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Group-III nitride on Si tandem solar cell structures, which are free from materials with high impacts on the environment such as group-III arsenides or phosphides, were successfully fabricated using the surfaceactivated bonding of nitride-based sub cells, which had been grown on (0001) GaN substrates, and (111) Si-based sub cells. The backsides of the GaN substrates had been polished and their averaged roughness was smaller than 1 nm prior to bonding. The open-circuit voltage (VOC) of the tandem cells was almost equal to the sum of VOC of the respective sub cells, while the conversion efficiency was limited by the properties of the nitride-based top cells. Nitride-on-(100) Si tandem cells were also fabricated and the enhancement in VOC was observed. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

This paper reports the application of InGaN films to a polymer electrolyte cells (PEFC) stainless steel (SS) separator for the first time. Using MOCVD, InGaN films with In content of 0-1 are grown at 600 °C on sapphire and stainless steel substrates. Sputtered TiN is used as a buffer for the InGaN growth on stainless steel. For InGaN on sapphire substrates, electrical resistivity and dissolution rate in a H2SO4 solution at 60 °C are measured, and films with In content in the range 0.2-0.5 are found to have both high electric conductivity and high corrosion resistance. The anti-corrosive properties of In0.3Ga0.7N/TiN/SS structures are also confirmed by the inductively coupled plasma spectrometry (ICP) chemical analysis of the H2SO4 solution in which the In0.3Ga0.7N/TiN/SS samples are dipped for 100 h. The levels of both Ga and In atoms are under the detection limits. The I-V characteristics of a single cell using an In0.3Ga0.7N/TiN/SS separator is measured in comparison with those using bare stainless steel or carbon separators. The performance of the cell with the In0.3Ga0.7N/TiN/SS separator is much superior to that with a bare stainless steel. Compared with the cell using the carbon separator, the cell using the In0.3Ga0.7N/TiN/SS separator has a little larger ohmic loss (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

This paper reports on a theoretical analysis of DC and RF performances of a novel In0.1Ga0.9N/InN HEMT. A very high cut-off frequency and attractive DC characteristics have been predicted. A simple analytical model is used to explain the DC characteristics. The cut-off frequency which has been found more than 0.6 THz for a gate length of 0.1 µm is explained with low-field mobility. The maximum drain current and transconductance have been found around 1000 mA/mm and 625 mS/mm, respectively. Similar analysis have been performed for the conventional AlGaN/GaN HEMT and compared with the proposed InGaN/InN HEMT. The predicted results are found to be in good agreement with previously published results. The calculated results show that InN-based HEMT has very high cut-off frequency and transconductance when compared with the conventional GaN-based HEMT for the same gate length. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Quantum confinement effect on 2DEG density and distribution on the dependence of an interfacial layer has been investigated in InGaN/InN HEMT using self-consistent solution of one dimensional Schrodinger-Poisson equation. It is shown that the addition of an i-InGaN interfacial layer leads to an increase in 2DEG density. The role of quantum confinement effect in the 2DEG distribution with an inserted i-InGaN interfacial layer is revealed. In the classical model, the insertion of an interfacial layer has a negligible effect on 2DEG distribution. While taking the quantum effect along with polarization effect into account, it is observed that i-InGaN interfacial layer has substantial influence on the 2DEG distribution in InGaN/InN HEMT; with electron spillover into the barrier is sharply reduced. The maximum 2DEG density after insertion of an i-InGaN interfacial layer has been found to be 1 × 1014 cm-2 which is higher than the conventional GaN-based HEMT (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

This paper describes the theoretical design and performance of a novel InGaN-based 1.55 μm quantum well laser. A careful analysis of the optical and modal gain, threshold behavior, optical output, and efficiency with cavity length has been carried out. The spectral range of the optical material gain is found to be narrow and modal gain varies sharply with changing the reflectivity of the partially transparent mirror. It is found that a very small voltage of 1.10 V is required to reach the threshold current. A very small threshold current of 5.1 mA is required to emit the light. The calculated values of threshold current and threshold voltage are lower than the reported values of the conventional 1.55 μm laser. An efficiency of around 59% is found for a cavity length of 300 μm keeping the reflectivity at 35% of one mirror and 100% of other. The above study indicates that the proposed InGaN-based 1.55 μm laser is very promising for the fabrication of future high performance laser (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

