Peak Carrier Concentration Research Articles

Dopant Activation Depth Profiling for Highly Doped Si:P By Scanning Spreading Resistance Microscopy (SSRM) and Differential Hall Effect Metrology (DHEM)

Heavily n-doped epitaxially grown Si layers are of great importance for source/drain (S/D) application in advanced node nMOS devices. For contact resistivity reduction, the dopant activation level is very important. Various techniques are being used to evaluate dopant activation in Si:P layers. Among these two methods are Scanning Spreading Resistance Microscopy (SSRM) and Differential Hall Effect Metrology (DHEM). SSRM uses an atomic force microscope equipped with a hard conductive probe that is scanned in contact mode on the cross-sectioned sample’s surface and measures spreading resistance. Measured resistance values as a function of depth are converted into resistivity and carrier concentration depth profiles using calibration measurements and conversion relationships. DHEM provides depth profiles of mobility, resistivity and carrier concentration through a semiconductor layer by making successive sheet resistance (Rs) and Hall voltage measurements using Hall effect/Van der Pauw techniques, as the electrically active thickness of the layer is reduced through successive oxidation steps. Controlled oxidation is achieved by electrochemical anodization. Data collected can then be processed to yield the depth profiles. In this contribution we have carried out SSRM and DHEM measurements on Si:P epi layers subjected to different processing conditions including annealing and ohmic contact fabrication. Secondary Ion Mass Spectrometry (SIMS) was used to measure the total (active + inactive) dopant profiles through the films. Effects of these processes on dopant diffusion and activation were studied and the results from DHEM and SSRM were compared.In-situ phosphorus (P) doped Si epi-layers were grown over 300mm diameter boron doped monitor wafers. While one set was kept as the reference, a second set was treated by a spike-annealing process at 1000 °C. In a third set a Ti/TiN contact fabrication process was carried out and the contact was removed before analysis. In the fourth set contact process was applied to the spike annealed wafer before removal of the Ti/TiN layers. Bulk sheet resistance measurements were made using 4-point probe (4PP) for all the samples. SSRM measurements were carried out at IMEC. Cross-shaped Van der Pauw test-patterns were formed on 8mmx8mm areas on the samples and DHEM measurements were performed at ALP. Figure below shows the dopant depth profiles obtained by SIMS and the carrier concentration profiles from DHEM and SSRM techniques for samples D02 (as deposited Si:P) and D03 (spike annealed). One can make some general observations from the data in this figure. The total dopant concentration as measured by SIMS is ~ 1.4E21/cm3. There is only a small difference in the total dopant distribution profiles (SIMS) between the as deposited sample and the spike annealed sample. However, the spike annealed sample shows much higher dopant activation as measured by DHEM. Carrier concentration is ~2.5x higher in sample D03 (~5E20/cm3) compared to sample D02 (~2E20/cm3). Activation levels measured by SSRM, however, are lower for both samples, and the peak carrier concentration value increases only slightly upon spike annealing, going from ~2E20/cm3 in sample D02 to ~2.2E20/cm3 in sample D03. DHEM clearly indicates the sharp interface between the p-type substrate and the n-type epi-layer and its depth calibration agrees well with the expected thicknesses of the epi layers. The tail of the SSRM data is much more graded. Data from other samples will be presented and discussed in the final manuscript. Figure 1

Tertiarybutylarsine damage-free thin-film doping and conformal surface coverage of substrate-released horizontal Si nanowires

Conformal damage-free doping is the holy grail for 3D semiconductor device structures, such as those used in multi-gate and nanowire-based field effect transistors (FETs). The shape, dimension, pitch, and spacing of parallel conduction paths introduce increased complexity in a number of ways, but particularly in the area of intentional impurity introduction for doping. To this end, gas-phase doping using tertiarybutylarsine (TBA) was employed to dope silicon-on-insulator (SOI) thin films basedcircular transfer length measurement (CTLM) devices with top silicon thicknesses down to 4.5 nm, and substrate-released horizontal Si nanowires. Dopant incorporation was observed with a peak active carrier concentration of ~7 × 1019 cm−3 after a 1050 °C rapid thermal anneal (RTA). An optimisation study showed that dopant incorporation is similar for varying exposure times to TBA gas, while increased exposure can cause roughening of the Si due to etching. Structural analysis by cross-sectional transmission electron microscopy (XTEM) and Energy-dispersive X-ray spectroscopy (EDX) showed conformal formation of an As-rich surface oxide on free standing nanowires, without surface etching or crystal damage, making this process promising for future gate-all-around (GAA) transistor architectures.

