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We present a simple and robust method to acquire quantitative maps of compositional fluctuations in nanostructures from low magnification high-angle annular dark field (HAADF) micrographs calibrated by energy-dispersive X-ray (EDX) spectroscopy in scanning transmission electron microscopy (STEM) mode. We show that a nonuniform background in HAADF-STEM micrographs can be eliminated, to a first approximation, by use of a suitable analytic function. The uncertainty in probe position when collecting an EDX spectrum renders the calibration of HAADF-STEM micrographs indirect, and a statistical approach has been developed to determine the position with confidence. Our analysis procedure, presented in a flowchart to facilitate the successful implementation of the method by users, was applied to discontinuous InGaN/GaN quantum wells in order to obtain quantitative determinations of compositional fluctuations on the nanoscale.

The light output of 530 nm green GalnN/GaN light-emitting diodes on sapphire has been nearly doubled by the insertion of a 130-nm GalnN underlayer (UL) between the n-GaN electron injection layer and the quantum-well (QW) active region. Under variation of the alloy composition, best results were obtained for an x = 6.3% Ga1-xInxN UL. By low-temperature depth-resolved cathodoluminescence spectroscopy, an interplay of the impurity-related donor-acceptor pair recombination, the UL, and the QW emission has been observed. We propose that the resonance and level alignments between the defect and UL levels reroute excitation toward radiative recombination in the QWs.

We report the growth and structural characteristics of green and yellow (529-576 nm) GaInN/GaN light emitting diodes (LEDs) grown on two types of c-plane substrates - bulk GaN and sapphire. In this longer wavelength range, depending on the substrate, we find different strain relaxation mechanisms within the GaInN/GaN quantum well (QW) region. In optimized epitaxy, structures on sapphire that contain a low density of threading dislocations (TDs) within the n-GaN show virtually no generation of additional misfit dislocations (MDs) (

The junction temperature of homoepitaxial green and blue GaInN/GaN quantum well light emitting diode (LED) dies is analyzed by micro-Raman, photoluminescence, cathodoluminescence mapping, and forward-voltage methods and compared to finite element simulations. Dies on GaN substrate and sapphire were analyzed under variable drive current up to 200 mA (246 A/cm2). At 100 mA, dies on bulk GaN remain as cool as 355 K (83 °C) while dies on sapphire heat up to 477 K (204 °C). The efficiency droop and spectral line shift in green LEDs with increasing current density can now be separated into electrical and thermal contributions.

GaInN/GaN light emitting diodes are the primary choice for efficient green light emitters, yet their performance limiting factors have yet to be identified. Here we perform a low-temperature luminescence study in depth-resolved cathodoluminescence. A series of LEDs with a combination of blue and green light emitting quantum wells exhibiting low and high electroluminescence output power were compared. In all samples, a band of donor-acceptor pair recombination was identified at 77 K. In the samples with lowest performance, such transitions located within the active region of the quantum wells had a particular strong contribution to the spectrum. In LEDs of higher performance, such luminescence was substantially suppressed within those layers. We argue that the higher performance of LEDs without such donor acceptor transition bands may be associated with the absence of corresponding dopant impurities and indicate a higher epitaxial perfection within the active quantum well region.

We demonstrate homoepitaxial growth of GaInN/GaN-based green (500–560 nm) light emitting diodes (LEDs) on a-plane and m-plane quasi-bulk GaN prepared by hydride vapor phase epitaxy (HVPE). We find that in order to achieve an emission peak wavelength beyond 500 nm, a minimum InN-fraction of ∼14% is needed for both, a- and m-plane quantum wells (QWs), while ∼8% are enough for c-plane-oriented QWs. Besides increasing the InN-fraction in these non-polar QWs, widening the QW also proves to effectively shift the emission to longer wavelengths without loosing efficiency with the benefit of maintaining a low InN-fraction.

