Al-implanted 4H-SiC Research Articles

Ohmic Contacts on p-Type Al-Implanted 4H-SiC Layers after Different Post-Implantation Annealings.

This paper reports on the electrical activation and Ohmic contact properties on p-type Al-implanted silicon carbide (4H-SiC). In particular, the contacts were formed on 4H-SiC-implanted layers, subjected to three different post-implantation annealing processes, at 1675 °C, 1175 °C, and 1825 °C. Under these post-implantation annealing conditions, the electrical activation of the Al dopant species increased from 39% to 56%. The Ti/Al/Ni contacts showed an Ohmic behavior after annealing at 950 °C. The specific contact resistance ρc could be lowered by a factor of 2.6 with the increase of the post-implantation annealing temperature. The result can be useful for application in device fabrication. Moreover, the dependence of ρc on the active acceptor concentration followed the thermionic field emission model, with a barrier height of 0.63 eV.

Defects and electrical properties in Al-implanted 4H-SiC after activation annealing**Project supported by the National Key Research and Development Program of China (Grant No. 2016YFB0100601).

The defects and electrical properties in Al-implanted 4H-SiC after activation annealing (1600 °C–1800 °C) are investigated. High temperature annealing can reduce the ion implantation-induced damage effectively, but it may induce extended defects as well, which are investigated by using Rutherford backscattering spectroscopy (RBS/C), secondary ion mass spectroscopy (SIMS), and transmission electron microscopy (TEM) analyses. According to the ratio of the channeled intensity to the random intensity in the region just below the surface scattering peak (Xmin) and RBS/C analysis results, the ion implantation-induced surface damages can be effectively reduced by annealing at temperatures higher than 1700 °C, while the defects near the bottom of the ion-implanted layer cannot be completely annealed out by high temperature and long time annealing process, which is also demonstrated by SIMS and TEM analyses. Referring to the defect model and TEM analyses, an optimized annealing condition can be achieved through balancing the generation and elimination of carbon vacancies in the ion implanted layers. Furthermore, the electrical and surface properties are also analyzed, and the hole concentration, mobility, and resistivity are obtained through the Hall effect. The optimized activation annealing conditions of 1800 °C/5 min are achieved, under which the lower defects and acceptable electrical properties are obtained.

Raman Characterization of Carrier Concentrations of Al-implanted 4H-SiC with Low Carrier Concentration by Photo-Generated Carrier Effect

In this work, 4H SiC samples with a multilayer structure (shallow implanted layer in a lowly doped n-type epitaxial layer grown on a highly doped thick substrate) were investigated by Raman scattering. First, Raman depth profiling was performed to identify characteristic peaks for the different layers. Then, Raman scattering was used to characterize the carrier concentration of the samples. In contrast to the conventional Raman scattering measuring method of the Longitudinal Optical Plasmon Coupled (LOPC) mode, which is only suitable to characterize carrier concentrations in the range from 2 × 1016 to 5 × 1018 cm−3, in this work, Raman scattering, which is based on exciting photons with an energy above the band gap of 4H-SiC, was used. The proposed method was evaluated and approved for different Al-implanted samples. It was found that with increasing laser power the Al-implanted layers lead to a consistent redshift of the LOPC Raman peak compared to the peak of the non-implanted layer, which might be explained by a consistent change in effective photo-generated carrier concentration. Besides, it could be demonstrated that the lower concentration limit of the conventional approach can be extended to a value of 5 × 1015 cm−3 with the approach presented here.

