Deep level electronic structure of ZnSe/GaAs heterostructures

We performed 1—2 keVcathodoluminescence measurements and He-Ne and HeCd excited photoluminescence studies of ZnSe/GaAs( 100) heterostructures grown by molecular beam epitaxy. Our goal was to investigate the deep level electronic structure and its connection with the heterojunction band offsets. We observed novel deep level emission features at 0.8, 0.98, 1.14, and 1.3 eV which are characteristic of the ZnSe overlayer and independent in energy of overlayer thickness. The corresponding deep levels lie far below those of the near-bandedge features commonly used to characterize the ZnSe crystal quality. The relative intensity and spatial distribution of the deep level emission was found to be strongly affected by the Zn/Se atomic flux ratio employed during ZnSe growth. The same flux ratio has been shown to influence both the quality of the ZnSe overlayer and the band offset in ZnSe/GaAs heterojunctions. In heterostructures fabricated in Se-rich growth conditions, that minimize the valence band offset and the concentration of Se vacancies, the dominant deep level emission is at 1.3 eV. For heterostructures fabricated in Zn-rich growth conditions, emission by multiple levels at 0.88,0.98, and 1.14 eV dominates. The spectral energies and intensities of deep level transitions reported here provide a characteristic indicator of ZnSe epilayer stoichiometry and near-interface defect densities.     

Optical gain spectra of InGaAsP/InP double heterostructures

We report the first optical gain measurements on InGaAsP/ InP double heterostructures with composition corresponding to an emission wavelength of about 1.3 μm at 300 K. At optical pumping levels of about 1 MW/cm²the maximum gain values of the best samples available are 800 cm^-1at 2 K, 500 cm^-1at 77 K, and 200 cm^-1at 300 K. We conclude from low-intensity luminescence and absorption spectra, that the laser transition corresponds to free-carrier recombination between band-tail states, which are present even in not intentionally doped material. Although these tail states result in rather broad low-intensity luminescence, narrow gain spectra comparable to those of GaAs/GaAlAs double heterostructures are obtained.     

Study of the misfit dislocations in semiconductor heterostructures by density functional TB molecular dynamics and path probability methods

The atomic and electronic structures of semiconductor heterostructures are studied using the density-functional tight-binding molecular dynamics simulations. Atomic structures of misfit dislocations both edge type 1/2 〈110〉 (001) and 60° dislocations in the semiconductor heterostructures, like Ge/Si(001), InAs/GaAs and InP/GaAs(001) systems are studied by using order of N [O(N)] calculational method. The path probability method in the statistical physics is used to study the influence of the interface disorder on the atomistic properties of the semiconductor heterostructures. It is shown that the compositional intermixing influences quite significantly on the electronic and thermodynamic properties of semiconductor heterostructures, especially on the critical layer thickness h_c for the generation of misfit dislocations.     

Determination of the density of states in quantum wells and quantum dot arrays by the capacitance-voltage method

The possibility of determining the electronic density of states in quantum wells and quantum dot arrays in heterostructures from the capacitance-voltage curve is investigated. In heterostructures fluctuations of the composition and geometrical dimensions play an important role. It is shown that to reconstruct the exact density of states from the measured capacitance-voltage curve is impossible, because this problem is ill-posed from the mathematical point of view. An approximate method is proposed for solving the problem, involving the determination of a “reduced” density of states. It is shown that the reduced density of states is close to the true density if the characteristic energy scale governing the variation of the latter is much greater than the thermal energy kT. The proposed method is used to find the density of states in the conduction band of a quantum well in an In_0.22Ga_0.78As/GaAs heterostructure. Fiz. Tekh. Poluprovodn. 33, 1246–1252 (October 1999)     

Suppression of electron magnetotunneling between parallel two-dimensional GaAs/InAs electron systems by the correlation interaction

Magnetotunneling between two-dimensional GaAs/InAs electron systems in vertical resonant tunneling GaAs/InAs/AlAs heterostructures is studied. A new-type of singularity in the tunneling density of states, specifically a dip at the Fermi level, is found; this feature is drastically different from that observed previously for the case of tunneling between two-dimensional GaAs tunnel systems in terms of both the kind of functional dependence and the energy and temperature parameters. As before, this effect manifests itself in the suppression of resonant tunneling in a narrow range near zero bias voltage in a high magnetic field parallel to the current direction. Magnetic-field and temperature dependences of the effect’s parameters are obtained; these dependences are compared with available theoretical and experimental data. The observed effect can be caused by a high degree of disorder in two-dimensional correlated electron systems as a result of the introduction of structurally imperfect strained InAs layers.     

