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.     

Gaussian distribution of inhomogeneous barrier height in tungsten/4H-SiC (000-1) Schottky diodes

The Gaussian distribution model have been used to analyze the anomalies observed in tungsten (W)/4H-SiC current voltage characteristics due to the barrier inhomogeneities that prevail at the metal-semiconductor interface. From the analysis of the forward I-V characteristics measured at elevated temperatures within the range of 303-448 K and by the assumption of a Gaussian distribution (GD) of barrier heights (BHs), a mean barrier height of 1.277 eV, a zero-bias standard deviation σ₀ = 0.092 V and a factor T₀ of 21.69 K have been obtained. Furthermore the modified Richardson plot according to the Gaussian distribution model resulted in a mean barrier height and a Richardson constant (A^∗) of 1.276 eV and 145 A/cm² K², respectively. The A^∗ value obtained from this plot is in very close agreement with the theoretical value of 146 A/cm² K² for n-type 4H-SiC. Therefore, it has been concluded that the temperature dependence of the forward I-V characteristics of the W/4H-SiC contacts can be successfully explained on the basis of a thermionic emission conduction mechanism with Gaussian distributed barriers. In addition, a comparison is made between the present results and those obtained previously assuming the pinch-off model.     

Thermal behavior Spice study of 6H-SiC NMOS transistors

Silicon carbide is a material that is undergoing major advances associated with a broad scope in the field of electronics. The main properties of silicon carbide such as its high thermal conductivity and high band gap make it a material suitable for use in high-temperature and high-power applications. In this Spice study, the thermal behavior of 6H-SiC NMOS transistors is analyzed through their conductance and transconductance changes with temperature in the range −200 to 700 °C. The performances in two basic applications, current mirrors and differential amplifiers, are compared to similar circuits with silicon transistors. The results show that the 6H-SiC NMOS transistors can be used up to 700 °C, while those based on silicon transistors are limited to around 160 °C.     

Growth of oriented diamond film on single crystalline 6H-SiC substrates

The bias-enhanced nucleation (BEN) technique in hot-filament chemical vapor deposition (HF-CVD) has been applied to single crystalline 6H-SiC substrates for the deposition of oriented diamond. The results of scanning electron microscopy (SEM) showed that on (000 ) face not only oriented diamond with relationship (111) Dia.//(000 )6H-SiC and 〈110〉Dia.//(11 0)6H-SiC, but also high nucleation density (>10⁹ cm⁻²) have been achieved. In the case of deposition on (0001) face of 6H-SiC under the same experimental conditions, although the nucleation density of diamond was enhanced, however, oriented diamond was not found. Diamond nucleation density is higher on (0001) face than that on (000 ) face. The differences in diamond oriented nucleation and nucleation density on these two faces are attributed to the difference of their specific free surface energy. The experimental results have shown that the 6H-SiC substrate surfaces are etched by the accelerated H-ions during BEN process, and many micro-triangular crystals with the faces of the kind {01 4} are formed on the substrate surface. Diamonds nucleate on the top of the micro-triangular crystals. Micro-Raman spectrum shows a strong feature of diamond crystals at 1334 cm⁻¹.     

No trace of divacancies at room temperature in germanium

Alpha-particle irradiated n⁺p-mesa diodes of Ge were investigated by conventional deep level transient spectroscopy and high-resolution Laplace deep level transient spectroscopy. The electronic and annealing properties of the observed hole traps were studied. It is concluded that none of the observed traps which are stable at room temperature are related to the divacancy. These results are consistent with previous optical studies. They are, however, in disagreement with recent numerical density function calculations which predict a stable divacancy at room temperature with band-gap levels in the lower half of the band gap.     

Leakage current and defect characterization of pn-source/drain diodes

A good control of the transient enhanced dopant diffusion is needed for MOSFET scaling down to the sub 50 nm regime. Carbon ion implant is known to significantly suppress the transient enhanced boron diffusion. However, carbon implantation is also reported to increase diode leakage current. This paper investigates the impact of ion implantation and annealing conditions during source/drain extension formation on leakage current behavior of boron/phosphorous diodes of PFET transistors. Analyzing the leakage current it is difficult to distinguish between the influence of the increased electric field due to the reduced diffusion and possible additional trap centers in the space charge region. This distinction can be made by electrical characterization, as shown in this paper. The leakage current mechanism is found to be trap assisted tunneling with phonon interaction. The corresponding trap energy within the band gap is 0.58 ± 0.10 eV. The carbon concentration in the space charge region measured by SIMS is below the detection limit. Also in electrical measurements, which are more sensitive, no significant influence of carbon related traps is observed. The leakage current is increased by the application of a Flash Anneal additionally to a Rapid Thermal Anneal for recrystallization of the silicon substrate.     

