Final Thesis
Spin Dependent recombination in GaNAs
Yuttapoom Puttisong
LITH-IFM-A-EX--09/2187—SE
Examiner: Irina Buyanova, Linköping University
Division of Functional Electronic Materials Department of Physics, Chemistry and Biology
Linköping University, Sweden Linköping 2009
Upphovsrätt
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Abstract
Spin filtering properties of novel GaNAs alloys are reported in this thesis. Spin-dependent recombination (SDR) in GaNAs via a deep paramagnetic defect center is intensively studied. By using the optical orientation photoluminescence (PL) technique, GaNAs is shown to be able to spin filter electrons injected from GaAs, which is a useful functional property for integratition with future electronic devices. The spin filtering ability is found to degrade in narrow GaNAs quantum well (QW) structures which is attributed to (i) acceleration of band-to-band recombination competing with the SDR process and to (ii) faster electron spin relaxation in the narrow QWs. Ga interstitial-related defect centers have been found to be responsible for the SDR process by using the optically detected magnetic resonance (ODMR) technique. The defects are found to be the dominant grown-in defects in GaNAs, commonly formed during both MBE and MOCVD growths. Methods to control the concentration of the Ga interstitials by varying doping, growth parameters and post-growth treatments are also examined.
Acknowledgement
I grateful to my supervisor, Prof. Irina Buyanova and Prof. Weimin Chen for letting me be a part of this interesting project, for always opening the door to have fruitful discussions and for their support and encouragement.
I would like to thank Xingjun Wang for all valuable time we spend in labs, for sharing the knowledge in semiconductor spintronics as well as for teaching me experimental techniques during my work on the thesis.
I also would like to thank my opponent, Huan-Hung Yu for intensively discussing my thesis.
I am also grateful to Shula Chen, Shun-Kyun Lee, Daniel Dagnelund, Jan Beyer and Deyong Wang for family-like research atmosphere, I am kind of working in a very lovely family. I further thank Arne Eklund for technical assistance and Lejla Vrazalica for her help with administrative matters.
Finally, I am grateful to my mother for always supporting me during all her life. I cannot stand in this place without you.
Yuttapoom Puttisong
Tables of contents
Introduction 8
Chapter One: Fundamental electronic structure of GaNAs 9
The empirical band anti-crossing (BAC) model...9
Pseudopotential LDA calculation...10
Band alignment in GaNAs/GaAs hetero-structures...11
Strain-induced splitting of the valence band...11
Confinement effect-induced lh-hh splitting in a QW system...12
Radiative recombination process in GaNAs...13
Non-radiative (NR) recombination...14
Chapter Two: Spin dynamics 15
Optical orientation and spin polarization...15
Spin relaxation...17
Chapter Three: Spin Dependent Recombination (SDR) 20
General principles of SDR...20
Optical orientation in the presence of a deep paramagnetic center...21
SDR ratio...22
Two spin pools picture...23
Physical realization of an efficient spin dependent-recombination process...24
Chapter Four: Experimental Approach 27
Optical orientation PL spectroscopy...27
Magnetic resonance technique...30
Chapter Five: Experimental Results Defect Engineering for Spin Filtering Effect in GaNAs 37
Defect engineered spin filter from a low dimensional semiconductor structure; spin filtering effect in QW structures...37
Effects of doping on the formation of the Gai-interstitial paramagnetic centers...46
Effects of growth techniques on the defect formation...50
Summary 53
Introduction
Diluted-nitrides (i.e. N containing III-V ternary and quaternary alloys) have emerged as a subject of considerable theoretical and experimental research efforts because of their unique and fascinating electronic properties. Unlike conventional ternary III-V alloys, such as AlGaAs, GaInAs, etc, where the band gap energy of the alloy can be approximated as a weighted linear average of the parental compounds, the dilute nitrides exhibit a huge bowing in the band gap energy. Consequently, GaInNAs has been considered as a key material for long wavelength lasers emitting at the optical-fiber communication wavelength window (1300-1550 nm) [1]. Unfortunately, from numerous optical studies it has been concluded that the radiative efficiency of dilute nitrides rapidly degrades with incorporation of nitrogen as a result of N-induced formation of efficient non radiative (NR) defect centers. This efficient NR recombination currently prevents efficient utilization of dilute nitrides in light emitting devices. On the other hand, the recent study by X.J. Wang et al. [2] provides different perspectives on the role of defect-mediated recombination in GaNAs. It shows a possibility to utilize this material as an efficient semiconductor spin-filter which operates at room temperature and does not require using a ferromagnetic metal (or diluted magnetic semiconductors) or applying a magnetic field. It has been shown that the spin filtering effect in GaNAs relies on existence of a paramagnetic defect and selective recombination under Pauli exclusion principle. Therefore, the NR defects decremented in terms of optical properties were found to have an important role in spintronics application.
The work presented in this thesis focuses on detailed characterization of the SDR process in GaNAs alloys. The thesis is organized as follows. In the first chapter we attempt to give a brief review on the present knowledge of electronic properties of GaNAs, mainly from experimental perspectives. The second chapter is devoted to physical mechanisms which govern spin relaxation in semiconductors and to optical methods of generating spin polarization. Basic principles of the spin-dependent recombination (SDR) are introduced in chapter three. Chapter four describes characterization techniques utilized in this work. Finally, experimental results and summary are presented in chapter five and in the final part of the thesis, respectively.
Chapter One:
Fundamental electronic structure of GaNAs
Early absorption measurements [3] [4] [5] [6] have unambiguously demonstrated that GaN𝑥As1−𝑥 is a direct band gap semiconductor, similar to parental GaAs and GaN. However, instead of the expected blue shift from the GaAs band gap with N incorporation, the GaNAs alloy has shown a considerable red shift in the fundamental absorption edge (see figure 1.1.) This was accompanied by the splitting of the conduction band (CB) of GaNAs into two subbands, as demonstrated by the electro- [7] and photo [8] -reflectance measurements. The lower one is usually denoted as E− and represents the CB edge of the alloys, whereas the upper subband is denoted by E+. The E− and E+energies depend on the concentration of nitrogen [N]. With increasing the nitrogen composition, the E− subband shifts towards lower energies while the E+ position increases with [N]. Photo-reflectance measurements have also shown that a spin-orbit splitting energy (∆0) remains independent of the nitrogen concentration.
Two theoretical approaches, i.e. the so-called band-anti-crossing (BAC) model [9] and local-density approximation (LDA) calculations [10], [11], [12] are usually considered to describe the formation of the E+ and E− states under the presence of nitrogen.
The empirical band anti-crossing (BAC) model
According to this model, the formation of the E− and E+states is induced by an interaction between the delocalized CB states of the Γ character and the localized nitrogen-related state EN (EN= 1.65 eV in GaAs.) A magnitude of the splitting is mainly determined by
an inter-band matrix element VMN. The E− sub-band is delocalized, conduction band-like while the E+ state is derived from the localized EN state. The dispersion relation is then given by:
Figure 1.1: Compositional dependence of the band gap energy of III-V-N alloys at room temperature. Lines show result from model calculation. Local density approximation and dielectric model is used.