N- and P- type Doping in Al-rich AlGaN and AlN

Significant challenges in point defect control in AlGaN epitaxy has precluded commercialization of AlGaN based devices. Si is typically employed as n-type dopant in AlGaN and exhibits a low activation energy (<50 meV) in AlxGa1-xN with x<0.8. However, Si doped AlGaN exhibits a “knee behavior” resulting in a conductivity and carrier concentration maxima at a specific Si concentration. Hence a high doping limit exists for Si in AlGaN that lowers the maximum achievable carrier concentrations that are necessary for AlGaN based power electronics. Similarly, a low doping limit (a minimum achievable carrier concentration with a corresponding maximum mobility) exists in AlGaN similar to that in GaN which precludes implementation of AlGaN power electronics that require low doped drift regions. Along the same lines, p-type doping is achieved with Mg, but high activation energies preclude its technological implementation. Following results in GaN, compensating point defects are also expected to play a major role in determining the doping efficiency of Mg, even if methods to “reduce” its activation energy are found. Hence a major “point defect problem” exists in AlGaN that needs to be solved for implementation of AlGaN technology. In this work, we update on three different achievements: (1) identifying and controlling point defects for efficient n-type doping in AlGaN, (2) implantation as a method for achieving high n-type conductivity in AlN and (3) the possibility of high conductivity p-type AlGaN. All these achievements are built upon a point defect control (PDC) framework based on a systematic chemical potential control (CPC) and defect quasi Fermi level control (DQFL). In CPC, the growth environment variables are related to the defect formation energy by determining and controlling the Al,Ga,N and impurity chemical potentials. This helps optimize the growth environment accordingly for minimal point defect incorporation or generation. In DQFL control, the defect quasi Fermi level, the Fermi level describing the probability of the defect level being occupied/unoccupied, is modified by introducing excess minority carriers (by above bandgap illumination). This leads to an increase in the compensating point defect formation energy, reducing their concentration. Both control schemes provide for a theoretical framework that provides a quantitative relationship between point defect formation energies and growth process parameters. CPC based targeted reduction requires knowledge of the defects responsible and accordingly, we identify the point defects responsible for knee behavior in n-type AlGaN as most likely vacancy-silicon complexes and low doping limit as CN. Note that threading dislocations and vacancy-oxygen complexes are also expected to be compensating and contribute to the low doping limit. Utilizing the vacancy-silicon complex and CN in Al0.7Ga0.3N as a case study, we demonstrate control over the knee behavior (improving the peak carrier concentration by impeding the formation VIII-SiIII complexes) and low doping limit (achieving lower carrier concentrations by reducing the compensating impurity (CN) density) in Al0.7Ga0.3N by controlling the chemical potentials of III/N and growth temperature with excellent agreement with quantitative theoretical predictions. These concepts are further used in Si implanted AlN where a reduction in the formation of vacancy-silicon complexes during implantation annealing is necessary. This was achieved by implementing DQFL control during the implantation annealing stage. Such process led to an increase of nearly 3 orders of magnitude in the conductivity of n-type AlN at room temperature. Finally, after recognizing the importance of point defect reduction during doping, significant achievements in p-type AlGaN were realized. Lower Mg activation energies than those previously reported were observed leading to significant p-type conductivity in AlGaN. All these results support our PDC framework and the utility of formation energies from equilibrium formalism typically employed in DFT in predicting defect incorporation in non-equilibrium and high temperature MOCVD growth.

