Electron accumulation and doping in InN and InGaN alloys
Linhart, W. M. (2012) Electron accumulation and doping in InN and InGaN alloys. PhD thesis, University of Warwick.
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InN and group III nitride materials have attracted great interest due to their potential applications for
optoelectronic devices, as the range of band gaps cover the ultra-violet to the near infrared. InN and
all In-rich InxGa1−xN alloys exhibit a surface electron accumulation layer. This is due to the unusually
low conduction band minimum (CBM) at the Brouillon zone centre (Γ-point) with respect to the charge-
neutrality level. Electron accumulation has been observed at the surface of almost all n-type and p-type
InN, making proof of p-type doping of this material very difficult. Routine characterization of p-type
conductivity of Mg-doped samples using single-field Hall effect is prevented by the presence of a surface
inversion space-charge layer, and hence the surface electron-rich region dominates the measurements. In
this thesis, the results of investigations on non-polar InN surfaces, Mg-doped InN surfaces and a range of
InxGa1−xN alloys across the composition entire range are presented.
Considerable improvement of the quality of a- and m-plane InN thin films has been achieved
using free standing GaN substrates in conjunction with a GaN buffer layer and grown by PAMBE. Using a
combination of infrared reflectivity (FTIR), x-ray photoemission spectroscopy (XPS) and electrochemical
capacitance voltage (ECV) measurements, the surface space charge properties of these samples have been
investigated. The surface Fermi level has been determined to be lower than previously observed on non-
cleaved InN samples. Additionally a high carrier concentration has been found on the non-polar InN, close
to the interface with the GaN buffer layer, associated with unintentionally incorporated oxygen impurities.
The increased concentration of oxygen impurities near the InN/GaN interface, confirmed by secondary ion
mass spectrometry (SIMS), is due to the relatively low growth temperature (380 - 450 ◦C) used to produce
the non-polar InN films.
XPS has been also used in the investigations of Mg-doped InN. A significant lowering of the
surface Fermi level has been observed with increasing Mg-doping for the highest Mg concentration (>
1 × 1019 cm−3) indicating a highly desirable reduction in the degree of surface electron accumulation.
While for moderate Mg concentrations the surface Fermi level is at the previously determined ‘universal’
value of ~ 1.4 eV above the valence band maximum, for [Mg]=1.2×1020 cm−3, a value of 0.83 eV is found.
As a consequence, for [Mg]> 1 × 1019 cm−3 the donor surface state density increases while the surface
electron density decreases enormously, resulting in a transition from electron accumulation to almost just
hole depletion layer. This reduction of electron accumulation in high Mg-doped InN can be improved by
additional surface treatment, therefore results of a series of sulfur treated Mg-doped InN sample are also
reported in this thesis.
Finally, the electronic properties of InxGa1−xN alloys with a composition range of 0.20 >= x >= 1.00
have been investigated, using XPS and FTIR. The transition from electron accumulation to electron
depletion has been observed at a composition of x = 0.20, while for x >= 0.20 an increasing electron
accumulation with decreasing Ga fraction has been observed.
|Item Type:||Thesis or Dissertation (PhD)|
|Subjects:||Q Science > QC Physics|
|Library of Congress Subject Headings (LCSH):||Nitrides -- Electric properties, Nitrides -- Surfaces, Gallium nitride, Semiconductors|
|Official Date:||May 2012|
|Institution:||University of Warwick|
|Theses Department:||Department of Physics|
|Supervisor(s)/Advisor:||McConville, Christopher Francis, 1959|
|Extent:||xviii, 136 leaves : ill., charts|
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