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We have performed Shubnikov-de Haas (SdH) and Hall effect measurements to investigate the electronic transport properties of polarisation induced 2DEGs in Al x Ga 1-x N/GaN heterostructures with alloy compositions between x = 0.10 and 0.35 and sheet carrier concentrations of up to n s = 1.05 × 10 13 cm -2 . From SdH measurements of 2DEGs with sheet carrier concentrations of 2.1 × 10 12 and 4.6 × 10 12 cm -2 , effective electron masses were determined to be 0.24 and 0.207m 0 , respectively. In addition, angle resolved SdH measurements were performed to evaluate the effective g-factor from the angle of zero oscillation amplitude. Including the measured electron masses, the g-factors were calculated to be 2.11 and 2.47, respectively. In order to identify the main electron scattering mechanism we determined the ratio between transport- and quantum scattering times as a function of sheet carrier concentration. We observe a significant decrease of the τ t /τ q ratio with increasing 2DEG carrier concentration, indicating a transition from a dominant small angle to large angle scattering mechanism when n s exceeds 7 × 10 12 cm -2 .

Undoped AlGaN/GaN structures are used to fabricate high electron mobility transistors (HEMTs). Using the strong spontaneous and piezoelectric polarization inherent in this crystal structure a two-dimensional electron gas (2DEG) is induced. Three-dimensional (3-D) nonlinear thermal simulations are made to determine the temperature rise from heat dissipation in various geometries. Epitaxial growth by MBE and OMVPE are described, reaching electron mobilities of 1500 and 1700 cm/sup 2//Ns, respectively, For electron sheet density near 1/spl times/10/sup 13//cm/sup 2/, Device fabrication is described, including surface passivation used to sharply reduce the problematic current slump (dc to rf dispersion) in these HEMTs. The frequency response, reaching an intrinsic f/sub t/ of 106 GHz for 0.15 /spl mu/m gates, and drain-source breakdown voltage dependence on gate length are presented. Small periphery devices on sapphire substrates have normalized microwave output power of /spl sim/4 W/mm, while large periphery devices have /spl sim/2 W/mm, both thermally limited. Performance, without and with Si/sub 3/N/sub 4/ passivation are presented. On SiC substrates, large periphery devices have electrical limits of 4 W/mm, due in part to the limited development of the substrates.

Plasma-induced damage often reduces the electrical and optical performance and the lifetime of compound semiconductor devices. We have investigated the effect of oxygen and methane reactive ion etching plasma on the electrical characteristics of nominally undoped GaN/Al 0.25 Ga 0.75 N/GaN high mobility heterostructures on sapphire substrates grown by plasma-induced molecular beam epitaxy and metalorganic chemical vapor deposition (MOCVD). The electrical transport properties of the two-dimensional electron gas in the AlGaN/GaN heterostructures were investigated by a combination of capacitance–voltage profiling and Hall effect measurements. The performance degradation of the heterostructures is attributed to the reduction of the carrier density and mobility due to ion bombardment, which is causing a creation of surface and deep acceptor states. After rapid thermal annealing (RTA) at temperatures between 400°C and 800°C, the electrical properties of the heterostructures exposed at moderated plasma power density and bias were mostly recovered. However, samples exposed at high power density lost the significant part of the electron sheet carrier concentration unrecoverable even after RTA at 800°C.

In Si doped AlN, a large persistent photoconductivity is found for temperatures below 60 K after exposure to light with photon energies above 1.5 eV. Simultaneously, a persistent electron spin resonance signal is observed with an isotropic g factor of 1.9885 due to an effective mass donor state, while no spin resonance signal is detectable after cooling the sample in the dark. Both observations show that Si undergoes a $\mathrm{DX}$-like metastability in this material. Based on the experimental findings, a detailed configuration diagram is proposed.

