A Quantum Computer (QC) is a device that utilizes the principles of Quantum Mechanics to perform computations. Such a machine would be capable of accomplishing tasks not achievable by means of any conventional digital computer, for instance factoring large numbers. Currently it appears that the QC architecture based on an array of spin quantum bits (qubits) embedded in a solid-state matrix is one of the most promising approaches to fabrication of a scalable QC. However, the fabrication and operation of a Solid State Quantum Computer (SSQC) presents very formidable challenges; primary amongst these are: (1) the characterization and control of the fabrication process of the device during its construction and (2) the readout of the computational result. Magnetic Resonance Force Microscopy (MRFM) - a novel scanning probe technique based on mechanical detection of magnetic resonance - provides an attractive means of addressing these requirements. The sensitivity of the MRFM significantly exceeds that of conventional magnetic resonance measurement methods, and it has the potential for single electron spin detection. Moreover, the MRFM is capable of true 3D subsurface imaging. These features will make MRFM an invaluable tool for the implementation of a spin-based QC. Here we present the general principles of MRFM operation, the current status of its development and indicate future directions for its improvement.

Several solid state quantum computer schemes are based on the manipulation of electron and nuclear spins of single donor atoms in a solid matrix. The fabrication of qubit arrays requires the placement of individual atoms with nanometer precision and high efficiency. In this article we describe first results from low dose, low energy implantations and our development of a low energy (<10 keV), single ion implantation scheme for ³¹P^q+ ions. When ³¹P^q+ ions impinge on a wafer surface, their potential energy (9.3 keV for P¹⁵⁺) is released, and about 20 secondary electrons are emitted. The emission of multiple secondary electrons allows detection of each ion impact with 100% efficiency. The beam spot on target is controlled by beam focusing and collimation. Exactly one ion is implanted into a selected area avoiding a Poissonian distribution of implanted ions.

InAs quantum dot infrared photodetectors based on bound-to- bound intraband transitions in undoped InAs quantum dots are reported. AlGaAs blocking layers were employed to achieve low dark current. The photoresponse peaked at 6.2 micrometers . At 77 K and -0.7 V bias the responsivity was 14 mA/W and the detectivity, D*, was 10¹⁰ cmHz^1/2/W. By introducing InGaAs cap layers, a QDIP with bias-controllable two-color characteristic was demonstrated.

Self-organized quantum dots are being incorporated in the active regions of interband lasers, modulators, and infrared detectors and sources. The unique hot-carrier relaxation rates in quantum dots play an important role in defining the device characteristics. We have conducted extensive theoretical and experimental studies of carrier dynamics in In(Ga)As/Ga(Al)As self-organized quantum dots grown by molecular beam epitaxy. Experimental techniques used include small and large-signal modulation of lasers and femtosecond pump-probe spectroscopy. It is found that the intersubband electron relaxation rates, which are strongly temperature dependent, are determined by electron-hole scattering in the dots. Theoretical calculations also show that electron-hole scattering is the dominant mechanism for the relaxation of hot carriers. It is also found that a phonon bottleneck exists in the dots for very weak excitations. The implications of these results on device performance will be discussed.

GaAs and InGaAs (311) surfaces may be spontaneously corrugated with a height and a period controlled by the film composition, strain and polarity of the substrate. Using image-processed high-resolution transmission electron microscopy we found that both GaAs - AlAs interfaces in short-period superlattices (SPSL) grown on (311)A GaAs substrates are corrugated with a height of 1 nm and a lateral periodicity of 3.2 nm. The same lateral periodicity is also revealed for SPSL grown on (311)B surfaces, but the corrugation height and the degree of order are strongly reduced in this case. A strong optical anisotropy (up to 60%) is found in photoluminescence (PL) spectra for SPSLs grown on (311)A surface and not for (311)B-grown SPSLs. We observed a strong mixing between (Gamma) and X states in the conduction band for the SPSLs grown on (311)A surface which allowed realization of bright PL at room temperature at 650 nm. (311)B and (100)GaAs-AlAs SPSLs grown side by side demonstrated only weak long-wavelength PL due to disorder- induced states. (311)A SPSLs demonstrate also a slow carrier relaxation with characteristic LO-phonon scattering times in excess of 10 ps. Corrugated SLs are particularly advantageous for polarization stabilized surface emitting lasers, bright-red AlGaAs lasers and far infrared emitters and detectors.

The effect of interdiffusion on the luminescence of InGaAs quantum dots grown by metal organic chemical vapor deposition with various In compositions was studied. The samples were subjected to thermal annealing with and without spin-on-glass. Up to 250 meV blueshifts and narrowing of the linewidths by up to 60 meV were observed in samples that were annealed without the dielectric cap. The effects of diffusion were seen at lower annealing temperatures for the samples with higher In content. The spin-on-glass created additional blueshift above that of simple thermal annealing. However, Ga-doped spin-on-glass suppressed significantly any additional intermixing other than that of simple thermal annealing. Strain is also a factor in the blueshift and narrowing of the photoluminescence. These results suggest that a range of bandgap energies could be achieved by selective area interdiffusion of the quantum dot samples.

