Junctions In Ge Research Articles

Disorder and cavity evolution in single-crystalline Ge during implantation of Sb ions monitored in-situ by spectroscopic ellipsometry

Ion implantation has been a key technology for the controlled surface modification of materials in microelectronics and generally, for tribology, biocompatibility, corrosion resistance and many more. To form shallow junctions in Ge is a challenging task. In this work the formation and accumulation of shallow damage profiles was studied by in-situ spectroscopic ellipsometry (SE) for the accurate tracking and evaluation of void and damage fractions in crystalline Ge during implantation of 200-keV Sb + ions with a total fluence up to 1016 cm−2 and an ion flux of 2.1 × 1012 cm−2s−1. The consecutive stages of damage accumulation were identified using optical multi-layer models with quantitative parameters of the thickness of modified layers as well as the volume fractions of amorphized material and voids. The effective size of damaged zones formed from ion tracks initiated by individual bombarding ions can be estimated by numerical simulation compared with the dynamics of damage profiles measured by ion beam analysis and ellipsometry. According to our observations, the formation of initial partial disorder was followed by complete amorphization and void formation occurring at the fluence of about 1 × 1015 cm−2, leading to a high volume fraction of voids and a modified layer thickness of ≈200 nm by the end of the irradiation process. This agrees with the results of numerical simulations and complementary scanning electron microscopy (SEM) measurements. In addition, we found a quasi-periodic time dependent behavior of amorphization and void formation represented by alternating accelerations and decelerations of different reorganization processes, respectively. For the understanding and prevention of adverse void formation and for controlled evolution of subsurface nanocavities or cellular surface texture the in-situ monitoring of the dynamics of structural damage accumulation by the developed SE method is essential.

(Invited) Issues with n-type Dopants in Germanium

The last decade has seen considerable interest in the use of germanium for high-speed low-power electronics. Despite the demonstration of high performance p-channel Ge transistors in planar and non-planar device technology, fabrication of n-channel Ge transistors faces a number of scientific and technological challenges which hinder the development of CMOS logic circuits based entirely on Ge. Major challenge towards the establishment of Ge deep submicron n-channel MOSFETs technology is the control of the fast vacancy-mediated n-type dopant diffusion in Ge, which prevents the formation of shallow (xj<20nm) and highly activated (active n-type dopant concentration ≥ 1020cm-3) n+/p junctions [1,2]. Additional issues in meeting this target are constituted by: (i) the significant simultaneous out-diffusion trend of n-type dopants and (ii) the Ge thermal etching during post-implantation annealing, which causes significant Ge substrate loss at temperatures above 500oC, thus imposing the presence of a capping layer on Ge surface [3]. Up to now, a variety of approaches have been followed to suppress n-type dopant diffusion in Ge, the most representative being [1,2]: (a) co-implants with impurities such as carbon, fluorine or nitrogen (b) co-implants of two n-type dopants (e.g arsenic with antimony) (c) bulk interstitial injection by Ge self-implantation or GeOx clusters dissolution and (d) high temperature-short time non-conventional annealing schemes (Flash Lamp Annealing, Laser Annealing, Microwave Annealing). More recently, the ability of surface capping layers to modify the near surface Ge vacancy concentration has been demonstrated [4]. This revealed “capping layer engineering” as an additional supporting technique to control n-type dopant diffusion in Ge. During the last years, our work in the field has been devoted to the beneficial action of co-implanted nitrogen in suppressing low energy-implanted phosphorous in and out-diffusion in Ge [4-6]. The action of nitrogen is based on its individual anomalous diffusion [Figure(1)] and the simultaneous formation of complexes with phosphorous atoms [Figure(2)]. The aim of this work is to review the field comparing the above mentioned approaches, with particular emphasis on the action of nitrogen as phosphorous diffusion blocker. A series of experiments performed in low energy/high dose N2/P co–implanted (100) Ge as function of the post-implantation annealing scheme and temperature, the surface capping layer stoichiometry and the level of simultaneous interstitial injection in the substrate will be presented and discussed. The experiments aspire to reveal the microscopic origin of nitrogen anomalous diffusion in Ge, the role of nitrogen in suppressing the damage caused in Ge substrate by non-conventional annealing schemes such as FLA [Figures (3)-(4)], as well as processing issues, limitations and strategies towards the formation of shallow and highly activated n+/p junctions in Ge based on N/P co-implants.

