Bulk In0 Research Articles

High-responsivity InAs quantum well photo-FET integrated on Si substrates for extended-range short-wave infrared photodetector applications

Low-intensity light detection necessitates high-responsivity photodetectors. To achieve this, we report In0.53Ga0.47As/InAs/In0.53Ga0.47As quantum well (InAs QW) photo-field-effect-transistors (photo-FETs) integrated on a Si substrate using direct wafer bonding. Structure of the InAs QW channel was carefully designed to achieve higher effective mobility and a narrower bandgap compared with a bulk In0.53Ga0.47As, while suppressing the generation of defects due to lattice relaxations. High-performance 2.6 nm InAs QW photo-FETs were successfully demonstrated with a high on/off ratio of 105 and a high effective mobility of 2370 cm2/(V·s). The outstanding transport characteristics in the InAs QW channel result in an optical responsivity 1.8 times greater than InGaAs photo-FETs and the fast rising/falling times. Further, we experimentally confirmed that the InAs QW photo-FET can detect light in the short-wavelength infrared (SWIR; 1.0–2.5 μm) near 2 μm thanks to bandgap engineering through InAs QW structures. Our result suggests that the InAs QW photo-FET is promising for high-responsivity and extended-range SWIR photodetector applications.

Morphological characterization of GaAs islands grown on InGaAs by droplet epitaxy

We investigated droplet epitaxy of Ga(As) on In0.53Ga0.47As as a method to extract Ga-adatom diffusion lengths with the aim of better understanding the underlying cause of phase separation in bulk In0.53Ga0.47As. However, instead of the expected morphology of a dot and a halo (the radius of which is directly related to adatom diffusion length), we found the deposited Ga etched into the underlying material, forming pits, tracks, and mounds with jagged edges. To understand the cause of this morphology, we varied several growth conditions including the substrate temperature, the Ga effective thickness, and depositing with/without As crystallization. Effective thickness and substrate temperature investigations revealed expected density trends in spite of the unusual morphology. This allowed us to compare densities of In halos (from our previous work) and Ga mounds, revealing a longer adatom diffusion length for In than Ga. We believe that the etching of the underlying In0.53Ga0.47As material led to scavenged In incorporating into the Ga droplet resulting in a low-In-content InGa alloy droplet. The irregular morphology then arose from competing In- and Ga-adatom diffusion lengths in Ga-rich conditions. Finally, we co-deposited In and Ga on In0.53Ga0.47As and found that instead of a uniform alloyed droplet morphology, the two group-III species formed separate In halos and Ga mounds. The inability of In and Ga to form an alloyed droplet under the growth conditions explored suggests that In-and Ga-adatoms may self-segregate under bulk growth conditions as well, leading to phase separation.

Toward deterministic construction of low noise avalanche photodetector materials

Over the past 40+ years, III-V materials have been intensively studied for avalanche photodetectors, driven by applications including optical communications, imaging, quantum information processing, and autonomous vehicle navigation. Unfortunately, impact ionization is a stochastic process that introduces noise, thereby limiting sensitivity and achievable bandwidths, leading to intense effort to mitigate this noise through the identification of different materials and device structures. Exploration of these materials has seen limited success as it has proceeded in a largely ad hoc fashion due to little consensus regarding which fundamental properties are important. Here, we report an exciting step toward deterministic design of low-noise avalanche photodetector materials by alternating the composition at the monolayer scale; this represents a dramatic departure from previous approaches, which have concentrated on either unconventional compounds/alloys or nanoscale band-engineering. In particular, we demonstrate how to substantially improve upon the noise characteristics of the current state-of-the art telecom avalanche multipliers, In0.52Al0.48As grown on InP substrates, by growing the structure as a strain-balanced digital alloy of InAs and AlAs layers, each only a few atomic layers thick. The effective k-factor, which has historically been considered a fundamental material property, was reduced by 6–7× from k = 0.2 for bulk In0.52Al0.48As to k = 0.05 by using the digital alloy technique. We also demonstrate that these “digital alloys” can significantly extend the photodetector cutoff wavelength well beyond those of their random alloy counterparts.

