Semiconductor Optoelectronic Devices Research Articles

Making germanium, an indirect band gap semiconductor, suitable for light-emitting devices**Invited talk at the 7th International Workshop on Advanced Materials Science and Nanotechnology IWAMSN2014, 2-6 November, 2014, Ha Long, Vietnam.

Germanium (Ge) is a group-IV indirect band gap semiconductor but the difference between its direct and indirect band gap is only 140 meV. It has been shown that when Ge is subjected to a tensile strain and a heavy n-doping level, room-temperature photoluminescence (PL) can be greatly enhanced. Among these two factors, achieving a heavy n-doping level in Ge (i.e., electron concentrations higher than 1 × 1019 cm−3) is a challenge since the solubility of most group-V elements (P, As, Sb) in Ge is very low. We report here Ge growth on silicon substrates using molecular beam epitaxial (MBE) technique. To enhance the n-doping level in Ge, a specific n-doping process based on the decomposition of the GaP compound has been implemented. The GaP decomposition allows producing P2 molecules, which have a higher sticking coefficient than that of P4 molecules. We show that phosphorus doping at low substrate temperatures followed by flash thermal annealing are essential to get a high doping level. We have obtained an activate phosphorus concentration up to 2 × 1019 cm−3 and room-temperature PL measurements reveal an intensity enhancement up to 50 times. This result opens a new route for the realization of group-IV semiconductor optoelectronic devices.

Thermal Regime of High-power Laser Diodes

We discuss the design and application perspectives of different crystal, ceramic and composite-type submounts with thermo-compensating properties as well as submounts from materials with high thermal conductivity for overcoming thermal problem in high-power laser diodes (LD) and improving thermal management of other high-power optoelectronic and electronic semiconductor devices. Thermal fields in high-power laser diodes were calculated in 3 D thermal model at CW operation for some heatsink designs taking into account the experimental dependence of laser total efficiency against pumping current in order to extend the range of reliable operation up to thermal loads 20-30W and corresponding output optical power up to 15-20W for 100μm stripe laser diodes.

Analysis and design of resistance-wire heater in MOCVD reactor

Metal organic chemical vapor deposition (MOCVD) is a key equipment in the manufacturing of semiconductor optoelectronic devices and microwave devices in industry. Heating system is a vital part of MOCVD. Specific heating device and thermal control technology are needed for each new reactor design. By using resistance-wire heating MOCVD reaction chamber model, thermal analysis and structure optimization of the reactor were developed from the vertical position and the distance between coils of the resistance-wire heater. It is indicated that, within a certain range, the average temperature of the graphite susceptor varies linearly with the vertical distance of heater to susceptor, and with the changed distances between the coils; furthermore, single resistance-wire heater should be placed loosely in the internal and tightly in the external. The modulate accuracy of the temperature field approximately equals the change of the average temperature corresponding to the change of the coil position.

Electron concentration dependence of optical band gap shift in Ga-doped ZnO thin films by magnetron sputtering

Ga-doped ZnO (GZO) thin films were deposited on glass substrates by a radio frequency magnetron sputtering technique. The optical properties of the deposited GZO films were evaluated using an optical transmission measurement. The optical band gap increased from 3.32eV to 3.45eV with the increasing carrier density from 2.0×1020cm−3 to 3.24×1020cm−3. Based on the experimental results, the optical band gap as a function of carrier density is systematically investigated with four available theoretical models taken into consideration. The blueshift of the optical band gap in GZO films can be well interpreted with a complex model which combines the Burstein–Moss effect, the band gap renormalization effect and the nonparabolic nature of conduction band. In addition, the BM contribution is almost offset by the BGR effect in both conduction band and valence band due to the approximate equality between electron and hole effective masses in GZO films with a nonparabolic conduction band. The tunability of optical band gap in GZO thin films by carrier density offers a number of potential advantages in the development of semiconductor optoelectronic devices.

Charge transport mechanisms of graphene/semiconductor Schottky barriers: A theoretical and experimental study

Graphene has been proposed as a material for semiconductor electronic and optoelectronic devices. Understanding the charge transport mechanisms of graphene/semiconductor Schottky barriers will be crucial for future applications. Here, we report a theoretical model to describe the transport mechanisms at the interface of graphene and semiconductors based on conventional semiconductor Schottky theory and a floating Fermi level of graphene. The contact barrier heights can be estimated through this model and be close to the values obtained from the experiments, which are lower than those of the metal/semiconductor contacts. A detailed analysis reveals that the barrier heights are as the function of the interface separations and dielectric constants, and are influenced by the interfacial states of semiconductors. Our calculations show how this behavior of lowering barrier heights arises from the Fermi level shift of graphene induced by the charge transfer owing to the unique linear electronic structure.

