KEYWORDS: Indium gallium nitride, Infrared radiation, Near field scanning optical microscopy, Dielectrics, Near field optics, Heterojunctions, Infrared imaging, Spectroscopy, Imaging spectroscopy, Super resolution microscopy
Group III-V semiconductor nanostructures have been at the forefront of numerous
applications in high-power, high frequency optical and optoelectronic devices.
Although, significant progress has been made in fabrication and characterization of
these materials, there are still challenges in the formation of compositional uniform
indium-rich ternary epilayers, embedded in wide bandgap III-N’s. For example,
nanoscale lateral compositional inhomogeneities at the growth surface lead to bulk
phase segregations will reduce the structural quality of the semiconductor
heterostructures both in macro and nanometer scales if not controlled through the
process parameter space at the surface. Studying and understanding the fundamental
physical and structural properties at the nanoscale level and correlating the findings
with processing parameters is essential to mitigate compositional fluctuations in
multinary III-N compounds. In this work we introduce infrared scattering type
scanning near-field microscopy (s-SNOM) for spectroscopic study of nanoscale
optical properties of InGaN epilayers on GaN- or InN templates. S-SNOM possesses
spatial resolution of few nanometers (~15 nm) far below the diffraction limit and
allows spectroscopic imaging of simultaneous chemical and structural information
correlated with morphology. We correlate s-SNOM near-field amplitude and phase
optical contrasts at infrared frequencies to the dielectric constants and growth
parameters of InN/InGaN heterostructures and/or single nanoparticles. We observed
that both the real and imaginary dielectric function values of mono-/bi-layers of
InN/InGaN can be extracted from s-SNOM data. By performing nano-spectroscopy
on lithographically patterned samples, we also show that self-assembled InGaN
nanoparticles have similar dielectric function values as that of thin film InGaN.
This contribution presents results on the structural and optoelectronic properties of InN layers grown on AlN/sapphire
(0001) templates by Migration-Enhanced Plasma Assisted Metal Organic Chemical Vapor Deposition (MEPAMOCVD).
The AlN nucleation layer (NL) was varied to assess the physical properties of the InN layers. For ex-situ
analysis of the deposited structures, Raman spectroscopy, Atomic Force Microscopy (AFM), and Fourier Transform
Infrared (FTIR) reflectance spectroscopy have been utilized. The structural and optoelectronic properties are assessed by
Raman-E2 high FWHM values, surface roughness, free carrier concentrations, mobility of the free carriers, and high
frequency dielectric function. This study focus on optimizing the AlN nucleation layer (e.g. temporal precursor
exposure, nitrogen plasma exposure, plasma power and AlN buffer growth temperature) and its effect on the InN layer
properties.
This paper presents optoelectronic and structural layer properties of InN and InGaN epilayers grown on sapphire templates by Migration-Enhanced Plasma Assisted Metal Organic Chemical Vapor Deposition (MEPA-MOCVD). Real-time characterization techniques have been applied during the growth process to gain insight of the plasma-assisted decomposition of the nitrogen precursor and associated growth surface processes. Analyzed Plasma Emission Spectroscopy (PES) and UV Absorption Spectroscopy (UVAS) provide detection and concentrations of plasma generated active species (N*/NH*/NHx*). Various precursors have been used to assess the nitrogen-active fragments that are directed from the hollow cathode plasma tube to the growth surface. The in-situ diagnostics results are supplemented with ex-situ materials structures investigation results of nanoscale structures using Scanning Near-field Optical Microscopy (SNOM). The structural properties have been analyzed by Raman spectroscopy and Fourier transform infrared (FTIR) reflectance. The Optoelectronic and optical properties were extracted by modeling the FTIR reflectance (e.g. free carrier concentration, high frequency dielectric constant, mobility) and optical absorption spectroscopy. The correlation and comparison between the in-situ metrology results with the ex-situ nano-structural and optoelectronic layer properties provides insides into the growth mechanism on how plasma-activated nitrogen-fragments can be utilized as nitrogen precursor for group III-nitride growth. The here assessed growth process parameter focus on the temporal precursor exposure of the growth surface, the reactor pressure, substrate temperature and their effects of the properties of the InN and InGaN epilayers.
The dependency of the structural and optoelectronic properties of InN thin films grown by high-pressure chemical
vapor deposition technique on the group V/III molar precursor ratio has been studied. X-ray diffraction, Raman
spectroscopy, and IR reflectance spectroscopy have been utilized to study local- and long-range structural ordering as
well as optoelectronic properties of the InN epilayers grown on crystalline sapphire substrates. The investigated InN
epilayers were grown with group V/III molar precursor ratio varying from 900 to 3600, while all other growth
parameters were kept constant. For a group V/III precursor ratio of 2400, the full width-half maximum of the Raman
E2(high) mode and XRD (0002) Bragg reflex exhibit minimums of 7.53 cm⁻¹ and 210 arcsec, respectively, with
maximized grain size and reduced in-plane strain effect. FTIR data analysis reveals a growth rate of 120 nm/hr, a carrier
mobility of 1020 cm²V⁻¹s⁻¹, and a free carrier concentration of 1.7×1018 cm⁻³ for a V/III ratio of 2400. The Raman
analysis indicate that non-polar E2(high) mode position remains unaffected from a changing V/III ratio; whereas, polar
A1(LO) mode position significantly changes with changing V/III ratio. Optical analysis also suggests that LO-phonon
correlates with free carrier concentration (ne) and TO-phonon correlates with free carrier mobility (μ) in the InN
epilayers.
The influence of structural and optoelectronic properties of InN epilayers on the duration of initial nucleation
has been studied. High pressure chemical vapor deposition (HPCVD) has been utilized to deposit InN epilayers on
GaN/sapphire (0001) templates at a reactor pressure of 15 bar. The initial nucleation period was varied between 10 s and 60 s, leaving all other growth parameters constant. The structural properties of the grown samples have been investigated by X-ray diffraction (XRD) spectroscopy and Raman spectroscopy. The optoelectronic properties were analyzed by Fourier transform infra-red (FTIR) spectroscopy. The layer thickness, free carrier concentration and void fraction were obtained by simulating IR spectra, using multi-layer stack model for epilayers and Lorentz-Drude model for dielectric function. Raman, X-ray diffraction (XRD) and void fraction calculation results suggest that the optimum nucleation time is between 10 - 20 s. However, simulation results revealed that the free carrier concentration of the bulk layer does not show any significant dependency on the duration of initial nucleation.
Results on the achievable growth temperature as a function of the reactor pressure for the growth of InN by high-pressure CVD are presented. As the reactor pressure was increased from 1 bar to 19 bar, the optimal growth temperature raised from 759°C to 876°C, an increase of 6.6 °C/bar. The InN layers were grown in a horizontal flow channel reactor, using a pulsed precursor injection scheme. The structural and optical properties of the epilayers have been investigated by Raman spectroscopy, X-ray diffraction, and IR reflectance spectroscopy.
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