Low-temperature Molecular Beam Epitaxial Growth Research Articles

(Invited) Si/SiGe Resonant Interband Tunnel Diodes Grown by Large-Area Chemical Vapor Deposition

Si/SiGe resonant interband tunnel diodes were successfully technology transferred from low-temperature molecular beam epitaxial growth to chemical vapor deposition (CVD) on 200-mm diameter p-doped silicon wafers in a production style reactor. The resonant interband tunnel diode structure consists of a p+-i-n+ diode that incorporates vapor phase doped δ-doping to enhance quantum mechanical confinement and tunneling probability. The tunneling barrier thickness was varied from 2 nm to 8 nm and a record high peak-to-valley current ratio of 5.2 for a CVD process is reported for a 6 nm barrier thickness. The CVD reactor is an active integral tool within a 200mm full CMOS process line and is suggestive that Si/SiGe RITDs could be seamlessly integrated into a full hybrid CMOS process flow.

Substrate temperature changes during molecular beam epitaxy growth of GaMnAs

Our band gap spectroscopy measurements reveal a remarkably big increase of the substrate temperature during the low-temperature molecular beam epitaxy growth of GaMnAs layers. With the help of numerical simulations we explain the effect as a consequence of changing absorption/emission characteristics of the growing epilayer. We discuss possibilities for reducing the substrate temperature variations during the growth.

Strain and hole-density dependence of hole mobility in strained-Ge modulation-doped structures

The strain and hole-density dependence of hole mobility was studied in compressively strained Ge channel modulation-doped structures. Ge channels with large strain (grown on SiGe buffers with Ge compositions down to 43%) were fabricated without any relaxation by low-temperature molecular beam epitaxy growth. It was found that the mobility monotonically increased with increasing strain. Since this increase in mobility was accompanied by an increase in hole density, the enhanced screening effect of the impurity scattering was thought to be a dominant mechanism for mobility enhancement due to strain. This mechanism was confirmed by backgating measurements, where the mobility increased with increasing hole density with the power of 0.5–0.7. It was also found that the inverted structure contributed to mobility enhancement particularly for highly strained structures, and the highest mobilities of 20 000 and 2000 cm2 (V s)−1 at 8 K and room temperature were obtained for Ge channels with a strain of around 2%, indicating that Ge channels with extremely high strain are very promising structures for high performance CMOS applications.

A hybrid epitaxy method for InAs on GaP

The interface formation mechanism during the molecular-beam epitaxy (MBE) of InAs∕GaP has been studied with the aid of the In–Ga–P phase diagram. It is discovered that an initial dissolution and crystallization process similar to liquid phase epitaxy (LPE) may happen at sufficiently high temperature, resulting in a graded composition at the interface. Consequently, “parasitic LPE∕MBE” is the name for this hybrid form of MBE. High-resolution TEM images confirm the existence of the interfacial layer in the sample grown at high temperature. The graded interface smears out the band offset and leads to a nonrectifying heterojunction. Low-temperature (LT) MBE growth can turn off the LPE component, enabling the growth of an abrupt interface. Based on this “LPE∕MBE” model, a LT MBE technique is developed to grow an abrupt InAs∕InGaP interface for heterojunction power Schottky rectifiers. The LT InAs∕InGaP heterojunction demonstrates nearly ideal Schottky rectifier characteristics, while the sample grown at high temperature shows resistive ohmic characteristics. The LT InAs∕InGaP Schottky diode also demonstrates good stability with respect to anneal temperature, similar to the InAs∕GaP heterojunctions.

Growth temperature and dopant species effects on deep levels in Si grown by low temperature molecular beam epitaxy

Deep-level transient spectroscopy measurements were performed in order to investigate the effects of substrate growth temperature and dopant species on deep levels in Si layers during low-temperature molecular beam epitaxial growth. The structures studied were n+-p junctions using B doping for the p layer and p+-n junctions using P doping for the n layer. While the density of hole traps H1 (0.38–0.41 eV) in the B-doped p layers showed a clear increase with decreasing growth temperature from 600 to 370 °C, the electron trap density was relatively constant. Interestingly, the minority carrier electron traps E1 (0.42–0.45 eV) and E2 (0.257 eV), found in the B-doped p layers, are similar to the majority carrier electron traps E11 (0.48 eV) and E22 (0.269 eV) observed in P-doped n layers grown at 600 °C. It is hypothesized that these dominating electron traps are associated with pure divacancy defects and are independent of the dopant species.

