High Resolution Laplace Deep Level Research Articles

Identification of the donor and acceptor states of the bond-centered hydrogen–carbon pair in Si and diluted SiGe alloys

The electrical and structural properties of two levels (E90 and H180) in diluted n- and p-type Si1 − xGex alloys (0 ≤ x ≤ 0.070) are investigated by high-resolution Laplace deep level transient spectroscopy measurements and first-principles calculations. By exploiting the presence of Ge atoms close to a substitutional C atom, we show that E90 and H180 belong to the same C–H pair (labeled CH1BC) with H in a bond-centered configuration (C—HBC—Si). The relative energies of the various configurations of the CH pair are calculated, and the complete vibrational spectra in the lowest-energy structures for each charge state are predicted.

Identification of titanium-hydrogen complexes with up to four hydrogen atoms in silicon

Using high-resolution Laplace deep level transient spectroscopy studies, several TiH-related complexes (E40′, E170, E170′, and E260) were observed in wet-chemically etched and H-plasma treated n-type Si. We assign E40′ and E170 to two different configurations of Ti with one H atom. Both of them are shown to behave as single donor states with an activation enthalpy of EC − 0.07 eV (E40′) and EC − 0.34 eV (E170) in the upper half of Si. E170′ with an activation energy of EC − 0.37 eV is correlated with the donor state of the TiiH2 defect, whereas E260 is attributed to the donor state of TiiH3. Besides the TiH defects, the presence of electrically inactive TiH4 is reported. No titanium-hydrogen-related levels were observed in p-type Si.

Isolated substitutional cobalt and Co-related complexes in silicon

Two dominant peaks at EC − 0.39 eV and EV + 0.46 eV previously assigned to substitutional cobalt are shown to belong to different defects by high-resolution Laplace Deep Level Transient Spectroscopy. We assign the level in the upper half of the band gap to substitutional Cos, whereas the level in the lower half is attributed to a CoB pair. No electrically active levels which belong to interstitial Coi was found. Besides the dominant defects, a number of minor DLTS peaks were observed. We correlate these peaks with H-related defects and will also discuss their origin.

No trace of divacancies at room temperature in germanium

Alpha-particle irradiated n +p-mesa diodes of Ge were investigated by conventional deep level transient spectroscopy and high-resolution Laplace deep level transient spectroscopy. The electronic and annealing properties of the observed hole traps were studied. It is concluded that none of the observed traps which are stable at room temperature are related to the divacancy. These results are consistent with previous optical studies. They are, however, in disagreement with recent numerical density function calculations which predict a stable divacancy at room temperature with band-gap levels in the lower half of the band gap.

High resolution Laplace deep level transient spectroscopy studies of electron and hole traps in n‐type GaN

AbstractDeep Level Transient Spectroscopy (DLTS) and Laplace DLTS (LDLTS) have been applied to MOVPE‐grown n‐type GaN, grown at 1020 °C on c‐plane sapphire. DLTS measured up to 600 K initially recorded three peaks due to electron emission. However, when the next rate window was sampled immediately afterwards, the DLTS scan showed a large negative peak which dominated the scan at 410K. The polarity of the feature means that this is due to hole emission. An electron trap initially present at 400 K in the DLTS spectrum gradually disappeared up to a measurement temperature of 520 K; beyond this the hole trap was observed. It then remained detectable by DLTS for up to one week, although cooling under bias decreased its intensity severely. LDLTS reveals that this hole trap is not a simple point defect as it has three emission rates. It is discussed in the context of the VGa‐ON(x) defect, where different emission rates present could reflect VGa complexed with different numbers of O atoms. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Alloy shift of “no-germanium” iron-related electronic levels in unstrained silicon-germanium alloys

We present a study of alloy-induced shifts of the electronic levels associated with isolated interstitial iron $({\mathrm{Fe}}_{i})$ and iron-aluminum pairs $({\mathrm{Fe}}_{i}\mathrm{Al})$, in which iron is sitting in the first-nearest (stable configuration) or second-nearest interstitial site (metastable) within an unstrained bulk SiGe matrix. For all of these three configurations of iron, high-resolution Laplace deep level spectroscopy reveals sitting information as it is sensitive to local atomic configurations. In the present work, we focus, however, our attention on the alloy configurations not involving Ge atoms in the first-nearest shell of host atoms surrounding iron. It has been found that the alloy-induced electronic level shifts of these ``no-germanium'' configurations are about $0.98x\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, where $x$ is the Ge fraction of the alloy, for all three forms of iron. If it is assumed that the iron-related no-germanium electronic levels do not shift on the absolute energy scale upon alloying, then the relaxed and unstrained $\mathrm{Si}∕\mathrm{Ge}$ heterojunction must be of type II with the band offsets $\ensuremath{\Delta}{E}_{v}=0.98\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ and $\ensuremath{\Delta}{E}_{c}=0.57\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. These results are discussed in the context of experimental values derived from strained systems.

