Edge Dislocations Triggered Surface Instability in Tensile Epitaxial

Dec 1, 2016 - Understanding the semiconductor surface and its properties including surface stability, atomic morphologies, and even electronic states ...
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Edge Dislocations Triggered Surface Instability in Tensile Epitaxial Hexagonal Nitride Semiconductor Jianpeng Cheng,† Xuelin Yang,*,† Jie Zhang,† Anqi Hu,† Panfeng Ji,† Yuxia Feng,† Lei Guo,† Chenguang He,† Lisheng Zhang,† Fujun Xu,† Ning Tang,† Xinqiang Wang,†,‡ and Bo Shen*,†,‡ †

State Key Laboratory of Artificial Microstructure and Mesoscopic Physics, School of Physics, Peking University, Beijing 100871, P. R. China ‡ Collaborative Innovation Center of Quantum Matter, Beijing 100871, P. R. China ABSTRACT: Understanding the semiconductor surface and its properties including surface stability, atomic morphologies, and even electronic states is of great importance not only for understanding surface growth kinetics but also for evaluating the degree to which they affect the devices’ performance. Here, we report studies on the nanoscale fissures related surface instability in AlGaN/GaN heterostructures. Experimental results reveal that edge dislocations are actually the root cause of the surface instability. The nanoscale fissures are initially triggered by the edge dislocations, and the subsequent evolution is associated with tensile lattice-mismatch stress and hydrogen etching. Our findings resolve a long-standing problem on the surface instability in AlGaN/GaN heterostructures and will also lead to new understandings of surface growth kinetics in other hexagonal semiconductor systems. KEYWORDS: surface instability, nanoscale fissures, edge dislocations, hexagonal nitrides, TEM

1. INTRODUCTION III-nitride semiconductors have a wide range of modern applications, including that for light-emitting diodes (awarded with the Noble Prize in Physics), lasers diodes, and power transistors.1−9 Understanding the nitride surface and its properties including surface stability, atomic morphologies, and even electronic states is of great importance for evaluating the degree to which they affect the devices’ performance, especially for power electronics.10,11 As a common sense, the surface morphologies of semiconductors are determined by growth kinetics,12 substrate miscuts,13 dislocations,14,15 and strain16 in the material system. For III-nitride materials, they are usually grown on foreign substrates, such as SiC, sapphire, and Si substrates, because native bulk GaN substrates are not available at low cost yet.17 Therefore, high-density threading dislocations including screw-type and edge-type dislocations are generated as a result of large lattice mismatch and thermal expansion coefficient mismatch between GaN films and these substrates.18,19 It is believed that each type of the threading dislocations should play different roles on the surface morphologies of nitride materials. For example, it has been well-known for many years that the atomic step pinning and spiral hillocks are linked with the emergence of screw-type dislocations at the GaN surface. That is well explained by the classical model of Burton, Cabrera, and Frank (BCF).20 However, there are few reports on the correlation between surface morphologies and edge-type dislocations which have an even larger density than that of screw-type dislocations. On the other hand, nitride quantum structures such as heterostructures and quantum wells usually sustain a certain © XXXX American Chemical Society

amount of stress due to lattice mismatch between the two adjacent layers. Different from the case in cubic semiconductors,21 the mismatch stress in hexagonal nitrides cannot be completely relaxed by misfit dislocations due to the absence of an effective primary slip system.22,23 There always exists residual stress in the structure. Under that condition, the surface morphologies of these hexagonal structures are unstable and would exhibit some new features.24 One example is the nanoscale fissures observed on the surface of AlGaN/GaN heterostructures.25−29 That kind of surface instability is believed to affect the gate leakage current and current collapse effect, which are two of the most important limitations for AlGaN/GaN high electron mobility transistors (HEMTs)30−33 Elimination of those nanoscale fissures is of course a key to further improve the performance of HEMTs. However, their formation mechanisms have not been fully understood yet. In general, the fissures or crack-like surfaces are formed through surface diffusion due to the stress-induced atom chemical potential variation.34 For III-nitride materials, as mentioned above, there are a larger number of dislocations. Whether the nanoscale fissures originate from the surrounding dislocations where atoms may have higher chemical potential and what are their correlations with dislocation types still remain controversial.25−29 Therefore, more systematic characterizations and in-depth understanding of the surface of AlGaN/GaN Received: September 4, 2016 Accepted: November 21, 2016

