Scanning electron microscopy as a flexible technique for investigating the properties of UV-emitting nitride semiconductor thin films Download: 806次
1. INTRODUCTION
The scanning electron microscope (SEM) is a very powerful tool for investigating and imaging a wide range of material properties spanning topography, structure, composition, and light emission [1–
In this paper we describe the SEM techniques of EBSD, ECCI, WDX, and CL hyperspectral imaging and illustrate the capability (and also the shortcomings) of each technique. To this end we present our recent results on the use of these non-destructive techniques to obtain information on the topography, crystal misorientation, defect distributions, composition, doping, and light emission from a range of UV-emitting nitride semiconductor structures. We also give examples where combining the techniques provided useful complementary information.
In comparison to their visible cousins, UV LEDs based on nitride semiconductor thin films exhibit poor electro-optical properties with external quantum efficiencies typically no more than 10% for wavelengths less than 350 nm [18]. Their ultimate performance is presently limited by the structural quality of AlN and AlGaN thin films, which limits the achievable internal quantum efficiency, and by low doping efficiencies, low carrier injection efficiencies, and poor light extraction [19]. Key to improving the performance of UV LEDs, and the main motivation for the research described in this paper, is (1) the understanding and control of extended defects, such as grain boundaries, threading dislocations, partial dislocations, and stacking faults and their influence on light emission; (2) the control of doping; and (3) control of the alloy composition of AlGaN.
To carry out our measurements we use a range of SEMs equipped with both commercial and bespoke detection systems. In the work reported here we have used an FEI Sirion 200 Schottky field emission gun SEM (Sirion SEM) equipped with an in-house developed ECCI system. We have also used an FEI Quanta 250 Schottky field emission gun environmental/variable pressure SEM (Quanta SEM) equipped with an Oxford Instruments Nordlys EBSD detector and forescatter diodes for EBSD and ECCI measurements, respectively, and an in-house developed CL hyperspectral imaging system. The CL system utilizes a Schwarzschild-type reflecting objective to collect the emission from a sample inclined at 45°, allowing the collection of light with wavelengths
2. CHARACTERIZATION TECHNIQUES IN THE SCANNING ELECTRON MICROSCOPE
2.1 A. Structural Characterization: Electron Backscatter Diffraction and Electron Channeling Contrast Imaging
The scanning electron microscopy techniques of EBSD and ECCI exploit diffraction of backscattered electrons or diffraction of the incident electron beam, respectively, to provide information on the structural properties of materials rapidly and non-destructively with a spatial resolution of tens of nanometers from large areas of the surface of a sample (of order
ECCI micrographs may be produced when a sample is placed so that a plane or planes are at, or close to, the Bragg angle with respect to the incident electron beam. Any deviation in crystallographic orientation or in lattice constant due to local strain will produce a variation in contrast in the resultant ECCI micrograph. The micrograph is constructed by monitoring the intensity of backscattered or forescattered electrons using an electron-sensitive diode as the electron beam is scanned over the sample. Extremely small changes in orientation and strain are detectable, revealing, for example, low angle tilt and rotation boundaries and atomic steps, and enabling extended defects, such as dislocations and stacking faults to be imaged [6,8
2.2 B. Determining Composition and Doping Concentration: Wavelength Dispersive X-Ray
When a high-energy electron beam strikes a material, it can result in the ejection of an inner shell electron. This hole can then be filled through relaxation of an electron from a higher energetic state. The energy lost by the electron can result in the emission of a characteristic X-ray. The X-ray energies are specific to the atomic structure of an element and its energy levels [29]. The detection of these X-rays therefore allows elemental identification, which is widely used for compositional analysis of materials [2].
In WDX the X-rays emitted from the material are dispersed to different angles depending on their energy using a diffracting crystal. The detector, generally a gas proportional counter or scintillation counter, only detects X-rays of one energy and the angle of diffracting crystal has to be changed to record a whole X-ray spectrum, similar to a monochromator for light detection. Quantitative measurements of the chemical composition of identified elements are generally carried out by WDX in an EPMA, which is similar to an SEM, but a dedicated machine with numerous WDX spectrometers containing different diffracting crystals in order to cover a range of X-ray energies. For quantitative analysis the results of the sample under investigation have to be compared against standards of known composition, and matrix corrections need to be applied taking additional effects (e.g., material density, absorption, energy loss) into account that could lead to errors in the results [1]. The spatial resolution mainly depends on the size of the excitation volume, which is defined by the electron beam energy. While a spatial resolution of the order of a few hundred nanometers is achievable, there is a trade-off between the highest spatial resolution and the necessary, minimum beam energy for excitation of X-rays [30].
