Multi-quantum Well Heterostructures Research Articles

Defect Tolerance of Intersubband Transitions in Nonpolar GaN/(Al,Ga)N Heterostructures: A Path toward Low-Cost and Scalable Mid- to Far-Infrared Optoelectronics

We report on the impact of structural defects on mid-infrared intersubband (ISB) properties of $\mathrm{Ga}\mathrm{N}/(\mathrm{Al},\mathrm{Ga})\mathrm{N}$ heterostructures grown by ammonia molecular beam epitaxy (${\mathrm{NH}}_{3}$ MBE). Twenty-period $\mathrm{Ga}\mathrm{N}/(\mathrm{Al},\mathrm{Ga})\mathrm{N}$ multi-quantum-well (MQW) heterostructures are grown on co-loaded a-plane freestanding $\mathrm{Ga}\mathrm{N}$ substrates and heteroepitaxial a-plane $\mathrm{Ga}\mathrm{N}$ on r-plane sapphire templates (a-$\mathrm{Ga}\mathrm{N}$/r-sap) for three different quantum-well (QW) widths (3.0, 3.3, and 3.7 nm). Co-loaded structures grown on freestanding a-plane with no basal-plane stacking faults (BSFs), prismatic stacking faults (PSFs), and partial dislocations (PDs), with low threading dislocation (TD) densities of about ${10}^{5}\phantom{\rule{0.25em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$ are compared with those grown on a-$\mathrm{Ga}\mathrm{N}$ templates on $(10\overline{1}2)$ r-sapphire with BSF, PSF, PD, and TD densities of about 4 \ifmmode\times\else\texttimes\fi{} ${10}^{5}$ to ${10}^{6}\phantom{\rule{0.25em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$, 5 \ifmmode\times\else\texttimes\fi{} ${10}^{3}$ to 2 \ifmmode\times\else\texttimes\fi{} ${10}^{4}\phantom{\rule{0.25em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$, about 9 \ifmmode\times\else\texttimes\fi{} ${10}^{10}$ to 2 \ifmmode\times\else\texttimes\fi{} ${10}^{11}\phantom{\rule{0.25em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$, and about ${10}^{10}\phantom{\rule{0.25em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$, respectively. Fourier-transform infrared absorption spectroscopy indicates ISB transition energies in the range of about 250--300 meV (wavelength range 4.1--4.8 \textmu{}m) for MQWs with different QW widths. The ISB absorption spectra indicate about 5% smaller transition energies and only about 10%--20% larger spectral linewidths for structures grown on a-$\mathrm{Ga}\mathrm{N}/$r-sapphire templates compared with those on freestanding $\mathrm{Ga}\mathrm{N}$ substrates. The strong defect tolerance in the nonpolar a-plane ISB structures could be due to the nature of defects and their energy levels with respect to the conduction-band minima, which do not affect the ISB properties. Our results pave the way toward the production of low-cost scalable nonpolar III-nitride MQW heterostructures for a variety of passive and active optical materials and devices based on intersubband transitions.

Design and modeling of third-harmonic plasmonic metasurfaces coupled to multi-quantum well structures

In this paper, we analyze plasmonic nonlinear metasurfaces based on multi-quantum well (MQW) substrates designed for the third-harmonic generation (THG) nonlinear process. MQW plasmonic metasurfaces have become one of the most noticeable structures in the implementation of nonlinear processes due to the high conversion efficiency for low pump intensities. We propose a semi-analytical algorithm to model TH metasurfaces using the Lorentz reciprocity theorem. Also, in order to predict the electromagnetic behavior of these structures accurately, the THG saturation effects are taken into account. In the next step, two MQW heterostructures and an efficient plasmonic resonator are designed for THG at 30 THz fundamental frequency. In our best design, power conversion efficiency up to 2.1×10−3% is achieved by applying pump intensity below the optical damage threshold (about 200 kW/cm2). The proposed TH metasurfaces can find application used in spectroscopy and as mid-infrared optical sources.

Carrier dynamics in Ga(NAsP)/Si multi-quantum well heterostructures with varying well thickness

Time-resolved photoluminescence (TR-PL) measurements have been performed in Ga(NAsP)/(BGa)(AsP) multi-quantum well heterostructures (MQWHs) with different well thicknesses. The studied structures have been pseudomorphically grown on Si substrates by metal organic vapor phase epitaxy (MOVPE) with an N content of about 7%. Experimental results reveal a shortening in the PL decay time with increasing QW thickness, meanwhile, accompanied by a decrease in the PL intensity. We attribute this behavior to an increasing non-radiative recombination rate for broader QWs which arises from an increasing number of defects in the QW material. The emission-energy distribution of the PL decay time is studied at various temperatures. The PL decay time strongly depends on the emission energy at low temperatures and becomes emission-energy-independent close to room temperature. This is discussed in terms of the carrier localization in the studied structures.

