Comparative study of the band-offset ratio of conventionally strained and strain-compensated InGaAs/GaAs QW lasers

In order to achieve good high temperature laser performance, it is essential to have very deep electron wells. InGaAs system on GaAs substrate suffers from poor temperature characteristics due to the electron overflow over the rather small conduction band offset. By means of the Harrison's model, we investigate the effect of the strain compensation on band alignments of InGaAs/GaAs laser system and show that strain compensation improves the band alignments of this laser system. The use of GaAsP or InGaP barrier instead of GaAs barrier results the strain-compensated laser system having better band alignment than that of the conventionally strained InGaAs. Therefore, high temperature operation has been anticipated in these laser systems with strain compensated barriers due to better electron and hole confinement as a result of the increased band offset and a more favorable band offset ratio.     

Change in band configuration of quantum wells from type-II to type-I by increasing Sb composition x

InGaAs/AlAsSb systems lattice matched to InP have two distinguishable features: a high conduction band offset and type-II band configuration. Although, the former results in a large intersubband transition (ISBT) at conduction band, the latter makes it difficult to use the effective interband transition (IBT). To overcome this latter problem, the effect of the Sb was investigated because Sb would act as a band modulator from type-II to type-I. In_0.57Ga_0.43As_1-xSb_x/AlAs_0.48Sb_0.52 single quantum well (SQW) samples with various Sb compositions x, were grown on GaAs substrates via AlAs_0.48Sb_0.52 buffer layer. Their photoluminescence (PL) properties were examined to identify their band configurations. When the excitation laser power was increased, the PL property of In_0.57Ga_0.43As SQW sample, showed a larger blue shift than that of ones. This indicates that the band configuration modulates from type-II to type-I when the antimonide composition is larger than 0.13. These findings indicate that new functional devices can be fabricated using a combination of IBT and ISBT.     

InAs–GaAs self-assembled quantum dot lasers: physical processes and device characteristics

The gain characteristics of InAs–GaAs self-assembled quantum dot lasers are studied using two complementary techniques. The modal gain is derived from a measurement of the normal incidence, inter-band photoconductivity. For a device containing a single layer of dots the maximum modal gain of the ground state transition is found to be insufficient for lasing action. As a consequence lasing occurs for excited state transitions, which have a larger oscillator strength, with the precise transition being dependent upon the device cavity length. The second technique uses the Hakki–Paoli method to determine the spectral and current dependence of the gain. A quasi-periodic modulation of the below threshold gain is observed. This modulation is shown to be responsible for the form of the lasing spectra, which consist of groups of lasing modes separated by non-lasing spectral regions. Possible mechanisms for this behaviour are discussed.     

Selective area growth of GaInNAs/GaAs by MOVPE

Selective area growth (SAG) of GaInNAs/GaAs systems has been studied by metalorganic vapor-phase epitaxy (MOVPE) for the first time. This also includes a comparative study of SAG of the GaInAs/GaAs. The patterns consisted of various filling factors (F). The band gap changes and the growth morphology have been investigated. A red-shift observed for SAG GaInAs is 100 nm with respect to the planar GaInAs which can be attributed to both In enrichment and quantum well (QW) thickness enhancement. Selectively grown GaInNAs structures exhibit a maximum wavelength of 1.3 μm, corresponding to a red-shift of 80 nm with respect to the planar GaInNAs. Atomic force microscopy (AFM) scans reveal a three-dimensional growth behaviour for SAG GaInNAs unlike SAG GaInAs. This can be related to a certain amount of phase separation or strain that are often the signatures of N incorporation. The cathodoluminescence (CL) intensities (spectral line width) for SAG GaInNAs are larger (smaller) than those for SAG GaInAs at low F's but smaller (larger) at high F's. This indicates that at low F's, GaInAs has degraded due to very high strain but certain amount of strain compensation occurs in GaInNAs.     

