Kei-Yu Ko

Void formation, electrical activation, and layer intermixing in Si‐implanted GaAs/AlGaAs superlattices

Direct experimental evidence is presented for the correlation between void formation, dopant electrical activation, and layer intermixing in GaAs/AlGaAs superlattices (SLs). Maximum layer intermixing is observed in the regions of maximum carrier concentration and no or little void formation in Si‐implanted and annealed SLs. In SLs implanted at room temperature, Si activation and layer intermixing enhancement are severely inhibited in the near‐surface region where voids are formed. However, when implantation is carried out at 250 °C, both the suppression of Si activation and layer intermixing enhancement in the near‐surface region are reduced, concurrent with a decrease in void density.

Distribution mechanism of voids in Si-implanted GaAs

Voids, formed by the condensation of an excess of implantation‐induced vacancies, have been recently identified as the defect directly responsible for dopant diffusion and electrical activation anomalies in Si‐implanted and annealed GaAs and GaAs/AlGaAs superlattice materials. Depending on the implanted dose, voids can be distributed either throughout the implanted region or in two bands. We have examined the origin of this void distribution difference. In the as‐implanted sample associated with the latter case, a buried continuous band of amorphous GaAs has formed. GaAs formed by the recrystallization of amorphous GaAs does not contain excess vacancies and therefore cannot form voids. However, on either side of the amorphous layer, the excess vacancies can condense to form the observed banded distribution of voids. In the as‐implanted sample associated with the former case, a continuous amorphous GaAs layer did not form, and therefore, upon annealing, voids are seen throughout the implanted region.

Compensation phenomena in GaAs implanted with 1 MeV silicon ions

We have investigated the nature of the compensation phenomena that limit carrier activation in GaAs implanted with 1 MeV Si ions to a dose of 3 × 1015ions/cm2, and subsequently annealed at 900°C for 10 s. The depth profiles of the Si implants and the net carrier concentration, measured using secondary ion mass spectrometry and electrochemical capacitance-voltage profiling, respectively, reveal a large disparity in the region around the maximum of the Si distribution. Structural characterization, using ion channeling and transmission electron microscopy, reveals the formation of a buried band of interstitial dislocation loops in approximately the same region where the carrier concentration is highly compensated. However, previous works have shown that the remnant loops are not directly responsible for the deactivation of Si. The apparent paradox is resolved with the help of photoluminescence measurements as a function of depth performed on samples bevel-etched into a wedgelike shape. The photoluminescence experiments show that Si atoms sitting on As sites are mainly responsible for the compensation of the Si donors. Indeed self-compensation is to be expected due to the amphoteric nature of Si in GaAs. In addition, emissions corresponding to gallium-vacancy complexes, arsenic-vacancy complexes, as well as other implantation induced residual defects are observed. It is argued that a fraction of the vacancies and interstitials generated during the implantation collapse separately upon annealing giving rise to either vacancy-type defects or interstitial-type defects, respectively. The effect of these defects on the carrier concentration is discussed.

Void Formation and Its Effect on Dopant Diffusion and Carrier Activation in Si-Implanted GaAs

GaAs samples, implanted with 220 keV Si to doses ranging from 3×1013 to 1×1015 cm-2 and annealed at 850°C were studied. Using transmission electron microscopy (TEM), voids were found in samples with implant doses ≥3×1014 cm-2 after an annealing time as short as 5 s. In the same region where voids were found, capacitance-voltage measurements showed abnormaly low electron concentrations. Also in the same region, secondary ion mass spectrometry (SIMS) measurements showed anomalies in the Si concentration profiles and required the interpretation that a Si redistribution process had occurred. At high Si implant doses, the onset of void formation, the abnormaly low electron concentration, and the Si accumulation anomaly are concurrent. Based on these results, we conclude that voids inhibit the Si electrical activity and lead to the Si diffusion anomaly.

Correlation of Void Formation with the Reduction of Carrier Activation and Anomalous Dopant Diffusion in Si-Implanted GaAs

We report the study of high-dose Si-implanted GaAs containing doses ranging from 1×10 14 to 1×10 15 cm -2 and with subsequent anneals at 850°C for 1 hour. At doses ≥ 3×10 14 cm -2 , a severe reduction of carrier concentration and anomalous Si diffusion are observed in the near-surface region. In the same region, small, near-spherical voids are found by transmission electron microscopy. In contrast, for samples implanted with doses ≤ 1×10 14 cm -2 , voids are not found, and both normal carrier activation and Si diffusion profiles are observed. The concurrent onset of these three phenomena in the same region in high-dose samples leads us to conclude that the severe reduction of carrier concentration and anomalous Si diffusion are attributable to the formation of voids.

