In this chapter, the ways of in situ doping of Nd into III-nitrides are discussed. A detailed analysis of their structural properties is presented. The optical properties of the materials are studied by photoluminescence and combined excitation-emission spectroscopy (CEES), which allowed the identification of the crystal-field levels of the Nd ion in the material and distinguished the respective optical transitions from those that are due to the appreciable coupling to localized and lattice phonons. Magnetic properties are studied using alternating gradient magnetometer and reveal a room-temperature magnetization. The chapter includes a detailed description on how magneto-optical measurements can be applied to CEES. Such measurements reveal g-factors for a large number of crystal-field states and a remarkable asymmetry in transition strength as magnetic fields are applied parallel or antiparallel to samples with the growth direction along the c-axis of GaN.

Ni films on (0001) and (0001¯) InN exhibited different reaction kinetics upon annealing at 673K. Structural and chemical analysis using grazing incidence X-ray diffraction, transmission electron microscopy, and X-ray energy dispersive spectrometry indicated that an interfacial reaction did not occur between the Ni film and the In-polar (0001) InN layer. However, the N-polar face reacted with Ni to form the Ni3InNx ternary phase with an anti-perovskite structure. The difference in reactivity for Ni on In-face and N-face InN indicates that polarity alters the reaction and may also affect interactions between other metals and group III-nitride semiconductors.

We present luminescence spectroscopy measurements of in situ Nd doped AlN grown by plasma-assisted molecular beam epitaxy. A Nd concentration as high as 0.08 at. % is incorporated into the host material. The Nd incorporation efficiency within the AlN matrix is found to be highly sensitive to the Al flux but independent of the substrate temperature (between 800 °C to 950 °C). Photoluminescence, photoluminescence excitation, and combined excitation-emission spectroscopy (CEES) spectra are used to identify the Stark sublevels of the following manifolds: 4I9/2, 4I11/2, 4I13/2, 4F3/2, 4F5/2, 2H9/2, 4F7/2, 4S3/2, 4G5/2, and 4G7/2. A main Nd incorporation site and two minority sites are identified using CEES measurements.

The optical and structural properties of AlGaN active regions containing nanoscale compositional inhomogeneities (NCI) grown on low dislocation density bulk AlN substrates are reported. These substrates are found to improve the internal quantum efficiency and structural quality of NCI-AlGaN active regions for high Al content alloys, as well as the interfaces of the NCI with the surrounding wider bandgap matrix, as manifested in the absence of any significant long decay component of the low temperature radiative lifetime, which is well characterized by a single exponential photoluminescence decay with a 330ps time constant. However, room temperature results indicate that non-radiative recombination associated with the high point defect density becomes a limiting factor in these films even at low dislocation densities for larger AlN mole fractions.

We present the Stark energy sublevels of Nd3+ ions in GaN grown by plasma-assisted molecular beam epitaxy as determined by luminescence spectra. We correlate the photoluminescence spectra with transitions from the F43/2 excited state to the I49/2, I411/2, and I413/2 multiplets of the Nd3+ ion for above and below bandgap excitation, with the strongest emission occurring at 1.12 eV (1106 nm). We determine a splitting of the F43/2 excited state to be 4.1 meV. From photoluminescence excitation spectra, we also identify the Stark sublevels of the upper states F45/2, H29/2, F47/2, S43/2, G27/2, and G45/2. Photoluminescence excitation spectra reveal an optimal excitation energy of 1.48 eV (836 nm). Site-selective spectroscopy studies using combined excitation-emission spectroscopy with confocal microscopy indicate enhanced substantial doping at the Ga site.

In this paper we report on the characterization of n - Al 0.51 Ga 0.49 N active regions and the fabrication of ultraviolet LEDs that contain self-assembled, nanometer-scale compositional inhomogeneities ( NCI - AlGaN ) with emission at ~290 nm. These active regions exhibit reduced integrated photoluminescence intensity and PL lifetime relative to 320 nm NCI - AlGaN active regions that have significantly lower AlN mole fraction, despite having more than an order of magnitude fewer threading dislocations, as measured by transmission electron microscopy. This behavior is attributed to nonradiative recombination associated with the presence of a larger density of point defects in the higher Al content samples. The point defects are ameliorated somewhat by the lower density of NCI AlGaN regions in the higher Al content samples, which leads to a larger concentration of carriers in the NCI and concomitant reduced radiative lifetime that may account for the high observed peak IQE (~ 25%). Prototype flip chip double heterostructure-NCI- ultraviolet light emitting diodes operating at 292 nm have been fabricated that employ a 50% NCI - AlGaN active region.

