Donor Acceptor Pair Peak Research Articles

Study of optical properties of high-resistivity CdMnTe:In single crystals before and after H2 atmosphere annealing

In order to investigate the passivation effects of hydrogen, high-resistivity indium doped cadmium manganese telluride (CMT:In) single crystals were annealed in H2 atmosphere. The results indicated the concentration of Mn element and the density of Te inclusions did not change after annealing. Both the resistivity and IR transmittance increased because of the reduction of the carrier concentration. The intensity of (D0,X) peak and (D0,h) peak decreases remarkably because of the passivation of the indium donor. The donor–acceptor pair peak (DAP) peak disappeared, which was due to the dissociation of [VCd − InCd] complexes and the passivation of the impurities by hydrogen. Moreover, Dcomplex peak consisted of two peaks. The intensities of D1 peak increased and D2 peak decreased because the passivation of hydrogen on the donor and dislocation, respectively. The best condition of passivation for H2 atmosphere annealing is 573 K for 30 h.

Photoluminescence of CuInSe2/GaN and CuInSe2/InN

The power and temperature dependent photoluminescence (PL) of epitaxially grown In-rich CuInSe2 (CIS) and Cu-rich CIS deposited on N-polar GaN and InN were investigated in this paper. The In-rich CIS/GaN has two PL emissions characterized by a donor-acceptor pair (DAP) peak at 0.92 eV and an excitonic peak at 1.08/1.1 eV. On the other hand, the Cu-rich CIS/GaN has four PL emissions characterized by two DAP peaks at 0.86 eV and 0.94 eV, free-to-bound recombination peak at 0.97 eV and an excitonic peak at 1.03 eV. Identification of these defects in the CIS absorber layer is crucial for the improvement of the device efficiency. The PL emission of the CIS/InN closely resembles that of the Cu-rich CIS/GaN. For all the samples, the PL intensity increased with excitation power while the PL intensity decreased with temperature. The obtained power coefficients and activation energies support the proposed mechanism causing the luminescence. The defects present in CIS/GaN and CIS/InN were also identified. The results from this study are consistent with those of CIS single crystals and show that the CIS absorber layer can be incorporated with III-nitride materials, and its absorption can be extended beyond the spectrum covered by plain CIS.

Shallow acceptor state in ZnO realized by ion irradiation and annealing route

Activation of shallow acceptor state has been observed in ion irradiated and subsequently air annealed polycrystalline ZnO material. Low temperature photoluminescence (PL) spectrum of the sample exhibits clear signature of acceptor bound exciton (ABX) emission at 3.360 eV. The other two samples, pristine and ion irradiated (without annealing), however, do not show acceptor related PL emission in the nearby energy region. Electron transition from shallow donor (most probable site is interstitial zinc for undoped ZnO) to such newly formed shallow acceptor state creates new donor-acceptor pair (DAP) luminescence peak ∼ 3.229 eV. ABX and DAP peak energy positions confirm that the acceptor is N related. The acceptor exciton binding energy has been estimated to be 180 ± 15 meV which is in conformity with earlier reports. The activation of shallow acceptors without any source of atomic nitrogen can only be possible through diffusion of molecular nitrogen inside the sample during annealing. The N2 molecules get trapped at bulk defect sites incorporated by ion irradiation and subsequent annealing. X-ray diffraction (XRD) and Raman spectroscopic (RS) investigation have been employed to probe the changing defective nature of the ZnO samples. Irradiation induced increased disorder has been detected (both by XRD and RS) which is partially removed/modified by annealing at 300 °C. Simultaneous activation of molecular nitrogen acceptor in purposefully defective ZnO is the key finding of this work. Results presented here provide a simple but controlled way of producing shallow acceptor state in ZnO. If optimized through suitable choice of ion, its energy and fluence as well as the annealing temperature, this methodology can trigger further scope to fabricate devices using ZnO epitaxial thin films or nanowires.

