Dual junction GaInP/GaAs solar cells grown on metamorphic SiGe/Si substrates with high open circuit voltage

Dual junction GaInP/GaAs solar cells have been grown and fabricated on Si substrates using relaxed, compositionally graded SiGe buffer layers that provide a nearly lattice-matched low threading dislocation Ge surface for subsequent cell growth. The dual junction cells on SiGe/Si displayed high open circuit voltages in excess of 2.2 V, compared to 2.34 V for control cells on GaAs, that are consistent with maintaining the 1.8/spl times/10/sup 6/ cm/sup -2/ threading dislocation density throughout the cell structure. Even with total current output limited by large grid coverage and high reflectance, total area AM1.5G efficiency is 16.8%, with active area efficiency at 18.6%. The high V/sub oc/ establishes that SiGe metamorphic buffers are viable for integrating III-V multijunction cells on Si in a monolithic process.     

A novel strained Si/sub 0.7/Ge/sub 0.3/ surface-channel pMOSFET with an ALD TiN/Al/sub 2/O/sub 3//HfAlO/sub x//Al/sub 2/O/sub 3/ gate stack

Proof-of-concept pMOSFETs with a strained-Si/sub 0.7/Ge/sub 0.3/ surface-channel deposited by selective epitaxy and a TiN/Al/sub 2/O/sub 3//HfAlO/sub x//Al/sub 2/O/sub 3/ gate stack grown by atomic layer chemical vapor deposition (ALD) techniques were fabricated. The Si/sub 0.7/Ge/sub 0.3/ pMOSFETs exhibited more than 30% higher current drive and peak transconductance than reference Si pMOSFETs with the same gate stack. The effective mobility for the Si reference coincided with the universal hole mobility curve for Si. The presence of a relatively low density of interface states, determined as 3.3 /spl times/ 10/sup 11/ cm/sup -2/ eV/sup -1/, yielded a subthreshold slope of 75 mV/dec. for the Si reference. For the Si/sub 0.7/Ge/sub 0.3/ pMOSFETs, these values were 1.6 /spl times/ 10/sup 12/ cm/sup -2/ eV/sup -1/ and 110 mV/dec., respectively.     

0.6-eV bandgap In/sub 0.69/Ga/sub 0.31/As thermophotovoltaic devices grown on InAs/sub y/P/sub 1-y/ step-graded buffers by molecular beam epitaxy

Single-junction, lattice-mismatched (LMM) In/sub 0.69/Ga/sub 0.31/As thermophotovoltaic (TPV) devices with bandgaps of 0.60 eV were grown on InP substrates by solid-source molecular beam epitaxy (MBE). Step-graded InAs/sub y/P/sub 1-y/ buffer layers with a total thickness of 1.6 /spl mu/m were used to mitigate the effects of 1.1% lattice mismatch between the device layer and the InP substrate. High-performance single-junction devices were achieved, with an open-circuit voltage of 0.357 V and a fill factor of 68.1% measured at a short-circuit current density of 1.18 A/cm/sup 2/ under high-intensity, low emissivity white light illumination. Device performance uniformity was outstanding, measuring to better than 1.0% across a 2-in diameter InP wafer indicating the promise of MBE growth for large area TPV device arrays.     

High-performance In/sub 0.53/Ga/sub 0.47/As thermophotovoltaic devices grown by solid source molecular beam epitaxy

In/sub 0.53/Ga/sub 0.47/As-based monolithic interconnected modules (MIMs) of thermophotovoltaic (TPV) devices lattice-matched to InP were grown by solid source molecular beam epitaxy. The MIM device consisted of ten individual In/sub 0.53/Ga/sub 0.47/As TPV cells connected in series on an InP substrate. An open-circuit voltage (V/sub oc/) of 4.82 V, short-circuit current density (J/sub sc/) of 1.03 A/cm/sup 2/ and fill factor of /spl sim/73% were achieved for a ten-junction MIM with a bandgap of 0.74 eV under high intensity white light illumination. Device performance uniformity was better than 1.5% across a full 2-in InP wafer. The V/sub oc/ and J/sub sc/ values are the highest yet reported for 0.74-eV band gap n-p-n MIM devices.     

