K. Klosek

Influence of substrate nitridation temperature on epitaxial alignment of GaN nanowires to Si(111) substrate

An arrangement of self-assembled GaN nanowires (NWs) grown by plasma-assisted molecular beam epitaxy on a Si(111) substrate is studied as a function of the temperature at which the substrate is nitridized before GaN growth. We show that the NWs grow with the c-axis perpendicular to the substrate surface independently of nitridation temperature with only a slight improvement in tilt coherency for high nitridation temperatures. A much larger influence of the substrate nitridation process on the in-plane arrangement of NWs is found. For high (850 °C) and medium (450 °C) nitridation temperatures angular twist distributions are relatively narrow and NWs are epitaxially aligned to the substrate in the same way as commonly observed in GaN on Si(111) planar layers with an AlN buffer. However, if the substrate is nitridized at low temperature (~150 °C) the epitaxial relationship with the substrate is lost and an almost random in-plane orientation of GaN NWs is observed. These results are correlated with a microstructure of silicon nitride film created on the substrate as the result of the nitridation procedure.

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

The growth mode and structural and optical properties of novel type of inclined GaN nanowires (NWs) grown by plasma-assisted MBE on Si(001) substrate were investigated. We show that due to a specific nucleation mechanism the NWs grow epitaxially on the Si substrate without any Si(x)N(y) interlayer, first in the form of zinc-blende islands and then as double wurtzite GaN nanorods with Ga-polarity. X-ray measurements show that orientation of these nanowires is epitaxially linked to the symmetry of the substrate so that [0001] axis of w-GaN nanowire is directed along the [111]Si axis. This is different from commonly observed behavior of self-induced GaN NWs that are N-polar and grow perpendicularly to the surface of nitridized silicon substrate independently on its orientation. The inclined NWs exhibit bright luminescence of bulk donor-bound excitons (D(0)X) at 3.472 eV and exciton-related peak at 3.46 eV having a long lifetime (0.7 ns at 4 K) and observable up to 50 K.

Enhanced catalyst-free nucleation of GaN nanowires on amorphous Al2O3 by plasma-assisted molecular beam epitaxy

We report on plasma-assisted molecular beam epitaxial growth of GaN nanowires (NWs) on Si(111) substrates with a thin amorphous Al2O3 buffer layer deposited by atomic layer deposition. Comparison of nucleation kinetics shows that presence of amorphous Al2O3 buffer significantly enhances spontaneous nucleation of GaN NWs. Slower nucleation was observed on partially amorphous silicon nitride films. No growth of NWs was found on sapphire substrate under the same growth conditions which we explain by a low density of defects on monocrystalline substrate surface where NWs may nucleate. Our finding shows that tuning of substrate microstructure is an efficient tool to control rate of self-induced nucleation of GaN NWs.

Kinetics of self-induced nucleation and optical properties of GaN nanowires grown by plasma-assisted molecular beam epitaxy on amorphous AlxOy

Nucleation kinetics of GaN nanowires (NWs) by molecular beam epitaxy on amorphous AlxOy buffers deposited at low temperature by atomic layer deposition is analyzed. We found that the growth processes on a-AlxOy are very similar to those observed on standard Si(111) substrates, although the presence of the buffer significantly enhances nucleation rate of GaN NWs, which we attribute to a microstructure of the buffer. The nucleation rate was studied vs. the growth temperature in the range of 720–790 °C, which allowed determination of nucleation energy of the NWs on a-AlxOy equal to 6 eV. This value is smaller than 10.2 eV we found under the same conditions on nitridized Si(111) substrates. Optical properties of GaN NWs on a-AlxOy are analyzed as a function of the growth temperature and compared with those on Si(111) substrates. A significant increase of photoluminescence intensity and much longer PL decay times, close to those on silicon substrates, are found for NWs grown at the highest temperature proving ...

