The effects of oxide traps on the MOS capacitance

The trapping of electrons and holes at a semiconductor surface by traps located in the oxide adjacent to the semiconductor has been considered. It is shown that the effective capture cross section of an oxide trap viewed by a carrier at the semiconductor surface is reduced by a factor which increases exponentially with the distance the trap is located from the interface. A pseudo-Fermi function in this position variable is developed which gives the probability that a trap will be filled (or emptied) in a measurement time, Tm. The trapping kinetics developed in the first part of the paper are applied to yield the full frequency and bias dependence of an MOS capacitor for an arbitrary spatial and energy trap distribution. Specific examples are given and the problem of voltage hysteresis is dealt with quantitatively. The conclusion is that very little information about the energy distribution and capture cross sections of the oxide traps is obtained from the analysis of MOS-capacitance curves.     

多晶MAPbBr_(3)薄膜中束缚载流子热发射复合过程北大核心CSCD

有机-无机卤化铅钙钛矿(organic inorganic lead halide perovskite,OLHP)半导体材料内部的陷阱是影响OLHP的光电性能的重要因素。为了理解多晶的甲胺溴基钙钛矿((Methylammonium)PbBr_(3),MAPbBr_(3))薄膜中陷阱对光生载流子复合的影响,本文采用了时间分辨微波光电导(time resolved microwave conductivity,TRMC)技术探究了多晶MAPbBr_(3)薄膜的光生载流子复合动力学过程。实验测量结果表明多晶MAPbBr_(3)薄膜的载流子复合过程包括自由载流子复合与束缚载流子的热发射复合两部分。其中,与束缚载流子热发射复合相关的能级远离连续带,且对应的能级深度约为0.6 eV,分布宽度约为89.2 meV。本文同时利用变激发波长TRMC对比实验,分析浅束缚光生电子与导带光生电子复合过程的差异。相比于导带上的电子,实验结果表明浅束缚电子跃迁到深束缚能级的概率更大。     

Assessment of capture cross sections and effective density of electron traps generated in silicon dioxides

Generation of acceptor-like electron traps in gate oxides is an important source for device instability. Despite previous efforts, capture cross sections are not unambiguously determined, and there is confusion on how many capture cross sections genuinely exist. Neither is the dependence of trap density for a given capture cross section on stress level clear. The objective of this paper is to fill this knowledge gap by investigating electron-trapping kinetics. There are a number of obstacles for such an investigation including the simultaneous occurrence of trapping and trap generation, stability of trapping, and effects of positive charges. Through careful selection of experimental conditions and testing samples, the authors have been able to overcome these obstacles. In particular, their recent work in this area has allowed them to develop a new method for correcting the effect of positive charges. After removing all uncertainties, the authors are able to identify a capture cross section as large as 10/sup -13/-10/sup -14/ cm/sup 2/ for the generated acceptorlike trap. It will be shown that electron trapping follows the first-order model, and there is also a smaller capture cross section in the region of 10/sup -15/-10/sup -16/ cm/sup 2/. To the best of their knowledge, for the first time, the authors will show that the density of the larger trap increases with stress, but the density of the smaller trap clearly saturates.     

Optimization of recombination levels and their capture cross section in power rectifiers and thyristors

Single level recombination statistics have been used to determine the influence of lifetime on the characteristics of power rectifiers and thyristors. It is shown that the ratings of rectifiers can be optimized by maximizing the ratio of the high level to the low level lifetime. An optimization criterion relating the recombination center location to its capture cross section ratio for holes and electrons is derived. The relationship is found to be a function of temperature but not dependent upon the resistivity of the base material. In the case of thyristors it is shown that the leakage current must also be considered in the optimization procedure. This leads to a criterion which relates the recombination center location to the resistivity and the capture cross section.In addition, presently used techniques of gold diffusion and electron irradiation are shown to produce levels which fit poorly into the optimization scheme. The work in this paper points out the need to gather more information regarding the energy levels and capture cross sections of other impurities in silicon.     