Near-infrared laser annealing of Ge layers epitaxially grown on Si for high-performance photonic devices

Near-infrared laser annealing of Ge epitaxial layers on Si is studied as a postgrowth annealing process to reduce the density of threading dislocations (TDs) in Ge. Laser annealing is performed using an exposure of 1.07-μm-laser light with the power density as large as 8 kW/cm2 and the duration as short as 10 ms. The TD density is successfully reduced from 6 × 108 to 2 × 108 cm−2 in a 600-nm-thick Ge layer grown by ultrahigh-vacuum chemical vapor deposition on Si. Vertical pin photodetectors of laser-annealed Ge show a significant reduction of dark leakage current as small as 20 mA/cm2 with a high photodetection efficiency. The short annealing time is effective to prevent the dopant diffusion in Ge together with the Ge-Si intermixing at the interface, being applicable to the annealing even after the formation of pin junctions in Ge. In terms of the reduction of thermal budget, the use of longer wavelength up to ∼1.6 μm has potential applications to preferential annealing of Ge layers on a Si platform due to the strong optical absorption in Ge despite the high optical transparency in Si.

Defect engineering for shallow n‐type junctions in germanium: Facts and fiction

An overview is given on the status of n‐type dopant activation and diffusion in Ge, based on a standard ion implantation and annealing scheme. Emphasis is on defect engineering approaches to optimize either or both parameters. A detailed discussion is given on the use of co‐implantation by neutral or other n‐type dopants. As a case study, the impact of the C ion implantation energy and dose on n‐type junctions in p‐Ge by P + C co‐implantation will be given. It is demonstrated that for fixed P implant conditions, there exist an optimum energy and dose for achieving a minimum junction depth by the formation of C–PV complexes. An alternative approach is the use of self‐interstitial management at the end‐of‐range of the P implantation. Finally, an overview is given of alternatives for obtaining shallow, highly activated n‐type junctions in Ge. They rely on: non‐standard implantation schemes, ultra‐short annealing methods or relying on in situ doped epitaxial deposition.

Suppression of segregation of the phosphorus δ-doping layer in germanium by incorporation of carbon

The successful formation of abrupt phosphorus (P) δ-doping profiles in germanium (Ge) is reported. When the P δ-doping layers were grown by molecular beam epitaxy (MBE) directly on Ge wafers whose surfaces had residual carbon impurities, more than a half the phosphorus atoms were confined successfully within a few nm of the initial doping position even after the growth of Ge capping layers on the top. On the other hand, the same P layers grown on Ge buffer layers that had much less carbon showed significantly broadened P concentration profiles. Current–voltage characteristics of Au/Ti/Ge capping/P δ-doping/n-Ge structures having the abrupt P δ-doping layers with carbon assistance showed excellent ohmic behaviors when P doses were higher than 1 × 1014 cm−2 and the capping layer thickness was as thin as 5 nm. Therefore, the insertion of carbon around the P doping layer is a useful way of realizing ultrashallow junctions in Ge.

(Invited) Smartphones: Driving Technology to More than Moore 3-D Stacked Devices/Chips and More Moore FinFET 3-D Doping with High Mobility Channel Materials from 20/22nm Production to 5/7nm Exploratory Research

3-D bulk-FinFET at 22nm node uses (551) 8o tapered Fin sidewall with 45o high tilt implantation for doping, eSiGe S/D for PMOS channel compressive strain and amorphous implant SPC (solid phase crystallization) dislocation defects for NMOS channel tensile strain. For 14nm node taller Fins with vertical sidewalls and partial recess embedded n+ S/D was added. 7/10nm node will use direct high mobility channel materials 50% SiGe to 100% Ge. To avoid CVD Ge or SiGe epi defects liquid phase crystallization (LPC) by laser melt annealing is an alternative using amorphous-Ge dose control deposition by ion implantation and at 5nm node gate-all-around (GAA) nano-wire. Residual implant damage into Ge material creates high level of acceptors up to 7E19/cm3 requiring high temperature annealing above 600oC to annihilate and establish stable p+ or n+ dopant activation for ultra-shallow junctions. For USJ n+ junctions in Ge, melt controlled junction depth using Sb dopant results in highest dopant activation >1E21/cm3 at sub-10nm junction depth. Si-capping layer offers lowest n+ Ge junction leakage and Sn co-implantation enables surface strain-Ge engineering for higher channel mobility.

