Nitride-based device structures for electronic and optoelectronic applications usually incor-porate layers of AlxGa1−xN, and n- and p-type doping of these alloys is typically required. Experimental results indicate that doping efficiencies in AlxGa1−xN are lower than in GaN. We address the cause of these doping difficulties, based on results from first-principles density-functional-pseudopotential calculations. For n-type doping we will discuss doping with oxygen, the most common unintentional donor, and with silicon. For oxygen, a DX transition occurs which converts the shallow donor into a negatively charged deep level. We present experimental evidence that oxygen is a DX center in AlxGa1−xN for x>∼0.3. For p-type doping, we find that compensation by nitrogen vacancies becomes increasingly important as the Al content is in-creased. We also find that the ionization energy of the Mg acceptor increases with alloy composition x. To address the limitations on p-type doping we have performed a comprehensive investigation of alternative acceptor impurities; none of the candidates exhibits characteristics that surpass those of Mg in all respects.
Two-dimensional electron gas transport properties have been investigated in nitride double-heterostructures. A striking effect has been observed that the two-dimensional electron gas mobility has been drastically enhanced in the AlGaN/GaN/AlGaN double-heterostructure, compared with that in the conventional AlGaN/GaN single-heterostructure. The observed mobility enhancement has been shown to be mainly due to the enhanced polarization-induced electron confinement in the double-heterostructure, and additionally due to the improvement of the interface roughness in the structure. Device operation of an AlGaN/GaN/AlGaN double-heterostructure field effect transistor has been demonstrated: a maximum transconductance of 180 mS/mm has been obtained for a 0.4 μm-gate-length device. In the double-heterostructure using InGaN channel, the increased capacity for the two-dimensional electron gas has been observed. The AlGaN/(In)GaN/AlGaN double-heterostructures are effective for improving the electron transport properties.
The deep level spectra in both p+-n homojunction and n-type Schottky GaN diodes are studied by deep level transient spectroscopy (DLTS) in order to compare the role of the junction configuration on the defects found within the n-GaN layer. Both majority and minority carrier DLTS measurements are performed on the diodes allowing the observation of both electron and hole traps in n-GaN. An electron level at Ec−Et=0.58 and 0.62 V is observed in the p+-n and Schottky diodes, respectively, with a concentration of ∼3−4×1014 cm−3 and a capture cross section of ∼1−5×10−15 cm2. The similar Arrhenius behavior indicates that both emissions are related to the same defect. The shift in activation energy is correlated to the electric field enhanced-emission in the p+-n diode, where the junction barrier is much larger. The p+-n diode configuration allows the observation of a hole trap at Et−Ev=0.87 eV in the n-GaN which is very likely related to the yellow luminescence band.
Behavior of the photoluminescence band at about 2.8 eV in heavily Mg doped GaN has been studied at different temperatures and excitation intensities. The 2.8 eV band is attributed to donor-acceptor transitions involving a Mg acceptor. The large blue shift of the band with increasing excitation intensity is explained by variation in the contribution of close and distant pairs to the luminescence. The red shift of the band with increasing temperature under high excitation intensity conditions results from thermal release of carriers from close pairs. The thermal activation energy of the deep donor, about 0.4 eV, is determined from the quenching of the 2.8 eV luminescence band at high temperatures.
High quality GaN films on AlN buffer layers were grown on Si(111) with a new, commercial, two-injector vertical rotating disk MOCVD reactor (CVD, Inc.). It was found that the geometry of the susceptor greatly affected the structural quality of the epilayers on Si. For the original susceptor geometry, though single crystal GaN films could be obtained, the films were dark gray in appearance with a rough morphology, and the best x-ray rocking curve FWHM was 2.33°. After modifying the susceptor geometry, transparent, mirror-like single crystal GaN films were obtained with the best x-ray rocking curve FWHM being 0.24°. Photoluminescence (PL) and infrared reflectance (IR) spectra of the grown films were compared. The film growth rate was found to increase with decrease of the growth pressure. A 2-D simulation of the flow, heat transfer, and chemical species transport in the reactor showed a more symmetric flow, larger velocity gradient, and lower upward velocity with the modified susceptor, which may be the main reason for the improvement of the structural quality of the films.
The behavior of H in p-GaN(Mg) at temperatures >400°C is modeled by using energies and vibration frequencies from density-functional theory to parameterize transport and reaction equations. Predictions agree semiquantitatively with experiment for the solubility, uptake, and release of the H when account is taken of a surface barrier.
The low-frequency noise in GaN-based Metal-Oxide-Semiconductor Heterostructure Field Effect Transistors (MOS-HFETs) and HFETs on sapphire and n-SiC substrates were studied. Hooge parameter at zero gate bias was calculated about 8 × 10−4 for both types of the devices. The AlGaN/GaN MOS-HFETs exhibited extremely low gate leakage current and much lower noise at both positive and negative gate biases. These features demonstrate the high quality of the SiO2/AlGaN heterointerface and feasibility of this technology for high-power microwave transmitter and high-power, high-temperature switches.
