C. R. Abernathy

Ultrahigh doping of GaAs by carbon during metalorganic molecular beam epitaxy

Recent advances in heterostructure bipolar transistor technology have created a need for p‐type doping at levels ≥1020 cm−3. Furthermore, such levels may eliminate the need for alloying during ohmic contact formation. We have achieved p‐type doping levels as high as 5×1020 cm−3 using an unconventional dopant, C, derived from the gaseous source chemical, trimethylgallium (TMG), during metalorganic molecular beam epitaxial (MOMBE) growth of GaAs. We have controllably achieved doping levels between 1019 and 5×1020 cm−3 by diluting the TMG flow with another metalorganic, triethylgallium (TEG). By utilizing the so‐called δ‐doping or atomic planar doping method we have also been able to grow C‐doped spikes with hole concentrations as high as 7×1019 cm−3, with a full width at half maximum of ∼50 A at 300 K. This doping level is the highest yet reported for planar doping, and the narrow width indicates that the C atoms are restricted to one or two atomic planes. By switching out the TMG, and switching in the TEG ...

Carbon doping of III-V compounds grown by mombe

Recent advances in heterostructure bipolar transistor (HBT) technology have created a need for p-type doping at levels ≥1020 cm-3. We have achieved p-type doping levels as high as 5×1020 cm-3 using C, which is introduced through the use of trimethylgallium (TMG) during metalorganic molecular beam epitaxy (MOMBE) growth of GaAs. By utilizing the atomic planar doping method, we have also been able to grow C-doped spikes with hole concentrations as high as 7×1019 cm-3, with a full width at half maximum of ∼50 A at 300 K. This level is among the highest reported for planar doping. By switching out the TMG, and switching in the triethylgallium (TEG) to continue to growth of C-free GaAs, we have grown sandwich-type structures with C levels of 1020 cm-3, which fall off within 210 A to C levels of <1017 cm-3. High temperature annealing of such structures reveals a C diffusion coefficient of <10-16 cm2 s-1 at 950°C, in agreement with other reports. The electrical properties of layers annealed at high temperatures appear to be influenced by the presence of strain arising from the high C concentration. X-ray diffraction patterns of 3 μm layers doped in excess of 1020 cm-3 show a lattice constant which corresponds roughly to that calculated by assuming a Vegard law mixture of GaAs and 0.7% GaC. Preliminary results of C-doping of InGaAs will also be discussed. Finally, the usefulness of carbon doping has been demonstrated in ohmic contact formation, Schottky barrier height enhancement in MESFETs and as the base layer in HBTs.

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...

High quality AlxGa1−xAs grown by organometallic vapor phase epitaxy using trimethylamine alane as the aluminum precursor

High quality AlxGa1−xAs has been grown by low‐pressure (30 Torr) organometallic vapor phase epitaxy (OMVPE) using a novel precursor, trimethylamine alane (TMAAl), as the aluminum source. The epilayers exhibited featureless surface morphology, very strong room‐temperature photoluminescence (PL), and excellent compositional uniformity (x=0.235±0.002 over a 40 mm diameter). The residual carbon incorporation, which determined the background doping, depended largely upon the choice of gallium precursor. Using triethylgallium, carbon incorporation could be largely suppressed ([C]≪1016 cm−3). The carbon‐related emission intensity was less than the bound exciton emission in low‐temperature (1.6 K) PL even at excitation powers as low as 50 μW cm−2. By sharp contrast, the use of trimethylgallium led to much higher C concentrations (2–5×1017cm−3). Under appropriate conditions, therefore, the use of TMAAl produces extremely high purity AlGaAs of superior quality to AlGaAs grown using conventional precursors.

Improved performance of carbon-doped GaAs base heterojunction bipolar transistors through the use of InGaP

Carbon‐doped GaAs/AlGaAs heterojunction bipolar transistors (HBTs) typically exhibit severe leakage at the base‐emitter interface which limits their utility for low‐current applications. Furthermore, the device breakdown voltage, and hence power handling capability, is limited due to the band gap of the GaAs collector material. In this letter we will demonstrate for the first time that both of these limitations can be overcome through the use of InGaP. Since InGaP is not readily doped with carbon, it does not suffer from compensation due to carryover of carbon from the GaAs base. Hence, the ideality factor of the base‐emitter junction improves from 1.3 to 1.09 when the conventional n‐AlGaAs emitter layer is replaced with n‐InGaP. Moreover, InGaP eliminates the crossover of the base and collector currents typically observed in heavily carbon doped GaAs HBTs. This results in the maintenance of gain even at very low collector currents. As a collector material, we have found that InGaP produces significantly ...

