Determination of energy density distribution and capture cross-section of interface states in the metal-nitride-oxide-semiconductor (MNOS) structure

The capacitance and conductance of a metal-nitride-oxide-semiconductor (MNOS) capacitor was measured as a function of voltage at various frequencies between 10 Hz and 200 kHz using a “lock-in” amplifier. The capacitance as a function of voltage was also measured at a frequency of 1 MHz as well as under quasi-static conditions. All the measurements were made at a temperature of 298°K. The high-frequency C-V measurements were used to establish the relation between the surface potential and gate voltage. The experimental data was analysed in terms of the theoretical models of Lehovec and of Deuling et al. to determine the energy density distribution of the interface states. The interface states density calculated from the quasi-static capacitance measurements agreed with those obtained from the low frequency capacitance results analysed in terms of Lehovec's model. However, in the energy range between 0.5 and 0.9 eV, the vlaues of the interface states energy density calculated from fitting the conductance data to the model of Deuling et al. differ from those obtained by the other two methods by as much as a factor of two due to the high density of interface states in the MNOS device. The analysis of conductance data also furnished the values of electron capture cross-section which decreases approximately exponentially with interface states energy, varying from 10^?13 cm² at 0.4 eV to 10^?19 cm² at 0.86 eV.     

The interface states analysis of the MIS structure as a function of frequency

The energy distribution of interface states (N_ss) and their relaxation time (τ) were of the fabricated the Al/SiO₂/p-Si (MIS) structures were calculated using the forward bias current-voltage (I-V), capacitance-frequency (C-f) and conductance-frequency (G-f) measurements. Typical ln[I/(1 − exp(−qV/kT)] versus V characteristics of MIS structure under forward bias show one linear region. From this region, the slope and the intercept of this plot on the current axis allow to determine the ideality factor (n), the barrier height (Φ_b) and the saturation current (I_S) evaluated to 1.32, 0.77 eV and 3.05 × 10⁻⁹ A, respectively. The diode shows non-ideal I-V behaviour with ideality factor greater than unity. This behaviour is attributed to the interfacial insulator layer at metal-semiconductor interface, the interface states and barrier inhomogeneity of the device. The energy distribution of interface states (N_ss) and their relaxation time (τ) have been determined in the energy range from (0.37 − E_v) to (0.57 − E_v) eV. It has been seen that the N_ss has almost an exponential rise with bias from the mid gap toward the top of valance band. In contrary to the N_ss, the relaxation time (τ) shows a slow exponential rise with bias from the top of the E_v towards the mid gap energy of semiconductor. The values of N_ss and τ change from 6.91 × 10¹³ to 9.92 × 10¹³ eV⁻¹ cm⁻² and 6.31 × 10⁻⁴ to 0.63 × 10⁻⁴ s, respectively.     

A study of deep traps at the SiO2/6H-SiC interface relying upon the nonequilibrium field effect

The nonequilibrium field effect associated with deep surface states at the SiO₂/6H-SiC interface has been observed and studied in a 6H-SiC MOSFET of depletion-accumulation type. An analysis of the relaxation of channel conductance at elevated temperatures upon filling of the surface traps with nonequilibrium carriers has shown that the energy distribution of the surface traps has the form of a narrow Gaussian peak in the upper half of the 6H-SiC band gap, with a peak energy E _C ?E _tm = 1.19eV, peak width ΔE _t ≈85 meV, and electron capture cross section σ_n≈10^?14 cm². These surface states are believed to have the fundamental nature of “oxidation defects” similar to P _b centers in the SiO₂-Si system (of dangling silicon bonds).     