First principles modeling of defects in the Al2O3/In0.53Ga0.47As system

Density functional theory paired with a first order many-body perturbation theory correction is applied to determine formation energies and charge transition energies for point defects in bulk In0.53Ga0.47As and for models of the In0.53Ga0.47As surface saturated with a monolayer of Al2O3. The results are consistent with previous computational studies that AsGa antisites are candidates for defects observed in capacitance voltage measurements on metal-oxide-semiconductor capacitors, as the AsGa antisite introduces energy states near the valence band maximum and near the middle of the energy bandgap. However, substantial broadening in the distribution of the GaAs charge transition levels due to the variation in the local chemical environment resulting from alloying on the cation (In/Ga) sublattice is found, whereas this effect is absent for AsGa antisites. Also, charge transition energy levels are found to vary based on proximity to the semiconductor/oxide interfacial layer. The combined effects of alloy- and proximity-shift on the GaAs antisite charge transition energies are consistent with the distribution of interface defect levels between the valence band edge and midgap as extracted from electrical characterization data. Hence, kinetic growth conditions leading to a high density of either GaAs or AsGa antisites near the In0.53Ga0.47As/Al2O3 interface are both consistent with defect energy levels at or below midgap.

Bandgap control for the lattice-matched InGaAs/InAlAs/InP single quantum wells

In0.53Ga0.47As/In0.52Al0.48As single quantum well (SQW) grown by molecular-beam epitaxy on the InP (001) substrate is investigated by using photoluminescence (PL) spectroscopy and time-resolved photoluminescence (TRPL). At the low temperature of 8 K and excitation intensity of 1 W/cm2, the emitting wavelength of QWs shifts from 1387 nm to 1537 nm when the well width increases from 5 nm to 50 nm, approaching the emitting wavelength of bulk In0.52Al0.48As. The emitting wavelength of QWs can be flexibly tailored by varying the thickness of the InGaAs layer.

Role of V-pits in the performance improvement of InGaN solar cells

We study the influence of V-pits on the overall conversion efficiency of bulk In0.12Ga0.88N based heterojunction solar cells grown by MOVPE. We show that V-pits significantly enhances the extraction of the photogenerated carriers in the InGaN absorber, resulting in a peak external quantum efficiency of 79% and a short circuit current density (twice the state of the art) of 2.56 mA/cm−2 under AM 1.5G conditions.

Extended hot carrier lifetimes observed in bulk In0.265±0.02Ga0.735N under high-density photoexcitation

We have investigated the ultrafast carrier dynamics in a 1 μm bulk In0.265Ga0.735N thin film grown using energetic neutral atom-beam lithography/epitaxy molecular beam epitaxy. Cathodoluminescence and X-ray diffraction experiments are used to observe the existence of indium-rich domains in the sample. These domains give rise to a second carrier population and bi-exponential carrier cooling is observed with characteristic lifetimes of 1.6 and 14 ps at a carrier density of 1.3 × 1016 cm−3. A combination of band-filling, screening, and hot-phonon effects gives rise to a two-fold enhanced mono-exponential cooling rate of 28 ps at a carrier density of 8.4 × 1018 cm−3. This is the longest carrier thermalization time observed in bulk InGaN alloys to date.

Study on the Ultrafast Carrier Dynamics in the Bulk In0.265GaN Thin Film

The concepts of third generation solar cells critically depend on the dynamics of ultrafast carrier relaxation and electron-phonon interactions on very short times scales. The hot carrier solar cell is a type of third generation cell which especially depends on the reduction in the energy relaxation rate in an absorber material. We investigated the ultrafast carrier dynamics in 1μm bulk In0.265GaN thin film grown by a new thin-film growth technique called energetic neutral atom-beam lithography/epitaxy (ENABLE) which was done by our collaborators in the Los Alamos National laboratory(LANL), US. Characterization including steady state photoluminescence (SSPL), time-resolved photoluminescence (TRPL) and transient absorption (TA) in the time scale of picoseconds have been measured and analysed. It indicates the overall relaxation time of our sample is about 22.42ps through the Maxwell-Boltzmann approximation. This indicates that the mechanisms are mediated by the Indium content. Moreover, the indium fluctuation introduced extrinsic energy states in the forbidden energy as observed through TRPL.

