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The luminescence efficiency of AlxGa1−xN quantum dots (QDs) and quantum wells (QWs), buried in AlN cladding layers and emitting in the ultraviolet range between 234 and 310 nm, has been investigated. The growth and optical properties have been done using similar aluminum composition (varying from 0.4 to 0.75) for both QDs and QWs. In order to compare as much as possible the optical properties, the QWs were fabricated with a growth time tuned such that the QW width is similar to the average height of the QDs. The photoluminescence (PL) showed emission ranging from 4 to 5.4 eV, putting into evidence differences in terms of full width at half maximum, PL intensity, and asymmetry of the line shape between QDs and QWs. The results show shorter wavelengths and a slightly narrower PL linewidth for QWs. To determine the light emission dependence with the electric field direction in the crystal, the evolutions of the emission diagrams for all samples were recorded along two orthogonal directions, namely, the "in-plane" (growth) and the "on-side" directions, from which the light emission was collected. For the whole QDs and QWs samples' series, the shapes of the emission diagram indicate emission in both in-plane and on-side directions, as evidenced by intra-valence band mixings caused by strain effects combined with the anisotropic Coulomb interactions that are particularly contributing to the polarization at wavelengths below 260 nm. Furthermore, the degree of polarization, determined for each sample, showed good agreement with results from the literature. [ABSTRACT FROM AUTHOR]

Non-polar m-plane GaN terahertz quantum cascade laser (THz-QCL) structures have been studied. One is traditional three-well resonant-phonon (RP) design scheme. The other is two-well phonon scattering injection (PSI) design scheme. The peak gains of 41.8 and 44.2 cm−1 have been obtained at 8.2 and 7.7 THz respectively at 300 K according to the self-consistent non-equilibrium Green's function calculation. Different from the usual GaAs two-well design, the upper and lower lasing levels are both ground states in the GaN quantum wells for the PSI scheme, mitigating the severe broadening effect for the excited states in GaN. To guide the fabrication of such devices, the doping effect on the peak gain has been analyzed. The two designs have demonstrated distinct doping density dependence and it is mainly attributed to the very different doping dependent broadening behaviors. The results reveal the possibility of GaN based THz-QCL lasing at room temperature. [ABSTRACT FROM AUTHOR]

For terahertz quantum cascade lasers employing the nonalignment injection scheme via direct-phonon resonance, this study presents the crucial effect of parasitic absorption in the narrow module architecture based on only two quantum wells. The optical gain can be restricted to small (or negative) values even at the low temperature. To address this issue, a suppression strategy by increasing the injection energy is used to isolate the relevant levels. The feasibility of this strategy is experimentally demonstrated by observing lasing at 194 K. [ABSTRACT FROM AUTHOR]

The correlation between the recombination dynamics of excitons and the internal quantum efficiency (IQE) of AlGaN-based UV-A multiple quantum wells (MQWs) was studied via photoluminescence (PL) and time-resolved PL (TRPL) spectroscopy. The probability ratio of the capture of excitons by nonradiative recombination centers (NRCs) and the radiative recombination of excitons was evaluated individually via two different experimental analyses. The IQE was evaluated via temperature- and excitation power density-dependent PL measurements and its dependence on excitation density was analyzed using a rate equation model based on the radiative and nonradiative recombination of excitons. Moreover, the radiative and nonradiative recombination lifetimes were evaluated via temperature-dependent TRPL measurements; furthermore, they were analyzed as functions of temperature and excitation energy density. The probability ratios obtained from the two individual analyses were in agreement. This quantitative agreement indicated that the analysis based on the radiative and nonradiative recombination processes of excitons, which included the process of filling NRCs, was valid for AlGaN-based UV-A MQWs. [ABSTRACT FROM AUTHOR]

A GaAs/AlGaAs‐based terahertz quantum cascade laser with a peak power of 1.31 W and average power of 52 mW is demonstrated. To realize a high output power, an effective way is to increase the electron density at the active region by using a high doping level. This work aims to optimize a design enabling to use a heavy doping level, by realigning the resonant tunneling which can be seriously degraded due to a band bending from this heavy local doping. With variable quantum well and barrier height, the design owns an improved parameter tuning freedoms, and a saturation of peak gain can be achieved at a relatively high doping concentration of 1.0 × 1017 cm−3, which is twice higher than the recently reported device. Nonequilibrium Green's function method including full carriers scattering effect is used to carry out the design optimization, mainly including the realignment of resonant tunneling and suppression of leakages via high‐energy confined levels. In the experiment, a 14 μm thick active layer for this design is grown and also large mesa size is developed. To ensure a quasi‐continuous wave (quasi‐CW) operation, the heat dissipation is also managed. [ABSTRACT FROM AUTHOR]