On the energy distribution of interface states and their relaxation time and capture cross section profiles in Al/SiO2/p-Si (MIS) Schottky diodes

The energy distribution profile of the interface states (N_ss) and their relaxation time (τ) and capture cross section (σ_p) of metal-insulator-semiconductor (Al/SiO₂/p-Si) Schottky diodes have been investigated by using the high-low frequency capacitance and conductance methods. The capacitance-voltage (C-V) and conductance-voltage (G/ω-V) characteristics of these devices were investigated by considering series resistance (R_s) effects in a wide frequency range (5 kHz-1 MHz.). It is shown that the capacitance of the Al/SiO₂/p-Si Schottky diode decreases with increasing frequency. The increase in capacitance especially at low frequencies results form the presence of interface states at Si/SiO₂ interface. The energy distributions of the interface states and their relaxation time have been determined in the energy range of (0.362-E_v)-(0.512-E_v) eV by taking into account the surface potential as a function of applied bias obtained from the measurable C-V curve (500 Hz) at the lowest frequency. The values of the interface state density (N_ss) ranges from 2.34 × 10¹² to 2.91 ×  10¹² eV⁻¹/cm², and the relaxation time (τ) ranges from 1.05 × 10⁻⁶ to 1.58 × 10⁻⁴ s, showing an exponential rise with bias from the top of the valance band towards the mid-gap.     

Deep levels related to gallium atom clusters in GaAs

The pseudopotential method and supercell (8×8×8) approach were used to study localized electron states introduced by tetrahedral clusters of gallium atoms into the band gap of GaAs. With increasing cluster size, the Fermi energy (E _F ) rapidly reaches its limiting value close to the Schottky barrier height at the planar metal-semiconductor interface. The gap between the completely filled and empty size-quantization levels is 0.06 eV for the largest cluster of 159 gallium atoms. The energy position and “tails” of metal-induced gap states in the vicinity of E _F are governed by the outermost layers of Ga_As antisite defects.     

Surface states on the <Emphasis Type="Italic">n</Emphasis>-InN-electrolyte interface

Dependences of differential capacitance of the electrolyte-n-InN (0001) contact on the bias voltage are studied. Their analysis of the basis of a model similar to a model of the MIS structure shows that the energy spectrum of surface states of InN above the conduction band bottom can be represented by two, relatively narrow, bands of deep levels described by the Gaussian distribution. Parameters of these bands are as follows: the average energy counted from the conduction band bottom, ΔE ₁ ≈ 0.15 eV and ΔE ₂ ≈ 0.9 eV; and the mean-square deviation, ΔE ₁ ≈ 0.15–0.25 eV and ΔE ₂ ≈ 0.05–0.1 eV. The total density of states in the bands are (1–2.5) × 10¹² and (0.2–4) × 10¹² cm^–2.     

Interface-state characteristics of GaN/GaAs MIS capacitors

Detailed results of the capacitance voltage, conductance voltage and transient capacitance analysis on GaN/GaAs MIS capacitor are presented. It has been found that the low frequency capacitance rises for deep-depletion biases for both n- and p-type GaAs. Transient capacitance analysis has resulted in bulk life time of a few nanosec which is expected for direct band gap semiconductors like GaAs. The interface state density distribution as obtained from the conductance technique showed a rise in the interface state density around 0.30 eV below E_c and 0.55 eV above E_ν of GaAs. The minimum interface state density is around 8 × 10¹⁰/cm² eV.     

Transformation of interface states in silicon-on-insulator structures under annealing in hydrogen atmosphere

Changes induced by annealing the spectrum of states on a Si/SiO₂ interface obtained by direct bonding and on a Si(substrate)/〈thermal SiO₂〉 interface in silicon-on-insulator (SOI) structures were investigated by charge-related deep-level transient spectroscopy. The structures were formed by bonding silicon wafers and slicing one of the wafers along a plane weakened by hydrogen implantation. The SOI structures were annealed at 430°C for 15 min in hydrogen, which corresponded to the conventional mode of passivation of the Si/SiO₂-interface states. The passivation of interface states by hydrogen was shown to take place for the Si/〈thermal SiO₂〉 interface, as a result of which the density of traps substantially decreased, and the continuous spectrum of states was replaced by a band of states in the energy range E _c=0.1–0.35 eV within the entire band. For the traps on the bonded Si/SiO₂ interface, the transformation of the centers occurs; namely, a shift of the energy-state band is observed from E _c=0.17–0.36 to 0.08–0.22 eV. The trapping cross section decreases by about an order of magnitude, and the density of traps observed increases slightly.     

