Leakage currents and dielectric breakdown of Si1−x−yGexCy thermal oxides

A detailed study of the leakage currents and dielectric wear-out of thermal oxides grown on Si_1−xGe_x, Si_1−yC_y and Si_1−x−yGe_xC_y epilayers to determine their quality and reliability for Si_1−x−yGe_xC_y MOS technology is presented. After applying electrical stress to the samples, we have determined the conduction mechanisms and the dependence of leakage currents upon epilayer composition (Ge and C content). Conduction takes place mainly via Fowler-Nordheim tunneling injection. Ge and C introduce traps in the oxide which assist injection and thus lower the effective height of the tunneling barrier. We have also monitored the oxide reliability, focusing on time-dependent dielectric breakdown (TDDB). The nature of trapped charge in the oxide depends on the initial epilayer composition. We have found that the formation of defects induced by the presence of C leads to extrinsic oxide failure. While the presence of Ge in the oxide does not seem to introduce significant differences with respect to Si breakdown statistics, C in the oxide truly modifies the statistical profile.     

Charge transport mechanisms in anisotype n-TiO2/p-Si heterostructures

Anisotype n-TiO₂/p-Si heterojunctions are fabricated by the deposition of a TiO₂ film on a polished poly-Si substrate using magnetron sputtering. The electrical properties of the heterojunctions are investigated and the dominant charge transport mechanisms are established; these are multi-step tunneling recombination via surface states at the metallurgical TiO₂/Si interface at low forward biases V and tunneling at V > 0.6 V. The reverse current through the heterojunctions under study is analyzed within the tunneling mechanism.     

Charge-transport mechanisms in heterostructures based on TiO2:Cr2O3 thin films

n-TiO₂:Cr₂O₃/p-Si anisotype heterostructures are fabricated by the deposition of a TiO₂: Cr₂O₃ film by electron-beam evaporation onto a polished polycrystalline silicon substrate. Their electrical properties are studied and the dominant charge-transport mechanisms are determined: multistage tunneling-recombination mechanism involving surface states at the TiO₂: Cr₂O₃/Si metallurgical interface under small forward biases and tunneling at biases exceeding 0.8 V. The reverse currents through the heterostructures under study are analyzed in terms of the single-stage tunneling mechanism of charge transport.     

On the direct currents through interface states in metal-semiconductor contacts

The direct currents through interface states in metal-semiconductor majority-carrier contacts are calculated analytically. A transition is observed with forward bias voltage from metal-controlled to semiconductor-controlled occupancy of the interface states. Calculations for the Au-n-type silicon system indicate that interface state currents are comparable with band currents for densities ? 10¹¹ cm^?2 eV^?1 (oxide thickness 15 Å) or 10¹³ cm^?2 eV^?1 (10 Å).     

Current-voltage characteristics of silicon metallic-silicide interfaces

The general quantum and electronic theory of the metal-semiconductor contacts, proposed in previous works, is applied to silicon-metallic silicide interfaces in order to calculate their current-voltage characteristics. The analysis takes into account the actual potential profile due both to the semiconductor depletion layer and to the electric dipole created, around the metal-semiconductor interface (MSI), by the quantum mechanical tunneling of the metal free electrons into the semiconductor and by the metal conduction band bending. The current across the MSI, ascribed to the thermionic assisted tunneling, is calculated by taking into account the anisotropy of the effective masses, the many valley-structure of the semiconductor energy bands and the quantum mechanical reflection and tunneling through the energy barrier by means of the generalized transmission probability of Kemble. The results shown by the analysis, which excludes explicity the image-force lowering of the energy barrier height, are the reduction of the height and width of the barrier itself and (hence) the increase of its “transparency” to the thermionic current produced by the increase of the reverse bias voltage and/or of the semiconductor impurity concentration. The effects of such properties of the energy barrier on the current-voltage characteristics of the MSI are the absence of a true reverse saturation current, an ideality factor n greater than 1 and a value of the energy barrier height, deduced from the forward current-voltage characteristics, lower than that true and than that obtained from the measured of the junction capacitance vs the bias voltage. The analysis, applied to interfaces between n-type silicon and the metallic silicides RhSi, ZrSi₂, PtSi and Pd₂Si, yields numerical values which agree well with the experimental ones obtained by several authors on the same contacts which, when it is necessary to eliminate field-enhancement at the electrode periphery and leakage currents, incorporate a guard ring. Effectively such a guard ring and the absence of intervening layers of oxide and of other contaminants in the silicon-metal silicide contacts allow one to acquire experimental data more easy to interpret quantitatively than those relative to other contact types.     

