Strained Multiple Quantum Wells Research Articles

GaAsBi/GaAs MQWs grown by MBE using a two-substrate-temperature technique

Eleven periods of GaAsBi/GaAs multiple quantum wells (MQWs) on (100) GaAs substrates were grown by molecular beam epitaxy (MBE) using the two-substrate-temperatures (TST) technique. In the TST technique, the substrate temperature was set at TGaAsBi = 350 °C for GaAsBi layers and TGaAs = 550 °C for GaAs layers for the growth of MQWs. The structural and optical characterization of GaAsBi/GaAs MQWs using TST technique was carried out for growth optimization by changing As4/Ga beam equivalent pressure (BEP) ratio. Atomic force microscope observation shows high As4/Ga BEP ratio supply can prevents roughening of the GaAsBi layer surface of MQWs. High resolution x-ray diffraction analysis results reveal Bi incorporation enhancement with increasing As4/Ga BEP ratio. The reciprocal space mapping shows highly strained MQWs layers can be grown without relaxation with respect to the GaAs substrate at optimized growth conditions. At room temperature 1.22 μm photoluminescence emission was obtained from GaAs0.962Bi0.038/GaAs MQWs grown using TST technique.

850-nm VCSELs With p-Type $\mathrm {{\delta }}$ -Doping in the Active Layers for Improved High-Speed and High-Temperature Performance

In this paper, we study the influence of p-type modulation doping on the dynamic/static performance of high-speed 850-nm VCSELs with highly strained multiple quantum wells. The studied device structure has a 3/ $2~\lambda $ asymmetric cavity design, which can let the internal transit time of injected carriers be as short as that of 1/ $2~\lambda $ cavity design and further improve its performance in terms of speed and output power for high single-mode operation. Our proposed VCSEL structure with p-type doping shows superior modulation speed with an output power comparable with that of the un-doped reference device under room temperature operation. Furthermore, when the operating temperature reaches 85 °C, there is a significant improvement in both the modulation speed and maximum power of the p-doped structures. According to our simulation, this can be attributed to the change in the quasi-Fermi levels of the injected carriers after the addition of p-doping in the active layers, which minimizes the electron leakage under high-temperature operation.

MgZnO/MgO strained multiple-quantum-well nanocolumnar films: Stress-induced structural transition and deep ultraviolet emission

MgZnO/MgO strained multiple-quantum-wells (S-MQWs) with different structures are grown by pulsed laser deposition. The S-MQW films are composed of closely arranged, vertically oriented nanocolumns. Z-contrast scanning transmission electron microscopy observations and line-scan compositional analysis reveal that the nanocolumns are compositionally modulated along their length and have a MQW structure. A coherent epitaxial relationship with a sharp interface is established between MgZnO and MgO layers with large lattice mismatch. It is worth noting that the MgZnO layer undergoes a structural transition from hexagonal to cubic phase with its thickness decreasing. The calculations reveal that a large in-plane compressive stress dominates such an interesting phase transition process, stabilizes the low Mg-content MgZnO alloy in the anomalous cubic phase, and also leads to a broadening of the band gap. As a result, the wavelength-tunable deep-ultraviolet emission in the range of 261–314 nm is obtained from these S-MQW nanocolumnar films. The 261 nm is known to be the shortest emission wavelength ever reported for MgZnO material.

InAs 0.45P 0.55/InP strained multiple quantum wells intermixed by inductively coupled plasma etching

The intermixing effect on InAs 0.45P 0.55/InP strained multiple quantum wells (SMQWs) by inductively coupled plasma (ICP) etching and rapid thermal annealing (RTA) is investigated. Experiments show that the process of ICP etching followed RTA induces the blue shift of low temperature photoluminescence (PL) peaks of QWs. With increasing etching depth, the PL intensities are firstly enhanced and then diminished. This phenomenon is attributed to the variation of surface roughness and microstructure transformation inside the QW structure during ICP processing.

