Host Semiconductor Research Articles

Beating the thermodynamic limit with photo-activation of n-doping in organic semiconductors.

Chemical doping of organic semiconductors using molecular dopants plays a key role in the fabrication of efficient organic electronic devices. Although a variety of stable molecular p-dopants have been developed and successfully deployed in devices in the past decade, air-stable molecular n-dopants suitable for materials with low electron affinity are still elusive. Here we demonstrate that photo-activation of a cleavable air-stable dimeric dopant can result in kinetically stable and efficient n-doping of host semiconductors, whose reduction potentials are beyond the thermodynamic reach of the dimer's effective reducing strength. Electron-transport layers doped in this manner are used to fabricate high-efficiency organic light-emitting diodes. Our strategy thus enables a new paradigm for using air-stable molecular dopants to improve conductivity in, and provide ohmic contacts to, organic semiconductors with very low electron affinity.

Calculation of the electronic structure and exchange interaction in the InSb and GaAs semiconductors codoped with Mn and Ni

Density functional theory calculations have been used to study the electronic structure of Mn-doped, Ni-doped, and Mn/Ni-codoped InSb and GaAs semiconductors. The ferromagnetic transition energy has been calculated using a multiscale method in which exchange interaction is calculated by the Hartree–Fock exact atomic method and is then included as a Hubbard parameter in calculation of the electronic structure of the material. The present calculation results demonstrate that, in all cases, there is hybridization of the impurity d states with the valence band of the host semiconductor. The contributions of the Ni and Mn dopants are approximately additive.

Experimental and theoretical study on the structural, electrical and optical properties of tantalum-doped ZnO nanoparticles prepared via sol-gel acetate route

Pure and Ta-doped ZnO were prepared by the sol-gel method in acetic medium using different annealing temperature treatments. The effects of low Ta doping on the crystallinity and electrical properties of ZnO nanoparticles were analyzed by means of X-ray diffraction and micro-Raman, UV–visible, and impedance spectroscopies.We also performed first principles calculations in order to study the predicted changes in the structural, vibrational and electronic properties induced by the inclusion of the impurities, and to complement the experimental measurements.We showed that above thermal treatments at 600°C the precursor samples take the hexagonal wurtzite ZnO phase with high crystallinity. For the doped samples, we found that the synthesizing method has good Ta doping efficiency of the ZnO host structure. Also, Ta doping substantially decreases the resistivity compared to pure ZnO. These results confirm that Ta impurities can substitute Zn atoms and act as donor impurities in the host semiconductor.

Cocatalyzing Pt/PtO Phase-Junction Nanodots on Hierarchically Porous TiO2 for Highly Enhanced Photocatalytic Hydrogen Production.

Phase-junctions between a cocatalyst and its semiconductor host are quite effective to enhance the photocatalytic activity and are widely studied, while reports on the phase-juncted cocatalyst are still rare. In this work, we report the deposition of the Pt/PtO phase-juncted nanodots as cocatalyst via NaOH modification of an interconnected meso-macroporous TiO2 network with high surface area and inner-particle mesopores to enhance the performance of photocatalytic H2 production. Our results show that NaOH modification can largely influence Pt/PtO phase-juncted nanodot formation and dispersity. Compared to the TiO2 nanoparticles, the hierarchically meso-macroporous TiO2 network containing 0.18 wt % Pt/PtO phase-juncted cocatalyst demonstrates a highest photocatalytic H2 rate of 13 mmol g-1 h-1 under simulated solar light, and possesses a stable cycling activity without obvious decrease after five cycles. Such high H2 production performance can be attributed to both the phase-juncted Pt/PtO providing more active sites while PtO suppresses the undesirable hydrogen back reaction, and the special hierarchically porous TiO2 network with inner-particle mesopores presenting short diffusion path lengths for photogenerated electrons and enhanced light harvesting efficiency. This work suggests that Pt/PtO phase-juncted cocatalyst on hierarchically porous TiO2 nanostructures is a promising strategy for advanced photocatalytic H2 production.

