Brian Fluegel

Measuring long-range carrier diffusion across multiple grains in polycrystalline semiconductors by photoluminescence imaging

Thin-film polycrystalline semiconductors are currently at the forefront of inexpensive large-area solar cell and integrated circuit technologies because of their reduced processing and substrate selection constraints. Understanding the extent to which structural and electronic defects influence carrier transport in these materials is critical to controlling the optoelectronic properties, yet many measurement techniques are only capable of indirectly probing their effects. Here we apply a novel photoluminescence imaging technique to directly observe the low temperature diffusion of photocarriers through and across defect states in polycrystalline CdTe thin films. Our measurements show that an inhomogeneous distribution of localized defect states mediates long-range hole transport across multiple grain boundaries to locations exceeding 10 μm from the point of photogeneration. These results provide new insight into the key role deep trap states have in low temperature carrier transport in polycrystalline CdTe by revealing their propensity to act as networks for hopping conduction.

Spatially resolved photoluminescence in partially ordered GaInP2

Scanning confocal microscopy combined with high-resolution spectroscopy is used to probe the spatial variations in the low-temperature (5.0 K) photoluminescence (PL) of partially ordered GaInP2 with a spatial resolution of 0.7 μm. We observe large regions (1–2 μm) wherein the excitonic PL is suppressed up to a factor of four (“defect-rich” regions) when compared to unaffected areas. These defect-rich regions show a commensurate enhancement in the lower energy below gap emission. The spatial extent of this effect is inconsistent with the picture that the low-energy emission originates solely at the antiphase boundaries of the ordered domains and therefore must originate from other defects within the ordered domain as well.

Direct-Indirect Crossover in GaxIn1-xP Alloys

The energy and composition of the direct to indirect bandgap crossover in GaxIn1-xP significantly influences its potential for optoelectronic devices, such as solar cells and light emitting diodes, however considerable discrepancies still remain in the literature with regard to the precise value of the crossover composition xc. We revisit this issue in GaxIn1-xP films epitaxially grown on GaAs substrates. Observation of concurrent yet distinct direct and indirect transitions in Ga0.719In0.281P at 2 K using time integrated and time resolved photoluminescence studies places the crossover very near the composition xC = 0.71.

Shubnikov-de Haas measurement of electron effective mass in GaAs1−xBix

Magnetic field and temperature dependent resistivity measurements on n-type GaAs1-xBix epitaxially grown films show clear Shubnikov de Haas oscillations in the range 0 ≤ x ≤ 0.0088. An overall decrease in the electron effective mass is observed for this range of compositions. Accounting for the known giant bandgap bowing and giant spin orbit bowing, the measured changes in the effective mass are in qualitative agreement with perturbation theory applied to these energy band changes, confirming that bismuth mainly perturbs the valence band. The stronger compositional dependence of the measured mass is attributed to effects from the bismuth isolated state.

Electronic Raman scattering as an ultra-sensitive probe of strain effects in semiconductors

Semiconductor strain engineering has become a critical feature of high-performance electronics because of the significant device performance enhancements that it enables. These improvements, which emerge from strain-induced modifications to the electronic band structure, necessitate new ultra-sensitive tools to probe the strain in semiconductors. Here, we demonstrate that minute amounts of strain in thin semiconductor epilayers can be measured using electronic Raman scattering. We applied this strain measurement technique to two different semiconductor alloy systems using coherently strained epitaxial thin films specifically designed to produce lattice-mismatch strains as small as 10−4. Comparing our strain sensitivity and signal strength in AlxGa1−xAs with those obtained using the industry-standard technique of phonon Raman scattering, we found that there was a sensitivity improvement of 200-fold and a signal enhancement of 4 × 103, thus obviating key constraints in semiconductor strain metrology.

Ordering Induced Direct-Indirect Transformation in Unstrained GaxIn1−xP for 0.76<=x<=0.78

GaxIn1−xP alloys grown by metalorganic vapor phase epitaxy (MOVPE) are known to exhibit spontaneous long-range ordering that results in a modification of the alloy electronic band structure. Using time resolved and time integrated photoluminescence studies at 9 K, we demonstrate that the change in alloy ordering in GaxIn1−xP alloys can transform the conduction to valence band optical transition from direct to indirect for a given Ga concentration. This finding may enable sequential growth of alternate layers of high bandgap direct and indirect semiconductor alloys with similar lattice constants, opening various possibilities for device applications.

Mysterious absence of pair luminescence in gallium phosphide bismide

Gallium phosphide bismide (GaP1−xBix) epilayers with x up to 1.0% were grown via molecular beam epitaxy and their photoluminescence spectra were investigated at low temperatures. Surprisingly, the emission spectrum of the GaP1−xBix epilayers was fully described by isolated bismuth-bound exciton recombination at the A and B lines (2.232 and 2.229 eV, respectively) together with their phonon replicas, without a need for any description of recombination from bismuth pair or cluster states. These observations contrast with the typical behavior of energy transfer to lower-lying nitrogen pair states in GaP1−yNy at similar impurity concentrations and offer insights into the electronic structure evolution of GaP1−xBix.

Probing carrier lifetimes at dislocations in epitaxial CdTe

Dark line defects arising from dislocations in epitaxial CdTe films are known to strongly limit the overall performance of optoelectronic devices. However, their effect on carrier diffusion length and lifetime in the material immediately surrounding dislocations is not well quantified. We apply a photoluminescence imaging technique to directly measure these parameters in a CdTe/MgCdTe double heterostructure. Radiative recombination is reduced by up to 85% within 5 µm of the dislocation. Additionally, the carrier diffusion length and lifetime decrease by ~50 and ~80%, respectively.

Precise Determination of the Direct–Indirect Band Gap Energy Crossover Composition in AlxGa1-xAs

MBE grown AlxGa1-xAs samples, with x in the range 0.282–0.424, were studied by using time-integrated and time-resolved photoluminescence spectroscopy, photo-modulation reflectance spectroscopy and photoluminescence excitation spectroscopy. The direct and indirect band gap transitions are observed simultaneously in two AlxGa1-xAs samples with x=0.387 and 0.396. An exact determination of the direct–indirect crossover composition at low temperatures is found at a direct band gap energy of 2.059 eV, which corresponds to a aluminum content of [Al]=38.4±0.3%.

Photogenerated plasmons in GaAs1- xBix

Light scattering measurements in the dilute isoelectronically doped alloy GaAs1−xBix reveal a large free electron population photogenerated by continuous-wave laser excitation at low temperature. Low-temperature time-resolved photoluminescence of the bismuth related near-band-gap states show carrier lifetimes of several nanoseconds. The authors attribute this to trapping of photoexcited holes at bismuth pair or cluster states located near the valence band maximum.