Film Properties of MoSi/sub 2/ and Their Application to Self-Aligned MoSi/sub 2/ Gate MOSFET

Molybdenum silicide (MoSi/sub 2/) gate technology has been extensively investigated in conjunction with MOS device performance and reliability. Features of the MoSi/sub 2/) gate technology are to realize a low resistivity of 1 X 10/sup -4/ Omega · cm for both gate and interconnection, and to give rise to higher reliability under both positive and negative bias stress of 2 MV/cm at 250/spl deg/C. Problems on the ohmic contact between MoSi/sub 2/ and single-crystal substrates are not completely solved yet, particularly when the device is processed at high temperature after MoSi/sub 2/ deposition.     

Growth of Si and Ge nanostructures on Si substrates using ultrathin SiO/sub 2/ technology

Using scanning reflection electron microscopy and a high-temperature scanning tunneling microscopy (STM), we study the growth processes of Si and Ge nanostructures on Si substrates covered with ultrathin SiO/sub 2/ films. Si windows are formed in the ultrathin SiO/sub 2/ films by irradiating focused electron beams used for SREM or field emission electron beams from STM tips before or during heating samples. Ge nanoislands are grown only at the Si window positions by depositing Ge on the samples and by subsequent annealing of them. Moreover, Ge nanoislands about 7 nm in size and ultrahigh density (>10/sup 12//cm/sup 2/) are grown on the ultrathin SiO/sub 2/ films. These nanoislands can be manipulated by STM when they are separated from Si substrates by the ultrathin SiO/sub 2/ films. Si, Ge, Ge/Si and Si/Ge/Si nanoislands can also be grown on the Si windows by selective growth using Si/sub 2/H/sub 6/ and GeH/sub 4/ gases. These nanoislands are found to be stable on the Si windows during high-temperature annealing. These results indicate that ultrathin SiO/sub 2/ technology is useful for growing Si and Ge nanostructures on given areas.     

Improved Schottky barrier on n-Sb/sub 2/S/sub 3/ films chemically deposited with silicotungstic acid (solar cells)

The first fabrication of low cost Schottky barrier solar cells on chemically deposited polycrystalline n-Sb/sub 2/S/sub 3/ thin films is reported. It is observed that in the films deposited with silicotungstic acid and annealed, the Schottky barrier height ( phi /sub b/) of the Au/n-Sb/sub 2/S/sub 3/ junctions is considerably improved from 0.54 to 0.76 eV. The ideality factor n decreased from 2.32 to 1.08 and the reverse-saturation current density J/sub 0/ from 3.2*10/sup -6/ to 1.5*10/sup -9/ A cm/sup -2/. Under AM1 illumination, the improved diode exhibited a conversion efficiency of approximately 3%.<>     

Si/SiGe n-MODFETs on thin SiGe virtual substrates prepared by means of He implantation

Si/SiGe n-type modulation-doped field-effect transistors grown on a very thin strain-relieved Si/sub 0.69/Ge/sub 0.31/ buffer on top of a Si(100) substrate were fabricated and characterized. This novel type of virtual substrate has been created by means of a high dose He ion implantation localized beneath a 95-nm-thick pseudomorphic SiGe layer on Si followed by a strain relaxing annealing step at 850/spl deg/C. The layers were grown by molecular beam epitaxy. Electron mobilities of 1415 cm/sup 2//Vs and 5270 cm/sup 2//Vs were measured at room temperature and 77 K, respectively, at a sheet carrier density of about 3/spl times/10/sup 12//cm/sup 2/. The fabricated transistors with Pt-Schottky gates showed good dc characteristics with a drain current of 330 mA/mm and a transconductance of 200 mS/mm. Cutoff frequencies of f/sub t/=49 GHz and f/sub max/=95 GHz at 100 nm gate length were obtained which are quite close to the figures of merit of a control sample grown on a conventional, thick Si/sub 0.7/Ge/sub 0.3/ buffer.     

