2.3 Investigations of metal contacts to amorphous evaporated Ge films

Amorphous Ge films are used as non-injecting ohmic contacts to high-resistivity n-silicon radiation detectors, but the function of this contact is not yet fully explored. One part problem is the role of the metal films used as external contacts to the amorphous film. In this paper we investigate the function of different contacting metals, such as Au, Al, Cr by measuring the I-V-characteristics of sandwich structures with two metals on both sides of the amorphous evaporated Ge film (of typical thickness 1 μm). It was found that while the symmetric CrGeCr structure (also AuGeAu) had low resistance (leading to resistivity values of ?4 × 10⁴ωcm for the Ge film), the unsymmetric structures e.g. AlGeCr and AlGeAu, showed higher resistance. This was again found for the AlGeAl structure, which also showed some polarity dependence. Rutherford backscattering was used to investigate diffusion effects between the layers; also the microstructure of the Ge films was investigated by electron microscopy.     

The effect of growth conditions on the structural and electrical properties of the SiGe heteroepitaxial system

The present study is a logical continuation of previous investigations of the SiGe heteroepitaxial system (HES). The primary object of this work was to define the conditions for the formation of fragmentary structure (FS) in the SiSi_eGe system, where Si_e designates a homoepitaxial silicon film.It is shown that, after the Ge film deposition, decrease of the HES cooling rate to 2 °C min^?1 makes it possible to obtain the HES without FS for a Ge film thickness d of approximately 3 μm and to improve the heterodiode I–V characteristics. The distribution of defects through the film thickness has also been studied.     

Investigation of germanium films and GeSi interface structure by transmission electron microscopy

The structure of germanium films (d≌0.3 μm) evaporated onto a silicon substrate and the GeSi interface have been investigated by transmission electron microscopy (TEM). Ge was evaporated in a vacuum of approximately 10^?6 Torr onto (111) Si. Epitaxial growth was observed at substrate temperatures T_s>500°C. The films grown at T_s = 520°–600°C (low temperature epitaxy) were characterized by a high density of stacking faults (SF), microtwin lamellae and dislocations which lay normal to the film surface. In this case the interface dislocations were regular and the dislocation density was equal to (3–4)×10¹⁰ cm^?2. At T_s = 700°–850°C (high temperature epitaxy) few stacking faults were discovered. Dislocations in the interior of the film formed three-dimensional networks as a result of interactions of different slip systems and dislocation climb. The formation of misfit dislocations was apparent at the interface. The region T_s = 630°–700°C is an intermediate region.     

The growth and transformation of Pd2Si on (111), (110) and (100) Si

Thin Pd films on (111), (110), (100) and amorphous Si substrates form [001] fiber textured Pd₂Si in the temperature range 100°–700°C. The degree of texture is a function of substrate orientation, increasing in the order amorphous Si, (100) Si, (110) Si and (111) Si. Only on the (111) Si substrate is the Pd₂Si film epitaxially oriented. Temperature-dependent growth on this orientation can be characterized by [001] textured growth, epitaxial azimuth orientation at the Si interface and progressive layer by layer formation of the mosaic crystal to the thin film surface.During Pd deposition, rapid non-diffusion-controlled growth of epitaxial Pd₂Si on (111) Si occurs at substrate temperatures of 100° and 200°C. An unidentified palladium silicide of low crystallographic symmetry forms during Pd deposition onto a 50°C substrate. The diffusion-controlled growth of Pd₂Si on (111) Si follows a t^0.5 dependence. The velocity constant is

$\text{k = 7 × 10}{}^{\mathrm{?2}}\text{exp}\text{?}\text{29200±800}\text{RT}\text{cm}{}^2\text{/}\text{sec}$

Palladium deposited on 100°C (111) Ge substrates reacts during deposition to form epitaxially oriented Pd₂Ge. However, growth of this phase at higher temperatures results in a randomly oriented film. The transformation of Pd₂Ge to PdGe is kinetically controlled. After a 15 min anneal at 560°±10°C in N₂ only PdGe is detectable on (111) Ge.The high temperature stability of thin film Pd₂Si is controlled by time- temperature kinetics. For a given annealing cycle, the nucleation and growth rates of the PdSi phase are inversely related to the crystalline perfection of Pd₂Si. Decreasing transformation rates follow the order (100), (110), (111) Si. formation of thin film Pd₂Si occurs by the formation of PdSi and subsequent growth of Si within the PdSi phase. After a 30 min N₂ anneal, initial transformation occurs at 735°C on (100) Si, 760°C on (110) Si and 840°C on (111) Si. Extended high temperature annealing produces a two-phase structure of highly twinned and misoriented Si and small PdSi grains that penetrate as much as 3 μm into the Si.     