Electrical characterization of ensemble of GaN nanowires grown by the molecular beam epitaxy technique

High quality Schottky contacts are formed on GaN nanowires (NWs) structures grown by the molecular beam epitaxy technique on Si(111) substrate. The current-voltage characteristics show the rectification ratio of about 103 and the leakage current of about 10−4 A/cm2 at room temperature. From the capacitance-voltage measurements the free carrier concentration in GaN NWs is determined as about 1016 cm−3. Two deep levels (H200 and E280) are found in the structures containing GaN NWs. H200 is attributed to an extended defect located at the interface between the substrate and SiNx or near the sidewalls at the bottom of the NWs whereas E280 is tentatively assigned to a gallium-vacancy- or nitrogen interstitials-related defect.

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 NWs 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 XDPs 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.

Structural and optical characterization of GaN nanowires

Optical properties of GaN nanowires (NWs) grown by plasma-assisted molecular beam epitaxy on Si(111) substrates were studied with the use of micro-Raman spectroscopy and photoluminescence. Two types of NWs, hereafter labeled as A and B, grown with different values of Ga flux were studied. Morphology of the samples was probed by high resolution scanning electron microscopy. It was found that large Ga flux has led to a partial coalescence of nanowires in sample A. Reduction of Ga flux during growth of sample B resulted in an ensemble of separated nanowires. Micro-Raman and photoluminescence spectra were taken under illumination of 325 nm He-Cd laser line. Micro-Raman data reveal hexagonal phase of GaN NWs as well as a slight strain of Si substrate. Photoluminescence data yield that ensembles of separated NWs in sample B are defect free, whereas the spectra for coalesced wires in sample A exhibit both broad yellow luminescence band and defect-related band centered around 3.36 eV. Moreover, it was found that ...

Influence of AlN layer on electric field distribution in GaN/AlGaN/GaN transistor heterostructures

Distribution of built-in electric field in GaN/AlGaN/GaN transistor heterostructures without and with AlN layer was studied theoretically (solving the Schrodinger and Poisson equation) and experimentally (measuring contactless electroreflectance (CER) spectra and analyzing the AlGaN-related Franz-Keldysh oscillation). It is shown that the AlN layer changes very strongly the distribution of electric field in such heterostructures. This change can be very well predicted if the surface boundary conditions for self-consistent Schrodinger-Poisson calculations in GaN/AlGaN/GaN heterostructures are known. These conditions can be determined/verified by CER measurements of AlGaN-related Franz-Keldysh oscillation, which depends on the built-in electric field in AlGaN layer.

Structural and Chemical Characterization of Al(Ga)N/GaN Quantum Well Structures Grown by Plasma Assisted Molecular Beam Epitaxy

Growth of high quality GaN/AlN heterostructures by plasma assisted molecular beam epitaxy (PAMBE) is possible with excess of Ga on the surface. During growth of AlN this additional Ga acts as surfactant and improves mobility of the Al adatoms on the growing surface, at the possible cost of Ga segregation and creation of mixed AlGaN interlayer. Scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) were used to determine chemical composition of high crystallographic quality GaN-AlN multilayer structure. It was shown that segregation occurs at AlN-GaN heterointerfaces, while GaN-AlN interfaces have abrupt stepwise change of the chemical composition. HRTEM results show creation of trench defects at the periphery of growing AlN islands in the case of nonoptimized growth.

An influence of the local strain on cathodoluminescence of GaN/AlxGa1−xN nanowire structures

Near-band-edge excitonic emission shift is investigated as a measure of the local strain in GaN nanowires with single AlxGa1−xN sections of various Al contents (x = 0.0, 0.22, 0.49, 1.0). Complementary data obtained by spatially and spectrally resolved cathodoluminescence spectroscopy and imaging of individual nanowires at low temperature, high resolution X-ray diffraction, and transmission electron microscopy are used to determine the correspondence between the cathodoluminescence emission energy and the strain in the GaN core of the nanowire surrounded by the AlxGa1−xN shell formed during the growth of AlxGa1−xN sections by catalyst-free plasma-assisted molecular beam epitaxy. In majority of nanowires, the blue-shift of GaN cathodoluminescence follows the evolution expected for the GaN core under uniaxial compressive strain along the axis of the structure.