Characterization of surface states in HCl-grown oxides using MOS transient currents

A fast transient current (TC) technique has been developed for the characterization of majority carrier charge emission from surface states using MOS capacitors excited by a voltage step-function. This technique, with appropriate choice of initial and final biasing conditions, allows a rapid determination of the density of surface states (N_ss) and their capture cross section values (σ_n) in preselected regions of band gap using suitable temperature ambients. A low temperature (113°K) was used for regions close to the bottom of conduction band and room temperature and moderately low temperatures were used for the mid-gap region. Results of transient current measurements were compared with those obtained from thermally stimulated current and low frequency C-V measurements. The MOS devices were fabricated using [100] oriented n-type (6–8 Ω-cm) silicon on n⁺ substrates with HCl added to the oxidizing ambient. The detectability limit of the TC technique has been found to be approximately 1 × 10¹⁰ cm^?2 eV^?1 for the device area used.     

Determination of the energy distribution of interface traps in MIS systems using non-steady-state techniques

Experimental techniques are described for determining the energy distribution of interface traps at the semiconductor-insulator interface of MIS devices. The device used here was an MNOS capacitor in which the semiconductor was n-type. The first technique which is described is that of measuring the thermally stimulated currents. The method consists of biasing the capacitor into the accumulation mode at a low temperature thereby filling the traps at the semiconductor oxide interface. The device is then biased into the deep-depletion mode in which state the traps remain filled because the temperature is too low to allow the electrons to be thermally excited out of the traps. The temperature of the device is then raised at a uniform rate, and the current associated with the release of electrons from the trap is monitored. The shape of the I?T characteristic is a direct image of the interface trap distribution is a broad peak with a maximum at 0·35 eV below the bottom of the conduction band, and of height approximately 6 × 10¹³ cm^?2eV^?1. The experiments were carried out at two heating rates (0·1°K/sec and 0·01°K/sec), and the trap densities so obtained were identical.The second method consists of biasing the device into the accumulation mode at a fixed temperature thereby filling the traps at the silicon-silicon oxide interface. It is then short-circuited and the non-steady state transient current associated with the release of electrons from the interface traps is monitored. The energy distribution of the interface traps in the upper half of the forbidden gap is shown to be readily obtained from the transient currents, and is found to be identical to that obtained using the thermal technique.     

Deep levels of platinum in silicon

Platinum has received renewed attention of late from device engineers as a means of lifetime-control in silicon. The energy levels assigned to platinum are not well defined, however, and ambiguities exist in the literature. In this work platinum was introduced into n- and p-type silicon and the energy levels and concentrations studied by thermally stimulated current and capacitance techniques. p]Schottky barrier diodes were used to study electron emission from two levels found in the upper half of the band gap. The use of Schottky barriers eliminated the problem of process-induced defect introduced by high concentration p⁺ diffusions. Phosphorus-diffused n⁺ - p diodes were used to study hole emission from three levels found in the lower half of the band gap. Platinum concentration versus distance profiles were obtained from thermally stimulated capacitance measurements. Experimental results indicate that the platinum concentration follows the boron concentration distribution near the junction in p-type silicon.     

Spatial Localization of Carrier Traps in 4H-SiC MOSFET Devices Using Thermally Stimulated Current

Carrier traps in 4H-SiC metal–oxide–semiconductor (MOS) capacitor and transistor devices were studied using the thermally stimulated current (TSC) method. TSC spectra from p-type MOS capacitors and n-channel MOS field-effect transistors (MOSFETs) indicated the presence of oxide traps with peak emission around 55 K. An additional peak near 80 K was observed due to acceptor activation and hole traps near the interface. The physical location of the traps in the devices was deduced using a localized electric field approach. The density of hole traps contributing to the 80-K peak was separated from the acceptor trap density using a gamma-ray irradiation method. As a result, hole trap density of N _t,hole = 2.08 × 10¹⁵ cm⁻³ at 2 MV/cm gate field and N _t,hole = 2.5 × 10¹⁶ cm⁻³ at 4.5 MV/cm gate field was extracted from the 80-K TSC spectra. Measurements of the source-body n ⁺–p junction suggested the presence of implantation damage in the space-charge region, as well as defect states near the n ⁺ SiC substrate.     

