A. E. Yunovich

Electrical properties and luminescence spectra of light-emitting diodes based on InGaN/GaN heterostructures with modulation-doped quantum wells

The distribution of charged centers N(w), quantum efficiency, and electroluminescence spectra of blue and green light-emitting diodes (LED) based on InGaN/AlGaN/GaN p-n heterostructures were investigated. Multiple InGaN/GaN quantum wells (QW) were modulation-doped with Si donors in GaN barriers. Acceptor and donor concentrations near the p-n junction were determined by the heterodyne method of dynamic capacitance to be about NA ≥ 1 × 1019 cm−3 ≫ ND ≥ 1 × 1018 cm−3. The N(w) functions exhibited maxima and minima with a period of 11–18 (±2–3 nm) nm. The energy diagram of the structures has been constructed. The shifts of spectral peaks with variation of current (J=10−6–3×10−2 A) are smaller (13–12 meV for blue and 20–50 meV for green LEDs) than the corresponding values for the diodes with undoped barriers (up to 150 meV). This effect is due to the screening of piezoelectric fields in QWs by electrons. The dependence of quantum efficiency on current correlates with the charge distribution and specific features in the current-voltage characteristics.

Luminescence spectra and efficiency of GaN-based quantum-well heterostructure light emitting diodes: Current and voltage dependence

Luminescence spectra and quantum yield in light emitting diodes (LEDs) based on InGaN/AlGaN/GaN heterostructures with multiple quantum wells (MQWs) were studied in the range of currents J=10−6–10−1 A. Minor spread in the quantum yield at operating currents (±15% at J≈10 mA) was observed in these LEDs, which were fabricated by Hewlett-Packard. The spread is due to differences in the current and voltage dependences of the diode emission intensity, caused by differences in the charged center distribution across the space-charge region (SCR) of the structures and in the role of the tunnel current component at low voltages. In the diodes with a thin (≲120 nm) SCR, a tunnel emission band was observed for J≲100 µA; the peak energy of this band ℏωmax=1.92–2.05 eV corresponds to the voltage applied. At low currents (J=0.05–0.5 mA), the spectral position of the main peak ℏωmax=2.35–2.36 eV is independent of the voltage and is determined by the radiative transitions between the localized states. At J>1 mA, this band shifts with the current (ℏωmax=2.36–2.52 eV). Its shape corresponds to the model for the occupation of states in the two-dimensional energy band tails, which are caused by the microscopic potential fluctuations. The four parameters in this model are related to the calculated energy band diagram of the MQW structure.

Luminescence and electrical properties of InGaN/AlGaN/GaN light emitting diodes with multiple quantum wells

Luminescence spectra of light-emitting diodes based on InGaN/AlGaN/GaN heterostructures with multiple quantum wells are studied for currents in the range J=0.15 µA-150 mA. The comparatively high quantum efficiency for low J(Jmax=0.5–1 mA) is a consequence of a low probability for the nonradiative tunnel current. The current-voltage characteristics J(V) are studied for J=10−12–10−1 A; they are approximated by the function V=ϕk+mkT· [1n(J/J0)+(J/J1)0.5] + J · Rs. The portion of V∞(J/J1)0.5 and measurements of the dynamic capacitance indicate that i-layers adjacent to the active layer play an important role. The spectra are described by a model with a two-dimensional density of states with exponential tails in multiple quantum wells. The rise in T with increasing J is determined from the short-wavelength decay of the spectrum of the blue diodes: T=360–370 K for J=80–100 mA. An emission band is observed at 2.7–2.8 eV from green diodes at high J; this band may be explained by phase separation with different amounts of In in the InGaN.

Changes in the luminescent and electrical properties of InGaN/AlGaN/GaN light-emitting diodes during extended operation

Changes in the luminescence spectra and current-voltage and capacitance-voltage characteristics of light-emitting diodes based on InGaN/AlGaN/GaN heterostructures were investigated as functions of operating time during extended use. Sample blue and green light-emitting diodes with InGaN single quantum-well active layers were examined during operating times of 102−2×103 h at currents up to 80 mA. An increase in the efficiency at the working currents (15 mA) was observed in the first stage of aging (100–800 h) followed by a decrease in the second stage. The greatest changes in the spectra were observed at low currents (<0.15 mA). Studies of the distribution of charged acceptors in the space-charge region showed that their concentration grows in the first stage and falls in the second. Models explaining the two stages of aging are proposed: 1) activation of Mg acceptors as a result of destruction of residual Mg-H complexes, and 2) formation of N donor vacancies. A model of subthreshold defect formation by hot electrons injected into the quantum wells is discussed.

Luminescence spectra, efficiency, and color characteristics of white-light-emitting diodes based on p-n InGaN/GaN heterostructures with phosphor coatings

The luminescence spectra, efficiency, and color characteristics of white-light-emitting diodes fabricated from p-n InGaN/AlGaN/GaN blue-light-emitting heterostructures grown on SiC substrates and coated with yellow-green phosphors based on the rare-earth-doped yttrium-aluminum garnets were studied. The efficiency of blue-emitting diodes is as high as 22% at a current of 350 mA and a voltage of 3.3 V. The white-emitting diodes have luminous efficiency as high as 40 lm/W and luminous flux up to 50 lm at 350 mA.

