GaAs Wells Research Articles

Energy states in ZnSe-GaAs heterovalent quantum structures

We investigate the energy states in ZnSe-GaAs heterovalent quantum structures. In the ZnSe-GaAs heterovalent heterostructures, the band offsets are controllable, and therefore different offsets can artificially be put at both sides of the GaAs wells through the control of the growth sequence forming the interfaces, which brings unique characteristics on the heterovalent quantum structures. The characteristic features are examined by the optical-absorption measurement. Dependence of the absorption edge on the offsets and the well width reveals the presence of an electric field; by changing the valence-band offset at one side of the GaAs well from 0.6 to 1.1 eV, while keeping that at the other side constant at 0.6 eV, we observe that the absorption edge energies are shifted toward lower energy and that the degree of the redshift depends on the GaAs well width. These experimental results are well explained by the theoretical calculation of the Poisson and Schr\odinger equations. On the other hand, the barrier width dependence exhibits the anomalous behavior that cannot be expressed by the theoretical analyses and is interpreted in terms of the charge transfer within the barrier region.

Excitonic photoluminescence in symmetric coupled double quantum wells subject to an external electric field

The effect of an external electric field F on the excitonic photoluminescence (PL) spectra of a symmetric coupled double quantum well (DQW) is investigated both theoretically and experimentally. We show that the variational method in a two-particle electron-hole wave function approximation gives a good agreement with measurements of PL on a narrow DQW in a wide interval of F including flat-band regime. The experimental data are presented for an MBE-grown DQW consisting of two 5 nm wide GaAs wells, separated by a 4 monolayers (MLs) wide pure AlAs central barrier, and sandwiched between Ga_{0.7}Al_{0.3}As layers. The bias voltage is applied along the growth direction. Spatially direct and indirect excitonic transitions are identified, and the radius of the exciton and squeezing of the exciton in the growth direction are evaluated variationally. The excitonic binding energies, recombination energies, oscillator strengths, and relative intensities of the transitions as functions of the applied field are calculated. Our analysis demonstrates that this simple model is applicable in case of narrow DQWs not just for a qualitative description of the PL peak positions but also for the estimation of their individual shapes and intensities.

Valley-Mixing Effects in AlAs/GaAs (001) Superlattices

The effects of ΓX-valley mixing in (AlAs)n(GaAs)n (001) superlattices (SLs) on the electronic properties are theoretically investigated versus the SL period (n = 1–25) and the band offsets. The calculations are based on the empirical sp3s* tight-binding model, which includes only nearest-neighbour interactions. The results show that the highest state of the valence band (VB) is always confined to the GaAs slabs, whereas the bottom state of the conduction band (CB) shows different behaviours as it is sensitive to band-mixing effects. It is due to these effects in the ultrathin-layer SLs (n ≤ 8) that the electrons become localized in the AlAs X-valley and the heterostructure becomes of type-II. Whereas, for thick-layer SLs (n ≥ 9), the GaAs wells become completely separated. As a result, the electrons are localized in the GaAs slabs and the SL is of type-I. The estimated critical layer thickness is about 26 Å (nc = 9), which is consistent with the photoreflectance experiments. The valley-mixing effects are shown to be efficient only for small conduction band offset (CBO ≤ 200 meV) and are essential for the existence of type-II behaviour. The relevance of our work to the photonic device applications is furtherly discussed.

Multibarrier heterostructure GaAs/AlAs switch

A multibarrier heterostructure GaAs/AlAs current switching diode has been proposed and investigated by Monte Carlo particle simulations. The switching phenomenon is based on electron tunneling and thermoemission from the GaAs wells to the AlAs barriers, electron drift across the thin AlAs barriers followed by a subsequent impact ionization in the undoped GaAs layers. The calculated switching voltage is close to 100 V for the diode involving four AlAs barriers at 300 K lattice temperature. The estimated switching time is in the order of 10 ps.

