GaAs solar cells grown on Si substrates for space use

GaAs single‐junction and InGaP/GaAs multi‐junction thin‐film solar cells fabricated on Si substrates have great potential for high‐efficiency, low‐cost, lightweight and large‐area space solar cells. Heteroepitaxy of GaAs thin films on Si substrates has been examined and high‐efficiency GaAs thin‐film solar cells with total‐area efficiencies of 18·3% at AM0 and 20·0% at AM 1·5 on Si substrates (GaAs‐on‐Si solar cells) have been fabricated. In addition, 1‐MeV electron irradiation damage to GaAs‐on‐Si cells has been studied. The GaAs‐on‐Si cells are found to show higher end‐of‐life efficiency than the conventional GaAs cells fabricated on GaAs substrates (GaAs‐ on‐GaAs cells) under high‐fluence 1‐MeV electron irradiation of more than 1 × 10¹⁵ cm⁻². The first space flight to make use of them has been carried out. Forty‐eight 2 × 2 cm GaAs‐on‐Si cells with an average AM0 total‐area efficiency of 16·9% have been evaluated in the Engineering Test Satellite No.6 (ETS‐VI). The GaAs‐on‐Si cells have been demonstrated to be more radiation‐resistant in space than GaAs‐on‐GaAs cells and 50, 100 and 200‐μm‐thick Si cells. These results show that the GaAs‐on‐Si single‐junction and InGaP/GaAs‐on‐Si multi‐junction cells have great potential for space applications. Copyright © 2001 John Wiley & Sons, Ltd.     

GaAs/AlGaAs heterojunction Pnp bipolar transistors grownon (100) Si by molecular beam epitaxy

GaAs/AlGaAs Pnp heterojunction bipolar transistors (HBTs) were fabricated and tested on (100) Si substrates for the first time. A common-emitter current gain of β=8 was measured for the typical devices with an emitter area of 50×50 μm² at a collector current density of 1×10⁴ A/cm² with no output negative differential resistance up to 280 mA, highest current used. A very high base-collector breakdown voltage of 10 V was obtained. Comparing the similar structures grown on GaAs substrates, the measured characteristics clearly demonstrate that device grade hole injection can be obtained in GaAs on Si epitaxial layers despite the presence of dislocations     

A high-gain GaAs/AlGaAs n-p-n heterojunction bipolar transistor on(100) Si grown by molecular beam epitaxy

High-gain GaAs/AlGaAs n-p-n heterojunction bipolar transistors (HBT's) on Si substrates grown by molecular beam epitaxy (MBE) have been fabricated and tested. In this structure, an n⁺-InAs emitter cap layer was grown in order to achieve a nonalloyed ohmic contact. Typical devices with an emitter dimension of 50×50 μm² exhibited a current gain as high as 45 at a collector current density of 2×10³ A/cm² with an ideality factor of 1.4. This is the highest current gain reported for HBT's grown on Si substrates. Breakdown voltages as high as 10 and 15 V were observed for the emitter-base and collector-base junctions respectively. The investigation on devices with varying emitter dimensions demonstrates that much higher current gains can be expected     

A dc and microwave comparison of GaAs MESFET's on GaAs and Si substrates

In order to assess GaAs on Si technology, we have made a performance comparison of GaAs MESFET's grown and fabricated on Si and GaAs substrates under identical conditions and report the first microwave results. The GaAs MESFET's on Si with 1.2-µm gate length (290-µm width) exhibited transconductances (gm) of 180 mS/mm with good saturation and pinchoff whereas their counterparts on GaAs substrates exhibited gmof 170 mS/mm. A current gain cut-off frequency of 13.5 GHz was obtained, which compares with 12.9 GHz observed in similar-geometry GaAs MESFET's on GaAs substrates. The other circuit parameters determined from S-parameter measurements up to 18 GHz showed that whether the substrate is Si or GaAs does not seem to make a difference. Additionally, the microwave performance of these devices was about the same as that obtained in devices with identical geometry fabricated at Tektronix on GaAs substrates. The side-gating effect has also been measured in both types of devices with less than 10-percent decrease in drain current when 5 V is applied to a pad situated 5 µm away from the source. The magnitude of the sidegating effect was identical to within experimental determination for all side-gate biases in the studied range of 0 to -5 V. The light sensitivity of this effect was also very small with a change in drain current of less that 1 percent between dark and light conditions for a side gate bias of -5 V and a spacing of 5 µm. Carrier saturation velocity depth profiles showed that for both MESFET's on GaAs and Si substrates, the velocity was constant at 1.5 × 10⁷cm/s to within 100-150 Å of the active layer-buffer layer interface.     