2.3 C. Light Emission: Cathodoluminescence Hyperspectral Imaging
The absorption of energetic electrons in a semiconductor results in the generation of excess charge carriers, and the radiative recombination of these carriers results in the phenomenon of CL [16]. The material’s intrinsic luminescence properties are influenced by crystal structure, composition, and strain, while additional bands are introduced by defects. While comparable techniques (photoluminescence, electroluminescence) also reveal such information, the higher spatial resolution of CL allows further data to be obtained, such as the mapping of individual extended defects which produce dark spots due to the presence of non-radiative recombination [31,32]. The spatial resolution of CL imaging is strongly dependent on the excitation volume, the diffusion length of the material, and the structure under investigation. If the material under investigation contains structures which can localize the carriers, such as quantum wells or defects, the spatial resolution can be of the order of 10 nm [33,34]. Extending the technique beyond simple intensity imaging and into the hyperspectral imaging mode allows the technique to be used to map energy shifts and peak widths, and to deconvolve overlapping spectral peaks [35]. The spectral resolution is defined by the spectrometer used, the ruling of the grating, and the slit width. For the results presented in this paper, the spectral resolution was typically better than 2 nm. Moreover, CL is not limited to the visible spectrum, but can be used out into the deep UV, and a particular advantage of the technique when working with UV materials, such as AlN (room temperature bandgap
3. RESULTS
3.1 A. ECCI of a c -Plane Thin Film
Figure
Techniques have also been developed to identify dislocation types [11,24,40]. Nitride semiconductors contain three types of TDs, namely screw-, edge-, and mixed-type dislocations. To identify the TD type, it is possible to apply the “invisibility criteria” used in transmission electron microscopy (TEM) [11]. In simple terms, dislocations are invisible in an ECCI or TEM micrograph if they do not distort the plane which diffracts the incident electron beam. The invisibility criteria are satisfied for screw dislocations where
For this AlGaN/GaN thin film the average TD density was determined to be
3.2 B. ECCI and EBSD Mapping of a c -plane AlN Thin Film Overgrown on a Nanopatterned Sapphire Substrate
Figure
Fig. 2. (a) SE image of nPSS, (b) schematic of overgrowth of AlN on nPSS, and (c) ECCI micrograph from an AlN thin film. Inset is on the same scale but with higher resolution.
The motivation for the growth of AlN on nPSS is to produce high-quality (low dislocation density) AlN/sapphire templates for the manufacture of high-performance UV LEDs. The reduction of dislocation densities from
Figure
The variation in gray scale in the ECCI micrograph of Fig.
As discussed in the previous section, while ECCI reveals misorientations between sub-grains, it does not provide a measure of the magnitudes and directions of the misorientations. To obtain quantitative information, EBSD data were acquired from the same sample at an electron beam energy of 20 keV using the Quanta SEM again in low vacuum mode. Figure
Fig. 3. EBSD maps from the AlN/nPSS thin film: (a) grain reference orientation deviation (GROD) map and (b) GROD axis map relative to the sample normal (c -axis, [0001] direction]) where the colors denote direction of in-plane rotation (i.e., around the c -axis). The red regions are rotated in the opposite direction to the blue regions as indicated.
These results illustrate how ECCI and EBSD can provide complementary structural information. ECCI allows fast determination of dislocation densities and their distribution and reveals the presence of sub-grains; Fig.
3.3 C. CL Imaging and ECCI of a Semi-Polar (11-22) GaN Thin Film Overgrown on GaN Microrods on m -Sapphire
UV LEDs produced from semi-polar nitride semiconductor thin films promise higher performance than those produced from their polar counterparts, due to reduction of piezoelectric and spontaneous polarization fields. Unfortunately, semi-polar nitride semiconductor thin films are often of poor quality with a high density of structural defects; in particular stacking faults, in addition to threading dislocations [50
Fig. 4. (a) Schematic of semi-polar GaN microrod template and overgrowth, indicating the distribution of stacking faults on the surface of the sample and the crystallographic directions. (b) ECCI micrograph revealing stacking faults. (c) Example CL spectra from a dark stripe and a bright stripe, respectively. The boxes on (d) indicate where the spectra were extracted from the CL dataset. (d) Integrated CL intensity image of the GaN near band edge (NBE) emission (3.15–3.50 eV) on the same scale as (e) but not from the same area. (e) Higher resolution ECCI micrograph revealing dislocations. (f) Integrated CL intensity image of the GaN near band edge (NBE) emission (3.15–3.50 eV) on the same scale as (e) but not from the same area.
Figures
The dislocation density in the high-density regions is
In summary, comparison of the ECCI micrographs with the CL images shows that at room temperature, both stacking faults and dislocations lead to a significant reduction in the NBE luminescence intensity due to non-radiative recombination at these defects.
3.4 D. ECCI and CL Hyperspectral Mapping of a c -plane Si-Doped Thin Film Grown on a Stripe Patterned Epitaxially Laterally Overgrown AlN/Sapphire Template
The topography, type and distribution of dislocations, and light emission were investigated for a polar (
Figure
Fig. 5. (a) Schematic of the sample structure. for the top 1.6 μm layer. (b) Atomic force microscopy image of the sample surface. (c) ECCI micrograph (the black brackets indicate “stripes” of higher dislocation density in the coalescence region). (d) Topographic image. (c) CL near band edge (NBE) peak intensity map. (d) NBE CL peak energy map. Images (c) to (f) were acquired from approximately the same region of the sample. The white arrows indicate the apexes of the hillocks. The CL peak intensity and peak energy were extracted from hyperspectral data.
