Quantum confinement in superlattices affects shallow levels and band edges considerably (length scale of order 100 Å), but not deep levels (length scale of order 5 Å). Thus by band-gap engineering, one can move a band edge through a deep level, causing the defect responsible for the level to change its doping character. For example, the cation-on-anion-site defect in AlxGa1−xSb alloys is predicted to change from a shallow acceptor to a deep acceptor-like trap as the valence band edge passes through its T2 deep level with increasing At alloy content x. In a, Type-II superlattice, such as InAs/AlxGa1−xSb for x>0.2, where the conduction band minimum of the InAs should lie energetically below the antisite defect's T2 level in bulk AlxGa1−xSb, the electrons normally trapped in this deep level (when the defect is neutral) remotely dope the InAs n-type in the superlattice, leaving the defect positively charged. Thus a native defect that is thought of as an acceptor can actually be a donor and control the n-type doping of InAs quantum wells. The physics of such deep levels in superlattices and in quantum wells is summarized, and related to high-speed devices.

The study of electronic properties of GaAs/AlGaAs quantum wells (QWs) has traditionally been focused on intrinsic phenomena, in particular the free exciton behaviour. Defects and impurities have often been regarded as less relevant compared to the case of bulk semiconductors. Doping in QWs is important in many applications, however, and recently the knowledge about the structure of shallow donors and acceptors from optical spectroscopy has advanced to a level comparable to the situation in bulk semiconductors. A dramatic difference from the bulk case is the common occurrence of localisation effects due to interface roughness in QW structures. The recombination of bound excitons (BEs) differs drastically from bulk, BE lifetimes decrease with decreasing well thickness Lw, but increase with decreasing barrier thickness Lb (at constant Lw) below Lb=70Å. Exciton capture at impurities is a process which is strongly influenced by the localisation potentials from the interface roughness. The recombination process in doped QWs involves a nonradiative component, for shallow acceptors an excitonic Auger process has been identified. Deep nonradiative defects in the (MBE grown) QW as well as in the barrier material are manifested in measurements of the PL decay time vs temperature. In undoped multiple QWs the decay times vs T are consistent with thermal emission out of the well into the barrier, where nonradiative recombination via deep level defects occur. Nonradiative recombination in the well itself can be studied in electron-irradiated structures. Preliminary data also demonstrate the feasibility of hydrogen passivation of dopants as well as deep levels in the QW structures.

Cathodoluminescence spectroscopy is used to identify diffusion-associated III-V semiconductor defects and establish their role in AlGaAs/GaAs intrinsic and n-type impurity induced interdiffusion (Si, Ge, S, and Se) for various ambient conditions, As- and Ga-rich. These identifications involves the study of the temperature and composition dependence of these deep levels and their correlation with theoretical calculations. Our results reveal Column III vacancies and their complexes as the sole mediators of diffusion.

The Al-Ga inter-diffusion induced by Si FIB implantation and subsequent RTA were investigated in an Al0.3Ga0.7As/GaAs superlattice with equal 3.5 nm barrier and well widths. Si++ was accelerated to 50 kV as well as 100kV and implanted parallel to sample normal at doses ranging from 1013 to 1015/cm2. The effect of rapid thermal anneal of 10s at 950°C was characterized by SIMS technique. In the implanted region, the inter-diffusion as well as compositional mixing were significantly enhanced. An ion dose as low as 1×1014/cm2 results in a two-order of magnitude increase in the inter-diffusion coefficient, to a value of 4.5×10-14 cm2/sec, in contrast to 1.3×10-16 cm2/sec from RTA-only. This produces a mixing parameter of σ90%. A strong depth dependence of the mixing process was observed at implantation energy of 100keV with a pinch-off region being formed at certain depth. It is noticed that the depth where this enhancement occurred is not associated with either the maximum concentration of Si ions or of vacancies. Instead, it represents the positive maximum of the second derivative of the vacancy profile, which in turn represents the vacancy injection generated by presence of a transient vacancy concentration gradient. Based on this, a theoretical model was developed using vacancy injection as responsible for mixing.

