Multiquantum-well Structures Research Articles

Growth and characterization of GaSb/AlGaSb multi-quantum well structures on Si (0 0 1) substrates

GaSb/AlGaSb multi-quantum well (MQW) structures with an AlSb initiation layer and a relatively thick GaSb buffer layer grown on Si (0 0 1) substrates were prepared by molecular beam epitaxy (MBE). Transmission electron microscopy (TEM) images and high-resolution X-ray diffraction (XRD) patterns indicated definite MQW structures. The photoluminescence (PL) emission around 1.55 μm wavelength was observed for 10.34 nm GaSb/30 nm Al 0.6Ga 0.4Sb MQW structure at room temperature. Dependence of PL emission energy on GaSb well width was well explained by finite square well potential model.

Heteroepitaxy of InP on Silicon-on-Insulator for Optoelectronic Integration

Epitaxial lateral overgrowth of InP was performed on patterned silicon-on-insulator (SOI) and compared with that on Si substrates in a low pressure hydride vapor phase epitaxy system. The InP was characterized by cathodoluminescence. No red shift of peak wavelength was detected for InP/SOI indicating a negligible thermal strain. Additional low energy peaks were found in some regions with a granular structure on the SOI template. A subsequent growth of an InGaAsP/InP MQW (multi quantum well) structure (λ ~ 1.5 µm) was grown on the SOI template and on a planar InP reference sample by metal- organic phase epitaxy. The MQW was characterized by room temperature photoluminescence. A red shift of 35 nm with respect to the reference sample was attributed to the selective-area effect causing thicker wells and/or an increased indium content. Although the PL intensity was weaker than that obtained for the reference, the FWHMs were comparable.

Multiquantum well spin polarized current oscillator

We analyze nonlinear electron spin dynamics of a n-doped DC voltage biased semiconductor II–VI multi-quantum well structure (MQWS) having one or more of its wells doped with Mn. Even if normal contacts have been attached to this nanostructure, spin polarized current can be obtained provided one well is doped with magnetic impurities. We have studied the conditions for the system to exhibit static electric field domains and stationary current or moving domains and time-dependent oscillatory current. There are self-sustained current oscillations (SSCO) for nanostructures with four or more QWs. Moreover, SSCO may appear or not depending on the spin splitting induced by both, the exchange interaction and the external magnetic field. We calculate the minimal doping density needed to have SSCO, which is crucial to design a device behaving as a spin-polarized current oscillator.

Nitride-Based Dual-Stage MQW LEDs

Nitride-based light emitting diode (LED) with dual-stage multiquantum well (MQW) structure is proposed and fabricated. It was found that we could improve crystal quality, reduce reverse leakage current, and reduce forward voltage of the LED by inserting the electron emitter MQW structure. With current injection, it was found that measured output powers were 3.2 and for the conventional single-stage MQW LED and the dual-stage MQW LED, respectively. Furthermore, it was found that electrostatic discharge characteristics of the dual-stage MQW LED are better.

Photoluminescence characterization technique for resonant-tunneling structures based on a long-period GaAs/AlGaAs superlattice, applicable at different stages of fabrication

A technique is developed for the photoluminescence-spectroscopy characterization of resonant-tunneling structures based on a long-period GaAs/AlGaAs superlattice that can be used for quality evaluation at all the stages of fabrication, including molecular-beam epitaxy, photolithography, and annealing. Factors such as the small energy difference between the quantum confined states in wide quantum wells, which make the photoluminescence characterization of such structures more difficult are taken into account. The long-period multiquantum-well structures are promising for the development of a new kind of solid-state intersub-band-transition devices emitting the narrow band radiation in far infrared. Their potential is essentially based on the fact that the scattering and the decay of carriers in the lower quantum-confined states may or may not involve optical phonons. The technique works at both liquid-helium and room temperature. It helps one optimize the process conditions to fabricate high-quality wide-quantum-well structures with excellent uniformity and desired parameters.

Structural and optical properties of ZnO∕Mg0.1Zn0.9O multiple quantum wells grown on ZnO substrates

Zn O ∕ Mg 0.1 Zn 0.9 O multiquantum-well (MQW) structures were grown on ZnO substrates by molecular beam epitaxy. Abrupt interfaces and well/barrier width in the MQWs were confirmed by x-ray diffraction measurement and transmission electron microscopy. The transition energy of the localized exciton in the ZnO∕Mg0.1Zn0.9O MQWs with well/barrier width of 5∕8nm was found to be about 3.375eV at low temperature, consistent with theoretical calculation. The first subband energies in the conduction and valence band were calculated to be 19.2 and 5.4meV, respectively. The transition energy showed no shift with excitation power, indicating that the polarization-induced electric field is negligible in the ZnO∕Mg0.1Zn0.9O MQW structures.

