Paul J. Simmonds

Quantum Dot Resonant Tunneling Diode for Telecommunication Wavelength Single Photon Detection

The authors present a quantum dot (QD) based single photon detector operating at a fiber optic telecommunication wavelength. The detector is based on an AlAs∕In0.53Ga0.47As∕AlAs double-barrier resonant tunneling diode containing a layer of self-assembled InAs QDs grown on an InP substrate. The device shows an internal efficiency of about 6.3% with a dark count rate of 1.58×10−6ns−1 for 1310nm photons.

GaSb thermophotovoltaic cells grown on GaAs by molecular beam epitaxy using interfacial misfit arrays

There exists a long-term need for foreign substrates on which to grow GaSb-based optoelectronic devices. We address this need by using interfacial misfit arrays to grow GaSb-based thermophotovoltaic cells directly on GaAs (001) substrates and demonstrate promising performance. We compare these cells to control devices grown on GaSb substrates to assess device properties and material quality. The room temperature dark current densities show similar characteristics for both cells on GaAs and on GaSb. Under solar simulation the cells on GaAs exhibit an open-circuit voltage of 0.121 V and a short-circuit current density of 15.5 mA/cm2. In addition, the cells on GaAs substrates maintain 10% difference in spectral response to those of the control cells over a large range of wavelengths. While the cells on GaSb substrates in general offer better performance than the cells on GaAs substrates, the cost-savings and scalability offered by GaAs substrates could potentially outweigh the reduction in performance. By fu...

Metamorphic GaAsP buffers for growth of wide-bandgap InGaP solar cells

compressively-strained graded GaAsxP1�x buffers on GaP showed nearly-complete strain relaxation of the top layers and no evidence of trenches but possessed threading dislocation densities that were one order of magnitude higher. We subsequently grew and fabricated wide-bandgap InyGa1�yP solar cells on our GaAsxP1�x buffers. Transmission electron microscopy measurements gave no indication of CuPt ordering. We obtained open circuit voltage as high as 1.42 V for In0.39Ga0.61P with a bandgap of 2.0 eV. Our results indicate MBE-grown InyGa1�yP is a promising material for the top junction of a future multijunction solar cell.

Structural and optical properties of InAs/AlAsSb quantum dots with GaAs(Sb) cladding layers

We investigate the effect of GaAs1−xSbx cladding layer composition on the growth and properties of InAs self-assembled quantum dots surrounded by AlAs0.56Sb0.44 barriers. Lowering Sb-content in the GaAs1−xSbx improves the morphology of the InAs quantum dots and reduces cladding layer alloy fluctuations. The result is a dramatic increase in photoluminescence intensity from the InAs quantum dots, with a peak at 0.87 eV. The emission energy exhibits a cube root dependence on excitation power, consistent with the type-II band alignment of the quantum dots. The characteristics of this quantum dot system show promise for applications such as intermediate band solar cells.

Photoluminescence from In0.5Ga0.5P/GaP quantum dots coupled to photonic crystal cavities

We demonstrate room temperature visible wavelength photoluminescence from In0.5Ga0.5As quantum dots embedded in a GaP membrane. Time-resolved above band photoluminescence measurements of quantum dot emission show a biexpontential decay with lifetimes of ~200 ps. We fabricate photonic crystal cavities which provide enhanced outcoupling of quantum dot emission, allowing the observation of narrow lines indicative of single quantum dot emission. This materials system is compatible with monolithic integration on Si, and is promising for high efficiency detection of single quantum dot emission as well as optoelectronic devices emitting at visible wavelengths.

Self-assembled In0.5Ga0.5As quantum dots on GaP

We demonstrate the growth and luminescence of coherently strained In0.5Ga0.5As self-assembled quantum dots on GaP. Cross-sectional and planar-view transmission electron microscopy confirmed the dislocation-free nature of the In0.5Ga0.5As quantum dots and GaP cap layers. Intense photoluminescence from the quantum dots was measured at 80 K and was visible to the unaided eye in ambient lighting. The photoluminescence results show that emission energy can be controlled by varying the In0.5Ga0.5As deposition thickness. In combination with recent advances in the growth of GaP on Si, the In0.5Ga0.5As quantum dots demonstrated here could enable monolithic optoelectronic integration on Si.

Tuning quantum dot luminescence below the bulk band gap using tensile strain.

Self-assembled quantum dots (SAQDs) grown under biaxial tension could enable novel devices by taking advantage of the strong band gap reduction induced by tensile strain. Tensile SAQDs with low optical transition energies could find application in the technologically important area of mid-infrared optoelectronics. In the case of Ge, biaxial tension can even cause a highly desirable crossover from an indirect- to a direct-gap band structure. However, the inability to grow tensile SAQDs without dislocations has impeded progress in these directions. In this article, we demonstrate a method to grow dislocation-free, tensile SAQDs by employing the unique strain relief mechanisms of (110)-oriented surfaces. As a model system, we show that tensile GaAs SAQDs form spontaneously, controllably, and without dislocations on InAlAs(110) surfaces. The tensile strain reduces the band gap in GaAs SAQDs by ~40%, leading to robust type-I quantum confinement and photoluminescence at energies lower than that of bulk GaAs. This method can be extended to other zinc blende and diamond cubic materials to form novel optoelectronic devices based on tensile SAQDs.

Self-assembly on (111)-oriented III-V surfaces

We demonstrate the self-assembly of tensile strained GaP into three-dimensional dots on GaAs(111)A. Size and areal density of the dislocation-free GaP dots are readily tunable with both substrate temperature and deposition thickness. GaP dot growth obeys island scaling theory, allowing us to predict dot size distributions a priori.

Quantum transport in In0.75Ga0.25As quantum wires

In addition to quantized conductance plateaus at integer multiples of 2e2∕h, the differential conductance G=dI∕dV shows plateaus at 0.25(2e2∕h) and 0.75(2e2∕h) under applied source-drain bias in In0.75Ga0.25As quantum wires defined by insulated split gates. This observation is consistent with a spin-gap model for the 0.7 structure. Using a tilted magnetic field to induce Landau level crossings, the g factor was measured to be ∼9 by the coincidence method. This material, with a mobility of 1.8×105cm2∕Vs at a carrier density of 1.4×1011cm−2, may prove useful for further study of electron-electron interaction effects in quantum wires.

Tensile-strained growth on low-index GaAs

We present a comparative study of the growth of tensile-strained GaP on the four low-index surfaces of GaAs: (001), (110), (111)A, and (111)B. For each surface orientation we outline the growth conditions required for smooth GaAs homoepitaxy. We are able to predict the resulting surface morphology when GaP is deposited onto these four GaAs surfaces by considering the influence of surface orientation on tensile strain relief. GaP deposited on GaAs(001) forms extremely smooth, planar layers. In contrast, the elastic relief of tensile strain on both GaAs(110) and GaAs(111)A leads to the three-dimensional self-assembly of GaP into dislocation-free nanostructures. Similarities between tensile and compressive self-assembly suggest that the kinetics governing many aspects of self-assembled growth is independent of the sign of strain. We show that differences in self-assembly on GaAs(110) and (111)A are the result of unequal adatom diffusion lengths. Tensile-strained self-assembly also occurs on GaAs(111)B, altho...