GaAs Surface Layer Research Articles

Terahertz radiation from n-type GaAs with Be-doped low-temperature-grown GaAs surface layers

We investigate the performance of n-type GaAs plasmon emitters with surface layers grown by low-temperature molecular-beam epitaxy. The emitters have as-grown Be-doped low-temperature-grown GaAs layers on the top surface, which radiate terahertz transients with a power four times greater than reference emitters without low-temperature-grown (LTG) GaAs layers. The power generated by the structures with low-temperature-grown GaAs layers shows much weaker saturation at large excitation densities. The mechanism of the surface field screened by photoexcited holes has been discussed to explain the negative dip in the THz wave form when the emitter is coated by a LTG GaAs layer.

Strain analysis of a quantum-wire system produced by cleaved edge overgrowth using grazing incidence x-ray diffraction

A GaAs surface layer of 10 nm thickness was grown on the cleaved edge of an In0.1Al0.9As/Al0.33Ga0.67As multilayer in order to induce a lateral periodic strain modulation. We apply surface sensitive grazing incidence x-ray diffraction to distinguish between compositional/morphological and purely strain induced modulations. The experimentally determined strain profile is confirmed by finite-element model calculations. The GaAs layer is found to be purely strain modulated with an average lattice parameter change of (0.8±0.1)% with respect to relaxed GaAs.

Growth of c-GaN films on GaAs(100) using hot-wire CVD

Cubic gallium nitride (GaN) films were grown on nitrided layers of GaAs(100) by hot-wire chemical vapor deposition. The nitrided layer was also formed by NH x radicals generated on a tungsten hot-wire surface. Nitridation conditions for the growth of GaN with a cubic-type structure were investigated. As a result, GaN film with a preponderant cubic phase was grown on the GaAs surface layer nitrided at a substrate temperature of 550 °C, a filament temperature of 1200 °C and an ammonia (NH 3) pressure of 1 Torr.

Structure of porous surface layers of single-crystal GaAs(001) wafers from data of X-ray diffractometry and reflectometry

The surface morphology and structure parameters of the surface layers of the single-crystal GaAs(001) wafers subjected to He+ ion implantation (E = 300 keV, D = 1016 atoms/cm2) and subsequent electrochemical etching in a 0.5 M H2SO4 aqueous solution are investigated by X-ray reflectometry and double-crystal X-ray diffractometry. The strain and amorphization profiles over the layer thickness are determined from X-ray diffraction data. The density of surface layers, their thickness, and the changes of the surface relief upon etching are evaluated from the reflectometric data. The experimental parameters of the studied layers are compared with the theoretical distributions of implanted impurities and intrinsic point defects, which are calculated by the Monte Carlo method. A correlation is revealed between the layer thickness and the depth of the maximum defect concentration. It is found that the electrochemical etching predominantly occurs in strongly amorphized regions of the surface layer and does not lead to a change in the total layer thickness. The results of X-ray diffraction investigations are confirmed by scanning electron microscopy.

Threshold of inelastic strain formation in Si and GaAs surface layers under multiple pulsed laser irradiation

For the first time, a contactless local photoacoustic technique based on the spectroscopy of laser beam deviation was used to estimate maximal values of elastic shear strains ϕ0 in micrometer-sized surface regions of Si and GaAs; the values obtained fall in the range of 10−5 ϕ0 is demonstrated. Studying the diffuse and Raman scattering of light near the thresholds ϕ0 suggests that the early stage of the inelastic cyclic strain photoinduced in the surface layers is accompanied by the generation and spatial redistribution of point rather than extended defects (e.g., dislocations). A number of threshold values, such as the photoinduced increase in the temperature and the mean shear stresses that appear in the surface layers of Si and GaAs samples exposed to local submicrosecond radiation, are estimated. The physical nature of the low-threshold effects is discussed.