A

DOI: 10.1021/acsami.6b11124 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 1. AFM images of the AlGaN/GaN heterostructures (a) cooled in a hydrogen atmosphere for sample A, (b) cooled in a nitrogen atmosphere for sample B, and (c) with thinner AlGaN barrier thickness for sample D. AFM images of the GaN layers cooled in (d) nitrogen and (e) hydrogen atmospheres. (f) AFM image of sample A etched by KOH. (g) AFM image of the AlGaN/GaN heterostructures on SiC with low dislocation density (sample E). the 2 μm thick GaN buffer layer without the GaN cap layer. Sample A was cooled down in a hydrogen atmosphere, whereas samples B, C, and D were cooled down in a nitrogen atmosphere. For samples A and B, the only difference is the cooling down atmosphere, hydrogen for sample A and nitrogen for sample B. For samples B and D, the only difference is the thickness of the AlGaN barrier, 30 nm for sample B and 10 nm for sample D. Furthermore, sample E was grown on the SiC substrate with the same structure and cooling atmosphere as sample B. The full widths of half-maximum (fwhm) of the asymmetric plane rocking curves measured by high-resolution X-ray diffraction are 627 and 310 arcsec for samples B and E, respectively. Since the threading dislocation density is proportional to the square of fwhm values,36 the threading dislocation density in sample B is almost 4 times larger than that in sample E. Surface morphologies and conductive atomic force microscope (cAFM) images were obtained by a Bruker dimension icon with ScanAyst atomic force microscope. The sample was imaged with an FEI TECNAI Osiris TF-20 FEG/TEM operated at 200 keV in brightfield scanning transmission electron microscopy (STEM) mode and high-resolution TEM (HRTEM) mode. The focus ion beam (FIB) technique was applied to prepare TEM specimens. The Hall effect measurements in Van der Pauw configuration were taken in a Bio Rad Accent HL5500 Hall measurement system. Ohmic contacts were prepared by depositing Ti/Al/Ni/Au (20/150/35/100 nm) metal stacks, followed by a rapid thermal annealing at 850 °C for 35 s under a nitrogen atmosphere.

heterostructures are of paramount priority to further improve the device performance. In this work, we have investigated the formation and elimination mechanisms of nanoscale fissure related surface instability in AlGaN/GaN by using atomic force microscopy (AFM) and transmission electron microscopy (TEM). Contrary to previous reports, we demonstrate that the nanoscale fissure related surface instability is triggered by edge-type dislocations instead of screw-type dislocations. It is verified that the fissure formation initially originates from edgetype dislocations and its further elongation is associated with tensile lattice-mismatch stress and hydrogen etching. The surface fissure formation can then be suppressed by reducing the dislocation density and the stress. This newly discovered mechanism not only is applicable for deeper understanding of the surface of nitride systems (AlGaN/GaN, AlN/GaN, InGaN/InN) but also has important impacts on the understanding of surface morphologies for other hexagonal semiconductors.

