S. W. Downey

Stability of carbon and beryllium‐doped base GaAs/AlGaAs heterojunction bipolar transistors

GaAs/AlGaAs heterojunction bipolar transitors (HBTs) utilizing highly Be‐doped base layers display a rapid degradation of dc current gain and junction ideality factors during bias application at elevated temperature. For example, the gain of a 2×10 μm2 device with a 4×1019 cm−3 Be‐doped base layer operated at 200 °C with a collector current of 2.5×104 A cm−2 falls from 16 to 1.5 within 2 h. Both the base emitter and base collector junction ideality factors also rise rapidly during device operation, and this current‐induced degradation is consistent with recombination‐enhanced diffusion of Be interstitials producing graded junctions. By sharp contrast, devices with highly C‐doped (p=7×1019 cm−3) base layers operated under the same conditions show no measurable degradation over much longer periods (12 h). This high degree of stability is most likely a result of the fact that C occupies the As sublattice, rather than the Ga sublattice as in the case of Be, and also has a higher solubility than Be. The effect...

N‐ and P‐type dopant profiles in distributed Bragg reflector structures and their effect on resistance

We report the n‐ and p‐type dopant profiles obtained in AlAs/GaAs structures. When structures were doped with Si or C, targeted dopant profiles were achieved. However, when Be was used, significant redistribution of the dopant occurred, which resulted in an accumulation of Be on the GaAs side of the heterointerfaces, and a maximum incorporation of Be of ∼5×10−17 cm−3 in the AlAs layers. Similar results were obtained for Be dopant profiles in AlGaAs/AlAs quarter‐wave distributed Bragg reflectors (DBR). Be segregation and diffusion during growth resulted in a high electrical resistance in these DBR structures. The resistance was significantly reduced when the structure was doped with C.

Single Particle Characterization by Time-of-Flight Mass Spectrometry

We have built a particle analyzer capable of real-time detection and characterization of individual particles. Particle analysis is accomplished by pulsed laser ablation of a particle followed by time-of-flight mass spectrometry of the resulting atomic and molecular ions. The detected ions are characteristic of a particles molecular composition. Particles of different sizes (0.02–10 μm) and compositions (ammonium sulfate, silica, rubidium nitrate, and cesium nitrate) have been detected and characterized. Even refractory materials, e.g. silica, are easily detected.

Photoemission optogalvanic spectroscopy: An in situ method for plasma electrode surface characterization

Photoemission optogalvanic spectroscopy (POGS) is shown to be useful for plasma electrode surface characterization. A pulsed ultraviolet laser is used to induce photoemission from the electrode surface in a radio frequency plasma reactor and the increase in plasma current is detected. The photoemission process is first characterized in vacuum and then compared to that in several plasma gases using Al and Si electrodes. In vacuum, the laser‐induced photoemission signal is generally consistent with space‐charge‐limited current. When below the space‐charge limit, the magnitude of the photoemission signal depends upon laser wavelength and power, surface composition, and film thickness. The removal of SiO2 from Si and the contamination of Al in fluorine‐containing plasmas is monitored using this technique. A large increase in the POGS signal is observed as the oxide is removed from Si or when a fluorinated Al surface is exposed to an O2 plasma. The POGS signal decreases with fluorine exposure with both Al and ...

Resonance ionization mass spectrometry of AlxGa 1− xAs: depth resolution, sensitivity, and matrix effects

Resonance ionization mass spectrometry (RIMS) of neutral atoms sputtered from III-V compound semiconductors such as Al(x)Ga(1-x)As provides information that is complementary to secondary ion mass spectrometry with the added advantages of rejecting mass interferences, retaining good sensitivity, and reducing matrix effects. A GaAs sample, delta doped with Be, is used to measure depth resolution and Be secondary ion and atom yield. Because of the coupling of the pulsed RIMS lasers and continuous sputtering beam, duty cycle factors are used to determine the atom yield. A 3-D model of the geometrical overlap of laser and sputtered atoms is developed to ascertain the same utilization efficiency in RIMS. About 30% of the atoms sputtered in 1 micros are calculated to be in the laser beam. The atom yield was found to be near unity. The time-gated RIMS useful yield is ~2%. RIMS is used to minimize matrix effects in a depth profile of a Be-implanted AlAs/A1(0.2)Ga(0.8)As heterostructure and shows that Be diffuses from higher Al-containing layers at concentrations near 10(19) cm(-3). The atomization of As is shown to be affected by the Al content in a GaAs/Al(0.5)Ga(0.5)As structure.

