A. Katz

Carbon doping of III-V compounds grown by mombe

Recent advances in heterostructure bipolar transistor (HBT) technology have created a need for p-type doping at levels ≥1020 cm-3. We have achieved p-type doping levels as high as 5×1020 cm-3 using C, which is introduced through the use of trimethylgallium (TMG) during metalorganic molecular beam epitaxy (MOMBE) growth of GaAs. By utilizing the atomic planar doping method, we have also been able to grow C-doped spikes with hole concentrations as high as 7×1019 cm-3, with a full width at half maximum of ∼50 A at 300 K. This level is among the highest reported for planar doping. By switching out the TMG, and switching in the triethylgallium (TEG) to continue to growth of C-free GaAs, we have grown sandwich-type structures with C levels of 1020 cm-3, which fall off within 210 A to C levels of <1017 cm-3. High temperature annealing of such structures reveals a C diffusion coefficient of <10-16 cm2 s-1 at 950°C, in agreement with other reports. The electrical properties of layers annealed at high temperatures appear to be influenced by the presence of strain arising from the high C concentration. X-ray diffraction patterns of 3 μm layers doped in excess of 1020 cm-3 show a lattice constant which corresponds roughly to that calculated by assuming a Vegard law mixture of GaAs and 0.7% GaC. Preliminary results of C-doping of InGaAs will also be discussed. Finally, the usefulness of carbon doping has been demonstrated in ohmic contact formation, Schottky barrier height enhancement in MESFETs and as the base layer in HBTs.

Dry etching of thin-film InN, AlN and GaN

Smooth, anisotropic dry etching of InN, AlN and GaN layers is demonstrated using low-pressure (1-30 mTorr) CH4/H2/Ar or Cl2/H2 ECR discharges with additional DC biasing of the sample. The etch rates are in the range 100-400 AA min-1 at 1 mTorr and -150 V DC for Cl2/H2, while higher biases are needed to initiate etching in CH4/H2/H discharges. The presence of hydrogen in the gas chemistries is necessary to facilitate equi-rate removal of the group III and nitrogen etch products, leading to smooth surface morphologies.

Pt/Ti/p‐In0.53Ga0.47As low‐resistance nonalloyed ohmic contact formed by rapid thermal processing

Very low resistance nonalloyed ohmic contacts of Pt/Ti to 1.5×1019 cm−3 Zn‐doped In0.53Ga0.47As have been formed by rapid thermal processing. These contacts were ohmic as deposited with a specific contact resistance value of 3.0×10−4 Ω cm2. Cross‐sectional transmission electron microscopy showed a very limited interfacial reacted layer (20 nm thick) between the Ti and the InGaAs as a result of heating at 450 °C for 30 s. The interfacial layer contained mostly InAs and a small portion of other five binary phases. Heating at 500 °C or higher temperatures resulted in an extensive interaction and degradation of the contact. The contact formed at 450 °C, 30 s exhibited tensile stress of 5.6×109 dyne cm−2 at the Ti/Pt bilayer, but the metal adhesion remained strong. Rapid thermal processing at 450 °C for 30 s decreased the specific contact resistance to a minimum with an extremely low value of 3.4×10−8 Ω cm2 (0.08 Ω mm), which is very close to the theoretical prediction.

Properties of titanium nitride thin films deposited by rapid‐thermal‐low‐pressure‐metalorganic‐chemical‐vapor‐deposition technique using tetrakis (dimethylamido) titanium precursor

Titanium nitride (TiNx) thin films were deposited onto InP by means of the rapid‐thermal‐low‐pressure‐chemical‐vapor‐deposition (RT‐LPMOCVD) technique, using the tetrakis (dimethylamido) titanium (Ti(NMe2)4 or DMATi) complex as the precursor. Depositions were successfully carried out at temperatures below 550 °C, pressure range of 5–20 Torr and duration of 50 to 90 s, to give layer thicknesses up to 200 nm and growth rates in the range of 0.8 to 4.5 nm/s. These films had a stoichiometric structure and contained nitrogen and titanium in a ratio close to unity, but also contained a significant amount of carbon and oxygen. The elements were spread uniformly through the films, the nitrogen was Ti bounded, and the carbon was partially titanium bonded and organic bonded as well. The film resistivity was in the range of 400–800 μΩ cm−2; the stress was always compressive, in the range of − 0.5 × 109 to − 2 × 1010 dyne cm−2, and the film had a good morphology. These layers performed as an ohmic contact while depos...

