Search

In this study, we investigated the effect of different-size V pits on the opto-electrical performance of different-size InGaN-based green (520 nm) light-emitting diodes (LEDs). The size of V pits varied from 57 to 250 nm. Two different-size LEDs (10 and 300 µm) were fabricated with flip-chip structures. It was shown that at 4 A cm−2, the forward voltages of 300 µm-size and 10 µm-size LEDs were in the range of 2.32–2.82 V and 2.23–2.53 V, respectively. Compared with the LEDs with small V pits, the LEDs with large V pits produced higher output power at the high current region (>50 A cm−2), but lower output power at the low current region (

The influence of the sapphire substrate off-cut angle from c-plane orientation on the surface morphology of AlN/AlGaN layers and the performance of 310 nm light emitting diode structures has been investigated. Increasing the off-cut angle from 0.1° to 10° towards the m-plane leads to a change of the (Al,Ga)N growth mode, threading dislocation density, and emission characteristics of ultraviolet light emitting diodes (LEDs). The growth mode changed from step-flow with bilayer steps to step-bunching and finally to rough 3D growth. The lowest threading dislocation density has been observed for structures deposited on sapphire with an off-cut angle of 0.65°, whereas the emission power is the highest for structures grown on substrates with an off-cut of 0.48°. Electroluminescence measurements display single peaks for structures on substrates with off-cut angles of up to 0.65°. The emission peak full width at half maximum remains nearly constant (~9.5 nm) for structures with off-cut angels up to 0.21° and increases to 11.7 nm and 21.0 nm for 0.48° and 0.65°, respectively. The emission wavelength is almost unchanged by the use of off-cut angles up to 0.48° (311 nm) and increases to 319 nm for 0.65°. Larger off-cut angles cause multiple emission peaks indicating compositional inhomogeneity of the InAlGaN quantum wells.

In this paper we describe the scanning electron microscopy techniques of electron backscatter diffraction, electron channeling contrast imaging, wavelength dispersive X-ray spectroscopy, and cathodoluminescence hyperspectral imaging. We present our recent results on the use of these non-destructive techniques to obtain information on the topography, crystal misorientation, defect distributions, composition, doping, and light emission from a range of UV-emitting nitride semiconductor structures. We aim to illustrate the developing capability of each of these techniques for understanding the properties of UV-emitting nitride semiconductors, and the benefits were appropriate, in combining the techniques.

Heterogeneously integrated AlGaN epitaxial layers will be essential for future optical and electrical devices like thin-film flip-chip ultraviolet (UV) light-emitting diodes, UV vertical-cavity surface-emitting lasers, and high-electron mobility transistors on efficient heat sinks. Such AlGaN-membranes will also enable flexible and micromechanical devices. However, to develop a method to separate the AlGaN-device membranes from the substrate has proven to be challenging, in particular, for high-quality device materials, which require the use of a lattice-matched AlGaN sacrificial layer. We demonstrate an electrochemical etching method by which it is possible to achieve complete lateral etching of an AlGaN sacrificial layer with up to 50% Al-content. The influence of etching voltage and the Al-content of the sacrificial layer on the etching process is investigated. The etched N-polar surface shows the same macroscopic topography as that of the as-grown epitaxial structure, and the root-mean square roughness is 3.5 nm for 1 µm x 1 µm scan areas. Separated device layers have a well-defined thickness and smooth etched surfaces. Transferred multi-quantum-well structures were fabricated and investigated by time-resolved photoluminescence measurements. The quantum wells showed no sign of degradation caused by the thin-film process.

The current injection mechanisms for blue light emitting diodes (LEDs) with and without V pits were examined by controlling the bandgaps of InGaN quantum wells (QWs), which were changed by reducing the indium content. To identify the distribution of holes in the QWs, the electroluminescence of the LEDs was characterized by varying the positions of the QWs with the wider bandgap consecutively from n-cladding to p-cladding sides. For the LEDs without V pits, holes were injected through the top QWs (p-cladding side), while for the LEDs with V pits, holes were injected mainly through the bottom QWs (n-cladding side).

The height of the barrier around the AlGaN quantum well has a strong impact on the external quantum efficiency of UVC light emitting diodes (LEDs) as it affects the carrier confinement, the polarization fields, and the injection efficiency as well as the optical polarization and emission profile of the emitted light. The electro-optical properties such as emission wavelength, optical polarization, and light output power of AlGaN multiple quantum well (MQW) LEDs emitting around 270 nm with Al mole fraction in the Al x Ga1−x N barriers between x = 55% and x = 76% are investigated by electroluminescence measurements. In order to analyze the experimental results, 6-band kp method-based simulations as well as single band Schrodinger-Poisson drift-diffusion simulations have been conducted. It was found that for the same current density of 100 A cm−2 the on-wafer emission power reaches a maximum for an Al mole fraction of x = 67% in the Al x Ga1−x N barrier of the Al0.53Ga0.47N MQW (0.84 mW at 40 mA). Furthermore, the emission wavelength decreases and the fraction of transverse-electric polarized light emission increases with increasing Al mole fraction in the barrier. This is consistent with drift-diffusion and kp simulations, attributing the changes of the emission power primarily to changes in charge carrier injection and electrical confinement in the quantum wells rather than to changes in the optical polarization and light extraction.