Surface-modified GaAs terahertz plasmon emitter

We studied the THz emission from n-GaAs plasmon emitters modified by low-temperature-grown (LT) GaAs surface layers. The THz emission is increased since the LT GaAs pins the Fermi level at a midgap position, increasing the surface depletion field. For a THz emitter with a 70-nm-thick LT GaAs layer we observe without external fields a THz emission intensity of 140 nW. In addition, the long-term performance of the modified emitters is improved by the LT GaAs surface layer.

Matrix effects on the structural and optical properties of InAs quantum dots

InAs quantum dots (QDs) have been grown by molecular-beam epitaxy on different InGaAs or GaAs surface layers to investigate the effect of the matrix on the structural and optical properties of the QDs. The density of QDs directly grown on GaAs is 1.1×1010 cm−2, and increases to 2.3×1010 cm−2 for dots grown on a 1 nm InGaAs layer. Single-mirror light-emitting-diode (SMLED) structures with InAs QDs capped by InGaAs and grown on GaAs and InGaAs layers were fabricated to compare the electroluminescence efficiency between the two structures. The maximum external quantum efficiency for QDs on a GaAs structure is 1.1% while that for QDs on InGaAs is 1.3%. The corresponding radiative efficiency could be deduced to be 17.5% for QDs on GaAs and 21.5% for QDs on InGaAs, respectively.

Characterization of the surface layer of GaAs nitrided by high-density plasma

A high-density N2 plasma was generated after mode jump by a helical antenna surrounded by magnetic coils. In the wavelength region of 600–800nm, the emission lines from the N2 first positive system and that from N atoms (3p→3s) were also observed, in addition to the emission lines from the N2 second positive and N2+ first negative systems in the ultraviolet and violet regions. At the mode jump, the emission peak intensity from N atoms especially increased. Nitridation of GaAs was performed in the N2 plasma before and after mode jump. N 1s peak intensities from the surface layer nitrided in the N2 plasma after mode jump measured by X-ray photoelectron spectroscopy were 2–5 times larger than those nitrided before mode jump, which indicates the enhancement of the nitridation. From the spectroscopic ellipsometry, the thickness of nitrided layer was 3–11nm.

Composition of the surface layer of GaAs after laser annealing of the Al–GaAs system

Laser assisted doping of GaAs from an Al thin film, deposited on its surface, has been carried out. The unreacted aluminum, left on the irradiated surface, has been removed chemically after the irradiation and the surface of the substrate has been investigated by electron microscope and X-ray diffraction studies. It has been found that when the system Al–GaAs is irradiated with lower laser energy densities, the composition of the solid phase is in the private triangle Al–GaAs–AlAs. When the system Al–GaAs is irradiated with intermediate laser energy densities, the solid phase, which probably consists of Ga x Al 1− x As, has been found. When the system Al–GaAs is irradiated with high laser energy density small amounts of Ga x Al 1− x As are formed in the solid phase. The results are explained on the basis of the thermal annealing model for interaction of laser irradiation with the matter.

Thermal Stability of Indium Tin Oxide/n-GaAs Heterostructures with and without Sulfur Passivation

Thermally evaporated ITO/n-GaAs contacts with and without sulfur passivation are studied. It is determined that passivation improves the thermal stability (up to 700 °C annealing temperature) of contacts. This improvement is due to formation of thermally stable S-S, S-As, and S-Ga bonds at GaAs surface layer.