Pseudo-binary alloying system of ZnO-AgGaO2 reducing the energy band gap of zinc oxide

Pseudo-binary oxide semiconductor alloy films of (1-x)ZnO-x(AgGaO2)1/2 were fabricated using a conventional rf-magnetron sputtering. The wurtzite-type single phases were obtained in the wide composition range of x ≤ 0.33 because the terminal β-AgGaO2 that corresponds to the composition with x = 1 possesses a wurtzite-derived β-NaFeO2 structure. The energy band gap of ZnO decreased with increasing AgGaO2 concentration, falling to 2.55 eV at x = 0.33. This alloy system enables to use ZnO-based semiconductors in optoelectronic devices working in visible region.

Microchannel cooling device with perforated side walls: Design and modeling

We propose and analyze a novel two-phase microchannel cooling device that incorporates perforated side walls for potential use as an embedded thermal management solution for high heat flux semiconductor devices. A dense array of perforated side walls separate alternating liquid and vapor microchannels, allowing the vapor generated through evaporation of liquid supplied through micro-perforations to flow only in the dedicated vapor channels. By separating the liquid and vapor flows, these “perspiring” side walls enable us to circumvent flow instabilities and other challenges associated with conventional two-phase microchannel cooling while at the same time effectively take advantage of the large extended surface areas available in high-aspect-ratio microchannels. One implementation of our design is parametrically analyzed using finite element modeling, demonstrating the potential of our proposed device for handling high heat flux electronic and optoelectronic semiconductor devices.

Templated Chemically Deposited Semiconductor Optical Fiber Materials

Chemical deposition is a powerful technology for fabrication of planar microelectronics. Optical fibers are the dominant platform for telecommunications, and devices such as fiber lasers are forming the basis for new industries. High-pressure chemical vapor deposition (HPCVD) allows for conformal layers and void-free wires of precisely doped crystalline unary and compound semiconductors inside the micro-to-nanoscale-diameter pores of microstructured optical fibers (MOFs). Drawing the fibers to serve as templates into which these semiconductor structures can be fabricated allows for geometric design flexibility that is difficult to achieve with planar fabrication. Seamless coupling of semiconductor optoelectronic and photonic devices with existing fiber infrastructure thus becomes possible, facilitating all-fiber technological approaches. The deposition techniques also allow for a wider range of semiconductor materials compositions to be exploited than is possible by means of preform drawing. Gigahertz bandwidth junction–based fiber devices can be fabricated from doped crystalline semiconductors, for example. Deposition of amorphous hydrogenated silicon, which cannot be drawn, allows for the exploitation of strong nonlinear optical function in fibers. Finally, crystalline compound semiconductor fiber cores hold promise for high-power infrared light–guiding fiber devices and subwavelength-resolution, large-area infrared imaging.

Molecular beam epitaxial growth of Bi2Te3 and Sb2Te3 topological insulators on GaAs (111) substrates: a potential route to fabricate topological insulator p-n junction

High quality Bi2Te3 and Sb2Te3 topological insulators films were epitaxially grown on GaAs (111) substrate using solid source molecular beam epitaxy. Their growth and behavior on both vicinal and non-vicinal GaAs (111) substrates were investigated by reflection high-energy electron diffraction, atomic force microscopy, X-ray diffraction, and high resolution transmission electron microscopy. It is found that non-vicinal GaAs (111) substrate is better than a vicinal substrate to provide high quality Bi2Te3 and Sb2Te3 films. Hall and magnetoresistance measurements indicate that p type Sb2Te3 and n type Bi2Te3 topological insulator films can be directly grown on a GaAs (111) substrate, which may pave a way to fabricate topological insulator p-n junction on the same substrate, compatible with the fabrication process of present semiconductor optoelectronic devices.

