Germanium doping of cubic GaN grown by molecular beam epitaxy

Abstract

We present a study of germanium as an alternative to silicon for n-type doping of cubic GaN. We find that Ge is a well-suited donor impurity. Our layers were grown by plasma-assisted molecular beam epitaxy on 3C-SiC/Si (001) substrates. Germanium-doped layers were fabricated with donor concentrations ranging over several orders of magnitude up to 3.7 × 10 20 cm −3 . For comparison, silicon-doped layers with donor concentrations of up to 3.8 × 10 19 cm −3 were also grown. Incorporation of germanium into the cubic GaN layers was verified by time-of-flight secondary ion mass spectrometry. The crystalline quality of our layers was analyzed using high-resolution x-ray diffraction. Germanium-as well as silicon-doped layers with donor concentrations above 10 19 cm −3 exhibited an increase of the dislocation density with increasing dopant concentration. The surface topography of our layers was investigated by atomic force microscopy. Comparable values for the surface roughness were measured for germanium-as well as silicon-doped layers. Optical properties were investigated by photoluminescence spectroscopy at 13 K. Doping with silicon resulted in a spectrally slightly narrower luminescence than doping with germanium. Donor concentrations and carrier mobilities were determined by Hall effect measurements at room temperature and we observe 20% higher electron mobilities for Ge-doping compared to Si-doping in the case of high dopant concentrations.Published under license by AIP Publishing. https://doi.

Molecular Beam Epitaxy Growth and Characterization of Germanium‐Doped Cubic AlxGa1−xN

“…From these relations, a donor concentration of N Ge = 4 × 10 18 cm −3 is estimated for the Ge‐doped ( T Ge = 800 °C) c‐GaN sample, which is consistent with the actual Ge concentration N Ge = 6.0 × 10 18 cm −3 determined by TOF‐SIMS …”

“…The dislocation density does not rise for Ge cell temperatures up to 800 °C, which corresponds to a donor concentration of 1.5 × 10 19 cm −3 . Therefore, considering the structural degradation, the maximum reasonably achievable donor concentration in c‐Al 0.25 Ga 0.75 N is approximately one order of magnitude lower than in c‐GaN …”

“…Growing the metastable cubic zinc blende phase is one way to overcome this effect as these fields are not present here . The most common donor for cubic GaN (c‐GaN) is Si, but recently, we have introduced Ge as an alternative n‐type dopant for c‐GaN …”

Photoluminescence Line‐Shape Analysis of Highly n‐Type Doped Zincblende GaN

“…The zb‐GaN layers were deposited on a 3C‐SiC/Si (001) substrate by plasma‐assisted molecular beam epitaxy. The samples were doped by either Ge or Si, respectively . One unintentionally doped sample (u.i.d.)…”

“…Very essential for possible applications is the understanding of the n‐type doped material. To achieve free‐carrier concentrations exceeding 10 20 cm −3 , germanium was recently introduced as a donor instead of the standard donor silicon . However, for these free‐carrier concentrations, the Fermi energy is pushed high into the conduction band and both the nonparabolicity of the conduction band and many‐body interactions, such as band filling and band‐gap renormalization (BGR) become increasingly important and must be considered.…”