Low frequency noise spectroscopy in InAs/GaAs resonant tunneling quantum dot infrared photodetectors

We have studied the temperature dependence of low-frequency noise in InAs–GaAs resonant tunneling quantum dot infrared photodetectors (T-QDIPs). The noise in these devices has been investigated in the temperature range of 78–300 K. The noise spectrum showed a weak Lorentzian component superimposed upon the 1/f^γ spectrum. The change in the cut-off frequency of the Lorentzian was analyzed as a function of temperature. The activation energy of the trap associated with this Lorentzian was obtained as 0.155 eV, which is in good agreement with the energy of the lowest energy state in the quantum dot.     

Annealing behavior of InAs/GaAs quantum dot structures

We investigate the annealing behavior of InAs layers with different thicknesses in a GaAs matrix. The diffusion enhancement by strain, which is well established in strained quantum wells, occurs in InAs/GaAs quantum dots (QDs). A shift of the QD luminescence peak toward higher energies results from this enhanced diffusion. In the case of structures where a significant portion of the strain is relaxed by dislocations, the interdiffusion becomes negligible, and there is a propensity to generate additional dislocations. This results in a decrease of the QD luminescence intensity, and the QD peak energy is weakly affected.     

Electron capture in AlGaAs/AlAs/GaAs double-barrier quantum well structures: Tunneling versus intervalley scattering

We have investigated electron capture times in n-type GaAs/AlAs/Al_0.3Ga_0.7As double-barrier quantum well (DBQW) structures in an electric field along the growth direction. To this end, the time dependence of the photoluminescence (PL) originating from the GaAs wells and the Al_0.3Ga_0.7As barrier layers is analyzed numerically. Theoretical electron capture times are calculated for the investigated DBQW's for the case of several scattering mechanisms. Electron capture mainly occurs via Γ-X intervalley transfer under participation of X-point subbands located at the AlAs tunnel barriers. Good agreement between experiment and theory is obtained when assuming Γ-X backfolding, while impurity scattering, optical phonon-assisted Γ-tunneling, and deformation potential scattering give rise to much larger electron capture times.     

Origin of the enhancement of negative differential resistance atlow temperatures in double-barrier resonant tunneling structures

An explanation of the increased peak-to-valley current ratio for double-barrier resonant tunneling structures (DBRTSs) operated at low temperatures is proposed. It was found that this phenomenon is an inherent property of DBRTSs not caused by the suppression of thermionic current over barriers. The energy distributions of electrons at different temperatures result in variations of peak and valley currents     

Investigation of indium doping and incorporation in AlGaAs/GaAs double-barrier resonant tunnelling structures

In doping and incorporation in the barrier layers of AlGaAs/GaAs double-barrier resonant tunnelling structures (DBRTSs) have been studied. It was found that the peak-to-valley current ratio (PVCR) can be improved by the proper amount of In doping. This is attributed to the improvement in the quality of the AlGaAs barrier layers due to the high surface migration rate of In atoms that reduces group III vacancies. Also pseudomorphic In/sub x/(Al/sub 0.5/Ga/sub 0.5/)/sub 1-x/As/GaAs (x=0.12) strained-layer DBRTSs have been fabricated by incorporating a sufficient amount of In into the AlGaAs barrier layers. PVCRs as high as 27.5 at 77 K have been obtained. This is the first realisation of such DBRTSs with lattice-mismatched quaternary barrier layers.<>     

Capacitance studies of multilayer ensembles of InAs QDs in a GaAs matrix

Quasi-static capacitance characteristics of multilayer arrays of vertically coupled InAs quantum dots (QDs) in a GaAs matrix were analyzed on the assumption of a Gaussian energy distribution of the ground state of the QDs. An array of InAs QDs with a characteristic base size of about 20 nm and height of ～3 nm was ordered in the growth direction and had 3, 6, or 10 layers spaced by ～5 nm. It was found that, as the number of layers increases from 3 to 10, the average binding energy of the ground electron state grows from ～80 to ～120 meV and the root mean square deviation characterizing the energy distribution of the levels of this state decreases from ～30 to ～15 meV.     

P-N double quantum well resonant interband tunneling diode withpeak-to-valley current ratio of 144 at room temperature

The current-voltage characteristics of the P-N double quantum well resonant interband tunneling (RIT) diodes in InAlAs-InGaAs system have been improved in this letter. The peak-to-valley current ratio (PVCR) is as high as 144 at room temperature. As we know, this is the highest room temperature PVCR ever reported in any tunneling devices. Moreover, the influence of the central barrier thickness varying from 10 Å to 30 Å on the device characteristics is also studied     

Influence of MBE growth temperature on GaAs/AlAs resonant tunneling structures

Double barrier GaAs/AlAs tunneling structures with typical 2.5:1 room temperature peak-to-valley current ratios are examined using Deep Level Transient Spectroscopy. Deep level trap concentrations are found to be much higher in samples grown at 550° C compared to those grown at 650° C. For devices grown at 550° C, an impedance switch-ing effect due to a high concentration of deep levels is observed. The peak-to-valley ratio of the tunneling devices is largely unaffected by the growth temperatures in this range, indicating that higher growth temperatures can be employed to grow resonant tunnel-ing diodes than previously suggested in the literature.     

