J.H. Edgar, Z. Gu, and K. Taggart, Kansas State University, Department of Chemical. Engineering ... R.Witt, EBSD Analytical Inc., 2044 N 1100 E, Lehi, UT 84043.
Mater. Res. Soc. Symp. Proc. Vol. 892 © 2006 Materials Research Society
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Oxidation of Aluminum Nitride for Defect Characterization J.H. Edgar, Z. Gu, and K. Taggart, Kansas State University, Department of Chemical Engineering, Durland Hall, Manhattan, KS 66506-5102 J. Chaudhuri, L. Nyakiti, and R.G. Lee,Texas Tech University, Department of Mechanical Engineering, Lubbock, TX 79409 R.Witt, EBSD Analytical Inc., 2044 N 1100 E, Lehi, UT 84043 ABSTRACT The thermal oxidation of aluminum nitride was developed as a means to study defects in bulk aluminum nitride crystals. The oxidation kinetics was established for the dry oxidation of highly textured AlN polycrystals produced by sublimation-recombination crystal growth in a tungsten furnace. Despite seeding on polycrystalline tungsten, the grains were predominantly [0001] oriented as verified by electron backscattering diffraction (EBSD). The oxidation rate is dependent on the crystal’s orientation, polarity, stress, and surface condition, thus oxidation decorates grain boundaries, polishing scratches, and inversion domains by producing oxide layers of different thicknesses. Low temperature (800 °C) dry oxidation produced an amorphous oxide layer and generated a high density of defects (vacancies, stacking faults, and dislocations) in the nitride near the oxide/nitride interface, as observed by cross-sectional transmission electron microscopy. In contrast, high temperature oxidation (1000 °C) produced a crystalline oxide layer, and left the nitride free of observable defects. INTRODUCTION Despite the progress achieved in the performance of group III nitride devices, there remains tremendous potential for improving device efficiency, power, and lifetime and for creating entirely new devices by employing native GaN and AlN substrates. The advantages of these substrates include better lattice constant and coefficient of thermal expansion match, reduced defect densities and background impurity concentrations, and the ability to support epitaxy on any crystals plane, including nonpolar planes [1,2]. Currently, these advantages remain largely unrealized due to the difficulties associated with the bulk crystal growth of GaN and AlN; the extremely high nitrogen vapor pressure of GaN, and the high thermal stability of AlN. Bulk AlN crystals are particularly attractive for supporting Al-rich AlxGa1-xN epitaxial layers for ultraviolet emitters and detectors [1]. Steady progress has been reported over the past ten years in producing bulk AlN crystals via the sublimation-recombination technique. Durable materials (tungsten and tantalum carbide) have been identified that are capable of withstanding high temperatures necessary for crystal growth [3-5]. The dependence of the growth rate on process parameters (temperature, temperature gradient, and pressure) has been modeled theoretically and verified experimentally [6]. To realize the full benefit of AlN substrates, their defect densities must be as low as possible, so the epitaxial layers they support can have similarly low defect densities. Mymrin et al [2]
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predicts an order of magnitude improvement in the internal quantum efficiency of UV LEDs by reducing the defect densities from a value of 1010 cm-2 to 108 cm-2. The lowest defect density reported for the best quality bulk AlN crystal has been less than 103 cm-2 [7]. In the present work, the oxidation of AlN crystals is explored as a simple method of producing a visual map of defects in bulk AlN crystals. An oxidation reaction is usually the first step involved with selective defect etching, and its rate is enhanced in the vicinity of dislocations [8]. Furthermore, the oxidation rate depends on the crystal orientation and polarity, impurity concentration, and stress. The initial oxidation rate of nitrogen polar (0001) AlN is approximately 25% faster than on aluminum polar crystals [9]. Thus, with oxidation conditions appropriate for producing oxide films with thicknesses that produce destructive interference, the oxide will have different colors that correspond to the properties of the individual AlN grains. This technique can provide complementary information to x-ray topography, defect selective etching, and transmission electron microscopy.
EXPERIMENTAL DETAILS The AlN crystals studied in this report were grown by the sublimation recombination technique in a high temperature tungsten furnace within a tungsten crucible and retort. The source was aluminum nitride powder initially containing 1 wt% oxygen and 0.06 wt% carbon as the main impurities. The source temperature was 2000 °C, and the crystal growth zone was approximately 20 °C lower, as estimated from previously measured axial temperature profiles performed without the retort or crucible in place. The closed system was pressurized at room temperature to 760 torr with pure nitrogen; the pressure increased to ~900 torr as the furnace was heated to the growth temperature. The polycystal was randomly nucleated on the polycrystalline tungsten lid. The polycrystal was produced in two runs, the first 39 hours in length, and the second 48 hours in length. The AlN powder source was replenished between runs. The polycrystalline boule was cut into nominally 1 mm thick wafers using a diamond wire saw. The wafers were subsequently double side polished. Two wafers, one near the beginning of growth, and the second near the end of growth, were studied by microdiffraction, so changes in the microstructure as a function of length along the boule could be monitored. Microdiffraction was performed by electron backscattering diffraction, for mapping the orientation of the individual grains, and for statistical analysis of the grain size. The EBSD data was collected on a Philips XL-30 FEG SEM with the accelerating voltage set at 20kV and an electron beam diameter of 5 nm. This corresponds to a beam current of about 2.5nA. The AlN was oxidized under flowing dry oxygen at atmospheric pressure. For oxidation at 1000 °C for 6 hours, oxides that are approximately 280 nm and 340 nm are produced on the Al- and N-polarity (0001) surfaces respectively [9]. A high resolution field emission analytic TEM JEOL 2010F, at 200kV and 0.24 nm point resolution at the center for Microanalysis of materials (CMM), University of Illinois, UrbanaChampaign, IL, was used for HRTEM study. All the image analysis was performed using a GATAN digital micrograph software.
