GaInN Layer Research Articles

Growth and Microstructure Analyses of Semipolar AlInN Epitaxial Layers on a Fully Relaxed Semipolar {112¯2} GaInN/GaN/m‐plane Sapphire Template

Semipolar {} AlInN layers with thicknesses of ≈0.4 μm are grown on a fully relaxed semipolar Ga0.9In0.1N/GaN/m‐plane sapphire template by metalorganic chemical vapor deposition. The grown AlInN layers are confirmed to have relatively flat surfaces of less than 1.5 nm in root mean square roughness and high InN mole fractions ranging from 0.306 to 0.444, which are close to alloy compositions lattice matched to the underlying semipolar {} Ga0.9In0.1N layer. The microstructure analyses reveal that the AlInN layers are mostly relaxed for the underlying GaInN layer and have large fluctuations in lattice strains or lattice spacings, which might have caused many dislocations with different types from the basal‐plane stacking faults existing in the underlying GaN and GaInN layers. When compared to the growth of c‐plane AlInN layers, it is found that the InN incorporation rate into the AlInN alloys is enhanced using the semipolar {} GaInN template. Most noteworthy is that the relatively flat surfaces are realized for the semipolar AlInN layers despite their high InN mole fractions greater than 30%, submicrometer thickness, and many dislocations.

ТЕРАГЕРЦОВІ КОЛИВАННЯ В InN-ДІОДАХ ГАННА З ДОВЖИНОЮ АКТИВНОЇ ОБЛАСТІ 1 мкм та з GaInN ВАРИЗОННИМ ШАРОМ

Subject and Purpose. The InN Gunn diode is known as the device capable of generating powerful oscillations atfrequencies above 300 GHz. A possible way for increasing both the microwave power and the cutoff frequency of the Gunn diode is to employ graded-gap semiconductors. The subject of this research is the process for generating electrical oscillations in InN and graded-gap GaInN Gunn diodes that involve resistive contacts at the cathode and the anode, and possess a 1-μm long active region. The research is aimed at suggesting an optimized structure for the graded-gap GaInN diode to obtain a maximum microwave power and maximum frequency of the oscillations, while consuming the lowest possible amount of DC power. Methods and Methodology. А hydrodynamic simulation has been performed of transport of electrons in graded-gap semiconductors, and an integro-differential equation analyzed concerning voltage drop across elements of the related RLC circuit. Results.The power spectra of oscillations have been analyzed for a variety of parameters of both the Gunn diode and the RLC circuit. The frequency dependences of the oscillatory power, characteristic of different electron concentrations, provide evidence for the possibility of obtaining considerable microwave powers at frequencies above 300 GHz through the use of graded-gap GaInN diodes. Conclusion. The results that have been obtained clearly confirm the expected practicality of using a graded GaInN layer in the InN diode for increasing the power of microwave oscillations, reducing the necessary level of the DC power, and restraining the dependence of the output characteristics on the electron density. The highest power of oscillations has been demonstrated by the InN diode with a 0.1 µm long graded-gap layer of GaInN. Meanwhile, the oscillation frequency generated in that diode is somewhat lower than in the InN diode. A compromise between the values of generated power and the oscillation frequency has been reached in the diode with a graded-gap GaInN layer of 0.9 µm in length. In addition, the latter structure requires the lowest level of DC power for effectuating microwave generation at the higher feasible frequencies.

Energy transport analysis in a Ga0.84In0.16N/GaN heterostructure using microscopic Raman images employing simultaneous coaxial irradiation of two lasers

Anisotropic heat transport in a Ga0.84In0.16N/GaN-heterostructure on a sapphire substrate is observed from microscopic Raman images obtained by utilizing coaxial irradiation of two laser beams, one for heating (325 nm) in the GaInN layer and the other for signal probing (325 nm or 532 nm). The increase in temperatures of the GaInN layer and the underlying GaN layer is probed by the 325-nm and 532-nm lasers, respectively, by analyzing the shift in the Raman peak energy of the higher energy branch of E2 modes. The result reveals that energy diffuses across a considerable length in the GaInN layer, whereas the energy transport in the perpendicular direction to the GaN layer is blocked in the vicinity of misfit dislocations on the heterointerface. This simultaneous irradiation of two lasers for heat generation and probing is effective in the microscopic analysis of energy transport through heterointerfaces.

GaInN-based tunnel junctions with graded layers

We demonstrated low-resistivity GaInN-based tunnel junctions using graded GaInN layers. A systematic investigation of the samples grown by metalorganic vapor phase epitaxy revealed that a tunnel junction consisting of a 4 nm both-sides graded GaInN layer (Mg: 1 × 1020 cm−3) and a 2 nm GaN layer (Si: 7 × 1020 cm−3) showed the lowest specific series resistance of 2.3 × 10−4 Ω cm2 at 3 kA/cm2 in our experiment. The InN mole fraction in the 4 nm both-sides graded GaInN layer was changed from 0 through 0.4 to 0. The obtained resistance is comparable to those of standard p-contacts with Ni/Au and MBE-grown tunnel junctions.