Raman Spectroscopy Characterization of Ion Implanted 4H-SiC

Raman spectroscopy and sheet resistance measurements were used to study the preparation processes of low-resistance p-type 4H-SiC by Al ion implantation with ion doses of 2.45×1012 - 9.0×1014 cm-2 and annealing treatment with temperatures of 1700 - 1900 °C. Greatly different from the LOPC (longitudinal optical phonon-plasmon coupled) Raman mode found from the sample of doping 4H-SiC during epitaxial growth, no significant influence on the surface concentration could be found for the longitudinal optical (LO) mode of Al-implanted 4H-SiC samples. When the Al surface concentration is larger than around 1018 cm-3, it was found that the intensity of the LO+ Raman peak (~ 980 - 1000 cm-1) increases and its full width at half maximum (FWHM) drops with the increase of surface concentration after annealing treatment. Moreover, for surface concentrations above 1018 cm-3, the LO+ Raman peak showed a left shift towards the LO peak, which could be related to the increase of free carrier concentration in the Al-implanted 4H-SiC samples. After higher annealing temperatures of 1800 °C and 1900 °C, the crystallinity of Al-implanted 4H-SiC was found to be improved compared to annealing at 1700 °C for surface concentrations larger than 1018 cm-3, which is consistent with the results of sheet resistance measurements.

Determination of Compensation Ratios of Al-Implanted 4H-SiC by TCAD Modelling of TLM Measurements

The prediction of the compensation induced hole concentration reduction in implanted Al regions is a key parameter in developing high power SiC devices. Hall effect measurements are commonly used to determine the compensation ratio of Al implanted regions. Due to the fact that this measurement method is rather complex, an approximate method was developed by using transfer length method structure measurements in combination with a TCAD simulation model. The determined compensation ratios from this work’s simulation and from Hall effect measurements from literature show consistent compensation ratios. Based on this data a fit function was derived which allows for estimating the compensation ratio for a wide Al concentration range.

Analysis of Trapping Effects on the Forward Current–Voltage Characteristics of Al-Implanted 4H-SiC p-i-n Diodes

The forward current–voltage characteristics (IF–VF) of aluminum (Al)-implanted 4H-SiC p-i-n diodes are investigated by means of a numerical simulation study that takes into account both intrinsic and doping-induced deep defects, namely, the Z1/2 and EH6/7 centers inside the drift region and an electrically active trap concentration inside the anode region due to the Al+ ion implantation process. From the experimental results, the fundamental electric parameters of several samples were extracted at different regions of diode operation and used for comparison. The modeling analysis reveals that Z1/2 and EH6/7 centers reduce the effective carrier lifetimes and increase the recombination rate in the drift region determining the slope of the IF curve in the recombination and diffusion regimes. In addition, a defect density that becomes comparable to the epilayer doping concentration introduces an apparent shunt resistance effect at low-medium biases and at the same time has a noticeable impact on the diode series resistance at voltages higher than 2.7 V. A detrimental effect on the series resistance is also observed in dependence of the trap concentration in the anode region that increases the diode’s internal resistance as a consequence of the carrier mobility decrease. Above the IF curve knee, the diode current is largely dominated by the electron injection into the anode since the concentration of free holes for conduction is strongly limited in turn by the incomplete activation of the ion-implanted impurities and the trap activity.

Analysis of the Forward I–V Characteristics of Al-Implanted 4H-SiC p-i-n Diodes with Modeling of Recombination and Trapping Effects Due to Intrinsic and Doping-Induced Defect States

In this paper, the impact of silicon carbide intrinsic defect states, such as Z1/2 and EH6/7 centers, on the forward current–voltage curves of aluminum (Al)-implanted 4H-SiC p-i-n diodes is investigated by means of a physics-based device simulator. During the simulations, an explicit carrier trap effect due to an electrically active defect concentration produced by the Al+ ion implantation process in the anode region was also taken into account. The obtained current–voltage characteristics are compared with those measured experimentally for several samples at different current levels. It is found that intrinsic defect densities as high as the epilayer doping may lead to undesirable device properties and instability of the forward bias behavior. The diode ideality factor and the series resistance increase with the increase of defects and could be controlled by using high-purity epi-wafers. Furthermore, due to their location in the bandgap and capture cross-sections, the impact of Z1/2 centers on the device electrical characteristics is more severe than that of EH6/7 centers.

