Neutral Donor-bound Exciton Research Articles

Erratum: Neutral-donor–bound-exciton complexes in ZnO crystals [Phys. Rev. B57, 12 151 (1998)

Erratum: Neutral-donor–bound-exciton complexes in ZnO crystals [Phys. Rev. B<b>57</b>, 12 151 (1998)

Optical spectroscopy of Si-related donor and acceptor levels in Si-doped GaN grown by hydride vapor phase epitaxy

The optical properties of n-type GaN grown by hydride vapor phase epitaxy, with intentional Si doping levels ranging from nominally undoped to ND−NA=4×1017 cm−3, are investigated using low temperature photoluminescence. We identify free and neutral donor-bound exciton transitions and two-electron satellites (TES) at 1.7 K. The energy difference between the principal neutral donor-bound exciton peak and its TES yields a Si donor binding energy of 22 meV. The intensity of the Si-related TES increases with increasing Si concentration. The Si donor is much shallower than the two residual donors, which have binding energies of 28 and 34 meV. This result suggests that the main residual donors in this material (and possibly in many layers grown by metal organic chemical vapor deposition and metal organic molecular beam epitaxy as well) are not Si. Silicon doping also introduces an acceptor level with a binding energy of about 224 meV.

Hydrogenation and Annealing Effects on GaN Epilayers Grown on Sapphire Substrates

Photoluminescence (PL) measurements have been carried out to investigate the hydrogenation and annealing effects of unintentionally doped n-type GaN epitaxial layers grown on sapphire substrates by plasma-assisted molecular beam epitaxy. When the as-grown GaN/sapphire heterostructure is annealed at 800°C, a new peak corresponding to the neutral donor-bound excitons appears. After hydrogenation, the yellow peak appeared for the as-grown GaN/sapphire heterostructure annealed at 800°C disappears. When the hydrogenated GaN/sapphire heterostructure is annealed at 950°C, three peaks appears. These results indicate that the crystallinity of the unintentionally doped n-type GaN epilayers grown on sapphire substrates is improved by hydrogenation and annealing.

Photoluminescence of Cd1—xZnxTe Crystals Grown by High-Pressure Bridgman Technique

Photoluminescence (PL) spectra of detector-grade Cd1—xZnii xTe crystals with x = 0.07 to 0.14 were measured as a function of the excitation laser power and over the temperature range from 12 to 295 K. Two bound exciton transitions at the energies of 13 and 24 meV below the band-gap Eg and a number of phonon replicas dominate the low-temperature PL spectra. The transition at the energy of Eg — 13 meV is attributed to a neutral donor-bound exciton D0X. The excitation power and temperature dependence of the peak at the energy of Eg — 24 meV show that it is of excitonic origin with binding energy of 13 meV. The intensity of this A0X peak is found to depend on x, which suggests that it is related to structural defects. In some samples a donor–acceptor pair (DAP) transition at the energy of Eg – 60 meV is observed. The PL spectra also exhibit a weak, defect-related broad peak at the energy of about Eg — 200 meV. The PL of a large number of samples from different boules and locations in the boules is compared. The DAP-peak intensity was observed to depend strongly on the boule.

Luminescence properties of ZnSe films grown by hot wall epitaxy

The photoluminescence (PL) properties of ZnSe films grown by hot wall epitaxy are reported. The PL spectra show clear neutral donor-bound exciton peak; donor acceptor pair (DAP) peak, conduction band to acceptor (CA) peak, and their phonon replicas until fourth order. The conduction band to acceptor peak and it's phonon replicas exist until room temperature. From the ratio of PL intensities of DAP and CA peaks and their replicas, we obtain the Huang-Rhys factor S = 0.58, in agreement with other experiments for acceptor-bound exciton transitions. From the temperature dependence of PL intensities we derive the activation energy of thermal quenching process for the DAP transitions as about 7 meV.

Magnetoluminescence and Resonant Electronic Raman Scattering Investigation of Donors and Excitons in Hydride Vpe and Mocvd GaN

AbstractThe donor and exciton states in ultra high-quality heteroepitaxial GaN grown by hydride vapor phase epitaxy (HVPE) and metalorganic chemical vapor deposition (MOCVD) on sapphire substrates are investigated using low temperature photoluminescence (PL), reflectance, magnetospectroscopy in fields up to 12 T, and resonant electronic Raman scattering (RERS). The A free exciton is confirmed to have a binding energy of about 26.4 meV, independent of strain in the material. Bound n=2 exciton peaks are distinguished in the PL spectrum by their thermalization and sample dependence. The Si donor is shown to have a binding energy of about 21 meV using Si-doped HVPE samples grown at Epitronics. Up to five additional residual donor species are observed when comparing various HVPE and MOCVD samples. Pronounced temperaturedependence of the two-electron satellites is observed, suggesting the existence of unresolved excited rotator states of the neutral donor-bound exciton. Highly resolved magnetic splitting patterns are observed in the two-electron satellites. A nonperturbative theory of these donor splittings is developed, including anisotropy. Resonant electronic Raman scattering from residual donors is reported, and yields improved linewidths compared to PL.

