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The effect of different carbon doping techniques on the dynamic behavior of GaN-on-Si buffer was investigated. Intentional doping using a hydrocarbon precursor was compared with the more common autodoping technique. Breakdown and dynamic behavior of processed devices indicate that extrinsic carbon doping delivers better dynamic properties for the same blocking voltage capabilities. Modeling and simulations have revealed that charge transport across the GaN buffer is the main limiting factor during the buffer discharge process.

Nonpolar m-plane InGaN/GaN light emitting diode structures with quantum well (QW) were grown on LiAlO2 (100) substrates. Their photoluminescence and electroluminescence were investigated at different excitation levels. It was shown that in QW of InGaN with the pronounced InGaN band tailing, the populating of both the first and the second valence subbands of InGaN takes place due to tunneling of charge carriers and, therefore, a lower degree of polarization of the electroluminescence was observed in comparison with photoluminescence. The differences in luminescence properties under optical and electrical excitation are discussed.

The optical and structural properties of m-plane GaN layers grown by metal organic vapor phase epitaxy on LiAlO 2 (100) substrates were investigated. Temperature-dependent and time-resolved photoluminescence (PL), X-ray diffraction and Raman scattering measurements were performed to analyze the correlation of the sample properties with the density of I 1 -type basal-plane stacking faults (BSFs). Electron channeling contrast imaging was used to reveal and calculate the density of BSFs reaching the surface of an m-plane GaN/LiAlO 2 layer. It was shown that a local increase of BSF density in the investigated samples results in a rise of the total PL efficiency at low temperatures because of the localization of excitons at BSFs and, therefore, a suppression of their diffusion to nonradiative centers. Parameters of time decay and temperature quenching of the BSF-related PL band were determined. A correlation of both e xx and e zz strain components with the BSFs and crystal mosaicity was observed, and possible reasons of this correlation are discussed.

Polarized photoluminescence (PL) of a series of m -plane InGaN/GaN multiple quantum well heterostructures (xIn = 5-30%) was measured in a wide temperature range (T = 10-580 K). A pronounced S-shape behaviour of PL band position was observed for all samples which is an evidence of a high degree of localization of photogenerated carriers. From temperature dependences of the PL degree of polarization, the energy difference between the two highest valence subbands ΔE in InGaN was estimated to be increasing from ∼70 meV to ∼160 meV with rising indium content from 5% to 30%. The high-temperature spectral difference between the polarized PL components was substantially lower than ΔE which is caused by imperfect polarization of optical transitions to different valence subbands. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Quaternary barrier layers for GaN-based high-electron-mobility transistors (HEMT) have recently been a focus of interest because of the possible lattice-matched growth to GaN. This results in a reduction of strain-related defects, while having the option of adjusting the bandgap separately. A further benefit of the quaternary approach is the possibility to achieve high polarization and high carrier mobility simultaneously. This may improve the performance of such devices beyond what is possible with ternary barrier layers. In this work, we report on growth and characterization of AlxInyGa1−x−yN barrier layers within the range of 16% to 56% Al, 2% to 45% In, and 20% to 82% Ga deposited on conventional GaN buffer layers on sapphire. We present an effective way to change the composition of quaternary layers and discuss the influence of tensile and compressive strain on structural and electrical properties. From high-resolution x-ray diffraction (HRXRD), Rutherford backscattering spectroscopy (RBS), and wavelength-dispersive x-ray spectroscopy (WDX), we determined the compositions and strain states of the AlInGaN layers. The bandgaps (Eg) were obtained by spectroscopic ellipsometry (SE). Hall and van der Pauw measurements on thin heterostructure layers yielded high mobilities in excess of 1550 cm2/V s and 5350 cm2/V s at room temperature and 77 K, respectively.