Ordering of conduction band states in thin-layer superlattices

The electronic structures of thin-layer superlattices (SLs) are investigated versus the SL layer thicknesses (m, n) and the band offsets. The calculations are based on the empirical tight-binding model, which includes only nearest-neighbour interactions. Particular attention is given to the effect of the interface parametrization on the SL electronic properties. This is done, mainly, by varying the band offsets over a sufficiently broad range. The results show that the existence of type-II behaviour in the ultrathin-layer SLs necessitates a large valence band offset and small conduction band offset . Providing that these offset values are achieved, it is found that the highest state of the valence band is always confined to the GaAs slabs whereas the bottom state of the conduction band shows different behaviours as it is sensitive to band-mixing effects. It is due to these mixing effects that most of the ultrathin-layer SLs (with ) behave as type-II heterostructures, where the electrons are localized in the AlAs valley. The rest of the ultrathin-layer SLs behave as type-I heterostructures with a direct bandgap at the point, whenever the GaAs slabs are thick enough to make the electron confinement energy small in the GaAs wells. For thick-layer SLs, our results suggest the existence of a critical barrier thickness, beyond which the GaAs wells become completely decoupled and the SL behaves as a type I heterostructure. The estimated critical layer thickness, for the crossover from type-I to type-II behaviour, is for the SLs with m = n when using VBO = 0.56 eV. This -value is consistent with the photoreflectance experiments. The relevance of our work to photonic device applications is discussed further.

Photocurrent resonances in short-period AlAs/GaAs superlattices in an electric field

The photocurrent was measured as a function of the external electric field in short-period AlAs/GaAs superlattices for various photon energies. Transport resonances, whose positions do not depend on the photon energy, were observed in these dependences together with optical resonances due to interband transitions in Wannier-Stark levels. It is shown that the transport resonances are due to tunneling of photoelectrons from the p-GaAs contact region into the first level in GaAs wells located 2–5 lattice periods from the contact layer.

Mean free path of ballistic electrons in GaAs/AlGaAs superlattices

The mean free path of ballistic electrons in GaAs/AlGaAs superlattices was measured using the technique of hot electron spectroscopy in magnetic fields perpendicular to the growth direction. We utilize the fact that the total effective path of an injected hot electron is a function of the applied magnetic field. For a superlattice with 6.5 nm GaAs wells and 2.5 nm GaAlAs barriers we measure a mean free path of 80 nm. The experimental results of a ten-period SL sample are compared to a fully three-dimensional calculation of the transmission including interface roughness with island sizes of 10 nm. We demonstrate that the observed mfp is limited due to interface roughness scattering for temperatures up to 50 K.

A Study of Semiconductor Quantum Structures by Microwave Modulated Photolumenescence

ABSTRACTMixed types I - type II multiple quantum wells (QW) structures consist of alternating narrow- and wide GaAs wells, separated by AlAs barriers. Transfer of electrons from the narrow- to the wide well results in the formation of a two-dimensional electron gas (2DEG) in the wide wells and a hole gas (2DHG) in the narrow ones. The present study investigated the effect of these gases on the various photoluminescence (PL) bands. The study utilized two modulations techniques: double beam PL and microwave modulated PL (MMPL), offering high-resolution spectroscopy, control of the 2DEG density and effective temperature. The results showed that the formation of a low density 2DEG in the wide wells cause the formation of trions. However, a large density of excess electrons makes mutual collisions with other photo-generated species, causing the dissociation of the trions and excitons. In addition, electrons transfer through the barrier gives rise to barrier-well indirect recombination emission.

Wannier–Stark effect in strongly coupled three quantum well structures and superlattices

The Wannier–Stark effect has been investigated in strongly coupled three quantum well structures and 23.5 period superlattices consisting of 25 A wide GaAs wells separated by Al0.25Ga0.75As barriers of the same width. A blue shift associated with the Wannier–Stark localization of heavy holes states is observed for the first time, followed by a second blue shift at higher electric fields due to the electron localization. This is contrary to the widely reported single blue shift of the order of (Δe+Δhh)/2, where Δe and Δhh are the electron and heavy hole miniband widths respectively, produced by Wannier–Stark localization in superlattices.

Monte Carlo study of multibarrier heterostructure switch

A multibarrier heterostructure GaAs/AlAs current switching diode has been investigated by Monte Carlo method. The switching phenomenon is based on electron tunnelling and thermoemission from the GaAs wells to the AlAs barriers, electron drift across the thin AlAs barriers followed by a subsequent impact ionisation in the undoped GaAs layers. The calculated switching voltage is close to 100 V for the diode involving four AlAs barriers at 300 K lattice temperature. The estimated switching time is in the order of 10 ps.

