High Mobility Transistors Research Articles

GSMBE growth of InGaP/(In)GaAs modulation-doped heterostructures and their applications to HEMT and HHMT

In this paper we report for the first time gas-source molecular beam epitaxy of both n- and p-channel InGaP/(In)GaAs modulation-doped heterostructure. The sheet density dependence of the two-dimensional hole gas (2DHG) mobility of p-type In 0.49Ga 0.51P/GaAs structures was investigated by Van der Paul Hall measurement at 300 and 77 K. The 2DHG densities of 2.78×10 12 and 1.02×10 12 cm −2 with mobilities of 191 and 2831 cm 2/V s at 300 and 77 K, respectively, for In 0.49Ga 0.51P/GaAs structures have been achieved. Low-temperature and high-magnetic field Hall measurements were also carried out. Longitudinal resistance of the structure shows two oscillations with different periods at a temperature of 0.3 K, indicating that two subbands have been occupied by holes in the GaAs channel. The sheet density for each subband was estimated to be 1.22×10 12 and 0.77×10 12 cm −2, respectively. Both high hole mobility transistors (HHMTs) and high electron mobility transistors (HEMTs) were demonstrated. HHMTs show a maximum DC transconductance of 35 mS/mm. The saturation current is 57 mA/mm at 300 K with V d=−3 V. Both extrinsic transconductance and saturation current of our novel In 0.49Ga 0.51P/GaAs HHMTs are well improved, compared with those of AlGaAs/GaAs and InGaAs/GaAs HHMTs. Enhanced-mode InGaP/InGaAs pseudomorphic HEMTs were achieved with a maximum DC transconductance of 250 mS/mm.

Vertical p-type high-mobility heterojunction metal–oxide–semiconductor field-effect transistors

We have fabricated a vertical p-channel metal–oxide–semiconductor field-effect transistor called high-mobility heterojunction transistor (HMHJT). Compared with a Si control device, reduced short channel effects, reduced floating body effect, and high drive current have been achieved with this device structure. A SiGe/Si heterojunction barrier at the source/bulk junction suppresses drain induced barrier lowering and bulk punchthrough, which are significant problems for sub-100 nm devices. A SiGe source also helps to reduce the charge built-up in the floating body. The higher mobility in a strained SiGe channel and the absence of a hetero-barrier between the source and channel result in higher drive current. The fabricated HMHJT has a 60% higher drive current at VDS=VGS−VT=−1.6 V, and a 70× lower off-state leakage current at VDS=−1.6 V and VGS=0.0 V, compared with the Si control device.

Polarization fields in nitride nanostructures: 10 points to think about

Macroscopic polarization, both of intrinsic and piezoelectric nature, is unusually strong in III–V nitrides, and the built-in electric fields in the layers of nitride-based nanostructures, stemming from polarization changes at heterointerfaces, have a major impact on the properties of single and multiple quantum wells, high mobility transistors, and thin films. The concepts involved in the theory and applications of polarization in nitrides have encountered some resistance in the field. Here we discuss critically 10 “propositions” aimed at clarifying the main controversial issues.

Structural properties of InN on GaN grown by metalorganic vapor-phase epitaxy

InN has been expected to be a suitable material for electronic devices such as high mobility transistors because of its small effective mass compared to other nitrides. Heteroepitaxial InN films were grown by metalorganic vapor-phase epitaxy. The films have been structurally characterized by triple-axis x-ray diffraction (XRD) analysis in terms of lattice-mismatch dependence and InN film thickness dependence, and Hall measurements have been performed. In the XRD measurement, ω and ω–2θ scans were used, and the degree of tilting (the linewidth of x-ray signal, Δωc) [(0002) reflection] and that of twisting (Δωa) [(101̄0) reflection] have been separated. In addition, the degree of distribution of lattice constant c (Δ2θc) [(0002) reflection] of InN films has been assessed. For study of the lattice-mismatch dependence, growth of InN films on GaN, AlN and directly on sapphire substrates was performed, and accordingly, Δωc was found to range from about 500 to 4000 arcsec, and Δ2θc from about 400 to 700 arcsec. Among those three kinds of samples, InN films grown on GaN showed the smallest Δωc and Δ2θc values. Observation of c- and a-lattice parameters has shown that the InN on GaN is affected by the residual strain. On the other hand, InN thickness dependence of XRD showed that Δωc was changed from about 700 to 500 arcsec, and Δ2θc from about 600 to 300 arcsec with increasing InN thickness from 400 to 2400 Å. In accordance with the thickness of InN, Δωa was found to change from about 2500 to 1700 arcsec. Moreover, it was found that the InN film less than 1200 Å thick is composed of grain islands with different crystalline orientation and that the growth mode changes at a thickness of about 1200 Å—and screw dislocations occur. It is found that the residual strain in InN films over 1200 Å thick is gradually released, resulting in almost the same orientation. This is reflected in the reduction of the mosaicity, the proceeding of relaxation and the surface morphology. Selection of GaN for the underlying layer of the InN film has been shown to lead to structural improvement of the epitaxial InN film. In fact, InN film with a thickness of 2400 Å grown on GaN has a Hall mobility of about 700 cm2/V s even at an electron carrier concentration of 5×1019 cm−3. This value corresponds to that for GaAs at the same impurity concentration.

