Search

New optical techniques have been implemented to compare LED properties with fundamental models. Voltage-dependent, contacted electroreflectance measurements and step-recovery measurements are reported for an InGaN/GaN single quantum well LED. Electroreflectance spectra failed to exhibit a blue-shift at small reverse bias, as predicted by Schrodinger-Poisson quantum well band solutions, suggesting that excess charge trapped in localized states enhances screening of the polarization electric field. In contrast, at large reverse bias where the QW is depleted of charge, a predicted red-shift was observed. Electrical and optical responses of the LED to forward-to-reverse bias step-recovery measurements were self-consistently fit to a diffusion equation solution, providing values of minority-carrier hole transport parameters: τp ≈ 760 ns, Lp ≈ 590 nm, and μp ≈ 0.2 cm2/Vs. The long lifetime is suggestive of slow emission processes occuring through deep levels, and the small mobility is indicative of trap-modulated transport. Our results demonstrate the highly non-ideal nature of GaN-based LEDs grown on sapphire. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

To observe the effects of polarization fields and screening, we have performed contacted electroreflectance (CER) measurements on In0.07Ga0.93N/GaN single quantum well light emitting diodes for different reverse bias voltages. Room-temperature CER spectra exhibited three features which are at lower energy than the GaN band gap and are associated with the quantum well. The position of the lowest-energy experimental peak, attributed to the ground-state quantum well transition, exhibited a limited Stark shift except at large reverse bias when a redshift in the peak energy was observed. Realistic band models of the quantum well samples were constructed using self-consistent Schrodinger–Poisson solutions, taking polarization and screening effects in the quantum well fully into account. The model predicts an initial blueshift in transition energy as reverse bias voltage is increased, due to the cancellation of the polarization electric field by the depletion region field and the associated shift due to the quantum-confined Stark effect. A redshift is predicted to occur as the applied field is further increased past the flatband voltage. While the data and the model are in reasonable agreement for voltages past the flatband voltage, they disagree for smaller values of reverse bias, when charge is stored in the quantum well, and no blueshift is observed experimentally. To eliminate the blueshift and screen the electric field, we speculate that electrons in the quantum well are trapped in localized states.

A contacted electroreflectance technique was used to investigate AlGaN/GaN heterostructures and their intrinsic electric field-induced properties. By studying variations in the electroreflectance with applied field, spectral features associated with the AlGaN barrier, the two-dimensional electron gas at the interface, and bulk GaN were identified. Barrier-layer composition and electric field were determined from the AlGaN Franz–Keldysh oscillations. For a high mobility heterostructure grown on SiC, measured AlGaN polarization electric field and two-dimensional electron gas density approached values predicted by a standard bandstructure model. The two-dimensional electron gas produced a broad, field-tunable first derivative electroreflectance feature. With a dielectric function calculation, we describe the line shape and relative amplitude of the two-dimensional electron gas electroreflectance feature for a wide range of electron density and applied field values.

We present a unified picture of our studies of the nitrogen-specific properties of InGaAsN. Nitrogen-induced bandgap reduction and symmetry-breaking are examined with band-structure calculations and pressure-dependent optical measurements for an idealized, 'ordered' InGaAsN alloy. Including disorder and large conduction band random alloy fluctuations, we describe photoluminescence linewidths and electron mobilities and propose a N-induced mobility-edge. In transport and device experiments, extrinsic material properties (inhomogeneities, defects, etc) mask any intrinsic electron localization.

We describe the metal-organic chemical vapor deposition (MOCVD) growth of InAsSb/InAs and GaAsSb/GaAs(P) multiple quantum well (MQW) and InAsSb/InAsP and InAsSb/InPSb strained-layer superlattice (SLS) active regions for use in mid-infrared emitters. We also describe the growth and initial characterization of GaAsSbN/GaAs MQW structures. By changing the layer thickness and composition of the InAsSb SLSs and MQWs, we have prepared structures with low temperature (

We have investigated deep levels in 1.05 eV, Sn-doped, n-type In0.075Ga0.925As0.975N0.025 lattice-matched to GaAs. The samples were grown by metal–organic chemical vapor deposition. Capacitance–voltage measurements were used to determine the electron concentration in both as-grown and postgrowth annealed samples, and a decrease in net electron concentration of about 1.5×1017 cm−3 was observed following annealing. Deep level transient spectroscopy measurements are consistent with the presence of four majority-carrier electron traps, E1 (a broad distribution extending from the conduction band edge to approximately EC−0.2 eV), E2(0.36 eV), E3(0.34 eV), and E4(0.82 eV), as well as one minority-carrier hole trap, H1(0.71 eV), in our material. It is shown that E2 and H1, both of which are present in the annealed material only, are likely the same defect observed under conditions of electron and hole emission, respectively. Current–voltage-temperature measurements indicate a thermal activation energy of 0.35 eV ...

