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We present an analytical model describing misfit stresses relaxation in semipolar III-nitride heterostructures caused by misfit dislocations (MDs) originating from basal or prismatic slip and by sessile MDs. We analyze the critical thickness hc for the formation of MDs depending on crystal lattice mismatch and orientation of the semipolar growth plane. We explore transversely isotropic elasticity solutions to describe the relaxation processes in III-nitride semipolar heterostructures and compare the results for hc with those calculated within the isotropic elasticity approach. We find that the value of angle ϑ between the polar с-axis, and the direction of heterostructure growth has an impact on the realization of the stress relaxation mechanism by the generation of MDs originating either from basal or prismatic slip. A comparison of theoretical calculations with experimental data on the observation of MDs in the III-nitride heterostructures is provided. [ABSTRACT FROM AUTHOR]

The impact of proton irradiation on the threshold voltage (VT) of AlGaN/GaN heterostructures is systematically investigated to enhance the understanding of a primary component of the degradation of irradiated high electron mobility transistors. The value of VT was found to increase monotonically as a function of 1.8MeV proton fluence in a sub-linear manner reaching 0.63V at a fluence of 1×1014 cm-2. Silvaco Atlas simulations of VT shifts caused by GaN buffer traps using experimentally measured introduction rates, and energy levels closely match the experimental results. Different buffer designs lead to different VT dependences on proton irradiation, confirming that deep, acceptorlike defects in the GaN buffer are primarily responsible for the observed VT shifts. The proton irradiation induced VT shifts are found to depend on the barrier thickness in a linear fashion; thus, scaling the barrier thickness could be an effective way to reduce such degradation. [ABSTRACT FROM AUTHOR]

Smooth N-polar InGaN/GaN and AlGaN/GaN multiquantum wells (MQWs) and heterostructures were grown by metal organic chemical vapor deposition on (0001) sapphire substrates with misorientation angles of 2°–5° toward the a-sapphire plane. For all investigated structures the tendency toward formation of multiatomic steps at the film surface and at interfaces increased with increasing misorientation angle. Thereby the crystal misorientation led to a stronger degradation of the interface quality and periodicity of InGaN/GaN in comparison to the AlGaN/GaN MQWs. While the alloy composition of AlGaN films was unaffected by the misorientation, the indium mole fraction in the InGaN layers and the wavelength of the MQW related luminescence decreased with increasing misorientation angle. The properties of the two dimensional electron gas (2DEG), which formed at the upper interface of semi-insulating GaN/AlGaN/GaN heterostructures, were strongly anisotropic. Whereas the resistivity of the 2DEG measured perpendicular to the surface steps/undulations decreased with increasing misorientation angle, the resistivity measured in the parallel direction was significantly lower and unaffected by the crystal misorientation. Electron mobility values as high as 1800 cm2/V s were determined for conduction parallel to the surface steps/undulations. [ABSTRACT FROM AUTHOR]

Smooth N-polar GaN/AlxGa1-xN/GaN heterostructures with a different Al mole fraction were grown by metalorganic chemical vapor deposition on (0001) sapphire substrates with a misorientation angle of 4° toward the a-sapphire plane. The sheet electron density of the two-dimensional electron gas (2DEG), which formed at the upper GaN/AlxGa1-xN interface increased with an increasing Al-mole fraction in the AlxGa1-xN layer and increasing silicon modulation doping, similar to the observations for Ga-polar heterostructures. The transport properties of the 2DEG, however, were anisotropic. The growth on vicinal substrates led to the formation of well ordered multiatomic steps during AlxGa1-xN growth and the sheet resistance of the 2DEG parallel to the steps was about 25% lower than the resistance measured in the perpendicular direction. The fabricated devices exhibited a drain-source current, IDS, of 0.9 A/mm at a gate-source voltage +1 V. At a drain-source voltage of 10 V and IDS=300 mA/mm, current-gain and maximum oscillation frequencies of 15 and 38 GHz, respectively, were measured. [ABSTRACT FROM AUTHOR]

Smooth, high quality N-polar GaN films were realized by metal organic chemical vapor deposition (MOCVD) through growth on misoriented (0001) sapphire substrates and the development of a high temperature nucleation process. Misorientation angles from 0.5° to 4° toward the a and the m plane of the sapphire substrate were investigated. Whereas GaN films grown on substrates with a misorientation angle of only 0.5° or 1° exhibited high densities of hexagonal surface features as commonly observed for N-polar GaN films grown by MOCVD, smooth GaN layers were obtained on sapphire substrates with misorientation angles of 2° or larger. In addition, the structural and optical properties of the GaN films significantly improved with increasing misorientation angle, as evaluated by high resolution x-ray diffraction, atomic force microscopy, transmission electron microscopy, and photoluminescence measurements. The properties of GaN layers grown on (0001) sapphire with a misorientation of 4° were comparable to Ga-polar GaN films grown in the same reactor. [ABSTRACT FROM AUTHOR]