Study of Ni as a barrier metal in AuSn soldering application for laser chip/submount assembly

The possibility of replacing Pt in the Ti/Pt/Au base and traditionally used metallurgical structure by Ni, while bonding InP laser chip to a submount with AuSn (80% Au) solder, has been investigated. Various Ni‐based metal alloys have been prepared by evaporation. Reflow experiments were conducted in a chamber under forming gas‐controlled ambient. The Ti/Ni/AuSn system provided much longer surface local freezing duration compared to the Ti/Pt/AuSn system. Scanning electron microscopy analysis revealed a smoother surface morphology for the Ti/Ni/AuSn system after the metal refroze. Auger electron spectroscopy depth profiles indicated the formation of a Ni‐Sn‐Au interacted layer. The interaction took place in two steps: the first stage was the dissolution of Ni into the Au‐Sn liquid followed by precipitation of a Ni‐Sn‐Au intermetallic compound; the second stage was a solid‐state interdiffusion of Sn, Au, and Ni which occured in the interacted layer and in the original Ni layer. The latter step was a diffus...

Pt/Ti ohmic contacts to ultrahigh carbon‐doped p‐GaAs formed by rapid thermal processing

Increasing the concentration of the carbon dopants in p‐GaAs layers grown on semi‐insulating substrates to levels of 1×1020 to 5×1020 cm−3 enables the formation of an ohmic contact with low resistance using the refractory Pt/Ti metallization. These contacts showed ohmic behavior prior to any heat treatment with specific contact resistance as low as 7×10−6 Ω cm2 (0.08 Ω mm) for the lower doping level and 8×10−7 Ω cm2 (0.04 Ω mm) for the higher level. Small improvements in the specific resistance of the former contact were achieved by rapid thermal processing at a temperature of 450 °C for 30 s, which yielded a value of 4.9×10−6 Ω cm2. The electrical nature of the contact to the heavily doped GaAs was not affected by heat treatments at temperatures up to 450 °C. Rapid thermal processing of these contacts at higher temperatures, however, caused an increase in the contact resistance which was correlated to the expanded Ti/GaAs and Pt/GaAs interfacial reactions. Current‐voltage characteristics were found to be...

Advanced metallization schemes for bonding of InP-based laser devices to CVD-diamond heatsinks

Abstract Semiconductor devices, and in particular InP-based laser devices, are usually bonded on a mounting plate (called a submount or heatsink) or directly onto a package. This bonding assembly, which comprises the die-bonding metallic layers, the joint solder and the submount, serves the purpose of heat dissipator, mechanical support and electrical conductor. As such, the quality of both the bonding metallization and the submount are as important as the device die itself to assure the short- and long-term reliable operation of the electronic assembly. Due to its high thermal conductivity, diamond has been used as a very efficient thermal conductor and dissipator under electronic and photonic devices. This application has become even more attractive since the commercialization of the chemically vapor-deposited (CVD)-diamond, due to its lower cost and larger available surface area compared to natural diamond. Therefore it is only natural to use CVD-diamond as the material of choice for mass production of high-power InP-based laser diode submounts. The device is attached to the submount by a metallic bonding medium that contains a few layers, such as an adhesion layer (typically of titanium (Ti) adjacent to the submount), a barrier layer (typically of platinum (Pt)) and capped with hard solder (such as gold-tin (AuSn) eutectic alloy, which has a melting temperature of 278 °C). While offering optimum bonding conditions, the Ti/Pt/AuSn bonding system provides a high quality bond of the laser diode to the CVD-diamond submount. However, the extensive reaction of the AuSn solder with the Pt and Ti layers, even after short bonding periods of 5–10 s, may lead to mechanical deterioration of the bonded assembly, resulting in delamination of the metal and failure of the bond. This failure occurs mainly due to the thermodynamically reactive nature of the Pt-Sn couple, which reacts almost spontaneously even at room temperature. The reaction consumes the Pt layer and an appreciable amount of Sn, leading to the disappearance of the barrier layer and to dilution of Sn from the solder. The variation in the molten solder stoichiometry results in premature solidification of the solder through the bonding cycle after consumption of the entire Pt barrier layer, and dissolution of the Ti adhesion layer, as well. In order to maintain the good wetting performance of the AuSn solder to the Ti/Pt metals, but yet to improve the thermodynamic stability of the metallurgical bonding system, various other metals such as Ni, W, Cr and some of their alloys were evaluated as advanced alternatives to the Pt barrier layer. The quality of the evaluated metallurgical systems was judged upon wetting performance, thermodynamic stability and lack of premature freezing of the molten solder. The Ti/W/W(Ni 3 Sn 4 )/Ni 3 Sn 4 /Au multilayered structure was finally defined as the optimal and superior metallurgical scheme for the purpose of bonding laser chip to CVD-diamond submount. The time required until the first surface local freezing phenomenon was observed at the AuSn solder (with eutectic composition of 80 wt.% Au) on Ti/W/W(Ni 3 Sn 4 )/Ni 3 Sn 4 /Au structure while melted at 320 °C was 200 s, compared to 5–10 s at the Ti/Pt/Au/AuSn system. At the former system the solder was maintained melted at 320 °C for more than 1 h before it was completely frozen, which is much longer than the 30 s measured for the latter system. It was also observed that not only did the AuSn solder on the Ti/W/W(Ni 3 Sn 4 )/Ni 3 Sn 4 multilayer maintain its accurate stoichiometric composition through the entire bonding cycle, but it also provided excellent adhesion and integrity for the entire bonded assembly.

