Deep Donor-acceptor Pair Research Articles

Photo- and electro-luminescence studies of semipolar (112¯2) InxAl1−xN

InxAl1−xN (0.15 < x < 0.4) single layers (thickness = 300 nm) and light emitting diodes (LEDs) with InxAl1−xN (x = 0.24 or 0.28) quantum wells (QWs) were grown on (112¯2) free-standing GaN substrates by metal organic chemical vapor deposition. The emission characteristics of InxAl1−xN films and QWs were then investigated by photoluminescence (PL), cathodoluminescence (CL), and electroluminescence (EL). PL emission down at 1.87 eV (∼660 nm) was observed from In0.37Al0.63N at room temperature. However, its emission wavelength and spectral width showed almost no temperature-dependence, strongly suggesting the radiative recombination process in In0.37Al0.63N is heavily affected by the potential fluctuations associated with the defect levels such as deep donor–acceptor pairs (DAPs). For In0.23Al0.77N, the emission peaks at 3.24 and 2.4 eV were observed at 300 K but CL showed no evidence of compositional fluctuations. Given the wide spectral width and weak In clustering, PL at 3.24 eV is likely from a shallow DAP transition involving an impurity-related shallow donor and valence band. PL emission at 2.4 eV, on the other hand, behaved more like In0.37Al0.63N, suggesting significant deep DAP transitions. Both LEDs with In0.24Al0.76N and In0.28Al0.72N QWs showed a discrete blueshift of the emission peak with increasing injection current, which is very different from the gradual shift observed in InGaN LEDs. The high-energy peak was much narrower than the low-energy peak, suggesting it could be the near-band edge emission. Si doping of In0.24Al0.76N QWs lowered the injection current level at which the high-energy peak appears.

Non-radiative coupled donor-acceptor pair recombination in nanostructures based on nitrides and related phenomena

It has been clarified that several peculiarities of I-V characteristics at U < 1.5 V and low frequency noise dependences in InGaN/GaN nanostructures for green and blue spectral range as well as in Schottky barriers in AlGaN/GaN HEMT nanostructures are evoked by the non-radiative coupled deep donor-acceptor pair recombination. The strong contribution of the non-radiative donor-acceptor recombination in the degradation of the nanostructures properties has been confirmed experimentally.

Ag doping of Zn-In-S quantum dots for photocatalytic hydrogen evolution: Simultaneous bandgap narrowing and carrier lifetime elongation

Visible-light-driven photocatalytic hydrogen production has been an ongoing hot topic for clean and renewable energy, for which I–III–VI multinary metal sulfides play an important role owning to their unique ability of bandgap manipulation by composition. In this work, we present the controlled synthesis and hydrogen evolution property of a series of Ag:Zn-In-S quantum dots (QDs) in an effort to a more comprehensive understanding of Ag doping. With increasing ratio of Ag, the UV–vis absorption and photoluminescence wavelength was observed to be tuned in a wide range from light green to dark red. With increased Ag doping, dramatic increase (over 70 times) of the photocatalytic activity was observed (maximized with Ag:In:Zn ratio of 1.5:10:5). More importantly, time-resolved photoluminescence study reveals that this synergistic enhancement effect of the photocatalytic activity by controllable Ag doping was correlated with the simultaneous bandgap narrowing and carrier lifetime elongation, which contribute to the enhanced visible light absorption and charge separation, repectively. The enhanced charge separation upon Ag doping were further proved by photocurrent and electrochemical impedance spectra measurements. A mechanism is proposed for this synergistic effect of photocatalysis by suitable Ag doping, where the long-lived deep donor-acceptor pair states play an important role. Our results provide an interesting view and useful guideline for photocatalyst design using the narrow bandgap multinary sulfides.

Annealing effects on Cd0.96Zn0.04Te crystals with Te inclusions probed by photoluminescence spectroscopy

With effectively improved spectral resolution and signal-to-noise ratio, PL features are well resolved in the Te- and Cd-rich CdZnTe crystals and ascribed to the neutral donor-bound exciton (DX), acceptor-bound exciton (AX), A-center, and deep donor–acceptor pair. A detailed analysis indicates that (i) Cd pressure annealing drives the Te inclusions to the surface region of the crystal, destroys the inclusions by excess Cd atoms, and releases abundant donor-like and acceptor-like impurities; (ii) the PL features of the deep energy levels at 1.30–1.55 eV originate in the A-center consisting Cd vacancy and one type shallow donor, and the PL feature at 1.474 eV is the so-called SA luminescence of a donor–acceptor transition between an A-center and another shallow donor. A quantitative description of the recombination scheme is established, and by which the effects of the annealing are clarified.

