Deep level electronic structure of ZnSe/GaAs heterostructures

We performed 1—2 keVcathodoluminescence measurements and He-Ne and HeCd excited photoluminescence studies of ZnSe/GaAs( 100) heterostructures grown by molecular beam epitaxy. Our goal was to investigate the deep level electronic structure and its connection with the heterojunction band offsets. We observed novel deep level emission features at 0.8, 0.98, 1.14, and 1.3 eV which are characteristic of the ZnSe overlayer and independent in energy of overlayer thickness. The corresponding deep levels lie far below those of the near-bandedge features commonly used to characterize the ZnSe crystal quality. The relative intensity and spatial distribution of the deep level emission was found to be strongly affected by the Zn/Se atomic flux ratio employed during ZnSe growth. The same flux ratio has been shown to influence both the quality of the ZnSe overlayer and the band offset in ZnSe/GaAs heterojunctions. In heterostructures fabricated in Se-rich growth conditions, that minimize the valence band offset and the concentration of Se vacancies, the dominant deep level emission is at 1.3 eV. For heterostructures fabricated in Zn-rich growth conditions, emission by multiple levels at 0.88,0.98, and 1.14 eV dominates. The spectral energies and intensities of deep level transitions reported here provide a characteristic indicator of ZnSe epilayer stoichiometry and near-interface defect densities.     

Effect of ion damage on the electrical and optical behavior of p-type GaAs and InGap

The effect of ion damage generated by exposure to argon or hydrogen electron cyclotron resonance plasmas at various conditions was investigated for p-InGaP and p-GaAs. Room temperature photoluminescence (PL) and either capacitance-voltage or Hall measurements were performed to determine the effect of these various treatments on the efficiency of band edge recombination and the electrical compensation. The feasibility of damage removal was investigated by examining the electrical and optical behavior after annealing at various temperatures. For argon plasma exposed InGaP, restoration of the PL intensity to ∼30% of as-grown sample could be achieved at modest annealing temperatures of ∼600°C. For hydrogen plasma exposed carbon doped GaAs, on the other hand, almost 80 ∼ 90% of the PL intensity of the as-grown sample could be recovered at 600°C. For heavily C-doped GaAs (p ∼ 10²¹cm⁻³), there was significant degradation of the optical properties at annealing temperatures ≥600°C.     

Formation mechanism of InxGa1−xAs ohmic contacts to n-type GaAs prepared by radio frequency sputtering

The formation mechanisms of InAs/Ni/W ohmic contacts to n-type GaAs prepared by radio-frequency (rf) sputtering were studied by measuring contact resistances (R_c) using a transmission line method and by analyzing the interfacial structure mainly by x-ray diffraction and transmission electron microscopy. Current-voltage characteristics of the InAs/Ni/W contacts after annealing at temperatures above 600°C showed “ohmic-like behavior.” In order to obtain the “ohmic” behavior in the contacts, pre-heating at 300°C prior to high temperature annealing was found to be essential. The contacts showed ohmic behavior after annealing at temperatures in the range of 500∼850°C and contact resistance values of as low as ∼0.3Ω-mm were obtained. By analyzing the interfacial structures of these contacts, In_xGa_1−xAs layers with low density of misfit dislocations at the In_xGa_1−xAs and GaAs interface were observed to grow epitaxially on the GaAs substrate upon heating at high temperatures. This intermediate In_xGa_1−xAs layer is believed to divide the high energy barrier at the contact metal and GaAs interface into two low barriers, resulting in reduction of the contact resistance. In addition, Ni was found to play a key role to relax a strain in the In_xGa_1−xAs layer (introduced due to lattice mismatch between the In_xGa_1−xAs and GaAs) by forming an intermediate Ni_xGaAs layer on the GaAs surface prior to formation of the In_xGa_1−xxAs layer.     

