InxGa1 xN Research Articles

Growth and properties of In‐rich InGaN films grown on (0001) sapphire by RF‐MBE

AbstractWe have performed detailed investigations of the optical properties of strain‐free In‐rich InxGa1–xN (0.61 ≤ x ≤ 1.0) films that were grown directly on (0001) sapphire substrates by radio‐frequency plasma‐excited molecular beam epitaxy. In‐composition dependence of photoluminescence peak energy for the InxGa1–xN films, measured at room temperature, exhibited a monotonic and smooth decrease with the increase in the In‐composition, approaching to 0.66 eV of a strain‐free InN film that was also grown directly on the sapphire substrate. The energy bowing parameter, b was determined to be 1.8 eV using these strain‐free InxGa1–xN films. Structural properties of these films are also investigated in detail. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Optically pumped lasing and gain formation properties in blue Inx Ga1–x N MQWs

AbstractLasing and gain formation properties have been studied in an InxGaN1–x multiple quantum wells lasing at around 460 nm by employing the modified variable stripe length method (VSLM) and pump and probe spectroscopy (P&P). It was found that the spontaneous emission (Esp) appeared far below absorption edge (Ea) that is observed as a photo‐bleaching negative‐peak in P&P, indicating the formation of localized tail states. Lasing peaks appeared in between Esp and Ea with wide spectral distribution. This is consistent with the results where VSLM revealed the broad feature of optical gain spectra associated with rapid peak saturation of lasing even just above the threshold photo‐pumping power density. Such mechanism observed in In‐rich InxGa1–xN MQWs is contributed not only from the broad distribution of localized density‐of‐states but also from hot carrier distribution (determined by Maxwell–Boltzmann statistics), temperature of which is raised up due to long energy relaxation time to localized tail states. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Ordering in cubic AlxGa1–xN and InxGa1–xN alloys due to biaxial strain

Chemical ordering in cubic epitaxial AlxGa1–xN and InxGa1–xN layers grown on GaN are investigated by combining first-principles pseudopotential plane-wave total energy calculations, a local concentrationdependent cluster-based method, and Monte Carlo simulations. It is found that for the unstrained or fully relaxed layers there are no stable ordered structures. The energetics of the AlxGa1–xN and InxGa1–xN layers pseudomorphycally grown on fully relaxed GaN (001) buffers show that biaxial strain acts as driving force for chemical ordering in both alloys. It is found that strained InxGa1–xN alloy comprises stable ordered structures while AlxGa1–xN is barely stable, or at most metastable possibly because of a slow kinetics of relaxation. Our results are used to shed light on the understanding of the observed ordered phases formation in both alloys. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Phase separation and ordering in group-III nitride alloys

The electronic, structural, and thermodynamic properties of cubic (zinc blende) group-III nitride ternary InxGa1�xN and quaternary AlxInyGa1�xyN alloys are investigated by combining first-principles total energy calculations and cluster expansion methods. External biaxial strain on the alloys are taken into account in the calculations. While alloy fluctuations and strain effects play a minor role in the physical properties of AlGaN, due to the small lattice-mismatch, in the InGaN alloys we found a remarkable influence of strain on the phase separation. The effect of biaxial strain on the formation of ordered phases has been also investigated, and the results are used to clarify the origin of the light emission process in InxGa1�xN based optoelectronic devices. The dependence of the lattice constant and energy bandgap in quaternary AlxInyGa1�xyN alloys was obtained as function of the compositions x and y.

Photoluminescence of InGaN/GaN and AlGaN/GaN multiple quantum well structures: role of depletion fields and polarization fields

We report on low temperature photoluminescence (PL) in InxGa1–xN multiple quantum wells (MQWs) with x in the range 0.1 and highly Si doped barriers of In0.01Ga0.99N. The MQW was placed either in the near surface depletion field or at the n-side depletion field of a pn-junction. At low temperatures the PL and electroluminescence (EL) spectra are quite different at no, low, or reverse bias, the PL appearing at higher energy. At high forward bias a spectral component at the EL position appears in PL. This proves a strong influence of the depletion field on the optical spectra. Preliminary results are also reported for Al0.07Ga0.93N/GaN structures, with near surface MQWs with 5 QWs, both Si-doped and nominally undoped. These structures show clear spectral features related to nonequivalent QWs in a graded depletion field.