The impact of operation current on the degradation behavior of 310 nm UV LEDs is investigated over 1000 h of stress. It ranges from 50 to 300 mA and corresponds to current densities from 34 to 201 A/cm2. To separate the impact of current from that of temperature, the junction temperature is kept constant by adjusting the heat sink temperature. Higher current was found to strongly accelerate the optical power reduction during operation. A mathematical model for lifetime prediction is introduced. It indicates that lifetime is inversely proportional to the cube of the current density, suggesting the involvement of Auger recombination.

The growth mechanisms during metalorganic vapor phase epitaxy (11-22) oriented Al x Ga1-x N with x ∼ 0.80 on m-plane sapphire are studied with the intention of mitigating the expansion of misoriented grains, composed of the (1-10-3) crystal orientation and achieving a flat surface with only the (11-22) orientation. An increase in reactor pressure, metalorganic supply, and V/III ratio led to a decrease in the grain density from 1.0 × 109 cm−2 to 1.5 × 107 cm−2. By comparing different growth regimes, we found that the main factor suppressing the growth of the (1-10-3) orientation and decreasing the grain density in the AlGaN layers is the growth rate, which decreased with increasing reactor pressure, MO supply, and V/III ratio due to increasing pre-reactions in the gas phase. To mitigate pre-reactions even with lower growth rates, growth conditions with higher total flow and lower TMAl flow were chosen, yielding low growth rates of 0.13 μm h−1 and a grain density of 3.0 × 107 cm−2 at an aluminum mole fraction of 84%. To allow the growth of thick LED heterostructures we demonstrated that such a buffer can be overgrown with higher growth rate AlGaN, yielding a low grain density of 1.0 × 106 cm−2 and a smooth morphology with a rms roughness of 2.5 nm by avoiding misoriented crystal propagation during nucleation.

The influence of aluminum mole fraction of Al x Ga1-x N/Al y Ga1-y N multiple quantum wells (MQWs) on the optical polarization, light extraction efficiency (LEE) and external quantum efficiency (EQE) of deep ultra violet light emitting diodes in the wavelength range between 264 and 220 nm is investigated. The on-wafer EQE decreases from 0.6% to 0.00013% in this wavelength range. Polarization resolved photoluminescence and electroluminescence measurements show a change from dominant transverse-electric to dominant transverse-magnetic polarized light emission with increasing aluminum mole fraction in the MQW. The quantitative agreement with k·p calculations allow to ascribe this shift to a change of the characteristic of the Γ7+ valance band. Ray tracing simulations predict a reduction of the on-wafer LEE from 4% to 1.5%. Therefore the dramatic drop of the EQE in this wavelength range cannot be attributed to a drop in LEE and is most likely dominated by charge carrier injection and radiative recombination efficiency.

We report on AlGaN-based tunnel heterojunctions grown by metalorganic vapor phase epitaxy enabling fully transparent UVC LEDs by eliminating the absorbing p-AlGaN and p-GaN layers. Furthermore, the electrical characteristics can be improved by exploiting the higher conductivity of n-AlGaN layers as well as a lower resistance of n-contacts. UVC LEDs with AlGaN:Mg/AlGaN:Si tunnel junctions exhibiting single peak emission at 268 nm have been realized, demonstrating effective carrier injection into the AlGaN multiple quantum well active region. The incorporation of a low band gap interlayer enables effective tunneling and strong voltage reduction. Therefore, the interlayer thickness is systematically varied. Tunnel heterojunction LEDs with an 8 nm thick GaN interlayer exhibit continuous-wave emission powers >3 mW near thermal rollover. External quantum efficiencies of 1.4% at a DC current of 5 mA and operating voltages of 20 V are measured on-wafer. Laterally homogeneous emission is demonstrated by UV-sensitive electroluminescence microscopy images. The complete UVC LED heterostructure is grown in a single epitaxy process including in situ activation of the magnesium acceptors.

