Bistability of theDX center in GaAs and AlxGa1-xAs,and experimental tests for negativeU of theDX level

We summarize a large body of experimental and theoretical work, especially in Si-doped GaAs and Al _x Ga_1-x As, regarding the bistability of theDX center. There is good evidence that theDX center is just the simple donor, and that each donor can exist in either of two distinct lattice configurations, each with its own spectrum of bound electronic states. Generally, the substitutional configuration binds electrons in shallow hydrogenic states, but many observations also indicate a deep (highly localized) state ofA ₁ symmetry. These states are to be distinguished from bound states of a lattice-distorted configuration, the lowest-lying of which is the deepDX level. The occupation of theDX level in thermal equilibrium with the states of the conduction band can be reasonably well modeled by assuming thatDX is either a one-electron or a two-electron state, and we discuss the reasons for this ambiguity. However, we then show that such thermal equilibrium results are consistent with thermal capture and emission kineticsonly if we assume thatDX is a two-electron state. Our results thus support the model of Chadi and Chang in which the distorted configuration is stabilized by capture of two electrons. In other words, the defect exhibits negative effective correlation energy (negativeU).     

Confirmation of large lattice relaxation of theDX center by extended photo-ionization cross-section measurements

Although the large lattice relaxation model (LLR) for electron capture at the donor relatedDX center in Al_xGa_1-xAs has seen wide acceptance over the last 12 years, there have been some recent proposals which have attempted to explain the experimental data with models that only require small lattice relaxation (SLR). One key piece of evidence supporting LLR is the large observed difference (in the case of Si-doped Al_xGa_1-xAs) between the optical (∼1.4 eV) and thermal (∼0.2 eV) ionization energies. The SLR model proposed that the lowest energy optical ionization was a very weak process, and that the optical transition which had been observed previously is a transition to a higher band. These arguments were supported by photoconductivity data showing a finite photo-ionization rate at energies as low as 200 meV. To resolve this question we have measured the photo-ionization cross section over 7 to 8 orders of magnitude using a tunable infrared laser as a source. A consistent optical ionization energy of about 1.4 eV was observed for 4 samples of differing alloy compositions and doping levels.In no case was there any detectable photo- ionization below 0.8 eV. A detailed discussion of these experiments examines the difficulty in obtaining such a large dynamic range optical spectrum. Of particular relevance are the issues of ionization detection, and the brightness of purity of the optical source. A thorough review of these issues and their impact on previous studies of theDX center is presented.     

Analysis of the vacancy-interstitial model ofDX centers

I describe the vacancy-interstitial model ofDX centers, in which the substitutional donor atoms relax toward interstitial sites when they capture electrons. This appears to be the only known model ofDX centers which can account for the structure observed by Mooneyet al. in DLTS spectra of dilute Al _x Ga_1-x As alloys. By comparing its predictions with the DLTS and Hall data from Al _x Ga_1-x As alloys of low Al content one can establish values for many of the relevant parameters, such as the enthalpies and entropies (degeneracies) associated with sites surrounded by differing numbers of Al atoms. The data also show that the distribution of donor configurations among the four available neighboring interstitial sites does not attain thermal equilibrium among states of differing energies before being released, although transitions (through tunneling) among states of equal energy may be possible. The model is seen to be consistent with those treated theoretically by Chadi and Chang forDX and by Dabrowski and Scheffler for EL2 centers.     

Atomic structure ofDX centers: Theory

A structural model forDX centers in Al _x Ga_1-x As alloys which explains their unusual properties in terms of two distinct bonding configurations for donor impurities is proposed. The results of ourab initio self-consistent pseudopotential calculations showDX to be a “negative-U” defect center. It results from a large lattice relaxation which is different for a Group IV than a Group VI donor. The proposed structural model provides a satisfactory explanation of the properties ofDX centers.     

