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Heterojunction bipolar transistors (HBTs) are becoming increasingly important for highspeed electronic applications. This paper will discuss how the unique growth chemistry of metalorganic molecular beam epitaxy (MOMBE) can be used to produce high performance HBTs. For example, it has been well documented that MOMBE's ability to grow heavily doped, well-confined layers of either n- or p-type is a significant advantage for this device. This feature arises primarily from the ability to use gaseous dopant sources in the absence of interfacial gas boundary layers. While this is an advantage for doping, it can be a disadvantage in other areas such as AlGaAs purity or InGaP lattice matching. This paper will discuss how these difficulties can be overcome through the use of novel Al or Ga precursors thus allowing deposition of high quality GaAs-based HBTs. By using trimethylamine alane (TMAA), background impurity concentrations can be reduced substantially. Further improvements in purity require cleaner Ga precursors or alternatively novel Ga substitutes. The resulting reduction in compensation allows for the use of lower dopant concentrations in the AlGaAs thus producing significant improvement in the leakage behavior of the base-emitter junction. Even further improvement can be achieved through the use of InGaP. Using novel Ga precursors, such as tri-isobutylgallium (TIBG), the problems associated with the sensitivity of composition to growth temperature are greatly reduced, allowing for the reproducible deposition of devices containing InGaP emitter layers.
Process technologies for self-aligned AlGaAs/GaAs and lnGaP/GaAs heterojunction bipolar transistors (HBTs) as well as dry etching fabrication schemes for submicron AlGaAs/GaAs based field effect transistors (FETs) are presented. Multiple energy F+ and H+ ions were used to isolate the active devices for HBTs. The resistance of test wafers at 200 °C showed no change over periods of 50 days. Highly selective dry and wet etch techniques for InGaP/GaAs and AlGaAs/GaAs material systems were used to uniformly expose junctions. Reliability of the alloyed ohmic contact and feasibility of the non-alloyed ohmic contact metallizations for both p and n type GaAs layers will be discussed. The reproducible gate recess etching is one of the critical steps for AlGaAs/GaAs based FETs. The etching selectivity, damage, pre and post-clean procedure were studied in terms of device performance. A simple low temperature SiNx deposition and an etch-back process with optical stepper were used to demonstrate 0.1 μm Y-shape gate feature.
Over the past 10 years, heterojunction bipolar transistors (HBTs) have progressed to where integrated circuit (IC) products are being sold and foundry services are being commercially offered utilizing gallium arsenide (GaAs) based technology. We will discuss, here, an alternative HBT technology based on indium phosphide (InP). While this technology is less mature than its GaAs counterpart, it offers several attractive benefits in comparison with GaAs. These benefits are provided through several key material properties of InP and ternary compound semiconductors, eg. gallium indium arsenide (GaInAs), grown on the InP. We review the status of this npn HBT technology and present performance results which illustrate the benefits of the technology with respect to electronic applications. Finally, we present measured reliability data for this technology which shows outstanding projected lifetimes.
Pseudomorphic HEMT (PHEMT) devices have demonstrated superior performance at microwave and millimeter wave frequency ranges. They exhibit multi-functional characteristics such as high power, high efficiency and low noise over a broad frequency range (C-band through W-band). Because of their important, broad range of applications, the microwave industry has recently begun developing the PHEMT manufacturing technology in order to cope with the increasing demand for PHEMT insertion into microwave products. In this paper we will discuss the advantages of using PHEMT devices for microwave and millimeter wave applications and provide an overview of the published state-ofthe-art performance of PHEMT devices and monolithic microwave integrated circuits (MMICs) at microwave and millimeter wave frequencies. We will also present recent progress in Hughes PHEMT technology development including device epitaxial design and process enhancements. Finally, we will present recent data on PHEMT reliability.
Lattice matched InP HEMT has demonstrated superior gain and noise figure performance compared to the AlGaAs HEMT and PHEMT. The gain and noise figure advantages of the InP HEMT have been transferred to the excellent MMIC performance in the millimeter-wave region.