Due to changes in the spontaneous and piezoelectric polarization, AlGaN/GaN heterostructures exhibit strong polarization fields at heterointerfaces. For quantum wells, the polarization fields lead to a strong band bending and a redshift of the emission wavelength, known as quantum-confined Stark effect. In this paper the polarization fields of thin AlGaN layers in a GaN matrix were determined by evaluating the changes in the depletion region width in comparison to a reference sample without heterostructure using capacitance-voltage-measurements. The polarization fields for Al0.09Ga0.91N (0.6 +/- 0.7 MV cm(-1)), Al0.26Ga0.74N (2.3 +/- 0.6 MV cm(-1)), Al0.34Ga0.66N (3.1 +/- 0.6 MV cm(-1)), Al0.41Ga0.59N (4.0 +/- 0.7 MV cm(-1)) and Al0.47Ga0.53N (5.0 +/- 0.8 MV cm(-1)) heterostructures were determined. The results of the field strength and field direction of all samples are in excellent agreement with values predicted by theory and a capacitance-voltage based Poisson-carrier transport simulation approach giving experimental evidence for a nonlinear increasing polarization field with Al-concentration. (C) 2019 The Japan Society of Applied Physics

The impact of dislocations on surface topology as well as on quantum well emission in c-plane, semipolar, and nonpolar InGaN/GaN heterostructures is being analyzed by micro-photoluminescence and white-light-interferometry. V-pits with (101) and (10) side facets are identified in a (102) semipolar heterostructure. Hillocks formed by spiral growth around screw dislocations change from hexagonal to triangular to rectangular shape in polar, semipolar, and nonpolar heterostructures, respectively, reflecting the symmetry of the individual surface. The emission in semipolar quantum wells, grown homoepitaxially on bulk GaN substrates, show dark stripes aligned with misfit dislocations. For (112) and (201) orientation, these dark stripes are perpendicular and parallel, respectively, to surface striation.

The performance characteristics of optically pumped laser heterostructures emitting in the UV-C spectral range between 272 and 279 nm are investigated. The laser heterostructures were grown by metal-organic vapor phase epitaxy on (0001) planar AlN/sapphire, epitaxially laterally overgrown (ELO) AlN/sapphire, and bulk AlN substrates with threading dislocation densities ranging from 2×1010 to 104 cm-2. We found that the defect density strongly affects the laser performance. The lowest pulse threshold energy density of 50 mJ/cm2 under resonant optical pumping condition was obtained for an AlGaN multiple quantum well laser grown pseudomorphically on low defect density bulk AlN substrate. Lasing was also observed for AlGaN MQW heterostructures grown on ELO AlN/sapphire templates. The laser emission in all lasers was TE polarized. However, no lasing was observed for heterostructures grown on high defect density AlN/sapphire.

We report on metal–organic vapor phase epitaxy (MOVPE) of (0001)InN layers simultaneously grown on a-plane (112¯0) and c-plane (0001) sapphire substrates. The substrates were nitridated at temperatures from 500 °C to 1050 °C prior to the growth of c-plane InN layers. Nitridation determined the polarity, the crystallinity, and the surface morphology of the InN layers. Nitridation temperatures above 800 °C lead to N-polar InN layers, while nitridation temperatures from 700 °C to 750 °C produce mixed-polar InN layers, and nitridation temperatures from 500 °C to 650 °C produce In-polar InN layers. The roughness and crystallinity of the InN layers are correlated with the changes of polarity. The incorporation of nitrogen into the nitridation layers at different nitridation temperatures was measured. A strong N–Al bond signal after nitridation is correlated with N-polarity layers after overgrowth.

The defect structure and the homogeneity of 1–3 µm thick AlxGa1−xN layers grown on epitaxially laterally overgrown (ELO) AlN on patterned AlN/sapphire templates have been investigated in dependence on the miscut direction of the c-plane sapphire substrates, the etching depth into the sapphire and the Al concentration. It was found that shallowly etched AlN/sapphire templates with a 0.25° miscut toward the a-plane provide a smooth surface of ELO AlN and therefore a good Al homogeneity in the overgrown Al0.8Ga0.2N layer. The threading dislocation density in these layers is as low as 5×108 cm−2.

The electrical properties of different metal systems for ohmic contacts on the nitrogen-face of c-plane n-type GaN substrates are investigated. The metal contacts are compatible with the fabrication process and the packaging technology for group III-nitride laser diodes. The metal system Ti/Al/Mo/Ti/Ni/Au/Ti/Pt is determined as the best suitable, since it is ohmic already after annealing at a temperature of 450°C for 60 s. This annealing temperature is high enough to make the contact insensitive against later soldering on a heat-sink at 330°C. At the same time, the temperature is low enough that the Pd-based p-contact, previously annealed at 530°C, does not degrade. In addition, the Ti/W/Al and Pd/Ti/Al metal systems form low-resistance ohmic contacts, too, although they require a longer annealing time of several minutes or a higher temperature of 500°C.