Deep centers in Si-doped n-GaN samples grown on sapphire by reactive molecular beam epitaxy, using different ammonia flow rates (AFRs), have been studied by deep level transient spectroscopy. In addition to five electron traps, which were also found in n-GaN layers grown by both metalorganic chemical-vapor deposition and hydride vapor-phase epitaxy, two new centers C-1 (0.43-0.48 eV) and E-1 (0.25 eV) have been observed. C-1, whose parameters show strong electric-field effects and anomalous electron capture kinetics, might be associated with dislocations. E-1, which is very dependent on the AFR, exhibits an activation energy close to that of a center created by electron irradiation and is believed to be a defect complex involving V-N.
In this paper, we present a detailed study of high quality (110) ZnO films, epitaxially grown on R-plane sapphire substrates by metal-organic chemical vapor deposition (MOCVD). The epitaxial relationships are (112¯0) ZnO//(011¯2) Al2O3 and {0001} ZnO//[01¯11] A12O3 as confirmed by X-ray diffraction (θ-2θ, and φ-scan) and high-resolution cross-sectional transmission electron microscopy (HR-TEM). Low temperature photoluminescence (PL) indicates the ZnO thin films are almost strain free. Optical absorption and reflection measurements with linearly polarized light indicate a strong optical anisotropy. The polarization rotation towards the C-axis associated with the optical anisotropy is utilized to demonstrate an optically addressed ultra-fast, ultraviolet light modulator.
Low frequency noise measurements are a powerful tool for detecting deep traps in semiconductor devices and investigating trapping-recombination mechanisms. We have performed low frequency noise measurements on a number of photoconducting detectors fabricated on autodoped n-GaN films grown by ECR-MBE. At room temperature, the noise spectrum is dominated by 1/f noise and thermal noise for low resistivity material and by generation-recombination (G-R) noise for high resistivity material. Noise characteristics were measured as a function of temperature in the 80K to 300K range. At temperatures below 150K, 1/f noise is dominant and at temperatures above 150K, G-R noise is dominant. Optical excitation revealed the presence of traps not observed in the dark, at room temperature.
Ambient gas effect on the epitaxial lateral overgrowth (ELO) of GaN via metalorganic vapor-phase epitaxy (MOVPE) on a MOVPE-grown GaN (0001) / sapphire (0001) substrate with a SiO2 stripe mask has been studied by means of field-emission scanning electron microscopy (SEM) and high-resolution X-ray diffraction (XRD) analysis. Different ambient gases of nitrogen, hydrogen and their mixture (mixture ratio, hydrogen: nitrogen = 1: 1) affect the lateral overgrowth rate, the surface morphology and the crystalline tilting of ELO-GaN layers. XRD revealed that the ELO-GaN layer on the SiO2 mask aligned along the <11¯00> direction exhibited anisotropic crystalline tilting toward <112¯0>. For ELO-GaN growth in nitrogen ambient, the growth rate of the (0001) facet decreases, the lateral overgrowth rate increases and the tilting of the ELO-GaN layer increases, while no smooth surface is obtained, in comparison with ELO-GaN growth in hydrogen ambient. For the mixture ambient, a smooth surface with a fast lateral overgrowth rate is achieved and the dislocation density is not more than 107 cm-2, which is comparable to that in hydrogen ambient.
The incorporation of oxygen onto the (3×3) reconstructed surface of GaN(0001) has been studied using X-ray Photoelectron Spectroscopy (XPS). It was found that the (3×3) reconstruction corresponds to a fractional Ga adlayer atop a Ga terminated GaN surface. Our measurements indicate a surface coverage of 1.15 ± 0.2 monolayers of relaxed Ga on the surface. The binding energy separation between the relaxed surface Ga3d core level and bulk Ga3d level was measured to be 1.1 ± 0.1 eV. A metallic component extending from the bulk GaN valence band maximum out to 0 eV was also present in the XPS spectrum. The separation between the bulk valence band maximum and the Fermi level of the metallic component was found to be 2.1 ± 0.1 eV. The relaxation of the surface Ga was found to decrease with oxygen exposure indicating Ga-O bonding, with oxygen adsorption terminating at 1.3 ± 0.2 monolayers. The O1s core level was found to have a FWHM of 2.0 ± 0.1 eV.
The near-surface (400-500Å) of p-GaN exposed to high density plasmas is found to become more compensated through the introduction of shallow donors. At high ion fluxes or ion energies there can be type-conversion of this surface region. Two different methods for removal of the damaged surface were investigated; wet etching in KOH, which produced self-limiting etch depths or thermal annealing under N2 which largely restored the initial electrical properties.