Determination of interface trap capture cross sections usingthree-level charge pumping

A modified three-voltage-level charge pumping (CP) technique is described for measuring interface trap parameters in MOSFETs. Charge pumping (CP) is a technique for studying traps at the Si-SiO₂ interface in MOS transistors. In the CP technique, a pulse is applied to the gate of the MOSFET which alternately fills the traps with electrons and holes, thereby causing a recombination current I_cp to flow in the substrate. With this technique, interface trap capture cross sections for both electrons and holes may be determined as a function of trap energy in a single device. It is demonstrated that a modified three-level charge pumping method may be used to determine not only interface trap densities but also to capture cross sections as a function of trap energy. The trap parameters are obtained for both electrons and holes using a single MOSFET     

Localized temporary increase in material conductivity following impact ionization in a Gunn-effect domain

It has been found that raising the applied bias on a long sample containing a high-field domain can result in a temporary increase in material conductivity over the region of the sample traversed by the domain while the higher bias was applied. The effect can be explained in terms of intrinsic impact ionization within the domain in the presence of a deep impurity level. The measured decay time of the "memory" is consistent with a capture cross section of about 10^-15cm², for electrons at the deep level, indicating that the empty center is neutral. Conversely, the filled center will have a high capture cross section for holes. Following intrinsic impact ionization in the domain the mechanism is believed to involve the rapid capture of holes at the filled centers and a subsequent relatively slow return of the extra electrons to the empty centers. The deep impurity level is thus responsible for fixing the memory in position in the material and also for inhibiting the emission of recombination radiation during impact ionization.     

Deep-level defects and diffusion length measurements in low energy proton-irradiated GaAs

A detailed characterization of deep-level defects induced by low energy proton irradiation in n-GaAs LPE layer using AlGaAs-GaAs mesa structure has been carried out for several proton energies (i.e., 50 100 and 290 kev) and fluences (i.e., 10¹⁰, 10¹¹, and 10¹² p/cm²), using DLTS, SEM-EBIC, I-V and C-V measurement techniques. Important defect and recombination parameters such as density and energy level of electron and hole traps, thermal emission rates and capture cross sections for electrons and holes in each trap level as well as hole diffusion lengths in the n-GaAs LPE layer were deduced from these measurements. Hole diffusion lengths determined by the EBIC, DLTS, and I-V measurements are found in good agreement. It is shown that dark current under forward bias condition was dominated by recombination of electron-hole pairs via deep level defects in the junction space charge region of the diode. Significant carrier removal occurs for proton fluence greater than 10¹³p/cm². Research supported by NASA Langley Research Center under grant No. NSG-1425.     

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.     

Identification of Cu-related thermally stimulated current trap in undoped semi-insulating GaAs

In the thermally stimulated current spectra of semi-insulating GaAs, a unique trap T_a at 170K is sometimes observed. The activation energy and capture cross section of T_a are 0.43 eV and 3.7×10⁻¹⁵ cm², respectively. Based on a good correlation with the Cu-related photoluminescence emission at 1.36 eV and the Cu-related deep level transient spectroscopy hole traps HL4 and HB4, we argue that T_a is a Cu-related hole trap.     

The effects of deep-level defects on the electrical properties of Cd0.9Zn0.1Te crystals

The deep-level defects of CdZnTe (CZT) crystals grown by the modified vertical Bridgman (MVB) method act as trapping centers or recombination centers in the band gap, which have significant effects on its electrical properties. The resistivity and electron mobility-lifetime product of high resistivity Cd_0.9Zn_0.1Te wafer marked CZT1 and low resistivity Cd_0.9Zn_0.1Te wafer marked CZT2 were tested respectively. Their deep-level defects were identified by thermally stimulated current (TSC) spectroscopy and thermoelectric effect spectroscopy (TEES) respectively. Then the trap-related parameters were characterized by the simultaneous multiple peak analysis (SIMPA) method. The deep donor level (E_DD)dominating dark current was calculated by the relationship between dark current and temperature. The Fermi-level was characterized by current-voltage measurements of temperature dependence. The width of the band gap was characterized by ultraviolet-visible-infrared transmittance spectroscopy. The results show the traps concentration and capture cross section of CZT1 are lower than CZT2, so its electron mobility-lifetime product is greater than CZT2. The Fermi-level of CZT1 is closer to the middle gap than CZT2. The degree of Fermi-level pinned by EDD of CZT1 is larger than CZT2. It can be concluded that the resistivity of CZT crystals increases as the degree of Fermi-level pinned near the middle gap by the deep donor level enlarges.     