InGaN quantum well structures on GaN epilayers were exposed to 500 keV alpha particles to fluences above 1014 cm2 to probe the relative radiation tolerance of the epilayer and wells. Performance was estimated by the intensity of ion-beam induced luminescence. Two separate types of quantum well structures emitted at 470 and 510 nm prior to irradiation, and only small wavelength shifts were observed even with the highest alpha fluences. Complementary cathodoluminescence experiments showed that luminescence in the quantum wells is strongly influenced by charges injected deep into the GaN epilayer. The 500 keV alpha penetration depth was ~ 1 mum, so that defects were created at a faster rate in GaN compared to InGaN as alpha particles slowed and stopped within a minority carrier diffusion length of the quantum wells. However, the rate of luminescent decay was similar for both materials. Taken together with the cathodoluminescence data, this ion-beam induced luminescence comparison indicates that the quantum well luminescence decay rate is dominated by radiation-induced defects in the GaN epilayer. InGaN quantum wells are then demonstrated to be not more than a factor of ten more radiation sensitive than GaN, and may be substantially less sensitive than this upper bound.

We perform a transmission electron microscopy (TEM) characterization of blue light-emitting diode epiwafers grown homoepitaxially on a c-plane GaN substrate and compare with such grown on sapphire. We find a threading dislocation (TD) density as low as 2 × 10 8 cm -2 in homoepitaxial growth, which is 1/30 of that in sapphire-based material. A unique type of inverted pyramid defect with diameter ∼650 nm was observed. It originates from the homo-interface in edge-type TDs. TDs were also found to be generated within the quantum wells without a precursor TD defect. Otherwise, high-resolution TEM suggests extremely homogeneous quantum wells and barriers in terms of composition and well width. Besides plan-view TEM, wet chemical etching on n-type GaN was carried out in hot phosphoric acid to evaluate the TD density more quickly. The two methods prove to be equivalent with respect of determined TD densities.

We present a junction temperature analysis of GaInN/GaN quantum well (QW) light-emitting diodes (LEDs) grown on sapphire and bulk GaN substrate by micro-Raman spectroscopy. The temperature was measured up to a drive current of 250 mA (357 A/cm2). We find better cooling efficiency in dies grown on GaN substrates with a thermal resistance of 75 K/W. For dies on sapphire substrates we find values as high as 425 K/W. Poor thermal performance in the latter is attributed to the low thermal conductivity of the sapphire. Three-dimensional finite-element simulations show good agreement with the experimental results, validating our thermal model for the design of better cooled structures.

We analyze GaInN based light emitting diodes emitting in the range of 400-550nm using a new intensity-quantitative spectroscopic cathodoluminescence mapping method. Spectroscopic information of arbitrary sample locations is generated from sequences of quantitative image scans. From the temperature dependence of the intensity, we derive thermal activation energies of the dominant loss processes. Those compare well with the hole binding energies in the piezoelectric and quantized quantum well structures.

A charge profiling of GaInN/GaN hetero-polarization structures typical for high power 525 nm green light emitting diodes is presented. We identify three individual charge maxima of some 1.17 x 10 12 cm -2 electrons with a line width as narrow as 17 nm corresponding to three individual but widely separated quantum wells. The sheet charge density indicates negligible screening of the polarization charges. The voltage required to deplete each well is found to be ΔU = -0.4 ± 0.1 V and approximates the voltage drop across the polarization dipole of the quantum well. The modulation frequency dependence reveals a characteristic time constant of 1 μs for the dominant trapping and ionization process. Such analysis of high performance green light emitters proves a suitable tool for further device optimization.

InGaN/GaN multiple quantum well light emitting diode structures have been grown on sapphire substrates by metalorganic chemical vapor deposition. They are investigated, in this study, by high-resolution X-ray diffraction, high-resolution transmission electron microscopy, photoluminescence, and photoluminescence excitation. HR-XRD showed multiple satellite peaks up to 10th order due to the quantum well superlattice confinement effects. These indicate the high quality of layer interface structures of this sample. Excitation power-dependent photoluminescence shows that both piezoelectric field-induced quantum-confined Stark effect and band filling effect influence the luminescent properties of this sample. Temperature-dependent photoluminescence of this sample has also been studied. The peak position of the PL exhibits a monotonic red-shift and the full width at half maximum of the PL band shows a W-shaped temperature-dependent behavior with increasing temperature. From the photoluminescence excitation results, a large energy difference, so-called Stokes shift, between the band-edge absorption and emission was observed.