Spin effect on the resonant tunneling characteristics of a double-barrier heterostructures under longitudinal stresses

Theoretical research on electronic properties in mesoscopic condensed matter systems has focused primarily on the electron charge freedom degrees, while its corresponding spin freedom degrees have not yet received the same attention. Nevertheless nowadays there has been an increment in the number of electron spin-related experiments showing unique possibilities for finding novel mechanisms of information processing and transmission, opening ample fields of opportunities in the theoretical developed of new models. In this spirit we have calculated the resonant tunneling characteristics curves in double-barrier heterostructures of GaAs–Ga_1?x Al_xAs under external stress and considering two charges with spin half. The resonant tunneling study has been carried out by means of the diagrammatic techniques for nonequilibrium processes following the model proposed by Keldysh [Sov. Phys. JETP 20 (1965) 1018] also a simple one-band tight-binding Hamiltonian is adopted in the theoretical framework. We have compared our results of the spin tunneling with previous ones reported in literature.     

GaAs structures with InAs and As quantum dots produced in a single molecular beam epitaxy process

Epitaxial GaAs layers containing InAs semiconductor quantum dots and As metal quantum dots are grown by molecular beam epitaxy. The InAs quantum dots are formed by the Stranskii-Krastanow mechanism, whereas the As quantum dots are self-assembled in the GaAs layer grown at low temperature with a large As excess. The microstructure of the samples is studied by transmission electron microscopy. It is established that the As metal quantum dots formed in the immediate vicinity of the InAs semiconductor quantum dots are larger in size than the As quantum dots formed far from the InAs quantum dots. This is apparently due to the effect of strain fields of the InAs quantum dots upon the self-assembling of As quantum dots. Another phenomenon apparently associated with local strains around the InAs quantum dots is the formation of V-like defects (stacking faults) during the overgrowth of the InAs quantum dots with the GaAs layer by low-temperature molecular beam epitaxy. Such defects have a profound effect on the self-assembling of As quantum dots. Specifically, on high-temperature annealing needed for the formation of large-sized As quantum dots by Ostwald ripening, the V-like defects bring about the dissolution of the As quantum dots in the vicinity of the defects. In this case, excess arsenic most probably diffuses towards the open surface of the sample via the channels of accelerated diffusion in the planes of stacking faults.     

Deep-level transient spectroscopy in InAs/GaAs laser structures with vertically coupled quantum dots

Indium arsenide/gallium arsenide structures with vertically coupled quantum dots imbedded in the active zone of a laser diode are investigated by deep-level transient spectroscopy (DLTS), and the capacitance-voltage characteristics are analyzed. The DLTS spectrum was found to undergo significant changes, depending on the temperature of preliminary isochronous annealing of the sample, T _a <T _ac =245 K or T _a >T _ac , and on the cooling conditions, with a bias voltage V _b =0 or with an applied carrier pulse V _f >0. The changes are attributed to the onset of Coulomb interaction of carriers trapped in a quantum dot with point defects localized in the nearest neighborhoods of the quantum dots and also to the formation of a dipole when T _a <T _ac and cooling takes place with V _f >0, or to the absence of a dipole when T _a >T _ac and V _b =0. It is discovered that the tunneling of carriers from the deeper states of defects to the shallower states of quantum dots takes place in the dipole, and the carriers are subsequently emitted from the dots into bands. Fiz. Tekh. Poluprovodn. 31, 1249–1255 (October 1997)     

Formation of a step-free InAs quantum well selectively grown on a GaAs (111)B substrate

We investigated the possibility of forming a step-free quantum well structure. A step-free InAs monolayer was grown on a selectively grown mesa by controlling surface phases with in-situ monitoring of surface photo-absorption. We selectively grew a GaAs buffer at 800°C and cooled the sample keeping the (2×2)-like As stabilized surface. Atomic force microscopy (AFM) observation demonstrated that fully step-free surfaces were formed on the 8 μm wide mesa. Then, a monolayer-thick InAs was formed on this step-free surface and this InAs layer was capped by GaAs under the (2×2)-like condition. The quantum level of the step-free InAs layer was evaluated by spatially resolved photoluminescence (μPL) measurement. Uniform PL intensity and the lack of a double layer peak indicated the formation of a step-free InAs quantum well, which was in good agreement with AFM observation.     

