SNDM Study of the MOS Interface State Densities on the 3C-SiC / 4H-SiC Stacked Structure

. A stacked layer structure of 3C-SiC/ 4H-SiC has been implemented by simultaneous lateral epitaxy (SLE). The SLE, involving spontaneous nucleation of 3C-SiC(111) on the 4H-SiC(0001) surface followed by step-controlled epitaxy, facilitates the creation of a single-domain 3C-SiC layer with an epitaxial relationship to the underlying 4H-SiC, establishing a coherent (111)/


Introduction
The remarkable high voltage blocking capabilities and the reduced drift resistance exhibited by 4H-SiC Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFETs) can be attributed to its wider bandgap over that of other SiC polytypes.However, challenges in 4H-SiC emerge due to its high density of the MOS interface states, causing high channel resistances as well as the insufficient reliability of the gate dielectrics that impact their practical applications.Conversely, 3C-SiC shows a much lower density of the MOS interface states which results in an improved reliability of the gate dielectrics [1,2].Nevertheless, its actual implementation has been impeded thus far due to generation of electrically active extended defects within the 3C-SiC crystal lattice [3].These defects degrade the blocking capability, particularly under the influence of high electric fields [4,5].
Implementing an optimal MOSFET entails harmonious integration of the advantageous properties of both 3C-SiC and 4H-SiC, while concurrently compensating their individual drawbacks.By employing a stacked structure of 3C-SiC on 4H-SiC, together with the use of a thermal oxide (SiO2) film as the gate dielectrics on the 3C-SiC layer, a remarkable convergence of benefits becomes attainable.Notably, this methodology affords a substantial reduction in the state density at the SiO2 film interface, consequently leading to a remarkable decrease in the channel resistance, while high breakdown voltages and low drift resistances are ensured by the 4H-SiC layer under the 3C-SiC layer.
The augmentation of the long-term stability of the SiO2 film is also an integral outcome of this design, owing to the higher electron affinity of 3C-SiC.This characteristic, being higher than that of 4H-SiC by approximately 0.9 eV, contributes to an elevated potential barrier at the interface with SiO2 [1].Replacing the vicinal surface of 4H-SiC with 3C-SiC, intrinsically addresses the two prominent challenges, presenting a pivotal resolution.
To realize this idea, we have developed a novel epitaxial growth technique, described in the following section, which yields a high-quality 3C-SiC epitaxial layer on a 4H-SiC epitaxial layer.

Simultaneous Lateral Epitaxy (SLE)
As is well known, epitaxial growth of 3C-SiC on 4H-SiC substrate results in dense double positioning boundaries (DPB).This is because the basic unit cell of 4H-SiC includes hexagonal-site, which makes the stacking order of the cubic close-packed structure of 3C-SiC uncertain and generates twins.When twins coexist in the same basal plane, they generate DPBs, which forms one of the critical degradation factors in the device performance.
Controlling the stacking sequence on the topmost surface of 4H-SiC is one of the necessary conditions for eliminating twins within the 3C-SiC layer that is epitaxially grown on it.However, the twinning resolution of the 3C-SiC layer does not ensure DPB resolution.This is because the four Si-C molecular-layers (4 MLs) height step at the interface between the 3C-SiC and 4H-SiC layers causes a local phase shift in the stacking order of the 3C-SiC, which is originally 3 MLs period.To obtain a DPB-free 3C-SiC on 4H-SiC, therefore, its boundary must be strictly aligned in the basal plane.One practical method to achieve this has been developed by the NASA group.This method involves growing 3C-SiC after exposing the atomically-flat hexagonal SiC (0001) surface [6][7][8].
We also have succeeded in growing a DPB-free 3C-SiC(111) film on a 4H-SiC(0001) surface with inclined in the[112 � 0] direction [9,10].This approach is termed Simultaneous Lateral Epitaxy (SLE).The schematic depiction of the SLE is presented in Fig. 1.In the conventional step-controlled epitaxy, the homoepitaxial growth of 4H-SiC proceeds as the surface of the 4H-SiC substrate is uniformly inclined at a consistent angle (θ) from the basal plane, typically around 4 degrees, in a specific direction, typically in the [112 � 0] direction.This growth facilitates the lateral expansion of the 4H-SiC's specific structure across the entire surface.In contrast, SLE intentionally terminates the step-controlled epitaxy on the 4H-SiC in order to induce two-dimensional nucleation of 3C-SiC.