Fluorescent P-Type 4H-SiC Grown by PVT Method

Fluorescent SiC, which contains donor and acceptor impurities with optimum concentrations, can work as a phosphor for visible light emission by donor-acceptor-pair (DAP) recombination. In this work, 3 inch N-B-Al co-doped fluorescent 4H-SiC crystals are prepared by PVT method. The p-type fluorescent 4H-SiC with low aluminum doping concentration can show intensive yellow-green fluorescence at room temperature. N-B DAP peak wavelength shifts from 578nm to 525nm and weak N-Al DAP emission occurred 403/420 nm quenches, when the temperature increases from 4K to 298K. The aluminum doping induces higher defect concentration in the fluorescent crystal and decreases optical transmissivity of the crystal in the visible light range. It triggers more non-radiative recombination and light absorption losses in the crystal.

Enhancement of phosphors-solubility in ZnO by thermal annealing

We have demonstrated the effect of annealing temperature on the diffusion density of phosphors in zinc oxide. The P-dopant P430 was sprayed on ZnO pellets and annealed at different temperatures from 500 to 1000 °C with a step of 100 °C for one hour using a programmable furnace. The concentration of P was controlled by varying the annealing temperature and the maximum solubility of P (3% At) was achieved at annealing 800 °C determined by energy dispersive X-ray diffraction (EDX) measurements. X-ray diffraction (XRD) confirmed the hexagonal structure of ZnO and showed that the growth direction was along the c-axis. We observed a maximum up shift in the (002) plane at an annealing temperature of 800 °C, suggesting that P atoms replaced Zn atoms in the structure which results in the reduction of the lattice constant. Room temperature photoluminescence (PL) spectrum consists of a peak at 3.28 eV and related to band edge emission, but samples annealed at 800 and 900 °C have an additional donor acceptor pair peak at 3.2 eV. Hall effect measurements confirmed the p-type conductivity of the sample annealed at 800 °C.

Donor-acceptor-pair emission in fluorescent 4H-SiC grown by PVT method

Fluorescent SiC, which contains donor and acceptor impurities with optimum concentrations, can work as a phosphor for visible light emission by donor-acceptor-pair (DAP) recombination. In this work, 3 inch N-B-Al co-doped fluorescent 4H-SiC crystals are prepared by PVT method. The p-type fluorescent 4H-SiC with low aluminum doping concentration can show intensive yellow-green fluorescence at room temperature. N-B DAP peak wavelength shifts from 578nm to 525nm and weak N-Al DAP emission occurred 403/420 nm quenches, when the temperature increases from 4K to 298K. The aluminum doping induces higher defect concentration in the fluorescent crystal and decreases optical transmissivity of the crystal in the visible light range. It triggers more non-radiative recombination and light absorption losses in the crystal.

Enhancement of P Diffusion Density in Bulk ZnO for p-type Conductivity

In this paper, we have demonstrated the effect of annealing temperature on the diffusion density of phosphors in Zinc Oxide. The spin on p-dopant was sprayed on ZnO pellets and annealed at different temperatures from 500 to 1000 ̊C with a step of 100 ̊C for one hour using a programmable furnace. The concentration of P was controlled by varying the annealing temperature and the maximum solubility of P (14% At) was achieved at annealing 800 ̊C determined by Energy Dispersive X-Ray Diffraction (EDX) measurements. X-Ray Diffraction (XRD) confirmed the hexagonal structure of ZnO and showed that growth direction was along C-axis. We observed a maximum up shift in (002) plane at annealing temperature 800 ̊C, suggested that P atoms replaced Zn atoms in structure which results in the reduction of lattice constant. Room temperature photoluminescence (PL) spectrum consists of peak at 3.28eV and related to band edge emission, but samples annealed at 800 and 900 ̊C have an additional donor acceptor pair peak at 3.2 eV. Hot probe method confirmed the p-type conductivity of sample annealed at 800 ̊C.

Optical property analysis of high-resistivity CZT:In single crystals before and after annealing

In-doped CdZnTe:In (CZT:In) single crystals with high resistivity were annealed by an effective method containing Cd/Zn atmosphere annealing and Te atmosphere annealing. They were mainly measured by IR transmittance and Photoluminescence (PL) spectra. The results indicated Te inclusions were eliminated completely after annealing. Both of the resistivity and IR transmittance decreased notably after Cd/Zn atmosphere annealing, and then increased tremendously after Te atmosphere annealing. For PL measurements, (A0, X) disappeared, the intensity of (D0, X) peak increased, and FWHM was obviously reduced after annealing. These indicated the crystal quality was enhanced. The donor–acceptor pair peak decreased in annealed CZT:In crystal, which might be due to the dissociation of [VCd−InCd] complex and the remove of the impurities. Moreover, Dcomplex peak containing two peaks was caused by Cd vacancy-related (D1) and dislocation-related defects (D2) in as-grown crystal. However, after annealing, the intensities of D1 and D2 peaks decreased because of the compensation of Cd vacancies and the elimination of Te inclusions, respectively.