Luminescence decay kinetics in GaP:Bi and GaP:N

The lifetime of excitons bound to isoelectronic traps in GaP is studied as a function of temperature, excitation energy and intensity. The photoluminescence was excited both selectively and above the gap by using a pulsed, tunable dye laser with photon flux varying in the range of 10¹⁴–10¹⁹ photons/cm² per pulse. GaP:Bi shows a strong intensity dependence of τ(T) in the temperature range of 40–60 K due to the thermal activation of the electron. Similarly, GaP:N shows this behavior in the range of 20–100 K. In this case, two activation processes can be identified: release of the bound exciton into the free exciton band and dissociation of the exciton. The observed dependence of τ(T) on both excitation energy and intensity indicate that saturable deep traps (shunt paths) deplete the excess free carriers. These traps can be completely saturated in GaP:N while only partial saturation is achieved in GaP:Bi. The kinetic equations are written for GaP:Bi and solved numerically assuming quasiequilibrium conditions. Fitting this model to the experimental results yields the capture cross sections for carriers by Bi and by the deep traps as well as the concentration of the latter.     

High-purity semi-insulating 4H-SiC for microwave device applications

High-purity, semi-insulating (HPSI) 4H-SiC crystals with diameters up to 75 mm have been grown by the seeded sublimation technique without the intentional introduction of elemental deep-level dopants, such as vanadium. Wafers cut from these crystals exhibit homogeneous activation energies near mid gap and thermally stable semi-insulating (SI) behavior (>10⁹ ohm-cm) throughout device processing. Secondary ion mass spectroscopy, deep-level transient spectroscopy, optical admittance spectroscopy, and electron paramagnetic resonance data suggest that the SI behavior originates from several deep levels associated with intrinsic point defects. Micropipe densities in HPSI substrates have been demonstrated to be as low as 10 cm⁻² in 2-in. substrates, and the room-temperature thermal conductivity of this material is near the theoretical maximum of 5 W/cm·K for 4H-SiC. Devices fabricated on these HPSI wafers do not exhibit any substrate related back-gating effects and have power densities as high as 5.2 W/mm with 63% power added efficiency.     

Fine Structure of the long-wavelength edge of exciton-phonon absorption and hyperbolic excitons in silicon carbide of 6<Emphasis Type="Italic">H</Emphasis> polytype

The fine structure of the long-wavelength edge of the polarization spectra of exciton-phonon absorption in moderate-purity n-type 6H-SiC crystals with a concentration of uncompensated donors N_D?N_A=(1.7–2.0)×10¹⁶ cm^?3 at T=1.7 K was studied. The analysis of new special features found at the absorption edge and the reliable detection of the onset of exciton-phonon steps related to the emission of phonons from acoustical and optical branches allowed highly accurate determination of a number of important parameters such as the band gap, the exciton band gap, the exciton binding energy, and the energies of spin-orbit and crystal-field splitting of an exciton. For the first time, transitions with the emission of LA phonons to the 1S exciton state with an M₁-type dispersion law were detected in E ∥ Z(C) polarization (the electric-field vector is parallel to the optical axis of the crystal). This observation supports the previously predicted “two-well” structure of the conduction band minimum in 6H-SiC.     

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 carrier emission from donor-related centres in GaAs1-xPx and AlxGa1-xAs

From an experimental study of the electron thermal emission and capture rates of Te-related centres, donor-related (DX) traps have been identified in GaAsP and GaAlAs compounds. It is shown that under thermal excitation nonexponential transient waveforms markedly deviate from theory. The relation of the nonexponential degree with alloy composition and junction electric field has been studied. In addition, optical activated transients, a model based on trap charge dependent emission and capture coefficients, and the drawbacks of Deep Level Transient Spectroscopy (DLTS) techniques to study DX centres are presented.     