Naphtho[1,2-b:5,6-b′]dithiophene-Based Donor–Acceptor Copolymer Semiconductors for High-Mobility Field-Effect Transistors and Efficient Polymer Solar Cells

Two donor–acceptor (D–A) copolymers, PzNDTDTBT and PzNDTDTBO, using 4,9-bis(2-ethylhexyloxy)naphtho[1,2-b:5,6-b′]dithiophene as an electron-rich unit and benzodiathiazole (BT) or benzoxadiazole(BO) as an electron-deficient one, were designed and synthesized via a Pd-catalyzed Stille-coupling method. The acceptor units were varied from BT to BO for adjusting the energy levels and optimizing the structures of polymers. Both copolymers possess good solubility, high thermal stability, broad absorption, and low bandgap and exhibit not only high field-effect mobilities but also high photovoltaic properties. The hole mobilities reach 0.43 and 0.34 cm2 V–1 s–1 for PzNDTDTBT and PzNDTDTBO film, respectively. Bulk heterojunction solar cells fabricated by using PzNDTDTBT or PzNDTDTBO as electron donor and PC71BM as acceptor show a power conversion efficiency of 3.22% and 5.07%, respectively, under illumination of AM1.5G, 100 mW cm–2. Both the hole mobilities and PCEs are among the highest values in the current reports based on naphthodithiophene-based polymers, indicating that “zigzag” naphthodithiophene-based D–A copolymers are very promising for application as solution-processable organic semiconductors in optoelectronic devices.

Optical enhancement of the open-circuit voltage in high quality GaAs solar cells

The self-absorption of radiated photons increases the minority carrier concentration in semiconductor optoelectronic devices such as solar cells. This so-called photon recycling leads to an increase in the external luminescent efficiency, the fraction of internally radiated photons that are able to escape through the front surface. An increased external luminescent efficiency in turn correlates with an increased open-circuit voltage and ultimately conversion efficiency. We develop a detailed ray-optical model that calculates Voc for real, non-idealized solar cells, accounting for isotropic luminescence, parasitic losses, multiple photon reflections within the cell and wavelength-dependent indices of refraction for the layers in the cell. We have fabricated high quality GaAs solar cells, systematically varying the optical properties including the back reflectance, and have demonstrated Voc = 1.101 ± 0.002 V and conversion efficiencies of (27.8 ± 0.8)% under the global solar spectrum. The trends shown by the model are in good agreement with the data.

Transport Property Tuned by Gate Irradiation in ZnO Nanotetrapod Devices

The transport properties tuned by gate electron-beam irradiation was investigated for ohmic- and Schotty-contact-type semiconductor optoelectronic devices based on a single zinc oxide (ZnO) nanotetrapod. Measurements of the I–V characteristics and time-dependent current were conducted. The results indicate that, for both ohmic and Schottky contact devices, the electrical transport properties can be readily tuned by electron-beam irradiation at the gate leg of the tetrapod, with favorable repeatability and reversibility. The response for the Schottky-contact-type device was obviously greater than that for the ohmic-contact-type device, and the closer the irradiated position approached the crystal nucleus, the larger the current response became. A probable mechanism is proposed and discussed. The ZnO nanotetrapod could potentially be used as a detector in irradiation environments.

Exciton Scattering Mechanism in a Single Semiconducting MgZnO Nanorod

Excitonic phenomena, such as excitonic absorption and emission, have been used in many photonic and optoelectronic semiconductor device applications. As the sizes of these nanoscale materials have approached to exciton diffusion lengths in semiconductors, a fundamental understanding of exciton transport in semiconductors has become imperative. We present exciton transport in a single MgZnO nanorod in the spatiotemporal regime with several nanometer-scale spatial resolution and several tens of picosecond temporal resolution. This study was performed using temperature-dependent cathodoluminescence and time-resolved photoluminescence spectroscopies. The exciton diffusion length in the MgZnO nanorod decreased from 100 to 70 nm with increasing temperature in the range of 5 and 80 K. The results obtained for the temperature dependence of exciton diffusion length and luminescence lifetime revealed that the dominant exciton scattering mechanism in MgZnO nanorod is exciton-phonon assisted piezoelectric field scattering.

High power LED package with vertical structure

A LED package structure with vertical geometry of W5II is proposed for high power optoelectronic semiconductor devices. To provide an efficient and easy method of assembling high-power LEDs to sockets of the fixtures, a vertical design referenced from the typical Edison screw base found in US-based lamp systems is used in the structure. The thermal simulation of the structure, fabrication processes and thermal characteristic are evaluated. Thermal resistance measurements are performed to characterize the thermal performance of W5II, and the optical and electrical characteristics of W5II are also verified. The good heat dissipation of W5II could be utilized to enhance the reliability and thermal fatigue capability of high-power LED packages.