“Inverted hut” structure of Si–Ge nanocrystals studied by extended x-ray absorption fine structure method

Local structure around Ge in Si/Ge superlattices containing the “inverted hut” nanocrystals has been investigated by using the extended x-ray absorption fine structure (EXAFS) technique. In contrast to the usual nanometer-sized Ge “hut clusters” commonly grown on top of Si layers using the conventional Stranski–Krastanow self-organized growth mode, SiGe-alloy nanocrystals can be formed beneath the Ge wetting layer and grown into the Si layer in Si/Ge superlattices prepared in a low-temperature molecular beam epitaxy growth mode, and exhibit inverted hut nanocrystal structures regularly spaced along the Si/Ge interface. The EXAFS results obtained with varying Ge wetting layer thickness provide a direct evidence that intermixing of Ge and Si atoms takes place in a zone of about 1–3 monolayers on each side of the Si/Ge interface. The intermixing of constituent atoms allows a mechanism other than the usual formation of misfit dislocations to release the strain energy resulted from lattice mismatch between Si and Ge at the interface.

Low-temperature molecular beam epitaxial growth of GaAs and (Ga,Mn)As

GaAs and (Ga,Mn)As were grown by low-temperature (LT) MBE. In this paper we report the growth condition dependence of properties of LT GaAs and (Ga,Mn)As. Transient reflectivity measurements showed that the carrier lifetime in LT GaAs can be modified by changing the V/III ratio. The Curie temperature of (Ga,Mn)As, determined by magneto-transport measurements, was also shown to be a function of V/III ratio.

Conducting (Si-Doped) Aluminum Nitride Epitaxial Films Grown by Molecular Beam Epitaxy

ABSTRACTAs a member of the III-V nitride semiconductor family, AlN, which has a direct energygap of 6.2eV, has received much attention as a promising material for many applications. However, despite the promising attributes of AlN for various semiconductor devices, research on AlN has been limited and n-type conducting AlN has not been reported. The objective of this research was to understand the factors impacting the conductivity of AlN and to control the conductivity of this material through intentional doping. Prior to the intentional doping study, growth of undoped AlN epilayers was investigated. Through careful selection of substrate preparation methods and growth parameters, relatively low-temperature molecular beam epitaxial growth of AlN films was established which resulted in insulating material. Intentional Si doping during epilayer growth was found to result in conducting films under specific growth conditions. Above a growth temperature of 900°C, AlN films were insulating, however, below a growth temperature of 900°C, the AlN films were conducting. The magnitude of the conductivity and the growth temperature range over which conducting AlN films could be grown were strongly influenced by the presence of a Ga flux during growth. For instance, conducting, Si-doped, AlN films were grown at a growth temperature of 940°C in the presence of a Ga flux while the films were insulating when grown in the absence of a Ga flux at this particular growth temperature. Also, by appropriate selection of the growth parameters, epilayers with n-type conductivity values as large as 0.2 Ω−1 cm−1 for AlN and 17 Ω−1cm−1 for Al0.75Ga0.25N were grown in this work for the first time.

Influence Of Growth Conditions On The Thermal Quenching Of Photoluminescence From Sige/Si Quantum Structures

AbstractIn this work we study effects of growth temperature and use of surfactant during growth on thermal quenching of photoluminescence (PL) from SiGe/Si quantum wells (QWs) grown by molecular beam epitaxy (MBE). We show that although all investigated structures demonstrate intense and sharp excitonic emissions from the SiGe QWs at liquid helium temperature, thermal quenching of this PL critically depends on the growth conditions. In particular, the use of low (⁤ 550°C) growth temperatures or employing Sb as a surfactant during high temperature (620°C) growth considerably degrades the PL thermal quenching behaviour by introducing some competing quenching processes with low activation energies of about 5 meV. The optimum growth conditions judging from the PL thermal behaviour are realised during high temperature growth without surfactant (620°C). Even higher growth temperature is shown to be required during surfactant mediated growth to improve the thermal quenching behaviour. From optically detected magnetic resonance (ODMR) studies, the competing quenching processes are attributed to a thermal activation of non-radiative defects introduced during either low-temperature MBE growth or during surfactant-mediated growth.