MOVPE growth studies of Ga(NAsP)/(BGa)(AsP) multi quantum well heterostructures (MQWH) for the monolithic integration of laser structures on (001) Si-substrates

High-quality, pseudomorphically strained Ga(NAsP)/(BGa)(AsP)-multiple quantum well heterostructures (MQWH) have been deposited on exactly oriented (001) Si-substrate by metal organic vapour phase epitaxy (MOVPE) in a wide temperature range between 525°C and 700°C. The individual atomic incorporation efficiencies, growth rates as well as nanoscale material properties have been clarified by applying detailed high-resolution X-ray diffraction (HR-XRD), photoluminescence (PL) spectroscopy and high-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) studies. An almost constant N-incorporation efficiency is obtained for a wide growth temperature range from 550°C up to 650°C. The P-incorporation is steadily increasing with increasing growth temperature reaching values at high temperatures in excess of the applied gas phase ratio. While the lower interface from the binary GaP- to the quaternary Ga(NAsP)-material system is very sharp, the upper interface is significantly rougher with a roughness scale of ±0.43nm in quantum well thickness variation at a growth temperature of 525°C. This roughness scale increases steadily with increasing growth temperature. No indication of any phase separation effects is detected in the Ga(NAsP)-material system even at the highest growth temperature of 700°C. The obtained experimental results are briefly discussed with respect to the anticipated metastable character of the novel dilute-nitride Ga(NAsP)-material system grown lattice-matched to (001) Si-substrate.

AlGaAs/GaAs multiquantum-well heterostructures for long-wavelength (8–10 μm) IR photodetectors

We have studied AlGaAs/GaAs multiquantum-well heterostructures grown by molecular beam epitaxy in an STE-3532 setup (SemiTEq, St. Petersburg), which are intended for long-wavelength IR photodetectors operating on inter-subband transitions. Quantum wells (QWs) in the obtained heterostructures are highly homogeneous and possess sharp heteroboundaries, which is confirmed by the photoluminescence spectra and dark current-voltage characteristics of photodetectors based on these heterostructures. The photodetectors exhibit sensitivity in the atmospheric transparency window (8–10 mm) and possess parameters that make possible their use in large-format photodetector arrays for a new generation of long-wavelength IR camera systems.

High room-temperature optical gain in Ga(NAsP)/Si heterostructures

We analyze the modal gain of Ga(NAsP) multi quantum-well heterostructures pseudomorphically grown on (001) silicon substrate by metal-organic vapor-phase epitaxy. Using the variable stripe length method, we obtain high modal gain values up to 78 cm−1 at room temperature that are comparable to the values of common high quality III-V laser material. We find good agreement between experimental results and theoretically calculated gain spectra obtained using a microscopic model. The results underline the high potential of Ga(NAsP) as an active material for directly electrically pumped lasers on silicon substrate.

Novel Ga(NAsP)-Based Heterostructures for the Integration of Optoelectronic Functionalities on (001) Si-Substrate

The novel direct band gap, dilute nitride Ga(NAsP)-material system allows for the first time the monolithic integration of a III/V laser material lattice matched to Si substrate. This lattice-matched approach offers the possibility for a high-quality, low defect density integration of a III/V-laser material potentially leading to long-term stable laser devices on Si-substrate. The broad area laser structures consist of pseudomorphically strained active Ga(NAsP)/(BGa)(AsP) multi-quantum-well heterostructures (MQWHs) embedded in thick doped (BGa)P waveguide layers, grown by a specific low-temperature metal organic vapour phase epitaxy (MOVPE) process on (001) Si-substrate. This paper will present and discuss the current status of the material and device optimization to realise electrical injection laser diodes as a basis for Si-photonics based optoelectronic integrated circuits (OEICs) with novel functionalities.

Monolithic integration of Ga(NAsP)/(BGa)P multi-quantum well structures on (0 0 1) silicon substrate by MOVPE

The pseudomorphic integration of an active III/V laser material onto Si substrates requires precise strain management of the entire several-μm-thick structure at both room and growth temperature. The incorporation of B into (BGa)P allows for the deposition of tensilely strained layers on the Si substrate in order to strain compensate the highly compressively strained Ga(NAsP) active material system. It is shown that the incorporation efficiency of B under the chosen conditions is unity and that (BGa)P layers with B concentrations high enough to allow for the strain compensation of an active Ga(NAsP) laser material on Si can be grown with excellent structural quality. Therefore, Ga(NAsP)/(BGa)P multi-quantum well heterostructures (MQWHs) with high-crystal quality as determined by high-resolution X-ray diffraction (XRD) were pseudomorphically grown on (0 0 1) Si substrates. The potential of this material combination to serve as an active optoelectronic device on Si is demonstrated by efficient room-temperature photoluminescence observed from the integrated Ga(NAsP) MQWH.