An overview of early studies on persistent photoconductivity and other properties of deep levels in GaAsP: The effect of deep levels on light emitting devices

Laser studies carried out at the University of Illinois in 1966 showed that the shortest wavelength at which lasing could be achieved in GaAsP was dependent on then-type dopant. In particular sulfur doping was found to restrict lasing to longer wavelengths. It was suggested that this could be due to the effect of energy levels associated with the higher conduction band minima. This led in 1967 to an investigation of the effect of Te and S donor levels on the properties of GaAsP near the direct-indirect crossover. Samples throughout the composition range were studied using Hall effect measurements from 55 to 400° K and resistivity measurements under hydrostatic pressure between 0 and 7 kbar at 300, 195, and 77° K. The data on Te-doped samples fit the standard energy band models, but S doping was found to exhibit dramatic persistent photoconductivity and other compositional effects similar to the “DX-center” effects later observed in AlGaAs and other materials. This paper summarizes these results on GaAsP:S, and gives a brief overview of other early investigations in compound semiconductors where energy levels associated with higher lying minima were studied and where, in some cases, nonequilibrium effects were observed. A later study on GaAsP:S is also described which shows the effect of S-doping on LED performance. Finally, some of the implications that the existence of deep levels of this type have on light emitting device performance in other alloy systems is discussed.     

Electron paramagnetic resonance and optically-detected magnetic resonance of donors in AlxGa1-xAs

The Electron Paramagnetic Resonance (EPR) and Optically-Detected Magnetic Resonance (ODMR) work on Si-donors in Al _{x Ga1- x} As is reviewed in the context of the shallow-deep bistability (DX) problem. Three donor states are important. Little work has been published on donors tied to theT-minimum. However, there are many results forX-donors. In AlAs/GaAs heterostructures, well-resolved spectra reveal a donor state comprised of independentX _x andX _y valleys with theXz valley unpopulated due to the hetero-epitaxial strain. As Al mole fraction decreases, intervalley coupling is evident from the line positions and linewidths. The published attempts to observe and identify the deep (relaxed) state are inconclusive. Some suggestions for future work are presented.     

Structure of DX-like centers in narrow-band IV–VI semiconductors doped with group-III elements

This paper presents a study of the structure of the IR reflectance spectra in the sub-gap region of lead telluride doped with indium and gallium and the Raman spectra in PbTe(In). In the Raman and reflectance spectra of PbTe(In), features are observed at a frequency of ω ₀⋍120 cm⁻¹, whose amplitude sharply increases at temperatures T below the temperature where delayed photoconductivity appears, T _c⋍25 K. A similar feature at a frequency of ω ₀⋍155 cm⁻¹ is also observed in PbTe(Ga), with the amplitude of the feature sharply increasing for T>T _c⋍80 K. An analysis of the resulting data makes it possible to conclude that, in contrast with classical DX centers in III–V semiconductors, the microscopic structure of the impurity centers in the two-electron (DX-like) ground state does not correspond to an impurity atom shifted from a lattice site, whereas the impurity atom is shifted from a lattice site for the metastable one-electron impurity state. Fiz. Tekh. Poluprovodn. 32, 679–683 (June 1998)     

The nature of the deep levels responsible for photoelectric memory in GaAs/AlGaAs multilayer quantum-well structures

The electron capture parameters and photoionization cross section of the unintentional deep levels, which are responsible for photoelectrical memory in GaAs/AlGaAs multilayer quantum-well structures, have been found from an analysis of the kinetics of the excess current during and after optical illumination of these structures. The dependence of the photoionization cross section on the photon energy, the capture cross section, and the energy barrier for capture of an electron from the bottom of the conduction band indicate that the unintentional deep levels are DX centers formed by the silicon impurity. These DX centers probably appear during growth of the structures as a result of silicon diffusion from the quantum wells along as-grown defects. Fiz. Tekh. Poluprovodn. 32, 1213–1218 (October 1998)     

Donor-acceptor recombination in short-period silicon-doped GaAs/AlAs superlattices

A low-frequency band is observed along with an exciton band in photoluminescence spectra of short-period GaAs/AlAs superlattices doped with Si in the barriers or in the barriers and wells. This band is ascribed to donor-acceptor recombination on the basis of the dependence of its frequency on the excitation intensity under cw excitation and on the time delay under pulsed excitation. Mainly type-II superlattices are investigated. The estimate E _A+E _D≈120 meV can be obtained from the peak energy of the donor-acceptor band with a very weak excitation intensity. The estimates E _A≈23 meV and E _D≈90 meV are obtained from the temperature dependence of the band intensity. It is suggested that the deep donor level is associated with a DX center in the AlAs layers. Fiz. Tekh. Poluprovodn. 32, 839–842 (July 1998)     