Photocurrent of SiGe/Si Strained Multiple Quantum-Wells Grown by UHV-CVD

The optical property was studied for the Si0.8Ge0.2/Si strained multiple quantum-well (MQW) structure grown by using ultra high vacuum chemical vapor deposition (UHV-CVD). The structural properties of the Si0.8Ge0.2/Si strained MQW were investigated using high-resolution X-ray diffraction (HR-XRD). The photocurrent spectrum of a Si0.8Ge0.2/Si strained MQW was measured at the room-temperature and the liquid-nitrogen temperature. For Si0.8Ge0.2/Si strained MQW, the transition peaks related to the MQW region observed in the photocurrent spectrum were preliminarily assigned to electron-heavy hole (e-hh) and electron-light hole (e-lh) fundamental excitionic transitions.

Optimization of InGaAsP–InP tensile strained multiple quantum-well structures emitting at 1.34 µm

A systematic investigation has been undertaken on the growth, characterization and performance of tensile-strained multiple-quantum-well (MQW) lasers emitting at 1.34 µm. Two photoluminescence peaks corresponding to n = 1 electron–light hole (E1–LH1) and n = 1 electron–heavy hole (E1–HH1) transitions are clearly observed in MQW structures. The improvements of tensile strained MQW lasers such as the internal quantum efficiency, internal loss and threshold current are attributed to the increased energy separation between LH1 and HH1 when the tensile strain and well width are larger. The relatively thick barrier layer is required to mitigate the strain relaxation between the quantum wells. The degradation of device performance by the excessive barrier height could be attributed to the exacerbated spatial non-uniformity of the carrier distribution in the valence band. The optimized tensile strained MQW lasers with 300 µm cavity length and ridge waveguide (RWG) structure exhibit a lower threshold current of 13 mA and a higher external differential efficiency per facet of 0.29 mWmA−1 at 25 °C.

1.55 µm InAsP/InGaAsP Strained Multiple-Quantum-Well Laser Diodes Grown by Solid-Source Molecular Beam Epitaxy

The growth of 1.55-µm-wavelength-range strained InAsP/InGaAsP multiple-quantum-well (MQW) structures by solid-source molecular beam epitaxy (SSMBE) using valved cracker cells is investigated. A low growth temperature or large V/III flux ratio is found to be favorable for attaining a high structural quality of highly strained MQW structures; the growth temperature has a critical effect on the sample's optical property. A measured photoluminescence (PL) full-width at half-maximum (FWHM) of 18.7 meV at room temperature is among the best reported to date for 1.55-µm-wavelength MQW laser structures. Finally, broad-area (BA) separate confinement heterostructure (SCH) MQW lasers were fabricated, and the threshold current density was measured to be 1.4 kA/cm2. This is the first report, to the best of the authors' knowledge, on a 1.55-µm-wavelength-range strained InAsP/InGaAsP MQW laser structure grown by SSMBE.

ALE growth and optical characterization of ZnSe/ZnS strained quantum well structures on Si substrate

The ZnSe/ZnS strained single quantum well (SSQW) and strained multiple quantum well (SMQW) structures were grown on (1 0 0)-oriented Si substrates at 150°C in a horizontal cold-wall quartz reactor by low-pressure metalorganic atomic layer epitaxy (ALE) at a pressure of 30 Torr. Dimethylzinc (DMZn), hydrogen sulfide (H 2S) and hydrogen selenide (H 2Se) were used as the reactants. X-ray diffraction (XRD), secondary ion mass spectromery (SIMS) and photoluminescence (PL) were used to characterize the strained quantum wells. The high-quality crystalline ZnS(1000 Å)/ZnSe(7l Å)/ZnS(1000 Å)/Si sub. SSQW structure exhibits a 32 meV full-width at half-maximum (FWHM) for the 7 K PL spectra. The PL peak emission energy dependence on the well thickness for the ZnSe/ZnS SSQW structure is in good agreement with predictions based on a parabolic well-strain mode. In addition, the well-thickness dependence of the PL peak energy and FWHM of SSQW structures are described in detail. The temperature dependence of the PL peak energy and FWHM of SMQW structures are also measured from 7 to 156 K using a He–Cd laser operated at a wavelength of 325 nm as an excitation source. Better understanding of the temperature-dependent behavior would be helpful in the optoelectronic device development.

