E. Haus

Control of GaN surface morphologies using plasma-assisted molecular beam epitaxy

The characteristic surface morphologies of GaN grown by plasma-assisted molecular beam epitaxy under various growth conditions have been investigated. Three growth regimes (one N stable and two Ga stable) are identified on a surface structure diagram (Ga/N ratio versus substrate temperature). The boundary between the N-stable regime (low Ga/N ratios) and the two Ga-stable regimes (high Ga/N ratios) is determined by the growth rate of the films and is constant over the range of substrate temperatures investigated. The boundary between the two Ga-stable regimes (the Ga-droplet regime and the intermediate regime) is determined by the formation of Ga droplets and has an Arrhenius dependence with substrate temperature. The characteristic morphologies of films grown within each of these regimes are investigated using atomic force microscopy and transmission electron microscopy. N-stable films have rough, heavily pitted morphologies. Films grown within the intermediate phase have areas of flat surface between la...

POLARIZATION-INDUCED CHARGE AND ELECTRON MOBILITY IN ALGAN/GAN HETEROSTRUCTURES GROWN BY PLASMA-ASSISTED MOLECULAR-BEAM EPITAXY

The formation of the two-dimensional electron gas (2DEG) in unintentionally doped AlxGa1−xN/GaN (x⩽0.31) heterostructures grown by rf plasma-assisted molecular-beam epitaxy is investigated. Low-temperature electrical-transport measurements revealed that the two-dimensional electron gas density strongly depends on both the thickness of the AlGaN layer and alloy composition. The experimental results agree very well with the theoretical estimates of the polarization-induced 2DEG concentrations. Low-temperature electron mobility was found to be much higher in the structures with lower electron sheet densities. Interface roughness scattering or alloy disorder scattering are proposed to be responsible for this trend. A maximum mobility of 51 700 cm2/V s (T=13 K) was obtained in the Al0.09Ga0.91N/GaN structure with a two-dimensional electron gas density of 2.23×1012 cm−2.

High mobility two-dimensional electron gas in AlGaN/GaN heterostructures grown by plasma-assisted molecular beam epitaxy

High quality AlGaN/GaN heterostructures have been grown by radio-frequency plasma-assisted molecular beam epitaxy on n-type GaN templates grown on sapphire by metal organic chemical vapor deposition. The unintentionally doped Al0.12Ga0.88N/GaN heterostructure exhibits a 77 K Hall mobility of 14 500 cm2/Vs and a 12 K mobility of 20 000 cm2/Vs (ns=5.0×1012 cm−2). A room temperature mobility of 1860 cm2/Vs (ns=4.8×1012 cm−2) was calculated for the two-dimensional electron gas channel using a two layer model from the measured mobility for the whole structure (template plus heterostructure). Magnetoresistance measurements at 4.2 K showed well-resolved Shubnikov–de Haas oscillations, which began at 2.6 T.

Mg doping of GaN layers grown by plasma-assisted molecular-beam epitaxy

GaN:Mg layers grown by plasma-assisted molecular-beam epitaxy at 650 °C are investigated. Secondary-ion-mass-spectroscopy measurements reveal uniform Mg doping profiles with very sharp boundaries. The amount of incorporated Mg atoms changes approximately linearly with incident Mg flux. Hall measurements on p-type GaN:Mg layers show that about 1%–2% of all Mg atoms are ionized at room temperature. The hole mobility depends strongly on the hole concentration, varying from μp=24 cm2/V s for p=1.8×1017 cm−3 to μp=7.5 cm2/V s for p=1.4×1018 cm−3. GaN p–n diodes with molecular-beam-epitaxy-grown p regions are analyzed using current–voltage measurements.

Characterization of an AlGaN/GaN two-dimensional electron gas structure

An AlxGa1−xN/GaN two-dimensional electron gas structure with x=0.13 deposited by molecular beam epitaxy on a GaN layer grown by organometallic vapor phase epitaxy on a sapphire substrate was characterized. X-ray diffraction maps of asymmetric reciprocal lattice points confirmed that the thin AlGaN layer was coherently strained to the thick GaN layer. Methods for computing the aluminum mole fraction in the AlGaN layer by x-ray diffraction are discussed. Hall effect measurements gave a sheet electron concentration of 5.1×1012 cm−2 and a mobility of 1.9×104 cm2/V s at 10 K. Mobility spectrum analysis showed single-carrier transport and negligible parallel conduction at low temperatures. The sheet carrier concentrations determined from Shubnikov–de Haas magnetoresistance oscillations were in good agreement with the Hall data. The electron effective mass was determined to be 0.215±0.006 m0 based on the temperature dependence of the amplitude of Shubnikov–de Haas oscillations. The quantum lifetime was about one...