E±= E𝑔± [( E𝑔− E𝑁 2+ 4𝑉𝑀𝑁2 )1/2− |E𝑔 − E𝑁|]/2,
where E𝑔 is the bandgap of the parental III-V compound before alloying it with N. VMN is the interaction term which depends on nitrogen composition:
𝑉𝑀𝑁 = 𝐶𝑀𝑁𝑥1/2,
Here x refers to nitrogen composition and 𝐶𝑀𝑁 is a constant equal to 𝐶𝑀𝑁 = 2.7eV for GaNAs.
This model is empirical since it only considers the interaction of the CB states with the N level related to an isolated substitutional N atom. It neglects mixing of the CB states introduced by nitrogen and also a complexity of formed nitrogen centers. However, the BAC model provides a simple, analytical expression to describe the GaNAs electronic properties, including the position of E±, a temperature induced shift of the band-gap, an electron effective mass, etc. The position of E± as a function of nitrogen concentration is shown in figure 1.2.
Pseudopotential LDA calculation
The more complicated theory based on pseudopotential provides more general physical explanation of the existence of the sub-bands. It shows that the formation of the E−and E+ states is induced by a strong perturbation by nitrogen of host states resulting in symmetry breaking. The degenerate L and X CB minima are now split into a1 and t1 states. Mixing between the a1(Γ) and a1(L) conduction band states leads to the formation of E−. On the other hand, the
E+ state originates from a weighted average of a1(L) and a1(N) states. According to this model, the interaction between the CB and EN is small and E+ exhibits the L-like characterer. However pseudopotential LDA calculations require a substantial effort and the numerical results are difficult to use.
Figure 1.2; positions of E+ and E− in
GaNAs alloy according to the empirical band-anti-crossing model. E𝐿 refers to the position of the L CB minimum relative to the top of the VB in GaAs.
Band alignment in GaNAs/GaAs hetero-structures
Band alignment in GaNAs/GaAs heterostructures is of importance for optoelectronic and spintronic applications. Both type-I and type-II band alignment in GaNAs/GaAs quantum wells (QWs) has been concluded as schematically illustrated in figure 1.3. However, the type-I band alignment seems to be more probable based on several experimental observations [13]. Firstly, according to the time-resolve photoluminescence measurements, recombination lifetime of electron-hole pairs in the GaNAs QWs is of the same order of magnitude as in bulk GaAs. Since in the type-II structures the recombination lifetime is expected to be longer due to spatial separation of electron-hole pairs, the GaNAs/GaAs structures have the type-I alignment. Secondly, it has been found that the lowest energy photoluminescence (PL) originates from the CB-light hole (lh) transitions, based on the PL polarization measurements in GaNAs/GaAs QWs. This means that holes participating in the recombination should be located in the GaNAs QW as the uppermost VB states in GaAs barriers are heavy hole (hh) states.
Strain-induced splitting of the valence band
At the Γ point (k =0) the top VB states has a total angular momentum 𝐽 = 3/2, due to spin orbit interaction. At this point, the Jz=±3/2 and Jz=±1/2 states are degenerate. But if the
bulk material is subjected to compressive or tensile stain, this degeneracy will be lifted. Indeed, let us consider the in-plane biaxial strain. In this case the in-plane deformation energy (say in a xy plane) will be different from that a z-direction. Hence, symmetry of the system is reduced and the degeneracy is lifted [14]. The result is a splitting between the hh (Jz=±3/2) and the lh (Jz=±1/2) states. Therefore, if a lattice constant of an epilayer material is smaller than that of a
Figure 1.3; schematics of type I and type II band alignment in GaNAs/GaAs QW system. The arrows show dominant recombination transitions, i.e. direct in space of the type I transitions (the solid line) and indirect in space for the type II transitions (the dashed line). Recombination rate is higher in the type I QW which is preferred for optoelectronic applications in terms of high radiative efficiency.
substrate, i. e. when the epilayer is under tensile strain, the VB lh states will lie above the hh states. On the other hand, if the system is under compression, the lh states lie below the hh states (see figure 1.4.)
GaAs is a common substrate for GaNAs. The lattice constant of GaNAs is smaller than that of GaAs and biaxial tensile stain dominates. Thus, in the strained GaNAs the lh VB states usually lie above the hh states.
Figure 1.4; fundamental semiconductor band structure under tensile and compressive strain.The degeneracy of the VB states is lifte and the, lh and hh states are spit.
Confinement effect-induced lh-hh splitting in a QW system
Due to the confinement effect, energy levels in quantum wells are separated into discrete levels. The separation between adjacent levels is determined by a size of the quantum well and by values of effective masses of carriers. By solving the Schrödinger equation for the QW structure it can be shown that the energy positions of levels in the QW are inversely proportional to the QW size and also to effective masses of the carriers [14]. Thus, for a fixed size and a fixed quantum number, the lh VB states, which have the smaller effective mass, will be pulled further down in energy as compared with the heavy hole states. If the system is unstrained, the degeneracy between the lh and hh states is lifted and the lh states have higher energy than the hh states (see figure 1.5.)
lh hh so E(p) tension Eg lh hh so E(p) ∆ unstrained Eg lh hh so E(p) compression p Eg
Radiative recombination process in GaNAs
As mentioned above, one of the promising applications of the III-V-N alloys is for the near-infrared light emitters. Therefore, understanding origin of radiative recombination processes in these materials is of importance. Previous optical measurements have shown that introduction of nitrogen induces strong localization effects. As a result, low temperature PL in these materials is due to localized exciton (LE) recombination [13]. This assignment was based on several experimental observations. (i) A strong red shift of the PL maximum is observed with increasing temperature and exhibits the so-called S-shape behavior (see figure 1.6). The physical explanation of this behavior is as follows. When temperature is increased, carriers in the localized excitonic states can be excited into the delocalized states in CB. The S-shape point is the transition from the localized states to the delocalized states. (ii) A blue shift of the PL maximum is observed when excitation power is increased as a result of filling of higher energy states within the localized states.
700 750 800 850 900 950 1000 1050 1100 1150 6 K 60 K 120 K 180 K 240 K 300 K P L I n te n s it y ( N o rm a li z e d ) wavelength (nm) GaNAs, N = 0.54%
Figure 1.6; PL spectra from GaNAs as a function of temperature. Eg n = 1 n = 2 hh, n = l lh, n = 1
Figure 1.5; quantum size effect. Due to quantum confinement the lh states have higher energy than the hh states. Optical transitions in the well follow selection rules, ∆n=0, ∆m=±1.
