P-type 4H-SiC Epitaxial Layers Research Articles

Quantitative Analysis of Piezoresistive Characteristic Based on a P-type 4H-SiC Epitaxial Layer.

In this work, the piezoresistive properties of heavily doped p-type 4H-SiC at room temperature were investigated innovatively. It was verified by a field emission scanning electron microscope (FESEM), X-ray diffraction (XRD), and laser Raman spectroscopy (LRS) that the crystal quality of the epitaxial layer was good. The doping concentration and thickness of the epitaxial layer were measured by secondary ion mass spectrometry (SIMS) to be ~1.12 × 1019 cm−3 and ~1.1 µm, respectively. The 4H-SiC cantilever beam along crystal orientation was fabricated, and the fixed end of the cantilever beam was integrated with longitudinal and transverse p-type 4H-SiC piezoresistors. A good ohmic contact was formed between Ni/Ti/Al/Au and a p-type 4H-SiC piezoresistor under nitrogen environment annealing at 1050 °C for 5 min. The free end of the cantilever beam was forced to cause strain on the p-type 4H-SiC piezoresistor, and then the resistances were measured by a high precision multimeter. The experimental results illustrated that longitudinal and transverse gauge factors (GFs) of the p-type 4H-SiC piezoresistors were 26.7 and −21.5, respectively, within the strain range of 0–336µε. In order to further verify the electro-mechanical coupling effect of p-type 4H-SiC, the piezoresistors on the beam were connected to the Wheatstone full-bridge circuit and the output changes were observed under cyclic loading of 0–0.5 N. The measuring results revealed that the transducer based on the 4H-SiC piezoresistive effect exhibited good linearity and hysteresis, which confirmed that p-type 4H-SiC has the potential for pressure or acceleration sensing applications.

Carrier Lifetimes in Lightly-Doped p-Type 4H-SiC Epitaxial Layers Enhanced by Post-growth Processes and Surface Passivation

We investigated limiting factors of carrier lifetimes and their enhancement by post-growth processes in lightly-doped p-type 4H-SiC epitaxial layers (N A ∼ 2 × 1014 cm−3). We focused on bulk recombination, surface recombination, and interface recombination at the epilayer/substrate, respectively. The carrier lifetime of 2.8 μs in an as-grown epilayer was improved to 10 μs by the combination of VC-elimination processes and hydrogen annealing. By employing surface passivation with deposited SiO2 followed by POCl3 annealing, a long carrier lifetime of 16 μs was obtained in an oxidized epilayer. By investigating carrier lifetimes in a self-standing p-type epilayer, it was revealed that the interface recombination at the epilayer/substrate was smaller than the surface recombination on a bare surface. We found that the VC-elimination process, hydrogen annealing, and surface passivation are all important for improving carrier lifetimes in lightly-doped p-type epilayers.

Comparative Study of p-Type 4H-SiC Grown on n-Type and Semi Insulating 4H-SiC Substrates

Comparative study of p-type 4H-SiC epitaxial layers grown simultaneously on two different 4H-SiC substrates, namely n-type and semi-insulating have been done by different structural, optical and electrical experimental techniques.

Effect of Boro-Silicate Glass (BSG) Gate Dielectric with Antimony Surface Doping on Channel Transport of 4H-SiC Mosfets