We report on the growth of nominally undoped GaN/AlxGa1−xN/GaN (x

Two dimensional electron gases in Alcursivₑ chiGa₁₋cursivₑ chiN/GaN based heterostructures, suitable for high electron mobility transistors, are induced by strong polarization effects. The sheet carrier concentration and the confinement of the two dimensional electron gases located close to the AlGaN/GaN interface are sensitive to a large number of different physical properties such as polarity, alloy composition, strain, thickness, and doping of the AlGaN barrier. We have investigated these physical properties for undoped and silicon doped transistor structures by a combination of high resolution x-ray diffraction, atomic force microscopy, Hall effect, and capacitance-voltage profiling measurements. The polarization induced sheet charge bound at the AlGaN/GaN interfaces was calculated from different sets of piezoelectric constants available in the literature. The sheet carrier concentration induced by polarization charges was determined self-consistently from a coupled Schrodinger and Poisson equation solver for pseudomorphically and partially relaxed barriers with different alloy compositions. By comparison of theoretical and experimental results, we demonstrate that the formation of two dimensional electron gases in undoped and doped AlGaN/GaN structures rely both on piezoelectric and spontaneous polarization induced effects. In addition, mechanisms reducing the sheet carrier concentrations like nonabrupt interfaces, dislocations, and the possible influence of surface states on the two dimensional electron gases will be discussed briefly. © 2000 American Institute of Physics.

Combined electron beam induced current and transmission electron microscopy measurements have been performed on both undoped and Si-doped AlGaN epitaxial films with aluminium contents x ranging from x=0 to x=0.79. TEM analysis shows the distribution and type of structural defects, the content of cubic phase and the decomposition phenomena strongly to depend on the Al content of the layers. The influence of the cubic phase on the electronic properties of the samples is discussed taking into account the high piezoelectric effect present in these alloys. The diffusion length of minority carriers has been determined by EBIC measurements and a lower limit of the lifetime has been estimated.

AlGaN epitaxial films have been grown on sapphire by plasma-induced molecular beam epitaxy (MBE) over the entire composition range from GaN to AlN. Structural and optical properties of the alloys have been investigated by X-ray diffraction (XRD), transmission electron and atomic force microscopy, Raman scattering, ellipsometry, optical transmission, and subgap absorption spectroscopy. Electron spin resonance has been used to study the dependence of intrinsic paramagnetic defects on Al mole fraction. N- and p-type doping with Si and Mg, respectively is found to become increasingly difficult with increasing Al content because of a continuous shift of the donor and acceptor levels deeper into the bandgap. Apart from the use of AlGaN as cladding layers in light emitting diodes, applications in MODFET transistors, solar blind photodetectors, surface acoustic wave devices and Bragg reflectors appear interesting and will be discussed briefly.

The role of hydrogen in gallium nitride was studied on thin films of GaN on sapphire prepared at substrate temperatures in the range of 600 to 1100 °C. By using triethylgallium and ammonia as precursor and hydrogen and/or nitrogen as transport gases, we have observed a strong influence of molecular hydrogen on the deposition rate and the structural properties of epitaxial GaN. By elastic recoil detection analysis and thermal desorption measurements we were able to determine the total concentration of nitrogen, hydrogen and carbon in the bulk material. Isotope substitution of hydrogen by deuterium in the H2 carrier gas did not give rise to a noticeable deuterium incorporation, showing that the sources for hydrogen are the metalorganic precursor, ammonia or reaction products of both. Once incorporated, thermally activated hydrogen effusion from n-type GaN occurs with an activation energy of more than 3.9 eV. With the help of mass spectrometry we established hydrogen effusion from heavily magnesium-doped (2 at%) GaN at temperatures between 600 and 700 °C, which is the temperature range used for acceptor activation.

Sub-band-gap absorption of GaN grown by metal-organic chemical vapor deposition on sapphire was investigated by photothermal deflection spectroscopy (PDS), transmission measurements, and the constant photocurrent method (CPM). We determine acceptor binding energies in undoped GaN at 220 and about 720 meV. A comparison between absorption and CPM spectra yields the dependence of the quantum efficiency-mobility-lifetime-product ($\ensuremath{\eta}\ensuremath{\mu}\ensuremath{\tau}$) versus energy and gives relevant information about the excitation mechanisms. CPM spectra show a significantly smaller absorption (up to a factor of $\frac{1}{10}$) in the range between 3.0 and 3.3 eV as compared to PDS. This indicates that the majority of carriers excited with these photon energies have a relatively small $\ensuremath{\eta}\ensuremath{\mu}\ensuremath{\tau}$ product and thus do not contribute to the externally detected photocurrent. We propose that in this energy range the spectrum is dominated by interband absorption in isolated cubic-phase crystallites in the hexagonal matrix and by excitation of electrons from occupied acceptors into the conduction band of the main hexagonal crystal modification ($h$-GaN). Temperature-dependent photoluminescence measurements, excited with energies below and above the direct band gap of hexagonal GaN, confirm this interpretation and can be correlated with the subgap absorption detected by PDS. Transient photocurrent measurements show a persistent photoconductivity, which can also be explained by the existence of isolated cubic-phase inclusions.

