M.R. Leys

A study of 1/f noise in InP grown by CBE

Abstract The origin of low-frequency noise in InP was studied experimentally by measuring the noise of InP layers grown by chemical beam epitaxy (CBE). Such InP layers are unintentionally doped, but of varying purity and always of n -type conductivity. We performed noise measurements at temperatures from 77 to 300 K. Our results at 77 and 300 K prove that the mobility noise is exclusively due to the lattice scattering. The noise in InP is well characterized by a parameter α Latt , which equals 3 × 10 −3 at 300 K and 2.3 × 10 −3 at 77 K.

Interfacet surface diffusion in selective area epitaxy of III–V semiconductors

In this letter, we discuss the interfacet diffusion of group-III species from {111} B facets to the (100) plane in planar selective area epitaxy. In general, this leads to enhanced vertical growth at the edges of the (100) surface. From such edge profiles, several groups have extracted adatom “diffusion lengths” of ∼1 μm. This is a factor of 100 larger than reported diffusion lengths obtained by reflective high energy electron diffraction, scanning tunneling microscopy, or growth-rate analysis. We show that these values are severely overestimated and that edge profiles only give information on the propagation velocity of macrosteps.

A chopped quantum-well polarization-independent interferometric switch at 1.53 /spl mu/m

We have theoretically designed and realized a phase shifter for a low-loss Mach-Zehnder interferometric switch. The phase shifter is based on 0.85% tensile strained InGaAs-InP chopped quantum-well material. We realized a Mach-Zehnder interferometric switch with polarization-independent switching voltages as low as 3.3/spl plusmn/0.05 V at 1525 nm for a switch with a 4-mm-long phase shifting section. The wavelength sensitivity of the switch is 0.036 V/nm for TE and 0.053 V/nm for TM polarization. Calculations of the electro-refraction in the -0.85% strained chopped quantum-well (QW) material based on the 4/spl times/4 Luttinger-Kohn Hamiltonian show that the electro-refraction due to the quantum-confined Stark effect (QCSE) for TM polarization is equal to the sum of the mutually comparable QCSE electro-refraction and the Pockels effect for TE polarization in waveguides along the [11~0] axis. Our first-principle model for calculating the electro refraction is an accurate design tool for predicting device performance in complicated layer structures. The shortest possible phase shifter with a <-25 dB crosstalk penalty due to electro-absorption unbalance can be as short as 2.2 mm. This compact switch is predicted to have a 6-V switching voltage and a 15-nm window for polarization-independent switching with a <-25-dB crosstalk penalty. With a slight increase of the strain, this chopped QW material can be used for polarization independent switching around 1550 nm.

X-ray interference effect as a tool for the structural investigation of GaInAs/InP multiple quantum wells

We present x-ray diffraction (XRD) investigations of the structure of nominally lattice-matched GaInAs/InP multiple quantum well (MQW) structures grown by chemical beam epitaxy (CBE). To obtain information about the individual MQW layers and the interface structure we make use of the x-ray interference effect between two layers of equal lattice constant but different layer thickness separated by ultrathin strained (interfacial) layers. This effect predicted by the dynamical diffraction theory provides a powerful tool to quantitatively investigate ultrathin single quantum well structures and monolayer thin interfaces as well as MQW structures. For a given switching sequence during CBE growth, we determine the interface structure of GaInAs/InP MQW structures within the limits given by XRD theory. Additionally we found that an As gradient from the GaInAs quantum well layers into the InP barrier layers is present. The influence of the substrate off-orientation, the growth rate, and the group V flux in the InP barriers on the total amount of strain incorporated into the InP layers is shown. The obtained results indicate that the mechanism of As incorporation into InP layers is similar to the mechanism observed for the As incorporation into (qua)ternary GaxIn1−xAsyP1−y layers.We present x-ray diffraction (XRD) investigations of the structure of nominally lattice-matched GaInAs/InP multiple quantum well (MQW) structures grown by chemical beam epitaxy (CBE). To obtain information about the individual MQW layers and the interface structure we make use of the x-ray interference effect between two layers of equal lattice constant but different layer thickness separated by ultrathin strained (interfacial) layers. This effect predicted by the dynamical diffraction theory provides a powerful tool to quantitatively investigate ultrathin single quantum well structures and monolayer thin interfaces as well as MQW structures. For a given switching sequence during CBE growth, we determine the interface structure of GaInAs/InP MQW structures within the limits given by XRD theory. Additionally we found that an As gradient from the GaInAs quantum well layers into the InP barrier layers is present. The influence of the substrate off-orientation, the growth rate, and the group V flux in the InP...