Effect of GaAs surface pretreatment on electrical properties of MBE-ZnSe/GaAs substrate interfaces

Interface properties of MBE-grown ZnSe/GaAs substrate systems formed on variously pretreated GaAs surfaces, which include standard chemically etched (5H₂SO₄:1H₂O₂: 1H₂O), (NH₄)₂S_x-, NH₄I-, and HF-pretreated surfaces, are investigated by capacitance-voltage (C-V) and deep level transient spectroscopy (DLTS) measurements. A HF-pretreated and annealed ZnSe/p-GaAs sample showed marked reduction of interface state density, N_ss, with N_ss,_min below 4 x 10¹¹cm^-2 eV^-1 near E_c- E_FS= 1.0 eV. The value is about one order of magnitude smaller than that of the standard chemically etched interface, and comparable to (NH₄)₂S_x- pretreated interface. Nevertheless, C-V characteristics of ZnSe/nGaAs samples, which were measured for the first time, indicate that interface Fermi level, E_FS, is not completely unpinned due to the interface states located above the midgap. A consistent result was obtained by DLTS method in determining E_FS position. The influence of N_ss distribution on vertical current conduction is also analyzed. It is found that U-shaped interface states with N_ss(E) > 1 x 10¹³ cm^-2 eV^-1 above the midgap may cause an excess voltage drop larger than a few volts at the interface.     

An impurity band in Hg<Subscript>3</Subscript>In<Subscript>2</Subscript>Te<Subscript>6</Subscript> crystals doped with silicon

The influence of silicon impurity on the energy-band spectrum in the Hg₃In₂Te₆ semiconductor compound, which incorporated a high concentration of stoichiometric vacancies, was studied on the basis of the results of electrical and optical measurements. It is shown that silicon impurity forms an impurity band of donor states whose density can be approximated by a Gaussian distribution with a peak at E_c-0.29 eV. The emergence of the impurity band is accompanied with the formation of a quasi-continuous spectrum of localized states in the band gap (E_g=0.74 eV); the density of these states is shown to increase as the doping level increases. All states merge into a continuous band if the impurity concentration N_Si>4.5×10¹⁷ cm^?3. Experimental data are explained on the basis of the effect of impurity self-compensation, in which case donor impurity states arise simultaneously with acceptor states of defects.     

Interface states and deep-level centers in silicon-on-insulator structures

Deep-level centers in a split-off silicon layer and trap levels were studied by deep-level transient spectroscopy both at the Si/SiO₂ interface obtained by direct bonding and also at the Si(substrate)/〈thermal SiO₂〉 interface in the silicon-on-insulator structures formed by bonding the silicon wafers and delaminating one of the wafers using hydrogen implantation. It is shown that the Si/〈thermal SiO₂〉 interface in a silicon-on-insulator structure has a continuous spectrum of trap states, which is close to that for classical metal-insulator-semiconductor structures. The distribution of states in the upper half of the band gap for the bonded Si/SiO₂ interface is characterized by a relatively narrow band of states within the range from E _c −0.17 eV to E _c −0.36 eV. Furthermore, two centers with levels at E _c −0.39 eV and E _c −0.58 eV are observed in the split-off silicon layer; these centers are concentrated in a surface layer with the thickness of up to 0.21 μm and are supposedly related to residual postimplantation defects. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 35, No. 8, 2001, pp. 948–953. Original Russian Text Copyright ? 2001 by Antonova, Stano, Nikolaev, Naumova, Popov, Skuratov.     