Ni-Al-Ti Ohmic Contacts on Al Implanted 4H-SiC

This study shows that an Al-Ti bilayer with an Al to Ti atomic ratio suitable forohmic contacts on p-type 4H-SiC can be covered by a Ni film during the high temperature alloying process, without altering the ohmic nature, while eliminating a detrimental contact morphology caused by the presence of molten Al-Si during alloying. On 1×1020 cm-3 Al-implanted 4H-SiC layer, the RT specific contact resistance of this Ni-Al-Ti contact measured by TLM-C method is (3 ± 1)×10-6 Ωcm2.

Al-implanted on-axis 4H-SiC MOSFETs

In this paper, the impact of temperature and time stress on gate oxide stability of several multi-implanted and epitaxied 4H-SiC nMOSFET is presented. The oxide layer was processed under a rapid thermal process (RTP) furnace. The variation of the main electrical parameters is shown. We report the high quality and stability of such implanted MOSFETs, and point out the very low roughness effect of the on-axis-cut sample. Particularly, in the best case, effective channel mobility (μfe) overcomes 20 cm2.V−1.s−1 at 300 °C for a channel length of 12 μm, which is very encouraging for implantation technology. Starting from 200 °C, the apparent increase of the μfe peak of the MOSFET ceases and tends to saturate with further temperature increase. This is an indication of the potential of MOSFETs built on on-axis substrates. Thus, starting from the real case of an implanted MOSFET, the global purpose is to show that the electrical performance of such an on-axis-built device can tend to reach that of the ideal case, i.e. epitaxied MOSFET, and even overcome its electrical limitation, e.g. in terms of threshold voltage stability at high temperature.

Microstructure and Transport Properties in Alloyed Ohmic Contacts to P-Type SiC and GaN for Power Devices Applications

The formation of good Ohmic contacts to p-type silicon carbide (SiC) and gallium nitride (GaN) is an important physical and technological concern, because of the difficulty to find metals with low Schottky barriers to p-type wide band gap materials, and due to the high ionization energies of p-type dopant impurities. Typically, to overcome these issues, alloyed metallic compounds are used. In this work, the electrical properties of alloyed Ohmic contacts to p-type (Al-implanted) 4H-SiC and p-type (Mg-doped epilayers) GaN are presented and correlated with their microstructure. The impact of the surface preparation and annealing conditions are discussed, reporting the cases of Al/Ti contacts to p-SiC and Au/Ni contacts to p-GaN. The electrical characterization as a function of temperature allowed to define the dominant transport mechanism and to determine the barrier heights.

Limiting mechanism of inversion channel mobility in Al-implanted lateral 4H-SiC metal-oxide semiconductor field-effect transistors

The mechanism limiting the channel mobility in metal-oxide-semiconductor field-effect transistors (MOSFETs) fabricated in Al-implanted 4H-SiC is discussed comparing different post-implantation annealings. In spite of the improved interfacial morphology in carbon capped samples during annealing, the observed reduction of the mobility (from 40 to 24 cm2 V−1 s−1) suggests that interfacial roughness does not significantly impact the transport in the channel. Furthermore, the temperature dependence of the mobility demonstrates that Coulomb scattering is the main degradation mechanism due to the presence of trapped charges at the SiO2/SiC interface.

Structural and transport properties in alloyed Ti/Al Ohmic contacts formed on p-type Al-implanted 4H-SiC annealed at high temperature

In this paper, the transport properties of alloyed Ti/Al Ohmic contacts formed on p-type Al-implanted silicon carbide (4H-SiC) were studied. The morphology of p-type implanted 4H-SiC was controlled using a capping layer during post-implantation activation annealing at 1700 °C. The different morphological conditions do not affect the macroscopic electrical properties of the implanted SiC (such as the sheet resistance or the mobility). On the other hand, the improved morphology of implanted SiC allows us to achieve a flatter Ti/Al surface and a lower specific contact resistance. The temperature dependence of the specific resistance of the contacts was studied to obtain physical insights into the carrier transport mechanism at the metal/SiC interface. The fit comparing several models shows that thermionic field emission is the dominant transport mechanism through the metal/SiC interface, and that a reduction in the barrier height from 0.51 to 0.46 eV is associated with the improvement of the Ohmic properties. Transmission electron microscopy analysis showed the presence of a laterally inhomogeneous microstructure of the metal/SiC interface. The reduction in the barrier height could be correlated with the different microstructures of the interfacial region.

