Magnetic resonance studies of InGaN-based quantum well diodes

Photoluminescence (PL) based optically detected magnetic resonance (ODMR) studies as well as electroluminescence detected and electrically detected magnetic resonance (ELDMR and EDMR, respectively) measurements of In_xGa_1−xN quantum wells were performed. In the ODMR, two PL-enhancing resonances were observed: an electron resonance and a hole resonance. The electron resonance is consistent with expectations for the g value in bulk In_xGa_1−xN for x ≈ 0.4 but deviates significantly in an x≈0.3 sample. Possible reasons for this include the effects of strain and confinement. The hole resonance is qualitatively similar to observations in Mg-doped GaN, but more isotropic in the x ≈ 0.3 diode than in the x ≈ 0.4 sample. We measure relatively long radiative lifetimes (as long as ∼0.2 ms) in the ODMR which facilitate the observation of the resonances and indicate that the electron and hole are spatially separated either by potential fluctuations within the quantum well or by the trapping of the hole at an acceptor in the player of AlGaN whch serves as one of the confining barriers. In the EDMR and ELDMR experiments, the signal is primarily due to a reduction in the nonradiative recombination at resonance. While the ODMR is alwyas emission-enhancing, the ELDMR is luminescence-quenching, supporting the notion that techniques are probing different centers.     

Metastable states in InGaN/GaN MQW structures doped with Sm, Eu, and Eu + Sm

Measurements of the microphotoluminescence (microPL) spectra of InGaN/GaN:Sm and InGaN/GaN:Eu quantum well (QW) structures show that the action of a magnetic field gives rise to Van Vleck paramagnetism for Eu³⁺ and Sm³⁺. The macrophotoluminescence (macroPL) spectra recorded after measuring the microPL spectra of InGaN/GaN QW structures doped with Sm or Eu + Sm at a high excitation level (>10²³ photons cm^?2 s^?1) in magnetic fields contain no QW emission lines which are present in the macroPL spectra recorded before these microPL measurements. This is indicative of the presence of photoinduced defects. Annealing of the InGaN/GaN:Sm and InGaN/GaN:(Eu + Sm) structures reduces the concentration of photoinduced defects.     

Molecular beam epitaxial HgCdTe material characteristics and device performance: Reproducibility status

Extensive material, device, and focal plane array (FPA) reproducibility data are presented to demonstrate significant advances made in the molecular beam epitaxial (MBE) HgCdTe technology. Excellent control of the composition, growth rate, layer thickness, doping concentration, dislocation density, and transport characteristics has been demonstrated. A change in the bandgap is readily achieved by adjusting the beam fluxes, demonstrating the flexibility of MBE in responding to the needs of infrared detection applications in various spectral bands. High performance of photodiodes fabricated on MBE HgCdTe layers reflects on the overall quality of the grown material. The photodiodes were planar p-on-n junctions fabricated by As ion-implantation into indium doped, n-type, in situ grown double layer heterostructures. At 77K, diodes fabricated on MBE Hg_1−xCd_xTe with x ≈ 0.30 (λ_co ≈ 5.6 μm), x ≈ 0.26 (λ_co ≈ 7 μm), x ≈ 0.23 (λ_co ≈ 10 μm) show R₀A products in excess of 1 x 10⁶ ohm-cm², 7 x 10⁵ ohm-cm², and 3 x 10² ohm-cm², respectively. These devices also show high quantum efficiency. As a means to assess the uniformity of the MBE HgCdTe material, two-dimensional 64 x 64 and 128 x 128 mosaic detector arrays were hybridized to Si multiplexers. These focal plane arrays show an operability as high as 97% at 77K for the x ≈ 0.23 spectral band and 93% at 77K for the x ≈ 0.26 spectral band. The operability is limited partly by the density of void-type defects that are present in the MBE grown layers and are easily identified under an optical microscope.     

Growth of InGaN HBTs by MOCVD

The design and growth of GaN/InGaN heterojunction bipolar transistors (HBTs) by metalorganic chemical vapor deposition (MOCVD) are studied. Atomic-force microscopy (AFM) images of p⁺InGaN base layers (∼100 nm) deposited under various growth conditions indicate that the optimal growth temperature is limited to the range between 810 and 830°C due to a trade-off between surface roughness and indium incorporation. At these temperatures, the growth pressure must be kept above 300 Torr in order to keep surface pit density under control. An InGaN graded-composition emitter is adopted in order to reduce the number of V-shaped defects, which appear at the interface between GaN emitter and InGaN base and render an abrupt emitter-base heterojunction nearly impossible. However, the device performance is severely limited by the high p-type base contact resistance due to surface etching damage, which resulted from the emitter mesa etch.     

