JOURNAL OF ELECTRONIC MATERIALS
	ABSTRACTS Volume 25, Number 11, November 1996 This Month Featuring: Proceedings from the symposium on Materials Science of Contacts, Metallization and Interconnects from the 1996 TMS Annual Meeting, Anaheim, CA, February 4-8. View November 1996 Table of Contents.

SPECIAL ISSUE PAPERS

• Extremely low contact and film resistance;
• Excellent thermal stability during processing, packaging, and device operation;
• Low interdiffusion;
• Smooth surface and sharp interface; and
• Compatibility with interconnect materials.

J. NARAYAN
M.O. ABOELFOTOH
J. SANCHEZ

Guest Editors

Cu₃Ge Ohmic Contacts to n-Type GaAs
S. OKTYABRSKY,¹ M.O. ABOELFOTOH,¹ J. NARAYAN,¹ and J.M. WOODALL²
1--Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC 27695-7916. 2--School of Electrical Engineering, Purdue University, West Lafayete, IN 47907-1285.

KEY WORDS Contact resistivity, Cu-Ge alloys, GaAs, high resolution transmission electron microscopy (HRTEM), ohmic contact

We show that Cu-Ge alloys prepared by depositing sequentially Cu and Ge layers onto GaAs substrates at room temperature followed by annealing at 400°C form a low-resistance ohmic contact to n-type GaAs over a wide range of Ge concentration that extends from 20 to 40 at.%. A contact resistivity of (4-6) x 10^-7 cm² is obtained on n-type GaAs with doping concentrations of ~1 x 10¹⁷ cm^-3. The contact resistivity is affected only slightly by varying the Ge concentration in the range studied and is not influenced by the deposition sequence of the Cu and Ge layers. In addition, the contacts are electrically stable during annealing at 450°C after contact formation. Structure and properties of Cu-Ge contact layers having lower and higher Ge concentrations from the stoichiometric Cu₃Ge composition are compared. High-resolution transmission electron microscopy and x-ray diffractometry have been used to study the ordering in the ₁-Cu₃Ge (average lattice parameters: a_o = 5.30Å, b_o = 4.20Å, c_o = 4.56Å) which is responsible for orthorhombic distortion of the parent hexagonal -phase. The results suggest that the formation of the and ₁-Cu₃Ge phases creates a highly doped n⁺-GaAs surface layer which leads to the low contact resistivity.

Microstructure and Chemistry of Cu-Ge Ohmic Contact Layers to GaAs
S. OKTYABRSKY, M.O. ABOELFOTOH, and J. NARAYAN
Department of Materials Science and Engineering, North Carolina StateUniversity, Raleigh, NC 27695-7916.

KEY WORDS GaAs, microstructure, ohmic contacts, transmission electron microscopy (TEM)

We report systematic studies of microstructure and chemistry of Cu-Ge alloyed ohmic contacts to n-GaAs with very low specific contact resistivity ((4-6) x 10^{-7 .}cm² for n~1 x 10¹⁷ cm^-3). Using transmission electron microscopy, x-ray microanalysis, and secondary ions mass spectroscopy, we investigated chemistry of phase formation, crystal structure, and mechanism of ohmic contact formation in Cu-Ge alloyed layers with Ge concentration in the range of 0-40 at.%. Layers with Ge deficiency to form -phase (average composition Cu₅Ge) reveal the formation of a nonuniform intermediate layer of hexagonal -Cu₃As phase which grows epitaxially on Ga{111} planes of GaAs. In this case, released Ga diffuses out and dissolves in the alloyed layer stabilizing -phase, which is formed in the structures with average Ge concentration as low as 5 at.%. Unique properties of the contact layers, namely low specific contact resistivity, high thermal stability, interface sharpness, and high contact layer uniformity are related to the formation of an ordered orthorhombic ₁^-Cu₃Ge phase. In the alloyed layer with Ge concentration >25 at.%, no phases due to the chemical reactions with GaAs in the interface region were found demonstrating the chemical inertness of the ₁^-Cu₃Ge ordered phase with respect to GaAs. This results in sharp interfaces and uniform chemical composition, the characteristics needed for superior contacts.

