Subcollector Layer Research Articles

Fabrication of InP/InGaAs DHBTs with Buried SiO2 Wires

In this paper, we report the fabrication and device characteristics of InP/InGaAs double heterojunction bipolar transistors (DHBTs) with buried SiO 2 wires. The SiO 2 wires were buried in the collector and subcollector layers by metalorganic chemical vapor deposition toward reduction of the base-collector capacitance under the base electrode. A current gain of 22 was obtained at an emitter current density of 1.25 MA/cm 2 for a DHBT with an emitter width of 400 nm. The DC characteristics of DHBTs with buried SiO 2 wires were the same as those of DHBTs without buried SiO 2 wires on the same substrate. A current gain cutoff frequency (f T ) of 213 GHz and a maximum oscillation frequency (f max ) of 100 GHz were obtained at an emitter current density of 725 kA/cm 2 .

Si-related defects in InGaP/GaAs heterojunction bipolar transistors

A mechanism of current gain degradation caused by highly Si-doped n-GaAs sub-collector layer in InGaP/GaAs heterojunction bipolar transistor was investigated. Current gain decreased with increasing carrier concentration in the sub-collector layer grown at a V/III ratio of 20, suggesting that Si-related defects act as a source of reduction in current gain. We suggest that Ga interstitial, which is generated together with Ga vacancy, is the origin to reduce current gain. It was found that current gain increased with decrease in the V/III ratio of the sub-collector layer from 20 to 1.3. Carbon atomic concentration was 2×10 16 cm −3 for the V/III ratio of 20, while it increased up to 2×10 18 cm −3 for the V/III ratio of 1.3. We suggest that formation of Ga interstitial and carbon complexes in the sub-collector layer prevent Ga interstitial diffusing into the base layer, which leads to current gain degradation.

The Effect of n-GaAs Carrier Concentration on Current Gain in InGaP/GaAs Heterojunction Bipolar Transistors

The effect of n-GaAs carrier concentration in the Si-doped sub-collector layer on InGaP/GaAs heterojunction bipolar transistor (HBT) was investigated. Current gain slightly decreased with carrier concentration increased from 5×1017 to 2×1018 cm-3, while it dramatically decreased with carrier concentration increased over 2×1018 cm-3. It was also found that current gain with a high carrier concentration of 4×1018 cm-3 decreased with annealing time. Gummel plots revealed that the reduction in current gain is caused by creation of recombination centers in the base layer. We suggest the diffusion of Ga interstitial, which is generated together with Ga vacancy, is the origin of current gain degradation during the post-growth of the sub-collector layer.

Monolithic Integration of InP HBTs and Uni-Traveling-Carrier Photodiodes Using Nonselective Regrowth

This paper describes the monolithic integration of InP HBTs and uni-traveling-carrier photodiodes (UTC-PDs) by nonselective regrowth. HBTs are fabricated from nonselectively regrown device layers and UTC-PD subcollector layers, which are grown first on a 3-in InP substrate. This makes it possible to optimize the layer design for the HBTs and UTC-PDs independently and minimize the interconnection between them. The fabricated HBTs have a collector thickness of 200 nm, and they show an f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">t</sub> of 260 GHz and an f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</sub> of 320 GHz at a collector current density of 2.5 mA/mum <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . The standard deviations of the f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">t</sub> and f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</sub> across the wafer are 1.7% and 4.4%, respectively. The length of the interconnection between the HBTs and UTC-PDs can be made as small as 10 mum without any degradation of the regrown-HBT performance. The UTC-PDs fabricated on the same wafer exhibit a 3-dB bandwidth of 100 GHz and an output voltage of 1.0 V. There is no drawback in the performance of either device, as compared with that of discrete devices. We also demonstrate 100-GHz optical-input divide-by-two optoelectronic integrated circuits (OEICs) consisting of InP HBTs and a UTC-PD using this technique. These results indicate that the nonselective regrowth is promising for application toward over 100-Gb/s OEICs.

