Molecular Beam Epitaxy Growth Conditions Research Articles

Anomalous As desorption from InAs(100) 2×4

Arsenic desorption from the InAs(100) 2×4 surface is investigated through temperature programmed desorption (TPD), isothermal desorption, and reflection high-energy electron diffraction. TPD area analysis indicates that the As coverage, θAs, of the InAs 2×4 structure is 0.7 monolayers. The As TPD spectrum shows two desorption features; a broad tail from 300 to 400 °C and a distinct peak at 430 °C. The 430 °C peak is well described in terms of a first-order-desorption kinetics with a preexponential factor ν of 1.5×1019 s−1. This extremely high ν value explains well the narrow window of optimal molecular beam epitaxy growth conditions for InAs.

Wafer-scale temperature mapping for molecular beam epitaxy and chemical beam epitaxy

A technique for the measurement of the variation in surface temperature across GaAs and InP wafers under normal molecular beam epitaxy and chemical beam epitaxy growth conditions is described. It is based on measuring heat treatment induced changes in the thickness of an epitaxial layer. The thickness map of a given slice, generated by scanning reflectance spectrometry, is compared with that obtained after a subsequent heat treatment in the deposition system. The thickness difference map obtained by subtracting the two data files is a measure of the evaporative loss of material due to the additional heat treatment. Since the rate of loss of material is exponentially dependent on the surface temperature (activation energy ∼3.7 eV), small temperature variations (<0.2 °C) can be readily measured. The high spatial resolution (<200 μm point-to-point), the whole wafer mapping capability and the use of a differential technique to eliminate thickness variations due to nonthermal causes, enables the effects of substrate mounting and back surface preparation to be clearly discerned.

Comparison of pnp and npn InGaAlAs/InGaAs heterojunction bipolar transistors

The molecular-beam epitaxial growth conditions and characteristics of npn and pnp heterojunction bipolar transistors in the InGaAlAs/InGaAs material system are described. Direct-current current gains of more than 15 000 have been obtained for npn heterojunction bipolar transistors (HBTs). pnp HBTs have a larger breakdown voltage compared to equivalent npn HBTs. A novel pnp HBT structure in which a strained hole filter is placed just above the base to inject light holes preferentially is proposed. The high speed performance of pnp HBTs is at present limited by emitter series resistance.

Molecular-beam epitaxy investigation of interface disorder effects in magnetic II–VI quantum wells

The present article describes the results of a combined theoretical and experimental investigation into the quality of the interfaces of II–VI quantum well structures grown by molecular-beam epitaxy (MBE). Detailed information is presented on the dilute magnetic semiconductor system CdTe/Cd1−xMnxTe in which the CdTe forms the well and the Cd1−xMnxTe forms the barrier. Structures are grown routinely in which both the photoluminescence (PL) and photoluminescence excitation (PLE) linewidths are narrow (∼1–2 meV). This is indicative of high quality material, a feature which is confirmed by the x-ray data in which Pendellösung fringes can be seen. However, in spite of this, it is found that although the same growth conditions are nominally employed quantum well structures are obtained which show either (i) different discernible structures in the PL and the PLE spectra, or (ii) marked differences in the Stokes’ shift between the PLE and PL from one sample to the next. A related feature concerns observations of an asymmetrical magnetic field splitting of the heavy-hole exciton states in the barriers. The results of theoretical calculations of the exciton energy levels and their associated linewidths are presented. A comparison of theory with the experimental observations shows that the above effects can be accounted for in terms of interface disorder and magnetic field dependent interface potentials. Thus in case (i) above, the relative intensity of the components in the PLE is different from that in the PL. This is consistent with large island growth in the plane of the well, large here meaning that the island diameters exceed those of the exciton diameter (∼140–150 Å). Observations of a Stokes’ shift, even though the absorption and emission lines are narrow, can be accounted for theoretically if the concept of smaller scale disorder is introduced, i.e., island sizes that are small compared with the diameter of the exciton. Finally, results are presented which show a remarkable asymmetry in the Zeeman splitting of the heavy-hole exciton lines. It is shown that this can be accounted for by introducing deep, short-range interface potentials (∼1 or 2 monolayers). The latter are magnetic field dependent, the source of the asymmetry being attributed to band-gap renormalization effects. The implications of all these results for MBE growth conditions are described.

Stoichiometry Related Phenomena in Low Temperature Grown GaAs

AbstractThe growth of GaAs at low temperatures (LT-GaAs) at or below 250 °C, under standard Molecular Beam Epitaxy (MBE) growth conditions, usually results in a massive incorporation of excess As in the lattice which then totally dominates the electrical and optical characteristics of the as grown material. We report on new phenomena associated with the growth of GaAs at 250 °C and we show, for the first time, data on highly electrically active doped material. By careful control of the growth conditions, namely As4/Ga flux ratios, material in which total defect concentrations of less than 1017 cm-3, well below the huge 1020 cm-3 that is normally obtained in LT-GaAs, can be achieved thereby demonstrating that high quality GaAs can in effect be grown at extremely low temperatures.