Transport and interface states in high-κ LaSiOx dielectric

The paper presents the results of capacitance-voltage, conductance-frequency and current-voltage characterization in the wide temperature range (140-300 K) as well as results of low temperature (5-20 K) thermally stimulated currents (TSC) measurements of metal-oxide-semiconductor (MOS) structures with a high-κ LaSiO_x dielectric deposited on p- and n-type Si(1 0 0) substrate. Interface states (D_it) distribution determined by several techniques show consistent result and demonstrates the adequacy of techniques used. Typical maxima of interface states density were found as 4.6 × 10¹¹ eV⁻¹cm⁻² at 0.2 eV and 7.9 × 10¹¹ eV⁻¹cm⁻² at 0.77 eV from the silicon valence band. The result of admittance spectroscopy showed the presence of local states in bandgap with activation energy E_a = 0.38 eV from silicon conductance band, which is in accord with interface states profile acquired by conductance method. Low-temperature TSC spectra show the presence of shallow traps at the interface with activation energies ranging from 15 to 32 meV. The charge carrier transport through the dielectric film was found to occur via Poole-Frenkel mechanism at forward bias.     

Modeling of direct tunneling current through interfacial oxide and high-K gate stacks

In this paper, we present a computationally efficient model to calculate the direct tunneling current from an inverted p-type (1 0 0) Si substrate through interfacial SiO₂ and high-K gate stacks. This model consists of quantum mechanical calculations for the inversion layer charge density and a modified WKB approximation for the transmission probability. The modeled direct tunneling currents agree well with a self-consistent model and experimental data. For the same effective oxide thickness (EOT) of 2 nm, the direct tunneling current of a HfO₂ high-K dielectric (6.4 nm, K_f=25) overlaying a 1 nm thermal oxide is reduced by four orders of magnitude compared with a pure SiO₂ film at low gate voltages. The effects of interfacial oxide thickness, dielectric constant and barrier height on the direct tunneling current have also been studied as a function of gate voltages.     

Electron tunneling through a double barrier in a reverse-biased metal-oxide-silicon structure

A number of effects in metal/(tunnel-thin SiO₂)/p ⁺-Si structures associated with electron tunneling from the valence band of bulk Si into a metal have been studied. The tunneling occurs through two successively arranged tunnel-transparent barriers: that of the depleted space charge region in Si and the SiO₂ barrier, with the possible intermediate involvement of a quantum well formed by the Si conduction band. The current-voltage characteristics of the structures are calculated in terms of a simple model that considers these mechanisms for the purely depletion mode, i.e., with the inversion layer charge neglected. The relationship between the structure parameters (p-Si doping level and oxide thickness) and the relative contributions of nonresonant and resonant (via quantum-well levels in the Si conduction band) tunneling to the overall current through an MOS structure is discussed. The conditions most favorable for the observation of resonance effects are formulated.     