Non-radiative (NR) recombination
The radiative efficiency of III-V-N compounds rapidly degrades when nitrogen composition is increased. The observed degradation is commonly attributed to poor structural quality of the N-containing alloys and also to increasing concentrations of non-radiative (NR) defects [15]. For optoelectronic applications, radiative luminescence has to be utilized. Fortunately, according to previous studies the radiative efficiency can be improved by post-growth treatments [13]. Rapid thermal annealing has also been employed to narrow a spectral width of the PL spectrum and to increase the PL intensity.
Spin-dependent recombination
GaNAs shows fascinating spin dynamics of carriers. Specifically, an apparent spin relaxation time has been shown to dramatically increase with increasing temperature and is longer than several ns at room temperature. Long spin polarization life-time in diluted nitrides can be explained by the spin-dependent recombination (SDR) model initially developed by Weisbush and Lampel in AlGaAs [16]. As will be discussed later, this unusual spin dynamics is due to a large concentration of the NR paramagnetic centers induced by the presence of nitrogen.
Chapter Two: Spin dynamics
The success of spintronics relies on the ability to create, control, maintain and manipulate spin orientation over practical time and length scales. Below we will discuss how spin orientation can be created and detected in semiconductors by optical means and also physical mechanisms which govern losses of this orientation.
Optical orientation and spin polarization
During all physical processes a total momentum must be conserved. Therefore, optical excitation with circularly polarized light can provide spin orientation of carriers in a semiconductor. The total spin of electron and hole must be equal to the angular momentum of an absorbed photon. Photons of right or left polarized light have a projection of the angular momentum on the direction of their propagation (helicity) equal to +1 or −1, respectively. Linearly polarized photons are in a superposition of these two states. When a circularly polarized photon is absorbed its angular momentum must be distributed between the photoexcited electron in the conduction band and hole in the valence band. Probability of optical transitions within the dipole approximation is described by a transition matrix element [17]:
𝐷𝑖𝑓 = 𝑓 𝐷 𝑖 ,
where 𝐷 is the dipole moment operator and | 𝑖, 𝑓 refer to an initial and final states, respectively. Wavefunction of the VB holes is p-like whereas the CB electrons have the s-like wavefunction. Due to these symmetry properties the only matrix elements not equal to zero are:
𝑆 𝐷𝑥 𝑋 = 𝑆 𝐷𝑦 𝑌 = 𝑆 𝐷𝑧 𝑍 ,
where | 𝑆 refers to the conduction band state (S symmetry) and | 𝑋, 𝑌, 𝑍 refers to the p-type coordinate parts of the Bloch amplitudes, which transforms as the coordinates x, y, z. Matrix elements for different interband transitions are summarized in table 2.1. Two dipoles rotating clockwise and counter-clockwise in a plane perpendicular to wave vector k (which is usually defined as the direction of the wave vector of light propagating perpendicularly to the semiconductor layer and normally defined to be z-axis) corresponds to hh-c transitions. The lh-c transitions and SO-c transitions correspond to two dipoles oscillating along k and two dipoles rotating in the plane perpendicular to k.
Band Initial (VB) Final (CB) ½ -1/2 hh lh SO +3/2 −3/2 +1/2 −1/2 +1/2 −1/2 − 1/2(𝑥 + 𝑖𝑦) 0 − 2/3 𝑧 1/6(𝑥 − 𝑖𝑦) − 1/3 𝑧 − 1/3(𝑥 − 𝑖𝑦) 0 1/2(𝑥 − 𝑖𝑦) − 1/6(𝑥 + 𝑖𝑦) − 2/3 𝑧 − 1/3(𝑥 + 𝑖𝑦) 1/3 𝑧
Table 2.1; matrix elements of dipole moment for different interband transitions, x and y are unit vector along a plane perpendicular to the momentum k, z is a unit vector along k.
Figure 2.1; selection rules and relative intensities of transitions. I+and I -are the intensities of right-and left polarized emission, respectively.
Since matrix elements of dipole transitions are not equal for the transitions involving lh and hh, relative intensities of these transitions are also different. Therefore, optical absorption of the 100% circularly polarized light in the strain-free material will generate 50% of electron spin polarization in CB, if only the hh and lh VB states are involved – see Figure 2.1. For example, if the pumping light is σ+ –polarized the photogenerated electrons in the conduction band will be preferentially generated in the -1/2 spin state (see figure 2.1) (Note, the two spin states in the conduction band will be populated equally if the photon energy sufficiently exceeds Eg+Δ, where Δ is the spin-orbit splitting). If both photo-excited electrons and holes retain their spin orientations without spin relaxation, the reverse PL process should lead to 100 % σ+polarization
+1/2 -1/2 -3/2 +3/2 -1/2 +1/2 I+ Emission I - I+ I - VB lh hh +1/2 -1/2 -3/2 +3/2 -1/2 +1/2 σ+ σ+ Absorption: σ+ light ∆mj= +𝟏 lh VB hh
in optical detection. If complete spin relaxation occurs between the hole states, e.g. due to strong hh-lh mixing, the polarization degree of optical detection should decrease down to 25 %. The latter is defined as in equation 2.1
𝑃𝑐= I+− I – (I++ I -),
where I+ and I - are the intensities of +- and --polarized emission, respectively. The absolute value of the polarization degree will be twice higher in the strained structures where the degeneracy of the VB states is lifted and only one of the VB states is populated. A sign of the PL polarization will depend on whether the hh or lh states are involved. For example, the PL will be
+
- polarized for the hh transitions, whereas it will be --polarized if the lh states are involved. Spin relaxation of the CB electrons will cause a reduction of the PL polarization degree. Since circular polarization of emission is directly linked to the spin orientation of electrons in the conduction band created via optical orientation, circular polarization can give the information about how much electrons are polarized. We would like to point out that no PL polarization is expected under linear excitation (σx) as equal populations of spin-up and spin down electrons will be generated.
Spin relaxation
Central issue for developing spintronic devices is how to preserve spin orientation of carriers. Understanding of spin relaxation mechanisms is, therefore, required. Spin relaxation, i.e., disappearance of initial non-equilibrium spin polarization, can be generally understood as a result of the action of fluctuating in time magnetic fields. In most cases, these are not real magnetic fields, but rather “effective” magnetic fields originating from the spin–orbit, or, sometimes, exchange interactions. The magnetic field causes spin precession around the field direction. However, as the latter randomly changes in time, the initial spin information will be completely lost after several changes.
Two main parameters describing spin relaxation are spin precession frequency in random magnetic field, ω, and its correlation time 𝜏𝑐, i.e., the time during which the field may be roughly considered as constant. These two parameters are the most commonly used to explain any mechanism of spin relaxation.
Elliot-Yafet Mechanism [17, 18]
The electrical field, accompanying lattice vibrations or the electric field of charged impurities is transformed to an effective magnetic field via the spin–orbit interaction. Thus momentum relaxation is usually accompanied by spin relaxation. For phonons, the correlation Eq. 2.1
time is on the order of the inverse frequency of a typical thermal phonon. Spin relaxation by phonons is normally rather weak, especially at low temperatures.