This paper describes a novel AlInN/InN metal-oxide-semiconductor heterojunction field effect transistor (MOSHFET) as a promising candidate for future high performance microwave devices. The transport properties such as channel mobility and velocity-field characteristics of the proposed AlInN/InN MOSHFET are calculated using Monte Carlo simulation. The DC and high frequency performances are assessed using an analytical model. The highest mobility is found to be 8.1×104 cm2V–1s–1 at a sheet carrier concentration of 8.79×1012cm–2. The peak drift velocity obtained for the device is 8.3×107 cm/s at the field of 66 kV/cm. The maximum cut-off frequency, transconductance and drain current have been found 160 GHz, 280 mS/mm, and 879 mA/mm, respectively, for a gate length of 0.25 μm. The calculated results are in good agreement with the published results of GaN MOSHFET (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

This paper describes the 2DEG properties of InGaN/InN/InGaN double channel high electron mobility transistor (DHEMT). These include an in-depth theoretical investigation of 2DEG sheet carrier concentration and mobility. The 2DEG mobility is calculated using ensemble Monte Carlo taking into account all relevant scatterings. The calculated sheet carrier concentration reaches as high as 1.77×1014 cm-2 for DHEMT. While for single channel high electron mobility transistor (SHEMT) the calculated sheet carrier concentration was found to be 7.06×1013 cm-2. The maximum total 2DEG mobility is found to be around 12×104 and 8×104 cm2V-1s-1, respectively, for DHEMT and SHEMT for a sheet carrier concentration of ns = 5×1012 cm-2 at 77 K. Thus, the 2DEG sheet carrier concentration and mobility for dual channel is higher than conventional single channel HEMTs. The high values of 2DEG sheet carrier concentration and mobility strongly suggest that the InGaN/InN/InGaN based double channel HEMT is very promising for the fabrication of high performance high speed device (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

This paper reports the effect of step-graded interlayers on MOVPE InGaN epi-layer quality. Multilayer epitaxial structures consisting of Inx Ga1–xN layers with various compositions have been successfully grown. HRXRD measurements reveal that the insertion of step-graded interlayers significantly reduces the twist while little effects are observed on tilt. Decreasing of twist with increasing the number of interlayers is found to be remarkable up to a certain value and then maintain nearly the same even with increasing the In composition. Different types of dislocation density have also been studied using the X-ray diffraction analysis. Dislocation densities, particularly the edge dislocation density, decrease considerably with the insertion of interlayers for higher In composition. An edge dislocation of 3 × 1010 cm–2 is obtained for a smple with 0.25 In composition, that seems to come from the GaN layer beneath the step-graded interlayers (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In this paper, we review and discuss the recent advances in InN-based solar cells. Before the discussion on InN-based solar cells, two major losses in a solar cell, transparency loss and excess excitation loss, are explained and the effectiveness of a multi-junction tandem cell in reducing the both losses simultaneously is described. Then, the status and issues for multi-junction tandem solar cells using the conventional III-V materials (InGaP/InGaAs systems) are reviewed briefly in order to understand the advantages of InN-based material as a multi-junction solar cell material. It is shown that main problem in the conventional III-V multi-junction solar cell is the current mismatching between sub-cells, which reduces significantly the conversion efficiency of the tandem cells. The main advantage of InGaN and InAlN as a multi-junction solar cell is that a wanted band-gap between 0.7 and 2.5 eV is realized by changing only their composition. This means that current matching between sub-cells will be easily achieved using InGaN and InAlN systems. After the status of InGaN solar cell research is summarized, the results on the MOVPE growth of InGaN with an In content up to 0.4, Mg doping using Cp2Mg, and the formation of an n+-p homo-junction are described. The insertion of step-graded InxGa1-xN interlayer between the GaN and InGaN epilayer is also shown to be effective to improve the epilayer quality. The study on InAlN solar cell is on the stage of the growth of In-rich InAlN. Single-crystalline InAlN films with an Al content from 0 (Eg = 0.7 eV) to 0.43 (2 eV) have been successfully grown by employing atmospheric-pressure MOVPE. The study on Mg-doping for InAlN will be the next step. Finally, we will summarize and discuss the challenges in InGaN and InAlN solar cells. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

