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We have studied the influence of hydrostatic pressure on the light emission from cubic In0.1Ga0.9N. A qualitative difference between pressure dependence of photoluminescence peak energies for cubic and wurtzite symmetry InGaN/GaN quantum wells (QWs) was found. Cubic samples revealed magnitude of dEE/dP of 26–30 meV/GPa, practically independent of the QW width. Previous studies of the hexagonal InGaN/GaN structures showed that with increasing QW width dEE/dP changed between about 30 meV/GPa and 0 meV/GPa. This different behavior of two types of QWs can be explained by the lack of built-in electric field (along growth direction) in case of cubic structures. To describe pressure evolution of the optical transitions in cubic InGaN/GaN QWs and thick epitaxial layer, we use a simple k×p model based on the linear theory of elasticity. To reproduce the experimental data, it is necessary to invoke presence of In-rich fluctuations in the cubic In0.1Ga0.9N samples.

We have measured the interband optical absorption of a free-standing sample of Ga0.96In0.04As0.99N0.01 in a wide energy range from 1 to 2.5 eV. We found that the fundamental absorption edge is shifted by 150 meV towards lower energies, and the absorption coefficient measured at higher energies exhibits substantial reduction comparing to that of GaAs. By removing the GaAs substrate, we were able to get an experimental insight into the interband optical transitions and the density of state in this material. The changes can be understood within the band anticrossing model predicting the conduction band splitting. New absorption edges associated with optical transitions from the spin-orbit split off band to the lower conduction subband (1.55 eV) and from the top of the valence band to the upper subband (1.85 eV) are observed.

We studied the InGaN laser diode, emitting in the blue region of the spectrum and characterized by a negative characteristic temperature T0. The corresponding decrease in the threshold current with the increase in temperature for this device is caused by the increased distance between the electron-blocking layer and the quantum wells. Because of the non-monotonic temperature dependence of laser parameters, we can demonstrate a correlation of the degradation rate with nonradiative part of the total device current. This result indicates the potential importance of the recombination processes occurring outside of the active region for the reliability of InGaN laser diodes.

We have studied the pressure and temperature dependence of the absorption edge of a 4-μm-thick layer of the alloy Ga0.92In0.08As0.985N0.015. We have measured the hydrostatic pressure coefficient of the energy gap of this alloy to be 51 meV/GPa, which is more than a factor two lower than that of GaAs (116 meV/GPa). This surprisingly large lowering of the pressure coefficient is attributed to the addition of only ∼1.5% nitrogen. In addition, the temperature-induced shift of the edge is reduced by the presence of nitrogen. We can explain this reduction by the substantial decrease of the dilatation term in the temperature dependence of the energy gap.

The energies of photo- and electroluminescence transitions in InxGa1−xN quantum wells exhibit a characteristic “blueshift” with increasing pumping power. This effect has been attributed either to band-tail filling, or to screening of piezoelectric fields. We have studied the pressure and temperature behavior of radiative recombination in InxGa1−xN/GaN quantum wells with x=0.06, 0.10, and 0.15. We find that, although the recombination has primarily a band-to-band character, the excitation-power induced blueshift can be attributed uniquely to piezoelectric screening. Calculations of the piezoelectric field in pseudomorphic InxGa1−xN layers agree very well with the observed Stokes redshift of the photoluminescence. The observed pressure coefficients of the photoluminescence (25–37 meV/GPa) are surprisingly low, and, so far, their magnitude can only be partially explained.

We report the results of a comparison study of photoluminescence (PL) from an In0.15Ga0.85N/GaN multiple-quantum-well (MQW) sample and an In0.11Ga0.89N thick epitaxial-layer sample, which have very similar band-gap energies. Large hydrostatic pressures were used for our investigations. The PL emissions in both samples were found to shift linearly to higher energy with applied pressure. In the MQW sample, the pressure response of the InGaN is dominated by the GaN layers, which leads to a significantly weaker pressure dependence as compared to the epilayer sample. Our results yield a pressure coefficient of 2.8×10−3 eV/kbar for the In0.15Ga0.85N/GaN MQW sample and 4.0×10−3 eV/kbar for the In0.11Ga0.89N epilayer. An abrupt decrease of PL intensity in both samples was observed at pressures above 100 kbar, indicating the carriers involved in the radiative recombination processes in the samples originate primarily from the adjacent GaN layers.