Ohmic nanocontacts to GaAs using undoped and p-doped layers of low-temperature-grown GaAs

The development and characterization of high-performance nanocontacts to n-GaAs are reported. The nanocontacts can be made to both undoped and p-doped low-temperature-grown GaAs (LTG:GaAs) cap layers. The geometry of the nanocontact is well characterized and requires the deposition of a 4 nm single-crystalline Au cluster onto an ohmic contact structure which features a chemically stable LTG:GaAs surface layer prepared using an ex situ chemical self-assembly technique. A self-assembled monolayer of xylyl dithiol (HS–CH2–C6H4–CH2–SH) is required to provide mechanical and electronic tethering of the Au cluster to the LTG:GaAs surface. For the case of an undoped LTG:GaAs cap layer, a specific contact resistance of 1×10−6 Ω cm2 and a current density of 1×106 A/cm2 have been measured from scanning tunneling microscopy. When a p-doped LTG:GaAs cap layer is used, the corresponding values are 1×10−7 Ω cm2 and 1×107 A/cm2, respectively. Improved surface stability as evidenced by a lower oxidation rate for p-doped LTG:GaAs provides a natural explanation for the higher-quality ohmic contact properties of the nanocontact to the p-doped LTG:GaAs cap layer.

Nanoelectronic device applications of a chemically stable GaAs structure

We report on nanoelectronic device applications of a nonalloyed contact structure which utilizes a surface layer of low-temperature grown GaAs as a chemically stable surface. In contrast to typical ex situ ohmic contacts formed on n-type semiconductors such as GaAs, this approach can provide uniform contact interfaces which are essentially planar injectors, making them suitable as contacts to shallow devices with overall dimensions below 50 nm. Characterization of the native layers and surfaces coated with self-assembled monolayers of organic molecules provides a picture of the chemical and electronic stability of the layer structures. We have recently developed controlled nanostructures which incorporate metallic nanoclusters, a conjugated organic interface layer, and the chemically stable semiconductor surface layers. These studies indicate that stable nanocontacts (4 nm×4 nm) can be realized with specific contact resistances less than 1×10−6 Ω cm2 and maximum current densities (1×106 A/cm2) comparable to those observed in high quality large area contacts. The ability to form stable, low resistance interfaces between metallic nanoclusters and semiconductor device layers using ex situ processing allows chemical self-assembly techniques to be utilized to form interesting nanoscale semiconductor devices. This article will describe the surface and nanocontact characterization results, and will discuss device applications and novel techniques for patterning close-packed arrays of nanocontacts and for imaging the resulting structures.

Interaction of GaSe with GaAs(111): Formation of heterostructures with large lattice mismatch

We have studied the epitaxial growth of GaSe, a layered van der Waals material, on GaAs, a zinc-blende-structure semiconductor. This heterostructure exhibits a 6% lattice mismatch, and is a prototypical example of van der Waals epitaxy, where the weak van der Waals interaction allows the misfit to be accommodated without the formation of electronically active defects. GaSe was supplied to the growing surface from a single GaSe Knudsen cell. Reflection high energy electron diffraction and x-ray photoemission spectroscopy studies of the nucleation of GaSe indicate Se reacts with the GaAs surface to remove the surface dangling bonds prior to GaSe formation. This is followed by the oriented growth of stoichiometric GaSe layers, that are rotationally aligned with the underlying GaAs substrate. The termination of the GaAs dangling bonds most likely occurs by Se substitution for As in the surface layer of GaAs(111) B and by direct bonding of Se to surface Ga on GaAs(111) A surfaces. In addition, photoemission measurements indicate that the subsurface Se uptake into the GaAs(111) A lattice is higher than that in the (111) B lattice.

Circular dichroism in the angular distribution of core-level photoelectrons from GaAs (111)

The circular dichroism of angle-resolved photoelectron spectra was investigated for core-levels from clean and hydrogen exposed GaAs (111) surfaces. It was found that the circular dichroism of Ga-3d and As-3d photoelectrons shows clear dependence on the emission angle of the photoelectrons and the sample surface. It is proposed that the experimental results may be interpreted primarily in terms of the interference of photoelectrons affected by the anisotropic field in the surface layer of GaAs (111).

