B. E. Weir

Totally relaxed GexSi1-x layers with low threading dislocation densities grown on Si substrates

We have grown compositionally graded GexSi1−x layers on Si at 900 °C with both molecular beam epitaxy and rapid thermal chemical vapor deposition techniques. Triple‐crystal x‐ray diffraction reveals that for 0.10<x<0.53, the layers are totally relaxed. GexSi1−x cap layers grown on these graded layers are threading‐dislocation‐free when examined with conventional plan‐view and cross‐sectional transmission electron microscopy. Electron beam induced current images were used to count the low threading dislocation densities, which were 4×105±5×104 cm−2 and 3×106±2×106 cm−2 Eq. 2×106 cm−2 for x=0.23 and x=0.50, respectively. Photoluminescence spectra from the cap layers are identical to photoluminescence from bulk GexSi1−x.

Extremely high electron mobility in Si/GexSi1−x structures grown by molecular beam epitaxy

A modulation‐doped Si/GexSi1−x structure was fabricated in which a thin Si layer is employed as the conduction channel for the two‐dimensional electron gas. The strained heterostructure is fabricated on top of a low threading dislocation density, totally relaxed, GexSi1−x buffer layer with a linearly graded Ge concentration profile. The mobility of the two‐dimensional electron gas as determined from Hall measurements was 1600 cm2/Vu2009s at 300 K and 96u2009000 cm2/Vu2009s at 4.2 K. Recently, a 4.2 K mobility of 125u2009000 cm2/Vu2009s was observed from a similar sample.

Performance of gain-guided surface emitting lasers with semiconductor distributed Bragg reflectors

The performance limitations of gain-guided vertical cavity surface emitting lasers (VCSELs) which use epitaxially grown semiconductor distributed Bragg reflectors (DBRs) are discussed. The light-current (L-I) characteristics and emission wavelength of such lasers are examined as a function of temperature and time under continuous wave (CW) and pulsed operation. The authors observed a sharp roll-over in the CW L-I characteristics which limits the maximum output power. The threshold current under CW operation is found to be lower than that obtained under pulsed conditions. Several microseconds long delay in lasing turn-on is also observed. It is shown quantitatively that these anomalies are a consequence of severe heating effects. It is shown that reduction of the series resistance and threshold current density can lead to significant improvements in the power performance of VCSELs. >

Mechanically and thermally stable Si‐Ge films and heterojunction bipolar transistors grown by rapid thermal chemical vapor deposition at 900 °C

Traditional techniques for growing Si‐Ge layers have centered around low‐temperature growth methods such as molecular‐beam epitaxy and ultrahigh vacuum chemical vapor deposition in order to achieve strain metastability and good growth control. Recognizing that metastable films are probably undesirable in state‐of‐the‐art devices on the basis of reliability considerations, and that in general, crystal perfection increases with increasing deposition temperatures, we have grown mechanically stable Si‐Ge films (i.e., films whose composition and thickness places them on or below the Matthews–Blakeslee mechanical equilibrium curve) at 900u2009°C by rapid thermal chemical vapor deposition. Although this limits the thickness and the Ge composition range, such films are exactly those required for high‐speed heterojunction bipolar transistors and Si/Si‐Ge superlattices, for example. The 900u2009°C films contain three orders of magnitude less oxygen than their limited reaction processing counterparts grown at 625u2009°C. The fi...

Si(100)‐(2×1)boron reconstruction: Self‐limiting monolayer doping

A (2×1) surface reconstruction distinct from the clean Si(100)‐(2×1) surface is formed by depositing boron onto silicon in ultrahigh vacuum. Overgrowth of epitaxial silicon at low temperature preserves a (2×1) superstructure of substitutional boron. Hall‐effect measurements at 4.2 K show complete electrical activity for boron coverages of 1/2 monolayer, but additional boron above 1/2 monolayer is not electrically active.