Progress of light emission efficiency in the green and deep green spectral range is crucial for the success of all solid state lighting. Yet, power performance in GaInN/GaN quantum well light emitting diodes (LEDs) in the green still lag far behind the development of red and blue. A key signature is a steep roll-off in emission power when the wavelength is increased from 460 nm in the blue to 525 nm in the green and 550 nm in the deep green. The physical reason needs to be identified to overcome such limitations. A meaningful study requires a large base of sample data to meliorate the effects of sample variations. We analyze the data of hundreds of growth runs for several distinct epi development stages. In particular, when comparing data for two different epi optimization concepts we find a strong variation of their respective roll-off slopes in the green. These findings show that the roll-off slopes are not universal and can be controlled at a high performance level by advanced epi optimization. We find that an optimization of the quantum well smoothness leads to much weaker roll-off behavior and boosts performance of newly optimized green LED dies (2.4 mW at 20 mA, unpackaged, 527 nm dominant). Our results should open pathways on how to further extend power performance of deep green LEDs. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The density of states optical absorption edge in polarized Ga1-xInxN/GaN quantum wells (x ≤0.14) is studied through photoluminescence excitation spectroscopy and effective mass theory. Within the vicinity of the main absorption edge in the range of 3.0–3.3 eV several extrema are resolved. On the basis of experimental polarization field values and established parameters a model of the electronic band structure in the quantum wells is derived. Both predicted interband transition energies as well as relative matrix elements provide a good description of the observations and a level assignment. We thus identify a dominance of e1lh1 and e1hh1 transitions with increasing competition by e1hh2 for higher x. Due to the strong polarization effects, the latter is an allowed transition and paired with the high lying ionization threshold to the valence band. These results provide a quantitative basis for the further identification of the electronic band structure in polarized heterostructures.

The absorption properties in hetero-polarization GaN/Al x Ga 1-x N (x = 0.06) quantum well structures are studied in reflection, photoreflection, and photoluminescence excitation spectroscopy and compared with the results of hand structure calculations. Above the energy of the main luminescence transitions we observe three distinct absorption thresholds. From Franz-Keldysh oscillations in the absorption spectra we directly derive the value of the acting electric field within the barriers. Upon this field strength we base a calculation of the electronic band structure and interband transition energies. The results suggest that the observed absorption edges are the AlGaN band edge and two quantized levels involving the crystal-field spilt-off hole.

Uniaxial wurtzite group-III nitride heterostructures are subject to large polarization effects with significant consequences for device physics in optoelectronic and transport device applications. A central aspect for the proper implementation is the experimental quantification of polarization charges and associated fields. In modulated reflection spectroscopy of thin films and heterostructures of AlGaInN we observe pronounced Franz-Keldysh oscillations that allow direct and accurate readings of the field strength induced by polarization dipoles at the heterointerfaces. In piezoelectric GaInN/GaN quantum wells this dipole is found to induce an asymmetry in barrier heights with a respective splitting of interband transitions. This splitting energy appears to reflect in the transitions of spontaneous and stimulated luminescence in the well. From these experiments the polarization dipole is identified as controllable type-II staggered band offset between adjacent barrier layers which can extend the flexibility in AlGaInN bandstructure design. The derived field values can serve as important input parameters in the further interpretation of the entire system.

The optical properties and their relation to the electronic bandstructure in Si-doped GaInN/GaN multiple quantum well structures typical for near UV laser diode devices are investigated in photoluminescence, photoluminescence excitation and photoreflection spectroscopies. A set of four evenly spaced levels N0 ··· N3 is identified covering the spectral range from the lowest luminescence band (N3) to the GaN barrier energy (N0). The splitting of N3 and N2 appears as a discrete splitting rather than a continuous localization process. The level splitting is well described by a Stark-like ladder defined by the multi interface bandoffset ΔE = FeLz across the piezoelectric GaN/GaInN/GaN structure.