In this work, we investigate the effect of Boro-silicate glass (BSG) gate dielectric in conjunction with Antimony (Sb) surface doped channels of lateral 4H-SiC MOSFETs. It has been reported that BSG gated 4H-SiC MOSFETs have a high channel mobility of ~100 cm2/V∙s for a wide range of surface transverse electric fields along with a large threshold voltage VT of ~5V in lightly doped p-epitaxial layers [1,2]. For vertical power MOSFETs with heavily doped p-wells, it is expected that BSG would cause an even further increase of VT (~10 V) which would be undesirable. Therefore in this work, our goal for combining the Sb surface doping process (demonstrated in our earlier works [3,4] and presented at ECSCRM 2014 [4]), with BSG gate dielectric was two-fold: (i) Tune VT to adequate value with high sub-threshold slope. (ii) Achieve high low-field channel mobility by Sb counter-doping while retaining the high-field mobility characteristics of BSG. The results from our experiments indicate that these goals were achieved. The 'Sb+BSG' process results in significant improvement of both low-field channel mobility to ~180cm2/V∙s (due to addition of Sb surface doping) and high-field channel mobility to ~90 cm2/V∙s (due to the BSG gate dielectric) along with a tuned threshold voltage of ~2V and a steeper sub-threshold slope. Lateral MOSFETs were fabricated on the Si-face of p-type 4H-SiC epitaxial layers doped at ~1x1016cm-3. Sb was implanted in the channel region with 80 keV at room temperature with dose of 2.5x1013cm-2, which results in a Gaussian profile with a depth of around 30nm below the surface of SiC. This was followed by post-implantation activation annealing at 1650°C using a graphitic carbon cap layer. Next, dry oxidation at 1150°C for 10 hours was performed followed by post-oxidation annealing using a planar diffusion source (Techneglas, GS-139) composed of boric oxide (B2O3) in a gas mixture of Ar (50sccm) and O2 (5sccm) at 950°C for 30 mins. Samples that received only boron annealing are referred to as 'BSG only' and samples underwent both Sb counter-doping and boron annealing are referred to as 'Sb+BSG'. Results for these samples are compared with standard NO-annealed devices in Table Ⅰ. The SIMS result in Fig. 1 shows B distributes throughout the oxide with a concentration of ~1x1022cm-3 and decreases as it reaches SiC. Threshold voltage and sub-threshold slope were characterized by Id-Vg measurement at room temperature and field-effect mobility was extracted from the transconductance of Id-Vg curve. Linear and log scale of Id-Vg curves in Fig. 2 and Fig. 3 demonstrate that 'Sb+BSG' tunes the threshold voltage to a more desirable value of ~2V along with a better sub-threshold slope than standard NO annealing. Fig. 4 shows a significant mobility improvement for 'Sb+BSG' at both high field due to the BSG passivation effect and low field due to the Sb counter-doping effect with a peak value of ~180cm2/V∙s compared to 'BSG only' with a peak mobility of ~140cm2/V∙s. In order to investigate the boron passivation effect on interface traps, C-V and constant capacitance deep level transient spectroscopy (CCDLTS) measurements were performed on the companion BSG capacitors. The interface trap density of 'NO' was determined to be ~2.5 times higher than that of 'BSG' for very shallow energy traps (<0.2 eV) from C-V measurements and ~1.5 times higher for the energy trap distributions centered at 0.15 eV and 0.39 eV by CCDLTS, as shown in Figs. 5 and 6. The mechanism of boron passivation has been suggested to be oxide stress relaxation by the reduction of required oxygen bonds due to the occupation of Si site by B [5]. In this presentation, further details of the nature of transport and mobility behavior in BSG gated channels will be presented as a function of temperature.

Simultaneous ohmic contacts to p- and n-type 4H-SiC by phase segregation annealing of co-sputtered Pt-Ti

