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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
Conferenze divulgative presso Scuole /Enti
No full textLe sfide dei nuovi materiali
(D. Cavalcoli, B. Fraboni)
Quali e quante sono le applicazioni dei nuovi materiali? Solo per citare alcuni esempi:
Lampadine a pellicola. Pellicole flessibili saranno in uso prima nei display dei telefonini ed
entreranno presto anche nelle case, in sostituzione delle normali lampadine. Energia solare.
La quantità di energia che arriva dal sole è circa 10000 volte il consumo globale di energia.
Come utilizzare al meglio questa enorme riserva energetica? Quali saranno i materiali che
miglioreranno l’efficienza e diminuiranno i costi dell’energia solare fotovoltaica?
Nanoelettronica. L’attuale microelettronica è in crisi... come potremo “aiutarla”? In che
modo la ricerca sta cercando di risolvere i problemi relativi all’eccessiva miniaturizzazione
dei dispositivi elettronici
Surface Photovoltage Spectroscopy of Semiconductor Nanostructures
No full textSurface Photovoltage Spectroscopy (SPS) [1] is a valuable method for the non-contact and non-destructive investigation of several semiconductor bulk properties, such as optical band-gap, defect-related optical transitions, minority carrier diffusion length and lifetime, Van Hove singularities or phase inhomogeneities. Moreover, SPS allows for obtaining a detailed picture of the electronic structure of surfaces and interfaces, of quantum structures such as quantum wells and superlattices.
In SPS, changes in band bending (both at the free semiconductor surface and at buried interfaces) are monitored as a function of external illumination.
In the present contribution the physical principles of the method and the experimental details will be discussed, together with several applications concerning a wide variety of materials, alloys and low dimensional semiconductor structures.
SPS has been applied to study hydrogenated nanocrystalline silicon films (nc-Si:H) [2]. nc-Si:H is a very complex material made by Si nano-crystals dispersed in hydrogenated amorphous Si (a-Si:H) matrix that is very promising for photovoltaic applications. Notwithstanding its interesting applications, many issues regarding its electronic and optical properties and the influence of structural defects on them are not completely understood yet. The SPS analyses of nc-Si:H films have allowed for the determination of defect states, energy gap and Urbach tails in the films. Several characteristics of the films were obtained: slight n-type conductivity, optical gap around 1.5 eV, Urbach tails around 50meV and the presence of intra-gap transitions relevant to defective states. Optical transitions (around 1.86 eV and 1.14 eV) were also detected and attributed to the optical gap of the amorphous phase and crystalline phases, respectively.
Another application will be shown, relevant to Cd1–xZnxTe radiation detectors studied by SPS [3]. SPV spectra have been obtained on semi-insulating Cd1-xZnxTe samples with varying Zn concentration. A reliable and very accurate determination of the energy gap in Cd1-xZnxTe (with x ranging from 0 to 6.76%) has been achieved, needed to assess the exact Zn concentration which significantly affects the transport and lattice parameters of the alloys. It was also found that the electron-hole recombination at the surface has a strong effect on the determination of band gap energy through SPV spectra. Moreover a band structure feature relevant the Γ8 splitting of the valence band in CdTe has been found.
A final application will concern Surface photovoltage spectroscopy of AlInN based semiconductor heterostructures. The SPS has allowed for the determination of energy gap in buried semiconductor layers and of intra-gap defect states.
References:
[1] L. Kronik Y. Shapira Surf. Interface Anal. 31, 954 (2001)
[3] A. Cavallini, D. Cavalcoli, M. Rossi, A. Tomasi, S. Pizzini, D. Chrastina, G. Isella. PHYSICA. B, Cond. Matt 401-402, 519 - 522. (2007).
[2] D. Cavalcoli, B. Fraboni, and A. Cavallini J Appl Phys 103, 043713 (2008)
Band bowing and Si donor levels in InGaN layers investigated by surface photo voltage spectroscopy
No full textBowing parameter and intra gap states have been measured on good quality undoped and n-type Si doped InGaN layers, by surface photovoltage spectroscopy (SPS). Bowing parameter calculations
have been done with consideration of strain and relaxed nature of InGaN layers. Si donor levels have been measured by SPS on layers with different indium content. Free carrier concentration and
Si donor level variations as a function of In content have been analysed. It has been found that the interaction between In and Si plays a fundamental role in Si doping process in InGaN
Electronic states related to dislocations in silicon
No full textDislocations and impurities in silicon, even though studied since many years, are now subject of a renewed
interest. Moreover, many question related to dislocation- related electronic states remain still unsolved.
The present contribution reviews several results, obtained by the authors, on dislocation impurity
interactions and their effects on the electronic properties of defect states in silicon. Dislocations introduced
by plastic deformation and oxygen precipitation in p-type Czochralski (Cz) silicon have been investigated
by junction spectroscopy methods. A deep hole trap, named T1, has been associated to dislocationrelated
impurity centers, while additional deep traps have been related to contamination by grown-in transition
metals and to clusters involving oxygen atoms. Moreover, experimental results obtained by junction
spectroscopy assessed the existence of dislocation related shallow states. These were found to be located
at 70 and 60 meV from the valence and conduction band edge, respectivel
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
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
Surface Photovoltage Spectroscopy Analyses of Cd1-x ZnxTe
No full textCd1-xZnxTe alloys have been studied by Surface Photovoltage Spectroscopy (SPS) and Energy Dispersive Spectroscopy (EDS). The analyses of surface photovoltage spectra have been perfomed at near- and above- band gap energies.
Surface recombination effects on the surface photovoltage have been investigated. SPS analyses of Cd1-xZnxTe alloys with different surface conditions have shown that the surface recombination velocity significantly affects the SPS determination of the material band gap. Accounting for this and preparing the surfaces accordingly, SPS spectra of Cd1-xZnxTe samples have allowed an accurate determination of the optical band gap as a function of the Zn concentration, determined via EDS analyses. The local increases in the density of states associated with band structure features in Cd1-xZnxTe alloys have been investigated by SPS spectra in the above- band gap energy range
Electrical activity of deep traps in high resistivity CdTe: Spectroscopic characterization
No full textThe electrical compensation processes of high resistivity CdTe is controlled by deep levels. We have characterized the electrical activity of deep traps by means of three different and complementary spectroscopic methods: photoinduced current transient spectroscopy, surface photovoltage spectroscopy, and space charge limited current analyses. The aim is twofold: to achieve a thorough characterization of the deep trap properties and to assess the potentiality and limitations of the three experimental techniques by a cross correlation of the results obtained with each one of them. We have obtained a direct quantitative estimate of the major deep trap concentration, and we have assessed the sensitivity limit in deep-level detection for surface photovoltage spectroscopy
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
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