Absorption spectrum of a weakly n-doped semiconductor quantum well

We calculate, as a function of temperature and conduction band electron density, the optical absorption of a weakly n-doped, idealized semiconductor quantum well. In particular, we focus on the absorption band due to the formation of a charged exciton. We conceptualize the charged exciton as an itinerant excitation intimately linked to the dynamical response of itinerant conduction band electrons to the appearance of the photo-generated valence band hole. Numerical results for the absorption in the vicinity of the exciton line are presented and the spectral weights associated with, respectively, the charged exciton band and the exciton line are analyzed in detail. We find, in qualitative agreement with experimental data, that the spectral weight of the charged exciton grows with increasing conduction band electron density and/or decreasing temperature at the expense of the exciton.Comment: 5 pages, 4 figure

Magnetic field dependence of the energy of negatively charged excitons in semiconductor quantum wells

A variational calculation of the spin-singlet and spin-triplet state of a negatively charged exciton (trion) confined to a single quantum well and in the presence of a perpendicular magnetic field is presented. We calculated the probability density and the pair correlation function of the singlet and triplet trion states. The dependence of the energy levels and of the binding energy on the well width and on the magnetic field strength was investigated. We compared our results with the available experimental data on GaAs/AlGaAs quantum wells and find that in the low magnetic field region (B<18 T) the observed transition are those of the singlet and the dark triplet trion (with angular momentum $L_z=-1$), while for high magnetic fields (B>25 T) the dark trion becomes optically inactive and possibly a transition to a bright triplet trion (angular momentum $L_z=0$) state is observed.Comment: 9 pages, 10 figures submitted to Phys. Rev.

Excitons and charged excitons in semiconductor quantum wells

A variational calculation of the ground-state energy of neutral excitons and of positively and negatively charged excitons (trions) confined in a single-quantum well is presented. We study the dependence of the correlation energy and of the binding energy on the well width and on the hole mass. The conditional probability distribution for positively and negatively charged excitons is obtained, providing information on the correlation and the charge distribution in the system. A comparison is made with available experimental data on trion binding energies in GaAs-, ZnSe-, and CdTe-based quantum well structures, which indicates that trions become localized with decreasing quantum well width.Comment: 9 pages, 11 figure

Reversal of the Charge Transfer between Host and Dopant Atoms in Semiconductor Nanocrystals

We present ab initio density functional calculations that show P (Al) dopant atoms in small hydrogen-terminated Si crystals to be negatively (positively) charged. These signs of the dopant charges are reversed relative to the same dopants in bulk Si. We predict this novel reversal of the dopant charge (and electronic character of the doping) to occur at crystal sizes of order 100 Si atoms. We explain it as a result of competition between fundamental principles governing charge transfer in bulk semiconductors and molecules and predict it to occur in nanocrystals of most semiconductors.Comment: 4 pages, 4 figures (3 in color), 2 table

The opposite of Dante's hell? The transfer of ideas for social housing at international congresses in the 1850s–1860s

With the advent of industrialization, the question of developing adequate housing for the emergent working classes became more pressing than before. Moreover, the problem of unhygienic houses in industrial cities did not stop at the borders of a particular nation-state; sometimes literally as pandemic diseases spread out 'transnationally'. It is not a coincidence that in the nineteenth century the number of international congresses on hygiene and social topics expanded substantially. However, the historiography about social policy in general and social housing in particular, has often focused on individual cases because of the different pace of industrial and urban development and is thus dominated by national perspectives. In this paper, I elaborate on transnational exchange processes and local adaptations and transformations. I focus on the transfer of the housing model of SOMCO in Mulhouse, (a French house building association) during social international congresses. I examine whether cross-national networking enabled and facilitated the implementation of ideas on the local scale. I will elaborate on the transmission and the local adaptation of the Mulhouse-model in Belgium. Convergences, divergences, and different factors that influenced the local transformations (personal choice, political situation, socioeconomic circumstances) will be taken into accoun

EXCITON PIÉGÉ PAR UNE PAIRE DONNEUR-ACCEPTEUR DANS UN SEMICONDUCTEUR

L'étude du système composé d'un exciton (X) et d'une paire donneur-accepteur (D+, A-) dans l'approximation de masses effectives et d'interactions coulombiennes montre qu'il existe une valeur critique R0 de la distance donneur-accepteur en dessous de laquelle il n'y a pas de liaison stable. Des bornes supérieure et inférieure sont assignées à cette distance critique en fonction des masses effectives de l'électron et du trou. L'influence des effets de polarisation électronique et ionique est aussi examinée. Les résultats obtenus sont comparés aux données expérimentales dans le cas de ZnSe(In-Li).An exciton (X) bound to a donor-acceptor pair (D+, A-) is studied in the approximation of effective masses and coulombic interactions, and it is shown that there is a critical value R0 of the donor-acceptor separation below which stable binding does not occur. Upper and lower limits of this critical separation are defined as functions of the electron and hole effective masses. The influence of additional electronic and ionic polarization effects is also considered. The results obtained are compared to the experimental data available for ZnSe(In-Li)

Ionized donor impurity in a semiconductor quantum well

We present the results of a variational calculation of the ground state energy of a (D+,X) complex in a quantum well with finite potential barriers as a function of the depth and the width of the well as well as the ratio [MATH] of the electron and hole effective masses. We use the envelope function approximation. We apply Our results to the system GaAs/Ga1-xAlxAs with x = 0.15 and x = 0.30. It appears that the coulombic correlation energy goes through a minimum for a well thickness in the order of 50 Angströms. This minimum is comprized between the values obtained in the 2D and 3D limit cases. Furthermore, it appears that only the heavy hole states give rise to a stable binding energy for all values of the well thickness. The light hole states give rise to stable binding only in the case of large thicknesses