Reduction of phosphorus diffusion in germanium by fluorine implantation

The control of phosphorus (P) diffusion in germanium (Ge) is essential for the realisation of ultrashallow n-type junctions in Ge. This work reports a detailed study of the effect of fluorine (F) co-implantation on P diffusion in Ge. P and F profiles were characterized by secondary ion mass spectroscopy. The ion implantation damage was investigated using cross sectional transmission electron microscopy. It is shown that F co-implantation reduces the implanted P profile width and reduces both intrinsic and extrinsic P diffusion in Ge. A defect mediated mechanism for the strong influence of F co-implantation on P diffusion in Ge is proposed and invokes the formation of FnVm clusters in the F-amorphized Ge layer. A fraction of these FnVm clusters decorate the interstitial type end-of-range defects in the re-grown Ge layer and the rest react during re-growth with interstitial germanium atoms diffusing back from the amorphous crystalline interface. The Ge vacancies are then annihilated and mobile interstitial F is released and out diffuses from the surface. This results in a re-grown Ge layer which has a low vacancy concentration and in which the P diffusion rate is reduced. These results open the way to the realization of enhanced Ge n-type devices.

Problems of N-type Doped Regions in Germanium, their Solutions, and How to Beat the ITRS Roadmap

In this paper the authors discuss the issues surrounding the formation of ultrashallow n-type junctions in Ge. In general the n-type dopants are relatively difficult to activate and diffuse quickly, leading to high access resistances and limited capability to reduce the device dimensions, respectively. Sheet resistance (Rs) was calculated for n-type box-like profiles in Si and Ge. To achieve ITRS Roadmap targets in Si an active doping concentration of 1.5-2×1020 cm-3 is required, but due to the enhanced electron mobility it is predicted approximately 6×1019 cm-3 is sufficient in Ge.

High-Performance $\hbox{GeO}_{2}/\hbox{Ge}$ nMOSFETs With Source/Drain Junctions Formed by Gas-Phase Doping

We reveal that the MOVPE-based gas-phase doping can yield lower arsenic diffusion constant and lower leakage current n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> /p junctions in Ge compared with conventional ion-implantation doping. Thus, the gas-phase doping is quite effective for realizing high-performance Ge n-channel MOSFETs. By using gas-phase doping for source/drain junction formation, the (100) GeO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /Ge nMOSFETs have achieved high electron mobility of 1020 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /V·s while maintaining low junction leakage current and high I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">on</sub> /I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">off</sub> ratio of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> . Furthermore, the (110) GeO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /Ge nMOSFETs have also shown high electron mobility and high I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">on</sub> /I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">off</sub> ratio.

Implantation, Diffusion, Activation, and Recrystallization of Gallium Implanted in Preamorphized and Crystalline Germanium

We investigated the as-implanted profiles, electrical activation, diffusion, and recrystallization of gallium implanted in germanium samples through the combination of secondary-ion mass spectrometry, transmission electron microscopy, and sheet resistance measurement. Because of their high activation level without preamorphization, low activation temperature , and absence of diffusion (up to ), Ga junctions in crystalline Ge are very promising candidates for implementation in germanium technology. In the amorphous Ge phase, an increased diffusivity of Ga was observed at temperatures above .

Lifetime and leakage current considerations in metal-doped germanium

This paper discusses the impact of transition metal (TM) impurities in germanium on the electrical material and device characteristics, i.e., the recombination and generation lifetime and the leakage current in a p–n junction. As will be shown, most relevant data have been obtained in the fifties and sixties from Hall effect and lifetime measurements, while there is a lack of accurate Deep Level Transient Spectroscopy (DLTS) results on metal-doped Ge. It will be shown that the most efficient lifetime killers are Fe, Ni and most likely Co, while Cu, which is the most notorious contaminant, has little impact on the minority carrier lifetime. Owing to the asymmetry in the capture cross sections, n-type material will generally have a lower lifetime than p-Ge for the same concentration of substitutional metal centers. Due to the presence of near mid-gap acceptor levels, most TMs will be efficient leakage current generators at room temperature. However, the expected maximum values for the leakage current density are one to two decades smaller than what is typically found in state-of-the-art ion-implanted shallow p–n junctions in Ge.

