T. Mei

Atomic nitrogen doping and p-type conduction in SnO2

We report the atomic N-doped SnO2 films with p-type conduction grown via reactive sputtering at high nitrogen partial pressure. From the high-resolution x-ray photoelectron spectroscopy (XPS) and x-ray diffraction patterns, it is deduced that the N 1s with binding energy of 397 eV could be attributed to the atomic N in the SnO2 films. In addition, the results of Hall effect measurement indicate that the atomic N incorporated substitutionally at O sites act as acceptors, which is responsible for the p-type conduction of the N-doped SnO2 films. It is believed that these findings should stimulate further research on p-type SnO2 films and SnO2-based ultraviolet optoelectronic devices.

Near- and mid-infrared detection using GaAs∕InxGa1−xAs∕InyGa1−yAs multiple step quantum wells

A dual-band multiple-quantum-well infrared photodetector capable of simultaneously detecting wavelengths near 0.9 μm and 10 μm has been fabricated using GaAs∕InGaAs step quantum wells. The detection of the near (0.82–0.95 μm)- and mid (9–11 μm)-infrared wavelength bands was achieved using interband and intersubband transitions. The measured peak responsivities of the near- and mid-infrared bands were 0.4 A∕W and 1 A∕W, respectively, at 0.8 V bias across the device. The broken symmetry of the step quantum well allows transitions from the ground states of heavy and light holes to the first-excited electron state allowing the photoexcited carriers to be efficiently collected. The estimated values of the detectivities for near- and mid-infrared bands at 40 K and 0.8 V bias are approximately 4.5×109cm(Hz)1∕2∕W and 1.1×1010cm(Hz)1∕2∕W, respectively.

Evaluation of thermal parameters of bolometer devices

Thermal conductance G, thermal capacitance H, and the time constant τ are major parameters to be determined in the design of bolometer devices and, therefore, it is an important task to evaluate these thermal parameters experimentally for design verification. In this work, a technique has been developed to determine these parameters with high accuracy by convenient electrical measurement where a high-voltage short-duration pulse was used. The method also incorporates operation conditions in the measurement and thus reveals the bias dependency of the thermal parameters. The approach was experimentally confirmed by using metal–film microbolometer devices with different values of both G and H. Test results were in a good agreement with the theoretical analysis of the measurement technique.

Photoluminescence enhancement by inductively coupled argon plasma exposure for quantum-well intermixing

The exposure of InGaAs/InGaAsP quantum-well (QW) structures to argon (Ar) plasma in an inductively coupled system has been studied. An increase in photoluminescence (PL) intensity without PL peak shift was observed for 5-min Ar plasma exposure compared to the as-grown sample. The exposure creates point defects, and upon rapid thermal annealing produces intermixing between barriers and QWs, resulting in the blueshift of QWs. A selective intermixing using a 200-nm-thick of SiO2 layer as an intermixing mask exhibited a differential band-gap blueshift of 86 nm, with a differential linewidth broadening of 0.3 nm between masked and unmasked section. The improvement of PL intensity in combination with selective intermixing process can pave the way for high-quality hybrid photonic and optoelectronic integrated circuits.

Thermally induced diffusion in GaInNAs∕GaAs and GaInAs∕GaAs quantum wells grown by solid source molecular beam epitaxy

The effects of the thermal annealing induced diffusion on the photoluminescence (PL) of a GaAs∕GaInAs∕GaAs∕GaInNAs∕GaAs quantum well (QW) structure grown by solid source molecular beam epitaxy are studied. The PL experimental results in conjunction with the numerical quantum-mechanical modeling that predicts the changes in the QW confining potential with group-III atomic diffusion, have been used to obtain the values for diffusion coefficient. The activation energies of GaInAs∕GaAs QW (ED,GIA) were found to be between 0.49to0.51eV, while that of GaInNAs∕GaAs QW (ED,GINA) showed comparable values of between 0.6 to a 0.67eV, as annealing time increases from 10to30s. The ED,GIA and ED,GINA values are attributed to the same interstitial diffusion mechanism.