In the case of charge impurities, scattering by an impurity center causes a change of effective magnetic field and, thus, a change of mean spin direction. As a result, spin relaxation time can be expressed as the function of scattering time and scattering angle. Thus, spin relaxation rate is proportional to impurity concentration.
Dyakonov-Perel Mechanism [20]
The Dyakonov-Perel mechanism is due to spin orbit splitting of the conduction band due to bulk inversion asymmetry (BIA) in non-centro-symetric semiconductors. In this case the precision frequency is dependent on momentum of electron, ω=Ω(p). An electron with a certain momentum will experience an effective magnetic field caused by spin-orbit splitting and hence starts its precession motion around an effective magnetic field axis. If the electron takes long time before relaxing to a lower momentum state, electron will precess long enough to forget an initial spin state. So, the spin relaxation time is mostly controlled by momentum relaxation time at a certain momentum,
1
𝜏s~Ω 2 𝑝 𝜏p,
where Ω(p) has three components along crystals axis,
Ωx(p)~px(py2− pz2), Ωy(p)~py(pz2− px2), Ωz(p)~pz(px2− py2).
In contrast to the Elliott–Yafet mechanism, now the spin rotates not during, but between the collisions. Accordingly, the relaxation rate increases when the impurity concentration decreases.
Bir-Aronov-Pikus Mechanism [21]
Bir-Aronov-Pikus mechanism is mainly caused by the exchange interaction between electrons in the conduction band and holes in the valence band. According to this mechanism, spin relaxation rate is proportional to a number of holes. In other words, spin relaxation of an electron in the CB is contributed by exchange interaction with all electrons in the VB.
Spin relaxation of holes in the valance band
In the VB, spin relaxation time is mainly controlled by the spitting between the lh- and hh states. One can say that the hole “spin” J is rigidly fixed with respect to its momentum p, and
because of this, momentum relaxation leads automatically to spin relaxation. For this reason, normally it is virtually impossible to maintain an appreciable non-equilibrium polarization of bulk holes and their spin direction is absolutely random.
Chapter Three: Spin Dependent Recombination (SDR)
General principles of SDR
As was mentioned in chapter one, GaNAs shows fascinating spin dynamics of carriers at room temperature. Long spin polarization life-time in diluted nitrides can be explained within the framework of the SDR model initially developed by Weisbush and Lampel in AlGaAs [16]. The SDR mechanism is due to the well known Pauli principle which states that two electrons cannot have the same spin orientation in the same orbital state. The key point in SDR is the existence of a deep paramagnetic center which possesses an unpaired electron before trapping a CB electron. As a consequence, if the photogenerated electron in the CB and the resident electron on the deep center have the same spin, the photo-generated electron cannot be captured by the center. The SDR effect can be explained as the following:
1. In the absence of photoexcitation, an electron at a deep defect level is not spin polarized. The center can only capture a photogenerated electron from the CB with a spin antiparallel to the spin of the electron present at the center (figure 3.1 a)).
2. Once the deep paramagnetic center is occupied by two electrons, no more electrons can be captured by this center until one of electrons (of either spin) recombine with a VB hole (figure 3.1 b)).
3. Even though the capture process of the CB electron by the center is spin-selective, recombination of electrons trapped by the center with the VB holes is not. Thus, after a few circles electrons left on the center become dynamically spin-polarized.
4. When the center is polarized, it acts as a spin-selective filter. The photogenerated CB electron with a spin direction opposite to that of the center is immediately captured and recombines with the VB holes. As a result, only the electrons with spins parallel to that of the center are left in the CB. (Figure 3.1 c)).
5. The same spin orientation of conduction and defect electrons prevents the former from being captured by the defect, resulting in higher concentrations of free carriers. The only remaining recombination channel under these conditions is band-to-band recombination- see figure 3.1 d).
Figure 3.1 a); photo-generated electrons are created by circular polarization excitation; populations of the spin up and spin down states are not equal due to optical selection rules. Electrons with a spin direction opposite to that of a paramagnetic center are immediately captured. Due to the Pauli exclusion principle, capture of the electrons with opposite spin is blocked. b) Once the center forms a singlet state, the capture process is stopped. One of the captured electrons with either spin recombines with a VB hole. The paramagnetic center is again ready to capture electrons. c) After some circles the electron trapped by the paramagnetic center is dynamically polarized. Any electron with the spin direction opposite to that of the trapped electron is extracted from the CB which results in complete spin polarization of the CB electrons. d) The same spin orientation of conduction and defect electrons blocks the defect-related recombination channel. Carrier recombination is only possible as a result of band-to-band transitions.
Optical orientation in the presence of a deep paramagnetic center
Figure 3.2 demonstrates effects of the SDR process on the band-to-band recombination. If the excitation light is linearly polarized, photogenerated electrons are not polarized. Thus, the capture process by the center is efficient. Under these conditions the recombination via the center efficiently competes with the band-to-band recombination and hence the corresponding PL intensity is low. On the other hand, when the excitation light is circularly polarized, the electrons are photo-generated with preferential spin orientation. Therefore, the electron trapped by the centers can be dynamically polarized and, hence, the capturing process is blocked. Therefore, the band-to-band recombination is more efficient, resulting in the higher intensity of the PL signal.
VB
CB
3.1 d)VB
CB
3.1 c)Forbidden
VB
CB
3.1 b)STOP
P
𝜎
+VB
CB
3.1 a)
Figure 3.2; optical excitation and recombination transitions observed under linear (the left part of the Figure) and circular (the right part of the Figure) excitation, respectively.
SDR ratio
An SDR ratio is used as an indicator of the SDR process since in this case a total PL intensity under linear excitation always lower than that under circular excitation. The SDR ratio is defined as in equation 3.1:
𝑆𝐷𝑅 𝑟𝑎𝑡𝑖𝑜 =𝐼𝜎+𝑜𝑟 𝜎− 𝐼𝜎𝑥 ,
where, 𝐼𝜎+𝑜𝑟 𝜎− and 𝐼𝜎𝑥 are total PL intensities detected under the circular and linear excitations
respectively. When the SDR process dominates, the SDR ratio will be higher than one. This ratio indicates to what degree the capture of the photogenerated electrons by the center can be blocked when the centers are dynamically polarized and, hence, the SDR ratio is proportional to the spin polarization of the CB electrons.