Thermal-field forward current in GaN-based surface-barrier structures

The voltage and temperature dependences of the capacitance and forward current in surface-barrier Ni-n-GaN structures are experimentally studied. The results are compared with the Padovani-Stratton thermofield emission theory. It is established that, in a temperature range of 250–410 K, the forward current of the Ni-n-GaN surface-barrier structures (the electron density in GaN is ～10¹⁷ cm^?3) is caused by a thermofield emission of electrons, whose energy is ～0.1 eV below the potential-barrier top.     

Schottky-barrier capacitance measurements for deep level impurity determination

The time dependence of a Schottky-barrier capacitance due to thermal excitation of trapping centres has been studied. An expression for the junction capacitance is derived which is not restricted to any special range of reverse bias nor to a special relation between shallow and deep impurity concentration. The concentration ratio of shallow to deep centres is calculated from the values of the capacitance at zero and infinite time. From a capacitance vs. time plot the trap emission rate for electrons e_n is obtained. Their energetic level within the forbidden band-gap is determined from the temperature dependence of e_n as well as from the capacitance-time variation. Experimental studies which do confirm the calculations were carried out on gold contacts on oxygen-doped n-type GaAs. Representative results of the investigated samples were: shallow donor density 3 × 10¹⁵ cm^?3, trap density 9·8 × 10¹⁵ cm^?3, electron emission rate 6 × 10^?2 sec^?1, energetic level 0·68 eV and capture cross section 7 × 10^?16 cm².     

Interphase interactions and features of structural relaxation in TiB<Subscript>x</Subscript>-<Emphasis Type="Italic">n</Emphasis>-GaAs (InP,GaP, 6<Emphasis Type="Italic">H</Emphasis>-SiC) contacts subjected to active treatment

The results of experimental studies of interphase interactions in TiB_x-n-GaAs (GaP, InP, 6H-SiC) contacts stimulated by external effects are described. These effects are rapid thermal annealing at temperatures as high as 1000°C, microwave treatment at f=2.5 GHz, and ⁶⁰Co γ radiation in the range of doses 10⁵–10⁷ rad. Possible thermal and athermal relaxation mechanisms of internal stresses are considered. It is shown that thermally stable TiB_x-n-GaAs (GaP, InP, 6H-SiC) interfaces can be formed.     

Establishment of specific features of electron localization at <Emphasis Type="Italic">U</Emphasis><Superscript>−</Superscript> centers in semiconductors from thermally stimulated currents

The specific features of thermally stimulated currents in semiconductors containing U ^? centers are treated theoretically in comparison with the corresponding features for semiconductors containing defects with a positive correlation energy or single-level centers. It is shown that the analysis of the curves of thermally stimulated currents normalized to the maximum current in respect to the shape and to the presence or absence of a shift of the temperature maximum of the curve in relation to the initial occupancy provides an extra necessary criterion for identifying defects with different parameters of binding of electrons. The results can be used in analyzing experimental data on thermally stimulated currents that exhibit features inexplicable in the context of the often used model of single-level centers.     

Investigations on the damage caused by ion etching of SiO2 layers at low energy and high dose

The radiation damage caused by argon ion bombardment during ion etching of thermally grown SiO₂ films at an energy below 1 keV and a dose of about 10¹⁸ cm^?2 has been studied by evaluating MOS C-V curves, FET characteristics, as well as Rutherford ion backscattering spectra. The bombarded samples revealed that ion beam etching in this energy range causes a damaged layer of 5–10 nm thickness at the single crystal silicon surface. Moreover, traces of metal atoms are found in the damaged layer together with argon atoms (≈ 10²¹cm^?3).     

Current dependence of capacitance of germanium p +-p junctions in the temperature range of 290–330 K

The current dependence of differential capacitance of germanium p ⁺-p junctions with the p-region resistivity of 45, 30, and 10 Ω cm is investigated in the temperature range of 290–350 K. It is shown that the current dependence of the p ⁺-p-junction capacitance varies with an increasing junction temperature. At the temperature of 290 K, the capacitance decreases with an increasing reverse current, changes sign from positive to negative, and increases with the forward current. At 330 K, the capacitance decreases to the lowest positive value with an increasing reverse current and changes sign to negative with increasing the forward current. At 310 K, the p ⁺-p-junction capacitance can change the sign from positive to negative with increasing the forward and reverse current. It is assumed that the positive and negative p ⁺-p-junction capacitance is caused by the change in the junction-region charge by the external voltage.     