SiSiO2 interface states based on optically activated conductance technique

Experimental technique recently developed by Poon and Card has been used to determine the energy distribution of the interface states at the SiSiO₂ interface using AlSiO₂nSi structure with an oxide thickness $\text{?38}\text{A}\text{?}$. The distribution obtained in the band gap of silicon was distorted U-shaped. The distortion in the lower half of the band gap was more pronounced. Surface state density in the structure studied was of the order of 10¹²/cm² eV.     

Analysis of interface states and series resistance of MIS Schottky diodes using the current-voltage (I-V) characteristics

The purpose of this paper is to analyze interface states in Al/SiO₂/p-Si (MIS) Schottky diodes and determine the effect of SiO₂ surface preparation on the interface state energy distribution. The current-voltage (I-V) characteristics of MIS Schottky diodes were measured at room temperature. From the I-V characteristics of the MIS Schottky diode, ideality factor (n) and barrier height (Φ_B) values of 1.537 and 0.763 eV, respectively, were obtained from a forward bias I-V plot. In addition, the density of interface states (N_ss) as a function of (E_ss-E_v) was extracted from the forward bias I-V measurements by taking into account both the bias dependence of the effective barrier height (Φ_e), n and R_s for the MIS Schottky diode. The diode shows non-ideal I-V behaviour with ideality factor greater than unity. In addition, the values of series resistance (R_s) were determined using Cheung’s method. The I-V characteristics confirmed that the distribution of N_ss, R_s and interfacial insulator layer are important parameters that influence the electrical characteristics of MIS Schottky diodes.     

The structural and electrical properties of the SrTa2O6/In0.53Ga0.47As/InP system

The structural and electrical properties of SrTa₂O₆(SrTaO)/n-In_0.53GaAs_0.47(InGaAs)/InP structures where the SrTaO was grown by atomic vapor deposition, were investigated. Transmission electron microscopy revealed a uniform, amorphous SrTaO film having an atomically flat interface with the InGaAs substrate with a SrTaO film thickness of 11.2 nm. The amorphous SrTaO films (11.2 nm) exhibit a dielectric constant of ∼20, and a breakdown field of >8 MV/cm. A capacitance equivalent thickness of ∼1 nm is obtained for a SrTaO thickness of 3.4 nm, demonstrating the scaling potential of the SrTaO/InGaAs MOS system. Thinner SrTaO films (3.4 nm) exhibited increased non-uniformity in thickness. From the capacitance-voltage response of the SrTaO (3.4 nm)/n-InGaAs/InP structure, prior to any post deposition annealing, a peak interface state density of ∼2.3 × 10¹³ cm⁻² eV⁻¹ is obtained located at ∼0.28 eV (±0.05 eV) above the valence band energy (E_v) and the integrated interface state density in range E_v + 0.2 to E_v + 0.7 eV is 6.8 × 10¹² cm⁻². The peak energy position (0.28 ± 0.05 eV) and the energy distribution of the interface states are similar to other high-k layers on InGaAs, such as Al₂O₃ and LaAlO₃, providing further evidence that the interface defects in the high-k/InGaAs system are intrinsic defects related to the InGaAs surface.     

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.     

Study of photocapacitance in diodes fabricated from silicon doped with vanadium

The levels of vanadium in the band gap of n-and p-Si were determined using photocapacitance measurements. It is shown that vanadium introduces levels only in the upper half of the band gap of n-Si; these levels have ionization energies of about E _c ?0.21 eV, E _c ?0.32 eV, and E _c ?0.52 eV. By contrast, V levels are located both in the upper and lower halves of the p-Si band gap: E _c ?0.26 eV, E _v +0.52 eV, E _v +0.42 eV, and E _v +0.31 eV. It is ascertained that the photoionization cross sections of all vanadium levels are larger for electrons than for holes. It is shown that the concentration of electrically active vanadium centers in n-and p-Si depends on both the concentration of shallow-level impurities and the time of vanadium diffusion into Si.     