Detection of traps induced and activated by high field stress in an N-channel VDMOSFET transistor using current deep level transient spectroscopy (CDLTS)

Commercial VDMOSFETs transistors were subjected to positive and negative high field stress. A new model of current deep level transient spectroscopy (CDLTS) characterization is adopted in a research of defects induced and activated by electrical stress. This model is based on pulse width scan instead of classical temperature scan. The band gap is scanned by varying the pulse base level. Positive and negative high field stresses were applied for different periods ranging from 30 to 120 min. After each stress period, activation energies and capture cross sections of detected traps were estimated. Different defects were detected and we have distinguished the doping levels and interface states from deep levels located in the forbidden band gap.     

Current transport mechanisms and trap state investigations in (Ni/Au)-AlN/GaN Schottky barrier diodes

The current transport mechanisms in (Ni/Au)-AlN/GaN Schottky barrier diodes (SBDs) were investigated by the use of current-voltage characteristics in the temperature range of 80-380 K. In order to determine the true current transport mechanisms for (Ni/Au)-AlN/GaN SBDs, by taking the J_s(tunnel), E₀, and R_s as adjustable fit parameters, the experimental J-V data were fitted to the analytical expressions given for the current transport mechanisms in a wide range of applied biases and at different temperatures. Fitting results show the weak temperature dependent behavior in the saturation current and the temperature independent behavior of the tunneling parameters in this temperature range. Therefore, it has been concluded that the mechanism of charge transport in (Ni/Au)-AlN/GaN SBDs, along the dislocations intersecting the space charge region, is performed by tunneling.In addition, in order to analyze the trapping effects in (Ni/Au)-AlN/GaN SBDs, the capacitance-voltage (C-V) and conductance-voltage (G/ω-V) characteristics were measured in the frequency range 0.7-50 kHz. A detailed analysis of the frequency-dependent capacitance and conductance data was performed, assuming the models in which traps are located at the heterojunction interface. The density (D_t) and time constants (τ_t) of the trap states have been determined as a function of energy separation from the conduction-band edge (E_c − E_t) as D_t≅(5-8)×10¹², respectively.     

Compensation and trapping in CdZnTe radiation detectors studied by thermoelectric emission spectroscopy, thermally stimulated conductivity, and current-voltage measurements

The thermal ionization energies of traps and their types, whether electron or hole traps, were measured in commercial CdZnTe crystals for radiation detectors. The measurements were done between 20 and 400K using thermoelectric emission spectroscopy (TEES) and thermally stimulated conductivity (TSC). For reliable results, indium ohmic contacts had to be used instead of gold Schottky contacts. For filling of the traps, photoexcitation was done at zero bias, at 20K, and at wavelengths which gave the maximum bulk photoexcitation. In agreement with theory, the TSC current was found to be on the order of times or even larger than the TEES current, where V is the applied bias in TSC and ΔT is the applied temperature difference in TEES. Large concentrations of hole traps at 0.1 and 0.6 eV were observed and a smaller concentration of electron traps at 0.4 eV was seen. The deep traps cause compensation in the material, which is desirable, but they also cause carrier trapping that degrades the spectral response of radiation detectors made from the material.     

Electrical and optical properties of Mn-doped Hg3In2Te6 crystals

The effect of Mn impurities on the properties of Hg₃In₂Te₆ crystals is studied by electrical and optical measurements. It is shown that, despite the high dopant concentration (1 × 10¹⁹ cm^?3), the electron concentration remains the same as that in an undoped crystal (～10¹³ cm^?3 at 300 K). At the same time, narrowing of the band gap from 0.74 to 0.7 eV is observed. From an analysis of the absorption spectra, it is found that the absorption edge is formed by optical transitions involving density-of-states (DoS) tails and that two acceptor- and donor-type impurity bands are formed in the band gap. The two bands are described by a Gaussian distribution of the DoS, with an energy gap between the peaks of E ₀ = E _d ⁰ ? E _a ⁰ = 0.4 eV. The total donor and acceptor concentration N _d + N _a and the degree of compensation K = N _a /N _d → 1 are determined. Such compensation is responsible for pinning of the Fermi level near the middle of the band gap and for quasi-intrinsic conductivity at temperatures T ≥ 300 K.     

