Si Valence Band Edge Research Articles

Structural Properties of Al-O Monolayers in SiO2 on Silicon and the Maximization of Their Negative Fixed Charge Density.

Al2O3 on Si is known to form an ultrathin interfacial SiO2 during deposition and subsequent annealing, which creates a negative fixed charge ( Qfix) that enables field-effect passivation and low surface recombination velocities in Si solar cells. Various concepts were suggested to explain the origin of this negative Qfix. In this study, we investigate Al-O monolayers (MLs) from atomic layer deposition (ALD) sandwiched between deliberately grown/deposited SiO2 films. We show that the Al atoms have an ultralow diffusion coefficient (∼4 × 10-18 cm2/s at 1000 °C), are deposited at a constant rate of ∼5 × 1014 Al atoms/(cm2 cycle) from the first ALD cycle, and are tetrahedral O-coordinated because the adjacent SiO2 imprints its tetrahedral near-order and bond length into the Al-O MLs. By variation in the tunnel-SiO2 thickness and the number of Al-O MLs, we demonstrate that the tetrahedral coordination alone is not sufficient for the formation of Qfix but that a SiO2/Al2O3 interface within a tunneling distance from the substrate must be present. The Al-induced acceptor states at these interfaces have energy levels slightly below the Si valence band edge and require charging by electrons from either the Si substrate or from Si/SiO2 dangling bonds to create a negative Qfix. Hence, tunneling imposes limitations for the SiO2 and Al2O3 layer thicknesses. In addition, Coulomb repulsion between the charged acceptor states results in an optimum number of Al-O MLs, i.e., separation of both interfaces. We achieve maximum negative Qfix of ∼5 × 1012 cm-2 (comparable to thick ALD-Al2O3 on Si) with ∼1.7 nm tunnel-SiO2 and just seven ALD-Al2O3 cycles (∼8 Å) after optimized annealing at 850 °C for 30 s. The findings are discussed in the context of a passivating, hole-selective tunnel contact for high-efficiency Si solar cells.

Strong Fermi-Level Pinning at Metal/n-Si(001) Interface Ensured by Forming an Intact Schottky Contact with a Graphene Insertion Layer.

We report the systematic experimental studies demonstrating that a graphene layer inserted at metal/n-Si(001) interface is efficient to explore interface Fermi-level pinning effect. It is confirmed that an inserted graphene layer prevents atomic interdiffusion to form an atomically abrupt Schottky contact. The Schottky barriers of metal/graphene/n-Si(001) junctions show a very weak dependence on metal work-function, implying that the metal Fermi-level is almost completely pinned at charge neutrality level close to the valence band edge of Si. The atomically impermeable and electronically transparent properties of graphene can be used generally to form an intact Schottky contact for all semiconductors.

Formation of an N-pair at the Si(001)/α–quartz interface

We conduct a first-principles total energy calculation of the atomic and electronic structures of the N defects near the Si(001)/α–quartz interface. We find that the N atoms preferentially form a pair structure at the Si surface immediately below the interface to minimize the system energy. A silicon-induced gap state (SIGS), which is predominantly localized at the interface, is formed just above the valence band edge of bulk Si. The energy level of the SIGS shifts toward the bulk Si valence band edge as the N pairs accumulate at the interface, indicating that the energy gap in the system increases. The atomic and electronic structures of the Np defects is consistent with the previous core-level spectroscopy and spectroscopic charge pumping results.

Controllably Interfacing with Ferroelectric Layer: A Strategy for Enhancing Water Oxidation on Silicon by Surface Polarization.

Silicon (Si) is an important material in photoelectrochemical (PEC) water splitting because of its good light-harvesting capability as well as excellent charge-transport properties. However, the shallow valence band edge of Si hinders its PEC performance for water oxidation. Generally, thanks to their deep valence band edge, metal oxides are incorporated with Si to improve the performance, but they also decrease the transportation of carriers in the electrode. Here, we integrated a ferroelectric poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] layer with Si to increase the photovoltage as well as the saturated current density. Because of the prominent ferroelectric property from P(VDF-TrFE), the Schottky barrier between Si and the electrolyte can be facially tuned by manipulating the poling direction of the ferroelectric domains. The photovoltage is improved from 460 to 540 mV with a forward-poled P(VDF-TrFE) layer, while the current density increased from 5.8 to 12.4 mA/cm(2) at 1.23 V bias versus reversible hydrogen electrode.

