Table of contents

Semiconductor microcavities, in which strong coupling of excitons to confined photon modes leads to the formation of exciton–polariton modes, have increasingly become a focus for the study of spontaneous coherence, lasing and condensation in solid state systems. This review discusses the significant experimental progress to date, the phenomena associated with coherence which have been observed and also discusses in some detail the different theoretical models that have been used to study such systems. We consider both the case of non-resonant pumping, in which coherence may spontaneously arise, and the related topics of resonant pumping, and the optical parametric oscillator.

The effect of high-κ dielectric HfO₂ films on 200 mm diameter p-type silicon substrates was investigated and compared with conventional dielectric material, SiO₂. We employed all-optical characterization methods using terahertz (THz) time-domain spectroscopy and visible cw pump/THz probe spectroscopy. Measurements were performed on two sets of samples, each set containing both HfO₂ and SiO₂ coated wafers with varying thickness of oxide layer. One set had a protective coating of either photoresist or Si₃N₄ deposition above the oxide layer which in turn was on a heavily doped p+ layer on p-type silicon. The samples exhibited similar linear transmission in the THz frequency range studied. However, when using an Ar+-ion laser as an optical pump source, differential measurements (with and without the pump source) showed that the transmission was far smaller for the high-κ dielectric HfO₂ films than its conventional dielectric SiO₂ counterparts, indicating the presence of large photo-generated carrier density in the p-type substrate.

A super-junction (SJ) SOI-LDMOS with a step doping surface-implanted n-type layer is proposed and optimized which allows high breakdown voltage (BV) and low on-resistance (R_on). The proposed structure overcomes the field effect action in conventional SJ SOI-LDMOS devices, thus achieving the charge compensation between the n and p pillars as well as a near uniform electric field distribution in the drift region in the off-state. The surface-implanted layer also provides a low current path in the on-state. In addition, the analysis result of surface doping distribution is obtained which is a power tool for device designers to provide accurate first-order design schemes. The simulation results show that an increase in the off-state BV by 55% and a reduction of the specific on-resistance by 37.4% are obtained for the proposed device when compared with those of the conventional one.

We investigate the electron spectrum and optical response of quantum dots (QDs) produced from narrow-band semiconductors with the inverted band structure (the negative sign of the energy gap). It is shown that in addition to usual size-quantized states which are located above the gap edge in normal band materials, there exist states residing inside the gap (analogous of the Tamm states) in inverted band structures. Such states manifest themselves in the far infrared part of emission and absorption spectra, measurement of which allows us to extract information about the QD surface.

Comprehensive and systematical comparisons of temperature-dependent characteristics of In_0.42Al_0.58As/In_0.46Ga_0.54As metamorphic heterostructure field-effect transistors (MHFETs) with various Schottky gate alloys are studied and demonstrated. The influence of the Schottky barrier height on the impact ionization effect and its associated device performance are also investigated. Better dc and microwave characteristics can be obtained by using the higher metal work function of gate alloys, e.g., Ti/Au, Ni/Au and Pt/Au. In particular, the device with a Pt/Au gate alloy shows the superior device performance in breakdown voltage, threshold voltage, maximum transconductance, output conductance, voltage gain and microwave properties at room temperature. Furthermore, the device with a Ti/Au gate alloy shows the thermally stable performance in threshold voltage, maximum transconductance, output conductance and voltage gain over a wide operating temperature range (from 300 to 510 K). Consequently, the studied devices with appropriate Schottky gate contacts provide the promise for high-speed and high-temperature electronic applications.