Films of aluminum nitride (AlN) with thicknesses in the range from 200 to 3600 A have been deposited at 1050°C by low-pressure MOCVD. Using an alternative precursor, tritertbutylaluminium (But3Al), and ammonia (NH3), we have grown AlN on sapphire (c-Al2O3). At a growth rate of 0.35 μm/h, the FWHM of the rocking curve measured by X-ray diffraction was below 320 arcsec. Therefore, we used the thin AlN films as buffer layers for the deposition of gallium nitride (GaN) at 950°C using triethylgallium (Et3Ga). The improved structural and optical properties of GaN were verified by XRD and Raman, photothermal deflection and photoluminescence spectroscopies. The influence of the buffer layer thickness on the optical properties such as the sub-bandgap absorption, the Raman-active phonons and photoluminescence are presented.

Hexagonal InN and InGaN are interesting wide-bandgap semiconductors for optoelectronic applications. The first example of an air-stable, fairly volatile, and low melting precursor, (N3)In[(CH2)3NMe2]2 (1), to deposit crystalline InN at low temperatures (350−450 °C) is presented.

Intentionally grown GaN inversion domain boundaries (IDBs) of lateral polarity heterostructures have been spectroscopically imaged at low temperature using high spatial resolution photoluminescence. It is shown that the IDBs are not only optically active, but are more than an order of magnitude brighter than the GaN bulk material. Our findings are in agreement with calculations predicting that IDBs should not adversely affect near-band-gap photoluminescence due to the absence of midgap electronic states. Typical linewidths are on the order of 10–20 meV, however, features less than 0.6 meV are observed. The boundary emission is found to be neither spectrally nor spatially uniform. Also, a strong polarization dependence of the IDB photoluminescence is measured and determined to be oriented parallel to the boundary between GaN of N- or Ga-face polarity.

The radiative recombination in Ga-face Al 0.30 Ga 0.70 N/GaN single heterostructures (SHs) was studied by photoluminescence (PL) measurements. An energy shift of the excitonic transitions toward higher energies was observed, indicating the presence of residual compressive strain in the GaN layer. In addition to these exciton lines, a broad band energetically localized between the exciton lines and the LO-phonon replica was noticed in the undoped SH. From its energy position, excitation power dependence, as well as temperature behaviour, we have attributed this luminescence to the H -band (HB), which is representative of the two-dimensional electron gas (2DEG) recombination.

The electronic properties and the electron affinities of AlxGa1−xN(0001) surfaces were investigated by ultraviolet photoemission spectroscopy (UPS) over the whole composition range. The samples were prepared by N-ion sputtering and annealing. Surface cleanliness and stoichiometry were monitored with x-ray photoemission spectroscopy. Samples with high aluminum content showed traces of oxygen which could not be removed by further cleaning cycles. However, we have evidence that the oxygen is located in the bulk and not at the surface. From the UP spectra the ionization energies and electron affinities as a function of composition x were determined. A decrease in electron affinity with increasing aluminum content was found, but the electron affinity remains positive for all x. Thus, earlier predictions of negative electron affinity for high aluminum content were not confirmed.

High-quality bulk InGaAs and InAlAs/InGaAs heterostructures have been grown on InP substrates with different orientation using molecular-beam epitaxy. It was found that the electrical and structural properties were strongly dependent on growth temperature and substrate misorientation. The electrical and structural properties of the film were investigated by high-resolution x-ray diffraction, Nomarski microscope, and Hall measurements. Full-width at half-maximum of 380 arcsec for bulk InGaAs on (111)B and 70 arcsec on (111)B misoriented by 1° towards 〈211〉 InP substrates were measured by x-ray diffraction. The room temperature electron Hall mobility for bulk InGaAs of 5100 cm2/V s, doped with Si concentration at the mid-1017/cm3, and two-dimensional electron gas mobility of 11 200 cm2/V s, and sheet density of 3.0×1012/cm2 for InAlAs/InGaAs heterostructures on (111)B misoriented by 1° towards 〈211〉 InP substrates were achieved.