In 535-565 nm green and deep green LED dies on c-plane sapphire, we correlate the structural morphology with photoluminescence performance. Two classes of material with active region roughness of 4 nm and 0.4 nm (RMS) are analyzed in transmission electron microscopy. In the rough material, a high density of edge-type dislocations is identified that originates within the quantum wells (QWs). They initiate V-defects that exhibit {101} growth facets with a second set of narrow QWs and barriers. As V-defects widen with progressive growth, the width of QW and barriers on the c-plane increases. In the smooth material, however, no V-defects can be found. QWs and barriers are highly uniform and homogenous throughout the structure. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We investigate pn-junction heating in 525 nm green GaInN/GaN quantum well light emitting diode dies by micro-Raman and photoluminescence spectroscopy. We compare dies grown on GaN and sapphire substrates of different sizes at current densities up to 267 A/cm2. Analysis of the experiments was supported by realistic three-dimensional finite-element simulations of the thermal properties. Sapphire based LED dies reach a junction temperature as high as 240 °C at a current density of 133 A/cm2, while those on bulk GaN only heat to 83 °C. This corresponds to a thermal resistance of 428 K/W and 63 K/W, respectively. By help of this calibration, the role of junction temperature in the spectral light output performance is identified. From the numerical simulation, dimensions and thermal conductivity of the substrate are identified as the dominant control parameters. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Ga 1~x In x N/GaN multiple quantum well heterostructures and thin Ga 1~x In x N/GaN films (0.03)x)0.22) are studied by photoreflection and photoluminescence spectroscopy at room temperature. In the thin single films we find strong interaction of heavy hole and light hole valence bands throughout the composition range. In MQW structures a splitting of those bands due to quantization is observed and accompanied by a decreasing interaction for increasing InN-fraction. In parallel the Stokes shift of the photoluminescence vanishes as the coupling of the valence band decreases. ( 1998 Elsevier Science B.V. All rights reserved.

The in-plane effective electron mass (${\mathit{m}}_{\mathrm{\ensuremath{\parallel}}}$) in narrow ${\mathrm{Ga}}_{0.47}$${\mathrm{In}}_{0.53}$As/InP quantum wells is strongly dependent on the quantization energy. Cyclotron resonance in a series of quantum wells with well widths down to 15 \AA{} reveals a mass enhancement of up to 50% (${\mathit{m}}_{\mathrm{\ensuremath{\parallel}}}$=0.065${\mathit{m}}_{0}$) over the bulk value of ${\mathrm{Ga}}_{0.47}$${\mathrm{In}}_{0.53}$As. This effect is caused by the nonparabolicity of the conduction band and wave function penetration into the barrier material. Our experimental findings are in good agreement with calculations performed within the framework of k\ensuremath{\cdot}p theory. We obtain an easy-to-use relation between the mass and the quantization energy ${\mathit{m}}_{0}$/${\mathit{m}}_{\mathrm{\ensuremath{\parallel}}}$(\ensuremath{\epsilon})=(1-1.96\ensuremath{\epsilon}/eV)/0.044. \textcopyright{} 1996 The American Physical Society.

We investigate the absorption spectrum and the distribution of radiative decay times for two-dimensional excitons in semiconductor quantum wells, as affected by an adiabatic white-noise random potential. Such a potential could result from atomic-scale fluctuations in the composition of an alloy semiconductor, or in quantum well thickness. We find in general that the shortest radiative decay time is directly proportional to the inhomogeneous width of the exciton line in absorption, itself proportional to the correlation parameter for the random potential.

Cubic GaInN/GaN multi-quantum wells (MQWs) are grown on V-grooved Si {100} substrates. The crystal phase is confirmed by electron-back scatter diffraction and cathodoluminescence shows room temperature MQW peak emission at 498 nm.

The spin-splitting factor ${\mathit{g}}^{\mathrm{*}}$ of the electrons at the very bottom of the conduction band in strained ${\mathrm{Ga}}_{\mathit{x}}$${\mathrm{In}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As/InP type-I quantum wells is reported. Experimental proof of quantum confinement-dependent anisotropy of ${\mathit{g}}^{\mathrm{*}}$ is given. Changing the alloy composition at fixed quantization, equivalent to introducing compressive and tensile strain, changes ${\mathit{g}}^{\mathrm{*}}$. The values of ${\mathit{g}}^{\mathrm{*}}$ perpendicular to the quantum-well plane can be explained in a model calculation. Apparently however, no quantitative theory on which to base the calculation of the anisotropic spin splitting is at present available.

