P. K. Kandaswamy

GaN/AlN short-period superlattices for intersubband optoelectronics: A systematic study of their epitaxial growth, design, and performance

We have studied the effect of growth and design parameters on the performance of Si-doped GaN/AlN multiquantum-well (MQW) structures for intersubband optoelectronics in the near infrared. The samples under study display infrared absorption in the 1.3–1.9 μm wavelength range, originating from the photoexcitation of electrons from the first to the second electronic level in the QWs. A commonly observed feature is the presence of multiple peaks in both intersubband absorption and interband emission spectra, which are attributed to monolayer thickness fluctuations in the quantum wells. These thickness fluctuations are induced by dislocations and eventually by cracks or metal accumulation during growth. The best optical performance is attained in samples synthesized with a moderate Ga excess during the growth of both the GaN QWs and the AlN barriers without growth interruptions. The optical properties are degraded at high growth temperatures (>720 °C) due to the thermal activation of the AlN etching of GaN. Fr...

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.

Tunnel-injection quantum dot deep-ultraviolet light-emitting diodes with polarization-induced doping in III-nitride heterostructures

Efficient semiconductor optical emitters in the deep-ultraviolet spectral window are encountering some of the most deep rooted problems of semiconductor physics. In III-Nitride heterostructures, obtaining short-wavelength photon emission requires the use of wide bandgap high Al composition AlGaN active regions. High conductivity electron (n-) and hole (p-) injection layers of even higher bandgaps are necessary for electrical carrier injection. This approach requires the activation of very deep dopants in very wide bandgap semiconductors, which is a difficult task. In this work, an approach is proposed and experimentally demonstrated to counter the challenges. The active region of the heterostructure light emitting diode uses ultrasmall epitaxially grown GaN quantum dots. Remarkably, the optical emission energy from GaN is pushed from 365 nm (3.4 eV, the bulk bandgap) to below 240 nm (>5.2 eV) because of extreme quantum confinement in the dots. This is possible because of the peculiar bandstructure and band alignments in the GaN/AlN system. This active region design crucially enables two further innovations for efficient carrier injection: Tunnel injection of carriers and polarization-induced p-type doping. The combination of these three advances results in major boosts in electroluminescence in deep-ultraviolet light emitting diodes and lays the groundwork for electrically pumped short-wavelength lasers.

Internal quantum efficiency of III-nitride quantum dot superlattices grown by plasma-assisted molecular-beam epitaxy

We present a study of the optical properties of GaN/AlN and InGaN/GaN quantum dot (QD) superlattices grown via plasma-assisted molecular-beam epitaxy, as compared to their quantum well (QW) counterparts. The three-dimensional/two-dimensional nature of the structures has been verified using atomic force microscopy and transmission electron microscopy. The QD superlattices present higher internal quantum efficiency as compared to the respective QWs as a result of the three-dimensional carrier localization in the islands. In the QW samples, photoluminescence (PL) measurements point out a certain degree of carrier localization due to structural defects or thickness fluctuations, which is more pronounced in InGaN/GaN QWs due to alloy inhomogeneity. In the case of the QD stacks, carrier localization on potential fluctuations with a spatial extension smaller than the QD size is observed only for the InGaN QD-sample with the highest In content (peak emission around 2.76 eV). These results confirm the efficiency o...

Strain relaxation in short-period polar GaN/AlN superlattices

We have investigated the strain relaxation mechanisms in short-period polar GaN/AlN superlattices deposited by plasma-assisted molecular-beam epitaxy, and designed to display intersubband transitions at 1.55 μm. In a first stage, we have identified the growth conditions to minimize strain relaxation, using a Ga excess to reduce the (0001) surface free energy of both GaN and AlN. Under these growth conditions, crack propagation is not observed, even for the tensile-strained superlattices grown on GaN templates. The initial misfit relaxation in the vicinity of the buffer occurs by the formation of a-type dislocations. The final strain state of the superlattice, reached after 10–20 periods, is independent of the substrate (either GaN or AlN templates). Once the steady-state conditions are reached, we observe a periodic partial relaxation of quantum wells and barriers. High-resolution transmission electron microscopy indicates that the periodic relaxation can be related to the presence of basal and prismatic ...

Tunnel-injection GaN quantum dot ultraviolet light-emitting diodes

We demonstrate a GaN quantum dot ultraviolet light-emitting diode that uses tunnel injection of carriers through AlN barriers into the active region. The quantum dot heterostructure is grown by molecular beam epitaxy on AlN templates. The large lattice mismatch between GaN and AlN favors the formation of GaN quantum dots in the Stranski-Krastanov growth mode. Carrier injection by tunneling can mitigate losses incurred in hot-carrier injection in light emitting heterostructures. To achieve tunnel injection, relatively low composition AlGaN is used for n- and p-type layers to simultaneously take advantage of effective band alignment and efficient doping. The small height of the quantum dots results in short-wavelength emission and are simultaneously an effective tool to fight the reduction of oscillator strength from quantum-confined Stark effect due to polarization fields. The strong quantum confinement results in room-temperature electroluminescence peaks at 261 and 340 nm, well above the 365 nm bandgap of bulk GaN. The demonstration opens the doorway to exploit many varied features of quantum dot physics to realize high-efficiency short-wavelength light sources.

Effect of doping on the mid-infrared intersubband absorption in GaN/AlGaN superlattices grown on Si(111) templates

We report on the effect of Si doping on the mid-infrared intersubband absorption in GaN/AlGaN superlattices. For increasing doping levels, interband luminescence displays a blueshift and a broadening of the band edge caused by the screening of the internal electric field and band-filling effects. The intersubband absorption energy is mainly governed by many-body effects like exchange interaction and depolarization shift, which increase the e1–e2 subband separation. The ISB blueshift induced by many-body effects can be more than 50% of the e1–e2 transition energy.

Midinfrared intersubband absorption in GaN/AlGaN superlattices on Si(111) templates

We report on the observation of midinfrared intersubband absorption in Si-doped GaN/AlGaN superlattices grown by plasma-assisted molecular-beam epitaxy on semi-insulating GaN-on-Si(111) templates. TM-polarized absorption attributed to transition between the first two electronic levels in the quantum wells peaked in the range from 2 to 9 μm. The relative spectral width remains around 20% in the whole midinfrared spectral range. Doping is predicted to have a large influence on the intersubband absorption energy due to screening of polarization-induced internal electric field.

Intersubband Transition-Based Processes and Devices in AlN/GaN-Based Heterostructures

We report on the physics, epitaxial growth, fabrication, and characterization of optoelectronic devices based on intersubband transitions in the AlN/GaN material system. While in 1999, only results of optical absorption experiments could be shown, photodetectors and modulators with operation frequencies beyond 10 GHz as well as optically pumped light emitters have been demonstrated recently. This is the reason for a comprehensive report on the most important properties of such devices. Beside some basic theoretical considerations, we will concentrate on the fabrication and characterization of modulators, switches, photodetectors, and light emitters. At the end of this paper, an outlook to future trends and developments in this emerging field will be given.

Suppression of nonradiative processes in long-lived polar GaN/AlN quantum dots

We present a temperature-dependent time-resolved photoluminescence study of the nonradiative processes in polar GaN/AlN quantum dots and quantum wells. The photoluminescence decay times of quantum wells drop above 50 K due to the presence of nonradiative recombination centers. In contrast, the three-dimensional carrier confinement in quantum dots efficiently suppresses nonradiative processes up to room temperature, even for radiative decay times reaching the microsecond range.