The review of electromodulation techniques such as photo- and electroreflectance spectroscopy is presented and illustrated using spectra of AlGaN/GaN heterostructures. By using these techniques it is possible to analyze the Franz-Keldysh oscillations to obtain the value of electric field near the surface or at interface. When additionally a constant voltage is applied to the structure, it is possible to obtain the charge distribution or the gradient of polarization present in the sample, which is of the origin of 2D electron gas at the AlGaN/GaN interface. The polarization is discussed assuming two contributions: spontaneous and piezoelectric polarization. Piezoelectric part of polarization can be obtained from X-ray diffraction, providing information about strain in the structure. Influence of light and voltage on 2D electron gas properties and depletion layer width is demonstrated using photocapacitance-voltage measurements. All results are discussed and compared with self-consistent calculations of potentials and electric fields in the structure.
We present optical and electrical measurements made on GaN/AlGaN photodetector structure capable to detect three UV ranges, tuned by external voltage. The highest band at energy of about 3.85 eV is nearly independent of bias applied to the Schottky contact. Photosensitivity of the second band at about 3.65 eV changes strongly with the bias. Signal in this range increases about 20 times when the bias changes from 0 V to -4 V. Photosensitivity of the third band (3.4 eV) appears for strong reverse bias (-3 V). Characteristics of the detector are in qualitative agreement with numerical model, however deep centers present in the AlGaN layers cause quantitative discrepancies. The concentration of defects of the order of 10^{16} cm^{-3} was estimated from current transients.
Due to its peculiar properties graphene is a good candidate for sensor materials. Therefore, it is important to study influence of different fluids on graphene layer. The presented studies showed pinning of NaCl microcrystals to graphene surface after immersing graphene in NaCl solution and subsequent careful rinsing with distilled water. The atomic force microscopy images revealed presence of many NaCl-related structures over 100 nm high on graphene surface. The electron spin resonance spectrum for magnetic field perpendicular to the graphene layer consisted of several lines originating from NaCl. The pinning of NaCl microcrystals resulted in increase of electron scattering, as confirmed by the Raman spectroscopy (the increase of intensity of D and D' bands) and weak localization measurement (the decrease of coherence length).
We present investigations of GaInN/GaN/AlGaN structure containing cavity designed so that the electric field inside it can be changed by illumination. Numerical calculations show that illumination can change carrier distributions and consequently change the field and potential. The electric field influences properties of a quantum well placed in the cavity. We confirmed experimentally that the electric field controlled by external bias or by optical pumping, can change energy and occupation of electronic states in the quantum well. The quantum well energy could be changed of about 80 meV by voltage and 15 meV by illumination.
GaN/AlGaN photodetector that exhibits new interesting property is presented. Its spectral sensitivity depends upon bias voltage. Under positive or low negative bias the detector is sensitive mainly to the ultrafiolet radiation absorbed by AlGaN layer 3.7-3.8 eV. Under negative bias U_B below -4 V, the detector is sensitive mainly to the radiation absorbed by GaN (3.4-3.6 eV). The effect can be explained based on numerical calculations of the electric field and potential profiles of this structure. The damping of GaN signal is attributed to activity of 2D electron gas formed on the GaN/AlGaN interface by spontaneous polarization. The reappearing of the signal is attributed to tunneling of holes through AlGaN, stimulated by a high electric field.
In this paper using scanning electron microscope, contactless microwave electronic transport and the Raman spectroscopy we studied the properties of graphene deposited on GaN nanowires and compared it with the graphene deposited on GaN epilayer. The Raman micro-mapping showed that nanowires locally change the strain and the concentration of carriers in graphene. Additionally we observed that nanowires increase the intensity of the Raman spectra by more than one order of magnitude.
Optical absorption and Raman scattering studies of few-layer epitaxial graphene obtained by high temperature annealing of carbon terminated face of 4H-SiC(000-1) on-axis substrates are presented. Changing the pressure and annealing time, different stages of the graphene formation were achieved. Optical absorption measurements enabled us to establish average number of graphene layers covering the SiC substrate. Raman scattering experiments showed that integrated intensity of the characteristic 2D peak positively correlated with the number of graphene layers deposited on the SiC substrate. The spectral width of the 2D peak was found to decrease with the number of the deposited graphene layers.
Photocurrent spectroscopy and Kelvin force microscopy have been used in order to determine charge, field, and potential distributions in spontaneously grown superlattice. The spectra show that light can generate currents and potentials in both directions depending on photon energy. A numerical model made for superlattice of periodλ_{SL} = 33 nm shows that electric field in superlattice oscillates coherently with Al content. The oscillations of electric field explain the different directions of photocurrent. The electric field can also separate electrons and holes, making carrier lifetimes longer and lowering excitation intensity threshold for occupation inversion.
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