We report on the metal organic vapour phase epitaxy (MOVPE) and characterization of n-type InGaN films for application in direct solar water splitting. The 40 nm thick films exhibit an In fraction of 15% and show good structural quality and a surface root mean square (RMS) roughness of only 0.7 nm. An electron concentration of 9 × 1018 cm-3 at a mobility of 185 cm2/Vs is assessed. The emission peak in photoluminescence (PL) spectra taken at room temperature (RT) is at 2.8 eV, which is consistent to transmission data and the expected bandgap value for this composition. Illumination of the InGaN electrode with a Xe lamp in a 1 M NaOH electrolyte solution at zero external voltage induces a photocurrent (PC) of 0.13 mA/cm2 with a pronounced overshoot in the first few seconds, which we explain by the discharging of a Helmholtz bilayer capacitor. The PC linearly increases with applied voltage while the estimated total conversion efficiency peaks at 0.6 V. Although hydrogen bubbles visibly evolve from the Pt counter electrode, total conversion efficiencies are limited to 0.05%, which is ascribed to incomplete utilization of the optical excitation spectrum and low thickness of both the InGaN film and the space charge region at the semiconductor surface. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Optically induced carrier dynamics was investigated in highly excited m -plane GaN films grown by metalorganic vapor phase epitaxy (MOVPE) on LiAlO 2 substrates and compared to a c -plane GaN reference. Photoluminescence (PL) spectra showing spontaneous and stimulated emission (SE) were investigated at room temperature in both backscattering and lateral geometries, the latter using two different orientations of the excitation stripe with respect to the m -plane GaN c -axis. A lower SE threshold was observed for the m -plane GaN film, which may be attributed to a different defect structure and in-plane strain. The threshold in the m -plane GaN layer is the lowest when using stripe excitation along [0 0 0 1], which is caused by a structural anisotropy in the film. The maximum degree of polarization for SE and spontaneous emission in the m -plane GaN film was found to be 0.47 and 0.23, respectively. The light-induced transient grating technique (LITG) provides carrier lifetimes of 40 and 380 ps for the m -plane and c -plane orientation, respectively, and reveals SE threshold values comparable to the ones obtained by PL.

We highlight the challenges of GaN growth on (1 1 0)-oriented Si substrates with respect to the substrate-induced anisotropic strain state. A 5:4 lattice matching relationship, which is valid for AlN and Si(1 1 1), is also expected for AlN on Si(1 1 0) along the [1 1 –2 0]AlN/[1 −1 0]Si direction. This reduces the effective lattice mismatch for AlN on Si(1 1 0) along [1 1 –2 0] AlN /[1 −1 0] Si from 23.4% to −1.3% (compressive), while in the [1 −1 0 0] AlN /[0 0 1] Si direction, the mismatch value remains at 0.7% (tensile). The opposite signs of the two mismatch values lead to an anisotropic strain state of the deposited AlN and GaN films, which makes it difficult to balance the tensile strain induced by thermal mismatch. For a simple growth scheme using 1 μm GaN on top of a 40 nm AlN layer, this situation is reflected in an aspherical wafer curvature, anisotropic X-ray diffraction (XRD) Ω scan full width at half maximum (FWHM) values, greater surface roughness and cracking, predominantly occurring parallel to the [1 1 –2 0] direction. The anisotropic XRD FWHM may be explained by a strain-induced anisotropic nucleation mechanism for GaN on AlN. Also, a partial relaxation of the GaN film during growth caused by very high tensile strain along [1 –1 0 0] AlN /[0 0 1] Si is assumed to contribute to the XRD anisotropy. The insertion of an AlN/GaN superlattice structure in between the AlN and GaN films inhibits this relaxation process and leads to a smoother surface morphology with less visible cracks.

We report about the optical and structural characterization of m-plane InGaN/GaN multiple quantum well (MQW) structures. All samples were grown on 2-inch LiAlO2 substrates in AIXTRON MOVPE reactors. By lowering the MQW growth temperature, an increase of an indium fraction from 5% to 30% is achieved. High-resolution X-ray diffraction (HRXRD) reciprocal space mapping reveals fully strained growth of the quantum wells. The photoluminescence (PL) peak full width half maximum (FWHM) values increase from ∼130 to ∼240 meV at room temperature when the indium fraction rises from 10% to 30% . Also, PL peaks show good wavelength stability for indium contents up to 10% when the excitation intensity is varied by two orders of magnitude. However, at 20% indium and above, a clear excitation intensity-induced peak shift of around 50 meV is detected which is likely caused by indium segregation. Cross-section transmission electron microscopy (TEM) hints at indium fluctuation near extended defects. We conclude that an improved growth process for nonpolar InGaN/GaN MQW is necessary to achieve high-performance long-wavelength light-emitting devices.

