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.     

Valence band offsets in strained GaAs1−xPx/GaAs heterojunctions

Valence band offsets at [100]-oriented heterojunctions between tensile-strained GaASj_1−xP_x and unstrained GaAs are studied experimentally and theoretically. Light-hole (LH) and heavy-hole (HH) offsets are first extracted from the well-width dependence of valence subband splittings observed in luminescence spectra of tensile-strained GaAs_1−xP_x/GaAs quantum wells of various compositions (x = 0.06,0.09, and 0.19). This data is then combined with results from two other laboratories, yielding a set of 30 independent experimental offset values for junctions with compositions throughout the range 0.06≤x ≤0.32. The data are found to be highly consistent, with linear fits δE_LH = −140x (meV) and δE_HH= −401x (meV) describing the measured offsets to within less than 5 meV on average. Experimental results are then compared with theoretical predictions for the GaAs_1−x P_x/GaAs system obtained from a tight-binding model for strained heterojunctions. Predictions from the tight-binding calculations are found to lie within experimental scatter for the LH offsets, which define the valence band edge in these heterostructures, while magnitudes of the tight-binding HH offsets exceed measured values by ~20% on average.     

Accurate determination of the band-offset of a quantum semiconductor structure from its capacitance-voltage profile: Application to an InP/Ga0.47In0.53As/InP single quantum well

We discuss the possible analysis of an electron distribution obtained by capacitance-voltage profiling for the determination of the conduction band offset of a single quantum well. We show that, for this method which requires only relatively light experimental equipment, a nonconsuming computational time interpretation can be set up within a quite satisfactory degree of accuracy. As an application, we report the study of a lattice matched InP/Ga_0.47In_0.53As/InP quantum well for which we get ΔE_c = (200±10) meV, in good agreement with other measurements upon this system.     

Capacitance-voltage profiling and thermal evolution of the conduction band-offset of unstrained Ga0.47In0.53As/InP single quantum well

We report the results of capacitance-voltage (C-V) and Deep Level Transient Spectroscopy (DLTS) measurements performed upon a Ga_0.47In_0.53As/InP quantum well structure. At room temperature, a conduction-band offset ΔE_c=(200±10)meV and charge densities σ_I=±(3±1)*10¹¹ times the electronic charge per cm² have been measured from C-V experiments. At lower temperature (T≤150K) we have observed an important decrease of the band-offset, considerably larger than a pure thermal effect. We have shown that the explanation lies in the presence of a high concentration of deep traps located at the well-barrier interfaces. Two species A and B have been detected through DLTS experiments with activation energies E_tA=90 meV and E_tB=195 meV, respectively. The filling of these trap levels at low temperature lowers the band offset from 200 to 120 meV, owing to band repulsion effects.     

Temperature dependence of the photoluminescence properties and band gap energy of InxGa1−xAs/GaAs quantum wells

The effective band gap energy of In_xGa_1−xAs/GaAs strained quantum wells (QWs) is investigated by photoluminescence spectroscopy (PL) in the range 12–295 K. The temperature dependence of the band gap energy of strained QWs correlates well with that of bulk In_xGa_1−xAs of similar composition. Deviations from the band gap variation of bulk material at low temperatures (12–90 K) are interpreted in terms of exciton localization. The differences ΔE(12 K) between the measured PL peak energies and the expected transition energies at 12 K (obtained by simulating the measured temperature dependence of the PL peak positions by the well-known Varshni relation) are suggested to be closely related to the Stokes shifts that often exist between PL and PL excitation spectra of QWs. A linear relation is found between the PL full-width at half-maximum measured at 12 K and ΔE for a range of QWs prepared under different growth conditions. Excitonic recombination is inferred to be dominant in the PL transitions at the highest temperatures investigated—even at room temperature.     

Photoluminescence spectroscopy of localized excitons in Si1−xGex

We have recently found that high quantum efficiency can be achieved in strained Si_1−xGe_x alloy layers through the elimination of nonradiative channels. We observed a photoluminescence process in SiGe grown on 〈100232A; silicon by rapid thermal chemical vapor deposition, which was attributed to free excitons localized by random fluctuations in alloy composition. The external quantum efficiency of this process was measured directly for a single Si_0.75Ge_0.25 quantum well and found to be extraordinarily high, about 11.5 ± 2%. In this paper, we present additional data on the localized exciton photoluminescence, including temperature dependence, time decay curves, and effects of sample annealing.     

