R. K. Bauer

Structural changes of the interface, enhanced interface incorporation of acceptors, and luminescence efficiency degradation in GaAs quantum wells grown by molecular beam epitaxy upon growth interruption

A comparison of the luminescence of GaAlAs/GaAs/GaAlAs quantum wells (QW) grown by molecular beam epitaxy with and without a 1–100 s interruption of the growth at the interfaces is presented. Well widths of 2 and 5 nm are studied as model systems. The luminescence from the noninterrupted samples consists of Gaussian shaped doublets. A line shape theory is outlined, allowing for the first time a determination of the distribution of the microscopic chemical and crystallographic disorder of the interfaces from the spectra. All samples were grown under nominally identical conditions and all show the same Gaussian distribution of the interface position. The full width at half maximum of the distribution function is 1.25 A. The interfaces in these samples are believed to have an island‐like character with a typical island size of much less than 17 nm. Interruption of the growth by a few seconds changes the luminescence line shape function qualitatively. The spectrum splits into doublets, with doublet fine struc...

Kinetics of island formation at the interfaces of AlGaAs/GaAs/AlGaAs quantum wells upon growth interruption

Abstract Surface roughness disappears and formation of islands of monolayer height occurs at GaAs surfaces upon interruption of the growth of AlGaAs/GaAs/AlGaAs quantum wells (QWs) at the respective interfaces. Both processes are directly visualized by the changes of the shape of luminescence spectra of such QWs. A detailed theory of the shape of QW luminescence which is presented here for the first time allows a quantitative determination of interface roughness and of island formation. The kinetics at and the properties of GaAs AlGaAs and AlGaAs GaAs interfaces are found to be completely inequivalent at the present growth conditions of 620° C substrate temperature and 0.28 nm/s growth rate.

Binary AlAs/GaAs versus ternary GaAlAs/GaAs interfaces: A dramatic difference of perfection

Large differences between the structural and chemical modifications binary AlAs and ternary Al0.4Ga0.6As barriers of GaAs quantum wells (QWs) upon undergoing growth interruption are found in an investigation of luminescence of type I and type II QWs. There are no increased trap incorporation but rapid smoothing and formation of interface islands larger than the diameter of type I excitons but smaller than the diameter of type II excitons is observed on the AlAs surface. Chemical lattice images obtained by high‐resolution transmission electron microscopy visualize directly the structure of the interfaces. Areas without monoatomic steps in the interface plane extending more than 0.1 μm are observed at the top surface. The bottom interfaces show structure on the scale of the exciton diameter in agreement with the luminescence results. Superimposed to the in‐plane variations of the interface positions is a grading extending over 2–4 monolayers in growth direction.

Interface roughness and charge carrier recombination lifetimes in GaInAs/InP quantum wells grown by LP-MOVPE

Abstract GaInAs/InP quantum wells (QWs) with widths between 14 A and 1000 A are grown by LP-MOVPE. The lineshape of the 1.5 K photoluminescence spectrum is predominantly Gaussian. Using a recently developed detailed theory of quantum well lineshapes the interface roughness and the in-plane statistical variation of quantum well thickness is determined. The interface roughness is found to be independent of L z , thus demonstrating that the morphology of the growth surface becomes stable after growth of a few monolayers and is controlled by desorption and adsorption processes. The lateral homogeneity of the well thickness L z , measured in terms of the variation of its mean value, is extremely good, e.g. for L z = 14 A is ΔL z = 0.15 A across 1 cm 3 . The first detailed L z dependent time resolved measurements ever reported show multi-exponential decays. Interband and intrasubband recombination and relaxation times are determined using least-square iterative reconvolution. The lifetime of the X (e, hh) transition decreases with decreasing L z . This is due to the increase of the carrier localization similar to the behaviour discovered earlier for GaAs QWs.

Enhancement of nonradiative interface recombination in GaAs coupled quantum wells

The recombination dynamics of GaAs multiple quantum wells as a function of barrier widths LB are studied in a semiconductor in the range between LB=0.87 nm (superlattice) and LB=18.1 nm (uncoupled wells) by means of cathodo‐ and photoluminescence. With decreasing LB the nonradiative recombination rate is found to be drastically enhanced, where‐as the radiative recombination probability decreases. Thus a pronounced decrease of the quantum efficiency results. The controlled variation of the barrier width is found to be decisive for an unambiguous identification of the origin of the traps which are responsible for the nonradiative processes: Comparison with a theoretical calculation shows that they are localized at the heterointerfaces and not in the barriers.