EuTe/PbTe Superlattices: Mbe Growth and Optical Characterization

ABSTRACTThe heteroepitaxial growth of EuTe on PbTe (111) by molecular beam epitaxy (MBE) was investigated using in situ reflection high energy electron diffraction (RHEED). As a function of substrate temperature and Te2 flux rate, the resulting EuTe (111) surfaces exhibit several different surface reconstructions corresponding to Te-stabilized or Eu-stabilized surfaces. The Eustabilized surface shows a (2√3 × 2√3)R30° surface reconstruction. Because of the strain induced tendency for 3D islanding, only in a narrow window of MBE growth parameters perfect 2D layer-by-layer heteroepitaxial growth exists. Using such optimized MBE growth conditions, we have fabricated a series of PbTe/EuTe superlattices. In such superlattices the wide band gap EuTe layers act as barriers and the narrow band gap PbTe as quantum wells. The superlattices were investigated by high resolution x-ray diffraction, showing their high structural perfection. Modulated low temperature Fourier transform infrared reflection measurements were performed in order to determine the confined energy levels in the PbTe quantum wells. The measurements indicate that mini-subbands are formed in the PbTe quantum wells with a mini-band width of 22 meV in agreement with envelope function calculations.

Molecular beam epitaxial growth of InGaAlAs/InGaAs heterojunction bipolar transistors on highly resistive low temperature InAlAs epilayers

In0.52Al0.48As epilayers grown by molecular beam epitaxy (MBE) at low temperatures typically possess resistivities of more than 105 Ω cm. We propose the use of such highly resistive epilayers to vertically isolate different devices on InP substrates. We demonstrate that high performance In0.52Ga0.24Al0.24As/In0.53Ga0.47As heterojunction bipolar transistors can be grown on top of such high resistivity buffer In0.52Al0.48As layers. Such HBTs have dc current gains of 1000 and emitter-base pn junctions with ideality factors of 1.05. However, the MBE growth conditions required to obtain such good material are quite critical. We describe these conditions and also the electrical properties of both the HBTs and the resistive InAlAs layers.

Behavior of excess As in nonstoichiometric Si-doped GaAs

The Si doping efficiency (≡[e−]/[Si]) in homoepitaxial GaAs has been measured as a function of molecular beam epitaxial growth conditions (As pressure, substrate temperature, and growth rate) and post-growth thermal annealing conditions. The doping efficiency decreases from one under optimal growth conditions to almost zero: (1) exponentially with inverse substrate temperature below 400 °C and (2) as a power law in the As4-to-Ga beam-equivalent fluence ratio above unity. The doping efficiency of nonoptimally grown films increases from almost zero to near one upon post-growth thermal annealing above 650 °C with slower than exponential kinetics. These changes are attributed to charge trapping into and release from, respectively, excess-As-related defects, which are incorporated during growth and removed by out-diffusion during annealing.

Growth of InAlAs/InGaAs and InGaAlAs/InGaAs heterojunction bipolar transistors on Si-implanted InP substrates by molecular beam epitaxy

We describe procedures to grow InAlAs/InGaAs and InGaAlAs/InGaAs heterojunction bipolar transistors (HBTs) on Si-implanted InP substrates by molecular beam epitaxy (MBE). The combined effects of ion implantation and annealing necessitates a significant modification of MBE growth conditions in order to obtain high quality epilayers and heterojunctions. Approximately 50% higher As4 flux is needed, especially during the initial heat cleaning step, to obtain layers with good surface morphology and HBTs with dc characteristics similar to those of HBTs grown on regular InP substrates. InGaAlAs/InGaAs HBTs grown under modified conditions on implanted and annealed InP have dc current gains of 2000 at a current density of 2 kA/cm2 . Similar MBE growth conditions can be used to grow other minority-carrier lifetime sensitive devices directly on implanted and annealed InP wafers.

Effect of molecular-beam epitaxy growth conditions on GaAs–AlGaAs heterojunction bipolar transistor performance: Beryllium incorporation and device reliability

We have studied the effect of molecular-beam epitaxy (MBE) growth conditions on the performance and reliability of npn GaAs–AlGaAs heterojunction bipolar transistors (HBTs). Wafers grown under normal conditions yield devices with high current gain, but suffer premature failure due to interstitial beryllium diffusion from the base into the graded AlGaAs emitter. Wafers grown with high arsenic/gallium flux ratio and reduced substrate temperature during base deposition have high current gain, and are extremely reliable. Mean time to failure, as defined by a 10% reduction in original current gain, is ≳ 108 h at 125 °C junction temperature for devices grown using conditions optimized for device reliability. We believe the ability to produce high reliability HBTs by MBE is vital to future applications of this technology.

