Growth Studies on Quaternary AlInGaN Layers for HEMT Application

Quaternary barrier layers for GaN-based high-electron-mobility transistors (HEMT) have recently been a focus of interest because of the possible lattice-matched growth to GaN. This results in a reduction of strain-related defects, while having the option of adjusting the bandgap separately. A further benefit of the quaternary approach is the possibility to achieve high polarization and high carrier mobility simultaneously. This may improve the performance of such devices beyond what is possible with ternary barrier layers. In this work, we report on growth and characterization of Al _x In _y Ga_1−x−y N barrier layers within the range of 16% to 56% Al, 2% to 45% In, and 20% to 82% Ga deposited on conventional GaN buffer layers on sapphire. We present an effective way to change the composition of quaternary layers and discuss the influence of tensile and compressive strain on structural and electrical properties. From high-resolution x-ray diffraction (HRXRD), Rutherford backscattering spectroscopy (RBS), and wavelength-dispersive x-ray spectroscopy (WDX), we determined the compositions and strain states of the AlInGaN layers. The bandgaps (E _g) were obtained by spectroscopic ellipsometry (SE). Hall and van der Pauw measurements on thin heterostructure layers yielded high mobilities in excess of 1550 cm²/V s and 5350 cm²/V s at room temperature and 77 K, respectively.     

Preparation and Thermoelectric Properties of Ag<Subscript>0.5</Subscript>In<Subscript>0.5−<Emphasis Type="Italic">x</Emphasis></Subscript>Pb<Subscript>5</Subscript>Sn<Subscript>4</Subscript>Te<Subscript>10</Subscript>

A series of compounds with composition Ag_0.5In_0.5−x Pb₅Sn₄Te₁₀ (x = 0.05 to 0.20) were prepared by slowly cooling the melts of the corresponding elements, and the effect of In content on the thermoelectric transport properties of these compounds has been investigated. Results indicate that the compounds’ electronic structure is sensitive to In content, and that the carrier concentration of these compounds at room temperature increases from 4.86 × 10¹⁸ cm⁻³ to 3.85 × 10²¹ cm⁻³ as x increases from 0.05 to 0.20. For these compounds, electrical conductivity decreases and Seebeck coefficient increases with increasing In content. Ag_0.05In_0.03Pb_0.5Sn_0.4Te₁₀ shows very low lattice thermal conductivity, and has a maximum dimensionless figure of merit ZT of 1.2 at 800 K.     

Nearly Perfect Electrical Activation Efficiencies from Silicon-Implanted Al<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Ga<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>N with High Aluminum Mole Fraction

Electrical activation studies of Al _x Ga_1−x N (x = 0.45 and 0.51) implanted with Si for n-type conductivity have been made as a function of ion dose and anneal temperature. Silicon ions were implanted at 200 keV with doses ranging from 1 × 10¹⁴ cm⁻² to 1 × 10¹⁵ cm⁻² at room temperature. The samples were subsequently annealed from 1150°C to 1350°C for 20 min in a nitrogen environment. Nearly 100% electrical activation efficiency was successfully obtained for the Si-implanted Al_0.45Ga_0.55N samples after annealing at 1350°C for doses of 1 × 10¹⁴ cm⁻² and 5 × 10¹⁴ cm⁻² and at 1200°C for a dose of 1 × 10¹⁵ cm⁻², and for the Al_0.51Ga_0.49N implanted with silicon doses of 1 × 10¹⁴ cm⁻² and 5 × 10¹⁴ cm⁻² after annealing at 1300°C. The highest room-temperature mobility obtained was 61 cm²/V s and 55 cm²/V s for the low-dose implanted Al_0.45Ga_0.55N and Al_0.51Ga_0.49N, respectively, after annealing at 1350°C for 20 min. These results show unprecedented activation efficiencies for Al _x Ga_1−x N with high Al mole fractions and provide suitable annealing conditions for Al _x Ga_1−x N-based device applications.     

Ohmic contacts ton-type In0.5Ga0.5P

The contact properties of alloyed Ni/Au-Ge/Mo/Au metallization to npoststagger+In_0.5Ga_0.5P epilayers grown by gas-source molecular beam epitaxy on GaAs substrates are reported. A minimum specific contact resistance of 10⁻⁵ Ωcm² was obtained forn = 2 × 10¹⁹ cm⁻³ material after alloying at 360° C for 20 sec. Above this temperature outdiffusion of lattice elements and reactions of the metallization with the In_0.5Ga_0.5P lead to severe morphological changes and degraded contact properties. From the temperature dependence of the contact resistance, thermionic emission was identified as the predominant current transport mechanism in these contacts.     