Suppression of current by light in p-Si-n +-ZnO-n-ZnO-Pd diode structures

The mechanism of charge transport in an anisotype p-Si-n ⁺-ZnO-n-ZnO-Pd heterostructure with Schottky contact was studied. Photoelectric characteristics of this heterostructure were analyzed. The observed dependence I ∝ V ³ is attributed to the double injection of charge carriers in the drift approximation. A suppression (decrease) of the forward current by irradiation with photons in the wavelength range of λ=0.7–1.2 μm was observed. This effect is accounted for by special features of the trapping of photogenerated (excess) minority charge carriers (holes) and their recombination with thermally equilibrium electrons. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 35, No. 4, 2001, pp. 479–481. Original Russian Text Copyright ? 2001 by Slobodchikov, Salikhov, Russu, Malinin.     

Au/SnO2/n-Si (MOS) structures response to radiation and frequency

Metal-insulator-semiconductor (MOS) structures with insulator layer thickness of 290 Å were irradiated using a ⁶⁰Co (γ-ray) source and relationships of electrical properties of irradiated MOS structures to process-induced surface defects have been investigated both before and after γ-irradiation. The density of surface state distribution profiles of the sample Au/SnO₂/n-Si (MOS) structures obtained from high-low frequency capacitance technique in depletion and weak inversion both before and after irradiation. The measurement capacitance and conductance are corrected for series resistance. Series resistance (Rs) of MOS structures were found both as function of voltage, frequency and radiation dose. The C(f)-V and G(f)-V curves have been found to be strongly influenced by the presence of a dominant radiation-induced defects. Results indicate interface-trap formation at high dose rates (irradiations) is reduced due to positive charge build-up in the semiconductor/insulator interfacial region (due to the trapping of holes) that reduces the flow rate of subsequent holes and protons from the bulk of the insulator to the Si/SnO₂ interface. The series resistance decreases with increasing dose rate and frequency the radiation-induced flat-band voltage shift in 1 V. Results indicate the radiation-induced threshold voltage shift (ΔV_T) strongly dependence on radiation dose rate and frequency.     

Mechanism of formation of the response of a hydrogen gas sensor based on a silicon MOS diode

Experimental data on the dependence of the flat-band voltage and relaxation time for the capacitance of the space-charge region in an MOS diode (Pd-SiO₂-n-Si) on the hydrogen concentration in a hydrogen/air gaseous mixture are discussed. It is assumed that variation in the flat-band voltage U _fb in an MOS structure with the thickness d = 369 nm subjected to a hydrogen/air gaseous mixture can be accounted for by the formation of dipoles in the Pd-SiO₂ gap due to polarization of hydrogen atoms (H _a ). An analytical expression describing the dependence of variation in the flat-band voltage ΔU _fb on the hydrogen concentration \(n_{H_2 } \) was derived. In MOS structures with d ≤ 4 nm (or MOS diodes), the value of ΔU _fb is mainly controlled by passivation of the centers responsible for the presence of the surface acceptor-type centers at the SiO₂-n-Si interface by hydrogen atoms. Analytical expressions describing the dependences of ΔU _fb and the capacitance relaxation time in the space-charge region on \(n_{H_2 } \) are derived. The values of the density of adsorption centers and the adsorption heat for hydrogen atoms at the Pd-SiO₂ and SiO₂-n-Si interfaces are found.     

‘Modulation effect by intense hole injection’ in epitaxial silicon Schottky-barrier-diodes

The forward characteristics of different epitazial n-n⁺ silicon Schottky barrier diodes have been studied up to high current densities. Modulation has been observed in these experiments, that means an increase in charge carrier density in the series resistance region, i.e. in the epitaxial n-layer. By appropriate analysis of the forward characteristics the carrier density can be determined as a function of the current density. In accordance with the predictions of Scharfetter we find that the modulation increases with the barrier height but decreases with rising donor density. Furthermore it is shown how modulation is affected by recombination centres. The modulation effect is attenuated by irradiation with 1·5 MeV electrons and in a similar manner by doping the epitaxial layer with gold.     

