The electrical and structural properties of AlN/GaN heterostructures grown by molecular beam epitaxy on sapphire are compared with those of AlGaN/GaN heterostructures. The structural characteristics as assessed by x-ray diffraction show little difference but the electron density in the two-dimensional electron gas is about twice higher for AlN/GaN structures with only slightly lower mobility than in AlGaN/GaN. By proper choice of the Fe doping in GaN(Fe) and the thickness of unintentionally doped GaN layers, the composite buffer of the structure can be made semi-insulating. The current through the AlN/GaN structures is determined by tunneling through the AlN barrier and is much higher than that for AlGaN/GaN films due to the lower thickness of AlN compared to AlGaN. Increasing the thickness of AlN from 3 to 4 nm decreases the leakage current by about an order of magnitude.

The effects of 10MeV electron irradiation on AlGaN∕GaN and AlN∕GaN heterojunctions grown by molecular beam epitaxy are reported. The irradiation increases the resistivity of the GaN buffer due to compensation by radiation defects with levels near Ec−1eV and decreases the mobility of the two-dimensional electron gas (2DEG) near the AlGaN∕GaN (or AlN∕GaN) interface. The bulk carrier removal rate in the GaN buffer is the same for both types of structures and similar to carrier removal rates for undoped n-GaN films. In structures with a density of residual donors of ∼1015cm−3, irradiation with electron doses of ∼5×1015cm−2 renders the buffer semi-insulating. The 50% degradation of the 2DEG conductivity happens at several times higher doses (close to 3×1016cm−2 versus 6.5×1015cm−2) for AlN∕GaN than for AlGaN∕GaN structures, most likely because of the lower thickness of the AlN barrier.

The electrical properties of AlGaN∕GaN high electron mobility transistor structures grown on composite GaN(Fe)∕GaN buffers by molecular beam epitaxy were reported. The concentration of Fe in the GaN(Fe) layer ranged from 8×1016to3×1017cm−3 as established by secondary ion mass spectrometry. The thickness of the undoped GaN layer of the buffer was varied from 2.2to4.1μm. For thinner buffers and higher Fe concentration, the buffer was semi-insulating, with the Fermi level pinned near Ec-0.57eV. For thicker buffers and lower Fe concentration, the top part of the buffer was conducting. Admittance spectra measured in conducting buffers also showed a prominent contribution from Ec-(055–0.6)eV electron traps. Despite the universal prominence of these traps in all our films, the behavior of their concentration with Fe doping and with increased distance from the GaN (Fe)∕GaN boundary is not compatible with the assumption that they are due to substitutional Fe acceptors. Possible compensation mechanisms in the studied structures were discussed.

Electrical photoelectrical, and microcathodoluminescence properties were measured on doped p-GaN superlattices prepared by molecular beam epitaxy on c-plane sapphire substrates. The photosensitivity of such superlattices is at least two orders of magnitude higher and the microcathodoluminescence intensity is about five times lower than for similarly grown uniformly doped p-GaN films. The difference is explained by the spatial separation of the photoexcited electrons and holes by the superlattice potential. The sheet carrier concentration in the superlattice is shown to be several times (6.6×1012cm−2 vs 2×1013cm−2) lower and the effective hole mobility almost two times higher (20cm2∕Vs vs 12cm2∕Vs) than for bulk p-GaN samples with similar doping.

Semi-insulating GaN(Fe) films grown by molecular beam epitaxy (MBE) were characterized by measuring electrical properties, deep-level spectra, Fe distribution profiles, microcathodoluminescence (MCL) spectra, electron-beam-induced current, and MCL imaging. The films were high-quality GaN(Fe) with Fe concentration from similar to 3 x 10(16) to similar to 3 x 10(17) cm(-3). The resistivity of GaN(Fe) buffers was > 10(5) Omega cm, with the Fermi level pinned near E-c 0.5 eV. The buffer quality was characterized for Si-doped GaN and AlGaN/GaN transistor structures grown by MBE on GaN(Fe). In contrast to the reported results for growth by metallorganic chemical vapor deposition, Fe distribution profiles did not show long tails extending into the layers grown on top of GaN(Fe). No features attributed to Fe were observed in lightly doped n-GaN grown on GaN(Fe) buffers. The n-GaN films showed electron mobility of > 500 cm(2)/V s. AlGaN/GaN transistor structures grown on GaN(Fe) buffers showed two-dimensional electron gas mobility > 1900 cm(2)/V s at 300 K, with a sheet density similar to 1 x 10(13) cm(-2) and a good pinch-off and a low interdevice leakage. (c) 2007 The Electrochemical Society.

The origin of hysteresis in capacitance-voltage (C-V) characteristics was studied for Schottky diodes prepared on AlGaN∕GaN transistor structures with GaN (Fe) buffers. The application of reverse bias leads to a shift of C-V curves toward higher positive voltages. The magnitude of the effect is shown to increase for lower temperatures. The phenomenon is attributed to tunneling of electrons from the Schottky gate to localized states in the structure. A technique labeled “reverse” deep level transient spectroscopy was used to show that the deep traps responsible for the hysteresis have activation energies of 0.25, 0.6, and 0.9eV. Comparison with deep trap spectra of GaN buffers and Si doped n-GaN films prepared on GaN buffers suggests that the traps in question are located in the buffer layer.

