Christos Thomidis

AlGaN based deep ultraviolet light emitting diodes with high internal quantum efficiency grown by molecular beam epitaxy

We report the development of AlGaN based deep ultraviolet light emitting diodes (UV-LEDs) by molecular beam epitaxy. By growing the AlGaN well layer under Ga-rich conditions to produce strong potential fluctuations, internal quantum efficiency of a quantum well structure emitting at 300 nm was found to be 32%. By combining such Ga-rich growth condition in the active region with polarization field enhanced carrier injection layers, deep UV-LEDs emitting at 273 nm were obtained with output power of 0.35 mW and 1.3 mW at 20 mA continuous wave and 100 mA pulsed drive current, respectively. The maximum external quantum efficiency was 0.4%.

Ultrafast all-optical switching with low saturation energy via intersubband transitions in GaN/AlN quantum-well waveguides.

A fiber-optic pump-probe setup is used to demonstrate all-optical switching based on intersubband cross-absorption modulation in GaN/AlN quantum-well waveguides, with record low values of the required control pulse energy. In particular, a signal modulation depth of 10 dB is obtained with control pulse energies as small as 38 pJ. Such low power requirements for this class of materials are mainly ascribed to an optimized design of the waveguide structure. At the same time, the intersubband absorption fully recovers from the control-pulse-induced saturation on a picosecond time scale, so that these nonlinear waveguide devices are suitable for all-optical switching at bit rates of several hundred Gb/s.

Nonlinear optical waveguides based on near-infrared intersubband transitions in GaN/AlN quantum wells

We present the design, fabrication, and characterization of III-nitride quantum-well waveguides optimized for nonlinear-optical switching via intersubband transitions. A dielectric structure consisting of an AlN lower cladding and a GaN cap layer allows minimizing the propagation losses while maintaining a large modal overlap with the quantum-well active layer. A strong nonlinear saturation of the intersubband absorption near 1.55 mum is demonstrated at record low input powers for these materials; in particular, a 3-dB saturation pulse energy of less than 10 pJ with 240-fs pulses is measured. Combined with the well established sub-picosecond recovery lifetimes of intersubband absorption in III-nitride quantum wells, these results are very promising for all-optical switching applications in future ultrafast fiber-optic communication networks.

Ultraviolet electroabsorption modulator based on AlGaN/GaN multiple quantum wells

An ultraviolet electroabsorption modulator based on AlGaN∕GaN quantum wells is demonstrated. Enhanced excitonic absorption in the quantum wells at around 3.48eV was achieved using a Schottky contact to partially cancel the polarization-induced electric fields in the quantum well layers. A change in the absorption coefficient greater than 4×104cm−1 was obtained for a modest reverse bias of 10V. The observed blueshift in the exciton energy was smaller than that predicted by theoretical calculations. This is accounted for by variations in the background carrier concentration in the wells with reverse bias.

Enhanced internal quantum efficiency and light extraction efficiency from textured GaN∕AlGaN quantum wells grown by molecular beam epitaxy

GaN∕Al0.2Ga0.8N multiple quantum wells (MQWs) were grown by molecular beam epitaxy on randomly textured and atomically smooth (0001) GaN templates. Smooth and textured GaN templates were deposited on (0001) sapphire substrates by varying the III∕V ratio and the substrate temperature during growth by the hydride vapor-phase epitaxy method. We find that the MQWs replicate the texture of the GaN template, which was found to have a Gaussian distribution. The peak photoluminescence intensity from the textured MQWs is always higher than from the smooth MQWs and for GaN (7nm)∕Al0.2Ga0.8N (8nm) MQWs, it is 700 times higher than that from similarly produced MQWs on smooth GaN templates. This result is attributed partly to the enhancement in light extraction efficiency and partly to the enhancement in internal quantum efficiency. The origin of the increase in internal quantum efficiency is partly due to the reduction of the quantum-confined Stark effect, since the polarization vector intersects the quantum well (QW...