A study of aging effects on the electron transport properties of AlGaN/GaN heterostructures grown on sapphire and silicon carbide substrates using temperature dependent Hall effect measurements is presented with the focus on the variations in the mobility, carrier concentration, and resistivity of these structures over time. The transport parameters for several of these structures were measured at various times after the initial measurement, including times exceeding one year. These studies show that these parameters are not stable in most of the samples. The most common effects are an increase in carrier concentration and a decrease in mobility. Changes on the order of several tens of percent were common. Since surface oxidation and other variations in surface states were suspected as possible sources of the observed changes, one sample was treated with HCl and KOH and measured again after each treatment. However, we were unable to recover the initial results although further changes in the electrical properties were observed. The study suggests that surface effects can have profound effects on the properties of the two-dimensional electron gas in these heterostructures. [ABSTRACT FROM AUTHOR]

We have used the Shubnikov-de Haas and the Hall effects to investigate the effect of subband gap illumination on the transport properties of a very high mobility, μ=54 000 cm[sup 2]/V s at T=1.2 K, Al[sub 0.09]GaN[sub 0.91]/GaN heterostructure. We have found that this illumination resulted in a photocurrent that was persistent at low temperatures. This photocurrent, which led to an increase in the carrier density, was used to study the dependence of the mobility and the quantum scattering time on the carrier density. Unlike in previous studies of persistent photoconductivity in GaN and AlGaN/GaN by other researchers, we have found that the mobility did not always increase with the carrier density as a result of illumination. For small duration of illumination, the mobility increased with the carrier density. However additional illumination resulted in a decrease of the mobility despite the increase in the carrier density. We believe that screening is responsible for the initial increase in the mobility, while ionization of defects in GaN, interface charges, and alloy disorder may be responsible for the eventual decrease in the mobility. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We calculate the critical thickness for misfit dislocation (MD) formation in lattice mismatched semipolar and nonpolar III-nitride wurtzite semiconductor layers for the case of MDs originated from prismatic slip (PSMDs). It has been shown that there is a switch of stress relaxation modes from generation of basal slip originated MDs to PSMDs after the angle between c-axis in wurtzite crystal structure and the direction of semipolar growth reaches a particular value, e.g., ~70° for Al0.13Ga 0.87N GaN (h0...1) semipolar heterostructures. This means that for some semipolar growth orientations of III-nitride heterostructures biaxial relaxation of misfit stress can be realized. The results of modeling are compared to experimental data on the onset of plastic relaxation in AlxGa1-xN GaN heterostructures. [ABSTRACT FROM AUTHOR]

In this letter, we report on the interfacial abruptness of GaN/AlN/GaN heterostructures with pulsed laser atom probe tomography (APT). The samples were grown by plasma-assisted molecular beam epitaxy (MBE) under both metal-rich and N-rich conditions on both Ga-polar (0001) GaN templates and N-polar ([formula]) GaN substrates. An NH3 assisted MBE technique was involved to grow similar Ga-polar and N-polar structures on GaN:Fe substrates and GaN/Al2O3 templates, respectively, for a comparison study. We find in all cases the interface with net positive polarization charge was chemically abrupt, whereas the interface with net negative polarization charge was diffuse. We discuss the implications on device design and performance. These data validate the efficiency of APT in studying interfaces for better performance in devices. [ABSTRACT FROM AUTHOR]

Growth of InGaN/GaN light-emitting devices on nonpolar or semipolar planes offers a viable approach to reducing or eliminating the issues associated with polarization-related electric fields present in c-plane III-nitride heterostructures. Although progress in device performance has been rapid since the introduction of high-quality free-standing nonpolar and semipolar GaN substrates, a full appreciation of the materials challenges unique to nonpolar and semipolar III-nitride semiconductors has been slower to emerge. Only recently have researchers begun to understand issues such as the origins of the pyramidal hillocks typically observed on nominally on-axis m-plane GaN films, the effects of m-plane substrate misorientation on surface morphology and device performance, the mechanics of anisotropic cracking in tensile strained m-plane AlGaN films, the formation of basal-plane stacking faults in long-wavelength m-plane InGaN quantum wells, and the mechanisms for stress relaxation in semipolar AlGaN and InGaN films. In this paper, we review the materials and growth issues unique to high-performance nonpolar and semipolar light-emitting devices grown on high-quality free-standing GaN substrates and provide an outlook for the opportunities and challenges that lie ahead. [ABSTRACT FROM AUTHOR]