The anisotropic film properties of m-plane GaN deposited by metal organic vapour phase epitaxy (MOVPE) on LiAlO 2 substrates are investigated. To study the development of layer properties during epitaxy, the total film thickness is varied between 0.2 and 1.7 μm. A surface roughening is observed caused by the increased size of hillock-like features. Additionally, small steps which are perfectly aligned in (1 1 −2 0) planes appear for samples with a thickness of ∼0.5 μm and above. Simultaneously, the X-ray rocking curve (XRC) full width at half maximum (FWHM) values become strongly dependent on incident X-ray beam direction beyond this critical thickness. Anisotropic in-plane compressive strain is initially present and gradually relaxes mainly in the [1 1 −2 0] direction when growing thicker films. Low-temperature photoluminescence (PL) spectra are dominated by the GaN near-band-edge peak and show only weak signal related to basal plane stacking faults (BSF). The measured background electron concentration is reduced from ∼10 20 to ∼10 19 cm −3 for film thicknesses of 0.2 μm and ∼1 μm while the electron mobilities rise from ∼20 to ∼130 cm 2 /V s. The mobilities are significantly higher in [0 0 0 1] direction which we explain by the presence of extended planar defects in the prismatic plane. Such defects are assumed to be also the cause for the observed surface steps and anisotropic XRC broadening.

We report on growth as well as structural and optical properties of m -plane InGaN/GaN multiple quantum wells (MQWs) grown on LiAlO 2 substrate by metal–organic vapor phase epitaxy. During the growth, surface nitridations of LiAlO 2 substrate and Mg-doped InGaN buffer layers were employed. Pure m -plane and two-orientation (mixture of c - and m -plane) samples were prepared and verified by X-ray diffraction and Raman scattering. Atomic force microscopy (AFM) image of our m -plane surface morphology reveals a “fabric”-like pattern, i.e., stripes running perpendicular to each other. Photoluminescence from the m -plane MQW has polarization anisotropy, which can be attributed to the anisotropic in-plane strain. In two-orientation sample, we found that this optical polarization property survives; yet it is weakened. This indicates that polarized emission is a robust optical property in our samples. Therefore InGaN/GaN MQWs on LiAlO 2 have potential for application in efficient polarized light emitters.

In recent years, GaN grown on silicon substrates (GaN-on-Si) managed to offer a commercial solution for harvesting the advantages of GaN-based devices whilst benefiting from the well-established silicon-based infrastructure. Although having an upper hand when it comes to cost, GaN-on-Si suffers from a vertical breakdown voltage which was found to be limited by the silicon substrate [1]. Earlier investigations showed that this breakdown is related to the formation of an inversion channel at the interface between the nucleation layer and the silicon substrate [2, 3]. This inversion layer acts as a source for electron injection during the application of high positive voltages relative to the substrate, thus leading to a premature breakdown. In this work we report on the characterization of this inversion channel and demonstrate that it can be suppressed by optimizing the growth conditions of the AlN nucleation layer. To obtain a deeper understanding of the inversion layer properties, two MOCVD AlN nucleation samples were grown on highly p-doped Silicon substrates, growth temperatures between 800 °C and 950 °C, while the desorption temperature was held at 950 °C. Vertical structures were then etched, with annealed Ti/Al/Ni/Au as ohmic contact to the silicon and Ni/Au Schottky contact to the AlN.

We study the oxygen incorporation during growth of m-plane GaN films on LiAlO2 substrates by metal organic vapour phase epitaxy (MOVPE). A broad peak at ∼550 nm in photoluminescence spectra is probably caused by oxygen impurities. Secondary ion mass spectroscopy (SIMS) confirms this finding and reveals a concentration in the order of 1019 cm-3 which is consistent to the measured n-type doping level. A general slow decrease of the oxygen concentration in growth direction is observed. A c-plane GaN film grown on sapphire co-loaded together with a piece of LiAlO2 shows a slightly increased doping concentration compared to a c-plane GaN reference. However, the achieved concentration of ∼1017 cm-3 is much lower compared to m-plane GaN films. Growth of GaN on a backside-coated GaN/LiAlO2 template does neither suppress the impurity incorporation. This implies that oxygen incorporation via the gas phase is not the dominant factor. The SIMS oxygen profile of this GaN film on the template shows a strong peak at the position of growth interruption indicating the formation of a surface oxide. We assume that the m-plane GaN surface has a large affinity for oxygen at the deposition temperature which leads to the high oxygen impurity level. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report on a new planetary hotwall research system with independent control of the ceiling and susceptor temperatures. This allows a study of the influence of an actively heated ceiling during a MOCVD process. It was found that the temperature profile across the substrate becomes more uniform when the ceiling temperature is increased while maintaining the same susceptor surface temperature. The parasitic coating on the ceiling was found to be stable and no source of particles up to 1050 °C. The influence of the ceiling temperature on the depletion profile and the possibility to manipulate it was investigated. Good thickness homogeneity of 1.35% (1·σ) on a 4″ sapphire wafer was achieved with excellent XRD results of 206 arcsec for the (0002) reflex and 272 arcsec for the (10-12) reflex for a 4.7 µm thick GaN film. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