Quantum Confinement and Interface States in ZnO Nanocrystalline Thin-Film Transistors

The experimental current–voltage ( ${I}_{D}$ – ${V}_{G}$ ) and capacitance–voltage ( ${C}$ – ${V}_{G}$ ) characteristics of the bottom-gate/top-contact ZnO thin-film transistors (TFTs) are analyzed accounting for quantum confinement and interface state effects. All the measurements are performed on the same TFTs composed of a 45-nm-thick nanocrystalline ZnO channel, indium tin oxide gate electrode, and a 21-nm-thick Al2O3 gate insulator. Interface state density (Dit) derived from the combined high–low frequency capacitance method reveals a large Dit near the conduction band edge. The TFT characteristics are simulated using a Schrodinger–Poisson model and compared to a semiclassical model. The Schrodinger–Poisson model calculates a lower C in accumulation by correctly accounting for the peak carrier concentration being several nanometers away from the ZnO/Al2O3 interface. The fit error in ${C}$ – ${V}_{G}$ in strong accumulation is only 1.2% compared to 4% in the semiclassical simulations. All simulations are performed using 21 nm of Al2O3. An effective mobility ( $\mu _{\mathrm{ eff}}$ ) that increases linearly with gate voltage is derived from the ratio of the experimental ${I}_{D}$ and the simulated mobile charge per unit area of the channel. Ignoring quantum confinement overestimates the channel charge and hence underestimates $\mu _{\mathrm{ eff}}$ by 40%. An additional Dit profile, at the top interface of the ZnO, is proposed in our analysis to explain the ${I}_{D}$ – ${V}_{G}$ characteristics in the subthreshold region.

High level active n+ doping of strained germanium through co-implantation and nanosecond pulsed laser melting

Obtaining high level active n+ carrier concentrations in germanium (Ge) has been a significant challenge for further development of Ge devices. By ion implanting phosphorus (P) and fluorine (F) into Ge and restoring crystallinity using Nd:YAG nanosecond pulsed laser melting (PLM), we demonstrate 1020 cm−3 n+ carrier concentration in tensile-strained epitaxial germanium-on-silicon. Scanning electron microscopy shows that after laser treatment, samples implanted with P have an ablated surface, whereas P + F co-implanted samples have good crystallinity and a smooth surface topography. We characterize P and F concentration depth profiles using secondary ion mass spectrometry and spreading resistance profiling. The peak carrier concentration, 1020 cm−3 at 80 nm below the surface, coincides with the peak F concentration, illustrating the key role of F in increasing donor activation. Cross-sectional transmission electron microscopy of the co-implanted sample shows that the Ge epilayer region damaged during implantation is a single crystal after PLM. High-resolution X-ray diffraction and Raman spectroscopy measurements both indicate that the as-grown epitaxial layer strain is preserved after PLM. These results demonstrate that co-implantation and PLM can achieve the combination of n+ carrier concentration and strain in Ge epilayers necessary for next-generation, high-performance Ge-on-Si devices.

Functionalization of SiO2 Surfaces for Si Monolayer Doping with Minimal Carbon Contamination.

Monolayer doping (MLD) involves the functionalization of semiconductor surfaces followed by an annealing step to diffuse the dopant into the substrate. We report an alternative doping method, oxide-MLD, where ultrathin SiO2 overlayers are functionalized with phosphonic acids for doping Si. Similar peak carrier concentrations were achieved when compared with hydrosilylated surfaces (∼2 × 1020 atoms/cm3). Oxide-MLD offers several advantages over conventional MLD, such as ease of sample processing, superior ambient stability, and minimal carbon contamination. The incorporation of an oxide layer minimizes carbon contamination by facilitating attachment of carbon-free precursors or by impeding carbon diffusion. The oxide-MLD strategy allows selection of many inexpensive precursors and therefore allows application to both p- and n-doping. The phosphonic acid-functionalized SiO2 surfaces were investigated using X-ray photoelectron spectroscopy and attenuated total reflectance Fourier transform infrared spectroscopy, whereas doping was assessed using electrochemical capacitance voltage and Hall measurements.