Structural and optical properties of an In xGa 1− xN/GaN nanostructure

The structural and optical properties of an In x Ga 1− x N/GaN multi-quantum well (MQW) were investigated by using X-ray diffraction (XRD), atomic force microscopy (AFM), spectroscopic ellipsometry (SE) and photoluminescence (PL). The MQW structure was grown on c-plane (0 0 0 1)-faced sapphire substrates in a low pressure metalorganic chemical vapor deposition (MOCVD) reactor. The room temperature photoluminescence spectrum exhibited a blue emission at 2.84 eV and a much weaker and broader yellow emission band with a maximum at about 2.30 eV. In addition, the optical gaps and the In concentration of the structure were estimated by direct interpretation of the pseudo-dielectric function spectrum. It was found that the crystal quality of the InGaN epilayer is strongly related with the Si doped GaN layer grown at a high temperature of 1090 °C. The experimental results show that the growth MQW on the high-temperature (HT) GaN buffer layer on the GaN nucleation layer (NL) can be designated as a method that provides a high performance InGaN blue light-emitting diode (LED) structure.

Nitride‐based heterostructures grown by MOCVD for near‐ and mid‐infrared intersubband transitions

AbstractIntersubband (ISB) optical absorption in different nitride‐based heterostructures grown by metal‐organic chemical vapour deposition (MOCVD) is reported. The role of indium in AlInN/GaN multi‐quantum wells (MQWs) is investigated. At high concentration (15%) AlInN is quasi lattice‐matched to GaN and no cracks appear in the structure. At very low indium concentration (∼2%) the material quality is improved without decreasing the ISB transition wavelength with respect to the case of indium‐free structures. Different mechanisms of strain relaxation in pure and 2% indium‐doped AlN/GaN MQW structures are also investigated. ISB transition wavelengths of 2 μm for AlN/GaN MQWs, and 3 μm for AlInN/GaN MQWs with 15% of In are achieved. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

In situ determination of InGaAs and GaAsN composition in multiquantum-well structures

Metal-organic vapor phase epitaxial growth of InGaAs/GaAs and GaAsN/GaAs multiquantum-well (MQW) structures was monitored by in situ reflectometry at 635 nm using a normal incidence reflectance setup. The reflectance signal is found to change significantly during both quantum-well (QW) and barrier growth regions. A matrix method is used to calculate the theoretical reflectance curve and comparing the theoretical curves to the measured ones the complex refractive index of the ternary alloys are derived. Consequently, when the behavior of the complex refractive indices of InGaAs and GaAsN is known as a function of composition, the composition of all the QWs in the MQW strucure can be determined in situ.

Structural and optical properties of Si/SiO 2 multi-quantum wells

Structural and optical properties of Si/SiO 2 multi-quantum wells (MQW) were investigated by means of Raman scattering and photoluminescence (PL) spectroscopy. The MQW structures were fabricated on a quartz substrate by remote plasma enhanced chemical vapour deposition (RPECVD) of alternating amorphous Si and SiO 2 layers. After layer deposition the samples were subjected to heat treatments, i.e. rapid thermal annealing (RTA) and furnace annealing. Distinct PL signatures of confined carriers evidenced formation of Si-nanocrystals (nc-Si) in annealed samples. Analyses of Raman spectra also show presence of nc-Si phase along with amorphous-Si (a-Si) phase in the samples. The strong influence of the annealing parameters on the formation of nc-Si phase suggests broad possibilities in engineering MQW with various optical properties. Interestingly, conversion of the a-Si phase to the nc-Si phase saturates after certain time of furnace annealing. On the other hand, thinner Si layers showed a disproportionately lower crystalline volume fraction. From the obtained results we could assume that an interface strain prevents full crystallization of the Si layers and that the strain is larger for thinner Si layers. The anomalous dependence of nc-Si Raman scattering peak position on deposited layer thickness observed in our experiments also supports the above assumption.

