Undoped Epilayers Research Articles

Nondestructive and Quantitative Evaluation of a GaAs Epitaxial Layer Covered With a Silicon Nitride Insulating Thin Film After a Highly Accelerated Temperature and Humidity Stress With the Use of Photoreflectance Spectroscopy

We demonstrate that photoreflectance spectroscopic measurements of Franz-Keldysh oscillations are applicable to nondestructively quantify the changes in surface Fermi level and surface recombination velocity of semiconductors after reliability stress tests. The present sample, a two-layer homoepitaxial structure consisting of a top undoped GaAs epilayer and a bottom n-type GaAs epilayer, were initially covered with a silicon nitride insulating thin film. This sample was subjected to the highly accelerated temperature and humidity stress. Initially, we estimated the built-in electric field strength from the Franz-Keldysh oscillations. We found that the built-in electric field strength is enhanced after the acceleration stress. Next, we performed the numerical analysis of the built-in electric field strength as a function of probe-light power density. From the above-mentioned numerical analysis, we clarified that the surface recombination velocity of the top undoped GaAs epilayer is enhanced by a factor of two. In contrast, the surface Fermi level is hardly changed. We discuss this phenomenon from the viewpoint of the formation of the surface defects that enhance the surface recombination velocity.

A fast approach to measuring the thickness uniformity of a homoepilayer grown on to any type of silicon wafer

Optical reflection spectroscopy techniques offer a non-destructive and fast method of measuring the thickness of silicon (Si) epilayers, enabling very fast thickness uniformity mapping across the full surface of epiwafers up to 450 mm in diameter. However, their use for undoped or low doped epilayers has traditionally been constrained by a dependence on high levels of substitutional doping in the Si wafer, at values of approximately 5 × 1019 cm−3. Whilst the high dopant concentration of this wafer creates the necessary reflectance boundary for optical reflection, their commercial availability is mainly limited to the (001) surface orientation only. Optical reflectance techniques are therefore also limited in use to this orientation. In this article, an approach to measure the thickness of a Si epilayer on any Si wafer, independent of its crystallographic orientation, doping type and value, using Fourier transform infrared reflection spectroscopy is proposed and demonstrated. Because the use of non-destructive optical reflection spectroscopy is already common and well-understood within both industry and academia, this technique could easily be implemented within existing industrial and research fabrication facilities. Furthermore, this approach could be adapted, with further work, to suit other semiconductor materials and other optical reflection techniques.

Deep and shallow electronic states associated to doping, contamination and intrinsic defects in ε-Ga2O3 epilayers

Deep and shallow electronic states associated to doping, contamination and intrinsic defects in ε-Ga2O3 epilayers

Electron drift velocity in wurtzite ZnO at high electric fields: Experiment and simulation

The hot-electron effect on electron transport in nominally undoped ZnO epilayers is studied at electric fields up to 430±50 kV/cm applied with 3 ns voltage pulses. The transient measurements do not demonstrate any change in the electron density at the fields up to 320 kV/cm. The deviation from Ohm’s law can be approximated by the linear dependence of the current on the electric field valid at the moderate fields (from 50 to 250 kV/cm). The model calculations based on the Boltzmann kinetic equation are used to demonstrate that the differential mobility at the moderate fields is almost independent of the scattering on the charged point defects. The electron drift velocity is estimated from the experimental values on the differential conductivity and the current density together with the calculated differential mobility as a reference for calibration. The drift velocity reaches 2.7±0.3×107 cm/s at the field of ∼320 kV/cm and approaches the theoretical limit predicted by the known Monte Carlo simulations.

