Comparison of current-voltage characteristics of n- and p-type 6H-SiC Schottky diodes

Current-voltage (I–V) characteristics of n- and p-type 6H−SiC Schottky diodes are compared in a temperature range of room temperature to 400°C. While the room temperature I–V characteristics of the n-type Schottky diode after turn-on is more or less linear up to ∼100 A/cm², the I–V characteristics of the p-type Schottky diode shows a non-linear behavior even after turn-on, indicating a variation in the on-state resistance with increase in forward current. For the first time it is shown that at high current densities (>125 A/cm²) the forward voltage drop across p-type Schottky diodes is lower than that across n-type Schottky diodes on 6H−SiC. High temperature measurements indicate that while the on-state resistance of n-type Schottky diodes increases with increase in temperature, the on-state resistance of p-type Schottky diodes decreases with increase in temperature up to ∼330 K.     

N-type doping of 4H-SiC with phosphorus Co-implanted with C or Si

Phosphorus has been shown to be a much better dopant than nitrogen in 4H−SiC for heavily doped n-type implantation. In this paper, the effect of co-implantation of phosphorus with carbon or silicon is studied. The implanted layers are characterized by an analytical technique (secondary ion mass spectrometry). Electrical measurements include sheet resistance and Hall measurements as well as forward and reverse I–V characterization of the resulting n+/p rectifiers. The effect of co-implantation of P/C and P/Si on the electrical activation of phosphorus has been monitored. After 1700°C anneal, respective sheet resistance values of 111 and 132 ohm/square were measured. Forward characteristics of these diodes are observed to obey a generalized Sah-Noyce-Shockley multiple level recombination model with four shallow levels and one deep level.     

Electronic properties of boron in p-type bulk 6H-SiC

The electronic properties of boron in bulk 6H-SiC have been studied by temperature dependent Hall effect, thermal admittance spectroscopy, and optical absorption. A single acceptor level located between 0.27 and 0.35 eV above the valence band is associated with boron on a silicon lattice site. The deep nature of this acceptor level prevents complete thermal activation of the level at room temperature and thus carrier concentration measurements at this temperature will not give the total boron concentration. A spread in the measured activation energy for boron is reported. Measurement of optical absorption is suggested as a nondestructive measure of boron concentration. No evidence for the D-center was observed in this material.     

Surface morphology of 6H-SiC after thermal diffusion

Diffusion of aluminum into 6H-SiC has been carried out in the temperature range of 1800–2100°C. Aluminum carbide (Al₄C₃) was used for a p-type impurity source; the diffused surface exhibited good stoichiometry and surface morphology. A thin-layer graphite film was developed to protect the wafer surface from deterioration during the high-temperature diffusion process. A high-resolution optical microscope (HROM) and atomic force microscopy (AFM) were employed to evaluate the surface morphology of the diffused samples. The protective graphite layer significantly decreased the surface roughness. X-ray photoelectron spectroscopy (XPS) was used to identify the Si/C ratio near the surface regions. Very little surface graphitization occurred during diffusion. In addition, secondary ion-mass spectroscopy (SIMS) was used to investigate the influence of the thin graphite film on the diffusion properties in SiC. There were no significant differences in doping profiles in the samples with and without the graphite film.     

Planar 4H- and 6H-SiC p–n diodes fabricated by selective diffusion of boron

Graphite film was used as a protective and selective mask for realizing diffusion of boron in SiC. Secondary ion mass spectroscopy was employed to identify the diffusion profile in SiC. No significant difference between diffusion profiles in 4H-SiC and 6H-SiC was found. Planar p–n diodes with local p-type emitter regions were fabricated in 4H-SiC and 6H-SiC based on this process. The current density versus voltage (J–V) curves of the formed diodes exhibited good rectification characteristics. The 4H-SiC p–n diodes had much lower forward voltage drops and much less temperature dependence in comparison with 6H-SiC p–n diodes.     

Shallow acceptor levels in 4H- and 6H-SiC

Shallow impurities are the principal means of affecting the electrical properties of semiconductors in order to induce desired characteristics. They can be used to isolate a region by introducing carriers of opposite charge, or they can be used to enhance the number of carriers of a particular type. Thermal admittance spectroscopy has been used to determine the activation energies of the principal p-type dopants, Al and B, in 4H and 6H-SiC, and temperature dependent Hall effect measurements were used to study the shallow B acceptors in 6H-SiC. The accept or species B and Al occupy inequivalent lattice sites in the Si sublattice, and would be expected to exhibit distinct energy levels for each site in analogy to the well known donor energy levels of N. Activation energies for B in 6H-SiC were found to be E_h=E_v+0.27 eV, E_k1=E_v+0.31 eV, and E_k2=E_v+0.38 eV. Al acceptors in 4H-SiC were found to exhibit two energy levels at E_h=E_v+0.212 eV and E_k=E_v+0.266 eV.     