AlInN is being considered to replace AlGaN as barrier material in high-electron mobility transistors (HEMT) mainly due to the possibility to downscale the barrier layer thickness at coexistent higher drain current density. However, the stability of surface conditions which can strongly influence the electrical properties of the device has to be assured. In situ deposition of a SiN passivation layer by metal-organic vapour phase epitaxy (MOVPE) is therefore a promising candidate to improve the device structure. In this paper, heterostructures with thin AlInN barrier passivated by in situ deposition of SiN layers of different thicknesses are presented. As expected, deterioration of the sheet resistance (RSh) was observed for the reference sample without SiN passivation layer by continuously performed RSh mappings of the investigated wafer. This change is noticeably reduced for samples with in situ SiN passivation layer. A comparison of two wafers, stored either with or without air exposure, reveals oxidation as presumable reason for the deterioration. Further, results from RSh measurements on processed wafers indicate an enhanced RSh stability during process for SiN-capped samples. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The line shape of X-ray diffraction (XRD) rocking curves of GaN layers grown epitaxially on (0001) oriented sapphire substrates is analyzed. Measurements performed with double- and triple-crystal setup show a q-3 and q-4 intensity decay, respectively, as expected for peak broadening dominated by randomly distributed dislocations. A model developed in [2], based on a restricted random dislocation distribution is fitted to the entire peak shape and used to extract dislocation densities and correlation lengths for edge and screw type threading dislocations. Parameters extracted by double- and triple-crystal x-ray diffraction measurements agree well with each other but still must be verified by systematic cross-sectional TEM measurements. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We used metal organic vapour phase epitaxy (MOVPE) to deposit m-plane GaN layers on LiAlO 2 substrates and studied the influence of the V/III ratio during growth. All films exhibit a special surface pattern with pyramidal hillocks elongated in [11-20] direction of GaN and small steps perfectly aligned in the (11-20) plane. For higher V/III ratios, the roughness is reduced while holes on the surface are generated. Polarization-dependent low-temperature (LT) photoluminescence (PL) spectra show GaN excitonic peaks at ∼3.5 eV with a high degree of polarization (DoP) of ∼0.9. Another peak at 3.47 eV is much less intense compared to the excitonic peak and probably related to basal plane stacking faults (BSF). Anisotropic in-plane compressive strain which slightly relaxes for a higher V/III ratio can be detected by high-resolution X-ray diffraction (HRXRD). XRD reciprocal space maps of the symmetric (1-100) reflection show relatively weak broadening for scans taken in the [0001] direction and strong broadening for the perpendicular direction. Anisotropic room temperature electron mobilities are found with average values of 108 and 117cm 2 /Vs in [11-20] and [0001] direction of GaN, respectively. We explain the higher values in c-axis direction by the presence of extended planar defects in the prism plane which may be also responsible for the observed surface steps and anisotropic broadening of the XRD RSM.

We deposited pure m -plane GaN (1 1¯ 0 0) layers on LiAlO 2 (1 0 0) substrates by MOVPE using Mg-doped InGaN buffer layers of various thickness. These sealing layers are grown under nitrogen ambient and help to improve the film coalescence while reducing the oxygen background doping of the GaN films. We show that for an increasing thickness of the InGaN:Mg buffer layer, the highly strained GaN layer slightly relaxes. An improved surface morphology can be achieved for thicker InGaN:Mg buffers while the defect density clearly increases above a certain thickness, as it can be seen in X-ray rocking curve measurements. In temperature-dependent photoluminescence spectroscopy, we observe not only peaks due to DAP transitions at an energy of around 3.3 eV, but also a defect-related peak at 3.43 eV as well as a peak at 3.49 eV connected to free and bound excitonic transitions. It is interesting to note that for an increasing thickness of the InGaN:Mg buffer we detect a higher intensity of the excitonic transition while the defect-related signal seen at 3.43 eV is getting more pronounced. It might be possible that during the film relaxation basal plane stacking faults are introduced into the layer.

