High aspect ratio GaN fin microstructures with non- polar sidewalls by continuous mode MOVPE

Three-dimensional GaN microand nanorods with high aspect ratio have recently gained substantial interest in LED research, due to their reduced defect density, their non-polar sidewalls and their increased active area. Here, we present an alternative geometry for high aspect ratio 3D nanostructures: vertically standing GaN “walls”, so called GaN fins. With high aspect ratios, these GaN fins exhibit the same interesting characteristics as their rod counterparts mentioned above. Beyond that, due to their geometry, the respective material analysis and device https://doi.org/10.24355/dbbs.084-201912181044-0 2 processing can be expected to be less complex. We are able to demonstrate the highly reproducible selective area growth of these fins by continuous mode MOVPE. Fin heights of more than 50 μm (aspect ratios of nearly 14) could be achieved and growth rates are as high as 22.8 μm/h in the beginning of the growth. The sidewalls are smooth non-polar <11-20> a-planes, suitable for optoelectronic devices due to the missing quantum-confined Stark effect and less edge effects compared to rods. We investigate the influence of pattern orientation and geometry on the fin morphology. Moreover, the influence of silane flow, which is known to enhance the vertical growth rate, and other growth parameters are systematically explored. INTRODUCTION GaN is the material of choice for solid state lighting since many years. Conventional light emitting diodes (LEDs) based on the (Al,In,Ga)N material system are generally grown in a planar geometry using c-oriented thin film deposition. These planar GaN, AlGaN and InGaN films are characterized by a huge density of threading dislocations due to lattice mismatch to each other and to the underlying growth substrate, typically sapphire, SiC or silicon. The polar c-orientation of the layers leads to high piezoelectric fields in the multi quantum well (MQW) region. In spite of these properties, very high quantum efficiencies can be reached in LEDs for certain indium concentrations, but both high defect density and piezoelectric effects are viewed as being disadvantageous for certain applications like laser diodes or higher indium content quantum wells. The m-plane quantum wells of three-dimensional (3D) microor nanorods in a core-shell geometry are both defect free and free of piezoelectric effects1. In addition, the large sidewall surface allows for increasing the light emitting area of LED devices without increasing the necessary substrate area2, potentially reducing production cost per lumen substantially and increasing the luminous density. https://doi.org/10.24355/dbbs.084-201912181044-0 3 Three-dimensional (3D) GaN structures are usually either directly grown by MOVPE in a selective area growth (SAG) process in order to reduce the dislocation density due to the small footprint3, or deep etched into a GaN thin film in order to release its strain4. 3D n-GaN structures have already been used as the basis for core-shell LEDs with MQW and p-GaN shell layers5. These QWs do not exhibit the quantum-confined Stark effect (QCSE)6, since they are grown on non-polar moriented sidewalls. As already mentioned in literature, there are several advantages of core-shell rod LEDs over conventional planar LEDs7,8. In order to exploit these interesting advantages, the 3D geometry does not necessarily need to be rod-like, but could also be in a so-called fin shape, including a few distinct advantages in comparison to microrod structures. These fin structures are long, thin and high GaN walls with non-polar sidewalls and aspect ratios (height to width as well as length to height) much larger than one. The growth of III-nitrides in fine wall-shaped structures, mainly by molecular beam epitaxy (MBE), was already shown by Kishino and Kikuchi9. MOVPE growth of these fin structures was demonstrated by T.-W. Yeh et al.10 and by A. K. Rishinaramangalam et al11. Due to the small footprint of fins and due to defect annihilation at the sidewall surfaces, lower defect densities can be expected as compared to planar GaN layers. In addition, core-shell fin LEDs have, under certain circumstances, an even larger active area ratio (AAR), i.e. active area (equivalent to surface area) to substrate area in comparison with core-shell rod LEDs: For hexagonally shaped rod LEDs in a hexagonal pattern AAAAAArrrrrr = 6 √3 HH∗DD PP2 with considering the height H, diameter D (defined as two times the circumradius of the hexagonal rod) and pitch P (defined as distance between the centers of neighboring structures) of the columns, respectively. If the end facets of the fins are neglected, for fin LEDs it is AAAAAAffffff = 2HH PP with the height H and pitch P of the fins, respectively. Taking the same diameter D (for rods) or width W (for fins) and https://doi.org/10.24355/dbbs.084-201912181044-0 4 the same height and pitch for rods and fins, there is an enhancement in the AAR of AAAAAAFFFFFF