Photoluminescence excitation spectroscopy of InAs0.67P0.33/InP strained single quantum wells

The optical emission characteristics of biaxially compressed InAs _x P_{1− x} /InP strained single quantum well (QW) structures, with nominal compositionx=0.67, have been investigated using photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopies. The highly strained QWs exhibit intense and narrow PL in the 0.9–1.5 μm wavelength range, similar to the lattice-matched InGaAs(P)/InP system. The 20 K PLE spectra exhibit well-resolved features attributed ton=1 heavy hole (E1H1) and light hole (E1L1) transitions in the 1.0–1.5 μm wavelength range. In addition, features attributed to transitions betweenn=2 electrons and heavy holes (E2H2), and betweenn=1 electrons and unconfined holes (E1Hf), were observed. The energy splitting between the heavy-hole and light-hole bands was found to be a sensitive measure of the band offsets in the system. The best prediction of this splitting was obtained for a valence band offset of δE _V ∼0.25δE _G . This value of band offset was in agreement with the energy position of the E1Hf transition. The observed transition energies were also compared with the results of a finite square well model, taking into account the effects of strain, and the results offer further support for the band offset assignment. This study indicates that the InAsP system may be advantageous for application in strained-layer optoelectronic devices operating in the 1.3–1.6 μm wavelength range.     

Growth of GaAs1−xPx/GaAs and InAsxP1−x/InP strained quantum wells for optoelectronic devices by gas-source molecular beam epitaxy

In this paper we show that pseudomorphically strained heterostructures of InAs _x P_1−x /InP may be an alternative to lattice-matched heterostructures of In_1−x Ga _x As _y P_1−y /InP for optoelectronic applications. We first studied the group-V composition control in the gas-source molecular beam epitaxy (GSMBE) of the GaAs_1-x P _x /GaAs system. Then we studied GSMBE of strained InAs _x P_1−x /InP multiple quantum wells with the ternary well layer in the composition range 0.15 <x < 0.75. Structural and optical properties were characterized by high-resolution x-ray rocking curves, transmission electron microscopy, absorption and low-temperature photoluminescence measurements. High-quality multiple-quantum-well structures were obtained even for highly strained (up to 2.5%) samples. The achievement of sharp excitonic absorptions at 1.06, 1.3 and 1.55μm at room temperature from InAs _x P_1−x /InP quantum wells suggests the possibility of long-wavelength optoelectronic applications.     

P-N junction observations on Pb-salt diode lasers using an ebic-mode SEM

A scanning electron microscope (SEM) operating in the elec-tron- beam induced current (EBIC) mode has been used to determine the depths and uniformities of p- n junctions in a variety of Pb- salt diode lasers. Pb- salt materials with 20K band gap values ranging from 44 meV to 450 meV have been investigated. This includes the ternary alloy systems Pb_1−xCd_xS(with 0 ≤ x ≤ 0.048), PbS_1-xSe_x (with 0 ≤ x ≤ l), Pb_1−xSn_xSe (with O ≤ x ≤ O.10),and Pb_1-xSn_xTe (with x = 0.25). With typical carrier concentrations in the 10¹⁸ – 10¹⁹ cm^{− 3} range, the junction depths were found to be independent of temperature between 77K and 300K. Small band gap devices exhibited EBIC signals strongly dependent on T, while devices with band gap values above 130 meV at 77K exhibited relatively little such temperature dependence to 300K. This non- destructive technique is capable of providing junction depth and uniformity information of all the Pb-salt materials, and is useful for estimating minority car-rier diffusion lengths at various temperatures. The method is also useful for investigating some of the more complex confinement heterostructures in these materials.     