Self-Assembled Heterostructures: Selective Growth of Metallic Nanoparticles on V2 -VI3 Nanoplates.

Precise control of the selective growth of heterostructures with specific composition and functionalities is an emerging and extremely challenging topic. Here, the first investigation of the difference in binding energy between a series of metal-semiconductor heterostructures based on layered V2 -VI3 nanostructures is investigated by means of density functional theory. All lateral configurations show lower formation energy compared with that of the vertical ones, implying the selective growth of metal nanoparticles. The simulation results are supported by the successful fabrication of self-assembled Ag/Cu-nanoparticle-decorated p-type Sb2 Te3 and n-type Bi2 Te3 nanoplates at their lateral sites through a solution reaction. The detailed nucleation-growth kinetics are well studied with controllable reaction times and precursor concentrations. Accompanied by the preserved topological structure integrity and electron transfer on the semiconductor host, exceptional properties such as dramatically increased electrical conductivity are observed thanks to the pre-energy-filtering effect before carrier injection. A zigzag thermoelectric generator is built using Cu/Ag-decorated Sb2 Te3 and Bi2 Te3 as p-n legs to utilize the temperature gradient in the vertical direction. Synthetic approaches using similar chalcogenide nanoplates as building blocks, as well as careful control of the dopant metallic nanoparticles or semiconductors, are believed to be broadly applicable to other heterostructures with novel applications.

Designing quantum resonant scatterers at subwavelength scale

For an isotropic quantum resonant scatterer, such as a quantum dot embedded in host semiconductors, we propose a method to achieve resonant electron scattering by taking physical quantities into account through the second-order expansion. All needed physical information for spherical harmonic channels in anomalous quantum resonant scattering is revealed in a parameter space as the size of quantum scatterer is comparable to the de Broglie wavelength of incoming matter wave. Our results provide the guideline to realize quantum resonant scatterers with state-of-the-art semiconductor heterostructure technology.

Exchange-only singlet-only spin qubit

We propose a feasible and scalable quantum-dot-based implementation of a singlet-only spin qubit which is to leading order intrinsically insensitive to random effective magnetic fields set up by fluctuating nuclear spins in the host semiconductor. Our proposal thus removes an important obstacle for further improvement of spin qubits hosted in high-quality III-V semiconductors such as GaAs. We show how the resulting qubit could be initialized, manipulated, and read out by electrical means only, in a way very similar to a triple-dot exchange-only spin qubit. Due to the intrinsic elimination of the effective nuclear fields from the qubit Hamiltonian, we find an improvement of the dephasing time $T_2^*$ of several orders of magnitude as compared to similar existing spin qubits.

How Important is the Host (Semiconductor Nanoparticles) Identity and Absolute Band Gap in Host-Sensitized Dopant Photoluminescence?

This work reports the host (semiconductor nanoparticles) sensitized dopant (lanthanides, Ln) photoluminescence in near band gap matched Sn(Ln)O2 and Zn(Ln)S [Ln = Sm, Tb] nanoparticles to address the importance of the nanoparticle identity and absolute band gap in the underlying process. While the sensitization was evident in the Sn(Sm)O2 and Zn(Tb)S nanoparticles, the same was not observed in the Sn(Tb)O2 and Zn(Sm)S nanoparticles. This observation stresses the importance of nanoparticle identity as the determining factor in realizing the host-sensitized dopant photoluminescence and provides important insight into developing novel doped inorganic nanoparticle-based optical materials.