Interplay between the magnetic and transport properties in the III-V diluted magnetic semiconductorGa1−xMnxAs

Using a low-temperature molecular-beam epitaxy growth procedure, ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{As}$ --- a III-V diluted magnetic semiconductor --- is obtained with Mn concentrations up to $x\ensuremath{\sim}9%.$ At a critical temperature ${T}_{c}$ (${T}_{c}\ensuremath{\approx}50\mathrm{K}$ for $x=0.03--0.05$), a paramagnetic to ferromagnetic phase transition occurs as the result of the interaction between $\mathrm{M}\mathrm{n}\ensuremath{-}h$ complexes. Hole transport in these compounds is strongly affected by the antiferromagnetic exchange interaction between holes and Mn $3d$ spins. A model for the transport behavior both above and below ${T}_{c}$ is given. Above ${T}_{c},$ all materials exhibit transport behavior which is characteristic for systems near the metal-insulator transition. Below ${T}_{c},$ due to the rising spontaneous magnetization, spin-disorder scattering decreases and the relative position of the Fermi level towards the mobility edge changes. When the magnetization has reached its saturation value (below $\ensuremath{\sim}10\mathrm{K}$) variable-range hopping is the main conduction mechanism. The negative magnetoresistance is the result of the expansion of the hole wave functions in an applied magnetic field.

Influence of growth conditions on the formation of deep photoluminescence bands in MBE-grown Si layers and SiGe/Si quantum structures

Influence of the growth parameters, i.e. growth temperature and ion bombardment, on the quality of SiGe/Si quantum structures and Si thin films, grown by molecular beam epitaxy (MBE), is studied using photoluminescence (PL) spectroscopy, optically detected cyclotron resonance and high-resolution X-ray diffraction measurements. The bombardment of even a low flux of Si + ions, as present in the MBE growth with biased substrates, is shown to degrade essentially the quality of epitaxial structures due to creation of both point-like and extended defects. The low temperature MBE growth primarily results in a quenching of SiGe-related emissions relative to the substrate-related PL due to formation of competing non-radiative channels associated, presumably, with vacancy-type complex defects.

Quantitative study of the decay of intensity oscillations in transient layer-by-layer growth.

An analytical formula for the diffraction from an initial transient layer-by-layer growth front has been derived. The approach utilizes recently developed dynamic scaling models which describe the growth-induced roughening evolution at the late stage of growth. The results can be applied to both out-of-phase and non-out-of-phase diffraction conditions. At the out-of-phase conditions, the Bragg peak intensity is given by ${\mathit{I}}_{\mathrm{Bragg}}$\ensuremath{\propto}${\mathit{e}}^{\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\pi}}2}$${\mathit{w}}^{2}$[1+cos(2\ensuremath{\pi}〈h〉)], which oscillates with the growth of the film thickness 〈h〉 but decays with the increase of the interface width w due to the growth-induced roughening. The derived diffraction formula is consistent with the peak intensity oscillation obtained from a transient layer-by-layer process observed in low-temperature molecular-beam epitaxy growth of Si/Si(111).

Low temperature growth of PbTe and of PbTe/ Pb1− xEuxTe multi-quantum wells by molecular beam epitaxy

A systematic study of molecular beam epitaxy (MBE) of PbTe and Pb 1- x Eu x Te on BaF 2 (111) at substrate temperatures between 350 and 70°C is presented. Reflection high-energy electron diffraction (RHEED) intensity oscillations with more than 200 oscillation periods are observed even for substrate temperatures as low as 95°C, signifying the persistence of two-dimensional layer-by-layer growth at surprisingly low growth temperatures. In addition, for low temperature MBE growth of the ternary Pb 1- x Eu x Te, the evolution of RHEED intensity oscillations depends strongly on the beam flux composition, i.e., the Te 2 to Eu beam flux ratio, and a Te-stabilized, as well as Eu-stabilized layer-by-layer growth mode exists. A series of PbTe epitaxial layers, grown at various substrate temperatures from 350 to 120°C, was characterized by high resolution X-ray diffraction and temperature dependent Hall effect measurements. High quality epitaxial layers were obtained for substrate temperatures as low as 160°C, whereas for lower temperatures a drastic broadening of the X-ray diffraction peaks and a strong decrease of the low temperature Hall mobilities is observed. In PbTe/Pb 1- x Eu x Te multi-quantum well structures grown at various substrate temperatures, no broadening of the superlattice satellite diffraction peaks is observed when the substrate temperature is decreased down to 200°C. The high quality of the samples grown at substrate temperatures very much below the usual MBE temperatures is based on a precise control of the beam flux composition. Under such conditions, MBE of IV-VI compounds can be carried out at very low substrate temperatures, whenever technologically necessary.