The growth conditions for GaInAsSb and AlGaAsSb using metal-organic chemical vapor deposition in an high speed rotating disk reactor are described. Trimethylindium, triethylgallium, arsine, and trimethylantimony were used as precursors for the growth of GaInAsSb. Triethylgallium, ethyldimethylamine alane, triethylantimony, and arsine were the precursors used for the growth of AlGaAsSb. These materials were doped both n- and p-type using a mixture of diethyltellurium and diethylzinc as sources. An optimum growth temperature of 520°C was determined for the growth of GaInAsSb. Growth at this temperature yielded a root-mean-square (rms) surface roughness of 0.142 nm. AlGaAsSb could be grown over the range of 500–600°C with somewhat rougher surfaces (rms>0.7 nm). The photoluminescence was found to correlate with surface roughness, increasing with smoother surfaces. AlGaAsSb mesa isolated diodes were prepared and characterized. These diodes showed good current-voltage characteristics with breakdown voltages greater than −6 V.

The authors report a measurement of the variation of the value of the linewidth of an excitonic transition in InGaAsN alloys (1 and 2% nitrogen) as a function of hydrostatic pressure using photoluminescence spectroscopy. The samples were grown by metal-organic chemical vapor deposition and the photoluminescence measurements were performed a 4K. The authors find that the value of the excitonic linewidth increases as a function of pressure until about 100 kbars after which it tends to saturate. This change in the excitonic linewidth is used to derive the pressure variation of the reduced mass of the exciton using a theoretical formalism which is based on the premise that the broadening of the excitonic transition is caused primarily by compositional fluctuations in a completely disordered alloy. The variation of the excitonic reduced mass thus derived is compared with that recently determined using a first-principles band structure calculation based on local density approximation.

We report on the metal-organic chemical vapor deposition (MOCVD) of strained layer superlattices (SLSs) of InAsSb/InPSb as well as mid-infrared optically pumped lasers grown using a high-speed rotating disk reactor (RDR). The devices contain AlAsSb cladding layers and strained, type-I, InAsSb/InPSb active regions. By changing the layer thickness and composition of InAsSb/InPSb SLSs, we have prepared structures with low-temperature ( < 20 K) photoluminescence wavelengths ranging from 3.4 to 4.8 μm. We find a variation in band gap of 0.272-0.324 eV for layer thicknesses of 9.0-18.2 nm. From these data we have estimated a valence band offset for the InAsSb/InPSb interface of about 400 meV. The optical properties of the superlattices revealed an anomalous low-energy transition that can be assigned to an antimony-rich interfacial layer in the superlattice. An InAsSb/InPSb SLS, laser was grown on an InAs substrate with AlAs 0.16 Sb 0.84 cladding layers. A lasing threshold and spectrally narrowed laser emission were seen from 80-200 K, the maximum temperature where lasing occurred. The temperature dependence of the SLS laser threshold is described by a characteristic temperature, T o = 72 K, from 80 to 200 K.

The compound semiconductor system InGaAsN exhibits many intriguing properties which are particularly useful for the development of innovative high efficiency thin film solar cells and long wavelength lasers. The bandgap in these semiconductors can be varied by controlling the content of N and In and the thin films can yet be lattice-matched to GaAs. In the present work, x-ray absorption fine structure (XAFS) and grazing incidence x-ray scattering (GIXS) techniques have been employed to probe the local environment surrounding both N and In atoms as well as the interface morphology of InGaAsN thin films epitaxially grown on GaAs. The soft x-ray XAFS results around nitrogen K-edge reveal that N is in the sp{sup 3} hybridized bonding configuration in InGaAsN and GaAsN, suggesting that N impurities most likely substitute for As sites in these two compounds. The results of In K-edge XAFS suggest a possible trend of a slightly larger coordination number of As nearest neighbors around In atoms in InGaAsN samples with a narrower bandgap whereas the In-As interatomic distance remains practically the same as in InAs within the experimental uncertainties. These results combined suggest that N-substitution of the As sites plays an important role of bandgap-narrowing while in the meantime counteracting the compressive strain caused by In-doping. Grazing incidence x-ray scattering (GIXS) experiments verify that InGaAsN thin films can indeed form very smooth interfaces with GaAs yielding an average interfacial roughness of 5-20{angstrom}.

${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{As}}_{1\ensuremath{-}y}{\mathrm{N}}_{y}$ is a semiconductor alloy system with the remarkable property that the inclusion of only 2% nitrogen reduces the band gap by more than 30%. In order to help understand the physical origin of this extreme deviation from the typically observed nearly linear dependence of alloy properties on concentration, we have investigated the pressure dependence of the excited state energies using both experimental and theoretical methods. We report measurements of the low-temperature photoluminescence of the material for pressures between ambient and 110 kbar. We also describe a density-functional-theory-based approach to calculating the pressure dependence of low lying excitation energies for low-concentration alloys. The theoretically predicted pressure dependence of the band gap is in excellent agreement with the experimental data. Based on the results of our calculations, we suggest an explanation for the strongly nonlinear pressure dependence of the band gap that, surprisingly, does not involve a nitrogen impurity band.