A1. Morphological stability A1. Stresses A1. Surface processes A3. Metalorganic vapour phase epitaxy B1. Nitrides B2. Semiconducting III-V materials abstract We have investigated on the growth of InGaN layers and quantum dots (QDs) on GaN (0001) by metal- organic vapour phase epitaxy. For indium contents above 15% strained QDs form after a certain layer thickness (Stranski Krastanov mode) with typical heights of 2-3 nm. The QD density increases fast during further deposition until saturation above 10 10 cm −2 . Growing thicker layers results in larger but relaxed structures and leave only a small growth window for strained QDs. The data indicate that QDs form when the accumulated strain energy calculated from indium content and layer thickness exceeds a critical value which is independent from the indium content. The same behaviour was found for the critical thickness of relaxation, indicating that relaxation occurs via the relaxation of large QDs with heights above 3 nm.

The substrate miscut orientation of c-plane sapphire has an impact on the lateral growth and coalescence of AlN during epitaxial lateral overgrowth of patterned AlN/sapphire templates. A faster and more homogeneous coalescence was observed for substrates with a miscut orientation of 0.25° toward the m -direction compared to 0.25° toward the a -direction of the sapphire. A reduction of V/III ratio during overgrowth leads to an increase of lateral growth rate and therefore to a faster coalescence. The layer thickness at coalescence was clearly determined by in-situ reflectance measurements during the overgrowth. The coalescence can be controlled by carefully choosing the parameters influencing lateral growth like substrate miscut, process temperature and V/III ratio.

We report on the growth of semipolar ( 11 2 ¯ 2 ) AlGaN in the entire composition range on ( 10 1 ¯ 0 ) sapphire by metal–organic vapor phase epitaxy. Growth rates increase linearly with the metal–organic supply and have been realized up to 2.5 μ m / h . The Al content determined by photoluminescence and transmission measurements depends linearly on the Al/Al+Ga ratio in the gas phase. The growth rate and the Al incorporation are higher for ( 11 2 ¯ 2 ) AlGaN than for (0001) AlGaN. This is attributed to different adatom mobility and desorption. The in-plane relationship of ( 11 2 ¯ 2 ) AlGaN is [ 1 ¯ 100 ] AlGaN ∥ [ 1 2 ¯ 10 ] Sap . and [ 1 ¯ 1 ¯ 23 ] AlGaN ∥ [0001] Sap . , as typical for semipolar nitrides on m-plane sapphire. For Al contents up to 60% triangle-like structures dominate the surface. For Al contents above 70% it transforms into an undulation along [ 1 ¯ 100 ] and additional dot-like structures. The smoothest surface morphologies with a rms roughnesses of 2 nm were achieved for Al mole fractions of ∼ 50 % .

The paper reports the analysis of (In)AlGaN-based UV-B LEDs degradation under constant current stress, and investigates the impact of defects in changing the devices electro-optical performance. The study is based on combined electro-optical characterization, deep-level transient- (DLTS) and photocurrent spectroscopy. UV-B LEDs show a decrease of the optical power during stress, more pronounced at low measuring current levels, indicating that the degradation is related to an increase of Shockley-Read-Hall (SRH) recombination. DLTS measurements allowed the identification of three defects, in particular one ascribed to Mg-related acceptor traps presence. Photocurrent spectroscopy allows the localization of the defects close to the mid-gap.

Detailed knowledge of the dopant concentration and composition of wide band gap AlxGa1−xN layers is of crucial importance for the fabrication of ultra violet (UV) light emitting diodes (LEDs). This paper demonstrates the capabilities of wavelength dispersive X-ray (WDX) spectroscopy in accurately determining these parameters and compares the results with those from high resolution X-ray diffraction (HR-XRD) and secondary ion mass spectrometry (SIMS). WDX spectroscopy has been carried out on different silicon-doped wide bandgap AlxGa1−xN samples (x between 0.80 and 1). This study found a linear increase in the Si concentration with the SiH4/group-III ratio, measuring Si concentrations between 3×1018 cm−3 and 2.8×1019 cm−3, while no direct correlation between the AlN composition and the Si incorporation ratio was found. Comparison between the composition obtained by WDX and by HR-XRD showed very good agreement in the range investigated, while comparison of the donor concentration between WDX and SIMS found only partial agreement, which we attribute to a number of effects.

We report on the use of micro-LED arrays, consisting of a matrix of interconnected micrometer-size light-emitting diodes (LEDs), to ensure uniform current injection, reduced series resistance, and improved heat extraction in LEDs emitting in the ultraviolet wavelength region. With the help of experiments and simulations, we show that, in both 380- and 300- to 325-nm LEDs, a greater than twofold decrease in the thermal resistance of the device, as compared with a conventional large area contact, is possible with the use of micro-LED arrays. The thermal resistance was found to linearly decrease with the size of the individual micro-LED, indicating that the improved heat dissipation is mainly due to the division of the heat source from one large area for the square contact into smaller pixels, thus allowing a sufficient area for the dissipation of the generated heat.