Composition-dependent photoluminescence properties of CuInS2/ZnS core/shell quantum dots

CuInS2/ZnS (CIS/ZnS) core/shell quantum dots (QDs) with various Cu/In ratios were synthesized using the hot-injection method, and their photoluminescence (PL) properties were investigated by measuring steady-state and time-resolved PL spectroscopy. The emission peak of the CIS/ZnS QDs were tuned from 680 to 580nm by decreasing the Cu/In precursor ratio from 1/1 to 1/9. As the Cu/In ratio decreases, the PL lifetimes and PL quantum yields (QYs) of CIS/ZnS core/shell QDs increased firstly and then decreased. Two dominant radiative recombination processes were postulated to analyze composition-dependent PL properties, including the recombination from a quantized conduction band to deep defects state and donor-acceptor pair (DAP) recombination. The decrease of PL efficiency resulted from high density defects and traps, which formed at the interface between CIS core and ZnS shell due to the large off-stoichiometry composition. The PL intensity and peak energy for CIS/ZnS core/shell QDs as a function of temperature were also provided. The thermal quenching further confirmed that the PL emission of CIS/ZnS QDs did not come from the recombination of excitons but from the recombination of many kinds of intrinsic defects inside the QDs as emission centers.

Luminescence in ZnSe scintillation crystals co-doped with oxygen and aluminum

Zinc selenide scintillation crystals isoelectronically doped with aluminum and oxygen by introducing Al2O3 into the charge were grown, and their emission was studied under photo- and X-ray- excitation, and comparison of their emission properties to those of the conventional scintillator ZnSe(Te) was accomplished. It is demonstrated that this technique for oxygen introduction enables the formation of oxygen-related deep donor–acceptor pairs (DAPs) acting as radiative recombination centers in ZnSe(O,Al) up to the densities exceeding the density of the corresponding Te-related DAPs in ZnSe(Te) at the doping level of 1wt%. ZnSe(O,Al) exhibits a higher photoluminescence intensity than that in ZnSe(Te) because of more efficient light absorption, while the photoluminescence quantum yield and the emission intensity under X-ray excitation show an opposite trend.

Improved Leakage Current, Output Power, and Electrostatic Discharge Characteristics of GaN LEDs by Chemical Etching

We report the effect of chemical etching of p-GaN using molten KOH:NaOH solution on leakage currents, light output power, and electrostatic discharge (ESD) characteristics of GaN light-emitting diodes (LEDs). Photoluminescence and capacitance-voltage measurement indicated that a deep donor-acceptor pair (DDAP) was densely concentrated near the p-GaN surface region (∼ 18 nm) and the defects were effectively removed by a chemical etching process, resulting in a remarkable reduction of defect-assisted leakage current on the forward and reverse bias, and improved light output power due to enhanced injection efficiency in etched GaN LEDs. The negative-voltage ESD characteristics of etched GaN LEDs were also improved due to the decrease in DDAP defects near the surface region of p-GaN.

Gradient Doping of Mg in p-Type GaN for High Efficiency InGaN–GaN Ultraviolet Light-Emitting Diode

The performance of a InGaN-GaN multiple quantum-well (MQW) ultraviolet (UV) light-emitting diode (LED) with an emission of 385 nm was enhanced by a gradient doping of Mg in the p-GaN layer. The optical output power was enhanced by 21% at an input current of 20 mA compared to that of a UV LED with a uniformly doped p-GaN layer. The improved performance of the UV LED could be attributed to the decrease in diffusion of Mg into MQW and the suppression of electron transport from the conduction band of the MQW to the acceptor level of the deep donor acceptor pair bands in the p-GaN layer by a gradient doping of Mg in p-GaN layer.

Control of the thin film properties of Cu(In,Ga)Se2 using water vapor introduction during growth

The effects of water vapor introduction during growth on Cu(In,Ga)Se2 (CIGSe) thin film properties, specifically the electrical and photoluminescence (PL) properties have been studied. Increases in the hole carrier density and conductivity with water vapor introduction were observed for all [Ga]∕[In+Ga] composition ratios. The PL spectra observed from CuGaSe2 (CGSe) showed an annihilation of deep donor-acceptor pair emissions related to Se vacancies with water vapor introduction. In addition, the Na content in the CIGSe layers as well as the O content was found to increase. These results suggest that the mechanism behind the variation observed in the electrical and PL properties and consequent cell improvement is largely attributable to a decrease in the Se-vacancies-induced donor defect density and an enhancement of Na effects.

