Introduction
In addition to applications in optoelectronics[Reference Nakamura and Fasol1], GaN and related alloys are of interest for applications in high temperature, high frequency and high power electronic devices[Reference Binari2]. In many of these devices, the AlGaN layer is an active part of the device structure and the structural properties of this layer can affect the basic device characteristics. This has been demonstrated and inferred for example in the work of Shiojima, et al [Reference Shiojima, Woodall, Eiting, Grudowski and Dupuis3] who observed lowering of the Schottky barrier height and assigned it to the presence of the structural defects in the n-doped layer. In this work, several AlGaN/GaN HFET structures are analyzed using transmission electron microscopy (TEM).
Experimental
The GaN and the AlGaN layers were grown by low pressure MOCVD in a modified EMCORE D125 reactor system. Trimethylgalium (TMGa) and trimethylaluminum (TMAl) were used as column III precursors, while hydrogen-diluted silane (SiH4) was used as the n-type dopant. High purity ammonia (NH3) was used as the N source. The Al2O3 substrates were cleaned for 10 minutes in 10:1:1 H2SO4:H2O2:DI H2O and then for 10 minutes in 2:9 HF:HNO3 The cleaning was followed by a DI water rinse. Substrates were then loaded into the system and, after prepumping to a base pressure of 10−7 torr, were loaded into growth chamber. After a H2 bake at 1050°C for 5 minutes, the substrates were cooled down and 25 nm-thick buffer layers were grown at 530°C. The wafers were then heated to 1050°C again and ∼2 μm of GaN was grown in NH3, H2, and TMGa. A rapid switch to TMAl, TMGa, NH, and H2 followed to grow undoped 30% AlGaN layers, as well as doped 30% and 45% AlGaN layers. The samples were subsequently cooled to room temperature in H2 and NH3 TEM cross-sections were prepared along [11-20], and [Reference Nakamura and Fasol1-Reference Nakamura and Fasol1 0 0] directions normal to the growth direction [0001]. Tilting (bright field, weak beam and dark field) experiments were performed in the side entry stage JEOL 200 CX, while high resolution imaging was perform on JEOL ARM operating at 800 kV.
Results
Figure 1 shows the low magnification image of the cross-section of an undoped 30% AlxGa1−xN film. The AlxGa1−xN layer was about 30 nm thick. Some undulation of the thickness of this layer was observed. These undulations were associated with the presence of the V-shaped defects, some of which extended almost halfway into the layer as shown in Fig. 2, Adding dopant to the AlxGa1−xN layer while preserving the aluminum concentration at x=30% resulted in overall increase of the density of the V-shaped defects. Otherwise the morphology of the doped film was very similar to that of the undoped AlxGa1−xN film. This is illustrated in Fig. 2 and Fig.3. By comparison, further increase in the concentration of Al to x=45% resulted in an increased density of the V-shaped defects, as well as in the changed morphology of the growth front from 2D to 3D. This is illustrated in Fig.5 and Fig.6
Fig 1: Weak beam g=[0002] image of an undoped 30% AlGaN layer (sample 1170) layer on about 1.7-2 μ GaN layer on a sapphire substrate. Note presence of V-shaped defects. In many cases these defects appear to be associated with the threading dislocations.
Fig 2: High resolution image of an undoped AlGaN layer (sample 1170) layer on ∼1.7-20μm GaN on a sapphire substrate showing V-shaped defects. Note that the growth between V-shaped defects is planar (2D). The V- shaped defect to the left initiated about 200 nm away from the AlGaN /GaN interface.
Fig 3: TEM bright field image of the cross-section of a doped 30% AlGaN layer on ∼2 μm GaN on a sapphire substrate (sample 1134B).
Fig 4: High resolution TEM image of the cross-section of a doped 30% AlGaN layer on ∼2 μm GaN on a sapphire substrate (sample 1134B).
Fig5: Bright field TEM image of the cross-section of a doped 45% AlGaN layer on ∼2 μm GaN on a sapphire substrate (sample 1129A).
Fig 6: High resolution TEM image of a doped 45% AlGaN layer on ∼2 μm GaN on a sapphire substrate (sample 1129A). Careful analysis of the growth front between V-shaped defects indicates 3D growth.
Summary and Conclusion
The density of the V-shaped defects increases with Al and dopant content. This is illustrated in Fig. 7 which summarizes the TEM results. This dependence on theAl concentration and on the dopant composition in the growing AlGaN film indicates that the poisoning of the ledges takes place and promotes V-shape defect formation. The local composition inhomogeniety due to poisoning of the ledges could be responsible for the observed lowering of the Schottky barrier height in n-doped AlGaN/Ni Schottky diode [Reference Shiojima, Woodall, Eiting, Grudowski and Dupuis3]. The role of threading dislocations in the formation of the V-shaped defects has to be further studied, because many, but not all, V-shaped defects were found in the proximity of these dislocations. The formation of the V-shaped defects presents a challenge for AlGaN/GaN electronic devices, as these defects will impact many electrical parameters and reduce device reliability. Therefore methods need to be developed to reduce the V-shaped defect density in AlGaN/GaN heterostructure.
Fig7: Frequency of the observation of the V-shaped defect as a function of the separation between two consecutive V defects is the highest for a doped sample with highest Al content and the lowest for the undoped lower Al content sample.
Acknowledgements
Work at E.O. Lawrence Berkeley National Laboratory was supported by the Director, Office of Basic Science, Materials Science Division, U.S. Department of Energy, under contract No. DE AC03-76SF00098. The use of the facilities at the National Center for Electron Microscopy at E.O. Lawrence Berkeley National Laboratory is greatly appreciated.
Work at the The University of Texas at Austin was in part supported by the ONR Contract N00014-95-1-1302 monitored by J. C. Zolper and NSF under Grants DMR-9312947 and CHE-89-20120.
