Double pulse doped InGaAs/AlGaAs/GaAs pseudomorphic high-electron-mobility transistor heterostructures

Double pulse doped (δ-doped) InGaAs/AlGaAs/GaAs pseudomorphic high-electron-mobility transistor (HEMT) heterostructures were grown by molecular-beam epitaxy using a multiwafer technological system. The room-temperature electron mobility was determined by the Hall method as 6550 and 6000 cm²/(V s) at sheet electron densities of 3.00 × 10¹² and 3.36 × 10¹² cm⁻², respectively. HEMT heterostructures fabricated in a single process feature high uniformity of structural and electrical characteristics over the entire area of wafers 76.2 mm in diameter and high reproducibility of characteristics from process to process.     

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.     

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.     

A study of chemical beam epitaxy of GaAs using tris-dimethylaminoarsenic

The growth of GaAs by chemical beam epitaxy using triethylgallium and trisdimethylaminoarsenic has been studied. Reflection high-energy electron diffraction (RHEED) measurements were used to investigate the growth behavior of GaAs over a wide temperature range of 300–550°C. Both group III- and group Vinduced RHEED intensity oscillations were observed, and actual V/III incorporation ratios on the substrate surface were established. Thick GaAs epitaxial layers (2–3 μm) were grown at different substrate temperatures and V/III ratios, and were characterized by the standard van der Pauw-Hall effect measurement and secondary ion mass spectroscopy analysis. The samples grown at substrate temperatures above 490°C showed n-type conduction, while those grown at substrate temperatures below 480°C showed p-type conduction. At a substrate temperature between 490 and 510°C and a V/III ratio of about 1.6, the unintentional doping concentration is n ∼2 × 10¹⁵ cm⁻³ with an electron mobility of 5700 cm²/V·s at 300K and 40000 cm²/V·s at 77K.     

Influence of layer structure on performance of AlGaN/GaN high electron mobility transistors before and after passivation

Intentionally undoped and three different, doped layer structures are used to investigate properties of AlGaN/GaN high electron mobility transistors (HEMTs) before and after SiN passivation. For unpassivated devices, the drain current, transconductance, cutoff frequency, and microwave output-power increase with increased doping level, in spite of an increase in the gate-leakage current. After passivation, an overall performance improvement of all devices occurs. The passivation-induced sheet charge decreases from 2×10¹² cm⁻² in undoped structures to ∼0.7×10¹² cm⁻² in higher doped structures and performance improvement with passivation is less pronounced for higher doped devices. However, the output power of unpassivated and passivated devices on higher doped structures is much higher than that on the undoped-passivated counter-part. These results underline an advantage of the doped layer structure for the preparation of high-performance AlGaN/GaN HEMTs.     

High temperature MOVPE growth of GaAs/AlGaAs device structures with tertiarybutylarsine

The performance characteristics of epitaxial structures suitable for optoelectronic and electronic devices were investigated. These were fabricated by MOVPE using tertiary-butylarsine, a non-hydride arsenic source. Minority carrier diffusion lengths of 5μm at 3 × 10¹⁸/cm³ and 2μm at 2 × 10¹⁹/cm³ were achieved inp-type GaAs. Recombination velocities at the GaAs/AlGaAs interface are reduced to 1 × 10³ cm/sec by processing under appropriate conditions. Electron mobilities of 4000 cm²/V-sec inn-type (2 × 10¹⁷/cm³) layers resulted in transconductances of 120 mS/mm in 1.5μm gate depletion mode MESFETs. The above values are comparable to those obtained with arsine in this work and others reported in the literature.     

OMVPE growth of p-AlGaAs/GaAs heterojunctions using diethylberyllium

We report on the OMVPE growth of modulation doped p-type Al_0.43Ga_0.57As(Be)/GaAs heterojunctions which exhibit a two-dimensional hole gas (2DHG). Hole mobilities de-termined by Hall or cyclotron resonance measurements at 300, 77, and 4 K were 394, 3750, and 21200 cm²/V bs s respectively for a sheet carrier density of about 4.5 × 10¹¹ cm^s−2. Beryllium doping of Al_xGa_1−xAs using diethylberyllium is characterized by Hall measurements, secondary ion mass spectrometry, and photoluminescence. The depen-dence of free carrier concentrationvs AlAs% forp ⁺ layers of Al_xGa_1−xAs_x,x = 0–0.5, is determined. A free carrier concentration greater than 1 × 10¹⁸ cm^s−3 is achieved forx = 0–0.43 with no carrier freeze-out down to 77 K.     

