Ge Dangling Bonds Research Articles

First-principles study of Ge dangling bonds with different oxygen backbonds at Ge/GeO2 interface

Calculations based on the density functional theory predict the formation of gap states near the Ge conduction-band minimum, due to the presence of the dangling bonds of Ge1+ and Ge2+ atoms at the Ge/GeO2 interface. Additionally, gap states near the valence-band maximum appear in the presence of threefold Ge0+ dangling bonds at the Ge/GeO2 interface. To further discuss the electrical property of the GeO2 near the interface, the oxygen-vacancy-related defect in GeO2 is calculated with bulk oxide structure. The result shows that the thermodynamical-transition level of +2/0 is 5.11 eV above the valence-band maximum of GeO2. The high transition level energy indicates the +2 charge state is a possible source for positive fixed-charges in defective GeO2.

First-principles study of Ge dangling bonds in GeO2 and correlation with electron spin resonance at Ge/GeO2 interfaces

The g-tensors of dangling bonds at defective Ge atoms in GeO2 are computed using density functional theory. The isotropic g-values of these defects are found to increase with the number of Ge backbonds. By comparing these calculations with the isotropic g-value of a Ge-related defect at Ge/GeO2 interfaces, recently observed by electron spin resonance (ESR) experiments, we tentatively identify this defect as a Ge2O≡Ge• center, i.e., a dangling bond on a Ge atom backbonded to two Ge atoms and one O atom, likely present near the Ge/GeO2 interface. The interaction of this defect with molecular hydrogen is investigated using first-principles molecular dynamics simulations. Our simulations predict that the Ge dangling bond can be hardly passivated by H2 molecules, in agreement with the electron spin resonance study.

Theoretical Study of Ge Dangling Bonds in GeO2 and Correlation with ESR Results at Ge/GeO2 Interfaces

The g-tensors of dangling bonds at defective Ge atoms in GeO2 are computed using density functional theory. The isotropic g-values of these defects are found to increase with the number of Ge backbonds. By comparing these calculations with the isotropic g-value of a Ge-related defect at Ge/GeO2 interfaces, recently observed by electron spin resonance and electrically detected magnetic resonance experiments, we tentatively identify this defect as a Ge2O≡Ge• center, i.e. a dangling bond on a Ge atom backbonded to two Ge atoms and one O atom, likely present near the Ge/GeO2 interface. The interaction of this defect with molecular hydrogen is investigated using first-principles molecular dynamics simulations. Our simulations predict that the Ge dangling bond can be hardly passivated by H2 molecules, in agreement with the electron spin resonance study.

Theoretical Study of Ge Dangling Bonds in GeO2 and Correlation with ESR Results at Ge/GeO2 Interfaces

not Available.

Energetics of hydrogen in GeO2, Ge, and their interfaces

The energetics of interstitial hydrogen in GeO2, Ge, and their interfaces are calculated using a hybrid density functional. We find that interstitial atomic hydrogen unexpectedly behaves as a donor in GeO2, like H in ZnO but unlike H in SiO2. At Ge:GeO2 interfaces, the donor electron can transfer to the lower lying Ge conduction band, which stabilises the O-H bond/donor complex. This reduces the probability of passivating Ge dangling bond (Pb) centers at Ge:GeO2 interfaces.

Passivation of Germanium Surfaces by a Quinhydrone–Methanol Solution Treatment

The bulk lifetime of minority carriers, τb, in germanium (Ge) is one of the most important material parameters in the design and fabrication of Ge-based solar cells. Surface passivation of Ge substrates is indispensable to the accurate evaluation of τb. We have found that a newly developed wet chemical treatment, quinhydrone–methanol (Q/M) solution treatment, effectively passivates Ge surfaces, achieving a surface recombination velocity (S) of less than 20 cm/s. It has been clarified that oxide layers containing the molecules from the Q/M solution were formed on the Ge surfaces treated with the Q/M solution. Ge dangling bonds (DBs) are thought to be terminated by these oxygen related molecules. Band bending in the Ge substrates induced by the oxide layers adsorbed on the Q/M-treated Ge surface can contribute to the reduction in S. We also briefly discuss the properties of a recombination center found in the bulk of highly resistive Ge substrates.

