Bound excitons

Secondly, the spectral region corresponding to the donor and acceptor bound excitons is analyzed. Extrinsic properties are related to dopants or defects, which usually create discrete electronic states in the band gap, and therefore influence both optical absorption and emission processes. In theory, excitons could be bound to neutral or charged donors and acceptors. A basic assumption in the description of the bound exciton states for neutral donors and acceptors is a dominant coupling of the like particles in the bound exciton states. These two classes of bound excitons are by far the most important cases for direct band gap materials. In high quality bulk ZnO, the neutral shallow donor-bound exciton (DBE) often dominates because of the presence of donors due to unintentional impurities and/or shallow donor-like

defects. In samples containing acceptors, the acceptor-bound exciton (ABE) is observed. The recombination of bound excitons typically gives rise to sharp lines with

650 (D 4X a photon energy characteristic to each defect.

The low-temperature PL spectra are dominated by several bound excitons in the range from 3.348 to 3.374 eV for bulk ZnO sample, as seen in Fig. 2-8. The prominent lines are the A excitons bound to neutral donors that are positioned at 3.3598 (D⁰1XA), 3.3605 (D⁰2XA), 3.3618 (D⁰3XA), 3.3 ⁰ A), and 3.3664 (D⁰5XA) eV. Based on the energy separation between the FX_Aⁿ⁼¹(Γ ) and the DBE peaks, ₅ we concluded that the binding energies of the DBEs related to the different donors range from 10 to 20 meV.

Fig. 2-8 Bound excitonic region of the 10 K PL spectrum for the ZnO single crystal. [14]

On the high-energy side of the neutral DBE, transitions between 3.3664 and 3.3724 eV have been attributed to the excited states or excited rotator states of the neutral-donor-bound excitons. These excited states are analogous to the rotational states of the H molecule. In Figure 2-8, we also seen the relatively strong emission 2

line at 3.3724 eV (D⁰2 BX ) that is attributed to the transition due to the B-free exciton bound to the same main neutral donor. The energy separation between this peak and the main peak at 3.3605 eV (D

B

rticular donor could vary from sample to sample as well as its capture cross section.

2.2.2

02XA) is about 12 meV, which is consistent with the energy splitting of the A- and B-free exciton lines. In the lower energy part of the PL spectrum, ABEs at 3.3564 (A⁰1XA), 3.3530 (A⁰2XA), and 3.3481 eV (A⁰3XA) are also observed. However, relative peak intensities of the particular donor-related exciton lines show some differences from sample to sample. This is because the concentration of the pa

.3 Two-electron satellites [14]

Next, two electron satellites related to the donor-bound excitons are discussed.

Another characteristic of the neutral-donor-bound exciton transition is the two-electron satellite (TES) transitions in the spectral region of 3.32–3.34 eV.

These transitions involve radiative recombination of an exciton bound to a neutral donor, leaving the donor in the excited state. In the effective mass approximation, the energy difference between the ground-state neutral DBEs and their excited states (TES) can be used to determine the donor binding energies (the donor excitation energy from the ground state to the first excited state equals to 3/4 of the donor binding energy, ED) and catalog the different species present in the material. The spectral region for the expected two-electron satellite transitions is shown in Fig. 2-9 for the ZnO single crystal. The main peak at 3.3224 eV (D⁰2XA)2e is the excited state associated with the most intense neutral DBE at 3.3605 eV (D⁰2XA). The shoulder seen at about 3.3268 eV (D⁰3XA)2e on the high-energy side of the main TES peak is related to the excited state of the donor whose ground state emission is at 3.3618 eV

(D⁰3XA). A weak emission at 3.3364 eV (D⁰4XA)2e is also attributed to the TES transition of the neutral donor whose ground state is at 3.3650 eV (D⁰4XA). From the separation of the ground state and the corresponding excited states, we were able to calculate the donor binding energies as 51 meV for the donor at 3.3605 eV, 47 meV for the donor at 3.3618, and 38 meV for the donor at 3.3650 eV.

