Session N26 - Wide-Band-Gap Semiconductors IX: Defects and Impurities in Semiconductors.
MIXED session, Thursday morning, March 20
Room 4300A, Conv. Center

[N26.01] Electron Paramagnetic Resonance Studies of ZnSe Epilayers

Electron paramagnetic resonance (EPR) studies were performed on a series of ZnSe samples grown by MBE at West Virginia University. Samples include undoped ZnSe, ZnSe:N, and ZnSe:H. EPR spectra taken at 8 K and 9.45 GHz as the magnetic field was rotated in the plane from [100] to [010] showed an isotropic signal at g=2.0027 with a line width of 5.8 G in many of the epilayers. We associate this signal with a singly ionized selenium vacancy center based on the small shift from the free spin g value. A similar EPR spectrum in ZnS has been assigned to singly ionized sulphur vacancies [1]. The appearance of the EPR signal in ZnSe correlated with an increase in the Zn/Se beam equivalent pressure ratio (during growth) in undoped films and with an increase in nitrogen concentration in N-doped films. We conclude that the singly ionized selenium vacancy may be a dominant point defect in many MBE-grown ZnSe layers and may play a role in compensation mechanisms in heavily nitrogen-doped ZnSe thin films. Work supported by the NSF/WV EPSCoR program (NSF Grant OSR- 9255224). 1 J. Schneider and A. Rauber, Solid State Commun. 5, 779 (1967).

[N26.02] Pressure-Photoluminescence Study of the Zn Vacancy and Donor Zn-Vacancy Complexes in ZnSe

We report photoluminescence (PL) results to 65kbar (at 8K) on n-type electron irradiated ZnSe containing high densities of isolated Zn vacancies (V_Zn) and donor-V_Zn complexes (A-centers).^1 Isotropic pressure is applied using a diamond-anvil cell with He medium, and laser excitations above and below the ZnSe bandgap (2.82eV) are employed. The 1 atm. spectra exhibit excitonic lines, shallow donor-acceptor pair (DAP) peaks, and two broad bands due to DAP transitions between shallow donors and deep acceptor states at A-centers (2.07eV) or V_Zn (1.72eV). At all pressures, these broad bands are prominent only for sub-gap excitation, which results in: i) A-center PL at energies above the laser line, and ii) strong enhancement of the first LO-replica in the shallow DAP series compared to 3.41eV UV excitation. This suggests that sub-gap excitation produces long-lived metastable acceptor states. The broad PL bands shift to higher energy with pressure faster than the ZnSe direct gap, indicating that compression causes the A-center and V_Zn deep acceptor levels to approach the hole continuum. This behavior is similar to that found by our group for P and As deep acceptor levels in ZnSe, supporting the view that deep substitutional defects often resemble the limiting case of a vacancy. ^1D. Y. Jeon, H. P. Gislason, G. D. Watkins Phys. Rev. B 48, 7872 (1993); we thank G. D. Watkins for providing the samples.

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[N26.03] ODMR Studies of DAP-Luminescence in Heavily N-Doped ZnSe

Optically-Detected Magnetic Resonance (ODMR) performed on MBE-grown N-doped ZnSe reveals two defects. The resonances were detected on DAP emission at 24 GHz on two samples grown on GaAs substrates. The first sample (p about 10^17 cm^-3) exhibits luminescence-decreasing signals around g = 2. These are identical with resonances reported previously(T.A. Kennedy et al., Appl. Phys. Lett. 65, 1112 (1994)) and assigned to a Se-vacancy pair. The second sample (p about 3x10^17 cm^-3) exhibits PL which peaks at 2.52 eV.(C. Kothanadaraman et al., Appl. Phys. Lett. 67, 3307 (1995)) A new luminescence-increasing signal is observed on this band with g_parallel=1.998 and g_perp=2.010. This signal is very broad with FWHM = 5.5 mT. Since the width is independent of modulation frequency, it is likely to be from either strain or local electric fields. The spectrum is tentatively assigned to N-acceptors. Further work will be done to look for the ODMR of the donors recombining with the N-acceptors. Work at NRL supported by the Office of Naval Research.

[N26.04] Effects of Simple and Complex Defects on the Structural and Electronic Properties of ZnSe.

