Verlag des Forschungszentrums Jülich
JUEL-3781
Lantier, Roberta
GaN/Al(Ga)n Heterostructures: MBE Growth, Electronic Properties and Polarization Fields
150 S., 2000
The demand of higher and higher storage density in digital data processing applications
lead in the last decade to an increased interest in the development of injection lasers
operating at short wavelengths. The group III-nitrides has revealed to be the most successfull
class of semiconductors for optoelectronic applications for wavelengths from 200 to
650 nm. Beside this, the nitrides are moving first promising steps also in the field of
radiofrequency devices: AlGaN/GaN HEMTs grown on seminsulating SiC show very high
power density at frequencies up to 50 GHz and an excellent thermal and chemical stability.
The aim ofthis work was to achieve experimentally the knowledge ofthe energy band
scheme along several nitride heterostructures and the surfaces electronic properties, thus
providing a set of basic parameters needed for the design of heterostructure devices.
The first step for the realization of GaN/Al(Ga)N heterostructures and for the determination
of their band scheme was the optimization of the GaN and AlGaN deposition by
means of MBE on 6H-SiC substrates. To this phase belonged the preparation of the substrate,
a systematic change of the growth parameters and a consequent control of structural,
optical and electrical properties by means of different experimental techniques (XRD, PL,
AFM, Hall).
The determination of the band offset at various nitride interfaces, including the ones to the
SiC substrate was obtained in situ by means of monochromatized XPS, after the heterojunction
was grown by MBE. The systematic analysis of the overlayer core level position
in dependence on the deposited thickness showed the presence of electric fields of the
order of a MV /cm in the thin (1 - 6 nm) nitride layers. These high intemal fields arise
from the high polarization ( spontaneous and piezoelectric) of the nitrides, because of their
hexagonal symmetry. The interplay of surface states, as evidenced by adsorption of activated
hydrogen, was responsible of a partial reduction of the field intensity with respect
to the predicted value. Because of the internal fields, the measured values of the valence
band offsets (VBO) strongly depend on the thickness and the heterojunction VBO was
extracted through extrapolation at zero thickness. By means of this systematic study a set
of VBOs determined by XPS is provided, which for the first time takes into account the
presence of the high polarization fields.
The measured valence band offsets, as well as the observed surface level pinning in thick
layers were used for modeling heterostructures, where the polarization fields were utilized
as design parameters. The first type consists of about 30 nm AlGaN on GaN(0001). In
this case the discontinuity of the polarizations of the two materials induces a positive fixed
charge at the interface, which attracts electrons with the formation of a high density 2-
dimensional electron gas (ns ~ 1013cm-2) , without modulation doping, also favoured
also by the relatively high band discontinuity. The presence of such a high conductive
channel was confirmed by temperature dependent Hall measurements on some optimized
GaN/ AlO.25Ga0.75N heterostructures and represents the first step for a further controlled
development of optimized HEMT devices. As second application of the polarization fields
as design parameters were Al(Ga)N/GaN superlattices prepared by MBE. By an appropriate
choice of the GaN well and of the strained AlN barrier thickness we could control
the fields in the triangular quantum well and consequently the energy of the dominant
emission in the PL spectra.
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