The ECCI micrograph reveals that the patterned template leads to a modulation of the dislocation density with higher dislocation densities around the ELO coalescence boundaries. Analysis reveals an average TD density of
Comparison of the ECCI micrograph of Fig.
Comparing the ECCI micrograph with the CL NBE peak intensity image in Fig.
Comparing Figs.
The origin of the redshift will be the subject of further study, for example, WDX mapping experiments (as described in the next section for a similar sample) could allow changes in the fractions of AlN and GaN to be mapped.
3.5 E. Composition and Doping in AlGaN
WDX analysis allows the straightforward and non-destructive analysis of both major and minor elements in surface layers with thicknesses typically greater than approximately 100 nm [63,64]. Here we describe mapping of the AlN and GaN content across topographic surface features in MOVPE grown AlGaN epilayers, as well as measurement of the concentration of Si-dopants in GaN epilayers, using a comparison with secondary ion mass spectrometry (SIMS) data. A calibration curve method is described, allowing the WDX data to be converted to doping densities in the range
The AlGaN layers were mapped in the JEOL EPMA, using an acceleration voltage of 15 kV and a beam current of 40 nA, by stepping the sample underneath a focused electron beam at normal incidence and using individual spectrometers to measure the intensities of Ga
Fig. 6. WDX maps of the intensities of (a) Ga (left) and (c) Al (right) X-rays, and (b) a backscattered electron image (center) of a micrometer-scale region of a c -plane AlGaN sample, with an average AlN content of 81%. The scale bar for X-ray intensities applies to both WDX maps, although with different absolute values.
A range of Si-doped GaN epilayers grown by MOVPE were investigated, with thicknesses in excess of 500 nm. WDX measurements were performed in the JEOL EPMA, using an acceleration voltage of 10 kV and a beam current of 40 nA for the major elements. The current (400 nA) and counting time are increased considerably for measurement of the trace elements (Si) in order to maximize the statistical accuracy. SIMS data were available from a small subset of these. At this acceleration voltage, 90% of the beam energy is deposited in the first 460 nm of a GaN sample, according to Monte Carlo simulations using the CASINO software [65]. The Ga
Figure
Fig. 7. (a) Semi-log plot showing the measured Si content in the GaN layers, calibrated using the points where SIMS data is available (red data points). (b) Long qualitative scan for Si for the sample with lowest measured Si content , using a TAP crystal showing the WDX Si peak.
Figure
4. SUMMARY AND CONCLUSIONS
In summary, we have illustrated the capabilities of a range of non-destructive SEM techniques that can be used to provide complementary information on material properties encompassing topography, structure, composition, and light emission down to the nanoscale. Recent general availability of environmental/variable pressure SEMs has made the characterization of wide bandgap and therefore resistive materials such as AlN and AlGaN in the SEM far more accessible. We have shown that EBSD and ECCI can provide valuable information on misorientations and on extended defects, such as dislocations and stacking faults. We have also shown that WDX can be used to investigate both composition and doping in nitride semiconductor layers. If ECCI is combined with CL, the influence of extended defects on light emission can be investigated. In conclusion the SEM is a very useful tool to investigate UV-emitting nitride semiconductor thin films. The data associated with all figures in this paper may be accessed [66].
5 Acknowledgment
Acknowledgment. The authors would like to acknowledge financial support of the Engineering and Physical Sciences Research Council, UK via Grant No. EP/J015792/1, “Nanoscale characterisation of nitride semiconductor thin films using EBSD, ECCI, CL and EBIC”; Grant No. EP/M015181/1, “Manufacturing nano-engineered III-nitrides”; and Grant No. EP/P015719/1, “Quantitative non-destructive nanoscale characterisation of advanced materials”. Work at Ferdinand-Braun-Institute and TU Berlin was partially supported by the German “Federal Ministry of Education and Research” (BMBF) within the “Advanced UV for Life” consortium and the “German Research Foundation” (DFG) within the “Collaborative Research Center 787”.
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Article Outline
C. Trager-Cowan, A. Alasmari, W. Avis, J. Bruckbauer, P. R. Edwards, B. Hourahine, S. Kraeusel, G. Kusch, R. Johnston, G. Naresh-Kumar, R. W. Martin, M. Nouf-Allehiani, E. Pascal, L. Spasevski, D. Thomson, S. Vespucci, P. J. Parbrook, M. D. Smith, J. Enslin, F. Mehnke, M. Kneissl, C. Kuhn, T. Wernicke, S. Hagedorn, A. Knauer, V. Kueller, S. Walde, M. Weyers, P.-M. Coulon, P. A. Shields, Y. Zhang, L. Jiu, Y. Gong, R. M. Smith, T. Wang, A. Winkelmann. Scanning electron microscopy as a flexible technique for investigating the properties of UV-emitting nitride semiconductor thin films[J]. Photonics Research, 2019, 7(11): 11000B73.