Instability of one-dimensional dangling-bond wires on H-passivated C(001), Si(001), and Ge(001) surfaces

We investigate the instability of one-dimensional dangling-bond (DB) wires fabricated on the H-terminated C(001), Si(001), and Ge(001) surfaces by using density-functional theory calculations. The three DB wires are found to show drastically different couplings between charge, spin, and lattice degrees of freedom, resulting in an insulating ground state. The C DB wire has an antiferromagnetic spin coupling between unpaired DB electrons, caused by strong electron–electron interactions, whereas the Ge DB wire has a strong charge-lattice coupling, yielding a Peierls-like lattice distortion. For the Si DB wire, the antiferromagnetic spin ordering and the Peierls instability are highly competing with each other. The physical origin of such disparate features in the three DB wires can be traced to the different degree of localization of 2 p, 3 p, and 4 p DB orbitals.

(Invited) Identification of Intrinsic Point Defects at GexSi1-x/Oxide Interfaces by ESR Probing

We report on the obvervation by electron spin resonance (ESR) of two paramagnetic Ge dangling bond (DB)-type defects at Ge/oxide interfaces. One, detected in SiO2/(100)GexSi1-x/SiO2/Si hetero-structures grown by the condensation method, exhibits monoc-linic-I (C2v) symmetry, with principal g values g1=2.0338(3), g2=2.0386(6), g3=2.0054 and g3 axis (DB) direction 24{plus minus} 2{degree sign} off a <111> direction towards the [100] interface normal. The defect reaches maximum densities for x~0.7, to disappear for x{less than or equal to}0.45 and x{greater than or equal to}0.87. It is suggested to concern a Ge Pb1-type center, i.e., not a trigonal basic Ge Pb(0)-type center (Ge3≡Ge•), thus exposing a unique interface mismatch adaptation as function of substrate Ge fraction. The second one, observed at the interface of epitaxially grown Ge3N4/(111)Ge entities with nm-thin Ge3N4 layers, shows trigonal (C3v) symmetry characterized by g//~2.0023 and g⊥~2.0032. Based on comparision of its salient ESR features with previous theoretical results, the defect is suggested to concern the Ge K center.

Experimental and theoretical investigation of defects at (1 0 0) Si 1−xGe x/oxide interfaces

The identification of a nontrigonal Ge dangling bond at SiO 2/Si 1− x Ge x /SiO 2 heterostructures and its electrical activity are discussed, both from experimental and theoretical points of view. This dangling bond is observed from multifrequency electron-spin resonance experiments performed at 4.2 K, for typical Ge concentrations in the range 0.4 ≤ x ≤ 0.85. The electrical activity of this defect is revealed from capacitance–voltage characteristics measured at 300 and 77 K, and is found to behave like an acceptor defect. First-principles calculations of the electronic properties of this Ge dangling bond indicate that its energy level approaches the valence band edge of the Si 1− x Ge x layer as the Ge content increases, confirming its acceptor-like nature.

Defect formation and annihilation at [formula omitted] interfaces

The stability and dynamics of defects at Ge – GeO 2 interfaces are key factors for the operation of Ge-based devices. Here we present the results of extensive first-principles calculations on creation mechanisms and transformations of defects at the Ge – GeO 2 boundary. We find that, similar to the case of Ge P b centers, reactions between interfacial divalent Ge atoms and hydrogen or fluorine do not lead to passivation of the Ge dangling bonds. Moreover, the insertion of extra oxygen atoms in the vicinity of P b and divalent Ge defects can lead to new defect complexes. The results reveal key differences with respect to the traditional Si – SiO 2 electronic system.

Morphology and defect properties of the Ge–GeO2 interface

Ge-based devices, currently being pursued as replacement of their Si counterparts, typically contain a germanium oxide layer next to the substrate. Here we show using first-principles calculations that, in contrast to Si technology, hydrogenation and fluorination are not effective ways of passivating Ge–GeO2 interfacial Pb centers with a Ge dangling bond (DB). Moreover, we identify DB geometries that differ from the Pb structures and we find that the Ge–GeO2 boundary has a higher atomic-scale roughness than the Si–SiO2 interface. These key differences in interface morphology and defect properties are consistent with experimental data.

Electronic properties of Ge dangling bond centers at Si1−xGex/SiO2 interfaces

Comparison between densities of paramagnetic Ge dangling bond defects and shallow acceptor traps at interfaces of the condensation-grown Si1−xGex layers (0.28≤x≤0.93) with thermal SiO2 as a function of Ge fraction, x, reveals quantitative agreement. Moreover, defect densities detected in both magnetic resonance and electrical experiments exhibit reversible passivation-depassivation behavior with respect to hydrogen indicating observation of the same defect (Ge Pb1 center). The corresponding energy level is estimated to lie at 0.35±0.10 eV above the valence band in bulk Si, which makes these defects behave as shallow acceptors in Ge-rich Si1−xGex.

