U. Reddy

Observation of a negative differential resistance due to tunneling through a single barrier into a quantum well

We have observed a negative differential resistance (NDR) in a single‐barrier tunneling structure in which electrons tunnel from a doped semiconductor emitter layer into a quantum well (QW) layer and subsequently drift laterally to a specially designed contact. Pronounced NDR is seen already at room temperature and at 77 K the peak to valley (PTV) ratio in current is more than 2:1. Our results lend support to a recent hypothesis by Luryi [Appl. Phys. Lett. 47, 490 (1985)] that the NDR in double‐barrier tunneling structures is not related to a resonant enhancement of the tunneling probability at selected electron energies, but rather originates from tunneling into a system of electron states of reduced dimensionality. For comparison we have also fabricated a QW structure with two tunneling barriers, in which the parameters of the emitter barrier and the QW are identical to those in the single‐barrier structure. In the double‐barrier structure we have obtained current densities as high as 4×104 A/cm2 and a ...

Transmission and photoreflectance spectra in highly strained InGaAs-GaAs multiple quantum wells

Abstract Low temperature optical transmission spectra (TS) and room temperature photoreflectance (PR) spectra have been measured to investigate several In x Ga 1− x AS GaAs strained multiple quantum wells (MQWs) grown by molecular beam epitaxy (MBE). Sharp PR features indicating excellent optical quality of these samples were observed. The excitonic transitions up to 3C-3H are observed. The notation nC-mH(L) is used to indicate the transitions related to the n-th conduction and m-th valence heavy (light) hole subbands. The band-to-band transitions are also observed in both TS and PR spectra, which, having step-like structure in TS spectra, are identified as 1C-1L transitions. The calculated transition energies, taking into account both the strain and the quantum well effects, are in good agreement with the TS and PR spectra. This shows that room temperature PR is a powerful and convenient tool to investigate the strained layer MQWs. By fitting the experimental results (TS and PR), a heavy-hole valence band discontinuity of 30% was obtained for 0.13 ≤ × ≤ 0.193. We conclude that the light holes are in the GaAs barrier region (Type II MQWs) and the light hole transition 1C-1L is a sensitive parameter in determining the band offset in GaAs GaAs strained layer MQWs.

Investigation of GaAs/(Al,Ga)As multiple quantum wells by photoreflectance

Room‐temperature photoreflectance has been used to investigate a series of GaAs/(Al,Ga)As multiple quantum‐well structures. In addition to the allowed (as high as n=5) and symmetry forbidden transitions, we have observed transitions involving the so‐called ‘‘unconfined’’ states, which have received less attention so far. We have examined these transitions more carefully by studying a low barrier multiple quantum‐well structure and observed transitions as far as 200 meV beyond the barrier gap. By using a simple two‐band tight‐binding model, the energies and matrix elements for these unconfined transitions were calculated and shown to agree with the experimental values determined by fitting the photoreflectance spectra to the theoretical line‐shape expression.

Photoreflectance, absorption, and nuclear resonance reaction studies of AlxGa1−x As grown by molecular‐beam epitaxy

The photoreflectance (PR) spectra of bulk AlxGa1−xAs alloys with x<=0.45 were studied. The observed line shapes from different samples suggest that the PR technique is very sensitive to the material quality, surface condition, and the background impurities. The energy gap derived from the PR spectra compared well to that obtained from the absorption spectra. The relationship between the energy gap and the Al mole fraction value x was established through the nuclear resonance reaction analysis. The electric field near the surface was calculated from the periodicity of Franz–Keldysh oscillations observed in many of the samples. From our analysis, we believe that the number of oscillations shown in PR spectra corresponds to sample quality, in general. We also believe that the low-field-like line shape is mainly caused by the fluctuation of Al distribution along the growth direction. An additional feature related to the impurity transition was also observed in the spectra.

InGaAs/InAlAs hot‐electron transistor

Using InGaAs for the base and InAlAs for the emitter and collector barriers, we have fabricated the first hot‐electron transistor in this material system. We have shown that 1.6% of the injected hot electrons can be transported ballistically through a 0.3‐μm‐thick In0.53Ga0.47As plus 800‐A‐thick InAlAs barrier layer at 77 K giving rise to an average mean free path of 920 A. An energy spread of 130 meV was observed for the ballistic electrons injected at about 700 meV above the thermal equilibrium conditions. The values of collector barrier heights measured are in reasonable agreement with those deduced independently from thermionic emission studies in InGaAs gate, InAlAs/InGaAs capacitor structures.

Patterned Quantum Dot Molecule Laser Fabricated by Electron Beam Lithography and Wet Chemical Etching

Quantum dot (QD) semiconductor lasers have demonstrated desirable properties when compared with traditional quantum well (QW)-based lasers such as ultra-low threshold current density and reduced temperature sensitivity [1]. Typically multiple layers of QDs must be stacked in order to provide enough gain to overcome cavity losses and achieve laser threshold. Due to strain fields created by the first QD layer, the QDs in subsequently grown layers tend to self-align with QDs in the first layer, resulting in vertical electronic coupling of the QD layers for small spacer layer thicknesses [2]. The result is effectively an array of QD “molecules”, which have also been proposed as potential building blocks for solid state quantum information processing applications [3].

Narrow Linewidth Surface-Etched DBR Lasers: Fundamental Design Aspects and Applications

Surface-etched distributed Bragg reflector (SE-DBR) semiconductor lasers show potential for use in a variety of communications and spectroscopy applications requiring compact, single-mode, narrow linewidth, and tunable laser sources. This approach eliminates contamination issues associated with forming gratings by regrowth in Al-containing structures by using a single growth step and etching the grating after the entire epitaxy is grown. This paper reviews progress in the development of SE-DBR lasers using InGaAs-GaAs-AlGaAs and GaAs-AlGaAs separate confinement heterostructure. Fabrication techniques, design optimization studies, and device performance characteristics are overviewed. Applications of SE-DBRs toward multiwavelength arrays, THz generation via optical heterodyning, and alkali spectroscopy are discussed.

Directed self-assembly of InAs quantum dots on nano-oxide templates

We describe the growth and characterization of InAs quantum dots on SiO2 patterned GaAs by metal organic chemical vapor deposition. Arrays of quantum dots with densities as high as 1.8×1010 cm−2 fabricated by electron beam lithography are demonstrated. A process consisting of dry and wet etching to minimize etch damage is developed. As the mask diameter increases, the nucleation transitions from single dots to multidot clusters. We achieve more uniform size and shape distributions of dots on patterned regions relative to unpatterned dots as revealed by structural characterization and room temperature photoluminescence emission spectra.

Investigation of GaAs/(Al,Ga)As multiple quantum wells grown on Ge and Si substrates by molecular-beam epitaxy

We report photoreflectance studies on GaAs/(Al,Ga)As multiple quantum wells grown on Si and Ge substrates. The sharp spectral features observed from various subband transitions indicate that good epilayer quality can be obtained on nonpolar substrates using suitable growth techniques. The experimental results agree well with calculated values based on the envelope function approximation, when the effect of residual strain resulting from the large difference in thermal expansion between GaAs and Si is taken into account.

Experimental verification of reduced intersubband scattering in ordered nanopore lattices

A photoluminescence study of emission from a periodically perforated quantum well at 77 K is presented. Good agreement is observed between numerical predictions and experimental results. The effects of pore diameter on peak emission wavelength and relative emission from second excited subbands are analyzed. The results are found to be consistent with predictions of reduced intersubband scattering rate in nanopore lattices due to the reduced wave function overlap between the initial and final states arising from the in-plane periodicity.