Properties of the main Mg-related acceptors in GaN from optical and structural studies

The luminescent properties of Mg-doped GaN have recently received particular attention, e.g., in the light of new theoretical calculations, where the deep 2.9 eV luminescence band was suggested to be the main optical signature of the substitutional MgGa acceptor, thus, having a rather large binding energy and a strong phonon coupling in optical transitions. We present new experimental data on homoepitaxial Mg-doped layers, which together with the previous collection of data give an improved experimental picture of the various luminescence features in Mg-doped GaN. In n-type GaN with moderate Mg doping (<1018 cm−3), the 3.466 eV ABE1 acceptor bound exciton and the associated 3.27 eV donor-acceptor pair (DAP) band are the only strong photoluminescence (PL) signals at 2 K, and are identified as related to the substitutional Mg acceptor with a binding energy of 0.225 ± 0.005 eV, and with a moderate phonon coupling strength. Interaction between basal plane stacking faults (BSFs) and Mg acceptors is suggested to give rise to a second deeper Mg acceptor species, with optical signatures ABE2 at 3.455 eV and a corresponding weak and broad DAP peak at about 3.15 eV. The 2.9 eV PL band has been ascribed to many different processes in the literature. It might be correlated with another deep level having a low concentration, only prominent at high Mg doping in material grown by the Metal Organic Chemical Vapor Deposition technique. The origin of the low temperature metastability of the Mg-related luminescence observed by many authors is here reinterpreted and explained as related to a separate non-radiative metastable deep level defect, i.e., not the MgGa acceptor.

Photoluminescence analysis of high-resistivity CdZnTe:In single crystals obtained by annealing

High-resistivity In-doped CdZnTe (CZT:In) single crystals were obtained by annealing under Te atmosphere. Photoluminescence spectrum was used to investigate annealed CZT:In single crystals. The results indicated that (D0, X) peak which is related to the crystal quality appeared after annealing. The donor–acceptor pair peak DAP mainly originated from the complex consisted of the donor In+Cd and the acceptor [In+CdV2−Cd]−. The intensity of DAP peak decreased, which might suggest that the impurities were removed. Moreover, Dcomplex peak containing two peaks for as-grown CZT:In consisted of Cd vacancy-related (D1) and dislocation-related (D2) defects. However, the intensity of D2 peak was very low because of the elimination of Cd inclusions after annealing. And the intensity of D1 peak increased because of the increase of Cd vacancies by evaporating for a long time.

Low-Temperature Photoluminescence Study of CdTe:In Crystals Annealed in Molten Bismuth

We used a low-temperature photoluminescence (PL) technique to investigate CdTe:In crystals after annealing in molten bismuth (Bi). The two annealed samples showed different resistivities after the treatment. For both samples, we observed very strong emissions in the excitonic spectral region and revealed fine structures of exciton emissions in the PL spectrum. In the sample with high resistivity, we found one ionized donor-bound exciton peak, (D+,X), that we ascribed to incorporated Bi atoms occupying Cd sites in the CdTe. The temperature dependence of the (D+,X) peak emission had an associated activation energy of 3.59 meV for the exciton bound to this ionized donor. Meanwhile, a donor–acceptor pair peak at 1.5315 eV, which was absent from the PL of the low-resistivity sample, suggested the likelihood of some Bi atoms occupying Te sites in the high-resistivity sample. Our findings highlight the need for detailed investigation of annealing conditions to ensure precise control of the electrical properties of the material during annealing in molten Bi.