Observation of a paramagnetic defect at the epitaxial Ge 3N 4/(111)Ge interface by electron spin resonance

A multifrequency electron spin resonance (ESR) study of Ge 3N 4/(111)Ge entities with nm thick epitaxial Ge 3N 4 layers, of subcritical thickness for mismatch relaxation, grown in a nitrogen plasma reveals the presence of an anisotropic paramagnetic interface center of trigonal symmetry. Building on the analysis of its specific ESR properties, including magnetic field angular mapping data and in the light of previous insight, the signal is tentatively ascribed to the interfacial •Ge N 3 center. The defect occurrence is discussed within the specific interface matching, without misfit dislocations. As an interfacial Ge dangling bond defect, it may operate as an inherent electrically detrimental trap.

High permittivity materials for oxide gate stack in Ge-based metal oxide semiconductor capacitors

In the effort to ultimately shrink the size of logic devices towards a post-Si era, the integration of Ge as alternative channel material for high-speed p-MOSFET devices and the concomitant coupling with high permittivity dielectrics (high- k) as gate oxides is currently a key-challenge in microelectronics. However, the Ge option still suffers from a number of unresolved drawbacks and open issues mainly related to the thermodynamic and electrical compatibility of Ge substrates with high- k gate stack. Strictly speaking, two main concerns can be emphasized. On one side is the dilemma on which chemical/physical passivation is more suitable to minimize the unavoidable presence of electrically active defects at the oxide/semiconductor interface. On the other side, overcoming the SiO 2 gate stack opens the route to a number of potentially outperforming high- k oxides. Two deposition approaches were here separately adopted to investigate the high- k oxide growth on Ge substrates, the molecular beam deposition (MBD) of Gd 2O 3 and the atomic layer deposition (ALD) of HfO 2. In the MBD framework epitaxial and amorphous Gd 2O 3 films were grown onto GeO 2-passivated Ge substrates. In this case, Ge passivation was achieved by exploiting the Ge 4+ bonding state in GeO 2 ultra-thin interface layers intentionally deposited in between Ge and the high- k oxide by means of atomic oxygen exposure to Ge. The composition of the interface layer has been characterized as a function of the oxidation temperature and evidence of Ge dangling bonds at the GeO 2/Ge interface has been reported. Finally, the electrical response of MOS capacitors incorporating Gd 2O 3 and GeO 2-passivated Ge substrates has been checked by capacitance–voltage measurements. On the other hand, the structural and electrical properties of HfO 2 films grown by ALD on Ge by using different oxygen precursors, i.e. H 2O, Hf(O t Bu) 2(mmp) 2, and O 3, were compared. Exploiting O 3 as oxidizing precursor in the ALD of HfO 2 is shown to play a beneficial role in efficiently improving the electrical quality of the high- k/Ge interface through the pronounced formation of a GeO 2-like interface layer. In both cases, carefully engineering the chemical nature of the interface by the deliberate deposition of interface passivation layers or by the proper choice of ALD precursors turns out to be a key-step to couple high- k materials with Ge.

First-principles study of the electronic properties of Ge dangling bonds at (100)Si1−xGex/SiO2 interfaces

First-principles calculations of the electronic properties of (100)Si1−xGex/SiO2 structures, with a Ge dangling bond at the interface, are reported. It is found that the defect level associated with this dangling bond approaches the valence band edge of the Si1−xGex substrate as the Si concentration is reduced, mainly due to the narrowing of the energy bandgap of the alloy. These results suggest that these dangling bonds likely behave as acceptor-type defects at Ge-rich (100)Si1−xGex interfaces.

Nontrigonal Ge dangling bond interface defect in condensation-grown(100)Si1−xGex/SiO2

Multifrequency electron-spin-resonance (ESR) study has revealed a nontrigonal Ge dangling bond (DB)-type interface defect in ${\text{SiO}}_{2}/(100){\text{Ge}}_{x}{\text{Si}}_{1\ensuremath{-}x}/{\text{SiO}}_{2}/\text{Si}$ heterostructures grown by the condensation method. The center, exhibiting monoclinic-I $({C}_{2v})$ symmetry, with principal $g$ values ${g}_{1}=2.0338\ifmmode\pm\else\textpm\fi{}0.0003$, ${g}_{2}=2.038\text{ }6\ifmmode\pm\else\textpm\fi{}0.000\text{ }6$, and ${g}_{3}=2.005\text{ }4$ and lowest $g$ value (DB) direction $24\ifmmode\pm\else\textpm\fi{}2\ifmmode^\circ\else\textdegree\fi{}$ off a $⟨111⟩$ direction toward the [100] interface normal, is observed in maximum densities for $x\ensuremath{\sim}0.7$, the signal disappearing for $x\ensuremath{\le}0.45$ and $x\ensuremath{\ge}0.93$. Neither $\text{Si}\text{ }{P}_{b}$ type nor trigonal Ge dangling bond defects is observed, enabling unobscured spectral analysis. Based on its ESR parameters, including $g$ matrix and symmetry, it is suggested to concern a $\text{Ge}\text{ }{P}_{b1}$-type center, that is, not a trigonal basic $\text{Ge}\text{ }{P}_{b(0)}$-type center $({\text{Ge}}_{3}\ensuremath{\equiv}{\text{Ge}}^{\ifmmode\bullet\else\textbullet\fi{}})$, thus exposing a unique interface mismatch healing as function of substrate Ge fraction. Its properties are discussed within the context of the thus far elusive role of interfacial Ge DB defects in Ge insulator structures, encompassing theoretical inferences.