Characterization of InP/GaInAs/InP heterostructures grown by organometallic vapor phase epitaxy for high-speed p-i-n photodiodes

Organometallic vapor phase epitaxy at atmospheric pressure is used to grow InP/Ga0.47In0.53As/InP p-i-n photodiode structures designed for high speed operation. Growth of high purity Ga0.47In0.53As and abrupt InP/GaInAs heterointerfaces is combined to make the fastest reported front-side illuminated GaInAs p-i-n detectors. High resolution transmission electron microscopy (HRTEM) examination of the InP/GaInAs interfaces indicates that the transitions are abrupt to about 4 monolayers or less. No misfit dislocations are detected at the InP substrate/GaInAs epitaxial layer interface or the GaInAs/InP epitaxial interface. The purity of n−-GaInAs is evaluated using photoluminescence at 1.5 K and Hall measurements at 77 K. The full width at half maximum (FWHM) of the GaInAs photoluminescence is 1.5 MeV at 1.4 K, the best value reported to date. The Hall mobility is as high as 64,000 cm2/V·s for n = 5×1014cm−3 at 77K. The lowest dark current measured is 0.15 nA at −4 V for a 50 μ m diameter photodiode with a 25 γm bonding pad or 6×10−6 A/cm2. This is among the best values reported to date. The bandwidth of a packaged photodiode with a 25 γm diameter photosensitive region is 17–20 GHz.

Growth kinetics, impurity incorporation, defect generation, and interface quality of molecular‐beam epitaxy grown AlGaAs/GaAs quantum wells: Role of group III and group V fluxes

A systematic knowledge of the influence of molecular‐beam epitaxy growth parameters on the properties of AlGaAs/GaAs quantum wells grown under realistic growth conditions is important in order to obtain optimal performance of modern electronic and optoelectronic devices. Photoluminescence (PL) was used to investigate the influence of the Ga‐controlled growth rate in the range below standard growth rates of 1 μm/h down to 0.1 μm/h, and of the As:Ga beam equivalent pressure ratio in the range of 10 to 60, on the growth kinetics, the interface quality, and the impurity incorporation, at a substrate temperature Ts =620 °C. As compared to in situ reflection high‐energy electron diffraction (RHEED) measurements, where no sample rotation is possible, PL has the advantage that realistic growth conditions can be used. A careful line shape analysis, together with infrared and time‐resolved PL measurements gives information on the interface roughness, the impurity incorporation, and the deep trap concentration. Non‐...

Reduction of trap concentration and interface roughness of GaAs/AlGaAs quantum wells by low growth rates in molecular beam epitaxy

A simultaneous reduction of interface roughness and of impurity and trap incorporation in GaAs/AlGaAs quantum wells is observed for a decrease of the molecular beam epitaxy growth rate below its standard value 1 μm/h, down to 0.1 μm/h, at a substrate temperature of 620 °C. Thus, layer quality is drastically improved at low, nonstandard growth rates. Incorporation of impurities from the background is observed to induce a transition from two‐ to three‐dimensional growth. The conclusions are based on a detailed study and line shape analysis of quantum well luminescence.

Coupling Induced Enhancement of Interface Recombination in GaAs Multiple Quantum Well Structures

The variation of luminescence transients of GaAs multiple quantum wells as a function of barrier widths is studied in the range between 0.87nm (superlattice) and 18.1 nm (uncoupled wells) by means of cathodo- and photoluminescence. With decreasing barrier width the nonradiative recombination rate is found to be drastically enhanced whereas the radiative recombination probability decreases. Thus a pronounced decrease of the quantum efficiency results. The controlled variation of the barrier width is found to be decisive for an unambigous identification of the origin of the traps: Comparison with a theoretical calculation shows that they are localized at the heterointerfaces and not in the barriers.

Impact of MBE-growth rate on optical properties of AlGaAs/GaAs quantum well structures

Abstract The results of the first systematic study of the correlation of the light emission from GaAs QWs, 2 nm, 5 nm and 8 nm wide, on the growth rate r in the range from 0.1 μm/h to 1 μm/h at a growth temperature of 620 °C are reported here. Emission energy positions, lineshapes and intensities are found to strongly vary with the growth rate. In particular, we observe that nonstandard low growth rates lead to much less interface roughness, a reduction of impurity and trap concentrations and higher quantum efficiency. For low growth rates the Ga desorption rate depends on growth rate. For higher growth rates it saturates due to the build up of an equilibrium vapor pressure at the surface.