The difference of Si doping efficiency in GaN and AlGaN in GaN‐based HBT structure

An AlGaN/GaN HBT structure was grown by low-pressure metalorganic chemical vapor deposition (MOCVD) on sapphire substrate. From the high-resolution X-ray diffraction and transmission electron microscopy (TEM) measurements, it was indicated that the structure is of good quality and the AlGaN/GaN interfaces are abrupt and smooth. In order to obtain the values of Si doping and electronic concentrations in the AlGaN emitter and GaN emitter cap layers, Secondary Ion Mass Spectroscopy (SIMS) and electrochemical CV measurements were carried out. The results showed that though the flow rate of silane (SiH4) in growing the AlGaN emitter was about a quarter of that in growing GaN emitter cap and subcollector layer, the Si sputtering yield in GaN cap layer was much smaller than that in the AlGaN emitter layer. The electronic concentration in GaN was about half of that in the AlGaN emitter layer. It is proposed that the Si, Al co-doping in growing the AlGaN emitter layer greatly enhances the Si dopant efficiency in the AlGaN alloy. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Surface roughness in sulfur ion-implanted InP with molecular beam epitaxy regrown double-heterojunction bipolar transistor layers

We report on deep ion-implantation of sulfur into InP substrates to replace the epitaxial subcollector layer of double-heterojunction bipolar transistors. Using optimized implantation conditions of 350keV energy and 1×1015cm−2 dose, we achieved a subcollector sheet resistance of 15Ω∕square. Under well-controlled regrowth conditions a root-mean-square roughness of 12Å is measured from DHBT epitaxial layers grown on implanted InP substrates, comparable to DHBT epitaxial layers grown on n+ epiready unimplanted substrates. We observe a pronounced increase in surface roughness of epitaxial layer beyond a threshold ion dose, depending on implantation energy. Large-area DHBT devices result with sulfur-ion implanted subcollector shows similar characteristics compared to devices fabricated on n+-doped InP substrates.

Fabrication of heterojunction bipolar transistors with buried subcollector layers for reduction of base-collector capacitance by molecular beam epitaxy regrowth

A process for fabrication of heterojunction bipolar transistors (HBTs) with selectively buried subcollectors by molecular beam epitaxy (MBE) regrowth is described. This process can be used to reduce parasitic base-collector capacitance of HBTs and improve the speed of these devices. In situ etching by iodine prior to the regrowth was used for the first time to improve the quality of the substrate epilayer interface in a semiconductor device grown by MBE. The secondary ion mass spectroscopy depth profiles of regrown HBT structures suggest that the in situ surface cleaning by molecular iodine was not sufficient to remove all contamination from the substrate epilayer interface and that the microwave performance of HBTs fabricated by this process may have been affected by that contamination. The dc performance of devices which were fabricated by our process was not affected, however, by the contamination and was comparable to the dc performance of conventional HBTs. Our results suggest that the described process for fabrication of HBTs with selectively buried subcollectors by MBE regrowth would be feasible in conjunction with a more effective surface treatment.

Planar InGaP/GaAs HBTs for high speed optoelectroniccircuit applications

A fully planar self-aligned HBT fabrication process based on selective chemical beam epitaxial (CBE) regrowth of both the extrinsic base and subcollector layers is reported. Using this technology, cutoff frequencies of 50 and 70 GHz for fT and fmax, respectively, have been achieved for In0.49Ga0.51P/GaAs HBTs with 9 × 4 µm2 emitter-base junction area. High speed integrated circuits for lightwave communications including a 10 Gbit/s decision circuit and an 18 GHz dynamic frequency divider were successfully fabricated using these planar HBTs.

Fabrication and characteristics of a GaInP/GaAs heterojunction bipolar transistor using a selective buried sub-collector

A C-doped GaInP/GaAs heterojunction bipolar transistor (HBT) with a selective buried sub-collector has been fabricated by two growth steps. The active HBT region was made on the selective buried sub-collector layer with minimum overlap of the extrinsic base and the sub-collector region resulting in substantial reduction of the base-collector capacitance. The experiment shows that the base-collector capacitance is reduced to about half of that of a conventional HBT while the base resistance remains unchanged resulting in a 40-50% increase in the maximum oscillation frequency. Both DC and RF characteristics are investigated and compared with a conventional HBT. A current gain of 40, cutoff frequency of 50 GHz and maximum oscillation frequency of 140 GHz were obtained for the GaInP/GaAs HBT. It is demonstrated that the selective buried sub-collector provides an effective means for enhancing RF performance of an HBT.