A study of the process of decomposition of supersaturated GaAs:Fe solid solution by scanning probe microscopy

Using an atomic-force microscope, the decomposition of the supersaturated solid solution of iron-doped GaAs (GaAs:Fe) is studied. GaAs:Fe samples were obtained in the course of high-temperature diffusion of Fe into GaAs and subsequent annealing at a temperature by 200°C below the doping temperature. The measurements are performed for transverse cleavages along the cleavage planes of the GaAs:Fe wafers. It is shown that, during annealing of the GaAs:Fe samples, the supersaturated alloy decomposes with the formation of particles of the second phase from ∼50 nm to ∼1 μm in size. The particles of the second phase possess ferromagnetic properties at room temperature.     

P-type doping of GaAs by carbon implantation

The electrical properties of C-implanted <100> GaAs have been studied following rapid thermal annealing at temperatures in the range from 750 to 950°C. This includes dopant profiling using differential Hall measurements. The maximum p-type activation efficiency was found to be a function of C-dose and annealing temperature, with the optimum annealing temperature varying from 900°C for C doses of 5 × 10¹³ cm⁻² to 800°C for doses ≥5 × 10¹⁴cm⁻². For low dose implants, the net p-type activation efficiency was as high as 75%; while for the highest dose implants, it dropped to as low as 0.5%. Moreover, for these high-dose samples, 5 × 10¹⁵ cm⁻², the activation efficiency was found to decrease with increasing annealing temperature, for temperatures above ∼800°C, and the net hole concentration fell below that of samples implanted to lower doses. This issue is discussed in terms of the amphoteric doping behavior of C in GaAs. Hole mobilities showed little dependence on annealing temperature but decreased with increasing implant dose, ranging from ∼100 cm²/V·s for low dose implants, to ∼65 cm²/V·s for high dose samples. These mobility values are the same or higher than those for Be-, Zn-, or Cd-implanted GaAs.     

The interactions between Si/Co films and GaAs(001) substrates

Interfacial reactions of Si/Co films on (001) oriented GaAs substrate, in the temperature range 300–700°C for 30 min, have been investigated using a combination of x-ray diffraction, Auger electron spectroscopy, and transmission electron microscopy. Cobalt starts to react with GaAs and Si at 380°C by formation of Co₂GaAs, and Co₂Si phases, respectively. At 420°C, the entire layer of Co is consumed, and the layer structure is observed with the sequence Si/CoSi/CoGa(CoAs)/Co₂GaAs/GaAs. Contacts produced in this annealing regime are rectifying and the Schottky barrier heights increase from 0.69 eV(as-deposited state) up to 0.81 eV (420°C). In the subsequent reaction, CoSi grows at the expense of the decompositions of CoGa and CoAs at 460°C. In addition, the ternary phase also is decomposed and only the CoSi phase remains upon the GaAs surface at 600°C. Contacts produced at higher temperature regime (>460°C) have low barriers. The interface between CoSi and GaAs is stable up to 700°C. The results of interfacial reactions can be understood from the calculated Si−Co−Ga−As quaternary phase diagram.     

Neutron-induced trapping levels in aluminum gallium arsenide

Deep level transient spectroscopy (DLTS) measurements have been performed on a variety of Al_xGa_1-xAs p-n junctions prior to and following a series of fast neutron irradiations at room temperature and subsequent isochronal anneals. In contrast with electron and proton irradiated GaAs, neutron irradiation produces a single, broad featureless DLTS band which is a majority carrier trap in both n and p type material. The characteristics of this neutron-induced trap are relatively independent of growth method, dopant type and concentration. In GaAs, the thermal emission energies of the trap are 0.58 to 0.68 eV depending on the particular junction. These energies increase with Al content to 0.94 eV at 20% Al. The trap introduction rate, which also increases with Al content, is 0.7 cm_-1 in GaAs. Isochronal annealing to temperatures as high as 400‡C results in a smaller FWHM of the DLTS band, a shift in the peak to higher temperatures, and a modest decrease in magnitude. Above 400‡C the magnitude decreases rapidly, suggesting a similarity with the antisite defect, As_Ga, which has been observed to anneal in this range.     

