Terahertz luminescence of GaAs-based heterostructures with quantum wells under the optical excitation of donors

Spontaneous emission from selectively doped GaAs/InGaAs:Si and GaAs/InGaAsP:Si heterostructures is studied in the frequency range of ～3–3.5 THz for transitions between the states of the two-dimensional subband and donor center (Si) under the condition of excitation with a CO₂ laser at liquid-helium temperature. It is shown that the population inversion and amplification in an active layer of 100–300 cm^?1 in multilayered structures with quantum wells (50 periods) and a concentration of doping centers N _D ≈ 10¹¹ cm^?2 can be attained under the excitation-flux density 10²³ photons/(cm² s).     

Electrical and optical properties of near-surface AlGaAs/InGaAs/AlGaAs quantum wells with different quantum-well depths

A series of AlGaAs/InGaAs/AlGaAs quantum-well heterostructures with different quantum-well depths and approximately the same concentrations of two-dimensional electrons is grown by molecular-beam epitaxy. The built-in electric field in the grown samples is determined from the photoreflectance data and, on this basis, the energy-band structure in the quantum-well region is calculated. It is found that the highest mobility μ_e of two-dimensional electrons is attained in the sample with a barrier-layer thickness of L _b = 11 nm. Measurements of the photoluminescence spectra and the band-structure calculations demonstrate that, as the quantum well becomes closer to the surface, the doping profile broadens due to diffusion and segregation processes. The nonmonotonic dependence of μ_e on the distance between the surface and the quantum well is explained.     

Electroluminescent properties of heterostructures with GaInNas quantum wells

GaInNAs quantum wells were grown by metal-organic vapor-phase epitaxy. In order to improve the optical properties, the GaNAs barriers were incorporated on both sides of the quantum well; these barriers compensated the elastic stresses. Characteristics of the optical transitions were assessed from the measurements of photoluminescence and photocurrent. In order to fabricate light-emitting diodes, nonalloyed ohmic contacts based on heavily δ-doped layers were used. Electroluminescence was observed at a wavelength of ～1.2 μm at temperatures of 77 and 300 K; the electroluminescence intensity depended linearly on the injection current if the latter exceeded a certain threshold value.     

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.     

High-speed long-wavelength optical modulation in InGaAs/InAlAs multiple quantum wells

A long-wavelength optical modulator has been fabricated which makes use of an electroabsorption effect in multiple quantum wells (MQWs). Here, InGaAs/InAlAs MQWs are prepared in a PIN configuration using molecular beam epitaxy (MBE). The rise time of the detected pulse modulation signal has been measured at 190 ps. This response level has been attributed to the detecting system response and RC time constant, and not to such intrinsic effects as carrier lifetime.     

Lateral photoconductivity of Ge/Si heterostructures with Ge quantum dots

The spectral dependences of the lateral photoconductivity of Ge/Si heterostructures with Ge quantum dots are studied. The photoresponse of the Ge/Si structures with Ge nanoclusters is detected in the range 1.0–1.1 eV at T = 290 K, whereas the photocurrent in the single-crystal Si substrate is found to be markedly suppressed. This result can be attributed to the effect of elastic strains induced in the structure on the optical absorption of Si. At temperatures below 120 K, the heterostructures exhibit photosensitivity in the spectral range 0.4–1.1 eV, in which the Si single crystal is transparent. The photocurrent in this range is most likely due to the transitions of holes from the ground states localized in the quantum dots to the extended states of the valence band.     

The cyclotron resonance of holes in InGaAs/GaAs heterostructures with quantum wells in quantizing magnetic fields

The spectra of the cyclotron resonance of holes in the InGaAs/GaAs selectively doped heterostructures with quantum wells are studied in pulsed magnetic fields as high as 50 T at 4.2 K. The previously observed effect of the inverted (compared with the results of the single-particle calculation of the Landau levels) ratio of the spectral weight of two split components of the line of the cyclotron resonance, which is attributed to the effects of the exchange interaction of holes, is confirmed. It is found that the ratios of intensities of the components of the line of the cyclotron resonance profoundly differ on the ascending and descending branches of the magnetic field pulse, which may be associated with a long time of the spin relaxation of holes between the two lowest Landau levels, which constitute tens of milliseconds.     