Doping Mn2+ in Lead Halide Perovskite Nanocrystals: Successes and Challenges

Mn2+ ions doped in high-energy absorbing semiconductor host nanocrystals take away the exciton energy and result in spin-polarized d–d emission. For the last three decades this has been widely studied on group II–VI semiconductors. Recently, the doping has been extended to CsPbX3 perovskite nanocrystals. Although the optical transition follows a similar principle, in which the exciton energy is transferred to dopant Mn d-state, doping in perovskite also revealed several new fundamental aspects of doping and dopant-induced new optical properties. Here, anions which mostly tune the band gap controlled the fate of the appearance of Mn emission. Also, the doping process was observed to be different than traditional growth doping. Hence, in perovskite host nanocrystals, while some aspects of Mn doping are found to be in agreement with previous findings, some new facts also surfaced. Combining all these facts, this Perspective focuses on the journey of Mn doping from group II–VI semiconductors to lead halide pe...

Enhancing the efficiency of the intermediate band solar cells by introducing: carrier losses, alloying and strain

A detailed balance model is used to determine the efficiency of intermediate band solar cell including carrier losses from the intermediate band. The effect of the energy gap of the host semiconductor is examined as a function of the intermediate band position in the energy gap and the host semiconductor energy gap. Generally the optimal intermediate band level decreases within the energy gap to mitigate the carrier losses, and carrier losses are less detrimental to small energy gap materials. We therefore focus on the role of carrier losses in wide bandgap semiconductor intermediate band solar cell systems, such as the GaN semiconductor with an Mn impurity band. Experimentally Mn acceptor level in the GaN energy gap is 1.8 eV above the valence band, which is 199 meV off the ideal intermediate band and reduces the efficiency to 21.36%. We demonstrate how carrier losses can be introduced into the system to shift the optimum IB position. Introducing carrier losses shifts the optimal intermediate band position to 1.8 eV above the valence band and increases the efficiency to 23.41%. We compare this to the effect of alloying GaN and introducing biaxial strain to shift the effective position of the Mn impurity band on the efficiency.

Molecular Doping and Trap Filling in Organic Semiconductor Host–Guest Systems

We investigate conductivity and mobility of different hosts mixed with different electron-withdrawing guests in concentrations ranging from ultralow to high. The effect of the guest material on the mobility and conductivity of the host material varies systematically with the guests’ LUMO energy relative to the host HOMO, in quantitative agreement with a recently developed model. For guests with a LUMO within ∼0.5 eV of the host HOMO the dominant process governing transport is the competition between the formation of a deep tail in the host DOS and state filling. In other cases, the interaction with the host is dominated by any polar side groups on the guest and changes in the host morphology. For relatively amorphous hosts the latter interaction can lead to a suppression of deep traps, causing a surprising mobility increase by 1–2 orders of magnitude. In order to analyze our data, we developed a simple method to diagnose both the presence and the filling of traps.

Structural, dielectric and ferromagnetic properties of nano-crystalline Co-doped SnS

Doping-induced tuning of host semiconductors properties offers an efficient way to realize the desired physical properties. In this study, we have synthesized pure and Co-doped SnS-diluted magnetic semiconductors with x Co = 0.00–0.10, by employing a very simple and low-cost chemical co-precipitation technique. The structural properties, determined through X-ray diffraction, have confirmed single-phase orthorhombic structure and have depicted nano-crystallites. Near-edge X-ray absorption fine structure spectroscopy has revealed a Co-doping-induced shift in the absorption edge towards lower energy, indicating a change in the Co oxidation state. Surface morphology observed through scanning electron microscopy indicates nano-structured surface. Dielectric properties measured by impedance analyzer (LCR meter) in the frequency range 1 kHz–20 MHz depict that grown materials respond to the electromagnetic radiations suggesting potential device applications. Complex impedance spectroscopy illustrates the dominance of grain resistance. The magnetic properties observed by vibrating sample magnetometer reveal room temperature ferromagnetism, and Co ions inducing tuning of the ferromagnetism suggests potential applications in the data storage devices.

