Capacitance-voltage study of heterostructures with InGaAs/GaAs quantum wells in the temperature range from 10 to 320 K

Heterostructures with single strained InGaAs/GaAs quantum wells have been studied by measuring the capacitance-voltage characteristics in a wide range of temperatures and test signal frequencies. Based on the analysis of experimental capacitance-voltage characteristics, a temperature shift of the peak in the apparent profile of a majority carrier’s concentration is revealed and a quantitative model of this phenomenon is proposed. The effect of incomplete impurity ionization on the experimentally found quantum well’s charge is determined. It is established by numerical simulation and fitting of capacitance-voltage characteristics that the conduction band’s discontinuity for heterostructures with strained In _x Ga_{1 − x} As/GaAs quantum wells (x = 0.225) remains constant and equal to 172 ± 10 meV at temperatures from 320 to 100 K.     

Observation of the pyroelectric effect in strained piezoelectric InGaAs/GaAs quantum-wells grown on (111) GaAs substrates

We investigated the temperature dependence of the piezoelectric constant e₁₄, i.e. the pyroelectric effect, of various strained InGaAs/GaAs single- and multi-quantum wells embedded in p-i-n structures grown on (111)B GaAs substrates and diodes made from these structures. Both photoreflectance spectroscopy and differential photocurrent spectroscopy were applied to obtain e₁₄ over the temperature range 11-300 K. The values of e₁₄ for In_xGa_1−xAs quantum well layers with x=0.12-0.21 were observed to increase with temperature, which is contrary to the expected dependence, and the strain-induced components of the pyroelectric coefficients were quantitatively determined. The dependence of the pyroelectric coefficient on In fraction is discussed.     

Band-edge line-up in GaAs/GaAsN/InGaAs heterostructures

Triple GaAs/GaAsN/InGaAs heterostructures were grown by MBE on GaAs substrates, and their optical properties were studied. The band-edge line-up in GaAs/GaAsN and InGaAs/GaAsN heterostructures was analyzed by correlating the experimental photoluminescence spectra with the known parameters of the band diagram in (In,Ga)As compounds. It is shown that a GaAs/GaAsN heterojunction is type I, while an In_xGa_1?x As/GaAsN heterojunction can be type I or type II, depending on the In content x.     

Influence of the cap layer thickness on photoluminescence properties in strained InGaAs/GaAs single quantum wells

The influence of the GaAs cap layer thickness on the luminescence properties in strained In_0.20Ga_0.80As/GaAs single quantum well (SQW) structures has been investigated using temperature-dependent photoluminescence (PL) spectroscopy. The luminescence peak is shifted to lower energy as the GaAs cap layer thickness decreases, which demonstrates the effect of the GaAs cap layer thickness on the strain of InGaAs/GaAs single quantum wells (SQW). We find the PL quenching mechanism is the thermal activation of electron hole pairs from the wells into the GaAs cap layer for the samples with thicker GaAs cap layer, while in sample with thinner GaAs cap layer exciton trapping on misfit dislocations is dominated.     

Critical layer thickness of strained-layer InGaAs/GaAs multiple quantum wells determined by double-crystal x-ray diffraction

Double-crystal x-ray diffraction (DCXD) is shown to reveal the onset of relaxation in strained-layer InGaAs/GaAs multiple quantum well (MQW) structures. The MQW structures contain 10 nm thick In_0.16Ga_0.84As quantum wells and 55 nm thick GaAs barrier layers. As the number of periods in the structure was increased from to 3 to 15, the x-ray rocking curves were characterized by increasing distortion of superlattice interference fringes, broadening of superlattice peaks, and reduction in peak intensity. The x-ray diffraction data are correlated with an asymmetric crosshatched surface pattern as observed under Nomarski contrast microscopy. By using DCXD and Nomarski microscopy, the onset of strain relaxation in InGaAs/GaAs MQW structures was established for samples with various GaAs barrier layer thicknesses. For MQW structures in which the thickness of the barrier layers is the same or greater than that of the strained quantum wells, the critical layer thickness can be calculated according to the Matthews and Blakeslee force-balance model with dislocation formation by the single-kink mechanism.     

Effect of structural parameters on InGaAs/InAlAs 2DEG transport characteristics

The effect of structural parameters on the transport characteristics from 15 to 300 K of molecular beam epitaxy-grown InGaAs/InAlAs two dimensional electron gas structures lattice-matched to InP is determined. The InAlAs buffer layer thickness was varied from 1000 to 10,000Å. One sample also incorporated a InGaAs/InAlAs superlattice. The buffer layer thickness and structure had almost no effect on the mobility or sheet density. The InAlAs spacer layer was varied from 25 to 200Å. Increases in the InAlAs spacer thickness resulted in a monotonically decreasing sheet density and a peak in the mobility versus spacer thickness at 100Å. The highest 77 K mobility was 66,700 cm²/V/sds withN _D =1.2×10¹² cm^?2. The effect of illumination and temperature on the sheet concentration in these structures as well as on “bulk” InAlAs:Si was much smaller than in Al _x Ga_1?x As/GaAs structures or “bulk” Al _x Ga_1?x As, forx?0.30, indicating that devices based on this material system will not be characterized by many of the device instabilities observed in the AlGaAs/GaAs system.     

