The development of new chemically based growth techniques has opened the range of possible GaN applications. This paper reviews some of the challenges in the chemically based growth of GaN and related materials. Ammonothermal-based growth, hydride vapor phase epitaxy and metal organic vapor phase epitaxy (MOVPE) are chemically complex systems wherein the underlying mechanisms of growth are not well understood at present. All these systems require substantial experimental and theoretical efforts to determine the nature and kinetics of GaN growth. In the case of metal organic vapor phase epitaxy, the application of computational techniques based on density functional theory have augmented the more conventional experimental approaches to determining the growth chemistry. These chemical reaction schemes, when combined with computational thermal-fluid models of the reactor environment, provide the opportunity to predict growth rates, uniformity and eve ntually materials properties.

We report on the effect of strain induced polarization fields in AlGaN/GaN heterostructures due to the incorporation of Si dopant ions in the lattice. By Si-doping (Al)GaN, a contraction of the wurtzite unit cell can occur leading to strain in doped AlGaN/GaN heterostructures such as high electron mobility transistors (HEMTs). In a typical modulation doped AlGaN/GaN HEMT structure, the Si-doped AlGaN supply layer is separated from the two-dimensional electron gas channel by an undoped AlGaN spacer layer. This dopant-induced strain, which is tensile, can create an additional source of charge at the AlGaN:Si/AlGaN interface. The magnitude of this strain increases as the Si doping concentration increases and the AlN mole fraction in the AlGaN decreases. Consideration of this strain should be given in AlGaN/GaN HEMT structure design.

We report on the effect of strain induced polarization fields in AlGaN/GaN heterostructures due to the incorporation of Si dopant ions in the lattice. By Si-doping (Al)GaN, a contraction of the wurtzite unit cell can occur leading to strain in doped AlGaN/GaN heterostructures such as high electron mobility transistors (HEMTs). In a typical modulation doped AlGaN/GaN HEMT structure, the Si-doped AlGaN supply layer is separated from the two-dimensional electron gas channel by an undoped AlGaN spacer layer. This dopant-induced strain, which is tensile, can create an additional source of charge at the AlGaN:Si/AlGaN interface. The magnitude of this strain increases as the Si doping concentration increases and the AlN mole fraction in the AlGaN decreases. Consideration of this strain should be given in AlGaN/GaN HEMT structure design.

Edge emission characteristics of optically pumped GaN-AlGaN double heterostructures and quantum wells are examined. The samples, which were grown by metalorganic vapor phase epitaxy, are photoexcited with light from a pulsed nitrogen laser. The pump light is focused to a narrow stripe on the sample surface, oriented perpendicular to a cleaved edge, and the edge luminescence is collected and analyzed. We first compare emission characteristics of highly excited GaN-AlGaN double heterostructures grown simultaneously on SiC and sapphire substrates. Polarization resolved spectral properties of edge luminescence from both structures is studied as a function of pump intensity and excitation stripe length. Characteristics indicative of stimulated emission are observed, particularly in the sample grown on SiC. We then present results demonstrating laser emission from a GaN-AlGaN separate-confinement quantum-well heterostructure. At high pump intensities, band edge emission from the quantum well exhibits five narrow (∼1 Å) modes which are evenly spaced by 10Å to within the resolution of the spectrometer. This represents the first demonstration of laser action in a GaN-based quantum-well structure.

A model for the growth of gallium nitride in a vertical metalorganic vapor phase epitaxy reactor is presented. For a mixture of non-dilute gases, the flow temperature and concentration profiles are predicted. The results show that the growth of GaN epilayers is through an intermediate adduct of TMG and ammonia. Growth rates are predicted based on simple reaction mechanisms and compared with those obtained experimentally. Loss of adduct species due to polymerization leads to lowering in growth rate. An attempt to quantify loss of reacting species is made based on experimentally observed growth rates.