Electron and Hole Capture Cross-Sections of Fe Acceptors in GaN:Fe Epitaxially Grown on Sapphire

Carrier trapping of Fe³⁺/Fe²⁺ deep acceptors in epitaxially grown GaN:Fe on sapphire was studied by time-resolved photoluminescence. For the investigated Fe doping levels on the order of 10¹⁸ cm⁻³, the luminescence decay times are strongly dependent on the Fe concentration, indicating that Fe centers act as predominant nonradiative recombination channels. Linear dependence of the decay time on the iron concentration allows estimation of the electron capture cross-section for the Fe³⁺ ions, which is equal to 1.9 × 10⁻¹⁵ cm². The upper bound for the cross-section of the hole capture of Fe²⁺ was evaluated as 1 × 10⁻¹⁵ cm².     

Electrical properties of molecular beam epitaxial GaAs grown at 300–450°C

We use the Hall effect and a new charge-transfer technique to study molecular beam epitaxial GaAs grown at the low substrate temperatures of 300–450°C. Layers grown from 350–450°C are semi-insulating (resistivity greater than 10⁷ Ω-cm), as grown, because of an As_Ga-related donor (not EL2) at E_C-0.65 eV. The donor concentrations are about 2×10¹⁸ cm⁻³ and 2×10¹⁷ cm⁻³ at growth temperatures of 300 and 400°C, respectively, and acceptor concentrations are about an order of magnitude lower. Relatively high mobilities (∼5000 cm²/V s) along with the high resistivities make this material potentially useful for certain device applications.     

Process parameter dependence of impurity-free interdiffusion in GaAs/AlxGa1−xAs and InxGa1−yAs/GaAs multiple quantum wells

The dependence of the impurity-free interdiffusion process on the properties of the dielectric cap layer has been studied, for both unstrained GaAs/Al_xGa_1−xAs and pseudomorphic In_y Ga_1−yAs/GaAs MQW structures grown by molecular beam epitaxy. The influence of the cap layer thickness, composition, and deposition technique on the degree of interdiffusion were all systematically investigated. Electron-beam evaporated SiO₂ films of varying thickness, chemical-vapor-deposited SiO_xN_y films of varying composition, and spin-on SiO₂ films were used as cap layers during rapid thermal annealing (850-950°C). Photoluminescence at 10K has been employed to determine the interdiffusioninduced bandgap shifts and to calculate the corresponding Al-Ga and In-Ga interdiffusion coefficients. The latter were found to increase with the cap layer thickness (e-beam SiO₂) up to a limit determined by saturation of the outdiffused Ga concentration in the SiO₂ caps. A maximum concentration of [Ga] = 4–7 ×10¹⁹ cm⁻³ in the SiO₂ caps was determined using secondary ion mass spectroscopy profiling. Larger band-edge shifts are also obtained when the oxygen content of SiO_xN_y cap layers is increased, although the differences are not sufficiently large for a laterally selective interdiffusion process based on variations in cap layer composition alone. Much larger differences are obtained by using different deposition techniques for the cap layers, indicating that the porosity of the cap layer is a much more important parameter than the film composition for the realization of a laterally selective interdiffusion process. For the calculated In_0.2Ga_0.8As/GaAs interdiffusion coefficients, activation energies E_A and prefactors D_o were estimated to ranging from 3.04 to 4.74 eV and 5 × Kh⁻³ to 2 × 10⁵ cm²/s, respectively, dependent on the cap layer deposition technique and the depth of the MQW from the sample surface.     

