Controlling the polarity of polar semiconductors on nonpolar substrates offers a wealth of device concepts in the form of heteropolar junctions. A key to realize such structures is an appropriate buffer-layer design that, in the past, has been developed by empiricism. Understanding the basic processes that mediate polarity, however, is still an unsolved problem. We present results on the structure of buffer layers for group-III nitrides on sapphire by transmission electron microscopy. We show that it is the conversion of the sapphire surface into a rhombohedral aluminum-oxynitride layer that converts the initial N-polar surface to Al polarity. With the various AlxOyNz phases of the pseudobinary Al2O3-AlN system and their tolerance against intrinsic defects, typical for oxides, a smooth transition between the octahedrally coordinated Al in the sapphire and the tetrahedrally coordinated Al in AlN becomes feasible. Based on these results, we discuss the consequences for achieving either polarity and shed light on widely applied concepts in the field of group-III nitrides like nitridation and low-temperature buffer layers.
Defect incorporation in AlGaN is dependent on the defect formation energy and hence on associated chemical potentials and the Fermi level. For example, the formation energy of CN in Al/GaN varies as chemical potential difference (µN- µC) and -EF (Fermi level). Here, we demonstrate a systematic point defect control by employing the defect formation energy as tool by (a) chemical potential control and (b) Fermi level control. Chemical potential control (µN and µC) with a case study of C in MOCVD GaN is reported. We derive a relationship between growth parameters, metal supersaturation (i.e. input and equilibrium partial pressures) and chemical potentials of III/N and impurity atoms demonstrating successful quantitative predictions of C incorporation as a function of growth conditions in GaN. Hence growth environment necessary for minimal C incorporation within any specified constraints may be determined and C is shown to be controlled from >1E19cm-3 to ~1E15 cm-3. Fermi level control based point defect reduction is demonstrated by modifying the Fermi level describing the probability of the defect level being occupied/unoccupied i.e. defect quasi Fermi level (DQFL). The DQFL is modified by introducing excess minority carriers (by above bandgap illumination). A predictable (and significant) reduction in compensating point defects (CN, H, VN) in (Si, Mg) doped AlGaN measured by electrical measurements, photoluminescence and secondary ion mass spectroscopy (SIMS) provides experimental corroboration. Further, experiments with varying steady state minority carrier densities at constant illumination prove the role of minority carriers and DQFL in defect reduction over other influences of illumination that are kept constant.
This contribution will present the structural and optoelectronic properties of GaN/AlGaN heterostructures
grown by Metal Organic Chemical Vapor Deposition (MOCVD) on GaN/sapphire templates. The target
parameters for the materials heterostructures have been modeled for utilization in Avalanche Photodiode
Detector Structures (APD) operating in the near and deep UV region. Optical modeling has improved
absorption within the heterojunction as well as maximized light trapping within the device. Electronic
modeling has determined the optimal dopant concentrations for maximum impact ionization rate, as well as
tolerance to defects and unintentional doping. This application will require advances in the defect densities,
surface morphology, and interfaces. Surface morphological and structural properties of GaN/AlGaN
heterostructures are analyzed by Atomic Force Microscopy, Raman spectroscopy, and X-ray diffraction. The
optoelectronic properties (phonon structures, free carrier concentrations, and carrier mobility) as well as layer
thickness information, are determined by Fourier Transform Infrared Reflectance spectroscopy. A correlation
of interfacial defects (type and concentration) with microscopic structural properties, surface morphology,
and optoelectronic properties (free carrier concentration and high-frequency dielectric function) is discussed.
The maximum achievable reflectivity of current III-nitride Bragg reflectors in the UV-C spectral range is limited due to plastic relaxation of thick multilayer structures. Cracking due to a large mismatch of the thermal expansion and lattice constants between Al<sub>x</sub>Ga<sub>1-x</sub>N/Al<sub>y</sub>Ga<sub>1-y</sub>N alloys of different composition and the substrate at the heterointerface is the common failure mode. Strain engineering and strain relaxation concepts by the growth on a strain reduced Al<sub>0.85</sub>Ga<sub>0.15</sub>N template and the implementation of low temperature interlayers is demonstrated. A significant enhancement of the maximum reflectivity above 97% at a resonance wavelength of 270 nm due to an increase of the critical thickness of our AlN/Al<sub>0.65</sub>Ga<sub>0.35</sub>N DBRs to 1.45 μm (25.5 pairs) prove their potential. By comparing the growth of identical Bragg reflectors on different pseudo-templates, the accumulated mismatch strain energy in the DBR, not the dislocation density provided by the template/substrate, was identified to limit the critical thickness. To further enhance the reflectivity low temperature interlays were implemented into the DBR to partially relief the misfit strain. Relaxation is enabled by the nucleation of small surface domains facilitating misfit dislocation injection and glide. Detailed structural and optical investigations will be conducted to prove the influence of the LT-AlN interlayers on the strain state, structural integrity and reflectivity properties. Coherent growth and no structural and optical degradation of the Bragg mirror properties was observed proving the fully applicability of the relaxation concept to fabricate thick high reflectivity DBR and vertical cavity laser structures.
A point defect control scheme is demonstrated, to control point defects during the growth of doped wide bandgap
semiconductors. First the theoretical description of this new concept is presented, second GaN:Mg is used as a model
system and as an experimental example to show its feasibility. It can be shown that above bandgap UV-light illumination
during the growth can reduce the passivation and compensation of Mg acceptors in GaN:Mg. The amount of hydrogen
impurities, that usually passivates Mg at doping concentrations around Mg:2x10<sup>19</sup> cm<sup>-3, </sup>is significantly reduced by UVillumination. The resistivity of samples grown with UV is similar to the resistivity of post-growth annealed samples. No post growth annealing was needed. In contrast samples that are doped below Mg:<1x10<sup>18</sup> cm<sup>-3 </sup>become n-type conductive when the samples are grown with UV illumination. This observation suggests a reduced incorporation of Mg acceptors due to the UV light. At low Mg doping concentrations the native donor incorporation by O donors dominates the conductivity over Mg acceptors. UV-illumination therefore reduces compensation of donors by Mg acceptors. Thus, these observations support the concept of UV illumination as a way to control the Fermi level of different charged point defects to control compensation in doped semiconductors.
The growth, fabrication, and properties of GaN/AlN/sapphire with periodically poled surface polarity for
second harmonic generation are investigated. The periodic inversion of the surface polarity is achieved by the
growth of a thin AlN buffer layer and subsequent partial removal by using either wet etching with potassium
hydroxide (KOH) or reactive-ion etching (RIE). GaN growth on these substrates by MOCVD leads to Gapolar
GaN on the AlN buffer and N-polar GaN on the bare sapphire. Using atomic force microscopy and
scanning electron microscopy, it is demonstrated that a sufficient combination of H<sub>2</sub> and NH<sub>3</sub> surface
treatment before the growth of the GaN layers removes surface defects introduced by RIE etching. Thus,
films with comparable quality and properties independent of the etching technique could be grown. However,
in contrast to RIE etching, the interfaces between the Ga-polar and N-polar GaN is rough if KOH etching is
applied. Thus, it is concluded that MOCVD in combination with RIE etched AlN/sapphire substrates can be a
versatile process to fabricate GaN with periodically poled surface polarity as desired for UV light generation
via frequency doubling.
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.