Electro- and photoluminescence spectra of high-brightness light-emitting AlGaN/InGaN/GaN single-quantum-well structures are studied over a broad range of temperatures and pumping levels. Blue shift of the spectral peak position was observed along with an increase of temperature and current. An involvement of band-tail states in the radiative recombination was considered, and a quantitative description of the blue temperature-induced shift was proposed assuming a Gaussian shape of the band tail.

We have measured photoluminescence and electroluminescence in two different types of high-brightness single-quantum-well light emitting diodes manufactured by Nichia Chemical Industries with InxGa1−xN active layers (x=0.45 and x=0.15), under hydrostatic pressures up to 8 GPa. We discovered that the pressure shift of the primary luminescence peak in each diode is very small: 12 and 16 meV/GPa for the green and blue diodes, respectively. The observed pressure coefficients are much lower than those characteristic of the energy gap in GaN (≈40 meV/GPa) or the energy gap in InN (≈33 meV/GPa). This kind of behavior is usually associated with recombination processes involving localized states. These localized states may be associated either with band tails (arising from In fluctuations in the active layer or from high density of defects), and/or with localized excitons of various types.

Electrical and optical properties of Nichia double‐heterostructure blue light‐emitting diodes, with In0.06Ga0.94N:Zn, Si active layer, are investigated over a wide temperature range from 10 to 300 K. Current–voltage characteristics have complex character and suggest the involvement of various tunneling mechanisms. At small voltages (and currents), the peak wavelength of the optical emission shifts with the applied bias across a large spectral range from 539 nm (2.3 eV) up to 443 nm (2.8 eV). Light emission takes place even at the lowest temperatures, indicating that a complete carrier freeze‐out does not occur.

We investigated the pressure behavior of yellow luminescence in bulk crystals and epitaxial layers of GaN. This photoluminescence band exhibits a blueshift of 30±2 meV/GPa for pressures up to about 20 GPa. For higher pressure we observe the saturation of the position of this luminescence. Both effects are consistent with the mechanism of yellow luminescence caused by electron recombination between the shallow donor (or conduction band) and a deep gap state of donor or acceptor character.

We demonstrate AlGaN-cladding-free laser diodes (LDs), operating in continuous wave (CW) mode at 482 nm grown by plasma-assisted molecular beam epitaxy (PAMBE). The maximum CW output power was 230 mW. LDs were grown on c-plane GaN substrates obtained by hydride vapor phase epitaxy. The PAMBE process was carried out in metal-rich conditions, supplying high nitrogen flux (ΦN) during quantum wells (QWs) growth. We found that high ΦN improves quality of high In content InGaN QWs. The role of nitrogen in the growth of InGaN on (0001) GaN surface as well as the influence of LDs design on threshold current density are discussed.

We demonstrate graded-index-separate-confinement-heterostructure InGaN laser diodes (GRINSCH) grown by metal organic vapor phase epitaxy. In this type of structure, the optical mode is confined close to the active region by cladding layers with linearly changing Al content. The virtue of this structure lies in simultaneous improvement of injection efficiency through the funneling effect and mode confinement. After a careful modeling we demonstrate that GRINSCH InGaN laser indeed possess very encouraging properties. Low threshold current density of around 3.5 kA/cm2 accompanied by high differential gain (dG/dJ = 14.93 cm kA−1) make these devices an interesting alternative for the classical step-index structure.

We have determined thermal stability of various InGaN laser diodes emitting in the spectral range of 390-436 nm. Their characteristic temperature T0 increases steeply with the increasing emission wavelength, reaching the value of around 230 K. Our analysis of current-light characteristics and activation energy of electroluminescence proves that this behavior is predominantly related to the thermal escape of electrons from quantum wells, which depends critically on their depth.

We demonstrate InGaN laser diodes with substantially reduced thickness of their bottom AlGaN cladding grown on plasmonic GaN substrate. The electron concentration in plasmonic substrate grown by ammonothermal method was of the order of 1020 cm−3, which corresponds to the refractive index reduction by 0.7% when comparing to undoped GaN. We were able to reduce the thickness of AlGaN bottom claddings from the initial 800 nm down to 400 nm without any worsening of laser threshold current or near-field patterns. The results are in agreement with the optical modeling of the InGaN laser diode structure fabricated on plasmonic GaN substrate.

We have studied two types of InGaN laser diodes emitting at 410 and 440 nm. Each device was characterized by measuring light-current characteristics in two geometries for which the light was collected: along the resonator and perpendicularly to the cavity. In the first configuration, the 410 nm device displays no reduction of differential efficiency while 440 nm laser shows evidence of droop. In the perpendicular configuration both devices show the pronounced droop. We associate the suppression of the droop for 410 nm laser in the “along cavity” configuration with the appearance of the stimulated recombination.

We report on electrical and optical properties of InGaN violet laser diodes, grown on high-pressure grown, almost dislocation free GaN substrates. In these structures, mechanical strain has been reduced by a partially relaxed AlN mask with stripe shaped windows. The dislocation density of the mask was 1010∕cm2. In the window areas, where we located the laser stripes, the dislocation density was below 3×106∕cm2. We fabricated three types of laser diodes with aluminum composition in the cladding of 8%, 13%, and 16%. All devices remained crack-free. The laser threshold current decreased with increasing Al composition in the cladding.

We have investigated the aging processes in InGaN laser diodes fabricated by metal organic vapor phase epitaxy on low-dislocation-density, high-pressure-grown bulk gallium nitride crystals. The measured threshold current turned out to be a square root function of aging time, indicating the importance of diffusion for device degradation. The differential efficiency, in contrast, was roughly constant during these experiments. From these two observations we can conclude that the main reason for degradation is the diffusion-enhanced increase of nonradiative recombination within the active layer of the laser diode. Additionally, microscopic studies of the degraded structures did not reveal any new dislocations within the active area of the aged diodes, thus identifying point defects as a source of nonradiative processes.

The inherent problem in III-nitride technology is the cracking of AlGaN layers that results from lattice mismatch between AlGaN and GaN. In case of thin substrates (30–90μm), such as, bulk GaN grown by the high-pressure/high-temperature method, the bowing of AlGaN∕GaN strained structures becomes an additional problem. To eliminate cracking and bowing, AlGaN layers were grown on GaN substrates with an AlN mask patterned to form 3–15μm wide windows. In the 3μm window, the AlGaN layer was not cracked, although its thickness and Al composition exceeded critical values for growth on nonpatterned substrates. Dislocation density in the windows was of 5×106∕cm2.

By using an atypically wide quantum well (95A) in the active layer of InGaN violet light emitting laser diode, we managed to fabricate a device characterized by very high thermal stability of the threshold current. The characteristic T0 temperature was measured to be 302K, which is the highest reported value up to date. After thermal cycling of the device, T0 drops down to the lower value of 220K. The very high value of T0 in our devices is accompanied by anomalous temperature behavior of the device slope efficiency. The slope efficiency improves with increasing temperature, reaches a maximum and then gradually decreases. This behavior we interpret as the competition between a regular increase of the thermal carrier escape and an improvement of carrier capture efficiency with an opposite temperature dependence. The latter mechanism we tentatively attribute to the temperature quenching of the ballistic transport related carrier leakage from the active region of the laser diode.

Photocurrent spectroscopy and hydrostatic-pressure-dependent electroluminescence are used to show that heavy 1×1019cm−3 Si doping of quantum barriers is sufficient to achieve full screening of polarization-induced electric fields (PIEFs) in nitride light emitting diodes (LEDs) and laser diodes (LDs) with InGaN quantum wells. Furthermore, it is shown that at currents close to lasing threshold in nitride LDs injected charge alone is sufficient to achieve full screening of PIEFs. In contrast, full screening at low currents can only be accomplished via Si doping of quantum barriers.