InGaN System Research Articles

Kinetically controlled composition of III-V ternary nanostructures

Controlling the composition of ternary III-V and III-nitride nanomaterials such as vertical nanowires, horizontal nanowires, nanosheets, and nanomembranes grown by different epitaxy techniques is essential for band gap engineering and fabrication of nanoheterostructures with tunable properties. Herein, we investigate the diffusion-induced growth process of III-V ternary materials in different geometries including planar layers, nanomembranes, and horizontal and vertical nanowires grown by selective area epitaxy or with a catalyst droplet on top and derive a rather general equation connecting the composition of ternary solid with the composition of vapor. The form of this vapor-solid distribution remains identical for a wide range of geometries, while the coefficients entering the equation contain thermodynamic factors, kinetic constants of the material transport, and geometrical parameters of the growth template. General properties of the vapor-solid distribution are investigated with respect to material constants, growth condition, and geometry, including the interplay of thermodynamics and growth kinetics leading to the suppression of the miscibility gaps in InGaAs and InGaN systems. A good correlation of the model with the data on the compositions of InGaAs, InGaP, and AlGaAs materials grown by different methods is demonstrated. Overall, these results give a simple analytical tool for understanding the compositional trends and compositional tuning of III-V ternary nanostructures, which should work equally well for Si-Ge and II-VI material systems.

34‐2: Invited Paper: Analysis of Temperature Effect of RGB Mini/Micro LED Chips

Mini/Micro light‐emitting‐diode (LED) emissive display has attracted wide range of attentions and has a good development prospect. One of the big challenges of Mini/Micro LED display is that the attenuation degree of luminous intensity of RGB chips with increasing working temperatures are different. In this work, we deeply analyzed this phenomenon and found out the factors contributing to that. It is found out that the attenuation degree of luminous intensity is directly related to the current density and material system of the chips. Moreover, the defect related non‐radiative recombination process and the different band structures between AlGaInP system and InGaN system are regarded as the main factors contributing to the different attenuation degree of luminous intensity with increasing working temperature of RGB chips.

Auger recombination in AlGaN quantum wells for UV light-emitting diodes

We show that the often observed efficiency droop in AlGaN quantum well heterostructures is an internal carrier loss process, analogous to the InGaN system. We attribute this loss process to Auger recombination, with C = 2.3 × 10−30 cm6 s−1; a similar value found commonly in InGaN-based devices. As a result, the peak internal quantum efficiency (IQE) of our structures is limited to 66%. These values were obtained by resonant excitation (time-resolved) photoluminescence (PL), avoiding common error sources in IQE measurements. The existence of strong Auger recombination implies that simple methods employed for IQE determination, such as temperature-dependent PL, may lead to erroneous values. Auger losses will have to be considered once the challenges regarding carrier injection are solved.

Deterministic optical polarisation in nitride quantum dots at thermoelectrically cooled temperatures

We report the successful realisation of intrinsic optical polarisation control by growth, in solid-state quantum dots in the thermoelectrically cooled temperature regime (≥200 K), using a non-polar InGaN system. With statistically significant experimental data from cryogenic to high temperatures, we show that the average polarisation degree of such a system remains constant at around 0.90, below 100 K, and decreases very slowly at higher temperatures until reaching 0.77 at 200 K, with an unchanged polarisation axis determined by the material crystallography. A combination of Fermi-Dirac statistics and k·p theory with consideration of quantum dot anisotropy allows us to elucidate the origin of the robust, almost temperature-insensitive polarisation properties of this system from a fundamental perspective, producing results in very good agreement with the experimental findings. This work demonstrates that optical polarisation control can be achieved in solid-state quantum dots at thermoelectrically cooled temperatures, thereby opening the possibility of polarisation-based quantum dot applications in on-chip conditions.

