Molecular beam epitaxy growth methods of wavelength control for InAs/(In)GaAsN/GaAs heterostructures

We discuss the molecular beam epitaxy (MBE) growth methods of emission wavelength control and property investigations for different types of InAs/(In)GaAsN/GaAs heterostructures containing InGaAsN quantum-size layers: (1) InGaAsN quantum wells deposited by the conventional mode in a GaAs matrix, (2) InAs quantum dots deposited in a GaAsN matrix or covered by an InGaAs(N) layer, and (3) InAs/InGaAsN/GaAsN strain-compensated superlattices with quantum wells and quantum dots. The structures under investigation have demonstrated photoluminescence emission in a wavelength range of ～1.3-1.8?μm at room temperature without essential deterioration of the radiative properties.     

Room-temperature photoconductivity in the 1–2.6 µm range in InAs/GaAs heterostructures with quantum dots

We have studied the room-temperature photoconductivity in the wavelength range 1–2.6 μm in InAs/GaAs heterostructures with quantum dots (QDs). Specific features of these heterostructures grown using the metalorganic vapor phase epitaxy (MOVPE) were an increase in the amount of InAs during the formation of a sheet of QDs and the use of alternating low-and-high-temperature regimes during their overgrowth with a GaAs barrier layer. For the first time, the MOVPE-grown multilayer InAs/GaAs heterostructures with quantum dots exhibited photoluminescence in a wavelength range of up to 1.6 μm and the photoconductivity up to 2.6 μm at room temperature. The heterostructures exhibited a room-temperature voltage sensitivity of 3 × 10³ V/W (within a Si-plate filter bandwidth) and a specific detectivity of 9 × 10⁸ cm Hz^1/2 W^?1.     

Photoluminescence from multilayer InAs/GaAs structures with quantum dots in the 1.3–1.4 μm wavelength range

The optical properties of multilayer heterostructures with quantum dots were studied for InAs/GaAs systems obtained by the combined method of molecular-beam epitaxy and submonolayer migration-stimulated epitaxy. Using these structures, it is possible to obtain the room-temperature photoluminescence with maximum emission in the wavelength range from 1.3 to 1.4 μm.     

InAs/GaAs nanostructures grown on patterned Si(001) by molecular beam epitaxy

The potential benefit from the combination of the optoelectronic and electronic functionality of III-V semiconductors with silicon technology is one of the most desired outcomes to date. Here we have systematically investigated the optical properties of InAs quantum structure embedded in GaAs grown on patterned sub-micron and nanosize holes on Si(001). III-V material tends to accumulate in the patterned sub-micron holes and a material depletion region is observed around holes when GaAs/InAs/GaAs is deposited directly on patterned Si(001). By use of a 60?nm SiO(2) layer and patterning sub-micron and nanosize holes through the oxide layer to the substrate, we demonstrate that high optical quality InAs nanostructures, both quantum dots and quantum wells, formed by a two-monolayer InAs layer embedded in GaAs can be epitaxially grown on Si(001). We also report the power-dependent and temperature-dependent photoluminescence spectra of these structures. The results show that hole diameter (sub-micron versus nanosize) has a strong effect on the structural and optical properties of GaAs/InAs/GaAs nanostructures.     

InAs/GaAs multiple quantum dot structures grown by LP-MOVPE

Structures with self-organised InAs quantum dots in a GaAs matrix were grown by the low pressure metal–organic vapour phase epitaxy (LP-MOVPE) technique. Photoluminescence and atomic force microscopy were used as the main characterisation methods for the growth optimisation. The properties of multiple-stacked quantum dot structures are influenced by the thickness of the GaAs separation layers (spacers) between quantum dot-containing InAs layers, by the InAs layer thickness, by arsine partial pressure during growth, and by group III precursor flow interruption time.     

