Aristo Yulius

Quasi-direct UV/blue GaP avalanche photodetectors

GaP avalanche photodiodes, with thin device layers have been processed, utilizing both p-i-n and recessed window p-i-n structures, as well as a Schottky structure. The results showed low dark currents, good quantum efficiency (QE), and high gains up to 10/sup 3/, with good uniformity across the wafer. The peak QE at 440 nm indicated /spl Gamma/-valley absorption, rather than band-edge absorption. The recess window photodiodes exhibited enhanced UV detection as a result of reduced absorption and recombination in the undepleted p-layer. Additionally, the Schottky structure demonstrated potential for further enhanced UV detection, by employing a thin semitransparent contact.

A hybrid epitaxy method for InAs on GaP

The interface formation mechanism during the molecular-beam epitaxy (MBE) of InAs∕GaP has been studied with the aid of the In–Ga–P phase diagram. It is discovered that an initial dissolution and crystallization process similar to liquid phase epitaxy (LPE) may happen at sufficiently high temperature, resulting in a graded composition at the interface. Consequently, “parasitic LPE∕MBE” is the name for this hybrid form of MBE. High-resolution TEM images confirm the existence of the interfacial layer in the sample grown at high temperature. The graded interface smears out the band offset and leads to a nonrectifying heterojunction. Low-temperature (LT) MBE growth can turn off the LPE component, enabling the growth of an abrupt interface. Based on this “LPE∕MBE” model, a LT MBE technique is developed to grow an abrupt InAs∕InGaP interface for heterojunction power Schottky rectifiers. The LT InAs∕InGaP heterojunction demonstrates nearly ideal Schottky rectifier characteristics, while the sample grown at high t...

Drift dominated InP photodetectors with high quantum efficiency

We present a drift dominated InP N-I-P photodetector, which can more efficiently collect the carriers generated by photon absorption by drift created by an electric field throughout the active layer instead of by diffusion associated with normal PN or PIN junction diodes and results in high and flat spectral response from UV to near infrared. It also promises to be an avalanche photodiode with high responsitivity, high gain-bandwidth product, and low noise.

High Quality Epitaxially-grown InAs on GaP Substrates

InAs is a very attractive narrow bandgap semiconductor because it has a high electron mobility, has a high electron drift velocity (Vsat ≈ 1 x 10 cm/s), and is very easy to make non-alloyed ohmic contact on it. However, due to its nature of having a narrow bandgap, there is no semi-insulating InAs substrate exists. This makes isolation between devices extremely difficult, if not impossible. On the other hand, GaP is a wide bandgap semiconductor and it’s lattice-matched to silicon, which makes it a perfect candidate for a substrate and large scale integration. Since there is about 11% lattice-mismatch between InAs and GaP, the process of growing InAs on GaP is pretty challenging. Here, we show that we are able to get high quality InAs grown on GaP substrates. The InAs electron mobility is improved to about 12,000 cm/V-s and the background electron concentration is reduced to about 3 x 10 cm as measured by Hall effect. The InAs surface has a smooth morphology free of cross-hatches with the RMS surface roughness value of about 14Å as measured by AFM. The surface pit density is also reduced by one order of magnitude to about 10 cm. The growth parameters as well as the benefits of the StrainedLayer Superlattices (SLS) which lead to the improvements above will be covered in details.

Efficient drift dominated photodiodes using defected materials

Lattice-mismatched crystal growth can result in a high density of defects, which usually degrades the performance of optoelectronic devices. We demonstrate a drift dominated photodiode, which has high responsivity even with defected material. A drift dominated InP grown on GaP substrate photodiode, with the 8% lattice mismatch, has been developed for InP on Si application. Meanwhile, a conventional p-i-n InP on GaP photodiode was also made as a control sample. Results show that with the same defected material, the drift dominated device has much higher quantum efficiencies than those of the p-i-n device, especially at short wavelengths. The internal quantum efficiencies of the drift dominated InP on GaP photodiode are higher than 70% in UV and visible region.

A high performance photodetector using a novel drift dominated structure in defected materials

By integrating InP photodiodes with Si, we can take advantage of the low cost and robustness of large Si substrates. However, the major challenge of this strategy is the high density of dislocations in InP grown on Si, due to the 8% lattice mismatch and large difference in thermal expansion coefficient. Large concentrations of dislocations act as recombination centers which greatly deteriorates the performance of the InP photodiodes. We have developed InP photodiodes whose photo-active regions have large electric fields in order to achieve high quantum efficiencies, even with defected material. We use a GaP substrate as the first step since GaP is lattice matched to Si, which could be used as a buffer layer between InP and Si. We compared two different structures: a normal p-i-n structure and a drift dominated structure.

Reliable contacts to two-dimensional conduction layers

For many experiments and device applications, electrical contacts to a two-dimensional conduction layer must remain reliable under repeated temperature cycling between 300 and 77 K or lower. This work introduces the use of a silicon-doped InAs contact to the AlGaAs/GaAs two-dimensional electron gas which demonstrates exceptional reliability under such temperature cycling. The noise spectrum of AlGaAs/GaAs contacted with silicon-doped InAs shows almost no dependence on bias current; this fact can be used to improve the performance of device applications such as Hall sensors. In addition, this work introduces an alternative two-dimensional conduction structure, highly mismatched InAs/GaP. InAs/GaP contacted with Ti/Au shows reliability equal to AlGaAs/GaAs contacted with silicon-doped InAs. The InAs/GaP material may be more desirable for some applications because of the lower temperature dependence of its electronic properties and potentially easier integration with silicon-based microelectronics.

Ballistic transport and Andreev resonances in Nb/In superconducting contacts to InAs and LTG-GaAs

Abstract We have formed superconducting contacts in which Cooper pairs incident from a thick In layer must move through a thin Nb layer to reach a semiconductor, either InAs or low temperature grown (LTG) GaAs. The effect of pair tunneling through the Nb layer can be seen by varying the temperature through the critical temperature of In. Several of the In/Nb–InAs devices display a peak in the differential conductance near zero-bias voltage, which is strong evidence of ballistic transport across the NS interface. The differential conductance of the In/Nb–LTG-GaAs materials system displays conductance resonances of McMillan–Rowell type. These resonant levels exist within a band of conducting states inside the energy gap, formed from excess As incorporation into the LTG-GaAs during growth. Electrons propagating in this band of states multiply reflect between the superconductor and a potential barrier in the GaAs conduction band to form the conductance resonances. A scattering state theory of the differential conductance, including Andreev reflections from the composite In/Nb contact, accounts for most qualitative features in the data.