Strain-relaxed In0 Research Articles

Nanometer scale fabrication and optical response of InGaN/GaN quantum disks

In this work, we demonstrate homogeneously distributed In0.3Ga0.7N/GaN quantum disks (QDs), with an average diameter below 10 nm and a high density of 2.1 × 1011 cm−2, embedded in 20 nm tall nanopillars. The scalable top-down fabrication process involves the use of self-assembled ferritin bio-templates as the etch mask, spin coated on top of a strained In0.3Ga0.7N/GaN single quantum well (SQW) structure, followed by a neutral beam etch (NBE) method. The small dimensions of the iron cores inside ferritin and nearly damage-free process enabled by the NBE jointly contribute to the observation of photoluminescence (PL) from strain-relaxed In0.3Ga0.7N/GaN QDs at 6 K. The large blueshift of the peak wavelength by over 70 nm manifests a strong reduction of the quantum-confined Stark effect (QCSE) within the QD structure, which also agrees well with the theoretical prediction using a 3D Schrödinger equation solver. The current results hence pave the way towards the realization of large-scale III-N quantum structures using the combination of bio-templates and NBE, which is vital for the development of next-generation lighting and communication devices.

Photoconductivity of Er-doped InAs quantum dots embedded in strain-relaxed InGaAs layers with 1.5 µm cw and pulse excitation

We fabricated a photoconductive antenna structure utilizing Er-doped InAs quantum dot layers embedded in strain-relaxed In0.35Ga0.65As layers on a GaAs substrate. Mesa-shaped electrodes for the antenna structure were formed by photolithography and wet etching in order to suppress its dark current. We measured the photocurrent with the excitation of ∼1.5 µm cw and femtosecond pulse lasers. Compared with the dark current, the photocurrent was clearly observed under both cw and pulse excitation conditions and almost linearly increased with increasing excitation power in a wide range of magnitudes from 10 W/cm2 to 10 MW/cm2 order.

Control of threading dislocations by strain engineering in GaInP buffers grown on GaAs substrates

High quality strain-relaxed In0.3Ga0.7As layers with threading dislocation density about 2×106cm−2 and root-mean-square surface roughness below 8.0nm were obtained on GaAs substrates using compositionally undulating step-graded Ga1−xInxP (x=0.48–0.78) buffers. The transmission electron microscopy results reveal that the conventional step-graded GaInP buffers produce high density dislocation pile-ups, which are induced by the blocking effect of the nonuniform misfit dislocation strain field and crosshatched surface on the gliding of threading dislocations. In contrast, due to strain compensation, insertion of the tensile GaInP layers decreases the surface roughness and promotes dislocation annihilation in the interfaces, and eventually reduces the threading dislocation density. This provides a promising way to achieve a virtual substrate with the desired lattice parameter for metamorphic device applications.

Strongly Enhanced Four-Wave Mixing Signal from GaAs/AlAs Cavity with InAs Quantnm Dots Embedded in Strain-Relaxed Barriers

Strong ultrafast four-wave mixing signals at ∼1.5 µm were demonstrated in a GaAs/AlAs multilayer cavity containing self-assembled InAs quantum dots (QDs) embedded in strain-relaxed In0.35Ga0.65As barriers. Time-resolved optical measurements using 100 fs pulses with 100 kHz repetition rate were carried out in the various excitation powers at room temperature. Strongly enhanced four-wave-mixing signals were observed for a cavity with two InAs QDs layers inserted in a half-wavelength (λ/2) cavity layer compared with that of a GaAs λ/2 cavity that had no QDs, in the whole range of excitation power (0.3–2 mW). For a low excitation power below 0.6 mW, the four-wave-mixing signals were about two orders of magnitude larger than that of the GaAs λ/2 cavity owing to the large nonlinearity of the InAs QDs.

