Marc E. Sherwin

The optimization of InxGa1-xAs and InP growth conditions by CBE

Abstract Minimization of the number of experiments needed to fully characterize and optimize the growth of epitaxial material is the first important step in realizing state of the art device structures. While widely used in some fields such as chemical engineering, response surface modeling (RSM) has been little used in crystal growth applications. Using RSM, input parameters such as substrate temperature hydride injector temperature and V III ratio, were simultaneously adjusted to characterize the crystal growth process. This technique identified interactions among parameters, minimized the number of experiments necessary to understand and optimize the process, and minimized the variability of the growth process. RSM has been applied to the CBE growth of InGaAs and InP with the purpose of generating an operating point at which both good surface morphology and high mobility material can be produced. Although the best 77 K InP mobility was 70,000 cm 2 /V⋯s, in order to improve the surface quality the input parameters were changed so that the final mobility was 37,000 cm 2 /V⋯s. Although the quality of the InGaAs layers showed a dependence on the reactor history, there did not appear to be any sensitivity to variations made in the operating conditions. The best 77 K InGaAs mobility was 62,500 cm 2 /V⋯s.

Investigation and optimization of InGaAs/InP heterointerfaces grown by chemical beam epitaxy using spectroscopic ellipsometry and photoluminescence

Spectroscopic ellipsometry (SE) has been used to investigate transition layers for InGaAs/InP heterointerfaces. For the case of InGaAs on InP, we have found that the samples can be best modeled by a strained InxGa1-xAs film with the possible presence of a thin interface region (15Å). We are unable to conclusively determine the existence of such a thin transition region. For InP on InGaAs, we find clear indications of As contamination in the bulk film, and that the addition of a thin interface region of In0.75Gao0.25As0.5P0.5 improves both the numerical fit and shape of the dielectric response curves, especially around E1 and E1+ Δ1 where the effects of a transition region are most pronounced. However, difficulties in modeling the dielectric response of the contaminated InP film make identification of an interface transition region only speculative at this point. Multiple single quantum well structures have also been grown and analyzed with 7K photoluminescence. The quality of the quantum wells shows strong dependence on the gas switching sequence used at the heterointerfaces. The best switching sequence produced a 0.5 nm well with a 7K FWHM of only 12.3 meV. Multiple quantum wells have also been grown to investigate the uniformity and repeatability of our system. Twenty period MQWs with a well width of 1.6 nm display a 14K FWHM of 7.9 meV.

Theoretical and experimental studies of the effects of compressive and tensile strain on the performance of InP-InGaAs multiquantum-well lasers

The authors have studied, both theoretically and experimentally, the effects of biaxial strain upon the performance characteristics of broad-area InP-InGaAsP-In/sub x/Ga/sub 1-x/As (0.33 >

F/sub max/-enhancement in CBE-grown InAlAs/InGaAs HEMT's using novel self-aligned offset-gate technology

Maximum frequency of oscillation (f/sub max/) to cutoff frequency (f/sub T/) ratios up to 2.7 and good microwave characteristics have been achieved by applying an offset self-aligned gate technology to InAlAs/InGaAs HEMTs (high electron mobility transistors) grown by CBE (chemical beam epitaxy). The microwave G/sub m//G/sub ds/ (output conductance) ratio for the devices was 23.5 using L/sub gd/ (gate to drain length)=0.4 mu m while the corresponding f/sub max/ was 251 GHz. The approach described combines the advantages of self-aligned and offset gate technologies.<<ETX>>

