Bibliothek

Notizen: The effect of the ambient conditions in the growth chamber of the molecular beam epitaxy machine during the growth of GaAs/Al0.35Ga0.65As structures was investigated. Both growth-interrupted (120 s at each heterointerface) and uninterrupted surfaces and interfaces were evaluated using a growth temperature of 580 °C. Two ambient conditions were studied: (a) ∼1×10−10 Torr O2; and (b) ultrahigh vacuum (UHV, ∼5×10−11 Torr, with no intentional introduction of contaminants). A striking difference was observed in both the 1.7 K photoluminescence (PL) spectra of single quantum well (SQW) structures and UHV scanning tunneling microscopy (STM) of surfaces, which were grown under ambient condition (a) as opposed to (b). When consecutive growth-interrupted SQW samples were grown with different well widths (25 and 28 A(ring)) under condition (a), the emission energy splitting into several peaks was observed, indicating discrete thicknesses of the well. However, the peak energies shifted as the laser spot was scanned across each sample. Additionally, the peak energy shifted from sample to sample for the same nominal well width.On the other hand, when SQW samples were grown under condition (b), no variation in the emission energy was observed as the laser was scanned across the sample, or from sample to sample for a given well width. Furthermore, the PL observations are supported by UHV-STM results. UHV-STM images indicated a very rough surface with large islands containing small terraces on top (a bimodal distribution) for condition (a). Conversely, when samples were grown under condition (b), only large islands were observed. For growth interrupted GaAs surfaces, 400 A(ring)×600 A(ring) islands were observed, and for Al0.35Ga0.65As, they were 150 A(ring)×400 A(ring), with a one-monolayer step in between islands. These data are consistent with abrupt interfaces with only a single-mode distribution for growth-interrupted surfaces. On the other hand, UHV-STM images of uninterrupted GaAs surfaces grown under condition (b) showed islands that were 40–60 A(ring) across. Photoluminesce spectra of a similarly grown SQW sample showed only a single broad emission line, consistent with an interface configuration of many steps which are smaller than the exciton diameter. The results show that interface roughness is sensitive to background O2.

Notizen: Experimental and theoretical results of Mg-doped superlattices consisting of uniformly doped AlxGa1−xN, and GaN layers are presented. Acceptor activation energies of 70 and 58 meV are obtained for superlattice structures with an Al mole fraction of x=0.10 and 0.20 in the barrier layers, respectively. These energies are significantly lower than the activation energy measured for Mg-doped bulk GaN. At room temperature, the doped superlattices have free-hole concentrations of 2×1018 cm−3 and 4×1018 cm−3 for x=0.10 and 0.20, respectively. The increase in hole concentration with Al content of the superlattice is consistent with theory. The room temperature conductivity measured for the superlattice structures is 0.27 S/cm and 0.64 S/cm for an Al mole fraction of x=0.10 and 0.20, respectively. X-ray rocking curve data indicate excellent structural properties of the superlattices. We discuss the origin of the enhanced doping, including the role of the superlattice and piezoelectric effects. The transport properties of the superlattice normal and parallel to the superlattice planes are analyzed. In particular, the transition from a nonuniform to a uniform current distribution (current crowding) occurring in the vicinity of contacts is presented. This analysis provides a transition length of a few microns required to obtain a uniform current distribution within the superlattice structure. © 2000 American Institute of Physics.

Notizen: GaN/InGaN light emitting diodes (LEDs) grown on sapphire substrates have current transport along the lateral direction due to the insulating nature of the substrate. The finite resistance of the n-type GaN buffer layer causes the pn junction current to be nonuniform and "crowd" near the edge of the contact. The current-crowding effect is analyzed both theoretically and experimentally for p-side-up mesa structure GaN/InGaN LEDs. The calculation yields an exponential decay of the current distribution under the p-type contact with a characteristic current spreading length, Ls. It is shown that GaN/InGaN LEDs with high p-type contact resistance and p-type confinement layer resistivity have a relatively uniform current distribution. However, as the p-type GaN conductivity and p-type ohmic contact conductivity is improved, significant current crowding near the contact edge will occur. The current crowding effect is analyzed experimentally in GaN/InGaN LEDs emitting in the blue spectral range. Experimental results show the light intensity decreasing with distance from the contact edge. A current spreading length of Ls=525 μm is found, in good agreement with theory. © 2001 American Institute of Physics.

Notizen: This article investigates steady-state nonequilibrium conditions in metal–oxide–semiconductor (MOS) capacitors. Steady-state nonequilibrium conditions are of significant interest due to the advent of wide-gap semiconductors in the arena of MOS (or metal–insulator–semiconductor) devices and due to the scaling of oxide thickness in Si technology. Two major classes of steady-state nonequilibrium conditions were studied both experimentally and theoretically: (i) steady-state deep depletion and (ii) steady-state low level optical generation. It is found that the identification and subsequent understanding of steady-state nonequilibrium conditions is of significant importance for correct interpretation of electrical measurements such as capacitance–voltage and conductance–voltage measurements. Basic implications of steady-state nonequilibrium conditions were derived for both MOS capacitors with low interfaces state density Dit and for oxide semiconductor interfaces with a pinned Fermi level. Further, a photoluminescence power spectroscopy technique is investigated as a complementary tool for direct-gap semiconductors to study Dit and to monitor the interface quality during device fabrication. © 1997 American Institute of Physics.

