Hole Ground State Research Articles

Acoustic switching of quantum states in semiconductors

The interaction between solitary elastic strain pulses (acoustic solitons) and localized holes in low-dimensional silicon structures is studied theoretically. It is shown that a soliton propagating through a region of hole localization converts it from one quantum state to another characterized by a different projection of the angular momentum. This effect originates in splitting of the ground hole state, which is degenerate in the absence of a perturbation, by the elastic strain. A detailed microscopic calculation of the acoustic switching of quantum mechanical states is carried out for holes localized at a quantum dot or at a shallow impurity acceptor in a quantum well. It is shown that the acoustic soliton amplitude required for complete reversal of the projected angular momentum of a hole corresponds to the amplitude of typical experimentally produced strain pulses.

Effect of strain and variable mass on the formation of antibonding hole ground states in InAs quantum dot molecules

Using four-band $k\ensuremath{\cdot}p$ Hamiltonians, we study how biaxial strain and position-dependent effective masses influence hole tunneling in vertically coupled InAs/GaAs quantum dots. Strain reduces the tunneling and hence the critical interdot distance required for the ground state to change from bonding to antibonding. The reduced spin-orbit interaction in the GaAs matrix, which we account for using position-dependent Luttinger parameters, has the opposite effect. This compensation results in the critical distance being little affected. The possibility to induce the bonding-to-antibonding transition using longitudinal magnetic fields is also investigated. Luttinger-Kohn Hamiltonian predicts a magnetic enhancement of the heavy hole-light hole coupling which, in turn, leads to such transition. No such effect is, however, observed in magnetophotoluminescence experiments. An alternative implementation of the magnetic field in the envelope function Hamiltonian is given which retrieves the experimental behavior.

Dielectric mismatch in finite barrier cubic quantum dots

We study exciton states in cubic quantum dots with finite potential barrier in the presence of dielectric mismatch. The dependences of electron and hole ground states as well as exciton binding energy and oscillator strength are found as a function of quantum dot size and dielectric constant of quantum dot and surrounding matrix. The developed theoretical approach has been applied to analyze the peculiarities of exciton spectra in CdTe quantum dots embedded in a variety of dielectric matrices.

Polarized emission from GaN/AlN quantum dots subject to uniaxial thermal interfacial stresses

The authors have studied the excitation-dependent optical properties of GaN/AlN self-assembled quantum dots (QDs) grown on Si(111) substrates. Ensembles of QDs were subject to various external stress configurations that resulted from the thermal expansion coefficient mismatch between the GaN/AlN layers and the Si(111) substrate and ranged from in-plane uniaxial stress, primarily along the ⟨11-20⟩ directions, to in-plane biaxial stress, having magnitudes ranging from 15 to 30 kbars. The authors have exploited microcracks that form during the postgrowth cooling as stressors in order to create the highly localized regions of uniaxial stress over limited regions of the samples. Cathodoluminescence (CL) measurements of the excitonic transitions exhibit an in-plane linear polarization anisotropy in close proximity to microcracks that is strongly dependent on the e-beam current used to excite the QD ensemble. The excitonic transition energy in varying proximity to the microcracks was studied with CL wavelength imaging. Some aspects of the carrier recombination are inferred from the temperature dependence of the integrated CL intensity. CL activation energy (AE) imaging is used to study changes in the AE for the thermal quenching of the luminescence in close proximity to the microcracks, and correlations are obtained with changes in the QD excitonic transition energy and changes in the average AlN band edge energies relative to the ground electron and hole states in the QD. Localized CL spectroscopy of the QDs exhibits emissions from both the ground and excited states, whose relative contributions depend on the level of excitation and temperature. Experimental results indicate that the polarization anisotropy vanishes at high temperatures (∼300 K) with an increasing excitation of the QDs, while the anisotropy decreases more slowly with excitation at low temperatures (∼60 K). A theoretical modeling of the effect of carrier filling on the polarization anisotropy and the excitonic transition energy was performed, as based on three-dimensional self-consistent solutions of the Schrödinger and Poisson equations using the 6×6 k⋅p and effective mass methods for the calculation of the e-h wave functions. The authors attribute carrier filling and a thermal excitation of holes into higher energy QD hole states during excitation to account for the observed gradual decrease in the polarization anisotropy with an increasing electron-hole pair excitation density at T=300 K.