High Resolution transmission electron microscopy (HRTEM) and electroreflectance (ER) were used to explain the role of point defects in the molecular beam epitaxy (MBE) grown PIN structure containing five coupled (50Å/28Å) GaAs/AlxGa1−xAs quantum wells with x = 0.25. The ER data were taken at 300K and 77K for energies from 1.4 to 2.1 eV from which sub-band energy transitions were determined. Data at 300K showed three transitions whereas four were readily resolved at 77K. HRTEM data determined the uniformity of both the wells and barriers to be within ± 2Å, which neither caused appreciable broadening nor a decrease in the transition probability. However, the data at different temperatures suggest that point defects may be responsible for the decrease in the transition probability.

Identifying and understanding the effects of impurities and defects in quantum dots, wires, and multiple wells is important for the development of nanostructures with good optical properties. Simple model calculations are presented to show when and how shallow impurities affect the radiative recombination of confined electron-hole pairs. Results for nanostructures are compared with results for bulk systems. Qualitative differences between bulk and confined systems are described.

Due to the reduced size of quantum devices, defects play a dominant role in their electrical behaviour. We theoretically show that defects can change the electrical behaviour of a single barrier or a quantum well. For the barrier and the well, we have performed a quantum calculation of a defect assisted tunneling effect based on the WKB theory to describe the wave functions and on the Oppenheimer approach for the determination of the tunneling probability. The main effects are : appearance of a negative differential resistance (NDR) in the currentvoltage characteristics (I-V) of the single barrier and an apparent lowering of the band offset of the quantum well. In the latter case, we have experimentally measured this effect on an AlInAs-GaInAs-AlInAs well, using admittance spectroscopy. The apparent lowering is associated with a native defect located in the barrier, a defect studied using the DLTS technique.

The effects of low-energy electron irradiation on the two-dimensional electron gases (2DEG's) in AlGaAs/GaAs heterostructures have been investigated. Not only the electron mobility of the 2DEG's but also the two-dimensional (2D) carriers are found to be reduced by the electron irradiation with the incident energies between 3.5 k and 8 keV and the electron dose of 1×1016 and 1×1017 /cm2. The degraded mobility and the removed carriers by the low-energy electron irradiation are shown to recover by isochronal annealing to some extent, but not completely below 400°C. It is also found that considerable amount of scatterers which are created by an electron irradiation at room temperature are also created by an irradiation at 90 K. Comparing the experimental results with the Monte Carlo simulation, we speculate that the mobility degradation and the 2D carrier compensation are partly caused by the formation of complex defects in the GaAs buffer layer which are due to the excitations of core electrons of As, and that the mobility is further degraded by the formation of short-range scatterers in the heterointerface.

Magnetooptical studies have been performed on the shallow Be acceptor confined in the central region of narrow GaAs/AlGaAs quantum wells (QWs) with the magnetic field along the growth direction. The magnetic field dependence of the acceptor transition between the 1S(Г6) hh-like ground state and the excited hh-like 2S(Г6) state has been investigated by means of two independent techniques: Two-hole transitions of the acceptor bound exciton (BE) and resonant Raman scattering. The 1S(Г6) – 2S(Г6) transition energy as a function of the magnetic field has been measured for central acceptors in QWs of widths in the range 50 – 150 Å. The energy levels for the 1S ground states and 2S excited states of the confined acceptor with a magnetic field as a perturbation have also been calculated. These calculations predict a larger splitting between the mj=+3/2 and mj=−3/2 components of the acceptor 1S(Г6) ground state in comparison with the corresponding splitting of the excited 2S(Г6) state. The experimental results are in good agreement with the theoretical predictions derived without any fitting parameters. Furthermore, the Zeeman splitting of the acceptor BE emission has been measured and it is concluded that the J = 5/2 BE state is lowest in energy, similar to shallow acceptor BEs in bulk GaAs.

Far infrared spectroscopic studies of electron-density- and magnetic-field-dependenceof manyelectron effects on silicon donor impurities confined in GaAs quantum wells are presented. At low excess electron densities, transitions from D− singlet and triplet states are observed. Temperature- and polarization-dependence measurements show that the relative absorption strengths of various spectroscopic features (D0, D−, and CR) are in qualitative agreement with a statistical calculation in thermal equilibrium in high magnetic fields. At large excess electron densities, the “D−” transition energy shifts to higher energy when electron density is increased, and the magnetic-field dependence of the transition energy exhibits discontinuities in slope at integer filling factors. The spectroscopic features and their relation to excess free carriers and the role of screening and correlation are presented and discussed.

