T Sass

One-dimensional heterostructures in semiconductor nanowhiskers

We report on the growth of designed heterostructures placed within semiconductor nanowhiskers, exemplified by the InAs/InP material system. Based on transmission electron microscopy, we deduce the interfaces between InAs and InP to be atomically sharp. Electrical measurements of thermionic emission across an 80-nm-wide InP heterobarrier, positioned inside InAs whiskers 40 nm in diameter, yield a barrier height of 0.6 eV. On the basis of these results, we propose new branches of physics phenomena as well as new families of device structures that will now be possible to realize and explore.

High peak-to-valley ratios observed in InAs/InP resonant tunneling quantum dot stacks

Resonant tunneling was observed through single InAs quantum dot (QD) stacks embedded in InP barriers with peak-to-valley ratios as high as 85 at 7 K. Negative differential resistance in the current–voltage [I(V)] characteristics was obtained up to a point above the temperature of liquid nitrogen. These features were observed in measurements on low-density QD stacks, in which a macroscopic ohmic contact covered less than 150 QD stacks. Due to the design of the structure, the upper QD in the stack has the function of a zero-dimensional emitter. Electrons easily fill the upper dot, whereas tunneling through the entire structure is only allowed when two states in the dots align energetically, resulting in sharp resonant tunneling peaks with high peak-to-valley ratios.

Oxidation and reduction behavior of Ge/Si islands

We have investigated the oxidation/reduction behaviour of dome-shaped three-dimensional islands of Ge on Si(001) grown by UHV-CVD at 620°C. The oxidation was done by exposing the surfaces to a steam of H2O in N2. The reduction was done in H2, which at T<800 °C selectively reduces GeO2 only. The results of the oxidation/reduction processes under varying conditions were analyzed by high-resolution transmission electron microscopy. We found that the selective reduction of such structures does not result in a perfect recovery of the former Ge dots, but results in phase-segregated Ge-enrichments. In most cases, these enrichments show epitaxial relationship to the underlying Si substrate. These structures are therefore of potential interest for lateral epitaxial overgrowth of the SiO2 by Ge, using the reduced Ge dots as the seeds for epitaxy (Less)

Designed emitter states in resonant tunneling through quantum dots

Resonant tunneling through a single layer of self-assembled quantum dots (QDs) is compared to tunneling through two layers of vertically aligned (stacked) dots. The difference can be viewed as going from a two-dimensional emitter to a zero-dimensional emitter. The temperature dependence of current peaks originating in tunneling through individual QDs and individual stacks is used to clarify this point. In addition, we show that the statistical size distribution of self-assembled quantum dots causing the inhomogeneous broadening in luminescence experiments can be analyzed in a resonant tunneling experiment

Luminescence polarization of ordered GaInP/InP islands

The luminescence polarization properties of GaInP islands have been investigated. The islands, which form during overgrowth of InP quantum dots, were studied using scanning tunneling luminescence (STL) and photoluminescence (PL). STL from these islands shows emission at an energy below the main emission peak of the bulk GaInP. The linear PL polarization anisotropy was measured at low temperature, for which the emission from the islands shows high polarization anisotropy. The intensity maximum for the emission occurs for light polarized parallel to the elongation of the islands. The observed linear PL polarization anisotropy indicates the presence of highly ordered domains of GaInP in the islands.

Metalorganic vapor phase epitaxy-grown GaP/GaAs/GaP and GaAsP/GaAs/GaAsP n-type resonant tunnelling diodes

We have studied GaP/GaAs/GaP and GaAsxP1−x/GaAs/GaAsxP1−x double-barrier resonant tunnelling diodes grown by metalorganic vapor phase epitaxy. We find that GaP tensile strained barriers in GaP/GaAs/GaP diodes may be grown with a barrier thickness below the critical thickness of about 12 monolayers. However, a corrugation of the strained barrier is observed by transmission electron microscopy. This variation may explain the low peak-to-valley ratio of the diodes (about 2). In contrast, GaAsxP1−x/GaAs/GaAsxP1−x resonant tunnelling diodes have been grown with a homogeneous thickness of the barriers, consequently showing a substantially improved electrical performance compared to the GaP diodes with peak-to-valley ratios >5.

Strain in GaP/GaAs and GaAs/GaP resonant tunnelling heterostructures

We studied the morphology of GaP/(001)GaAs and GaAs/(001)GaP heterostructures grown by metal-organic vapour-phase epitaxy and found wire-like surface undulations elongated in the [110] direction. We attribute this elongation to anisotropic lateral growth rates in the [110] and [110] directions, which are due to a different roughness of monolayer surface steps. In III-V materials grown by molecular beam epitaxy. such surface corrugations are usually elongated in [110]. We explain this difference by the two growth methods having inverted ratios of lateral growth rates in [110] and [110]. Resonant tunnelling diodes fabricated from the GaP/GaAs heterostructures showed very symmetric I-V characteristics. Their peak-to-valley ratio was limited to 2. most probably due to the corrugation of the GaP barriers

Thin layers of GaInP, GaP and GaAsP in metalorganic vapour phase epitaxy-grown resonant tunnelling diodes

We have studied the epitaxial growth and electrical performance of Al-free, GaAs-based, resonant tunnelling diodes (RTDs) including thin barriers of GaInP, GO, or GaAsxP1-x. n-Type tunnelling diodes have been fabricated and the symmetry in the current-voltage (I-V) characteristics, as well as the peak-to-valley ratios, are found to be sensitive probes for the inter-face quality in the heterostructures. For GaInP RTDs, we show that the introduction of Gap intermediate layers is crucial for the realisation. of a useful tunnelling current. RTDs including thin barriers (less than about 10 monolayers (ML)) of Gap are realised, but the strong mismatch between the materials limit the useful thickness. Finally, RTDs with GaAslambdaP1-x alloys are fabricated showing the best peak-to-valley ratio of the diodes (about 5), as well as a symmetric I-V characteristics. The electrical data are further compared to studies by transmission electron microscopy (TEM) in the various material systems. (Less)

Site control of InAs quantum dots on a patterned InP surface: As/P exchange reactions

In this paper, we present the effect of annealing temperature and annealing time on InAs site-controlled quantum dot growth. Individual InAs quantum dots formed by self-assembling have been positioned into holes, created by partial overgrowth of electron beam induced nano-carbon deposits by metal organic vapor phase epitaxy. As/P exchange reactions produce material sufficient for selective dot nucleation in the holes. Results, showing that As/P exchange reactions occur even when capping the dots with InP are presented.

The effect of GaP in Al-free, GaAs-based resonant tunnelling diodes

We have studied the role of GaP intermediate layers in GaInP/GaAs double barrier resonant tunnelling structures. Measurements of the current-voltage characteristics show an asymmetric behaviour which was improved in diodes including thicker GaP intermediate layers. From cross-sectional transmission electron microscopy investigations we have observed a fluctuation in the thickness of the GaP layers at the GaInP to GaAs transition to which we attribute the asymmetric characteristics. Finally, we demonstrate a simplified structure using only GaP layers as barriers. These new diodes show more symmetric I-V characteristics than the GaInP/GaAs tunnelling diodes.