Inverted Ge islands in {111} faceted Si pits—a novel approach towards SiGe islands with higher aspect ratio

A detailed study of the so far unexplored Ge island nucleation on Si (001) substrates patterned with {111} faceted pyramidal pits is reported. The pits are defined by an anisotropic wet-chemical etch through a SiNx hard mask. Due to the self-limiting of the wet etch, an extremely uniform pit pattern is achieved. On these substrates, Ge layers were grown by solid source molecular beam epitaxy at various growth temperatures TGe. For TGe=550 °C, Ge fills the pits in the form of inverted {111} pyramids with a rounded apex and a (001)-oriented top surface. These islands have aspect ratios much larger than upright pyramids and domes that are usually obtained on substrates with cylindrical pits fabricated by lithographic techniques and reactive ion etching. Based on the experimentally determined shape, three-dimensional (3D) energy level calculations in an envelop function approach have been performed for the inverted pyramids. They show that, due to the orientation of the pyramid square base along ⟨110⟩ directions, the overlap between the hole ground states and electron states confined to the Si conduction band valleys perpendicular to the growth direction is much larger than for upright SiGe domes and {105} pyramids. For elevated growth temperatures around 700 °C, already a Ge coverage of five monolayers induces a strong Si transfer into the {111} pits, effectively converting them towards {1 1 10} faceted ones.

Molecular double core hole electron spectroscopy for chemical analysis

We explore the potential of double core hole electron spectroscopy for chemical analysis in terms of x-ray two-photon photoelectron spectroscopy. The creation of deep single and double core vacancies induces significant reorganization of valence electrons. The corresponding relaxation energies and the interatomic relaxation energies are evaluated by complete active space self-consistent field (CASSCF) calculations. We propose a method on how to experimentally extract these quantities by the measurement of single ionization potentials (IPs) and double core hole ionization potentials (DIPs). The influence of the chemical environment on these DIPs is also discussed for states with two holes at the same atomic site and states with two holes at two different atomic sites. Electron density difference between the ground and double core hole states clearly shows the relaxations accompanying the double core hole ionization. The effect is also compared to the sensitivity of single core hole IPs arising in single core hole electron spectroscopy. We have demonstrated the method for a representative set of small molecules LiF, BeO, BF, CO, N2, C2H2, C2H4, C2H6, CO2, and N2O. The scalar relativistic effect on IPs and on DIPs are briefly addressed.

Hole Band Mixing in CdS and CdSe Quantum Dots and Quantum Rods

Using a six-band k·p Hamiltonian for wurtzite lattice, we study the hole ground state symmetry and composition in spherical quantum dots and elongated quantum rods made of CdS and CdSe. The ground state crossovers which occur when changing the nanocrystal size and shape are well understood in terms of hole band mixing. Contrary to previous belief, the quantum rod ground state crossover with increasing length is shown not to occur at a fixed aspect ratio. The geometry and composition that maximize the spin purity and the intensity of linearly polarized light emission are elucidated. The six-band wurtzite Hamiltonian results for CdSe are compared to those obtained with quasi-cubic four-band and one-band Hamiltonians, and the performance of these simplified Hamiltonians is discussed.

G-factor tuning in self-assembled quantum dots

The possibility of electrical tuning of exciton g-factors in self-assembled InAs/GaAs quantum dots is explored theoretically by means of a tight-binding-like effective bond-orbital approach. The electron g-factor in the dots of various sizes is found to exhibit very little change over a broad range of the field strength. In contrast, the ground hole state in the dots of high aspect ratio is seen very sensitive to the applied field, its g-factor even changes the sign with the field. The distinct behavior of the electron and hole g-factors in the presence of electric field is explained in terms of nonzero envelope orbital angular momentum carried by the hole states.

Lateral electric field effects on quantum size confinement in cylindrical quantum dot under parabolic potential

Within the effective mass approximation, we investigated theoretically the ground-state energy of a single particle and the binding energy of the neutral donor impurity (D 0) affected by a lateral electric field in a parabolic quantum dot (QD). The results show that the electron and the hole ground-state energy and the band to band transition energies shift to lower values (red shift) by increasing the field intensity. The quantum Stark shift (QSS) for the electron increases rapidly in the quasi spherical QD (QSQD) by increasing the lateral field, whereas for the hole it increases monotony. In the cylindrical QDs (CQDs), we found that the QSS for electron and hole increase monotonically. The quantum size, lateral electric field and impurity position effect on the binding energy of neutral donor (D 0) is studied. Unexpected behavior of D 0 in quantum well limit (QW), the binding energy of D 0 is increasing (blue shift) with increasing QD radius R at the presence of a lateral electric field. It appears that for a fixed size of the QD, the off-center binding energy decreases when the impurity ion is displaced from the center to the QD borders, while it is shifted to lower energy with increasing the field.