A 5-10 GHz octave-band AlGaAs/GaAs HBT-Schottky diode down-converter MMIC

The authors describe an AlGaAs/GaAs heterojunction bipolar transistor (HBT) X-band down-converter monolithic microwave integrated circuit (MMIC) which integrates a double double-balanced Schottky mixer and five stages of HBT amplification to achieve greater than 30 dB conversion gain over an RF bandwidth from 5 to 10 GHz. In addition, an output IP3 as high as +15 dBm has been achieved. The Schottky diodes are constructed from the existing N/sup $/collector and N/sup +/ subcollector layers of the HBT molecular beam epitaxy (MBE) device structure. A novel HBT amplifier topology employing active feedback which provides wide bandwidth in a compact area is used for the RF, LO, and IF amplifier stages. The complete down-converter MMIC is realized in a 3.6/spl times/3.4 mm/sup 2/ area, is self-biased through a 6 V supply, and consumes 530 mW. This MMIC represents the highest complexity X-band down-converter MMIC demonstrated using GaAs HBT-Schottky diode technology.

Growth temperature dependence of Zn diffusion in [formula omitted] heterojunction bipolar transistor structures grown by metalorganic chemical vapor deposition

We investigate the behavior of zinc impurities in InPInGaAs heterojunction bipolar transistor (HBT) structures grown by a low pressure metalorganic chemical vapor deposition (MOCVD) technique. When using this technique, zinc is anomalously diffused in InPInGaAs HBTs with a heavily Si-doped subcollector layer (2 × 1019 cm−3) when the growth temperature before the base-layer growth is higher or lower than around 600°C. On the other hand, an abrupt zinc profile in the same HBT structure is realized when the growth temperature of the subcollector layer is 600°C. Two kinds of point defect reactions which depend strongly on the growth temperature are proposed. In one, excess group III interstitials which are produced in a heavily Si-doped subcollector layer are easily removed and, in the other, the equilibrium concentration of charged group-III vacancies decreases as the growth temperature increases. According to this model, complete suppression of an undesired Zn diffusion can be realized by using a growth interruption technique for obtaining the point defect equilibrium before a base-layer growth at 550°C when the growth temperature of the subcollector layer is either lower or higher than 600°C. Thus, either interrupting the growth for an appropriate period of time before the base layer growth or using an appropriate growth temperature for the subcollector is essential for obtaining an abrupt Zn profile in an HBT structure with a heavily doped subcollector layer.

Suppression of abnormal Zn diffusion in InP/InGaAs heterojunction bipolar transistor structures

It is known that highly n+-doped subcollector layers in InP/InGaAs heterojunction bipolar transistor structures lead to drastically enhanced zinc diffusion in the subsequently grown base layer. We show that this abnormal zinc diffusion can be suppressed by interrupting growth before the zinc-doped layer is grown. It is speculated that this growth interruption allows excess nonequilibrium group III self-interstitials coming from the n+-doped subcollector layer to disappear before they have a chance to enhance zinc diffusion in the base layer.

Non-equilibrium point defect phenomena influencing beryllium and zinc diffusion in GaAs and related compounds

Beryllium and zinc are the main p-type dopants used for the fabrication of devices based on GaAs or related III-V materials. Both elements are substitutionally dissolved on the group III sublattice and diffuse via the “kick-out” mechanism which involves group III self-interstitials. Non-equilibrium concentrations of these self-interstitials have a strong influence on the diffusivities of Be and Zn with often drastic consequences on device behavior especially if Be or Zn is used to realize narrow base regions in heterojunction bipolar transistors (HBTs). Various situations in which non-equilibrium point defects play a role for Be and Zn diffusion are discussed such as: in-diffusion of these dopants from an outside source, diffusion of grown-in dopants, self-interstitial generation by Fermi level surface pinning of highly n+-doped emitter cap or subcollector layers in HBTs, or recom bination-enhanced beryllium diffusion during device operation. Finally, we will comment on the diffusion behavior of carbon, which is dissolved on the group V sublattice in GaAs, is much less sensitive to non-equilibrium point defect, and, therefore, is increasingly used to replace Be and Zn as p-type dopants.

Selective growth of InGaAs/InP heterojunction bipolar transistors with a buried subcollector

We demonstrate the fabrication of an InGaAs/InP heterojunction bipolar transistor (HBT) with a reduced base-collector overlap area using selective epitaxy techniques. The process uses planarizing growth by chloride-transport vapor-phase epitaxy (VPE) to create a buried subcollector layer, followed by selective regrowth by organometallic VPE. The quality of the regrowth over the planarized substrates is evident from the dc characteristics of the HBT which show an ideality factor η=1.05 for the base-emitter junction.