A pseudomorphic Ga0.1In0.9P/InP MESFET grown by low pressure metalorganic chemical vapor deposition(LP-MOCVD) has been fabricated and characterized. The results indicated a transconductance of 66.7 ms/mm and a saturation drain current (Idss) of 55.6 mA have been achieved; furthermore, the Schottky barrier on InGaP as high as 0.67eV can be obtained using Pt2Si as the gate material. For comparison, a conventional InP MESFET with 5μm gate length has also been fabricated on InP epitaxial layer grown by low pressure metalorganic chemical vapor deposition on Fe-doped semi-insulating InP substrate. The transconductance and Idss were found to be 46.7 mS/mm and 43.1 mA at zero gate, respectively, for the depletion mode n-channel MESFET with Au as the gate metal; whereas, for the MESFET using Pt2Si as the gate metal, a transconductance of 40.3 mS/mm and a saturation drain current of 41.1 mA at zero gate bias have been obtained. The results indicated that Ga0.1In0.9P/lnP MESFET has better performance than InP MESFET because of higher energy gap of Ga0.1In0.9P.
Nanoscale gated quantum wires in GaAs MODFET material with the conduction channel and gates in the plane of the 2DEG have been fabricated and studied. Electron beam lithography was used for mask definition followed by flood exposure to low energy argon ions (150 eV) for pattern transfer into the 2DEG. Compared to metal top-gate designs the in-plane design simplifies fabrication and reduces device capacitance, promising ultra-fast operation. This method of pattern transfer produced devices having channel-to-gate isolation of 1014 Ω and breakdown fields above 106 V/cm at 4.2 K. In addition to exhibiting standard FET characteristics, including gating to pinchoff, the devices showed significant negative differential resistance (NDR) in the saturation region.
There is considerable interest in InGaAs/GaAs strained quantum well lasers for applications within the 0.9 to 1.1 μm wavelength range, such as high power lasers for Efdoped fiber amplifiers and rare-earth-ion solid state lasers, obtaining blue-green laser emission by frequency doubling, and optoelectronic integrated circuits. Epitaxial growth of these structures by organometallic vapor phase epitaxy, molecular beam epitaxy, and gas source molecular beam epitaxy will be discussed. The relative merits of AlGaAs and InGaP cladding layers will be examined with respect to growth challenges, laser processing and performance, and device reliability. Several device structures which provide transverse and lateral confinement will be reviewed. Reduction of the transverse far-field angle, which improves fiber coupling efficiency, can be accomplished through the use of periodic index separate confinement and depressed index cladding heterostructures. The performance of ridge waveguide lasers, which require accurate control of the ridge height, can be improved through the incorporation of etch-stop layers, either InGaP or AlAs, in the AlGaAs cladding layer. Nonplanar growth over mesas etched into the substrate is a convenient method to obtain buried heterostructure lasers. Carbon-doped planar InGaAs/AlGaAs lasers, using CC14 as an extrinsic dopant, have been fabricated with an impurity-induced layer disordering process. Microcylinder lasers have been fabricated out of InGaAs/InGaP layer structures.
We discuss the characteristics of MOMBE based selective area epitaxy useful in the preparation of optoelectronic devices. Selective area epitaxy, a process in which epitaxy is restricted only to the areas opened in a suitably prepared dielectric mask, offers a powerful method of preparing high performance devices, varying the thickness and composition of the grown layers simply by controlling the width of the open areas and monolithically integrating different device types on common substrates. Lasers, heterostructure bipolar transistors, and optoelectronic integrated circuits based on InGaAs/InP system and relying on selective area epitaxy are described.
GaInAs/GaAs/GaInP multiquantum well laser structures have been grown by chemical beam epitaxy (CBE) using conventional sources (hydrides as group V element sources). Large area lasers were photolitographically defined and mounted for continuous wave (CW) measurements. CW output power levels of 600 mW at 25°C are reported from 100 μm wide, 300 μm long laser diodes without any facet treatment. At these levels, the delivered current is 2A, with an associated voltage of less than 1.7 V. The characteristic temperature of the structure is 95 K.
