meV Donor Research Articles

Correlation between small polaron tunneling relaxation and donor ionization in Ga2O3

Pulsed laser deposition is employed to fabricate as-grown amorphous and post-growth annealed crystalline β-Ga2O3 films. The films annealed at temperatures above 600 °C are found to exhibit a pure monolithic phase with a bandgap of 4.7 eV. The thermally activated donor ionization and dielectric relaxation of these films are systematically investigated by temperature-dependent DC and AC conductivity measurements, and complex electric modulus analysis. A donor level at ∼180 meV below the conduction band edge and a small polaron tunneling (SPT) relaxation with an activation energy of ∼180 meV are observed in the as-grown amorphous Ga2O3 film but not in the monolithic β-Ga2O3 film. The SPT occurs between donor sites with its thermal relaxation of polarization being associated with the thermal ionization of the donor state. Thermal annealing of the amorphous films removes the 180 meV donors as well the corresponding SPT relaxation.

Study of point defects in wide-bandgap Cu2CdGeS4 microcrystals by temperature and laser power dependent photoluminescence spectroscopy

We present temperature and laser power dependent photoluminescence (PL) study of high quality wide-bandgap Cu2CdGeS4 microcrystals. At T = 10 K three PL bands were detected at about 1.919 eV (#1), 1.855 eV (#2) and 1.748 eV (#3). The temperature and laser power dependencies indicate that the properties of PL bands can be explained by donor- acceptor pair model, where the #1 and #2 bands result from a recombination between distant pairs involving the same shallow acceptor VCu with EA ≈ 30 meV and different deep donor defects. The #3 PL band originates from the deep donor-deep acceptor pair recombination where the depth of deep acceptor defect is more than 157 meV. Detailed analysis of the PL spectra show the absence of deep potential or band gap fluctuations in this material making it suitable for photovoltaic applications.

Incomplete Ionization of a 110\u2009meV Unintentional Donor in \u03b2-Ga2O3 and its Effect on Power Devices

Understanding the origin of unintentional doping in Ga2O3 is key to increasing breakdown voltages of Ga2O3 based power devices. Therefore, transport and capacitance spectroscopy studies have been performed to better understand the origin of unintentional doping in Ga2O3. Previously unobserved unintentional donors in commercially available (bar{2}01) Ga2O3 substrates have been electrically characterized via temperature dependent Hall effect measurements up to 1000 K and found to have a donor energy of 110 meV. The existence of the unintentional donor is confirmed by temperature dependent admittance spectroscopy, with an activation energy of 131 meV determined via that technique, in agreement with Hall effect measurements. With the concentration of this donor determined to be in the mid to high 1016 cm−3 range, elimination of this donor from the drift layer of Ga2O3 power electronics devices will be key to pushing the limits of device performance. Indeed, analytical assessment of the specific on-resistance (Ronsp) and breakdown voltage of Schottky diodes containing the 110 meV donor indicates that incomplete ionization increases Ronsp and decreases breakdown voltage as compared to Ga2O3 Schottky diodes containing only the shallow donor. The reduced performance due to incomplete ionization occurs in addition to the usual tradeoff between Ronsp and breakdown voltage.

Evidence for Native-Defect Donors inn-Type ZnO

Recent theory has found that native defects such as the O vacancy V(O) and Zn interstitial Zn(I) have high formation energies in n-type ZnO and, thus, are not important donors, especially in comparison to impurities such as H. In contrast, we use both theory and experiment to show that, under N ambient, the complex Zn(I)-N(O) is a stronger candidate than H or any other known impurity for a 30 meV donor commonly found in bulk ZnO grown from the vapor phase. Since the Zn vacancy is also the dominant acceptor in such material, we must conclude that native defects are important donors and acceptors in ZnO.

On the nitrogen vacancy in GaN

The dominant electrically active defect produced by 0.42 MeV electron irradiation in GaN is a 70 meV donor. Since only N-sublattice displacements can be produced at this energy, and since theory predicts that the N interstitial is a deep acceptor in n-type GaN, we argue that the 70 meV donor is most likely the isolated N vacancy. The background shallow donors, in the 24–26 meV range, actually decrease in concentration, probably due to interactions with mobile N interstitials that are produced by the irradiation. Thus, the recent assignment of a photoluminescence (PL) line as an exciton bound to a 25 meV N-vacancy donor is incompatible with our results. Moreover, we do not observe that PL line in our sample.