DX centers in AlxGa1-xAs bulk alloy,AlAs/GaAs ordered,and disordered superlattices

Properties of theDX centers in Al_0.5Ga_0.5As bulk alloy (b-AL), (AlAs)₂ (GaSa)₂ ordered superlattice (o-SL) and (AlAs) _m (GaAs) _n disordered superlattice (d-SL) (m = 1, 2, 3,n = 1, 2, 3) with the same macroscopic composition were measured and compared. By deconvolution of deep level transient spectroscopy (DLTS) spectrum due to theDX center, we have found a decrease in the number of separate peaks in DLTS spectrum in an intentionally atomic ordered arrangement. Visiting Scholar of the Japan Society for the Promotion of Science. On leave from Department of Electrical Engineering, San Jose State University, San Jose, California 95192-0084, USA.     

Charging of deep-level centers and negative persistent photoconductivity in modulationdoped AlGaAs/GaAs heterostructures

The relaxation kinetics of persistent photoconductivity in AlGaAs/GaAs modulation-doped heterostructures due to charging of EL2-and DX-centers is investigated over a wide range of temperatures and excitation photon energies. The light-induced charging of these deep centers was found to lead to accumulation of positive and negative localized charges, which give rise to positive and negative persistent photoconductivities, respectively. These positive and negative charges are accumulated in different parts of the heterostructure. Their different characteristic times, and the different temperature dependences of these times, result in nonmonotonic time and temperature dependences of the persistent photoconductivity. Charging of EL2-centers in the GaAs buffer layer leads to negative persistent conductivity in the temperature range 180–300 K, while the negative photoconductivity observed at the temperatures below 180 K is caused by excited states of DX-centers in the n ⁺-AlGaAs. Fiz. Tekh. Poluprovodn. 33, 68–74 (January 1999)     

Electrical properties of nickel in silicon

Electrical activity and energy levels as well as diffusion properties of nickel in silicon have not yet been reliably established. In this paper, we investigated the diffusion and the electrical properties of nickel in silicon to confirm that nickel is electrically active and introduces one acceptor and one donor level by combined measurements of Hall coefficient and DLTS, and measurements of the distribution of electrically active nickel in various silicon diodes by DLTS. The former experiments show that bothn- andp- type silicon are compensated by nickel and that nickel introduces an acceptor level ofE _c-0.47 ± 0.04 eV and a donor level ofE _v +0.18 ± 0.02 eV. The concentrations of these two levels are almost identical over the diffusion temperatures from about 800 to 1100° C, indicating that these donor and acceptor levels are due to different charge states of the same nickel center. In the distribution measurements of electrically active nickel in silicon diodes, we inspected how nickel can be observed by DLTS. It was found that the nickel diffusion intop- n junction is rather complicated, the distribution profiles of nickel in the vicinity of thep- n junction being markedly influenced by an additional heating at elevated temperatures after the nickel diffusion. This gives evidence that the difference in silicon devices used in various studies could give rise to different results.     

Measurement of angular emission trajectories for magnetron-sputtered tantalum

Sputtering target crystallographic orientation affects film deposition uniformity through the emission trajectories of the sputtered atoms. Understanding the relationship between the crystallographic orientation of the sputtering target and the emission trajectory for the sputtered atoms is important for being able to accurately model film thickness uniformity and step coverage. This paper describes the measurement of atom sputter trajectory from a tantalum single crystal. A quartz crystal monitor is used to detect sputter atom flux as a function of angle. The angular sputter emission measurements are consistent with emission from tantalum single crystals along the 〈111〉 close-packed direction as well as the second close-packed 〈100〉 direction. Best fit of the experimental data occurs when the ratio of the emission probabilities in the 〈111〉 and 〈100〉 directions (P〈111〉/P〈100〉) is 1.4, and the ratio of the major and minor axis for the sputter emission ellipsoid (the b/a ratio) is 1.5.     