Instability in Low-Temperature Molecular-Beam Epitaxy Growth of Si/Si(111)

Using high-resolution low-energy electron diffraction we studied in situ low-temperature molecular-beam epitaxial growth of Si/Si(111). At $\ensuremath{\sim}275 \ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ and a deposition rate of $\ensuremath{\sim}7$ bilayers/min, a roughening evolution occurs after an initial transient layer-by-layer growth. It shows dynamic scaling characteristics with a roughness exponent $\ensuremath{\alpha}\ensuremath{\approx}1$ and an interface growth exponent $\ensuremath{\beta}=\frac{1}{4}$. More importantly, the average local slope is found to increase with time as $\ensuremath{\sim}\sqrt{\mathrm{ln}(t)}$. However, such a roughening behavior does not exist either at temperatures higher than 350 \ifmmode^\circ\else\textdegree\fi{}C or under a slower deposition rate of $\ensuremath{\sim}1$ bilayer/min. The phenomena are consistent with a statistical model of linear diffusion dynamics.

A TEM study of low-temperature Ge growth on Ge(001)2×1

One approach to a solution of the doping problem in the molecular beam epitaxy (MBE) growth of the group IV semiconductors, e.g., segregation, low incorporation, and control of the dopants, is to lower the growth temperatures. It has been found that the room temperature Si deposition becomes amorphous after growth of a limiting epitaxial thickness which increases rapidly with the growth temperature. However, the mechanism for this structural transition is not well understood. In the present paper, we report the preliminary results of a study on the mechanism of the low temperature MBE growth of Ge on Ge(001)2×1 over the temperature range of 20-100 °C at deposition rates R=0.05 and 0.1 nm s−1 in an MBE system which has a bass pressure of 5xl0−11 Torr which increases to about 2xl0−9 Torr during deposition. The structural transitions were investigated using a combination of in-situ reflection high-energy electron diffraction (RHEED) and post-deposition high resolution cross-sectional transmission electron microscopy (XTEM).

In0.5Ga0.5As/InAlAs Modulation-doped Field Effect Transistors on GaAs Substrates Grown by Low-Temperature Molecular Beam Epitaxy

We report on the lattice-mismatched growth and electrical properties of In0.5Ga0.5As/InAlAs modulation-doped heterostructures on GaAs substrates by molecular beam epitaxy using a wide-gap InAlAs graded buffer layer and low-temperature Molecular Beam Epitaxial growth at about 350°C. A drastic reduction in residual carrier accumulation has been achieved, and a high electron mobility at room temperature of 11100 cm2/V·s was obtained with an electron concentration of 3×1012/cm2. It was found that the cause of residual carrier accumulation is related to the growth interruption or temperature rise after the growth of the graded buffer layer. The fabricated Modulation-doped Field Effect Transistors showed low output conductance, good pinch-off characteristics and no kink effect. The higher transconductance of 270 mS/mm was also obtained.

Low temperature MBE growth of high quality modulation doped AlGaAs/GaAs MQWs for ultra-high-speed laser diodes

Highly Be-modulation-doped ( p=2×10 19 cm -3) AlGaAs/ GaAs MQWs without interface disorder for ultra-high-speed laser diodes have been grown by molecular beam epitaxy (MBE). The MQWs were grown at low substrate temperatures below 600°C to avoid diffusion of highly doped Be during growth. Structures of the modulation doped MQW were analyzed by high resolution SIMS and TEM analysis, which showed that no disordering took place at the MQW region. Photoluminescent peak wavelength of the modulation doped MQW was shifted to 13 meV lower energy side from that of an undoped MQW at 77 K as a result of energy band bending. The modulation doped MQW laser, though grown at such low growth temperatures, showed stable room temperature operation with extremely high relaxation oscillation frequency up to 30 GHz, which is about twice as high as previous top data.