NiGe-Based Ohmic Contacts to n-Type GaAs
MASAKI FURUMAI,¹ TAKEO OKU,¹ HIDENORI ISHIKAWA,¹ AKIRA OTSUKI,² YASUO KOIDE,¹ TETSUO OIKAWA,³ and MASANORI MURAKAMI⁴
1--Department of Materials Science and Engineering, Kyoto University, Sakyo-ku, Kyoto 606-01, Japan. 2--Department of Materials Science and Engineering, Kyoto University, Sakyo-ku, Kyoto 606-01, Japan. Present address: Sumitomo Electric Industries, Osaka, Japan. 3--JEOL Ltd, 1-2 Musashino 3-chome, Akishima, Tokyo 196, Japan. 4--Department of Materials Science and Engineering, Kyoto University, Sakyo-ku, Kyoto 606-01, Japan.

KEY WORDS NiGe, n-type GaAs, ohmic contact

Refractory NiGe ohmic contacts which have excellent thermal stability and smooth surface have been developed. To apply these contacts to the future very large scale integration GaAs devices, reduction of the contact resistance (R_c) of the NiGe contacts is mandatory. In the present paper, in order to obtain a guideline for the R_c reduction, the formation mechanism of the NiGe contacts was investigated. The NiGe contacts were found to have two different ohmic contact formation mechanisms. These mechanisms suggested that facilitation of heavy doping at the GaAs surface and/or in the Ge layer was very effective to reduce the R_c values of the NiGe contacts. Experimentally, the R_c reduction was demonstrated by adding a small amount of third elements. Direct doping elements (Sn, Sb, and Te) and indirect doping elements (Pd, Pt, Au, Ag, and Cu) were chosen as the third elements. In additon, the effect of addition of In, which forms a low barrier layer between metal and GaAs, was investigated. The contact resistance of these NiGe-based contacts were close to 0.3 mm, and they provided smooth surface and shallow reaction depth. Finally, the NiGe-based contacts were compared with the conventional AuGeNi contact.

Metal Contacts to Gallium Arsenide
W.O. BARNARD, G. MYBURG, F.D. AURET, S.A. GOODMAN, and W.E. MEYER
Physics Department, University of Pretoria, Pretoria, South Africa.

KEY WORDS GaAs, ohmic contacts, Ru Schottky contacts

In this paper, some aspects that determine the properties of Schottky and ohmic contacts to GaAs are discussed. For Schottky barrier diodes (SBD), we present results of a comprehensive study involving 41 different metals. We pay special attention to Ru and show that its thermal and chemical stability makes it ideal for use in devices operating above room temperature and for experiments involving annealing. Further, we discuss the effect of different metallization methods on SBD properties and show that methods which use energetic particles, such as electron beam deposition and sputter deposition, often result in inferior SBD properties--the consequence of electrically active defects introduced by the energetic particles at and close to the semiconductor surface. The advantages of using Ru as contact material to GaAs are that it forms high quality, thermally stable Schottky contacts to n-GaAs and thermally stable ohmic contacts with low specific contact resistance to p-GaAs. The versatile applicability of Ru contacts makes them extremely important for future use in devices such as heterojunction bipolar transistors and solid state lasers.

Ohmic Contacts and Schottky Barriers to n-GaN
Z. FAN,¹ S.N. MOHAMMAD,¹ W. KIM,¹ Ö. AKTAS,¹ A.E. BOTCHKAREV,¹ K. SUZUE,¹ and H. MORKODz K. DUXSTAD,³ and E.E. HALLER³
1--University of Illinois at Urbana-Champaign, Materials Research Laboratory & Coordinated Science Laboratory, 104 S. Goodwin Ave., Urbana, IL 61801. 2--University of Illinois at Urbana-Champaign, Materials Research Laboratory & Coordinated Science Laboratory, 104 S. Goodwin Ave., Urbana, IL 61801. On sabbatical at Wright Laboratory under URRP program funded by AFOSR. 3--University of California, Department of Materials Science and Lawrence Berkeley National Laboratory, Berkeley, CA 94720.