Capacitance-voltage (C-V), capacitance-frequency (C-f), admittance spectroscopy, deep trap spectra, and far infrared reflectance measurements were performed on undoped and N-doped ZnO films deposited on sapphire by molecular beam epitaxy. The results show existence of a heavily doped n+ layer near the interface with the substrate. The presence of these layers explains the large difference between the electron concentrations measured in the films by Hall effect and C-V profiling or calculated from the plasma minimum frequency in reflectance. C-V data obtained at low temperatures show a prominent persistent photocapacitance in the films. Admittance spectra were dominated by electron traps with ionization level EC−0.3eV commonly observed in ZnO crystals grown by all techniques.

Undoped n-GaN grown by two different metallorganic chemical vapor deposition (MOCVD) techniques, standard MOCVD and epitaxial lateral overgrowth, and Mg-doped p-GaN prepared by hydride vapor phase epitaxy and molecular beam epitaxy were irradiated with fast reactor neutrons to the high fluence of 1018 cm−2. In such heavily irradiated samples the Fermi level is shown to be pinned in a narrow interval of Ec−(0.8−0.95) eV, irrespective of the starting sample properties. The Fermi level pinning position correlates with the measured Schottky barrier height in n-type GaN. The results are interpreted from the standpoint of the existence of the charge neutrality level in heavily disordered material. Based on published theoretical calculations and on deep level transient spectroscopy (measurements and lattice parameter measurements in irradiated material), it is proposed that the Fermi level could be pinned between the gallium-interstitial-related deep donors near Ec−0.8 eV and nitrogen-interstitial-related acceptors near Ec−0.9 eV

Electrical properties, admittance, and microcathodoluminescence spectra are compared for p-GaN samples grown by hydride vapor phase epitaxy (HVPE) and by molecular beam epitaxy (MBE). The former are characterized by a high 300K hole concentration and a weak temperature dependence of conductivity. The latter samples show strongly temperature-activated conductivity due to ionization of Mg acceptors. The main effects of neutron irradiation were similar for the p-HVPE and the p-MBE materials: a compensation of p-type conductivity starting with neutron fluences exceeding 2×1016cm−2 and conversion to high resistivity n type with the Fermi level pinned near Ec-(0.8–0.9)eV after irradiation with high doses of 1018cm−2. For the heavily neutron irradiated p-HVPE samples, a strong increase was observed in the c-lattice parameter which indicates an important role for interstitial-type defects.

A comparison is reported of the electrical, photoelectrical, and microcathodoluminescence properties of bulk p-GaN films and p-type p-GaN(Mg)∕GaN superlattices grown by molecular beam epitaxy. It is shown that the microcathodoluminescence intensity is much lower and the photocurrent (both on samples with indium Ohmic contacts and on Au Schottky diodes) much higher for the doped superlattices due to the spatial separation of photoexcited electrons and holes in the doped and undoped layers and corresponding increase of the lifetime in comparison to the bulk samples. The in-plane conductivity in the doped superlattices is not enhanced significantly relative to conventional p-GaN, but the hole mobilities are increased (from 12to20cm2∕Vs).

This paper reviews of some of the progress made in the development of ZnO-based light emitting diodes (LEDs). n-ZnO/p-AlGaN-based heterostructures have been successfully for the fabrication of UV emitting LEDs that have operated at temperatures up to 650K, suggesting an excitonic origin for the optical transitions. RF-plasma-assisted molecular beam epitaxy has been used to grow epitaxial Cd x Zn 1-x O films on GaN/sapphire structure. These films have a single-crystal wurtzite structure as demonstrated by structural and compositional analysis. High quality Cd x Zn 1-x O films were grown with up to x=0.78 mole fraction as determined by RBS and SIMS techniques. Optical emission ranging from purple (Cd 0.05 Zn 0.95 O) to yellow (Cd 0.29 Zn 0.71 O) was observed. Compositional fluctuations in a Cd 0.16 Zn 0.84 O films were not detected by spatially resolved CL measurements, although intensity fluctuation with features of ∼0.5 μm diameter were seen on the intensity maps. Time resolved photoluminescence shows multi-exponential decay with 21 psec. and 49±3 psec. lifetimes, suggesting that composition micro-fluctuations may be present in Cd 0.16 Zn 0.84 O film.

A new type of defects in Mg doped p-GaN films was detected and studied by means of microcathodoluminescence (MCL) imaging and electron beam induced current (EBIC) imaging in scanning electron microscope. The defects consist of small (about 10 μm) regions in which the MCL intensity is increased and the EBIC signal is decreased. Such behavior is explained by local increase of the hole density due to decreased concentration of compensating native donor defects. The density of these regions of enhanced hole concentration becomes higher and the characteristic dimensions lower in the samples with lower crystalline perfection which suggests diffusion controlled nucleation around nucleation sites. The defects of the above described nature are only detected in heavily Mg doped samples with the Mg concentration exceeding some 1019 cm−3 which is most likely related to less efficient formation of compensating donor defects in less heavily doped samples.