Electron mobility in selectively Si-doped GaAs/N–Al0.3Ga0.7As quantum well heterostructures with super-flat interfaces grown on (411)A GaAs substrates by molecular beam epitaxy

Reduced roughness of GaAs/Al0.3Ga0.7As heterointerfaces formed on (411)A substrates by Molecular Beam Epitaxy (MBE) was confirmed by measuring Two Dimensional Electron Gas (2DEG) mobility in selectively Si doped GaAs/N–Al0.3Ga0.7As Quantum Well (QW) structures with 4, 6, 8, or 50-nm thick GaAs wells. Below a QW thickness (Lw) of 8 nm, the 2DEG mobility at 15 K was mainly determined by interface roughness scattering and rapidly decreases with decreasing well thickness. The 2DEG in the (411)A selectively doped QW sample with Lw=4 nm has 34% higher mobility (μ=32 500 cm2/Vs) at 15 K than that of the (100) sample simultaneously grown (24 200 cm2/Vs). Every (411)A sample with Lw≤8 nm has 24–34% higher 2DEG mobility than the (100) sample simultaneously grown. This result indicates that smoother GaAs/Al0.3Ga0.7As interfaces were realized on the (411)A substrates. The (411)A sample with the 50-nm thick well shows 84% higher 2DEG mobility than the (100) sample simultaneously grown and its mobility is not determined by interface roughness scattering but ionized impurity scattering, which may suggest that MBE growth of GaAs and AlGaAs on the (411) GaAs substrates reduces unintentional impurity incorporation.

Carrier transport and photoluminescence affected by Γ–X resonance in GaAs–AlAs type-I superlattices

We observed two peaks (fast and slow components) in time-resolved photocurrents for type-I GaAs/AlAs superlattices (SLs). The calculations of the envelop function for the SLs showed that the X 1 state in the AlAs barriers resonates with the Γ 2 state in the adjacent GaAs wells at the bias voltage where the slow components become larger. The photoluminescence measurements revealed that some electrons transfer from the Γ 1 to X 1 states and flow through the X 1– Γ 2–X 1– Γ 2 channel at the near X 1– Γ 2 resonance voltage. The slow component in the transient photocurrents originates from the X 1– Γ 2–X 1– Γ 2 electron transport.

Influence of growth conditions for GaAs quantum wells and composition fluctuations in Al(Ga)As barriers on exciton and electron localization

Quantum wells (QWs) with short-period superlattice barriers are grown by molecular-beam epitaxy in two-dimensional island nucleation or step-flow growth mode. A pronounced roughening of step edges as imaged by atomic force microscopy for samples grown either at high As 4-partial pressure or on misoriented substrates with As-terminated steps is observed. Micro-photoluminescence spectra of the GaAs wells and their barriers reveal a rich fine structure. The narrow lines are attributed to exciton localization by potential fluctuations at the interface. Since the fine structure is observed in ternary barriers too, it is concluded that compositional fluctuations at QW interfaces are crucial. The interface roughness correlation lengths are measured by diffuse X-ray scattering. Another localization effect is connected with the appearance of heavy-mass X-electrons in the short-period superlattice barriers on both sides of the QW. Thus, their high screening capability results in unexpected high mobilities at high carrier densities. Transport correlation lengths are compared with those of the interface roughness.

Valence-band structure and optical absorption inp-type GaAs–Al0.3Ga0.7As multi-quantum-well infrared photodetectors under an electric field

Hole structure of a GaAs–Al0.3Ga0.7Asp-type multiple quantum well (MQW) subjected to an electric field parallel to the growth axis is studied using the envelope-function approximation and taking into account the valence subband mixing. The system considered in this work consists of five GaAs wells and six thick Al0.3Ga0.7As barriers. The valence subband structure and the optical-absorption coefficient are calculated as functions of the electric-field strength for various doping levels. The subband structure is shown to be nonparabolic and anisotropic in the plane of the layers with a four-fold symmetry. The spin splitting due to the lack of specular symmetry of quantum wells is a growing function of the electric-field strength. The calculated optical absorption is in good agreement with the experimental spectra.

How good is the polarization selection rule for intersubband transitions?

Using GaAs based quantum well infrared photodetectors (QWIPs) with either GaAs or InGaAs wells, we experimentally investigate the accuracy of the polarization selection rule for conduction band intersubband transitions. We employ a device structure and a light coupling geometry where the parasitic light scattering is negligible. The experiments imply that the selection rule is followed to an accuracy of 0.2% for a 8.1 μm QWIP with GaAs wells; this degrades to 3% for a 4.6 μm QWIP with In0.1Ga0.9As wells.

