We investigate the magneto-optical properties of excitons bound to single stacking faults in highpurity GaAs. We find that the two-dimensional stacking fault potential binds an exciton composed of an electron and a heavy-hole, and confirm a vanishing in-plane hole g-factor, consistent with the atomic-scale symmetry of the system. The unprecedented homogeneity of the stacking-fault potential leads to ultra-narrow photoluminescence emission lines (with full-width at half maximum 80 µeV) and reveals a large magnetic non-reciprocity effect that originates from the magnetoStark effect for mobile excitons. These measurements unambiguously determine the direction and magnitude of the giant electric dipole moment ( e · 10 nm) of the stacking-fault exciton, making stacking faults a promising new platform to study interacting excitonic gases.Introduction. The stacking fault (SF), a planar, atomically thin defect, is one of the most common extended defects in zinc-blende, wurtzite, and diamond semiconductors. A fundamental understanding of the SF potential is important for determining how the defect affects semiconductor device performance [1, 2], engineering heterostructures based on crystal phase [3][4][5], and providing a new twodimensional (2D) platform for fundamental physics [6,7]. Here we report on excitons bound to large-area, single SFs in high-purity GaAs, a unique system where SFs are easily isolated with far-field optical techniques. The atomic smoothness of the potential and extreme perfection of the surrounding semiconductor result in ultra-high optical homogeneity ( 80 µeV). This enables optical resolution of the SF exciton fine-structure and thus direct measurement of the giant built-in dipole moment ( e · 10 nm) via the magnetoStark effect. These results indicate that the extremelyhomogeneous SF potential may be promising for studies of many-body excitonic physics, including coherent phenomena [8-10], spin currents [11], superfluidity [12], long-range order [13][14][15][16][17], and large optical nonlinearities [18][19][20].Stacking fault photoluminescence. Figure 1(a) shows a spectrally resolved confocal scan of SF structures in a GaAs epilayer, excited with an above band-gap laser (1.65 eV, 1.5 K) [21]. The image is colored red, green or blue according to three characteristic emission bands shown in Fig. 1e. The narrow-band PL at 1.493 and 1.496 eV originates from excitons, electron-hole pairs, bound to the 2D SF potential [22,23]. The sample consists of a 10 µm GaAs layer on 100 nm AlAs on a 5 nm/5 nm AlAs/GaAs (10×) superlattice grown directly on a semi-insulating (100) GaAs substrate. Stacking fault structures nucleate near the substrateepilayer interface during epitaxial growth [21].The physical origin of the potential can be understood from the atomic structure of the SF defect: the lattice-plane ordering in the [111] direction of zinc-blende is modified

X-band accelerator structures meeting the Next Linear Collider (NLC) design requirements have been found to suffer vacuum surface damage caused by radio frequency (RF) breakdown, when processed to high electric-field gradients. Improved understanding of these breakdown events is desirable for the development of structure designs, fabrication procedures, and processing techniques that minimize structure damage. RF reflected wave analysis and acoustic sensor pickup have provided breakdowns localization in RF structures [1] [2]. Particle contaminations found following clean autopsy of four RF-processed travelling wave structures, have been catalogued and analyzed. Their influence on RF breakdown, as well as that of several other material-based properties, will be discussed.

In an effort to locate the cause(s) of high electricfield breakdown in x-band accelerating structures, we have cleanly-autopsied (no debris added by post-operation structure disassembly) an RF-processed structure. Macroscopic localization provided operationally by RF reflected wave analysis and acoustic sensor pickup was used to connect breakdowns to autopsied crater damage areas. Surprisingly, the microscopic analyses showed breakdown craters in areas of low electric field. High currents induced by the magnetic field on sharp corners of the input coupler appears responsible for the extreme breakdown damage observed.

X-band accelerator structures meeting the Next Linear Collider (NLC) design requirements have been found to suffer vacuum surface damage caused by radio frequency (RF) breakdown, when processed to high electric-field gradients. Improved understanding of these breakdown events is desirable for the development of structure designs, fabrication procedures, and processing techniques that minimize structure damage. RF reflected wave analysis and acoustic sensor pickup have provided breakdowns localization in RF structures [1] [2]. Particle contaminations found following clean autopsy of four RF-processed travelling wave structures, have been catalogued and analyzed. Their influence on RF breakdown, as well as that of several other material-based properties, will be discussed.

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