The suitability of nanoscale non-planar FinFETs and classical planar single and double gate SOI MOSFETs for rf applications is examined via extensive 3D device simulations and detailed interpretation. It is shown that although nanoscale FinFETs achieve higher values of intrinsic dc gain (nearly 20 dB higher than planar SG devices), they also present higher gate capacitance that severely undermines their rf performance. We also show that at large values of drain currents, well-designed conventional planar single and double gate SOI MOSFETs attain higher values of cut-off frequency compared to FinFETs, whereas at lower drain currents, a well-aligned planar double gate SOI MOSFET is the optimal structure. The reason for higher parasitic capacitance in FinFETs as compared to planar MOSFETs is examined in detail. An assessment of the impact of back gate misalignment on the rf performance of a 25 nm gate length planar double gate MOSFET indicates that a misalignment of 12 nm towards the source end is acceptable to give superior performance to a FinFET. The importance of source/drain extension region engineering in nanoscale FinFETs for ultra-low voltage analogue applications is also investigated. RF figures of merit for planar and vertical MOS devices are also compared based on layout-area calculations. The paper provides valuable design insights for optimizing device parameters for nanoscale planar and vertical MOSFETs.

The extraction of the equivalent circuit parameters for bipolar transistors is undertaken via high-frequency S-parameter and low-frequency noise measurements. The extraction is exhaustively detailed here for a double purpose. First, it highlights the not-so-justified approximations so often made for the extraction of the extrinsic base–collector capacitance. Second, it emphasizes the similarity of the values obtained for series resistances in both the low-frequency and high-frequency ranges.

Bloch-like surfaces that describe all possible spin states and, therefore, can be used to visualize spin precession and spin dynamics in external fields are presented for HgTe, a material with inverted energy bands. The spin and orbital surfaces for both free electrons and holes are shown, in general, to be spheroids, with the rotational axis parallel to a charge-carrier wave vector. The lengths of the principal axes of the spheroids depend on the energy band index and wave vector magnitude. At some wave vectors the surfaces may reduce to a line or a disc in the spin space. The corresponding graphs are presented.

We examine effects of free-carrier absorption produced by non-equilibrium electrons and holes injected in the waveguide on characteristics of high-power AlGaAs-based laser diodes emitting light at 808 nm. The carrier transport in the laser heterostructures is studied theoretically, using the drift-diffusion numerical approach. On the basis of simulations, a relation between the current density and non-equilibrium carrier concentrations in the waveguides is found. The internal optical losses of the waveguide modes and their effect on the differential quantum efficiency of the laser diodes are estimated from the computed carrier concentrations. Some approaches aimed at reducing the free-carrier absorption and, thus, improving the laser performance are discussed. The theoretical predictions are compared with available data to validate the theoretical model and justify the conclusions coming from the simulations.

Metal organic chemical vapour deposition (MOCVD) grown n-type GaN on sapphire substrates was irradiated with 100 MeV Au⁷⁺ ions, varying the fluence as 1 × 10¹², 1 × 10¹³ and 5 × 10¹³ ions cm⁻² at room temperature. The irradiated samples were characterized by x-ray diffraction (XRD), atomic force microscopy (AFM), Raman scattering, photoluminescence and UV–visible optical absorption spectrum. XRD analysis reveals a huge lattice disorder for fluence beyond 1 × 10¹³ ions cm⁻². This is observed from the increase in the FWHM and decrease in the intensity of the GaN (0 0 0 2) peak. The sapphire peak totally vanishes for higher fluence. Raman modes E₂ (high) and A₁ (LO) are observed at 568 cm⁻¹ and 749 cm⁻¹ respectively. For the fluence of 1 × 10¹² ions cm⁻² the E₂ (high) shifts to lower frequency and a further increase in fluence results in a broader less intense E₂ mode. Additional modes around 100 cm⁻¹ and 220 cm⁻¹ were observed and the origin of those modes is discussed. We also observed the Boson peak, the intensity of which increases as the fluence increases. AFM images show the roughness of GaN increases with increasing ion fluences. At a fluence of 5 × 10¹³ ions cm⁻², a new type of ditch and dike structure was observed. These structures were distributed over the irradiated GaN surface. UV–visible absorption spectrum shows that the band gap energy decreases with increasing ion fluences and also that the band edge is broadened. The results are discussed by the complementary use of these studies.

High-quality C-doped GaN buffers grown on sapphire substrates were employed for the fabrication of high-power AlGaN/GaN heterojunction field effect transistors (HFETs). The fabricated device exhibited a very high breakdown voltage (BV) over 1350 V and low specific on-resistance (AR_DS(ON)) of 3.4 mΩ cm². This result is very close to the 4H-SiC limit and a record achievement for GaN-based HFETs realized on sapphire substrates, to the best of our knowledge.