Chemically Filled and Au-Coupled BiSbS3 Nanorod Heterostructures for Photoelectrocatalysis

The synergistic effects of foreign ion incorporation into a semiconductor host crystal lattice can produce new functional properties. This concept has been adopted in the design of various energy materials for light harvesting, charge transport, and energy storage applications. Going beyond the traditionally used group II–VI and III–V semiconductor nanostructures, herein, 1D materials involving Bi(III) and Sb(III) sulfides are reported. Upon Sb dilution into the Bi2S3 lattice, exciting new material properties, including induction of localized surface plasmon resonance (LSPR) and a drastic change to the 1D crystal growth pattern, were observed. The presence of the Sb(III) precursor along with Bi(III) led to nanotubes with controlled length as the ultimate product, and their transformation to nanorods via chemical filling emerged as a new fundamental mechanism of crystal growth. Due to its slow thermal decomposition rate, the Sb(III) precursor dominantly filled these tubes, resulting in graded alloy Bi1.09S...

Resonant optical control of the spin of a single Cr atom in a quantum dot

A Cr atom in a semiconductor host carries a localized spin with an intrinsic large spin to strain coupling particularly promising for the development of hybrid spin-mechanical systems and coherent mechanical spin driving. We demonstrate here that the spin of an individual Cr atom inserted in a semiconductor quantum dot can be controlled optically. We first show that a Cr spin can be prepared by resonant optical pumping. Monitoring the time dependence of the intensity of the resonant fluorescence of the quantum dot during this process permits to probe the dynamics of the optical initialization of the Cr spin. Using this initialization and read-out technique we measured a Cr spin relaxation time at T=5 K of about 2 microseconds. We finally demonstrate that, under a resonant single mode laser field, the energy of any spin state of an individual Cr atom can be independently tuned by using the optical Stark effect.

Mn2+-Doped Lead Halide Perovskite Nanocrystals with Dual-Color Emission Controlled by Halide Content.

Impurity doping has been widely used to endow semiconductor nanocrystals with novel optical, electronic, and magnetic functionalities. Here, we introduce a new family of doped NCs offering unique insights into the chemical mechanism of doping, as well as into the fundamental interactions between the dopant and the semiconductor host. Specifically, by elucidating the role of relative bond strengths within the precursor and the host lattice, we develop an effective approach for incorporating manganese (Mn) ions into nanocrystals of lead-halide perovskites (CsPbX3, where X = Cl, Br, or I). In a key enabling step not possible in, for example, II-VI nanocrystals, we use gentle chemical means to finely and reversibly tune the nanocrystal band gap over a wide range of energies (1.8-3.1 eV) via postsynthetic anion exchange. We observe a dramatic effect of halide identity on relative intensities of intrinsic band-edge and Mn emission bands, which we ascribe to the influence of the energy difference between the corresponding transitions on the characteristics of energy transfer between the Mn ion and the semiconductor host.

Diluted magnetic semiconductors with narrow band gaps

We propose a method to realize diluted magnetic semiconductors (DMS) with p- and n-type carriers by choosing host semiconductors with a narrow band gap. By employing a combination of the density function theory and quantum Monte Carlo simulation, we demonstrate such semiconductors using Mn-doped BaZn2As2, which has a band gap of 0.2 eV. In addition, we found a new non-toxic DMS Mn-doped BaZn2Sb2, of which the Curie temperature Tc is predicted to be higher than that of Mn-doped BaZn2As2, the Tc of which was up to 230 K in the recent experiment.

Giant Excitonic Exchange Splittings at Zero Field in Single Colloidal CdSe Quantum Dots Doped with Individual Mn2+ Impurities.

Replacing a single atom of a host semiconductor nanocrystal with a functional dopant can introduce completely new properties potentially valuable for "solotronic" information-processing applications. Here, we report successful doping of colloidal CdSe quantum dots with a very small number of manganese ions-down to the ultimate limit of one. Single-particle spectroscopy reveals spectral fingerprints of the spin-spin interactions between individual dopants and quantum-dot excitons. Spectrally well-resolved emission peaks are observed that can be related to the discrete spin projections of individual Mn2+ ions. In agreement with theoretical predictions, the exchange splittings are enhanced by more than an order of magnitude in these quantum dots compared to their epitaxial counterparts, opening a path for solotronic applications at elevated temperatures.