We investigate the influence of inhomogeneous broadening on the optical polarization properties of high-inclination semipolar and nonpolar ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}$/GaN quantum wells. Different planar m-plane and $(20\overline{2}\overline{1})$ samples were grown (including core-shell microrods) and have been characterized by excitation-dependent polarization-resolved confocal micro-photoluminescence. The measured degree of linear polarization (DLP) is compared to theoretical predictions obtained by Fermi-Dirac statistical filling of the electronic band structure calculated by the $\mathbf{k}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbf{p}$ envelope function method. We show that our measured DLP at room temperature, as well as values reported by other groups, are systematically higher than the theoretical predictions. We propose to solve this discrepancy between theory and experiment by introducing inhomogeneous broadening in our calculations. Considering indium content fluctuations and the localization lengths of electrons and holes, different effective broadenings are applied to different subsets of subbands. We thereby show that inhomogeneous broadening leads to an increase of the DLP at room temperature. Furthermore, the dependence of the optical properties on the excitation density is better reproduced. Looking at the DLP as a function of the temperature gives us insight into the thermalization dynamics of charge carriers.

The suitability of materials for deep ultraviolet (DUV) waveguides concerning transmittance, fabrication, and coupling properties is investigated and a fused silica core/ambient air cladding waveguide system is presented. This high refractive index contrast system has far better coupling efficiency especially for divergent light sources like LEDs and also a significantly smaller critical bending radius compared to conventional waveguide systems, as simulated by ray-tracing simulations. For the fabrication of 300-ffm-thick multimode waveguides a hydrouoric (HF) acid based wet etch process is compared to selective laser etching (SLE). In order to fabricate thick waveguides out of 300-ffm-thick silica wafers by HF etching, two masking materials, LPCVD silicon nitride and LPCVD poly silicon, are investigated. Due to thermal stress, the silicon nitride deposited wafers show cracks and even break. Using poly silicon as a masking material, no cracks are observed and deep etching in 50 wt% HF acid up to 180 min is performed. While the masked and unmasked silica surface is almost unchanged in terms of roughness, notching defects occur at the remaining polysilicon edge leading to jagged sidewalls. Using SLE, waveguides with high contour accuracy are fabricated and the DUV guiding properties are successfully demonstrated with propagation losses between 0.6 and 0:8 dB=mm. These values are currently limited by sidewall scattering losses.

The crystalline perfection of n-type AlxGa1 − xN layers with Al content higher than x > 0.4 on epitaxial laterally overgrown AlN was investigated by high resolution X-ray diffraction, photoluminescence (PL), cathodoluminescence (CL) and resistivity mapping. The critical layer thickness of AlGaN is drastically increased compared to planar AlN/sapphire templates. CL mapping reveals compositional variations caused by different Ga incorporation during growth over steps resulting from step bunching of the AlN. With increasing Al content the homogeneity is improved. N-type conductivity was achieved for AlGaN with Al content of up to 82%. Temperature dependent PL measurements of AlGaN/AlN multi quantum wells (MQW) emitting at 230 nm show that the internal quantum efficiency improves by a factor of about two for MQWs on defect reduced laterally overgrown AlN compared to planar AlN/sapphire templates. The data show the usefulness of defect reduced AlN templates as quasi-substrates for AlGaN layer structures with high Al content that can be employed in light emitting diodes emitting in the deep ultraviolet.

The growth of AlGaN, GaN and InGaN layers on ( 20 2 ¯ 1 ) GaN substrates was investigated by metal-organic vapor phase epitaxy. All layers exhibit undulations along [ 10 1 ¯ 4 ] with a period length between 20 nm and 45 nm. Under certain growth conditions, GaN exhibits bunching of the undulations leading to an increase of the undulation period and amplitude. This is also observed in the ternary alloys InGaN and AlGaN. Strong impact of the growth temperature and reactor pressure on the surface roughness and photoluminescence properties is found for GaN layers, with the smoothest layers obtained at low growth temperatures and low reactor pressures. Atomic scale interpretation suggests that the undulations originate from ( 10 1 ¯ 0 ) and ( 10 1 ¯ 1 ) microfacets.

We report on the growth of semipolar ( 11 2 ¯ 2 ) AlN by metal-organic vapor phase epitaxy on ( 10 1 ¯ 0 ) sapphire. High temperature nitridation and nucleation at a V/III ratio of 1050 provide single phase ( 11 2 ¯ 2 ) AlN with mirror like crack free surfaces, even for layer thicknesses larger than 1 μ m . Increasing reactor pressure results in lower growth rates because of gas phase particle formation. The in-plane relationship is [ 0001 ] Sap . ∥ [ 1 ¯ 1 ¯ 23 ] AlN and [ 1 2 ¯ 10 ] Sap . ∥ [ 1 1 ¯ 00 ] AlN and therewith similar to MBE grown AlN and MOVPE and HVPE grown GaN on m -plane sapphire. The ( 11 2 ¯ 2 ) AlN reflex is anisotropically broadened in XRD. The surface shows an undulation along [ 1 1 ¯ 00 ] as typical for ( 11 2 ¯ 2 ) oriented nitride epilayers. Transmission spectroscopy measurements exhibit a sharp absorption edge at 5.95 eV, which is 130 meV shifted towards lower energies compared to c -plane AlN.