2. EXPERIMENTAL SECTION AlGaN/GaN heterostructures labeled as samples A, B, C, and D were grown on 650 μm thick 100 mm p-type Si(111) substrates by metal− organic chemical vapor phase deposition (MOCVD) with a close coupled showerhead 3 × 2 in. reactor (Aixtron CCS). Trimethylgallium, trimethylaluminum, and ammonia were used as precursors for gallium, aluminum, and nitrogen, respectively. Hydrogen was used as the carrier gas. The stress control methods and detailed growth parameters can be found in our previous work.35 For samples A and B, a 30 nm thick Al0.25Ga0.75N barrier layer was grown on the 2 μm thick GaN buffer layer without the GaN cap layer. For sample C, a 30 nm thick Al0.25Ga0.75N barrier layer was grown on the 2 μm thick GaN buffer layer with a 2 nm thick GaN cap layer. For sample D, a thin Al0.25Ga0.75N barrier layer with the thickness of 10 nm was grown on

3. RESULTS AND DISCUSSION Figure 1a,b shows the AFM images of the AlGaN surface for sample A (cooled in a hydrogen atmosphere) and sample B (cooled in a nitrogen atmosphere), respectively. In sample A, the surface features nanoscale fissure networks. Similar morphologies have been reported and were attributed to the tensile lattice-mismatch stress in the AlGaN layer grown on thick GaN layer.25,27 However, in sample B, it is found that the B

DOI: 10.1021/acsami.6b11124 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 2. (a) The schematic of the voltage setup during the scanning. (b) C-AFM image of the sample B surface, and (c) the corresponding AFM image.

Figure 3. (a) Cross-sectional bright-field STEM image of sample B along the [11̅00] zone axis. The inset gives the diffraction pattern from the sample. Images under two beam conditions with (b) g = [0002] and (c) g = [112̅0]. The insets are the enlarged images in the red squares. (d−f) High-resolution TEM images around dislocations 1, 2, and 3, respectively.

fissure size and density are significantly reduced. As mentioned above, the growth conditions for both samples are the same. The only difference is the cooling down atmosphere, hydrogen for sample A and nitrogen for sample B. That indicates that tensile stress is not the only factor for fissure network formation. In other words, the hydrogen etching during the cooling down process should also play an important role. With the aid of hydrogen, GaN can dissolve into Ga atom and ammonia.37 This chemical reaction means that the surface atoms have higher chemical potential in a hydrogen atmosphere than that in a nitrogen atmosphere. As a result, the short nanoscale fissures observed on the surface of sample B can be mainly attributed to the tensile stress. This is also consistent with the result that the nanoscale fissure density is further reduced in sample D (Figure 1c) with a lower strain energy due to its thinner AlGaN barrier thickness of 10 nm. If there is no tensile stress, there would be no short fissure at the beginning and thus no fissure network could be formed even with hydrogen etching. This is in agreement with the fissure-free surface observed on two GaN samples which were cooled in

either a nitrogen (Figure 1d) or a hydrogen atmosphere (Figure 1e). Thus, the formation of the fissure network in sample A can be divided into two steps. The first step is that short fissures are initially induced by the tensile stress. The second step is that these short fissures enlarge and elongate with the aid of hydrogen etching, and finally the fissure network is formed. In order to gain further insights into the origins of the fissures, sample A was wet etched with molten KOH at 210 °C for 2 min. After cleaning with deionized water and drying in nitrogen ambient, the surface morphology was checked by AFM, as shown in Figure 1f. It is found that the fissures evolve into grooves and each groove contains one dark pit. Since dislocations are more vulnerable to the molten KOH,38 the dark pits in the AFM image correspond to the dislocation positions. That means the nanoscale fissures might be correlated with the dislocations. This correlation is further confirmed by the fact that the fissure density is smaller in the lower dislocation density sample E (Figure 1g) grown on the SiC substrate compared with that in sample B (Figure 1b) C

DOI: 10.1021/acsami.6b11124 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 4. (a) Plan-view TEM image of the AlGaN/GaN heterostructure. The upper left and lower right insets show the Fourier-filtered HRTEM image and the corresponding electron diffraction pattern, respectively. (b) Atomic schematic of edge-type dislocation. (c, d) Evolution of fissure formation through chemical bond breakage.