We report the Stark energy sublevels of Nd3+ ions in GaN grown by plasma-assisted molecular beam epitaxy as determined by luminescence spectra. The photoluminescence spectra is correlated with transitions from the F-4(3/2) doublet state to the I-4(9/2), I-4(11/2), and I-4(13/2) multiplets of the Nd3+ ion for above and below bandgap excitation, with the strongest emission occurring at 1.12 eV (1106 nm). A splitting of the F-4(3/2) excited state is determined to be 4.1 meV. From photoluminescence excitation spectra, we also identify the Stark sublevels of the excited states F-4(5/2), H-2(9/2), F-4(7/2), S-4(3/2), (2)G(7/2), and (4)G(5/2). Photoluminescence excitation spectra exhibit an optimal excitation energy of 1.48 eV (836 nm). Site-selective spectroscopy studies using combined excitation-emission spectroscopy with confocal microscopy imply enhanced substantial doping at the Ga site. In addition, optical loss and gain measurements of a GaN:Nd waveguide excited above bandgap indicate an internal loss coefficient of similar to 6.5 +/-2 cm(-1) and a net gain of similar to -2 +/- 2 cm(-1). (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

In this paper we report - on first InGaN-based light emitting structures grown by hydride vapour phase epitaxy (HVPE). InGaN layers and multi layer InGaN/InGaN epitaxial structures were grown on GaN/sapphire template substrates and characterized, InN content in the InGaN layers was varied from 5 to 35 mol. %. Thickness of InGaN layers was controlled from 10 nm to 2 microns. Density of treading dislocations in the InGaN layers was estimated to be in the 109 cm(-2) range. X-ray diffraction measurements and transmission electron microscopy data confirmed a formation of InGaN/InGaN superlattice structures. Light emitting diode epitaxial wafers were fabricated by HVPE deposition of n-type InGaN layers and multi layer structures on p-type GaN template substrates. Depending on InN content in the InGAN light emitting regions, peak electroluminescence wavelength varied from 450 to 510 nm. Results of material characterization are reported. Advantages of the proposed upside down LED configuration and future applications of HVPE to grow InGaN layers and structures are discussed. (c) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

We observe polarized terahertz emission from nonpolar gallium nitride due to an in-plane polarization terminated by stacking faults. A measured flip of the THz waveform polarity agrees with carrier transport in an in-plane electric field.

We provide an investigation of in situ doping of GaN with the RE element Nd by plasma assisted-molecular beam epitaxy (PA-MBE). GaN epilayers are grown on c-plane sapphire and free standing GaN substrates and the Nd doping is controlled by an effusion cell. The ideal growth conditions for Nd incorporation maintaining crystal quality in GaN were investigated. The optical absorption characteristics indicate that the GaN:Nd epilayer remains transparent at the Nd emission wavelength of interest. For the highest Nd effusion cell temperatures, Rutherford backscattering and secondary ion mass spectrometry data indicate ˜5 at. % in epilayers grown on c-plane sapphire. X-ray diffraction found no evidence of phase segregation up to ˜1 at. % Nd. The highest luminescence intensities correspond to a doping range 0.05-1 at. %, with the strongest emission occurring at 1.12 eV (1107 nm). We also present the Stark energy sublevels of Nd3+ ions in GaN as determined by luminescence spectra. Photoluminescence excitation spectra reveal an optimal excitation energy of 1.48 eV (836 nm). We correlate the photoluminescence spectra with transitions from the 4F3/2 excited state to the 4I9/2, 4I11/2, and 4I13/2 multiplets of the Nd3+ ion for above (325nm) and below (836nm) bandgap excitation. Spectral correlation of the Nd emission multiplets in addition to site-selective spectroscopy studies using combined excitation-emission spectroscopy with confocal microscopy indicate enhanced substantial doping at the Ga site compared to other techniques (ion implantation and co-sputtering).

In this paper we demonstrate the electrical and optical effects of negative polarization charge at the n-InGaN/p-GaN interface on the performance of single heterostructure n-InGaN/p-GaN LEDs with p-side down. This negative polarization charge at the interface leads to a reduced barrier for hole injection from p-GaN to n-InGaN, with a hole accumulation layer forming within the InGaN near the n-InGaN/p-GaN interface. Electrons encounter a significant barrier for injection from the n-InGaN into p-GaN, compounded by the heterobarrier, making p-GaN behave like an effective electron blocking layer. We show that the combination of 2D hole-gas formation on the n-InGaN side of the hetero-interface and enhancement of the electron barrier to transport across this interface may reduce efficiency droop at high current density without the need for an AlGaN electron blocking layer.