Improvements in the Quasi-static Capacitance–Voltage Characterization of Semiconductor–Insulator Interface States (Si/SiO2)

The quasi-static capacitance–voltage determination of the electron density of states as a function of energy at the semiconductor–insulator interface is addressed. The respective effects are analyzed of random and systematic errors in a measured capacitance–voltage characteristic on the interface-state distribution derived. The random errors show up as fluctuations that grow indefinitely in magnitude as the energy approaches either band edge. Systematic errors are manifested in under- and overshoots near the band edges. The most important are the systematic errors associated with the estimation of the insulator capacitance and the identification of the relationship between the semiconductor surface potential and the applied voltage. A method for minimizing the errors is proposed. It is noted that the method should enable one to substantially expand the accessible energy range and to significantly improve the accuracy to which the density of states is evaluated. These advantages are confirmed by an experiment on the Si/SiO₂ interface. It is found that the energy range can be made as wide as about 0.9 eV and the accuracy of the semiconductor surface potential can be improved to about 0.1 meV, so that the integrated density of states can be determined to within about 5 × 10⁷ cm^–2. It is inferred from the experimental data that the interface states are concentrated near the conduction band edge and are due to positive oxide fixed ions rather than P _bcenters. The ions should act as electron traps involved in tunnel electron exchange with the conduction band of the silicon.     

Photo-thermal probing of Si-SiO2 surface centers—II: Experiment

The results of photo-thermal probing measurements are presented and interpreted to characterize the Si-SiO₂ surface center photoresponse and to provide information relevant to the evaluation of existing surface state models. Data is first presented to indisputably confirm the facts that surface center photoemission had indeed been observed and that the photoresponse could be isolated from competing relaxation mechanisms. The subsequent presentation is devoted primarily to an examination and analysis of photovoltage vs time data characterizing the surface center response. From the analysis it is concluded that two distinct types of surface centers are quasi-continuously distributed in energy over the central portion of the Si band gap, with both types of states acting as if they were positioned right at the Si-SiO₂ interface. The feature distinguishing the two types of states, referred to as A-states and B-states, is a widely different photocapture cross section at any given band gap energy, measured photocapture cross sections being on the order of 10^?19 cm² and 10^?20 cm² for A- and B-states, respectively. B-states, which exhibit the longer photorelaxation time constant, dominate the response in the upper portion of the band gap, while A-states dominate the response below approximately E_v + 0.3 eV. Finally, the photocapture cross section of each type of state was found to increase systematically toward the band edges due to a Lucovsky-type energy dependence.     

Photocapacitance of deep levels in GaAs:Cr and GaAs:O

Photocapacitance measurements have been applied to characterize deep impurities present in bulk-grown single crystals of n-type GaAs:O and GaAs:Cr. Three principal defects in GaAs:O have levels located at (E_c ? 0.79 eV), (E_v + 0.40 eV) and (E_c ? 0.46 eV); the first of these corresponds to the level commonly associa with oxygen and agrees well with the Lucovsky model for photoionization spectrum. The Cr level in GaAs:Cr does not follow the Lucovsky model and appears to undergo “lattice relaxation” during optical transitions.     

Gold traps in 〈100〉 silicon-silicon dioxide interfaces

The effects of gold on the trapping states at the silicon-silicon dioxide interface have been studied. A theory is presented which allows the ‘static’ low frequency C-V characteristic of a MOS capacitor, with arbitrary interface trap distribution, to be determined. The quasi-static technique of Kuhn, which measures the displacement current response to a slowly varying linear voltage, is subsequently used to obtain experimental curves which are correlated with theory. It is found that at low temperatures (? 230°K) the technique resolves pronounced structure in the interfacial trap distribution that is not apparent at room temperature.To assess the effects of varying amounts of gold, various diffusion times at 900°C were used on n-type silicon wafers of (100) orientation. It was found that interface trap density increased with extended diffusion time, but the energy distribution remained essentially the same, exhibiting pronounced maxima at energies E_v + 0.36 eV and E_v + 0.63 eV. As well as producing peaks in the distribution, the gold diffusions resulted in an increase in interface state density by approximately a factor of 5 from the control devices.