Si - doped semiconductor structures have been of great interest because of their technological utility in electronic and photonic devices [1,2]. In these structures, a layer of Si atoms provides electron and gives rise to quantum subbands. By this means, a two-dimensional electron gaz can be obtained by planar doping of GaAs at high donors concentration. Hence there is great interest in a good un- derstanding of Si - doped as a representative example of those devices. Theoretical studies of the above sys- tems usually neglect possible effects of disorder due to the random distribution of impurities in order to simplify the analysis. Indeed, currently available techniques allow for an optimal control of the growing heterostructures, thus justifying the assumption that the ionized impurity atoms are homogeneously distributed inside the - doped layers. This approximation has recently been shown to be correct in the high density limit . A number of re- searches have considered this limit within different ap- proaches, like the Thomas-Fermi , local density ap- proximation (LDA)  and Hartree methods . These previous works show that in the absence of external fields the Thomas-Fermi semiclassical approach is equivalent to a self-consistent formulation over a wide range of doping concentrations . The effects of applied electric field have recently been considered in the case of single and periodically Si - doped GaAs [7,8] by using a gener- alized Thomas-Fermi formalism. The electric field de- pendence of the intersubband optical absorption is also interesting for potential device applications. Intersubband
tioning the blocked-impurity band concept, 3 conversely, as an example of generator there are phosphorus or gallium impurities in silicon. 4 If a shallow impurity is placed in a QW, then its intersublevel transitions can be tuned in a con- trolled way to the desired THz frequency. It is advantageous to use ␦ doping as it prevents the extension of impurity en- ergy levels resulting from any distribution of the dopant at- oms along the growth direction of the QWs. The structures prepared in such a way seem to be very attractive for fabri- cation of the THz detectors and emitters. 5–7 In this respect, the Be ␦ -doped GaAs/ AlAs multiple quantum well 共MQW兲 system is of special interest as it gives the maximum possible confinement for the acceptor states in the valence band and, thus, the maximum possible tuning range for the dipole- allowed 1s-2p transition of the acceptor. Usually, such in- traacceptor transitions are studied at low doping densities 共⬍3 ⫻ 10 16 cm −3 兲 to avoid impurity band effects smearing
A series of Be d -doped GaAs/AlAs MQWs were grown by molecular beam epitaxy with doping at the quantum-well center, on a semi-insulating ~100! GaAs substrate in a VG V80 H reactor equipped with all solid sources. The growth of the layers was performed under exact stoichiometric condi- tion using the technique of stoichiometric low-temperature growth, 5 which ensures high quality optical materials even at relatively low growth temperatures. Under these conditions, the quantum-well structures were grown at 550 or 540 °C and without interruptions at the quantum well interfaces, which ensured negligible diffusion of the Be d layers. Prior to the growth of the MQWs a 3000 Å GaAs buffer layer was grown. Each of multiple-quantum well structures investi- gated contained a same 50 Å wide AlAs barrier, while every GaAs well layer was d doped at the well center with Be acceptor atoms. The doping level and the main characteris- tics of each sample are summarized in Table I.
Recently, the possible applications of inter-subband tran- sitions in beryllium doped GaAs/AlAs QWs for developing THz sensors and sources has stimulated extensive scientific interest. Acceptor binding energy in d-doped GaAs/AlAs multiple QWs have been studied, 23 the effect of quantum well confinement on acceptor state lifetime has been investi- gated, 24 and the dynamics of intra-acceptor level scattering have been examined using far-infrared pump-probe measure- ments. 25 Application of modulation spectroscopy has allowed estimation of internal built-in electric fields and broadening mechanisms, 26 discrimination between the origin of optical transitions below and close to the Mott transition, and demon- stration that with increasing doping level phase space filling effects dominate over the Coulomb screening. 27 Photoluminescence (PL) and time-resolved PL spectra studies have also demonstrated acceptor-impurity induced effects in the (PL) line shapes dependent on the QW widths 28 and allowed one to investigate possible mechanisms for the carrier radiative recombination, both above and below the Mott metal-insulator transition. 29
in a semi-infinite array 380 Å thick placed 400 Å behind the line ~ see the inset to Fig. 4 ! . For illustrative purposes, we have assumed that the 2.2 3 10 11 cm 2 2 electrons in the 2DEG come from impurities closest to the line, i.e., that all the impurities in the depletion region will be positively charged, though it is far from certain that this is the case in practice. The remainder of the impurities in the array have a 50% probability of being either positive or negative. It is imme- diately obvious from Fig. 4 that the potential due to the ran- dom array of both positive and negative charges ~ dotted line ! has considerably larger fluctuations than the array of positive charges in the depletion region ~ broken line ! , and that the former fluctuations are reproduced almost exactly in the total potential ~ solid line ! . This is true despite the fact that deple- tion region charges are closer than the array of mixed charges. It is also worth noting that increasing the size of the depletion region does not alter this general picture, it just increases the overall potential without, on average, changing the size of the fluctuations. Given that the fluctuations due to the array of positive charges, which are affected only by the randomness in the impurity position, are considerably smaller than the fluctuations due to mixed charges, which contains contributions from both position and charge, we conclude that the dominant scattering in modulation-doped GaAs/Al x Ga 1 2 x As heterojunctions from remote impurities is
