Channel Mobility In 4H-SiC MOSFETs Research Articles

Influences of pre-oxidation nitrogen implantation and post-oxidation annealing on channel mobility of 4H-SiC MOSFETs

Detailed investigations on the pre-oxidation nitrogen implantation and post-oxidation annealing processes for the improvement of channel mobility and specific on-resistance in 4H-SiC MOSFETs are reported. The results indicate that higher temperature and time for NO nitridation annealing process lead to higher field-effect channel mobility and lower specific on-resistance attributed to the good interface properties of SiC and SiO2 of MOSFETs. Furthermore, more nitrogen implantation energy and dose improved the channel mobility and specific on-resistance due to higher Id and less fixed charge, traps, and defects at the SiO2/SiC interface. The threshold voltages and blocking performance of MOSFETs are influenced by pre-oxidation nitrogen implantation conditions attributed to changing of counter doping mechanism in the DMOSFET p-well.

Borosilicate Glass (BSG) as Gate Dielectric for 4H-SiC MOSFETs

In this work, we investigate the effect of borosilicate glass (BSG) as gate dielectric on dielectric/4H-SiC interface traps and channel mobility in 4H-SiC MOSFETs. The interface trap characterization by C−ψs analysis and I-V characterization show lower fast interface trap density (Dit) as well as significant improvement of channel field-effect mobility on devices with BSG than that on devices with standard NO anneal. In addition, the results indicate interface trap density decreases with increasing B concentration at the interface of BSG/4H-SiC, which in turn, results in higher channel mobility.

Effects of antimony (Sb) on electron trapping near SiO2/4H-SiC interfaces

To investigate the mechanism by which Sb at the SiO2/SiC interface improves the channel mobility of 4H-SiC MOSFETs, 1 MHz capacitance measurements and constant capacitance deep level transient spectroscopy (CCDLTS) measurements were performed on Sb-implanted 4H-SiC MOS capacitors. The measurements reveal a significant concentration of Sb donors near the SiO2/SiC interface. Two Sb donor related CCDLTS peaks corresponding to shallow energy levels in SiC were observed close to the SiO2/SiC interface. Furthermore, CCDLTS measurements show that the same type of near-interface traps found in conventional dry oxide or NO-annealed capacitors are present in the Sb implanted samples. These are O1 traps, suggested to be carbon dimers substituted for O dimers in SiO2, and O2 traps, suggested to be interstitial Si in SiO2. However, electron trapping is reduced by a factor of ∼2 in Sb-implanted samples compared with samples with no Sb, primarily at energy levels within 0.2 eV of the SiC conduction band edge. This trap passivation effect is relatively small compared with the Sb-induced counter-doping effect on the MOSFET channel surface, which results in improved channel transport.

High Channel Mobility 4H-SiC MOSFETs by Antimony Counter-Doping

Channel mobility of >100 cm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{2}$ </tex-math></inline-formula> V <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{-1}$ </tex-math></inline-formula> s <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{-1}$ </tex-math></inline-formula> has been obtained on enhancement mode 4H-SiC MOSFETs using an antimony (Sb) doped surface channel in conjunction with nitric oxide (NO) postoxidation annealing. Temperature dependence of the channel mobility indicates that Sb, being an n-type dopant, reduces the surface electric field while the NO anneal reduces the interface trap density, thereby improving the channel mobility. This letter highlights the importance of semiconductor/dielectric materials processes that reduce the transverse surface electric field for improved channel mobility in 4H-SiC MOSFETs.

Challenges for energy efficient wide band gap semiconductor power devices

Wide band gap semiconductors, and in particular silicon carbide (4H-SiC) and gallium nitride (GaN), are very promising materials for the next generation of power electronics, to guarantee an improved energy efficiency of devices and modules. As a matter of fact, in the last decade intensive academic and industrial research efforts have resulted in the demonstration of both 4H-SiC MOSFETs and GaN HEMTs exhibiting VB2/Ron performances well beyond the silicon limits. In this paper, some of the present scientific challenges for SiC and GaN power devices technology are reviewed. In particular, the topics selected in this work will be the SiO2/SiC interface passivation processes to improve the channel mobility in 4H-SiC MOSFETs, the current trends for gate dielectrics in GaN technology and the viable routes to obtain normally-off HEMTs.

A Study on Pre-Oxidation Nitrogen Implantation for the Improvement of Channel Mobility in 4H-SiC MOSFETs

Detailed investigations on the pre-oxidation nitrogen implantation process for the improvement of channel mobility in 4H-SiC MOSFETs are reported. Comparisons with conventional thermally nitrided gate oxides are highlighted. The results of a nitrogen dose dependence study indicate that higher N implantation doses lead to higher peak field-effect mobilities but lower threshold voltages. This apparently correlates with the interface trap density near the conduction band of 4H-SiC, as previous work suggests, but an alternate mechanism associated with counter doping of the MOSFET p-well via N implantation is proposed here. Analysis of the experimental results suggests that the counter-doping mechanism is more likely to be the dominant effect.