We present the first successful optically detected magnetic resonance experiments (ODMR) on strained and unstrained GaxIn1−xAs/InP type-I quantum wells. The resonances attributed to electrons are in general anisotropic and detailed results are given for the composition range 0.4 < x < 0.6. Extrapolating to the binary end points, i.e. InAs and GaAs close agreement with existing results is found. The anisotropy is explained within a simple model using subband calculations. It also explains why single-sided p-type modulation doping reduces the electron-hole wavefunction overlap, increases the radiative life-times and allows for optical detection of magnetic resonance. A detailed discussion why hole resonances can be ruled out is given.

In absence of piezoelectric polarization along the growth axis, a- and m-plane green GaInN light emitting diodes manifest stable emission wavelength -- independent of the injection current density. The shift of the dominant wavelength is less than 8 nm when varying the forward current density from 0.1 to 38 A/cm2. Furthermore, the light emitted from the growth surface of such non-polar structures shows a very degree of linear polarization. This is attributed to a strong valance band splitting in such anisotropically strained wurtzite GaInN quantum wells . Such light emitting diodes show a high potential for energy efficient display applications.

Microwave detection of the Shubnikov–de Haas (SdH) effect as a contact‐free characterization technique for different types of two‐dimensional semiconductor structures is explored in the low magnetic field region. The detection technique and the data analysis are described. The character and relevance of the single‐particle relaxation time that can be detected by this technique are distinguished from the usual transport scattering time. The measured values of the carrier concentration and single‐particle relaxation time agree with electrical measurements, while the problem of making contacts on the structure is avoided. Uncertainties in the analysis for the single‐particle relaxation time are discussed. Cyclotron resonance, optically detected cyclotron resonance, and magneto‐photoluminescence are applied as other contact‐free techniques on the same samples. The results and suitability of these techniques are compared with the microwave detection of the SdH effect.

Very often classical characterization techniques (Hall effect, current transport) fail because of the inability to make contacts for electrical measurements. However, basic semiconductor properties such as the effective masses of electrons and holes, scattering times, the dominating scattering mechanism and carrier densities in two-dimensional systems can be obtained by contactless advanced microwave and far-infrared techniques.

Received 2 September 2009; revised manuscript received 29 January 2010; published 22 March 2010 The creation of symmetrical pairs of inclined dislocations was observed in the GaInN/GaN quantum wells QWs of c-axis grown green light-emitting diodes LEDs on low-defect density bulk GaN substrate, but not in green LEDs on sapphire substrate with high threading dislocation TD density. Pairs of dislocations start within 20 nm of the same QW and incline 18° – 23° toward two opposite 11 ¯ 00 directions or in a 120° pattern. We propose that in the absence of TDs, partial strain relaxation of the QWs drives the defect formation by removal of lattice points between the two dislocation cores. In spite of those inclined dislocation pairs, the light output power of such green LEDs on GaN is about 25% higher than in LEDs of similar wavelength on sapphire.

Experimental data obtained on undoped, ${\mathrm{Ga}}_{0.47}$${\mathrm{In}}_{0.53}$As/InP, single quantum wells using the far-infrared, optically-detected cyclotron resonance technique show a strong increase of the in-plane effective mass of electrons with increasing quantum confinement. The experimental results are compared with a model calculation in which conduction-band nonparabolicity and wave-function penetration into the barrier material have been taken into account. The roughness of the surface between the quantum well and the barrier material is proposed to be the reason for the decrease in electron scattering time from 1.1 ps (1000 \AA{}) to 120 fs (80 \AA{}).

The green spectral region provides a formidable challenge for energy efficient light emitting diodes. In metal organic vapor phase epitaxy we developed GaInN/GaN quantum well material suitable for 500 - 580 nm LEDs by rigorous defect reduction and thrive for alloy uniformity. We achieve best results in homoepitaxy on polar c-plane, and non-polar a -plane and m -plane bulk GaN. By the choice of crystal orientation, the dipole of piezoelectric polarization in the quantum wells can be optimized for highest diode efficiency. We report progress towards the goal of reduced efficiency droop at longer wavelengths.

