Photoreflectance spectroscopy of a Ga0.62In0.38N0.026As0.954Sb0.02/GaAs single quantum well tailored at 1.5 μm

Optical properties of a Ga_0.62In_0.38As_0.954N_0.026Sb_0.02/GaAs single quantum well (SQW) tailored at ∼1.5 μm have been investigated by photoreflectance (PR) spectroscopy. The identification of the optical transitions was carried out in accordance with theoretical calculations, which were performed within the framework of the usual envelope function approximation. Using this method, four confined states for both electrons and heavy holes have been found and the optical transitions between them have been determined. The obtained result corresponds to a conduction band offset ratio close to 80%. In addition, the effect of ex situ annealing has been investigated. Lineshape analysis of the PR transitions shows that one of the phenomena responsible for the blueshift of QW transitions is the change in the nitrogen nearest-neighbour environment from Ga-rich to In-rich environments.     

Optimization of GaInNAs quantum-well vertical-cavity surface-emitting laser emitting at 2.33 μm

In the present paper, a comprehensive computer simulation is used to determine optimal structure of the InP-based GaInNAs quantum-well (QW) active region and to investigate a possibility of reaching room-temperature (RT) continuous-wave (CW) single-fundamental-mode 2.33-μm operation of vertical-cavity surface-emitting laser (VCSEL) with such an active region. From among various considered InP-based active regions, the one with the Ga_0.15In_0.85N_0.015As_0.985/Al_0.138Ga_0.332In_0.530As QW, i.e. with barriers lattice matched to InP, seems to be optimal for the 2.33-μm VCSEL performance. Its QW material is chosen for the required long-wavelength emission whereas its barrier is expected to ensure promising laser performance at room and higher temperatures. The latter is mostly connected with the QW conduction band offset equal in the above active region to as much as 413 meV, which is much larger than those of its possible lattice matched to InP competitors, e.g. 276 meV for the Ga_0.47In_0.53As barrier and 346 meV for the Ga_0.327In_0.673As_0.71P_0.29 one. Our simulation reveals that from among various considered structures, a VCSEL with a 4-μm-diameter tunnel junction and two 6-nm Ga_0.15In_0.85N_0.015As_0.985/Al_0.138Ga_0.332In_0.530As QWs exhibits the lowest calculated threshold current of 0.88 mA. Its promising RT CW performance suggests that it may represent a very interesting alternative to GaSb-based VCSELs.     

Photoreflectance spectroscopy of semiconductor structures at hydrostatic pressure: A comparison of GaInAs/GaAs and GaInNAs/GaAs single quantum wells

The pressure dependence of optical transitions in Ga_0.64In_0.36As/GaAs and Ga_0.64In_0.36N_0.01As_0.99/GaAs single quantum well (SQW) structures were studied in photoreflectance (PR) spectroscopy. In order to apply high hydrostatic pressure, up to ∼11 kbar, the liquid-filled clamp-pressure cell with a sapphire window for optical access has been adopted in the PR set-up with the so called ‘bright configuration’. It has been found that the linear hydrostatic pressure coefficient for the ground state transition are equal to 8.6 and 7.3 meV/kbar for the GaInAs/GaAs and GaInNAs/GaAs SQWs, respectively. This result shows that the incorporation of only 1% of N atoms into GaInAs/GaAs leads to ∼15% decrease in the pressure coefficient. In addition, a non-linearity in the pressure dependence of the ground state transition has been resolved for the GaInNAs/GaAs SQW.     