Light-emitting tunneling nanostructures based on quantum dots in a Si and GaAs matrix

InGaAs/GaAs and Ge/Si light-emitting heterostructures with active regions consisting of a system of different-size nanoobjects, i.e., quantum dot layers, quantum wells, and a tunneling barrier are studied. The exchange of carriers preceding their radiative recombination is considered in the context of the tunneling interaction of nanoobjects. For the quantum well-InGaAs quantum dot layer system, an exciton tunneling mechanism is established. In such structures with a barrier thinner than 6 nm, anomalously fast carrier (exciton) transfer from the quantum well is observed. The role of the above-barrier resonance of states, which provides “instantaneous” injection into quantum dots, is considered. In Ge/Si structures, Ge quantum dots with heights comparable to the Ge/Si interface broadening are fabricated. The strong luminescence at a wavelength of 1.55 μm in such structures is explained not only by the high island-array density. The model is based on (i) an increase in the exciton oscillator strength due to the tunnel penetration of electrons into the quantum dot core at low temperatures (T < 60 K) and (ii) a redistribution of electronic states in the Δ₂-Δ₄ subbands as the temperature is increased to room temperature. Light-emitting diodes are fabricated based on both types of studied structures. Configuration versions of the active region are tested. It is shown that selective pumping of the injector and the tunnel transfer of “cold” carriers (excitons) are more efficient than their direct trapping by the nanoemitter.     

A mechanism of charge transport in electroluminescent structures consisting of porous silicon and single-crystal silicon

Electroluminescent structures that emit in the visible region of the spectrum and are based on porous silicon (por-Si) formed on the p-Si substrate electrolytically using an internal current source are fabricated. The photoluminescent and electroluminescent properties, as well as the current-and capacitance-voltage characteristics of the structures are studied. Electroluminescence is observed only if the forward bias voltage is applied to the structure; the electroluminescence mechanism is based on the injection and is related to the radiative recombination of electrons and holes in quantum-dimensional Si nanocrystals. The injection of holes is controlled by the condition of their accumulation in the space-charge region of p-Si and by a comparatively low concentration of electronic states at the por-Si/p-Si interface. The charge transport in por-Si is caused by the direct tunneling of charge carriers between the quantum-mechanical levels, which is ensured by an appreciable number of quantum-dimensional Si nanocrystals. The leakage currents are low as a result of a small variance in the sizes of Si nanocrystals and the absence of comparatively large nanocrystals.     

Current-voltage characteristics of Al/SiO2/p-Si MOS tunnel diodes with a spatially nonuniform oxide thickness

The effect of nonuniform distribution of the insulator thickness on the behavior of Al/SiO₂/p-Si MOS tunnel structures with a (1–4)-nm-thick insulator is studied. The character and magnitude of the effect depend on the applied bias. In any conditions, the nonuniformity of the SiO₂ thickness enhances the total through currents as compared to those flowing across a uniform oxide layer of the same nominal thickness. Further, the potential of the inversion layer changes in the inversion mode. The calculations performed take into account the tunnel transport between the Si conduction band and the metal, that between the Si valence band and the metal (including in the inversion mode, the resonant transport, which is less clearly pronounced because of the thickness nonuniformity), and the band-to-band tunneling in the semiconductor.     

Heterojunction solar cells of SnO2/Si

Heterojunction solar cells (HJSC’s), fabricated by electron beam evaportaion of SnO₂ films onto monocrystalline and polycrystalline Si substrates, show conversion efficiencies as high as 9.9%, fill factors of 0.64, and open circuit voltages of 525 mV under AMI simulated irradiation. The SnO₂, an n-type semiconductor, acts as a transparent window to solar irradiation and as an antireflection coating of the Si, and it provides the band bending in the Si necessary for photovoltaic conversion. The SnO₂ films, nominally 50 nm thick, have conductivities of the order of 10³(Ω-cm)⁻¹ so that the film makes a good electrical contact between the junction and the metallic front contacts. Measurements of C⁻²-V and I-V characteristics are consistent with heterojunction theory, and the data imply an electron affinity of the SnO₂ of approximately 0.8 eV greater than the electron affinity of Si. This value limits the open circuit voltage of HJSC’s made on p-type substrates to values too small for useful photovoltaic conversion. The predominant dark current mechanism of units of n-type substrates at room temperature and forward bias in the range of 0.3−0.5 V is electron-hole recombination in the transition region. The experimentally determined activation energy is 0.51 eV, approximately E_g/2. At forward voltages below 0.3 V, multistep tunneling via interband states predominates. The photocurrent apparently depends on interface states through which the photogenerated holes in the Si recombine with electrons in the SnO₂.     