Photoluminescence and electroreflectance studies of modulation-doped pseudomorphic AlGaAs/InGaAs/GaAs quantum wells

In this study, we describe the correlations between the photoluminescence (PL) spectra and electrical properties of pseudomorphic modulation-doped AlGaAs/InGaAs/GaAs quantum wells (MDQWs) grown by molecular beam epitaxy. In MDQWs, the presence of a large sheet carrier density contributes significantly to the PL linewidth. At low temperatures (4.2 K), free carrier induced broadening of the PL linewidth is influenced by the material quality of the structure. At higher temperatures (77 K), differences in the material quality do not affect the linewidth significantly, and under these conditions the PL linewidth is a good measure of the sheet carrier density. The ratio of the 77 K to 4.2 K PL linewidths provides useful information about the crystalline quality of the MDQW structures as illustrated by the correlation with 77 K Hall mobility data and a simple model. We present results of Electron Beam Electroreflectance (EBER) to characterize MDQWs and undoped quantum wells in the AlGaAs/InGaAs/GaAs material system. Several transitions have been observed and fitted to excitonic Lorentzian lineshapes, providing accurate estimates of transition energy and broadening parameter at temperatures of 96 K and 300 K.     

MOVPE InGaAs/InP grown directly on GaAs substrates

We have demonstrated the feasibility of growth of InP/InGaAs structures directly on GaAs using atmospheric pressure metal organic vapour phase epitaxy. Growth conditions and the equipment used are described along with some preliminary measurements on the GaAs/InP/InGaAs wafers. P-N junctions were formed in these materials to evaluate diode characteristics.     

Effect of external electric field on the probability of optical transitions in InGaAs/GaAs quantum wells

The effect of external electric field on interband optical transitions in single In_xGa_{1 ? x} As/GaAs quantum wells is studied by electroreflectance spectroscopy. A procedure is suggested for separating the contribution of particular exciton transitions to the complicated modulation spectrum. Nontrivial field dependences of the probability of optical transitions forbidden by the symmetry are observed experimentally. The data are compared with the corresponding theoretical dependences. The strength of the internal electric field in the region of the quantum well is determined from Frantz-Keldysh’s oscillations. Under certain electric fields, the probability of transitions forbidden with no field is higher than the probability of transitions allowed by the symmetry.     

Chemical beam epitaxy and laser-modified chemical beam epitaxy of InGaAs using tris-dimethylaminoarsenic

The growth of In_xGaj_1−xAs (x = 0.13–0.25) on GaAs by chemical beam epitaxy (CBE) and laser-modified CBE using trimethylindium (TMIn), triethylgallium (TEGa), and tris-dimethylaminoarsenic (TDMAAs) has been studied. Reflection high-energy electron diffraction measurements were used to investigate the growth behavior of InGaAs at different conditions. X-ray rocking curve and lowtemperature photoluminescence (PL) measurements were used to characterize the InGaAs/GaAs pseudomorphic strained quantum well structures. Good InGaAs/GaAs interface and optical property were obtained by optimizing the growth condition. As determined by the x-ray simulation, laser irradiation during the InGaAs quantum well growth was found to enhance the InGaAs growth rate and reduce the indium composition in the substrate temperature range studied, 440–500°C, where good interfaces can be achieved. These changes, which are believed to be caused by laser-enhanced decomposition of TEGa and laser-enhanced desorption of TDMAAs, were found to depend on the laser power density as well. With laser irradiation, lateral variation of PL exciton peaks was observed, and the PL peaks became narrower.     

Stimulated emission in heterostructures with double InGaAs/GaAsSb/GaAs quantum wells,grown on GaAs and Ge/Si(001) substrates

We report the observation of stimulated emission in heterostructures with double InGaAs/GaAsSb/GaAs quantum wells, grown on Si(001) substrates with the application of a relaxed Ge buffer layer. Stimulated emission is observed at 77 K under pulsed optical pumping at a wavelength of 1.11 μm, i.e., in the transparency range of bulk silicon. In similar InGaAs/GaAsSb/GaAs structures grown on GaAs substrates, room-temperature stimulated emission is observed at 1.17 μm. The results obtained are promising for integration of the structures into silicon-based optoelectronics.     