Temperature dependence of photoluminescence from InAsP/InP strained quantum well structures grown by metalorganic chemical vapor deposition

In this article, we report the growth and characterization of InAsP/InP strained single quantum well (SSQW), strained single quantum well (SSQW) stack, and strained multiple quantum well (SMQW) structures on (1 0 0)-oriented InP substrates grown by metalorganic chemical vapor deposition. Double-crystal X-ray diffraction, photoluminescence (PL) and transmission electron microscope (TEM) are used to characterize the strained quantum wells. The high-quality crystalline InAs y P 1 −y (72Å)/InP SSQW structure with y⩽0.36 exhibits a 9.9 meV full-width at half-maximum (FWHM) of 10 K PL spectra. The peaks in the PL spectra for SSQW stack structure with a well thickness of 8, 14, and 35 Å vanish above 100, 150, and 296 K, respectively, presumably due to the decrease of photons yielded by electron–hole recombination in thinner quantum well regions on increasing the temperature. The PL peak emission energy dependence of well thickness in the InAsP/InP SSQW stack structure is in good agreement with the calculated results. In addition, the variations of the PL peak energy and FWHM in all the InAsP/InP SSQW, SSQW stack, and SMQW structures are described in detail.

MOCVD growth of strained multiple quantum well structure for 1.3 μm InAsP/InP laser diodes

In this article, we describe the growth and characterization for 1.3 μm InAsP/InP strained multiple quantum well (SMQW) laser diodes (LDs) with separate confinement heterostructure grown at 580°C by metalorganic chemical vapor deposition. The grown strained single quantum well (SSQW) stack and strained multiple quantum well (SMQW) structures are characterized using double-crystal X-ray diffraction and photoluminescence (PL) to confirm the structural and optical qualities for practical device applications. The InAsP/InP SSQW stack grown at 580°C appears to be extremely abrupt, uniform, free of misfit dislocations and narrow PL half width. Although the InAsP/InP SMQWs grown at 580°C maintain its structural integrity throughout the deposition sequence, the slightly broader PL half width for InAsP/InP SMQW structure is attributed to the dislocations resulted from a large net strain. Laser emission can be achieved by using the InAsP/InP SMQWs and the lasing wavelength is in a good agreement with our designed structure. The experimental data of broad-area and ridge waveguide LDs are described in detail.

Influence of growth temperature of InAsP/InP strained multiple quantum wells grown by metalorganic chemical vapor deposition

In this article, we report the influence of growth temperature on the luminescent and structural properties of InAsyP1−y/InP strained multiple quantum wells (SMQWs) and strained single quantum wells (SSQWs) grown by metalorganic chemical vapor deposition (MOCVD). The strained quantum wells are characterized by high-resolution transmission electron microscope (TEM), photoluminescence (PL), and double-crystal x-ray diffraction (DC-XRD). An AsH3/(AsH3+PH3) gas flow ratio of 0.50% and 1.48% at 580 and 650 °C growth temperatures, respectively, will result in an InAsP layer with y=0.3 solid composition. The experimental PL emission energies at 10 K at different well thicknesses for the InAsyP1−y/InP SSQWs grown at 580 and 650 °C are in well agreement with the trend of the calculated curves. The TEM lattice image of an InAsP/InP SSQW grown at 580 °C on the order of two monolayers has been demonstrated. The InAsP/InP SSQW structure grown at 580 °C appears to be extremely abrupt, uniform, free of misfit dislocations, and narrow PL linewidth. Besides, the growth of InAsP/InP SMQWs at 580 °C maintains its structural integrity throughout the deposition sequence with smooth interface and well-defined periodicity. However, the InAsP/InP SSQWs or SMQWs exhibit an adverse property at 650 °C growth temperature. From the above results, the lower growth temperature is necessary for the InAsP/InP SMQW growth by MOCVD.

CBE growth of 2%-strained InAsP MQWs for 1.5 μm wavelength laser diodes

Structural and optical properties of the compressively strained multiple quantum wells (MQWs), where InAsP well layer strain ranges from 1.9–2.15%, have been studied. Transmission electron microscopy reveals that 1.9%-strained MQWs have flat interfaces whereas 2.15%-strained MQWs have wavy interfaces as growth proceeds. The 1.9%-strained MQW photoluminescence (PL) intensities stay quite strong in the barrier tensile strain range of 0–0.4% but are reduced by 90% in the 0.6–0.9% range, i.e. strain compensation is not useful in the highly strained MQWs. A new type of MQW structure, where a thin compositional-graded InAs x P 1− x layer is inserted between the well and the barrier layers, provides six-well MQWs having high PL intensities. These results show that the shearing stress at the well–barrier interface rather than net strain plays an important role in growing the 1.9%-strained MQWs. We have fabricated the buried heterostructure MQW lasers emitting at a 1.5 μm wavelength light with a low threshold current of 6.5 mA and a large differential gain of 9.3×10 −16 cm 2.

