Te Donors Research Articles

Real-time band structure changes of GaAs during continuous dynamic compression to 5 GPa

Real time changes of the GaAs band structure were determined using time-resolved photoluminescence (PL) spectroscopy, with nanosecond resolution, in single-event continuous compression experiments. Continuous compression to 5 GPa over 150 ns was achieved by impacting fused silica buffers preceding the GaAs crystals. PL spectra and compression wave profiles were measured simultaneously for uniaxial strain compression along the [100] orientation. Below 3 GPa, PL peaks from Te donors and Zn acceptors showed a blueshift upon compression, consistent with a widening of the band gap. At 3 GPa, the PL intensity decreased abruptly, due to a direct-to-indirect transition.

Influence of excited states of a deep substitutional dopant on majority-carrier concentration in semiconductors

The density $({N}_{A})$ and energy level $({E}_{A})$ of an acceptor in a $p$-type wide-band-gap semiconductor (e.g., SiC, GaN, and diamond) are determined by a least-squares fit of the charge neutrality equation to the temperature dependence of the hole concentration $p(T)$ using the Fermi-Dirac distribution function for acceptors that does not consider the influence of the excited states of the acceptor. The ${N}_{A}$ obtained this way is, however, much higher than the concentration of acceptor atoms determined by secondary ion mass spectroscopy. Because ${E}_{A}$ is far from the valence-band maximum $({E}_{V})$ and the Fermi level is between ${E}_{A}$ and ${E}_{V}$, the influence of the excited states of the acceptor on $p(T)$ should not be ignored. A distribution function including the influence of excited states, which is derived from the viewpoint of the microcanonical ensemble, not the grand canonical ensemble, leads to reliable ${N}_{A}$ and ${E}_{A}$. The situation in $n$-type Te-doped ${\mathrm{Al}}_{0.6}{\mathrm{Ga}}_{0.4}\mathrm{Sb}$ is the same, because the Te donor level is far from the conduction-band minimum for large Al mole fraction. Finally, the excited states of a substitutional dopant with a deep energy level are found to enhance the ionization of the dopant, even though they were expected to suppress the ionization because they acted as a trap according to the distribution function derived from the viewpoint of the grand canonical ensemble.

Doping and defect control of ferromagnetic semiconductors formed by ion implantation and pulsed-laser melting

We demonstrate a simple but effective route for the synthesis of ferromagnetic semiconductors that combines Mn ion implantation (II) and pulsed-laser melting (PLM). By this process Ga 1− x Mn x As films exhibiting a ferromagnetic Curie temperature ( T C) exceeding 130 K have been synthesized. These materials possess electrical and magnetic characteristics similar to molecular beam epitaxy-grown (MBE-grown) films that are tunable through experimental parameters such as implantation dose, laser energy fluence, and co-doping with Te donors. We have also used II–PLM to form ferromagnetic single-crystalline Ga 1− x Mn x P films. These films display magnetic properties similar to their arsenide counterparts despite remaining non-metallic and becoming strongly insulating at low temperatures. Unlike as-grown MBE films, both Ga 1− x Mn x P and Ga 1− x Mn x As formed by II–PLM do not contain Mn at interstitial sites; instead, non-substitutional Mn atoms are found to be randomly located, perhaps as clusters. As a result, post-PLM thermal annealing at temperatures higher than 300 °C leads to a decrease in T C as Mn atoms leave substitutional sites.

Competition of N-passivation and Te-passivation in hydrogenation of Te-doped (Ga,In)(N,As)

(Ga,In)(N,As) lattice matched to GaAs with a band gap of 1 eV is employed as active material in high-efficiency III–V solar cells. Te-doped Ga 0.934In 0.066N 0.023As 0.977 layers were grown by metal-organic vapor-phase epitaxy on (1 0 0) GaAs. The samples were highly doped n-type with carrier concentrations ranging from about 10 17–10 19 cm −3. Pieces of the samples were hydrogenated with H-doses of 10 18 ion/cm 2. The optical and electrical properties of the samples before and after hydrogenation were studied by low-temperature photoluminescence and magnetotransport. In undoped samples hydrogen is known to form N–H complexes which strongly reduce the local perturbation of the lattice due to nitrogen and thus reverse the N-induced global changes of the band structure. Combined analysis of photoluminescence and transport measurements on Te-doped samples, however, indicates a competition between N–H formation and passivation of the Te donor favoring the latter. Hardly any band structure changes due to hydrogenation are observed in these Te-doped samples, instead a strong reduction of the free-carrier concentration is observed after hydrogenation.