Study of Zn,Zn−Cd diffusion in In x Ga1−x As

 In this report,the diffusion of Zn,Zn-Cd in In_xGa_(1-x)As is investigated using ZnAs_2 and ZnAs_2+Cd as diffusion sources. The effect of the diffusion temperature,diffusion time,a variety of the diffusion source and composition x of the material on the relation of the(X_j-t~(1/2))are given.The diffusion velocity X_j~2/t of Zn in In_xGa_(1-x)As is faster than that of Zn-Cd in In_xGa_(1-x)As,and at 500-600℃,the surface acceptor concentration is from 1×10~(19)to 2×10~(20)cm~(-3),which is higher than that of Zn in InP.Reduction of contact resistance by use of In_xGa_(1-x)As contact layer for 1.3μm LED can be expected.     

Electrical Activation Studies of Silicon-Implanted Al<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Ga<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>N with Aluminum Mole Fraction of 11% to 51%

Electrical activation studies of Si-implanted Al _x Ga_1−x N with an Al mole fraction of 11% to 51% have been carried out as a function of ion dose and annealing temperature. The Al _x Ga_1−x N samples were implanted at room temperature with Si ions at 200 keV in doses ranging from 1 × 10¹⁴ cm⁻² to 1 × 10¹⁵ cm⁻², and subsequently annealed from 1100°C to 1350°C for 20 min in a nitrogen environment. The maximum electrical activation efficiencies for the Al _x Ga_1−x N samples with an Al mole fraction less than 40% were obtained for samples implanted with the highest Si dose of 1 × 10¹⁵ cm⁻². On the other hand, for the Al _x Ga_1−x N samples with an Al mole fraction more than 40%, nearly perfect activation efficiencies of 99% and 100% were obtained for the samples implanted with the lowest Si dose of 1 × 10¹⁴ cm⁻². The mobility of the Si-implanted Al _x Ga_1−x N samples increased with increasing annealing temperature in spite of the increased number of ionized donors and thus increased impurity scattering, indicating that a greater amount of lattice damage is being repaired with each successive increase in annealing temperature. These results provide suitable annealing conditions for Si-implanted Al _x Ga_1−x N-based devices with an Al mole fraction from 11% to 51%.     

Three-Stage Thin-Film Superlattice Thermoelectric Multistage Microcoolers with a Δ<Emphasis Type="Italic">T</Emphasis><Subscript>max</Subscript> of 102 K

Thin-film Bi₂Te₃- and Sb₂Te₃-based superlattice (SL) thermoelectric (TE) devices are an enabling technology for high-power and low-temperature applications, which include low-noise amplifier cooling, electronics hot-spot cooling, radio frequency (RF) amplifier thermal management, and direct sensor cooling. Bulk TE devices, which can pump heat loads on the order of 10 W/cm², are not suitable in these applications due to their large size and low heat pumping capacity. Recently, we have demonstrated an external maximum temperature difference, ΔT _max, as high as 58 K in an SL thin-film p–n couple. This state-of-the-art couple exhibited a cold-side minimum temperature, T _cmin, of −30.9°C. We regularly attain ΔT _max values in excess of 53 K, in spite of the many significant electrical and thermal parasitics that are unique to thin-film devices. These measurements do not use any complex thermal management at the heat sink to remove the heat flux from the TE device’s hot side. We describe here multistage SL cooling technologies currently being developed at RTI that can provide useful microcooling cold-side temperatures of 200 K. This effort includes a three-stage module employing independently powered stages which produced a ΔT _max of 101.6 K with a T _cmin of −75°C, as well as a novel two-wire three-stage SL cascade which demonstrated a T _cmin of −46°C and a ΔT _max of nearly 74 K. These RTI modules are only 2.5 mm thick, significantly thinner than a similar commercial three-stage module (5.3 mm thick) that produces a ΔT _max of 96 K. In addition, TE coolers fabricated from these thin-film SL materials perform significantly better than the extrapolated performance of similar thickness bulk alloy materials.     