Quantum Wires Structures Were Fabricated By Patterning Quantum Well Samples With Electron Beam Lithography And Various Wet Chemical Etching Procedures. Wire Structures With 800Å Wire Width Were Achieved By Wet Etching In Nh4Oh / H2O2 / H2O (20:7:973). These Samples Were Characterized By Scanning Electron Microscopy (Sem), Photoluminescence (Pl), And Polarization-Dependent Photoluminescence Excitation (Ple) Measurements. The Pl Spectra Show Significantly Strongpr Peaks Than That Taken From An En-Etched Quantum Well Sample. A Wire Width Of 400Å Was Estimated From The Blue Shift Of Pl Peaks. A 22% Anisotropy Was Observed From Polarization-Dependent Ple Spectra, Further Corroborating The Existence Of Two-Dimensional Quantum Confinement.

We investigate the occurrence of impurity density of states of hydrogenic impurities placed on the axis of a cylinder quantum-well wire of GaAs/GaAlAs structure. The effects of disorder are taken into account in the calculation. It is shown that for a specific radius of the wire, the peak energy of the density of states as a function of the impurity concentration increases very fast enlarging the bandwidth. The impurity bands are considered for the observed binding energy and radius as well as for the impurity concentration of experimental interest.

Recently, it was found that semi-insulating behavior of undoped InP can be realized by high pressure annealing of undoped high purity InP. In the present work, studies related with the achievement of the semi-insulating state are reviewed. Purification of raw materials, effect of native defects, effect of high pressure annealing, contamination of Fe are discussed. The semi-insulation mechanism is explained by the Shockley diagram. The semi-insulating state is supposed to be achieved by the annihilation of shallow donors (presumably phosphorus vacancies) and the compensation of the residual donors with a small amount of Fe deep acceptor.

Low temperature, resonantly excited photoluminescence (PL) has proven to be the method of choice for impurity identification in GaAs. InP has suffered from insufficient impurity binding energy data to benefit similarly. We will report results of selectively-excited donor-acceptor pair spectroscopy for acceptor identification in InP. Ion implantation doping of high purity InP is used for generation of known impurity samples. Progress toward a complete database of acceptor binding energies in InP is reported. We will discuss the results of high magnetic field low temperature resonant photoluminescence spectroscopy for donor identification in InP. The success of donor ion implantation studies will be included. This data should provide direction for efforts in growing high purity InP by MOCVD and gas source MBE.

>100> -semiinsulating InP was implanted with 600 keV Se-ions at temperatures between 300K and 425K with an ion dose of 1 ×1014 cm−2. After capping the samples with about 120 nm siliconoxynitride annealing was performed at 700°C up to 975°C using a graphite strip heater system. The annealed samples were analyzed with Rutherford backscattering, electron microscopy and conventional Hall measurements. The results show, that a strong correlation exists between defects remaining after annealing (for instance dislocations, loops, microtwins) and the measured electrical properties. An implantation temperature ≦ 395K and annealing at least at 800°C for 50 s is necessary to obtain high performance electrically active layers. The activation of selenium in InP can be well described using a simple thermodynamical model. The model yields an activation energy of EA = (1.0 ± 0. 1) eV which can be understood as the energy necessary to split-up selenium-vacancy-complexes and a diffusion energy of Ed = (2.0 ± 0.2) eV representing material transport of the semiconductor material.

Deep levels in MOCVD grown p-InP on GaAs substrates have been investigated by Deep Level Transient Spectroscopy (DLTS). The effect of hydrogenation on the electrical activity of these levels has been studied through a combination of DLTS and Photoluminescence (PL) measurements. DLTS measurements indicate a drop of trap density from σ 5 × 1014 cm−3 to σ 1 × 1012 cm−3 after hydrogenation. Annealing at 400°C reactivated only the dopants, while temperatures above 600°C were necessary for deep-level reactivation. This combined with a logarithmic dependence on fill pulse time, indicate that at least one broad DLTS peak is associated with dislocations. The PL the DLTS results show that the dislocation related traps are passivated by hydrogen, preferentially over the dopants and that a wide annealing window exists for dopant reactivation.

