S. K. Lyo

Photonic band gap quantum well and quantum box structures: A high‐Q resonant cavity

We have tested a series of high‐Q photonic band gap (PBG) resonant cavities in the mm‐wave regime and achieved a cavity‐Q of 2.3×104, the highest value reported among all two‐ and three‐dimensional PBG cavities. We have also systematically varied the size and reflectivity of such cavities to study their effect on cavity properties such as cavity modal frequency, linewidth, and cavity Q value. We show that the resonant frequencies can be tuned throughout the PBG regime and that linewidths (or equivalently Q value) can be varied over two orders of magnitude (i.e., a Q value from ∼2.7×102 to 2.3×104).

Planar quantum transistor based on 2D–2D tunneling in double quantum well heterostructures

We report on our work on the double electron layer tunneling transistor (DELTT), based on the gate-control of two-dimensional -- two-dimensional (2D-2D) tunneling in a double quantum well heterostructure. While previous quantum transistors have typically required tiny laterally-defined features, by contrast the DELTT is entirely planar and can be reliably fabricated in large numbers. We use a novel epoxy-bond-and-stop-etch (EBASE) flip-chip process, whereby submicron gating on opposite sides of semiconductor epitaxial layers as thin as 0.24 microns can be achieved. Because both electron layers in the DELTT are 2D, the resonant tunneling features are unusually sharp, and can be easily modulated with one or more surface gates. We demonstrate DELTTs with peak-to-valley ratios in the source-drain I-V curve of order 20:1 below 1 K. Both the height and position of the resonant current peak can be controlled by gate voltage over a wide range. DELTTs with larger subband energy offsets ({approximately} 21 meV) exhibit characteristics that are nearly as good at 77 K, in good agreement with our theoretical calculations. Using these devices, we also demonstrate bistable memories operating at 77 K. Finally, we briefly discuss the prospects for room temperature operation, increases in gain, and high-speed.

Determination of energy‐band dispersion curves in strained‐layer structures

Simultaneous measurement of both the conduction‐ and valence‐band dispersion curves in single strained‐layer structures is presented. These measurements rely on the application of recent observations regarding breaking of the usual selection rules for interband magnetoluminescence transitions in modulation‐doped structures. Low‐temperature magneto‐luminescence data for three representative InGaAs/GaAs n‐type single‐strained quantum well structures are presented. For energies approaching 50 meV above the band gap, we find that the conduction band is parabolic with an effective mass of 0.071m0. Over the same energy range, the valence bands are highly nonparabolic.

Lasing threshold in quantum well surface‐emitting lasers: Many‐body effects and temperature dependence

The geometry of quantum well surface‐emitting lasers has several important consequences. The ultrashort (∼1 μm) vertical cavity defines longitudinal modes with energy separation greater than the bandwidth of spectral gain. The optical confinement of these modes can approach unity. To achieve lasing, high carrier densities (∼1012 cm−2) in the quantum well are required. The confined carriers interact through enhanced many‐body exchange which influences both the lasing wavelength and threshold characteristics. Indeed, the exchange interaction can facilitate the lasing process. We theoretically and experimentally study the role of the short cavity and exchange interaction on the cw lasing threshold as a function of temperature. In constrast to edge emitters, the lasing threshold in these surface emitters exhibits a well‐defined minimum at a particular temperature. The temperature of the minimum can be designed by merely changing the active layer thickness.

Room temperature midinfrared electroluminescence from InAs quantum dots

We demonstrate room temperature midinfrared electroluminescence from intersublevel transitions in self-assembled InAs quantum dots. The dots are grown in GaAs/AlGaAs heterostructures designed to maximize current injection into dot excited states while preferentially removing electrons from the ground states. As such, these devices resemble quantum cascade lasers. However, rigorous modeling of carrier transport through the devices indicates that the current transport mechanism for quantum dot active regions differs from that of quantum-well-based midinfrared lasers. We present the calculated energy states and transport mechanism for an intersublevel quantum dot emitter, as well as experimental electroluminescence data for these structures.

High-efficiency, continuous-wave, epitaxial surface-emitting laser with pseudomorphic InGaAs quantum wells

We report the first continuous‐wave (cw) photopumped operation of surface‐emitting lasers comprising pseudomorphic InGaAs quantum wells. The lasers were grown by molecular beam epitaxy and incorporate epitaxial quarter‐wave AlAs/GaAs mirrors surrounding an active region. In the active region, 50 A InGaAs quantum wells are distributed with half‐wave periodicity to center on cavity standing wave maxima. Lasing is observed from 78 to 250 K in the spectral range 920–950 nm, where the GaAs substrate is transparent. Thresholds were as low as 1.5×104 W/cm2, and overall (differential) output power efficiency was as high as 35% (85%) with up to 60 mW in a low divergence beam. Both periodic gain and biaxial compressive layer strain contribute to the reduced lasing threshold. The laser gain length is only 550 A (11 quantum wells). The possibility of surface‐emitting lasing in single quantum wells is discussed.

Negative differential conductance in two-dimensional electron grids

Negative differential conductance has been observed in grid-shaped surface superlattices, realized in a high mobility two-dimensional electron system. The current-voltage characteristics vary with the modulation strength, indicating that the two-dimensional electronic transport properties can be manipulated in a controllable way. Theoretical modeling yields reasonable agreement with the experimental data.

Strongly reduced exciton transfer between parallel quantum wires

Exciton transfer between two parallel GaAs V-groove quantum wires or two planar quantum wells separated by AlGaAs barriers ranging from 5.5 nm to 20 nm thickness is studied by photoluminescence and photoluminescence excitation spectroscopy. It is found that the transfer is strongly reduced between the widely spaced quantum wires as compared with quantum wells. This observation is supported by model calculations, which yield strong dimensionality dependence of the photon-exchange transfer.

Correlation between photoluminescence data and device performance of p -channel strained-layer materials

The 4-K photoluminescence spectrum and room temperature transconductance for modulation dopedp-type GaAs/(In,Ga)As dual-channel strained-quantum-well field-effect transistors with comparable dopant and 2-D carrier concentrations were studied. All gate sizes were nominally 300 μm wide by 1 μm long. The best sample has a peak normalized extrinsic transconductancegmoat room temperature of 31 mS/mm and a 4K photoluminescence linewidth of 6 meV. Depending upon the sample,gmovaried from about 0.5 to 31 mS/mm while the 4-K photoluminescence linewidth decreased from 26 to 6 meV. The low-temperature photoluminescence linewidth and room temperature transconductance were correlated. These results indicate that photoluminescence spectroscopy can be used for screening wafers for potential device peformance before processing.

Magneto-optical studies of compound semiconductors

Abstract A simultaneous determination of both the conduction and valence-band dispersion curves (and masses) from a single compound semiconductor quantum-well structure using magnetoluminescence is discussed. Data from InGaAs/GaAs strained-single-quantum wells and GaAs/AlGaAs lattice matched quantum wells are presented. The conduction bands are found to be parabolic for densities approaching 1 × 1012cm−2. However, the valence bands are highly nonparabolic and strongly affected by heavy-hole light-hole mixing.