Magnetic properties of donors in GaAsP

The magnetic properties of hydrogenic Te donors and bistable,DX-like S donors in GaAS _x P_1-x have been investigated. Te shows an EPR signal after cooling down, both in the dark. The EPR signal from S appears only after illumination. S does not show an EPR signal after cooling down in the dark, but after illumination when the hydrogenic state of this donor is populated. Static magnetic susceptibility measurements show however that the ground state of S in GaAs _x P_1-x is paramagnetic even though it does not produce an EPR signal. This is in agreement with a one-electron ground state of this bistable donor.     

Implantation of sodium ions into germanium

The donor properties of Na atoms introduced by ion implantation into p-Ge with the resistivity 20–40 Ω cm are established for the first time. Na profiles implanted into Ge (the energies 70 and 77 keV and the doses (0.8, 3, 30) × 10¹⁴ cm⁻²) are studied. The doses and annealing temperatures at which the thermoprobe detects n-type conductivity on the sample surface are established. After implantation, the profiles exhibit an extended tail. The depth of the concentration maximum is in good agreement with the calculated mean projected range of Na ions R _p . Annealing for 30 min at temperatures of 250–700°C brings about a redistribution of Na atoms with the formation of segregation peaks at a depth, which is dependent on the ion dose, and is accompanied by the diffusion of Na atoms to the surface with subsequent evaporation. After annealing at 700°C less than 7% of the implanted ions remain in the matrix. The shape of the profile tail portions measured after annealing at temperatures 300–400°C is indicative of the diffusion of a small fraction of Na atoms into the depth of the sample.     

Annealing studies of photoluminescence spectra from multiple Er3+ centers in er-implanted GaN

A study of selectively excited photoluminescence (PL) at ∼ 6K in Er-im planted GaN as a function of annealing temperature (400–1000°C) has detected nine different Er³⁺ centers with distinct ∼ 1540 nm ⁴I_13/2 → ⁴I_15/2 Er³⁺ PL spectra and different activation temperatures. However, most of the optically active implanted Er atoms are incorporated at annealing temperatures as low as 400°C on a single type of center for which PL can only be excited efficiently by direct intra-4f shell absorption and is not strongly pumped by either above-gap or broad-band below-gap absorption. This strongly suggests that this high-concentration Er³⁺ center is an isolated, isoelectronic center consistent with Er³⁺ substituted on a Ga site. In contrast, a very small fraction of the Er atoms that form a variety of Er-defect/impurity complexes dominate the Er³⁺ emission spectra excited by above-gap and broad-band below-gap absorption because of their larger cross sections for both carrier capture and optical absorption.     

Investigation of energy levels of Er-impurity centers in Si by the method of ballistic electron emission spectroscopy

The method of ballistic electron emission spectroscopy is used for the first time to study the energy spectrum of Er-impurity complexes in Si. The features are observed in the ballistic electron spectra of mesa diodes based on p ⁺-n ⁺ Si structures with a thin (∼30 nm) p ⁺-Si:Er surface layer in the region of ballistic-electron energies eV _t lower than the conduction-band-edge energy E _c in this layer. They are associated with the tunnel injection of ballistic electrons from the probe of the scanning tunnel microscope to the deep donor levels of the Er-impurity complexes in the p ⁺-Si:Er layer with subsequent thermal excitation into the conduction band and the diffusion to the p ⁺-n ⁺ junction and the direct tunneling in it. To verify this assumption, the ballistic-electron transport was simulated in the system of the Pt probe, native-oxide layer SiO₂-p ⁺-Si:Er-n ⁺, and Si substrate. By approximating the experimental ballistic-electron spectra with the modeling spectra, the ground-state energy of the Er complex in Si was determined: E _d ≈ E _c − 0.27 eV. The indicated value is consistent with the data published previously and obtained from the measurements of the temperature dependence of the free-carrier concentration in Si:Er layers.     

