Studies on Carbon as Alternative P-Type Dopant for Gallium Nitride
- PDF / 70,507 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 72 Downloads / 139 Views
Downloaded from https:/www.cambridge.org/core. Columbia University Libraries, on 27 Jun 2017 at 16:16:02, subject to the Cambridge Core terms of use, available at https:/www.cambridge.org/core/terms. https://doi.org/10.1557/S1092578300002994
method and predict an ionization energy of 60 meV for the beryllium acceptor in wurtzite GaN [6]. Group IV elements can form acceptors or donors in GaN depending on whether they are located on the nitrogen or gallium sublattice, respectively [7]. As it is known from GaAs, this amphoteric behaviour critically depends on the growth conditions [8]. Especially carbon has received a considerable interest as a possible p-type dopant in GaN. It should preferentially occupy nitrogen sites due to its similarity to nitrogen in terms of atomic radius and electronegativity. Moreover, it has proved to be a highly efficient acceptor in GaAs featuring a shallow acceptor level and high solubility as well as a low diffusion coefficient in this material. P-type conductivity of carbon doped GaN was observed for cubic material [9] whereas little is known for this dopant in wurtzite GaN. Theoretical considerations predict an excellent solubility of this element in GaN [1, 10] and no segregation was found in layers grown by molecular beam epitaxy (MBE) doped with a concentration of 2 x 1020 cm-3 [11]. Furthermore, the lack of d-electrons in carbon are assumed to result in a shallower acceptor level compared to magnesium [12]. In this paper, a study on the behaviour of carbon in wurtzite GaN is presented with the intention to check if this element can be an alternative p-type dopant to magnesium. Experiment GaN films were synthesized in an EPI 930 MBE system equipped with an EPI Unibulb radio frequency (rf) nitrogen plasma source. All layers were grown with a V/III flux ratio close to unity. Layers with thickness of about 1 µm were deposited on sapphire (0001) after a nitridation step and the growth of 40 nm undoped GaN which should guarantee similar nucleation conditions for all layers under investigation. The growth rate and growth temperature were 640 nm/h and 750oC, respectively. Hydrogen passivation of acceptors is not expected to occur since annealing experiments of magnesium doped samples did not change the electrical properties of these films. Carbon doping was realized by use of a resistively heated graphite filament. The density of the incorporated carbon in GaN was calculated via the growth and C doping of GaAs layers, assuming a carbon sticking coefficient of unity for both materials and an acceptor ionization ratio of unity at RT for GaAs. This results in maximum nominal carbon concentrations of about 2 x 1020 cm-3 in the GaN samples under investigation, limited by the maximum heating power of approx. 550 W for the carbon filament. In the following, an unintended background carbon concentration in nominally undoped GaN layers is neglected. Free carrier concentrations as well as mobility’s were measured by Hall-effect in van der Pauw geometry assuming a Hall factor of unity. Photoluminesc
Data Loading...