20-nm enhancement-mode metamorphic GaAs HEMT with highly doped InGaAs source/drain regions for high-frequency applications

In this article, the DC and RF performance of a SiN passivated 20-nm gate length metamorphic high electron mobility transistor (MHEMT) on GaAs substrate with highly doped InGaAs source/drain (S/D) regions have investigated using the Synopsys TCAD tool. The 20-nm enhancement-mode (E-mode) MHEMT device also features δ-doped sheets on either side of the In_0.53Ga_0.47As/InAs/In_0.53Ga_0.47As channel which exhibits a transconductance of 3100 mS/mm, cut-off frequency (f_T) of 740 GHz and a maximum oscillation frequency (f_max) of 1040 GHz. The threshold voltage of the device is found to be 0.07 V. The room temperature Hall mobilities of the 2-dimensional sheet charge density are measured to be over 12,600 cm²/Vs with a sheet charge density larger than 3.6 × 10¹² cm^?2. These high-performance E-mode MHEMTs are attractive candidates for sub-millimetre wave applications such as high-resolution radars for space research, remote atmospheric sensing, imaging systems and also for low noise wide bandwidth amplifier for future communication systems.     

MOCVD grown CdZn Te/GaAs/Si substrates for large-area HgCdTe IRFPAs

Large-area HgCdTe 480×640 thermal-expansion-matched hybrid focal plane arrays were achieved by substituting metalorganic chemical vapor deposition (MOCVD)-grown CdZnTe/GaAs/Si alternative substrate in place of bulk CdZnTe substrates for the growth of HgCdTe p-on-n double-layer heterojunctions by controllably-doped mercury-melt liquid phase epitaxy (LPE). (100) CdZnTe was grown by MOCVD on GaAs/Si using a vertical-flow high-speed rotating disk reactor which incorporates up to three two-inch diameter substrates. Layers having specular surface morphology, good crystalline structure, and surface macro defect densities <50 cm⁻² are routinely achieved and both the composition uniformity and run-to-run reproducibility were very good. As the composition of the CdZnTe layers increases, the x-ray full width at half maximum (FWHM) increases; this is a characteristic of CdZnTe grown by VPE techniques and is apparently associated with phase separation. Despite a broader x-ray FWHM for the fernary CdZnTe, the FWHM of HgCdTe grown by LPE on these substrates decreases, particularly for [ZnTe] compositions near the lattice matching condition to HgCdTe. An additional benefit of the ternary CdZnTe is an improved surface morphology of the HgCdTe layers. Using these silicon-based substrates, we have demonstrated 78K high-performance LWIR HgCdTe 480×640 arrays and find that their performance is comparable to similar arrays fabricated on bulk CdZnTe substrates for temperatures exceeding approximately 78K. The performance at lower temperatures is apparently limited by the dislocation density which is typically in the low-mid 10⁶ cm⁻² range for these heteroepitaxial materials.     

Si-implantation into GaAs grown on Si

200 keV Si implantations were performed in the dose range of 5 × 10¹² − 1 × 10¹⁴ cm⁻² in GaAs grown on Si. For comparison implants were also performed in GaAs layers grown on GaAs substrates. Implanted layers were annealed by both furnace and halogen lamp rapid thermal anneals. Significantly lower donor activations were observed in GaAs layers grown on Si substrates than in the layers grown on GaAs substrates. Extremely low dopant activations were obtained for Be implants in GaAs grown on Si. Photoluminescence and photoreflectance measurements were also performed on the implanted material.     

Large-area uniform OMVPE growth for GaAs/AIGaAs quantum-well diode lasers with controlled emission wavelength

The properties of GaAs and AlGaAs epilayers grown in a vertical rotating-disk OMVPE reactor operated at reduced pressure (0.2 atm) are extremely uniform. For substrate rotation at 500 rpm, the thickness uniformity is ±1% for thick epilayers and ±2% for quantum wells 3−10 nm thick. The coefficient of variation in aluminum composition is 1.8 × 10⁻³ or less. For broad-area GRIN-SCH diode lasers containing a single-quantumwell active layer, the threshold current density and differential quantum efficiency are highly uniform. The laser emission wavelength is precisely controlled by adjusting the active layer thickness and composition. For 175 devices distributed over a 16-cm² wafer containing a 10-nm-thick Al_0.07Ga_0.93As active layer, the total variation in emission wavelength is 3.0 nm. For all of these devices and for test devices from nine additional wafers, the wavelengths range from 803.5 to 807.4 nm.     