Fig. 2-9 10 K PL spectrum in the TES re on of the main bound exciton lines. [14]

tified t this point, but they may be related to the excitons bound to structural defects.

gi

From the separation between the A-free exciton and the ground-state neutral DBEs, we can also determine the binding energies of these excitons as 16.5 meV (for 3.3605 eV), 15.3 meV (for 3.3618 eV), and 12.1 meV (for 3.3560 eV). According to the empirical Haynes rule, the binding or localization energy of the DBEs is proportional to the binding energy of the corresponding donor. Indeed, this relation is clearly seen in the inset of Fig. 2-9. Additionally, there are two additional peaks at 3.332 and 3.313 eV on both sides of the main TES lines, which could not be iden a

2.2.2.4 LO-phonon replicas [14]

As indicated in Fig. 2-10, the bump at the higher energy side of the spectrum labeled as 1LO (FXA) has a peak around 3.306 eV, which is the expected value for the 1LO-phonon replica of the free exciton peak (about 71 meV apart from the FX_Aⁿ⁼¹ free-exciton peak). Although weak, second and third order LO phonon replicas labeled as 2LO (FXA) and 3LO (FXA) are also observed in the PL spectrum. It should be noted that LO-phonon replicas occur with a separation of 71–73 meV, which corresponds to the LO-phonon energy in ZnO.

Fig. 2-10 10 K PL spectrum in the region where DAP transition and LO-Phonon replicas are expected to appear. [14]

The peak at 3.2898 is the first LO-phonon replica of both 3.3618 and 3.3605 eV lines, whereas the first LO-phonon replica of 3.3650 eV line is seen as a shoulder on the high-energy side of this intense peak. Resolving the second and higher order LO replicas is even harder because the energy position (3.218–3.223 eV) falls in the

spectral region where the DAP transition and its LO phonon replicas are expected to appear (will be described in the following section). In fact, we observed a radiative recombination peak at 3.217 eV that is attributed to the DAP (labeled as DAP in Fig.

2-10 along with its first, second, and third LO-phonon replicas at 3.145, 3.073, and 3.001 eV, respectively). We can resolve at least two closely spaced peaks at 3.2898 and 3.2920 eV, which are about 72 meV apart from the main neutral DBE lines at 3.3605 and 3.3650 eV. Moreover, LO-phonon replicas are expected to be roughly two orders of magnitude less intense than the neutral DBE lines due to donor-related bound exciton lines couple only weakly with the optical phonons. The relatively broad peak around 3.280 eV is the first LO-phonon replica associated with the most intense ABE line (3.3564 eV). This is indicated as 1LO (A⁰X) in Fig. 2-10.

Finally, first, second, and third order LO-phonon replicas of the TES lines are also clearly observed in the PL spectra. These peaks are labeled as 1LO, 2LO, and 3LO-TES and they are positioned at 3.252, 3.182, and 3.112 eV, respectively.

2.2.3

the ionization energy Defect emission [1,18]

Besides exciton-related emissions in the photon energy range of 3.25–3.4 eV, PL spectrum of ZnO usually contains a sharp peak at about 3.22 eV followed by at least two LO-phonon replicas. This emission has been attributed to the DAP transitions involving a shallow donor and a shallow acceptor. This conclusion is based mostly on a characteristic transformation of the DAP emission lines to similar, but shifted, emission lines arising from transitions from the conduction band to the same shallow acceptor (e-A transitions) with increasing temperature. The DAP line quenches and gives way to the e-A line at elevated temperatures due to thermal ionization of the shallow donors. From the position of the e-A and DAP lines,

of th

defect types and concentrations resulting in different lumi

or ~1.8–1.9 eV) has een less commonly observed than green and yellow emissions.

e unintentional shallow acceptor in ZnO can be estimated.

In addition to UV excitonic emission peak, ZnO commonly exhibits luminescence in the visible spectral range with dissimilar emission wavelengths due to intrinsic or extrinsic defects. The origin of these emissions, including three different types of defects (green at ~2.3 eV, yellow at ~2.1 eV, and red at ~1.8 eV), has been controversial, especially the green emission. Assignment of various defect emissions to the specific transitions in ZnO is often complicated by the presence of multiple emissions and broad emission peaks containing contributions from multiple transitions. Different fabrication conditions (pressure, temperature, flow rate, etc.) resulted in different

nescence spectra.