High-quality AlGaAs/GaAs heterostructures have been grown on on-axis (111)B GaAs substrates using molecular-beam epitaxy to study the effect of As4 pressure on the material qualities. It was found that the electrical and structural properties were strongly dependent on the As4 pressure. The quality of the heterostructure was investigated by Hall effect, atomic force microscopy, and high-resolution x-ray diffraction measurements. The electron Hall mobility dropped from 7200 and 90 000 cm2/V s to 930 and 1340 cm2/V s at room temperature and 77 K, respectively, as As4 pressure was increased from 4.9×10−6 to 1.1×10−7 Torr. The same trend was observed at the root-mean-square roughness of the AlGaAs/GaAs heterostructures measured by atomic force microscopy, and the intensity and full width at half-maximum of the x-ray diffraction curve.

High-quality AlGaN/GaN heterostructures have been grown on sapphire substrates by plasma-assisted molecular-beam epitaxy. Polarization effects are exploited to achieve a two-dimensional electron-gas sheet density of 8.8×1012 cm−2 and greater on intentionally undoped material with a measured room-temperature mobility as high as 1478 cm2/V s. Transistors were then fabricated from this material, yielding a unity current gain frequency of 50 GHz and a unity power gain frequency of 97 GHz. By increasing the buffer layer thickness, output powers of 1.88 W/mm at 4 GHz with an efficiency of 34% were achieved. These results prove that the polarization effects in the nitrides are as enormous as theory predicts. The key to the improved mobility and operation of the devices of the all-molecular-beam-epitaxy-grown material, the AlN nucleation layer, will be discussed.

The present and predicted limits on microwave power performance of undoped AlGaN/GaN HEMTs are presented, based on measured frequency response and drain-source breakdown voltage, both as functions of gate length. The spontaneous and piezoelectric polarization that induce the 2DEG in these HEMTs are covered. Process methods, including Si/sub 3/N/sub 4/ passivation are included. Thermal simulation results are shown for heat dissipation that limits channel temperature to 300/spl deg/C. Microwave cw power density limits of 12.5 W/mm at 10 GHz are predicted for class A operation on thick SiC substrates.

Ionization energies and electron affinities of clean AlxGa1−xN(0001) surfaces were investigated by ultraviolet photoemission spectroscopy over the whole composition range. The samples were cleaned with cycles of N+-ion sputtering and annealing partly within a Ga atom flux. The ionization energy is measured as 6.5 eV and is almost independent of the aluminum content in good agreement with the general chemical trend. The electron affinity decreases linearly with composition from 3.1 eV for GaN to 0.25 eV for AlN. No evidence for negative electron affinity at AlN(0001) surfaces was found. Adsorption of oxygen at room temperature leads to a significant increase of the ionization energy and electron affinity. With AlN(0001) surfaces, an oxygen uptake of 0.6 monolayers is observed after exposures of 108 Langmuirs and the ionization energy increases by approximately 2 eV.

Ga-face AlGaN/GaN heterostructures with different sheet carrier concentrations have been studied by photoluminescence and Raman spectroscopy. Compared to bulk GaN, an energy shift of the excitonic emission lines towards higher energies was observed, indicating the presence of residual compressive strain in the GaN layer. This strain was confirmed by the shift of the E2 Raman line, from which biaxial compressive stresses ranging between 0.34 and 1.7 GPa were deduced. The spontaneous and piezoelectric polarizations for each layer of the heterostructures have been also calculated. The analysis of these quantities clarified the influence of the residual stress on the sheet electron concentration (ns). Possible causes for the discrepancies between the calculated and experimentally determined sheet carrier densities are briefly discussed. Andres.Cantarero@uv.es ; Gema.Martinez@uv.es

Combined electron beam induced current and transmission electron microscopy (TEM) measurements have been performed on both undoped and Si-doped AlGaN epitaxial films with aluminum contents x ranging from x = 0 to x = 0.79, in order to correlate the electrical and structural properties of the films. The diffusion length of holes in the films ranges between 0.3 and 15.9 mu m, and the estimated lifetime of holes for doped samples varies between 0.2 ns and 16 mu s. Different effects contribute to the observed increase in the diffusion length with increasing aluminum content. Among others, dislocations seem to be active as nonradiative recombination sites, and phase separation and decomposition as observed by TEM in Al-rich alloys lead to the formation of a spatially indirect recombination path due to the piezoelectric field in the films. Potential fluctuations associated with these phase irregularities could also give rise to electron induced persistent conductivity contributing to the increase of the diffusion length. From our experimental observations, we conclude that the silicon dopants are partially activated in Al-rich alloys, and do not influence significantly the values of the diffusion length of holes in these samples.