Experimental results on reflectivity (R), PL, and PR spectra of ANA14 and IQE14 structures are given in Figure 3. Calculated PR spectra are also included in the figure. The R spectra are given just for information and not to be included in the discussion. PL signal begins to rise from the fundamental band edge of bulk GaAs and peaks at about the combined excitonic transition region. Details of the excitonic transitions are smeared out. However, in the PR spectra, the fundamental band gaps of bulk GaAs cap layer and Al x Ga 1 −x As barrier
Chapter 4 discusses the factors controlling the amphoteric incorporation of Si on various GaAs surface orientations and the results of two analytical techniques available for its experimental determination in our samples. It begins with an overview of basic semiconductor theory, describing the GaAs crystal and energy band structure, allowed energy states and the Fermi function. These concepts are then developed into relationships for carrier concentrations in undoped and doped material. Compensation in silicon-doped GaAs is described and the effect of structure and growth conditions on the silicon incorporation is commented on. This follows on to a review of how carrier mobility is affected by different scattering mechanisms present in compensated material. Hall measurements on the planar doped substrates are detailed, with both room and liquid nitrogen temperature results being presented. The liquid nitrogen temperature results are used to determine the acceptor binding energy and a variation between surfaces is evident. Lastly, analysis of the compensation ratios for n- and p-type material is undertaken.
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Nonlinear charge transport parallel to the layers of p -modulation-doped GaInNAs/GaAs quantum wells (QWs) is studied both theoretically and experimentally. Experimental results show that at low temperature, T = 13 K, the presence of an applied electric field of about 6 kV/cm leads to the heating of the high mobility holes in the GaInNAs QWs, and their real-space transfer (RST) into the low-mobility GaAs barriers. This results in a negative differential mobility and self-generated oscillatory instabilities in the RST regime. We developed an analytical model based upon the coupled nonlinear dynamics of the real-space hole transfer and of the interface potential barrier controlled by space-charge in the doped GaAs layer. Our simulation results predict dc bias-dependent self- generated current oscillations with frequencies in the high microwave range.
increases with increasing T , and therefore, the estimated carrier concentration n decreases with increasing T , as shown in the inset of Figure 2b, where d is the thickness of the measured GaPSb film. It can be observed that n changes rapidly for T < 40 K, while only a slight de- crease of n can be seen at higher T . Neither activation of carriers to the conduction band nor electron–electron interaction effects can explain such results as both would tend to decrease the Hall slope with increasing T . As suggested by our previous study on a Si delta- doped GaAs single quantum well  and the work per- formed by Yildiz et al. on Si delta-doped GaAs , the observation of non-monotonic T dependence of n , that is, it first decreases and then increases with increasing T , can be regarded as a piece of evidence that there is a crossover from VRH conduction to the activation one with increasing T since the VRH conduction would affect the estimation of Hall slope at low T and results in a reduction of n . According to our results, the decrease of n with increasing T as shown in the inset of Figure 2b, we are able to deduce that the VRH conduc- tion coexists with the activation one over the whole measurement range. To further support this argument, the two-band conduction model [20-22] is used to analyze the R ( T ) results shown in Figure 1a. In this model, one can expect that the longitudinal conductance
wash in 1% diluted HF. The wafer was then returned to the reactor for overgrowth. 100 nm p-doped GaAs was overgrown to in ﬁ ll and planarize the index-coupled DFB grating, before 600 nm n-doped GaInP ( lattice-matched to GaAs ) opto-electro- nic con ﬁ nement layer, and 20 nm of GaAs completed the overgrowth. Planarization of the gratings is important to ensure high quality GaInP can be grown upon the grating, to prevent corrugation of the waveguide and to simplify grating coupling calculation. In order to in ﬁ ll and planarize the grating, the GaAs layer was grown at a higher temperature than is typically used for GaAs. This imposes a minimum thickness limitation on the GaAs layer in order to adequately planarize the surface prior to GaInP growth. Thinner GaAs layers, such as those used pre- viously [ 2 ] and incorporated in our initial design, were defective in planar areas on test overgrowth samples. Although higher quality overgrowth was observed in the grating areas, this would not be suitable for future integrated devices, which would require components to be processed within these planar areas. Over- growth quality was signi ﬁ cantly improved by using a thicker GaAs planarization layer. A dark- ﬁ eld 002 TEM, recorded for a cross-section along the grating, is also shown in ﬁ gure 2 ( a ) , demonstrating high quality in ﬁ ll and planarization of the InGaP grating with subsequent n-doped InGaP growth above, using the modi ﬁ ed thickness of GaAs for in ﬁ ll and planarization.