Impact of Surface Morphology on the Electrical Properties of Al/Ti Ohmic Contacts on Al-Implanted 4H-SiC

This paper reports on the impact of the surface morphology on the properties of Ti/Al Ohmic contacts fabricated on Al-implanted 4H-SiC. In particular, the surface roughness of the Al-implanted regions after annealing at 1700 °C was strongly reduced by the using a protective carbon capping layer during annealing (the surface roughness decreased from 9.0 nm to 1.3 nm). In this way, also the morphology and the specific contact resistance of Ti/Al Ohmic contacts formed on the implanted regions could be improved. The electrical and morphological data were correlated with the structural properties of the reacted metal layer and of the metal/SiC interfacial region.

Influence of Heating and Cooling Rates of Post-Implantation Annealing Process on Al-Implanted 4H-SiC Epitaxial Samples

We report on topographical, structural and electrical measurements of aluminum-implanted and annealed 4H-SiC epitaxial samples. The influence of heating-up and cooling-down temperature rates on the SiC surface roughness, the crystal volume reordering and the dopant electrical activation was particularly studied. A higher heating-rate was found to preserve the rms roughness for annealing temperatures lower than 1700°C, and to improve the sheet resistance whatever the annealing temperature due to a better dopant activation (except for 1600°C process, which induced a dark zone in the sample volume). A complete activation was calculated for an annealing at 1700°C during 30 minutes, with a ramp-up at 20°C/s. Rising the cooling-down rate appeared to increase the sheet resistance, probably due to a higher concentration of point defects in the implanted layer.

AI 이온 주입된 p-type 4H-SiC에 형성된 Ni/AI 오믹접촉의 전기 전도 특성

Ni/Al (`/` denotes deposition sequence) contacts were deposited on Al-implanted 4H-SiC for ohmic contact formation, and the conduction properties were characterized and compared with those of Ni-only contacts. The thicknesses of the Ni and Al thin film were 30 nm and 300 nm, respectively, and the films were sequentially deposited bye-beam evaporation without vacuum breaking. Rapid thermal anneal (RTA) temperature was varied as follows : , , and . The specific contact resistivity of the Ni contact was about , However, with the addition of Al overlayer, the specific contact resistivity decreased to about , almost irrespective of RTA temperature. X-ray diffraction (XRD) analysis of the Ni contact confirmed the existence of various Ni silicide phases, while the results of Ni/Al contact samples revealed that Al-contaning phases such as , , , and were additionally formed as well as the Ni silicide phases. Energy dispersive spectroscopy (EDS) spectrum showed interfacial reaction zone mainly consisting of Al and Si at the contact interface, and it was also shown that considerable amounts of Si and C have diffused toward the surface. This indicates that contact resistance lowering of the Ni/Al contacts is related with the formation of the formation of interfacial reaction zone containing Al and Si. From these results, possible mechanisms of contact resistance lowering by the addition of Al were discussed.