Several processes affect the dynamic behavior of the photogenerated carriers. Firstly, the depletion rate of free carriers is determined by their capture by the centers. This process is spin-dependent. Secondly, the recombination between the trapped electrons and the VB holes is also essential. Since the VB holes rapidly loose their spin orientation, this process is spin independent. The band to band recombination also contributes to carrier dynamics. All these processes can be described by the following coupled nonlinear rate equations 3.2 [2]:
𝑑𝑛± 𝑑𝑡 = −𝛾𝑒𝑛±𝑁∓− 𝑛±− 𝑛∓ 2𝜏𝑠 + 𝐺±− 𝑛± 𝜏𝑑, 𝑑𝑁± 𝑑𝑡 = −𝛾𝑒𝑛∓𝑁±− 𝑁±− 𝑁∓ 2𝜏𝑠𝑐 + 1 2𝛾𝑝𝑁↑↓, 𝜎+ VB CB 𝜎+ 𝜎𝑥 VB CB 𝜎− 𝜎+ Eq. 3.1
𝑑𝑝
𝑑𝑡 = −𝛾𝑝𝑁↑↓+ 𝐺++ 𝐺−−
𝑛+− 𝑛− 𝜏𝑑 ,
𝑁𝑐 = 𝑁↑↓+ 𝑁++ 𝑁−, 𝑝 = 𝑛++ 𝑛−+ 𝑁↑↓,
where 𝑛± denotes the number of photogenerated electrons with spin up and down respectively. 𝑁± is the number of the paramagnetic centers with a single spin up/down electron, 𝑁↑↓ is the number of the centers that already form a singlet state and 𝑁𝑐 is the total number of the defect centers contributing to the spin filtering process. 𝛾𝑒(𝛾 ) is a capture coefficient for electrons (holes), which is characteristic for the defect. 𝜏𝑠 (𝜏𝑠𝑐) is spin relaxation time of the free (trapped) electrons. The density of free holes is denoted by p. 𝐺± is the photo-generation rate of the spin up and spin down electrons. τd denotes the free carrier decay time, including all radiative and spin-independent non-radiative recombination channels except that via the paramagnetic center.
The SDR process is power dependent. Before the defects start to spin filter the free electrons they need to be dynamically polarized. This is only possible if the number of the photogenerated electrons is higher than the number of the defect centers.
Two spin pools picture
Let us simplify description of the SDR process by considering two spin pools as schematically shown in Figures 3.4 a) and b). Suppose that the CB electrons can be subdivided into two sub groups, spin up and spin down pools. In the SDR process, once the center is polarized, the pool with the spin orientation opposite to the center will be depleted. The capture process is very fast initially, however, when all the centers are occupied, the capture process will stop. Therefore, this decay process of the CB electrons is controlled by how fast the trapped electrons recombine with the holes.
After one pool is depleted, another decay process will start. This decay process is controlled by spin relaxation. As shown in figure 3.4 c) if the electron flips its spin, it will be transferred to another pool and quickly depleted by the center (see figure 3.4 d)). Thus, the PL decay contain two components related to the hole capture and electron spin relaxation.
Figure 3.4 a) and b); the idea of two spin pools, the deep paramagnetic center acts as a spin filter depleting electrons in the spin up pool, decay time of this process is mainly controlled by hole recombination life time. c) and d). After spin up pool is empty the PL decay time is controlled by spin relaxation which causes spin flips to the other pool.
Physical realization of an efficient spin dependent-recombination process
Since the proposal of SDR, slightly enhanced electron polarization Pe in e.g. AlGaAs and
GaAs by optical orientation has been demonstrated at low temperatures and was attributed to an SDR process via deep-level defects. However, it is only until very recently that a giant Pe was
achieved at RT in Ga(In)NAs [22, 23]. From cw- and time-resolved PL, it was shown that very high degree of circular PL polarization can be achieved in optical orientation experiments in these alloys, in sharp contrast with the parental GaAs. Moreover, according to the time resolved PL measurements [22], this circular polarization in GaInNAs remained practically constant within the measurement window of 2 ns in GaNAs QW whereas it rapidly decreased with a
UP DOWN
Capture by center
Capture by hole
HOLE Spin flip due to spin relaxation
UP DOWN HOLE 100% spin polarization 3.4 c) UP DOWN Capture by hole HOLE SLOW 3.4 b) UP DOWN Capture by center HOLE FAST 3.4 a) 3.4 d)
characteristic time of 50 ps in the N-free QW (see figure 3.5). This indicated that nitrogen incorporation caused an apparent increase of the spin relaxation time.
The physical mechnism behind this finding as being due to the SDR process was suggested by V.K. Kalevich et. al. [23] who found that the strong PL polarization is accompanied by the strong SDR ratio and that both of them can be suppressed in transvered magnetic fileds.
Figure 3.5 a); PL and PL polarization of the N-free and N-containing QW sample with N=0.6%. b) Decays of the circular polarization detected from the same samples as in a) [22].
Figure 3.6 a); calculated energy levels associated with the electronic and nuclear spin states of the Gai2+ defect. The
allowed ESR transitions (mS=±1 and mI=0) occur when the electron spin splitting matches the microwave photon
energy, and are marked by the vertical lines. b)Typical ODMR spectra by monitoring the total intensity of the BB PL from an RTA-treated GaN0.021As0.979 epilayer, obtained at 3K under x and + excitation at 850 nm. The
microwave frequency used is 9.2823 GHz. A simulated ODMR spectrum of the identified Gai defect (denoted by Gai
-C) is also shown. From Ref.[2]
In order to identify the SDR-active defects and thus to provide the first unambiguous proof that the strong Pe can indeed be generated by SDR, a combination of optical orientation
with the optically detected magnetic resonance (ODMR) technique was employed [2]. ODMR is known to be sensitive to SDR, especially if SDR acts as a dominant carrier recombination channel, and also to be able to identify chemical nature of defects in semiconductors. A Gai2+ self-interstitial was unambiguously identified as the common core of the defects responsible for the monitored SDR, based on a hyperfine structure – see Figure 3.6 This study has shown a promising way to construct a spin filter by introducing a suitable defect center to a semiconductor material.
Chapter Four: Experimental Approach
Optical orientation PL spectroscopy
PL spectroscopy is a contactless and non-destructive method of probing the electronic structure of a semiconductor. The principle of the PL measurements is simple: when a semiconductor sample is optically excited with a photon having energy above the band gap of the semiconductor, electrons and holes are created, usually in the near surface region by absorption of the excitation light. These photogenerated electrons and holes diffuse into the bulk and at the same time, they relax and recombine via various channels. Some of the most important applications of the PL spectroscopy are listed below:
Bandgap determination: Radiative transitions in semiconductors can occur between states in the conduction and valence bands, with the energy difference equal to the bandgap energy. This can be used to determine the bandgap energy of the semiconductor provided that the origin of the PL transitions is proven to be the band-to-band recombination.
Impurity levels and defect detection: Radiative transitions in semiconductors often involve localized defect levels. The PL energy associated with these levels can be used to identify specific defects, and the PL intensity, if calibrated, can be used to determine their concentration.
Recombination mechanisms: The return to equilibrium, known as recombination, can involve a radiative recombination process. Properties of the corresponding PL such as its line shape, dependences of the PL intensity on photo-excitation power and temperature can be used to understand the origin of the radiative recombination.