Luminescence studies of GaN grown on GaN and GaN/AlN buffer layers by metalorganic chemical vapor deposition

A detailed study of the luminescence properties of GaN layers grown by metalorganic chemical vapor deposition on sapphire substrates with GaN/AlN double buffer layers or GaN single buffer layers was carried out with photoluminescence and cathodoluminescence (CL) spectroscopy. It was discovered that the use of the double buffer layer resulted in an improved surface morphology, but also increased the strain in the samples relative to samples grown on single GaN buffer layers. Free exciton (A exciton), neutral donor-bound exciton, and acceptor-bound exciton photoluminescence peaks were observed for GaN films grown on GaN, AlN, and GaN/AlN buffer layers. Acceptor free-to-bound luminescence was also observed and the thermal activation energy of the acceptors was measured. From these data we are able to determine the acceptor binding energy, EA, to be 231.5 meV and the donor binding energy ED to be 29.5 meV. An exciton peak, the acceptor free-to-bound luminescence, and an unidentified lower energy peak were observed with CL. CL imaging also allowed us to correlate luminescence with surface features of the films. e-beam annealing of both n-type and p-type films was investigated.

Photoluminescence, Reflectance, and Magnetospectroscopy of Shallow Excitons in GaN

ABSTRACTWe report several new aspects of the excitonic properties of heteroepitaxial GaN grown on sapphire or 6H-SiC. In particular, we observed the n = 2 free exciton associated with both A and B excitons (which are distinct from the n = 1 C exciton) using reflectance and 1.7 K photoluminescence. We also studied the behavior of the n = 2 A-exciton using magnetoluminescence in fields up to 12 T. The large diamagnetic shift and splitting positively confirm the identification, yielding an exciton binding energy of about 26.4 meV. Several previous identifications of the n = 2 free exciton yielding a smaller exciton binding energy are probably in error, based on our results. We have also detected the two-electron replica of the neutral donor-bound exciton for the first time in GaN and observed its splitting pattern in magnetic fields up to 12 T. This feature is 22 meV below the principal neutral donor-bound exciton peak, independently of strain shifts in the overall spectrum. It yields a precise donor binding energy of 29 meV for the shallow residual donor in material grown by metalorganic chemical vapor deposition and gas-source molecular beam epitaxy, considerably smaller than that of the residual donor reported earlier in hydride vapor phase epitaxial material (about 35.5 meV).

Ground and excited states associated with donor bound-excitons in high purity GaAs

Four emission lines have been observed in high purity GaAs, which are associated with excitons bound to a shallow neutral donor. The lowest energy line is the ground state; the three higher energy lines are due to excited states of the neutral donor-bound exciton. The excited states consist of a rotation of light and heavy holes around the donor as was previously observed in InP. The experimentally observed energy ordering of the two exited states associated with the light-hole was previously found to be reversed from that predicted by theory for the case of InP. In the case of GaAs, the present work shows that the ordering of these two states agrees with the ordering predicted by theory. Two additional excited states were also observed; they may be associated with the next higher rotational quantum state ( l = 2).

Preparation and Properties of (ZnS)3(ZnSe)42 Ordered Alloys Fabricated by Plasma-Enhanced Low-Temperature Growth Technique

Gallium-doped ZnSe and ordered alloys, (ZnS)3(ZnSe)42, were grown on a GaAs substrate. Low-temperature growth (T g=200° C) of ZnSe and ZnS films was performed by hydrogen radical-enhanced chemical vapor deposition (HRCVD) using triethylgallium (TEGa) as the dopant source. Atomic hydrogen generated by RF plasma is used to enhance the formation of the depositing species. Two-dimensional ordered structures on the atomic scale were achieved by atomic layer epitaxy (ALE). In photoluminescence (PL) measurements of doped ZnSe films, the emission intensity attributed to the neutral donor-bound exciton increases with increasing TEGa flow rates. Strong blue PL emission was observed for the doped ZnSe film with optimized TEGa flow rate at room temperature. In X-ray diffraction (XRD) spectra of the ordered alloys, satellite peaks due to the layered structure were observed. A carrier concentration of 3×1016 cm-3 and a Hall mobility of 154 cm2/Vs were obtained for the ordered alloy, (ZnS)3(ZnSe)42, at room temperature.