Electronic structure and optical properties of bulk In0.53Ga0.47As for near-infrared photocathode

In0.53Ga0.47As is a new near-infrared material for negative electron affinity photocathodes. Geometry optimization combined generalized gradient approximation based on the density functional theory is adopted to calculate the bulk In0.53Ga0.47As. Lattice constant of bulk In0.53Ga0.47As after geometry optimization is compared with the theoretical one based on the Vigard law, which proves the reliability of the parameters of the geometry optimization. The energy bands, electronic structures and optical properties of the bulk ternary In0.53Ga0.47As for near-infrared photocathode are analyzed. Electron transition way which is closely related with the photoemission is analyzed from the energy bands and the density of electrons. The dielectric function and the optical absorption coefficient are made clear based on the partial density of states. It is found that optical absorption coefficient of bulk In0.53Ga0.47As is bigger than that of GaAs in the near-infrared band, which is in keeping with the experiments. The analysis provides a theoretical foundation to design a better near-infrared negative electron affinity photocathode from the point of microscopic atoms.

Plasma‐assisted molecular beam epitaxy of strain‐compensated a‐plane InGaN/AlGaN superlattices

AbstractStrain‐compensated InGaN/AlGaN structures can enable the growth of thick layers of InGaN epitaxial films far beyond the critical thickness for InGaN grown pseudomorphically to GaN. In this paper, we demonstrate the epitaxial growth of high‐quality strain‐compensated a‐plane In0.12Ga0.88N/Al0.19Ga0.81N superlattices up to 5 times thicker than the critical thickness on free‐standing a‐plane GaN substrates by plasma‐assisted molecular beam epitaxy (PA‐MBE). The superlattices consist of 50 to 200 periods of 10 nm thick In0.12Ga0.88N and 6 nm thick Al0.19Ga0.81N layers. The structures are characterized using a double crystal X‐ray diffractometer, asymmetric reciprocal space mapping, and atomic force microscopy. We use X‐ray diffraction to determine the strain, composition, degree of relaxation, and superlattice period of our samples. The structural characteristics of periodic structures containing from 50 to 200 periods are compared to single layer, uncompensated In0.12Ga0.88N films. A 100 period structure exhibited only 15% relaxation compared to 69% relaxation for the bulk In0.12Ga0.88N film grown with the same total InGaN thickness but without strain‐compensating layers. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Co-doping of InxGa1−xAs with silicon and tellurium for improved ultra-low contact resistance

We report Te doping of bulk In0.53Ga0.47As up to 2.6×1019cm−3 without saturation effects, and structural characterization and contact resistances between metal and epitaxial regrowth for four structures: Si doped and Si and Te co-doped n+ InAs regrowth on a 10nm In0.53Ga0.47As channel, Si doped and Si and Te co-doped n+ In0.53Ga0.47As regrowth on a 7nm In0.53Ga0.47As channel. We observe that the contact resistance for the Si doped and Si and Te co-doped n+ InAs regrowth on the 10nm In0.53Ga0.47As channel is 9.9Ωμm (3.9Ωμm2) and 6.6Ωμm (2.3Ωμm2), and the contact resistance for the Si doped and Si and Te co-doped n+ In0.53Ga0.47As regrowth on the 7nm In0.53Ga0.47As channel is 8.5Ωμm (2.3Ωμm2) and 6.8Ωμm (1.9Ωμm2). The improvement in contact resistance comes from improvements in electron mobility consistent with Te improving material quality as a surfactant.

Comparison of High Field Electron Transport in GaAs, InAs and In<sub>0.3</sub>Ga<sub>0.7</sub>As

An ensemble Monte Carlosimulation is used to compare high field electron transport in bulk GaAs, InAs and In0.3Ga0.7As. In particular, velocity overshoot and electron transit times are examined. We find the steady state velocity of the electrons is the most important factor determining transit time over distances longer then 0.2 μm. Over shorter distances velocity overshoot effects in InAs and In0.3Ga0.7 As at high fields are comparable to those in GaAs. We estimate the minimum transit time across a 1 μm InAs sample to be about 4.2 ps. Similar calculations for In0.3Ga0.7As yield 6 ps (for GaAs yield 10 ps). Calculations are made using a nonparabolic effective mass energy band model, Monte Carlo simulation that includes all of the major scattering mechanisms. The band parameters used in the simulation are extracted from optimized pseudopotential band calculations to ensure excellent agreement with experimental information and ab initio band models.