Material quality: In general, competing NR processes are associated with localized defect levels, whose presence is harmful to material quality and subsequent device performance. As the NR processes compete with PL, material quality can be evaluated by quantifying an amount of radiative recombination.
A typical PL set-up can be divided into three parts; an excitation side, a cryostat, and a detection side.
Excitation side: An excitation source is used to create photogenerated electron-hole pairs. In our experiments, a tunable Ti-sapphire laser was used for these purposes. Resonance cavity conditions in a Ti-sapphire rod can be adjusted to tune the laser wavelength within the range of 750-1100 nm. Long Wavelength Past (LWP) and Short Wavelength Past (SWP) filters were used to avoid any leakage of stray laser light.
A focusing lens was placed close to a cryostat window. Since the SDR process is sensitive to the excitation power, the laser beam was focused. For this purpose, a fine adjustable lens holder was used.
Cryostat: PL measurements are often performed at low measurement temperatures, i.e around T = 3 - 6 K. This can be achieved by using liquid He which flows into a cryostat where a sample is placed. The cryostat usually consists of four chambers. A vacuum chamber with vacuum in the range of 10−6 mbar is used to thermally isolate other chambers from the environment. The 2nd chamber contains liquid nitrogen to pre-cool the sample chamber down to the temperature of around 160 K. The 3rd chamber will be filled with liquid He. The helium chamber and the sample chamber are linked via a needle valve. The He flow between these chambers can be controlled by using a mechanical pump. The temperature can be adjusted in the range of 2-9 K by changing the flow rate. To achieve temperatures from 10 to 300 K, a heater is used.
Detection side: The PL signal from a sample can be collected and focused by two lenses on an entrance slit of a monochromator. A photodetector will then register the PL emission. Two types of detector were used in this work, i.e. a Si charge-coupled device (CCD) and a Ge detector. The former is sensitive in the spectral range of 300 – 1050nm, whereas the later can be used to detect emissions with longer wavelengths up to 1600 nm. By scanning the monochromator through the desired energy range, and registering the intensity, a PL spectrum is obtained.
Lock-in technique
The PL signals were detected using the lock-in technique. By modulating the intensity of the excitation light beam at a certain frequency and using a lock-in amplifier, stray light, that is not connected to the PL emission, will be discriminated – see Figure 4.1.
Figure 4.1; lock-in amplifier technique; only a signal in-phase with reference is amplified and noise can be subtracted.
The PL set up can be easily modified for optical orientation measurements by placing appropriate polarizers in detection and excitation optical paths. At the excitation side, the excitation beam is circularly polarized by installing a linear polarizer followed by a quarter wave plate. If optical axis of the quarter wave plate is rotated ±45 degree from the optical axis of incoming linearly polarized light, right-handed (σ+) and left-hand (σ -) polarized light (see figure 4.2) will be produced. On the detection side, a quarter wave plate combined with a linear polarizer are again used to monitor a polarization state of the PL emission. The experimental set-up used for optical orientation measurements is shown schematically in figure 4.3.
Figure 4.3; photoluminescence set up for optical orientation PL measurements, the set up is a typical PL set up with retarders installed to produce/ detect circularly polarized light.
Ar+ laser Ti: sapphire Linear polarizer 𝜆/4 retarder Focusing lens Lock-in amplifier Chopper sample
Cryostat Collection lens Focusing lens
Linear
retarder
Monochrometer Detector
Computer
Figure 4.2; the principle of a retarder. The principle of retarder; the figure represents quarter wave plate retarder, in case that linearly polarized input is deviated 45 degree from optical axis, the output will be circularly polarization according from the chance in phase delay speed when light passes though the retarder.
Magnetic resonance technique
Electron Paramagnetic Resonance (EPR)
EPR is a spectroscopic method1 which can be used to study paramagnetic systems, i.e. systems containing one or several unpaired electron. An unpaired electron has an intrinsic spin angular momentum S with an associated magnetic moment 𝒎 = −𝜇𝐵𝒈𝒆𝑺 where 𝑔𝑒 is the electron g-factor and 𝜇𝐵 is the Bohr magneton. If the unpaired electron is placed in a static magnetic field B, the so-called Zeeman interaction energy between the applied field and the magnetic moment is given by the classical expression E = -m·B. This expression can be represented by the spin Hamiltonian
𝑯 = 𝜇𝐵𝒈𝒆∙ 𝑺 ∙ 𝑩.
There are two allowed directions of the electron spin S=1/2, parallel or antiparallel to the direction of the static magnetic field B, which is applied along the z-axis. These can be represented by the two spin states |12, 12 and, |12, − 12 respectively, having the corresponding spin quantum number 𝑚𝑠 = ±1/2. The degeneracy of the energy level of the electron will thus be removed in the presence of the external field as shown in Figure 4.4, in which the level is split into two sublevels. In order to induce a transition between two sublevels, an electromagnetic field with energy quanta hν with the time-dependent component perpendicular to the static magnetic field is applied. In thermal equilibrium the population difference between the two spin states is given by the Boltzmann distribution
𝑁−
𝑁+= exp(−𝑔𝑒𝜇𝐵𝐵𝑧/𝑘𝐵𝑇),
𝑁− and 𝑁+ are the populations of the different spin states. At normal temperature (T< 300 K), a slight difference in the relative population between the two states is anticipated, and hence it is possible to induce a transition. If an energy quantum hν is absorbed the following condition must be satisfied according to Planck’s law
𝜈 = 𝜇𝐵𝒈𝒆∙ 𝑺 ∙ 𝑩.
This relation defines the basic resonance condition in the EPR experiment. The energy separation between the two sublevels depends on the magnitude of the applied magnetic field. Note that the
1 It is also reffered as Electron Spin Resonance (ESR).
Eq. 4.1
Eq. 4.2
value of the g-factor in a semiconductor deviates from the free electron value ge = 2.0023. This is
due to the effect of the orbital angular momentum L of the unpaired electron. The g-value is usually anisotropic, i.e. the magnitude depends on the direction of the static magnetic field relative to the orientation of the paramagnetic centre.
Figure 4.4; microwave induced transition and corresponding EPR signal of two cases: a) S=1/2, I=0 and b) S=1/2, I=1/2.
In practice, an EPR setup basically consists of several essential parts such as a cavity where the sample is placed, a magnet to produce the static magnetic field and an electromagnetic source, usually with a fixed frequency in the range of 9-95 GHz. The resonance occurs when the energy separation between two states caused by an applied magnetic field is identical to the microwave energy – Equation 4.3. In order to increase the sensitivity of the spectrometer, a small AC component is added to a DC magnetic field. This results in an AC modulated EPR signal, which can be detected using a lock-in amplifier. Consequently, the EPR spectrum is recorded as the first derivative of microwave intensity dI/dB reflected from the cavity versus the magnetic field B (see Figure 4.4.)