Sticking and Desorption Coefficients of As4 and As2 During Group V and Group III Controlled MBE Growth

ABSTRACTReflection High Energy Electron Diffraction (RHEED) oscillations under arsenic and gallium-controlled Molecular Beam Epitaxy (MBE) growth conditions have been used to measure the sticking and desorption coefficients of As2 and As4. The coefficients are obtained from measurements of the arsenic incorporation rates. Comparisons are made with measurements obtained from desorption rates using modulated beam mass spectroscopy. The transition from gallium to arsenic-controlled growth is observed to occur after excess gallium atoms accumulate on the surface. The maximum intrinsic arsenic sticking coefficients occur when the maximum number of gallium atoms can be incorporated for a given arsenic flux. The intrinsic maximum arsenic sticking coefficients are found to be 0.75 and 0.50 for As2 and As4, respectively. During galliumcontrolled growth, the arsenic sticking coefficients are independent of substrate temperature as long as the sticking coefficient of gallium is equal to one. However, a temperature dependent maximum gallium-controlled arsenic sticking coefficient exists. It can be measured by the maximum Ga to As4 flux ratio that produces specular film surfaces. During gallium-controlled growth, the Ga to As flux ratios are shown to be equal to the gallium-controlled arsenic sticking coefficients. The activation energy for arsenic desorption during arsenic-controlled growth conditions was measured as -0.50 eV for independent As4 and As2 incident fluxes. During gallium-controlled growth with incident As4 fluxes, an activation energy for arsenic desorption of -0.70 eV was measured for the maximum gallium-controlled arsenic sticking coefficients.

Surface diffusion length during MBE and MOMBE measured from distribution of growth rates

The growth rates of the layers, which were grown both by usual solid-source molecular beam epitaxy (MBE) and by metalorganic molecular beam epitaxy (MOMBE) using trimethylgallium (TMGa) and metal As source materials, on a mesa-etched (001) GaAs surface, were measured by in-situ scanning microprobe reflection high-energy electron diffraction (μ-RHEED) from the period of the RHEED intensity oscillation in real time. The diffusion length of the surface adatoms was determined by the gradient of the variation of the growth rate. The obtained values of the diffusion lengths were of the order of a micrometer under the usual growth conditions both in MBE and in MOMBE. The diffusion length in the case of MOMBE was larger than that of a Ga atom in the case of MBE.

Optical spectroscopy of (001) GaAs and AlAs under molecular-beam epitaxy growth conditions

Reflectance-difference (RD) studies were performed on variously reconstructed (001) GaAs and AlAs surfaces. The spectra of the (2×4) and (4×2) reconstructions on (001) GaAs show prominent features due to electronic transitions between lone-pair orbitals and dimer states, as previously identified by theoretical calculations. The spectra of the c(4×4) reconstructions on (001) GaAs and AlAs show similar features that we also interpret in terms of surface dimer excitations. These dimer features provide a capability of obtaining real-time, in situ information of dynamics on polar surfaces.

Role of electric fields in enhancing the doping of semiconductors during epitaxial growth

A novel technique is described to enhance the doping in semiconductors by the application of external electric fields during crystal growth. We show that this technique can enhance the doping efficiency, and suppress self-compensation processes in novel growth techniques such as molecular-beam epitaxy (MBE). An obvious application of this technique is to enhance the doping of wide band gap II–VI semiconductors, where doping in both n- and p-types is usually not possible to achieve because of extensive self-compensation. The physics of the electric field assisted doping process can be described in two parts. First, the external electric field produces a change in the band bending at the growth surface and alters the carrier concentrations near the surface region. This influences doping near the surface region. Second, this enhanced surface doping concentration can be kinetically buried by low temperature growth processes. In our calculations, the dopants are modeled as charged, mobile species that are free to diffuse and drift under electric fields. In the case of MBE growth, we solve for the equilibrium of these species in a moving coordinate frame that travels with the growth front. We have specifically applied our analysis to Li donors in n-type ZnTe. Our results indicate that excellent improvements in the doping concentrations could be obtained under normal MBE growth conditions, with the application of substantial electric fields. We expect that our analysis, and the proposed electric field assisted doping technique will play an important role in the effort to overcome compensation, and achieve selective doping in wide band gap II–VI semiconductors.