We report about the MOVPE growth and characterisation of Al0.90In0.10N single layers and multiple quantum well structures (MQW) containing 8 periods of Al0.90In0.10N/AlN quantum wells. All samples show good quality in high-resolution X-ray diffractometry (HRXRD) evaluation, superlattice fringes up to the 2nd order can be seen for the MQW sample. In optical transmission experiments, an absorption edge for the Al0.90In0.10N single layer is detected at 4.9 eV. Due to the low intensity of emission and limited sensitivity of our setup in this spectral region, photoluminescence (PL) emission above 4 eV is only visible for the MQW sample. Both PL and cathodoluminescence (CL) measurements show an emission peak at longer wavelength for the Al0.90In0.10N/AlN MQW sample. When increasing the excitation current density in CL, the emission at lower wavelength is clearly enhanced, while it is slightly reduced at higher wavelength. We believe this to be a saturation effect of a defect-related emission. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report on the growth and characterisation of 190 nm thick Al0.83In0.17N layers and Al0.83In0.17N/GaN high electron mobility transistor (HEMT) structures on GaN/sapphire. High-resolution X-ray diffraction rocking curve measurements (HRXRD) exhibit full width at half maximum (FWHM) values of 68 and 175 arcsec for the GaN (0002) and (10-15) reflections, respectively. The corresponding Al0.83In0.17N reflections yield 321 and 582 arcsec on 190 nm thick layers. XRD simulations of HEMT structures match well with measured data and with the intended heterostructure stack. Layer thickness fringes up to the 3rd order indicate a good heterointerface and enable precise barrier thickness determination. Further, no spinodal decomposition is detected for AlInN in both symmetrical and asymmetrical reciprocal space map measurements. Electrical characterisations of the HEMT at room temperature yielded a charge carrier density of ns = 2 x 1013 cm–2 and a mobility of µ = 1460 cm2/Vs underlined by processed devices demonstrating maximum drain current densities of 1.2 A/mm at VGS = 2 V and extrinsic transconductances of 243 mS/mm (LG = 1 µm, WG = 50 µm, 5 µm source-drain spacing). (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Lattice-matched Al0.83In0.17N/GaN high electron mobility transistors (HEMTs) grown by MOCVD on sapphire substrate were investigated. The transport properties of the 2DEG at room temperature were characterised by sheet carrier concentrations up to 2.0×1013 cm–2 at a mobility of 1450 cm2/Vs. Unpassivated devices with gate lengths of LG = 1 µm and a gate-drain spacing of 5 µm exhibit maximum drain current densities of 1.2 A/mm at VGS = 2 V and extrinsic transconductances of 243 mS/mm, respectively. The small-signal behaviour is characterised by a current gain cut-off frequency fT = 13 GHz and a maximum frequency of oscillation fmax= 29 GHz, respectively. Pulsed I-V measurements from different quiescent bias points reveal dispersion effects resulting in a partial drain current collapse. A SiN passivation was found not to alleviate these effects. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We present a study of growth and properties of m -plane GaN/InGaN/AlInN structures on LiAlO2 substrates grown by metal organic vapour phase epitaxy (MOVPE). A buffer structure, including an m -plane AlInN interlayer prior to GaN growth, has been developed. Quantum well structures on top of this buffer showed absence of polarization-induced electric fields verified by room temperature photoluminescence (RT PL) measurements with different excitation intensities. Different samples with peak emission wavelength between 433 nm and 495 nm exhibited stable peak position even for high excitation intensities of 500 kW/cm2. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Back-gated measurements on conductive silicon substrates have been performed to investigate the effect of stress voltage on the dynamic behaviour of GaN-on-silicon (GaN-on-Si) transistors. Two comparable samples were studied with the only difference being the vertical dislocation density. Results show a clear correlation between dislocation density and the ability of the GaN buffer to dynamically discharge under high stress conditions.

