D. J. Wolford

Pressure dependence of GaAs/AlxGa1−xAs quantum‐well bound states: The determination of valence‐band offsets

We report experiment and theory on the pressure dependence of quantum‐well bound states formed in the GaAs/AlxGa1−xAs heterostructure system. Using MQW’s and SL’s of various barrier compositions x, we trace in photoluminescence (8 K), and in full‐scale pseudopotential calculation, the pressure‐induced evolution of the lowest spatially confined states within the wells. With increasing pressure Γ‐confined states follow the shift to higher energies of the direct GaAs band gap. At critical pressures a crossing occurs between these Γ bound states and the barrier indirect X states. Here, Γ intensities plunge and new emission tracking the X edges appears. Confirmed in wave function calculation, these new transitions occur across the heterointerface, between X‐confined electrons within the AlxGa1−xAs and Γ‐confined holes within the GaAs. Arising from valence‐offset‐induced staggered band alignment, critical pressures for observation of these states decrease with increasing Al mole fraction. We thus obtain, with p...

Dependence of the AlxGa1−xAs band edge on alloy composition based on the absolute measurement of x

The absolute determination of the Al concentration, x, in epitaxial layers of AlxGa1−xAs was carried out using a nuclear reaction technique. This technique utilizes the narrow resonances found in the 27Al(u2009p,γ)Si28 reaction, together with Rutherford backscattering measurements, to obtain accurate values of the alloy composition. The AlxGa1−xAs band edge was measured on these samples through low‐temperature photoluminescence (2 K) measurements. An improved value of the direct edge (Γ) on composition was determined to be EΓg =1.512 +1.455x(eV) within a ±0.3% limit. The direct‐to‐indirect transition was found to occur at an Al concentration of x≂0.37±0.015, lower than previously reported for He temperatures.

Intrinsic recombination and interface characterization in ‘‘surface‐free’’ GaAs structures

We have conducted a thorough photoluminescence (PL) and PL time‐decay study of the interfacial passivating effects of metalorganic chemical vapor deposition prepared Al0.3Ga0.7As, n+ ‐doped GaAs, and Na2S surface barriers on epitaxial, high‐purity (n−) GaAs structures. We observe 300‐K radiative lifetimes, in such 10‐μm structures, of 2.5 μs, 800 ns, and 150 ns, respectively. These are to be compared with lifetimes of ∼2 ns for a single, ‘‘bare’’ GaAs surface with an Al0.3Ga0.7As rear surface barrier, and ∼0.5 ns for unpassivated epitaxial GaAs. Accompanying radiative efficiencies are 103–104 higher in all of these epibased structures, and 102 higher for Na2S, than for corresponding bare GaAs surfaces. Further, from detailed PL lifetime studies versus GaAs thickness, we find the lowest interfacial recombination velocities reported for any GaAs/AlxGa1−xAs structure, to date, of ≲40 cm/s, and, correspondingly, 0–1800 cm/s for n+/n−/n+ all‐GaAs homostructures. Thus, virtually ‘‘surface‐free’’ structures are ...

Photoexcited carrier lifetimes and spatial transport in surface‐free GaAs homostructures

We show that both the radiative efficiencies and lifetimes of photoexcited carriers in epitaxial GaAs may be enhanced by 3–4 orders of magnitude by the preparation of n+, doped layers at surface and substrate interfaces. Samples were prepared by organometallic vapor phase epitaxy, with n‐region thicknesses of 3–10 μm, and narrow layers Si‐doped to n+ concentrations of 5×1018 cm−3. Time‐resolved luminescence in such structures, under both surface and bulk (near‐band‐edge) excitation conditions, reveal near‐edge‐excitonic or band‐to‐band‐dominated recombination spectra, with carrier lifetimes ranging from 1.5 ns at 1.5 K to nearly 1 μs at room temperature. This is in contrast to the subnanosecond lifetimes typical in conventionally prepared GaAs, but is comparable to the best reported for high‐purity liquid phase epitaxy prepared GaAs/AlxGa1−xAs double heterostructures. The spatial distributions of photoexcited carriers in these structures are observed to expand by over an order of magnitude during their 1 ...

Minority‐carrier recombination kinetics and transport in ‘‘surface‐free’’ GaAs/AlxGa1−xAs double heterostructures

We have measured room‐temperature band‐to‐band recombination decay kinetics in superior quality GaAs heterostructures, and have observed the longest lifetime (2.5 μs) observed for any GaAs/AlxGa1−xAs structure to date. Additionally, using a novel time‐resolved optical photoluminescence imagining technique, analogous to the Haynes–Shockley experiment, we have also measured room‐temperature minority‐carrier transport in this series of ‘‘surface‐free’’ GaAs/Al0.3Ga0.7As double heterostructures, measurements only possible in high‐quality samples with long lifetimes and intense photoluminescence. We find the transport to be diffusive with diffusion lengths of ≳100 μm. Further, we find, for thick structures, minority‐carrier transport is hole‐dominated ambipolar diffusion, as expected for high‐purity n‐type material. However, for thinner structures, we find that the minority‐carrier transport is time dependent, changing from ambipolar diffusion at early times, as in thick structures, to electron‐dominated diffu...