Galvanomagnetic properties of two-dimensional electron gases in strain relaxed InxGa1−xAs(x<0.40) heterostructures grown by molecular beam epitaxy on GaAs substrates

We have investigated, as a function of indium content x, the galvanomagnetic and Shubnikov de Haas (SdH) properties of two-dimensional electron gases (2DEG) formed at lattice matched, strain relaxed InAlAs/InGaAs heterojunctions. These were grown by molecular beam epitaxy on GaAs misoriented substrates with a two degree offcut toward the nearest (110) plane. Variable temperature resistivity and Hall measurements indicate an increase in the electron sheet density n_s from 0.78×10¹²cm⁻² for x=0.15 to 1.80×10¹² cm⁻² for x=0.40 at 300K, and from 0.75×10¹²cm⁻² to 1.67×10¹²cm⁻² at T=1.6K. The room temperature electron mobility, measured along the in plane [110], direction is independent of indium content and equals approximately 9500 cm²/Vs. For T<50K, the mobility is independent of temperature decreasing with increasing x from 82000 cm²/Vs for x=0.15 to 33000 cm²/Vs for x=0.40. The ratios (τ_t/τ_q) at 1.6K between the electron relaxation time τ_t and the single particle relaxation time τq, for the strain relaxed specimens, as well as for pseudomorphically strained Al_0.35Ga_0.65As/In_0.15Ga_0.85As structures grown on GaAs substrates, and In_0.52Al_0.48As/In_0.53Ga_0.47As heterostructures grown lattice matched on InP substrates. Such a study indicates the presence of inhomogeneities in the 2DEGs of the strain relaxed specimens which appear to be related to the process of strain relaxation. Such inhomogeneities, however, have little effect on the electron relaxation time τ_t which, at low temperatures, is limited principally by alloy scattering.     

In0.2Ga0.8As/GaAs quantum well laser with C doped cladding and ohmic contact layers

Carbon doping in Al_xGa_1−xAs was achieved using different approaches. The moderate growth temperature of 650°C was employed to grow C bulk-doped Al_xGa_1−xAs with a high Al mole fraction. The hole-density was altered using different V/III ratios. The trimethylaluminum (TMAl) was used as an effective C δ-doping precursor for growth of C δ-doped pipi doping superlattices in Al_xGa_1−xAs. the average hole-density of C δ-doped pipi superlattices was greater than 2−3 × 10¹⁹ cm⁻³. Zn-free GRINSCH In_0.2Ga_0.8As/GaAs laser structures were then grown using the C bulk-doped Al_xGa_1−xAs and C δ-doped pipi superlattice as a cladding and ohmic contact layer, respectively. The ridge waveguide laser diodes were fabricated and characterized to verify flexibility of these two doping approaches for device structures.     

Low pressure metalorganic chemical vapor deposition of InP and related compounds

The low pressure metalorganic chemical vapor deposition epitaxial growth and characterization of InP, Ga_0.47In_0.53 As and Ga_xIn_1-xAs_yP_1-y, lattice-matched to InP substrate are described. The layers were found to have the same etch pit density (EPD) as the substrate. The best mobility obtained for InP was 5300 cm² V⁻¹S⁻¹ at 300 K and 58 900 cm² V⁻¹ S⁻¹ at 77₂K, and for GaInAs was 11900 cm² V⁻¹ S⁻¹ at 300 K, 54 600 cm² V⁻¹ S⁻¹ at 77 K and 90 000 cm V⁻¹S⁻¹ at 2°K. We report the first successful growth of a GaInAs-InP superlattice and the enhanced mobility of a two dimensional electron gas at a GaInAs -InP heterojunction grown by LP-MO CVD. LP MO CVD material has been used for GaInAsPInP, DH lasers emitting at 1.3 um and 1.5 um. These devices exhibit a low threshold current, a slightly higher than liquid phase epitaxy devices and a high differential quantum efficiency of 60%. Fundamental transverse mode oscillation has been achieved up to a power outpout of 10 mW. Threshold currents as low as 200 mA dc have been measured for devices with a stripe width of 9 um and a cavity length of 300 um for emission at 1.5 um. Values of T in the range 64–80 C have been obtained. Preliminary life testing has been carried out at room temperature on a few laser diodes (λ = 1.5μm). Operation at constant current for severalthousand hours has been achieved with no change in the threshold current.     

Thermoelectric Performance of Zn and Nd Co-doped In<Subscript>2</Subscript>O<Subscript>3</Subscript> Ceramics

Polycrystalline In₂O₃ ceramics co-doped with Zn and Nd were prepared by the spark plasma sintering (SPS) process, and microstructure and thermoelectric (TE) transport properties of the ceramics were investigated. Our results indicate that co-doping with Zn²⁺ and Nd³⁺ shows a remarkable effect on the transport properties of In₂O₃-based ceramics. Large electrical conductivity (~130 S cm⁻¹) and thermopower (~220 μV K⁻¹) can be observed in these In₂O₃-based ceramic samples. The maximum power factor (PF) reaches 5.3 × 10⁻⁴ W m⁻¹ K⁻² at 973 K in the In_1.92Nd_0.04Zn_0.04O₃ sample, with a highest ZT of ~0.25.     