High electron mobility in (InAs)n(GaAs)n short period superlattices grown by MOVPE for high-electron mobility transistor structure

(InAs)_n(GaAs)_n short period superlattices (SPSs) have been successfully grown by a continuous MOVPE process on InP substrates. Their structural, optical, and electrical properties have been studied. The periodic structures have been confirmed by x-ray measurements and (InAs)₁(GaAs)₁ SPSs have been clearly observed by transmission electron microscopic characterization. The optical quality of the material has been tested by 2K photoluminescence and excitonic recombinations have been observed. Mobilities as high as 10700 cm².V⁻¹.s⁻¹ and 64000 cm². V⁻¹.s⁻¹ for a sheet concentration of 3 × 10¹² cm⁻² have been obtained at 300K and 77K, respectively.     

Performance comparison of AlGaAs,GaAs and InGaP tunnel junctions for concentrated multijunction solar cells

Four tunnel junction (TJ) designs for multijunction (MJ) solar cells under high concentration are studied to determine the peak tunnelling current and resistance change as a function of the doping concentration. These four TJ designs are: AlGaAs/AlGaAs, GaAs/GaAs, AlGaAs/InGaP and AlGaAs/GaAs. Time‐dependent and time‐average methods are used to experimentally characterize the entire current–voltage profile of TJ mesa structures. Experimentally calibrated numerical models are used to determine the minimum doping concentration required for each TJ design to operate within a MJ solar cell up to 2000‐suns concentration. The AlGaAs/GaAs TJ design is found to require the least doping concentration to reach a resistance of <10⁻⁴ Ω cm² followed by the GaAs/GaAs TJ and finally the AlGaAs/AlGaAs TJ. The AlGaAs/InGaP TJ is only able to obtain resistances of ≥5 × 10⁻⁴ Ω cm² within the range of doping concentrations studied. Copyright © 2010 John Wiley & Sons, Ltd.     

High-energy proton irradiation effects on AlGaN/GaN high-electron mobility transistors

AlGaN/GaN high-electron mobility transistors (HEMTs) show decreases in extrinsic transconductance, drain-source current threshold voltage, and gate current as a result of irradiation with 40 MeV protons at doses equivalent to decades in low-earth orbit. The data are consistent with the protons creating deep electron traps that increase the HEMT channel resistance. Postirradiation annealing at 300°C was able to restore ∼70% of the initial g_m and I_DS values in HEMTs receiving proton doses of 5×10¹⁰ cm⁻².     

Deep states in silicon δ-doped GaAs

The density and electron trapping cross section of deep states in silicon δ-doped GaAs were investigated by means of measurements of the voltage and temperature dependences of the impedance of a Schottky contact to the structure. It was observed that density-of-states tails appear in the band gap when the silicon density in the d-layer exceeds 6×10¹² cm⁻². In our structures the energy characterizing the penetration depth of a tail was in the range 20–100 meV. The characteristic electron trapping cross section of deep states in δ-layers was of the order of 10⁻¹⁷ cm². It was shown that saturation of the electron density in the δ-layer with increasing Si density is due to self-compensation of Si. Fiz. Tekh. Poluprovodn. 32, 733–738 (June 1998)     

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.     

MeV energy sulfur implantation in GaAs and InP

Room temperature and elevated temperature sulfur implants were performed into semi-insulating GaAs and InP at variable energies and fluences. The implantations were performed in the energy range 1–16 MeV. Range statistics of sulfur in InP and GaAs were calculated from the secondary ion mass spectrometry atomic concentration depth profiles and were compared with TRIM92 values. Slight in-diffusion of sulfur was observed in both InP and GaAs at higher annealing temperatures for room temperature implants. Little or no redistribution of sulfur was observed for elevated temperature implants. Elevated temperature implants showed higher activations and higher mobilities compared to room temperature implants in both GaAs and InP after annealing. Higher peak electron concentrations were observed in sulfur-implanted InP (n ≈ 1 × 10¹⁹ cm⁻³) compared to GaAs (n ≈ 2 × 10¹⁸ cm⁻³). The doping profile for a buried n⁺ layer (n ≈ 3.5 × 10¹⁸ cm⁻³) of a positive-intrinsic-negative diode in GaAs was produced by using Si/S coimplantation.