This paper reports the atmospheric-pressure MOVPE growth of In-rich InAlN. All InAlN films prepared here (Al content:0∼ 0.43) do not show phase separation. The incorporation of Al in InAlN is decreased with increasing growth temperature. A decrease in Al content is also observed for films grown at a position farther from the up-stream end of the susceptor. The marked decrease in the Al content along the gas flow direction seems to be caused by the shortage of TMA supply at the downstream by the parasitic reaction of TMA. A single-crystalline InAlN film with an Al content of 0-0.43 is successfully grown by adjusting growth temperature and TMA/(TMI+TMA) molar ratio. FWHM of X-ray rocking curve for InAlN is increased with increasing Al content. The carrier concentrations in InAlN films are comparable to that in InN (1-5 × 1019 cm–3). All the single-crystalline InAlN films with an Al content of 0-0.3 show a photoluminescence at room temperature. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The Mg doping for MOVPE InN has been studied using biscyclopentadienyllmagnesium CP2Mg as a Mg source. The InN samples grown near the upstream end of the 18 cm long susceptor have very small grain size and contain a high level of C and H contamination. The XPS analysis of the deposits obtained by the supply of CP2Mg together with NH3 reveals that new Mg compounds are formed near the upstream end of the susceptor. The new compounds are adducts of CP2Mg and NH3, because they can not be formed when only CP2Mg is supplied. The adduct formation seems to be a cause for the very small grain size and the high levels of C and H contamination. The adduct formation is suppressed at positions with a distance more than 9 cm from the upstream end. This means that designs of reactor and susceptor are very important for successful Mg doping of InN with CP2Mg. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

This paper reports the crystallographic degradation of MOVPE InN during the growth. Using FWHMs of X-ray rocking curve, tilt ((0002)) and twist ((10-10)) angle distributions are evaluated and effects of the major growth parameters, such as growth temperature, growth time and with/without GaN buffer in the degradation, are revealed. With increasing either thickness of grown InN or growth temperature up to 600 °C, the tilt angle distribution is markedly increased, indicating the crystallographic degradation of grown films. The use of a GaN buffer reduces such degradation. Since the twist angle distribution is scarcely changed by such growth parameters, the destruction of InN crystals during growth and annealing is concluded to be anisotropic. The trends of the crystallographic degradation revealed here are in good agreement with those for the electrical and optical degradation previously reported. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