Al 이온 주입된 p-type 4H-SiC에 형성된 Ni/Ti/Al Ohmic Contact의 전기적 특성

Ni/Ti/Al multilayer system (`/`denotes the deposition sequence) was tested for low-resistance ohmic contact formation to Al-implanted p-type 4H-SiC. Ni 30 nm / Ti 50 nm / Al 300 nm layers were sequentially deposited by e-beam evaporation on the 4H-SiC samples which were implanted with Al (norminal doping concentration

Effect of Surface Orientation and Off-Angle on Surface Roughness and Electrical Properties of p-Type Impurity Implanted 4H-SiC Substrate after High Temperature Annealing

We investigate the effect of surface orientation and off-angle for Al-implanted 4H-SiC samples after high temperature annealing. The samples are obtained from a 4H-SiC (0001) substrate 8° off-angled (Si-face 8°off), and (000-1) substrates 8° (C-face 8°off), 4° (C-face 4°off) and less than 1° off-angled (C-face ~1°off). An n-type epitaxial layer is deposited on all substrates. Multiple implantations of Al+ (30~200keV) are carried out at 600°C. The total dose is 8.6 × 1015 cm-2. The Al-implanted samples are annealed in Ar ambient at 1580°C, 1700°C and 1800°C for 30s using the hybrid super rapid thermal annealing (HS-RTA) equipment. In this study, sheet resistance (Rs), free carrier concentration (Ns), Hall mobility (μ) and root-mean square roughness (Rrms) are used to evaluate the Al-implanted samples after high temperature annealing. Rs for all Al-implanted samples after annealing at 1800°C for 30s is around 18k/. Rrms for the Al-implanted C-face samples after annealing at 1800°C increases with increasing off-angle. Rrms for the Al-implanted Si-face 8°off sample after annealing increases with annealing temperature. Rrms for the C-face ~1°off Al-implanted sample after annealing at 1800°C is lower than that for the Si-face 8°off Al-implanted sample after annealing at 1700°C, moreover Rs for the C-face ~1°off sample after annealing at 1800°C is about 10% of that for the Si-face 8°off Al-implanted sample after annealing at 1700°C. It is shown that the C-face ~1°off sample is useful to fabricate a p+ region with low Rs and low Rrms. If C-face 4H-SiC is used to fabricate devices, devices made on C-face 4H-SiC with low off angle are expected to decrease any problems caused by increase of surface roughness after high temperature annealing (~1800°C).

Impact of EBAS annealing on sheet resistance reduction for Al-implanted 4H-SiC(0001)

AbstractIn this paper, we demonstrate that high temperature and short time EBAS annealing is effective to obtain low sheet resistance without surface roughening in heavily Al-implanted 4H-SiC (0001) samples (Al concentration: 1.0 × 1021 /cm3, thickness: 0.3 microns, total dose: 2.6 × 1016 /cm2). The sheet resistance and rms surface roughness of the sample annealed at 1800 °C for 0.5 min is estimated to be 4.8k ohm/sq. and 0.82 nm, respectively. Also, we discuss the advantage of EBAS annealing for the suppression of surface roughening during annealing.

Investigation of Two-Stage Activation Annealing of Al-Implanted 4H-SiC Layers

Investigation of Two-Stage Activation Annealing of Al-Implanted 4H-SiC Layers

Effects of implantation temperature on damage accumulation in Al-implanted 4H–SiC

Damage accumulation in 4H–SiC under 1.1 MeV Al22+ irradiation is investigated as a function of dose at temperatures from 150 to 450 K. Based on Rutherford backscattering spectroscopy and nuclear reaction analysis channeling spectra, the damage accumulation on both the Si and C sublattices have been determined, and a disorder accumulation model has been fit to the data. The model fits indicate that defect-stimulated amorphization is the primary amorphization mechanism in SiC over the temperature range investigated. The temperature dependence of the cross section for defect-stimulated amorphization and the critical dose for amorphization indicate that two different dynamic recovery processes are present, which are attributed to short-range recombination and long-range migration of point defects below and above room temperature, respectively. As the irradiation temperature approaches the critical temperature for amorphization, cluster formation has an increasing effect on disorder accumulation, and ion flux plays an important role on the nature and evolution of disorder. Dislocation loops, which are mostly formed under high ion flux, act as sinks for point defects, thereby reducing the disorder accumulation rate.