A new electron-trapping model for the gate insulator of insulated gate field-effect transistors

In the present paper, a new model for electron trapping kinetics in the gate insulator of an insulated gate field-effect transistor (IGFET) is proposed. This model includes a continuous variation of the trapping cross section, σ_o, as a function of the number of filled traps,N _D . The dependency of σ_o is believed to be related physically to the annihilation, or buildup of coulombic charge, which effect has heretofore been neglected in first-order trapping kinetics that describe the entire defect concentration range. The result is that in order to model the experimental data fewer classes of trap cross sections are needed. AsN _D traps fill, the trapping cross section, σ_o, is assumed to be reduced by a factor (1 -N _D /N _T ) whereN _T is the total number of available traps per unit area. This decrease in δ_o is consistent, physically, with a concept of increasing repulsion of carriers as traps fill. This new model also indicates that the number of injected electrons needed to populate 99% of the total traps is about 20 times greater than that predicted by the existing first-order trapping kinetics model. Comparisons between the results of the new model and the first-order trapping kinetics model applied to experimental defect data are also given.     

Spectra of persistent photoconductivity in InAs/AlSb quantum-well heterostructures

Persistent photoconductivity at T = 4.2 K in AlSb/InAs/AlSb heterostructures with two-dimensional (2D) electron gas in InAs quantum wells is studied. Under illumination by IR radiation (?ω = 0.6–1.2 eV), positive persistent photoconductivity related to the photoionization of deep-level donors is observed. At shorter wavelengths, negative persistent photoconductivity is observed that originates from band-to-band generation of electron-hole pairs with subsequent separation of electrons and holes by the built-in electric field, capture of electrons by ionized donors, and recombination of holes with 2D electrons in InAs. It is found that a sharp drop in the negative photoconductivity takes place at ?ω > 3.1 eV, which can be attributed to the appearance of a new channel for photoionization of deep-level donors in AlSb via electron transitions to the next energy band above the conduction band.     

Energy level of minority carrier trap centers induced by light illumination in B-doped Cz-Si solar cells

The conversion efficiency of boron (B)-doped Czochralski silicon (Cz-Si) solar cells decreased by light illumination or minority carrier injection. Defects are induced by illumination and they act as trap centers, shorten the minority carrier (electron) lifetime. The energy level of this minority carrier trap center was determined by analyzing the open-circuit voltage (V_OC) changes as a function of substrate temperature. When substrate temperature is low, all electrons which are captured by the trap centers recombine with holes and they do not contribute to the generation of electric power. However, as the substrate temperature is increasing, some of the captured electrons are thermally excited to the conduction band before recombination. Hence, the lifetime of minority carriers are improved and V_OC is recovered. Based on this result, the energy level of trap center induced by light illumination is estimated to be 0.26 eV, which corresponds to the boron–oxygen-related defect (E_C-0.26 eV).     

PLhenomenological model of grain-boundary trapping states in polycrystalline silicon under optical illumination

We have developed a new phenomenological model of grain boundary (GB) recombination, carrier transport and electrostatics under assumptions of Gaussian energy distributions of GB interface states, and unequal capture cross-sections of these GB states for electrons and holes in polycrystalline materials. Calculations have been performed of the recombination current density and the recombination velocity at grain boundaries in polycrystalline silicon for four different energy distributions. The results indicated that the recombination velocity at the grain boundary strongly depends on the location of the trap energy level. The assumption of GB interface states with a discrete (δ-function) distribution has been found to be inappropriate.     