AlGaN LEDs emitting < 230 nm UV light were fabricated on sapphire substrates. We employed a quantum well (QW) with an extremely thin barrier to enhance the quantum confinement of holes, wherein the calculation showed that the topmost valence subband became X ± i Y -like and increased the transverse-electric polarized emission. Additionally, we modified the Al composition of the spacer layer situated between the QW and an electron-blocking layer, which significantly improved the current-injection efficiency. The combination and optimization of these structures produced an LED emission of 228-nm UV light with an output power of 1.4 mW at 150 mA. [ABSTRACT FROM AUTHOR]

Internal quantum efficiency (IQE) and stimulated emission properties of AlGaN-based UV-C multiple quantum wells grown on c-plane sapphire substrates were assessed using photoluminescence spectroscopy. The IQEs were estimated to be 53% at room temperature and 16% at 750 K. Furthermore, optically pumped stimulated emission was clearly observed at room temperature. The threshold excitation power densities were estimated to be 13 kW/cm2 at 10 K and 69 kW/cm2 at room temperature. The temperature dependence of the threshold excitation power density suggested that the mechanism of optical gain formation changed from excitonic transition to degenerated electron–hole plasma between 200 and 250 K. [ABSTRACT FROM AUTHOR]

The influence of residual impurities in quaternary InAlGaN active layers on the emission efficiency of 350 nm band ultraviolet light-emitting diodes on GaN substrates has been investigated. Secondary ion mass spectrometry and capacitance-voltage measurements have revealed that a large amount of oxygen is incorporated into the InAlGaN active layers owing to relatively low growth temperatures for Al-content epitaxial films. The increase of the InAlGaN growth temperature from 780 to 830 °C results in both the residual oxygen level decrease from 1×1018 to 4×1017 cm-3 and the output power improvement from 0.6 to 2.6 mW at 100 mA. It is also found that devices containing lower oxygen concentration in the InAlGaN active layers demonstrate a higher electroluminescence intensity ratio of the band-edge emission from the well layers to the donor-acceptor-pair emission from the p-type layers at low temperatures. These experimental results, in conjunction with numerical calculations, suggest that the reduction in the oxygen impurity level in the InAlGaN active layers has an effect on suppressing the electron leakage current into the p-type layers, and thus improving the internal quantum efficiency of InAlGaN-based ultraviolet light-emitting diodes. [ABSTRACT FROM AUTHOR]

In order to realize 250–350-nm-band high-efficiency deep ultraviolet (UV) emitting devices using group-III-nitride materials, it is necessary to obtain high-efficiency UV emission from wide-band-gap (In)AlGaN. The use of the In-segregation effect, which has already been used for InGaN blue emitting devices, is quite effective for achieving high-efficiency deep UV emission. We have demonstrated high-efficiency UV emission from quaternary InAlGaN-based quantum wells in the wavelength range between 290 and 375 nm at room temperature (RT) using the In-segregation effect. Emission fluctuations in the submicron region due to In segregation were clearly observed for quaternary InAlGaN epitaxial layers. An internal quantum efficiency as high as 15% was estimated for a quaternary InAlGaN-based single quantum well at RT. Such high-efficiency UV emission can even be obtained on high threading-dislocation density buffer layers. A comparison of electroluminescence is made between light-emitting diodes (LEDs) with InAlGaN, AlGaN, and GaN active regions fabricated on SiC substrates with emission wavelengths between 340 and 360 nm. The emission intensity from the quaternary InAlGaN UV-LED was more than one order of magnitude higher than that from the AlGaN or GaN UV-LEDs under RT cw operation. We therefore fabricated 310–350-nm-band deep UV-LEDs with quaternary InAlGaN active regions. We achieved submilliwatt output power under RT pulsed operation for 308–314-nm LEDs. We also demonstrated a high output power of 7.4 mW from a 352-nm quaternary InAlGaN-based LED fabricated on a GaN substrate under RT cw operation. The maximum external quantum efficiency (EQE) of the 352-nm InAlGaN-based LED was higher than that obtained for an AlGaN-based LED with the same geometry. From these results, the advantages of the use of quaternary InAlGaN in 350-nm-band UV emitters were revealed. [ABSTRACT FROM AUTHOR]

Influence of Undoped-AlGaN Final Barrier of MQWs on the Performance of Lateral-Type UVB LEDs 17 mW semiconducting aluminium gallium nitride (AlGaN)-based ultraviolet-B (UVB) light-emitting diodes (LEDs) at 300 nm emission grown on AlN template are expected to be useful for production of vitamin D3 in the human body as well as for killing of tomato mosaic virus (ToMV) in tomato. UV-light power is improved in AlGaN UVB LED using optimized undoped-AlGaN final barrier of the quantum well. [Extracted from the article]

We demonstrated room-temperature (RT) intense ultraviolet (UV) emission with wavelength in the range of 300-340 nm from In[sub x1]Al[sub y1]Ga[sub l-x1-yl]N/In[sub x2]Al[sub y2]Ga[sub 1-x2-y2]N multiple-quantum wells (MQWs) fabricated on SiC by metalorganic vapor phase epitaxy. We found that the UV emission is considerably enhanced upon introducing approximately 5% of In into A1GaN. Maximally efficient emission was obtained at 318 nm from the fabricated In[sub 0.05]Al[sub 0.34]Ga[sub 0.61]N/In[sub 0.02]Al[sub 0.60]Ga[sub 0.38]N three-layer MQW when the QW thickness was approximately 1.4 nm. The intensity of 320 nm band emission from the InA1GaN-based MQWs was as strong as that of 410 nm band emission from InGaN-based QWs at RT. We observed emission fluctuations of submicron size in cathode luminescence images of In[sub x1]Al[sub y1]Ga[sub l -x1-y1]N/In[sub x2]Al[sub y2]Ga[sub l-x2-y2]N single QW which might be due to In segregation effect. The temperature dependence of photoluminescence emission for InA1GaN-based QWs was greatly improved in comparison with that of GaN- or A1GaN-based QWs. [ABSTRACT FROM AUTHOR]

We demonstrated efficient ultraviolet (UV) photoluminescence (PL) with the wavelength ranging from 230 to 280 nm from Al[sub x]Ga[sub 1-x]N(AlN)/Al[sub y]Ga[sub 1-y]N multiquantum wells (MQWs) grown on SiC by metalorganic vapor phase epitaxy. We systematically investigated the PL intensity of the AlGaN-based MQWs with wide-band-gap AlGaN barriers as functions of QW thickness and Al content of barriers. Single-peak, efficient PL emission was obtained at 282-234 nm at 77 K from Al[sub x]Ga[sub 1-x]N(AlN)/Al[sub y]Ga[sub 1-y]N 5-layer MQWs with approximately 1.5-nm-thick active layers by changing the Al content of the AlxGa[sub 1-x]N barriers from 53% to 100%. The efficiency of the deep-UV emission from AlGaN-based QWs was as high as that of blue emission from InGaN-based QWs at 77 K. [ABSTRACT FROM AUTHOR]

Reports on the experimental determination of the photoluminescence mechanism in a set of InGaN quantum wells. Photoluminescence as a function of the quantum well width; Influence of a piezoelectric field induced by strain; Causes of the abnormal photoluminescence shifts and temperature dependencies.

Estimates the carrier capture time for separate confinement heterostructure single quantum-well lasers by spontaneous emission spectra. Measurement of spontaneous emission from the optical confinement layers; Dominance of hole capture time for the emission increase; Attainment of additional transport time for multiquantum well lasers.

Highlights • GaN-based THz-QCL active region was grown by MOCVD on Si substrate. • A GaN/AlGaN QC active layer consisting of two-QW for one-period was grown. • Samples were evaluated by HR-XRD and cross-sectional TEM. • FWHM of the XRD satellite peaks became narrower when growth temperature decreased. Abstract GaN/AlGaN THz quantum-cascade laser (QCL) structures were grown on AlGaN/Si templates by a metal-organic chemical vapor deposition (MOCVD). We aim to fabricate a double metal waveguide (DMW) based GaN THz QCL through removing the Si substrate by wet etching. The DMW is essential for realizing low cavity loss and a very high optical confinement factor (99%). We grew 100 periods of GaN/Al 0.21 Ga 0.79 N two quantum wells (QW) type active region on Al 0.08 Ga 0.92 N/Si templates. Samples were evaluated by cross-sectional transmission-electron microscopy (TEM) and high-resolution X-ray diffraction (HR-XRD). We found that the full width at half maximum (FWHM) of the XRD satellite peaks became narrower when the growth temperature decreased, and the steepness of the QC layer hetero-interfaces has been improved. [ABSTRACT FROM AUTHOR]

Nonradiative recombination (NRR) processes through defect states and their temperature dependence in UV-B AlGaN MQW sample on sapphire substrate grown by MOCVD technique have been studied by photoluminescence (PL) spectroscopy. We detected NRR centers by adding a below-gap excitation light with photon energies from 0.93 eV to 1.46 eV on an above-gap excitation light of 4.66 eV. All the BGE energies decreased PL intensity at 25 K, and the most-distinct quenching is observed by 1.27 eV BGE at the same BGE photon number density. The temperature-dependent PL intensity for the BGE energy of 1.27 eV is interpreted by three NRR centers. The one-level model dominates over that of two-level model in the temperature range 58 K < T < 88 K. The two-level model prevails in other region of temperature. The combination of one-level and two-level models is consistent with the spectral peak-energy shift as a function of temperature. • Investigates NRR processes in UV-B AlGaN MQW by adding below-gap excitation light. • Detects NRR centers by observing PL-intensity change due to below-gap excitation. • Identifies correlation of T -dependent NRR processes with T -dependent PL peak-energy. • Interpret T -dependent NRR processes by a model based on SRH statistics. [ABSTRACT FROM AUTHOR]