GaN-based heterostructures are widely used nowadays in many applications such as high brightness light emitting diodes (LEDs), semiconductor lasers, photodetectors, transistors, solar cells, etc. An advantage of AlxInyGa1-x-yN compound over other wide-gap materials is potential ability to cover spectral region from deep ultraviolet (UV) (∼198 nm corresponding to AlN) to infrared (∼1,770 nm corresponding to InN). Creating of effective optoelectronic devices operating in deep UV region and finding of low-cost and practically feasible substrates for GaN heteroepitaxy are the main objectives in GaN-based technology today. In the present work, several examples of fabrication of GaN-based electronic devices are considered, i. e. blue InGaN/GaN LED (∼475 nm) grown on silicon substrate; blue InGaN/GaN LED emitting polarized light (∼450 nm, polarization degree ∼0.42) grown on LiAlO2 substrate; high sensitive metal-semiconductor-metal AlGaN/GaN UV photodetector (η ∼ 4×107 A/W); AlGaN-based UV LED (300–340 nm); optically pumped AlGaN-based UV lasers emitting at 303 and 295 nm with laser threshold of ∼1 MW/cm2.

In this paper we report on the growth and charcterization of quaternary AlInGaN/AlN/GaN heterostructures, spanning a range from 26% to 82% AlN, 4% to 18% InN and 0% to 70% GaN in the barrier layer. All samples were grown in AIXTRON metal organic vapor phase epitaxy (MOVPE) reactors on 2″ sapphire substrates. Sample characterisation was performed using high resolution X-ray diffraction (HRXRD), X-ray reflection (XRR), photoluminescence spectroscopy (PL), atomic force microscopy (AFM) and Rutherford backscattering (RBS). Inductive sheet resistance measurements and Hall measurements in van der Pauw geometry provided electrical characterization. AFM measurements show closed layers, in HRXRD measurements pendellosung oscillations are visible even in asymmetric reciprocal space maps. Mobility and sheet carrier concentration reach state of the art values currently achieved for ternary structures showing the high degree of tunability in the quaternary nitride system. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In this paper, we investigate the influence of the nitridation of LiAlO2 substrates on the growth of m-plane (1-100) GaN layers by metal-organic vapour phase epitaxy (MOVPE). Before thin film deposition, we performed an in-situ substrate pretreatment by exposing the wafer to NH3 for different times between no pretreatment and 300 s. The properties of subsequently grown layers show a significant dependency on this nitridation step. We find that this procedure is essential for obtaining pure m-plane GaN films and has a beneficial effect on the X-ray rocking curve (XRC) full width at half maximum (FWHM) value, which decreases by almost two orders of magnitude. Deposited layers with NH3 pretreatment also exhibit much smoother surfaces with a reduction of the root mean square (RMS) roughness value from ∼20 to ∼6 nm. Additionally, the nitridation greatly increases the GaN band edge emission intensity in room temperature (RT) photoluminescence (PL) spectroscopy. Furthermore, we compare the sensitivity of the substrate against water for uncoated LiAlO2 wafers with and without nitridation process. While the untreated surface shows a clear roughening when dipped into de-ionized (DI) water for 5 min, we can see no significant impact on the nitridated substrate surface. This indicates a change in surface composition which protects the sensitive substrate surface and provides good conditions for the nucleation of high-quality m-plane GaN films. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report on structural and device properties of AlInN/AlN/GaN transistor heterostructures grown by metal organic vapour phase epitaxy (MOVPE) on 2″ sapphire substrates with AlInN barriers of thicknesses between 4 nm and 10 nm. The In content and thickness of the thin AlInN barrier is shown to be well determinable by high-resolution X-ray diffraction (HRXRD). Room temperature Hall measurements yielded similar mobility between 1400 cm2V–1s–1 and 1520 cm2V–1s–1 on all samples and increasing sheet carrier concentration ns with rising barrier thickness resulting in a minimum sheet resistance value of 200 Ohm/□. The effect of surface passivation with Si3N4 on the electrical properties is investigated and found to strongly increase sheet carrier concentration ns of the two-dimensional electron gas (2DEG) to values above 2×1013cm–2. Characterization of transistors with gate length Lg of 1.5 μm produced from the grown samples reveals high transconductance (gm) and a maximum drain current (ID) of 300 mS/mm and ∼1 A/mm, respectively. For the sample with 4.6 nm barrier thickness, a reduced gate leakage current (IGL) and a absolute value of the threshold voltage (Vth) of -1.2 V is detected. Radio frequency (RF) measurements of passivated samples lead to maximum current gain cut-off frequencies ft of 11 GHz and maximum oscillating frequencies fmax of 25 GHz. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report on deposition and properties of m-plane GaN/InGaN/AlInN structures on LiAlO2 substrates grown by metal organic vapor phase epitaxy (MOVPE). At first, two different buffer structures, one of them including an m-plane AlInN interlayer, were investigated concerning their suitability for the subsequent coalesced single-phase m-plane GaN growth. A series of quantum well structures with different well thickness based on one of these buffers showed absence of polarization-induced electric fields verified by room temperature photoluminescence (RT PL) measurements at different excitation intensities. Furthermore, polarization-resolved PL measurements revealed a high degree of polarization (DoP) of the emitted light with an intensity ratio of 8:1 between light polarized perpendicular and parallel to the c-axis.

