Short-period Strained-layer Superlattices Research Articles

Low-temperature growth of ZnSe-based pseudomorphic structures by hydrogen-radical-enhanced chemical vapor deposition

We investigated the growth of crystals with pseudomorphic structures such as ZnSe-ZnS x Se 1− x strained-layer superlattices (SLSs) and short-period SLSs (ZnSe) m-(ZnS) m , by means of hydrogen-radical-enhanced chemical vapor epitaxy (HRCVD). Atomic layer epitaxy (ALE) was realized by a self-limiting action in depositing the Zn monolayer and by removal of an excess amount of Se with the aid of atomic hydrogen in depositing the Se monolayer. Evidence obtained from measurements of photoluminescence and X-ray diffraction spectra led us to the conclusion that both pseudomorphic structures, in good order, are successfully made by this technique at a low temperature, 200 ° C, under precise control of chemical reactions on the growing surface.

Optical nonlinearities and ultrafast charge transport in all-binary InAs/GaAs strained hetero n-i-p-i’s

We use picosecond differential spectroscopy to temporally and spectrally resolve the formation and decay of nonlinearities and space-charge fields in a hetero n-i-p-i that contains quantum wells in the intrinsic regions that are composed of all-binary InAs/GaAs short-period strained-layer superlattices. The evolution of the optical response is determined by competition between excitonic bleaching and the excitonic shift caused by screening of the built-in electric field of the n-i-p-i. The relative contributions of the two resulting optical nonlinearities are complicated functions of fluence, time, and wavelength, with the detailed dynamics determined by thermionic emission from the wells, picosecond charge transport over nanometer dimensions, screening, and recombination. At low fluences, excitonic bleaching is the source of an ultrafast nonlinear response that can be turned on and off in <10 ps. This initial excitonic bleaching gives way to a blue shift of the exciton as the carriers escape the wells in ∼3 ps and drift to screen the built-in field in <10 ps. The blue shift persists until the carriers recombine nonexponentially on microsecond time scales. At higher fluences, excitonic bleaching and the blue shift are observed simultaneously, since only a fraction of the carriers are required to screen the field and the wells remain partially occupied. On the time scale of ∼10 ns, the bleaching contribution disappears as the carriers within the wells recombine, leaving only the persistent blue shift.

InAs/InP short-period strained-layer superlattices grown by atomic layer epitaxy

InAs/InP short-period strained-layer superlattices were successfully grown by atomic layer epitaxy (ALE), for the first time, using trimethylindium (TMIn), arsine (AsH 3) and phosphine (PH 3) as sources. ( InAs) m ( InP) 1( m=2−4) structures were grown on InAs (100) substrates at 365°C. Structural characterization was done using X-ray diffraction measurements and many satelite peaks confirming the formation of designed superstructures were detected. We clarified that self-limiting growth still exists at the InAs/InP interfaces. Results indicate that, despite the 3.2% lattice mismatch. ALE forms the growth layer-by-layer from the first layer of heterointerfaces and is a powerful method of fabricating structures with atomically controlled heterointerfaces.

(ZnSe) m-(ZnTe) n short-period strained layer superlattices prepared by atomic layer epitaxy

(ZnSe) m -(ZnTe) n short-period strained layer superlattices (SLSs) were grown by atomic layer epitaxy using the molecular beam epitaxy technique. Transmission electron microscopy, X-ray diffraction and Raman scattering measurements were carried out to evaluate the grown SLSs. It was proved that the atomic layer epitaxy enabled growth of fine short-period superlattice structures which have abrupt interfaces with at most one-monolayer steps.

Optical studies of InAs/GaAs on GaAs short-period strained-layer superlattices grown by MBE and MEE

We have grown InAs/GaAs short-period strained-layer superlattices (SPSLSs) by both conventional molecular beam epitaxy (MBE) and migration enhanced epitaxy (MEE). Our efforts have been directed towards the optimization of growth parameters including shutter times, growth interrupt times, and substrate temperatures for both growth techniques. We have investigated several structures with different numbers and thicknesses of InAs and GaAs layers. Optical measurements, including absorbance, CW photoluminescence (PL), and time resolved PL, have been employed to determine the quality of the samples. We have grown samples with 50 repetitions of the SPSLS separated by 20 nm GaAs barrier layers which have exhibited excellent 5 K CW PL peaks with full width at half maximum (FWHM) of less than 9 meV. In addition, several samples have shown room temperature exciton features in absorbance spectra. A synchronously mode-locked Styryl 9 dye laser has been used in conjunction with a streak camera to perform time resolved PL measurements, thereby determining carrier lifetimes in the samples. We will discuss our results, comparing the growth conditions as well as the different SPSLS structures, in terms of the optical data that we have obtained.