KEY WORDS GaN, ohmic contacts, Schottky barriers, Ti/Al/Ni/Au

Gallium nitride is a highly promising wide bandgap semiconductor with applications in high power electronic devices and optoelectronic devices. For these devices to be realized, metallization, both ohmic and rectifying must be available. In this manuscript, we discuss the properties of ohmic contacts and Schottky barriers on n-type GaN. The most recent ohmic metallization scheme involves Ti/Al based composites, namely Ti/Al/Ni/Au (150Å/2200Å/400Å/500Å) preceded by a reactive ion etching (RIE) process which most likely renders the surface highly n-type. With annealing at 900°C for 30 s, contacts with specific resistivity values less than _s = 1 x 10^-7 cm² for a doping level of 4 x 10¹⁷ cm^-3 were obtained. Schottky barriers with Ti, Cr, Pd, Au, Ni, and Pt have been reported; however, we will concentrate here on Pt based structures as they yield a large barrier height of 1.1 eV. Both capacitance-voltage and current-voltage analyses have been carried out as a function of temperature to gain insight into the current conduction processes involved. Attention must now be turned to the modifications needed to render these contacts reliable.

Ohmic Contacts to n-Type GaN
STEPHEN MILLER and PAUL H. HOLLOWAY
Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611-6400.

KEY WORDS GaN, ohmic contacts, titanium

Ohmic contacts to n-GaN using Ag, Au, TiN, Au/Ti, Au/Mo/Ti, and Au/Si/Ti have been studied. The Fermi level of GaN appears to be unpinned, and metals and compounds with work functions less than the electron affinity resulted in ohmic contacts. Reactively sputter deposited TiN was ohmic as deposited. However, Au/Ti, Au/Mo/Ti, and Au/Si/Ti required heat treatments to form ohmic contacts, with the best being an RTA at 900°C. Ag and Au were shown to diffuse across the GaN surface at T>500°C; therefore, they are unstable, poor ohmic contact metallizations as single metals. The other contact schemes were thermally stable up to 500°C for times of 30 min.

Investigation of SnPbAg Solder for Die Attach of GaAs Devices
J.M. PARSEY, JR.,¹ K. KYLER,¹ W. CRONIN,¹ P. MOBLEY,¹ B.L. SCRIVNER,¹ S. VALOCCHI,¹ L. SUE,¹ J. MOHR,² K. MONARCH,² B. KELLER,² and L.-P. LAI³
1--Motorola Communications Products Laboratory, Tempe, AZ 85284. 2--Motorola MCL, Mesa, AZ 85202. 3--Motorola, Austin, TX 75091.

KEY WORDS Die-attach, GaAs devices, metallization, SnPbAg solder

Solder-based die-attach processes used to affix GaAs devices to heat spreading carriers were investigated. The microstructures of the solder materials were assessed, with a focus on void formation, and the response of the solder and backsurface metallization and carrier plating to the temperature cycles of die attachment and reflow processes. Voided regions were found in all solder joints, with a dramatic sensitivity to temperature cycles. Gold-tin alloy phases were found to dominate the microstructure of the solder for all configurations evaluated. The total thermal budget was a critical issue in the formation and transformation of various phases, as expected for low melting point alloys. NiVAu and TiPtAu backmetal systems were investigated to determine their suitability for die attachment.