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Growing and Forming of Semiconductor Layers As shown in Fig. 1 a), the finite step-controlled epitaxy can be achieved by forming grooves parallel to the [11 � 00] direction at specific locations on the 4H-SiC surface.This groove intercepts the step flowing from upstream ([1 � 1 � 20] direction) during the step-controlled epitaxy process and enlarges an atomically-flat wide-terrace (WT) downstream ([112 � 0] direction) adjacent to the groove (Fig. 1 b).
The WT expands progressively by increasing the supersaturation of the precursors as the epitaxial growth proceeds, and finally, when a critical width for spontaneous SiC nucleation (wn) is reached, a two-dimensional nucleus of 3C-SiC (η3c) is formed in the central region of WT (Fig. 1 c).Since the η3c generates spontaneously, its crystal structure becomes that of 3C-SiC.
Once η3c is formed on WT, it provides new atomic-height steps and expands the 3C-SiC structure in the [112 � 0] direction.At the same time, 4H-SiC grows laterally from the edge of the narrowterraces (NT) on the misoriented surface of the 4H-SiC layer.As a result, the interface structure between the 3C-SiC and 4H-SiC layers propagates parallel to the basal plane, forming thereby a coherent interface suppressing the DPB generation.
As long as the groove acts as an obstacle to the step-controlled epitaxy, the WT expansion following the η3c formation continues, so that the 3C-SiC layer expands in the [112 � 0] direction with increasing thickness (Fig. 1 d).The width of the resulting 3C-SiC area (W3c) is determined geometrically with the epitaxial layer thickness (tepi) as below.
As schematically shown in Fig. 2, the distinctive feature of SLE is that 4H-SiC and 3C-SiC surfaces can coexist on the same flat surface and that the 3C-SiC surface can be extended to the desired width.For example, MOSFET cells can be arranged on the 3C-SiC area to reduce the channel resistance, and the adjacent 4H-SiC area can be used to fabricate a Schottky Barrier Diodes (SBDs) as for high-voltage freewheeling.SLE is attractive in that it provides flexibility in the device designing and simplifies the device fabrication process [9].

Experimental
This idea of SLE was verified experimentally according to the following procedure.A commercially available single-crystal 4H-SiC wafer was employed as a substrate.The wafer surface inclines 4 degrees from the (0001) orientation to the [112 � 0] direction.Multiple parallel grooves were formed on the wafer surface by femtosecond laser processing with a beam diameter of 3 μm, an output of 500 mW, and a repetition rate of 100 kHz [11,12].The grooves are roughly aligned in the [11 � 00] direction, and the groove period is 100 um with a typical depth of 4.5 um and a width of approximately 20 um.
After the laser processing, the 4H-SiC surface was subjected to a surface treatment called Step-Alignment ® .This procedure is essential for specifying the stacking order on the WT top-surface to suppress the onset of twinning of 3C-SiC as well.
Then a 2.2 um-thick SiC layer was grown epitaxially in a SiH4 + C3H8 + H2 atmosphere.During the growth process, the donor concentration of the SiC layer was adjusted to 1×10 16 cm -3 by adding nitrogen to the atmosphere.In contrast to the conventional SiC epitaxy, no high-temperature H2 Solid State Phenomena Vol.362 etching was performed prior to the growth in order to preserve the specific stacking order on 4H-SiC surface generated by Step-Alignment ® .
The epitaxial layer formed by SLE was characterized as follows: The surface of the SLE-grown layer was observed by Scanning Electron Microscope (SEM) to distinguish different areas of the crystal structure.Electron backscatter diffraction (EBSD) was then performed to obtain pole figure (PF) and inverse pole figure (IPF) maps for both the 3C-SiC and 4H-SiC regions.The PF identifies the crystal structure of each distinguished area.The IPF maps show the orientation of 3C-SiC and 4H-SiC lattice planes on the specified area, which clarifies the stacking structure of 3C-SiC and 4H-SiC.
Finally, the interface state density (Dit), which strongly affects the MOSFET performance, was measured by forming a thermal oxide film on the SLE-grown SiC substrate.The formation of a thermal oxide (SiO2) film onto the SLE-grown surface, which serves as the gate dielectric, facilitates a comparison of the Dit for both 3C-SiC and 4H-SiC.The 24 nm-thick SiO2 thin film was thermally grown by H2 + O2 pyrogenic oxidation at 1323 K and atmospheric pressure for 4 hours.
Then, Scanning Nonlinear Dielectric Microscopy (SNDM) was employed to determine the Dit on both the 3C-SiC and 4H-SiC areas.SNDM is a kind of scanning probe microscope, and has a detection sensitivity for the capacitance changes as small as 10 -22 F. In this investigation, a local deep level transient spectroscopy (local-DLTS) using the time-resolved SNDM technique was executed to measure Dit without fabricating MOS capacitors [13,14].This enables the acquisition of a twodimensional Dit distribution (Dit map), with a lateral resolution of approximately 30 nm.

