**2.1 Fundamental concept of DLTS**

### **2.1.1 Capacitance transient**

490 Solar Cells – New Aspects and Solutions

On the other hand, similar electrical properties were obtained in InGaAsN grown by metalorganic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE) despite the large difference in the density of residual impurities, which excludes them as a main cause of low mobilities and short minority carrier lifetimes. For that, lattice defects, essentially related to the N atom, were expected to be the main reason of such degradation. Several theoretical and experimental studies have investigated carrier traps in InGaAsN films. Theoretically, using the first principles pseudo-potential method in local density approximation, four N-related defects were proposed: (AsGa-NAs)nn, (VGa-NAs)nn, (N-N)As, and (N-As)As ( Zhang & Wei, 2001). While the two first structures were supposed to have lower formation probabilities, the two split interstitials (N-N)As, and (N-As)As were suggested to compensate the tensile strain in the film and to create two electron traps at around 0.42 and 0.66 eV below the conduction band minimum (CBM) of InGaAsN with a band gap of 1.04 eV, respectively (Zhang & Wei, 2001). Experimentally, the ion beam analysis provided a quantitative evidence of existence of N-related interstitial defects in GaAsN (Spruytte et al., 2001; Ahlgren et al., 2002; Jock, 2009). Furthermore, several carrier traps were observed in GaAsN and InGaAsN using deep level transient spectroscopy (DLTS). A deep level (E2/H1), acting as both an electron and a hole trap at 0.36 eV below the CBM, was observed (Krispin et al., 2001). Other electron traps in GaAsN grown by MBE were recorded: A2 at 0.29 eV and B1 at 0.27 eV below the CBM of the alloy (Krispin et al., 2003). In addition, a well known electron trap at 0.2 0.3 eV and 0.3 0.4 eV below the CBM of *p*-type and *n*-type GaAsN grown by MOCVD were observed, respectively (Johnston et al., 2006). Although the importance of these results as a basic knowledge about lattice defects in GaAsN and InGaAsN, no recombination center was yet experimentally proved and characterized. Furthermore, the main cause of high background doping in p-type films was

Chemical beam epitaxy (CBE) has been deployed (Yamaguchi et al., 1994; Lee et al., 2005) to grow (In)GaAsN in order to overcome the disadvantages of MOCVD and MBE. It combines the use of metal-organic gas sources and the beam nature of MBE. (In)GaAsN films were grown under low pressure and low temperature to reduce the density of residual impurities and to avoid the compositional fluctuation of N, respectively. Furthermore, a chemical N compound source was used to avoid the damage of N species from N2 plasma source in MBE. Although we obtained high quality GaAsN films gown by CBE, the diffusion length of minority carriers is still short (Bouzazi et al., 2010). This indicates that the electrical properties of GaAsN and InGaAsN films are independent of growth method and the problem may be caused by the lattice defects caused by N. Therefore, it is necessary to investigate these defects and their impact on the electrical properties of the film. For that, this chapter summarizes our recent results concerning lattice defects in GaAsN grown by CBE. Three defect centers were newly obtained and characterized. The first one is an active non-radiative *N*-related recombination center which expected to be the main cause of short minority carrier lifetime. The second lattice defect is a N-related acceptor like-state which greatly contributes in the background doping of p-type films. The last one is a shallow

To characterize lattice defects in a semiconductor, several techniques were used during the second half of the last century. Between these methods, we cite the thermally stimulated

not completely revealed.

radiative recombination center acceptor-like state.

**2. Deep level transient spectroscopy** 

To fully understand DLTS, it is worth to have a basic knowledge of capacitance transients arising from the SCR of Schottky contacts or *p+-n*/*n+-p* asymmetric junctions. If a pulse voltage is applied to one of these device structures that is originally reverse-biased, the SCR width decreases and the trap centers are filled with carriers (majority or minority depending on the structure). When the junction is returned to reverse bias condition, the traps that remains occupied with carriers are emptied by thermal emission and results in a transient decay. The capacitance transients provide information about these defect centers. Here, we restrain our description to a *p+-n* junction where the p-side is more much heavily doped than the *n*-side, which gives the SCR almost in the low doped side.

The causes of change in capacitance depend on the nature of applied voltage. In case of reverse biased voltage, the junction capacitance, due to the change in SCR width, is dominant. However, when the applied voltage is forward biased, the diffusion capacitance, due to the contribution of minority carrier density, is dominant. The basic equation governing the capacitance transient in the *p+-n* junction is expressed by

$$\mathbf{C(t)} = A \sqrt{\frac{\varepsilon \mathbf{z}\_b e N\_{\mathrm{D}}}{2(V\_{\mathrm{R}} + V\_{\mathrm{s}})}} \left[ 1 - \frac{N\_r}{2N\_{\mathrm{D}}} \exp\left(-\frac{t}{\tau}\right) \right] = C\_o \left[ 1 - \frac{N\_r}{2N\_{\mathrm{D}}} \exp\left(-\frac{t}{\tau}\right) \right] \tag{1}$$

where A is the contact area, *V*b is the built-in potential, 0 is the permittivity of the semiconductor material, and e is the elementary charge of an electron. *C*0, *N*T, *N*D, and 

Investigation of Lattice Defects in GaAsN

**2.2.2 Double carrier pulse DLTS** 

biased junction to fill the trap with holes.

*Vip*

*Vp*

0

*VR*

(*e*

**2.2.1 Isothermal capacitance transient spectroscopy** 

the first method, the medium trap density *<sup>T</sup> N xij* ( ) at a point *ij x* is given by

where Cij is the amplitude difference of the two capacitance transients.