单晶6H-SiC纳米线的可控掺杂及光学特性

采用有机前驱体制备纳米材料工艺,制备出不同Al掺杂浓度的6H-SiC纳米线(Al/6H-SiC).用HRTEM、EDX、XRD等对纳米线进行了表征,发现随着起始材料中异丙醇铝含量的增加,所制备的纳米线中的Al浓度也在增加,最高可达到1.25%, HRTEM显示晶格间距为0.26 nm和0.25 nm,对应为6H-SiC的(101)和(102)面间距,Si、C原子比为1∶1.拉曼光谱得到这种6H-SiC的声子能量为100 meV,由吸收光谱带边吸收外推计算得到Al/6H-SiC纳米线光学带隙,掺杂浓度越大,吸收边红移越大.     

Effect of processing conditions on inversion layer mobility and interface state density in 4H−SiC MOSFETs

N-channel, inversion mode MOSFETs have been fabricated on 4H−SiC using different oxidation procedures, source/drain implant species and implant activation temperature. The fixed oxide charge and the field-effect mobility in the inversion layer have been extracted, with best values of 1.8×10¹² cm⁻² and 14 cm²/V-s, respectively. The interface state density, D_it close to the conduction band of 4H−SiC has been extracted from the subthreshold drain characteristics of the MOSFETs. A comparison of interface state density, inversion layer mobility and fixed oxide charges between the different processes indicate that pull-out in wet ambient after reoxidation of gate oxide improves the 4H−SiC/SiO₂ interface quality.     

Two-step rapid thermal diffusion of boron into silicon using a boron nitride solid diffusion source

A two-step rapid thermal diffusion process of boron into silicon using a boron nitride solid diffusion source is described. During the first step, HBO² glass is transferred onto the silicon wafer from the diffusion source by keeping the temperature of the silicon wafer at 750° C while the diffusion source is at about 900° C. Boron is, then, diffused into the silicon wafer from HBO² glass at 1000° C or 1100° C in N₂ during the second step. Extremely shallow junctions with junction depths of about 20 nanometers and sheet resistances of about 350 ohms/sq can be achieved with this method as well as relatively deep junctions with junction depths of about 175 nanometers and sheet re-sistances of about 55 ohms/sq. When the diffusion is performed at 1100° C, both the junction depth and electrically active boron concentration at the surface increase as the ambient gas is changed from N₂ to O₂ while the sheet resistance decreases. A boron rich layer which has high resistivity is not formed at the surface when the diffusion is performed at 1100° C in O₂ ambient. This work was supported by Ministry of Science and Technology, Korea     

钒掺杂形成半绝缘6H-SiC的补偿机理

研究了钒掺杂生长半绝缘6H-SiC的补偿机理.二次离子质谱分析结果表明,非故意掺杂生长的6H-SiC中,氮是主要的剩余浅施主杂质.通过较深的钒受主能级对氮施主的补偿作用,得到了具有半绝缘特性的SiC材料.借助电子顺磁共振和吸收光谱分析,发现SiC中同时存在中性钒(V4+)和受主态钒(V3+)的电荷态,表明掺入的部分杂质钒通过补偿浅施主杂质氮,形成受主态钒,这与二次离子质谱分析结果相吻合.通过对样品进行吸收光谱和低温光致发光测量,发现钒受主能级在6H-SiC中位于导带下0.62eV处.     

钒掺杂形成半绝缘6H-SiC的补偿机理

研究了钒掺杂生长半绝缘6H-SiC的补偿机理.二次离子质谱分析结果表明,非故意掺杂生长的6H-SiC中,氮是主要的剩余浅施主杂质.通过较深的钒受主能级对氮施主的补偿作用,得到了具有半绝缘特性的SiC材料.借助电子顺磁共振和吸收光谱分析,发现SiC中同时存在中性钒(V4 )和受主态钒(V3 )的电荷态,表明掺入的部分杂质钒通过补偿浅施主杂质氮,形成受主态钒,这与二次离子质谱分析结果相吻合.通过对样品进行吸收光谱和低温光致发光测量,发现钒受主能级在6H-SiC中位于导带下0.62eV处.     