An attempt to design long-wavelength (>2 μm) InP-based GaInNAs diode lasers

In the present paper, anticipated performance characteristics of various InP-based GaInNAs quantum-well (QW) active regions are determined with the aid of our comprehensive computer model for various sets of parameters (temperature, carrier concentration, QW thickness). It is evident from this analysis that the compressively strained InP-based Ga_0.12In_0.88N_0.02As_0.98/Ga_0.275In_0.725As_0.6P_0.4 QW structure may offer expected lasing emission. Its maximal optical gain of over 2150 cm^?1 has been determined at room temperature for the wavelength of about 2815 nm for the QW thickness of 10 nm and the carrier concentration of 5×10¹⁸ cm^?3. Therefore, the above InP-based QW structure may be successfully applied in compact semiconductor laser sources of the desired radiation of wavelengths longer at room temperature than even 2800 nm. Similar GaAs-based devices emit radiation of distinctly shorter wavelengths, whereas GaSb-based ones avail themselves of more expensive substrates as well as exhibit lower thermal conductivities and worse carrier confinements.     

Contactless electroreflectance of GaNyAs1−y/GaAs multi quantum wells: The conduction band offset and electron effective mass issues

Interband transitions in GaN_yAs_1−y/GaAs multi quantum well (MQW) samples with y=0.012 and 0.023 have been studied by contactless electroreflectance spectroscopy (CER). Optical transitions related to absorption in the GaAs barriers and in the GaN_yAs_1−y/GaAs QWs have been observed and analyzed. The GaAs related transition exhibits clear Franz-Keldysh oscillations with the period corresponding to the built-in electric field of 14 and 17 kV/cm for samples with y=0.012 and 0.023, respectively. The portion of the CER spectrum related to absorption in the GaN_yAs_1−y/GaAs QW exhibits two clear resonances which are attributed to optical transitions between the ground and excited states confined in the QWs. The resonance attributed to the ground state transition is associated with absorption between the first light- and heavy-hole subbands and the first electron subband (11L and 11H) while the resonance attributed to the excited state transition is associated with absorption between the second heavy-hole subband and the second electron subband (22H). The energies of the 11H and 22H transitions have been matched with those obtained from theoretical calculations performed within the effective mass approximation. Thus, the GaN_yAs_1−y/GaAs QWs are type-I structures with a conduction band offset, Q_C, between 70 and 80%. Moreover, the incorporation of N atoms into GaAs is found to cause a significant increase in the electron effective mass. The determined values of electron effective mass for GaN_yAs_1−y/GaAs QW with y=0.012 and 0.023 are 0.105m₀ and 0.115m₀, respectively.     

Investigation of deep levels of AsGa and GaAs antistructural defects and InAs and SbGa heteroantistructural defects in GaAs using a 4×4×4 expanded unit cell technique

A procedure is developed for calculation of the electron-energy spectrum of crystals with point defects using a 128-atom periodic cluster and with consideration of local defect symmetry. Electron-energy spectra are calculated for GaAs with As_Ga, Sb_Ga, Ga_As, and In_As defects. The calculated results for deep levels agree with observations. Deep-level electron-density distribution is discussed as a function of center type. The spinorbital orbital splitting of localized levels is estimated. Tomsk State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 13–21. August, 1996.     

Comparison of the band alignment of strained and strain-compensated GaInNAs QWs on GaAs and InP substrates

We present a comparison of the band alignment of the Ga_1−xIn_xN_yAs_1−y active layers on GaAs and InP substrates in the case of conventionally strained and strain-compensated quantum wells. Our calculated results present that the band alignment of the tensile-strained Ga_1−xIn_xN_yAs_1−y quantum wells on InP substrates is better than than that of the compressively strained Ga_1−xIn_xN_yAs_1−y quantum wells on GaAs substrates and both substrates provide deeper conduction wells. Therefore, tensile-strained Ga_1−xIn_xN_yAs_1−y quantum wells with In concentrations of x0.53 on InP substrates can be used safely from the band alignment point of view when TM polarisation is required. Our calculated results also confirm that strain compensation can be used to balance the strain in the well material and it improves especially the band alignment of dilute nitride Ga_1−xIn_xN_yAs_1−y active layers on GaAs substrates. Our calculations enlighten the intrinsic superiority of N-based lasers and offer the conventionally strained and strain-compensated Ga_1−xIn_xN_yAs_1−y laser system on GaAs and InP substrates as ideal candidates for high temperature operation.     