An experimental study has been made on the correlation between the fabrication conditions for 3c-SiC/Si (111) template and electrical and crystallographic properties of InN films grown on the templates. The template has been prepared by C+-ion implantation into Si (111). Although the 3c-SiC layers show a large (170 arcmin) FWHM of XRC (111) diffraction, InN films with a considerably small (about 50 arcmin) FWHMs for (0002) and (10-10) diffraction are grown on the layers. A lower implantation dose brings a smaller XRC FWHMs for 3c-SiC. The post implantation annealing is found to be effective to improve the quality of the 3c-SiC layer and, therefore, to improve the quality of InN films. Hall mobility in InN is found to be markedly increased by decreasing FWHM of InN (0002) diffraction. The best data of carrier concentration and Hall mobility are 6.2×1018 cm–3 and 630 cm2/Vs, respectively. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We have investigated quantitatively on the origins of residual electrons in InN layers to make clear some roles of oxygen incorporation for band-gap widening. It has been found out that a linear relation was observed between oxygen and residual electron concentrations for InN layers grown by RF-MOMBE using TMIn source, although the residual electron concentration is super-linearly dependent on oxygen concentration for InN layers grown by RF-MBE using metal In source. The experimental results strongly indicate that oxygen atoms and/or nitrogen vacancies induced by oxygen incorporation are major origins of the residual carrier concentrations. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Polarity control of MOVPE InN on sapphire (0001) was investigated. Two kinds of polarity InN films on sapphire substrate by MOVPE have been prepared by controlling the substrate nitridation process and/or the LT-GaN buffer annealing process. Surface of the nitrided sapphire is a crucial factor in determining the polarity of InN film. N-polarity InN film was grown on the 1000 °C-nitrided substrate without LT-GaN buffer; while In-polarity InN film was grown on the 900 °C-nitrided substrate. In the case of InN growth with a LT-GaN buffer, buffer annealing is a crucial issue in determining the polarity of InN film. N- and In-polarity InN films were obtained on the long-time-annealed buffer and the short-time-annealed buffer, respectively. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We have investigated about a control technique of oxygen contamination into the InN layers by simultaneous irradiation of Ga beam during RF-MOMBE growth using the combination of the TMIn and the RF-plasma nitrogen sources. Red shifts of the band gap energy and the improvement of the electrical properties have been achieved by the Ga beam irradiation. The suppresssion mechanism of the oxygen contamination has been discussed from the experimental results of the InN growth by the RF-MOMBE with the Ga beam irradiation. The present results strongly indicate that the simultaneous irradiation of the Ga beam would be useful to suppress the oxygen contamination into the InN layers during the growth. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

