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Thermionic emission from the 2DEG assisted by image-charge-induced barrier lowering in AlInN/AlN/GaN heterostructures
No full textThe effect of image charges on current transport mechanisms investigated at the nanoscale in Al1􀀀xInxN/GaN heterostructures was studied. Current–voltage (I–V) measurements were performed locally using a conductive AFM-tip as a nanoprobe and the conduction mechanism was modeled to explain the observed behavior. This model suggests that current transport is controlled by thermionic emission (TE) of the two-dimensional electron gas (2DEG) across the potential barrier at the heterointerface, where the image charges generated by the 2DEG induce a barrier lowering at the Al1􀀀xInxN/GaN interface, enhancing electron transport. This
barrier lowering depends on the 2DEG characteristics, such as 2DEG density n2D, first subband energy E0 and the average distance x0 of the 2DEG from the interface. By fitting the
experimental I–V curves with the present model the 2DEG density was evaluated. The obtained results were in very good agreement with the Hall measurements
Defect investigation in Al0.87In0.13N/AlN/GaN heterostructures by scanning force microscopy methods
No full textAlInN/AlN/GaN heterostructures were characterized by atomic force microscopy (AFM) in semi-contact and conductive mode. These indium-related alloys contain threading dislocations (TDs) with a density around 108 ~109cm-2, originating from the GaN (0001)
substrate grown on sapphire. The TDs, with screw or mixed components, terminate at the
surface of overgrown layers as V-defects. Using semi-contact AFM (phase-imaging) mode, we
traced sites of indium segregation at the V-defects. These sites in V-defects were found to be
highly conductive by current-AFM and could be a possible cause for the leakage current in
Schottky diodes
Defective State Studies in III-Nitride Alloys by Surface Photovoltage Spectroscopy
No full textHigh quality ternary (InGaN) and quaternary (AlInGaN) alloys have attracted a great attention in the last 20 years due to the large progress in epitaxial growth techniques which has led to the demonstration of highly efficient optical devices and solar cells [1], high electron mobility transistors [2], and, recently, photo-electrochemical (PEC) devices for water splitting cells [3]. These alloys show a bandgap tunable with composition, covering the whole spectrum from the infrared (InN, Eg ∼ 0.7 eV) to the deep ultraviolet (AlN, Eg ∼ 6.2 eV). However, despite extensive research on these systems, several issues such as polarity control, role of strain, relaxation mechanisms, and In-content on dislocations and interface defects are still debated. As electron-hole recombination mechanisms at defects and interfaces strongly affect the efficiency of devices based on these alloys, the study and understanding of these issues is mandatory.
The present contribution aims to investigate the role of doping, misfit and threading dislocations, and V pits in InxGa1-xN/GaN and AlxInyGa1-x-yN/GaN alloys with varying In concentrations. The work is focused on the results of electrical and optical characterization by surface photovoltage (SPV) spectroscopy. These results are compared with transmission electron microscopy analysis (TEM), light-assisted Kelvin probe force microscopy (KPFM), and deep level transient spectroscopy (DLTS) analyses in order to obtain a comprehensive picture of the carrier recombination at interface and defect states.
The SPV signal is due to the illumination-induced change in the equilibrium surface potential, which is related to charge transfer and/or redistribution within the device. Surface photovoltage spectroscopy analyses surface potential as a function of the energy of incident photons. In SPV spectroscopy, electronic transitions are detected as changes in the slope of the spectra, which correspond to band-to-band transitions or intra-band transitions at the surface or interface of a multi-layered structure [4,5].
Figure 1 shows an example of such spectra on InxGa1-xN/GaN heterostructures, with different In content and Si doping level. The main features in the spectra allow for the extraction of the band gap energy both in the InxGa1-xN top layer and in the GaN substrate. Other features, like the minima indicated by the black arrow, allow us to investigate interface recombination phenomena. The comparison with other characterization results has allowed us to clarify the role of In content and misfit dislocations on electron-hole recombination mechanisms and to characterize the defects acting as strong recombination centres in ternary and quaternary GaN-based alloys.