First‐Principles Calculation of Structural and Electronic Properties of Wurtzite Al x Ga 1— x N, In x Ga 1— x N, and In x Al 1— x N Random Alloys

Using first-principles method, we studied the influence of alloying on the structural and electronic properties of the AlxGa1—xN, InxGa1—xN, and InxAl1—xN random alloys in the wurtzite structure. For AlxGa1—xN, the lattice constants a and c are found to follow the Vegard law, while for both InxGa1—xN and InxAl1—xN the lattice parameters, a, exhibit an upward bowing of —0.09 and —0.16 A, respectively. The calculated band structures for the three alloys show a direct-gap, and exhibit a downward bowing of 0.71, 1.7, and 4.09 eV for AlxGa1—xN, InxGa1—xN, and InxAl1—xN, respectively.

Temperature Dependence of Optical Absorption Spectra in In x Ga 1— x N Epitaxial Layers

Optical properties of InxGa1—xN epitaxial layers with indium compositions of x = 0.02, 0.05, and 0.09 have been studied by means of optical absorption (OA) and photoluminescence (PL) spectroscopy. The clear peak due to the absorption of InxGa1—xN ternary alloys was observed up to 300 K for the first time. As a result, temperature dependence of Stokes shift was investigated. The Stokes shift was estimated to be 51 meV for the sample with x = 0.05. With increasing temperature up to about 50 K, the Stokes shift slightly increased. With further increasing temperature from 50 to 100 K, the Stokes shift conversely decreased. Above 100 K, the Stokes shift was independent of temperature and indicated almost a constant value up to 300 K. The Stokes shift at 300 K was estimated to be 44 meV. This temperature dependence of Stokes shift was a characteristic common to all of the samples used in the present work.

Comment on ?Band Gap of InN and In-Rich InxGa1?xN Alloys (0.36 < x < 1)?

physica status solidi (b)Volume 233, Issue 3 p. R8-R9 Rapid Research Note Comment on “Band Gap of InN and In-Rich InxGa1—xN Alloys (0.36 < x < 1)” B.R. Nag, B.R. Nag brn@cucc.ernet.in Institute of Radio Physics and Electronics, 92 Acharya Prafulla Chandra Road, Kolkata-700009, IndiaSearch for more papers by this author B.R. Nag, B.R. Nag brn@cucc.ernet.in Institute of Radio Physics and Electronics, 92 Acharya Prafulla Chandra Road, Kolkata-700009, IndiaSearch for more papers by this author First published: 26 September 2002 https://doi.org/10.1002/1521-3951(200210)233:3<R8::AID-PSSB99998>3.0.CO;2-ICitations: 8AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article.Citing Literature Volume233, Issue3October 2002Pages R8-R9 RelatedInformation

Single Chip White LEDs

The operation principles of the various single chip luminescence conversion (LUCO) white LEDs, that can be realized with blue or ultraviolet (UV) LED chips, are briefly reviewed. In order to demonstrate the feasibility of the so far almost unexplored case of UV pumped tricolor LUCO white LEDs, InxGa1—xN single quantum well LED layer sequences were grown by MOCVD and processed into near-UV LED pump chips. Europium doped oxide and sulfide phosphors emitting in the blue, the green and the red spectral range were used for UV to visible conversion. With white blends prepared from these phosphors, housed white LEDs with high color quality were realized.

Electroluminescence Mapping of InGaN-based LEDs by SNOM

Electroluminescence (EL) mapping has successfully been performed for InxGa1—xN single quantum well (SQW)-based light emitting diodes (LEDs) by employing scanning near-field optical microscopy (SNOM) at room temperature. The relative EL intensity fluctuates in the range from 1 to 4 with the spatial scale less than a few hundred nanometers. Clear correlation has been found between EL peak wavelengths and EL intensities where the regions of long wavelength correspond to those of strong EL intensity, suggesting that the injected carriers redistribute toward local potential minima within InxGa1—xN SQWs.