A new concept, Phase Segregation Annealing (PSA), was investigated for implementing simultaneous ohmic contacts (SOCs) to p- and n-type 4H-SiC. Test structures with selected ratio compositions of co-sputtered Pt:Ti contacts were fabricated in p-type 4H-SiC epitaxial layers having aluminum acceptor concentrations, Na = 2 × 1019, 7 × 1019, and 2.5 × 1020 cm−3, and a nitrogen doped n-type epitaxial layer having donor concentration, Nd = 7 × 1018 cm−3. The ratios of the co-sputtered Pt-Ti metallization were 80:20, 50:50, and 30:70 at. %. After rapid thermal annealing (RTA) ranging between 800 and 1200 °C in vacuum and confirming SOCs by linear current-voltage (I-V) measurement, the specific contact resistance (ρc) values were extracted using the Transfer Length Measurement method. SOCs were realized with the Pt80:Ti20 composition starting from 1000 °C, and the Pt30:Ti70 composition from 1100 °C, with both exhibiting eutectic and segregated phases. The Pt50:Ti50 composition produced no SOC and eutectic and segregated phases were absent. The Pt80:Ti20 composition had the lowest pair of average ρc values of 7 × 10−5 Ω cm2 and 7.3 × 10−4 Ω cm2 on the highest doped p-type and the n-type samples after RTA at 1000 °C, respectively. Auger electron spectroscopy and focused ion beam field emission scanning electron microscopy with energy dispersive x-ray spectroscopy indicated distinct phase segregation via the eutectic-liquidus-eutectic transitions, the coalescence of likely Pt3Si and Pt2Si binary phases, and solid phases of Ti3Si, Ti5Si3, and TiC, with all the active phases maintaining intimate contact to both the p- and n-type 4H-SiC surfaces. The SOC formation was attributed to the disparate work functions of these phases, which was in good agreement with the proposed PSA model.

Oxidation-induced majority and minority carrier traps in n- and p-type 4H-SiC

We investigated majority and minority carrier traps in lightly doped n- and p-type 4H-SiC epitaxial layers before and after thermal oxidation using deep level transient spectroscopy and minority carrier transient spectroscopy. We detected oxidation-induced new minority carrier traps, HO1 (EV + 0.36 eV) and HO2 (EV + 0.54 eV) for n-type 4H-SiC, and EO1 (EC − 0.59 eV) and EO2 (EC − 0.84 eV) for p-type 4H-SiC after thermal oxidation at 1300 °C. After subsequent Ar annealing at 1550 °C, the HO1, HO2, and EO1 centers disappeared, whereas the concentration of the EO2 center increased. The properties of these carrier traps are discussed.

Low specific contact resistance on epitaxial p-type 4H-SiC with a step-bunching surface**Project supported by the Key Specific Projects of Ministry of Education of China (Grant No. 625010101), the National Natural Science Foundation of China (Grant No. 61234006), the Natural Science Foundation of ShaanXi Province, China (Grant No. 2013JQ8012), the Doctoral Fund of Ministry of Education of China (Grant No. 20130203120017),

This paper reports the performances of Ti/Al based ohmic contacts fabricated on highly doped p-type 4H-SiC epitaxial layer which has a severe step-bunching surface. Different contact schemes are investigated based on the Al:Ti composition with no more than 50 at.% Al. The specific contact resistance (SCR) is obtained to be as low as 2.6 × 10−6 Ω·cm2 for the bilayered Ti(100 nm)/Al(100 nm) contact treated with 3 min rapid thermal annealing (RTA) at 1000 °C. The microstructure analyses examined by physical and chemical characterization techniques reveal an alloy-assisted ohmic contact formation mechanism, i.e., a high degree of alloying plays a decisive role in forming the interfacial ternary Ti3SiC2 dominating the ohmic behavior of the Ti/Al based contact. Furthermore, a globally covered Ti3SiC2 layer with (0001)-oriented texture can be formed, regardless of the surface step bunching as well as its structural evolution during the metallization annealing.

Enhancement of carrier lifetime in lightly Al-doped p-type 4H-SiC epitaxial layers by combination of thermal oxidation and hydrogen annealing

We investigated the enhancement of carrier lifetime in lightly Al-doped p-type 4H-SiC epilayers (NA ≃ 2 × 1014 cm−3) by postgrowth processing. A carrier lifetime of 2.8 µs in an as-grown epilayer is increased to 5.1 µs by carbon vacancy elimination, i.e., thermal oxidation at 1400 °C for 48 h. It reaches 10 µs by subsequent hydrogen annealing at 1000 °C for 10 min. The carrier lifetime in the as-grown epilayer is also increased to 4.0 µs by only hydrogen annealing. These results suggest that, in addition to carbon vacancy, there is another lifetime killer in p-type SiC, which cannot be eliminated by thermal oxidation but can be passivated by hydrogen annealing.

