H. H. Cheng

Mid-infrared electroluminescence from a Ge/Ge0.922Sn0.078/Ge double heterostructure p-i-n diode on a Si substrate

We report the observation of mid-infrared room-temperature electroluminescence from a p-i-n Ge/Ge0.922Sn0.078/Ge double heterostructure diode. The device structure is grown using low-temperature molecular beam epitaxy. Emission spectra under various injection current densities in the range of 318u2009A/cm2–490u2009A/cm2 show two distinct profiles peaked at 0.545u2009eV (2.275u2009μm) and 0.573u2009eV (2.164u2009μm), corresponding to indirect and direct bandgaps of the Ge0.922Sn0.078 active layer, respectively. This work represents a step forward towards the goal of an efficient direct-bandgap GeSn light-emitting device on a Si substrate by incorporating higher Sn content of 7.8% in a diode structure that operates at lower current densities.

GeSn-based p-i-n photodiodes with strained active layer on a Si wafer

We report an investigation of GeSn-based p-i-n photodiodes with an active GeSn layer that is almost fully strained. The results show that (a) the response of the Ge/GeSn/Ge heterojunction photodiodes is stronger than that of the reference Ge-based photodiodes at photon energies above the 0.8 eV direct bandgap of bulk Ge (<1.55u2009μm), and (b) the optical response extends to lower energy regions (1.55–1.80u2009μm wavelengths) as characterized by the strained GeSn bandgap. A cusp-like spectral characteristic is observed for samples with high Sn contents, which is attributed to the significant strain-induced energy splitting of heavy and light hole bands. This work represents a step forward in developing GeSn-based infrared photodetectors.

Sn-based Ge/Ge0.975Sn0.025/Ge p-i-n photodetector operated with back-side illumination

We report an investigation of a GeSn-based p-i-n photodetector grown on a Ge wafer that collects light signal from the back of the wafer. Temperature dependent absorption measurements performed over a wide temperature range (300u2009K down to 25u2009K) show that (a) absorption starts at the indirect bandgap of the active GeSn layer and continues up to the direct bandgap of the Ge wafer, and (b) the peak responsivity increases rapidly at first with decreasing temperature, then increases more slowly, followed by a decrease at the lower temperatures. The maximum responsivity happens at 125u2009K, which can easily be achieved with the use of liquid nitrogen. The temperature dependence of the photocurrent is analyzed by taking into consideration of the temperature dependence of the electron and hole mobility in the active layer, and the analysis result is in reasonable agreement with the data in the temperature regime where the rapid increase occurs. This investigation demonstrates the feasibility of a GeSn-based photodio...

Temperature-dependent electroluminescence from GeSn heterojunction light-emitting diode on Si substrate

The electroluminescence from a Ge/GeSn/Ge p–i–n light-emitting diode on Si was investigated under different temperatures ranging from 25 to 150 K. The diode was operated at a low injection current density of 13 A/cm2. We obtained no-phonon- and phonon-assisted replicas in emission spectra. Also, the relationship between indirect bandgap energy and temperature was investigated. The temperature-dependent bandgap energy followed Varshnis empirical expression with α = 4.884 × 10−4 eV/K and β = 130 K.

Disorder-induced enhancement of indirect absorption in a GeSn photodetector grown by molecular beam epitaxy

We report an investigation on the absorption mechanism of a GeSn photodetector with 2.4% Sn composition in the active region. Responsivity is measured and absorption coefficient is calculated. Square root of absorption coefficient linearly depends on photon energy indicating an indirect transition. However, the absorption coefficient is found to be at least one order of magnitude higher than that of most other indirect materials, suggesting that the indirect optical absorption transition cannot be assisted only by phonon. Our analysis of absorption measurements by other groups on the same material system showed the values of absorption coefficient on the same order of magnitude. Our study reveals that the strong enhancement of absorption for the indirect optical transition is the result of alloy disorder from the incorporation of the much larger Sn atoms into the Ge lattice that are randomly distributed.