Electrical and microstructural characteristics of Ge/Cu ohmic contacts to n-type GaAs

It is shown that Cu–Ge alloys prepared by depositing sequentially Cu and Ge layers onto GaAs substrates at room temperature followed by annealing at 400 °C form a low-resistance ohmic contact to n-type GaAs over a wide range of Ge concentration that extends from 15 to 40 at. %. The contacts exhibit a specific contact resistivity of 7 × 10−7 Ω cm2 on n-type GaAs with doping concentrations of 1 × 1017 cm−3. The contact resistivity is unaffected by varying the Ge concentration in the range studied and is not influenced by the deposition sequence of the Cu and Ge layers. Cross-sectional high-resolution transmission electron microscopy results show that the addition of Ge to Cu in this concentration range causes Cu to react only with Ge forming the ξ and ε1–Cu3Ge phases which correlate with the low contact resistivity. The ξ and ε1–Cu3Ge phases have a planar and structurally abrupt interface with the GaAs substrate without any interfacial transition layer. It is suggested that Ge is incorporated into the GaAs as an n-type impurity creating a highly doped n+-GaAs surface layer which is responsible for the ohmic behavior. n-channel GaAs metal-semiconductor field-effect transistors using ohmic contacts formed with the ξ and ε1–Cu3Ge phases demonstrate a higher transconductance compared to devices with AuGeNi contacts.

Properties of Al, (AuGe)-GaAs contacts fabricated by combining ion-beam treatment and metallization

because many questions connected with the modification of the structure and chemical composition of a GaAs surface still have not been investigated. The characteristic features of the formation of contacts with both barrier ~Al! and Ohmic ~AuGe! metallizations integrating the aluminum or AuGe euctectic deposition processes with preliminary IBT were investigated using a combination of electrophysical, optical, and secondary-emission methods. The chemical composition of a AuGe‐GaAs heterostructure was analyzed using layer-by-layer ion etching, while the characteristic features of the interphase interactions in Al‐GaAs contacts were studied during formation of the contacts starting with submonolayer coatings. Figure 1 displays the distribution of the atomic components in a AuGe‐GaAs contact before ~a! and after ~b! IBT of a GaAs surface with Ar 1 ions (E5200 eV). One can see that the destruction which occurs in the near-surface region of GaAs during bombardment and is accompanied by the generation of defects in the semiconductor lattice stimulates Ge diffusion in the GaAs surface layer. This is also promoted by the active absorption of Ga by the Au film, observed in these structures under these conditions. At the same time, the changes in the As profiles are smaller, and considering the poor solubility of As in Au and the absence of intermetallic compounds of gold with arsenic, the observed blurring of the As profile is most likely due to the morphology of the film coating. The changes noted in the microstructure and atomic composition of the interphase boundary ~IPB! of the contact lead to a change in its electric characteristics ~see Table I!. The observed decrease in the contact resistance R under optimal ion bombardment conditions is a consequence of Ge diffusion in the surface region of GaAs. Although a quite high potential barrier ~0.5 eV! is formed at the AuGe‐GaAs interface, the increase in the donor impurity concentration ~Ge! in the surface layer of GaAs results in a narrower barrier for current flow in the contact and a larger tunneling component of the current. The weak effect of the ion beam on R under other bombardment conditions and even its increase are apparently due to compensation effects due to Au diffusion in GaAs or the formation of an amorphized layer, whose contribution to the contact resistance is determined by its thickness and conductivity. The ion-bombardment-stimulated changes in the parameters of surface-barrier structures ~see Table I! are accompanied by a more complicated interphase interaction mechanism, primarily due to the greater role of chemical effects. In this case, the dominant processes are Al reduction processes in the oxide phases of the GaAs surface and the formation of intermetallic phases with the semiconductor components. 3 This results in a more homogeneous phase, which replaces the initial heterogeneous oxide layer on the GaAs surface. In the process, Ga diffuses to the surface of the metal film, and this changes the defect composition of the near-surface layer of the semiconductor. An investigation of the photoluminescence ~PL! spectra confirmed the radical changes produced in the defect composition of the semiconductor layer in contact with the metal by these processes. It was found that the generated defects influence the magnitude of the Schottky barrier, but they play a larger role in the processes associated with current flow. As their density increases, the Schottky barrier undergoes degradation.