Interfacial microstructure and electrical properties of the Pt/Ti ohmic contact in p-In0.53Ga0.47As formed by rapid thermal processing

The interfacial microstructure and electrical properties of the Pt/Ti ohmic contact to p‐In0.53Ga0.47As (Zn: 5×1018 cm−3) formed by rapid thermal processing (RTP) were intensively studied. Significant interdiffusion of Ti, In, and As across the interface, driven by RTP, occurred at temperatures of, or above, 350 °C for a heating duration of 30 s. A minimum specific contact resistance (9.0×10−6 Ω cm2) was achieved after heating at 450 °C. Cross‐sectional transmission electron microscopy of this sample revealed an interfacial reaction zone with complicated microstructure, and the dominant interfacial compound was identified to be InAs. Further increase in RTP temperature resulted in a change in the microstructure, and degradation of the contact resistance. The temperature‐dependence characteristic of the specific contact resistance of as‐deposited Pt/Ti contact to InGaAs revealed a thermionic‐emission‐dominated carrier‐transport mechanism with an effective barrier height φb, of 0.13 V. RTP treatment to the ...

Au/Pt/Ti contacts to p-In0.53Ga0.47As and n-InP layers formed by a single metallization common step and rapid thermal processing

We have demonstrated the viability of depositing a thick Au bonding pad on top of Pt/Ti contacts on both p‐InGaAs and n‐InP within a single evaporation prior to heat treatment. This eliminates the usual post‐sinter Au plating process. In particular, Au (500 nm)/Pt (60 nm)/Ti (50 nm) common contacts to Zn‐doped 5×1018 cm−3 p‐In0.53Ga0.47As and S‐doped 1×1018 cm−3 n‐InP were formed within a single pumpdown electron‐gun evaporation and subsequently a single sintering process by means of rapid thermal processing. The lowest resistivity of these ohmic contacts were found to be 0.11 and 0.13 Ω mm (5.5×10−7 and 8×10−6 Ω cm2) for the p and n contacts, respectively. These values were achieved as a result of heating at 450 °C for 30 sec. This heat treatment caused a limited reaction at the Au‐Pt and Pt‐Ti interfaces, which did not lead to any significant intermixing of the Ti and Au. Thus, no significant indiffusion of the Au thorough the Pt barrier was observed and contact degradation did not occur. The stress of the as‐deposited trilayer structure on InP was found to be 3×108 dyne cm2 tensile and increased to about 2×109 dyn cm2 as a result of the rapid thermal processing at 450 °C

Stress measurements of Pt/Ti/InP and Pt/Ti/SiO2/InP systems : in situ measurements through sintering and after rapid thermal processing

The stresses induced in an evaporated Pt(75 nm)/Ti(50 nm) bilayer metallization scheme on InP and SiO2 (300 nm)/InP substrates, as well as the stress in a SiO2 layer (300 nm) on an InP substrate, were measured in situ during sintering at temperatures of 25 to 500 °C and after rapid thermal processing (RTP) at temperatures of 400, 450, and 500 °C for 30 s. The as‐deposited highly tensile Pt/Ti bilayer structure on InP (5×109 dyn cm−2) was found to be stress‐free when heated to 400 °C and to have relatively low tensile stress after cooling back to room temperature. The as‐deposited Pt/Ti/SiO2 structure on InP was found to be only moderately tensile stressed (3×109 dyn cm−2) and became more tensile as a result of heating to 500 °C (5×109 dyn cm−2). The high tensile stress was preserved even after cooling back to room temperature. This is mostly due to the tendency of the plasma‐enhanced chemical vapor deposited (PECVD) SiO2 layer to undergo densification and switch its as‐deposited compressive stress (−2.5×1...