Low Frequency Noise in 4H-SiC MOSFETs

Low-frequency noise in 4H-SiC MOSFETs has been measured for the first time. At drain currents varying from deep subthreshold to strong inversion, the 1/f (flicker) noise dominated at frequencies 1 - 105 Hz. The dependence of relative spectral noise density, , on drain current Id (at a constant drain voltage Vd) differs qualitatively from that in Si MOSFETs. In Si MOSFETs, ~ 1/ in strong inversion, whereas tends to saturate in sub-threshold. In 4H-SiC MOSFETs under study, ~ 1/ over the whole range of currents from deep sub-threshold to strong inversion. Similar noise behavior is often observed in poly- or a-Si TFTs. The effective channel mobility in 4H-SiC MOSFETs, 3 - 7 cm2/Vs, is also as low as that in TFTs. Both noise behavior and transport properties of 4H-SiC MOSFETs are explained, analogously to TFTs, by a high density of localized states (bulk and interface) near the conduction band edge in the ion implanted p-well.

Effect of Graphite Cap for Implant Activation on Inversion Channel Mobility in 4H-SiC MOSFETs

The effects of using a graphite capping layer during implant activation anneal on the performance of 4H-SiC MOSFETs has been evaluated. Two sets of samples, one with the graphite cap and another without, with a gate oxide process consisting of a low-temperature deposited oxide followed by NO anneal at 1175°C for 2hrs were used for characterization. Various device parameters, particularly threshold voltage, subthreshold slope, field-effect mobility, inversion sheet carrier concentration and Hall mobility have been extracted for the two processes.

High Channel Mobility of 4H-SiC MOSFETs Fabricated by Over-Oxidation of the N-/Al-Coimplanted Surface Layer

Conventional MOSFETs and Hall-bar MOSFETs are fabricated side by side by over-oxidation of N-implanted or N-/Al-coimplanted 4H-SiC layers. It is demonstrated that the N-/Al-coimplanted MOSFETs possess a positive threshold voltage at room temperature and reach high values of the channel mobility. The effective electron mobility and Hall mobility in Hall-bar MOSFETs are 31 cm2/Vs and 150 cm2/Vs, respectively, indicating a high density of interface traps in spite of the excellent high mobility values.

Atomistic Scale Modeling of Factors Affecting the Channel Mobility in 4H-SiC MOSFETs

We have identified the crystal planes of 4H-SiC, interfacing with gate oxide, which will lead to minimum defect density and lowest interface corrugations. The atomic surface density, surface energy, and surface roughness of various crystal planes of 4H-SiC have been computationally characterized using Molecular Dynamics simulations. We have also investigated the screening distances of defects in SiO2 and SiC using a multiscale approach.

High Inversion Channel Mobility of 4H-SiC MOSFETs Fabricated on C(000-1) Epitaxial Substrate with Vicinal (Below 1º) Off-Angle

SiC power MOSFETs are expected to be normally-off type fast switching devices. The on-resistance of SiC power MOSFETs is much higher than the value predicted from the physical properties of SiC. This is caused by the low channel mobility due to high interface state density (Dit). We have already reported that 4H-SiC MOSFETs on the C(0001 _ ) face had higher inversion-channel mobility. However, there is the SiO2/SiC interface roughness problem in SiC MOSFETs. There are many steps at the SiO2/SiC interface because a high off-angle is necessary for SiC epitaxial growth. These steps might make SiO2/SiC interfaces rough, which leads to reduction of channel mobility. In this work, we have investigated the effect of the SiO2/SiC interface roughness caused by the off-angle on the inversion channel mobility of 4H-SiC MOSFETs fabricated on the C(0001 _ ) face. The inversion-channel mobility of MOSFETs fabricated on the 4H-SiC C(0001 _ ) face substrate with the vicinal off-angle(0.8°) is higher than that of MOSFETs fabricated on the 4H-SiC C(0001 _ ) face substrate with the 8° off-angle. Reduction of the off-angle is very useful for improvement of channel mobility. A C(0001 _ ) epitaxial substrate with the vicinal off-angle would be suitable for SiC DMOSFETs.

Effective Normal Field Dependence of Inversion Channel Mobility in 4H-SiC MOSFETs on the (11-20) Face

Effective Normal Field Dependence of Inversion Channel Mobility in 4H-SiC MOSFETs on the (11-20) Face

Influence of the Crystalline Quality of Epitaxial Layers on Inversion Channel Mobility in 4H-SiC MOSFETs

Influence of the Crystalline Quality of Epitaxial Layers on Inversion Channel Mobility in 4H-SiC MOSFETs

The Effects of Post-Oxidation Anneal Conditions on Interface State Density Near the Conduction Band Edge and Inversion Channel Mobility for SiC MOSFETs

ABSTRACTResults are reported for the passivation of interface states near the conduction band edge in n-4H-SiC using post-oxidation anneals in nitric oxide, ammonia and forming gas (N2/5%H2). Anneals in nitric oxide and ammonia reduce the interface state density significantly, while forming gas anneals are largely ineffective. Results suggest that interface states in SiO2/SiC and SiO2/Si have different origins, and a model is described for interface state passivation by nitrogen in the SiO2/SiC system. The inversion channel mobility of 4H-SiC MOSFETs increases with the NO annealing.

High channel mobility in inversion layers of 4H-SiC MOSFETs by utilizing (112~0) face

A dramatic improvement of inversion channel mobility in 4H-SiC MOSFETs was successfully achieved by utilizing the (112~0) face: 17 times higher (95.9 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /Vs) than that on the conventional (0001) Si-face (5.59 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /Vs). A low threshold voltage of MOSFETs on the (112~0) face indicates that the (112~0) MOS interface has fewer negative charges than the (0001) MOS interface. Small anisotropy of channel mobility in 4H-SiC MOSFETs (μ/sub (11~00)//μ/sub (0001)/=0.85) reflects the small anisotropy in bulk electron mobility.