Super-flat (411)A interfaces and uniformly corrugated (775)B interfaces in GaAs/AlGaAs and InGaAs/InAlAs heterostructures grown by molecular beam epitaxy

Extremely flat interfaces, i.e. effectively atomically flat interfaces over a wafer-size area were realized in GaAs/AlGaAs quantum wells (QWs) grown on (411)A GaAs substrates by molecular beam epitaxy (MBE). These flat interfaces are called as “(411)A super-flat interfaces”. Besides in GaAs/AlGaAs QWs, the (411)A super-flat interfaces were formed in pseudomorphic InGaAs/AlGaAs QWs on GaAs substrates and in pseudomorphic and lattice-matched InGaAs/InAlAs QWs on InP substrates. GaAs/AlGaAs resonant tunneling diodes and InGaAs/InAlAs HEMT structures with the (411)A super-flat interfaces were confirmed to exhibit improved characteristics, indicating high potential of applications of the (411)A super-flat interfaces. High density, high uniformity and good optical quality were achieved in (775)B GaAs/(GaAs)_m(AlAs)_n quantum wires (QWRs) self-organized in a GaAs/(GaAs)_m(AlAs)_n QW grown on (775)B GaAs substrates by MBE. The QWRs were successfully applied to QWR lasers, which oscillated at room temperature for the first time as QWR lasers with a self-organized QWR structure in its active region. These results suggest that MBE growth on high index crystal plane such as (411)A or (775)B is very promising for developing novel semiconductor materials for future electron devices.     

Optical investigations of strained InGaAs quantum wells

InGaAs/GaAs MOCVD-grown quantum wells have been investigated. Photoluminescence (PL) measurements have shown heavy-hole-related excitonic transitions within the temperature range from 10 to 100 K for all samples. In room-temperature photoreflectance (PR), sharp heavy- and light-hole excitonic transitions in the quantum wells have been observed. The transition energies obtained have been compared with values derived from theoretical considerations using the envelope function model including lattice-mismatch-related stress. The heavy- and light-hole transitions have been identified as excitonic transitions of types I and II respectively. © 1997 John Wiley & Sons, Ltd.     

Growth and characterization of (111)B InGaAs/GaAs multi-quantum well PIN diode structures

High quality piezoelectric strained InGaAs/GaAs multi-quantum well structures on (111)B GaAs substrates have been grown by solid-source molecular beam epitaxy in a PIN configuration. 10K photoluminescence (PL) shows narrow peaks with widths as low as 3 meV for a 25-period structure while room temperature (RT) PL shows several higher order peaks, normally forbidden, indicating breaking of inversion symmetry by the piezoelectric field. Furthermore, both the 10K PL peak position and the form of the RT PL spectra depend on the number of quantum wells within the intrinsic region, suggesting that the electric-field distribution is altered thereby. Diodes fabricated from these structures had sharp avalanche breakdown voltages (V_bd) and leakage currents as low as 8 × 10⁻⁶ A/cm² at 0.95 V_bd, indicating quality as high as in (100) devices. On leave from: Department de Ingeniera Electronica, Universidad Politecnica de Madrid, Madrid, Spain.     

Piezoelectric field determination in strained InGaAs quantum wells grown on [111]B GaAs substrates by differential photocurrent

The measurement of the differential photocurrent (DP) generated in PIN GaAs diodes with embedded strained InGaAs quantum wells has allowed us to determine the fields both in the GaAs barriers, as well as in the InGaAs strained quantum wells. Based on an electroreflectance (ER) setup, this technique relies on the generated photocurrent and does not require detection of the reflected light. The field in the GaAs barriers is obtained from DP Franz–Keldysh oscillations at photon energies above the GaAs bandgap, while the piezoelectric field in the wells is deduced from a phase change in the DP spectra under flat-band conditions in the quantum wells (QW). With optical power illuminations of a few nW, the in-well screening in these structures becomes negligible. With just 35 mV_RMS of modulating signal added to the diode bias and without photovoltaic effects due to the bias control of the sample, uncertainties in field calculations are reduced. The piezoelectric field values obtained at room temperature in MQW structures with 10 wells of 12 and 18% In content are close to those obtained by similar related techniques. For higher In content (20, 25 and 30%), single quantum well structures were used, also allowing the confirmation of pyroelectric behavior of the structures. Besides, direct comparisons of simultaneously grown [100] and [111] samples clearly reveal the earlier relaxation of the first ones when their QW absorption feature disappears.     

Structural and optical properties of GaAs-based heterostructures with Ge and Ge/InGaAs quantum wells

GaAs-based heterostructures with Ge and Ge/InGaAs quantum wells are grown by laser-assisted sputtering. Structural and optical studies of the heterostructures are carried out. A broad photoluminescence line is observed in the wavelength range from 1300 to 1650 nm. The line corresponds to indirect transitions in the momentum space of the Ge quantum wells and to transitions between the In_0.28Ga_0.72As and Ge layers, indirect in coordinate space, but direct in momentum space.     