Deep Levels in P-Type 4H-SiC Induced by Low-Energy Electron Irradiation

We have characterized deep levels in as-grown and electron irradiated p-type 4H-SiC epitaxial layers by the current deep-level transient spectroscopy (I-DLTS) method. A part of the samples were irradiated with electrons in order to introduce defects. As a result, we found that electron irradiation to p-type 4H-SiC created complex defects including carbon vacancy or interstitial. Moreover, we found that observed deep levels are different between before and after annealing, and thus annealing may change structures of defects.

Excess Carrier Lifetime in p-Type 4H-SiC Epilayers with and without Low-Energy Electron Irradiation

We measured excess carrier lifetimes in as-grown and low-energy electron-irradiated p-type 4H-SiC epitaxial layers by the microwave photoconductivity decay method. The carrier lifetime increases with the excitation density in the epilayers. This dependence suggests that the dominant recombination center in the epilayers has a larger capture cross section for electrons than for holes. The low-energy electron irradiation reduces the carrier lifetime in the epilayers, and 1000 °C annealing increases the carrier lifetime in the electron-irradiated samples.

Deep levels in iron doped n- and p-type 4H-SiC

Deep levels were detected in Fe-doped n- and p-type 4H-SiC using deep level transient spectroscopy (DLTS). One defect level (EC–0.39 eV) was detected in n-type material. DLTS spectra of p-type 4H-SiC show two dominant peaks (EV + 0.97 eV and EV + 1.46 eV). Secondary ion mass spectrometry measurements confirm the presence of Fe in both n- and p-type 4H-SiC epitaxial layers. The majority of the capture process for Fe1, Fe2, and Fe3 is multi-phonon emission assisted. These three detected peaks are suggested to be related to Fe.

Epitaxial Growth and Defect Control of SiC for High-Voltage Power Devices

Recent progress in fast epitaxial growth and defect control of silicon carbide (SiC) toward development of high-voltage power devices is reviewed. In chemical vapor deposition of 4H-SiC on off-axis (0001), a high growth rate of 85 µm/h and a low background doping of 1×1013 cm−3 are achieved. Conversion of basal-plane dislocations to threading edge dislocations and generation of stacking faults during epitaxial growth are discussed. Deep levels in as-grown n-type and p-type 4H-SiC epitaxial layers have been investigated. A lifetime-killing defect, Z1/2 center, can be almost eliminated by thermal oxidation, which leads to significant increase in carrier lifetimes. The obtained carrier lifetimes are long enough to fabricate 10 kV-class bipolar devices. Control of carrier lifetimes by low-energy electron irradiation is demonstrated.

Characterization of the Excess Carrier Lifetime of As-Grown and Electron Irradiated Epitaxial p-Type 4H-SiC Layers by the Microwave Photoconductivity Decay Method

We measured the excess carrier lifetimes in as-grown and electron irradiated p-type 4H-SiC epitaxial layers with the microwave photoconductivity decay (-PCD) method. The carrier lifetime becomes longer with excitation density for the as-grown epilayer. This dependence suggests that e ≥h for the dominant recombination center, where e andh are capture cross sections for electrons and holes, respectively. In contrast, the carrier lifetime does not depend on the excitation density for the sample irradiated with electrons at an energy of 160 keV and a dose of 1×1017 cm-2. This may be due to the fact that recombination centers with e &lt;&lt;h were introduced by the electron irradiation or due to the fact that the acceptor concentration was decreased significantly by the irradiation.