Metal Silicides: Active Elements of ULSI Contacts
C.M. OSBURN,¹ J.Y. TSAI,² and J. SUN¹
1--Department of Electrical and Computer Engineering, North Carolina State University, Raleigh, NC 27695-7911. 2--Department of Electrical and Computer Engineering, North Carolina State University, Raleigh, NC 27695-7911. Current address: Research and Development, LSI Logic, Inc., Santa Clara, CA 95054.

KEY WORDS Contact resistance, metal oxide semiconductor field-effect transistor (MOSFET) device, metal silicides

As device dimensions scale to the 0.1 µm regime, the self-aligned silicide (SALICIDE) contact technology increasingly becomes an integral part of both the ultra-shallow junction and the metal oxide semiconductor field-effect transistor device itself. This paper will discuss the effect of silicide materials and formation processes on silicide stability, junction consumption, the ability to accurately profile shallow junctions, and contact resistance in series with the channel. The use of silicides as diffusion sources (SADS) provides an important pathway toward optimization of silicide technology. Diffusion of boron and arsenic from nearly epitaxial layers of CoSi₂, formed from bilayers of Ti and Co, offer good silicide stability, ultra-shallow, low-leakage junctions, and low contact resistance.

Epitaxial TiN Based Contacts for Silicon Devices
R.D. VISPUTE and J. NARAYAN
Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC 27695-7916.

KEY WORDS Cu/TiN/Si(100), domain matching epitaxy, Schottky barrier, TiN/Si(100)

We have grown high quality epitaxial TiN/Si(100) and Cu/TiN/Si(100) heterostructures by pulsed laser deposition. The epitaxial TiN films have the same low (15 µ-cm) resistivity as TiSi₂ (C-54) phase with excellent diffusion barrier properties. In addition, Schottky barrier height of TiN was close to that of TiSi₂ (0.6-0.7 eV). Auger and Raman spectroscopy revealed that the films were stoichiometric TiN and free from oxygen impurities. The x-ray diffraction and transmission electron microscope (TEM) results showed that the TiN films deposited at 600°C were single crystal in nature with epitaxial relationship <100>TiN|| <100>Si. The Rutherford backscattering channeling yield for TiN films was found to be in the range of 10-13%. The epitaxy of Cu on TiN was found to be cube-on-cube, i.e., <100>Cu||<100>TiN||<100>Si. The Cu/TiN and TiN/Si interfaces were found to be quite sharp without any indication of interfacial reaction. The growth mechanism of copper on TiN was found to be three-dimensional. We discuss domain matching epitaxy as a mechanism of growth in these large lattice mismatch systems, where three lattice constants of Si(5.43Å) match with four of TiN(4.24Å) and seven units of Cu(3.62Å) match with six of the TiN. Thus, for next generation of device complementary metal oxide semiconductor structures, Cu/TiN/Si(100) contacts hold considerable promise, particularly since Cu is a low resistivity metal (1.6 µ-cm) and is considerably more resistant to electromigration than Al. The implications of these results in the fabrication of advanced microelectronic devices are discussed.

Schottky Barrier Heights on IV-IV Compound Semiconductors
F. MEYER,¹ M. MAMOR,¹ V. AUBRY-FORTUNA,¹ P. WARREN,² S. BODNAR,² D. DUTARTRE,² and J.L. REGOLINI²
1--IEF, CNRS URA 22, Bât. 220, Université Paris Sud, 91405 Orsay Cedex, France. 2--France Telecom CNET BP 98, 38243 Meylan Cedex, France.