Photoexcited carrier diffusion dependence of differential reflection dynamics in [formula omitted] ( x ≤ 0.35) strained-multiple-quantum wells

Using the pump-probe technique, we have observed time-resolved differential reflection in InAs x P 1−x InP ( x ≤ 0.35) strained-multiple-quantum wells (SMQWs) and also examined the photoluminescence spectra for the samples studied. The experimental results show that barrier height, interface roughness and quantum-well width influence strongly the differential reflection dynamics. From the experimental results, we have demonstrated that photoexcited carrier diffusion in cap layer and barriers along the direction perpendicular to the sample surface plays a dominant role in determining the differential reflection dynamics.

Low-drive-voltage MQW electroabsorption modulator for optical short-pulse generation

This paper reports on InGaAsP-InGaAsP tensile strained MQW electroabsorption (EA) modulators with a high modulation efficiency of 35 GHz/V that generate optical short pulses. We studied and optimized the multiple-quantum-well (MQW) structural parameters, barrier height, and well number, thickness, and strain in the absorption layer to ensure high attenuation efficiency and generate low duty cycle pulses. Low TE/TM polarization sensitivity was obtained by controlling strain. Stable, nearly transform-limited optical pulse trains with a narrow pulsewidth of 3.6 ps are generated by applying a 20-GHz sinusoidal modulation voltage (6 V/sub pp/) to the EA modulator. This achieves a very small pulse duty cycle of 7.2%.

Precise control of island formation using overgrowth technique on cleaved edges of strained multiple quantum wells

We propose an approach to control the position and the size of semiconductor islands formed by the Stranski–Krastanov growth mode. The method is to perform overgrowth on a cleaved edge of strained multiple quantum wells (SMQW), which give periodically modulated strain to the epitaxial layer. Proper choice of the growth conditions results in the formation of islands selectively on one of the constituents of the SMQW. The usefulness of this method was demonstrated by overgrowing Ge islands on the cleaved edges of Si0.8Ge0.2/Si SMQW. The Ge islands were selectively formed on the Si layer at 500 °C and on the Si0.8Ge0.2 layer at 600 °C. The former reflects the smaller critical thickness of Ge on Si, and the latter is driven by the minimization of strain energy owing to the smaller lattice mismatch between Ge and Si0.8Ge0.2.

Growth and Characterization of InAsxP1−x/InP Strained Multiple Quantum Wells by Gas Source Molecular Beam Expitaxy

AbstractInAsxPi.x/InP (10 period 50/100Å with x=0.25–0.79) pseudomorphically strained multiple quantum wells (SMQWs) were grown by gas source molecular beam expitaxy (GSMBE) at 470°C and characterized by cross-sectional transmission electron microscope (XTEM), double crystal x-ray diffraction (DCXRD), and optical spectroscopy. The structural analysis demonstrates that excellent control of the sharp interface and limited As-P interdiffusion can be achieved by GSMBE growth. XTEM images of these SMQWs display no misfit dislocations, and DCXRD scans reveal high order superlattice satellite peaks. Photoluminescence (PL) and transmission measurements were performed for all SMQWs to evaluate crystal quality. Only slight degradation in luminescence was observed as the As composition increased. Based on the three-band Kane model which includes the lattice strain, the transition energies of SMQWs were calculated using the conduction-band offset (Qc=δEc/δEg) as an adjustable parameter. The best fit of measured and calculated interband transition energies suggests that Qc is independent of As composition and is 0.70±0.05. Finally, a growth kinetics model based on the Langmuir equation was derived to realize the As/P incorporation ratio in the InAsP materials. Theoretical results show good agreement with experimental data.