In experiments conducted nearly 20 years ago, the spontaneous emission from single atoms was modified using electromagnetic cavities. In a condensed matter analogy to a single atom, we demonstrate that the spontaneous emission from an isolated InAs quantum dot can be modified as well. The single quantum dot spontaneous emission is coupled with high efficiency to a single, polarization-degenerate cavity mode using a compact, semiconductor resonator structure. The quantum dot is embedded in a planar epitaxial microcavity, which is processed into a post of submicron diameter. The single quantum dot spontaneous emission lifetime is reduced from the noncavity value of 1.3 ns to 280 ps, resulting in a single-mode spontaneous emission coupling efficiency of 78%. It is believed that this structure will be useful in triggered photons sources for quantum cryptography.

Different approaches to the design of a genuinely temperature-insensitive quantum dot (QD) laser are proposed. Suppression of the parasitic recombination outside the QDs, which is the dominant source of the temperature dependence of the threshold current in the conventional design of a QD laser, is accomplished either by tunneling injection of carriers into the QDs or by bandgap engineering. Elimination of this recombination channel alone enhances the characteristic temperature T₀ above 1000 K. Remaining sources of temperature dependence (recombination from higher QD levels, inhomogeneous line broadening, and violation of charge neutrality in QDs) are studied. Tunneling injection structures are shown to offer an additional advantage of suppressed effects of inhomogeneous broadening and neutrality violation.

We present a mesoscopic theory for the spatio-temporal carrier- and light field dynamics in quantum dot lasers based on spatially resolved semiconductor Bloch equations describing the dynamics of electrons and holes in each quantum dot. The Bloch equations are dynamically coupled to spatially resolved wave equations for the counterpropagating light fields and to a diffusion equation describing the carrier dynamics in the wetting layer of the quantum dot laser. These quantum dot Maxwell-Bloch equations (QD-MBEs) self-consistently consider the dynamic changes in the carrier distributions and the inter-level dipoles together with the spatially varying carrier-light field dynamics. Intradot scattering via emission and absorption of phonons, as well as the scattering with the carriers and phonons of the surrounding wetting layer are dynamically included on a mesoscopic level. Spatial fluctuations in size and energy levels of the quantum dots and irregularities in the spatial positioning of the quantum dots in the laser structure are simulated via statistical methods. Numerical simulations on the basis of the QD-MBEs reveal a complex carrier dynamics and a characteristic interplay of spontaneous and stimulated emission. For a specific set of QD-parameters the results of the modeling allow an analysis and interpretation of, e.g., saturation effects and dynamic pulse shaping in quantum dot lasers.

The influence of two monolayer (ML)-thick AlAs under- and overlayers on the formation and properties of self-assembled InAs quantum dots (QDs) has been studied using transmission electron microscopy, photoluminescence (PL) and electroluminescence. The main purpose of this work was to achieve high internal quantum efficiency of the active medium and temperature stability of the laser diodes. Single and multiple layers of 2.0-2.4ML InAs QDs with various combinations of under- and overlayers were grown on GaAs (001) substrate by molecular beam epitaxy inside a AlAs/GaAs short-period superlattice. It was found that a 2.4-ML InAs QD layer with GaAs underlayer and 2-ML AlAs overlayer exhibited the lowest QD surface density of 4.2x10¹⁰ cm^-2 and the largest QD lateral size of about 19 nm as compared to the other combinations of cladding layers. This InAs QD ensemble has also shown the highest room temperature PL intensity with a peak at 1210 nm and the narrowest linewidth, 34 meV. Fabricated edge-emitting lasers using triple layers of 2.2-ML InAs QDs with AlAs overlayer demonstrated 120 A/cm² threshold current density and 1230 nm emission wavelength at room temperature. Excited state QD lasers have shown high thermal stability of threshold current up to 130 degree(s)C.

The microstructural, luminescence properties and photoresponse of multilayer Ge(Si) quantum dots grown on Si (100) substrates are studied. The strain and composition of the dots are studied by synchrotron-radiation x-ray. The dots are found to be Si_0.58Ge_0.42 alloy with 50% strain relaxed in average. The photoluminescence from the dots is observed up to room temperature. The thermal stability of the quantum dots is studied. P-i-n structures are grown with Ge(Si) dots embedded in the i-layer for photodetection investigation. The photoresponse wavelength of Ge(Si) dots covers the wavelength range of 1.3-1.52 mm and relatively high external quantum efficiency is obtained.

We report our recent results on characterization of GaAs/AlAs superlattices exhibiting evidence of a quasi- indirect transition between minibands. These structures have potential applications in semiconductor optical amplifiers with greatly-reduced cross-talk at high bit rates and Q-switched lasers.

Self-assembled InP/InAs/InP quantum wires have been successfully stacked for 10 vertical periods and characterized based on photoluminescence (PL) studies. Compared with single-period quantum wires, unique behaviors appear in the PL spectra and some fundamental effects have been observed. Through the detailed analyses of the PL shapes, linewidths, and polarizations at different pump wavelengths, pump intensities and sample temperatures, it is evidenced that the wire width and subband energy gradually decrease while the average wire thickness increases from the bottom period to the top one, period by period. Meanwhile, the average wire width gradually decreases. In addition, from the bottom to the top period the size fluctuation within each period decreases. Furthermore, above certain temperature or pump intensity all the quantum wires are vertically coupled among one another. Following these results, new growth conditions have been suggested, which can be essential to improving the optical quality of these self-assembled quantum wires.