The authors report on recent progress and improvements in the metal-organic chemical vapor deposition (MOCVD) of mid-infrared InAsSb emitters using a high speed rotating disk reactor (RDR). The devices contain AlAsSb claddings and strained InAsSb active regions. These emitters have multi-stage, type 1, InAsSb/InAsP quantum well active regions. A semi-metal GaAsSb/InAs layer acts as an internal electron source for the multistage injection lasers and AlAsSb is the electron confinement layer. These structures are the first MOCVD multi-stage devices. Growth in an RDR was necessary to avoid the previously observed Al memory effects found in a horizontal reactor. Broadband LED`s produced 2 mW average power at 3.7 {micro}m and 80 K and 0.1 mW at 4.3 {micro}m and 300 K. a multi-stage, 3.8--3.9 {micro}m laser structure operated up to T = 180 K. At 80 K, peak-power > 100 mW/facet and a high slope efficiency (48%) were observed in these gain guided lasers.

We describe the metal-organic chemical vapor deposition (MOCVD) of InAsSb/InAs multiple quantum well (MQW) and InAsSb/InAsP strained-layer superlattice (SLS) active regions for use in mid-infrared emitters. We have made gain-guided, injection lasers using undoped, p-type AlAs0.16Sb0.84 for optical confinement and both strained InAsSb/InAs MQW and InAsSb/InAsP SLS active regions. The lasers and light emitting diodes (LED) utilize the semi-metal properties of a p-GaAsSb/n-InAs heterojunction as a source for electrons injected into active regions. A multiple-stage LED utilizing this semi-metal injection scheme is reported. Gain-guided, injected lasers with a strained InAsSb/InAs MQW active region operated up to 210 K in pulsed mode with an emission wavelength of 3.8–3.9 μm and a characteristic temperature of 29–40 K. We also present results for both optically pumped and injection lasers with InAsSb/InAsP SLS active regions. The maximum operating temperature of an optically pumped 3.7 μm SLS laser was 240 K. An SLS LED emitted at 4.0 μm with 80 μW of power at 300 K.

We describe the metal-organic chemical vapor deposition growth of InAsSb/InAsP strained-layer superlattice (SLS) active regions for use in mid-infrared emitters. These SLSs were grown at 500{degrees}C, and 200 torr in a horizontal quartz reactor using TMIn, TESb, AsH{sub 3},and PH{sub 3}. By changing the layer thickness and composition we have prepared structures with low temperature ({le}20K) photoluminescence wavelengths ranging from 3.2 to 4.4 {mu}m. Excellent performance was observed for an SLS LED and both optically pumped and electrically injected SLS lasers. An optically pumped, double heterostructure laser emitted at 3.86 {mu}m with a maximum operating temperature of 240 K and a characteristic temperature of 33 K. We have also made electrically injected lasers and LEDs utilizing a GaAsSb/InAs semi-metal injection scheme. The semi-metal injected, broadband LED emitted at 4 {mu}m with 80 {mu}W of power at 300K and 200 mA average current. The InAsSb/InAsP SLS injection laser emitted at 3.6 gm at 120 K.

The authors have grown AlSb and AlAs/sub x/Sb/sub 1-x/ epitaxial layers by metal-organic chemical vapour deposition (MOCVD) using trimethylamine or ethyldimethylamine alane, triethylantimony and arsine. These layers were successfully doped p- or n-type using diethylzinc or tetraethyltin, respectively. The authors have examined the growth of AlAs/sub x/Sb/sub 1-x/ using temperatures of 500 to 600/spl deg/C, pressures of 65 to 630 torr, V/III ratios of 1-17, and growth rates of 0.3 to 2.7 /spl mu/m/hour in a horizontal quartz reactor. The authors have also grown gain-guided, injection lasers using AlAsSb for optical confinement and a strained InAsSb/InAs multiquantum well active region using MOCVD. The semi-metal properties of a p-GaAsSb/n-InAs heterojunction were utilised as a source for injection of electrons into the active region of the laser. In pulsed mode, the laser operated up to 210 K with an emission wavelength of 3.8-3.9 /spl mu/m. The dependence of active region composition on wavelength was determined. The authors also report on the 2-colour emission of a light-emitting diode with two different active regions to demonstrate multistage operation of these devices.