Defects Related to Sb-Mediated Ge Quantum Dots

We report on a specific defect, which may form during the growth of Stranski-Krastanov surfactant-mediated Ge/Si (100) islands. Transmission electron microscopy reveals that these loop-like defects are local and could be represented by a missing plane of Ge atoms inside some of Ge islands. This specific defect may generate an electrically active trap within the Si band gap at about 0.3 eV above the Si valence band edge. Deep level transient spectroscopy reveals that at least 1 % of Ge islands may include such defects.

Modulation of the effective work function of TiN metal gate for PMOS application

It is important to find a way to modulate the work function of TiN metal gate towards the valence band edge of Si, which can meet the lower threshold voltage requirement of p-type metal—oxide—semiconductor (MOS) transistor. In this work, effects of TiN thickness, post-deposition annealing (PDA), oxygen incorporation and N concentration variation on the work function of TiN metal gate in MOS structures are systematically investigated. It can be found that the work function positively shifts at the initial stage as the thickness of the TiN layer increases and stabilizes at such a thickness. PDA at N2 ambience with a trace of O2 can also cause a positive shift in the work function of TiN metal gate. The same tendency can be observed when oxygen is incorporated into TiN. Finally, increasing the N concentration in TiN can also positively shift the work function. All these measures are effective in modulating the TiN metal gate so that it is more suitable for PMOS application.

Electrical properties of pSi/[6,6] phenyl-C61 butyric acid methyl ester/Al hybrid heterojunctions: Experimental and theoretical evaluation of diode operation

In this work, we analyze electrically the Al/p-Si/[6,6] phenyl-C61 butyric acid methyl ester/Al hybrid heterojunction. The barrier height at the p-Si/PCBM interface corresponding to the difference between Si valence band edge and the lowest unoccupied molecular orbital energy level of PCBM is studied with current-voltage (J-V) and capacitance-voltage (C-V) methods and determined to be ≃0.55 eV. This value is in agreement with the onset energy of spectrally resolved photocurrent measurements presented in a previous publication [Matt et al., Adv. Mater. 22, 647 (2010)]. For the J-V characteristics, a thorough model based on an interface generation-recombination current is proposed. All relevant energy levels for this model are obtained experimentally. As origin of the large reverse current, the thermal generation of charge carriers throughout the Si depletion region is identified by the thermal activation measurements.

Compensation of oxygen defects in La-silicate gate dielectrics for improving effective mobility in high-k/metal gate MOSFET using oxygen annealing process

Oxygen incorporation for compensation of oxygen defects is investigated with La-silicate dielectrics in directly contacted with the Si substrate. The amount of oxygen is controlled by the temperature of annealing in oxygen atmosphere (oxygen annealing) and the thickness of the gate electrode. The positive shift in flatband voltage ( V FB) by oxygen incorporation is an experimental evidence for defects compensation in La-silicate dielectrics. Optimum oxygen annealing provides the V FB shift toward positive direction without increasing equivalent oxide thickness (EOT). Although the oxygen annealing degrades the interfacial property at La-silicate/Si interface, subsequent forming gas annealing (FGA) can recover the interfacial property. It is experimentally revealed that the positive V FB shift of La-silicate dielectrics is stable even after subsequent FGA. The supplied oxygen in La-silicate is expected to maintain even after reducing process. Movement of Fermi level toward the Si valence band edge caused by oxygen incorporation is successfully observed by XPS. Moreover, no chemical reaction between La-silicate and Si substrate by oxygen annealing are confirmed from TEM observation and analyses of X-ray photoelectron spectra. It is experimentally demonstrated that effective hole mobility can be improved without increase in EOT by combination of oxygen annealing and FGA.