Doping Mechanism in Transparent, Conducting Tantalum Doped ZnO Films Deposited Using Atomic Layer Deposition

The effect of tantalum (Ta) addition to ZnO films has been investigated. The films are synthesized using atomic layer deposition. The precursor for Ta is pentakis‐dimethylamino Ta (PDMAT) whereas, diethyl Zn (DEZ) is used for ZnO deposition with H2O as an oxidant. The surface reactions are studied using in situ downstream quadrupole mass spectrometry. X‐ray photoelectron spectroscopy and photoluminescence show that at low Ta doping, oxygen vacancies are eliminated from the film, whereas at higher Ta doping, Zn vacancies are formed. These observations are supported by density functional theory calculations. Hall measurements show the lowest resistivity for a 30 nm Ta doped ZnO film to be 4 × 10−3 Ω cm with a peak carrier concentration of 1.2 × 1020 cm−3. The ideal cycle ratio used to achieve these properties is 40:1 of (DEZ+H2O):(PDMAT+H2O) pulses. Optical measurements show an average transmittance of 85% in the visible regime. The efficiency of Ta doping the ZnO, which represents the fraction of Ta5+ contributing to conductivity, is found to be at least 6.3%.

Impact of Phosphorus Implantation on the Electrical Properties of SiO&lt;sub&gt;2&lt;/sub&gt;/4H-SiC Interfaces Annealed in N&lt;sub&gt;2&lt;/sub&gt;O

In this work, the combined effect of a shallow phosphorus (P) pre-implantation and of a nitridation annealing in N2O on the properties of the SiO2/4H-SiC interface has been investigated. The peak carrier concentration and depth extension of the electrically active dopants introduced by the nitridation and by the combination of P pre-implantation and nitridation were determined by high resolution scanning capacitance microscopy (SCM). Macroscopic capacitance-voltage (C-V) measurements on metal oxide semiconductor (MOS) capacitors and nanoscale C-V analyses by SCM allowed to quantify the electrical effect of the donors introduced underneath the SiO2/4H-SiC interface. Phosphorous pre-implantation and subsequent high temperature electrical activation has been shown not only to produce an increased doping in the 4H-SiC surface region but also a better homogeneity of surface potential with respect to the use of N2O annealing only.

(Invited) Dopant Activation and Deactivation in InGaAs during Sub-Millisecond Thermal Annealing

Dopant activation of In0.53Ga0.47As was determined following laser spike annealing (LSA) at temperatures up to 900 oC with dwells from 250 μs to 2000 μs. Temperatures of the InGaAs under LSA were measured using Pt thermistors and calibrated to the known decomposition temperature of a simple polymer. Using spatially resolved probes, temperature dependent activation levels were determined by scanning across a single laser stripe. Raman analysis of the LO phonon peak and the LO phonon-plasmon coupling mode (LOPCM) peak were used to monitor crystal quality and carrier concentrations following anneals of Si+ implanted InGaAs. Compared to furnace annealing, LSA was found to increase active carrier concentration and extend the temperature limit before damage. CAPRES 4-point measurements were similarly used to determine carrier activation as a function of annealing temperature. For a 1014 cm-2 Si implant dose at 20keV, a minimum sheet resistance of 135 Ω/sq was obtained for a 1000 μs dwell at 845oC, with a peak carrier concentration of 7.7x1018 cm-3. Figure 1

(Invited) Dopant Activation and Deactivation in InGaAs during Sub-Millisecond Thermal Annealing

Dopant activation of Si implanted In0.53Ga0.47As (InGaAs) following laser spike annealing (LSA) was studied at temperatures up to 1000oC with dwells from 250 μs to 2000 μs. Temperatures of the InGaAs under LSA were directly measured using Pt thermistors and calibrated to the known decomposition temperature of a simple polymer. Using spatially resolved probes with single laser stripe scans, the temperature dependent activation and damage behaviors were determined. Compared to furnace annealing, LSA was found to increase active carrier concentration and extend the temperature limit before damage. CAPRES 4-point measurements were used to determine carrier activation while Raman analysis of the LO phonon peak and LO phonon-plasmon coupling mode (LOPCM) peak was used to monitor crystal quality and carrier concentrations. For a 1014 cm-2 Si implant at 20keV, a minimum sheet resistance of 135 Ω/sq was obtained for a 1000 μs dwell at 845oC, with a peak carrier concentration near 8x1018 cm-3.