Sub-300 nm optically pumped ultraviolet lasers were realized on low-defect-density (0001) AlN substrates fabricated from single crystalline AlN boules. The AlxGa1–xN/AlyGa1–yN hetero-structures were grown by metal-organic vapor phase epitaxy near atmospheric pressure. The high structural quality of the pseudomorphically deposited films was confirmed by X-ray reciprocal space mappings and time-resolved photoluminescence (PL) studies of the multiple quantum well emission. The initial PL-decay times for a sample emitting at 267 nm were 0.87 ns and 1.14 ns for T = 295 K and T = 14 K, respectively. Laser resonators with a length of about 1 mm were formed by cleaving the AlN crystal to obtain m-plane mirror facets. Lasing was demonstrated for various wavelengths between 267 and 291 nm with the threshold power density as low as 126 kW/cm2 for the shortest attempted wavelength. The laser emission was TE polarized for all emission wavelengths. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We compare InN layers grown directly on c -plane (0001) sapphire and on (0001) GaN templates using metal-organic vapor phase epitaxy. InN grown on nitridated c -plane sapphire showed N-polarity, while InN grown on c -plane GaN templates showed In-polarity. N-polar and In-polar InN layers showed different surface morphology and crystallinity. N-polar InN was smoothest when grown at higher growth temperatures, while In-polar InN was smoothest at intermediate growth temperatures. Growth mode of the N-polar InN on sapphire is Frank-Van der Merwe, while growth mode of the In-polar InN on GaN templates is Volmer-Weber. Electrical properties were similar for both layers. Lowtemperature photoluminescence of In-polar InN layers is shifted to higher energies most likely due to biaxial compressive stresses. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The polarization dependent photoluminescence at low temperatures of strained semipolar and nonpolar InGaN quantum wells was studied as a function of the emission wavelength. We found for semipolar QWs that the maximum of the spectral resolved optical polarization is either red- or blue-shifted with respect to the maximum of the emission. In contrast, the nonpolar emission exhibits no clear maximum. We assign all effects to an inhomogeneous broad- ening of the emission caused by indium fluctuations and explain this behavior here in the light of the optical polarization switching. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Growth mechanism of InGaN quantum dots on GaN was studied during metalorganic vapor phase epitaxy in a temperature range of 600–750 °C in a horizontal reactor. Atomic force microscopy and high resolution X-ray diffraction show that InGaN quantum dots on GaN are formed according to the Stranski–Krastanov growth mode. When growth temperature was varied between 600 and 750 °C, a 2–3-dimensional growth transition was observed below 650 °C due to the increase in indium content. While the layer thickness was varied, similar transitions were observed with critical thicknesses of ∼2.7 nm and ∼4 nm at growth temperatures of 625 °C (In 0.31 Ga 0.69 N) and 675 °C (In 0.22 Ga 0.78 N), respectively. InGaN quantum dots with a density of ∼10 10 cm −2 , an average diameter of ∼40 nm and average height of ∼4 nm were obtained. Reciprocal space maps from the asymmetric (10–15) reflection show that all the InGaN samples are fully strained. Strong green emission at 534 nm was observed from InGaN grown at 675 °C.

Heteroeptaxial growth of semipolar GaN on (101¯0) sapphire by metal-organic vapor phase epitaxy has been investigated using high temperature nucleation layers in order to provide large GaN templates for semipolar light emitters. The nitridation layer and nucleation conditions were optimized for phase pure {112¯2} layer orientation, minimum surface roughness and improved crystal quality. Single phase {112¯2} GaN layers were obtained by controlling the nitridation conditions and thus nitridation layer thickness. The crystal quality was improved by high temperature growth. Basal plane stacking faults were identified as the dominating defects by transmission electron microscopy. Photoluminescence measurements revealed that these defects are type I1. A layer roughness of 4 nm was measured by atomic force microscopy.

The development of ultraviolet AlGaN multiple quantum well (MQW) light emitting diodes (LEDs) in the wavelength range between 239 and 217 nm is presented. The effects of aluminum composition in the MQW active region and of the underlying AlxGa1���xN:Si current spreading layer on the emission characteristics and operating voltages are investigated. A strong reduction in output power is observed with decreasing emission wavelength which is partly attributed to light absorption within the underlying AlxGa1���xN:Si. Additionally, a reduced carrier injection efficiency is identified as the root cause for the reduced emission power with decreasing emission wavelength. Emission powers at a dc current of 20 mA between 310 and 0.15 ��W have been achieved for LEDs emitting between 239 and 217 nm. The maximum light output in pulsed mode operation of these LEDs ranged between 4.6 mW and 3.6 ��W, respectively.

