Multi-quantum Wells Sample Research Articles

A Novel Way to Fill Green Gap of GaN-Based LEDs by Pinning Defects in Nanorod Array.

Nanorod array and planar green-emission InGaN/GaN multi-quantum well (MQW) LEDs were fabricated by lithography, nano-imprinting, and top-down etching technology. The defect-pinning effect of the nanostructure was found for the first time. The ratio of the bright regions to the global area in the panchromatic CL images of green MQW samples increased from 30% to about 90% after nano-fabrication. The overall luminous performance significantly improved. Throughout temperature-dependent photoluminescence (TDPL) and time-resolved PL (TRPL) measurements, the migration and recombination of carriers in the MQWs of green LEDs were analyzed. It was proved that nanostructures can effectively prevent carriers from being captured by surrounding nonradiative recombination centers. The overall PL integral intensity can be enhanced to above 18 times. A much lower carrier lifetime (decreasing from 91.4 to 40.2 ns) and a higher internal quantum efficiency (IQE) (increasing from 16.9% to 40.7%) were achieved. Some disputes on the defect influence were also discussed and clarified.

Effects of selective area intermixing on InAlGaAs multiple quantum well laser diode

In this work, we report on laser diodes with intermixed InAlGaAs multi-quantum well (MQW) as the active gain medium. Using an impurity-free vacancy-enhanced disordering technique, several silicon nitride-capped InAlGaAs MQW samples are intermixed to different levels by varying the rapid-annealing temperatures from 730 °C to 830 °C. The intermixed laser diodes operate at wavelengths spanning from 1520 nm to 1400 nm, while exhibiting an increase in threshold current from 25 mA (as-grown) to 45 mA, and a decrease in slope efficiency from 0.101 W A−1 (as-grown) to 0.068 W A−1 as the intermixing extent is increased. For a sample that was annealed at 770 °C while only covered with a silicon dioxide film, the lasing wavelength is 1543 nm compared to 1550 nm for the as-grown laser wavelength, and the lasing threshold current is 26 mA with a slope efficiency of 0.08 W A−1.

Photoluminescence related to Gd3+:N-vacancy complex in GaN:Gd multi-quantum wells

The photoluminescence of Gd-doped GaN multi-quantum wells (MQWs) is presented and discussed considering the formation of a Gd3+:Nitrogen-vacancy (N-vacancy) complex. A lower energy photoluminescence peak was observed for the Gd-doped GaN MQW sample with respect to the main peak assigned to a neutral donor bound exciton (D0X) of the undoped GaN MQW sample. The X-ray absorption near edge structure spectrum observed at Gd LIII-edge indicates a nitrogen vacancy adjacent to the Gd substituting the Ga ion in Gd-doped GaN MQW sample. Local stresses around the Gd dopants in Gd-doped GaN matrix generated due to the larger diameter of the Gd3+ ion with respect to the Ga3+ ion can be relieved by the creation of vacancies. The lower formation energy of N-vacancies in GaN matrix introduce them as a preferred candidate to relieve the generated stresses. A Gd3+:N-vacancy complex consisting of a Gd3+ ion and the created nitrogen vacancy adjacent to the Gd3+ dopant is likely to form in GaN:Gd matrix. The lower photoluminescence peak energy observed in the Gd-doped GaN MQW sample is assigned to the recombination of an exciton captured at the Gd3+:N-vacancy complex forming a small polaron-like state. A model is presented considering the small exciton-polaron population in defect sites captured around the Gd3+ ions in the Gd-doped GaN.

Investigation of Mn incorporation in fifteen-period InGaAs/GaAs quantum well system

A ferromagnetic ordering with a Curie temperature of 50 K of fifteen layer of InGaMnAs/GaAs multi quantum wells (MQWs) structure grown on high resistivity (100) p-type GaAs substrates by molecular beam epitaxy (MBE) was found. It is likely that the ferromagnetic exchange coupling of sample with Curie temperature of 50 K is hole-mediated resulting in Mn substituting In or Ga sites. Temperature and excitation power dependent PL emission spectra of InGaMnAs MQWs sample grown at temperature of 170 °C show that an activation energy of Mn ion on the first quantum confinement level in InGaAs quantum well is 36 meV and impurity Mn is partly ionized. It is found that the activation energy of 36 meV of Mn ion in the QW is lower than the activation energy of 110 meV for a substitutional Mn impurity in GaAs. These measurements provide strong evidence that an impurity band existing in the bandgap due to substitutional Mn ions and it is the location of the Fermi level within the impurity band that determines Curie temperature.