Relationship between misfit-dislocation formation and initial threading-dislocation density in GaInN/GaN heterostructures

The dependence of the critical thickness for the introduction of misfit dislocations in a GaInN/GaN heterostructure system on the dislocation density in the underlying GaN layer was investigated using in situ X-ray diffraction (XRD), ex situ scanning electron microscopy, and transmission electron microscopy analyses. The critical thickness for the introduction of misfit dislocations in the GaInN layer was found to significantly depend on the dislocation density in the underlying GaN layer. Notably, on the basis of the in situ XRD results, a reliable critical thickness for obtaining misfit-dislocation-free growth was determined.

Fabrication of AlInN/AlN/GaInN/GaN heterostructure field‐effect transistors

AbstractWe report on the electrical properties of AlInN/GaInN heterostructures fabricated with InN molar fractions of 0 to 0.6 in the GaInN layer. High‐density two‐dimensional electron gases are formed near the interfaces of AlInN/AlN/GaInN at InN molar fractions of 0.3 and 0.6. The Al0.82In0.18N/AlN/Ga0.4In0.6N/GaN heterostructure field‐effect transistors exhibited static characteristics. The maximum drain‐source current reached a value of 0.26 A/mm (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Novel system for X-ray CTR scattering measurement on in-situ observation of OMVPE growth of nitride semiconductor heterostructures

A novel system for in-situ X-ray CTR scattering measurement on in-situ observation of group-III nitride semiconductor growth was constructed by setting an OMVPE reactor at the sample position of the X-ray diffractometer which was developed for in-situ measurement of growing semiconductor crystals. For in-situ observation of the growth of semiconductor, firstly, it is necessary to investigate the capabilities of the new system. In this work, after the safety check of the OMVPE reactor with thin Be windows, the capability of the new system as an X-ray diffractometer and a growth reactor of group-III nitride semiconductors was tested. After the GaInN layer was deposited successfully, the X-ray CTR scattering measurement was conducted at the growth temperature. The different CTR spectra measured at the growth temperature and at room-temperature after the growth were obtained. It indicates that the status of the crystal at growth conditions can be analyzed by using the new system. Therefore, the new system can be developed for in-situ observation of the OMVPE growth of the group-III nitride semiconductors.

GaInN/GaN p‐i‐n light‐emitting solar cells

AbstractGaInN/GaN p‐i‐n double‐heterojunction structures were grown by metal‐organic vapor phase epitaxy on c‐plane sapphire substrates, and light‐emitting solar cells with different GaInN active layer thicknesses were fabricated. The thickness of the GaInN active layer was varied from 100 to 400 nm, while the thickness of both n‐type and p‐type GaN layers was kept constant. A semitransparent ohmic contact to p‐type GaN was formed by electron‐beam evaporation of Ni/Au (5 nm/5 nm). The film thickness of p‐GaN was 50 nm. The external quantum efficiency (EQE) of the device with a 400 nm GaInN layer exceeded 60% at a wavelength of approximately 380 nm. The transparency of the Ni/Au electrode was 68%. Therefore, the internal quantum efficiency of this device exceeded 95%. Note that this device emitted green light when a forward voltage was applied. The electroluminescence peak wavelength was approximately 525 nm, which was much longer than the EQE peak wavelength. The origin of this large emission shift is discussed. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Realization of high‐crystalline‐quality and thick GaInN films

AbstractWe report on the growth of a completely relaxed, high‐ crystalline‐quality, and thick GaInN layer on a grooved m‐plane GaN template using sidewall epitaxial lateral overgrowth (SELO) technology. We grew GaInN‐based multiple quantum wells (MQWs) on this GaInN layer, and the PL intensity was compared with that for the same GaInN/GaN MQWs on m‐plane GaN grown by SELO. The PL peak intensity obtained from the GaInN/GaN MQW on the high‐crystalline‐quality thick GaInN on grooved GaN was approximately 1.7 times higher than that on the SELO m‐plane GaN template. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Improvement in crystalline quality of thick GaInN on m‐plane 6H‐SiC substrates using sidewall epitaxial lateral overgrowth

AbstractWe report the fabrication of an epitaxial lateral overgrown (ELO) GaInN layer on a high crystalline‐quality‐grooved GaN template, which is improved by a sidewall ELO (SELO) technology. The photoluminescence peak intensity of ELO‐grown GaInN layer on the SELO GaN underlying layer is twice as high as that of ELO‐grown GaInN layer on the m‐plane GaN template. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