Strongly confined quantum wire states in strained T-shaped GaAs/InAlAs structures

The confinement energy of T-shaped quantum wires (QWRs), which were fabricated by the cleaved edge overgrowth technique in a way that the QWRs form at the intersection of In_0.2Al_0.8As stressor layers and the overgrown (1 1 0) GaAs quantum well (QW), is examined using micro-photoluminescence spectroscopy. Photoluminescence (PL) signals from individual QWRs can be spatially resolved, since the strained films are separated by 1 μm wide Al_0.3Ga_0.7As layers. We find that due to the tensile strain being transmitted to the QW, the confinement energy of the QWRs rises systematically up to 40 meV with increasing thickness of the stressor layers. By reducing the excitation power to 0.1 μW the QWR PL emission occurs 48 meV redshifted with respect to the QW. All QWR peaks exhibit smooth lineshapes, indicating the absence of pronounced exciton localization.     

Experimental investigation of the CMN matrix element in the band anticrossing model for GaAsN and GaInAsN layers

The temperature dependence of bandgap energy of GaAs_0.98N_0.02 and Ga_0.95In_0.05As_0.98N_0.02 compounds has been analysed in order to determine the C_MN matrix element of the band anticrossing model. We have found that the element equals 2.48 and 2.60 eV for GaAs_0.98N_0.02 and Ga_0.95In_0.05As_0.98N_0.02 layers, respectively. When the same value has been assumed for annealed layers, an increase in the energy of the resonant nitrogen level E_N has been obtained. Two possible mechanisms leading to the increase in this energy are discussed in this work.     

Optical properties of narrow high quality GaAs/AlGaAs quantum wells grown by MOVPE

We report optical characterization of high quality quantum well (QW) structures grown by metal-organic vapour-phase epitaxy (MOVPE). Thin QW layers of GaAs of thicknesses between 20 Å and 80 Å inserted between Al_0.36Ga_0.64 As confining layers as well as nominally 20 Å QW's in Al_xGa_1−xAs with varying x have been studied. Exciton confinement energies exceeding 250 meV and a FWHM of 6 meV for the thinnest QW have been observed. The photoluminescence (PL) data allows the observation of monolayer fluctuations in the QW widths and indicates an interface abruptness of about one atomic layer. Photoluminescence excitation spectroscopy allows electronic excited states to be seen.     

Classical and quantum hall effect measurements in GaInNAs/GaAs quantum wells

We have performed magneto-transport experiments in modulation-doped Ga_0.7In_0.3N_yAs_1−y/GaAs quantum wells with nitrogen mole fractions 0.4%, 1.0% and 1.5%. Classical magnetotransport (resistivity and low-field Hall effect) measurements have been performed in the temperatures between 1.8 and 275 K, while quantum Hall effect measurements in the temperatures between 1.8 and 47 K and magnetic fields up to 11 T.The variations of Hall mobility and Hall carrier density with nitrogen mole fractions and temperature have been obtained from the classical magnetotransport measurements. The results are used to investigate the scattering mechanisms of electrons in the modulation-doped Ga_0.7In_0.3N_yAs_1−y/GaAs quantum wells. It is shown that the alloy disorder scattering is the major scattering mechanism at investigated temperatures.The quantum oscillations in Hall resistance have been used to determine the carrier density, effective mass, transport mobility, quantum mobility and Fermi energy of two-dimensional (2D) electrons in the modulation-doped Ga_0.7In_0.3N_yAs_1−y/GaAs quantum wells. The carrier density, in-plane effective mass and Fermi energy of the 2D electrons increases when the nitrogen mole fraction is increased from y=0.004 to 0.015. The results found for these parameters are in good agreement with those determined from the Shubnikov-de Haas effect in magnetoresistance.     

