Role of contacts in characterization of deep traps in semi-insulating GaAs by thermally stimulated current spectroscopy

The role of contacts in characterization of traps in semi-insulating (SI) GaAs by thermally stimulated current (TSC) methods has been demonstrated by comparing alloyed In and soldered In contacts. Alloyed In contacts, which have an ohmic characteristic, assure high sensitivities in both TSC and temperature dependent photocurrent (PC), and both are important for determining the trap concentrations in SI GaAs. On the other hand, soldered In contacts, which act like Schottky barriers, cause a significant reduction of both PC and TSC, particularly at low temperatures, and can lead to a misinterpretation of TSC results.     

Characterization of deep centers in undoped semi-insulating GaAs substrates by normalized thermally stimulated current spectroscopy: Comparison of 100 and 150 mm wafers

A well-established characterization method for investigating deep traps in semi-insulating (SI) GaAs is thermally stimulated current (TSC) spectroscopy; however, TSC is not considered to be a quantitative technique because it involves carrier mobility, lifetime, and geometric factors, which are either unknown or poorly known. In this paper, we first show how to quantify a TSC spectrum, by normalizing with infrared (hv = 1.13 eV) photocurrent, and then apply this method (called NTSC) to study the lateral uniformity of the main deep centers across the diameters of undoped SI GaAs wafers. The wafers used in the study include both the standard 100 mm sizes and the new 150 mm variations, and are grown by both the low and high pressure liquid encapsulated Czochralski techniques. The results reveal that the 150 mm wafers have a worse NTSC uniformity for the main traps and a higher degree of compensation, as compared these parameters for the 100 mm wafers. In addition, nonuniformities related to the electric field effects on both the TSC spectrum and the low temperature photocurrent are found in the 150 mm wafer grown by the low pressure technique.     

Effects of Electron Irradiation on Deep Centers in High-Purity Semi-Insulating 6H-SiC

Many point-defect–related centers have been investigated in electron-irradiated 6H-SiC by deep-level transient spectroscopy (DLTS). Most of them are believed to be related to vacancies. Our DLTS studies on deep centers produced by electron-irradiation (EI) in conductive epi-6H-SiC are in agreement with the literature data. However, for semi-insulating SiC, DLTS cannot be used for trap studies so we have applied thermally stimulated current (TSC) spectroscopy. At least nine TSC traps have been observed in high-purity/semi-insulating (HPSI) 6H-SiC. To understand the nature of these centers, 1-MeV EI and postirradiation annealing at 600°C were applied to the sample. The TSC spectroscopy and 4.2 K photoluminescence (PL) have been used to study the effects of EI and annealing on the centers in HPSI 6H-SiC. It was found that (1) some of the major EI-induced DLTS centers in conductive 6H-SiC, such as ED1, E₁/E₂, E_i, Z₁/Z₂, and L₉, have TSC counterparts even in as-grown HPSI 6H-SiC; (2) EI-induced TSC centers in HPSI 6H-SiC are due to point defects, which have been confirmed by typical PL lines (such as S, L, and V lines); and (3) the concentration of the 1.1-eV center, which controls material conductivity, can be increased by 1-MeV EI and decreased by 600°C annealing.     

High-purity semi-insulating 4H-SiC grown by the seeded-sublimation method

The growth of high-purity, semi-insulating (HPSI) 4H-SiC crystals has been achieved using the seeded-sublimation growth technique. These semi-insulating (SI) crystals (2-inch diameter) were produced without the intentional introduction of elemental deep-level dopants, such as vanadium, and wafers cut from these crystals possess room-temperature resistivities greater than 10⁹ Ωcm. Based upon temperature-dependent resistivity measurements, the SI behavior is characterized by several activation energies ranging from 0.9–1.5 eV. Secondary ion mass spectroscopy (SIMS) and electron paramagnetic resonance (EPR) data suggest that the SI behavior originates from deep levels associated with intrinsic point defects. Typical micropipe densities for wafers were between 30 cm⁻² and 150 cm⁻². The room-temperature thermal conductivity of this material is near the theoretical maximum of 5 W/m K for 4H-SiC, making these wafers suitable for high-power microwave applications.     

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.     

Deep centers in undoped semi-insulating InP

Undoped semi-insulating (SI) InP samples, subjected to one-step and multi-step wafer annealing, and lightly and normally Fe-doped SI InP samples without annealing have been characterized by thermally stimulated current (TSC) spectroscopy. A dominant deep center at 0.63 eV is found in all samples and is undoubtedly due to iron. Two prominent TSC traps, T_b (0.44 eV) and T_d (0.33 eV), found in undoped SI InP, are thought to be related to the phosphorus antisite P_In, and traps at low temperatures, like T_e* (0.19 eV), to the phosphrus vacancy V_P.     

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.     

