Benefits of chemical beam epitaxy for micro and optoelectronic applications

This paper reviews the benefits that Chemical Beam Epitaxy (CBE) growth technique can bring to micro and optoelectronic devices. The characteristics of the technique are first underlined for a better understanding of the specific advantages it offers compared to other growth techniques. The first one is the low growth temperature, which pushed the thickness limit in strained InGaAsP multi-quantum well structures. Carbon doping of GaAs and In_0.53Ga_0.47As, and its application to GaAs and InP based npn heterojunction bipolar transistors is another important contribution of CBE which has resulted in successful device development. The last developments in surface cleaning and in situ etching with new amine and chloride sources are presented. The capabilities of CBE for selective and uniform growth on partially masked substrates are illustrated through examples of device planarization and integration. Finally, the most recent improvements in the technology of CBE equipment and their impact on CBE production capabilities are presented.     

CBE growth of GaAs/GaAs, GaAs/Si and AlGaAs/GaAs using TEG, AsH3 and amine-alane precursors

The growth of high quality AlGaAs by CBE has been limited by the high levels of carbon and oxygen contamination. The use of alane based precursors offers a significant reduction in such contamination. We report for the first time the CBE growth of Al_xGa_1−xAs from triethylgallium, dimethylethylamine-alane and arsine, and compare with. growth from triethylgallium, trimethylamine-alane and arsine. Some preliminary results of work on the CBE growth of GaAs on silicon will also be reported.     

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.     

p-Type GaAs doped by diiodomethane (CI2H2) in molecular beam epitaxy, metalorganic molecular beam epitaxy, and chemical beam epitaxy

Highly p-type carbon-doped GaAs epitaxial layers were obtained using diiodomethane (CI₂H₂) as a carbon source. In the low 10¹⁹ cm⁻³ range, almost all carbon atoms are electrically activated and at 9×10¹⁹ cm⁻³, 91% are activated. The carbon incorporation efficiency in GaAs layers grown by metalorganic molecular beam epitaxy (MBE) and chemical beam epitaxy (CBE) is lower than that by MBE due to the site-blocking effect of the triethylgallium molecules. In addition, in CBE of GaAs using tris-dimethylaminoarsenic (TDMAAs), the carbon incorporation is further reduced, but it can be increased by cracking TDMAAs. Annealing studies indicate no hydrogenation effect.     

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.     

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.     

Dependence of InP and GaAs chemical beam epitaxy growth rate on substrate orientations; applications to selective area epitaxy

In order to optimize the shape of chemical beam epitaxy (CBE) selective area growth, growth rates on (100), (111)B, (111)A and (110) substrate orientations have been examined for GaAs and InP materials. (111)B GaAs growth rate appears to be drastically enhanced at low V/III ratio, which has been applied to grow selective GaAs patterns limited by vertical sidewalls. Concerning InP, high growth rates were obtained on all orientations. This was used to perfectly fill a rectangular groove by selective embedded InP growth.     

High efficiency GaAs thin film solar cells by peeled film technology

p-GaAs/n-GaAs thin film concentrator solar cells were fabricated by Peeled Film Technology. This is the first paper that reports the concentration characteristics of thin film solar cells. The energy conversion efficiency of thin film solar cells at a concentration ratio of 109 is 9.4% and the output power density is 0.82 W/cm² · n-Ga_1?xAlxAs/p-GaAs heterojunction thin film solar cells were also fabricated. The initial heterojunction thin film solar cell with a Al mole fraction of 0.5 showed an efficiency of up to 13.5% (AM 1.5). It is proposed that Multi-Peeled Film Technology will give numerous GaAs thin films by selective etching of (GaAl)As/GaAs multi-layered structures.     