Electrical characterization of ultrathin oxides of silicon grown by N2O plasma assisted oxidation

The ultrathin (2.0–3.5 nm) oxides of silicon have gained renewed importance in view of ultra large scale integration (ULSI) of the silicon devices. In the present investigation, the ultrathin oxides are grown on (100) oriented p-type single side polished silicon using N₂0 plasma assisted oxidation in a PECVD reactor at 200°C. The oxide growth as a function of oxidation time is studied. The oxidation growth conforms to the reaction limited regime. In order to understand the electrical quality of Si/ultrathin SiO₂ interface, Al-thin SiO₂-Si tunnel capacitors are fabricated and their capacitance-voltage (C-V) and current-voltage (I–V) characteristics are studied. The effect of annealing on these oxides (termed as “post oxidation annealing”) has also been studied. The C-V characteristics of tunnel capacitors with “as grown” oxide showed a frequency dependence, possibly due to the presence of large fast interface state density. These fast interface states are observed to decrease with increasing oxidation time. The tunnel capacitors that the oxides undergone “post oxidation annealing” (POA) at 350°C in N₂ ambient for 20 minutes have shown practically no frequency dependence of the C-V characteristics; this observation along with the data on I-V characteristics confirms that POA reduces the interface state density considerably. The forward and reverse currents of POA capacitors are observed to decrease considerably indicating the reduction in the trap assisted tunneling transport process across the tunnel insulator.     

Simulating Tunneling Electron Transport in the Semiconductor–Crystalline Insulator–Si(111) System

Tunneling carrier transport through a thin insulator (e.g., CaF₂) layer between a Si(111) substrate and a semiconductor gate is theoretically investigated. Along with the conservation of a large transverse wave vector of tunneling particles, the limitation imposed on the availability of states in the gate is taken into account. Due to this limitation, the tunneling currents at low insulator bias are weaker than in an analogous structure with a metal gate electrode. The same feature leads to a change in the shape of the energy distribution of tunneling electrons, both in transport between the substrate and gate conduction bands and during the Si(111) conduction band–gate valence band transfer.     

The Surface Oxide Transistor (SOT)

A new type of surface barrier transistor has been investigated in which two junctions are employed each consisting of a thin conducting oxide or insulator between a metal and a semiconductor. The transistor behaviour of a structure composed of two such MIS contacts and a third base contact has been studied as a function of oxide thickness and contact separation distance. Two distinct classes of transistor action are observed each depending on a different mechanism of current transfer through the insulator of the individual formed diodes. The distinguishing feature between these two classes was found to be common base current gain, h_FB. Devices characterized by h_FB > 1 were noted to involve current conduction through the oxide via tunneling processes while those with h_FB < 1 generally involved conduction in Schottky barrier-like diodes.     

Electrical properties of anisotype n-CdO/p-Si heterojunctions

An n-CdO/p-Si heterojunction is fabricated by the deposition of a thin cadmium-oxide film with n-type conductivity onto a polished polycrystalline p-Si wafer by the spray-pyrolysis technique. The I-V characteristics of the heterostructure are measured at different temperatures. It is established that the current through the investigated heterostructure at the forward bias 3kT/e < V < 0.5 V is formed by tunneling-recombination processes with the participation of surface states at the CdO/Si interface and at V > 0.5 V, by tunneling through the space-charge region. The dominant mechanisms of current transport at reverse bias are the Frenkel-Pull emission and tunneling with the participation of energy levels formed by surface states.     

