Since 2011 NaMLab is belonging to a small but elite group of bulk‑GaN growers. It is cooperating onsite with Freiberger Compound Materials (FCM) in Freiberg and owns a modified and optimized state-of-the-art vertical HVPE reactor. The main focus of the research and development is the growth of several millimeters thick GaN crystals (Fig. 1 and 2). So far, crack-free 2“ crystals can be reliably grown. Besides scaling, the optimization of the lattice properties, e.g. the lattice bow, and the electrical properties, which range between n-type and semi-insulating, is of major scientific interest.
The latter is achieved by intentional doping, i.e. to enhance the n-type character or to suppress the unintentional doping (“UID” with charge carrier concentrations lower than 5 x 1016 cm-3), that is inevitably present after growth. Here, the vertical HVPE reactor has four different possibilities for introducing a variety of dopants ranging from solid-state dopants to various gaseous precursors which have been investigated thoroughly in the last years.
To achieve a distinct n-type conduction, mainly silicon (Si) and germanium (Ge) were used. A gaseous dopant is utilized for Si, while a solid state method can also be applied for Ge.Here, solid germanium is introduced to the reactor and an in-situ chemical reaction is used to form a gaseous chemical compound. This involved growing n-type GaN:Ge crystals, which was confirmed by SIMS and room-temperature Hall measurements. A closer investigation of the dopant incorporation reveals an improved lateral distribution of the dopant along the crystal surface, with the doping concentration oscillating perpendicular to the investigated growth direction.
The introduction of a bubbler setup also allowed Ge incorporation via GeCl4 which provides a improved control on the doping concentration. With this, higher charge carrier concentrations in the range of 2 x 1018 cm-3, compared to Si with around 8 x 1017 cm-3, could be achieved. In both cases, the distribution of the charge carrier concentration perpendicular to the crystal surface is very uniform. The distribution parallel to the surface is more inhomogeneous for Ge-doped than for Si-doped GaN (Fig 3). In the future the uniformity improvements for Ge doping will be of particular interest since higher charge carrier concentrations can be achieved for this dopant.
Other doping methods like doping with metal organic precursors or soluble solids in a liquid gallium source to achieve semi-insulating GaN:Fe via Cp2Fe (ferrocene) or GaN:Mn via solid Mn have been established and investigated.
The doped crystals are also monitored with respect to their lattice and mechanical properties in order to investigate the influence of the dopants.
Freiberger Compound Materials GmbH, Freiberg (Germany)