Structural properties of ZnSy Se1−yZnSe/GaAs (001) heterostructures grown by photoassisted metalorganic vapor phase epitaxy

ZnS_ySe_1−yZnSe/GaAs (001) heterostructures have been grown by photoassisted metalorganic vapor phase epitaxy, using the sources dimethylzinc, dimethylselenium, diethylsulfur, and irradiation by a Hg arc lamp. The solid phase composition vs gas phase composition characteristics have been determined for ZnSy_ySe_1−y grown with different mole fractions of dimethylselenium and different temperatures. Although the growth is not mass-transport controlled with respect to the column VI precursors, the solid phase composition vs gas phase composition characteristics are sufficiently gradual so that good compositional control and lattice matching to GaAs substrates can be readily achieved by photoassisted growth in the temperature range 360°C ≤ T ≤ 400°C. ZnSe/GaAs (001) single heterostructures were grown by a two-step process with ZnSe thicknesses in the range from 54 nm to 776 nm. Based on 004 x-ray rocking curve full width at half maximums (FWHMs), we have determined that the critical layer thickness is h_c ≤200 nm. Using the classical method involving strain, lattice relaxation is undetectable in layers thinner than 270 nm for the growth conditions used here. Therefore, the rocking curve FWHM is a more sensitive indicator of lattice relaxation than the residual strain. For ZnS_ySe_1−y layers grown on ZnSe buffers at 400°C, the measured dislocation density-thickness product Dh increases monotonically with the room temperature mismatch. Lower values of the Dh product are obtained for epitaxy on 135 nm buffers compared to the case of 270 nm buffers. This difference is due to the fact that the 135 nm ZnSe buffers are pseudomorphic as deposited. For ZnS_ySe_1−y layers grown on 135 nm ZnSe buffers at 360°C, the minimum dislocation density corresponds approximately to room-temperature lattice matching (y ∼ 5.9%), rather than growth temperature lattice matching (y ∼ 7.6%). Epitaxial layers with lower dislocation densities demonstrated superior optical quality, as judged by the near-band edge/deep level emission peak intensity ratio and the near band edge absolute peak intensity from 300K photoluminescence measurements.     

Processing and reconstruction effects on Al-GaAs(100) barrier heights

We have used low-energy electron diffraction, soft x-ray photoemission, and cathodoluminescence (CLS) spectroscopies to investigate the effects of the GaAs(100) surface geometry and composition on the formation of electrically active interface states at Al-GaAs(100) contacts. Clean GaAs(100) surfaces in the 350 to 620°C annealing temperature range undergo large compositional, structural, and electronic changes with temperature. Aluminum thin film deposition induces new discrete deep level CLS features between ∼0.80 and 1.20 eV photon energy, whose properties depend sensitively and systematically on surface annealing temperature and/or reconstruction. Fermi level (E_F) stabilization energies at these interfaces span the range from 0.58 eV above the valence band maximum (E_v) for an arsenic-rich starting surface to 0.46 eV above E_v for a gallium-rich starting surface. Correlation between the aluminum-induced deep level energies and the interface E_F position suggests an important role of localized bandgap states in determining the interface barrier height. The sensitivity of these states to starting surface composition and reconstruction may open new possibilities for tailoring Schottky barrier properties.     

Blue/green luminescence based on Zn(S)Se/GaAs heterostructures

Substantial advances have been realized in the aim to achieve blue–green light emitting devices based on Zn(S)Se wide band gap II–VI semi-conductor materials. Two light emitting diodes p on n and n on p heterostructures were grown on GaAs substrate by molecular beam epitaxy. The active layer was a single ZnCdSe quantum well, with ZnSSe guiding layers and ZnSe cladding layers. p-GaInP, p-AlGaAs and p-CdZnSe buffer layers were deposited at the p-ZnSe/GaAs interface to reduce the valence band offset in the case of n on p heterostructures. Electrical and optical properties were investigated using current voltage, capacitance voltage, electroluminescence, photoluminescence and photocurrent measurements at room temperature. Blue–green luminescence centered at 516.7 nm is observed. The highest luminescence intensity is observed under 7 V forward bias. Photoluminescence spectrum shows two wide peaks at 2.2 and 1.9 eV energies. These energies are attributed to the transitions between ZnSe and GaAs conduction bands and the deep level at E_v−0.6 eV. Absorption process from ZnSe and ZnSSe conduction bands to the shallow nitrogen acceptor level (2.6 and 2.8 eV, respectively) have been observed using photocurrent measurements. From these results we present a band alignment diagram which confirms the presence of the two levels at 0.1 and 0.6 eV from the valence band of ZnSe.     