InGaAs---GaAs pseudomorphic heterostructure transistors prepared by MOVPE

In this paper, we will demonstrate two new InGaAs-GaAs pseudomorphic heterostructure transistors prepared by MOVPE technology, i.e. InGaAs-GaAs graded-concentration doping-channel MIS-like field effect transistors (FET) and heterostructure-emitter and heterostructure-base (InGaAs-GaAs) transistors (HEHBT). For the doping-channel MIS-like FET, the graded In_0.15Ga_0.85As doping-channel structure is employed as the active channel. For a 0.8 × 100 μm² gate device, a breakdown voltage of 15 V, a maximum transconductance of 200 mS/mm, and a maximum drain saturation current of 735 mA/mm are obtained. For the HEHBT, the confinement effect for holes is enhanced owing to the presence of GaAs/InGaAs/GaAs quantum wells. Thus, the emitter injection efficiency is increased and a high current gain can be obtained. Also, due to the lower surface recombination velocity of InGaAs base layers, the potential spike of the emitter-base (E-B) junction can be reduced significantly. This can provide a lower collector-emitter offset voltage. For an emitter area of 4.9 × 10⁻⁵ cm², the common emitter current gain of 120 and the collector-emitter offset voltage of 100 mV are obtained.     

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.     

Molecular dynamics simulation of (100)InGaAs/GaAs strained-layer relaxation processes

Molecular dynamics simulations on In_1−xGa_xAs/GaAs(100) systems are performed showing the dynamics of threading dislocations in the overlayers and the formation of misfit dislocations at the heterojunction interface. The developed code, using a modified Tersoff potential, simulates the threading dislocation dynamics in the InGaAs overlayer, and also the formation of interface misfit dislocations. Values for critical thicknesses are predicted and the atomic structure of the dislocation cores are determined.     

Very uniform epitaxy

A comprehensive evaluation of the T-shaped reactor, in view of its use as a production tool for 1300 nm optoelectronic devices, was carried out. The material obtained was of excellent quality and exhibited a very high degree of uniformity and good reproducibility, despite the difficulties associated with the control of two composition ratios, particularly with the As/P one. The extension to other compositions is obvious. Ternary and quasi-ternary materials ( InGaAs, InGaAlAs ) can be handled with even greater ease, not to speak about binaries and quasi-binaries ( GaAs, GaAlAs ). The system lends itself naturally to extension toward larger wafer sizes. A multi-wafer system based on the same principles can also be implemented ; very recently, such a system was proposed by Frijlink [16].     

Reaction kinetics for the CBE growth of GaAs from triethylgallium; computer modelling studies incorporating recent surface spectroscopic data

A new model for the decomposition of triethylgallium on GaAs(100), with kinetic parameters derived from the results of surface science experiments, is presented. Deficiencies of early models are corrected by including surface coverage and site blocking effects, and lateral interactions between absorbed DEG species are included. The model successfully predicts variations in the rate of CBE growth of GaAs with substrate temperature, and addresses effects induced by variations in arsenic overpressure. This dependence of growth rate on the arsenic flux is modelled by computing the steady state concentrations of absorbed arsenic as a function of temperature and As₂ and TEG flux. Excess arsenic is shown to inhibit GaAs growth by blocking sites for TEG absorption.     

Anisotropy in InGaAs/GaAs heterostructures grown by low-pressure MOVPE and CBE

InGaAs/GaAs heterostructures grown on (001) substrates by low-pressure MOVPE exhibit a measurable anisotropy in their structural, optical and electrical properties. This anisotropy occurs in structures which have undergone partial or complete strain relaxation and it can be strongly reduced by using slightly misoriented substrates. A comparison with similar structures grown by CBE indicates that this anisotropy is less important. This study suggests that strain relaxation is achieved by a combination of several mechanisms whose relative importance depends on the orientation of the substrate and on growth temperature which varies with the growth technique.     

