Analysis Of Franz-Keldysh Oscillations Research Articles

Photoreflectance spectroscopy of delta-doped GaAs layers

Delta-doped GaAs layers have been studied by photoreflectance spectroscopy. The built-in electric fields and interband transition energies in the semiconductor structure have been evaluated from analysis of Franz-Keldysh oscillations. The delta-doped region is found to have an increased interband transition energy, which is interpreted in terms of the Burstein-Moss effect and carrier photogeneration.

Electroreflectance investigations of quantum confined Stark effect in GaN quantum wells

In this paper we present room temperature electroreflectance studies of GaN quantum wells (QWs) with different well width. The electroreflectance measurements were performed with external voltage applied to the structure therefore it was possible to tune the electric field inside QW up to its completely screening and furthermore even reversing it. The analysis of QW spectral lines showed the Stark shift dependence on applied voltage and well width reaching about 35 meV for highest voltage and widest well width. It was possible to obtain the condition of zero electric field in QW. Both broadening and amplitude of QW lines are minimal for zero electric field and increases for increasing electric field in QW. The energy transition is maximum for zero electric field and for increasing electric field it decreases due to Stark effect. Neither amplitude and broadening parameter nor energy transition does not depend on the direction of electric field. Only parameter that depends on the direction of electric field in QW is phase of the signal. The analysis of Franz-Keldysh oscillations (FKOs) from AlGaN barriers allowed to calculate the real electric field dependence on applied voltage and therefore to obtain the Stark shift dependence on electric field. The Stark shift reached from −12 meV to −35 meV for 450 kV/cm depending on the well width. This conditions were established for highest forward voltages therefore this is the value of electric field and Stark shift caused only by the intrinsic polarization of nitrides.

Optical investigation of an AlGaN/GaN interface with the presence of a two-dimensional electron gas

In this paper we presented the study of an AlGaN/GaN heterostructure with the presence of a two-dimensional electron gas (2DEG) by using X-ray reciprocal space mapping (RSM), reflectance (R), photoluminescence (PL), electroreflectance (ER) and photoconductivity (PC) measurements. Some important properties such as alloy composition, strain status and layer thickness of Al x Ga 1− x N layer and carrier concentration of 2DEG are determined. The structure strain was analyzed by asymmetric RSM. We observed that the AlGaN layer grown on GaN layer was fully strained. The band gap energies of GaN and Al x Ga 1− x N are identified by PL and R spectrum, from which the Al composition x is determined to be 0.13. Using a semitransparent gate, an external dc bias was applied to control the carrier concentration of 2DEG underneath the Al x Ga 1− x N layer. We have also performed the ER and PC measurements at various external dc biases. In the ER spectra, not only the energy gap transitions of GaN and Al x Ga 1− x N but also the 2DEG feature and the Franz–Keldysh oscillations (FKOs) were observed. Through the analysis of FKOs, the strength of surface electric field and the 2DEG concentration can be determined. The optical absorption phenomenon in the AlGaN/GaN interface observed by the bias-dependent PC spectroscopy are presented and discussed.

Contactless electroreflectance approach to study the Fermi level position in GaInNAs/GaAs quantum wells

A fruitful approach to study the Fermi level position in GaInNAs/GaAs quantum wells (QWs) has been proposed in this paper. This approach utilizes contactless electroreflectance (CER) spectroscopy and a very simple design of semiconductor structures. The idea of this design is to insert a GaInNAs quantum well (QW) into a region of undoped GaAs layer grown on n-type GaAs substrate. The possible pinning of the Fermi level in the GaInNAs QW region modifies band bending in this system. In CER spectra both QW transitions and GaAs-related Franz-Keldysh oscillations (FKOs) are clearly observed. The analysis of QW transitions allows one to determine the band gap discontinuity at GaInNAs/GaAs interface whereas the analysis of FKOs allows one to determine the built-in electric field in the GaAs cap layer, and, finally, one is able to find the Fermi level pinning in GaInNAs QW region.

Optical study of the AlGaN/GaN high electron mobility transistor structures

We present an optical study on three Al x Ga 1-x N/GaN high electron mobility transistor structures, which were grown by metalorganic chemical vapor deposition (MOCVD) on sapphire substrates, with different Al compositions using contactless electroreflectance (CER), piezoreflectance (PzR) and surface photovoltage spectroscopy (SPS). The Al composition x can be determined from the band gap energy of Al x Ga 1-x N layer observed in CER and PzR spectra. The determination of the built-in electric field strength in the barrier layer (F) has been done by the analysis of Franz-Keldysh oscillations (FKOs) above the Al x Ga 1-x N band gap in CER spectra. A broad feature appeared at energies above the band gap of GaN was observed in each CER, PzR and SPS spectrum. We believe that it is related to the two-dimensional electron gas (2DEG). The energy of the broad 2DEG minimum roughly corresponds to the Fermi energy plus the band gap. The Hall-effect measurements have also been carried out in van der Pauw geometry at room temperature to reveal the sheet carrier density (n s ). For the sample with lowest alloy composition (x = 0.07) the analysis of the FKOs yields F= 316 kV/cm at room temperature. When the temperature was cooled down to 50 K the electric field strength decreases to 188 kV/cm. This result can be explained by the change of piezoelectric polarization.

Piezoelectric Field Influence on GaN/Al x Ga 1—x N Quantum Well Optical Properties

The absorption and luminescence properties of hetero-polarization GaN/AlxGa1—xN (x = 0.12 and 0.165) quantum well (QW) structures are studied by photoreflectivity, photoluminescence excitation spectroscopy (PLE), and photoluminescence at low temperature. The QW transition energy as a function of well thickness exhibits a quantum-confined Stark effect (QCSE) due to the presence of a strong built-in electric field (piezoelectricity and spontaneous polarization). An electric field strength of 120 kV/cm in the barrier and between 600 and 800 kV/cm in the well are obtained from the analysis of Franz-Keldysh oscillations in photoreflectivity spectra. These values are in good agreement with results from the fit of the QW transition energy versus the thickness, using the electric field as a parameter.