was taken along the [11̅00] GaN zone axis, as shown in Figure 3a. Three dislocations are detected in the TEM specimen, labeled as dislocation 1 (D1), 2 (D2), and 3 (D3). In order to identify the Burgers vectors of these dislocations, images with g = [0002] and [112̅0] diffraction vectors were taken, as shown in Figure 3b,c, respectively. It is clearly seen that D2 and D3 are edge-type dislocations with the Burgers vectors b = 1/3[112̅0]. The part of D1 in the GaN layer shows contrast in both images with g = [0002] and [112̅0]. However, the part of D1 in the AlGaN layer only shows contrast in the image with g = [0002]. These phenomena imply that the AlGaN/GaN interface changes the characteristic of D1, and eventually D1 evolves into a screw-type dislocation. From Figure 3d−f, we can clearly see that the pits do not locate at the top of screw-type dislocations, but locate at the top of edge-type dislocations. Therefore, the TEM results further confirm that fissures are triggered by edge-type dislocations instead of screw-type dislocations. It should be emphasized that this mechanism has never been reported in III-nitride materials. A plan-view TEM image of sample B is shown in Figure 4a. Nanoscale fissures are indicated by purple arrows in the image. From the corresponding electron diffraction pattern and Fourier-filtered HRTEM image (the insets), these fissures were confirmed to elongate along ⟨112̅0⟩ directions, which can generate the steady {11̅00} cleavage planes.40 We employ an 8atom structure to build the model of edge dislocations located around the low-angle grain boundary. Three oblique lines are added around the edge dislocation core, as shown in Figure 4b. Due to the presence of the extra half atom plane as labeled by the middle red line, a stress field is generated near the dislocation. Atoms below the dislocation core sustain a compressive stress, whereas atoms above the dislocation core sustain a tensile stress. This tensile stress can lead to the stretched atomic bonds. Additional tensile stress comes from

grown on the Si substrate. Therefore, we can conclude that these nanoscale fissures originate from the threading dislocations. Furthermore, we find that the fissures usually appear on the surface terraces, as shown in Figure 1b. Considering the fact that edge-type dislocations usually locate at the surface terraces while screw-type dislocations usually locate at the area where surface steps disappear,15 the fissures on the AlGaN/ GaN surface are thus related to edge-type dislocations. From the viewpoint of electrical properties, the effects of edge-type and screw-type dislocations on leakage currents are different. We can also identify the dislocation types from their contribution to leakage current. The vertical leakage current path in sample B was measured by c-AFM (conductive atomic force microscope). Figure 2a shows the schematic of the voltage setup during the scanning. The Si substrate was grounded, and a DC bias of −3 V was applied on the metal coated AFM tip. Figure 2b,c shows the c-AFM image and corresponding AFM image of the sample B surface, respectively. Two circular leakage current paths are observed, as shown by the red arrows in Figure 2b. The corresponding surface morphologies are shown by the red triangles in Figure 2c. We did not find leakage current paths at the short fissure positions. It has been reported that pure screw dislocations are solely responsible for the leakage paths, while edge dislocations are not conductive.39 Therefore, we can conclude that the fissures originate from edge dislocations instead of screw dislocations. In order to further confirm the fact that the nanoscale fissure related surface instability is triggered by edge-type dislocations, cross-sectional TEM experiments are performed for sample B. To obtain more detailed information about the fissures, the TEM specimen has to come from a selected area including fissures and to be cut in some specific directions. The brightfield image in scanning transmission electron microscopy mode D

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Figure 5. (a) XRD reciprocal space mapping of sample C. (b) AFM image in 2 × 2 μm2 scanned area of sample C. (c) I−V characteristics of Schottky diodes fabricated on sample A (without GaN cap layer) and sample C (with GaN cap layer).

effective to obtain a more stable surface, which is of great importance for AlGaN/GaN heterostructures in power electronics applications. A promising way is to use low density bulk GaN substrates even it is not a cost-effective solution at the present time. Looking beyond AlGaN/GaN heterostructures, the mechanisms are also applicable for other nitride heterostructures such as AlN/GaN, InGaN/InN, and even other hexagonal semiconductor (ZnO- and CdS-based materials) systems.

