Kelvin Probe Force Microscopy Studies

The growth of GaN nanowires having a polar, wurtzite structure on nonpolar Si substrates raises the issue of GaN nanowire polarity. Depending on the growth procedure, coexistence of nanowires with different polarities inside one ensemble has been reported. Since polarity affects the optical and electronic properties of nanowires, reliable methods for its control are needed. In this work, we use Kelvin probe force microscopy to assess the polarity of GaN nanowires grown by plasma-assisted Molecular Beam Epitaxy on Si(111) substrates. We show that uniformity of the polarity of GaN nanowires critically depends on substrate processing prior to the growth. Nearly 18% of nanowires with reversed polarity (i.e., Ga-polar) were found on the HF-etched substrates with hydrogen surface passivation. Alternative Si substrate treatment steps (RCA etching, Ga-triggered deoxidation) were tested. However, the best results, i.e., purely N-polar ensemble of nanowires, were obtained on Si wafers thermally deoxidized in the growth chamber at ~1000 °C. Interestingly, no mixed polarity was found for GaN nanowires grown under similar conditions on Si(111) substrates with a thin AlOy buffer layer. Our results show that reversal of nanowires’ polarity can be prevented by growing them on a chemically uniform substrate surface, in our case on clean, in situ formed SiNx or ex situ deposited AlOy buffers.

GaN Nanowire Array for Charge Transfer in Hybrid GaN/P3HT:PC71BM Photovoltaic Heterostructure Fabricated on Silicon

We demonstrate that a GaN nanowire array can be used for efficient charge transfer between the organic photovoltaic layer and silicon in a Si/GaN/P3HT:PC₇₁BM inverted hybrid heterostructure. The band alignment of such a material combination is favorable to facilitate exciton dissociation, carrier separation and electron transport into Si. The ordered nature of the GaN array helps to mitigate the intrinsic performance limitations of the organic active layer. The dependence of photovoltaic performance enhancement on the morphology of the nanostructure with nanowire diameters 30, 50, 60, 100 and 150 nm was studied in detail. The short circuit current was enhanced by a factor of 4.25, while an open circuit voltage increase by 0.32 volts was achieved compared to similar planar layers.

Selective area formation of GaN nanowires on GaN substrates by the use of amorphous Al x O y nucleation layer

Examples are presented that application of amorphous Al _x O _y nucleation layer is an efficient way of controlling spatial distribution of GaN nanowires grown by plasma-assisted molecular beam epitaxy. On GaN/sapphire substrates Al _x O _y stripes induce formation of GaN nanowires while a compact GaN layer is formed outside the stripes. We show that the ratio of nanowire length h to the thickness of the compact layer d can be tailored by adjusting impinging gallium and nitrogen fluxes. Calculations of the h/d aspect ratio were performed taking into account dependence of nanowire incubation time on the growth parameters. In agreement with calculations we found that the value of h/d ratio can be increased by increasing the N/Ga flux ratio in the way that the N-limited growth regime determines nanowire axial growth rate while growth of compact layer remains Ga-limited. This ensures the highest value of the h/d aspect ratio. Local modification of GaN growth kinetics caused by surface diffusion of Ga adatoms through the boundary separating the Al _x O _y stripe and the GaN/sapphire substrate is discussed. We show that during the nanowire incubation period gallium is transported out of the Al _x O _y stripe, which delays nanowire nucleation onset and leads to reduced length of GaN nanowires in the vicinity of the stripe edge. Simultaneously the growth on the GaN/sapphire substrate is locally enhanced, so the planar GaN layers adopts a typical edge shape of mesa structures grown by selective area growth. Ga diffusion length on a-Al _x O _y surface of ∼500 nm is inferred from our results.

Effects of the surface preparation and buffer layer on the morphology, electronic and optical properties of the GaN nanowires on Si

The role of Si (111) substrate surface preparation and buffer layer composition in the growth, electronic and optical properties of the GaN nanowires (NWs) synthesized via plasma-assisted molecular beam epitaxy is studied. A comparison study of GaN NWs growth on the bare Si (111) substrate, silicon nitride interlayer, predeposited AlN and GaO _x buffer layers, monolayer thick Ga wetting layer and GaN seeding layer prepared by the droplet epitaxy is performed. It is demonstrated that the homogeneity and the morphology of the NW arrays drastically depend on the chosen buffer layer and surface preparation technique. An effect of the buffer and seeding layers on the nucleation and desorption is also discussed. The lowest NWs surface density of 14 μm^-2 is obtained on AlN buffer layer and the highest density exceeding the latter value by more than an order of magnitude corresponds to the growth on the 0.3 ML thick Ga wetting layer. It is shown, that the highest NWs mean elongation rate is obtained with AlN buffer layer, while the lowest elongation rate corresponds to the bare Si (111) surface and it is twice as lower as the first case. It is found, that use of AlN buffer layer corresponds to the most homogeneous NWs array with the smallest length dispersion while the least homogeneous array corresponds to the bare Si substrate. Vertically aligned GaN NWs array on the wide bandgap GaO _x semiconductor buffer layer grown by plasma-enhanced chemical vapor deposition demonstrates its potential for electronic applications. Photoluminescence (PL) study of the synthesized samples is characterized by an intense optical response related to the excitons bound to neutral donors. The highest PL intensity is obtained in the sample with AlN buffer layer.