An investigation of strained layer superlattices in which GaxIn1ixAs multilayer structures have been grown lattice mismatched to InP substrates are reported. The photoluminescence properties of these layers were studied under the in∞uence of microwave irradiation (24 GHz). Using the optically detected impact ionisation technique it is possible to separate and identify the difierent recombinations in the quantum wells. The advantage of this technique compared to standard PL is demonstrated. The ability to form ternary and quaternary alloys in connection with properties like high mobility and direct band gap has made the III-V semiconductors particularly important for optoelectronic and device applications. The wavelength range to 1.9 „m is obtained by strained layer superlattices (SLS) of GaxIn1ixAs grown on InP. When the layer thickness of the lattice-mismatched GaInAs is less than a critical value hc the mismatch strain is accomodated elastically and misflt dislocations are avoided. It is very interesting to study the efiects of the grown-in strain on electrical and optical properties[1] (e.g. lifting of the degeneracy of the valence band). The GaInAs/InP SLS structures allow both to study the biaxial strain induced valence band splitting in tension and in compression depending on the Gallium concentration.

The authors report on photothermal deflection spectroscopy (PDS) experiments for InGaAs/InP heterostructures. The apparatus is simple, yet sufficiently sensitive to register the heating by photon absorption for a single quantum well on a substrate with considerable thermal mass (thickness approximately=400 mu m). The PDS measurement accounts for the non-radiative relaxation after optical excitation. The extent to which the spectral dependence of the PDS signal at room temperature follows the absorption spectrum allows an estimation of the fraction of non-radiative processes. The large dynamic range of PDS and the ease of calibration of the signal permit one to measure quantum well absorption in the proper units.

We report on the low-frequency current and light noise in 515 nm green GaInN/GaN quantum well LEDs. The current noise was the superposition of the 1/f and the generation-recombination (GR) noise. The characteristic time of the GR process was found to be proportional to the reciprocal current for the entire current range. This dependence is the characteristic for the monomolecular non-radiative recombination .The dominance of the nonradiative recombination is in agreement with the measured low external quantum efficiency (EQE)

Optical properties of green emission Ga0.80In0.20N/GaN multi-quantum well and light emitting diode have been investigated by using photoluminescence, cathodoluminescence, electroluminescence, and photoconductivity. The temperature dependent photoluminescence and cathodoluminescence studies show three emission bands including GaInN/GaN quantum well emission centered at 2.38 eV (~ 520 nm). The activation energy of the non-radiative recombination centers was found to be ~ 60 meV. The comparison of photoconductivity with luminescence spectroscopy revealed that optical properties of quantum well layers are strongly affected by the quantum-confined Stark effect.

We report nonlinear optical investigation of green emission GaInN/GaN multi-quantum structures grown along c- and m-axes on sapphire and bulk GaN substrates, respectively. Under intense pulsed photo excitation, we observed strong superluminescence near the lasing condition in c-plane grown quantum well structures with full width at half maximum of 6 nm. The superluminescence couples out of the edge of the sample in a mode pattern consistent with gain in a high mode of the waveguide. The wavelength of the superluminescence is 474 nm. The threshold intensity of superluminescence was found to be 156 kW/cm2. Increasing pump intensity leads to a strong photoluminescence blueshift as large as 380 meV in samples grown along the c-axis on sapphire substrate, while under the same excitation conditions, the blue shift for the m-axis grown structure on bulk GaN substrate is less than 10 meV. The large emission blueshift is hereby attributed to the internal piezoelectric field in the c-axis grown structure. We observe a gain value of 20 cm-1 together with internal absorption losses of 2.3 – 6.0 cm-1 for the superluminescent samples.