AlGaN/GaN high electron mobility transistors (HEMTs) using HfO₂ as a surface passivation layer and metal–oxide–semiconductor HEMTs (MOS-HEMTs) using HfO₂ as gate oxide have been investigated and compared with the regular HEMTs. In MOS-HEMTs, the HfO₂ gate dielectric is also used for passivation simultaneously. Our measurements have shown that both passivated HEMTs and MOS-HEMTs outperformed the regular HEMTs in dc, high-frequency and pulsed-mode operations, with MOS-HEMTs exhibiting the best characteristics, including the highest drain current, the lowest gate leakage current, the largest gate voltage swing, the highest cut-off frequencies and the best immunity to current collapse. In addition, the decrease in transconductance of MOS-HEMTs relative to HEMTs is as low as 8.7%, most probably a consequence of the high-k value of HfO₂. Our results thus indicate the great potential of HfO₂/AlGaN/GaN MOS-HEMTs for high-frequency and high-power applications.

Experimental photoluminescence (PL) spectra and structural properties of the ultrathin InGaN insertions in an AlGaN matrix grown on a sapphire substrate were investigated. The PL emission mechanism was shown to be governed by the quantum dot (QD)-like indium-rich areas with size about 3 nm, which was determined from high resolution transmission electron microscopy image analysis. The local indium composition in QDs of the samples was estimated as about 26% using the suggested model of PL transition energy, which accounts for quantum confinement energy, spontaneous and piezoelectric fields.

The frequency dependence of the capacitance in accumulation of the metal–Ta₂O₅/SiO₂–Si structures was studied in the frequency range from 20 Hz to 1 MHz. The capacitances at low and high frequencies differ significantly even in accumulation where the interface states do not affect the measurement. The experimentally observed frequency dependence was explained by the different conductivities of the insulating layers, the bulk Ta₂O₅ and the interfacial SiO₂, due to the difference in the corresponding conduction mechanisms. A five-element equivalent circuit model was proposed for explaining the observed dependences. A qualitative agreement of the theoretical to the experimental results is observed almost over the entire measurement range.

The deep levels in semi-insulating CdTe crystals grown by vertical gradient freezing technique were studied using thermoelectric effect spectroscopy (TEES) and photo-induced current transient spectroscopy (PICTS). The measurement of TEES spectra in the temperature range 90–400 K was performed at different heating rates. The positions of the levels and their capture cross-sections were obtained by using the heating rate method. It was found that near midgap levels in samples doped with shallow donors (Cl, In) have a low value of capture cross section (≈10⁻¹⁵ − 10⁻¹⁷ cm²) and are hole traps. Samples doped with a deep donor (Ge, Sn) have a much higher capture cross section of the midgap level (≈10⁻¹³ cm²), which acts as an electron trap. Comparison of the TEES results with the PICTS method has shown that while the evaluated values of ionization energies are comparable using both methods, the PICTS technique gives systematically approximately two orders of magnitude higher capture cross sections than TEES.

Hall effect measurements on undoped Al_0.25Ga_0.75N/GaN heterostructures grown by a metalorganic chemical vapour deposition (MOCVD) technique have been carried out as a function of temperature (20–350 K) and magnetic field (0–1.5 T). Magnetic field dependent Hall data were analysed using the quantitative mobility spectrum analysis (QMSA) technique. The mobility and density within the two-dimensional electron gas (2DEG) at the Al_0.25Ga_0.75N/GaN interface and within the underlying GaN layer were successfully separated by QMSA. Mobility analysis has been carried out using both the measured Hall data at a single field and the extracted data from QMSA. Analysis of the temperature-dependent mobility of 2DEG extracted from QMSA indicates that the interface roughness and alloy disorder scattering mechanisms are the dominant scattering mechanisms at low temperatures while at high temperatures only polar optical phonon scattering is the dominant mechanism. Al_0.25Ga_0.75N/GaN interface related parameters such as well width, deformation potential constant and correlation length were also accurately obtained from the fits of the simple analytical expressions of scattering mechanisms to the 2DEG mobility.