GaAs/AlAs Multilayer Cavity with Er-Doped InAs Quantum Dots Embedded in Thin Strain-Relaxed In0.45Ga0.55As Barriers for Ultrafast All-Optical Switches

A GaAs/AlAs multilayer cavity with a λ/2 AlAs cavity layer, which includes Er-doped InAs quantum dots (QDs) embedded in a thin strain-relaxed In0.45Ga0.55As barrier, was fabricated. Structural and optical properties were characterized by scanning electron microscopy and optical reflection measurements, respectively. We found that the cavity quality of the Er-doped QD cavity was improved by reducing the thickness of the strain-relaxed In0.45Ga0.55As barriers in the λ/2 AlAs cavity layer. Furthermore, time-resolved optical measurements were performed to study the relaxation time of the photogenerated carriers in the Er-doped QD cavity. A full width at half maximum of a 1 ps with a large reduction of slowly decaying carriers was obtained for the Er-doped QD cavity with a thin strain-relaxed In0.45Ga0.55As barrier.

Ultrafast photocarrier relaxation processes in Er-doped InAs quantum dots embedded in strain-relaxed InGaAs barriers

Time-resolved transmission change of Er-doped InAs quantum dots (QDs) embedded in strain-relaxed In0.45Ga0.55As barriers has been studied using different excitation wavelengths (1.4–1.55μm) to understand the QD-size dependence of the photocarrier relaxation. Each measured temporal profile was well reproduced by a sum of three exponential decays. Ultrafast (∼1.6ps) and fast (∼6–9ps) components are dominant in the initial stage of decay, and then the slow (∼70–130ps) component due to the radiative recombination in the QDs appears to be seen. The first two components come from photocarrier relaxation into the nonradiative centers related to Er dopants or the lattice relaxation. The difference is considered to result from whether the nonradiative centers are positioned inside or outside the QDs. The decay time for the outside case becomes shorter as the excitation wavelength is shortened, which might be due to the weaker confinement of photocarriers in the smaller-size QDs. Suppression of the QDs contributing to the radiative recombination is also more significant for the smaller-size QD excitation.

Marked reduction in photocarrier lifetime by erbium doping into self-assembled InAs quantum dots embedded in strain-relaxed InGaAs barriers

Effects of erbium doping on photocarrier lifetime have been investigated for self-assembled InAs quantum dots (QDs) embedded in strain-relaxed In0.35Ga0.65As barriers grown on (100) GaAs substrates by molecular beam epitaxy. Time-resolved transmission change measurements were performed on 20-layer stacks of the Er-doped InAs QDs at room temperature using 0.1ps laser pulses with the center wavelength of 1.5μm. Fast and slow decay components were observed in the temporal profile of each Er-doped InAs QD sample. The slow decay component attributed to the radiative recombination process in the QDs was well suppressed by increasing Er-doping density. Moreover, the decay time of the fast component due to the nonradiative process was markedly reduced by the incorporation of Er dopants during the QD formation. A decay time of 3.5ps was obtained for the Er-doped InAs QD sample with a sheet density of incorporated Er dopants per QD layer of 2.7×1013cm−2.

GaAs/AlAs Multilayer Cavity with InAs Quantum Dots Embedded in Strain-Relaxed Barriers for Planar-Type Optical Kerr Gate Switches

A GaAs/AlAs multilayer cavity containing self-assembled InAs quantum dots (QDs) embedded in strain-relaxed In0.35Ga0.65As barriers has been studied for planar-type optical Kerr gate switches. The multilayer cavity structure with smooth GaAs/AlAs interfaces was successfully grown on a (001) GaAs substrate by molecular beam epitaxy despite the lattice strain relaxation induced in the half-wavelength (λ/2) cavity layer. Time-resolved optical measurements at the cavity mode (λ= 1.46 µm) were performed by a pump–probe method at room temperature. Although only two layers of the InAs QDs were inserted into the λ/2 cavity layer, a large transmission change caused by the absorption saturation in the resonant QDs was clearly observed. The temporal profile was dominated by a fast (∼16 ps) decay component, which comes from carrier relaxation into the nonradiative centers arising from the lattice strain relaxation. We have also shown that an ultrafast response time (<1 ps) of the strongly enhanced optical Kerr signal is determined by the photon lifetime in the multilayer cavity.