Polarity control during molecular beam epitaxy growth of Mg-doped GaN

Mg doping has been found in some situations to invert growth on Ga-face GaN to N-face. In this study, we clarified the role the Ga wetting layer plays in rf plasma molecular beam epitaxy of GaN when Mg doping, for [Mg] from ∼2×1019 to ∼1×1020 cm−3 corresponding to the useful, accessible range of hole concentrations of p∼1017–1018 cm−3. Structures were grown in the N-rich and Ga-rich growth regime for single Mg doping layers and for multilayer structures with a range of Mg concentrations. Samples were characterized in situ by reflection high-energy electron diffraction and ex situ by atomic force microscopy, transmission electron microscopy, convergent beam electron diffraction, and secondary ion mass spectroscopy. Growth on “dry” surfaces (without a Ga wetting layer) in the N-rich regime completely inverted to N-face upon exposure to Mg. No reinversion to Ga-face was detected for subsequent layers. Additionally, Mg was seen to serve as a surfactant during this N-rich growth, as has been reported by others...

The role of growth conditions on the p-doping of GaN by plasma-assisted molecular beam epitaxy

Mg-doped GaN layers were grown by plasma-assisted molecular beam epitaxy under a range of conditions. The growth temperatures and III/V ratio were altered and the resultant films were analyzed using atomic force microscopy, Hall effect measurements, and secondary ion mass spectroscopy (SIMS). The growth temperatures ranged from 600°C to 700°C. The III/V ratios were varied from N-rich (less than unity) to Ga-rich conditions (greater than unity). AFM showed that N-rich (III/V ratio 1) yielded smoother morphologies and exhibited p-type conductivity. The trend in morphology with changing III/V ratio is consistent with previous studies of the MBE growth of unintentionally doped GaN. SIMS measurements show that Mg incorporation was negligibly affected by the III/V ratio but exponentially decreased with increasing growth temperature.

Characterization of Nitrides by Electron Paramagnetic Resonance (EPR) and Optically Detected Magnetic Resonance (ODMR)

Abstract We will highlight our recent work on the properties of residual defects and dopants in GaN heteroepitaxial layers and on the nature of recombination from InGaN single quantum well (SQW) light emitting diodes (LEDs) through magnetic resonance techniques. Electron paramagnetic resonance (EPR) and optically detected magnetic resonance (ODMR) were performed on undoped (highly resistive and n-type) and intentionally doped (Si, Mg, or Be) GaN films grown by a variety of techniques (MOCVD, MBE, and HVPE) in order to obtain general trends and behavior. Through the spin-Hamiltonian parameters, these methods can reveal symmetry information, the character of the wave function and (ideally) the chemical identity of the defect. In addition, low temperature EPR intensities can be used to determine the neutral acceptor or donor concentrations without the need for contacts or the high temperatures required for Hall effect measurements. The ODMR was performed on both bandedge (mainly shallow donor–shallow acceptor recombination) and deep (visible and near-IR) PL bands. In spite of the radically different (non-equilibrium) growth techniques, many of the same defects were found in the various samples. Finally, earlier ODMR studies of recombination from Nichia InGaN ‘green’ and ‘blue’ LEDs were extended to include shorter (‘violet’) and longer (‘amber’) wavelength LEDs and an undoped 30 A In0.3Ga0.7N/GaN heterostructure. The results provide evidence for spatially separated electrons and holes in the optically-active 30 A InGaN layers under low photoexcitation conditions, likely due to localization at different potential minima in the x–y planes and/or the large strain-induced piezoelectric fields parallel to the growth direction.

Persistent photoconductivity study in a high mobility AlGaN/GaN heterostructure

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 cm2/V s at T=1.2 K, Al0.09GaN0.91/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...

Electron Transport in AlGaN/GaN Heterostructures Grown by Plasma-Assisted Molecular Beam Epitaxy

High quality AlGaN/GaN heterostructures have been grown by rf plasma-assisted molecular beam epitaxy (MBE) on n-type GaN templates grown on sapphire by metalorganic chemical vapor deposition (MOCVD). By optimizing the AlGaN thickness and the Al content, record low temperature mobilities have been achieved. Temperature and magnetic field dependent Hall effect, and Shubnikov-de Haas oscillations, were used to probe the two-dimensional electron gas (2DEG) with consistent results. The unintentionally doped Al0.09Ga0.91N heterostructures exhibit a measured 77 K Hall mobility of 24,000 cm2/Vs (nsh=2.5×1012), 12 K mobility of 52,000 cm2/Vs and ~4 K mobility of 60,000 cm2/Vs (nsh=2.25×1012), all records for this material system. The magnetic field dependent Hall effect revealed carriers from the bulk GaN freeze out and that a single carrier system is dominant below 80 K.