J. M. Woodall

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.

An In 0.15 Ga 0.85 As/GaAs pseudomorphic single quantum well HEMT

This letter describes high electron mobility transistors (HEMTs) utilizing a conducting channel which is a single In<inf>0.15</inf>Ga<inf>0.85</inf>AS quantum well grown pseudomorphically on a GaAs substrate. A Hall mobility of 40 000 cm²/V.s has been observed at 77 K. Shubnikov-de Haas oscillations have been observed at 4.2 K which verify the existence of a two-dimensional electron gas at the In<inf>0.15</inf>Ga<inf>0.85</inf>As/GaAs interface. HEMTs fabricated with 2-µm gate lengths show an extrinsic transconductance of 90 and 140 mS/mm at 300 and 77 K, respectively-significantly larger than that previously reported for strained-layer superlattice In<inf>x</inf>Ga<inf>1-x</inf>As structures which are nonpseudomorphic to GaAs substrates. HEMTs with 1-µm gate lengths have been fabricated, which show an extrinsic transconductance of 175 mS/mm at 300 K which is higher than previously reported values for both strained and unstrained In<inf>x</inf>Ga<inf>1-x</inf>As FETs. The absence of Al<inf>x</inf>Ga<inf>1-x</inf>As in these structures has eliminated both the persistent photoconductivity effect and drain current collapse at 77 K.

EFFICIENT VISIBLE ELECTROLUMINESCENCE AT 300°K FROM Ga1‐xAlxAs p‐n JUNCTIONS GROWN BY LIQUID‐PHASE EPITAXY

Efficient visible light emitting diodes have been fabricated from Ga1‐xAlxAs. Epitaxial layers were obtained by a modified solution growth technique. External quantum efficiencies of up to 3.3% have been measured at room temperature on diodes, which had their emission at 1.70 eV. The switching time for the light emission at 300°K was measured to be 60 nsec.

Highly carbon‐doped p‐type Ga0.5In0.5As and Ga0.5In0.5P by carbon tetrachloride in gas‐source molecular beam epitaxy

Highly carbon‐doped, highly p‐type Ga0.5In0.5As and Ga0.5In0.5P epilayers were grown by gas‐source molecular beam epitaxy (GSMBE) using carbon tetrachloride (CCl4). Growth temperatures slightly below conventional values were used to increase the carbon incorporation, and a short‐duration post‐growth anneal near the growth temperature was necessary in order to obtain the highest hole concentrations, which were p=3×1019 cm−3 for Ga0.5In0.5As and p=5×1018 cm−3 for Ga0.5In0.5P. This is the first report of significant p‐type carbon doping for Ga0.5In0.5P and the highest concentration from carbon doping yet reported for both ternary compounds. Reversible acceptor passivation from hydrogen species in the growth environment is a plausible explanation for the annealing behavior.Highly carbon‐doped, highly p‐type Ga0.5In0.5As and Ga0.5In0.5P epilayers were grown by gas‐source molecular beam epitaxy (GSMBE) using carbon tetrachloride (CCl4). Growth temperatures slightly below conventional values were used to increase the carbon incorporation, and a short‐duration post‐growth anneal near the growth temperature was necessary in order to obtain the highest hole concentrations, which were p=3×1019 cm−3 for Ga0.5In0.5As and p=5×1018 cm−3 for Ga0.5In0.5P. This is the first report of significant p‐type carbon doping for Ga0.5In0.5P and the highest concentration from carbon doping yet reported for both ternary compounds. Reversible acceptor passivation from hydrogen species in the growth environment is a plausible explanation for the annealing behavior.

High carbon doping efficiency of bromomethanes in gas source molecular beam epitaxial growth of GaAs

Carbon tetrabromide (CBr4) and bromoform (CHBr3) have been studied as carbon doping sources for GaAs grown by gas source molecular beam epitaxy (GSMBE) with elemental Ga and thermally cracked AsH3. Hole concentrations in excess of 1×1020 cm−3 have been measured by Hall effect in both CBr4‐ and CHBr3‐doped GaAs, which agrees closely with the atomic C concentration from secondary‐ion mass spectrometry, indicating complete electrical activity of the incorporated carbon. The GaAs growth rate is unaffected by the CBr4 and CHBr3 fluxes over the range of dopant flow investigated. The efficiencies of carbon incorporation from CBr4 and CHBr3 are, respectively, 750 and 25 times that of trimethylgallium (TMG), which is commonly employed as a carbon doping source in metalorganic MBE (MOMBE). The sensitivity of carbon incorporation to varying substrate temperature and V/III ratio has been observed to be significantly reduced with CBr4 and CHBr3 from that obtained under similar growth conditions with TMG in MOMBE.

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.

High-speed 1.3 mu m GaInAs detectors fabricated on GaAs substrates

High-speed interdigitated metal-semiconductor-metal detectors have been fabricated on non-lattice-matched, semi-insulating, GaAs substrates using two GaInAs layers of differing indium concentrations to accommodate most of the lattice mismatch by interface misfit dislocations. Bandwidths as high as 3 GHz were measured with none of the detrimental low-frequency gain usually observed in this type of device. This is attributed to the inhibition of the surface trapping of photoinduced carriers by a graded pseudomorphic layer at the surface.<<ETX>>

NOVEL LOW-RESISTANCE OHMIC CONTACT TO N-TYPE GAAS USING CU3GE

We show that e1‐Cu3Ge forms a low‐resistance ohmic contact to n‐type GaAs. The e1‐Cu3Ge contact exhibits a planar and abrupt interface and contact resistivity of 6.5×10−7 Ω cm2 which is considerably lower than that reported for Ge/Pd and AuGeNi contacts on n‐type GaAs with similar doping concentrations (∼1×1017 cm−3). The contact is electrically stable during annealing at temperatures up to 450 °C. We also show that in the Ge/Cu/n‐type GaAs system, the contact remains ohmic over a wide range of Ge concentration that extends from 15 to 40 at. %. n‐channel GaAs metal–semiconductor field‐effect transistors using the e1‐Cu3Ge ohmic contacts demonstrate a higher transconductance compared to devices with Ge/Pd and AuGeNi contacts.

Photoemission spectroscopy of GaAs:As photodiodes

We report on photoemission measurements of molecular‐beam‐epitaxy‐grown GaAs p‐i‐n structures, in which the optically active insulating GaAs layer contains As precipitates (GaAs:As). GaAs:As is formed by low‐temperature growth of GaAs at 225 °C, followed by an anneal at 600 °C. Layers grown in this way have been reported to be sensitive to subband‐gap light. The measured barrier height of 0.7 eV, extracted from a well‐behaved Fowler plot, indicates that the mechanism for photodetection involves arsenic clusters embedded in GaAs acting as internal Schottky barriers.

Photoreflectance characterization of an InP/InGaAs heterojunction bipolar transistor structure

We have measured the photoreflectance (PR) spectrum at 300 K from a lattice‐matched InP/InGaAs heterojunction bipolar transistor structure grown by gas‐source molecular beam epitaxy. From the observed Franz–Keldysh oscillations we have evaluated the built‐in dc electric fields and associated doping profiles in the n‐InGaAs collector and n‐InP emitter regions. These donor concentrations are in agreement with capacitance‐voltage and secondary ion mass spectroscopy determinations, but are considerably lower than the intended values from the growth parameters. We have thereby detected a failure of the Si doping source which occurred during material growth using this contactless, nondestructive, and rapid characterization method. The energy of the InGaAs band gap from the PR spectrum also verifies the lattice‐matched nature of the system, further demonstrating the diagnostic and process control value of the PR technique.