Optical and Magnetic Properties of Ten-Period InGaMnAs/GaAs Quantum Wells

Ten layers of InGaMnAs/GaAs multiquantum wells (MQWs) structure were grown on high resistivity (100) p-type GaAs substrates by molecular beam epitaxy (MBE). A presence of the ferromagnetic structure was confirmed in the InGaMnAs/GaAs MQWs structure, and have ferromagnetic ordering with a Curie temperature, T C=50 K. It is likely that the ferromagnetic exchange coupling of the sample with T C=50 K is hole-mediated resulting in Mn substituting In or Ga sites. PL emission spectra of the InGaMnAs MQWs sample grown at a temperature of 170 °C show that an activation energy of the Mn ion on the first quantum confinement level in InGaAs QW is 32 meV and impurity Mn is partly ionized. The fact that the activation energy of 32 meV of Mn ion in the QW is lower than an activation energy of 110 meV for a substitutional Mn impurity in GaAs, indicating an impurity band existing in the bandgap due to substitutional Mn ions.

MOVPE growth of Ga(AsBi)/GaAs multi quantum well structures

This paper summarizes results of the epitaxial growth of Ga(AsBi) by metal organic vapor phase epitaxy (MOVPE) using all-liquid group V precursors. Ga(AsBi)/GaAs multi quantum well (MQW) samples are grown on GaAs (001) substrates at temperatures as low as 375°C and 400°C using triethylgallium (TEGa), tertiarybutylarsine (TBAs) and trimethylbismuth (TMBi) as precursors. High resolution x-ray diffraction (HR-XRD), transmission electron microscopy (TEM) as well as atomic force microscopy (AFM) measurements show that MQW structures with good crystalline quality are realized. Under specific growth conditions, the Bi droplet formation can be avoided completely. The incorporated Bi-content is limited depending on the growth temperature used. Surplus Bi segregates at the surface and incorporates into the subsequent GaAs barrier when the Bi supply is stopped. The MQW samples show room temperature photoluminescence (PL) already after growth. A redshift and a decreasing PL signal intensity with increasing Bi fraction is observed.

Photocurrent characteristics in p‐i‐n diodes with built‐in coupled or uncoupled multi‐quantum wells

AbstractWe studied photocurrent characteristics in AlGaAs p‐i‐n diodes, where coupled or uncoupled GaAs/AlGaAs multi‐quantum wells (MQWs) were embedded in their intrinsic layers, to investigate how the electronic–coupling in the MQWs affects the device performance of MQW‐based solar cells. Measurements were done at 77 K to suppress thermal processes. When carriers were generated only in QWs, the photocurrent (PC) in the coupled MQW sample decreased substantially as soon as the forward‐bias voltage V is raised above ∼0.5 V and became almost zero at V ∼1 V. By contrast, in the uncoupled MQW sample the PC did not exhibit such a step‐like decrease but only gradually decreased up to V ∼1.2 V. This step‐like loss in PC observed in the coupled MQW structure is presumably attributable to the accumulation of photogenerated carriers in its MQW region. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

An effect of As flux on GaAs/AlAs quantum wells: A combined photoluminescence and reflection high-energy electron diffraction study

Photoluminescence (PL) measurements and in situ reflection high-energy electron diffraction (RHEED) observations were used to optimize GaAs/AlAs multi-quantum-well (MQW) structures grown by molecular beam epitaxy (MBE) under different As fluxes. PL peaks were identified with computer simulations, intensity behavior as a function of temperature, and with previous results in literature. The room temperature PL intensity from our MQW samples first increases with the As flux and then decreases as the flux is increased. Combining our RHEED and PL observations, we propose that an optimum As flux for growth of GaAs/AlAs structures is close to the special As flux at which the GaAs(1 0 0) surface reconstruction changes from ( 2 × 4 ) to ( 4 × 2 ). The reasons for the findings are discussed.

Improvement of photoluminescence by Si delta‐doping in GaN barrier layer of GaN/InxGa1–xN multi‐quantum wells

AbstractWe report on the effect of Si delta doping in the barriers of multi‐quantum wells (MQWs) grown by metalorganic chemical vapor deposition (MOCVD). The room temperature photoluminescence (PL) intensity of the MQW sample with Si delta doped barriers was improved by 33 times compared to that of a MQW sample without Si delta doped barriers. The temperature dependent PL measurement showed slow quenching of the integrated PL intensity of the MQW sample grown by using Si delta doping process as temperature increased compared to that of normal MQW sample. This was attributed to the effective injection of electron from the Si delta doped layer into quantum well (QW) layer as temperature increased and also the higher hole confinement resulted from the higher band offset. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of thermal damage on optical and structural properties of In 0.08Ga 0.92N/In 0.02Ga 0.98N multi-quantum wells grown by MOCVD

Effects of thermal damage on the optical and structural properties of InGaN/InGaN multi-quantum wells (MQWs) were investigated by using photoluminescence (PL), high-resolution X-ray diffraction (HRXRD), and atomic force microscopy (AFM). The five-period In 0.08Ga 0.92N/In 0.02Ga 0.98N MQWs were grown on sapphire using metalorganic chemical vapor deposition (MOCVD). To investigate the influence of thermal damage on InGaN MQWs, the MQW sample was heated to the high-temperature for p-type GaN layer growth and cooled down and its structural and optical properties were compared with the MQW sample cooled down without high-temperature thermal treatment. The surface morphology of InGaN MQWs changes from a spiral to a step structure after the high-temperature treatment, which is induced by the desorption and the surface migration of In and Ga adatoms during the high-temperature treatment. In HRXRD (0 0 0 4) ω / 2 θ scan, the intensity fringes between high-order satellite peaks disappear, which suggests that the interface quality of InGaN MQWs be also deteriorated by high-temperature treatment. Temperature dependence of PL measurement shows that InGaN MQWs without high-temperature thermal treatment had higher internal quantum efficiency and lower localization effect.