Vacancy Clusters in Germanium

Fast neutron irradiation of germanium has been used to study vacancy reactions and vacancy clustering in germanium as a model system to understand ion implantation and the vacancy reactions which are responsible for the apparently low n-type doping ceiling in implanted germanium. It is found that at low neutron doses (~1011cm-2) the damage produced is very similar to that resulting from electron or gamma irradiation whereas at higher doses (> 1013cm-2) the damage is similar to that resulting from ion implantation as observed in the region near the peak of a doping implant. Electrical measurements including CV profiling, spreading resistance, Deep- Level Transient-Spectroscopy and high resolution Laplace Deep-Level Transient-Spectroscopy have been used in conjunction with positron annihilation and annealing studies. In germanium most radiation and implantation defects are acceptor like and in n-type material the vacancy is negatively charged. In consequence the coulombic repulsion between two vacancies and between vacancies and other radiation-induced defects mitigates against the formation of complexes so that simple defects such as the vacancy donor pair predominate. However in the case of ion implantation and neutron irradiation it is postulated that localized high concentrations of acceptor like defects produce regions of type inversion in which the vacancy is neutral and can combine with itself or with other radiation induced acceptor like defects. In this paper the progression from simple damage to complex damage with increasing neutron dose is examined.

Alpha-particle irradiation-induced defects in n-type germanium

Deep level transient spectroscopy and high-resolution Laplace deep level transient spectroscopy were used to investigate alpha-particle irradiation-induced defects in n-type Ge. It is proposed that there is no electrically active divacancy level in the upper half of the band gap. A dominant peak has been observed at alpha-particle doses higher than about 6×1010cm−2. The electronic and annealing properties of the defect have been investigated. The defect is suggested to be a multivacancy complex.

Annealing studies of cluster defects in ion-implanted silicon using high resolution DLTS

High resolution Laplace deep level transient spectroscopy (LDLTS) has been applied to investigate the annealing behaviour of small cluster defects in n-type Si. The Si was implanted with either Ge or Si, with energies 1500keV and 850keV respectively, and doses of 1×1010cm−2. The low dose ensured that there was a minimum of carrier removal due to deep defect states after implantation. Defect states in the as-implanted samples were attributed to VO pairs, divacancies and very small interstitial cluster defects, after detailed depth profiling. LDLTS of Ge+ and Si+ implanted silicon shows that there are three closely spaced deep levels associated with these clusters, with energies in the region of Ec-400meV. Samples were then isochronally annealed in very small temperature intervals up to 560K, in situ in our high temperature measurement cryostat, and the LDLTS re-examined as a function of annealing temperature. A new deeper energy level emerges as the cluster-related signal reduces, and it is suggested that this new trap is a major recombination centre, by comparison with current–voltage data.

Electric field enhancement of electron emission from deep level traps in Ge crystals

Effects of electric field on electron emission rates have been studied for a number of deep level traps in electron-irradiated Ge crystals. Static electric fields have been induced by the application of a reverse bias to Au–Ge Schottky barriers. High-resolution Laplace deep level transient spectroscopy has been used to study electron emissions from double acceptor states of the vacancy–oxygen center and vacancy–group-V-impurity atom (Sb or Bi) pairs. It is found that in the range of moderate electric fields [ ( 2 – 5 ) × 10 4 V / cm ] the electric-field enhancement of electron emission from the above traps can be described in the frame of a phonon-assisted tunneling model. This mechanism is characterized by the following dependence of electron emission rate on electric field E: e ( E ) / e ( 0 ) = exp ( E 2 / E ch 2 ) , where E ch is characteristic field strength, which depends on tunneling time τ 2 . The values of E ch and τ 2 have been determined and from the analysis of the values obtained conclusions on the adiabatic potential structures of the traps have been drawn. In the range of low electric fields ( < 2 × 10 4 V / cm ) deviations from the phonon-assisted tunneling mechanism of the electric-field enhancement of electron emission have been observed for the traps studied. It appears that in this range of electric fields the e( E) dependencies can be described in the frame of the Poole–Frenkel mechanism rather than phonon-assisted tunneling.

Piezospectroscopic studies of the re-orientation process of defect complexes

The stress-induced electronic level splitting pattern can provide information regarding the symmetry of a defect complex. If the components of the piezospectroscopic tensor are known it is also possible to deduce information about the lattice relaxation in the vicinity of the defect. Based on a complete piezospectroscopic analysis of the vacancy–oxygen complex in silicon we confirm that this defect has an orthorhombic symmetry in the stable configuration while in the unstable configuration of the saddle point on the reconfiguration trajectory we demonstrate that it has trigonal symmetry. The piezospectroscopic description of static defects is, in some cases, inadequate to deal with some defect complexes when they are observed at relatively high temperatures. This is often the case for the high-resolution Laplace deep level transient spectroscopy (DLTS) measurements. At very low temperatures the vacancy–oxygen–hydrogen complex has monoclinic symmetry while at the temperatures used by us for Laplace DLTS measurements the observed symmetry is orthorhombic due to fast hydrogen jumps. A similar re-orientation (position averaging) process occurs for the divacancy in silicon. However, this is true only for the divacancy in the single negative charge state while the symmetry of the complex in the double negative state is stable trigonal. We discuss how these re-orientation processes can be identified from the observed piezospectroscopic parameters for both cases.