Room-temperature self-organised In/sub 0.5/Ga/sub 0.5/As quantum dot laser on silicon

The first room-temperature operation of In/sub 0.5/Ga/sub 0.5/As quantum dot lasers grown directly on Si substrates with a thin (/spl les/2 /spl mu/m) GaAs buffer layer is reported. The devices are characterised by J/sub th//spl sim/1500 A/cm/sup 2/, output power >50 mW, and large T/sub 0/ (244 K) and constant output slope efficiency (/spl ges/0.3 W/A) in the temperature range 5-95/spl deg/C.     

10-gb/s operation of an In/sub 0.53/Ga/sub 0.47/As p-i-n photodiode on metamorphic InGaP buffered semi-insulating GaAs substrate

The SONET OC-192 receiving performance of In/sub 0.53/Ga/sub 0.47/As p-i-n photodiode grown on linearly graded metamorphic In/sub x/Ga/sub 1-x/P buffered GaAs substrate is reported. With a low-cost TO-46 package, such a device exhibits a frequency bandwidth up to 8 GHz, a bit-error rate (BER) of 10/sup -9/ at 10 Gb/s, a sensitivity of -17.8 dBm, and a noise equivalent power of 3.4/spl times/10/sup -15/ W/Hz/sup 1/2/ owing to its ultralow dark current of 3.6/spl times/10/sup -7/ A/cm/sup 2/. Eye diagram analysis at 10 Gb/s without transimpedance amplification reveals a statistically distributed Q-factor of 8.21, corresponding to a minimum BER of 1.1/spl times/10/sup -16/ at receiving power of -6 dBm.     

A 183 GHz f/sub T/ and 165 GHz f/sub max/ regrown-emitter DHBT with abrupt InP emitter

Small-area regrown emitter-base junction InP/In-GaAs/InP double heterojunction bipolar transistors (DHBT) using an abrupt InP emitter are presented for the first time. In a device with emitter-base junction area of 0.7 /spl times/ 8 /spl mu/m/sup 2/, a maximum 183 GHz f/sub T/ and 165 GHz f/sub max/ are exhibited. To our knowledge, this is the highest reported bandwidth for a III-V bipolar transistor utilizing emitter regrowth. The emitter current density is 6/spl times/10/sup 5/ A/cm/sup 2/ at V/sub CE,sat/ = 1.5 V. The small-signal current gain h/sub 21/ = 17, while collector breakdown voltage is near 6 V for the 1500-/spl Aring/-thick collector. The emitter structure, created by nonselective molecular beam epitaxy regrowth, combines a small-area emitter-base junction and a larger-area extrinsic emitter contact, and is similar in structure to that of a SiGe HBT. The higher f/sub T/ and f/sub max/ compared to previously reported devices are achieved by simplified regrowth using an InP emitter and by improvements to the regrowth surface preparation process.     

La/sub 2/O/sub 3//Si/sub 0.3/Ge/sub 0.7/ p-MOSFETs with high hole mobility and good device characteristics

We have studied high-k La/sub 2/O/sub 3/ p-MOSFETs on Si/sub 0.3/Ge/sub 0.7/ substrate. Nearly identical gate oxide current, capacitance density, and time-dependent dielectric breakdown (TDDB) are obtained for La/sub 2/O/sub 3//Si and La/sub 2/O/sub 3//Si/sub 0.3/Ge/sub 0.7/ devices, indicating excellent Si/sub 0.3/Ge/sub 0.7/ quality without any side effects. The measured hole mobility in nitrided La/sub 2/O/sub 3//Si p-MOSFETs is 31 cm/sup 2//V-s and comparable with published data in nitrided HfO/sub 2//Si p-MOSFETs. In sharp contrast, a higher mobility of 55 cm/sup 2//V-s is measured in La/sub 2/O/sub 3//Si/sub 0.3/Ge/sub 0.7/ p-MOSFET, an improvement by 1.8 times compared with La/sub 2/O/sub 3//Si control devices. The high mobility in Si/sub 0.3/Ge/sub 0.7/ p-MOSFETs gives another step for integrating high-k gate dielectrics into the VLSI process.     