Observation of direct and phonon-assisted indirect transitions inGaAs/GaxAl1−xAsmultiquantum wells under hydrostatic pressure

Using photomodulated transmission spectroscopy we investigated the interband transitions in ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{G}\mathrm{a}}_{1\ensuremath{-}x}{\mathrm{Al}}_{x}\mathrm{As}$ multiquantum wells as a function of hydrostatic pressure up to 50 kilobar. A number of spectral structures, associated with the \ensuremath{\Gamma}, $L,$ and $X$ critical points in GaAs wells and ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Al}}_{x}\mathrm{As}$ barriers were observed, and calculations were performed to help understanding their origins. The pressure coefficients of the direct transitions in the quantum wells fall between those of GaAs and ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Al}}_{x}\mathrm{As}$ bulk. The observation of phonon-assisted indirect transitions is made easy by photomodulated transmission technique, and by the band folding effect in the multiple-quantum-well systems. The indirect transitions derived from the $L$ conduction subbands and \ensuremath{\Gamma} valence subbands have been observed in wide pressure ranges.

Photoluminescence study on twenty GaAs/Al0.3Ga0.7As tilted T-shaped quantum wires fabricated by glancing-angle molecular beam epitaxy

Twenty GaAs/Al0.3Ga0.7As tilted T-shaped quantum wires (T-QWRs) on a (111)B facet were uniformly fabricated with a two-step growth of molecular beam epitaxy (MBE), which consists of a glancing-angle MBE of GaAs/Al0.3Ga0.7As multi-quantum well layer with 20 GaAs wells (a well width of Lw=6.1 nm) on reverse-mesa stripes on a (001) GaAs substrate and MBE overgrowth of a GaAs/Al0.3Ga0.7As single-quantum well with Lw=6.3 nm on a (111)B facet. Full width at half maximum of a photoluminescence peak (λ=792 nm) from the tilted T-QWRs was as small as 8.7 meV at 28 K, which is comparable with those (6 meV,Reference 5 10 meVReference 10) of conventional GaAs/Al0.3Ga0.7As T-QWRs (6.8 nm/5.8 nm, 7 nm/7 nm) fabricated by the cleaved-edge overgrowth.

Optical Absorption in ZnSe-GaAs Heterovalent Quantum Structures

AbstractZnSe-GaAs(001) heterovalent quantum structures, multiple quantum wells (MQWs), were fabricated. Since the band offsets in this heterovalent heterostructure are controllable, different offsets can artificially be put at both sides of the GaAs wells, which may lead to modification of the electronic properties of the MQWs. This hypothesis was examined by the optical absorption measurement. Consequently, by changing the valence band offset at the GaAs-on-ZnSe interface from 0.6 to 1.1 eV, while keeping that at ZnSe-on- GaAs constant at 0.6 eV, the absorption edge energies of the MQWs shifted from 1.43 to 1.37 eV. This indicates the presence of an internal electric field and the capability of controlling the electronic properties of heterovalent MQWs with the completely same structural parameters. The numerical analyses of the Poisson and Schrddinger equations well explain the trend of this experimental result.

High-field domain formation in selectively doped N-type GaAs/AlGaAs multiple quantum well structures

We have studied the sequential tunneling of electrons in selectively doped GaAs/AlGaAs tight-binding multiple quantum wells (MQWs) using two structures with different doping profiles; one with Si doping in the AlGaAs barriers and the other with Si doping in the GaAs wells. In both cases, the current–voltage (I–V) measurements showed two series of current oscillations which persisted up to 200 K. TheI–Vcharacteristics also showed a hysteresis effect at increasing and decreasing voltage sweep. The voltage separations between the adjacent current oscillations in theI–Vcharacteristics of the structures with ohmic and Schottky contact metallization give a good measure of the energy difference between the excited states and the ground state in the quantum well, with good agreement with theoretical calculations. The process of sequential electron tunneling through the MQW structures is discussed in terms of the creation and extension of high-field domains (HFDs) due to the applied bias.

Optical anisotropy of (11N) GaAs/GaAlAs superlattices

The in-plane anisotropy of a series of (113), (115), (001) vicinal, and singular (001) oriented GaAs/Ga0.7Al0.3As superlattices (SL's) has been studied by reflectance difference spectroscopy (RDS) in the photon energy range covering both the E0 and the E1 energies of GaAs, and from 80 to 300 K. The polarity and the energy position of the observed RDS resonances near the E0 energy confirm that these resonances are indeed originated from the heavy hole (HH), the light hole (LH), and the Γ7 LH subbands in the GaAs wells. The transition strength anisotropy is in agreement with the multiband k * p model calculations. Sizable RD resonances have been observed in the (100) singular SL's at low temperatures, which are believed to be due to the HH and the LH exciton bound to anisotropic interface structural defects. The optical anisotropy of the SL's near the E1 critical energy of GaAs shows complicated resonance patterns. Some of the resonances occur below the E1 energy, which the simple effective-mass theory for the L-point states cannot explain.