Phase Separation In InGaN Research Articles

Phase separation in Zn-doped InGaN grown by metalorganic chemical vapor deposition

Zn-doped InGaN thin films were deposited on GaN/sapphire by metalorganic chemical vapor deposition, and studied by a combination of high-resolution X-ray diffraction (HR-XRD), micro-photoluminescence (PL) and secondary ion mass spectrometry (SIMS). Indium phase separation is studied comparatively. HR-XRD exhibits a GaN band and a single band from InGaN for samples without phase separation, but two InGaN bands corresponding to different x(In) for samples with phase separation. PL excitation power dependence measurements reveal 2 sets of InGaN PL emissions for samples with phase separation, but only 1 set for samples without phase separation. SIMS data showed that phase separated InGaN:Zn films possess a high Zn concentration near the InGaN–GaN interface and non-uniform distributions of In and Zn contents, which are in contrast with data from InGaN:Zn films with no In-phase separation.

CRYSTALLINE PHASE SEPARATION OF InGaN LAYER MATERIALS PREPARED BY METALORGANIC CHEMICAL VAPOR DEPOSITION

Zn -doped InGaN thin films were epitaxied on the top of 1-2 micron thick GaN grown on sapphire by metal organic chemical vapor deposition, and studied by a combination of high resolution X-ray diffraction (HR-XRD), micro-photoluminescence (PL) and secondary ion mass spectrometry (SIMS). HRXRD exhibits a GaN band and a single band from InGaN for samples without phase separation, but two or more InGaN bands corresponding to different x(In) for samples with phase separation. PL emissions from InGaN spread over a wider energy ranges and were modulated by the interference effects. Excitation power dependence measurements reveal 2-sets of PL emissions for samples with phase separation, but only 1-set for samples without phase separation. SIMS data showed that phase separated InGaN:Zn films possess a high Zn concentration near the InGaN-GaN interface and non-uniform distributions of In and Zn contents, which are in contrast with data from InGaN:Zn films with no In -phase separation. These interesting results are correlated to the growth process and microstructural properties.

Cubic III-nitrides—a key to understand radiative recombination in nitride heterostructures?

Besides the well known hexagonal (wurtzite) phase the group III-nitrides can also exist in the metastable cubic (zincblende) modification. One of the distinguished physical properties of the cubic nitrides is the absence of electric fields due to spontaneous polarization or piezoelectric effects in heterostructures. Therefore the cubic nitrides are supposed to be well suited to tackle the still open question of radiative recombination in InGaN/GaN quantum wells. In this paper we review recent experiments which demonstrate the importance of highly localized quantum dot-like structures formed by phase separation in InGaN for the optical properties of InGaN/GaN heterostructures.

Room-temperature random telegraph noise in luminescence from macroscopic InGaN clusters

We report on photoluminescence properties of individual macroscopically sized InGaN clusters that were formed in InGaN multiple quantum wells. Phase separation in InGaN results in the formation of clusters with a size of 1–2 μm with three different indium compositions. A small fraction (one in 100–1000) of the clusters shows random telegraph noise in luminescence at room temperature. Superlinear dependence of the luminescence switching rate on excitation intensity indicates that the switching is induced by the cooperation of multiple carriers.

A study of dislocations in InGaN/GaN multiple-quantum-well structure grown on ( [formula omitted]) sapphire substrate