Temperature-independent transport in high-mobility pentacene transistors

The charge-carrier transport mechanism in the organic semiconductor pentacene is explored using thin-film transistor structures. The variation of the field-effect mobility with temperature differs from sample to sample, ranging from thermally activated to temperature-independent behavior. This result excludes thermally activated hopping as the fundamental transport mechanism in pentacene thin films, and suggests that traps and/or contact effects may strongly influence the observed characteristics. These results also indicate that field-effect transistors may not be appropriate vehicles for illuminating basic transport mechanisms in organic materials.

Highly strained ln0.35Ga0.65As/GaAs layers grown by molecular beam epitaxy for high hole mobility transistors

We have grown highly strained In0.35Ga0.65As layers on GaAs substrates by molecular beam epitaxy to improve the performance of high hole mobility transistors (HHMTs). The mobility and sheet hole concentration of double side doped pseudomorphic HHMT structures at room temperature reached 314 cm2/V-s and 1.19 × 1012 cm−2, respectively. Photoluminescence measurements at room temperature show good crystalline quality of the In0.35Ga0.65As layers. This study suggests that the performance of HHMTs can be improved by using high-quality In0.35Ga0.65As layers for the channel of double side doped heterostructures pseudomorphically grown on GaAs substrates.

Transport Properties in Band-Tails of High Mobility Poly-Si TFTs

Electrical properties of high-mobility polycrystalline silicon thin film transistors (poly-Si TFTs) are measured over the wide temperature range from 300 K to 1.5 K in order to clarify the role of band-tail states in the grain boundaries (GBs). Two distinct regimes of weak and strong localization of electrical transport have been identified below and above the carrier trap state density N st≈1.0×1012 cm-2 in the GBs for high-mobility poly-Si TFTs. Poly-Si TFTs with a smaller carrier trap state density N st (≈6×1011 cm-2) show metallic behavior where the mobility increases as temperature decreases. Poly-Si TFTs with larger N st (≈1.5×1012 cm-2) show semiconducting behavior of mobility. At low temperatures, on the former poly-Si TFT the resistance changes due to the weak-localization effect and on the latter poly-Si TFT due to the variable-range hopping (VRH) in the regime of strong localization. In both regimes, negative magnetoresistances have been observed and interpreted by the quantum interference effect. All of these features are attributed to the band-tail states decaying exponentially from the band-edge, based on the observations of irregular GB structure in the poly-Si film from transmission electron microscope images and the roughness at the SiO2/poly-Si interface seen on atomic force microscope images.

Scattering of conduction electrons by interface roughness in semiconductor heterostructures

A fluctuation transport theory is applied to describe the mobility limiting effect of interface roughness scattering in semiconductor heterostructures. As an example we consider a high mobility transistor and compare electron scattering by interface roughness with Coulomb scattering processes by ions and dipole fluctuations. We show that the dynamical resistivity measurement provides detailed information about the autocorrelation of the interface morphology.

Partial and perfect dislocation nucleation at the onset of stress relaxation in In0.60Ga0.40As active layers of high mobility transistors grown on InP

The morphology of compressive InxGa1−xAs/In0.52Al0.48As layers grown on (100)-InP substrates by molecular beam epitaxy was observed by transmission electron microscopy. A preliminary analysis of the network of misfit dislocations at the interface in layers with a thickness of 0.5 μm and xIn between 54% and 63% led to a further study of the onset of stress relaxation for layers with composition xIn=60% and thickness ranging from 5 to 25 nm. A critical thickness was found for plastic relaxation at 20 nm<tc<25 nm. Following a model of excess stress, a mechanism for the nucleation of dislocations according to the sequence 90°partial→60°perfect→30°partial is proposed.