Annealing experiments of the GaP based dilute nitride Ga(NAsP)

AbstractThe post‐growth annealing behaviour of Ga(NAsP) multi quantum well heterostructures (MQWHs) grown pseudomorphically strained to GaP substrate has been investigated. The optical properties as well as the structural crystal quality of the novel dilute nitride show an obvious dependence on the applied annealing temperature. Photoluminescence (PL) measurements reveal a step‐like blue shift of the PL peak position in line with an increase of PL intensity with rising annealing temperature. The PL line width decreases to a mini‐ mal value for an optimized heating temperature around 800 °C. This annealing behaviour of the Ga(NAsP)/GaP‐MQWHs up to 850 °C is quite typical for a dilute nitride, however, the functional dependence of the integrated intensity above 850 °C is unusual. The increase of the PL line width above 850 °C suggests a deterioration of the crystalline MQW quality, but transmission electron microscopy (TEM) and high resolution X‐ray diffraction (XRD) prove the opposite. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

MOVPE growth conditions of the novel direct band gap, diluted nitride Ga(NAsP) material system pseudomorphically strained on GaP-substrate

Multi-quantum well heterostructures (MQWHs) in the novel dilute nitride Ga(NAsP)/GaP material system have been grown pseudomorphically strained on GaP-substrate by metal organic vapour phase epitaxy (MOVPE). The group V-incorporation has been clarified as a function of growth temperature, chemical composition, gas phase V/V-ratios and macroscopic strain. An overall complex incorporation behaviour of the group V-atoms is observed. The precise adjustment of the V/V competition processes on the crystal surface allows, however, for a control of the solid composition of the Ga(NAsP)/GaP material system.

Luminescence investigations of the GaP-based dilute nitride Ga(NAsP) material system

Multi-quantum well heterostructures (MQWHs) of the novel Ga(NAsP)/GaP material system have been grown, pseudomorphically strained to GaP-substrate. The crystalline perfection is verified by transmission electron microscopy (TEM). For As-concentrations in excess of about 70%, a direct band structure and adequate luminescence efficiency for laser device application is observed. Temperature-dependent photoluminescence (PL) investigations show the influence of carrier localisation and non-radiative recombination processes typical for dilute nitride materials. With rising N content in the active material, the emission wavelength shifts towards longer wavelength, leading to Ga(NAs)/GaP MQW structures with photon energies below the indirect band gap of silicon (Si). At the same time the luminescence intensity drops due to an increase in non-radiative carrier traps and/or structural degradation.

Scanning photoluminescence spectroscopy in InAs∕InGaAs quantum-dot structures

Spatially-resolved photoluminescence (PL) spectroscopy was performed at different temperatures on self-assembled InAs quantum dots embedded into MBE-grown In0.15Ga0.85As∕GaAs multiquantum-well heterostructures. Strong inhomogeneity of the PL intensity is observed by mapping samples with different In∕Ga composition of the InxGa1−xAs capping layers (0.1⩽x⩽0.2). Two different behaviors in the quantum-dot PL maps are observed: (1) a reduction of the PL intensity is accompanied by a gradual “blue” shift of the luminescence maximum at 300K and “red” shift at 80K, and (2) PL intensity variation occurs at a stable peak position of the PL maximum. Two separate mechanisms are suggested to account for the observed intensity variation of the quantum-dot luminescence.

Differential Gain Analysis of InGaAs/InGaAsP/InP Multiquantum Well Lasers with 1.55 <FONT FACE=Symbol>m</font>m Emission Wavelength

We present measurements of the differential gain of multiquantum well (MQW) broad area lasers emitting at 1.55<FONT FACE="Symbol">m</font>m wavelength, taking into account the nonuniform stimulated emission caused by filamentation within the optical cavity. The differential gain was determined from measurements of the threshold current density, considering the losses due to the inhomogenous carrier density distribution, as an apparent leakage current effect in the output power-current characteristics. The lasers were grown by low pressure MOCVD and incorporate an InGaAs/InGaAsP/InP separate confinement MQW active region. The results are compared with previous data reported on similar MQW heterostructures.

Visible electroluminescence from crystallized a-Si:H/a-SiN x:H multiquantum well structures

Visible photoluminescence (PL) in crystallized a-Si:H/a-SiN x:H multiquantum well (MQW) structures is reported for the first time. In this paper new results are presented on visible electroluminescence (EL) from these novel luminescent structures. The MQW heterostructures consisting of 40 layers were formed by PECVD method and then crystallized by Ar ion laser annealing technique. The crystallized samples with well layer thickness, L S = 4 nm, showed an intense visible PL peaked ∼ 2.0 eV. The experimental EL cells are of the form of semitransparent metal/crystallized a-Si:H/a-SiN x:H MQWs/p +-a-Si:H/quartz substrate. When the forward current density reaches a certain value, stable visible (orange) light is observed by the naked eye. A possible explanation of this light emission is the radiative recombination due to the carriers injection into quantized states in nano-size Si crystallites within the Si potential wells.

New photoconductive gain mechanism by electric field modulation in multiquantum-well heterostructures

A new type of photoconductivity in semiconductor heterojunctions has been observed. The structures are InP/In0.47Ga0.53As forward-biased multiquantum-well p-n junctions grown by chemical-beam epitaxy. This phenomenon manifests itself in a photocurrent proportional to the dark differential conductance. This effect is associated with the partial screening of the electric field in the wells induced, at constant bias, by photogenerated carriers in the wells. The resulting increase of the field in the barriers leads to enhanced injection and photocurrent gain.