T. G. Andersson

Molecular beam epitaxy growth and characterization of InSb layers on GaAs substrates

The authors have investigated InSb layers grown heteroepitaxially on GaAs(100) substrates by molecular beam epitaxy (MBE). The dependence of electron mobilities on the MBE-growth conditions was investigated. The best room temperature mobility, 55000 cm2 V-1 s-1 for a 2 mu m thick layer, was obtained for a growth temperature of 420 degrees C with an antimony over indium ratio of 1.4. The 14.6% lattice mismatch between epilayer and substrate gives rise to threading dislocations and microtwins, as evidenced by transmission electron microscopy. The defects are shown to reduce the mobility for thin samples. One of the most interesting results of the work is the evidence of an electron accumulation layer at the InSb(100) surface. This result is obtained from temperature-dependent Hall measurements which exhibited two singularities in the carrier concentration versus temperature plot. Calculations of the Hall constant considering parallel conduction is successfully used to model this temperature dependence. The MBE-grown InSb layers are shown to have an unintentional acceptor background. The authors also investigated n-type doping using silicon. It is shown that the measured low temperature carrier concentrations and mobilities in undoped samples are considerably influenced by compensation effects.

A multiple-state memory cell based on the resonant tunneling diode

The device consists primarily of several molecular-beam-epitaxy (MBE-) grown GaAs/(AlGa)As resonant tunneling diodes connected in parallel. This device exhibits multiple peaks in the I-V characteristic. When a load resistor is connected, the circuit can be operated in a multiple stable mode. With this concept, implementation of three-state and four-state memory cells are made. In the three-state case the operating points at voltages V/sub 0/=0.27 V, V/sub 1/=0.42 V, and V/sub 2/=0.53 V represent the logic levels 0, 1, and 2. Similarly for the four-state memory cell the logic levels voltages are V/sub 0/=0.35 V, V/sub 1/=0.42 V, V/sub 2/=0.54 V, and V/sub 3/=0.59 V. A suggestion of an integrated device structure using this concept is also presented.<<ETX>>

Cathodoluminescence and electron beam induced current study of partially relaxed AlGaAs/GaAs/InGaAs heterojunction phototransistors under operating conditions

We have studied time-resolved cathodoluminescence (CL) and electron beam induced current (EBIC) on AlGaAs/GaAs/InGaAs heterojunction phototransistors under operating conditions, i.e., at room temperature and under bias. Devices from four wafers, with a different amount of lattice relaxation, were tested. It is shown that the CL intensity increases more than one order of magnitude as the voltage is increased and the current gain of the device turns on. The voltage dependence of the CL signal is analogous to the current–voltage curve of the transistor. The buildup in CL intensity was found to be much less in devices with low current gain showing that the CL intensity correlates to the electrical gain of the device. Time resolved CL showed two distinct CL decay times, one very short, a few nanoseconds, and one long, of the order of microseconds. This indicates that two fundamental recombination processes are present, which we attribute to a spatially direct recombination between carriers in the base and a sp...

Morphology and size distributions of islands in discontinuous films

We have investigated sizes and shapes of islands in discontinuous gold films. These were UHV deposited in an electric field parallel with the surface of the glass substrates. Films with average thickness t in the range 10⩽t⩽45A have been studied. The two‐dimensional island shapes are represented by ellipses where, from experiments, both the major and minor axes obey log‐normal distribution functions. These facts can be reconciled with a statistical model of island coalescence. Empirically, the median island diameter is approximately proportional to t, whereas the geometric standard deviation σ falls in the range 1.25⩽σ⩽1.30, irrespective of thickness. We define a mean eccentricity ē as an average over the ratio between major and minor axes; as expected ē increases with increasing t. Supplementing the size determinations with evaluations of area fraction of the substrate covered with metal we find the three‐dimensional form of the islands to be accurately approximated by prolate spheroids (symmetry axes pa...