Single step quantum well intermixing with multiple band gap control for III-V compound semiconductors

We report a quantum well intermixing technique based on Ar plasma induced damage on both GaAs- and InP-based materials with single-step multiple band gap creation across a substrate. A quantum well structure with multiplewidths serves as a sensitive tool to probe the damage created by Ar plasma. The analysis reveals that the surface defects were created up to a certain depth and propagated deeper into the material upon subsequent annealing. A simple and reliable way to obtain a controlled multiple band gap was achieved by using the spatial defect modulated intermixing. Eight band gap levels were realized across a single chip of quantum well laser structure with a linear relationship to the fraction of the open area under plasma exposure. This simple approach can be implemented at a postgrowth level to a wide range of material systems to achieve multiple band gaps, suitable for photonic integration.

Argon plasma exposure enhanced intermixing in an undoped InGaAsP∕InP quantum-well structure

Intermixing in an undoped InGaAsP∕InP quantum well structure enhanced by near-surface defects generated using inductively coupled argon plasma is temperature dependent. The group III sublattice interdiffusion can be four times as fast as that of the group V sublattice for the annealing temperature lower than 600°C, and a maximum band gap redshift of 50nm is obtained in experiment. Blueshift is obtained at 700°C when the group V sublattice interdiffusion becomes appreciable.

GaAs/AlGaAs quantum well intermixing using high-density argon plasma

We report the Ar plasma induced quantum well intermixing technique for bandgap modification of GaAs/AlGaAs quantum well structures. The application of Ar inductively coupled plasma enhances the intermixing via the energetic ion bombardment mechanism and the effect of high plasma density. A bandgap shift up to 39.6 meV (20.5 nm) with a linewidth broadening of 4.2 meV (1.5 nm) was obtained. The bandgap shift was observed to saturate with increasing plasma exposure time in good agreement with the theoretical model that most beneficial point defects are introduced at the early exposure stage. To further reveal the mechanism of intermixing, the plasma-induced damage distribution range was investigated after exposure and subsequent annealing using a structure of GaAs/AlGaAs quantum wells of different well widths as a highly sensitive damage probe. The result indicates that the point defects are created near the sample surface during Ar plasma exposure and upon annealing they propagate downwards from the surface to promote quantum well intermixing to a depth beyond ~2.32 µm.

Halftoning band gap of InAs/InP quantum dots using inductively coupled argon plasma-enhanced intermixing

Inductively coupled argon plasma-enhanced intermixing of InAs∕InP quantum dots grown on InP substrate is investigated. Intermixing is promoted by the near-surface defects generated by plasma exposure in annealing at a temperature of 600°C for 30s. The annealing results in a maximum differential band-gap blueshift of 106nm but a thermal shift of only 10nm. Band-gap halftones are obtained by controlling the amount of near-surface defects via wet chemical etching on the plasma-exposed InP cap layer. No degradation of quantum-dot crystal quality due to the process has been observed as evidenced by photoluminescence intensity.

Growth of p-type GaAs/AlGaAs"111… quantum well infrared photodetector using solid source molecular-beam epitaxy

A p-type GaAs∕AlGaAs multi-quantum-well infrared photodetector (QWIP) was fabricated on a GaAs (111)A substrate by molecular-beam epitaxy using silicon as dopant. The same structure was also grown on a GaAs (100) wafer simultaneously to compare the material and structural properties. It was found that Si acts as a p-type dopant in the GaAs (111)A sample while it is n-type in the GaAs (100) counterpart. The growth rate was found to be appreciably enhanced for GaAs (111)A compared with that of GaAs (100) orientation, while the Al composition in the barriers was found to be 20% smaller for a (111) orientation which results in a smaller barrier height. A peak responsivity of 1mA∕W with a relatively wide wavelength response (Δλ∕λp∼53%) was observed for the GaAs (111)A QWIP, mainly due to the location of the excited state far above the barrier. The photoresponse also showed a relatively strong normal incident absorption probably originating from the mixing of the conduction and valence Bloch states. The optimiz...