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RESULTS AND DISCUSSION A typical polycrystalline AlN boule produced as described in the experimental section is shown in Figure 1(a). This boule was 6 mm thick, so the crystal growth rate was approximately 70 µm/h. A wafer cut from such a boule is shown in Figure 1(b). The wafer was transparent with a uniform amber color. The side of the wafer facing the source material was more difficult to polish, and its final surface was rougher than the other side. Previously, we determine that the (0001) surfaces of spontaneously nucleated crystals facing the source material were primarily Alpolar [10]. Thus, the Al-polar surface is more difficult to polish than the N-polar surface. This is similar to the case of GaN, for which the group III polar side is also more difficult to polish than the N-polar side [11,12].
Figure 1 (a) Polycrystalline AlN boule, 25 mm in diameter, and 6 mm thick grown at 2000 °C (b) A polished slice from this boule. Cracks are evident, apparently due to the mismatch of the coefficient of thermal expansion of the AlN and the tungsten. An orientation map from wafer 2, positioned approximately 2 mm from the base of the boule is shown in Figure 2. Even though no seed crystal was employed to orient the growth, the majority of grains were (0001) oriented. Furthermore, 99.6% of the grains were oriented with their [0001] axis deviating at an angle less than 15° from the perpendicular to the surface. The number of grains with other orientations is negligible. For this slice, the area average grain size based on a criterion of an in-plane rotational misorientation around the c-axis of 5° between grains was 930 square microns. For a slice approximately 2 mm closer to the top of the boule, the average area grain size was 987 square microns. This confirmed the visual appearance that the grains increase in size as the boule grew larger. This was probably due to grain expansion and to crystal growth at slightly higher temperatures, as the crystal surface grew into warmer regions of furnace.
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Figure 2 Electron backscattering diffraction orientation map of the AlN crystal with a 15° tolerance. The total area of the crystal in this image is 7.4 mm by 6.7 mm. The majority of the grains are oriented with their c-axis nearly perpendicular to the surface. An AlN polycrystalline wafer is shown before and after oxidation in Figure 3(a) and 3(b) respectively. Before oxidation, individual grains can not be distinguished; after oxidation, the individual grains are clearly evident and the grain boundaries become much easier to identify. At higher magnification, decoration of the remnant polishing scratches is also apparent. The oxidized AlN resembles image produced by microdiffraction. From comparison with the microdiffraction results, it appears that the abrupt changes in color correspond to grain boundaries with tilt orientation changes greater than 10 °. The more gradual changes in color within the clearly decorated grains boundaries correspond to grain boundaries that are less than 5°. Color changes within the individual grains may be due to variations in strain.
(a) (b) Figure 3 Optical micrograph of a polycrystalline AlN sample (a) before oxidation and (b) after oxidation. Oxides produced by the thermal oxidation of single crystals of AlN at two temperatures are shown in cross-sectional TEM images in Figure 4 [13]. The oxidation of AlN at 800 °C
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proceeds by first producing a disordered layer of AlN containing a high concentration of oxygen. Thus, low temperature oxidation can be employed to selectively introduce defects into AlN. An amorphous aluminum oxide is produced on the top surface of the AlN. In contrast, the oxidation of AlN at 1000 °C produces an abrupt oxide/nitride interface, with very few defects in the underlying AlN. At this temperature, two crystalline oxides are produced: a mixture of gamma and alpha Al2O3 is formed on the top of an alpha Al2O3 layer. The AlN surface was (0001) and the epitaxial relationship between Al2O3 and AlN was (0001) AlN // (01 1 1 ) Al2O3 and [1 1 00] AlN // [01 1 1] Al2O3.
(a)
(b)
Figure 4 Cross-sectional TEM of thermal oxides on AlN single crystals. (a) The oxide/nitride interface region produced by oxidation at 800 °C. (b) The oxide/nitride interface region produced by oxidation at 1000 °C.
CONCLUSIONS We demonstrate that the oxidation of AlN can be employed to delineate defects in AlN. The oxidation of AlN polycrystalline wafers highly textured with a (0001) orientation produced a mosaic of colors corresponding to different grain orientations. Grain boundaries and remnant polishing scratches became readily apparent after oxidation. The structure of the oxide and the effects of oxidation on the nitride are dependent on the oxidation conditions employed. A low oxidation temperature produces an amorphous oxide and disorders the nitride, while a high oxidation temperature produces a crystalline oxide.
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ACKNOWLEDGEMENTS This work was support from grants from the National Science Foundation (DMR-0408874) and the Office of Naval Research (N00014-02-1-0290). The TEM work was carried out in the Center for Microanalysis of Materials, University of Illinois, which is partially supported by the U.S. Department of Energy under grant DEFG02-91-ER45439.
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