Turn-on voltage investigation of GaAs-based bipolar transistors with Ga/sub 1-x/In/sub x/As/sub 1-y/N/sub y/ base layers

The fundamental lower limit on the turn on voltage of GaAs-based bipolar transistors is first established, then reduced with the use of a novel low energy-gap base material, Ga/sub 1-x/In/sub x/As/sub 1-y/N/sub y/. InGaP/GaInAsN DHBTs (x/spl sim/3y/spl sim/0.01) with high p-type doping levels (/spl sim/3/spl times/10/sup 19/ cm/sup -3/) and dc current gain (/spl beta//sub max//spl sim/68 at 234 /spl Omega///spl square/) are demonstrated. A reduction in the turn-on voltage over a wide range of practical base sheet resistance values (100 to 400 /spl Omega///spl square/) is established relative to both GaAs BJTs and conventional InGaP/GaAs HBTs with optimized base-emitter interfaces-over 25 mV in heavily doped, high dc current gain samples. The potential to engineer turn-on voltages comparable to Si- or InP-based bipolar devices on a GaAs platform is enabled by the use of lattice matched Ga/sub 1-x/In/sub x/As/sub 1-y/N/sub y/ alloys, which can simultaneously reduce the energy-gap and balance the lattice constant of the base layer when x/spl sim/3y.     

A novel InGaP/Al/sub x/Ga/sub 1-x/As/GaAs CEHBT

A new and interesting InGaP/Al/sub x/Ga/sub 1-x/As/GaAs composite-emitter heterojunction bipolar transistor (CEHBT) is fabricated and studied. Based on the insertion of a compositionally linear graded Al/sub x/Ga/sub 1-x/As layer, a near-continuous conduction band structure between the InGaP emitter and the GaAs base is developed. Simulation results reveal that a potential spike at the emitter/base heterointerface is completely eliminated. Experimental results show that the CEHBT exhibits good dc performances with dc current gain of 280 and greater than unity at collector current densities of J/sub C/=21kA/cm/sup 2/ and 2.70/spl times/10/sup -5/ A/cm/sup 2/, respectively. A small collector/emitter offset voltage /spl Delta/V/sub CE/ of 80 meV is also obtained. The studied CEHBT exhibits transistor action under an extremely low collector current density (2.7/spl times/10/sup -5/ A/cm/sup 2/) and useful current gains over nine decades of magnitude of collector current density. In microwave characteristics, the unity current gain cutoff frequency f/sub T/=43.2GHz and the maximum oscillation frequency f/sub max/=35.1GHz are achieved for a 3/spl times/20 /spl mu/m/sup 2/ device. Consequently, the studied device shows promise for low supply voltage and low-power circuit applications.     

High frequency performance of Si/sub 1-x/Ge/sub x//Si/sub 1-y/Ge/sub y//Si/sub 1-x/Ge/sub x/ HBTs

The results of a theoretical study of the performance of high speed SiGe HBTs is presented. The study includes a group of SiGe HBTs in which the Ge concentration in the base is 20% higher than that in the emitter and collector (i.e. y=x+0.2). It is shown that the composition dependences of f/sub T/ and the F/sub max/ are non-monotonic. As the Ge composition in the emitter and collector layers is increased, f/sub T/ and f/sub max/ first decrease, then remain constant and finally increase to attain their highest values.<>     

Ultralow leakage In/sub 0.53/Ga/sub 0.47/As p-i-n photodetector grown on linearly graded metamorphic In/sub x/Ga/sub 1-x/P buffered GaAs substrate

A novel top-illuminated In/sub 0.53/Ga/sub 0.47/As p-i-n photodiodes (MM-PINPD) grown on GaAs substrate by using linearly graded metamorphic In/sub x/Ga/sub 1-x/P (x graded from 0.49 to 1) buffer layer is reported. The dark current, optical responsivities, noise equivalent power, and operational bandwidth of the MM-PINPD with aperture diameter of 60 /spl mu/m are 13 pA, 0.6 A/W, 3.4/spl times/10/sup -15/ W/Hz/sup 1/2/, and 7.5 GHz, respectively, at 1550 nm. The performances of the MM-PINPD on GaAs are demonstrated to be comparable to those of a similar device made on InGaAs-InP substrate.     