We report on the fabrication and evaluation of flip-chipped double heterostructure UVLEDs operating at 320 nm that employ a bulk (80nm) AlGaN active region containing nano-scale compositional inhomogeneities (NCI), deposited by plasma-assisted molecular beam epitaxy. The devices were deposited on AlGaN templates grown by hydride vapor phase epitaxy and contain electron and hole injection layers grown by metalorganic chemical vapor deposition. A packaged, 300 mu m x 300 urn device with 3% internal quantum efficiency has a peak output power of 0.56 mW at 90 mA DC current that corresponds to an external quantum efficiency of 0.15%. Nearly constant external quantum efficiency is observed for DC current density up to 100 A/cm(2), suggesting that the NCI regions effectively suppress the deleterious effects of polarization fields at low injection currents that should inhibit radiative recombination in double heterostructure devices.

Most III-V nitride light emitting diodes have an n-down structure with Ga polarity. In such a device, the active layer is grown on top of the n-cladding layer and the p-type cladding layer is grown on top of the active layer. We have analyzed the band structure of such a device and found a reduced effective conduction band barrier due to the positive spontaneous and piezoelectric polarization charge, resulting in large electron overshoot and necessitating the introduction of the commonly employed electron blocking layer. On the other hand, the polarization charge at the corresponding interface for a p-side down device with Ga polarity is negative, resulting in a significant enhancement of the electron barrier and the existence of a 2D hole gas near the interface. These are beneficial to the performance of single heterejunction LEDs.

We report on the properties of high quality HVPE InN and on successful subsequent MBE growth of InN layers with improved, characteristics on HVPE InN template substrates. InN layers were grown by HVPE on GaN/sapphire HVPE templates. The (00.2) XRD rocking-curve of the best InN layer (RC) had the FWHM of about 375 arc see, being the narrowest XRD RCs ever reported for HVPE InN. Transmission Electron Microscopy (TEM) revealed that at the GaN/InN interface, the threading dislocations that come from GaN were transmitted into the InN layer. We estimated the dislocation density in HVPE grown InN to be in the low 10(9) cm(-2) range. Reflection high energy electron diffraction (RHEED) confirmed monocrystalline structure of the InN layers surfaced layers photoluminescence (PL) showed edge emission, around 0.8 eV. Hall measured free electron concentration was in the range of 10(19) -10(20) cm(-3) and electron mobility was similar to 200 cm(3)/V s. MBE growth of InN was performed on the HVPE grown InN template substrate demonstrating the improvement of material quality in the case of homo-epitaxial growth of InN. Demonstration of the high quality HYPE InN materials opens a new way for InN substrate development.

We probe the electric field in In-face and N-face InN/InGaN multiple quantum wells (MQWs) using low temperature intensity-dependent photoluminescence (PL) under continuous-wave laser excitation at 900 nm. The In-face structure consists of 30 periods of 2.5 nm thick InN wells and 24 nm thick In0.92Ga0.08N barriers. The N-face sample consists of 25 periods of 1.5 nm InN wells and 15 nm thick In0.88Ga0.12N barriers. At low excitation power, the PL peak energy from both In-face and N-face MQWs is redshifted relative to that from bulk InN. As excitation power increases, we observe a 50 and 44 meV blueshift of the PL peak energy from the In-face and N-face MQWs, respectively. The blueshift is a result of the optically induced screening of the built-in electric field. From the measured blueshift, we calculate a minimum change of the electric field to be ∼0.4 MV/cm and ∼0.6 MV/cm in the In-face and N-face well regions, respectively. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The effects of negative polarization charge at the n-InGaN∕p-GaN interface on the performance of hydride vapor phase-epitaxy deposited single heterostructure n-InGaN∕p-GaN LEDs with p-side down are investigated. The strong peak emission wavelength blueshift and concomitant superlinear increase in light output as the injection current increases below 25A∕cm2 are characteristic of radiative tunneling. We show that the combination of two-dimensional hole gas formation on the n-InGaN side of the heterointerface and enhancement of the electron barrier to transport across this interface results in only ∼10% efficiency droop up to 500A∕cm2 without implementation of an AlGaN electron-blocking layer or a second heterointerface.

Abstract : Most III-V nitride light emitting diodes have an n-type down structure with Ga polarity. In such a device, the active layer is grown on top of the n-cladding layer and the p-type cladding layer is grown on top of the active layer. We have analyzed the band structure of such a device and found a reduced effective conduction band barrier due to the positive spontaneous and piezoelectric polarization charge, resulting in large electron overshoot and necessitating the introduction of the commonly employed electron blocking layer. On the other hand, the polarization charge at the corresponding interface for a p-type down device with Ga polarity is negative, resulting in a significant enhancement of the electron barrier and the existence of a 2D hole gas near the interface. These are beneficial to the performance of single heterojunction LEDs. We have fabricated and tested such a device with a peak efficiency above 100A/sq cm and only approx. 10% droop up to 500A/sq cm, which was achieved without the implementation of an AlGaN electron-blocking layer or a second hetero-interface.