Abnormal redistribution of Zn in InP/InGaAs heterojunction bipolar transistor structures

High n+ doping (≳1019 cm−3 of Si) in the subcollector of InP/InGaAs heterojunction bipolar transistor structures is observed to induce an anomalously high Zn diffusivity and an associated broadening of the base layers. It is proposed that due to Fermi level surface pinning and a long time constant for the recovery of point defect equilibrium the subcollector acts as a continuously operating source of group III interstitials which in turn diffuse into the subsequently grown base region and enhance Zn diffusion via the kick-out mechanism. In this sense, highly n+ doped grown-in subcollector layers may be considered as having the effect of a ‘‘time bomb’’ in terms of generating undesirable excess point defects during subsequent further crystal growth and device processing.

InGaAs/InP HBTs with a buried subcollector fabricated by selective epitaxy

The authors demonstrate the fabrication of a novel HBT (heterojunction bipolar transistor) with a buried subcollector layer using selective growth techniques. The effective base-collector overlap area is drastically reduced with respect to a conventional device. The process consists of two distinct steps: (1) definition of a buried subcollector layer using chloride-transport VPE (vapor-phase epitaxy), and the subcollector is created by planarizing growth over an InP:Fe substrate with etched openings; and (2) regrowth, using organometallic VPE, of the collector, base, and emitter layers. The requirements of excellent planarization and high-quality regrown layer material are both achieved. Chloride-transport VPE allows a very high degree of planarization with very little excess growth on top of the substrate. This feature makes the selective growth process self-limiting and relatively independent of mask layout, a common problem with many selective epitaxy approaches.

Application of Localized Zn Diffusion across Heterointerfaces for the Realization of a Compact High‐Speed Bipolar Driver Circuit on InGaAsP / InP

In this paper a model is proposed explaining the mechanism of Zn diffusion across heterointerfaces. The model is based on the interstitial‐substitutional diffusion mechanism. Employing a repetitive algorithm Zn diffusion across heterointerfaces has been simulated such that the evolution of Zn diffusion in time can be calculated. As an application Zn diffusion across three heterointerfaces involved in the fabrication of invertible double heterostructure bipolar transistors (DHBT) (Zn from the gas phase at 823 K) has been numerically calculated. Good agreement between simulated results and experimental data was achieved. The invertible DHBTs were used to build a compact driver circuit with the three transistors involved being interconnected by a common subcollector layer. A current modulation capability up to 2.6 Gbit/s with output currents of more than 50 mA and an ac transconductance of 120 mS were obtained.

Carbon-doped base GaAs-AlGaAs HBT's grown by MOMBE and MOCVD regrowth

High-quality GaAs-AlGaAs heterojunction bipolar transistors (HBTs) in which the carbon-doped base layers (p=10/sup 10/-10/sup 20/ cm/sup -3/, 400-800 AA thick) and Sn-doped collector and subcollector layers are grown by metalorganic molecular-beam epitaxy (MOMBE) and a subsequent regrowth using metalorganic chemical vapor deposition (MOCVD) is used to provide the n/sup +/ AlGaAs emitter and GaAs/InGaAs contact layers are discussed. A current gain of 20 was obtained for a base doping of 10/sup 19/ cm/sup -3/ (800 AA thick) in a 90- mu m-diameter device, with ideality factors of 1.0 and 1.4 for the base-collector and emitter-base junctions, respectively, demonstrating the excellent regrowth-interface quality. For a base doping of 10/sup 20/ cm/sup -3/ (400 AA thick), the current gain decreased to 8.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

P-n-p heterojunction bipolar transistors with buried subcollector layers

The buried-layer technology was applied to the fabrication of high-speed p-n-p AlGaAs/GaAs heterojunction bipolar transistors (HBTs). The subcollector layer was selectively implanted prior to the epitaxial growth of the rest of the device structure thereby eliminating the need for deep mesa isolation. Devices with 2*10- mu m/sup 2/ emitter fingers and 100-nm base thickness had common-emitter current gains of 15 and cutoff frequencies of 17 GHz.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>