KEY WORDS Si1-x-yGexCy, Schottky barrier height, strain, tungsten

The variations of Schottky barrier heights on Si_1-x-yGe_xC_y films with composition and strain have been investigated and compared to those expected for the band gap energy. The barrier on n-type does not depend on composition and strain. This independence suggests that the Fermi level at the interface between tungsten and Si_1-x-yGe_xC_y alloys (x 0) is pinned relative to the conduction-band. For Si_1-xGe_x, the barrier on p-type follows the same trends as the band gap. For the ternary alloys, the variations of the barrier on p-type seem to be too large to be only due to a variation of the band-gap. In addition, we have investigated the influence of the deposition conditions of the sputtered-W-gate on the barrier to silicon and Si_1-xGe_x. Our results show that the barrier on n-type-Si and p-type-Si_1-xGe_x-films increases when the stress retained in the W-films changes from compressive to tensile as the deposition pressure increases. The absence of change in the barrier height of W to p-type-silicon and n-type-Si_1-xGe_x-films suggests that the Fermi level at the interface with Si is pinned relative to the valence-band while it is pinned relative to the conduction when Ge is added.

Microstructures and Electrical Properties of SrRuO₃ Thin Films on LaAlO₃ Substrates
F. CHU,¹ Q.X. JIA,¹ G. LANDRUM,² X.D. WU,¹ M. HAWLEY,¹ and T.E. MITCHELL¹
1--Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, NM 87545. 2--Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, NM 87545. Present address: Department of Chemistry and Materials Science Center, Cornell University, Ithaca, NY 14853-1301.

KEY WORDS Conductive SrRuO3, LaAlO3 substrates, microstructures, thin films

Conductive SrRuO₃ thin films have been deposited using pulsed laser deposition on LaAlO₃ substrates at different substrate temperatures. Structural and microstructural properties of the SrRuO₃/LaAlO₃ system have been studied using x-ray diffraction, scanning electron microscopy, and scanning tunneling microscopy. Electrical properties of SrRuO₃ thin films have been measured. It was found that the film deposited at 250°C is amorphous, showing semiconductor-like temperature dependence of electrical conductivity. The film deposited at 425°C is crystalline with very fine grain size (100~200Å), showing both metallic and semiconductor-like temperature dependence of electrical conductivity in different temperature regions. The film deposited at 775°C shows a resistivity of 280 µ.cm at room temperature and a residual resistivity ratio of 8.4. Optimized deposition conditions to grow SrRuO₃ thin films on LaAlO₃ substrates have been found. Possible engineering applications of SrRuO₃ thin films deposited at different temperatures are discussed. Bulk and surface electronic structures of SrRuO₃ are calculated using a semi-empirical valence electron linear combination of atomic orbitals approach. The theoretical calculation results are employed to understand the electrical properties of SrRuO₃ thin films.

LaNiO₃ and Cu₃Ge Contacts to YBa₂Cu₃O_7-x Films
D. KUMAR,¹ R.D. VISPUTE,¹ O. ABOELFOTOH,¹ S. OKTYABRSKY,¹ K. JAGANNADHAM,¹ J. NARAYAN,¹ P.R. APTE,² and R. PINTO²
1--Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC 27695-7916. 2--Tata Institute of Fundamental Research, Bombay, India.

KEY WORDS Contact resistance, Cu3Ge, high temperature superconductors, LaNiO3, YBa2Cu3O7-x

Metallization of high-T_c superconductors using low resistivity metal oxides and Cu-Ge alloys has been investigated on high quality pulsed laser deposited epitaxial YBa₂Cu₃O_7-x (YBCO) films. Epitaxial LaNiO₃ (LNO) thin films have been grown on YBCO films at 700°C using pulsed laser deposition. The specific resistivity of LNO was measured to be 50 µ-cm at 300K which decreases to 19 µ-cm at 100K indicating good metallicity of the LNO films. The contact resistance of LNO-YBCO thin film interface was found to be reasonably low (of the order of 10^-4 -cm² at 77K) which suggests that the interface formed between the two films is quite clean and LNO can emerge as a promising metal electrode-material to YBCO films. A preliminary investigation related to the compatibility of Cu₃Ge alloy as a contact metallization material to YBCO films is discussed. The usage of other oxide based low resistivity materials such as SrRuO₃ (SRO) and SrVO₃ (SVO) for metallization of high-T_c YBCO superconductor films is also discussed.