Room-temperature luminescence study on the effect of Mg activation annealing on p-GaN layers grown by MOCVD

An Mg-doped p-GaN layer was grown by the metalorganic chemical vapor deposition method. The dissociation extent of hydrogen-passivated Mg acceptors in the p-GaN layer through Mg activation annealing was estimated by using room-temperature cathodoluminescence (CL) spectroscopy. The CL measurement revealed that the CL spectra intensities tend to increase with increasing the activation annealing temperature. The sample annealed at 925 °C showed the most intense emission and the narrowest width among the emission peaks. Consequently, it was the most excellent dissociation extent of Mg–H complexes caused by the Mg activation annealing. The hole concentration under this optimum condition was 1.3×10 17 cm −3 at room temperature. The photoluminescence (PL) measurement showed a 2.8 eV band having characteristically a broad peak in heavily Mg-doped GaN at room temperature. By analyzing the PL results, we learned that this band was associated with the deep donor–acceptor pair (DAP) emission rather than with the emission caused by the transition from the conduction band to deep acceptor level. The four emission peaks in the resolved 2.8 eV band were emitted by transiting from deep donor levels of 0.14, 0.26, 0.40, and 0.62 eV below the conduction band to the shallow Mg acceptor level of 0.22 eV above the valence band.

Vertical Bridgman growth and annealing effect of Bi doped ZnSe single crystal

In order to prepare p-type ZnSe, bismuth (Bi) as an acceptor dopant was doped into ZnSe single crystal by vertical Bridgman method. Photoluminescence (PL) spectra measured at 4.2 K on as-grown crystal showed the strong shallow donor–acceptor pair (DsAP) emission, deep donor–acceptor pair (DdAP) emission and weak I1Bi emission line. To enhance the activation ratio of Bi in ZnSe single crystals, ZnSe:Bi was annealed in saturated zinc and selenium atmosphere at different temperatures. In the case of annealing in the range of 700–800 °C, DdAP emission disappeared and the intensities of DsAP and I1Bi emission relatively increased. From the dependence of PL spectra on excitation light intensity and the temperature, the acceptor activation energy and deep donor level were estimated about 103 meV and 34 meV, respectively. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of annealing in N2 atmosphere on net acceptor concentration in ZnSe:N grown by MOCVD

Abstract Annealing effect on net acceptor concentration in ZnSe:N is investigated. ZnSe:N homo-epitaxial layer was grown at 823 K by MOCVD using ammonia (NH 3 ) as a dopant source. Photoluminescence (PL) spectra measured on as-grown layer exhibited the strong deep donor–acceptor pair (D d AP) emission and the weak I 1 N emission line. In order to enhance the activation of nitrogen in ZnSe epitaxial layer, sample was annealed at the 823 K in nitrogen (N 2 ) and hydrogen (H 2 ) atmosphere. Only the annealing in nitrogen atmosphere increased I 1 N emission intensity indicate the activation of nitrogen acceptor. And net acceptor concentration was estimated to be 3 × 10 17 cm −3 by C – V measurements. This activation mechanism is interpreted as hydrogen is released from N–H bonds during annealing in nitrogen atmosphere.

Mechanism for forming the red emission band of ZnSe&lt;Te&gt; scintillation crystals

This paper describes the results of studies of the luminescence properties of ZnSe crystals containing the isovalent impurity Te. Taking into account defect-formation mechanisms in combination with modulation-spectroscopy methods made it possible to determine the conditions under which recombination processes via local deep levels or donor-acceptor pairs are dominant.

Blue emission band in compensated GaN:Mg codoped with Si

Codoping of p-type GaN with Mg and Si was investigated to determine its effect on deep level luminescence. By codoping with Si, the absolute intensity of the 2.8-eV blue luminescence band, previously tentatively attributed to deep donor-acceptor-pair (DAP) emission, decreased by more than an order of magnitude. The observed decrease is attributed to the reduction of the concentration of deep donor nitrogen vacancy complexes. The dependence of the emission peak position on hole concentration was investigated. A blueshift was observed with increasing carrier concentration. The shift of the blue band with carrier concentration and excitation intensity is explained semiquantitatively by a potential fluctuation model indicating its importance in DAP recombination.

Compensation of nitrogen acceptor in ZnSe : N/ZnSe grown by MOCVD

It has been proved that the net acceptor concentration as high as 2×10 17 cm −3 can be obtained when ZnSe : N homoepitaxial layers grown by metalorganic chemical vapor deposition (MOCVD) are annealed in molten zinc. However, it was found that the emission intensity of two sets of donor–acceptor pair (DAP), shallow DAP (D sAP) and deep DAP (D dAP), increased with the increase in annealing temperature. In order to clarify the compensation effect, the study on the origin and the concentration of deep donor related to the D dAP emission should be carried out. The ionization energies of acceptor and deep donor were estimated to be 101±1 and 44±5 meV, respectively, by examining the dependence of their emission peaks with the excitation intensity. Furthermore, the upper limit of the deep donor concentration was estimated to be 1×10 16 cm −3. This is one magnitude smaller than the net acceptor concentration obtained from the annealed ZnSe : N homoepitaxial layer in our previous paper. By studying on the behaviors of deep donor, the main reason to lead to a low net acceptor concentration in the ZnSe : N epitaxial layers is not attributed to deep donor complex containing nitrogen atoms, but hydrogen passivation, although a part of doped-nitrogen atoms combine with other defects to form complicated defects.