The EPR technique has been successfully applied to study the electronic structure and identification of defects in semiconductors. In a more general form, Equation 4.1 can be rewritten as
𝑯 = 𝜇𝐵𝒈𝒆∙ 𝑺 ∙ 𝑩 + 𝑺 ∙ 𝑫 ∙ 𝑺 + 𝑺 ∙ 𝑨𝒊 𝒊
∙ 𝑰𝒊.
Here 𝑺 and 𝑰𝒊 represent an effective electronic spin and a nuclear spin of a defect or ligand atom i. the anisotropy of the g-tensor and D-tensor reflects the symmetry of the crystal
E E +1/2 -1/2 -1/2 +1/2 b) dB dI B B mS=+1/2 mS=-1/2 B dB dI S=1/2 S=1/2 B mS = -1/2 mI I=1/2 a) mS=+1/2 Eq. 4.4
lattice and the defect. The first term describes the usual linear term of electron Zeeman interaction. The second term introduces a fine structure, i.e. zero-field splitting, important only for S > 1/2. The most important information in EPR experiments is usually obtained from the hyperfine coupling, which is due to the interaction between the magnetic moments of the effective electronic spin S and the nuclear spin Ii, which is described by the third term in
Equation 4.4. The nuclear Zeeman term and the other higher order terms have not been included in Equation 4.4 due to their negligible effects in most cases of EPR and ODMR investigations.
Let us consider a simple case (S=1/2) with an isotropic g-factor and an isotropic central hf interaction which is small compared to the electron Zeeman interaction. Equation 4.4 can be solved by a perturbation theory in the first order and for energies one obtains
𝐸 = 𝑔𝜇𝐵𝐵𝑚𝑆+ 𝐴𝑚𝐼𝑚𝑆,
with eigenfunctions Ψ = | 𝑚𝐼, 𝑚𝑆 . For S=1/2, I=1/2 there are now four energy levels instead of two without the hf interaction because of the mS = ±1/2 and mI = ±1/2 quantum numbers.
Application of a microwave field induces the EPR transitions with the selection rule ∆𝑚𝑠 = ±1, ∆𝑚𝐼 = 0.
Instead of one transition observed in the case of I=0, there are now two lines with the separation between them ∆B=A/(gμB).
In general, there are 2I+1 lines due to hf splitting. By analyzing the hf pattern obtained from EPR spectra we are able to identify the chemical and electronic properties of the defect and its surroundings. One excellent example of this can be found in ref. [15] where two Ga interstitials were ambiguously identified based on the analysis of the hf interaction. Gallium consists of two isotopes, 69Ga with 60.4% abundance and 71Ga with 39.6% abundance. Both isotopes have a nuclear spin I=3/2. The hf interaction of each isotope with an unpaired electron gives rise to four transitions with the contribution to the relative intensity following the ratio of natural abundances. Due to the difference in their nuclear magnetic moment, the two Ga isotopes give rise to slightly different hf splittings leading to a characteristic hf structure shown in Figure 4.5. This is the signature of the hf interaction involving a Ga atom.
Though EPR shows a great potential in defect indentification, it still has some limitations. First of all, a paramagnetic ground state is required. Some defects have zero-spin ground state and thus cannot be detected in EPR. Secondly, no information on carrier recombination related to defects can be obtained by EPR. And thirdly, the sensivity is low due to microwave detection. Fortunately, the first limitation can be overcome if another charge state of the same defect can be reached by changing the Fermi level position, e.g by doping with shallow impurities or by Eq. 4.5
illuminating samples. The last two limitations could only be resolved if another methods or extended techniques are used. A combination of EPR with optical detection methods such as PL is a solution to the problems.
Optically Detected Magnetic Resonance
Optically detected magnetic resonance is a combination of EPR and PL. The technique is based on the fact that the recombination processes are spin-dependent. When the microwave field induces transitions between two Zeeman sublevels that have different recombination rates or polarizations, the total PL intensity or its polarization can be changed. ODMR spectrum is obtained by a measurement of this change versus magnetic field (Figure 4.6). Figure 4.7 shows a schematic illustration of an ODMR setup.
ODMR measurements not only preserve all the potentials from conventional EPR and PL but also add more advantages. It is more sensitive due to higher sensitive optical detection over the microwave detection. This advantage makes ODMR suitable for studies of thin films, layered and quantum structures. The radiative recombination spectrum from PL measurements of a deep level defect often shows up as a broad featureless band, from that, very little information can be obtained. By measuring the spectral dependence of the ODMR signal, the ODMR spectrum can be assigned to the relevant PL spectrum of a specific defect, which is suitable for studies of carrier recombination processes and for assigning them to corresponding defects.
Typical ODMR set up is shown in Figure 4.7. The sample is placed in a microwave cavity inside a cryostat to obtain liquid He temperature. Liquid He is continually supplied by a He transfer tube which is connected to a liquid He reservoir. A static magnetic field is provided by a magnetic coil. A microwave field with a frequency of about 9.5 MHz (X-band) is generated
3200 3400 3600 (a) (b) (c) 95GHz O D M R Int ens it y (a. u. ) Magnetic field (mT)
Figure 4.5; (a) simulation at 95GHz of the hf splitting arising from the interaction between an unpaired electron and the nuclear spin of a Ga atom. The contribution from (b) 69Ga and (c) 71Ga.
in the cavity by a microwave generator through a waveguide. The microwave frequency is always kept constant and the static magnetic field can be swept from 100 to 10000 Guass by applying a current to the magnetic coil. Optical detection is used. For these purposes, the PL emission from the sample is excited by a laser (Ti-Sapphire for GaNAs) and is detected by a detector without a monochromator. The PL signal is detected by selecting proper LWP and SWP filters.
Figure 4.7; schematic illustration of an ODMR set-up.
Ar+ laser Ti: sapphire Linear polarizer 𝜆/4 retarder Focusing lens Lock-in amplifier Chopper
Collection lens Focusing lens
Detector
Computer Microwave
generator
Chopper
Static magnetic field
E B B S=1/2 𝑛1 1 𝑛2 2 k2 k1 ∆I
Figure 4.6; principle of an ODMR experiment. Microwave field induces a transition between two sublevels. The ODMR signal is obtained as a change of total light ∆I~∆k∆n.
n1, n2 populations of the spin at sublevels.
∆k= k1- k2 the difference in recombination rates of
two sublevels.
∆n number of carriers transferred between two sublevels due to microwave field.
Chapter Five: Experimental Results
Defect Engineering for Spin Filtering Effect in GaNAs
According to previous findings, GaNAs is a very promising material for spin filtering which can be accomplished at room temperature and without application of an external magnetic field. However, the fundamental knowledge on how to control and optimize this ability is still lacking. The purpose of this work is to understand effects of growth parameters and structural design on the formation of deep centers responsible for SDR aiming at optimization of spin filtering. First of all, the effect of quantum confinement on the spin filtering ability is examined. This is performed under conditions of electron injection from GaAs barrier layers, i.e. under conditions relevant to device applications. Secondly, optimization of the fabrication conditions (i.e. growth temperature and post-growth annealing) for efficient formation of spin-filtering defects is performed. Effects of doping on the formation of these defects are also analyzed. And finally, defect formation during different epitaxial processes, such as molecular beam epitaxy growth (MBE) and metal-organic chemical vapor deposition (MOCVD), is also studied.