Polarization charge properties of low-temperature atomic layer deposition of AlN on GaN

Low-temperature (<250 °C) plasma-enhanced atomic layer deposition (PEALD) is used to fabricate aluminum nitride (AlN)/gallium nitride (GaN) heterojunctions for tunnelling contacts in devices where selective contacts are required, such as GaN metal–oxide semiconductor field-effect transistors. AlN is grown on GaN templates with via low-temperature plasma-enhanced ALD, and compared in order to extract their surface two-dimensional electron gas concentration. A peak electron density of >2 × 1013 cm−2 was observed for an approximately 4.5 nm AlN thickness on GaN wafers with a higher initial doping concentration while for the lightly doped GaN samples, a peak carrier concentration of 1.9 × 1013 cm−2 was observed for a thickness of 5.7 nm AlN. Polarization is strongest near the AlN critical thickness of strain relaxation for low-temperature deposition methods. This suggests that greater initial dopant concentrations are more conducive to enhanced polarization characteristics under these low growth temperatures because the critical thickness is realized at lower thicknesses. This approach also yields a low contact resistance of 0.45 Ω mm with Al contacts.

Nanofabrication processes for innovative nanohole-based solar cells

The ability to form ordered nanostructures at the wafer level with low cost methodologies has represented a challenge in the last decade in many research fields spanning from nanoelectronics to photovoltaics (PVs). For the latter application the nanostructures have demonstrated interesting capabilities for exploiting the quantum effects in terms of efficient visible light absorption. To fabricate ordered nanostructures many solutions have been proposed but they provide feature densities lower than 109 cm−2 or present high fabrication costs. We propose a wafer level and low-cost Lithography based on block CoPolymers self-assembling (LCP), which allows the formation of nanofeatures controlled down to 10 nm and density higher than 5 × 1010 cm−2. We propose to use this technique to form radial junctions in nanoholes for solar cells. The approach is similar to that of the nanowires, i.e., it decouples the optical path of the visible photons from the electrical path of the carriers, but since the one-dimensional (1D) structures are embedded inside the bulk of the wafer the structure is more robust and allows easier implementation. To form the junction inside the nanoholes a novel strategy based on the deposition of monolayers of dopant-containing molecules is proposed. This technique allows to obtain shallow and controlled junction depths with peak carrier concentrations of about 1019 cm−3 for both n- and p-type doping.

High carrier concentration in high Al‐composition AlGaN‐channnel HEMTs

AbstractHigh Al‐composition (Al = 51%) and low Al‐composition (Al = 20%) AlGaN‐channel high‐electron‐mobility transistors (HEMTs) on AlN layers with very high carrier concentration were demonstrated. In two types of HEMTs, 2‐dimensional electron gases (2DEG) were clearly observed and peak carrier concentration and sheet carrier concentration were approximately 1020 cm‐3 and higher than 2 × 1013 cm‐2, respectively. From the X‐ray diffraction (XRD) measurements, it was observed that the AlGaN channel layers on the AlN layers were partially relaxed and the degree of relaxation decreased with increasing Al‐composition of AlGaN channel layers. Therefore the misfit dislocations in the high Al‐composition HEMTs were considered to be lower than those in the low Al‐composition HEMTs. Furthermore, it was revealed that very high Al‐composition of AlGaN barrier layers can be grown coherently on the AlGaN channel layers in consequence of the partly relaxation of the AlGaN channel layers, which of lattice constants of a‐axis were smaller than that of fully relaxed AlGaN channel layers. Therefore very high carrier concentration of 2DEG can be obtained in spite of the high Al‐composition of AlGaN channel layers. We considered that this high carrier concentration of 2DEG was necessary to demonstrate high Al‐composition AlGaN‐channel HEMTs. (© 2012 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Analysis of Optical Properties of GaN/AlGaN Quantum Well Ultra-Violet Laser Diode Using 6X6 Hamiltonian