Transmission electron microscope (TEM) and X-ray diffraction (XRD) measurements performed on an InGaN/GaN multiple-quantum-well (MQW) structure deposited on ( 1 1 2 ̄ 0 ) and (0 0 0 1) sapphire substrates have been investigated. The grown MQW deposited on ( 1 1 2 ̄ 0 ) sapphire substrate are still oriented along the (0 0 0 1) direction due to an unusual growth mechanism, which is confirmed by XRD measurement. TEM measurements of the initial stage of the high-temperature GaN growth after the low-temperature buffer GaN layer on ( 1 1 2 ̄ 0 ) sapphire are different from the TEM results obtained for GaN on (0 0 0 1) sapphire substrate, unlike the XRD patterns which were identical. In addition, a further TEM study indicates that the sample on ( 1 1 2 ̄ 0 ) sapphire substrate shows higher threading dislocation density than that on (0 0 0 1) sapphire substrate, a fact that is related to the initial stage of the high-temperature GaN layer. Moreover, the sample deposited on ( 1 1 2 ̄ 0 ) sapphire substrate shows a higher density of V-defects incorporating a threading dislocation. The plane-view images of scanning electron microscopy also show that there is a higher density of small dark spots appearing in the paired form compared with that on (0 0 0 1) sapphire substrate, which generally correspond to screw dislocations. It is generally accepted that V-defects and screw dislocations result in phase separation in InGaN around these defects and finally give rise to a strong exciton-localization effect. This result can explain why an enhanced exciton-localization effect was recently observed in the InGaN/GaN MQW deposited on ( 1 1 2 ̄ 0 ) sapphire substrate.

Evidence for Phase Separation in InGaN by Resonant Raman Scattering

Evidence for Phase Separation in InGaN by Resonant Raman Scattering

Group-III Nitride Quantum Heterostructures Emitting in the whole Visible Range

ABSTRACTGroup-III nitride quantum wells (QWs) and quantum dots (QDs) have been grown by molecular beam epitaxy (MBE). Their optical properties are shown to be governed by the presence of a huge internal polarization field. For example, GaN/AlN QDs emit from the blue to the orange due to the giant quantum confined Stark effect (QCSE) induced by a built- in electric field of 4 MV/cm. Another consequence of the QCSE is to drastically reduces the oscillator strength of the ground state transition and thereby to increase by several orders of magnitude its radiative lifetime. Despite the very large density of dislocations in nitride layers, which induce non-radiative recombinations, carrier localization enhances the photoluminescence (PL) efficiency. This is demonstrated by GaN/AlN QDs grown on silicon substrates exhibiting strong PL intensity at room temperature. InGaN/GaN QWs with In composition of 20% also display 300 K PL peaking through the whole visible spectrum (0.4-0.66 νm). This is achieved by varying the QW thickness from 1.5 to 5.5 nm, the red-shift resulting from an internal electric field of 2.5 MV/cm. For InGaN/GaN QWs emitting at 2.8-2.9 eV, the PL efficiency at 300 K is larger than 10 %. This is ascribed to carrier localization, which is not due to InGaN phase separation that would form QDs, as revealed by transmission electron microscopy. Another origin of the carrier localization in InGaN/GaN QWs is then discussed.

Study of Phase-Separated InGaN Grown by Metalorganic Vapor Phase Epitaxy

InGaN thin films were grown by low-pressure metalorganic vapor phase epitaxy (MOVPE) and characterized by photoluminescence (PL) with variable excitation intensity and temperature. The surface morphology was investigated by Atomic Force Microscopy (AFM). All the samples show dominant peaks at around 2.9 eV and extra peaks or shoulders at 2.8 eV at 6 K. We concluded that the low energy peak is due to the localized near-band edge transition from the phase-separated InGaN mesoscopic structure with high In content. The strong luminescence of the low energy peak at room temperature is due to the quantum confinement enhancement in the form of nanostructures or quantum dots. AFM images showed that phase-separated InGaN samples have inverted hexagonal pits which are formed by In segregation on (101_1) surfaces.

Phase separation in InGaN/GaN multiple quantum wells and its relation to brightness of blue and green LEDs

InGaN/GaN multiple quantum wells (MQW) and high-brightness MQW light-emitting diodes were grown by production scale metal-organic chemical vapor deposition (MOCVD). We have found that the extent of InGaN phase separation in InGaN MQWs depends strongly on growth conditions. Multiple peaks in photoluminescence (PL) of InGaN/GaN MQWs and electroluminescence (EL) of MQW LEDs can be observed at room temperature. In the presence of InGaN phase separation, photoluminescence of MQW is red-shifted with respect to the expected wavelength calculated for the apparent indium composition determined by X-ray diffraction (XRD). We have determined that InGaN phase separation is necessary for high brightness electroluminescence in LEDs. Under optimal growth conditions, MQWs with very well-defined XRD satellite peaks and PL in the wavelength range of (450–520 nm) can be achieved. High-brightness LEDs emitting at 480 nm have been successfully fabricated with an output power well better than 2 mW at 20 mA and with a forward voltage less than 4 V.