Comprehensive analysis of the self-assembled formation of GaN nanowires on amorphous Al x O y : in situ quadrupole mass spectrometry studies

A comprehensive description of the self-assembled formation of GaN nanowires (NWs) by plasma-assisted molecular beam epitaxy (PAMBE) on amorphous-Al _x O _y buffered Si is presented. The incubation time that precedes the formation of GaN NWs is analyzed as a function of the growth parameters using line-of-sight quadrupole mass spectrometry. We found that the incubation time follows an Arrhenius-type temperature dependence as well as an inverse power law with respect to the Ga flux. Our results reveal a weaker dependence of the incubation time on the Ga flux and faster nucleation on amorphous-Al _x O _y in comparison to conventional nitridated Si substrates. In addition, an unprecedented analysis of the dependence of the incubation time on the N flux demonstrates a stronger dependence of the incubation time on the N than on the Ga flux. Our results are summarized in growth diagrams to visualize the impact of the growth parameters on the incubation time. The diagrams can also be used to predict the incubation time for so far unexplored growth conditions. Finally, we measured the desorbing Ga flux upon the nucleation stage to determine the growth parameters that result in effective N-rich conditions as required for the self-assembled formation of GaN NWs. These original measurements were combined with the knowledge gained on the incubation time to create a growth map that illustrates the different growth regimes that can be obtained when GaN is grown on an amorphous-Al _x O _y buffer layer, regardless of the host substrate. Such a map provides a useful guide to induce the growth and control the morphology of GaN NW ensembles on amorphous-Al _x O _y . Results presented in this work allow to conclude that amorphous-Al _x O _y is preferred over nitridated Si as it enables shorter incubation times as well as a wider range of growth parameters to induce the self-assembled formation of GaN NWs in PAMBE.

Quantified hole concentration in AlGaN nanowires for high-performance ultraviolet emitters

p-Type doping in wide bandgap and new classes of ultra-wide bandgap materials has long been a scientific and engineering problem. The challenges arise from the large activation energy of dopants and high densities of dislocations in materials. We report here, a significantly enhanced p-type conduction using high-quality AlGaN nanowires. For the first time, the hole concentration in Mg-doped AlGaN nanowires is quantified. The incorporation of Mg into AlGaN was verified by correlation with photoluminescence and Raman measurements. The open-circuit potential measurements further confirmed the p-type conductivity, while Mott-Schottky experiments measured a hole concentration of 1.3 × 1019 cm-3. These results from photoelectrochemical measurements allow us to design prototype ultraviolet (UV) light-emitting diodes (LEDs) incorporating the AlGaN quantum-disks-in-nanowire and an optimized p-type AlGaN contact layer for UV-transparency. The ∼335 nm LEDs exhibited a low turn-on voltage of 5 V with a series resistance of 32 Ω, due to the efficient p-type doping of the AlGaN nanowires. The bias-dependent Raman measurements further revealed the negligible self-heating of devices. This study provides an attractive solution to evaluate the electrical properties of AlGaN, which is applicable to other wide bandgap nanostructures. Our results are expected to open doors to new applications for wide and ultra-wide bandgap materials.

Reflectance and fast polarization dynamics of a GaN/Si nanowire ensemble

Optical phenomena in an ensemble of high-quality GaN nanowires (NWs) grown on a Si substrate have been studied by reflectance and time-resolved luminescence. Such NWs form a structure that acts as a virtual layer that specifically reflects and polarizes light and can be characterized by an effective refractive index. In fact we have found that the NW ensembles of high NW density (high filling fraction) behave rather like a layer of effective medium described by the Maxwell Garnett approximation. Moreover, light extinction and strong depolarization are observed that we assign to scattering and interference of light inside the NW ensemble. The wavelength range of high extinction and depolarization correlates well with transverse localization wavelength estimated for such an ensemble of NWs, so we suppose that these effects are due to Anderson localization of light. We also report results of time-resolved measurements of polarization of individual emission centers including free and bound excitons (D⁰X_A, 3.47 eV), inversion domain boundaries (IDB, 3.45 eV) and stacking faults (SF, 3.42 eV). The emission of the D⁰X_A and SF lines is polarized perpendicular to GaN c-axis while the 3.45 eV line is polarized along the c-axis which supports a hypothesis that this line is emitted from IDBs. Time-dependent depolarization of luminescence is observed during the first 0.1 ns after excitation and is interpreted as the result of interaction of the emission centers with hot particles existing for a short time after excitation.