Microstructure Mapping of Interconnects by Orientation Imaging Microscopy
DAVID P. FIELD¹ and DAVID J. DINGLEY²
1--TexSEM Laboratories, Inc., 226W 2230N #120, Provo, UT 84604. 2--TexSEM Laboratories, Inc., 226W 2230N #120, Provo, UT 84604. On leave from H.H. Wills Physics Laboratory, University of Bristol, UK.

KEY WORDS Interconnect lines, metallization, orientation imaging microsocpy (OIM), scanning electron microscopy (SEM)

Orientation imaging microscopy (OIM) provides a detailed description of the variation in crystallographic structure over the surface of single crystal or polycrystalline bulk materials. This technique has been used in the investigation of interconnect lines and contact pad material in a number of aluminum metallizations on silicon oxide/silicon substrates. The specimens were examined in a scanning electron microscope fitted with apparatus for collecting electron back-scatter diffraction patterns (EBSPs). OIM scans were performed over various regions of each specimen. The technique has provided information regarding the local texture variations between different regions of a thin film structure for both patterned and unpatterned metallizations. It was concluded that significant modification of the microstructure may occur subsequent to the initial metal deposition and that this modification can differ between the unpatterned regions and interconnect lines of varying widths.

REGULAR ISSUE PAPERS

KEY WORDS Amorphous indium oxide, metal-insulator transition, optical absorption, oxygen diffusion, photoreduction, secondary ion mass spectrometry (SIMS), transparent conductors

A stable increase by as much as 10⁸ in the conductivity of amorphous indium oxide to 10^{3 -1}cm^-1 can be achieved by ultraviolet photoreduction. This treatment also increases the absorption coefficient, (h), by up to a factor of 10³ for h <1.5 eV due to free carrier absorption and causes a 0.1 eV shift of the absorption edge to the blue. These changes are controlled by the Fermi level, E_F, which is presumably determined by doping due to oxygen vacancies. A diffusion constant D >3 x 10^-12 cm²/s for oxygen at 300K is determined from a constant flow experiment. Oxygen diffusion is verified by secondary ion mass spectrometry with ¹⁸O. The functions (h) and (T) are simulated as E_F is varied using a simple density of states model appropriate for amorphous semiconductors. These simulations qualitatively agree with the experimental data if transitions from the conduction band tail to the conduction band are assumed to be forbidden.

Microstructures of Parylene-N Thin Films and the Effect on Copper Diffusion
G.-R. YANG,^1,2 D. MATHUR,^1,3 X.M. WU,^1,2 S. DABRAL,^1,4,5 J.F. MC DONALD,^1,4 T.-M. LU^1,2
and H. BAKHRU⁶
1--Center for Integrated Electronics, Rensselaer Polytechnic Institute, Troy, NY 12180. 2--Physics Department. 3--Chemical Engineering Department. 4--Electrical Engineering Department. 5--Presently at Intel Corporation. 6--Physics Department, State University of New York, Albany, NY 12222.

KEY WORDS Cu diffusion, films, microstructure, parylene-N

Using scanning electron microscopy, the microstructure of annealed N-type parylene films on silicon substrate was observed and compared to the as-deposited film. The diffusion of copper through the parylene-N film was studied and correlated to the microstructure. A web-like microstructure was observed on annealing parylene-N to a temperature of 300°C and higher. This microstructure differed from the as-deposited homogeneous and continuous structure at room temperature. The web-like structure observed is proposed to be a fibrillar crystalline structure embedded in an amorphous matrix. X-ray diffraction studies supported this view and showed that the crystalline structure was the phase. Also, when the film was annealed at 300 and 350°C, a thin continuous layer was formed at the surface of the web-like parylene-N film. In contrast, no such thin layer was observed when the annealing was performed with a copper overlayer. Based on this observation, a two-stage annealing process was carried out to reduce the copper diffusion into parylene-N, pre-annealing, before copper deposition and post-annealing after copper deposition. The results, as judged from Rutherford backscattering spectroscopy indicate that the thermal stability for copper diffusion into parylene-N films can be increased by 50°C (from 300 to 350°C) using pre-annealing. Experimental data shows that a minimum pre-anneal temperature of 250°C for 1 h is required for this purpose.