Defect spectra in epitaxial CuInSe 2 grown by MOVPE

For investigations of the defect spectra of CuInSe 2 epitaxial thin films are grown on GaAs (0 0 1) wafers using metal organic vapor phase epitaxy. The photoluminescence spectra for p-type Cu-rich ([Cu]/[In]>1.05) CuInSe 2 are dominated by one donor acceptor pair transition at 0.972 eV. For slightly Cu-poor and stoichiometric samples a free to bound transition at 0.992 eV is observed. Also an exciton emission could be detected at E FX=l.032 eV indicating a bandgap of E G=1.038 eV at 10 K. These results can be combined in a defect model for CuInSe 2 containing two acceptors states with 40 and 60 meV and a compensating 6 meV donor state.

The path to ZnO devices: donor and acceptor dynamics

Hall-effect, photoluminescence (PL), and electron paramagnetic resonance (EPR) measurements have been performed in single-crystal ZnO samples annealed in air at 25, 550, 750, and 950 °C, for 30 min each. A 37 meV donor is dominant in the unannealed sample, but nearly disappears during the higher temperature anneals, and is replaced by a 67 meV donor. The 37 meV donor is responsible for a donor-bound-exciton PL line at (3.3631 ± 0.0002) eV, which is dominant in the unannealed sample. The EPR measurements show that NO centers appear in the 750 °C and 950 °C anneals, and they are probably responsible for an increase in the acceptor concentration found from the Hall-effect results. A PL emission at (3.3570 ± 0.0002) eV, assigned in the literature as an acceptor-bound exciton, may involve NO complexes.

The electronic properties of native interstitials in ZnO

To investigate the role of interstitial geometry, we reported ab initio calculations of octahedrally and tetrahedrally native interstitials in ZnO, using the full-potential linear muffin-tin orbital method. The results show that both zinc interstitials could contribute to the native n-type conduction in ZnO, meanwhile, the zinc vacancy and octahedral oxygen interstitial may contribute to the p-type conduction in ZnO. We also suggest that the 31 and 61 meV donors found by temperature-dependent Hall experiment originate from tetrahedral- and octahedral zinc interstitials, respectively, based on their charge distribution. It is noticeable that tetrahedral interstitials cause stronger interaction between interstitials and its neighboring atoms than that of octahedral interstitials.

Optical spectroscopy of bulk GaN crystals grown from a Na–Ga melt

Colorless transparent platelet and prismatic GaN crystals up to 3–4 mm, grown from a Na–Ga melt (0.6–0.7 mol fraction of Na) at temperatures of 700–800 °C in a modest (5 MPa) pressure of N2, are characterized using Raman scattering, room and low temperature photoluminescence, and reflectance. They exhibit sharp free and bound exciton luminescence features (down to 0.22 meV full width at half maximum), including multiple excited states. Residual Mg and Zn acceptors and a 33.6 meV donor (possibly ON) are identified. Raman spectra suggest free carrier concentrations down to the low to mid 1016 cm−3 range.

Negatively Charged Donor Centers in Ultrathin ZnSe:N Layers

Pseudomorphically grown p-ZnSe:N layers being as thin as 100, 20, 10 and 5 nm embedded in ZnMgSSe barriers were investigated by means of photoluminescence and spin-flip-Raman (SFR) scattering spectroscopy. A new donor-acceptor-pair (DAP) band has been observed involving a very shallow donor with a binding energy of (19 ± 2) meV and the well-known N acceptor on Se site. Further evidence for a new donor center is given by transitions in angle-dependent SFR scattering measurements which correspond to an isotropic (donor-like) g-value of (1.49 ± 0.03). The 19 meV donor is interpreted as the negatively charged state of the usually found 52 meV deep donor in ZnSe:N. This is supported by the temperature behavior of the DAP band and, further, by its pronounced appearance for resonant excitation into the barriers quite similar to the behavior of trions.