5.1 Defect engineered spin filter from a low dimensional semiconductor structure; spin filtering effect in QW structures
In this set of measurement, PL and ODMR measurements under optical orientation were performed for a set of GaNAs multiple quantum well (MQW) structures. Parameters of the samples are summarized in table 5.1.1. The measurements were performed at room temperature (300 K) for optical PL orientation and at 6 K for ODMR. Excitation laser beam was aligned parallel to a growth direction.
Sample QW width GaAs barrier width GaAs buffer layer GaAs capping layer Growth method Growth Temperature Substrate Nitrogen composition (%) GaAs band-(300 K) GaNAs peak (300K) 2521 30 Å 7-period 202 Å 2500 Å 500 Å MBE 420 C Semi-insulating GaAs 1.6 875.5 nm 956.5 Nm 2522 50 Å 7-period 202 Å 2500 Å 500 Å MBE 420 C Semi-insulating GaAs 1.6 875.5 nm 987.0 Nm 2523 70 Å 7-period 202 Å 2500 Å 500 Å MBE 420 C Semi-insulating GaAs 1.6 875.5 nm 997.0 Nm
Table 5.1.1; parameters of the structures, all samples contain 7-periods GaN0.016As0.984/GaAs QWs grown on a semi-insulating GaAs substrate with a GaAs capping layer.
The excitation wavelength was 827 nm. Under these conditions most of the carriers participating in the band-to-band recombination in the GaN0.016As0.984 QWs are injected from GaAs, since a total thickness of the GaAs barriers is much larger than that of the GaNAs QWs. For GaAs, such excitation can be used for optical orientation as the spin-orbit--split VB states do not participate in the absorption process and a preferential spin orientation of the CB electrons is created. Circular polarization and the SDR ratio are detected via the GaNAs- related band-to-band PL emission, to determine the ability of this material to spin filter the injected electrons.
Figure 5.1.1; PL spectra of the GaNAs/GaAs QW samples. The low energy PL band originates from the band-to-band emission in the GaNAs QWs.
PL spectra of the investigated samples are shown in figure 5.1.1. The strong band-to-band emission was observed at room temperature for all samples, allowing the SDR measurements. The results of these measurements are summarized in figure 5.1.2, taking as an example the 2521 sample. The SDR ratio of 1.23 at the PL peak position and a circular polarization degree of 12% were observed for the excitation power W of 200 mW.
2524 90 Å 7-period 202 Å 2500 Å 500 Å MBE 420 C Conducting GaAs 1.6 875.5 nm 1003.5 nm 800 850 900 950 1000 1050 1100 2521 N = 1.6% 30 A 2522 N = 1.6% 50 A 2523 N = 1.6% 70 A 2524 N = 1.6% 90 A P L I nt en si ty ( A rb it ar y un it ) wavelength (nm)
Full spectrum 2521, 2522 and 2523 multi-QW(300 K)
exc. wavelength 827 nm Exc. power = 200 mW
Figure 5.1.2; results of optical orientation measurements performed for the 2521 sample.
Fig. 5.1.3; power dependences of the SDR ratio (a) and the circular polarization degree (c-d) measured for the investigated QW samples. 0 50 100 150 200 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 a) 2521 L = 30 A 2522 L = 50 A 2523 L = 70 A 2523 L = 90 A c ir c u la r p o la ri z a ti o n c ir c u la r p o la ri z a ti o n c ir c u la r p o la ri z a ti o n excitation power (mW) excitation power (mW) excitation power (mW) S D R r a ti o excitation power (mW) -50 0 50 100 150 200 250 -0.10 -0.05 0.00 0.05 0.10 b) Exc: 0 50 100 150 200 0 5 10 15 20 c) Exc:+ 0 50 100 150 200 250 0 -5 -10 -15 -20 d) Exc: 900 960 1020 900 960 1020 900 960 1020 900 960 1020 1.0 1.2 1.4 1.6 -5 0 5 10 15 20 -20 -15 -10 -5 0 5 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3
P
L
I
nt
en
si
ty
(
A
rb
.U
ni
ts
)
Exc: x 2521 T: 300 K C ir cu la r p ol ar iz at io n C ir cu la r p ol ar iz at io n Exc: Det: + Exc: Laser power: 200 mW Excitation wavelength : 827 nm Det: + C ir cu la r p ol ar iz at io n Exc:+ Wavelength, (nm) S D R R at io ( / Det: + L = 30 ABoth SDR ratio and circular polarization degree decrease with decreasing excitation power, as shown in figure 5.1.3. The same trend was observed for all investigated structures: the SDR ratio and the PL polarization degree increase with W and saturate at high excitation powers. This can be explained by dynamic polarization of paramagnetic defect centers. When W is low, a number of photogenerated electrons is lower than a number of deep paramagnetic centers. Therefore, the centers are not polarized and photogenerated electrons are rapidly depleted by the centers. After increasing the excitation power the number of electrons becomes high enough to polarize the centers and the SDR process dominates. Further slight increase of W will not affect the center polarization and the SDR ratio and spin polarization saturate.
Also obvious from Figure 5.1.3, the saturation value of the SDR ratio increases with increasing width of the QW. Possible explanations for this effect are as follows.
1. The effect of band-to-band recombination; quantum confinement- induced enhancement of the band-to-band recombination rate in narrow QWs.
In narrow QW structures, strong confinement-induced overlap of electron and hole wave functions promotes the efficient band-to-band recombination. This would suppress importance of the defect-related recombination and the SDR process and would degrade the spin filtering.
2. The effect of electron spin relaxation; acceleration of electron spin relaxation in narrow QWs.
Spin relaxation usually accelerates in narrow QWs, promoted via the Dyakonov-Perel mechanism [20]. This would diminish the ability of the injected electrons to polarize the defect centers which would, in turn, lead to a reduction of the SDR ratio.
To identify chemical nature of the deep paramagnetic defects responsible for the SDR process in the investigate samples and to determine the origin of the observed degradation of the spin filtering efficiency in the narrow QWs, ODMR measurements were performed. A typical ODMR spectrum is shown in Figure 5.1.4 a). Similar spectra were also detected from other structures. The ODMR spectrum shows the following two distinct features originated from different defects. The first one is a single strong line situated in the middle of the ODMR spectrum, with a g-value close to 2. Due to a lack of hyperfine (HF) structure, unfortunately, the chemical nature of the corresponding defect cannot be identified. Below we shall simply refer to it as the “unknown 1” defect. The second feature of the ODMR spectra consists of a complicated pattern of lines spreading over a wide field range. Such multiple ODMR lines arise from a high