Relaxation of InGaN thin layers observed by X-ray and transmission electron microscopy studies

Double-crystal and triple-axis x-ray diffractometry and transmission electron microscopy are used to characterize the microstructure, strain, and composition of InGaN layers grown on GaN by metalorganic chemical vapor deposition (MOCVD). Three different samples with increasing In concentration have been studied, all grown on GaN deposited on sapphire either with GaN or AlN buffer layers. It was found that InGaN layers with nominal 28% and 40% InN content consist of two sub-layers; the first sub-layer is pseudomorphic with the underlying GaN with lower In content than nominal. The top sub-layer is fully relaxed with a high density of planar defects and In content close to the nominal value. This is in contrast to a common assumption applied to InGaN quantum wells that ‘quantum-dot like areas’ are formed with different In content. The sample with the nominal indium concentration of 45% does not exhibit any intermediate strained layer, is fully relaxed and the In concentration (43.8%) agrees well with the nominal value.     

Energy of impurity resonance states in lead telluride with different contents of thallium impurity

The Hall factor in PbTe is calculated with regard to resonance scattering of holes at thallium impurity. At low temperatures (T ≈ 77 K) and relatively low Tl content (N _Tl ≈ 0.5 at %), the calculated value substantially differs from unity. Using the corrected Hall factor, the energy of resonance levels is refined. In contrast to the results obtained previously, the energy of the impurity resonance states appeared independent of thallium concentration up to 2 at %.     

Impurity centers in silicon doped with rare-earth impurities of dysprosium, holmium, erbium, and ytterbium

The results of an investigation of the donor centers in Czochralski-grown silicon ion-implanted with rare-earth impurities of Dy, Ho, Er, and Yb are presented. The formation of three groups of dominant donors with ionization energies less than 0.2 eV in silicon after annealing at 700 and 900 °C is discussed. The shallow donors at ≈E _c −40 meV are interpreted as thermal donors containing oxygen and intrinsic defects. The two other groups of donor states are identified as centers containing rare-earth ions. Fiz. Tekh. Poluprovodn. 33, 649–651 (June 1999)     

Characteristics of Green Light-Emitting Diodes Using an InGaN:Mg/GaN:Mg Superlattice as p-Type Hole Injection and Contact Layers

Mg-doped InGaN/GaN p-type short-period superlattices (SPSLs) are developed for hole injection and contact layers of green light-emitting diodes (LEDs). V-defect-related pits, which are commonly found in an InGaN bulk layer, can be eliminated in an InGaN/GaN superlattice with thickness and average composition comparable to those of the bulk InGaN layer. Mg-doped InGaN/GaN SPSLs show significantly improved electrical properties with resistivity as low as ∼0.35 ohm-cm, which is lower than that of GaN:Mg and InGaN:Mg bulk layers grown under optimized growth conditions. Green LEDs employing Mg-doped InGaN/GaN SPSLs for hole injection and contact layers have significantly lower reverse leakage current, which is considered to be attributed to improved surface morphology. The peak electroluminescence intensity of LEDs with a SPSL is compared to that with InGaN:Mg bulk hole injection and contact layers.     

Characterization of MOVPE-grown (Al, In, Ga) N heterostructures by quantitative analytical electron microscopy

The low pressure metalorganic vapor phase epitaxy growth of wurzite (Al, In, Ga)N heterostructures on sapphire substrates is investigated by quantitative analytical scanning transmission electron microscopy techniques like atomic number (Z-) contrast imaging and convergent beam electron diffraction (CBED). Especially (In, Ga)N quantum wells of different thicknesses as well as superlattices were analyzed with respect to defects, chemical composition variations, interface abruptness and strain (relaxation) effects. The interfaces in In_0.12Ga_0.88N/GaN quantum wells appear to be asymmetric. Additionally, we found composition variations of Δx_In≥0.03 within the InGaN quantum wells. The application of electron diffraction techniques (CBED) yields quantitative information on strain and relaxation effects. For the case of 17 nm thick InGaN quantum wells, we observed relaxation effects which are not present in the investigated thin quantum wells of 2 nm thickness. The experimentally obtained diffraction patterns were compared to simulations in order to get values for strain within the quantum wells. Additionally, the influence of dislocations on the digression of superlattices is investigated.     

Influence of hydrogen on local phase separation in InGaN thin layers and properties of light-emitting structures based on them

Results of studies of hydrogen addition during the growth of thin (∼2–3 nm) InGaN layers on their structural properties and properties of light-emitting structures that contain InGaN/GaN heterostructures in the active region are reported. It is shown that, with the known effect of a decrease in the average content of In, hydrogen addition leads to varying the local phase separation in the InGaN layers. Hydrogen addition during the growth of the InGaN layers initially causes suppression of the local phase separation, while hydrogen addition during interruptions of the growth after deposition of the InGaN films leads to a decrease in the size of the formed local In-enriched regions and to a certain increase in the local content of the In atoms.     

