K. Jandieri

Resonant electron tunneling through defects in GaAs tunnel diodes

Current-voltage characteristics of GaAs tunnel diodes are studied experimentally and theoretically. In theoretical calculations contributions of three different transport mechanisms are considered: direct tunneling processes, nonresonant multiphonon tunneling processes via defects, and resonant tunneling processes through defects. The comparison between theoretical results and experimental data reveals resonant tunneling as the dominant transport mechanism at voltages corresponding to the peak current. At higher voltages this mechanism is replaced by nonresonant tunneling, which is in its turn replaced by over-barrier transport at even larger voltages.

The Thermal Resistance of High-Power Semiconductor Disk Lasers

We present a model for the simulation of the thermal resistance of flip-chip bonded vertical-external-cavity surface-emitting lasers based on the finite-element method. Therefore, we take on and deepen precedent models with regard to three modifications. Our model for the first time comprises the complete heat removal, incorporates temperature-dependent heat conductivity of the diamond heat spreader and features the consideration of the exact pump distribution. The simulations are accompanied by an extensive experimental investigation of four gain chips. Thereby, a high accuracy of our simulations is confirmed. In addition, we use our model in order to investigate the influence of a ternary distributed Bragg reflector, which lacks in pump light absorption and the subsequent additional heating. Recently, this model was used to push the output power of vertical-external-cavity surface-emitting lasers beyond 100 W.

Influence of spatial and temporal coherences on atomic resolution high angle annular dark field imaging.

Aberration-corrected (scanning) transmission electron microscopy ((S)TEM) has become a widely used technique when information on the chemical composition is sought on an atomic scale. To extract the desired information, complementary simulations of the scattering process are inevitable. Often the partial spatial and temporal coherences are neglected in the simulations, although they can have a huge influence on the high resolution images. With the example of binary gallium phosphide (GaP) we elucidate the influence of the source size and shape as well as the chromatic aberration on the high angle annular dark field (HAADF) intensity. We achieve a very good quantitative agreement between the frozen phonon simulation and experiment for different sample thicknesses when a Lorentzian source distribution is assumed and the effect of the chromatic aberration is considered. Additionally the influence of amorphous layers introduced by the preparation of the TEM samples is discussed. Taking into account these parameters, the intensity in the whole unit cell of GaP, i.e. at the positions of the different atomic columns and in the region between them, is described correctly. With the knowledge of the decisive parameters, the determination of the chemical composition of more complex, multinary materials becomes feasible.

Interface morphology and composition of Ga(NAsP) quantum well structures for monolithically integrated LASERs on silicon substrates

Highly efficient light sources are the remaining item required for the realization of optoelectronically integrated circuits on exactly oriented Si(0 0 1). Here, we present—using transmission electron microscopy—an investigation on the structure and stability of Ga(NAsP), which is a direct bandgap semiconductor. It is shown that Ga(NAsP) can be grown on Si(0 0 1) substrates at a wide range of growth temperatures. No sign of defect formation and phase separation is observed even for the highest growth temperatures used. The interfaces of the quaternary alloys with the GaP barriers roughen significantly with increasing growth temperature. On the contrary, the material deposited at high temperatures is more homogeneous than the one deposited at low temperatures. This is highly surprising as dilute nitride III/V alloys are commonly thought to be metastable. This is resolved by density functional theory calculations, which show that Ga(NAsP) becomes significantly more stable when grown on substrates which have a smaller lattice constant than the equilibrium lattice constant of the alloy. This stability together with the strong room-temperature photoluminescence shown by all samples, make the Ga(NAsP) material system highly promising for laser applications on Si substrates.

Thermal quenching of photoluminescence in Ga(AsBi)

We report on a comparative experimental and theoretical study of the thermal quenching of the photoluminescence (PL) intensity in Ga(AsBi)/GaAs heterostructures. An anomalous plateau in the PL thermal quenching is observed at intermediate temperatures under relatively low excitation intensities. Theoretical analysis based on a well-approved approach shows that this peculiar behavior points at a non-monotonous density of states (DOS) in the disorder-induced band tails with at least two-energy-scales. While in previous studies carried out at relatively high excitation intensities a single-energy-scale was sufficient to fit the thermal quenching of the PL in Ga(AsBi), our study at lower excitation intensities proves that two-energy-scales of disorder contribute to the thermal quenching of the PL. Possible energy shapes of the DOS, which can fit experimental data, are revealed.