The same structures were then grown using tertiarybutylarsine (TBAs) and tertiarybutylphosphine (TBP). The large area laser diodes were characterized under pulsed conditions. For a 300 μm long cavity, threshold current density of 390 A/cm2 and external quantum efficiency of 0.6 W/A (2 facets) were obtained, demonstrating the suitability of TBP and TBAs as substitutes of arsine and phosphine in chemical beam epitaxy for laser fabrication.
GaAs-InGaAs quantum well laser structures were fabricated using a 5 MeV O+ implant (∼1015 cm−2 dose) to disorder the quantum well for optical isolation upon post-implant annealing. End-of-range disorder is placed in the underlying substrate, and consisted of small dislocation loops. Electrical isolation was provided by a subsequent multiple energy (40-300 keV) O+ implant scheme. Masking for both implant steps was obtained using a lift-off Au deposition. This fully planar process is considerably simpler than the Si diffusion process for quantum well disordering that is commonly employed for 0.98 gim laser fabrication. A discussion will be given of the relative advantages and disadvantages of the two processes, with particular emphasis on reliability issues.
In this paper we report a modified Kroemer's analysis for the determination of the band offset (ΔEc) of single quantum well (SQW) structures from simple C-V measurements. The experimental carrier profile from an MOVPE grown pseudomorphic GaAs/InGaAs/GaAs strained SQW structure shows a sharp accumulation peak bounded by depletion regions on either side. The full width at half maximum of the accumulation peak is comparable to the width of the quantum well. The value of ΔEC obtained from C-V measurement is in good agreement with the values determined by simulation and photoluminescence measurements. DLTS measurements on our SQW samples do not show any peaks which is contrary to the published reports. We believe that it is necessary to carefully isolate the role of interface states, before assigning a DLTS peak to emission from the quantum well.
A study of material and optical characteristics of InGaAsPAnGaP lasers grown on GaAs (100) substrate, Operating at λ = 0.8μm was carried out. The main features of InGaAsPAnGaP lasers over the conventional AlGaAs/aCAs; ones were not only stable epitaxial layer growth due to the aluminum-free material, but also strong resistance to the degradation. The other benefits of InGaAsP/inGaP laser over AlGaAs/GaAs one was the quantum-well structure growth by a simple version of liquid-phase epitaxy (LPE), which might be irnispensable to the high power operation of a semiconductor laser. The separate confinement heterostructure single quantum weil(SCH-SQW) InGaAsP/nGaP lasers were fabricated for the material charaterization of quaternary systen. The local temperature rise at the mirror facet of a InGaAsP/InGaP laser, which is the barometer of a reliable operation, was less than 30°C under even above 500 mW CW operation. We presumably attribute this low local temperatur rise of the quaternary system to the fact that it has thenodynamically stable minor facet. The surface analysis of InGaAsP layer by ESCA showed that the formation of indium oxide would prevent the elemental segragation constituting dangling orbital, which is common to A1GaAs material systems.
A planar self-aligned process for fabricating integrated lasers and modulators is described. This process employs a native oxide of AlxGa1−xAs to form the waveguide structure and dielectric passivation layer. Wet oxidation of AlxGa1−xAs is being investigated to determine possible processing parameters that result in good quality oxides and a reliable fabrication process. Variations in the mechanical properties were observed with changes in processing parameters.
In this presentation the various technology steps for the monolithic integration of GaAs quantum well lasers with Double Pulse Doped AlGaAs/GaAs/AlGaAs Quantum Well (DPDQW) E/D HEMT electronics on a single substrate in one process run are described. All layers are grown by molecular beam epitaxy. The laser structure, consisting of three 74 Å GaAs quantum wells between two AlGaAs cladding layers, are grown on top of the electronic structure. The laser mesas and contact areas are defined by a combined wet and dry etch process. Apart from the transistor gates which are exposed by electron beam lithography, all lithography steps are performed using contact printing. A two layer metallization is used to interconnect the devices whereby air-bridges are used to connect the laser mesas to the electronics. First results showed laser action of laser diodes of area 3 x 300 μm2 at a threshold current of less than 60 mA, as well as the operation of different electronic devices on wafers which have been processed in this way. These include a laser diode driver, and an optoelectronic receiver with a MSM photo diode, both devices operating at a data rate of 5 Gbit/sec. These results indicate that the process sequence described is suitable for the integration of laser diodes and HEMT electronics.