This paper reports the residual tensile stress in GaN grown on 3c-SiC/Si template formed by C+-ion implantation into Si(111). A GaN layer is grown using MOVPE method with the conventional two-step growth. The residual tensile stress is dependent on the fabrication conditions of the template, such as substrate temperature for implantation, acceleration voltage of C+-ions, temperature and time for the post-implantation annealing of Si wafers. It is found that the substrate temperature during implantation is a predominant factor. The residual stress in GaN layer is reduced from 1.3 to 0.45 GPa when the substrate temperature during implantation is changed from 400 to 600 oC. The stress is also decreased by increasing accelerating voltage for implantation and post-implantation annealing time. The residual tensile stress of 0.45 GPa is the lowest one ever reported for GaN on Si. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The influence of substrate polarity on the growth of InN film by MOVPE was investigated using bulk GaN as a substrate. Single-crystalline In- and N-polarity InN films were obtained on Ga- and N-polarity GaN substrate, respectively. Significant difference of the morphologies between the In- and N-polarity InN films was found. For the In-polarity InN film, the morphology was similar to that grown on sapphire substrate. The film surface was consisted of grains with small facets. In contrast, for the N-polarity InN film, the surface was consisted of large hexagonal shape crystal grains with flat surface. The grain size was about 2 μm in diameter on the average, and two-dimensional growth was enhanced obviously for each crystal grain. The influence of the growth temperature on the morphology, polarity, and optical property was also investigated. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The deterioration of MOVPE InN during the growth and the post-growth annealing processes is investigated. Photoluminescence (PL) measurement with excitation sources with a different wavelength is mainly used to detect deterioration of InN. This is aimed to get in-depth information about the quality of InN since the penetration depth of an excitation source is different depending on its wavelength. A serious deterioration, which is the decomposition of InN, is found for InN annealed in the NH3 flow after the growth. The deterioration occurs predominantly near the substrate interface. On the contrary, the deterioration of InN during growth is scarcely observed. Since the InN films grown on sapphire have an In-polarity, the deterioration during the post-growth annealing occurs at the N-surface. Such deterioration is suppressed by the supply of TMI together with NH3 (InN growth mode). From these facts, hydrogen produced by the decomposition of NH3 seems to be responsible for the deterioration since hydrogen is consumed to form CH4 when TMI is supplied. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Infrared reflectance measurements of a series of InN thin films have been performed and attempt to derive carrier concentration and other physical constants for InN thin film characterization. Fitting calculations are performed by use of the dielectric function equation based on phonon-plasmon coupling model. Longitudinal and transverse optical phonon frequencies, plasma frequency and their damping parameters can be derived from fitting. From those results, electrical and phonon properties of InN and characterization of films are discussed. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Films of InN have been grown on sapphire with a GaN buffer by the atmospheric-pressure MOVPE and the effects of GaN buffer etching with a KOH solution have been studied. With increasing etching time, the number of GaN grains is decreased and, as a result, the grain size of InN grown on the buffer is markedly increased. The grain size of InN grown on the 10 min-etched GaN buffer is larger than that of films grown without GaN buffer. When the GaN etching time is less than 5 min, the number of InN grains is smaller by about one order of magnitude than that of GaN buffer, showing that one InN grain is grown on about ten GaN grains on average. For the etching time of 8-10 min, the number of InN grains becomes approximately the same as that of GaN grains, indicating that under this condition one GaN grain acts as a nucleation center for one InN grain and no InN nucleation occurs in the areas without GaN grains on the sapphire substrate. The diffusion length of InN species on the sapphire substrate is estimated to be approximately 1 μm or more. In spite of the enhanced grain growth of InN films grown on the etched buffers, their electrical properties are a little worse compared with those grown without etching. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The electrical and optical properties for atmospheric-pressure MOVPE InN have been studied as a function of NH3/TMI molar ratio during the growth. Residual carrier (electron) concentration is decreased with increasing NH3/TMI molar ratio. PL peak energy is also shifted to a low energy side with increasing NH3/TMI molar ratio. Hall mobility of InN is almost independent on NH3/TMI molar ratio and is rather decreased at a high NH3/TMI molar ratio. The highest mobility is obtained for a sample grown at a relatively low NH3/TMI molar ratio. The grain size of the InN films is decreased with increasing NH3/TMI molar ratio. The lowest carrier concentration of 4.5×1018 cm–3 and highest mobility of 1100 cm2/Vs obtained here are the best data for MOVPE InN ever reported. Based on these results, the most probable candidate for donors and the dominant carrier-scattering mechanism for atmospheric-pressure MOVPE InN are discussed. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

This paper reports on the catalyst-assisted metal-organic vapor phase epitaxy (CA-MOVPE) growth of InN with an independent control of catalyst temperature. When the catalyst temperature is increased above the growth temperature of InN, the grain size becomes large and full width at half maximum (FWHM) of X-ray rocking curve (XRC) is drastically decreased The migration of precursors is highly enhanced and the crystal quality is improved by CA-MOVPE with an independent heating system of the catalyst. This CA-MOVPE system will open the possibility of the growth of high-quality InN and In containing alloys especially with the higher In content (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

This paper reports on the MOVPE growth of InN and InxGa1-xN with x up to ∼0.4 on AlN/Si (111) substrates. Good quality InN films are grown after a pretreatment of the substrate surface at 1000 °C in NH3. While pretreatment of the substrate surface in N2results the In droplet formation. Single-crystalline InxGa1-xN films with x up to ∼0.4 are successfully grown on Si (111) substrates without phase separation and metallic In incorporation by changing the growth temperature and TMI/(TMI+TEG) molar ratio. Comparative study between the AlN/Si (111) and GaN/α-Al2O3(0001) substrates shows that the quality of InN, GaN and InGaN with low In contents depends on the quality of the underlying layer. While the quality of InGaN at the intermediate In content range hardly depends on of the underlying layer/substrate. These works will allow us towards the fabrication of the future InGaN-Si tandem cell using MOVPE. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

