Growth of GaAs/AlGaAs HBTs by MOMBE (CBE)

In this paper, we will discuss how the unique growth chemistry of MOMBE can be used to produce high speed GaAs/AlGaAs heterojunction bipolar transistors (HBTs). The ability to grow heavily doped, well-confined layers with carbon doping from trimethylgallium (TMG) is a significant advantage for this device. However, in addition to high p-type doping, high n-type doping is also required. While elemental Sn can be used to achieve doping levels up to 1.5×10¹⁹ cm^-3, severe segregation limits its use to surface contact layers. With tetraethyltin (TESn), however, segregation does not occur and Sn doping can be used throughout the device. Using these sources along with triethylgallium (TEG), trimethylamine alane (TMAA), and AsH₃, we have fabricated Npn devices with 2 μm×10 μm emitter stripes which show gains of ≥ 20 with either ƒ_t = 55 GHz and ƒ_max = 70 GHz or ƒ_t = 70 GHz and ƒ_max = 50 GHz, depending upon the structure. These are among the best RF values reported for carbon doped HBTs grown by any method, and are the first reported for an all-gas source MOMBE process. In addition, we have fabricated a 70 transistor decision circuit whose performance at 10 Gb/s equals or exceeds that of similar circuits made from other device technologies and growth methods. These are the first integrated circuits reported from MOMBE grown material.     

A comparative study of GaAs- and InP-based HBT growth by means of LP-MOVPE using conventional and non gaseous sources

Low-pressure metalorganic vapor phase epitaxy (LP-MOVPE) growth of carbon doped (InGa)P/GaAs and InP/(InGa)As heterojunction bipolar transistors (HBT) is presented using a non-gaseous source (ngs-) process. Liquid precursors TBAs/TBP for the group-V and DitBuSi/CBr₄ for the group-IV dopant sources are compared to the conventional hydrides AsH₃/PH₃ and dopant sources Si₂H₆/CCl₄ while using TMIn/TEGa in both cases. The thermal decomposition of the non gaseous sources fits much better to the need of low temperature growth for the application of carbon doped HBT. The doping behavior using DitBuSi/CBr₄ is studied by van der Pauw Hall measurements and will be compared to the results using Si₂H₆/CCl₄. Detailed high resolution X-ray diffraction (HRXRD) analysis based on 004 and 002 reflection measurements supported by simulations using BEDE RADS simulator enable a non-destructive layer stack characterization. InGaP/GaAs HBT structures designed for rf-applications are grown at a constant growth temperature of T_gr=600°C and at a constant V/III-ratio of 10 for all GaAs layers. P-type carbon concentrations up to P = 5·10¹⁹cm⁻³ and n-type doping concentrations up to N = 7·10¹⁸cm⁻³ are achieved. The non self-aligned devices (A_E = 3·10 μm²)_show excellent performance, like a dc-current gain of B_max = 80, a turn on voltage of V_offset = 110 mV (Breakdown Voltage V_CEBr,0 > 10 V), and radio frequency properties of f_T/f_max = 65 GHz/59 GHz.

In the non-gaseous source configuration the strong reduction in the differences of V/III-ratios and temperatures during HBT structure growth enable easier LP-MOVPE process control. This is also found for the growth InP/InGaAs HBT where a high dc-current gain and high transit frequency of f_T= 120 GHz are achieved.     

The role of hydrogen in low-temperature MOVPE growth and carbon doping of In0.53Ga0.47As for InP-based HBT

Hydrogen radicals are decisive for the low-temperature growth and carbon doping of In_0.53Ga_0.47As in LP-MOVPE. This is demonstrated for the growth of highly p-type doped In_0.53Ga_0.47As layers with CCl₄ as dopant source. Perturbed angular correlation measurements (PAC) were used to investigate the passivation of acceptors by hydrogen in low-temperature grown In_0.53Ga_0.47As. Based on the above analysis an InP-based layer stack is developed which employs low-temperature growth of the base layer, high-temperature growth of the remaining HBT layers, and an in situ post-growth annealing under TMAs/N₂ ambient.     

