Growth and Critical Layer Thickness Determination of Indium Gallium Nitride Films Grown on Gallium Nitride
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2003-09-18
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Abstract
Many of the physical properties between strained epilayers and relaxed epilayers can be different. A critical layer thickness (CLT) can be determined by measuring these properties and observing the transition point at which they change. The main purpose of this dissertation is to determine the CLT of In[subscript x]Ga[subscript 1-x]N /GaN material system primarily by determining the onset of photoluminescent band-edge emission red-shift as a function of InxGa1-xN thickness between strained and relaxed InxGa1-xN films. The CLT determination by this method for both In[subscript x]Ga[subscript 1-x]N/GaN single heterostructures and In[subscript x]Ga[subscript 1-x]N/GaN double heterostructures were in good agreement and the CLT for InGaN layers where approximately 25 nm, 40 nm, and 80 nm for InN compositions of 16%, 10%, and 5%, respectively.
In addition, the optical band gaps as a function of In[subscript x]Ga[subscript 1-x]N over 0≤x≤0.25 for strained and relaxed films were determined. Band-gaps deduced from optical transmission measurement techniques were in good agreement with the optical band-gaps determined by PL emission measurements for very thick relaxed In<sub>x</sub>Ga<sub>1-x</sub>N films. The composition for very thin strained In[subscript x]Ga[subscript 1-x]N was assumed to be the same as the composition for the relaxed In[subscript x]Ga[subscript 1-x]N. The band-gap's dependence on InN mole fraction, x, for strained and relaxed In[subscript x]Ga[subscript 1-x]N films was fit to a parabolic function with strained and relaxed film bowing parameters of 2.1259 eV and 2.7503 eV, respectively, using a relaxed InN band-gap endpoint of 0.77 eV (3.4159 eV and 4.112 eV, respectively, using a relaxed InN band-gap endpoint of 1.89 eV). The bowing parameter, especially, the bowing parameter for the relaxed In[subscript x]Ga[subscript 1-x]N film was compositionally dependent (b=f(x)) with a bowing parameter of 3.76 eV and 2.58 eV (using a relaxed InN band-gap endpoint of 0.77 eV) or 5.02 eV and 3.97 eV (using a relaxed InN band-gap endpoint of 1.89 eV) for composition regions 0.05<x<0.12 and 0.12<x<0.25, respectively. This was attributed to compositional inhomogeneity that was found to increase with increased composition. Furthermore, the band gap difference between the strained and relaxed films was also found to increase with composition.
Methods for achieving high quality InxGa1-xN and GaN epilayers by metal organic chemical vapor deposition (MOCVD) and characterizing the properties of these compounds is presented. In addition, metal semiconductor metal (MSM) photodectors were fabricated with back to back Schottky diodes on anIn[subscript x]Ga[subscript 1-x]N/GaN structure. Turn-on wavelengths were found to increase between 370 nm and 430 nm by varying the indium mole fraction in the InxGa1-xN active layer from x=0 to x=0.13. Schottky contacts became increasingly leaky and dark current increased substantially for InxGa1-xN layers exceeding the CLT which is most likely associated with CLT electronic defects such as surface traps due to rough surfaces and recombination via mid-band-gap traps within the In[subscript x]Ga[subscript 1-x]N layer.
For a comparison to determining the CLT in III-nitride compounds, GaP/GaAs, a typical III-V zincblende compound system, was investigated and the CLT was experimentally determined to be within 100 Å and 140 Å with a variety of experimental techniques. All the experimental methods used exceeded the CLT value of 16 Å predicted by the force balance Matthews Blakeslee model. This indicated that the CLT could be exceeded for growth of films at low temperature two-dimensional (2D) atomic layer epitaxy (ALE) growth.
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netal organic chemical vapor deposition, gallium phosphide, photodetectors, bowing parameter, band gap dependence on composition, indium gallium nitride, critical layer thickness
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PhD
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Electrical Engineering