Effects of C incorporation on the luminescence properties of ZnSe grown by metalorganic chemical vapor deposition

The effects of C incorporation in heteroepitaxial ZnSe grown on GaAs by metalorganic chemical vapor deposition (MOCVD) with methylallylselenide (MASe), diethylselenide (DESe), and hydrogen selenide (H 2Se) selenium precursors together with dimethylzinc (DMZn) are investigated. The effects of growth temperature and VI/II ratio are determined. In material grown at 480°C and above, all of the low temperature photoluminescence (PL) peaks are generally shifted several meV lower in energy than in material grown in the 300–425°C range. In particular, the light hole component of the neutral donor-bound exciton shifts down to around 2.792 eV, where it was incorrectly identified in previous work as an exciton bound to a complex involving C. Detailed PL and reflectance measurements give conclusive evidence for this interpretation. The peak shifts are attributed to increased thermal mismatch strain resulting from the higher growth temperature used with MASe and DESe. However, the VI/II ratio strongly affects the strain in samples grown at 500°C using DESe, suggesting that heavy C incorporation at the ZnSe/GaAs interface may stimulate the thermal relaxation. The mechanism might involve the formation of C-related microprecipitates near the interface, which then nucleate misfit dislocations. We conclude that C from the growth precursors does not produce any new PL peaks above 1.4 eV in MOCVD ZnSe. Moreover, the room temperature PL efficiency is not degraded in C-contaminated material, suggesting that C is not an important non-radiative recombination center in ZnSe. The incorporation of moderate C levels is found to have little effect on the optoelectronic properties of MOCVD ZnSe.

The influence of impurity concentration on exciton photoluminescence in GaAs and InP

The authors report an investigation of the exciton photoluminescence (PL) spectra of high-purity nominally n-type GaAs and InP layers, grown by metalorganic vapour phase epitaxy. The results are discussed in terms of exciton-polariton kinetics. The ratio of the integrated emission intensities of neutral donor-bound exciton (D0, X) transitions and free exciton-polariton (FX) transitions rDF=I(D0, X)/I(FX) shows a sublinear dependence upon neutral donor concentration nD0 deduced from 77 K Hall measurements. This relationship is particularly useful for estimating carrier concentrations in high-purity materials, which cannot be characterized by Hall measurements due to depletion.

Electronic properties in Ga-doped CdTe layers grown by metalorganic vapor phase epitaxy

Electronic properties in Ga-doped (100) CdTe layers on (100) GaAs grown by atmospheric pressure metalorganic vapor phase epitaxy were studied. Triethylgallium was used as a dopant source. The source materials were dimethylcadmium (DMCd) and diethyltelluride (DETe). The effects of the DETe/DMCd (VI/II) ratio on the electrical properties were evaluated by Hall measurements. Electron concentration (300 K) was controlled from 3.5×1014 cm−3 to 2.5×1016 cm−3 by the VI/II ratio in the range 0.5 to 2. Higher growth temperature lowered the electron concentration. High electron mobility of 630 cm2/V s (300 K) was obtained for a growth temperature of 375 °C and a VI/II ratio of 2. Good correspondence was observed between electrical and photoluminescence (PL) properties. Both intensity and linewidth of a neutral-donor bound-exciton (D0,X) emission at 1.5932 eV increased with the electron concentration. The ionization energy of the Ga donor was estimated to be about 18 meV from electrical and PL properties. A Ga incorporation mechanism was deduced on the basis of the experimental results.

Chlorine-related photoluminescence lines in high-resistivity Cl-doped CdTe

High-resistivity Cl-doped CdTe crystals grown by the gradient-freeze method were studied by high-resolution photoluminescence (PL) and electrical measurements. A sharp PL line ascribed to a Cl-related defect was observed at 1.5903 eV in all of the high-resistivity crystals doped with different amounts of chlorine. A broad line around 1.586 eV was clearly observed in highly doped crystals. The intensities of these lines increased with the amount of chlorine. Annealing under Cd-saturated atmosphere eliminated the 1.5903 eV line and intensified the neutral donor-bound exciton line together with its two-electron transition. The high-resistivity crystals were also converted to n-type with low resistivity by the Cd annealing. These results show that the 1.5903 eV line is a key emission line for the high-resistivity, and suggest that the line is associated with Cd-vacancy/chlorine complex, probably in the form of V CD-Cl.

