V. N. Pavlovskii

Blue InGaN/GaN multiple-quantum-well optically pumped lasers with emission wavelength in the spectral range of 450–470 nm

Optically pumped lasing in the wavelength range of 450–470 nm in InGaN/GaN multiple-quantum-well heterostructures grown by metalorganic vapor phase epitaxy was achieved and investigated. The energy and power per pulse of the laser were 80 nJ and 10 W correspondingly for one facet at room temperature. The far-field patterns of the laser emission consisted of three light spots near the angles of +30°, −15°, and −45°. The highest operating temperature was 450 K. The photoluminescence and photoluminescence excitation spectrum structures suggest that the quantum dots inside the quantum wells are involved in the recombination mechanism.

Luminescence and lasing in InGaN∕GaN multiple quantum well heterostructures grown at different temperatures

It was found that the decrease of the InGaN∕GaN multiple quantum well (MQW) growth temperature from 865 to 810 °C leads to a MQW emission wavelength shift from the violet to the green spectral region. The lowering of the growth temperature also promotes a decrease of the MQW photoluminescence (PL) intensity at high excitation and a disappearance of the excitonic features from the low-temperature reflection and PL spectra of GaN barriers and claddings. The laser threshold dependence on Tg is not monotonic, with the lowest value of 270kW∕cm2 at Tg=830°C. High-temperature annealing (900 °C, 30 min) leads to a twofold increase of the PL efficiency only from the InGaN QWs grown at the lowest temperature. The results allow one to explain the laser threshold behavior in terms of the heterostructure quality, the defect concentration, In clusterization, and the piezoelectric field dependence on the MQW growth temperature.

Multiple quantum well InGaN/GaN blue optically pumped lasers operating in the spectral range of 450-470 nm

Lasing under optical pumping by N 2 -laser radiation in InGaN/GaN multiple quantum well heterostructures grown in AIXTRON MOVPE reactors was achieved and investigated in the wavelength range of 450-470 nm. The laser operation wavelength depends most strongly on V/III ratio during quantum well barrier growth. The total energy and power per pulse of the laser were 300 nJ and 40 W, respectively, with differential quantum efficiency of 3% at room temperature. The laser threshold increases exponentially with increasing operation wavelength which is mainly due to the decreasing efficiency of the spontaneous emission and due to an increase of its spectral width.

Influence of UV Light-Assisted Annealing on Optical Properties of InGaN/GaN Heterostructures Grown by MOVPE

The influence of ultraviolet light-assisted annealing on the optical properties of InGaN/GaN single quantum wells (SQW), and thick single and double heterostructures (SH, DH) was investigated. It was shown that annealing promotes an increase of the photoluminescence (PL) intensity from the active layers of the SQW and double heterostructures by as much as one order of magnitude. This is effected mainly by a diminishing of the defect concentration and a smoothing of the potential fluctuations in the upper cladding layer. The operating temperature of the DH lasers was lifted from 220 up to 300 K after annealing. Using nitrogen laser irradiation during annealing led both to a PL efficiency increase and to a phase separation in the SH with a thick active layer.

Irregular spectral position of E || c component of polarized photoluminescence from m-plane InGaN/GaN multiple quantum wells grown on LiAlO2

Polarized temperature dependent photoluminescence (PL) and room temperature (RT) photocurrent spectra of m-plane InGaN/GaN multiple quantum wells grown on LiAlO2 with In content xIn = 5%-30% were studied. As expected, higher xIn leads to larger strain in the wells and enhances both the splitting between the two highest valence subbands and the RT PL degree of polarization. At low temperatures, an irregular red-shift of the PL component with polarization E || c relative to E ⊥ c is observed, which is ascribed to the contribution of recombination of holes localized at band tails within the second highest valence subband.

High‐Temperature Lasing in InGaN/GaN Multiquantum Well Heterostructures

Lasing, stimulated emission and photoluminescence in InGaN/GaN multiquantum well (MQW) heterostructures were investigated in a wide temperature interval from the liquid nitrogen temperature up to 600 K. Laser action was achieved up to T = 585 K in a heterostructure consisting of ten quantum wells with a width of 10 nm. Laser thresholds at T = 78, 300, and 585 K were 25, 100, and 550 kW/cm2, respectively. The pulse energy of the InGaN/GaN MQW laser at room temperature was 600 nJ and the average power was 0.6 mW. The characteristic temperature value of T0 = 164 K was derived from the temperature dependence of the lasing threshold.

Time‐ and Temperature‐Resolved Photoluminescence of GaN:Mg Epitaxial Layers Grown by MOVPE

Time-integrated and time-resolved photoluminescence (PL) spectra as well as the luminescence transients of moderately doped GaN:Mg samples grown by MOVPE were studied between 80 K and 380 K at pulse excitation by a nitrogen laser beam in order to clarify the mechanism of the large blue shift of the 2.8 eV PL band above room temperature. Based on the performed study, the new band at 3.05 eV dominating in PL spectra above room temperature is attributed to the donor-to-valence band recombination. The corresponding donor ionization energy is about 290 meV. The blue shift of the spectra is therefore explained as a result of ionization of shallow acceptor states involved together with deep donors in donor-acceptor recombination forming the 2.8 eV band below room temperature.

Luminescence and lasing in GaN epitaxial layers and InGaN/GaN quantum well heterostructures under optical and electron-beam excitation

Abstract Interrelation between stimulated and excitonic emission intensity of GaN epitaxial layers and yellow luminescence intensity as well as correlation between photoluminescence and laser properties of InGaN based multiple quantum well heterostructures was investigated. It was found among all studied undoped GaN epitaxial layers that the higher intensity of the yellow luminescence and so the higher concentration of the yellow luminescence related centres the higher is the excitonic, electron–hole plasma and stimulated emission intensity. It was shown that a small Stokes shift and a high ratio of the luminescence intensity from InGaN quantum well layers to the photoluminescence intensity from GaN barrier layers indicate high laser quality of the multiple quantum well heterostructures. The lowest full width at half maximum of the laser line was 0.04 nm, the highest operating temperature was 585 K, the lowest threshold was 100 kW cm −2 , the highest characteristic temperature was 164 K and the highest wavelength was 442.5 nm. The far-field patterns of the laser emission from the MQW lasers consist of two approximately symmetrical high brightness spots localized at angles α =±30–35°.

Determination of active region overheating temperature of GaN-based light emitting diodes promising for laser media pumping

Overheating temperature of active region of LEDs made of Bridgelux chips was determined using long-wavelength shift of the EL spectra as well as the voltage reduction at constant current. It was shown that direct liquid active cooling of LED chip reduces drastically its thermal resistance that is promising for pumping of laser active media.

Influence of InGaN/GaN/Si electroluminescent heterostructure design and quantum well thickness on their luminescent and laser properties

Employing a strain-reducing layer stack between Si substrate and MQW layers as well as optimization of quantum well thicknesses of InGaN/GaN/Si MQW electroluminescent heterostructures led to disappearance of cracks and pinholes, to reduction of the laser threshold down to 75 kW/cm2 at 433 nm under N2 laser emission excitation.