Sn-based waveguide p-i-n photodetector with strained GeSn/Ge multiple-quantum-well active layer

We report on Sn-based p-i-n waveguide photodetectors (WGPD) with a pseudomorphic GeSn/Ge multiple-quantum-well (MQW) active layer on a Ge-buffered Si substrate. A reduced dark-current density of 59u2009u2009mA/cm2 was obtained at a reverse bias of 1xa0V due to the suppressed strain relaxation in the GeSn/Ge active layer. Responsivity experiments revealed an extended photodetection range covering the O, E, S, C, and L telecommunication bands completely due to the bandgap reduction resulting from Sn-alloying. Band structure analysis of the pseudomorphic GeSn/Ge quantum well structures indicated that, despite the stronger quantum confinement, the absorption edge can be shifted to longer wavelengths by increasing the Sn content, thereby enabling efficient photodetection in the infrared region. These results demonstrate the feasibility of using GeSn/Ge MQW planar photodetectors as building blocks of electronic-photonic integrated circuits for telecommunication and optical interconnection applications.

Diode-like electrical characteristics of SiGe wrinkled heterostructure operating under both forward and reverse bias

We report the electrical behaviour of heterostructure channels with spatially deformed wrinkle patterns at the edge. Instead of the linear current–voltage relationship, a diode-like current–voltage trace is observed under both forward and reverse bias. Analysing the position-dependent strain and energy levels of the wrinkled heterostructure shows that the energy minimum transforms from a two-dimensional plane at the heterointerface to a one-dimensional trajectory at the wrinkled edge characterized by a potential. When a voltage is applied, the carriers at the left and right electrodes travel through a one-dimensional potential, analogously to how carriers move across a potential in the p-n junction, resulting in diode-like electrical characteristics. This work represents a step forward in developing the wrinkled structure for electronic devices.

Investigation of Ge1-xSnx/Ge quantum-well structures as optical gain media

An efficient Si-based laser is one of the most important components for photonic integrated circuits to break the bottleneck of data transport over optical networks. The main challenge is to create gain media based on group-IV semiconductors. Here we present an investigation of using low-dimensional Ge1-xSnx/Ge quantum-well (QW) structures pseudomorphically grown on Ge-buffered Si substrates as optical gain media for efficient Si-based lasers. Epitaxial growth of Ge1-xSnx/Ge QW structures on Ge-buffer Si substrate was carried out using low-temperature molecular beam epitaxy techniques. The light emission properties of the grown Ge1-xSnx/Ge QW structure were studied using photoluminescence spectroscopy, and clear redshifts of emission peaks were observed. Theoretical analysis of band structures indicates that Ge1-xSnx well sandwiched by Ge barriers can form type-I alignment at Г point with a sufficient potential barrier height to confine carriers in the Ge1-xSnx well, thereby enhancing efficient electron-hole direct recombination. Our calculations also show that the energy difference between the lowest Г-conduction subband and L conduction subband can be reduced with increasing Sn content, thereby enabling optical gain. These results suggest that Ge1-xSnx/Ge QW structures are promising for optical gain media to develop efficient Si-based light emitters.

Electrooptical Properties of InGaAs/GaAs Strained Single Quantum Wells

The electrooptical properties of (100) and (111)B InGaAs/GaAs strained single quantum wells have been studied by the photoreflectance spectroscopy at various temperatures and under different biases. There is a Si δ-doping sheet under the GaAs surface to screen the surface built-in electric field from the quantum well. The spectral features consist of the excitonic interband transitions in the quantum well, and the Franz-Keldysh oscillations from the band edge transitions of the GaAs barrier near the surface. The piezoelectric field tilts the (111)B single quantum well toward the δ-doping and causes subband filling by the electrons from the δ-doping centers. With a proper external bias to repel electrons from the tilted quantum well, the excitonic interband transition in the (111)B system is enhanced.