Showing papers by "Stacia Keller published in 2003"

Polarization effects in AlGaN/GaN and GaN/AlGaN/GaN heterostructures

Abstract: The influence of AlGaN and GaN cap layer thickness on Hall sheet carrier density and mobility was investigated for Al0.32Ga0.68N/GaN and GaN/Al0.32Ga0.68N/GaN heterostructures deposited on sapphire substrates. The sheet carrier density was found to increase and saturate with the AlGaN layer thickness, while for the GaN-capped structures it decreased and saturated with the GaN cap layer thickness. A relatively close fit was achieved between the measured data and two-dimensional electron gas densities predicted from simulations of the band diagrams. The simulations also indicated the presence of a two-dimensional hole gas at the upper interface of GaN/AlGaN/GaN structures with sufficiently thick GaN cap layers. A surface Fermi-level pinning position of 1.7 eV for AlGaN and 0.9–1.0 eV for GaN, and an interface polarization charge density of 1.6×1013–1.7×1013 cm−2, were extracted from the simulations.

Memory Effect and Redistribution of Mg into Sequentially Regrown GaN Layer by Metalorganic Chemical Vapor Deposition

Abstract: Mg redistribution into a subsequently regrown GaN epilayer by metalorganic chemical vapor deposition (MOCVD) is studied. Dopant profiles from secondary ion mass spectrometry (SIMS) on n–p–n GaN samples have been analyzed. The regrowth study in a Mg-free reactor reveals that a Mg-rich film is present on MOCVD as-grown GaN:Mg base layers and can be removed by an acid etch, and that a slow Mg decay into the sequentially regrown GaN results from this Mg-rich surface film. We believe the commonly seen Mg memory effect in MOCVD causes the accumulation of Mg on the surface. From a MOCVD regrowth on n–p–n GaN grown by molecular beam epitaxy (MBE), the Mg diffusion constant is calculated to be about 3 ×10-15 cm2/s at 1160°C for Mg concentrations between 5 ×1017 cm-3 and 1 ×1019 cm-3. The roles of memory effect, surface segregation, and diffusion associated with Mg are addressed.

High conductivity modulation doped AlGaN/GaN multiple channel heterostructures

Abstract: A methodology for the design of modulation doped AlGaN/GaN multiple channel heterostructures is presented. Doping is utilized in conjunction with polarization effects to achieve high carrier mobility and high sheet carrier density in each channel, while maintaining a low energy barrier for majority carrier transfer between channels. Several eight-period Si-doped Al0.22Ga0.78N/GaN heterostructures were grown by metalorganic chemical vapor deposition, according to the methodology. Sheet electron densities around 7.7×1013 cm−2, and room temperature electron mobilities as high as 1200 cm2/V s were measured. Applications for the structure include lateral current spreading layers in III-nitride visible light and UV emitters and detectors, and high conductance source and drain access regions in AlGaN/GaN high electron mobility transistors.

Non-planar Selective Area Growth and Characterization of GaN and AlGaN

Abstract: Non-planar selective area growth of GaN and Al0.1Ga0.9N was performed by metalorganic chemical vapor deposition, on selectively etched GaN samples masked with SiO2 and AlN. Simultaneous growth on bottom surface and etched side walls occurred, resulting features extending in height above the etched mesa near the mask edge. For GaN, a significant mask-diffusion enhanced growth rate was observed, while for Al0.1Ga0.9N the enhancement was insignificant. The Al composition was investigated near the mask edge, with spectrally resolved cathodoluminescence microscopy, and was found to be lower within 0.6 µm of the mask edge. The electrical properties of GaN was investigated using transmission line model (TLM) structures, near and far from the mask edge. The sheet conductance was found to be greatly enhanced near the mask edge, related to enhanced n-type impurity incorporation during the non-c-plane growth in this region. The choice of mask material, SiO2 or AlN, was also shown to influence the sheet conductance.

Non-polar (Al,B,In,Ga)N quantum well and heterostructure materials and devices

Abstract: A method for forming non-polar (Al,B,In,Ga)N quantum well and heterostructure materials and devices. Non-polar (11{overscore (2)}0) a-plane GaN layers are grown on an r-plane (1{overscore (1)}02) sapphire substrate using MOCVD. These non-polar (11{overscore (2)}0) a-plane GaN layers comprise templates for producing non-polar (Al, B, In, Ga)N quantum well and heterostructure materials and devices.

Observation of huge nonlinear absorption enhancement near exciton resonance in GaN

Abstract: Huge excitonic enhancement of two-photon absorption near the exciton-transition energy was observed in the GaN system. The peak value of the nonlinear absorption coefficient is at least 1500 cm/GW, corresponding to an enhancement factor of >100. The room temperature exciton dephasing time is also obtained based on a spectral analysis to be ∼150 fs in bulk GaN, which is close to the exciton ionization time.

Stimulated emission and ultrafast carrier relaxation in AlGaN'GaN multiple quantum wells

Abstract: x50.2 and 0.3, respectively. The threshold density for SE ( I th .100 mJ/cm 2 ) was found to be independent of x. Room-temperature, time-resolved, differential transmission measurements mapped the carrier relaxation mechanisms for above barrier energy excitation. Photoexcited carriers are observed to relax into the QWs in ,1 ps, while carrier recombination times as fast as 30 ps were measured. For excitation above I th , SE is shown to deplete carriers in the barriers through a cascaded refilling of the QW state undergoing SE. Similar behavior is seen in an Al 0.3Ga0.7N/GaN MQW grown with a N-rich atmosphere, but the relaxation phenomena of all AlGaN MQWs are significantly faster than observed in InGaN MQWs of similar structure. © 2003 American Institute of Physics. @DOI: 10.1063/1.1581385#

Oxygen doping of c‐plane GaN by metalorganic chemical vapor deposition

Abstract: Oxygen doping of c-plane GaN was investigated by metalorganic chemical vapor deposition (MOCVD), using O2 as the oxygen precursor The oxygen incorporation into GaN was strongly affected by the surface orientation of the GaN film At an O2 partial pressure of 10 Pa, growth on a smooth (0001) GaN surface resulted in an oxygen concentration of 8 × 1016 cm−3 However, the oxygen incorporation increased drastically to concentrations up to 3 × 1019 cm−3, when the growth was performed on rough surfaces containing other crystal planes than (0001) GaN Such surfaces include (0001) GaN with hexagonal pits, or the hexagonal islands formed during the initial stage of growth of GaN films on sapphire substrate Furthermore, above a certain critical O2 partial pressure, which depended on the growth temperature, oxygen itself caused a perturbation of the smooth GaN growth on (0001) surfaces, leading to the formation of hexagonal pits during growth The highest oxygen concentration attained during smooth growth was 35 × 1017 cm−3 The results imply, that oxygen is not a suitable dopant for the growth of uniform n+ (0001) GaN layers (© 2003 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim)

NON-POLAR (A1,B,In,Ga) QUANTUM WELL AND HETEROSTRUCTURE MATERIALS AND DEVICES

Abstract: A method for forming non-polar (A1,B,In,Ga)N quantum well and heterostructure materials and devices. Non-polar (1120) a-plane GaN layers are grown on an r-plane (1102) sapphire substrate using MOCVD. These non-polar (1120) a-plane GaN layers comprise templates for producing non-polar (A1,B,In,Ga)N quantum well and heterostructure materials and devices.