Voltage dependence of the differential capacitance of a p +-n junction

The dependences of the differential capacitance and current of a p ⁺-n junction with a uniformly doped n region on the voltage in the junction region are calculated. The p ⁺-n junction capacitance controls the charge change in the junction region taking into account a change in the electric field of the quasi-neutral n region and a change in its bipolar drift mobility with increasing excess charge-carrier concentration. It is shown that the change in the sign of the p ⁺-n junction capacitance with increasing injection level is caused by a decrease in the bipolar drift mobility as the electron-hole pair concentration in the n region increases. It is shown that the p ⁺-n junction capacitance decreases with increasing reverse voltage and tends to a constant positive value.     

Pyroelectric properties of the wide-gap semiconductor CdS in the low-temperature region

Spontaneous polarization, thermally stimulated conductivity and depolarization are comprehensively studied in the range from 4.2 to 300 K on nonstoichiometric n-type CdS crystals grown from the gas phase in an argon atmosphere at T = 1450 K. The objects of study are initial samples and samples polarized by a weak electric field at T = 4.2 K. Sample polarization results in a decrease in the conductivity σ₃₃ due to restructurization of the entire energy level spectrum associated with the formation of donor-acceptor pairs. The latter processes also contribute to the temperature dependences of the spontaneous polarization and the pyroelectric effect, characterized by the formation of anomalies below 15 K and the formation of thermoelectret. The role of an uncontrollable oxygen impurity in the formation of CdS cationic conductivity above 270 K, associated with the decay of a fraction of donor-acceptor pairs, is discussed. In the temperature range from 20 to 250 K, the pyroelectric coefficient and spontaneous polarization are independent of external influences within experimental error; at T = 200 K, they are ΔP _s = ?(6.1 ± 0.2) × 10^?4 C/m² and γ _s = ?(4.1 ± 0.3) × 10^?5 C/m² K.     

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.     

C-V characteristics of ion implanted depletion IGFETs and buried channel CCDs

Capacitance voltage measurements of ion implanted devices for several circuit connections are presented and interpreted in terms of a simple constant profile approximation. Based on this model the device capacitance is described quantitatively in terms of the series combination of a p?n junction capacitance and a conventional MOS capacitance. It is shown that shallow and deep implants reveal characteristically distinctive C-V curves which provide an immediate rough estimate of the implant depth. Analysis of the model yields directly, important parameters for first order design purposes. It is also shown that the measurements provide a simple diagnostic technique to examine the physics of the implanted structure. The use of the simplified model is justified by the agreement between experimental and calculated values.     

Characteristics of RF reactive sputter-deposited Pt/SiO2/n-InGaN MOS Schottky diodes

All RF sputtering-deposited Pt/SiO₂/n-type indium gallium nitride (n-InGaN) metal–oxide–semiconductor (MOS) diodes were investigated before and after annealing at 400 °C. By scanning electron microscopy (SEM), the thickness of Pt, SiO_2, n-InGaN layer was measured to be ~250, 70, and 800 nm, respectively. AFM results also show that the grains become a little bigger after annealing, the surface topography of the as-deposited film was smoother with the rms roughness of 1.67 nm and had the slight increase of 1.92 nm for annealed sample. Electrical properties of MOS diodes have been determined by using the current–voltage (I–V) and capacitance–voltage (C–V) measurements. The results showed that Schottky barrier height (SBH) increased slightly to 0.69 eV (I–V) and 0.82 eV (C–V) after annealing at 400 °C for 15 min in N₂ ambient, compared to that of 0.67 eV (I–V) and 0.79 eV (C–V) for the as-deposited sample. There was the considerable improvement in the leakage current, dropped from 6.5×10⁻⁷ A for the as-deposited to 1.4×10⁻⁷ A for the 400 °C-annealed one. The annealed MOS Schottky diode had shown the higher SBH, lower leakage current, smaller ideality factor (n), and denser microstructure. In addition to the SBH, n, and series resistance (R_s) determined by Cheungs׳ and Norde methods, other parameters for MOS diodes tested at room temperature were also calculated by C–V measurement.     

On the temperature dependence of subthreshold currents in MOS electron inversion layers

It has been observed that the “n” factor which enters into the exponential dependence of drain current on gate voltage for MOS transistors operating in the weak inversion region, exhibits a significant temperature dependence. This effect is correlated with the increase of interface state density towards the band edges and the variation of the space-charge capacitance.