Effect of strained InGaAs step bunching on mobility and device performance in n-InGaP/InGaAs/GaAs pseudomorphic heterostructures grown by metalorganic vapor phase epitaxy

We studied the surface of thin strained InGaAs layers grown on GaAs and n-GaAs/n-GaP/InGaAs/GaAs pseudomorphic high elecron mobility transistor (HEMT) structures using atomic force microscopy (AFM). We observed large step bunching from layer-by-layer growth more readily in InGaAs than in GaAs, due to strain. AsH 3 annealing of the InGaAs surface induced step bunching movement, and reduced the surface free energy and strain. We investigated the effect of the InGaAs growth conditions on the heterointerface roughness in HEMTs by selectively etching n-GaAs and n-InGaP. We found that etched InGaAs differs from as-grown InGaAs and that the electron mobility in HEMT's is insensitive to InGaAs step bunching due to both the high sheet carrier density and the large step periodicity. Increasing InGaAs growth rate causes micro-alloy clustering, although growth proceeds layer-by-layer, which affects the n-GaAs cap growth mode and degrades the mobility. Tertiarybutylarsine can suppress step bunching because of its low decomposition temperature. We also investigated the effect of substrate misorientation and found that both the mobility and the quantum well characteristics were primarily affected by step type. We fabricated HEMTs with 1 μm long gate electrodes parallel and perpendicular to the step bunching. We found that the step bunching slightly influences the K-value around the threshold voltage in HEMTs.

Role of molecular beam epitaxy in the field of optoelectronics

This article reviews the role of molecular beam epitaxy (MBE) in the field of optoelectronics. In the past 25 years, this crystal growth technology has helped researchers to investigate important physical phenomena related to materials properties. In particular, success in engineering the band structure and refractive index was a key to the exploration and development of carrier transport devices such as high-mobility, bipolar and tunnel transistors, as well as optoelectronic elements such as LEDs, lasers, modulators and photodetectors. Because of its relatively simple growth process, excellent growth control and ability to characterize growth-related phenomena in situ, MBE initiated many important research fields such as GaAs on silicon, fast optical switches and detectors, and vertical cavity and edge-emitting quantum well lasers, particularly in the GaAs/AlGaAs materials system. In addition, relatively unexplored systems such as II/VI materials, nitrides or even Si/Ge heterostructures, are currently hot topics among MBE crystal growers. Over the years, the division of the roles of MBE as a research and prototyping tool and metal-organic vapor-phase epitaxy (MOVPE) as a production tool has become well established. However, for certain production processes, specifically compact disc and 980 nm Er-fiber pump lasers, MBE has been proved to be as good or better than MOVPE, even when high throughput is a necessity. New fields, such as devices based on very precise vertical Fabry-Perot cavity designs and low-temperature GaAs, are emerging in which the uniform thickness control, in situ monitoring capability and low-temperature growth ability of MBE offer clear superiority over MOVPE for eventual production.

Stability of interfaces in pseudomorphic ingaas hemts

Considerable progress is now reported on pseudomorphic InGaAs high mobility transistors (PM. HEMTs). However, structures which exhibit the best performances are thermodynamically metastable. The stabilities of commercially available devices have been studied. Investigations of both GaAs InGaAs and AlGaAs/InGaAs interface vicinities have been performed through deep level characterizations. Life-testing experiments have then been carried out in order to assess the stability of the InGaAs buried strained layer and related interfaces during operation. Deep level studies together with electrical parameter measurements indicate that interfaces of InGaAs strained quantum well HEMTs do not induce particular degradations after 3000 hours into biased ageing tests.

Investigation of pseudomorphic InGaAs hemt interfaces

Pseudomorphic InGaAs high mobility transistors (PM. HEMTs) which exhibit the best performances are thermodynamically metastable. Investigations on both GaAs/InGaAs and AlGaAs/InGaAs interface vicinities have been performed through deep level characterizations and lifetesting experiments. Results indicate that interfaces of InGaAs strained quantum well HEMTs do not suffer particular degradation after 3000 hours of biased aging tests.