Point defects and composition in hexagonal group-III nitride monolayers: A first-principles calculation

In this paper, we systematically investigate the structural, electronic and magnetic properties of hexagonal group-III nitride monolayers with point defects and alloying on the basis of first-principles calculations. Six typical point defects including three vacancies and three antisites are modeled in pure AlN, GaN and InN monolayers. The defect-induced modifications of band gaps and magnetic properties are demonstrated. The vacancy of nitrogen, with lowest formation energy, metalizes the semiconducting nitride monolayers. The defects losing single group-III atom introduce net magnetic moment to the systems, while others maintain non-magnetic. For ordered alloy monolayers, the AlGaN and InGaN systems are taken into consideration. The compositional variation is achieved by atomic substitution in supercells with different sizes. We find that the lattice constant and cohesive energy follow good linear relation with concentration while a slight bowing effect is observed for the band gap. These results provide a development in defective and alloy nitride monolayers and extend the potential applications.

Sub-μeVdecoherence-induced population pulsation resonances in an InGaN system

We report on high frequency resolution coherent nonlinear optical spectroscopy on an ensemble of InGaN disks in GaN nanowires at 300 K. Sub-$\mu$eV resonances in the inhomogeneously broadened third order ($\chi^{(3)}$) absorption spectrum show asymmetric line shapes, where the degree of asymmetry depends on the wavelength of the excitation beams. Theory based on the Optical Bloch Equations (OBE) indicates that the lineshape asymmetry is a result of fast decoherence in the system and the narrow resonances originate from coherent population pulsations that are induced by decoherence in the system. Using the OBE, we estimate that the decoherence time of the optically induced dipole (formed between the unexcited ground state the excited electron-hole pair) at room temperature is 125 fs, corresponding to a linewidth of ~10 meV. The decay time of the excitation is ~5-10 ns, depending on the excitation energy. The lineshapes are well fit with the OBE indicating that the resonances are characterized by discrete levels with no evidence of many body physics.

Temperature-dependent fine structure splitting in InGaN quantum dots

We report the experimental observation of temperature-dependent fine structure splitting in semiconductor quantum dots using a non-polar (11-20) a-plane InGaN system, up to the on-chip Peltier cooling threshold of 200 K. At 5 K, a statistical average splitting of 443 ± 132 μeV has been found based on 81 quantum dots. The degree of fine structure splitting stays relatively constant for temperatures less than 100 K and only increases above that temperature. At 200 K, we find that the fine structure splitting ranges between 2 and 12 meV, which is an order of magnitude higher than that at low temperatures. Our investigations also show that phonon interactions at high temperatures might have a correlation with the degree of exchange interactions. The large fine structure splitting at 200 K makes it easier to isolate the individual components of the polarized emission spectrally, increasing the effective degree of polarization for potential on-chip applications of polarized single-photon sources.

High brightness InGaN-based yellow light-emitting diodes with strain modulation layers grown on Si substrate

InGaN-based multiple quantum wells (MQWs) yellow light-emitting diodes (LEDs) were grown on Si substrate by metal organic vapor deposition. Blue MQWs were introduced as strain modulation layers for yellow MQWs. The LED chips emitted 72-mW yellow light with 566-nm dominant wavelength and 9.4 % external quantum efficiency (EQE) at 350 mA under room temperature, and it reached a peak EQE of 22.2 % at 0.7 mA. A comparison sample without strain modulation layers exhibited much weaker performance. The results reveal that long-wavelength emission of InGaN system is reliable if the strain of MQWs has been properly modulated.

Photoluminescence study of ZnCdO alloy

The photoluminescence (PL) properties of ZnCdO alloy are investigated at various temperatures. Each PL profile contains four distinct peaks: P1, P2, P3, and P4. Peak P4 is due to free-excitonic recombination. The temperature mapping of the P3 peak position shows an S-shaped shift similar to the InGaN system due to the inhomogeneous distribution of Cd in ZnCdO alloy. The peak P2 is attributed to the phonon replica of the localized excitonic emission (P3). The linewidth of the photoluminescence profiles of the ZnCdO alloy is shown to depend on the concentration of Cd.