Large Stark tuning of donor electron spin qubits in germanium

Donor electron spins in semiconductors make exceptional quantum bits because of their long coherence times and compatibility with industrial fabrication techniques. Despite many advances in donor-based qubit technology, it remains difficult to selectively manipulate single donor electron spins. Here, we show that by replacing the prevailing semiconductor host material (silicon) with germanium, donor electron spin qubits can be electrically tuned by more than an ensemble linewidth, making them compatible with gate addressable quantum computing architectures. Using X-band pulsed electron spin resonance, we measured the Stark effect for donor electron spins in germanium. We resolved both spin-orbit and hyperfine Stark shifts and found that at 0.4 T, the spin-orbit Stark shift dominates. The spin-orbit Stark shift is highly anisotropic, depending on the electric field orientation relative to the crystal axes and external magnetic field. When the Stark shift is maximized, the spin-orbit Stark parameter is four orders of magnitude larger than in silicon. At select orientations a hyperfine Stark effect was also resolved and is an order of magnitude larger than in silicon. We report the Stark parameters for $^{75}$As and $^{31}$P donor electrons and compare them to the available theory. Our data reveal that $^{31}$P donors in germanium can be tuned by at least four times the ensemble linewidth making germanium an appealing new host material for spin qubits that offers major advantages over silicon.

Thermoelectric properties of semiconductor-metal composites produced by particle blending

In the quest for more efficient thermoelectric material able to convert thermal to electrical energy and vice versa, composites that combine a semiconductor host having a large Seebeck coefficient with metal nanodomains that provide phonon scattering and free charge carriers are particularly appealing. Here, we present our experimental results on the thermal and electrical transport properties of PbS-metal composites produced by a versatile particle blending procedure, and where the metal work function allows injecting electrons to the intrinsic PbS host. We compare the thermoelectric performance of composites with microcrystalline or nanocrystalline structures. The electrical conductivity of the microcrystalline host can be increased several orders of magnitude with the metal inclusion, while relatively high Seebeck coefficient can be simultaneously conserved. On the other hand, in nanostructured materials, the host crystallites are not able to sustain a band bending at its interface with the metal, becoming flooded with electrons. This translates into even higher electrical conductivities than the microcrystalline material, but at the expense of lower Seebeck coefficient values.

Sol-gel auto combustion synthesis, electrical and dielectric properties of Zn1−xCoxO (0.0 ≤ x ≤ 0.36) semiconductor nanoparticles

In the present work, Co2+ ion doped zinc oxide nanoparticles Zn1−xCoxO (x = 0.00, 0.06, 0.12, 0.18, 0.24, 0.30 and 0.36 mol) were synthesized by sol-gel auto combustion method. The effects of heavily doped Co2+ ion concentration on the structural, electrical and dielectric properties were studied. The prepared nanoparticles were characterized by X-ray diffraction technique (XRD), transmission electron microscopy (TEM), selected area diffraction pattern (SAED) and Fourier transform infrared spectroscopy (FT-IR). The X-ray diffraction analysis revealed the formation of single phase having hexagonal wurtzite structure along with secondary phase (Co3O4). TEM analysis clearly showed the small agglomeration and spherical shape of nanoparticles. SAED pattern also confirms the hexagonal wurtzite structure with single crystalline nature. FTIR analysis showed the vibrational frequency band position of Zn–O shifted to higher frequency band with Co2+ ion increasing host semiconductor nanoparticles. The temperature dependent (300–400 K) DC resistivity of samples was studied by the standard two probe method. DC electrical resistivity was found to increase with increasing Co2+ content in ZnO matrix. The dielectric properties of pure and Co2+ doped ZnO nanoparticles were studied as a function of frequency and composition using the LCR–Q meter. All the dielectric parameters show dispersion and decreases with increase in Co2+ content. The observed dielectric behavior is explained on the basis of Maxwell–Wagner model and Koops phenomenological theory.