Superconducting contacts to a two-dimensional electron gas in GaAs/AlGaAs-heterostructures

The process of Sn/Ti-diffusion is shown to provide low-resistance (highly transmissive) contacts to the two-dimensional electron gas (2DEG) in GaAs/AlGaAs-heterostructures. At temperatures down to 80 mK the resistance stays very low. The temperature and gate-voltage dependence of the dV/dl-V-curves shows that Andreev reflection must take place in the sample. The measurements suggest that Ti is the relevant superconductor and not Sn. The fact that the bottom of the low-resistance region is very flat and does not show a peak near V=0 V means that the transmission of the NS (normal metal-superconductor) interface is very close to 1. Therefore these superconducting contacts seem to be very suitable for studying SNS (superconductor-normal metal-superconductor) junctions in the clean limit.<<ETX>>

A RHEED study of the dynamics of GaAs and AlGaAs growth on a (001) surface by MBE

Abstract In this paper we report on RHEED investigations of the damping and recovery of the specular-beam intensity during MBE growth. Our results on the damping behaviour during growth of GaAs and Al0.33Ga0.67As indicate that at a temperature of approximately 610°C the average step density on the surface is at a minimum. At this temperature the change from a (2 × 4) As-stabilized surface to the (1 × 1) (3 × 1) metal-rich surface takes place. We attribute the larger degree of damping at lower temperatures to insufficient ad-atom mobility and thus a high step density on the surface. At temperatures above 610°C we attribute the larger degree of damping to adsorption of gallium atoms on arsenic sites. A purely analytical procedure is used to calculate the recovery rate of the specular-beam intensity. Results are presented for growth termination at various temperatures and at various fractions of monolayer coverage, θGa. It can be concluded that the initial, fast recovery process on the arsenic-stabilized surface is due to Group III kinetics with activation energies generally greater than 1.5 eV. However, a lower activation energy is observed on termination of growth at 1 2 of a complete monolayer (θGa = 0.5), indicating the recovery process to be then limited by As4 physisorption. The second, slow step in the recovery has activation energies of the order of 5 eV. The initial recovery process on the gallium-rich surface is characterized by lower activation energies, of the order of 0.5 eV. The mathematical analysis presented here allows one to calculate the minimum growth interruption times to obtain smooth surfaces in a straightforward manner.

Large asymmetric Stark shift in GaxIn1–xAs/InP/InAsyP1–y composite quantum wells

Strong asymmetric Stark shift in excess of 115 meV of the lowest energy transition has been experimentally observed in composite GaxIn1−xAs/InP/InAsyP1−y quantum-well system. In this structure, we can independently control the confinement of electrons and holes by controlling the strain. The photoexcited electrons and holes are confined in spatially separated regions without the application of an electric field. Due to the large asymmetry in the structure, we observed large blueshifts and redshifts of the absorption edge with an applied electric field. All our measurements agree with the calculations within the framework of the Bir–Pikus strain Hamiltonian.

Electrorefraction in strained InGaAs/InP chopped quantum wells: Significance of the interface layers

We present a model for electrorefraction based on the quantum confined Stark effect (QCSE) in strained InGaAs/InP chopped quantum wells (CQWs) consisting of three 27 A InGaAs wells separated by 15 A InP barriers. The model fully takes into account the influence of the thin interface layers around each well. We experimentally verify the model on a InGaAs/InP CQW which combines a large 60 meV QCSE redshift at 11.7 V bias with waveguide transparency at 1.55 μm, which is two times larger than in a InGaAsP quaternary well. The calculated electroabsorption spectra of the CQWs are in good agreement with experiment. We finally applied the Kramers–Kronig transformations for calculating the switching voltage in a Mach–Zehnder switch employing CQWs in the phase shifting section. The model was found to be in good agreement with experiment for both polarizations.

Morphology of homo-epitaxial vicinal (100) III-V surfaces

Abstract Vicinal substrates are widely used to fabricate semiconductor devices. The different surface step types and their density have a strong influence on the surface structure. We have systematically studied this misorientation dependence on the morphology of (1xa00xa00)InP as a function of growth conditions in chemical beam epitaxy. On substrates containing no steps or only A-type steps we observe mirror-like surfaces, but many characteristic defects are always present. The defect density is successfully reduced by the introduction of B-type steps. When a sufficient density of only B steps is present, the surface is very smooth with almost no defects. Near any B “step up” edge, however, a ripple pattern is formed, which extends over tens of microns in the B step propagation direction. For substrates with mixed steps the ripple pattern is more pronounced and significant surface roughening occurs. Here, we present and discuss a general model for the morphology of (2×4) reconstructed vicinal (1xa00xa00) III–V surfaces. It gives a complete and quantitative description of these observations, including the dependence on growth conditions and thus provides a useful tool to study and optimize the growth process.

The temperature dependence of 1/f noise in InP

Noise spectra were measured on CBE grown InP samples in the frequency range from 1 Hz to 104 kHz at temperatures from 77 to 500 K. The experimental results show that 1f noise stems from the lattice scattering. The 1f noise in InP is well characterised by a parameter αLatt in this temperature range. The temperature dependence of αLatt was determined experimentally. Assuming that the number of phonons in a mode fluctuates with a 1f spectrum, we are able to derive a theoretical expression for αLatt in terms of the contributions by the acoustic phonons and the polar optical phonons. At low temperatures (<200 K), the temperature dependence of αLatt can be analysed by this model with two kinds of phonons. However, at temperatures above 200 K, the model does not lead to satisfactory results.