Determination of Donor Densities and Donor Levels in 3C-SiC Grown from Si2(CH3)6 Using Hall-Effect Measurements

Without any assumption of the number of types of impurities, the densities and energy levels of donors in undoped 3C-SiC grown from Si2(CH3)6 are precisely determined by the simple graphical method proposed here, using the temperature dependence of the majority-carrier concentration obtained by Hall-effect measurements. We detect at least three types of donors whose energy levels (ΔED) are 7–14 meV, 46–54 meV and 97–120 meV as measured from the conduction band, although it was reported that ΔED for nitrogen atoms decreased with an increase in the donor density from ∼50 meV to ∼15 meV. In addition to the ∼15 meV donor that was reported in undoped 3C-SiC grown from a mixture of SiH4 and C3H8, at least two donor levels are detected in undoped epilayers grown from Si2(CH3)6. From the viewpoints of donor density and compensation ratio, the quality of undoped 3C-SiC grown from Si2(CH3)6 is better than that of undoped 3C-SiC grown from a mixture of SiH4 and C3H8.

The gyromagnetic ratio of the compensating donor centre in nitrogen-doped ZnSxSe1-x

The nature of the so-called deep donor that appears at a depth of about 50 meV when ZnSe is doped with nitrogen remains a matter of controversy. The centre is of considerable importance since it appears to act as a compensating centre which is (at least in part) responsible for the difficulties of obtaining effective acceptor concentrations in excess of 1018 cm-3. Several models have been suggested, the most favoured of which is that of a selenium vacancy associated with a substitutional nitrogen. However, although very recent total energy calculations support such a model, no conclusive evidence for this identification exists. Electrons trapped by the centre have a gyromagnetic ratio of 1.38, in contrast to the value of 1.11 for electrons bound at the normal, shallow (26 meV) donors in this material. In order to obtain a better understanding of the centre, we have used spin-flip Raman scattering experiments to determine how this g-value depends on composition in the nitrogen-doped ternary alloy ZnSxSe1-x, for x in the range 0 to 0.1. Surprisingly, we find that the g-value remains constant, in contrast to that for the shallow donors, which increases by 0.12 over the same composition range. The results present a significant challenge for theoretical calculation, since any microscopic model has to be reconciled with this invariance.

Oxygen and Erbium related donor centers in Czochralski grown silicon implanted with erbium

The Hall effect measurements were conducted on Czochralski-grown silicon after implantation of erbium and two-step annealing at 700 °C and 900 °C. After the first step the formation of oxygen-related shallow donors was observed at E c in the range 20–40 meV and erbium-related donor centers at ≈E c -70 meV and ≈E c -120 meV. Along with the same oxygen-related shallow thermal donors and donor centers at ≈E c -70 meV, other donor centers at ≈E c -150 meV are formed following the 900°C anneal, instead of those at ≈E c -120 meV. The new donor states are of particular interest because of their possible involvement in the photoluminescence process. The obtained results for erbium-implanted silicon are compared with some fragmentary DLTS data found in the current literature on the donors with ionization energies less than 0.2 eV.

Compensation introduced by defect complexes in p-type ZnSe

Defect complexes in N-doped and As-doped ZnSe are studied by using the discrete-variational local-density-functional method within a cluster model. Based on the difference of formation energy between two complexes, it is found that the NSe–Zn–VSe complex is a more efficient acceptor compensator than the NSe–Znint complex in N-doped ZnSe, while the AsSe–Znint complex is a more efficient acceptor compensator than the AsSe–Zn–VSe complex in the As-doped ZnSe. The NSe–Zn–NSe complex with a 170 meV acceptor level and the NSe–NZn complex with an 88 meV donor level are respectively identified. The existence of donor states of N molecules in ZnSe is confirmed.

Oxygen aggregation in Czochralski-grown silicon heat treated at 450 °C under compressive stress

It has been established that the oxygen aggregation processes in Czochralski-grown silicon (Cz–Si) at 450 °C are strongly affected by high hydrostatic pressure. We observed the enhanced production of shallow thermal donors with ionization energies of 30–40 meV and deep donors at ≈EC−0.1 eV under a pressure of 1 GPa. In contrast, the concentration of the well-known double thermal donors was found to be much less than that in Cz–Si heat treated without stress. The latter effect may be associated with the involvement of self-interstitials in their formation. The enhanced production of other thermal donors is thought to be caused by increasing diffusivity of oxygen under the high stress.