Linear optical properties of quantum wells composed of all-binary InAs/GaAs short-period strained-layer superlattices

The linear optical properties of a variety of (InAs/GaAs)-GaAs multiple quantum well structures, where each well consists of a highly strained InAs/GaAs short-period superlattice, have been investigated in detail. The results attest to the improvement in material quality over previously reported structures of this type. Clearly resolved excitonic absorption peaks have been observed at room temperature in all samples. Photoluminescence and excitonic absorption linewidths at 15 K are less than 10 meV in each case, with the photoluminescence Stokes shifted by less than 1 meV. Temporally resolved photoluminescence measurements at 15 K indicate carrier lifetimes of 1.4–1.8 ns. Dramatic strain-related differences are observed when compared to random alloy InGaAs/GaAs quantum wells with an equivalent average indium mole fraction.

Direct interferometric measurements of quantum-confined Stark effect in InAs/GaAs short period strained-layer superlattice MQWs

The electrorefraction within and below the band edge of multiple (50) InAs/GaAs short period strained-layer superlattice quantum wells has been measured. An index change of 4×10−3 was measured near the excitonic resonance with an applied electric field of 27 kV/cm, corresponding to a quadratic electro-optic coefficient of −3×10−13 cm2/V2. These results agree with predictions from the Kramers-Kronig relations based on measurements of electroabsorption.

Electro- and Photoluminescence from Ultrathin SImGEn Superlattices

ABSTRACTP-i-n doped short-period SimGen strained layer superlattices (SLS) are grown on (100) silicon substrates by low temperature molecular beam epitaxy (300C°<∼Tg<∼400C°). The SLS's are grown with period lengths around 10 monolayers (ML) to a thickness of 250nm on a rather thin (50nm) homogeneous Si1−ybGeyb alloy buffer layer serving as strain symmetrizing substrate. Photoluminescence at T=5K is observed for various SimGen SLS samples, the strongest signal was found for a Si5 Ge5 SLS. Samples with identical SLS's but different buffer layer composition and thicknesses are grown to study the influence of strain on the PL. Electroluminescence (EL) at the same energy range is observed from mounted SimGen SLS mesa and waveguide diodes up to T=130K – for the first time reported in strain symmetrized short-period SimGen SLS. The intensity and peak positon of the EL signal was found to be dependent on the injected electrical power.

Structural Characterization of Short-Period SimGEn Superlattices by Transmission Electron Microscopy and X-Ray Diffraction

ABSTRACTHigh resolution and analytical transmission electron microscopy (TEM) and X-ray diffraction (XRD) were used to characterize short-period strained-layer Sim-Gen superlattices ( m monolayers Si, n monolayers Ge, total number of periods N≤ 145, total thickness ≃ 200 nm). The superlattices were grown by low-temperature molecular beam epitaxy (T = 300–400°C) on different SiGe alloy buffer layers on Si (100)substrates. The combination of these two methods shows that detailed informations can be obtained about superlattice periodicity, interface roughness, strain, and average composition.

P-type doping of gallium antimonide grown by molecular beam epitaxy using silicon

We report the first effective p-type doping of molecular beam epitaxy (MBE) grown GaSb using silicon. The samples were grown by molecular beam epitaxy and characterized by Hall-effect measurements and photoluminescence. Room-temperature hole concentrations ranging from 4.0×1015 to 4.3×1018 cm−3 were obtained. Photoluminescence (PL) spectra undergo considerable broadening with increasing doping concentration, consistent with an impurity banding picture. Furthermore, the MBE-grown samples display only one of the two PL features found in a melt-grown substrate and no other satellites, suggesting higher material purity. This may be a direct benefit from the use of an antimony cracker, ultrahigh vacuum conditions, and high-purity elemental sources. The short-period strained-layer superlattice buffering scheme employed may have also contributed to better structural quality.