Impact of carbon concentration on the interface density of states of metal-oxide Si<Subscript>1−x−y</Subscript> Ge<Subscript>x</Subscript>C<Subscript>y</Subscript> (strained) capacitors

We describe and model the electrical response of interface states of metal-oxide semiconductor (MOS) capacitors fabricated from Si_1−x−yGe_xC_y strained layers as a function of C concentration. We find that the introduction of Ge and C in the epilayers leads to anomalies in the capacitance-voltage curves in the form of kinks or plateaus. This behavior is explained by the presence of pronounced peaks on the density of interface states in the bandgap. Our results suggest an adequate Ge/C ratio of 40 minimizes the density of interface states. This ratio is different from the ratio of ∼10 required for stress compensation. Finally, we discuss the implications for the introduction of Si_1−x−yGe_xC_y strained layers to fabricate MOS devices.     

Electroluminescence of the unconfined heterostructure p-GaInAsSb/p-InAs at liquid-helium temperatures

The electroluminescence of the single unconfined type-II heterojunction p-GaInAsSb/p-InAs was investigated in the temperature range T=4.2–77 K. As the temperature was reduced below T=77 K, the luminescence bands with maxima at 311 meV (band A) and 384 meV (band B) were found to shift toward higher energies. At 4.2 K, the short-wave band split into two bands, B ₁ and B ₂. These results are explained in terms of a model involving recombinations of electrons from the conduction band to an acceptor level of InAs, and also recombinations of electrons and holes localized in self-consistent quantum wells on either side of the heterojunction. Fiz. Tekh. Poluprovodn. 31, 1216–1219 (October 1997)     

Materials characteristics of pseudomorphic high electron mobility transistor structures with InxGa1−xas single quantum well and GaAs-InxGa1−xas (0.25 < x < 0.4) thin strained superlattice active layers

Growth and characterization results are presented for pseudomorphic high electron mobility transistor structures with In_xGa_1-xAs single quantum well and GaAs(h ₁)In _x Ga_1−x As(h ₂) thin strained superlattice active layers where 0.25≤x ≤ 0.4. All of the samples were grown by molecular beam epitaxy. Hall effect at 77 K, photoluminescence at 2 K, in-situ reflection high energy electron diffraction, and transmission electron microscopy measurements are discussed. Critical layer thickness measurements are compared with the Matthews-Blakeslee theory. Photoluminescence transition energies are compared with a self-consistent solution to Schrodinger’s and Poisson’s equations.     

Photoluminescence characterization of UHV/CVD grown multi-quantum well structures

Ultra-high vacuum chemical vapor deposition is particularly suitable for growth of band gap engineered Ge_xSi_1?x structures, since abrupt epilayers with a very high degree of uniformity can be obtained. We have grown multi-quantum well structures over a range of well widths and have characterized them by photoluminescence spectroscopy. We have observed prominent quantum well-related features that shift to higher energy in samples with narrower well widths. Spectra taken from various points on a 75 mm wafer show a maximum variation of ±3 meV in position of the no phonon peak in the 35Å well multi-quantum well sample.     

Field screening in (111)B InAsP/lnP strained quantum wells

Low-temperature photoluminescence measurements were performed on InAsP/InP strained quantum wells grown on InP (lll)B substrates by gas-source molecular beam epitaxy. The emission energy was observed to increase as the pump-power density increased. This was attributed to the screening of the internal piezoelectric field by photo-generated carriers. The energy shift was as large as 35 meV for an InAs_0.28P_0.72/InP quantum well with a lattice mismatch of ~0.9%. A similar structure with a smaller strain showed saturation of the energy shift with increasing pump-power density. We performed a model calculation which includes the quantum confined Stark effect, and this saturation was correlated with a flat-band structure of the quantum well due to the nearly complete screening of the built-in electric field.     

Growth,characterization and modeling of InxGa1−xP stripes by selective-area MOCVD

A computational diffusion model is used to predict the lateral thickness and composition profiles of In_xGa_1−xP stripes grown by selective-area, atmospheric pressure metalorganic chemical vapor deposition. Standard profilometry is used to measure the thickness profiles of InP and GaAs stripes grown on SiO₂ patterned InP and GaAs substrates, respectively. The model is used to find self-consistent empirical diffusion parameters for the In and Ga components which yield fits to the measured thickness data. The InP and GaAs data is then used to predict the growth thickness profile of InGaP by a weighted sum of the predicted profiles of the InP and GaP binary constituents. InGaP composition profiles are calculated by taking the ratio of the InP deposited volume to the InGaP deposited volume predicted by the model at each of the simulation points. Predicted thickness profiles are verified by standard profilometry, and composition profiles are verified by secondary ion mass spectrometry imaging using a fast resistive anode encoding detector. It is found that the measured thickness and composition profiles agree well with the profiles predicted by the model, thus verifying that the model can be used for the InGaP material system. The derived empirical parameters are used to predict the thicknesses and compositions of selectively grown InGaP quantum wells as a function of oxide width.     