Reliable high-efficiency high-brightness laser diode bars at 940 nm

High-quality InGaAs/AlGaAs laser diode bars emitting at 940 nm have been fabricated by low-pressure metal-organic chemical vapor deposition (LP-MOCVD). Two hundred and ten Watts maximum continuous-wave output power and a maximum power conversion efficiency of 60% at an output power of 72 W have been demonstrated for a single 1-cm-wide laser bar. These bars exhibit a very good beam quality of 5.7°×27.2° (full-width at half-maximum). Reliability test have been carried out for over 2000 h at 58 W at room temperature. Under these conditions, the extrapolated lifetime is 100,000 h, which suggests that AlGaAs-based lasers of proper designs could have similar long-term reliability as their Al-free counterpart.     

Tilt of InGaN layers on miscut GaN substrates

We report on the crystallographic orientation of InGaN layers grown on GaN substrates with a miscut with respect to c ‐planes up to 2.5°. The samples were examined using high‐resolution X‐ray diffraction (HRXRD) and atomic force microscopy (AFM). Because of the large (up to about 2% in this study) lattice mismatch between InGaN and GaN, an additional tilt between the c lattice planes of InGaN and GaN was observed and explained by using the Nagai model [J. Appl. Phys. 45 , 3789 (1974)]. We observed that for part of the samples, this tilt is about 10% smaller compared to the one predicted by the model. The experimental data are important for understanding the microstructure of InGaN layers grown on substrates of non‐perfect morphology. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Near UV photoluminescence of Hg-doped GaN nanowires

Mass Hg-doped GaN nanowires with an average diameter of about 25 nm and lengths up to hundreds of micrometers are fabricated by chemical vapor reaction. The as-synthesized products have a single crystal phase and grow along the 0 0 1 direction. The growth of Hg-doped GaN nanowires is suggested for quasi vapor–solid mechanism (QVSM). In particular, for as large-scale GaN nanowires like films, a novel strong near ultraviolet PL spectrum appears with a doping Hg where the Hg-doped GaN nanowires are found to be responsible for the different characteristics; the PL mechanism is explained in detail.     

Formation of InAs quantum dots and wetting layers in GaAs and AlAs analyzed by cross-sectional scanning tunneling microscopy

We have used cross-sectional scanning-tunneling microscopy (X-STM) to compare the formation of self-assembled InAs quantum dots (QDs) and wetting layers on AlAs (1 0 0) and GaAs (1 0 0) surfaces. On AlAs we find a larger QD density and smaller QD size than for QDs grown on GaAs under the same growth conditions (500 °C substrate temperature and 1.9 ML indium deposition). The QDs grown on GaAs show both a normal and a lateral gradient in the indium distribution whereas the QDs grown on AlAs show only a normal gradient. The wetting layers on GaAs and AlAs do not show significant differences in their composition profiles. We suggest that the segregation of the wetting layer is mainly strain-driven, whereas the formation of the QDs is also determined by growth kinetics. We have determined the indium composition of the QDs by fitting it to the measured outward relaxation and lattice constant profile of the cleaved surface using a three-dimensional finite element calculation based on elasticity theory.     

Pressure dependence of the band-gap energy and the conduction-band mass for an n-type InGaAs/GaAs strained single-quantum well

We report the measurement of the pressure dependence for the band-gap energy and conduction-band mass for an 80 Å-wide n-type strained-single-quantum well at 4.2 K for pressures between 0 and 35 kbar and fields up to 30 T. The band-gap energy , at each pressure, was determined by extrapolating the magnetoluminescence “fan-diagram” to zero magnetic field. The pressure dependence of the band-gap energy was found to be quadratic with a linear term of about 10.3 meV/kbar and a small, , quadratic contribution. Analyses of the pressure-dependent 4.2 K magnetoluminescence data yield a conduction-band mass logarithmic pressure derivative

Full-size image

.     

Magnetooptical investigations of single self assembled In0.3Ga0.7As quantum dots

We present results from magnetooptical investigations of large elongated single self assembled In_0.3Ga_0.7As quantum dots with a low surface density of . Compared to conventional In_0.6Ga_0.4As quantum dots the dimension of the investigated dots is enlarged by nearly one order of magnitude using a low strain In_0.3Ga_0.7As nucleation layer. In addition, the exciton exhibits a smaller g-factor of 0–0.4 and a larger diamagnetic coefficient of 20– in Faraday geometry, reflecting the increased extension of the exciton wavefunction, with respect to In_0.6Ga_0.4As quantum dots. From power dependent investigations we observe biexciton binding energies ranging from 1.7 to 1.9 meV. Excited state emission appears typically 2–5 meV above the ground state which is consistent with the increased dimensions of the structure. Furthermore we find linear polarization degrees of up to 0.6 from exciton emission of the elongated quantum dot structure.     

