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Novel nitride-based heterostructures have been fabricated demonstrating two-dimensional hole gases as the basis for p-channel transistors. The carrier density in the 2DHG is adjusted between very high values of 2 × 1013 cm−2 and low values of 6 × 1011 cm−2 by the polarization difference, ΔP, between quaternary AlInGaN backbarriers and a GaN channel on top. Record mobilities for holes in GaN of 43 cm2 V−1 s−1 (median 30 cm2 V−1 s−1) are observed for a moderate 2DHG density of 1.3 × 1012 cm−2 (median 2.2 × 1012 cm−2).Heterostructures with different backbarrier compositions are processed to field effect transistors and show a systematic threshold voltage shift from positive to negative values according to the corresponding 2DHG density. It is shown for the first time that by appropriate polarization-engineering through changing the AlInGaN composition, both depletion and enhancement mode behaviour can be achieved for p-channel devices.Drain current densities |Id| above 40 mA mm−1 at a drain source voltage Vds of −10 V are achieved for heterostructures with high polarization differences, ΔP, between AlInGaN backbarrier and GaN channel. Reducing ΔP leads to decreasing on-state drain currents |Id| with a simultaneous reduction in off-state current. This results in very large on/off ratios of up to 108 for enhancement mode devices, demonstrating record performances and great potential for future applications. [ABSTRACT FROM AUTHOR]

We demonstrate the application of nitrogen (N) implantation in GaN as a current-blocking layer. In a first step, vertical current-blocking behavior was confirmed by processing quasi-vertical Schottky diodes with full-area N-implantation. The leakage current was only 10-6 Acm-2 in forward and reverse directions. Also, the regrowth of AlGaN/GaN heterostructure field-effect transistors on N-implanted and, for reference, non-implanted GaN templates is demonstrated. Even though a decrease in the mobility and sheet carrier density of the two-dimensional electron gas was observed, excellent off-state properties were achieved. Regrown devices exhibited leakage currents as low as 10-7 mAmm-1, showing very good quality of the regrowth interface. However, a detailed analysis with pulsed I-V and C-V measurements suggest an increased presence of traps due to regrowth, especially on N-implanted templates. [ABSTRACT FROM AUTHOR]

The reduction of the polarization charge in a GaN-based single-heterostructure field-effect transistor (HFET) by polarization engineering is proposed as a method for achieving normally off operation. The concept minimizes the dependence of the threshold voltage on the barrier layer thickness. Therefore, thicker gate dielectrics for suppression of gate leakage currents can be applied without a shift in threshold voltage. A polarization-reduced enhancement-mode (E-mode) InAlGaN/GaN HFET is presented and demonstrates the basic working principle. Also an insulated-gate device with only minor shift in threshold voltage compared to the HFET validates the new concept and demonstrates the advantages compared to commonly applied concepts for E-mode operation. [ABSTRACT FROM AUTHOR]

With the aim of establishing the mechanisms for spontaneous recombination and lasing, we have studied InGaN/GaN multiple quantum well heterostructures emitting in the 450 nm region and grown by organometallic vapor-phase epitaxy on silicon substrates using several mechanical stress-reducing AlN/AlGaN inserts. Photoluminescence (PL) excitation spectroscopy, the non-monoexponential nonequilibrium carrier relaxation kinetics, and x-ray diffractometry data indicate significant inhomogeneity of the InGaN solid solution in quantum wells of these structures. The dependences of the position of the photoluminescence spectra on the excitation level and the temperature, the large shift in the photoluminescence, gain, and lasing spectra relative to the absorption edge allow us draw the conclusion that the dominant contribution to spontaneous and stimulated recombination comes from nonequilibrium charge carriers localized in indium-rich InGaN clusters. [ABSTRACT FROM AUTHOR]

