E. Towe

Normal-incidence intersubband (In, Ga)As/GaAs quantum dot infrared photodetectors

Univ Virginia, Lab Opt & Quantum Elect, Charlottesville, VA 22903 USA; USA, Res Lab, Adelphi, MD 20783 USA; Chinese Acad Sci, Inst Semicond, Beijing 100083, Peoples R China

Semiconductor quantum-dot nanostructures: Their application in a new class of infrared photodetectors

Semiconductor quantum-dot nanostructures are interesting objects for fundamental as well as practical reasons. Fundamentally, they can form the basis of systems in which to study the quantum mechanics of electrons confined in zero-dimensional (0-D) space. In practice, the dots can be embedded in the active regions of a new class of electronic and optoelectronic devices with novel functionalities. This paper reviews the state-of-the-art in the use of these objects in infrared detectors. It describes the progress, challenges, and projections for continued development of normal-incidence intersublevel detectors operating in the spectral region between 6 and 20 /spl mu/m.

A five-period normal-incidence (In, Ga)As/GaAs quantum-dot infrared photodetector

We have measured the optical and electrical properties of a five-period normal-incidence (In, Ga)As/GaAs quantum-dot infrared photodetector. A primary intersubband transition peak is observed at the wavelength of 10.2 μm and a secondary one at 9.4 μm. Excellent electron transport and peak detectivity of 7×109 cm Hz1/2/W are achieved at 30 K, with a low bias responsivity of up to 70 mA/W at 0.6 V. We believe that an observed avalanche gain process is initiated by intersubband absorption in the quantum dots. The maximum responsivity due to this avalanche multiplication process is about 4 A/W at a bias of 1.0 V.

Photovoltaic quantum-dot infrared detectors

We demonstrate the operation of photovoltaic quantum-dot infrared detectors fabricated from (In, Ga)As/GaAs heterostructures. These detectors are sensitive to normal incidence light. At zero bias, we obtain a low-temperature (78 K) peak detectivity of 2×108 cm Hz1/2/W, with a responsivity of 1 mA/W at a wavelength of 13 μm for one of the devices. The photovoltaic effect in our detectors is a result of the intrinsic inversion asymmetry of the band structure of self-formed quantum dots. A compensation voltage of 18 mV is measured.

NEAR-FIELD OPTICAL BEAM INDUCED CURRENT MEASUREMENTS ON HETEROSTRUCTURES

We report near‐field optical beam induced current (NOBIC) measurements on semiconductor quantum well (QW) structures. A subwavelength fiber tip is coupled with a tunable laser source and scanned over a sample surface. The induced photocurrent reveals the compositional profile of quantum structures. Semiconductor QW structures were designed and fabricated by molecular beam epitaxy (MBE) to study the wavelength dependence and resolution capability of NOBIC. We demonstrated that the resolution of this technique strongly depends on the aperture size. For aperture sizes that allow for coupling of evanescent fields from the tip into the semiconductor as propagating fields, the resolution strongly depends on the excitation wavelength due to the variation of the optical penetration depth. For smaller apertures, the optical field remains evanescent in the semiconductor and resolution is essentially independent of the wavelength.

Polarization control of vertical-cavity surface-emitting lasers through use of an anisotropic gain distribution in [110]-oriented strained quantum-well structures

An analysis of the in-plane optical matrix elements connected with the gain distribution of (In,Ga)As-GaAs quantum-well structures on [110] GaAs substrates is presented. The in-plane gain distribution is found to be anisotropic-with a maximum directed along the [110]-[110] crystallographic axis. Optically-pumped vertical-cavity surface-emitting lasers on the [110] surface with these quantum wells in the cavity exhibit stable, well-defined polarization states; this stability is believed to be a consequence of the predicted anisotropic gain distribution on the [110] surface. Of the two orthogonal eigen polarizations observed, the one with the higher optical intensity, for a given pump power, was found to be stabilized along the [110] crystallographic axis; this is in agreement with the analysis. >

100-GHz resonant cavity enhanced Schottky photodiodes

Resonant cavity enhanced (RCE) photodiodes are promising candidates for applications in optical communications and interconnects where ultrafast high-efficiency detection is desirable. We have designed and fabricated RCE Schottky photodiodes in the (Al,In)GaAs material system for 900-nm wavelength. The observed temporal response with 10-ps pulsewidth was limited by the measurement setup and a conservative estimation of the bandwidth corresponds to more than 100 GHz. A direct comparison of RCE versus conventional detector performance was performed by high speed measurements under optical excitation at resonant wavelength (895 nm) and at 840 nm where the device functions as a single-pass conventional photodiode. A more than two-fold bandwidth enhancement with the RCE detection scheme was demonstrated.

Fabrication of high-speed resonant cavity enhanced Schottky photodiodes

We report the fabrication and testing of a GaAs-based high-speed resonant cavity enhanced (RCE) Schottky photodiode. The top-illuminated RCE detector is constructed by integrating a Schottky contact, a thin absorption region (In/sub 0.08/Ga/sub 0.92/As) and a distributed AlAs-GaAs Bragg mirror. The Schottky contact metal serves as a high-reflectivity top mirror in the RCE detector structure. The devices were fabricated by using a microwave-compatible fabrication process. The resulting spectral photo response had a resonance around 895 nm, in good agreement with our simulations. The full-width-at-half-maximum (FWHM) was 15 nm, and the enhancement factor was in excess of 6. The photodiode had an experimental setup limited temporal response of 18 ps FWHM, corresponding to a 3-dB bandwidth of 20 GHz.

Near-field optical studies of semiconductor heterostructures and laser diodes

Near-field optical microscopy and spectroscopy is emerging as a powerful tool for the investigation of semiconductor structures. Tunable excitation combined with sub-wavelength resolution is providing an unprecedented level of detail on the local optical properties of semiconductor structures. Recent near-field optical studies have addressed issues of laser diode mode profiling, minority carrier transport, near-field photocurrent response of quantum-well structures and laser diodes, imaging of local waveguide properties, and location and studies of dislocations in semiconductor thin films. We present results on the intrinsic resolution limitations of near-field photoconductivity in quantum-well heterostructures and demonstrate that the resolution depends strongly on the amount of evanescent and propagating field components in the semiconductor. Spectroscopic mode-profiling of high-power laser diode emission details the spatial dependence of multiple spectral modes. This paper presents an overview of NSOM techniques for semiconductor systems, its limitations, and present status.

Effective mass theory for III‐V semiconductors on arbitrary (hkl) surfaces

The effects of arbitrary substrate orientation on the electronic and optical properties of III‐V zinc‐blende semiconductors are considered. A unitary transformation matrix is used to rotate the 4×4 Luttinger valence band Hamiltonian, and the Bir‐Pikus strain Hamiltonian from the conventional (001) surface to any arbitrary (hkl) surface of interest. The effects of strain on several electronic and optical properties are examined. It is found that the strain‐induced change in the forbidden gap is largest for the (111) plane and other equivalent planes. Furthermore, the strain is also found to induce both a longitudinal and a transverse piezoelectric field. The longitudinal field reaches a maximum for the (111) surface and its other equivalent planes, while the transverse field reaches a maximum for the (110) surface and its other equivalent planes. The orientation‐dependence of the hole effective masses is also examined; it is found that the (111) surface, and other equivalent planes, exhibits the largest he...