I. P. Smirnova

High-power flip-chip blue light-emitting diodes based on AlGaInN

The design and fabrication of a high-power light-emitting diode chip that has an active-region area of 1 mm2 and emits at a wavelength of 460 nm are described. The chip structure is developed on the basis of numerical simulation and is intended for flip-chip assembly. The use of two-level interconnections for an n-type contact made it possible to obtain an unprecedentedly low series resistance (0.65 Ω) and a high uniformity of pump-current distribution. Light-emitting diodes based on the developed design operate in the continuous-wave mode in a current range of 0–2 A, and their highest emission power is 430 mW.

Reflecting p-contact based on thin ITO films for AlGaInN flip-chip LEDs

A reflecting contact to a p-GaN layer, used in fabrication of blue flip-chip light-emitting diodes, has been produced by deposition of thin indium tin oxide (ITO) films by electron-beam evaporation. The high reflectance of the contact, which exceeds that of a Ni/Ag contact, provides a 15–20% increase in the external quantum efficiency of light-emitting crystals. The forward voltage drops for crystals with an ITO(5 nm)/Ag(220 nm) contact are comparable with the corresponding values for crystals with a Ni(1.5 nm)/Ag(220 nm) contact. The specific resistance of the contact with an ITO layer is 3.7 × 10−3 Ω cm2. It is shown that, for ITO films produced by the given method, the optimal thicknesses providing the best electrical and optical characteristics of the crystals are in the range 2.5–5.0 nm.

Optimization of the deposition technique of thin ITO films used as transparent conducting contacts for blue and near-UV LEDs

Structural, optical, and electrical properties of indium-tin oxide (ITO) films produced by electron-beam evaporation, magnetron sputtering, and a combined method are studied. It is demonstrated that, despite the high transparency of the electron-beam-deposited ITO films in the visible spectral range, their conductivity and refractive index are noticeably lower than the respective parameters of ITO films deposited by magnetron sputtering. A technique for the fabrication of double-layer systems by the combination of these two deposition techniques is developed. As a result, we obtained the films, which possess the advantages of magnetron-sputtered layers and can be used as contact layers to p-type GaN in LEDs for the blue and nearultraviolet (near-UV) spectral ranges.

Comparison of the properties of AlGaInN light-emitting diode chips of vertical and flip-chip design using silicon as the a submount

Vertical and flip-chip light-emitting diode (LED) chips are compared from the viewpoint of the behavior of current spreading in the active region and the distribution of local temperatures and thermal resistances of chips. AlGaInN LED chips of vertical design are fabricated using Si as a submount and LED flipchips were fabricated with the removal of a sapphire substrate. The latter are also mounted on a Si submount. The active regions of both chips are identical and are about 1 mm2 in size. It is shown that both the emittance of the crystal surface in the visible range and the distribution of local temperatures estimated from radiation in the infrared region are more uniform in crystals of vertical design. Heat removal from flip-chips is insufficient in regions of the n contact, which do not possess good thermal contact with the submount. As a result, the total thermal resistances between the p-n junction and the submount both for the vertical chips and for flip-chips are approximately 1 K/W. The total area of the flip-chips exceeds that of the vertical design chips by a factor of 1.4.

AlGaInN-based light emitting diodes with a transparent p -contact based on thin ITO films

A method for obtaining transparent conductive ITO (indium-tin oxide) films aimed for use in light emitting diodes of the blue spectral range is developed. The peak external quantum efficiency of light-emitting diodes with a p-contact based on the obtained films reaches 25%, while for similar light-emitting diodes with a standard semitransparent metal contact, it is <10%. An observed increase in the direct voltage drop from 3.15 to 3.37 V does not significantly affect the possibility of applying these films in light-emitting diodes since the optical power of light-emitting diodes with a transparent p-contact based on ITO films exceeds that of chips with metal semitransparent p-contacts with a working current of 20 mA by a factor of almost 2.5. Light-emitting diodes with p-contacts based on ITO films successfully withstand a pumping current that exceeds their calculated working current by a factor of 5 without the appearance of any signs of degradation.

Raising the quantum efficiency of AlGaInN flip-chip LEDs by Reactive ion etching of the outer side of SiC substrates

A method for creating a light-scattering microprofile on the outer side of SiC substrates in order to diminish the light extraction loss caused by the effect of total internal reflection in light-emitting AlGaIn/GaN structures has been further developed. It is suggested to use thin photoresist layers as random masks for reactive ion etching of silicon carbide substrates. A microprofile with the required parameters has been obtained on the substrate surface by optimizing the etching modes, with the result that the external quantum efficiency of light-emitting crystals increased by more than 25%.

Blue flip-chip AlGaInN LEDs with removed sapphire substrate

Characteristics of AlGaInN LEDs with removed sapphire substrate are studied. To remove the substrate from a finished LED crystal mounted by the flip-chip method onto a silicon wafer, the laser lift-off technique was used. To raise the light output efficiency, a scattering profile was formed on the n-GaN surface by ion etching in a Cl2: Ar gas mixture. This resulted in the 25–30% increase in the external quantum efficiency of LEDs. The LEDs fabricated in this way demonstrate stable operation at drive currents of up to 300 mA with an optical power as high as 110 mW.

Technique for forming ITO films with a controlled refractive index

A new method for fabricating transparent conducting coatings based on indium-tin oxide (ITO) with a controlled refractive index is proposed. This method implies the successive deposition of material by electron-beam evaporation and magnetron sputtering. Sputtered coatings with different densities (and, correspondingly, different refractive indices) can be obtained by varying the ratio of the mass fractions of material deposited by different methods. As an example, films with effective refractive indices of 1.2, 1.4, and 1.7 in the wavelength range of 440–460 nm are fabricated. Two-layer ITO coatings with controlled refractive indices of the layers are also formed by the proposed method. Thus, multilayer transparent conducting coatings with desired optical parameters can be produced.

Use of double-layer ITO films in reflective contacts for blue and near-UV LEDs

The structural and optical properties of multilayer ITO/SiO2/Ag composites are studied. In these composites, the ITO (indium-tin oxide) layer is produced by two different methods: electron-beam evaporation and a combined method including electron-beam evaporation and subsequent magnetron sputtering. It is shown that the reflectance of the composite based on the ITO film produced by electron-beam evaporation is substantially lower. This can be attributed to the strong absorption of light at both boundaries of the SiO2 layer, which results from the complex surface profile of ITO films deposited by electron-beam evaporation. Samples with a film deposited by the combined method have a reflectance of about 90% at normal light incidence, which, combined with their higher electrical conductivity, makes these samples advantageous for use as reflective contacts to the p-type region of AlInGaN light-emitting diodes of the flip-chip design.

Calculation of the optimal architecture of a double-layer ITO film intended for use in reflective contacts in blue and near-UV LEDs

Reflection from multilayer coatings serving as contacts in AlInGaN light-emitting diodes (LEDs) is calculated in terms of a model based on the transfer-matrix method. ITO/SiO2/Ag composites with an ITO film constituted by two layers with different refractive indices are considered. The thicknesses of the ITO layers are varied within a prescribed range. The angular dependences of the reflectance for light of both polarizations are calculated, and the integrated reflectivity of a contact is obtained on this basis. The dependence of the integrated reflectivity of the contact and that of the reflectance of light incident perpendicularly to the plane of the contact on the thickness of the ITO layers are analyzed. As a result, the optimized architecture of a reflective contact to the p-GaN region of AlInGaN LED chips is suggested.