Analytic computation of the photocurrent density in a n-6H-SiC/p-Si/n-Si/p-Si0.8Ge0.2 multilayer solar cell

In this work we conceived a model of a multilayer solar cell composed by four layers of opposite conductivities: an n-type 6H-SiC used as a frontal layer to absorb high energy photons (energy gap equals 2.9 eV), a p-type Si layer, an n-type Si layer and a p-type SiGe back layer to absorb low energy photons (Si_0.8Ge_0.2 with an energy gap equal to 0.8 eV). The impurity concentration in every layer of the model is taken equal to 10¹⁷ cm⁻³ to ensure abrupt junctions inside the cell. The optical properties of the separate layers have been fitted and tabulated to be used for thin films devices numerical simulation. We developed the equations giving the minority carrier concentration and the photocurrent density in each abscissa of the model. We used Matlab software to simulate and optimize the layers thicknesses to achieve the maximum photocurrent generated under AM0 solar spectrum. The results of simulation showed that the optimized structure could deliver, assuming 10⁵ cm/s surface recombination velocity, a photocurrent density of more than 53 mA/cm², which represents 88.3% of the ideal photocurrent (59.99 mA/cm²) that can be generated under AM0 solar spectrum.     

Deep-level transient spectroscopy of radiation-induced levels in 6H-SiC

The methods of capacitance and current deep level transient spectroscopy are used to investigate single crystals of Leli n-SiC(6H) irradiated by 5-MeV electrons at doses of 10¹⁶–10¹⁸ cm⁻². Eleven deep levels belonging to the resulting radiation-induced intrinsic defects were observed in the energy range 0.18–1.44 eV from the bottom of the conduction band. Isochronous annealing of various samples showed that most of the observed defects were stable up to a temperature of ∼1000 °C. In addition, annealing of a deep level with ionization energy E _i=0.48–0.53 eV was observed in the temperature range 150–250 °C. It is believed that this center is caused by a vacancy in the carbon sublattice. Fiz. Tekh. Poluprovodn. 33, 1314–1319 (November 1999)     

The impurity optical absorption and conduction band structure in 6H-SiC

Absorption spectra of nitrogen-doped n-type 6H-SiC crystals were studied for two orientations of the light wave electric field (E) relative to the optical axis (C), E ∥ C and E ⊥ C, within the range from the near-infrared region to the fundamental band. For E ∥ C, a weak absorption band peak at 2.85 eV was investigated for the first time. All the absorption bands observed are attributed to donor (nitrogen) photoionization, as a result of which electrons are transferred to the conduction-band upper minima that correspond to different critical points of the Brillouin zone. A combined analysis of the new data obtained in this study, the previous experimental results concerning photoionization of nitrogen, and theoretical data on 6H-SiC conduction band structure made it possible to refine the arrangement and symmetry of the additional conduction-band extrema in the Brillouin zone.     

Two-dimensional analysis of the surface state effects in 4H-SiC MESFETs

Two-dimensional small-signal ac and transient analysis of surface trap effects in 4H-SiC MESFETs have been performed in this paper. The mechanism by which acceptor-type traps effect the transconductance and drain current changes has been discussed. The simulation results show that transconductance exhibits negative frequency dispersion behavior, which is caused by the charge exchange via the surface states existing between the gate-source and gate-drain terminals. The current degradation behavior is also observed due to acceptor-type traps, acting as electron traps, in MESFET devices. A detailed study involving the density, ionization and energy level of traps reveals conclusive results in the devices analyzed.     

Traps with near-midgap energies at the bonded Si/SiO<Subscript>2</Subscript> interface in silicon-on-insulator structures

Capture centers (traps) are studied in silicon-on-insulator (SOI) structures obtained by bonding and hydrogen-induced stratification. These centers are located at the Si/SiO₂ interface and in the bulk of the split-off Si layer. The parameters of the centers were determined using charge deep-level transient spectroscopy (Q-DLTS) with scanning over the rate window at fixed temperatures. Such a method allows one to study the traps near the Si midgap at temperatures near 295 K. It is shown that the density of traps with a continuous energy spectrum, which are located at the bonded Si/SiO₂ interface, decreases by more than four orders of magnitude at the mid-gap compared with the peak density observed at the activation energy E _a ≈0.2–0.3 eV. The capture centers are also found in the split-off Si layer of the fabricated SOI structures. Their activation energy at room temperature is E _a =0.53 eV, the capture cross section is 10^?19 cm², and the concentration is (0.7–1.7)×10¹³ cm^?3. It is assumed that these capture centers are related to deep bulk levels induced by electrically active impurities (defects) in the split-off Si layer close to the Si/SiO₂ interface.