Minority carrier MIS tunnel diodes and their application to electron- and photo-voltaic energy conversion—I. Theory

If the insulating layer in a metal-insulator-semiconductor (MIS) diode is very thin ($\text{<60}\text{A}\text{?}$ for AlSiO₂Si), measureable tunnel current can flow between the metal and the semiconductor. If the insulating layer is even thinner ($\text{<30}\text{A}\text{?}$), tunnel currents are so large that they can significantly disturb the semiconductor from thermal equilibrium. Under such conditions, MIS diodes exhibit properties determined by which of the following tunneling processes is dominant; tunneling between the metal and the majority carrier energy band in the semiconductor, between the metal and the minority carrier energy band, or between the metal abd surface state levels. In the present paper, minority carrier MIS tunnel diodes are analysed using a very general formulation of the tunneling processes through the insulator, transport properties in the semiconductor, and surface state effects. Starting from solutions for diodes with relatively thick insulating layers where the semiconductor is essentially in thermal equilibrium, solutions are obtained for progressively thinner insulating layers until non-equilibrium effects in the semiconductor are observed. It is shown that such minority carrier MIS tunnel diodes with very thin insulating layers possess properties similar to p-n junction diodes including exponential current-voltage characteristics which approach the “ideal diode” law of p-n junction theory. The theory adequately describes the observed properties of experimental devices reported in a companion paper. The diodes have application as injecting contacts, as photodiodes or elements of photodiode arrays, and as energy conversion devices employing the electron- or photo-voltaic effects.     

The barrier height change and current transport phenomena with the presence of interfacial layer in MIS Schottky barrier solar cells

In this comprehensive study, several interesting results which are different from those previous are reported. We find the barrier height decreases for n-type and increases for p-type when positive ions are introduced into the insulating layer. The increase of open circuit voltage can be traced to the suppression of the dark saturation current by the depletion field induced by the positive charge, and to the diminution of the majority tunneling current by the oxide potential barrier. The tunneling probabilities for majority and minority carriers are different; there are only a finite amount of majority carriers with thermionic energy greater than q(V_bi ? V_s) which can surmount the depletion potential and tunnel into the metal, whereas the photogenerated minority carriers derive kinetic energy in the depletion layer making tunneling easier. Transport coefficients for electrons to transmit from metal to semiconductor and from semiconductor to metal are different for the departure of built in potentials during illumination.     

The electrical characteristics of Sn/methyl-red/p-type Si/Al contacts

The junction characteristics of the organic compound methyl-red film (2-[4-(dimethylamino)phenylazo]benzoic acid) on a p-type Si substrate have been studied. The current-voltage characteristics of the device have rectifying behavior with a potential barrier formed at the interface. The barrier height and ideality factor values of 0.73 eV and 3.22 for the structure have been obtained from the forward bias current-voltage (I-V) characteristics. The interface state energy distribution and their relaxation time have ranged from 1.68 × 10¹² cm⁻² eV⁻¹ and 1.68 × 10⁻³ s in (0.73-E_v) eV to 1.80 × 10¹² cm⁻² eV⁻¹ and 5.29 × 10⁻⁵ s in (0.43-E_v) eV, respectively, from the forward bias capacitance-frequency and conductance-frequency characteristics. Furthermore, the relaxation time of the interface states shows an exponential rise with bias from (0.43-E_v) eV towards (0.73-E_v) eV.     

Space-charge and injection limited current in organic diodes: A unified model

Space-charge and injection limited currents in organic diodes have been analyzed using a unified model. Both currents have been modeled using the transport equations combined with a proper value of the free charge density at the metal–organic interface. The method has been applied to diodes with different organic materials, metal contacts and lengths of the organic materials. This unified model accurately reproduces published current–voltage curves for a variety of diode structures operating at different temperatures and voltages. The results of the method, in the injection regime, have been compared with the results of pure injection models developed by other authors. The reduction of the computational time and the number of parameters in our model are important advantages of our procedure. Moreover, it is an alternative where the injection models start to fail: at low applied voltages, close to the Ohmic regime, and for low heights of the energy barrier at the interface. The treatment is complemented with a compact model that relates the current density j with the free charge-carrier density at the interface p_f(0): p_f(0) = K₁j^m + K₂, where the parameter K₁ depends on the barrier height at the interface, m depends on the organic material and K₂ controls the flat zone at low currents to include the dependence with thermal carriers and impurities.