P. D. Wang

Optical characterization of submonolayer and monolayer InAs structures grown in a GaAs matrix on (100) and high‐index surfaces

We studied the optical properties of InAs/GaAs heterostructures with InAs average layer thickness ranging from 1 A [one‐third of a monolayer (ML)] to 4 ML grown on (100) and (311) surfaces. Extremely high optical quality was revealed for the structures with ultrasmall InAs coverage. We attribute the improvements to the first stage of InAs growth on the GaAs surface which we refer to as submonolayer epitaxy. Optical anisotropy found in photoluminescence (PL), as well as in PL excitation spectra indicates a highly anisotropic growth mode for InAs molecules on the GaAs (100) surface. An InAs/GaAs superlattice composed of submonolayer InAs exhibits greatly improved luminescence efficiency at room temperature and much better nonequilibrium carrier capture compared to either the (In,Ga)As alloy or an InAs/GaAs superlattice composed of monolayer‐thick InAs layers with the same average In composition.

Raman scattering of coupled longitudinal optical phonon‐plasmon modes in dry etched n+‐GaAs

We have carried out extensive Raman scattering investigations of the damage caused by the dry etching in GaAs. The heavily doped n+‐GaAs (2–3×1018 cm−3) allows the study of the coupled longitudinal optical (LO) phonon‐plasmon mode as a probe to assess the dry etch‐induced damage. Three etching techniques were used including conventional radio frequency (rf) reactive ion etching (RIE), ion beam etching (IBE), and electron cyclotron resonance radio frequency reactive ion etching (ECR‐RIE). It is demonstrated that the etched damage is confined to a few tens of nanometers after 20 nm of material is etched away. ECR‐RIE etching produces the smallest damage. It is found that in RIE etching, as etching proceeds, the depletion depth saturates while for purely physical etching (IBE) the depletion depth increases continuously, at least under the conditions used.

Optical spectroscopy of self-organized nanoscale hetero-structures involving high-index surfaces

Similar effects are responsible for self-organization of periodically corrugated surface structures and ordered dot arrays on crystal surfaces. Strain relaxation on facet edges may result in the appearance of periodically corrugated surfaces for lattice-matched growth. Strain relaxation on facet edges and island interaction via the strained substrate act as driving forces for the formation of ordered arrays of uniform, strained lattice-mismatched islands on a crystal surface. A pseudoperiodic square lattice is manifested for the InAs-GaAs(100) system. Less ordered dots are formed on the GaAs(100) surface with a 4 monolayer GaSb deposition. New experimental methods are applied for the characterization of faceted nanoscale structures. For GaAs-AlAs multilayer structures grown on (311)A substrates, interface corrugation results in optical anisotropy of the same sign as expected from the low symmetry growth direction, making the main origin of the anisotropy unclear. Our quantitative optical reflectance and reflectance anisotropy studies show that the interface corrugation plays an important role for thin (less than 4 nm) GaAs layers. Mesa arrays from samples with InAs quantum dots grown on (100) surface are fabricated. The photoluminescence intensity is found to depend only weakly on the mesa size (1000 nm to 250 nm). The estimated electron-hole pair capture time into the InAs dot at room temperature is less than 1 ps. We also found a weak dependence of the threshold current density on the deep mesa stripe width (down to 3 μm) in the case of room temperature operated quantum dot injection lasers.

CHARACTERIZATION OF PROCESS-INDUCED STRAINS IN GAAS/GA0.7AL0.3AS QUANTUM DOTS USING ROOM-TEMPERATURE PHOTOREFLECTANCE

Using contactless photoreflectance at 300 K, we have studied several GaAs/Ga0.7Al0.3As quantum dot arrays fabricated by reactive‐ion etching using SiCl4. The spectrum from a control sample that had no dots also was recorded. From the observed shifts of the fundamental conduction to heavyand of the process‐induced strain in the dots.

Very low damage etching of GaAs

A very low damage, anisotropic and selective reactive ion etching process has been developed using SiCl4 for etching GaAs which stops on extremely thin GaAlAs layer (4 monolayers 1.13 nm thick). Using low rf powers of ≤15 W, and hence low dc biases of 40 to ≤70 V, pressures of 8 mTorr and flow rates of 4–6 sccm, the damage was kept to a minimum value while maintaining very good anisotropy. Both the surface and sidewall damage were measured and the results were confirmed by evaluation of the performance of a metal–semiconductor field effect transistor (MESFET) with a recessed gate. Raman scattering studies of the etched surface of a heavily doped GaAs layer show that the surface damage thickness is only 3–4 nm after 2 min of etching. The damage depth increases and saturates at 9 nm after 4 min of etching (etch rate of ∼100 nm/min).Conductance measurements [S. Thoms, S. P. Beaumont, C. D. W. Wilkinson, J. Frost, and C. R. Stanley, in Microcircuit Engineering, edited by H. W. Lehman and Ch. Bleicher (North H...

