Doping of high-Al-content AlGaN grown by MOCVD (2014)

The high-Al-content AlxGa1-xN, x &gt; 0.70, is the principal wide-band-gap alloy system to enable the development of light-emitting diodes operating at the short wavelengths in the deep-ultraviolet, λ &lt; 280 nm. The development of the deep-ultraviolet light-emitting diodes (DUV LEDs) is driven by the social and market impact expected from their implementation in portable units for water disinfection and based on the damaging effect of the deep-ultraviolet radiation on the DNA of various microorganisms. Internationally, intense research and technology developments occur in the past few years, yet, the external quantum efficiency of the DUV LEDs is typically below 1%. One of the main material issues in the development of the DUV LEDs is the achievement of n- and ptype doped layers of high-Al-content AlxGa1-xN with low resistivity, which is required for the electrical pumping of the diodes. The doping process, however, becomes significantly more complex with increasing the Al content and the resistivity value can be as high as 101-102 Ω cm for n-type AlN doped by silicon, and 107-108 Ω cm for p-type AlN doped by magnesium. The present study is therefore focused on gaining a better understanding of the constraints in the doping process of the high-Al-content AlxGa1-xN alloys, involving mainly the silicon dopant. For this purpose, the epitaxial growth of the high-Al-content AlxGa1-xN and AlN by the implementation of the distinct hot-wall MOCVD is developed in order to achieve layers of good structural and morphological properties, and with low content of residual impurities, particularly oxygen and carbon. Substitutional point defects such as ON and CN may have a profound impact on the doping by their involvement in effects of n-type carrier compensation. The process temperature can be set from 1000 °C and up to 1400 °C in the present study, which is a principal advantage in order to optimize the material properties of the high-Al-content AlxGa1-xN and AlN. The epitaxial growth of the high-Alcontent AlxGa1-xN and AlN is largely performed on 4H-SiC substrates motivated by (i) the lattice mismatch of ~ 1% along the basal plane (the smallest among other available substrates including Si and sapphire), (ii) the good thermal conductivity of 3.7 W cm-1 K-1, which is essential to minimize the self-heating during the operation of any light-emitting diode, and (iii) the limited access to true-bulk AlN wafers. The Si doping is investigated over a large range of [Si] ~ 1×1017 cm-3 - 1×1020 cm-3. Only the high doping range of [Mg] ~ (1-3)×1019 cm-3 is targeted motivated by the large thermal ionization energy of this common acceptor (from 200 meV in GaN to about 630 meV in AlN). The material characterization involves extensive implementation of atomic force microscopy (AFM), x-ray diffraction (XRD), cathodoluminescence (CL), secondary ion mass spectrometry (SIMS), capacitancevoltage measurements, as well as measurements of the conductivity of the layers by contactless microwave-based technique. The possibility to perform electron paramagnetic resonance (EPR) measurements on the Si-doped high-Al-content AlxGa1-xN is essential in order to establish any effect of self-compensation of the shallow donor state of silicon through the related so-called DX state. The EPR measurements corroborate the study of the incorporation kinetics of silicon and oxygen at various process temperatures and growth rates. The outcome of this study is accordingly summarized and presents our understanding for (i) the complex impact of silicon and oxygen on the n-type conductivity of Al0.77Ga0.23N, which is the alloy composition at which a drastic reduction of the n-type conductivity of high-Al-content AlxGa1-xN is commonly reported (paper 1); (ii) the strain and morphology compliance during the intentional doping by silicon and magnesium, and its correlation with the resistivity in the highly doped layers of Al0.82Ga0.18N alloy composition (paper2); (iii) the n-type conductivity of highly-Si-doped Al0.72Ga0.28N layers as bound by the process temperature (paper 3); and (iv) the shallow donor or DX behavior of the Si dopant in conductive AlxGa1-xN layers, 0.63 ≤ x ≤ 1 (paper 4). It is noted that the measured n-type conductivity in reference layers of Al0.77Ga0.23N, alternatively Al0.72Ga0.28N, alloy composition is on par with the state-of-the-art values, i.e. ≤ 0.05 Ω cm, and 0.012 Ω cm, respectively. A room-temperature resistivity of 7 kΩ cm is measured in Mg-doped layers of Al0.85Ga0.15N alloy composition, which is superior to the state-of-art values (paper 5). The performance of the transport properties of the high-Al-content AlxGa1-xN layers is expected to improve with improvement of their material quality. This can be achieved by improvement of the crystalline quality of the AlN-on-SiC template and by the implementation of true-bulk AlN substrates. The AlN heteroepitaxial growth at the process temperatures of 1100-1200 °C is therefore investigated (paper 6). The lattice constants, structural and optical properties of true-bulk, homoepitaxial and heteroepitaxial AlN material grown at high process temperatures of up to 1400 °C is further reported (paper 7).