Quantum Confinement and Carrier Localization Effects in ZnO/Mg<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Zn<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>O Wells Synthesized by Pulsed Laser Deposition

The optical properties of ZnO/Mg _x Zn_1−x O (x = 0.17) quantum wells (QWs) grown on c-plane sapphire substrates by pulsed laser deposition are presented. A blueshift in the low-temperature photoluminescence (PL) of the QWs illustrates quantum confinement effects as a function of ZnO well widths in the range from 3 nm to 10 nm. Enhanced luminescence properties are observed with increasing quantum confinement. PL data indicate weak polarization effects associated with the heterojunctions. Temperature-dependent PL measurements indicate carrier/exciton localization with activation energy of approximately 4−5 meV, which are attributed to potential fluctuations at the well-barrier interface.     

Electron and hole spectra and selection rules for optical transitions in Ge1−x Six/Ge heterostructures

The electron and hole spectra in strained Ge_1−x Si_x/Ge heterostructures grown on a (111) plane have been investigated. It is shown that the structure of the conduction band in these structures can be determined by investigating the polarization of their photoluminescence. The selection rules for indirect optical transitions have been found. Fiz. Tekh. Poluprovodn. 31, 235–242 (February 1997)     

Valence band offset in HgTe/Hg1−xCdxTe superlattices

The valence band offset (Λ) between HgTe and CdTe has been determined by means of an optical investigation of (112)B oriented HgTe/Hg_1−xCd_xTe superlattices. Based on the fact that the difference in energy between the first heavy hole and the first light hole subband is to a good approximation due primarily to Λ, it has been shown that Λ=580±40 meV at 5K. In addition Λ has a significant temperature dependence with a linear coefficient of −0.34±0.02 meV/K, i.e., Λ is 480±40 meV at room temperature.     

Solid- and gas-source “hybrid” Si molecular beam epitaxy for a Si1−xGex/Si single quantum well electroluminescent device

A p-i-n diode for a Si_1−xGe_x/Si single quantum well (SQW) electroluminescent (EL) device was successfully fabricated by solid-source (SS) and gas-source (GS) “hybrid” Si molecular beam epitaxy (MBE). First, the undoped SQW layer was grown on a p-type Si(100) substrate by GSMBE using disilane (Si₂H₆) and germane (GeH₄). Then the n-type Si contact layer was regrown by SSMBE after transferring the sample through the air. A (2 × 1) reconstruction was observed on a GSMBE-prepared Si surface even after the sample was exposed to air for 15 h. The excellent quality of the EL p-i-n device was shown by the sharpest emission lines, ≈5.5 meV, ever reported in the EL spectra of an SiGe system. Linear polarization along the SQW plane was also observed for no-phonon replica of EL.     

Gas-Source molecular beam epitaxy and characterization of inGaAs/lnGaAsP quantum well structures on InP

In this paper, we report the effect of using a group-V residual source evacuation (RSE) time on the interfaces of InGaAs/lnGaAsP quantum wells (QWs) grown by gas-source molecular beam epitaxy. High-resolution x-ray rocking curve and low-temperature photoluminescence (PL) were used to characterize the material quality. By optimizing the RSE time, a PL line width at 15K as narrow as 6.6 meV is observed from a 2 nm wide single QW, which is as good as or better than what has been reported for this material system. Very sharp and distinct satellite peaks as well as Pendellosung fringes are observed in the x-ray rocking curves of In_xGa_1−xAs/InxGa_1−xAS_yP_1−y multiple QWs, indicating good crystalline quality, lateral uniformity, and vertical periodicity. Theoretical considerations of the PL linewidths of In_xGa_1−xAs/InxGa_1−xAS_yP_1−y single QWs show that for QW structures grown with the optimized RSE time, the PL linewidth is mainly due to alloy scattering, whereas the contribution from interface roughness is small, indicating a good interface control.