Photoluminescence and lasing at a wavelength of λ = 510–530 nm (green spectral region) in Cd(Zn)Se/ZnMgSSe structures with a different design of the active region are studied in a wide range of temperatures and nitrogen laser pump intensities. A minimal lasing threshold of 10kW/cm2, a maximal external quantum efficiency of 12%, and a maximal output power of 20 W were obtained for the structure with the active region composed of three ZnSe quantum wells with fractional-monolayer CdSe inserts. The lasers exhibited a high temperature stability of the lasing threshold (characteristic temperature T0 = 330 K up to 100°C). For the first time, an integrated converter composed of a green Cd(Zn)Se/ZnMgSSe laser optically pumped by a blue InGaN/GaN laser that is grown on a Si (111) substrate and incorporates multiple quantum wells is suggested and studied. © 2004 MAIK “Nauka / Interperiodica”. [ABSTRACT FROM AUTHOR]

A lattice-matched InAlGaN/GaN heterostructure with a barrier-layer thickness of 4 nm has been grown and passivated in situ with a 63-nm SiN by metal–organic chemical vapor deposition. Enhancement-mode heterostructure field-effect transistors have been realized by a fluorine-based surface treatment after the local removal of the SiN. The threshold voltage and transconductance were 0.65 V and 250 mS/mm, respectively, for a 1-\mu\m gate-length device. The benefits of an in situ SiN passivation are demonstrated: first, the stabilization of the barrier material and prevention from oxidation and second, the improvement of the device characteristics by reduced source resistance and reduced trapping effects. [ABSTRACT FROM AUTHOR]

It was found that the decrease of the InGaN/GaN multiple quantum well (MQW) growth temperature from 865 to 810 °C leads to a MQW emission wavelength shift from the violet to the green spectral region. The lowering of the growth temperature also promotes a decrease of the MQW photoluminescence (PL) intensity at high excitation and a disappearance of the excitonic features from the low-temperature reflection and PL spectra of GaN barriers and claddings. The laser threshold dependence on Tg is not monotonic, with the lowest value of 270 kW/cm2 at Tg=830 °C. High-temperature annealing (900 °C, 30 min) leads to a twofold increase of the PL efficiency only from the InGaN QWs grown at the lowest temperature. The results allow one to explain the laser threshold behavior in terms of the heterostructure quality, the defect concentration, In clusterization, and the piezoelectric field dependence on the MQW growth temperature. [ABSTRACT FROM AUTHOR]

Quaternary AlInGaN layers were grown on conventional GaN buffer layers on sapphire by metal organic vapour phase epitaxy at different surface temperatures and different reactor pressures with constant precursor flow conditions. A wide range in compositions within 30-62% Al, 5-29% In, and 23-53% Ga was covered, which leads to different strain states from high tensile to high compressive. From high-resolution x-ray diffraction and Rutherford backscattering spectrometry, we determined the compositions, strain states, and crystal quality of the AlInGaN layers. Atomic force microscopy measurements were performed to characterize the surface morphology. A critical strain value for maximum In incorporation near the AlInGaN/GaN interface is presented. For compressively strained layers, In incorporation is limited at the interface as residual strain cannot exceed an empirical critical value of about 1.1%. Relaxation occurs at about 15 nm thickness accompanied by strong In pulling. Tensile strained layers can be grown pseudomorphically up to 70 nm at a strain state of 0.96%. A model for relaxation in compressively strained AlInGaN with virtual discrete sub-layers, which illustrates the gradually changing lattice constant during stress reduction is presented. [ABSTRACT FROM AUTHOR]

An integral optically-pumped laser converter comprising an ultra-low-threshold green (∼540 nm) CdSe quantum dot laser chip pumped by a blue InGaN/GaN quantum well heterostructure laser grown on Si (111) substrates has been fabricated and studied. The maximum achieved quantum efficiency and pulse output power in green are as high as 14% and 3 W, respectively. [ABSTRACT FROM AUTHOR]

This work investigates the electrical and optical properties of hybrid GaN/pentacene heterostructures. We identify bipolar transport in GaN/pentacene by I-V and C-V measurements. Further results show a much stronger photocurrent under AM1.5 illumination of GaN/pentacene/Au structures than that of simple GaN/Au Schottky diodes. The higher photocurrent can be explained with the additional absorption of low-energy photons by pentacene. We introduce an interface model describing the behaviour of photogenerated charge carriers pairs for the case of Ga-polar n-GaN and pentacene. These results demonstrate the enhancement of inorganic device performance by integration of organic semiconductors. [ABSTRACT FROM AUTHOR]