In this study thin AlxGa1−xN nucleation layers on sapphire were patterned and overgrown by plasma-induced molecular beam epitaxy (PIMBE) and metalorganic chemical vapor deposition (MOCVD) to obtain adjacent regions of GaN and AlGaN/GaN heterostructures with different polarities. The role of polarity on the structural and electrical properties of epitaxial layers and AlGaN/GaN heterostructures was investigated for samples grown on patterned AlN or GaN nucleation layers. Epitaxial GaN and AlGaN/GaN heterostructures grown on Al-face AlN or N- face GaN nucleation layers were found to be Ga-face or N-face, respectively, independent of the technique used for the overgrowth.

Micro-Raman spectroscopy has been applied to isotopically pure GaiN (i = 14, 15) epitaxial films in which structural disorder was introduced by ion implantation or mechanical damage. A pair of Raman peaks appearing at about 300 (300) and 667 (646) cm—1 is consistent with the anion mass 14 (15) suggesting disorder activation of the silent B1 modes, where the low-frequency mode is related to the Ga vibration (no N isotope shift) and the high-frequency mode to the N vibration (full N isotope shift). Isotopically pure Al0.5Ga0.5iN indicates respective activation of the silent B1 modes due to alloying disorder and confirms a two-mode behavior of the E2 mode. In agreement with recent IR measurements and theoretical predictions, we additionally provide evidence for an E1(TO) two-mode behavior in Raman scattering as obtained from polarisation dependent measurements on AlxGa1—xN epilayers.

Thin films of GaN on c-plane sapphire were grown by plasma-enhanced molecular beam epitaxy (PEMBE). The influence of different growth conditions on the quality of the epitaxial layers was studied by x-ray diffraction (XRD), atomic force microscopy (AFM) and Hall measurements. For low deposition temperatures, the growth of a thin buffer layer of AlN results in a decrease of the XRD rocking curve full width at half maximum (FWHM) but also in poorer quality in electronic and optical properties. Samples of 3μm thickness with 570 arcsec FWHM in the XRD rocking curve, a near band gap PL-emission FWHM at 5 K of 7 meV, charge carrier densities of ne = 2 × 1017 cm−3, and Hall mobilities of 270 cm2/Vs at 300 K were grown without a buffer layer. A comparison of the morphology and XRD rocking curves with those of GaN films deposited by metalorganic chemical vapour deposition (MOCVD) shows that the two methods have different growth mechanisms.

Transmission electron microscopy (TEM) investigations of metal organic vapor phase deposition grown AlxGa1−xN/GaN heterostructures on Si(111) containing an AlN high-temperature buffer layer have been carried out. The structural properties at the interface and in the epilayer as well as the electronic properties suitable for a high electron mobility transistor (HEMT) were analyzed and compared with systems grown on Al2O3(0001). High resolution TEM (HRTEM) at the AlN/Si(111) interface reveals a 1.5–2.7 nm thick amorphous SiNx layer due to the high growth temperature of TAlN=1040 °C. Therefore, a grain-like GaN/AlN region extending 40–60 nm appears and it is subsequently overgrown with (0001) orientated GaN material because of geometrical selection. The residual strain at the AlN/Si(111) interface is estimated to be er=0.3±0.6% by Fourier filtering of HRTEM images and a moire fringe analysis. This indicates almost complete relaxation of the large mismatch f(AlN/Si)=+23.4% which seems to be supported by the S...

Ga14N/Ga15N/Ga14N isotope heterostructures are used to study nitrogen self-diffusion by secondary-ion mass spectrometry and thermally activated decomposition. After interdiffusion of Ga14N and Ga15N layers at temperatures between 770°C and 970°C the diffusion profiles are measured. The isotope heterostructures are particularly well suited for self-diffusion studies because the diffusion takes place at the interface inside the GaN crystal, and therefore the analysis is free from perturbations such as surface electric fields, mechanical stress or chemical potential gradients. The temperature dependence of the nitrogen self-diffusion coefficient (D) in hexagonal GaN was determined to be 1600 cm-2 s-1 exp [(-4.1±0.4) eV/k B T], leading to a self-diffusion entropy S SD of about 10k B. The nitrogen flux through an isotope interface is compared with the nitrogen loss from a free GaN surface in vacuum above the decomposition temperature, to obtain information about the diffusion kinetics relevant for epitaxial growth and high temperature device applications.