Excitation Wavelength Dependence of Carrier Relaxation in Self-Assembled InAs Quantum Dots Embedded in Strain-Relaxed In0.35Ga0.65As Barrier Layers

Excitation wavelength dependence (λ= 1350–1550 nm) of carrier relaxation was investigated for self-assembled InAs quantum dots (QDs) embedded in strain-relaxed In0.35Ga0.65As barriers grown on GaAs(100) substrates. Fast (τ∼12–17 ps) and slow (τ∼350 ps) decay components were observed in the time-resolved transmission change measurements by a pump-probe method. Decay time of the fast component becomes faster as the excitation wavelength is shortened, indicating that photo-generated carries in the smaller-size QDs more easily escape into the nonradiative centers arising from the crystal defects related to the lattice-mismatch. In addition, the slow decay component attributed to the radiative recombination process was strongly suppressed for the shorter wavelength excitation.

Cathodoluminescence study of InGaN MQW laser diodes using laser lift‐off technique

In this paper, spatially integrated or spatially resolved cathodoluminescence (CL) spectroscopy were employed to evaluate the structural and optical changes of InGaN multiple quantum well (MQW) structure before and after Laser lift-off (LLO) process. As for a partially strain-relaxed In0.1Ga0.9N MQW laser diode (LD) structure with cracking in underlying epitaxial layers, LLO did not cause the blue shift in CL spectra as expected, but rather the red shift induced by the indium diffusion from its abundant regions around cracks. This result could be proved by the monochromated scanning CL images taken at 390 nm and 408 nm not having the same obvious complementary as those of original ones. Compared with calculated laser fluence for LLO, this phenomenon could be mainly attributed to the easy penetration of laser energy via cracks and consequent irradiation absorption within MQWs. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Ion beam analysis of defects and strain in swift heavy ion irradiated InGaAs/GaAs heterostructures

Analysis of defects by Channeling in strain relaxed In0.18Ga0.82As/GaAs heterostructures before and after swift heavy ion irradiation has been reported. Energy dependence of dechanneling parameter has been analyzed which indicates a thickness dependence of defects, involving dislocations (for 36 and 96nm) and stacking faults (for 60nm). The dislocation density reduces upon irradiation and the possibilities for the same have been discussed in detail. The cross-sectional transmission electron microscopy (TEM) analysis indicates the presence of stacking faults in 60nm and dislocations in 96nm irradiated samples complementing the dechanneling studies. Angular scans along off-normal axis have been carried out for strain analysis. A strong strain relaxation as a function of thickness is observed from the strain measurements. The strain values are almost same after irradiation compared with unirradiated ones. The flux distribution of channeled ions at smaller thicknesses is discussed in detail.

Strain-Relaxed In0.1Al0.25Ga0.65As ∕ In0.22Ga0.78As ∕ In0.1Al0.25Ga0.65As HEMT

A GaAs-based high-electron mobility transistor with a quaternary strain-relaxed layer (SR-HEMT) has been fabricated successfully. The lower lattice mismatch between and yields high-electron mobility and good carrier confinement. Experimental results indicate that the SR-HEMT exhibits a peak of , a maximum drain-current density of , a high breakdown voltage of , a cut-off frequency of , a maximum oscillation frequency of , a small-signal power gain of , and a saturated power of at with the gate dimension of . Furthermore, the SR-HEMT exhibits a temperature-insensitive threshold voltage shift of .

Development of Cross-Hatch Morphology During Growth of Lattice Mismatched Layers

ABSTRACTAn approach is developed for understanding the cross-hatch morphology in lattice mismatched heteroepitaxial film growth. It is demonstrated that both strain relaxation associated with misfit dislocation formation and subsequent step elimination (e.g. by step-flow growth) are responsible for the appearance of nanoscopic surface height undulations (0.1-10 nm) on a mesoscopic (∼100 nm) lateral scale. The results of Monte Carlo simulations for dislocation- assisted strain relaxation and subsequent film growth predict the development of cross-hatch patterns with a characteristic surface undulation magnitude ∼50 Å in an approximately 70% strain relaxed In0.25Ga0.75As layers. The model is supported by atomic force microscopy (AFM) observations of cross-hatch morphology in the same composition samples grown well beyond the critical thickness for misfit dislocation generation.