张应变GaAs层引入使InAs/InP量子点有序化排列的机理研究

采用ＬＰ－ＭＯＶＰＥ技术,在（００１）ＩｎＰ衬底上生长的ＩｎＡｓ／ＩｎＰ自组装量子点是无序的。为了解决这个问题,在ＩｎＰ衬底上先生长张应变的ＧａＡｓ层,然后再生长ＩｎＡｓ层,可得到有序化排列的量子点。本文对张应变ＧａＡｓ层引入使量子点有序化排列的机理进行了分析,为生长有序化、高密度,均匀性好自组装量子点提供了依据。     

Room-temperature photoluminescence at 1.55 μm from heterostructures with InAs/InGaAsN quantum dots on GaAs substrates

Room-temperature photoluminescence (PL) at 1.55 μm from heterostructures with InAs/InGaAsN quantum dots (QDs) grown by MBE on GaAs substrates is demonstrated for the first time. The effect of nitrogen incorporated into InAs/InGaAsN QDs on the PL wavelength and intensity was studied. The integral intensity of PL from the new structure with InAs/(In)GaAsN QDs is comparable to that from a structure with InGaAsN quantum wells emitting at 1.3 μm.     

Morphology and composition of InAs/GaAs quantum dots

Surface compositional maps of self-organized InAs/GaAs quantum dots were obtained with laterally resolved photoemission spectroscopy. We found a surface In concentration of about 0.85 at the center of the islands which decreases to 0.75 on the wetting layer. Comparison with concentration values found in the core of similar dots suggests a strong In segregation on the topmost surface layers of the dots and on the surrounding wetting layer. Furthermore, the morphological properties of the dots such as size and density have been measured with plan-view transmission electron microscopy and low energy electron microscopy.     

Optimizing the spacer layer thickness of vertically stacked InAs/GaAs quantum dots

Vertically stacked multilayers of self-organized InAs/GaAs quantum dots (QDs) structures with different GaAs intermediate layer thicknesses varying between 2.8 and 17 nm are grown by solid source molecular beam epitaxy (SSMBE) and investigated by photoluminescence spectroscopy (PL). For 17 nm thick GaAs spacer, the PL spectra show two well separated features attributed to the formation of two QDs family with a bimodal size distribution indicating no correlation between the dots in different layers. In the meanwhile, the structures having thinner spacer thickness demonstrate single PL peaks showing an enhancement of high energy side asymmetrical broadening when increasing the excitation power. The corresponding emission energies exhibit a red shift when the spacer layer thickness decreases and correlated with the enhancement of the vertical electronic coupling as well as the rise of the QD's size in the upper layers induced by the build up of the strain field along the columns. The spacer thickness of 8.5 nm is found to yield the best optical properties.     

Growth of InAs Quantum Dots on GaAs (511)A Substrates: The Competition between Thermal Dynamics and Kinetics

The growth process of InAs quantum dots grown on GaAs (511)A substrates has been studied by atomic force microscopy. According to the atomic force microscopy studies for quantum dots grown with varying InAs coverage, a noncoherent nucleation of quantum dots is observed. Moreover, due to the long migration length of In atoms, the Ostwald ripening process is aggravated, resulting in the bad uniformity of InAs quantum dots on GaAs (511)A. In order to improve the uniformity of nucleation, the growth rate is increased. By studying the effects of increased growth rates on the growth of InAs quantum dots, it is found that the uniformity of InAs quantum dots is greatly improved as the growth rates increase to 0.14 ML s^?1. However, as the growth rates increase further, the uniformity of InAs quantum dots becomes dual‐mode, which can be attributed to the competition between Ostwald ripening and strain relaxation processes. The results in this work provide insights regarding the competition between thermal dynamical barriers and the growth kinetics in the growth of InAs quantum dots, and give guidance to improve the size uniformity of InAs quantum dots on (N11)A substrates.     