Photothermal deflection spectroscopy (PDS) is used to study the absorption of GaN/InGaN/GaN double heterostructures in the energy range from 0.6 to 3.8 eV. The heterostructures containing 1–77-nm-thick InGaN single quantum wells were deposited by chemical vapour deposition from organometallic precursors. They are measured to investigate the absorption coefficient, bandgap, indium concentration and fluctuation of the quantum wells. A bandgap increase of hexagonal In x Ga1-x N (x≈0.14) of 60 meV is observed with decreasing well thicknesses from 15 to 1 nm. The distribution of In-concentration of the In x Ga1-x N layers was estimated from the slope of the absorption coefficient versus photon energy for energies below the bandgap and found to be Gaussian with a full width of half maximum of Δx=0.03.

Carrier concentration profiles of two-dimensional electron gases are investigated in wurtzite, Ga-face AlₓGa₁₋ₓN/GaN/AlₓGa₁₋ₓN and N-face GaN/AlₓGa₁₋ₓN/GaN heterostructures used for the fabrication of field effect transistors. Analysis of the measured electron distributions in heterostructures with AlGaN barrier layers of different Al concentrations (0.15

Patterned etching of GaN films was achieved with laser‐induced thermal decomposition. High‐energy laser pulses are used to locally heat the film above 900 °C, causing rapid nitrogen effusion. Excess gallium is then removed by conventional etching. At exposures of 0.4 J/cm2 with 355 nm light, etch rates of 50–70 nm per pulse were obtained. Illumination with an interference grating was used to produce trenches as narrow as 100 nm.

The wurzite group-III nitrides InN, GaN, and AlN are tetrahedrally coordinated direct band gap semiconductors having a hexagonal Bravais lattice with four atoms per unit cell. As a consequence of the noncentrosymmetry of the wurzite structure and the large ionicity factor of the covalent metal–nitrogen bond, a large spontaneous polarization oriented along the hexagonal c-axis is predicted. In addition, group-III nitrides are highly piezoelectric. The strain induced piezoelectric as well as the spontaneous polarizations are expected to be present and to govern the optical and electrical properties of GaN based heterostructures to a certain extent, due to the huge polarization constants which are one of the most fascinating aspects of the nitrides. In this paper we will present theoretical and experimental results demonstrating how polarization induced electric fields and bound interface charges in AlGaN/GaN, InGaN/GaN and AlInN/GaN heterostructures lead to the formation of two-dimensional carrier gases suitable for the fabrication of high power microwave frequency transistors.

AlGaN/GaN HEMTs are suitable for high power applications because of their high breakdown voltages and high currents. However, the large signal performance of these devices is not as good as one would expect from the DC performance of the device. The large signal performance of the device is improved by depositing a thin layer of SiN by PECVD. In this work silicon nitride (SiN) was deposited in a variety of conditions on AlGaN/GaN HEMT structures on sapphire substrates to study the impact of the quality of SiN on the DC, breakdown, small signal and large signal performance of the device. A maximum increase in the output power from 1.3 W/mm to 2.3 W/mm at 4 GHz, 20 V bias was observed with a SiN passivation film, grown in NH/sub 3/ rich conditions and which gave the least etch rate of 25 nm/min in buffered oxide etch (6:1). This increase in power is mainly due to the increase in DC current. This is clearly shown by plotting the RF dynamic loadline for an unpassivated and passivated device.

Two dimensional hole and electron gases in wurtzite GaN/AlxGa1-xN/GaN heterostructures are induced by strong polarization induced effects. The sheet carrier concentration and the confinement of the two dimensional carrier gases located close to one of the AlGaN/GaN interfaces are sensitive to a high number of different physical properties such as polarity, alloy composition, strain, thickness and doping. We have investigated the structural quality, the carrier concentration profiles and electrical transport properties by a combination of high resolution x- ray diffraction, Hall effect and C-V profiling measurements. The investigated heterostructures with N- and Ga-face polarity were grown by metalorganic vapor phase or plasma induced molecular beam epitaxy covering a broad range of alloy compositions and barrier thickness. By comparison of theoretical and experimental results we demonstrate that the formation of two dimensional hole and electron gases in GaN/AlGaN/GaN heterostructures both rely on the difference of the polarization between the AlGaN and the GaN layer. In addition the role of polarity on the carrier accumulation at different interfaces in n- and p-doped heterostructures will be discussed in detail