Results and Discussion
Stacked Structure of 3C-SiC and 4H-SiC.Fig. 3 shows an SEM image of the surface of the SLE-grown layer.On the right of the laser-processed groove, which is in the downstream of the step-flow, a bright band of 32 μmwidth and an adjacent dark-band of 41 μm-width are observed.The relationship between the 32 μm-width and the 2.2 μm-thick of the SLEgrown layer corresponds to Eq. 1.
The EBSD observations confirm that the bright band found in the SEM image of Fig. 3 corresponds to the single-domain 3C-SiC surface and the dark band to the 4H-SiC surface as described below.The PFs on the bright band (area-a) and the dark band (area-b) are shown in Fig. 4 a and b, respectively.The PFs on area-a (Fig. 4 a) correspond to that of 3C-SiC, indicating that the {11 � 0} and {100} planes orient with a three-fold symmetric relationship, and the (111) plane orients in the normal direction.On the other hand, the arrangement of the PFs on area-b (Fig. 4 b) correspond to that of 4H-SiC, indicating that the {11 � 00} and {112 � 0} planes orient keeping a six-fold symmetric relationship, and the (0001) plane orients in the normal

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Growing and Forming of Semiconductor Layers direction.This suggests that the single-domain 3C-SiC(111) layer is epitaxially grown on 4H-SiC(0001).

Fig. 1 .
Fig. 1.Schematic of SLE growth: a) Groove in the [11 � 00] direction terminates the step-controlled epitaxial growth of 4H-SiC; b) Atomically flat wide-terrace (WT) is formed adjacent to the groove; c) When the width of the WT exceeds a critical value for spontaneous SiC nucleation (wn), 2-D nucleus of 3C-SiC (η3c) is generated on the terrace, which provides new steps toward downstream.At the same time, 4H-SiC grows laterally from the step of narrow-terrace (NT); d) Additional η3c is generated on WT of 3C-SiC, and it further promotes stepcontrolled epitaxial growth of 3C-SiC.

Fig. 2 .
Fig. 2. Schematic cross-sectional structure of SLE growth layer: 3C-SiC layer stacks on 4H-SiC layer forming a coherent interface aligned in the basal plane.The width of 3C-SiC layer (W3c) is determined by SLE layer thickness (tepi) and tilt angle (θ) from the basal plane.

Fig. 3 .
Fig. 3. SEM image of the surface of SLE-grown SiC epitaxial layer: a 32 μm-wide bright band is found on the right of the groove as well as a dark band adjacent to it on the right.The area-a on the bright band, the area-b and d on the dark band, and area-c on the boundary between the bright and dark bands are delineated and related to the result of EBSD observations.