Temperature‐dependent transition from two‐dimensional to three‐dimensional growth in highly strained InxGa1−xAs/GaAs (0.36≤x≤1) single quantum wells

Transition from two‐dimensional to three‐dimensional growth mode has been investigated by photoluminescence in highly strained InxGa1−xAs/GaAs (0.36≤x≤1) single quantum wells. The structures were grown by molecular beam epitaxy from 410 to 590 °C. The critical layer thickness based on this transition decreased as the growth temperature increased. This behavior was well described by the single‐kink Matthews model [J. Vac. Sci. Technol. 12, 126 (1975)] including the simplest expression of the Peierls–Nabarro friction stress [J. Appl. Phys. 41, 3800 (1970)].

Quantitative evolution studies of particle separation, size and shape for vapour-deposited ultrathin gold films on glass substrates

Abstract Quantitative results concerning the evolution of particle size, coverage, edge-to-edge separation and shape for ultrathin gold films on glass substrates are reported. These structural features of the particles deposited in ultrahigh vacuum were determined using a Quantimet 720 image analyser. The results from the frequency plots of the edge-to-edge separation and the form factor reveal that the liquid-like coalescence successively changes into an ordinary coalescence process with the onset at a mean film thickness of approximately 3 nm. The two processes are discussed in terms of Ostwald ripening and particle mobility, and future investigations are outlined to clarify the dependence of the observed coalescence processes on residual gases in the vacuum system.

Molecular beam epitaxy growth and characterization of InxGa1 − xAs (0.57 ⩽ x ⩽ 1) on GaAs using InAlAs graded buffer

Abstract In x Ga 1 − x As ( x ⩾ 0.57) thin layers were grown on GaAs by molecular beam epitaxy using an InAlAs graded buffer. Surface roughness of the graded buffer decreased with reduced buffer thickness and growth temperature. The InGaAs layers grown on a graded buffer revealed a larger tilting angle, a higher residual strain and electron mobility than those grown directly on GaAs. Increasing the graded buffer thickness above 1.2 μm did not improve the transport properties of InAs. The InAs mobility increased with layer thickness. This was explained by assuming two conducting channels, given μ 300 = 1.8 × 10 4 cm 2 /(V s) and μ 77 = 5.1 × 10 4 cm 2 /(V s) for bulk InAs.

Self‐consistent analysis of electric field‐dependent intersubband transitions for a thin layer inserted quantum well

A self‐consistent theoretical analysis, using both the Schrodinger and Poisson equations, is made to calculate the electric field‐dependent intersubband electron transition for a thin layer inserted in a quantum well. In a 100‐A‐wide square quantum well a layer is embedded giving a deeper wall (or a barrier). This layer is carefully adjusted in its position and composition to optimize the dependence of intersubband transition on the external electric field. The analysis shows that the structure with a narrow well thickness between 30 and 45 A, located at the side of the wide well, has optimal properties in terms of large near‐linear Stark shifts associated with high oscillator strengths and a wide range of voltage tunability in the transition wavelength.

Critical layer thickness in InzxGa1−xAs/GaAs quantum wells studied by photoluminescence and transmission electron microscopy

Abstract Critical layer thickness (CLT), in strained InxGa1−xAs/GaAs single and multiple quantum wells, has been investigated by photoluminescence and transmission electron microscopy techniques. A carefull and detailed analysis of the experimental data shows that a transition from two-dimensional to three-dimensional growth occurs before development of strain released misfit dislocations for x>0.3. The CLT, which is based on this change, is found to decrease with In content and has a break at x=0.35.

The effect of the first GaN monolayer on the nitridation damage of molecular beam epitaxy grown GaN on GaAs(001)

The initial molecular beam epitaxy growth of GaN on GaAs(001) was studied by real-time monitoring of the (3×3) surface reconstruction and its transition to an unreconstructed (1×1). Various growth conditions were established by variation of the V/III ratio, i.e., the Ga flux. We characterized the effect of the first two strained GaN monolayers: a N-terminated GaN (3×3) monolayer and a second unreconstructed (1×1) monolayer. A series of samples were grown under N-rich, Ga-rich, and near-stoichiometric growth conditions. The resulting morphology of the interface region was analyzed by high-resolution scanning electron microscopy, Auger-electron spectroscopy, and double crystal x-ray diffractometry. The N-rich and Ga-rich conditions resulted in extensive defect formation due to the nitridation damage of the GaAs substrate. The extent of this was found to be determined by the properties of the first GaN monolayer. The surface roughness under optimum growth conditions could be as low as ∼20 nm, defined by nano...