DX centers in II-VI semiconductors and heterojunctions

Measurements of the photoconductivity and Hall effect in Ga-doped ZnSe indicate that Ga donors form DX states in ZnSe. When the photocarriers remain in the ZnSe:Ga layer, the photoconductivity is persistent up to T_a= 100K, due to a barrier to recapture the photocarriers, E_c ≈ 0.3eV. Under certain growth conditions, there is a large conduction band offset at the heterojunction with the GaAs substrate. The photocarriers are trapped at the interface, causing an enhancement of the annealing temperature to T_a≈350K. We discuss the implications of these results to device applications.     

Specific Features and Nature of the 890 nm Photoluminescence Band Detected in SiO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> Films after Low-Temperature Annealing

A band with a peak at 890 nm is detected in the photoluminescence spectra of SiO _x (x ≈ 1.3) films deposited by thermal evaporation of SiO and annealed in air at 650–1150°C. The 890-nm band appears after low-temperature (∼650°C) annealing and exhibits a number of features: (i) as the annealing temperature is elevated to 1150°C, the position of the band peak remains unchanged, whereas the intensity increases by two orders of magnitude; (ii) the effects of the annealing atmosphere (air, vacuum) and the excitation wavelength and power density on the intensity of the 890-nm band differ from the corresponding effects on the well-known bands observable in the ranges 600–650 and 700–800 nm; and (iii) the photoluminescence decay is first fast and then much slower, with corresponding lifetimes of ∼9 and ∼70 μs. The observed features are inconsistent with the interpretation of photoluminescence observed in SiO _x so far. Specifically, the earlier observed photoluminescence was attributed to transitions between the band and defect states in the matrix and between the states of band tails, transitions inside Si nanoclusters, and intraion transitions in rare-earth impurity ions. Therefore, we consider here the possibility of attributing the 890-nm band to transitions in local centers formed by silicon ions twofold- and/or threefold-coordinated with oxygen; i.e., we attempt to interpret the 890-nm band in the same manner as was done for luminescence in SiO₂ glasses and films slightly deficient in oxygen.     

Role of interface chemistry and growing surface stoichiometry on the generation of stacking faults in ZnSe/GaAs

The existence of Zn-As and vacancy-contained Ga-Se interfacial layers are suggested by transmission electron microscopy of Zn-and Se-exposed (or - reacted) ZnSe/GaAs interfaces, respectively. A very low density of faulted defects in the range of ∼10⁴cm² was obtained in samples with Zn passivation on an Asstabilized GaAs-(2 × 4). However, the density of As precipitates increases as the surface coverage of c(4 × 4) reconstruction increased on the Zn-exposed Asstabilized GaAs-(2 × 4) surface and this is associated with an increase of the density of extrinsic-type stacking faults bound by partial edge dislocations with a core structure terminated on additional cations. On the other hand, densities of extrinsic Shockley-and intrinsic Frank-type stacking faults are of ∼5 × 10⁷/cm² in samples grown on Se-exposed Ga-rich GaAs-(4 × 6) surfaces. Annealing on this Se-exposed Ga-rich GaAs-(4 × 6) generated a high density of vacancy loops (1 × 10⁹/cm²) and an increase of the densities of both Shockley-and Frank-type stacking faults (>5 × 10⁸/cm²) after the growth of the films. Furthermore, we have studied the dependence of the generation and structure of Shockley-type stacking faults on the beam flux ratios in samples grown on Zn-exposed As-stabilized GaAs-(2 × 4) surfaces. Cation-and anion-terminated extrinsic-type partial edge dislocations were generated in samples grown under Zn-and Se-rich conditions, respectively. However, an asymmetric distribution on defect density under varied beam flux ratios (0.3 ≤ P_Se/P_Zn ≤ 10) is obtained.     