New low contact resistance triple capping layer enabling very high Gm InAIAs/lnGaAs HEMTs

It has been demonstrated that a highly doped (Si:3 × 10¹⁹ cm^-3) triple capping layer consisting of n⁺−In_0.53Ga_0.47As, n⁺−In_0.52Al_0.48As, and n⁺-In_0.53Ga_0.47As can remarkably reduce the parasitic source resistance in InP-based high electron mobility transistors (HEMTs). The analysis of the source resistance revealed that the resistance element at the n⁺−In_0.53Ga_0.47As/un−In_0.52Al_0.48As/un-In_0.53Ga_0.47As channel heterointerfaces was as large as 70% of the source resis-tance when nonalloyed ohmic electrodes were used. The highly doped triple capping layer reduces the resistance contribution of vertical conduction between the capping layer and 2DEG channel. A low source resistance of 0.57 Ωmm and a low contact resistivity of 3 × 10⁻⁵ Ωcm² were obtained for the HEMTs with the highly doped triple capping layer, which were 60% lower and one order of magnitude smaller than those for the HEMTs with a conventional single capping layer doped 5 × 10¹⁸ cm⁻³, respectively. These values were also 70 and 30% lower than those for the HEMTs with a highly doped (3 × 10¹⁹ cm⁻³) single capping layer, respectively. The low source resistance brings high peak extrinsic transconduc-tance of 1 S/mm for a device with 0.4 μm long gate, which was 42% higher than the previously reported HEMTs with the same gate length.     

Visible light-emitting diodes grown on optimized ∇x[InxGa1−x]P/GaP epitaxial transparent substrates with controlled dislocation density

Epitaxial transparent-substrate light-emitting diodes (ETS-LEDs) have been fabricated on optimized graded buffers of In_xGa_1−xP on GaP (∇_x[In_xGa_1−x]P/GaP) that feature controlled threading dislocation densities of 3×10⁶ cm⁻². The ETSLEDs show increasing efficiency from 575 nm to 655 nm, in marked contrast to previous reports where performance drops above 600 nm, and feature the lowest spectral widths ever reported in ∇_x[In_xGa_1−x]P/GaP. The improvement over earlier reports is attributed to large mean dislocation spacings in optimized ∇_x[In_xGa_1−x]P/GaP, which are an order of magnitude greater than the mean carrier diffusion length. A slight performance decline remains at 655 nm, but the overall performance of this first generation of ETS-LEDs is promising.     

Electron mobility and drift velocity in selectively doped InAlAs/InGaAs/InAlAs heterostructures

An increase in the electron mobility and drift velocity in high electric fields in quantum wells of selectively doped InAlAs/InGaAs/InAsAs heterostructures is obtained experimentally via controlling the composition of semiconductors forming the interface. The electron mobility at the interface in the In_0.8Ga_0.2As/In_0.7Al_0.3As metamorphic structure with a high molar fraction of In (0.7–0.8) is as high as 12.3 × 10³ cm² V⁻¹ s⁻¹ at room temperature. An increase in the electron mobility by a factor of 1.1–1.4 is attained upon the introduction of thin (1–3 nm) InAs layers into a quantum well of selectively doped In_0.53Ga_0.47As/In_0.52Al_0.48As heterostructures. A maximal drift velocity attains 2.5 × 10⁷ cm/s in electric fields of 2–5 kV/cm. The threshold field F _th for the intervalley Γ-L electron transfer (the Gunn effect) in the InGaAs quantum well is higher than in the bulk material by a factor of 2.5–3. The effect of two- to threefold decrease in the threshold field F _th in the InGaAs quantum well is established upon increasing the molar fraction of In in the InAlAs barrier, as well as upon the introduction of thin InAs inserts into the InGaAs quantum well.     

Materials characteristics of pseudomorphic high electron mobility transistor structures with InxGa1−xas single quantum well and GaAs-InxGa1−xas (0.25 < x < 0.4) thin strained superlattice active layers

Growth and characterization results are presented for pseudomorphic high electron mobility transistor structures with In_xGa_1-xAs single quantum well and GaAs(h ₁)In _x Ga_1−x As(h ₂) thin strained superlattice active layers where 0.25≤x ≤ 0.4. All of the samples were grown by molecular beam epitaxy. Hall effect at 77 K, photoluminescence at 2 K, in-situ reflection high energy electron diffraction, and transmission electron microscopy measurements are discussed. Critical layer thickness measurements are compared with the Matthews-Blakeslee theory. Photoluminescence transition energies are compared with a self-consistent solution to Schrodinger’s and Poisson’s equations.     