Computational Study of the Electronic Performance of Cross-Plane Superlattice Peltier Devices

ABSTRACTWe use a state-of-the-art non-equilibrium quantum transport simulation code, NEMO-1D, to address the device physics and performance benchmarking of cross-plane superlattice Peltier coolers. Our findings show quantitatively how barriers in cross-plane superlattices degrade the electrical performance, i.e. power factor. The performance of an In0.53Ga0.47As/In0.52Al0.48As cross-plane SL Peltier cooler is lower than that of either a bulk In0.53Ga0.47As or bulk In0.52Al0.48As device, mainly due to quantum mechanical effects. We find that a cross-plane SL device has a Seebeck coefficient vs. conductance tradeoff that is no better than that of a bulk device. The effects of tunneling and phase coherence between multi barriers are examined. It is shown that tunneling, SL contacts, and coherency only produce oscillatory behavior of Seebeck coefficient vs. conductance without a significant gain in PF. The overall TE device performance is, therefore, a compromise between the enhanced Seebeck coefficient and degraded conductance.

Mitigation of Voltage Defect for High-Efficiency InP Diode Lasers Operating at Cryogenic Temperatures

The power conversion efficiency of cryogenically cooled InP-based diode lasers is limited by excess electrical voltage caused by the freeze-out of holes at low temperature. Hall-effect measurements are performed to determine the ionization energy of Zn in bulk InP and In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.90</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.10</sub> As <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.24</sub> P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.76</sub> (the values obtained are 18.6 and 11.6 meV, respectively). A laser design with an InGaAsP p-cladding layer shows a large decrease in the 77 K voltage defect relative to a more traditional InP design. Peak conversion efficiency of 73% and >10-W maximum power are reported at 1493 nm from a single 200-μm stripe laser operating at 77 K.

Temperature dependence of the photoluminescence emission from InxGa1−xAs quantum wells on GaAs(311) substrates

We studied the photoluminescence (PL) properties of In0.2Ga0.8As/GaAs quantum well structures grown by molecular beam epitaxy on (311)-oriented substrates. The structure consists of three quantum wells (QWs) of 100, 50, and 25 Å nominal thickness. The temperature dependence of the PL emission from the QWs in the range of 5–250 K is reported. Three models by Varshni, Viña, and Pässler, respectively, were employed to analyze the variation in the excitonic energy transitions as a function of temperature. We compared the excitonic behavior with the band gap temperature dependence of bulk In0.2Ga0.8As and found a difference, as opposed to the unstrained AlGaAs/GaAs system. We attributed this difference to the modification of the QW energy levels by the quantum confinement Stark effect, which is temperature and well width dependent.

Magnetotransport in modulation-doped In0.53Ga0.47As/In0.52Al0.48As heterojunctions: in-plane effective mass, quantum and transport mobilities of 2D electrons

Shubnikov–de Haas (SdH) and Hall effect measurements, performed in the temperature range between 3.3 and 20 K and at magnetic fields up to 2.3 T, have been used to investigate the electronic transport properties of lattice-matched In0.53Ga0.47As/In0.52Al0.48As heterojunctions. The spacer layer thickness (tS) in modulation-doped samples was in the range between 0 and 400 Å. SdH oscillations indicate that two subbands are already occupied for all samples except for that withtS= 400 Å. The carrier density in each subband, Fermi energy and subband separation have been determined from the periods of the SdH oscillations. The in-plane effective mass (m*) and the quantum lifetime (τq) of 2D electrons in each subband have been obtained from the temperature and magnetic field dependences of the amplitude of SdH oscillations, respectively. The 2D carrier density (N1) in the first subband decreases rapidly with increasing spacer thickness, while that (N2) in the second subband, which is much smaller thanN1 , decreases slightly with increasing spacer thickness from 0 to 200 Å. The in-plane effective mass of 2D electrons is similar to that of electrons in bulk In0.53Ga0.47As and show no dependence on spacer thickness. The quantum mobility of 2D electrons is essentially independent of the thickness of the spacer layer in the range between 0 and 200 Å. It is, however, markedly higher for the samples with a 400 Å thick spacer layer. The quantum mobility of 2D electrons is substantially smaller than the transport mobility which is obtained from the Hall effect measurements at low magnetic fields. The transport mobility of 2D electrons in the first subband is substantially higher than that of electrons in the second subband for all samples with double subband occupancy. The results obtained for transport-to-quantum lifetime ratios suggest that the scattering of electrons in the first subband is, on average, forward displaced in momentum space, while the electrons in the second subband undergo mainly large-angle scattering.

