Light-emitting diodes of “Warm” white luminescence on the basis of p-n heterostructures of the InGaN/AlGaN/GaN type coated with phosphors made of yttrium-gadolinium garnets

Semiconductors - Electroluminescence spectra and color characteristics of light-emitting diodes of white luminescence based on p-n heterostructures of the InGaN/AlGaN/GaN type with blue emission...     

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(J _max=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/J ₀)+(J/J ₁)^0.5] + J · R _s. The portion of V∞(J/J ₁)^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. Fiz. Tekh. Poluprovodn. 33, 445–450 (April 1999)     

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 V _th>8–10 V, i.e., ∼3E _g , (E _g 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. Fiz. Tekh. Poluprovodn. 32, 63–67 (January 1998)     

Nitrogen divacancies — The possible cause of the “yellow band” in the luminescence spectra of GaN

A strong analogy is demonstrated between the well-known impurity complex NN₁ in GaP consisting of a pair of nearest-neighbor isovalent nitrogen impurity atoms in the nitrogen-doped gallium phosphide lattice (GaP: N), and the divacancy complex of nearest-neighbor vacancies in the nitrogen sublattice of gallium nitride. This divacancy or complexes of this divacancy with impurities may be the cause of the “yellow band” in the luminescence spectra of GaN. This work was presented at a session of the Electrochemical Society (Paris, September 1997 by A. E. Yunovich, in Proceedings of the Second Symposium on III–V Nitride Materials and Processes, Electrochemical Society (Pennington, New Jersey, 1998), Vol. 98-02, p. 258. Fiz. Tekh. Poluprovodn. 32, 1181–1183 (October 1998)     

Electroluminescence spectra of ultraviolet light-emitting diodes based on <Emphasis Type="Italic">p</Emphasis>-<Emphasis Type="Italic">n</Emphasis>-heterostructures coated with phosphors

The electroluminescence spectra of light-emitting diodes based on p-n heterostructures of the InGaN/AlGaN/GaN type are studied in the near-ultraviolet spectral region (360–405 nm). The spectra are peaked at the wavelengths 385 and 395 nm, and the intensity of emission falls exponentially with the photon energy in the shorter-wavelength and longer-wavelength regions. The emitters in the green and yellow spectral regions based on these light-emitting diodes coated with silicate phosphors are studied. The luminescence spectra of phosphors have the Gaussian shape and maximums in the range from 525 to 560 nm. The color characteristics of emitters depend on the ratios of intensities of the ultraviolet and yellow-green bands. The possibilities of fabrication of light-emitting diodes of visible luminescence based on ultraviolet light-emitting diodes that excite colored phosphors are discussed.     

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

p?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, E _g ^* , is determined at 2.76–2.78 eV. The experimental dependence of E _g ^* 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.     

Luminescence spectra of blue and green light-emitting diodes based on multilayer InGaN/AlGaN/GaN heterostructures with quantum wells

The luminescence spectra of blue and green light-emitting diodes based on In_xGa_1−x N/Al_yGa_1−y N/GaN heterostructures with a thin (2–3 nm) In_xGa_1−x N active layer have been investigated in the temperature and current intervals 100–300 K and J=0.01–20 mA, respectively. The spectra of the blue and green light-emitting diodes have maxima in the interavals ℏω_max=2.55–2.75 eV and ℏω_max=2.38–2.50 eV, respectively, depending on the In content in the active layer. The spectral intensity of the principal band decreases exponentially in the long-wavelength region with energy constant E ₀=45–70 meV; this is described by a model that takes into account the tails of the density of states in the two-dimensional active region and the degree of filling of the tails near the band edges. At low currents radiative tunneling recombination with a voltage-dependent maximum in the spectrum is observed in the spectra of the blue diodes. A model of the energy diagram of the heterostructures is discussed. Fiz. Tekh. Poluprovodn. 31, 1055–1061 (September 1997)     

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.