A. Vardi

Near infrared quantum cascade detector in GaN∕AlGaN∕AlN heterostructures

A quantum cascade detector in the GaN/AlGaN/AlN material system was implemented. The design takes advantage of the large internal field existing in the nitrides in order to generate the essential saw tooth energy level structure. The device operates in the near IR spectral range with a room temperature responsivity at λ=1.7μm of 10mA∕W (1000V∕W) at zero bias. The spectroscopic measurements are in good agreement with simulations.

GaN/AlGaN intersubband optoelectronic devices

This paper reviews recent progress toward intersubband (ISB) devices based on III-nitride quantum wells (QWs). First, we discuss the specific features of ISB active region design using GaN/AlGaN materials, and show that the ISB wavelength can be tailored in a wide spectral range from near- to long infrared wavelengths by engineering the internal electric field and layer thicknesses. We then describe recent results for electro-optical waveguide modulator devices exhibiting a modulation depth as large as 14 dB at telecommunication wavelengths. Finally, we address a new concept of III-nitride QW detectors based on the quantum cascade scheme, and show that these photodetectors offer the prospect of high-speed devices at telecommunication wavelengths.

Room temperature demonstration of GaN∕AlN quantum dot intraband infrared photodetector at fiber-optics communication wavelength

We fabricated a communication wavelength photodetector based on intraband transition in GaN∕AlN self-assembled quantum dot heterostructures. The quantum dot photodetector is based on in-plane transport and has a room temperature spectral peak responsivity of 8mA∕W at wavelength of 1.41μm. We use multipass waveguide geometry to show that the polarization sensitive optical absorption spectrum of the heterostructure is nearly the same as its photocurrent spectral response. This establishes that the detector’s response is due to the presence of quantum dots in its active layer. We use photoluminescence, transmission, and intraband photocurrent spectroscopy to consistently describe the alignment between the energy levels of the quantum dots and that of the wetting layer.

High-speed operation of GaN/AlGaN quantum cascade detectors at λ≈1.55 μm

We demonstrate room-temperature, high-speed operation of GaN/AlGaN quantum cascade detectors. The devices are processed as square mesas with 50 Ω coplanar access lines. Frequency response measurements were performed under illumination by a modulated laser diode emitting at λ=1.55 μm. The electrical response exhibits a first-order filter frequency response. For 17×17 μm2 (25×25 μm2) detectors the −3 dB cutoff frequency is 11.4 GHz (6.5 GHz). S-parameter analysis confirms that the cutoff frequency is extrinsically limited and that the speed of the device can be further increased by reducing the device size.

Femto-second electron transit time characterization in GaN/AlGaN quantum cascade detector at 1.5 micron

The ultra fast carrier dynamic in GaN/AlGaN quantum cascade detector was investigated using a time-resolved bias-lead monitoring technique. It is demonstrated that the intrinsic speed limitation, governed by the carrier transit time, is smaller than 1 ps, corresponding to a frequency cut off above 200 GHz.

Interlevel transitions and two-photon processes in Ge/Si quantum dot photocurrent

Photocurrent spectra due to interlevel transitions of holes in Ge/Si quantum dots show several peaks in the range of 60–300 meV, which superlinearly increase with bias, indicating release of carriers by tunneling. The relative peak intensity drastically changes with applied voltage, its polarity, and the measurement system. Lower energy peaks, at 69 and 86 meV, are observed only with a Fourier transform IR (FTIR) spectrometer. The 69 and 86 meV transitions excite holes into intermediate levels from which they are re-excited to shallow levels in a two-photon process. This is observed with FTIR only where the sample is simultaneously exposed to a wide range of energies. Direct band-to-band excitation at 1.25 eV increases the midinfrared signals by orders of magnitude by pumping the intermediate levels. Placing dopants in the barrier greatly increases photocurrent intensity and reduces noise. One-dimensional and three-dimensional numerical analyses confirm our findings.

Polarization-independent intersubband based GaInAsN quantum-well photodetector with dominant detection at 1.42 μm

We report on a room temperature polarization-independent intersubband photocurrent (PC) in quantum-well infrared photodetector based on a GaInAsN/GaAs standard multiple-quantum-well structure. The dominant room temperature PC is peaked at 1.42 μm with peak responsivity of 2 A/W and exhibits similar intensities in TM and TE polarizations. The structure’s energy levels were analyzed using a ten band k⋅p model. As a result of this analysis the 1.42 μm dominant PC transition is attributed to a transition from the fundamental E1− electron level into the localized quasicontinuum state formed by the unconfined E2+ electron energy level.

Stranski–Krastanov growth of GaN quantum dots on AlN template by metalorganic chemical vapor deposition

GaN/AlN quantum dots (QDs) grown on sapphire/GaN substrate have been realized by low-pressure metalorganic chemical vapor deposition. The influence of the growth conditions, postgrowth annealing ambience, and roughness of AlN template surface has been investigated. QDs with 3–10 nm height, 40–100 nm diameter, and density from 1×1011 down to 1×108 cm−2 were achieved by adjusting the growth temperature, V/III ratio, and growth time. For the formation of QDs, the postgrowth annealing in H2 is more favorable than that in NH3; however, it results in a bad surface of wetting layer and therefore is not suitable for device fabrication. When rough-surface templates were used, small-sized dots with the same density as obtained on smooth-surface templates could be found besides some large clusters. An intense photoluminescence peak of QDs is observed at 15 K and up to room temperature.

Fermi edge singularity observed in GaN/AlGaN heterointerfaces

We observe sharp spectral lines, at energies which are higher than the bulk GaN band gap, in the photoluminescence and photoluminescence excitation spectra of GaN/AlGaN heterointerfaces grown by molecular beam epitaxy. The spectra and their temperature dependence are in accord with the Fermi edge singularity expected for two dimensional electron gas systems. The associated localized hole energy in the AlGaN interface side was extracted directly from the spectra.

Negative photoconductivity due to intraband transitions in GaN/AlN quantum dots

In-plane photoconductivity (PC) measurements in a GaN/AlN quantum dots (QDs) layer show a TM-polarized infrared (IR) peak, at 0.9 eV, and a visible-UV (vis-UV) peak, at 2.8 eV. Based on the energy and polarization dependence, the 0.9 eV is associated with the polarized S to Pz intraband transition within the QDs. The IR PC turns from positive PC (PPC) to negative PC (NPC) as temperature is raised, increasing exponentially from 50 to 300 K. Vis-UV radiation renders PPC at all temperatures. Combined with vis-UV radiation, the IR PC is negative even at low temperatures. Based on these observations, we suggest a model in which IR excited carriers in the QD layer are coupled to deep levels (DLs) in the AlN barrier and turn immobile, i.e., NPC is observed. Vis-UV radiation re-excites them into the QDs, resulting in PPC. At lower temperatures coupling into the DL becomes inefficient, thus, IR excitation results in PPC. This model was translated into rate equations. Simulations based on these rate equations repro...