Simultaneous Increase in Seebeck Coefficient and Conductivity in a Doped Poly(alkylthiophene) Blend with Defined Density of States

The Seebeck coefficient, a defining parameter for thermoelectric materials, depends on the contributions to conductivity of charge carriers at energies away from the Fermi level. Highly conductive materials tend to exhibit conductivity from carriers close to the Fermi level. In this article, we propose polymer blends in which ground state hole carriers, created by doping a minor additive component, are mainly at an orbital energy set below the hole energy of the major component of the blend. Transport, however, is expected to occur through the major component. This leads to a regime in which hole conductivity and Seebeck coefficient may be increased in parallel. While the absolute conductivity of the composite, and thus ZT, are not particularly high, this work demonstrates a route for designing thermoelectric materials in which increases in Seebeck coefficient and conductivity do not cancel each other.

Signatures of antibonding hole ground states in exciton spectra of vertically coupled quantum dots in an electric field

We study exciton energy spectra in a pair of vertically coupled self-assembled quantum dots in external electric field. We perform a systematic comparison of the four-band Luttinger Kohn modeling producing an antibonding hole ground state with the single valence-band approximation in which the hole ground state has the bonding character. We find that the single-band approximation remains relevant for description of the electric field dependence of the photoluminescence spectrum for interdot barrier thickness of 7 nm or larger. We explain that for thinner barriers the antibonding character of the hole orbital can be deduced from the ground state recombination probability as a function of the electric field.

Linker Dependence of Energy and Hole Transfer in Neutral and Oxidized Multiporphyrin Arrays

The excited-state photodynamics of the neutral and one-electron-oxidized forms of five porphyrin dyads were studied in benzonitrile containing tetrabutylammonium hexafluorophosphate as the supporting electrolyte. Each dyad contains a zinc porphyrin (Zn) and a free base porphyrin (Fb) joined by a linear biphenylene (Phi(2)), terphenylene (Phi(3)), quaterphenylene (Phi(4)), diphenylbutadiyne (L), or phenylethyne (E) linker (ZnFbPhi(2), ZnFbPhi(3), ZnFbPhi(4), ZnFbL, ZnFbE). The findings along with recent results on the neutral and oxidized forms of ZnFb dyads containing a diphenylethyne or phenylene linker (ZnFbU, ZnFbPhi) and steric hindrance to porphyrin-linker internal rotation at one or both ends of a diarylethyne linker (ZnFbD, ZnFbP, ZnFbB) give insights into the effects of linker characteristics (length, orbital energies, orbital overlap with the porphyrins) on the rate constants for excited-state energy transfer, excited-state hole transfer, and ground-state hole transfer. Analysis of the results is aided by density functional theory molecular orbital calculations and Forster energy-transfer calculations. Although the rate constants for linker-mediated through-bond excited-state energy transfer can be modulated significantly using a number of molecular design criteria (e.g., linker characteristics, interplay between porphyrin orbital characteristics, and linker attachment site), ground-state hole transfer, which also occurs via a linker-mediated through-bond electron-exchange mechanism, is primarily affected by the free-energy driving force for the process as dictated by the redox characteristics of the interacting porphyrins. The insights gained from this study should aid in the design of next-generation multichromophore arrays for solar energy applications.

Spin polarization of the neutral exciton in a single InAs quantum dot at zero magnetic field