High-Resolution X-Ray Diffractometry Investigation of Interface Layers in GaN/AlN Structures Grown on Sapphire Substrates

GaInN is an important wide band gap material with applications in short wavelength optoelectronic devices. The GaInN layer is often grown on a sapphire substrate, with low-temperature-deposited AlN and thick GaN used as buffer layers. The growth regime consists of many steps, each of which contributes to the overall properties of the device. The aim of our high-resolution X-ray diffraction experiments, conducted at the Photon Factory (Tsukuba, Japan), was to investigate the structural quality of the AlN buffer layer, which affects the final properties of the device. Reciprocal space mapping was used to study samples (having various layer thicknesses) from each stage of the growth process. Analysis of the experimental data provides parameters such as mosaic block dimensions and orientation, lattice strain distribution, and layer thickness.

Investigations about series resistance of MOVPE grown GaN laser structures

In order to reduce the total series resistance of an AlGaInN laser structure, we have exchanged the GaN:Mg contact layer normally covering a laser structure by GaInN:Mg and have investigated the influence of several growth conditions on its electrical properties. We found a significant decrease of the series resistance for a GaInN layer grown at 800°C with hydrogen as carrier gas which results, at lower current densities, in a voltage drop of at least 2 V compared to GaN contact layers. Our studies show that not only the changed growth conditions, but indeed the In content plays a major role for these improvements, although it is only about 2%. Laser structures grown on SiC wafers show minimum total differential resistivities below 1.5×10 −4 Ω cm 2.

Electron microscopy analyses of microstructures in ELO-GaN

Several topical results of electron microscope analyses for microstructures in epitaxial lateral overgrown (ELO)-GaN are reported. (1) Dislocations lying on (0 0 0 1), or horizontal dislocations (HDs), are generated in ELO-GaN layers overlying on a mask of a-SiO 2. The HDs have a shape of loop or semi-loop, and the morphology suggests that the HDs are generated through a multiplication mechanism with the assistance of internal stress. (2) On GaInN/GaN, a pit is formed at the end of a threading dislocation (TD) of any Burgers vector, a , c or a+ c . The density and distribution of TDs in ELO-GaN can be estimated by observing these growth pits after a subsequent deposition of a thin GaInN layer on the ELO-GaN. The critical thickness for the formation of pits has a dependency upon the concentration of In in GaInN and the Burgers vector. (3) It was demonstrated that electron backscattering diffraction pattern (EBSP) analyses can estimate a two-dimensional distribution of c-axis orientation of ELO-GaN for a wide area with an accuracy of 0.2° or better.

Atomic scale characterization of GaInN/GaN layers grown on sapphire substrates with low-temperature deposited AlN buffer layers

The GaInN/GaN layers grown on the sapphire substrates treated with and without nitridation process have been studied by X-ray CTR scattering measurements. Crystalline quality of the samples with different thicknesses of low-temperature (LT) deposited AlN buffer layers has been investigated. For the samples without the nitridation process, when the buffer layer was 70 nm thick, many defects were generated in the buffer layer and the crystalline quality of the layers on the buffer layer was degraded. On the other hand, for the samples with the nitridation process, the crystalline quality of the GaInN layer was the best when the LT-AlN buffer layer was 70 nm thick.

Effects of misfit dislocations and AlN buffer layer on the GaInN/GaN phase diagram of the growth mode

The thickness-composition phase diagrams of the growth modes were determined for the GaInN-on-GaN (GaInN/GaN) and the GaInN-on-AlN-on-GaN (GaInN/AlN/GaN) structures. For this determination, the strain energy was calculated by considering the stress relaxation due to introduction of misfit dislocations, the surface energy was estimated from bonding enthalpy of the nearest-neighbor bonds on the surface, and the interface energy was estimated by considering both effects of the dangling bonds due to lattice misfit and the abrupt transition of bonding species at the heterointerface. From these phase diagrams, it was found that the layer-by-layer growth such as the Frank–van der Merwe mode was very difficult to obtain for the epitaxial growth of GaInN on GaN when the InN fraction is large. The Volmer–Weber mode is dominant in the phase diagram of the GaInN/GaN structures. The influence of an AlN buffer layer with a larger surface energy was studied by introducing an AlN layer between the GaInN layer and the GaN substrate. It was known that the layer-by-layer growth could be more easily obtained if misfit dislocations were introduced and an AlN layer was used as a buffer.