Spontaneously laterally ordered (GaP)2/(InP)2 short period superlattices (SPS) grown by Molecular Beam Epitaxy (MBE) on nominal (100) GaAs substrates have been studied by photoluminescence (PL) spectroscopy. The samples studied included SPS comprising 110 pairs of (GaP)2/(InP)2 (total thickness σ90 nm) and multiquantum well structures in which quantum wells comprising 12 pairs of (GaP)2/(InP)2 SPS layers (thickness σ10 nm) are alternated with lattice-matched GaInP random alloy barrier layers. The 5K PL spectra include a σ1760 meV nearband edge band, and a much broader, lower energy (σ1670 meV) luminescence band that exhibits an unusual fatiguing behavior; its intensity diminishes monotonically during continuous illumination by the exciting light. This fatigued PL state is metastable at low temperatures. In the quantum well structure, although the relative intensity of the lower energy band is significantly weaker in comparison to the higher one, the fatiguing behavior still exists. However the fatiguing rate is slower in quantum well structures than that observed in the thick SPS film.

Deep levels in lattice matched Ga0.51In0.49P/GaAs heterostructure have been investigated by thermal-electric effect spectroscopy(TEES) and temperature dependent conductivity measurements. Four samples were grown by molecular beam epitaxy with various phosphorus (P2) beam equivalent pressure(BEP) of 0.125, 0.5, 2, and 4×10−4 Torr. We report for the first time, to our knowledge, an electrical observation of phosphorus vacancy point defects in the GaInP/GaAs material system. The phosphorus vacancies, Vp. behave as an electron trap which is located at EC−0.28±0.02 eV. We have found that this trap dominates the conduction band conduction when T> 220K, and is responsible for the variable-range hopping conduction when T < 220K. Its concentration decreases with the increasing phosphrous BEP. Successive rapid thermal annealing showed that its concentration increases with the increasing annealing temperature. Another electron trap at EC−0.51eV was also observed only in samples with P2 BEP less than 2×10−4 Torr. Its capture cross section is 4.5×10−15 cm 2 as obtained from the illumination time dependent TEES spectra.

Deep levels and luminescence in SiGe/Si heterostructures and quantum wells have been investigated. We have studied the effects of Be- and B-doping on the luminescent properties of Si1−xGex/Si single and multiquantum wells. No new levels, or enhancement of luminescence, from that in undoped samples, is detected in samples which are selectively doped in the well-regions, implying that the observed luminescence in the undoped quantum wells is a result of alloy disordering. Slight enhancement of luminescence is observed in disordered wells and in quantum wires made by electron beam lithography and dry etching. Deep levels have been identified and characterized in undoped Si1-xGex alloys. Hole traps in the p-type layers have activation energies ranging from 0.029-0.45 eV and capture cross sections (σ∞ ) ranging from 10−9 to 10−20 cm2. Possible origins of these centers are discussed. Some possibilities of obtaining enhanced electro-optic coefficients in SiGe/Si heterostructures are discussed.

Carrier trapping and recombination activity have been studied with DLTS and EBIC in RTCVD grown compositionally graded Ge0.3Si0.7/Si heterostructures. DLTS peak height is found to vary with applied bias, and the bias conditions used indicate that at least one peak is present in the homoepitaxial Si buffer layer and perhaps the substrate as well. Variations in EBIC contrast as a function of reverse bias, and DLTS fill pulse experiments both indicate that the DLTS peaks observed are dislocation related. Moreover, the bias dependent decrease in DLTS peak height is observed to occur at different rates for different peaks, indicating a possible connection between certain DLTS peaks and dislocation orientation or type. Activation energies of one electron trapping center and one hole trapping center add up to roughly the expected bandgap in a relaxed GexSi1−x, alloy with x ≦ 0.3, indicating that the electron and hole trapping centers observed with DLTS may, in fact, be associated with the R-G center observed by EBIC.