Threading dislocations, stacking faults, and associated partial dislocations significantly degrade the optical and electrical properties of materials such as non-polar III-nitride semiconductor thin films. Stacking faults are generally difficult to detect and quantify with existing characterization techniques. We demonstrate the use of electron channeling contrast imaging in the scanning electron microscope to non-destructively reveal basal plane stacking faults terminated by partial dislocations in m-plane GaN and InGaN/GaN multiple quantum well structures grown on γ-LiAlO2 by metal organic vapor phase epitaxy.

Using X-ray reflectivity (XRR) measurements and high-resolution transmission electron microscopy (HRTEM), we demonstrate that a thin AlN layer is formed on top of a γ-LiAlO2(100) substrate by a nitridation process using NH3 in a metal organic vapor phase epitaxy (MOVPE) reactor. This thin layer is identified to be monocrystalline, M-plane AlN, enabling the growth of nonpolar M-plane GaN on γ-LiAlO2(100) by MOVPE. HRTEM reveals that this thin nitridation layer is grown pseudomorphically without any detectable misfit dislocations.

Polarized temperature dependent photoluminescence (PL) and room temperature (RT) photocurrent spectra of m-plane InGaN/GaN multiple quantum wells grown on LiAlO2 with In content xIn = 5%-30% were studied. As expected, higher xIn leads to larger strain in the wells and enhances both the splitting between the two highest valence subbands and the RT PL degree of polarization. At low temperatures, an irregular red-shift of the PL component with polarization E || c relative to E ⊥ c is observed, which is ascribed to the contribution of recombination of holes localized at band tails within the second highest valence subband.

The development of GaInN quantum well structures with nonpolar crystal orientation for light-emitting diodes and semiconductor lasers is currently one of the main foci of III-nitride-based optoelectronics research. One of the advantages of nonpolar orientations is the absence of polarization fields perpendicular to the quantum well plane. As a consequence, radiative recombination rates are higher compared to quantum wells on polar surfaces. However, due to high densities of threading dislocations and basal plane stacking faults in the case of heteroepitaxially grown nonpolar layers, and due to band gap inhomogeneities in the GaInN quantum wells, characterization of radiative and nonradiative recombination mechanisms is a complex challenge. So far, most published data about band gap fluctuations, charge carrier localization and internal quantum efficiency in nonpolar quantum wells are ambiguous. Here, we present temperature and excitation power density-dependent photoluminescence data featuring multiple characteristics related to strong charge carrier localization in m-plane (1–100) GaInN quantum wells. Thermally activated redistribution of charge carriers between localization sites in these quantum wells is weaker than in polar c-plane ones. The localization strength increases with higher indium concentration in the quantum wells. In the heteroepitaxially grown quantum well structures, the internal quantum efficiency is reduced even at low temperatures (T = 10 K) and especially for m-plane quantum wells with high indium mole fractions.

Polarized in-plane and cross-sectional Raman spectra have been used to determine the crystal orientation of m-plane GaN grown on γ-LiAlO2(100) (LAO) using a three-step metalorganic vapor phase epitaxy process. The epitaxial relationship is found to be GaN(11¯00)∥LAO(100) and GaN[112¯0]∥LAO[001]. However, the stripes on the GaN surface are oriented parallel to [0001], i.e., perpendicular to the one found on striated m-plane GaN surfaces in previous studies. This unusual orientation is attributed to the changes in the Ga adatom kinetics due to the presence of a 2-nm-thick interlayer observed at the GaN/LAO interface in transmission electron microscopy.