High resolution electrical studies of vacancy-rich and interstitial-rich regions in ion-implanted silicon

A combination of high resolution Laplace deep level transient spectroscopy (LDLTS) and direct capture cross-section measurements has been used to investigate whether deep electronic states related to interstitial-type defects introduced by ion implantation originated from point or extended defects, prior to any annealing. n-type silicon was implanted with doses of 1×109 cm−2 of silicon, germanium, or erbium, and comparison was made with proton- and electron-irradiated material. When measured by LDLTS at 225 K, the region of the implant thought to contain mostly vacancy-type defects exhibited a complex spectrum with five closely spaced defect-related energy levels, with energies close to EC-400 meV. The region nearer the tail of the implant, which should be dominated by interstitial-type defects, exhibited a simpler LDLTS spectrum with three closely spaced levels being recorded, again with energies centered on EC-400 meV. Annealing at 180 °C did not completely remove any of the defect peaks, suggesting that the energy levels were not due to the simple vacancy-phosphorus center. Direct electron capture cross-section measurements revealed that the defects in the tail of the implanted volume, prior to any annealing, were not simple point defects, as they exhibited nonexponential capture properties. This is attributed to the presence of extended defects in this region. By contrast, defects with the same activation energy in proton- and electron-irradiated silicon exhibited point-defect-like exponential capture.

High resolution deep level transient spectroscopy studies of the vacancy-oxygen and related defects in ion-implanted silicon

We have carried out high resolution Laplace deep level transient Spectroscopy (DLTS) and conventional DLTS on silicon implanted with very low doses of either silicon, germanium, erbium, or ytterbium, and compared the results to those from electron-irradiated silicon. DLTS spectra of all the samples initially look very similar, and a peak at 95 K appears in all spectra which may be due to the vacancy-oxygen (VO) defect. We have carried out detailed measurements of the capture cross section and activation energy of this defect using Laplace DLTS. We show that, when the mass of the implanted ion exceeds that of silicon, the defect has a much smaller electron capture cross section than that expected for the VO defect, and a smaller activation energy. Hydrogen has been introduced, either by wet chemical processing or plasma, to all samples to observe the hydrogen–VO interactions resulting in VOH. By using high resolution DLTS we are able to establish that, after hydrogenation, the VOH defect exists with an identical emission rate in the silicon-implanted silicon and the electron-irradiated silicon, but not in the silicon implanted with heavier ions. We conclude that the peak at 95 K in the DLTS spectra in the case of the heavier ions is due to a different defect, confirming earlier reports in the literature. This defect is negatively charged, unlike VO, which is acceptor-like. We are also able to observe VOH in samples where VO is not present, after these samples have been annealed. We attribute this to release of V and H atoms from other defects during annealing.

High resolution Laplace DLTS studies of defects in ion-implanted silicon

We have used high resolution Laplace deep level transient spectroscopy (LDLTS) to investigate defects in n-type silicon caused by implantation of Si, Ge or Er with doses of the order of 1×10 9 cm −2 . These are compared with defects created in proton irradiated n-type silicon. Unlike the simple proton irradiated case, LDLTS spectra of ion implanted silicon show that there are many emission rates associated with defects with energies in the region of E c−400 meV. We have carried out annealing studies and Laplace DLTS depth profiling and show that the complex spectra measured from a region less than half way through the implant simplify as the profile is moved through the implant and towards the tail. Annealing studies show that these defects survive an anneal that should remove the E-centre.

High resolution DLTS of hydrogen reactions with defects in erbium-implanted silicon

High resolution Laplace Deep Level Transient Spectroscopy (LDLTS) has been applied to n and p type silicon implanted with low doses of either erbium or silicon, and spectra are compared to those from electron irradiated silicon. Hydrogen has been introduced by wet chemical etching. Prior to annealing n-type electron irradiated Si exhibits the deep level VOH at E C — 311 meV, but in the ion implanted n-type Si this peak is largely obscured in DLTS by a defect peak at E C — 400 meV. LDLTS overcomes this problem and shows that VOH is present in the n-type Si implanted Si, but not in Er implanted Si. In the Er implanted samples, defect migration is occurring beyond the implanted ion range, and in the p-type Er implanted Si hydrogen passivates the divacancy and the C IO I defect. The passivation of the latter state is not stable, and the C IO I deep state starts to re-appear after a 1 h anneal at 320 K.