Quantitative analysis of strain relaxation in GexSi1−x/Si(110) heterostructures and an accurate determination of stacking fault energy in GexSi1−x alloys

We report a quantitative theoretical and experimental analysis of strain relaxation in GexSi1−x/Si(110) heterostructures. It is shown that above a critical composition, the critical thickness for edge a/6〈112〉 Shockley partial dislocations is less than that for 60° a/2〈110〉 total dislocations. The net (excess) stress is greater on the edge a/6〈112〉 dislocations for epilayer thicknesses, h<hx, but greater on the 60° a/2〈110〉 dislocations for h≳hx. The sensitive calculated dependence of hx upon the stacking fault energy per unit area γ allows an experimental determination of γ=65±10 mJu2009m−2 for x∼0.3 in GexSi1−x.

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 Ωu2009mm (5.5×10−7 and 8×10−6 Ωu2009cm2) for the p and n contacts, respectively. These values were achieved as a result of heating at 450u2009°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 dyneu2009cm2 tensile and increased to about 2×109 dynu2009cm2 as a result of the rapid thermal processing at 450u2009°C

Pt/Ti/n‐InP nonalloyed ohmic contacts formed by rapid thermal processing

Low resistance nonalloyed ohmic contacts of e‐gun evaporated Pt/Ti to S doped n‐InP 5×1017, 1×1018, and 5×1018 cm−3 have been fabricated by rapid thermal processing. The contacts to the lower doped substrates (5×1017 and 1×1018 cm−3) were rectifying as‐deposited as well as after heat treatment at temperatures lower than 350u2009°C. Higher processing temperatures stimulated the Schottky to ohmic contact conversion with minimum specific contact resistance of 1.5×10−5 and 5×10−6 Ωu2009cm2, respectively, as a result of rapid thermal processing at 450u2009°C for 30 s. Heating at a temperature of 550u2009°C again yielded a Schottky contact. The contact to the 5×1018 cm−3 InP was ohmic as deposited with a specific contact resistance value of 1.1×10−4 Ωu2009cm2. Supplying heat treatment to the contact caused a decrease of the specific contact resistance to a minimum of 8×10−7 Ωu2009cm2 as a result of rapid thermal processing at 450u2009°C for 30 s. In all cases, this heat treatment caused a limited interfacial reactions between the Ti and t...

Microstructure and contact resistance temperature dependence of Pt/Ti ohmic contact to Zn‐doped GaAs

Ohmic contacts to p‐GaAs, Zn doped at 5×1018 and 1×1019 cm−3, with the best specific contact resistance of 1.2×10−5 and 2.4×10−6 Ωu2009cm2, respectively, have been formed with deposited layers of Ti and Pt. These layers, which were sequentially evaporated and then rapid thermally processed at 450u2009°C for 30 s, contained an interfacial layer constituted mainly of the TiAs phase adjacent to the substrate and the TixGa1−x solid solution in between it and the Ti layer. In addition, a significant amount of the Pt3Ti intermetallic was found at the Ti/Pt interface. The same metallization scheme, applied to 1×1018 cm−3 Zn‐doped GaAs, produced a Schottky contact for the as‐deposited and heat‐treated samples. The temperature dependence characteristic of the specific contact resistance of the as‐deposited Pt/Ti contacts to 5×1018 and 1×1019 cm−3 Zn‐doped GaAs revealed a thermionic emission dominated carrier transport mechanism with an apparent barrier height of about 0.118 and 0.115 eV, respectively. This mechanism remai...

Carbon reactions in reactive ion etched silicon

Reactive ion etching (RIE) of silicon creates interstitial defects in the near surface region which diffuse into the bulk material and are trapped at substitutional carbon sites. Photoluminescence (PL), current-voltage (I–V), and Rutherford backscattering (RBS) measurements show that silicon etched in a CF4 + 8%O2 or SF6 + 8%O2 plasma consists of two distinct regions, adisplacement damage region extending 1000Å from the surface and apoint defect reaction region which can extend to depths > 1 μm. The size of the point defect reaction region is determined by diffusion limited trapping of the interstitial silicon generated during the RIE resulting in the formation ofCi - Oi orCs-Ci defect pairs. The long range diffusion rate of the interstitial defects is enhanced by the plasma during the RIE processing, and by a recombination enhanced reaction path.