Photoluminescence of InGaAs/GaAs strained-layer quantum well structures under high pressure

Abstract

Measurements of the photoluminescence (PL) of strained In_0.2Ga_0.8As/GaAs and In_0.15Ga_0.85As/GaAs quantum well structures together with the PL from bulk GaAs, in a diamond anvil cell show that the pressure coefficient of the ground confined state in the wells depends upon well width (L_Z). In the thinnest wells, the coefficient is closer to that of the bulk GaAs (10.7 meV/kbar), as expected. However, in the widest wells the coefficients tend to values (9.5meV/kbar for the 15% alloy and 9.1meV/kbar for the 20% alloy) that are significantly lower than the pressure coefficient of unstrained In_0.53Ga_0.47As (10.9meV/kbar). It is found that the low pressure coefficients can not be explained by the change in uniaxial stress with pressure due to a difference in bulk moduli between the barrier and well.     

Interband emission energy in a dilute nitride quaternary semiconductor quantum dot for longer wavelength applications

Excitonic properties are studied in a strained Ga_1−xIn_xN_yAs_1−y/GaAs cylindrical quantum dot. The optimum condition for the desired band alignment for emitting wavelength 1.55 µm is investigated using band anticrossing model and the model solid theory. The band gap and the band discontinuities of a Ga_1−xIn_xN_yAs_1−y/GaAs quantum dot on GaAs are computed with the geometrical confinement effect. The binding energy of the exciton, the oscillator strength and its radiative life time for the optimum condition are found taking into account the spatial confinement effect. The effects of geometrical confinement and the nitrogen incorporation on the interband emission energy are brought out. The result shows that the desired band alignment for emitting wavelength 1.55 µm is achieved for the inclusion of alloy contents, y=0.0554% and x=0.339% in Ga_1−xIn_xN_yAs_1−y/GaAs quantum dot. And the incorporation of nitrogen and indium shows the red-shift and the geometrical confinement shows the blue-shift. And it can be applied for fibre optical communication networks.     

Influence of N cluster states on band dispersion in GaInNAs quantum wells

We use a modified band-anticrossing (BAC) model to investigate the band dispersion in a GaN_xAs_1-x/AlGaAs quantum well (QW) as a function of hydrostatic pressure. The band edge mass increases considerably more quickly with pressure than in the case of a GaAs/AlGaAs QW, and the subband separation also decreases significantly. We predict that the strong anticrossing interaction between the GaAs host conduction band and isolated N levels will inhibit tunnelling through the QW for a range of energy above the isolated N levels. The energy of N resonant states depends strongly on details of the local environment, giving a broader calculated distribution of N states in GaInNAs compared to GaNAs.     

Integrated MQW intermixed InGaAsP/InP waveguide photodiodes

A multi-wavelength Quantum well (QW) waveguide photodiode (PD) have been designed for Coarse Wavelength Division Multiplexing (CWDM) systems in which spatial tailoring of the bandgap with post growth F implanted QW intermixing of InGaAsP/InP multi QWs for the integration have been considered. Two separate structures with different composition but same well widths are necessary to detect all CWDM wavelengths. For In_0.5995Ga_0.4005As_0.8521P_0.1479 well there is a 12 channel coverage from 1,270 to 1,490 nm and for the In_0.5540Ga_0.4460As_0.9489P_0.0511 well the 14 channel spans from 1,350 to 1,610 nm. A carrier tunneling time of 20 ps along with the transit time limited bandwidth of 86 GHz gives a 3 dB bandwidth of 41 GHz by optimizing the i-MQW thickness and dopant concentration of the different layers of the waveguide. A maximum efficiency of 22% with insertion loss of 0.4–23 dB has been obtained.     