Maskless epitaxial overgrowth of AlGaN on structured AlN templates was performed and the impact of stripe orientation on the lateral growth of AlGaN was studied. AlN/sapphire templates were patterned into stripes with 1.5 μ m wide bars and a period of 3 μ m . AlGaN and AlN growths were performed on the patterned templates. Two stripe orientations were investigated: stripes parallel to the [ 1 1 ¯ 0 0 ] and parallel to the [ 1 1 2 ¯ 0 ] directions. Coalescence was achieved for both stripe orientations. AlGaN layers grown on stripes oriented along the [ 1 1 ¯ 0 0 ] direction show a flat and closed surface in contrast to overgrown stripes oriented along the [ 1 1 2 ¯ 0 ] direction where the surface is facetted and rough. The Al content is strongly dependent on the growth facet and varies between 25% at the sidewalls and 50% on the c-facet. The overgrowth of stripes parallel to the [ 1 1 ¯ 0 0 ] direction with AlN shows coalescence and flat surfaces. The X-ray rocking curve full width at half-maximum of the ( 1 0 1 ¯ 2 ) reflection is reduced from 1000 arcsec to 500 arcsec for the overgrown AlN. Temperature dependent photoluminescence measurements of AlGaN:Si grown on these coalesced AlN templates also indicate a dislocation density reduction via this method.

The growth rates and aluminium contents of Al x Ga 1− x N layers grown in a close-coupled showerhead reactor were investigated as a function of growth pressure and chamber height during metal-organic vapour phase epitaxy. The data show strong non-linear dependencies due to nanoparticle formation in the gas-phase. Good agreement between the experimental data and modeling results is obtained when the contribution of both Ga- and Al-containing species to the gas-phase particle formation is considered.

Wafer bowing has a strong impact on the wavelength homogeneity of InGaN based light emitters due to the strong temperature dependence of the In incorporation. Using in-situ curvature measurements and pyrometry we have studied the impact of growth conditions on the lateral and vertical temperature distribution across the wafer and on wafer bow on sapphire and GaN substrates. From the pyrometry signal at 950 nm the temperature of the substrate holder can be deduced. Using 400 nm pyrometry the surface temperature was determined for sufficiently thick GaN layers. While total flow and satellite rotation were found to have no impact, the total pressure affects the vertical temperature profile and thus the wafer bow. However, the effect is small and can only be used for fine tuning. To compensate for the strain of the AlGaN cladding layer in laser structures precurved GaN substrates are shown to be advantageous. Photoluminescence confirmed a spatially uniform surface temperature distribution during the deposition of the InGaN quantum well active region with a peak wavelength around 500 nm and a deviation as low as 3.5 nm on flat sapphire substrates and precurved GaN substrates.

InGaN/GaN multiple quantum well structures emitting in the blue/green wavelength region were grown by metal organic vapor phase epitaxy. By reducing the quantum well growth time the influence of the quantum well thicknesses between 3.8 and 1.1 nm on the indium incorporation and the distribution of indium in the quantum wells in growth direction were investigated. X-ray diffraction measurements show that the average indium mole fraction in the quantum wells decreases with reducing quantum well width due to a delay in the indium incorporation at the barrier/well interface. Quantitative analysis reveals a segregation length of about 2 nm as a measure of the graded region in growth direction. Cathodoluminescence imaging reveals that the spatial variation of the wavelength is reduced with decreasing quantum well thickness down to 1.7 nm. Reducing the width of the quantum well further results in an increase of the spatial wavelength variation.

In this paper the internal electric fields of nearly lattice matched InAlN/GaN heterostructures were determined. Pin-diodes containing InAlN/GaN heterostructures grown on sapphire substrates by metalorganic vapour phase epitaxy were fabricated by standard lithography and metallization techniques. To determine the polarization fields in the InAlN quantum wells capacitance-voltage-measurements were performed on the pin-diodes. To reduce the measurement error, the heterostructure thicknesses were accurately determined by transmission electron microscopy. Large polarization fields, which correspond mainly to the spontaneous polarizations, for In0.15Al0.85N ( MV cm−1), In0.18Al0.82N ( MV cm−1) and In0.21Al0.79N ( MV cm−1) quantum wells were observed. The results of the internal field strength and field direction are in excellent agreement with values predicted by theory and a CVM-based coupled Poisson/carrier transport simulation approach.