HYDROGEN IN CARBON-DOPED GAAS GROWN BY METALORGANIC MOLECULAR BEAM EPITAXY

Atomic profiles show that hydrogen is incorporated in GaAs:C that has been grown by metalorganic molecular beam epitaxy. The hydrogen concentration has been found to be about 5% of the carbon concentration for our growth conditions. An infrared absorption study shows that this hydrogen is involved in stable C‐H complexes. At the lower C concentrations (<1019 cm−3) the CAs‐H complex is the dominant species involving C and H. At higher C concentrations new complexes involving C and H appear.

Growth of GaAs and AlGaAs by metalorganic molecular beam epitaxy using tris‐dimethylaminoarsenic

Due to the extreme toxicity of AsH3, safer alternatives for III–V epitaxy are highly desirable. In addition, the AsH3 molecule is too stable to decompose on the wafer surface at the temperature and pressure conditions normally used during growth by metalorganic molecular beam epitaxy (MOMBE). This requires the use of high‐temperature cracker cells to decompose the AsH3 to elemental As prior to entry to the growth chamber and as a result leads to significant As buildup within the chamber. In this letter we report for the first time MOMBE growth at low temperatures (≤525 °C) using a novel As precursor, tris‐dimethylaminoarsenic (DMAAs) without precracking. Specular surface morphologies were obtained over a wide range of growth temperatures, 375–525 °C, for both GaAs and AlGaAs. Carbon concentrations measured by SIMS analysis in GaAs layers deposited from triethylgallium were lower than those obtained using a similar flux of AsH3, while carbon was reduced more than two orders of magnitude in films grown with...

Dry etching of thin-film InN, AlN and GaN

Smooth, anisotropic dry etching of InN, AlN and GaN layers is demonstrated using low-pressure (1-30 mTorr) CH4/H2/Ar or Cl2/H2 ECR discharges with additional DC biasing of the sample. The etch rates are in the range 100-400 AA min-1 at 1 mTorr and -150 V DC for Cl2/H2, while higher biases are needed to initiate etching in CH4/H2/H discharges. The presence of hydrogen in the gas chemistries is necessary to facilitate equi-rate removal of the group III and nitrogen etch products, leading to smooth surface morphologies.

Passivation of carbon-doped GaAs layers by hydrogen introduced by annealing and growth ambients

Carbon acceptors in GaAs epitaxial layers grown from metalorganic sources are often partially passivated by hydrogen following growth. Here we examine heavily C‐doped GaAs epilayers grown by metalorganic molecular beam epitaxy and metalorganic vapor phase epitaxy by infrared absorption, secondary ion mass spectrometry, and Hall measurements. The concentration of passivated C has been determined by calibrating the intensity of infrared absorption due to C‐H complexes. We have investigated the sources of H in the layers and have found that H2 in the growth and annealing ambients is especially effective in passivating C. A brief anneal in an inert ambient at temperatures above 550 °C is sufficient to activate C acceptors that are passivated by H.

Reversible changes in doping of InGaAlN alloys induced by ion implantation or hydrogenation

Carrier concentrations in doped InN, In0.37Ga0.63N, and In0.75Al0.25N layers are reduced by both F+ ion implantation to produce resistive material for device isolation, and by exposure to a hydrogen plasma. In the former case, post‐implant annealing at 450–500 °C produces sheet resistances ≳106 Ω/⧠ in initially n+ (7×1018–3×1019 cm−3) ternary layers and values of ∼5×103 Ω/⧠ in initially degenerately doped (4×1020 cm−3) InN. The evolution of sheet resistance with post‐implant annealing temperature is consistent with the introduction of deep acceptor states by the ion bombardment, and the subsequent removal of these states at temperatures ≲500 °C where the initial carrier concentrations are restored. Hydrogenation of the nitrides at 200 °C reduces the n‐type doping levels by 1–2 orders of magnitude and suggests that unintentional carrier passivation occurring during cool down after epitaxial growth may play a role in determining the apparent doping efficiency in these materials.