Some investigations of the heterojunctions in AIVBIV systems

Heterojunctions in the Sn/Si and Pb/Si systems are observed for larger temperature ranges than have previously been reported. Reflection high energy electron diffraction and depth selective Mössbauer spectroscopy investigations are carried out together with conventional electrical measurements for the Sn/Si system in an attempt to explain the observed inverse layer at the Sn-Si boundary. The contact of tin and lead with silicon of different resistivities is discussed. A new crystalline state of tin is observed when tin is evaporated onto (111)Si. It is diamond cubic with $\text{a = 5.60±0.06}\text{A}\text{?}$ and the isometric shift δ has a very high positive value (+4.42±0.34 mms^?1>) relative to SnO₂. For longer heating times there is a decrease of δ at 400°C to +2.75±0.34 mm s^-1 relative to SnO₂. The reasons for the difference in the electrical behaviour below and above 400 °C of the Sn/Si heterojunctions are discussed.     

LuMnGe2, un nouveau type structural apparente a ceux de TiMn(Fe)Si2, ZrMnSi2 et ZrFeSi2; ScMnGe2, un nouvel isotype de TiMn(Fe)Si2

LuMnGe₂ has been studied by single - crystal X-ray diffraction analysis. The structure is of a new type with space group Cmmm and Z = 12: a = 5.466(2), b = 18.519(7), $\text{c}\text{= 8.173(3)}\text{A}\text{?}$, D_x = 9.03 Mg m^?3, μ(AgKα) = 31 mm^?1, F(000) = 1919, R = 0.030 for 593 independant reflexions (R_w = 0.034). The LuMnGe₂ structure type had been predicted and completes a large structural family TT′Si(Ge)₂ where T = Nb, Ti, Zr, Hf, Sc, Lu and T′ = Cr, Mn, Re, Fe, Co, where ScMnGe₂ is another new member, isostructural with TiMn(Fe)Si₂     

Ionic conductivity of amorphous mNaFnMF3 thin films (M=Al,Cr)

Evaporated 0.5 to 0.7 μm thick thin films of mNaFnAlF₃ or mNafnCrF₃ were prepared in a 1 mPa vacuum on glass substrates kept at room temperature. Chemical analysis and AC conductivity measurement revealed that in each case studied, a high ionic conductivity plateau existed for the composition range $\text{0.1 <}\text{n}\text{(m+n)}\text{< 0.4}$. The maximum conductivities at room temperature were σ = 1 × 10^?6 S/mm for NaFAlF₃ and σ = 2 × 10^?5 S/m for NaFCrF₃. Those high conductivities were discussed in terms of arrangement of MF₆ octahedra.     

Growth and characterization of group-III impurity-doped semiconducting BaSi2 films grown by molecular beam epitaxy

Ga- or In-doped BaSi₂ films were grown on Si(111) by molecular beam epitaxy (MBE). The Ga-doped BaSi₂ showed n-type conductivity. The electron concentration and resistivity of the Ga-doped BaSi₂ depended on the Ga temperature; however, the electron concentration and resistivity could not be controlled properly. In contrast, the In-doped BaSi₂ showed p-type conductivity and its hole concentration was controlled in the range between 10¹⁶ and 10¹⁷ cm^− 3 at RT.     

Forming n-p junctions based on p-CdHgTe with low charge carrier density

The first results on the formation of n ⁺-p photodiodes with a long-wavelength boundary at λ_0.5 = 10.6 μm based on p-CdHgTe epilayers with a hole density of 3.3 × 10¹⁴ cm^?3 are presented. The CdHgTe layers were obtained by molecular beam epitaxy and exhibited p-type conduction in the as-grown state. The hole density and mobility were determined by means of the mobility spectrum technique. The photodiode structures were prepared by B⁺ ion implantation into as-grown p-CdHgTe epilayers without additional annealing. The R ₀ A product for the n ⁺-p photodiodes obtained is about 20 Θ cm² at 77 K.     

Effect of silicidation on silicon-based thin film resistors in SiGe integrated circuits

The resistance variation of the silicon–germanium (SiGe) thin film resistor caused by the fabrication process of SiGe integrated circuits (ICs) was investigated. The SiGe resistor and the Si resistor were made of the thin films identical with the p-type SiGe base layer and the n-type Si emitter layer of the SiGe hetero-junction bipolar transistor, respectively. The range of the resistance value of the SiGe resistor was much larger than that of the Si resistor, and an abnormally high resistance of the SiGe resistor was often observed. Ti–B precipitates and Ti(Si_1−x Ge _x )₂ protrusions were created as the result of the Ti silicidation of the p-type SiGe layer, whereas no precipitates and protrusions were generated in the case of the n-type Si layer. It was confirmed by scanning electron microscopy that the nonuniform resistance of the SiGe resistor was induced by the removal of the protrusions and underlying field oxides in the contact window. Resistance uniformity of the SiGe resistor was much improved by increasing the contact size. The simulation result of the detrimental influence of the resistance change on ICs indicated that the fabrication process and the structure of the thin film resistor should be optimized for enhancing IC reliability.     

