Naoki Furuhata

High-f/sub max/ AlGaAs/InGaAs and AlGaAs/GaAs HBT's with p/sup +//p regrown base contacts

The present paper describes a new approach to fabricating high performance HBTs with low base resistance. Their base contact resistance is reduced by using MOMBE selective growth in the extrinsic base region-a key process in the fabrication of high-f/sub max/ AlGaAs/InGaAs and AlGaAs/GaAs HBTs. A p/sup +//p regrown base structure, which consists of a 40-nm-thick graded InGaAs strained layer and a heavily C-doped regrown contact layer, is used for the AlGaAs/InGaAs HBTs to reduce both their base transit time and base resistance, while preventing aluminum oxide incorporation at the regrowth interface. An h/sub fe/ of 93, an f/sub T/ of 102 GHz, and an f/sub max/ of 224 GHz are achieved for a 1.6-/spl mu/m/spl times/4.6-/spl mu/m HBT, together with reduced base push-out effects and improved reliability. AlGaAs/GaAs HBTs with an 80-nm-thick uniform base layer that have high f/sub max/ values ranging from 140-216 GHz are also fabricated using the selective growth technique. These results confirm the high potential of the proposed HBTs, especially for microwave and millimeter-wave applications. >

Chemical dry etching of GaAs and InP by Cl2 using a new ultrahigh‐vacuum dry‐etching molecular‐beam‐epitaxy system

Damage and contamination‐free chemical dry etching of (100)GaAs and (100)InP by Cl2 was demonstrated using a new ultrahigh‐vacuum dry‐etching molecular‐beam‐epitaxy (MBE) system. This system consists of a combined etching chamber, an MBE chamber, and a sample preparation chamber, all at ultrahigh vacuum. A mirrorlike surface was obtained after etching at substrate temperatures ranging from 300 to 400 °C for GaAs, and from 200 to 400 °C for InP. In situ reflection high‐energy electron diffraction observations were accomplished for GaAs, with a mirrorlike surface after etching, and (2×4) surface reconstruction was observed. Results show that a smooth surface was formed at an atomic level.

Cl2 chemical dry etching of GaAs under high vacuum conditions: crystallographic etching and its mechanism

Cl2 chemical dry etching for GaAs substrates of {111}A, {111}B, {110} and {100} orientations was accomplished under high vacuum conditions. The etch rate for different substrate orientations was {111}B > {110} = {100} > {111}A for temperatures below 450° C, and was nearly equal for temperatures above 450° C. For {111}B, {110} and {100} substrates, the etch rate depends strongly on the substrate temperature above 450° C and below 150° C. Two activation energies for etching (10.0 kcal/mol below 150° C and 16.0 kcal/mol above 450° C) were obtained. Between 150 and 450° C, the etch rate depends weakly on the substrate temperature. However, for {111}A substrate, the etch rate increased monotonically with increasing substrate temperature above 300° C. The activation energy corresponds to that for the other substrates above 450° C. These results are caused by the surface chemical reaction of GaAs/Cl2. By using these etching properties, a vertical side wall was fabricated without ion bombardment.

Ohmic contacts to p‐GaAs with p+/p regrown structures formed by metalorganic molecular beam epitaxy

Excellent ohmic contacts to p‐GaAs are fabricated using selective growth by metalorganic molecular beam epitaxy. Specific contact resistance of about 5×10−8 Ω cm2 is achieved, without any heat treatment, at AuMn/Au and Ti/Pt/Au metal contacts, formed on p+‐GaAs layers heavily carbon‐doped to 4.4×1020 cm−3. Regrown contacts with planar and lateral p+/p structures are fabricated to clarify interface contact resistivities. A fairly low value of 7.1×10−8 Ω cm2 is established, using an equivalent circuit model, for the lateral contacts to thin p‐GaAs layers, reasonably independent of its thicknesses in the range of 9.5–95 nm. These results, in addition to excellent growth selectivity, have confirmed prospects for practical use.

Heavily Si‐doped GaAs grown by metalorganic chemical vapor deposition

Heavily Si‐doped GaAs layers were grown by a metalorganic chemical vapor deposition method using disilane (Si2H6) as a silicon dopant source gas. The grown layers were characterized by the van der Pauw, secondary ion mass spectroscopy, and low‐temperature Fourier transformation infrared spectroscopy (FTIR) method. The carrier concentration has no growth temperature dependence from 550 to 700 °C temperature range. However, it has temperature dependence below 550 °C and above 700 °C. The carrier concentration of Si‐doped GaAs is usually saturated at 6×1018 cm−3 level. Further, Si doping makes the carrier concentration decrease. By using the low‐temperature FTIR method, absorption bands for SiGa, SiAs, SiGa‐SiAs pair, and lower energy bands (374 and 369 cm−1) were observed in heavily Si‐doped GaAs. The peak intensity for SiGa is smaller than that for SiAs, and the peak heights at 374 and 369 cm−1 are relatively larger than that for the other peaks. These facts suggest that, in heavily Si‐doped GaAs, a part o...