氙光作用后半绝缘4H-SiC外延片本征缺陷的ESR谱特性

The intrinsic defects in epitaxial semi-insulating 4H-SiC prepared by low pressure chemical vapor deposition （LPCVD） are studied by electron spin resonance （ESR） with different illumination times. The results show that the intrinsic defects in as-grown 4H-SiC consist of carbon vacancy （Vc） and complex-compounds-related Vc. There are two other apexes presented in the ESR spectra after illumination by Xe light, which are likely to be Vsi and VcCsi. Illumination time changes the relative density of intrinsic defects in 4H-SiC; the relative density of intrinsic defects reaches a maximum when the illumination time is 2.5 min, and the ratio of Vc to complex compounds is minimized simultaneously. It can be deduced that some Vsi may be transformed to the complex-compounds-related Vc because of the illumination.     

ESR characters of intrinsic defects in epitaxial semi-insulating 4H-SiC illuminated by Xe light

The intrinsic defects in epitaxial semi-insulating 4H-SiC prepared by low pressure chemical vapor de-position (LPCVD) are studied by electron spin resonance (ESR) with different illumination times. The results show that the intrinsic defects in as-grown 4H-SiC consist of carbon vacancy (V_C) and complex-compounds-related V_C.There are two other apexes presented in the ESR spectra after illumination by Xe light, which are likely to be V_(Si) and V_CC_(Si). Illumination time changes the relative density of intrinsic defects in 4H-SiC; the relative density of intrinsic defects reaches a maximum when the illumination time is 2.5 min, and the ratio of V_C to complex compounds is minimized simultaneously. It can be deduced that some V_(Si) may be transformed to the complex-compounds-related V_C because of the illumination.     

Influence of 4H-SiC semi-insulating substrate purity on SiC metal-semiconductor field-effect transistor performance

The performances of silicon carbide (SiC) metal-semiconductor field-effect transistors (MESFETs) fabricated on conventional V-doped semi-insulating substrates and new V-free semi-insulating substrates have been compared. The V-free 4H-SiC substrates were confirmed by secondary ion mass spectrometry (SIMS). X-ray topography revealed significantly fewer micropipes and low-angle boundaries in V-free semi-insulating substrates than in conventional V-compensated substrates. Deep-level transient spectroscopy (DLTS) indicated that the spectra signals observed in conventional V-doped substrates were either reduced or disappeared in V-free substrates. The intrinsic deep levels in V-free substrates to make semi-insulating properties were also observed in DLTS spectra. Under various DC and RF stresses, SiC MESFETs fabricated on new V-free semi-insulating substrates showed superior device performance and stability.     

高电流增益的碳化硅双极型晶体管

A novel 4H-SiC BJT of high current gain with a suppressing surface traps effect has been proposed. It is effective to improve the current gain due to the lower electrons density in the surface region by extending the emitter metal to overlap the passivation layer on the extrinsic base surface. The electrons trapped in the extrinsic base surface induce the degeneration of Si C BJTs device performance. By modulating the electron recombination rate, the novel structure can increase the current gain to 63.2% compared with conventional ones with the compatible process technology. Optimized sizes are an overlapped metal length of 4 m, as well as an oxide layer thickness of 50 nm.     

Investigations of 3C-SiC inclusions in 4H-SiC epilayers on 4H-SiC single crystal substrates

Synchrotron white beam x-ray topography (SWBXT) and Nomarski optical microscopy (NOM) have been used to characterize 4H-SiC epilayers and to study the character of triangular inclusions therein. 4H-SiC substrates misoriented by a range of angles from (0001), as well as (1 100) and (1120) oriented substrates were used. For epilayers grown on substrates misoriented by 3.5° from (0001) toward <1120>, the triangular inclusions were identified as consisting of two 3C-SiC structural configurations which are related to each other by a 180° rotation about the [111] axis. The epitaxial relationships between the 3C inclusions and the 4H-SiC epilayers (or substrates) were also determined. No evidence was found for the nucleation of 3C-SiC inclusions at superscrew dislocations (along the [0001] axis) in the 4H-SiC substrates. Increasing the off-axis angle of the substrates from 3.5 to 6.5° was found to greatly suppress the formation of the triangular inclusions. In the case of substrates misoriented by 8.0° from (0001) toward <1120>, the triangular inclusions were virtually eliminated. The crystalline quality of 4H-SiC epilayers grown on the substrates misoriented by 8.0° from (0001) was very good. For the (1100) and (1120) samples, there is no indication of 3C-SiC inclusions in the epilayers. Possible formation mechanisms and the morphology of 3C-SiC inclusions are discussed.     