Paramagnetic Ge dangling bond type defects at (1 0 0)Si 1−xGe x/SiO 2 interfaces (Invited Paper)

The work addresses the occurrence of Ge dangling bond type point defects at Ge x Si 1− x /insulator interfaces as evidenced by conventional electron spin resonance (ESR) spectroscopy. Using multifrequency ESR, we report on the observation and characterization of a first nontrigonal Ge dangling bond (DB)-type interface defect in SiO 2/(1 0 0)Ge x Si 1− x /SiO 2/(1 0 0)Si heterostructures (0.27 ⩽ x ⩽ 0.93) manufactured by the condensation technique, a selective oxidation method enabling Ge enrichment of a buried epitaxial Si-rich SiGe layer. The center, exhibiting monoclinic-I ( C 2v ) symmetry is observed in highest densities of ∼7 × 10 12 cm −2 of Ge x Si 1− x /SiO 2 interface for x ∼ 0.7, to disappear for x outside the ]0.45–0.87[ interval, with remarkably no copresence of Si P b-type centers. Neither are trigonal Ge DB centers observed, enabling unequivocal spectral analysis. Initial study of the defect passivation under annealing in molecular H 2 has been carried out. On the basis of all data the defect is depicted as a Ge P b1-type center, i.e., distinct from a trigonal basic Ge P b(0)-type center (Ge 3 Ge ). The modalities of the defect’s occurrence as unique interface mismatch healing defect is discussed, which may widen our understanding of interfacial DB centers in general.

Rashba Effect of the Tl-covered Ge(111) Surface

We studied electronic structure and Rashba-type spin splitting in the (3×1) and (1×1) structures on Tl/Ge(111) using angle-resolved photoelectron spectroscopy (ARPES) and first-principles calculation incorporating the spin-orbit interaction. The calculations showed that Tl behaves as a monovalent ion and surface states involved with the Tl-Ge bond are formed mainly by the Tl 6p orbital and the Ge dangling bond. The surface states show k-dependent spin splitting up to ∼200 and ∼800 meV for the (3×1) and (1×1) structures, respectively. The occupied surface states were observed by ARPES. Their spin splittings were predicted to be 150−200 meV, while these were not resolved in the measurement. The largest splitting of ∼800 meV is calculated for the unoccupied states at K− on the (1×1) structure. The wavefunction is strongly localized in the Tl overlayer, indicating that the heavy core potential of Tl is essential for the large spin splitting.

Evidence of dangling bond electrical activity at the Ge/oxide interface

The nature of the defects present at the Ge/oxide interface has been investigated by electrically detected magnetic resonance spectroscopy. In vacuo atomic oxygen exposure has been exploited to generate germanium oxide with variable composition according to the oxidation temperature. Two different kinds of interfacial defects have been identified, which correspond to the Ge dangling bonds and to an oxygen-related defect.

Ge dangling bonds at the (100)Ge/GeO2 interface and the viscoelastic properties of GeO2

The electronic energy band structure of a (100)Ge/GeO2 interface with a Ge3≡Ge• center (Ge dangling bond) is calculated using the density functional theory. The defect level associated with this dangling bond is found to lie near the middle of the Ge band gap. Consequently, the Ge dangling bond at this interface should be paramagnetic when the Fermi level is near the midgap, and it should thus be observed by electron spin resonance (ESR), which appears to be in contradiction with the results reported by [Afanas’ev et al., Appl. Phys. Lett. 87, 032107 (2005)]. We point out that the density of Ge3≡Ge• centers at the (100)Ge/GeO2 interface is possibly at/below the ESR detection limit due to the viscoelastic properties of GeO2, owing to a better stress relaxation at/near the interface, compared to its silicon counterpart.