Acceptor and donor doping of AlxGa1−xN thin film alloys grown on 6H-SiC(0001) substrates via metalorganic vapor phase epitaxy

Thin films of Si-doped Al_xGa_1−xN (0.03≤x≤0.58) having smooth surfaces and strong near-band edge cathodoluminescence were deposited at 0.35–0.5 μm/h on on-axis 6H-SiC(0001) substrates at 1100°C using a 0.1 μm AlN buffer layer for electrical isolation. Alloy films having the compositions of Al_0.08Ga_0.92N and Al_0.48Ga_0.52N exhibited mobilities of 110 and 14 cm²/V·s at carrier concentrations of 9.6×10¹⁸ and 5.0×10¹⁷ cm⁻³, respectively. This marked change was due primarily to charge scattering as a result of the increasing Al concentration in these random alloys. Comparably doped GaN films grown under similar conditions had mobilities between 170 and ∼350 cm²/V·s. Acceptor doping of Al_xGa_1−xN for x≤0.13 was achieved for films deposited at 1100°C. No correlation between the O concentration and p-type electrical behavior was observed.     

Schottky barrier height dependence on the metal work function for p-type 4H-silicon carbide

We investigated Schottky barrier diodes of several metals (Ti, Ni, and Au) having different metal work functions to p-type 4H−SiC (0001) using I–V and C–V characteristics. Contacts showed excellent Schottky behavior with stable ideality factors of 1.07, 1.23, and 1.06 for Ti, Ni, and Au, respectively, in the range of 24°C to 300°C. The measured Schottky barrier height (SBH) was 1.96, 1.41, and 1.42 eV for Ti, Ni, and Au, respectively, in the same temperature range from I–V characteristics. Based on our measurements for p-type 4H−SiC, the SBH (φ_Bp) and metal work functions (φ_m) show a linear relationship of φ_Bp = 4.58 − 0.61φ_m and φ_Bp = 4.42 − 0.54φ_m for I–V and C–V characteristics at room temperature, respectively. We observed that the SBH strongly depends on the metal work function with a slope (S ≡ φ_Bp/φ_m) of 0.58 even though the Fermi level is partially pinned. We found the sum of the SBH (φ_Bp + φ_Bn = E_g) at room temperature for n-and p-type 4H−SiC to be 3.07 eV, 3.12 eV, and 3.21 eV for Ti, Ni, and Au, respectively, using I–V and C–V measurements, which are in reasonable accord with the Schottky-Mott limit.     

Modification of 4H-SiC and 6H-SiC(0001)<Subscript>Si</Subscript> surfaces through the interaction with atomic hydrogen and nitrogen

The interaction of 4H-SiC(0001)_Si and 6H-SiC(0001)_Si surfaces with atomic hydrogen and atomic nitrogen produced by remote radio-frequency plasmas is investigated. The kinetics of the surface modifications is monitored in real time using ellipsometry, while chemical modifications of the surface are characterized using x-ray photoelectron spectroscopy (XPS). Film morphological properties are assessed with atomic force microscopy (AFM). A two-stage substrate preparation procedure is described that effectively removes oxygen from the SiC surface at low (200°C) temperature. In the first step, the SiC surface is etched with an HCl/HF acid solution as an alternative to the conventional HF(1%)-H₂O solution. The HCl/HF etch provides effective hydrogen passivation of the SiC surface. In the second step, the SiC surface is exposed to atomic hydrogen that selectively interacts with residual oxygen. In addition, the temperature dependence of the nitridation of SiC surfaces has also been investigated. It is found that interaction of SiC surfaces with atomic hydrogen at 200°C provides clean, smooth, and terraced surfaces suitable for epitaxial growth. In contrast, SiC surface exposure at high temperature (750°C) to atomic hydrogen and nitrogen results in very rough and disordered Si-rich surfaces. Finally, we find that the 4H-SiC surface is more reactive than the 6H-SiC surface to both species studied, independent of temperature. Surface geometry and electronic factors responsible for the observed reactivities are discussed.     