Very shallow p+-n junction formation by low-energy BF+2 ion implantation into crystalline and germanium preamorphized silicon

Very low energy (6 keV) BF+2 ion implantation has been used to form very shallow (≤1000 Å) junctions in crystalline and Ge+ preamorphized Si. Low-temperature furnace annealing was used to regrow the crystal, and rapid thermal annealing was used for dopant activation and radiation damage removal. In preamorphized samples, Ge+ implantation parameters were found to have an influence on B diffusion. Our results show that for temperatures higher than 950 °C, B diffusion, rather than B channeling, becomes the dominant mechanism in determining the junction depth. Computer simulations of the profiles show regions of retarded and enhanced B diffusion, which depend on surface and end-of-range damage, respectively.

Threshold energy effect on avalanche breakdown voltage in semiconductor junctions

The band bending for avalanche breakdown in semiconductor junctions and its temperature dependence are predicted taking account of threshold energy effects on the ionization process in semiconductors. Where experimental results exist, the theoretical predictions and experimental results are in excellent agreement. In the high electric field region inclusion of both bulk and boundary threshold energy effects is essential. The predictions were based on exact solutions in the nonlocalized ionization coefficient formulation developed by Okuto and Crowell who showed that ionization coefficients as usually understood are functions of both electric field and position in a device. Predictions for abrupt and p- i- n junctions in Ge, Si, GaAs and GaP are presented.

Effect of junction curvature on breakdown voltage in semiconductors

The effect of the radius of curvature of the metallurgical junction (or junction curvature) on avalanche breakdown voltage is herein computed numerically for spherical and cylindrical p- n junctions in Ge, Si, GaAs and GaP. Three types of impurity distribution have been considered: abrupt, linearly graded and composite. For the composite distribution the space charge terminates in a graded region on one side of the junction and in a uniformly doped region on the other side. The results, which are presented graphically, show that for abrupt junctions the breakdown voltage, V B , decreases with decreasing radius of curvature, r j , the effect being larger for spherical junction than for cylindrical junction. Typically, for Si abrupt junctions with a background doping of 10 15 cm −3, V B is 330 V for a plane junction, 80 V for a cylindrical junction with r j = 1 μ, and 39 V for a spherical junction with the same junction radius. For linearly graded junctions however, V B is essentially independent of r j . For composite junctions, at small impurity gradient V B is nearly equal to that of linearly graded junctions; at large impurity gradient, however, V B is determined by r j and the background doping. The results presented here can be applied to junctions formed by planar technology where the edge of the junction has cylindrical and/or spherical geometry with radius of curvature approximately equal to the junction depth. It is further suggested that the radius of curvature is an important design parameter in high power and higher frequency semiconductor devices as well as in integrated circuits.

Hot Carriers in Microplasmas and their Radiation in Germanium and Silicon

AbstractOn the basis of Chambers' equation, assuming the mean free path of carriers to be independent of energy, the form of the distribution function for high electric fields is obtained. Using the distribution function, the spectrum of light emitted from reverse biased junctions in Ge and Si is computed. It is assumed that this emitted light origins from bremsstrahlung of “hot” carriers in the Coulomb field of charged centres. Good conformity of the theoretical curves with the experimental results of CHYNOWETH and GUMMEL, CHYNOWETH and MCKAY, DAVIES and STORM is obtained for both germanium and silicon junctions. This agreement is best for field intensities of the order of 105 to 106 V/cm (calculated for an assumed mean free path of 200 Å) and lattice temperatures giving microplasmas in the region 350 to 700 °K.

Junctions in vapor-grown germanium

The closed-tube process for the epitaxial vapor growth of Ge by the Ge-I 2 disproportionation reaction has the advantage of ease of tending and high purity. The open-tube process offers more flexibility in doping and better thickness control. In either case, the Ge produced is comparable in most respects to melt-grown Ge and is suitable for device applications. Alloyed and diffused junctions in vapor-grown Ge are equivalent to similar junctions in melt-grown Ge. Junctions grown from the vapor are not always similar to junctions grown from the melt; for example, impurity concentration gradients may be varied more widely. Because of the relatively low temperatures involved and the growth mechanism itself, various kinds of masking and matrix formation are feasible.

Preparation of P-N Junctions in Ge and Si by the Evaporation Method and their Photoelectric Characteristics

Preparation of P-N Junctions in Ge and Si by the Evaporation Method and their Photoelectric Characteristics