In this paper, we report on observations of the operation of secondary slip systems to relieve lattice mismatch stress in semipolar InGaN/GaN heterostructures. Two-dimensional arrays of misfit dislocations were observed. Consistent with previous reports, primary relaxation occurred along the projected c direction via primary slip on the (0001) basal plane. In addition, evidence for secondary relaxation was detected in cathodoluminescence spectroscopy, high resolution x-ray diffraction, and transmission electron microscopy (TEM) studies. The secondary misfit dislocations were determined by TEM to have a-type Burgers vectors a/3 <[formula]> and line directions along <[formula]>, consistent with prismatic slip on one of the m-type planes inclined with respect to the [formula] growth surface. Evidence of an additional slip system with approximate misfit line direction of type <[formula]> is also given. [ABSTRACT FROM AUTHOR]

Smooth N-polar GaN films and GaN/AlGaN/GaN heterostructures with properties comparable to Ga-polar GaN-on-silicon films were grown by metal organic chemical vapor deposition on (111) silicon substrates with a misorientation angle of 3.5° toward the [formula] direction. For heterostructures with a sheet electron density of 9×1012 cm-2 an electron mobility of 1500 cm2/V s was derived for transport parallel to the surface steps which formed as a result of the substrate misorientation. [ABSTRACT FROM AUTHOR]

Deep levels were observed using capacitance deep level optical spectroscopy (DLOS) in an AlGaN/GaN heterostructure equivalent to that of a heterojunction field effect transistor. Band gap states were assigned to either the AlGaN or GaN regions by comparing the DLOS spectra in accumulation and pinch-off modes, where the former reflects both AlGaN- and GaN-related defects, and the latter emphasizes defects residing in the GaN. A band gap state at Ec-3.85 eV was unambiguously identified with the AlGaN region, and deep levels at Ec-2.64 eV and Ec-3.30 eV were associated with the GaN layers. Both the AlGaN and GaN layers exhibited additional deep levels with large lattice relaxation. The influence of deep levels on the two-dimensional electron gas sheet charge was estimated using a lighted capacitance-voltage method. [ABSTRACT FROM AUTHOR]

Local electronic properties in a molecular-beam-epitaxy-grown Al[sub x]Ga[sub 1-x]N/GaN heterostructure field-effect transistor epitaxial layer structure are probed using depth-resolved scanning capacitance microscopy. Theoretical analysis of contrast observed in scanning capacitance images acquired over a range of bias voltages is used to assess the possible structural origins of local inhomogeneities in electronic structure, which are shown to be concentrated in areas where Ga droplets had formed on the surface during growth. Within these regions, there are significant variations in the local electronic structure that are attributed to variations in both Al[sub x]Ga[sub 1-x]N layer thickness and Al composition. Increased charge trapping is also observed in these regions. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Plasma-assisted molecular-beam epitaxy is used to grow a set of two-dimensional electron-gas AlN/GaN structures with AlN barrier thicknesses varied between 24 and 50 Å. The density of the two-dimensional electron gas formed at the GaN/AlN interface increases from 1.51x10[sup 13] cm[sup -2] for the AlN barrier width of 24 Å to 3.65x10[sup 13] cm[sup -2] for the AlN barrier width of 49 Å. The increase in the electron sheet density is accompanied by a decrease in electron mobility related to tensile strain relaxation and enhanced interface roughness scattering. It is shown that room-temperature sheet resistances below 200 Ω/&square; can be achieved in AlN/GaN high electron mobility transistor structures with 35-45 Å AlN barriers. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Presents a correction to the author's name for the article, entitled `(Ga, Mn)As as a digital ferromagnetic heterostructure,' published in the volume 77 issue of the `Applied Physics Letters' magazine'.

(Ga,Mn)As digital ferromagnetic heterostructures are grown by incorporating submonolayer planes of MnAs into GaAs using molecular beam epitaxy. Structural and magnetic measurements indicate single-crystalline superlattice structure and ferromagnetic order with Curie temperatures (T[sub C]) up to 50 K. By varying the spacing between neighboring Mn layers, we observe that T[sub C] initially decreases with increasing spacer thickness, followed by a regime with weak dependence on the spacer thickness. The persistence of ferromagnetism for interlayer spacings of at least 200 ML (∼560 Å) suggests that the individual Mn layers are ferromagnetic. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The origin of the two-dimensional electron gas (2DEG) in AlGaN/GaN heterostructure field effect transistors is examined theoretically and experimentally. Based on an analysis of the electrostatics, surface states are identified as an important source of electrons. The role of the polarization-induced dipole is also clarified. Experimental Hall data for nominally undoped Al[sub 0.34]Ga[sub 0.66]N/GaN structures indicate that ∼1.65 eV surface donors are the actual source of the electrons in the 2DEG, which forms only when the barrier thickness exceeds 35 Å. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]