Impact ionization luminescence of InGaN/AlGaN/GaN p-n-heterostructures

The luminescence spectra of InGaN/AlGaN/GaN p-n heterostructures with reverse bias sufficient for impact ionization are investigated. The injection luminescence of light-emitting diodes with such structures was examined earlier. A strong electric field is present in the InGaN active layer of the heterostructures, and for small reverse bias the tunneling component of the current predominates. Avalanche breakdown commences at voltages Vth>8–10 V, i.e., ∼3Eg, (Eg is the width of the band gap) in the absence of lightly doped structures. The luminescence spectra have a short-wavelength edge corresponding to the width of the GaN band gap (3.40 eV) and maxima in the region 2.60–2.80 eV corresponding to the maxima of the injection luminescence spectra in the active layer. The long-wavelength edge of the spectra in the region 1.7–1.8 eV may be associated with deep recombination levels. Mechanisms of recombination of the hot electron-hole plasma in the strong electric fields of the p-n heterostructures are discussed.

Emission spectra of InGaN/AlGaN/GaN quantum well heterostructures: Model of the two-dimensional joint density of states

The luminescence spectra of light emitting diodes based on InGaN/AlGaN/GaN heterostructures with multiple quantum wells are analyzed in the context of a model of the two-dimensional density of states in the active region. The model accounts for the potential fluctuations, the statistics of occupation of the wells with charge carriers, and specific features of the extraction of radiation from the structure. The model describes the position of the maxima of the spectra and the exponential decline of the emission intensity in the short-and long-wavelength regions as well as the modification of the spectra under variations in the current. The problems of limitations of the model and the physical meaning of the parameters are discussed. The examples of approximation of the spectra of blue light emitting diodes based on InGaN/AlGaN/GaN heterostructures show the necessity of determining the temperature in the active region independently and taking into account the interference in the planar structure. The differences of the shape of the spectra from that obtained in the simple model depend not only on the properties of the quantum wells but also on the nonuniformities in the distribution of In in InGaN.

Aging of InGaN/AlGaN/GaN Light-Emitting Diodes

Changes in luminescent spectra, current-voltage, and capacitance-voltage characteristics were studied versus time of working at forward currents for light-emitting diodes based on InGaN/AlGaN/GaN heterostructures. The samples of blue and green LEDs with a single quantum well InGaN active layers were studied during 10 2 -2.10 3 hours at currents 30–80 mA. An increase in efficiency at the first stage and a decrease one at the second stage were detected. The main changes were detected at low currents in blue diodes for tunnel components of current and radiation. Activation of acceptors Mg because of residual H atoms are going out of complexes Mg-H and creation of donor type defects are proposed as models for two stages. A model of injection stimulated (non-thermal) underthreshold creation of defects can explain the results.

Electroreflectance spectra of InGaN/AlGaN/GaN quantum-well heterostructures

Abstractp−n InGaN/AlGaN/GaN heterostructures with InGaN/AlGaN multiple quantum wells are studied by electroreflectance spectroscopy. The structures are grown by metal—organic epitaxy and arranged with the p region in contact with the heat sink. Light is incident on and reflected from the structures through the sapphire substrate. To modulate the reflectivity, rectangular voltage pulses and a dc reverse bias are applied to the p−n junction. A line corresponding to interband transitions in the region of InGaN/AlGaN multiple quantum wells is observed in the electroreflectance spectra. The peak of this line is shifted to shorter wavelengths from the peak of injection luminescence of the light-emitting diode structures. The low-field model developed by Aspnes is used to describe the electroreflectance spectra. By choosing the parameters of the model to fit the experimental data, the effective band gap of the active region of the structure, Eg*, is determined at 2.76–2.78 eV. The experimental dependence of Eg* on the applied voltage is attributed to the effect of piezoelectric fields in the InGaN quantum wells. In the electroreflectance spectra, an interference pattern is observed in the wide spectral range from 1.4 to 3.2 eV. The interference is due to the dependence of the effective refractive index on the electric field.

Energy Diagram and Recombination Mechanisms in InGaN/AlGaN/GaN Heterostructures with Quantum Wells

Electroluminescence spectra of GaN based LEDs are analyzed quantitatively using a model of 2D density of states with band tails. Calculations take into account an energy diagram with given band offsets between AlGaN, InGaN and GaN layers. The model describes spectral shapes with four fitting parameters in a wide range of currents and intensities with a good accuracy. Fluctuations of well thickness (heterointerface roughness), of In content, of charged impurities and piezoelectric fields are discussed for an evaluation of the exponential tail parameter of 2D density of states in the active region. Spectral maxima move with current changes due to the filling of the band tails and redistribution of current carriers between regions with a larger probability of radiative recombination. 2D concentrations and lifetimes of minority carriers in active layers are evaluated from the parameters of the model.