We have grown AlSb and ALAsxSb1-x epitaxial layers by metalorganic chemical vapor deposition (MOCVD) using trimethylamine or ethyldimethylamine alane, triethylantimony, and arsine. These layers were successfully doped p- or n-type using diethylzinc or tetraethyltin, respectively. We examined the growth of AlAsxSb1-x using temperatures of 500 to 600‡C, pressures of 65 to 630 Torr, V/III ratios of 1-17, and growth rates of 0.3 to 2.7 Μm/h in a horizontal quartz reactor. We have also grown gain-guided, injection lasers using AlAsSb for optical confinement and a strained InAsSb/lnAs multi-quantum well active region using MOCVD. The semi-metal properties of a p-GaAsSb/n-InAs heterojunction are utilized as a source for injection of electrons into the active region of the laser. In pulsed mode, the laser operated up to 210K with an emission wavelength of 3.8-3.9 Μm. The dependence of active region composition on wavelength was determined. We also report on the two-color emission of a light-emitting diode with two different active regions to demonstrate multi-stage operation of these devices.

We have grown AlSb and AlAs x sb 1-x epitaxial layers by metal-organic chemical vapor deposition (MOCVD) using trimethylamine alane or ethyldimethylamine alane, triethylantimony, and arsine. These layers were successfully doped p- or n-type using diethylzinc or tetraethyltin, respectively. We examined the growth of AlAS x Sb 1-x using temperatures of 500-600°C, pressures of 65-630 Torr, V/III ratios of 1-17, and growth rates of 0.3-2.7 μm/h in a horizontal quartz reactor. We have also fabricated gain-guided, injection lasers using AlAs x Sb 1-x for optical confinement and a strained InAsSb/InAs multi-quantum well active region grown using MOCVD. In pulsed mode, the laser operated up to 210 K with an emission wavelength of 3.8-3.9 μm.

The goal of this Technical Specification is to provide a guideline for manufacturers of photovoltaic (PV) modules to produce modules that, once the design is proven to meet the quality and reliability requirements, replicate the design on an industrial scale without compromising its consistency with the requirements.

Forward-to-reverse bias step-recovery experiments were performed on an InGaN∕GaN single-quantum-well light-emitting diode grown on sapphire. With the quantum well sampling the minority-carrier hole density at a single position, the optical emission displayed a two-stage decay. Using a solution to the diffusion equation to self-consistently describe both the optical and electrical recovery data, we estimated values for the hole lifetime (758±44ns), diffusion length (588±45nm), and mobility (0.18±0.02cm2∕Vs) in GaN grown on sapphire. This low value of the minority-carrier mobility may reflect trap-modulated transport, and the lifetime is suggestive of slow capture and emission processes occurring through deep levels.

Ultraviolet light-emitting diodes (LEDs) have been grown by metalorganic vapor phase epitaxy using AlN nucleation layers and thick n-type Al0.48Ga0.52N current spreading layers. The active region is composed of three Al0.36Ga0.64N quantum wells with Al0.48Ga0.52N barriers for emission at 290 nm. Devices were designed as bottom emitters and flip-chip bonded to thermally conductive submounts using an interdigitated contact geometry. The ratio of quantum well emission to 330 nm sub-band gap emission is as high as 125:1 for these LEDs. Output power as high as 1.34 mW at 300 mA under direct current operation has been demonstrated with a forward voltage of 9.4 V. A peak external quantum efficiency of 0.18% has been measured at an operating current of 55 mA.

Strained-layer superlattices (SLS's) with biaxially compressed ${\mathrm{InAs}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sb}}_{\mathit{x}}$ were characterized using magnetophotoluminescence and compared with unstrained ${\mathrm{InAs}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sb}}_{\mathit{x}}$ alloys. Holes in the SLS exhibited a decrease in effective mass, approaching that of the electrons. In the two-dimensional limit, a large increase in the Auger threshold energy accompanies this strain-induced change in SLS valence-band symmetry. Correspondingly, the activation energy for nonradiative recombination in the SLS's displayed a marked increase compared with that of the unstrained alloys. Strained-layer superlattices and alloy activation energies are in agreement with estimated Auger threshold energies.

InAs{sub 1-x}Sb{sub x}/InAs strained-layer superlattice (SLS) semiconductors and thick epitaxial layers of InAs{sub 1-x}Sb{sub x} were grown under a variety of conditions by metal-organic chemical vapor deposition on InAs substrates. The III/V ratio was varied from 0.026 to 1.0 over a temperature range of 475--525C, at pressures of 200 to 660 torr and growth rates of 0.75 to 3.0 {mu}m/hour. The composition of the ternary can be predicted from the input gas molar flow rates using a thermodynamic model. At lower temperatures, the thermodynamic model must be modified to take account of the incomplete decomposition of arsine and trimethylantimony. These layers were characterized by optical microscopy, SIMS, and x-ray diffraction. The optical properties of these SLS`s were determined by infrared photoluminescence and absorption measurements. The PL peak energies of the alloys` and the SLS`s are consistently lower than the previously reported values for the bandgap of InAs{sub 1-x}Sb{sub x} alloys.