Crystal growth and structure determination of silicon telluride Si2Te3

Large single crystals of Si₂Te₃, which is the only stable phase in the SiTe system, were successfully grown by the Bridgman technique and by sublimation. Si₂Te₃ crystallizes in the trigonal space group P3?1c with a = 7.430 $\text{A}\text{?}\text{, C}\text{= 13.482}\text{A}\text{?}$, and z = 4. Using 856 independent intensities the structure was refined to R = 0.070. As compared to the GaS layer structure, Si₃Te₃ contains Si₂ units with SiSi bond distances of about 2.3 Å. In Si₂Te₃, however, only $\text{1}\text{4}$ of the Si₂ units run parallel to the c-axis direction, the others exhibit more complex orientations. Each Si is tetrahedrally coordinated by 3 Te and 1 Si, having SiTe bond lengths of about 2.55 Å. Each Te is bonded to only 2 Si atoms with SiTeSi bond angles around 93°.     

Structural perfection of the GeSi and SiGe heteroepitaxial systems

The structural perfection of heteroepitaxial systems (HESs) of the SiGe type (germanium films deposited onto silicon substrates) and of the “reverse” type (Si films on Ge substrates) has been investigated by X-ray diffraction and metallographical methods. It has been found that the real structures of these HESs are very different. The fragmentation previously observed in the SiGe HES is not found in the reverse system, but thick Ge substrates (d_Ge ～ 1 mm) appear to be plastically deformed throughout their thickness. A mutual correspondence of the damaged areas, inherent in the fragmentary structure (FS) in the SiGe system, is absent in the reverse system. Annealing does not influence the SiGe HES. The nature of the FS and the peculiarities of the deformation of both systems are discussed.     

De nouveaux stannures ternaires de rhodium et d'elements des terres rares : TR2Rh3Sn5 OU TR = Y,Gd, Tb,Dy, Ho. structure cristalline et proprietes electriques et magnetiques de Y2Rh3Sn5

New ternary stannides are reported : RE₂Rh₃Sn₅ (RE = Y, Gd, Tb, Dy, Ho). Y₂Rh₃Sn₅ has been studied by single - crystal X-ray diffraction analysis. Its structure is of a new type with space group Cmc2₁ and Z = 4 : a = 4,387(2), b = 26,212(4), $\text{c}\text{= 7,1550(8)}\text{A}\text{?}$, D_x = 8,71 Mgm^?3, μ(AgK_α) = 17 mm^?1, F(000) = 1851, R = 0,045 for 478 independant reflexions (R_w = 0,046). This structure is characterized by a tridimensionnal lattice of RhSn covalent bonds with two very different sites for yttrium. Y₂Rh₃Sn₅ is diamagnetic and semimetallic according to its electrical properties.     

MnGeP<Subscript>2</Subscript> Thin Films Grown by Molecular Beam Epitaxy

Growth conditions for MnGeP₂ thin films have been investigated by using molecular beam epitaxy (MBE) method. Mn and Ge were evaporated by K-cells, and P₂ was supplied by decomposing tertialybutylphosphine (TBP). GaAs (001) and InP (001) single crystals were used as substrates. An X-ray diffraction peak, which can be assigned to (008) peak of MnGeP₂, was observed at nearly the same position as the (004) peak of GaAs. The lattice constant of the MnGeP₂ thin film was determined to be 1.13 nm assuming its crystal structure is a c-axis oriented chalcopyrite type structure. Secondary phases such as GeP, MnGe _x and MnP were observed for beam fluxes of Mn and Ge as high as 1×10^?8 Torr.     

p-type doping of ZnO by means of high-density inductively coupled plasmas

A custom-designed inductively coupled plasma assisted radio-frequency magnetron sputtering deposition system has been used to fabricate N-doped p-type ZnO (ZnO:N) thin films on glass substrates from a sintered ZnO target in a reactive Ar + N₂ gas mixture. X-ray diffraction and scanning electron microscopy analyses show that the ZnO:N films feature a hexagonal crystal structure with a preferential (002) crystallographic orientation and grow as vertical columnar structures. Hall effect and X-ray photoelectron spectroscopy analyses show that N-doped ZnO thin films are p-type with a hole concentration of 3.32 × 10¹⁸ cm^− 3 and mobility of 1.31 cm² V^− 1 s^− 1. The current-voltage measurement of the two-layer structured ZnO p-n homojunction clearly reveals the rectifying ability of the p-n junction. The achievement of p-type ZnO:N thin films is attributed to the high dissociation ability of the high-density inductively coupled plasma source and effective plasma-surface interactions during the growth process.     