Time-resolved cathodoluminescence of Mg-doped GaN

The optical properties of GaN films doped with magnesium concentrations of the order of 1018 and 1019cm−3 have been analyzed by picosecond time-resolved cathodoluminescence spectroscopy. Two luminescence bands in the vicinity of ∼3.45 and ∼3.27eV are observed to increase in intensity with Mg concentration. They have been identified as related to acceptor-bound exciton and donor-acceptor pair (DAP) transitions, respectively. Time-resolved cathodoluminescence spectra, taken at low temperatures (∼4K), show that the lifetime of the DAP peak (3.27eV) and of what appears to be its first longitudinal-optical phonon replica at ∼3.19eV decrease with increasing Mg concentration. These peaks differ in their recombination kinetics, strongly suggesting the existence of an underlying broad emission band which may be related to Mg.

Nitrogen-related recombination mechanisms in p-type ZnO films grown by plasma-assisted molecular beam epitaxy

The recombination mechanisms of nitrogen-related emissions in p-type ZnO films were investigated by photoluminescence (PL) measurements. The enhanced peak at 3.260 eV was confirmed as a donor-acceptor pair (DAP) transition and the emission around 3.310 eV was assigned to the free electron to acceptor (FA) recombination by studying the temperature evolution of DAP and FA luminescence, calculating the energy level of the corresponding nitrogen acceptor, and measuring the decay time of DAP transition. By comparing the PL spectra of the samples with various hole concentrations, it can be found that as the hole concentration increases, the DAP peak significantly dominates the spectra. In addition, the acceptor binding energy is estimated to be about 120 meV from the FA transition, which is in good agreement with the value calculated by a hydrogenic acceptor model.

Photoluminescence analysis of Cd 1−xZn xTe single crystals annealed by a two-step method

Cd 1− x Zn x Te (CZT) single crystals were annealed by a two-step method, including a vapor-environment step and a liquid-environment step in sequence. Photoluminescence spectra were used to investigate the effects of annealing on the properties of CZT. After annealing the full width at half maximum of the donor-bound exciton (D 0,X) peak was reduced, and the free-exciton emission was weakened. Meanwhile, the intensity of the donor–acceptor pair peak was greatly decreased. In addition, the deep defect-related emission band D complex disappeared after the annealing, which was distinct for as-grown CZT wafers. The investigation confirms that the two-step annealing can compensate for cadmium vacancies and possibly reduce the impurities from CZT wafers.

Electrical and optical activation studies of Si-implanted GaN

Comprehensive and systematic electrical and optical activation studies of Si-implanted GaN were made as a function of ion dose and anneal temperature. Silicon ions were implanted at 200 keV with doses ranging from 1×1013 cm−2 to 5×1015 cm−2 at room temperature. The samples were proximity-cap annealed from 1050°C to 1350°C with a 500-A-thick AlN cap in a nitrogen environment. The optimum anneal temperature for high dose implanted samples is approximately 1350°C, exhibiting nearly 100% electrical activation efficiency. For low dose (≤5×1014 cm−2) samples, the electrical activation efficiencies continue to increase with an anneal temperature through 1350°C. Consistent with the electrical results, the photoluminescence (PL) measurements show excellent implantation damage recovery after annealing the samples at 1350°C for 20 sec, exhibiting a sharp neutral-donor-bound exciton peak along with a sharp donor-acceptor pair peak. The mobilities increase with anneal temperature, and the highest mobility obtained is 250 cm2/Vs. The results also indicate that the AlN cap protected the implanted GaN layer during high-temperature annealing without creating significant anneal-induced damage.

Photoluminescence of mechanically polished ZnO

The effects of mechanical polishing on the photoluminescence (PL) from each polar face of wurtzite-structure ZnO are presented. Differences observed for the 4.2K PL of a mechanically polished surface when compared to that of a chemomechanically polished surface include broadened bound-exciton peaks, hot-exciton luminescence, and a donor-acceptor pair peak at 3.2108eV. Analysis of this donor-acceptor pair peak results in estimated donor and acceptor ionization energies of 52±10 and 230±10meV, respectively, with a mean separation distance between pairs of approximately 3–4nm. The donors and acceptors are attributed to point defects introduced by dislocation motion during the polishing process and identified as octahedral Zn interstitials and Zn vacancies, respectively.

