Doping of high-Al-content AlGaN grown by MOCVD

Abstract

The high-Al-content Al x Ga 1-x N, x > 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 < 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/surface 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 high-Al-content Al x Ga 1-x N layers with low resistivity, which is required for the electrical pumping of the diodes. The doping process becomes significantly more complex with increasing the Al content and the electrical resistivity is as high as 10 1 -10 2 Ω . cm for n-type AlN doped by silicon, and 10 7 -10 8 Ω cm for p-type AlN doped by magnesium.The present study was therefore focused on gaining a better understanding of the constraints in the doping process of the high-Al-content Al x Ga 1-x N alloys, involving mainly silicon. For this purpose, the epitaxial growth of the high-Al-content Al x Ga 1-x N and AlN by implementing the distinct hot-wall MOCVD was 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 O N and C N may have a profound impact on the doping by their involvement in effects of ntype carrier compensation. The process temperature was varied between 1000 and up to 1400 °C, which is a principal advantage in order to optimize the material properties of the high-Al-content Al x Ga 1-x N and AlN. The epitaxial growth of the high-Al-content Al x Ga 1-x N and AlN was 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 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 Al 0.77 Ga 0.23 N, representative for the alloy composition at which a drastic reduction of the n-type conductivity of Al x Ga 1-x N is commonly reported (paper 1); (ii) the strain and morphology compliance during the intentional iv doping by silicon and magnesium, and its correlation with the resistivity of highly doped layers of Al 0.82 Ga 0.18 N alloy composition (paper2); (iii) the n-type conductivity of highly Si-doped Al 0.72 Ga 0.28 N as bound by the process temperature (p...

Status of Growth of Group III-Nitride Heterostructures for Deep Ultraviolet Light-Emitting Diodes

“…The effective activation energy was determined to be 29 meV in Mg-doped AlxGa1-xN/AlN superlattices with an average Al composition of 0.62, and 18 meV with an average Al composition of Kakanakova-Georgieva et al estimated a hole concentration of~10 14 cm −3 for Mg-doped Al 0.85 Ga 0.15 N layers, from the a room temperature resistivity of 7 kΩ cm by assuming a hole mobility of 2 cm 2 V −1 s −1 , when grown using a hot wall MOCVD system which purportedly leads to growth conditions for reduced compensation by native defects [108]. The same authors also reported a resistivity of 2 kΩ cm and 60 Ω cm for Al 0.70 Ga 0.30 N and Al 0.60 Ga 0.40 N, respectively, by using the hot wall MOCVD reactor with optimized V/III ratio [109]. If the same hole mobility of 2 cm 2 V −1 s −1 is assumed, the hole concentrations in the Al 0.…”

Status of Growth of Group III-Nitride Heterostructures for Deep Ultraviolet Light-Emitting Diodes

“…[100] The same authors also reported a resistivity of 2 kΩ cm and 60 Ω cm for Al0.70Ga0.30N and Al0.60Ga0.40N, respectively, by using the hot wall MOCVD reactor with optimized V/III ratio. [101] If the same hole mobility of 2 cm 2 V -1 s -1 is assumed, the hole concentrations in the Al0.70Ga0.30N and Al0.60Ga0.40N layers are deduced to be ~3.5×10 14 cm -3 and ~1×10 16 cm -3 , respectively. Through the optimization of growth conditions, highly conductive p-type Mg-doped Al0.70Ga0.30N thin films were realized by employing a high V/III ratio and moderate Mg concentration by Kinoshita et al [102] Such conditions were found to effectively suppress self-compensation by the formation of nitrogen vacancy complexes, which are considered to be the compensating donors.…”