Molecular-beam epitaxial growth of p- and n-type ZnSe homoepitaxial layers

Undoped, p- and n-type ZnSe homoepitaxial layers have been successfully grown by molecular beam epitaxy on dry-etched substrates. ZnSe substrates were etched with a BCl 3 plasma to eliminate polishing damage. The full width at half maximum of an X-ray rocking curve for a homoepitaxial layer has reached 21 arc sec. N-doped ZnSe homoepitaxial layers grown with the active-nitrogen doping technique have shown p-type conduction which is the evidence of p-type conduction in ZnSe. The n-type homoepitaxial layers have been grown with Cl doping. The Cl-doped homoepitaxial layers have shown high Hall mobilities relative to heteroepitaxial layers. These undoped, N-doped and Cl-doped layers have exhibited strain-free photoluminescence properties; free exciton emission, neutral acceptor-bound exciton emission and neutral donor-bound exciton emission showed a single peaks at 2.804, 2.7931 and 2.7980 eV, respectively.

Effect of Residual Strain on the Splitting of Excitonic Luminescence Lines in Epitaxially Grown ZnSe/GaAs

Polarization characteristics for excitonic luminescence lines of epitaxially grown ZnSe on GaAs demonstrate the splitting of the neutral donor-bound exciton as well as the free exciton due to the residual strain in the epilayer. The strain is interpreted well by the misfit of the calculated lattice constants between ZnSe and GaAs.

Design and operation of a valved solid-source As2 oven for molecular beam epitaxy

The design and operation of a valved solid-source of arsenic for molecular beam epitaxy is described. The addition of a throttle valve to a solid-charge arsenic source allows the arsenic flux to be varied rapidly and controllably during epitaxial growth with a time constant of &amp;lt;1 s. In addition to the novel valve, the arsenic source assembly includes a large reservoir, a thermal cracking zone for the production of As2, and provisions for adding an exit aperture ionization gauge for feedback flux control. Low-temperature photoluminescence from undoped GaAs grown with this source exhibited a neutral donor-bound exciton peak with full width at half-maximum of 0.128 meV, and lightly silicon-doped GaAs exhibited a mobility of 73 000 cm2/V s at 77 K, corresponding to ND =1.3×1015/cm3 and NA =2.0×1014/cm3.

Observation of the donor-bound light-hole free exciton in AlGaAs-GaAs quantum wells

The neutral donor-bound light-hole free exciton has been observed in AlxGa1−xAs-GaAs multi-quantum wells (MQWs). The transition was observed using selective excitation photoluminescence spectroscopy on two nominally 350-Å-wide AlxGa1−xAs-GaAs MQW structures in the energy region between the light-hole and heavy-hole free-exciton transitions, where no other intrinsic transitions exist. The neutral donor-bound heavy-hole free-exciton transition was also observed in both MQW structures. A third 150-Å-wide MQW structure which showed no evidence of a donor-bound heavy-hole free exciton also showed no evidence of a donor-bound light-hole free exciton.

Identification of donors in GaAs by resonantly excited high-field magnetospectroscopy

The use of magnetospectroscopy of donor-bound excitons in the low-temperature (1.7 K) photoluminescence spectrum of GaAs for the identification of shallow donor species in low-doped epitaxial GaAs is described. The selective excitation of individual donor species in excited final-state (2 <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</i> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-</sup> , 2 <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</i> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sup> ) transitions by tuning a dye laser into resonance with a corresponding component of the principal neutral donor-bound exciton lines is discussed. Individual components of the principal neutral donor-bound exciton peaks corresponding to different donor species are directly resolved in luminescence for the first time. Individual donors are also resolved in the ionized donor-bound excitation peaks, and Haynes' rule relationships are determined for both neutral and ionized donor-bound excitons. Shallow acceptors are identified in high-fields from the principal neutral acceptor-bound exciton peaks; the central-cell corrections to the exciton localization energies are found to be increased by the field.

Annealing behavior of bound exciton lines in high quality CdTe

Annealing behavior of shallow donor-bound and acceptor-bound exciton lines in CdTe crystals grown by the Bridgman method was investigated by high-resoluton photoluminescence measurements at 4.2 K. The intensity of the neutral acceptor-bound exciton line (A0X) at 1.5896 eV, which was commonly observed in p-type CdTe, drastically decreased by annealing under Cd saturated atmosphere. By further subsequent annealing under Te saturated atmosphere, the A0X line recovered again. On the other hand, the intensity of the neutral donor-bound exciton line increased by annealing under Cd saturated atmosphere and decreased by the further annealing under Te saturated atmosphere. It was found that this change in the photoluminescence lines by the annealing process was reversible. These results strongly suggest that the A0X line at 1.5896 eV can be ascribed to the recombination of excitons trapped at Cd-vacancy/donor-impurity complexes.