S. V. Kondratenko

Effect of carrier capture by deep levels on lateral photoconductivity of InGaAs/GaAs quantum dot structures

Having used thermally stimulated conductivity (TSC) technique, we identified deep electron traps that produce strong effects on charge carrier transport and photoconductivity in InGaAs/GaAs quantum dot (QD) structures. The values of deep levels below the conduction band of GaAs at 0.16, 0.22, and 0.35 eV are obtained from the analysis of the shapes of TSC curves after the excitation with the quanta energy hv = 0.9, 1.2, and 1.6 eV. The level 0.16 eV in depth is an effective electron trap that provides crossing of lateral conductivity with a high-resistance mode and, therefore, causes a high photocurrent sensitivity of about 3 A/W at 77 K with excitation by interband transitions in QDs. We determined the charge density of electrons captured by the (Ec – 0.16 eV) level to be 2 × 10−6 C/cm2 at 77 K that induces electric field ∼ 105 V/cm in a vicinity of QDs. The state at Ec – 0.22 eV is shown to be related to the recombination center that can hold non-equilibrium holes over a long time under the condition th...

Deep level centers and their role in photoconductivity transients of InGaAs/GaAs quantum dot chains

The in-plane photoconductivity and photoluminescence are investigated in quantum dot-chain InGaAs/GaAs heterostructures. Different photoconductivity transients resulting from spectrally selecting photoexcitation of InGaAs QDs, GaAs spacers, or EL2 centers were observed. Persistent photoconductivity was observed at 80 K after excitation of electron-hole pairs due to interband transitions in both the InGaAs QDs and the GaAs matrix. Giant optically induced quenching of in-plane conductivity driven by recharging of EL2 centers is observed in the spectral range from 0.83 eV to 1.0 eV. Conductivity loss under photoexcitation is discussed in terms of carrier localization by analogy with carrier distribution in disordered media.

Photoelectric properties of the metamorphic InAs/InGaAs quantum dot structure at room temperature

We present the study of optical and photoelectric properties of InAs quantum dots (QDs) grown on a metamorphic In0.15Ga0.85As buffer layer: such nanostructures show efficient light emission in the telecom window at 1.3 μm (0.95 eV) at room temperature. We prepared a sample with vertical geometry of contacts isolated from the GaAs substrate. The structure is found to be photosensitive in the spectral range above 0.9 eV at room temperature, showing distinctive features in the photovoltage and photocurrent spectra attributed to QDs, InAs wetting layer, and In0.15Ga0.85As metamorphic buffer, while a drop in the photoelectric signal above 1.36 eV is related to the GaAs layer. No effect of defect centers on the photoelectrical properties is found, although they are observed in the absorption spectrum. We conclude that metamorphic QDs have a low amount of interface-related defects close to the optically active region and charge carriers can be effectively collected into InAs QDs.

Photovoltage Performance of Ge/Si Nanostructures Grown on Intermediate Ultrathin SiOX Layers

An enhanced photovoltage is reported to occur in Ge/Si structures with a SiOx layer having a thickness of 0.5-2 nm and placed between a Si substrate and Ge nanoislands. The effect is interpreted in terms of an increased separation distance for photoexcited electrons and holes occurring in the stress fields generated in the oxidized Ge/SiOx/Si structure. The electron-hole separation is modeled utilizing finite-element method techniques, and a good agreement between the experimentally observed enhancement and the computationally increased inter-charge distance is obtained. It is also found that insertion of the oxide layer accelerates the photovoltage decay. This result is interpreted in terms of competing processes, involving the direct recombination of the separated electrons and holes and multi-trapping behavior typical of disordered systems caused by Ge islands.

Intensity-dependent nonlinearity of the lateral photoconductivity in InGaAs/GaAs dot-chain structures

Photoelectric properties of laterally correlated multilayer InGaAs/GaAs quantum dots (QDs) heterostructures are studied. The response of the photocurrent to increasing excitation intensity is found to be nonlinear and varying with excitation energy. The structures are photosensitive in a wide range of photon energies above 0.6 eV. The spectral dependence of the photoconductivity (PC) is caused by strong interaction between the bulk GaAs and the lower energy states of the wetting layer, the QDs, as well as the defect states in the GaAs band gap. In particular, a mechanism for the participation of deep electron trap levels in the photocurrent is clarified. These structures also demonstrate a high sensitivity of up to 10 A/W at low excitation intensities. However, at higher excitation intensities, the sensitivity reduces exhibiting a strong spectral dependence at the same time. The observed sublinear PC dependence on excitation power results from a direct electron-hole recombination both in the QDs and in Ga...