Generation and recombination mechanisms at heterojunction interfaces are quantitatively discussed for lattice-matched (AlGaAs/GaAs) and lattice-mismatched (InGaAs/InP) systems. The effect of increased interface recombination on photon recycling and carrier diffusion through the interface region are estimated through a calculation based on the ambipolar diffusion equation. Experimental photoluminescence power dependencies, revealing information about generation and recombination mechanisms, fit well with calculated photoluminescence intensities. Lifetimes and interface extent can be determined by these fits.
Unusually large transverse magnetic component has been observed at energy corresponding to the edge of heavy hole band in the optical emission from unstrained and strained layer single quantum well lasers above threshold condition. The existing model of Fermi sea shake-up is inadequate to explain the enhancement in the TM component beyond lasing threshold. Our results indicate that under lasing conditions the directional properties of the emitting dipole arising from electronic transitions to the heavy hole band is modified such that the dipole moment has equal projections in x, y and z directions.
Two techniques for fabricating through-wafer via holes in 2-4 mil thick GaAs substrates were examined. In the first, Ni or thick photoresist masks were used for patterning 30 μm diameter vias by ECR-rf dry etching using low pressure (10-20 mTorr), low bias (– 150V) BCI3/C12 discharges. Microwave enhancement of these discharges produced faster etch rates but a greater degree of isotropic material removal at a given pressure. Reducing the process pressure produces extremely anisotropic features with high aspect ratio. The BC13-to-C12 ratio must be kept to ≥5:1 to maintain the anisotropy. A novel laser drilling technique was also examined - in this case a Q-switched beam with high energy density was used to ablate material in each pass of the beam, producing a via in approximately 40 passes. This is a maskless procedure capable of producing any desired via hole pattern, but currently there is no selectivity for ablating GaAs over a front-side metal film.
In this paper we will describe applications of a low temperature SiNx for the novel fabrication of lasers and FET's. Sidewall roughness which appears on dry etched semiconductor laser mesas is a common problem in laser fabrication. Protecting the sidewall with a low temperature PECVD SiNx can greatly reduce laser mesa roughness that occurs during dry etching of the mesa. Another application uses low temperature SiNx to extend the resolution of standard optical replication. Submicron gate fingers in field effect transistors can be fabricated by using this low-temperature SiNx deposition. By depositing SiNx on the photoresist gate pattern and etching back the SiNx leaving a sidewall, this will reduce the opening of gate features. Submicron gate length MESFETs have been demonstrated with this technique which showed comparable results to conventional submicron MESFETs fabricated with E-beam direct writing.
Narrow (1 μm), deep (3.5 μm) laser mesas have been formed on 2”φ InP wafers using stepper lithography and dry etching techniques for both dielectric and semiconductor patterning. Contrast enhancement techniques produce excellent edge acuity and vertical sidewalls on the initial photoresist lines. Pattern transfer to the underlying SiO2 regrowth mask is achieved by ECR SF6/Ar dry etching at 1 mTorr and –100V, conditions which also retain the verticality of the mesa. The semiconductor is etched using an ECR Cl2/CH4/H2/Ar discharge at 0.3 mTorr and –80V, with the sample held at ∼ 150°C. The etch rate under these conditions is ∼1 μm/min, with a selectivity of ≥10:1 for the semiconductor over the dielectric mask. The smooth etched surface and low degree of damage make this process ideal for epitaxial regrowth. The uniformity of each process step is also acceptable (≤7%). Comparison of the elevated temperature Cl2/CH4/H2/Ar mixture with the more conventional room temperature CH4/H2 plasma chemistry will be given.