High carbon doping of Ga1−xInxAs (x≈0.01) grown by molecular beam epitaxy

We have grown layers of Ga_1−xIn_xAs:C (x ≈ 0.01) on (100) GaAs by molecular beam epitaxy. As C source a graphite filament was used. Structures coherent with the substrate were obtained by adjusting properly the In and C concentrations. With simultaneous incorporation of In and C the strain is compensated and, consequently, the defect density is reduced. A maximum hole concentration value of p = 6×10¹⁹ cm⁻³ was achieved, which is twice higher than the saturation value of C doping of GaAs produced under the same conditions. There is evidence that this value is not in the saturation limit. The product of the hole density times the mobility increases, so the resistance decreases with higher C doping. Raman spectra show that the C_As peak broadens and shifts to lower frequencies for increasing concentration of indium. In H-passivated samples, Raman spectroscopy shows that C_As is surrounded by Ga atoms only. Indium atoms are thus present only in the second group III shell.     

Wavelength selective materials modification of bulk As2S3 and As2Se3 by free electron laser irradiation

In this study we report first measurements of wavelength-selective infrared-induced materials modification of bulk As₂S₃ and As₂Se₃. These materials are currently being considered as candidate materials for infrared optical fiber transmission in the range of 1–10 μm. Our study is aimed at modifying oxygen, hydrogen and carbon impurities bound to chalcogenide constituent elements in the materials to reduce absorption. Tunable infrared radiation from the W.M. Keck Free Electron Laser (FEL) at Vanderbilt was used to excite specific vibrational modes, S–O–H and CH_x modes in bulk As₂S₃ and Se–H, CH_x and S–H₂ modes in bulk As₂Se₃. Changes in vibrational mode amplitudes are monitored by measuring the intensity of the Fourier transform infrared (FTIR) spectra before and after irradiation at appropriate wavelengths. By tuning wavelengths to hydrogen vibrational modes, we find evidence that hydrogen is released and/or redistributed athermally. In particular, following irradiation at specific resonant wavelengths, vibrational mode amplitudes as monitored by FTIR associated with CH_x are significantly reduced in bulk As₂S₃ and As₂Se₃ samples. In As₂S₃, the changes in CH_x modes are reversed by heat treatment at 115°C for 35 min in nitrogen atmosphere.     

Hydrogen passivation of residual acceptors in GaSb single crystals

Undoped GaSb single crystals were grown using the Czochralski method without encapsulant in an atmosphere of ionized hydrogen. It has been found that the resistivity increased by more than one order of magnitude (0.6–0.8 Ω cm) in comparison with the crystals grown under molecular hydrogen atmosphere only. A certain asymmetry in acceptor and donor passivation is assumated, because the Hall concentration does not change along the direction of crystal growth ((3.3–3.5) × 10¹⁷ cm^-3) and it is comparatively higher. Donors are passivated more than acceptors, which should be confirmed by increasing resistivity and decreasing mobility.     

Growth of heavily C-doped GaAs/AlGaAs MQW structures by MOVPE for 2–3 μm normal incidence photodetectors

The growth and intersubband optical properties of high quality heavily doped p-type GaAs/AlGaAs multiple quantum well (MQW) structures are reported. The MQWs were fabricated by the atmospheric pressure metalorganic vapor phase epitaxy process using liquid CCl₄ to dope the wells with C acceptors (N_a ≈ 2 × 10¹⁹ cm⁻³). A constant growth temperature was maintained for the entire structure while different V/III ratios were used for the well and barrier regions. By this process it is possible to achieve both high C doping densities in the wells and to simultaneously obtain good quality AlGaAs barriers. Fourier transform infrared spectroscopy measurements on heavily doped 10-period MQW structures reveal a new absorption peak at 2 μm with an effective normal incidence absorption coefficient of 4000 cm⁻¹. Photocurrent measurements on mesa-shaped diodes show a corresponding peak at 2.1 μm. The photodiodes exhibit a symmetrical current-voltage characteristic and a low dark current, which are indicative of a high quality MQW structure and a well-controlled C doping profile. The 2 μm absorption represents the shortest wavelength ever reported for any GaAs/AlGaAs or InGaAs/AlGaAs MQW structure and should be very useful for implementing multicolor infrared photodetectors.     