The incorporation of In on the non-polar, piezoelectric-free (001) facet of cubic (c-) GaN epitaxially grown over a Si(001) substrate by metal-organic vapor phase epitaxy is reported. Relying on a hexagonal (h-) to c-phase transformation during epitaxy on an 800 nm-wide, Si(111)-faceted v-groove patterned into the substrate, the GaN epilayer at cross sectional view retains a triangular c-phase inside a chevron-shaped h-phase that results in a top surface bounded by a (001) facet parallel to Si(001) at the center and ( 11¯01) facets at both edges. A stack of five, ∼3 nm-thick, InxGa1−xN/GaN quantum wells (QWs) was deposited on the double-phased top surface. The c-phase region up to the QWs keeps extremely small misfit (∼0.002) to the fully relaxed h-GaN underneath it and is in tensile stress implying undefected by the h-c phase interface. The In incorporation on a strained non-polar (001) of c-GaN is comparable with that on totally relaxed semi-polar ( 11¯01) of h-GaN without noticeable adatom migration ac...

Ion-induced damage and intermixing was evaluated in InGaN/GaN multi-quantum wells (MQWs) using 35 keV N(+) implantation at room temperature. In situ ion channeling measurements show that damage builds up with a similar trend for In and Ga atoms, with a high threshold for amorphization. The extended defects induced during the implantation, basal and prismatic stacking faults, are uniformly distributed across the quantum well structure. Despite the extremely high fluences used (up to 4 × 10(16) cm(-2)), the InGaN MQWs exhibit a high stability against ion beam mixing.

Achieving p-type conduction in GaN has poised the material a power semiconductor on all fronts. Record conversion efficiencies and power densities in LEDs render it a platform of GW installation base. After commercial success as a blue light emitter, the emphasis now rests on the expansion of its capacity as a full spectrum direct emitter --- and absorber. Green and amber LEDs will allow full control of lighting quality and specificity in photochemistry. In early results we find evidence of temperature performance crucially superior to those of the incumbent. Next, we explore the same GaInN/GaN quantum wells as a concentration solar cell converter. While definitely limited to the short wavelength range so far, we find respectably high spectral conversion efficiencies and outstanding temperature stability. Such performance, however, is just early indication of what else lies ahead in terms of power electronics in the form of lateral and vertical transistor. In early results we monolithically integrate LED and transistor.

We have identified piezoelectric fields in strained GaInN/GaN quantum well p-i-n structures using the quantum-confined Stark effect. The photoluminescence peak of the quantum wells showed a blueshift with increasing applied reverse voltages. This blueshift is due to the cancellation of the piezoelectric field by the reverse bias field. We determined that the piezoelectric field points from the growth surface to the substrate and its magnitude is 1.2 MV/cm for Ga0.84In0.16N/GaN quantum wells on sapphire substrate. In addition, from the direction of the field, the growth orientation of our nitride epilayers can be determined to be (0001), corresponding to the Ga face.

InGaN/GaN multiple quantum well (MQW) light emitting diode (LED) structures with blue and green light emissions have been grown on sapphire substrates by metalorganic chemical vapor deposition. They are investigated by high-resolution X-ray diffraction (HR-XRD), high-resolution transmission electron microscopy (HR-TEM), photoluminescence (PL) and photoluminescence excitation (PLE). HR-XRD showed multiple satellite peaks up to 10th order due to the quantum well superlattice confinement effects. HR-TEM determined the MQW structures and parameters, indicating the high quality of layer interfaces of these LED samples. Excitation power-dependent PL predicates that both piezoelectric field-induced quantum-confined Stark effect and band filling effect influence the luminescent properties. Temperature-dependent PL shows that the QW PL emission peak exhibits a monotonic red-shift and that the full width at half maximum of the PL band shows a W-shaped temperature-dependent behavior with increasing temperature. From the PLE results, a large energy difference, so-called quantum confined Stokes shift, between the band-edge absorption and emission was observed. Penetrating TEM revealed the V-shape defects, and quantum dot-like structures within the InGaN well region, which leads to intense light emissions from these MQW LEDs.