Influence of substrate misorientation and growth temperature on N incorporation in InGaAsN/GaAs quantum wells grown on (1 1 1)B GaAs

In this work, the effects of the substrate misorientation in the nitrogen incorporation in InGaAsN (1 1 1)B p–i–n diode structures grown by molecular beam epitaxy are discussed. The (1 1 1)B surfaces misoriented towards [2–1–1] are found to be more suitable to enhance the optical quality of the samples. We also found that the nitrogen incorporation is highly dependent on the growth temperature as well as on the V–III flux ratio. In addition to this, the optical properties and crystal quality of these structures depend strongly on the nitrogen content, as in the case of similar samples grown on (1 0 0) surfaces. High nitrogen contents (up to 3%) in InGaAsN layers grown on two different misoriented (1 1 1)B GaAs substrates are reported. Besides, low-temperature photoluminescence emission wavelengths longer than 1.4 μm are achieved using (1 1 1)B misoriented substrates.     

Electron-induced dissociation of SiH complexes in hydrogenated Si-doped GaAs. Application to the fabrication of microstructures

We study the role of hot electron injection in the dissociation of the SiH complexes which appear in n-type Si-doped GaAs epilayer exposed to a hydrogen or deuterium plasma. Firstly, the results recently obtained in room-temperature aging experiments on hydrogenated or deuterated Schottky diodes submitted to high bias voltage are summarized and the role of hot carriers in the dissociation of donors and the observed isotope effect are described. Then, it is shown that SiH dissociation can also be achieved using hot carriers injected into the semiconductor by electron beam. Such electron-beam effects are finally used for the fabrication and characterization by cathodoluminescence of micronic conductive GaAs structures.     

Critical layer thickness of GaIn(N)As(Sb) QWs on GaAs and InP substrates for (001) and (111) orientations

The aim of this work is to examine the effect of dilute nitride and/or antimonite on the critical layer thickness of GaInAs quantum wells on GaAs and InP substrates by means of Matthews and Blakeslee force model. The study provides a comparison of the critical layer thickness of the related GaIn(N)As(Sb) QWs in (001) and (111) orientation. Our calculations indicate the importance of antimonite and the proper usage of it with dilute nitrides in order to tailor the active layer thickness and emission wavelength of quantum well laser devices.     

{1 1 1} Quantum wells of dilute nitrides grown on GaAs by molecular beam epitaxy

Dilute nitrides are promising alloys in view of extending potential micro- and opto-electronics applications of GaAs technology. Orientation effects on nitrogen incorporation in GaAs have been scarcely addressed. Here, GaAsN on (1 0 0) and on As(B)- and Ga(A)-rich (1 1 1) substrates was grown by molecular beam epitaxy at different substrate temperatures. Nitrogen content measured by secondary ion mass spectrometry as a function of the growth temperature highlights the influence of orientation on nitrogen incorporation. Furthermore, thermal annealing is shown to improve the optical quality of GaAsN quantum wells whatever their substrate orientations.     

Studies of electro-optical properties and band alignment of InGaPN/GaAs heterostructures by photoreflectance and photoluminescence

Photoreflectance (PR) and photoluminescence (PL) spectra are measured for a series of In_0.54Ga_0.46P_1−yN_y/GaAs heterostructures at temperatures ranging from 25 to 300 K. The redshifts of the PR and PL peaks indicate that the band gap of InGaPN is dramatically reduced as nitrogen is incorporated. The transition energies of the band edge at various temperatures are measured and least-squares fitted to the Varshni equation. With N incorporation, the PL peak energy exhibits a particular behavior with temperature, which is not observed in PR spectra. This is attributed to carrier localization at low temperatures resulting from N clusters in the samples. In addition, the emergence of additional peaks in PR spectra as N is incorporated implies that the band alignment switches from type I to type II, due to the lowering of the conduction band, thus forming a two-dimensional electron gas (2DEG) in the interface region between InGaPN and GaAs. The number of confined levels in the 2DEG is found to increase with N concentrations.     

