Adam Wood

Increased bismuth concentration in MBE GaAs1−xBix films by oscillating III/V flux ratio during growth

The authors have examined bismuth concentration profiles in GaAs1−xBix films grown by molecular beam epitaxy using high angle annular dark field imaging (Z-contrast imaging) in an aberration-corrected scanning transmission electron microscope in conjunction with x-ray diffraction. Samples were grown with a gradient in each of the component fluxes, and therefore, the III/V ratio across the substrate. Rotating the sample during growth exposed the growth surface to an oscillating III/V flux ratio. Sinusoidal [Bi] profiles resulted in the growth direction, the wavelength and number of which were consistent with the growth rate and the rate of substrate rotation. However, the magnitude of [Bi] in the observed fluctuations was greater than the maximum [Bi] achieved using the same Bi flux and Ga/As flux ratios in steady-state conditions on a stationary substrate, suggesting that varying the III/V flux ratio during growth promotes the incorporation of Bi in GaAs1−xBix films. A proposed qualitative model for how t...

Unexpected bismuth concentration profiles in metal-organic vapor phase epitaxy-grown Ga(As1−xBix)/GaAs superlattices revealed by Z-contrast scanning transmission electron microscopy imaging

A set of GaAs1−xBix/GaAs multilayer quantum-well structures was deposited by metal-organic vapor phase epitaxy at 390 °C and 420 °C. The precursor fluxes were introduced with the intent of growing discrete and compositionally uniform GaAs1−xBix well and GaAs barrier layers in the epitaxial films. High-resolution high-angle annular-dark-field (or “Z-contrast”) scanning transmission electron microscopy imaging revealed concentration profiles that were periodic in the growth direction, but far more complicated in shape than the intended square wave. The observed composition profiles could explain various reports of physical properties measurements that suggest compositional inhomogeneity in GaAs1−xBix alloys as they currently are grown.

Design and characterization of thick InxGa1-xAs metamorphic buffer layers grown by hydride vapor phase epitaxy

Thick InxGa1-xAs metamorphic buffer layers (MBLs) grown by hydride vapor phase epitaxy (HVPE) were studied. Relationships between MBL properties and growth parameters such as grading rate, cap layer thickness, final xInAs, and deposition temperature (TD) were explored. The MBLs were characterized by measurement of in-plane residual strain (???), surface etch pit density (EPD), and surface roughness. Capping layer thickness had a strong effect on strain relaxation, with thickly capped samples exhibiting the lowest ???. EPD was higher in samples with thicker caps, reflecting their increased relaxation through dislocation generation. ??? and EPD were weakly affected by the grading rate, making capping layer thickness the primary structural parameter which controls these properties. MBLs graded in discrete steps had similar properties to MBLs with continuous grading. In samples with identical thickness and 10-step grading style, ??? increased almost linearly with final xInAs, while total relaxation stayed relatively constant. Relaxation as a function of xInAs?could be described by an equilibrium model in which dislocation nucleation is impeded by the energy of the existing dislocation array. EPD was constant from xInAs?= 0 to 0.24 then increased exponentially, which is related to the increased dislocation interaction and blocking seen at higher dislocation densities. RMS roughness increased with xInAs?above a certain strain rate (0.15%/?m); samples grown below this level possessed large surface hillocks and high roughness values. The elimination of hillocks at higher values of xInAs?is attributed to increased density of surface steps and is related to the out-of-plane component of the burgers vector of the dominant type of 60? dislocation. TD?did not affect ??? for samples with a given xInAs. EPD tended to increase with TD, indicating dislocation glide likely is impeded at higher temperatures.

Impact of Sb Incorporation on MOVPE-Grown “Bulk” InGaAs(Sb)N Films for Solar Cell Application

We have investigated the impacts of Sb incorporation on the microstructural, optical, electrical, and carrier dynamics properties of bulk InGaAsSbN films in a comparative study of InGaAsN and InGaAsSbN materials grown by metal-organic vapor phase epitaxy (MOVPE). These films were grown at the relatively high temperature of 600 °C and annealed at 800 °C for 30 min. Transmission electron microscopy studies indicate compositional and structural homogeneity of the InGaAsN and InGaAsSbN films. Low-temperature time-resolved photoluminescence measurements of the MOVPE-grown InGaAsN film show a longer minority carrier lifetime (~40 ns) than observed for the InGaAsSbN film (~26 - 27 ns). In addition, single-junction solar cells with an InGaAsN (InGaAsSbN) base layer exhibit an open-circuit voltage of 0.64 (0.58) V, a short-circuit current of 17.13 (16.89) mA/cm2, a fill factor (FF) of 77.55 (74.29)%, and an efficiency of 8.57 (7.31)%. Sb incorporation in InGaAsN adversely affects solar cell performance due to a reduced minority carrier lifetime correlated with the formation of defects and narrow depletion region width resulting from a higher background carbon impurity level.

Quantum-cascade-laser active regions on metamorphic buffer layers

Strained-layer superlattice (SL) structures have been grown by metalorganic vapor phase epitaxy (MOVPE) on metamorphic buffer layers (MBLs) for application in intersubband-transition devices, such as quantum cascade lasers. Using the MBL as an adjustable lattice-parameter platform, we have designed relatively-low-strain quantum-cascade-laser structures that will emit in the 3.0-3.5 μm wavelength range while suppressing carrier leakage from the upper laser level. Thick (10-12 μm) compositionally-graded, hydride-vapor-phase-epitaxy (HVPE)-grown MBL structures are employed. To improve the planarity of the MBL surface, we employ chemical mechanical polishing (CMP) followed by wet chemical etching prior to the growth of the SL/device structures. We find that the wet-chemical etching step is crucial to remove residual damage introduced during CMP. 20-period InxGa1-xAs (wells)/AlyIn1-yAs (barriers) SLs grown on the MBLs are characterized by x-ray diffraction (XRD). Intersubband electroluminescence emission is observed in the 3.5 μm wavelength range from devices employing such SL structures.