Transport and Photoelectric Effects in Structures with Ge and SiGe Nanoclusters Grown on Oxidized Si (001)

Crystalline germanium nanoclusters (NCs) are grown by a molecular-beam epitaxy technique on chemically oxidized Si (100) surface at 700oC. Deposition of silicon on the surface with Ge nanoclusters leads to surface reconstruction and formation of polycrystalline diamond-like Si coverage, while nanoclusters core becomes tetragonal SiGe alloy. Possible mechanisms for nanoclusters growth are discussed. Selective photoexcitation of Ge or SiGe nanoclusters or space-charge layer of underlying Si allows to observe two non-equilibrium steady-states with higher and lower conductivity values as compared to the equilibrium one. The persistent photoconductivity (PPC) behaviour was observed after excitation of electron-hole pairs in Si (001) substrate. This effect may be attributed to spatial carrier separation by macroscopic fields in the depletion layer of the near-surface Si. Decreasing of surface conductivity, driven by optical recharging of NCs and Si/SiO2 interface states, is observed in the spectral range from 0.6 to 1.0 eV. Conductivity drop is discussed in the terms of hole accumulation by Ge-NC states enhancing the local-potential variations and, therefore, decreasing the surface conductivity of p-Si.

Effect of Ge Nanoislands on Lateral Photoconductivity of Ge-SiOX-Si Structures

The results of the experimental studies of the effect of nanoislands on the lateral photoconductivity in structures with Ge nanoislands formed on the SiOx layer using molecular beam epitaxy are reported. It is shown that nanoislands increase the surface recombination rate and affect the fundamental absorption edge of c-Si. The generation of lateral photocurrent in the range 0.8 – 1.0 eV was observed due to transitions between tails in the density of states of the near-surface c-Si, which is described by Urbach dependence. It was shown that the absorption spectrum of nanoislands is typical for the disordered Ge and is due to transitions between density-of-states tails of the valence and conductance bands. The mechanism is proposed of lateral photoconductivity involving the non-equilibrium charge carriers, generated in Ge nanoislands. It is suggested that the optical absorption and lateral photocurrent in Ge-SiOx-Si structures are affected by fluctuations of the surface potential in the near-surface region of c-Si, fluctuations of the Si band gap width and by effects of disorder in Ge nanoislands.

Bipolar Effects in Photovoltage of Metamorphic InAs/InGaAs/GaAs Quantum Dot Heterostructures: Characterization and Design Solutions for Light-Sensitive Devices

The bipolar effect of GaAs substrate and nearby layers on photovoltage of vertical metamorphic InAs/InGaAs in comparison with pseudomorphic (conventional) InAs/GaAs quantum dot (QD) structures were studied. Both metamorphic and pseudomorphic structures were grown by molecular beam epitaxy, using bottom contacts at either the grown n+-buffers or the GaAs substrate. The features related to QDs, wetting layers, and buffers have been identified in the photoelectric spectra of both the buffer-contacted structures, whereas the spectra of substrate-contacted samples showed the additional onset attributed to EL2 defect centers. The substrate-contacted samples demonstrated bipolar photovoltage; this was suggested to take place as a result of the competition between components related to QDs and their cladding layers with the substrate-related defects and deepest grown layer. No direct substrate effects were found in the spectra of the buffer-contacted structures. However, a notable negative influence of the n+-GaAs buffer layer on the photovoltage and photoconductivity signal was observed in the InAs/InGaAs structure. Analyzing the obtained results and the performed calculations, we have been able to provide insights on the design of metamorphic QD structures, which can be useful for the development of novel efficient photonic devices.

Photocurrent spectroscopy of Ge nanoclusters grown on oxidized silicon surface

Germanium (Ge) nanoclusters are grown by a molecular-beam epitaxy technique on chemically oxidized Si(100) surface at 700ºC. Evidence for long-term photoinduced changes of surface conductivity in structures with Ge nanoclusters (NCs) grown on silicon oxide is presented. Photoexcitation NCs or Si by quanta with different energy allows observing two non-equilibrium steady-states with excess and shortage of conductivity values as compare to equilibrium one. The persistent photoconductivity (PPC) behaviour was observed after interband excitation of electron-hole pairs in Si(001) substrate. This effect may be attributed to spatial carrier separation of photoexcited electron-hole pairs by macroscopic fields in the depletion layer of near-surface Si. Photoquenching of surface conductivity, driven by optical recharging of Ge NC’s and Si/SiO2 interface states, is observed. Conductivity decay is discussed in the terms of hole`s accumulation by Ge-NC states enhancing the local-potential variations and, therefore, decreasing the surface conductivity of p-Si.

Effect of Ge-Nanoislands on the Low-Frequency Noise in Si/SiOx/Ge Structures

Low-frequency noise of the structures with Ge-nanoclusters of rather high surface density grown on the oxidized silicon surface is investigated for the first time. It was revealed that the 1/f γ noise, where γ is close to unity, is the typical noise component. Nevertheless, the 1/f γ noise sources were found to be distributed nonuniformly upon the oxidized silicon structure with Ge-nanoclusters. The noise features revealed were analyzed in the framework of widely used noise models. However, the models used appeared to be unsuitable to explain the noise behavior of the structures studied. The physical processes that should be allowed for to develop the appropriate noise model are discussed.