Ferromagnetism inGa1−xMnxP: Evidence for Inter-Mn Exchange Mediated by Localized Holes within a Detached Impurity Band

We report an energy gap for hole photoexcitation in ferromagnetic Ga(1-x)Mn(x)P that is tunable by Mn concentration (x < or = 0.06) and by compensation with Te donors. For x approximately 0.06, electrical transport is dominated by excitation across this gap above the Curie temperature (TC) of 60 K and by thermally activated hopping below TC. Magnetization measurements reveal a moment of 3.9 +/- 0.4 muB per substitutional Mn while the large anomalous Hall signal demonstrates that the ferromagnetism is carrier mediated. In aggregate these data indicate that ferromagnetic exchange is mediated by holes localized in a Mn-derived band that is detached from the valence band.

Ionization of deep Te donor in Te-doped Al0.6Ga0.4Sb epilayers

The temperature dependence of the electron concentration n(T) for a Te-doped AlxGa1−xSb epilayer with x=0.2 or x=0.6 is obtained from Hall-effect measurements. The density ND and energy level ΔED of Te donors are determined by the graphical peak analysis method (i.e., free-carrier concentration spectroscopy) from the n(T). Since the donor level of Te is shallow in Al0.2Ga0.8Sb, the Fermi–Dirac distribution function, which does not include the influence of the excited states of the Te donors, can be applied to determining ND and ΔED. In Al0.6Ga0.4Sb, on the other hand, a proposed distribution function including this influence is elucidated to be necessary to the determination of ND and ΔED, because Te acts as a deep donor. Moreover, the excited states of the Te donors in Al0.6Ga0.4Sb are found to enhance the ionization efficiency of the Te donors at elevated temperatures.

Semi‐insulating CdTe with a minimum deep level doping

Quasichemical formalism is used to study self-compensation and reactions of point defects during annealing and successive cooling with aim to find a technique to prepare semi-insulating CdTe (SICT) with minimized deep level doping. We present a theoretical model which provides SICT also in 7N or less purity material with deep level density below the limit 1013 cm–3 which is demanded for detector grade CdTe. The principle of the method is based on a proper thermal treatment with the low temperature dwell and the defect reaction between shallow acceptor Cd vacancies and shallow donor Te antisite. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Anomalous Hall effect in insulatingGa1−xMnxAs

We have investigated the effect of doping by Te on the anomalous Hall effect in Ga1-xMnxAs (x = 0.085). For this relatively high value of x the temperature dependence of resistivity shows an insulating behavior. It is well known that in Ga1-xMnxAs the Mn ions naturally act as acceptors. Additional doping by Te donors decreases the Curie temperature and increases the anomalous Hall resistivity. With increasing Te concentration the long-range ferromagnetic order in Ga1-xMnxAs eventually disappears, and paramagnetic-to-spin glass transition is observed instead. The critical concentration of holes required for establishing ferromagnetic order in Ga1-xMnxAs (x = 0.085) has been estimated by using the magnetic polaron percolation theory proposed by Kaminski and Das Sarma [Phys.Rev.Lett. 88, 247202 (2002)].

Optical properties of tellurium-dopedInxGa1−xAsySb1−y epitaxial layers studied by photoluminescence spectroscopy

Controlled doping of quaternary alloys ofInxGa1−xAsySb1−y with tellurium is fundamental to obtain the n-type layers needed for the development ofoptoelectronic devices based on p–n heterojunctions. InGaAsSb epitaxial layers weregrown by liquid phase epitaxy and Te doping was obtained by incorporating smallSb3Te2 pellets in the growth melt. The tellurium doping levels were in the range1016–1017 cm−3. We have used low-temperature photoluminescence (PL) spectroscopy to study theinfluence of the Te donor levels on the radiative transitions shown in the PL spectra. ThePL measurements were done by exciting the samples with the 448 nm line of an Ar ion laserwith varying excitation powers in the range from 10 to 200 mW. For the low-doped samplethe PL spectrum showed a narrow exciton-related peak centred at around 610 meV with afull width at half maximum (FWHM) of about 7 meV which is evidence of the goodcrystalline quality of the layers. For higher Te doping, the PL spectra show thepresence of band-to-band and donor-to-acceptor transitions which overlap as the Teconcentration increases. The peak of the PL band shifts to higher energies as Tedoping increases due to a band-filling effect as the Fermi level enters into theconduction band. From the peak energy of the PL spectra, and using a model thatincludes the band-filling and band-shrinkage effects due to the carriers, we haveestimated the effective carrier concentration due to doping with Te in the epilayers.

Pressure dependence of donor excitation spectra in AlSb

We have investigated the behavior of ground- to bound excited-state electronic transitions of Se and Te donors in AlSb as a function of hydrostatic pressure. Using broadband far-infrared Fourier transform spectroscopy, we observe qualitatively different behaviors of the electronic transition energies of the two donors. While the pressure derivative of the Te transition energy is small and constant, as might be expected for a shallow donor, the pressure derivatives of the Se transition energies are quadratic and large at low pressures, indicating that Se is actually a deep donor. In addition, at pressures between 30 and 50 kbar, we observe evidence of an anticrossing between one of the selenium electronic transitions and a two-phonon mode.