Optical properties of heterostructures with deep AlSb/InAs0.84Sb0.16/AlSb quantum wells

Using the Kane model, the energy of the dimensional quantization levels, absorption coefficient, and radiative-recombination rate are calculated for interband optical transitions between different dimensional quantization subbands in a heterostructure with a deep AlSb/InAs_0.86Sb_0.14/AlSb quantum well with regard to and without regard for the spin-orbit interaction. It is shown that the corrections introduced by the spin-orbit interaction in calculating these quantities are no larger than a few tens of percent even at spin-orbit interaction constants exceeding the band gap and account for the nonparabolicity in the calculation of the energy of dimensional quantization levels and absorption coefficient is much more important than account for the spin-orbit interaction. In calculation of the radiative-recombination rate, both these effects should be taken into account.     

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.     

Structural and optical properties of heterostructures with InAs quantum dots in an InGaAsN quantum well grown by molecular-beam epitaxy

Results obtained in a study of the structural and optical properties of GaAs-based heterostructures with InAs quantum dot layers overgrown with InGaAsN quantum wells are presented. Transmission electron microscopy has been applied to analyze how the thickness of the InGaAsN layer and the content and distribution of nitrogen in this layer affect the size of nanoinclusions and the nature and density of structural defects. It is shown that the size of InAs nanodomains and the magnitude of the lattice mismatch in structures containing nitrogen exceed those in nitrogen-free structures. A correlation between the luminescence wavelength and the size and composition of nanodomains is demonstrated. Furthermore, a correlation between the emission intensity and defect density in the structure is revealed.     

Electron properties of surface InGaAs/InAlAs quantum wells with inverted doping on InP substrates

The electron-transport and optical properties of heterostructures with a surface InGaAs/InAlAs quantum well in the cases of inverted δ doping with Si atoms (below the quantum well) and of standard δ doping (above the quantum well) are compared. It is shown that, in the case of inverted doping, the two-dimensional electron density in the quantum well is increased in comparison with the case of the standard arrangement of the doping layer at identical compositions and thicknesses of other heterostructure layers. The experimentally observed features of low-temperature electron transport (Shubnikov–de Haas oscillations, Hall effect) and the photoluminescence spectra of heterostructures are interpreted by simulating the band structure.     

Effect of hydrogen on the properties of Pd/GaAs/InGaAs diode structures with quantum wells

The effect of hydrogen on photoelectric properties and photoluminescence of Pd/GaAs/InGaAs diode structures with quantum wells (QWs) was investigated. The dependence of the structure characteristics on the thickness of the GaAs anodic oxide layer is revealed, and the optimum oxide thickness for the fabrication of hydrogen sensors is determined. It is established that the existence of metal bridges in a thin oxide layer has a significant influence on the I-V curves of the structures. It is shown that the presence of QWs leads to an increase in the structure’s sensitivity to hydrogen. Using the QWs as local defect probes, formation of the defects resulting from the deposition of a Pd electrode both on natural and on anodized GaAs surface is studied. It is found that defects in the QWs of the diode structures can be passivated by introduction of atomic hydrogen through the Pd electrode upon exposure of the structures to an atmosphere of molecular hydrogen.     

Persistent photoconductivity in InAs/AlSb heterostructures with double quantum wells

Effects of persistent photoconductivity in InAs/AlSb heterostructures with the cap GaSb layer and two-dimensional electron gas in the InAs double quantum wells at T = 4.2 K are studied. From Fourier analysis of the Shubnikov-de Haas oscillations, electron concentrations in each quantum well are determined at various wavelengths of illumination and pronounced asymmetry of the structure caused by the built-in electric field is demonstrated explicitly. The self-consistent calculations of the energy profile of the double quantum well are performed and the concentrations of ionized donors on both sides of the well are determined, which provided concretization of the previously suggested mechanism of bipolar persistent photoconductivity in such structures.     

On the process of hole trapping in Ge/Si heterostructures with Ge quantum dots

Admittance spectroscopy is used to determine the cross sections and energy levels of holes in Ge/Si heterostructures with Ge quantum dots. The structures are grown by molecular-beam epitaxy. It is established that, in layers of quantum dots produced at low growth temperatures T _g ≤ 450°C, the capture cross section for hole trapping into quantum dots exponentially increases with increasing hole binding energy (the Meyer-Neldel rule), with the same characteristic energy ～25 eV independent of T _g . It is shown that the Meyer-Neldel rule is violated in structures grown at higher temperatures or in samples treated in hydrogen plasma. In the case of nanoclusters synthesized at low temperatures, the experimental results suggest that charge-carrier trapping into Ge quantum dots proceeds via the electron-phonon mechanism with the participation of structural defects.     