There is very great interest in altering the electronic and consequent optical properties of wide-band-gap semiconductors through the use of selected additives(dopants). These effects depend on the way in which the additive enters the lattice structure, and the resulting possible lattice relaxation effects and their repercussions. For that reason, we have studied the energetics of site selection and lattice relaxation for selected additives, such as As and N, in ZnSe using a full-potential, linear combination of muffin-tin-orbitals total energy calculation with an atomic force routine. We have also found the relaxation around a vacancy of either Zn or Se. In addition, we have considered selected combinations of dopants and defects(complexes) to discover possible compensation mechanisms inhibiting the desired doping in this material. We have obtained results for the location the additive atoms assume within the unit cell of ZnSe and the lattice response of ZnSe in these various configurations.

[N26.05] Photoluminescence Identification of Frenkel Pairs on the Zn-Sublattice of ZnTe

Irradiation of single crystal ZnTe by 2.5 MeV electrons \textitin situ at 4.2 K produces a broad luminescence band at 1.0 \mum and a weaker partially overlapping band at 1.15 \mum. ODMR studies of the weaker band have revealed close vacancy-interstitial Frenkel pairs on the Te sublattice which anneal thermally in various stages, the last and dominant pair disappearing at 120 K.(B. Ittermann et al, Mat. Sci. Forum 143-7, 423 (1994).) We here report on the 1.0 \mum band which shows no ODMR and anneals at 100 K in a stage unrelated to the ODMR pairs. By study of its optical polarization properties vs orientation of the bombarding electron beam, we conclude that it arises from displacements on the Zn sublattice. It appears insensitive to recombination-enhanced annealing processes, in contrast to the Te sublattice pairs, which can be annihilated by optical injection at T < 35 K.

[N26.06] Doping Problems in Wide Bandgap Materials Revisited

This abstract was not submitted electronically.

[N26.07] Stabilization of Ternary Semiconductors via Formation of Ordered Arrays of Defect Pairs,

We showed, using the general potential linearized augmented plane wave (LAPW) method and a supercell approach that defects in ternary semiconductors such as CuInSe_2 are qualitatively different from conventional binary semiconductors such as ZnSe: (i) Neutral cation vacancy in CuInSe_2 has considerably lower, and sometimes negative formation energies than ZnSe. (ii) Oppositely charged defect pairs (e.g., V_Cu^- and In_Cu^++) tend to bind, with binding energies as large as -2.6 eV/pair and (iii) such neutral pairs order crystallographically with additional binding of -0.8 eV/pair forming ordered defect arrays. We identified the origin for the formation of ordered arrays as the Madelung binding of the charged constituents of the defect pairs. While isolated defects in CuInSe_2 tend to create many deep levels in the band gap, the formation of such defect pairs and subsequently their ordered arrays, remove such levels from the band gap, resulting in an increase of the band gap. Our results explain the existence of various forms of the Cu-In-Se systems and the fact that despite the large defect concentrations, Cu-In-Se systems are often good device materials for low cost solar cells.

\vskip0.1cm Supported under DOE/EE contract No. DE-AC36-83CH10093

[N26.08] Pressure Dependence of Shallow Donors in AlSb

We have studied ground to bound excited state transitions associated with Se and Te donors in AlSb under large hydrostatic pressure using IR absorption spectroscopy. A modified Merrill-Basset diamond anvil cell was used to apply pressure of a few GPa to samples of AlSb which were moderately doped (~10^16 cm^-3) with either Se or Te. Although both donors are in the DX configuration at these pressures, they were converted to the metastable shallow donor state by illumination at low temperatures with a red light emitting diode. At atmospheric pressure, the Se donors show two distinct peaks at 946 cm^-1 and 1121 cm^-1, corresponding to ground to bound excited state transitions, as well as a large continuum absorption. The Te donors show only one distinct peak at 496 cm^-1 and a large continuum absorption. Upon the application of hydrostatic presure, the Se peaks shift rapidly downwards in energy with a quadratic dependence. At low pressures, the shift is ~ -12 cm^-1/kbar, roughly equal to the shift of the bandgap in pressure. In addition, some interaction with the AlSb two-optical phonon mode is observed at higher pressures. In contrast, the Te peak decreases in energy linearly with pressure at a rate of 1 cm^-1/kbar. The implications of these results will be discussed. Work supported by USNSF DMR 94-17763 at Berkeley and DMR 93-03186 at Purdue.