Interface states and the transport of two-dimensional interface excitons in AlGaAs/GaAs structures

Using an unique combination of time and spatially resolved all optical photoluminescence measurement techniques, we have studied the properties of intrinsic excitons localized in quantum interface potentials in symmetric GaAs/Al0.3Ga0.7As double heterostructures. We find the transport of these interface excitons to be extremely sensitive to the time‐dependent, spatially varying conduction, and valence band potentials near the heterointerface. The observed dynamics have been modeled, relying heavily on our previous kinetic results, which proved conclusively that H‐band emission in our samples arises from the radiative recombination of quasi‐two‐dimensional excitons localized at the heterointerfaces, and find a simple electrostatic model to be in excellent quantitative agreement with experiment.

Comparison of transport, recombination, and interfacial quality in molecular beam epitaxy and organometallic vapor‐phase epitaxy GaAs/AlxGa1−xAs structures

We have studied free‐carrier recombination and transport in GaAs structures prepared by different epitaxial growth techniques and with different ‘‘surface barriers’’ including molecular beam epitaxy (MBE) and organometallic vapor‐phase epitaxy (OMVPE) prepared undoped, symmetric GaAs/Al0.3Ga0.7As double heterostructures and these same structures after etch removing the top Al0.3Ga0.7As layer and repassivating with Na2S. We find 300‐K lifetimes of ≥2.5 μs (350 ns), and interface recombination velocities of 40 cm/s (250 cm/s) for our OMVPE (MBE) structures. Identical measurements for Na2S and bare surfaces yield interface recombination velocities of 5500 cm/s and 34u2009000 cm/s, respectively. Free‐carrier transport in both types of structures is diffusive with hole mobilities of ∼350 cm2/Vu2009s.

Long‐range, minority‐carrier transport in high quality ‘‘surface‐free’’ GaAs/AlGaAs double heterostructures

G. D. Gilliland, D. J. Wolford, T. F. Kuech, and J. A. Bradley Citation: Applied Physics Letters 59, 216 (1991); doi: 10.1063/1.105970 View online: http://dx.doi.org/10.1063/1.105970 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/59/2?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Temperature dependence of the minority carrier lifetime in GaAs/AlGaAs double heterostructures J. Appl. Phys. 78, 4808 (1995); 10.1063/1.360740 Minoritycarrier recombination kinetics and transport in ‘‘surfacefree’’ GaAs/Al x Ga1x As doubleheterostructures J. Appl. Phys. 73, 8386 (1993); 10.1063/1.353407 The effects of majority carrier diffusion on the minority carrier radiative lifetime in thin layer surface freeGaAs/AlGaAs structures J. Appl. Phys. 70, 5404 (1991); 10.1063/1.350197 Effects of prelayers on minoritycarrier lifetime in GaAs/AlGaAs double heterostructures grown by molecularbeam epitaxy Appl. Phys. Lett. 45, 1227 (1984); 10.1063/1.95107 Minoritycarrier lifetimes and internal quantum efficiency of surfacefree GaAs J. Appl. Phys. 49, 6103 (1978); 10.1063/1.324530

Electronic structure of quantum-well states revealed under high pressures

Abstract We report on electronic and optical properties under pressure of GaAs Al x Ga 1− x As multi-quantum-wells and superlattices versus well-width and composition x. Photoluminescence measurements are used together with full-scale pseudopotential simulation of electronic structure. Λ 1e -Λ 1hh transition intensity thresholds mark level degeneracy with the Al x Ga 1−x As X-band edge, shifted in the heterostructures by valence-band offset-induced staggered band alignment. In SLs, indirect-gap spatially quantized electron states formed within the subsidiary X bands are observed experimentally and modelled theoretically. These new X-derived states are located within the Al x Ga 1−x As and optical transitions occur across both k-space and the hetero-interface. We thus obtain direct optical measure of the GaAs Al x Ga 1− x As band offsets, giving Δ E v ∼- (0.32 ± 0.02) Δ E g Γ across the alloy system. Intervalley “mixing” connecting the quantized electron states of differing k-value is also explored, as crossings between them are induced under pressure. Energy levels, transition energies and intensities, radiative lifetimes, level perturbations (anticrossings), and oscillator strengths have been obtained with good agreement between experiment and theory. We show that coupling between the familiar zone-center quantum-well states and the new zone-edge states is significant and observable, and must be taken into account for full description of quantum-well states in multi-valley semiconductors.

Measurement of absolute Al concentration in AlxGa1−xAs

A technique has been developed to measure the absolute Al concentration x in AlxGa1-xAs. The technique involves simultaneous measurements of the 27Al(p, γ)28Si resonant nuclear reaction and Rutherford backscattering (RBS), and comparisons with an AlAs reference sample. A detailed description of the experimental procedure is given. Samples analyzed in this study were prepared by LPE and MOVPE growth on GaAs substrates, with epitaxial layer thicknesses in the range 1–3 μm and Al concentrations in the range 0.10