Improvement of crystal quality and laser characteristics by zero net strain structure

Zero net strain structure is very useful for strained multiple quantum well (MQW) lasers for which many numbers of quantum wells are required. We demonstrate the effectiveness of the zero net strain structure to prevent the degradation of the crystal quality up to 15 wells, which cannot be realized by the conventionally strained MQW (well: 1.5% compressive strain, 30 Å thick). We also show that the zero net strain structure has finite critical thickness and dislocations other than 60° misfit dislocations are introduced into the zero net strain structure beyond the critical thickness. Narrower distribution of threshold current than that of the conventionally strained lasers, high relaxation oscillation frequency of 5.5 GHz/ √mW and high differential gain of 6.3 X 10 -16 cm 2 have been realized for the 1.55 μm laser with zero net strain MQW with 15 wells by the suppression of degradation of crystal quality.

Strain-induced effects on the performance of AlGaInP visible lasers

The strain-induced reduction of the threshold current density (J/sub th/) of AlGaInP visible lasers was investigated. Strained multiple-quantum-well (SMQW) and unstrained MQW AlGaInP broad contact lasers were fabricated using low-pressure organometallic vapor phase epitaxy (OMVPE). To evaluate the strain-induced effects, AlGaInP compressively strained quantum wells were introduced into the active region of the SMQW laser, making the energy band structure of the SMQW laser almost the same as that of the MQW one. Comparing the J/sub th/ of both lasers at the same lasing wavelength showed that, for cavity lengths between 300 and 700 mu m, more than a 4% decrease in J/sub th/ was obtained by incorporating the SMQW structures, and the maximum reduction of 6% was achieved at 500 mu m. In addition, it was shown that, in the SMQW laser, the considerable decrease in the transparency current density J/sub 0/ contributes effectively toward J/sub th/ reduction. >

Characterization of GaAs/GaAsP strained multiple quantum wells grown by gas-source molecular beam epitaxy

GaAs/GaAs1−xPx strained multiple quantum well (SMQW) structures with x ranging from 0 to 0.5 were prepared by gas-source molecular beam epitaxy. High-resolution x-ray rocking curves and transmission electron microscopy show the multilayered GaAs/GaAsP structures to possess good periodicity and crystalline quality. Photoluminescence and photoluminescence excitation spectra show sharp and distinct peaks that are attributed to the various interband excitonic transitions in quantum wells. An energy-level calculation based on an envelope function model suggests Qv=ΔEv/ΔEg=0.42 for strained GaAs/GaAs0.61P0.39 quantum well structure. Moreover, a 90-meV valence-band offset for a GaAs(76.5 Å)/GaAs0.68P0.32(76 Å) SMQW structure is determined from photoluminescence measurements under hydrostatic pressures.

Growth and characterization of InAs microclusters in InP and InAsP/InP heterostructures by low pressure MOCVD using tertiarybutylarsine

InAs microclusters embedded in an InP matrix and InAs x P 1– x /InP (0.1 ≤ x≤0.3) strained multiple quantum wells (SMQW) have been prepared by LP-MOCVD using tertiarybutylarsine (TBAs) as a substitute for AsH 3. The relatively large cracking efficiency of TBAs resulted in low As/In ratios (from 4.3 to 8.8) for InAs/InP structures and low As/P ratios (from 1×10 -3 to 3.5×10 -3 for InAs x P 1– x /InP (0.1 ≤ x ≤0.3). High-resolution X-ray diffraction (HRXRD) and photoluminescence (PL) show that the TBAs-grown material is of high quality. The HRXRD patterns of InAs microclusters embedded in the InP matrix demonstrate that the InAs unit cell is tetragonally distorted in accordance with the elasticity theory. For InAs microclusters embedded in the InP matrix, the 7 K emission PL peak shifts from 1.35 to 1.39 eV when the InAs coverage is reduced from 0.8 to 0.2 monolayers, a clear indication of a lateral size effect. Picosecond time resolved PL shows a weak coupling of excitons confined by these InAs microclusters. We report also the results of a study of the interface roughness of InAs x P 1– x /InP heterostructures. HRXRD, PL, absorption and photoconductivity (PC) measurements for InAsP/InP SMQW reveal that the InAsP/InP interface is very sensitive to growth procedure. For nonoptimal growth procedures a large density of interface states is created, probably as a consequence of compositional modifications within the interface region. The optical measurements clearly resolve the fundamental intersubband transitions in the InAsP/InP SMQW, including heavy hole-light splittings, even at room temperature.