Improved Radiative Efficiency using Self-Formed GaInN/GaN Quantum Dots Grown by Molecular Beam Epitaxy

GaInN/GaN heterostructures have been grown by molecular beam epitaxy (MBE) on c-plane sapphire substrates. The growth of Ga1—xInxN (x > 12%) alloy has been extensively studied. At low V/III ratio, the growth undergoes a Stranski-Krastanov transition giving rise to the formation of three-dimensional (3D) islands. On the other hand, a high V/III ratio promotes the two-dimensional (2D) layer-by-layer growth regime. The optical properties of Ga1—xInxN (x > 12%)/GaN heterostructures made from either a 3D GaInN layer or a 2D GaInN layer are similar. This indicates that the mechanism responsible for the peculiar optical properties of Ga1—xInxN (x > 12%)/GaN heterostructures is intrinsically due to self-formed quantum dots (QDs) that may arise from In clustering in the GaInN alloy. Actually, the properties of the GaInN/GaN QDs grown by MBE are very close to those grown by metal-organic chemical vapor deposition (MOCVD). As for MOCVD material, a high radiative efficiency is obtained correlatively with a giant Stokes shift and a very large carrier lifetime. The high efficiency of GaInN/GaN QDs is confirmed by gain measurements performed at room temperature. Finally, light-emitting diodes (LEDs) based on GaInN QDs have been fabricated. They demonstrate improved performances compared to LED based on homogeneous GaInN alloys.

Characterization of initial growth stage of GaInN multi-layered structure by X-ray CTR scattering method

Two types of samples were investigated by X-ray crystal truncation rod (CTR) and X-ray reflectivity measurements. One type was grown on sapphire substrate with nitridation process before deposition of low-temperature (LT)-AlN buffer layer. The other type was grown on sapphire substrate without the nitridation process. With the nitridation process, crystalline AlN was formed on the sapphire substrate and the LT-AlN buffer layer became crystalline. However, the crystalline quality of GaN and GaInN layer on the crystalline LT-AlN buffer layer was poor. It suggests that the lattice-mismatching between the crystallized AlN and GaN degrades the crystalline quality of GaN and GaInN layers. On the other hand, without the nitridation process, the LT-AlN buffer layer was amorphous or poor crystalline layer and the crystalline quality of GaN and GaInN on the LT-AlN buffer layer was good. It indicates that the amorphous or poor crystalline LT-AlN buffer layer works well as a buffer layer.

Growth characteristics of GaInN on (0 0 0 1)sapphire by plasma-excited organometallic vapor phase epitaxy

Growth characteristics of GaInN on (0 0 0 1)sapphire substrate by plasma-excited organometallic vapor phase epitaxy (OMVPE) are investigated. GaInN with whole In composition is obtained at 680°C, which is at least 100°C higher than reported temperature for conventional OMVPE using NH 3. The distribution coefficient of In scarcely depends on the growth temperature, which indicates that dissociation of In–N bond is compensated even at high growth temperatures. This specific feature is attributed to high reaction rate of excited nitrogen species in plasma-excited N 2. The compositional inhomogeneity of the GaInN layer is also characterized.

GaInN/GaN-Heterostructures and Quantum Wells Grown by Metalorganic Vapor-Phase Epitaxy

The dependence of the In-incorporation efficiency and the optical properties of MOVPE-grown GaInN/GaN-heterostructures on various growth parameters has been investigated. A significant improvement of the In-incorporation rate could be obtained by increasing the growth rate and reducing the H2-partial pressure in the MOVPE reactor. However, GaInN layers with a high In-content typically show an additional low energy photoluminescence peak, whose distance to the band-edge increases with increasing In-content. For GaInN/GaN quantum wells with an In-content of approximately 12%, an increase of the well thickness is accompanied by a significant line broadening and a large increase of the Stokes shift between the emission peak and the band edge determined by photothermal deflection spectroscopy. With a further increase of the thickness of the GaInN layer, a second GaInN-correlated emission peak emerges. To elucidate the nature of these optical transitions, power-dependent as well as time-resolved photoluminescence measurements have been performed and compared to the results of scanning transmission electron microscopy.

Large-Area Growth of InGaN/AlGaN Using In Situ Monitoring

ABSTRACTA newly developed Metalorganic Chemical Vapor Deposition (MOCVD) reactor for processing batches of seven 50mm wafers per run at deposition temperatures up to 1600°C is introduced. The substrates are individually rotated by means of gas bearings utilizing high purity gas for operation. In order to achieve the maximum uniformity on a wafer the uniformity of the growth temperature was maximized. Pyrometric measurements revealed a temperature uniformity across the susceptor of ±4°C at a temperature of 1300°C. Growth of GaN and GaInN produced uniform layers regarding composition and thickness. On a 50mm diameter wafer the standard deviation for the thickness of a GaN layer is 6% and the standard deviation for the composition of a GaInN layer as determined by photoluminescence is 2%.