Growth of carbon-doped base GaAs/AlGaAs HBT by gas-source MBE using TEG, TEA, TMG, AsH3, and Si2H6

High-performance carbon-doped-base GaAs/AlGaAs heterobipolar transistors (HBTs) were grown by gas-source MBE using only gaseous sources including dopant sources. The AlGaAs emitter layer was doped with Si from uncracked SI₂H₆ (n = 9 × 10¹⁷ cm^-3), and the base layer (92.5 nm) was doped with carbon from TMG (p = 4 × 10¹⁹ cm^-3). From SIMS analysis it was confirmed that a well-defined emitter-base junction with sharp carbon profile was obtained. The base-current ideality factor from the Gummel plot was 1.47, and the emitter-base junction ideality factor was 1.12. A high DC current gain of 53 was obtained at a current density of 4 × 10⁴ A/cm². The device characteristics of our carbon-doped HBTs were found to be stable under current stress.     

双甜菜碱为客体包合物的三维氢键网格的构筑

利用硫脲和双甜菜碱制备了一种新型的包合物 [^-OOCCH₂N⁺(CH₃)₂]₂(CH₂)₃·4(NH₂)₂CS,用X射线单晶衍射方法测定其晶体结构。结果表明,晶体属三斜晶系,P1ˉ空间群, 其中a=0.884 5(2) nm,b=0.936 7(2) nm,c=0.946 4(3) nm,α=91.591(2)°,β=91.591(2)°,γ=91.591(2)°,Z=1,R₁=0.039 9, wR₂=0.100 6(I>2σ(I))。在标题化合物的晶体结构中,硫脲分子通过N-H…S氢键肩并肩相连形成四聚体,客体分子的羧基通过N-H…O氢键连接硫脲四聚体形成氢键层,客体阳离子部分夹在相邻的氢键层中,形成三明治晶体结构。     

Investigations on the effect of InSb and InAsSb step-graded buffer layers in InAs0.5Sb0.5 epilayers grown on GaAs (0 0 1)

We report the structural and electrical properties of InAsSb epilayers grown on GaAs (0 0 1) substrates with mid-alloy composition of 0.5. InSb buffer layer and InAs_xSb_1−x step-graded (SG) buffer layer have been used to relax lattice mismatch between the epilayer and substrate. A decrease in the full-width at half-maximum (FWHM) of the epilayer is observed with increasing the thickness of the InSb buffer layer. The surface morphology of the epilayer is found to change from 3D island growth to 2D growth and the electron mobility of the sample is increased from 5.2×10³ to 1.1×10⁴ cm²/V s by increasing the thickness of the SG layers. These results suggest that high crystalline quality and electron mobility of the InAs_0.5Sb_0.5 alloy can be achieved by the growth of thick SG InAsSb buffer layer accompanied with a thick InSb buffer layer. We have confirmed the improvement in the structural and electrical properties of the InAs_0.5Sb_0.5 epilayer by quantitative analysis of the epilayer having a 2.09 μm thick InSb buffer layer and 0.6 μm thickness of each SG layers.     

The incorporation behavior of arsenic and antimony in GaAsSb/GaAs grown by solid source molecular beam epitaxy

GaAsSb ternary epitaxial layers were grown on GaAs (0 0 1) substrate in various Sb₄/As₂ flux ratios by solid source molecular beam epitaxy. The alloy compositions of GaAs_1−ySb_y were inferred using high-resolution X-ray symmetric (0 0 4) and asymmetric (2 2 4) glance exit diffraction. The non-equilibrium thermodynamic model is used to explain the different incorporation behavior between the Sb₄ and As₂ under the assumption that one incident Sb₄ molecule produces one active Sb₂ molecule. It is inferred that the activation energy of Sb₄ dissociation is about 0.46 eV. The calculated results for the incorporation efficiency of group V are in good agreement with the experimental data.     

Extremely flat interfaces in GaAs/AlGaAs quantum wells with high Al content(0.7) grown on GaAs (411)A substrates by molecular beam epitaxy

Effectively atomically flat interfaces over a macroscopic area (200 μm diameter) have been achieved in GaAs/Al_0.7Ga_0.3As quantum wells (QWs) with well widths of 3.6-12 nm grown on (411)A GaAs substrates by molecular beam epitaxy (MBE) for the first time. A single and very narrow photoluminescence peak (FWHM, full width at half maximum, is 6.1 meV) was observed at 717.4 nm for the QW with a well width of 3.6 nm at 4.2 K. The linewidth is comparable to that of growth-interrupted QWs grown on (100)-oriented GaAs substrates by MBE. A 1.5 μm thick Al_0.7Ga_0.3As layer with good surface morphology also could be grown on (411)A GaAs substrates in the entire growth temperature region of 580-700°C, while rough surfaces were observed in Al_0.7Ga_0.3As layers simultaneously grown on (100) GaAs substrates at 640-700°C. These results indicate that the surface of GaAs and Al_0.7Ga_0.3As grown on the (411)A GaAs substrates are extremely flat and stable on the (411)A plane.     