A number of different hypotheses have been proposed to explain the green emission, such as transition between singly ionized oxygen vacancy and photoexcited hole, transition between electron close to the conduction band and a deeply trapped hole at V_o⁺⁺, surface defects, etc. While green emission is typically associated with oxygen deficiency, yellow/orange emission is associated with excess oxygen. The yellow/orange defect emission observed in ZnO synthesized by a hydrothermal method is typically assigned to interstitial oxygen, although other hypotheses such as dislocation related luminescence centers and the DAP-type transitions including a shallow donor and the Li acceptor dominate at low temperatures have been proposed.

The assignment of the emission to interstitial oxygen has been confirmed by reduction of this emission after annealing in a reducing environment. Unlike green emission, yellow emission is not significantly influenced by the surface modifications. On the other hand, red-orange emission (peak position at ~640–680 nm

b

2.2.4

ere a new collective phase is form

Biexciton and exciton-exciton scattering [6,19]

The PL features as mentioned above are under low excitation intensity. With increasing excitation intensity we reach the so-called intermediate density regime.

There are, e.g., elastic and inelastic scattering processes between excitons and (at higher temperature) between excitons and free carriers due to excitons density is so high that they start to interact with each other in the regime. These scattering processes may lead to a collision-broadening of the exciton resonances and to the appearance of new luminescence bands, to an excitation-induced increase of absorption, to bleaching, or to optical amplification, i.e., to gain or negative absorption depending on the excitation conditions. Another group of coherent and incoherent interaction processes in this intermediate density regime involves transitions to the excitonic molecule or biexciton. The biexciton is a quasiparticle which consists of two electrons and two holes. If we pump the sample even harder, we leave the intermediate and arrive at the high density regime, where the excitons lose their identity as individual quasiparticles and wh

ed which is known as the electron-hole plasma.

In the system with high density of excitons are created in crystal, the biexciton decay can be explained by the following channels indicating in Fig. 2-11(a). (1) As a result of the biexciton decay a lightlike state mt of the LPB and an excitonlike state of the LPB are formed. On reaching the crystal surface the lightlike polariton escapes as a luminescence photon mt. (2) The biexciton causes a lightlike state of the LPB and a longitudinal exciton state. Reaching the crystal surface the lightlike polariton escapes as a photon ml. Figure 2-11(b) diagramed the inelastic exciton-exciton scattering process. As a result of such scattering, one exciton is

excited into a higher state (n = 2, 3, 4,...,∞), while the other exciton loses its kinetic

= 60meV is the exciton binding energy of ZnO, and kT is the thermal ener .

ex

Eb

gy

Fig. 2-11 Inelastic scattering processes in the intermediate density regime: biexciton decay (a) and elastic exciton-exciton scattering (b). [6]

in

Next, we give an example experimentally related to PL characteristic features of both processes. Figure 2-12 shows PL spectra obtained under various excitation densities in ZnO epitaxial thin films. The lowest curve shows a low-excitation PL spectrum obtained using a continuous wave (cw) He–Cd laser (λ = 325 nm). PL due to a free exciton is observed at 3.370 eV and is denoted by X. There are two peaks at the lower energy side of the X band which are due to bound excitons (I2 : 3.362 eV, I4 : 3.357 eV). The PL band at 3.309 eV denoted by X–1LO is a 1-LO phonon-assisted

radiative recombination of free excitons. As the excitation intensity increases, a new PL peak, denoted M, appears at 3.35 eV. With a further increase in excitation intensity above 470 W/cm², a second peak, denoted P, emerges at ~3.32 eV [Fig.

2-12(d)]. The P band shifts toward the lower energy side as the excitation intensity increases further. According to previous statements, the P band originates from inelastic scattering between excitons. The energy of such scattering is roughly located between 3.318 (n = 2) and 3.303 eV (n = ∞) according to the expression (2.16).