Interface structure of GaAs/AlAs superlattices grown on (113) surfaces: Raman scattering studies

Abstract We present an analysis of the influence of the type of interface upon the Raman response of (113) oriented GaAs/AlAs superlattices. The atomic displacements are calculated using the bond charge model. Theoretical Raman spectra are obtained using the bond polarizability model. Spectra taken in the z(xy) z configuration, with z ∥ [113], x ∥ [1 1 0] and y ∥ [33 2] are very sensitive to the presence of additional inplane periodicities. We have compared the theoretical Raman response with the experimental Raman spectra from samples grown by Molecular Beam Epitaxy. Although growth conditions favoured corrugated interfaces in these samples no characteristic feature of a perfectly corrugated interface in the experimental spectra is observed. Instead spectra are fitted better taken cationic mixing at the interface into account.

Process‐induced strains in dry etched semiconductor nanostructures studied by photoreflectance

This article reports a photoreflectance study of the process‐induced strains in both dry etched CdTe/Cd0.875Mn0.125Te and GaAs/Al0.3Ga0.7As nanostructures patterned by electron beam lithography. The results show that compressive strains can be introduced in both the dry etched nanostructures and the layers underneath the etched surfaces due to the introduction of defect complexes and/or crystographic damage inflicted in the fabrication process. The effect of post dry etch thermal annealing on the strains in the dry etched nanostructures has also been studied.

Photoluminescence intensity and multiple phonon Raman scattering in quantum dots: evidence of the bottleneck effect

Abstract We report a systematic study of photoluminescence efficiency in dry etched quantum well dots. Luminescence degradation is observed on decreasing the quantum dot lateral size. Furthermore, luminescence quenching is more pronounced in quantum dots with a thicker quantum well than those with a thinner one. This effect is attributed to the reduced carrier relaxation in 0D system. Resonant Raman scattering and “hot” exciton luminescence have also been observed in dry etched GaAs AlGaAs quantum dots in which quantum confinement effects are found with decreasing dot sizes. Up to 4th-order multiphonon processes have been observed through photoluminescence and photoluminescence excitation in quantum dots. These results are direct evidence of the bottleneck effect. Lateral patterning of quantum wells strongly localises excitons in three-dimensional quantum confinement regime (0D system). The observation of resonant Raman scattering and hot exciton luminescence up to high temperature demonstrates the high stability of localised excitons.

Magnetically confined plasma reactive ion etching of GaAs/AlGaAs/AlAs quantum nanostructures

An electron cyclotron resonance reactive ion etching machine has been successfully run under low magnetic field conditions. We call this the magnetically confined plasma condition. Under this condition, a new process using SiCl4 with a small amount of O2 has been developed for etching nanometer scale structures on GaAs, AlGaAs, and AlAs multilayer materials. The effects of the percentage of O2, the rf power, the microwave power, and the flow rate are described. 100 nm quantum dots have been etched on multiple quantum well materials to a depth of about 1 μm. Vertical and smooth sidewalls were obtained on these nanostructures. Poly‐methylmethacrylate (PMMA) electron beam resist can be used directly as a dry etch mask, and the selectivity between GaAs and PMMA can be as high as 28:1. Raman spectroscopic studies showed that the process induced no detectable damage to the surface for an etch depth of 130 nm and only very little damage for deeper etching.

Shear strains in dry etched GaAs/AlAs wires studied by high resolution x-ray reciprocal space mapping

We have fabricated GaAs/AlAs quantum wires and quantum dots by means of molecular beam epitaxy, electron beam lithography, and subsequent reactive ion etching using SiCl4 and O2. The nominal periods are 300 nm and 350 nm for both wire and dot samples. High resolution x-ray reciprocal space maps of the 350 nm samples exhibit not only satellites corresponding to a periodicity of 350 nm but also additional satellites corresponding to a period of three times 350 nm, whereas there are no such extra peaks in the maps of the 300 nm samples. These secondary satellites are shown to be associated with a discretization effect in electron beam writing. Moreover, we found, that the shear strain in the wires has a distinct influence on the intensities of these weak extra satellites. Hence, they provide a sensitive means for the assessment of shear strains in elastically relaxed quantum wires.