This report shows the effect of Cp2Mg supply on MOVPE growth behavior of InN. At low growth temperature (∼600 °C), the formation of adducts occurred, which degenerates the crystal quality. With increasing the growth temperature, the adduct formation was suppressed because the decomposition of Cp2Mg was enhanced and thus the incorporation of carbon into the film was suppressed. The use of Cp2Mg during InN growth increases the growth rate in the lateral direction. Non-doped InN film grown on GaN buffer has an In-face of the top side. On the other hand, the inversion domains seems to be formed in the highly Mg-doped InN. Tilt distribution decreases from 65 to 30 arcmin with the increase of Cp2Mg/TMI molar ratio 0 to 0.06. The donor is produced in highly Mg-doped MOVPE-grown InN (Cp2Mg/TMI molar ratio > 0.005). Therefore, the effect of Cp2Mg supply on MOVPE growth behavior of InN is found to improve a macro-scale crystal quality but also produces the donor (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Low-temperature (∼600 °C) MOVPE growth of GaN is investigated using a catalyst to enhance NH3 decomposition rate. As a catalyst, Ir is employed in this study. Significant differences in electrical properties between samples grown with and without catalyst are observed. The sample grown at 600 °C without catalyst shows a high resistivity (106 Ωcm) and a low carrier concentration (1011 cm-3). The low carrier concentration is seemed to be due to the compensation by the carbon impurity known as a deep level acceptor, since the activation energy for such sample is closed to that for carbon-doped GaN. The employment of the catalyst results in the improvement of crystalline quality of low-temperature grown GaN. As a result of this study, we have found that the catalyst-assisted MOVPE has significant effects on the suppression of impurity (mainly carbon) incorporation and the improvement of crystalline quality of GaN grown films. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We have estimated the longitudinal effective mass (m‖) of electron in n-type InN films by Raman scattering. The samples were grown by MOVPE (metal organic vapor phase epitaxy) with free carrier concentration of n =6.7×1018-9.9×1018 cm-3 according to Hall measurement. A weak Raman signal observed at ∼430 cm-1 at room temperature was sharpened and shifted to higher frequency toward the A1(TO)-phonon mode at 447 cm-1 with increasing n. This mode was assigned to the lower branch (L-) of the longitudinal-optic-phonon-plasmon-coupled (LOPC) mode. The line shape was carefully analyzed by a semi-classical line-shape fitting analysis assuming deformation potential and electro-optic coupling mechanisms for the light scattering process. A line-shape fitting analysis was conducted by adjusting three major parameters; electron density, effective mass and plasmon damping rate. The analysis well reproduced values of electron density and mobility deduced by Hall measurement. Electron effective mass of m‖*/m0 = 0.05 (±0.01) was also obtained as the best-fit parameter. The result agrees well with previous data obtained by other optical methods. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