Phase separation in InGaN grown by metalorganic chemical vapor deposition

We report on phase separation in thick InGaN films with up to 50% InN grown by metalorganic chemical vapor deposition from 690 to 780 °C. InGaN films with thicknesses of 0.5 μm were analyzed by θ–2θ x-ray diffraction, transmission electron microscopy (TEM), and selected area diffraction (SAD). Single phase InGaN was obtained for the as-grown films with <28% InN. However, for films with higher than 28% InN, the samples showed a spinodally decomposed microstructure as confirmed by TEM and extra spots in SAD patterns that corresponded to multiphase InGaN.

Suppression of phase separation in InGaN due to elastic strain

The effect of elastic strain in epitaxial InGaN layers coherently grown on GaN wafers on spinodal decomposition of the ternary compound is examined. The effect results in considerable suppression of phase separation in the strained InGaN layers. To predict correctly the position of the miscibility gap in the T-x diagram it is important to take into account the compositional dependence of the elastic constants of the ternary compound. The contribution of the elastic strain to the Gibbs free energy of InGaN is calculated assuming uniform compression of the epitaxial layer with respect to the underlying GaN wafer. The interaction of binary constituents in the solid phase is accounted for on the base of regular solution model. The enthalpy of mixing is estimated using the Valence Force Field approximation. The strain effect becomes stronger with increasing In content in the InGaN. As a result the miscibility gap shifts remarkably into the area of higher InN concentration and becomes of asymmetrical shape. Various growth surface orientations and the type of crystalline structure (wurtzite or sphalerite) provide different effects of the elastic strain on phase separation in ternary compounds.

RT-CW Operation of InGaN multi-quantum-well structure laser diodes

The continuous-wave operation of InGaN multi-quantum-well (MQW) structure laser diodes (LDs) was demonstrated at room temperature with a threshold current of 25 mA, a threshold voltage of 5.8 V, an output power of 30 mW and a high operating temperature of 100°C. The energy differences between the absorption and the emission energy of the InGaN MQW structure LDs were as large as 220 meV at RT. A deep localized state (the localization energy is >100 meV) was formed in the InGaN well layer due to the InGaN phase separation during the growth. Both the spontaneous emission and the stimulated emission of the LDs originated from these deep localized energy states. The far field pattern showed a higher order transverse mode of the entire 5-μm-thick epitaxial layer stack, with air and sapphire as the upper and lower cladding layers, respectively.

Apparatus for HREEL and molecular beam spectroscopy of surfaces: A Gussoni, G Maloberti, L Risso, M Rocca, B Vincenzi and U Valbusa, Dipartimento di Fisica dell'Università di Genova and GNSM, Unità di Genova, Italy

Immiscibility of InGaN hinders epitaxial growth of high-quality AlGaN/InGaN heterojunction, which could have superior performances than AlGaN/GaN in view of high-speed devices. AlGaN/InGaN/GaN double heterostructures have been grown on silicon (111) substrate using plasma assisted molecular beam epitaxy. All growth conditions for each sample have been kept identical except the InGaN channel thickness. Alloy inhomogeneity has been found to occur in the InGaN channel by high resolution (HR) X-ray diffractometer (XRD) and cross-sectional HR- transmission electron microscopy (TEM). This non-uniformity of alloy causes reduced indium incorporation with a decrease of channel thickness along with a thin InN binary alloy. Capacitance-voltage (C-V) profile has revealed non-uniformity of alloy and spatial position of InN in the channel due to variation of band-offset and carrier confinement. Unconventional two-step profile has been obtained for the heterostructure. Higher capacitance at near zero bias corroborates the formation of InN at AlGaN/InGaN due to larger band offset. Conductance-voltage (G-V) profiles further validate mapping of InGaN phase separation in terms of carrier trapping. Lower effect of trapping has been identified due to low bandgap InN formation at the interface. Effect of epilayer relaxation on the phase separation has also been discussed in terms of threading dislocation and V-defects.