The Quantum well structures have exhibited significant utility in the fabrication of advanced laser devices. The Gallium nitride semiconductor and its alloy particularly AlGaN based quantum structures are having important applications in optical data storage systems and the visible displays. Due to tailoring of wide band gap energy the spectrum obtained is from visible to ultraviolet wavelength range. We had thoroughly investigated the influence of Aluminum mole fraction variation in AlxGa1-xN under a biased condition for GaN/AlGaN based quantum heterostructure optical properties. Here, we had used 6X6 Hamiltonian to realize these properties. The 6X6 Hamiltonian has been chosen to include the many body effect in the calculation and to enhance the accuracy of the optimized results. The paper is focused to reveal the Aluminum mole fraction dependence of near and far filed intensities, peak optical gain, carrier concentration, and optical confinement factor. The effective index method has been used in determination of the optical field intensity in the near and far regimes. The variation in Aluminum mole fraction produces disparity in carrier concentration; hence, we have obtained the spontaneous emission and optical gain as a function of photon energy for different carrier density. The piezoelectric effect on GaN quantum well due to AlGaN barriers has been included through Poisson equation. This Poisson equation has been solved in a self-consistent manner along with Schrödinger and subsequently carrier concentrations have been deduced with a high accuracy using our simulation tools developed in MATLAB.

Constraints on micro-Raman strain metrology for highly doped strained Si materials

Ultraviolet (UV), low penetration depth, micro-Raman spectroscopy, and high-resolution x-ray diffraction (HRXRD) are utilized as complementary, independent stress characterization tools for a range of strained Si samples doped by low energy (2keV) Sb ion implantation. Following dopant implantation, good agreement is found between the magnitudes of strain measured by the two techniques. However, following dopant activation by annealing, strain relaxation is detected by HRXRD but not by micro-Raman. This discrepancy mainly arises from an anomalous redshift in the Si Raman peak position originating from the high levels of doping achieved in the samples. This has serious implications for the use of micro-Raman spectroscopy for strain characterization of highly doped strained Si complementary metal-oxide semiconductor devices and structures therein. We find a direct correlation between the Si Raman shift and peak carrier concentration measured by the differential Hall technique, which indicates that UV micro-Raman may become a useful tool for nondestructive dopant characterization for ultrashallow junctions in these Si-based materials.

Characterization of free-standing GaN substrate grown through hydride vapor phase epitaxy with a TiN interlayer

Large-scale GaN free-standing substrate was obtained by hydride vapor phase epitaxy directly on sapphire with porous network interlayer. The bottom surface N-face and top surface Ga-face showed great difference in anti-etching and optical properties. The variation of optical and structure characteristics were also microscopically identified using spatially resolved cathodoluminescence and micro-Raman spectroscopy in cross-section of the GaN substrate. Three different regions were separated according to luminescent intensity along the film growth orientation. Some tapered inversion domains with high free carrier concentration of 5 × 10 19 cm −3 protruded up to the surface forming the hexagonal pits. The dark region of upper layer showed good crystalline quality with narrow donor bound exciton peak and low free carrier concentration. Unlike the exponential dependence of the strain distribution, the free-standing GaN substrate revealed a gradual increase of the strain mainly within the near N-polar side region with a thickness of about 50 μm, then almost kept constant to the top surface.