-GaAs/substrate interface is opposite to that in the rest of the MBE-grown structure (see the Fig. 3). Hence, the excitation in the n + -GaAs layer and substrate (above 1.36 eV) gives an opposite PV signal to that from the QDs, WL, and buffers. The same applies to the excita- tion from EL2 defects (above 0.72 eV) of the GaAs sub- strate and especially EL2-like defects in n + -GaAs/GaAs strained region [46, 57]. Contribution of the substrate and n + -GaAs to the total PV signal is essentially stronger than that of the upper MBE layers, and the negative signal of PV is generally observed at lower excitation energies, while the impact of InGaAs layers and nanostructures appears as valleys on the respective spectral curves in Fig. 2. This is clearly seen by comparing the QDs, WL, and buffer spectral bands on the PV curves of the struc- tures contacted to MBE buffers with the valleys in spectra of the substrate-top-contacted samples. For the higher energies, however, the excitation is absorbed closer to the sample surface not reaching the deeper MBE layers and substrate, which is the main source of negative signal. Hence, the PV signal becomes positive at larger energies. So, the presence of electrically active si-substrate leads to the competition between the spectral components related
In summary, vertical energy transfer for InAs/GaAs QD pair structures with and without an AlGaAs barrier was compared. Low-temperature PL measurements show that the QD peaks shift to the blue and the relative PL intensities of the two QD layers change as a result of adding the AlGaAs barrier. In addition, the dependen- cies of the intensity ratios on excitation laser intensity and wavelength are very different with the AlGaAs bar- rier. TRPL measurements give a carrier tunneling time from the seed layer QDs to the top layer QDs of 380 ps. However, the carrier tunneling time increases to 780 ps due to the Al 0.5 Ga 0.5 As barrier. These results help in the
Deep cooling of electron and nuclear spins is equivalent to achieving polarization degrees close to 100% and is a key requirement in solid state quantum information technologies [1–7]. While polarization of individual nuclear spins in dia- mond  and SiC  reaches 99% and beyond, it has been limited to 50-65% for the nuclei in quantum dots [8–10]. Theoretical models have attributed this limit to formation of coherent ”dark” nuclear spin states [11–13] but experi- mental verification is lacking, especially due to the poor accuracy of polarization degree measure- ments. Here we measure the nuclear polarization in GaAs/AlGaAs quantum dots with high accu- racy using a new approach enabled by manipula- tion of the nuclear spin states with radiofrequency pulses. Polarizations up to 80% are observed – the highest reported so far for optical cooling in quantum dots. This value is still not limited by nuclear coherence effects. Instead we find that optically cooled nuclei are well described within a classical spin temperature framework . Our findings unlock a route for further progress to- wards quantum dot electron spin qubits where deep cooling of the mesoscopic nuclear spin en- semble is used to achieve long qubit coherence [4, 5]. Moreover, GaAs hyperfine material con- stants are measured here experimentally for the first time.
shown in Fig. 2 did not exhibit any distinct peaks that would correspond to LH and HH transitions. X-ray dif- fraction of the annealed GaAsSbN nanowires (not shown here) exhibited only (111) GaAs peak and did not exhibit any other distinct peak as reported previously  on as- grown nitride core-shell nanowires. This can be consid- ered as evidence of a lattice-matched GaAs/GaAsSbN/ GaAs core-shell structure, which suggests that any contri- bution of a strain component to the splitting of HH and LH is negligible. We, therefore, speculate and assign the additional low-temperature feature to the differences in the electron-phonon interaction with the HH and LH excitons becoming more pronounced for lower dimen- sional structures  although the energy splitting be- tween HH and LH excitons may be small.