Notizen: We propose and demonstrate a novel approach to the coating of semiconductor laser facets. In this approach, processed semiconductor lasers are cleaved in a high-vacuum system immediately followed by coating of the vacuum-exposed facet with a very thin Si layer (≤100 A(ring)) and a large band gap dielectric (Al2O3) layer. The Si layer is sufficiently thin to avoid the formation of quantized bound states in the Si. GaAs coated with thin Si and Al2O3 have a higher luminescence yield and a lower surface recombination velocity than bare GaAs surfaces as well as GaAs surfaces coated with Al2O3 only. A surface recombination velocity of 3×104 cm/s has been obtained using a modified dead layer model for the Si/Al2O3 sample. It is also shown that lasers which are cleaved in vacuum and subsequently coated with Si and Al2O3 have improved properties including an increased threshold for catastrophic optical damage. © 1996 American Institute of Physics.

Notizen: Properties of Ga2O3 thin films deposited by electron-beam evaporation from a high-purity single-crystal Gd3Ga5O12 source are reported. As-deposited Ga2O3 films are amorphous, stoichiometric, and homogeneous. Excellent uniformity in thickness and refractive index was obtained over a 2 in. wafer. The films maintain their integrity during annealing up to 800 and 1200 °C on GaAs and Si substrates, respectively. Optical properties including refractive index (n=1.84–1.88 at 980 nm wavelength) and band gap (4.4 eV) are close or identical, respectively, to Ga2O3 bulk properties. Reflectivities as low as 10−5 for Ga2O3/GaAs structures and a small absorption coefficient (≈100 cm−1 at 980 nm) were measured. Dielectric properties include a static dielectric constant between 9.9 and 10.2, which is identical to bulk Ga2O3, and electric breakdown fields up to 3.6 MV/cm. The Ga2O3/GaAs interface demonstrated a significantly higher photoluminescence intensity and thus a lower surface recombination velocity as compared to Al2O3/GaAs structures. © 1995 American Institute of Physics.

Notizen: The competition between band gap and the 2.2 eV (yellow) luminescence of epitaxial GaN is studied for excitation densities ranging from 5×10−6 to 50 W/cm2. The ratio of the peak intensities of the band gap-to-yellow luminescence changes from 4:1 to 3000:1 as the excitation density is increased by 7 orders of magnitude. At room temperature, the band gap luminescence linewidth is 2.3kT, close to the theoretical minimum of 1.8kT. A model is developed describing the intensity of the two radiative transitions as a function of the excitation density. This model is based on bimolecular rate equations and takes into account shallow impurities, deep levels, and continuum states. The theoretically predicted dependences of the two different luminescence channels follow power laws with exponents of 1/2, 1 and 3/2. Thus the intensity of the yellow luminescence does not saturate at high excitation densities. These dependences are in excellent agreement with experimental results. The relevance of the results for optoelectronic GaN devices is discussed. It is shown that the peak intensity of the yellow luminescence line is negligibly small at typical injection currents of light-emitting diodes and lasers. © 1996 American Institute of Physics.

Notizen: Electron transport in quantum well modulation δ doped on either the normal or the inverted side has revealed the major cause of the long-puzzling inferior transport characteristics of the inverted interface. For growth conditions optimized for best transport with normal-side doping, we find migration of the Si dopant toward the inverted interface during growth to be the primary reason for the reduced inverted well mobility. This new understanding has allowed us to grow modulation-doped inverted quantum wells of unprecedented quality having electron mobilities as high as 2.4×106 cm2/V s at 4.2 K and 3.0×106 cm2/V s at 1.0 K.

Notizen: A major problem in group IV molecular beam epitaxy (MBE) is the difficulty to incorporate and control dopants due to the low incorporation probability and strong segregation in Si at typical growth temperatures. It is demonstrated here that growth at low temperatures yields a solution to this doping problem making thermal, coevaporative doping with excellent control possible in Si MBE without the need for any post-growth annealing. Unity incorporation and activation of Sb with concentrations reaching 5×1019 cm−3 are achieved for epitaxial growth of Si on Si(100) at temperatures of 325 °C. Hall electron mobilities in the films are close to bulk values indicating the high quality of the films. Capacitance-voltage measurements on Sb δ-doped films have full widths at half maximum of (approximately-less-than)50 A(ring), the narrowest Sb-doping profiles in Si determined with an electrical technique.

Notizen: Semitransparent thin silver films are employed as both the top mirrors and electrodes for GaAs vertical-cavity surface-emitting lasers. The semitransparent silver films allow the emission of light from the top epitaxial side. Quarter-wave AlAs/AlxGa1−xAs stacks are used as the bottom n-type mirrors. Light output versus excitation current measurements yields an efficiency of 0.76 mW/mA from the top silver mirror side, which corresponds to an external differential quantum efficiency of 54% at a lasing wavelength of 0.88 μm. The internal differential quantum efficiency is estimated to be (approximately-greater-than)94%. An optical output power of 10 mW is obtained at a pulsed excitation current of 72 mA.