Indium-surfactant-assisted growth of high-mobility AlN/GaN multilayer structures by metalorganic chemical vapor deposition

AlN/GaN single and multilayer structures with various AlN and GaN layer thicknesses were grown by metalorganic chemical vapor deposition. Step flow growth of AlN was achieved using trimethylindium as a surfactant. Defect formation in the AlN layer could be largely prevented for AlN layers thinner than 2.9 nm. In the multiquantum-well samples, which consisted of five (AlN/GaN) periods, a two-dimensional electron gas (2DEG) was formed at the interface between the GaN base layer and the first AlN barrier layer. When the thickness of the AlN barrier layer in the multiquantum well was increased from 0.9 to 2.6 nm at a constant GaN well thickness of 5 nm, the sheet carrier density of the 2 DEG increased from 5×1012 to 2.1×1013 cm−2 and the electron mobility measured at 77 K decreased from 11780 to 3140 cm2/V s. The effect of the GaN well thickness was also investigated.

Effect of the nuclear hyperfine field on the 2D electron conductivity in the quantum Hall regime

The effect of the nuclear hyperfine interaction on the dc conductivity of 2D electrons under quantum Hall effect conditions at filling factor v= 1 is observed for the first time. The local hyperfine field enhanced by dynamic nuclear polarization is monitored via the Overhauser shift of the 2D conduction electron spin resonance in AlGaAs/GaAs multiquantum-well samples. The experimentally observed change in the dc conductivity resulting from dynamic nuclear polarization is in agreement with a thermal activation model incorporating the Zeeman energy change due to the hyperfine interaction. The relaxation decay time of the dc conductivity is, within experimental error, the same as the relaxation time of the nuclear spin polarization determined from the Overhauser shift. These findings unequivocally establish the nuclear spin origins of the observed conductivity change.

Quantum hall effect in Ge 1-XSi X/(111)Ge MQW samples of non-standard six-point contact geometry

We investigate the Quantum Hall Effect (QHE) in selectively boron-doped GeSi/(111)Ge multi-quantum well (MQW) samples of non standard geometry with six-point contacts disconnected from the sample edges. The Hall resistance shows a well-pronounced quantisation that is consistent with integral QHE (IQHE) value (h/ve 2) at temperature T = 4.2K. When the temperature was lower than 2.5K, unusual peaks on the R xy QHE plateaux were observed. We explain this behaviour by non-equilibrium population of Landau levels that results from the separation of current contacts from the edge of the sample.

Spectroscopic ellipsometry of Si1-xGex multi-quantum wells

Spectroscopic Ellipsometry (SE) has been used to characterise a range of strained Si1-xGex/Si multi-quantum well (MQW) samples (x≊0.2) using recently acquired strained Si1-xGex reference dielectric function spectra. SE results for thickness and composition compared well with those from double crystal X-ray diffraction (DCXRD) and cross-sectional transmission electron microscopy (XTEM) for well widths above 50Å. For thinner wells quantum confinement and/or interface effects may be important, invalidating the use of 'bulk' strained Si1-xGex reference dielectric function spectra. Models and procedures appropriate for fitting simulated SE spectra to those measured on MQW samples are discussed in detail. Improvements are seen in fits to some experimental spectra by assuming the period thickness (barrier + well) varies with depth in the MQW.

Measurement of 'material' parameters in multi-quantum-well structures

Multi-quantum-well (MQW) structures can be regarded as new materials whose properties are determined by the well and barrier thicknesses Lz and LB and the well depth (which is composition dependent). Measurements of Lz, LB and x are reported for 16 MQW samples grown by molecular beam epitaxy in the AlxGa1-xAs system. Results from X-ray diffraction, photoluminescence excitation spectroscopy and transmission electron microscopy are found to compare well with each other and with values predicted from MBE growth conditions.

Extrinsic photoluminescence from GaAs quantum wells

Extrinsic photoluminescence has been studied from both undoped and p-type doped single and multiquantum well (MQW) GaAsAl xGa 1−xAs heterostructures grown by molecular beam epitaxy. For undoped MQW samples this luminescence is attributed to n = 1 electrons recombining with shallow neutral acceptors (probably carbon) near the well center whereas for single well samples the acceptors (probably carbon) are frequently concentrated in a thin GaAs layer at the first grown heterointerface. Ionization energies obtained from these data are in reasonable agreement with the calculations of Bastard. With Be-doped samples one can also see luminescence due to ground state heavy hole excitons bound to Be neutral acceptors. These data provide estimates of the dissociation energy of the heavy hole exciton neutral acceptor complex.