A review of surface structures of bare and adsorbate-covered GaN (0001) and (000) surfaces is presented, including results for In, Mg, Si, and H adsorbates. Emphasis is given to direct determination of surface structure employing experimental techniques such as scanning tunneling microscopy, electron diffraction, and Auger electron spectroscopy, and utilizing first principles computations of the total energy of various structural models. Different surface stoichiometries are studied experimentally by varying the surface preparation conditions (e.g. Ga-rich compared to N-rich), and the stoichiometry is included in the theory by performing calculations for various chemical potentials of the constituent atoms. Based on the work reviewed here, surface reconstructions for plasma-assisted molecular beam epitaxy growth of GaN (0001) and (000) surfaces are fairly well understood, but reconstructions for reactive molecular beam epitaxy or for metal-organic vapor phase epitaxy (both involving H, at moderate and high temperatures, respectively) are less well understood at present.
Vibrational spectroscopy, photoluminescence, and optically detected electron paramagnetic resonance (ODEPR) have been used to characterize the defects produced in undoped and Sidoped GaN by the implantation of hydrogen. Several new vibrational bands were found near 3100 cm−1 in GaN that had been implanted with protons. These frequencies are close to those predicted for VGa−Hn complexes, leading to the tentative assignment of the new lines to VGa defects decorated with different numbers of H atoms. The proton implantation also produces an infrared PL band centered at 0.95 eV and the ODEPR spectrum labeled LE1, both of which were seen previously for electron-irradiated GaN.
Exposure of wurtzite GaN films grown on Si-polar 6H-SiC(0001) to magnesium during molecular beam epitaxy (MBE) has been studied. In the nitrogen rich regime of MBE growth, GaN films are known to grow with rough morphology. We observe on GaN(0001) that small doses of Mg act as a surfactant, smoothing out this roughness. An interpretation of this surfactant behavior is given in terms of electron counting arguments for the surface reconstructions. Previously, we have reported that larger doses of Mg lead to inversion of the Ga-polar GaN film to produce N-polar GaN. Several Mg-related reconstructions of the resulting GaN(000 ) surface are reported.
Fabricating device structures from the III-N wide bandgap semiconductors requires anisotropoic dry etching processes that leave smooth surfaces with stoichiometric composition after transferring high-resolution patterns with vertical sidewalls. The purpose of this article is to describe results obtained by a new low-damage dry etching technique that provides an alternative to the standard ion-enhanced dry etching methods in meeting these demands for processing the III-N materials.
We have studied crystal structure and associated defects in GaN films grown on sapphire under nitrogen-deficient conditions by metalorganic chemical vapor deposition (MOCVD) and pulsed laser deposition (PLD). The structural quality of the PLD films grown at 750 °C was comparable with those grown by MOCVD at 1050 °C having threading dislocations density of about 1010 cm−2 at a film thickness 150-200 nm. Microstructure of the PLD films grown at temperatures above 780°C was found to be similar to that of nitrogen-deficient MOCVD films indicating the loss of nitrogen due to thermal decomposition of the nitride layers. Nitrogen-deficient MOCVD and PLD films exhibit polycrystalline structure with a mixture of cubic zinc-blende and wurtzite hexagonal GaN grains retaining tetragonal bonding across the boundaries and hence the epitaxial orientations and polarity. Renucleation of the wurtzite phase at different {111} planes of cubic GaN results in a rough and faceted surface of the film. Most of the stoichiometric films displayed (0001) Ga-face polarity, but the renucleated inclined wurtzite grains grew in the opposite N-face polarity. The major defects related to the cubic structural metastability are stacking faults and microtwins which being nuclei of the metastable cubic phase have an extremely low energy. We elucidate that the cubic phase is more stable under the nitrogen deficiency and, therefore, can exist without decomposition at higher nitrogen vacancy concentrations in the material.
The characteristics of dry etching of the AlGaInN materials system in different reactor types and plasma chemistries are reviewed, along with the depth and thermal stability of etch-induced damage. The application to device processing for both electronics and photonics is also discussed.
The epitaxial growth of zinc-blende (cubic) GaN and InGaN on GaAs with a common cleavage plane and readily high-quality, low-cost wafers may be considered as an alternative approach for the future realization of cleaved laser cavities. To obtain detailed information about the potential of cubic GaN and InGaN for device applications we performed optical gain spectroscopy accompanied by time-integrated and time-dependent photoluminescence measurements at 2 K and 300 K. From intensity-dependent gain measurements, the identification of the gain processes was possible. For moderate excitation levels, the biexciton decay is likely to be responsible for a gain structure at 3.265 eV in cubic GaN [10]. For the highest pump intensities, the electron- hole-plasma is the dominant gain process, providing gain values up to 200 cm −1. Furthermore cubic GaN samples with different cavity lengths from 250 to 600 µm were cleaved to investigate the influence of the sample geometry on the gain mechanisms. In these samples increased gain values up to 150 cm −1 as well as lower threshold excitation densities were observed, indicating the potential of cubic GaN for device applications. The results of GaN will be compared with intensity-dependent gain measurements on InGaN samples, grown on GaAs with varying In-content. The observed gain mechanisms in cubic InGaN will be discussed in detail.