The work presented in this thesis deals with optical studies of semiconductor quantum dots (QDs) in the InGaAs material system. It is shown that for self-assembled InAs QDs, the interaction with the surrounding GaAs barrier and the InAs wetting layer (WL) in particular, has a very large impact on their optical properties. The ability to control the charge state of individual QDs is demonstrated and attributed to a modulation in the carrier transport dynamics in the WL. After photo-excitation of carriers (electrons and holes) in the barrier, they will migrate in the sample and with a certain probability become captured into a QD. During this migration, the carriers can be affected by exerting them to an external magnetic field or by altering the temperature. An external magnetic field applied perpendicular to the carrier transport direction will lead to a decrease in the carrier drift velocity since their trajectories are bent, and at sufficiently high field strength become circular. In turn, this decreases the probability for the carriers to reach the QD since the probability for the carriers to get trapped in WL localizing potentials increases. An elevated temperature leads to an increased escape rate out of these potentials and again increases the flow of carriers towards the QD. These effects have significantly different strengths for electrons and holes due to the large difference in their respective masses and therefore it constitutes a way to control the supply of charges to the QD. Another effect of the different capture probabilities for electrons and holes into a QD that is explored is the ability to achieve spin polarization of the neutral exciton (X0). It has been concluded frequently in the literature that X0 cannot maintain its spin without application of an external magnetic field, due to the anisotropic electron – hole exchange interaction (AEI). In our studies, we show that at certain excitation conditions, the AEI can be by-passed since an electron is captured faster than a hole into a QD. The result is that the electron will populate the QD solely for a certain time window, before the hole is captured. During this time window and at polarized excitation, which creates spin polarized carriers, the electron can polarize the QD nuclei. In this way, a nuclear magnetic field is built up with a magnitude as high as ~ 1.5 T. This field will stabilize the X0 spin in a similar manner as an external magnetic field would. The build-up time for this nuclear field was determined to be ~ 10 ms and the polarization degree achieved for X0 is ~ 60 %. In contrast to the case of X0, the AEI is naturally cancelled for the negatively charged exciton (X-) and the positively charged exciton (X+) complexes. This is due to the fact that the electron (hole) spin is paired off in case of X- (X+). Accordingly, an even higher polarization degree (~ 73 %) is measured for the positively charged exciton. In a different study, pyramidal QD structures were employed. In contrast to fabrication of self-assembled QDs, the position of QDs can be controlled in these samples as they are grown in inverted pyramids that are etched into a substrate. After sample processing, the result is free-standing AlGaAs pyramids with InGaAs QDs inside. Due to the pyramidal shape of these structures, the light extraction is considerably enhanced which opens up possibilities to study processes un-resolvable in self-assembled QDs. This has allowed studies of Auger-like shake-up processes of holes in single QDs. Normally, after radiative recombination of X+, the QD is populated with a ground state hole. However, at recombination, a fraction of the energy can be transferred to the hole so that it afterwards occupies an excited state instead. This process is detected experimentally as a red-shifted luminescence satellite peak with an intensity on the order of ~ 1/1000 of the main X+ peak intensity. The identification of the satellite peak is based on its intensity correlation with the X+ peak, photoluminescence excitation measurements and on magnetic field measurements.

Rectification and stability of a single molecular diode with controlled orientation

In the molecular electronics field it is highly desirable to engineer the structure of molecules to achieve specific functions. In particular, diode (or rectification) behaviour in single molecules is an attractive device function. Here we study charge transport through symmetric tetraphenyl and non-symmetric diblock dipyrimidinyldiphenyl molecules covalently bound to two electrodes. The orientation of the diblock is controlled through a selective deprotection strategy, and a method in which the electrode-electrode distance is modulated unambiguously determines the current-voltage characteristics of the single-molecule device. The diblock molecule exhibits pronounced rectification behaviour compared with its homologous symmetric block, with current flowing from the dipyrimidinyl to the diphenyl moieties. This behaviour is interpreted in terms of localization of the wave function of the hole ground state at one end of the diblock under the applied field. At large forward current, the molecular diode becomes unstable and quantum point contacts between the electrodes form.

Deep-level transient spectroscopy studies of light-emitting diodes based on multiple-quantum-well InGaN/GaN structure

Capacitance–voltage (C–V) and deep-level transient (DLTS) spectroscopies has been applied to study the carrier emission from states of the quantum-well superlattice (QWSL) in a p–n InGaN/GaN heterostructure. Changes in the DLTS spectra of this structure strongly depend on bias-on–bias-off cooling conditions and the applied bias Ub. It is believed that these changes are determined by (i) built-in piezoelectric field that leads to spatial separation of the electron and hole ground-state wave functions of the SL, defined as a manifestation the quantum-confined Stark effect and (ii) localization of the electron wave functions in separate QWs, named the Wannier-Stark localization.

Bandgap calculation in Si quantum dot arrays using a genetic algorithm

In this work we present a fast and accurate genetic algorithm to determine the envelope functions and eigenenergies of the ground states of electrons and holes in low-dimensional complex semiconductor structures. We have developed the theoretical formalism of the algorithm in a general way in order to make it easy to include arbitrary nonparabolic and anisotropic band profiles in the calculations. From these results, calculation of the bandgaps of nanostructures can be carried out efficiently. Besides presenting and testing the algorithm, we calculate the ground state of electron and holes in two-dimensional quantum dot arrays, taking nonparabolicity and anisotropy into account.