Ultraviolet light emitting diodes (LEDs) have been grown using metalorganic vapor phase epitaxy, while monitoring the 550 nm reflected light intensity. During nucleation of GaN on sapphire, the transition from three-dimensional (3D) grain growth to two-dimensional (2D) coalesced growth was intentionally delayed in time by lowering the NH3 flow during the initial high temperature growth. Initially, when the reflectance signal is near zero, the GaN film is rough and composed of partly coalesced 3D grains. Eventually, the reflected light intensity recovers as the 2D morphology evolves. For 380 nm LEDs grown on 3D nucleation layers, we observe increased light output. For LEDs fabricated on GaN films with a longer recovery time an output power of 1.3 mW at 20 mA current was achieved.

Deep-level transient spectroscopy (DLTS) studies on both p-type unintentionally doped and n-type (Si-doped), 1.05 eV band gap InGaAsN grown by molecular-beam epitaxy are reported. Two majority-carrier hole traps were observed in p-type material, H3′ (0.38 eV) and H4′ (0.51 eV), and no evidence was found for the presence of minority-carrier electron traps. In n-type material, we observed a shallow distribution of electron levels, E1′, as well as a deep electron trap E4′ (0.56 eV) and a deep hole trap H5′ (0.71 eV). All DLTS peaks observed were broad and are thus consistent with continuous defect distributions and/or groups of closely spaced discrete energy levels in the band gap. Comparison of the spectra to previously reported spectra of metalorganic chemical vapor deposition-grown InGaAsN of the same composition revealed some similarities and some differences, suggesting that some of the observed deep levels are due to intrinsic physical sources, whereas others are specific to the growth technique used.

A contacted electroreflectance technique was used to characterize the electronic properties of AlGaN/GaN heterostructures and field-effect transistors. By studying variations in the electroreflectance with applied electric field, spectral features associated with the AlGaN barrier, the two-dimensional electron gas at the interface, and bulk GaN were observed. The AlGaN barrier displayed Franz–Keldysh oscillations which provided estimates of barrier-layer composition and electric field. The electron gas Fermi energy was determined from analysis of the broad two-dimensional electron gas feature.

To gain insight into the nitrogen-related defects of InGaAsN, nitrogen vibrational mode spectra, Hall mobilities, and minority carrier diffusion lengths are examined for InGaAsN (1.1 eV band gap) grown by molecular beam epitaxy (MBE). Annealing promotes the formation of In–N bonding, and lateral carrier transport is limited by large scale (≫mean free path) material inhomogeneities. Comparing solar cell quantum efficiencies with our earlier results for devices grown by metalorganic chemical vapor deposition (MOCVD), we find significant electron diffusion in the MBE material (reversed from the hole diffusion in MOCVD material), and minority carrier diffusion in InGaAsN cannot be explained by a “universal,” nitrogen-related defect.

Piezomodulated reflectivity is used to probe the electronic structure of GaInP[sub 2] epilayers grown by organometallic chemical-vapor epitaxy on misoriented [001] GaAs substrates. The epilayers, grown at different growth temperatures using 2[degree] and 6[degree] misoriented substrates, exhibit various degrees of ordering. Our study provides information on the fundamental gap, crystal-field splitting, and spin-orbit splitting for temperatures ranging from 6 to 250 K.

InP1-xSbx was grown by metalorganic chemical vapor deposition under a variety of conditions. The V/III vapor phase ratio was varied from 327 to 12 over a temperature range of 450–525°C, at pressures of 77 to 660 Torr and growth rates of 0.30 to 2.0 μm/h. For some samples, the V/III vapor phase ratio was optimized for InP1-xSbx lattice matched to InAs to give smooth morphologies and lattice matching with a minimized variation of composition versus changes in the TMSb/(TMSb+PH3) ratio. Examination by transmission electron microscopy indicates that the materials are not single phase, but have undergone a spinodal decomposition, which is consistent with thermodynamic calculations. When our specimen compositions are corrected for strain relaxation, the infrared photoluminescence measurements indicate a lower energy photoluminescence peak for these materials than had been previuosly reported in the literature. Infrared absorption and photoconductivity measurements indicate no sharp band edge as would be expected for a direct gap semiconductor. The observed broad spectra are consistent with alloy decomposition.

Using a double-modulation, infrared photoluminescence technique, the authors have examined a variety of long-wavelength, HgCdTe materials under development for focal-plane array applications. Near-bandgap photoluminescence was easily observed in 'good quality' HgCdTe at 77 K, and they demonstrate the feasibility of using this technique for routine characterization of HgCdTe prior to device processing. Information regarding minority carrier lifetime, compositional uniformity, and native defects is obtained from photoluminescence data.

Strained single quantum well InGaSb/AlGaSb structures for field-effect transistor applications have been grown by molecular beam epitaxy. Modulation-doped p-type structures have been characterized by a variety of techniques, including Hall effect, Shubnikov-de Haas measurements, and cyclotron resonance. These structures exhibit improved hole transport in comparison to similar GaSb/ AlGaSb structures as a result of strain-splitting of the valence band. P-channel field-effect transistors fabricated in this system exhibited a maximum transconductance of 51 and 161 mS/mm at 300 and 77K, respectively, for a 1.2 μm gate-length device.