Shallow donors in epitaxial GaN

Photoluminescence (PL) of GaN is commonly dominated by the annihilation of excitons bound to a 34.5 meV deep donor appearing at 3.472 eV (for strain free GaN at 4 K). With PL we were able to resolve two additional donor bound exciton transitions. The excitons have localization energies of 3.7 ± 0.3 and 11.3 ± 05 meV, respectively. Using Haynes rule the respective donors lie 56.5 and 18.5 meV below the conduction band. The applicability of Haynes rule could nicely be confirmed by IR absorption where the 1s-2p transition of the 34.5 and 58 meV donor are observed. The issue of residual donors in GaN will be discussed in this context.

Deep-center photoluminescence in nitrogen-doped ZnSe.

In this paper, we report optical studies of nitrogen-doped ZnSe epilayers grown by molecular-beam epitaxy. Photoluminescence spectra of the donor-acceptor pair region at different temperatures and different carrier concentrations show that two donors are present in the samples: residual shallow donors with activation energy 26 meV and deep donors with activation energy of 4 meV previously assigned to a ${\mathrm{V}}_{\mathrm{Se}}$-Zn-${\mathrm{N}}_{\mathrm{Se}}$ complex. In the exciton region, we observe an emission at 2.765 eV, which shows an increased intensity when the epilayer is compensated by the deep donor. We therefore propose that this transition may be related to a deep-donor bound exciton.

Defect Studies in n-Type GaN Grown by Molecular Beam Epitaxy

AbstractGaN thin films grown by molecular beam epitaxy (MBE) were characterized by Hall effect measurements in the temperature range from 80 K to 500 K and by photoluminescence spectroscopy (PL) at 2 K and at 300 K. These films were grown by MBE utilizing either electron cyclotron resonance (ECR) plasma activated nitrogen gas or thermally cracked hydrogen azide (HN3) as the source of chemically reactive nitrogen. The electrical properties of the GaN films grown by ECR plasma assisted MBE were found to vary with growth parameters, dominated either by shallow donors with activation energies (ΔE)in the range between 10 meV and 30 meV or deep donor levels (ΔE; > 500 meV). GaN grown by (HN3) gas-source MBE exhibited metallic conduction and electron mobilities <1 cm2/Vs. However, these films displayed sharp photoluminescence lines at 3.360 eV and 3.298 eV and no deep level related luminescence, whereas only broad deep level related emission was observed in the PL spectra of the ECR plasma assisted MBE grown GaN films.

Deep centre photoluminescence in nitrogen doped ZnSe

In this paper, we report optical studies of nitrogen doped ZnSe epilayers grown by molecular beam epitaxy. Photoluminescence spectra of the donor-acceptor pair region at different temperatures and different carrier concentrations show that two donors are present in the samples, residual shallow donors with activation energy 26 meV and deep donors with activation energy of 46 meV previously assigned to a VSe-Zn-NSe complex. In the exciton region we observe a new emission at 2.765 eV and the intensity increases when the epilayer is compensated by the deep donor. We therefore propose that this transition is related to a deep donor bound exciton. Excitation power dependent photoluminescence measurements show the presence of two deep transitions separated by 23 meV at low excitation intensities which we propose are due to deep donor-acceptor pairs.

Properties Of crystalline 3C-SIC Grown From Methyl Trichlorosilane

ABSTRACTCubic silicon carbide grown by thermal decomposition of methyl trichlorosilane in hydrogen has been studied in infrared (IR) transmission, cyclotron resonance (CR), and photoluminescence (PL). Sample characteristics include: The nitrogen concentration, determined from CR and IR can be of the order of 1015 cm-3 or less. Ionized impurity concentrations determined from the linewidths of CR and IR spectra can be less than 7 × 1014 cm-3. Low temperature electron mobilities from CR are near 105 cm2 v-1 s-1. Carrier freeze-out is consistent with either uncompen-sated donors with activation energy near 50 meV or heavily compensated donors with 20 meV activation energy. The latter choice, however, is excluded by the low ionized impurity concentration. These high quality samples show little or no evidence of the apparently shallow (< ∼20 meV) donor which usually dominates the low temperature conductivity of 3C-SiC grown on Si. Nitrogen donor excitation spectra are in excellent agreement with effective mass theory. Only nitrogen donors at 54.2 meV and an unidentified effective-mass donor at 47.8 meV are observed. If material with the characteristics described above can be reliably reproduced then there is substantial expectation that high quality electronic devices based on 3C-SiC can be fabricated.