Methods of controlling the emission wavelength in InAs/GaAsN/InGaAsN heterostructures on GaAs substrates

Studies of the properties of InGaAsN compounds and methods of controlling the emission wavelength in InAs/GaAsN/InGaAsN heterostructures grown by molecular beam epitaxy on GaAs substrates are reviewed. The results for different types of heterostructures with quantum-size InGaAsN layers are presented. Among those are (1) traditional InGaAsN quantum wells in a GaAs matrix, (2) InAs quantum dots embedded in an (In)GaAsN layer, and (3) strain-compensated superlattices InAs/GaAsN/InGaAsN with quantum wells and quantum dots. The methods used in the study allow controllable variations in the emission wavelength over the telecommunication range from 1.3 to 1.76 μm at room temperature.     

Electronic spectra of quasi-regular Fibonacci systems: Analysis of simple 1D models

The electronic spectra of quasi-regular systems grown following the Fibonacci sequence are investigated via simple one-dimensional tight-binding, one-band models. Different models are considered and the influence of the model parameters and the number of atoms entering the different blocks on the electronic spectrum are discussed.     

The Stark shift of the hole states in separate InAs/GaAs quantum dots grown on (100) and (311)A GaAs substrates

Deep-level transient spectroscopy is used to study charge-carrier emission from the states of separate quantum dots in InAs/GaAs p-n heterostructures grown on (100)-and (311)A-oriented GaAs substrates in relation to the reverse-bias voltage U. It is established that the structures under consideration exhibit different bias-voltage dependences of the Stark shift for the energy levels of the quantum-dot states on the value of U.     

Electronic state mixing in X x and X y valleys in AlAs/GaAs (001)

The X _x-X _y mixing of X-valley electronic states in AlAs/GaAs(001) heterostructures is analyzed. General, structure symmetry-related constraints on the parameters of a matrix matching the envelopes are derived. A model describing the X _x-X _y mixing is proposed. The obtained structure of the matching matrix differs essentially from those used previously. The parameters of the model are determined on the basis of pseudopotential calculations. Both the model and pseudopotential calculations have been carried out for the level dispersion in AlAs X quantum wells, the electronic spectra of (AlAs)_M(GaAs)_N(001) superlattices, and the X-electron transmission coefficient for structures with a single X-well and two X-barriers. The good agreement of the results of both sets of calculations shows the efficiency of the proposed model for envelopes with parameters determined in the study.     

Population-inversion and gain estimates for a semiconductor TASER

We have investigated a solid-state design advanced (see Soref et al. in SPIE Proceedings, vol.3795, p.516, 1999) to achieve a terahertz-amplification-by-the-stimulated-emission-of-radiation (TASER). The original design was based on light-to heavy-hole intersubband transitions in SiGe-Si heterostructures. This work adapts the design to electron intersubband transitions in the more readily available GaAs/Ga _1-xAl_xAs material system. It is found that the electric-field induced anti-crossings of the states, derived from the first excited state with the ground states of a superlattice in the Stark-ladder regime, offers the possibility of a population inversion and gain at room temperature     

Germanium quantum dots in an unstrained GaAs/ZnSe/Ge/ZnSe heterosystem

Arrays of Ge quantum dots in unstrained GaAs/ZnSe/Ge heterostructures were obtained by molecular-beam epitaxy for the first time. Their spatial parameters are examined by scanning tunneling microscopy, and their electronic structure is studied by Raman spectroscopy.     

A first-principles study of transport properties of a gallium arsenide nanoribbon-based molecular device

GaAs nanoribbon based molecular device is investigated using density functional theory. The electronic transport properties of GaAs nanoribbon are discussed in terms of density of states, electron density, transmission spectrum and transmission pathways. The applied bias voltage increases the peak maximum in the valence band and the conduction band. The electron density is found to be more on the arsenic sites than in gallium sites across GaAs nanoribbon. The transmission spectrum provides the insight to the transmission of electrons at different energy intervals across GaAs nanoribbon. The transmission pathways give the visualization of possible path for the electrons, when the bias voltage is varied between the electrodes. The transmission pathways get modified with the applied bias voltage. The result of the present study gives clearer vision of enhancing the electronic transport properties of GaAs nanoribbon which is used in optoelectronic devices.     