Real-time compositional analysis of submicrometre particles

A particle analyser is described that simultaneously detects and characterizes 10 mu m diameter particles independent of particle composition in real time. No previous instrument has been able to perform these functions simultaneously. Our design uses pulsed laser ablation of particles followed by time-of-flight mass spectrometric analysis of the resulting ions. The ion intensity is related to particle size at least for small particles. Thus particle size information is obtained concurrently with the other information.

Evidence of very strong inter-epitaxial-layer diffusion in Zn-doped GaInPAs/InP structures

Zn-doped InP and GaInPAs layers were grown by OrganoMetallic Vapor-Phase Epitaxy (OMVPE). The epitaxial films consist of a primary GaInPAs/InP epitaxial layer and a secondary InP/GaInAs epitaxial layer. We present evidence that the redistribution of Zn acceptors in the primary epitaxial layer is strongly influenced by the Zn doping concentration in the secondary epitaxial layer. Rapid redistribution of Zn acceptors in the primary epitaxial layer occurs if the Zn doping concentration in the secondary epitaxial layer exceeds a critical concentration ofNZn≅3×1018cm−3. The influence of the growth temperature on this effect is also presented.

Improved Precision in a Resonance Ionization Mass Spectrometer by the Use of Stark-Shifted Spectral Lines as a Probe for Extraction Field

High-lying (Rydberg) electronic levels of sputtered Na atoms are spectrally shifted by the strong ion extracting electric field (⩾12 kV/cm) present in a resonance ionization mass spectrometer (Stark effect). The Stark-shifted lines are a beneficial diagnostic to probe the ion sources local electric field, which is related to sample alignment and hence ion transmission efficiency. Changes in electric field of 1% can be detected spectrally, so the sample position is controlled to ⩽ 10 μm by this technique. Results show that by the use of the Na spectrum as an alignment aid prior to sputter depth profiling, the precision of the RIMS instrument, as measured by the reproducibility of either the peak of the integrated signals from Be-implanted GaAs, is improved to about ±3%—at least a fourfold improvement over the best previous results. Spatial variations in the Be-ion implant dose over a GaAs wafer may be detected because of improved precision.

Comparison of Current-Induced Migration of Be and C in GaAs/AlGaAs HBTs

We contrast the stability under bias-aging conditions of GaAs/AlGaAs HBTs utilizing highly Be- or C-doped base layers. Devices with Be doping display a rapid degradation of dc current gain and junction ideality factor. At 200°C, a 2 × 10 μm 2 Be-doped device (4 × 10 19 cm −3 base doping) operated at a current density of 2.5 × 10 4 A. cm −2 shows a decrease in gain from 16 to 1.5 within 2h. Under the same conditions a C-doped device with even higher base-doping (7 × 10 19 cm −3 ) is stable over periods of 36h, the longest time we tested our structures. The degradation of Be-doped devices is consistent with the mechanism of recombination-enhanced diffusion of interstitials into the adjoining layers. Similar results are obtained with Zn-doped devices. Since C occupies the As sub-lattice rather than the Ga sublattice as with Be and Zn, it is not susceptible to reaction with Ga interstitials injected during growth or bias-aging.

Depth profiling resonance ionization mass spectrometry of electronic materials

Resonance ionization mass spectrometry (RIMS) of neutral atoms sputtered from semiconductors is used to provide quantitative information about the major and minor constituents. If the photoionization process is demonstrated to be saturated, RIMS signals can be related to the absolute concentrations of many elements in semiconductors such as Si and GaAs. RIMS signals are demonstrated to be nearly element-independent, that is, equal concentrations of impurities such as Be, Al, and Co in Si give equivalent signals. However, partitioning into various quantum states and velocities of sputtered atoms must be considered when comparing interelement signals on an absolute basis. Three- photon ionization is shown to be useful in reducing some background ionization effects and detecting high ionization potential non-metal elements.