Using low-temperature cathodoluminescence spectroscopy, we study the properties of N- and Al-polar AlN layers grown by molecular beam epitaxy on bulk AlN{0001}. Compared with the bulk AlN substrate, layers of both polarities feature a suppression of deep-level luminescence, a total absence of the prevalent donor with an exciton binding energy of 28 meV, and a much increased intensity of the emission from free excitons. The dominant donor in these layers is characterized by an associated exciton binding energy of 13 meV. The observation of excited exciton states up to the exciton continuum allows us to directly extract the Γ5 free exciton binding energy of 57 meV.
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Heteroepitaxy of N-polar AlN on C-face 4H-SiC: Structural and optical properties
To date, it has remained challenging to achieve N-polar AlN, which is of great importance for high power, high frequency, and high temperature electronics, acoustic resonators and filters, ultraviolet (UV) optoelectronics, and integrated photonics. Here, we performed a detailed study of the molecular beam epitaxy and characterization of N-polar AlN on C-face 4H-SiC substrates. The N-polar AlN films grown under optimized conditions exhibit an atomically smooth surface and strong excitonic emission in the deep UV with luminescence efficiency exceeding 50% at room temperature. Detailed scanning transmission electron microscopy (STEM) studies suggest that most dislocations are terminated/annihilated within ∼200 nm AlN grown directly on the SiC substrate due to the relatively small (1%) lattice mismatch between AlN and SiC. The strain distribution of AlN is further analyzed by STEM and micro-Raman spectroscopy, and its impact on the temperature-dependent deep UV emission is elucidated.
more » « less- Award ID(s):
- 2026484
- NSF-PAR ID:
- 10494103
- Publisher / Repository:
- American Institute of Physics
- Date Published:
- Journal Name:
- APL Materials
- Volume:
- 11
- Issue:
- 12
- ISSN:
- 2166-532X
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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N-polar AlGaN is an emerging wide-bandgap semiconductor for next-generation high electron mobility transistors and ultraviolet light emitting diodes and lasers. Here, we demonstrate the growth and characterization of high-quality N-polar AlGaN films on C-face 4H-silicon carbide (SiC) substrates by molecular beam epitaxy. On optimization of the growth conditions, N-polar AlGaN films exhibit a crack free, atomically smooth surface (rms roughness ∼ 0.9 nm), and high crystal quality with low density of defects and dislocations. The N-polar crystallographic orientation of the epitaxially grown AlGaN film is unambiguously confirmed by wet chemical etching. We demonstrate precise compositional tunability of the N-polar AlGaN films over a wide range of Al content and a high internal quantum efficiency ∼74% for the 65% Al content AlGaN film at room temperature. Furthermore, controllable silicon (Si) doping in high Al content (65%) N-polar AlGaN films has been demonstrated with the highest mobility value ∼65 cm2/V-s observed corresponding to an electron concentration of 1.1 × 1017 cm−3, whereas a relatively high mobility value of 18 cm2/V-s is sustained for an electron concentration of 3.2 × 1019 cm−3, with an exceptionally low resistivity value of 0.009 Ω·cm. The polarity-controlled epitaxy of AlGaN on SiC presents a viable approach for achieving high-quality N-polar III-nitride semiconductors that can be harnessed for a wide range of emerging electronic and optoelectronic device applications.
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BAlN films were grown by flow-rate modulation epitaxy on AlN. Figure 1 shows x-ray diffraction (XRD) peaks of 3-µm AlN/(0001) sapphire template layer and 45-nm BAlN layer at 2θ angles of 36.146o and 36.481o, corresponding to c-lattice constants of 4.966 and 4.922Å, respectively. The BAlN XRD peak is very clear and distinct given the small thickness, indicating good wurtzite crystallinity. It is not possible to directly calculate the B content from XRD alone because of uncertainty of the lattice parameters and strain. However, based on the angular separation of the XRD peaks and c-lattice constant difference, the B content is estimated to be ~7% [ ], which is considerably higher than those of high-quality wurtzite BAlN layers reported before [ , , ]. To obtain the accurate B content, Rutherford backscattering spectrometry (RBS) measurements are being made. Figures 2(a)-(b) show a high-resolution cross-sectional transmission electron microscopy (TEM) image with a magnification of 150 kx taken at a-zone axis ([11-20] projection) and diffraction pattern after fast-Fourier transform (FFT). A sharp interface between the AlN and BAlN layers is observed. In addition, the BAlN film exhibits a highly ordered lattice throughout the entire 45nm thickness without the polycrystalline columnar structures found in previous reports [1, ]. The FFT image confirms a wurtzite structure oriented along c-axis. Figure 3 shows a 5×5 µm2 atomic force microscopy (AFM) image of BAlN layer surface. The root-mean-square (RMS) surface roughness is ~1.7nm. Surface macro-steps were found on the surface due to longer diffusion length of group-III atoms than the expected step terrace width. This indicates there is potential to lower the growth temperature to create smoother surfaces while maintaining crystallinity which has been observed for AlN [ ]. In summary, a high-quality wurtzite BAlN layer with relatively high B content ~7% was demonstrated by MOCVD. Refractive index will be measured to facilitate design of distributed Bragg reflector (DBR) for deep UV vertical-cavity surface-emitting laser (VCSEL).more » « less
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Secondary‐ion mass spectrometry (SIMS) is used to determine impurity concentrations of carbon and oxygen in two scandium‐containing nitride semiconductor multilayer heterostructures: Sc
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A top-illuminated deep-ultraviolet Al0.6Ga0.4N p-i-n avalanche photodiode (APD) structure was designed and grown by metalorganic chemical vapor deposition on an AlN bulk substrate and on two different quality AlN/sapphire templates, and APDs were fabricated and tested. The APD devices with a circular diameter of 20 μm have demonstrated a distinctive reverse-bias breakdown behavior. The reverse breakdown voltage of the APDs is approximately −140 V, which corresponds to a breakdown electric field of 6–6.2 MV/cm for the Al0.6Ga0.4N material as estimated by Silvaco TCAD simulation. The APDs grown on the AlN bulk substrate show the lowest leakage current density of <1 × 10−8 A/cm2(at low reverse bias) compared to that of the devices grown on the AlN templates. From the photocurrent measurement, a maximum gain (current limited) of 1.2 × 104is calculated. The average temperature coefficients of the breakdown voltage are negative for APD devices fabricated from both the AlN bulk substrate and the AlN templates, but these data show that the coefficient is the least negative for the APD devices grown on the low-dislocation-density AlN bulk substrate.