High quantum efficiency ultraviolet/blue AlGaN∕InGaN photocathodes grown by molecular-beam epitaxy

Enormous technological breakthroughs have been made in optoelectronic devices through the use of advanced heteroepitaxial-semiconductor crystal-growth techniques. This technology is being extended toward enhanced ultraviolet/blue single-photon detection through the design and fabrication of atomically tailored heteroepitaxial GaAlN∕GaInN photocathode device structures. The AlGaN∕InGaN system is ideal because the band gap can be tailored over an energy range from 0.8 to 6.2 eV and epitaxial thin-film layers can be grown directly on optically transparent sapphire substrates. Although a single p-type GaN layer activated with cesium can produce reasonably high quantum efficiency in the ultraviolet wave band, a more complex design is necessary to achieve high levels extending into the blue region. In the present work, band-gap engineering concepts have been utilized to design heterostructure photocathodes. The increased level of sophistication offered by this approach has been exploited in an attempt to precisely control photoelectron transport to the photocathode surface. Thin heterostructure layers designed for transmission-mode detection were fabricated by molecular-beam epitaxy. A quantum efficiency of 40% at 250 nm was achieved using a thin, compositionally graded GaN∕InGaN layer, epitaxially grown on a sapphire substrate. Further improvements are anticipated through continued optimization, defect reduction, and more complex photocathode designs.

UV Emission Mechanisms in Quaternary AlInGaN Epilayers and Multiple Quantum Wells

AbstractWe present the results on investigation and analysis of photoluminescence (PL) dynamics of quaternary AlInGaN epilayers and AlInGaN/AlInGaN multiple quantum wells (MQWs) grown by a novel pulsed metalorganic chemical vapor deposition (PMOCVD). The emission peaks in both AlInGaN epilayers and MQWs show a blueshift with increasing excitation power density. The PL emission of quaternary samples is attributed to recombination of carriers/excitons localized at band-tail states. The PL decay time increases with decreasing emission photon energy, which is a characteristic of localized carrier/exciton recombination due to alloy disorder. The obtained properties of AlInGaN materials grown by a PMOCVD are similar to those of InGaN. This indicates that the AlInGaN system is promising for ultraviolet applications such as the InGaN system for blue light emitting diode and laser diode applications.

Phonons in III–V nitrides: Confined phonons and interface phonons

Phonons in III–V nitrides are examined experimentally for dimensionally confined systems and for alloys of InGaN with a view towards understanding the phonon modes of these systems. Results are compared with the predictions of Loudon's model for uniaxial semiconductors. The modes of the InGaN system are compared with those of the AlGaN ternary alloy. The first Raman measurements of interface phonons in binary GaN–AlN superlattices are presented.

Theoretical predictions of unstable two-phase regions in wurtzite group-III-nitride-based ternary and quaternary material systems using modified valence force field model

Group-III (B, Al, Ga, and In)-nitride quaternary alloys and group-III (Al, Ga, and In)-nitride-based mixed anion (As, P, and Sb) quaternary alloys are useful for blue and green light emitting devices and high-temperature, high-power, and high-frequency electronic devices. It is known that these alloys are very difficult to grow in certain compositional regions. The thermodynamical stability of these alloys is studied with respect to an unstable two-phase region in the phase field. The unstable two-phase region is predicted based on a strictly regular solution model. The interaction parameter used in our model is obtained analytically using the valence force field (VFF) model modified for wurtzite structures. The calculated interaction parameters, which are required to predict the unstable two-phase region, agree well with experimental results for related alloy systems. The modified VFF model can also be used to predict the microscopic crystal structure, such as first neighbor anion–cation bond lengths. In order to verify our calculations, we compare the calculated and experimental results for the first neighbor anion–cation bond lengths in the InGaN system. The calculated results agree well with the experimental results. From our calculation results, the unstable two-phase regions for four A1−x−yBxCyD type group-III-nitride quaternary alloys and nine A1−xBxC1−yDy type group-III-nitride mixed anion quaternary alloys are calculated. The predicted unstable two-phase regions agree well with experimentally observed regions of phase separation in ternary alloys, which suggests our model calculations can provide useful guidance in ternary and quaternary systems where there is no experimental data.