the lattice mismatch between the 30 nm thick AlGaN barrier layer and 2 μm thick GaN buffer layer. Thus, the total tensile stresses induced by the dislocation and lattice mismatch result in the breakage of the stretched crystal bonds, and eventually the formation of the short fissures, as shown in Figure 4c. The stress distribution around the fissure then changes, and thus chemical bonds below the dislocation core begin to break to form the fissure shown in Figure 4d. The atom chemical potentials at the edge of the short fissures increase due to the broken bonds. When the sample surface is exposed to a hydrogen atmosphere, atoms at the edge of the short fissures with higher chemical potential are more unstable and thus being first etched. Finally, the short fissures enlarge and form the fissure networks, as shown in Figure 1a. According to the mechanisms we proposed above, both the lattice-mismatch tensile stress and the dislocation tensile stress are the necessary conditions for the fissures formation. In order to eliminate that kind of surface instability, we can eliminate either effect of the stress. Sample C (capped with a GaN layer and cooled down in a nitrogen atmosphere) is designed to eliminate the effect of the lattice-mismatch tensile stress on the surface layer. From the XRD reciprocal space mapping shown in Figure 5a, we can see that the AlGaN barrier layer has the same a lattice constant of GaN due to its pseudo-growth on the GaN buffer layer. Thus, the GaN cap layer on the AlGaN layer is almost strain-free. The single effect left of dislocation stress is not enough to form fissures on the surface. As a result, sample C displays a smooth surface with clear atomic steps (Figure 5 b). In order to evaluate the surface morphology on electrical properties, we performed current−voltage (I−V) measurements. Figure 5c shows the reverse leakage current density of the fabricated Schottky diodes on sample C and sample A (uncapped). Without employing any extra surface treatments, the Schottky diodes on sample C exhibit an ultralow reverse leakage current density of 2 × 10−5 A/cm2 at a gate bias of −30 V, which is about 3 orders of magnitude lower than that of the Schottky diodes on sample A. In addition, the maximum electron mobility at room temperature in sample C can be up to 2260 cm2/(V s) at a sheet charge density of 1.0 × 1013 cm−2. These indicate that the high quality fissure-free surface can significantly improve the electrical properties of AlGaN/GaN heterostructures. Another approach to eliminate the nanoscale fissure related surface instability is to eliminate the dislocations. However, completely eliminating dislocations is not possible for IIInitride materials. Nevertheless, based on the fissure formation mechanisms, we believe that the reduction of dislocation density (especial the edge-type dislocation density) should be

4. CONCLUSIONS In summary, we have investigated the surface instability in AlGaN/GaN heterostructures. It is revealed that the fissure related surface instability is initially triggered by edge-type dislocations. With the assistance of lattice-mismatch stress and hydrogen etching, the short fissures will evolve into fissure networks on the AlGaN surface. The experimental results indicate that the surface instability can be suppressed either by decreasing the stress or by decreasing dislocation density. Our findings suggest that edge dislocations are actually the root cause of nanoscale fissure formation and resolve a long-standing problem on the surface instability in AlGaN/GaN heterostructures. The present work will also lead to new understandings of surface growth kinetics in other hexagonal semiconductor systems and will be of great interest for advancing optoelectronic devices.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (X.Y.). *E-mail: [email protected] (B.S.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Key Research and Development Program (Nos. 2016YFB0400104 and 2016YFB0400201), the National Natural Science Foundation of China (Nos. 61574004, 61306110, 61521004, 11634002, and 61361166007), the National High-Tech Research and Development Program of China (Nos. 2014AA032606 and 2015AA016801), Beijing Municipal Science and Technology Project (No. Z151100003315002), and Open Project from Key Laboratory of Nanodevices and Applications, Suzhou Institute of Nano-Tech and Nano-Bionics, CAS (15ZS05). We are grateful to Prof. Weikun Ge for his critical reading of the manuscript. E

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DOI: 10.1021/acsami.6b11124 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX