Tracking Dislocations in SiC Epitaxial Layers by UVPL Imaging

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1 The 5th International Symposium on Advanced Science and Technology of Silicon Materials (JSPS Si Symposium), Nov , 2008, Kona, Hawaii, USA Tracking Dislocations in SiC Epitaxial Layers by UVPL Imaging R. E. Stahlbush *, B.L. VanMil K.X. Liu, K.K. Lew, R.L. Myers-Ward, D.K. Gaskill, C.R. Eddy, Jr. Naval Research Laboratory, Washington, DC Abstract The recently developed technique of UVPL imaging has been used to track the path of basal plane dislocations (BPDs) in SiC epitaxial layers and to develop growth processes that reduce BPD density in the epitaxy. Experiments have involved ex situ and in situ epitaxial growth interrupts. For the former, each of several epitaxial growths was stopped after ~20 μm, and a UVPL image was collected. For the latter, changing the gas flow interrupted the growth and the BPDs were imaged at the end of the growth. The series of ex situ BPD images made it possible to observe BPD glide during growth. In some cases BPD glide in the basal plane can be many mm and is shown to cause the defect alternately called half-loop array or pair array. At both ex situ and in situ growth interrupts BPDs are converted to threading edge dislocations (TEDs). The conversion efficiency strongly depends on growth and interrupt conditions. The best results were obtained using in situ interrupts and achieved a 98% BPD reduction with a final density <10/cm 2. Introduction SiC power devices are in the process of replacing their Si-based counterparts. The superior material properties of SiC for power devices include a 3X larger bandgap, an 8X higher breakdown field and a 3X higher thermal conductivity. These large improvements alter circuits and systems possible using SiC devices. Power systems based partially or fully on SiC devices have higher efficiency, are smaller and lighter and have reduced cooling requirements. However, SiC technology is immature and continued improvements in the quality of SiC wafers and epitaxy are necessary for it to reach its full potential. The understanding and reduction of basal plane dislocations (BPDs) have been a primary focus of work to improve SiC technology. BPDs cause degradation in a number of power devices including PiN diodes, BJTs and MOSFETs [1-4]. While there have been dramatic improvements in reducing the BPD concentration in epitaxial layers, further improvements are still needed [5]. The problems are more pronounced for higher voltage parts that require thicker epitaxy. At the start of the epitaxial growth on a SiC substrate, ~90% of the BPDs are converted to the benign threading edge dislocations (TEDs) [6], but a higher conversion is needed. Altering the surface morphology of the wafer by KOH etching or dry etching a hexagonal pattern have been successfully used to increase BPD conversion [5,7]. Growing on wafers with lower offcut angles and higher C/Si ratios can also increase BPD conversion [8]. Each of these processes has tradeoffs due to overall epitaxial quality and/or added processing complexity. The recently developed UVPL imaging technique makes it possible to map the dislocations and other extended defects in SiC epitaxial layers, and it has been particularly productive in tracking the path of BPDs through the epitaxy [9]. A combination of interrupted growths and UVPL wafer mapping is used to track changes to BPDs in the epitaxy during the growth process. Two types of changes to BPDs during growth and interrupt processes have been investigated. The first is the result of BPD glide in their basal plane during epitaxial growth. One consequence can be the formation of the defect alternately known as half-loop arrays or pair arrays [10]. The second change to the BPDs is their tendency to convert to TEDs due to a growth interruption. A variety of interrupt conditions have been studied and BPD to TED conversion percentages have ranged from 5% to 98% [11,12]. Experimantal Details The BPDs in the epitaxial layers were imaged using the newly developed ultraviolet photoluminescence (UVPL) mapping technique [13]. It non-destructively provides images of BPDs as well as other dislocations and extended defects throughout the thickness of the epitaxy and over the whole area of the wafer. It uses the 364 nm and 351 nm lines of an Ar-ion laser to excite electron-hole pairs; penetration depths are 120 μm and 50 μm, respectively. Individual images ~1 mm 2 with micron scale resolution were collected with a liquid nitrogen cooled CCD. Collection is limited to the 800 to 1000 nm to increase the contrast of dislocations above the background. The final full-wafer image was assembled after image processing to minimize optical system non-uniformities and to align the individual images.

2 The epitaxial layers were grown in a Aixtron/Epigress VP508 hot-wall chemical vapor deposition reactor using gas foil rotation. They were low doped or unintentionally doped, low /cm 3 or <10 14 /cm 3, respectively. The growth rate was 10 μm/hr and they were grown at 1580 C with a 1.55 C/Si ratio set by the propane/silane flow rates [14]. The growths were on quartered 3 inch wafers with 8 offcuts. Results from two types of growth sequences are presented. The first had separate 20 μm growths. The first growth included a 5 μm buffer layer grown at 5 μm/hr doped ~2x10 18 /cm 2. After each layer, the sample was removed from the reactor and a wafer map was collected. The second sequence had two growths of 20 μm without a buffer and the sample remained in reactor for the complete sequence. The growth was interrupted by stopping the silane flow. Half-Loop Array Formation Plan view images of five BPDs from ex situ growth interruptions are shown in Fig. 1: (a) after the first 20 μm of n - growth and (b) after the second 20 μm layer. The offcut direction is to the left. Thus, the left end of each BPD is the point where it enters the n - layer from the n + buffer and the right end is at the top of the SiC. The basal planes are tilted 8 so the expected left-to-right distances of the BPDs are 140 μm in (a) and 280 μm in (b), which agrees with the images. As discussed below, three of the BPDs, labeled T, are converted (or turned) to TEDs at the interrupt. The two BPDs labeled C that continue through the full 40 μm thickness illustrate that during epitaxial growth the BPD can glide perpendicular to the offcut direction. Fig. 1(c) is an overlay of the first two images. Note the path of the two continuing BPDs. Their paths through the first 20 μm of the epitaxy has moved after the second epitaxial growth due to the BPD glide in the basal plane. It is also apparent that the BPD entrance point into the n - epitaxy has negligible movement. Notice that the shape of the dislocations in Fig. 1 show that the BPDs are not perfect dislocations in the images. As grown, most of the BPDs from an arc within the epitaxial layer due to their glide. The curved lines in Fig. 1 are the path of the perfect BPD before exposure to UV light. The electron-hole recombination at each BPD that occurs during the UVPL exposure is sufficient to cause an initial rapid faulting along the segment of the BPD within the epitaxial layer. The curved boundary of the fault is a C-core partial dislocation along the initial perfect BPD path. The other two boundaries are Si-core and C-core partials along the < > directions [15]. The Si-core partials are the bright horizontal lines and the C-core partials are the faint straight lines originating at the entry point of the BPD into the n - layer. Its other end is at the Si-core partial. The initial faulting is rapid compared to the UV exposure time. Furthermore, once the Fig. 1. UVPL images (a-c) and 3D schematics (d-e) showing the faulted BPDs. Image (a) is after the initial 20 μm growth and (b) is after the second 20 μm growth. An overlay of (a) and (b) is shown in (c). moving partials lie along the energetically favorable < > directions their subsequent movement with UV exposure is significantly slower. Thus, the initially moving partials are captured as straight lines in the UVPL images. Three dimensional schematics of stacking faults are shown in Figs. 1(d) and 1(e) for the continuing and turned BPDs, respectively. The bottom, darker plane is at the bottom of the n - epitaxial layer and the lighter, semitransparent plane is at the top of the epitaxy. The shaded stacking fault is bounded by the curved partial dislocation and the straight Si-core and C-core partials along < > directions.

3 A consequence of this BPD glide can be the formation of a defect alternately known as a half-loop array or as a pair array. The formation of a pair array is captured in the two images shown in Figs. 2. The pair array contains an array of half loop dislocations that each has a short basal plane segment that forms Shockley faults in the presence of electron-hole injection and expands until it spans the n - epitaxial layer [10]. Four BPDs, A-D are shown in Fig. 2(a) after the first 20 μm growth. In Fig. 2(b), after the second 20 μm growth, BPDs A and D continue without significant movement of the portions of the BPDs in the first 20 μm (likely due to pinning by 1c screw dislocations). In contrast, BPDs B and C exhibit glide of μm. As the ends of BPDs B and C glide along the SiC surface in opposite directions, short segments of BPDs are left behind. Subsequent growth covers these segments and two TEDs continue the dislocation to the new surface. Each gliding BPD produces an array of half-loops that terminates at the surface. BPD B is gliding towards the bottom of Fig. 2 and towards the substrate to the left in the figure. The first BPD segment it leaves is the topmost followed by the three more segments. The opposite is true of BPD C. It is gliding towards the top of Fig. 2 and forms the array starting at its bottom element. This formation process explains the observation that all of the basal plane segments are on the same plane [10]. The portion of BPD C that extends from its position entering the epitaxial layer shown in Fig. 2(a) to the position seen in Fig. 2(b) is not visible because it lies near the buffer layer and dislocations at the heavily doped buffer are not visible by UVPL. In subsequent growths the portion of BPD B that is gliding toward the substrate continues that glide and is not visible after the third growth. Transmission x-ray topography also shows that these portions of the BPD are often close to the substrate/epi interface [16]. The Burgers vectors of BPDs B and C as well as the depths of the half-loops bottoms can be determined from images of the stacking faults (SFs) originating from the BPD segments of the half-loops. Fig. 3(a) is a magnified view of the array and Fig. 3(b) shows the combination of Si- and C-core partial dislocations bounding the stacking faults, which were induced by electron-hole pairs introduced by UV illumination - at higher intensity and for a longer time than used for typical imaging. A schematic of the image is shown in Fig. 3(c). It shows the Burgers vectors of the partial dislocations as well as the original perfect dislocation [10]. Note that the Burgers vectors of BPDs B and C are in opposite directions, which is consistent with their glide during growth being in opposite directions in response to stress. The depth of the pair arrays can be calculated from the point at which the stacking faults intersect the SiC surface: d = L/tan(8 ) where d is their depth, L is the left-to-right distance in Fig. 3(a) between a half-loop and where the stacking fault intersects the surface. Half-loops from BPD B and C are about 2 and 10 μm deep, respectively. As the half-loops in each array are formed, one at a time, each successive half loop is not as deep. This shows in Figs. 2 and 3 as Fig. 2. Four BPDs (a) after 20 μm and (b) 40 μm of growth. BPDs B and C glide and form the circled pair arrays. C B B pl B pl B p B pr B pr Fig. 3. Images of pair arrays (a) before (from Fig. 2(b)) and (b) after SFs expand. The expanding SFs have a rhombic shape. The bright sides are Si-core partials and the faint lines are C-core. Schematic of (b) shown in (c). Dashed lines show where SFs intersect the surface after further expansion. B pl and B pr are the Burgers vectors of the partial dislocations expanding to the left and right. B p is the Burgers vector of the original perfect BPD. B p c

4 a rightward displacement the later elements of the array. As already mentioned, the interfacial part of the pair array is often not visible in the UVPL images. The presumption is that in most cases this portion of the BPD is close enough to the substrate to quench its luminescence. An example in which the interfacial BPD in visible in the UVPL images and further illustrates the BPD glide is shown in Fig. 4. The n - epitaxial layer is 100 μm thick and was grown without interruptions on an 8 offcut wafer. The figure illustrates the overall structure of the pair array defect. Figure 4(a) shows the complete BPD for one pair array defect, labeled 1, and parts of two others, labeled 2 and 3. A sketch of BPD 1 is drawn in Fig. 4(b). As illustrated in Fig. 1, the BPD position at the epi/substrate interface does not appear to move. Thus, as the BPD glides, it tends to leave an interfacial dislocation, which is on the left side of the image. The Fig. 4. Examples of pair array defect: (a) is a UVPL image and (b) is a schematic of the BPD #1 path (faulted areas are shaded). In (a) only BPD #1 is completely shown. gliding part of the BPD arcs to the growth surface. Parts of this arc have started to fault during the UV exposure and the stacking faults are shown in gray in Fig. 4(b). In some, but not all cases, the BPD leaves short segments at the growth surface as its end glides along the surface. This is the part of the pair array that was first noticed [10] and shows as a sequence of dots in the UVPL image. Note that the interfacial part of the BPD is pinned due to a threading screw dislocation (TSD). As the BPD glides down in the image, the interfacial part of the BPD extends between the pinning point and the arced portion of the BPD. Similar pinning is discussed above for Fig. 2. Also note that the gliding BPD did not leave the short BPD segments along the full trajectory of its intersection with the growth surface. The BPD dots start in the middle of the image and extend downward about 200 μm. BPD to TED Conversion at Growth Interrupts The ability of UVPL mapping to track the path of BPDs through epitaxial layers has also been employed to develop a new, easily implemented process to reduce BPD concentration in the epitaxy. The tendency of BPDs to be converted into TEDs is mentioned above and is shown in Fig. 1. In the area shown in the figure, three of the five BPDs are converted to TEDs at the ex situ interrupt. Three ex situ interrupts on the same quarter wafer had conversion percentages over the whole area ranging from 32 to 36% for each interrupt [9]. The ex situ interrupts showed that the number of BPDs extending into each new epitaxial layer decreased and no new ones were introduced. This confirms that all of the BPDs in our samples originated from the substrate. (This ignores a very small fraction of the BPDs that are caused by locally high stress from a defect such as a micropipe.) These results show that it is also possible to determine which BPDs are converted to TEDs during in situ growth interrupts. The population of BPDs after a growth with one in situ interrupt has two distinct lengths. As illustrated in Fig. 1, the length of BPDs projected along the offcut direction of the ones that continue through both layers of the growth are twice as long as the ones that change from BPDs to TEDs at the beginning of the second growth assuming the same thickness before and after the interrupt. The advantage of in situ interruptions is they are performed without removing the wafer from the epitaxial growth chamber, which significantly decreases the time and complexity of the process. In situ growth interruptions are implemented by stopping the silane flow. BPD conversion has been optimized by varying growth and interrupts parameters. These include the interrupt duration as well as the temperature and propane flow during the interrupt. The best results yield a 98% conversion for a single in situ interrupt and a BPD concentration less than 10/cm 2 after the interrupt. Summary Examples of using the newly developed UVPL imaging to understand and modify BPDs in SiC epitaxy are presented here. By combining UVPL imaging and interrupted epitaxial growths ex situ and in situ two aspects of BPD evolution during growth have been investigated. First, the glide of some BPD during growth is shown to be responsible for the formation of pair or half-loop arrays. Second, UVPL imaging made it possible to optimize in

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