Calculations of carrier localization inInxGa1−xN

The electronic structures of cubic InGaN systems are calculated using an atomistic empirical pseudopotential method. Two extreme cases are studied. One is a pure InN quantum dot embedded in a pure GaN matrix, another is a pure ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}$ alloy without clustering. We find hole localizations in both cases. The hole wave function starts to be localized as soon as a few In atoms segregate to form a small cluster, while the electron wave function only becomes localized after the number of In atoms in the quantum dot becomes larger than 200. The hole state is also strongly localized in a pure ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}$ alloy, on top of randomly formed (110) directioned In-N-In chains. Using one proposed model, we have calculated the hole energy fluctuation, and related that to photoluminescence linewidth. The calculated linewidth is about 100 meV, close to the experimental results. Wurtzite InGaN is also studied for optical anisotropies. We find that in both quantum dot and pure alloy, the polarization is in the $\mathrm{xy}$ plane perpendicular to the c axis of the wurtzite structure.

V-shaped defects connected to inversion domains in AlGaN layers

Thick AlGaN layers and AlGaN/GaN superlattices have been grown on GaN using metalorganic chemical vapor deposition. Cross-sectional transmission electron microscopy has been used to show that V-shaped surface pits on these samples differ from similar features observed in the InGaN system. Inversion domains and segregated Al are found in the middle of each V pit, and superlattice layers are observed to follow the pit sidewalls.

Theoretical analysis of unstable two-phase region and microscopic structure in wurtzite and zinc-blende InGaN using modified valence force field model

A model to predict material characteristics of the InGaN ternary system, which is useful for blue and green light emitting and laser diodes, with respect to an unstable two-phase region in the phase field and the first neighbor anion–cation bond length is developed. The unstable region is analyzed using a strictly regular solution model. The interaction parameter used in the analysis is obtained from a strain energy calculation using the valence force field (VFF) model, modified for both wurtzite and zinc-blende structures to avoid overestimation of the strain energy. The structural deviation from an ideal wurtzite structure in GaN and InN is also taken into account in our model. The critical temperatures found in our analysis for wurtzite InGaN and zinc-blende InGaN are 1967 and 1668 K, respectively. This suggests that, at typical growth temperatures around 800 °C, a wide unstable two-phase region exists in both wurtzite and zinc-blende structures. The modified VFF model can also predict the microscopic crystal structure, such as first neighbor anion–cation bond lengths. In order to validate our calculation results for zinc-blende structures, we compare the calculated and the experimental results in terms of the interaction parameter and the first neighbor anion–cation bond lengths in the InGaAs system. For the wurtzite structure, we compare the calculated and the experimental results for the first neighbor anion–cation bond lengths in the InGaN system. The calculated results agree well with the experimental results.

Differences in Design Considerations between InGaN and Conventional High-Brightness Light-Emitting Diodes

Based on the escape cone concepts, high-brightness light-emitting diodes (LEDs) have been analyzed. In AlGaAs or InGaAlP LEDs, photon absorption in the ohmic region under the electrode is known to be significant. Thus, ins general, a thick window layer (WL) and a transparent substrate (TS) would minimize photon shielding by the electrodes and considerably improve photon output coupling efficiency. However, the schemes do not seem to be necessary in InGaN system. Photon absorption in ohmic contact to a wide bandgap semiconductor such as GaN may be negligible and, as a result, the significant photon shielding by the electrodes will not degrade the photon output coupling efficiency so much. The photon output coupling efficiency estimated in InGaN LEDs is about 2.5 - 2.8 times that of the conventional high-brightness LED structures based on both WL and TS schemes. As a result, the extenal quantum efficiency in InGaN LEDs is as high as 9% despite the presumably very low internal quantum efficiency.