Pressure behaviour of self-assembled In0.55Al0.45As/Al0.5Ga0.5As quantum dots with multi-modal distribution in size

The pressure behaviour of In0.55Al0.45As/Al0.5Ga0.5As self-assembled quantum dots (QDs) has been studied at 15 K in the pressure range of 0-1.3 GPa. The atomic force microscopy image shows that the QDs have a multi-modal distribution in size. Three emission peaks were observed in the photoluminescence (PL) spectra, corresponding to the different QD families. The measured pressure coefficients are 82, 94 and 98 meV GPa-1 for QDs with average lateral size of 26, 52 and 62 nm, respectively. The pressure coefficient of small QDs is about 17% smaller than that of bulk In0.55Al0.45As. An envelope-function calculation was used to analyse the effect of pressure-induced change of barrier height, effective mass and dot size on the pressure coefficients of QDs. The -X state mixing was also included in the evaluation of the reduction of the pressure coefficients. The results indicate that both the pressure-induced increase of effective mass and -X mixing respond to the decrease of pressure coefficients, and the -X mixing is more important for small dots. The calculated -X interaction potentials are 15 and 10 meV for QDs with lateral size of 26 and 52 nm, respectively. A type-II alignment for the X conduction band is suggested according to the pressure dependence of the PL intensities. The valence-band offset was then estimated as 0.15±0.02.

Graded-composition buffer layers using digital AlGaAsSb alloys

We describe step-graded digital-alloy buffers using alternate layers of Al0.5Ga0.5As and Al0.5Ga0.5As0.65Sb0.35 grown on GaAs substrates by molecular beam epitaxy. The buffers consist of three sets of superlattices with AlGaAs/AlGaAsSb layer thicknesses of 7.7/2.3 nm, 5.4/4.6 nm, and 3.1/6.9 nm, respectively, terminating in a lattice constant equal to that of bulk In0.32Ga0.68As. Transmission electron micrographs show that most of the misfit-generated dislocations lie near the steps in pseudoalloy composition, and atomic force micrographs indicate a rms surface roughness of 3.6 nm. A 20.5-period lattice-matched InGaAs/InAlAs reflector stack grown on such a buffer has a peak reflectivity of 98% near 1.3 μm. These buffers provide potentially useful substrates for optoelectronic device applications near 1.3 μm using strained InGaAs active regions.

Electron velocity-field characteristics of In0.52Al0.48As

Theoretical results of electron transport in n-type In0.52Al0.48As are presented. The transport properties of this important semiconductor were obtained using the Monte Carlo method. In particular, velocity-electric field characteristics for different temperatures and doping concentrations in bulk In0.52Al0.48As are calculated for the first time. Physical parameters for In0.52Al0.48As (which is lattice-matched to InP and In0.53Ga0.47As) were obtained based on interpolation of available experimental and theoretical results for InAs, AlAs, and In0.75Al0.25As. Our study suggests that In0.52Al0.48As has electron transport properties which are comparable to and complimentary with those of other materials lattice-matched to InP.

OMVPE growth of In0.53Ga0.47As on InP using tertiarybutylarsine

We have successfully grown bulk In0.53Ga0.47As on InP using tertiarybutylarsine (TBA), trimethylindium and trimethylgallium. The growth temperature was 602° and the V/III ratio ranged from 19 to 38. Net carrier concentrations were 2 – 4 × 1015 cm-3, n-type, with a peak 77 K mobility of 68,000 cm2/V. sec. Increasing compensation was observed in In0.53Ga0.47As grown at higher V/III ratios. PL spectra taken at 5 K revealed strong near bandgap emission at 0.81 eV—with the best sample having a FWHM of 2.5 meV. At lower energies, donor-acceptor pair transitions were evident. Strong and sharp 5 K PL emission was observed from InP/In0.53Ga0.47As/InP quantum wells grown with TBA.