Fig. 2-12 PL spectra of a ZnO epilayer for various excitation intensities of (a) 2.9, (b) 15, (c) 150, (d) 930, and (e) 4700 W/cm² at 77 K. [19]

The integrated PL intensity of the M band increases superlinearly with increasing excitation density. The asymmetric spectral shape and nonlinear excitation intensity dependence strongly suggest that the M band is due to a biexciton state. The binding energy of the biexciton is estimated to be 15 meV, which agrees well with the

reported value, 14.7 meV. The kinetics of excitons with increase in excitation intensity would be qualitatively understood as follows. As the excitation intensity increases, the probability of association of excitons becomes frequent, which leads to the formation of biexcitons. With further increase in excitation intensity, the kinetic energy of some of the excitons becomes higher than the biexciton binding energy, which would enhance inelastic exciton–exciton scattering. Hence, the M band is gradually taken over by the P band, as the excitation intensity increases further.

References

[1] Ü. Özgür, Ya. I. Alivov, C. Liu, A. Teke, M. A. Reshchikov, S. Doğan, V.

Avrutin, S.-J. Cho, and H. Morkoç, J. Appl. Phys. 98, 041301 (2005).

[2] R. Loudon, Adv. Phys. 50, 813 (2001).

[3] M. A. Stroscio and M. Dutta, “Phonons in Nanostructures” (Cambridge university press, United Kingdom 2001).

[4] J. M. Calleja and M. Cardona, Phys. Rev. B 16, 3753 (1997).

[5] A. P. Jephcoat, R. J. Hemley, H. K. Mao, R. E. Cohen, and M. J. Mehl, Phys. Rev.

B 37, 4727 (1988).

[6] C. F. Klingshirn, “Semiconductor Optics“ (Springer, Berlin, 1997).

[7] M. Ueta, H. Kanzaki, K. Kobayashi, Y. Toyozawa, and E. Hanamura, “Excitonic Processes in Solids“ (Springer-Verlag, Berlin, 1984).

[8] K. J. Button, D. R. Cohn, M. Ortenbert, B. Lax, E. Mollwo, and R. Helbig, Phys.

Harsch, Phys. Rev. B 60, 2340 (1999).

[16] K. Thonke, Th. Gruber, N. Teofilov, R. Schönfelder, A. Waag, and R. Sauer,

Physica B 308–310, 945 (2001).

[17] C. Boemare, T. Monteiro, M. J. Soares, J. G. Guilherme, and E. Alves, Physica B 308–310, 985 (2001).

[18] A. B. Djurišić, Y. H. Leung, K. H. Tam, L. Ding, W. K. Ge, H. Y. Chen, and S.

Gwo, Appl. Phys. Lett. 88, 103107 (2006).

[19] A. Yamamoto, K. Miyajima, T. Goto, H. J. Ko, and T. Yao, J. Appl. Phys. 90, 4973 (2001).

Chapter 3 Experimental procedures and characterization techniques

Incorporating Mg into ZnO has been obtained by molecular beam epitaxy [1,2], metalorganic vapor-phase epitaxy [3], and pulsed laser deposition [4-8]techniques etc.

However, these methods are expensive and require high vacuum and formation controlling conditions. Compared with these methods, the sol-gel process is an attractive technique for compound semiconductors preparation because of its simplicity, low costs, and ease of composition control.[9,10] In particular, it has the potential to produce samples with large areas and complicated forms on various substrates. In this research, the vapor-phase transport via a vapor-liquid-solid (VLS) mechanism [11,12] and sol-gel method were used to fabricate MgZnO alloys. The detailed growth mechanisms and characterization techniques of the alloys are discussed as follows.