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.     

Ion-implantation and activation behavior of Si in MBE-Grown GaAs on Si substrates for GaAs MESFET's

The suitability of MBE-grown GaAs layers on Si substrates has been studied for ion-implanted GaAs MESFET technology. The undoped as-grown GaAs layers had a carrier concentration below 10¹⁴cm^-3. Uniform Si ion implants into 4-µm-thick GaAs layers on Si were annealed at 900°C for 10 s, using a rapid-thermal-annealing (RTA) system. Both the activation and the doping profile were similar to those obtained in bulk semi-insulating GaAs under similar conditions. The SIMS profiles of Si and As atoms near the GaAs/Si heterointerface were identical before and after the RTA process, indicating negigible interdiffusion during the implant activation. Dual implants of a shallow n+ layer and an n-channel layer were used to fabricate GaAs MESFET's with a recess-gate technology. Selective oxygen ion implantation was used for device isolation. The maximum transconductance obtained was 135 mS/ mm compared to typical values of 150-180 mS/mm obtained in our laboratory on GaAs substrates in similar device structures.     

⟨110⟩-oriented GaAs MESFETs

GaAs metal semiconductor field-effect transistors (MESFETs) have been successfully fabricated on molecular-beam epitaxial (MBE) films grown on the off-axis (110) GaAs substrate. The (110) substrates were tilted 6° toward the (111) Ga face in order to produce device quality two-dimensional MBE growth. Following the growth of a 0.4-μm undoped GaAs buffer, a 0.18-μm GaAs channel with a doping density of 3.4×10¹⁷ cm^-3 and a 0.12-μm contact layer with a doping density of 2×10¹⁸ cm^-3, both doped with Si, were grown. MESFET devices fabricated on this material show very low-gate leakage current, low output conductance, and an extrinsic transconductance of 200 mS/mm. A unity-current-gain cutoff frequency of 23 GHz and a maximum frequency of oscillation of 56 GHz have been achieved. These (110) GaAs MESFETs have demonstrated their potential for high-speed digital circuits as well as microwave power FET applications     

Characterization of single quantum wells on GaAs/Si grown bymetalorganic chemical vapor deposition

The use of Al_0.5Ga_0.5As/Al_0.55Ga _0.45P intermediate layers and in-situ thermal cycle annealing are shown to be effective in obtaining smooth and sharp heterointerfaces for Al_0.3Ga_0.7As/GaAs single quantum wells (SQWs) grown on Si substrates, as a result of an initial two-dimensional growth and a reduction in threading dislocation density. The best lasing characteristics are obtained in Al_0.3Ga_0.7As/GaAs SQW laser diodes fabricated using the above structure in comparison to those fabricated using the more conventional two-step growth technique     

Threshold-voltage control for GaAs MESFETs using the thermal-wavetechnique

Thermal-wave measurements are a very effective tool in the control of the threshold voltage V_th of GaAs MESFETS with a very thin (100-nm) active layer. A linear correlation is given between the thermal-wave signals and implantation into semi-insulating (SI) undoped LEC-grown GaAs substrates at 50 keV. In addition, the relation between the threshold voltage of GaAs MESFETs fabricated by direct Si ⁺ implantation at 50 keV and dosage was experimentally obtained. Using these two results, the threshold voltage is easily predicted just after implantation by thermal-wave measurements and adjusted to the desired value by additional implantation     

Molecular beam epitaxial GaAs/Al0.2Ga0.8Asp-channel field-effect transistors on (311)A facets of patterned (100)GaAs obtained by silicon doping

P-channel and n-channel heterostructure field effect transistors (HFETs) have been simultaneously fabricated by one-step molecular beam epitaxial growth of Si-doped Al_0.2Ga_0.8As/GaAs heterostructures on patterned (100) GaAs substrates. The p-HFETs were made on the etched (311)A facets and the n-HFETs on the planar (100) surface. A transconductance value of 23 mS/mm at 300 K for a p-HFET with a 1.1×50-μm gate is measured. The same size n-HFET made with the same structure and same level of Si doping has a transconductance value of 250 mS/mm at room temperature     

Application of low temperature GaAs to GaAs/Si

Low Temperature grown GaAs (LT-GaAs) was incorporated as a buffer layer for GaAs on Si (GaAs/Si) and striking advantages of this structure were confirmed. The LT-GaAs layer showed high resistivity of 1.7 × 10⁷ ω-cm even on a highly defective GaAs/Si. GaAs/Si with the LT-GaAs buffer layers had smoother surfaces and showed much higher photoluminescence intensities than those without LT-GaAs. Schottky diodes fabricated on GaAs/Si with LT-GaAs showed a drastically reduced leakage current and an improved ideality factor. These results indicate that the LT-GaAs buffer layer is promising for future integrated circuits which utilize GaAs/Si substrates.     

