R. Enderlein

First principles studies of point defects and impurities in cubic boron nitride

The full-potential linear augmented-plane-wave (FLAPW) method within the frameworks of the local density approximation and the large unit cell approach was used to carry out self-consistent calculations of vacancies, substitutional and interstitial Be impurity in cubic boron nitride (c-BN). It has been found that the Be substitutional impurity replacing B leads to a shallow acceptor level just above the top of the valence band. This result makes Be a typical p-type dopant in c-BN. Be replacing N leads to occupied and empty energy levels at the middle gap region, indicating an amphoteric character for this center. Be at tetrahedral interstitial sites induce deep levels at the upper part of the band gap comprising two electrons and four holes for each center. The role played by these findings on the interpretation of experimental data taken on Be-doped c-BN is discussed.

Photoreflectance spectra from GaAs HEMT structures reinvestigated: solution of an old controversy

An old controversy concerning the origin of the short-period oscillations in the photoreflectance (PR) spectra of GaAs high electron mobility transistor (HEMT) structures is solved. Combining experimental and theoretical PR studies, it is demonstrated that these oscillations are not generated in the channel region, but are due to an ionized acceptor plane at the buffer/substrate interface which causes a weak homogeneous electric field in the buffer layer. The interface sheet charge density is obtained from the PR oscillation period. The utilization of PR spectroscopy for measuring charge densities at interfaces is of interest beyond the scope of this work.

Miniband structures and effective masses of n-type -doping superlattices in GaN

Self-consistent electronic structure calculations of n-type -doping superlattices (SLs) in GaN are performed using effective-mass theory and local density functional theory. The electron bulk mass of cubic GaN is obtained from ab initio full potential linear augmented plane wave (FLAPW) band-structure calculations. For the SLs, self-consistent potentials, subband levels, miniband dispersions and Fermi-level positions are calculated, assuming sheet donor concentrations up to and periods between 50 A and 400 A which cover the whole range between an isolated-well and SL regime. The miniband effective masses exhibit strong dependence on the SL wavevector. The confinement effects in GaN n-type -doping SLs are strong enough to be seen experimentally in transport and optical properties. A comparison of the results obtained for GaN with -doped layers in other III-V compounds, such as GaAs, is made.

Parameters of the Kane model from effective masses : Ambiguities and instabilities

The problem of calculating the parameters of the Kane model from experimental effective masses is reinvestigated. Conditions for the applicability of the Kane and Luttinger-Kohn models are derived in terms of experimental effective masses. It is found that different definitions of the Luttinger parameters are used within the Kane model. Some of the parameters of the Kane model are very sensitive to small changes of effective masses. The reason for this unexpected instability is analyzed, and its effects on bulk band structures and energy levels from effective mass theory are discussed.

Photoreflectance investigations of semiconductor device structures

The application of photoreflectance (PR) spectroscopy to metal-semiconductor field effect transistor and high electron mobility transistor structures is demonstrated. The line shape analysis relies on a new method for calculating PR spectra and electric field profiles of heterostructures with weakly inhomogeneous layers. The method allows inhomogeneous built-in electric field distributions below the surface to be detected. It can be used to determine characteristic parameters of device structures and to detect quantum states confined to potential wells. Differences between PR spectra from nominally identical or similar structures are attributed to different interface qualities.

Comparative studies of photoluminescence from n and p δ doping wells in GaAs

Abstract Luminescence spectra for n and p δ doping wells in GaAs are calculated. The stationary electron and hole states of the wells are obtained self-consistently by means of the multiband effective mass theory. The overlap integrals of the electron and hole envelope functions are crucial for the luminescence intensities. Those for light holes are larger than those for heavy holes, and those for p δ doping wells exceed those for n δ doping wells by almost two orders of magnitude. This explains the experimental findings on luminescence from such wells.

Stability of native defects in cubic boron nitride

Abstract The electronic structure and formation energies of the native defects boron and nitrogen vacancies and nitrogen antisite defect in cubic BN have been calculated with the use of the full-potential linear augmented-plane-wave method within the local density and large unit cell approximations. We show that the nitrogen vacancy and the nitrogen antisite defect have high formation energies in B-rich or N-rich and p-type or n-type material. These centers are unlikely to occur even at the high temperature conditions assumed for the samples growth. For the nitrogen vacancy to be ascribed to the high resistivity n-type effect and to the dominant electron paramagnetic line observed in c-BN samples, a pertubation is required, for example by an impurity, to lower the formation energy of the defect. Besides, the calculations show that the deep acceptor level, with activation energy at about 1 eV, observed in n-type or intrinsic c-BN crystals, can be ascribed to the negatively charged boron vacancy.

Hole band structure of p-type delta-doping quantum wells in silicon

Abstract The subband structure of p-type delta-doping wells in Si is calculated within self-consistent six-component envelope function theory, taking the heavy hole, light hole and spin-orbit split bulk bands into account. The subband dispersion shows a pronounced anticrossing behavior, being enhanced by the small splitting between heavy and light hole levels and the strong subband anisotropy parallel to the doping layers. For sheet doping concentrations above 10 13 cm −2 , subbands derived from the spin-orbit split band compete with heavy and light hole subbands. A comparison with experimental results shows good agreement.

Optical characterization of GaAs/AlAs multiple quantum wells interfaces

Abstract The investigation of optical properties in two wide GaAs/AlAs multiple quantum wells is reported. Molecular beam epitaxy growth interruption method was used for both samples. The growth conditions were the same for both, except for the substrate temperature. The excitonic peaks in the photoluminescence spectra were observed to be overlapping. Additional information was required for a correct interpretation. This was provided by combining photoluminescence excitation measurements with calculated excitonic energies. The calculation was made by using an envelope function approach, and takes into account weakly strained GaAs layers. From this analysis the presence of different wells whose widths vary by one monolayer is suggested. The upper limit in the fluctuations of the well-width was estimated for both samples.

Investigation of the photoluminescence linewidth broadening in symmetric and asymmetric InGaAsGaAs n-type δ-doped quantum wells

We have performed a systematic study of photoluminescence (PL) line shape in n-type σ-doped In 0.15 Ga 0.85 As/GaAs quantum wells (6 QWs). Samples grown by molecular beam epitaxy with well widths of 60, 100, 300 A, with sheet doping concentration ranging from 10 11 to 10 13 cm -2 , were analysed. Two positions were considered for the doping planes, at the centre (symmetric) and at the edge (asymmetric) of the QW. The luminescence spectrum from the symmetric 60 A δ QW exhibits a dominating broad structure that at low excitation condition and low temperature has an almost rectangular form. The square shape of this spectrum is explained by the collective recombination of electrons from the conduction band with thermalized photogenerated holes. For the symmetric L W = 300 A samples, the PL spectra display an almost symmetric peak followed by a small shoulder on the higher or lower energy side depending on the carrier concentrations. For the asymmetric 300 A δ QW sample two distinct peaks were observed. They were correlated to transitions involving the fundamental and first excited electronic subbands.