Transport in strain relaxed In0.15Al0.85As/In0.17Ga0.83As heterojunctions

The electrical and galvanomagnetic properties of the two-dimensional electron gas (2DEG) in δ-doped and modulation doped strain–relaxed In0.15Al0.85As/In0.17Ga0.83As heterojunctions have been determined, as a function of sheet carrier density, by temperature dependent resistivity Hall and 1.6 K Shubnikov de Haas (SdH) oscillatory magnetoresistance measurements. The structures were grown by molecular-beam epitaxy on misoriented (001) GaAs substrates; strain relaxation, confirmed by double crystal x-ray diffraction, was obtained by means of step graded InxGa1−xAs buffer layers with x=0.10, and x=0.15 steps of 0.3 μm in thickness. Low temperature (1.6 K) SdH measurements confirm the presence of well behaved 2DEG layers with negligible parallel conduction, and single band occupancy for sheet carrier densities ranging from 7.4×1011 cm−2 to 1.0×1012 cm−2, and respective mobilities of 7.5×104 cm2/V s and 6.1×104 cm2/V s. Low field resistivity and Hall measurements suggest a transition, at a sheet carrier density of approximately 1012 cm−2, from an alloy scattering dominated mobility, to a mobility limited by ionized impurity scattering. The excellent transport properties of the 2DEG channels suggests the use of strain relaxation by means of appropriate buffer layers for heterostructure devices grown on gallium arsenide substrates.

Effect of crystalline defects on electrical properties of heavily Si-doped strain-relaxed In0.5Ga0.5As layers grown by molecular beam epitaxy on GaAs

Heavily Si-doped, strain-relaxed In0.5Ga0.5As layers are often used as contact layers on N+ GaAs to form nonalloyed ohmic contacts. The present work studies the electrical properties of such In0.5Ga0.5As layers grown on undoped GaAs buffer layers by molecular beam epitaxy both as-grown and after anneals at temperatures between 300 and 430 °C. It is found that in the as-grown condition, there is a region of about 150 Å extending into the In0.5Ga0.5As layer contiguous to the In0.5Ga0.5As/GaAs interface where both the electron concentration and mobility decrease toward the interface. With postgrowth anneal, the electron concentration in this region will be further reduced and the width of the region will be increased. The associated activation energy is estimated to be 0.99±0.41 eV. This phenomenon can be fully explained in terms of defect structural changes during anneal. This work provides strong evidence to indicate that the as-grown In0.5Ga0.5As layers are not in thermodynamic equilibrium and will continue to relax when thermal energy is provided.

Magnetotransport properties of two-dimensional channels in pseudomorphic and strain relaxed In0.17Ga0.83As heterojunctions

The electrical and galvanomagnetic properties of the two-dimensional electron gas present in pseudomorphic In0.17Ga0.83As/Al0.35Ga0.65As and in strain-relaxed In0.17Ga0.83As/In0.15Al0.85As modulation δ-doped heterojunctions were measured as a function of temperature using conventional Hall and resistivity measurements and 1.6 K Shubnikov de Haas oscillatory magnetoresitance measurements, which revealed single subband occupancy. Typical mobilities of the pseudomorphic specimens are, μ(300 K)=9.3×103 cm2/V s and μ(1.6 K)=5.51×104 cm2/V s for electron densities, ns=1.07×1012 cm−2 and 9.78×1011 cm−2, respectively. The strain-relaxed structures have higher electron mobilities, μ(300 K)=104 cm2/V s and μ(1.6 K)=7.61×104 cm2/V s for ns=9.37×1011 cm−2 and 8.67×1011 cm−2. Although the total scattering time τq calculated from the magnetic field-dependent amplitudes of their Shubnikov de Haas oscillations, is half that of the pseudomorphic structures, their low temperature, classical relaxation time τc is greater by 30%.