References
[1] S. Zhou et al, (2018) Scientific Reports 8, 11053
[2] H. Li et al, (2018) Appl. Phys. Lett. 112, 073501
[3] J.K. Sheu et al, (2017) Sol. Energy Mater. Sol. Cells 166, 86-90
[4] L. Kronik, Y. Shapira, (1999) Surf. Sci. Rep. 37, 1-206
[5] D. Cavalcoli, M.A. Fazio, (2019) Mat. Sci. in Sem. Processing 92, 28-3
Nano-scale Electrical Characterization of advanced semiconductors
No full textResume : Atomic Force Microscope is well-known, widely used technique for the topographic
analysis of nanostructured materials. The use of the AFM tip as a probe of electrical properties
allows enormous insights into material functionality at the nanoscale. In the present contribution
two different cases relevant to the application of Conductive AFM to semiconducting thin films will
be reported and discussed. First of all the electrical properties of intrinsic, B and P doped nc-Si:H
(hydrogenated nanocrystalline Si) films grown by LEPECVD (Low Energy Plasma Enhanced
Chemical Vapor Deposition) investigated by C-AFM will be dealt with. Notwithstanding this
material is receiving an increased interest in the last years due to its promising photovoltaic
properties, its electrical conduction at the nanoscale is still strongly debated. The maps present a
clear evidence of enhanced conduction in the nanocrystals, while the (mainly amorphous)
disordered tissue surrounding the nano-grains is mostly nonconductive. Thus, it can be concluded
that the electrical conduction likely occurs via the conductive nanocrystals. The second case is the
application of C-AFM to the investigation of lattice-matched Al0.84In0.16N/AlN/GaN
heterostructures. These structures are interesting due to their applications as high electron
mobility transistors (HEMTs) as due to the strong piezoelectric field a 2D electron gas density
builds up at the GaN/AlN interface. Conductive and phase contrast AFM maps show several
features that can be related to indium segregation at V-defects in InAlN. V-defects are directly
associated with threading defects in wurtzite films
Structural and local electrical properties of AlInN/AlN/GaN heterostructures
No full textGaN layers and Al1xInxN/AlN/GaN heterostructures have been studied by scanning probe microscopy methods. Threading dislocations(TDs), originating from the GaN(0001) layer grown on sapphire,
have been investigated. Using Current-Atomic Force Microscopy(C-AFM)TDs have been found to be highly conductive in both GaN and AlInN, while using semi-contact AFM (phase-imagingmode) indium segregation has been traced at TDs in AlInN/AlN/GaN heterostructures. It has been assessed that In segregation is responsible for high conductivity at dislocations in the examined heterostructures
Investigation of the properties of In-related alloys by AFM
No full textMOCVD grown Al1-xInxN/AlN/GaN heterostructures
have been characterized by atomic force microscopy(
AFM) in semi-contact and conductive mode. The
surface of In-related alloys consist of grain-like structures
indicating step flow and 3D-growth with TDs of density
equal to ~108/cm2, the origin of which is mostly attributed
to lattice mismatch between GaN and sapphire.
These TDs with screw or mixed components terminate at
the surface of overgrown layers as V-defects, which are
six-facetted inverted pyramidal structures. Strain relaxation
mechanism, formation of cracks and its propagation
to the surface of the samples have been also investigated.
With phase-imaging (in semi-contact AFM), we have
traced sites of indium segregation in the V-defects, surface-
relaxation and crack propagation in In-related alloys.
These sites in V-defects and cracks were found to
be highly conductive by current-AFM either due to the
presence of In-segregation or due to lowering of potential
barrier as a consequence of strain-relaxation. They may
be the possible dominant cause of leakage current in
Schottky diodes
Indium segregation in AlInN/AlN/GaN heterostructures
No full textAlInN/AlN/GaN heterostructures were characterized by atomic force microscopy. V-defects and channels were observed. In phase-contrast mode, these features were found related to inhomogeneities associated with In-segregation and/or In-diffusion and Al-rich surface reconstruction. The electrical characterization via conductive atomic force microscopy showed enhanced conductivity regions related to In-rich traces within channels and V-defects
Electrical properties of dislocations in III-Nitrides
No full textResearch on GaN, AlN, InN (III-N) and their alloys is achieving new heights due their high potential applications in photonics and electronics. III-N semiconductors are mostly grown epitaxially on sapphire, and due to the large lattice mismatch and the differences in the thermal expansion coefficients, the structures usually contain many threading dislocations (TDs). While their structural properties have been widely investigated, their electrical characteristics and their role in the transport properties of the devices are still debated. In the present contribution we will show conductive AFM studies of TDs in GaN and Al/In GaN ternary alloys to evidence the role of strain, different surface polarity and composition on their electrical properties. Local I-V curves measured at TDs allowed us to clarify their role in the macroscopic electrical properties (leakage current, mobilities) of III-N based devices. Samples obtained by different growers (AIXTRON, III-V Lab) were studied. The comparison between the results obtained in the different alloys allowed us to understand the role of In and Al on the TDs electrical properties
Two-dimensional electron gas properties by current-voltage analyses of Al0.86In0.14N/AlN/GaN heterostructures
No full textWe report on the current transport properties of AlInN/AlN/GaN high electron mobility transitors with different AlN interlayer thickness. We determined the two-dimensional electron gas (2DEG) properties directly from simple current-voltage measurements, carried out with two Schottky contacts in a planar back-to-back configuration. A model has been developed to straightforwardly extract the 2DEG electrical properties from room-temperature current-voltage curves, and we correlated them to the effects of varying AlN thickness. The 2DEG properties calculated from current-voltage analyses are in very good agreement with results obtained with standard Hall measurements
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