Pavel Yu. Bokov

Unintentional indium incorporation into barriers of InGaN/GaN multiple quantum wells studied by photoreflectance and photoluminescence excitation spectroscopy

InxGa1–xN/GaN single and multi quantum well (MQW) structures with x ≈ 0.13 were investigated optically by photoreflectance, photoluminescence excitation spectroscopy, and luminescence. Clear evidence of unintentional indium incorporation into the nominal GaN barrier layers is found. The unintentional In content is found to be around 3%. Inhomogeneous distribution of In atoms occurs within the distinct quantum well (QW) layers, which is commonly described as statistical alloy fluctuation and leads to the characteristic S-shape temperature shift of emission energy. Furthermore, differences in emission energy between the first and the other QWs of a MQW stack are found experimentally. This effect is discussed with the help of model calculations and is assigned to differences in the confining potential due to unwanted indium incorporation for the upper QWs.

Impact of AlN Spacer on Metal?Semiconductor?Metal Pt?InAlGaN/GaN Heterostructures for Ultraviolet Detection

We report on metal–semiconductor–metal (MSM) photodetectors (PDs) fabricated on InAlGaN/GaN two-dimensional electron gas (2DEG) heterostructures. Electrical and photodetection properties were compared in two structures with and without an AlN spacer between the barrier (InAlGaN) and the GaN. The presence of the spacer hugely reduces the leakage current, allowing biasing at higher voltages. In photodetection, gain is obtained in both structures at a high bias. The photocurrent transient behavior revealed a faster response for excitation energy close to the GaN band edge than for energy above the barrier band edge. The fabrication and improvement of this type of device can lead to integration with the already mature high-electron-mobility transistor (HEMT) technology.

Linear Electro-Optic Effect in Electroreflectance Spectra of AlGaN/InGaN/GaN Light Emitting Diodes Structures

The linear electro-optic effect in InGaN/AlGaN/GaN pn-heterostructures for light emitting diodes, grown by metal–organic chemical vapor deposition on sapphire substrates and flip-chip mounted, was studied by electroreflectance spectroscopy. Interference fringes, whose parameters depend on the DC voltage applied on the pn-junction, were observed in electroreflectance spectra. Data analysis, based on a calculation of the built-in electric field in the depletion layer and linear electro-optic effect, yielded the linear electro-optic coefficient r13 of 22±6 pm/V for hexagonal In0.12Ga0.88N.

Photoreflectance study of indium segregation in the InGaAs quantum well

The electron-hole states in the molecular beam epitaxy grown GaAs/In0.5Ga0.5As quantum well, placed into space charge region, have been studied by photoreflectance spectroscopy. The energies of electrons and holes have been calculated in the envelope function model including deformation-induced changes in the band structure of quantum well. It is shown that the best accordance between experimental and theoretical data is achieved in the case of band offset Q = ;Ec/;Ev = 0.62/0.38 at GaAs/In0.5Ga0.5As heterojunction. The most intense transition is observed in this case between 1 electron and 1 light hole energy. This fact is connected with the indium segregation in the GaAs/In0.5Ga0.5Asquantum well. In this case one could obtain the flat bands in the quantum well and realize the parity selection rules for the rectangular potential. The model of the nonsymmetrical rectangular potential is applied to describe the energies of electronic levels in the quantum well. The segregation parameters have been calculated from the segregation-induced shift of the energies of interband transitions in the GaAs/In0.5Ga0.5As quantum well.

A Role of the Built-in Piezoelectric Field in InGaN/AlGaN/GaN Multiple Quantum Wells in the Electroferlectance Experiments

The influence of built in piezoelectric field in the light-emitting diodes based on InGaN/AlGaN/GaN heterostructures on the electroreflectance spectra have been studied. The structures were grown by MOCVD technology and «flip-chip» mounted. Light was emitted or reflected through sapphire substrate. The built in electric field in the structure was modulated by pulses of reverse bias from -6 to +1 V applied to the contacts of diode. Observed blue shift of spectral line from InGaN/GaN multiply quantum wells region with the increasing reverse bias voltage has been explained as the result of lowering of the electric field in the quantum well.

Electroreflectance spectra of InGaN/AlGaN/GaN p-n-heterostructures

Electroreflectance (ER) spectra of InGaN/AlGaN/GaN p-n- heterostructures with multiple quantum wells (MQW) are studied. Structures with MQW InGaN/GaN were grown for blue LEDs by MOCVD technology and “flip-chip” mounted. The ER spectral maxima correspond to the high energy side of electroluminescence spectral line. The ER spectra caused by Franz-Keldysh effect are approximated by Aspnes theory. The ER spectra in a range 400 ÷ 800 nm have interference bands caused by the change of refraction index in the structure.

Room temperature photoreflectance investigation of undoped and doped GaAs/AlGaAs quantum well structures

The room temperature photoreflectance (PR) investigation of optical transitions in Al0.2Ga0.8As/GaAs/Al0.2Ga0.8As single and coupled quantum wells is presented. The structures were grown by molecular beam epitaxy for different barrier thickness and quantum well width. Three kinds of spectral features were observed in PR spectra: sharp line connected with GaAs band gap (1.42 eV), Frantz-Keldysh oscillations near Al0.2Ga0.8As band gap (1.71 eV) and features originated from electron-hole transitions in quantum well. The energies of observed transitions have been compared with the results of envelope function calculations.