Vibrational Properties of Cubic AlxGa1?xN and InxGa1?xN Ternary Alloys

The phonon dispersion curves and phonon density of states of the cubic AlxGa1—xN and InxGa1—xN alloys were calculated within the framework of a valence force field method and an effective charge approach for the long-range Coulomb interaction. The valence-force parameters and effective charges for the cubic GaN, AlN and InN binary compounds were fitted to the phonon frequencies at high-symmetry points in the Brillouin zone measured by Raman spectroscopy and calculated by ab initio methods. The scaling factor approximation was used to obtain the valence-force parameters and effective charges for the alloys. A good agreement between our calculations and recent Raman data for the alloys is achieved.

Photoluminescence Excitation Spectroscopy of InxGa1xN/GaN Multiple Quantum Wells with Various In Compositions

Luminescence properties of InxGa1—xN/GaN multiple quantum wells (MQWs) with various In compositions have been studied by means of photoluminescence excitation (PLE) spectroscopy. The clear peak due to the absorption of InxGa1—xN quantum wells was observed in the PLE spectrum of the MQW sample with x < 0.01 at 4 K, and the Stokes shift was estimated to be 63 meV. It was found from temperature-dependent PLE measurements that the Stokes shift was independent of temperature up to 300 K. This result suggests that the large Stokes shift cannot be explained only by the effect of carrier localization due to compositional fluctuation.

Polaron Effective Mass and Binding Energy in Semiconducting InxGa1?xN

Polaron binding energy and effective mass are calculated for bulk hexagonal InxGa1—xN with the use of a dielectric continuum Fröhlich-like electron–phonon Hamiltonian. The calculation includes the evaluation of polaronic corrections up to second order in the coupling constants. The result obtained for the electron effective mass in hexagonal GaN is in very good agreement with recent experimental findings. The relevance of having reliable experimental values for the InN conduction band effective mass and optical dielectric constant in order to clarify the real contribution of second-order corrections to the binding energy and the anisotropy of the polaron mass in this material is discussed. The case of cubic alloys is briefly considered as well.

Impact of Internal Electric Field and Localization Effect on Quantum Well Excitons in AlGaN/GaN/InGaN Light Emitting Diodes

Strained InxGa1—xN quantum wells (QWs) on thick GaN base layers were investigated to verify the importance of localized QW excitons in their spontaneous emission mechanisms. A strength of the internal piezoelectric field (FPZ) across the QW increases with increasing x up to 1.4 MV/cm for x = 0.25, since the in-plain strain increases. For the QWs with the well thickness L greater than 3 nm, FPZ dominates the emission peak energy due to the quantum-confined Stark effect. Absorption spectra of both hexagonal and cubic InGaN QWs exhibited a broad band-tail regardless of the presence of FPZ normal to the QW plane. The luminescence peak energy of the 3 nm thick QWs was higher than the bandgap energy of the unstrained bulk crystal for x < 0.15, showing that doping of Si in barriers or injection of carriers effectively screens the field. The emission lifetime increased with increasing monitoring wavelength. Also, a temperature-induced change in the luminescence peak energy decreased with increasing x. The real-space variation of the luminescence peak energy was confirmed by the spatially-resolved monochromatic cathodoluminescence mapping method. The localization depth increases with increasing x. The carrier localization is confirmed to originate from the effective bandgap inhomogeneity due to a fluctuation of the local InN mole fraction, which is enhanced by the large and composition-dependent bowing parameter of InGaN material.

Radiative Recombination Dynamics of Carriers in InxGa1-xN Epitaxial Layers Revealed by Temperature Dependence of Time-Resolved Photoluminescence Spectra

The radiative recombination process of InxGa1–xN epitaxial layers was investigated by means of the temperature dependence of time-resolved photoluminescence spectra. Two emission components separated by about 40 meV were clearly observed. At 6 K, the higher- and lower-energy components had decay-time constants of 30 and 540 ps, respectively. The energy transfer of carriers between the two levels was observed. With increasing temperature, the decay-time constant of the higher-energy component was almost constant of about 30 ps, whilst that of the lower-energy component decreased from 540 to 20 ps. On the basis of a strong electron–phonon interaction the contribution of a polaron state of electrons to the recombination process of the higher-energy component is proposed.