We report quantitative ERD (Elastic Recoil Detection) measurements for the determination of the composition and distribution of the elements in nitride heterostructures. The investigated samples were MOCVD (molecular chemical vapor deposition) and PIMBE (plasma induced molecular beam epitaxy) grown HEMT (high electron mobility transistor) structures from Cornell University. We present the measured Al distribution parallel to the growth direction of AlxGa1-xN layers of a thickness of about 10 to 30 nm and an Al concentration of x = 0.1 to 0.7. The results are compared with HRXRD (high resolution X-ray diffraction) measurements.

Scanning electron microscopy, micro-Raman, and photoluminescence (PL) measurements are reported for Mg-doped GaN films grown on (0001) sapphire substrates by low-pressure metalorganic chemical vapor phase deposition. The surface morphology, structural, and optical properties of GaN samples with Mg concentrations ranging from 1019 to 1021 cm−3 have been studied. In the scanning micrographs large triangular pyramids are observed, probably due to stacking fault formation and three-dimensional growth. The density and size of these structures increase with the amount of magnesium incorporated in the samples. In the photoluminescence spectra, intense lines were found at 3.36 and 3.31 eV on the triangular regions, where the presence of cubic inclusions was confirmed by micro-Raman measurements. The excitation dependence and temperature behavior of these lines enable us to identify their excitonic nature. From our study we conclude that the interface region between these defects and the surrounding wurtzite GaN could be responsible for PL lines. Ana.Cros@uv.es ; cantarer@uv.es ; Gema.Martinez@uv.es

We investigate the incorporation of magnesium in gallium nitride (GaN) films deposited on (0 0 0 1) sapphire (c-Al 2 O 3 ) substrates by low-pressure metalorganic chemical vapor-phase deposition (MOCVD). The growth, structure and optical properties of GaN samples with a dopant concentration ranging from 10 17 to 10 21 cm −3 have been studied using transmission electron microscopy and micro-Raman spectroscopy. The growth rate and sample quality are strongly influenced by the amount of magnesium incorporated in the sample, with the inclusion of cubic GaN on the otherwise hexagonal material and a disruption of the two-dimensional growth for the highest dopant concentrations. The presence of the cubic phase is accompanied by the formation of large triangular islands due to a local increase of growth rate for a dopant concentration exceeding 10 20 cm −3 .

AIN thin films were grown on c-plane sapphire by metalorganic chemical vapor deposition from tritertiarybutylaluminum and ammonia at 1050°C. These films exhibit a full width at half maximum of the 002 X-ray rocking curve below 200 arcsec indicating high epitaxial quality. By measuring asymmetric reflections, a structural disorder of the lattice mainly due to edge dislocations can be observed. For further investigations, atomic force microscopy and photothermal deflection spectroscopy were performed. In order to study the effect of increasing AIN layer thickness on the optical and structural properties of GaN in an AIN/GaN heterostructure, AIN thin films with increasing thickness ranging from 0.02 to 0.36 μm were used as sublayers for the deposition of 0.75 μm thick GaN layers. Photoluminescence, micro- Raman and X-ray diffraction measurements confirm the relaxation of biaxial compressive stress in the GaN layers due to different thermal expansion coefficients by increasing AIN layer thickness. The pressure dependence of the band gap shift was determined as 24 meV/GPa for biaxial compressive stress. Our results indicate that the growth of AIN with metal organic chemical vapor deposition from tritertiarybutylaluminum and ammonia is a promising method for obtaining high quality epitaxial films.

Films of GaN and related materials can be processed by methods that invoke thermal decomposition, induced by intense illumination with a pulsed laser. At elevated temperatures, the nitride semiconductors undergo decomposition, with the effusion of nitrogen gas. We exploit this mechanism as an alternative to etching for the patterning of nitride films and for the opening of buried interfaces. Films of GaN have been etched to a depth of 1 μm in less than three seconds. This interface decomposition allows in particular the separation of nitride films from transparent growth substrates such as sapphire.