Transport properties of thermal oxide films grown on polycrystalline silicon—Modeling and experiments

A theoretical model considering the effects of Fowler-Nordheim tunneling, image-force lowering, first-order trapping kinetics, impact ionization, and asperity-induced field enhancement has been developed to investigate the ramp-voltage-stressed I-V characteristics of the oxide films thermally grown on the polycrystalline silicon. From the ramp-voltage-stressed I-V measurements, the important physical parameters such as average field-enhancement factor, effective total trapping density, trap capture cross section, recombination capture cross section, and dielectric breakdown field can be extracted. Under a ramp voltage stress, it is shown that the serious asperity effect can lead to a larger leakage current and a weaker dielectric breakdown field, but the serious trapping effect may reduce the leakage current and enlarge the dielectric breakdown field. Moreover, dry O2oxidation at a higher temperature and steam oxidation at a lower temperature can result in a better quality poly-oxide because the asperity-induced field enhancement is weakened and the electron trapping effect is slightly increased. Besides, high-temperature dry O2oxidation can result in a smaller asperity effect as compared with steam oxidation, and the quality of the poly-oxide is deteriorated when the poly-Si substrate is heavily doped because the asperity effect is enhanced.     

On the process of hole trapping in Ge/Si heterostructures with Ge quantum dots

Admittance spectroscopy is used to determine the cross sections and energy levels of holes in Ge/Si heterostructures with Ge quantum dots. The structures are grown by molecular-beam epitaxy. It is established that, in layers of quantum dots produced at low growth temperatures T _g ≤ 450°C, the capture cross section for hole trapping into quantum dots exponentially increases with increasing hole binding energy (the Meyer-Neldel rule), with the same characteristic energy ～25 eV independent of T _g . It is shown that the Meyer-Neldel rule is violated in structures grown at higher temperatures or in samples treated in hydrogen plasma. In the case of nanoclusters synthesized at low temperatures, the experimental results suggest that charge-carrier trapping into Ge quantum dots proceeds via the electron-phonon mechanism with the participation of structural defects.     

A new model for the post-stress interface trap generation inhot-carrier stressed p-MOSFETs

A new insight into the post-stress interface trap (N_it) generation in hot-electron stressed p-MOSFETs is presented. N_it generation is suppressed for positive oxide field but enhanced for negative oxide field. This observation provides strong support for a two-carrier model, involving the recombination between trapped electrons and inversion holes. While post-stress interface instability has generally been associated with hole trapping and hydrogen transport, our results clearly show the importance of electron traps on the long term stability of the Si-SiO₂ interface, and that the two-carrier model provides a consistent explanation for post-stress N_it generation in p-MOSFETs stressed under hot-electron injection     

Trap-limited bimolecular recombination in poly(3-hexylthiophene): Fullerene blend films

Trap-limited bimolecular recombination in poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PC₆₁BM) blend films has been investigated by using photo-induced charge extraction by linearly increasing voltage (photo-CELIV) method. The bimolecular recombination rate is strongly dependent on the photoexcitation density, the PC₆₁BM composition and the thermal annealing process, but it slightly depends on the thickness of the blend film. The results show that the trap-limited bimolecular recombination is strongly affected by the distribution of the density of trap state (trap DOS). The higher trap-limited bimolecular recombination rate means the trap DOS centered at lower energy which is beneficial to charge carriers transportation, due to the lower activation energy and faster release rate. On the other hand, the trap-limited bimolecular recombination rate is mainly controlled by the slower species of charge carriers in the blend film when the transport of electrons and holes are strongly unbalanced, and the recombination rate will increase when the transport of electrons and holes becomes more balanced.     

Electrical characterization of self-assembled In0.5Ga0.5As/GaAs quantum dots by deep level transient spectroscopy

We have investigated electron emission from self-assembled In_0.5Ga_0.5As/GaAs quantum dots (QDs) grown by molecular-beam epitaxy (MBE). Through detailed deep level transient spectroscopy comparisons between the QD sample and a reference sample, we determine that trap D, with an activation energy of 100 meV and an apparent capture cross section of 5.4×10⁻¹⁸ cm², is associated with an electron quantum level in the In_0.5Ga_0.5As/GaAs QDs. The other deep levels observed, M1, M3, M4, and M6, are common to GaAs grown by MBE.