Picosecond room temperature excitonic-absorption saturation and four-wave mixing in InAs/GaAs short-period strained-layer superlattices

We report picosecond nonlinear optical measurements in InAs/GaAs short-period strained-layer superlattices. Under resonant excitation near 1 μ m at room temperature, strong bleaching of the n=1 heavy-hole excitonic absorption peak and large associated degenerate four-wave mixing signals were observed. The spectral dependence of the absorption change and refractive index change per photo-generated electron-hole pair were extracted directly from the measured differential transmission and wavelength-dependent four-wave mixing diffraction efficiency respectively. The measured refractive index change is consistent with that predicted indirectly by a Kramers-Kronig transformation of the measured wavelength-dependent absorption change per carrier pair.

Preparation and characterization of high quality inGaAs/GaAs strained multi-quantum wells grown by MBE

InGaAs/GaAs strained multi-quantum wells were prepared in our molecular beam epitaxy (MBE) system. Intensity oscillations in reflection high energy electron diffraction (RHEED) were used to determine layer thickness, indium composition, and the optimum growth temperature. By introducing a short period strained layer superlattice as the graded layer, the mismatched strain between the GaAs buffer layer and the subsequent heterostructures were buffered, and the fluctuation of indium beam flux due to the thermal disturbance was reduced. The high quality InGa/GaAs quantum wells were characterized by X-ray rocking curves, photoluminescence (PL) spectra, and transmission electron microscopy (TEM).

Variation in frequencies of confined longitudinal-optical phonon modes due to changes in the effective force constants near heterojunction interfaces

A linear-chain model with variable force constants for the couplings at heterojunctions is used to calculate the spectrum of confined longitudinal-optical (LO) phonon frequencies in a short-period strained-layer superlattice. Even for the case of a superlattice with only two atomic monolayers in each superlattice layer, it is demonstrated that the frequencies of confined LO-phonon modes are only weakly dependent on variations in interfacial force constants.

Confined optical phonons in GaAs/GaP strained layer superlattices

In this letter we report on Raman characterization of GaAs/GaP short period strained layer superlattices grown on GaAs (100) substrates by Atomic layer Molecular Beam Epitaxy (ALMBE). Confined optical phonons are observed in this highly strained system. The experimental results are compared with a calculation based on the equivalent wave-vector model.

Determination of in-plane lattice parameters of InAs/AlAs strained-layer superlattices by X-ray precession camera

A determination of the lattice parametersa∥ andb∥ of InAs/AlAs short-period strained-layer superlattices grown by Atomic Layer Molecular Beam Epitaxy (ALMBE) on GaAs(001) substrates has been performed by means of an X-ray precession camera using copper radiation. Spots belonging to the superlattice are clearly differentiated from those of the substrate, which confirms that they are partly decoupled from each other. It was also possible to resolve the lattice spots of InAs or In0.8Ga0.2 As decoupling buffer layers grown between the substrate and the superlattice. This technique proves to be very useful to characterize, in a very short time and with a reasonable resolution, highly mismatched epitaxial systems in which lattice parameters parallel to the interface play a crucial role in the understanding of the growing behaviour.

Structural and optical studies on molecular beam epitaxially grown (InAs) m(AlAs) m and (InAs) m(GaAs) m short-period strained layer superlattices

Binary consisting (InAs) m (AlAs) m and (InAs) m (GaAs) m short-period strained layer superlattice InP substrates were grown and their structural and optical properties were examined. Mirror-like surface morphologies were obtained as long as their total thicknesses were less than a critical value. X-ray diffraction measurements showed the formation of atomically ordered periodic superstructure. Photoluminescence spectra measured at 77 K revealed good cry stalline quality in spite of about 7% lattice mismatch between the binary layers. The peak energy changes greatly on (InAs) m (IlAs) m according to the layer thickness, while it varies by a small amount on (InAs) m (GaAs) m structures, which reflects to the layer band gap discontinuities between InAs and AlAs. These structures can be substituted for InAlAs and InGaAs alloys.