Reproducible growth of InAsxSb1−x/InSb strained-layer superlattice photodiodes by low pressure metalorganic chemical vapor deposition (MOCVD) has resulted from the use of Zn doped InAsxSb1−x step-graded buffer layers. These photodiodes show improved performance with response to 10 μm and detectivities of 4 × 109 cm Hz 1 2 /W at 6 μm. Dislocation formation in InAsxSb1−x buffer layers grown on InSb substrates by MOCVD is shown to be reproducibly enhanced by p-type doping at levels exceeding the intrinsic carrier concentration at 475°C using diethylzinc. Carrier concentrations up to 6 × 1018 cm-3 were obtained. The Zn doped buffer layers have proven to be reproducibly crack free for InAsxSb1−x step graded buffer layers with a final composition of x=0.12 lattice matched to a strained layer superlattice (SLS) with an average composition of x=0.09.

Ordering on the {111} planes of the group-V sublattice (CuPt type) is demonstrated in molecular-beam-epitaxy grown ${\mathrm{InAs}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sb}}_{\mathit{x}}$ (x\ensuremath{\approxeq}0.4) alloys and strained-layer superlattices (SLS's) using transmission electron diffraction. Accompanying ordering, a significant reduction of the band gap of the alloy was observed through infrared photoluminescence and photoconductive response measurements. An ordered SLS displayed a photoresponse at longer wavelength than the ordered alloy, due to a type-II band offset of the ordered constituents of the SLS. The band-gap reduction caused by ordering in these samples indicates that ${\mathrm{InAs}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sb}}_{\mathit{x}}$ alloys can effectively span the 8--12-\ensuremath{\mu}m atmospheric window for long-wavelength, infrared-detector applications.

Electron and hole transport in compensated, InGaAsN ({approx} 2% N) are examined through Hall mobility, photoconductivity, and solar cell photoresponse measurements. Short minority carrier diffusion lengths, photoconductive-response spectra, and doping dependent, thermally activated Hall mobilities reveal a broad distribution of localized states. At this stage of development, lateral carrier transport appears to be limited by large scale (>> mean free path) material inhomogeneities, not a random alloy-induced mobility edge.

Near-field scanning optical microscopy (NSOM) was used to study cleaved edges of GaAs solar cell devices. Using visible light for excitation, the NSOM acquired spatially resolved traces of the photocurrent response across the various layers in the device. For excitation energies well above the band gap, carrier recombination at the cleaved surface had a strong influence on the photocurrent signal. Decreasing the excitation energy, which increased the optical penetration depth, allowed the effects of surface recombination to be separated from collection by the pn junction. Using this approach, the NSOM measurements directly observed the effects of a buried minority carrier reflector/passivation layer.

Time-resolved photoluminescence spectroscopy has been used to investigate carrier decay dynamics in a InxGa1−xAs1−yNy (x∼0.03, y∼0.01) epilayer grown on GaAs by metal organic chemical vapor deposition. Time-resolved photoluminescence (PL) measurements, performed for various excitation intensities and sample temperatures, indicate that the broad PL emission at low temperature is dominated by localized exciton recombination. Lifetimes in the range of 0.07–0.34 ns are measured; these photoluminescence lifetimes are significantly shorter than corresponding values obtained for GaAs. In particular, we observe an emission energy dependence of the decay lifetime at 10 K, whereby the lifetime decreases with increasing emission energy across the PL spectrum. This behavior is characteristic of a distribution of localized states, which arises from alloy fluctuations.

We report on the metal-organic chemical vapor deposition (MOCVD) of mid- infrared InAsSb/InPSb optically pumped lasers grown using a high speed rotating disk reactor (RDR). The devices contain AlAsSb claddings and strained, type 1, InAsSb/InPSb active regions. By changing the layer thickness and composition of InAsSb/InPSb SLSs, we have prepared structures with low temperature (

Long-wavelength magnetotransmission and photoluminescence measurements were performed on InAs{sub 0.13}Sb{sub 0.87}/InSb strained-layer superlattices (SLS's). The energies and reduced effective masses of several interband optical transitions were obtained from these experiments. SLS's with different layer thicknesses produced self-consistent results. With these data, the type-II band offset and band-edge strain-shift parameters were accurately determined. Consistent with a type-II offset, an extremely small, in-plane effective-mass hole ground state was observed, thus proving that the hole quantum well is located in the biaxially compressed, InSb layer. Nonparabolicity, suggesting valence-band anticrossing, was also observed.

GaAsSb/InGaAs superlattices of varying thicknesses have been grown nominally lattice matched to InP and characterised by X-ray diffraction, transmission electron microscopy. Hall measurement, and photoluminescence. These structures exhibit net Hall electron densities of ∽5×10 15 cm -3 at room temperature. X-ray diffraction results demonstrate the existence of an intrinsic interfacial strain between these two materials. Low-temperature photoluminescence in the range 2.3–2.8 μm is consistent with the expected type II band alignment in this system, and allows determination of the band offsets between these materials.