Spatial ordering of self-organized InGaAs/AlGaAs quantum disks on GaAs (311)B substrates

We report the control of self-organization of In_xGa_1−xAs/AlGaAs quantum disks on GaAs (311)B surfaces using a novel technique based upon lithography-defined SiN dot arrays. A strained InGaAs island array selectively grown using the SiN dots provides periodic strain field. When the pitch of lateral ordering corresponds with the period of the strain field, self-organized quantum disks stacked on the InGaAs islands are precisely arranged just as the buried SiN dot array. The spacing of the array element is 250–300 nm (x = 0.3) and around 150 nm (x = 0.4). Vertical alignment by strain is achieved at a very thick (95 nm) separating layer. Characterization using atomic force microscopy reveals the size-fluctuation of disk is dramatically improved with spatial ordering.     

AP-MOVPE of InGaAs on GaAs (0 0 1): Analysis of in situ reflectivity response

We have investigated in situ monitoring of growth rate and refractive index by laser reflectometry during InGaAs on GaAs (0 0 1) substrate growth in atmospheric pressure metalorganic vapour-phase epitaxy (AP-MOVPE). The indium solid composition (x_In^s) was varied by changing the substrate temperature or the indium vapour composition (x_In^v). The refractive index of InGaAs alloys as a function of temperature and composition was quantified and compared which that of GaAs for 632.8 nm wavelength by simulation of experimental reflectivity responses. Composition analyses were carried out by high-resolution X-ray diffraction (HRXRD) and optical absorption (OA). The layers thicknesses were estimated by scanning electron microscopy (SEM) observations. The temperature dependence of InGaAs growth rate has been investigated in the temperature range 420-680 °C using trimethylgallium (TMGa), trimethylindium (TMIn) and arsine (AsH₃) sources. It shows Arrhenius-type behaviour with an apparent activation energy E_a of 0.62 eV (14.26 kcal/mol). This value is close to that determinate in the AP-MOVPE of GaAs.     

100‐period, 1.23‐eV bandgap InGaAs/GaAsP quantum wells for high‐efficiency GaAs solar cells: toward current‐matched Ge‐based tandem cells

Major challenges for InGaAs/GaAsP multiple quantum well (MQW) solar cells include both the difficulty in designing suitable structures and, because of the strain‐balancing requirement, growing high‐quality crystals. The present paper proposes a comprehensive design principle for MQWs that overcomes the trade‐off between light absorption and carrier transport that is based, in particular, on a systematical investigation of GaAsP barrier effects on carrier dynamics that occur for various barrier widths and heights. The fundamental strategies related to structure optimization are as follows: (i) acknowledging that InGaAs wells should be thinner and deeper for a given bandgap to achieve both a higher absorption coefficient for 1e‐1hh transitions and a lower compressive strain accumulation; (ii) understanding that GaAs interlayers with thicknesses of just a few nanometers effectively extend the absorption edge without additional compressive strain and suppress lattice relaxation during growth; and (iii) understanding that GaAsP barriers should be thinner than 3 nm to facilitate tunneling transport and that their phosphorus content should be minimized while avoiding detrimental lattice relaxation. After structural optimization of 1.23‐eV bandgap quantum wells, a cell with 100‐period In_0.30GaAs(3.5 nm)/GaAs(2.7 nm)/GaAsP_0.40(3.0 nm) MQWs exhibited significantly improved performance, showing 16.2% AM 1.5 efficiency without an anti‐reflection coating, and a 70% internal quantum efficiency beyond the GaAs band edge. When compared with the GaAs control cell, the optimized cell showed an absolute enhancement in AM 1.5 efficiency, and 1.22 times higher efficiency with 38% current enhancement with an AM 1.5 cut‐off using a 665‐nm long‐pass filter, thus indicating the strong potential of MQW cells in Ge‐based 3‐J tandem devices. Copyright © 2013 John Wiley & Sons, Ltd.     

Transient negative photocurrent and out-of-well dipole kinetics in novel piezoelectric InGaAs/GaAs MQW pin diodes

In 111B InGaAs/GaAs pin structures with a multiple quantum well (MQW) embedded region, the average internal field in the active MQW region can be tailored to obtain device configurations with a negative average field (NAF), opposite to the built-in field. In 111 NAF diodes, carriers photogenerated at the wells become trapped early at the potential minima located at the ends of the active region thus creating an electric dipole. In this work, in 111 NAF devices with a 0.17 In mole fraction layers, by using time-resolved photocurrent and a novel optical-pump electrical-probe techniques, we report the presence of a negative transient photocurrent, a direct quantitative evidence of such dipole formation, and we present measurements of its extinction kinetics at room temperature.