A. C. Warren

Arsenic precipitates and the semi‐insulating properties of GaAs buffer layers grown by low‐temperature molecular beam epitaxy

Arsenic precipitates have been observed in GaAs low‐temperature buffer layers (LTBLs) used as ‘‘substrates’’ for normal molecular beam epitaxy growth. Transmission electron microscopy has shown the arsenic precipitates to be hexagonal phase single crystals. The precipitates are about 6±4 nm in diameter with a density on the order of 1017 precipitates per cm3. The semi‐insulating properties of the LTBL can be explained in terms of these arsenic precipitates acting as ‘‘buried’’ Schottky barriers with overlapping spherical depletion regions. The implications of these results on LTBL resistivity stability with respect to doping and anneal temperature will be discussed as will the possible role of arsenic precipitates in semi‐insulating liquid‐encapsulated Czochralski‐grown bulk GaAs.

Formation of arsenic precipitates in GaAs buffer layers grown by molecular beam epitaxy at low substrate temperatures

We have grown film structures by molecular beam epitaxy which include GaAs buffer layers grown at low substrate temperatures (250 °C). The film structures have been examined using transmission electron microscopy. The layers grown at normal temperatures (600 °C) were free of defects or clusters. In contrast, the layer which was grown at low substrate temperatures contained precipitates which have been identified as hexagonal arsenic. The density of the arsenic precipitates is found to be very sensitive to the substrate temperature during growth.

Unpinned (100) GaAs surfaces in air using photochemistry

We have unpinned the Fermi level at the surface of both n‐ and p‐type (100) GaAs in air. Light‐induced photochemistry between GaAs and water unpins the surface Fermi level by reducing the surface state density. Excitation photoluminescence spectroscopy shows a substantial decrease in both surface band bending and surface recombination velocity in treated samples, consistent with a greatly reduced surface state density (≂1011 cm−2). Capacitance‐voltage measurements on metal‐insulator‐semiconductor structures corroborate this reduction in surface state density and show that the band bending may be controlled externally, indicating an unpinned Fermi level at the insulator/GaAs interface. We discuss a possible unpinning mechanism.

Subpicosecond, freely propagating electromagnetic pulse generation and detection using GaAs:As epilayers

Using GaAs epilayers with arsenic precipitates (GaAs:As) as the photoconductive material in a broad‐band optoelectronic terahertz beam system, we have generated and detected freely propagating, subpicosecond electromagnetic pulses. The receiver signal gave a measured integrated pulse width of 0.71 ps. Fast photoconductive rise times have been achieved which are characteristic of good mobility GaAs. In addition, the material exhibits a short ‘‘effective’’ carrier lifetime of several ps due to the embedded, closely spaced (about 20 nm) arsenic precipitates.

High-mobility modulation-doped SiGe-channel p-MOSFETs

A novel subsurface SiGe-channel p-MOSFET is demonstrated in which modulation doping is used to control the threshold voltage without degrading the channel mobility. A novel device design consisting of a graded SiGe channel, an n/sup +/ polysilicon gate, and p/sup +/ modulation doping is used. A boron-doped layer is located underneath the graded and undoped SiGe channel to minimize process sensitivity and maximize transconductance. Low-field hole mobilities of 220 cm/sup 2//V-s at 300 K and 980 cm/sup 2//V-s at 82 K were achieved in functional submicrometer p-MOSFETs.<<ETX>>

Substrate temperature dependence of arsenic precipitate formation in AlGaAs and GaAs

GaAs epilayers which are grown by molecular‐beam epitaxy under ‘‘normal’’ group III–V fluxes but at very low substrate temperatures contain as much as 1% excess arsenic. Upon annealing these epilayers at a temperature of 600 °C, the excess arsenic forms precipitates. We have undertaken a systematic study of the substrate growth temperature dependence of this incorporation of excess arsenic in both GaAs and Al0.3Ga0.7As epilayers. The substrate growth temperature was varied in increments of 25 °C from 225 to 375 °C after every 0.25 μm of film growth for a GaAs and an Al0.3Ga0.7As epilayer. Both epilayers were grown using a dimer arsenic source and a group V to total group III beam equivalent pressure ∼20. After growth the films were annealed for 1 h in the As2 flux at a temperature of 600 °C. Cross‐sectional samples were than prepared by the ion thinning technique and examined by transmission electron microscopy (TEM). Both epilayers contained arsenic precipitates; this is the first observation of arsenic ...

1.3- mu m P-i-N photodetector using GaAs with As precipitates (GaAs:As)

The fabrication of a GaAs detector which operates in the 1.3- to 1.5- mu m optical range is reported. The detector is a P-i-N photodiode with an intrinsic layer composed of undoped GaAs which was grown at 225 degrees C and subsequently annealed at 600 degrees C. This growth process has been demonstrated to produce a high density of As precipitates in the low-temperature grown region, which the authors show to exhibit absorption through internal photoemission. The internal Schottky barrier height of the As precipitates is found to be 0.7 eV, leading to reasonable room-temperature responsivity out to around 1.7 mu m.<<ETX>>

Formation of two‐dimensional arsenic‐precipitate arrays in GaAs

GaAs epilayers were grown by molecular beam epitaxy under normal conditions, except a substrate temperature of 250 °C was used instead of the normal 600 °C. This results in an excess of arsenic of about 1.5% in the epilayer. The epilayers also contained regions that were delta doped with silicon, beryllium, and indium. Samples were annealed for 30 s at 600, 700, and 800 °C to investigate the effects of the Si, Be, and In impurities on the precipitation of the excess As. It was found that the As precipitates form preferentially on planes of Si while forming preferentially between planes of Be. The isoelectronic impurity In appeared to have no effect on the precipitation process.

GaAs buffer layers grown at low substrate temperatures using As2 and the formation of arsenic precipitates

Abstract We have grown GaAs layers by molecular beam epitaxy at low substrate temperatures (250°C) using the dimer arsenic source As 2 . Following a one hour anneal at 600°C, the GaAs layers were examined with transmission electron microscopy. The GaAs layers contained arsenic precipitates of average diameter 100 A and density of 10 17 cm −3 .

Arsenic cluster dynamics in doped GaAs

We have studied the formation of As precipitates in doped GaAs structures that were grown by molecular beam epitaxy at low substrate temperatures and subsequently annealed. We find that the As precipitates form preferentially on the n side of such fabricated GaAs pn junctions. As the coarsening process proceeds, there is a gradual increase in the amount of As in precipitates in the n‐GaAs region and a decrease in the p‐GaAs region; the depletion region between the pn junction becomes free of As precipitates. These observations can be understood qualitatively based on the charge states of the As interstitial and using thermodynamic arguments in which the crystal attempts to minimize the chemical potential during the anneal. The presence of the excess As results in a stable Be profile even to anneals of 950 °C. Finally, a temperature cycling technique to grow arbitrarily thick GaAs epilayers containing As precipitates was demonstrated.