For a set of p-GaN films and p-AlGaN/GaN modulation doped superlattices hole mobilities were measured together with atomic force microscope (AFM) imaging of the surface. It is shown that the apparent mobilities are strongly affected by the parameters of the mosaic structure of the films. Multifractal analysis (MFA) of the AFM images is shown to be a very useful instrument for quantitative comparison of various mosaic structures and for identifying the structures with low mobilities. A good correlation between such MFA parameters as the Renyi dimension and the order index degree and the measured mobilities is reported.

The results of microcathodoluminescence (MCL) and deep levels transient spectroscopy (DLTS) studies of AlGaN/GaN modulation doped field effect transistor (MODFET) structures and modulation doped and undoped AlGaN/GaN superlattices (SLs) grown by metalorganic chemical vapor deposition are presented. A strong blue shift in the energy position of the band-edge MCL peak in modulation doped SLs compared to undoped SLs is observed and attributed to screening of the built-in piezoelectric field in AlGaN/GaN SLs by the high density of two-dimensional electron gas in the GaN quantum wells. The dominant traps in AlGaN/GaN MODFETs and doped SLs with Al composition in the barrier close to 28% were found to be the electron traps with activation energy 0.9 eV located near the AlGaN/GaN interface. In undoped SLs similar traps were observed and their energy position was found to strongly depend on the Al composition in the barrier and on the strength of the electric field in space charge region of the Schottky diode used for DLTS measurements.

GaN/SiC and Al0.25Ga0.75 N/SiC heterojunction diodes were fabricated using Al/Ti for p-ohmic contact to the SiC and Ti/Al/Pt/Au for n-ohmic contact to the GaN and AlGaN. Annealing at 850 °C for 20 s (GaN) or 120 s (AlGaN) was required for achieving specific contact resistances in the 10−6Ωcm2 range. The reverse breakdown voltage showed a negative temperature coefficient in both types of sample, with value ∼5.5±2.5×10−3 V/K. The I–V characteristics of both heterojunctions show evidence of tunneling via defect states.

The magnetization of p-GaN or p-AlGaN/GaN superlattices was measured after implantation with high doses (3-5 x 10(16) cm(-2)) of Mn, Fe, or Ni and subsequent annealing at 700-1000 degreesC. The samples showed ferromagnetic contributions below temperatures ranging from 190-250 K for Mn to 45-185 K for Ni and 80-250 K for Fe. The use of superlattices to enhance the hole concentration did not produce any change in ferromagnetic ordering temperature. No secondary phase formation was observed by x-ray diffraction, transmission electron microscopy, or selected area diffraction pattem analysis for the doses we employed. (C) 2002 American Vacuum Society.

The magnetization of p-GaN or p-AlGaN/GaN superlattices was measured after implantation with high doses (3–5×1016 cm−2) of Mn, Fe, or Ni and subsequent annealing at 700–1000 °C. The samples showed ferromagnetic contributions below temperatures ranging from 190–250 K for Mn to 45–185 K for Ni and 80–250 K for Fe. The use of superlattices to enhance the hole concentration did not produce any change in ferromagnetic ordering temperature. No secondary phase formation was observed by x-ray diffraction, transmission electron microscopy, or selected area diffraction pattern analysis for the doses we employed.

AlN/GaN multiple quantum wells (MQWs) were grown on sapphire substrates by plasmaassisted molecular beam epitaxy. Growth temperature, III/V ratio, growth rate, and other growth parameters were optimized for the buffer layer and the MQWs, separately. The growth of AlN buffer was kept as Al-rich as possible while the formation of Al droplets was avoided. A GaN buffer layer was also tried but proved to be inferior to AlN buffer probably due to its larger surface roughness, higher dislocation density, and larger lattice mismatch with the AlN barrier layers in the MQWs. Very flat surfaces with a RMS roughness of 0.7nm were observed by atomic force microscopy (AFM) on the samples with both AlN buffer layer and 20 MQWs deposited under the optimized growth conditions. Abrupt interfaces and excellent periodicities of the MQWs were confirmed by X-ray diffraction (XRD) and reflectivity measurements with MQWs’ satellite peaks clearly visible up to the 10th order. Room-temperature intense ultraviolet (UV) photoluminescence (PL) emission with wavelength in the range of 320-350nm was also observed from the MQWs with well width ranging from 1.0 to 1.5nm. These MQW structures can potentially be used for UV light emitters and quantum cascade lasers.

Effects of UV/O 3 or deuterium plasma treatment, of annealing in air at 550 °C, of annealing in N 2 at 500 °C and various combinations of these treatments on leakage current and resistance in the forward direction of GaN/InGaN multiquantum-well light emitting diodes MQW LEDs were studied. It was shown that the best results are achieved with 5 minutes long UV/O 3 treatment. LED structures thus prepared showed effects of strong tunneling in their I-V characteristics. The space charge region was shown to be located in the GaN/InGaN superlattice SL. Passing of moderately high forward current through the structure for several hours enhanced the overall tunneling through the structure and facilitated faster tunneling between the layers in the GaN/InGaN SL.