Si面6H-SiC衬底外延石墨烯的异常硝酸掺杂效应

比较研究了Si面、C面SiC外延石墨烯以及CVD石墨烯的硝酸掺杂效应。结果表明对C面SiC外延石墨烯和CVD石墨烯,硝酸掺杂显现出p型掺杂特性使其方阻降低,这是由于硝酸与石墨烯氧化还原反应过程中电荷转移所导致;而对于Si面SiC外延石墨烯,硝酸掺杂则显现出n型掺杂特性使其方阻增大,这种异常掺杂效应是由于硝酸与石墨烯发生氧化还原反应释放出的热量使悬挂键吸附的氧脱附,增强了石墨烯与衬底之间的耦合效应,从而使得电子浓度增大,电子的迁移率显著减小。     

Anomalous oxidation rate in 6H-SiC depending on the partial pressure of O2 and H2O

On the thermal oxidation of silicon carbide, the partial pressure of H₂O affects the oxidation rate very differently from silicon. We have found that the oxide thickness shows the bell-shape as the function of the relative partial pressure of H₂O, called the p parameter. The main reason for this is that the diffusion limiting process diminishes according to the reduction of p while the linear rate constant does not depend on p strongly until p=0.15. The reduction of the oxide thickness below p=0.15 is explained by the abrupt reduction of the linear rate constant. In addition, the interface quality for oxides grown in the different p has been investigated. For the reduction of the surface states, the large p (over p=0.8) is favorable.     

Electrical characteristics of a 6H-SiC epitaxial layer grown by chemical vapor deposition on porous SiC substrate

Porous SiC (PSC) has been proposed as a buffer layer for reducing defects in epitaxial SiC layers. In this study, electrical characteristics of a 6H-SiC epitaxial layer grown by chemical vapor deposition on a porous SiC substrate (SiC-on-PSC) have been compared to those simultaneously grown on a standard SiC substrate (SiC-on-STD). Schottky barrier diodes (SBDs) have been fabricated on both epitaxial layers and then investigated with temperature-dependent current-voltage (I-V), capacitance-voltage (C-V), and deep-level transient spectroscopy (DLTS) measurements. The SBDs on both SiC-on-PSC and SiC-on-STD show about the same I-V and C-V characteristics, and at least four electron traps, i.e., B (0.75 eV), C (0.63 eV), D (0.40 eV), and E (0.16 eV), can be identically found in both SBDs by DLTS measurements. Thus, we conclude that the electrical quality of SiC-on-PSC is comparable to that of SiC-on-STD, and that the higher breakdown voltages observed in SBDs on SiC-on-PSC are not obviously related to a different defect structure.     

6H-SiC单极功率器件性能的温度关系

基于碳化硅材料电离系数和迁移率的温度依赖性 ,利用有效电离系数的 Fulop近似 ,推出了 6 H- Si C单极性功率器件击穿电压和比导通电阻的温度依赖性解析表达式 .理论预言的击穿电压和临界电场与先前的实验结果基本一致 (误差小于 10 % ) ,验证了理论模型的适用性     

Al+ and B+ implantations into 6H-SiC epilayers and application to pn junction diodes

Aluminum (Al) and boron (B) ion implantations at room temperature into n-type 6H-SiC epilayers have been investigated. Rutherford backscattering spectroscopy (RBS) channeling measurements revealed larger lattice damage in Al⁺ implantation at a given total implantation dose. A nearly perfect electrical activation ratio (>90%) could be attained by high-temperature annealing at 1600°C for Al⁺ and 1700°C for B⁺ implantations. Mesa pn junction diodes formed by either Al⁺ or B⁺ implantation with a 1×10¹⁴ cm⁻² dose exhibited high blocking voltages of 950∼1070 V, which are 80∼90% of the ideal value predicted for the diode structure. The forward current can clearly be divided into two components of diffusion and recombination currents. B⁺-implanted diodes showed higher breakdown voltage on average but poor forward conduction. Comparison of the performance of Al⁺ and B⁺-implanted diodes is discussed.     

Zn doping of InP, InAsP/InP, and InAsP/InGaAs heterostructures through metalorganic vapor phase diffusion (MOVPD)

Zinc incorporation by post-growth metalorganic vapor phase diffusion (MOVPD) is used to achieve high p-doping, which is desirable for the fabrication of photodiodes. Diethylzinc (DEZ) is used as precursor and Zn is diffused into InP and InAs_0.6P epitaxial layers grown by low pressure metalorganic vapor phase epitaxy (MOVPE) on different substrate orientations, enabling the investigation of the dislocation density on the Zn incorporation. Diffusion depths are measured using cleave-and-stain techniques, resistivity measurements, electrochemical profiling, and secondary ion mass spectroscopy. High hole concentrations of, respectively, 1.7 10¹⁹ and 6 10¹⁸ cm⁻³, are obtained for, respectively, InAs_0.60P and InP. The diffusion coefficients are derived and the Zn diffusion is used for the fabrication of lattice-mismatched planar PIN InAsP/InGaAs photodiodes.