3.1 Synthesis mechanism

3.1.1 Vapor-Liquid-Solid (VLS) method

Among all vapor based methods, the VLS methods seem to be the most successful for fabricating nanowires with single crystalline structures and in relatively large quantities. A typical VLS process starts with the dissolution of gaseous reactants into nano-sized liquid droplets of catalyst metal while the liquid droplets are supersaturated with the guest material, followed by nucleation and growth of single crystalline nanorods and then nanowires. The one-dimensional growth is mainly induced and dictated by the liquid droplets, whose size remains essentially unchanged

during the entire process of nanowire growth. In the sense, each of liquid droplets serves as a soft template to strictly limit the lateral growth of an individual nanowire.

As a major requirement, there should exist a good solvent capable of forming liquid alloy with the target material, ideally they should be able to form eutectic compounds.

The formation procedure of one-dimensional nanostructure in the VLS method is shown in Fig. 3-1 [13], which demonstrates the formation of semiconductor nanowire using metal catalyst. The reactant metal vapor which could be generated by the thermal evaporation is condensed to the catalyst metal to form a liquid alloy nanocluster as the temperature is low. Nanowires grown after the liquid metal alloys become supersaturated and continue as long as the metal nanoclusters remain in a liquid state. Growth of nanowires will be terminated as the temperature reduces to the point that the metal nanoclusters solidify.

metal

catalysts alloy liquid

vapor

nanowire

Time

Fig. 3-1 VLS method. [13]

3.1.2 Sol-gel process

A colloid is a suspension in which the dispersed phase is so small (~1-1000 nm) that the gravitational force is negligible and interactions are dominated by the short-range forces, such as Van der Waals attraction and surface charge. Sol-gel synthesis has two ways to prepare solution. One way is the metal-organic route with metal alkoxides in organic solvent; the other way is the inorganic route with metal

salts in aqueous solution. It is much cheaper and easier to handle than metal alkoxides, but their reactions are more difficult to control. The metal-organic route uses metal alkoxides in organic solvent. The inorganic route is a step of polymerization reactions through hydrolysis and condensation of metal alkoxides M(OR)^Z, where M = Si, Ti, Zr, Al, Sn, Ce, and OR is an alkoxy group and Z is the valence or the oxidation state of the metal. First, hydroxylation upon the hydrolysis of alkoxy groups:

The second step, polycondensation process, leads to the formation of branched oligomers and polymers with a metal oxygenation based skeleton and reactive residual hydroxyl and alkoxy groups. There are 2 competitive mechanisms:

(1) Oxolation: formation of oxygen bridges:

XOH

(2) Olation: formation of hydroxyl bridges when the coordination of the metallic center is not fully satisfied (N - Z > 0):

Figure 3-2 presents a schematic of the routes that one could follow within the scope of sol-gel processing.[14] A sol is a colloidal suspension of solid particles in a liquid, whereas, an aerosol is a colloidal suspension of particles in a gas (the suspension may be called a fog if the particles are liquid and a smoke if they are solid) and an emulsion is a suspension of liquid droplets in anther liquid. All of these types of colloids can be used to generate polymers or particles from which ceramic

materials can be made. In the sol-gel process, the precursors (starting compounds) for preparation of a colloid consist of a metal or metalloid element surrounded by various ligands.

Fig. 3-2 Schematic of the rotes that one could follow within the scope of sol-gel processing.

Metal alkoxides are members of the family of metalorganic compounds, which have an organic ligand attracted to a metal or metalloid atom. Metal alkoxides are popular precursors because they react readily with water. The reaction is called hydrolysis, because a hydroxy ion becomes attached to the metal atom. This type of reaction can continue to build larger and larger molecules by the process of polymerization. A polymer is a huge molecule (also called a macromolecule) formed from hundreds or thousands of units called monomers. If one molecule reaches macroscopic dimensions so that it extends throughout the solution, the substance is said to be gel. The gel point is the time (or degree of reaction) at which the last bound is formed that completes this giant molecule. It is generally found that the process begins with the formation of fractal aggregates that they begin to impinge on one anther, then those clusters link together as described by the theory of percolation. The gel point corresponds to the percolation threshold, when a single

cluster (call the spanning cluster) appears that extends throughout the sol; the

cluster (call the spanning cluster) appears that extends throughout the sol; the