Ga/sub 0.8/In/sub 0.2/As/GaAs/Ga/sub 0.51/In/sub 0.49/P buried ridge structure single quantum well laser emitting at 0.98 mu m

The first fabrication is reported of a buried ridge structure Ga/sub 0.8/In/sub 0.2/As/GaAs/Ga/sub 0.51/In/sub 0.49/P laser emitting at 0.98 mu m grown by two-step low pressure metal organic chemical vapour deposition. The width of the ridge is 2 mu m. Laser output power greater than 40 mW with a threshold current of 30 mA has been measured. A typical quantum efficiency of eta =60% was obtained without any facet coating. Excellent homogeneity and uniformity have been achieved over a wafer area of 10 cm/sup 2/.<>     

High quality GaAs qrowth by MBE on Si using GeSi buffers and prospects for space photovoltaics

III–V solar cells on Si substrates are of interest for space photovoltaics since this would combine high performance space cells with a strong, lightweight and inexpensive substrate. However, the primary obstacles blocking III–V/Si cells from achieving high performance to date have been fundamental material incompatibilities, namely the 4% lattice mismatch between GaAs and Si, and the large mismatch in thermal expansion coefficient. In this paper, we report on the molecular beam epitaxial (MBE) growth and properties of GaAs layers and single junction GaAs cells on Si wafers which utilize compositionally graded GeSi intermediate buffers grown by ultra‐high vacuum chemical vapor deposition (UHVCVD) to mitigate the large lattice mismatch between GaAs and Si. GaAs cell structures were found to incorporate a threading dislocation density of 0.9–1.5×10 cm⁻², identical to the underlying relaxed Ge cap of the graded buffer, via a combination of transmission electron microscopy, electron beam induced current, and etch pit density measurements. AlGaAs/GaAs double heterostructures were grown on the GeSi/Si substrates for time‐resolved photoluminescence measurements, which revealed a bulk GaAs minority carrier lifetime in excess of 10 ns, the highest lifetime ever reported for GaAs on Si. A series of growths were performed to assess the impact of a GaAs buffer layer that is typically grown on the Ge surface prior to growth of active device layers. We found that both the high lifetimes and low interface recombination velocities are maintained even after reducing the GaAs buffer to a thickness of only 0.1 μm. Secondary ion mass spectroscopy studies revealed that there is negligible cross diffusion of Ga, As and Ge at the III–V/Ge interface, identical to our earlier findings for GaAs grown on Ge wafers using MBE. This indicates that there is no need for a buffer to ‘bury’ regions of high autodoping, and that either pn or np configuration cells are easily accommodated by these substrates. Preliminary diodes and single junction AlGaAs heteroface cells were grown and fabricated on the Ge/GeSi/Si substrates for the first time. Diodes fabricated on GaAs, Ge and Ge/GeSi/Si substrates show nearly identical I–V characteristics in both forward and reverse bias regions. External quantum efficiencies of AlGaAs/GaAs cell structures grown on Ge/GeSi/Si and Ge substrates demonstrated nearly identical photoresponse, which indicates that high lifetimes, diffusion lengths and efficient minority carrier collection is maintained after complete cell processing. Copyright © 2000 John Wiley & Sons, Ltd.     

Double heterojunction bipolar transistor in Al/sub x/Ga/sub 1-x/As/GaAs/sub 1-y/Sb/sub y/ system

Double heterojunction bipolar transistors based on the Al/sub x/Ga/sub 1-x/As/GaAs/sub 1-y/Sb/sub y/ system are examined. The base layer consists of narrow band gap GaAs/sub 1-y/Sb/sub y/ and the emitter and collector consist of wider band gap Al/sub x/Ga/sub 1-x/As. Preliminary experimental results show that AlGaAs/GaAsSb/GaAs DHBTs exhibit a current gain of five and a maximum collector current density of 5*10/sup 4/ A/cm/sup 2/.<>     

High-efficiency GaAs/Ge monolithic tandem solar cells

Tandem cells of GaAs grown by metalorganic chemical vapor deposition (MOCVD) on thin Ge to address both higher efficiency and reduced weight are discussed. GaAs/Ge monolithic tandem cells of 4-cm ² area have been produced with independently verified efficiencies up to 21.7% (AM0, one sun, 25°C, total area). Under AM1.5 global conditions, efficiencies are up to 24.3%. These are believed to be the highest one-sun efficiencies reported for GaAs/Ge cells, and the highest efficiency for a two-terminal monolithic tandem cell