The self-assemble GaN:Mg inverted hexagonal pyramids formatted by photoelectrochemical wet-etching process

Small sized self-assembled inverted hexagonal pyramids consisting of GaN:Mg and InGaN/GaN multi-quantum-well (MQW) structures were formed using photoelectrochemical wet etching. Lateral etching, bottom-up etching, and anisotropic etching are the formation mechanism of the pyramids during the etching process sequentially. The dimension of these inverted hexagonal pyramids was measured as 245 nm in width and 184 nm in height, and the angle between the top GaN:Mg surface and the pyramid sidewall was calculated at about 56.3°. Due to the strain relief in the nano-disk MQW structure we induced an emission peak of photoluminescence at the tip of the inverted hexagonal pyramid which had a strong blue shift of 244 meV at 100 K.

Optical characterization of digital alloy In0.49Ga0.51P∕In0.49(Ga0.6Al0.4)0.51P multi-quantum-wells grown by molecular beam epitaxy

An In0.49Ga0.51P∕In0.49(Ga0.6Al0.4)0.51P multi-quantum-well (MQW) structure grown by molecular beam epitaxy using a digital alloy method was parametrically investigated by photoluminescence (PL) measurement performed in a temperature range of 10–290K. The PL peak energies did not change with increasing temperature up to 60K, while the PL peak energy monotonously decreased with increasing temperature beyond 60K. From the curve fit of the linewidth full width at half maximum of the PL peak, it was observed that the homogeneous broadening of In0.49Ga0.51P∕In0.49(Ga0.6Al0.4)0.51P MQW with digital alloy barriers due to scattering by longitudinal optical phonons was smaller than that of InGaAs∕InGaAlAs MQW with digital alloy barriers. This is in accordance with the existence of a relatively weak phonon-related PL peak in the PL spectrum of InGaAlP digital alloy, as compared with InGaAlAs digital alloy. The fit of the integrated PL intensity shows the occurrence of a nonradiative recombination process with an activation energy E1=24.4meV up to 60K. On the other hand, the process of nonradiative recombination with an activation energy E2=109meV occurred above 60K, which is in good agreement with one-half of the calculated total confinement energy ΔE of the electron-hole pair in the quantum well (∼108meV). The In0.49Ga0.51P∕In0.49(Ga0.6Al0.4)0.51P MQW structure with digital alloy barriers has larger activation energy (E2=109meV) than In0.49Ga0.51P∕In0.49(Ga0.6Al0.4)0.51P MQW (E2=90meV) with analog alloy barriers. Therefore, the thermal emission of carriers into the barrier can be reduced at temperatures above 60K due to the high effective barrier height.

Resonant tunneling effect in metal-semiconductor-metal ultraviolet detectors grown with AlGaN∕GaN multi-quantum-well interlayer

I - V and C-V curves of metal-semiconductor-metal ultraviolet detector grown with AlGaN∕GaN multi-quantum-well (MQW) interlayer are found to oscillate with applied bias voltage. A simple model is proposed to explain the oscillation phenomena of both curves. Resonant tunneling of polarization induced charges through the barriers of the nitride based MQW structure terminated with back-to-back configuration Schoktty contacts may play a key role in the oscillating phenomena.

Study of the optical and structural properties of multi quantum well structures grown on high-index surfaces

The MOVPE overgrowth of high [01¯1]-oriented ridges confined at sides by facets related to {n11} crystallographic planes is reported. We studied the influence of the side tilt on the thickness of the AlGaAs and GaAs epitaxial layers grown under the condition of the kinetic growth mode. The multi quantum well (MQW) structures were prepared on the sides of ridges tilted at 54.7°, 45° and 30° to (100). The sidewall surface morphologies before and after epitaxial growth were evaluated and compared. We observed no tendency towards planarization towards a neighbouring high-index crystallographic plane, such as (211) and (311). We also showed that the quantum wells of the MQW structure make a smooth transition over the edge between the top surface and the facet as both AlGaAs and GaAs grew at similar rates on the surfaces.

MOCVD of InGaN‐based light emitting structures on silicon substrates with strain optimized buffer layers using long Al pre‐deposition

AbstractGaN/InGaN multi quantum wells (MQWs) were grown on silicon (Si) substrates by Metalorganic Vapor Phase Epitaxy (MOVPE). A structure of layers consisting of AlN, AlGaN and GaN was used for subsequent deposition of the MQW structures on Si substrates. The Al pre‐deposition time before growing the buffer structure and its effect on the formation of cracks was investigated. The emitted wavelength of the structure was changed by varying the thickness of the Mg‐doped AlGaN electron blocking layer, by reducing the growth temperature of the MQW structure and by increasing the QW width of the structure. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Growth of thin AlInN∕GaInN quantum wells for applications to high-speed intersubband devices at telecommunication wavelengths