Monolithic integration of Si MOSFET's and GaAs MESFET's

Integration of Si MOSFET's and GaAs MESFET's on a monolithic GaAs/Si (MGS) substrate has been demonstrated. The GaAs MESFET's have transconductance of 150 mS/mm for a gate length of 1 µm, and the Si MOSFET's have transconductance of 19 mS/mm for a gate length of 5 µm and an oxide thickness of 800 Å. These characteristics are comparable to those for devices fabricated on separate GaAs and Si substrates.     

Direct growth of CdZnTe/Si substrates for large-area HgCdTe infrared focal plane arrays

Direct epitaxial growth of high-quality 100lCdZnTe on 3 inch diameter vicinal {100}Si substrates has been achieved using molecular beam epitaxy (MBE); a ZnTe initial layer was used to maintain the {100} Si substrate orientation. The properties of these substrates and associated HgCdTe layers grown by liquid phase epitaxy (LPE) and subsequently processed long wavelength infrared (LWIR) detectors were compared directly with our related efforts using CdZnTe/ GaAs/Si substrates grown by metalorganic chemical vapor deposition (MOCVD). The MBE-grown CdZnTe layers are highly specular and have both excellent thickness and compositional uniformity. The x-ray full-width at half-maximum (FWHM) of the MBE-grown CdZnTe/Si increases with composition, which is a characteristic of CdZnTe grown by vapor phase epitaxy, and is essentially equivalent to our results obtained on CdZnTe/GaAs/Si. As we have previously observed, the x-ray FWHM of LPE-grown HgCdTe decreases, particularly for CdZnTe compositions near the lattice matching condition to HgCdTe; so far the best value we have achieved is 54 arc-s. Using these MBE-grown substrates, we have fabricated the first high-performance LWIR HgCdTe detectors and 256 x 256 arrays using substrates consisting of CdZnTe grown directly on Si without the use of an intermediate GaAs buffer layer. We find first that there is no significant difference between arrays fabricated on either CdZnTe/Si or CdZnTe/GaAs/Si and second that the results on these Si-based substrates are comparable with results on bulk CdZnTe substrates at 78K. Further improvements in detector performance on Si-based substrates require a decrease in the dislocation density.     

MBE growth and device applications of lattice-matched and strained heterostructures on (n11)-oriented and patterned substrates

The epitaxy of lattice-matched and strained semi-conducting films on patterned and misoriented substrates has led to new growth phenomena, material properties and device applications. Our work on InP- and GaAs-based heterostructures on (111)- and (311)-oriented substrates and strained heterostructures on planar and patterned (small area) substrates is described in this paper. The possibility of reliable and reproducible p-type doping of (311)A GaAs by Si during molecular-beam epitaxial growth and the application of such doping in the realization of high-performance electronic devices have been investigated. It is seen that p-type doping up to a free hole concentration of 4 × 10¹⁹ cm⁻³ is obtained at low ( 500°C) growth temperature and high As₄ flux. The incorporation of Si atoms into electrically active As sites is at least 95%. n-p-n heterojunction bipolar transistors grown by all-Si doping exhibit excellent current voltage characteristics and a common emitter current gain β = 240. Doped channel p-type heterojunction field-effect transistors have transconductance g_m = 25 mS/mm. We have experimentally and theoretically studied piezo-electric field effects in InP-based In_xGa_{1 − x}As/In_0.52Al_0.48As pseudomorphic quantum wells grown by molecular-beam epitaxy on (111)B InP substrates. The electro-optic coefficients of this material were measured and found to be much larger than that of GaAs. We have also investigated the consequences of altered growth modes on the epitaxy of highly strained InGaAs on patterned small area (001) GaAs substrates. Al_0.15Ga_0.85As/In_0.25Ga_0.75As pseudomorphic modulation-doped field-effect transistors and strained In_xGa_{1 − x}As/GaAs p-i-n photodiodes have been fabricated on patterned (100)-GaAs substrates and characterized. Compared with devices made on planar substrates, small area growth improves the dc transconductance by 40% and current gain cutoff frequency by 50% in the transistors. Photodiodes grown in small recesses (30 μm) exhibit 2–4 times higher quantum efficiency than those on planar substrates.     