Optical characterization of bulk ZnSeTe solid solutions

Optical characterization was performed on wafers sliced from crystals of ZnSe, ZnTe, and ZnSe1−xTex(0&amp;lt;x&amp;lt;0.4) grown by physical vapor transport. Energy band gaps at room temperature were determined from optical transmission measurements on 11 wafers. A best fit curve to the band gap versus composition x data gives a bowing parameter of 1.45. This number lies between the value of 1.23 determined previously on ZnSeTe bulk crystals and the value of 1.621 reported on ZnSeTe epilayers. Low-temperature photoluminescence (PL) spectra were measured on six samples. The spectra of ZnSe and ZnTe were dominated by near band edge emissions and no deep donor-acceptor pairs were observed. The PL spectrum exhibited a broad emission for each of the ZnSe1−xTex samples, 0.09&amp;lt;x&amp;lt;0.39. For x=0.09, this emission energy is about 0.2 eV lower than the band gap energy measured at low temperature. As x increases the energy discrepancy gradually decreases and reduces to almost zero at x=0.4. The single broad PL emission spectra and the spectra measured as a function of temperature were interpreted as being associated with the exciton bound to Te clusters because of the high Te content in these samples.

Magneto-Optical Characterization of the Nitrogen-Related Impurities in p-Type ZnSe Epilayers

The impurity properties of nitrogen-doped ZnSe epilayer structures in the doping concentration range from 2 × 1017 up to 1.5 × 1018 cm–3 have been investigated by means of photoluminescence (PL) experiments under high magnetic field up to 15 T at liquid helium temperature. The main feature is the pronounced decrease in intensity of the deep-donor–acceptor pair (DdAP) transition as the magnetic field is applied in Faraday configuration. This transition involves a deep level of a nitrogen-related complex center and a nitrogen acceptor. This inhibition of the DdAP line is very pronounced (≈︂70%) for samples with a doping density below the compensating limit [N] = 1 × 1018 cm–3, and it is very weak for samples above this limit. This difference of regime is tentatively explained by the model of wave function shrinkage, giving the donor state radius as a determined parameter. Calculations within a simple model strongly suggest correlations between donor levels beyond the compensating limit.

Photoluminescence and Raman study of compensation effects in Mg-doped GaN epilayers

The compensation of Mg-doped GaN is systematically studied by low-temperature photoluminescence and Raman spectroscopy using a series of samples with different Mg concentrations. Strongly doped samples are found to be highly compensated in electrical measurements. The compensation mechanism is directly related to the incorporation of Mg. Three different deep donor levels are found at 240±30, 350±30, and 850±30 meV from the conduction band, each giving rise to deep unstructured donor-acceptor pair emission.

Atomic nitrogen doping in p-ZnSe molecular beam epitaxial growth with almost 100% activation ratio

An almost 100% activation ratio {(NA-ND)/[N]} for a nitrogen-doped ZnSe molecular beam epitaxy (MBE) layer with the highest net acceptor concentration (NA-ND) of 1.2×1018 cm−3 was obtained using a high-power rf plasma source. Even at this high NA-ND value, a 4.2 K photoluminescence spectrum shows bound exciton emission and deep donor–acceptor pair emission with well-resolved phonon replicas. The high activation in nitrogen doping could be ascribed to the generation of the predominant atomic nitrogen and to the suppressed extraction of nitrogen ions and excited neutral nitrogen molecules due to the structure of the orifice placed between the MBE growth chamber and the plasma discharge tube of the high-power plasma source.

Molecular beam epitaxial growth of P-ZnSe:N using a novel plasma source

We have studied the p-type doping in ZnSe molecular beam epitaxial growth using a novel high-power (5 kW) radio frequency (rf) plasma source. The effect of growth conditions such as the rf power, the Se/Zn flux ratio and the growth temperature on p-ZnSe:N was investigated. The net acceptor concentration (NA—ND) of around 1 × 1018 cm−3 was reproducibly achieved. The activation ratio ((NA—ND)/[N]) of p-ZnSe:N with NA—ND of 1.2 × 1018 cm−3 was found to be as high as 60%, which is the highest value so far obtained for NA—ND ∼ 1018 cm−3. The 4.2K photoluminescence spectra of p-ZnSe:N grown under the optimized growth condition showed well-resolved deep donor-acceptor pair emissions even with high NA—ND.