Au-Assisted Substrate-Faceting for Inclined Nanowire Growth

We study the role of gold droplets in the initial stage of nanowire growth via the vapor-liquid-solid method. Apart from serving as a collections center for growth species, the gold droplets carry an additional crucial role that necessarily precedes the nanowire emergence, that is, they assist the nucleation of nanocraters with strongly faceted {111}B side walls. Only once these facets become sufficiently large and regular, the gold droplets start nucleating and guiding the growth of nanowires. We show that this dual role of the gold droplets can be detected and monitored by high-energy electron diffraction during growth. Moreover, gold-induced formation of craters and the onset of nanowires growth on the {111}B facets inside the craters are confirmed by the results of Monte Carlo simulations. The detailed insight into the growth mechanism of inclined nanowires will help to engineer new and complex nanowire-based device architectures.

Droplet epitaxy mediated growth of GaN nanostructures on Si (111) via plasma-assisted molecular beam epitaxy

We demonstrate that the use of a GaN seeding layer prepared prior to the growth of epitaxial GaN on Si (111) can lead to the formation of oriented arrays of Y-shaped nanoislands and nanowires and affects the surface density of the nanostructures.

Selective-area growth of single-crystal wurtzite GaN nanorods on SiOx/Si(001) substrates by reactive magnetron sputter epitaxy exhibiting single-mode lasing

Selective-area growth (SAG) of single-crystal wurtzite GaN nanorods (NRs) directly onto Si(001) substrates with un-etched native SiOx amorphous layer, assisted by a patterning TiNx mask fabricated by nanosphere lithography (NSL), has been realized by reactive magnetron sputter epitaxy (MSE). The GaN NRs were grown vertically to the substrate surface with the growth direction along c-axis in the well-defined nano-opening areas. A 5-step structural and morphological evolution of the SAG NRs observed at different sputtering times depicts a comprehensive growth model, listed in sequence as: formation of a polycrystalline wetting layer, predominating c-axis oriented nucleation, coarsening and coalescence of multi-islands, single NR evolution, and finally quasi-equilibrium crystal shape formation. Room-temperature cathodoluminescence spectroscopy shows a strong GaN bandedge emission with a uniform luminescence across the NRs, indicating that the SAG NRs are grown with high quality and purity. In addition, single-longitudinal-mode lasing, attributed to well-faceted NR geometry forming a Fabry-Pérot cavity, was achieved by optical pumping, paving a way for fabricating high-performance laser optoelectronics using MSE.

Self-assembled growth of GaN nanowires on amorphous Al x O y : from nucleation to the formation of dense nanowire ensembles

We present a comprehensive description of the self-assembled nucleation and growth of GaN nanowires (NWs) by plasma-assisted molecular beam epitaxy on amorphous Al x O y buffers (a-Al x O y ) prepared by atomic layer deposition. The results are compared with those obtained on nitridated Si(111). Using line-of-sight quadrupole mass spectrometry, we analyze in situ the incorporation of Ga starting from the incubation and nucleation stages till the formation of the final nanowire ensemble and observe qualitatively the same time dependence for the two types of substrates. However, on a-Al x O y the incubation time is shorter and the nucleation faster than on nitridated Si. Moreover, on a-Al x O y we observe a novel effect of decrease in incorporated Ga flux for long growth durations which we explain by coalescence of NWs leading to reduction of the GaN surface area where Ga may reside. Dedicated samples are used to analyze the evolution of surface morphology. In particular, no GaN nuclei are detected when growth is interrupted during the incubation stage. Moreover, for a-Al x O y , the same shape transition from spherical cap-shaped GaN crystallites to the NW-like geometry is found as it is known for nitridated Si. However, while the critical radius for this transition is only slightly larger for a-Al x O y than for nitridated Si, the critical height is more than six times larger for a-Al x O y . Finally, we observe that in fully developed NW ensembles, the substrate no longer influences growth kinetics and the same N-limited axial growth rate is measured on both substrates. We conclude that the same nucleation and growth processes take place on a-Al x O y as on nitridated Si and that these processes are of a general nature. Quantitatively, nucleation proceeds somewhat differently, which indicates the influence of the substrate, but once shadowing limits growth processes to the upper part of the NW ensemble, they are not affected anymore by the type of substrate.

High-resolution X-ray diffraction analysis of strain distribution in GaN nanowires on Si(111) substrate

In this work, the influence of micro- and macro-deformation profiles in GaN nanowires (NWs) on the angular intensity distribution of X-ray diffraction are studied theoretically. The calculations are performed by using kinematical theory of X-ray diffraction and assuming the deformation decays exponentially from the NW/substrate interface. Theoretical modeling of X-ray scattering from NWs with different deformation profiles are carried out. We show that the shape of the (002) 2θ/ω X-ray diffraction profile (XDP) is defined by initial deformation at the NW's bottom and its relaxation depth given by the decay depth of the exponential deformation profile. Also, we demonstrate that macro-deformation leads to XDP shift, whereas micro-deformations are the cause of XDP's asymmetry and its symmetrical broadening. A good correlation between calculated and experimental XDP from self-assembled GaN NWs on Si(111) substrate was achieved by taking into account all parameters of micro- and macro-deformation profiles.