Suk Chung

Direct observation of basal-plane to threading-edge dislocation conversion in 4H-SiC epitaxy

The propagation behavior of basal plane dislocations from off-oriented 4H-SiC substrates into homoepitaxial layers has been investigated using transmission electron microscopy (TEM), secondary electron microscopy (SEM), and chemical etching. Cross-sectional TEM shows that basal plane dislocations in the substrate are dissociated into pairs of partial dislocations separated by a stacking fault with a width of about 40 nm. Near the substrate/epilayer interface, where most of the basal plane dislocations convert to threading edge dislocations, the two partials constrict before converting. Threading edge segments are inclined by about 20° from the c-axis toward the down-step direction. It is concluded that the critical and limiting step of the dislocation conversion process is constriction of the dissociated partials. Growth surface morphology at the emergence point of the basal plane dislocation was imaged using SEM and is thought to play an important role in the constriction.

Determination of the Inelastic Mean-Free-Path and Mean Inner Potential for AlAs and GaAs Using Off-Axis Electron Holography and Convergent Beam Electron Diffraction

The mean-free-paths for inelastic scattering of high-energy electrons (200 keV) for AlAs and GaAs have been determined based on a comparison of thicknesses as measured by electron holography and convergent-beam electron diffraction. The measured values are 77 +/- 4 nm and 67 +/- 4 nm for AlAs and GaAs, respectively. Using these values, the mean inner potentials of AlAs and GaAs were then determined, from a total of 15 separate experimental measurements, to be 12.1 +/- 0.7 V and 14.0 +/- 0.6 V, respectively. These latter measurements show good agreement with recent theoretical calculations within experimental error.

Electronic structure analysis of threading screw dislocations in 4H–SiC using electron holography

Off-axis electron holography has been used to map the electrostatic potential distribution of threading screw dislocations in differently n-doped 4H–SiC epitaxial layers. Observed phase contrast indicated the presence of a negatively charged dislocation core. Comparison between experimental and simulated potential profiles indicated that the density of trapped charges increased for a higher doped epilayer. Assuming a single level of the trap at the core, the ionization energy of the trap was calculated to be 0.89±0.22 eV.

Off-axis electron holographic potential mapping across AlGaAs/AlAs/GaAs heterostructures

The electrostatic potential profile across AlGaAs/AlAs/GaAs heterostructures containing 1-μm-thick n-doped (or p-doped) AlGaAs layers is measured using off-axis electron holography. Simulations of the potential profiles assuming no unintentional impurities in the undoped regions of the samples show small discrepancies with experiment. Revised simulations reproduce the measurements accurately, when a p-layer with an 8.4×1011 cm−2 acceptor density is included at the buffer/substrate interface to simulate the presence of unintentional carbon impurities.

Quantitative Analysis of Dopant Distribution and Activation Across p-n Junctions in AlGaAs/GaAs Light-Emitting Diodes Using Off-Axis Electron Holography

Off-axis electron holography has been used to measure the electrostatic potential profile across the p-n junction of an AlGaAs/GaAs light-emitting diode with linearly graded triangular AlGaAs barriers. Simulations of the junction profile showed small discrepancies with experiment when the nominal dopant concentrations of Si and Be impurities were used. Revised simulations reproduced the measurements reasonably using reduced dopant levels that reflected the efficiency of dopant activation. Band-edge diagrams simulated with the nominal and revised dopant concentrations were also compared in terms of the effect that activation efficiency had on the AlGaAs barrier shape and carrier transport. It is concluded that electron holography measurements combined with modeling offer device designers and growers a helpful tool for analyzing and confirming doping profiles in complex heterostructures.

Secondary electron dopant contrast imaging of compound semiconductor junctions

Secondary electron imaging combined with immersion lens and through-the-lens detection has been used to analyze various semiconductor junctions. Dopant contrast imaging was applied for multi-doped 4H–SiC, growth-interrupted n+/p and n/n+ homoepitaxial interfaces, and an AlGaAs/GaAs p-n junction light-emitting diode structure. Dopant contrast was explained by the local variation in secondary electron escape energies resulting from the built-in potential difference. The effect of varying electron affinity on contrast for the heterostructures is also discussed. The contrast profile of the n-doped AlGaAs compared reasonably well to the simulated valence bandedge energy using a previously determined efficiency of dopant ionization.

Quantitative dopant profiling of p-n junction in InGaAs/AlGaAs light-emitting diode using off-axis electron holography

The electrostatic potential profile across the p-n junction of an InGaAs light-emitting diode with linearly graded AlGaAs triangular barriers has been measured using off-axis electron holography. Simulations of the junction profile show small discrepancies with experimental measurements in the region of the p-and n-doped AlGaAs barriers, which are located away from the InGaAs quantum wells. Revised simulations reproduce the measurements reasonably when a carrier-trap density of 6×1016cm−3 in the AlGaAs barriers is subtracted from the dopant concentrations. The presence of oxygen impurities is considered as the most likely reason for the reduction in doping efficiency.