Realization of high mobility p-type co-doped ZnO: AlN film with a high density of nitrogen-radicals

High mobility p-type ZnO:AlN thin films have been efficiently realized by utilizing pre-activated nitrogen (N) plasma sources with an inductively coupled dual target co-sputtering system. High density of N-plasma-radicals was generated with an additional RF power applied through a ring-shaped quartz-tube located inside the chamber during co-sputtering process. The AlN codoped ZnO film shows excellent p-type behavior with a high mobility and a hole concentration of 154 cm^2°V^− 1s^− 1 and about 3 × 10^18°cm^− 3 at 600 °C, respectively. Electrical properties of p-n homo-junction devices based on p-type ZnO film are also discussed.     

Hot Carrier Velocities in Doped and in Ultra-pure Germanium Crystals at Millikelvin Temperatures

The velocity laws of electrons and holes in germanium single crystals at millikelvin temperatures are determined as a function of the electric field in the 〈001〉 orientation, based on time-of-flight measurements in cryogenic coplanar grid Ge detectors. Results obtained in two n-type crystals (|N _a ?N _d |<10¹⁰ cm^?3 and doped to 10¹¹ cm^?3) are compared with the experimental data from previous investigations, and shown to be consistent with Monte-Carlo simulations of carrier transport.     

Nitrogen doping of ZnTe for the preparation of ZnTe/ZnO light-emitting diode

Nitrogen-doped p-type zinc telluride (p-ZnTe) films are prepared by sputtering in a mixture of nitrogen/argon plasma. The effect of doping level N (ratio of N₂ flow rate to that for the mixed gas) in the range 0–10 % on the films properties is investigated. Heterojunction diodes are prepared on stainless steel flexible substrate from the p-ZnTe (doping level, N = 5 %) and solution-grown n-ZnO films. The junction parameters and light-emission properties of diodes are investigated. Doping level beyond N = 1 %, changes the cubic crystal structure of ZnTe to hexagonal and reduces the size of crystallites. At the doping level N = 2–4 %, films with the highest hole density of 2.5 × 10¹⁸ cm^?3 and lowest band gap energy of 1.4 eV are obtained. The diodes junction built-in potential and the donor density in n-ZnO films are found to be in the range 0.4–0.7 V and 1.5 × 10¹⁷–1.4 × 10¹⁸ cm^?3, respectively. Diodes exhibit electroluminescence in the UV and visible regions due to the band edge and defect emissions in ZnO.     

Progress in a-Si:H/c-Si heterojunction emitters obtained by Hot-Wire CVD at 200 °C

In this work, we investigate heterojunction emitters deposited by Hot-Wire CVD on p-type crystalline silicon. The emitter structure consists of an n-doped film (20 nm) combined with a thin intrinsic hydrogenated amorphous silicon buffer layer (5 nm). The microstructure of these films has been studied by spectroscopic ellipsometry in the UV-visible range. These measurements reveal that the microstructure of the n-doped film is strongly influenced by the amorphous silicon buffer. The Quasy-Steady-State Photoconductance (QSS-PC) technique allows us to estimate implicit open-circuit voltages near 700 mV for heterojunction emitters on p-type (0.8 Ω·cm) FZ silicon wafers. Finally, 1 cm² heterojunction solar cells with 15.4% conversion efficiencies (total area) have been fabricated on flat p-type (14 Ω·cm) CZ silicon wafers with aluminum back-surface-field contact.     

AlN codoping and fabrication of ZnO homojunction by RF sputtering

The ZnO homojunction fabricated from undoped and 1 mol% AlN doped (codoped) ZnO targets by RF magnetron sputtering has been reported. The grown films on Si (100) substrate have been characterized by X-ray diffraction (XRD), Energy dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), Photoluminescence (PL) and Hall measurements. The increase of d-space value (compared with unstressed bulk) found from XRD for AlN codoped ZnO film supports the formation of p-ZnO due to the N incorporation. The presence of N in the film has been confirmed by EDS and XPS analysis. Further, the p-conductivity in AlN codoped ZnO has been evidenced by low temperature PL (donor-acceptor-pair emission) and room temperature PL (red shift in near-band-edge emission). Hall measurement shows that 1 mol% AlN codoped ZnO has the hole concentration of 3.772 × 10¹⁹ cm⁻³. The fabricated homojunction with 1% AlN doped ZnO (p-type) and undoped ZnO (n-type) exhibits a typical rectification behavior with high breakdown voltage, and rectification ratio, 13.4. The junction parameters such as ideality factor, barrier height and series resistance have also been calculated for the fabricated p-n junction. The energy band diagram has been proposed for the fabricated homojunction.