Optimization of Saturation Current Density of PECVD SiN Coated Phosphorus Diffused Emitters Using Neural Network Modeling
L. CAI, S. HAN, G. MAY, S. KAMRA, T. KRYGOWSKI, and A. ROHATGI
School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, GA 30332.

KEY WORDS Interface state density, plasma enhanced chemical vapor deposition (PECVD), photo-assisted annealing, SiN

Emitter surface passivation by low temperature plasma enhanced chemical vapor deposition (PECVD) silicon nitride is investigated and optimized in this paper. We have found that the saturation current density of a 90 ± 10 /sq phosphorus diffused emitter with N_s 3 x 10¹⁹ and X_j 0.3 µm can be lowered by a factor of eight by appropriate PECVD silicon nitride deposition and photo-assisted anneal. PECVD silicon nitride deposition alone reduces the emitter saturation density (J_oe) by about a factor of two to three, and a subsequent photo-anneal at temperatures 350°C reduces J_oe by another factor of three. In spite of the larger flat band shift for direct PECVD silicon nitride coating, the silicon nitride induced surface passivation is found to be about a factor of two inferior to the thermal oxide plus PECVD silicon nitride passivation due to higher interface state density at the SiN/SiO₂ interface compared to SiO₂/Si interface. A combination of statistical experimental design and neural network modeling is used to show quantitatively that lower radio frequency power, higher substrate temperature, and higher reactor pressure during the PECVD deposition can reduce the J_oe of the silicon nitride coated emitter.

Deep Levels, Electrical and Optical Characteristics in SnTe-Doped GaSb Schottky Diodes
J.F. CHEN, N.C. CHEN, and H.S. LIU
Department of Electrophysics, National Chiao Tung University, Hsinchu, Taiwan, Republic of China.

KEY WORDS Deep levels, GaSb Schottky diodes, Hall measurement, photoluminescence (PL), SnTe dopants

The GaSb layers investigated were grown directly on GaAs substrates by molecular beam epitaxy (MBE) using SnTe source as the n-type dopant. By using admittance spectroscopy, a dominant deep level with the activation energy of 0.23-0.26 eV was observed and its concentration was affected by the Sb₄/Ga flux ratio in the MBE growth. A lowest deep-level concentration together with a highest mobility was obtained for GaSb grown at 550°C under a Sb₄/Ga beam equivalent pressure (BEP) ratio around 7, which should correspond to the lowest ratio to maintain a Sb-stabilized surface reconstruction. In the Hall measurement, an analysis of the temperature-dependent mobility shows that the ionized impurity concentration increases proportionally with the sample's donor concentration, suggesting that the ionized impurity was introduced by an SnTe source. In addition, optical properties of an undoped p-, a lightly and heavily SnTe-doped GaSb layers were studied by comparing their photoluminescence spectra at 4.5K.

LETTER SECTION

KEY WORDS Invar material, laser welding, semiconductor laser packaging, thin film coating

The effect of a thin film coating of Au on the joint strength, weld width, and penetration depth in laser welding techniques for Invar-Invar packages is investigated experimentally. It is found that the joint strength, weld width, and penetration depth are strongly dependent on the coating of Au thickness on the Invar material. The welded joints with thick Au coating show narrower weld width, shallower penetration, and hence less joint strength than those of the packages with thin Au coating. The increase in both the thermal conductivity and the vapor volume in the welded joints as the coating of Au thickness increases are the possible mechanisms for the reduction. Detailed knowledge of the effect of thin film coatings of Au on the welded materials, which gives both the highest joint strength and good adhesion, is essential for practical design and fabrication of reliable optoelectronic packaging.