Photoelectrochemical capacitance-voltage measurements of 4H-SiC

Photoelectrochemical capacitance-voltage (PC-V) measurements have been successfully made on 4H-SiC material. The 0.05 M KOH solution used as a Schottky contact etches the material with an average roughness of about one percent of the total average etch depth at an etch rate of 1 μm/h. This capability allows the carrier concentration profile of thicker layers with different doping levels to be analyzed. The PC-V measurements are compared with secondary ion mass spectroscopy. The results show agreement with nitrogen doping in the top epitaxial layer and in the substrate. The PC-V measurement also sees an unknown species diffusing out of the substrate into the nominally undoped buffer layer.     

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.     

Investigation of lateral spreading current in the 4H-SiC Schottky barrier diode chip

Lateral current spreading in the 4H-SiC Schottky barrier diode(SBD)chip is investigated.The 4H-SiC SBD chips with the same vertical parameters are simulated and fabricated.The results indicate that there is a fixed spreading resistance at on-state in current spreading region for a specific chip.The linear specific spreading resistance at the on-state is calculated to be 8.6 Ω/cm in the fabricated chips.The proportion of the lateral spreading current in total forward current(Psp)is related to an-ode voltage and the chip area.Psp is increased with the increase in the anode voltage during initial on-state and then tends to a stable value.The stable values of Psp of the two fabricated chips are 32％and 54％.Combined with theoretical analysis,the pro-portion of the terminal region and scribing trench in a whole chip(Ksp)is also calculated and compared with Psp.The Ksp val-ues of the two fabricated chips are calculated to be 31.94％and 57.75％.The values of Ksp and Psp are close with each other in a specific chip.The calculated Ksp can be used to predict that when the chip area of SiC SBD becomes larger than 0.5 cm2,the value of Psp would be lower than 10％.     

SIMS analysis of nitrided oxides grown on 4H-SiC

This paper shows for the first time, physical evidence of nitrogen incorporation at the oxide-SiC interface as a result of post-oxidation annealing in nitric oxide (NO). Using secondary ion mass spectroscopy (SIMS) analysis, the location and shape of the nitrogen profile is seen to be almost identical to that found in oxide-silicon interfaces. Close examination of oxygen and carbon SIMS profiles and atomic force microscope scans also indicate a sharper interface when annealing is done using NO compared to inert gases such as N₂ or argon, possibly due to the removal of carbon clusters which form at the interface during oxidation. As in the case of silicon, NO annealing shows great promise as a processing step in the production of device quality gate oxides on SiC.     

钛酸钡微晶粉粒的热刺激电流

研究了铁电微晶粉粒的热刺激电流及其铁电相变。对三种不同化学成分的钛酸钡粉料与硅油的混合物进行了热刺激电流测量。钛酸钡粉粒与硅油的混合物有很明显的热刺激电流(TSC)且表现出和铁电相变有关的明显的峰,显示出与粒径存在一定的关系;得出了样品的表观极化强度曲线。证明了当粉料与硅油混合后,用介电谱观测不到的粉料的铁电–顺电相变,用 TSC 方法可以观察到。     

Study of defect levels in CdTe using thermoelectric effect spectroscopy

We have studied the defect levels in as grown and post growth processed cadmium telluride (CdTe) using thermoelectric effect spectroscopy (TEES) and thermally stimulated current (TSC) techniques. We have extracted the thermal energy (E_th) and trapping cross section (σ_th) for the defect levels using the initial rise and variable heating rate methods. We have identified 10 different defect levels in the crystals. Thermal ionization energy values obtained experimentally were compared to theoretical values of the transition-energy levels of intrinsic and extrinsic defects and defect complexes in CdTe determined by first-principles band-structure calculations. On the basis of this comparison, we have associated the observed ionization levels with various native defects and impurity complexes.     

4H-SiC低压热壁CVD同质外延生长及特性

为了获得高质量4H-SiC外延材料,研制出一套水平式低压热壁CVD(LP-HWCVD)生长系统,在偏晶向的4H-SiC Si(0001)晶面衬底上,利用"台阶控制生长"技术进行了4H-SiC的同质外延生长,典型生长温度和压力分别为1500℃和1.3×103Pa,生长速率控制在1.0μm/h左右.采用Nomarski光学显微镜、扫描电镜(SEM)、原子力显微镜(AFM)、X射线衍射、Raman散射以及低温光致发光测试技术,研究了4H-SiC的表面形貌、结构和光学特性以及用NH3作为n型掺杂剂的4H-SiC原位掺杂技术,并在此基础上获得了4H-SiC p-n结二极管以及它们在室温及400℃下的电致发光特性,实验结果表明4H-SiC在Si不能工作的高温环境下具有极大的应用潜力.     

Advanced SPICE-Modeling of 4H-SiC MESFETs

A modified drain source current suitable for simulation program with integrated circuit emphasis （SPICE） simulations of SiC MESFETS is presented in this paper. Accurate modeling of SiC MESFET is achieved by introducing three parameters in Triquint＇s own model （TOM）. The model, which is single piece and continuously differentiable, is verified by measured direct current （DC） I-V curves and scattering parameters （up to 20 GHz）.