Influence of metal/n‐InAs/interlayer/n‐GaAs structure on nonalloyed ohmic contact resistance

We have investigated in detail the influence of interlayer structures on nonalloyed ohmic contact resistance (ρc), in terms of the crystalline defects and the potential barrier at the interlayer/GaAs interface. The interlayer structures are a graded‐band‐gap InAs/GaAs strained‐layer superlattice (graded SLS), a graded‐band‐gap InGaAs, and conventional SLSs without graded band gaps. A two‐layer transmission line model indicates that the barrier resistance in the interlayer highly depends on the interlayer structure: ≤5×10−8 Ω cm2 for the graded SLS and graded InGaAs interlayers and 10−5–10−6 Ω cm2 for the conventional SLS interlayers. To explain the large dependence of the interlayer structure, first, the density and distribution of the misfit dislocations and stacking faults caused by the large lattice mismatch between InAs and GaAs have been investigated in detail by high‐resolution transmission electron microscopy. In the graded SLS and conventional SLS interlayers, the influence of the high‐density dep...

Selective growth mechanism of GaAs in metalorganic molecular beam epitaxy

GaAs selective growth mechanism was investigated, based on Ga species behavior using trimethyl gallium (TMG) or triethyl gallium (TEG) as the Ga source. Selective growth, using TMG, was obtained at a substrate temperature as low as 400°C. In the case of TEG use, the selective growth temperature was above 600°C. Furthermore, selective Ga deposition on a GaAs substrate with a SiO2 mask was carried out, using only an alkyl Ga source without arsenic. The selective Ga deposition was realized at 350°C for TMG use and above 500°C for TEG use. These results show that the Ga species related to TMG is easier to desorb from a SiO2 surface than that related to TEG, and that the desorption temperature for the Ga species is strongly correlative to the selective growth temperature. Consequently, the difference in selective growth temperature may be caused by the desorption rate for the Ga species, originating from the alkyl Ga source. Moreover, the difference in Ga species desorption rates, between GaAs surface and SiO2 surface, will contribute to the selective growth.

High-f/sub max/ AlGaAs/InGaAs and AlGaAs/GaAs HBTs fabricated with MOMBE selective growth in extrinsic base regions

The present paper reports AlGaAs/GaAs and AlGaAs/InGaAs heterojunction bipolar transistors (HBTs) with excellent microwave performance. An fT of 102 GHz and an fof 224 GHz are achieved, using selective growth of heavily C-doped GaAs layers in exmnsic base regions, in combination with a 40-nm-thick compositionally-graded-InGaAs base structure. These are by far the highestf, andfever reported for HBTs fabricated with selective growth.

Improvement in Electrical Properties at an n-GaAs/n-GaAs Regrown Interface Using Ammonium Sulfide Treatment

Electrical properties in an n-GaAs/n-GaAs interface regrown by molecular beam epitaxy (MBE) were remarkably improved using ammonium sulfide [(NH4)2Sx] treatment prior to regrowth. Reflection high-energy electron diffraction observations indicate that GaAs native oxide is removed by this treatment at a 500°C substrate temperature. This is 100°C lower than the temperature for removal of a native oxide by conventional thermal annealing in MBE. Transmission line model measurement shows that contact resistance at the (NH4)2Sx-treated interface is 1.8×10-6 Ωcm2, while it is 6.0×10-5 Ωcm2 without this treatment. Capacitance-voltage measurement and secondary-ion mass spectroscopy show that this reduction, in contact resistance at the regrown interface, is due to sulfur atoms in the interface behaving as donors (at a carrier concentration of 2×1018 cm-3); therefore, they compensate impurities such as carbon or oxygen in the interface. These results reveal that (NH4)2Sx-treatment before regrowth is useful for improving device performances; that is, reducing source resistance for field-effect transistors fabricated by n+-GaAs selective growth.

Selective-area metalorganic molecular beam epitaxy of GaAs using metalorganic chloride gallium precursors

Selective-area metalorganic molecular beam epitaxy at low growth temperatures using diethylgallium chloride (DEGaCl) and dimethylgallium chloride (DMGaCl) was investigated and significantly different results from those with metalorganic vapor-phase epitaxy were obtained. In the DEGaClAs4 and DmGaClAs4 systems, growth rates were extremely low or zero because DEGaCl and DMGaCl on the substrate surface decomposed to GaCl, which then desorbed very quickly. Therefore, these precursors and triethylgallium (TEGa) were used simultaneously to obtain a high growth rate and improve the selectivity in the TEGaAs4 system. In both the TEGa/DEGaCl/As4 and TEGa/DMGaCl/As4 systems, high growth rates over 1 μm/h and perfect selectivity were obtained at 500°C. Even at a low temperature of about 400°C, selectivity was perfect when DMGaCl was added: DMGaCl was more effective than DEGaCl. DMGaCl irradiation only prior to growth improves the selectivity in the TEGaAs4 system; therefore, cleaning the mask surface with chlorine-containing species was found to be important in both systems. In the TEGa/DMGaCl/As4 system, moreover, enhanced desorption of TEGa by interaction with methyl radicals supplied by DMGaCl was also found to improve selectivity. This was indicated by the experimental results in which selectivity was improved by adding trimethylgallium or trimethylindium to the TEGaAs4 system.