would nonetheless be enough to block access of HF to films under 10 nm, can effectively suppress the underet- ching of the AlAs layer. In accordance with our findings, the thicker the sacrificial layer gap and the faster the HF etching rate, the less likely the ESE takes place. In the suppression regime for thinner AlAs layers, for higher intensities of illumination the photogeneration of holes is more pronounced leading to a faster subsequent passivation (and reaching of the ESE limit with time). For thicker AlAs layers, higher illumination can still lead to changes at the GaAs substrate interface  but does not hinder the etching of the AlAs.
taminations and native oxides. For environmental con- trol, the microscope was placed into a closed box with the relative humidity around 45%.The local oxide pat- terns were generated on n- and p-type GaAs(100) and GaAs(711), respectively, with a doping concentration of approximately 10 19 cm -3 , at room temperature during the experiments. The oxide structures were formed electro- chemically on the GaAs reactive surface by applying a negative bias voltage between the sample surface and the AFM probe. The electrical field was then created between the native oxide layer and the substrate, which caused the oxyanions (OH-) to drift through the oxide film [3-6]. During the AFM local oxidation in contact mode, the tip applied bias was varied in the range of 5 to 15 V and the tip loading force was modulated from approximately 60 nN to approximately 180 nN. The scan speed was fixed to 6.028 μ m/s, during the process.
Self-assembled quantum dots (QDs) have been intensively studied over the past decades in both fundamental and application fields. To date, several systems have exhibited great optical properties and find their applications, such as laser diodes  and optical detectors . The InAs/GaAs should undoubtedly be the most widely studied one among these systems. In recent years, room temperature emission of InAs QD laser around 1.3 lm for the fiber optical communication waveband  and optical absorption at 8–12 lm for the long-wavelength infrared detecting  had been achieved by means of employing a so-called dots-
We have made a preliminary estimate for the perform- ance of GaInP/GaAs/GaInNAs/Ge SC under concentrated sunlight at AM1.5D using GaInP/GaAs/Ge parameters from reference . When compared to 1-sun results, the benefit of using a GaInNAs junction starts to be signifi- cant at concentrated sunlight. We estimate that GaInP/ GaAs/GaInNAs triple-junction SCs operated at a concen- tration of 300 times have up to 3- to 6-percentage point higher efficiencies than GaInP/GaAs/Ge SCs. The situ- ation gets even more favorable for using GaInNAs when four-junction devices are considered. Our calculations show that the efficiency can be further improved by ap- proximately 3.5 percentage points compared with a GaInP/GaAs/GaInNAs triple-junction device by adding the fourth junction.
theoretically relaxes back to its own lattice constant. This is ideally how using GaP for strain balancing should work. One can see by looking at the left (under balanced) or the right (over balanced), how too little or too much GaP can immediately alter the strain energy in the stack. Since these diagrams only represent a single repeat unit of a much higher order superlattice, a layer of InAs will inevitably be grown on top of the last layer of GaAs. In the left or the right case, the behavior of this secondary InAs layer will behave differently than the first (shown). And the difference will propagate up through the superlattice. If this difference is small, the effects may not compound significantly enough in only 5 layers, but may still be present and could not show up until 10 or more layers are grown. It is this added constraint that indicates the criticality of getting this layer thickness correct.
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In the course of my thesis I have worked with many collaborators without whom many of the results in this thesis would have been impossible. The work began with Ge/Si bonding for triple- junction cells with funding from Tecstar, and while working with Tecstar I was assisted in solar cell growth and design by Charlie Chu and Peter Iles. Following Emcore’s acquisition of Tecstar, GaAs growth was continued at Spectrolab, and the assistance of Richard King and others there is greatly appreciated. The growth of test structures on InP/Si bonded samples was performed by Mark Wanlass at the National Renewable Energy Laboratory (NREL). Optical characterization of GaAs and InGaAs on Ge/Si and InP/Si, respectively, were conducted by Richard Ahrenkiel, also from NREL. The spectroscopic studies of hydrogen in Ge and InP would not have been possible without the equipment and insight of Yves Chabal and his research group at Rutgers University. The transmission electron micrographs in this thesis were made by Carol Garland at Caltech.
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