Characterization of ion-implanted 4H-SiC Schottky barrier diodes

Ion-implantation layers are fabricated by multiple nitrogen ion-implantations (3 times for sample A and 4 times for sample B) into a p-type 4H-SiC epitaxial layer. The implantation depth profiles are calculated by using the Monte Carlo simulator TRIM. The fabrication process and the I-V and C-V characteristics of the lateral Ti/4H-SiC Schottky barrier diodes (SBDs) fabricated on these multiple box-like ion-implantation layers are presented in detail. Measurements of the reverse I-V characteristics demonstrate a low reverse current, which is good enough for many SiC-based devices such as SiC metal–semiconductor field-effect transistors (MESFETs), and SiC static induction transistors (SITs). The parameters of the diodes are extracted from the forward I-V and C-V characteristics. The values of ideality factor n of SBDs for samples A and B are 3.0 and 3.5 respectively, and the values of series resistance Rs are 11.9 and 1.0 kΩ respectively. The values of barrier height φB of Ti/4H-SiC are 0.95 and 0.72 eV obtained by the I-V method and 1.14 and 0.93 eV obtained by the C-V method for samples A and B respectively. The activation rates for the implanted nitrogen ions of samples A and B are 2% and 4% respectively extracted from C-V testing results.

Observation of stacking faults formed during homoepitaxial growth of p-type 4H-SiC

Threading dislocations and their transformation into stacking faults (SFs) are observed in p-type 4H-SiC epitaxial layers by high voltage transmission electron microscope. Homoepitaxial growth and in situ aluminum doping of 4H-SiC epitaxial layers are carried out using the organosilicon precursor bistrimethylsilylmethane (C7H20Si2 and the metal-organic precursor trimethylaluminum (C3H9Al), and the free hole concentration of the most heavily aluminum-doped epitaxial layers is &amp;gt;1021 cm−3. Threading dislocations are formed at the interface between the epitaxial layer and the substrate. However, the density of these threading dislocations decreases toward the epitaxial layer surface with their transformations to SFs.

Fabrication and characteristics of lateral Ti/4H-SiC Schottky barrier diodes

This paper describes the fabrication and characteristics of the lateral Ti/4H-SiC Schottky barrier diodes (SBDs). SBDs are fabricated by nitrogen ion implantation into p-type 4H-SiC epitaxial layer. The implant depth profile is simulated using the Monte Carlo simulator TRIM. Measurements of the reverse I-V characteristics demonstrate a low reverse current, that is good enough for many SiC-based devices such as SiC metal-semiconductor field-effect transistors and SiC static induction transistors. The parameters of the diodes are extracted from the forward I-V characteristics. The barrier height b of Ti/4H-SiC is 0.95 eV.

Characterization of p-type 4H-SiC epitaxial layer grown on (11-20) substrate

ABSTRACTGrowth and characterization of p-type 4H-SiC epitaxial layers grown on (11-20) substrates are reported. P-type 4H-SiC epilayers with smooth surface morphology have been grown on (11-20) substrates by low-pressure, hot-wall type CVD with SiH4–C3H8–H2–TMA system. The doping concentration can be controlled in the range from about 1×1016cm−3 to 1×1019cm−3. Anisotropy of the crystalline quality is observed by x-ray diffraction measurement. P-type epilayers, in which near band-gap emissions are dominated and D-A pair peak is not observed, are obtained. Hole mobility of (11-20) epilayers is smaller than that of (0001) epilayers probably due to the lack of crystalline quality compared to (0001) epilayers. The results of both low-temperature photoluminescence and the temperature dependence of Hall effect measurements indicate that the boron concentration as undoped impurity in (11-20) epilayer is lower than that of (0001) epilayer. This may be caused by the smaller incorporation efficiency of boron into (11-20) epilayer than that of (0001) epilayer.

Electrical characteristics and structural properties of ohmic contacts to p-type 4H-SiC epitaxial layers

Epitaxial films grown by low-temperature liquid phase epitaxy on p-type 4H-SiC were used as strongly doped subcontact layers for making low-resistance contacts to the p-type material. The layers had a bulk resistivity of ∼0.02 Ω · cm and an aluminum atom concentration of ∼1.5×1020 cm−3. The absence of polytype inclusions and the distinct crystalline quality of the strongly doped subcontact layers was confirmed by x-ray diffraction methods. Ohmic contacts with resistivities less than 10−4 Ω · cm2 were prepared by depositing and then annealing multilayer metal mixtures containing Al and Ti. The structural properties and energy characteristics of the resulting ohmic contacts are discussed.