7.2: Integrated Organic Semiconductor Optoelectronic Devices as Real‐time and Indicator‐free Biosensor

AbstractA biosensor specific to nitric oxide is realized by integrating an organic light‐emitting diode, a photodetector, and a sensing hydrogel film. Two organic filters are also used to reduce the influence of the excitation light on the output photocurrent of the photodetector. The response time is less than 5 min.

Integrated semiconductor optoelectronic devices for real-time and indicator-free detection of aqueous nitric oxide

A real-time and indicator-free sensor specific to nitric oxide (NO) is realized for the first time by integrating a sensing hydrogel film, a top-emitting polymer light-emitting diode (PLED), and a silicon photodetector. The top-emitting PLED is utilized to excite the sensing film specific to NO, and the photoluminescence of the sensing film is transformed into electric signal by the photodetector. The influence of the excitation light on the out-put photocurrent of the photodetector is diminished by two delicately selected organic filters based on their absorption spectra. Lead phthalocyanine is deposited on a top-emitting PLED as a filter for generating an excitation light without overlapping the emission maximum of the sensing hydrogel film. Poly(3-hexylthiophene) is placed in front of the photodetector as another filter to remove the excitation light. This sensor demonstrated a fast photocurrent response to NO with response time within 5 min.

Dynamic isomers engaged fabrication of copper sulfide rattle-type structures and their optoelectronic properties

We demonstrated a new strategy for the preparation of the cubic phase Cu1.8S rattle-type structure via a dynamic isomers engaged solution route. The isomerization of thiourea into ammonium thiocyanate under solvothermal condition results in the formation of Cu1.8S and CuSCN in initial products. The transformation of CuSCN into Cu1.8S causes detachment between the core and shell. Studies show that the homogeneity and size of the rattle-type structure can be controlled by varying the concentration of precursor and PVP. These novel Cu1.8S rattle-type structures exhibit photoconductivity and may have potential applications in semiconductor optoelectronic devices.

Molecular beam epitaxy growth of GaAs on an offcut Ge substrate

Molecular beam epitaxy growth of GaAs on an offcut Ge (100) substrate has been systemically investigated. A high quality GaAs/Ge interface and GaAs film on Ge have been achieved. High temperature annealing before GaAs deposition is found to be indispensable to avoid anti-phase domains. The quality of the GaAs film is found to strongly depend on the GaAs/Ge interface and the beginning of GaAs deposition. The reason why both high temperature annealing and GaAs growth temperature can affect epitaxial GaAs film quality is discussed. High quality In0.17Ga0.83As/GaAs strained quantum wells have also been achieved on a Ge substrate. Samples show flat surface morphology and narrow photoluminescence line width compared with the same structure sample grown on a GaAs substrate. These results indicate a large application potential for III—V compound semiconductor optoelectronic devices on Ge substrates.

Evaluation of Static Performance of Optoelectronic Semiconductor Devices under X-Rays Irradiation

In this research, optoelectronic devices consisted of an infrared light emitting diode and a phototransistor with no special handling or third party-packaging were irradiated to ionizing radiation utilizing x-rays. It was found that the devices under test (DUTs) undergo performance degradation in their functional parameters during exposure to x-rays. These damaging effects are depending on their current drives and also the Total Ionizing Dose (TID) absorbed. The TID effects by x-rays are cumulative and gradually take place throughout the lifecycle of the devices exposed to radiation.

(Invited) Epitaxial Growth of III-Nitrides on Silicon Substrates

Hetero epitaxial structures with strained layers are widely used in electronic and optoelectronic semiconductor devices. Over the last decades the group III-Nitride semiconductor materials have been extensively used for light emitting applications and are nowadays emerging as promising candidates for high power electronic applications. As no native substrates are available for the III-Nitrides, growth is performed on foreign substrates such as sapphire, silicon carbide or silicon. III-Nitride epi layers grown on Silicon substrates offer a lower cost technology compared to other substrates because of the scalability towards larger wafer size and the leverage on Si technology. However, the large thermal mismatch between III-Nitrides and Silicon requires advanced deposition schemes to allow compensating for the huge tensile thermal stresses and to reduce the dislocation density of these materials.