This paper reports the deterioration of nitrogen (N)-face of wurtzite InN in the NH3 ambient near the growth temperature. Using an atmospheric-pressure MOVPE system, both In-polar and N-polar InN are grown by controlling the nitridation temperature of a sapphire substrate. The N-face has a much larger etching rate (40 nm/h) compared with that of the In-face (14 nm/h). From the PL measurement from the front surface and the rear surface through the sapphire substrate, it is shown that the N-face of both In- and N-polar films shows a marked intensity reduction and a large blue shift of PL spectrum. These facts show that the N-face of InN is very unstable and deteriorates in the NH3 ambient. The deterioration becomes severer at a temperature higher than 550 °C. Since such a PL shift is due to the Burstein-Moss effect, the result indicates that the high density of donor-type defects are preferentially introduced on the N-face of InN. Further investigations will be needed to clarify whether such instability of N-face in the NH3 ambient is related to the poorer quality of MOVPE InN. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Mosaicity control is necessary for improvement of crystal quality of III-V nitride semiconductor materials, especially for InN. However, the mosaicity control for both of the tilt and the twist angle fluctuations has not been achieved still now in the direct growth of InN on the sapphire substrate by nitridation. Moreover, the role of nitridation process has not been so much clear from the view point of the mosaicity control. In this paper, we propose a new nitridation technique, two-step nitridation, combined with the high- and the low- temperature nitridation to reduce the mosaicity and also to make clear the roles of the nitridation process in the InN growth. As the result of the new nitridation technique, the considerably improvement of the tilt and the twist angle fluctuations has been successfully achieved in the direct growth of InN without any buffer layer. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We have found that the material quality of MOVPE InN can be markedly improved after the annealing in the air at around 300 °C. By the annealing in the air, carrier concentration is reduced by about one order of magnitude. In accordance with the carrier reduction, PL intensity is increased and PL peak energy is shifted to the lower energy side by about 0.06 eV for the film annealed for 3 h. The reduction of carrier concentration is also conformed by the shift of LO phonon-plasmon coupled mode in the Raman spectrum. The FWHM of the E2 (high) mode is decreased, indicating that the crystalline quality is slightly improved by the annealing. Since the FWHM of X-ray rocking curve is not changed after the annealing, the improvement by the annealing is concluded not to be in macroscopic scale but microscopic scale. No improvements are found for the samples annealed in the N2 flow. No data that show the chemical oxidation of InN are also found. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In the InN growth on sapphire substrates, it is difficult to control both of tilt and twist angle fluctuations at same time. It is necessary to understand initial growth stage such as the role of nitridation process to improve the mosaicity. Low-temperature nitridation technique brings the drastically improvement of the tilt angle fluctuation, although the twist angle fluctuation becomes worse. Such experimental results strongly indicate that there is some trade-off relation between the tilt and the twist angle fluctuations as a function of the nitridation condition such as the nitridation time. In this paper, we discuss about such trade-off relation in the direct growth of InN on the nitridation sapphire substrates and also propose a simple model of initial nitridation process to explain it. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Micro optical reflecance measurements of InN films have been performed at middle and near infrared region. Spectrum line shape fitting calculation based on theoretical formula which takes account of the coupling between LO phonons and plasmons have been also performed. Small area reflectance spectra (50 μm × 50 μm) at every points in the field of microscope vision show both similar and different line shapes by wavenumber location. The different part is mainly responsible for plasmon relaxation time which directly correspond to the carrier mobility. Comparison among contour image map of the fitting parameters and visible image is also presented. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We analyzed the atomic structure around Mg atoms in MOCVD- and MBE-grown Mg-doped InN using Mg K-edge X-ray absorption fine-structure (XAFS) measurements. Our experimental data closely fit to the simulated data in which Mg atoms occupy the substitutional sites of In atoms. From this result, we conclude that Mg atoms essentially occupy not N atoms sites but In atoms sites, meaning that Mg atoms can act as acceptors in InN. We believe that observations of p-type conductivity are prevented by problems such as carrier compensation and electron accumulation at the surface. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

This paper indicates an evidence for the deterioration of the MOVPE InN during the growth at high temperature (∼650 °C). It is noted that the deterioration occurs near the interface and InN film becomes porous layer during the further growth. The porous layer has high electron density. The rate-limiting process of N-face InN decomposition depends on atomic hydrogen. The atomic hydrogen produced by the decomposition of NH3 is responsible for the deterioration of InN film. The crystal quality of InN improves with decreasing the porous layer which is important for MOVPE InN. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

This paper reports the effect of oxygen supply on MOVPE InN growth at high temperature (∼650 °C). It is noted that the deterioration occurs near the interface, and InN film becomes porous layer during growth. When we supply oxygen in the reactor (O2/NH3 ratio is 0∼0.018%), atomic hydrogen is reduced by reacting oxygen and the porous layer is decreased. Our study reveals that the porous layer is produced by atomic hydrogen. It is important to reduce the atomic hydrogen in the reactor. The oxygen suppresses the deterioration of the film during growth. The carrier concentration increases with the increase of oxygen supply. However, not only oxygen but also defect density due to oxygen is responsible for a higher carrier concentration. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)