This paper reports on the device characteristics of strain-compensated multi-quantum wells (SC-MQWs) GaAsN/InGaAs single junction solar cell based on this very promising InGaAs material. Limitations of lattice mismatch could be overcome because the compressive strain in the InGaAs layers matches the tensile strain in the GaAsN layers. GaAsN/InGaAs SC-MQWs solar cells, lattice matched to GaAs, are proposed as a means of extending the long-wavelength absorption to make a competitive candidate in a cascade solar cell structure. We demonstrated that this GaAsN/InGaAs SC-MQWs solar cell could achieve a high short-circuit current density of about 20.87 mA cm⁻², under AM1.5G illumination, which is higher than that of the InGaNAs cell. The GaAsN/InGaAs SC-MQWs structures show many of the characteristics required to make it a candidate for the next generation of multi-junction solar cells, which means this design can be used as the third junction in future generation ultrahigh-efficiency three- and four-junction devices.

Highly efficient orange organic electroluminescent devices based on Re(CO)₃Cl-bathophenanthroline have been fabricated. A device with 9 wt% shows the highest efficiencies of 13.8 cd A⁻¹ (luminance efficiency), 8.69 lm W⁻¹ (power efficiency) and 5.24% (external quantum efficiency). Maximum luminance over 4000 cd m⁻² is obtained. By discussing the mechanisms, it is believed that trapping contributes mostly to these relatively much higher efficiencies.

Using measured amplified spontaneous emission data, we have derived and analysed the carrier distribution of a five-layer tunnelling injection quantum-dot structure at temperatures of 300 K and 350 K. The results are consistent with the direct injection of electrons from the injector well into a subset of lower energy dot states. The carrier distribution spectra contain features which suggest that dots of a particular size within the ensemble are preferentially populated leading to a reduced spectral broadening of the emission.

Continuous chips removed by single-point diamond turning of single crystal silicon have been investigated by means of scanning electron microscopy/transmission electron microscopy and micro-Raman spectroscopy. Three different chip structures were probed with the use of electron diffraction pattern: (i) totally amorphous lamellar structure, (ii) amorphous structure with remnant crystalline material and (iii) partially amorphous together with amorphous with remnant crystalline material. Furthermore, micro-Raman spectroscopy from the chips remained in the cutting tool rake face depicted different silicon phases. We have found, from a detailed analysis of the debris, five different structural phases of silicon in the same debris. It is proposed that material removal mechanisms may change along the cutting edge from shearing (yielding lamellar structures) to extrusion. Shearing results from structural changes related to phase transformation induced by pressure and shear deformation. Extrusion, yielding crystalline structures in the chips, may be attributed to a pressure drop (due to an increase in the contact area) from the tool tip towards the region of the cutting edge where the brittle-to-ductile transition occurs. From this region upwards, pressure (stress) would be insufficient to trigger phase transformation and therefore the amorphous phase would not form integrally along the chip width.

We have fabricated polysilicon thin film transistors (TFTs) using sputtered HfO₂ gate dielectric and SiGe as source/drain. The polysilicon film is formed by low temperature furnace recrystallization of amorphous silicon. The effective dielectric constant of the HfO₂ in the TFTs is 17.7, and the transconductance of the TFTs is about 4.5 times that of those using SiO₂ as gate dielectric. The use of SiGe as source/drain requires a lower thermal budget than the conventional technique of impurity implantation in the polysilicon film.

In this work, the recessed source/drain (ReS/D) ultra-thin body (UTB) SOI MOS transistor is investigated in detail. Results reveal that the ReS/D structure provides UTB devices with much lower source/drain series resistance than the elevated source/drain (E-S/D) one and thereby alleviates the critical requirement for low contact resistivity of sub-50 nm generations. On the other hand, the ReS/D devices do not exhibit the degraded short channel effect immunity compared to the E-S/D devices. The design guidelines and window for the ReS/D devices are also suggested in terms of the simulation results. It is thus demonstrated that the ReS/D approach is capable of accelerating SOI device scaling down to 10 nm node.