Low-frequency noise in as-fabricated and degraded blue InGaAs/GaN LEDs

Study of the spectral noise density and its dependence on current density in as-fabricated and degraded blue light-emitting diodes (LEDs) based on InGaN/GaN quantum-well structures are reported. It is shown that defects are generated nonuniformly in the course of degradation, being concentrated along extended defects penetrating into the active region of LEDs. It is demonstrated that the decrease in the external quantum efficiency in the course of aging is due to the enhancement of charge transport uniformity, which leads to the formation of shunts and local overheating regions. Typically, in blue LEDs, these effects are responsible for the ambiguous development of the degradation process, which hinders prognostication of LED service life. The effect of noise suppression is observed in a narrow current density range (10⁻² to 10⁻¹ A cm⁻²) corresponding to the onset of radiative recombination.     

Formation of composite InGaN/GaN/InAlN quantum dots

Composite InGaN/GaN/InAlN quantum dots (QDs) have been formed and studied. The structural properties of thin InAlN layers overgrown with GaN have been analyzed, and it is shown that 3D islands with lateral sizes of ∼(20–30) nm are formed in structures of this kind. It is demonstrated that deposition of a thin InGaN layer onto the surface of InAlN islands overgrown with a thin GaN layer leads to transformation of the continuous InGaN layer to an array of isolated QDs with lateral sizes of 20–30 nm and heights of 2–3 nm. The position of these QDs in the growth direction correlates with that of InAlN islands.     

Electrical and luminescent properties and the spectra of deep centers in GaMnN/InGaN light-emitting diodes

Electrical and electroluminescent properties were studied for GaN/InGaN light-emitting diodes (LEDs) with the n-GaN layer up and with the top portion of the n layer made of undoped GaMnN to allow polarization modulation by applying an external magnetic field (so-called “spin-LEDs”). The contact annealing temperature was kept to 750°C, which is the thermal stability limit for retaining room-temperature magnetic ordering in the GaMnN layer. Measurable electroluminescence (EL) was obtained in these structures at threshold voltages of ∼15 V, with a lower EL signal compared to control LEDs without Mn. This is related to the existence of two parasitic junctions between the metal and the lower contact p-type layer and between the GaMnN and the n-GaN in the top contact layer.     

Shallow–Deep InGaN Multiple-Quantum-Well System for Dual-Wavelength Emission Grown on Semipolar ($$ 11\bar{2}2 $$) Facet GaN

We report growth and characterization of a shallow–deep InGaN/GaN multiple-quantum-well (MQW) system for dual-wavelength emission grown on semipolar (11[`2]2 11\bar{2}2 ) facet GaN. Structural and optical properties of the InGaN multiple-quantum-well system were investigated by scanning electron microscopy (SEM), cross-sectional scanning transmission electron microscopy (XSTEM), photoluminescence (PL), photoluminescence excitation (PLE), and time-resolved photoluminescence (TRPL) measurements. Cross-sectional transmission electron microscopy (XTEM) revealed that the growth rate of the InGaN well layers on the (0001) flat top microfacet (~500 nm) was about six times as fast as on the (11[`2]2 11\bar{2}2 ) inclined facet, whereas the growth rate of GaN barrier layers on the (0001) flat top facet was roughly 4.5 times as large as that on the (11[`2]2 11\bar{2}2 ) facet. A room-temperature PL spectrum showed dual-wavelength light emission of the shallow–deep InGaN multiple-quantum-well system situated at 2.720 eV (455 nm) and 2.967 eV (418 nm). The Stokes shifts between the two PL peaks and the two “effective bandgaps” were ~260 meV in energy for the deep quantum wells and ~233 meV for the shallow quantum wells. The TRPL decay demonstrated the short radiative recombination lifetime on the order of several nanoseconds in the InGaN MQW system. Realization of the shallow–deep InGaN multiple-quantum-well system with emission wavelength controllability would be useful to achieve III-nitride-based multicolor light-emitting devices for displays.     

Recent issues in fabrication of Ag-clad BSCCO superconductors

Long lengths of mono-and multifilament Ag-clad BSCCO superconductors were fabricated by the powder-in-tube technique. Critical current density (J_c) up to 12,000 A/cm² has been achieved in an 850 m long multicore conductor. Long length conductors were formed into pancake-shaped coils by the wind-and-react approach. Test magnets were then fabricated by stacking the pancake coils and connecting them in series. The magnets were characterized as a function of applied magnetic field at various temperatures. A test magnet, fabricated with ≈770 m of BSCCO tape, generated fields of ≈1 T at 4.2K and ≈ 0.6 T at 27K, both in an applied background field of 20 T. Additionally, the strain tolerance of both mono-and multifilament conductors at 77K in 0.5 T applied field has been studied. We observed that multifilament conductors have better strain tolerance than monofilament tapes, retaining more than 90% of the initial critical current (at 0.5 T) with strain ≥1%.     