Substrate effect on the electronic structure in GaxIn1−xAsySb1−y/GaSb and GaxIn1−xAsySb1−y/InAs

We present a theoretical study of the substrate effect on electronic structure in cubic Ga_xIn_1−xAs_ySb_1−y lattice-matched to GaSb and InAs. Our calculations are based on the empirical pseudopotential formalism within the virtual crystal approximation where the effect of disorder is taken into account. We show that the electronic band structure of the quaternary alloy Ga_xIn_1−xAs_ySb_1−y is altered by the change of substrate for the entire range of alloy compositions x. Moreover, we find that at Ga concentrations (x≤0.8), the ionicity is less important when Ga_xIn_1−xAs_ySb_1−y is lattice-matched to GaSb instead of InAs. The information will be useful for the choice of substrate in the composition range 0–1 and hence for the determination of the lattice-matching conditions for Ga_xIn_1−xAs_ySb_1−y quaternary materials.     

Photoreflectance investigation of hydrogenated (InGa)(AsN)/GaAs heterostructures

The optical response of as grown and hydrogenated In_0.32Ga_0.68As_1-yN_y/GaAs single quantum wells (y = 0, 0.027) has been investigated from T = 80 K to room temperature by photoreflectance. Three excitonic spectral features detected in the N free sample shift to lower energy in the N containing sample and back to higher energy upon H irradiation of the N containing sample. In the hydrogenated sample, a progressive change with increasing temperature of the nature of the lowest energy transition from an excitonic to a band-to-band character has been explained in terms of an increasing release of carriers from traps formed by H and N clusters. A reduction in the oscillator strength of the lowest energy transition and an increase in the binding energy of the heavy-hole exciton have been explained in terms of an increase in the electron effective mass upon N introduction into the In_xGa_1-xAs lattice. Received 23 June 2002 Published online 19 November 2002     

Optimization of optical characteristics of In0.29Ga0.71As0.99N0.01/GaAs straddled nano-heterostructure

Designing of a nanoscale Quantum Well (QW) heterostructure with a well thickness of ～60?Å is critical for many applications and remains a challenge. This paper has a detailed study directed towards designing of In_0.29Ga_0.71As_0.99N_0.01/GaAs straddled nanoscale-heterostructure having a single QW of thickness ～60?Å and optimization of optical and lasing characteristics such as optical and mode gain, differential gain, gain compression, anti-guiding factor, transparency wavelength, relaxation oscillation frequency (ROF), optical power and their mutual variation behavior. The outcomes of the simulation study imply that for the carrier concentration of ～2?×?10¹⁸cm^?3 the optical gain of the nano-heterostructure is of 2100?cm^?1 at the wavelength is of 1.30?μm. Though the obtained gain is almost half of the gain of InGaAlAs/InP heterostructure, but from the wavelength point of view the InGaAsN/GaAs nano-heterostructure is also more desirable because the 1.30?μm wavelength is attractive due to negligible dispersion in the silica based optical fiber. Hence, the InGaAsN/GaAs nano-heterostructure can be very valuable in optical fiber based communication systems.     

Electronic structure of GaxIn1-xAsySb1-y quaternary alloy by recursion method

Summary  This work reports the electronic structure of GaInAsSb quaternary alloy by recursion method. A five-orbital sp₃s^* per atom model was used in the tight-binding representation of the Hamiltonian. The local density of states (LDOS), integrated density of states (IDOS) and structural energy (ST.E) were calculated for Ga, In, As and Sb sites in Ga_0.5 In_0.5 As_0.5 Sb_0.5 and GaInAsSb lattice matched to GaAs and the same alloy lattice matched to GaSb. There are 216 atoms in our cluster arranged in a zincblend structure. The results are in good agreement with available information about the alloy.     

Theory of Raman spectra of correlated substitutional alloys

A method for computing the Raman spectra of substitutional crystalline III-V alloys, combining Monte Carlo and Recursion methods, is presented and applied to In_0.5Ga_0.5As_0.5Sb_0.5.