Applications of MBMS and surface spectroscopic techniques in the study of reaction mechanisms in CBE; investigations of the reactivity of tritertiarybutylgallium and triisobutylgallium as alternative precursors for epilayer growth

Current approaches to the study of reaction mechanisms in CBE and investigations of the potential of triisobutylgallium and tritertiarybutylgallium as novel CBE precursors are reviewed. Surface spectroscopic techniques indicate that adsorbed iso-butyl radicals decompose to produce gaseous butene and hydrogen at significantly lower temperatures than in the corresponding process for ethyl radicals on GaAs, resulting in lowered growth temperatures and low temperature C incorporation levels in comparison to the results obtained with triethylgallium. A β-methyl migration occurs at higher temperatures causing C to deposit irreversibly on the surface in the presence of Al. Lowered temperatures for the β-hydride elimination reaction are also observed for adsorbed tertiarybutyl radicals and the absence of β-methyl groups avoids the facile C deposition process seen for iso-butyl. These potential advantages associated with tertiarybutyl ligands cannot be realized straightforwardly in CBE using tritertiarybutylgallium however, since steric crowding effects result in the inefficient total dissociation of the adsorbing precursor molecule.     

Low-temperature characteristics of the current gain of GaN/InGaN double-heterojunction bipolar transistors

We investigated the temperature dependence of the current gain of npn-type GaN/InGaN double-heterojunction bipolar transistors (DHBTs) in the low-temperature region. The current gain increased with decrease in device temperature due to the reduction of the recombination current in the p-type base layer. The current gain reached as high as 5000 at 40 K, which is the highest among nitride-based HBTs. For conventional HBTs made of InP or GaAs, the current gain decreased with decreasing device temperature. However, no reduction of the current gain was observed in this study, suggesting that the minority carrier mobility in the p-type InGaN base layer has negative temperature dependence, presumably because the ionized impurity scattering is relatively unaffected owing to the carrier freezeout and the high activation energy of Mg in the p-InGaN base layer.     

Effects of pressure and temperature on RD detected growth oscillations

We report on the vacuum chemical epitaxy (VCE) growth of GaAs from triethylgallium and arsine at varying partial pressures of arsine and hydrogen. In situ, monolayer growth oscillations were, for the first time, detected in a hydrogen environment using reflectance difference (RD). These results offer the possibility to link surface mechanisms occuring during chemical beam epitaxy (CBE) with those taking place in metalorganic vapour phase epitaxy (MOVPE) and may lead to the observation of growth oscillations also during MOVPE. Finally, the behaviour of the RD signal as a function of substrate temperature is studied over a wider temperature interval than has previously been reported, giving further information about surface processes.     

CBE growth of high-quality AlGaAs/GaAs heterostructures for HEMT applications

AlGaAs/GaAs heterostructures were grown by chemical beam epitaxy using triethylgallium, triisobutylaluminium and pure arsine in flow control mode with hydrogen as carrier gas. For substrate temperatures of 580°C and V/III ratios of 10, high quality AlGaAs layers are obtained; heterostructures show abrupt and smooth interfaces. Modulation doping with silicon evaporated from a conventional effusion cell gives two-dimensional electron gases with carrier densities up to 1×10¹² cm^-2. Mobilities of 70000 cm²/V·s are obtained at 77 K for carrier densities of 4×10¹¹ cm^-2. The lateral homogeneity of the heterostructures in layer thickness, composition and doping level is excellent. Perfect morphology with defect densities of about 100 cm^-2 is observed. High electron mobility transistors (gate length 0.3 nm) fabricated from quantum well structures show a transconductance of about 380 mS/mm.     

Characterization of MOVPE-grown p-InGaAs/n-InP interfaces

The low-pressure MOVPE-grown p-InGaAs-on-n-InP and n-InP-on-p-InGaAs diodes were characterized by I–V and C–V measurements to study the effects of the growth conditions on the heterointerface. The obtained band discontinuity in the conduction band ΔE_c ranged from 0.19 to 0.32 eV. It was found that ΔE_c was very sensitive to the growth interruption at the InP/InGaAs heterointerface. The n-InP-on-p-InGaAs diodes tend to show higher ΔE_c than the p-InGaAs-on-n-InP diodes. The decreased ΔE_c at InGaAs-on-InP heterointerface might be attributed to the graded layer of InGaAsP formed by intermixing at the interface. It is concluded from the estimated ΔE_c that the InP-on-InGaAs heterointerface is more abrupt than the InGaAs-on-InP heterointerface. An improvement of the InGaAs-on-InP heterointerface is mandatory for fabrication of high-performance double heterojunction bipolar transistors with InP collector layers.