Impurity centers of tin in glassy arsenic chalcogenides

¹¹⁹Sn atoms produced by radioactive decay of ¹¹⁹Sb impurity atoms in the structure of As _x S_{1 − x} and As _x Se_{1 − x} glasses are stabilized in the form of Sn²⁺ and Sn⁴⁺ ions at arsenic sites and correspond to ionized states of the amphoteric two-electron center with negative correlation energy (Sn²⁺ is an ionized acceptor, and Sn⁴⁺ is an ionized donor), whereas the neutral state of the Sn³⁺ center is unstable. The fraction of Sn⁴⁺ states increases with chalcogen content in glass. ¹¹⁹Sn atoms produced by radioactive decay of ^119m Te impurity atoms in the structure of As _x S_{1 − x} and As _x Se_{1 − x} glasses are stabilized at chalcogen sites (they are electrically inactive) and arsenic sites, and the fraction of arsenic atoms decreases with the chalcogen content in glass.     

Use of tertiarybutylphosphine for OMVPE growth of (AlxGa1-x)o.51 In0.49P

This paper reports the results of atmospheric pressure organometallic vapor phase epitaxial growth of (Al _x Ga_1-x )_{0.51 0.49}P thin films using tertiarybutylphosphine (TBP) as the phosphorus source. The trimethylalkyls were used as group III sources. The growth temperature was 680° C. It was observed that V/III ratio dramatically affected the surface morphology and photoluminescence (PL). The epilayers grown at a V/III ratio lower than 60 had rough surfaces and weak PL emission. An input V/III ratio larger than 70 was required to obtain good surfaces and strong PL emission. Good quality (Al_xGa_1-x )_{0.51 0.49}P epilayers forx ≤ 0.62 were obtained at a V/III ratio of 85. The surface morphologies were smooth except for the occurrence of dense oval-shaped hillocks forx > 0.42. The Al distribution coefficient using TBP is the same as for phosphine (PH₃), which was used as the phosphorus source in previous AlGalnP growth. No parasitic reactions between TBP and trimethylalkyls were observed. 10 and 300 K PL emission was observed for all epilayers withx ≤ 0.62. However, the PL peak energy did not follow the band gap, as obtained for (Al_xGa_1-x )_{0.51 0.49}P epilayers grown using PH₃. The PL peak energy at both 10 and 300 K increased with increasingx forx ≤ 0.35, and then became nearly constant with further increases inx. In this region the PL is believed to be from a process involving a deep energy level, induced by an impurity from the TBP, bound to theX conduction band minimum. It was concluded that TBP has the potential to replace PH₃ for OMVPE growth of Al-containing compounds, although the purity needs to be improved.     

Effects of S,Si, or Fe dopants on the diffusion of Zn in InP during MOCVD

Diffusion of Zn in InP during growth of InP epitaxial layers has been investigated in layer structures consisting of Zn-InP epilayers grown on S-InP and Fe-InP substrates, and on undoped InP epilayers. The layers were grown by metalorganic chemical vapour deposition (MOCVD) atT = 625° C andP = 75 Torr. Dopant diffusion profiles were measured by secondary ion mass spectrometry (SIMS). At sufficiently high Zn doping levels ([Zn] ≥8 × 10¹⁷ cm⁻³) diffusion into S-InP substrates took place, with accumulation of Zn in the substrate at a concentration similar to [S]. Diffusion into undoped InP epilayers produced a diffusion tail at low [Zn] levels, probably associated with interstitial Zn diffusion. For diffusion into Fe-InP, this low level diffusion produced a region of constant Zn concentration at [Zn] ≈ 3 × 10¹⁶ cm⁻³, due to kick-out of the original Fe species from substitutional sites. We also investigated diffusion out of (Zn, Si) codoped InP epilayers grown on Fe-InP substates. The SIMS profiles were characterised by a sharp decrease in [Zn] at the epilayer-substrate interface; the magnitude of this decrease corresponded to that of the Si donor level in the epilayer. For [Si] ≫ [Zn] in the epilayer no Zn diffusion was observed; Hall measurements indicated that the donor and acceptor species in those samples were electrically active. All these results are consistent with the presence of donor-acceptor interactions in InP, resulting in the formation of ionised donor-acceptor pairs which are immobile, and do not contribute to the diffusion process.