Photoconductivity of InAs/GaAs structures with InAs nanoclusters in the near-infrared region

InAs/GaAs multilayered heterostructures containing dense arrays of the low-defect partially relaxed InAs nanoclusters larger than defect-free quantum dots are fabricated by metal-organic chemical vapor deposition in an atmospheric-pressure reactor. The structures have intense photoconductivity in the wavelength range of 1–2 μm at room temperature. The detectivity of fabricated prototypes of photodetectors is D* = 10⁹ cm Hz^1/2 W⁻¹. The relaxation time of photoconductivity at a wavelength of 1.5 μm is less than 10 ns.     

Self-organization of quantum dots in multilayer InAs/GaAs and InGaAs/GaAs structures in submonolayer epitaxy

The experimental results of RHEED and scanning tunneling microscopy investigations of multilayer structures of InGaAs/GaAs quantum dots, obtained by submonolayer epitaxy on singular and vicinal GaAS (100) substrates, are reported. The results presented show that spatial ordering of nano-objects exists in multilayer structures for InAs and heteroepitaxial InGaAs layers. Fiz. Tekh. Poluprovodn. 33, 733–737 (June 1999)     

Effects of layer design on the performance of InAs/AlSb/GaSb resonant interband tunneling diodes on GaAs substrates

The room temperature current-voltage characteristics of InAs/AlSb/GaSb resonant interband tunneling diodes (RITDs), grown by chemical beam epitaxy on GaAs substrates, are reported as a function of the InAs buffer layer thickness and different interface configurations. The peak-to-valley current ratio (PVCR) improved from 1 to 12 as the buffer thickness was increased from 0 to 500 nm and the density of dislocations caused by the lattice mismatch of ∼7% decreased. No significant improvement was seen for a buffer thickness beyond 500 nm. Dislocation-free RITDs, grown lattice-matched on InAs substrates, show PVCRs of approximately 16. The InAs/AlSb Interfaces in these structures can be either InSb-like or AlAs-like and the interface can have a very strong effect on the diode performance. Unlike the case in InAs/AlSb field effect transistor structures, an AlAs-like interface results in better PVCRs in the diodes. Details of the results of this study are presented.     

(InAs)/sub 1//(GaAs)/sub 4/ superlattice strained quantum well laser at 980 nm

The first successful MBE growth of (InAs)/sub 1//(GaAs)/sub 4/ short-period superlattice (SPS) strained quantum well lasers emitting at approximately 980 nm is reported. The SPS consists of six periods of one and four monolayers of InAs and GaAs, respectively. The measured threshold current density was approximately 100 A cm/sup -2/ for 0.96 mm long lasers. Devices have operated up to 170 degrees C with a characteristic temperature T/sub 0/ of 148 K. The (InAs)/sub m//(GaAs)/sub n/ superlattice is an ordered counterpart of the InGaAs random alloy, and provides an alternative method fabricating high speed electronic and photonic devices.<>     

Photoelectric properties of GaAs/InAs heterostructures with quantum dots

The capacitive photovoltage and photoconductivity spectra of GaAs/InAs heterostructures with quantum dots is discussed. For these structures, which were fabricated by metallorganic gas-phase epitaxy, the photosensitivity spectrum has a sawtoothed shape in the wavelength range where absorption by the quantum dots takes place, which is characteristic of a δ-function-like density of states function. The spectra also exhibit photosensitivity bands associated with the formation of single-layer InAs quantum wells in the structure. An expression is obtained for the absorption coefficient of an ensemble of quantum dots with a prespecified size distribution. It is shown that the energy distribution of the joint density of states, the surface density of quantum dots, and the effective cross section for trapping a photon can all be determined by analyzing the photosensitivity spectrum based on this assumption. Fiz. Tekh. Poluprovodn. 31, 1100–1105 (September 1997)     

The engineering and properties of InAs quantum dot molecules in a GaAs matrix

Arrays of InAs quantum dot (QD) molecules in the GaAs matrix, which consist of pairs of vertically aligned InAs QDs, have been synthesized by molecular beam epitaxy. A study of the resulting structures by transmission electron microscopy demonstrated that the vertically aligned QDs are equal in size. Photoluminescence measurements revealed that the spectra of the samples under study contain bands corresponding to electronic states in QD molecules.     

Wannier-Stark states in a superlattice of InAs/GaAs quantum dots

Electron and hole emission from states of a ten-layer system of tunneling-coupled vertically correlated InAs/GaAs quantum dots (QDs) is studied experimentally by capacitance—voltage measurements and deep-level transient spectroscopy. The thickness of GaAs interlayers separating sheets of InAs QDs was ≈3 nm, as determined from transmission electron microscope images. It is found that the periodic multimo-dal DLTS spectrum of this structure exhibits a pronounced linear shift as the reverse-bias voltage U _r applied to the structure is varied. The observed behavior is a manifestation of the Wannier—Stark effect in the InAs/GaAs superlattice, where the presence of an external electric field leads to the suppression of coupling between the wave functions of electron states forming the miniband and to the appearance of a series of discrete levels called Wannier—Stark ladder states.