Bandgap engineering of InSb by N incorporation by metal-organic chemical vapor deposition

Bandgap engineering of InSb by N incorporation by metal-organic chemical vapor deposition

Optimization of the interfacial misfit array growth mode of GaSb epilayers on GaAs substrate

Optimization of the interfacial misfit array growth mode of GaSb epilayers on GaAs substrate

Interfacial Misfit Array Technique for GaSb Growth on GaAs (001) Substrate by Molecular Beam Epitaxy

Undoped GaSb epilayers, deposited at low growth temperature (440°C), have been grown on GaAs (001) substrate with 2° offcut towards [110], by a molecular beam epitaxy system. Interfacial misfit array (IMF) growth mode has been used in order to impede the propagation of the threading dislocations through the GaSb epilayer. Under optimized growth parameters, both transmission electron microscopy (TEM) measurements and high-resolution x-ray diffraction (HRXRD) revealed the presence of a periodic array of pure 90° edge dislocations along [110]. Furthermore, HRXRD shows a full width at half maximum of a 2-μm-thick GaSb epilayer peak as low as 195 arcsec. In addition, the GaSb layer is found to be 99.8% relaxed, with a residual strain of 1.4 × 10−4. Moreover, based on TEM measurements, the dislocations spacing or the period of the IMF was found to be 5–5.2 nm.

II-VI Material Integration With Silicon for Detector and PV Applications

ABSTRACTHeteroepitaxial growth of high-quality II-VI-alloy materials on Si substrates is a well-established commercial growth process for infrared (IR) detector devices. However, it has only recently been recognized that these same processes may have important applications for production of high-efficiency photovoltaic devices. This submission reviews the process developments that have enabled effective heteroepitaxy of II-VI alloy materials on lattice-mismatched Si for IR detectors as a foundation to describe recent efforts to apply these insights to the fabrication of multijunction Si/CdZnTe devices with ultimate conversion efficiencies >40%. Reviewed photovoltaic studies include multijunction Si/CdZnTe devices with conversion efficiency of ∼17%, analysis of structural and optoelectrical quality of undoped CdTe epilayer films on Si, and the effect that a Te-rich growth environment has on the structural and optoelectronic quality of both undoped and As-doped heteroepitaxial CdTe.

Stratigraphy of a diamond epitaxial three-dimensional overgrowth using doping superlattices

The selective doped overgrowth of 3D mesa patterns and trenches has become an essential fabrication step of advanced monolithic diamond-based power devices. The methodology here proposed combines the overgrowth of plasma-etched cylindrical mesa structures with the sequential growth of doping superlattices. The latter involve thin heavily boron doped epilayers separating thicker undoped epilayers in a periodic fashion. Besides the classical shape analysis under the scanning electron microscope relying on the appearance of facets corresponding to the main crystallographic directions and their evolution toward slow growing facets, the doping superlattices were used as markers in oriented cross-sectional lamellas prepared by focused ion beam and observed by transmission electron microscopy. This stratigraphic approach is shown here to be applicable to overgrown structures where faceting was not detectable. Intermediate growth directions were detected at different times of the growth process and the periodicity of the superlattice allowed to calculate the growth rates and parameters, providing an original insight into the planarization mechanism. Different configurations of the growth front were obtained for different sample orientations, illustrating the anisotropy of the 3D growth. Dislocations were also observed along the lateral growth fronts with two types of Burger vector: b011¯=12[011¯] and b112=16[112]. Moreover, the clustering of these extended defects in specific regions of the overgrowth prompted a proposal of two different dislocation generation mechanisms.

Field-Plated Ga2O3MOSFETs With a Breakdown Voltage of Over 750 V

Depletion-mode field-plated Ga2O3 metal–oxide–semiconductor field-effect transistors were demonstrated for the first time. Substantial enhancement in breakdown voltage was achieved with a gate-connected field plate. The device channels, formed by selective-area Si ion implantation doping of an undoped Ga2O3 epilayer, were electrically isolated by the highly resistive epilayer without mesa etching. Effective surface passivation and high Ga2O3 material quality contributed to the absence of drain current collapse. The transistors exhibited an off-state breakdown voltage of 755 V, a high drain current on/off ratio of over $10^{9}$ , and stable high temperature operation against 300°C thermal stress.