Optical study of implantation damage recovery from Si-implanted GaN

Comprehensive and systematic optical activation studies of Si-implanted GaN grown on sapphire substrates have been made as a function of ion dose and anneal temperature. Silicon ions were implanted at 200 keV with doses ranging from 1×10 13 to 5×10 15 cm −2 at room temperature. The samples were proximity cap annealed from 1250 to 1350 °C with a 500-Å-thick AlN cap in a nitrogen environment. The results of photoluminescence measurements made at 3 K show a very sharp neutral-donor-bound exciton peak along with a sharp donor–acceptor pair peak after annealing at 1350 °C for 20 s, indicating excellent implantation damage recovery. The results also indicate the AlN cap protected the implanted GaN layer very well during high temperature annealing without creating any significant anneal-induced damage. This observation is consistent with the electrical activation results for these samples.

Optical Activation Behavior of Ion Implanted Acceptor Species in GaN

Ion implantation is used to investigate the spectroscopic properties of Mg, Be, and C acceptors in GaN. Activation of these species is studied using low temperature photoluminescence (PL). Low dose implants into high quality undoped hydride vapor phase epitaxial (HVPE) material are used in conjunction with high temperature (1300 °C) rapid thermal anneals to obtain high quality spectra. Dramatic, dose-dependent evidence of Mg acceptor activation is observed without any co-implants, including a strong, sharp neutral Mg acceptor-bound exciton and strong donor-acceptor pair peaks. Variable temperature measurements reveal a band-to-acceptor transition, whose energy yields an optical binding energy of 224 meV. Be and C implants yield only slight evidence of shallow acceptor-related features and produce dose-correlated 2.2 eV PL, attributed to residual implantation damage. Their poor optical activation is tentatively attributed to insufficient vacancy production by these lighter ions. Clear evidence is obtained for donor-Zn acceptor pair and acceptor-bound exciton peaks in Zn-doped HVPE material.

Effect of hydrogenation on the electrical and optical properties of p-type Zn 1− xMg xS ySe 1− y epitaxial layers grown on GaAs (100) substrates

Capacitance–voltage ( C– V) and photoluminescence (PL) measurements have been carried out to investigate the hydrogenation effects on the electrical and the optical properties in p-type Zn 0.92Mg 0.08S 0.13Se 0.87 epilayers grown on n-type GaAs (100) substrates by molecular beam epitaxy. The results of the high-resolution transmission electron microscopy showed that the Zn 0.92Mg 0.08S 0.13Se 0.87/GaAs heterostructure had a high-quality heterointerface with sharp interface. After hydrogenation, the acceptor–carrier concentration of the p-type Zn 0.92Mg 0.08S 0.13Se 0.87 decreased due to acceptor neutralization resulting from complexes forming between the hydrogen atoms and the Zn or Se vacancies. After hydrogenation, the PL spectrum showed new donor-bound exciton and free electron-to-acceptor peaks, and the peak position of the donor–acceptor pair peak shifted to higher energy due to hydrogenation neutralization of the acceptor impurities. These results indicate that the crystallinity of the p-type Zn 0.92Mg 0.08S 0.13Se 0.87 epilayers grown on n-type GaAs substrates is improved by hydrogenation and that hydrogenated Zn0 .92Mg 0.08S 0.13Se 0.87 films grown on GaAs substrates hold promise for potential applications as cladding layers in optoelectronic devices operating in the blue–green spectral region.

Optical Activation Behavior of Ion Implanted Acceptor Species in GaN

AbstractIon implantation is used to investigate the spectroscopic properties of Mg, Be, and C acceptors in GaN. Activation of these species is studied using low temperature photoluminescence (PL). Low dose implants into high quality undoped hydride vapor phase epitaxial (HVPE) material are used in conjunction with high temperature (1300 °C) rapid thermal anneals to obtain high quality spectra. Dramatic, dose-dependent evidence of Mg acceptor activation is observed without any co-implants, including a strong, sharp neutral Mg acceptor-bound exciton and strong donor-acceptor pair peaks. Variable temperature measurements reveal a band-to-acceptor transition, whose energy yields an optical binding energy of 224 meV. Be and C implants yield only slight evidence of shallow acceptor-related features and produce dose-correlated 2.2 eV PL, attributed to residual implantation damage. Their poor optical activation is tentatively attributed to insufficient vacancy production by these lighter ions. Clear evidence is obtained for donor-Zn acceptor pair and acceptor-bound exciton peaks in Zn-doped HVPE material.