Effect of group II impurity and group III native defect on disordering Cu–Pt type ordered structures in In0.5(AlxGa1−x)0.5P layers

Cu–Pt ordering is studied with cross-sectional transmission electron microscopy in p-type, n-type, and nominally undoped InAlP and In(AlGa)P layers. These different doping conditions allow us to investigate the effect of acceptor doping with magnesium and donor doping with tellurium on removing Cu–Pt ordering in In(AlGa)P. Even more significant, however, is the ability to investigate the effect of native group III self-diffusion in p-type, n-type, and nominally undoped InAlP and In(AlGa)P layers. Our data indicate that the rapidly diffusing Mg acceptor species is less effective on removing Cu–Pt ordering than the relatively slow diffusing Te donor species. Also, our results indicate that the native group III defects and group III self-diffusion play a more important role in removing Cu–Pt ordering than either group II acceptor diffusion or group VI donor diffusion.

A New Family of Chalcogen Bearing Macrocycles: Synthesis and Characterization of N 4 O 2 E 2 (E = Se,Te) Type Compounds

A new series of 24- and 28-membered macrocyclic systems associated with "hard" (N and O) and "soft" (Se or Te) donor atoms have been developed via template free (2 + 2) condensation reactions of bis(aminoalkyl)selenides/tellurides, {NH 2 (CH 2 ) n } 2 E (E = Se, Te; n = 2,3) with 2,6-diacetyl-4-methylphenol. A macroacycle, Se{(CH 2 ) 2 N=C(CH 3 )C 6 H 2 (OH)(CH 3 )C=O(CH 3 )} 2 , has also been obtained. These compounds have been characterized by ESMS, IR, and 1 H, 13 C, and 77 Se NMR spectroscopy.

Low-temperature scanning tunneling microscopy of subsurface shallow dopants: depth dependence of the corrugation for the GaAs(110) surface

We investigated the corrugations induced by subsurface donor and acceptor atoms in low-temperature STM images of the (110) surface of doped GaAs single crystals. When the impurities are screened by Friedel charge-density oscillations, the height distribution for Si and Te donors exhibits an oscillatory behavior with four maxima being observable. We argue that these maxima do not correspond to dopant atoms in four different subsurface layers. The maxima rather correspond to four periods in the oscillatory dependence of the corrugation height on the depth of the impurity, which has been predicted by Kobayashi [Phys. Rev. B 54, 17 029 (1996)]. When the charged impurities are not screened, the height of the hillocks induced by donors, and the depth of the depressions induced by acceptors reveal an exponentially decaying distribution.

Linear electro-optic reflectance modulated spectra of GaAs (001) around E 1 and E 1+Δ 1

We report on the determination of the linear electro-optic (LEO) reflectance modulated spectra of GaAs (101). LEO spectra were obtained from photoreflectance (PR) measurements. Experiments were carried out in an energy range around the E 1 and E 1+Δ 1 interband transitions. Two samples were employed, an undoped homepitaxial GaAs and a GaAs bulk crystal doped with Te donors in the low 10 17/cm 3 range. We show that, although LEO spectra amplitudes for the two samples differ for one order of magnitude, their line shape is essentially the same. We further report on reflectance-difference (RD) measurements aimed to obtain LEO spectra. We employed the same GaAs samples for, both, PR and RD measurements in order to contrast the information provided by these two techniques.

Surfactant effects of dopants on ordering in GaInP

The use of surfactants for control of specific aspects of the VPE growth process is beginning to be studied for both the elemental and III/V semiconductors. The objective is to change the characteristics of the material grown epitaxially by the addition of a surfactant during growth. Most reported surfactant effects for semiconductors relate to some detail of the morphology of the growing films. For ordered semiconductor alloys the effects can be much more dramatic, including major changes in the electrical and optical properties. Since the bandgap energy is dependent on the microscopic arrangement of the atoms in an alloy with a fixed composition, the change in order parameter induced by the surfactant translates into a marked change in the bandgap energy. This paper presents the results of a study of the effects of n-type (Te and Si), p-type (Zn), and isoelectronic (Sb) dopants on the ordering process in GaInP grown by OMVPE. All of the dopants studied were found to decrease or eliminate ordering; however, the mechanisms are quite different. The donor Te apparently affects the adatom attachment kinetics at steps on the (001) surface, a surfactant effect. On the other hand the donor Si was found to decrease the degree of order by an entirely different mechanism, attributed to an increase in the Ga and In diffusion coefficients in the bulk. It apparently does not involve the surface. Disordering due to the acceptor Zn was found to occur by the same mechanism. The isoelectronic impurity Sb is found to act as a surfactant and to decrease the order parameter by changing the surface reconstruction, eliminating the [110]-P dimers that provide the thermodynamic driving force for formation of the CuPt structure during growth.