Limitations of selective epitaxial growth conditions in gas-source MBE using Si 2H 6

The limiting conditions of selective epitaxial growth (SEG) on SiO 2 patterned Si(001) substrate were studied for Si gas-source molecular beam epitaxy (MBE) by use of 100% Si 2H 6. In the initial stage of growth, epitaxial Si was selectively grown on a Si surface in the temperature range of 500 to 850°C. On the other hand, polycrystalline Si nucleation on a SiO 2 surface was intimately related to the impinging density of the Si 2H 6 molecules on SiO 2, so that SEG was limited by the supply gas volume. Under optimum SEG conditions such as substrate temperature of 700°C and Si 2H 6 flow rate of 60 SCCM, a SEG layer could be deposited at a rate as high as 645 Å/min.

Elemental boron doping behavior in silicon molecular beam epitaxy

Boron-doped Si epilayers were grown by molecular beam epitaxy (MBE) using an elemental boron source, at levels up to 2×1020 cm−3, to elucidate profile control and electrical activation over the growth temperature range 450–900 °C. Precipitation and surface segregation effects were observed at doping levels of 2×1020 cm−3 for growth temperatures above 600 °C. At growth temperatures below 600 °C, excellent profile control was achieved with complete electrical activation at concentrations of 2×1020 cm−3, corresponding to the optimal MBE growth conditions for a range of Si/SixGe1−x heterostructures.

Molecular dynamics and quasidynamics simulations of the annealing of bulk and near-surface interstitials formed in molecular-beam epitaxial Si due to low-energy particle bombardment during deposition

Molecular dynamics and quasidynamics simulations, utilizing the Tersoff many-body potential, were used to investigate the relaxation, diffusion, and annihilation of split and hexagonal interstitials resulting from 10 eV Si irradiation of (2×1)-terminated Si(001). The interstitials were created in layers two through six. Stable atomic configurations and total potential energies were derived as a function of site symmetry and layer depth. The interstitial Si atoms were allowed to diffuse and total potential energy changes calculated. Lattice configurations along each path, as well as the starting configurations, were relaxed and minimum energy diffusion paths derived. The results showed that the minimum energy paths were toward the surface and generally involved tetrahedral sites. Calculated interstital migration activation energies were always less than 1.4 eV and were much lower in the near-surface region than in the bulk. Thus, the defects could easily be annealed out over time periods corresponding to less than that required for monolayer deposition under typical molecular-beam epitaxial film growth conditions.

Independence of peak current from emitter spacer layer width in AlGaAs/GaAs resonant tunneling diodes

The peak current for the negative differential resistance region of AlGaAs/GaAs resonant tunneling diodes is shown to behave independently of the width of a low-doped emitter spacer layer if an isolated accumulation region forms upstream from the first AlGaAs barrier. The effect of the voltage drop across the emitter spacer layer is shown to be minor. These results appear to confirm that electron transport through these resonant tunneling diodes is a two-step process. In addition, a radial dependency of device performance attributed to molecular beam epitaxy growth conditions is noted.

Molecular-beam epitaxial growth of metastable Ge1−xSnx alloys

Substrate-stabilized, metastable, single-crystal Ge1−xSnx films can be grown by molecular-beam epitaxy (MBE). We have grown for the first time single crystal Ge1−xSnx alloys on lattice matched GaSb (with x=0.5) and InP (with x=0.26) substrates up to a thickness of 0.3 μm. Reflection high-energy electron diffraction (RHEED) observations and x-ray measurements show that even at very small lattice mismatch (less than 0.05%), single crystal Ge1−xSnx films cannot be grown thicker than 0.3 μm. Our x-ray results suggest that the critical thickness of α-Sn and Ge1−xSnx single crystal films is mainly determined by a phase transition mechanism, and the dislocation generation equivalent critical thickness is an overestimate. Under practical MBE growth conditions, it is very difficult to grow thick films, due to the sensitivity of the critical thickness to composition fluctuations. We have shown that even under an exact lattice match between substrate and film, the critical film thickness is limited.

I n s i t u scanning microprobe reflection high-energy electron diffraction observation of GaAs surfaces during molecular-beam epitaxial growth

A specially designed molecular-beam epitaxy (MBE) system with scanning microprobe reflection high-energy electron diffraction (RHEED) has been developed. The incident electron beam is designed to focus down to 10 nm in diameter and scan the surface of the substrate under MBE growth conditions. Using the RHEED intensity variation produced by the scanning of the incident beam, we obtain a microscopic image, i.e., which is the scanning reflection electron microscope (SREM) image, highly sensitive to the surface structure. The SREM images of GaAs(001) using the specular beam spot revealed granular features over the entire surfaces of MBE-grown GaAs layers which come from undulations of the surface. The undulation wavelength was typically 0.1–0.5 μm and developed along [1̄10] direction during MBE growth at 570 °C. At higher substrate temperatures, little development of the undulation was observed. As an overall feature of MBE growth, two-dimensional layer-by-layer growth was observed over the whole surface having undulations.