The waveguide strain and the surface morphologies of AlGaN-based laser heterostructures emitting in the deep UV spectral range have been investigated. In particular, the impact of the AlGaN heterostructure design on the strain relaxation as well as the effect of the growth temperature on the surface morphology were explored. We found strain-induced plastic relaxation for laser heterostructures with 130 nm thick Al0.45Ga0.55N waveguide layers, whereas pseudomorphic growth was obtained for laser heterostructures with Al0.70Ga0.30N waveguide layers. Optically pumped lasing near 280 nm was demonstrated for the coherently grown laser heterostructures. A strong correlation of the surface morphology with the waveguide growth conditions was also observed. Low growth temperatures of 900 °C lead to a high density of V-pits originating from dislocations. By increasing the growth temperatures to 1070 °C the V-pit density significantly decreases, resulting in a more than two-fold reduction of the threshold power density of optically pumped lasers.

The degradation behavior of ultraviolet-B light emitting diodes (UV-B LEDs) emitting near 310 nm has been investigated and a method to localize the degradation effects is presented. Measurements of the electro-optical characteristics of UV-B LEDs, during a 200 h constant-current degradation study, showed an initial fast decrease in the optical power accompanied by a decrease in the drive voltage and an increase in the capacitance. Furthermore, by using a specially designed contact geometry, it was possible to separate the degradation of the electrical properties of the p-layers and p-contacts from the degradation of the active region and n-side of the LED heterostructure. Our investigations show that the initial changes in capacitance and voltage can be attributed to changes in the p-side and at the p-contact of the LED, which can be explained by an activation of Mg dopants.The degradation behavior of ultraviolet-B light emitting diodes (UV-B LEDs) emitting near 310 nm has been investigated and a method to localize the degradation effects is presented. Measurements of the electro-optical characteristics of UV-B LEDs, during a 200 h constant-current degradation study, showed an initial fast decrease in the optical power accompanied by a decrease in the drive voltage and an increase in the capacitance. Furthermore, by using a specially designed contact geometry, it was possible to separate the degradation of the electrical properties of the p-layers and p-contacts from the degradation of the active region and n-side of the LED heterostructure. Our investigations show that the initial changes in capacitance and voltage can be attributed to changes in the p-side and at the p-contact of the LED, which can be explained by an activation of Mg dopants.

We show that the often observed efficiency droop in AlGaN quantum well heterostructures is an internal carrier loss process, analogous to the InGaN system. We attribute this loss process to Auger recombination, with C = 2.3 × 10−30 cm6 s−1; a similar value found commonly in InGaN-based devices. As a result, the peak internal quantum efficiency (IQE) of our structures is limited to 66%. These values were obtained by resonant excitation (time-resolved) photoluminescence (PL), avoiding common error sources in IQE measurements. The existence of strong Auger recombination implies that simple methods employed for IQE determination, such as temperature-dependent PL, may lead to erroneous values. Auger losses will have to be considered once the challenges regarding carrier injection are solved.

The growth of semipolar GaN on (101¯0) (m-plane) sapphire was investigated by metal-organic vapor phase epitaxy (MOVPE). Most of the layers were polycrystalline with {112¯2}, {101¯3¯} and {101¯0} as the dominant surface orientations. For incomplete nitridation and recrystallization the {101¯3¯} orientation was dominant. However, the {101¯3¯} surface is rough due to twinned crystallites. Growth at lower temperatures and with reduced gas flow velocity results in predominantly {112¯2} oriented smooth layers. The orientation was also found to be influenced by the nitridation and recrystallization process.

We have investigated the modification of the surface electric field (SEF) of the GaAs (001)-c(4 × 4) surface during the adsorption of cyclopentene and 1,4-cyclohexadiene molecules by reflectance anisotropy spectroscopy (RAS) in the spectral range from 1.5 to 5.0 eV. At around 3eV the RAS line shape originates from the so-called linear electro-optic effect (LEO). The amplitude of the LEO oscillation scales linearly with the SEF within the RAS penetration depth and is thus a measure for it. For the separation of the LEO from the RAS spectra several different methods are described in the literature. Here, we present a modified method which allows the LEO separation in particular for interfaces between GaAs and organic molecules. The results obtained this way show a significant increase of the LEO effect upon molecule adsorption indicating a higher SEF. The observed changes of the surface electronic properties are probably related to a modification of surface strain upon molecule adsorption.

Different technologies have been evaluated in order to create vertical and smooth facets for GaN-based laser diodes on non-and semipolar substrates. Laser assisted scribing and cleaving proved to be a suitable method to create c-, a- and m-plane facets for devices grown on nonpolar, polar and semipolar GaN planes. The RMS roughness measured by atomic force microscopy was found to be less than 1 nm. Semipolar facets showed tilted or multifaceted cleavage planes. Dry etching using inductively coupled plasma etching (ICP) resulted in vertical facets for devices grown on polar c-plane GaN, while the facets of semipolar (1122) GaN on sapphire exhibited a slight tilt depending on the crystal orientation. Wet chemical postprocessing of semipolar facets can improve the tilt angle but suffers from a roughening of the facets due to selective dislocation etching. By using a focussed ion beam (FIB) it is possible to create vertical and smooth facets independent of the crystal orientation.