We have fabricated AlxGa1−xN/GaN heterostructures with high two-dimensional electron gas (2DEG) mobilities and high sheet carrier densities by metalorganic vapor phase epitaxy (MOVPE). The 2DEG sheet density and mobility exhibit a compositional dependence on the Al fraction of the electron donor layer. The highest mobility (5750 cm2/Vs at 16K) was measured in a sample with x=0.15 that had a sheet carrier density of 8.5×1012 cm−2. The undoped AlxGa1−xN layers have low background carrier concentrations and can be intentionally doped n-type using SiH4. The effect of intentional n-type doping of the AlxGa1−xN donor layer on the electrical properties of the 2DEG was studied in structures that included an undoped AlxGa1−xN spacer layer of varying thickness. Higher 2DEG mobilities were obtained when a 100Å thick undoped layer was included in the structure due to spatial separation of the 2DEG from ionized impurities in the doped AlxGa1−xN. These initial results demonstrate that the electrical properties of AlxGa1−xN/GaN heterostructures can be controlled by intentional doping and appropriate layer design.

Impurities at heterointerfaces can alter the interfacial structure resulting in changes in physical, electrical and optical properties. We present a study of the interfacial roughness of GaAs/A1xGa1-xAs superlattices which were grown using controlled addition of oxygen at the interface. The interfacial properties were characterized by x-ray diffraction. The morphology of the surface was determined by Atomic Force Microscopy (AFM). X-ray diffraction measurements, both θ-2θ and rocking curves, were used to analyze the correlated and uncorrelated component of the interfacial roughness. A strong difference in the interfacial roughness was observed depending on whether the intentional oxygen incorporation occurred at the GaAs-to-A1GaAs interface or at both interfaces. When oxygen is incorporated at both interfaces, the x- ray reflectivity of the superlattice is decreased considerably resulting from a much higher interfacial roughness. The substrate miscut has a significant effect on RMS roughness, correlated roughness and its correlation length when oxygen is incorporated at the GaAs-to-A1xGa1-xAs interface.

Atomic force microscopy (AFM) has been used to investigate the atomic-scale mechanisms of growth of GaAs by metal organic vapor phase epitaxy (MOVPE). The influence of impurities such as silicon, oxygen and carbon on the small-scale periodic structure as well as on the large-scale features has been studied. The growth front of GaAs epitaxial films grown on vicinal GaAs (100) substrates exhibits periodic structure except for the Si doped GaAs grown on semi-insulating vicinal GaAs (100) substrates. The periodicity on the surface breaks down when oxygen, silicon or carbon concentration exceeds 1018cm−3. These impurities may preferentially attach at the step edges resulting in reduction of the mobility of the steps. At higher impurity concentrations, the motion of the growth front gets pinned on the surface resulting in a disruption of the step flow mode of growth. The wide terraces on Si doped GaAs grown on semi-insulating substrate is proposed as the kinetic limited step bunching during the step flow mode of growth.

MOVPE growth experiments have been used to evaluate the role of gas phase and surface chemistry in the silicon doping of GaAs films grown using SiH4 and Si2H6 dopant sources and tertiarybutylarsine (t-C4H9AsH2 or TBAs) as the Group V source. The use of TBAs with SiH4 results in a greater Si doping efficiency and weaker dependence on growth temperature than is typically observed with AsH3. Gas phase pyrolysis studies combined with reactor residence time experiments indicate that heterogeneous chemistry is responsible for this enhanced Si doping process. TBAs has much less of an effect on Si incorporation from Si2H6, a doping process controlled by homogeneous Si2H6 chemistry. Increased Si doping from Si2H6 with TBAs can be attributed to contributions from the enhanced doping efficiency of SiH4, a product of Si2H6 decomposition. The influence of TBAs on SiH4 doping chemistry was found to improve the doping uniformity of GaAs films grown in our reactor, from +/-12% for films grown with AsH3to +/-5% for TBAs-based material.