Deep-level transient spectroscopy of radiation-induced levels in 6H-SiC

The methods of capacitance and current deep level transient spectroscopy are used to investigate single crystals of Leli n-SiC(6H) irradiated by 5-MeV electrons at doses of 10¹⁶–10¹⁸ cm⁻². Eleven deep levels belonging to the resulting radiation-induced intrinsic defects were observed in the energy range 0.18–1.44 eV from the bottom of the conduction band. Isochronous annealing of various samples showed that most of the observed defects were stable up to a temperature of ∼1000 °C. In addition, annealing of a deep level with ionization energy E _i=0.48–0.53 eV was observed in the temperature range 150–250 °C. It is believed that this center is caused by a vacancy in the carbon sublattice. Fiz. Tekh. Poluprovodn. 33, 1314–1319 (November 1999)     

High resistivity LT-In0.47Ga0.53P grown by gas source molecular beam epitaxy

Low-temperature (LT) growth of In_0.47Ga_0.53P was carried out in the temperature range from 200 to 260°C by gas source molecular beam epitaxy using solid Ga and In and precracked PH₃. The Hall measurements of the as-grown film showed a resistivity of ∼10⁶ Ω-cm at room temperature whereas the annealed film (at 600°C for 1 h) had at least three orders of magnitude higher resistivity. The Hall measurements, also, indicated activation energies of ∼0.5 and 0.8 eV for the asgrown and annealed samples, respectively. Double-crystal x-ray diffraction showed that the LT-InGaP films had ∼47% In composition. The angular separation, Δθ, between the GaAs substrate and the as-grown LT-InGaP film on (004) reflection was increased by 20 arc-s after annealing. In order to better understand the annealing effect, a LT-InGaP film was grown on an InGaP film grown at 480°C. While annealing did not have any effect on the HT-InGaP peak position, the LT-InGaP peak was shifted toward the HT-InGaP peak, indicating a decrease in the LT-InGaP lattice parameter. Cross-sectional transmission electron microscopy indicates the presence of phase separation in LT-InGaP films, manifested in the form of a “precipitate-like” microstructure. The analytical scanning transmission electron microscopy analysis of the LT-InGaP film revealed a group-V nonstoichiometric deviation of ∼0.5 at.% P. To our knowledge, this is the first report about the growth and characterization of LT-InGaP films.     

Electron microscopy of GaAs Structures with InAs and as quantum dots

An electron-microscopy study of GaAs structures, grown by molecular-beam epitaxy, containing two coupled layers of InAs semiconductor quantum dots (QDs) overgrown with a thin buffer GaAs layer and a layer of low-temperature-grown gallium arsenide has been performed. In subsequent annealing, an array of As nanoinclusions (metallic QDs) was formed in the low-temperature-grown GaAs layer. The variation in the microstructure of the samples during temperature and annealing conditions was examined. It was found that, at comparatively low annealing temperatures (400–500°C), the formation of the As metallic QDs array weakly depends on whether InAs semiconductor QDs are present in the preceding layers or not. In this case, the As metallic QDs have a characteristic size of about 2–3 nm upon annealing at 400°C and 4–5 nm upon annealing at 500°C for 15 min. Annealing at 600°C for 15 min in the growth setup leads to a coarsening of the As metallic QDs to 8–9 nm and to the formation of groups of such QDs in the area of the low-temperature-grown GaAs which is directly adjacent to the buffer layer separating the InAs semiconductor QDs. A more prolonged annealing at an elevated temperature (760°C) in an atmosphere of hydrogen causes a further increase in the As metallic QDs’ size to 20–25 nm and their spatial displacement into the region between the coupled InAs semiconductor QDs.     