The design of an ECR plasma system and its application to InP grown by CBE

An electron cyclotron resonance (ECR) plasma system has been designed for the purpose of using an excited beam of gases during CBE growth. The system was designed to use hydrogen, nitrogen and argon. An ECR plasma system has the ability to ignite a low pressure and low temperature plasma with very low ion energies, which should minimize any damage to the growing layer. The motivation behind using a plasma during growth is the ability of atomic hydrogen to remove contaminants from the growing layer and to enhance the decomposition of organometallic precursors at low substrate temperatures. InP grown with a hydrogen plasma showed an n-type background carrier concentration of 6.0X1016 cm-3, with a rough surface and a strong photoluminescence peak at 1.378 eV. A control sample grown with excess hydrogen but without the plasma had a background carrier concentration of 1.0X1015 cm-3, a 77 K mobility of 65,000 cm2/V·s and a very weak photoluminescence peak at 1.378 eV. The most likely cause for the layer degradation during plasma growth is an intrinsic defect such as an antisite defect or a vacancy. The n-type nature of the layer and the relatively high carrier concentration would seem to exclude the possibility of carbon or any other unintentional impurities.

InGaAs/InP hot electron transistors grown by chemical beam epitaxy

In this letter, we report on the dc performance of chemical beam epitaxy grown InGaAs/InP hot electron transistors (HETs). The highest observed differential β (dIC/dIB) is over 100. The HETs have Pd/Ge/Ti/Al shallow ohmic base contacts with diffusion lengths less than 300 A. Furthermore, we also demonstrated ballistic transport of electrons in an InGaAs/InP HET by obtaining an energy distribution of electrons with ∼60 meV full width at half maximum. The measured conduction band discontinuity of InGaAs/InP is 250.3 meV, which is 39.8% of the band gap difference.

InAlAs/InGaAs/InP sub-micron HEMTs grown by CBE

The lnAlAs/lnGaAs/InP high electron mobility transistor (HEMT) lattice matched to lnP offers excellent high frequency, low noise operation for MMICs and low-noise amplifiers. The lnP channel in the InP/InAlAs HEMT offers the advantages of improved high field velocity and higher breakdown voltages (the potential for higher power applications) over InGaAs channel HEMTs. InAlAs has been grown for the first time by CBE using TMAA producing InGaAs/lnAlAs and lnP/InAlAs HEMTs. Sub-micron InGaAs/InAlAs HEMTs with planar Si doping have been fabricated with f~values of 150 G1-Izand f,,t,, values of 160 GHz. This device showed excellent pinch-off characteristics, with a maximum transconductance of 890 mS/mm. The planar doped InGaAs channel HEMT had a higher f, than a similar uniformly doped device. However, the non-optimized structure of the planar doped device resulted in a large output conductance of 120 mS/mm, limiting f~,,,for that device. A sub-micron InP channel device was grown with a quantum well channel and double-sided planar Si doping. A sheet charge density of 4.4x 1012 cm2 and associated room temperature mobility of 2800 cm2/Vs were achieved; however, the saturation current was low. The most likely causes for this are diffusion of the planar doping beneath the channel and the poor quality of the InP on InAlAs interface at the bottom of the quantum well channel.

The growth of InGaAsP by CBE for SCH quantum well lasers operating at 1.55 and 1.4 μm

InGaAsP has been grown by CBE at compositions of 1.1, 1.2 and 1.4 μm for the development of MQW-SCH lasers. The observed incorporation coefficients for TMI and TEG show strong temperature sensitivity while the phosphorus and arsenic incorporation behavior is constant over the substrate temperature range explored, 530 to 580°C setpoint. For higher substrate temperatures the growth rate increases with the largest growth rates occurring for the 1.4 μm quaternary. Low temperature photoluminescence indicates the possibility of compositional grading or clustering for the 1.1 μm material and also for the 1.2 μm material grown at the lowest substrate temperature. The final laser structure was grown with the InP cladding regions grown at 580°C with the inner cladding and active regions grown at 555°C. Using this approach we have successfully grown MQW-SCH lasers with the composition of the active InxGa1−xAs ranging from x=0.33 to x=0.73. Threshold current densities as low as 689 A/cm2 have been measured for an 800 μm×90 μm broad area device with x=0.68.