*<sup>T</sup> ij*

*N x*

Grown by Chemical Beam Epitaxy Using Deep Level Transient Spectroscopy 493

ICTS is used to analyze the profiling of lattice defects in the SCR of the semiconductor. It can be done through three different methods. The first one is obtained by fixing *V*R and varying *V*<sup>p</sup> to build difference of transients among the SCR of the device. The second method is evaluated by measuring at constant *V*p and varying *V*R. The last option is obtained by varying *V*R and *V*p, where the profiling analysis is also possible without building difference of transients. Using

*D ij*

*C LL*

*N A C*

 2 2 0

DC-DLTS is used in asymmetric *n+-p* or *p+-n* junctions (Khan et al., 2005). It aims to check whether a trap is a recombination center or not. As shown in Fig. 1, two pulsed biases are applied to the sample, in turn, to inject majority and minority carriers to an electron trap. At the initial state, the junction is under reverse bias, and the energy level *E*T of the trap is higher than the Fermi level (*E*Fn).When the first pulse voltage is applied to the sample, *E*Fn is higher than *E*T, which allows the trap to capture electrons. During the second reverse biased pulse, with a duration *tip*, holes are injected to the SCR from the *p*side of the junction. After the junction pulse is turned off, electrons and holes are thermally emitted. The amount of trapped carriers can be observed as a change in the DLTS peak height of the trap. If the trap captures both electrons and holes, the DLTS maximum of the corresponding level decreases compared with that in conventional DLTS. Such a decrease is explained by the electronhole

*h*) recombination process, which indicates that the level is a recombination center.

*tp*

(2) *Majority carriers' injection*

(1)*Quiescent reverse bias*

*p*<sup>+</sup> *n*

Fig. 1. Basic concept of capture and thermal emission processes from an electron trap located at an energy level *E*T in *p+-n* junction. A saturating injection pulse is applied to the reverse

*p*<sup>+</sup> *p n* <sup>+</sup> *n*

*tip* 

(3) *Minority carriers' injection*

(4)*Beginning of thermal emission*

*p*<sup>+</sup> *n*

*R Pi Pj*

3 22

2 ( ) ( ) ( ) (9)

denote the junction capacitance at reverse bias, the density of filled traps under steady state conditions, the ionized donor concentration, and the time constant that gives the emission rate, respectively. The change in capacitance after the recharging of traps is given by

$$
\Delta \mathbf{C} = \mathbf{C}\_o \sqrt{1 - \frac{N\_r}{N\_o}} \tag{2}
$$

In most cases of using transient capacitance, the trap centers form only a small fraction of the SCR impurity density, i.e., *N*T << *N*D. Hence, using a first-order expansion of Eq. (2) gives

$$\left|\Delta\mathbf{C}\right| = \mathbf{C}\_{0}\left|\mathbf{1} - \mathbf{N}\_{\mathrm{T}}\{\mathbf{2}\mathbf{N}\_{\mathrm{D}}\} \right| \equiv \mathbf{C}\_{0}\mathbf{N}\_{\mathrm{T}}\{\mathbf{2}\mathbf{N}\_{\mathrm{D}}\} \tag{3}$$

Thus, the trap concentration calculates from the capacitance change C is expressed by

$$N\_{\rm T} = 2\frac{\Delta \mathcal{C}}{\mathcal{C}\_0} N\_{\rm D} \tag{4}$$

Note that Eq. (3) assumes that NT << ND and the traps are filled throughout the total depletion width. To be more accurate, NT should be adjusted to NTadj according to [30]

$$N\_{\text{Taj}} = 2 \frac{\Delta \mathcal{C}}{\mathcal{C}\_0} N\_D \frac{W\_\text{\textdegree}}{L\_1^2 - L\_2^2} \tag{5}$$

where *W*R is the total SCR at reverse bias voltage *V*R, *L*1 = *W*R - , *L*2 = *W*p - , and

$$
\mathcal{A} = (\frac{2\varepsilon\varepsilon\_0}{e^2 N\_D} (E\_{\parallel} - E\_{\parallel}))^{\natural 2} \tag{6}
$$

where *W*p, *E*F, and *E*T denote the SCR at *V*p, the Fermi level, and the trap energy level.

### **2.1.2 Thermal emission of carriers from deep levels**

The emission rates for electrons and holes are given, respectively by

$$
\sigma\_n = \sigma\_n \upsilon\_{\text{shr}} N\_c \exp\left(-\frac{E\_{\text{CRM}} - E\_r}{kT}\right) \tag{7}
$$

$$\sigma\_{\nu} = \sigma\_{\nu} \upsilon\_{a\wp} N\_{\upsilon} \exp\left(-\frac{E\_r - E\_{\text{VRM}}}{KT}\right) \tag{8}$$

where *n, N*c, and *vthn* are the thermal capture cross section, the density of states, and the thermal velocity of holes, respectively. *p, Nv*, and *vthp* are the same parameters for holes. *ECBM*, *EVBM*, and *ET* are the energy levels of the conduction band minimum, the valence band maximum, and the trap, respectively.

#### **2.2 Other DLTS related techniques**

The isothermal capacitance transient spectroscopy (ICTS) and the double carrier pulse DLTS (DC-DLTS) are two DLTS related methods. They are used to obtain the density profiling of lattice defects and to check whether they act as recombination centers or not, respectively.