A novel approach for the complete removal of threading dislocations from ZnSe on GaAs (001)

Here we demonstrate a novel approach to the complete removal of threading dislocations in ZnSe on GaAs (001). This approach, which we call patterned heteroepitaxial processing (PHP), involves post-growth patterning and thermal annealing. Eyitaxial layers of ZnSe on GaAs (001) were grown to thicknesses of 2000–6000 A by photoassisted metalorganic vapor phase epitaxy (MOVPE). Following growth, layers were patterned by photolithography and then annealed at elevated temperatures under flowing hydrogen. Threading dislocation densities were determined using a bromine/methanol etch followed by microscopic evaluation of the resulting etch pit densities. We found that as-grown layers contained more than 107 CM-2 threading dislocations. The complete removal of threading dislocations was accomplished by patterning to 70 gm by 70∼tm square regions followed by thermal annealing for 30 minutes at temperatures greater than 5000C. Neither post-growth annealing alone nor post-growth patterning alone had a significant effect. The effectiveness of this approach dminishes significantly below 500 C so that annealing at 400 C produces no measurable effect. We propose that the underlying mechanism for dislocation removal is the thermally activated glide of dislocations to the sidewalls of patterned regions, as promoted by sidewall image forces.     

Annealing temperature dependence of electric conduction and capacitance dispersion in nitrogen-implanted GaAs

The annealing behavior of nitrogen-implanted GaAs samples has been investigated by secondary ion mass spectroscopy, current-voltage (I-V) and capacitance-frequency (C-F) measurements. The I-V data show that the conductivity of as-implanted samples is dominated by variable-range hopping between defect states below 300 K. The implanted layer becomes highly resistive after annealing. The activation energy of the resistance is found to increase from 0.2 eV for as-implanted samples to 0.71 eV for 950°C-annealed samples. Significant capacitance dispersion is observed over frequency for implanted samples. Based on a proposed equivalent circuit, the high-frequency capacitance dispersion is shown to be the result of resistance-capacitance (RC) time constant effects. The increase of activation energy of the resistance can be explained by the creation of deep traps by high temperature annealing. Traps at 0.69 eV and 0.82 eV are detected for 700°C and 950°C-annealing, respectively.     

Shallow and Deep Centers in As-Grown and Annealed MgZnO/ZnO Structures with Quantum Wells

Shallow and deep centers in ZnO(P)/MgZnO/ZnO/MgZnO/ZnO(Ga) structures grown by pulsed laser deposition on sapphire were studied before and after annealing in oxygen atmosphere at high temperatures of 850°C to 950°C. In both as-grown and annealed structures, microcathodoluminescence spectra in the near-bandgap region demonstrate a blue-shift by 0.13 eV compared with bulk ZnO films, indicating carrier confinement in the MgZnO/ZnO/MgZnO quantum well (QW). Annealing strongly decreases the concentration of shallow uncompensated donors from ~10¹⁷ cm⁻³ to ~10¹⁶ cm⁻³ and makes it possible to probe the region of the QW by capacitance–voltage (C–V) profiling. This profiling confirms charge accumulation in the QW. The dominant electron traps in the as-grown films are the well-known traps with activation energies of 0.3 eV and 0.8 eV. After annealing, the electron traps observed in the structure have activation energies of 0.14 eV, 0.33 eV, and 0.57 eV, with the Fermi level in the n-ZnO(P) pinned by the 0.14-eV traps. The annealing also introduces deep compensating defects that decrease the intensity of band-edge luminescence and produce a deep luminescence defect band at 2.2 eV. In addition, a defect vibrational band becomes visible in Raman spectra near 650 cm⁻¹. No conversion to p-type conductivity was detected. The results are compared with the data for the structures successfully converted to p-type, and possible reasons for the observed differences are discussed.     

Formation and reconstruction of Se nanoislands at the surface of thin epitaxial ZnSe layers grown on GaAs substrates

Strained epitaxial ZnSe layers are grown on GaAs substrates by the method of vapor-phase epitaxy from metal-organic compounds. It is found that Se nanoislands with a density of 10⁸ to 10⁹ cm^–2 are formed at the surface of such layers. It is established that an increase in the size of Se islands and a decrease in their density take place after completion of growth. Annealing in a H₂ atmosphere at a temperature higher than 260°C leads to the disappearance of Se islands and to a decrease in the surface roughness. It is shown that annealing does not lead to deterioration of the structural perfection of the epitaxial ZnSe films; rather, annealing gives rise to a decrease in the intensity of impurity–defect luminescence and to an increase in the intensity of intrinsic radiation near the bottom of the exciton band.