Photoluminescence of anti-modulation-doped GaAs/AlGaAs single quantum well structures exposed to hydrogen plasma

Techniques of low-temperature photoluminescence (PL), photoluminescence excitation, and photoreflectance were used to study the effect of hydrogen plasma treatment at 260°C on antimodulation Si-doped GaAs/AlGaAs heterostructures with near-surface single quantum wells (QWs) grown by molecular-beam epitaxy. It was found that, in the case of excitation below the AlGaAs band gap, exciton PL from the QW is quenched due to an increase in the electric field in the structure. The reason for this is that the passivation of surface states by hydrogen results in the Fermi level depinning from the midgap of the nominally undoped p-type GaAs cap layer, while shallow-level impurities present in the layers of the structure are not neutralized (their complexes with hydrogen dissociate under the influence of illumination and strong electric fields).     

Photoresponse study of normal incidence detection in p-type GaAs/AIGaAs multiple quantum wells

We studied p-type GaAs/AIGaAs multiple quantum well (MQW) materials as a possible alternative to the current n-type GaAs/AIGaAs MQWs for infrared detection. The advantage of p-type MQWs is that absorption of infrared radiation at normal incidence is not selection rule forbidden as it is for the n-type. We have verified that significant photoresponse occurs at normal incidence in p-type MQWs. We studied changes in the photoresponse spectrum as a function of well width and temperature. The MQW heterostructures were designed to use bound to continuum intersubband absorption in the GaAs valence band and to have a peak photoresponse near 8 μm. The photoresponse spectrum was compared to the first theoretical model of the bound to continuum absorption in p-type GaAs/ AlGaAs MQWs. The theoretical absorption curve was found to be in good qualitative agreement with the experimental results.     

Resonant tunneling dynamics of heterostructures based on semiconductors with two-valley spectra

Time-dependent characteristics of electron resonant tunneling in quantum heterostructures based on GaAs and AlAs are investigated using the two-valley model. Analysis of the phases of the transmitted and reflected waves yields analytic expressions for tunneling and reflection times. The solution to the problem incorporates the decay of Γ-and X-valley resonant states in these heterostructures. Numerical solution of the nonstationary Schrödinger equation is used to demonstrate the tunneling of wave packets through double-barrier heterostructures.     

分子束外延生长的GaAs-Al_xGa_(1-x)As多层异质结构中的“精细低维调制条纹”的观察

应用透射式电子显微镜观察了GaAs-Al_xGa_(1-x)As多层异质结结构中的“精细低维调制条纹”。在邻近GaAs-Al_xGa_(1-x)As超晶格层的缓冲层中和与这缓冲层邻近的GaAs-Al_xGa_(1-x)As超晶格层的小区域中发现了等宽度的“精细低维调制条纹”,其宽度为9.1(?)的GaAs条纹,12(?)的Al_xGa_(1-x)As条纹。文中介绍了用显微密度计获得的这些条纹的密度分布结果。同时还给出了GaAs-Al_xGa_(1-x)As 多层异质结结构的晶格像和用X射线能量散射谱技术获得的成分定量分析结果。     

Effect of carrier localization on the optical properties of MBE-grown GaAsN/GaAs heterostructures

The optical properties of GaAsN/GaAs heterostructures grown by molecular-beam epitaxy with different nitrogen content in the layers have been studied. The optical properties of GaAsN layers in the growth conditions under study are defined by the carrier recombination via localized states related to a strong composition inhomogeneity in the solid solution. The increasing of the nitrogen content raises the composition inhomogeneity and the carrier localization energy.     

GaAs-GaAlAs heterostructure lasers on semi-insulating substrates

Two structures of GaAs-GaAlAs heterostructures were fabricated on semi-insulating substrates. Low-threshold single-mode operation was achieved. The integration of a laser with an active electronic device (Gunn oscillator) on the same chip of GaAs was demonstrated.     

量子干涉效应在红外探测器能带结构设计中的应用

利用量子干涉效应,提出设计GaAs／AlxGa1-xAs量子阱红外探测器能带结构的方法。此法比K-P方法简便易行,而且随着势垒宽度的增加,两种方法所得结果趋于一致。这表明新方法更适合计算具有较宽势垒超晶格的电子态。另外,用新方法对超晶格样品的光电流谱进行了分析,所得结果比K-P方法更接近实验值。由此证实了量子干涉效应引起的电子能态的存在和新方法在量子阱红外探测器能带结构设计中的实用性。