High-performance AlGaAs/GaAs SDHTs and ring oscillators grown byMBE on Si substrates

High-performance AlGaAs/GaAs selectively doped heterojunction transistors (SDHTs) and 19-stage oscillators fabricated on silicon substrates are discussed. Epitaxial layers of AlGaAs/GaAs were grown by MBE on Si substrates. The mobility of two-dimensional electron gas (2DEG) in the SDHTs was as high as 53000 cm²/V-s at 77 K for a sheet charge density of 10×1¹² cm^-2. For 1-μm-gate-length devices, maximum transconductances of 220 and 364 mS/mm were measured at 300 and 77 K, respectively, for the SDHTs. A minimum propagation delay time of 27 ps/stage at room temperature was obtained for a 19-stage direct-coupled FET logic ring oscillator with a power dissipation of 1.1 mW/stage. The propagation delay time was reduced to 17.6 ps/stage at 77 K. From microwave S-parameter measurements at 300 K, current gain and power gain cutoff frequencies of 15 and 22 GHz, respectively, were measured. These results are comparable to those obtained for SDHT technology on GaAs substrates     

GaAs MESFET's fabricated on monolithic GaAs/Si substrates

GaAs MESFET's have been fabricated for the first time on monolithic GaAs/Si substrates. The substrates were prepared by growing single-crystal GaAs layers on Si wafers that had been coated with a Ge layer deposited by e-beam evaporation. The MESFET's exhibit good transistor characteristics, with maximum transconductance of 105 mS/mm for a gate length of 2.1 µm.     

High speed III-V electrooptic waveguide modulators at λ-1.3μm

Traveling wave GaAs electrooptic waveguide modulators at a wavelength of 1.3 μm with bandwidth in excess of 20 GHz have been developed and characterized. The design and characteristics of both p-i-n modulators in microstrip configuration and Schottky barrier on n ^--GaAs/semi-insulating (SI) GaAs in the coplanar strip configuration modulators are discussed. It is shown that microwave loss and slowing on n⁺ GaAs substrates will limit the bandwidth of the microstrip modulator to less than 10 GHz for a device 8 mm in length. Modulators with bandwidths in excess of 10 GHz are fabricated on SI GaAs substrates     

Microwave properties of self-aligned GaAs/AlGaAs heterojunction bipolar transistors on silicon substrates

(Al,Ga)As/GaAs heterojunction transistors (HBT's) grown on Si substrates have been characterized at microwave frequencies and have been found to perform extremely well. For emitter dimensions of 4 × 20 µm², current gain cutoff frequenciesf_{T} = 30GHz and maximum oscillation frequenciesf_{max}of 11.5 GHz have been obtained in a mesa-type structure. These values compare very well with thef_{T} = 40GHz andf_{max} = 26GHz which are the highest reported for HBT's on GaAs substrates in a nonmesa structure with an emitter width of ∼ 1.5 µm. These results clearly demonstrate the potential of HBT's in general at microwave frequencies, as well as the viability of GaAs on Si technology.     

X-Band MMIC amplifier on GaAs/Si

A single-stage MMIC feedback amplifier fabricated on GaAs epitaxially grown on high-resistivity Si (6 × 10³Ω.cm) will be described. The GaAs active and buffer layers were grown on 4°-off-     

Laser Annealed and Thermal Annealed Refractory Ohmic Contacts to GaAs

Ohmic contacts to n-type GaAs have been developed for high-temperature device applications up to 300°C. Refractory metallizations were used with epitaxial Ge layers to form the contacts TiW/Ge/GaAs, Ta/Ge/GaAs, Mo/Ge/GaAs, and Ni/Ge/GaAs. Contacts with high dose Si or Se ion implantation (1012 to 1014/cm2) of the Ge/GaAs interface were also investigated. The purpose of this work was to develop refractory ohmic contacts with low specific-contact resistance (~10-6 ?cm2 on 1 x 1017cm-3GaAs) which are free of imperfections, resulting in a uniform n+ doping layer. The contacts were fabricated on epitaxial GaAs layers (n = 2 x 1016 to 2 x 1017 cm-3) grown on n+ ( 2 x 1018 cm-3) or semi-insulating GaAs (at strates. Ohmic contact was formed by both thermal annealing ( at temperatures up to 700°C) and laser annealing (pulsed Ruby). Examination of the Ge/GaAs interface revealed Ge migration into GaAs to form an n+layer. Under optimum laser anneal conditions, the specific contact resistance was in the range 1-5 x 10-6 ?-cm2 (on 2 x 1017cm-3GaAs). Thermally annealed TiW/Ge had a contact resitivity of 1 x 10-6 ? cm2 on 1 x 1017 cm-3 GaAs under optimum anneal conditions. The contacts also showed improved thermal stability over conventional Ni/AuGe contacts at temperatures above 300°C.