MOVPE grown Ga1−xInxAs solar cells for GaInP/GaInAs tandem applications

Lattice-mismatched Ga_1−xIn_xAs solar cells with an absorption edge between 900 and 1150 nm have been grown on GaAs substrates. Different graded Ga_1−xIn_xAs buffer layers and solar cell structures were evaluated to achieve a good electrical performance of the device. External quantum efficiencies comparable to our best GaAs solar cells were measured. The best 1 cm² cell with a bandgap energy of 1.18 eV has an efficiency of 22.6% at AM1.5g and a short circuit current density of 36.4 mA/cm². To our knowledge, this is the highest reported efficiency for a Ga_0.83In_0.17As solar cell.     

Thermoelectric Properties of an Al-Doped In-Sn-Te-Based Alloy

In-Sn-Te-based alloys usually have low electrical and thermal conductivity. In the present work we substituted Al for In in an In-Sn-Te alloy and prepared an (In_1.9Al_0.1Te₃)_0.08(SnTe)_0.92 alloy by spark plasma sintering. Substitution of Al for In favors the formation of indium impurity levels in this structure and accounts for the decrease of the band gap (E _g) and much of the increase of mobility and electrical conductivity. The thermal conductivity decreases from 1.72 W K⁻¹ m⁻¹ to 1.44 W K⁻¹ m⁻¹ with temperature, while that of the (In₂Te₃)_0.08(SnTe)_0.92 alloy increases from 2.29 W K⁻¹ m⁻¹ to 3.50 W K⁻¹ m⁻¹. The thermoelectric figure of merit (ZT) of the sample increases with measurement temperature, and the highest ZT value of 0.28 was obtained at 668 K, being a factor of 4.5 greater than the maximum ZT value for the Al-free (In₂Te₃)_0.08(SnTe)_0.92 alloy at 510 K.     

Electric and photoelectric properties of n-GaxIn1−x N/p-Si anisotypic heterojunctions

We have developed a technology for producing n-type Ga_xIn_1−x N/p-Si heterostructures by combined pyrolysis of indium and gallium monoammoniate chlorides, making it possible to obtain heterolayers with composition varying over wide limits (from GaN up to InN). The composition and basic electric and optical characteristics of nitride films were determined. The electric and photoelectric properties of the heterostructures with Ga_xIn_1−x N films of different composition were investigated. It was shown that the anisotypic heterojunction n-Ga_xIn_1−x N/p-Si is a promising photosensitive element for detecting visible-range radiation. The maximum values of the specific detectivity were D*=1.2×10¹¹ Hz^1/2·W⁻¹ at 290 K. A band diagram of the heterojunction was constructed. Fiz. Tekh. Poluprovodn. 32, 461–465 (April 1998)     

The preparation of Ga-rich GaxIn1−xSb alloy crystals

Radially homogeneous bulk single crystals of Ga_xIn_1−xSb have been grown by the Czochralski technique in the range 0.9 < x < 1. High seed rotation rate and appropriate shaping of the crystal boule are essential for the suppression of interface breakdown phenomena. Electron microprobe analyses of the first to freeze ends of the crystal necks produced by Czochralski pulling reveal effective distribution coefficients close to the corresponding equilibrium distribution coefficients. The etch pit density on (111) A faces of Czochralski pulled Ga_xIn_1−xSb crystals is < 10⁴cm⁻². Their electrical properties are similar to those of zone leveled Ga_xIn_1−xSb crystals.     

Temperature-dependent minority-carrier lifetime measurements of red AlGaAs light emitting diodes

Electroluminescent decay and internal quantum efficiency measurements are made as a function of temperature on two double heterostructure AIGaAs light emitting diodes (LEDs) that emit in the visible (red) portion of the spectrum. The electroluminescent lifetimes increase by more than a factor of ten and the internal quantum efficiency falls by a factor of three as the temperature is raised from 90 to 400K. By analyzing the data with a model that accounts for the transfer with increasing temperature of the minority-carrier electrons from the direct-gap to the indirect-gap minima in the p-type active layer of these near-crossover LEDs, values for the radiative and nonradiative lifetimes as a function of temperature are obtained. A fit to the radiative-lifetime data results in an estimate of 1.3 × 10⁻¹⁰ cm³s⁻¹ for the room-temperature radiative recombination coefficient of Al_0.39Ga_0.61As, which is very similar to values reported for GaAs. The nonradiative lifetimes are found to be nearly independent of temperature from 220 to 400K and provide upper limits of 940 and 1250 cms⁻¹ for the interface recombination velocities of the two samples. These values are roughly an order of magnitude lower than any previously reported values for high-Al-content (x > 0.3) Al_xGa_1−xAs heterostructures.