Structural and infrared absorption properties of self-organized InGaAs/GaAs quantum dots multilayers

Self-organized InGaAs/GaAs quantum dots (QDs) stacked multilayers have been prepared by solid source molecular beam epitaxy. Cross-sectional transmission electron microscopy shows that the InGaAs QDs are nearly perfectly vertically aligned in the growth direction [100]. The filtering effect on the QDs distribution is found to be the dominant mechanism leading to vertical alignment and a highly uniform size distribution. Moreover, we observe a distinct infrared absorption from the sample in the range of 8.6–10.7 μm. This indicates the potential of QDs multilayer structure for use as infrared photodetector.     

Electrical properties and luminescence spectra of light-emitting diodes based on InGaN/GaN heterostructures with modulation-doped quantum wells

The distribution of charged centers N(w), quantum efficiency, and electroluminescence spectra of blue and green light-emitting diodes (LED) based on InGaN/AlGaN/GaN p-n heterostructures were investigated. Multiple InGaN/GaN quantum wells (QW) were modulation-doped with Si donors in GaN barriers. Acceptor and donor concentrations near the p-n junction were determined by the heterodyne method of dynamic capacitance to be about N _A ≥ 1 × 10¹⁹ cm^?3 ? N _D ≥ 1 × 10¹⁸ cm^?3. The N(w) functions exhibited maxima and minima with a period of 11–18 (±2–3 nm) nm. The energy diagram of the structures has been constructed. The shifts of spectral peaks with variation of current (J=10^?6–3×10^?2 A) are smaller (13–12 meV for blue and 20–50 meV for green LEDs) than the corresponding values for the diodes with undoped barriers (up to 150 meV). This effect is due to the screening of piezoelectric fields in QWs by electrons. The dependence of quantum efficiency on current correlates with the charge distribution and specific features in the current-voltage characteristics.     

Persistent photoeffects in p-i-n GaAs/AlGaAs heterostructures with double quantum wells

Abrupt changes in the capacitance between the p and n regions were observed in a planar p-i-n GaAs/AlGaAs heterostructure with two tunneling-coupled quantum wells exposed to laser irradiation (λ=633 nm). These changes can be caused by variations in both temperature (in the vicinity of T～2 K) and the dc voltage applied to the structure. A memory effect was detected; this effect manifested itself in the long lifetime of anomalies observed after the illumination had been turned off. Self-consistent calculations of distributions of charge and electric field were performed for the structures that contained a donor impurity in AlGaAs layers; this impurity is responsible for origination of DX centers that give rise to persistent photoconductivity. It is demonstrated that abrupt changes in capacitance can occur in such a structure, and the values of the parameters required for origination of these jumps are determined.     

Structural characterization of InGaAs/InAlAs quantum wells grown on 0 (111)-InP substrates

Recent studies on HEMT structures according to their growth orientation have attracted much interest from the viewpoint of physics and novel device applications. However, the optimization of their properties needs a high degree of crystalline quality. In this work, the structural characteristics of the layers present on InGaAs/InAlAs HEMT structures grown on (111)-InP substrates have been analyzed by atomic force and transmission electron microscopies. The presence of a strained quantum well induces a defect structure and surface morphology quite different from those observed in similar samples without the quamtum well. These results show that an accurate control of the growth conditions is necessary to obtain acceptable structural quality for (111) devices.     

Cyclotron resonance in doped and undoped InAs/AlSb heterostructures with quantum wells

Backward-wave tubes are used to study the spectra of cyclotron resonance in the range of 150–700 GHz in the AlSb/InAs/AlSb heterostructures with quantum wells and with an electron concentration in a two-dimensional electron gas ranging from 2.7 × 10¹¹ to 8 × 10¹² cm^?2 at 4.2 K. A significant increase in the cyclotron mass (from 0.03m₀ to 0.06m₀) is observed as the electron concentration (and, correspondingly, the Fermi energy) increases, which is typical of semiconductors with a nonparabolic dispersion relation. The results obtained are in satisfactory agreement with theoretical calculations of cyclotron masses at the Fermi level in the context of the simplified Kane model.