In this article, we report on growth of AlInN∕GaInN multi-quantum-wells (MQWs) with high Al content (93%) by rf-plasma-assisted molecular-beam epitaxy on (0001) GaN/sapphire templates and on bulk GaN crystals. A series of samples with a barrier thickness of 3nm and with different well thicknesses of 1.5–3nm was grown. The wells were doped with Si at a concentration of 5×1019cm−3. Structures grown on (0001) GaN-based substrates are crack-free, as demonstrated by Nomarski contrast and scanning electron microscopy measurements. X-ray diffraction mapping of a and c lattice parameters shows that AlInN∕GaInN MQWs are fully strained and have up to 7% indium in the barriers and up to 10% In in the quantum wells. These structures exhibit intersubband absorptions at room temperature at a wavelength in the range of 2.45–1.52μm. The AlInN∕GaInN strain-compensated MQW structures, having good quality, are very attractive for ultrahigh-bit-rate telecommunication applications at 1.55μm wavelengths. In addition, because of their low average refractive index, they could be used as thick cladding layers for optical waveguides.

Fabrication InGaN Nanodisk Structure in GaN Reverse Hexagonal Pyramid

Small self-assembled inverted hexagonal pyramids consisting of GaN:Mg and InGaN/GaN multi-quantum-well (MQW) structures were fabricated using photoelectrochemical wet etching. Lateral etching, bottom-up etching, and anisotropic etching are the sequential formation mechanism of such pyramids during the wet etching. The inverted hexagonal pyramids were measured to be 245 nm in width and 184 nm in height, and the angle between the top GaN:Mg surface and the pyramid sidewall was calculated to be about 56.3°. Due to the strain relief in the nano-disk MQW structure, we induced an emission peak of photoluminescence at the tip of the inverted hexagonal pyramids, which had a strong blue shift of 244 meV measured at 100 K.

The wavelength of the responsive peak resulted from electron interferences for multi-quantum well infrared detectors

The wavelength of the responsive peak for GaAs/Al 0.3Ga 0.7As multi-quantum well (MQW) infrared detectors was measured to be λ = 6.29 μ m at 77 K. Based on the theory of electron interference, the results calculated for the MQW structure indicate that there may be a series of separated conduction states above the barriers, on which the electrons can propagate along the direction perpendicular to well/barrier interfaces. For the quantum well parameters given in this article, the wavelength of light absorbed is calculated to be 6.29 μm when optical excitations occur for which electrons transit from Fermi level in quantum wells to the first conduction state E 1 above the barriers. The theoretical value for the responsive peak wavelength is in very good agreement with the measurement result. This shows that the wavelength of the responsive peak resulted from electron interferences for the MQW infrared detectors.

High-power GaN-based blue-violet laser diodes with AlGaN∕GaN multiquantum barriers

AlGaN ∕ GaN multiquantum barriers (MQBs) were introduced into violet AlInGaN laser diodes with an InGaN multiquantum-well structure, resulting in drastic improvements in lasing performance. Comparing with conventional AlGaN single electron blocking layer (EBL), lower threshold current of 32mA and higher slope efficiency of 1.12W∕A at room temperature has been achieved by using the AlGaN∕GaN multiquantum barrier. This improvement implies that p-type AlGaN∕GaN MQBs are more effective in suppressing the overflow of electrons than p-type AlGaN single EBL. Effective barrier heights of the MQBs should be higher than the single EBL due to the quantum effect of MQBs and the enhancement of p-type doping efficiency. Additionally, the effect of strain on InGaN multiquantum wells from the single EBL can be reduced by using the AlGaN∕GaN MQBs structure.

Carrier loss channels for non-collected carrier in InAsxP1−x∕InP multiquantum well solar cells

We present here the direct observation of majority carrier (electron) defects generation from chemical-beam epitaxy grown InAsxP1−x∕InP multiquantum-well solar cell structures by deep level transient spectroscopy. Four dominant majority carrier levels E1, E2, E3, and E4 at energy position EC−0.17eV, EC−0.25eV, EC−0.57eV, and EC−0.75eV below the conduction-band edge have been observed. Detailed analysis of double-correlation deep level transient spectroscopy (DDLTS) measurements shows that traps E1, E2, E2, and E4 are located at the well-barrier interfacial zone or/and extended over the whole region occupied by the quantum wells. The midgap center E3 has large capture cross sections and is found to exhibit significant dependence of the thermal emission rates on electric field, from DDLTS measurements. These characteristics indicate that the electron trap E3 at EC−0.57eV acts as a strong recombination center and could be responsible for the main carrier loss channel for non-collected carrier in the device.