Features of free carrier and exciton recombination, diffusion, and photoluminescence in undoped and phosphorus-doped diamond layers

Features of free carrier and exciton recombination, diffusion, and photoluminescence in undoped and phosphorus-doped diamond layers

Suppressing Al memory effect on CVD growth of 4H-SiC epilayers by adding hydrogen chloride gas

The Al memory effect during the growth of Al-doped 4H-SiC by the hot-wall chemical vapor phase epitaxy method was investigated. To suppress unintentional incorporation of Al impurities during succeeding growth, a technique was developed by employing HCl-assisted “site-competition” growth. Three methods of introducing HCl, namely, HCl flushing before growth, HCl addition during growth, and the combination of the two preceding methods, have been performed and the Al suppression effects corresponding to the input C/Si ratio were studied separately. It is found that lowering the C/Si ratio reduces Al incorporation for all methods of introducing HCl and using the combination of HCl flushing and HCl addition is highly effective. Optimizing growth rate, temperature, and pressure can further improve Al suppression efficiency; thus, a highly abrupt change in Al distribution between Al-doped and undoped epilayers with Al concentration differences of more than five orders of magnitude was obtained, e.g., a steep change from 1020 to 1014 cm−3.

Magnetic, electrical and structural properties of annealed ferromagnetic (Zn,Sn)As2:Mn thin films on InP substrates: comparison with undoped ZnSnAs2

We report the magnetic and electrical properties in Mn-doped and undoped (Zn,Sn)As2 epilayers, that were annealed at slightly higher temperatures than the growth temperature. (Zn,Sn)As2 :Mn were epitaxially grown on InP (001) substrates at 300°C, and showed room-temperature ferromagnetism. The hole concentration, saturation magnetization and Curie temperature were measured and evaluated as a function of annealing temperature. The Curie temperature had a tendency to slightly increase at annealing temperatures up to 340°C, and completely disappeared at 400°C. The ferromagnetism could be attributed to hole-mediated ferromagnetism resulting from Mn ion substitutions at both the II-group Zn and IV-group Sn sites, especially from the large solubility of Mn2+ substitution at Zn sites. The disappearance of ferromagnetism may be explained by several types of mechanisms : migration of mobile interstitial Mn atoms, diffusion of substitutional Mn ions to the surface, substitution of interstitial Mn atoms on Zn vacancies, and formation of MnAs clusters. It is noteworthy that the growth of magnetic semiconductor thin films from substrate lattice matching is essential for avoiding magnetic secondary phases such as MnAs clusters.

Effect of Hydrogen and Nitrogen Carrier Gas Ratio on the Structural and Optical Properties of AlInGaN Alloy

Undoped AlInGaN epilayers on GaN templates with different hydrogen (H2) and nitrogen (N2) carrier gas ratios (1:8, 2:8, and 3:8 as samples 1, 2 and 3, respectively) were grown. When the flow ratio of H2 and N2 rises from 1:8 to 3:8, an indium composition decrease from 3% to 1.2% is observed while the aluminum content stays constant at any flow ratio. Due to the quantum-dot-like effect, photoluminescence intensity is enhanced in the sample with the low carrier gas flow ratio of H2/N2. However, the potential well caused by indium uneven distribution is nonuniform, which is more severe in the sample with carrier gas flow ratio 1:8. The process of carrier transfer from shallow to deep potential wells would be more difficult to accomplish, resulting in the reduction of the photoluminescence intensity. This is found to be consistent with the carriers' lifetime with the help of time-resolved photoluminescence.

Probing the nature of carrier localization in GaInNAs epilayers by optical methods

Photoluminescence (PL), optical pumping, and reflectance studies of nominally undoped and p-type GaInNAs epilayers are presented. The PL peak energy of the nominally undoped sample exhibits an S-shaped dependence on temperature for T < 50 K. This is attributed to recombination of bound excitons localized on traps. The energy of the PL circular-polarization maximum coincides with the energy of the free-exciton related reflectance feature at all temperatures. In heavily p-type samples the S-shaped temperature-dependence of the PL energy disappears, and the PL peak and circular polarization maximum coincide with the reflectance feature at all temperatures, indicating that the PL is free-exciton-like.