High power ultraviolet light emitting diodes based on GaN/AlGaN quantum wells produced by molecular beam epitaxy

In this paper, we report on the growth by molecular beam epitaxy and fabrication of high power nitride-based ultraviolet light emitting diodes emitting in the spectral range between 340 and 350nm. The devices were grown on (0001) sapphire substrates via plasma-assisted molecular beam epitaxy. The growth of the light emitting diode (LED) structures was preceded by detailed materials studies of the bottom n-AlGaN contact layer, as well as the GaN∕AlGaN multiple quantum well (MQW) active region. Specifically, kinetic conditions were identified for the growth of the thick n-AlGaN films to be both smooth and to have fewer defects at the surface. Transmission-electron microscopy studies on identical GaN∕AlGaN MQWs showed good quality and well-defined interfaces between wells and barriers. Large area mesa devices (800×800μm2) were fabricated and were designed for backside light extraction. The LEDs were flip-chip bonded onto a Si submount for better heat sinking. For devices emitting at 340nm, the measured diffe...

Sequential tunneling transport characteristics of GaN/AlGaN coupled-quantum-well structures

Vertical electronic transport in periodic GaN/AlGaN multiple-quantum-well structures grown on free-standing GaN substrates is investigated. Highly nonlinear current-voltage characteristics are measured, displaying a clear transition from a high-resistance state near zero applied bias to a low-resistance state as the voltage is increased. The measurement results, including their temperature dependence and the variations in turn-on voltage with subband structure and bias polarity are in full agreement with a picture of sequential tunneling through the ground-state subbands of adjacent coupled quantum wells. Scattering-assisted tunneling due to interface roughness or structural defects appears to be the dominant transport mechanism. The potential role of photon-assisted tunneling is also investigated.

Molecular beam epitaxy growth of AlGaN quantum wells on 6H-SiC substrates with high internal quantum efficiency

The authors report the development of high internal quantum efficiency AlN/AlGaN/AlN double heterostructures and AlGaN/AlN multiple quantum wells (MQWs) grown on 6H-SiC and 4H-SiC substrates of various miscuts by plasma-assisted molecular-beam epitaxy. The authors find that the luminescence spectra for identical MQWs show a single peak across the gap, with a wavelength that is redshifted by ∼20 nm as the excess Ga during growth of the wells increases. The internal quantum efficiency of the double heterostructures emitting at 250 nm is found to be 43%, and that of the multiple quantum wells emitting at 245 nm is 68%. These results suggest that AlGaN alloys on SiC substrates are capable of producing deep-ultraviolet emitters with high efficiency. The authors propose that these results can be accounted for by the introduction of lateral band structure potential fluctuations due to the changing of the growth mode from physical vapor phase epitaxy to liquid phase epitaxy (LPE) as the excess gallium increases. ...

Investigation of excitons in AlGaN/GaN multiple quantum wells by lateral photocurrent and photoluminescence spectroscopies

We report on the investigation of excitons in Al0.2Ga0.8N/GaN multiple quantum wells (MQWs) by lateral photocurrent, and photoluminescence (PL) spectroscopies over the temperature range from 9 to 300 K. The MQWs were deposited homoepitaxially by plasma-assisted molecular-beam epitaxy on a (0001) GaN template grown by hydride vapor phase epitaxy. Excitonic peaks in the photocurrent spectra due to the bulk GaN template and the MQW structure were observed up to room temperature. The PL excitonic peak in the MQWs was strongly Stokes shifted over the whole temperature range, a result attributed to recombination via disorder-induced excitonic band-tail states in the MQWs, due primarily to well/barrier interface roughness fluctuations. A theoretical calculation estimates these fluctuations to be 0.7±0.2 monolayers. The temperature dependence of the Stokes shift indicates that, at higher temperatures, the excitons in the PL experiments are in thermal equilibrium with the lattice before recombining. At lower tempe...

Deep UV-LEDs with high IQE based on AlGaN alloys with strong band structure potential fluctuations

In this paper we review our progress in developing AlGaN-based deep UV LEDs with internal quantum efficiency (IQE) in excess of 50%. This is accomplished by growing the active region of the LEDs by plasma-assisted MBE under a growth mode which promotes the introduction of deep band structure potential fluctuations in the wells beyond the statistical ones due alloy disorder. AlGaN-based deep UV-LEDs emitting in the wavelength range from 320 nm to 265 were grown by this method and fabricated into devices. By combining high IQE AlGaN QWs in the active region with polarization field enhanced carrier injection layers, unpackaged deep UV-LEDs emitting at 295 nm and 273 nm were obtained with optical output power of 0.35 mW and 1.8 mW at 20 mA continuous wave and 100 mA pulsed drive current, respectively. The maximum external quantum efficiency of these devices was calculated to be 0.4%, a result consistent with the low extraction efficiency of only 1-2%.