Time-resolved photoluminescence studies can provide useful information for the development of InGaN/GaN heterostructures for long wavelength visible emitters. In this paper, we present results of time-resolved photoluminescence from samples grown using two different approaches to achieve green emission from InGaN/GaN MQWs. In one approach, samples, with high indium incorporation, were grown on a high quality AlN substrate to achieve green emission. The resulting photoluminescence decay of the green luminescence is long-lived and non-exponential. Quantitative analysis showed that the decay has a stretched-exponential characteristic, typical of InGaN/GaN MQW with potential fluctuation along the growth plane. This carrier localization, in a structure with low defect density, proves to be an effective means to achieve green emission. In another approach, a piezoelectric Stark-like ladder effect is used. In this case, a methodical layer-by-layer growth homogeneity optimization process was adopted to achieve an optical transition below the electron to heavy-hole (e1hh1) transition when the quantum well is subjected to the strong piezoelectric polarization dipole. This approach has proven to be successful in achieving green luminescence on conventional sapphire substrates. The resulting photoluminescence decay at 14 K, of a sample grown by this approach, is single exponential and shorter in duration than the decay observed in the first approach. This exponential decay is consistent with previous AFM studies that revealed a homogeneous active region.

Increasing emission power and efficiency in green light emitting diodes is one of the big challenges towards all-solidstate lighting. The prime challenge lies in the combination of extension of wavelength from 470 nm blue to 525 nm green while maintaining the emission power level. Commonly a steep decrease in power is observed. In a broad development effort we have been able to ameliorate that decrease significantly and obtain bare die performance at 525 nm of 1.6 mW at 20 mA for 350x350 µm 2 dies. Here we discuss critical die performance and wafer yield aspects of our optimization approach to the active layer of the GaInN/GaN quantum wells.

A detailed modeling of the electronic bandstructure of GaInN alloys and GaInN/GaN heterostructures typically used for high efficiency light emitting diodes is of high relevance for future improvements. Here we are exploring opportunities to accurately quantify the carrier dynamics under forward and reverse voltage bias. In GaInN/GaN LED-type heterostructures we observe distinct steps in the junction capacitance as a function of bias voltage within the depletion regime. Up to three individual steps can be identified that correspond to alternating ranges of capacitive and resistive impedances. Our analysis suggests that we are quantitatively monitoring the electron concentration in each individual quantum well. The pronounced clarity of the data reveals a high level of epitaxial perfection and spatial homogeneity across the entire area of the junction.

Using magnetic and magneto-optical methods, the authors were able to determine various material parameters of the strained system GaInAs in InP. To investigate some of these parameters, they have grown various modulation doped single quantum wells. The quantum well composition of Ga/sub x/In/sub 1-x/As was varied from x = 0.4 to 0.55 and the well width from 1 to 15 nm. The effective in-plane electron masses were measured for the various compositions using cyclotron resonance and temperature dependent Shubnikov-de Haas measurements (SdH). The dependency of the carrier mobility of n- and p-modulation doped samples on the well width and the composition were investigated by the use of SdH and Hall measurements at 4 K and 77 K. Optical detected magnetic resonance investigations were carried out to determine the hole/electron g-factors. >

Photoreflection and photoluminescence spectroscopies have been used to identify details of the electronic bandstructure in GaInN/GaN strained heterostructures and multiple quantum well structures. Franz-Keldysh oscillations in the ternary layers are identified in both systems revealing large piezoelectric fields of 240 kV/cm (x=0.079, thin film) and 0.65 MV/cm (x=0.187). From spatially resolved luminescence a very narrow distribution /spl Delta/E=28 meV of the bandgap energy is derived (x=0.187). From the variation of the field with strain a piezoelectric coefficient dP/sub z//d/spl epsiv//sub zz/=0.3 C/m/sup 2/ is obtained corresponding to e/sub 14/=0.1 C/m/sup 2/ and an equilibrium polarization of GaN of P/sub eq/=43 mCm/sup 2/ is extrapolated in qualitative agreement with recent calculations.

The composition dependence of the in‐plane conduction band effective mass in strained 15‐nm‐thick lattice‐mismatched Ga1−xInxAs/InP single quantum wells was determined by conventional cyclotron and optically detected cyclotron resonance techniques. Our results are in agreement with a self‐consistent calculation taking into account effects due to nonparabolicity, confinement, strain, and finite two‐dimensional carrier densities.