P-type InGaN across the entire alloy composition range

A systematic investigation on Mg doped and undoped InGaN epilayers grown by plasma-assisted molecular beam epitaxy has been conducted. Single phase InGaN alloys across the entire composition range were synthesized and Mg was doped into InxGa1−xN (0.1 ≤ x ≤ 0.88) epilayers up to ∼1020/cm3. Hall effect, thermopower, and electrochemical capacitance voltage experimental results demonstrate the realization of p-type InGaN across the entire alloy composition range for properly Mg doped InGaN. Hole densities have been measured or estimated to be in the lower ∼1018/cm3 range when the net acceptor concentrations are in the lower ∼1019/cm3 range across the composition range.

MOVPE growth of AIIIBV‐N semiconductor compounds for photovoltaic applications

AbstractThe present work presents the influence of the growth parameters on the structural and optical properties of undoped GaAsN epilayers and triple quantum wells 3×InGaAsN/GaAs obtained by atmospheric pressure metal organic vapour phase epitaxy APMOVPE. The structures were examined using high resolution X‐Ray diffraction HRXRD, contactless electroreflectance CER, photocurrent PC and Raman RS spectroscopies. The influence of the growth temperature and the gas phase composition on the material quality and alloy composition of the investigated structures as well as the growth and calibration characteristics are presented and discussed. (© 2012 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Effect of Bi alloying on the hole transport in the dilute bismide alloy GaAs1−xBix

We studied the effect of Bi incorporation on the hole mobility in the dilute bismide alloy GaAs${}_{1\ensuremath{-}x}$Bi${}_{x}$ using electrical transport (Hall) and photoluminescence (PL) techniques. Our measurements show that the hole mobility decreases with increasing Bi concentration. Analysis of the temperature-dependent Hall transport data of $p$-type GaAsBi epilayers along with low-temperature PL measurements of $p$-doped and undoped epilayers suggests that Bi incorporation results in the formation of several trap levels above the valence band, which we attribute to Bi-Bi pair states. The decrease in hole mobility with increasing Bi concentration can be explained as being caused by scattering at the isolated Bi and the Bi-Bi pair states. We also observed a decrease in hole concentration with Bi incorporation. We believe that Bi${}_{\mathrm{Ga}}$ heteroantisite defects compensate the acceptors, thus reducing the effective hole concentration.

Structural and optical properties of GaInP grown on germanium by metal-organic chemical vapor deposition

Structural and optical properties of Si-doped as well as nominally undoped GaInP epilayers grown on Germanium substrates by metal-organic chemical vapor deposition have been investigated by high resolution transmission electron microscope and photoluminescence (PL). Si incorporation results in an increased inner band PL transition and a blue shift of PL energy with increasing temperature, which arises from the trapping states around Ge-GaInP interface due to Ge diffusion to GaInP epilayer as well as Si doping. For the inter band PL transition, a competition between the emission processes near the band edge and in the ordered GaInP domains is responsible for the inverted S shape temperature dependence of PL peaks. By analyzing the time-resolved PL results, we attribute this emission near the ordered states to the localized states due to the potential fluctuation, which is induced by the compositional inhomogeneity of Ga and In in the partially ordered GaInP.

Comparison of the crystalline quality of homoepitaxially grown CVD diamond layer on cleaved and polished substrates

AbstractA transmission electron microscopy (TEM) study, comparing homoepitaxial diamond layers grown on different substrate surface preparation states and crystallographic orientation is presented. Quality of epilayers grown on {111} cleaved surface and on {001} polished one is evaluated in terms of crystalline defects. Focused ion beam (FIB‐dual beam) sample preparations are observed by TEM using the {111}, {400} and {022} reflections of the [011] pole. Burgers vector analysis allowed to conclude the presence of 60° dislocations oriented along all the different 〈110〉 directions in the cleaved sample, while no dislocations are observed on the polished one. A complete dislocation analysis is performed for that sample. The presence of dislocations in an undoped (i.e. fully lattice matched) epilayer is surprising; and is probably induced by atomic steps, generated by the cleavage process. This notes down the importance of the substrate if high quality diamond epilayers want to be grown.