[N26.09] Observation of non-Pinch-Off Current Saturation in InAs/AlSb Single Quantum Well FETs

Current saturation is a commonly observed phenomenon in all the field-effect transistors (FETs). It is mainly attributed to the ``pinch-off'' effect and velocity saturation. Here we report an InAs/AlSb single quantum well(QW) FET in which the current saturation is from neither pinch-off, nor velocity saturation. The transistors are fabricated by using MBE-grown InAs/AlSb heterostructures. The 120 ÅInAs QW is designed to be close to surface and sandwiched by a 50 ÅAlSb top barrier and a 500 Åbottom barrier. Therefore, the accumulation of electrons in the QW may come from surface donors and deep donors in the AlSb barriers. In equilibrium, those donors above Fermi level are all ionized. The ionized donors, however, can trap the energetic electrons which are accelerated by the lateral electric field. The effective carrier concentration is reduced and the drain current will decrease. The direct evidence of electron trapping in the AlSb barrier has been observed from the hysteresis loops in the source-drain current, where the loop size and direction depend on the polarity of the gate bias and temperature. We believe that the current saturation in this transistor is a result of trapping and de-trapping of free carriers by the ionized donors near and above the quantum well Fermi level.

[N26.10] The Amphoteric States of Hydrogen in Crystalline GaAs

The behavior of Hydrogen in crystalline GaAs is examined with total energy calculations, in which the ab initio pseudo-potentials are used in a supercell crystal geometry, by Car-Parrenello method. The static equilibrium sites for H are determined, and the charged states are studied according to the Fermi-level position. If the relaxation in the whole crystal is allowed, H is a stable shallow donor around the bond-centered site (BC) in p-type material, while it is a stable shallow acceptor around the interstitial site surrounded by four nearest Ga host atoms (T_Ga) in n-type material. The results explain the experimentally observed hydrogen passivation effects of dopants in both p-type and n-type GaAs. The diffusion paths of H are also discussed.

[N26.11] Isotope splitting of the zero-phonon line of Fe^2+ in cubic semiconductors

A theoretical study of the isotopic-mass dependence of the internal transitions of Fe^2+ at a cation site in a cubic zinc-blende semiconductor is presented. The model used is based on crystal-field theory, including spin-orbit and spin-spin interactions and a dynamic Jahn-Teller coupling between the ^5 \Gamma_5 excited manifold of Fe^2+ and a phonon of \Gamma_5 symmetry. The effect of the dynamic Jahn-Teller interaction on the local vibronic states is calculated as a function of the isotopic mass using a model similar to that of Colignon et al. (D. Colignon, E. Kartheuser, S. Rodriguez, and M. Villeret, Phys. Rev. B 51), 4849 (1995). The theoretical results are compared with the observed isotopic shifts (K. Pressel, K. Thonke, A. Dörnen, and G. Pensel, Phys. Rev. B 43), 2239 (1991). of the zero-phonon line in InP:Fe^2+ originating from optical transitions between the vibronic \Gamma_1 ground state and \Gamma_5 excited states.

Work supported by NSF Grant No. DMR 93-03186, The Nuffield Science Foundation (U.K.) and by the North Atlantic Treaty Organization (Research Grant No. 960666).

[N26.12] Decay Kinetics of Growth-Induced Alignment of Carbon-Containing Defects in AlGaAs: Motion of C in the Alloy

We have recently discovered that the first neighbor shell of C\(_\mboxAs\) in Ga\(_\mboxx\)Al\(_\mbox1-x\)As is aligned in some samples following growth by metalorganic molecular beam epitaxy.(J.-F. Zheng et al., Bull. Am. Phys. Soc. 41), 340 (1996). (J.-F. Zheng et al., Proc. Fall MRS Symp. on Defects in Electronic Materials, Boston, Dec., 1996.) The C atom's Al neighbors preferentially lie in a particular \110\ plane perpendicular to the (001) growth surface. This alignment was detected from the polarization properties of the vibrational absorption bands assigned to C\(_\mboxAs\) and also bands assigned to C\(_\mboxAs\)-H complexes. Here we report studies of the decay kinetics of the growth-induced alignment upon thermal annealing. Presumably, the loss of alignment is due to motion of the C atom. Thus the decay kinetics provide a novel source of information about the motion of C in AlGaAs. \mbox Supported by ONR Grant No. N00014-94-1-0117.

[N26.13] \lambda-Effect and Trap Concentration in Deep Levels of LT-GaAs

This abstract was not submitted electronically.

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