The adsorption of cesium on clean n- and p-GaN(0001)-1×1 surfaces at 150 K was investigated using x-ray photoemission spectroscopy, photoemission spectroscopy with monochromatized He I radiation ultraviolet photoelectron spectroscopy (UPS) and a Kelvin probe (contact potential difference, CPD). The CPD measurements gave work functions of 3.88±0.15 and 3.6±0.15 eV for clean n- and p-GaN(0001) surfaces, respectively. The widths of UPS energy distribution curves yield an ionization energy of 6.8±0.15 eV. Thus, depletion and inversion layers exist at clean surfaces of n- and p-GaN(0001) surfaces, respectively. As a function of Cs coverage, the work function displays the well-known behavior in that it first decreases, passes through a minimum, and eventually reaches a value of 2.1 eV, the work function of metallic cesium. In the submonolayer coverage regime, the ionization energy decreases by 2.3±0.15 eV. At clean p-GaN(0001) surfaces the vacuum level lies by only 0.3 eV above the conduction-band minimum in th...

High quality Ga-face and N-face AlGaN/GaN based heterostructures have been grown by plasma induced molecular beam epitaxy. By using Ga-face material we are able to fabricate conventional heterojunction field effect transistors. Because the N-face material confines electrons at a different heterojunction, the resulting transistors are called inverted. The Ga-face structures use a high temperature AIN nucleation layer to establish the polarity. Structures from these materials, relying only on polarization induced interface charge effects to create the two-dimensional electron gases, are used to confirm the polarity of the material as well as test the electrical properties of the layers. The resulting sheet concentrations of the two dimensional electron gases agree very well with the piezoelectric theory for this materials system. Hall mobilities of the two-dimensional gases for the N-face structures are as high as 1150 cm2/Vs and 3440 cm2/Vs for 300 K and 77 K respectively, while the Ga-face structures yield room temperature mobilities of 1190 cm2/Vs. Both structures were then fabricated into transistors and characterized. The inverted transistors, which were fabricated from the N-face material, yielded a maximum transconductance of 130 mS/mm and a current density of 905 mA/mm. Microwave measurements gave an ft of 7 GHz and an fmax of 12 GHz for a gate length of 1 μm. The normal transistors, fabricated from the Ga-face material, produced a maximum transconductance of 247 mS/mm and a current density of 938 mA/mm. Microwave measurements gave an ft of 50 GHz and an fmax of 97 GHz for a gate length of 0.25 μm. This shows that using plasma induced molecular beam epitaxy N-face and Ga(A1)-face AlGaN/GaN heterostructures can be grown with structural and electrical properties very suitable for high power field effect transistors.

We present results on the thermal stability as well as the thermally induced hydrogen, hydrocarbon, and nitrogen–hydrogen effusion from thin films of Group III nitrides prepared by low‐pressure chemical vapor deposition from organometallic precursors. We have deposited amorphous, polycrystalline, and epitaxial InN, GaN, and AIN films on (0001) Al2O3 substrates using the chemical reaction of azido[bis(3‐dimethylamino)propyl]indium, triethylgallium, and tritertiarybutylaluminium with ammonia. The substrate temperature was varied between 400 °C and 1100 °C. The elemental composition, in particular its dependence on the growth temperature, was investigated by elastic recoil detection analysis (ERDA). The influence of growth rate and crystallite size on the concentration of surface adsorbed hydrocarbons and carbon oxides is determined by a combination of ERDA and thermal desorption measurements. In addition, the stability of and the nitrogen flux from the InN, GaN, and AIN surfaces was determined by x‐ray diffraction and thermal decomposition experiments.

Nominally undoped GaN/Al x Ga1-x N/GaN high electron mobility transistors were grown by plasma-induced molecular beam epitaxy in order to study the formation and electric transport properties of two-dimensional electron gases. By depositing an AlN nucleation layer on sapphire substrates before the growth of the GaN buffer layer, we were able to change the polarity of the wurtzite films from N- to Ga-face. The change in polarity causes a change in the sign of the spontaneous and piezoelectric polarization directed along the c-axis of the strained AlGaN barrier. The sign and the gradient in polarization at one of the GaN/AlGaN interfaces is mainly responsible for the generation and confinement of the two-dimensional electron gas. Ga- and N-face heterostructures with mobilities up to 1050 and 1200 cm2/Vs, respectively, and sheet carrier concentrations of up to 1.2×1013 cm-2 at room temperature were realized. Transistors processed from heterostructures with both polarities show maximum source-drain currents between 800 and 850 mA/mm and a transconductance of up to 250 mS/mm.