Towards nondestructive carrier depth profiling

As indicated by the International Technology Roadmap for Semiconductors (ITRS) (http://public.itrs.net/), obtaining accurate information on the electrically active dopant profile for sub-30nm structures is a key issue. Presently, however, there is no conventional destructive (probe based) technique available satisfying the ITRS targeted depth (3%) and carrier level (5%–10%) reproducibility and accuracy. In this work, we explore the promising capabilities of a nondestructive photomodulated reflectance technique, based on the local detection of variations in the reflectivity of the sample due to thermal and plasma (excess carrier) effects as can be generated by a modulated pump laser such as in the therma-probe (TP) system. Experimental data obtained on chemical vapor deposition grown layers will be discussed with respect to junction depth and active dopant (carrier) level sensitivity. A methodology to decouple the junction depth and peak carrier concentration information for unknown samples based on the conventionally available TP signals will be presented. Also simulations and theoretical insights relating to the methodology will be discussed.

Electrical properties of n-type layers formed in GaN by Si implantation

Silicon ions have been implanted in undoped GaN layers epitaxially grown on an AlN/(0001)-oriented Al2O3 substrate. Carrier-concentration and mobility profiles for Si-implanted layers have been examined by differential Hall-effect measurements. It is shown that highly doped n-type layers with a peak carrier concentration of 2.1×1020cm−3 are formed after rapid thermal annealing at 1250°C for 30s. Comparison with SIMS profiles indicates that a very high electrical activation of Si atoms located near the surface is achieved after annealing. In a back tail part of the profile, the electrical activity is limited, which will be due to lattice defects remaining after annealing and acting as carrier trapping centers and/or compensation centers. Values of carrier mobilities are identical to those previously reported for n-type epitaxial GaN layers doped with Si during crystal growth.

Silicon implantation in epitaxial GaN layers: Encapsulant annealing and electrical properties

Silicon ions have been implanted into GaN layers epitaxially grown on an AlN/(0001)-Al2O3 substrate to a dose of 1×1015/cm2. The Si-implanted GaN has been rapid thermal annealed at temperatures between 800 and 1250 °C. Thermal pits are formed in the surface during annealing at 1200 °C, resulting in deterioration of the morphology. The surface deterioration is effectively suppressed by using a 140-nm-thick Si3N4 film as an encapsulant during annealing up to 1250 °C. The electrical activation process for Si atoms has an activation energy of 3.1 eV in the range of 1000 to 1250 °C. Carrier concentration and mobility profiles for n-type layers formed by Si implantation have been examined by differential Hall-effect measurements. A very high electrical activity for implanted Si atoms of 86% can be achieved, and a highly doped n-type layer with a peak carrier concentration of 6×1019/cm3 is formed after annealing at 1250 °C. Electrons generated from Si atoms located near the end of range are trapped by defects remaining after annealing at 1250 °C.

Sub-0.5-µm gate doped-channel field-effect transistors with HEMT-like channel using thermally reflowed photoresist and spin-on glass

In this paper we report on a new sub-0.5-µm gate-length field-effect transistor (FET) processing technique by using conventional i-line optical lithography. The key methodology is to thermally reflow the patterned photoresist upon two-step spin-coated spin-on glass (SOG). According to this new process, the deposited gate metal has its final length and thickness separately determined by taped resist profile and SOG thickness. Furthermore, undercutting formed during isotropic etch SOG film is beneficial to the subsequent lift-off process, achieving high fabrication yield. The implemented gate length is as short as 0.41 µm. Then it was successfully applied to fabrication of a newly designed hetero-doped-channel FET (HDCFET) with digital-graded InxGa1−xAs multilayer forming a HEMT-like channel. This digital-graded InxGa1−xAs channel by changing x values from 0.1 to 0.2 has most electrons closer to the gate metal. The measured sheet carrier density and mobility are 4.3 × 1012 cm−2 and 3560 cm2 V−1 s−1, respectively, while the peak carrier concentration is larger than 1 × 1019 cm−3. A fabricated 0.41 × 100 µm2 HDCFET exhibits the maximum transconductance of 370 mS mm−1 with an output current larger than 535 mA mm−1 and ft(fmax) of 26 (32) GHz.