Fluctuations of the peak current of tunnel diodes in multi-junction solar cells

Interband tunnel diodes are widely used to electrically interconnect the individual subcells in multi-junction solar cells. Tunnel diodes have to operate at high current densities and low voltages, especially when used in concentrator solar cells. They represent one of the most critical elements of multi-junction solar cells and the fluctuations of the peak current in the diodes have an essential impact on the performance and reliability of the devices. Recently we have found that GaAs tunnel diodes exhibit extremely high peak currents that can be explained by resonant tunnelling through defects homogeneously distributed in the junction. Experiments evidence rather large fluctuations of the peak current in the diodes fabricated from the same wafer. It is a challenging task to clarify the reason for such large fluctuations in order to improve the performance of the multi-junction solar cells. In this work we show that the large fluctuations of the peak current in tunnel diodes can be caused by relatively small fluctuations of the dopant concentration. We also show that the fluctuations of the peak current become smaller for deeper energy levels of the defects responsible for the resonant tunnelling.

Compositional dependence of the band gap in Ga(NAsP) quantum well heterostructures

We present experimental and theoretical studies of the composition dependence of the direct band gap energy in Ga(NAsP)/GaP quantum well heterostructures grown on either (001) GaP- or Si-substrates. The theoretical description takes into account the band anti-crossing model for the conduction band as well as the modification of the valence subband structure due to the strain resulting from the pseudomorphic epitaxial growth on the respective substrate. The composition dependence of the direct band gap of Ga(NAsP) is obtained for a wide range of nitrogen and phosphorus contents relevant for laser applications on Si-substrate.

Non-Onsager mechanism of long-wave photogeneration in amorphous selenium at high electric fields

The quantum efficiency of the free-carrier-photogeneration in amorphous selenium avalanche blocking structures is studied experimentally in a wide range of wavelengths (380–600 nm) at high electric fields (10−112.5Vμm−1). While at comparatively small excitation wavelengths (up to ≃540nm), our experimental results are consistent with the Onsager theory of electron-hole pair dissociation [L. Onsager, Phys. Rev. 54, 554 (1938)], at larger wavelengths (540–600 nm) and high electric fields Onsager theory fails to explain our results. The reason for the failure of the Onsager approach is discussed and an alternative theoretical tool is adopted to account for the experimental observations.

Quantitative characterization of the interface roughness of (GaIn)As quantum wells by high resolution STEM.

The physical properties of semiconductor quantum wells (QW), like (GaIn)As/GaAs, are significantly influenced by the interface morphology. In the present work, high angle annular dark field imaging in (scanning) transmission electron microscopy ((S)TEM), in combination with contrast simulation, is used to address this question at atomic resolution. The (GaIn)As QWs were grown with metal organic vapor phase epitaxy on GaAs (001) substrates under different, precisely controlled conditions. In order to be able to compare different samples, a carefully applied method to gain reliable results from high resolution STEM micrographs was used. The thickness gradient of the TEM samples, caused by sample preparation, was compensated by the intensity of group V atomic columns, where no alloying takes place. After that, the In concentration map was plotted for the investigated regions based on the intensity of the group III atomic columns. The composition maps show that the Indium distribution across the quantum well is not homogeneous. The growth temperature of the QW can greatly influence the composition fluctuation and the interface morphology, with higher growth temperature resulting in larger composition fluctuations in the QWs and slightly wider interfaces, i.e. larger In-segregation. Growth interruptions are shown to significantly homogenize the elemental depth profile especially along the (GaIn)As/GaAs interface and hence have a positive effect on interface smoothness.

Influence of surface relaxation of strained layers on atomic resolution ADF imaging

Surface relaxation of thin transmission electron microscopy (TEM) specimens of strained layers results in a severe bending of lattice planes. This bending significantly displaces atoms from their ideal channeling positions which has a strong impact on the measured annular dark field (ADF) intensity. With the example of GaAs quantum wells (QW) embedded in a GaP barrier, we model the resulting displacements by elastic theory using the finite element (FE) formalism. Relaxed and unrelaxed super cells served as input for state of the art frozen phonon simulation of atomic resolution ADF images. We systematically investigate the dependencies on the sample´s geometric parameters, i.e. QW width and TEM sample thickness, by evaluating the simulated intensities at the atomic column´s positions as well as at the background positions in between. Depending on the geometry the ADF intensity can be affected in a range several nm from the actual interface. Moreover, we investigate the influence of the surface relaxation on the angular distribution of the scattered intensity. At high scattering angles we observe an intensity reduction at the interface as well as in the GaP barrier due to de-channeling. The amount of intensity reduction at an atomic column is directly proportional to its mean square displacement. On the contrary we find a clearly increased intensity at low angles caused by additional diffuse scattering. We discuss the implications for quantitative evaluations as well as strategies to compensate for the reduced intensities.