Depth measurement of doped semiconductors using the Hall technique

A new method using the Hall technique to determine the change in surface layer thickness of doped semiconductors is presented. An equation to calculate the semiconductor thickness change has been determined by comparing the difference in Hall measured sheet carrier concentration and mobility before and after a change in surface layer thickness. Experiments were conducted using a wet chemical digital etch to remove n-type GaAs surface layers having an incremental etch depth control of approximately 15 Å in thickness, and the resulting thickness changes were calculated by the Hall technique and measured with a mechanical profilometer. This Hall measurement technique was able to measure changes in surface layer thickness of less than 100 Å, and the accuracy of this new technique compared favorably with mechanical profilometer measurements. The new Hall technique method provides accurate measurements of minute thickness changes, and is more accurate than mechanical profilometers for thickness changes less than 150 Å.

Inhibited oxidation in low-temperature grown GaAs surface layers observed by photoelectron spectroscopy

The surface oxidation characteristics of a GaAs layer structure consisting of a thin (10 nm) layer of low-temperature-grown GaAs (LTG:GaAs) on a heavily n-doped GaAs layer, both grown by molecular beam epitaxy, have been studied using x-ray photoelectron spectroscopy (XPS). Between the layer growth and XPS characterization, the unannealed LTG:GaAs sample and a control sample without the LTG:GaAs surface layer were exposed to the atmosphere. The rate of surface oxidation in the sample with a LTG:GaAs surface layer was significantly lower than the oxidation rate of the control sample. This direct observation of inhibited oxidation confirms the surface stability of comparable structures inferred from earlier electrical measurements. The inhibited surface oxidation rate is attributed to the bulk Fermi-level pinning and the low minority carrier lifetime in unannealed LTG:GaAs.

Cu3Ge ohmic contacts to n-type GaAs

We show that Cu-Ge alloys prepared by depositing sequentially Cu and Ge layers onto GaAs substrates at room temperature followed by annealing at 400°C form a low-resistance ohmic contact to n-type GaAs over a wide range of Ge concentration that extends from 20 to 40 at.%. A contact resistivity of (4-6) x 10-7 Ω cm2 is obtained on n-type GaAs with doping concentrations of∼ 1 x 1017 cm-3. The contact resistivity is affected only slightly by varying the Ge concentration in the range studied and is not influenced by the deposition sequence of the Cu and Ge layers. In addition, the contacts are electrically stable during annealing at 450°C after contact formation. Structure and properties of Cu-Ge contact layers having lower and higher Ge concentrations from the stoichiometric Cu3Ge composition are compared. High-resolution transmission electron microscopy and x-ray diffractometry have been used to study the ordering in the e1-Cu3Ge (average lattice parameters: a0= 5.30A, b0= 4.20A, c0= 4.56A) which is responsible for orthorhombic distortion of the parent hexagonal ζ-phase. The results suggest that the formation of theξ and e-Cu3Ge phases creates a highly doped n+-GaAs surface layer which leads to the low contact resistivity.

Arsenic precipitation from thin surface layers of low-temperature grown GaAs

Arsenic precipitation from a thin, 100-nm surface layer of GaAs grown at low temperature (LT) by molecular beam epitaxy is investigated. The precipitate depth distribution is examined for different rapid thermal annealing cycles. It is found that the precipitate distribution can tail a long distance into the underlying stoichiometric GaAs layer, depending on the peak annealing temperature. The distribution for an 800 °C anneal is virtually unaffected by a prior low temperature ‘‘soak’’ at 600 °C, thus showing that the precipitation is insensitive to the initial point defect concentrations in this temperature range. The relevance of these results to the precipitation process and to the use of thin LT layers in device applications is discussed.