Surface Chemistry of Prototypical Bulk II-VI and III-V Semiconductors and Implications for Chemical Sensing.

In the presence of an electric field, the dielectric constant of a semiconductor exhibits Franz–Keldysh oscillations (FKO), which can be detected by modulated reflectance. Although it could be a powerful and simple method to study the electric fields/charge distributions in various semiconductor structures, in the past it has proven to be more complex. This is due to nonuniform fields and impurity induced broadening, which reduce the number of detectible Franz–Keldysh oscillations, and introduce uncertainties into the measurement. In 1989, a new structure, surface–undoped–doped (s‐i‐n+/s‐i‐p+) was developed, which allows the observation of a large number of FKOs and, hence, permitting accurate determination of electric fields. We present a review of the work on measuring electric fields in semiconductors with a particular emphasis on microstructures using the specialized layer sequence. We first discuss the general theory of modulation techniques dwelling on the approximations and their relevance. The cas...

Franz–Keldysh oscillations in modulation spectroscopy

The α-phase Bi(2)O(3) (α-Bi(2)O(3)) is a crucial and potential visiblelight photocatalyst material needless of intentional doping on accommodating band gap. The understanding on fundamental optical property of α-Bi(2)O(3) is important for its extended applications. In this study, bismuth oxide nanowires with diameters from tens to hundreds nm have been grown by vapor transport method driven with vapor-liquid-solid mechanism on Si substrate. High-resolution transmission electron microscopy and Raman measurement confirm α phase of monoclinic structure for the as-grown nanowires. The axial direction for the as-grown nanowires was along . The band-edge structure of α-Bi(2)O(3) has been probed experimentally by thermoreflectance (TR) spectroscopy. The direct band gap was determined accurately to be 2.91 eV at 300 K. Temperaturedependent TR measurements of 30-300 K were carried out to evaluate temperature-energy shift and line-width broadening effect for the band edge of α-Bi(2)O(3) thin-film nanowires. Photoluminescence (PL) experiments at 30 and 300 K were carried out to identify band-edge emission as well as defect luminescence for the α-Bi(2)O(3) nanowires. On the basis of experimental analyses of TR and PL, optical characteristics of direct band edge of α-Bi(2)O(3) nanowires have thus been realized.

The study of optical band edge property of bismuth oxide nanowires α-Bi2O3.

Pseudomorphic Ga1−xInxN/GaN single heterostructures in the composition range 0<x<0.2 have been investigated by photoreflectance and photoluminescence spectroscopy. Strong Franz–Keldysh oscillations near the band gap of the ternary film are observed and attributed to a large constant piezoelectric field of up to 0.63 MV/cm. This allows an accurate determination of the electric field. A significant redshift between the optical band gap from photoreflectance and the luminescence maximum is observed. Luminescence is proposed to originate in the indirect transitions between the electric field tilted band edges in GaInN. The presence of this field is expected to dominate the bandstructure and the recombination and transport processes in strained nitride structures. We find no evidence for large inhomogeneities or phase separation in this material.

https://www.rpi.edu/~wetzel/Preprints/APLWetzelPiezoFKOJ.Appl.Phys.85(7),3786-91(1999).pdf

Piezoelectric Franz-Keldysh effect in strained GaInN/GaN heterostructures

Recently bendable and flexible optoelectronics have attracted more attentions on display technology and concaved or curvy photovoltaic devices owing to the flexibility and softy in layered materials. The bending photoluminescence (BPL) and surface photovoltaic effect of a thin multilayer InSe 2D crystal are demonstrated here by using PL and surface photovoltage (SPV) experiments. The BPL result of InSe (t ≈ 30 nm) shows an enhancement in light intensity with respect to that of the other flat PL measurement owing to the spreading of emission solid angle for the irradiant Se-In-In-Se basic units under concaved upward condition. The band edge structure of multilayer InSe is analyzed. The SPV measurement of an In mesh-coated InSe microplate demonstrates that the Schottky-type solar cell converts sunlight starting from ≈1 eV with the auxiliary of surface states on InSe. An initially formed In-InSe Schottky solar cell (surface area of ≈2 mm2 and thickness of ≈85 μm) is also tested. The testing result shows a photovoltage of V ≈ 24 mV can be generated from the Schottky solar cell under the illumination of Hg lamp with power density of P ≈ 0.83 mW cm−2. All the experimental results demonstrate flexible light-emission capability and “1 eV” photoelectric conversion capacity existed in the 2D multilayer e-InSe.

Bending Photoluminescence and Surface Photovoltaic Effect on Multilayer InSe 2D Microplate Crystals

Modulation spectroscopy is a powerful method for the study and characterization of a large number of semiconductor configurations, including bulk/thin film, microstructures (heterojunctions, quantum wells, superlattices, quantum dots), surfaces/interfaces and actual device structures in addition to semiconductor growth/processing. Furthermore, the influence of external perturbations such as temperature, electric fields, hydrostatic pressure, uniaxial stress, etc. can be investigated. This optical technique utilizes a very general principle of experimental physics, in which a periodically applied perturbation (either to the sample or probe) leads to sharp, derivative-like spectral features in the optical response of the system. Because of the richness of the derivative-like spectra, the information in the lineshape fits, room temperature performance and relative simplicity of operation this method is becoming increasingly more important as a tool to study these materials and structures. This article will review developments in the field during the last decade.

H. Shen

Papers

Modulation spectroscopy of semiconductors: bulk/thin film, microstructures, surfaces/interfaces and devices