Neural Model for Left-Handed CPW Bandpass Filter Loaded Split Ring Resonator

Compact left-handed coplanar waveguide (CPW) bandpass filter loaded split ring resonator (SRR) is presented in this paper. The proposed filter exhibits a quasi-elliptic function response and its circuit size occupies only 12 × 11.8 mm² (≈0.21 λg × 0.20 λg). Also, a simple circuit model is given and the parametric study of this filter is discussed. Then, with the aid of NeuroModeler software, a five-layer feed-forward perceptron neural networks model is built up to optimize the proposed filter design fast and accurately. Finally, this newly left-handed CPW bandpass filter was fabricated and measured. A good agreement between simulations and measurement verifies the proposed left-handed filter and the validity of design methodology.     

The magnetoplasma effect in single crystals of antimony at T⩾80 K

Transmission spectra of infrared laser radiation (λ=10.6 μm) passed through samples consisting of two symmetric halves of an antimony single crystal separated by a small gap are investigated in pulsed magnetic fields B⩽20 T at temperatures T⩾80 K. The magnetoplasma effect was observed for the magnetic induction B≈15 T, with change in the transmission close to 100%. The magnetoplasma relaxation time has been determined. The possibility of using such objects as IR optical valves with response time not worse than 10⁻⁴ s is demonstrated. Fiz. Tekh. Poluprovodn. 32 1318–1319 (November 1998)     

Electrical characterization of very-narrow-gap bulk HgCdTe single crystals by variable magnetic field hall measurements

Variable-magnetic-field Hall measurements (0 to 1.5 T) are performed on very-narrow-gap bulk-grown Hg_1−xCd_xTe single crystals (0.165 ≤ x ≤ 0.2) at various temperatures (10 to 300K). The electron densities and mobilities are obtained within the one-carrier (electrons) approximation of the reduced-con-ductivity-tensor scheme. The present data together with the selected data set reported by other workers exhibit a pronounced peak when the electron mobility is plotted against the alloy composition x-value which has been predicted to be due to the effective-mass minimum at the bandgap-crossing (E_g ≈ 0). The observed position (x ≈ 0.165), height (≈4 x 10² m²Vs), and width (≈0.01 in x) of the mobility-peak can be explained by a simple simulation involving only ionized-impurity scattering. A lower bound of the effective mass is introduced as a fitting parameter to be consistent with the finiteness of the observed electron mobility and is found to be of the order of 10⁻⁴ of the mass of a free electron.     

Electron spin resonance study of defects in CVD-grown 3C-SiC irradiated with 2MeV protons

Electron spin resonance (ESR) was used to study defects induced by 2MeV-proton irradiation in cubic silicon carbide (3C-SiC) epitaxially grown on Si substrates by chemical vapor deposition. A new ESR signal labeled T5 was observed at temperatures lower than ≈100 K in Al doped, p-type 3C-SiC epilayers irradiated. The T5 signal has anisotropic g-values of g₁ = 2.0020 ± 0.0001, g₂ = 2.0007 ± 0.0001,and g₃ = 1.9951 ± 0.0001. The principal axes of the g-tensor were found to be along the 〈100〉 directions, indicating that the T5 center has D₂ symmetry. Isochronal annealing of the irradiated epilayers showed that the T5 center was annealed at temperatures around 150° C. A tentative model is discussed for the T5 center.     

Exciton wavefunction coupled surface plasmon resonance for In-rich InGaN film with perforated aluminum cylindrical micropillar arrays

The optical characterization of excitons coupled with surface plasmon resonance (SPR) for InGaN/GaN heterostructures with perforated cylindrical micropillar arrays is investigated. We analyze the optical characteristics of excitons coupled with SPR for InGaN/GaN heterostructures with perforated cylindrical micropillars, as shown in measurements of the photoluminescence (PL) spectra over a broad range of temperatures between 20 and 300 K. From the temperature-dependent PL spectra, we observe the better SPR coupling effects, resulting in less carrier confinement in the InGaN energy band. The magnitude of the redshift of the emission peak shown by the sample with the coated aluminum (Al) pattern is larger than that shown by the sample with no metal film. This was due to the presence of more exciton coupling surface plasmons within the Al/InGaN interface. The enhancement of the PL intensity of the sample with the deposited Al pattern film can be attributed to a stronger SPR coupling interaction with the excitons. The experimental results indicate that a perforated Al cylindrical micropillar array can significantly affect carrier confinement, enhancing the quantum efficiency of Al/In-rich InGaN heterostructures due to the interaction of the SPR coupling effect between the InGaN quantum dot-like region and the Al film.