Lasing and optical properties of ZnMgSSe/ZnSe-, ZnMgSSe/ZnSSe/ZnSe-, and ZnMgSSe/ZnMgSSe/ZnSe-based single- and multiple-quantum-well heterostructures grown by metalorganic vapor phase epitaxy were studied, and the characteristics were found to depend on the excitation intensity I[sub exc], temperature, and well width. Laser action under transverse optical pumping was achieved only for well widths L[sub z]>=4 nm and optical confinement factors Γ>0.04. In separate confinement heterostructures, lasing with the lowest threshold (I[sub thr]=10–30 kW/cm[sup 2] at T=78 K) was achieved and device characteristics were studied up to T=577 K. © 1998 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Abstract: Early passivated AlGaN/GaN heterostructure field effect transistors (HFETs) have shown superior device performance compared to their unpassivated counterparts. The opening of gate trenches in the passivation layer by a fluorine based dry-etch process is the crucial step to obtain high-performance devices. Even though oxygen addition to this process has been reported, its effect on the device characteristics has not been studied yet. In this paper, we report the impact of oxygen addition to a fluorine based plasma on two-dimensional electron gas (2DEG) properties as well as on device characteristics of passivated AlGaN/GaN HFETs. It is shown that oxygen addition reduces the fluorine-induced degradation of the 2DEG density. Yet, recovery of the 2DEG density is prevented. Additionally, it is shown that oxygen addition increases the current collapse and therefore decreases the RF performance. [Copyright &y& Elsevier]

Abstract: We highlight the challenges of GaN growth on (110)-oriented Si substrates with respect to the substrate-induced anisotropic strain state. A 5:4 lattice matching relationship, which is valid for AlN and Si(111), is also expected for AlN on Si(110) along the [11–20]AlN/[1−10]Si direction. This reduces the effective lattice mismatch for AlN on Si(110) along [11–20]AlN/[1−10]Si from 23.4% to −1.3% (compressive), while in the [1−100]AlN/[001]Si direction, the mismatch value remains at 0.7% (tensile). The opposite signs of the two mismatch values lead to an anisotropic strain state of the deposited AlN and GaN films, which makes it difficult to balance the tensile strain induced by thermal mismatch. For a simple growth scheme using 1μm GaN on top of a 40nm AlN layer, this situation is reflected in an aspherical wafer curvature, anisotropic X-ray diffraction (XRD) Ω scan full width at half maximum (FWHM) values, greater surface roughness and cracking, predominantly occurring parallel to the [11–20] direction. The anisotropic XRD FWHM may be explained by a strain-induced anisotropic nucleation mechanism for GaN on AlN. Also, a partial relaxation of the GaN film during growth caused by very high tensile strain along [1–100]AlN/[001]Si is assumed to contribute to the XRD anisotropy. The insertion of an AlN/GaN superlattice structure in between the AlN and GaN films inhibits this relaxation process and leads to a smoother surface morphology with less visible cracks. [Copyright &y& Elsevier]

Abstract: The influence of MQW growth temperature as well as of the post-epitaxial annealing of InGaN-based heterostructures grown by MOCVD on their optical properties at high excitation levels has been studied, with the focus on the use of these heterostructures as laser active media. The laser thresholds demonstrated non-monotonic behavior with deposition temperature , correlating with the intensity dependence of photoluminescence (PL) under low CW. The optimum values of for lowest threshold, corresponding to the highest PL intensity at low excitation, were between 830 and 850°C. The PL intensity at high excitation increases monotonically with , having no correlation with laser thresholds. This could be explained by a consideration of the relation between the average value of potential fluctuations and the energy spectrum of non-radiative centers, both in the upper GaN layer and in the MQW, and by the change of this relation with the excitation level. [Copyright &y& Elsevier]