Magnesium-doped InGaAs using (C2H5C5H4)2Mg: application to InP-based HBTs

Heavily magnesium-doped p-type-InGaAs layers on InP(100) substrates were successfully grown, for the first time, by low-pressure metalorganic chemical vapor deposition (MOCVD) using bis-ethylcyclopentadienyl-magnesium, (C₂H₅C₅H₄)₂Mg (EtCp₂Mg), as organometallic precursor for the Mg. It was experimentally verified that the room-temperature hole concentration of Mg into InGaAs increased with increase of the V/III ratio and decrease of the growth temperature. A maximum hole concentration of over 4 × 10¹⁹ cm⁻³ was obtained. The diffusion coefficient of Mg in InGaAs was experimentally derived to be 10⁻¹² cm²/s at 800°C, which was comparable to that of Be. Finally, InP/InGaAs heterojunction bipolar transistors (HBTs) with Mg-doped bases were fabricated successfully. Measured maximum current gain was about 320 with a 90 nm thick base and a sheet resistance of the base layer of 1.28 kΩ/sq.     

Epitaxial growth of CuInSe2 single crystal by halogen transport method

CuInSe₂ single crystals were epitaxially grown on (001) GaP, (001) GaAs, and (110) GaP by the halogen transport method. The orientation relationships in the growth on the (001) and (110) faces were (a) [001]_CuInSe2 [001]_sub and [100]_CuInSe2 [100]_sub (c-axis orientation growth), and (b) [110]_CuInSe2 [110]_sub and [001]_CuInSe2 [001]_sub, respectively. On (001) InP, the orientation relationships between the layer and substrate consist of two sets: (c) [100]_CuInSe2 [001]_sub and [001]_CuInSe2 [010]_sub, and (d) [100]_CuInSe2 [001]_sub and [001]_CuInSe2 [ 00]_sub (a-axis orientation growth). The above results, i.e., c-axis growth on (001) GaP and (001) GaAs and a-axis growth on (001) InP, could be explained by a criterion of the minimum lattice mismatch between grown layers and substrates. A series of growth experiments on (001) GaAs indicated that appropriate gas etching of the substrate surface and growth temperature were required for obtaining twin-free single-crystal epitaxial CuInSe₂.     

Crystal growth of metal fluorides for CO2 laser operation I. The necessity of the RAP approach

The 10 μm transparency of halides addresses directly its oxide content. The conversion from oxide to fluoride must have ΔG0. HF(g) is different from the other hydrogen halides because its ΔG of formation is larger than that of H₂O(g). Decrease in the hydrolysis of RF₃ (R=rare earth) influences the crystal phase stability and the tendency of the crystal to crack. The base-to-acid behavior of +3 rare earths provides quantitative support. HF(g) and RAP (reactive atmosphere process) CF₄(g) formations of YF₃ from Y₂O₃ are given in detail. This indifference to HF(g) is reported also in the congruent growth of CaF₂, SrF₂, and BaF₂. The explanation is seen in ZrF₄, HfF₄, and ThF₄. Although RAP does not use HF(g), it permits the congruent growth of good-quality single crystals with very low H₂O(g) content.     

Czochralski grown Ga2O3 crystals

We report on the successful growth of β-Ga₂O₃ single crystals using the Czochralski method. Model calculations show that the gas phase consists of Ga₂O, GaO or Ga independent of the ratio of oxygen and Ar or N₂. We find that for growing single crystals the evaporation has to be suppressed by a finite amount of oxygen. A CO₂/Ar gas atmosphere was found to meet this requirement.     

Structural investigation of gallium oxide (β-Ga2O3) nanowires grown by arc-discharge

Gallium oxide nanowires were synthesized by electric arc discharge of GaN powders mixed with a small amount of Ni and Co. The crystal structure of nanowires was determined by multi-channel X-ray diffractometry (MC-XRD), FT-Raman spectroscopy and transmission electron microscopy (TEM). The analyzed results clearly show that the synthesized nanowires are monoclinic gallium oxide (β-Ga₂O₃). Final morphology and microstructure of β-Ga₂O₃ nanowires were changed depending on the presence of the transition metals into the nanowires. The β-Ga₂O₃ nanowires grown by the assistance of transition metals demonstrate a smooth edge surface while containing twin defects at the center. The transition metals have enhanced the step growth of nanowires. However, in the case of the β-Ga₂O₃ nanowires, where the transition metals are not shown on the surface, the nanowires demonstrate rather thin and long shapes with amorphous gallium oxide layers on the nanowire surface.     