Energy band gaps for the GaxIn1−xAsyP1−y alloys lattice matched to different substrates

A pseudopotential formalism within the virtual crystal approximation in which the effects of composition disorder are involved is applied to the Ga_xIn_1−xAs_yP_1−y quaternary alloys in conditions of lattice matching to GaAs, InP and ZnSe substrates so as to predict their energy band gaps. Very good agreement is obtained between the calculated values and the available experimental data for the alloy lattice matched to InP and GaAs. The alloy is found to be a direct-gap semiconductor for all y compositions whatever the lattice matching to the substrates of interest. The (Γ→Γ) band-gap ranges and the ionicity character are found to depend considerably on the particular lattice-matched substrates suggesting therefore that, for an appropriate choice of y and the substrate, Ga_xIn_1−xAs_yP_1−y could provide more diverse opportunities to obtain desired band gaps, which opens up the possibility of discovering new electronic devices with special features and properties.     

Nonlinear absorption and refractive index of Brillouin scattered mode in piezoelectric semiconductor plasmas by an applied magnetic field

With the aid of hydrodynamic model, a detailed analytical investigation is made of the stimulated Brillouin scattering (SBS) of the Stokes component of the scattered wave in piezoelectric-doped semiconductor plasma subjected to a magnetostatic field. The origin of the SBS process lies in the third-order nonlinear optical susceptibility arising due to the induced nonlinear current density and acoustic perturbations internally generated due to crystal properties such as piezoelectricity and electrostriction. Using the coupled mode theory of plasmas the effective refractive index and absorption coefficient are determined via the effective susceptibility. The influence of piezoelectricity, magnetostatic field and doping concentration has been explored. The analysis has been applied to both noncentrosymmetric and centrosymmetric crystals. Numerical estimates are made for n-type InSb crystal duly irradiated by a frequency doubled 10.6 μm CO₂ lasers. Results are found to be well in agreement with available literature. The analysis establishes that a large nonlinear refractive index and small absorption coefficient can easily be obtained under moderate excitation intensity in piezoelectric doped magnetized semiconducting crystal, which proves its potential as candidate material for the fabrication of cubic nonlinear devices.     

A theoretical investigation of the band alignment of type-I direct band gap dilute nitride phosphide alloy of GaN_xAs_yP_(1-x-y)/GaP quantum wells on GaP substrates

The GaP-based dilute nitride direct band gap material Ga（NAsP） is gaining importance due to the monolithic integra- tion of laser diodes on Si microprocessors. The major advantage of this newly proposed laser material system is the small lattice mismatch between GaP and Si. However, the large threshold current density of these promising laser diodes on Si substrates shows that the carrier leakage plays an important role in Ga（NAsP）/GaP QW lasers. Therefore, it is necessary to investigate the band alignment in this laser material system. In this paper, we present a theoretical investigation to optimize the band alignment of type-I direct band gap GaNxAsyP1-x-y/GaP QWs on GaP substrates. We examine the effect of nitrogen （N） concentration on the band offset ratios and band offset energies. We also provide a comparison of the band alignment of type-I direct band gap GaNxAsyP1-x-y/GaP QWs with that of the GaNxAsyP1-x-y/Al2Ga1-2P QWs on GaP substrates. Our theoretical calculations indicate that the incorporations of N into the well and AI into the barrier improve the band alignment compared to that of the GaAsP/GaP QW laser heterostructures.     

Study of post-growth annealing and Ga coverage effects in low-density GaAs/AlGaAs quantum dots grown by modified droplet epitaxy

We present a detailed analysis of the Ga coverage and of the post-growth annealing effects on the optical properties of very-low-density self-assembled GaAs/AlGaAs quantum dots grown by modified droplet epitaxy. Through theoretical calculation of the QD electronic states, including thermally activated Al–Ga interdiffusion processes, we were able to relate our spectroscopic observations to QD structural properties.