自组织InAs/GaAs与InGaAs/GaAs量子点生长及退火情况的比较

本文采用MBE进行InAs/GaAs与InGaAs/GaAs量子点的生长,利用RHEED进行实时监测,并利用RHEED强度振荡测量生长速率。对生长的InAs/GaAs和InGaAs/GaAs两种量子点生长过程与退火情况进行对比,观察到当RHEED衍射图像由条纹状变为网格斑点时,InAs所需要的时间远小于InGaAs;高温退火下RHEED衍射图像恢复到条纹状所需要的时间InAs比InGaAs要长。     

Electrical characterization of InAs/GaAs quantum dot structures

The electrical properties of InAs quantum dots (QD) in InAs/GaAs structures have been investigated by space charge spectroscopy techniques, current–voltage and capacitance–voltage measurements. Au/GaAs/InAs(QD)/GaAs Schottky barriers as well as ohmic/GaAs/InAs(QD)/GaAs/ohmic structures have been prepared in order to analyze the apparent free carrier concentration profiles across the QD plane, the electronic levels around the QD and the electrical properties of the GaAs/InAs(QD)/GaAs heterojunction. Accumulation and/or depletion of free carriers at the QD plane have been observed by Capacitance–Voltage (C–V) measurements depending on the structure parameters and growth procedures. Similarly, quantum dot levels which exhibit distributions in energy have been detected by Deep Level Transient Spectroscopy (DLTS) and Admittance Spectroscopy (AS) measurements only on particular structures. Finally, the rectification properties of the InAs/GaAs heterojunction have been investigated and the influence of the related capacitance on the measured capacitance has been evidenced.     

Piezoelectric effects in InAs/GaAs(N11) self-assembled quantum dots

The effects of a piezoelectric field on the spectroscopic properties of strained InAs/GaAs self-assembled quantum dot (QD) heterostructures grown on (N11) substrates with A or B termination are presented. An increasing blue shift of photoluminescence (PL) band was observed with increasing excitation density. The PL blue shift of (N11) quantum dots measured at the highest excitation grows with 1/N and shows an asymmetric dependence on whether the substrate has A or B termination. We attributed the blue shift of the photoluminescence band to the screening of the piezoelectric field by the photo-generated carriers, leading to a reduction of the piezoelectric induced quantum confined Stark effect.     

InAs quantum dots capped by GaAs,In0.4Ga0.6As dots,and In0.2Ga0.8As well

We have fabricated and characterized three types of InAs quantum dots (QDs) with different InxGa1-xAs capping layers. Post-growth atomic force microscopy measurements show that the In0.2Ga0.8As/InAs structure has a smooth surface (dot-in-well structure), whereas the In0.4Ga0.6As/InAs structure revealed large QDs with a density similar to that underneath InAs QDs on GaAs (dot-in-dot). With increasing In mole fraction of the capping layer and increasing In0.4Ga0.6As thickness, the energy position of the room-temperature photoluminescence (PL) peak is red-shifted. The quantum dot-in-dot structure emits stronger room-temperature PL than does the quantum dot-in-well structure. With a spatially distributed strain in the InAs quantum dot, we have solved the three-dimensional Schr?dinger equation by the Green's function theory for the eigenvalues and eigen wave functions. It is concluded that the ground state increases its wave function penetration into the low-barrier InxGa1-xAs capping layer so that its energy position is red-shifted. The reduced PL peak intensity of the dot-in-well (compared with GaAs covered dots) is due to the reduced overlapping between the ground state and the extended states above the GaAs barrier. The overlapping reduction in the dot-in-dot is over compensated for by the reduced relaxation energy (full width at half-maximum), indicating the importance of the sample quality in determining the PL intensity.     

Effect of high energy proton irradiation on InAs/GaAs quantum dots: Enhancement of photoluminescence efficiency (up to ∼7 times) with minimum spectral signature shift

We demonstrate 7-fold increase of photoluminescence efficiency in GaAs/(InAs/GaAs) quantum dot hetero-structure, employing high energy proton irradiation, without any post-annealing treatment. Protons of energy 3–5 MeV with fluence in the range (1.2–7.04) × 10¹² ions/cm² were used for irradiation. X-ray diffraction analysis revealed crystalline quality of the GaAs cap layer improves on proton irradiation. Photoluminescence study conducted at low temperature and low laser excitation density proved the presence of non-radiative recombination centers in the system which gets eliminated on proton irradiation. Shift in photoluminescence emission towards higher wavelength upon irradiation substantiated the reduction in strain field existed between GaAs cap layer and InAs/GaAs quantum dots. The enhancement in PL efficiency is thus attributed to the annihilation of defects/non-radiative recombination centers present in GaAs cap layer as well as in InAs/GaAs quantum dots induced by proton irradiation.     