CBE growth of GaAs/GaAlAs HBTs using the new DEAlH-NMe3 precursor and all-gaseous dopants

This paper describes the first reported use of diethylaluminium hydride-trimethylamine adduct (DEAlH-NMe₃) for the growth of GaAs/GaAlAs power heterojunction bipolar transistors (HBTs) by chemical beam epitaxy (CBE). This precursor possesses a significantly higher vapour pressure than the more conventionally used triethylaluminium (TEA), and leads to much less stringent requirements for bubbler and gas-line heating, and also much-improved GaAs/GaAlAs heterojunction definition when no carrier gas is employed. The use of all-gaseous n- and p-type dopants offers significant technological advantages in CBE, and the current paper also provides the first report of the use of hydrogen sulphide for n-type doping of CBE-grown GaAlAs HBT emitter regions. In conclusion, DC and RF data obtained from the heterojunction bipolar transistors fabricated to date are described. A DC gain of 40 has already been measured and encouraging early data obtained from RF-probed devices are also presented.     

Replacement of hydrides by TBAs and TBP for the growth of various III–V materials in production scale MOVPE reactors

Besides the standard group V precursors AsH₃ and PH₃, so-called alternative precursors like TBAs and TBP (tertiary-butyl-arsine and tertiary-butyl-phosphine) are more and more important in today's MOVPE processes. A lot of publications have demonstrated that these precursors can be successfully used for the growth of different III–V materials. In this study we want to demonstrate that TBAs and TBP can be used as the group V precursor in a complete family of production scale reactors. It is shown that these precursors can be used for the growth of InP-based as well as for GaAs-based materials. The reactors that have been employed are medium scale reactors (AIX 200/4; 1 × 2 inch, 3 or 4 inch or 3 × 2 inch capability) and large scale Planetary Reactors®, in particular the AIX 2400 system (15 × 2 inch or 5 × 4 inch). Materials that have been grown are (Al)GaInP on GaAs and GaInAsP on InP. The lower cracking energy of these precursors compared to PH₃ and AsH₃ allows one to use lower growth temperatures and lower V/III ratios, particularly in combination with the high cracking efficiencies of the used reactors. For the growth of GaInAsP on InP, the consumption of TBP and TBAs is up to 8 times lower than using PH₃ and AsH₃. GaInP on GaAs could be grown with a V/III ratio as low as 25 in a Planetary Reactor®. Good crystalline quality is demonstrated by DCXD (e.g. for GaInP: FWHM = 35 arcsec, substrate 32 arcsec). PL intensity and growth rate are not affected by using the alternative precursors. The compositional uniformity is similar to layers grown with arsine and phosphine (e.g. 1.5 nm uniformity for GaInAsP (λ = 1.5 μm) on 2 inch; approximately 1 nm uniformity for GaInP) [1,2]. The purity of the grown layers depends mainly on the quality of the TBP and TBAs. Using high purity TBP, InP revealed background carrier concentration in the mid 10¹⁴ cm⁻³ regime. Our investigation shows that TBP and TBAs can replace phosphine and arsine in state of the art MOVPE reactors. Both for single and multi-wafer production MOVPE reactors these compounds can be used successfully for the growth of the entire material spectrum in the Al---Ga---In---As---P system.     

Characteristic crystal growth from amorphous WO3 film by vacuum heating

Selective growth of WO₂, W and WO_3−x crystals from amorphous WO₃ film by vacuum heating at 400–900°C was clarified. The grown WO_3−x crystals were incommensurate structure based on crystallographic share structure. The growth process of WO₂ crystal in the amorphous film was directly observed at high temperature in the electron microscope. The growth front of the WO₂ crystal consumes WO₃ microcrystallites with various orientations. The growth speed of the WO₂ depended on WO₃ microcrystallites orientation. The origin of the wavy growth front of WO₂ was due to an orientation dependence of the WO₃ microcrystallites.