AAAAAArrrrrrrr = PP √3DD . For a pitch of more than 1.7 times the diameter or width, the AAR for fins is larger than that for rods. This prerequisite is fulfilled in most core-shell rod LED designs, since the outer shell layers, e.g. the p-GaN, need additional space in between the structures. Furthermore, the structures should have enough distance between each other to couple out light with high efficiency. If micro-grained optical converter materials filled into the 3D-ensemble are used for white light applications, even more space is required between the rods or fins. Besides the greater AAR of fins, the vertical current density in the fin LED core is less than in a rod core, assuming comparable active area and driving conditions. This is due to a larger footprint area per active area for fins than for rods. Thus, the current density in the base area is reduced and less problems regarding heating can be expected in fin LEDs. For rod LEDs the footprint ratio FFAArrrrrr = √3DD 8HH , while for fin LEDs it holds FFAAffffff = WW 2HH . Taking again the same diameter or width and the same height for rods and fins, there is an enhancement of FFAAffFFFF FFAArrrrrr = 4 √3 ≈ 2.31. Another advantage of GaN fins is their much easier fabrication: Less effort is needed for lithography masks since a precise pattern profile is necessary in one dimension only, which is less critical for nanoimprint lithography and allowing laser interference methods to be used. Also, the analysis of single fin LEDs is much easier. E.g. by cleaving the samples perpendicular to the fin longitudinal direction, a cross-sectional view into the structures can easily be achieved, which is extremely difficult for rod geometries. Also, GaN fins have higher mechanical stability than rods and therefore are less sensitive to further processing steps. Unlike GaN fins, rod structures offer six large edge lines between the sidewall facets. Therefore, boundary effects, like e.g. an increased indium content at the edges between two non-polar planes due to different strain relaxation12,13 https://doi.org/10.24355/dbbs.084-201912181044-0 5 could become an issue. In this respect, fins can be expected to be much more homogeneous. For the same reason, the current conducting path inside the n-GaN core could be more uniformly for GaN fins10. Last but not least, the fins provide relatively large non-polar surfaces, each of several thousand μm2, at a low cost. These surfaces could be viewed as “vertical a-plane GaN pseudo substrate” with low defect density, which can be used to study the properties of non-polar GaN without the necessity to use very expensive a-oriented GaN substrates. Such fins could be applied for LEDs10,11, but also for laser structures, Fin-FETs or HEMTs, waveguides14 and much more. GaN fins, also called nanowalls or nanosheets in other publications, with {1-100} non-polar sidewall planes and aspect ratios of two or more have already been fabricated by pulsed growth mode selective area in MOVPE10,11. In this article, we demonstrate the growth of GaN fins with {11-20} non-polar sidewalls by continuous growth mode selective area MOVPE. The growth rates are enhanced compared to pulsed growth mode and thus large vertical dimensions of the fins are shown. Aspect ratios of almost 14 could be achieved, the highest ever reported for bottom-up GaN fin geometries. The dependencies on pattern orientation, pattern geometry and growth conditions are described. EXPERIMENTAL SECTION The fin structures were grown by SAG on 2” wafer templates of two different kinds, either SiOx on GaN on sapphire or SiOx directly on sapphire. The 30 nm thick SiOx masking layer was structured by photolithography. Either a lift-off process with an image reversal lithography step and SiOx evaporation or a positive lithography step with subsequent dry etching of the SiOx (by inductively coupled plasma, using SF6) is applied. The photolithography is done with a 300 nm thick layer of AZ 5214E. Both techniques resulted in very comparable finlike structures. The lithography mask consists of different patterns of line openings with lengths of 10 μm, 25 μm, 50 μm, 100 μm, 250 μm, 500 μm and 1000 μm, with pattern widths (Wp) of 0.5 https://doi.org/10.24355/dbbs.084-201912181044-0 6 μm, 1 μm, 1.5 μm and 2 μm and pitches of three, six and nine times the width (see Figure 1). These patterns are repeatedly rotated by 15° relative to each other, arranged in a circle with six patterns of lines parallel to the a-direction (resulting in fins with m-plane sidewalls), six patterns of lines parallel to the m-direction (resulting in fins with a-plane sidewalls) and additional twelve patterns oriented in directions in between these two. Besides of this main circle of patterns, at the rim of the photolithography mask are structures with even longer line openings and with smaller rotation angles relative to mand a-direction. The orientation of the lines is given as the angle to the sapphire wafer flat, taking into account the 30° twist of GaN gr