Relationship Between Electrical Activation and Residual Defects in MeV Si Implanted GaAs.

We have investigated the relationship between electrical activation and residual defects in GaAs implanted with 1 MeV Si ions to a fluence of 3×10 15 cm −2 and subsequently annealed, using rapid thermal annealing (RTA) for 10 seconds, at temperatures up to 1050°C. Hall measurements show n-type activation that increases in magnitude with increasing annealing temperature to reach a sheet carrier concentration of 2.6×10 14 cm −2 after RTA at 1050°C. Two main types of extended defects, perfect and partial dislocation loops, had previously been identified in the as-implanted samples. In addition, a large number of discrete point defect complexes are certainly created but escape detection. We show here a correlation between the annealing of the defects and the recovery of the electrical transport properties. Most of the point defects anneal out between room temperature and 700°C, and it is in this temperature regime that the transport properties show their most significant improvement. For annealing temperatures between 700°C and 1050°C, the mobility of the carriers decreases slightly while the carrier concentration gradually approaches saturation. Concurrently, the loops grow in size while decreasing in number and eventually anneal out. However, after annealing at 1050°C, while most of the extended defects have disappeared, only about 9% of the implanted atoms had been activated, suggesting that residual point defect centers control the activation of Si dopants in MeV implanted GaAs.

High temperature thermally stable implant isolation for GaAs via void formation

A new method of forming thermally stable high‐resistivity regions is developed for device isolation in GaAs. For Al+‐implanted epitaxial‐layer structures, the sheet resistivity increases by about six orders of magnitude from the as‐grown values, after annealing in the 700–900 °C range. This increase in resistivity is shown to correlate with the formation of voids. The creation of high resistivity via void formation is different from the conventional damage‐induced isolation by H or O implantation. This type of isolation becomes ineffective once the lattice is annealed at high temperatures due to the annealing out of lattice damage between 400 and 700 °C. In contrast, voids are stable at high temperatures. The potential advantages of using such defects for device isolation will be discussed.

Anomalous electrical activation in Si-implanted GaAs/AlGaAs superlattices

Electrical activation in Si-implanted and annealed GaAs/AlGaAs superlattices (SLs) has been studied as a function of implantation temperature and dose. Carrier concentration, measured by the electrochemical capacitance-voltage profiling, is shown to be strongly dependent on implantation temperature. When SLs are implanted at room temperature or lower, electrical activation is severely suppressed in the near-surface region, where the carrier concentration is one to two orders of magnitude lower than the maximum (1–3) × 1018/cm3 observed near the implant peak position. Similar deactivation behavior was observed in Si-implanted GaAs crystals, but the disparity between the minimum and maximum of the carrier concentration was significantly less than one order of magnitude. The minimum in carrier concentration profiles is explained in terms of carrier deactivation by the near-spherical voids of 25–200 A in diameter which are formed only in the near-surface regions, as revealed by transmission electron microscopy analysis. The larger carrier deactivation in the SL materials is attributed to the combined effects of electron confinement by the AlGaAs into the GaAs regions together with the preferential formation of voids in the GaAs layers.

On the Distribution Mechanism of Voids in Si-Implanted GaAs

Voids, formed by the condensation of an excess of implantation-induced vacancies, have been recently identified as the defect directly responsible for dopant diffusion and electrical activation anomalies in Si-implanted and annealed GaAs and GaAs/AlGaAs superlattice materials. Depending on the implanted dose, voids can be distributed either throughout the implanted region or in two bands. We have examined the origin of this void distribution difference. In the asimplanted sample associated with the latter case, a buried continuous band of amorphous GaAs has formed. GaAs formed by the recrystallization of amorphous GaAs does not contain excess vacancies, and therefore cannot form voids. However, on either side of the amorphous layer, the excess vacancies can condense to form the observed banded distribution of voids. In the as-implanted sample associated with the former case, a continuous amorphous GaAs layer did not form and therefore, upon annealing, voids are seen throughout the implanted region.