Te doping of GaInP: Ordering and step structure

The donor Te has been added to GaInP during organometallic vapor phase epitaxial growth using the precursor diethyltelluride. In agreement with previous studies, the addition of high Te concentrations leads to the elimination of the CuPt ordering observed in undoped layers. The degree of order is estimated from the low temperature photoluminescence peak energy to decrease from 0.5 at Te concentrations of &amp;lt;2×1017 cm−3 to 0 for Te concentrations of &amp;gt;6×1017 cm−3. This is verified by transmission electron diffraction studies, which show the elimination of the 1/2{111} superlattice spots at high Te doping levels. A remarkable change in the surface structure is found to accompany this decrease in ordering: The surfaces become much smoother. Step bunching is observed to disappear for the vicinal GaAs substrates, misoriented from (001) by 3° in the B direction, and three-dimensional island (or mound) formation is eliminated for the singular (001) substrates. A qualitative model is presented explaining this behavior based on the effect of Te on the step structure and the bonding at step edges, both of which affect the adatom sticking at steps.

Long-Living Excited State of the Te Donor in GaP

The kinetics and temperature dependences (5 to 50 K) of the extrinsic photoconductivity (PC) in microwave (MCW) and DC electric fields in GaP doped with Te ( 2 × 1017 cm−3) have been studied. A slow relaxation (≈︂10−2 s) of MCW PC, in contrast to a fast DC PC relaxation was observed at 5 K. The steady-state MCW PC exceeded the DC PC by 2 to 3 orders of magnitude at 5 to 16 K, and dropped by subsequent increase of temperature to 35 K. In the same temperature range a slow relaxation of absorption of the room-temperature background radiation, induced by GaP excitation was observed. In the range 45 to 90 meV it increased, but decreased in the higher photon energy range. The obtained results are explained by accumulation of nonequilibrium electrons in the long-living excited state 1S(Γ3) of the Te donor in GaP.

Long-lived excited state of Te donors in GaP

The kinetics of the photoresponses in constant and microwave electric fields and the variation of the absorption of background radiation in GaP doped with Te (2×1017 cm−3) upon impurity excitation at 5–50 K are investigated. The lifetime of the excited state of the Te donors is determined (∼10−2 s). It is shown that the results presented are consistent with the model of carrier accumulation in long-lived impurity excited states in semiconductors. These results are compared with the results previously obtained for diamond-structure semiconductors.

Fermi energy pinning at the surface during growth of n- and p-type GaAs

The Te donor concentration has been used as a probe to infer the surface carrier concentration during growth by organometallic vapor phase epitaxy. With the partial pressure of Te held constant during the growth of an npn structure, the Te concentration is found to remain nearly unchanged across the p-n junctions. This result is used to show that the carrier concentration at the growing surface shifts by a negligible amount as the growth is switched between n- and p-type GaAs. When combined with previous results for p-type GaAs, we conclude that the Fermi energy at the (100)-oriented surface remains pinned between 100 and 200 meV below the intrinsic Fermi energy, independent of the type or the concentration of dopant used.

Asymmetric diorganotellurides containing substituted pyridyl groups—synthesis and ligation with palladium(II) and platinum(II). Crystal structure of [Pd{4-MeC 6H 4Te CH 2CH 2-2-(C 5H 4N)} Cl 2] · 2CHCl 3

A (N,Te,N) ligand 2-(2-pyridoethyletelluro)ethylpyridine ( 1) has been synthesized by reacting in situ generated Na 2Te with 2-(2-chloroethyl)pyridine. The Pd II and Pt II complexes of stoichiometry [MCl· 1]Cl have been studied. The deshielding of ortho protons of pyridine and CH 2Te signal (up to 0.5 ppm) indicate that 1 ligates in a tridentate mode. The red shift (20–25 cm −1) in ν(TeC(alkyl)) supports the involvement of Te in coordination. Molecular weights and Λ M values of these complexes support their ionic nature (1 : 1 electrolyte). The reactions of 2-(chloromethyl)pyridine and 2,6-bis(chloromethyl)pyridine with ArTeNa (Ar = 4-MeOC 6H 4) could not produce (N,Te) and Te,N,Te) ligands comprising pyridyl groups. The CHCl 3 present as a solvent in these reactions reacted with ArTeNa resulting in (ArTe) 2CH 2. To understand the behaviour of Te donor site vis-à-vis pyridyl group the single crystal structure of Pd{4-MeC 6H 4TeCH 2CH 2-2-(C 5H 4N)}Cl 2 · 2CHCl 3 was solved. An endo, puckered chelate ring is formed by (N,Te) ligand. The trans influence of the pyridyl group appears to be lower or comparable to that of tellurium.