Dimensionality of Excitons in InGaN-Based Light Emitting Devices

Temperature dependence of radiative and non-radiative recombination times has been investigated in InxGa1–xN based light emitting devices by employing time-resolved luminescence spectroscopy. The mean In-composition (x value) assessed in this study is 10%, 20% and 30% whose emissions at 300 K correspond to near ultraviolet (390 nm), violet (422 nm) and blue (471 nm), respectively. It was found that the degree of exciton localization was enhanced with increasing In-composition in InxGa1–xN active layers, and that the zero-dimensional feature was revealed best of all in the sample with x = 30%, where radiative recombination time was almost constant (4 to 6 ns) in the temperature range from 20 to 300 K. The internal quantum efficiency of this sample was estimated to exceed 70% at 300 K. It is likely that such high efficiency is a result of zero-dimensionality because capture cross-sections to non-radiative recombination centers are greatly reduced once excitons are trapped at deep localization centers.

Photoluminescence and Gain of MBE Grown Cubic InxGa1—xN/GaN Heterostructures

We report on the molecular beam epitaxy of cubic InxGa1—xN/GaN heterostructures with different In contents. High resolution X-ray diffraction (HRXRD) measurements gave the In content in the layers. Reciprocal space mapping of the symmetrical (002) and asymmetrical (113) reflection reveal that cubic InxGa1—xN with low In content (x < 0.2) is single phase whereas layers with higher In content show evidence of phase separation. The photoluminescence (PL) of the InxGa1—xN layers was measured at 2 K. The full width at half maximum of the PL is as low as 200 meV for single phase layers whereas the emission is significantly broadened for phase separated InxGa1—xN. Optical gain measurements revealed a maximum gain of about 70 cm—1 from InxGa1—xN (x = 0.07) and a decrease of the gain with increasing In content.

Observation of Phonon Modes in Bulk InGaN Films by Raman Scattering

Thick InxGa1—xN layers were grown by hot wall epitaxy on sapphire substrates without thick GaN buffer layers, and the E2 phonon mode was clearly observed by a non-resonant Raman spectroscopy for the first time. In the measured range of In content, x< 0.07, the phonon frequency showed a linear variation with the In content between the corresponding values for pure GaN and InN compounds.

MOVPE Growth of In-Rich InxGa1xN (0.5 &lt; x &lt; 1) Films on α-Al2O3(0001)

This paper reports MOVPE growth of In-rich InxGa1—xN films with x = 0.5 to 1.0 on α-Al2O3(0001) at temperatures from 550 to 700 °C. The θ–2θ X-ray diffraction analysis shows that single phase films of x = 0.5 to 1 are successfully grown by lowering the growth temperature to 550 °C. With decreasing x, the surface roughness of the films is decreased and striped features appear on the surface. When the growth temperature is increased up to 600 °C or higher, a single phase film is obtained only for x > 0.8. Even a film with multi-peaks in the X-ray diffraction pattern shows no rings in the RHEED pattern. A film grown at 550 to 600 °C with x ≈︂ 0.5 has additional spots in its RHEED pattern. Such a pattern is very similar to that expected for a mixture of wurtzite and zincblende crystals.

Determination of Optical Constants for Cubic InxGa1—xN Layers

Spectroscopic ellipsometry studies have been carried out in the energy range from 1.5 to 4.0 eV in order to determine the complex refractive indices for cubic InGaN layers with various In contents. Studying single GaN films we prove that for the analysis of optical data a parametric dielectric function model can be used. Its application to the InGaN layers yields the refractive index and extinction coefficient as function of photon energy as well as the compositional dependence of the fundamental absorption edge at room temperature. From the latter a bowing parameter of 1.4 eV is deduced.