Oxygen vacancy levels and interfaces of Al 2O 3

We have calculated the oxygen vacancy levels in Al 2O 3 using first principles methods. They are found to lie just below midgap in the oxide, equivalent to below the Si valence band edge when aligned to the silicon band structure. This low energy accounts for the behaviour of Al 2O 3 in oxide capping layers and as an insulator in future Flash memory.

Impact of nitrogen incorporation on interface states in (100)Si∕HfO2

The influence of nitrogen incorporation on the energy distribution of interface states in the (100)Si∕HfO2 system and their passivation by hydrogen has been studied as compared to N-free samples. The nitrogen in the (100)Si∕HfO2 entity is found to increase the trap density, most significantly, in the upper part of the Si band gap, in which energy range N prevents passivation of interface traps by hydrogen. At the same time, passivation of fast interface traps in the lower part of the band gap proceeds efficiently, provided the thickness of the nitrogen-containing interlayer is kept within a few monolayers. The minimal interface trap density below the midgap achieved after passivation in H2 is determined by the presence of slow N-related states, likely located in the insulator. As inferred from capacitance-voltage and ac conductance analysis, the lowest density of electrically active defects [(8–9)×1010eV−1cm−2 at 0.4–0.5eV from the top of the Si valence band edge] is achieved both in the N-free and N-containing (100)Si∕HfO2 structures after post-deposition anneal at 800°C in N2+5% O2 followed by passivation in molecular hydrogen at 400°C for 30min.

Effect of pulsed laser action on hole-energy spectrum ofGe∕Siself-assembled quantum dots

Space-charge spectroscopy has been used to study the hole energy spectrum of an array of Ge quantum dots (QD's) coherently embedded in a Si matrix and subjected to a ruby laser $(\ensuremath{\lambda}=694\phantom{\rule{0.3em}{0ex}}\mathrm{nm})$ nanosecond irradiation ex situ. The laser energy density in a single pulse was near the melting threshold of the Si surface. The number of laser pulses was varied from 1 to 10, and the duration of each pulse was $80\phantom{\rule{0.3em}{0ex}}\mathrm{ns}$. From the capacitance-voltage characteristics, temperature- and frequency-dependent admittance measurements, the energies of holes confined in Ge QD's were determined. The pulsed laser annealing was found to result in a deepening of the hole energy level relative to the bulk Si valence band edge and in a decrease of the hole energy dispersion. After the treatment with ten laser pulses, the spread of the hole energies due to varying sizes of the QD's within the ensemble was reduced by a factor of about 2. The obtained results give evidence for a substantial reduction of the QD's size dispersion and for a narrowing distribution of the hole energy levels stimulated by nanosecond laser irradiation. A possible explanation of the improved uniformity of QD's sizes involves dissolving small size Ge QD's in a Si matrix by pulsed laser melting of the Ge nanoclusters and their subsequent intermixing with surrounding solid Si.

Difficulties of the microscopic theory of leakage current through ultra‐thin oxide barriers: point defects

AbstractWe use a combination of first‐principles density functional theory and non‐perturbative scattering theory to investigate the effect of point defects on the hole leakage current through ultra thin oxide films. In particular, we consider O vacancies and B interstitial in monoclinic HfO2; five different defects are considered: (1) O vacancy at a three‐ and (2) a four‐coordinated O site located in the HfO2 region, (3) O vacancy along a Hf–O–Si bond and (4) along a Si–O–Si bond at the Si/HfO2 interface, and (5) an interstitial Batom in the HfO2 region. We find that the neutral bulk vacancies and an interface vacancy along theSi–O–Si bond have little impact on the leakage current. On the contrary, an interface vacancy along the Hf–O–Si bridge and an interstitial B atom in the HfO2 region introduce states in the Si band gap and in the region just below the Si valence band edge thus strongly enhancing the leakage current at a low bias. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Chemical design of direct-gap light-emitting silicon.