Effect of the growth parameters on the electron structure of quantum dots in InGaAs/GaAs heterostructures

The optical and structural properties of heterostructures with quantum dots (QDs) in the InAs/GaAs system overgrown with an InGaAs solid solution were studied. The QD layers were obtained using different molecular beam deposition techniques: molecular beam epitaxy versus submonolayer migration-stimulated epitaxy. The photoluminescence peaks in the spectra of samples with overgrown QD layers occur in the wavelength range from 1.18 to 1.32 μm. It was found that the growth conditions also influence the electronic structure of QDs.     

Thermal stability of the peak emission wavelength in multilayer InAs/GaAs QDs capped with a combination capping of InAlGaAs and GaAs

The effect of rapid thermal annealing (RTA) on the optical properties of a 10 layer stacked InAs/GaAs quantum dot (QD) heterostructure where the QDs are overgrown with a combination of quaternary InAlGaAs and GaAs capping have been investigated. TEM micrographs showed that the shape of the QDs is preserved for annealing temperatures up to 800 degrees C. The peak emission wavelength of the investigated heterostructures remains stable on annealing at temperatures upto 750 degrees C, which is unusual in QD samples. This phenomenon is attributed due to the suppression of the strain-enhanced intermixing in such structures. One of the reasons behind such suppression is the strain driven phase separation of Indium from the overgrown quaternary alloy, which maintains an In rich region across the QD periphery thereby checking the out-diffusion of Indium from the dots. The overlapping vertical strain from the under lying dot layers in the QD stack also maintains a strain relaxed state at the QD base, thereby preventing the material mixing at the base of the pyramidal QDs. This stability of wavelength is of paramount importance in optoelectronic devices where the design is based on the emission wavelength of the active region.     

Photoluminescence in the 1.55 μm wavelength range in the InGaAs/GaAs system with quantum dots and wells

It is demonstrated that longwave room-temperature photoluminescence (up to 1.65 μm) can be obtained using InGaAs/GaAs heterostructures of two types grown by low-temperature molecular beam epitaxy: (i) with InAs quantum dots formed at a low growth velocity and (ii) with an In_0.5Ga_0.5As quantum well grown in excess of the group III elements.     

Growth of InAs quantum dots on focussed ion beam implanted GaAs(100)

The growth of self-assembled InAs quantum dots on implantation doped GaAs was studied. Be and Si ions were implanted in a combined ion implantation/molecular beam epitaxy process to generate p- and n-type GaAs, respectively. The quality of the InAs quantum dots was investigated by photoluminescence spectroscopy and scanning electron microscopy. By employing an in situ annealing step before re-growth it was possible to fabricate high quality InAs quantum dots on ion doped GaAs for Be doses up to 1.4×10¹⁴ cm⁻². The sheet resistance of the Be doped GaAs was as low as 1 kΩ at 300 K and 0.6 kΩ at 4.2 K, respectively. Only for rather low Si doses up to 5×10¹³ cm⁻² acceptable photoluminescence could be detected. The sheet resistance for these doses was 1 kΩ at 300 K and 1.7 kΩ at 4.2K.     

Photoluminescence and magnetophotoluminescence of circular and elliptical InAs/GaAs quantum dots

Photoluminescence, magnetophotoluminescence, and atomic force microscopy were used for the characterization of MOVPE prepared InAs/GaAs quantum dots. Significant differences in the behaviour of the first excited photoluminescence transition in magnetic field are explained by the different lateral shape of quantum dots. While the first excited luminescence peak of circular quantum dots splits with increasing magnetic field into two peaks, no splitting occurs for quantum dots with elliptic shape, only small red shift is observed. Theoretical calculations of energy levels in InAs/GaAs quantum dots with circular and elliptical shape with different elongations are presented and compared with experimental results.