Spectroscopic access to single-hole energies in InAs/GaAs quantum dots

We present a purely spectroscopic way to determine single-particle energy level splittings in individual InAs/GaAs QDs. The method is based on a combination of $\ensuremath{\mu}$-photoluminescence spectroscopy $(\ensuremath{\mu}\text{PL})$ with resonant excitation and $\ensuremath{\mu}$-photoluminescence excitation spectroscopy $(\ensuremath{\mu}\text{PLE})$ of charged QDs. The approach allows elimination of all contributions from few-particle interactions such that true single-particle energy level distances are determined. For the present InAs/GaAs QDs, showing ground-state recombination energies between 1.23 to 1.27 eV, the splitting between the hole ground state $({h}_{0})$ and first excited state $({h}_{1})$ is $\ensuremath{\Delta}({h}_{0}{h}_{1})=(27.1\ifmmode\pm\else\textpm\fi{}1.8)\text{ }\text{meV}$. The splitting between the first and second excited hole states is $\ensuremath{\Delta}({h}_{1}{h}_{2})=(10.8\ifmmode\pm\else\textpm\fi{}0.3)\text{ }\text{meV}$. These values are in good agreement with results from eight-band $\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ calculations on lens-shaped InAs/GaAs QDs. Theoretical investigations identify unambiguously the heavy-hole-light-hole coupling as the decisive parameter leading to a nonzero ${h}_{1}\ensuremath{-}{h}_{2}$ splitting.

Probing Ground‐state Hole Transfer Between Equivalent, Electrochemically Inaccessible States in Multiporphyrin Arrays Using Time‐resolved Optical Spectroscopy

A new strategy is described and implemented for determining the rates of hole-transfer between equivalent porphyrins in multiporphyrin architectures. The approach allows access to these rates between sites that are not the most easily oxidized components of the array. The specific architectures investigated with this new strategy are triads consisting of one zinc porphyrin (Zn) and two free base porphyrins (Fb). The triads employ a diphenylethyne linker (ZnFbFbU) and a phenylene linker (ZnFbFbPhi). The zinc porphyrin is selectively oxidized to produce Zn(+)FbFb, the free base porphyrins are excited to produce the excited-state mixture Zn(+)Fb*Fb and Zn(+)FbFb*, and the subsequent dynamics are monitored by ultrafast absorption spectroscopy. The system evolves by a combination of energy- and hole-transfer processes involving (adjacent and nonadjacent) zinc and free base porphyrin constituents that are complete within 100 ps of excitation; the rate constants of many of these processes are derived from prior studies of the oxidized forms of the benchmark dyads (ZnFbU and ZnFbPhi). One of the excited-state decay channels produces the metastable state ZnFbFb(+) that decays to a second metastable state ZnFb(+)Fb by the target hole-transfer process, followed by rapid hole transfer to produce the Zn(+)FbFb thermodynamic ground state of the system. The rate constant for hole transfer between the free base porphyrins in the oxidized ZnFbFb triads is found to be (0.5 ns)(-1) and (0.6 ns)(-1) across phenylene and diphenylethyne linkers, respectively. These rate constants are comparable to those recently measured, using a related but distinct strategy, for ground-state hole transfer between zinc porphyrins in oxidized ZnZnFb triads. The two complementary strategies provide unique approaches for probing hole transfer between equivalent sites in multiporphyrin arrays, with the choice of method being guided by the particular target process and the ease of synthesis of the necessary architectures.

Selective oxidation of poly-Si with embedded Ge nanocrystals in Si/SiO2/Ge(NCs)/poly-Si structure for memory device fabrication

Selective oxidation of poly-Si in Si/SiO2/Ge(NCs)/poly-Si structure was proposed as the method of tunable synthesis of a control insulator in a memory device with Ge nanocrystals (NCs). Different behavior of oxidation resulting in partial and total oxidation of poly-Si was investigated using spectral ellipsometry and capacitance–voltage (CV) techniques. The ∼2.1 V memory window corresponding to the electron and hole charging/discharging processes in NCs was obtained in Si/SiO2/Ge(NCs)/SiO2 structure. To compare the experimental and calculated CV characteristics the energies of the electron and hole ground states were determined.

Experimental and theoretical investigations of optical properties of GaN/AlGaN MQW nanostructures. Impact of built-in polarization fields

AbstractWe report the results from detailed optical spectroscopy from MOCVD grown GaN/AlGaN multiple quantum wells (MQWs), as opposed to most previous studies where MBE was employed by means of photoluminescence (PL) technique. In this paper we will present theoretical and experimental results demonstrating how polarization induced electric fields and bound interface charges in GaN/AlGaN MQWs affect the emission peak energy, PL line shape, as well as the emission line width. Theoretically estimated fields in this work are consistent with experimental data. Transition energy of the heavy hole and electron ground state Ee-hh in GaN/AlGaN MQWs were calculated and it is found that it stays in good agreement with the experimental data.