Thermal processing of strained GaInAs/GaAs high hole mobility transistor structures

We have studied the resistance to thermal processing of a realistic strained-layer device structure: a GaAs/GaInAs p-type modulation-doped field-effect transistor layer. The integrity of the structure was monitored using the photoluminescence from the strained quantum well in the active region of the structure. No evidence of mixing or strain relaxation was observed when samples were annealed at 750 °C for 5 h. At higher temperatures, 900 °C and above, mixing is observed and values for the interdiffusion constants and the activation energy obtained.

Strained InGaAs/InAiAS High Eledron Mobility Transistors

The direct current (dc) and microwave performance of InP-based In(x),Gal-Js/Zno~52Alo,48As strained high electron mobility transistcrs are described. Its design is based on theoretical and experimental studies incl'rlding iow- and high-field transport characterisation of heterostructures with different strains. Mobilities as high as 13,900; 74,000 snd 134,000 cm2/Vs are measured at 300,77. and 4.2 K in a heterostructure with the value ofxbeing 0.65. Shubnikov-de Haas measurements indicate that the change inthe effective mass with increasing In is not sigrfificant in n-HEMTs and isnot responsible for the enhancement in mobilities. We believe that the improvement results from reduced alloy scattering, reduced inter sub band scattering, and reduced impurity scattering, all of which result from ahigher conduction-band offset and increased carrier confinement in thetwo-dimensiona e ectron gas. The high field eiectron velocities have been measured in these samples using pulsed current-voltage and pulsed Hai:measurements. A monotonic increese in velocities is osbzmed both at 300and 77 K with an increase of In content in the channel. Velocities of1.55 x 10' and 1.87 x 10' cm/s are measured at 300 and 77 K, respectively,in a Ino.asGao3 ~ s N n o . sr~slAo m ~o ciulation-doped heterostructure In 1.4pm gate HEMTs, the maximuni intrinsic dc transconductor is 700 rnS/mmwhen x = 0.65. The highest values of fr and f,, are 45 GHz and 60 GHzrespectively. These figures clearly indicate their superiority over identical lattice-matched devices, primarily resulting from increased first sub-band confinement in the pseudomorphic quantum well channel. It is expected that by reducing gate lengths tc submicron dimensions and making some growth-reiated changes in heterostructure design, the best millimetre wavesolid-state low-noise amplifiers and oscillators can be realised.

High-mobility CMOS transistors fabricated on very thin SOS films

It is reported that the mobility of CMOS transistors fabricated on very thin silicon-on-sapphire (SOS) films is a function of the film growth rate. Transistors with mobilities nearly as high as those obtained on 1.0- mu m-thick films have been fabricated on SOS films 0.2 mu m thick that have been grown at growth rates above 4 mu m/min. >

Rapid isothermal processing

The physics and technology of a relatively new, short-time, thermal processing technique, namely rapid isothermal processing (RIP), based on incoherent sources of light for the fabrication of semiconductor devices and circuits, are reviewed in this paper. Low-cost, minimum overall thermal budget, low-power consumption, and high throughput are some of the attractive features of RIP. The discussion of RIP, in the context of other thermal processes, history, operating principles, different types of RIP systems, various applications of RIP using single processing steps, and novel applications of RIP, including in situ processing and multistep processing, is described in detail. Current trends are in the direction of RIP-dominated silicon integrated circuit fabrication technology that can lead to the development of the most advanced three-dimensional integrated circuits suitable for applications such as parallel processing and radiation hardening. RIP is not only a superior alternative to furnace processing, but it is also the only way to perform certain crucial steps in the processing of compound semiconductor devices such as high-mobility transistors, resonant tunneling devices, and high-efficiency solar cells. Development of more accurate temperature measurement techniques and theoretical studies of heat transfer and other fundamental processes are needed. Dedicated equipment designed for a specific task coupled with in situ processing capabilities will dominate the future direction of RIP.

Quantized Hall effect in gated AlGaAs/GaAs heterostructures: Localization as a function of number density and magnetic field

We report two-terminal magnetoresistance measurements on a short gated GaAsAlGaAs HEMT (High Electrical Mobility Transistor) sample for magnetic fields to 18.5 Telsa and temperatures near 0.5 K. A gated sample was used, permitting variation of the 2D charge density over a wide range. Additional measurements were made on gated Van Der Pauw squares that are consistent with the short gate FET results. Using the localization model, the experimental data shows that in the Quantum Hall Effect (QHE) region, the fraction of localization within a Landau level remains unchanged at different magnetic fields and is also unchanged over a wide range of 2D electron number density ( N s ). This is in contrast with the observed large dependence of mobility on N s at the same temperatures in the same sample.