Quantum confinement effect has been the dominant approach guiding the search for light-emitting silicon. By first-principles total energy calculations of Si delta-doped with oxygen layers, however, we show that this generally accepted mechanism is ineffective. Instead, a different mechanism is proposed based on defect engineering where the oxygen atoms chemically shift the valence band edge of Si away from the zone center, resulting in the desired optical functionality. Our calculation suggests new design principles for light-emitting silicon as well as providing the microscopic origin for the observed strong luminescence and band gap increase in the SiO(001) multi-delta-layers.

Comparison of Trapping States at SiO2/Si Interfaces on si(100), (110), and (111) Prepared by Plasma-Assisted Oxidation and Oxide Deposition, and by Exposure to Atomic H Prior to Oxidation and Deposition

ABSTRACTS1O2 layers, ∼ 15 nm thick, were formed on Si (100), (110), and (111) surfaces by a low-temperature, 200–300°C, plasma-assisted oxidation/deposition process sequence. The distribution of the trap density at the S1O2/Si interface, and in the Si band-gap, Dit, was deduced from capacitance-voltage, C-V, measurements. The values of Dit at midgap varied with crystal orientation in a similar way as in thermally grown oxides. In addition, the low-temperature oxides displayed a characteristic and intrinsic dangling bond state, the so-called Pb center, at ∼0.3 eV above the Si valence-band edge. Samples exposed to atomic H prior to the oxidation and deposition steps, showed an increase in Dit due to a roughening of the Si surfaces, which produced additional broad bands of defect states, at ∼0.5 and ∼0.7 eV above the Si valence-band edge.

The nature of the trapped hole annealing process

Post-irradiation positive-bias annealing and negative-bias reverse annealing of trapped positive charge in MOS oxides were studied as a function of temperature. Below 125 degrees C, only a slight increase in the positive annealing response was observed, consistent with a tunneling process to a trap level below the Si valence band edge. A much large increase in the trapped-hole annealing rate observed above this temperature is discussed in terms of two possible mechanisms: (1) tunneling to an excited state above the Si conduction band and (2) thermal detrapping of holes to the oxide valence band. The annealing of positive oxide-trapped charge appears to proceed at least partly by a compensation process called negative-bias reverse annealing. This effect is only slightly increased at elevated temperatures in hardened oxides, where it is significant even at room temperature. However, in a soft oxide, which did not exhibit significant negative bias reverse annealing at room temperature, the effect was observed at elevated temperatures. Two unexpected results related to the interface trap density are discussed: the late-time saturation value of the radiation-induced interface trap density increases with annealing temperature if positive bias is maintained, and the interface trap density oscillates with alternating bias at elevated temperatures. >

Reversibility of trapped hole annealing

Annealing under negative bias of metal-oxide-semiconductor field-effect transistors (MOSFETs) previously irradiated and annealed under positive bias is studied as a function of oxide field and time. Using three different sets of samples ranging from hard to soft, it is possible to observe a considerable reversal of the oxide trapped charge component in the two harder sets and essentially none in the soft set. These results are interpreted in terms of electrons tunneling back and forth from the Si substrate to electron traps associated with the simple oxygen vacancy (an E' center) in the oxide. Some of these compensating electrons do reform the Si-Si bonds broken by the trapped holes. The results are consistent with a large body of previous work. In addition, the data suggest that the energy level of the hole trap is below the valence band edge of Si.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Electronic states and Schottky barriers at Pd 2Si/Si(111) interfaces

It is possible to account for measured Pd/Si Schottky barrier heights (0.71-0.73 eV) by assuming that the interface between Si and Pd 2Si includes a hexagonal Si transition region induced by the presence of Pd impurities. With the Fermi level pinned slightly above the Si valence band edge by hydridized PdSi states at the interface, as calculations indicate, the Schottky barrier height is determined primarily by the band gap of hexagonal Si, which is about 0.85 eV. Since Pd atoms are larger than Si atoms, isolated substitutional and interstitial Pd atoms are not readily incorporated into the Si lattice. We believe that the Pd impurities form triangular clusters at Si vacancies. Since these clusters are more easily accommodated in hexagonal than in cubic Si, the Si stacking sequence changes from cubic to hexagonal, giving rise to the transition region.