The growth of InAlP using trimethyl amine alane by chemical beam epitaxy

The growth of InAlP and related compounds such as InGaP lattice matched to GaAs has attracted a great deal of interest for optoelectronic devices emitting in the range from 638 to 700 nm and for electronic devices such as the heterojunction bipolar transistor. Although some gas source MBE work has been performed in this material system, very little CBE work has been done, largely attributable to the lack of a suitable aluminum source. This is the first report of trimethyl amine alane (TMAA) being used to grow InAlP. TMAA offers advantages of less carbon incorporation and less oxygen sensitivity compared to triethyl aluminum, tri-isobutyl aluminum, or trimethyl aluminum. Trimethyl amine alane has been used to grow AlGaAs HBTs and more recently to grow InAlAs/InGaAs HEMTs by CBE. One of the principal strengths of CBE is its ability to handle phosphorus based compounds efficiently, offering excellent interface control. InAlP films with carbon concentrations below 7×1017 cm-3 lattice matched |δaa|<2×10-3 with good surface morphology have been grown. Double crystal X-ray diffraction exhibits a single epi-peak with a full width at half max of 46 arc sec with multiple Pendellosung fringes. The epitaxial films are semi-insulating, completely depleted for thicknesses up to 1.6 μm. Oxygen levels measured by secondary ion mass spectroscopy are comparable to levels measured in InAlAs films (∼2.5×1018 cm-3) lattice matched to InP. The likely source of this oxygen is the hydride precursor as has been shown for the growth of InAlAs. As the substrate temperature is raised, the films become increasingly indium-rich. Breaking the growth rate down into its constituent binaries indicates an enhanced TMI incorporation rate. The quality of the films as measured by X-ray full width at half max and the surface morphology is extremely sensitive to substrate temperature. A very narrow window exists for the growth of good quality material in the range from 535 to 545°C.

Parametric investigation of InGaAs/InAlAs HEMTs grown by CBE

The InAlAs/InGaAs high electron mobility transistor offers excellent high frequency, low noise operation for amplifiers. While this material system has been grown primarily by conventional MBE, other growth techniques have been examined for improved throughput. The flexibility of chemical beam epitaxy offers semi-infinite sources, good source stability, efficient phosphorus utilization, and extended uptime (more than 560 growth runs over 1.5 years). However, CBE has only recently been shown to produce excellent quality InAlAs suitable for the growth of InAlAs/InGaAs HEMTs [1]. This is the first parametric investigation of the properties of InAlAs/InGaAs HEMTs grown by CBE. A series of lattice matched, pulse doped HEMTs have been grown in which the dopant dose, spacer layer, and channel thickness were systematically varied. Low field 300 K Hall mobilities as high as 8700 cm2/V·s for a sheet carrier concentration of 3x1012 cm-2have been measured. This mobility is somewhat lower than uniformly doped HEMTs, which have shown mobilities over 10,000 cm 2/V·s at room temperature. A figure of merit, the low field conductivity, has been correlated among the device structure, gateless saturation currents, and DC and microwave device performance. Its applicability as a rough predictor of device performance will be discussed. For a given spacer thickness, the mobility improves as the pulse dose is decreased up to a mobility somewhat below that for uniformly doped structures. As the dopant to channel thickness is increased, this saturated mobility also increases. Secondary ion mass spectroscopy has shown no increase in carbon or oxygen levels at the dopant pulse. This has led to speculation that interface scattering at the top InAlAs/InGaAs interface may be important; however, initial SIMS results do not conclusively show intermixing of the Group III elements at this interface. It is possible that a reduction in the substrate temperature during growth may improve any interface roughness. Results of this modification in growth conditions shall be reported. Self-aligned 0.15 μm HEMTs fabricated from these layers have shown external DC transconductances over 1000mS/mm, unity current gain cutoff frequencies as high as 190 GHz and unity power gain frequencies above 300 GHz. These results and those of more conventional 0.1 μm gate length HEMTs demonstrate the potential of InAlAs/InGaAs HEMTs grown by CBE.