Diethylselenium and tetraethyltin were investigated as n-type dopants for InSb. Carrier concentrations between 6 {times} 10{sup 15} and 4 {times} 10{sup 18} cm{sup -3} with 77 K mobilities ranging from 75, 000 to 10,000 cm{sup 2}/Vs were achieved for Sn doped InSb. Triethylantimony and trimethylindium were used to grow InSb over the temperature range of 410 {minus} 470 C with a decrease in the p-type background occurring at 410 C. Diodes were prepared using tetraethyltin, trimethylindium and trimethylantimony at 470 C. The resulting current-voltage characteristics are improved over those of diodes grown previously using dimethyltellurium. SIMS measurements indicate a diffusion coefficient for Sn in InSb at 470 C that is approximately 1 {times} 10{sup -15} cm{sup 2}/s. 8 refs., 4 figs.

The optical gain spectra for compressive-strained and lattice-matched GaInNAs/GaAs quantum wells are computed using a microscopic laser theory. From these spectra, the peak gain and carrier radiative decay rate as functions of carrier density are determined. These dependences allow the study of lasing threshold current density for different GaInNAs/GaAs laser structures.

Deep level transient spectroscopy (DLTS) measurements were utilized to investigate deep level defects in metal-organic chemical deposition (MOCVD)-grown unintentionally doped p-type InGaAsN films lattice matched to GaAs. The as-grown material displayed a high concentration of deep levels distributed within the bandgap, with a dominant hole trap at E{sub v} + 0.10 eV. Post-growth annealing simplified the deep level spectra, enabling the identification of three distinct hole traps at 0.10 eV, 0.23 eV, and 0.48 eV above the valence band edge, with concentrations of 3.5 x 10{sup 14} cm{sup {minus}3}, 3.8 x 10{sup 14} cm{sup {minus}3}, and 8.2 x 10{sup 14} cm{sup {minus}3}, respectively. A direct comparison between the as-grown and annealed spectra revealed the presence of an additional midgap hole trap, with a concentration of 4 x 10{sup 14} cm{sup {minus}3} in the as-grown material. The concentration of this trap is sharply reduced by annealing, which correlates with improved material quality and minority carrier properties after annealing. Of the four hole traps detected, only the 0.48 eV level is not influenced by annealing, suggesting this level may be important for processed InGaAsN devices in the future.

The design, growth by metal-organic chemical vapor deposition, and processing of an In{sub 0.07}Ga{sub 0.93}As{sub 0.98}N{sub 0.02} solar Al, with 1.0 ev bandgap, lattice matched to GaAs is described. The hole diffusion length in annealed, n-type InGaAsN is 0.6-0.8 pm, and solar cell internal quantum efficiencies > 70% arc obwined. Optical studies indicate that defects or impurities, from InGAsN doping and nitrogen incorporation, limit solar cell performance.

We report the molecular beam epitaxial growth of AlxGa1-xAsySb1-y (0.0 ≤ x ≤ 1.0) on undoped, liquid-encapsulated Czochralski, (100) oriented InAs substrates. The degree of lattice mismatch was determined by x-ray diffraction. The lattice matched materials (y ≈ 0.08 + 0.08x) were characterized by low temperature photoluminescence, electro-reflectance, and capacitance-voltage measurements. The experimental bandgap energies agree with earlier experimental results for AlxGa1-xSb, and also with a self-consistent first principles pseudopotential model. The capacitance-voltage measurements indicate background acceptor concentrations for the unintentionally-doped epitaxial layers of about 2 × 1015 cm-3 atx = 1.0 to 5 × 1016 cm-3 atx ≈ 0.0.

The ability to confine and control light in three dimensions would have important implications for quantum optics and quantum-optical devices: the modification of black-body radiation, the localization of light to a fraction of a cubic wavelength, and thus the realization of single-mode light-emitting diodes, are but a few examples1,2,3. Photonic crystals — the optical analogues of electronic crystal — provide a means for achieving these goals. Combinations of metallic and dielectric materials can be used to obtain the required three-dimensional periodic variations in dielectric constant, but dissipation due to free carrier absorption will limit application of such structures at the technologically useful infrared wavelengths4. On the other hand, three-dimensional photonic crystals fabricated in low-loss gallium arsenide show only a weak ‘stop band’ (that is, range of frequencies at which propagation of light is forbidden) at the wavelengths of interest5. Here we report the construction of a three-dimensional infrared photonic crystal on a silicon wafer using relatively standard microelectronics fabrication technology. Our crystal shows a large stop band (10–14.5 μm), strong attenuation of light within this band (∼12 dB per unit cell) and a spectral response uniform to better than 1 per cent over the area of the 6-inch wafer.

Lasers and light-emitting diodes with multistage, type I InAsSb/InAsP quantum well active regions are reported. These ten stage, cascaded devices were grown by metalorganic chemical vapor deposition. The broadband light-emitting diodes produced high average powers, >2 mW (∼80 K, 3.7 μm) and >0.1 mW (∼300 K, 4.3 μm). A 3.8–3.9 μm laser structure operated up to T=180 K. At 80 K, peak power >100 mW and a slope efficiency of 48% (4.8% per stage) were observed in our gain guided lasers. The slope efficiency was strongly dependent on cavity length, and analysis of efficiency data suggests an internal differential quantum efficiency >1 and a loss coefficient ⩾100 cm−1.

The properties of InAsSb/InAsP strained-layer superlattice (SLS), midwave infrared materials, and devices are reported. SLSs were grown by metal-organic chemical vapor deposition and characterized by magnetophotoluminescence and x-ray diffraction. Excellent performance was observed for an SLS light emitting diode (LED) and an optically pumped SLS laser. The semimetal injected, broadband LED emitted at 4 μm with 80 μW of power at 300 K and 200 mA average current. The laser displayed 3.86 μm emission at 240 K, the maximum operating temperature of the laser, and a characteristic temperature of 33 K.

GaN-based microwave power amplifiers have been identified as critical components in Sandia's next generation micro-Synthetic-Aperture-Radar (SAR) operating at X-band and Ku-band (10-18 GHz). To miniaturize SAR, GaN-based amplifiers are necessary to replace bulky traveling wave tubes. Specifically, for micro-SAR development, highly reliable GaN high electron mobility transistors (HEMTs), which have delivered a factor of 10 times improvement in power performance compared to GaAs, need to be developed. Despite the great promise of GaN HEMTs, problems associated with nitride materials growth currently limit gain, linearity, power-added-efficiency, reproducibility, and reliability. These material quality issues are primarily due to heteroepitaxial growth of GaN on lattice mismatched substrates. Because SiC provides the best lattice match and thermal conductivity, SiC is currently the substrate of choice for GaN-based microwave amplifiers. Obviously for GaN-based HEMTs to fully realize their tremendous promise, several challenges related to GaN heteroepitaxy on SiC must be solved. For this LDRD, we conducted a concerted effort to resolve materials issues through in-depth research on GaN/AlGaN growth on SiC. Repeatable growth processes were developed which enabled basic studies of these device layers as well as full fabrication of microwave amplifiers. Detailed studies of the GaN and AlGaN growth of SiC were conducted and techniques to measure the structural and electrical properties of the layers were developed. Problems that limit device performance were investigated, including electron traps, dislocations, the quality of semi-insulating GaN, the GaN/AlGaN interface roughness, and surface pinning of the AlGaN gate. Surface charge was reduced by developing silicon nitride passivation. Constant feedback between material properties, physical understanding, and device performance enabled rapid progress which eventually led to the successful fabrication of state of the art HEMT transistors and amplifiers.

Vertical-cavity lasers emitting at 1.3 /spl mu/m are extremely attractive for high bandwidth fiber communications where it is advantageous to operate at the dispersion minimum of silica optical fiber. To date, VCSELs based on pseudomorphic GaAs materials have achieved lasing emission only slightly longer than 1.3 /spl mu/m. We report the first 1.3 /spl mu/m selectively oxidized VCSELs using InGaAsN quantum wells which operate at continuous wave at and above room temperature.

Based on the wurtzite crystal structure, large (MV/cm) polarization-induced electric fields are known to exist in InGaN single quantum wells (SQWs) grown perpendicular to the GaN c-axis, and these fields may impact optical device performance due to the quantum-confined Stark effect (QCSE). In general, the QCSE has experimentally been found to be smaller than the theoretical value expected for a coherently strained InGaN QW, and subsequently the InGaN/GaN QW polarization field is often under-estimated as well. In this study, we measure the QCSE in modulation-doped, InGaN/GaN SQW LEDs. The well-behaved capacitance-voltage (majority-carrier) characteristics of these devices allow us to unambiguously determine the applied field with bias. With this analysis, we de-couple the QCSE from the QW polarization field and show that although the applied field approaches the opposing QW polarization field theoretical value (i.e., flatband), the QCSE remains too small. We propose a localized-hole picture of the InGaN QW which explains our optical and electrical measurements.

Gain‐guided, injection lasers using AlAsSb for optical confinement and a strained InAsSb/InAs multiquantum well active region were grown by metalorganic chemical vapor deposition. The semi‐metal properties of a p‐GaAsSb/n‐InAs heterojunction are utilized as a source for injection of electrons into the active region of the laser. In pulsed mode, the laser operated up to 210 K with an emission wavelength of 3.8–3.9 μm. We also report on the two‐color emission of a light‐emitting diode with two different active regions to demonstrate multistage operation of these ‘‘unipolar ’’ devices.